Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes

I. The Asymmetric Total Synthesis of Apratoxin D

II. Studies in the Gold(I)-Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes

III. Synthesis and Application of Multidentate Ligands Toward the Realization of Fluxional Mechanocatalysis

Bradley David Robertson

Department of Chemistry Duke University

Date:______Approved:

______Ross A. Widenhoefer, Supervisor

______Steven W. Baldwin

______Stephen L. Craig

______Patrick Charbonneau

Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University

2015

ABSTRACT

I. The Asymmetric Total Synthesis of Apratoxin D

II. Studies in the Gold(I)-Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes

III. Synthesis and Application of Multidentate Ligands Toward the Realization of Fluxional Mechanocatalysis by

Bradley David Robertson

Department of Chemistry Duke University

Date:______Approved:

______Ross A. Widenhoefer, Supervisor

______Steven W. Baldwin

______Stephen L. Craig

______Patrick Charbonneau

An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University

2015

Abstract

Apratoxin D, recently isolated from two species of cyanobacteria, L. majuscula and L. sordida, exhibits highly potent in vitro cytotoxicity against H-460 human lung cancer cells with an IC50 value of 2.6 nM. The potent biological activity exhibited by apratoxin D combined with its intriguing molecular architecture has led to the pursuit of its asymmetric total synthesis. Studies toward and completion of the first asymmetric total synthesis of apratoxin D are reported. Key transformations include a Kelly thiazoline formation, Paterson anti-aldol and an Evans syn-aldol. The synthesis was completed in 2.1% total yield over 31 steps from (R)-citronellic acid.

Cationic gold (I) complexes are highly efficient catalysts for the cycloisomerization of 1,6-enynes, a transformation capable of providing a great amount of structural complexity from simple starting materials. The in situ spectroscopic analysis of the catalytic cycloisomerization of a 7-phenyl-1,6-enyne, as well as the tandem gold/silver-catalyzed cycloaddition/hydroarylation of 7-aryl-1,6-enynes is described. The cycloaddition/hydroarylation reaction provides 6,6- diarylbicyclo[3.2.0]heptanes in good yield under mild conditions. Experimental observations point to a mechanism involving gold-catalyzed cycloaddition followed by silver-catalyzed hydroarylation of a bicyclo[3.2.0]hept-1(7)-ene intermediate.

The control of bond scission and formation by mechanocatalysis has potential in a variety of applications, including biomedical devices, mechanical sensors and self-

healing materials. The synthesis and study of C2-symmetric bis(phosphine) ligands with applications toward mechanocatalysis is described. Additionally, the synthesis and study of a tetradentate ligand designed toward mechanochemical activation of a latent catalytic complex is reported. These studies have allowed further development in the design of transition metal complexes capable of activation by mechanical force.

To my family and friends.

Contents

Abstract ...... iv

List of Tables ...... xi

List of Figures ...... xii

List of Schemes ...... xiv

Acknowledgements ...... xvii

1. The Asymmetric Total Synthesis of Apratoxin D ...... 1

1.1. Background and Introduction ...... 1

1.1.1. Macrocyclic natural products ...... 1

1.1.2. Isolation and biological activity of the apratoxins ...... 3

1.1.3. Biological activity of the apratoxins and structure-activity relationships ...... 5

1.1.4. Previous synthetic studies ...... 8

1.1.4.1 Syntheses of the apratoxins and common strategies ...... 8

1.1.4.2 Forsyth’s synthesis of apratoxin A ...... 9

1.1.4.3 The Takahashi/Doi synthesis of apratoxin A ...... 13

1.1.5. Apratoxin D ...... 17

1.1.5.1 Background ...... 17

1.1.5.2 Retrosynthetic analysis of apratoxin D ...... 18

1.2. Results and Discussion ...... 22

1.2.1. Synthesis of polyketide from 3-butenal ...... 22

1.2.2. Synthesis of polyketide fragment via ACC alkylation of an advanced intermediate ...... 25

vii

1.2.3. Synthesis of polyketide fragment from (R)-citronellic acid ...... 26

1.2.4. Tripeptide incorporation, thiazoline formation and macrocyclization ...... 29

1.2.5. Summary and conclusions ...... 31

1.3. Experimental ...... 32

1.3.1. General methods ...... 32

2. Studies in the Gold(I)-Catalyzed Cycloisomerization of 7-Aryl-1,6-enynes ...... 56

2.1. Introduction ...... 56

2.1.1. Homogeneous gold catalysis ...... 56

2.1.2. Mechanistic considerations in the gold(I)-catalyzed cycloisomerization of 1,6-enynes ...... 60

2.1.2.1 Cyclobutenes in the cycloisomerization of 1,6-enynes ...... 61

2.1.2.2 Molecular complexity from the trapping of carbenoid intermediates in the cycloisomerization of 1,n-enynes ...... 64

2.1.3. Project goals and scope ...... 65

2.2. Results and Discussion ...... 67

2.2.1. In situ spectroscopic analysis of the conversion of 2.12 to 2.13...... 67

2.2.2. Trapping of cyclobutene 2.25 via hydroarylation ...... 71

2.2.3. Nucleophile scope in the hydroarylation of 3.12...... 73

2.2.4. Mechanistic considerations in the hydroarylation of 2.12...... 75

2.2.5. Summary and conclusions ...... 78

2.3. Experimental ...... 80

2.3.1. General methods ...... 80

2.3.2. In situ spectroscopic analysis of the conversion of 2.12 to 2.13 ...... 81

viii

2.3.2.1 Spectroscopic analysis of the gold-catalyzed conversion of 2.12 to 2.13 81

2.3.2.2 In situ spectroscopic snalysis of the gold/triflic acid catalyzed conversion of 2.12 to 2.13...... 82

2.3.3. Gold complexes ...... 82

2.3.4. 1,6-Enynes ...... 83

2.3.5. 6,6-Diaryl-bicyclo-[3.2.0]-heptanes ...... 87

2.3.6. Optimization and control experiments ...... 104

2.3.7. X-ray crystal structure of 2.48 ...... 109

3. Synthesis and Application of Multidentate Ligands Toward Realization of Fluxional Mechanocatalysis ...... 113

3.1. Introduction ...... 113

3.1.1. Mechanocatalysis ...... 113

3.1.2. Bidentate ligands in transition metal-catalyzed chemical transformations 116

3.1.3. Polydimethylsiloxane (PDMS) polymer system ...... 120

3.1.4. Project goals ...... 123

3.2. Results and Discussion ...... 127

3.2.1. Intramolecular hydrosilylation of alkenes catalyzed by rhodium- bis(phosphine) complexes ...... 127

3.2.2. Asymmetric intramolecular allylic alkylation catalyzed by palladium- PHOX complexes...... 131

3.2.3. Further directions ...... 142

3.3. Experimental ...... 143

3.3.1. General Methods ...... 143

3.3.2. Synthesis of bis(phosphine) ligands and rhodium complexes ...... 144

3.3.3. Hydrosilylation procedure...... 145

3.3.4. Synthesis of tetradentate complex 3.36...... 146

3.3.5. Synthesis of bipyridine 3.34...... 147

3.3.6. Synthesis of ester 3.47 and silyl ether 3.49...... 149

References ...... 150

Biography ...... 162

List of Tables

Table 1.1. Biological activity of the apratoxins and apratoxin analogs...... 7

Table 2.1. Effect of gold, silver, and Brønsted acid on the cycloisomerization/ hydroarylation of 2.12 with 1,3,5-trimethoxybenzene (TMB)...... 72

Table 2.2. Cycloisomerization/hydroarylation of 7-aryl-1,6-enynes catalyzed by a mixture of 2.27 (5 mol %) and AgSbF6 (20 mol %) in CH2Cl2 at 25 °C for 1.5 h [Z = C(CO2Me)]...... 74

Table 2.3. Sample and crystal data for 2.48...... 111

Table 2.4. Data collection and structure refinement for 2.48...... 112

Table 3.1. Effect of ligand dihedral angle (θ) on yield and enantiomeric excess in the hydrogenation of methyl acetoacetate...... 118

Table 3.2. Effect of ligand identity on palladium-mediated asymmetric Heck reaction of 2,3-dihydrofuran and phenyl triflate...... 120

Table 3.3. Ranking of PDMS swelling in various solvents based on the solubility parameter S...... 123

Table 3.4. Effect of ligand identity on enantiomeric excess (% ee) of the intramolecular hydrosilylation of olefins 3.17 and 3.19 ...... 130

Table 3.5. Enantioselective intramolecular Tsuji allylation reported by Stoltz.159 ...... 133

Table 3.6. Effect of linker length on geometry of PHOX-bpy tetradentate Pd(0) complex...... 135

Table 3.7. Effect of ligand identity on Pd(0)-catalyzed intramolecular Tsuji-allylation of allyl enol carbonate 3.39...... 137

Table 3.8. Attempts at incorporation of the 5-pentenyl moiety into 3.47 and 3.49...... 141

List of Figures

Figure 1.1. Apratoxins A-H (1.1-1.8) and apratoxin A sulfoxide (1.9). Structurally unique features are highlighted...... 5

Figure 1.2. 1H NMR spectrum of apratoxin D 1.4...... 54

Figure 1.3. Literature 1H NMR spectrum of apratoxin D 1.4...... 55

Figure 2.1. A) Examples of commonly used ligands in Au(I) catalysis. B) Chloride abstraction from [LAuCl] complexes with Ag(I) salts possessing a weakly-/non- coordinating counterion to produce catalytically active Au(I) species...... 58

Figure 2.2. The Dewar-Chatt-Duncanson model of binding modes in Au(I)-catalysis. ... 59

Figure 2.3. Concentration versus time plot for the cycloisomerization of 2.12 ([2.12]0 = 100 mM) to 2.13 via 2.25 catalyzed by a mixture of (2.2)AuCl (2 mol %) and AgSbF6 (2 mol %) in CD2Cl2 at 25 °C...... 68

Figure 2.4. Concentration versus time plot for the cycloisomerization of 2.12 ([2.12]0 = 100 mM) to 2.13 catalyzed by a mixture of (2.2)AuCl (2 mol %), AgSbF6 (2 mol %), and HOTf (5 mol %) in CD2Cl2 at 25 °C...... 69

Figure 2.5. 1H NMR analysis 2.36-d3 in toluene-d8 at 25 °C...... 89

Figure 2.6. 13C{1H} NMR analysis 2.36-d3 in toluene-d8 at 25 °C...... 90

Figure 2.7. 1H NMR spectrum of exo-2.45 in toluene-d8 at 25 °C...... 94

Figure 2.8. 1H–1H NOESY NMR analysis exo-2.45 in toluene-d8 at 25 °C...... 95

Figure 2.9. 1H NMR spectrum of endo-2.45 in toluene-d8 at 25 °C (right spectrum) and 80 °C (left spectrum)...... 96

Figure 2.10. 1H–1H NOESY NMR analysis of exo-2.47 in toluene-d8 at 25 °C...... 99

Figure 2.11. Concentration versus time plot for the formation and consumption of 2.13 (o) in the reaction of 2.12 (¡; 100 mM) and 1,3,5-trimethoxybenzene (2 equiv) catalyzed by a 1:4 mixture of 2.27 (5 mM) and AgSbF6 (20 mM) in CH2Cl2 at 25 °C...... 106

xii

Figure 2.12. Concentration versus time plot for the formation and consumption of 2.13 (o) in the reaction of 2.12 (¡; 100 mM) and 1,3,5-trimethoxybenzene (2 equiv) catalyzed by a 1:1 mixture of 2.28 (5 mM) and HOTf (5 mM) in CH2Cl2 at 25 °C...... 107

Figure 2.13. ORTEP diagram of 2.48 with ellipsoids shown at the 50% probability level...... 110

Figure 3.1. Activation of latent catalysts as demonstrated by Sijbesma et al. A) Transesterification of vinyl acetate with benzyl alcohol catalyzed by an ultrasound- activated, polymer-supported N-heterocyclic carbene 3.2. B) Ring-closing metathesis of diethyl diallylmalonate catalyzed by ultrasound-activated Ru-alkylidene 3.4.A...... 116

Figure 3.2. Bidentate ligands in catalysis. A) Representative examples of bidentate ligands in metal-catalyzed reactions. B) Correlation between dihedral angle (θ) of C2- symmetric ligands, corresponding bite angle (β), and the tunable bis(phosphine) ligand TunePhos...... 117

Figure 3.3. General structure of polydimethylsiloxane and Pt-catalyzed crosslinking. . 121

Figure 3.4. Strategies in the in the realization of fluxional mechanocatalytic systems. A) Force-induced perturbation of dihedral angle θ in a chiral bis(phosphine) incorporated into a PDMS framework. B) Activation of a latent catalytic complex by mechanical force. C) Crosslinking of a ligand into the PDMS system...... 126

xiii

List of Schemes

Scheme 1.1. Forsyth’s retrosynthetic dissection of 1.1...... 9

Scheme 1.2. Forsyth’s synthesis of polyketide fragment 1.11...... 10

Scheme 1.3. Synthesis of azide 1.10...... 12

Scheme 1.4. Forsyth’s Staudinger-aza-Wittig and macrocyclization to complete apratoxin A (1.1)...... 13

Scheme 1.5. The Takahashi/Doi retrosynthetic disconnection of 1.1...... 14

Scheme 1.6. The Takahashi/Doi synthesis of polyketide fragment 1.11...... 15

Scheme 1.7. Protecting group exchange, thiazoline formation and synthesis of alcohol 1.29...... 16

Scheme 1.8. Completion of 1.1 by Takahashi and Doi...... 17

Scheme 1.9. Retrosynthetic dissection of apratoxin D (1.4)...... 19

Scheme 1.10. Initial retrosynthetic dissection of 1.44...... 20

Scheme 1.11. Retrosynthetic dissection of 1.44 to give methacrolein...... 21

Scheme 1.12. Retrosynthesis of 1.44 leading to (R)-citronellic acid 1.55...... 21

Scheme 1.13. Synthesis of α,β-unsaturated lactone 1.47 from glyoxal 1.56...... 23

Scheme 1.14. Synthesis of aldehyde 1.50 from 1.47...... 24

Scheme 1.15. Synthesis of ketone 1.50 from methacrolein...... 26

Scheme 1.16. Synthesis of aldehyde 1.54 from (R)-citronellic acid...... 26

Scheme 1.17. Synthesis of aldehyde 1.50 from aldehyde 1.54...... 28

Scheme 1.18. Completion of the polyketide moiety...... 28

Scheme 1.19. Synthesis of carboxylic acid 1.42 from 1.44...... 30

Scheme 1.20. Completion of apratoxin D...... 31

xiv

Scheme 2.1. Potential pathways and intermediates for enyne cycloisomerization catalyzed by electrophilic noble metal complexes...... 61

Scheme 2.2. A) Blum’s PtCl4-mediated synthesis of cyclobutene 2.6 and subsequent trapping with oxygen. B) Recent synthesis of a similar dione 2.9 by Shin...... 62

Scheme 2.3. Isolation of bicyclo[3.2.0]hept-5-enes 2.10 and 2.11 from the Au(I)-catalyzed cyclization of 1,6-enynes 2.8 and 2.9 by Echavarren...... 63

Scheme 2.4. Spectroscopic detection of gold bicyclo[3.2.0]heptene complex 2.13 in the gold-catalyzed cycloisomerization of 2.12 to 2.13...... 64

Scheme 2.5. Trapping of carbenoid intermediates by nucleophiles.68 ...... 65

Scheme 2.6. Mechanism of the acid-catalyzed conversion of 2.14 to 2.15 under catalytic conditions...... 66

Scheme 2.7. Improved procedure for the synthesis of 2.13 from the gold-catalyzed cycloisomerization of 2.12...... 71

Scheme 2.8. Gold/silver-catalyzed hydroarylation of 2,4,6-trideutero-1,3,5-trimethoxybenzene (TMB-d3)...... 76

Scheme 2.9. Proposed mechanism for the conversion of 2.12 and TMB to 2.26 via gold/silver tandem catalysis...... 78

Scheme 3.1. Irradiation of stiff stilbene (Z)-3.11 with light (λ = 365 nm) prompts Z→E isomerization to provide a 68:23:9 mixture of (Z)-3.11/(E)-3.11/(E)-3.11’...... 119

Scheme 3.2. Synthesis of [Rh(bisphosphine)(NBD)]ClO4- complexes 3.14-3.16.a ...... 128

Scheme 3.3. Synthesis of chiral bis(phosphine) 3.22 and Rh(I) complex 3.23 from (S)- MeO-BIPHEP 3.7...... 131

Scheme 3.4. Synthesis of PHOX/bpy tetradentate ligand 3.36 from 2-bromobenzoyl chloride...... 136

Scheme 3.5. Synthesis of alcohol 3.44 from 5,5’-dimethyl-2,2’-bipyridine 3.41...... 138

Scheme 3.6. Retrosynthetic dissection for incorporation of the pendant alkene and phosphine moieties from bromofluorobenzene 3.47...... 139

Scheme 3.7. Synthesis of methyl ester 3.47 and silyl ether 3.49 from 4-bromo-2- fluorobenzoyl chloride 3.45...... 140

Scheme 3.8. Future plans for synthesis of tetradentate ligand 3.55 and polymer incorporation...... 143

xvi

Acknowledgements

First and foremost, I would like to thank my advisor, Ross Widenhoefer. You helped me out when I was in a very difficult position and I will forever be indebted to you for that. I admire your love of pure science and have a deep respect for your level of scientific rigor. It is something I will carry with me throughout my career.

I would also like to thank the members of my committee, Professors Baldwin,

Craig and Charbonneau. You were willing to fight for me when I needed it, provide guidance when I needed it, and give me a kick from behind when I needed it. Thank you for your motivation and support throughout the years.

To my wife, Casey. You are amazing. Thank you for putting up with me throughout this journey, especially during the last few months. I apologize that this was due the day after our first anniversary.

To my family, especially my parents, Jim and Michele, thank you for your undying love and support. You always encouraged me to follow my dreams and helped me get through this when times got really tough. It’s been a long road and I wouldn’t have been able to do it without you. You’re easily the best parents I’ve ever had.

Finally, to my friends, particularly Scott Sauer, Myron Evans and Stephen Payne, and all of my former labmates, thank you for maintaining my sanity. You made this a whole lot less painful than it could’ve been. Thanks for your friendship.

xvii

1. The Asymmetric Total Synthesis of Apratoxin D

Portions of this chapter have been published: Robertson, B. D.; Wengryniuk, S.

E.; Coltart, D. M. Org. Lett. 2012, 14, 5192-5195; Dey, S.; Wengryniuk, S. E.; Tarsis, E. M.;

Robertson, B. D.; Zhou, G.; Coltart, D. M. Tetrahedron Lett. 2015, 56, 2927-2929.

1.1. Background and Introduction

1.1.1. Macrocyclic natural products

Cyanobacteria are a truly amazing source of complex bioactive natural products.

Billions of years of evolution have provided these unicellular organisms with an exceptional ability to produce a wide variety of biologically active compounds. These compounds are commonly secondary metabolites of cellular processes evolved for the utilization in defense mechanisms. The study of bioactive compounds produced by these organisms can be traced as far back as the 1930s, but the modern study of cyanobacteria began with the pioneering work by Richard E. Moore with his discovery of the majusculamides in 1977.1 The resulting boom in research has identified an enormous variety of compounds known to be toxic to both humans and aquatic life.

These organisms produce an array of structurally diverse compounds, such as fatty acid derivatives, terpenes, saccaharides, peptides, polyketides and lipopeptides.2 The motivations for the study of these compounds can be traced back to both the negative effect these compounds can have on aquatic life, as evidenced by the massive negative

effects produced by algal blooms, and the tireless search for new treatments for human disease.3

One common structural feature of larger natural products is the macrocycle (a ring structure of ≥12 atoms). The macrocyclic structure permits a level of preorganization such that specific functionalities within the molecule can interact with biological targets over a large area. Macrocycles can interact with multiple binding sites within a biological target without a significant entropic penalty compared to that of small molecules. Therefore, they have been used to target biological features that are much more difficult to disrupt with small molecules, such as protein-protein interactions.4 Macrocycles can be considered the smallest examples of biomolecules that possess individual subdomains. However, since they are not structurally rigid, they exhibit a fine balance between preorganization of structure and the flexibility to interact with the 3D surfaces of their biological targets. Because of their unique structural features and the large number of biologically active compounds within this class, the study of macrocycles has garnered a significant amount of attention. Indeed, macrocycles are essential features of >100 drugs currently on the market for the treatment of human disease.5

1.1.2. Isolation and biological activity of the apratoxins

The apratoxins are a family of macrocyclic depsipeptides isolated from Lyngbya species of cyanobacteria. This group of complex natural products is composed of apratoxins A—H and apratoxin A sulfoxide (Figure 1.1). Structurally, the family is characterized by discrete polypeptide and polyketide portions joined by thiazoline and ester linkages. Apratoxin A (1.1) was the first compound discovered in this family from collections of Lyngbya majuscula in the Apra Harbor, Guam, in 2001.6 Structural determination identified a polypeptide domain that consists of a highly methylated tetrapeptide moiety, D-proline—N-methyl-L-isoleucine—N-methyl-L-alanine—O- methyl-L-tyrosine, and a modified vinylogous D-cysteine residue in which the thiol has condensed on the adjacent amide carbonyl to form a thiazoline ring. The polyketide domain possesses stereogenic centers at C34, C35, C37 and C39. Soon after the initial report of the isolation and structural determination of apratoxin A, the structures of apratoxins B (1.2) and C (1.3) were reported by the same group.7 Isolated from the same species of cyanobacteria near the western Pacific islands of Guam and Palau, the structure of apratoxin B differs only from 1.1 in the terminus of the polyketide domain, the tert-butyl group having been exchanged for an iso-propyl group. Apratoxin C differs from 1.1 only in the state of methylation at the isoleucine residue. In 2008, apratoxin D

(1.4) was isolated by Gerwick and coworkers from samples Lyngbya majuscula and

Lyngbya sordida in the waters near the coast of Papua New Guinea.8 It was determined to

be structurally similar to 1.1, however, it possessed a novel terminus to the polyketide domain, being extended by two carbons and possessing an additional stereogenic center at C40. Apratoxin E (1.5) was isolated by Luesch and coworkers from Lyngbya bouillonii in 2008.9 It lacks a methyl group at C34 and is unsaturated between C34 and C35, having been dehydrated. Additionally, the vinylogous cysteine moiety lacks a methyl group at

C28 and has been saturated between C28 and C29. Apratoxins F (1.6) and G (1.7), reported in 2010 by Gerwick and coworkers, were discovered from samples of Lyngbya bouillonii collected in the central Pacific Palmyra Atoll.10 In both compounds, the proline residue has been exchanged for an N-methylalanine. Additionally, apratoxin G possesses an N-methylvaline residue in place of the isoleucine residue. Apratoxin H (1.8) and apratoxin A sulfoxide (1.9) were both reported by McPhail and coworkers in 2013 from Moorea producens (formerly known as Lyngbya majuscula) in the Red Sea.11 In apratoxin H, the proline ring has expanded to a pipecolic acid residue, and apratoxin A sulfoxide possesses an oxidized thiazoline.

Ala R O-Me-Tyr 1 O O Ile N N N N OMe OMe O HN O HN Pro N O O N O O 28 29 O O O OH N O OH N 34 39 37 35 S moCys S R 2 Polyketide 1.4 apratoxin D R1 R2 1.1: apratoxin A Me Me 1.2: apratoxin B H Me R1 1.3: apratoxin C Me H O N O N OMe N O HN N N O O OMe O HN N O O O O OH N O O N S S R1 1.5 apratoxin E 1.6: apratoxin F Me 1.7: apratoxin G H

O O N N N N OMe OMe O HN O HN N O O N O O

O O O OH N O OH N S S O

1.8 apratoxin H 1.9 apratoxin A sulfoxide

Figure 1.1. Apratoxins A-H (1.1-1.8) and apratoxin A sulfoxide (1.9). Structurally unique features are highlighted.

1.1.3. Biological activity of the apratoxins and structure-activity relationships

While the apratoxins were discovered to be structurally interesting, the members of the family also displayed a high level of bioactivity against various lines of cancer

cells (Table 1.1). Apratoxin A (1.1) inhibits cellular proliferation (IC50 ≤ 10 nM) in a variety of cell lines, including LoVo, KB, HeLa, HCT116, HT29 and U20S cells.6,12

Apratoxin B (1.2) shows decreased activity, implying that the methylation state of the isoleucine residue is conformationally important.7 However, the polyketide terminus does not appear to be important, as apratoxin C (1.3) and D (1.4) are both nanomolar inhibitors of proliferation in LoVo, KB (for 1.3) and H-460 (for 1.4) cells. Apratoxin D

(1.4) displays a high level of activity against H-460 lung cancer cells (IC50 = 2.6 nM).8

Dehydration of C34—C35 and saturation of C28—C29 leads to greatly reduced activity, as evidenced by apratoxin E (1.5).9 Structurally similar residues to proline (N-Me-alanine in 1.6 and 1.7, pipecolic acid in 1.8) and isoleucine (N-Me-Ile→N-MeVal in 1.7) are tolerated, as evidenced by the retention of bioactivity in apratoxins F (1.6), G (1.7) and H

(1.8).10,11 Oxidation of the thiazoline sulfur is detrimental to activity, as apratoxin A sulfoxide (1.9) is greater than one order of magnitude less potent (IC50 = 90 nM in H-460 cells).

Table 1.1. Biological activity of the apratoxins and apratoxin analogs.

cell line [IC50 (nM)]

KB H-460 HeLa HCT116 HT29 U20S compound LoVo (colon) (breast) (lung) (cervical) (colon) (colon) (bone) 1.1 0.36 0.52 10 1.21 1.4 10

1.2 10.8 21.3

1.3 0.73 1

1.4 2.6

1.5 72 21 59

1.6 2 36.7

1.7 14

1.8 3.4

1.9 89.9

dehydro- 85.1 37.6 1.1 1.1- 9.7 oxazoline 1.3- 920 oxazoline C37-epi- 1.3- >10000

oxazoline C37- desmethyl >10000 -1.3- oxazoline

Additionally, a variety of apratoxin A analogs have been isolated or synthesized.

It is clear that the presence of the C35 hydroxyl is important, as dehydro-1.1 shows greatly reduced activity.7 Ma and coworkers have synthesized a variety of analogs structurally related to 1.1.13 While the oxazoline analog of 1.1 appears to retain activity

(IC50 = 9.7 nM in HeLa cells), this result does not seem to be uniform, as 1.3-oxazoline loses activity (920 nM). The stereochemical configuration of C37 and the presence of the

methyl group are clearly very important, as all activity is lost in C37-epi-1.3-oxazoline and its C37-desmethyl derivative (both IC50 >10000 nM in HeLa cells). It has been determined that 1.1 induces G1-phase cell cycle arrest and apoptosis.12,14 The G1-phase of the cell cycle is the period in which organelles are synthesized for cell growth and eventual division.

1.1.4. Previous synthetic studies

1.1.4.1 Syntheses of the apratoxins and common strategies

The first synthesis of a member of the apratoxin family was completed by

Forsyth and Chen, with the initial report of the total synthesis of apratoxin A in 2003.15,16

Later syntheses were completed by Takahashi and Doi,17-19 and Ma et al.13 The total synthesis of apratoxin C was reported by Doi and coworkers in 2014,20 and Luesch et al. reported the total synthesis of an apratoxin A/E hybrid in 2011.21 All reported syntheses share two strategies in common. First, macrocyclization is completed between the isoleucine and proline residues in order to avoid difficulties with macrolactonization between the proline residue and the sterically congested C39 hydroxyl group. Second, the thiazoline moiety is constructed in the final steps of the syntheses in order to avoid undesired side reactions with this sensitive functionality.

1.1.4.2 Forsyth’s synthesis of apratoxin A

Forsyth’s retrosynthetic analysis led to dissection of the molecule at the proline- isoleucine amide bond in order to avoid difficulties with late-stage macrolactonization at the sterically congested C39 hydroxyl group. The group envisioned late-stage thiazoline formation via an intramolecular Staudinger-aza-Wittig reaction, targeting α- azidothioester 1.10 (Scheme 1.1). Further deconstruction leads to polyketide–prolyl ester

1.11 and triamide 1.12.

O N N N OMe O HN MeO O N O O O O N NBoc O HN O OH N O N O S 3 O TESO O OMe 1.1 S 1.10

N NBoc MeO O O O N O O TBSO O + HN OH PMBO O 1.11 OMe HS 1.12

Scheme 1.1. Forsyth’s retrosynthetic dissection of 1.1.

Forsyth’s synthesis began with chiral homoallylic alcohol 1.13, which was derived from a Brown allylation of pivaldehyde (Scheme 1.2).22 Acylation with acrylic

acid mediated by N-methyl-2-chloropyridinium iodide23 provided diene 1.14. Ring- closing metathesis24 provided an α,β-unsaturated δ-valerolactone, and the C37 stereocenter was set by a substrate-controlled conjugate addition of a higher-order

Gilman-type methyl organocuprate to provide 1.15.25 The resulting saturated lactone was reduced with LiAlH4 and selective protection of the primary alcohol as the tert- butyldimethylsilyl (TBS) ether yielded 1.16. Yamaguchi esterification26 with N-Boc-L- proline and subsequent deprotection of the primary alcohol with tetrabutylammonium fluoride (TBAF) provided primary alcohol 1.17. Ley oxidation [tetrapropylammonium perruthenate (TPAP) oxidant with N-methylmorpholine-N-oxide (NMO) cooxidant] provided aldehyde 1.18. A Paterson anti-aldol27 with chiral benzoyloxypentanone 1.19 set the C34-C35 stereogenic centers. Oxidative cleavage of the β-hydroxyketone provided carboxylic acid 1.11 as the completed polyketide fragment.

Cl N I O O OH Me O O acrylic acid, Et3N 1. Grubb's I, 97% 1. LiAlH4, 83% 71% 2. Me CuCNLi , 86% 2. TBSCl, 1.13 2 2 1.14 1.15 imidazole, 98% NBoc 1. N-Boc-Pro-OH OH OTBS O Yamaguchi reagent, 91% TPAP, NMO, 89% O OH 2. TBAF, 88% 1.16 O 1.17 NBoc NBoc NBoc 1. 1.19 OBz O O O Cy BCl, Me NEt 1. K2CO3, MeOH O O 2 2 O TBSO O O OH O

2. TBSOTf, 2. NaIO4, MeOH/ H OH 2,6-lutidine H2O 1.18 74% (2 steps) 1.20 OBz 1.11 75% (2 steps)

Scheme 1.2. Forsyth’s synthesis of polyketide fragment 1.11. 10

C-Me-tripeptide 1.21 was assembled in a C→N manner using traditional Boc- strategy peptide couplings. The latent modified cysteine surrogate 1.22 was coupled with tripeptide portion, providing 1.23 (Scheme 1.3). The protected primary alcohol was converted to the primary thiol 1.24 over a three-step sequence. Thioester formation and cleavage of the para-methoxybenzyl (PMB) ether delivered alcohol 1.25. Conversion to the azide 1.26 under Mitsunobu conditions with diphenylphosphoryl azide (DPPA) set the stage for the Staudinger-aza-Wittig reaction. Prior to thiazoline formation, the C35 protecting group had to be exchanged to the smaller triethylsilyl ether due to difficulties with deprotection after macrocyclization. Thus, treatment with HF-pyridine followed by triethylsilyl trifluoromethanesulfonate (TESOTf) gave TES-ether 1.10.

OMe OMe

PMBO PyAOP, Et N, 76% TBSO O 3 O CO H + 2 N N 1.22 N PMBO N NH2 O CO2Me TBSO NH O CO2Me

1.21 O 1.23 OMe

1. HF-pyridine O 2. DIAD, PPh3, AcSH 1. 1.11, DPPA, PPh3, 97% N PMBO 3. K2CO3, MeOH N 2. DDQ, 93% HS NH O CO2Me

O 1.24

N N MeO O MeO O O O N O O N NBoc DPPA, PPh3, NBoc HN DIAD, 97% HN O O HO O N3 O O TBSO O OMe O OR O OMe S S 1.25 1.26 R = TBS 1. HF-pyridine 1.10 R = TES 2. TESOTf, 2,6-lutidine, 86% (2 steps)

Scheme 1.3. Synthesis of azide 1.10.

Treatment of 1.10 with triphenylphosphine successfully installed the thiazoline without issue to provide 1.27 (Scheme 1.4). Removal of the Boc group on the N-terminus followed by saponification of the C-terminus provided macrocyclization precursor 1.28.

Macrocyclization with PyAOP coupling reagent and deprotection of the C35 alcohol successfully provided apratoxin A 1.1 in 28 steps (longest linear sequence and 3.1% overall yield.

N N MeO O O O O N MeO N O NBoc O PPh , 63% HN 3 NBoc HN O O OMe N O O TESO O 3 O OMe O TESO N S S

1.10 1.27

N O HO N 1. TBSOTf, 2,6-lutidine O O 1. PyAOP, iPr NEt 2. TBAF, 86% (2 steps) NH HN 2 3. LiOH 73% (2 steps) O OMe 1.1 O 2. HF-pyridine, 65% O TESO N S

1.28

Scheme 1.4. Forsyth’s Staudinger-aza-Wittig and macrocyclization to complete apratoxin A (1.1).

1.1.4.3 The Takahashi/Doi synthesis of apratoxin A

The Takahashi and Doi synthesis of apratoxin A provided the overall protecting group/thiazoline formation strategies for all following syntheses.28 Similar disconnections to those of Forsyth were followed, with the exception of the thiazoline formation and protecting group strategy. Apratoxin A 1.1 was deconstructed into tripeptide 1.30 and polyketide-modified cysteine 1.29 (Scheme 1.5). The intended thiazoline formation would be performed on compound 1.31 using Kelly’s biomimetic thiazoline synthesis.29 They employed an Fmoc protecting group at the N-terminus

(proline), allyl groups at the C-termini (vinylogous cysteine and isoleucine), and a 2,2,2- trichloroethylcarbonate (Troc) protecting group at the C35 hydroxyl.

OMe O N PMB O N N AllylO2C O NH2 N 1.30 O HN N O O NFmoc CO2H

O O O OH N O OH N S S 1.1 1.29

NFmoc

O Troc TrtS O O O

N CO2Allyl H 1.31

Scheme 1.5. The Takahashi/Doi retrosynthetic disconnection of 1.1.

