Total Synthesis of Verruculogen

Part I: Synthetic Studies in Sesquiterpenes,

Part II: Total Synthesis of Verruculogen

A thesis presented

Dane Holte

The Scripps Research Institute Graduate Program

in partial fulfillment of the requirements

for the degree of

Doctor of Philosophy

in the subject of

Chemistry

for

The Scripps Research Institute

La Jolla, California

August 2014

iii [page # place holder for thesis acceptance form]

Dedicated to my brother, because you’re a pretty alright brother.

v Acknowledgements

I have to start by acknowledging my parents, Don and Sophia, without your constant love

and support over 27 years I couldn’t have made it to where I am now. Thank you

both. And to Dylan: isn’t education fun?

Phil, the opportunity to work in your lab, and under your guidance, has been one of the

most significant events in my life. Thank you for giving me the freedom to go off on

tangents and tackle the problems that I think are most intriguing, and thank you for

bringing me back to the big picture and keeping me on track.

To my committee, Professors Ryan Shenvi, Julius Rebek, Jin-Quan Yu, Valery Fokin,

and Carlos Guerrero: thank you all for the advice over the past five years.

To my undergraduate mentors: SonBinh, the ISP program truly shaped how I think about

science, your advice has always been appreciated. OPP, I appreciate you giving me a

summer job doing synthesis when I needed one, and supporting me when I moved to

a new lab and to graduate school. Karl, and the rest of the Scheidt lab, the most

important lessons I learned about doing science I picked up from you all during my

last two years at Northwestern, these skills have helped carry me through graduate

school.

Seth Bitter, Andrew Candelaresi, and Doug Gray: I appreciate you all dealing with my

infrequent emails and phone calls. I’m sorry I’ve dropped off the face of the planet. I

aspire to be less busy in the future, we should hang out. Having you all as friends has

been wonderful.

vi Matt Beirbaum: I’m sorry, and please don’t make me say I’m sorry again, because I’m at

least as, if not more, sorry than you are; and Gene Lee.com: until our next adventure,

stay hungry, eat well.

To my climbing partners, Jeeves, Soviet, Matt Grieving, and Matt Weiss: You’re all

prolific scientists, climbers, and friends. I’d gladly let you hold me on a string a few

hundred feet in the air. Climbing has kept me sane, and your various outlooks on life

and science have really made me think.

Kit and Nick, you were great first year roommates and Scripps hasn’t been the same

without you. Your advice over the years, both chemical and otherwise, has been

really helpful to me.

Jamal: You’ve been a huge influence. My political awareness has been vastly improved,

and my rate of recycling is at an all time high. Living with you for two years was

great. Also: it gets better (or so I hear, I wouldn’t know).

Erik Wold: I feel prepared to pursue an alternate career in avante garde Mexican food

technology. Lets get a burrito ASAP; and Sarah: gaming with you has brought out a

competitive edge that I was unaware I had.

Will: When its time to party Will will party hard… and then stay up talking about science

until at least 3 am. Your genuine curiosity about everything is inspiring.

Brady: Scripps hasn’t been the same since u been gone. I need a home cooked meal, a

belly full of beer, and a good laugh. I should also be running more. I’ve gotten fat in

your absence.

Hans: considering you didn’t speak to me for the first two years I was in Baran Lab, we

became excellent friends. Thanks for the good music, good beers, and subtle humor.

vii Dan: You brought the heat, and candy, when it was needed most; and Kanny, you and

Dan continued to bring the heat. You’ve both kept me calm.

Emily: You’ve been like a lab sister to me. Having you as a staple in lab for the past five

years has been comforting. You’re someone I can count on, and I’m envious of your

productivity; and Paul: you were a great friend inside of lab and out. I learned a lot

from you during my first few years, and have always enjoyed talking about music,

even when on shuffle.

Brandon: Thanks for the all the adventures and going deep into cocktails with me.

Yellowknife remains on the bucket list. Rejoice because you’re trying your best!

Matt Villaume: You’ve helped me more than you know. LOL?

Art: “Yes I like piña coladas, and getting caught in the rain…”

Nathan: Your wisdom levels are high. I always appreciate your sound, sage-like advice.

Steve: So long and thanks for all the booze.

Florina: You are a willful woman.

Rodrigo: Being hoodmates with you has been fun. I’m glad we could finish this together.

Yoshi: Thanks for teaching me a lot when I first started, and thanks for keeping this ship

afloat these days.

Tim: Thanks for all your help during my early years. Your intelligence, productivity, and

enthusiasm for chemistry is boundless.

Ian: Even though I’ve tried to keep it loud, lab has been too quiet without you.

Taycoh: Is it too early for a drink?

Dr. Darryl David Dixon and Janice, when I die, you can have my car. This document is

legally as good as a will, right?

viii Shun and Abraham: I learned so much from you two, you deserve your own category.

Yuki, Eddie, Yeh: I hereby apologize for being loud and obnoxious. I don’t know how

you put up with me.

Danny Götz: I learned more from you about doing thorough science than anyone else.

Thanks so much for everything; it was truly a pleasure working, writing, and hanging

out with you. Magical kugelschreiber!

Jochen: I owe you a great deal of thanks, your energy kept me motivated for a final push.

Feng Yu, Ippei, and Klement: it was a pleasure to work with each of you.

Baran Labbers not mentioned (past and present): I’m so sorry I didn’t mention you. There

are 35,436 people in this lab and you’ve all helped me a lot. Grad school has been so

much fun because all the people in the lab have been like, awesome, bro.

Gillian: Come down on the street and dance with me.

To all my friends:

None of this is going anywhere – pretty soon we’ll all be old,

and no one left alive will really care about our glory days, when we sold our souls.

But if you’re all about the destination, then take a flight.

We’re going nowhere slowly, but we’re seeing all the sights.

And we’re definitely going to hell, but we’ll have all the best stories to tell.

ix TABLE OF CONTENTS

Acknowledgements ...... v

Table of Contents ...... ix

List of Figures ...... xiii

List of Schemes ...... xiv

List of Tables ...... xvi

List of Abbreviations ...... xvii

Abstract ...... xxii

Part I – Synthetic Studies in Sesquiterpenes ...... 1

Chapter 1. Progress Toward the Total Synthesis and Transannular Cyclizations of α-

Humulene ...... 2

1.1 Background, Biosynthesis, and Cyclase Phase ...... 3

1.2 Humulene ...... 5

1.3 Attempts at the Transannular Cyclizations of Humulene ...... 8

1.4 Progress Toward the Total Synthesis of Humulene ...... 14

1.5 Experimental ...... 31

1.6 Spectra ...... 44

Chapter 2. An Approach to Mimicking the Sesquiterpene Cyclase Phase by Nickel-

Promoted Diene/Alkyne Cooligomerization ...... 76

2.1 Introduction to Oxidative Cyclization in the Context of Terpene Synthesis ....77

2.2 Planning and Historical Context ...... 79

2.3 Butadiene Cooligomerizations ...... 85

x 2.4 Innate Reactivity of 4Z,7Z,10E-Cyclodecatrienes ...... 94

2.5 Isoprene Cooligomerizations ...... 95

2.6 Delayed Installation of C1 Units onto 4Z,7Z,10E-Cyclodecatriene 2.20c ...... 99

2.7 Conclusion ...... 101

2.8 Experimental ...... 111

2.9 Spectra ...... 137

Chapter 3. Macrocyclic Sesquiterpenes: The Fallout ...... 294

3.1 On the Mechanism, Reactivity, and Selectivity of the Ni-Catalyzed [4+4+2]

Cycloadditions of Dienes and Alkynes ...... 295

3.2 Scalable, Enantioselective Synthesis of Germacrenes and Related

Sesquiterpenes Inspired by Terpene Cyclase Phase Logic ...... 300

Part II – Verruculogen and Fumitremorgin A ...... 319

Chapter 4. Total Syntheses of Demethoxyverruculogen and Verruculogen ...... 320

4.1 Introduction ...... 321

4.2 Pennicilium verruculosum ...... 324

4.3 Installation of the Peroxide ...... 327

4.4 Synthesis of Demethoxyverruculogen ...... 336

4.5 Efforts Toward Scalable Production of 6-Methoxytryptophan ...... 341

4.6 First Generation Total Synthesis of Verruculogen ...... 344

4.7 Experimental ...... 351

4.8 Spectra ...... 372

xi Distribution of Credit ...... 417

Curriculum Vitae ...... 419

xii List of Figures1

Figure 1.1: Selected examples of terpene natural products ...... 3

Figure 1.2: The many faces of humulene ...... 6

Figure 1.3 Humulene crystal structures ...... 7

Figure 2.1: Reaction analysis flowchart for nickel catalyzed cooligomerizations ...... 88

Figure 3.1: Gibbs free energy profiles: cooligomerization ...... 296

Figure 3.2: Gibbs free energy profiles: [2+2+2] cycloadditions ...... 298

Figure 3.3: Gibbs free energy profile: butadiene/2-butyne cooligomerization ...... 299

Figure 4.1: Initial spore growth of Pennicilium verruculosum ...... 325

1 Figure and Scheme titles have been abbreviated for clarity.

xiii List of Schemes

Scheme 1.1: Abridged biosynthesis of terpene natural products ...... 4

Scheme 1.2: General biosynthesis of humulene ...... 9

Scheme 1.3: Selective functionalizations of humulene ...... 10

Scheme 1.4: Reactivity of bromohumulene 1.20 ...... 12

Scheme 1.5: Reactivity of miscellaneous humulene derivatives ...... 13

Scheme 1.6: Previous total syntheses of humulene ...... 16

Scheme 1.7: Retrosynthesis and stability of humulene ...... 17

Scheme 1.8: Synthesis of ring closing metathesis precursor 1.41 ...... 19

Scheme 1.9 Alternate ring closing metathesis strategies ...... 21

Scheme 1.10: Ring contraction approach to humulene and aza-Wacker ...... 23

Scheme 1.11: Initial experiments toward an allyl-allyl coupling reaction ...... 24

Scheme 1.12: Allyl-allyl macrocyclic coupling reaction ...... 25

Scheme 1.13 Attempted allylboronate olefin coupling ...... 26

Scheme 2.1: Oxidative cyclization of dienes onto metallacycles ...... 77

Scheme 2.2: Retrosynthetic analysis of eudesmane natural ...... 78

Scheme 2.3: Synthesis of cyclic carbon skeletons from butadiene by nickel catalysis ....81

Scheme 2.4: A controlled Ni0-catalyzed cooligomerization of isoprene ...... 82

Scheme 2.5: Catalytic Cycle as Proposed by Wilke ...... 84

Scheme 2.6: Selected butadiene/alkyne cooligomerization products ...... 87

Scheme 2.7: General Reactivity of 4Z,7Z,10E-Cyclodecatriene 2.20c ...... 95

Scheme 2.8: Isoprene cooligomerization model study ...... 96

Scheme 2.9: Installation of “methyls” on a macrocycle derived from butadiene ...... 100

xiv Scheme 3.1: Retrosynthetic outline ...... 302

Scheme 3.2: Enantioselective synthesis of the coupling partner 3.29 ...... 304

Scheme 3.3: Various cyclization attempts on substrates 3.30, 3.32, and 3.33 ...... 306

Scheme 3.4: Attempts at an asymmetric NHK reaction ...... 307

Scheme 3.5: Synthesis of germacrane-type sesquiterpenes ...... 310

Scheme 3.6: Transannular cyclization to other sesquiterpenes ...... 313

Scheme 4.1: Biosynthesis of verruculogen (4.1) and fumitremorgin A (4.2) ...... 322

Scheme 4.2: Literature precedent for installation of a cis-diol ...... 323

Scheme 4.3: Studies on the stabliliy of natural verruculogen (4.1) ...... 326

Scheme 4.4: Retrosynthetic analysis of demethoxyverruculogen (4.18) ...... 328

Scheme 4.5: Synthesis of prenylated compounds for late stage peroxidation ...... 330

Scheme 4.6: Mukaiyama hydration reactions ...... 331

Scheme 4.7: Preparation of protected prenyl peroxides ...... 333

Scheme 4.8: Installation of the first eight-membered ring endoperoxide 4.50 ...... 333

Scheme 4.9: Acylation of TBDPS-peroxide 4.46 ...... 336

Scheme 4.10: Permutations of reactions to access demethoxyverruculogen (4.18) ...... 338

Scheme 4.11: An unusually stable peroxyacetal ...... 339

Scheme 4.12: Dihydroxylation of indole 4.65 ...... 339

Scheme 4.13: Four possible routes to 6-methoxytryptophan (4.67) ...... 343

Scheme 4.14: Endoperoxide on a substrate derived from methoxytryptophan ...... 345

Scheme 4.15: Total synthesis of verruculogen (4.1) ...... 347

xv List of Tables

Table 2.1: Ni0-catalyzed butadiene/alkyne cooligomerization ...... 91

Table 2.2: Ni0-catalyzed isoprene/alkyne cooligomerization ...... 98

Table 3.1: Key ring closure of 3.29 via an umpolung allylation ...... 308

xvi List of Abbreviations

Å = angstrom

Ac = acetyl

AcOH = acetic acid acac = acetylacetonate

AIBN = azobis(iso-butyronitrile) b = broad

BF3OEt2 = boron trifluoride diethyl etherate

BHT = butylated hydroxytoluene = 2,6-bis(1,1-dimethylethyl)-4-methylphenol n-BuLi = n-butyl lithium

ºC = degrees celcius

CAN = ammonium cerium nitrate cat. = catalyst or catalytic

Cbz = carboxybenzyl

Cy = Cyl = cyclohexyl

δ = chemical shift d = doublet dba = dibenzylideneacetone

DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene

DDQ =2,3-dichloro-5,6-dicyano-1,4-benzoquinone

DIBAL = diisobutyl aluminum hydride o-DCB = ortho-dichlorobenzene

DCM = dichloromethane (CH2Cl2)

xvii DMAP = (4-dimethylamino)pyridine

DMF = N,N’-dimethylformamide

DME = 1,2-dimethoxyethane

DCE = 1,2-dichloroethane

DMP = Dess–Martin periodinane

DMS = dimethyl sulfide

DMSO = dimethylsulfoxide

ESI-TOF = electrospray ionization-time of flight

EDC = 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide

EtOAc = ethyl acetate

EtOH = ethanol equiv = equivalents g = gram hν = UV irradiation

HRMS = high resolution mass spectrometry

HMDS = hexamethyldisilazide

HPMA = hexamethylphosphoramide

Hz = hertz

IBX = ortho-iodoxybenzoic acid

IR = infrared imid = imidazole

NaHMDS = sodium hexamethyldisilazide

LC/MS = liquid chromatography/mass spectrometry

xviii LAH = lithium aluminum hydride

LDA = lithium diisopropylamide

LiHMDS = lithium hexamethyldisilazide

L-selectride = lithium tr1.(sec-butyl)borohydride

m = multiplet

M = molar

M+ = molecular ion

m-CPBA = meta-chloroperoxybenzoic acid

MeOH = methanol

mg = milligram

modp = tert-butyl (Z)-3-hydroxy-4-morpholino-4-oxobut-2-enoate mp = melting point

MOM = methoxy methyl ether

Ms = methansulfonyl

µL = microliter

NBS = N-bromosuccinimide

NCS = N-chlorosuccinimide

NMR = nuclear magnetic resonance

OPP = pyrophosphate

[O] = oxidant

PhH = benzene (C6H6)

PhMe = toluene (C7H8)

PMA = phosphomolybdic acid

xix ppm = parts per million

PTLC = preparative thin layer chromatography

py. = pyridine (C5H5N)

q = quartet

Rf = retention factor

s = singlet

t = triplet

tBuMePhos = 2-D1.tert-butylphosphino-2'-methylbiphenyl

TBS = tert-butyldimethysilyl

TBDPS = tert-butyldiphenylsilyl

TEA = triethylamine (Et3N)

TEMPO = 2,2,6,6-tetramethyl-1-piperdinyloxy free radical

TES = triethylsilyl

Tf = trifluoromethanesulfonate

TFA = trifluoroacetic acid

TFAA = trifluoroacetic anhydride

TIPS = triisopropylsilyl

p-TsOH = para-toluenesulfonic acid

THF = tetrahydrofuran (C4H8O)

TLC = thin layer chromatography

TMS = trimethylsilyl

TMSOTf = trimethylsilyltrifluoromethane sulfonate

Ts = para-toluenesulfonyl

xx ZrCl4 = Zirconium Tetrachloride

xxi Abstract

The two phase approach to terpene synthesis has been established as a powerful

approach to thinking about terpene retrosynthesis in the Baran Laboratory.

Retrosynthetically, this approach mimics Nature, removing oxidation states to a parent

skeleton (oxidase phase), and then “un-cyclizing” these skeletons to simple precursors

(cyclase pahse). Part I of this thesis is dedicated to the cyclase phase approaches toward

sesquiterpene macrocycles in low oxidation states. This section begins with studies on the

synthesis and cyclizations of humulene, an eleven-membered macrocycle. From there, it

transitions into a nickel-catalyzed cooligomerization methodology aimed at producing

ten-membered macrocycles from dienes and alkynes. To this end, the germacrene family

of natural products was pursued. This section is wrapped up with computational evidence

that supports the observations regarding the nickel-catalyzed diene/alkyne

cooligomerization. Finally, to conclude the terpene section of this thesis, a second

strategy toward germacrene sesquiterpenes is discussed, utilizing a Pd-catalyzed

allylation.

In Part II of this document, the total synthesis of the natural product verruculogen

is pursued. Verruculogen is a highly oxidized indole alkaloid diketopiperazine natural

product which is a tremorigenic mycotoxin. To this end, a method for forming a unique

eight-membered ring containing a peroxyhemiaminal functionality is uncovered, and the

order of steps required to synthesize the natural product is established. This lays the groundwork for a fully optimized synthesis of Verruculogen.

xxii

PART I

Synthetic Studies in Sesquiterpenes

CHAPTER 1

Progress Toward the Total Synthesis and

Transannular Cyclizations of α-Humulene

2 1.1 Background, Biosynthesis, and Cyclase Phase

In perhaps the most dramatic example of divergent synthesis, Nature utilizes

1 simple five-carbon (C5) building blocks to fashion tens of thousands of natural products

known collectively as terpenes and possessing a diverse array of physical and biological

properties (for selected examples of both simple and complex terpene structures, see

Figure 1.1).2,3 Terpenes have long been utilized as flavors and fragrances, from floral to

citrus and peppermint. The direct use of isoprene in the form of natural rubber,

polyisoprene (1.1, Figure 1.1), by the ancient Mesoamericans dates back to as early as

4 1600 B.C. Today the majority of isoprene is obtained from the C5 cracking fractions of

petroleum refining and is mostly used in the production of synthetic polymers.5

OH Me H Me Me Me OH n Me Me Me Me Me Me H Me Me HO cis-1,4-polyisoprene (1.1) (natural rubber) humulene (1.2) illudol (1.3)

AcO O Me OH Me Me NHBz O Me Me Me H H OH Me Me Me Ph O OH Me H H Me H OH Me Me H O Me OH OH OH OH OH Me BzO AcO germacrene A (1.4) pygmol (1.5) vinigrol (1.6) Taxol® (1.6)

Figure 1.1: Selected examples of terpene natural products: polyisoprene (natural rubber, 1.1) humulene (1.2), illudol (1.3), germacrene A (1.4), dihydrojuneol (1.5), vinigrol (1.6), and Taxol®(1.7).

Organic chemists have held a long-term fascination with terpenoids because of their structural diversity, challenging molecular frameworks, and opportunities for the invention of methods and strategies in organic synthesis.1,6 Some of the earliest work in the Baran laboratory involved the synthesis of prenylated indole alkaloids,7 and our first work on pure terpenes, namely vingrol (1.6), was published in 2008.8 We have held a

3 longstanding interest in the syntheses of both simple, e.g. pygmol (1.5)9 and extremely

complex, e.g Taxol® (1.7),10 terpene natural products.

Me Me Me cyclase phase monoterpenes Me OPP Me Me OPP DMAPP (1.8) geranyl pyrophosphate (1.10) OPP IPP (1.9) Me

OPP Me

OPP dimerization Me Me Me higher order squalene terpenoids Me OPP farnesyl pyrophosphate (1.11)

cyclase phase

Me Me H H Me Me Me Me Me

Me Me Me !-bisabolene (1.12) germacrene A (1.4) humulene (1.2)

Scheme 1.1: Abridged biosynthesis of terpene natural products, with emphasis on sesquiterpene natural products.

Our interests in terpene natural products led us to deeply consider their biosyntheses.1 In Nature, γ,γ-dimethylallyl pyrophosphate (DMAPP, 1.8) and isopentenyl

pyrophosphate (IPP, 1.9) are first oligomerized to relatively simple linear precursors like

geranyl pyrophosphate (1.10) and farnesyl pyrophosphate (1.11, Scheme 1.1). Additional

oligomerizations with IPP (1.9) can lead to higher-order linear terpenes.11 Following

prenyl chain extension, carbocationic cyclizations can afford monocyclic compounds,

such as β-bisabolene (1.12), germacrene A (1.4), and humulene (1.2). After building the carbocyclic skeleton, Nature employs both oxidase enzymes and non-enzymatic oxidation and rearrangement processes to take the unadorned carbocycles to various oxidation states.

Artificially mimicking the cyclase phase of terpene biosynthesis can inspire the invention of new methodologies, since working with carbogenic frameworks containing

4 minimal functionality limits the chemist’s toolbox of synthetic strategies. The first goal

of my Ph.D. was to mimic Nature’s approach for rapidly building molecular complexity

in the context of macrocyclic sesquiterpene natural products.

1.2 Humulene

Sesquiterpenes, comprised of three isoprene units (C15), have seen a myriad of synthetic endeavors due to their molecular complexity and wide biological activities.

Two representative sesquiterpene macrocycles, humulene (1.2) and germacrene (1.4), have rich synthetic histories,12-14 yet could still benefit from being viewed under a new light. At the onset of my Ph.D. work, I sought to view these natural products under the scrutiny of a two-phase strategy: cyclase and oxidase phases. Taking inspiration from

Nature, the cyclase phase aims to rapidly assemble a minimally oxidized carbogenic precursor, which can be carried forward into the oxidase phase, adorning the molecule with requisite oxidations. The Baran laboratory anticipated that approaching synthesis in this manner would provide meaningful quantities of these natural products and will also encourage the development of new cyclization and oxidation methodologies that, after development, could then be applied to more complex di- and triterpenes.

Humulene (1.2) is an eleven-memebered macrocyclic sesquiterpene primarily isolated from the oil of hops and the oil of cloves. Studies on humulene date back to

1951,15 and it was isolated as early as 1895.16 One of the reasons humulene remains so

fascinating is that despite over 100 years of research, I would argue that there is still a

poor understanding of its structure and chemistry – for reasons I will detail below. To

5 date, there is no comprehensive review on this topic, but I will cover a few of the most

interesting aspects here.

2 2 2 3 Me 3 Me 3 Me Me 5 Me 5 Me 5 Me 5 Me 10 6 Me 10 Me 6 Me 10 6 10 9 9 11 9 Me 9 Me Me

2 Me 6 2 Me 6 2 6 3 5 3 5 5 3 5 Me Me Me Me 9 9 11 9 10 Me 10 10 Me 10 9 Me Me Me Me Me Me

2E,5E,9E-humulene, "-humulene (1.13) #-humulene (1.14) 2E,5E,9Z-humulene (1.15) !-humulene (1.2)

Figure 1.2: The many faces of humulene, α- (1.2), β- (1.13), γ- (1.14) and E,E,Z- humulene (1.15).

In many of the early papers on humulene, there is a great deal of confusion over the identity of the molecule, both in nomenclature and structure. Several early papers refers to humulene as didymocarpene,16 and in multiple publications it has been referred

to as α-caryophyllene.17,18 It should be noted that β-caryophyllene is the current common name for a bicyclic isomer of humulene.19 Throughout this document, I use “humulene”

to refer to the IUPAC compound (1E,4E,8E)-2,6,6,9-tetramethylcycloundeca-1,4,8- triene, α-humulene (1.2, Figure 1.2), although β- and γ-humulene (1.13 and 1.14, respectively) are positional olefin isomers of humulene20,21 and the Z-olefin isomers (e.g.

E,E,Z-humulene 1.15) are biosynthetically relevant.22 In keeping with my preferred literature numbering (as there are multiple in use), not IUPAC numbering, I have defined the geminal dimethyl carbon to be carbon 1. Having worked with these compounds for a

long time, I still find the colloquial nomenclature confusing, however the IUPAC

nomenclature is tedious.

In addition to the early naming issues, the first years of humulene research were

dedicated to structure elucidation through chemical degradation.23 This was eventually

6 solved, but not before improperly assigning the molecule.18 Even after the chemical formula was assigned through degradation and confirmed with 1H NMR,24 the exact orientation of the double bonds remained a debatable issue: trans,trans,cis25 (e.g. 1.15)

versus all-trans stereochemistry (e.g 1.2).26 However, this debate was put to rest when an x-ray crystal of a humulene silver nitrate adduct was obtained.27 Interestingly, I was able to reproduce this experiment, obtaining the crystal 1.16 shown below (Figure 1.3a).

a. Ag O O Ag O N O O O O Crystal structure of a silver Ag Ag Ag nitrate humulene complex N O Me O Me Me Me 1.16

Me Me Me Me rotate molecule Me 3 "olefin flips" Me Me Me Me Me Me Me Me Me Me Me humulene (1.2) "ent-humulene" (1.2b)

Me Me Me Fujita's humulene crystal Me structure utilizing the crystalline sponge method single "olefin flip" 1.2a

Figure 1.3: a. Humulene-silver nitrate complex (1.16) crystal structure; b. Humulene (1.2) conversion into “ent-humulene” (1.2b).

In addition to the silver nitrate–humulene complex 1.16, a crystal structure was

obtained28 by the Fujita group using their “crystalline sponge” method.29 Through careful

analysis of these two crystal structures, several interesting observations can be made. In

humulene, the olefin π-systems face inward/outward (known as a “peripheral olefin”).

Thus, any reagent that reacts with the olefin reacts from the outer face, not from inside

the ring. On the other hand, transannular cyclizations occur from the inside of the ring. In

addition, at room temperature, humulene’s alkenes can undergo a “flip” as depicted in

Figure 1.3b. For this reason, some early humulene literature suggests that it does have an

7 optical activity, as humulene (1.2) and “ent-humulene” (1.2a) are distinct structures when it cannot undergo ring flipping (low temperature).30 It is possible that nature makes two distinct humulene enantiomers in the presence of enzymatic macrocyclization, but that optical activity is degraded under ambient conditions. Interestingly, both 5,6- epoxyhumulene (1.19, Scheme 1.3) and 9,10-epoxyhumulene show substantial optical activity.31 These details are likely to have stereochemical implications during any

transannular cyclization event.

The conformations of the alkenes in humulene have their own nomenclature

(further complicating that issue as well!) and a significant amount of computational work

has been done in this area.32 Indeed, it is hypothesized that different ring flipped conformations of humulene can give rise to different humulene derived terpene skeletons.33,34 Similarly, each of the three humulene oxides have been studied experimentally, and have been shown to react differently during transannular cyclization.35 In fact, humulene triepoxide 1.18 exists as several stable, isolable, compounds which have been characterized.36,37 However, further discussion on these subtle details remains out of the scope of my thesis, and the simplistic description

provided in Figure 1.3b provides enough detail. These structural aspects of humulene

made it all the more intriguing as a target for synthesis and cyclization.

1.3 Attempts at the Transannular Cyclizations of Humulene

When I joined the laboratory, I was handed Scheme 1.2, which is a simplified

version of humulene-derived natural products biosynthesis, where oxidations have been

omitted. My notes from Phil during this time include his draft for a single-figure paper. In

8 the proposed Figure 1a I would prepare grams of humulene and in Figure 1b I would have a starburst from humulene accessing a multitude of natural product skeletons – simple! Even as a naïve first year student this seemed ambitious, but I had hope that I could uncover new substrate and reagent controlled cyclizations.

H Me Me Me Me Me a path b Me f path g Me Me H Me b Me Me Me g Me Me Me Me humulene (1.2) protoilludane cerapicane sterpurane

path a path f

Me Me Me Me Me Me Me i path c Me path i Me Me Me d h Me c Me Me Me Me illudane hirsutane cucumane

path d path h

Me Me Me Me Me path e e Me Me Me Me Me Me Me Me lactarane marasmade pleurotellane

Scheme 1.2: General biosynthesis of humulene derived natural products and my initial goal.

It is worth mentioning that Matsumoto and others have produced a substantial body of literature on the “controlled” cyclizations of humulene, however they will not be reviewed in detail at this juncture. A noteworthy example is shown in Scheme 1.3, wherein 17 products are observed when humulene is subjected to aqueous H2SO4

(conditions d).38 Many of these “more selective” transformations are simply low yielding

or lengthy syntheses, a few examples are cited here in lieu of a published review.39-42

Jumping right into the thick of it, I purchased five grams of humulene (1.2) from

Sigma-Aldrich and commenced my investigations (Scheme 1.3). There is only one

selective one-step cyclization of humulene known in the literature, namely the reaction of

9 humulene montmorillonite K10 in DCM to afford isodaucane skeleton 1.17 (conditions

a).43 This reaction was first reproduced, and then other solvents and additives were screened in attempts to change this cyclization, but reactions either proceeded solely to isodaucane 1.17 or did not proceed at all. Humulene triepoxide 1.18 and humulene-5,6- oxide 1.19 were both prepared through stoichiometric control of the m-CPBA (conditions b and c). It is also worth noting that humulene undergoes an air oxidation to humulene-

5,6-oxide when left under ambient conditions for ~weeks.44 Humulene undergoes a

chemoselective allylic bromination with NBS in MeCN to afford bromohumulene 1.20

(conditions e). The reactivity with enophile 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD)

is similar, reacting exclusively with the 5,6-olefin to afford PTAD-adduct 1.21

(conditions f). It is noteworthy that all functionalizations of humulene typically begin

with the 5,6 double bond due to a slight torsion of the double bond, making it more

reactive than the 2,3 or the 9,10 double bond.34

Me Me Conditions Screened: 1 Me 5 g. ~170 reactions Acids: 2 2 Me 6 a. K10 Clay 3 5 attempted Me no selective TiCl4; BF3!OEt2; FeCl3; ZnCl2; 6 cyclizations 1 9 CuBr2; AgOTf; Et2SBr!SbCl5Br DCM 10 Me 10 9 (60%) Me Me 1.2 Unselective functionalizations: 1.17 1 b. m-CPBA (10 equiv), f. PTAD (1 equiv), oxymercuration; O2; SeO2; DCM DCM dihydroxylations; Mukaiyama Ph (48%) d. H2SO4 (54%) hydration; Br ; Kharasch reactions (aq.) 2 c. m-CPBA e. NBS, O N O (1 equiv), MeCN Transition Metals: O DCM (66%) N NH O Me 17 products Me RCM catalysts; Crabtree's catalyst; Me (92%) Me AuCl3; Au(PPh3)Cl; silver salts O Me Me Me 1.18 MeO Me Br 1.21 Other: Me Me µw; "; h#; BuLi Me Me 1.19 Me 1.20

Scheme 1.3: Selective functionalizations of humulene.

In addition to these selective reactions, myriad unselective reactions were

investigated (conditions g). The acid catalyzed transannular cyclizations were

investigated with a number of acids, in a number of solvents, but none of them proved

10 reasonably selective (See Scheme 1.3, right). Indeed, most of these reactions were a

complete mess, or did not react at all. A number of straightforward olefin

functionalization reactions also turned out to be unselective with regard to the three

olefins in humulene. Several transition metal catalysts known to have reactivity with

olefins were investigated, however, the majority of these led to no reaction. Finally, humulene was inert to photolysis and strong base, and decomposed when heated or pyrolyzed.

Given the large number of reactions that decomposed, or were wholly unselective under cyclization acid catalyzed conditions, I turned my attention to a prefunctionalization/cyclization strategy. To this end, bromohumulene 1.20 can be prepared from humulene (Scheme 1.3, conditions e). In general, bromohumulene 1.20 is primed for SN2’ reactivity. Although lithium aluminum hydride led to a mixture of

reduction products, favoring hydride delivery through a SN2’ pathway, dimethylmalonate was reacted selectively in an SN2’ fashion. Several other nonobvious nucleophiles were trapped while attempting other cyclizations: Et3B led to ethylation, a TEMPO adduct was

isolated, and the nitrate of silver nitrate also added into the olefin selectively (Scheme

1.4, conditions a). In addition to these successful functionalizations, a number of

attempted reactions failed to afford cyclized products or decomposed under the reaction

conditions. Similar to the reaction of humulene with Montmorillonite K10 clay (see

Scheme 1.3, conditions a),43 the action of boron trifluoride diethyl etherate on bromohumulene 1.20 afforded the same isodaucane, bicyclo[5.3.0]decane, skeleton

previously observed, albeit with unconjugated olefins (1.22, conditions c). Interestingly,

photolyzing azobisisobutyronitrile and tributyltin hydride led to decomposition

11 (conditions b), but heating tributyltin hydride to reflux in benzene provided a clean reduction of the bromide functionality to afford hydrocarbon 1.23 (conditions d).

g. Pd2(dba)3 Me Me 6 Br Me a. SN2' or Pd(PPh3)4 Me Me Me no reaction II Pd Ln Me X Me Me Conditions Screened: 1.20 1.26 #; NaOCHO; NEt3; K2CO3 X = -H, -Et, -CH(CO2Me)2, -ONO2,-TEMPO b. failed e. m-CPBA, f. 2,3-butadione, DCM cyclizations or UV h!, MeCN c. BF3"OEt2, d. Bu3SnH, 1.25 selective DCM PhH Conditions Screened: Br Br AIBN, Bu3SnH; silver salts; h! O Me Me 2 Me Me 3 + Me 9 Me 10 Br O Me Me Me 1.24 1.25 Me

Me Me 1.22 1.23

Scheme 1.4: Reactivity of bromohumulene 1.20.

Although poor chemoselectivity is generally not a desirable outcome, when

bromohumulene (1.20) was treated with a molar equivalent of m-CPBA, both the 2,3- and

the 9,10-epoxide products 1.24 and 1.25, respectively, could be obtained in acceptable yields on small scale, utilizing preparatory TLC to separate the isomers (Scheme 1.4, conditions e). Although this line of thought was not extensively pursued, it had been hoped that the two epoxides could be cyclized to different skeletons under acidic conditions. Interestingly, using 2,3-butanedione as a photosensitizer, air could be used to selectively form the 9,10-epoxide 1.25 (conditions f).45 The rationale for this selectivity

remains unclear.

Perhaps the most interesting compound synthesized from bromohumulene 1.20

occurred when it was reacted with zero-valent palladium. An oxidative insertion takes

place to form a stable organopalladium species 1.26 (conditions g). However, this

compound remains elusive as it is an oil, and a crystal was never formed. However,

organopalladium 1.26 can be chromatographed and characterized by both NMR and

12 HRMS. Additionally, tetrakis(triphenylphosphine)palladium(0) and

tris(dibenzylidinediacetone)dipalladium-chloroform adduct produce the same product,

1 which seems to be free of any ligand (dba or PPh3) by H NMR. For this reason, the most likely “ligands” are the olefins of humulene itself. Since this compound is remarkably stable to thermal conditions and reductive elimination attempts, and importantly, is an oil, it is hypothesized that the palladium atom is sequestered within a humulene “cup” and is coordinated to the olefins.

H Me Me TMSOTf, 10 9 Me 7 2,6-lutadine, PhMe 2 OH MeO (81%) 6 Me Me 1.27 Me Conditions Screened: Me 1.19 Sc(OTf)2; Cp2TiCl decomposition

2,3-butadione, Conditions Screened: Me Br Br 0 UV h", Me In ; SiO2 Me Me decomposition Me MeCN O 1.20 Me 1.25

Conditions Screened: Ph Ph !; ClSO3H; F3CSO3H; ZnCl2; Me2AlCl; EtAlCl2; AgSbF6; Sc(OTf)2; O N O O N O Yb(OTf)2; Dy(OTf)3; Eu(OTf)3; Sm(OTf) ; Sn(OTf) ; Ce(OTf) Me N NH m-CPBA, Me N NH 3 2 4 Me Me decomposition DCM O Me (64%) Me 1.21 1.28

Scheme 1.5: Reactivity of miscellaneous humulene derivatives.

A few other noteworthy reactions were conducted. The reaction of 5,6-humulene oxide (1.19) with TMSOTf and 2,6-lutidine was known to afford the fused tricyclic cyclopropane 1.27 (Scheme 1.5).46 This reaction was reproduced successfully, but

treating the same epoxide 1.19 with scandium triflate led to no reaction. Hypothesizing

that Ti(III) could radically open the epoxide and then cyclize transannularly,47,48 this

reaction was attempted, albeit unsuccessfully.

13 PTAD-adduct 1.21 could also be epoxidized selectively, simply utilizing m-

CPBA to afford expoxide 1.28. Compared to the conversion of bromohumulene 1.20 into epoxides 1.24 and 1.25, perhaps the bulky PTAD group changes the conformation of the macrocycle enough as to provide selectivity. Epoxide 1.28 was screened against acids extensively in order to attempt a selective cyclization. However, in most of these cases the reactions were not at all selective.

Despite the multitude of reactions conducted on humulene or closely related

derivatives no novel selective cyclizations were discovered. However, at the same time as

Section 1.2 was being conducted, I was working toward a total synthesis of humulene.

Ultimately, transannular cyclization chemistry was never explored in full detail, since a nickel-promoted diene/alkyne cooligomerization methodology (the subject of Chapter 2) took precedent.

1.4 Progress Toward the Total Synthesis of Humulene

There have been seven total syntheses conducted since 1967. The first,49 from the

Corey lab, utilizes a Ni(CO)4-mediated reductive coupling of bis-allylbromide 1.29

(Scheme 1.6). Under these conditions, the C7-C8 bond can be forged (yield not reported).

After this penultimate step, the Z olefin of 2,3-isohumulene 1.30 is photolytically

isomerized to the requisite E olefin (yield not reported), to afford humulene (1.2) in 8-

steps, longest linear sequence. Following Corey’s seminal synthesis, Bari et al. produced

a synthesis containing a similar key step.50 In this instance, all E-bisallylbromide 1.31

was macrocyclized under the influence of Ni(CO)4 in “good yield.” This synthesis is

slightly longer at 10-steps, however it avoids the final isomerization utilized by Corey.

14 The Yamamoto group published a 13-step synthesis of humulene in 1977 that

relied on a Tsuji-Trost disconnection to forge the C1-C11 bond.51 In the key step, the discrete sodium salt of 1,3-diketone 1.32 was formed first, followed by reaction with

Pd(PPh3)4 and 1,3-bis(diphenylphosphino)propane (dppp) to give the macrocycle 1.33 in

45% yield. Notably, the success of this reaction was highly dependent on the solvent

(THF/HMPA) and the use of dppp as ligand. Without dppp up to 25% of the 22-

membered ring was formed. The observation of macrocyclic dimerization being a

significant byproduct attests to the inherent strain of the eleven-membered macrocycle.

The shortest approach to date was published in 1982 by McMurry and Matz and

52 relied on his namesake reaction for the key step. The McMurry coupling with TiCl3 and

zinc-copper couple was conducted on keto aldehyde 1.34 and proceeded in 60% yield,

forming the olefin at C9-C10. In only five steps (four longest linear) McMurry prepared

humulene in 29% overall yield, a synthesis that remained unmatched for 20 years.

In 1983, the Takahashi group published the longest synthesis to date (14-steps,

longest linear sequence, 4% overall yield), yet it is noteworthy for having the highest

yielding macrocyclization reaction.53 The key anionic alkylation proceeded in a remarkable 80% yield when nitrile 1.35 was deprotonated with NaHMDS and reacted

intramolecularly with the pendant alkyl chloride. The C7-C8 bond was forged to give the

macrocycle 1.36 similar to Corey (1967) and Bari. To date, this remains the highest yielding macrocyclization reaction en route to humulene.

15 SnBu3 Me Corey 1967: Me Me Ni(CO)4 Me Me Me (4 equiv) Me Me Me Me 1 7 Me Me O 11 Br 10 Me 8 NMP, 50 ºC MeO SO Me OHC 1.29 1.30 Me 2 Me 9 Br 1.38 McMurray 1982: TiCl (6 equiv), Zn-Cu, 1.34 Corey 1993: 3 PhSeSePh, DME, 85 ºC Me2AlCl, (60%) C6H12, h! DCM, PhMe OCO Me TMSO 2 Corey 2002: O (37%) 3 4 Me Pd2(dba)3 Me Me (10 mol%), Me Me Me Bari 1976: dppf, THF, 70 ºC 2 Me Me Ni(CO)4 Me 1.39 Me 1.40 Me 3 (5 equiv) Me 1 Me Me 11 7 DMF, 50 ºC 7 Br O Me 8 ("good yield") 1.31 Me 8 humulene (1.2) Me Br Yamamoto 1977: Me NaH, THF; then MeO2C Suginome 1984: Pd(PPh3)4 (20 mol%), Pd(PPh ) (20 mol%), dppp,HMPA, THF, 66 ºC 1.33 3 4 (45%) Me NaOH (4 M, aq.), PhH, 80 ºC O (32%) Me Me B(Sia)2 1 Me Takahashi 1983: Br/Cl MeO2C Me Me 3 4 11 NaHMDS Me Me Me Cl AcO Me 7 Me Me THF, 55 ºC Me 1.32 Me 8 OEE (80%) Me Me 1.35 NC 1.36 NC OEE 1.37

Scheme 1.6: Previous total syntheses of humulene.

The Suginome group disconnected the C3-C4 bond in their synthesis, and were

able to produce humulene in 10% overall yield in only six steps.54 The key

macrocyclization reaction on bromoalkenylborane 1.37 afforded humulene in 32% under

aqueous basic conditions with Pd(PPh3)4 in refluxing benzene.

The two most recent syntheses were both from the Corey group as well. In his

1993 synthesis, Corey et al. disconnected the biomimetic C1-C11 bond

retrosynthetically, affording a decorated farnesol precursor (See Scheme 1.1 for

biosynthesis).55 In a forward sense, farnesol was carried forward 8-steps to afford allyltin

species 1.38 (Scheme 1.6). Under Lewis acidic conditions (Me2AlCl) macrocyclic ring closure afforded humulene in 37% yield. A decade later, Corey published the highest yielding synthesis of humulene to date.12 In the forward sense, the C3-C4 bond is assembled through a Tsuji-Trost reaction in which a trimethylsilyl enol ether attacks the

16 activated allylic carbonate 1.39 intramolecularly. The macrocyclization in this reaction

affords a 44-52% yield, and the overall sequence proceeds in ~40% yield.

Given the precedent of the aforementioned syntheses (Scheme 1.6), the bar was

set quite high for my own work. There were a few requirements that needed to be met.

First of all, the synthesis needed to be short, certainly less than ten steps, but realistically

closer to five. Secondly, the synthesis needed to have a high yielding macrocyclization

step, above 50% yield. Thirdly, it would be desirable to achieve a synthesis with an

interesting or completely novel macrocyclization. Finally, in order to fulfill Phil Baran’s dreams, it needed to be conducted on gram scale.

a. b. Conditions Screened: G1, PhMe (0.001 M), 110 ºC; G2, PhMe (0.001 M), 110 ºC; Me Me Me Me Me Me Me Me HG2, PhMe (0.001 M), 110 ºC 10 Me Me no reaction 9 X ring closing metathesis Me Me 1.2 1.41

Ring Closing Metathesis (RCM) Catalysts Utilized: Me Me N N Me Me Me Me N N Me Me P(Cy)3 Cl P(Cy) 3 Me Me Me Me Ru Cl Cl Cl Cl Ru Ru Ru Cl Cl Cl Me O Ph Ph O P(Cy)3 P(Cy)3 Me Grubbs Catalyst, 1st Grubbs Catalyst, 2nd Hoyveyda-Grubbs 1st Hoyveyda-Grubbs 2nd Generation (G1, 1.42) Generation (G2, 1.43) Generation (HG1, 1.44) Generation (HG2, 1.45)

Scheme 1.7: a. Retrosynthetic disconnection of humulene; b. Stability of humulene control studies.

Given the strict requirements established above, in our first strategy toward humulene (1.2) ring-closing metathesis (RCM) was employed as the key disconnection

(Scheme 1.7a). At the onset of this strategy, it was envisioned that other RCM disconnections could and would be explored, and the C9-C10 bond was simply chosen as the first based on the perceived ease of synthesis of linear precursor 1.41. Utilizing commercial humulene, several control experiments were first conducted to show that

17 humulene was stable to Grubbs-type metathesis catalysts and did not undergo ring-

opening, polymerization, or degradation, even under elevated temperatures (Scheme

1.7b).

The key linear alkene 1.41 was synthesized in a straightforward manner from

isobutyronitrile and isoprene oxide (1.43, Scheme 1.8). In a forward sense,

isobutyronitrile was deprotonated with freshly prepared lithium diisopropylamide and

allylated with allyl bromide to afford nitrile 1.44 in 81% yield. The resulting nitrile could

be reduced at low temperature with DIBAl-H, giving aldehyde 1.45. Aldeyhde 1.45 could

be carried forward as a solution in hexanes and was propargylated under Ohira-Bestmann

conditions.56 Because the initial concentration of aldehyde was approximated by 1H

NMR, the exact stoichiometry for the reaction was challenging to obtain. However, the robustness of the Ohira-Bestmann propargylation allowed the aldehyde to be “titrated” with diazo phosphonate 1.46, the Ohira-Bestmann reagent. Once again, the low boiling nature of the resulting product, alkyne 1.47, required a clever isolation procedure. In this case, the methanolic solution could be diluted with water and extracted multiple times with pentane. The pentane could then be partially removed in vacuo to give a solution of alkyne 1.47 in reasonable purity. Both of the previous two reactions remain unoptimized, and if this synthetic route had been pursued further, better methods of purification, namely fractional distillation, could have been used to obtain analytically pure samples.

18 O O P OMe Br Me OMe CN CN CHO N LiN(i-Pr)2, DIBAl-H 1.46 2 Me Me Me Me Et2O, 0 ºC Me hexanes Me K2CO3, MeOH Me Me (81%) 1.44 1.45 (~quant.) 1.47

K2CO3, Na2SO3, DBU, 1. MsCl, NEt3, CuI, pentane, Me Me CuI (5 mol%), Me Me DCM/THF, –40 ºC Me Me O DMF MgCl + OH Br (53%) THF –30 ! rt 2. LiBr, 0 ºC (76 % overall) 1.50 1.43 (84–92%) 1.51 1.49

Me Me 0 Me Me Me Me Me Me (NH4)SO4, Li , NH3 Me Me Me t-BuOH, Et2O Me Conditions Screened: 1.41 1.48 humulene (1.2) G1 (1.42), C6D6 (0.002 M), rt ! 75 ºC; G2 (1.43), C D (0.002 M), rt ! 75 ºC; 6 6 1H NMR changed HG2 (1.45), C6D6 (0.002 M), rt ! 75 ºC

Me Me Me Me

Me Me Me Me dimer observed by LC/MS 1.52

Scheme 1.8: Synthesis of ring closing metathesis precursor 1.41.

