University of Nevada, Reno Efficient Synthesis of Bicyclo[4.4.0]Decanes

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University of Nevada, Reno

Efficient Synthesis of Bicyclo[4.4.0]decanes by Tandem Double Diels-Alder

A thesis submitted in partial fulfillment of the requirements for the degree of

Bachelor of Science in Chemistry and the Honors Program

Amanda O’Loughlin

Wesley Chalifoux, Ph.D., Thesis Advisor

May 2016 UNIVERSITY OF NEVADA THE HONORS PROGRAM RENO

We recommend that the thesis prepared under our supervision by

AMANDA O’LOUGHLIN

entitled

Efficient Synthesis of Bicyclo[4.4.0]decanes by Tandem Double Diels-Alder

be accepted in partial fulfillment of the requirements for the degree of

BACHELOR OF SCIENCE, CHEMISTRY

Wesley Chalifoux, Ph.D., Thesis Advisor

Tamara Valentine, Ph.D., Director, Honors Program

May, 2016 i

ABSTRACT

Polycyclic structures are frequently found in nature with specific and significant biological activity and their efficient synthesis is an important area of research. Economical syntheses have high significance in the pharmaceutical industry due to the potential to reduce step- count, waste streams, time, and money. This project’s tandem [4+2] cyclization of alkynes generates bicyclo[4.4.0]decane (cis-decalin) products in a succinct manner. The reaction includes the production of four new carbon-carbon bonds, two rings, and quaternary or vicinal quaternary centers in one pot. Successful preliminary reactions creating complex cis-decalin molecules were performed, and show promise for the synthesis of a large scope of products found in high-interest biologically active structures.

ACKNOWLEDGMENTS

Special thanks to my advisor, Dr. Wesley Chalifoux, for his knowledge, guidance, and support, and to graduate student, Jade Horton, who I worked closely with on this project for her mentorship, dedication, and knowledge.

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TABLE OF CONTENTS

Page Abstract………………………….……………...……………………..……..………..…...i

Acknowledgments……….………………………………..….…….…………....………..ii

Table of Contents………………………….……………….…….…………..…………...iii

List of Tables…………………………………………………………...……….….…...... iv

List of Figures……….…………….……….…………………………..……..….………..v

List of Schemes……………….………………………………………..…………………vi

Introduction..………………….…………………………….………..………….……...…1

1. cis-Decalin Substructure……………………………..……………………………2

2. Biological Relevance…….……………..………………………….....………..….3

3. Biosynthetic Pathways of cis-Decalins………………………………...... ……...7

4. Literature Syntheses…………………….…….…………….…………....…...…...9

5. Diels-Alder Reaction…………………………………….…….…….….……...... 12

6. Methodology: Tandem Double Diels-Alder...…………….…….….…...... ….…..14

7. Results ………………….……………....………….………....………...... 15

8. Discussion and Conclusion…………….…………………………...... …….21

9. Future Direction: Total Synthesis of Valerane…………………………...….…...22

References……………………….…………………………………………………...…..25

LIST OF TABLES

Table Page

[1]: Reaction scope of ynones with

2,3-dimethyl-1,3-butadiene………………………………………………………….17

[2]: Eli Lilly Screening Results………………………………………………………….18

LIST OF FIGURES

Figure Page

[1]: Isoprene unit…………….…………………………………………....…………….…1

[2]: cis- and trans-Decalin conformations………………………….……………………..3

[3]: Natural products with biological activity containing cis-decalin backbone………….4

[4]: Neutral, normal, inverse demand Diels-Alder……………………....………….…...12

[5]: Molecular orbital diagram where Lewis acid coordination

decreases the HOMO-LUMO gap……………...……………….….……….….……13

[6]: Diels-Alder reaction with alkyne dienophile to create

asymmetric products………….…….…...…….………………………………….....14

[7]: Proposed double Diels-Alder of conjugated alkynes to yield

bicyclo[4.4.0]decanes……………………………………………………………….14

[8]: 1H NMR study monitoring conversion of ynone 24a to

bicyclized product 26a……………………………………………………....……....16

[9]: Srikrishna and colleagues’ 13 step total synthesis of valerane……………………...22

[10]: Proposed five step total synthesis of valerane via proposed

tandem double Dies-Alder………….……………………………………………...23 vi

LIST OF SCHEMES

Scheme Page

[1]: Biosynthetic [4+2] cyclization of cis-decalin solanapyrone A

via enzyme Sol5…….…….…….………………………………………………….....8

[2]: Enantioselective synthesis of cis-decalin 18 using organocatalysis and sulfonyl

