Application of the Diels-Alder Reaction

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APPLICATION OF THE DIELS-ALDER REACTION

TO THE SYNTHESIS OF NATURAL PRODUCT-

LIKE LIBRARIES

John Reed

School of Chemistry Faculty of Science UNSW Australia

A thesis submitted in fulfilment of the requirements for the degree of Master of Science in Chemistry January 2017

PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet

Surname or Family name: Reed

First name: John Other name/s:

Abbreviation for degree as given in the University calendar: MSc

School: Chemistry Faculty: Science

Title: Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Abstract 350 words maximum: (PLEASE TYPE)

This thesis is focussed on the use of the diene-regenerative Diels-Alder reaction to assemble bicyclic molecular frameworks that are commonly found in bioactive natural products. Chapter 1 examines the history of natural products in pharmaceuticals, and introduces the concept of natural product derivatisation to improve biological activity. The development of focussed libraries of compounds to discover potent natural product derivatives is contrasted with combinatorial chemistry, using two illustrative case studies. The diene-regenerative Diels-Alder reaction is introduced, and key literature examples of this transformation are discussed. Chapter 2 describes the application of the diene-regenerative Diels-Alder reaction to the synthesis of functionalised 1,2,3,4-tetrahydroisoquinolines bearing aromatic substituents at the C(5)-position. This strategy provides access to a number of compounds that would be difficult to generate using existing methods for 1,2,3,4-tetrahydroisoquinoline synthesis. The diene-regenerative Diels-Alder reaction was tolerant to a broad range of substrates in terms of both sterics and electronic character. Furthermore, the reaction was used to prepare a 1,2,3,4-tetrahydroisoquinoline containing three contiguous synthetic handles in the form of an aryl silane, aryl bromide and aryl ether. In Chapter 3, the use of the diene-regenerative Diels-Alder reaction in the preparation of carbocylic compounds is discussed. Ten 1,5,6,7,8,8a- hexahydronaphthalenes were prepared in this manner. One of these compounds was selected as a model substrate and subjected to a variety of conditions to illustrate the broad spectrum of reactivity these 1,5,6,7,8,8a-hexahydronaphthalenes exhibit. This enabled the synthesis of a variety of highly functionalised molecules containing a decalin scaffold. A summary of the work described in this thesis, and the future directions for the project, is provided in Chapter 4. Full experimental procedures are listed in Chapter 5.

Declaration relating to disposition of project thesis/dissertation

I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation.

I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only).

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A[GrabPPLICATION your reader’s attention OF with aTHE great quote DIELS from the-A documentLDER or use R thisEACTION space to emphasize a key point. To place this text box anywhere on the page, just drag it.] TO THE SYNTHESIS OF NATURAL PRODUCT-

LIKE LIBRARIES

John Reed

School of Chemistry Faculty of Science UNSW Australia

A thesis submitted in fulfilment of the requirements for the degree of Master of Science in Chemistry January 2017

This thesis is dedicated to May, who has been sorely missed since December.

Ye! Who behold perchance this simple urn,

Pass on – it honours none you wish to mourn.

To mark a friend’s remains these stones arise;

I never knew but one – and here he lies.

-Lord Byron, “Epitaph to a Dog”

ORIGINALITY STATEMENT

I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged.

Signed:......

Date:...... 23/01/2017

iii

I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis of dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the revisions of the Copyright Act 1968. I retain all propriety rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation.

I also authorise University microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International.

I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.

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iv Thesis Title Goes Here - Your Name – Month Year

ACKNOWLEDGEMENTS

I would like to thank Associate Professor Jonathan Morris for supervising me for the duration of this degree. The guidance you have provided for my research has been perfectly complemented by the thought-provoking questions you have asked, and the helpful answers you have provided.

Thanks also to the members of the Morris group, particularly Steve for convincing me to join the group, and to Jono and Joana for collecting mass spectrometry data for me. There are many aspects of life in the Morris group that I will miss, none more so than the maturity displayed by each and every member.

Thanks to the members of the NMR facility in the Mark Wainwright Analytical Centre. In particular, thanks to Don for helping me with some of the more sophisticated NMR experiments and providing easy-to-understand explanations of the underlying theory behind them.

A massive shout out to the Normo’ boys. We finally got it! Thanks for making every summer Saturday unbelievably fun. I could write a whole other thesis on the hilarious moments that occurred over the last three seasons, both on and off the field…

Thanks to all family and friends, especially those who were stuck living with me over the last two years. I would have driven myself crazy a long time ago without you.

Finally, a special thanks to Veronica. I can’t wait to begin life in Europe together.

ABSTRACT

This thesis is focussed on the use of the diene-regenerative Diels-Alder reaction to assemble bicyclic molecular frameworks that are commonly found in bioactive natural products.

Chapter 1 examines the history of natural products in pharmaceuticals, and introduces the concept of natural product derivatisation to improve biological activity. The development of focussed libraries of compounds to discover potent natural product derivatives is contrasted with combinatorial chemistry, using two illustrative case studies. The diene-regenerative Diels-Alder reaction is introduced, and key literature examples of this transformation are discussed.

Chapter 2 describes the application of the diene-regenerative Diels-Alder reaction to the synthesis of functionalised 1,2,3,4-tetrahydroisoquinolines bearing aromatic substituents at the C(5)-position. This strategy provides access to a number of compounds that would be difficult to generate using existing methods for 1,2,3,4-tetrahydroisoquinoline synthesis. The diene-regenerative Diels-Alder reaction was tolerant to a broad range of substrates in terms of both sterics and electronic character. Furthermore, the reaction was used to prepare a 1,2,3,4-tetrahydroisoquinoline containing three contiguous synthetic handles in the form of an aryl silane, aryl bromide and aryl ether.

In Chapter 3, the use of the diene-regenerative Diels-Alder reaction in the preparation of carbocylic compounds is discussed. Ten 1,5,6,7,8,8a-hexahydronaphthalenes were prepared in this manner. One of these compounds was selected as a model substrate and subjected to a variety of conditions to illustrate the broad spectrum of reactivity these 1,5,6,7,8,8a-hexahydronaphthalenes exhibit. This enabled the synthesis of a variety of highly functionalised molecules containing a decalin scaffold.

A summary of the work described in this thesis, and the future directions for the project, is provided in Chapter 4. Full experimental procedures are listed in Chapter 5.

CONTENTS

1 INTRODUCTION ...... 1

1.1 THE ROLE OF NATURAL PRODUCTS ...... 1 1.1.1 Natural Product Pharmaceuticals ...... 1 1.1.2 The Synthesis and Derivatisation of Natural Products ...... 4 1.1.3 An Alternative Drug Discovery Paradigm ...... 8

1.2 THE DIENE-REGENERATIVE DIELS-ALDER REACTION ...... 11

1.3 WORK DESCRIBED IN THIS THESIS ...... 16

1.4 CHAPTER 1 REFERENCES ...... 17 2 A NEW STRATEGY FOR THE SYNTHESIS OF FUNCTIONALISED TETRAHYDROISOQUINOLINES ...... 23

2.1 THE CHEMISTRY AND BIOLOGY OF TETRAHYDROISOQUINOLINES ...... 23 2.1.1 A Profoundly Bioactive Scaffold ...... 23 2.1.2 Synthetic Aspects of Tetrahydroisoquinolines ...... 24 2.1.3 The Naphthylisoquinoline Alkaloids ...... 29

2.2 TARGET COMPOUNDS AND RETROSYNTHETIC ANALYSIS...... 30

2.3 SYNTHESIS OF A COMMON PYRONE PRECURSOR...... 32 2.3.1 Methods to Access Pyrones ...... 32 2.3.2 Attempts to Synthesise a Useful Pyrone ...... 34

2.4 ELABORATION OF THE SCAFFOLD TO THE PYRONE DIELS-ALDER PRECURSOR ...... 36

2.5 INVESTIGATIONS INTO THE PYRONE DIELS-ALDER REACTION ...... 39

2.6 EXPANDING THE SCOPE OF THE STRATEGY ...... 46

2.7 SUMMARY ...... 54

2.8 CHAPTER 2 REFERENCES ...... 55 3 ACCESSING FUNCTIONALISED DECALINS USING THE DIENE-REGENERATIVE DIELS- ALDER REACTION ...... 59

3.1 INTRODUCTION: EMBELLISTATIN, 12-DEOXYHAMIGERONE AND TEO3.1 ...... 59

3.2 SYNTHESIS OF A PYRONE PRECURSOR ...... 65

3.3 STRATEGIES FOR THE DIVERSIFICATION OF THE DIENOPHILE...... 67

3.4 APPLICATION OF THE DIENE-REGENERATIVE DIELS-ALDER REACTION ...... 72

3.5 MANIPULATION OF THE DIHYDROAROMATIC SCAFFOLD ...... 75

3.6 SUMMARY ...... 85

3.7 CHAPTER 3 REFERENCES ...... 86 4 CONCLUSIONS AND FUTURE WORK ...... 89

vii

4.1 CONCLUSIONS ...... 89

4.2 FUTURE WORK ...... 90

4.3 CHAPTER 4 REFERENCES ...... 92 5 EXPERIMENTAL ...... 93

5.1 GENERAL EXPERIMENTAL ...... 93

5.2 EXPERIMENTS DESCRIBED IN CHAPTER 2 ...... 94

5.3 EXPERIMENTS DESCRIBED IN CHAPTER 3 ...... 115

5.4 CHAPTER 5 REFERENCES ...... 135

viii

LIST OF ABBREVIATIONS AND ACRONYMS

Å Ångstrom LUMO Lowest unoccupied molecular Ac Acetyl orbital AIBN Azobisisobutyronitrile μW Microwave aq Aqueous m Multiplet Ar Aryl M Molar BARF Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate M+ Parent ion BHT Butylated hydroxytoluene mbar Millibar Bn Benzyl m-CPBA meta-chloroperbenzoic acid bpy 2-2’-bipyridine Me Methyl br Broad MHz Megahertz n-Bu Primary butyl min minutes oC Degrees Celsius Mp Melting Point calc. Calculated m/z Mass to charge ratio cm Centimetres NBS N-Bromosuccinimide cm-1 Wavenumbers NMR Nuclear magnetic resonance cod 1,5-Cyclooctadiene NOESY Nuclear Overhauser effect COSY Correlation spectroscopy spectroscopy Cp Cyclopentadienyl o-DCB ortho-dichlorobenzene m-CPBA meta-Chloroperbenzoic acid ppm Parts per million δ Chemical shift in parts per million ppy 2-Phenylpyridine d Doublet q Quartet DIBAL Diisobutylaluminium hydride QSAR Quantitative Structure-Activity DME 1,2-Dimethoxyethane Relationship DMF N,N-Dimethylformamide quant. Quantitative yield DMSO Dimethylsulfoxide Red-Al Sodium bis(2-methoxyethoxy) dr Diastereomeric ratio aluminium hydride dtbbpy 4,4-Di(tert-butyl)-2,2’-bipyridine s Singlet ESI Electrospray ionisation SCE Saturated Calomel electrode Et Ethyl t Triplet FDA US Food and Drug Administration TBAF Tetrabutylammonium fluoride FTIR Fourier Transform Infrared TBS tert-Butyldimethylsilyl g Grams TEBAC Benzyltriethylammonium GASP Genetic Algorithm Similarity Program chloride GRP78 Glucose-regulated protein-78 TFA Trifluoroacetic acid h Hours THF Tetrahydrofuran HIV Human Immunodeficiency Virus TLC Thin layer chromatography HMBC Heteronuclear multiple bond correlation Ts para-Toluenesulfonyl HOMO Highest occupied molecular orbital W Watts HRMS High resolution mass spectrometry hv Visible light Hz Hertz IC50 Half maximal inhibitory concentration L Litre

ii Chapter 1: Introduction 1 INTRODUCTION

1.1 The Role of Natural Products The metabolic processes common to all domains of life produce an incredibly vast array of molecules, each of which plays a role in the preservation and propagation of the proprietor species. It is customary to describe these compounds as either primary or secondary metabolites.1,2

Primary metabolites – proteins, lipids, carbohydrates, and nucleic acids – are molecules that are essential to ongoing survival.3 They are ubiquitous in all domains of life, and are frequently found with a high degree of similarity between species. As such, they can be considered the bare minimum required for life. In contrast, the secondary metabolites are usually small molecules that are specific to a certain genus or species. While they are not necessarily essential for survival and replication, they often display a profound biological activity that imbues the concomitant organism with an evolutionary advantage. Such uses include defence mechanisms (tetrodotoxin), growth regulation (gibberellins), sexual stimulation (steroids), and vascular regulation (prostaglandins).4

The wide range of biological activities displayed by natural products is a result of the diversity exhibited by their chemical structures, particularly of the secondary metabolites. For over a century the synthesis of these structures has been, and continues to be, explored by organic chemists. The reasons for such endeavours range from structural confirmation and development of new methodology to providing sufficient quantities of the target compound to enable pharmaceutical development.

1.1.1 Natural Product Pharmaceuticals For millennia, human societies have relied on extracts from terrestrial flora to provide remedies against many different diseases and maladies. Cuneiform tablets from the ancient Sumerians, some of the earliest examples of writing, describe the medicinal uses of certain plants.5 Other societies have similarly detailed herbal and plant based remedies. Notably, the Ebers and Edwin Smith Papyri from Ancient Egypt display empiricism in the discovery of these medicines, taking them beyond the realm of witchcraft and sorcery.6 Modern-day knowledge now recognises that such plants contain bioactive metabolites that are responsible for the therapeutic effects reported by the Ancient Egyptians.

John Reed - January 2017 1 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

The opiate alkaloids, found in the opium poppy Papaver somniferum, have enjoyed a storied history from the Sumerians to the present day.7,8 Cultivated throughout much of Asia and the Middle East, the plant was revered by the Egyptian Pharaohs, yet reviled by the medieval Inquisition. Constituents of the poppy such as morphine (1.1a) and heroin (1.1b) are currently used as potent analgesics, and are also abused recreationally (Figure 1.1).

Other traditional medicines, having undergone studies to determine the active constituents, are now recognisable as clinically approved therapeutics. Quinine (1.2), the first effective treatment for malaria, is an alkaloid isolated from Cinchona tree bark. When afflicted by fever, a common symptom of malaria, the Quechua people of South America consumed a beverage made from this bark that would relieve the fever. Cinchona bark was brought to Europe by Jesuit missionaries, and by the 17th century was incorporated in various therapeutics.9

By the mid-20th century, it was recognised that quinine treatment resulted in a number of side effects such as nausea, fever, confusion and hallucinations.10 The adverse effects associated with quinine, coupled with the advent of drug-resistance, necessitated the development of new treatments against malaria. In 1967, the Chinese government established Project 523, a research programme to identify a new treatment for malaria from plants.11 Tu Youyou, a researcher involved in the project, famously archived the different treatments prescribed by traditional Chinese medicine practitioners across the whole country in her magnum opus, A Collection of Single Practical Prescriptions for Anti-Malaria. The course of this research led her to isolate artemisinin (1.3) from sweet wormwood (Artemisia annua), and this compound was found to be a potent anti-malarial agent. It is currently the gold standard treatment for Plasmodium falciparum infections, while a number of synthetic derivatives of the compound are included on the World Health Organisation’s list of essential medicines. The importance of this work was recognised when Tu Youyou shared in the award of the 2015 Nobel Prize in Medicine and Physiology.

Figure 1.1: Bioactive natural products from traditional medicines Understanding the basis of traditional medicines has led to an era of widespread interest in the chemistry and biology of natural products. Aided by ever-improving isolation and spectroscopic techniques, novel metabolites are routinely identified from both marine and terrestrial organisms and tested for biological activity. Demonstrating the importance of this research, investigations by

2 John Reed - January 2017 Chapter 1: Introduction

Newman and Cragg revealed that 48.6 % of the FDA approved cancer therapies are natural products or directly derived from natural products.12 The authors also comment on the success of natural products in other fields of medicine, in particular, noting the dependence on natural products of anti-infective treatments.

The age of antibiotics began in 1928 with the discovery by Alexander Fleming that Penicillium moulds would prevent the growth of bacteria. Subsequent isolation of the active ingredients, now known as penicillins G (1.4a) and V (1.4b), and development of large-scale production processes resulted in saving hundreds of thousands of lives during World War II alone (Figure 1.2). Importantly, it paved the way for identification of new antibiotic agents. Among these, tetracycline (1.5), a natural product from actinobacteria of the genus Streptomyces, is notable due to its effectiveness against both gram-negative and gram-positive bacteria.

Figure 1.2: Naturally produced antibiotics penicillin G (1.4a), penicillin V (1.4b) and tetracycline (1.5) The 1966 identification of paclitaxel (1.6) as the cytotoxic ingredient in samples of Pacific yew tree bark led to approval of the compound as a treatment for ovarian cancer 26 years later (Scheme 1.1). It has since become the highest-selling cancer drug ever manufactured, with global sales exceeding $1 billion per year.13 The impact that paclitaxel has had on human health cannot be understated. More than just treating millions of patients worldwide, it is a frontline drug for a number of aggressive, and previously untreatable, cancers such as advanced metastatic breast cancer, epithelial ovarian cancer, and pancreatic cancer.14–19 Before paclitaxel even reached the market, it was recognised that global demand for the drug would make extraction from the Pacific yew bark an unsustainable production method. This problem was redressed when it was shown that 10- deacetylbaccatin (1.7), abundant in the leaves of both the Pacific and European yew trees, could be converted to paclitaxel in only four steps.20 Whereas isolating paclitaxel from the bark of the Pacific yew is fatal to the tree, the leaves can be harvested and 10-deacetylbaccatin obtained without killing the tree, and thus, allows continued access to the compound.

John Reed - January 2017 3 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 1.1: Paclitaxel is accessed from the more abundant 10-deacetylbaccatin. Image freely obtained from Walter Siegmund under Creative Commons 4.0 licensing: https://creativecommons.org/licenses/by-nc-nd/4.0/legalcode Penicillin, tetracycline and paclitaxel represent the set of natural products that have had, and continue to have, a profound impact on human health. Moreover, the structural complexity exhibited by these compounds has made them popular targets for synthetic chemists. Indeed, the syntheses of these compounds represent important milestones in the development of organic chemistry.

1.1.2 The Synthesis and Derivatisation of Natural Products While natural products have proven to be worthy therapeutics, minor structural amendments are sometimes necessary to provide a drug fit for human consumption. In some cases, the natural product is not potent enough or, through the acquisition of drug resistance, no longer affects the target disease. For example, less than four years after industrial scale production began in 1943, the first penicillin-resistant microbes were observed. These microbes produce enzymes known as β-lactamases, which are able to catalytically hydrolyse the β-lactam group in penicillin. Meticillin (1.8) was developed by the Beecham group in 1959 as a treatment for penicillin-resistant Staphylococcus aureus infections (Figure 1.3). Like penicillin, its mode of action involves the disruption of bacterial cell wall synthesis, however, by switching the benzyl group of penicillin G to a bulkier ortho-dimethoxyphenyl group, the developers were able to prevent β-lactamase catalysed hydrolysis.

Figure 1.3: The ß-lactamase stable meticillin The development of new reaction methodology over the past century has allowed the synthesis of natural products with ever-increasing complexity, as well as enabling the generation of structural analogues with a view to improving the biological profile of these compounds. Indeed, it is in the pursuit of complex targets that new reactions are often discovered.21,22 The powerful antitumour activity displayed by halichondrin B (1.9), a polyether macrolide isolated from the marine sponge Halichondria okadai, caused a wave excitement (Scheme 1.2).23 Naturally occurring sources of

4 John Reed - January 2017 Chapter 1: Introduction halichondrin B are scarce, however, and the startling complexity of the molecule that confers its unique biological activity and makes it so interesting to study renders synthetic approaches to it impractical on a commercial scale. The first total synthesis of halichondrin B, reported in 1992 by Kishi and co-workers, accessed the compound in less than 3 % overall yield, with a 38-step longest linear sequence.24

Scheme 1.2: Kishi’s synthesis of halichondrin B; inset: generalised Nozaki-Hiyama-Kishi reaction scheme Crucial to this venture were five iterations of the Nozaki-Hiyama-Kishi reaction - a Ni(II)/Cr(II)- mediated coupling reaction between an aldehyde and a vinyl, aryl or allyl halide.25–28 The use of the reaction in synthesising fragments 1.10 and 1.11, as well as the coupling of fragments 1.10 and 1.12, all introduced stereochemical information that was retained throughout the rest of the synthetic sequence and in the target compound. The reaction mechanism, as originally proposed by Nozaki and co-workers, begins with the chromium(II) species reducing the nickel(II) to nickel(0). Oxidative addition of the nickel(0) species to the allyl/vinyl halide followed by transmetallation with the newly formed chromium(III) salt provides an organochromium compound that reacts selectively

John Reed - January 2017 5 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries with aldehydes. The successful application of the Nozaki-Hiyama-Kishi coupling in the halichondrin B total synthesis paved the way for an expansion of the reaction’s utility. In particular, efforts were directed at devising methods to use only a catalytic amount of the potentially toxic chromium species (previously required in stoichiometric amounts),29–31 as well as developing enantioselective variants.30,32,33 More importantly, the Kishi group also carried out biological testing on a number of intermediates from their halichondrin B synthesis. They discovered that the right half of the molecule (vide supra) displayed almost exactly the same antitumour activity as the entire compound. These studies also elucidated the mechanism by which halichondrin B and its derivatives attack cancer cells. Further optimisation of the fragment lead to the FDA approval of eribulin (1.13) as a treatment for metastatic breast cancer (Figure 1.4). The story of halichondrin B highlights the utility of natural product synthesis with regards to both reaction development (the Nozaki-Hiyama-Kishi reaction) and pharmaceutical development. Without Kishi and co-workers undertaking this laborious endeavour, it is unlikely that eribulin would be clinically available today.

H2N OH OMe

O O O O

O O O O Me O

1.13 Figure 1.4: Derived from halinchondrin B, eribulin has been successfully used in the clinic Recently, the Burke laboratory has disclosed the results of its investigations into the basis of the antifungal activity displayed by amphotericin B (1.14), a natural product isolated from Streptomyces nodosus bacteria (Figure 1.5). This polyene macrolide has been used clinically for the treatment of fungal and protozoan infections since 1966, yet remarkably, there is very little evidence to suggest that any sort of pathogen resistance to amphotericin B has developed. In some cases, amphotericin B is the only effective treatment, particularly in immunocompromised patients.34 Its effectiveness, however, is undermined by its not inconsiderable toxicity, which limits the amount of the drug that can be safely administered to a patient.35 Commonly, a holding pattern ensues, where the condition neither improves nor deteriorates. A considerable amount of evidence has accumulated to show that amphotericin B self-assembles in lipid membranes, creating pore-like structures that mimic ion conducting channels.36–38 It was widely thought that the unregulated ion flux caused by these channels was responsible for both the antifungal activity and the human toxicity, intrinsically linking the two.39–41 Burke and co-workers were able to prove that the toxicity

6 John Reed - January 2017 Chapter 1: Introduction of amphotericin B to both human and fungi is a result of its ability to bind sterols, and the channel formation actually contributes very little.42,43

Crucial for channel formation Facilitates ion transport OH OH Me O 3 OH Crucial for channel 8 formation HO O OH OH OH OH O O 35 Me 41 Required for self-assembly O Me O O Me Crucial for antifungal activity HO OH 1.14 NH3 Figure 1.5: Important structural features of amphotericin and their roles Guided by computational results, the Burke group synthesised a number of amphotericin B analogues with the key functional groups systematically deleted, and compared the biological activity with the natural product. From these experiments, it was discovered that oxidation at C8, C35, or C41 was crucial for the compounds ability to form channels, and the C3-hydroxyl group was necessary to facilitate ion transport.42–44 Intriguingly, they also found that removal of the C8- or C35-hydroxyl groups, or the C41-carboxylic acid had little to no effect on the fungicidal activity, while removal of the β-mycosamine tail rendered the compound completely useless. An elegant nuclear magnetic resonance experiment indicated that amphotericin B formed aggregates on the outside of a cells phospholipid bilayer that would absorb sterols from the membrane, in much the same way that a sponge absorbs water (Figure 1.6).45 This phenomenon was directly visualised using tunnelling electron microscopy.

Figure 1.6: Visualisation of amphotericin B aggregates absorbing sterols outside a cells phospholipid bilayer. Figure adapted with permission from Gray et al.43 Furthermore, the researchers realised that the main sterols being absorbed differed in human (cholesterol) and fungal (ergosterol) cells. This point of differentiation raised the possibility of a synthetic analogue that would bind ergosterol but not cholesterol, thereby reducing the human toxicity while maintaining the fungicidal activity. To this end, derivatising the C41-carboxylate to a urea resulted in a marked decrease in human toxicity while exhibiting potent anti-fungal activity.46

John Reed - January 2017 7 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

With the aid of computational evidence, the authors rationalised this result by postulating that the carboxylate interacts with the C3’-amino group, forcing amphotericin B into a conformation that permits binding to both ergosterol and cholesterol. By disrupting this interaction, the molecule could move into a lower-energy conformation that favoured ergosterol binding.

While an amphotericin B analogue is yet to reach the market, the urea-based derivatives are presently in clinical trials. Such developments serve to highlight the importance of natural product research. Studies into halichondrin B not only helped bring to a light a powerful new coupling reaction, but resulted in a synthetic derivative being clinically approved. Similarly, by determining the interactions between amphotericin B and human and fungal cells, Burke and co-workers have been able to design an analogue with a better therapeutic index, which may replace the existing antifungal agent in the clinic.

1.1.3 An Alternative Drug Discovery Paradigm While natural products continue to provide waypoints in the search for new therapeutics, a number of other strategies have evolved. For a number of years, combinatorial libraries, chemistry’s version of the infinite monkey theorem, became one of the hottest topics in drug discovery.47 This approach sought to deliver new biologically active compounds by synthesising extremely large libraries of compounds (in order of thousands to millions of members) in the hope that one of them would display excellent activity.48 Unfortunately, despite over two decades of keen interest, the only compound developed from a combinatorial library to gain FDA approval is sorafenib (1.15), a tyrosine kinase inhibitor (Figure 1.7).12

Figure 1.7: Sorafenib is the only FDA approved drug that was discovered through combinatorial screening One hypothesis for this is that the compounds generated in combinatorial libraries display little diversity in terms of their physicochemical properties.49 In particular, there is a distinct lack of chirality and structural rigidity in combinatorial libraries: two properties abundantly present in natural products and clinically approved drugs, as illustrated in a report from Feher and Schmidt (Figure 1.8). Their study also compared combinatorial libraries with natural products and clinically approved drugs according to parameters such as the presence of complex ring systems and heteroatoms, and found that combinatorial libraries consistently failed to access the chemical space occupied by natural products and marketed therapeutics.

8 John Reed - January 2017 Chapter 1: Introduction

Figure 1.8: A principle component analysis of combinatorial libraries (red), natural products (blue), and clinically approved drugs (green) according to various physicochemical parameters. Figure adapted with permission from Feher and Schmidt.49 As researchers have realised that the scattergun approach taken by combinatorial chemistry libraries has been largely ineffective, more focussed libraries, where a deliberate effort has been made to maximise chemical diversity with fewer compounds, have become common. The concepts behind these so called diversity-oriented syntheses (DOS) have been formalised, in particular, by Schreiber and co-workers.50,51 Chief among these is the build-couple-pair strategy as a means of combining small, stereochemically-rich building blocks in as many ways as possible using a minimum number of chemical transformations.51–54 One of the key differences between combinatorial chemistry and a diversity-oriented synthesis is the types of reactions used in the different strategies. Combinatorial chemistry relies on coupling reactions such as amide bond formation or the Suzuki reaction to connect building blocks in a linear fashion.55 On the other hand, DOS projects explore a wider variety of reactions such that two building blocks can be reacted in different ways to create a large number of different products.