In order to prepare internal alkyne 1.48, bromide 1.49 was prepared from 3- chloro-2-methylpropene and isoprene oxide (1.43). Isoprene oxide can either be purchased, or prepared on large scale through an epoxidation of isoprene with peracetic acid and NaHCO3 with BHT as a radical trap. 3-Chloro-2-methylpropene was

transformed into its corresponding Grignard reagent 1.50 with activated magnesium and,

under copper catalysis, it reacts with isoprene oxide (1.43) in a 1,4-sense to give alcohol

1.51. Interestingly, if two equivalents of copper iodide were added to the Grignard

reagent prior to its addition to the epoxide, the major product isolated was a 1,2-addition

product (not shown). This observed reactivity is likely due to the fact that in the catalytic

reaction copper iodide acts as a mild Lewis acid whereas, upon the formation of the

stoichiometric cuprate from the Grignard, MgX2 (X = Cl or I) is liberated, a stronger

Lewis acid leading to a more SN1-like mechanism, and the cuprate can add to the more

19 stable tertiary carbocation. The small amount of 1,2-type product in the catalytic variant

of this reaction corroborates this hypothesis.

Alcohol 1.51, or (E)-2,6-dimethylhepta-2,6-dien-1-ol, which I have dubbed “un- geraniol,” can be converted to the corresponding bromide 1.49, via the methansulfonyl activated alcohol (Scheme 1.8). Following a Corey procedure,57 the alcohol is treated with methansulfonyl chloride in DCM/THF at –40 ºC in the presence of triethylamine.

The activated alcohol is then warmed to 0 ºC and lithium bromide displaces the activated alcohol to form bromide 1.49. Bromide 1.49 is somewhat unstable to long term storage so it is generally utilized in the next reaction quickly. In this reaction, a solution of terminal alkyne 1.47 is mixed with allyl bromide 1.49 in the presence of potassium carbonate, sodium sulfite, DBU, and copper iodide to afford skipped enyne 1.48.58 The internal alkyne 1.48 was easily reduced under dissolving metal conditions, using a procedure optimized for internal alkynes with long carbon chains, to afford the polyene 1.41.59

Gratifyingly, internal alkyne 1.48 and polyene 1.41 were separable by flash column chromatography with distilled hexanes as eluent.

With polyene 1.41 in hand, I was prepared to attempt the key ring closing metathesis step. At high dilution (0.002 M), Grubbs (1.42 and 1.43) and Grubbs–

Hoyveda (1.44 and 1.45) type ruthenium catalysts (Scheme 1.7) were slowly heated to reflux and monitored closely by 1H NMR. However, the only observable product was

homodimerization of the Type I olefin (monosubstituted).60 Dimer 1.52 was observed by

carefully monitoring changes in the splitting pattern of olefins by 1H NMR and also by

LC/MS. However, this product remained unisolated. This homodimerization was

unsurprising given that monosubstituted Type I olefins are known to undergo rapid

20 homodimerization. The desired reaction, however, was not completely without precedent:

the cross metathesis of Type I and Type III (1,1-disubstituted) olefins has been shown to

proceed in high yield and good E/Z ratio.60 However, requiring an 11-membered ring

containing three E olefins to macrocylize was perhaps too demanding. In retrospect, altering the monosubstitued Type I olefin to a 1,1-dimethyl trisubstituted (Type III) olefin could have prevented the homodimerization. However, this line of thought was never pursued in the laboratory.

Conditions Screened: CuSO4, NH2NH2!H2O; Pd/CaCO3, quinoline, H2; Me Me Me Me Me Pd/BaSO4, quinoline, H2 Me Me Me Me Me Me 1.48 1.53 1.30 Me

Co2(CO)8, DCM HG1 (1.44), DCM (60%)

(OC) Co Co(CO) 3 3 G2 (1.43), PhH, 60 ºC (OC)3Co Co(CO)3 Me Me Me Me Me Me Me 1.55 Me Me Me 1.54 Me 1.56 Me

Scheme 1.9: Alternate ring closing metathesis strategies.

With remaining internal alkyne 1.48 in hand, several other end game strategies were investigated (Scheme 1.9). First, it was hypothesized that a syn-reduction of the internal alkyne, giving rise to Z olefin 1.53 would undergo more facile macrocyclization.

The constitutional isomer of humulene (1.30) had been previously prepared by Corey,

who also showed that the Z olefin could be photolytically isomerized to afford humulene

(1.2), albeit in an unreported yield.49 However, under the conditions screened, diimide reductions and quinoline poisoned palladium hydrogenations in various solvents, a clean reduction was never observed. Instead, either no reaction (diimide) or multiple products

(Pd/H2) were obtained.

21 A single reaction was attempted in which internal alkyne 1.48 was subjected to

RCM conditons using Grubbs-Hoyveda 1st generation catalyst (1.44) in DCM, however no reaction was observed. Notably, this reaction was not run at elevated temperature, and homodimerization was not observed to any appreciable extent. In an attempt to access the same macrocyclic alkyne 1.54, an alkyne protecting group was employed. First, internal alkyne 1.48 was subjected to dicobalt octacarbonyl, affording stable cobalt complex 1.55.

It is known that these cobalt complexes significantly alter the geometry of the alkyne, making them more E-olefin-like.61 Indeed, this strategy has been previously used to

render challenging macrocyclizations containing alkynes possible; Magnus’ studies on

enediyne cores are one noteworthy example.62 However, upon heating the substrate with

Grubbs 2nd generation catalyst (1.43) in benzene, no macrocyclization product (1.56) was observed. At this point, the ring closing metathesis route was abandoned.

With the realization that the former route would be unproductive, a second synthetic route was undertaken. We envisioned exploiting a larger macrocyclic template, that would have less strain, followed by a ring contraction to form our desired macrocycle, humulene (1.2). After considering thio- and aza- templates, macrocyclic azo compound 1.57 was chosen as a target, photolysis of which could lead to humulene

(Scheme 1.10a).

To this end, the synthesis of azo-macrocycle 1.57 was attempted with farnesol as the starting material. Farnesyl bromide (1.58) needed to be freshly prepared from farnesol with phosphorous tribromide in DCM/Et2O at low temperature due to its instability

(reaction not shown). Farnesyl bromide (1.58) was immediately reacted with protected hydrazine 1.59 to afford hydrazine 1.60 if the reaction was worked up in less than 30

22 minutes. However, if the reaction was kept under basic conditions for an extended period

of time, base promoted elimination of the nosyl protecting group proceeded readily to

give azo compound 1.61. Compound 1.61 is unable to be isolated and rapidly undergoes tautomerization to hydrazone 1.62. Alternatively, if the isolated hydrazine 1.60 was stoichiometrically deprotonated with NaHMDS, the nosyl group could be cleanly eliminated, which also immediately tautomerized to hydrazone compound 1.62. Under no conditions could the reverse isomerization be effected (hydrazine 1.62  azo 1.61), rendering the macrocyclic aza-ene reaction unachievable. Base, heat, metals, and oxidants were all screened in attempts to promote this macrocyclic aza-ene reaction

(Scheme 1.10a).

H a. Ns N N Boc 1.59 H Me Me Me Me Me Me K2CO3, MeCN, H NaHMDS N Me Br 30 min Me N Boc E,E,E-farnesyl bromide (1.58) (99%) 1.60 Ns

~hours (<48%)

Me Me Me Me H Me Me H Me h! [O] aza-ene N Me Me N Boc -N 2 N NH 1.61 Me Me Me Me Me N HN humulene (1.2) Me 1.57 Me 1.63 rapid tautomerization Conditions Screened: d8-PhMe, 110 ºC; NaHMDS, PhMe, 110 ºC; Cu(OTf) , DCM; Me Me Me 2 H N [O] of C1-C2 double bond; Me 1 9 N Boc 10 protecting group alteration 1.62

b. O Pd or Cu Me Me catalysis Me Me + R1 N Me OH Me OH R2 geraniol (1.64) O R2 = -phthalimide (1.67) R1 = -H (1.65) -N-aminophthalimide (1.68) -NH2 (1.66)

Scheme 1.10: a. Ring contraction approach to humulene; b. Development of aza- Wacker.

Following this failure, an aza-Wacker route was briefly explored in the context of a model system (Scheme 1.10b). In this attempted transformation, geraniol (1.64) was

23 reacted with phthalamide (1.65) or N-aminophthalamide (1.66) in the presence of

palladium and/or copper catalysis. After screening ~20 reactions, amination of geraniol to

afford compound 1.67 or 1.68 was never observed.

B2pin2, cat., p-TsOH Me Me Ph OH Ph Bpin + Me OCO2Me DMSO, MeOH, 50 ºC 1.70 1.69 (71%)

Pd2(dba)3, PPh3, cat. THF, 60 ºC

Me Me PhSe Pd SePh 1.71 Cl Me 1.72 Ph

Scheme 1.11: Initial experiments toward an allyl-allyl coupling reaction.

A substantial amount of time (~one year) was spent on several other routes that

followed biomimetic approaches similar to Corey’s 1993 synthesis,55 namely starting

from farnesol.55 In the one instance, an allyl-allyl coupling was envisioned, similar to the

chemistry of Morken and Miyaura.63,64 To initially test the palladium catalyzed allyl-allyl

coupling (Scheme 1.11), the methyl carbonate of geraniol (1.69) and cinnamyl boronate

1.70 were readily prepared. Cinnamyl boronate 1.70 was prepared in 71% yield from

cinnamyl alcohol utilizing the palladium pincer complexes (1.71) developed by

Szabó.65,66 The allyl boronate was then be reacted with geraniol methyl carbonate 1.69 in

the presence of Pd2(dba)3 and PPh3 at 50 ºC, and the desired coupled product 1.72 was obtained in acceptable yield.

a. Me Me Me Me Me Me Me Me Me

Me OH Me OH Me OH OH OBoc OCO Me 1.73 1.74 2 1.75

Me Me Me Me Me Me Me Me Me

Me OH Me OAc Me OCO2Me O O 1.76 1.77 OCO2Me 1.78

b. Me Me Me Me Me Me

Me OCO2Me Me Bpin Bpin 1.79 OCO2Me 1.80

c. Me Me Me Me Pd2(dba)3, PPh3, Me Me + Me Me OCO Me Me Bpin Me 2 THF, 60 ºC 1.69 1.81 1.82 Me Me

Scheme 1.12: a. Substrates prepared for allyl-allyl macrocyclic coupling reaction; b. Substrates that could not be prepared; c. Failed gernayl-geranyl coupling.

Following this success, a series of farnesol derivatives (1.73–1.78) were prepared

(Scheme 1.12a), typically following Corey’s humulene synthesis with slight alterations.55

Notably, I was unable to prepare mixed boronate/protected alcohol compounds (e.g, 1.79

or 1.80), which would have been ideal in this reaction (Scheme 1.12b). However, it was entirely plausible that the alcohol or activated alcohols in compounds 1.73–1.78 could

form a pinacolborane derivative in situ and undergo the desired coupling reaction.

Despite extensive experimentation, none of the palladium catalyzed macrocyclic allyl-

allyl couplings attempted with substrates 1.73–1.78 showed any product. The final nail in

the coffin of this route came when I took a step back and conducted the intermolecular

control reaction between geraniol methyl ester 1.69 and pinacol borane 1.81 (prepared in

a similar manner to allylborane 1.70, Scheme 1.11). Despite the success of the test

reaction (Scheme 1.11), geraniol methyl ester 1.69 and pinacol borane 1.81 failed to

couple under identical conditions (Scheme 1.12c).

25 A final attempt was made utilizing farnesol as a starting material, however this

approach lacked significant literature precedence (Scheme 1.13). In this approach, I

sought to do a direct coupling of an allylboronate with an olefin. Despite stepping back

and working on a model system of cinnamyl boronates 1.70 and 1.83 and 2-methyl-2-

butene (1.84), this reaction was never realized and the coupled product 1.85 was never

observed, even in trace amounts (Scheme 1.13a). Nevertheless, I also screened a number

of cyclization attempts on farnesyl boronates 1.86 and 1.87 (Scheme 1.13b).

a. Ph Bpin ~30 reactions screened: 1.70 Me or + metals, oxidants, acids Me Me Ph 1.84 1.85 Me Me Ph BF3K 1.83

b. Me Me Me Me Me Me

Me Bpin Me BF3K 1.86 1.87

Scheme 1.13: a. Attempted allylboronate olefin coupling; b. Macrocyclic substrates.

Following the work on the ring closing metathesis route (Scheme 1.7–Scheme

1.9), all of my efforts had focused on synthesizing humulene from farnesol. This was

because both the prior literature (Scheme 1.6) and Professor Phil Baran had dictated that

my synthesis had to be short, high yielding, and gram-scale. Assembling small molecule

terpene fragments simply took too long. For this reason, I had my mind fixed on farnesol

as the only starting material. During this period, numerous ideas were thrown out simply because they were more than a few steps long. During this time, I began to consider other

simpler terpene starting materials; perhaps there was something besides farnesol that could be rapidly assembled into terpene skeletons. These explorations are the subject of

Chapter 2.

1 Maimone, T. J.; Baran, P. S. Nat. Chem. Biol. 2007, 3, 396–407.

2 Breitmaier, E. Terpenes: Flavors, Fragrances, Pharmaca, Pheromones; Wiley-VCH:

Weinheim, Germany, 2006; p 1–9, 24–50.

3 Sacchettini, J. C.; Poulter, C. D. Science 1997, 277, 1788–1789.

4 Hosler, D.; Burkett, S. L.; Tarkanyan, M. J. Science 1999, 284, 1988–1991.

5 Weitz, H. M.; Loser, E. Isoprene. In Ullmann's Encyclopedia of Industrial Chemistry;

Wiley-VCH: Weinheim, Germany, 2002; p 1–20.

6 For a review that focuses on the syntheses of germacrane sesquiterpenes, see: Minnaard,

A. J.; Wijnberg, J. B. P. A.; de Groot, A. Tetrahedron 1999, 55, 2115–2146.

7 Baran, P. S.; Richter, J. M. J. Am. Chem. Soc. 2004, 126, 7405–7451.

8 Maimone, T. J.; Voica, A. F.; Baran, P. S. Angew. Chem. Int. Ed. 2008, 47, 3054-3056.

9 Chen, K.; Baran, P. S. Nature, 459, 824–828.

10 Mendoza, A.; Ishihara, Y.; Baran, P. S. Nature Chem. 2012, 4, 21–25.

11 Kellogg, B. A.; Poulter, C. D. Curr. Opin. Chem. Biol. 1997, 1, 570–578.

12 There are currently no comprehensive reviews on humulene; in lieu of listing myriad references the most recent total synthesis and one recent cyclization paper are included.

See the references therein for more detail: (a) Hu, T.; Corey, E. J. Org. Lett. 2002, 4,

2441–2443. (b) Hanson, J. R.; Hitchcock, P. B.; Macias-Sanchez A. J.; Mobbs, D. J. J.

Chem. Res. 2004, 465–467.

13 Minnaard, A. J.; Wijnberg, J. B. P. A.; de Groot, A. Tetrahedron, 1999, 55, 2115–

2146.

14 Adio, A. M. Tetrahedron 2009, 65, 1533–1552.

15 Clemo, G. R.; Harris, J. O. J. Chem. Soc. 1951, 22–25.

16 Chapman, A. C. J. Chem. Soc. 1895, 54, 780.

17 Hildebrand, R. P.; Sutherland, M. D. Australian J. Chem. 1961, 14, 272–275.

18 Fawcett, R. W.; Harris, J. O. J. Chem. Soc. 1954, 2669–2673.

19 Collado, I. G.; Hanson, J. R.; Macías-Siânchez, A. J. Nat. Prod. Rep. 1998, 187-204.

20 Benešová, V.; Herout, V.; Šorm, F. Collect. Czech. Chem. Commun. 1961, 26, 1832–

1838.

21 Smedman, L. Å.; Zavarin, E.; Teranishi, R. Phytochemistry, 1969, 8, 1547–1570.

22 Shirahama, H.; Arora, G. S.; Osawa, E.; Matsumoto, T. Tetrahedron Lett. 1983, 24,

2869–2872.

23 Clemo, G. R.; Harris, J. O. J. Chem. Soc. 1951, 22–25.

24 Dev, S. Tetrahedron Lett. 1959, 1, 12–15.

25 Hendrickson, J. B. Tetrahedron, 1959, 7, 82–89.

26 Sutherland, M. D.; Waters, O. J. Australian J. Chem. 1961, 14, 596–605.

27 McPhail, A. T.; Sim, G. A. J. Chem. Soc. (B) 1966, 112–120.

28 Unpublished, personal communication with Professor Makato Fujita.

29 Inokuma, Y.; Yoshioka, S.; Ariyoshi, J.; Arai, T.; Hitora, Y.; Takada, K.; Matsunaga,

S.; Rissanen, K.; Fujita, M. Nature, 495, 461–466.

30 Dev, S. Anderson, J. E.; Cormier, V.; Damodaran, N. P. Roberts, J. D. J. Am. Chem.

Soc. 1968, 90, 1246–1248.

31 Damodaran, N. P.; Dev, S. Tetrahedron, 1968, 24, 4123–4132.

32 Tashkhodzhaev, B. Chem. Nat. Compounds 1997, 33, 213–220.

33 Shirahama, H.; Ōsawa, E.; Matsumoto, T. J. Am. Chem. Soc. 1980, 102, 3208–3213.

34 Neuenschwander, U.; Czarniecki, B.; Hermans, I. J. Org. Chem. 2012, 77, 2865–2869.

35 Hayano, K.; Shirahama, H. Bull. Chem. Soc. Jap. 1996, 69, 459–464.

36 Hayano, K.; Mochizuki, K. Heterocycles, 1997, 45, 1573–1578.

37 Hayano, K.; Ito, N.; Sato, M.; Takishima, M.; Mochizuki, K. Heterocycles, 1998, 48,

1747–1752.

38 Baines, D.; Forrester, J.; Parker, W. J. Chem. Soc. Perkin Trans. I, 1974, 1598–1603.

39 Misumi, S.; Ohfune, Y.; Furusaki, A.; Shirahama, H.; Matsumoto, T. Tetrahedron Lett.

1976, 33, 2865–2868.

40 Misumi, S.; Matsushima, H.; Shirahama, H.; Matsumoto, T. Chem. Lett. 1982, 855–

858.

41 Takatsu, T.; Ito, S.; Kan, T.; Shirahama, H.; Matsumoto, T. Synlett 1989, 40–42.

42 Blake, A. J.; Gladwin, A. R.; Pattenden, G.; Smithies, A. J. J. Chem. Soc., Perkin

Trans. 1 1997, 1167–1170.

43 Khomenko, T. M.; Zenkovets, G. A.; Barkhash, V. A. Russ. J. Org. Chem., 1997, 33,

595–600.

44 Pickett, J. A.; Sharpe, F. R.; Peppard, T. L. Chem. and Ind. 1977, 1, 30–31.

45 Shepherd, J. P. (Celanese Corporation) Photochemical Epoxidation. U. S. Patent,

US4383904 A. May 17, 1983.

46 Shirahama, H.; Murata, S.; Fujita, T.; Chhabra, B. R.; Noyori, R.; Matsumoto, T. Bull.

Chem. Soc. Jpn., 1982, 55, 2691–2692.

47 Barrero, A. F.; Oltra, J. E.; Cuerva, J. M.; Rosales, A. J. Org. Chem. 2002, 67, 2566–

2571.

48 Barrero, A. F.; del Moral, J. F. Q.; Herrador, M. M.; Loayza, I.; Sánchez, E. M.;

Arteaga, J. F. Tetrahedron 2006, 62, 5215–5222.

49 Corey, E. J.; Hamanaka, E. J. Am. Chem. Soc. 1967, 89, 2758–2759.

50 Vig, O. P.; Ram, B.; Atwal, K. S.; Bari, S. S. Indian J. Chem. 1976, 14B, 855–857.

51 Kitagawa, Y.; Itoh, A.; Hashimoto, S.; Yamamoto, H.; Nozaki, H. J. Am. Chem. Soc.

1977, 99, 3864–3867.

52 McMurry, J. E.; Matz, J. R. Tetrahedron Lett. 1982, 23, 2723–2724.

53 Takahashi, T.; Kitamura, K.; Tsuji, J. Tetrahedron Lett. 1983, 24, 4695–4698.

54 Miyaura, N.; Suginome, H.; Suzuki, A. Tetrahedron Lett. 1984, 25, 761–764.

55 Corey, E. J.; Daigneault, S.; Dixon, B. R. Tetrahedron Lett. 1993, 34, 3675–3678.

56 Müller, S.; Liepold, B.; Roth, G. J.; Bestmann, H. J. Synlett, 1996, 521–522.

57 Corey, E. J.; Hahl, R. W. Tetrahedron Lett. 1989, 30, 3023–3026.

58 Bieber, L. W.; da Silva, M. F. Tetrahedron Lett. 2008, 48, 7088–7090.

59 Brandsma, L.; Nieuwenhuizen, W. F.; Zwikker, J. W.; Maeorg, U. Eur. J. Org. Chem.

1999, 775–779.

60 Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003,

125, 11360–11370.

61 Sly, W. G. J. Am. Chem. Soc. 1959, 81, 18–20.

62 Magnus, P.; Annoura, H.; Harling, J. J. Org. Chem. 1990, 55, 1709–1711.

63 Zhang, P.; Brozek, L. A.; Morken, J. P. J. Am. Chem. Soc. 2010, 132, 10686–10688.

64 Ishiyama, T.; Ahiko, T.; Miyaura, N. Tetrahedron Lett. 1997, 37, 6889–6892.

65 Selander, N.; Szabó, K. J. J. Org. Chem. 2009, 74, 5695–5698.

66 Selander, N.; Szabó, K. J. Chem. Rev. 2010, 111, 2048–2076.

30 1.5 Experimental

General procedures. All reactions were carried out under a nitrogen atmosphere with

dry solvents under anhydrous conditions, unless otherwise noted. Dry tetrahydrofuran

(THF), triethylamine (TEA), dichloromethane (DCM), and toluene were obtained by passing commercially available pre-dried, oxygen-free formulations through activated alumina columns. Yields refer to chromatographically and spectroscopically (1H NMR)

homogeneous materials, unless otherwise stated. Reagents were purchased at the highest

commercial quality and used without further purification, unless otherwise stated.

Reactions were monitored by thin-layer chromatography (TLC) carried out on 0.25 mm

E. Merck silica gel plates (60F-254) using UV light as visualizing agent and an acidic

mixture of p-anisaldehyde or an acidic mixture of phosphomolybdic acid/cerium sulfate

(Seebach’s stain), and heat as developing agents. NMR spectra were recorded on a

Bruker DRX 600, DRX 500, or an AMX 400 spectrometer and were calibrated using

1 residual solvent as an internal reference (CDCl3: 7.26 ppm for H NMR and 77.16 ppm

for 13C NMR). The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, b = broad, a = apparent. IR spectra were recorded on a Perkin-Elmer Spetrum BX spectrometer. High-resolution

mass spectra (HRMS) were recorded on an Agilent Mass spectrometer using ES1.TOF

(electrospray ionization-time of flight). Melting points (m.p.) are uncorrected and were

recorded on a Fisher-Johns 12-144 melting point apparatus. Optical rotations were

obtained on a Perkin-Elmer 431 Polarimeter. The UCSD small molecule X-ray facility

collected and analyzed all X-ray diffraction data.

31 Me Me Me Isodaucane 1.17: Solid Montmorillonite K10 clay (120 mg) was added to

humulene (34 µL, 0.15 mmol) in DCM (5.9 mL, 0.025 M). The reaction Me mixture was stirred at room temperature until judged complete by TLC. The solids were filtered, and the organic phase was concentrated in vacuo. Isoducane 1.17 was isolated by

1 preparatory TLC (hexanes): Rf = 0.81 (hexanes); H NMR (500 MHz, CDCl3) δ 5.92 (s, 1

H) 2.72 (hept, J = 6.8 Hz, 1 H), 2.21 (dd, J = 9.0, 5.1 Hz, 2 H), 2.14 (q, J = 6.8, 5.9 Hz, 2

13 H), 1.79 (s, 3 H), 1.68 – 1.47 (m, 6 H), 0.98 –0.94 (m, 9 H); C NMR (151 MHz, CDCl3)

δ 143.4, 139.1, 137.3, 119.3, 50.2, 42.5, 40.1, 37.1, 27.9, 27.7, 27.2, 24.8, 22.0, 21.7,

21.0.

MeO Humulene-6,7-oxide 1.19: m-CPBA (127 mg, 1.05 equiv) in DCM (5 Me Me Me mL) added to humulene (100 mg, 0.489 mmol) in DCM (14.6 mL, 0.025

M) over 30 minutes. After 30 minutes, the reaction mixture was transferred to a

separatory funnel and washed with NaHCO3 (sat. aq.) and H2O. The organic phase was dried over MgSO4 and concentrated in vacuo and humulene-6,7-oxide 1.19 was purified by flash column chromatography (silica gel, hexanes to 5% EtOAc:hexanes) to afford a

1 clear oil (99 mg, 92%): Rf = 0.50 (5% EtOAc:hexanes); H NMR (400 MHz, CDCl3) δ

5.28 (ddd, J = 15.2, 9.9, 5.2 Hz, 1 H), 5.15 (d, J = 15.9 Hz, 1 H), 5.04 – 4.95 (m, 1 H),

2.62 – 2.49 (m, 2 H), 2.28 – 2.05 (m, 3 H), 2.00 (dd, J = 13.7, 9.2 Hz, 1 H), 1.87 (dd, J =

14.0, 5.5 Hz, 1 H), 1.72 – 1.59 (m, 2 H), 1.56 (s, 3 H), 1.31 (s, 3 H), 1.11 (s, 3 H), 1.08 (s,

3 H).

32 O MeO Humulene trioxide 1.18: Humulene (11 µL, 0.05 mmol) was added to a Me O Me Me solution of mCPBA (123 mg, 10 equiv) in DCM (0.5 mL, 0.1 M). The reaction was judged complete by TLC in less than 5 minutes. The organic phase was

washed with NaHCO3 (sat. aq.) and H2O. The organic phase was dried over MgSO4 and

concentrated in vacuo and humulene trioxide 1.18 was purified by flash column chromatography (silica gel, DCM to 5% MeOH:DCM) to afford a waxy solid (6.1 mg,

1 48%): Rf = 0.18 (20% EtOAc:hexanes); H NMR (500 MHz, CDCl3) δ 2.93 (dt, J = 10.7,

2.5 Hz, 1 H), 2.90 – 2.86 (m, 1 H), 2.77 – 2.69 (m, 2 H), 2.68 – 2.62 (m, 1 H), 2.33 (d, J

= 2.3 Hz, 1 H), 2.30 – 2.21 (m, 2 H), 2.17 (ddd, J = 13.5, 5.6, 2.4 Hz, 1 H), 1.74 – 1.57

(m, 3 H), 1.38 (s, 3 H), 1.36 (s, 3 H), 1.10 (s, 3 H), 0.87 (s, 3 H).

Bromohumulene 1.20: Humulene (225 L, 0.97 mmol) was added to Me Br µ Me Me freshly recrystallized N-bromosuccinimide (174 mg, 1 equiv) in MeCN

(38.8 mL, 0.025 M) at room temperature. The reaction was complete in less than 5 minutes and was then concentrated in vacuo. The crude residue was brought up in hexanes and filtered through celite, rinsing with hexanes until the filtrate showed no product by TLC. The filtrate was concentrated in vacuo and purified by gravity column chromatography with a tall, thin plug of silica gel (hexanes) to afford bromohumulene

1 1.20 (148 mg, 66%): Rf = 0.33 (hexanes); H NMR (400 MHz, CDCl3) δ 5.19 (s, 1 H),

5.12 (s, 1 H), 5.11 (s, 1 H), 5.05 (s, 1 H), 4.84 (d, J = 10.2 Hz, 1 H), 4.23 (t, J = 7.2 Hz, 1

H), 2.85 (dd, J = 12.5, 2.7 Hz, 1 H), 2.66 – 2.59 (m, 1 H), 2.31 (q, J = 6.7, 6.1 Hz, 2 H),

2.08 (t, J = 6.4 Hz, 2 H), 2.00 (dd, J = 13.4, 11.3 Hz, 1 H), 1.76 (d, J = 14.5 Hz, 1 H),

1.55 (s, 3 H), 1.10 (s, 3 H), 1.06 (s, 3 H).

Ph Humulene-PTAD adduct 1.21: A solution of 4-phenyl-1,2,4-

O N O Me N NH triazoline-3,5-dione (17.2 mg, 1 equiv) in DCM (1 mL) was added Me

Me to a solution of humulene (22 µL, 0.098 mmol) in DCM (3 mL) at

0 ºC. The reaction was done instantaneously, was concentrated in vacuo, and purified by flash column chromatography (hexanes to 20% EtOAc:hexanes) to afford humulene-

1 PTAD adduct 1.21 (20 mg, 54%): Rf = 0.72 (40% EtOAc:hexanes); H NMR (400 MHz,

CDCl3) δ 7.57 – 7.50 (m, 2 H), 7.46 (td, J = 7.1, 1.9 Hz, 2 H), 7.36 (t, J = 7.3 Hz, 1 H),

5.27 (d, J = 15.7 Hz, 1 H), 5.19 (s, 1 H), 5.13 – 5.09 (m, 2 H), 4.88 (d, J = 10.4 Hz, 1 H),

4.51 (dd, J = 10.1, 2.5 Hz, 1 H), 2.81 (dd, J = 12.0, 5.2 Hz, 1 H), 2.51 (t, J = 10.8 Hz, 1

H), 2.17 – 1.97 (m, 5 H), 1.79 – 1.73 (m, 2 H), 1.58 (s, 3 H), 1.11 (s, 3 H), 1.08 (s, 3 H).

Me Me Br Brominated Isodaucane 1.22: BF3OEt2 (17 µL, 1 equiv) was added to

Me bromohumulene 1.20 (38 mg, 0.134 mmol) in DCM (5.4 mL, 0.025 M)

at 0 ºC and was warmed slowly to room temperature. After the starting material was

consumed by TLC, the organic phase was washed sequentially with H2O and NaCl (sat. aq.), dried over MgSO4, and concentrated in vacuo. Brominated isodaucane 1.22 was

partially purified (~50%) by column chromatography (silica gel, hexanes): 1H NMR (500

MHz, C6D6) δ 5.43 (dt, J = 7.0, 3.1 Hz, 1 H), 3.70 (d, J = 9.3 Hz, 1 H), 3.62 (d, J = 9.5

Hz, 1 H), 3.02 (d, J = 16.4 Hz, 1 H), 2.82 (hept, J = 6.8 Hz, 1 H), 2.61 (dd, J = 16.4, 3.0

Hz, 1 H), 2.26 – 2.14 (m, 2 H), 1.73 – 1.65 (m, 1 H), 1.61 – 1.26 (m, 4 H), 1.00 (d, J =

6.8 Hz, 3 H), 0.97 (s, 3 H), 0.95 (d, J = 6.8 Hz, 3 H), 0.91 (m, 1 H assigned by 2D); 13C

34 NMR (151 MHz, C6D6) δ 141.7, 136.9, 136.8, 132.0, 128.2, 128.1, 127.9, 50.3, 41.8,

38.8, 36.0, 28.1, 27.1, 27.1, 25.0, 24.0, 21.7, 21.3.

Me Me Isohumulene 1.23: To bromohumulene 1.20 (10 mg, 0.035 mmol) in

Me benzene (1.16 mL, 0.3 M) was added tributyltin hydride (28 µL, 3 equiv) and the reaction mixture was heated to 70 ºC until the starting material was consumed by

TLC. The reaction mixture was concentrated in vacuo and isohumulene 1.23 was purified

1 by preparatory TLC (hexanes): Rf = 0.55 (hexanes); H NMR (500 MHz, CDCl3) δ 5.10 –

5.06 (m, 2 H), 4.95 (t, J = 7.3 Hz, 1 H), 4.80 (s, 1 H), 4.68 (d, J = 1.0 Hz, 1 H), 2.60 (d, J

= 5.0 Hz, 2 H), 1.96 (t, J = 6.1 Hz, 2 H), 1.91 (d, J = 7.5 Hz, 2 H), 1.87 (t, J = 6.6 Hz, 2

H), 1.61 (dt, J = 12.6, 6.5 Hz, 2 H), 1.53 (s, 3 H), 1.08 (s, 6 H); 13C NMR (126 MHz,

CDCl3) δ 148.8, 140.1, 134.0, 125.4, 125.4, 108.9, 42.2, 40.4, 37.7, 37.4, 29.7, 25.1, 16.8.

O Me Br Me Br Epoxides 1.24 and 1.25: m-CPBA (5.4 mg, 1.05 Me + Me O Me Me equiv) was added to bromohumulene 1.20 (6 mg) in

DCM (0.84 mL, 0.025 M) at 0 ºC. When starting material was consumed by TLC, the

solvent was removed in vacuo and epoxides 1.24 and 1.25 were separated on preparatory

TLC (10% EtOAc:hexanes).

1 Epoxide 1.24: Rf = 0.41 (10% EtOAc:hexanes); H NMR (400 MHz, CDCl3) d 5.33 (s, 1

H), 5.20 (s, 1 H), 4.99 (d, J = 11.5 Hz, 1 H), 4.11 (dd, J = 10.0, 4.7 Hz, 1 H), 2.97 (dd, J

= 12.8, 2.8 Hz, 1 H), 2.79 – 2.67 (m, 1 H), 2.43 – 2.06 (m, 6 H), 1.89 (d, J = 14.5 Hz, 1

H), 1.71 (t, J = 11.3 Hz, 1 H), 1.63 (s, 3 H), 1.09 (s, 3 H), 0.73 (s, 3 H).

35 1 Epoxide 1.25: Rf = 0.28 (10% EtOAc:hexanes); H NMR (400 MHz, CDCl3) δ 5.42 –

5.29 (m, 2 H), 5.19 (s, 1 H), 5.08 (s, 1 H), 4.38 (dd, J = 9.8, 5.3 Hz, 1 H), 2.89 (dd, J =

12.4, 4.6 Hz, 1 H), 2.70 (dd, J = 12.3, 8.6 Hz, 1 H), 2.42 (d, J = 9.6 Hz, 1 H), 2.46 – 2.27

(m, 1 H), 2.19 (ddd, J = 19.3, 9.9, 4.6 Hz, 1 H), 2.00 (dt, J = 13.6, 4.8 Hz, 1 H), 1.58 (d,

J = 14.1 Hz, 1 H), 1.36 (dd, J = 13.6, 10.0 Hz, 1 H), ~1.2 (hidden, 1 H), 1.28 (s, 3 H),

1.17 (s, 3 H), 1.10 (s, 3 H).

Me Me Humulene palladium complex 1.26: Tris(dibenzylideneacetone)- II Pd Ln Me dipalladium(0)-chloroform adduct (~1 mg, 10 mol%) was added to

humulene (2 mg, 0.007 mmol) in THF (0.28 mL, 0.025 M) at room temperature under an

atmosphere of argon. The reaction was heated to 66 ºC and changed color from red to

yellow. When humulene was observed to be consumed by TLC the reaction mixture was

concentrated in vacuo and loaded directly on to preparatory TLC and palladium complex

1 1.26 was isolated as a yellow film: Rf = 0.65 (15% EtOAc:hexanes); H NMR (400 MHz,

CDCl3) δ 5.31 (ddd, J = 15.0, 8.7, 5.9 Hz, 1 H), 5.22 (d, J = 16.4 Hz, 1 H) 5.04 (dd, J =

9.7, 5.4 Hz, 1 H), 4.30 (d, J = 11.4 Hz, 1 H), 3.69 (s, 1 H), 3.27 (s, 1 H), 2.96 (t, J = 11.4

Hz, 1 H), 2.77 (dd, J = 13.6, 6.2 Hz, 1 H), 2.19 (ddd, J = 12.7, 10.0, 4.0 Hz, 1 H), 2.09 –

2.03 (m, 1 H), 2.02 – 1.94 (m, 2 H), 1.81 (dd, J = 13.6, 5.6 Hz, 1 H), 1.33 (s, 3 H), 1.09

13 (s, 3 H), 1.07 (s, 3 H); C NMR (151 MHz, CDCl3) δ 142.8, 126.2, 125.0, 83.1, 61.7,

41.1, 40.5, 40.1, 28.9, 28.0, 26.4 (carbons assigned by 2D, not all carbons assigned);

HRMS calcd for C15H23Pd [M-Ln] 309.0835 (100%), 311.0839 (96.6%), 308.0851

(81.7%), found (ESI-TOF) 308.0847, 309.0834, 311.0835, 350.1091 [M-Ln+H2O+Na].

36 Ph Epoxide 1.28: m-CPBA (6.4 mg, 1 equiv) in DCM (0.25 mL) in

O N O Me N NH added to humulene-PTAD adduct 1.21 (10 mg, 0.026 mmol). The Me O Me reaction is judged complete by TLC in 15 minutes and is washed

with H2O. The aqueous phases are extracted with DCM (3x) and the organic phases are combined and dried over MgSO4. Epoxide 1.28 is purified by preparatory TLC (50%

1 EtOAc:hexanes) (6.6 mg, 64%): Rf = 0.24 (40% EtOAc:hexanes); H NMR (600 MHz,

CDCl3) δ 8.07 (s, 1 H), 7.98 (d, J = 7.7 Hz, 1 H), 7.58 (d, J = 8.1 Hz, 1 H), 7.54 (d, J =

7.8 Hz, 1 H), 7.49 (t, J = 7.7 Hz, 1 H), 7.41 (dt, J = 15.4, 7.6 Hz, 2 H), 5.57 (d, J = 15.7

Hz, 1 H), 5.34 (ddd, J = 15.9, 10.0, 5.7 Hz, 1 H), 5.28 (s, 1 H), 5.15 (s, 1 H), 4.73 (d, J =

10.7 Hz, 1 H), 2.88 (dd, J = 12.3, 5.6 Hz, 1 H), 2.58 (t, J = 11.0 Hz, 1 H), 2.43 (d, J = 9.1

Hz, 1 H), 2.24 (t, J = 12.6 Hz, 1 H), 2.05 (d, J = 14.0 Hz, 1 H), 1.70 (t, J = 14.2 Hz, 1 H),

1.56 (d, J = 14.2 Hz, 1 H), 1.46 (dd, J = 14.2, 9.2 Hz, 1 H), 1.34 (s, 3 H), 1.19 (s, 3 H),

13 1.12 (s, 3 H); C NMR (151 MHz, CDCl3) δ 154.0, 152.2, 141.3, 140.8, 134.8, 133.8,

131.2, 130.4, 130.0, 129.3, 128.5, 125.4, 124.9, 114.5, 64.0, 60.2, 53.9, 39.6, 37.6, 35.7,

33.8, 31.7, 30.9, 29.8, 25.8, 24.1, 22.8, 16.7, 14.3.

Nitrile 1.44: Isobutyronitrile (6.5 mL, 72.3 mmol) was added dropwise to a CN Me Me solution of lithium diisopropylamide (79.5 mmol, 1.1 equiv) in Et2O (28 mL)

and was stirred for 1 hour at 0 ºC. Allyl bromide (6.9 mL, 1.1 equiv) in Et2O (32 mL) was added via syringe pump over 2 hours at 0 ºC. The reaction mixture was warmed to room temperature and stirred for 24 hours. The reaction was then cooled to 0 ºC, slowly

quenched with H2O (32 mL), and extracted with Et2O (3 x 100 mL). The organic phases were combined, washed with HCl (2 M, 100 mL), NaHCO3 (sat. aq., 100 mL), H2O (100

37 mL), and NaCl (sat. aq., 100 mL), dried over MgSO4, and concentrated in vacuo. Nitrile

1 1.44 was isolated as a brown oil (6.4 g, 81%): Rf = 0.23 (5% EtOAc:hexanes); H NMR

(400 MHz, CDCl3) δ 5.87 (ddt, J = 16.9, 10.2, 7.3 Hz, 1 H), 5.27 – 5.13 (m, 2 H), 2.28

(ddd, J = 7.3, 1.3, 0.9 Hz, 2 H), 1.34 (s, 6 H).

CHO Aldehyde 1.45: Nitrile 1.44 (6.4 g, 58.6 mmol) in hexanes was cooled to –78 Me Me ºC. DIBAL-H (66.8 mL, 1 M in hexanes, 1.14 equiv) was added slowly and the reaction mixture was stirred at –78 ºC for 6 hours. Excess DIBAL-H was quenched

with ethyl formate (1.5 mL) and the reaction was warmed to 0 ºC. NH4Cl (sat. aq., 4 mL) was added, followed by HCl (10% aq.) until aluminum salts dissolved, and the organic

layer was separated and stirred with H2SO4 (10% aq.) for 30 min. All aqueous layers were

combined and extracted with Et2O. All organic layers were then combined and washed with NaCl (sat. aq.), dried over MgSO4, and Et2O was removed in vacuo. Aldehyde 1.45

1 was isolated as a ~50% solution in hexanes: H NMR (500 MHz, CDCl3) δ 9.49 (d, J =

0.6 Hz, 1 H), 5.71 (ddtd, J = 15.4, 10.4, 7.4, 0.5 Hz, 1 H), 5.12 – 5.03 (m, 2 H), 2.22 (d, J

= 7.4 Hz, 2 H), 1.06 (s, 6 H).

Alkyne 1.47: A mixture of aldehyde 1.45 (~1.5 g, ~50% in hexanes) and Me Me

K2CO3 (~2 equiv) in MeOH (100 mL) was treated with Ohira–Bestmann reagent 1.46

(2.7 g, 14.1 mmol) in MeOH (80 mL). After the reaction was complete by TLC, NH4Cl

(50% sat.) was added and the reaction mixture was extracted with minimal pentanes (6 x

25 mL, extracted until extracts didn’t show a TLC spot with KMnO4). Volume

concentrated by half in vacuo to afford alkyne 1.47 as a 1:117 solution in pentane: Rf =

38 1 0.34 (hexanes); H NMR (400 MHz, CDCl3) δ 6.01 – 5.84 (m, 1 H), 5.13 – 5.03 (m, 2 H),

2.18 (d, J = 7.5 Hz, 2 H), 2.11 (s, 1 H), 1.53 (s, 6 H).

Me Me Alcohol 1.51: Prior to the reaction set up, CuI was freshly dried under OH vacuum at 110 ºC overnight, magnesium turnings were washed sequentially with HCl (1

M), water, and then acetone, and THF was freshly distilled from a ketyl still. Magnesium turnings (1.26 g, 51.8 mmol, 5.8 equiv) were flamed dried under vacuum and allowed to cool under an argon atmosphere. THF (14.9 mL) was added and the reaction was placed in a room temperature water bath. A small amount of I2 in THF was added to activate the

magnesium. After all the I2 color dissipated, 3-chloro-2-methylpropene (1.74 mL, 17.68 mmol, 2 equiv) was then added to the magnesium turnings slowly, monitoring for exotherm during the formation of the Grignard reagent. The Grignard reagent was allowed to stir at room temperature for 1 hour and was transferred by cannula to a mixture of 2-methyl-2-vinyloxirane (0.87 mL, 8.91 mmol) and CuI (85 mg, 5 mol%) in

THF (18.6 mL) at –30 ºC. After the addition of the Grignard reagent was complete, the reaction was allowed to come to room temperature and was stirred for 3 hours. The reaction mixture was quenched with NH4Cl (sat. aq.) and was extracted with Et2O (3x).

The organic phases were then combined, washed sequentially with HCl (1 M), Na2HCO3

(sat. aq.), H2O, and NaCl (sat. aq.), dried over MgSO4, filtered, and concentrated in

vacuo. Flash column chromatography (silica gel, 15% EtOAc:hexanes) afforded pure

alcohol 1.51 (1.25 g, 92%): Rf = 0.29 (20% EtOAc:hexanes, product stains green in p-

1 anisaldehyde); H NMR (400 MHz, CDCl3) δ 5.43 – 5.38 (m, 1 H), 4.73 – 4.68 (m, 2 H),

39 4.00 (s, 2 H), 2.21 – 2.15 (m, 2 H), 2.09 – 2.03 (m, 1 H), 1.74 – 1.72 (m, 3 H), 1.69 –

1.67 (m, 3 H).

Me Me Bromide 1.49: Alcohol 1.51 (500 mg, 3.57 mmol) in DCM (16 mL) and Br THF (8 mL) was cooled to –40 ºC. Methanesulfonyl chloride (0.33 mL, 1.2 equiv)

followed by NEt3 (0.60 mL, 1.2 equiv) were added and the reaction was stirred at –40 ºC

for 2 h. The reaction was warmed to 0 ºC and diluted with THF (30 mL), and lithium

bromide (3.1 g, 10 equiv) was added. After the reaction was complete by TLC, mesyl

alcohol: Rf = 0.41 (20% EtOAc:hexanes), the crude mixture was partitioned between 10%

EtOAc:hexanes and washed with 5:1 NaCl (sat. aq.)/NaHCO3 (sat. aq.). Bromide 1.49

was purified by flash column chromatography (silica gel, distilled hexanes): Rf = 0.90

1 (20% EtOAc:hexanes); H NMR (400 MHz, CDCl3) δ 5.60 (t, J = 6.8 Hz, 1 H), 4.73 (s, 1

H), 4.68 (s, 1 H), 3.97 (s, 2 H), 2.18 (q, J = 7.2 Hz, 2 H), 2.07 (q, J = 8.0, 6.8 Hz, 2 H),

1.77 (s, 3 H), 1.72 (s, 3 H).

Me Me Alkyne 1.48: Bromide 1.49 (275 mg, 1.35 mmol) was added to a Me Me

solution of alkyne 1.47 (crude solution in pentane) in DMF (0.9

mL, 1 M). K2CO3 (124 mg, 1 equiv), Na2SO3 (57 mg, 0.5 equiv), DBU (1 drop), CuI (3.5 mg, 5 mol%) were added sequentially and the reaction was stirred vigorously overnight.