Nazarov reagent 16…………………….…………………………..……….…………9

[3]: Transannular Diels-Alder (TADA) reaction with macrocycle 19 to generate

cis-decalin 20………..…………………………………..……….……...... …….….10

[4]: Intramolecular Heck reaction using palladium catalyst and chiral ligand

(R)-BINAP to produce cis-decalin 21...... 11

[5]: Intramolecular alkylation of ketone enolate to generate cis-decalin 23…….…….....11

[6]: Preliminary experiment of Lewis acid-catalyzed tandem double

Diels-Alder cyclization of α,β-unsaturated ynone 24a to produce bicyclic

product 26a………………………….………………………………..……..…...…..15

[7]: Lewis acid catalyzed Diels-Alder of 27 to make monocyclized

intermediate 28……………………………………………………………………...19

[8]: Proposed addition of two different dienes by utilizing the rate

difference between the cycloadditions to generate an asymmetric product……...…20

[9]: Lewis acid catalyzed tandem double Diels-Alder of ynone 24a with

1 equiv. of 2,3-dimethyl-1,3-butadiene and excess 1,3-butadiene to

produce asymmetric bicyclic product 30…………………………………………....20

INTRODUCTION

The specificity required in the structure of effective drugs presents many challenges for the synthesis of highly desirable pharmaceuticals.1 The development of efficient routes to complex targets is a large and significant area of research due to the potential to reduce step-count, waste streams, time, and money. Synthetic efficiency is measured by the number of steps, yield, cost, and time; these are important considerations in the drug design process. Pharmaceutical companies look for syntheses that will help them to arrive at complex molecules in the shortest and most reliable manner possible. Therefore, these types of economical syntheses have the potential to be used for large scale drug production.

There are a number of terpenoid (and specifically sesquiterpenoid) natural products that have been found to have beneficial effects in the treatment of human diseases.2

Terpenoids are a class of natural products, often produced by plants, that are derived from isoprene units (Figure 1) and contain oxygen functional groups.3 Sesquiterpenoids are a subclass of terpenoids that have three isoprene units and 15 carbon atoms.3 Some terpenoids are found to have anticancer,4 antifungal,5-8 antitumor9, antibacterial,6,8 and antiparasitic10 properties, and continue to be important compounds for drug discovery.2 An economical synthesis of such structures has been a challenge in the past due to high step- counts and large expenses but should be possible via our “one-pot” methodology. A “one- pot” synthesis is a general term used to describe reactions that occur simultaneously or sequentially, and without isolation of intermediate products.11

This thesis focuses on the development of a methodology for the rapid synthesis of cis-decalin products with low step-count and high yield, as well as the exploration of a 2 scope of successful alkyne and diene substrates. We examine the biological activity of these compounds and investigate the total synthesis of compounds in the valerane family of natural products. The relative simplicity in the structure of valerane makes it a good starting point to prove that short total syntheses of this class of molecules is possible using our methodology. Valerane, and many terpenoid natural products with biological activity, contain a cis-decalin substructure. Compounds with the cis-decalin substructure are the focus of this thesis due to their reactivity, which will be discussed in Section 1.

1. cis-Decalin Substructure

The cis-decalin motif is a bicyclic fused ring system in which the substituents on the bridgehead carbons are oriented in the same plane of space. The opposite of cis is trans, in which the substituents on the bridgehead carbons are not in the same plane of space. Cis and trans molecules are diastereomers because they have the same molecular and structural formulas, but are not mirror images of each other. Diastereomers can have different physical properties, chemical properties, and reactivities.