To illustrate, Panek and co-workers developed a strategy whereby hydroxyalkyl esters are cyclodimerised to give macrodiolides resembling the natural products pyrenophorol, conglobatin and elaiophylin (Scheme 1.3).56 A number of different substrates were used, each bearing a variety of substituents and stereochemical elements, which find their way to the final compounds. Impressively, with a couple of exceptions, the authors were able to synthesise the dimers in upwards of 65 % yields, with little of the corresponding cyclotrimers observed.

John Reed - January 2017 9 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 1.3: Synthesis of diverse 14- and 16-membered macrodiolides This example also introduces another concept at the forefront of current library design: privileged structures.57,58 Polyketide macrolides and macrodiolides have become a well-known class of natural products due to the profound biological activity exhibited by many of the members.59 For example, there are fifteen different clinically approved macrolide antibiotics,60 while others have displayed potential as therapeutic agents against certain respiratory illnesses,61,62 as well as anti- cancer activity.63 Consequently, Panek and co-workers viewed the 14- and 16-membered macrocycles as a core structure around which they could build non-natural functionality and still hope to have a good hit rate of biologically potent molecules.

This process of identifying a privileged scaffold and building functionality around it has been formalised by Waldman.64 His application of a solid-phase synthesis strategy has seen the development of library based around the ubiquitous decalin motif (Scheme 1.4).65,66 Drawing inspiration from naturally occurring compounds such as nakijichinon C (1.16), nootkaton (1.17) and dysidiolide (1.18), a range of cis-fused decalins, and 8,8a-dehydrodecalins, were produced.

Scheme 1.4: A focussed library of compounds inspired by decalin containing natural products

10 John Reed - January 2017 Chapter 1: Introduction

We believe that there is still plenty of scope to investigate similar drug discovery approaches. The success of such a project would hinge on the identification of a potent privileged scaffold, as well as the design of a synthetic strategy that allows quick access to the scaffold and diverse functionality to be introduced in a minimum number of steps. To this end, multi-bond forming reactions present a great starting point when designing such syntheses. We feel that with an astute choice of scaffold, reactions like the Diels-Alder cycloaddition and its variants are ideal candidates for such an endeavour. Such an approach has met with considerable success from other research groups.67–72

1.2 The Diene-Regenerative Diels-Alder Reaction First described in 1931, the diene-regenerative Diels-Alder reaction has been used to access a wide-variety of molecular architectures.73 In broad terms, the diene-regenerative Diels-Alder reaction is a tandem reaction involving a Diels-Alder cycloaddition followed by a retro-Diels-Alder reaction to extrude a small molecule and regenerate the diene moiety (Scheme 1.5). When an olefinic dienophile is used, a dihydroaromatic product is formed. Conversely, when an alkyne is used as the dienophile, an aromatic ring is generated. General examples include the tetrazine Diels-Alder reaction, the triazine Diels-Alder reaction, and the pyrone Diels-Alder reaction.74,75 In the context of this thesis, only the pyrone Diels-Alder reaction is discussed.

Scheme 1.5: Intramolecular diene-regenerative Diels-Alder reaction in the context of a 2-pyrone substrate The diene-regenerative Diels-Alder reaction has been extensively used in the synthesis of complex natural products. Notably, the strategy has been employed as an effective method for synthesis of bent aromatic rings. In their total synthesis of (±)-haouamine (1.19), Baran and Burns described the difficulty associated with preparing one of the phenol rings that forms part of the macrocycle (Scheme 1.6).76 The strain imposed by the macrocycle forces the aromatic ring into a pseudo-boat configuration that thwarted attempts to effect macrocyclisation via transition-metal mediated biaryl- bond formation, Witkop photocyclisation, or intramolecular alkylation. The authors then hypothesised that a non-aromatic conformational mimic of this structure might enable completion of the macrocycle. If the mimic could subsequently be aromatised then, the target molecule would be prepared. To this effect, pyrone 1.20 was prepared in 3 steps from bromide 1.21 and stannane 1.22 via a Stille coupling followed by N-alkylation and replacement of the methyl ethers with acetate protecting groups.

John Reed - January 2017 11 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 1.6: Construction of a bent aromatic ring via a diene-regenerative Diels-Alder reaction enables the total synthesis of (±)-haouamine. Reagents and conditions: (a) 1.25, Pd(PPh3)4, CuI, PhMe, reflux, 12 h; (b) 10:1 CH2Cl2/TFA, rt, 3 h; 4-tosyloxybutyne, K2CO3, CH3CN, reflux, 6 h; (c) BBr3, CH2Cl2, -78 oC→rt, 1:1 Ac2O/pyr, 3 h; (d) BHT, o-DCB, 250 oC, μW, 10 h; (e) K2CO3, MeOH, rt, 30 min Effecting the Diels-Alder cycloaddition produced a highly strained intermediate with a completed macrocycle. Extrusion of carbon dioxide regenerates the diene giving the bent aromatic ring so desired. The authors found that conventional heating of the substrate failed to deliver the desired Diels-Alder adduct, however, upon irradiation in a microwave reactor, the product could be obtained, along with some recovered starting material. Deprotection of the acetate groups from the Diels-Alder adduct provided the target compound. A similar approach was taken by Beaudry and Zhao in their synthesis of (±)-cavicularin (1.23): a macrocyclic, conformationally chiral natural product with a similarly bent aromatic ring (Scheme 1.7).77

Scheme 1.7: Beaudry’s end-game approach to (±)-cavicularin. Reagents and conditions: (a) BHT, o-DCB, 250 oC, μW, 8 h; (b) BBr3, CH2Cl2

12 John Reed - January 2017 Chapter 1: Introduction

Drawing inspiration from Baran’s haouamine synthesis, Beaudry identified the diene-regenerative Diels-Alder reaction as likely method for the construction of the non-planar B ring. Interestingly, it was found that the regioselectivity could be modulated by using isomeric vinyl sulfones as the dienophile. Where the terminal alkene 1.24 gave clean conversion to the natural product, the internal (E)-alkene 1.25 exclusively produced an unnatural regioisomer (Scheme 1.8). When the vinyl sulfone was replaced with a terminal alkyne, as Baran had done in his work, a mixture of the two isomers was formed.

Scheme 1.8: Control of Diels-Alder regioselectivity by isomeric vinyl sulfones Further studies on this system allowed the authors to develop an enantioselective diene- regenerative Diels-Alder reaction. Catalysed by a cinchona alkaloid, this variant gave access to (+)-cavicularin as a single conformational enantiomer.78,79

In their second generation, chiral pool synthesis of (+)-pseudodeflectusin (1.26), Kobayashi and co-workers employed a diene-regenerative Diels-Alder cycloaddition between 4-bromo-3-hydroxy- 2H-pyran-2-one and chiral alkyne 1.27 to construct the aromatic ring, around which the lactone and furanone rings are built (Scheme 1.9).80 Previous attempts at generating the core via an intermolecular Diels-Alder reaction between 2-methoxyfuran and an α,β-unsaturated δ-lactone failed, which lead them to try the diene-regenerative Diels-Alder approach.

Scheme 1.9: (+)-Pseudodeflectusin total synthesis. Reagents and conditions: NaH, 1.30, 1,4-dioxane, 200 oC, 5 h Complete regioselectivity for the cycloaddition could be achieved by first treating the hydroxypyrone with sodium hydride. The resultant sodium salt was then heated to 200 oC in a sealed tube with the chiral alkyne to give the desired Diels-Alder adduct in 78 % yield. The regioselectivity of the reaction can be attributed to the in situ transesterification of the ethyl ester with the deprotonated pyrone.

John Reed - January 2017 13 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

The subsequent intramolecular diene-regenerative Diels-Alder reaction produces a four-membered lactone that is then ring opened by the stereogenic alcohol.

The diene-regenerative Diels-Alder reaction has also been used to prepare dihydroaromatic compounds. Martin and co-workers developed a synthetic route to the indole alkaloids (±)- reserpine and (±)-α-yohimbine (Scheme 1.10).81 Pyrone 1.28 was heated at reflux in xylenes to effect the intramolecular [4+2] cycloaddition. Upon extrusion of carbon dioxide, the bicyclic dihydroaromatic amide 1.29 was obtained in 93 % yield, and this scaffold could be used to access both alkaloids.

Scheme 1.10: Accessing various alkaloids through the diene-regenerative Diels-Alder reaction The Stoltz group viewed the transtaganolide family of natural products as accessible by trapping the bridged-bicyclic adduct of a pyrone Diels-Alder reaction (Scheme 1.11).82,83 While initial attempts only resulted in isolation of the undesired diene-regenerated product, the authors found that reducing the reaction temperature to 90 oC gave them enough of a window to obtain the initial adduct before carbon dioxide was extruded.

Scheme 1.11: Progress towards the transtaganolide natural products via a pyrone Diels-Alder reaction Snyder and co-workers recently disclosed their studies on the preparation of fused polycyclic heterocycles using the diene-regenerative Diels-Alder reaction.84 4,6-Dichloropyrone (1.30) could be elaborated to generate a diverse range of compounds based on indoline and hydroindoline scaffolds (Scheme 1.12). Furthermore, this strategy was used to complete a total synthesis of ∆7- mesembrenone (1.31), and formal syntheses of mesembrine (1.32) and gracilamine (1.33). A notable feature of this report is the use of tri-substituted dienophiles in some of the hydroindoline examples. These serve to highlight the robust nature of the pyrone Diels-Alder reaction.

14 John Reed - January 2017 Chapter 1: Introduction

Scheme 1.12: Pyrone Diels-Alder routes to indolines, hydroindolines and various natural products Prior work within the Morris groups has applied the diene-regenerative Diels-Alder reaction to the synthesis of the bicyclic core of the marine natural products embellistatin, hamigerone and TEO3.1 (Scheme 1.13).85,86 A crucial result of these studies was that the cycloaddition proceeds diastereoselectively.

Scheme 1.13: Synthesis of the bicyclic core of embellistatin, hamigerone and TEO3.1. The use of the diene-regenerative Diels-Alder reaction extends far beyond the realm of natural product synthesis.87–96 Such applications are not discussed as the selected natural product examples provide enough illustration of the scope and utility of the transformation.

John Reed - January 2017 15 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

1.3 Work Described in this Thesis This thesis is an account of the studies undertaken to design a reliable method for the synthesis of functionalised tetrahydroisoquinolines and to explore further applications of this strategy to organic synthesis (Scheme 1.14). The aims of this thesis were thus:

 Employ the diene-regenerative Diels-Alder reaction as the key step in synthesising diverse tetrahydroisoquinolines  To use the same strategy to synthesise related dihydroaromatic products, and to explore the possible manipulations of the regenerated diene in these compounds

The synthetic procedures developed throughout this work will provide access to bioactive scaffolds inspired by natural products.

Scheme 1.14: Summary scheme of this project

16 John Reed - January 2017 Chapter 1: Introduction

1.4 Chapter 1 References (1) Firn, R. D.; Jones, C. G. Mol. Microbiol. 2000, 37 (5), 989–994. (2) Williams, D. H.; Stone, M. J.; Hauck, P. R.; Rahman, S. K. 2004. (3) Glazer, A. N.; Nikaido, H. Microbial biotechnology : fundamentals of applied microbiology; Cambridge Univ Press, 2007. (4) Mann, J. Natural Products: their chemistry and biological significance; Pearson Education, 1994. (5) Thompson, R. C. Assyrian Medical Tests; Oxford University Press, 1923. (6) Serageldin, I. Glob. Cardiol. Sci. Pract. 2013, 4, 395–404. (7) Norn, S.; Kruse, P. R.; Kruse, E. Dansk Med. Årb. 2005, 33, 171–184. (8) Kłys, M.; Maciów-Głab, M.; Rojek, S. Arch. Med. sa̧dowej i Kryminol. 2013, 63 (3), 226– 235. (9) Rocco, F. Quinine: malaria and the quest for a cure that changed the world; NY: Perennial: New York, 2004. (10) Taylor, W. R. J.; White, N. J. Drug Saf. 2004, 27 (1), 25–61. (11) Jianfang, Z. A Detailed Chronological Record of Project 523 and the Discovery and Development of Qinghaosu (Artemisinin); Strategic Book Publishing, 2013. (12) Newman, D. J.; Cragg, G. M. J. Nat. Prod. 2012, 75 (3), 311–335. (13) Ganem, B.; Franke, R. R. J. Org. Chem. 2007, 72 (11), 3981-3987. (14) Bishop, J. F.; Dewar, J.; Toner, G. C.; Smith, J.; Tattersall, M. H.; Olver, I. N.; Ackland, S.; Kennedy, I.; Goldstein, D.; Gurney, H.; Walpole, E.; Levi, J.; Stephenson, J.; Canetta, R.; J. Clin. Oncol. 1999, 17 (8), 2355–2364. (15) Sledge, G. W.; Neuberg, D.; Bernardo, P.; Ingle, J. N.; Martino, S.; Rowinsky, E. K.; Wood, W. C. J. Clin. Oncol. 2003, 21 (4), 588–592. (16) Jassem, J.; Pieńkowski, T.; Płuzańska, A.; Jelic, S.; Gorbunova, V.; Mrsic-Krmpotic, Z.; Berzins, J.; Nagykalnai, T.; Wigler, N.; Renard, J.; Munier, S.; Weil, C. J. Clin. Oncol. 2001, 19 (6), 1707–1715. (17) Priyadarshini, K.; Keerthi Aparajitha, U. Med. Chem. 2012, 2 (7), 139. (18) Khanna, C.; Rosenberg, M.; Vail, D. M. J. Vet. Intern. Med. 2015, 29 (4), 1006–1012. (19) Kumar, S.; Mahdi, H.; Bryant, C.; Shah, J. P.; Garg, G.; Munkarah, A. Int. J. Womens. Health 2010, 2, 411–427. (20) Bristol Myers Squibb, Semi-synthesis of paclitaxel using dialkyldichlorosilanes, US 6242614 B1, June 5, 2001. (21) Nicolaou, K. C. Proc. Natl. Acad. Sci. 2004, 101 (33), 11928–11928. (22) Nicolaou, K. C.; Snyder, S. A. Proc. Natl. Acad. Sci. 2004, 101 (33), 11929–11936. (23) Hirata, Y.; Uemura, D. Pure Appl. Chem. 1986, 58 (5). (24) Aicher, T. D.; Buszek, K. R.; Fang, F. G.; Forsyth, C. J.; Jung, S. H.; Kishi, Y.; Matelich, M. C.; Scola, P. M.; Spero, D. M.; Yoon, S. K. J. Am. Chem. Soc. 1992, 114 (8), 3162–3164.

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(25) Okude, Y.; Hirano, S.; Hiyama, T.; Nozaki, H. J. Am. Chem. Soc. 1977, 99 (9), 3179–3181. (26) Takai, K.; Kimura, K.; Kuroda, T.; Hiyama, T.; Nozaki, H. Tetrahedron Lett. 1983, 24 (47), 5281–5284. (27) Takai, K.; Tagashira, M.; Kuroda, T.; Oshima, K.; Utimoto, K.; Nozaki, H. J. Am. Chem. Soc. 1986, 108 (19), 6048–6050. (28) Jin, H.; Uenishi, J.; Christ, W. J.; Kishi, Y. J. Am. Chem. Soc. 1986, 108 (18), 5644–5646. (29) Fürstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118 (49), 12349–12357. (30) Bandini, M.; Cozzi, P.; Melchiorre, P.; Umani-Ronchi, A. Angew. Chem. Int. Ed. Engl. 1999, 38 (22), 3357–3359. (31) Durandetti, M.; Périchon, J.; Nédélec, J.-Y. Tetrahedron Lett. 1999, 40 (51), 9009–9013. (32) Chen, C.; Tagami, K.; Kishi, Y. J. Org. Chem. 1995, 60 (17), 5386–5387. (33) Sugimoto, K.; Aoyagi, S.; Kibayashi, C. J. Org. Chem. 1997, 62 (8), 2322–2323. (34) Moen, M. D.; Lyseng-Williamson, K. A.; Scott, L. J. Drugs 2009, 69 (3), 361–392. (35) Perfect, J. R.; Dismukes, W. E.; Dromer, F.; Goldman, D. L.; Graybill, J. R.; Hamill, R. J.; Harrison, T. S.; Larsen, R. A.; Lortholary, O.; Nguyen, M.-H.; Pappas, P. G.; Powderly, W. G.; Singh, N.; Sobel, J. D.; Sorrell, T. C. Clin. Infect. Dis. 2010, 50 (3), 291–322. (36) Andreoli, T. E.; Monahan, M. J. Gen. Physiol. 1968, 52 (2), 300–325. (37) Cass, A.; Finkelstein, A.; Krespi, V. J. Gen. Physiol. 1970, 56 (1), 100–124. (38) Ermishkin, L. N.; Kasumov, K. M.; Potzeluyev, V. M. Nature 1976, 262 (5570), 698–699. (39) Bolard, J. Biochim. Biophys. Acta - Rev. Biomembr. 1986, 864 (3-4), 257–304. (40) Hartsel, S. C.; Hatch, C.; Ayenew, W. J. Liposome Res. 1993. (41) Hartsel, S.; Bolard, J. Trends Pharmacol. Sci. 1996, 17 (12), 445–449. (42) Palacios, D. S.; Anderson, T. M.; Burke, M. D. J. Am. Chem. Soc. 2007, 129 (45), 13804– 13805. (43) Gray, K. C.; Palacios, D. S.; Dailey, I.; Endo, M. M.; Uno, B. E.; Wilcock, B. C.; Burke, M. D. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (7), 2234–2239. (44) Palacios, D. S.; Dailey, I.; Siebert, D. M.; Wilcock, B. C.; Burke, M. D. Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (17), 6733–6738. (45) Anderson, T. M.; Clay, M. C.; Cioffi, A. G.; Diaz, K. A.; Hisao, G. S.; Tuttle, M. D.; Nieuwkoop, A. J.; Comellas, G.; Maryum, N.; Wang, S.; Uno, B. E.; Wildeman, E. L.; Gonen, T.; Rienstra, C. M.; Burke, M. D. Nat. Chem. Biol. 2014, 10 (5), 400–406. (46) Endo, M.; Cioffi, A.; Burke, M. Synlett 2015. (47) Bensaude-Vincent, B.; Newman, W. R. The artificial and the natural : an evolving polarity; MIT Press, 2007. (48) Corbett, P. T.; Leclaire, J.; Vial, L.; West, K. R.; Wietor, J.-L.; Sanders, J. K. M.; Otto, S. Chem. Rev. 2006, 106 (9), 3652–3711. (49) Feher, M.; Schmidt, J. M. J. Chem. Inf. Comput. Sci. 2003, 43 (1), 218–227. (50) Tan, D. S. Nat. Chem. Biol. 2005, 1 (2), 74–84. (51) Burke, M. D.; Schreiber, S. L. Angew. Chemie Int. Ed. 2004, 43 (1), 46–58. (52) Spring, D. R. Org. Biomol. Chem. 2003, 1 (22), 3867.

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(53) Galloway, W. R. J. D.; Isidro-Llobet, A.; Spring, D. R. Nat. Commun. 2010, 1 (6), 1–13. (54) Fitzgerald, M. E.; Mulrooney, C. A.; Duvall, J. R.; Wei, J.; Suh, B.-C.; Akella, L. B.; Vrcic, A.; Marcaurelle, L. A. 2012. (55) Brown, D. G.; Boström, J. 2015. (56) Su, Q.; Beeler, A. B.; Lobkovsky, E.; Porco, J. A.; Panek, J. S. Org. Lett. 2003, 5 (12), 2149– 2152. (57) Welsch, M. E., Snyder, S. A., Stockwell, B. R. Curr. Opin. Chem. Biol. 2010, 14 (3), 347. (58) Kim, J.; Kim, H.; Park, S. B. J. Am. Chem. Soc. 2014, 136 (42), 14629–14638. (59) Hamilton-Miller, J. M. Bacteriol. Rev. 1973, 37 (2), 166–196. (60) Ritzau, M.; Heinze, S.; Fleck, W. F.; Dahse, H. M.; Gräfe, U. J. Nat. Prod. 1998, 61 (11), 1337–1339. (61) Schultz, M. J. J. Antimicrob. Chemother. 2004, 54 (1), 21–28. (62) López-Boado, Y. S.; Rubin, B. K. Curr. Opin. Pharmacol. 2008, 8 (3), 286–291. (63) Bailly, S.; Pocidalo, J. J.; Fay, M.; Gougerot-Pocidalo, M. A. Antimicrob. Agents Chemother. 1991, 35 (10), 2016–2019. (64) Kumar, K.; Wetzel, S.; Waldmann, H. In The Practice of Medicinal Chemistry; Elsevier, 2008; pp 187–209. (65) Röttger, S.; Waldmann, H. European J. Org. Chem. 2006, 2006 (9), 2093–2099. (66) Yoshida, M.; Hedberg, C.; Kaiser, M.; Waldmann, H. Chem. Commun. 2009, 34 (20), 2926. (67) Buller, F.; Mannocci, L.; Zhang, Y.; Dumelin, C. E.; Scheuermann, J.; Neri, D. Bioorg. Med. Chem. Lett. 2008, 18 (22), 5926-5931. (68) Graven, A.; St. Hilaire, P. M.; Sanderson, S. J.; Mottram, J. C..; Coombs, G. H.; Meldal, M. J. Comb. Chem. 2001, 3 (5), 441-452. (69) Frankowski, K. J.; Ghosh, P.; Setola, V.; Tran, T. B.; Roth, B. L.; Aube , J. ACS Med. Chem. Lett. 2010, 1 (5), 189–193. (70) Wu, P.; Petersen, M. Å.; Petersen, R.; Flagstad, T.; Guilleux, R.; Ohsten, M.; Morgentin, R.; Nielsen, T. E.; Clausen, M. H. RSC Adv. 2016, 6 (52), 46654–46657. (71) Brummond, K. M.; You, L. Tetrahedron 2005, 61 (26), 6180–6185. (72) Kormann, C.; Heinemann, F. W.; Gmeiner, P. Tetrahedron 2006, 62 (29), 6899–6908. (73) Diels, O.; Alder, K. Justus Liebigs Ann. Chem. 1931, 490, 257–266. (74) Diels, O., Alder, K. Justus Liebigs Ann. Chem. 1928, 460, 98–122; Afarinkia, K.; Vinader, V.; Nelson, T. D.; Posner, G. H. Tetrahedron 1992, 48 (42), 9111–9171. (75) Prokhorov, A. M.; Kozhevnikov, D. N. Chem. Heterocycl. Compd. 2012, 48 (8), 1153–1176. (76) Baran, P. S.; Burns, N. Z. J. Am. Chem. Soc. 2006, 128 (12), 3908–3909. (77) Zhao, P.; Beaudry, C. M. Org. Lett. 2013, 15 (2), 402–405. (78) Zhao, P.; Beaudry, C. M. Angew. Chem. Int. Ed. Engl. 2014, 53 (39), 10500–10503. (79) Zhao, P.; Beaudry, C. Synlett 2015, 26 (14), 1923–1929.

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(80) Sato, Y.; Kuramochi, K.; Suzuki, T.; Nakazaki, A.; Kobayashi, S. Tetrahedron Lett. 2011, 52 (5), 626–629. (81) Martin, S. F.; Rueger, H.; Williamson, S. a.; Grzejszczak, S. J. Am. Chem. Soc. 1987, 109 (9), 6124–6134. (82) Nelson, H. M.; Stoltz, B. M. Org. Lett. 2008, 10 (1), 25–28. (83) Nelson, H. M.; Murakami, K.; Virgil, S. C.; Stoltz, B. M. Angew. Chem. Int. Ed. Engl. 2011, 50 (16), 3688–3691. (84) Gan, P.; Smith, M. W.; Braffman, N. R.; Snyder, S. A. Angew. Chem. Int. Ed. Engl. 2016, 55 (11), 3625–3630. (85) Lundy, S. Synthetic Approaches to the Bicyclic Core of TEO3.1, Hamigerone and Embellistatin, PhD Thesis, University of Canterbury, 2007. (86) Nash, J. Application of the Diels-Alder reaction towards the total synthesis of embellistatin and natural product inspired libraries, PhD Thesis, The University of New South Wales, 2013. (87) Loupy, A.; Maurel, F.; Sabatié-Gogová, A. Tetrahedron 2004, 60 (7), 1683–1691. (88) Delaney, P. M.; Browne, D. L.; Adams, H.; Plant, A.; Harrity, J. P. A. Tetrahedron 2008, 64 (5), 866–873. (89) Kirkham, J. D.; Butlin, R. J.; Harrity, J. P. A. Angew. Chem. Int. Ed. Engl. 2012, 51 (26), 6402–6405. (90) Crépin, D. F.; Harrity, J. P. A. Org. Lett. 2013, 15 (16), 4222–4225. (91) Crépin, D. F. P.; Harrity, J. P. A.; Jiang, J.; Meijer, A. J. H. M.; Nassoy, A.-C. M. A.; Raubo, P. J. Am. Chem. Soc. 2014, 136 (24), 8642–8653. (92) Guney, T.; Lee, J. J.; Kraus, G. A. Org. Lett. 2014, 16 (4), 1124–1127. (93) Lee, J. J.; Pollock III, G. R.; Mitchell, D.; Kasuga, L.; Kraus, G. A. RSC Adv. 2014, 4 (86), 45657–45664. (94) Juranovič, A.; Kranjc, K.; Perdih, F.; Polanc, S.; Kočevar, M. Tetrahedron 2011, 67 (19), 3490–3500. (95) Zhang, Y.; Herndon, J. W. J. Org. Chem. 2002, 67 (12), 4177–4185. (96) Afarinkia, K.; Bearpark, M. J.; Ndibwami, A. J. Org. Chem. 2003, 68 (19), 7158–7166.

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John Reed - January 2017 21 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

22 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

2 A NEW STRATEGY FOR THE SYNTHESIS OF FUNCTIONALISED TETRAHYDROISOQUINOLINES

2.1 The Chemistry and Biology of Tetrahydroisoquinolines

2.1.1 A Profoundly Bioactive Scaffold 1,2,3,4-Tetrahydroisoquinoline (2.1) is a structural motif found in many bioactive small molecules (Figure 2.1). The chemical structures built around this scaffold are highly diverse and give rise to a wide range of biological activities. It is noteworthy that these compounds include secondary metabolites that have been used therapeutically for centuries in their naturally occurring form and synthetic drugs that have received clinical approval.

Figure 2.1: 1,2,3,4-tetrahydroisoquinoline and some bioactive derivatives

John Reed - January 2017 23 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

The biosynthesis of many tetrahydroisoquinoline alkaloids has been established by a number of studies, revealing that they derive from the amino acid phenylalanine (2.2, Scheme 2.1).1–3 Enzymatic dihydroxylation followed by decarboxylation gives the key biosynthetic precursor dopamine (2.3) which can then be elaborated in a number of ways. The isoquinoline structure is completed by Pictet-Spenglerases which catalyse the condensation of functionalised phenethylamines with aldehydes and subsequent cyclisation.4 Recent progress in the field of isoquinoline biosynthesis has identified some of the genes responsible for these pathways.5

Scheme 2.1: Biosynthesis of naturally occurring isoquinolines

2.1.2 Synthetic Aspects of Tetrahydroisoquinolines Investigations into the biological activity of tetrahydroisoquinolines have been complemented by a significant amount of progress on the chemistry of the system. Much of this work has focussed on the elaboration of the scaffold to generate a high degree of structural complexity.