After 21 hours, small portions of alkyne 1.47, K2CO3, Na2SO3, and DBU were “titrated”

into the reaction until bromide 1.49 was fully consumed. The crude reaction mixture was

diluted with Et2O and washed with HCl (1 M). The aqueous phase was extracted with

Et2O and the organic fractions were combined, washed with NaCl (sat. aq.), dried over

40 MgSO4, and concentrated in vacuo. Alkyne 1.48 was purified by flash column

chromatography (silica gel, hexanes) as a clear oil (166 mg, 53%): Rf = 0.33 (hexanes);

1 H NMR (400 MHz, CDCl3) δ 6.02 – 5.86 (m, 1 H), 5.47 – 5.38 (m, 1 H), 5.09 – 5.02 (m,

2 H), 4.72 (s, 1 H), 4.70 (s, 1 H), 2.84 (s, 1 H), 2.20 – 2.02 (m, 6 H), 1.73 (s, 3 H), 1.66

(s, 3 H), 1.18 (s, 6 H).

Me Me Me Me Polyene 1.41: Alkyne 1.48 (20 mg, 0.087 mmol), t-BuOH (0.2

mL, 24 equiv), and ammonium sulfate (138 mg, 12 equiv) were added to condensed NH3

0 (~1.7 mL) at –50 ºC. To this was slowly added a solution of Li in NH3. When alkyne was fully consumed, the reaction was quenched with HCl (1 M), the organic phases were separated and blown dry and polyene 1.41 was purified by preparatory TLC (hexanes) as

1 a film (~50% conversion): Rf = 0.66 (hexanes); H NMR (400 MHz, CDCl3) δ 5.76 (ddt,

J = 14.6, 10.5, 7.3 Hz, 1 H), 5.41 (d, J = 15.6 Hz, 1 H), 5.30 – 5.21 (m, 1 H), 5.14 (t, J =

6.5 Hz, 1 H), 5.02 – 4.94 (m, 2 H), 4.75 – 4.65 (m, 2 H), 2.64 (d, J = 6.6 Hz, 2 H), 2.14

(q, J = 7.0 Hz, 2 H), 2.09 – 1.99 (m, 4 H), 1.73 (s, 3 H), 1.58 (s, 3 H), 0.97 (s, 6 H).

(OC)3Co Co(CO)3 Me Me Cobalt complex 1.55: Alkyne 1.48 (5 mg, 0.022 mmol) was

Me Me subjected to dicobalt octacarbonyl (17 mg, 1.1 equiv) in DCM

(0.5 mL, 0.1 M) under an argon atmosphere. The reaction was complete after 4 hours and was purified by flash column chromatography to afford cobalt complex 1.55 (7.2 mg,

1 60%): Rf = 0.54 (hexanes); H NMR (500 MHz, CDCl3) broad, but included for

completeness, see below; this compound was characterized by LC/MS, however due to

41 this experiment being conducted by a shoddy first-year graduate student, that data has

been lost in the annals of time.

Me Me Me H Hydrazine 1.60: Farnesyl bromide (5 mg, 0.018 mmol) N Me N Boc Ns was added as a solution in MeCN (0.11 mL) to protected

hydrazine 1.58 (5.6 mg, 1 equiv) and K2CO3 (12 mg, 5 equiv) in MeCN (0.70 mL). The

reaction mixture was filtered after 30 min and concentrated in vacuo. Hydrazine 1.60 was

purified by to flash column chromatography (silca gel, 5% EtOAc:hexanes): Rf = 0.61

1 (20% EtOAc:hexanes); H NMR (400 MHz, CDCl3) δ 8.36 (d, J = 8.8 Hz, 2 H), 8.11 (d,

J = 8.8 Hz, 2 H), 5.24 (t, J = 7.4 Hz, 1 H), 5.15 – 5.06 (m, 2 H), 4.22 (d, J = 6.8 Hz, 2 H),

2.18 – 2.04 (m, 6 H), 2.06 – 1.97 (m, 2 H), 1.72 (s, 3 H), 1.65 (s, 3 H), 1.63 (s, 6 H), 1.31

(s, 9 H).

Me Me Me H E/Z Imines 1.62: Farnesyl bromide (107 mg, 0.375 N Me N Boc mmol) was added as a solution in MeCN to protected hydrazine 1.59 (119 mg, 1 equiv)

and K2CO3 (259 mg, 5 equiv) in MeCN (3.75 mL total volume). The reaction mixture was stirred for several hours under argon, diluted with Et2O and washed with H2O. The aqueous phases were extracted with Et2O, and the organic phases were combined, dried

over MgSO4, and concentrated. Flash column chromatography (10-20% EtOAc:hexanes)

1 afforded E/Z imines 1.62 as an inseparable mixture: Rf = 0.27 (20% EtOAc:hexanes); H

NMR (400 MHz, d4-MeOD) δ 7.87 (ap t, J = 9.7 Hz, 2 H), 6.10 – 5.91 (m, 2 H), 5.17 –

5.03 (m, 4 H), 2.29 (ap t, J = 7.2 Hz, 2 H), 2.23 – 2.12 (m, 2 H), 2.09 – 2.03 (m, 4 H),

42 2.00 – 1.95 (m, 4 H), 1.88 (d, J = 1.4 Hz, 3 H), 1.86 (d, J = 1.3 Hz, 3 H), 1.66 (ap d, J =

1.4 Hz, 6 H), 1.61 (ap d, J = 1.3 Hz, 6 H), 1.59 (s, 6 H), 1.50 (s, 9 H), 1.49 (s, 9 H).

Me Me Polyene 1.72: Geraniol methyl carbonate 1.69 and cinnamyl borane Me Ph 1.70 were added to Pd2(dba)3 and PPh3 in THF at 60 ºC under rigorously air free conditions. The reaction was maintained at 60 ºC overnight. After 12

hours, the reaction was diluted with Et2O, washed with H2O, dried over MgSO4, and

concentrated. The crude reaction mixture was flushed through a silica plug to afford the

polyene product 1.72 in sufficient purity to identify it by NMR: 1H NMR (400 MHz,

CDCl3) δ 7.39 – 7.16 (m, 5 H), 6.51 – 6.31 (m, 2 H), 6.18 (dt, J = 15.5, 7.5 Hz, 1 H), 5.79

(dd, J = 17.5, 10.8 Hz, 1 H), 5.13 – 5.05 (m, 1 H), 5.04 (dd, J = 10.8, 1.4 Hz, 1 H), 4.94

(dd, J = 17.5, 1.4 Hz, 1 H), 3.56 (d, J = 6.6 Hz, 1 H), 2.25 – 2.19 (m, 2 H), 1.67 (s, 3 H),

1.59 (s, 3 H), 1.02 (s, 3 H).

43 1.6 Spectra

Me Me Me

Me Me

Me Me Me Me

O Me Me Me Me

O Me Me O O Me Me

Br Me Me Me

O NH Ph N N O Me Me Me

Br Me Me Me

! Br Me Me Me

' ( ! % % & $ ! " # ! " Br Me Me Me

Me Me Me

Br Me O Me Me

n L II Pd Me Me Me

! n L II Pd Me Me Me

O NH Ph N

Me O

NH Ph N N

Me Me

CN Me Me

CHO hexanes + Me Me

Me pentane + Me

Br Me OH Me Me

Me Me Me Me

Me Me Me

3 Me Co(CO) Me Co 3 Me (OC)

Boc

N H

N Ns

Me Me Me Me

Boc N H N Me

Ph Me Me

CHAPTER 2

An Approach to Mimicking the Sesquiterpene Cyclase Phase by Nickel-Promoted

Diene/Alkyne Cooligomerization

2.1 Introduction to Oxidative Cyclization in the Context of Terpene Synthesis

At this juncture in my Ph.D., I was tasked with covering 10 years of the journal

Organometallics in one succinct group meeting. In a single week I only made it through

the years 1990–1997, just fewer than 40,000 pages, but one repeated observation

completely changed the direction of my project: the oxidative cyclization of dienes (e.g. butadiene, 2.1) onto a variety of metals to form metallacycles like 2.2 (Scheme 2.1a).

a. oxidative cyclization + MLn M Ln

2.1 2.2

b. Me Ln Me Me M Me + MLn MLn

2.3 2.4 Me 2.5

c. Ni0 H Me Me Me R Biomimetic Me Me Me Cyclase Phase R germacrene A (2.6) Me

Scheme 2.1: a. and b. Oxidative cyclization of dienes onto metallacycles; c. Retrosynthesis of germacrenes based on an oxidative cyclization.

Similarly, the controlled oligomerization of isoprene (Scheme 2.1b) could

drastically simplify the synthesis of terpenes used in the medicine, perfumery, flavor, and

materials industries. Given my previous year and a half working with humulene (1.2), I

aspired to add a three carbon fragment to organometallic intermediate 2.5. However,

given the precedent that is highlighted in Section 2.2, the germacrene skeleton 2.6 was

considered a more apt initial target for this methodology. As such, this project was an effort to cooligomerize butadiene (2.1) or isoprene (2.3) with alkynes in a controlled

77 fashion by zero-valent nickel catalysis building off the classic studies by Günther Wilke

and coworkers. The general retrosynthetic idea is outlined in Scheme 2.1c.

A recapitulation of the biosynthesis of sesquiterpene natural products is a significant challenge in the laboratory. As a modest step in that direction, the Baran laboratory previously outlined a two-phase biomimetic approach to terpenes consisting of the efficient construction of a minimally oxidized terpenoid followed by stepwise, chemoselective oxidations (Scheme 2.2, left).1,2

OH H HO Me OHMe Me pygmol (2.7)

Oxidase Phase Me

H H Me Me

Me OH Me Me Me

dihydrojunenol (2.8) germacrene A (2.6)

Previously Biomimetic Published Cyclase Phase Cyclase Phase

O O Me Me + Me Me Me 2.9 2.10 Me isoprene (3 x 2.3)

Scheme 2.2: Retrosynthetic analysis of eudesmane natural products utilizing a two-phase approach to terpenes: Direct comparison between the previously published, target- oriented method (left) and the proposed biomimetic route (right).

Although our previous approach to the eudesmane sesquiterpenes (e.g., pygmol

(2.7) and dihydrojunenol (2.8)) is efficient and scalable (Scheme 2.2, left),2,3 it is

strategically inferior to Nature’s union of simple five-carbon building blocks. In further contrast to biosynthesis, the target-oriented synthesis used to access dihydrojunenol (2.8) from ketone 2.9 and aldehyde 2.10 does not permit divergent entry to numerous

78 scaffolds. Thus, a route from a simple C5 unit, isoprene (2.3), to macrocyclic terpene natural products was pursued. Such a plan would rapidly forge the all-carbon germacrane skeleton (2.6), in the lowest possible oxidation state (Scheme 2.2, right). In principle, such a structure could then be processed to a multitude of different terpenes, including the eudesmanes. This strategy would represent an ideal entry to many terpene families, as it does not require the divergent prefunctionalization of a five-carbon fragment (like the conversion of mevalonic acid pyrophosphate into DMAPP (1.8) and IPP (1.9) in Nature4) but would rely on the feedstock chemical isoprene (2.3) as the sole starting material.

Attempts to achieve this goal through a closely related diene/alkyne co-oligomerization are described in this article.

2.2 Planning and Historical Context

“[Nickel] catalysis may also be expected to be used in the partial syntheses of

natural products (e.g. terpenes from isoprene).” (Paul Heimbach, 1973)5

Isoprene is underutilized in terpene total synthesis, often because more advanced

terpenoid starting materials are available (e.g., limonene, farnesol, or steroid skeletons).

Additionally, there is a lack of methods to rapidly process isoprene into synthetically useful building blocks. The initial inspiration for harnessing the reactivity of isoprene arose from the well-established fact that dienes react with a variety of transition metals through an oxidative cyclization to form metallacyclopentenes (Scheme 2.1a-c). These

metallacycles are known to be key intermediates in the oligomerization of dienes. If

successfully controlled, a cyclo-oligomerization approach could rapidly furnish

79 macrocyclic terpene natural products, like the simple sesquiterpenes germacrene (2.6) or

humulene (1.2),6 which currently require multi-step total syntheses.

Identification of the metal–ligand system with the best precedence for rapidly

building medium-sized rings was the first priority. In principle, any metal capable of two- electron redox chemistry with a high affinity for π-donating ligands would be able to catalyze isoprene oligomerization. Indeed, a plethora of transition metals have been used to promote diene oligomerization,7,8 the most prominent of which are Pd,9-15 Fe,16-18

Co,16,19 Cr,20-22 Ti,21-24 Ru,25 and Zr.23

Eventually, the chemistry of Günther Wilke and Paul Heimbach using zero-valent nickel for the oligomerization of dienes piqued our interest.5,26-28 Utilizing dienes, Wilke and coworkers have established methods for the creation of 4,29-31 5,32-34 6,35 8,29-31 10,36-38

12,39 and >12-membered rings5,40 (Scheme 2.3a). In particular, the synthesis of 10-

41,42 membered ring systems by cooligomerization with a C2 component, either alkenes or alkynes,37,38,43,44 has been developed in great detail (vide infra). There have been extensive

studies on the effects of solvent, temperature, pressure, ligands, and substrates (for both

the diene and cooligomerization partner).45 In the case of cyclo-cooligomerization with ethylene, Wilke was able to achieve 78% crude yield on multi-kilogram scale.46 This seems ideal, as we had hoped that all of the skeletons targeted in this research program would be carried on to further oxidase-phase studies that require scalable access to the lowly oxidized germacrene scaffold. Heimbach’s striking quote above was particularly inspiring in this regard. To our knowledge, despite Wilke’s and Heimbach’s pioneering studies on the Ni0-catalyzed cyclo-oligomerization, chemists have not explored this prospect in the context of natural product synthesis.47-50

80 To further expand the concept of alkyne/diene cooligomerization, the catalytically

formed metal-bound C8-intermediate 2.11 (Scheme 2.3b) was identified as a reactive building block with differentiated termini51 with the potential to be intercepted by a

variety of C1, C2, and C3 units to selectively access caryophyllane, germacrane, and

humulane frameworks, respectively (Scheme 2.3a). Establishing such a

cooligomerization as a method for terpene synthesis would require a great expansion of

Wilke’s methodology – already well worked out for butadiene – by judicious choice and fine-tuning of both the catalytic system and cooligomerization components.

a. 4 >12 n 5 (cembrane family)

refs. 39–41 refs. 36–38 refs. 12, 47 12

ref. 42 ref. 46 + Ni0 6 C3

11 C2 refs. 43–45 refs. 36–38 C1

(humulane family)

8 10 9

(caryophyllane family) (germacrane family)

b. via: 0 " Me Me " C -C Me Ni 1 3 C -C ring 2 8 9 11 systems 1 C8-intermediate "geranyl synthon" (2.11)

Scheme 2.3: a. Synthesis of monocyclic carbon skeletons from butadiene by nickel catalysis; b. The nickel-bound C8-intermediate 2.11 as a “geranyl synthon.”

Zero-valent nickel stands out for its pronounced tendency to form medium-sized rings due to its innate size.52 In the absence of ligands, Ni0 will engage three butadiene

(2.1) molecules and rapidly form 1,5,9-trans,trans,trans-cyclododecatriene (CDT)

catalytically and in greater than 80% yield.53,54 Although handling Ni0 complexes requires

81 an inert atmosphere, there are reasonably stable zero-valent nickel sources such as

55 Ni(cod)2 which can be purchased or easily prepared in multigram quantities. In addition,

II the respective Ni precursors (e.g., Ni(acac)2) are air-stable, inexpensive, and are easily converted in situ to catalytically active Ni0 species by a variety of common reducing agents.56

If successfully adapted to isoprene, the application of this method employing

simple alkynes 2.12a-2.12c could produce the germacrane skeleton in a single step

(2.13a-2.13c, Scheme 2.4). Sophisticated choice of the alkyne “coupling” partner should

afford different functional handles on the terpenoid backbone and thus allow access to

related sesquiterpenes like β-elemene (2.14), α-eudesmol (2.15), or constunolide (2.16) within a minimum number of synthetic manipulations.

one step synthesis of germacrane skeleton by regio- and stereocontrolled cooligomerization

Me Me Me OH Me OH 2.12a 2.12b 2.12c

Me Me Me Ni0 Ni0 Ni0

1 1 1 10 10 10

Me Me Me 7 Me 7 OH 7 4 4 4 OH Me Me Me Me Me Me 2.13a 2.13b 2.13c

divergent elaboration to related sesquiterpene frameworks

Me Me Me H H H Me OH Me O H H Me Me Me Me O "-elemene (2.14) !-eudesmol (2.15) constunolide (2.16)

Scheme 2.4: Development of a controlled Ni0-catalyzed cooligomerization of isoprene (2.3): Rapid access to germacrene-derived natural products.

82 The field of catalytic C–C coupling processes has undergone a stunningly rapid

development since the early work of Wilke and his coworkers. Thus, a reinvestigation to

overcome some of the limitations that have to date precluded the application of Ni0- catalyzed ring-forming processes of isoprene to significantly more complex terpenoid frameworks was pursued. Several issues needed to be addressed in order to realize this plan; these are highlighted within the catalytic cycle of cooligomerization proposed by

Wilke and coworkers (Scheme 4). 57 In the first step, two isoprene units must be

oxidatively dimerized regioselectively, in a head-to-tail fashion, as found in Nature. In an experiment with stoichiometric nickel, Wilke has shown that the regioselectivity in the formation of 2.17 can be altered by employing a suitable ligand. Strikingly, the use of

PMe3 led to a 50:50 mixture of tail-to-tail and head-to-tail dimers, while changing the

i phosphine ligand to PPh3 or P Pr3 yielded exclusively the desired coupling product with head-to-tail connectivity.57,58 Similarly, van Leeuwen et al. have demonstrated modest control of regioselectivity in the cyclodimerization of isoprene through extensive exploration of phosphine ligands.59

Second, the hapticity in the key intermediate 2.18 (previously characterized by X- ray diffractometry33,60-62) has to be controlled since it determines the regioselective

outcome of the alkyne incorporation.58,63 The Ni0 intermediate 2.18 has been shown to

exist as the η1,η3-complex (as drawn) irrespective of the nature of the phosphine ligand.57

Since the two reactive positions in key intermediate 2.18 are clearly differentiated – both

sterically and electronically51 – regioselective incorporation of unsymmetrically

disubstituted alkynes was deemed a worthy pursuit.

83 The most serious drawback of the alkyne/diene cooligomerization as originally

reported by Wilke is that the cyclodecatrienes are always obtained as the 4Z,7Z,10E

isomers,64 whereas most naturally occurring germacrenes feature two endocyclic 4E,10E

double bonds.65 It was conjectured that fine-tuning the catalytic system using modern ligands capable of stabilizing low-valent transition metals and/or the addition of suitable additives might resolve this issue.66 As an alterative, the possibility of delaying the olefin

isomerization to a subsequent transformation was considered.67,68

product stability, Me product isolation Ni0 + L + 2 1 1 10 R head-to-tail Me regioselectivity 7 2 4 R

Me 2.13 Me Ni L reductive oxidative elimination cyclization Me Me 2.17 linear vs. cyclic dimers, higher order oligomers, polymers

olefin E/Z geometry hapticity Me Me Me

Ni Ni L Me L R2 R1 2.19 2.18

R1 regioselective alkyne incorporation, ligand 2.12 dissociation alkyne alkyne Me 2 insertion R coordination

Ni L Me R1 R2

Scheme 2.5: Catalytic Cycle as Proposed by Wilke, and Difficulties (blue) Associated with the Cooligomerization of Isoprene and Alkynes.

Additionally, if intermediate 2.19 reenters the catalytic cycle, the

cooligomerization reaction has the potential to produce higher-order oligomers and

polymers, both linear and cyclic.69 For simple systems, Wilke was able to optimize the ligand, temperature, and reactant ratio in order to partially suppress such nonproductive pathways.37,38,70

84 Finally, germacrene sesquiterpenes are notoriously unstable to acidic conditions

(leading to cyclized products), thermal conditions (leading to Cope rearrangements),71

and can often exist as conformationally stable isomers at ambient temperatures.72-74

All of the above issues lead to further serious, albeit technical obstacles since the copolar nature of the products creates problems with regard to purification of crude reaction mixtures and subsequent analysis of inseparable mixtures of isomers. Because

Heimbach conducted most of his work with butadiene 2.1 (instead of isoprene 2.3) and symmetric alkynes, many of the above difficulties were avoided. Furthermore, his research group often simplified the resulting mixture of crude products by hydrogenation or Cope rearrangement.75,76 Our proposal, utilizing isoprene and unsymmetrical alkynes

2.12, and accounting for all possible combinations of E and Z olefins, leads to 32 possible

isomers 77 (as opposed to only six possible isomers for the cooligomerization of symmetric alkynes with butadiene). While Wilke and coworkers usually purified complex product mixtures by gas chromatography and/or spinning-band distillation on preparative scale,36,44 this was deemed unsuitable for the synthesis of terpenoid natural

products in multigram quantities. Despite these tremendous obstacles, the benefits of

such a simple and rapid synthesis were still enticing, and thus research endeavors were

carried out.

2.3 Butadiene Cooligomerizations

Initial forays utilized butadiene as a means to probe the scope of alkyne

38 substitution. Wilke’s reported procedure was repeated at first: Ni(cod)2 (3.4 mol%),

PPh3 (3.4 mol%), butadiene (5 equiv.), and 4-octyne (2.12d), neat, and sealed in a

85 pressure vessel at room temperature for ~18 hours (Scheme 2.6a). With similar dialkyl

alkynes, Wilke reported a ~90% yield of 10-membered rings based on GC which was

immediately distilled to afford the respective Cope products.78 In our hands, macrocycle

2.20d could be isolated (along with trace impurities) by silica gel chromatography, eluting with hexanes, in 83% yield. As anticipated, 4Z,7Z,10E-cyclodecatriene 2.20d was conformationally flexible at room temperature and required low-temperature NMR for reliable structure elucidation; this turned out to be a recurring issue for the analysis of any

4Z,7Z,10E-cyclodecatriene system. 2.20d underwent quantitative Cope rearrangement at

120 ºC to give divinylcyclohexene 2.21.79 It was found that simply increasing the catalyst loading from 3.4 mol% to 10 mol% shortened the necessary reaction time. The use of a pressure vessel was essential for the reaction to proceed quickly and with good conversion, as previously demonstrated by Wilke. Unfortunately, the calibrated and heatable glass autoclave routinely used by Wilke to monitor internal pressure, volume loss, and temperature over the course of the reactions was not available to us.30,37,44,80

Although volume loss was observed during the reactions, its quantification was not

possible. To obtain comparable results, reactions were typically stopped after ~16–20 h.

86 a. Ni(cod)2 Me Me (10 mol %), Me + Me PPh (10 mol %) Me 3 120 ºC Me (5 equiv) (~quant.) 2.12d 2.20d 2.21

CO2Me CO2Me CO2Me b. Ni(cod) 2 MeO C CO Me MeO C MeO C CO2Me (10 mol %), 2 2 2 2 + + + MeO C PPh (10 mol %) 2 3 MeO C CO Me MeO C MeO C (5 equiv) 2.12e 2 2 2 2 CO2Me CO2Me CO2Me 2.22 2.23 2.24 (trace)

c. Ni(cod) 2 OTBS TBSO OTBS OTBS (10 mol %), + + PPh3 (10 mol %) (40 equiv) OTBS 2.12f 2.20f 2.25

d. Ni(cod) 2 OH HO OH (10 mol %), + +

PPh3 (10 mol %) OH HO (5 equiv) 2.12c 2.20c 2.26 + 2.27

OH Scheme 2.6: Major products of selected butadiene/alkyne cooligomerization reactions.

After a preliminary substrate screen, several interesting, albeit undesired, side

products that plagued these reactions were identified. Specifically, the use of alkynes

with conjugated electron-withdrawing groups (e.g., 2.12e) gave exclusively [2+2+2],

Reppe-type cycloadducts (e.g., 2.22, Scheme 2.6b). 81 , 82 This well-precedented cycloaddition demonstrates one background reaction that occurred with many different substrates (cf. formation of 2.25, Scheme 2.6c). Many reactions also afforded hetero-

[2+2+2] products, in which two alkynes and a single butadiene molecule had reacted

(e.g., rac-2.23 and 2.24, Scheme 2.6c). In addition, linear products were observed repeatedly (e.g., 2.27, Scheme 2.6d).

As expected, the isolation and structure elucidation of these repeatedly occurring motifs was challenging, but their characterization enabled the establishment of guidelines for the analysis of the conducted cyclo-cooligomerization reactions (Figure 2.1). Because product purification was extraordinarily tedious, rapid analysis of crude reaction mixtures

87 by means of GC-MS and/or crude NMR according to the flowchart shown in Figure 2.1 was crucial.

Figure 2.1: A flowchart that demonstrates the method of reaction analysis, illustrated with selected gas chromatograms representative for reactions belonging to categories A– E.

Immediately, after running the reactions, crude GC/MS analysis quickly showed

if the reaction fit into category A or B: no reaction at all or too complex to be practical. In both of these cases, the reactions were abandoned at that point. If the reaction appeared to show a limited number of products, but the desired product was not the main component by gas chromatography (category C), the reaction was worked up. The crude reaction mixture was first analyzed by 1H and 13C NMR. If crude analysis was insufficient and

88 separation was possible by silica gel chromatography, purification was undertaken and

the obtained products were reanalyzed. Frequently, isomeric mixtures were still

inseparable and were analyzed in a “semi-pure” state by 1D and 2D NMR, as well as

HRMS.

In the event that the product was the major component after crude analysis

(categories D1, D2, and E), purification was attempted by silica gel chromatography and preparative thin-layer chromatography, often in combination and repeatedly. It is worth noting that the selection of alkyne substrate was at least partially influenced by the projected facility of purification. Nonpolar alkynes such as isopropylacetylene (2.12a)

could rapidly lead to germacrene (2.6), but unless the reaction performed perfectly – in

terms of yield, regio- and stereoselectivity – completely intractable mixtures of nonpolar

compounds would result. In addition, distillation was hampered by the facile [3,3]-Cope

rearrangement. On the other hand, 10-membered ring systems with polar functional

groups were more easily handled and separated. Nevertheless, even for these more polar

products, secondary derivatization (e.g., removal of the protecting group from protected

alcohol 2.20f) was often necessary for purification (category D2).

With a working reaction in hand, namely butadiene/4-octyne (2.12d, Scheme

2.6a), the substrate scope was explored (Table 2.1). It was quickly determined that bulky

substituents on the alkyne inhibited the reaction, presumably due to steric clash around

the metal center (2.12g-2.12i, entries 1–3, Table 2.1). Carboxylic acid 2.12j did not react

(entry 4, Table 2.1), perhaps due to either chelation to Ni0 or the instability of zero-valent nickel in the presence of protic substrates. Allylic alkyne 2.12k resulted in a complex, inseparable mixture (entry 5, Table 2.1). As briefly stated above, alkynes with electron-

89 withdrawing substituents (2.12e and 2.12l, entries 6 and 7, Table 2.1) reacted exclusively

in [2+2+2] fashion, which is well documented in the literature.81 Under standard conditions, substrates 2.12b, 2.12m, and 2.12n (entries 8–10, Table 2.1) also primarily underwent [2+2+2] cyclotrimerization. This was not entirely unexpected, as Reppe-type trimerizations were generally observed as a background reaction. The use of excess butadiene (20 equiv.) with protected alcohol 2.12n (entry 11, Table 2.1) suppressed the

[2+2+2] reaction. Unfortunately, the primary 10-membered ring was only observed in trace amounts by GC-MS; instead, the follow-up Cope product predominated. In the presence of homopropargyl alcohol (2.12c, entry 12, Table 2.1) the primary product was

found to be a linear adduct between two alkynes and a single butadiene (see Section 2.8).

This may be due to the ability of the free alcohol to coordinate to the nickel center

bringing two alkynes closer to the reactive metal.83,84 Simple alkyl alkynes 2.12a, 2.12d,

or 2.12o are incorporated efficiently into 10-membered rings, as previously demonstrated

by Wilke (entries 13–15, Table 2.1). In the case of 1-octyne (2.12o), the crude mixture

was distilled directly and analyzed as its respective Cope-rearranged product (structure

not shown, see Section 2.8). It is worth mentioning that the reaction with 1-octyne

(2.12o) is the first reported case of successful incorporation of a terminal alkyne into a

10-membered ring system under Wilke’s reported conditions. Utilizing

isopropylacetylene (2.12a), the product was isolated following derivatization by

cycloaddition with 1,1-dibromoformaldoxime (see Section 2.8). To the best of our

knowledge, α-branched alkyl alkynes have not been previously used in the Ni0-catalyzed cooligomerization with dienes. To fully suppress the formation of linear side products (cf. entry 12, Table 2.1), the unsymmetrically disubstituted alkynes 2.12p and 2.12q were

90 investigated (entries 16–17, Table 2.1). In each case, the desired product was obtained

and crude analysis of the reaction mixture was sufficient for structural elucidation.

Product 2.20p (entry 16, Table 2.1) was isolated as a 1.6:1 mixture of regioisomers (with respect to the alkyne incorporation) as determined by low-temperature 2D-NMR.85 This encouraging result indicated that regiochemical incorporation of the alkyne might be even more effectively controlled by judicious choice of the substrate.

Although surprising at first glance, it was observed that disubstituted products

2.20p and 2.20q (entries 16 and 17, Table 2.1) underwent Cope rearrangement at unusually low temperatures. It is difficult to deduce simple rules for the propensity of the

10-membered macrocycles to rearrange, but there are two trends, both of which are

supported by theses from the Heimbach group.42 First, 4Z,7Z,10E-cyclodecatrienes

undergo Cope rearrangement at a lower temperature (room temperature to 60 ºC) than

4Z,10E-cyclodecadienes (~150 ºC). Second, the ease of [3,3]-rearrangement correlates

with increasing bulkiness and number of the substituents on the olefinic carbons. This is,

however, extremely dependent on the position of the substituents. These trends are a

result of the orbital overlap of the π-systems as determined by the conformation of the macrocycle, which is strongly influenced by the substitution pattern of the 10-membered ring. A significant driving force for the rearrangement is the strain release of the macrocycle.

Table 2.1: Ni0-catalyzed butadiene/alkyne cooligomerization.a 1 2 2 10 R R Ni(cod)2, PPh3 + R1 7 1 2.12 4 R 2.20

entry alkyne R1 R2 product outcomeb

1 2.12g CH2CH2OTBS TBS 2.20g A 2 2.12h CH2CH2OTBS TBDPS 2.20h A

91 3 2.12i CH2CH2OTBS TIPS 2.20i A 4 2.12j (CH2)4CO2H H 2.20j A 5 2.12k CH2CH=CH2 H 2.20k B c 6 2.12e CO2Me CO2Me 2.20e C c 7 2.12l CO2Et H 2.20l C c 8 2.12m CH2CH2Cl H 2.20m C d c 9 2.12b C(CH3)2OH H 2.20b C c 10 2.12n C(CH3)2OTMS H 2.20n C d 2.12n 2.20n e 11 C(CH3)2OTMS H C f 12 2.12c CH2CH2OH H 2.20c C 13g 2.12d nPr nPr 2.20d D2, Eh 14 2.12o nHex H 2.20o D2 15 2.12a iPr H 2.20a D2 16 2.12p CH2CH2OTBS Me 2.20p D1 17 2.12q CH2CH2OTBS TMS 2.20q D1 g h 18 2.12f CH2CH2OTBS H 2.20f D2, E a Reagents and conditions: 5:1 butadiene/alkyne, Ni(cod)2 (10 mol%), PPh3 (10 mol%), rt, 16 h. bRefer to Figure 2.1 for reaction outcomes. cMain product: [2+2+2] adducts. d20:1 butadiene/alkyne. eMain product: Cope; GC/MS shows traces of 10-membered ring. fMain product: linear coupling products (2.27, see Section 2.8). g40:1 butadiene/alkyne. hThe macrocyclic product was characterized both directly and after derivatization.

Based on the above results, terminal alkyne 2.12f (entry 18, Table 2.1) appeared to be an ideal cooligomerization partner since: (1) The alkyne is electronically and sterically more differentiated than 2.12p and 2.12q (entries 16 and 17, Table 2.1) and was thus expected to be incorporated with higher regioselectivity; (2) the respective 10- membered ring 2.20f is less prone to undergo Cope rearrangement under the reaction conditions, therefore enabling its isolation; and (3) the desired product 2.20f would contain a suitably functionalized side chain for elaboration into terpene natural products when applied to a cooligomerization reaction with isoprene (2.3).

Under the standard conditions, the desired product 2.20f was observed, albeit with

a substantial number of higher-order oligomeric side products and Reppe-type

cycloadducts. In order to suppress these side reactions, optimization of the

butadiene/alkyne 2.12f cooligomerization was carried out. An extensive ligand screen

92 including mono- and bidentate phosphines (e.g., P(2-furyl)3, PBu3, SPhos, and rac-

BINAP), amines (NPh3), NHCs (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride), and arsines (AsPh3) led to either inhibition or no significant improvement when

compared to the routine ligand (PPh3). However, two general trends were deduced: (1)

The only ligands that afforded the desired product were derivatives of PPh3 (e.g., P(3-

MeOPh)3 or P(4-ClPh)3) and (2) the use of bidentate ligands (e.g., dppp or rac-BINAP) showed no consumption of alkyne. Changing the reaction temperature from room temperature to 40 ºC led to a more complex mixture, while decreasing it to 4 ºC led to a lowered reaction rate without improving the reaction outcome. Finally, the diene/alkyne ratio was modulated in the hope of inhibiting the formation of both [2+2+2] products and other nonproductive pathways. A decreased ratio of 2:1 resulted in reduced formation of

10-membered ring and more side products, while a 40:1 stoichiometry sufficiently suppressed side reactions involving multiple alkynes. Similarly, diluting the reaction with

toluene, Et2O, or THF (5:1 diene/alkyne, concentration: 0.5 M) led to comparable results.

Finally, the optimized conditions (vide supra) afforded the desired product 2.20f in 23% isolated yield (980 mg product). Most remarkably, macrocycle 2.20f was shown to be a single regioisomer by 1D and 2D NMR studies, demonstrating the excellent selectivity of the alkyne insertion step (see 2.18 → 2.19, Scheme 2.5). With this result, it was anticipated that substrate 2.12f could lead to terpene natural products if cooligomerized with isoprene instead of butadiene.

93 2.4 Innate Reactivity of 4Z,7Z,10E-Cyclodecatrienes

With 4Z,7Z,10E-cyclodecatriene 2.20f in hand, some of its general reactivity was explored. Macrocycle 2.20f was first deprotected with TBAF to generate free alcohol

2.20c (Scheme 2.7). From this common intermediate, various derivatizations were carried out. A transannular cyclization with acetic acid afforded the 6,6-fused bicyclic system rac-2.28. While the constitution of macrocycle 2.20f had been fully elucidated, the

regiochemical outcome of the alkyne insertion remained unclear due to the complexity of

the NMR spectra, even employing low temperature experiments. After extensive efforts

to synthesize crystalline derivatives of 2.20c or 2.20f (e.g., by epoxidation with excess m-

CPBA and esterification with various substituted benzoyl chlorides), single crystals

suitable for X-ray crystallographic analysis were finally obtained from the dibrominated derivative 2.29 (Scheme 2.7). The crystal structure confirmed the constitution, previously

determined by NMR experiments (see Section 2.8), and unambiguously proved the

regiochemical outcome of the alkyne insertion: Thus, the macrocycles 2.20c and 2.20f

are 4Z,7Z,10E-cyclodecatrienes with a side chain in the 7-position.

Crabtree’s catalyst effected a selective hydrogenation, unexpectedly reducing the most strained 10E-olefin rather than the olefin proximal to the alcohol to yield diene 2.30.

Continued hydrogenation led to the expected 7Z-double bond being reduced, affording

macrocycle rac-2.31. In contrast, Wilke had demonstrated the reduction of both

38 disubstituted olefins with H2/Raney-Ni. Not surprisingly, under thermal conditions, the

10-membered ring 2.20c underwent [3,3]-Cope rearrangement to give divinylcyclohexene rac-2.32. Additionally, Heimbach and coworkers have shown that

4Z,7Z,10E-cyclodecatrienes can be further processed to saturated hydrocarbons and

94 diketones by selective hydrogenation and ozonolysis, respectively.86,87 As such, a variety of synthetically useful building blocks can be obtained from 4Z,7Z,10E-cyclodecatrienes,

as exemplified by some selected reactions of macrocycle 2.20c (Scheme 2.7).

H 1 10 H 100 ºC H2SO4, AcOH + (for R = TBS) 7 (for R = TBS) OTBS OR OAc H 4 OAc 2.20f: R = TBS H rac-2.32 AcO rac-2.28 2.20ad (80%) TBAF (46%) (28%) (97%) 2.20c: R = H 4-BrPhNCO (4 equiv), Et N, Crabtree, H2 3 (for R = H) DMAP (cat.), DCM (38%) (for R = H)

1 1 10 10 8 O O 7 + Ar 7 OH OH O N N H 4 H Ar rac-2.31 2.30 2.29: Ar = 4-BrPh (~30%) (~60%)

Scheme 2.7: General Reactivity of 4Z,7Z,10E-Cyclodecatriene 2.20c.

2.5 Isoprene Cooligomerizations

The reaction of simple test substrate 4-octyne (2.12d) with isoprene (2.3) afforded

a crude mixture of 10-membered ring products, which was distilled to give two regioisomeric Cope products (rac-2.33 and meso-2.33’, 1:1 ratio, Scheme 2.8) as determined by 1H, 13C, and APT NMR. The structural elucidation of vinylcyclohexenes rac-2.33 and meso-2.33’ provided indirect proof of the constitution of the primary products: rac-2.33 arises from the desired 4,10-dimethylcyclodecatriene 2.13d (head-to- tail connectivity), while meso-2.33’ traces back to the respective 1,4-regioisomer 2.13d’

(tail-to-tail connectivity). From the obtained data, the presence of the head-to-tail 1,5- dimethyl regioisomer cannot be excluded since it converges to yield the same Cope product (rac-2.33) as the 4,10-dimethylcyclodecatriene 2.13d. The fact that the third possible regioisomer (i.e., with head-to-head coupled isoprene subunits) was not observed, demonstrated that the regioselectivity in the initial oxidative dimerization of

95 88 two C5 building blocks can indeed be controlled – at least to a certain extent. The head- to-tail connectivity and 4Z,10E-geometry of 2.13d was corroborated by exhaustive epoxidation of the crude reaction mixture and subsequent X-ray crystallographic analysis of triepoxide product (rac-2.34, Scheme 2.8b). The reaction of 2.13d with excess m-

CPBA afforded a mixture of three components, two of which were determined to be diastereomers by X-ray crystallography, both arising from the desired head-to-tail coupled 10-membered ring 2.13d.89 At this point, the regioselectivity problem arising

from the alkyne insertion was successfully overcome (cf. 2.20f), and the formation of

regioisomeric mixtures (head-to-tail selectivity) during the oxidative dimerization of

isoprene was partially solved (cf. 2.13d).

a. Me Me + Me (5 equiv) 2.3 2.12d

Ni(cod)2 (10 mol%), PPh3 (10 mol %), 16 h

1 Me 10 Me 1 10 Me Me + 4 Me 5 4 Me Me 5 2.13d Me 2.13d' + 1,5-dimethyl- regioisomer? distillation

Me Me H Me Me 10 1 + 4 Me 4 H Me Me H rac-2.33 Me meso-2.33'

b. H O 1 Me 10 O Me 4 5 Me Me O H rac-2.34

Scheme 2.8: a. Isoprene cooligomerization model study; b. Crystal structure of triepoxide rac-2.34.

96 After successfully applying Wilke’s reaction conditions to isoprene/4-octyne

(2.12d) the substrate scope was explored in further detail (Table 2.2). Substrates 2.12r-

2.12u (entries 1–4, Table 2.2) were not reactive under these standard conditions. This can

be rationalized for the carboxylic acid and the diol, in agreement with the results obtained

with butadiene (entries 4 and 12, Table 2.1): Coordination events and/or the presence of

an acidic proton may inhibit or shut down the reactivity of the catalytically active nickel

species. This hypothesis was substantiated by testing methyl ester 2.12v which was fully

consumed, but in unproductive fashion (entry 5, Table 2.2). As opposed to the

observations for butadiene, simple terminal alkyl alkynes 2.12a, 2.12o, and 2.12w

(entries 6–8, Table 2.2) typically resulted in complex mixtures. Similarly, alkyl, phenyl,

and oxygenated unsymmetrically disubstituted alkynes (2.12n, 2.12q, and 2.12x-2.12ac,

entries 9–16, Table 2.2) were consumed, but also resulted in inseparable mixtures. As

expected from the results obtained for butadiene (cf. Table 2.1), substrates 2.12l and

2.12b (entries 17 and 18, Table 2.2) showed [2+2+2] adducts as the primary products.

Disappointingly, the substrates which had provided the most promising results for

butadiene, namely alkynes 2.12p and 2.12f (entries 19 and 20, Table 2.2), yielded

mixtures of [2+2+2], linear, Cope, and other unidentifiable products. Protected alcohol

2.12p (entry 19, Table 2.2) afforded a Cope product as the major component (see Section

2.8), demonstrating the intermediacy and thermal instability of the desired 10-membered

ring. Being aware that dilution had significantly improved the butadiene/2.12f

cooligomerization, a dropwise addition of protected alcohol 2.12f was undertaken (entry

20, Table 2.2). As expected, the reaction performed poorly in the absence of a pressure

vessel, but there was a noticeable increase in product formation by GC/MS analysis.

97 Unfortunately, the mixture was still too complex to allow for isolation, and further

attempts to improve the reaction were met with failure. In order to amend these results, a variety of additives were screened, including Brønsted (e.g., AcOH, Montmorillonite

i K10) and Lewis acids (e.g., MgBr2·Et2O, TiCl2( OPr)2), bases (e.g., DBU, Cs2CO3), co-

metals (e.g., PtCl4, AgNO3), solvents (e.g., DMSO, hexafluoroisopropanol), and salts

(e.g., KI, Bu4NBr). However, none of these led to any improvement in the reaction.

Table 2.2: Ni0-catalyzed isoprene/alkyne cooligomerization.a 1 2 10 R 2 Me R Ni(cod)2, PPh3 + Me 7 1 R1 4 R 2.12 Me 2.13

entry alkyne R1 R2 product outcomeb 1 2.12r TMS H 2.13r A 2 2.12s TMS TMS 2.13s A c 3 2.12t (CH2)2CO2H H 2.13t A

4 2.12u CH2OH CH2OH 2.13u A

5 2.12v (CH2)4CO2Me H 2.13v B 6 2.12w Et H 2.13w B 7 2.12o nHex H 2.13o B 8 2.12a iPr H 2.13a B 9 2.12x iPr Et 2.13x B 10 2.12y Ph Bu 2.13y Bd

11 2.12n C(CH3)2OTMS H 2.13n B

12 2.12z CH2OTHP H 2.13z B

13 2.12aa CH2CH2OTHP H 2.13aa B

14 2.12ab CH2CH2OMe H 2.13ab B

15 2.12ac CH2CH2OTMS H 2.13ac B

16 2.12q CH2CH2OTBS TMS 2.13q B e f 17 2.12l CO2Et H 2.13l C g f 18 2.12b C(CH3)2OH H 2.13b C h 19 2.12p CH2CH2OTBS Me 2.13p C e,g f,i 20 2.12f CH2CH2OTBS H 2.13f C 21 2.12d nPr nPr 2.13d D2 a Reagents and conditions: 5:1 isoprene/alkyne, Ni(cod)2 (10 mol%), PPh3 (10 mol%), 60 ºC, 16 h. bRefer to Figure 2.1 for reaction outcomes. cToluene was used as a cosolvent (0.5 M). dTrace Cope product observed. eAlkyne was added dropwise; not under pressure. fMain product: [2+2+2] adduct. g40:1 butadiene/alkyne. hMain product: Cope product. iMain product: Linear trimer (see Section 2.8).

98 Despite all of the substrates and conditions explored, none of the

cooligomerizations employing isoprene proceeded with sufficient selectivity. Even when

utilizing the conditions optimized for butadiene (vide supra), the isoprene/alkyne 2.12f

cooligomerization resulted in a complex mixture. Furthermore, the reaction of isoprene

with one of the simplest substrates, 1-octyne (2.12o), revealed a general limitation: While the butadiene/1-octyne (2.12o) system had provided a clean reaction to form the desired

10-membered ring, the outcome was completely different with isoprene, in which case the desired macrocyclic product 2.13o was not observed. Instead, formation of different

isomers of the rearranged Cope product predominated. This showcases an inherent

problem related to the use of isoprene as the cooligomerization partner, since substituted

4,10-dimethyl-4Z,7Z,10E-cyclodecatrienes – the desired products – can be expected to undergo Cope rearrangement under the reaction conditions (60 ºC). For this reason, any optimization cannot overcome the main obstacle: The thermal instability of substituted

4,10-dimethyl-4Z,7Z,10E-cyclodecatrienes. While significantly lowering the reaction

temperature may preclude Cope rearrangement, the rate of the reaction would then

become the limiting factor and would prevent its synthetic utility.

2.6 Delayed Installation of C1 Units onto 4Z,7Z,10E-Cyclodecatriene 2.20c

With the inherent flaw in the isoprene/alkyne cooligomerization revealed, the butadiene system 2.20c (obtained by deprotection of macrocycle 2.20f, Scheme 2.7) was

reevaluated. It was envisioned that subsequent installation of the “missing” methyl

groups on macrocycle 2.20c could still enable access to germacrene natural products

(Scheme 2.9). Most importantly, this transformation would have to proceed with high

99 regio- and chemoselectivity, to afford only one out of four possible regioisomers (rac-

2.35–rac-2.38, Scheme 2.9). Epoxidation followed by subsequent SN2 reaction with a methyl nucleophile was ruled out because backside attack on the σ* orbitals is blocked by the macrocycle. Utilizing a cycloaddition strategy would install the necessary carbon atom while maintaining the oxidation state of the endocyclic olefin. Cycloadditions with

1,1-dibromoformaldoxime have been shown to proceed with high regioselectivity at room temperature,90 which was crucial to avoid Cope rearrangement. The intended

endgame would then involve fragmentation of the resulting isoxazoline moieties to the

bis-β-hydroxynitrile rac-2.39 with Raney-Ni91,92 or TMSCl/NaI.93 Following elimination

of the 2º alcohols, a vinyl nitrile cross-coupling based closely on the work of Dankwardt

could be employed in order to access the lowly oxidized germacrene skeleton 2.40.94

HO N N Br N Br Br N Br Br, O O H H O 1 1 H NaHCO3, 1 + H H 7 HO 4 7 OH N H EtOAc H OH H 5 7 OH (41%) Br H rac-2.37 O H rac-2.38 1 rac-2.42 1:1 mixture O 10 Br Br , N N Br (58%) NaHCO3, + 7 OH EtOAc 4 HO 5 2.20c N N N O Br N O H O Br Br Br, Br H 10 H 10 10 NaHCO3, H 7 OH + H H H 7 7 OH 4 rac-2.36 5 OH EtOAc Br H H (47%) O rac-2.41 (21%) N O H rac-2.35 1:1 mixture desired regioisomer (minor) desired regioisomer N Br

HO 10 CN Me 10 7 4 OR H 4 7 OH Me 2.40 H rac-2.39 NC OH

Scheme 2.9: Attempted installation of missing germacrene carbons on a macrocycle derived from butadiene.