Their synthesis began from chiral β-hydroxyketone 1.32, derived from a proline- catalyzed aldol reaction between acetone and pivaldehyde (Scheme 1.6). Protection of the C39 hydroxyl as the PMB ether, Grignard reaction with allylmagnesium bromide and protection of the resulting alcohol provided 1.33. Palladium-mediated alkene isomerization to the internal olefin and removal of the acetate protecting group yielded primary allylic alcohol 1.34. The C37 stereogenic center was installed by asymmetric hydrogenation of the allylic alcohol to give 1.35. In a manner analogous to Forsyth, the primary alcohol was oxidized to the aldehyde via Swern oxidation, and the aldehyde was reacted in a Paterson anti-aldol to set the C34-C35 stereogenic centers in 1.36. This 14

intermediate was advanced to the polyketide fragment 1.11 in steps highly similar to those of Forsyth.

1. PMBimidate, TfOH, 90% 1. PdCl (MeCN) OH O 2. AllylMgBr PMBO OAc 2 2 PMBO 3. Ac2O, pyridine, DMAP 2. K2CO3, MeOH OH 50% (4 steps) 1.32 1.33 1.34

Ru[(S)-BINAP](OAc)2, PMBO PMBO OH O H2 (100 atm) 1. (COCl)2, DMSO, 96% OH 100% 2. 1.19, Cy2BCl, Me2NEt 1.35 1.36 OBz

NBoc

O O TBSO O

OH 1.11

Scheme 1.6. The Takahashi/Doi synthesis of polyketide fragment 1.11.

Polyketide fragment 1.11 and 1.37 were condensed with HATU peptide coupling reagent to provide amide 1.38 (Scheme 1.7). In order to simplify completion of the synthesis, the N-terminal protecting group (Boc→Fmoc) and the C35 protecting group

(TBS→Troc) was exchanged in a four-step sequence to give 1.39 and set the stage for thiazoline formation. Treatment of 1.39 with triphenylphosphine oxide and trifluoromethanesulfonic anhydride induced thiazoline formation per the Kelly protocol to yield 1.40. Subsequent treatment with Zn/NH4OAc to remove the Troc group and

Pd(PPh3)4 and N-methylaniline as an allyl trapping agent revealed the targeted acid 1.29.

TrtS NBoc NBoc H2N CO2Allyl O 1.37 O TrtS O TBSO O O TBSO O HATU, iPr2NEt, 85%

OH N CO2Allyl H 1.11 1.38

NFmoc

1. TBAF, 95% O Troc 2. TMSOTf, 2,6-lutidine TrtS P(O)Ph , Tf O O O O 3 2 3. FmocOSu, iPr2NEt, 96% N CO2Allyl 4. TrocCl, pyridine, 84% H 1.39

NFmoc NFmoc CO2Allyl CO2H

O Troc O O O N 1. Zn, NH4OAc, 90% O OH N S S 2. PdPPh3)4, N-Me-aniline, 95% 1.40 1.29

Scheme 1.7. Protecting group exchange, thiazoline formation and synthesis of alcohol 1.29.

Completion of the synthesis proceeded smoothly. Coupling of acid 1.29 and 1.30 provided macrocyclization precursor 1.41 (Scheme 1.8). Sequential deprotection of the

N- and C-termini and HATU-mediated led to the successful synthesis of 1.1 in 25 steps

(longest linear sequence) and 4.6% total yield.

NFmoc CO2H O O N PMB O OH N N HATU, iPr2NEt, 71% + S AllylO2C O NH2 1.29 1.30

N O AllylO N 1. Pd(PPh ) , O O 3 4 N-Me-aniline NFmoc HN 2. Et2NH O OMe 1.1 O 3. HATU, iPr2NEt O OH N 53% (3 steps) S

1.41

Scheme 1.8. Completion of 1.1 by Takahashi and Doi.

1.1.5. Apratoxin D

1.1.5.1 Background

Lung cancer is a profoundly devastating disease. It is the deadliest form of cancer for both men and women and is responsible for 1.3 million deaths world wide, annually. More people die of lung cancer than breast, colon, and prostate cancers combined. Presently, only three in ten patients diagnosed with lung cancer are cured.30

There are two general types of lung cancer, small cell lung carcinomas (SCLC) and non- small cell lung cancers (NSCLC). Of the two, NSCLC tumors are particularly difficult to treat, as they are insensitive to chemotherapy. Adenocarcinoma is the most common type of NSCLC (accounting for 40% of all cases), and current therapies are only effective if the cancer is detected early on, which, unfortunately, is rarely the case. Late stage

cancer patients (Stage IIIB and beyond) require intensive treatment regiments that lead to increased incidence of severe side effects and, moreover, these treatments are typically only palliative, with no hope of a cure.31 Unfortunately, there is a serious dearth of treatments available for these late stage lung cancer patients and, as a result, the survival rate for Stage IIIB patients and beyond is less than 5%. This exceptionally poor prognosis poignantly underscores the need for the development of new therapeutic treatments.

Apratoxin D (1.4) was recently isolated from two species of cyanobacteria, L. majuscula and L. sordida.8 It exhibits highly potent in vitro cytotoxicity against H-460 human lung cancer cells with an IC50 value of 2.6 nM. All known apratoxins are potent inhibitors of cancer cell growth (IC50 < 72 nM). The potent biological activity exhibited by the apratoxins in general, combined with their intriguing molecular architecture has drawn considerable attention from the synthetic community. Given the biological importance of apratoxin D, it is desirable to develop a synthetic route toward this compound and structural analogs. Herein, we report our studies toward and the completion of the first asymmetric total synthesis of apratoxin D (1.4).

1.1.5.2 Retrosynthetic analysis of apratoxin D

Our plan for the synthesis of apratoxin D is shown in Scheme 1.9.

Macrocyclization would be achieved by coupling between the proline and isoleucine residues, thereby avoiding late-stage esterification of the sterically congested C39

hydroxyl.15 Coupling of tripeptide 1.30 and carboxylic acid 1.42 would set the stage for the macrocyclization event. Formation of the thiazoline moiety of 1.42 would be achieved by using Kelly’s procedure,29 which would require the preparation of D- cysteine-derived intermediate 1.37 and polyketide fragment 1.44.

OMe O N PMB O N peptide AllylO2C O NH2 N coupling macrolactamization N 1.30 O HN N O O NFmoc CO2H

O O O OH N O OH N S S 1.42 1.4 Apratoxin D Kelly thiazoline formation

NBoc NBoc

TrtS O O O TBSO O TrtS O TrocO O 34 37 35 OH H2N CO2Allyl N CO2Allyl H 1.44 1.37 1.43 amidation

Scheme 1.9. Retrosynthetic dissection of apratoxin D (1.4).

The polyketide domain 1.44 offers a unique synthetic challenge. While we could utilize prior syntheses as guidelines for protection strategies and late-stage manipulations, the novelty of the polyketide tail in 1.4 required a new strategy for early- stage synthesis of the polyketide fragment, notably the stereogenic center at C40 not present other members of the apratoxin family. We envisioned three possible routes to

synthesize the polyketide domain and set the relative configuration and absolute stereochemistry at C37 and C39. In our initial synthetic approach (Scheme 1.10), the

C34—C35 stereogenic centers would be installed with a Paterson anti-aldol, the proline ester introduced with a Yamaguchi esterification, and the C37 stereogenic center would be installed with a substrate-controlled conjugate addition on 1.47 in the manner of

Forsyth. Fragment 1.48 would be reached from an Evans syn-selective aldol with 3- butenal, which would be leveraged to install the C41 tert-butyl group, giving rise to the neopentyl moiety.

lactone NBoc Yamaguchi reduction O substrate-controlled esterification conjugate addition O O TBSO O OH OH O 35 35 37 34 OH

1.44 Paterson 1.45 1.46 anti-aldol acylation Grubbs Kambe-Terao O O ring-closing coupling metathesis O O O 39 39 H 40 1.49 1.48 1.47 Evans syn-aldol

Scheme 1.10. Initial retrosynthetic dissection of 1.44.

As an alternative approach, polyketide 1.44 can be constructed in a similar manner, with the C37 absolute stereochemistry being derived from our recently developed ACC bisalkylation of hydrazone 1.51 derived from ketone 1.52 (Scheme

1.11).32

Yamaguchi Lemieux-Johnson NBoc NBoc ACC esterification oxidation bisalkylation O O OH O O O O BnO 35 35 34 OH 35 H N 1.50 ACC 1.44 Paterson 1.51 anti-aldol

Evans syn-aldol O BnO 37 39 H 37 1.52 O 1.53 Kambe-Terao Lemieux-Johnson coupling oxidation

Scheme 1.11. Retrosynthetic dissection of 1.44 to give methacrolein.

Finally, we envisioned the configuration of the C37 stereogenic center could be established from the chiral pool (Scheme 1.12). Therefore, aldehyde 1.54 would be the target for use in the Evans syn-aldol reaction. Aldehyde 1.54 could be derived from commercially available (R)-citronellic acid (1.55).

NBoc NBoc Yamaguchi esterification O O TBSO O O O O O OBn 35 39 39 34 OH 40 H H 37 1.50 1.54 1.44 Paterson Kambe-Terao Evans anti-aldol coupling syn-aldol O

39 37 OH 1.55

Scheme 1.12. Retrosynthesis of 1.44 leading to (R)-citronellic acid 1.55.

1.2. Results and Discussion

1.2.1. Synthesis of polyketide from 3-butenal

In our initial route (Scheme 1.13), synthesis of the polyketide fragment 1.44 began from a commercially available aqueous solution of glyoxal 1.56. A tin-mediated

Barbier reaction with allyl bromide in THF/H2O under ultrasound gave diol 1.57, which was oxidatively cleaved with sodium periodate in biphasic DCM/H2O to give 3-butenal

1.49 as a solution in DCM in ca. 50% yield on multigram scale.33 Reaction of 3-butenal with acyl oxazolidinone 1.58 under the Crimmins modification of the Evans syn-aldol conditions gave β-hydroxyimide 1.59 in 90% yield as a single diastereomer after silica gel chromatography.34,35 The hydroxy group was protected as the triethylsilyl (TES) ether prior to reductive cleavage of the chiral auxiliary to give primary alcohol 1.61 in 89% yield. Subsequent conversion of the primary alcohol to the tosylate 1.62 set the stage for introduction of the tert-butyl group under slightly modified conditions (cat. 10% CuCl2,

25% 1-phenyl-1-propyne)36 of the cross-coupling recently developed by Terao and

Kambe (1.62 → 1.63).36,37 Deprotection of the secondary hydroxyl group with tetrabutylammonium fluoride (1.63 → 1.64) and acylation with acryloyl chloride provided diene 1.48. Treatment with Grubbs 2nd generation catalyst gave α,β- unsaturated δ-valerolactone 1.47.

O Br OH NaIO O H Sn, 4 H THF/H2O, CH2Cl2/H2O, H O OH Ultrasound 0 °C → rt, 1.56 1.57 1.49 < 50%

TiCl , ( )-Sparteine, O O 4 − O O OH OTES 1.49 NaBH4, H2O O N O N RO CH2Cl2, 0 °C, 90% THF, 0 °C → rt, 89% Bn Bn TsCl, 1.58 1.59 R = H 1.61 R = H 2,6-Lutidine, TESOTf Pyridine, 1.60 R = TES CH2Cl2, 0 °C, 97% 1.62 R = Ts 89%

O O O t-BuMgCl, 10% CuCl2, OR 25% Ph Me i-Pr2NEt, Cl O Grubbs 2nd gen. O THF, reflux, 48 h t-Bu CH2Cl2, 0 °C → rt, 70% CH Cl , reflux, 97% t-Bu 2 2 t-Bu 1.63 R = TES TBAF, 1.48 1.47 1.64 R = H THF, 75% (2 steps)

Scheme 1.13. Synthesis of α,β-unsaturated lactone 1.47 from glyoxal 1.56.

Conjugate addition to this lactone provided methylated product in 95% yield as a single diastereomer following column chromatography. Origins of selectivity involve the avoidance of a steric interaction between the incoming nucleophile and the bulky R- group (Scheme 1.14, inset), resulting in trans-addition of the incoming organocuprate relative to the C5 substituent.25 It is energetically favorable for the R-group to adopt the axial orientation due to the minimization of Gauche interactions and the lack of 1,3- diaxial interactions in the system. This addition is analogous to a similarly described situation involving the stereoelectronic control of conjugate additions to 5-alkyl-2- cyclohexenones.38

"Me:" O O R Me2CuCNLi2 H O O O H H O O Et2O, −78 °C, H t-Bu t-Bu 95% H O R 1.47 1.46 H "Me:"

OH OTBS 1) N-Boc-Proline, i-Pr2NEt, t-Bu 2,4,6-trichlorobenzoyl chloride, THF LiAlH4 2) DMAP, Toluene, 93% Et2O, 0 °C, 94% 1.45 R = H TBSCl, Imidazole 1.65 R = TBS CH2Cl2, 93%

Boc N O Boc Boc O OR 4Å mol. sieves, N O N O Pr4NRuO4, NMO t-Bu O O O TBSO O CH2Cl2 t-Bu H t-Bu OH 1.66 R = TBS TBAF, 1.67 R = H THF, >99% 1.50 1.44

Scheme 1.14. Synthesis of aldehyde 1.50 from 1.47.

Having δ-valerolactone 1.46 in hand, the synthesis of the polyketide fragment continued with reductive opening of the lactone using LiAlH4, giving diol 1.45 in 94% yield (Scheme 1.14). The primary alcohol was selectively protected in the presence of the secondary alcohol using TBSCl and imidazole providing 1.65. This allowed for selective incorporation of the proline ester at the C39 hydroxyl group via a Yamaguchi esterification, analogous to the early strategy of Forsyth.15,16 Deprotection using TBAF gave primary alcohol 1.67 and Ley oxidation to aldehyde 1.50 set the stage for completion of the polyketide fragment.

Unfortunately, the initial route was plagued by supply issues. Synthesis of 3- butenal 1.49 in sufficient quantities and purity hampered the eventual supply of

aldehyde 1.50, and a well-known shortage of (-)-sparteine in 201039,40 made it necessary to explore other routes concurrently. Attempted Evans aldol reaction using 1.58 and 1.49 failed because of complications with organotin residues being carried through from the

Barbier reaction.

1.2.2. Synthesis of polyketide fragment via ACC alkylation of an advanced

intermediate

As an alternative route, the synthesis of 1.4 was pursued through ketone 1.52.

Our plan hinged on the successful α,α-bisalkylation of ACC hydrazone 1.51 (Scheme

1.11), which required ketone 1.52. The synthesis of 1.52 began with a syn-selective Evans aldol addition between N-acyl oxazolidinone 1.58 and methacrolein 1.53 (Scheme

1.15). Reductive removal of the auxiliary from the aldol product (1.68), followed by benzylidene acetal formation, furnished compound 1.69 in high yields over the two steps.41 Next, the regioselective reductive ring opening of 1.69 was conducted using

DIBAL-H,42 which was followed by tosylation of the resulting alcohol to give compound 1.71. Installation of the t-Bu group was achieved using the Terao–Kambe cross-coupling reaction (1.71 → 1.72).37 Finally, Lemieux–Johnson oxidation of 1.72 yielded the desired methyl ketone (1.52) in 91% yield. Ketone 1.52 was later advanced to aldehyde 1.50 by other members of the Coltart group.43

Ph O O 1. n-Bu2BOTf, i-Pr2NEt O O OH DCM, 0 °C 1. LiBH4, H2O, Et2O O O O N O N 2. 1.53, –78 °C → 0 °C, 87% 2. PhCH(OMe)2, CSA Bn Bn 1.58 1.68 1.69

t-BuMgCl, CuCl , 1. OsO , NMO OBn OBn 2 OBn 4 DIBAL-H, DCM 1-phenyl-1-propyne 2. NaIO4, 91% 74% (three steps) RO THF, reflux, 86% O 1.52 1.70 R = H 1.72 TsCl, pyridine, 85% 1.71 R = Ts

1.50

Scheme 1.15. Synthesis of ketone 1.50 from methacrolein.

1.2.3. Synthesis of polyketide fragment from (R)-citronellic acid

Construction of polyketide fragment 1.44 began with the synthesis of chiral, nonracemic aldehyde 1.54, which would be derived from commercially available (R)- citronellic acid. Reduction to the corresponding alcohol (1.73), protection as the benzyl ether and selenium-mediated oxidation44 gave allylic alcohol 1.75, which was then subjected to ozonolysis with dimethylsulfide workup to produce the desired chiral aldehyde 1.54 in 58% total yield for 4 steps (Scheme 1.16).

O OH OBn LiAlH4 NaH, BnBr, Bu4NI

OH Et2O, 0 °C to rt THF, 93% (2 steps) 1.55 1.73 1.74 1. H O , PhSeSePh, 2 2 OH OBn O OBn MgSO4, CH2Cl2 1. O3, MeOH, -78 °C 37 2. t-BuOOH, 80% 2. Me2S, -78 °C to rt H 1.75 78% 1.54

Scheme 1.16. Synthesis of aldehyde 1.54 from (R)-citronellic acid.

With an effective route to aldehyde 1.54 secured, we undertook the preparation of advanced carboxylic acid 1.44 (Scheme 1.17). To do so, N-acyl oxazolidinone 1.58 and 1.54 were engaged in an Evans syn-aldol addition.45 Silica gel chromatography of the product mixture gave the desired stereoisomer (1.76) in 83% yield, which was then converted to silyl ether 1.77. Reductive removal of the oxazolidinone auxiliary provided alcohol 1.78. With 1.78 in hand, we used adapted conditions for a cuprate-based alkylation of electrophiles with t-butyl Grignard reagents recently reported by Kambe and Terao.37 We were pleased to find that these conditions worked remarkably well in the case of tosylate 1.79, reliably providing compound 1.80.

We next removed the TES protecting group from 1.80 to enable incorporation of the C39 proline residue via the Yamaguchi procedure.26 This was followed by hydrogenolysis of the benzyl ether to give alcohol 1.67. Oxidation of 1.67 provided aldehyde 1.50, as required for the planned anti-selective Paterson aldol27 using chiral α-benzoyloxy ketone 1.19.

n-Bu2BOTf, i-Pr2NEt, O O CH Cl , 0 ºC, then O O OR OBn 2 2 NaBH4, THF TESO OBn 1.54, –78 to 0 ºC H2O (87%) O N O N 83% RO Bn Bn 1.78 R = H TsCl, Et N, DMAP, 1.58 1.76 R = H TESOTf, 2,6-lutidine, 3 1.79 R = p-Ts CH2Cl2, 0 ºC to rt 1.77 R = TES CH2Cl2, 0 ºC 97% 89%

1) N-Boc-L-Pro, 2,4,6-Trichlorobenzoyl chloride, i-Pr NEt, THF, DMAP, t-BuMgCl, CuCl2 2 1-Phenyl-1-propyne OR OBn Toluene (99%) THF, reflux 2) H2, Pd-C, MeOH (93%)

1.80 R = TES TBAF, THF; 1.81 R = OH 79% (from 1.79)

Boc Boc N O N O TPAP, NMO O OH 4 Å MS O CH2Cl2 CHO

1.50 1.67

Scheme 1.17. Synthesis of aldehyde 1.50 from aldehyde 1.54.

In the event, the aldol addition produced compound 1.82 in both excellent yield and diastereoselectivity. A sequence of TBS protection, benzoate hydrolysis, and oxidation produced carboxylic acid 1.44.

Boc Boc N O N O

Cy2BCl, Me2NEt 1) K CO , MeOH O O OR O 2 3 O OTBS Et2O, 1.50 2) NaIO4, THF, –78 ºC to –20 ºC t-BuOH, H2O CO2H 82% OBz OBz 1.44 1.19 1.82 R = H TBSOTf, 2,6-Lutidine 1.83 R = TBS CH2Cl2, –50 ºC 66% (from 1.50)

Scheme 1.18. Completion of the polyketide moiety.

1.2.4. Tripeptide incorporation, thiazoline formation and macrocyclization

As indicated above, we planned to generate the required thiazoline moiety via the Kelly procedure, which, in this instance, would be conducted on compound 1.43 (Scheme 1.19). Preparation of 1.43 began with the coupling of carboxylic acid 1.44 and compound 1.37 (previously prepared by Takahashi and Doi).28 At this stage the Boc protecting group on the proline residue was exchanged for an Fmoc group

(Boc removal after thiazoline formation gave C34—C35 dehydrated product) and to simplify the pending macrocyclization event in a practical sense. In addition, the TBS group on the C37 hydroxyl was exchanged for a Troc group in order to aid in protecting group removal under gentle conditions following thiazoline formation, giving compound 1.85. Compound 1.85 smoothly underwent cyclization to produce the desired thiazoline (1.86) in excellent yield. Direct removal of the Troc group from 1.86 was followed by deprotection of the C-terminal carboxylic acid, providing advanced intermediate 1.42.

TrtS

HN CO Allyl Boc 2 O 1.37 N 1) TBSOTf, 2,6-Lutidine, CH2Cl2 HATU TBS TrtS 2) TBAF, THF, 0 °C i-Pr2NEt O O O 3) FmocOSu, i-Pr2NEt, THF/CH2Cl2, 0 °C to rt CH2Cl2 4) Cl3CCH2OC(O)Cl, Pyridine, DMAP, CH2Cl2 1.44 N CO2Allyl 87% H 88% 1.84

Fmoc N O Fmoc CO2Allyl N O Troc TrtS Ph3PO, O O O Tf2O, O OR N CH2Cl2, 0 °C N CO2Allyl H S 1.85

1.86 R = Troc Zn, NH4OAc, THF, H2O 1.87 R = H 91% (two steps)

Pd(PPh ) , Fmoc CO2H 3 4 O PhNHMe, THF N O OH N S

1.42

Scheme 1.19. Synthesis of carboxylic acid 1.42 from 1.44.

With compound 1.42 in hand, we embarked on the final steps of the synthesis of apratoxin D (Scheme 1.20). Thus, 1.42 was coupled to the tripeptide 1.3028 using HATU and Hünig’s base, giving 1.88 in 93% yield over two steps. At this stage, conversion of the allyl ester to the corresponding carboxylic acid was achieved by treatment with

Pd(PPh3)4 and N-methylaniline. Removal of the Fmoc moiety from the proline residue was followed by HATU-mediated macrolactamization, which successfully provided apratoxin D 1.4 in 2.1% yield over 29 steps (longest linear sequence) from (R)-citronellic acid 1.55. The spectrometric data obtained for our synthetic material matched in all respects with the data reported for its isolation (see section 1.3).8

O N PMB N AllylO C O HN 2 O Fmoc N O

1.30, HATU O OH N i-Pr2NEt, CH2Cl2 1.42 S 93% (from 1.87) 1.88

O N PMB 1) Pd(PPh3)4, THF N N-Methylaniline O HN N O O 2) Et2NH, MeCN 3) HATU, i-Pr2NEt O CH Cl 2 2 O OH N 23% S

Apratoxin D 1.4

Scheme 1.20. Completion of apratoxin D.

1.2.5. Summary and conclusions

In conclusion, we have achieved the first asymmetric total synthesis of apratoxin

D, a compound that exhibits highly potent cytotoxicity against H-460 human lung cancer cells. Evaluation of the cytotoxicity of our synthetic material against a range of cancer cell lines, as well as related SAR studies, are underway and will be described in due course.

1.3. Experimental

1.3.1. General methods

Unless stated to the contrary, where applicable, the following conditions apply:

Reactions were carried out using dried solvents (see below) and under a slight static pressure of Ar (pre-purified quality) that had been passed through a column (5 x 20 cm) of Drierite. Glassware was dried in an oven at 120 °C for at least 12 h prior to use and then either cooled in a desiccator cabinet over Drierite or assembled quickly while hot, sealed with rubber septa, and allowed to cool under a stream of Ar. Reactions were stirred magnetically using Teflon-coated magnetic stirring bars. Teflon-coated magnetic stirring bars and syringe needles were dried in an oven at 120 °C for at least 12 h prior to use then cooled in a desiccator cabinet over Drierite. Hamilton microsyringes were dried in an oven at 60 °C for at least 24 h prior to use and cooled in the same manner.

Commercially available Norm-Ject disposable syringes were used. Dry benzene, toluene,

Et2O, CH2Cl2, THF, MeCN, and DME were obtained using an Innovative Technologies solvent purification system. All other dry solvents were of anhydrous quality purchased from Aldrich. Commercial grade solvents were used for routine purposes without further purification. Et3N, pyridine, i-Pr2NEt, 2,6-lutidine, i-Pr2NH, and TMEDA were distilled from CaH2 under a N2 atmosphere prior to use. Flash column chromatography was performed on silica gel 60 (230–400 mesh). 1H and 13C NMR spectra were recorded on a Varian Mercury 400 MHz spectrometer or Varian INOVA 500 MHz

spectrophotometer at ambient temperature. All 1H chemical shifts are reported in ppm

(δ) relative to TMS; 13C shifts are reported in ppm (δ) relative to CDCl3 (77.16). MS data were obtained using an Agilent 1100 Series liquid chromatography-electrospray ionization mass spectrometer. Chiral HPLC was performed on a 4.6 x 250 mm Chiralcel

OD-H column (Chiral Technologies) or a 4.6 mm x 250 mm Chiralpak AD-H column, using UV detection. Compounds 1.19,27 1.30,28 1.37,28 1.49,33 1.58,45 and 1.7046 were synthesized using published procedures.

(R)-Citronellol 1.73. To a stirred suspension of lithium aluminum hydride (3.0 g,

79 mmol) in Et2O (400 mL) at 0 °C was added via cannula a solution of (R)-citronellic acid (9.85 g, 57.9 mmol) in Et2O (50 mL). The reaction was stirred for 15 min at 0 °C, warmed to rt and stirred an additional 2 h. The reaction was cooled to 0 °C before being carefully quenched with H2O (20 mL) and stirred 15 min. The resulting suspension was filtered and the filtercake was washed with EtOAc. The filtrate was dried over MgSO4, filtered and concentrated under reduced pressure. The product was isolated as a colorless oil (9.03 g) and used without further purification.

(R)-Citronellol benzyl ether 1.74. To a stirred suspension of NaH (60% dispersion in mineral oil, 5.76 g, 144 mmol) in THF (200 mL) at 0 °C was added dropwise via cannula a solution of (R)-citronellol 1.73 (9.03 g, ~57 mmol) in THF (50

mL). The resulting mixture was stirred at 0 °C for 30 min (gas evolution) before Bu4NI

(211 mg, 0.571 mmol) and BnBr (7.16 mL, 60.3 mmol) were added. The reaction mixture was allowed to gradually warm to rt while stirring overnight. The reaction was cooled to 0 °C and carefully quenched with small ice chips. The mixture was diluted with

EtOAc and the separated aqueous phase was extracted with EtOAc (x3). The combined organic phases were washed with H2O, brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (gradient,

0:100 EtOAc-hexanes to 5:95 EtOAc-hexanes) gave 1.74 (13.27 g, 93% for two steps) as a colorless oil. Spectroscopic data was identitical to previously reported.47

Allylic alcohol 1.75. To a stirred solution of PhSeSePh (25.3 g, 80.9 mmol) in

CH2Cl2 (270 mL) at 0 °C was slowly added H2O2 (30% in H2O, 8.26 mL, 80.9 mmol). The mixture was stirred vigorously at 0 °C (precipitate formation) for 30 min before MgSO4

(13.5 g) was added and stirring continued at the same temperature for an additional 30 min. (R)-Citronellol benzyl ether 1.74 (13.28 g, 53.9 mmol) was added and the mixture was allowed to warm to rt with stirring for 6 h. The mixture was cooled to 0 °C and chilled t-BuOOH (70% in H2O, 41.3 mL, 289 mmol) was added. The reaction mixture was stirred at rt for 20 h before being filtered (filtercake washed with Et2O). The filtrate was concentrated under reduced pressure. The residue was dissolved in Et2O (300 mL), washed twice with 5% aq. Na2CO3 (200 mL, 100 mL) and twice with water (100 mL). The

ether solution was then added dropwise to a vigorously stirred solution of 10% aq.

FeSO4 (300 mL). The organic phase was separated and washed with water, sat. aq.

NaHCO3, water and brine, successively. The organic phase was dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel

(20:80 EtOAc-hexanes) gave 1.75 (11.30 g, 80%) as a pale yellow oil. Spectroscopic data was identitical to previously reported.47

Aldehyde 1.54. To a stirred solution of allylic alcohol 1.75 (1.82 g, 6.94 mmol) in

MeOH (85 mL) was added a saturated solution of Sudan III in MeOH until the solution had a definitive red color (~2 mL). The solution was cooled to -78 °C and ozone was bubbled into the solution until discoloration of the dye was observed. A stream of N2 was then bubbled into the solution at -78 °C for 15 min to purge the solution of excess ozone. Me2S (1.5 mL) was added dropwise at -78 °C and the reaction mixture was slowly warmed to rt with stirring overnight. The reaction mixture was then concentrated under reduced pressure. Flash chromatography over silica gel (10:90 EtOAc-hexanes) gave 1.54

(1.12 g, 78%) as a pale yellow oil, which was placed under high vacuum for 2 h and immediately used in the subsequent reaction. 1H NMR (CDCl3, 500 MHz): δ 9.74 (s, 1 H),

7.36-7.26 (m, 5 H), 4.49 (s, 2 H), 3.52 (t, J= 5.5 Hz, 2 H), 2.46-2.42 (m, 1 H), 2.32-2.22 (m, 1

H), 1.70-1.51 (m, 4 H), 0.98 (d, J= 6.0 Hz, 3 H); 13C NMR (CDCl3, 125 MHz): δ 202.9, 138.5,

128.5, 127.8, 127.7, 73.1, 68.1, 51.1, 36.7, 25.5, 20.2; ESI-MS m/z calcd for [C13H18O2 + Na]+:

229.1, found: 229.2.

syn-Aldol 1.76. To a stirred solution of acyl oxazolidione 1.5845 (1.04 g, 4.45 mmol) in CH2Cl2 (10 mL) at 0 °C was added dropwise n-Bu2BOTf (1.0 M in CH2Cl2, 4.67 mL, 4.67 mmol) and stirred 10 min. i-Pr2NEt (0.930 mL, 5.34 mmol) was then added and the reaction was stirred at the same temperature for 1 h. The mixture was cooled to –78

°C before a solution of aldehyde 1.54 (1.01 g, 4.90 mmol) in CH2Cl2 (15 mL) was added dropwise via cannula. Stirring was continued at –78 °C for 3 h before gradually warming to 0 °C with stirring for an additional 3 h. The reaction was quenched by the addition of 0.1 M pH 7 phosphate buffer (7.5 mL) followed by MeOH (10 mL) at 0 °C.

After stirring for 5 min, a solution of 30% aqueous H2O2 (7.5 mL) in MeOH (15 mL) was added dropwise and stirred at the same temperature for 1 h before being concentrated under reduced pressure. The concentrate was diluted with Et2O, the phases separated and the aqueous phase extracted with Et2O (x3). The combined organic phase was washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (35:65 EtOAc-hexanes) gave 1.76 (1.63 g,

83%) as a viscous, colorless oil; 1H NMR (400 MHz, CDCl3): δ 7.36-7.25 (m, 8H), 7.20 (d, J

= 6.9 Hz, 2H), 4.69 (m, 1H), 4.50 (d, J = 2.9 Hz, 2H), 4.25-4.17 (m, 2H), 4.09 (br m, 1H), 3.72

(dd, J = 2.6, 7.0 Hz, 1H) 3.58-3.47 (m, 2H), 3.25 (dd, J = 3.2, 13.4 Hz, 1H), 2.97 (br s, 1H),

2.78 (dd, J = 9.5, 13.4 Hz, 1H), 1.86-1.73 (m, 2H), 1.52-1.33 (m, 3H), 1.24 (d, J = 7.0 Hz, 3H),

0.95 (d, J = 6.6 Hz, 3H); 13C NMR (125 MHz, CDCl3): δ 177.6, 153.1, 138.6, 135.2, 129.5,

129.1, 128.5, 127.8, 127.6, 127.5, 73.1, 69.4, 68.7, 66.3, 55.2, 42.4, 41.2, 37.9, 36.1, 27.1, 20.7,

10.6; ESI-MS m/z calcd for [C26H33NO5 + Na]+: 462.2, found: 462.3.

Triethylsilyl ether 1.77. To a stirred solution of 1.76 (1.54 g, 3.50 mmol) and 2,6- lutidine (0.49 mL, 4.2 mmol) in CH2Cl2 (15 mL) at 0 °C was added TESOTf (0.830 mL,

3.67 mmol) and the reaction was stirred for 30 min at the same temperature. The reaction was quenched by the addition of MeOH (2 mL) and resulting mixture was concentrated under reduced pressure. The residue was taken up in Et2O and the organic phase was washed with sat. aq. NH4Cl. The aqueous phase was extracted with Et2O (x3) and the combined organic phase was dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (10:90 EtOAc-hexanes) gave 1.77

(1.72 g, 89%) as a colorless oil; 1H NMR (500 MHz, CDCl3): δ 7.35-7.25 (m, 8H), 7.22 (d, J

= 6.9 Hz, 2H), 4.57 (m, 1H), 4.49 (d, J = 6.2 Hz, 2H), 4.15-4.08 (m, 3H), 3.85 (dd, J = 4.2, 6.9

Hz, 1H), 3.55-3.47 (m, 2H), 3.29 (dd, J = 3.1, 13.4 Hz, 1H), 2.76 (dd, J = 9.7, 13.3 Hz, 1H),

1.71-1.63 (m, 2H), 1.61-1.56 (m, 1H), 1.46-1.39 (m, 1H), 1.38-1.32 (m, 1H), 1.18 (d, J = 6.9

Hz, 3H), 0.95 (d, J = 6.3 Hz, 3H), 0.93 (t, J = 7.9 Hz, 9H), 0.56 (q, J = 7.5 Hz, 6H); 13C NMR

(125 MHz, CDCl3): δ 175.2, 153.3, 138.8, 135.6, 129.6, 129.1, 128.5, 127.8, 127.6, 127.5, 73.1,

71.1, 68.5, 66.1, 56.1, 43.8, 43.4, 37.8, 37.2, 26.7, 20.5, 11.0, 7.1, 5.3; ESI-MS m/z calcd for

[C32H47NO5Si + Na]+: 576.3, found: 576.5.