The structural motif of cis-decalins has its advantages and disadvantages. Because the bridgehead substituents are cis, the two six-membered rings form chair conformations and the entire molecule adopts a tent-like shape. This tent-like shape creates a gauche interaction between the atoms on the bottom face and causes the cis-decalin to be less stable than the trans-decalin.12 An advantage of the cis-isomer is that its faces have different reactivity – the top (convex) face is more “exposed” and therefore more reactive than the bottom (concave) face (Figure 2). This difference in reactivity can be a beneficial tool for selectively synthesizing one diastereomer over the other.12 Important biological 3 implications of various cis-decalins are discussed in Section 2, and these implications along with the reactivity of this type of molecule, make cis-decalins attractive synthetic targets.

2. BIOLOGICAL RELEVANCE

The cis-decalin substructure is found in many natural products with biological and pharmaceutical implications. Many decalins found in nature, including the ones in Figure

3, contain complex groups and are highly functionalized, contributing to their structural and functional diversity. Because of their range of biological activities, along with the decalin motif serving as a basic template for constructing entire molecules, cis-decalins are attractive biosynthetic targets.13 Figure 3 below shows nine natural products, with their cis- decalin substructure highlighted in blue. These decalins have been shown to be active against one or more infectious agents such as bacteria, fungi, parasites, and rapid-growing cells such as cancer and tumors.4-6,8-10

cis-Decalin 1: 5,6-dihydroxycadinan-3-ene-2,7-dione is one of the bioactive constituents in Eupatorium adenophorum,5 a flowering plant found in Mexico and Central

America. The plant is also known as crofton weed or sticky snakeroot. 5,6- dihydroxycadinan-3-ene-2,7-dione has expressed fungal growth inhibition of phytopathogens: Rhizoctonia solani, Sclerotium rolfsii, Fusarium oxysporum, and 4

Macrophomina phaseolina.5 With its moderate activity against these different plant pathogens, and significantly higher fungal growth inhibition against R. solani, this cis- decalin has high potential to act as an antifungal agent.

cis-Decalin 2: meiogynin A is extracted from the bark of Meiogyne cylindrocarpa, a flowering shrub in the Annonaceae family that is native to Malaysia.4 It shows potent activity against the protein Bcl-xL. Bcl-xL is an antiapoptotic protein that is involved in the “regulation of cell death in many eukaryotic systems”.4 This protein is over-expressed in prostate cancer progression and is reported by Litaudon and colleagues to contribute to 5 multidrug resistance.4 Thus agents that show inhibition of this protein, like meiogynin A, may be useful for preventing or treating cancer. Meiogynin A has an inhibition constant of

Ki=10.8 ± 3.1 μM. This low inhibition constant means only a small concentration of meiogynin A is needed to produce one-half the maximum inhibition, indicating the high potency of this molecule.4

Both cis-Decalin 3: aignopsanoic acid and cis-decalin 5: methyl aignopsanoate A are cis-decalin sesquiterpenes found in the Indo Pacific marine sponge Cacospongia

10 mycofijiensis. With IC50 values (concentration required to inhibit a given biological process by 50%) of 6 μg/mL and 16 μg/mL, respectively, aignopsanoic acid and methyl aignopsanoate A display moderate inhibition of Trypanosoma brucei. T. brucei is a parasite transmitted by the tsetse fly in Sub-Saharan Africa. This parasite is responsible for Human

African trypanosomiasis or “sleeping sickness”.10

cis-Decalin 4: ageline B, isolated from Pacific sea sponge Agelas sp. in Palau,

Western Caroline Island, demonstrates antifungal and antibacterial activity.8 Among the various bacteria ageline B inhibits is Staphylococcus aureus, a gram-positive bacteria responsible for skin and respiratory tract infections known as “Staph infections”. Ageline

B also exhibits cytotoxic activity against the fungus Candida utilis, which is involved in many chronic urinary tract infections.8

cis-Decalin 6: 15-hydroxy-T-muurolol is isolated from the marine-derived bacteria

Streptomyces sp. M4919 after extracting the bacteria from a sample of sand off the Qingdao coast of China. According to Ding and colleagues, this sesquiterpene exhibited weak