100 In practice, 1,1-dibromoformaldoxime afforded the desired isoxazoline rac-2.41, unfortunately as the minor regioisomer (2.7:1 ratio, Scheme 8). Regioisomers rac-2.41 and rac-2.42 were separated and subjected to the same [3+2]-cycloaddtion conditions.95

In both cases the respective diadducts (rac-2.35–rac-2.38) were formed with no observed regioselectivity (1:1 ratio). Although good chemoselectivity was achieved (i.e. the trisubstituted olefin remained intact), it proved impossible to alter the regioselectivity in either cycloaddition by changing the solvent, temperature, or base. Thus, the dicycloadducts were not elaborated to natural products because of a lack of regiocontrol during the dipolar cycloaddition. More importantly, this sequence deviated significantly from the initial proposal of an ideal approach, namely utilizing isoprene for the total synthesis of terpenes.

2.7 Conclusion

This chapter discussed my extensive studies aiming to mimic Nature’s cyclase phase of terpene synthesis. Wilke’s and Heimbach’s pioneering studies in both diene oligomerization and organonickel chemistry were vital in our efforts. Dr. Daniel Götz and

I have detailed a comprehensive list of their original publications related to diene

(co)oligomerization, many of which are not easily available, and efforts to translate and summarize details from the original German texts were made. Reinvestigating their work in a more modern context, an attempt was made to push the limits of their methods to the total synthesis of terpene natural products. The ambitious goal was to establish a preparatively useful method for the creation of simple, yet synthetically challenging

101 terpenes such as the germacrenes, in two to three steps by a controlled Ni0-mediated isoprene/alkyne cooligomerization. Lasting lessons from these studies include: gram scal

1. It was realized through experimental findings that the vision of terpene synthesis from isoprene/alkyne cooligomerization, in the present form, is inherently flawed. Most significantly, the products, 4Z,7Z,10E-cyclodecatrienes, are thermally unstable under the reaction conditions. The number of regio- and stereoisomers and inseparable side products led to tedious and often impossible separations that resulted in serious analytical issues.

2. Despite the complexity of the reaction mixtures, several repeatedly occurring side products from the diene/alkyne cooligomerization were isolated and fully elucidated.

This allowed for the establishment of a rapid qualitative analysis of crude reaction mixtures.

3. In spite of intense efforts, the outcome of the diene/alkyne cooligomerization was only minimally tunable by variation of the reaction conditions. For example, the ligand was incapable of significantly altering the outcome of the reaction – in terms of both olefin geometry and regioselective incorporation of isoprene or alkyne. Ultimately, it was observed that the reaction is contingent primarily upon the alkyne substrate.

4. With respect to butadiene, notable advances in substrate scope were achieved. For the first time, both terminal and α-branched alkynes were successfully incorporated into the

4Z,7Z,10E-cyclodecatriene frameworks. Most remarkably, full regiocontrol was obtained in the incorporation of the unsymmetrical alkyne 2.12f, as unambiguously established by the crystallographic analysis of macrocycle 2.29.

102 5. Despite its propensity to undergo Cope rearrangement, a crystal structure of 4,10- dimethyl-4Z,7Z,10E-cyclodecadiene 2.13d was obtained, as the triepoxide rac-2.34. This

directly confirmed the product structure, solid-state conformation, and olefin geometry

for the first time.

Although ultimately, terpenes could not be produced from isoprene due to several

intractable factors, the cooligomerization of alkynes with butadiene may still be useful

for total syntheses in a different context. Such studies, as well as the utilization of the

bulk chemical isoprene as a building block for terpene synthesis, continue to be of

interest in our laboratory.

After this project was concluded, it was published in the Journal of Organic

Chemistry.96 Following publication, we were contacted by the Ken Houk laboratory at the

University of California, Los Angeles and began a collaboration to explain our published observations on the nickel catalyzed [4+4+2] cycloadditions of dienes and alkynes. This

project is discussed briefly in Chapter 3.1. The Baran Laboratory’s interest in germacrene

sesquiterpenes did not die with this publication, it then morphed into an entirely different

synthesis, spearheaded by my coworkers and me.97 This project is discussed in Chapter

3.2.

1 Ishihara, Y.; Baran, P. S. Synlett 2010, 12, 1733–1745.

2 Chen, K.; Baran, P. S. Nature 2009, 459, 824–828.

3 Chen, K.; Ishihara, Y.; Morón Galán, M.; Baran, P. S. Tetrahedron 2010, 66, 4738–

4744.

103

4 Spurgeon, S.L.; Porter, J. W. Conversion of acetyl coenzyme A to isopentenyl pyrophosphate. In Biosynthesis of Isoprenoid Compounds; John Wiley & Sons: New

York, 1981; v 1, p 1–93.

5 Heimbach, P. Angew. Chem. Int. Ed. 1973, 12, 975–989.

6 Hu, T.; Corey, E. J. Org. Lett. 2002, 4, 2441–2443 and references cited therein.

7 For selected examples see refs. 8–24.

8 Wilke, G.; Bogdanovic, B.; Hardt, P.; Heimbach, P.; Keim, W.; Kröner, M.; Oberkirch,

W.; Tanaka, K.; Steinrücke, E.; Walter, D.; Zimmermann, H. Angew. Chem. Int. Ed.

1966, 5, 151–164.

9 Neilan, J. P.; Laine, R. M. Cortese, N.; Heck, R. F. J. Org. Chem. 1976, 41, 3455–3460.

10 Keim, W.; Röper, M.; Schieren, M. J. Mol. Catal. 1983, 20, 139–151.

11 Röper, M.; He, R.; Schieren, M. J. Mol. Catal. 1985, 31, 335–343.

12 Jolly, P. W. Angew. Chem. Int. Ed. 1985, 24, 283–295.

13 Goliaszewski, A.; Schwartz, J. Tetrahedron Lett. 1985, 5779–5789.

14 Hoberg, H.; Minato, M. J. Organomet. Chem. 1991, 406, C25–C28.

15 Maddock, S. M.; Finn, M. G. Organometallics 2000, 19, 2684–2689.

16 Misono, A.; Uchida, Y.; Hidai, M.; Ohsawa, Y. Bull. Chem. Soc. Jpn. 1966, 39, 2425–

2429.

17 Buchkremer, J. Ph.D. Thesis, University of Bochum, 1973.

18 tom Dieck, H.; Dietrich, J. Angew. Chem. Int. Ed. 1985, 24, 781–783.

19 Kopp, W. Ph.D. Thesis, University of Bochum, 1970.

20 Wilke, G.; Kröner, M. Angew. Chem. 1959, 71, 574.

21 Selbeck, H. Ph.D. Thesis, University of Bochum, 1972.

104

22 Wilke, G.; Briel, H. Makromol. Chem. 1963, 69, 18–40.

23 Yasuda, H.; Nakamura, A. Angew. Chem. Int. Ed. 1987, 26, 723–742.

24 Morikawa, H; Kitazume, S. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18, 254–258.

25 Itoh, K.; Masuda, K.; Fukahori, T.; Nakano, K.; Aoki, K.; Nagashima, H.

Organometallics 1994, 13, 1020–1029.

26 Wilke, G. Angew. Chem. Int. Ed. 1988, 27, 185–206.

27 Heimbach, P. J. Synthetic Org. Chem. 1973, 31, 300–312.

28 Wilke, G.; Eckerle, A. Cyclooligomerizations and Cyclo-co-oligomerizations of 1,3-

Dienes. In Applied Homogeneous Catalysis with Organometallic Compounds: A

Comprehensive Handbook in Two Volumes; Cornils, B.; Herrmann, W., Eds.; Wiley-

VCH: Weinheim, Germany, 1996; Vol. 1, p 368–382.

29 Heimbach, P.; Hey, H.-J. Angew. Chem. Int. Ed. 1970, 9, 528–529.

30 Hey, H.-J. Ph.D. Thesis, University of Bochum, 1969.

31 Wiese, W. Ph.D. Thesis, University of Bochum, 1969.

32 Kiji, J.; Masuy, K.; Furukawa, J. Bull. Chem. Soc. Jap. 1971, 44, 1956–1961.

33 Barnett, B.; Büssemeier, B; Heimbach, P.; Jolly, P. W.; Krüger, C.; Tkatchenko, I.;

Wilke, G. Tetrahedron Lett. 1972, 15, 1457–1460.

34 For Ni0-catalyzed construction of 5-membered rings from 1,5-cyclooctadiene, see:

Nüssel, H.-G. Ph.D. Thesis, University of Bochum, 1970.

35 Benn, R.; Büssemeier, B.; Holle, S.; Jolly, P. W.; Mynott, R.; Tkatschenko, J.; Wilke,

G. J. Organomet. Chem. 1985, 279, 63–86.

36 Heimbach, P.; Wilke, G. Liebigs Ann. Chem. 1969, 727, 183–193.

37 Brenner, W.; Heimbach, P.; Wilke, G. Liebigs Ann. Chem. 1969, 727, 194–207.

105

38 Brenner, W.; Heimbach, P.; Ploner, K.-J.; Thömel, F. Liebigs Ann. Chem. 1973, 1882–

1892.

39 Bogdanovic, B.; Heimbach, P.; Kröner, M.; Wilke, G.; Brandt, J. Liebigs Ann. Chem.

1969, 727, 143–160.

40 Miyake, A.; Kondo, H.; Nishino, M.; Tokizane, S. Special Lectures, XXIII. Internat.

Congr. Pure Appl. Chem.; Butterworth: London, 1971; v 6, p 201.

41 Meyer, R.-V. Ph.D. Thesis, University of Bochum, 1973.

42 Buchholz, H. A. Ph.D. Thesis, University of Bochum, 1971.

43 Büssemeier, B. Ph.D. Thesis, University of Bochum, 1973.

44 Thömel, F. Ph.D. Thesis, University of Bochum, 1970.

45 The majority of these studies are only published in detail in the form of several Ph.D. dissertations from the Heimbach group. They were graciously made available by The

University of Bochum. These documents can be obtained by emailing the corresponding author (P.S.B.). Furthermore, many of the published studies are not easily available and are in German, however, the primary literature is comprehensively referenced in this publication.

46 Heimbach, P.; Wilke, G. Liebigs Ann. Chem. 1969, 727, 183–193.

47 For examples of intramolecular Ni0 catalyzed [4+4] cycloaddition in total synthesis, see refs. 48 and 49.

48 Wender, P. A.; Snapper, M. L. Tetrahedron Lett. 1987, 28, 2221–2224.

49 Wender, P. A.; Ihle, N. C.; Correia, C. R. D. J. Am. Chem. Soc. 1988, 110, 5904–5906.

106

50 For an example of Ni0 catalyzed isoprene oligomerization, affording complex mixtures

of terpenes, see: Bogatian, M. V.; Bogatian, G.; Udrea, S.; Corbu, A. C.; Chiraleu, F.;

Maganu, M.; Deleanu, C. Rev. Roum. Chem. 2006, 51, 735–741.

51 Baker, R.; Cook, A. H.; Crimmin, M. J. J. Chem. Soc., Chem. Commun. 1975, 727–

728.

52 Bogdanovic, B.; Kröner, M.; Wilke, G. Liebigs Ann. Chem. 1966, 699, 1–23.

53 Oenbrink, G.; Schiffer, T. Cyclododecatriene, Cyclooctadiene, and 4-

Vinylcyclohexene. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH:

Weinheim, Germany, 2009; p 1–3.

54 A similar, although mechanistically distinct, phenomenon regarding the

oligomerization of ethylene is called the “nickel effect:” Fischer, K.; Jonas, K.; Misbach,

P.; Stabba, R.; Wilke, G. Angew. Chem. Int. Ed. 1973, 12, 943–953.

55 Although this is a published procedure, we have reproduced the procedure for generating Ni(cod)2 in the Supporting Information. See also: Ozawa, F. Group 10 (Ni, Pd,

Pt) Metal Compounds. In Synthesis of Organometallic Compounds; Komiya, S.,Ed.; John

Wiley & Sons Ltd., 1997; p 249–303.

56 For an early, extensive study on the reduction of Ni(acac)2 with organoaluminum

reagents, see: Misbach, P. Ph.D. Thesis, University of Bochum, 1969.

57 For a discussion of the catalytic cycle as originally proposed by Wilke and coworkers, see ref. 35.

58 For detailed investigations on the complexity of conformational isomerism in π-allyl nickel systems, see: Schenkluhn, H. Ph.D. Thesis, University of Bochum, 1971.

59 van Leeuwen, P. W. N. M.; Roobeek, C. F. Tetrahedron, 1980, 37, 1973–1983.

107

60 For crystallographic and IR studies on related butadiene-derived systems, see refs. 61 and 62.

61 Taube, R.; Gehrke, J.-P.; Böhme, P.; Köttnitz, J. J. Organomet. Chem. 1990, 395, 341–

358.

62 Brown, J. M.; Golding, B. T.; Smith, M. J. J. Chem. Soc. D, Chem. Commun. 1971,

1240–1241.

63 For theoretical work on hapticity in butadiene dimerization, see: Tobisch, S.; Ziegler,

T. J. Am. Chem. Soc. 2002, 124,13290–13301.

64 In order to simplify the comparison of our 4Z,7Z,10E-macrocycles to germacrene natural products, they are consistently numbered in the aforementioned fashion, resulting in numbering and E/Z descriptors which differ from the preferred IUPAC nomenclature.

65 There are examples of (4E,10Z), (4Z,10E), (4Z,10Z) germacrene natural products,

however they are in the minority, for several examples, see: Adio, A. M. Tetrahedron

2009, 65, 1533–1552.

66 A different metal may also change the outcome, for example it is known that titanium catalysts produce primarily 1,5,9-cis,trans,trans-cyclododecatriene, whereas nickel and chromium afford the respective all-trans isomer: Weber, H.; Ring, W.; Hochmuth, U.;

Franke, W. Liebigs Ann. Chem. 1965, 681, 10–20.

67 Schreiber, S. L.; Santini, C. J. Am. Chem. Soc. 1984, 106, 4038–4039.

68 Corey, E. J.; Hamanaka, E. J. Am. Chem. Soc. 1967, 89, 2758–2759.

69 Fleck, W. Ph.D. Thesis, University of Bochum, 1971.

70 Brille, F.; Heimbach, P.; Kluth, J.; Schenklunh, H. Angew. Chem. Int. Ed. 1979, 18,

400–401.

108

71 For a comprehensive overview, see the review included in ref. 65.

72 For NMR studies, see: Faraldos, J. A.; Wu, S.; Chappell, J.; Coates, R. M. Tetrahedron

2007, 63, 7733–7742.

73 For implications in synthesis, see for example: Kodama, M.; Yokoo, S.; Matsuki, Y.;

Ito, S. Tetrahedron Lett. 1979, 20, 1687–1690.

74 Tashkhodzhaev, B; Abduazimov, B. K. Chem. Nat. Compd. 1997, 33, 382–388.

75 Heimbach, P. Angew. Chem. Int. Ed. 1966, 5, 961.

76 Heimbach, P.; Brenner, W. Angew. Chem. Int. Ed. 1966, 5, 961–962.

77 There are 64 theoretical isomers, half of which are degenerate. If one assumes exclusive head-to-tail coupling of isoprene, this number reduces to 16 possible isomers.

On the other hand, if the 4Z,7Z,10E-olefin geometry is obtained selectively, there are only 4 possible isomers.

78 No yield was given for the Cope rearrangements.

79 The assignment of the cis-relationship of the substituents at the 5 and 10 positions is based on previous results from extensive studies in the Heimbach laboratory. See ref. 44 and references cited therein.

80 Obtained from Ingenieurbüro SFS Zürich, Switzerland by G. Wilke.

81 Electron-poor alkynes have been incorporated into 10-membered rings in a related

stoichiometric experiment at low temperature: Büssemeier, B.; Jolly, P. W.; Wilke, G. J.

Am. Chem. Soc. 1974, 94, 4726–4727.

82 Meriwether, L. S.; Colthup, E. C.; Kennerly, G. W.; Reusch, R. N. J. Org. Chem. 1961,

26, 5155–5163.

83 Mori, T.; Nakamura, T.; Kimura, M. Org. Lett. 2011, 13, 2266–2269.

109

84 Baker, R.; Crimmin, M. J. J. Chem. Soc., Perkin Trans. 1 1979, 1264–1267.

85 Despite complete characterization, unambiguous assignment of the major component from the 1.6:1 mixture was not possible. Based on the X-ray analysis obtained for 33, the likely major/minor structures are proposed in the Supporting Information.

86 Heimbach, P. Angew. Chem. Int. Ed. 1966, 5, 961.

87 Heimbach, P.; Brenner, W. Angew. Chem. Int. Ed. 1966, 5, 961–962.

88 The fact that the 5,10-dimethyl regioisomer was not observed is in full agreement with

previous results from the Heimbach group, see ref. 44.

89 The third triepoxide product was not fully characterized.

90 See, e.g.: Maimone, T. J.; Shi, J.; Ashida, S.; Baran, P. S. J. Am. Chem. Soc. 2009, 131,

17066–17067.

91 Curran, D. P. J. Am. Chem. Soc. 1983, 105, 5826–5833.

92 Cheng, G.; Wang, X.; Chu, R.; Shao, C.; Xu, J.; Hu, Y. J. Org. Chem. 2011, 76, 2694–

2700.

93 Kociolek, M. G.; Kalbarczyk, P. K. Synth. Comm. 2004, 34, 4387–4394.

94 For benzonitrile cross-couplings, see: Miller, J. A.; Dankwardt, J. W. Tetrahedron Lett.

2002, 44, 1907–1910. Preliminary studies in our group have shown that vinyl-nitriles can

also be cross-coupled under Dankward’s conditions.

95 The dicycloadduct can be accessed in one step by utilizing excess dipole, but this results in the inability to separate the regioisomers.

96 Holte, D.; Götz, D. C. G.; Baran, P. S. J. Org. Chem. 2012, 77, 825–842.

97 Foo, K.; Usui, I.; Götz, D. C. G.; Werner, E. W.; Holte, D.; Baran, P. S. Angew. Chem.

Int. Ed. 2012, 51, 11491–11495.

110 2.8 Experimental

General. All reactions were carried out under an argon atmosphere with dry solvents under anhydrous conditions, unless otherwise noted. Dry tetrahydrofuran (THF), triethylamine (NEt3), toluene, dichloromethane (DCM), and diethyl ether (Et2O) were

obtained by passing commercially available pre-dried, oxygen-free formulations through

activated alumina columns. Reagents were purchased at the highest commercial quality

and used without further purification unless otherwise stated. Dibromoformaldoxime was

synthesized according to a literature-known procedure.1 Reactions were monitored by thin-layer chromatography (TLC), carried out on 0.25 mm silica gel plates (60F-254) using UV light as visualizing agent and either (1) p-anisaldehyde in ethanol/aqueous

H2SO4/CH3CO2H or (2) 2% KMnO4 in 4% aqueous sodium bicarbonate and heat as developing agents. Nuclear magnetic resonance (NMR) spectra were calibrated using

1 residual undeuterated solvent signals as an internal reference (CHCl3: 7.26 ppm for H

NMR, 77.16 ppm for 13C NMR). 1D and 2D NMR spectra were recorded at room

temperature unless otherwise stated. All NMR peak assignments given in the Supporting

Information were determined by 1H, 13C, APT, COSY, HMQC, and/or HMBC experiments. The following abbreviations were used to assign NMR signal multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad. High resolution mass spectra (HRMS) were recorded on an LC/MSD TOF mass spectrometer by electrospray ionization time of flight reflectron experiments (ESI-TOF). For IR spectra the major absorbance bands are reported in wavenumbers. Melting points (m.p.) are uncorrected. GC/MS analyses were carried out applying the following method: Agilent

111 19091S-433 column (30 m x 320 µm x 0.25 µm); H2 30 mL/min, air 400 mL/min, He 25

mL/min; 50 °C for 2.25 min, ramp to 300 °C (60 ºC/min), and hold at 300 ºC for 4 min.

Preparation of Bis(1,5-cyclooctadiene)nickel(0). Since the outcome of the Ni(cod)2

formation is strongly dependant on the quality of Ni(acac)2, the latter was first

azeotropically dried in refluxing toluene (130 ºC) with a Dean-Stark apparatus to remove

trace amounts of water. The flask was cooled to ~90 ºC and filtered through Celite while

hot. Toluene was removed in vacuo to afford anhydrous Ni(acac)2 (forest green solid).

Freshly dehydrated Ni(acac)2 (2.0 g, 7.78 mmol) was dissolved in toluene (4.8 mL, if not fully soluble the azeotropic distillation should be repeated) in a flame dried Schlenk flask.

1,5-cyclooctadiene (4.78 mL, 38.9 mmol, 5 equiv) was added and the mixture degassed by either three freeze-pump-thaw cycles or by sonication for 15 min with concomitant vigorous argon bubbling. The degassed homogenous mixture was cooled to –78 ºC and butadiene (~0.5 mL, 5.91 mmol, 0.76 equiv) was condensed into the flask. The solution was warmed to –10 ºC and AlEt3 (0.9 M solution in toluene, 19.5 mL, 2.25 equiv) added

dropwise. The reaction was then allowed to come to room temperature and stirred

overnight (8-10 h). The resulting red-brown suspension was cooled to –30 ºC and the

supernatant carefully removed by cannula to give a yellow solid, which was washed with

degassed Et2O (the suspension was allowed to equilibrate to –30 ºC between washings).

Analytically pure Ni(cod)2 (1.24 g, 4.51 mmol, 58%) was obtained as an air-sensitive yellow solid, dried in vacuo in the Schlenk flask and stored under argon (preferentially inside a glove box) at low temperatures. In general, yields ~60% Ni(cod)2 were obtained

112 1 13 on up to 10 g scale. H NMR (C6D6, 400 MHz) δ = 4.30 (s, 8 H), 2.08 (s, 16 H) ppm; C

NMR (C6D6, 400 MHz) δ = 89.7 (CH), 30.9 (CH2) ppm.

1 2 0 2 10 R R Ni(cod)2, PPh3 General Procedure for Ni -Catalyzed + R1 7 1 2.12 4 R 2.20 Cooligomerization of Butadiene (2.1) and

Alkynes 2.12: A 35 mL pressure vessel and stir bar were flame-dried and brought into a glove box. Freshly prepared Ni(cod)2 (79.0 mg, 0.29 mmol, 10 mol%) and PPh3 (75.0 mg, 0.29 mmol, 10 mol%) were added to the reaction flask.2 The vessel was sealed with a septum, brought out of the glove box, connected to a Schlenk line and cooled to –78 ºC.

Under an argon atmosphere, 1,3-butadiene (~1.25–10 mL, 14.4–115 mmol, 5–40 equiv) was condensed into the flask and the alkyne 2.12 (2.87 mmol) was added. Under a steady flow of argon, the septum was rapidly exchanged with a screw cap. The reaction was allowed to come to room temperature and stirred for 18 h. For safety reasons the reactions were conducted behind an explosion-proof blast shield. The reaction mixture was cooled to –78 ºC, uncapped, and excess 1,3-butadiene was allowed to evaporate at room temperature. Subsequently, the reaction was worked up according to one of the following two methods:

(A) Wilke’s standard workup procedure.

The residue was taken up in EtOAc (50 mL) and washed subsequently with 5 M HCl, 5% aqueous H2O2, saturated aqueous sodium bicarbonate solution, water, and finally brine.

The organic phase was dried over magnesium sulfate, filtered, and concentrated in vacuo.

Purification by flash chromatography then followed, if necessary and if the crude mixture was deemed separable (i.e., categories C, D1, D2, and E3).

113 (B) Modified workup procedure.

The residue was suspended in hexanes, loaded directly onto a plug of silica gel and eluted

sequentially with hexanes or appropriate hexanes/EtOAc mixtures. The solvent was

removed in vacuo and the crude mixture thus obtained was further purified by flash

chromatography, if necessary and if it was deemed separable (i.e., categories C, D1, D2,

and E3).

1 2 0 10 R 2 General Procedure for Ni -Catalyzed Me R Ni(cod)2, PPh3 + Me 7 1 R1 4 R 2.12 Me 2.13 Cooligomerization of Isoprene (2.3) and

Alkynes 2.12: A 35 mL pressure vessel equipped with a stir bar was flame-dried. In a glove box freshly prepared Ni(cod)2 (79.0 mg, 0.29 mmol, 10 mol%) and PPh3 (75.0 mg,

0.29 mmol, 10 mol%) were added to the reaction flask. The vessel was sealed with a septum, brought out of the glove box, and connected to a Schlenk line. Under an argon atmosphere, isoprene (1.5–12 mL, 14.4–115 mmol, 5–40 equiv) and alkyne 2.12 (2.87 mmol) were added.2 Under a steady flow of argon, the septum was rapidly exchanged with a screw cap, the mixture heated to 60 ºC and stirred for 18 h. The reaction mixture was then cooled to room temperature and excess isoprene was allowed to evaporate under a flow of nitrogen. The remaining crude material was subsequently worked up by either method A or B (vide supra).

Synthesis of 10-Membered Ring Systems: Isolation and Characterization of Primary

Macrocycles, Secondary Derivatives, and Undesired Side Products.

114 CO2Me + Isolation of [2+2+2] cycloadducts MeO C 2 2.12e

Ni(cod)2, PPh3 from the reaction of alkyne 2.12e and

CO2Me CO2Me CO2Me 3 MeO2C CO2Me MeO2C MeO2C butadiene (category C ): The standard + +

MeO2C CO2Me MeO2C MeO2C

CO2Me CO2Me CO2Me procedure was followed using dimethyl 2.22 rac-2.23 2.24 acetylenedicarboxylate (2.12e, 1.62 g, 11.5 mmol) and 1,3-butadiene (~5 mL, 57.3 mmol,

5.0 equiv) to give a brown viscous oil. GC/MS analysis of the obtained crude mixture allowed for the tentative assignment of the above three homo- and hetero-[2+2+2] cycloadducts 2.22–2.24 as the main products (for GC trace see Section 2.9).

CO2Et CO2Et Isolation of [2+2+2] cycloadducts CO2Et CO2Et Ni(cod)2, PPh3 + + 2.12l 2.S1 and 2.S2 from the reaction of CO2Et CO2Et 2.S1 2.S2 alkyne 2.12l and butadiene (category

C3). The standard procedure was followed using alkyne 2.12l (1.13 g, 11.5 mmol) and

1,3-butadiene (~5 mL, 57.3 mmol, 5.0 equiv) to give a yellow oil. The main components were isolated in analytically pure form by preparative TLC (silica gel, hexanes:EtOAc,

12:1). GC/MS analysis and 1H NMR of the obtained products allowed for the unambiguous assignment of the [2+2+2] cycloadducts 2.S1 and 2.S2 (GC trace given in

Section 2.9). OH + 2.12c Ni(cod)2, PPh3

2 1 OH Isolation of the linear cooligomer 2.27 (category 3

4 5 7 9 11 13 15 3 HO 6 8 10 12 14 16 C ). The reaction was performed according to the 2.27 (main product, isolated) H standard procedure, however, with altered + OH OH H 2.20c (not isolated) rac-2.26 (not isolated) stoichiometry: Homopropargyl alcohol 2.12c (806

115 mg, 0.87 mL, 11.5 mmol), 1,3-butadiene (~5.0 mL, 57.3 mmol, 5.0 equiv), Ni(cod)2 (106

mg, 0.39 mmol, 3.3 mol%), PPh3 (100 mg, 0.39 mmol, 3.3 mol%). Workup A afforded a pale yellow oil. TLC of the obtained crude mixture showed four major components along with several minor side products. This was confirmed by GC/MS analysis, which revealed a fairly clean and selective reaction with full consumption of starting material giving 1,5-cyclodecadiene, Cope product rac-2.26, traces macrocycle 2.20c (the latter two were inseparable and were tentatively assigned by 1H and 13C NMR of a crude

mixture), and one further, unknown compound. Subsequently, an aliquot (20 mg, crude)

of the crude mixture was purified by preparative TLC (silica gel, EtOAc/hexanes, 1:1)

yielding the main product, diol 2.27, in analytically pure form. Its structure was fully

1 elucidated by 2D NMR experiments. Rf = 0.76 (silica gel, hexanes:EtOAc, 1:1); H NMR

(500 MHz, CDCl3) δ = 5.80 (ddt, J = 17.1, 10.4, 6.7 Hz, 1 H, H15), 5.71 (t, J = 7.3 Hz, 1

H, 8-H), 5.50 – 5.34 (m, 2 H, 10-H, 11-H), 4.99 (dd, J = 17.3, 1.3 Hz, 1 H, 16-H-trans),

4.94 (dt, J = 10.1, 1.1 Hz, 1 H, 16-H-cis) 3.79 – 3.73 (m, 4 H, 1-H, 5-H), 2.95 (t, J = 6.7

Hz, 2 H, 9-H), 2.63 (t, J = 6.2 Hz, 2 H, 2-H), 2.37 (t, J = 5.9 Hz, 2 H, 6-H), 2.00 (m, 4 H,

13 12-H, 14-H), 1.44 (dt, J = 14.4, 7.4 Hz, 2 H, 14-H) ppm; C NMR (125 MHz, CDCl3)

δ = 138.9 (C-15), 137.7 (C-8), 131.4 (C-11), 127.5 (C-10), 119.8 (C-7), 114.6 (C-16),

91.7 (C-3), 80.4 (C-4), 61.5 (C-5), 61.4 (C-1), 40.5 (C-6), 33.9 (C-9), 33.4 (C-14), 32.1

(C-12), 28.8 (C-13), 24.1 (C-2) ppm; IR (ATR, neat) νmax: 3335, 2923, 2853, 1640, 1455,

1431, 1377, 1329, 1260, 1178, 1090, 1041, 967, 909, 845, 801, 699 cm-1; HRMS calcd

+ for C16H24O2 249.1849 [M+H] , found (ESI-TOF) 249.1852.

116 Me Direct characterization of 2.20d (category E3). The standard procedure

Me 2.20d was followed using 4-octyne (2.12d, 316 mg, 0.42 mL, 2.87 mmol) and

1,3-butadiene (~10 mL, 115 mmol, 40 equiv) to give the colorless oil 2.20d (518 mg,

2.37 mmol, 83%; this material contains 9% Cope product meso-2.21 and 12% of unknown impurities according to GC/MS analysis) applying workup procedure B. Rf =

1 0.72 (silica gel, hexanes); H NMR (500 MHz, CDCl3, –50 ºC) δ = 5.47 (t, J = 10.2 Hz, 1

H), 5.28 (ddd, J = 15.1, 10.4, 4.5 Hz, 1 H), 5.18 (m, 2 H), 3.22 (t, J = 12.5 Hz, 1 H), 2.92

(dd, J = 15.1, 10.8 Hz, 1 H), 2.57 (d, J = 14.6 Hz, 1 H), 2.22 (ddd, J = 24.6, 12.0, 5.8 Hz,

2 H), 2.11 – 1.95 (m, 3 H), 1.91 (dd, J = 23.0, 11.5 Hz, 1 H), 1.79 (m, 1 H), 1.63 (td, J =

11.9, 4.0 Hz, 1 H), 1.52 (q, J = 11.5 Hz, 1 H), 1.41 – 1.27 (m, 3 H), 1.24 – 1.17 (m, 1 H),

13 0.91 – 0.84 (m, 6 H) ppm; C NMR (125 MHz, CDCl3, –50 ºC) δ = 134.5, 132.5, 132.4,

131.8, 129.4, 122.8, 38.7, 37.7, 35.6, 32.2, 31.4, 26.2, 22.4, 21.7, 14.9, 14.5 ppm; IR

(ATR, neat) νmax: 3007, 2956, 2928, 2867, 1453, 1376, 1202, 1183, 1123, 1088, 970, 950,

- + 911, 842, 813, 738, 704 cm ; HRMS calcd for C16H26 219.2107 [M+H] , found (ESI-

TOF) 219.2107.

H Me Characterization of 2.20d in form of its respective Cope product

Me H 3 meso-2.21 (meso-2.21, category D2 ). Macrocycle 2.20d (10.0 mg, 45.8 µmol) was

heated neat to 120 ºC for 1 h. The reaction afforded the clear yellow oil,

divinylcyclohexene meso-2.21 in quantitative yield (9.90 mg, 45.3 µmol, 99%). Rf = 0.66

1 (silica gel, hexanes); H NMR (400 MHz, CDCl3) δ = 5.79 (ddd, J = 17.0, 10.6, 7.5 Hz, 2

H), 5.01 (app ddd, app J = 8.6, 2.0, 0.8 Hz, 2 H), 4.98 (d, J = 0.8 Hz, 2 H), 2.41 (dd, J =

11.8, 4.9 Hz, 2 H), 2.17 – 1.87 (m, 8 H), 1.44 – 1.32 (m, 4 H), 0.88 (t, J = 7.3 Hz, 6 H)

117 13 ppm. C NMR (125 MHz, CDCl3) δ = 140.6, 128.7, 114.4, 41.9, 35.2, 33.5, 21.7, 14.4

ppm; IR (ATR, neat) νmax: 3075, 2957, 2929, 2900, 2871, 1639, 1465, 1455, 1434, 1377,

-1 •+ 4 994, 967, 910, 719 cm ; GC/MS calcd for C16H26 218.2035 [M] , found (FID ) 218.2.

+ Me Characterization of 2.20o after Cope rearrangement 2.12o Ni(cod)2, PPh3 (category D23). The standard procedure was followed using

Me 1-octyne (2.12o, 316 mg, 0.42 mL, 2.87 mmol) and 1,3- 2.20o (not isolated)

! H butadiene (~1.25 mL, 14.4 mmol, 5.0 equiv). After workup

Me H A the crude mixture containing 2.20o was distilled directly 2.S3 to afford the Cope product 2.S3 (racemic mixture) as a pale yellow oil (564 mg, 2.58

1 mmol, 70% over two steps). Rf = 0.66 (silica gel, hexanes); H NMR (500 MHz, CDCl3)

δ = 5.85 – 5.76 (m, 2 H), 5.38 – 5.35 (m, 1 H), 5.04 – 4.98 (m, 4 H), 2.49 – 2.37 (m, 2 H),

2.22 – 2.19 (m, 1 H), 2.18 – 2.15 (m, 1 H), 2.06 – 1.82 (m, 4 H), 1.43 – 1.22 (br m, 8 H),

13 0.88 (t, J = 6.9 Hz, 3 H) ppm; C NMR (150 MHz, CDCl3) δ = 140.5, 140.4, 136.5,

119.0, 114.5, 114.5, 41.7, 41.2, 37.9, 32.3, 31.9, 29.3, 29.2, 27.8, 22.8, 14.3 ppm; IR

(ATR, neat) νmax: 3076, 2956, 2925, 2856, 1639, 1457, 1437, 1378, 994, 910, 819, 722

-1 •+ 4 cm ; GC/MS calcd for C16H26 218.2035 [M] , found (FID ) 218.2.

Me Characterization of 2.20a in form of its Ni(cod) , PPh + 2 3 Me Me Me dihydroisoxazole derivative 2.S4 (category 2.12a 2.20a (not isolated)

HO N 3 Br 14 N D2 ). The standard procedure was followed O H Br Br 1 9 8 , 2 10 KHCO3 7 12 3 H 11 Me using alkyne 2.12a (196 mg, 0.29 mL, 2.87 4 6 5 Me13 dihydroisoxazole derivative 2.S4 mmol) and 1,3-butadiene (~1.25 mL, 14.4

118 mmol, 5.0 equiv) to afford a crude mixture (486 mg) containing 2.20a after workup A.

An aliquot of this mixture (35 mg) was brought up in EtOAc (2.0 mL) and NaHCO3 (100

mg, 1.19 mmol, excess) and dibromoformaldoxime (40 mg, 0.20 mmol, approx. 1.0

equiv) were added sequentially. The reaction was stirred at room temperature for 30 min

and additional dibromoformaldoxime (40 mg, 0.20 mmol, approx. 1.0 equiv) was added.

The reaction mixture was washed with H2O, brine, dried over MgSO4, and concentrated

in vacuo. Preparative TLC (silica gel, hexanes:EtOAc 20:1) afforded the above

dihydroisoxazole derivative 2.S4 (racemic mixture) as the only isolable regioisomer

(colorless oil, 11.3 mg, 37.9 µmol, 18% estimated yield over two steps based on alkyne

5 1 2.12a). Rf = 0.50 (silica gel, hexanes:EtOAc, 20:1); H NMR (600 MHz, CDCl3) δ =

5.77 (ddd, J = 10.8, 6.6, 6.0 Hz, 1 H, H-5), 5.34 – 5.24 (m, 2 H, H-4, H-8), 4.43 (d, J =

13.9 Hz, 1 H, H-10), 3.21 (t, J = 12.0 Hz, 1 H, H-6), 3.09 (t, J = 12.7 Hz, 1 H, H-1), 2.83

(t, J = 12.7 Hz, 1 H, H-9), 2.71 – 2.64 (m, 1 H, H-9), 2.44 – 2.31 (m, 3 H, H-3, H-6, H-

11), 2.12 – 2.07 (m, 1 H, H-3), 1.93 (t, J = 13.9 Hz, 1 H, H-2), 1.59 – 1.53 (m, 1 H, H-2),

13 1.07 – 1.03 (m, 6 H, H-12, H-13) ppm; C NMR (150 MHz, CDCl3) δ = 150.4 (C-7),

146.8 (C-14), 129.7 (C-5), 128.1 (C-4), 114.6 (C-8), 89.6 (C-10), 49.8 (C-1), 34.5 (C-11),

30.3 (C-2), 29.9 (C-9), 27.5 (C-6), 23.7 (C-3), 23.1 (C-12), 22.0 (C-13) ppm; IR (ATR,

neat) νmax: 3011, 2959, 2924, 2866, 1611, 1571, 1466, 1447, 1381, 1356, 1294, 1251,

1130, 1100, 1082, 1045, 1009, 999, 963, 919, 878, 825, 799, 758, 705 cm-1; HRMS calcd

+ for C14H20BrNO 298.0801 [M+H] , found (ESI-TOF) 298.0799.

Me 8 OTBS Direct characterization of 2.20p in crude form + 7 OTBS Me 3 2.20p: Inseparable mixture of regioisomers (category D1 ). The standard procedure was

119 followed using alkyne 2.12p (569 mg, 2.87 mmol) and 1,3-butadiene (~10 mL, 115

mmol, 40 equiv). In order to establish the constitution of the isolated component(s), the

following protocol was undertaken: After the standard workup procedure A, GC/MS

analysis of the crude mixture revealed a fairly clean reaction, with almost all starting

material consumed after 18 h. Based on the GC/MS chromatogram, it was determined

that the reaction contained 10-membered ring 2.20p (tR = ~6.0 min, broad) and traces of

Cope products (tR = ~5.9 min). Column chromatography (silica gel, silica gel, hexanes → hexanes:EtOAc 100:1) afforded the main product of the reaction, 2.20p (576 mg, 1.89 mmol, 66%), as an inseparable mixture of two components. 13C and APT NMR excluded the presence of Cope product (no olefinic methylene groups with a characteristic chemical shift of ~115 ppm). Evidence for the formation of 10-membered ring was demonstrated by the resolution of the 1H NMR at –54 ºC: Upon cooling the sample, the

broad signals of the CH2 groups of the ring system became sharp due to the slowed down

molecular motion of the conformationally flexible macrocycle. Observing two methylene

carbons adjacent to an oxygen, two methyl groups joined to sp2 carbons, and twelve olefinic carbons, the reaction was determined to be a 1.6:1 mixture (cf. integrals of the two methyl groups attached to olefinic carbons C-7 or C-8) of regioisomers, which coeluted during column chromatography and GC analysis. Further characterization of the ring system and partial assignment of substructures of the two regioisomers was possible by full 2D NMR analysis (COSY, HMQC, HMBC) at low temperature. Chromatograms and spectra are included in the Section 2.9. 2.20p (mixture of two regiosiomers): Rf =

1 0.75 (silica gel, DCM:Et2O, 2:1); H NMR (500 MHz, CDCl3, –54 ºC) δ = 5.48 (dt, J =

11.3, 10.4 Hz, 1 H), 5.12 – 5.34 (m, 3 H), 3.53 (t, J = 6.5 Hz, 2 H), 3.20 – 3.40 (m, 2 H),

120 2.90 –3.03 (m, 1 H), 2.57 (q, J = 13.5 Hz, 3 H), 2.17 – 2.41 (m, 3 H), 1.85 – 2.14 (m, 4

H), 1.69 (s, 3 H), 1.52 (q, J = 11.5 Hz, 1 H), 0.86 (s, 9 H), 0.043 (br s, 6 H) ppm; 13C

NMR (150 MHz, CDCl3, rt) δ = 132.1, 131.7, 130.1, 129.8, 128.0, 123.3, 62.5, 41.0,

40.3, 38.2, 34.9, 32.8, 26.5, 26.1 (two signals), 22.6, –5.1 (two signals) ppm; IR (ATR, neat) νmax: 3007, 2952, 2928, 2895, 2857, 1471, 1462, 1445, 1382, 1361, 1253, 1089,

-1 1005, 971, 937, 913, 833, 812, 773, 731, 703, 661 cm ; HRMS calcd for C19H34OSi

307.2452 [M+H]+, found (ESI-TOF) 307.2449.

TMS OTBS Direct characterization of 2.20q in crude form + OTBS TMS 3 2.20q: Inseparable mixture of regioisomers and (category D1 ). The standard procedure was coeluting Cope products (not shown) followed with alkyne 2.12q (736 mg, 2.87 mmol) and 1,3-butadiene (~1.25 mL, 14.4

mmol, 5.0 equiv). Different from the standard procedure the reaction mixture was stirred

for 96 h (due to the slower reaction rate for the sterically more demanding alkyne) with

monitoring by GC/MS, the final alkyne 2.12q (tR = 4.62 min) to product 2.20q (tR = 6.20 min, broad) ratio being ~65:35. The GC/MS analysis of the crude mixture revealed at least four peaks showing product mass (m/z = 307.2, [M–tBu]+). Following workup A, the crude material was subjected to column chromatography (silica gel, hexanes → hexanes:EtOAc 9:1) to remove unreacted starting material, PPh3, and PPh3O. The obtained mixture was analyzed by 1H and 13C NMR. 1H NMR revealed traces of Cope product (characteristic olefinic methylene groups with a chemical shift of ~115 ppm) as well as the typical broadening associated with the conformational flexibility of the desired 10-membered ring system. The 13C NMR showed at least four methylene carbons

bonded to oxygen, indicating four major products which incorporated alkyne 2.12q.

121 Further, unambiguous identification and assignment proved unsuccessful, but based on

the obtained data it is likely that the main components comprise the two regioisomers

(with regard to the alkyne incorporation) of the expected macrocycle 2.20q and the

follow-up Cope product. The obtained spectral data and GC traces are provided in the

Section 2.9.

3 2.20f Direct characterization of 2.20f (category E ). The standard OTBS

procedure was followed with Ni(cod)2 (395 mg, 1.44 mmol, 10 mol%),

PPh3 (377 mg, 1.44 mmol, 10 mol%), alkyne 2.12f (2.65 g, 14.4 mmol), and 1,3- butadiene (~50 mL, 576 mmol, 40 equiv) to afford crude 2.12f (after workup B), which was further purified by flash chromatography (silica gel, hexanes → hexanes:EtOAc

100:1). After removal of traces 1,5-cyclooctadiene by application of vacuum overnight analytically pure 2.20f was obtained as a colorless oil (980 mg, 3.35 mmol, 23%). Rf =

1 0.69 (silica gel, DCM:Et2O, 2:1); H NMR (500 MHz, CDCl3, –54 ºC) δ = 5.46 (t, J =

10.9 Hz, 1 H), 5.39 – 5.28 (m, 2 H), 5.24 – 5.15 (m, 2 H), 3.65 (td, J = 9.5, 5.0 Hz, 1 H),

3.56 (q, J = 8.9 Hz, 1 H), 3.25 (t, J = 12.5 Hz, 1 H), 2.79 – 2.63 (m, 2 H), 2.29 – 2.23 (m,

2 H), 2.21 – 2.14 (m, 1 H), 2.08 – 1.99 (m, 2 H), 1.94 (q, J = 11.6 Hz, 1 H), 1.54 (q, J =

11.6 Hz, 1H), 0.84 (s, 9 H), 0.02 (s, 6 H) ppm; 13C NMR (125 MHz, CDCl3, –54 ºC) δ =

138.7, 131.4, 131.4, 129.4, 123.8, 123.6, 62.7, 41.9, 32.6, 31.4, 31.0, 30.4, 25.9, 18.5, –

5.4 ppm; IR (ATR, neat) νmax: 3007, 2928, 2894, 2856, 1471, 1462, 1435, 1387, 1360,

1253, 1091, 1005, 970, 932, 914, 833, 811, 772, 742, 706, 664 cm-1; HRMS calcd for

+ C18H32OSi 293.2295 [M+H] , found (ESI-TOF) 293.2289.

122 Characterization of 2.20f after deprotection 2.20f 2.20c TBAF OTBS OH to form 2.20c (category D23). Protected alcohol 2.20f (760 mg, 2.60 mmol) was dissolved in THF (15 mL, 0.17 M). TBAF in

THF (1.0 M, 2.86 mL, 2.86 mmol, 1.1 equiv) was added dropwise over 15 min at room temperature. The reaction was stirred at room temperature until it was judged to be complete by TLC (ca. 30 min). The mixture was neutralized with 1.0 N HCl and extracted with Et2O. The organic layer was washed with water and brine, dried over

MgSO4, and concentrated in vacuo. The crude material was purified by column chromatography (silica gel, hexanes → DCM/hexanes 2:1) to give 2.20c as a colorless oil

1 (448 mg, 2.51 mmol, 97%). Rf = 0.25 (silica gel, DCM); H NMR (500 MHz, CDCl3, –

50 ºC) δ = 5.49 – 5.41 (m, 2 H), 5.35 (ddd, J = 15.3, 10.4, 4.7 Hz, 1 H), 5.27 – 5.18 (m, 2

H), 3.71 – 3.54 (m, 2 H), 3.28 (t, J = 12.6 Hz, 1 H), 2.85 – 2.68 (m, 2 H), 2.33 – 2.23 (br m, 3 H), 2.18 (br s, 1 H), 2.11 – 2.02 (m, 2 H), 1.95 (dd, J = 11.9, 11.5 Hz, 1 H), 1.56 (q,

13 J = 11.6 Hz, 1 H) ppm; C NMR (125 MHz, CDCl3, –50 ºC) δ = 138.3, 131.2, 131.1,

129.6, 125.3, 124.0, 59.7, 41.4, 32.7, 31.0, 29.5, 25.9 ppm; IR (ATR, neat) νmax: 3335,

3007, 2928, 2885, 2856, 1471, 1437, 1361, 1252, 1194, 1041, 869, 833, 773, 706, 666

-1 + cm ; HRMS calcd for C12H18O 179.1430 [M+H] , found (ESI-TOF) 179.1429.