Alcohol 1.78. To a stirred solution of 1.77 (1.70 g, 3.07 mmol) in THF (70 mL) at 0

°C was added a solution of NaBH4 (579 mg, 15.3 mmol) in H2O (18.5 mL). The resulting solution was stirred 10 min at 0 °C before being allowed to gradually warm to rt with stirring overnight. The reaction was quenched by the addition of sat. aq. NH4Cl (20 mL) and stirred at rt for 1 h. The separated aqueous phase was extracted with EtOAc (2 x 25 mL). The combined organic phase was washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (15:85

EtOAc-hexanes) gave 1.78 (1.02 g, 87%) as a colorless oil; 1H NMR (500 MHz, CDCl3): δ

7.34-7.32 (m, 3H), 7.30-7.26 (m, 2H), 4.50 (d, J = 4.1 Hz, 2H), 3.94 (m, 1H), 3.67 (m, 1H),

3.57-3.46 (m, 3H), 2.60 (br s, 1H), 1.89 (m, 1H), 1.66 (m, 2H), 1.52-1.47 (m, 1H), 1.43-1.39

(m, 1H), 1.34-1.28 (m, 1H), 0.95 (t, J = 7.9 Hz, 9H), 0.92 (d, J = 6.5 Hz, 3H), 0.80 (d, J = 7.1

Hz, 3H), 0.61 (q, J = 7.9 Hz, 6H); 13C NMR (125 MHz, CDCl3): δ 138.7, 128.5, 127.8, 127.6,

73.7, 73.2, 68.5, 66.5, 40.5, 39.1, 37.0, 26.8, 20.5, 11.3, 7.0, 5.3; ESI-MS m/z calcd for

[C22H40O3Si + Na]+: 403.3, found: 403.3.

Tosylate 1.79. To a stirred solution of 1.78 (1.135 g, 2.982 mmol) in CH2Cl2 (15 mL) at 0 °C was added Et3N (0.75 mL, 5.4 mmol), TsCl (682 mg, 3.58 mmol) and DMAP

(18 mg, 0.15 mmol) and the resulting mixture was allowed to warm to rt and stirred overnight. The reaction mixture was quenched by the addition of H2O (10 mL) and the aqueous layer was extracted with EtOAc (3 x 25 mL). The combined organic phase was washed with 10% HCl (2 x 5 mL), saturated aqueous NaHCO3 and brine, dried over

MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (12:88 EtOAc-hexanes) gave 1.79 (1.54 g, 97%) as a colorless oil; 1H NMR (500

MHz, CDCl3): δ 7.78 (d, J = 8.3 Hz, 2H), 7.35-7.26 (m, 7H), 4.48 (d, J = 1.9 Hz, 2H), 4.00

(dd, J = 6.9, 9.2 Hz, 1H), 3.87-3.81 (m, 2H), 3.51-3.43 (m, 2H), 2.44 (s, 3H), 1.91 (m, 1H),

1.61-1.38 (m, 4H), 1.16-1.10 (m, 1H), 0.87 (t, J = 7.9 Hz, 9H), 0.85 (d, J = 6.4 Hz, 3H), 0.78

(d, J = 6.9 Hz, 3H), 0.49 (m, 6H); 13C NMR (125 MHz, CDCl3): δ 144.7, 138.6, 133.3, 129.9,

128.4, 128.0, 127.7, 127.6, 73.1, 69.6, 68.3, 41.3, 37.3, 36.9, 26.6, 21.7, 20.0, 9.9, 7.0, 5.2; ESI-

MS m/z calcd for [C29H46O5SSi + Na]+: 557.3, found: 557.3.

Alcohol 1.81. To a stirred solution of 1.79 (1.475 g, 2.758 mmol), CuCl2 (37 mg,

0.27 mmol) and 1-phenyl-1-propyne (86 µL, 0.69 mmol) in THF (6 mL) was added a solution of t-BuMgCl (0.7 M in THF, 7.9 mL, 5.5 mmol). The resulting mixture was heated to reflux and stirred 24 h before being cooled to rt. The reaction was quenched by the addition of sat. aq. NH4Cl and stirred 30 min before being diluted with EtOAc. The separated aqueous phase was extracted with EtOAc (x2). The combined organic phase was washed with brine, dried over MgSO4, filtered and concentrated under reduced

pressure. Flash chromatography over silica gel (5:95 EtOAc-hexanes) gave silyl ether

1.80 as a partially purified pale yellow oil, which was used in the subsequent reaction without further purification. To a stirred solution of 1.80 (955 mg, ~2.3 mmol) in THF (7 mL) at rt was added a solution of TBAF (1.0 M in THF, 4.5 mL, 4.5 mmol). The reaction was allowed to stir until complete by TLC (~6 h). The mixture was diluted with Et2O (10 mL) and H2O (5 mL). The separated aqueous phase was extracted with Et2O (3 x 10 mL).

The combined organic phase was washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (20:80

EtOAc-hexanes) gave 1.81 (666 mg, 79% for 2 steps) as a colorless oil; 1H NMR (500

MHz, CDCl3): δ 7.36-7.32 (m, 4H), 7.30-7.25 (m, 1H), 4.50 (d, J = 3.6 Hz, 2H), 3.59-3.47 (m,

3H), 1.80-1.72 (m, 2H), 1.57-1.51 (m, 2H), 1.44-1.29 (m, 4H), 1.03 (dd, J = 6.7, 13.9 Hz, 1H),

0.94 (d, J = 6.6 Hz, 3H), 0.90-0.88 (m, 12H); 13C NMR (125 MHz, CDCl3): δ 138.6, 128.5,

127.8, 127.7, 74.5, 73.2, 68.9, 47.7, 41.8, 36.3, 34.6, 31.1, 30.0, 27.4, 20.9, 16.4; ESI-MS m/z calcd for [C20H34O2 + Na]+: 329.3, found: 329.3.

Alcohol 1.3.1. To a stirred solution of N-Boc-L-proline (1.17 g, 5.44 mmol) and i-

Pr2NEt (1.51 mL, 8.67 mmol) in THF (40 mL) was added 2,4,6-trichlorobenzoyl chloride

(1.02 mL, 6.52 mmol) and the reaction was stirred at rt for 3 h. The reaction mixture was then filtered through a pad of silica gel and the filtrate was concentrated under reduced pressure. The residue was dissolved in toluene (40 mL) and transferred to a solution of

alcohol 1.81 (610 mg, 1.99 mmol) in toluene (40 mL). DMAP (486 mg, 3.98 mmol) was added and the mixture was stirred at rt for 3 h. The reaction mixture was diluted with

EtOAc and washed with saturated aqueous NaHCO3 and brine. The combined aqueous phases were then extracted with EtOAc (x3). The combined organic phase was washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (17:83 EtOAc-hexanes) gave the proline ester 1.3.1 (988 mg, 99%) as a colorless oil; 1H NMR (400 MHz, CDCl3, mixture of rotamers): δ 7.34-7.32

(m, 4H), 7.29-7.25 (m, 1H), 4.97-4.91 (m, 1H), 4.49 (s, 2H), 4.31 (dd, J = 3.4, 10.7 Hz, 0.4H),

4.26 (dd, J = 3.4, 10.7 Hz, 0.6H), 3.57-3.34 (m, 4H), 2.23-2.11 (m, 1H), 2.00-1.91 (m, 1H),

1.91-1.83 (m, 2H), 1.76-1.68 (m, 2H), 1.66-1.56 (m, 1H), 1.56-1.20 (m, 13H), 1.04-0.83 (m,

16H); 13C NMR (125 MHz, CDCl3, mixture of rotamers): δ 173.0, 172.8, 154.3, 154.1, 138.7,

128.5, 127.8, 127.6, 127.5, 79.9, 79.6, 78.1, 77.8, 73.1, 73.0, 68.8, 68.5, 59.4, 59.3, 47.5, 47.1,

46.6, 46.3, 38.9, 38.7, 36.7, 36.4, 32.2, 32.1, 31.0, 30.0, 28.5, 27.2, 26.9, 24.4, 23.4, 20.4, 20.3,

16.9, 16.7; ESI-MS m/z calcd for [C30H49NO5 + Na]+: 526.4, found: 526.5.

Alcohol 1.67. To a solution of proline ester 1.3.1 (418 mg, 0.830 mmol) in MeOH

(5 mL) was added Pd/C (10 wt %, 181 mg, 0.17 mmol) and the flask was purged with H2.

The mixture was stirred under H2 (balloon pressure) overnight. The reaction mixture was filtered through a plug of celite and the filtrate was concentrated under reduced pressure. Flash chromatography over silica gel (25:75 EtOAc-hexanes) gave 1.67 (320 mg,

93%) as a viscous colorless oil; 1H NMR (500 MHz, CDCl3, mixture of rotamers): δ 4.99-

4.96 (m, 1H), 4.29-4.25 (m, 1H), 3.72-3.66 (m, 1H), 3.66-3.58 (m, 1H), 3.56-3.45 (m, 1H),

3.43-3.37 (m, 1H), 2.92 (br s, 1H), 2.25-2.15 (m, 1H), 2.05-1.83 (m, 3H), 1.82-1.63 (m, 3H),

1.60-1.53 (m, 1H), 1.45 (s, 6H), 1.43 (s, 3H), 1.40 (d, J = 13.9 Hz, 1H), 1.28 (t, J = 13.3 Hz,

1H), 1.10 (m, 1H), 1.02 (dd, J = 1.0, 13.8 Hz, 1H), 0.97-0.92 (m, 6H), 0.89 (s, 3H), 0.88 (s,

6H); 13C NMR (125 MHz, CDCl3, mixture of rotamers): δ 173.1, 154.5, 80.2, 80.0, 78.0,

76.6, 60.8, 60.4, 59.4, 59.0, 47.5, 46.8, 46.3, 46.1, 39.6, 39.5, 38.1, 37.7, 33.2, 32.6, 31.2, 31.1,

31.0, 30.1, 30.0, 28.6, 28.5, 26.3, 25.1, 24.5, 23.4, 20.5, 20.3, 18.1, 16.9; ESI-MS m/z calcd for

[C23H43NO5 + Na]+: 436.3, found: 436.4.

Aldehyde 1.50. To a stirred solution of 1.67 (534 mg, 1.29 mmol) and powdered 4

Å molecular sieves (650 mg) in CH2Cl2 (15 mL) were sequentially added 4- methylmorpholine-N-oxide (227 mg, 1.94 mmol) and tetrapropylammonium perruthenate (22.9 mg, 0.065 mmol) and the mixture was stirred at rt for 2 h. The reaction mixture was filtered through a pad of silica gel and the filtrate was concentrated under reduced pressure to give aldehyde 1.50 as a colorless oil. The residue was placed under high vacuum for 2 h before being used in the subsequent reaction without further purification; 1H NMR (400 MHz, CDCl3, mixture of rotamers) δ 9.73 (s, 0.4 H), 9.67 (d,

0.6 H, J= 3.2 Hz), 4.96-4.88 (m, 1 H), 4.28 (apparent d, 1 H, J= 9.2 Hz), 3.57-3.36 (m, 2 H),

2.64 (dd, 0.6 H, J= 4.0, 15.2 Hz), 2.57 (dd, 0.4 H, J= 5.2 Hz, 16.8 Hz), 2.27-2.17 (m, 2 H),

2.09-1.88 (m, 5 H), 1.75-1.70 (m, 1 H), 1.44, 1.43 (2 s, for a total of 9 H), 1.35-1.25 (m, 2 H),

1.01-0.94 (m, 7 H), 0.89, 0.88 (2 s, for a total of 9 H); 13C NMR (CDCl3, 200 MHz): δ 203.9,

202.1, 173.5, 79.8, 59.3, 59.2, 50.4, 49.8, 47.4, 46.8, 46.7, 46.4, 38.7, 38.3, 32.9, 32.5, 31.1, 31.0,

29.9, 28.6, 25.1, 24.6, 24.5, 23.4, 20.9, 20.6, 17.5, 16.9; ESI-MS m/z calcd for [C23H41NO5 +

Na]+: 434.3, found: 434.3.

anti-Aldol TBS-ether 1.83. To a stirred solution of chlorodicyclohexylborane (1.0

M in hexanes, 2.3 mL, 2.3 mmol) in Et2O (5 mL) at 0 °C was added Me2NEt (0.31 mL, 2.86 mmol) and stirred for 10 min. To this solution was transferred via cannula a solution of

(R)-2-benzoyloxy-3-pentanone 1.1927 (372 mg, 1.80 mmol) in Et2O (5 mL) and the resulting mixture was stirred at 0 °C for 1.5 h before being cooled to –78 °C. To this solution was transferred via cannula a solution of aldehyde 1.50 (~1.29 mmol) in Et2O (5 mL) and the reaction was stirred at –78 °C for 6 h. The reaction was allowed to gradually warm to –20 °C while stirring for an additional 20 h. The reaction was quenched by the addition of MeOH (10 mL), 0.1 M pH 7 phosphate buffer (10 mL) and 30% aq. H2O2 (10 mL) at –20 °C, warmed to rt and stirred 1 h. The reaction mixture was diluted with

CH2Cl2 and the aqueous phase was extracted with CH2Cl2 (x3). The combined organic phase was washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (15:85 EtOAc-hexanes) gave aldol product 1.82 as a highly viscous colorless oil. To a solution of this residue (~1.29

mmol) and 2,6-lutidine (0.38 mL, 3.26 mmol) in CH2Cl2 (25 mL) at –50 °C was added dropwise TBSOTf (0.59 mL, 2.57 mmol). The reaction was allowed to stir at the same temperature for 2 h before being diluted with Et2O (20 mL) and quenched with saturated aqueous NH4Cl (10 mL). The separated aqueous phase was extracted with Et2O (x3) and the combined organic phase was dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (gradient 15:85 to 30:70 EtOAc- hexanes) gave 1.83 (627 mg, 66% for 3 steps) as a viscous colorless oil as well as unprotected aldol product 1.82 (208 mg, 26%); 1H NMR (500 MHz, CDCl3, mixture of rotamers): δ 8.07 (d, J = 7.9 Hz, 2H), 7.58 (m, 1H), 7.47 (t, J = 7.8 Hz, 2H), 5.44 (q, J = 6.9

Hz, 1H), 4.90 (m, 1H), 4.31 (dd, J = 2.8, 6.0 Hz, 0.3H), 4.24 (dd, J = 2.8, 6.0 Hz, 0.7H), 4.08

(m, 1H), 3.55-3.36 (m, 2H), 3.08 (m, 1H), 2.17 (m, 1H), 1.96 (m, 1H), 1.87 (m, 2H), 1.71 (m,

1H), 1.65 (m, 1H), 1.54-1.50 (m, 3H), 1.48-1.40 (m, 9H), 1.40-1.22 (m, 3H), 1.16 (m, 3H),

1.10-0.96 (m, 3H), 0.95-0.90 (m, 6H), 0.88-0.82 (m, 18H), -0.01 (s, 3H), -0.05 (s, 3H); 13C

NMR (125 MHz, CDCl3, mixture of rotamers): δ 209.0, 172.8, 165.9, 154.3, 154.1, 133.5,

133.4, 130.0, 129.7, 129.6, 128.6, 110.1, 79.9, 79.5, 78.4, 78.2, 74.6, 70.3, 69.8, 59.5, 59.3, 48.8,

48.7, 48.2, 47.6, 46.5, 46.3, 41.3, 40.2, 39.9, 31.2, 31.0, 30.9, 30.2, 29.9, 29.0, 28.7, 28.6, 26.0,

25.8, 24.3, 23.5, 19.0, 18.7, 18.1, 16.1, 16.0, 15.9, 11.6, 11.2, -4.3; ESI-MS m/z calcd for

[C41H69NO8Si + Na]+: 754.5, found: 754.6.

Polyketide carboxylic acid 1.44. To a stirred solution of 1.83 (158 mg, 0.216 mmol) in MeOH (5 mL) was added K2CO3 (45 mg, 0.33 mmol). The reaction was stirred at rt until complete by TLC (~2 h). The mixture was then diluted with CH2Cl2 (25 mL) and washed with H2O (5 mL). The separated aqueous phase was extracted with CH2Cl2

(3 x 10 mL) and the combined organic phase was dried over MgSO4, filtered and concentrated under reduced pressure. The residue was dissolved in t-BuOH (4 mL) and

H2O (2 mL) and to this solution was added NaIO4 (231 mg, 1.08 mmol). The mixture was allowed to stir at rt for 16 h. Saturated aq. NH4Cl (5 mL) was added and the mixture was diluted with CH2Cl2. The separated aqueous phase was extracted with CH2Cl2 (3x) and the combined organic phase was dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (30:70 EtOAc-hexanes) gave 1.44

(106 mg, 82% for 2 steps) as a white foam; 1H NMR (400 MHz, CDCl3, mixture of rotamers): δ 4.90 (m, 1H), 4.35 (dd, J = 3.0, 8.6 Hz, 0.4H), 4.25 (dd, J = 3.0, 8.6 Hz, 0.6H),

4.07 (m, 1H), 3.56-3.34 (m, 2H), 2.69 (m, 1H), 2.19 (m, 1H), 2.01-1.83 (m, 3H), 1.74 (m, 1H),

1.62 (m, 1H), 1.56-1.48 (m, 2H), 1.46 (s, 3.5H), 1.43 (s, 5.5H), 1.35-1.21 (m, 3H), 1.17 (d, J =

8.0 Hz, 3H), 1.08-0.96 (m, 1H), 0.96-0.91 (m, 6H), 0.91-0.85 (m, 18H), 0.11 (s, 1H), 0.10 (s,

5H); 13C NMR (100 MHz, CDCl3, mixture of rotamers): δ 178.9, 178.3, 172.8, 172.6, 154.4,

154.0, 80.0, 79.8, 78.3, 78.0, 71.5, 71.1, 59.5, 59.2, 48.1, 47.2, 46.6, 46.3, 45.7, 45.5, 41.9, 41.6,

39.8, 39.1, 31.6, 31.2, 31.1, 31.0, 30.2, 30.0, 29.9, 28.9, 28.6, 26.1, 26.0, 25.9, 24.3, 23.5, 19.3,

19.0, 18.1, 16.5, 15.9, 11.8, 11.6, -4.3, -4.4, -4.5; ESI-MS m/z calcd for [C32H61NO7Si + Na]+:

622.4, found: 622.4.

Amide 1.84. To a stirred solution of 1.44 (181 mg, 0.302 mmol) in CH2Cl2 (3 mL) was added i-Pr2NEt (0.11 mL, 0.63 mmol) and HATU (149 mg, 0.392 mmol). The mixture was stirred for 5 min before a solution of amine 1.3728 (208 mg, 0.383 mmol) in CH2Cl2 (3 mL) was transferred to this solution via cannula. The reaction was stirred overnight at rt before being concentrated under a stream of nitrogen. Flash chromatography over silica gel (27:73 EtOAc-hexanes) gave 1.84 (270 mg, 87%) as a white foam; 1H NMR (500 MHz,

CDCl3, mixture of rotamers): δ 7.37 (d, J = 8.0 Hz, 6H), 7.28-7.24 (m, 6H), 7.22-7.19 (m,

3H), 6.91 (d, J = 8.2 Hz, 0.1H), 6.85 (d, J = 8.2 Hz, 0.15H), 6.77 (d, J = 8.1 Hz, 0.3H), 6.70 (d,

J = 8.1 Hz, 0.45H), 6.44 (dd, J = 1.6, 9.0 Hz, 1H), 5.98-5.89 (m, 1H), 5.31 (dd, J = 1.5, 11 Hz,

1H), 5.23 (dd, J = 1.5, 11 Hz, 1H), 4.92-4.85 (m, 1H), 4.79-4.67 (m, 1H), 4.65-4.58 (m, 2H),

4.30 (dd, J = 3.2, 8.6 Hz, 0.4H), 4.24 (dd, J = 3.2, 8.6 Hz, 0.6H), 3.75 (br s, 1H), 3.55-3.35 (m,

2H), 2.45-2.36 (m, 2H), 2.35-2.29 (m, 1H), 2.25-2.10 (m, 1H), 2.00-1.92 (m, 1H), 1.92-1.83

(m, 2H), 1.80-1.65 (m, 4H), 1.63-1.47 (m, 1H), 1.45 (s, 4H), 1.42 (s, 5H), 1.41-1.23 (m, 4H),

1.23-1.15 (m, 4H), 1.06-0.93 (m, 1H), 0.94-0.85 (m, 24H), 0.09 (s, 3.5H), 0.07 (s, 2.5H); 13C

NMR (125 MHz, CDCl3, mixture of rotamers): δ 173.9, 172.8, 167.3, 154.0, 144.7, 140.0,

139.6, 132.4, 129.7, 128.1, 126.9, 118.2, 79.9, 78.1, 77.9, 72.9, 72.6, 67.0, 66.0, 65.5, 59.5, 59.2,

48.2, 46.7, 46.6, 46.3, 44.0, 38.9, 38.7, 37.2, 36.4, 31.1, 30.2, 30.0, 28.6, 26.6, 26.1, 24.8, 23.5,

20.9, 20.7, 20.3, 18.1, 16.4, 16.2, 15.9, 15.4, 13.2, 8.1, 4.1, 0.1, -2.0, -4.0, -4.1, -4.4; ESI-MS m/z calcd for [C60H88N2O8SSi + Na]+: 1047.6, found: 1047.6.

Alcohol 1.3.2. To a solution of Boc-carbamate 1.84 (69.9 mg, 68.2 µmol) and 2,6- lutidine (32 µL, 0.28 mmol) in CH2Cl2 (1.5 mL) under argon was added TBSOTf (31.3 µL,

0.136 mmol). The resulting solution was stirred at rt for 1 h before being quenched with saturated aqueous NH4Cl. The aqueous phase was extracted with CHCl3 (x3) and the combined organic phase was dried over MgSO4, filtered and concentrated under reduced pressure to give the crude TBS-carbamate, which was used subsequently without further purification.

To a solution of the residue in THF (1.5 mL) at 0 °C was added TBAF (1.0 M in

THF, 0.17 mL, 0.17 mmol). The reaction was stirred at the same temperature for 30 min before being quenched with sat. aq. NaHCO3 and diluted with EtOAc. The phases were separated and the aqueous phase was extracted with EtOAc (x3). The combined organic phase was washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure to give the crude amino alcohol, which was used subsequently without further purification.

To a solution of the crude amino alcohol and i-Pr2NEt (15.4 µL, 88.4 µmol) in

CH2Cl2 (1 mL) and THF (0.5 mL) at 0 °C was added FmocOSu (25.3 mg, 75.0 µmol). The resulting solution was allowed to warm to rt and stirred 1 h. The reaction was quenched

by the addition of 1M HCl (1 mL) and the aqueous layer was extracted with EtOAc (x3).

The combined organic phase was washed with saturated aqueous NaHCO3 and brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (35:65 EtOAc-hexanes) gave 1.3.2 (64.2 mg, 91% for 3 steps) as a white foam; 1H NMR (500 MHz, CDCl3, mixture of rotamers): δ 7.77-7.74 (m,

2H), 7.66-7.56 (m, 2H), 7.43-7.13 (m, 19H), 6.96 (d, J = 7.6 Hz, 0.75H), 6.93 (d, J = 7.6 Hz,

0.25H), 6.42 (d, J = 9.1 Hz, 1H), 5.97-5.71 (m, 1H), 5.33-5.13 (m, 1H), 4.98-4.88 (m, 1H),

4.66-4.56 (m, 2H), 4.56-4.44 (m, 2H), 4.44-4.33 (m, 2H), 4.33-4.19 (m, 3H), 3.70-3.47 (m,

4H), 2.46-2.37 (m, 1H), 2.37-2.24 (m, 1H), 2.24-2.16 (m, 1H), 2.16-2.07 (m, 1H), 2.03-1.86

(m, 4H), 1.79-1.65 (m, 4H), 1.65-1.56 (m, 1H), 1.56-1.45 (m, 1H), 1.44-1.31 (m, 2H), 1.15 (d,

J = 7.1 Hz, 3H), 1.12-1.02 (m, 1H), 0.99-0.90 (m, 6H), 0.88 (s, 7H), 0.86-0.74 (m, 2H); 13C

NMR (125 MHz, CDCl3, mixture of rotamers): 175.6, 172.5, 167.2, 155.2, 144.7, 144.6,

144.1, 143.9, 141.4, 139.9, 132.4, 130.1, 129.7, 128.1, 128.1, 128.0, 127.9, 127.3, 127.2, 126.9,

125.3, 125.2, 120.1, 120.0, 118.1, 77.4, 77.2, 76.9, 76.8, 71.4, 67.9, 67.1, 65.5, 59.6, 47.3, 47.2,

46.7, 46.1, 46.0, 41.0, 38.3, 36.2, 35.9, 35.8, 33.8, 31.0, 30.1, 30.0, 25.7, 24.4, 19.9, 18.1, 15.4,

13.1; ESI-MS m/z calcd for [C64H76N2O8S + Na]+: 1055.5, found: 1055.5.

Troc-Carbonate 1.85. To a solution of alcohol 1.3.2 (56.8 mg, 55.0 µmol), pyridine

(26.7 µL, 0.330 mmol) and DMAP (single crystal, cat.) in CH2Cl2 (1.5 mL) at 0 °C was added TrocCl (22.7 µL, 0.165 mmol). The mixture was stirred for 5 min at the same

temperature before warming to rt with stirring for an additional 1 h. The reaction was quenched by the addition of 1 M HCl and the aqueous phase was extracted with EtOAc

(x3). The combined organic phase was washed with saturated aqueous NaHCO3 and brine, dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (30:70 EtOAc-hexanes) gave 1.85 (64.5 mg, 97%) as a white foam; 1H NMR (500 MHz, CDCl3, mixture of rotamers): δ 7.80-7.66 (m, 2H), 7.63-

7.55 (m, 2H), 7.45-7.14 (m, 19H), 6.43-6.19 (m, 1H), 6.02-5.79 (m, 1H), 5.40-5.13 (m, 2H),

5.03-4.71 (m, 3H), 4.70-4.51 (m, 3H), 4.51-4.35 (m, 3H), 4.31-4.17 (m, 2H), 3.73-3.60 (m,

1H), 3.60-3.43 (m, 1H), 2.55-2.42 (m, 1H), 2.42-2.32 (m, 1H), 2.32-2.20 (m, 1H), 2.20-2.07

(m, 1H), 2.03-2.84 (m, 3H), 1.80-1.67 (m, 4H), 1.67-1.41 (m, 5H), 1.40-1.20 (m, 3H), 1.19-

1.06 (m, 2H), 1.03-0.89 (m, 7H), 0.89-0.80 (m, 9H); 13C NMR (125 MHz, CDCl3, mixture of rotamers): δ 172.6, 172.4, 171.8, 167.2, 154.9, 154.7, 153.9, 153.7, 144.8, 144.6, 144.5, 144.5,

144.2, 144.0, 143.9, 141.4, 141.1, 139.6, 139.4, 132.3, 130.3, 129.7, 129.6, 128.2, 128.1, 128.1,

128.0, 127.8, 127.4, 127.3, 127.2, 127.2, 127.1, 127.0, 126.9, 126.0, 125.6, 125.3, 125.2, 120.1,

120.1, 120.0, 118.4, 118.2, 95.0, 95.0, 79.1, 78.4, 77.9, 77.4, 76.6, 76.4, 68.0, 67.6, 67.3, 67.2,

65.7, 65.5, 60.5, 59.7, 59.4, 47.4, 47.1, 46.6, 46.5, 45.3, 45.0, 38.4, 37.9, 37.5, 36.8, 36.1, 35.7,

32.8, 32.0, 31.3, 31.0, 30.1, 30.0, 26.1, 25.6, 24.8, 24.4, 23.5, 19.5, 19.2, 17.2, 14.3, 13.7, 13.4,

13.1; ESI-MS m/z calcd for [C67H77Cl3N2O10S + Na]+: 1229.4, found: 1229.4.

Thiazoline 1.87. To a solution of triphenylphosphine oxide (89.2 mg, 0.321 mmol) in CH2Cl2 (0.5 mL) at 0 °C was added Tf2O (27.0 µL, 0.160 mmol) and the mixture was stirred 15 min (white precipitate formation). A solution of 1.85 (64.5 mg, 53.4 µmol) in CH2Cl2 (1 mL) was added via cannula and the reaction was stirred 1 h at 0 °C before being quenched by the addition of sat. aq. NaHCO3. The aqueous phase was extracted with CHCl3 (x 3) and the organic phase was dried over MgSO4, filtered and concentrated under reduced pressure to give the crude thiazoline 1.86. To a solution of this residue in

THF (2.5 mL) and 1.0 M aqueous NH4OAc (0.5 mL) was added freshly activated zinc dust (70 mg, 1.07 mmol). The flask was covered with foil and the mixture was stirred vigorously at rt for 24 h. The reaction mixture was filtered through a plug of cotton before being partitioned between EtOAc and brine. The layers were separated and the aqueous phase was extracted with EtOAc (x3). The combined organic phase was dried over MgSO4, filtered and concentrated under reduced pressure. Flash chromatography over silica gel (31:69 EtOAc-hexanes) gave 1.87 (37.7 mg, 91% for 2 steps) as a white foam; 1H NMR (500 MHz, CDCl3): δ 7.75 (d, J = 7.5 Hz, 2H), 7.68-7.55 (m, 2H), 7.39 (t, J =

7.3 Hz, 2H), 7.34-7.27 (m, 2H), 6.84-6.74 (m, 1H), 6.01-5.84 (m, 1H), 5.38-5.08 (m, 2H),

5.01-4.91 (m, 1H), 4.70-4.63 (m, 1H), 4.63-4.56 (m, 1H), 4.50-4.21 (m, 4H), 3.78 (br s, 1H),

3.72-3.59 (m, 2H), 3.59-3.29 (m, 3H), 3.01-2.86 (m, 1H), 2.77-2.50 (m, 1H), 2.35-2.09 (m,

1H), 2.09-1.89 (m, 6H), 1.77-1.55 (m, 3H), 1.55-1.28 (m, 3H), 1.28-1.14 (m, 4H), 1.01-0.91

(m, 7H), 0.87 (s, 6H), 0.83 (s, 3H); 13C NMR (125 MHz, CDCl3): δ 176.4, 172.7, 167.4, 154.9,

144.3, 144.1, 141.4, 140.9, 140.5, 140.3, 132.4, 129.8, 129.4, 127.7, 127.2, 127.1, 125.6, 125.4,

125.4, 120.0, 118.2, 78.0, 77.0, 74.3, 71.7, 71.6, 68.0, 67.8, 65.6, 59.7, 59.6, 47.5, 47.4, 47.1,

46.6, 46.4, 46.3, 46.0, 45.2, 41.7, 39.9, 39.8, 38.6, 37.8, 37.7, 36.8, 35.9, 32.9, 32.3, 31.4, 31.0,

30.1, 29.9, 25.9, 25.2, 24.7, 23.5, 20.0, 19.6, 18.0, 16.9, 16.5, 15.8, 13.2; ESI-MS m/z calcd for

[C45H60N2O7S + Na]+: 795.4, found: 795.4.

Cyclization precursor 1.88. To a solution of allyl ester 1.87 (5.6 mg, 7.2 µmol) and

N-methylaniline (1.9 µL, 18 µmol) in THF (0.5 mL) was added Pd(PPh3)4 (0.8 mg, 0.7

µmol). The reaction was stirred at rt for 30 min before being concentrated under a stream of nitrogen. Flash chromatography over silica gel (5:95 MeOH-CH2Cl2) gave the acid 1.42 as a white foam. The residue was azeotroped with toluene (x2) and CH2Cl2 (x2). To a solution of 1.42 and tripeptide 1.3028 (14.4 µmol) in CH2Cl2 (0.5 mL) was added i-Pr2NEt

(5.0 µL, 29 µmol) and HATU (5.5 mg, 14 µmmol). The reaction was stirred overnight before being concentrated under a stream of nitrogen. Flash chromatography over silica gel (75:25 EtOAc-hexanes) gave 1.88 as a pale yellow foam (7.8 mg, 93% for 2 steps); 1H

NMR (500 MHz, CDCl3, mixture of rotamers): δ 7.77-7.74 (m, 2H), 7.66-7.57 (m, 2H),

7.41-7.37 (m, 2H), 7.32-7.28 (m, 2H), 7.11-7.04 (m, 2H), 6.80-6.75 (m, 2H), 6.56-6.44 (m,

1H), 6.37-6.18 (m, 1H), 5.94-5.82 (m, 1H), 5.44-5.08 (m, 5H), 5.01-4.92 (m, 2H), 4.66-4.55

(m, 2H), 4.49-4.16 (m, 4H), 3.76 (s, 3H), 3.70-3.60 (m, 1H), 3.60-3.49 (m, 1H), 3.42-3.26 (m,

1H), 3.06-2.95 (m, 3H), 2.95-2.81 (m, 3H), 2.81-2.63 (m, 3H), 2.34-2.19 (m, 1H), 2.16-2.00

(m, 2H), 2.00-1.83 (m, 5H), 1.78-1.56 (m, 4H), 1.50-1.34 (m, 2H), 1.30-1.24 (m, 7H), 1.24-

1.19 (m, 3H), 1.01-0.91 (m, 10H), 0.91-0.84 (m, 11H), 0.83 (s, 3H); 13C NMR (125 MHz,

CDCl3, mixture of rotamers): δ 212.8, 204.8, 200.9, 174.7, 173.9, 172.8, 167.2, 164.0, 154.0,

144.9, 144.6, 139.6, 132.4, 130.4, 129.7, 128.1, 126.9, 118.2, 115.6, 110.1, 79.9, 72.9, 72.6, 67.0,

65.5, 59.4, 58.0, 48.2, 46.7, 46.5, 46.3, 44.0, 43.8, 38.9, 36.4, 35.9, 32.1, 31.1, 30.0, 28.6, 26.6,

26.1, 24.4, 23.5, 20.3, 18.1, 16.4, 16.2, 15.9, 13.2, 0.1, -1.8, -4.1, -4.4; ESI-MS m/z calcd for

[C66H91N5O11S + Na]+: 1184.6, found: 1184.7.