9 cytotoxic activity against many human tumor cell lines (IC50= 6.7 μg/mL). 6

cis-Decalin 7: T-muurolol is one of the most active constituents isolated from essential oil of the tree leaf of Calocedrus formosana.7 Cheng and colleagues collected leaves from this indigenous tree in the central mountain region of Taiwan and were able to isolate more than 50 compounds.7 When eight of the compounds were tested against four wood-rot fungi: L. sulphureus, P. coccineus, L. betulina, and T. versicolor, T-muurolol possessed higher antifungal activity compared to the monoterpene compounds. T-muurolol also showed 100% mortality against the termite Coptotermes formosanus, therefore displaying its potential for fungicide and termiticide development.7

cis-Decalin 8: (+)-α-muurolene and cis-decalin 9: (+)-6-hydroxy-α-muurolene are isolated from the soft coral species of the Heteroxenia sp. genus off the shores of Mindoro

Island, Philippines by Edrada and colleagues.6 They are both active against the fungus

Cladosporium cucumerinum and the bacterium Bacillus subtilis (located in soil and the human gastrointestinal tract).6 (+)-6-Hydroxy-α-muurolene is lethal to brine shrimp with a

6 median lethal concentration (LC50) of 6.49 μg/mL.

These nine compounds represent a small number of the biologically-interesting cis- decalins that are found in nature.14 The compounds have a wide range of biological activity and they are diverse with complex functional groups. However, their common cis-decalin backbones can be used as a framework for constructing molecules with even more diversity and biological relevance. The biosynthetic pathway of natural cis-decalin products is a good starting point for developing an efficient method of similar products and is discussed in Section 3.

3. BIOSYNTHETIC PATHWAY OF cis-DECALINS

Understanding how these products are formed in nature provides insight into efficient methods for the synthesis of similar complex molecules. Many natural resources that produce decalin substructures are fungi and bacteria, synthesizing functionalized structures with extensive biological activities. Although not fully understood, most cis- and trans-decalins are formed in nature by an enzyme-catalyzed intramolecular Diels-Alder reaction.13

An example of this pathway is seen in the biosynthesis of solanapyrone A. The formation of the cis-decalin core of solanapyrone A is catalyzed by Sol5, a flavoprotein oxidoreductase15 (Scheme 1). The enzyme “oxidizes the hydroxymethyl group on the pyrone ring”16 of 10 to form intermediate 11. The raised highest occupied molecular orbital

(HOMO) energy of the diene and the lowered lowest unoccupied molecular orbital

(LUMO) energy of the dienophile allow the exo [4+2] cycloaddition to take place while the enzyme is bound to the intermediate. This cycloaddition results in the formation of major cis-decalin product 12, solanapyrone A. Sol5 is responsible for both the oxidation and cycloaddition in this biosynthetic reaction.16

This mechanism provides us with valuable information about the formation of cis- decalins in nature. Being one of the few natural syntheses that favors the cis product over trans, the biosynthesis of solanapyrone A demonstrates how difficult cis-decalins are to generate, even in nature. Although the cis product, solanapyrone A, has a less energetically favored final configuration, it is the major product because the transition state that occurs to produce the cis configuration is lower in energy than the trans transition state.15

As discussed in Section 6, the proposed method of this thesis eliminates a competition of transition states because the formation of a trans product is not possible.

In addition to recognizing advantages and disadvantages of the biosynthesis of cis- decalins, valuable information can be gathered from studying previous methods in the literature on synthetically producing cis-decalins. In Section 4, literature syntheses with a few different approaches to construct cis-decalins are reviewed.

4. LITERATURE SYNTHESES

The biological activity of many cis-decalins makes them interesting targets in organic synthesis. A number of different approaches have been explored in an attempt to synthetically manufacture these compounds. A few of these approaches include Nazarov cyclizations,17 transannular Diels-Alder reactions,18 intramolecular Heck reactions,19,20 and intramolecular alkylations of ketone enolates.21 Two of the following syntheses are enantioselective.17,19,20 Enantiomers, like diastereomers discussed in Section 1, are a subgroup of stereoisomers; enantioselectivity is when the formation of one enantiomer is favored over the other.22 Enantiomeric excess (ee) is a percentage of how enantiomerically pure a sample is: where 100% ee is purely one enantiomer, and 0% ee is an equal mixture of both enantiomers.