Cross-dehydrogenative coupling (CDC) reactions have recently become popular for the formation of C(sp3)-C(sp3) bonds at centres α to amines.6–8 An important example of this is the recent synthesis of the antimalarial agent (R)-praziquantel (2.4), reported by the Todd group (Scheme 2.2).9 This improved access to (R)-praziquantel has achieved one of the World Health Organisation’s long-standing goals.10

Scheme 2.2: Cross-dehydrogenative couplings of tetrahydroisoquinolines. Reagents and conditions: (a) DDQ, CH3NO2, rt, 5 min; (b) RANEY® nickel, H2, rt, 4 h; (c) cyclohexane carbonyl chloride, DMAP, Et3N, CH2Cl2, 0 oC, 4 h; (d) CAN, MeCN/H2O, 0 oC, 5 min; (e) chloroacetyl chloride, NaOH, CH2Cl2, rt, 30 min, then TEBAC, reflux, 2 h The cross-dehydrogenative coupling method is characterised by oxidation of the amine to give an iminium cation to which a nucleophile can be introduced. In the context of tetrahydroisoquinoline

24 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines chemistry, the stability of the benzylic radical directs exclusive formation of 3,4- dihydroisoquinolinium over the 1,4-isomer. The Stephenson group has extended these principles to include the use of photoredox catalysis.11,12 The redox active photosensitizers [Ru(bpy)3]Cl2 and

[Ir(ppy)2(dtbbpy)]PF6 undergo a metal-to-ligand charge transfer when irradiated with visible light as a consequence of the relatively high energy d-orbitals of the respective metals and the low-lying π* orbitals of the pyridyl/bipyridyl ligands (Scheme 2.3). Oxidative quenching of this excited species

(for example, with diethyl bromomalonate) gives a Ru3+ species. This powerful oxidant (E1/2II/III = +1.29 V vs SCE) reacts with tertiary amines via a single electron transfer from the nitrogen’s lone pair to give a cationic amino radical and regenerate Ru2+. Hydrogens geminal to this radical amine have bond dissociation energies as little as 17 kcal mol-1 compared to >90 kcal mol-1 for the neutral amine species.13 Thus, radical abstraction of such a hydrogen readily occurs to leave an electrophilic iminium cation, as with the prior cross-dehydrogenative coupling examples.

Scheme 2.3: Redox cycle of Ru(bpy)32+ and reactivity with tetrahydroisoquinolines Nucleophiles that have been introduced in this manner include the cyanide anion, nitroalkanes, isobutene, malonates, indole, copper acetylides and 2-(trimethylsiloxy)furan (which generates a α,β-unsaturated γ-lactone).

Functionalisation of the C-H bonds at the other benzylic site have also been reported, utilizing a variety of strategies.14–16 Bruneau’s recent endeavours have resulted in a method for the functionalisation at the C(4) position under mild conditions activated by a ruthenium complex (Scheme 2.4). Compared to previous reports on the functionalisation of this site, which have required the use of a strong base and/or toxic chromium reagents, this methodology is certainly more benign and expands the range of tolerated coupling partners. The authors explore the use of heterocyclic aromatic carbaldehydes as coupling partners to furnish a methylene linkage to the tetrahydrosioquinoline. No explanation of the observed regioselectivity was offered, although

John Reed - January 2017 25 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries subsequent reports from the same research group have revealed similar reactivity profiles on piperidine-based substrates.17,18

Scheme 2.4: Ruthenium catalysed C-H activation at the 4-position of tetrahydroisoquinolines Clearly, such protocols have exploited the natural reactivity of the benzylic positions compared to the other α-amino carbon centre. This makes the report by Lahm and Opatz on the selective C(3) alkylation of tetrahydroisoquinolines all the more remarkable.19 Attaching a benzo[d]oxazole group to the secondary amine directs C-H activation with various terminal olefins under catalysis by

[Ir(cod)2]BF4 or [Ir(cod)2]BARF exclusively to the 3-position (Scheme 2.5). Previously, functionalisation at this position has only been reported in low yields under forcing conditions, and only after the 1-position had been functionalised (i.e. only 1-substituted and 1,3-disubstituted products were detected).19

Scheme 2.5: Uniquely regioselective C-H functionalisations. Reagents and conditions: (a) 2-chlorobenzoxazole, Et3N, THF, 70 oC, 2 h; (b) [Ir(cod)2]BF4 or [Ir(cod)2]BARF (7 mol %), H2CCHR, dimethoxyethane, 85 oC, 4-48 h; (c) LiAlH4, THF, reflux, 48 h The authors hypothesised that this unique regioselectivity might be a result of the tetrahydroisoquinoline’s C(8)-hydrogen providing enough steric hindrance to force the intermediate iridium-hydrido complex to the other side of the amine, although no evidence is provided for this. To date, no studies confirming or rejecting this mechanism have been published. The unexpected discovery of a benzoxazole directing group that is easily installed and removed on the secondary amine of tetrahydroisoquinolines has provided synthetic chemists with a new reactive site to explore and exploit.

An exhaustive review of methods for the functionalisation of tetrahydroisoquinolines is outside the scope of this discussion; however, these examples have been selected to illustrate the tools at the disposal of a modern synthetic chemist. From these, it is apparent that all four sp3 centres of the

26 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines tetrahydroisoquinoline scaffold can be readily functionalised. Substitution on the aryl ring, however, is less advanced. It is possible that this is a consequence of the relatively few reliable methods available for the synthesis of tetrahydroisoquinolines. The most established methods for tetrahydroisoquinoline synthesis are the Pictet-Spengler reaction and the related Bischler- Napieralski reaction (Scheme 2.6). Both reactions proceed via an iminium intermediate, which undergoes an electrophilic aromatic substitution to furnish the cyclised product. In the case of the case of the Bischler-Napieralski reaction, an alternative mechanism, involving a nitrilium ion has been proposed.20 In both cases, the rate-limiting step is the attack of the aromatic system on the iminium species. A strongly electron-donating group, positioned either ortho or para to the point of cyclisation, increases the rate of this reaction. Without such a substituent, the reaction is highly unfavourable, but can be achieved either through the use of superacids or by forming an N- acyliminium ion (a superior electrophile than the iminium ion).20,21

Scheme 2.6: Mechanisms for (a) the Pictet-Spengler reaction and (b) the Bischler-Napieralski reaction While functionalising the aromatic portion of tetrahydroisoquinolines is by no means a dead end, it is somewhat surprising that more modern, reliable methods for tetrahydroisoquinoline synthesis, capable of tolerating a wider variety of functional groups on the aryl ring, have not become commonplace. Recent creative approaches include the palladium-catalysed domino Heck-aza- Michael reaction, which simultaneously allows functionalisation at the C(1) position (Scheme 2.7).22–24 This approach cleverly introduces an electron-poor olefin ortho to an ethylene linked carbamate or sulfonamide. The carbamate/sulfonamide then reacts with the olefin as an aza- Michael nucleophile to form the C(1)-N bond of the tetrahydroisoquinoline. The obvious drawback of this approach is the necessity for an electron-deficient alkene; however, standard functional group interconversions mean that these can then be manipulated in a number of ways.

John Reed - January 2017 27 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 2.7: Synthesis of tetrahydroisoquinolines via a domino Heck-aza-Michael reaction Another interesting example was reported as part of an recent investigation into the diclofensine scaffold (2.5), an anti-depressant developed in the 1970s that never received clinical approval. Having developed an asymmetric synthesis of suitable (benzylamino)ethanols, Panda and co- workers were able to convert these to the corresponding tetrahydroisoquinolines via an intramolecular Friedel-Crafts alkylation (Scheme 2.8). Notably, this synthesis maintained the inbuilt C(3)-stereocentre, while diastereoselectively generating a new stereocentre at C(4) of the tetrahydroisoquinoline.

Scheme 2.8: Synthesis of tetrahydroisoquinolines via a Friedel-Crafts alkylation strategy The last example provides an insight as to how new methods for tetrahydroisoquinoline synthesis can be discovered. The key step in the strategy is the formation of the C(4)-C(4a) bond of the tetrahydroisoquinoline framework. This is not an obvious disconnection to make when performing a retrosynthetic analysis. Nevertheless, by pursuing this unlikely approach, the authors have been able to develop a highly useful synthesis, complementary to existing methods. The continued discovery of new methods like this one is contingent upon researchers making such novel disconnections when planning syntheses.

28 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

2.1.3 The Naphthylisoquinoline Alkaloids The naphthylisoquinoline alkaloids are a class of natural products identifiable by the presence of a functionalised naphthalene system connected to an isoquinoline moiety by a biaryl bond. Molecules of this nature have only been found in the Ancistrocladaceae and Dioncophyllaceae plant families - plants which have been used throughout the Indochina region as a remedy for malaria.25 It has been determined that much of this activity is due to the constituent naphthylisoquinoline alkaloids.

In particular, dioncopeltine A (2.6) and dioncophylline B (2.7) have displayed IC50 values of 55 nM and 173 nM respectively against certain chloroquine resistant Plasmodium falciparum cells, as well as curative properties when administered to mice infected with Plasmodium berghei parasites (Table 2.1).26,27 In a subsequent paper, Bringmann and co-workers described a Quantitative Structure-Activity Relationship (QSAR) analysis of a wide range of naphthylisoquinoline alkaloids.28 Two proprietary molecular alignment programs, FlexS and Genetic Algorithm Similarity Program (GASP), were used for this study and the results from each compared were compared against each other.29–32 To no great surprise, it was identified that the N-atom of the tetrahydroisoquinoline moiety was crucial to the antiplasmodial activity. The authors recommended that analogue development should look towards making this functional group more exposed. Furthermore, they discovered that the presence of an H-bond donor, H-bond acceptor, or excessively hydrophobic group on the C(6)-atom is detrimental to activity, leading to the supposition that a hydrogen is the most appropriate group at this position.

Table 2.1: Activity of selected naphthylisoquinoline alkaloids against certain malarial parasites

Alkaloid IC50 (μM)* Survival Rate†

Dioncopeltine A (2.6) 0.0553 100 %

Dioncophylline B (2.7) 0.173 83 %

Dioncophylline A 2.28 50 %

N-Methyldioncophylline A 14.7 Data not reported

Ancistrobrevine D 24.8 Data not reported

Ancistrocladine 62.2 Data not reported

*In vitro IC50 values against the K1 chloroquine resistant Plasmodium falciparum strain †In OF1 mice, 7 days after infection with Plasmodium berghei. Control group survival rate was 33 % While these results are rudimentary, they serve to highlight that through the synthesis of unnatural analogues, a greater understanding of the biological target can be achieved, more detailed structure-activity relationships can be identified, and an improved therapeutic agent can be

John Reed - January 2017 29 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries designed. The lack of further literature on this matter suggests that there is a bottleneck preventing further development. Indeed, in their QSAR analysis, Bringmann and co-workers admit that application of their model is limited to a very a specific range of naphthylisoquinoline alkaloids. Other compounds, such as simplified analogues or synthetic precursors whose biological activities have also been tested, do not fit the model. Better predictive models require access to more diverse representative compounds, in turn this will allow the design of structurally simplified analogues with comparable activity. Consequently, new methods to synthesise generalised tetrahydroisoquinolines, that can incorporate some of the functionality displayed by the naphthylisoquinoline alkaloids (eg. a hindered biaryl bond), have the potential to unlock new knowledge on the mode of action of these compounds and ultimately be used in the synthesis of an improved therapeutic agent.

2.2 Target Compounds and Retrosynthetic Analysis In general terms, the aim of this chapter was to develop a broadly applicable method to synthesise functionalised tetrahydroisoquinolines. Given the Morris research group’s ongoing interest in the synthesis of naphthylisoquinoline alkaloids, we decided that this research should focus on the synthesis of compounds with biaryl linkages, akin to those found in the natural products (see Table 2.1 for sample structures).33–36 Thus, our target compounds were defined by three points of variation (Figure 2.2). Firstly, the procedure had to be tolerant to a number of electronically dissimilar aryl groups. Secondly, we wanted to be able to incorporate different amines on the tetrahydroisoquinoline scaffold, and finally we wanted to be able to vary the substituents on the aromatic ring of the tetrahydroisoquinoline.

Figure 2.2: Representative target compounds Given the prevalence of biaryl linkages at the C(5)-position in the naphthylisoquinoline alkaloids, our first efforts were to look at similarly substituted tetrahydroisoquinolines. While substitution at other positions is not uncommon, this seemed the most natural starting point when our goal of

30 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines exploring the tolerance of other groups at the remaining positions on the aromatic ring was taken into account.

Utilising different amines in the protocol was also an important requirement. Aubé and co-workers have shown that amines contained within polycyclic scaffolds provide useful synthetic handles to build diverse libraries in the context of drug discovery.37,38 This strategy proved successful in quickly generating large numbers of compounds with drug-like character as opposed to combinatorial libraries, which all too often fail to display the pharmacokinetic parameters found in many drugs.39– 41 While this project is not geared as a medicinal chemistry undertaking, we thought that if we could successfully incorporate different amines, it would open up the possibility of this strategy being used to generate medicinal chemistry libraries in the future.

Our retrosynthetic analysis aimed to satisfy these constraints, while still being able to access the tetrahydroisoquinoline scaffold rapidly (Scheme 2.9). Accordingly, we made the novel double- disconnection through the C(5)-C(6) and C(4a)-C(8a) bonds. It was our view that these bonds could be simultaneously constructed using the diene-regenerative Diels-Alder reaction. Further disconnections can be made on the requisite Diels-Alder substrates whereby a Sonogashira coupling reaction is used to install the aryl groups and form what will eventually be the biaryl bond. Alkylation of commercially available amines with mesylated 3-butyn-1-ol provides the amine tether that can be attached to the pyrone either with another N-alkylation on a pseudobenzylic halide or a reductive amination on the corresponding aldehyde.

Scheme 2.9: Retrosynthetic analysis of target tetrahydroisoquinolines This retrosynthesis provides a highly modular way to access the tetrahydroisoquinoline scaffold, rapidly breaking down the complexity of the system. Indeed, once the appropriate pyrone bearing either an aldehyde or a halogen has been synthesised, the final tetrahydroisoquinolines can be accessed in as little as two steps (if the entire amine tether with an aryl group is pre-fabricated). It was envisaged, however, that it would be more efficient to install the amine tether on the pyrone sans the aryl group (Scheme 2.10). This would provide a common precursor that can be used to access multiple target compounds in a divergent manner, thereby reducing the total number of reactions needed to generate the library.

John Reed - January 2017 31 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 2.10: Planned synthetic strategy for accessing 5-aryl-1,2,3,4-tetrahydroisoquinolines To this end, the first step towards these target molecules would be to synthesise an appropriate pyrone with either a halomethyl group or a carbaldehyde at the C(6)-position. In terms of looking at substituents on the pyrone, a C(4)-methyl group was arbitrarily chosen. It was hoped that the 1H NMR shift of this methyl group would provide a distinctive peak with which to monitor reactions and simplify characterisation at the early stages of the project.

2.3 Synthesis of a Common Pyrone Precursor

2.3.1 Methods to Access Pyrones There are numerous methods to synthesise 2-pyrones, many of which are detailed in a recent review on the matter.42 A number of strategies have looked to synthesise an acyclic carboxylic acid or ester that can be cyclised to generate the O-C(6) bond, completing the pyrone scaffold. Larock and co-workers developed a two-step procedure involving a Sonogashira reaction between various terminal alkynes and either (Z)-3-iodoacrylates or o-iodobenzoates to give (Z)-2-en-4-ynoates or o-alkynyl-benzoates respectively (Scheme 2.11).43 Treatment of these with electrophiles such as iodine monochloride yield the corresponding 2-pyrones or isocoumarins.

Scheme 2.11: Electrophilic cyclisation of (Z)-2-en-4-ynoates gives 2-pyrones A similar coupling-lactonisation procedure was established by Abarbi and co-workers who detailed the synthesis of pyrones and isocoumarins via the treatment of stannylated allenes with (Z)-3-

32 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines iodovinylic acids or 2-iodobenzoic acids in the presence of a palladium catalyst and tetrabutylammonium bromide (Scheme 2.12).44

Scheme 2.12: Addition of iodovinylic acids to allenyl-stannanes gives 2-pyrones Jiang and co-workers described an attractive palladium-catalysed oxidative annulation between acrylic acid and a range of internal alkynes (Scheme 2.13).45 This synthesis is notable not just because of its simplicity, but also because of its mild conditions and the fact that it uses O2 as a stoichiometric oxidant, and water is the only byproduct. Naturally, regioselectivity issues arise when unsymmetrical alkynes are used. The authors show, however, that the more electron-rich substituent on the alkyne is more likely to end up as the C(6)-substituent (i.e. the R group).

Scheme 2.13: Oxidative annulation of acrylic acid and terminal alkynes gives a range of 2-pyrones

A different approach was taken by Liebeskind and Wang.46 They were able to prepare a range of 3,4,6-trisubstituted pyrones via the palladium-catalysed carbonylation of substituted 4-chloro-2- cyclobutenones (Scheme 2.14). The pyrone is formed after the addition of a tributyltin reagent and subsequent rearrangement.

Scheme 2.14: Synthesis of 2-pyrones from 4-chloro-2-butenones The Schreiber lab reported the intramolecular synthesis of 2-pyrones from propargyl propiolates using sequential alkyne activation with a gold catalyst (Scheme 2.15).47 The reaction proceeds via oxocarbenium intermediate 2.8. Deprotonation of this intermediate gives 5-vinyl-2-pyrones. Alternatively, the intermediate can be trapped with a nucleophile to give a diverse range of 5- substituted pyrones. Further developments of this strategy resulted in an intermolecular variant where propiolic acids are coupled with terminal alkynes to give 2-pyrones in one step.48

John Reed - January 2017 33 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 2.15: Sequential alkyne activation of propargyl propiolates yields 2-pyrones The conversion of diynes to pyrones has also been reported by Louie and co-workers, although their approach exploits a [2+2+2] cycloaddition with CO2, catalysed by nickel(0) with an N- heterocyclic carbene ligand (Scheme 2.16).49

Scheme 2.16: Nickel(0) catalysed [2+2+2] cycloaddition of CO2 and diynes

2.3.2 Attempts to Synthesise a Useful Pyrone Previous work on another project within the Morris research group had used a method developed by Negishi and co-workers involving a zinc-catalysed cyclisation of (Z)-2-en-4-ynoic acids (Scheme 2.17).50 The reaction displays selectivity for the 6-endo-dig closure over the 5-exo-dig alternative. Given the experience in the group, it was felt that this strategy could be used to access the pyrones we required for our synthetic sequence. We thus felt that acid 2.10 could be converted to 6- hydroxymethyl-pyrone 2.11. Subsequent oxidation of the allylic alcohol using manganese(II) dioxide would provide the desired carbaldehyde 2.12.

Scheme 2.17: Zn(II) catalysed synthesis of 2-pyrones Unfortunately, synthesis of pyrone 2.11 using this methodology proved to be troublesome (Scheme 2.18). Conjugate addition of the iodide anion to ethyl but-2-ynoate (2.13) gave vinyl iodide 2.14 exclusively as the desired (Z)-isomer in quantitative yield. This could be coupled with propargyl alcohol under Sonogashira conditions to give the conjugated ester 2.15. However, attempts to hydrolyse this ester only led to complete decomposition of the material.

34 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

Scheme 2.18: Initial attempts to access a suitably functionalised 2-pyrone. Reagents and conditions: (a) NaI, AcOH, reflux, 2 h; (b) propargyl alcohol, Pd(PPh3)Cl2, CuI, Et3N, 50 oC, 16 h; (c) LiOH, H2O/MeOH/THF (1:1:1), rt, 10 h; (d) ZnBr2, THF, rt, 48 h This problem could be circumvented by switching the order of the Sonogashira/ester hydrolysis sequence. Accordingly, hydrolysis of the ester 2.14 followed by Sonogashira coupling with propargyl alcohol provided acid 2.10, albeit in 23 % yield over the two steps. Upon treatment with zinc(II) bromide in THF, only the 5-exo-dig cyclisation product, furanone 2.16, was formed in 68 % yield. This result can perhaps be rationalised by the hydroxyl group holding the zinc(II) cation in closer proximity to C(5), thereby directing attack of the carboxylic acid to the C(4) end of the alkyne. Alternatively, the alcohol might cause the alkyne to be polarised in a contrasting manner to substrates without such a group.

While it may have been possible to devise a set of conditions that would favour formation of the pyrone over the furanone, it was felt that this approach was, on the whole, unsatisfactory due to the number of steps and the low yield of the hydrolysis and Sonogashira coupling sequence. As such, a new strategy was required. While some of the chemistry described in section 2.3.1 may allow access to the desired molecules, the report describing the synthesis of 6-(chloromethyl)-4- methyl-2H-pyran-2-one (2.17) in two steps from readily available chloroacetyl chloride and ethyl 3,3-dimethylacrylate caught our attention.51 The reaction proceeds via a Friedel-Crafts type acylation of the dimethyl acrylate, followed by acid-catalysed lactonisation to give the pyrone (Scheme 2.19). This pyrone was functionally equivalent to the carbaldehyde 2.12 in that we anticipated it could alkylate amines, as opposed to undergoing a reductive amination. Pleasingly, the conditions reported by Song and co-workers were reproducible, giving access to the desired pyrone in 62 % yield across two steps. Furthermore, these conditions proved to be very scalable, allowing 2.17 to be synthesised on a multi-gram scale in a very short time.

John Reed - January 2017 35 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

HO Me O O CO Et d,e Me 2 a,b O c O N O Me Me HN Cl Cl Cl 2.17 2.19

2.18a

Scheme 2.19: Synthesis of a key intermediate. Reagents and conditions: (a) AlCl3, CH2Cl2, reflux, 3 h; (b) H2SO4, AcOH, 80 oC, 1 h; (c) 2.18a, NaI, K2CO3, EtOH, rt, 16 h; (d) CH3SO2Cl, Et3N, CH2Cl2, 0 oC, 2 h; (e) benzylamine (5 eq), rt, 4 h At the same time, a representative amine tether 2.18a was synthesised from the commercially available 3-butyn-1-ol. Activation of the alcohol with methanesulfonyl chloride permitted a substitution reaction with benzylamine to occur, to give the desired secondary amine in 87 % yield across two steps, following chromatographic purification. To test the suitability of pyrone 2.17 as a stepping stone to our target tetrahydroisoquinolines, it was reacted with secondary amine 2.18a in the presence of potassium carbonate and a catalytic amount of sodium iodide in ethanol. These conditions furnished the desired product (2.19) in 93 % yield. Pleasingly, at no point was over- alkylation of the amine observed, allowing for easy separation of the product from the reaction mixture. Clearly, pyrone 2.17 was a highly useful intermediate due to its ease of synthesis and the simplicity with which it could be further functionalised.

2.4 Elaboration of the Scaffold to the Pyrone Diels-Alder Precursor In line with the aim of incorporating different amines into the scaffold, pyrone 2.17 was used to alkylate three other secondary amines (2.18c-d) that had been prepared from a commercially available primary amine and 3-but-yn-1-yl methanesulfonate. The primary amines were chosen on the basis that they displayed varying steric and electronic characteristics. The results of these experiments are summarised in Table 2.2. Pleasingly, all amines gave good to excellent yields throughout the sequence.

36 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

Table 2.2: Sequential alkylation of primary amines with but-3-yn-1-yl methanesulfonate and 2.17. Reagents and conditions: (a) 3-butyn-1-yl methanesulfonate, neat, rt, 4 h; (b) 2.17, NaI, K2CO3, EtOH, rt, 16 h

Amine Step (a) Yield (%) Step (b) Yield (%) Product

91 93

76 87

89 81

82 91

Having successfully attached a number of different amine tethers onto the pyrone system, attention then turned to the coupling of these alkynes with various aryl iodides under Sonogashira conditions. Our first foray into this chemistry involved the use of 4-iodopyridine as the coupling partner. It was anticipated that the electron deficient aryl iodide would couple more readily with the terminal alkyne than other, more electron-rich, substrates might.

The initial catalytic system was set up as follows:52

 5 mol % Pd(PPh3)2Cl2, 5 mol % CuI  Triethylamine as both base and solvent (concentration of the alkyne substrate = 0.2 M)

John Reed - January 2017 37 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

After stirring at room temperature overnight, the previously yellow suspension had turned into a viscous, black mixture. A small aliquot of the reaction mixture was removed with a syringe and partitioned between ethyl acetate and saturated aqueous ammonium chloride solution in a 4 mL vial. TLC analysis of the organic extract indicated the presence of a small amount of a new, more polar product, in addition to a substantial amount of both starting materials. The addition of another

5 mol % of both Pd(PPh3)2Cl2 and CuI followed by degassing by standard freeze-pump-thaw technique failed to induce much more conversion (by TLC analysis) after 6 hours. Rather than trying to drive this reaction to completion, it was thought that a greater insight would be gained by starting the reaction anew, with more forcing conditions.*

To this effect, the reaction was repeated with double the amount of Pd(PPh3)2Cl2 (10 mol %). Furthermore, the reaction was heated to 50 oC and stirred overnight. As before, a small aliquot was subjected to a mini-work up and analysed by thin layer chromatography. While there still appeared to be some of the pyrone starting material present in the reaction mixture, it appeared that all the 4-iodopyridine had been consumed. Consequently, the reaction was quenched with saturated ammonium chloride solution; a thick, brown oil was obtained after work up. Analysis of the crude product by 1H NMR spectroscopy revealed that the proton on the terminal alkyne had disappeared (Figure 2.3). Furthermore, in the 1H NMR spectrum of the starting material 2.19, coupling between the terminal proton and the nearest methylene group (J = 2.7 Hz) can be clearly seen. This coupling was not present in the 1H NMR spectrum of the product.

Figure 2.3: 1H NMR analysis of the coupling of 2.19 (blue) with 4-iodopyridine.

______

*It should be noted that this reaction was worked up with ethyl acetate and saturated aqueous ammonium chloride solution. 1H NMR spectroscopic analysis of the crude organic extract showed no indication of any homo-coupled alkyne substrate, as could happen if the copper(I) catalyst oxidises to copper(II) and promotes Glaser coupling.52

38 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

The main conclusion drawn from this analysis was that the alkyne had now, most likely, been functionalised and was no longer terminal. The major product was purified by chromatographic separation and full spectroscopic analysis (1H NMR, 13C NMR, HRMS, FTIR) revealed it to indeed be the functionalised alkyne 2.23 (Scheme 2.20). Unfortunately, the product had only been isolated in 23 % yield, indicating that further refinement of the synthetic procedure was required.

Two ideas were offered to explain the poor performance of these Sonogashira reactions. Firstly, the substrates were not overly soluble in triethylamine. The reactions therefore required relatively dilute conditions, thus slowing the reaction. Secondly, due to the high amount of triethylamine present, there was a greater scope for oxidation to the corresponding imine and the associated cross-reactivity to occur. In principle, both of these issues could be corrected by using a better solvent and subsequently reducing the number of equivalents of triethylamine. Pleasingly, conducting the reaction in DMF with 10 equivalents of triethylamine proved to be very effective and even allowed the catalyst loading to be reduced to 1 mol % Pd(PPh3)2Cl2 and 0.5 mol % CuI. Analysis by TLC indicated complete consumption of both starting materials after stirring at room temperature overnight.