Me17 Isolation of the main product S6 Ni(cod) , 9 16 OTBS 2 Me 1 10 8 13 Me PPh3 2 + Me Me 7 12 3 4 11 Si Me 5 14 6 O from the reaction of alkyne 2.12p 2.12p 18 Me Me Me Me15 2.S6, only isolated 3 component from complex with isoprene (category C ). The mixture

standard procedure was followed with alkyne 2.12p (569 mg, 2.87 mmol) and isoprene

(1.5 mL, 14.4 mmol, 5.0 equiv). Workup A afforded a complex product mixture showing

123 >10 products by TLC, while GC revealed the crude material to contain ~20 components.

Despite the complexity of the crude mixture, isolation of the title compound was

achieved by column chromatography (silica gel, hexanes:EtOAc, 250:1) followed by 1H

NMR analysis of single column fractions. The constitution of the Cope product 2.S6 was unambiguously established by 2D NMR experiments (see Section 2.9).5 1H NMR (600

MHz, CDCl3) δ = 4.76 (s, 2 H, H-2, H-3), 4.64 (d, J = 9.2 Hz, 2 H, H-2, H-3), 3.61 (t, J =

7.5 Hz, 2 H, H-12) 2.51 – 2.44 (m, 2 H, H-6, H-10), 2.33 – 2.27 (m, 1 H, H-11), 2.26 –

2.20 (m, 1 H, H-11), 2.17 – 2.06 (m, 4 H, H-5, H-9), 1.73 (s, 3 H, H-18), 1.72 (s, 3 H, H-

17), 1.65 (s, 3 H, H-16), 0.89 (s, 9 H, H-15), 0.05 (s, 6 H, H-13) ppm; 13C NMR (150

MHz, CDCl3) δ = 148.1 (C-1, C-4), 127.5 (C-8), 125.9 (C-7), 110.3 (C-2, C-3), 61.9 (C-

12), 42.8 (C-5), 42.5 (C-10), 37.2 (C-11), 36.1 (C-9), 34.1 (C-6), 26.1 (C-15), 23.9 (C-

18), 23.8 (C-17), 19.0 (C-16), 18.5 (C-14), –5.1 (C-13) ppm; IR (ATR, neat) νmax: 3084,

2954, 2928, 2857, 1643, 1462, 1375, 1361, 1253, 1087, 1005, 887, 834, 811, 775, 733,

-1 + 662 cm ; HRMS calcd for C21H38OSi 335.2765 [M+H] , found (ESI-TOF) 335.2763.

9 8 OTBS 7 Isolation of a main component 2.S7 OTBS Ni(cod)2, 2 5 10 TBSO 3 6 Me PPh3 + 1 11 4 12 from the reaction of alkyne 16f with 2.12f OTBS 2.S7, only isolated component from complex 3 mixture isoprene (category C ). Reaction performed as in standard procedure with altered stoichiometry: Protected alcohol (2.12f,

2.11 g, 11.5 mmol), isoprene (5.7 mL, 57.3 mmol, 5.0 equiv), Ni(cod)2 (106 mg, 0.39

6 mmol, 3.3 mol %), PPh3 (100 mg, 0.39 mmol, 3.3 mol %). Workup A afforded a brown

oil, which was judged to be a mixture of >6 components by TLC and GC analysis. An

aliquot (20 mg) of the crude material was purified by preparative TLC (silica gel,

124 hexanes, plate run three times; then hexanes:EtOAc, 150:1, plate run three times)

yielding S7 as the only product that could be isolated in analytically pure form. Rf = 0.21

1 (silica gel, 2% EtOAc in hexanes); H NMR (600 MHz, CDCl3) δ = 6.04 (s, 1 H, H-5),

5.37 (s, 1 H, H-4), 5.02 (s, 1 H, H-4), 3.79 – 3.67 (m, 6 H, H-1, H-10, H-12), 2.63 (t, J =

7.0 Hz, 2 H, H-11), 2.57 (t, J = 7.4 Hz, 2 H, H-9), 2.35 (t, J = 6.9 Hz, 2 H, H-2), 0.90 –

0.88 (m, 36 H), 0.07 (s, 6 H), 0.05 (s, 6 H), 0.04 (s, 6 H) ppm; 13C NMR (150 MHz,

CDCl3) δ = 142.5 (C-6), 136.7 (C-5), 118.5 (C-3), 118.0 (C-4), 94.1 (C-8), 81.3 (C-7),

63.0 (C-12), 62.2 (C-1), 62.0 (C-10), 43.3 (C-2), 38.8 (C-11), 26.8 (tBu), 26.7 (tBu), 24.3

(C-9), 18.5, –5.1 ppm; IR (ATR, neat) νmax: 2955, 2926, 2855, 1724, 1463, 1380, 1361,

-1 1253, 1096, 1005, 938, 834, 812, 775, 721, 667 cm ; HRMS calcd for C30H60O3Si3

553.3923 [M+H]+, found (ESI-TOF) 553.3912.

Me Characterization of 2.13d after Cope Me Pr Ni(cod) , PPh Me + 2 3 Me Pr 2.12d Me rearrangement to form rac-33 and 2.13d (not isolated) + 2.13d' (not shown) 17 17 3 Me Me meso-33’ (category D2 ). The H H 1 2 8 Me 2 8 Me ! + 3 7 standard procedure with workup A was 3 7 Me Me 4 H Me Me rac-2.33 18 meso-2.33' followed using alkyne 2.12d (316 mg,

0.42 mL, 2.87 mmol) and isoprene (~1.5 mL, 14.4 mmol, 5.0 equiv). The obtained crude

material containing macrocycle 2.13d was distilled directly to afford a 1:1 mixture of

rac-2.33 and meso-2.33’ as determined by analysis of 1H, 13C and APT NMR. In the

following only the characteristic proton NMR signals are listed (full spectral data

1 provided in Supporting Information). Rf = 0.67 (silica gel, hexanes); H NMR (500 MHz,

CDCl3) δ = 6.02 (dd, J = 17.1, 11.4 Hz, 1 H, H-4 in rac-2.33), 4.97 (s, 1 H), 4.94 (dd, J =

125 8.0, 1.7 Hz, 1 H), 4.80 – 4.75 (m, 3 H), 4.71 (d, J = 2.4 Hz, 1 H), 4.67 (s, 2 H), 1.74 (s, 6

H, H-17, H-18 in rac-2.33’), 1.71 (s, 3H, H-17 in rac-2.33) ppm; 13C NMR (125 MHz,

CDCl3) δ = 148.3, 147.8, 144.0 (CH, C-4 in rac-2.33), 129.6, 129.5, 128.6, 112.2, 112.0,

110.2 (CH2, C-2, C-3 in meso-2.33’), 51.6, 44.5, 42.7, 38.5, 35.3, 35.1, 34.9, 33.6, 26.1,

23.9, 23.0, 21.8, 21.7 (two signals), 14.5, 14.4 ppm; IR (ATR, neat) νmax: 3082, 2957,

-1 2930, 2870, 1639, 1454, 1374, 1006, 910, 888, 738 cm ; GC/MS calcd for C18H30

246.2348 [M]•+, found (FID4) 246.3.

1 10 Me Characterization of 2.13d after Pr Me Ni(cod)2, PPh3 + Me Pr 4 Me 2.12d 5 Me exhaustive epoxidation yielding 2.13d (not isolated) 3 H O triepoxide rac-2.34 (category D2 ). The 1 Me mCPBA 10 O Me 4 5 Me standard procedure was followed with Me O H rac-2.34 alkyne 2.12d (316 mg, 0.42 mL, 2.87 mmol) and isoprene (~1.5 mL, 14.4 mmol, 5.0 equiv). After workup A an aliquot of the crude mixture containing 2.13d (500 mg, crude) was brought up in DCM (20 mL) and 70% mCPBA (3.00 g, 12.2 mmol, ~6.0 equiv) was added. The reaction was allowed to stir overnight at room temperature. Excess mCPBA was quenched with saturated aqueous NaS2O3 and the mixture was extracted with DCM.

The organic layer was washed with half-saturated aqueous NaHCO3, H2O, brine, and dried over MgSO4. The crude material was purified by column chromatography (silica gel, hexanes:EtOAc, 9:1) to afford triepoxide rac-2.34 (105 mg, 0.36 mmol; fully characterized) along with a second diastereomer (characterized by X-ray, see Supporting

Information). Crystals suitable for X-ray diffraction were obtained by slow evaporation of solution of rac-2.34 in Et2O layered with hexanes. Rf = 0.25 (silica gel, 15% EtOAc in

126 1 hexanes); H NMR (400 MHz, CDCl3) δ = 2.81 (d, J = 8.4 Hz, 1 H), 2.70 (d, J = 14.9 Hz,

1 H), 2.65 (dt, J = 8.0, 2.4 Hz, 1 H), 2.45 (d, J = 15.6 Hz, 1 H), 2.19 – 2.11 (m, 2 H), 2.02

(ddd, J = 14.1, 10.5, 5.2 Hz, 1 H), 1.72 – 1.34 (m, 11 H), 1.33 (s, 3 H), 1.26 (s, 3 H), 1.01

13 – 0.97 (m, 6 H) ppm; C NMR (100 MHz, CDCl3) δ = 69.9, 65.9, 65.4, 61.7, 61.4, 58.6,

44.7, 37.1, 33.9, 32.2, 30.3, 24.2, 22.8, 18.2, 17.9, 17.1, 14.6, 14.5 ppm; IR (ATR,

neat) νmax: 2962, 2932, 2872, 1458, 1383, 1289, 1255, 1158, 1135, 1109, 1066, 1045,

-1 1006, 958, 912, 872, 842, 798, 762, 731, 670 cm ; HRMS calcd for C18H30O3 295.2268

[M+H]+, found (ESI-TOF) 295.2265.

Divergent Derivatization of 24f and 24c.

Ring fusion to yield a 6,6-bicyclic system (rac- 2.20f

OTBS 2.28). Macrocycle 2.20f (50.0 mg, 0.17 mmol) H2SO4, AcOH

H 1 9 was dissolved in glacial acetic acid (1 mL, 0.17 8 2 10 O 7 12 3 5 + 4 6 13 14 OAc 11 O Me 16 H M) and H2SO4 (1 drop) was added. The mixture Me 15 O 2.20ad rac-2.28 O was stirred at room temperature for 18 h, diluted with H2O, neutralized with saturated aqueous NaHCO3, and extracted with DCM. The

organic layer was dried over MgSO4 and concentrated in vacuo. Purification of the crude

material by preparative TLC (silica gel, DCM:hexanes, 3:2) afforded the fused 6,6-

bicyclic system rac-2.28 as a colorless oil (22.0 mg, 78.5 µmol, 46%) and uncyclized

macrocycle 2.20ad as an oil (10.4 mg, 47.2 µmol, 28%).

1 rac-2.28: Rf = 0.10 (DCM:hexanes, 1:1); H NMR (500 MHz, CDCl3) δ = 5.37 (dd, J =

2.0, 1.5 Hz, 1 H, H-8), 4.89 (dt, J = 11.0, 5.0 Hz, 1 H, H-4), 4.14 (t, J = 7.0 Hz, 2 H, H-

127 12), 2.30 – 2.21 (m, 4 H, H-5, H-9, H-11), 2.03 (s, 6 H, H-14, H-16), 1.85 – 1.67 (m, 5 H,

H-3, H-6, H-9, H-10), 1.57 (ddd, J = 24.0 , 12.5, 4.5 Hz, H-3), 1.41 – 1.18 (m, 3 H, H-1,

13 H-2) ppm; C NMR (125 MHz, CDCl3) δ = 171.2 (C-13), 170.7 (C-15), 130.8 (C-7),

121.7 (C-8), 75.6 (C-4), 63.1 (C-12), 36.9 (C-11), 36.0 (C-5), 33.2 (C-10), 31.5 (C-9),

25.9 (C-1), 25.6 (C-3), 24.1 (C-6), 23.8 (C-2), 21.5 (C-14), 21.1 (C-16) ppm; IR (ATR,

-1 neat) νmax: 2924, 2857, 1736, 1435, 1363, 1240, 1092, 1031, 973, 910 cm ; HRMS calcd

+ for C16H24O4 281.1747 [M+H] , found (ESI-TOF) 281.1742.

1 2.20ad: Rf = 0.38 (DCM:hexanes, 1:1); H NMR (500 MHz, CDCl3) δ = 5.52 – 5.44 (m,

1 H), 5.43 – 5.33 (m, 2 H), 5.32 – 5.19 (m, 2 H), 4.13 (t, J = 7.0 Hz, 2 H), 3.30 (br s, 1

H), 2.76 (br s, 2 H), 2.35 – 2.10 (m, 4 H), 2.04 (s, 3 H) ppm; 13C NMR (125 MHz,

CDCl3) δ = 171.2, 131.4, 131.3, 131.2, 129.8, 124.5, 124.1, 63.5, 38.0, 32.9, 31.0, 26.4,

21.1 ppm; IR (ATR, neat) νmax: 3007, 2957, 2925, 2855, 1741, 1436, 1381, 1364, 1236,

-1 + 1036, 971, 913, 842, 793, 708 cm ; HRMS calcd for C14H20O2 221.1536 [M+H] , found

(ESI-TOF) 221.1536.

Br O O Preparation of the crystalline derivative 2.29. Et3N (0.23

O N N H 2.29 mL, 1.68 mmol, 6.0 equiv) and DMAP (8.6 mg, 0.07

Br mmol, 25 mol%) were added to a solution of 2.20c (50 mg,

0.28 mmol) in DCM (3 mL, 0.09 M). 4-Bromobenzyl isocyanate (221 mg, 1.12 mmol,

4.0 equiv) was added and the resulting suspension was stirred for 18 h at room temperature. The mixture was acidified with 1 N HCl and extracted with DCM and

EtOAc. The organic layers were washed with 1 N HCl, H2O, and brine, dried over

MgSO4, and concentrated in vacuo. Purification of the crude product by preparative TLC

128 (silica gel, DCM:hexanes, 1:1) afforded 2.29 (61.0 mg, 0.11 mmol, 38%) as a white

foam. Single crystals suitable for X-ray crystallographic analysis were obtained by slow

evaporation of a solution of 2.29 in Et2O:hexanes (1:1). Rf = 0.41 (silica gel,

1 DCM:hexanes, 1:1); H NMR (500 MHz, CDCl3) δ = 10.87 (br s, 1 H), 7.55 (d, J = 7.0

Hz, 2 H), 7.43 (s, 4 H), 7.07 (d, J = 7.5 Hz, 2 H), 5.37 – 5.26 (m, 2 H), 5.24 – 5.17 (m, 2

H), 5.13 (t, 4.5 Hz, 1 H), 4.21 (br s, 2 H), 3.19 (br s, 2 H), 2.68 (br s, 2 H), 2.22 (t, J = 5.0

13 Hz, 2 H), 2.02 (br s, 4 H) ppm; C NMR (125 MHz, CDCl3) δ = 155.6, 151.3, 136.8,

136.0, 132.4, 132.1, 131.2, 130.6, 125.3, 124.3, 122.6, 121.6, 116.8, 66.3, 37.9, 33.0,

30.5, 26.3 ppm; IR (ATR, neat) νmax: 3220, 3104, 3002, 2924, 2855, 1720, 1688, 1598,

1541, 1487, 1439, 1397, 1378, 1312, 1300, 1270, 1238, 1183, 1133, 1115, 1103, 1072,

1016, 1004, 993, 969, 925, 904, 833, 774, 743, 705, 688 cm-1; HRMS calcd for

+ C26H26Br2N2O3 573.0383 [M+H] , found (ESI-TOF) 573.0395.

1 1 10 Crabtree, 10 OH 8 OH Hydrogenation of 2.20c to afford OH H2 + 7 7 H 2.20c 2.30 rac-2.31 2.30 and rac-2.31.

[Ir(cod)(PCy3)(py)]PF6 (4.50 mg, 5.59 µmol, 10 mol%) was added to a solution of

macrocycle 2.20c (10.0 mg, 56.1 µmol) in DCM. The reaction vessels was placed in a

hydrogenation apparatus, charged with H2 (150 psi), and stirred at room temperature for

18 h. The crude mixture was filtered through a silica plug (eluting with DCM) and the

organic phase evaporated to dryness. Preparative TLC (silica gel, DCM) afforded

analytically pure 2.30 (6.1 mg, 33.8 µmol, 60%) and rac-2.31 (3.1 mg, 17.0 µmol, 30%).

1 2.30: Rf =0.43 (silica gel, DCM); H NMR (600 MHz, CDCl3) δ = 5.74 (app q, J = 9.3

Hz, 1 H, H-5), 5.29 (app dd, J = 9.4, 8.9 Hz, 1 H, H-4), 5.08 (t, J = 8.4 Hz, 1 H, H-8),

129 3.72 (d, J = 5.7 Hz, 2 H, H-12), 2.78 (d, J = 8.4 Hz, 2 H, H-6), 2.38 – 2.30 (m, 6 H, H-3,

H-9, H-11), 1.56 – 1.50 (m, 2 H), 1.27 – 1.22 (m, 4 H) ppm; 13C NMR (150 MHz,

CDCl3) δ = 136.1 (C-7), 130.7 (C-4), 127.7 (C-8), 127.5 (C-5), 60.4 (C-12), 40.8 (C-11),

30.4, 30.1, 28.3 (C-9), 27.9 (C-6), 27.4 (C-3), 20.6 ppm; IR (ATR, neat) νmax: 3335, 3007,

2958, 2921, 2852, 1465, 1443, 1377, 1044, 1025, 935, 877, 864, 796, 761, 710 cm-1;

+ HRMS calcd for C12H20O 181.1587 [M+H] , found (ESI-TOF) 181.1592.

1 rac-2.31: Rf =0.43 (silica gel, DCM); H NMR (600 MHz, CDCl3) δ = 5.47 – 5.38 (m, 2

H), 3.75 – 3.68 (m, 2 H), 2.35 (br s, 1 H), 2.14 (br s, 2 H), 1.76 (br d, J = 6.6 Hz, 1 H),

13 1.62 – 1.37 (m, 12 H), 1.24 (m, 2 H); C NMR (150 MHz, CDCl3) δ = 131.2, 128.1,

61.6, 37.4, 32.9, 30.3, 29.9, 27.9, 27.5, 26.1, 25.7, 20.9 ppm; IR (ATR, neat) νmax: 3333,

-1 2957, 2922, 2853, 1463, 1377, 1048, 888, 789, 722 cm ; HRMS calcd for C12H22O

183.1743 [M+H]+, found (ESI-TOF) 183.1752.

H rac-2.32 Cope rearrangement to divinylcyclohexene rac-2.32. Macrocycle

OTBS H 2.20f (20.0 mg, 68.4 µmol) was heated neat to 100 ºC in a sealed tube

for 3 h. After purification of the crude mixture by column chromatography (silica gel,

hexanes:EtOAc, 99:1) rac-2.32 was obtained as a clear, pale yellow oil (16.0 mg, 54.7

1 µmol, 80%). H NMR (500 MHz, CDCl3) δ = 5.84 – 5.74 (m, 1 H), 5.41 (s, 1 H), 5.03 –

4.98 (m, 3 H), 3.66 (t, J = 7.0 Hz, 2 H), 2.48 – 2.36 (m, 2 H), 2.23 – 2.10 (m, 4 H), 2.01 –

13 1.89 (m, 2 H), 0.89 (s, 9 H), 0.05 (s, 6 H) ppm; C NMR (125 MHz, CDCl3) δ = 140.3

(two signals), 133.5, 121.2, 114.6, 114.5, 62.4, 41.6, 41.2, 41.1, 32.8, 29.5, 26.1, 18.5, –

5.1 ppm; IR (ATR, neat) νmax: 3076, 2955, 2928, 2889, 2857, 1639, 1472, 1463, 1436,

130 1387, 1361, 1254, 1094, 995, 911, 833, 812, 774, 717, 661 cm-1; HRMS calcd for

+ C18H32SiO 293.2295 [M+H] , found (ESI-TOF) 293.2287.

Delayed Installation of C1 Units onto 24c.

N O 13 Br Monocycloadducts rac-2.41 and rac-2.42. HO H 1 rac-2.41 9 8 N 2 10 7 12 3 H 11 Br Br OH , 4 6 Macrocycle 2.20c (100 mg, 0.56 mmol) was 5 KHCO3 + OH Br 13 N O dissolved in EtOAc (5.6 mL, 0.1 M). NaHCO3 2.20c H 1 rac-2.42 9 8 2 10 7 12 3 H 11 OH 4 6 5 (282 mg, 3.36 mmol, 6.0 equiv.) followed by dibromoformaldoxime (115 mg, 0.56 mmol, 1.0 equiv) were added at room temperature and the reaction was stirred under ambient atmosphere. Additional dibromoformaldoxime

(115 mg, 0.56 mmol, 1.0 equiv) and NaHCO3 (94.0 mg, 1.12 mmol, 2.0 equiv) were added in intervals of 1 h until TLC analysis indicated full consumption of starting

7 material. The reaction was quenched with H2O and extracted with EtOAc. The organic layer was washed with H2O and brine, dried over MgSO4, filtered, and concentrated in

vacuo. Column chromatography (silica gel, DCM:Et2O, 100:1 → 10:1) afforded the

desired monocycloadduct rac-2.41 (35 mg, 21%) and its undesired regioisomer rac-2.42

(98 mg, 58%).

1 rac-2.41: Rf = 0.48 (silica gel, DCM:Et2O, 10:1); H NMR (600 MHz, CDCl3) δ = 5.68

(td, J = 10.7, 6.4 Hz, 1 H, H-5), 5.44 (td, J = 11.0, 5.5 Hz, 1 H, H-4), 5.11 (dd, J = 11.4,

5.7 Hz, 1 H-8), 4.30 (ddd, J = 13.9, 11.5, 2.4 Hz, 1 H, H-1), 3.78 – 3.70 (m, 2 H, H-12),

3.29 (t, J = 11.9 Hz, 1 H-6), 3.24 (ddd, J = 13.8, 5.4, 2.4 Hz, 1 H, H-10), 2.87 (ddd, J =

14.9, 11.4, 5.6 Hz, 1 H, H-9), 2.59 (ddd, J = 14.9, 5.8, 2.6 Hz, 1 H, H-9), 2.53 – 2.44 (m,

131 1 H, H-3), 2.39 – 2.27 (m, 3 H, H-6, H-11), 2.14 (ddd, J = 13.3, 8.5, 4.1 Hz, 1 H, H-3),

2.08 (tt, J = 13.8, 2.4 Hz, 1 H, H-2), 1.88 (dddd, J = 13.8, 10.8, 4.2, 3 Hz, 1 H, H-2) ppm;

13 C (150 MHz, CDCl3) δ = 145.5 (C-13), 140.5 (C-7), 129.6 (C-4), 126.8 (C-5), 120.2 (C-

8), 83.3 (C-1), 60.5 (C-12), 56.7 (C-10), 40.0 (C-11), 32.1 (C-2), 27.7 (C-6), 26.7 (C-9),

22.8 (C-3); IR (ATR, neat) νmax: 3410, 3014, 2957, 2922, 2853, 1715, 1644, 1571, 1466,

1377, 1289, 1180, 1118, 1107, 1084, 1045, 979, 916, 887, 802, 760, 720 cm-1; HRMS

+ calcd for C13H18BrNO2 300.0594 [M+H] , found (ESI-TOF) 300.0602.

1 rac-2.42: Rf = 0.38 (silica gel, DCM:Et2O, 10:1); H NMR (600 MHz, CDCl3) δ = 5.78

(td, J = 10.8, 6.3 Hz, 1 H, H-5), 5.38 – 5.29 (m, 2 H, H-4, H-8), 4.43 (dt, J = 13.5, 2.8 Hz,

1 H, H-10), 3.75 (dd, J = 8.1, 3.6 Hz, 2 H, H-12), 3.26 (t, J = 11.9 Hz, 1 H, H-6), 3.08 (t,

J = 12.0 Hz, 1 H, H-1), 2.86 (ddd, J = 15.0, 11.1, 3.6 Hz, 1 H, H-9), 2.70 (ddd, J = 14.9,

5.2, 2.0 Hz, 1 H, H-9), 2.43 – 2.31 (m, 4 H, H-3, H-6, H-11), 2.15 – 2.09 (m, 1 H, H-3),

13 1.95 (t, J = 13.9 Hz, 1 H, H-2), 1.56 (m, 1 H, H-2) ppm; C NMR (150 MHz, CDCl3)

δ = 146.9 (C-13), 140.6 (C-7), 129.0 (C-4), 128.4 (C-5), 119.8 (C-8), 89.3 (C-10), 60.6

(C-12), 49.9 (C-1), 40.0 (C-11), 30.1 (C-2), 29.9 (C-9), 27.9 (C-6), 23.8 (C-3) ppm; IR

(ATR, neat) νmax: 3390, 3010, 2922, 2862, 1719, 1657, 1572, 1466, 1379, 1305, 1251,

-1 1128, 1072, 1043, 1007, 927, 879, 798, 772, 720 cm ; HRMS calcd for C13H18BrNO2

300.0594 [M+H]+, found (ESI-TOF) 300.0597. N O 13 Br H 1 9 8 2 10 7 12 HO 3 H 11 4 N 5 OH H 6 N O H rac-2.35 O Dicycloadducts rac-2.35 and rac-2.36. Br Br Br, 14 H N Br KHCO3 + H OH N Monocycloadduct rac-2.41 (30.1 mg, 0.10 O Br rac-2.41 H 1 9 8 10 7 12 H 11 4 mmol) was dissolved in EtOAc (1 mL, 0.1 M). 5 OH H 6 Br H rac-2.36 O N NaHCO3 (33.6 mg, 0.40 mmol, 4.0 equiv) was

132 added to the reaction followed by dibromoformaldoxime (41.6 mg, 0.20 mmol, 2.0

equiv). Repeated additions of base (16.8 mg, 0.20 mmol, 2.0 equiv) and

dibromoformaldoxime (20.8 mg, 0.10 mmol, 1.0 equiv) were continued over 22 h until

the reaction was judged to be complete by TLC. The mixture was diluted with H2O and extracted with EtOAc. The organic was washed with brine, dried over MgSO4, and concentrated in vacuo. Preparative TLC (silica gel, DCM:Et2O, 9:1) afforded a 1:1 mixture (by 1H NMR) of the two cycloadducts rac-2.35 and rac-2.36 (20.0 mg, 47.4

µmol, 47%). For full structural elucidation the crude mixture was further purified by preparative TLC (silica gel, DCM:Et2O, 9:1): The seemingly single PTLC spot was arbitrarily split in half affording enriched samples of the two regioisomers rac-2.35 and rac-2.36 which allowed for full assignment by 2D NMR.

1 rac-2.35: Rf = 0.41 (silica gel, DCM:Et2O, 9:1); H NMR (600 MHz, CDCl3) δ = 5.21 (t,

J = 8.5 Hz, 1 H, H-8), 4.51 (t, J = 9.1 Hz, 1 H, H-4), 4.19 (t, J = 12.8 Hz, 1 H, H-1), 3.84

– 3.74 (m, 2 H, H-12), 3.26 (d, J = 13.3 Hz, 1 H, H-10), 3.01 (t, J = 6.9 Hz, 1 H, H-5),

2.78 (dd, J = 15.3, 6.0 Hz, 1 H, H-6), 2.70 (m, 2 H, H-9), 2.45 (dt, J = 13.1, 4.0 Hz, 1 H,

H-11), 2.37 (dt, J = 14.9, 7.4 Hz, 1 H, H-11), 2.30 (dd, J = 14.0, 5.3 Hz, 1 H, H-3), 2.14

(m, 2 H, H-2, H-3), 2.05 (m, 1 H, H-2), 1.84 (d, J = 15.6 Hz, 1 H, H-6) ppm; 13C NMR

(150 MHz, CDCl3) δ = 145.6 (C-14), 145.0 (C-13), 139.6 (C-7), 122.1 (C-8), 87.3 (C-4),

82.1 (C-1), 60.4 (C-12), 56.1 (C-10), 52.7 (C-5), 39.0 (C-11), 30.7 (C-2), 26.5 (C-9), 24.9

(C-6), 22.6 (C-3) ppm; IR (ATR, neat) νmax: 3415, 2926, 2866, 1717, 1571, 1467, 1448,

1392, 1306, 1288, 1250, 1214, 1177, 1107, 1085, 1043, 1002, 952, 908, 872, 854, 800,

-1 + 731 cm ; HRMS calcd for C14H18Br2N2O3 420.9757 [M+H] , found (ESI-TOF)

420.9752.

133 1 rac-2.36: Rf = 0.37 (silica gel, DCM:Et2O, 9:1); H NMR (600 MHz, CDCl3) δ = 5.26

(dd, J = 10.9, 6.0 Hz, 1 H, H-8), 4.70 (t, J = 9.8 Hz, 1 H, H-5), 4.23 (t, J = 12.7 Hz, 1 H,

H-1), 3.83 – 3.72 (m, 2 H, H-12), 3.24 (d, J = 13.8 Hz, 1 H, H-10), 2.88 (t, J = 12.7 Hz, 1

H, H-6), 2.83 (m, 1 H, H-4), 2.74 – 2.68 (m, 2 H, H-9), 2.52 (d, J = 14.2 Hz, 1 H, H-6),

2.41 – 2.31 (m, 2 H-11), 2.12 (t, J = 14.5 Hz, 1 H), 1.98 (t, J = 15.1 Hz, 1 H), 1.66 (dd, J

13 = 16.2, 3.6 Hz, 1 H) ppm; C NMR (150 MHz, CDCl3) δ = 145.7, 145.6, 136.5 (C-7),

123.7 (C-8), 82.5 (C-5), 81.8 (C-1), 61.0 (C-12), 55.8 (C-10), 53.5 (C-4), 39.0 (C-11),

32.7, 28.3 (C-6), 26.5 (C-9), 20.7 ppm; IR (ATR, neat) νmax: 3432, 2923, 2855, 1718,

1570, 1467, 1448, 1379, 1287, 1270, 1253, 1214, 1176, 1108, 1085, 1044, 976, 952, 909,

-1 + 877, 853, 801, 733 cm ; HRMS calcd for C14H18Br2N2O3 420.9757 [M+H] , found (ESI-

TOF) 420.9738.

Br 13 N Dicycloadducts rac-2.37 and rac-2.38. O H 1 9 8 2 10 7 12 HO 3 H 11 4 N 5 OH H 6 Monocycloadduct rac-2.42 (91.0 mg, 0.30 Br N H rac-2.37 Br 14 O Br Br O H , N KHCO3 + mmol) was dissolved in EtOAc (3 mL, 0.1 H OH Br 13 N O rac-2.42 H 1 9 8 2 10 M). NaHCO3 (101 mg, 1.20 mmol, 4.0 equiv) 7 12 3 H 11 4 5 OH H 6 O H rac-3.38 was added to the reaction followed by N 14 Br dibromoformaldoxime (122 mg, 0.60 mmol, 2.0 equiv). Repeated additions of base (51.0 mg, 0.60 mmol, 2.0 equiv) and dibromoformaldoxime (61.0 mg, 0.30 mmol, 1.0 equiv) were continued over 22 h until the reaction was judged to be complete by TLC. The mixture was diluted with H2O and extracted with EtOAc. The organic was washed with brine, dried over MgSO4, and concentrated in vacuo. Purification of the crude mixture by

column chromatography (silica gel, DCM:Et2O, 50:1 → 2:1) afforded a 1:1 mixture (by

134 1H NMR) of the two cycloadducts rac-2.37 and rac-2.38 (52.0 mg, 0.12 mmol, 41%). For full structural elucidation an aliquot of the crude mixture was further purified by preparative TLC (silica gel, DCM:Et2O, 9:1): The seemingly single PTLC spot was arbitrarily split in half affording enriched samples of the two regioisomers rac-2.37 and rac-2.38 which allowed for full assignment by 2D NMR.

1 rac-2.37: Rf = 0.36 (silica gel, DCM:Et2O, 9:1); H NMR (600 MHz, CDCl3) δ = 5.53

(dd, J = 11.0, 5.5 Hz, 1 H, H-8), 4.74 (t, J = 10.0 Hz, 1 H, H-5), 4.45 (d, J = 13.4 Hz, 1 H,

H-10), 3.84 – 3.74 (m, 2 H, H-12), 3.06 (t, J = 12.8 Hz, 1 H, H-1), 2.91 – 2.68 (m, 4 H,

H-4, H-6, H-9), 2.54 (d, J = 14.2 Hz, 1 H, H-6), 2.43 – 2.33 (m, 2 H, H-11), 2.03 – 1.83

13 (m, 2 H, H-2, H-3), 1.70 – 1.57 (m, 2 H, H-2, H-3) ppm; C NMR (150 MHz, CDCl3) δ

= 145.8 (C-13), 145.4 (C-14), 135.9 (C-7), 123.7 (C-8), 88.3 (C-10), 82.7 (C-5), 61.1 (C-

12), 53.2 (C-4), 49.6 (C-1), 38.9 (C-11), 30.5 (C-2), 29.6 (C-9), 28.4 (C-6), 21.6 (C-3) ppm; IR (ATR, neat) νmax: 3404, 2922, 2855, 1728, 1570, 1464, 1447, 1378, 1325, 1303,

1263, 1210, 1167, 1126, 1084, 1043, 1010, 964, 907, 884, 855, 829, 801, 729 cm-1;

+ HRMS calcd for C14H18Br2N2O3 420.9757 [M+H] , found (ESI-TOF) 420.9771.

1 rac-2.38: Rf = 0.40 (silica gel, DCM:Et2O, 9:1); H NMR (600 MHz, CDCl3) δ = 5.45

(dd, J = 11.1, 5.7 Hz, 1 H, H-8), 4.50 – 4.42 (m, 2 H, H-4, H-10), 3.87 – 3.75 (m, 2 H, H-

12), 3.07 (t, J = 7.3 Hz, 1 H, H-5), 2.97 (t, J = 13.0 Hz, 1 H, H-1), 2.85 – 2.70 (m, 3 H,

H-6, H-9), 2.50 – 2.44 (m, 1 H, H-11), 2.41 (q, J = 7.7 Hz, 1 H, H-11), 2.27 – 2.21 (m, 1

H, H-3), 2.14 – 2.06 (m, 2 H, H-2, H-3), 1.86 (d, J = 15.3 Hz, 1 H, H-6), 1.72 (m, 1 H, H-

13 2) ppm; C NMR (150 MHz, CDCl3) δ = 145.7 (C-14), 145.2 (C-13), 138.9 (C-7), 122.2

(C-8), 88.3 (C-10), 87.1 (C-4), 60.6 (C-12), 52.7 (C-5), 50.5 (C-1), 38.9 (C-11), 29.6 (C-

9), 28.3 (C-2), 25.2 (C-6), 23.6 (C-3) ppm; IR (ATR, neat) νmax: 3421, 2922, 2854, 1730,

135 1571, 1464, 1378, 1301, 1254, 1127, 1088, 1047, 966, 948, 886, 853, 797, 728 cm-1;

+ HRMS calcd for C14H18Br2N2O3 420.9757 [M+H] , found (ESI-TOF) 420.9774.

136 2.9 Spectral Data (1D and 2D NMR) and GC Chromatograms

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167 168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195 S4

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211 212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

1 Rohloff, J. C.; Robinson III., J.; Gardner, J. O. Tetrahedron Lett. 1992, 33, 3113–3116.

292

2 When screening other conditions (e.g., ligands other than PPh3, additives, cosolvents,

…), any solids were added in the glove box and liquids were added through the septum under argon prior to the addition of alkyne.

3 For assignment of the categories of reaction outcome refer to Figure 3.

4 The compound did not ionize under ESI-TOF conditions.

5 In addition, several – partially inseparable – further components were obtained that could not be characterized due to a lack of material.

6 Alternatively, the reaction was run with a dropwise addition of 16f or with a 40:1 diene:alkyne ratio. Although there was a noticeable increase in macrocycle formation by

GC/MS, the mixture was still too complex for isolation.

7 The dipole, generated in situ, dimerizes and must be replenished for the reaction to go to completion.

293

CHAPTER 3

Macrocyclic Sesquiterpenes: The Fallout

3.1 On the Mechanism, Reactivity and Selectivity of Ni-Catalyzed [4+4+2]

Cycloadditons of Dienes and Alkynes1

Shortly after Dr. Daniel Götz and I published our work on mimicking cyclase phase of germacrene sesquiterpenes in the Journal of Organic Chemistry,2 we were contacted by Xin Hong and Professor Ken Houk, computational chemists at the

University of California, Los Angeles. They were excited to add computational evidence to explain some of the phenomena we had observed experimentally. In this section I will briefly discuss their findings, and their publication will be submitted in due course.

Geometry optimizations, frequencies, and thermal energy corrections were performed with the B3LYP functional, 6-31G(d) basis set for all main group elements, and SDD basis set for nickel, implemented in Gaussian 09.3 Energies were evaluated with

the M06 method,4 the 6-311+G(2d,p) basis set for all main group elements and SDD basis set for nickel. All reported free energies involve zero-point vibrational energy corrections and thermal corrections to Gibbs free energy at 298 K. Extensive conformational searches have been made to make sure that the most stable conformers are located. Computed structures are illustrated using CYLVIEW drawings.5

Investigating the mechanism of the [4+4+2] cycloaddition computationally, the

Houk group made a few key observations. First, only the formation of the 1Z,4Z,8E-

cyclododecatriene product through one s-cis and one s-trans butadiene was found to be

facile and exergonic. Additionally, competition between the desired cooligomerization

and alkyne [2+2+2] cyclization is dependent on the coordination of the alkynes to the Ni0

center. Using these observations as well as additional calculations the Houk group was

able to propose a complete mechanism for the catalytic cycle.

295 ! G

3.1 3.8 TS3.4

TS3.2

TS3.7

3.3 3.5 3.8

3.6 ! G

3.1-cc 3.8-cc

TS3.2-cc TS3.4-cc

3.1-cc

3.8-cc 3.6-cc ! G

TS3.4-tt 3.1-tt 3.8-tt

TS3.2-tt

3.1-tt 3.8-tt

3.6-tt

TS3.2 TS3.4 TS3.7

3.1 3.3 3.5 3.6 3.8

Figure 3.1: Gibbs free energy profiles of [Ni(PMe3)]-catalyzed cooligomerization between s-cis and s-trans butadienes and 2-butyne.

In order to explain the Z,Z,E outcome of macrocycle 3.8, the transition state

energies of oxidative cyclization were evaluated (Figure 3.1). Utilizing PMe3 as a model ligand, the lowest energy pathway for oxidative insertion is in fact that of one s-cis and

296 one s-trans butadiene (3.1) at 14.2 kcal/mol (TS3.2). Additionally, the pathway to

oxidative insertion product 3.3 is exergonic at -5.5 kcal/mol. The oxidative cyclization of two s-cis butadiene moieties (3.1-cc) has a significantly higher transition state barrier

(TS3.2-cc) at 26.0 kcal/mol. When considering oxidative cyclization of two s-trans dienes (3.1-tt) the barrier (TS3.2-tt) is only slightly higher to one s-cis and one s-trans at

15.9 kcal/mol, however the overall the energy of the energy of the resultant intermediate

(3.3-tt) is higher (9.6 kcal/mol) than the starting materials, and the subsequent transition

state for alkyne insertion (TS3.4-tt) is significantly higher at 31.2 kcal/mol. The overall reaction, the formation of nickel-bound macrocycle 3.8 from compound 3.1 is outlined at the bottom of Figure 3.1.

During our work on the cooligomerization, we often observed seemingly unpredictable amounts of [2+2+2] alkyne cooligomerization. The Houk group observed that the rate limiting step of the [2+2+2] cycloaddition was in fact, the oxidative cyclization and became favorable when electron deficient alkynes and alkynes bearing coordinating groups, such as hydroxyls, were utilized. In these cases, alkyne coordination was favorable to diene coordination. Obtaining large amounts of [2+2+2] cycloadducts for these substrates is in line with our experimental observations (Chapter 2).

297 ! G

TS3.11

3.12 TS3.14 3.10 TS3.16

3.9 3.13 3.15

3.10 to 3.17 : R1, R2 = Me (3.18) 3.17

! G

3.10b

3.13b 3.15b

3.10b to 3.17b ; R1, R2 = CO2Me (3.19) 3.17b

! G

3.10c 3.13c

3.15c

3.10c to 3.17c : R1 = CMe2OH ; R2 = H (3.20) 3.17c

TS3.11 TS3.14 TS3.16

3.9 3.10 3.12 3.13 3.15 3.17

Figure 3.2: Gibbs free energy profiles of [Ni(PMe3)]-catalyzed (2+2+2) cycloadditions with 2-butyne (3.18), dimethyl acetylenedicarboxylate (3.19), and 2-methyl-3-butynol (3.20).

Utilizing 2-butyne (3.18), both ligand exchange between butadiene and the alkyne

to complex 3.10 and oxidative cyclization (TS3.11) are uphill in energy, 27.2 kcal/mol total (Figure 3.2). As a result, this pathway is not operative, and the reaction favors the

298 cooligomerization pathway (Figure 3.1). On the other hand, the same step is exergonic by

18.0 kcal/mol utilizing dimethylacetylene dicarboxylate (3.19) and 9.0 kcal/mol using 2-

methyl-3-butyn-2-ol (3.20). As these alkyne coordinations are energetically favorable,

(2+2+2) pathways are competitive and benzene products 3.17b and 3.17c are observed

(Figure 3.2). Some chemical rationale can be used to explain these observations. Electron deficient alkynes such as dicarboxylate 3.19 have a stronger coordination to electron rich nickel(0) and complex 3.10c, derived from 2-methyl-3-butyn-2-ol (3.20) can hydrogen bond, increasing its stablility.

! G TS3.22

3.21

TS3.23 TS3.4 TS3.2

TS3.7

3.1 3.8

3.3

3.5

3.6 3.24

Figure 3.3: Gibbs free energy profile of [Ni(PMe3)]-catalyzed butadiene/2-butyne cooligomerization.

Bearing in mind these observations, a clearer picture of the mechanism emerges

than we could have deduced experimentally (Figure 3.3). Initially, two butadiene

moieties coordinate to the Ni(0) center (3.1) and undergo oxidative cyclization via TS3.2

299 to form nine-membered organometallic species 3.3. Coordination of 2-butyne (3.18) at this point leads to energetically uphill intermediate 3.21, with an even more unfavorable alkyne insertion TS3.22 - 45.0 kcal/mol barrier from intermediate 3.3. Alternatively, a

ligand exchange between PMe3 and 2-butyne (3.18) leads to TS3.4 which is only 25.1

kcal/mol higher in energy than intermediate 3.3. This lends credence to the observation

that we were unable to achieve any ligand control in this reaction. From eleven-

membered intermediate 3.5, immediate reductive elimination to TS3.23 is significantly

uphill. Alternatively, an association of PMe3 at this point leads to a more favorable

reductive elimination via TS3.7 to nickel associated product 3.8. Dissociation of product

cyclododecatriene 3.24 and reassociation of two butadiene moieties affords complex 3.1

turning over the catalytic cycle.

Despite the lack of “success” of the nickel-catalyzed cooligomerization to produce terpene natural products, we were able to uncover some interesting reactivity

that was backed up by computational evidence.

3.2 Scalable, Enantioselecitve Synthesis of Germacrenes and Related Sesquiterpenes

Inspired by Terpene Cyclase Phase Logic

Immediately following my work on the nickel catalyzed diene alkyne

cooligomerization, I remained working toward germacrene sesquiterpene natural

products albeit through another route. I was involved in the early part of this project,

investigating macrocyclizations, but not on the end of the project, including the

optimization of the final route and transannular cyclizations. In this section, I will clearly

delineate my role in the project. Supporting information (both spectra and experimental

300 descriptions) has not been supplied for this section of my thesis. The vast majority of this

information is available in the resultant publication in Angewante Chemie6 or Klement

Foo’s thesis. The few novel compounds that I synthesized during in this section are simple derivations of farnesol, and were not used further on this project.

As previously mentioned, the Baran laboratory has a longstanding interest in harvesting new chemical knowledge by learning from terpene biosynthesis. The straightforward construction of carbocyclic terpene backbones such as epoxy- germacrenol (3.25, Scheme 3.1a) in a low oxidation state (cyclase phase) followed by regio-, chemo-, and stereoselective oxidative modifications (oxidase phase) allow Nature to access a wide variety of related family members in a divergent manner.7-10 In 2012, Dr.

Daniel Götz and I reported our initial forays into the redox-economic synthesis of

terpenes that resemble 3.25 by oligomerization of unfunctionalized isoprene (Scheme

3.1b, path a).2 The lessons from these studies ultimately led us to a new approach that

utilizes the more advanced, yet readily available, C15 building block farnesol (3.26,

Scheme 3.1a) as a starting point. As such, my coworkers and I affected a short, efficient, scalable, and enantioselective synthesis of 3.25. Furthermore, we demonstrated how the key intermediate 3.25 can be processed not only to germacrane-type natural products by chemo- and stereoselective oxidations, but also to a variety of polycyclic sesquiterpene frameworks (like selinanes, guaianes, or elemenes, Scheme 3.1a, left) by acid-mediated transannular cyclization reactions.

301 a. Two-phase approach to germacrane-type sesquiterpenes.

Me Me Me O O HO OH Me O

Me cyclase/ Me oxidase cyclase Me phase Me Me phase Me Me O Me Me Me Me OH Me HO OH O OH Me OH Me Me epoxy-germacrenol (3.25): linchpin E,E-farnesol (3.26) in sesquiterpene (bio)synthesis Me OMe Me

O OH Me Me OHHO Me

b. Selected cyclase-phase routes to germacrane-type sesquiterpenes.

O Me Me CHO R Me Me Br previous work Me O Me Me 21% over 12 steps 33% over 5 steps from S-(-)-menthene a Ni(cod)2, from farnesol PPh3 f b TiCl , Zn/Cu, 3 CrCl3–LiAlH4, DME, 0 ºC (60%) DMF (42%) f a a Me e d a,b,c Me Me NaHMDS, DME, 85 ºC 1. NaH, dicyclohexano-18-crown-6, 80 ºC (67%) core germacrene 2.t-BuLi, Et2O, –78 ºC (44% overall) skeleton e c OTs Bu SnH, AIBN, OTHP d 3 PhH, 80 ºC (14%) Me Me MeO C Me 2 O Me OH SPh Me Br

31% over 11 steps from Br Me 27% over 4 steps (+)-dihydrolimonene from farnesol Me 8% over 8 steps from chiral pyrrolidine

Scheme 3.1: a. Retrosynthetic outline; b. Known cyclase-phase routes (letters indicate retrosynthetic disconnections).