Peña and colleagues reported an enantioselective synthesis of cis-decalins using organocatalysis and sulfonyl Nazarov reagents (Scheme 2).17 Using cyclic unsaturated

10 aldehydes 14 as starting materials, an iminium ion 15 is formed with the enantiomeric catalyst. This iminium ion then reacts with Nazarov reagent 16 to generate an enamine 17, which then proceeds to react with the α-β-unsaturated ketone from the intermediate. After elimination of the catalyst, Peña and colleagues obtained cis-decalin 18 in 52% yield and

96% ee.17

In the process of the total synthesis of (+)-phomopsidin, Suzuki and colleagues performed a transannular Diels-Alder reaction (TADA)18 to create cis-decalin 20 (Scheme

3). The macrocycle 19 was prepared via E-selective intramolecular Horner-Wadsworth-

Emmons reaction in a two-step yield of 78%. The macrocycle was then refluxed in toluene for one day to yield 20 as an inseparable mixture of two diastereomers (2:1) in 63% yield.18

A TADA reaction is a cyclic version of an intramolecular Diels-Alder. The TADA can be a beneficial tool in synthesizing complex, polycyclic molecules. However, TADA can be limited by its difficulty in predicting the stereochemical outcome of the [4+2] cyclization and the difficulty in preparing the macrocycle starting material 19.23

An intramolecular Heck reaction was employed by Shibasaki and colleagues to synthesize cis-decalin 21, using chiral ligand (R)-BINAP, silver carbonate base, and N- methyl-2-pyrrolidone (NMP)19,20 in good yield and modest enantioselectivity (Scheme 4).

The mechanism for this reaction involves an oxidative addition of a Pd(0) catalyst in order 11 to form a Pd(II) intermediate. This intermediate undergoes β-hydride elimination, releasing the Heck alkene product. A base is then used to convert the Pd(II) intermediate back to the

Pd(0) catalyst. However, “poor regio-selectivity in the β-hydride elimination step limits the use of the asymmetric Heck reaction for construction of tertiary stereocenters.”20

Using known intramolecular alkylations of ketone enolates, Fleming and colleagues21 were able to form cis-decalin 23 (Scheme 5). The β-siloxy unsaturated nitrile

22 was constructed via the addition of a Grignard reagent24 to form an enolate intermediate.

That intermediate was then reacted with tert-butyldimethylsilyl chloride (TBDMSCl) and

22 was collected in 66% yield. Fleming et al. then cyclized this β-siloxy unsaturated nitrile

21 with tetra-n-butylammonium fluoride (n-Bu4NF) to collect 23 in 90% yield.

Although these literature syntheses were able to produce their desired cis-decalin products, there are definite drawbacks. These drawbacks include difficulty in predicting the outcome of the cyclization, poor selectivity, and barriers in making the complicated 12 starting materials. Drawbacks like this make these approaches not ideal for large scale production that is required in the pharmaceutical industry. Examined in Section 5 is a type of reaction that is employed in the biosynthesis of solanapyrone A and is one that we believe would be a much better tool for synthesizing cis-decalins: The Diels-Alder

Reaction.

5. DIELS-ALDER REACTION

Discovered in 1928 by Otto Diels and Kurt Alder, the [4+2] cycloaddition is a concerted reaction between a 4π electron system (diene) and a 2π electron system

(dienophile) to create two new sigma bonds.25 The reaction is successful because the new sigma bonds that form are energetically more stable than the pi bonds of the diene and dienophile. The Diels-Alder (DA) reaction is “the best-known organic reaction that is widely used to construct, in a regio- and stereo-controlled way, a six-membered ring with up to four stereogenic centers”26 and has been used for decades as a favorable synthetic tool.

The simplest DA reaction involves neutral and symmetric starting materials to generate cyclic, symmetrical products (Figure 4a). The reactivity of the DA reaction depends on the energy difference between the HOMO of one component and the LUMO of the other component.26 The addition of electron-donating groups (EDG) and electron- withdrawing groups (EWG) to the components (Figure 4) causes the starting materials to become more reactive. The energy gap between the HOMO and LUMO is decreased, therefore accelerating the cycloaddition.