Apratoxin D 1.4. To a solution of cyclization precursor 1.88 (22.5 mg, 19.4 µmol) and N-methylaniline (6.3 µL, 58 µmol) in THF (1 mL) was added Pd(PPh3)4 (2.2 mg, 1.9

µmol). The reaction was stirred at rt for 30 min before being concentrated under a stream of nitrogen. Flash chromatography over silica gel (8:92 MeOH-CH2Cl2) gave the acid as a pale yellow residue. To a solution of the residue in MeCN (5 mL) was added diethylamine (2.5 mL) and the reaction was stirred at rt for 30 min before being concentrated under a stream of nitrogen. The resulting residue was azeotroped with toluene twice, CH2Cl2 twice and dried under high vacuum for 2 h. To a solution of this residue in CH2Cl2 (30 mL) at 0 °C was added i-Pr2NEt (70 µL, 0.402 mmol) and HATU

(21.8 mg, 57.3 µmol). The reaction was stirred at the same temperature for 30 min before warming to rt. After stirring for 24 h, the reaction was concentrated under a stream of nitrogen. Flash chromatography over silica gel (100% EtOAc) gave partially purified

product. Semi-preparative HPLC (Agilent ZORBAX StableBond-C18, 9.4 x 250 mm, 5

µm, isocratic 84:16 MeCN-H2O, tR = 15.5 min) gave apratoxin D 1.4 (3.3 mg, 23% for 3 steps) as a pale yellow residue, which matched literature data8; 1H NMR (600 MHz,

CDCl3): δ 7.16 (d, J = 8.6 Hz, 2H), 6.80 (d, J = 8.6 Hz, 2H), 6.34 (dd, J = 1.0, 9.6 Hz, 1H),

5.95 (d, J = 9.4 Hz, 1H), 5.24 (dt, J = 4.4, 9.2 Hz, 1H), 5.19 (d, J = 11.5 Hz, 1H), 5.04 (m, 1H),

5.00 (ddd, J = 2.0, 5.0, 12.6 Hz, 1H), 4.64 (d, J = 10.7 Hz, 1H), 4.22 (m, 1H), 4.16 (t, J = 7.6

Hz, 1H), 3.78 (s, 3H), 3.66 (m, 1H), 3.56 (m, 1H), 3.46 (dd, J = 8.8, 10.9 Hz, 1H), 3.30 (br q, J

= 6.2 Hz, 1H), 3.15-3.09 (m, 2H), 2.86 (dd, J = 4.7, 12.6 Hz, 1H), 2.81 (s, 3H), 2.70 (s, 3H),

2.64 (m, 1H), 2.25-2.14 (m, 3H), 2.06 (m, 1H), 1.95 (s, 3H), 1.89-1.83 (m, 2H), 1.76 (m, 1H),

1.67 (m, 1H), 1.52 (m, 1H), 1.32-1.24 (m, 3H), 1.22 (d, J = 6.7 Hz, 3H), 1.11 (m, 1H), 1.07 (d,

J = 6.8 Hz, 3H), 0.98 (d, J = 7.7 Hz, 3H), 0.94-0.91 (m, 11H), 0.87 (s, 9H); 13C NMR (150

MHz, CDCl3): δ 172.7, 170.3, 169.5, 158.6, 136.3, 130.6, 128.3, 113.8, 76.4, 75.4, 72.5, 71.6,

60.6, 59.7, 56.9, 55.3, 50.5, 49.1, 47.7, 45.6, 38.4, 38.3, 37.6, 37.2, 33.5, 31.8, 31.7, 30.4, 30.0,

30.0, 29.7, 29.2, 25.6, 24.6, 24.1, 19.7, 18.7, 16.7, 14.1, 13.9, 13.4, 9.2; ESI-HRMS m/z calcd for [C48H75N5O8S + H]+: 882.5409, found: 882.5414.

S"62% O N OMe N O HN N O O

O O OH N S 1.4$5

7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 δ (ppm)

Figure 1.2. 1H NMR spectrum of apratoxin D 1.4.

S"63%

LiteratureS5. 1H NMR 1spectraH NMR of apratoxin spectrum D (1 of) apratoxin D5

7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm (t1)

Figure 1.3. Literature 1H NMR spectrum of apratoxin D 1.4.

2. Studies in the Gold(I)-Catalyzed Cycloisomerization of 7-Aryl-1,6-enynes

Portions of this chapter have been published: Brooner, R. E. M.; Robertson, B. D.;

Widenhoefer, R. A. Organometallics, 2014, 33, 6466-6473; Robertson, B. D.; Brooner, R. E.

M.; Widenhoefer, R. A. Chem. Eur. J., 2015, 21, 5714-5717. All studies concerning the stoichiometric conversion of 2.12 to 2.14/2.15 were performed by Rachel E. M. Brooner in the Widenhoefer laboratory,48 as well as the initial studies on the catalytic conversion of

2.12 to 2.13 and the first example of hydroarylation of compound 2.14 with 1,3- dimethoxybenzene.49 X-ray crystal structure of compound 2.48 was solved and refined by Dr. Roger D. Sommer at North Carolina State University.

2.1. Introduction

2.1.1. Homogeneous gold catalysis

Homogeneous transition metal catalysis has played a vital role in the development of pharmaceuticals, novel materials and fine chemicals due in large part to the ability of transition metals to catalyze a wide range of efficient, selective and atom- economical organic transformations. Although the first reports of the use of gold in organic synthesis appeared in the 1930s,50-52 the noble metal was typically thought to be catalytically “dead” for the following 60 years.53 It wasn’t until 1998, with an initial report by Teles et al., that this doctrine was finally challenged. In the paper, Teles

described the addition of methanol to alkynes to produce acetals, catalyzed by homogeneous, cationic gold(I)-complexes.54 This report enlightened chemists to the catalytic powers of cationic gold species, and research in the area has expanded exponentially in the following two decades. As our understanding of gold catalysis developed, it became clear that these complexes had two synthetically powerful features both characterized by π-activation of unsaturated carbon-carbon bonds: a) the ability of gold complexes to efficiently catalyze the hydrofunctionalization of C-C multiple bonds, and b) the abililty of gold complexes to catalyze the cycloisomerization of 1,n-enynes.55-58

The early years of gold catalysis mostly involved the use of AuCl and AuCl3, but as research in the area advanced, there was a general shift toward the use of ligated gold(I) cationic species as a means to control the reactivity of gold complexes and prevent reduction of active species into inert Au(0).59 The majority of Au(I)-complexes display a linear dicoordinate geometry.60,61 The most commonly utilized ligands include monodentate, bulky phosphines (such as 2.1 and 2.2, Figure 2.1A) or N-heterocyclic carbenes (such as 2.3 and 2.4), but the electronic/steric nature of these ligands can be easily tuned depending on the application.62 In general, linear dicoordinated Au(I) complexes [LAuCl] are poorly reactive toward ligand association and must be activated prior to catalysis. The Au-Cl bond in these complexes is quite strong, and the activation energy for dissociative chloride abstraction has been calculated to be as high as 78-82 kcal/mol.63,64 Instead, activation of Au(I) catalysts typically involves the treatment of

[LAuCl] with AgX salts (where X is a weakly or non-coordinating counterion), leading to Ag-Cl coordination and increasing the leaving-group ability of the chloride counterion. Resulting associative ligand exchange and precipitation of insoluble AgCl generates active cationic catalyst LAuX (Figure 2.1B).65 Additionally, if the chloride abstraction is performed in a weakly coordinating solvent such as acetonitrile, the resulting complexes can be isolated and purified, providing a “silver free” source of cationic gold complexes [LAuNCMe]+X-.

A. 1 R1 R R2 R2 P N N R3 R3 R2 R2

2.1: R1 = Cy "CyJohnPhos" 2.3: R2 = R3 = Me "IMes" 2.2: R1 = tBu "JohnPhos" 2.4: R2 = iPr, R3 = H "IPr"

B. Ag(X) L Au Cl L Au X + AgCl (ppt)

------L = 2.1-2.4 X = SbF6 , BF4 , ClO4 , PF6 , TfO , Tf2N , etc.

Ag(X) L Au Cl L Au N Me X + AgCl (ppt) MeCN Figure 2.1. A) Examples of commonly used ligands in Au(I) catalysis. B) Chloride abstraction from [LAuCl] complexes with Ag(I) salts possessing a weakly-/non- coordinating counterion to produce catalytically active Au(I) species.

In general, the Dewar-Chatt-Duncanson model66,67 can be invoked in the complexation of Au(I) with C-C multiple bonds. This model separates two orbital interactions, a σ-donation component and a π-donation component, in the formation of a

metal-ligand bond. The two components can be explained based on orbital symmetry. In the σ-component, electron density from the filled π-orbital on the alkene/alkyne is donated into an empty d-orbital on Au of appropriate symmetry (Figure 2.2). The π- component, also known as backbonding, involves donation of electron density from a filled d orbital of appropriate symmetry on Au to the empty π*-orbital of the alkene/alkyne ligand. Due to the high electronegativity of Au relative to other transition metals (Pauling electronegativity value = 2.54), the sigma donation component often dominates in coordination of Au with alkenes/alkynes, rendering the unsaturated carbon ligand electrophilic.173-178 The balance between the two components can be mediated by the identity of the supporting ligand on Au.

AuL AuL H H H H H H ≡ H H H H H H + ≡ H H H H ∗ π → dz2 π → dxz

σ-donation π-backbonding Figure 2.2. The Dewar-Chatt-Duncanson model of binding modes in Au(I)-catalysis.

2.1.2. Mechanistic considerations in the gold(I)-catalyzed cycloisomerization of 1,6-enynes

The cycloisomerization of enynes catalyzed by electrophilic noble metals, in particular Pt(II) and Au(I), has attracted considerable attention as an expedient and atom-economical route to carbo- and heterocyclic compounds from simple starting materials.55,68-73 The electrophilic activation of enynes was first developed by Trost in the

1980s using palladium catalysts,74 and later the groups of Echavarren,75 Fürstner76 and

Toste77 reported the remarkable activity of Au(I) complexes in the cycloisomerization of

1,n-enynes. 1,6-Enynes undergo catalytic cycloisomerization to form a range of skeletal rearrangement products, including vinylcyclopentenes, alkylidenecyclohexenes, and bicyclo[4.1.0]heptenes (Scheme 2.1). These products are presumably formed via 5-exo-dig or 6-endo-dig cycloisomerization to form the highly delocalized cyclopropyl carbenoid complexes 2.A and 2.B, respectively, followed by skeletal rearrangement and/or elimination. The outcome of these reactions has been shown to be highly dependent on the identity of the subsituents on the enyne (R1 and R2, Scheme 2.1).68,72 Fürstner first posited the intermediacy of metal-stabilized nonclassical cyclopropylmethyl-, cyclobutyl, and homoallylic carbocations/carbenes that result from attack of the C=C moiety on a metal-coordinated C-C triple bond.78

R2 R2 Z Z R1 R1 H

M M 2 2 R 6-endo-dig R 5-exo-dig R2 Z Z Z 1 Z R 2 1 R R1 R1 R 2.B 2.A

M M R2 R2 M R2 R2 -M Z Z Z Z R1 R1 R1 R1 2.C.I 2.C.II 2.C.III

Scheme 2.1. Potential pathways and intermediates for enyne cycloisomerization catalyzed by electrophilic noble metal complexes.

2.1.2.1 Cyclobutenes in the cycloisomerization of 1,6-enynes

7-Aryl-1,6-enynes undergo cycloisomerization in the presence of Au(I),79-

81 Pt(II),82-84 or Rh(II)85 catalysts to form bicyclo[3.2.0]hept-6-enes, presumably via rearrangement of the cyclopropyl carbenoid complex 2.B (R2 = Ar) to the metal- stabilized cyclobutyl cation 2.C followed by 1,3-hydrogen migration and demetallation

(Scheme 2.1). Although intermediates of type 2.C had been invoked as intermediates in the cyclization of 1,6-enynes, proof of their existence was only anecdotally provided through trapping experiments. For example, Blum reported the recovery of dione 2.5 upon PtCl4-mediated cycloisomerization of allyl propynyl ether 2.5 under oxygen

(Scheme 2.2A).86 The cycloisomerization product underwent rapid polymerization when 61

oxygen was excluded, suggesting the oxidative trapping of intermediate of cyclobutene

2.6.86 Similar results have been recently observed in the case of gold(I)-catalysis by Shin et al. in the synthesis of diones of type 2.9 from 1,6-enynes 2.8 (Scheme 2.2B).87

O O Ph Ph Ph O PtCl4 O2 O O O O O 2.5 2.6 Ph 2.7 No O2

Polymerization

B. O O LAuCl O Ar AgSbF6 O BnN BnN O2 Ar 2.8 2.9

Scheme 2.2. A) Blum’s PtCl4-mediated synthesis of cyclobutene 2.6 and subsequent trapping with oxygen. B) Recent synthesis of a similar dione 2.9 by Shin.

In the first example of bicyclo[3.2.0]hept-5-enes being directly isolated from a reaction mixture, Echavarren reported synthesis of cyclobutene 2.10 from enyne 2.9 in

80% yield, and cyclobutene 2.11 from enyne 2.10 in 88% yield (Scheme 2.3). Their identities were confirmed by X-Ray crystal structure of the 2,4-dinitrophenylhydrazone derived from 2.11. In this example, Echavarren proposed a mechanism involving direct ring expansion of a 5-endo-dig product of type 2.A, but does not provide further spectulation into the origins of the products. While this was the first bicyclo[3.2.0]hept-5- ene recovered from a cycloisomerization reaction, the origin of the strained cyclobutene

in these particular examples is not entirely clear as they are limited to structures possessing the pendant ketone.

R O O LAu R Z [LAuNCPh]SbF6 O R Z Z Z = C(CO2Me), L = 2.4 2.8: R = Me H 2.10, 80% 2.11, 88% 2.9: R = Ph(p-NO2)

Scheme 2.3. Isolation of bicyclo[3.2.0]hept-5-enes 2.10 and 2.11 from the Au(I)-catalyzed cyclization of 1,6-enynes 2.8 and 2.9 by Echavarren.

The first direct experimental evidence of bicyclo[3.2.0]hept-5-ene intermediates was reported by the Widenhoefer lab in 2013.48 In this example, Brooner and

Widenhoefer selectively generated and spectroscopically characterized the gold bicyclo[3.2.0]hept-1(7)-ene complex 2.14 formed in the gold-catalyzed cycloisomerization of the 7-phenyl-1,6-enyne 2.12 (Scheme 2.4). Their analysis pointed to predominant contribution of the metallacyclopropane canonical form 2.14A with a lesser contribution of the cyclobutyl cation form 2.14B. Upon warming a solution of 2.14 to 25

°C, the complex converted to 2.15 with 96% selectivity via a 1,3-hydrogen migration mechanism (Scheme 2.4).

AuL 1 eq. LAuCl LAu AuL Ph Ph Ph Ph 1 eq. AgSbF6 Z Z Z Z CD2Cl2, −20 °C, 8 h 2.14A 2.14B 2.14C 2.12 Z = C(CO2Me)2 L = 2.2 −20 °C → 25 °C

Ph 5 % LAuCl Ph 5% AgSbF6 Z AuL Z CD2Cl2, 25 °C 2.15 2.13 Scheme 2.4. Spectroscopic detection of gold bicyclo[3.2.0]heptene complex 2.13 in the gold-catalyzed cycloisomerization of 2.12 to 2.13.

2.1.2.2 Molecular complexity from the trapping of carbenoid intermediates in the cycloisomerization of 1,n-enynes

In addition to the palette of substructures accessible through the cycloisomerization of enynes, many of the intermediates of these transformations can be trapped with a variety of heteroatom75,88 or carbon nucleophiles, including electron-rich arenes, either in an inter-89-91 or intramolecular92,93 fashion (Scheme 2.5). Thus, methoxy- substituted alkylidenecyclopentanes such as 2.17 or alkylidenecyclohexanes 2.19 can be recovered from treatment of 2.16 or 2.18 with cationic Au and MeOH. Additionally, trapping of similar intermediates with electron-rich arenes such as indole or 1,3- dimethoxybenzene can provide products such as 2.21, 2.22, and 2.24.

M H Nuc R R2 2 R1 R H-Nuc H Z R1 Z or Z R2 or Z R1 R1

2.A Nuc = ROH, Ar-H Nuc Nuc

MeO2C MeO C 2 [Au], MeOH MeO2C MeO2C H 2.16 2.17 OMe

MeO C [Au], MeOH 2 MeO2C MeO C 2 MeO2C OMe 2.18 2.19

N TsN [Au], H TsN NH NH + Ph H TsN Ph 2.20 Ph 2.21 2.22 H OMe

Ar NO2 [Au], OMe MeO2C MeO C 2 OMe MeO2C MeO2C 2.23 OMe 2.24 Scheme 2.5. Trapping of carbenoid intermediates by nucleophiles.68

2.1.3. Project goals and scope

Initial stoichiometric studies by Rachel Brooner in the Widenhoefer lab established that the mechanism of 1,3-hydrogen transfer step in the isomerization of 2.14 to 2.15 is both catalyzed by Brønsted acid and occurs outside the coordination sphere of gold (i.e. from free 2.25, Scheme 2.6).48,49 These results suggested that a similar pathway was operative in the gold(I)-catalyzed conversion of 2.12 to 2.13. Although it was previously determined that 2.14 was the only phosphine-containing species present

during the gold-catalyzed conversion of 2.12 to 2.13, the 31P NMR analysis employed in this experiment would not have revealed formation of free 2.25. Therefore, we sought to spectroscopically monitor the conversion of 2.12 to 2.13 in situ to evaluate the role of free

2.25 in the Au(I)-catalyzed cycloisomerization of 2.12. In addition to gaining insight into the role of 2.25 in the Au(I)-catalyzed cycloisomerization of 2.12, these studies led to an improved protocol for the conversion of 2.12 to cyclobutene 2.13.

LAu Ph Ph CD2Cl2, 25 °C Z Z AuL H H H 2.14 2.15

LAu+ LAu+

Ph Ph Ph H+ −H+ Z Z Z H H H H H H 2.25 2.13

Z = C(CO2Me)2 L = t-Bu2(o-biphenyl)P (2.2) Scheme 2.6. Mechanism of the acid-catalyzed conversion of 2.14 to 2.15 under catalytic conditions.

The identification of 2.14 as a local minimum on the reaction coordinate in the conversion of 2.12 to 2.13 suggested that these intermediates might be susceptible to nucleophilic trapping. Guided by this hypothesis, we have developed and herein report the gold/silver-catalyzed tandem cycloaddition/hydroarylation of 7-aryl-1,6-enynes to form 6,6-diarylbicyclo-[3.2.0]heptanes. However, in contrast to our expectations, gold

was not involved in the hydroarylation event and, rather, our experimental observations point to a mechanism involving gold-catalyzed cycloaddition followed by silver- catalyzed hydroarylation of a bicyclo[3.2.0]hept-1(7)-ene intermediate.

2.2. Results and Discussion

2.2.1. In situ spectroscopic analysis of the conversion of 2.12 to 2.13.

To evaluate the role of free cyclobutene 2.25 in the gold-catalyzed cycloisomerization of 2.12, a CD2Cl2 solution of 2.12 (100 mM) and a catalytic 1:1 mixture of (2.2)AuCl (2 mol %) and AgSbF6 (2 mol %) was monitored periodically by 1H NMR spectroscopy at 25 °C (Figure 2.3). The disappearance of 2.12 displayed approximate first-order kinetics with a half-life of ~82 min (see section 2.3). The concentration of free 2.25 increased rapidly early in the reaction, reaching 12 mM at ~15% conversion without formation of significant amounts of 2.13 (≤1 mM) and reaching a maximum concentration of 14 mM after ~25% conversion (Figure 2.3). The concentration of 2.25 then decreased with concomitant formation of 2.13, which reached a maximum concentration of 22 mM after 6.5 h (≥98% conversion). The productive isomerization of 2.25 to 2.13 is presumably compromised by the dimerization/oligomerization of 2.25 and 2.13, which leads to the modest yield of 2.13 generated under these conditions (see section 2.3).

Ph 2 % LAuCl Ph Ph Z 2% AgSbF6 Z Z CD2Cl2, 25 °C H H 2.12 Z = C(CO2Me)2 2.25 2.13 L = 2.2

To evaluate the effect of exogenous Brønsted acid on the gold-catalyzed conversion of 2.12 to 2.13, a CD2Cl2 solution of 2.12 (100 mM) and a catalytic 1:1:2.5 mixture of (2.2)AuCl (2 mol %), AgSbF6 (2 mol %), and HOTf (5 mol %) was monitored periodically by 1H NMR spectroscopy at 25 °C (Figure 2.4). Disappearance of 2.12 in the presence of triflic acid displayed approximate first-order decay with a half-life of ~19 min (see section 2.3), which is approximately 4 times faster than the disappearance of 2.12 in the absence of triflic acid. Under these conditions, free 2.25 failed to

accumulate and, rather, 2.13 began to appear within the first ~5% conversion and reached a maximum concentration of 46 mM at 90% conversion (~1 h). At higher conversion, the concentration of 2.13 began to decrease, falling to 23 mM after 3.7 h to form byproducts consistent with a ~2:1:1 mixture of isomeric dimers of 2.13.

5% HOTf Ph 2 % LAuCl Ph Ph Z 2% AgSbF6 Z Z CD2Cl2, 25 °C H H 2.12 Z = C(CO2Me)2 2.25 2.13 L = 2.2

Both the rate and efficiency of the conversion of 2.25 to 2.13 under catalytic conditions increased significantly in the presence of catalytic amounts of triflic acid.

Triflic acid likewise increased the rate of consumption of enyne 2.12 under catalytic conditions, presumably due to the efficient removal of 2.25 from the reaction mixture.

Whereas the gold bicyclo[3.2.0]hept-1(7)-ene complex 2.14 is directly implicated as an intermediate in the gold-catalyzed conversion of enyne 2.12 to bicyclo[3.2.0]hept-1(7)- ene 2.13, our experiments rule out the involvement of the gold bicyclo[3.2.0]hept-6-ene complex 2.15 as an intermediate in the conversion of 2.25 to 2.13 and hence in the catalytic conversion of 2.12 to 2.13. Furthermore, it also appears unlikely that 2.15 forms as an off-cycle catalyst reservoir under reaction conditions, owing to the poor coordinating ability of 2.13 relative to that of the other available π-donor ligands in solution.48

With a better understanding of the reaction mechanism in the conversion of enyne 2.12 to cyclobutene 2.13, we sought to identify conditions to improve the catalytic yield of 2.13. Additionally, we suspected that excess TfOH and/or Ag(I) had a role in the decomposition 2.13 in the reaction mixture based on the increased yield of 2.13 from the cycloisomerization 2.12 when silver-free [(2.2)AuNCMe]SbF6 was used,79,80 the potential for silver-catalyzed dimerization of cyclobutenes,94 and the observation that 2.13 decomposed in the presence of silver and/or TfOH (Figure 2.4). Therefore, when 2.12 was treated with a 1:1 mixture of [(2.1)AuNCMe]SbF6 (2 mol %) and TfOH (2 mol %) in

CH2Cl2 at 25 °C for 3 h, cyclobutene 2.13 was isolated in 72% yield following workup and silica gel chromatography. This is the highest reported yield for 2.13 from the cycloisomerization of 2.12. In our hands, we were never previously able to synthesize

2.13 in >35% yield utilizing the reported conditions.79,80

[(2.1)AuNCMe]SbF6 (2 mol %) Ph Ph TfOH (2 mol %) Z Z CH2Cl2, 25 °C, 3 h

2.12 2.13 72 % yield Z = C(CO2Me) Scheme 2.7. Improved procedure for the synthesis of 2.13 from the gold-catalyzed cycloisomerization of 2.12.

2.2.2. Trapping of cyclobutene 2.25 via hydroarylation

As an extension of the prior work by Echavarren, we targeted electron-rich arenes as trapping agents for the gold bicycloheptyl cations generated via enyne cycloaddition owing to the precedence for the hydroarylative trapping of cyclopropyl carbenoid intermediates.89-91,95 In apparent support of our hypotheses, treatment of a 1:2 mixture of enyne 2.12 and 1,3,5-trimethoxybenzene (TMB) with a catalytic 1:1.1 mixture of [PCy2(o-biphenyl)AuCl] (2.27; 5 mol %) and AgSbF6 at 25 °C for 3 h formed bicyclo[3.2.0]heptane 2.26 in 82 % yield with ≥25:1 endo/exo selectivity (Table 2.1, entry 1).

However, subsequent experimentation revealed the critical role of silver in the hydroarylation event, which argued against the hydroarylation of a gold-stabilized bicycloheptyl cation. In particular, reaction of 2.12 with TMB catalyzed by the silver-free 71

gold complex [(PCy2)(o-biphenyl)AuNCMe]+SbF6− (2.28; 5 mol %) at 25 °C for 6 h formed bicyclo[3.2.0]hept-6-ene 2.13 in 47 % yield without formation of detectable quantities of 2.26 (entry 2). Further increasing the silver loading to 20 mol % in combination with either 2.27 or 2.28 (5 mol %) decreased the reaction time to approximately 1.5 h and increased the yield of 2.26 to 87-90% (entries 3-4). Conversely, AgSbF6 alone led to no detectable cycloaddition (entry 5).

Table 2.1. Effect of gold, silver, and Brønsted acid on the cycloisomerization/ hydroarylation of 2.12 with 1,3,5-trimethoxybenzene (TMB).

Au source (x mol %) AgSbF (y mol %) OMe OMe 6 Ph Ph Ph HOTf (z mol %) Z + Z + Z OMe CH2Cl2, 25 °C MeO OMe MeO 2.12 2.13 2.26

Z = C(CO2Me)2 Cy Cy Cy Cy SbF6 P AuCl P Au N Me

2.27 2.28 Au HOTf (mol time 2.13 2.26 entry Ag (mol %) source %) (h)a (%) (%) 1b 2.27 5.5 0 3 ≤2 82 2c 2.28 0 0 6 47 ≤2 3b 2.27 20 0 1.5 ≤2 90 4c 2.28 20 0 1.5 ≤2 87 5d none 20 0 24 ≤2 ≤2 6c 2.28 0 5 6 10 73 7c none 0 5 24 ≤2 ≤2 aReaction progress monitored by TLC (unless otherwise stated). bYield of isolated product. cYield determined by 1H NMR analysis. dReaction progress monitored by GC

2.2.3. Nucleophile scope in the hydroarylation of 3.12.

In the presence of a catalytic 1:4 mixture of 2.27 (5 mol %) and AgSbF6 (20 mol %),

1,6-enynes possessing a 7-(2-naphthyl) (2.29) or 7-(3,5-dimethylphenyl) (2.30) group, 4,4- gem-acetoxymethyl (2.31) or acetonide (2.32) groups, a 3-methyl (2.33) or 3-phenyl (2.34) group, or 3,3-gem-dimethyl (2.35) groups underwent efficient gold/silver-catalyzed cycloaddition/hydroarylation with TMB to form the corresponding bicycloheptanes 2.36-2.42 in 64–86 % yield with ≥25:1 endo/ exo selectivity (Table 2.2, entries 1–7). In addition to TMB, a number of mono-, di-, and trisubstituted arenes underwent gold/silver-catalyzed cycloaddition/ hydroarylation with 2.12 to form 6,6- diarylbicycloheptanes 2.43-2.48 in >80 % yield as mixtures of endo/exo diastereomers ranging from 1:7 in the case of 2,6-di-tert-butylphenol to ≥25:1 in the case of 3,5- dimethoxytoluene (Table 2.2, entries 9–13).

Table 2.2. Cycloisomerization/hydroarylation of 7-aryl-1,6-enynes catalyzed by a mixture of 2.27 (5 mol %) and AgSbF6 (20 mol %) in CH2Cl2 at 25 °C for 1.5 h [Z = C(CO2Me)].

entry enyne arene product [yield (%)]a endo/exob

OMe Ar OMe Ar Z Z OMe MeO OMe MeO 1 2.29 (Ar = 2-naphthyl) 2.36 (84%) ≥25:1

2 2.30 (Ar = 3,5-C6H3Me2) 2.37 (84%) ≥25:1 OMe OMe AcO Ph Ph 3 AcO AcO OMe ≥25:1 MeO OMe AcO 2.31 MeO 2.38 (80%) OMe OMe Ph O Ph O OMe ≥25:1 4 O O MeO OMe MeO 2.32 2.39 (64%) OMe Ph Ph Z Z 1 OMe R 2 R 1 MeO R R2 5 2.33 (R1 =Me, R2 = H) OMe 2.40 (79%, 2:1)c ≥25:1 6 2.34 (R1 = Ph, R2 = H) 2.41 (86% 3:1)c ≥25:1 7 2.35 (R1 = R2 = Me) MeO OMe 2.42 (77%) ≥25:1 Ph Ph OR Z Z OR

8 2.12 R = Me 2.43 1:1.8 9 2.12 R = H 2.44 1:2.5 OR RO Ph Ph Z Z OR OR 10 2.12 R = Me 2.45 (85%) 1:1.3 11 2.12 R = H 2.46 (72%) 1:3.6 OH Ph tBu tBu tBu 12 Z 2.12 OH 1:7

2.47 (86%) tBu OMe OMe Ph

13 2.12 Z ≥25:1 MeO Me Me MeO 2.48 (88%)

aYield of product isolated in >95% purity. bendo/exo ratio determined by 1H NMR analysis of the crude reaction mixture. cMixture of C2 diastereomers.

2.2.4. Mechanistic considerations in the hydroarylation of 2.12.

A number of additional experiments employing 2 and 1,3,5-trimethoxybenzene

(TMB) were performed to further clarify the role of silver and the nature of the intermediates involved in the hydroarylation event of catalytic cycloaddition/hydroarylation. For example, periodic analysis of the gold/silver- catalyzed cycloaddition/hydroarylation of 2.12 with TMB revealed that cyclobutene 2.13 accumulated slowly throughout the reaction, reaching a maximum relative concentration of approximately 6 % at 90 % conversion (see section 2.3), which is inconsistent with the intermediacy of 2.13 in the conversion of 2.12 to 2.26. Independent analysis of the silver- or gold/silver-catalyzed hydroarylation of 2.13 with TMB confirmed that hydroarylation of 2.13 was too slow to account for the formation of 2.26 in the reaction of 2.12 with TMB and also established AgSbF6 as a stand-alone catalyst for bicycloheptene hydroarylation [Eq. (1)].

OMe Ph OMe 2.27 (X mol %) Ph AgSbF6 (20 mol %) Z + Z (1) CH Cl , 25 °C OMe MeO OMe 2 2 24 h MeO 2.13 2.26

X = 5, t1/2 = 5 h, 63% yield X = 0, t1/2 = 5 h, 67% yield

Rather, hydroarylation of the bicyclo[3.2.0]hept-1(7)-ene intermediate 2.25 was implicated through the gold/silver-catalyzed reaction of 2.12 with 2,4,6-trideutero-1,3,5- trimethoxybenzene (TMB-d3; 97 % deuterium incorporation), which formed 2.26-d3 with

approximately 75 % deuterium incorporation at the C1 bridgehead position without detectable deuteration at C7 (Scheme 2.8).

~75% d D MeO OMe 2.27 (5 mol %) D Ph OMe Ph D D AgSbF6 (20 mol %) Z + Z D CH2Cl2, 25 °C, 1.5 h OMe MeO OMe 82% 2.12 D <5% d 97% d 2.26-d 97% d 3

Scheme 2.8. Gold/silver-catalyzed hydroarylation of 2,4,6-trideutero-1,3,5-trimethoxybenzene (TMB-d3).

Owing to the potential generation of Brønsted acid from gold/silver mixtures96-101 and the recent demonstration of gold/Brønsted acid tandem catalysis,102-106 including our own studies on the conversion of 2.12 to 2.13,107 we evaluated the potential role of

Brønsted acid in gold/silver-catalyzed enyne cycloaddition/ hydroarylation. Indeed, treatment of 2.12 and TMB with a 1:1 mixture of 2.28 and HOTf for 6 h at 25 °C formed 2.26 in 73 % yield and 2.13 in 10 % yield (by 1H NMR analysis; Table 2.1, entry 6).

Periodic analysis of a similar reaction mixture revealed that the relative concentration of 2.13 increased to approximately 20 % after 56 % conversion and then decreased to approximately 10 % at 95 % conversion, suggesting competitive hydroarylation of both 2.25 and 2.13 (see section 2.3).

Although the experiments described in the preceding paragraph established the viability of Brønsted acid-catalyzed pathways for bicycloheptene hydroarylation, the relevance of these pathways to the gold/silver-catalyzed cycloaddition/ hydroarylation of 2.12 is not clear. We previously established the participation of Brønsted acid in the gold-catalyzed cycloisomerization of 2.12 to 2.13, specifically the isomerization of 2.25 to 2.13.107 However, silver was not the apparent source of Brønsted acid in these transformations and rather, silver inhibited the conversion of 2.14 to 2.13.48 Similarly, attempts to generate Brønsted acid in the gold/silver-catalyzed reaction of 2.12 with

TMB through addition of catalytic amounts of water led to inhibition of the reaction.

Furthermore, the greater efficacy of AgSbF6 as a hydroarylation catalyst, even with only

0.1 mol % excess relative to 2.27 (Table 2.1, entry 2), as compared to HOTf (5 mol %) appears incongruent with hidden Brønsted acid catalysis in the former case. This outcome would require generation of stronger acid from gold/silver mixtures as compared to gold/HOTf mixtures, but acid strength in both cases should be leveled by the presence of Brønsted bases. We therefore propose a mechanism for the cycloaddition/hydroarylation of 2.12 with TMB involving gold-catalyzed cycloaddition of 2.12 followed by silver-catalyzed hydroarylation of 2.25, presumably via a silver- stabilized bicycloheptyl cation (Scheme 2.9). In this context, it is worth noting that although there is a growing body of work that documents the ability of silver salts to affect the outcomes of gold(I)-catalyzed transformations,108-116 in only one case have gold

and silver been shown to function orthogonally in two discrete steps of a tandem catalytic process in which the silver salt functions as a simple Lewis acid for carbonyl activation.117

Ph Ph + + Ph Au Ag Ar Z Z Ar H Z

2.12 2.25 2.26

Ag Ph Ar-H Z

Scheme 2.9. Proposed mechanism for the conversion of 2.12 and TMB to 2.26 via gold/silver tandem catalysis.

2.2.5. Summary and conclusions

In summary, in situ spectroscopic analysis of the gold-catalyzed cycloisomerization of 2.12 to 2.13 supported previous findings in our laboratory, which was consistent with a mechanism involving sequential gold-catalyzed cycloaddition of

2.12 to 2.25 followed by Brønsted acid catalyzed 1,3-hydrogen migration outside the coordination sphere of gold. Although gold(I)/Brønsted acid cascade catalysis has been previously documented, most notably in the enantioselective tandem hydroamination/transfer hydrogenation of alkynes to form primary amines,104,118,119 examples of gold/Brønsted acid cascade catalysis in the context of enyne cycloaddition are rare.120 Therefore, identification of the central role of Brønsted acid in

the catalytic conversion of 2.12 to 2.13 suggests that Brønsted acid catalysis might also participate in other enyne cycloaddition processes catalyzed by electrophilic noble-metal complexes. Furthermore, identification of the beneficial effect of Brønsted acid in the gold-catalyzed conversion of 2.12 to 2.13 points to the potential of combined transition metal/Brønsted acid catalyzed approaches for the cycloisomerization of 7-aryl 1,6- enynes and related substrates. Indeed, mechanistic understanding of the transformation under catalytic conditions has directly led to an improved preparation of cyclobutene

2.13.