O O

O a O N N Me Me 2.19 2.23 Scheme 2.20: Optimised conditions for the Sonogashira coupling of 2.12 with 4-iodopyridine. Reagents and conditions: (a) 4-iodopyridine, Et3N, Pd(PPh3)2Cl2, CuI, DMF, rt, 16 h Under these improved conditions, the desired product was isolated in 74 % yield. Another benefit to using DMF as the solvent was the ease with which the work-up and purification could be carried out. There was no formation of insoluble by-products; instead, if enough care was taken to remove all the DMF in the work-up, the crude product was obtained in upwards of 90 % purity. Having established a reliable method for the functionalisation of the terminal alkyne, it was decided that the diene-regenerative Diels-Alder reaction would be investigated and optimised on substrate 2.23 before any more analogues were generated.

2.5 Investigations into the Pyrone Diels-Alder Reaction Before attempting the diene-regenerative Diels-Alder reaction, a survey of the relevant literature was conducted to ascertain the most commonly effective conditions when it comes performing the

John Reed - January 2017 39 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries reaction. Specifically, examples of cycloadditions using an alkynyl dienophile would be considered relevant to this work.

Investigations conducted by Harrity and co-workers have resulted in a versatile synthetic route towards functionalised ortho-silylaryl boronates (Scheme 2.21).53,54 Oxidation of the boronic ester followed by treatment with triflic anhydride affords the corresponding ortho-silylaryl triflate: known aryne precursors.

Scheme 2.21: Functionalised benzyne precursors are accessible using the diene-regenerative Diels-Alder reaction. Reagents and conditions: (a) o-DCB, 180 oC, 18 h; (b) H2O2 (30 % w/v in H2O), Na2CO3, EtOH, rt; (c) Tf2O, i-Pr2NEt, CH2Cl2, rt, 16 h; (d) dienophile, CsF, CH3CN, rt, 18 h The initial reports on the matter detailed the use of ortho-dichlorobenzene as a high boiling solvent to enable the cycloaddition. Subsequent work from the same research group used mesitylene instead, as they strove to understand the regiochemical outcomes.55

Khatri and Samant reported the reaction between diethyl acetylenedicarboxylate and various 6- amino-2-pyrones (Scheme 2.22).56 Interestingly, their reported conditions are somewhat milder than many other similar transformations: the reaction time is considerably shorter than most and proceeds in refluxing toluene as opposed to xylenes or ortho-dichlorobenzene.

Scheme 2.22: 6-amino-2-pyrones undergo Diels-Alder cycloadditions surprisingly quickly. Reagents and conditions: (a) PhMe, reflux, 2 h This willingness to undergo the diene-regenerative Diels-Alder reaction can be attributed to both the highly electron-deficient alkyne as well as the electron-rich pyrone, courtesy of the 6-amino group. As such, it would be expected that other substrates would require more forcing conditions. This is exemplified by the reaction between dimethyl acetylenedicarboxylate and a variety of aryl- substituted pyrones as reported by Kim and co-workers (Scheme 2.23).57 The dienophile is just as activated towards a normal electron demand Diels-Alder reaction as in the previous example, however, the pyrones used bear no extra electron-donating functionality. Consequently, the reaction only goes to completion after 24 hours at 180 oC.

40 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

Scheme 2.23: Diene-regenerative Diels-Alder reactions between aryl-substituted pyrones and dimethyl acetylenedicarboxylate. Reagents and conditions: (a) p-xylene, 180 oC, 24 h Kitamura and co-workers opted to use a diene-regenerative Diels-Alder reaction approach to the synthesis of a hypervalent iodine-benzyne precursor 2.24 (Scheme 2.24).58 Their strategy required a reaction between bis(trimethylsilyl)acetylene and methyl 2-pyrone-5-carboxylate (2.25). Throughout their studies, they found that best results were obtained when the reaction was conducted neat in a sealed tube at 200 oC for 24 hours.

Scheme 2.24: A solvent-free intermolecular diene-regenerative Diels-Alder reaction. As discussed in the thesis introduction, work by Baran and Burns on the total synthesis of the marine natural product haouamine utilised a diene-regenerative Diels-Alder reaction to synthesise a highly strained aromatic ring (Scheme 1.6, p. 12).59 The authors opted to effect this transformation in a microwave reactor, in order to take advantage of the more efficient delivery of thermal energy.60 The product was obtained in only 21 % yield, along with 30 % recovered starting material, but this was nevertheless a step forward in terms of substrate complexity for the pyrone Diels-Alder reaction. Furthermore, the reaction displayed a remarkable preference (10:1) for the desired atropisomer. This methodology has since been used by Zhao and Beaudry for the synthesis of cavicularin (Scheme 1.7, p. 12).61–63 In this case, microwave irradiation gave the desired product in a much improved yield of 83 %. Haufe and co-workers have further explored the use of a microwave reactor to effect these cycloadditions, in the context of synthesising functionalised trifluoromethylbenzenes (Scheme 2.25).64 Pyrone 2.26 could be reacted with a variety of terminal alkynes under microwave irradiation to generate the corresponding aromatic compounds.

Scheme 2.25: Synthesis of functionalised trifluoromethylbenzenes under microwave irradiation. Reagents and conditions: (a) 150 W, 150 oC, 30 min – 3 h

John Reed - January 2017 41 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

While this method tolerated a wide variety of terminal alkynes, the authors were unable to obtain any desired product when the internal alkyne pent-3-yn-1-ol was used as the dienophile. Instead, only a complex mixture was produced.

From this review of the literature, two distinct approaches emerged with regards to effecting the transformation: a number of methods opted to reflux the substrate(s) in a high boiling solvent (b.p. ≥110 oC), or alternatively, the use of a microwave reactor has met with considerable success in reducing the required reaction times when compared with traditional thermal conditions. Incidentally, Loupy and co-workers undertook a comparative study on the efficiency of conventional heating and microwave irradiation in effecting the diene-regenerative Diels-Alder reaction with acetylenylic dienophiles.65 Through both experimentation and theoretical calculations, the authors reached an important conclusion regarding effects of microwave irradiation on the pyrone Diels- Alder reaction: while a more traditional diene reacts via a concerted mechanism, the more polarised pyrone substrates proceed through a non-symmetrical mechanism whereby opposite charges are observed on each end of the diene. The greater dipole moment of the pyrone is more susceptible to microwave irradiation, and therefore conducting the pyrone Diels-Alder reaction in a microwave reactor leads to a greater rate enhancement over conventional heating than for a less polarised diene.

Given these precedents, we envisaged that the diene-regenerative Diels-Alder reaction could be effected using a microwave reactor. To ensure a robust study, we decided to also explore a conventional heating method for the transformation. Thus, heating the pyrone 2.23 in xylenes at reflux led to the formation of a single new product according to TLC analysis (Scheme 2.26). After 48 hours, complete consumption of the starting material was observed. Isolation of the product, in 73 % yield, via chromatographic separation enabled characterisation using a suite of spectroscopic tools.

Scheme 2.26: Conventional heating effects a diene-regenerative Diels-Alder reaction. Reagents and conditions: (a) xylenes, reflux, 48 h

Several diagnostic resonances in the 1H NMR spectrum could be used to determine the products structure (Figure 2.4). The two olefinic protons of the pyrone had disappeared from the product’s

42 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines spectrum and, in their stead, two nearly overlapping singlets appeared downfield at 6.876 and 6.883 ppm, indicating the formation of an aromatic ring.

Figure 2.4: Comparison of the 1H NMR spectra of 2.23 (blue) and the reaction product after refluxing in xylenes for 48 hours (red)

The 1H NMR spectrum of 2.23 shows two triplets and two singlets between 2.61 and 3.79 ppm, each integrating to two protons, corresponding to the four methylene groups. In the spectrum of the product, these peaks disappear and are replaced by two peculiarly shaped multiplets at 2.64- 2.72 and 3.64-3.71 ppm, both of which integrate to four protons. The lineshape of these peaks is consistent with a more strained system such as a cyclic compound, as would be found in the desired product. A comparison of the IR spectra of the two compounds provides further support: a distinct carbonyl stretching peak is visible at 1719 cm-1 in the starting material’s spectrum, but was no longer present in the spectrum of the product. Finally, mass spectrometry returned a major m/z value of 315.1846, corresponding to the protonated product, giving us high confidence that our desired product had been formed.

Optimisation of the conversion of pyrone 2.23 to the tetrahydroisoquinoline 2.27 was carried out by screening a range of different conditions in a microwave reactor (Table 2.3). The first parameters to be modified were the reaction temperature and duration. Since our conventional heating reaction had proceeded at ca. 138 oC, and reports of microwave-assisted pyrone Diels-Alder reactions had temperatures of up to 250 oC, we surmised that the ideal reaction temperature would lie within these boundaries.

John Reed - January 2017 43 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Table 2.3: Attempts at optimising the reaction temperature and duration for the conversion of 2.23 to 2.27

Entry Solvent Temp. (oC) Time (h) Yield (%) Byproduct Yield (%)

1 o-DCB 150 4 0a -

2 o-DCB 150 10 0a -

3 o-DCB 150 16 0a -

4 o-DCB 200 4 0a -

5 o-DCB 250 4 0b -

6 DMF 150 4 0a -

7 EtOH 150 4 0a -

8 MeCN 150 4 0a -

o-DCB 9 (with 7 eq. BHT) 250 4 0 57 %

aOnly starting material recovered in near quantitative yield Unfortunately, the formation of the product was not observed under any of these conditions. Furthermore, while the reports from Baran and Beaudry had detailed the use of ortho- dichlorobenzene, we thought that a more polar solvent might be more appropriate since microwaves transmit heat onto dipoles more effectively. This might enable the reaction to be run at a lower set temperature level in the microwave reactor. As such, we elected to screen the reaction in three polar solvents (N,N-dimethylformamide, ethanol and acetonitrile). The safe-working limits of these solvents in the microwave reactor meant that these experiments could be run at temperatures no higher than 150 oC. No product was ever observed in these experiments. Clearly, any solvent effects were not enough to compensate for the drop in temperature. Puzzlingly, when the temperature was increased to 250 oC, a new product formed, which was clearly not the desired tetrahydroisoquinoline. Addition of the radical inhibitor butylated hydroxytoluene failed to curb the formation of this product. To understand what was happening in the reaction, determination of the structure of the byproduct was necessary.

The 1H NMR spectrum was very surprising as there was only one peak in the alkyl region: a singlet at 2.61 integrating to three protons (Figure 2.5). Every other peak in the spectrum was in the

44 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines aromatic region between 7.4 and 9.3 ppm. What was also noteworthy was the clean splitting between the aromatic peaks. This lead us to believe that the benzyl group, usually characterised by a broad, messy multiplet around 7.30-7.40 ppm, was no longer present.

Figure 2.5: 1H NMR of the unknown diene-regenerative Diels-Alder reaction byproduct

The 13C NMR spectrum (see Appendix) showed resonances for one carbon in the alkyl region and twelve different aromatic environments. The infrared spectrum did not display any strong, noteworthy peaks, however, the absence of a carbonyl stretching resonance was important. We concluded that the byproduct was most likely the fully aromatic, debenzylated isoquinoline 2.28 (Scheme 2.27). This was further supported by the mass spectrum of the compound, which showed major m/z ratios of 221.1079 ([M+H]+) and 243.0895 ([M+Na]+). We thought it was most likely that the desired tetrahydroisoquinoline 2.27 was formed first in the reaction and that the benzyl group had then been cleaved, potentially catalysed by trace amounts of acid, and the resulting tetrahydroisoquinoline subsequently oxidised by atmospheric oxygen to give isoquinoline 2.28. It was thought that the trace acid could be carbonic acid produced by the extruded carbon dioxide.

N N

O N N N Me Me Me 2.23 2.27 2.28 Scheme 2.27: Formation of the unwanted isoquinoline 2.28 occurs at elevated temperatures

John Reed - January 2017 45 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

If our hypothesis was true, then theoretically, we could slow down the undesired reaction pathway in one of two ways: by adding a scavenger to the reaction mixture to mop up any trace acid, thereby preventing ionic debenzylation, or by conducting the reaction in strict anaerobic conditions, potentially allowing the isolation of the debenzylated tetrahydroisoquinoline. The practical limitations of maintaining strict anaerobic conditions (the effectiveness of the seal on the microwave reactor vials is questionable, and the o-DCB is removed by blowing a stream of nitrogen gas over it) meant that the former was our most preferred option. To this effect, the experiment was repeated as before with one equivalent of Proton Sponge® added to the reaction mixture. Pleasingly, the desired tetrahydroisoquinoline 2.27 was isolated in 83 % yield, with identical spectroscopic data to the product from the conventional heating reaction.

2.6 Expanding the Scope of the Strategy Having developed efficient protocols for each step of the synthesis, attention then turned to applying the strategy to build a small library of aryl tetrahydroisoquinolines. Alkyne 2.19 was reacted with methyl 4-iodobenzoate under the Sonogashira coupling conditions developed previously (Scheme 2.28). After chromatographic purification, the desired internal alkyne 2.29 was obtained in 76 % yield.

Scheme 2.28: Synthesis of aryl tetrahydroisoquinoline 2.33. Reagents and conditions: (a) methyl 4-iodobenzoate, Pd(PPh3)2Cl2, CuI, Et3N, DMF, rt, 16 h; (b) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h Application of the microwave assisted Diels-Alder reaction conditions afforded the desired tetrahydroisoquinoline 2.30 in 71 % yield. In order to probe to effects of electronically different substituents on the dienophile, we decided to couple 2.19 with 4-iodoanisole. Under the conditions used to synthesise pyrones 2.23 and 2.29, 4-iodoanisole refused to participate. After stirring at room temperature for 16 h, no new spots were observable by TLC. In order to force the reaction, another portion of each catalyst was added, the solution was degassed, and the reaction mixture was heated to 80 oC for another 8 hours. Following this time, nearly all the alkyne starting material had been consumed by TLC analysis, and a new spot had formed slightly below where the starting material ran. Following work up and isolation by flash chromatography, this new product was identified to be the product of Glaser alkyne homo-coupling 2.31 (Table 2.4). The reaction was

46 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines repeated twice over, this time heating to 80 oC from the start, and also to 150 oC in order to try and force the reaction. In neither case did any reaction occur after 16 hours. When both reactions were left for longer, the homo-coupling product was observed.

A quick literature search revealed a report by Buchwald, Fu and co-workers describing the Suzuki and Sonogashira couplings of electron-rich aryl bromides.66 The authors employed a tri-tert- butylphosphine ligand for the palladium catalyst, which allowed even the highly electron-rich 4- bromo-N,N-dimethylaniline to be coupled with phenylacetylene. Subsequent work by Fu has shown that the bench stable salt tri-tert-butylphosphonium tetrafluoroborate is an interchangeable alternative to the less stable tri-tert-butylphosphine. As such we set up an experiment employing bis(benzonitrile)palladium(II) dichloride with tri-tert-butylphosphonium tetrafluoroborate and copper iodide in 1,4-dioxane. After 1 hour at room temperature, starting material was still observable by TLC and no new spots had formed, however, after another further 7 hours, all the starting material had been consumed but a complex mixture had formed, indicating that this catalytic setup was incompatible with our system.

Table 2.4: Attempts to induce the coupling of 2.19 with 4-iodoanisole to give 2.32

Entry Catalyst Base Solvent Temp. (oC) Duration (h) Yield (%)

Pd(PPh3)Cl2 1 Et3N DMF 80 oC 24 h 2.31 CuI Pd(PPh3)Cl2 2 Et3N DMF 150 oC 16 h N.R. CuI Pd(PhCN)2Cl2 1,4- 3 [(t-Bu)3PH]BF4 HN(i-Pr)2 RT 1 h N.R Dioxane CuI Pd(PhCN)2Cl2 1,4- 4 [(t-Bu)3PH]BF4 HN(i-Pr)2 RT 8 h Decomp. Dioxane CuI Pd(PPh3)Cl2 5 Et3N (20 eq.) - 80 oC 16 h Decomp. CuI Pd(PPh3)Cl2 6 Pyrrolidine DMF RT 16 h 59 % CuI

John Reed - January 2017 47 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Pleasingly, upon switching the base from triethylamine to pyrrolidine, clean conversion to a new product was observed under relatively mild conditions. Following workup and chromatographic purification, the new product was characterised and identified to be the desired functionalised alkyne 2.32. It is unclear why the reaction works in pyrrolidine, but not in triethylamine, however, the superiority of pyrrolidine as a base in Sonogashira couplings has been previously noted by other researchers.33,67–69 The Sonogashira coupling between 2.19 and 4-iodopyridine (see Scheme 2.28) was repeated using pyrrolidine as the base. Pleasingly, product 2.23 was formed in 81 % yield. Pyrone 2.32 was then converted to the corresponding tetrahydroisoquinoline 2.33 using the microwave-assissted conditions for the Diels-Alder reaction (Scheme 2.29), with the product being obtained in 74 % yield.

Scheme 2.29: Microwave-assisted synthesis of 2.33. Reagents and conditions: (a) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h Attention then turned structural diversity at the amine tether of pyrones 2.20-2.22, in order to expand the library of tetrahydroisoquinolines. Both 2.20 and 2.21 were coupled with 4-iodoanisole to give pyrones 2.34 and 2.35 respectively (Scheme 2.30). Using the microwave-assisted conditions, the diene-regenerative Diels-Alder reaction was successfully effected on these two substrates to give tetrahydroisoquinolines 2.36 and 2.37 in respective yields of 97 % and 44 %. The low yield of 2.37 could be attributed to the high reactivity of the furan moiety. No other product was isolated from the reaction mixture, however, it is possible that polymerised material could have been obtained.

Scheme 2.30:Synthesis of tetrahydroisoquinolines 2.37 and 2.38. Reagents and conditions: (a) 4-iodoanisole, Pd(PPh3)2Cl2, CuI, pyrrolidine, DMF, rt, 16 h; (b) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h

48 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

Similarly, 2.22 underwent a Sonogashira coupling with 4-iodopyridine to give pyrone 2.38 in 85 % yield (Scheme 2.31). This was then cyclised under microwave irradiation to give tetrahydroisoquinoline 2.39. When the Diels-Alder reaction was attempted on 2.38 without Proton Sponge® in the reaction mixture, exactly the same decomposition product (2.28) was obtained as when pyrone 2.23 was used, while pyrone 2.35 decomposed to the 4-anisole analogue. Interestingly, pyrone 2.34 never underwent this decomposition pathway, suggesting that the carbon adjacent to the nitrogen must be sp3 hybridised for the C-N bond cleavage to occur.

Scheme 2.30: Synthesis of tetrahydroisoquinoline 2.39. Reagents and conditions: (a) 4-iodopyridine, Pd(PPh3)2Cl2, CuI, pyrrolidine, DMF, rt, 16 h; (b) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h Having shown that this strategy was indeed tolerant of different amines, we then looked at using different pyrones in the sequence so that different substituents could be affixed to the aromatic ring of the tetrahydroisoquinoline. To this end, we would need to synthesise 6-halomethyl pyrones with differing substituents at other positions around the pyrone. While the Friedel-Crafts/lactonisation sequence had been successful in synthesising a 4-methyl pyrone, we were concerned that this chemistry might not transpose well with other groups. After reviewing the literature on pyrone synthesis, it became obvious that the simplest method to synthesise 6-halomethyl pyrones was using Schreiber’s gold catalysed procedure.48 Dickschat and co-workers had used this method to synthesise the simple 6-(bromomethyl)-pyran-2-one (2.40) from propiolic acid and propargyl bromide (Scheme 2.32).70 Pleasingly, this reaction was reproducible, and 2.40 underwent the SN2 reaction with secondary amine 2.18a to give pyrone 2.41 in 77 % yield.

Scheme 2.31: Application of Schreiber’s gold catalysed pyrone synthesis to access 2.41. Reagents and conditions: (a) (PPh3)AuCl, AgOTf, CH2Cl2, rt, 16 h; (b) 2.18a, NaI, K2CO3, EtOH, rt, 16 h Alkyne 2.41 was able to be coupled with 4-iodoanisole in 55 % yield to give pyrone 2.42 (Scheme 2.33). Upon microwave irradiation for 4 hours, the corresponding tetrahydroisoquinoline 2.43 was isolated in 85 % yield following purification by flash chromatography.

John Reed - January 2017 49 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 2.32: Synthesis of tetrahydroisoquinoline 2.43. Reagents and conditions: (a) 4-iodoanisole, Pd(PPh3)2Cl2, CuI, pyrrolidine, DMF, rt, 16 h; (b) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h Similarly, taking phenylpropiolic acid and propargyl bromide and stirring them with (triphenylphosphine)gold(I) chloride and silver(I) triflate in dichloromethane provided 6- (bromomethyl)-4-phenyl-pyran-2-one (2.44) as a white solid in 81 % yield (Scheme 2.34). Again, this was used to alkylate amine 2.18a, providing alkyne 2.45 as a thick yellow oil in 96 % yield. Coupling of 2.45 with both 4-iodopyridine and 4-iodoanisole was effected using the Sonogashira conditions developed previously to provide access to the Diels-Alder precursors 2.46 and 2.47.

O O O

OH aO b O Br N

Br 2.44 2.45 c

d O N N

2.48 X=N 2.46 X=N 2.49 X= C(OMe) 2.47 X= C(OMe)

Scheme 2.33: Synthesis of tetrahydroisoquinolines 2.48 and 2.49. Reagents and conditions: (a) (PPh3)AuCl, AgOTf, CH2Cl2, rt, 16 h; (b) 2.18a, NaI, K2CO3, EtOH, rt, 16 h; (c) 4-iodoanisole, Pd(PPh3)2Cl2, CuI, pyrrolidine, DMF, rt, 16 h; (d) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h Tetrahydroisoquinolines 2.48 and 2.49 were synthesised from pyrones 2.46 and 2.47 in yields of 93 % and 68 % respectively, using the microwave-assisted conditions to effect the diene- regenerative Diels-Alder reaction.

Having synthesised are variety of functionalised tetrahydroisoquinolines, we wanted to see how the sequence would handle more challenging substrates. In particular, we recognised that the naphthylisoquinoline alkaloids displayed sterically demanding, functionalised naphthalene systems attached to the tetrahydroisoquinoline moiety. We were interested in seeing whether we could include similarly bulky aryl groups in our Sonogashira/Diels-Alder sequence.

50 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

Our first venture on this path saw the coupling of pyrone 2.19 with1-iodonaphthalene (Scheme 2.35). Under the previously optimised coupling conditions utilising pyrrolidine as a base, the two fragments could be joined in 80 % yield to give pyrone 2.50 as a yellow oil.

Scheme 2.34: Synthesising more sterically demanding aryl tetrahydroisoquinolines. Reagents and conditions: (a) 1- iodonaphthalene, Pd(PPh3)2Cl2, CuI, pyrrolidine, DMF, rt, 16 h; (b) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h The diene-regenerative Diels-Alder reaction was induced on substrate 2.50 by microwave irradiation, thus giving tetrahydroisoquinoline 2.51 in 83 % yield. This marked a key milestone for the strategy as it had now been used to synthesise a ‘naphthylisoquinoline’, albeit one stripped of a lot of functionality from the natural products.

This shortcoming was partially redressed by employing a more hindered and electron rich substrate. The naphthalene groups on the natural products are usually characterised by two alkoxy groups and a methyl group at different positions around the ring system. We therefore thought that 2-iodo-1,3-dimethoxybenzene (2.52) might be a suitable model substrate to probe the limits of this strategy on (Scheme 2.36). The commercially available dimethylresorcinol was treated with n- butyllithium in THF at -78 oC to induce ortho-directed lithiation of the aromatic ring. Upon quenching with iodine and subsequent warming, the desired aryl iodide 2.52 was formed. This was isolated as a white solid in quantitative yield, with all spectroscopic data indicating a high degree of purity, following a simple aqueous workup.

Scheme 2.35: Pushing the boundaries of the synthetic strategy: an aryl tetrahydroisoquinoline displaying high steric hindrance and unfavourable electronics. Reagents and conditions: (a) n-BuLi, THF, 0 oC, 1 h, then I2, THF, 0 oC→rt, 1 h; (b) 2.52, Pd(PPh3)2Cl2, CuI, pyrrolidine, DMF, rt, 16 h; (c) BHT, Proton Sponge®, o-DCB, μW, 4 h

John Reed - January 2017 51 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Somewhat surprisingly, coupling of 2.19 and 2.52 was achieved on the first attempt using the previously described Sonogashira conditions providing pyrone 2.53 in 92 % yield. All that remained to be seen was whether the steric crowding around the alkyne would render the dienophile unreactive. Pleasingly, this was not the case, the dienophile proved to be reactive enough to undergo the cycloaddition, thus affording tetrahydroisoquinoline 2.54 as a yellow oil in 74 % yield following chromatographic purification. Plainly, the synthetic strategy has stood up to any test it has been subjected to, highlighting the versatility and robustness of the sequence.

As a final challenge for the project, we thought that if we could incorporate appropriate functional groups, we might be able to synthesise a tetrahydroisoquinoline with a number of different synthetic handles at various places around the scaffold. Hypothetically, these could then be used to couple on other fragments to generate even more molecular complexity. Given the prevalence of methods to functionalise the non-aromatic portion of tetrahydroisoquinolines (see section 2.1.2), we wanted these synthetic handles to be on the aromatic ring. Furthermore, we decided that there should be some degree of orthogonality in the synthetic handle, to enable the selective functionalisation of one over the others. We thus aimed to synthesise tetrahydroisoquinoline 2.55 (Figure 2.6). This could also be used as a benzyne precursor if treated with a fluoride source such as TBAF.

Figure 2.6: The tetrahydroisoquinoline 2.55 bears three contiguous synthetic handles and is a benzyne precursor In this regard, the commercially available 4-hydroxy-6-methylpyran-2-one was treated with trimethyl phosphate in the presence of potassium carbonate to install a methyl protecting group on the alcohol (Scheme 2.37).71 Borrowing the chemistry devised by Moreno-Mañas and co-workers, this pyrone was irradiated with visible light in the presence of N-bromosuccinimide and the radical initiator azobisisobutyronitrile (AIBN) in benzene.72 These conditions promoted radical allylic bromination on the C(6)-methyl group, while also allowing an electrophilic aromatic substitution reaction to occur. In this way, two bromine atoms were installed onto the scaffold to give 2.57 in 70 % yield.

52 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

O O O Br Br O a,b O c O Br N HO Me MeO MeO 2.56 2.57 2.58 d

Si(CH3)3

Si(CH3)3 O Br Br e O N N MeO MeO 2.55 2.59

Scheme 2.36: Synthesis of the functionalised tetrahydroisoquinoline 2.55. Reagents and conditions: (a) PO(OCH3)3, K2CO3, 150 oC, 1 h; (b) NBS, AIBN, C6H6, hv, rt, 16 h; (c) 2.18a, NaI, K2CO3, EtOH, rt, 16 h; (d) n-BuLi, HN(i-Pr)2, (CH3)3SiCl, THF, -78 oC→rt, 12 h; (e) BHT, Proton Sponge®, o-DCB, 250 oC, μW, 4 h Treatment of 2.57 with amine 2.18a facilitated a substitution reaction exclusively at the alkyl bromide to give pyrone 2.58 in 74 % yield. Careful deprotonation of the terminal alkyne using lithium diisopropylamide at -78 oC, followed by the introduction of chlorotrimethylsilane resulted in the formation of silyl alkyne 2.59 in 44 % yield, where all the functional groups that we desired to be on the final tetrahydroisoquinoline were present. Conducting the reaction at higher temperatures resulted in the formation of a number of other products, presumably due to reaction at the benzylic centres. Finally, 2.55 was generated in 36 % yield by subjecting 2.59 to microwave irradiation to effect the Diels-Alder cyclisation. No other products were isolated from this reaction, making it difficult to understand why the yield was so low.