Acids, according to the Lewis definition, are electron-deficient molecules or ions that act as an electron-pair acceptor and can coordinate with unshared pairs of electrons.27

This makes them useful catalysts for DA reactions as discovered by Yates and Eaton in

1960.28 Lewis acids (LA) catalyze DA cycloadditions by increasing the rate of reaction by coordinating to a carbonyl group. This coordination decreases the “influence of the oxygen lone pairs on the π-bond of the carbonyl group”, therefore lowering the LUMO of the dienophile (Figure 5) and allowing the reactions to be achieved at lower temperatures and pressures.29

Alkynes can also be used in DA reactions because the high electron density of their

π-bonds allow them to act as dienophiles. Lewis acids can be used to catalyze DA reactions using alkynyl ketones (ynones) to increase the rate of reaction. Along with increasing the rate, Lewis acids can also increase the enantioselectivity of the cycloaddition.26 14

The high reactivity of polyunsaturated alkynes and ynones make them appealing substrates in synthesis reactions. Utilizing alkynes as dienophiles can serve as a method of creating asymmetric cyclic products (Figure 6).

Alkynes have the potential to participate in tandem and multicomponent reactions, can be easily constructed from simple starting materials, and are underused in organic synthesis. This thesis looks into using relatively simple ynones as the dienophile in a tandem double Diels-Alder cycloaddition to produce a variety of cis-decalin products.

6. METHODOLOGY: TANDEM DOUBLE DIELS-ALDER

The idea behind this project was to utilize the high-reactivity of polyunsaturated alkynes (such as 24) to create complex monocyclic 25 and polycyclic products 26 that have the potential for further manipulations (Figure 7). The Diels-Alder cyclization between an electron-deficient ynone 24 and a diene to produce a “skipped” diene intermediate 25 contains a conjugated alkene structure (shown in red) that is still reactive enough to undergo another cyclization. With an alkyne’s capacity to undergo intra- and intermolecular tandem reactions, we envisioned that a second cyclization would be induced by a Lewis acid catalyst to produce the desired cis-decalin products 26.

This proposed reaction results in the formation of two rings, four new carbon- carbon bonds, a quaternary center (a carbon that is connected to four atoms other than hydrogen), and the potential for vicinal quaternary centers (two adjacent quaternary centers) all in one pot. This reaction will create products that have high functionality and potential for further manipulation. With this method, we believe a total synthesis of products that have cis-decalin motifs and quaternary or vicinal quaternary stereocenters, such as valerane (Figure 7) and compounds within the valerane family, will be achievable in significantly fewer steps.

7. RESULTS

As verification of the theory of the tandem double Diels-Alder cyclization, a preliminary reaction was performed by mixing one equivalent of ynone 24a with excess

2,3-dimethyl-1,3-butadiene in C6D6, followed by the addition of one equivalent of dimethylaluminum chloride (Scheme 6). With mesitylene as a reference, the reaction was monitored by proton nuclear magnetic resonance (1H NMR) spectroscopy and the complete conversion to monocyclized intermediate 25a was observed at room temperature in less than 10 minutes (Figure 8). The complete and clean conversion of intermediate 25a to bicyclic product 26a occurred after approximately 22 hours at room temperature, with

100% NMR yield.

24a

25a

26a

These preliminary results show promise for the success of future explorations of alkyne and diene substrates. With this in mind, we have begun to synthesize a small library of cis-decalin compounds using aryl substituted ynones and 2,3-dimethyl-1,3-butadiene shown in Table 1. The general procedure for synthesizing cis-decalins 26a-26e involved dissolving ynone 24a-24e (0.78 mmol) and 2,3-dimethyl-1,3-butadiene (4.0 mmol) in dichloromethane or dichloroethane (10 mL). The mixture was cooled to 0° C in an ice/water bath and allowed to stir while under a nitrogen atmosphere. Then the Lewis acid catalyst, either Me2AlCl or EtAlCl2 (0.78 mmol), was added slowly. After 30 minutes the reaction was allowed to warm to room temperature. For cis-decalins 26b-26e, the reaction was then heated to 50° C after 45 minutes. The reaction was allowed to stir for 23-24 hours 17

and was quenched with saturated aqueous NaHCO3 (4 mL), and dried over Na2SO4.

Evaporation of the solvent and purification of the residues by flash chromatography with pH 8.0 buffered silica, C-18 reverse phase silica, or by Kugelrohr distillation provided cis- decalins 26a-26e (Table 1).