We have also developed an effective tandem gold/silver-catalyzed protocol for the cycloaddition/hydroarylation of 7-aryl-1,6-enynes to form 6,6-diarylbicyclo-

[3.2.0]heptanes. Our experimental observations regarding the gold/ silver-catalyzed cycloaddition/hydroarylation of 2.12 with TMB point to a mechanism involving sequential gold-catalyzed conversion of 2.12 to 2.25 followed by silver-catalyzed hydroarylation of 2.25. These transformations represent the first examples involving efficient trapping of the bicyclo[3.2.0]hept-1(7)-ene intermediate generated via enyne cycloaddition to generate functionalized bicyclo[3.2.0]heptanes.

2.3. Experimental

2.3.1. General methods

Reactions were performed under a nitrogen atmosphere employing standard

Schlenk and glovebox techniques unless specified otherwise. NMR spectra were obtained on a Varian spectrometer operating at 500 MHz for 1H, 125 MHz for 13C, and

202 MHz for 31P at 25 °C unless noted otherwise. 13C NMR spectra were referenced relative to CD2Cl2 (δ 53.8) or CDCl3 (δ 77.16), 1H NMR spectra were referenced relative to residual CHDCl2 (δ 5.32) or tetramethylsilane (for CDCl3, δ 0.00), and 31P spectra were referenced to an external solution of triphenylphosphine oxide in CD2Cl2 (δ 26.9). NMR

Probe temperatures were calibrated using a neat methanol thermometer. LCMS was performed on an Agilent Technologies 1100 Series LC/MSD-Trap SL equipped with an

Agilent Zorbax C-18 with 3.5 mm particle 1 × 150 mm column. Flash column chromatography was performed employing 200-400 mesh silica gel 60 (EM). Thin layer chromatography (TLC) was performed on silica gel 60 F254. CD2Cl2 was dried over

CaH2 prior to use. Ether, methylene chloride, and THF were purified by passage through columns of activated alumina under argon. (PCy2o-biphenyl)AuCl (2.27),

AgSbF6, anhydrous acetonitrile, nitrobenzene, n-hexadecane, and reagents for enyne synthesis were obtained through major chemical suppliers and were used as received.

Dimethyl allylpropargylmalonate (2.3.1),121 7-phenyl-4,4-bis(methoxycarbonyl)-1,6- enyne (2.12),122 3,3-Bis(methoxycarbonyl)-6-phenyl-[3.2.0]-bicyclohept-6-ene (2.13),80 2.33,

2-allyl-2-(3-phenylprop-2-yn-1-yl)propane-1,3-diol (2.3.2),123 dimethyl 2-(1- methylallyl)malonate (2.3.3), dimethyl 2-(1-phenylallyl)malonate (2.3.4), dimethyl 2-(1,1- dimethylallyl)malonate (2.3.4),124 (3-bromoprop-1-yn-1-yl)benzene (2.3.5),125 and 1,3-5- trimethoxybenzene-2,4,6-d3 (TMB-d3)91 were prepared according to published procedures.

2.3.2. In situ spectroscopic analysis of the conversion of 2.12 to 2.13

2.3.2.1 Spectroscopic analysis of the gold-catalyzed conversion of 2.12 to 2.13

A suspension of (2.2)AuCl (7.9 mg, 1.5 × 10–2 mmol) and AgSbF6 (5.4 mg, 1.6 ×

102) in CD2Cl2(1.5 mL) was stirred in the dark for 15 min at ambient temperature. A portion (0.20 mL, 2.0 × 10–3 mmol) of this suspension was placed in an NMR tube containing a solution of 2.12 (28.6 mg, 0.100 mmol), and nitromethane (2.7 µL, 5.0 ×

102 mmol, internal standard) in CD2Cl2 (0.80 mL). The tube was inverted once, placed in the probe of an NMR spectrometer maintained at 25 °C, and monitored periodically by 1H NMR spectroscopy (Figure 2.3). The concentrations of 2.12, 2.13, and 2.25 were determined by integrating the resonances corresponding to the internal vinyl proton of 2.12 (δ 5.72), one of the diastereotopic C4 methylene protons of 2.25 (δ 1.68 (dd, J =

10.0, 12.8 Hz, 1 H)), and the vinylic C7 proton of 2.13 (δ 6.08 (s)) relative to nitromethane

(δ 4.31 (s)) in the 1H NMR spectrum. Obtaining an accurate integration of the δ 1.68 resonance of 2.25 was complicated by the appearance of a methylene resonance at δ 1.70

(dd, J = 8.7, 14.0 Hz, 1 H) associated with one of the isomeric dimers of 2.13 that partially obscured the δ 1.68 methylene resonance of 2.15. Accurate integration of the δ 1.68 methylene resonance of 2.25 was determined by utilizing an aromatic resonance associated with the byproduct at δ 6.91 (dd, J = 1.4, 8.3 Hz) that was twice the intensity of the δ 1.70 resonance by employing the relationship (integration of δ 1.68) = (integration of δ 1.68 + δ 1.70) – 1/2(integration at δ 6.91).

2.3.2.2 In situ spectroscopic snalysis of the gold/triflic acid catalyzed conversion of 2.12 to 2.13.

A suspension of (2.2)AuCl (9.8 mg, 1.8 × 10–2 mmol), AgSbF6 (6.6 mg, 1.9 ×

102 mmol), and TfOH (4.0 µL, 4.5 × 10–2 mmol) in CD2Cl2 (1.8 mL) was stirred in the dark for 15 min at ambient temperature. A portion (0.20 mL; 2.0 × 10–3 mmol of

(2.2)AuCl/AgSbF6, 5.0 × 10–3 mmol of TfOH) of this suspension was placed in an NMR tube containing a solution of 2.12 (28.6 mg, 0.100 mmol) and nitromethane (2.7 µL, 5.0 ×

10–2 mmol, internal standard) in CD2Cl2 (0.80 mL). The tube was inverted once, placed in the probe of an NMR spectrometer maintained at 25 °C, and monitored periodically by 1H NMR spectroscopy. The concentrations of 2.12, 2.13, and 2.25 were determined as described above (Figure 2.4).

2.3.3. Gold complexes

[(PCy2o-biphenyl)Au(NCMe)]+ SbF6– (2.28). Compound 2.28 was prepared by a procedure similar to that described by Echavarren.126 Anhydrous acetonitrile (8 mL) was 82

added to a mixture of (PCy2o-biphenyl)AuCl (230 mg, 0.395 mmol) and AgSbF6 (170 mg,

0.50 mmol). The reaction vessel was covered with foil, stirred at room temperature for 6 h, and concentrated under vacuum. The residue was taken up in CH2Cl2 (2 mL) and filtered through a short plug (~1 cm) of silica in a Pasteur pipette which was eluted with

CH2Cl2. The filtrate was concentrated under vacuum to give a gray solid. This solid was exposed to natural light for 24 h, giving a darker gray solid. This solid was taken up in

CH2Cl2, filtered through a plug of silica and eluted with CH2Cl2 as described above. The process was repeated until no further graying of the solid was apparent after exposure to sunlight (repeated 2-3 times). Recrystallization of the resulting white solid by slow diffusion of hexanes into CH2Cl2 gave 2.28 as colorless crystals (286 mg, 88%).

Spectroscopy of 2.28 matched the published data.79,126

2.3.4. 1,6-Enynes

7-(2-Naphthyl)-4,4-bis(methoxycarbonyl)-1,6-enyne (2.29). Pd(PPh3)2Cl2 (34 mg,

4.8 × 10–2 mmol) and CuI (18 mg, 9.5 × 10–2 mmol) were added to a solution of dimethyl allylpropargylmalonate 2.3.1 (401 mg, 1.91 mmol) and 2-iodonaphthalene (534 mg, 2.10 mmol) in triethylamine (20 mL) and the resulting solution was stirred at room temperature for 6 h. The reaction mixture was diluted with hexanes (50 mL) and filtered through a plug of Celite which was eluted with hexanes. The filtrate was concentrated and the resulting oily residue was chromatographed (SiO2; hexanes–EtOAc = 85:15) to

provide 2.29 as a pale yellow oil (617 mg, 96%). TLC (hexanes–EtOAc = 4:1): Rf = 0.40.

1H NMR (CDCl3): δ 7.88 (s, 1 H), 7.79–7.71 (m, 3 H), 7.48–7.39 (m, 3 H), 5.77–5.66 (m, 1

H), 5.28–5.14 (m, 2 H), 3.77 (s, 6 H), 3.08 (s, 2 H), 2.93 (d, J = 7.5 Hz, 2 H). 13C{1H} NMR

(CDCl3): δ 170.4, 133.0, 132.7, 131.9, 131.4, 128.6, 127.9, 127.8, 127.7, 126.6, 126.6, 120.5,

120.0, 84.6, 84.1, 57.4, 52.9, 36.9, 23.8. HRMS calcd (found) for C21H21NaO4 (M + Na+):

359.1254 (359.1258).

7-(3,5-Dimethylphenyl)-4,4-bis(methoxycarbonyl)-1,6-enyne (2.35). Enyne 2.35 was synthesized in 88% yield as a pale yellow viscous oil from 2.3.1 employing a procedure similar to that used to synthesize 2.29. TLC (hexanes–EtOAc = 4:1): Rf = 0.44.

1H NMR (CDCl3): δ 7.00 (s, 2 H), 6.91 (s, 1 H), 5.73–5.62 (m, 1 H), 5.24–5.12 (m, 2 H), 3.75

(s, 6 H), 3.01 (s, 2 H), 2.87 (d, J = 7.5 Hz, 2 H), 2.26 (s, 6 H). 13C{1H} NMR: δ 170.4, 137.8,

131.9, 130.0, 129.4, 122.8, 119.9, 84.0, 83.3, 57.3, 52.8, 36.8, 23.7, 21.1. HRMS calcd (found) for C19H22NaO4 (M + Na+): 337.1410 (337.1414).

2.31. Acetyl chloride (0.32 mL, 4.5 mmol) was added dropwise to a solution of

2.3.3 (468 mg, 2.03 mmol), Et3N (0.85 mL, 6.1 mmol) and DMAP (~10 mg) in CH2Cl2 (10 mL) at 0 °C. The solution was warmed to room temperature, stirred for 12 h, and quenched with saturated aqueous NaHCO3. The aqueous phase was extracted with

CH2Cl2 (3 × 10 mL) and the combined organic extracts were dried (MgSO4), filtered, and concentrated under vacuum. The oily residue was chromatographed (SiO2, hexanes–

EtOAc = 85:15) to give 2.31 as a colorless oil (582 mg, 92%). TLC (hexanes–EtOAc = 4:1):

Rf = 0.32. 1H NMR (CDCl3): δ 7.43–7.36 (m, 2 H), 7.31–7.27 (m, 3 H), 5.88–5.74 (m, 1 H),

5.22–5.11 (m, 2 H), 4.09 (s, 4 H), 2.49 (s, 2 H), 2.30 (d, J = 7.6 Hz, 2 H), 2.08 (s, 6 H).

13C{1H}NMR (CDCl3): δ 170.9, 132.2, 131.7, 128.4, 128.0, 123.5, 119.6, 85.0, 83.6, 65.8, 40.6,

36.4, 23.2, 21.0. HRMS calcd (found) for C19H22NaO4 (M + Na+): 337.1410 (337.1404).

2.32. Pyridinium p-toluenesulfonate (8 mg, 3 × 10–2 mmol) was added to a solution of 2.3.3 (150 mg, 0.651 mmol) and 2,2-dimethoxypropane (96 µL, 0.78 mmol) in acetone (10 mL). The resulting solution was stirred for 16 h, concentrated under vacuum and chromatographed (SiO2, hexanes–EtOAc = 9:1) to give 6d as a colorless oil (168 mg,

95%). TLC (hexanes–EtOAc = 4:1): Rf = 0.47. 1H NMR (CDCl3): δ 7.43–7.37 (m, 2 H),

7.31–7.26 (m, 3 H), 5.88–5.75 (m, 1 H), 5.22–5.11 (m, 2 H), 3.73 (d, J = 6.1 Hz, 4 H), 2.60 (s,

2 H), 2.23 (d, J = 7.6 Hz, 2 H), 1.44 (s, 6 H). 13C{1H} NMR (CDCl3): δ 132.5, 131.7, 128.3,

127.8, 119.1, 98.3, 86.5, 83.3, 66.9, 37.3, 36.0, 26.0, 23.4, 21.8. HRMS calcd (found) for

C18H22O2 (MH+): 271.1693 (271.1697).

3-Methyl-7-phenyl-4,4-bis(methoxycarbonyl)-1,6-enyne (2.33). A solution of dimethyl 2-(1-methylallyl)malonate (2.3.4) (374 mg, 2.01 mmol) in THF (5 mL) was added dropwise via cannula to a suspension of NaH (60 wt. % in mineral oil, 88 mg, 2.2 mmol) in THF (5 mL) at 0 °C. The resulting solution was warmed to room temperature and stirred for 1 h. The solution was cooled to 0 °C before the dropwise addition of 2.3.7

(468 mg, 2.4 mmol). The solution was warmed to room temperature, stirred for 16 h, and quenched with saturated aqueous NH4Cl (5 mL). The aqueous phase was extracted

with Et2O (3 × 20 mL) and the combined organic extracts were washed with brine, dried

(MgSO4), filtered, and concentrated under vacuum. The oily residue was chromatographed (SiO2, hexanes–EtOAc = 85:15) to give 2.33 as a colorless oil (555 mg,

92%). TLC (Hexanes–EtOAc = 4:1): Rf = 0.35. 1H NMR (CDCl3): δ 7.39-7.33 (m, 2 H), 7.29-

7.25 (m, 3 H), 5.80 (ddd, J = 8.7, 10.2, 17.0, 1 H), 5.22-5.06 (m, 2 H), 3.76 (s, 3 H), 3.75 (s, 3

H), 3.22-3.15 (m, 1 H), 3.02 (s, 1 H), 3.01 (s, 1 H), 1.20 (d, J = 6.9 Hz, 3 H). 13 C{1H} NMR

(CDCl3): δ 170.2, 170.1, 138.5, 131.6, 128.3, 128.0, 123.4, 116.8, 85.1, 83.5, 60.7, 52.5, 52.5,

41.4, 24.6, 16.5. HRMS calcd (found) for C18H20NaO4 (M + Na+): 323.1254 (323.1257).

3,7-Diphenyl-4,4-bis(methoxycarbonyl)-1,6-enyne (2.34). Enyne 2.34 was synthesized in 82% yield as a pale yellow viscous oil from 2.3.5 employing a procedure similar to that used to synthesize 2.33. TLC (hexanes–EtOAc = 4:1): Rf = 0.33. 1H NMR

(CDCl3): δ 7.42-7.36 (m, 2 H), 7.31-7.27 (m, 5 H), 7.26-7.22 (m, 3 H), 6.46 (ddd, J = 8.1, 10.3,

17.0, 1 H), 5.18-5.08 (m, 2 H), 4.35 (d, J = 8.1 Hz, 1 H), 3.77 (s, 3 H), 3.72 (s, 3 H), 2.98 (d, J

= 17.2 Hz, 1 H), 2.77 (d, J = 17.2 Hz, 1 H). 13C{1H} NMR (CDCl3): δ 170.0, 169.9, 138.7,

137.3, 131.7, 129.4, 128.4, 128.3, 128.1, 127.5, 123.4, 117.7, 84.9, 84.1, 61.8, 52.8, 52.7, 52.6,

25.5. HRMS calcd (found) for C23H22NaO4 (M + Na+): 385.1410 (385.1410).

3,3-Dimethyl-7-phenyl-4,4-bis(methoxycarbonyl)-1,6-enyne (2.35). Enyne 2.35 was synthesized in 77% yield as a pale yellow oil from 2.3.6 employing a procedure similar to that used to synthesize 2.33 with the exception that the alkylation reaction was heated to reflux for 18 h. TLC (hexanes–EtOAc = 9:1): Rf = 0.25. 1H NMR (CDCl3): δ 7.35-

7.30 (m, 2 H), 7.27-7.23 (m, 3 H), 6.22 (dd, J = 11.0, 17.2, 1 H), 5.10–5.02 (m, 2 H), 3.75 (s, 6

H), 3.02 (s, 2 H), 1.31 (s, 6 H). 13C{1H} NMR (CDCl3): δ 170.1, 144.1, 131.5, 128.2, 127.8,

123.7, 113.3, 86.9, 82.8, 64.2, 52.3, 42.2, 24.1, 24.0. HRMS calcd (found) for C19H22NaO4 (M

+ Na+): 337.1410 (337.1415).

2.3.5. 6,6-Diaryl-bicyclo-[3.2.0]-heptanes

2.36. A solution of AgSbF6 (13.7 mg, 4.00 × 10–2 mmol) in CH2Cl2 (0.5 mL) was added to a solution of enyne 2.12 (57.2 mg, 0.200 mmol), 1,3,5-trimethoxybenzene (67 mg, 0.40 mmol) and 2.27 (5.8 mg, 1.0 × 10–2 mmol) in CH2Cl2 (1.5 mL). The reaction vessel was covered with foil (critical), stirred for 1.5 h, quenched by addition of 0.1 M

Et3N in cyclohexane (0.5 mL) and concentrated under vacuum. The resulting residue was chromatographed (SiO2; hexanes–EtOAc = 7:3) to give 2.36 (81.5 mg, 90%, ≥25:1 ratio of endo/exo isomers) as a white solid. The relative configuration of 2.36 was assigned by analogy to 2.48 (see below). TLC (hexanes–EtOAc = 7:3): Rf = 0.28. 1H NMR (CDCl3): δ

7.59 (d, J = 7.3 Hz, 2 H), 7.22 (t, J = 7.7 Hz, 2 H), 7.09 (t, J = 7.3 Hz, 1 H), 6.07 (d, J = 2.3 Hz,

1 H), 6.01 (d, J = 2.3 Hz, 1 H), 3.76 (s, 3 H), 3.75 (s, 3 H), 3.71 (s, 6 H), 3.65 (s, 3 H), 3.58

(dq, J = 4.8, 8.1 Hz, 1 H), 2.99 (ddd, J = 4.4, 7.1, 11.4, 1 H), 2.68–2.54 (m, 3 H), 2.47 (ddd, J =

1.3, 7.9, 13.9, 1 H), 2.25 (dd, J = 9.3, 13.8, 1 H), 2.09 (dd, J = 2.9, 13.9 Hz, 1 H). 13C{1H}

NMR (CDCl3): δ 173.4, 172.8, 159.5, 159.4, 157.9, 149.5, 127.8, 126.7, 125.3, 115.6, 91.2, 90.8,

64.0, 55.5, 55.3, 55.1, 52.7, 52.6, 51.4, 46.0, 44.0, 41.1, 38.3, 34.8. HRMS calcd (found) for

C26H31O7 (MH+): 455.2064 (455.2076).

2.36-d3. Deuterated isotopomer 2.36-d3 was synthesized in 82% yield from reaction of 2.12 and 2,4,6-trideuterio-1,3,5-trimethoxybenzene-d3 (TMB-d3, 97% d) in

CD2Cl2 in foil-covered glassware that had been previously soaked overnight in 6 M DCl, washed with D2O, and treated with Me2SiCl2. 1H NMR analysis in toluene-d8 (which provides full resolution of the C5 and C7 proton resonances) revealed ~75% deuterium incorporation at the C5 bridgehead position, while 1H and 13C{1H} NMR analysis revealed no detectable deuteration at the C7 position (Figure 2.5 and Figure 2.6). 1H

NMR (400 MHz, toluene-d8): δ 7.77 (d, J = 7.5 Hz, 2 H), 7.20 (t, J = 7.4 Hz), 7.01 (t, J = 7.3

Hz, 1 H), 5.93 (s, 0.03 H), 5.85 (s, 0.03 H), 3.91 (dq, J = 5.9, 10.8 Hz, 0.24 H), 3.38 (s, 3 H),

3.32 (s, 3 H), 3.29 (s, 3 H), 3.21 (s, 6 H), 3.17–3.09 (m, 1 H), 3.00 (dd, J = 6.9, 14.0 Hz, 1 H),

2.86 (dd, J = 9.0, 12.3 Hz, 1 H), 2.77–2.68 (m, 1 H), 2.70–2.55 (m, 2 H), 2.40 (dd, J = 2.5, 13.8

Hz, 1 H). 13C{1H} NMR (CDCl3): δ 173.4, 172.8, 159.3, 157.8, 149.4, 127.8, 126.7, 125.3,

115.5, 90.7 (m), 64.0, 55.4, 55.3, 55.1, 52.7, 52.6, 51.4, 51.0 (t, J = 20.5 Hz, 45.9 (isotopic shift

= 131 ppb), 45.8, 44.0, 41.1, 38.3 (isotopic shift = 103 ppb), 38.2, 34.8 (isotopic shift = 140 ppb), 34.6.

Figure 2.5. 1H NMR analysis 2.36-d3 in toluene-d8 at 25 °C.

Figure 2.6. 13C{1H} NMR analysis 2.36-d3 in toluene-d8 at 25 °C.

2.43. The relative configuration of 2.43 was assigned by analogy to 2.45 (see below). TLC (hexanes–EtOAc = 7:3): Rf = 0.42. 1H NMR (CDCl3, 1.7:1 exo/endo): δ 7.36 (d,

J = 8.6 Hz, 1 H, exo + endo), 7.31–7.21 (m, 3 H, exo + endo), 7.14–7.06 (m, 2 H, exo + endo), 7.01 (d, J = 8.8 Hz, 1 H, exo + endo), 6.80 (d, J = 8.7 Hz, 2 H, endo), 6.79 (d, J = 8.9

Hz, 2 H, exo), 3.75 (s, 3 H, endo), 3.74 (s, 3 H, exo), 3.72 (s, 3 H, exo + endo), 3.63 (s, 3 H, endo), 3.61 (s, 3 H, exo), 3.48 (dq, J = 3.9, 8.3 Hz, 1 H, exo + endo), 2.94–2.79 (m, 2 H, exo + endo), 2.67–2.50 (m, 3 H, exo + endo), 2.15–2.01 (m, 2 H, exo + endo). 13C{1H} NMR

(CDCl3, 1.7:1 exo/endo): δ 172.8 (exo + endo), 172.2 (endo), 172.2 (exo), 157.5 (exo), 157.5

(endo), 149.8 (endo), 148.1 (exo), 141.5 (exo), 140.0 (endo), 128.4 (exo), 127.9 (endo), 127.6

(exo + endo), 126.8 (exo), 126.5 (endo), 125.6 (endo), 125.5 (exo), 113.8 (endo), 113.7 (exo),

64.4 (exo), 64.3 (endo), 55.3, 52.9, 52.7, 49.5 (exo), 49.2 (exo), 48.7 (endo), 48.6 (endo), 41.1

(endo), 41.0 (exo), 38.5 (exo), 38.5 (endo), 33.3 (exo), 33.3 (endo). HRMS calcd (found) for

C24H26NaO5 (M + Na+): 417.1672 (417.1672).

2.46. Viscous colorless oil, 83%, as a 2.5:1 ratio of exo/endo isomers. Relative configuration assigned by analogy to 2.45 (see below). TLC (hexanes–EtOAc = 7:3): Rf =

0.22. 1H NMR (CDCl3, 2.5:1 exo/endo): δ 7.35 (d, J = 7.3 Hz, 1 H, exo + endo), 7.28–7.19

(m, 3 H, exo + endo), 7.14–7.04 (m, 2 H, exo + endo), 6.93 (d, J = 8.6 Hz, 1 H, exo + endo),

6.71 (d, J = 8.7 Hz, 2 H, exo + endo), 5.38 (br s, 1 H, exo), 5.33 (br s, 1 H, endo), 3.72 (s, 3

H, exo + endo), 3.62 (s, 3 H, endo), 3.61 (s, 3 H, exo), 3.48 (dq, J = 4.3, 7.9 Hz, 1 H, exo + endo), 2.94–2.78 (m, 2 H, exo + endo), 2.66–2.48 (m, 3 H, exo + endo), 2.15–1.98 (m, 2 H,

exo + endo). 13C{1H} NMR (CDCl3, 2.5:1 exo/endo): δ 173.1 (exo), 173.0 (endo), 172.4

(endo), 172.2 (exo), 153.7 (exo), 153.6 (endo), 149.7 (endo), 148.1 (exo), 141.4 (exo), 139.9

(endo), 128.4 (endo), 128.4 (exo), 128.1 (endo), 127.8 (exo), 126.7 (exo), 126.5 (endo), 125.7

(endo), 125.5 (exo), 115.2 (endo), 115.2 (exo), 64.4 (exo), 64.3 (endo), 53.0 (exo), 53.0

(endo), 52.8 (endo), 52.8 (exo), 49.5 (exo), 49.2 (endo), 48.6 (endo), 48.6 (exo), 41.1 (endo),

41.0 (exo + endo), 38.5 (exo), 33.3 (exo), 33.2 (endo). HRMS calcd (found) for C23H24NaO5

(M + Na+): 403.1516 (403.1521).

2.45. Colorless, highly viscous oil, 85% as a 2:1 ratio of exo/endo isomers. The relative configuration of the exo and endo diastereomers of 2.45 were assigned as follows. Repeated chromatography from a large scale experiment (350 mg total product;

SiO2, hexanes–EtOAc = 4:1) provided a fractions enriched in the exo isomer (32.6 mg, exo/endo ≥ 50:1) and in the endo isomer (9.2 mg, endo/exo = 10:1). 1H NMR analysis of the major exo-isomer in toluene-d8 at 25 °C (Figure 2.5), which provides full resolution of the C5 bridgehead proton resonance, exhibited no evidence of hindered rotation of the exo-2,4-dimethoxyphenyl ring. 1H–1H NOESY NMR analysis revealed through-space interaction between the C5 bridgehead proton and the C6 proton of the 2,4- dimethoxyphenyl ring (Figure 2.8). 1H NMR analysis of the minor endo-isomer at 25 ºC in toluene-d8 revealed hindered rotation of the endo-2,4-dimethoxyphenyl ring as evidenced by broadening of the resonances corresponding to the C1, C2, C4, C5 and C7 protons on the bicyclo-[3.2.0]-heptane backbone (Figure 2.9). When the solution was

heated at 80 °C, these resonances sharpened to reveal the fine splitting associated with each resonance (Figure 2.9). TLC (hexanes–EtOAc = 4:1): Rf = 0.24. 1H NMR (exo isomer, toluene-d8, 25 ºC): δ 7.37 (d, J = 8.5 Hz, 1 H), 7.16 (d, J = 7.2 Hz, 2 H), 7.07 (t, J = 7.7 Hz, 2

H), 6.91 (t, J = 7.3 Hz, 1 H), 6.35 (dd, J = 2.5, 8.5 Hz, 1 H), 6.15 (d, J = 2.5 Hz, 1 H), 3.80 (dq,

J = 3.6, 8.3 Hz, 1 H), 3.37 (s, 3H), 3.36 (s, 3H), 3.19 (s, 3H), 3.04 (s, 3H), 2.87 (ddd, J = 3.5,

8.3, 11.9 Hz, 1 H), 2.78 (d, J = 7.6 Hz, 1 H), 2.76 (d, J = 7.8 Hz, 1 H), 2.69 (dq, J = 3.6, 7.9 Hz,

1 H), 2.62 (ddd, J = 1.3, 8.1, 13.4 Hz, 1 H), 2.34 (dd, J = 3.4, 13.4 Hz, 1 H), 2.23 (dd, J = 8.8,

13.9 Hz, 1 H). 1H NMR (endo isomer, toluene-d8, 80 °C): δ 7.39 (d, J = 8.4 Hz, 1 H), 7.10

(t, J = 7.5 Hz, 2 H), 7.01–6.93 (m, 3 H), 6.33 (dd, J = 2.4, 8.3 Hz, 1 H), 6.17 (d, J = 2.3 Hz, 1

H), 3.83–3.72 (br m, 1 H), 3.39 (s, 3 H), 3.38 (s, 3 H), 3.25 (s, 3 H), 3.13 (s, 3 H), 2.91–2.79

(m, 2 H), 2.71–2.55 (m, 3 H), 2.34 (dd, J = 8.4, 14.0 Hz, 1 H), 2.28 (dd, J = 3.2, 13.8 Hz, 1 H).

13C{1H} NMR (CDCl3, exo + endo): δ 173.1 (endo), 173.0 (exo), 172.5 (endo), 172.3 (exo),

159.5 (exo), 158.7 (exo), 149.1 (endo), 146.9 (exo), 129.0, 128.6, 127.9, 127.4, 127.3, 126.6,

126.5, 125.3, 124.9, 103.8 (endo), 103.4 (exo), 99.9 (exo), 99.0 (endo), 64.3, 55.4 (endo), 55.3

(exo), 55.0 (endo), 55.0 (exo), 52.8 (endo), 52.8 (exo), 52.6 (endo), 52.6 (exo), 47.8 (exo),

47.3 (endo), 47.0 (exo), 41.1 (exo), 38.7 (exo), 38.1 (exo), 33.3 (exo). HRMS calcd (found) for C25H29O6 (MH+): 425.1959 (425.1954).

Figure 2.7. 1H NMR spectrum of exo-2.45 in toluene-d8 at 25 °C.

Figure 2.8. 1H–1H NOESY NMR analysis exo-2.45 in toluene-d8 at 25 °C.

Figure 2.9. 1H NMR spectrum of endo-2.45 in toluene-d8 at 25 °C (right spectrum) and 80 °C (left spectrum).

2.46. Pale yellow foam, 72% as a 3.6:1 mixture of exo/endo isomers. TLC

(CH2Cl2–MeOH = 9:1): Rf = 0.36 (exo), 0.41 (endo). 1H NMR (CDCl3, exo, major rotamer):

δ 7.39 (d, J = 8.6 Hz, 1 H), 7.22 (t, J = 7.6 Hz, 2 H), 7.17 (d, J = 7.4 Hz, 2 H), 7.10 (t, J = 7.5

Hz, 1 H), 6.40 (dd, J = 2.6, 8.5 Hz, 1 H), 6.18 (d, J = 2.5 Hz, 1H), 5.79 (br s, 1 H), 5.10 (br s, 1

H), 3.69 (s, 3H), 3.66–3.58 (m, 1 H), 3.62 (s, 3 H), 2.82–2.57 (m, 4 H), 2.53 (d, J = 6.6 Hz, 1

H), 2.51 (d, J = 6.2 Hz, 1 H), 2.38 (ddd, J = 1.1, 8.1, 13.8 Hz, 1 H), 2.12 (dd, J = 3.2, 13.9, 1

H), 1.91 (dd, J = 9.2, 13.8, 1 H). 1H NMR (400 MHz, CDCl3, endo): δ 7.75 (d, J = 10.3 Hz, 1

H), 7.46–7.38 (m, 1 H), 7.36–7.09 (m, 4 H), 6.07–6.02 (m, 2 H), 4.72 (br s, 2 H), 3.76–3.70

(m, 3 H), 3.68–3.59 (m, 4 H), 2.88–2.65 (m, 3 H), 2.63–2.51 (m, 1 H), 2.52–2.41 (m, 1 H),

2.25–2.10 (m, 1 H), 2.08–1.93 (m, 1 H). 13C{1H} NMR (CDCl3, exo, major rotamer): δ 173.4,

172.5, 155.7, 155.1, 145.6, 128.5, 127.0, 126.6, 126.5, 126.0, 107.1, 104.7, 64.2, 53.1, 52.9, 47.5,

46.2, 41.1, 37.8, 37.6, 33.1. HRMS calcd (found) for C23H24NaO6 (M + Na+): 419.1465

(419.1479).

2.47. White foam, 86%, 7:1 ratio of exo/endo isomers. The relative configuration of the major and minor isomers were assigned by NOESY analysis (Figure 2.10). In particular, nOe signal enhancement was observed between the C5 bridgehead proton and the C2/C6 protons of the 3,5-di-tert-butylphen-4-ol ring of the major diastereomer that was absent in the minor diastereomer. TLC (hexanes–EtOAc = 4:1): Rf = 0.36. 1H

NMR (CDCl3, 7:1 exo/endo): δ 7.38 (d, J = 7.4 Hz, 1 H, endo), 7.25 (t, J = 7.7 Hz, 2 H, exo + endo), 7.16 (s, 2 H, exo), 7.12–7.06 (m, 3 H, exo + endo), 6.84 (s, 2 H, endo), 5.01 (br s, 1 H,

exo), 4.98 (br s, 1 H, endo), 3.71 (s, 3 H, endo), 3.70 (s, 3 H, exo), 3.62 (s, 3 H, endo), 3.59

(s, 3 H, endo), 3.40 (dq, J = 3.8, 8.4 Hz, 1 H, exo + endo), 2.98 (ddd, J = 3.7, 8.4, 11.8 Hz, 1

H, exo + endo), 2.90 (dq, J = 3.5, 8.1 Hz, 1 H, exo + endo), 2.63 (ddd, J = 1.1, 8.1, 13.9 Hz, 1

H, exo + endo), 2.58–2.48 (m, 2 H, exo + endo), 2.10 (d, J = 13.9 Hz, 2 H, exo + endo), 2.08

(dd, J = 5.8, 13.9 Hz, 1 H, exo + endo), 1.39 (s, 18 H, endo), 1.38 (s, 18 H, exo). 13C{1H}

NMR (CDCl3): δ 172.9 (endo), 172.9 (exo), 172.2 (endo), 172.2 (exo), 151.6 (exo), 151.5

(endo), 150.4 (endo), 148.5 (exo), 139.8 (exo), 138.2 (endo), 135.4 (endo), 135.3 (exo), 128.3

(endo), 128.2 (exo), 127.0 (exo), 126.5 (endo), 125.4 (endo), 125.3 (exo), 123.4 (endo), 123.2

(exo), 64.4 (endo), 64.4 (exo), 52.8 (exo), 52.7 (exo), 51.5 (endo), 50.2 (exo), 49.9 (endo),

49.3 (endo), 49.2 (exo), 41.2 (endo), 41.1 (endo), 41.1 (exo), 41.0 (exo), 38.7 (exo), 38.5

(endo), 34.6 (exo), 34.5 (endo), 33.6 (exo), 33.2 (endo), 30.5 (endo), 30.5 (exo). HRMS calcd (found) for C31H41O5 (MH+): 493.2949 (493.2950).

Figure 2.10. 1H–1H NOESY NMR analysis of exo-2.47 in toluene-d8 at 25 °C.