Having synthesised a small, focussed library of tetrahydroisoquinolines, a few conclusions could be drawn about the synthetic strategy:

 The diene-regenerative Diels-Alder reaction was tolerant to different electronic environments on the dienophile  The sequence was applicable to different amines, allowing different groups to be present on the nitrogen of the tetrahydroisoquinoline  Pyrones bearing C4 substituents could be used, enabling different functionality on the aromatic ring of the tetrahydroisoquinoline

From these conclusions we were able to extend the work to access more challenging compounds in terms of both sterics and electronics.

John Reed - January 2017 53 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

2.7 Summary A novel strategy for the synthesis of functionalised tetrahydroisoquinolines has been developed. The key transformation in this approach is the intramolecular diene-regenerative Diels-Alder reaction between a suitably functionalised 2-pyrone and an alkynyl dienophile. This strategy has displayed a high degree of tolerance to sterically and electronically diverse substrates, and pleasingly, provides access to a number of different compounds previously inaccessible from existing methods for tetrahydroisoquinoline synthesis.

All but one of the tetrahydroisoquinolines synthesised in this project contain a biaryl bond, a common structural feature that can often be challenging to construct. The strategy described in this chapter provides a relatively mild procedure for the synthesis of such bonds, by way of a Sonogashira coupling between an alkynyl functionalised pyrone and an aryl iodide, followed by the diene-regenerative Diels-Alder reaction.

Furthermore, it has been shown that the crucial diene-regenerative Diels-Alder reaction can be effected using conventional heating as well as microwave-assisted conditions. The latter allows access to the target compounds in a much shorter time and, in general, better yields.

It is anticipated that this approach to the synthesis of tetrahydroisoquinolines will find use in the development of medicinal chemistry libraries, providing valuable structure-activity relationships on compounds that other methods such as the Bischler-Napieralski or Pictet-Spengler reactions are unable to construct.

54 John Reed - January 2017 Chapter 2: A New Strategy for the Synthesis of Functionalised Tetrahydroisoquinolines

2.8 Chapter 2 References (1) Herbert, R. B. The Chemistry and Biology of Isoquinoline Alkaloids; Phillipson, J. D., Roberts, M. F., Zenk, M. H., Eds.; Proceedings in Life Sciences, 1985. (2) Bjorklund, J. A.; Frenzel, T.; Rueffer, M.; Kobayashi, M.; Mocek, U.; Fox, C.; Beale, J. M.; Groeger, S.; Zenk, M. H.; Floss, H. G. J. Am. Chem. Soc. 1995, 117 (5), 1533–1545. (3) Le, V. H.; Inai, M.; Williams, R. M.; Kan, T. Nat. Prod. Rep. 2015, 32 (2), 328–347. (4) Koketsu, K.; Minami, A.; Watanabe, K.; Oguri, H.; Oikawa, H. Curr. Opin. Chem. Biol. 2012, 16 (1-2), 142–149. (5) Baccile, J. A.; Spraker, J. E.; Le, H. H.; Brandenburger, E.; Gomez, C.; Bok, J. W.; Macheleidt, J.; Brakhage, A. A.; Hoffmeister, D.; Keller, N. P.; Schroeder, F. C. Nat. Chem. Biol. 2016, 12 (6), 419–424. (6) Scheuermann, C. J. Chem. Asian J. 2010, 5 (3), 436–451. (7) Yeung, C. S.; Dong, V. M. Chem. Rev. 2011, 111 (3), 1215–1292. (8) Li, C.-J. Acc. Chem. Res. 2009, 42 (2), 335–344. (9) Tsang, A. S.-K.; Ingram, K.; Keiser, J.; Hibbert, D. B.; Todd, M. H. Org. Biomol. Chem. 2013, 11 (30), 4921–4924. (10) Savioli, L.; Fenwick, A.; Rollinson, D.; Albonico, M.; Ame, S. M. Lancet 2015, 386 (9995), 739. (11) Freeman, D. B.; Furst, L.; Condie, A. G.; Stephenson, C. R. J. Org. Lett. 2012, 14 (1), 94– 97. (12) Beatty, J. W.; Stephenson, C. R. J. Acc. Chem. Res. 2015, 48 (5), 1474–1484. (13) Wayner, D. D. M.; Dannenberg, J. J.; Griller, D. Chem. Phys. Lett. 1986, 131 (3), 189–191. (14) Blagg, J.; Coote, S. J.; Davies, S. G.; Mobbs, B. E. J. Chem. Soc. Perkin Trans. 1 1986, 2257. (15) Kessar, S. V.; Singh, P.; Singh, K. N.; Venugopalan, P.; Kaur, A.; Mahendru, M.; Kapoor, R. Tetrahedron Lett. 2005, 46 (39), 6753–6755. (16) Boudiar, T.; Sahli, Z.; Sundararaju, B.; Achard, M.; Kabouche, Z.; Doucet, H.; Bruneau, C. J. Org. Chem. 2012, 77 (7), 3674–3678. (17) Yuan, K.; Jiang, F.; Sahli, Z.; Achard, M.; Roisnel, T.; Bruneau, C. Angew. Chem. Int. Ed. Engl. 2012, 51 (35), 8876–8880. (18) Sahli, Z.; Sundararaju, B.; Achard, M.; Bruneau, C. Green Chem. 2013, 15 (3), 775. (19) Lahm, G.; Opatz, T. Org. Lett. 2014, 16 (16), 4201–4203; Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani, N. Nature 1993, 366, 529–531. (20) Fodor, G.; Nagubandi, S. Tetrahedron 1980, 36 (10), 1279–1300; Yokoyama, A.; Ohwada, T.; Shudo, K. J. Org. Chem. 1999, 64 (2), 611–617. (21) Maryanoff, B. E.; Zhang, H.-C.; Cohen, J. H.; Turchi, I. J.; Maryanoff, C. A. Chem. Rev. 2004, 104 (3), 1431–1628. (22) Ferraccioli, R.; Carenzi, D.; Catellani, M. Tetrahedron Lett. 2004, 45 (37), 6903–6907. (23) Priebbenow, D. L.; Stewart, S. G.; Pfeffer, F. M. Org. Biomol. Chem. 2011, 9 (5), 1508– 1515. (24) Priebbenow, D. L.; Pfeffer, F. M.; Stewart, S. G. European J. Org. Chem. 2011, 9, 1632–

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1635. (25) Ruangrungsi, N.; Wongpanich, V.; Tantivatana, P.; Cowe, H. J.; Cox, P. J.; Funayama, S.; Cordell, G. A. J. Nat. Prod. 1985, 48 (4), 529–535. (26) François, G.; Bringmann, G.; Phillipson, J. D.; Assi, L. A.; Dochez, C.; Rübenacker, M.; Schneider, C.; Wéry, M.; Warhurst, D. C.; Kirby, G. C. Phytochemistry 1994, 35 (6), 1461– 1464. (27) Francois, G.; Timperman, G.; Eling, W.; Assi, L.; Holenz, J.; Bringmann, G. Antimicrob. Agents Chemother. 1997, 41 (11), 2533–2539. (28) Bringmann, G.; Rummey, C. J. Chem. Inf. Comput. Sci. 2003, 43 (1), 304–316. (29) Lemmen, C.; Lengauer, T. J. Comput. Aided. Mol. Des. 1997, 11 (4), 357–368. (30) Lemmen, C.; Lengauer, T.; Klebe, G. J. Med. Chem. 1998, 41 (23), 4502–4520. (31) Jones, G.; Willett, P.; Glen, R. C. J. Comput. Aided. Mol. Des. 1995, 9 (6), 532–549. (32) Jones, G.; Willett, P.; Glen, R. C. J. Mol. Biol. 1995, 245 (1), 43–53. (33) Bungard, C. Total Synthesis of the 7-3’-Linked Naphthylisoquinoline Alkaloid Ancistrocladidine, University of Cantebury, 2001. (34) Brusnahan, J. Total Synthesis of Ancistrotanzanine A, The University of Adelaide, 2009. (35) Toop, H. Taking the Lead from Natural Products: Developing Synthetic Protocols to Probe Biological Systems, UNSW Australia, 2014. (36) Dragutinovic, I. Honours Thesis, UNSW Australia, 2015. (37) Frankowski, K. J.; Hirt, E. E.; Zeng, Y.; Neuenswander, B.; Fowler, D.; Schoenen, F.; Aubé, J. J. Comb. Chem. 2007, 9 (6), 1188–1192. (38) Frankowski, K. J.; Ghosh, P.; Setola, V.; Tran, T. B.; Roth, B. L.; Aube , J. ACS Med. Chem. Lett. 2010, 1 (5), 189–193. (39) Spring, D. R. Org. Biomol. Chem. 2003, 1 (22), 3867. (40) Feher, M.; Schmidt, J. M. J. Chem. Inf. Comput. Sci. 2003, 43 (1), 218–227. (41) Ortholand, J.-Y.; Ganesan, A. Curr. Opin. Chem. Biol. 2004, 8 (3), 271–280. (42) Lee, J. S. Mar. Drugs 2015, 13 (3), 1581–1620. (43) Yao, T.; Larock, R. C. J. Org. Chem. 2003, 68 (15), 5936–5942. (44) Cherry, K.; Parrain, J.-L.; Thibonnet, J.; Duchêne, A.; Abarbri, M. J. Org. Chem. 2005, 70 (17), 6669–6675. (45) Yu, Y.; Huang, L.; Wu, W.; Jiang, H. Org. Lett. 2014, 16 (8), 2146–2149. (46) Liebeskind, L. S.; Wang, J. Tetrahedron 1993, 49 (25), 5461–5470. (47) Luo, T.; Schreiber, S. L. Angew. Chem. Int. Ed. Engl. 2007, 46 (43), 8250–8253. (48) Luo, T.; Dai, M.; Zheng, S.-L.; Schreiber, S. L. Org. Lett. 2011, 13 (11), 2834–2836. (49) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec, T. N. J. Am. Chem. Soc. 2002, 124 (51), 15188–15189. (50) Anastasia, L.; Xu, C.; Negishi, E. Tetrahedron Lett. 2002, 43 (32), 5673–5676. (51) Liu, Z.; Yao, Y.; Kogiso, M.; Zheng, B.; Deng, L.; Qiu, J. J.; Dong, S.; Lv, H.; Gallo, J. M.; Li, X.-N.; Song, Y. J. Med. Chem. 2014, 57 (20), 8307-8318.

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(52) Glaser, C. Ann. der Chemie und Pharm. 1870, 154 (2), 137–171. (53) Delaney, P. M.; Moore, J. E.; Harrity, J. P. A. Chem. Commun. 2006, 42 (31), 3323–3325. (54) Kirkham, J. D.; Delaney, P. M.; Ellames, G. J.; Row, E. C.; Harrity, J. P. A. Chem. Commun. 2010, 46 (28), 5154. (55) Kirkham, J.; Leach, A.; Row, E.; Harrity, J. Synthesis (Stuttg). 2012, 44 (13), 1964–1973. (56) Khatri, A.; Samant, S. Synthesis (Stuttg). 2014, 47 (03), 343–350. (57) Kim, E. S.; Kim, K. H.; Kim, S. H.; Kim, J. N. Tetrahedron Lett. 2009, 50 (36), 5098–5101. (58) Kitamura, T.; Wasai, K.; Todaka, M.; Fujiwara, Y. Synlett 1999, 1999 (6), 731–732. (59) Baran, P. S.; Burns, N. Z. J. Am. Chem. Soc. 2006, 128 (12), 3908–3909. (60) Hoz, A. de la; Loupy, A. Microwaves in Organic Synthesis; Wiley-VCH, 2012. (61) Zhao, P.; Beaudry, C. M. Org. Lett. 2013, 15 (2), 402–405. (62) Zhao, P.; Beaudry, C. M. Angew. Chem. Int. Ed. Engl. 2014, 53 (39), 10500–10503. (63) Zhao, P.; Beaudry, C. Synlett 2015, 26 (14), 1923–1929. (64) Kondratov, I. S.; Tolmachova, N. A.; Dolovanyuk, V. G.; Gerus, I. I.; Daniliuc, C.-G.; Haufe, G. European J. Org. Chem. 2015, 2015 (11), 2482–2491. (65) Loupy, A.; Maurel, F.; Sabatié-Gogová, A. Tetrahedron 2004, 60 (7), 1683–1691. (66) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett. 2000, 2 (12), 1729– 1731. (67) Liang, B.; Dai, M.; Chen, J.; Yang, Z. J. Org. Chem. 2005, 70 (1), 391–393. (68) Wolf, C.; Lerebours, R. Org. Biomol. Chem. 2004, 2 (15), 2161–2164. (69) Chinchilla, R.; Nájera, C. Chem. Rev. 2007, 107 (3), 874–922. (70) Wickel, S. M.; Citron, C. A.; Dickschat, J. S. European J. Org. Chem. 2013, 2013 (14), 2906–2913. (71) Hansen, C. A.; Frost, J. W. J. Am. Chem. Soc. 2002, 124 (21), 5926–5927. (72) Bacardit, R.; Moreno-Mañas, M.; Pleixats, R. J. Heterocycl. Chem. 1982, 19 (1), 157–160.

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58 John Reed - January 2017 Chapter 3: Accessing Functionalised Decalins Using the Diene-Regenerative Diels-Alder Reaction

3 ACCESSING FUNCTIONALISED DECALINS USING THE DIENE- REGENERATIVE DIELS-ALDER REACTION

3.1 Introduction: Embellistatin, 12-Deoxyhamigerone and TEO3.1 In 1997, the isolation of embellistatin (3.1), a polyketide natural product, from the fungal spores of Curvularia lunata was reported (Figure 3.1).1 It was revealed to be an inhibitor of microtubule polymerisation, a common target for anti-cancer therapeutics such as Taxol and the vinca alkaloids.2–5 It was subsequently isolated from another fungal species, Embellisia chlamydospora, and shown to also exhibit anti-angiogenic activity.6 Compounds that exhibit both inhibition of microtubule polymerisation and interfere with angiogenesis have displayed significant anti-tumour activity.7 It has been shown that the combination of these modes of action results in a synergistic effect against tumour growth.8

Figure 3.1: The structurally related secondary metabolites embellistatin (3.1), hamigerone (3.2a), TEO3.1 (3.2b), and antarones A (3.3) and B (3.4). The stereochemistry of the epoxide moieties in 3.2a and 3.2b, and the chiral quarternary carbon in 3.4 were not determined due to insufficient material. Around the same time as the original isolation report of embellistatin, Breinholt and co-workers discovered hamigerone (3.2a), another fungal metabolite, isolated from Hamigera avellanea, that bears a strong structural resemblance to embellistatin.9 The authors noted the ability of hamigerone to inhibit the growth of phytopathogenic fungi such as Pyricularia oryzae. Three other structurally related natural products have since been isolated: TEO3.1 (3.2b), and the antarones A (3.3) and B (3.4).10

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Interestingly, the unsaturated bicyclic carboskeleton of these compounds is a motif that can be found in many bioactive compounds (Figure 3.2). These include opiate alkaloids such as thebaine (3.5), steroids, both naturally occurring (ergosterol, 3.6) and synthetic (danazol, 3.7) and resin acids like levopimaric acid (3.8).

Figure 3.2: Other natural products sharing the bicyclic scaffold found in embellistatin and hamigerone The presence of this structural motif in such a wide array of biologically important compounds indicates that it is a privileged scaffold: a structure around which functionality can be built in the hope of discovering a novel bioactive compound.11,12 Further evidence for the potential of this scaffold is seen in the fact that variations of this chemical framework can be found in a number of other biologically active compounds (Figure 3.3). Such examples include another tubulin polymerisation inhibitor (phomopsidin, 3.9), an HIV integrase inhibitor (integramycin 3.10), both of which are cis-fused decalins, as well as the trans-fused decalins versipelostatin aglycone (3.11), a down-regulator of GRP78 expression, and the antibiotic tubelactomicin A (3.12).

Figure 3.3: Selected examples of bioactive compounds exhibiting a decalin core motif The prevalence of the decalin motif in bioactive molecules, and the wide variety of effects conferred by such compounds, leads to the belief that exploring the scaffold further with non-natural variations and functionality has the potential to discover novel therapeutic agents.13 As discussed in the introduction to the thesis, the Waldmann research group has investigated an approach based on the cis-fused decalins and 8-8a-dehydrodecalins found in nakijiquinone and dysidiolide families of natural products.14–16 Similar work has been reported by Peña and co-workers (Scheme 3.1).17 Building on a preliminary communication from Lavallée and Deslongchamps, they describe an

60 John Reed - January 2017 Chapter 3: Accessing Functionalised Decalins Using the Diene-Regenerative Diels-Alder Reaction enantioselective organocatalytic method for the synthesis of cis-fused decalins from a Nazarov reagent and various cyclic enals.18

Scheme 3.1: Enantioselective synthesis of cis-fused decalins using and organocatalyst Under these conditions, enantiomeric excesses of over 90 % were obtained. While the substrate scope reported by the authors is limited to four examples, the products obtained have a wide variety of functionality built in.

During their studies into perturbing the activity of protein kinase C, the Yli-Kauhaluoma group prepared a range of bridged, cis-fused decalins from maleic anhydride and cyclopentadiene using the Diels-Alder reaction (Scheme 3.2).19 Some of these compounds displayed binding activity to protein kinase C comparable with phorbol-12,13-dibutyrate, a known inhibitor.

Scheme 3.2: Accessing cis-fused decalins from the Diels-Alder reaction between cyclopentadiene and maleic anhydride. Reagents and conditions: (a) CH2Cl2, 0 oC→rt, 20 h; (b) sulfolene, phenothiazine, p-xylene, 165 oC, 2 d Both of these examples serve to illustrate not only the utility of the decalin framework, but also the great potential of multi-bond forming processes when applied to the synthesis of compound libraries. In both cases, the core scaffold is assembled rapidly and contains a variety of synthetic handles with which to introduce more functionality.

Along the same lines, we envisaged our own synthesis of a library of decalin-containing compounds. Such a project would require an efficient synthetic method to access decalin-based compounds that is amenable to the introduction of different substitution patterns and functional groups. Furthermore, changing the level of saturation around the decalin core would allow access to new chemical space. A synthesis that facilitates the manipulation of this feature is therefore highly desirable. To this effect, we feel that the diene-regenerative Diels-Alder reaction is a perfect candidate with which to generate such a library. In its most immediate manifestation, this provides hexahydronaphthalenes (Scheme 3.3). Further manipulation of the regenerated diene, through selective reductions, could provide access to a wider variety of compounds. Ideally, this would provide selective access to both cis- and trans-fused decalins. More complexity can be built into the scaffold through other reactions such as the Diels-Alder reaction or addition across one of the double bonds, for example an epoxidation reaction.

John Reed - January 2017 61 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

R' H

R R' H

R R'

O R' R' H H O R R R

H R' H

R R' H H R OH Scheme 3.3: Possible outcomes from our proposed synthetic strategy of diverse decalins using the diene- regenerative Diels-Alder reaction The use of the diene-regenerative Diels-Alder reaction to prepare decalin scaffolds is limited to only three examples. Corey and Watt’s synthesis of the copaene and ylangene natural products utilised this transformation to assemble the cis-fused decalin core (Scheme 3.4).20 The intermolecular cycloaddition proceeds with complete regioselectivity. The authors do not provide an explanation for this, though it is likely to be dictated by a steric argument.

O Me O Me Me Me O a b,c d-f O O O O O H H O H O CO2Me MeO2C MeO2C 3.139:1 dr 3.14

Me Me Me Me Me

H H H H Me -copaene -ylangene -copaene -ylangene Scheme 3.4: Synthesis of the copaene and ylangene natural products using the diene-regenerative Diels-Alder reaction. Reagents and conditions: (a) neat, 150 oC, 24 h; (b) (CH2OH)2, p-TsOH, C6H6, Dean-Stark, 12 h; (c) m- CPBA, CH2Cl2, rt, 48 h; (d) LiAlH4, THF, Et2O; (e) ethyl chloroformate, pyridine, 0 oC, 96 h; (f) DMSO, Ac2O, rt, 24 h Treatment of the bicyclic adduct (3.13) with ethylene glycol and tosic acid under Dean-Stark conditions enabled the protection of the ketone as a ketal. Subsequent epoxidation using m- chloroperbenzoic acid proceeded with good diastereoselectivity (dr: 9:1). A series of functional group interconversions transforms this into the bicyclic dienone (3.14). This substrate enabled the preparation of the unusual cyclic structure found in the copaene and ylangene natural products.

Only two other examples of using the pyrone Diels-Alder reaction to assemble a decalin scaffold with a regenerated diene have been reported.21,22 These come from the Stoltz total synthesis of the transtaganolide natural products, and the total synthesis of chatancin as reported by Zhao and

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Maimone, both of which are discussed in the Introduction (Scheme 1.11, p. 14). In both cases, regeneration of the diene was an unwanted reactive pathway as the desired product was the bridged lactone formed by the initial Diels-Alder cycloaddition. Application of the diene-regenerative Diels-Alder reaction in this context is therefore underutilised.

Furthermore, numerous examples exist of the regenerated diene moiety from a pyrone Diels-Alder reaction being used as a functional group for further derivatisation. Some of these have already been discussed in the Introduction (Martin’s and Yamaguchi’s synthesis of yohimbe alkaloids, Scheme 1.10, p. 14; Snyder’s synthesis of hydroindolines, Scheme 1.12, p. 15), or previously in this chapter (Corey’s synthesis of the copaene and ylangene natural products, vide supra).20,23,24

In some cases, hydrogenation of the Diels-Alder adduct can be performed with good stereocontrol, providing access to chiral sp3 centres. This is exemplified by Engelbert Ciganek’s synthesis of morphine derivatives lacking the B ring of the opiate alkaloids (Scheme 3.5).25 Hydrogenation of the dihydroaromatic adduct over palladium on carbon delivers hydrogen selectively to the opposite face to the furanyl substituents, thereby creating the trans-fused ring system seen. Interestingly, this is also the first example of a Diels-Alder reaction using a benzofuran as the dienophile.

Scheme 3.5: Synthesis of a morphine derivative using the diene-regenerative Diels-Alder reaction. Reagents and conditions: (a) 1,2,4-trichlorobenzene, 215 oC, 10 h; (b) H2, Pd/C, THF; (c) BH3•Me2S The π-system regenerated by the extrusion of carbon dioxide has also been used in strain- promoted electrocyclic ring expansion reactions. The reaction between 2-pyrone and 1,2,3- triphenylcyclopropene gives the bicyclic diene 3.15 which spontaneously isomerises to give the corresponding cycloheptatriene (Scheme 3.6).26

Scheme 3.6: Synthesis of a cycloheptatriene via a diene-regenerative Diels-Alder reaction/electrocyclic ring expansion tandem reaction Barton and co-workers further explored this strategy while synthesising substituted benzocyclooctatrienes from 2-pyrones and benzocyclobutene (Scheme 3.7).27,28 Again, the ring

John Reed - January 2017 63 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries strain of the tricyclic intermediate drives the electrocyclic ring opening to give the benzocyclooctatriene.

Scheme 3.7: Synthesis of substituted benzocyclooctatrienes from 2-pyrones and benzocyclobutene In one case, 3-carbamethoxy-2H-pyran-2-one (3.16) was reacted with a vinyl protected glucose derivative (Scheme 3.8).29 The resultant cyclohexadiene was then epoxidised selectively at the most distal olefin from the exocyclic ester. Further manipulations eventually led to the inositol 3.17.

O O O O O O O O O O MeO C O aO b 2 O O O O O O MeO C O MeO C 2 O 2 3.16 O 2.68:1 dr 2.74:1 dr c

OH O

HO O O O OH OH O HO d-g O O OH O H N HO O 2 O OH OH MeO2C HO HO 3.17 OH OH NH2 N3 Scheme 3.8: Synthesis of aminosugar 3.17 using the diene-regenerative Diels-Alder reaction. Reagents and conditions: (a) CH2Cl2, sealed tube, 100 oC, 48 h; (b) m-CPBA, CH2Cl2/H2O (3:1), rt, 24 h; (c) NaN3, NH4Cl, DME/EtOH/H2O (2:1:1), rt, 24 h; (d) TBSCl, imidazole, CH2Cl2, rt, 24 h; (e) DIBAL, PhMe, -78 oC, 5 h; (f) TFA/THF/H2O (2:1:1), rt, 4 h; (g) H2, Pd/C, MeOH, rt, 24 h Asymmetric induction from the sugar moiety enables the Diels-Alder reaction to proceed with moderate diastereoselectivity. This also gives a slight stereochemical bias to the outcome of the epoxidation reaction. Interestingly, hydrogenation occurs only from one face, providing one diastereomer in quantitative yield.

Komiyama and co-workers were able to synthesise 2-arylthio-2-cyclohexenone derivatives from 4- arylthio-3-hydroxy-pyran-2H-ones using the diene-regenerative Diels-Alder reaction (Scheme 3.9).30 The partial aromaticity of the pyrone means it favours the enol tautomer, thereby permitting the Diels-Alder reaction to occur. The product lacks any aromaticity and, consequently, reverts to the ketone tautomer after the extrusion of carbon dioxide.

64 John Reed - January 2017 Chapter 3: Accessing Functionalised Decalins Using the Diene-Regenerative Diels-Alder Reaction

Scheme 3.9: Synthesis of 2-arylthio-2-cyclohexenones using the diene-regenerative Diels-Alder reaction The regenerated diene can also undergo another cycloaddition with an appropriate dienophile. This is described by Kranjc and co-workers who performed a series of reactions between fused 2- pyrones and maleimides or maleic anhydride (Scheme 3.10).31 It was noted that the exo-endo selectivity for the second cycloaddition could be manipulated by changing the size of the ring fused to the pyrone.

Scheme 3.10: Double cycloadditions with fused 2-pyrones Previous work in the Morris group has established that the diene-regenerative Diels-Alder reaction can be used to construct the unsaturated decalin framework found in embellistatin (3.1), hamigerone (3.2a) and TEO3.1 (3.2b) (see Figure 3.1). As an extension of this work , we wanted to investigate whether this reaction could be used to synthesise simplified analogues of these natural products bearing different exocyclic substituents. Furthermore, we aimed to use the regenerated diene in these compounds as a synthetic handle to allow access to further diversified decalin structures. These could include both cis- and trans-fused decalins, or compounds with even more functionality built in.

3.2 Synthesis of a Pyrone Precursor Initial efforts towards the decalin scaffold focussed on accessing pyrone 3.18 (Scheme 3.11). It was felt that this could elaborated to the appropriate Diels-Alder precursors using a reductive coupling method.