Table 1. Reaction scope of ynones with 2,3-dimethyl-1,3-butadiene

24 R1 Conditions 26 % yield 24a Ph A 26a 60 24b 4-BrC H B 26b 45 6 4 24c 2-BrC6H4 B 26c 25

24d 4-MeOC6H4 B 26d 18

24e 2-MeOC6H4 B 26e 26

When determined by NMR with a standard, the yields of these aryl substituted compounds are high—almost quantitative (near 100%). However, isolation has been an issue due to partial loss of the compounds during purification (which is reflected in the yields listed in Table 1). We are in the process of finding a suitable isolation method to increase the yields of these compounds. These products have also been submitted to Eli

Lilly’s Open Innovation Drug Discovery program for biological screening. The products have been flagged as being potentially interesting biologically active molecules and some of the results of the screening are listed in Table 2.

Table 2. Eli Lilly Screening Results IL-17 PPI IL- 17 PCSK9 Tuberculosis IL-5 cis -decalin % Inhib % Inhib % Inhib % Inhib % Inhib @ 100µM @ 10µM @ 5µM @ 20µM @ 10µM 24a 27% 19% 30% ------

24b 24% ----- 32% ------24c 26% 27% 24% ------24d ------62%

24e 22% 14% ----- 11% -----

cis-Decalins 26a-26e have been shown by Eli Lilly to have some inhibition of various biological processes (Table 2). IL-17 PPI stands for Interleukin17A-Interleukin

17RA protein-protein interaction. The inhibition of this pathway, disrupts protein-protein interaction to prevent association of the signaling cytokine-receptor complex. This complex is involved in autoimmune disorders such as psoriasis, rheumatoid arthritis, and multiple sclerosis.30 With moderate inhibition of this pathway, cis-decalins 24a-24c and

24e may be possible beneficial compounds for the treatment of these diseases.

IL-17 is Interleukin 17, a pro-inflammatory cytokine produced by T-helper cells in the human body. The activity of IL-17 can result in various autoimmune disorders that involve chronic inflammation like asthma, lupus, and rheumatoid arthritis.31 PCSK9, or proprotein convertase subtilisin/kexin type 9, is an enzyme in humans that binds to the receptors for low-density lipoprotein (LDL) cholesterol. When PCSK9 is blocked, blood cholesterol levels can be lowered.32

Interleukin-5 (IL-5) is an enzyme that binds to the IL-5 receptor and stimulates B cell growth, increased immunoglobulin secretion, and is a key mediator in eosinophil 19 activation.33 Eosinophils are involved in several allergic diseases such as asthma and allergic rhinitis.33 cis-Decalin 24d shows significant inhibition of IL-5 of 62%, even at low concentrations of 10µM. These results show promise for the biological activity of this type of molecule and encourages us to extend this project further to diversify our compound scope.

By exploiting alkynyl ester substrates (where R1= OMe or OEt) we can extend the usefulness of the products formed. These products are useful because esters can easily be converted to other functional groups, which could quickly and easily expand the compound library. In an attempt to utilize an alkynyl ester as the dienophile in our tandem double

Diels-Alder, 1 equivalent of di-ester 27 was reacted with 1 equivalent of 2,3-dimethyl-1,3- butadiene in CH2Cl2 at 0 °C (Scheme 7). After 25 minutes, the monocyclized product 28 was obtained in 98% yield and pure.

After repeating this reaction with excess 2,3-dimethyl-1,3-butadiene to allow for the second cyclization to occur, we did not observe bicyclic product. We believe this to be due to low reactivity of the sterically-hindered tetra-substituted alkene in the monocyclized intermediate 28. Additional studies using ester substrates will need to be performed to determine the extent of this effect and to establish trends. Stronger Lewis acids such as

BCl3, BBr3, and TiCl4 will be tested for catalyzing the second cyclization. 20

Based on the preliminary NMR study that proved that the tandem double Diels-

Alder is possible (Figure 8), we know that the second cyclization is slower because the

“new” dienophile 25a is a trisubstituted alkene and is sterically hindered. The significant difference in rate between the conversion 24a to 25a and the second cyclization to 26a, will allow us to produce asymmetrical products. Specifically, the rate difference allows for the addition of a different diene that can participate in the second cyclization. The addition of a different diene allows for more control in the synthesis of a wide range of cis-decalin products 29 (Scheme 8).