2.48. White solid, 88%, as a ≥50:1 ratio of endo/exo isomers. Relative configuration determined by X-ray crystallography (see below). TLC (hexanes–EtOAc =

3:1): Rf = 0.27. 1H NMR (CDCl3): δ 7.61 (d, J = 7.4 Hz, 2 H), 7.21 (t, J = 7.8 Hz, 2 H), 7.09 (t,

J = 7.3 Hz, 1 H), 6.31 (s, 1 H), 6.24 (s, 1 H), 3.76 (s, 3 H), 3.72 (s, 3 H), 3.71 (s, 3 H), 3.64 (s, 3

H), 3.62–3.56 (m, 1 H), 3.06–2.98 (m, 1 H), 2.67–2.56 (m, 3 H), 2.46 (ddd, J = 1.4, 7.8, 13.9, 1

H), 2.27 (s, 3 H), 2.24 (dd, J = 9.5, 14.0 Hz, 1 H), 2.09 (dd, J = 2.7, 13.9 Hz, 1 H). 13C{1H}

NMR (CDCl3): δ 173.5, 172.8, 158.7, 157.1, 149.3, 137.2, 127.8, 126.8, 125.4, 119.9, 105.7,

104.9, 64.1, 55.5, 55.1, 52.7, 52.6, 51.6, 46.2, 44.1, 41.1, 38.4, 34.8, 22.0. HRMS calcd (found) for C26H30NaO6 (M + Na+): 461.1935 (461.1937).

2.36. Viscous colorless oil, 84%, as a ≥25:1 ratio of endo/exo isomers. Relative configuration assigned by analogy to 2.48. TLC (hexanes–EtOAc = 7:3): Rf = 0.26. 1H

NMR (CDCl3): δ 7.99 (d, J = 1.4 Hz, 1 H), 7.83–7.64 (m, 4 H), 7.44–7.31 (m, 2 H), 6.09 (d, J =

2.3 Hz, 1 H), 6.01 (d, J = 2.3 Hz, 1 H), 3.80 (s, 3 H), 3.78–3.74 (m, 1 H), 3.74 (s, 3 H), 3.73 (s,

3 H), 3.72 (s, 3 H), 3.67 (s, 3 H), 3.13–3.00 (m, 1 H), 2.76–2.59 (m, 3 H), 2.49 (dd, J = 6.3,

14.0 Hz, 1 H), 2.30 (dd, J = 9.3, 13.8 Hz, 1 H), 2.14 (dd, J = 2.5, 13.9 Hz, 1 H). 13C{1H} NMR

(CDCl3): δ 173.5, 172.9, 159.6, 159.5, 158.1, 146.7, 133.3, 131.8, 128.1, 127.5, 127.3, 126.7,

125.5, 125.0, 123.9, 115.4, 91.3, 90.9, 64.1, 55.5, 55.3, 55.2, 52.8, 52.6, 51.2, 46.2, 43.9, 41.1,

38.4, 34.9. HRMS calcd (found) for C30H33O7 (MH+): 505.2221 (505.2228).

2.37. Pale yellow solid, 84%, as a ≥25:1 ratio of endo/exo isomers. The relative configuration of 2.37 was assigned by analogy to 2.48. TLC (hexanes–EtOAc = 7:3): Rf =

0.32. 1H NMR (CDCl3): δ 7.18 (s, 2 H), 6.74 (s, 1 H), 6.07 (d, J = 2.3 Hz, 1 H), 6.01 (d, J = 2.3

Hz, 1 H), 3.76 (s, 3 H), 3.75 (s, 3 H), 3.72 (s, 6 H), 3.65 (s, 3 H), 3.56 (dq, J = 4.5, 7.7 Hz, 1

H), 2.97 (ddd, J = 4.4, 7.7, 12.1 Hz, 1 H), 2.69–2.52 (m, 3 H), 2.46 (ddd, J = 1.5, 8.1, 13.9 Hz,

1 H), 2.25 (s, 6 H), 2.22 (dd, J = 9.0, 13.5 Hz, 1 H), 2.09 (dd, J = 3.1, 14.0 Hz, 1 H). 13C{1H}

NMR (CDCl3): δ 173.5, 172.8, 159.4, 159.3, 157.9, 149.3, 136.9, 127.2, 124.6, 115.9, 91.3, 90.9,

100

64.0, 55.4, 55.3, 55.1, 52.7, 52.6, 51.5, 45.8, 44.1, 41.1, 38.4, 34.8, 21.8. HRMS calcd (found) for C28H35O7 (MH+): 483.2377 (483.2386).

2.38. Viscous colorless oil, 80%, as a ≥25:1 ratio of endo/exo isomers. The relative configuration of 2.38 was assigned by analogy to 2.48. TLC (hexanes–EtOAc = 3:1): Rf =

0.26. 1H NMR (CDCl3): δ 7.60 (d, J = 7.3 Hz, 2 H), 7.23 (t, J = 7.8 Hz, 2 H), 7.10 (t, J = 7.3

Hz, 1 H), 6.09 (d, J = 2.3 Hz, 1 H), 6.00 (d, J = 2.3 Hz, 1 H), 4.05 (d, J = 10.8 Hz, 1 H), 3.96

(d, J = 10.9 Hz, 1 H), 3.94 (d, J = 11.0 Hz, 1 H), 3.86 (d, J = 10.9 Hz, 1 H), 3.78 (s, 3 H), 3.76

(s, 3 H), 3.68 (s, 3 H), 3.65–3.57 (m, 1 H), 3.05–2.94 (m, 1 H), 2.62–2.50 (m, 2 H), 2.06 (s, 3

H), 2.01 (s, 3 H), 1.89 (dd, J = 7.9, 13.6 Hz, 1 H), 1.71 (dd, J = 6.9, 14.0 Hz, 1 H), 1.65 (dd, J =

9.4, 13.8 Hz, 1 H), 1.37 (dd, J = 2.0, 14.0 Hz, 1 H). 13C{1H} NMR (CDCl3): δ 171.4, 171.2,

159.4, 158.0, 149.6, 127.8, 126.7, 125.3, 116.0, 91.1, 90.8, 68.8, 66.5, 55.5, 55.3, 55.0, 50.7, 50.3,

46.5, 44.8, 39.1, 35.9, 34.5, 21.0. HRMS calcd (found) for C28H35O7 (MH+): 483.2377

(483.2381).

2.39. White solid, 64%, as a ≥25:1 ratio of endo/exo isomers. The relative configuration of 2.39 was assigned by analogy to 2.48. TLC (hexanes–EtOAc = 7:3): Rf =

0.38. 1H NMR (CDCl3): δ 7.61 (d, J = 7.4 Hz, 2 H), 7.23 (t, J = 7.7 Hz, 2 H), 7.10 (t, J = 7.3

Hz, 1 H), 6.07 (d, J = 2.3 Hz, 1 H), 6.01 (d, J = 2.3 Hz, 1 H), 3.77 (s, 3 H), 3.76 (s, 3 H), 3.72

(s, 3 H), 3.66 (s, 2 H), 3.57 (d, J = 11.2 Hz, 1 H), 3.55–3.49 (m, 1 H), 3.47 (d, J = 11.2 Hz, 1

H), 3.03 (ddd, J = 4.5, 7.6, 12.0 Hz, 1 H), 2.60–2.51 (m, 1 H), 2.47 (dd, J = 9.2, 12.0 Hz, 1 H),

2.05 (dd, J = 8.6, 13.1, 1 H), 1.86 (dd, J = 8.3, 13.6, 1 H), 1.53 (dd, J = 9.2, 13.9, 1 H), 1.40 (s, 3

101

H), 1.39 (s, 3 H), 1.28-1.20 (m, 2 H). 13C{1H} NMR (CDCl3): 159.5, 159.4, 158.1, 149.9, 127.8,

126.8, 125.3, 116.2, 97.7, 91.2, 90.8, 70.1, 69.0, 55.5, 55.3, 55.1, 50.5, 46.5, 46.4, 45.0, 40.5,

36.8, 34.2, 24.2, 23.8. HRMS calcd (found) for C27H35O5 (MH+): 439.2479 (439.2487).

2.40. Pale yellow viscous oil, 79%, as a ≥25:1 ratio of endo/exo isomers and a 2:1 ratio of C2 diasteromers (major isomer not determined). The relative configuration of the C6 stereocenter of 2.40 was assigned by analogy to 2.48 and 2.42. TLC (hexanes–

EtOAc = 7:3): Rf = 0.33. 1H NMR (CDCl3, 2:1 C2 diastereomers): δ 7.63–7.53 (m, 2 H, major + minor), 7.26–7.16 (m, 2 H, major + minor), 7.12–7.06 (m, 1 H, major + minor), 6.09

(d, J = 2.4 Hz, 1 H, major), 6.08 (d, J = 2.3 Hz, 1 H, minor), 6.01 (d, J = 2.3 Hz, 1 H, minor),

6.00 (d, J = 2.3 Hz, 1 H, major), 3.89–3.83 (m, 1 H, minor), 3.78 (s, 3 H, major), 3.75 (s, 3 H, minor), 3.75 (s, 3 H, minor), 3.74 (s, 3 H, major), 3.72 (s, 3 H, major), 3.70 (s, 3 H, minor),

3.68 (s, 3 H, major), 3.68 (s, 3 H, minor), 3.63 (s, 3 H, major), 3.62 (s, 3 H, minor), 3.33 (dq,

J = 4.5, 7.5 Hz, 1 H, major), 3.08–2.95 (m, 1 H, major + minor), 2.85–2.51 (m, 3 H, major + minor), 2.34 (dd, J = 7.8, 14.2 Hz, 1 H, major), 2.26 (dq, J = 3.5, 8.3 Hz, 1 H, major), 2.10

(dd, J = 8.6, 14.2 Hz, 1 H, minor), 0.96 (d, J = 7.2 Hz, 3 H, minor), 0.86 (d, J = 7.4 Hz, 3 H, major). 13C{1H} NMR (CDCl3, 2:1 C2 diastereomers): δ 173.6 (major), 172.9 (minor), 172.2

(minor), 171.5 (major), 159.5 (minor), 159.4 (major + minor), 159.4 (major), 158.0 (minor),

157.8 (major), 149.9 (minor), 149.4 (major), 127.8 (minor), 127.8 (major), 126.7 (major),

126.7 (minor), 125.3 (major), 125.3 (minor), 115.9 (major), 115.3 (minor), 91.4 (major), 91.1

(minor), 90.9 (major), 90.7 (minor), 67.3 (minor), 67.2 (major), 55.6 (major), 55.4 (minor),

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55.3 (major), 55.1 (minor), 55.0 (major), 52.7 (major), 52.4 (minor), 52.1 (major), 51.9

(minor), 51.0, 49.4 (major), 48.4 (minor), 45.5 (major), 45.4 (minor), 43.9, 42.7, 40.2, 40.0,

38.6, 36.7, 35.7, 17.0 (minor), 13.4 (major). HRMS calcd (found) for C27H33O7 (MH+):

469.2221 (469.2223).

2.41. Yellow solid, 86%, as a ≥25:1 mixture of endo/exo isomers and a 3:1 mixture of C2 diastereomers (major isomer not determined). The relative configuration of the C6 stereocenter of 2.41 was assigned by analogy to 2.48 and 2.42. TLC (hexanes–EtOAc =

7:3): Rf = 0.33. 1H NMR (CDCl3, 3:1 C2 diastereomers): δ 7.66 (d, J = 8.3 Hz, 1 H, minor),

7.61 (d, J = 8.2 Hz, 1 H, minor), 7.48 (d, J = 8.1 Hz, 1 H, major), 7.38–7.14 (m, 8 H, major + minor), 7.10 (t, J = 7.3 Hz, 1 H, major), 6.14–6.01 (m, 2 H, major + minor), 4.27 (d, J = 7.4

Hz, 1 H, minor), 4.21–4.08 (m, 1 H, major + minor), 3.91 (d, J = 9.2 Hz, 1 H, major), 3.78 (s,

3 H, minor), 3.76 (s, 3 H, major), 3.75 (s, 3 H, major), 3.72 (s, 3 H, minor), 3.60 (s, 3 H, minor), 3.42 (s, 3 H, major), 3.30 (s, 3 H, major), 3.16–3.04 (m, 1 H, major + minor), 3.13 (s,

3 H, minor), 3.09 (s, 3 H, minor), 3.03–2.88 (m, 1 H, major + minor), 2.79 (dd, J = 9.4, 14.6

Hz, 1 H, major + minor), 2.72 (dd, J = 8.5, 13.6 Hz, 1 H, major + minor), 2.52 (dd, J = 8.6

14.9 Hz, 1 H, minor), 2.17 (dd, J = 4.7, 14.6 Hz, 1 H, major + minor). 13C{1H} NMR (CDCl3, major diastereomer): δ 172.0, 171.9, 159.5, 159.3, 145.4, 139.9, 129.0, 128.4, 127.9, 127.7,

126.9, 126.7, 125.4, 120.6, 93.0, 91.7, 91.6, 69.0, 57.1, 55.6, 55.4, 55.3, 52.2, 51.8, 49.0, 41.0,

38.7, 34.8. HRMS calcd (found) for C32H34NaO7 (MH+): 553.2197 (553.2204).

2.42. White solid, 77%, as a ≥25:1 mixture of endo/exo isomers. The relative

103

configuration of 2.42 was assigned by analogy to 2.48. TLC (hexanes–EtOAc = 7:3): Rf =

0.33. 1H NMR (CDCl3): δ 7.56 (d, J = 7.4 Hz, 2 H), 7.22 (t, J = 7.8 Hz, 2 H), 7.09 (t, J = 7.3

Hz, 1 H), 6.10 (d, J = 2.3 Hz, 1 H), 6.01 (d, J = 2.3 Hz, 1 H), 3.78 (s, 3 H), 3.75 (s, 3 H), 3.68

(s, 3 H), 3.67 (s, 3 H), 3.63–3.56 (m, 1 H), 3.45 (s, 3 H), 3.07 (dd, J = 9.0, 13.4 Hz, 1 H), 2.78

(dd, J = 9.1, 15.3 Hz, 1 H), 2.60 (ddd, J = 4.0, 8.5, 13.1 Hz, 1 H), 2.41 (q, J = 8.5 Hz, 1 H),

2.28 (dd, J = 6.1, 15.3 Hz, 1 H), 1.18 (s, 3 H), 1.00 (s, 3 H). 13C{1H} NMR (CDCl3): δ 172.0,

171.9, 159.5, 159.4, 158.0, 150.3, 127.8, 126.5, 125.2, 115.0, 91.4, 90.7, 67.9, 55.7, 55.3, 55.1,

52.3, 51.8, 48.0, 47.9, 47.2, 45.3, 37.1, 36.8, 27.3, 21.8. HRMS calcd (found) for C28H35O7

(MH+): 483.2377 (483.2388).

2.3.6. Optimization and control experiments

Reaction of 2.12 with 1,3,5-trimethoxybenzene (TMB) catalyzed by 2.28 (Table

2.1, entry 2). A solution of 2.28 (4.1 mg, 5.0 × 10–3) in CH2Cl2 (0.5 mL) was added to a solution of enyne 2.12 (28.6 mg, 0.100 mmol), TMB (34 mg, 0.20 mmol), and nitrobenzene

(5.1 µL, 0.050 mmol) in CH2Cl2 (0.5 mL). The reaction vessel was covered with foil, the resulting solution was stirred at room temperature and was monitored periodically by

TLC. After 6 h, 2.12 had been fully consumed. The reaction was quenched by addition of

0.1 M Et3N in cyclohexane (0.5 mL) and concentrated under vacuum. 1H NMR analysis of the crude reaction mixture showed formation of 2.13 in 47% yield without detectable

104

quantities of 2.26, as determined by integration of the vinylic C7 proton resonance of

2.13 at δ 6.08 relative to the ortho proton resonance of nitrobenzene at δ 8.23.

Reaction of 2 with TMB catalyzed by AgSbF6 (Table 2.1, entry 5). A solution of

AgSbF6 (6.9 mg, 2.0 × 10–2 mmol) in CH2Cl2 (0.5 mL) was added to a solution of enyne

2.12 (28.6 mg, 0.100 mmol), TMB (34 mg, 0.20 mmol) and n-hexadecane (14.6 µL, 0.050 mmol) in CH2Cl2 (0.5 mL). The resulting solution was stirred at room temperature and was monitored by GC. After 72 h, no consumption of 2.12 and no formation of either

2.13 or 2.26 was detected.

GC analysis of the reaction of 2.12 with TMB catalyzed by 2.27/AgSbF6. A solution of AgSbF6 (10.6 mg, 3.08 × 10–2 mmol) in CH2Cl2 (0.5 mL) was added to a solution of enyne 2.12 (44.6 mg, 0.156 mmol), TMB (52.5 mg, 0.312 mmol), 2.27 (4.5 mg,

7.7 × 10–3 mmol) and n-hexadecane (22.8 µL, 7.78 × 10–2 mmol) in CH2Cl2 (1.06 mL). The resulting solution was stirred at room temperature and monitored periodically by GC

(Figure 2.11). Disappearance of 2.12 obeyed approximate pseudo-first-order kinetics over two half-lives with a half-life of ~20 min, reaching 90% conversion after 90 min. The concentrationof 2.13 increased slowly and steadily throughout the reaction, reaching 6 mM after 70 min and then decreasing slowly to ~4 % over the following 100 min.

105

100.0!

80.0!

60.0!

40.0!

concentration(mM) 20.0!

0.0! 0.0! 60.0! 120.0! 180.0! time (min)

Reaction of 2.12 with TMB catalyzed by HOTf (Table 1, entry 7). A solution of

HOTf (0.44 µL, 5.0 × 10–3) in CH2Cl2 (0.5 mL) was added to a solution of enyne 2.12 (28.6 mg, 0.100 mmol), TMB (34 mg, 0.20 mmol), and n-hexadecane (14.6 µL, 0.050 mmol) in

CH2Cl2 (0.5 mL). The resulting solution was stirred at room temperature and analyzed by GC. After 24 h, no consumption of 2.12 and no formation of either 2.13 or 2.26 was detected.

GC analysis of the reaction of 2.12 with TMB catalyzed by 2.28/AgSbF6. A solution of AgSbF6 (10.6 mg, 3.08 × 10–2) in CH2Cl2 (0.5 mL) was added to a solution of

106

enyne 2.12 (44.6 mg, 0.156 mmol), TMB (52.5 mg, 0.312 mmol), 2.28 (4.5 mg, 7.7 × 10–3 mmol) and n-hexadecane (22.8 µL, 7.78 × 10–2 mmol) in CH2Cl2 (1.06 mL). The resulting solution was stirred at room temperature and monitored periodically by GC (Figure

2.12). Disappearance of 2.12 obeyed approximate pseudo-first-order kinetics over two half-lives with a half-life of ~20 min, reaching 90% conversion after 90 min. The concentration of 2.13 increased slowly and steadily throughout the reaction, reaching 6 mM after 70 min and then decreasing slowly to ~4 % over the following 100 min.

Reaction of 2.13 with TMB catalyzed by 4a and AgSbF6 (eq 1). A solution of 107

AgSbF6 (6.9 mg, 2.0 × 10–2 mmol) in CH2Cl2 (0.5 mL) was added to a solution of 3 (28.6 mg, 0.100 mmol, entry 1), 1,3,5-trimethoxybenzene (34 mg, 0.20 mmol, entry 3), 4a (2.9 mg, 5.0 × 10–3 mmol), and n-hexadecane (14.6 µL, 0.050 mmol) in CH2Cl2 (0.5 mL). The resulting solution was stirred at room temperature and analyzed periodically by GC, which revealed a half-life for consumption of 2 of t1/2 ~ 5 h. After 24 h, 2 was fully consumed and the reaction mixture was treated with 0.1 M Et3N in cyclohexane (0.25 mL) and concentrated under vacuum. The resulting residue was chromatographed

(SiO2, hexanes–EtOAc = 3:7) to give 5a in 63% yield as a ≥50:1 mixture of exo and endo isomers as determined by 1H NMR analysis.

Reaction of 3 with TMB catalyzed by AgSbF6 (eq 1). A solution of AgSbF6 (6.9 mg, 2.0 × 10–2 mmol) in CH2Cl2 (0.5 mL) was added to a solution of cyclobutene 2.13 (28.6 mg, 0.100 mmol), 1,3,5-trimethoxybenzene (34 mg, 0.20 mmol) and n-hexadecane (14.6

µL, 0.050 mmol) in CH2Cl2 (0.5 mL). The resulting solution was stirred at room temperature and was monitored by GC as described above, which revealed a half-life for consumption of 2.12 of t1/2 ~ 5 h. After 24 h, 2.12 was fully consumed and the reaction mixture was treated with 0.1 M Et3N in cyclohexane (0.25 mL) and concentrated under vacuum. The resulting residue was chromatographed (SiO2, hexanes–EtOAc = 3:7) to give 2.26 in 67% yield as a ≥50:1 mixture of exo and endo isomers as determined by 1H

NMR analysis.

108

2.3.7. X-ray crystal structure of 2.48

Slow evaporation of an ether/hexane solution of 2.48 gave crystals suitable for X- ray analysis (Figure 2.13, Table 2.3 and Table 2.4). A colorless rod-like specimen of

C26H30O6, approximate dimensions 0.160 mm × 0.180 mm × 0.300 mm, was used for analysis. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using a triclinic unit cell yielded a total of 45384 reflections to a maximum θ angle of 34.34° (0.63 Å resolution), of which

9381 were independent (average redundancy 4.838, completeness = 99.8%, Rint = 3.99%,

Rsig = 3.24%) and 7099 (75.67%) were greater than 2σ(F2). The final cell constants of a =

8.5825(3) Å, b = 11.4821(4) Å, c = 12.2823(4) Å, α = 71.488(2)°, β = 80.525(2)°, γ =

79.979(2)°, volume = 1122.44(7) Å3, are based upon the refinement of the XYZ-centroids of 9950 reflections above 20 σ(I) with 4.853° < 2θ < 70.24°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.927. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.9730 and 0.9860. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group

P-1, with Z = 2 for the formula unit, C26H30O6. The final anisotropic full-matrix least- squares refinement on F2 with 294 variables converged at R1 = 4.03%, for the observed data and wR2 = 12.01% for all data. The goodness-of-fit was 1.057. The largest peak in the final difference electron density synthesis was 0.452 e/Å3 and the largest hole was –

109

0.241 e/Å3 with an RMS deviation of 0.049 e/Å3. On the basis of the final model, the calculated density was 1.297 g/cm3 and F(000), 468 e.

Figure 2.13. ORTEP diagram of 2.48 with ellipsoids shown at the 50% probability level.

110

Table 2.3. Sample and crystal data for 2.48.

Identific ation code rds145

Chemical formula C26H30O6 Formula weight 438.50 Temperature 110(2) K Wavelength 0.71073 Å Crystal size 0.160 × 0.180 × 0.300 mm Crystal habit colorless rod Crystal system triclinic Space group P-1 Unit cell dimensions a = 8.5825(3) Å α = 71.488(2)° b = 11.4821(4) Å β = 80.525(2)°

c = 12.2823(4) Å γ = 79.979(2)°

Volume 1122.44(7) Å3

Z 2 Density (calculated) 1.297 g/cm3 Absorption coefficient 0.091 mm–1 F(000) 468

111

Table 2.4. Data collection and structure refinement for 2.48.

Theta range for data collection 1.76 to 34.34° Index ranges –13<=h<=13, –18<=k<=12, –19<=l<=19 Reflections collected 45384 Independent reflections 9381 [R(int) = 0.0399] Coverage of independent 99.8% reflections Absorption correction multi-scan Max. and min. transmission 0.9860 and 0.9730 Structure solution technique direct methods Structure solution program SHELXS-97 (Sheldrick, 2008) Refinement method Full-matrix least-squares on F2 Refinement program SHELXL-2013 (Sheldrick, 2013)

Function minimized Σ w(Fo2 – Fc2)2 Data / restraints / parameters 9381/0/294 Goodness-of-fit on F2 1.057

Δ/σmax 0.001 Final R indices 7099 data; I>2σ(I) R1 = 0.0403, wR2 = 0.1135 all data R1 = 0.0558, wR2 = 0.1201

Weighting scheme w=1/[σ2(Fo2)+(0.0678P)2+0.0334P] where

P=(Fo2+2Fc2)/3 Largest diff. peak and hole 0.452 and –0.241 eÅ-3 R.M.S. deviation from mean 0.049 eÅ-3

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3. Synthesis and Application of Multidentate Ligands Toward Realization of Fluxional Mechanocatalysis

Compound (Z)-(S)-3.11 synthesized by Zachary S. Kean and compound 3.34 was synthesized by Bobin Lee in the Craig laboratory at Duke University. DFT calculations to aid in the design of tetradentate ligand 3.36 were performed by Zachary S. Kean.

3.1. Introduction

3.1.1. Mechanocatalysis

The use of mechanical force to manipulate reactivity of chemical bonds is of considerable interest, as it allows for regulation of bond formation and scission that does not typically occur under thermal or (photo)chemical control. The use of mechanical force to initiate or control the reaction of chemical bonds is termed mechanochemistry.127,128 The use of mechanochemistry extends to time immemorial, when our early ancestors would use the heat generated from the friction between two pieces of wood to initiate a combustion reaction. The first scientific recording of a mechanochemically induced reaction can be dated as far back as 300 BC, when

Theophrastus of Ephesus, a student of Aristotle, observed the formation of liquid mercury as a result of grinding cinnabar (mercury(II) sulfide) together with vinegar

(aqueous acetic acid) in a brass mortar.127 More recently, groups have become interested in the incorporation of catalysts into a flexible polymeric network, allowing for control of catalyst reactivity through transmittance of force through the polymer network. The first example of polymer mechanochemistry was described by Klibanov et al., in which 113

the activity of an enzyme was controlled by its connection to a nylon polymeric fiber.

Through stretching and unstretching of the nylon fiber, the reactivity of the enzyme was altered due to the highly interconnected nature of enzyme conformation and activity.

Thus, when the enzyme was mechanically stretched, the resulting change in enzyme conformation mitigated the catalytic ability of the enzyme. When the force was released, the enzyme returned to its natural conformation and catalysis was restored.129

More recently, Sijbesma et al. reported the activation of latent catalysts using ultrasound irradiation.130,131 In these examples, dissociation of transition metal- coordinated supporting ligand was achieved through the use of ultrasound. The treatment of a solution with ultrasound creates longitudinal sound waves that pass through the liquid, leading to localized expansion and contraction of the fluid and resulting collapse of cavitation bubbles. The collapse of these cavitation bubbles creates strong shear forces and leads to the uncoiling and stretching of polymer chains within the solution.132

In the first example of mechanocatalysis provided by Sijbesma, an N-heterocyclic carbene (NHC) catalyzed transesterification reaction was initiated by sonochemical irradiation of a solution containing the catalytically inactive bis(carbene) silver complex

3.1, vinyl acetate and benzyl alcohol (Figure 3.1A). Ultrasound irradiation leads to dissociation of the NHC ligand, providing active NHC catalyst 3.2, which successfully catalyzes the transesterification reaction. In the second example, latent bis(NHC)-Ru-

114

alkylidene ring-closing metathesis (RCM) catalysts 3.3 were activated via ultrasound to provide an open coordination site in activated complex 3.4, resulting in successful ring- closing metathesis of diethyl diallylmalonate (Figure 3.1B). In these RCM studies, it was also shown that after initiation of the reaction by ultrasound, conversion of starting material could be disrupted by discontinuation of irradiation and the reaction could be restarted by restoring irradiation. Although this “on/off” temporal control of a reaction has great potential in many applications, including biological materials, mechanical signal transduction and self-healing materials, there have been no additional reports in the literature of such mechanocatalysis since the initial report by Sijbesma.

115

A. O O + HO EtN N O n Ag+ ultrasound O EtN N n O 3.2 EtN N n O 3.1 O

EtO2C CO2Et ArN N O n ArN N Cl ultrasound O Ru n Cl Ph Cl Ru O Cl Ph ArN N n 3.3 3.4 EtO2C CO2Et

Ar = 2,6-i-Pr2-Ph

3.1.2. Bidentate ligands in transition metal-catalyzed chemical transformations

The use of chiral bidentate supporting ligands to effect chemical transformations has received considerable attention.133-137 With the initial report of the DIOP ligand

(Figure 3.2A, 3.5), Kagan introduced the concept of C2-symmetry in ligand design.138

Representative examples of chiral bidentate supporting ligands include the C2- symmetric bis(phosphines) DIOP, BINAP, MeO-BIPHEP (3.5-3.7), bis(oxazolines) BOX

(3.8), and the bidentate non-symmetric phosphino(oxazolines) (PHOX, 3.9), which have

116

all been used extensively in transition metal-catalyzed reactions (Figure 3.2A). More specifically, the identity of the coordinating atoms and/or the stereochemical nature of the ligand has been shown to have profound effects on reaction outcomes. In the case of bis(phosphine) ligands, reaction rate, yield and enantioselectivity have all been shown to be highly dependent upon the dihedral angle (θ) between the two aryl rings and, as a consequence, bite angle (βn) on the transition metal atom (Figure 3.2B). This is especially evident in ruthenium and palladium-mediated catalytic transformations, such as asymmetric hydrogenation and asymmetric allylic alkylation reactions.135,136,139,140

R R O PPh O O O 2 PPh2 MeO PPh2 PPh MeO PPh N N O PPh2 2 2 PPh2 N R R tBu (R,R)-3.5 (R,R)-DIOP (R)-3.6 (R)-3.7 3.8 3.9 (R)-BINAP (R)-MeO-BIPHEP BOX (R)-t-Bu-PHOX

Biaryl backbone O PAr2 θ (CH2)n PAr P P O 2 β

3.10 M (R)-Cn-TunePhos

117

In a pioneering study, Zhang et al. synthesized a variety of C2-symmetric MeO-

BIPHEP derivatives in which the 6,6’-oxygens were tethered with a linker of tunable length, which they termed TunePhos ligands (Figure 3.2B, 3.10).13 In an initial survey, they performed Ru-catalyzed hydrogenation on a variety of β-ketoesters to give chiral β- hydroxyesters, and showed that the asymmetric induction was dependent on the dihedral angle θ, and thus ligand bite angle β. Additionally, these ligands were later shown to effect asymmetric induction in several allylic alkylation reactions of allyl acetates with malonate derivatives.141

Table 3.1. Effect of ligand dihedral angle (θ) on yield and enantiomeric excess in the hydrogenation of methyl acetoacetate.

H2 (750 psi), [Ru(C6H6)Cl2]2 (0.5 mol %) O O Ligand (1.2 mol %) OH O

OMe MeOH, 60 °C, 20 h OMe (% ee)

MeO- ligand BINAP C1 C2 C3 C4 C5 C6 BIPHEP

θ (°) 87 87 60 74 77 88 94 106

% ee 98.4 97.9 90.9 90.8 97.7 99.1 97.1 96.5

More recently, Craig, Boulatov and Widenhoefer have shown the dependence of bite angle in an asymmetric Heck reaction using a chiral ligand tethered to a stiff stilbene photoswitch that functions as a molecular force probe.140 In their studies, an axially chiral (S)-MeO-BIPHEP derivative was used, in which the 6,6’-oxygens were 118

tethered by a stiff stilbene whose E/Z isomerism could be manipulated by photoisomerization. Photochemical irradiation (λ = 365 nm) of the stilbene prompts

Z→E isomerization about the double-bond, thus enacting a mechanical force on the bis(phosphine) ligand and leading to a change in dihedral angle θ, and correspondingly, bite angle β (Scheme 3.1).

O O

hν O O PPh2 (365 nm) (E)-3.11, 23% O O PPh2

O O (Z)-3.11 68%

(E)-3.11', 9%

Scheme 3.1. Irradiation of stiff stilbene (Z)-3.11 with light (λ = 365 nm) prompts Z→E isomerization to provide a 68:23:9 mixture of (Z)-3.11/(E)-3.11/(E)-3.11’.

Following the chromatographic isolation of the individual isomers (Z)-3.11, (E)-

3.11, and (E)-3.11’, the effect of each isomer as a ligand was examined in the palladium- mediated asymmetric Heck arylation of 2,3-dihydrofuran with phenyl triflate. In their studies, Craig et al. use the natural biaryl angle φ, as predicted by conformational energy minimization via DFT calculations. When a mixture of phenyl triflate, 2,3-dihydrofuran, iPr2NEt and catalytic Pd(OAc)2 was treated with (S)-MeO-BIPHEP (3.7; φ = 97°), the product 3.12 was recovered in 90% ee (Table 3.2, entry 1). When the same mixture was treated with the geometrically compressed ligand (Z)-3.11 (φ = 83°), the product 3.12 was

119

recovered in 96% ee (entry 2). Treatment of the mixture with the stretched ligand (E)-3.11

(φ = 106°) led to isolation of 3.12 in 79% ee (entry 3). Additionally, irradiation of the reaction mixture containing (E)-3.11 with 365 nm light gave 3.12 in 90% ee, resulting from the presumed 68:23:9 mixture of isomers as illustrated in Scheme 3.1. In these studies, Craig et al. illustrate that changes in reactivity of the complexes are a direct consequence of mechanical distortion of the ligand dihedral angle.

Table 3.2. Effect of ligand identity on palladium-mediated asymmetric Heck reaction of 2,3-dihydrofuran and phenyl triflate.

OTf ligand, Pd(OAc)2, iPr2NEt + O Benzene, 40 °C, O Ph O Ph O Ph O Ph 24 h (S)-3.12 (R)-3.12 (S)-3.13 (S)-3.13 natural (S)- dihedral Conv. Total (S) entry ligand 3.12/3.13 3.12 ee angle φ (%) products (%) (°) (S)-3.7 1 (MeO- 97 23 95/5 91 90 BIPHEP) 2 (Z)-3.11 83 55 97/3 96 96 3 (E)-3.11 106 95 98/2 88 79 (E)-3.11 4 Mixture 93 97/3 93 90 + 365nm

3.1.3. Polydimethylsiloxane (PDMS) polymer system

Polydimethylsiloxane (PDMS) is one of the most versatile and widely utilized polymers, finding applications from microfluidic,142-144 electrolithographic,145,146 and

120

biomedical engineering devices147,148 to pneumatically powered soft robotics149,150 and colorimetric materials.151 Its wide use can be traced to its chemical inertness, functional group tolerance152 and unique rheological properties, ranging from a thick pourable liquid (Figure 3.3, low n) to a thick rubbery semisolid (high n). The flexible polymer chains become loosely entangled following crosslinking, providing the material with highly elastic properties. PDMS has high stretchability153 and high mechanical strength.154 Although there are several methods of crosslinking, commercially available kits commonly used contain unmixed alkene-terminated and hydrosilane-terminated

PDMS chains. When the two chains are mixed together with a Pt catalyst, the polymer is crosslinked via hydrosilylation (Figure 3.3).

Si Si Si R O O R n

Si H [Pt] + Si Si H H

Figure 3.3. General structure of polydimethylsiloxane and Pt-catalyzed crosslinking.