Scheme 3.11: Initial strategy for decalin synthesis

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To access this pyrone, two options were considered: Negishi’s zinc(II) bromide catalysed cyclisation protocol and Schreiber’s gold(I) catalysed alkyne activation cascade reaction (Scheme 3.12).32,33

Scheme 3.12: Possible synthetic routes to pyrone 3.18 While the Schreiber procedure is attractive since it only requires one step from readily available precursors, it was found that five equivalents of the unactivated alkyne (1,7-octadiyne in this case), and a reasonably high catalyst loading (5 mol %) were required to achieve synthetically useful yields. On the other hand, while the Negishi procedure would require more steps to assemble the precursor, previous experience within the research group on similar systems demonstrated that it was a suitable method for medium to large scale preparation of these substrates.34,35

In order to gain rapid access to 3.18, the Schreiber strategy was implemented, however, it was recognised that as more of the material is needed, the Negishi protocol could be used to deliver a more substantial amount (Scheme 3.13). Thus, on a 1 mmol scale (70 mg propiolic acid, 531 mg 1,7-octadiyne), 3.18 could be obtained in 57 % yield, providing 100 mg of the pyrone. As the scale of the reaction was incrementally increased, the yield decreased. On a 2 mmol scale, 144 mg of product were obtained (41 % yield) and on a 5 mmol scale, 255 mg (29 %) were obtained. Furthermore, in generating the 255 mg of product on the 5 mmol scale reaction, 2.65 g of 1,7- octadiyne, 123 mg of (PPh3)AuCl and 64 mg of AgOTf were used. While some of the 1,7-octadiyne could be recovered and recycled through distillation, the precious metal catalysts were lost. Clearly, from a cost and atom efficiency point of view, this procedure was not suitable for scale up. As such, the Negishi strategy was also investigated.

Scheme 3.13: Comparison of a) Schreiber’s gold(I) catalysed procedure and b) Negishi’s zinc(II) bromide catalysed strategy in the synthesis of 3.18. Reagents and conditions: (a) (PPh3)AuCl, AgOTf, CH2Cl2, rt, 16 h; (b) NaI, AcOH, reflux, 2 h; (c) Pd(PPh3)2Cl2, CuI, Et3N, 50 oC, 16 h; (d) LiOH(aq), MeOH/THF (1:1), rt, 10 h; (e) ZnBr2, THF, rt, 48 h

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Treatment of ethyl propiolate with sodium iodide in refluxing acetic acid provided vinyl iodide 3.19 in quantitative yield with complete diastereoselectivity. Coupling of this iodide with 1,7-octadiyne under Sonogashira cross-coupling conditions provided ester 3.20, which could be hydrolysed to the acid and cyclised using zinc(II) bromide to give 3.18 in 83 % overall yield. In one sequence of reactions, 2.03 g of pyrone could be obtained, highlighting the scalability of this strategy.

With a substantial amount of pyrone 3.18 in hand, efforts towards the construction of the decalin scaffold, and the subsequent derivatisaiton, could now begin.

3.3 Strategies for the Diversification of the Dienophile It was envisaged that hydrometallation of the terminal alkyne of 3.18 would lead to an organometallic species that could participate in cross-coupling reactions (Scheme 3.14). This would enable the introduction of different groups onto the olefinic dienophile.

Scheme 3.14: Strategy for the conversion of pyrone 3.18 to Diels-Alder precursors Three options were considered for the hydrometallation of 3.18: hydrostannylation, hydroboration and hydrozirconation. Of these, hydrozirconation, using Schwartz’s reagent, appealed the most due to its noted selectivity for terminal alkynes and reliable regiochemical outcomes. Furthermore, the reactions of organozirconium reagents have been well studied, with some key examples presented here (Scheme 3.15).36–40

Scheme 3.15: Reactions of organozirconocene compounds

John Reed - January 2017 67 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Organozirconocene compounds can be used to access homologated aldehydes through the reaction with organic isonitriles. Insertion of the isonitrile carbenoid into the carbon-zirconium bond gives an imino metal carbenoid. Subsequent acid catalysed hydrolysis converts this to the aldehyde.41

The organometallic compound can be trapped with iodine, or other suitable electrophiles, to generate a vinyl iodide. The steric bulk of the cyclopentadienyl complex limits the reactivity of the species. As such, only unencumbered electrophiles can be used in this way. Transmetalation of the organozirconocene may open up a wider array of reactions to the organometallic intermediate. Numerous metals have been reported to undergo transmetalation with organozirconocene species, including aluminium, boron, copper, mercury, nickel, palladium, tin, and zinc.42 As one would expect, transmetalation with zinc gives organometallic compound that have been shown to undergo 1,2-addition to a variety of carbonyl compounds. Conversely, organocuprates can be accessed from the organozirconocene and used for 1,4-addition.

One interesting, and unusual, reaction occurs when the organozirconocene is treated with a catalytic amount of silver perchlorate. This results in abstraction of the chloride from the zirconium complex to generate a cationic, Lewis-acidic, organometallic complex. This complex can undergo 1,2-addition to aldehydes without transmetalation with another metal such as zinc. Furthermore, this complex activates the ring-opening of epoxides, which, following a 1,2-hydride shift to give an activated carbonyl compound, enables the addition of the organozirconocene reagent.43

Schwartz’ reagent was prepared by treating zirconocene dichloride with lithium aluminium hydride, according to Buchwald’s protocol.44 It was envisaged that the reaction between pyrone 3.18 and Schwartz’ reagent would enable the introduction of an ester group onto the dienophile using ethyl chloroformate. Pyrone 3.18 was treated with Schwartz’s reagent in THF at 0 oC before being allowed to warm to room temperature (Scheme 3.16). Analysis by TLC after 2 hours indicated consumption of the starting material and the formation of a new product, presumed to be the hydrozirconated species.

Scheme 3.16: Failed attempt to convert 3.18 to 3.21. Reagents and conditions: (a) Cp2ZrHCl, THF, 0 oC→rt, 2 h; (b) ethyl chloroformate, THF, 0 oC→50 oC, 16 h The susceptibility of organozirconates to both oxygen and water meant that no attempt was made to isolate or characterise this intermediate. Instead, ethyl chloroformate was added to the reaction

68 John Reed - January 2017 Chapter 3: Accessing Functionalised Decalins Using the Diene-Regenerative Diels-Alder Reaction mixture as a solution in THF. After stirring this solution for 16 hours, no reaction was observed by TLC. An aliquot of the reaction was removed and quenched with a saturated aqueous solution of sodium bicarbonate. The crude product obtained from this aliquot was analysed by 1H NMR (Figure 3.4). This spectrum clearly shows the disappearance of the terminal C-H resonance (t, 1.95 ppm) and the appearance of three new alkene resonance (two between 4.95-5.05 ppm, and a third from 5.73-5.85 ppm). The splitting pattern displayed by these resonances is consistent with a mono- substituted terminal alkene. We concluded that the alkyne had been hydrozirconated, but had failed to react with ethyl chloroformate. Upon quenching with water, the organozirconate had proto- demetalated leading to formation of the terminal alkene 3.22.

Figure 3.4: Comparison of the 1H NMR spectra of 3.18 (red) and the crude product obtained after reacting with Schwartz’ reagent and ethyl chloroformate (blue)

The remaining reaction mixture was heated to 50 oC. After 1 hour, this solution had changed to a deep purple colour, and analysis by TLC showed a strong baseline spot and only faint traces of any product. No attempt was made to isolate or characterise this material. It was hypothesised that ethyl chloroformate was not a powerful enough electrophile to react with the organozirconate. Literature examples of similar transformations displayed success when dimethylaluminium chloride was used to transmetalate the organozirconate.45,46 The smaller size of the organoaluminate enables reactions with electrophiles that would otherwise be precluded by the bulk of the zirconocene complex, as discussed earlier. Consequently, the reaction was repeated, but prior to the addition of ethyl chloroformate, a solution of dimethylaluminium chloride in hexanes was added at 0 oC and stirred for a further 2 hours (Scheme 3.17). Ethyl chloroformate was introduced at 0 oC and stirred for 16 hours after which analysis by TLC showed a new, more polar product. Following an aqueous work-up, the product was isolated and identified to be the desired ester 3.21.

John Reed - January 2017 69 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 3.17: Successful synthesis of ester 3.21. Reagent and conditions: (a) Cp2ZrHCl, THF, 0 oC→rt, 2 h; (b) ClAlMe2 (1 M in hexanes), 0 oC, 2 h; (c) ethyl chloroformate, THF, 0 oC→50 oC, 16 h Unfortunately, the product was only isolated in 8 % yield. This could be a result of the decomposition of Schwartz’s reagent, which is sensitive to air, moisture and light.40,47,48 It was hypothesised that an in situ formation of Schwartz’s reagent from its stable precursor, zirconocene dichloride, might be a more efficient and reliable method for accessing 3.21. Numerous procedures have been developed to implement this strategy, all of which revolve around the addition of a reducing agent to zirconocene dichloride, followed by the reagent.48–53 We thus decided to screen three reducing agents: lithium triethylborohydride, lithium tri-tert-butoxyaluminium hydride, and sodium bis(2- methoxyethoxy)aluminium hydride (Red-Al); in order to determine the optimal conditions for our system (Table 3.1). Using sodium bis(2-methoxyethoxy)aluminium hydride failed to generate any of the desired product, and only a small amount of the starting material was recovered. However, both lithium triethylborohydride and lithium tri-tert-butoxyaluminium hydride enabled conversion of pyrone 3.18 to 3.21, with lithium triethylborohydride giving a slightly better yield (47 %).

Table 3.1: Conversion of 3.18 to 3.21 using Schwartz’s reagent formed in situ. Reagents and conditions: (a) CP2ZrCl2, red. agent, THF, 0 oC→rt, 2 h; (b) ClAlMe2 (1 M in hexanes), 0 oC, 2 h; (c) ethyl chloroformate, THF, 0 oC, 16 h

Reducing Agent Yield (%)

Red-Al 0 %

LiEt3BH 47 %

Li(t-BuO)3AlH 41 %

We next looked at introducing a phenyl ring onto the dienophile using this method for synthesising Schwartz’s reagent in situ. Since it was unlikely that iodobenzene would couple with the organozirconate (or transmetalated organoaluminate) directly, we decided to attempt a palladium catalysed cross coupling to effect the transformation (Scheme 3.18). After in situ formation of Schwartz’s reagent using lithium triethylborohydride, pyrone 3.18 was added to form the organozirconocene. After 2 hours, iodobenzene, potassium carbonate and 5 mol % of tetrakis(triphenylphosphine)palladium(0) were introduced. The reaction mixture was then degassed via three freeze-pump-thaw cycles before being allowed to stir for 16 hours at room temperature.

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Following aqueous work up and chromatographic purification, styrene 3.23 was obtained in 41 % yield.

Scheme 3.18: Palladium catalysed coupling of 3.18 with iodobenzene. Reagents and conditions: (a) CP2ZrCl2, LiEt3BH, THF, 0 oC→rt, 2 h; (b) PhI, K2CO3, Pd(PPh3)4, rt, 2 h Attempts to apply this methodology with other coupling substrates, such as 4-iodotoluene and 4- iodoanisole, only ever led to the formation of alkene 3.22. It was thought that the desired compounds could be accessed from this alkene through an olefin cross-metathesis reaction with a suitable styrene. In order to test this hypothesis, we set about deliberately accessing a more substantial amount of 3.22 (Scheme 3.19). To this end, tosylation of 5-hexen-1-ol enabled a substitution reaction with lithium acetylide, ethylenediamine complex to give alkyne 3.24. Once more, the alkyne 3.24 could be elaborated to pyrone 3.22 in two ways: following Schreiber’s gold(I) catalysed procedure, or Negishi’s zinc(II) method. Both avenues were explored with the gold(I) catalysed synthesis allowing rapid access on a small scale, while Negishi’s protocol facilitated the preparation of 3.22 on a larger scale.

Scheme 3.19: Synthesis of pyrone 3.22. Reagents and conditions: (a) TsCl, Et3N, CH2Cl2, 0 oC→rt, 16 h; (b) LiCCH•H2N(CH2)2NH2, DMSO, rt, 16 h; (c) propiolic acid, (PPh3)AuCl, AgOTf, CH2Cl2, rt, 16 h; (d) 3.19, Pd(PPh3)2Cl2, CuI, Et3N, 50 oC, 16 h; (e) LiOH(aq), MeOH/THF (1:1), rt, 10 h; (f) ZnBr2, THF, rt, 48 h Pleasingly, upon applying Lipshutz’s conditions, alkene 3.22 underwent olefin cross-metathesis reactions with a range of terminal alkenes to give the internal alkenes 3.21, 3.23 and 3.26-3.28 (Scheme 3.20). In all cases, the desired E-alkene was obtained as the predominant isomer.54 The transformations could be effected with a loading of 2 mol % for the Grubbs generation II ruthenium catalyst and 3 mol % for the copper(I) iodide co-catalyst.

John Reed - January 2017 71 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Scheme 3.20: Diversification of pyrone 3.22 via an olefin cross-metathesis strategy Lipshutz notes that the copper(I) iodide has a dual role with the iodide anion stabilising the ruthenium catalyst, thereby extending the catalyst lifetime, and the copper acting as a phosphine scavenger that accelerates the reaction rate. This phosphine scavenger-mediated rate acceleration has been previously described, especially in the context of the Grubbs-Hoveyda-Blechert family of catalysts.55,56

Five more pyrones, bearing a methyl group at the C(4) position, were prepared using a similar strategy (Scheme 3.21). Ethyl 2-butynoate was treated with sodium iodide in refluxing acetic acid for two hours to give the vinyl iodide 3.29 in quantitative yield. A Sonogashira reaction was then used to couple 3.29 with alkyne 3.24, prepared previously. Hydrolysis of the resultant ester 3.30 followed by treatment with zinc(II) bromide in THF led to formation of the pyrone 3.31. A series of olefin metathesis reactions were carried out, using the conditions previously described, providing access to pyrones 3.32-3.36 in yields ranging from 42-90 %.

Scheme 3.21: Synthesis of five pyrones bearing a methyl group at the C(4) position. Reagents and conditions: (a) NaI, AcOH, reflux, 2 h; (b) 3.24, Pd(PPh3)2Cl2, CuI, Et3N, 50 oC, 16 h; (c) LiOH(aq), MeOH/THF (1:1), rt, 10 h; (d) ZnBr2, THF, rt, 48 h; (e) CH2CHR, Grubbs Gen. II catalyst, CuI, Et2O, 35 oC, 10 h

3.4 Application of the Diene-Regenerative Diels-Alder Reaction With these ten pyrone Diels-Alder substrates in hand, it was now possible to investigate the cycloaddition, and the myriad ways to functionalise the dihydroaromatic products. When comparing methods to effect the diene-regenerative Diels-Alder reaction on pyrone substrates 3.21, 3.23, 3.26-3.28, and 3.32-3.36, it was important to note the propensity of the dihydrobenzene products to undergo oxidation to the aromatic analogues.57,58 This has previously been observed by members of the Morris group on related systems (Scheme 3.22).34,35 While investigating a model system for the total synthesis of embellistatin, Sarah Lundy attempted to convert pyrone 3.37 to

72 John Reed - January 2017 Chapter 3: Accessing Functionalised Decalins Using the Diene-Regenerative Diels-Alder Reaction

3.38 by refluxing in xylenes. After 48 hours, three products were observed in the reaction mixture. These were identified to be the desired adduct 3.38, its isomer 3.39 and the aromatised analogue 3.40.

Scheme 3.22: Isomerisation and oxidation of dihydrobenze 3.38 It was shown experimentally that trace amounts of acid would catalyse the isomerisation of pure 3.38 to 3.39, which would readily undergo aerobic oxidation to 3.40. This decomposition pathway could be avoided by the use of Proton Sponge® as an additive in the Diels-Alder reaction, however, storage of the isolated product for extended periods of time was not possible. As such, the microwave conditions used to effect the transformation in Chapter 2 were not explored here. There was some doubt over the integrity of the inert atmosphere maintained inside the microwave vials and it was also believed that the time and effort needed to remove the ortho-dichlorobenzene solvent would provide too great a window for isomerisation and oxidation to occur. Our efforts were therefore focussed on achieving the diene-regenerative Diels-Alder using conventional heating under an inert atmosphere provided by a Schlenk manifold.

Pyrone 3.21 was heated in refluxing xylenes in the presence of both Proton Sponge® and the radical inhibitor butylated hydroxytoluene (Scheme 3.23). After 48 hours, analysis by TLC clearly indicated the consumption of all starting material and the formation of a less polar product. Spectroscopic characterisation of this product revealed it to indeed be the desired bicyclic compound 3.41, obtained as a racemic mixture.

Scheme 3.23: Conversion of pyrone 3.21 to the diene-regenerative Diels-Alder adduct 3.41 A suite of two-dimensional NMR experiments were performed in order to assign each resonance in the 1H and 13C NMR spectra (Figure 3.5). Two important resonances can be observed at 2.73 (tt) and 3.02 (dt). On the basis of the two-dimensional NMR experiments, these were assigned as the protons at C(8a) and C(1) respectively. The coupling constant between these protons is 12.5 Hz. The ~180o dihedral angle between diaxial protons dictates that the coupling constant between the two will be 10-13, according to the Karplus equation, whereas diequatorial protons or protons

John Reed - January 2017 73 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries in an axial/equatorial relationship have smaller coupling constants of around 2-5 Hz. The coupling constant between the H(8a) and H(1) protons can thus be taken as evidence of a diaxial relationship, confirming the trans orientation of the two protons, and therefore indicating that the stereochemistry of the dienophile has been retained through the reaction.

EtO C 2 H 8 2 7 1 8a 3 6 4a 4 5 3.41

H(1) H(8a)

Figure 3.5: 1H and 13C NMR spectra of 3.41 These conditions were then applied to the remaining nine pyrones to convert them to the corresponding tetradehydrodecalins 3.42-3.50 (Table 3.2), all obtained as racemic mixtures. The reaction tolerated a variety of functionality on the dienophile. Electron-deficient dienophiles (3.41 and 3.46) as well as electron-rich ones (3.43, 3.44, 3.48 and 3.49) successfully underwent the cycloaddition, as did more electron-neutral dienophiles (3.42, 3.45, 3.47 and 3.50). Moreover, the presence of the methyl group on the pyrone did not affect the reaction outcome in a significant way. The yields for these reactions were generally good, the only exception was 3.42, which was obtained in 63 % yield.

Compounds 3.41-3.50 all displayed susceptibility to isomerisation and oxidation on standing. Even when stored in a freezer at -20 oC, oxidation was still observed after a week. Nevertheless, all of the compounds could be isolated via chromatographic separation and characterised without taking any special precautions to exclude water or air.

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Table 3.2: Synthesis of 10 tetradehydrodecalin compounds via a diene-regenerative Diels-Alder reaction

Product Yield Product Yield

96 % 72 %

63 % 71 %

72 % 79 %

75 % 86 %

84 % 89 %

3.5 Manipulation of the Dihydroaromatic Scaffold With the goal of this chapter being to utilise the regenerated diene from these pyrone Diels-Alder reactions as a synthetic handle to access a greater variety of chemical space, we set about investigating which reactions were possible on this scaffold. 3.41 was arbitrarily chosen as a model substrate for these investigations.

The most obvious candidate when considering possible reactions of dienes was another Diels- Alder reaction. An intramolecular Diels-Alder reaction with a suitable dienophile would allow a rapid

John Reed - January 2017 75 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries increase in molecular complexity, while simultaneously generating another four stereocentres. A number of issues with the Diels-Alder reaction needed to be considered when planning this step. Firstly, the reaction has the potential to form eight different regio- and stereoisomers if an unsymmetrical dienophile is used (Scheme 3.24). To simplify analysis of the reaction outcome, it was decided to use a symmetrical alkene as this reduces the number of possible products to four.

Scheme 3.24: Possible isomers from the Diels-Alder cycloaddition between 3.41 and a generic, unsymmetrical dienophile Secondly, the electronic character of the dienophile has to be compatible with the diene. In this case, the diene is electron rich and the Diels-Alder reaction can be presumed to be a ‘normal electron demand’ Diels-Alder reaction. Therefore, to allow for the requisite orbital interactions between the diene’s highest occupied molecular orbital (HOMO) and the dienophile’s lowest unoccupied molecular orbital (LUMO), an electron-deficient dienophile would be best suited to the reaction. With these factors in mind, we decided to use maleic anhydride as the dienophile. Owing to the carbonyl groups on either side of the alkene, maleic anhydride is a very powerful dienophile in normal electron demand Diels-Alder reactions.

Pleasingly, upon heating a mixture of 3.41 and maleic anhydride in refluxing xylenes, a single, more polar product formed (Scheme 3.25). After 6 hours, the reaction had gone to completion and the product was isolated in 95 % yield following flash chromatography. Analysis of the 1H NMR spectrum quickly established that the product was a Diels-Alder adduct, but the question of which isomer had formed still remained and required more sophisticated analysis.

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CO2Et CO2Et

O O O O H H O O O H H EtO2C O O H xylenes 3.51a 3.51b reflux, 6 h O O O H H O 3.41 H H O O CO2Et CO2Et

3.51c 3.51d Scheme 3.25: The four possible products from the Diels-Alder reaction between 3.41 and maleic anhydride Two-dimensional correlation NMR experiments (COSY, HSQC, HMBC) were used to assign each resonance in the 1H NMR spectrum unambiguously (Figure 3.6).

H(5) H(4a) H(6) H(9*) H(7) H(8*) H(6*) H(8) H(7*)

H(9b)

H(10) H(3a) H(5) H(4) H(11) H(9)

Figure 3.6: Assignment of each resonance in the 1H NMR spectrum of 3.41. Atom numbering is in accordance with IUPAC priorities. Inset: the aliphatic region of the spectrum

With each resonance in the 1H NMR spectrum assigned, it was now possible to exploit the nuclear Overhauser effect to determine which protons exhibited through-space coupling, and therefore which protons were in close spatial proximity. It was hoped that this would shed some insight as to which isomer had been obtained. The most striking result from the NOESY experiment was the observation of a correlation between the H(5a) and H(9b) protons. This through-space interaction is only possible in the endo product 3.51c. This structure was confirmed by X-ray crystallography, after recrystallising the product from hexane (Figure 3.7). The regioselectivity of this reaction indicates that the bottom face of diene 3.41, as drawn, is the less hindered face. This has important implications when predicting the outcomes of other reactions on the diene.

John Reed - January 2017 77 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Figure 3.7: Structure of 3.51c confirmed by X-ray crystallography The next reaction we wanted to investigate was the selective epoxidation of one of the alkenes using the Prilezhaev reaction. These epoxidation reactions, where a peracid such as m-CPBA oxidises an olefin, proceed via a ‘butterfly’ transition state that delivers an oxygen atom suprafacially across the alkene (Scheme 3.26).

O H O H OH O O O O O O

Cl Cl Cl

Scheme 3.26: Mechanism of the Prilezhaev reaction with m-CPBA The regio- and stereoselectivity of the Prilezhaev reaction can often be predicted. Peracids are electrophiles and m-CPBA in particular displays good selectivity for the most electron-rich alkene in a system where multiple double-bonds are present. If the substrate contains stereochemical information, this can direct the epoxidation reaction to favour one diastereomer since the peracid prefers to approach the olefin from its least hindered face. Based on these considerations, we predicted that if diene 3.41 was reacted with m-CPBA, epoxidation would occur selectively at the tri-substituted alkene. This is more electron rich than the di-substituted alkene as a result of having an extra electron-donating alkyl group, and also because it is more distal from the electron- withdrawing ester group. From the results of the intermolecular Diels-Alder reaction described

78 John Reed - January 2017 Chapter 3: Accessing Functionalised Decalins Using the Diene-Regenerative Diels-Alder Reaction previously (vide supra), we expected that the epoxide would form on the bottom face of the molecule, as drawn, since this was apparently less sterically encumbered.

Pleasingly, upon treating 3.41 with one equivalent of m-CPBA in CH2Cl2 at 0 oC, a single product formed, with the reaction reaching completion after 6 hours (Scheme 3.27). Following flash chromatography, the product was isolated in 84 % yield and subjected to spectroscopic analysis.

m-CPBA EtO2C EtO2C EtO2C H CH2Cl2 H H 0 oC, 6h OR

O O 3.41 3.52a 3.52b Scheme 3.27: Epoxidation of 3.41 with m-CPBA gives a single isomer

The 1H NMR spectrum displayed two resonances in the alkene region (at 5.77 and 6.04 ppm). This suggested that the tri-substituted alkene had been epoxidised, since only one alkene resonance would be expected if the di-substituted alkene had reacted instead. Two-dimensional NMR experiments confirmed this regiochemical outcome through observation of a number of multiple- bond correlations (Figure 3.8). Important correlations highlighted by these experiments are the 3- bond 1H-13C couplings between the H(3) olefinic proton and the carbonyl carbon, and the H(2) proton and the quarternary carbon. A 1H-1H correlation between H(3) and H(4) can also be seen in the COSY spectrum providing further evidence for the proposed structure.

Figure 3.8: Important multiple bond NMR correlations exhibited by 3.52. 1H-1H correlations are shown in blue, while 1H-13C correlations are shown in red Ascertaining the stereochemistry of the epoxide was not possible using NOESY experiments as there were no through-space correlations that could be uniquely attributed to a single isomer. An examination of the coupling constants between the protons, however, was able to shed more light on the issue. From the 1H NMR spectrum, the coupling constant between H(1) and H(2) was determined to be 4.05 Hz. According to the Karplus equation, this would correspond to a dihedral angle between the two protons of approximately 60o (Figure 3.9). Using the Chem3D software, the dihedral angles for the two possible isomers were approximated, following energy minimisation using the MM2 calculations.

John Reed - January 2017 79 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Figure 3.9: Conformational analysis of 3.52a (left) and 3.52b (right) using Chem3D software and application of the Karplus equation to determine which isomer has been formed. The protons of interest are highlighted yellow. Image freely obtained from William Reusch under Creative Commons 4.0 licensing: https://creativecommons.org/licenses/by-nc-nd/4.0/legalcode According to the Chem3D software, the dihedral angles between H(1) and H(2) (shown in yellow) are 44.737o for 3.52a and 7.780o for 3.52b. According to the Karplus equation, protons with these dihedral angles can be expected to have coupling values of approximately 6-7 Hz and 10-11 Hz respectively. Compared with the experimental coupling constant (4.05 Hz), it seems highly likely that 3.52a is the product obtained from this reaction, where the m-CPBA has approached the alkene from the bottom face of the molecule as drawn. This result is in agreement with the predictions made regarding the regio- and diastereoselectivity of the reaction.

In a similar vein to the Prilezhaev reaction, treatment of diene 3.41 with one equivalent of potassium permanganate allowed the selective dihydroxylation of tri-substituted alkene (Scheme 3.28). This reaction proceeds through a concerted, suprafacial mechanism, meaning that both alcohols are installed on the same face of the double bond.

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Scheme 3.28: Dihydroxylation of 3.41 proceeds regio- and diastereoselectively After stirring in basic methanol/water for 30 minutes, a single product had formed. Analysis of the crude 1H NMR spectrum confirmed that the tri-substituted alkene had been dihydroxylated. Modelling the two possible stereoisomers indicated that the dihedral angles between the H(3) and H(4) protons on each isomer were not sufficiently different to enable assignment of relative stereochemistry at the C(4) and C(4a) carbon centres. Fortunately, through space correlations were detected between the H(4) proton and the H(6) and H(8) protons. Such interactions are only possible if the decalin framework is bent as a result of a cis-relationship between the bridgehead substituents (Figure 3.10). Thus, it can be confidently assumed that diol 3.53 has been formed.