In order to test the synthesis of asymmetrical products utilizing the slow reaction time of the second cyclization, one equivalent of ynone 24a was reacted with one equivalent of 2,3-dimethyl-1,3-butadiene in CH2Cl2 at 0 °C for 4 hours (Scheme 9). Then excess 1,3-butadiene was added at room temperature and allowed to react for 46 hours.

The asymmetric bicyclic product 30 was obtained in 39% yield. This exciting result demonstrates that the difference in reaction time between the two cyclization steps can be used to create unsymmetrical products. This tool will be important in the total synthesis of the natural product valerane, which will be examined in Section 9.

8. DISCUSSION AND CONCLUSION

Although there have been some setbacks, the future of this project is very promising. We now know that a tandem double Diels-Alder cyclization of activated alkynes is possible. This reaction produces four new carbon-carbon bonds and two rings in one-pot. We have seen in the NMR study that with terminal ynones, a complete and clean conversion of starting materials to bicyclized product occurs. We have determined spectroscopically that these bicyclic products are formed quantitatively. However, we have not yet identified an effective isolation method to obtain these products in high yields. A possible solution to this problem could be to use methods of ketone protection and ketone reduction in order to acquire the protected or reduced cis-decalin products in higher isolated yields.

Another challenge we have faced is the second cyclization step of the double Diels-

Alder because tri- and tetra-substituted alkenes tend to be much less reactive.34 The second cyclization is especially difficult with disubstituted ynones, esters, and di-esters. We are forming the monocyclized intermediate for these substrates almost immediately but are still struggling to find a suitable Lewis acid able to catalyze the second, more inhibited cyclization. Some solutions to be considered for addressing this problem are to continue screening for appropriate Lewis acids and also performing reactions with these substrates under high pressure and high heat conditions in order to give the second cyclization enough energy to overcome its barriers. Once the conditions are found that will allow the second cyclization to occur, we hope to utilize disubstituted ynones in order to create polycyclic molecules with vicinal quaternary centers in a single reaction step. 22

A future goal of this project is to implement our tandem double Diels-Alder method in the total synthesis of natural products such as valerane, as well as compounds within the valerane family. We believe that by using our method, we can drastically decrease the number of steps needed to synthesize biologically relevant cis-decalins. This reduction in step-count can be very benefical in the pharmaceutical industry where time, money, and waste production are big concerns. The proposed total synthesis of valerane using our tandem double Diels-Alder method is explored in Section 9.

9. FUTURE DIRECTION: TOTAL SYNTHESIS OF VALERANE

Although natural sesquiterpene valerane does not contain specifically interesting biological activity itself, it is of synthetic interest due to its structural relationship to other biologically active compounds. Also, its relative simplicity makes it a logical starting point for proving that a short total synthesis for this class of natural products is possible.

Currently, the shortest stereoselective synthesis of valerane is reported by

Srikrishna and colleagues, taking 13 steps from a commercially available chiral starting material, (R)-carvone.35 This method involves a ring cleavage, catalytic hydrogenation, reductive deoxygenation, and desulfurization (Figure 9).

We will be investigating the possibility of an enantioselective total synthesis with lower step-count, waste production, and environmental impact. Our proposed total synthesis will require only five steps, with steps one and two occurring in one-pot. These first two steps will involve the use of a chiral catalyst to control the stereogenic centers.

Step three involves hydrogenation from the “convex” face. Reduction of the ester and deoxygenation using standard conditions will provide valerane in short order (Figure 10).

Other work in our lab36 has shown the use of a monomeric BINOL-Al species to carry out enantioselective Diels-Alder reactions with a number of ynones and dienes in good yields and enantioselectivities. We believe the use of this catalyst protocol will allow us to control the stereochemistry in steps one and two. This methodology could pave the way toward succinct and efficient bicyclo[4.4.0]decane synthesis.

We are excited to carry out the tandem double Diels-Alder with various dienes and ynones to increase the scope of natural and unnatural cis-decalin molecules that can be 24 formed. The future of this project will include optimizing the isolation conditions, using alkynyl ester substrates, developing asymmetrical products, and working toward the total synthesis of valerane and other cis-decalin products.

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