In 2003, Whitesides et al. documented the compatibility of PDMS-based devices with organic solvents.155 In solvent-insoluble materials, such as crosslinked polymers, the solubility of the material in a given solvent is measured by the degree of swelling of

121

said material. The usefulness of PDMS in applications involving organic solvents is directly related to its solubility in a given solvent. In high-swelling solvents, PDMS is not a useful structural component, as its elastic properties degrade. In the study by

Whitesides, swelling ratios S (length of swollen PDMS/length of dry PDMS) are experimentally determined and solvents are ranked based on their ability to swell the polymer (Table 3.3). Low swelling solvents (1 < S < 1.1) included most alcohols, nitriles, disubstituted amides and sulfoxides. Reactions taking place in a PDMS system should target the use of these solvents.

122

Table 3.3. Ranking of PDMS swelling in various solvents based on the solubility parameter S.

Rank Solvent Swelling Rank Solvent Swelling

1 i-Pr2NH 21 n-PrOH

2 Et3N 22 Acetone 3 Pentane 23 Pyridine 4 Xylenes 24 EtOH

5 CHCl3 25 Dimethyl carbonate

6 Et2O 26 NMP 7 THF "High" 27 DMF 8 Hexanes 1.28

~~14 Benzene 34 MeNO2 15 PhCl 35 DMSO 16 DCM 36 Ethylene glycol 17 t-BuOH "Moderate" 37 Glycerol 18 2-Butanone 1.1 < S < 1.22 38 Water 19 EtOAc~~

~~20 Dioxane~~

~~3.1.4. Project goals~~

Due to the inhomogeniety of stretched PDMS, only about 1% of embedded mechanophores are activated under stress (>100 pN force).140 Therefore, to realize fluxional mechanocatalysis, catalysts embedded in a PDMS polymer network have two requirements: they must be activated by force, and they must undergo geometric change through applied force.

~~123~~

The objectives of this work are: 1) to extend the analysis of fluxional mechanocatalysis using molecular force probes (i.e. stiff stilbene photoswitch)140 to additional catalytic transformations in a PDMS polymer and 2) to develop a mechanically activated ligand for transition metal catalysis. Toward the former goal, we aim to incorporate a bis(phosphine) ligand into a PDMS framework and moderate its reactivity by transmission of mechanical force throughout the network (Figure 3.4A).

Prior analyses have shown that introduction of mechanical force on the ligand via the polymer network will have an observable outcome in reaction efficiency and/or enantiomeric excess via a mechanically induced change in dihedral angle.139,140 Toward the latter goal, we aim to synthesize and incorporate a latent catalyst that can be activated via mechanical force to provide the open coordination sites necessary for a catalytic reaction to occur (Figure 3.4B). Ideally, the elasticity of the PDMS can allow for restoration of original reactivity upon removal of the mechanical stimulus, an idea that has been recently demonstrated by Craig et al.151 Crosslinking of alkene-terminated ligands into the polymer network will allow for mechanical manipulation of these ligands (Figure 3.4C).

~~Ideal reactions for mechanocatalysis should possess the following characteristics:~~

~~a) An easily observable outcome that varies with the force applied (i.e.~~

~~enantiopurity, diastereomeric purity, reaction rate/yield).~~

~~124~~

~~b) High reaction rates so that reactions proceed at low temperature (to minimize~~

~~the decrease in viscosity observed when heating PDMS) and low catalyst~~

~~loading.~~

~~c) Practically simple to perform (ambient pressure/temperature, low air- and~~

~~moisture sensitivity).~~

~~d) Do not require the use of PDMS-swelling solvents.~~

~~Herein I describe efforts to synthesize the necessary ligands for polymer incorporation and demonstrate fluxional mechanocatalysis with these catalytic systems.~~

A stiff-stilbene-tethered bis(phosphine)-rhodium complex was synthesized and its activity demonstrated in a rhodium-catalyzed asymmetric intramolecular hydrosilylation, and bis(phosphine)-rhodium complex with suitable functionality for polymer incorporation was synthesized. Additionally, a tetradentate PHOX-bipyridyl ligand was synthesized and its activity determined in a palladium-catalyzed asymmetric intramolecular Tsuji allylation.

~~125~~

~~mechanical PPh2 O PPh2 PPh2 force θ θ O PPh ≡ 2 PPh2 PPh2~~

~~P N~~

~~P N Pd0 mechanical force Pd0~~

~~N N~~

~~N Latent Active~~

~~Si H [Pt] + + Si Si H H~~

~~= Ligand~~

~~126~~

~~3.2. Results and Discussion~~

~~3.2.1. Intramolecular hydrosilylation of alkenes catalyzed by rhodium- bis(phosphine) complexes~~

A survey of the literature for the desired reaction parameters first led us to the ruthenium-catalyzed intramolecular hydrosilylation of alkenes reported by Bosnich et al.156 The reaction possessed many of the desired characteristics for fluxional mechanocatalysis: it could be perfomed in acetone (non-swelling) solvent, required low catalyst loading, was efficient at ambient temperature and pressure and had a measureable outcome dependent on dihedral angle of the ligand used.

We began our studies with the synthesis of ligands to determine the effect of dihedral angle on the outcome of the reaction (Scheme 3.2). Chiral Rh(I)-bis(phosphine) complexes were synthesized by the treatment of [Rh(NBD)Cl]2 with AgClO4 in acetone, followed by filtration and treatment with ligands (S)-3.6, (S)-3.7 or (S)-(Z)-3.11 to give complexes 3.14-3.16 in 91-92% yield.157 Recrystallization of the complexes from Et2O provided solids that were spectroscopically pure by 1H and 31P NMR.

~~127~~

~~1. AgClO4, acetone, rt, 1 h [Rh(NBD)Cl]2 [Rh(L)(NBD)]ClO4 2. Ligand ((S)-3.6, 3.7 or (Z)-(S)-3.11)a~~

~~- - ClO4 ClO4~~

~~Ph2 Ph2 MeO P P Rh+ Rh+ MeO P P Ph2 Ph2~~

~~3.14 3.15 [Rh((S)-MeO-BIPHEP)(NBD)]ClO4 [Rh((S)-BINAP)(NBD)]ClO4 91% 92%~~

~~- ClO4~~

~~Ph2 O O P Rh+ O O P Ph2~~

~~3.16 [Rh((Z)-3.11)(NBD)]ClO4 92%~~

~~Scheme 3.2. Synthesis of [Rh(bisphosphine)(NBD)]ClO4- complexes 3.14-3.16.a a(Z)-(S)-3.11 synthesized by Zachary S. Kean in the Craig lab at Duke University~~

With 3.14-3.16 in hand, we employed these complexes as catalysts for the intramolecular hydrosilylation of olefins. Prior to each hydrosilylation reaction, the latent catalysts were activated by removal of the NBD group via hydrogenation to form the corresponding acetone-solvated complexes, revealing the necessary coordination sites for hydrosilylation, and the resulting complexes were treated with substrates in situ. When (di-iso-propyl)hydrosiloxane 3.17 was treated with activated complexes 3.14-

3.16 (2 mol %) in acetone at ambient temperature, efficient hydrosilylation was achieved within 2 h. The reaction mixtures were filtered through a plug of Florisil, concentrated and directly derivatized with PhLi in Et2O to give ring-opened product 3.18 to allow for measurement of % ee (HPLC). Reaction of 3.17 with (S)-MeO-BIPHEP complex 3.14 (θ =

~~128~~

~~87°) gave 3.18 in 1.2% ee (Table 3.4, entry 1), whereas treatment of the same compound with complex (S)-BINAP (θ = 87°) complex 3.15 provided 3.18 in 6.6% ee (entry 2).~~

~~Reaction of 3.15 with photoswitch complex 3.16 (θ = 83°) provided 1.0% ee (entry 3).~~

While reaction of diphenylhydrosiloxane 3.19 with complexes 3.14-3.16 was slower (all required stirring for 16 h), it showed a much higher dependence of % ee on ligand identity (entries 4-6). Treatment of 3.19 with (S)-MeO-BIPHEP complex 3.14 (θ = 87°) gave triphenylsilane 3.20 in 40.6% ee (entry 4), whereas treatment of the same compound with (S)-BINAP (θ = 88°) complex 3.15 provided 3.20 in 42% ee (entry 5). Reaction of 3.15 with photoswitch complex 3.16 (θ = 83°) provided 3.20 in 54.2% ee (entry 6). Although there is no consistent trend between ligand dihedral angle and product enantiopurity, it is clear that subtle conformational changes can account for varying outcomes with regard to enantiopurity. However, DFT calculations have not been performed to determine dihedral angles in complexes 3.14-3.16 relative to unbound ligands, and these calculations may provide a clearer explanation of the observed results.

~~129~~

~~Table 3.4. Effect of ligand identity on enantiomeric excess (% ee) of the intramolecular hydrosilylation of olefins 3.17 and 3.19 Precatalyst~~

~~H2 O PhLi Ph SiR2 R2Si OH Acetone, rt Et2O, rt, 1 h (R) H R2Si O 3.18: R = iPr 3.17: R = i-Pr 3.20: R = Ph 3.19: R = Ph~~

~~catalyst % ee entry substrate catalyst dihedral 3.18/3.20 angle (°) 1 3.17 3.14 87 1.2 2 3.17 3.15 88 6.6 3 3.17 3.16 83 1 4 3.19 3.14 87 40.6 5 3.19 3.15 88 42 6 3.19 3.16 83 54.2~~

~~aDetermined by HPLC on chiral stationary phase (Daicel CHIRALPAK AD-H). bAbsolute configuration assigned by analogy to Bosnich30~~

~~Next, we undertook the synthesis of a ligand capable of polymer incorporation.~~

~~To this end, demethylation of commerically available (S)-MeO-BIPHEP 3.7 with BBr3 in~~

~~CH2Cl2 gave diol 3.21 in 26% yield (Scheme 3.3). The poor yield of 3.21 was a result of phosphine oxidation upon reaction workup. Subsequent treatment of 3.21 with 5-bromo-~~

1-pentene and K2CO3 in refluxing acetone gave ligand 3.22 in 67% yield. The pendant alkenes allow for incorporation of the ligand into the PDMS polymer system via hydrosilylation. Additionally, Rh(I)-complex 3.23 was synthesized in 80% yield from ligand 3.22 employing a procedure similar to that used to synthesize complexes 3.14-

~~130~~

~~3.16. Bis(phosphine) (S)-3.22 and cationic Rh(I)-complex (S)-3.23 can both be incorporated into the PDMS framework, and studies are ongoing in the Craig lab.~~

~~MeO PPh2 BBr3 HO PPh2 MeO PPh HO PPh 2 CH2Cl2 2 –78 °C → rt, 16 h (S)-3.7 28% (S)-3.21~~

~~K2CO3, Br O PPh2 Acetone, reflux, 18 h O PPh2 67%~~

~~(S)-3.22~~

~~- ClO4 Ph2 1. AgClO4, acetone, rt, 1 h O P [Rh(NBD)Cl]2 Rh 2. (S)-3.22, 80% O P Ph2~~

~~(S)-3.23~~

~~Scheme 3.3. Synthesis of chiral bis(phosphine) 3.22 and Rh(I) complex 3.23 from (S)- MeO-BIPHEP 3.7.~~

~~3.2.2. Asymmetric intramolecular allylic alkylation catalyzed by palladium- PHOX complexes.~~

While initial results from the hydrosilylation studies were promising, we foresaw potential problems with this catalytic system. First, the catalysts required activation by hydrogenation prior to use, which may be difficult to achieve in the PDMS system.

Additionally, the reaction under study was a hydrosilylation reaction, identical to the reaction used to incorporate the ligand into the PDMS framework. Unreacted Si-H bonds in the polymer system during crosslinking may interfere with desired reactivity.

~~131~~

Although this may be mediated by treating the polymer with a reactive alkene prior to treatment with the reaction medium, we sought alternative strategies to realize mechanocatalysis in a PDMS system. As an alternative objective, we targeted the activation of a latent catalytic complex by mechanical force.

~~Identification of the intramolecular asymmetric Tsuji allylation developed by~~

Stoltz et al. appeared to be a promising candidate for the desired goal.158,159 In their initial survey of a variety of bidentate ligands, phosphino(oxazole) (PHOX) ligands were identified as highly active ligands for the palladium-catalyzed intramolecular enantioselective Tsuji allylation of allyl enol carbonates. Although the majority of reactions were performed in THF, the reaction could be performed in dioxane, toluene, benzene, Et2O, MTBE, iPr2O and EtOAc with little change in reactivity and selectivity.

The reaction proceeded regardless of ligand identity, but enantioselectivity depended strongly on the nature of the ligand (Table 3.5). The reaction was efficient at ambient temperature, was intramolecular, required low catalyst loading, could be performed in a variety of solvents, and had an easily measureable outcome. Stoltz later reported even more efficient reactivity in this system upon incorporation of electron-withdrawing substituent p-CF3 groups on the aryl groups of the phosphine (complete reaction within

~~10 min at 25 °C).160~~

~~132~~

~~Table 3.5. Enantioselective intramolecular Tsuji allylation reported by Stoltz.159 O~~

~~O O Pd2(dba)3, (5 mol %) O ligand (12.5 mol %) THF, 25 °C, time~~

~~% % entry ligand time yield ee 1 (R,R)-3.24 5 92 64 2 (R)-3.6 5 76 2 3 (R,R)-3.25 5 56 0 4 (R,R)-3.5 2 59 2 5 (R)-3.26 3 47 13 6 (R)-3.27 2 97 61 7 (S)-2.28 5 94 63 8 (S)-3.29 2 96 88~~

~~O O O PPh2 PPh Ph P NH HN PPh 2 2 2 P P PPh PPh2 O 2 (R,R)-DIOP (R,R)-Me-DUPHOS (R,R)-3.5 (R,R)-Trost ligand (R)-BINAP (R,R)-3.25 (R,R)-3.24 (R)-3.6~~

~~N OMe O PPh PPh 2 2 PPh2 N R~~

~~(R)-MOP (R)-QUINAP R = Bn: (S)-Bn-PHOX 3.28 (R)-3.26 (R)-3.27 R = t-Bu: (S)-tBu-PHOX 3.29~~

We envisioned that we could utilize the Stoltz allylation reaction as a test bed to develop a mechanically activated ligand by synthesizing a tetradentate ligand for Pd(0), in which two coordination sites could be revealed by application of external mechanical force. In consideration of ligand design, we targeted a latent tetradentate palladium(0) 133

~~complex in which two of the coordination sites would be occupied by a P,N-PHOX ligand, and two of the coordination sites would be occupied by an N,N-bipyridyl ligand.~~

Due to the softer nature of the phosphine, the N,N-bipyridyl ligand would require less mechanical energy to remove from coordination with the palladium center, leading to a catalytically competent PHOX-coordinated Pd(0) center with two open coordination sites (Figure 3.4B). The tendency of Pd(0) to adopt a tetrahedral geometry must be considered in the design of such a tetradentate ligand system.

To this end, we needed to first identify the optimal linker length to achieve coordination to Pd(0) in the required tetrahedral geometry. Using a variety of carbon linker lengths, bond lengths and angles in the tetradentate complex were calculated by conformational energy minimization in Spartan (work performed by Zachary S. Kean from the Craig lab at Duke University). The tetrahedral angle indicated in Table 3.6 should be minimized in order to ensure a perpendicular relationship and minimize off- plane pucker between the PHOX and the bipyridyl portions. This computational analysis suggested that the optimal tetrahedral geometry was achieved with a linker length of n = 5 (Table 3.6).

~~134~~

~~Table 3.6. Effect of linker length on geometry of PHOX-bpy tetradentate Pd(0) complex.~~

~~Ph2P N O Pd~~

~~(CH2)n N2 N1~~

~~Dihedral~~

Bite Bpy- Bite Angle P-Pd N(phox)- N1(bpy)- N2(bpy)- Angle Linker n Phox N- (Å) Pd (Å) Pd (Å) Pd (Å) Bpy N- Dihedral Pd-P (°) Pd-N (°) (°) 0 2.398 1.99 2.074 2.044 80.36 98.08 - 2 2.395 2.05 2.004 2.062 80.5 94.48 11.95 3 2.396 2.034 2.001 2.077 80.52 94.88 12.08 4 2.402 2.025 2.103 2.017 80.04 95.73 4.39 5 2.401 2.02 2.116 2.027 80.01 97.75 0.29

~~With this information in hand, we set out to synthesize a tetradentate complex for model studies. Synthesis of the tetradentate phosphinooxazoline (PHOX)/bipyridyl~~

~~(PHOX-bpy) ligand used in model studies ligand began with commericially available 2- bromobenzoyl chloride 3.30 following adapted procedures from Pfaltz (Scheme 3.4).161~~

~~Direct coupling of 3.30 with (L)-serine methyl ester hydrochloride gave amide 3.31 in~~

85% yield. Oxazoline formation was achieved with the use of diethylaminosulfur trifluoride (DAST) to give condensation product 3.32 in 87% yield.162 Subsequent reduction of the methyl ester with lithium aluminum hydride provided alcohol 3.33 in

~~82% yield, which set the stage for incorporation of the pendant bipyridyl moiety.~~

~~Alkylation of the PHOX portion with bipyridyl tosylate 3.34 proceeded with the use of~~

~~135~~

sodium hydride as a base to give 3.35 in 40% yield. Incorporation of the necessary phosphine at the final stage utilizing the modified Buchwald coupling conditions developed by Stoltz provided tetradentate ligand 3.35 in 39% yield for model studies in the asymmetric Tsuji allylation.160,163

~~CO Me HO 2~~

~~NH2 •HCl, Et3N H 1. DAST, CH2Cl2, –78 °C Cl N CO2Me MeOH, 0 °C, 2 h 2. K2CO3, –78 °C → rt Br O 85% Br O 87% OH 3.30 3.31~~

~~LiAlH4 O O 1. NaH, DMF Et O, 0 °C, 1 h 2 2. TsO N N Br N 82% Br N 5 3.32 CO2Me 3.33 OH 3.34a~~

~~Ph2PH, CuI (12.5 mol %), O 2,2'-DMEDA (87.5 mol %), O Br N Cs2CO3 Ph2P N O N N Toluene, 110 °C, 24 h 39% O N N 3.35 6 3.36 6 40%~~

~~aTosylate 3.34 provided by Bobin Lee from the Craig lab at Duke University~~

~~Scheme 3.4. Synthesis of PHOX/bpy tetradentate ligand 3.36 from 2-bromobenzoyl chloride.~~

With tetradentate ligand 3.36 in hand, we examined the activity of bidentate ligands (S)-t-Bu-PHOX 3.29 and bipyridine 3.37 compared to tetradentate ligand 3.36 on the outcome of the intramolecular Tsuji allylation. In an initial set of experiments, a mixture of allyl enol carbonate 3.39, Pd2(dba)3 (2.5 mol %; 5 mol % total [Pd]) and either

~~3.29 or 3.37 (6.25 mol %) in dioxane (the lowest “moderate” swelling organic solvent)~~

~~136~~

~~gave full conversion of the tetralone-derived starting material within 2 h at 25 °C (Table~~

3.7, entries 1-2). However, when the four coordination sites were fully occupied by the addition of 12.5 mol % 3.29/3.37 (entries 3-4), no detectable conversion was observed after 24 h (no observable product formation visible by TLC). Most importantly, when tetradentate 6.25 mol % 3.36 was used (entry 5), no product was detected after 24 h. This demonstrates that the allyl enol carbonate cannot displace the ligands on a coordinatively saturated Pd(0) center under the reaction conditions, successfully

~~“turning off” the reaction as desired. Additionally, employing a 1.25:1 mixture of~~

~~3.36/[Pd] successfully extinguishes catalyst reactivity.~~

~~Table 3.7. Effect of ligand identity on Pd(0)-catalyzed intramolecular Tsuji-allylation of allyl enol carbonate 3.39.~~

~~O O N N Ph2P N Ph P N 2 O N N t-Bu (S)-3.29 3.37 3.36 6 O~~

~~O O Pd2(dba)3 (2.5 mol %), O L (x mol %) Dioxane, rt, time~~

~~3.39 3.40~~

entry L mol % L conv (%)a time (h) 1 3.29 6.25 > 99 2 2 3.37 6.25 > 99 2 3 3.29 12.5 < 5 24 4 3.37 12.5 < 5 24 5 3.36 6.25 < 5 24 a>99 %: no 3.39 observable by TLC. b<5 %: no 3.40 observable by TLC 137

After successfully demonstrating the desired effect of ligand concentration on reaction conversion, we set out to synthesize a tetradentate ligand suitable for incorporation into a cross-linked PDMS polymer. Synthesis of the 2,2’-bipyridyl fragment began with LDA-mediated lithiation of 5,5’-dimethyl-2,2’-bipyridine164 and alkylation with 4-bromo-1-butene to give 3.42 in 45% yield (Scheme 3.5). Lithiation under the same conditions and reaction with bromopentanol tetrahydropyranyl ether followed by deprotection with TsOH in refluxing methanol gave alcohol 3.44 in 38% yield for two steps. This alcohol can be advanced to the tosylate, equivalent to 3.34

~~(Error! Reference source not found.), to take place in alkylation with the PHOX portion.~~

~~1. LDA, THF, N N -78 °C, 2 h N N 1. LDA, THF, -78 °C, 2 h 3 2. 2. OTHP Br Br 4 3.41 45% 3.42~~

~~THPO N N TsOH, MeOH, reflux, 24 h HO N N 5 3 5 38 % (2 steps) 4 3.43 3.44~~

~~Scheme 3.5. Synthesis of alcohol 3.44 from 5,5’-dimethyl-2,2’-bipyridine 3.41.~~

While the synthesis of the bipyridyl portion of the ligand was achieved without issue, the PHOX portion proved more challenging. The aryl ring required functionality to incorporate both the phosphine and the pendant alkene, therefore a synthesis directly analogous to 3.36 was not feasible. Instead, the pendant alkene was envisioned to be incorporated through aryl-alkyl cross-coupling, and the phosphine moiety was

~~138~~

~~envisioned to be incorporated by nucleophilic aromatic substitution of a fluorobenzene with KPPh2, revealing bromofluorobenzene 3.47 (Scheme 3.6).165,166~~

~~Cross-coupling~~

~~Br n KPPh2 n [M] O O O SNAr~~

~~Ph2P N F N F N~~

~~TBSO TBSO TBSO~~

~~F N 3.47 CO2Me~~

~~Scheme 3.6. Retrosynthetic dissection for incorporation of the pendant alkene and phosphine moieties from bromofluorobenzene 3.47.~~

~~In a manner similar to the synthesis of 3.33, 4-bromo-2-fluorobenzoyl chloride~~

~~3.45 was coupled with L-serine methyl ester to give amide 3.45 followed by DAST- mediated condensation to give oxazoline 3.46 in 58% yield for two steps. Reduction with~~

~~LiAlH4 and subsequent protection as the tert-butyldimethylsilyl ether provided 3.49 in~~

~~59% yield for two steps. Methyl ester 3.46 and silyl ether 3.49 were both used as substrates for attempted incorporation of the 5-pentenyl moiety.~~

~~139~~

~~1. DAST, CO2Me Br Br HO Br CH2Cl2, –78 C NH2 •HCl, Et3N H ° O Cl N CO2Me MeOH, 0 °C, 2 h 2. K2CO3, F N F O 93% F O –78 °C → rt OH CO Me 3.45 3.46 62% 3.47 2~~

~~Br Br~~

~~LiAlH4 O TBSCl, imidazole O~~

~~Et2O, 0 °C, 1 h F N CH2Cl2, rt, 18 h F N 59% (2 steps) OH OTBS 3.48 3.49~~

~~Scheme 3.7. Synthesis of methyl ester 3.47 and silyl ether 3.49 from 4-bromo-2- fluorobenzoyl chloride 3.45.~~

Despite multiple attempts under various reaction conditions, incorporation of the pendant 5-pentenyl moiety failed. Although a Negishi coupling on a highly related substrate has been reported,167 multiple attempts at Negishi coupling of 3.47 with 5- pentenylzinc bromide gave predominately isomerization of the internal alkene, which is a less viable substrate for PDMS incorporation (Table 3.8, entry 1). Attempts at Suzuki coupling with potassium (5-pentenyl)-trifluoroborate salt gave only recovered starting material (entries 2-3).168,169 Formation of the aryllithium or Grignard reagent from 3.49 only led to oligomerization of starting material (entries 4-5), resulting from nucleophilic aromatic substitution of the metalated species on the starting material, as confirmed by

~~1H NMR analysis of the purified product mixture.~~

~~140~~

~~Table 3.8. Attempts at incorporation of the 5-pentenyl moiety into 3.47 and 3.49.~~

~~Br [M] 2 O [M] O Conditions O~~

~~F N F N F N R R R~~

~~3.47, R = CO2Me 3.49, R = CH2OTBS~~

~~coupling entry substrate conditions result partner 2 mol % Pd(PPh3)4, alkene 2 ZnBr 1 CO Me THF/DMA, isomerization rt, 24 h~~

~~5 mol % Pd(dppf)Cl2,~~

~~2 2 3 BF3K 2 CO Me Cs CO , NR Dioxane/H2O, 80 °C, 24 h~~

~~5 mol % Pd(OAc)2, 9 mol %~~

~~2 BF3K 3 CO Me RuPhos, NR K3PO4, 3:1 PhMe/H2O, 80 °C, 24 h~~

~~1.1 eq. n-~~

~~2 Br 4 CH OTBS BuLi, THF, - oligomerization 78 °C 1 eq Mg0, 2 Br 5 CH OTBS oligomerization THF~~

~~RuPhos = dppf = PPh2 PCy2 Fe i-PrO O-i-Pr PPh2~~

~~141~~

~~3.2.3. Further directions~~

~~Due to complications with the aryl-alkyl coupling, an alternative synthetic strategy was devised (Scheme 3.8) that targets attachment of the pendant alkene to the~~

PHOX portion of the ligand through a benzylic alcohol moiety. Thus, commericially available 4-bromobenzonitrile 3.50 will be subjected to lithium-halogen exchange and formylated with N,N-dimethylformamide to give aldehyde 3.51, which will be subsequently reduced with NaBH4 and alkylated with 5-bromo-1-pentene to provide

3.53. Lewis-acid (ZnCl2) mediated condensation of a 1,2-aminoalcohol with the nitrile should provide oxazoline 3.53.170 Ortho-lithiation of 3.53 can be realized through the use of sec-BuLi/TMEDA171 followed by attack of Ph2PCl, giving phosphine 3.54. Deprotection with acid or fluoride ion and NaH-mediated alkylation of the pendant alcohol with tosylate in a manner analogous to synthesis of tetradentate 3.36 would provide tetradentate ligand 3.55, which is properly functionalized for incorporation into the

PDMS framework. Possibilities for incorporation of the ligand into PDMS include precomplexation of the tetradentate ligand with Pd prior to incorporation, or incorporation of the ligand as a crosslinker followed by treatment of the polymer system with the Tsuji allylation reaction mixture. Studies involving bidentate ligand 3.22, Rh(I)- complex 3.23 and synthesis of a tetradentate ligand of type 3.55 are ongoing in the

~~Widenhoefer and Craig labs at Duke University.~~

~~142~~

Br OHC 1.n-BuLi, THF, -78 °C, 15 min 1. NaBH4, MeOH 2. DMF, -78 °C → rt 2. NaH, THF, 0 °C, 1 h CN CN 3. 3. NH4Cl (aq) Br 3.50 3.51 0 °C → rt, 18 h HO OTBS 1. s-BuLi, TMEDA, NH2 2 O cat. ZnCl -78 °C → 0 °C 2 O 2 O PhMe, Δ 2. Ph2PCl CN N 3. [F-] or [H+] 3.52 3.53 OTBS

~~2 O O 2 O O NaH, DMF; 3.44 Ph2P N Ph P N 2 O N N OH 3.54 3.55 6 3~~

~~1. [Pd] 2. Incorporate into polymer~~

~~Scheme 3.8. Future plans for synthesis of tetradentate ligand 3.55 and polymer incorporation.~~

~~3.3. Experimental~~

~~3.3.1. General Methods~~

~~Reactions were performed under a nitrogen atmosphere employing standard~~

~~Schlenk and glovebox techniques unless specified otherwise. NMR spectra were obtained on a Varian spectrometer operating at 500 MHz for 1H, 125 MHz for 13C, and~~

~~Agilent Technologies 1100 Series LC/MSD-Trap SL equipped with an Agilent Zorbax C-~~

~~143~~

~~18 with 3.5 mm particle 1 × 150 mm column. Flash column chromatography was performed employing 200-400 mesh silica gel 60 (EM). Thin layer chromatography~~

(TLC) was performed on silica gel 60 F254. Acetone was dried over 3Å molecular sieves and distilled prior to use. Ether, methylene chloride, and THF were purified by passage through columns of activated alumina under argon. Reagents for ligand synthesis were obtained through major chemical suppliers and were used as received. Complexes 3.14,

~~3.15,157 (hydro)siloxanes 3.17 and 3.19,156 3.21,139 allyl enol carbonates,159 and alcohol~~

~~3.33172 were prepared according to published procedures.~~

~~3.3.2. Synthesis of bis(phosphine) ligands and rhodium complexes~~

Complex 3.16. Acetone (5 mL) was added to a mixture of [Rh(NBD)Cl]2 (6.4 mg, 0.014 mmol) and AgClO4 (5.8 mg, 0.028 mmol) and the resulting solution covered with foil and stirred for 1 h at room temperature. The solution was filtered under inert conditions and the filtrate was treated with 3.11 (25.0 mg, 0.028 mmol). The solution was stirred for

1 h before being partially concentrated under vacuum to ca. 0.5 mL. Et2O was carefully layered on top and the mixture was kept at 4-6 °C overnight. The solid was collected by filtration and washed with a small amount of Et2O to provide 3.16 as a red powder (31 mg, 92%). 1H NMR (CDCl3): δ 7.63-7.45 (m, 13H), 7.44-7.22 (m, 12H), 7.12 (d, J = 8.3 Hz,

~~5H), 7.02 (t, J = 7.8 Hz, 3H), 6.52 (d, J = 8.3 Hz, 2H), 6.25 (d, J = 8.1 Hz, 2H), 5.07 (s, 2H),~~

~~144~~

~~4.41 (s, 2H), 4.05 (s, 2H), 3.54-3.32 (m, 3H), 3.06-2.66 (m, 2H), 1.91-1.39 (m, 3H), 1.21 (t, J =~~

~~7.0 Hz, 1H). 31P NMR (CDCl3): δ 26.6, 25.7.~~

Bis(phosphine) 3.22. To a solution of K2CO3 (533 mg, 3.86 mmol) in acetone (25 mL) was added 5-bromo-1-pentene (0.34 mL) and (S)-3.21 (535 mg, 0.965 mmol). The resulting mixture was refluxed overnight under N2. The reaction mixture was filtered through a plug of Celite and the filtrate was concentrated under vacuum. The residue was chromatographed (SiO2, DCM-hexanes = 35:65) to give 3.22 as a white crystalline solid (445 mg, 67%). 1H NMR (CDCl3): δ 7.28-7.14 (m, 22H), 6.71 (d, J = 7.5 Hz, 2H), 6.66

~~(d, J = 8.1 Hz, 2H), 5.64 (ddt, J = 17.8, 9.5, 6.7 Hz, 2H), 4.91-4.86 (m, 3H), 4.86-4.82 (m, 2H),~~

~~3.63-3.49 (m, 2H), 3.32 (dt, J = 9.1, 5.9 Hz, 2H), 1.79 (tq, J = 14.4, 7.0 Hz, 4H), 1.35 (m, J =~~

~~7.1 Hz, 4H). 31P NMR (CDCl3): -13.7.~~

~~Complex 3.23 was synthesized in 80% yield as a red crystalline solid from 3.22 employing a procedure similar to that used to synthesize 3.16.~~

~~3.3.3. Hydrosilylation procedure.~~

~~The two-step procedure employed by Bosnich156 was used to synthesize compounds 3.18 and 3.20 employing 3.14-3.16. HPLC analysis of the arylated products revealed the ratio of R:S.~~

~~3.18. Daicel CHIRALPAK AD-H, 0.5% iPrOH/hexanes, 1 mL/min. Major (R-isomer) tr =~~

~~23.8 min, minor (S)-isomer tr = 22.1 min.~~

~~145~~

~~3.20. Daicel CHIRALPAK AD-H, 2% iPrOH/hexanes, 1 mL/min. Major (R-isomer) tr =~~

~~22.0 min, minor (S)-isomer tr = 17.1 min.~~

~~3.3.4. Synthesis of tetradentate complex 3.36.~~

3.35. To a solution of alcohol 3.33172 (237 mg, 0.925 mmol) in DMF (5 mL) was added NaH (60 wt. % in mineral oil, 40 mg, 1.0 mmol) and the resulting solution was stirred at room temperature for 1 h. Tosylate 3.34 was added in one portion and the resulting solution was stirred at room temperature for 48 h. The reaction was quenched with H2O (20 mL) and extracted with DCM (3 x 20 mL). The combined organic phase was dried (MgSO4), filtered and concentrated under vacuum. The residue was chromatographed (SiO2, Et3N-hexanes-EtOAc 1:50:50→1:25:75) provided 3.35 as a viscous colorless oil (190 mg, 40 %). 1H NMR (CDCl3): δ 8.51-8.45 (m, 2H), 8.25 (dd, J =

~~8.1, 4.8 Hz, 2H), 7.96-7.92 (m, 1H), 7.68 (dd, J = 7.5, 1.9 Hz, 1H), 7.65-7.57 (m, 3H), 7.42-~~

~~7.37 (m, 1H), 7.35-7.25 (m, 2H), 4.55-4.43 (m, 2H), 4.38-4.33 (m, 1H), 4.31 (s, 1H), 3.73 (dt,~~

~~J = 8.6, 4.0 Hz, 1H), 3.47 (m, 4H), 2.63 (q, J = 7.5 Hz, 2H), 2.38 (s, 3H), 1.71-1.51 (m, 4H),~~

~~1.37 (m, 4H).~~

~~Phosphine 3.36. To a Schlenk tube was added CuI (8.4 mg, 0.044 mmol), Ph2PH~~

(0.12 mL, 0.69 mmol), N,N’-dimethylethylenediamine (33 µL, 0.31 mmol) and toluene 5 mL and the resulting mixture was stirred for 20 min at room temperature. Cs2CO3 (433 mg, 1.33 mmol) was added followed by a solution of 3.33 (180 mg, 0.354 mmol) in

~~146~~

~~toluene (2 x 1 mL) via syringe. The mixture was heated to 100 °C for 16 h before being cooled and chromatographed directly (SiO2, Et3N-hexanes-EtOAc = 1:60:40) to provide~~

~~3.36 as a white solid (84 mg, 39%). 1H NMR (CDCl3): δ 8.48 (s, 2H), 8.28-8.22 (m, 2H),~~

~~7.90-7.83 (m, 1H), 7.60 (dd, J = 8.1, 1.8 Hz, 3H), 7.39-7.22 (m, 15H), 6.89-6.81 (m, 1H), 4.30-~~

~~4.19 (m, 1H), 4.19-4.12 (m, 1H), 4.02-3.89 (m, 1H), 3.39 (dt, J = 8.8, 4.3 Hz, 1H), 3.37-3.30~~

~~(m, 1H), 3.30-3.22 (m, 1H), 2.75 (t, J = 8.9 Hz, 1H), 2.64 (t, J = 7.6 Hz, 2H), 2.37 (s, 3H),~~

~~1.72-1.58 (m, 2H), 1.57-1.43 (m, 3H), 1.43-1.28 (m, 8H). 31P NMR (CDCl3): δ -4.1.~~

~~3.3.5. Synthesis of bipyridine 3.34.~~

3.42. To a solution of LDA (freshly prepared, 15 mmol) in THF (20 mL) at -78 °C was added dropwise via cannula a solution of 5,5’-dimethyl-2,2’-bipyridine (2.50 g, 13.6 mmol) in THF (40 mL + 8 mL rinse). The resulting black solution was stirred for 2 h at -

78 °C before gradually warming to rt. 4-Bromo-1-butene (2.76 mL, 27.2 mmol) was added and the mixture was stirred at room temperature for 72 h. The reaction mixture was quenched with H2O (100 mL) and extracted with Et2O (4 x 50 mL). The combined organics were washed with brine, dried (MgSO4), filtered and concentrated under vacuum. The residue was chromatographed (SiO2, Et3N-hexanes-EtOAc = 1:80:20) to give 3.42 as a yellow oil (1.443 g, 45%). 1H NMR (CDCl3): δ 8.49 (s, 2H), 8.25 (t, J = 7.8 Hz,

~~2H), 7.66-7.57 (m, 3H), 5.83 (m, 1H), 5.11-4.96 (m, 2H), 2.72-2.64 (m, 2H), 2.39 (s, 3H), 2.12~~

~~(q, J = 7.0 Hz, 2H), 1.76 (p, J = 7.5 Hz, 2H).~~

~~147~~

3.44. To a freshly prepared solution of LDA (6.75 mmol) in THF (10 mL) at -78 °C was added a solution of 3.42 (1.443 g, 6.055 mmol) in THF (20 mL). The resulting solution was stirred at -78 °C for 2 h before gradually warming to room temperature. 2-

(6-bromohexyloxy)tetrahydro-2H-pyran (2.7 mL, 12.3 mmol) was added and the reaction mixture was stirred at room temperature for 72 h. The reaction mixture was quenched with H2O (100 mL) and extracted with Et2O (4 x 50 mL). The combined organics were washed with brine, dried (MgSO4), filtered and concentrated under vacuum. 1H NMR analysis of the crude reaction mixture showed a mixture showed ~1:1 product:starting material, which was inseparable by TLC. The mixture was used directly without further purification.