Figure 3.10: Conformational analysis of 3.53 using Chem3D software. The H(4), H(6) and H(8) protons are highlighted in yellow

Another oxidant that has proven highly useful in reactions with dienes is singlet oxygen.59 Singlet oxygen (1O2) refers to the excited electronic state of diatomic oxygen, where the total quantum spin equals zero. Singlet oxygen can be produced from gaseous oxygen using light and a photosensitizer such as Rose Bengal (3.54). The reaction of singlet oxygen with a diene gives a Diels-Alder adduct (Scheme 3.29). The mechanism for this reaction has been well studied and is known to proceed through two mechanisms that give the same product: a concerted [4+2] cycloaddition, and a stepwise cheletropic addition followed by a rearrangement.

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Scheme 3.29: Prototypical reaction of a diene with singlet oxygen Having seen the Diels-Alder reaction, the Prilezhaev reaction, and the dihydroxylation all yield single diastereomers, we were hopeful that reacting 3.41 with singlet oxygen would demonstrate similar diastereoselectivity. After stirring 3.41 with a catalytic amount of Rose Bengal in dichloromethane in an atmosphere of oxygen for 12 hours at room temperature, two major products were observable by TLC, as well as a significant amount of starting material. The reaction was stirred for a further 24 hours after which, all starting material had been consumed. Following chromatographic separation, these products were characterised and their structures elucidated.

The two products were identified to be the diastereomeric Diels-Alder adducts 3.55a and 3.55b (Scheme 3.30). Nuclear Overhauser spectroscopy allowed the easy identification of each isomer, as one exhibited a strong correlation between the H(4) proton and the H(5) and H(7) protons. When the conformation of the two isomers are modelled, it can be seen that this interaction is only possible in the cis-fused 3.55a. Furthermore, the J coupling between the H(8) and H(8a) protons in 3.55b is 13.0 Hz. This indicates an axial-axial relationship between the two protons. H(8a) can only be positioned axially in a trans-fused ring system, thus the coupling constants provide further evidence for the stereochemical assignment. This was isolated in 35 % yield, while the trans-fused 3.55b was isolated in 28 % yield. We believe that this reaction exhibits poor diastereoselectivity because of the small size of the reagent (O2). This allows it to approach the diene from either face. In contrast, maleic anhydride and m-CPBA are relatively large, and are thus more susceptible to steric effects when approaching the substrate.

J = 13.0 Hz

EtO C EtO C EtO C H 2 H 2 H 2 H H a O O O O through space H H correlations detected 3.55a (35 %) 3.55b (28 %)

Scheme 3.30: Reaction of 3.41 with singlet oxygen. Reagents and conditions: (a) O2, Rose Bengal, CH2Cl2, rt, 36 h

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Owing to the small size of the singlet oxygen reagent, we believed that finding conditions for the endoperoxidation to only give one diastereomer would prove to be challenging. As such we elected not to optimise this reaction, and looked at new ways of manipulating the scaffold.

We next looked at reductions of the diene moiety via catalytic hydrogenation. As an initial experiment, pyrone 3.21 was converted to diene 3.41 by refluxing in xylenes (Scheme 3.31). Once the reaction had gone to completion, the xylenes were removed by distillation and the crude product was dissolved in methanol. Palladium on carbon was added and a balloon of hydrogen was fixed to the reaction. After stirring at room temperature for 12 hours, no starting material remained. The heterogeneous catalyst and the solvent were removed, and the crude product was analysed by 1H NMR spectroscopy. This spectrum displayed two quartet resonances between 4.10 and 4.40 ppm. This indicated that there were two products in the reaction mixture. Furthermore, a large number of multiplet resonances in the aliphatic region were observed, suggesting that a saturated decalin had indeed formed. The presence of a number of new resonances in the aromatic region made us believe that the second product was the result of catalytic dehydrogenation. This was supported by an analysis of the low resolution mass spectrum of the crude product. Major peaks were observed with m/z ratios of 205 and 233 corresponding to the aromatic product plus hydrogen and the saturated product plus sodium.

Scheme 3.31: Catalytic hydrogenation and dehydrogenation of 3.41. Reagents and conditions: (a) Proton Sponge®, BHT, xylenes, reflux, 48 h; (b) Pd/C, MeOH, rt, 12 h Performing the hydrogenation reaction in ethyl acetate returned a nearly identical mix of products, as such we decided that performing reaction using batch chemistry was not worth pursuing. Instead, we opted to use flow chemistry, performed on an H-Cube® Continuous-flow Hydrogenation Reactor. Flow hydrogenation presents a number of advantages when compared with traditional batch chemistry. The hydrogen is produced by electrolysis of water, meaning that no hydrogen cylinders or balloons are necessary. The reaction conditions are easily optimised as the temperature, pressure and flow rate are manipulated via a graphical user interface and a number of different heterogeneous catalysts can be installed in the flow reactor.

A trial reaction on the H-Cube® was conducted (Scheme 3.32). Using palladium on carbon as the catalyst, diene 3.41 was run through the flow reactor at 0.5 mL/min at a temperature of 30 oC and 30 mbar hydrogen pressure. After one cycle through the H-Cube® under these conditions, a

John Reed - January 2017 83 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries significant amount of starting material was recovered, but pleasingly, a single new product was also detected. After passing the crude reaction mix through the H-Cube® for a second time, all starting material was consumed. Analysis of the 1H NMR spectrum of the product indicated that this was the fully saturated decalin 3.56. The stereochemistry at the bridgehead was established using nuclear Overhauser spectroscopy. A strong through-space correlation was detected between the two bridgehead protons indicating that the product was the cis-fused decalin.

Scheme 3.32: Hydrogenation of diene 3.41 on the H-Cube® flow reactor gives decalin 3.56 as a single diastereomer A series of reactions were run on the H-Cube® to determine the optimal conditions for the hydrogenation (Table 3.3). For the initial catalyst screening, the reaction was performed in methanol at 50 oC and 50 mbar at a flow rate of 0.5 mL/min. Several catalysts were tested, with rhodium on carbon giving the best conversion. Increasing the pressure to 60 mbar failed to increase the conversion, however, setting the temperature to 60 oC led to complete consumption of the starting material. Other solvents were also screened with ethanol giving comparable results, however, non-protic solvents resulted in slightly lower conversion.

84 John Reed - January 2017 Chapter 3: Accessing Functionalised Decalins Using the Diene-Regenerative Diels-Alder Reaction

Table 3.3: Optimisation of the hydrogenation of 3.41 on the H-Cube® flow reactor

Catalyst Temperature Pressure Flow Rate Solvent Conversion (oC) (mbar) (mL/min) (%) Pd/C 50 50 0.5 MeOH 86

Pt/C 50 50 0.5 MeOH 81

PtO2 50 50 0.5 MeOH 87

Rh/C 50 50 0.5 MeOH 94

Ru/C 50 50 0.5 MeOH 76

RaNi 50 50 0.5 MeOH 38

Rh/C 50 60 0.5 MeOH 94

Rh/C 60 50 0.5 MeOH 100

Rh/C 60 50 0.5 EtOH 100

Rh/C 60 50 0.5 EtOAc 73

Rh/C 60 50 0.5 MeCN 59

Having optimised the conditions for the flow hydrogenation of 3.41, the convenience of this process was further highlighted by increasing the scale of the reaction to 1 mmol of product with no discernible decrease in yield observed.

3.6 Summary Having developed an efficient method to synthesise functionalised, unsaturated decalin based compounds using the diene-regenerative Diels-Alder reaction, various reactions were employed to add further functionality to these compounds. These include an intermolecular Diels-Alder reaction, oxidation reactions and hydrogenation. As such, this protocol gives access to a highly diverse range of molecules. Furthermore, the stereoselectivity of these reactions was highly predictable, meaning that the dihydroaromatic compounds such as 3.41 can be used as intermediates when planning syntheses of more complex molecules.

John Reed - January 2017 85 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

3.7 Chapter 3 References (1) Shintani, Y.; Hayashi, K.; Nozaki, Y. Jpn. Kokai Tokkyo Koho 1997, 15. (2) Hamel, E. Med. Res. Rev. 1996, 16 (2), 207–231. (3) Cragg. G. M.; Newman, D. J. J. Nat. Prod. 2003, 67 (2), 232-244. (4) Islam, M.; Iskander, M. Mini-Reviews Med. Chem. 2004, 4 (10), 1077–1104. (5) Kingston, D. G. I. J. Nat. Prod. 2009, 72 (3), 507-515. (6) Jung, H. J.; Shim, J. S.; Lee, H. B.; Kim, C.-J.; Kuwano, T.; Ono, M.; Kwon, H. J. Biochem. Biophys. Res. Commun. 2007, 353 (2), 376–380. (7) Chekler, E. L. P.; Kiselyov, A. S.; Ouyang, X.; Chen, X.; Pattaropong, V.; Wang, Y.; Tuma, M. C.; Doody, J. F. ACS Med. Chem. Lett. 2010, 1 (9), 488-492. (8) Varghese, H.; Mackenzie, L.; Groom, A.; Ellis, C.; Ryan, A.; Macdonald, I.; Chambers, A. Angiogenesis 2004, 7 (2), 157–164. (9) Breinholt, J.; Kjœr, A.; Olsen, C. E.; Rassing, B. R.; Rosendahl, C. N. Acta Chem. Scand. 1997, 51, 1241–1244. (10) Shiono, Y.; Seino, Y.; Koseki, T.; Murayama, T.; Kimura, K. Zeitschrift für Naturforsch. B 2008, 63 (7), 909–914. (11) Welsch, M. E.; Snyder, S. A.; Stockwell, B. R. Curr. Opin. Chem. Biol. 2010, 14 (3), 347. (12) Kim, J.; Kim, H.; Park, S. B. J. Am. Chem. Soc. 2014, 136 (42), 14629–14638. (13) Li, G.; Kusari, S.; Spiteller, M.; Stocking, E. M.; Williams, R. M. Nat. Prod. Rep. 2014, 31 (9), 1175–1201. (14) Röttger, S.; Waldmann, H. European J. Org. Chem. 2006, 9, 2093–2099. (15) Kumar, K.; Wetzel, S.; Waldmann, H. Practice of Medicinal Chemistry; 2008; 187–209. (16) Yoshida, M.; Hedberg, C.; Kaiser, M.; Waldmann, H. Chem. Commun. 2009, 34 (20), 2926- 2928. (17) Peña, J.; Silveira-Dorta, G.; Moro, R.; Garrido, N.; Marcos, I.; Sanz, F.; Díez, D. Molecules 2015, 20 (4), 6409–6418. (18) Lavallée, J.-F.; Deslongchamps, P. Tetrahedron Lett. 1988, 29 (40), 5117–5118. (19) Kiriazis, A.; Boije af Gennäs, G.; Talman, V.; Ekokoski, E.; Ruotsalainen, T.; Kylänlahti, I.; Rüffer, T.; Wissel, G.; Xhaard, H.; Lang, H.; Tuominen, R. K.; Yli-Kauhaluoma, J. Tetrahedron 2011, 67 (45), 8665–8670. (20) Corey, E. J.; Watt, D. S. 2002. (21) Nelson, H. M.; Stoltz, B. M. Org. Lett. 2008, 10 (1), 25–28. (22) Zhao, Y.-M.; Maimone, T. J. Angew. Chemie Int. Ed. 2015, 54 (4), 1223–1226. (23) Martin, S. F.; Rueger, H.; Williamson, S. a.; Grzejszczak, S. J. Am. Chem. Soc. 1987, 109 (9), 6124–6134. (24) Gan, P.; Smith, M. W.; Braffman, N. R.; Snyder, S. A. Angew. Chem. Int. Ed. Engl. 2016, 55 (11), 3625–3630. (25) Ciganek, E. J. Am. Chem. Soc. 1981, 103 (20), 6261-6262. (26) Barton, T. J.; Kippenhan, R. C. K.; Nelson, J. J. Am. Chem. Soc. 1974, 96 (7), 2272-2273. (27) Barton, J. W.; Lee, D. V.; Shepherd, M. K. J. Chem. Soc. Perkin Trans. 1 1985, 1407.

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(28) Barton, J. W.; Howard, J. A. K.; Shepherd, M. K.; Stringer, A. M. J. Chem. Soc. Perkin Trans. 1 1987, No. 0, 2443. (29) Afarinkia, K.; Haji Abdullahi, M.; Scowen, I. J. Org. Lett. 2010, 12 (23), 5564–5566. (30) Komiyama, T.; Takaguchi, Y.; Tsuboi, S. Synth. Commun. 2007, 37 (13), 2131–2136. (31) Kranjc, K.; Perdih, F.; Kocevar, M. J. Org. Chem. 2009, 74 (16), 6303–6306. (32) Anastasia, L.; Xu, C.; Negishi, E. Tetrahedron Lett. 2002, 43 (32), 5673–5676. (33) Luo, T.; Dai, M.; Zheng, S.-L.; Schreiber, S. L. Org. Lett. 2011, 13 (11), 2834–2836. (34) Lundy, S. Synthetic Approaches to the Bicyclic Core of TEO3.1, Hamigerone and Embellistatin, University of Canterbury, 2007. (35) Dwyer, R. Honours Thesis, The University of New South Wales, 2012. (36) Schwartz, J.; Labinger, J. A. Angew. Chemie Int. Ed. English 1976, 15 (6), 333–340. (37) Bertelo, C. A.; Schwartz, J. J. Am. Chem. Soc. 1975, 97 (1), 228-230. (38) Hart, D. W.; Blackburn, T. F.; Schwartz, J. J. Am. Chem. Soc. 1975, 97 (3), 679-680. (39) Hart, D. W.; Schwartz, J. J. Am. Chem. Soc. 1974, 96 (26), 8115-8116. (40) Wipf, P.; Jahn, H. Tetrahedron 1996, 52 (40), 12853–12910. (41) Negishi, E.; Swanson, D. R.; Miller, S. R. Tetrahedron Lett. 1988, 29 (14), 1631–1634. (42) Wipf, P.; Kendall, C. Chem. - A Eur. J. 2002, 8 (8), 1778. (43) Wipf, P.; Xu, W. J. Org. Chem. 1993, 58 (4), 825–826. (44) Buchwald, S. L.; LaMaire, S. J.; Nielsen, R. B.; Watson, B. T.; King, S. M. Org. Synth. 1993, 71, 77. (45) Lipshutz, B. H.; Lindsley, C. J. Am. Chem. Soc. 1997, 119 (19), 4555-4556. (46) Maddess, M. L.; Lautens, M. Org. Lett. 2005, 7 (16), 3557-3560. (47) Wipf, P. In Metallocenes in Regio- and Stereoselective Synthesis, Springer Berlin Heidelberg, 2004, 1–25. (48) Zhao, Y.; Snieckus, V. Org. Lett. 2013, 16 (2), 390-393. (49) Carr, D. B.; Schwartz, J. J. Am. Chem. Soc. 1979, 101 (13), 3521-3531. (50) Negishi, E.; Miller, J. A.; Yoshida, T. Tetrahedron Lett. 1984, 25 (32), 3407–3410. (51) Lipshutz, B. H.; Keil, R.; EIIsworth, E. L. Tetrahedron Lett. 1990, 31 (50), 7257–7260. (52) Wailes, P. C.; Weigold, H.; Schwartz, J.; Jung, C. In Inorganic Syntheses XIX; John Wiley & Sons, Inc., 2007, 223–227. (53) Zhao, Y.; Snieckus, V. Synfacts 2014, 10 (04), 0412–0412. (54) Voigtritter, K.; Ghorai, S.; Lipshutz, B. H. J. Org. Chem. 2011, 76 (11), 4697–4702. (55) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 791. (56) Garber, S. B.; Kingsbury, J. S.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168. (57) Noguchi, M.; Kakimoto, S.; Kawakami, H.; Kajigaeshi, S. Heterocycles 1985, 23 (5), 1085. (58) Noguchi, M.; Kakimoto, S.; Kajigaeshi, S. Chem. Lett. 1985, No. 2, 151–154. (59) Clennan, E. L.; Pace, A. Tetrahedron 2005, 61 (28), 6665–6691.

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88 John Reed - January 2017

Chapter 4: Conclusions and Future Work 4 CONCLUSIONS AND FUTURE WORK

4.1 Conclusions This thesis describes the efforts taken to apply modern synthetic chemistry to the design and synthesis of novel small molecules based on biologically active natural products. The core ring systems of these natural products were employed as molecular scaffolds around which functionality was built. In all cases, these scaffolds were assembled rapidly from 2-pyrone based starting materials using the diene-regenerative Diels-Alder reaction, highlighting the utility of multi-bond forming processes in synthetic chemistry.

Chapter 2 focussed on the development of a library of tetrahydroisoquinolines. This motif is found in a large number of bioactive natural products, including the naphthylisoquinoline alkaloids, which have demonstrated promising anti-malarial activity. This research provided access to functionalised tetrahydroisoquinolines not previously accessible using existing methods such as the Bischler- Napieralski reaction and the Pictet-Spengler reaction (Figure 4.1). Importantly, this methodology was applicable to sterically and electronically diverse substrates.

Figure 4.1: Tetrahydroisoquinolines synthesised using the diene-regenerative Diels-Alder reaction Chapter 3 involved the synthesis of ten carbocyclic compounds. These were accessed in two steps from pyrones 3.22 and 3.31 using a Grubbs generation II catalysed olefin cross-metathesis reaction

John Reed - January 2017 89 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries followed by an intramolecular diene-regenerative Diels-Alder reaction. These bicyclic compounds bore structural resemblance to the natural products embellistatin, hamigerone and TEO3.1, which have received interest from researchers as a result of their anti-cancer activity. Furthermore, it was shown that the diene moiety on these compounds could be used as a synthetic handle to access even more chemical space. In this regards, compound 3.41 was used as a lynchpin compound to synthesise the intermolecular Diels-Alder adduct 3.51c, epoxide 3.52a, diol 3.53 and the cis-fused decalin 3.56 (Scheme 4.1). 3.41 could also be used to access endoperoxides, however, this reaction was not selective.

Scheme 4.1: Compounds accessed from the lynchpin 3.41

4.2 Future Work Since the diene-regenerative Diels-Alder reaction has proven adept at constructing a wide variety of tetrahydroisoquinolines, it stands to reason that it could be used in the total synthesis of naphthylisoquinoline alkaloids such as Ancistroealaines A (4.1) and B (4.2) (Scheme 4.2). Employing the same retrosynthetic strategy used in chapter 2, it is hypothesised that these natural products can be accessed from the known aryl halide 4.3, amine 4.4, accessible from D-alanine using standard functional group interconversions, and either pyrone 4.5 or 4.6. Applying these steps in the forward direction leads to a highly convergent synthesis of Ancistroealaine A. Diastereoselective reduction of this imine, enabled by the asymmetric induction provided by the existing stereocentre, would provide Ancistroealaine B. Similar reductions have been reported in the literature previously.1–3

90 John Reed - January 2017 Chapter 4: Conclusions and Future Work

OMe OMe

OMe OMe Sonogashira known Coupling compound OMe OMe Me 4.3 Br Me Diastereoselective Me reduction MeO Me NH2 Me 4.4 MeO Me N 4.1 from D-Ala NH OMe Me O 4.2 MeO MeO O O OMe Me O Reductive O OR O Diene-Regenerative Amination Diels-Alder Reaction OMe Me OMe Me 4.5 4.6 Scheme 4.2: Retrosynthetic analysis of the natural products Ancistroealaine A (4.1) and Ancistroealaine B (4.2) There remains an enormous cache of chemistry with which to manipulate bicyclic dienes such as 3.41. Further exploring such reactions would enable access to a wider variety of compounds from the one lynchpin. With the exception of endoperoxide 3.55b, which was obtained with poor selectivity and in a low yield, compound 3.41 has provided access only to cis-fused decalins. Discovery of methods to access trans-fused decalins would be especially useful (Scheme 4.3). Similar transformations have been reported in the context of steroidal chemistry by Caglioti et al., who used a hydroboration/elimination protocol, and by Burgstahler et al., who employed a dissolving metal reduction.4,5 Other useful transformations to explore would include the selective oxidation of the di-substituted alkene.

Scheme 4.3: Further manipulations to make on 3.41

John Reed - January 2017 91 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

4.3 Chapter 4 References (1) Hoye, T. R.; Chen, M.; Mi, L.; Priest, O. P. Tetrahedron Lett. 1994, 35 (47), 8747–8750. (2) Lipshutz, B. H.; Keith, J. M. Angew. Chem. Int. Ed. 1999, 38 (23), 3530–3533. (3) Davis, F. A.; Mohanty, P. K.; Burns, D. M.; Andemichael, Y. W. Org. Lett. 2000, 2 (24), 3901–3903. (4) Caglioti, L.; Cainelli, G.; Maina, G. Tetrahedron 1963, 19 (6), 1057–1060. (5) Burgstahler, A. W.; Marx, J. N.; Zinkel, D. F. J. Org. Chem. 1969, 34 (6), 1550–1561.

92 John Reed - January 2017 Chapter 5: Experimental 5 EXPERIMENTAL

5.1 General Experimental All reactions were conducted under an inert atmosphere of dry nitrogen at atmospheric pressure, unless otherwise stated. Reagents and solvents were purchased from commercial sources and used without further purification, unless stated below.

Tetrahydrofuran and xylenes (as a mix of isomers) were freshly distilled from sodium and benzophenone under an inert atmosphere of argon. N,N-Dimethylformamide was dried sequentially over three batches of 4Ǻ molecular sieves for 24 hours at a time, before being stored over a fourth batch, under argon. N,N-Dimethylamine was removed from DMF by evacuation at ~0.1 mm/Hg for at least 30 minutes prior to use. Methanol was distilled from magnesium and stored over 3Ǻ molecular sieves under argon. Dichloromethane, triethylamine and benzylamine were distilled from calcium hydride immediately prior to use.

Analytical thin layer chromatography was conducted on Merck, aluminium-backed silica plates 60-

F254. Flash chromatography was performed using Grace Davison Discovery Sciences, Davisil LC60A 40 – 63 micron silica gel. Solvent was eluted using a Thomson SINGLE StEP pump at the flow rate recommended by the manufacturer. Solvent systems for elution consisted of petroleum spirit (boiling point 40-60 oC) and ethyl acetate in varying ratios to adjust the polarity. Concentration of products was performed under reduced pressure on a rotary evaporator at 30-35 oC. Residual solvent was removed under high vacuum (~0.1 mm/Hg).

Melting points were obtained on an OptiMelt Automated Melting Point System with Digital Image Processing Technology and are uncorrected. 1H, 13C and two dimensional NMR spectra were recorded at the Nuclear Magnetic Resonance Facility within the Mark Wainwright Analytical Centre at the University of New South Wales on a Bruker Avance III 300 (300 MHz), Bruker Avance III 400 (400 MHz,), Bruker Avance III 500 (500 MHz) or Bruker Aance III 600 (600 MHz), with data acquired and processed using TopSpin software. Chemical Shifts are expressed in parts per million (PPM).

The spectra were calibrated relative to the solvent resonances from CDCl3: 7.26 for 1H NMR, and 77.16 for 13C NMR. Infrared spectra were obtained on an Agilent Cary 630 FTIR spectrometer, with resonances reported in wavenumbers (cm-1). Mass spectrometry was performed at the Bioanalytical Mass Spectrometry Facility within the Mark Wainwright Analytical Centre at the

John Reed - January 2017 93 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

University of New South Wales on an Orbitrap LTQ XL ion trap mass spectrometer using an electrospray ionization source.

5.2 Experiments Described in Chapter 2 6-(Chloromethyl)-4-methyl-2H-pyran-2-one (2.17)

A one-neck round-bottom flask, equipped with a reflux condenser and drying tube filled with CaCl2, was charged with aluminium trichloride (5.85 g, 43.9 mmol) and CH2Cl2 (50 mL). Ethyl 3-methylbut- 2-enoate (2.5 g, 19.5 mmol) and chloroacetyl chloride (2.20 g, 19.5 mmol) were successively added in portions at room temperature, and the resultant solution heated at reflux. After 3 h, the reaction mixture was cooled to 0 oC and quenched with 2 M HCl solution (15 mL). The two phases were separated and the aqueous layer extracted with CH2Cl2 (3 x 20 mL). The combined organic layers were washed with sat. aq. NaHCO3 solution (30 mL) and brine (30 mL), dried over Na2SO4 and conc. in vacuo to give a translucent brown oil. The crude mixture of isomeric esters was heated at 50 oC in glacial acetic acid (25 mL) and sulfuric acid (98 %, 5 mL) for 2 hours. The reaction mixture was cooled to 0 oC and neutralised carefully with solid NaHCO3 until universal indicator paper showed pH ~7. The aqueous solution was extracted with CH2Cl2 (3 x 30 mL) and the combined organic extracts were washed with brine (30 mL), dried over Na2SO4, and concentrated in vacuo. Purification by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided the title compound as an off-white solid (1.92 g, 62 %). All spectroscopic data were in agreement with those previously reported.1

Mp: 143-144 oC

1H NMR (300 MHz, CDCl3) δ 2.16 (s, 3H), 4.26 (s, 2H), 6.06 (s, 1H), 6.16 (s, 1H); 13C NMR (75

MHz, CDCl3) δ 21.6, 41.2, 107.6, 113.3, 155.6, 157.8, 161.7.

6-(Bromomethyl)-2H-pyran-2-one (2.41)

Propiolic acid (238 mg, 3.4 mmol), propargyl bromide (2.54 g, 80 % w/w soln. in PhMe, 17.1 mmol), chloro(triphenylphosphne)gold(I) (85 mg, 0.17 mmol), silver triflate (44 mg, 0.17 mmol), and CH2Cl2 (15 mL) were added successively to a 21 mL capacity screw-top vial, and the resultant mixture was stirred at room temperature for 16 hours. The solvent was removed in vacuo to give the crude

94 John Reed - January 2017 Chapter 5: Experimental product. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a white solid (469 mg, 73 %). All spectroscopic data were in agreement with those previously reported.2

Mp: 68-70 oC

1H NMR (400 MHz, CDCl3) δ 4.15 (s, 2H), 6.27 (s, 1H), 6.29 (s, 1H), 7.26-7.31 (m, 1H); 13C NMR

(100 MHz, CDCl3) δ 26.9, 105.0, 116.3, 143.0, 159.4, 161.3.

6-(Bromomethyl)-4-phenyl-2H-pyran-2-one (2.44)

Phenylpropiolic acid (0.5 g, 3.4 mmol), propargyl bromide (2.54 g, 80 % w/w soln. in PhMe, 17.1 mmol), chloro(triphenylphosphne)gold(I) (85 mg, 0.17 mmol), silver triflate (44 mg, 0.17 mmol), and

CH2Cl2 (15 mL) were added successively to a 21 mL capacity screw-top vial, and the resultant mixture was stirred at room temperature for 16 hours. The solvent was removed in vacuo to give the crude product. Purification of the crude product by flash chromatography, eluting with 10 % EtOAc/petroleum spirit, provided a white solid (730 mg, 81 %).

Mp: 121-122 oC

1H NMR (400 MHz, CDCl3) δ 4.23 (s, 2H), 6.47 (d, J = 1.6 Hz, 1H), 6.62 (d, J = 1.6 Hz, 1H), 7.42-

7.58 (m, 5H); 13C NMR (100 MHz, CDCl3) δ 27.1, 105.3, 111.0, 126.8, 129.5, 131.1, 135.2, 154.9,

158.7, 162.1; FTIR (cm-1) 2930, 2865, 1715 (C=O), 1583; HRMS (ESI) m/z calc. C12H10BrO2 [M+H]+ 264.9859, found 264.9858.