For the goal of establishing a scalable, divergent, and broadly applicable entry to a variety of related sesquiterpene families, the germacrenes were identified as strategic key intermediates due to the following reasons: 1) germacrenes constitute a large class of cyclic terpenes, many of which have been shown to exhibit promising bioactivities and, in addition, are of growing importance in the perfumery industry; 2) germacrenes are

302 known to be a biosynthetic linchpin en route to various related terpene congeners; and 3) most importantly, many studies have demonstrated that germacrenes can be transformed into mono-, di-, and tricyclic sesquiterpene subclasses (e.g., the elemenes, cadinanes, eudesmanes, guaianes, and bourbonanes) with the "Achilles heel" of these approaches being the accessibility of germacrene precursors in quantity.11,12

Despite the industrial interest in terpene natural products, most of the naturally occurring germacrenes and their congeners are not easily available in bulk quantities due to a variety of challenges associated with their synthesis and/or isolation: germacrenes featuring a cyclodecadiene core are notoriously unstable to acidic and thermal conditions

(leading to cyclized and/or rearranged products) and can often exist as conformationally semistable isomers at ambient temperature, thus creating complications with regard to purification and product analysis.13

Although the total synthesis of sesquiterpenes has been an area of intense research

efforts for decades,14-18 there is still no reliable synthetic pathway to rapidly forge the ten- membered germacrene carbocycle. The most effective synthetic approaches published to date (Scheme 3.1b) suffer from serious drawbacks such as low overall yield, poor step economy, and/or are impractical to perform on scale. In addition, these syntheses either yield racemic products or start from chiral-pool starting materials. Whereas Nature's cyclase enzymes can efficiently overcome the entropic barrier for the formation of medium-sized carbocycles, direct ring closures from acyclic precursors remain a challenge in organic synthesis and are especially difficult to perform on larger scale (e.g., due to the need for high-dilution techniques). Finally, terpenes often occur in opposite

303 enantiomeric forms in various organisms and the programmed synthesis of either

antipode of germacrenes and their congeners is a problem that has yet to be solved.

Our synthesis started with the readily available C15 building block farnesol (3.26,

Scheme 3.2), which was transformed in two known steps (Sharpless epoxidation19-21 and

Parikh-Doering oxidation) to epoxy aldehyde 3.28.22 The crude material was used directly

in the next step, a regioselective chlorination with concomitant transposition of the

internal 10,11-double bond to the terminal position according to a protocol invented by

Tunge et al.23,24 The chlorinated epoxy aldehyde 3.29 was thus obtained in 63% overall yield in three steps on decagram-scale from farnesol (3.26), with only one chromatographic purification. The route is amenable to produce both enantiomers of 3.29 and the most efficient route to a cyclization precursor in the context of germacrene total synthesis.

4 Å MS, Ti(Oi-Pr)4 (10 mol%), SO py, i-Pr NEt, Me (+)-DET (12 mol%), Me 3! 2 Me PhSeCl, NCS, Me Cl Me Me 10 Me Me 11 t-BuOOH (4 equiv), O DMSO, DCM, 0 ºC O DCM O Me OH Me DCM, –50 ºC Me OH Me Me O Me Me O farnesol (3.26) ee = 90% (2S,3S)-3.27 (2R,3S)-3.28 (2R,3S)-3.29

- 63% over 3 steps - decagram-scale - enantiodivergent - no protecting groups - only one chromatography

Scheme 3.2: Enantioselective synthesis of the coupling partner 3.29 (shown for the (2R,3S)-enantiomer).

Given the ease with which epoxy chloro aldehyde 3.29 could be prepared, several derivatives were prepared during our preliminary macrocyclization explorations. Both these substrates and their macrocyclizations was where I made my primary contribution to the project. As seen in both Scheme 3.3 and Scheme 3.4, epoxy bromide 3.30 and

epoxy iodide 3.31 were a straightforward conversion from chloride 3.29. Additionally,

304 simply omitting the Sharpless asymmetric epoxidation, as in chloride 3.32, another

substrate to attempt to cyclize was obtained. Finally, dialdeyhde 3.33 was simple to make

from farnesol.

Utilizing iodide 3.30, a multitude of Nozaki-Hiyama-Kishi (NHK) and Barbier

conditions were investigated. However, in all of these cases, no reaction performed any

better (higher yielding or cleaner) than our initial stoichiometric conditions to give

macrocycle 3.34 in ~30% yield (blue diastereomer) (Scheme 3.3a). An interesting observation came from a Barbier-type cyclization with zinc metal. In this case, it seems that the diasteroselectivity had reversed from the stoichiometric chromium conditions affording macrocycle 3.35 as the major product (red diastereomer). However, in both cases, the reactions were not clean and only ~30% of the major product was obtained.

Utilizing the chloro enal 3.32 instead of the epoxide did not afford a clean reaction either.

The only observable product was isomerization product 3.36. However, this observation

helped explain why reactions with the enal 3.32 were messier than with the epoxy chloro

aldehyde 3.29, since both the E and Z isomer can undergo cyclization.

305 a. Me Me Me I O O 3.30 Conditions Screened: Barbier: 0 NHK: Zn , NH4Cl (sat. aq.), THF; 0 CrCl2 (4 equiv), THF; In , H2O, THF; 0 0 CrCl2 (7 mol%), Mn (1.7 equiv), TMS-Cl (2.4 equiv), THF; Li , THF; 0 0 CrCl2 (10 mol%), Mn (3 equiv), Zr(Cp)2Cl2 (1 equiv), 2,6- Sn ,HBr (aq.), Et2O; 0 lutadine (11 mol%), THF Mg ,Et2O; SmI2, THF

Me Me Me CrCl2 (4 equiv), I Me H + Me H O THF O O O 3.30 Me OH Me Me OH Me 3.34 (~30%) 3.35 (trace)

Me Me Me Zn0, NH Cl (sat. aq.), H H I 4 Me Me O + + ~10 other products O THF O O 3.30 Me OH Me Me OH Me 3.35 (26%) 3.34 (~5%)

Me Me Me Me Me Me b. 0 Zn , NH4Cl (sat. aq.), O Cl THF Cl 3.32 O also screened 10 other 3.36: major product Zn0 reactions

Me Me Me In0, Me H O H2O, THF Cl 3.32 Me OH Me

Me Me Me SmI2 (excess), H O Me CHO O THF, t-BuOH 3.33 Me OH Me

Scheme 3.3: a. and b. Various cyclization attempts on substrates 3.30, 3.32, and 3.33.

In a final attempt to improve this NHK macrocyclization, catalytic variants were investigated. It was hypothesized that the utilization of an enatiopure ligand could provide double diastereoinduction, or perhaps provide a cleaner outcome, going through a discreet organometallic species. To this end, bromide 3.37, iodide 3.38, and

epoxybromide 3.31 were screened against Kishi’s catalytic asymmetric conditions.25

However, in all cases, the reactions did not proceed as quickly or to full conversion, when

compared to the stoichiometric reactions (Scheme 3.4). Ultimately, the NHK route was abandoned in favor of a Pd-catalyzed umpolung allylation reaction uncovered by my coworkers.

306 Me Me Me

X CrCl2 (10 mol%), ligand (11 mol%), O 0 Mn (3 equiv), Zr(Cp)2Cl2 (1 equiv), 3.37: X = Br NEt3 (10 mol%), 2,6-lutadine (11 mol%), H 3.38: X = I or Me or Me Me Me Me O THF Me OH Me Me OH Me Br O O Ligands: 3.31 H H Me H Me H OMeH OMeH N N N N N N SO2Me SO2Me SO2Me SO2Me SO2Me SO2Me N Me N N Me N N Me N t-Bu t-Bu t-Bu O Me O O Me O O Me O

H H Me H Me H OMeH OMeH N N N N N N SO2Ph SO2Ph SO2Ph SO2Ph SO2Ph SO2Ph N Me N N Me N N Me N t-Bu t-Bu t-Bu O Me O O Me O O Me O

Scheme 3.4: Attempts at an asymmetric NHK reaction.

With the epoxy chloro aldehyde 3.29 in hand, cyclization conditions were

explored to furnish the desired medium-sized germacrane ring system. Inspired by

previous work of Shibuya et al.14 who performed a similar ring closure under NHK

conditions using CrCl3–LiAlH4 (Scheme 3.1, path b), the development of a robust, scalable, and diastereoselective cyclization was pursued. After an extensive screening of conditions, the intramolecular coupling of aldehyde 3.29 succeeded via a Pd-catalyzed

26 - 28 umpolung allylation reaction. Using Pd(PPh3)4 and Et2Zn in THF, we obtained intermediate 3.25 in varying yields of 10–30% (see Table 3.1, entries 1 and 2). It was

found that only 1.5 equiv. of Et2Zn (entry 2) is needed instead of 4 (entry 1). Interestingly, by switching the catalyst from Pd(PPh3)4 (20 mol %) to Pd(PPh3)2Cl2 (10 mol %) and adding K2CO3 to the reaction mixture, the reaction proceeded more cleanly, which

simplified the purification of the product (entries 3–6). Finally, changing solvents from

THF to DMF then to DMA gave the best yield of 42% for cyclized product 1 (entries 4

and 5). Attempts to vary the reductant from Et2Zn to Me2Zn (entry 7), Bu2Zn (entry 8),

Et3B, InI or SnCl2 also failed to improve the results.

307 With optimized conditions in hand, the reaction was ultimately performed on

gram-scale, by the dropwise addition of both substrate and Et2Zn to a solution of

PdCl2(PPh3)2 and K2CO3 in DMA, giving (+)-3.25 in 42% yield, as the only diastereomer,

opposite in stereoselectivity to that in the previously described conditions of Shibuya et

al.14 Overall, this diastereoselective ring closure strategy provides efficient access to the

optically active germacrane-type key intermediate 3.25 reproducibly on gram-scale. In

retrospect, it is not surprising that this ring closure was the only method that worked well,

given the pioneering findings of Trost on medium and macrocyclic ring formation via π-

allyl palladium catalysis.29-31

Table 3.1: Key ring closure of 3.29 via an umpolung allylation.

Me Cl Conditions Me OH Me Me Me Me Me O [gram-scale] O Me O Me OH O H (2R,3S)-3.29 (+)-3.25

# catalyst ligand reduct. solv. base 1[a] [b] [c] 1 Pd(PPh3)4 -- Et2Zn THF -- 19 [b] 2 Pd(PPh3)4 -- Et2Zn THF -- 10- 30

3 PdCl2(PPh3)2 -- Et2Zn THF K2CO3 13- 42

4 PdCl2(PPh3)2 -- Et2Zn DMF K2CO3 32 [d] 5 PdCl2(PPh3)2 -- Et2Zn DMA K2CO3 42 6 PdCl2(PPh3)2 -- Et2Zn DMA -- 33 7 PdCl2(PPh3)2 -- Me2Zn DMA K2CO3 0

8 PdCl2(PPh3)2 -- Bu2Zn DMA K2CO3 31 9 Pd(OAc)2 PPh3 Et2Zn DMA K2CO3 15

Standard conditions unless otherwise stated: cat. (10 mol %), ligand (20 mol %), K2CO3 (1.5 equiv), solvent (0.033 M overall), 3.29 (100–600 mg scale, 1.0 equiv., 0.05 M, slow addition over 1.5 h), reductant (1.5 equiv., slow addition over 1.5 h), T = 50 °C. [a]

Isolated yield. [b] Cat. (20 mol %). [c] 4 Equiv of Et2Zn. [d] Reproduced on six occasions, 42% was obtained for gram-scale.

With the supply problem solved, attention turned to exploring the innate reactivity of 3.25 and its conversion to other natural products. Thus, acetylation of (+)-3.25 with

308 DCC, DMAP and AcOH proceeded in quantitative yield to give (+)-3.39 (Scheme 3.5).

The stereochemistry of (+)-3.25 was unambiguously established from the X-ray crystallographic analysis of acetate 3.39, revealing the cis-relationship between the 6-OH group and isopropylidene group in the 7-position. Hydroboration followed by oxidative work up of acetate 3.39 furnished the desired diol product, which was immediately

oxidized (without further purification) with TEMPO and PhI(OAc)2 to 11,13-dihydro- epi-parthenolide (3.40) as a mixture of diastereomers.32 Converting 3.25 to acetate 3.39 is

necessary for the hydroboration step, since a <40% conversion was observed otherwise.

α-Bromination of lactone 3.40 with LDA and CBr4, followed by dehydrobromination with TBAF,33,34 gave rise to 7-epi-parthenolide (–)-3.41 (60% over 2 steps), an unnatural epimer of the biologically active 35 - 38 sesquiterpene lactone parthenolide. 39 , 40 While

parthenolide has been successfully isolated in quantity recently,41 none of its epimers

have been synthesized to date, and thus the biological activity of 3.41 has not yet been

explored. (–)-4-Hydroxyallohedycaryol (3.45) was obtained in a four-step sequence from

(+)-3.25 by: 1) directed epoxidation with VO(acac)2 to give bis-epoxide 3.42 (77% yield);

2) chemoselective reduction of the less-hindered epoxide to diol (+)-3.43 with LiAlH4; 3) mesylation of the 2º alcohol providing (+)-3.44 (44% over 2 steps); and 4) reductive elimination of the epoxy mesylate (+)-3.44 with lithium naphthalenide to afford (–)-3.45 in 65% yield, together with recovered diol 3.43 in 23% yield (Scheme 3.5).

Chemoselective hydrogenation of the isopropylidene side chain in (+)-3.25 using

Crabtree’s catalyst42,43 afforded (+)-shiromool44 (3.46) in good yield (65–84%, Scheme

3.5). It was found that the addition of 2,6-di-tert-butylpyridine was crucial to suppress side product formation. Epoxidation of (+)-3.46 with m-CPBA led to a 4: 1

309 diastereomeric mixture of (–)-1β,10α,4β,5α-diepoxy-7α(H)-germacran-6β-ol (3.47) and

(+)-3.48 in 84% combined yield, both of which are natural products.45,46 Acetylation of the 6-OH of (+)-3.46 with AcOH, DCC and DMAP furnished (+)-shiromool acetate47

48 (3.49) in quantitative yield. Reaction of (+)-3.49 with WCl6 and n-BuLi effected the removal of the 4,5-epoxide, followed by saponification of the acetate group with

49 K2CO3/MeOH at 50 ºC gave (–)-(1E,4E)-7αH-germacra-1(10),4-dien-6β-ol (3.50) in

82% yield over 2 steps. Oxidation of this natural product (–)-3.50 with IBX50 gave (+)-

acoragermacrone51-54 (3.51) in 90% yield.

MeSO2Cl, Et3N, Li-napth., Me Me Me Me Me Me O OH DCM, –5 ºC THF, –25 ºC Me OH Me O OH OH OH (44% over two steps) Me OMs Me (65% + 25% 3.43) Me Me (+)-3.43 (+)-3.44 (!)-4-hydroxyallohedycaryol (3.45)

LiAlH4, THF, 0 ºC

Me Me Me O O O Me O Me OH O 3.42 (!)-7-epi-parthenolide (3.41)

VO(acac)2, 1. LDA, THF, –78 ºC; t-BuOOH, [X-ray] then CBr4 DCM, 0 ºC 2. TBAF, THF (77%) (60% overall)

1. 9-BBN, THF; Me then EtOH, Me DCC, DMAP, Me Me NaOH (aq.) Me 4 6 11 Me O O O Me OH AcOH, DCM Me OAc 2. TEMPO, Me O (+)-3.25 (~quant.) PhI(OAc)2, DCM (+)-3.39 (77%) O 11,13-dihydro-epi- Crabtree's, parthenolide (3.40) 2,6-di-t-Bu-py., H2, DCM (65–84%) O O

Me Me Me Me Me Me m-CPBA, NaHCO3, O O O Me OH Me DCM, –5 ºC Me OH Me Me OH Me (+)-shiromool (3.46) (62% 3.47 and (!)-1#,10",4#,5"-diepoxy- (+)-3.48 22% 3.48) 7"(H)-germacran-6#-ol DCC, DMAP, (3.47) AcOH, DCM (~quant.)

1. WCl6, n-BuLi, THF, –60 ºC Me Me Me Me Me Me (86%) IBX, O Me OAc Me Me OH Me Me O Me 2. K2CO3, MeOH DMSO (+)-shiromool (95%) (!)-(1E,4E)-7"H-germacra- (90%) (+)-acoragermacrone (3.51) acetate (3.49) 1(10),4-dien-6#-ol (3.50)

Scheme 3.5: Synthesis of germacrane-type sesquiterpenes.

310

In addition to the successful syntheses of sesquiterpenes 3.41–3.51 incorporating the 10-membered carbocycle, common bicyclic sesquiterpene frameworks were also accessed by transannular cyclizations as depicted in Scheme 3.6. It is known that transannular cyclization of compounds such as 3.45, with the allohedycaryol framework, would give rise to cadinane-type sesquiterpenes. Depending on the acid used with (±)-

3.45, trichotomol55 (3.52) and its C10-epimer 3.53, δ-cadinene-11-ol56 (3.54) and γ-

cadinene-11-ol (3.55) were obtained in varying ratios (Scheme 3.6).57 The elemene skeleton can be accessed by a Cope rearrangement of acoragermacrone (3.51), giving a

21:4 mixture of shyobunone58 (3.56) and its epimer 3.57, respectively.51 Alternatively,

treating 3.51 with acid would afford selinane-type sesquiterpenes, acolamone59 (3.58;

55% yield) and isoacolamone (3.59; 23% yield).60 Likewise, the guaiane framework can

be accessed by treating shiromool acetate (3.49) with dry HCl gas in Et2O at 0 ºC to obtain a ~15:5:4 mixture of (+)-3.60a, (+)-3.61a and (+)-3.62a respectively, with a

preference for the exocyclic olefin.47 A similar outcome (5:2:1 mixture) was observed

with p-TsOH (0.27 equiv) as the acid catalyst in CH2Cl2 at room temperature. The

resulting major product (+)-3.60a was then converted to (+)-teucladiol 61 (3.60) by saponification. At this point, we set out to investigate whether different acids would affect the regioselectivity of this cyclization, and if the same transformation could be performed directly on (+)-shiromool (3.46), in order to obtain the natural products 3.61 and 3.63 selectively. Subjecting (+)-3.46 to catalytic p-TsOH gave a 3: 1 mixture of (+)-

3.61 and (+)-3.60, a selectivity opposite to that observed for the same reaction with

acetate 3.49. After further screening, it was found that adding Sc(OTf)3 to (+)-3.46 in

311 62 CH2Cl2 at 0 ºC gave a >20: 1 mixture of (+)-4β,6β-dihydroxy-1α,5β(H)-guai-9-ene

(3.61) and (+)-3.60 in 67% yield. A separate reaction of (+)-3.46 with (S)-(+)-1,1'-

binaphthalene-2,2'-diyl hydrogen phosphate (S)-3.65 as acid catalyst in CH2Cl2 at room temperature gave a 11:5:2 mixture of (+)-chrysothol63,64 (3.63; 49% yield), 3.61 and 3.60

(32% yield combined) respectively. Instead of the concomitant loss of proton after the

acid-induced epoxide opening, as in the case in the formation of 3.61 and 3.60, the ether

linkage in (+)-3.63 is formed by the trapping of the cation intermediate by the 6-OH group, which is not possible when (+)-shiromool acetate (3.49) is used as substrate.

Using the above conditions, the 4,8-ring system (as in 3.64a) was not observed, which could potentially arise from the Markovnikov opening of the 4,5-epoxide of (+)- shiromool (3.46). However, when dibenzyl phosphate was used as the acid for cyclization of (+)-3.46, (+)-3.64 was obtained, albeit with poor selectivity. Gratifyingly, using (+)-

3.49 instead of (+)-3.46 as substrate under the same conditions afforded acetate (+)-3.64a in 70% yield, whose stereochemistry was unequivocally verified by X-ray crystallographic analysis. Acetate (+)-3.64a was subsequently converted to (+)-3.64 in

78% yield upon treatment with K2CO3/MeOH at 50 ºC.

312 Elemene-type Selinane/Eudesmane-type Cadinane-type Guaiane-type Example of acid:

Me Me BnO O Me Me Me P Me O O BnO OH P O OH OH OH dibenzyl O Me O Me Me Me phosphate Me OR Me (S)-3.65 acoragermacrone (3.51) hydroxyallohedycaryol (3.45) (+)-shiromool (3.46) (+)-shiromool acetate (3.49)

R = H or Ac Me H OH Me Me H H H

Me Me Me H Me H Me Me H Me H Me Me O Me Me Me Me OH OR OH OR OH HO OR Me Me shyobunone (3.56) trichotomol (3.52) (+)-3.64 or acetate (+)-3.64a (+)-teucladiol (3.60) (+)-4!,6!-dihydroxy- and epi-shyobunone (3.57) and epi-trichotomol (3.53) or acetate (+)-3.60a 1",5!(H)-guai-9-ene (3.61) or acetate (+)-3.61a or H Me Me Me H

Me O Me H Me Me H Me Me H Me Me OH OAc H O Me OH OH Me Me acolomone (3.58) and #-cadinene-11-ol (3.54) and (+)-4!-hydroxy-6!-acetoxy- (+)-chrysothol (3.63) isoacolomone (3.59) $-cadinene-11-ol (3.55) [X-ray] 5!(H)-guai-1(10)-ene (3.62a)

Scheme 3.6: Transannular cyclization to cadinane-, selinane-, elemene- and guaiane-type sesquiterpenes.

The concise, scalable, enantioselective synthesis of epoxy-germacrenol 3.25 via a unique Pd-catalyzed macrocyclization presents a viable solution to the longstanding problem of forming the 10-membered germacrane ring system. It has also enabled the rapid formation of a range of germacrane-type sesquiterpenes 3.41–3.51, six of which are natural products. Of these compounds, 3.45, 3.46 and 3.51 can be converted in one single step to give cadinane-, elemene- or selinane-, and guaiane-type frameworks respectively.

In particular, we have demonstrated that the cyclization of shiromool 3.46 to the guaiane- type natural products 3.60, 3.61, and 3.63 can be performed selectively, by varying either the substrate or the acid used. Finally, the divergent pathways to sesquiterpenes illustrated here nicely complement the recent elegant studies of Winssinger and coworkers.65

313

1 The work in this section is quoted heavily from personal communication with Xin

Hong, Houk Laboratory, UCLA. Without his expertise in computational chemistry, I

would not be able to present his findings in such detail. The schemes were prepared by

him and used with permission. The writing is my own.

2 Holte, D.; Götz, D. C. G.; Baran, P. S. J. Org. Chem. 2012, 77, 825–842.

3 Gaussian 09, Rev. B.01: Frisch, M. J.; et al., Gaussian, Inc., Wallingford CT, 2010.

4 Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.

5 Legault, C. Y. CYLView, 1.0b; Universite´ de Sherbrooke, Canada, 2009;

http://www.cylview.org.

6 Foo, K.; Usui, I.; Götz, D. C. G.; Werner, E. W.; Holte, D., Baran, P. S. Angew. Chem.

Int. Ed. 2012, 51, 11491–11495.

7 Chen, K.; Baran, P. S. Nature 459, 824–282.

8 Chen, K.; Ishihara, Y.; Galán, M. M.; Baran, P. S. Tetrahedron 2010, 66, 4738–4744.

9 Ishihara, Y.; Baran, P. S. Synlett 2010, 1733–1745.

10 Maimone, T. J.; Baran, P. S. Nat. Chem. Biol. 2007, 77, 825–842.

11 Adio, A. M. Tetrahedron 2009, 65, 1533–1552.

12 Bülow, N.; König, W. A. Phytochemistry 2000, 55, 141–168.

13 Minnaard, A. J.; Wijnberg, J. B. P. A.; de Groot, A. Tetrahedron 1999, 55, 2115–2146.

14 Shibuya, H.; Ohashi, K.; Kawashima, K.; Hori, K.; Murakami, N.; Kitagawa, I. Chem.

Lett. 1986, 85–86.

15 Kitahara, T.; Mori, M.; Koseki, K.; Mori, K. Tetrahedron Lett. 1986, 27, 1343–1346.

16 Takahashi, T.; Nemoto, H.; Kanda, Y.; Tsuji, J.; Fukazawa, Y. Tetrahedron 1987, 43,

5499–5520.

314

17 McMurray, J. E.; Siemers, N. O. Tetrahedron Lett. 1994, 35, 4505–4508.

18 Jonas, D.; Özlü, Y.; Parsons, P. J. Synlett, 1995, 255–256.

19 Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974–5237.

20 Marshall, J. A.; Hann, R. K. J. Org. Chem. 2008, 73, 6753–6757.

21 Uyanik, M. Ishibashi, H.; Ishihara, K.; Yamamoto, H. Org. Lett. 2005, 7, 1601–1604.

22 Kotaki, T.; Shinada, T.; Kaihara, K.; Ohfune, Y.; Numata, H.; Org. Lett. 2009, 11,

5234–5237.

23 Mellegaard-Waetzig, S. R.; Wang, C.; Tunge, J. A. Tetrahedron 2006, 62, 7191–7198.

24 Tunge, J. A. ; Mellegaard-Waetzig, S. R. Org. Lett. 2004, 6, 1205–1207.

25 Wan, Z.-K.; Choi, H.-w.; Kang, F.-A.; Nakajima, K.; Demeke, D.; Kishi, Y. Org. Lett.

2002, 4, 4431–4434.

26 Masuyama, Y.; Takahara, J. P.; Kurusu, Y. J. Am. Chem. Soc. 1988, 110, 4473–4474.

27 Kimura, M.; Horino, Y.; Mukai, R.; Tanaka, S.; Tamaru, Y. J. Am. Chem. Soc. 2001,

123, 10401–10402.

28 Zhu, S.-F.; Qiao, X.-C.; Zhang, Y.-Z.; Wang, L.-X.; Zhou, Q.-L. Chem. Sci. 2011, 2,

1135–1140.

29 Trost, B. M. Angew. Chem. Int. Ed. 1989, 28, 1173–1192.

30 Trost, B. M.; Verhoeven, T. R. J. Am. Chem. Soc. 1979, 101, 1595–1597.

31 Trost, B. M.; Burns, A. C.; Bartlett, M. J.; Tautz, T.; Weiss, A. H. J. Am. Chem. Soc.

2012, 134, 1474–1477.

32 Azarken, R.; Guerra, F. M.; Moreno-Dorado, F. J.; Jorge, Z. D. Massanet, G. M.

Tetrahedron, 2008, 64, 10896–10905.

33 Higuchi, Y.; Shimona, F.; Ando, M. J. Nat. Prod. 2003, 66, 810–817.

315

34 Matsuo, K.; Ohtsuki, K.; Yoshikawa, T.; Shishido, K.; Yokotani-Tomita, K.; Shindo,

M. Tetrahedron 2010, 66, 8407–8419.

35 Ogura, M.; Cordell, G. A.; Farnsworth, N. R. Phytochemistry, 1978, 17, 957–961.

36 Fischer, N. H.; Weidenhamer, J. D.; Bradow, J. M. Phytochemistry, 1989, 28, 2315–

2317.

37 Fischer, N. H.; Weidenhamer, J. D.; Riopel, J. L.; Quijano, L.; Menelaou, M. A.

Phytochemistry 1990, 29, 2479–2483.

38 Marles, R. J.; Kaminski, J.; Arnason, J. T.; Pazas-Sanou, L.; Heptinstall, S.; Fischer, N.

H.; Crompton, C. W.; Kindack, D. G.; Awang, D. V. G. J. Nat. Prod. 1992, 55, 1044–

1056.

39 Govindachari, T. R.; Joshi, B. S.; Kamat, V. N.; Tetrahedron Lett. 1964, 5, 3927–3933.

40 Govindachari, T. R.; Joshi, B. S.; Kamat, V. N.; Tetrahedron 1965, 21, 1509–1519.

41 Zhai, J.-D.; Li, D.; Long, J.; Zhang, H.-L.; Lin, J.-P.; Qiu, C.-J.; Zhang, Q.; Chen, Y. J.

Org. Chem. 2012, 77, 7103–7107.

42 Crabtree, R. H. Acc. Chem. Res. 1979, 12, 331–337.

43 Crabtree, R. H.; Davis, M. W. J. Org. Chem. 1986, 51, 2655–2661.

44 Wada, K.; Enomoto, Y.; Munakata, K. Agric. Biol. Chem. 1970, 34, 946–953.

45 Sanz, J. F.; Marco, J. A. Phytochemistry 1991, 30, 2788–2790.

46 Zhu, Y.; Zhao, Y.; Huang, G.-D.; Wu, W.-S. Helv. Chim. Acta 2008, 91, 1894–1901.

47 Garcia-Granados, A.; Molina, A. Cabrera, E. Tetrahedron 1986, 42, 81–87.

48 Sharpless, K. B.; Umbreit, M. A.; Nieh, M. T.; Flood, T. C. J. Am. Chem. Soc. 1972,

94, 6538–6540.

49 Stahl-Biskup, E.; Laakso, I. Planta Med. 1990, 56, 464–468.

316

50 Frigerio, M.; Santagostino, M. Tetrahedron Lett. 1994, 35, 8019–8022.

51 Iguchi, M.; Niwa, M.; Nishiyama, A.; Yamamura, S. Tetrahedron Lett. 1973, 14,

2759–2762.

52 Niwa, M.; Nishiyama, A.; Igushi, M.; Yamamura, S. Bull. Chem. Soc. Jap. 1975, 48,

2930–2934.

53 Still, W. C. J. Am. Chem. Soc. 1977, 99, 4186–4187.

54 Takahashi, T.; Nemoto, H.; Tsuji, J.; Miura, I. Tetrahedron Lett. 1983, 24, 3485–3488.

55 Menezes, J. E. S. A.; Lemos, T. L. G.; Silveira, E. R.; Braz-Fihlo, R.; Pesso, O. D. L. J.

Braz. Chem. Soc. 2001, 12, 787–790.

56 Bohlmann, F.; Jakupovic, J.; Vogel, W. Phytochemistry 1982, 21, 1153–1154.

57 Kodama, M.; Shimada, K.; Takahashi, T.; Kabuto, C.; Itô, S. Tetrahedron Lett. 1981,

22, 4271–4274.

58 Iguchi, M.; Nishiyama, A.; Koyama, H.; Yamamura, S.; Hirata, Y. Tetrahedron Lett.

1968, 9, 5315–5318.

59 Niwa, M.; Nishiyama, A.; Iguchi, M.; Yamamura, S. Chem. Lett. 1972, 1, 823–826.

60 Niwa, M.; Iguchi, M.; Yamamura, S. Bull. Chem. Soc. Jap. 1976, 49, 3148–3154.

61 Bruno, M.; de La Torre, M. C.; Rodríguez, B.; Omar, A. A. Phytochemistry 1993, 34,

245–247.

62 Mahmoud, A. A. Phytochemistry 1997, 45, 1633–1638.

63 Ahmed, A. A.; Hegazy, M.-E. F.; Hassan, N. M.; Wojcinska, M.; Karchesy, J.; Pare, P.

W.; Mabry, T. J. Phytochemistry 2006, 67, 1547–1553.

64 Ono, M.; Yamashita, M.; Mori, K.; Masuoka, C.; Eto, M.; Kinjo, J.; Ikeda, T.;

Yoshimitsu, H.; Nohara, T. Food Sci. Technol. Res. 2008, 14, 499–508.

317

65 Valot, G.; Garcia, J.; Duplan, V.; Serba, C.; Barluenga, S.; Winssinger, N. Angew.

Chem. Int. Ed. 2012, 51, 5391–5394.

318

PART II

Verruculogen and Fumitremorgin A

319

CHAPTER 4

Total Syntheses of Demethoxyverruculogen and Verruculogen

320 4.1 Introduction

Verruculogen (4.1) belongs to a family of diketopiperazine indole alkaloids that

also includes the fumitremorgins (e.g. 4.7, 4.10–4.12), tryprostatins (e.g. 4.6, 4.8, 4.9),

and brevianamides (e.g. 4.5). The biosynthesis is reasonably well understood and is

depicted in Scheme 4.1.1-3 Brevianamide F (4.5) is formed from the enzymatic union of

L-tryptophan (4.3) and L-proline (4.4). Brevianamide F (4.5) is then prenylated with prenyltransferase FtmPT1/FtmB in the presense of dimethylallyl diphosphate (DMAPP) to give trypostatin B (4.6). Two different cytochrome P450 enzymes are responsible for converting trypostatin B (4.6) into demethoxyfumitremorgin C (4.7) and 6- hydroxytrypostatin B (4.8). 6-Hydroxytrypostatin B (4.8) is then converted into trypostatin A (4.9) by a methyl transferase and further converted to fumitremorgin C

(4.10) by the same cytochrome P450 (FtmP450-2/FtmE). A different cytochrome P450,

FtmP450-3/FtmG, is responsible for the dihydroxylation that converts fumitremorgin C

(4.10) into 12,13-dihydroxyfumitremorgin C (4.11). The prenyl transferase

FtmPT2/FtmH is responsible for the conversion of 12,13-dihydroxyfumitremorgin C

(4.11) into fumitremorgin B (4.12). The endoperoxide of verruculogen (4.1) has been

shown to be dependent on non-heme Fe(II) and α-ketoglutarate-dependent dioxygenase.

In 2012, verruculogen prenyltransferse (FtmPT3) was found to install the final prenyl

unit to form fumitremorgin A (4.2).4 It is likely that this final enzyme took longer to

elucidate because verruculogen is produced by many more species of fungus than fumitremorgin A (which is not produced by Pennicillium species at all).

321 H CO2H

NH2 O O N O H N H N H L-tryptophan (4.3) FtmPS/ N FtmPT1 / FtmP450-2 / FtmA FtmB HN H FtmE N + H HN H O HN N N O O H H N DMAPP PPi Me H Me HO2C H Me Me demethoxy- L-proline (4.4) brevianamide F (4.5) tryprostatin B (4.6) fumitremorgin C (4.7) FtmP450-1 / FtmC O O O H H N N N FtmP450-2/ FtmMT/ FtmE FtmD N HN H HN H MeO H MeO HO N O N O N O H H H Me FtmP450-3/ Me Me Me FtmG Me Me fumitremorgin C (4.10) tryprostatin A (4.9) 6-hydroxytryprostatin B (4.8)

O O O HO HO RO HO HO HO N N N FtmPT2 / FtmOx1 / E FtmH Fe(II)ascorbate D N A C N N MeO H B H MeO H MeO N O N O N O succinate H DMAPP PPi -ketoglutarate F Me ! + CO Me 2 Me Me O O Me Me Me Me Me DMAPP Me 12,13-dihydroxyfumitremorgin B (4.12) R = H, verruculogen (4.1) FtmPT3 fumitremorgin C (4.11) R = prenyl, fumitremorgin A (4.2) PPi

Scheme 4.1: Biosynthesis of verruculogen (4.1) and fumitremorgin A (4.2) and related diketopiperazine alkaloids.

From a synthetic standpoint, we were interested in verruculogen and

fumitremorgin A because of their hexacyclic structures containing an endoperoxide in an eight-membered ring (F ring, Scheme 4.1). Interestingly this unique macrocyclic endoperoxide existed in close proximity to an electron rich indole, a functionality that should be easily oxidized by peroxides. Several less obvious challenges include the 6- methoxy substituent on the indole A ring, an indole substitution pattern that is not commercially available, the C12-C13 cis-diol, and the 1,3-cis relationship of the substituents on the C-ring (cis-bonds shown in red, Scheme 4.1). Given that

322 diketopiperazine alkaloids have seen significant attention in the literature these were the

primary challenges at the onset of the project.

An excellent review of the syntheses of the fumitremorgins and verruculogens

was published by Hino and Nakagawa in 1997. 5 This review highlights the

aforementioned challenges and several approaches toward their solutions. Gratifyingly,

since Nakagawa’s review, Bailey and coworkers have successfully addressed the 1,3-cis

β-carboline by conducting a kinetically controlled cis-selective Pictet–Spengler reaction.6

The cis-diol on the C ring was troubling not because it hadn’t seen any synthetic effort, but because a multitude of groups had trouble efficiently installing it. Catalytic dihydroxylations of an olefin at this position proved low yielding (and significantly worse on substrates containing 6-methoxy substitution on the indole). Going through a bromohydrin to a diol led to undesired epimers. Despite this, the stoichiometric OsO4

mediated dihydroxylation of compound 4.13 to give diol 4.14 in 75% yield gave us hope

that this conversion could be effected (Scheme 4.2), 7 especially given new

dihydroxylation conditions that have been developed since 1987.

O O HOHO N N

OsO4, N H N H N O pyridine N O H 75% H Me Me HO HO Me Me 4.13 4.14

Scheme 4.2: Literature precedent for installation of a cis-diol on the pentacyclic C ring.

Bearing these things in mind, we wanted to be able to access the late stage

challenges associated with these molecules as quickly as possible. The Bailey synthesis

323 of demethoxyfumitremorgin C (4.7) was a good starting point, as it was both short (three-

steps) and high yielding (21% overall).6

4.2 Pennicilium verruculosum

At the onset of this project, I set out to purchase a sample of verruculogen (4.1) in order to ascertain its stability and reactivity under a variety of conditions. However, the samples I found were expensive, at $369 for 5 mg 8 or completely unavailable. 9

Pennicilium verruculosum is routinely used to produce biologically useful quantities of

verruculogen.10-13 The procedures seemed straightforward, but I hadn’t done any biology in years, so it was safe to say I had no idea what I was doing. However, after talking with two graduate students at The Scripps Research Institute, Van Vu (Marletta Laboratory) and Erik Wold (Schultz and Felding Laboratories), I was convinced that I would be able to “grow” an infinite amount of my natural product. I owe a great deal of thanks to both

Erik and Van for their help with all of my simple biology. So, I purchased a spore of

Pennicilium verruculosum from ATC14 and began, what I believe is, the only in-house

biology the Baran Laboratory has ever done.

To this end, the dry spore of Pennicilium verruculosum was hydrated and streaked

onto 12”x12” potato starch agar plates. After several weeks, the fungus had grown to fill

the entire plate (Figure 4.1). An aliquot of the fungus could be transferred using sterile

technique to a new agar plate to propagate the fungus and the remaining fungus and agar

could be transferred to a 4 L Erlenmeyer flask and extracted.15

324

Figure 4.1: Initial spore growth of Pennicilium verruculosum in a 1 L Erlenmeyer flask. Subsequent generations were grown on 12”x12” petri dishes.

Initially, samples were purified utilizing HPLC, however after several runs, checking by LC/MS and experimenting with TLC, it was found that simple column chromatography gave access to pure verruculogen. Indeed, extended exposure to trifluoroacetic acid led to decomposition of the natural product, so column chromatography turned out to be preferable to HPLC. After several extractions, a yield of

10-20 mg per 12”x12” plate was achieved and some basic stability studies were conducted (Scheme 4.3).

325 O O HO HOHO HO N N F. AcOH:H2O (9:1), BHT, DCM, A. H2, Pd/C (10%) N 0 ºC to 23 ºC N H no reactions MeO H MeO O G. Cu(OAc) , N O MeOH N 2 H O2, MeOH Me 4.15 Me H O O HO Me Me Me Me B. p-TsOH, 4.1 E. TFA, acetone CDCl3 O O HO C. 4-nitrobenz- D. BF OEt HO 3 2 HOHO N aldehyde, (10 mol%), N p-TsOH, PhMe, DCM, 0 ºC 100 ºC N H N MeO MeO H O N N O H Me decomposition no reaction H O O Me Me Me Me Me 4.16 12,13-dihydroxyfumitremorgin C tentatively assigned

Scheme 4.3: Studies on the stabliliy of natural verruculogen (4.1) produced by fungus.

Testing the stability of the verruculogen (4.1), it readily underwent hydrogenation of the peroxide bond and hydrolysis of the aminal to afford the alcohol 4.15 (conditions

A). This was also a useful compound to characterize, as the alcohol later proved to be a common decomposition product of all free peroxides during endoperoxidation attempts.

Two attempts were made at preparing acetals. In the first case, verruculogen was reacted with anhydrous tosic acid in acetone at room temperature (conditions B). After three hours, the starting material was mostly consumed, and LC/MS and NMR analysis showed an isomer of verruculogen which has tentatively been assigned to be the peroxyhemiaminal epimer 4.16. In the case of forming the acetal with p- nitrobenzaldehyde, no reaction was observed at room temperature, and upon heating to

100 ºC nonspecific decomposition resulted (conditions C). These experiments illustrate two interesting points. First of all, the acetal of the diol was unable to be formed under standard acetal forming conditions, which could limit the possibilities of protecting this functionality in a synthesis. Secondly, we were unable to perform an “acetal exchange”

326 with the peroxyhemiaminal functionality, which attests to its stability under mildly acidic

conditions. Similarly, reacting verruculogen with catalytic BF3OEt2 in DCM led to no

decomposition of the endoperoxide functionality (conditions D). However, the reaction of wet trifluoroacetic acid in deuterated chloroform leads to the decomposition of verruculogen into the olefin 4.17 (conditions E), so there is some instability of the endoperoxide to wet acids. Utilizing Dussault’s conditions,16 specifically AcOH, BHT,

H2O/DCM (conditions F), we attempted to hydrolyze verruculogen’s peroxyhemiaminal functionality to the free peroxide, but no reaction resulted from this single attempt.

Finally, the peroxide was exposed to Cu(OAc)2, O2, and MeOH , conditions that we were considering using for a late stage installation of the 6-methoxy substitutent (conditions

G), and there was no reaction, thus the peroxide seemed to tolerate copper.

These studies were useful for both basic reactivity, and the realization that verruculogen (4.1) was not terribly unstable. We were able to store, handle, react, and purify verruculogen under standard conditions. Most importantly from these studies was simply having access to verruculogen for authentic standards for LC/MS, NMR, and

TLC.

4.3 Installation of the Peroxide

Our first goal was to establish a method for introducing the peroxide, as we considered this to be the most challenging and least precedented step.17 To this end, a few

simple retrosynthetic disconnections were made (Scheme 4.4). Given the challenges associated with 6-substituted indoles, we immediately decided to ignore the methoxy

substituent, leaving this issue to deal with in the future (see Section 4.5). With our goal

327 now being the synthesis of demethoxyverruculogen (4.18), the diol on the C ring was removed retrosynthetically, as well as the peroxyhemiaminal, the functionality we assumed to be least stable on the molecule. This led to pentacycle 4.19, which was further retrosynthetically disconnected to demethoxyfumitremorgin C (4.7), removing the unstable peroxide. We had hoped that this peroxide could be installed at a late stage using

18 19 either a Mukaiyama hydration or an acid catalyzed (SN1) peroxidation. In either case, chemoselectivity between the prenyl and indole would be an issue that needed attention.

O O O HO HO N N N

C N late stage oxidation and N late stage peroxidation N H H H N O N O N O prenyl introduction H H Me HOO Me H O O Me Me Me Me Me Me demethoxyverruculogen (4.18) 4.19 demethoxyfumitremorgin C (4.7) diketopiperazine formation Bailey et al.

CO2Me CO2Me CO2Me Pictet-Spengler NH peroxidation NH NH2 N N H H N H ROO Me L-tryptophan methyl ester (4.3) 4.20 Me Me 4.21 Me

Scheme 4.4: Retrosynthetic analysis of demethoxyverruculogen (4.18).

In the event that peroxidation could not be effected at a late stage, an alternate retrosynthetic disconnection was possible. In this case, the peroxide was left intact, and the diketopiperazine moiety was removed to afford tetrahydrocarboline 4.20. From this intermediate, a peroxidation could be applied retrosynthetically to give trisubstituted olefin 4.21. Alternatively, the peroxide could be brought into the synthesis early, through a Pictet-Spengler reaction, which simplifies the molecule retrosynthetically to L- tryptophan methyl ester (4.3).

328 In the forward direction, Bailey et al. had developed an impressively efficient

route to demethoxyfumitrmorgin C (4.7, Scheme 4.5).6 Starting from L-tryptophan methyl ester (4.3), 3-methyl-2-butenal (4.22) was condensed onto the free amine in the presence of 3Å molecular sieves. This afforded imine 4.23, which was not isolated, but further diluted with CHCl3 and then treated with trifluoroacetic acid at low temperature

for 10 hours. Aqueous workup and chromatography afforded the cis diastereomer 4.21.

Although the reported diastereoselectivity was 6:1, we were typically unable to achieve

better than 2:1dr. We did not attempt to reoptimize this step at this time, because 1.) The

diastereomers may react differently and provide insight into the endoperoxide formation

and 2.) We would need to reoptimize for a substrate containing a 6-methoxy substituent.

Deviating slightly from Bailey’s conditions, the free amine 4.21 was then

acylated under Schotten-Baumann conditions with Fmoc-protected proline acid chloride

4.24 to afford amide 4.25.20,21 At this point, amide 4.25 could be deprotected and simultaneously cyclized to give demethoxyfumitremorgin C (4.7). This pentacycle could be N-prenylated by stoichiometrically deprotonating the indole N-H with sodium hydride and alkylating with prenyl bromide 4.26 to give N-prenylated compound 4.27. A second series of substrates could be synthesized by utilizing nitrosobenzene and zirconium tetrachloride to install an alkene on amide 4.25, forming α,β-unsaturated ester 4.28.

These conditions were originally developed in our laboratory for the stephacidin synthesis, but were not published until the full account. 22 Changing the Fmoc deprotection conditions to diethylamine in THF, conditions from Corey’s (+)-austamide synthesis,23 the Fmoc protecting group could be removed avoiding the use of both DMF and piperadine, which allowed for purification of pentacycle 4.29 by filtration.

329 Me CO2Me CO2Me CO2Me Me O O 4.22 N TFA (10 equiv), NH Fmoc N NH2 + N N Cl 3Å MS, CHCl3 H CHCl3 (0.033M) H H N -40 ºC, 10 h Me H Me 4.23 38%, 6:1 cis 4.21 4.24 L-tryptophan methyl Me Me ester (4.3) NaHCO3 (sat. aq.), DCM 76% O Me O N CO2Me Me Br N O N 4.26 piperadine (2 equiv), N H N H N O N H NFmoc NaH, DMF N O DMF, 40 ºC H Me (~quant.) H 66% Me Me Me Me Me Me Me 4.27 demethoxyfumitremorgin C (4.7) 4.25

PhNO, PhNO, ZrCl , DCM ZrCl , DCM 4 4 71% O Me O N CO2Me E N D Me Br O N 4.26 Et2NH (5 equiv), N R N H N O N H NFmoc NaH, DMF N O THF H Me (~65%) H 58% Me Me Me NaH (1 equiv): R = H (4.30) Me NaH (excess): R = prenyl (4.31) Me Me Me 4.29 4.28

Scheme 4.5: Synthesis of prenylated compounds for late stage peroxidation.