Alcohol 3.44. To a solution of a mixture 3.43 and 3.42 (ca. 6 mmol) in MeOH (50 mL) was added TsOH-H2O (585 mg, 3.07 mmol). The reaction mixture was heated to reflux for 24 h before being concentrated under vacuum. The residue was taken up in

~~EtOAc (100 mL) and washed with 10% aq. NaOH (10 mL). The aqueous phase was extracted once with EtOAc (25 mL). The combined organic phases were washed with~~

~~H2O (10 mL), brine, dried (MgSO4), filtered and concentrated under vacuum. The residue was chromatographed (SiO2, Et3N-hexanes-EtOAc = 1:50:50→1:25:75) provided~~

~~3.44 as a viscous oil (777 mg, 38% for 2 steps) as well as recovered 3.42. 1H NMR (CDCl3):~~

~~δ 8.49 (d, J = 2.4 Hz, 2H), 8.29-8.23 (m, 2H), 7.62 (dt, J = 8.1, 2.6 Hz, 2H), 5.83 (m, 1H),~~

~~5.15-4.96 (m, 2H), 3.75-3.59 (m, 4H), 3.42 (t, J = 6.8 Hz, 1H), 2.67 (q, J = 8.0 Hz, 3H), 2.12~~

~~148~~

~~(q, J = 7.1 Hz, 2H), 1.88 (dt, J = 13.8, 6.8 Hz, 2H), 1.76 (p, J = 7.4 Hz, 2H), 1.72-1.26 (m,~~

~~14H).~~

~~3.3.6. Synthesis of ester 3.47 and silyl ether 3.49.~~

~~3.46. Synthesized in a manner analogous to 3.31. Fluffy white powder, 93%. 1H~~

~~NMR (CDCl3): δ 7.97 (t, J = 8.4 Hz, 1H), 7.61 (br s, 1H), 7.43 (dd, J = 8.4, 1.8 Hz, 1H), 7.36~~

~~(dd, J = 11.2, 1.8 Hz, 1H), 4.92-4.85 (m, 2H), 4.09 (br s, 1H), 3.84 (s, 3H), 2.42 (br s, 1H).~~

3.47. Synthesized in a manner analogous to 3.32. Workup provided a residue that was chromatographed (SiO2, hexanes-EtOAc = 1:1) to give 3.47 as a white crystalline solid in 62% yield. 1H NMR (CDCl3): δ 7.84-7.77 (m, 1H), 7.39-7.35 (m, 1H), 7.35-7.33 (m,

~~1H), 4.98 (dd, J = 10.7, 8.1 Hz, 1H), 4.69 (td, J = 8.4, 0.4 Hz, 1H), 4.64-4.56 (m, 1H), 3.82 (s,~~

~~3H).~~

~~3.48. Synthesis in a manner analogous to 3.33. Workup provided product as a white, sticky solid, which was used without further purification.~~

3.49. To a crude mixture of 3.48 (approx. 4.0 mmol) in DCM (20 mL) was added imidazole (408 mg, 6.0 mmol) and TBSCl (723 mg, 4.80 mmol). The reaction mixture was stirred overnight at room temperature before being quenched with H2O (10 mL) and extracted with DCM (2 x 20 mL). The combined organic phase was dried (MgSO4), filtered and concentrated under vacuum. The residue was chromatographed (SiO2, hexanes-EtOAc = 85:15) to provide 3.49 as a colorless oil (931 mg, 59% for 2 steps).

~~149~~

~~References~~

~~1. Marner, F. J.; Moore, R. E.; Hirotsu, K.; Clardy, J. J. Org. Chem. 1977, 42, 2815-2819.~~

2. Tidgewell, K.; Clark, B. R.; Gerwick, W. H. 2.06 - the Natural Products Chemistry of Cyanobacteria. In Comprehensive Natural Products II, Liu, H.-W.; Mander, L., Eds. Elsevier: Oxford, 2010; pp 141-188.

~~3. Walsh, P. J.; Smith, S.; Fleming, L.; Solo-Gabriele, H.; Gerwick, W. H., Oceans and Human Health: Risks and Remedies from the Seas. Academic Press: 2011.~~

~~4. White, C. J.; Yudin, A. K. Nat. Chem. 2011, 3, 509-524.~~

~~5. Oyelere, A. K. Curr. Top. Med. Chem. 2010, 10, 1359-1360.~~

~~6. Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J.; Corbett, T. H. J. Am. Chem. Soc. 2001, 123, 5418-5423.~~

~~7. Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J. Bioorg. Med. Chem. 2002, 10, 1973- 1978.~~

~~8. Gutierrez, M.; Suyama, T. L.; Engene, N.; Wingerd, J. S.; Matainaho, T.; Gerwick, W. H. J. Nat. Prod. 2008, 71, 1099-1103.~~

~~9. Matthew, S.; Schupp, P. J.; Luesch, H. J. Nat. Prod. 2008, 71, 1113-1116.~~

~~10. Tidgewell, K.; Engene, N.; Byrum, T.; Media, J.; Doi, T.; Valeriote, F. A.; Gerwick, W. H. ChemBioChem 2010, 11, 1458-1466.~~

~~11. Thornburg, C. C.; Cowley, E. S.; Sikorska, J.; Shaala, L. A.; Ishmael, J. E.; Youssef, D. T. A.; McPhail, K. L. J. Nat. Prod. 2013, 76, 1781-1788.~~

~~12. Ma, D.; Zou, B.; Cai, G.; Hu, X.; Liu, J. O. Chem. Eur. J. 2006, 12, 7615-7626.~~

~~13. Ma, D. W.; Zou, B.; Cai, G. R.; Hu, X. Y.; Liu, J. O. Chem. Eur. J. 2006, 12, 7615-7626.~~

~~150~~

~~14. Luesch, H.; Chanda, S. K.; Raya, R. M.; DeJesus, P.; Orth, A. P.; Walker, J. R.; Belmonte, J. C. I.; Schultz, P. G. Nat. Chem. Bio. 2006, 2, 158-167.~~

~~15. Chen, J. H.; Forsyth, C. J. J. Am. Chem. Soc. 2003, 125, 8734-8735.~~

~~16. Chen, J.; Forsyth, C. J. Proc. Natl. Acad. Sci. USA 2004, 101, 12067-12072.~~

~~17. Doi, T.; Numajiri, Y.; Munakata, A.; Takahashi, T. Org. Lett. 2006, 8, 531-534.~~

~~18. Numajiri, Y.; Takahashi, T.; Doi, T. Chem. Asian J. 2009, 4, 111-125.~~

~~19. Doi, T.; Numajiri, Y.; Takahashi, T.; Takagi, M.; Shin-ya, K. Chem. Asian J. 2011, 6, 180-188.~~

~~20. Masuda, Y.; Suzuki, J.; Onda, Y.; Fujino, Y.; Yoshida, M.; Doi, T. J. Org. Chem. 2014.~~

~~21. Chen, Q. Y.; Liu, Y. X.; Luesch, H. ACS Med. Chem. Lett. 2011, 2, 861-865.~~

~~22. Brown, H. C.; Jadhav, P. K. J. Am. Chem. Soc. 1983, 105, 2092-2093.~~

~~23. Mukaiyama, T.; Usui, M.; Shimada, E.; Saigo, K. Chem. Lett. 1975, 1045-1048.~~

~~24. Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413-4450.~~

~~25. Pirkle, W. H.; Adams, P. E. J. Org. Chem. 1980, 45, 4117-4121.~~

~~26. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull. Chem. Soc. Jpn. 1979, 52, 1989-1993.~~

~~27. Paterson, I.; Wallace, D. J.; Cowden, C. J. Synthesis 1998, 639-652.~~

~~28. Doi, T.; Numajiri, Y.; Munakata, A.; Takahashi, T. Org. Lett. 2006, 8, 531-534.~~

~~29. You, S. L.; Razavi, H.; Kelly, J. W. Angew. Chem. Int. Ed. 2003, 42, 83-85.~~

~~30. U.S. National Library of Medicine: PubMed Health. http://www.ncbi.nlm.nih.gov/ pubmedhealth/PMH0004529 (accessed Mar 1, 2012).~~

~~31. National Cancer Institute: Surveillance Epidemiology and End Results. http://seer.cancer.gov/statfacts/html/lungb.html (accessed April 30, 2012).~~

~~151~~

~~32. Lim, D.; Coltart, D. M. Angew. Chem. Int. Ed. 2008, 47, 5207-5210.~~

~~33. Crimmins, M. T.; Kirincich, S. J.; Wells, A. J.; Choy, A. L. Synth. Commun. 1998, 28, 3675-3679.~~

~~34. Crimmins, M. T.; King, B. W.; Tabet, E. A.; Chaudhary, K. J. Org. Chem. 2001, 66, 894- 902.~~

~~35. Abbineni, C.; Sasmal, P. K.; Mukkanti, K.; Iqbal, J. Tetrahedron Lett. 2007, 48, 4259- 4262.~~

~~36. Zhou, G. “Methods for Direct Carbon-Carbon Bond Formation and Their Application to Natural Product Synthesis.” Ph.D. Thesis, Duke University, Durham, NC, 2009.~~

~~37. Terao, J.; Todo, H.; Begum, S. A.; Kuniyasu, H.; Kambe, N. Angew. Chem. Int. Ed. 2007, 46, 2086-2089.~~

~~38. Toromanoff, E. Tetrahedron 1978, 34, 2105-2111.~~

~~39. Azmanam, A. The Chemistry Blog. Where has all the (-)-sparteine gone? http://www.chemistry-blog.com/2010/06/11/where-has-all-the-sparteine-gone/ (accessed Jun 11, 2010).~~

40. Lowe, D. L. In The Pipeline. Sparteine and other fine chemical shortages. http://blogs.sciencemag.org/pipeline/archives/2010/06/16/sparteine_and_other_fi ne_chemical_shortages (accessed Sep 2, 2015).

~~41. Breit, B.; Zahn, S. K. J. Org. Chem. 2001, 66, 4870-4877.~~

~~42. Evans, D. A.; Allison, B. D.; Yang, M. G.; Masse, C. E. J. Am. Chem. Soc. 2001, 123, 10840-10852.~~

~~43. Dey, S.; Wengryniuk, S. E.; Tarsis, E. M.; Robertson, B. D.; Zhou, G. Q.; Coltart, D. M. Tetrahedron Lett. 2015, 56, 2927-2929.~~

~~44. Hori, T.; Sharpless, K. B. J. Org. Chem. 1978, 43, 1689-1697.~~

~~45. Gage, J. R.; Evans, D. A. Org. Synth. 1990, 68, 83-91.~~

~~152~~

~~46. Evans, D. A.; Rieger, D. L.; Jones, T. K.; Kaldor, S. W. J. Org. Chem. 1990, 55, 6260- 6268.~~

~~47. Yadav, J. S.; Kumar, G. G. K. S. N. Tetrahedron 2010, 66, 480-487.~~

~~48. Brooner, R. E. M.; Brown, T. J.; Widenhoefer, R. A. Angew. Chem. Int. Ed. 2013, 52, 6259-6261.~~

~~49. Brooner, R. E. M. “Mechanistic Studies of π-Activation Catalysis by Cationic Gold(I) and Brønsted-acid.” Ph.D. Thesis, Duke University, Durham, NC, 2013.~~

~~50. Kharasch, M. S.; Isbell, H. S. J. Am. Chem. Soc. 1931, 53, 3053-3059.~~

~~51. Kharasch, M. S.; Beck, T. M. J. Am. Chem. Soc. 1934, 56, 2057-2060.~~

~~52. Schwemberger, W.; Gordon, W. Chem. Zentralbl. 1935, 106, 514-514.~~

~~53. Schmidbaur, H. Naturwiss. Rundsch. 1995, 48, 443.~~

~~54. Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem. Int. Ed. 1998, 37, 1415-1418.~~

~~55. Furstner, A. Chem. Soc. Rev. 2009, 38, 3208-3221.~~

~~56. Shapiro, N. D.; Toste, F. D. Synlett 2010, 675-691.~~

~~57. Liu, L. P.; Hammond, G. B. Chem. Soc. Rev. 2012, 41, 3129-3139.~~

~~58. Rudolph, M.; Hashmi, A. S. K. Chem. Soc. Rev. 2012, 41, 2448-2462.~~

~~59. Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180-3211.~~

~~60. Schwerdtfeger, P.; Hermann, H. L.; Schmidbaur, H. Inorg. Chem. 2003, 42, 1334-1342.~~

~~61. Carvajal, M. A.; Novoa, J. J.; Alvarez, S. J. Am. Chem. Soc. 2004, 126, 1465-1477.~~

~~62. Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351-3378.~~

~~63. Preisenberger, M.; Schier, A.; Schmidbaur, H. J. Chem. Soc., Dalton Trans. 1999, 1645- 1650.~~

~~153~~

~~64. Couce-Rios, A.; Kovacs, G.; Ujaque, G.; Lledos, A. ACS Catalysis 2015, 5, 815-829.~~

~~65. Jia, M. Q.; Bandini, M. ACS Catalysis 2015, 5, 1638-1652.~~

~~66. Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71.~~

~~67. Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939-2947.~~

~~68. Obradors, C.; Echavarren, A. M. Acc. Chem. Res. 2014, 47, 902-912.~~

~~69. Furstner, A. Acc. Chem. Res. 2014, 47, 925-938.~~

70. Toullec, P. Y.; Michelet, V. Cycloisomerization of 1. In Computational Mechanisms of Au and Pt Catalyzed Reactions, Soriano, E.; Marco-Contelles, J., Eds. Springer: Verlag Berlin Heidelberg, 2011; pp 31-80.

~~71. Echavarren, A. M.; Jimenez-Nunez, E. Top. Catal. 2010, 53, 924-930.~~

~~72. Jimenez-Nunez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326-3350.~~

~~73. Michelet, V.; Toullec, P. Y.; Genet, J. P. Angew. Chem. Int. Ed. 2008, 47, 4268-4315.~~

~~74. Trost, B. M. Acc. Chem. Res. 1990, 23, 34-42.~~

~~75. Nieto-Oberhuber, C.; Munoz, M. P.; Bunuel, E.; Nevado, C.; Cardenas, D. J.; Echavarren, A. M. Angew. Chem. Int. Ed. 2004, 43, 2402-2406.~~

~~76. Mamane, V.; Gress, T.; Krause, H.; Furstner, A. J. Am. Chem. Soc. 2004, 126, 8654-8655.~~

~~77. Luzung, M. R.; Markham, J. P.; Toste, F. D. J. Am. Chem. Soc. 2004, 126, 10858-10859.~~

~~78. Furstner, A.; Szillat, H.; Gabor, B.; Mynott, R. J Am Chem Soc 1998, 120, 8305-8314.~~

~~79. Nieto-Oberhuber, C.; Lopez, S.; Echavarren, A. M. J. Am. Chem. Soc. 2005, 127, 6178- 6179.~~

~~80. Nieto-Oberhuber, C.; Perez-Galan, P.; Herrero-Gomez, E.; Lauterbach, T.; Rodriguez, C.; Lopez, S.; Bour, C.; Rosellon, A.; Cardenas, D. J.; Echavarren, A. M. J. Am. Chem. Soc. 2008, 130, 269-279.~~

~~154~~

~~81. Escribano-Cuesta, A.; Perez-Galan, P.; Herrero-Gomez, E.; Sekine, M.; Braga, A. A. C.; Maseras, F.; Echavarren, A. M. Org. Biomol. Chem. 2012, 10, 6105-6111.~~

~~82. Nieto-Oberhuber, C.; Lopez, S.; Jimenez-Nunez, E.; Echavarren, A. M. Chem. Eur. J. 2006, 12, 5916-5923.~~

~~83. Furstner, A.; Davies, P. W.; Gress, T. J. Am. Chem. Soc. 2005, 127, 8244-8245.~~

~~84. Matsuda, T.; Kadowaki, S.; Goya, T.; Murakami, M. Synlett 2006, 575-578.~~

~~85. Ota, K.; Lee, S. I.; Tang, J. M.; Takachi, M.; Nakai, H.; Morimoto, T.; Sakurai, H.; Kataoka, K.; Chatani, N. J. Am. Chem. Soc. 2009, 131, 15203-15211.~~

~~86. Blum, J.; Beerkraft, H.; Badrieh, Y. J. Org. Chem. 1995, 60, 5567-5569.~~

~~87. Patil, D. V.; Park, H. S.; Koo, J.; Han, J. W.; Shin, S. Chem. Commun. 2014, 50, 12722- 12725.~~

~~88. Nieto-Oberhuber, C.; Munoz, M. P.; Lopez, S.; Jimenez-Nuner, E.; Nevado, C.; Herrero-Gomez, E.; Raducan, M.; Echavarren, A. M. Chem. Eur. J. 2006, 12, 1677- 1693.~~

~~89. Amijs, C. H. M.; Ferrer, C.; Echavarren, A. M. Chem. Commun. 2007, 698-700.~~

~~90. Amijs, C. H. M.; Lopez-Carrillo, V.; Raducan, M.; Perez-Galan, P.; Ferrer, C.; Echavarren, A. M. J. Org. Chem. 2008, 73, 7721-7730.~~

~~91. Leseurre, L.; Chao, C. M.; Seki, T.; Genin, E.; Toullec, P. Y.; Genet, J. P.; Michelet, V. Tetrahedron 2009, 65, 1911-1918.~~

~~92. Chao, C. M.; Vitale, M. R.; Toullec, P. Y.; Genet, J. P.; Michelet, V. Chem. Eur. J. 2009, 15, 1319-1323.~~

~~93. Sethofer, S. G.; Mayer, T.; Toste, F. D. J. Am. Chem. Soc. 2010, 132, 8276-+.~~

~~94. Nevado, C.; Echavarren, A. M. Chem. Eur. J. 2005, 11, 3155-3164.~~

~~95. Bohringer, S.; Gagosz, F. Adv. Synth. Catal. 2008, 350, 2617-2630.~~

~~155~~

~~96. Cabrero-Antonino, J. R.; Leyva-Perez, A.; Corma, A. Chem. Eur. J. 2013, 19, 8627-8633.~~

~~97. Brooner, R. E. M.; Widenhoefer, R. A. Chem. Eur. J. 2011, 17, 6170-6178.~~

~~98. Kanno, O.; Kuriyama, W.; Wang, Z. J.; Toste, F. D. Angew. Chem. Int. Ed. 2011, 50, 9919-9922.~~

~~99. Dang, T. T.; Boeck, F.; Hintermann, L. J. Org. Chem. 2011, 76, 9353-9361.~~

~~100. Taylor, J. G.; Adrio, L. A.; Hii, K. K. Dalton Trans. 2010, 39, 1171-1175.~~

~~101. Cheon, C. H.; Kanno, O.; Toste, F. D. J. Am. Chem. Soc. 2011, 133, 13248-13251.~~

~~102. Wu, X.; Li, M. L.; Wang, P. S. J. Org. Chem. 2014, 79, 419-425.~~

~~103. Qian, D. Y.; Zhang, J. L. Chem. Eur. J. 2013, 19, 6984-6988.~~

~~104. Han, Z. Y.; Xiao, H.; Chen, X. H.; Gong, L. Z. J. Am. Chem. Soc. 2009, 131, 9182-+.~~

~~105. Horino, Y.; Takahashi, Y.; Nakashima, Y.; Abe, H. RSC Adv. 2014, 4, 6215-6218.~~

~~106. Wu, H.; He, Y. P.; Gong, L. Z. Adv. Synth. Catal. 2012, 354, 975-980.~~

~~107. Brooner, R. E. M.; Robertson, B. D.; Widenhoefer, R. A. Organometallics 2014, 33, 6466-6473.~~

~~108. Tarselli, M. A.; Chianese, A. R.; Lee, S. J.; Gagne, M. R. Angew. Chem. Int. Ed. 2007, 46, 6670-6673.~~

~~109. Li, H.; Widenhoefer, R. A. Org. Lett. 2009, 11, 2671-2674.~~

~~110. Wang, D. W.; Cai, R.; Sharma, S.; Jirak, J.; Thummanapelli, S. K.; Akhmedov, N. G.; Zhang, H.; Liu, X. B.; Petersen, J. L.; Shi, X. D. J. Am. Chem. Soc. 2012, 134, 9012- 9019.~~

~~111. Su, Y. J.; Zhang, Y. W.; Akhmedov, N. G.; Petersen, J. L.; Shi, X. D. Org. Lett. 2014, 16, 2478-2481.~~

~~112. Homs, A.; Escofet, I.; Echavarren, A. M. Org. Lett. 2013, 15, 5782-5785.~~

~~156~~

~~113. Schroder, F.; Tugny, C.; Salanouve, E.; Clavier, H.; Giordano, L.; Moraleda, D.; Gimbert, Y.; Mouries-Mansuy, V.; Goddard, J. P.; Fensterbank, L. Organometallics 2014, 33, 4051-4056.~~

~~114. Weber, D.; Gagne, M. R. Org. Lett. 2009, 11, 4962-4965.~~

~~115. Zhu, Y. Y.; Day, C. S.; Zhang, L.; Hauser, K. J.; Jones, A. C. Chem. Eur. J. 2013, 19, 12264-12271.~~

~~116. Weber, S. G.; Rominger, F.; Straub, B. F. Eur. J. Inorg. Chem. 2012, 2863-2867.~~

~~117. Xiao, Y. P.; Liu, X. Y.; Che, C. M. Beilstein J. Org. Chem. 2011, 7, 1100-1107.~~

~~118. Liu, X. Y.; Xiao, Y. P.; Siu, F. M.; Ni, L. C.; Chen, Y.; Wang, L.; Che, C. M. Org. Biomol. Chem. 2012, 10, 7208-7219.~~

~~119. Liu, X. Y.; Che, C. M. Org. Lett. 2009, 11, 4204-4207.~~

~~120. Grise, C. M.; Barriault, L. Org. Lett. 2006, 8, 5905-5908.~~

~~121. Miura, K.; Saito, H.; Fujisawa, N.; Hosomi, A. J. Org. Chem. 2000, 65, 8119-8122.~~

~~122. Park, K. H.; Chung, Y. K. Adv. Synth. Catal. 2005, 347, 854-866.~~

~~123. Park, J. H.; Cho, Y.; Chung, Y. K. Angew. Chem. Int. Ed. 2010, 49, 5138-5141.~~

~~124. Takeuchi, R.; Kashio, M. Angew. Chem. Int. Ed. 1997, 36, 263-265.~~

~~125. Kleinbeck, F.; Toste, F. D. J. Am. Chem. Soc. 2009, 131, 9178-+.~~

~~126. Nieto-Oberhuber, C.; Lopez, S.; Munoz, M. P.; Cardenas, D. J.; Bunuel, E.; Nevado, C.; Echavarren, A. M. Angew. Chem. Int. Ed. 2005, 44, 6146-6148.~~

~~127. Beyer, M. K.; Clausen-Schaumann, H. Chem. Rev. 2005, 105, 2921-2948.~~

~~128. Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Chem. Rev. 2009, 109, 5755-5798.~~

~~129. Berezin, I. V.; Klibanov, A. M.; Samokhin, G. P.; Martinek, K.; Klaus, M.; Ed., Methods in Enzymology. Academic Press: 1976; Vol. 44.~~

~~157~~

~~130. Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P. Nat. Chem. 2009, 1, 133-137.~~

~~131. Groote, R.; Jakobs, R. T. M.; Sijbesma, R. P. Polym. Chem. 2013, 4, 4846-4859.~~

~~132. Basedow, A. M.; Ebert, K. H. Adv. Polym. Sci. 1977, 12, 83-148.~~

~~133. Ghosh, A. K.; Packiarajan, M.; Cappiello, J. Tetrahedron: Asymmetry. 1998, 9, 1-45.~~

~~134. Hargaden, G. C.; Guiry, P. J. Chem. Rev. 2009, 109, 2505-2550.~~

~~135. McCarthy, M.; Guiry, P. J. Tetrahedron 2001, 57, 3809-3844.~~

~~136. McManus, H. A.; Guiry, P. J. Chem. Rev. 2004, 104, 4151-4202.~~

~~137. Pfaltz, A. Acc. Chem. Res. 1993, 26, 339-345.~~

~~138. Whitesell, J. K. Chem. Rev. 1989, 89, 1581-1590.~~

~~139. Zhang, Z. G.; Qian, H.; Longmire, J.; Zhang, X. M. J. Org. Chem. 2000, 65, 6223-6226.~~

~~140. Kean, Z. S.; Akbulatov, S.; Tian, Y. C.; Widenhoefer, R. A.; Boulatov, R.; Craig, S. L. Angew. Chem. Int. Ed. 2014, 53, 14508-14511.~~

~~141. Raghunath, M.; Zhang, X. M. Tetrahedron Lett. 2005, 46, 8213-8216.~~

~~142. Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974-4984.~~

~~143. McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H. K.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27-40.~~

~~144. McDonald, J. C.; Whitesides, G. M. Acc. Chem. Res. 2002, 35, 491-499.~~

~~145. Suo, Z. G. Acta Mech. Solida Sin. 2010, 23, 549-578.~~

~~146. Wang, Q. M.; Tahir, M.; Zang, J. F.; Zhao, X. H. Adv. Mater. 2012, 24, 1947-1951.~~

~~147. Fujii, T. Microelectron. Eng. 2002, 61-2, 907-914.~~

~~158~~

~~148. Khademhosseini, A.; Langer, R.; Borenstein, J.; Vacanti, J. P. Proc. Natl. Acad. Sci. USA 2006, 103, 2480-2487.~~

~~149. Laschi, C.; Mazzolai, B.; Mattoli, V.; Cianchetti, M.; Dario, P. Bioinspir. Biomim. 2009, 4.~~

~~150. Shepherd, R. F.; Ilievski, F.; Choi, W.; Morin, S. A.; Stokes, A. A.; Mazzeo, A. D.; Chen, X.; Wang, M.; Whitesides, G. M. Proc. Natl. Acad. Sci. USA 2011, 108, 20400- 20403.~~

~~151. Gossweiler, G. R.; Hewage, G. B.; Soriano, G.; Wang, Q. M.; Welshofer, G. W.; Zhao, X. H.; Craig, S. L. ACS Macro Lett. 2014, 3, 216-219.~~

~~152. Wong, I.; Ho, C. M. Microfluid. Nanofluidics 2009, 7, 291-306.~~

~~153. Lotters, J. C.; Olthuis, W.; Veltink, P. H.; Bergveld, P. J. Micromech. Microeng. 1997, 7, 145-147.~~

~~154. Schneider, F.; Fellner, T.; Wilde, J.; Wallrabe, U. J. Micromech. Microeng. 2008, 18.~~

~~155. Lee, J. N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 6544-6554.~~

~~156. Bergens, S. H.; Noheda, P.; Whelan, J.; Bosnich, B. J. Am. Chem. Soc. 1992, 114, 2121- 2128.~~

~~157. Tani, K.; Yamagata, T.; Akutagawa, S.; Kumobayashi, H.; Taketomi, T.; Takaya, H.; Miyashita, A.; Noyori, R.; Otsuka, S. J. Am. Chem. Soc. 1984, 106, 5208-5217.~~

158. Behenna, D. C.; Mohr, J. T.; Sherden, N. H.; Marinescu, S. C.; Harned, A. M.; Tani, K.; Seto, M.; Ma, S.; Novak, Z.; Krout, M. R.; McFadden, R. M.; Roizen, J. L.; Enquist, J. A.; White, D. E.; Levine, S. R.; Petrova, K. V.; Iwashita, A.; Virgil, S. C.; Stoltz, B. M. Chem. Eur. J. 2011, 17, 14199-14223.

~~159. Behenna, D. C.; Stoltz, B. M. J. Am. Chem. Soc. 2004, 126, 15044-15045.~~

~~160. Tani, K.; Behenna, D. C.; McFadden, R. M.; Stoltz, B. M. Org. Lett. 2007, 9, 2529-2531.~~

~~161. Franzke, A.; Pfaltz, A. Chem. Eur. J. 2011, 17, 4131-4144.~~

~~159~~

~~162. Phillips, A. J.; Uto, Y.; Wipf, P.; Reno, M. J.; Williams, D. R. Org. Lett. 2000, 2, 1165- 1168.~~

~~163. Gelman, D.; Jiang, L.; Buchwald, S. L. Org. Lett. 2003, 5, 2315-2318.~~

~~164. De Feyter, S.; Abdel-Mottaleb, M. M. S.; Schuurmans, N.; Verkuijl, B. J. V.; van Esch, J. H.; Feringa, B. L.; De Schryver, F. C. Chem. Eur. J. 2004, 10, 1124-1132.~~

~~165. Williams, J. M. J. Synlett 1996, 705.~~

~~166. Peer, M.; deJong, J. C.; Kiefer, M.; Langer, T.; Rieck, H.; Schell, H.; Sennhenn, P.; Sprinz, J.; Steinhagen, H.; Wiese, B.; Helmchen, G. Tetrahedron 1996, 52, 7547- 7583.~~

~~167. Huo, S. Q. Org. Lett. 2003, 5, 423-425.~~

~~168. Molander, G. A.; Yokoyama, Y. J. Org. Chem. 2006, 71, 2493-2498.~~

~~169. van den Hoogenband, A.; Lange, J. H. M.; Terpstra, J. W.; Koch, M.; Visser, G. M.; Visser, M.; Korstanje, T. J.; Jastrzebski, J. T. B. H. Tetrahedron Lett. 2008, 49, 4122- 4124.~~

~~170. Koch, G.; Lloydjones, G. C.; Loiseleur, O.; Pfaltz, A.; Pretot, R.; Schaffner, S.; Schnider, P.; Vonmatt, P. Recl. Trav. Chim. Pays-Bas. 1995, 114, 206-210.~~

~~171. Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33, 336-345.~~

~~172. Chen, R. J.; Fang, J. M. J. Chin. Chem. Soc. 2005, 52, 819-826.~~

~~173. Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. J. Am. Chem. Soc. 2009, 131, 6350- 6351.~~

~~174. Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. Chem. Commun. 2009, 42, 6451- 6453.~~

~~175. Brown, T. J.; Sugie, A.; Dickens, M. G.; Widenhoefer, R. A. Organometallics 2010, 29, 4207-4209.~~

~~176. Brown, T. J.; Widenhoefer, R. A. J. Organomet. Chem. 2011, 696, 1216-1220.~~

~~160~~

~~177. Brown, T. J.; Sugie, A.; Leed, M. G. D.; Widenhoefer, R. A. Chem. Eur. J. 2012, 18, 6959-6971.~~

~~178. Brooner, R. E. M.; Brown, T. J.; Widenhoefer, R. A. Chem. Eur. J. 2013, 19, 8276-8284.~~

~~161~~

~~Biography~~

~~Bradley Robertson was born July 12, 1986, in Sarnia, Ontario, Canada. In 1997, he and his family moved to Summerville, SC. He graduated summa cum laude from~~

~~Winthrop University in 2004, where he earned a B.S. in Chemistry and a B.S. in Biology.~~

~~Prior to enrolling at Winthrop, he was named a Winthrop Scholar, providing a full academic scholarship. While at Winthrop, he conducted research in the laboratory of~~

~~Professor Aaron Hartel, studying the utilization of the Brook rearrangement to synthesize silyl enol ethers from acyloin derivatives. It was during his studies with Dr.~~

~~Hartel when he developed a love for organic chemistry.~~

In the Fall of 2008, he began his graduate studies in the Department of Chemistry at Duke University. During his time at Duke, he was awarded the C. R. Hauser fellowship. Additionally, he completed a one-year internship in medicinal chemistry at

GlaxoSmithKline in Research Triangle Park, NC, from June 2012 to May 2013. He has published manuscripts in a variety of fields, including organic methodology, total synthesis of natural products, and organometallic chemistry. In February 2015, he began working at Chimerix, Inc., in Durham, NC, where he is involved in the synthesis of inhibitors of viral replication to benefit areas of unmet medical need.

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