Interestingly, demethoxyfumitremorgin (4.7) could not be converted into unsaturated pentacycle 4.29 under any conditions attempted, including utilizing zirconium tetrachloride and nitrosobenzene (the conditions that worked well on the tricyclic compound 4.25). Prenylation of pentacycle 4.29 proceeded smoothly to give N- prenyl compound 4.30, though it was discovered that excess base led to prenylation of the next most acidic position, namely the D/E ring junction to give polyprenylated product

4.31 (Scheme 4.5).

Given the successful synthesis of several substrates suitable for the subsequent step, a peroxide installation was the immediate goal. Simple allyl and prenyl-derived substrates (4.32–4.34, Scheme 4.6a) were readily peroxidized under Mukaiyama

24,25 conditions utilizing Co(modp)2, although 3-methyl-2-butenal (4.22) did not undergo

330 peroxide installation. Following this success, using the substrates prepared above

(Scheme 4.5), installation of the peroxide was attempted under various conditions based on Mukaiyama’s and Boger’s precedents.26 None of these reactions led to formation of

product.

a. O Me O Co(modp) (10 mol%), Co(modp)2 (10 mol%), 2 Me no reaction N Ph Me O N Ph O2, TESi-H, DCE, H O2, TESi-H, DCE, H 23 ºC, 12-24h O 23 ºC, 12-24h 4.32 OTES 4.35 4.22

Me O Me OEt Me O Me OEt Co(modp)2 (10 mol%), Co(modp)2 (10 mol%), Me O Ph Me OEt Me O Ph O Me OEt O2, TESi-H, DCE, O O2, TESi-H, DCE, OTES 23 ºC, 12-24h OTES 4.33 23 ºC, 12-24h 4.36 4.34 4.37 O t-Bu modp = N O OH O

O b. O N CO2Me CO2Me N O N N NAc H N H H N O N NFmoc N N O H H Me H Me Me Me Me 4.7 Me Me Me 4.25 4.38 Me Me 4.27 ~70 reactions Screened: Catalysts O Co(acac)2, Co(modp)2, Co(salen), NH4MoO4, O O N Mn(dpm)3, Fe(ClO4)3!6H2O, FeF3, Fe4[Fe(CN)3]3, N H N Fe(acac)3, Fe2(ox)3!6H2O, Fe(dpm)3, FeCl3!6H2O, N R Fe(dibm)3, Fe(SO2Tol)3 N N N O H H N N O Oxidants Me O H H H O2, t-BuOOH, H2O2 Me Me Me 4.29 4.39 Me Me 4.30 Me Me Solvents, Temperatures

Scheme 4.6: a. Simple Mukaiyama hydration reactions; b. Failed late-stage Mukaiyama hydration substrates, and a list of various conditions attempted.

In addition to the conditions shown in Scheme 4.6b, peroxide installation was

attempted with H2O2 under acidic conditions. These conditions were chosen because they are similar to the industrial synthesis of t-BuOOH. However, it was quickly realized that under these conditions the indole was rapidly oxidized and often formed spirocyclic compounds (not shown). Attempting to deactivate the indole through N-Boc protection

331 (not shown) or Gribble reduction 27 (substrate 4.39) did not lead to productive peroxidation either.

After many efforts, we were unable to peroxidize any “late stage” substrates, which led to a reevaluation of our strategy. Reviewing our previous retrosynthesis

(Scheme 4.4), the plan was changed to the installation of the requisite peroxide early in the synthesis. Utilizing the knowledge of peroxides obtained from our previous exploration, we decided a Mukaiyama oxidation was a straightforward way of installing

the necessary oxidation onto a prenyl unit. As shown in Scheme 4.7, we were able to

prepare a series of peroxides derived from 3-methyl-2-butenal (4.22). The corresponding

diethyl acetal 4.34 was prepared from the 3-methyl-2-butenal,28 as the Mukaiyama hydration did not proceed on the free aldehyde. Diethyl acetal 4.34 peroxidized smoothly with either triethyl silane or t-butyl dimethylsilane, however, bulkier silanes like triisopropyl silane and tert-butyl diphenyl silane did not allow the reaction to proceed.

This can be rationalized by the steric bulk of the silane preventing the cobalt hydride from forming, preventing catalytic initiation.

However, as a workaround for these bulkier silanes, triethyl silyl protected peroxide 4.37 could be deprotected with TBAF in THF to afford the free peroxide. After aqueous workup, this peroxide was carried forward without purification and reprotected as the t-butyl diphenyl silyl peroxide 4.41.

332 silane, O Me (EtO)3CH, KHSO4 EtO Me Co(modp)2 (5 mol%) EtO Me Me TFA O Me Me

EtO Me EtO OOR H OOR H Me EtOH O2, DCE H2O, CHCl3 4.22 (55%) 4.34 4.37: R = TES (72%) (ca. quant.) 4.42: R = TES 4.40: R = TBS (90%) 4.43: R = TBS

EtO Me Me 1. TBAF, THF EtO Me Me TFA O Me Me

EtO OOTES 2. TBDPSi-Cl, EtO OOTBDPS H2O, CHCl3 H OOTBDPS 4.37 imid., DCM 4.41 (ca. quant.) 4.44 (75% over two steps)

Scheme 4.7: Preparation of protected prenyl peroxides.

With these substrates in hand, it was time to attempt the requisite Pictet-Spengler reaction. First, the diethyl acetals 4.37, 4.40, and 4.41 were deprotected under acidic, biphasic conditions (Scheme 4.7) The aldehydes 4.42–4.44 could be used without purification (Scheme 4.8). Triethylsilyl peroxide 4.42 performed poorly in the Pictet-

Spengler reaction since, under prolonged acidic conditions, the protecting group was cleaved. After the protecting group fell off, a variety of side reactions occurred, affording a messy reaction. Both the t-butyl dimethyl silyl peroxide 4.43 and the tert-butyl diphenyl peroxide 4.44 underwent Pictet-Spengler reaction smoothly.

CO Me CO Me CO2Me 2 2 O Me Me TFA, 3Å MS AcCl, NaHCO3 (aq.) NH NAc NH2 + H OOR CHCl3, 0 ºC N DCM N N H H 4.3 H 4.42: R = TES Me Me ROO ROO 4.43: R = TBS Me Me 4.44: R = TBDPS R =TES (decomp.) 4.47: R = TBS (63%) 4.45: R = TBS (24%) 4.48: R = TBDPS (87%) 2:1 dr 4.46: R = TBDPS (67%) TBAF, AcOH, 3:1 dr DMF, 0 ºC ~85–97% O Me

CO2Me H Me 4.22 CO2Me NAc BF3!OEt2, 4Å MS N NAc DCE, 28 min N Me 42% H O O 4.49 Me Me Me Me HOO 4.50 Me

Scheme 4.8: Installation of the first eight-membered endoperoxide 4.50.

333 Based on the earlier work with olefin containing substrates (Scheme 4.5), we had two concerns about proceeding with the synthesis of pentacyclic systems. The first was that in any substrate containing and the N-Fmoc protecting group (e.g 4.25 or 4.28), the

NMR spectra were completely indecipherable, due to rotomeric structures. The second was that the pentacyclic systems were incredibly insoluble due to their extremely planar nature. Given these two observations, we decided to first begin with a model system and acylate the free amines 4.45 and 4.46 with acetyl chloride (Scheme 4.8). These acylations proceeded smoothly with both the t-butyldimethyl substrate 4.45 and the tert- butyldiphenyl substrate 4.46 reacting in acceptable yields to afford amides 4.47 and 4.48, respectively. Notably, in both the Pictet–Spengler reaction and the acylation, the substrates utilizing a TBDPS protecting group afforded higher yields. It is worth noting that, the diasteromers of TBS protected compound 4.45 were separable by silica gel chromatography, whereas TBDPS protected diastereomers 4.46 were not.

The silyl protecting groups on compounds 4.47 and 4.48 could be removed under acidic TBAF conditions in high yield (Scheme 4.8). The use of a molar equivalent of acetic acid in this reaction prevented quantitative epimerization of the stereocenter adjacent to the methyl ester. In both cases free peroxide 4.49 could be isolated and fully characterized. Although we were always careful when handling this compound, if it was stored at room temperature under vacuum, we did not observe any decomposition.

Leaving it in chloroform at room temperature under ambient conditions did, however, lead to nonspecific decomposition. Gratifyingly, this compound was a pleasant white solid, and a single crystal was grown which demonstrated the correct structure with the

1,3-cis-configuration and free peroxide poised near the indole.

334 Endoperoxidation of the free peroxide 4.49 under acidic conditions proceeded

relatively smoothly. A small amount of optimization was conducted on this substrate and,

although a variety of acids effected the reaction, boron trifluoride diethyl etherate was

chosen due to its ease of handling. The key optimization in raising the yield of this

reaction from ~10% to 42% was a control reaction which established that the product slowly decomposed under reaction conditions. Thus, the reaction was quenched at 15

minutes, when there was still starting material remaining, and a 42% yield was obtained.

If the reaction was stopped too early lower yields were obtained, and if the reaction was

allowed to proceed to full conversion, there was significant decomposition to the tertiary

alcohol (not shown).

Although the NMR of the endoperoxide 4.50 appeared as rotomers due to the amide bond, it was possible to fully characterize the structure (all 1D and 2D NMR available in Section 4.8), and it compared well to an NMR of natural verruculogen. The

diagnostic signal was the HMQC signal at the peroxyhemiaminal position. In

verruculogen (4.1), this 1H signal appears as a doublet at 6.83 ppm correlated with a 13C

signal at 86.0 ppm. In our synthetic endoperoxide, the rotomers were fully characterized

together and one 1H signal was a doublet at 6.71 ppm attached to a carbon at 86.8 ppm and the second 1H signal was nearly coincident at 6.71 ppm attached to a carbon at 86.4

ppm (See Section 4.8 for spectra).

When this reaction was run on larger scale, a solid was once again obtained, and

an X-ray quality crystal was produced. It was a good day when this crystal structure was

obtained and showed the correct connectivity and the correct diastereoselectivity

(Scheme 4.8). Despite the excellent NMR data, we were never 100% sure of the structure

335 due to the presence of rotomers, but the crystal structure was like a shining beacon of

hope in a sea of darkness.

At this point, with key step behind me, I naïvely assumed the project would

rapidly proceed to completion, but once again, as with all things in science, there was

much work to be done.

4.4 Synthesis of Demethoxyverruculogen

Given the success at installing the requisite endoperoxide on model system 4.49,

proceeding with the system with the full diketopiperazine was thought to be

straightforward. Utilizing free amine 4.46 (prepared in Scheme 4.8) an N-acylation under our previously elucidated Schotten-Baumann conditions proceeded smoothly to afford amide which was principally characterized by LC/MS (Scheme 4.9).

Fmoc N CO2Me ClOC CO2Me H O NH N NaHCO (sat. aq.), N 3 N H NFmoc H DCM H Me 76% TBDPSOO Me TBDPSOO Me 4.46 4.51 Me

Scheme 4.9: Acylation of TBDPS-peroxide 4.46.

After the synthesis of large quantities of amide 4.51, this substate then became an area of intense study for several months. The biggest issue with this molecule was the terrible NMR spectra that resulted from the amide bond and the Fmoc protecting group rotomers. While LC/MS and TLC provided essential data for moving forward, in order to actually characterize compounds they needed to be carried forward to a pentacyclic diketopiperazine. Another confounding issue at this point was the loss of material upon

336 deprotection: our key tricyclic compound 4.51 has a molecular weight of 876.14 g/mol and its doubly deprotected, cyclized diketopiperazine analog (not shown) has a molecular weight of 383.45 g/mol. This corresponds to 56% protecting group weight (a Baran laboratory nightmare). So even if we achieved excellent yields, material was quite precious.

Scheme 4.10 provides a comprehensive view of how we proceeded with this portion of the project. Our first observation proved consistent with our observations in

Scheme 4.5. When amide 4.51 was carried forward to the pentacycle 4.52, it was unable to be unsaturated to pentacycle 4.53 using the nitrosobenzene/ZrCl4 mediated olefination.

However, the unsaturated pentacycle 4.53 could be accessed by switching the sequence.

In this case, the amide 4.51 could be unsaturated first to α,β-unsaturated ester 4.54, and

then carried forward to the unsaturated pentacycle 4.53.

Utilizing both pentacycles 4.52 and 4.53, it was attempted to establish the

required endoperoxide through our previously uncovered conditions. However, no

endoperoxide was ever observed, even traces by LC/MS. Consistent with our previous

observation, if the free peroxide was left under the reaction conditions for extended

periods of time, it decomposed to the alcohol.

Interestingly, while working on the 6-methoxy system 4.63 (Scheme 4.11),

BF3OEt2 and diethyl acetal 4.34 as a latent aldehyde were reacted with the free peroxide

to afford the stable peroxyacetal 4.64 which could be chromatographed and fully

characterized. All four diastereomers were synthesized (accounting for the diastereomers which arose during the Pictet–Spengler). Unfortunately, compound 4.64 was unable to be processed into the endoperoxide under any conditions explored.

337 ) H H H 4.18 ( O N py. O O N N Me , 4 Me Me Me N O Me Me N N O O OsO O H 4.62 HO 4.60 O O O HO N O O N N H H H Me Me Me demethoxyverruculogen Me Me Me 2 2 THF THF (41%) HNEt HNEt N Fmoc N Fmoc Me Me 2 H 2 N Fmoc H PhNO, O (36%) O Me , Me 4 CO CO Me Me Me N N 2 H 4.59 O Me ZrCl DCM O CO O Me HO 4.61 N O O HO N N O 4.58 H H Me Me O N H Me Me Me Me %) %) MS MS MS Me Me Me 4Å 4Å ~50 4Å 74–90 Me Me ( Me (89%) ( , , , 2 2 2 3:2 DCE DCE (31%) =

OEt OEt OEt O O O 3 3 TBAF 3 TBAF TBAF dr

2. 1.

2. 2. BF BF 1. 1. BF N Fmoc N Fmoc py. N Fmoc 4.51 , 4 Me Me Me 2 2 H H 2 H O O O Me OsO Me Me CO CO CO Me Me N N Me N HO 4.56 4.54 HO N H N H N H PhNO, (66%) , 4 TBDPSOO TBDPSOO TBDPSOO ZrCl DCM 2 2 THF HNEt THF (89%) (??%) HNEt H H H PhNO, , 4 O O O N N N Me Me Me ZrCl DCM Me Me Me N N N O O O H HO py. , HO N N N H H H 4 4.52 4.57 4.53 (20%) OsO TBDPSOO TBDPSOO TBDPSOO

Scheme 4.10: Permutations of reactions in order to access demethoxyverruculogen (4.18)

338 O H N O EtO Me H N N H EtO Me MeO 4.34 N O N H MeO H Me N O HBF4, DCM H O Me 4.63 Me EtO O HOO Me 4.64 Me Me

Scheme 4.11: An unusually stable peroxyacetal.

When considering the dihydroxylation, the key piece of data was shown in

Scheme 4.2: the dihydroxylation of pentacyclic alcohol 4.13 in 75% yield. However,

when we subjected the tricyclic model system 4.65 to the reaction conditions, we only

observed dihydroxylation of the indole to give diol 4.66 (Scheme 4.12). While disappointing, this substrate was different from the one utilized in the precedented reaction, as it was tricyclic and conjugated to an ester instead of the amide of a

diketopiperazine. This gave us hope that other substrates would be sterically and

electronically more similar to 4.13.

CO Me CO Me 2 HO 2 OsO , py. NAc 4 NAc N (47%) N H H OH Me Me TBSOO TBSOO Me Me 4.65 4.66

Scheme 4.12: Dihydroxylation of indole 4.65 in the presence of an α,β-unsaturated ester.

For the sake of completeness, a stoichiometric dihydroxylation was attempted on

tricyclic α,β-unsaturated ester 4.54 and unsurprisingly, the only observed product was dihydroxylation of the indole (Scheme 4.10). Gratifyingly however, it proved possible to dihydroxylate pentacycle 4.53 on a test scale (no yield obtained). However, we

339 anticipated that we would not be able to close the endoperoxide due to 1.) competition

with the diol, and 2.) our inability to close the endoperoxide on either pentacycles 4.52,

4.53, or 4.63. As such, we did not pursue this reaction.

Having made forays into the dihydroxylation, it was time to reinvestigate the

endoperoxide formation (Scheme 4.10). Utilizing our principle starting material 4.51, the

endoperoxide 4.58 formed in an acceptable yield of 31% without any additional

optimization. Interestingly, utilizing the unsaturated tricyclic peroxide 4.54, the

unsaturated peroxyhemiaminal 4.59 could not be formed. There are clearly subtle

structural steric and strain effects allowing the endoperoxidation to take place considering

neither pentacycles nor α,β-unsaturated ester 4.54 would form the eight-membered endoperoxide. In order to unambigiously demonstrate that we formed the endoperoxide

4.58, it was carried forward to the pentacyclic diketopiperazine 4.60 in 41% yield, which proved to be characterizable.

With endoperoxide 4.58 in hand, the nitrosobenzene/zirconium tetrachloride olefination proceeded smoothly to give unsaturated ester 4.59. It was anticipated that this compound would dihydroxylate at the indole as we had previously observed, so the Fmoc protecting group was removed and the diketopiperazine was cyclized to give 4.62.

Hexacyclic compound 4.62 was characterized and carried forward under our previously utilized conditions, stoichiometric osmium tetroxide in pyridine. This reaction afforded demethoxyverruculogen 4.18 in low yield and minimal efforts were made to optimize, since the methoxy-substrate would need to be optimized anyway.

340 4.5 Efforts Toward Scalable Production of 6-Methoxytryptophan

In order to prepare verruculogen (4.1), we needed to obtain large amounts of 6-

methoxytryptophan (4.67) as our principle starting material (Scheme 4.13). Tryptophans with 6-substituents remain a challenging problem in the literature although there are several acceptable, but by no means ideal, solutions available. In working toward our synthesis, we principally investigated four different routes.

In route A (Scheme 4.13), we recapitulated a route utilized in the Baran laboratory for the synthesis of stephacidins that was based on a procedure from Merck.

22, 29 Although this route provided initial access to 6-methoxytryptophan (4.67) in

reasonable quantities (~2-3 grams prepared total) from inexpensive starting materials

4.68 and 4.70, it had a nine-step linear sequence that suffered from a somewhat

irreproducible key step. The tryptophan formation step, with which the members of my

committee may be intimately acquainted, was reported at 75% yield, which I have nearly

reproduced (~70%) on several occasions. Despite attempting to optimize this reaction,

there is some unknown variable that occasionally results in much lower than expected

yield (< 40%). One issue is certainly scale: reactions larger than 500 mg routinely

produced lower yields than reactions on less than 100 mg scale. But other possible issues

could be the batch of palladium, impurities in the reagents (1,4-

diazabicyclo[2.2.2]octane, tetrabutylammonium iodide, DMF), degree of degassing, and

the fact that the reaction was occasionally inhomogeneous. Also, we were unable to

conduct the synthesis with the methoxy substituent already on the molecule. The electron

rich nature of the aromatic ring caused the yield to drop off to traces. As such, the end of

the route involved a multitude of protecting group manipulations.

341 The next two routes (B and C, Scheme 4.13) that were investigated rely on

Rapoport’s Batcho-Leimgruber indole synthesis of 6-substituted indoles.30 Starting with

4-methyl-3-nitrophenol (4.73), this is actually the most efficient procedure for producing large quantities of 6-methoxyindole (4.74). This route is only three steps, yields 68%, and is chromatographically simple. In my own hands, I have produced ~30 g of 6- methoxyindole in this manner. The problem lies in the steps that come from converting the indole to the enantiopure tryptophan. In route B, indole is alkylated with L-serine, but requires a subsequent enzymatic resolution. This route was carried forward to the enzymatic resolution step, but we did not proceed further than this despite the anticipated

20% yield overall.

Route C used the same intermediate, 6-methoxyindole (4.74), but then relied on

the Schöllkopf auxiliary (4.77) to install the chiral center. This route too, proved amenable to scale, however, the eleven-step synthesis was long for our taste, despite its relatively good overall yield of 19% for the longest linear sequence. Several batches (~1-

2 grams) of 6-methoxytryptophan (4.67) were produced using this route.

342 A. Merck / Stephacidin route: 12 steps total, 9 longest linear, 6–12% overall

NH2 I 3 steps CO2Me CO2Me

HO NO2 93% TsO NH 2 Pd(OAc)2, TBAI, NHCbz 5 steps NH2 4.68 4.69 H Cbz DABCO, DMF 37% N O 3 steps N OH 30–70% TsO N MeO N HO2C MeO2C H H 4.72 4.67 56% 4.70 4.71

B. Batcho-Leimgruber/resolution route: 6 steps total, 6 longest linear, 20% overall

CO2Me Me 3 steps 3 steps NH2 68% N HO NO2 MeO ~30% H (including 4.73 4.74 MeO N enymatic H resolution) 4.67

C. Batcho-Leimgruber/Schöllkopf route: 11 steps total, 8 longest linear, 19% overall Br Me 6 steps

MeO Me CO2Me 29% N N HO NO2 MeO H n-BuLi, 3N HCl 4.73 4.75 Me NH2 OMe N NH2 THF, –78 ºC OMe THF, H2O, MeOH 80% 83% 3 steps N MeO N O Me H MeO N N Me H 4.67 OH Me 53% 4.78 OMe Me D-valine (4.76) 4.77

D. Iridium catalyzed C–H borolation route: 6-7 steps total, 6-7 longest linear, overall yield to be determined:

CO2Me CO2Me CO2Me [Ir(cod)OMe]2 (5 mol%), 1. Boc2O, NEt3, 1,10-phenanthroline (10 mol%), NH NHBoc NHBoc 2 DCM HBpin (0.25 equiv), B2pin2 (4 equiv),

2. NaHMDS, TIPS-Cl hexanes (0.35 M), 60 ºC, 16 h O N N Me B N H THF, –78 ºC TIPS 70–75%, (C6:C5 7.5–8.3:1) TIPS Me L-tryptophan 4.79 O 4.80 methyl ester (4.3) Me Me 1. oxone, THF, acetone, H2O 2. NaH, MeI, DMF

CO2Me CO2Me CO2Me

NH2 NH2 NHBoc + 3N HCl

MeO N MeO N EtOAc MeO N H TIPS 90% TIPS 4.63 4.82 (4.82:4.63 2:1) 4.81 TBAF THF 92%

Scheme 4.13: Four possible routes to 6-methoxytryptophan (4.67).

Finally, we have very recently been investigating a C–H activation strategy

toward 6-substituted tryptophans, the principle benefit of this route being the extremely

low cost of L-tryptophan (4.3). I’m pleased to write that within a week of the deadline for this thesis, my talented coworker Dr. Yu Feng finalized conditions for this C–H

343 activation (route D, Scheme 4.13). Namely, the doubly protected N-TIPS,N-Boc

tryptophan 4.79 is added to a premixed solution of iridium catalyst, 1,10-phenanthroline

ligand, catalytic HBpin, and several equivalents of B2pin2. Importantly, both protecting

groups are needed to solubilize the substrate in hexanes, as the reaction does not proceed

in more polar solvents. Additionally, it is essential for the catalyst, ligand, and boron

sources to be premixed. Finally HBpin is needed to form an iridium hydride, initiating the

catalytic cycle. Gratifyingly, this reaction proceeds to high conversion (95%), and has a

67% isolated yield on 1.5 gram scale. The ratio for C6 to C5 borylation is surprisingly

high at 7.5:1. Conversion of the pinacol borane 4.80 to the methoxy substituted tryptophan 4.81 proceeds in two steps, involving conversion to the phenol with oxone,

and subsequent methylation with MeI. The protecting groups are then removed in either one or two steps to afford spectroscopically identical 6-methoxytryptophan (4.67). This route remains to be optimized, and as these results are very new, it remains to be seen if this route is competitive with the prior three routes, but we are optimistic.

4.6 First Generation Total Synthesis of Verruculogen

Simultaneously with our efforts to produce large quantities of 6- methoxytrypophan methyl ester (4.67), we utilized our knowledge gained during the synthesis of demethoxyverruculogen (4.18) to work on the synthesis of verruculogen

(4.1). After working through all possible synthetic routes on demethoxyverruculogen

(Scheme 4.10), I had hope that, but did not expect that, the synthesis of the natural product, verruculogen would go well. Surprisingly, it did.

344 In a forward sense, 6-methoxytryptophan (4.67) was first tested in our Pictet-

Spengler reaction with trifluoroacetic acid and product was obtained, albeit in low yield

(Scheme 4.14). After looking at the messy TLC and crude NMR data, the reaction

seemed to be significantly less pure. Based on this decomposition we attempted to lower

the amount of acid (catalytic or 1 equiv) and temperature (–78 ºC to 0 ºC), but in each

case the reaction shut down (catalytic or –78 ºC) or exhibited the same amount of

decomposition (1 equiv or 0 ºC). Solving this problem came about somewhat

serendipitously. The aldehyde was allowed to stir with the free amine for several hours

and checking a 1H NMR aliquot for imine (not shown, similar to 4.23 in Scheme 4.5)

showed the formation of Pictet–Spengler product without the addition of acid. When the

reaction was left for extended periods (~48 hours) of time at 4 ºC, it proceeded to high

conversion of Pictet-Spengler products 4.83a and 4.83b. This enhanced reactivity is likely due to the more electron rich nature of the aromatic system.

CO2Me CO2Me CO2Me O 3Å MS NH AcCl, NaHCO (aq.) Me Me MeO 3 NH + NAc MeO 2 N MeO OOR H CHCl3, 2 d H DCM N N H H Me (95%) 4.67 4.43: R = TBS ROO 4.84 Me 4.44: R = TBDPS Me TBSOO 4.83a: R = TBS (48%) Me 2:1 dr (separable) 4.83b: R = TBDPS (97%) TBAF, AcOH, 2:1 dr (inseparable) DMF, 0 ºC O Me (52%)

CO2Me H Me CO2Me 4.22 NAc MeO BF3!OEt2, 4Å MS NAc MeO N N DCE, 0 ºC, 15 min H Me (23%) 4.85 O O Me HOO Me Me Me Me 4.86

Scheme 4.14: Installation of an endoperoxide on a substrate derived from 6- methoxytryptophan (4.67).

Taking a small diversion, since the actual route would involve four steps with rotomeric NMR spectra, a model system was briefly investigated. Pictet-Spengler product

345 4.83b was N-acylated with acetyl chloride to afford amide 4.84. The peroxide 4.84 could

be deprotected under acidic tetrabutylammonium flouride to afford the free peroxide

4.85. Which under our previously optimized endoperoxide conditions decomposed

rapidly. A quick optimization (amount of acid, temperature, and time) revealed that

lowering the reaction tempterature to 0 ºC allowed us to isolate our desired endoperoxide

4.86. This reactivity is similar to the enhanced reactivity we see in the Pictet–Spengler

reaction. The exact stereochemical outcome of the peroxyaminal remains undetermined,

but the NMR data compares well to endoperoxide 4.50.

With this result in hand, we proceeded to the real system, following our

previously established route (Scheme 4.10). Instead of acylating amine 4.83b with acetyl

chloride, Fmoc-proline acid chloride 4.24 was used to forge the amide bond in product

4.87 under Schotten-Baumann conditions (Scheme 4.15). The silyl protecting group is

then removed with acidic TBAF to afford free peroxide 4.88. This material underwent the

endoperoxide bond forming step in moderate yield to give endoperoxide 4.89. Our

previously developed conditions (nitrosobenzene, ZrCl4) effected the unsaturation

smoothly to give product 4.90. Although this reaction remains unoptimized, it seems to

stall at ~50% conversion. Neither adding additional ZrCl4 and nitrosobenzene nor

prolonged reaction times increased the conversion. With α,β-unsaturated ester 4.90 in hand, the Fmoc protecting group on compound 4.90 was removed by diethylamine and concomitantly cyclized to the diketopiperazine 4.91.

346 Fmoc CO2Me ClOC N CO2Me CO Me Fmoc 2 H H Fmoc NH 4.24 N TBAF, AcOH, H N MeO N MeO N N MeO H N DMF, 0 ºC N NaHCO3 (aq.), DCM H O O Me (83%) (75%) H TBDPSOO Me Me Me TBDPSOO Me HOO 4.83b 4.87 4.88 Me

O Me BF3OEt2, 4Å MS, DCE, 8 min, –30 ºC H Me (15%) O CO Me 3 2 CO Me N Fmoc 2 H Fmoc N H N N N HNEt , MeO ZrCl , PhNO, N MeO H 2 4 MeO N N N O O O THF DCM Me Me O Me O H O H O O H O Me Me Me Me Me Me Me Me Me 4.91 4.90 4.89

OsO4, py.

O O HO HO a HO N H HO Hb Hf N

N N MeO H MeO Hh N O O Hg N Hd He Me O Me H O Hc O O Me Me Me Me Me Me verruculogen (4.1) diagnostic protons observed by 1H NMR

Scheme 4.15: Total synthesis of verruculogen (4.1).

At this point, in our first generation synthesis, we had only a small amount (less

than a miligram) of our key endoperoxide 4.91. A successful test reaction on ~10µg

showed a small amount of product by LC/MS. So, all of the unsaturated product 4.91 was then subjected to stoichiometric OsO4 in pyridine. After 30 minutes, the reaction was

worked up under reducing conditions and direct comparison with an authentic

verruculogen sample by LC/MS showed product, albeit very little. No product could be

observed by TLC, but the area next to a cospot with authentic sample was removed and

filtered. Again product was observed by LC/MS, but an overnight proton NMR on the

600 MHz spectrometer was required to see the product. Even then it contained significant

impurities (possibly a diastereomer or dihydroxylated isomer). The final 1H NMR

347 spectrum showed six (or eight, depending on how liberal you are) key signals. This data,

coupled with a TLC cospot, and an LC/MS coelution, and identical mass spectrum lends

credence to the fact that verruculogen (4.1) was successfully synthesized. However, I would estimate the yield of the final step to be less than 10% (on less than 1 mg scale).

Given this “success,” there remains much work to be done. Namely, 1.) The final route to 6-methoxytryptophan remains unoptimized (route D, Scheme 4.13). Obtaining enough of this simple starting material has been an ongoing issue throughout the synthesis. 2.) The endoperoxide 4.89 formation yield is low, and certainly worth optimizing. Our initially developed conditions (boron trifluoride diethyletherate, room temperature) seem too harsh for the electron rich indole system. Because they were “good enough,” we have yet to revaluate this step. The most obvious optimization would be to explore milder acids and lower temperatures. 3.) Due to the scale of the final reaction, the

last dihydroxylation needs to be investigated in much greater detail. The low yield and

stoichiometric conditions leaves significant room for improvement. At the completion of

this thesis, this work is achievable and remains to be completed in the coming months.

1 Horak, R. M.; Vleggaar, R. J. Chem. Soc., Chem. Commun. 1987, 1568–1570.

2 Li, S.-M. Nat. Prod. Rep. 2010, 27, 57–78.

3 Li; S.-M. J. Antibiot. 2011, 64, 45–49.

4 Mundt, K.; Wollinsky, B.; Ruan, H.-L.; Zhu, T.; Li, S.-M. ChemBioChem 2012, 13,

2583–2592.

5 Hino, T.; Nakagawa, M. Heterocycles, 1997, 46, 673–704.

348

6 Bailey, P. D.; Cochrain, P. J.; Lorenz, K.; Collier, I. D.; Pearson, D. P. J.; Rosair, G. M.

Tetrahedron Lett. 2001, 42, 113–115.

7 Boyd, S. A.; Thompson, W. J. J. Org. Chem. 1987, 52, 1790–1794.

8 Verruculogen | CAS 12771-72-1 | Santa Cruz Biotech. http://www.scbt.com/datasheet-

204939-verruculogen.html (accessed July 14, 2014)

9 Verruculogen from Penicillium verruculosum ~95% | Sigma-Aldrich.

http://www.sigmaaldrich.com/catalog/product/sigma/v7755?lang=en®ion=US

(accessed July 14, 2014)

10 Cole, R. J.; Kirksey, J. W.; Moore, J. H.; Blankenship, B. R.; Diener, U. L.; Davis, N.

D. App. Microbiol. 1972, 24, 248–256.

11 Day, J. B.; Mantle, P. G.; Shaw, B. I. J. Gen. Microbiol. 1980, 117, 405–410.

12 Willingale, J.; Perera, K. P. W. C.; Mantle, P. G. Biochem. J. 1983, 214, 991–993.

13 Kato, N.; Suzuki, H.; Takagi, H.; Uramoto, M.; Takahashi, S.; Osada, H.

ChemBioChem 2011, 12, 711–714.

14 Penicillium verruculosum Peyronel, anamorph ATCC ® 24640™.

http://www.atcc.org/products/all/24640.aspx#culturemethod (accessed July 15, 2014)

15 For complete experimental details, see Section 4.7.

16 Dussault, P. H.; Portner, N. A. J. Am. Chem. Soc. 1988, 110, 6276–6277.

17 Dussault, P. H.; Lee, I. Q.; Lee, H.-J.; Lee, R. J.; Niu, Q. J.; Schultz, J. A.; Zope, U. R.

J. Org. Chem. 2000, 65, 8407–8414.

18 Isayama, S.; Mukaiyama, T. Chem. Lett. 1989, 573–576.

19 Sanchez, J.; Myers, T. N. "Peroxides and Peroxide Compounds, Organic Peroxides".

Kirk-Othmer Encyclopedia of Chemical Technology. Wiley-VCH Verlag GmbH & Co.

349

20 Schotten, C. Gesellschaft 1884, 17, 2544–2547.

21 Baumann, E. Gesellschaft 1886, 19, 3218–3222.

22 Baran, P.S.; Hafensteiner, B.D.; Ambhaikar, N. B.; Guerrero, C. A.; Gallagher, J. D.

Enantioselective Total Synthesis of Avrainvillamide and the Stephacidins J. Am. Chem.

Soc. 2006, 128, 8678–8693.

23 Baran, P. S.; Corey, E. J. J. Am. Chem. Soc. 2002, 124, 7904–7905.

24 Isayama, S. Bull. Chem. Soc. Jap. 1990, 63, 1305–1310.

25 Wang, J.; Morra, N. A.; Zhao, H.; Gorman, J. S. T.; Lynch, V.; McDonald, R.;

Reichwein, J. F.; Pagenkopf, B. L. Can. J. Chem. 2009, 87, 328–334.

26 Leggans, E. K.; Barker, T. J.; Duncan, K. K.; Boger, D. L. Org. Lett. 2012, 14, 1428–

1431.

27 Gribble, G. W. Chem. Soc. Rev. 1998, 27, 395–404.

28 North, J. T.; Kronenthal, D. R.; Pullockaran, A. J.; Real, S. D.; Chen, H. Y. J. Org.

Chem. 1995, 60, 3397–3400.

29 Chen, C.; Lieberman, D. R.; Larsen, R. D.; Verhoeven, T. R.; Reider, P. J. J. Org.

Chem. 1997, 62, 2676–2677.

30 Feldman, P. L.; Rapoport, H. Synthesis 1986, 735–737.

350 4.7 Experimental

obtained by passing commercially available pre-dried, oxygen-free formulations through

activated alumina columns. Reagents were purchased at the highest commercial quality

and used without further purification unless otherwise stated. Reactions were monitored

by thin-layer chromatography (TLC), carried out on 0.25 mm silica gel plates (60F-254)

using UV light as visualizing agent and either (1) p-anisaldehyde in ethanol/aqueous

H2SO4/CH3CO2H or (2) 2% KMnO4 in 4% aqueous sodium bicarbonate and heat as developing agents. Nuclear magnetic resonance (NMR) spectra were calibrated using

1 residual undeuterated solvent signals as an internal reference (CHCl3: 7.26 ppm for H

NMR, 77.16 ppm for 13C NMR). 1D and 2D NMR spectra were recorded at room

temperature unless otherwise stated. All NMR peak assignments given in the Supporting

19091S-433 column (30 m x 320 µm x 0.25 µm); H2 30 mL/min, air 400 mL/min, He 25

mL/min; 50 °C for 2.25 min, ramp to 300 °C (60 ºC/min), and hold at 300 ºC for 4 min.

351 O HOHO Natural verruculogen: Potatoes (900 g) were boiled in H2O (3 L) N

N MeO H for 1 hour. The resulting solution was filtered through cheesecloth N O

Me H O O Me Me and buffered with NaOAc/HCl to pH 4.8. The solution was heated Me in an autoclave and transferred to a clean hood. Agar and dextrose were added and the

substrate was plated onto 12”x12” square petri dishes. A spore of Penicillium

verruculosum was suspended in H2O for 3 hours then streaked onto the prepared potato

dextrose agar plates. The plates were transferred to a dark place at room temperature for

1–3 months. After the fungus had grown, the agar was chopped and transferred to a 4 L

Erlenmeyer flask. The solids were extracted with sonication using 10-20% MeOH:DCM

(3x). The combined extracts were washed with NaCl (sat. aq.) and the aqueous phase was

back extracted with DCM. The combined organic phases were dried with Na2SO4 and concentrated in vauco. The resulting yellow solid was dry loaded onto silica gel and purified by flash column chromatography (silica gel) (hexanes to 70% EtOAc:hexanes)

and verruculogen (4.1) was isolated as a white solid foam: Rf = 0.48 (70%

1 EtOAc:hexanes); H NMR (600 MHz, CDCl3) δ 7.90 (d, J = 8.7 Hz, 1 H), 7.36 (s, 1 H),

6.83 (d, J = 8.9 Hz, 1 H), 6.64 (d, J = 8.2 Hz, 1 H), 6.60 (s, 1 H), 6.05 (d, J = 10.0 Hz, 1

H), 5.66 (s, 1 H), 5.04 (d, J = 8.1 Hz, 1 H), 3.84 (s, 4 H), 3.65 (d, J = 6.5 Hz, 2 H), 2.53–

2.48 (m, 1 H), 2.13–2.02 (m, 2 H), 2.00 (s, 3 H), 1.74 (s, 3 H), 1.72 (s, 3 H), 1.68 (t, J =

13 11.9 Hz, 1 H), 1.01 (s, 3 H); C NMR (151 MHz, CDCl3) δ 170.9, 166.3, 156.5, 143.3,

136.4, 131.7, 128.5, 121.8, 121.1, 118.6, 109.5, 94.1, 86.0, 82.7, 82.3, 68.8, 58.9, 55.9,

+ 51.3, 49.0, 45.5, 29.2, 27.2, 25.8, 24.3, 22.8, 19.0; LC/MS calcd for C27H32N3O6 [M-OH]

494.2291, found (ESI-TOF) 512.3481 [M]+, 494.3384 [M-OH]+.

352 EtO Me Me Acetals 4.37 and 4:40: O2 was bubbled through a solution of olefin (1 EtO OOR equiv), Co(modp)2 (10 mol%), and silane (3 equiv) in DCE for 15 minutes. Then the

reaction mixture was stirred under static O2 pressure for 24 hours. The reaction was then concentrated and purified by silica gel column chromatography (hexanes).

1 4.37: H NMR (400 MHz, C6D6) δ 4.84 (t, J = 5.2 Hz, 1 H), 3.59 (dq, J = 9.4, 7.0 Hz, 2

H), 3.43 (dq, J = 9.4, 7.0 Hz, 2 H), 2.17 (d, J = 5.1 Hz, 2 H), 1.34 (s, 6 H), 1.13 (t, J = 7.0

Hz, 6 H), 1.06 (t, J = 7.9 Hz, 9 H), 0.74 (q, J = 8.0 Hz, 6 H).

1 4:40: H NMR (400 MHz, C6D6) δ 4.83 (t, J = 5.2 Hz, 1 H), 3.58 (dq, J = 9.3, 7.0 Hz, 2

H), 3.42 (dq, J = 9.4, 7.1 Hz, 2 H), 2.14 (d, J = 5.2 Hz, 2 H), 1.32 (s, 6 H), 1.13 (t, J = 7.0

Hz, 6 H), 1.02 (s, 9 H), 0.20 (s, 6 H).

EtO Me Me TBDPS acetal 4.41: TBAF (6.1 mL, 1 equiv, 1 M in THF) was added EtO OOTBDPS to TES-peroxide 4.37 (1.88 g, 6.13 mmol) in THF (61 mL, 0.1 M) at 0 ºC. The reaction

was complete in less than 5 minutes and was diluted with Et2O and washed with NH4Cl

(sat. aq.). The aqueous phase was extracted with Et2O (3x). The organic phases were

combined, washed with H2O and NaCl (sat. aq.), dried over MgSO4, and concentrated in vacuo. The free peroxide was then brought up in DCM. To this mixture was added imidazole (0.626 g, 1.5 equiv) and TBDPSi-Cl (2.07 mL, 1.3 equiv). The reaction as

stirred for 4 hours and then the solids were removed by filtration. The crude was

concentrated in vacuo and purified by column chromatography (hexanes–2%

EtOAc:hexanes) to afford TBDPS-protected peroxide 4.41 as a clear oil (2.23g, 84%): 1H

NMR (500 MHz, C6D6) δ 7.89 (d, J = 3.4 Hz, 4 H), 7.21 (dd, J = 4.0, 2.4 Hz, 6 H), 4.72

353 (t, J = 5.2 Hz, 1 H), 3.50 (dq, J = 8.6, 7.1 Hz, 2 H), 3.33 (tt, J = 9.0, 6.6 Hz, 2 H), 2.12 (d,

J = 5.1 Hz, 2 H), 1.27 (s, 6 H), 1.25 (s, 9 H), 1.08 (t, J = 7.0 Hz, 6 H).

O Me Me Aldehydes 4.42–4.44: TFA (1.5 equiv) is added to a stirred solution of H OOR

diethyl acetal (1 equiv) and H2O (40 equiv) in CHCl3 (0.1 M). After an hour, the reaction

is quenched with NaHCO3 (sat. aq.) and the organic phase is separated, dried over

MgSO4, and concentrated in vacuo. The aldehydes were pure without further purification.

1 4.43: H NMR (500 MHz, CDCl3) δ 9.84 (s, 1 H), 2.61 (d, J = 2.9 Hz, 2 H), 1.31 (s, 6 H),

0.93 (d, J = 0.8 Hz, 9 H), 0.14 (s, 6H).

1 4.44: H NMR (400 MHz, CDCl3) δ 9.73 (t, J = 2.9 Hz, 1 H), 7.78 – 7.65 (m, 4 H), 7.46

– 7.32 (m, 6 H), 2.62 (d, J = 3.0 Hz, 2 H), 1.26 (s, 6 H), 1.12 (s, 9H).

CO2Me Pictet–Spengler products 4.45 and 4.46: Aldehyde 4.43 or 4.44 (1.3 NH N H equiv), L-tryptophan methyl ester (4.3), and 3 Å molecular sieves were Me ROO Me stirred in CHCl3 (0.1 M) under argon for an hour. Then TFA was added quickly and the reaction was monitored by LC/MS. The reaction was stirred for 20-30 minutes and slowly quenched with NaHCO3. The biphasic mixture could be filtered through a coarse sintered glass frit and then the aqueous phase could be extracted with

DCM (3x). The combined organic phases were then dried over MgSO4 and concentrated

in vacuo. The products could be purified by column chromatography, and the diagnostic

data is reported below, see Section 4.8 for spectra:

1 4.45-major: (24%, ~2:1 dr); H NMR (500 MHz, CDCl3) δ 9.31 (s, 1 H), 7.46 (d, J = 7.7

Hz, 1H), 7.28 (d, 1 H), 7.13 (t, J = 7.3 Hz, 1 H), 7.07 (t, J = 7.4 Hz, 1 H), 4.30 (dd, J =

354 5.7, 2.8 Hz, 1 H), 3.87 – 3.80 (m, 4 H), 3.15 – 3.07 (m, 1 H), 2.90 – 2.77 (m, 1 H), 2.58

(dd, J = 15.5, 5.9 Hz, 1 H), 1.72 (dd, J = 15.5, 2.8 Hz, 1 H), 1.37 (s, 3 H), 1.36 (s, 3 H),

1.35 – 1.22 (m, 2 H), 1.02 (s, 9 H), 0.30 (s, 3 H), 0.28 (s, 3 H).

1 4.46-major: (67%, ~2.5:1 dr); H NMR (400 MHz, CDCl3) δ 8.64 (s, 1 H), 6.56 (d, J =

7.2 Hz, 1 H), 4.07 (dd, J = 5.6, 2.8 Hz, 1 H), 3.79 (s, 3 H), 2.76 (ddd, J = 15.0, 11.3, 2.5

Hz, 1 H), 2.37 (dd, J = 15.5, 5.9 Hz, 1 H), 1.72 (dd, J = 15.4, 2.8 Hz, 1H), 1.29 (s, 6 H),

1.18 (s, 9 H).

CO2Me N-Acyl tricycle 4.48: Acetyl chloride (130 µL, 5 equiv) was added to a NAc N H vigorously stirred solution of amine 4.46 in DCM (3.6 mL, 0.1 M) and Me TBDPSOO Me sat. aq. NaHCO3 (3.6 mL, 0.1 M). The reaction was stirred for 14 hours

and the phases were separated. The aqueous phase was extracted with DCM (3x), dried

over MgSO4, and concentrated in vacuo. The crude was purified by column

chromatography (15–30% EtOAc:hexanes) to afford amide 4.48 as a white solid foam

+ (186 mg, 87%): LC/MS calcd for C35H42N2O5Si [M + 1] 599.2863, found 599.2939 [M +

1]+. Procedure is the same for TBS protected amine.

CO2Me Free Peroxide 4.49: TBAF (0.22 mL, 1 M in THF) was added to a NAc N H solution of protected peroxide 4.48 (130 mg, 0.22 mmol) and AcOH Me HOO Me (12.6 µL, 1 equiv) in DMF (4.4 mL, 0.05 M) at 0 ºC. The reaction was

done within 5 minutes and was partitioned between H2O and EtOAc. The aqueous phase was extracted with EtOAc (3x). The organic phases were combined, dried over MgSO4,

washed sequentially with H2O and NaCl (sat. aq.), and concentrated in vacuo. The crude

355 was purified by column chromatography (30–50% EtOAc:hexanes) to afford free

1 peroxide 4.49 as a white solid (67.2 mg, 86%): Rf = 0.33 (50% EtOAc in hexanes); H

NMR (600 MHz, CDCl3) δ 9.14 (s, 1 H), 8.97 (br s, 1 H), 7.47 (d, J = 7.8 Hz, 1 H), 7.32

(d, J = 8.2 Hz, 1 H), 7.15 (ddd, J = 8.1, 7.0, 1.3 Hz, 1 H), 7.09 (ddd, J = 7.9, 6.8, 1.1 Hz, 1

H), 5.65 (d, J = 8.6 Hz, 1 H), 4.82 (d, J = 4.8 Hz, 1 H), 3.63 (s, 3 H), 3.41 (d, J = 15.5 Hz,

1 H), 2.75 (dd, J = 15.5, 6.2 Hz, 1 H), 2.25 (s, 3 H), 2.20 – 2.06 (m, 2 H), 1.36 (s, 3 H);

13 C NMR (151 MHz, CDCl3) δ 170.9, 136.3, 133.1, 126.5, 122.0, 119.4, 118.3, 111.2,

104.7, 83.2, 55.5, 52.7, 47.2, 44.9, 26.8, 23.2, 23.1, 23.0; HRMS calcd for C19H24N2O5 [M

+ 1]+ 361.1685, found 361.1906 [M + 1]+.

CO2Me Endoperoxide 4.50: Free peroxide 4.49 (33.9 mg, 0.094 mmol) was NAc N added to a stirred solution of 3-methyl-2-butenal (45 µL, 5 equiv) and Me O O Me Me Me 4 Å molecular sieves. To this was added BF3OEt2 (94 µL, 1 equiv) slowly, and the reaction was stirred at room temperature for 28 minutes. The acid was

then quenched with a few drops of 1:1 Et3N:MeOH and concentrated in vacuo. The crude

was purified by column chromatography (2–5% DCM:Et2O) to afford endoperoxide 4.50

1 rotomers as an off white solid (16.9 mg, 42%): H NMR (600 MHz, CDCl3) δ 7.59 – 7.51

(m), 7.22 – 7.08 (m), 6.74 – 6.68 (m), 6.47 (d, J = 10.2 Hz), 5.86 – 5.80 (m), 4.92 (d, J =

7.3 Hz), 4.85 – 4.79 (m), 4.69 (dt, J = 7.9, 1.4 Hz), 3.69 (s), 3.68 (s), 3.59 (dd, J = 15.6,

1.4 Hz), 3.43 (dd, J = 15.8, 3.3 Hz), 3.21 – 3.06 (m), 2.40 (s), 2.35 (s,), 2.21 (dd, J = 14.9,

1.5 Hz), 2.08 – 2.03 (m), 1.98 (d, J = 1.4 Hz), 1.95 (d, J = 1.3 Hz,), 1.80 (dd, J = 13.6,

10.1 Hz), 1.71 (s), 1.70 (d, J = 1.4 Hz), 1.66 (s), 1.62 (d, J = 1.4 Hz), 1.09 (s), 1.05 (s);

356 CO2Me Amide 4.51: Free amine 4.46 (50 mg, 0.09 mmol) was added to a O N N H NFmoc H vigorously stirred emulsion of acid chloride 4.24 (0.12 mmol) in TBDPSOO Me Me DCM (0.45 mL, 0.2 M) and sat. aq. NaHCO3 (0.45 mL, 0.2 M).

When the reaction was judged complete by LC/MS, the organic phase was separated and the aqueous phase was extracted with DCM (2x). The organic phases were collected, dried over MgSO4, and purified by column chromatography (30% EtOAc:hexanes).

Amide 4.51 was judged to be pure by LC/MS (69 mg, 87%): LC/MS calcd for

+ + + C53H57N3O7Si [M + H] 876.3966, found 876.4022 [M + H] , 898.3849 [M + Na] .

O Pentacycle 4.52: Et2NH (0.3 mL, 15 equiv) was added to protected H N

N H diastereomeric amines 4.51 (179 mg, 0.2 mmol) in THF (2 mL, 0.1 N O H Me TBDPSOO Me M) and the reaction was heated to 40 ºC for 5 hours. The crude was

concentrated on silica gel and purified by column chromatography (40% EtOAc:hexanes)

to afford pentacycle 4.52 as a 2:1 mixture of diastereomers: Rf = 0.23 (50%

EtOAc:hexanes), diagnostic 1H data reported below, see Section 4.8 for all spectra:

1 4.52-major: H NMR (600 MHz, CDCl3) δ 6.57 (d, J = 8.6 Hz, 1 H), 5.58 (dd, J = 10.2,

3.3 Hz, 1 H), 3.47 (dd, J = 15.7, 5.1 Hz, 1H), 3.00 (dd, J = 15.7, 11.9 Hz, 1 H), 1.59 (s, 3

H), 1.15 (s, 9 H), 0.78 (s, 3 H).

1 4.52-minor: H NMR (600 MHz, CDCl3) δ 8.21 (s, 1 H), 6.48 – 6.41 (m, 1 H), 6.01 –

5.96 (m, 1 H), 4.34 (dd, J = 11.4, 4.4 Hz, 1 H), 3.53 (dd, J = 15.6, 4.4 Hz, 1 H), 2.75 (dd,

J = 15.6, 11.3 Hz, 1H), 1.53 (s, 3 H), 1.18 (s, 9 H), 1.17 (s, 3 H).

357 CO2Me Fmoc Unsaturated Ester 4.54: ZrCl4 (99 mg, 1 equiv) and nitrosobenzene H N N N H O (114 mg, 2.5 equiv) were added at once to a solution of ester 4.51 Me TBDPSOO Me (374 mg, 0.43 mmol) in DCM (3 mL, 0.14 M). After 2 hours, the

reaction was quenched with a few drops of 1:1 Et3N:MeOH and concentrated and

purified by column chromatography (50% EtOAc:hexanes to 10% DCM:Et2O) to afford

α,β-unsaturated ester 4.54 (244 mg, 65%) and recovered starting material 4.54 (111 mg,

26%). The product was an off white solid foam: Rf = 0.59 (50% EtOAc:hexanes); LC/MS

+ + calcd for C53H55N3O7Si [M + H] 874.3809, found 874.3858 [M + H] , 896.3675 [M +

Na]+.

O Pentacycle 4.53: Et2NH (0.29 mL, 10 equiv) was added to a stirred N

N H solution of protected amine 4.54 (244 mg, 0.28 mmol) in THF (2.8 N O H Me TBDPSOO Me mL, 0.1 M) and was stirred at 40 ºC. The reaction was concentrated

and purified by flash column chromatography (50-66% EtOAc:hexanes) to afford

hexacycle 4.53 as an off-white solid (154 mg, 89%): Rf = 0.11 (50% EtOAc in hexanes);

1 H NMR (600 MHz, CDCl3) δ 7.95 – 7.91 (m, 2 H), 7.81 – 7.78 (m, 2 H), 7.61 (s, 1 H),

7.53 – 7.49 (m, 4 H), 7.47 – 7.42 (m, 1 H), 7.37 (t, J = 7.5 Hz, 2 H), 7.31 (s, 1 H), 7.08

(td, J = 7.4, 1.1 Hz, 1 H), 7.03 (ddd, J = 8.3, 7.1, 1.3 Hz, 1 H), 6.54 (d, J = 8.2 Hz, 1 H),

6.13 (dd, J = 9.0, 4.0 Hz, 1 H), 4.06 (dd, J = 10.2, 6.5 Hz, 1 H), 3.77 – 3.56 (m, 2 H),

2.50 – 2.38 (m, 1 H), 2.16 – 2.06 (m, 1 H), 2.02 – 1.91 (m, 3 H), 1.56 (s, 3 H), 1.17 (s, 9

13 H), 0.80 (s, 3 H); C NMR (151 MHz, CDCl3) δ 166.0, 160.2, 136.2, 136.0, 135.8, 135.3,

132.7, 132.1, 130.8, 130.4, 128.5, 128.1, 123.8, 122.4, 120.9, 118.2, 112.1, 111.8, 106.5,

358 83.1, 58.9, 47.2, 45.3, 45.1, 28.9, 27.4, 25.8, 25.4, 22.3, 19.7; LC/MS calcd for

+ + C37H41N3O4Si [M + H] 620.2866, found 620.2947 [M + H] .

O HOHO Diol 4.57: OsO4 (1 equiv) in pyridine was added to pentacyclic N

N H N O compound 4.53 (81.6 mg, 132 µmol) in pyridine (4.4 mL, 0.03 M) at 0 H Me TBDPSOO Me ºC. The reaction was stirred for 3 hours at 0 ºC and then quenched

with NaHSO3. After warming to room temperature, the reaction mixture was diluted with

H2O and DCM and extracted with DCM (3x). The organic phases were combined,

washed with H2O and NaCl (sat. aq.), and dried over MgSO4. The crude was purified by column chromatography (Et2O to 1% MeOH:Et2O to 5% MeOH:DCM) affording diol

4.57 (17.4 mg, 20%): 1H NMR (400 MHz, Chloroform-d) δ 8.57 (s, 1 H), 7.86 (d, J = 7.1

Hz, 4 H), 7.67 (d, J = 7.5 Hz, 1 H), 7.52 – 7.35 (m, 6 H), 7.04 – 6.95 (m, 3 H), 6.39 (d, J

= 7.2 Hz, 1 H), 5.93 (d, J = 5.8 Hz, 1 H), 5.15 (d, J = 8.6 Hz, 1 H), 4.89 (s, 1 H), 4.19

(dd, J = 9.8, 6.6 Hz, 1 H), 3.87 – 3.74 (m, 1 H), 3.64 (td, J = 10.2, 9.5, 4.5 Hz, 1 H), 3.35

(s, 1 H), 2.68 (d, J = 9.7 Hz, 1 H), 2.57 – 2.45 (m, 1 H), 2.43 (d, J = 5.8 Hz, 1 H), 2.24

(dd, J = 16.3, 3.0 Hz, 1 H), 2.14 – 1.90 (m, 4 H), 1.49 (s, 3 H), 1.19 (s, 9 H), 1.14 (s, 3

+ + H); LC/MS calcd for C37H43N3O6Si [M + H] 654.2921, found 654.2996 [M + H] ,

636.2893 [M – OH]+.

CO2Me Fmoc Endoperoxide 4.58: TBAF (80 µL, 1 M in THF) was added to a H N N N O solution of protected peroxide 4.51 (69 mg, 0.079 mmol), AcOH Me H O O Me Me Me (4.5 µL, 1 equiv), and DMF (1.6 mL, 0.05 M) at 0 ºC. The reaction

was done in less than 5 minutes and was diluted with H2O and extracted with EtOAc

359 (3x). The organic phases were dried over MgSO4 and concentrated in vacuo. The free

peroxide was purified by a short silica plug (30–50% EtOAc:hexanes) and was carried

forward immediately (37 mg, 74%).

The free peroxide (20 mg, 0.031 mmol) was brought up in DCE (320 µL). To the solution

was added 4 Å molecular sieves and 3-methyl-2-butenal under argon. BF3OEt2 (4 µL) was added last, and the reaction was stirred at room temperature for 13 minutes. The reaction was quenched with a few drops of 1:1 Et3N:MeOH and was concentrated in

vacuo. The crude was purified by column chromatography (20–40% EtOAc:hexanes) and

+ was characterized by LC/MS (6.8 mg, 31%): LC/MS calcd for C42H45N3O7 [M + H]

704.3258, found (ESI-TOF) 704.3597 [M + H]+, 726.3429 [M + Na]+.

O H Endoperoxide 4.60: Et2NH (10 µL, 10 equiv) was added to a stirred N

N H N O solution of protected amine 4.58 (6.8 mg, 0.0097 mmol) in THF (97

Me H O Me O Me Me µL, 0.1 M) and was stirred at room temperature. The reaction was

concentrated and purified by flash column chromatography (50 EtOAc:hexanes) to afford

1 hexacycle 4.60 as an off-white solid (41%): Rf = 0.19 (50% EtOAc in hexanes); H NMR

(600 MHz, CDCl3) δ 7.62 (d, J = 7.8 Hz, 1 H), 7.21 (dt, J = 7.2, 1.2 Hz, 1 H), 7.17 (dt, J

= 7.5, 1.2 Hz, 1 H), 7.12 (d, J = 7.8, 1 H), 6.72 (d, J = 8.1 Hz, 1 H), 6.20 (d, J = 10.4 Hz,

1 H), 5.09 (d, J = 8.0 Hz, 1 H), 4.22 (dd, J = 11.7, 4.8 Hz, 1 H), 4.17 (t, J = 8.3 Hz, 1 H),

3.67 – 3.62 (m, 3H), 3.02 (dd, J = 15.9, 12.1 Hz, 1 H), 2.44 (dtd, J = 13.5, 6.9, 3.2 Hz, 1

H), 2.26 (dddd, J = 12.9, 11.1, 9.3, 7.2 Hz, 1 H),2.13 – 2.06 (m, 2 H), 2.02 (s, 3 H), 2.02

13 – 1.95 (m, 1 H), 1.78 – 1.74 (m, 7 H), 1.01 (s, 3 H); C NMR (151 MHz, CDCl3) δ

169.5, 165.8, 143.2, 134.9, 134.5, 126.3, 122.1, 120.0, 118.9, 118.6, 109.6, 106.7, 86.0,

360 82.4, 77.4, 77.2, 76.9, 59.4, 56.8, 51.3, 50.1, 45.6, 29.8, 28.8, 27.3, 25.8, 24.5, 23.2, 22.1,

+ 19.0; HRMS calcd for C26H31N3O4 [M + H] 450.2315, found (ESI-TOF) 450.2546 [M +

+ H] ; [α]D = –22.5 (c 0.12, CHCl3).

CO2Me Fmoc a,b-unsaturated ester 4.59: ZrCl4 (33 mg, 1 equiv) and H N N N O nitrosobenzene (31 mg, 2.5 equiv) were added at once to a solution Me H O O Me Me Me of ester 4.58 (82 mg, 0.117 mmol) in DCM (0.83 mL, 0.14 M).

After 2 hours, the reaction was concentrated and purified by column chromatography

(20–40% EtOAc:hexanes) to afford α,β-unsaturated ester 4.59 (29.9 mg, 36 %): Rf = 0.54

+ (40% EtOAc:hexanes); LC/MS calcd for C42H43N3O7 [M + H] 702.3101, found (ESI-

TOF) 702.3280 [M + H]+, 724.3087 [M + Na]+.

O Hexacyclic compound 4.62: Et NH (45 µL, 10 equiv) was added to N 2

N H N O a stirred solution of protected amine 4.59 (29.9 mg, 0.043 mmol) in

Me H O Me O Me Me THF (430 µL, 0.1 M) and was subsequently warmed to 40 º C for 4 hours. The reaction was concentrated and purified by flash column chromatography (50–

80% EtOAc:hexanes) to afford hexacycle 4.62 (6.4 mg, 34%): Rf = 0.17 (50%

1 EtOAc:hexanes); H NMR (400 MHz, CDCl3) δ 7.64 – 7.54 (m, 1 H), 7.16 (s, 1 H), 7.16

– 7.11 (m, 2 H), 7.04 – 6.98 (m, 1 H), 6.72 (d, J = 10.8 Hz, 1 H), 6.62 (d, J = 8.0 Hz, 1

H), 5.05 (d, J = 7.8 Hz, 1 H), 4.13 – 3.98 (m, 2 H), 3.69 – 3.56 (m, 2 H), 2.35 (dq, J =

11.2, 6.5, 5.3 Hz, 1 H), 2.17 – 2.07 (m, 1H), 2.03 (t, J = 9.0 Hz, 1 H), 1.93 (d, J = 1.4 Hz,

3 H), 1.68 (d, J = 1.5 Hz, 3 H), 1.60 (s, 3 H), 0.93 (s, 3 H).

361 O HOHO Demethoxyverruculogen (4.18): OsO4 (75 µL, 0.06 M in pyridine) N

N H N O was added to hexacyclic compound 4.62 (2 mg, 4.47 µmol) dissolved

Me H O O Me Me Me in pyridine (375 µL). The reaction was stirred at 0 ºC for an hour and then quenched with NaHSO3 to hydrolyze the osmate esters. The biphase was diluted

with H2O and DCM and the layers were separated. The aqueous phase was extracted with

DCM (2x) and the organic phases were combined. The organic phases were washed with

H2O, NaCl (sat. aq.), dried over MgSO4, and concentrated in vacuo. The crude was

purified by preparatory TLC (70% EtOAc:hexanes) to afford diol 4.18: 1H NMR (600

MHz, CDCl3) δ 7.86 (d, J = 7.7 Hz, 1 H), 7.19 – 7.16 (m, 1 H), 7.14 (dd, J = 7.7, 6.6 Hz,

1 H), 7.10 – 7.08 (m, 1 H), 6.76 (d, J = 7.9 Hz, 1 H), 6.60 (d, J = 10.0 Hz, 1 H), 5.32 (dd,

J = 7.1, 1.2 Hz, 1 H), 5.12 (s, 1 H), 4.90 (s, 1 H), 4.70 (d, J = 8.1 Hz, 1 H), 4.22 (dd, J =

10.6, 6.3 Hz, 1 H), 3.88 (t, J = 10.4 Hz, 1 H), 3.68 – 3.57 (m, 3 H), 3.43 (d, J = 7.1 Hz,

1H), 3.35 (s, 1 H), 2.63 (dd, J = 15.0, 10.2 Hz, 1 H), 2.53 (dt, J = 12.1, 6.0 Hz, 1 H), 2.36

(d, J = 7.5 Hz, 1 H), 1.68 (s, 3 H), 1.65 (s, 3 H), 1.38 (s, 3 H), 1.13 (s, 3 H); LC/MS calcd

+ + for C26H31N3O6 [M + H] 482.2213, found (ESI-TOF) 482.2290 [M + H] , 464.2182 [M

– OH]+.

O H Peroxyacetal 4.64: HBF4 (3 drops, 50% in Et2O) was added to N

N MeO H free peroxide 4.63 in DCM. The reaction does not proceed in N O H Me O Me solution, so it was immediately concentrated on preparatory TLC EtO O

Me 1 and product was purified: H NMR (600 MHz, CDCl3) δ 9.64 (s, 1 Me H), 7.33 (d, J = 8.5 Hz, 1 H), 6.90 (d, J = 2.2 Hz, 1 H), 6.75 (dd, J = 8.5, 2.3 Hz, 1 H),

5.97 (d, J = 7.5 Hz, 1 H), 5.58 (d, J = 6.5 Hz, 1 H), 5.30 (t, J = 3.7 Hz, 1 H), 4.39 (dd, J =

362 12.0, 4.4 Hz, 1 H), 4.04 (dd, J = 9.5, 7.0 Hz, 1 H), 3.97 (dt, J = 12.6, 8.2 Hz, 1 H), 3.83

(s, 3 H), 3.75 (dd, J = 9.5, 7.0 Hz, 1 H), 3.57 (dd, J = 15.6, 4.4 Hz, 1 H), 3.45 (ddd, J =

13.3, 10.3, 3.6 Hz, 1 H), 2.78 (dd, J = 15.7, 11.4 Hz, 1 H), 2.49 (dt, J = 13.0, 6.6 Hz, 1

H), 2.41 (dd, J = 14.7, 7.7 Hz, 1H), 1.76 (s, 3 H), 1.73 (s, 3 H), 1.56 (s, 3 H), 1.33 (t, J =

13 7.1 Hz, 3 H); C NMR (151 MHz, CDCl3) δ 172.0, 165.5, 164.8, 157.1, 140.8, 138.1,

121.4, 120.9, 119.4, 109.7, 106.1, 103.6, 96.1, 82.3, 64.7, 61.3, 60.4, 56.6, 55.7, 47.6,

47.2, 45.8, 31.1, 29.8, 27.6, 26.5, 24.3, 22.4, 21.9, 19.8, 16.3, 15.0.

Four diastereomers of this compound have been isolated and characterized, but absolute

configuration has not been established. Only one is shown here.

CO2Me HO Diol 4.66: OsO4 (48µL, 0.4 M in pyridine) was added to conjugated NAc N H OH ester 4.65 (9.3 mg, 19 µmol) in pyridine (580 µL) at 0 ºC and the Me TBSOO Me 4.66 reaction was allowed to warm to room temperature over one hour. The

reaction was quenched with sat. aq. NaHSO3 after 4 hours and was allowed to stir for 30

minutes. The reaction was diluted with DCM and H2O and the organic phase was

separated. The aqueous phase was extracted with dichloromethane (2x) and the organic

phases were combined and dried over MgSO4. The crude was purified by preparatory

TLC (50% EtOAc:hexanes) to afford diol 4.66 (4.7 mg, 47%), the structure of which was

1 confirmed by 2D NMR: H NMR (600 MHz, CDCl3) δ 7.27 – 7.26 (m, 1 H), 7.18 (t, J =

7.4 Hz, 1 H), 6.81 (t, J = 7.5 Hz, 1 H), 6.63 (s, 1 H), 6.54 (d, J = 8.0 Hz, 1 H), 4.90 (s, 1

H), 4.61 (d, J = 8.0 Hz, 1 H), 4.08 (s, 1 H), 3.85 (d, J = 8.0 Hz, 1 H), 3.77 (s, 4 H), 2.94

(s, 1 H), 2.33 (s, 3 H), 1.78 (d, J = 15.4 Hz, 1 H), 1.18 (s, 3 H), 1.06 (s, 3 H), 1.02 (s, 9

13 H), 0.24 (s, 3 H), 0.21 (s, 3 H); C NMR (151 MHz, CDCl3) δ = 171.2, 163.7, 148.0,

363 132.3, 130.7, 127.6, 123.4, 122.9, 119.2, 108.8, 91.3, 80.9, 73.1, 60.1, 51.6, 35.3, 28.8,

+ 25.9, 25.5, 25.3, 22.9, 20.8, 17.5, -6.4; LC/MS calcd for C25H38N2O7Si [M + H]

507.2448, found (ESI-TOF) 507.1528 [M + H]+, 529.1333 [M + Na]+.

CO2Me Aryl boronic ester 4.80: To a flamed dried tube was added NHBoc

[Ir(cod)(OMe)]2 (100 mg, 5 mol%), 1,10-phenanthroline (54 mg, 10 Bpin N TIPS mol%), B2pin2 (3 g, 4 equiv), and freshly distilled hexanes (10 mL) under an argon

atmosphere. HBpin (76 µL, 0.25 equiv) was added at once, and the reaction mixture was

stirred for 3 minutes until it changed from green to black. Protected tryptophan 4.79 (1.5 g, 3.16 mmol) was added as a solution in hexanes and the reaction vessel was sealed and heated to 75 ºC got 36 hours. The reaction mixture was cooled and concentrated in vacuo.

The crude mixture was purified by column chromatography (5–9% EtOAc:hexanes) to

1 afford borylated product 4.80 (~1.3 g, 70%): Rf = 0.55 (20% EtOAc:hexanes); H NMR

(400 MHz, CDCl3) δ 7.92 (s, 1 H), 7.54 (d, J = 7.8 Hz, 1 H), 7.51 (d, J = 8.0 Hz, 1 H),

7.08 (s, 1 H), 5.04 (d, J = 8.1 Hz, 1 H), 4.63 (q, J = 6.2 Hz, 1 H), 3.62 (s, 3 H), 3.27 (d, J

= 5.5 Hz, 2 H), 1.69 (p, J = 7.6 Hz, 3 H), 1.43 (s, 9 H), 1.35 (s, 12 H), 1.14 (d, J = 7.0 Hz,

18 H).

CO2Me Amine 4.83a: A solution of 6-methoxtryptophan (4.67) (60 mg, NH MeO N H 0.24 mmol) in CHCl3 (2 mL) was added to a solution of aldehyde Me TBSOO Me 4.43 (78 mg, 1.4 equiv) and 3 Å molecular sieves in CHCl3 (4 mL)

364 at 0 ºC. The reaction was stirred for ~48 hours and monitored by LC/MS. When starting

material was consumed, the sieves were filtered and the material was concentrated onto

silica gel and purified by column chromatography (10% EtOAc:hexanes):

1 4.83a-major (34. 1 mg, 31%): Rf = 0.28 (30% EtOAc:hexanes); H NMR (400 MHz,

CDCl3) δ 8.56 (br s, 1 H), 7.33 (d, J = 8.4 Hz, 1 H), 6.81 – 6.72 (m, 2 H), 4.44 (br s, 1 H),

3.89 (t, J = 6.5 Hz, 1 H), 3.82 (s, 3 H), 3.75 (s, 3 H), 3.07 (dd, J = 15.3, 4.8 Hz, 1 H), 2.90

(dd, J = 15.8, 8.2 Hz, 1 H), 2.29 (d, J = 15.5 Hz, 1 H), 1.83 (dd, J = 15.4, 5.3 Hz, 1 H),

1.32 (s, 3 H), 1.26 (s, 3 H), 1.01 (s, 9 H), 0.26 (s, 3 H), 0.24 (s, 3 H).

1 4.83a-minor (18.3 mg, 17%): Rf = 0.38 (30% EtOAc:hexanes); H NMR (400 MHz,

CDCl3) δ 9.21 (s, 1 H), 7.33 (d, J = 8.5 Hz, 1 H), 6.79 (s, 1 H), 6.75 (d, J = 9.3 Hz, 1 H),

4.27 (br s, 1 H), 3.82 (s, 3 H), 3.81 (s, 3 H), 3.11 – 3.02 (m, 1 H), 2.78 (d, J = 13.2 Hz, 1

H), 2.54 (dd, J = 15.5, 6.2 Hz, 1 H), 1.71 (d, J = 14.0 Hz, 1 H), 1.36 (s, 3 H), 1.35 (s, 3

H), 1.02 (s, 9 H), 0.30 (s, 3 H), 0.27 (s, 3 H).

CO2Me Amine 4.83b: A solution of 6-methoxtryptophan (4.67) (15 mg, NH MeO N H 0.06 mmol) in CHCl3 (0.5 mL) was added to a solution of aldehyde Me TBDPSOO Me 2:1 mixture 4.44 (30 mg, 1.4 equiv) and 3 Å molecular sieves in CHCl3 (1 mL) at 0 ºC. The reaction was stirred for ~48 hours and monitored by LC/MS. When starting material was consumed, the sieves were filtered and the material was concentrated onto silica gel and purified by column chromatography (20% EtOAc:hexanes) to afford inseparable diastereomers 4.83b (34 mg, 97%), diagnostic data reported below, see

Section 4.8 for spectra:

365 1 4.83b-major: H NMR (400 MHz, CDCl3) δ 8.47 (s, 1 H), 7.81 (d, J = 6.6 Hz, 2 H), 6.71

– 6.63 (m, 1 H), 6.01 (d, J = 2.2 Hz, 1 H), 3.98 (dd, J = 5.8, 2.8 Hz, 1 H), 3.78 (s, 3 H),

3.72 (s, 3 H), 3.03 – 2.95 (m, 1 H), 2.71 (ddd, J = 14.5, 11.2, 2.6 Hz, 1 H), 2.32 (dd, J =

15.3, 6.1 Hz, 1H).

1 4.83b-minor: H NMR (400 MHz, CDCl3) δ 7.97 (s, 1 H), 7.73 (d, J = 6.8 Hz, 2 H), 6.71

– 6.63 (m, 1 H), 6.12 (d, J = 2.2 Hz, 1 H), 4.28 (t, J = 4.6 Hz, 1 H), 3.75 (s, 3 H), 3.73 (s,

3 H), 3.03 – 2.95 (m, 1 H), 2.80 (dd, J = 14.9, 8.6 Hz, 1 H), 2.21 (dd, J = 15.0, 4.5 Hz, 1

H), 1.82 (dd, J = 15.1, 4.4 Hz, 1 H).

CO2Me Amide 4.84: Acetyl chloride (30 µL, 5 equiv) was added to a NAc MeO N H vigorously stirred solution of free amine 4.83a (35.3 mg, 0.076 Me TBSOO Me

mmol) in sat. aq. NaHCO3 (1 mL) and DCM (1 mL). The reaction

was stirred vigorously until judged complete by TLC. The organic phase was removed

and the aqueous phase was extracted with DCM (2x). The organic phases were

combined, dried over MgSO4, and concentrated in vacuo. The crude white foam was of sufficient purity (> 80%) to carry forward to the next step (36.6 mg, 95% yield, 83% pure

1 by NMR): Rf = 0.20 (30% EtOAc:hexanes); H NMR (400 MHz, CDCl3) δ 9.43 (s, 1 H),

7.37 (d, J = 8.6 Hz, 1 H), 6.82 (d, J = 2.3 Hz, 1 H), 6.76 (dd, J = 8.6, 2.3 Hz, 1 H), 5.62

(d, J = 9.1 Hz, 1 H), 4.90 (d, J = 4.5 Hz, 1 H), 3.82 (s, 3 H), 3.63 (s, 3 H), 3.50 (d, J =

15.4 Hz, 1 H), 2.97 (ddd, J = 15.4, 6.0, 2.2 Hz, 1 H), 2.38 – 2.29 (m, 2 H), 2.26 (s, 3 H),

1.64 (s, 3 H), 1.33 (s, 3 H), 0.98 (9 H), 0.33 (s, 3 H), 0.30 (s, 3 H); 13C NMR (101 MHz,

CDCl3) δ 171.5, 170.2, 156.2, 137.1, 132.9, 121.1, 118.6, 108.7, 104.1, 95.2, 84.5, 55.8,

366 55.8, 52.6, 47.5, 45.2, 27.5, 26.3, 26.3, 23.7, 23.3, 23.3, 18.4, -4.7, -4.8; LC/MS calcd for

+ + C26H40N2O6Si [M + H] 505.2656, found (ESI-TOF) 505.2323 [M + H] .

CO2Me Peroxide 4.85: TBAF (73 µL, 1 M in THF) was added to a stirred NAc MeO N H mixture of silyl peroxide 4.83a (36.6 mg, 0.073 mmol), acetic acid Me HOO Me (4.2 µL, 1 equiv), and DMF (1.5 mL, 0.05 M) at 0 ºC. The reaction

was complete in less than 5 minutes, and was subsequently poured into sat. LiCl (aq.) and

EtOAc. The organic phase was separated and the aqueous phase was extracted with

EtOAc (2x). The organic phases were combined, washed sequentially with LiCl (sat. aq.)

twice, H2O, and NaCl (sat. aq.), dried over MgSO4, and concentrated in vacuo. The crude

was purified by column chromatography (30–50% EtOAc:hexanes) to afford free

1 peroxide 4.85 as a white solid (14.9 mg, 52%): H NMR (500 MHz, CDCl3) δ 8.98 (br s,

1 H), 871 (br s, 1 H), 7.35 (br s, 1 H), 6.85 (s, 1 H), 6.76 (dd, J = 8.4, 2.2 Hz, 1 H), 5.64

(br s, 1 H), 4.85 (d, J = 5.8 Hz, 1 H), 3.83 (br s, 3 H), 3.65 (s, 3 H), 3.43 (br d, J = 14.0

Hz, 1 H), 2.26 (s, 3 H), 2.18 – 2.11 (m, 2 H), 1.61 (s, 3 H), 1.37 (s, 3 H).

CO2Me Endoperoxide 4.86: Free peroxide 4.85 (7.5 mg, 19 µmol), 3- NAc MeO N methyl-2-butenal (8 mg, 5 equiv), and 4 Å molecular sieves were Me O O Me Me Me mixed in DCE (0.19 mL, 0.1 M) and cooled to 0 ºC. BF3OEt2 (1

equiv) was added as a stock solution. The reaction was quenched at 11 minutes with a

few drops of 1:1 Et3N:MeOH and was allowed to warm to room temperature. The reaction mixture was concentrated in vacuo and purified by preparatory TLC (66%

EtOAc:hexanes) to afford endoperoxide 4.86 (1.9 mg, 23%): 1H NMR is rotomeric

367 mixture assigned by comparison to 4.50, see section 4.8; LC/MS calcd for C25H32N2O6

[M + H]+ 457.2260, found (ESI-TOF) 457.2074 [M + H]+.

CO2Me Fmoc Amide 4.87: Free amine 4.83b (297 mg, 0.506 mmol) was H N N MeO N H O added to a rapidly stirred emulsion of Fmoc proline acid Me TBDPSOO Me chloride 4.24 (0.607 mmol, 1.2 equiv) in DCM (2.5 mL, 0.2 M) and sat. aq. NaHCO3 (2.5 mL, 0.2 M). The reaction was stopped after 5 hours when

judged complete by LC/MS. The phases were separated and the aqueous phase was

extracted with DCM (3x). The organic phases were combined and dried over MgSO4 and concentrated in vacuo. The crude was purified by column chromatography (20-40%

EtOAc:hexanes) to afford acylated amine 4.87 as an off white solid foam (380 mg, 83%):

+ LC/MS calcd for C54H59N3O8Si [M + H] 906.4071, found (ESI-TOF) 906.4169 [M +

H]+, 690.4136 [M + Na]+.

CO2Me Fmoc Free peroxide 4.88: TBAF (0.17 mL, 1 M in THF) was added H N N MeO N H O to a stirred mixture of silyl peroxide 4.87 (155 mg, 0.17 mmol), Me HOO Me acetic acid (12 µL, 1 equiv), and DMF (3.4 mL, 0.05 M) at 0 ºC.

The reaction was complete in less than 5 minutes, and was subsequently poured into sat.

LiCl (aq.) and EtOAc. The organic phase was separated and the aqueous phase was

extracted with EtOAc (2x). The organic phases were combined, washed sequentially with

LiCl (sat. aq.), H2O, and NaCl (sat. aq.), dried over MgSO4, and concentrated in vacuo.

The crude was purified by column chromatography (30–70% EtOAc:hexanes) to afford free peroxide 4.88 as an whitish-yellow solid (85.2 mg, 75%): Rf = 0.13 (40%

368 + EtOAc:hexanes); LC/MS calcd for C43H47N3O8 [M + H] 668.2894, found (ESI-TOF)

668.4314 [M + H]+, 690.4136 [M + Na]+.

CO2Me Fmoc Endoperoxide 4.89: Free peroxide 4.88 (10mg, 15 mol), 3- H N µ N MeO N O methyl-2-butenal (6 mg, 5 equiv), and 4 Å molecular sieves (7 Me H O O Me Me Me mg) were cooled to –30 ºC in DCE (100 µL). The temperature

was maintained below –20 ºC and BF3OEt2 (1 equiv) was added. The reaction was

quenched at 8 minutes with a few drops of 1:1 Et3N:MeOH and was allowed to warm to

room temperature. The reaction was concentrated in vacuo and purified by preparatory

TLC (50% EtOAc:hexanes) to give endoperoxide 4.89 (1.6 mg, 15%): Rf = 0.54 (50%

+ EtOAc:hexanes); LC/MS calcd for C43H47N3O8 [M + H] 734.3363, found (ESI-TOF)

734.4865 [M + H]+, 756.4714 [M + Na]+.

CO Me 3 2 Fmoc α,β-Unsaturated Ester 4.90: ZrCl4 (3.8 mg, 1 equiv) and H N N MeO N O nitrosobenzene (4.3 mg, 2.5 equiv) were added at once to a Me H O O Me Me Me solution of ester 4.89 (11.6 mg, 16.2 µmol) in DCM (110 µL,

0.14 M) at 0 ºC. The reaction was maintained at 0 ºC for two hours when it was judged

complete by TLC. The acid was quenched with a few drops of 1:1 Et3N:MeOH at 0 ºC, and then concentrated in vacuo. The crude mixture was purified by preparatory TLC

(60% EtOAc:hexanes) to afford α,β-unsaturated ester 4.90: LC/MS calcd for C43H45N3O8

[M + H]+ 732.3207, found (ESI-TOF) 732.4750 [M + H]+.

369 O Diketopiperazine 4.91: HNEt2 (3 drops) was added to a solution N

N MeO H of substrate 4.90 (~1 mg) in THF (100 µL) and the reaction was N O

Me H O Me O Me warmed to 40 ºC for 12 hours. The reaction was judged to be Me complete by LC/MS. The crude mixture was purified directly by preparatory TLC (70%

1 EtOAc:hexanes) to afford hexacyclic product 4.91: H NMR (600 MHz, CDCl3) δ 7.56

(d, J = 8.6 Hz, 1 H), 7.30 (s, 1 H), 6.88 (dd, J = 8.6, 2.2 Hz, 1 H), 6.78 (dd, J = 11.0, 1.3

Hz, 1 H), 6.63 (d, J = 8.1 Hz, 1 H), 6.57 (d, J = 2.2 Hz, 1 H), 5.14 (dt, J = 8.1, 1.4 Hz, 1

H), 4.16 (dd, J = 10.1, 6.4 Hz, 1 H), 3.85 (s, 3 H), 3.77 – 3.64 (m, 2 H), 2.47 – 2.42 (m, 1

H), 2.22 – 2.16 (m, 1 H), 2.16 – 2.08 (m, 2 H), 2.03 (d, J = 1.4 Hz, 3 H), 1.99 – 1.91 (m,

13 1 H), 1.78 (d, J = 1.4 Hz, 3 H), 1.69 (s, 3 H), 1.02 (s, 3H); C NMR (151 MHz, CDCl3) δ

165.9, 159.7, 156.9, 144.0, 136.4, 134.6, 122.2, 119.4, 119.1, 118.3, 111.0, 110.3, 106.8,

94.7, 86.4, 82.0, 59.1, 55.9, 49.6, 49.1, 45.3, 29.1, 27.3, 25.9, 24.5, 22.3, 19.0; LC/MS

+ + calcd for C27H31N3O5 [M + H] 478.2264, found (ESI-TOF) 478.2996 [M + H] .

Synthetic verruculogen (4.1): Compound 4.91 (less than 1 mg) O HO HO N

N was dissolved in anhydrous pyridine (440 µL) under an argon MeO H N O

Me H O O atmosphere and cooled to 0 ºC. OsO4 in pyridine (35µL, 0.03 M) Me Me Me

was added slowly. The reaction was stirred for 40 minutes, quenched with NaHSO3 (sat. aq.) at 0 ºC, and allowed to warm to room temperature. After 30 minutes, the biphasic mixture was diluted with DCM and water. The organic phase was separated and the aqueous phases were extracted with DCM (2x). All organic phases were combined,

washed with H2O and NaCl (sat. aq.), and dried over MgSO4. The solvent was removed

in vacuo and verruculogen (4.1) was isolated by PTLC (70% EtOAc:hexanes): Rf = 0.48

370 1 (70% EtOAc:hexanes); H NMR (600 MHz, CDCl3) δ 7.90 (d, J = 8.7 Hz, 1 H), 6.83 (d,

J = 8.9 Hz, 1 H), 6.64 (d, J = 8.2 Hz, 1 H), 6.60 (s, 1 H), 6.05 (d, J = 10.0 Hz, 1 H), 5.66

(s, 1 H), 5.04 (d, J = 8.1 Hz, 1 H), 4.49 (t, J = 8.2 Hz, 1 H) (only diagnostic protons

+ shown); LC/MS calcd for C27H32N3O6 [M – OH] 494.2291, found (ESI-TOF) 494.3367

[M – OH]+.

371

4.8 Spectra

O N

N O O HO O HO N

MeO

372

H O N Me Me N O

O HO O HO N H

MeO

373

OOTES Me Me O Et EtO

374

OOTBS Me Me

Et EtO

375

OOTBDPS Me Me O Et

EtO

376

OOTBS Me Me

377

OOTBDPS Me Me O H

378

Me 2 Me CO Me NH N H TBSOO

379

Me 2 Me CO Me NH N H TBDPSOO

380

Me 2 Me CO Me NAc

O N

381

Me 2 Me CO Me NAc O O N Me

382

Me 2 Me CO Me NAc O O N Me Me

383

Me 2 Me CO Me NAc O O N Me Me

384

NAc

385

H O N Me

Me N

N H

TBDPSOO

386

O N

Me N

N H

TBDPSOO

387

H O N Me Me N O H N H TBDPSOO

388

H O N

N O

N H

TBDPSOO

389

H O N Me Me N O

N H

TBDPSOO

390

H O N Me Me N O N H TBDPSOO

391

O N

Me N O

HO HO N H

TBDPSOO

392

O N

Me Me N O

Me Me

393

H O N Me Me N O O HO O HO N H Me

394

$ % $ & ' # $ " ! H O N Me Me N O O HO O HO N H Me Me

395

H O N Me Me N O O HO O HO N H Me Me

396

H O N Me Me N O H O O N H Me EtO Me MeO

397

O N

Me Me N O H

O O

N H Me

EtO Me MeO

398

H O N Me Me N O H O O N H Me EtO Me MeO

399

H O N Me Me N O H O O N H Me EtO Me MeO

400

H O N Me Me N O H O O N H Me EtO Me MeO

401

Me NAc

OH N H

TBSOO

402

Me 2 Me CO Me NAc OH N H

TBSOO

403

Me 2 Me CO Me NAc OH N H HO TBSOO

404

Me 2 Me CO Me NAc OH N H HO TBSOO

405

Me 2 NHBoc CO

TIPS

B O

O Me Me

Me Me

406

Me 2 Me CO Me NH N H TBSOO

MeO

407

Me 2 Me CO Me NH N H TBSOO

MeO

408

Me CO Me NH

N H

mixture

TBDPSOO 2:1

MeO

409

Me 2 Me CO Me NAc N H TBSOO

MeO

410

Me 2 Me CO Me NAc

N H

TBSOO

MeO

411

Me 2 Me CO Me NAc HOO N H

MeO

412

Me 2 Me CO Me NAc O O N Me Me MeO

413

H O N Me Me N O

MeO

414

H O N Me

N O

O O N H

MeO

415

h H O N Me Me N O e O HO H f O HO H N c a g H Me H H d H b Me H MeO

416 Distribution of Credit

Chapter 1: An early strategy toward the total synthesis and transannular

cyclizations of humulene was put forth Dr. Ke Chen and Professor Phil Baran in an NIH

grant before I joined the laboratory in 2009. As presented here, the ideas in Chapter 1

were conceived of entirely by Professor Phil Baran and me. I was solely responsible for

the experiments performed.

Chapter 2: The conceptual framework for chapter 2 was my own idea, namely

using an organometallic cooligomerization of dienes to form terpene macrocycles. Beyond

this initial retrosynthetic disconnection, the strategy and ideas for the project was

Professor Phil Baran’s, Dr. Daniel Götz’s, and my own. All of the experiments were

conducted by Daniel Götz and myself.

Chapter 3: In Section 3.1, the all of the computational work was conceived of and

executed by Xin Hong and Professor Ken Houk at the University of California, Los

Angeles. The Figures 3.1–3.3 were prepared by Xin Hong but their interpretation and

descriptions are mine. In Section 3.2, I was involved in an early strategy toward the ten-

membered macrocycles, specifically Schemes 3.3 and 3.4. The remainder of the work was

conceived of by Professor Phil Baran, and Drs. Klement Foo, Ippei Usui, Daniel Götz, and

Erik Werner. The experiments in the project were conducted by Klement Foo, Ippei Usui,

Daniel Götz, and Erik Werner.

Chapter 4: The general synthetic strategy for the total synthesis of Verruculogen

was conceived by Professor Phil Baran, Dr. Will Gutekunst, and myself in March 2012.

Will Gukekunst and I performed experiments on a strategy toward Verruculogen that did

417 not appear in this thesis. The experiments in this chapter were conducted by Dr. Yu Feng,

Jochen Zoller, and myself.

Professor Arnie Rheingold and Dr. Curtis Moore were responsible for all of the X- ray crystallography that appears in this work.

418 Curriculum Vitae

Dane Holte Ph.D. Candidate The Scripps Research Institute, 10550 N. Torrey Pines Rd. BCC-457 La Jolla, CA 92037, (858) 784-7371, [email protected]

Education

The Scripps Research Institute, La Jolla, California 2009 – 2014 (expected) National Science Foundation Graduate Research Fellow Advisor: Dr. Phil S. Baran

Northwestern University, Evanston, Illinois 2005 – 2009 BA Chemistry, Integrated Science Cumulative GPA: 3.37/4.0

Colerain High School, Cincinnati, Ohio 2001 – 2005 Cumulative GPA: 4.0/4.0 Valedictorian

Research Experience

Ph.D. Candidate, The Scripps Research Institute June 2009 – present Advisor: Dr. Phil S. Baran 1. Studies toward the total synthesis of the neurotropic mycotoxin verruculogen 2. Investigated the sesquiterpene cyclase phase through Ni0-assisted cyclooligomerization in the context of the total syntheses of humulene and germacrane natural products

Undergraduate Research Associate, Northwestern University April 2008 – June 2009 Advisor: Dr. Karl A. Scheidt Assisted in the development of an N-heterocyclic carbene and Lewis acid cooperative catalysis methodology to afford γ-lactams via a formal [3+2] cycloaddition of hydrazones and homoenolates

Undergraduate Research Associate, Northwestern University June 2007 – April 2008 Advisor: Dr. Owen Priest Developed a green inquiry-based laboratory currently in use in the Northwestern University undergraduate organic chemistry curriculum which explored the utility of Oxone® in Baeyer- Villiger oxidations

Undergraduate Research Associate, Northwestern University April 2006 – December 2006 Advisor: Dr. Farhad Yusef-Zadeh Conducted galactic center research involving the calculation of spectral index values of non- thermal radio filaments located near the black hole at the center of the Milky Way

Publications

Foo, K.; Usui, I.; Götz, D. C. G.; Werner, E. W.; Holte, D.; Baran, P. S. Scalable, Enantioselective

419 Synthesis of Germacrenes and Related Sesquiterpenes Inspired by Terpene Cyclase Phase Logic, Angew. Chem. Int. Ed. 2012, 51, 11491–11495.

Holte, D.; Götz, D. C. G.; Baran, P. S. An Approach to Mimicking the Sesquiterpene Cyclase Phase by Nickel-Promoted Diene/Alkyne Cooligomerization, J. Org. Chem. 2012, 77, 825–842.

Raup, D. E. A.; Cardinal-David, B.; Holte, D.; Scheidt, K. A. Cooperative catalysis by carbenes and Lewis acids in a highly stereoselective route to γ-lactams, Nature Chem. 2010, 2, 766–771.

Posters and Presentations

Holte, D.; Gutekunst, W. R.; Baran, P. S. Synthetic Studies Toward Verruculogen and Fumitremorgin, The Scripps Research Institute Symposium 2012, Poster Presentation

Holte, D; Götz, D. C. G.; Baran, P. S. An Approach to Mimicking the Sesquiterpene Cyclase- Phase via Isoprene Oligomerization, The Scripps Research Institute Symposium 2011, Poster Presentation

Professional Experience

Teaching Assistant (Classics in Total Synthesis) 2013 Chemistry Distinguished Lecture Series Organizer 2012 –2013

Honors/Awards

National Science Foundation Graduate Research Fellowship 2011 – present Amgen Graduate Fellowship 2010 – 2011 The Scripps Research Institute Dean’s Fellow 2009 Northwestern University Academic Year Undergraduate Research Grant 2009 Weinberg College Research Grant 2008 Northwestern University Summer Undergraduate Research Grant 2008 NASA Summer Research Grant through the Illinois Space Grant Consortium 2006 Robert C. Byrd Undergraduate Scholarship 2005 – 2009 Eagle Scout Award 2003

420