General procedure for the synthesis of secondary homopropargyl amines

A flame-dried flask was purged with nitrogen and charged with 3-butyn-1-ol (1.00 g, 14.3 mmol), triethylamine (2.98 mL, 2.17 g, 21.4 mmol) and CH2Cl2 (50 mL). The solution was cooled to 0 oC, and methanesulfonyl chloride (1.96 g, 17.1 mmol) was added dropwise over 5 minutes. After 1 h, the reaction was quenched with sat. aq. NaHCO3 solution (30 mL) and extracted with CH2Cl2 (3 x

20 mL). The combined organic extracts were washed with brine (30 mL), dried over Na2SO4, and concentrated in vacuo. The crude mesylate was diluted with neat primary amine (5 eq.) and stirred at rt for 16 h. The mixture was diluted with EtOAc (20 mL), washed with sat. aq. NaHCO3 solution

(20 mL) and brine (20 mL) and dried over Na2SO4. The solvent and excess amine were removed in vacuo to provide the crude product.

John Reed - January 2017 95 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

N-Benzylbut-3-yn-1-amine (2.18a)

The title compound was synthesised from 3-butyn-1-ol and benzylamine according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a yellow oil (2.07 g, 91 %). All spectroscopic data were in agreement with those previously reported.3

1H NMR (300 MHz, CDCl3) δ 1.75 (bs, 1H), 2.00 (t, J = 2.7 Hz, 1H), 2.42 (dt, J = 2.6 Hz, J = 6.59

Hz, 2H), 2.81 (t, J = 6.6 Hz, 2H), 3.83 (s, 2H), 7.24-7.34 (m, 5H); 13C NMR (75 MHz, CDCl3) δ 19.7, 47.4, 53.5, 69.7, 82.6, 127.2, 128.3, 128.6, 140.1.

N-(But-3-yn-1-yl)-4-methoxyaniline (2.18b)

O Me

N H The title compound was synthesised from 3-butyn-1-ol and p-anisidine according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a brown solid 1.90 g, 76 %).

1H NMR (400 MHz, CDCl3) δ 2.04 (t, J = 2.7 Hz, 1H), 2.49 (dt, J = 2.7 Hz, J = 6.6 Hz, 2H), 3.28 (t,

J = 6.6 Hz, 2H), 3.75 (s, 3H), 6.61-6.64 (m, 2H), 6.77-6.81 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 19.3, 43.7, 55.9, 70.1, 82.0, 114.9, 115.1, 141.9, 152.6; FTIR (cm-1) 3379 (N-H), 3282 (alkyne C-

H), 2933, 2830, 1617 (N-H), 1508; HRMS (ESI) m/z calc. C11H14NO [M+H]+ 176.1070, found 176.1068.

N-(Furan-2-ylmethyl)but-3-yn-1-amine (2.18c)

The title compound was synthesised from 3-butyn-1-ol and furfurylamine according to the general procedure described above. Purification of the crude product by bulb-to-bulb distillation (15 mmHg, 65 oC) provided a light yellow oil (1.90 g, 89 %).

1H NMR (400 MHz, CDCl3) δ 1.99 (t, J = 2.7 Hz, 1H), 2.39 (dt, J = 2.7 Hz, J = 6.6 Hz, 2H), 2.79 (t,

J = 6.6 Hz, 2H), 3.81 (s, 2H), 6.19 (m, 1H), 6.31 (m, 1H), 7.36 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 19.6, 45.9, 47.3, 69.7, 82.5, 107.1, 110.2, 142.0, 143.8; FTIR (cm-1) 3292 (alkyne C-H), 2915,

96 John Reed - January 2017 Chapter 5: Experimental

2834, 2115, 1672, 1503, 1457, 1337; HRMS (ESI) m/z calc. C9H12NO [M+H]+ 150.0913, found 150.0910.

N-(But-3-yn-1-yl)cyclopentylamine (2.18d)

The title compound was synthesised from 3-butyn-1-ol and cyclopentylamine according to the general procedure described above. Purification of the crude product by bulb-to-bulb distillation (15 mmHg, 65 oC) provided a light yellow oil (1.61 g, 82 %).

1H NMR (400 MHz, CDCl3) δ 1.29-1.33 (m, 2H), 1.50-1.60 (m, 2H), 1.66-1.73 (m, 2H), 1.82-1.88 (m, 2H), 1.98 (t, J = 2.6 Hz, 1H), 2.38 (dt, J = 2.6 Hz, J = 6.7 Hz, 2H), 2.76 (t, J = 6.7 Hz, 2H), 3.08

(p, J = 6.8 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 19.9, 24.2, 33.3, 46.9, 59.4, 69.5, 82.8; FTIR

(cm-1) 3304 (alkyne C-H), 2949, 2863, 2116, 1708, 1453, 1347; HRMS (ESI) m/z calc. C9H16N [M+H]+ 138.1277, found 138.1274.

General procedure for alkylation of secondary homopropargyl amines with 6- (chloromethyl)-4-methyl-2H-pyran-2-one

In a 40 mL capacity screw-top vial, sodium iodide (227 mg, 1.51 mmol) and potassium carbonate (1.26 g, 9.08 mmol) were successively added to a solution of 6-(chloromethyl)-4-methyl-2H-pyran- 2-one 2.17 (1.20 g, 7.57 mmol) in EtOH (23 mL) at room temperature. The secondary amine (1.2 eq, 9.08 mmol) was added in one portion, and the resultant mixture was warmed to 40 oC. After 16 h, the reaction mixture was cooled to room temperature, diluted with sat. aq. NaHCO3 solution (10 mL) and extracted with EtOAc (3 x 50 mL). The combined organic extracts were washed with brine

(20 mL), dried over Na2SO4, and concentrated in vacuo to provide the crude product.

6-((Benzyl(but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran-2-one (2.19)

The title compound was synthesised from compound 2.18a (1.45 g) and 6-(chloromethyl)-4-methyl- 2H-pyran-2-one (2.17) according to the general procedure described above. Purification of the

John Reed - January 2017 97 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous yellow oil (1.98 g, 93 %).

1H NMR (300 MHz, CDCl3) δ 1.99 (t, J = 2.7 Hz, 1H), 2.13 (d, J = 1.1 Hz, 3H), 2.40 (dt, J = 2.7 Hz, J = 7.1 Hz, 2H), 2.81 (t, J = 7.1 Hz, 2H), 3.49 (s, 2H), 3.73 (s, 2H), 5.96 (s, 1H), 6.20 (d, J = 1.2 Hz,

1H), 7.27-7.37 (m, 5H); 13C NMR (75 MHz, CDCl3) δ 17.7, 21.7, 52.9, 54.9, 58.5, 69.6, 82.8, 106.6, 111.5, 127.5, 128.6, 128.8, 138.5, 156.1, 162.4, 163.0; FTIR (cm-1) 3294 (alkyne C-H), 3027, 2916,

2834, 1722 (C=O), 1644, 1561, 1493; HRMS (ESI) m/z calc. C18H20NO2 [M+H]+ 282.1489, found 282.1482.

6-((But-3-yn-1-yl(4-methoxyphenyl)amino)methyl)-4-methyl-2H-pyran-2-one (2.20)

The title compound was synthesised from compound 2.18b (1.59 g) and 6-(chloromethyl)-4-methyl- 2H-pyran-2-one (2.17) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous dark-yellow oil (1.96 g, 87 %).

1H NMR (400 MHz, CDCl3) δ 2.01 (t, J = 2.7 Hz, 1H), 2.08 (d, J = 1.1 Hz, 3H), 2.48 (dt, J = 2.7 Hz, J = 7.2 Hz, 2H), 3.57 (t, J = 7.2 Hz, 2H), 3.76 (s, 3H), 4.27 (s, 2H), 5.96 (m, 2H), 6.66 (m, 2H), 6.83

(m, 2H); 13C NMR (100 MHz, CDCl3) δ 17.5, 21.8, 51.4, 53.5, 55.8, 70.2, 81.9, 105.2, 111.4, 114.6, 115.2, 141.2, 152.7, 156.3, 161.6, 162.8; FTIR (cm-1) 3278 (alkyne C-H), 2932, 1718 (C=O), 1644,

1561, 1510, 1440; HRMS (ESI) m/z calc. C18H19NNaO3 [M+Na]+ 320.1257, found 320.1254.

6-((But-3-yn-1-yl(furan-2-ylmethyl)amino)methyl)-4-methyl-2H-pyran-2-one (2.21)

The title compound was synthesised from compound 2.18c (1.35 g) and 6-(chloromethyl)-4-methyl- 2H-pyran-2-one (2.17) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous yellow oil (1.66 g, 81 %).

1H NMR (400 MHz, CDCl3) δ 1.99 (t, J = 2.7 Hz, 1H), 2.14 (d, J = 1.1 Hz, 3H), 2.38 (dt, J = 2.7 Hz, J = 7.2 Hz, 2H), 2.79 (t, J = 7.2 Hz, 2H), 3.53 (s, 2H), 3.78 (s, 2H), 5.97 (s, 1H), 6.22 (d, J = 3.3 Hz,

98 John Reed - January 2017 Chapter 5: Experimental

1H), 6.24 (d, J = 1.2 Hz, 1H), 6.31 (m, 1H), 7.37 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 17.9, 21.7, 50.3, 52.7, 55.0, 69.6, 82.6, 106.4, 109.2, 110.4, 111.5, 142.4, 151.6, 156.2, 162.3, 163.0; FTIR (cm-1) 3290 (alkyne C-H), 2916, 2834, 2115, 1716 (C=O), 1643, 1560, 1437; HRMS (ESI) m/z calc.

C16H18NO3 [M+H]+ 272.1281, found 272.1278.

6-((But-3-yn-1-yl(cyclopentyl)amino)methyl)-4-methyl-2H-pyran-2-one (2.22)

The title compound was synthesised from compound 2.18d (1.25 g) and 6-(chloromethyl)-4-methyl- 2H-pyran-2-one (2.17) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous yellow oil (1.79 g, 91 %).

1H NMR (400 MHz, CDCl3) δ 1.34-1.41 (m, 2H), 1.49-1.58 (m, 2H), 1.61-1.68 (m, 2H), 1.76-1.85 (m, 2H), 1.96 (t, J = 2.7 Hz, 1H), 2.14 (d, J = 1.1 Hz, 3H), 2.31 (dt, J = 2.7 Hz, J = 7.3 Hz, 2H), 2.81 (t, J = 7.3 Hz, 2H), 3.15 (m, 1H), 3.48 (s, 2H), 3.78 (s, 2H), 5.95 (s, 1H), 6.24 (d, J = 1.2 Hz, 1H);

13C NMR (100 MHz, CDCl3) δ 17.9, 21.7, 24.2, 29.9, 52.1, 53.1, 64.1, 69.3, 83.0, 106.1, 111.1, 156.4, 163.1, 164.2; FTIR (cm-1) 3271 (alkyne C-H), 2949, 2865, 2114, 1720 (C=O), 1642, 1560;

HRMS (ESI) m/z calc. C16H22NO2 [M+H]+ 260.1645, found 260.1641.

6-((Benzyl(but-3-yn-1-yl)amino)methyl)-2H-pyran-2-one (2.42)

In a 4 mL capacity screw-top vial, sodium iodide (14 mg, 93.4 µmol) and potassium carbonate (197 mg, 1.43 mmol) were successively added to a solution of 6-(bromomethyl)-2H-pyran-2-one (2.41) (180 mg, 952 µmol) in EtOH (3 mL) at room temperature. N-Benzylbut-3-yn-1-amine (2.18a) (182 mg, 1.14 mmol) was added in one portion, and the resultant mixture was warmed to 40 oC. After

16 h, the reaction mixture was cooled to room temperature, diluted with sat. aq. NaHCO3 solution (10 mL) and extracted with EtOAc (3 x 20 mL). The combined organic extracts were washed with brine (20 mL), dried over Na2SO4, and conc. in vacuo. Purification of the crude product, eluting with 30 % EtOAc/petroleum spirit, provided a yellow oil (195 mg, 77 %).

1H NMR (400 MHz, CDCl3) δ 1.99 (t, J = 2.7 Hz, 1H), 2.40 (dt, J = 2.7 Hz, J = 7.1 Hz, 2H), 2.81 (t, J = 7.1 Hz, 2H), 3.51 (s, 2H), 3.73 (s, 2H), 6.17 (d, J = 9.4 Hz, 1H), 6.35 (d, J = 6.6 Hz,1H), 7.25-

John Reed - January 2017 99 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

7.37 (m, 6H); 13C NMR (100 MHz, CDCl3) δ 17.7, 52.8, 55.0, 58.5, 69.6, 82.6, 103.5, 114.1, 127.5, 128.6, 128.8, 138.5, 143.6, 162.5, 164.2; FTIR (cm-1) 3292 (alkyne C-H), 2926, 2116, 1717 (C=O),

1452; HRMS (ESI) m/z calc. C17H18NO2 [M+H]+ 268.1332, found 268.1337.

6-((Benzyl(but-3-yn-1-yl)amino)methyl)-4-phenyl-2H-pyran-2-one (2.45)

In a 20 mL capacity screw-top vial, sodium iodide (28 mg, 189 µmol) and potassium carbonate (391 mg, 2.83 mmol) were successively added to a solution of 6-(bromomethyl)-4-phenyl-2H-pyran- 2-one (2.44) (500 mg, 1.89 mmol) in EtOH (5 mL) at room temperature. N-Benzylbut-3-yn-1-amine (2.18a) (360 mg, 2.26 mmol) was added in one portion, and the resultant mixture was warmed to 40 oC. After 16 h, the reaction mixture was cooled to room temperature, diluted with sat. aq.

NaHCO3 solution (10 mL) and extracted with EtOAc (3 x 20 mL). The combined organic extracts were washed with brine (20 mL), dried over Na2SO4, and conc. in vacuo. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a pale yellow oil (621 mg, 96 %).

1H NMR (300 MHz, CDCl3) δ 1.98 (t, J = 2.7 Hz, 1H), 2.45 (dt, J = 2.7 Hz, J = 7.0 Hz, 2H), 2.86 (t, J = 7.0 Hz, 2H), 3.57 (s, 2H), 3.78 (s, 2H), 6.37 (d, J = 1.7 Hz, 1H), 6.74 (t, J = 1.6 Hz, 1H), 7.24-

7.64 (m, 10H); 13C NMR (75 MHz, CDCl3) δ 17.8, 53.0, 55.2, 58.5, 69.7, 82.8, 103.6, 109.0, 126.9, 127.6, 128.6, 128.8, 129.3, 130.8, 136.0, 138.5, 155.4, 163.3, 163.5; FTIR (cm-1) 3296 (alkyne C-

H), 3057, 2915, 2821, 2114, 1709 (C=O), 1636, 1544; HRMS (ESI) m/z calc. C23H22NO2 [M+H]+ 344.1645, found 344.1640.

General procedure for the Sonogashira coupling of alkynyl-pyrones with aryl halides

To a flame dried Young’s tube, charged with nitrogen, was added Pd(PPh3)2Cl2 (8.77 mg, 12.5 µmol) and CuI (476 µg, 2.50 µmol). The flask was evacuated and refilled with nitrogen before a solution of aryl halide (250 µmol), alkynyl-pyrone (300 µmol), and freshly distilled pyrrolidine (205 µL, 178 mg, 2.50 mmol) in anhydrous DMF (1 mL) was added under a positive flow of nitrogen.

100 John Reed - January 2017 Chapter 5: Experimental

The reaction mixture was degassed by three freeze-pump-thaw cycles, refilled with nitrogen and left to stir at room temperature. The reaction was quenched with sat. aq. NH4Cl solution (5 mL) and extracted with EtOAc (3 x 10 mL). The combined organic extracts were washed with H2O (20 mL) and brine (20 mL), dried over Na2SO4, and conc. in vacuo to give the crude product.

6-((Benzyl(4-(pyridin-4-yl)but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran-2-one (2.23)

The title compound was synthesised from compound 2.19 (84 mg) and 4-iodopyridine (51 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a yellow oil (72 mg, 81 %).

1H NMR (400 MHz, CDCl3) δ 2.07 (d, J = 1.1 Hz, 3H), 2.64 (t, J = 7.0 Hz, 2H), 2.89 (t, J = 7.0 Hz, 2H), 3.52 (s, 2H), 3.77 (s, 2H), 5.96 (s, 1H), 6.18 (d, J = 1.1 Hz, 1H), 7.23-7.40 (m, 7H), 8.54 (d, J

= 4.0 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 18.8, 21.7, 52.6, 55.0, 58.6, 79.6, 93.9, 106.6, 111.6, 125.9, 127.7, 128.6, 128.8, 132.1, 138.5, 149.8, 156.1, 162.3, 162.9; FTIR (cm-1) 3026, 2914, 2821,

2223, 1719 (C=O), 1643, 1560, 1491; HRMS (ESI) m/z calc. C23H23N2O2 [M+H]+ 359.1754, found 359.1754.

Methyl 4-(4-(benzyl((4-methyl-2-oxo-2H-pyran-6-yl)methyl)amino)but-1-yn-1-yl)benzoate (2.29)

The title compound was synthesised from compound 2.19 (84 mg) and methyl 4-iodobenzoate (66 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 10 % EtOAc/petroleum spirit, provided a viscous yellow oil (79 mg, 76 %).

1H NMR (300 MHz, CDCl3) δ 2.05 (d, J = 1.1 Hz, 3H), 2.64 (t, J = 7.0 Hz, 2H), 2.89 (t, J = 6.9 Hz, 2H), 3.52 (s, 1H), 3.77 (s, 1H), 3.91 (s, 1H), 5.94 (s, 1H), 6.23 (d, J = 1.3 Hz, 1H), 7.28-7.41 (m,

John Reed - January 2017 101 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

5H), 7.44 (d, J = 8.6 Hz, 2H), 7.97 (d, J = 8.6 Hz, 2H); 13C NMR (75 MHz, CDCl3) δ 18.8, 21.7, 523, 52.8, 55.0, 58.6, 81.3, 91.6, 106.6, 111.5, 127.5, 128.6, 128.8, 129.3, 129.6, 131.6, 138.6, 156.1, 162.5, 162.9, 166.7; FTIR (cm-1) 2950, 2836, 2236, 1721 (C=O), 1646, 1562, 1435; HRMS (ESI) m/z calc. C26H26NO4 [M+H]+ 416.1856, found 416.1847.

6-((Benzyl(4-(4-methoxyphenyl)but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran-2-one (2.32)

The title compound was synthesised from compound 2.19 (84 mg) and 4-iodoanisole (59 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a yellow oil (57 mg, 59 %).

1H NMR (600 MHz, CDCl3) δ 2.04 (d, J = 1.1 Hz, 3H), 2.61 (t, J = 7.0 Hz, 2H), 2.85 (t, J = 7.0 Hz, 2H), 3.52 (s, 2H), 3.77 (s, 2H), 3.80 (s, 3H), 5.94 (s, 1H), 6.28 (d, J = 1.2 Hz, 1H), 6.81-6.83 (m,

2H), 7.26-7.40 (m, 7H); 13C NMR (150 MHz, CDCl3) δ 18.6, 21.7, 53.1, 55.0, 55.4, 58.5, 81.6, 86.7, 106.5, 111.4, 114.0, 115.9, 127.4, 128.6, 128.8, 133.0, 138.7, 156.3, 159.3, 162.7, 163.1; FTIR

(cm-1) 2908, 2834, 2056, 1720, 1641, 1561, 1507, 1439; HRMS (ESI) m/z calc. C25H26NO3 [M+H]+ 388.1907, found 388.1902.

6-((Benzyl(4-(naphthalene-1-yl)but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran-2-one (2.50)

The title compound was synthesised from compound 2.19 (84 mg) and 1-iodonaphthalene (64 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided an off-white foam (98 mg, 80 %).

1H NMR (400 MHz, CDCl3) δ 1.87 (d, J = 1.1 Hz, 3H), 2.80 (t, J = 6.8 Hz, 2H), 2.97 (t, J = 6.8 Hz, 2H), 3.56 (s, 2H), 3.82 (s, 2H), 5.91 (s, 1H), 6.28 (d, J = 1.2 Hz, 1H), 7.21-7.86 (m, 11H), 8.36 (d,

J = 7.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 19.0, 21.4, 53.3, 54.9, 58.6, 79.9, 93.4, 106.5, 111.3,

102 John Reed - January 2017 Chapter 5: Experimental

121.5, 125.3, 126.3, 126.4, 126.7, 127.4, 128.3, 128.4, 128.6, 128.8, 130.2, 133.3, 133.6, 138.6, 156.3, 162.5, 163.0; FTIR (cm-1) 3437, 3055, 2815, 2222, 1719 (C=O), 1642, 1560; HRMS (ESI) m/z calc. C28H26NO2 [M+H]+ 408.1958, found 408.1961.

6-((Benzyl(4-(2,6-dimethoxyphenyl)but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran-2-one (2.53)

The title compound was synthesised from compound 2.19 (84 mg) and 2-iodo-1,3- dimethoxybenzene (67 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a yellow oil (115 mg, 92 %).

1H NMR (400 MHz, CDCl3) δ 2.02 (d, J = 1.1 Hz, 3H), 2.76 (t, J = 6.8 Hz, 2H), 2.92 (t, J = 6.8 Hz, 2H), 3.57 (s, 2H), 3.80 (s, 2H), 3.84 (s, 6H), 5.91 (s, 1H), 6.36 (d, J = 1.2 Hz, 1H), 6.52 (d, J = 8.5

Hz, 2H), 7.15-7.43 (m, 6H); 13C NMR (100 MHz, CDCl3) δ 19.4, 21.5, 53.1, 55.0, 56.1, 58.5, 73.9, 97.0, 101.8, 103.5, 106.4, 111.2, 127.3, 128.5, 128.8, 129.2, 138.9, 156.4, 161.6, 163.0, 163.1;

FTIR (cm-1) 2934, 2835, 1720 (C=O), 1642, 1560, 1471; HRMS (ESI) m/z calc. C26H28NO4 [M+H]+ 418.2013, found 418.2009.

6-(((4-Methoxyphenyl)(4-(4-methoxyphenyl)but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran- 2-one (2.34)

The title compound was synthesised from compound 2.20 (89 mg) and 4-iodoanisole (59 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a viscous yellow oil (47 mg, 47 %).

John Reed - January 2017 103 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

1H NMR (400 MHz, CDCl3) δ 2.03 (d, J = 1.1 Hz, 3H), 2.69 (t, J = 7.1 Hz, 2H), 3.64 (t, J = 7.1 Hz, 2H), 3.76 (d, 3H), 3.79 (s, 3H), 4.31 (s, 2H), 5.95 (s, 1H), 6.00 (d, J = 1.3 Hz, 1H), 6.68-6.72 (m,

2H), 6.79-6.85 (m, 4H), 7.27-7.30 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 18.6, 21.7, 51.7, 53.4, 55.4, 55.8, 82.2, 85.8, 105.2, 111.3, 114.0, 114.4, 115.1, 115.6, 133.0, 141.4, 152.5, 156.4, 159.4, 161.7, 162.8; FTIR (cm-1) 2906, 2833, 2054, 1723 (C=O), 1644, 1561, 1512; HRMS (ESI) m/z calc.

C25H26NO4 [M+H]+ 404.1856, found 404.1862.

6-(((Furan-2-ylmethyl)(4-(4-methoxyphenyl)but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran- 2-one (2.35)

The title compound was synthesised from compound 2.21 (81 mg) and 4-iodoanisole (59 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a viscous yellow oil (50 mg, 53 %).

1H NMR (400 MHz, CDCl3) δ 2.06 (s, 3H), 2.59 (t, J = 7.1 Hz, 2H), 2.85 (t, J = 7.1 Hz, 2H), 3.56 (s, 2H), 3.80 (s, 3H), 3.82 (s, 2H), 5.95 (s, 1H), 6.24-6.32 (m, 3H, CH), 6.81 (m, 2H), 7.31-7.38 (m,

3H); 13C NMR (100 MHz, CDCl3) δ 18.9, 21.7, 50.4, 53.0, 55.0, 55.4, 81.6, 86.5, 106.3, 109.1, 110.4, 111.3, 114.0, 115.9, 133.0, 142.4, 151.8, 156.4, 159.3, 162.5, 163.0; FTIR (cm-1) 3056,

2932, 2835, 2309, 2096, 1718 (C=O), 1643, 1603, 1561, 1508; HRMS (ESI) m/z calc. C23H24NO4 [M+Na]+ 400.1519, found 400.1521.

104 John Reed - January 2017 Chapter 5: Experimental

6-((Cyclopentyl(4-(pyridin-4-yl)but-3-yn-1-yl)amino)methyl)-4-methyl-2H-pyran-2-one (2.38)

The title compound was synthesised from compound 2.22 (78 mg) and 4-iodopyridine (51 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a viscous yellow oil (72 mg, 85 %).

1H NMR (400 MHz, CDCl3) δ 1.32-1.41 (m, 2H), 1.41-1.57 (m, 2H), 1.57-1.69 (m, 2H), 1.75-1.88 (m, 2H), 2.06 (s, 3H), 2.53 (t, J = 7.1 Hz, 2H), 2.88 (t, J = 7.1 Hz, 2H), 3.16 (p, J = 7.9 Hz, 1H), 3.50 (s, 2H), 5.92 (s, 1H), 6.23 (s, 1H), 7.20 (d, J = 4.7 Hz, 2H), 8.50 (d, J = 4.7 Hz, 2H); 13C NMR (100

MHz, CDCl3) δ 18.9, 21.7, 24.1, 29.9, 51.7, 53.1, 64.0, 79.3, 94.1, 106.0, 111.1, 125.8, 132.0, 149.7, 156.3, 163.0, 164.1; FTIR (cm-1) 3044, 2949, 2865, 2224, 1718 (C=O), 1640, 1559; HRMS

(ESI) m/z calc. C21H24N2NaO4 [M+Na]+ 359.1730, found 359.1724.

6-((Benzyl(4-(4-methoxyphenyl)but-3-yn-1-yl)amino)methyl)-2H-pyran-2-one (2.42)

The title compound was synthesised from compound 2.41 (80 mg) and 4-iodoanisole (59 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 10 % EtOAc/petroleum spirit, provided a viscous yellow oil (51 mg, 55 %).

1H NMR (400 MHz, CDCl3) δ 2.60 (t, J = 7.0 Hz, 2H), 2.87 (t, J = 7.0 Hz, 2H), 3.55 (s, 2H), 3.77 (s, 2H), 3.80 (s, 3H), 6.16 (dd, J = 0.9 Hz, J = 9.4 Hz, 1H), 6.42 (dd, J = 1.0 Hz, J = 6.6 Hz, 1H), 6.82

(m, 2H), 7.22-7.41 (m, 8H); 13C NMR (100 MHz, CDCl3) δ 18.7, 53.2, 55.1, 55.4, 58.5, 81.6, 86.7, 103.5, 114.0, 114.1, 115.9, 127.5, 128.6, 128.8, 133.0, 138.7, 143.7, 159.4, 162.6, 164.5; FTIR

(cm-1) 3028, 2928, 2834, 1731 (C=O), 1634, 1556; HRMS (ESI) m/z calc. C24H24NO3 [M+H]+ 374.1751, found 374.1745.

John Reed - January 2017 105 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

6-((Benzyl(4-(pyridin-4-yl)but-3-yn-1-yl)amino)methyl)-4-phenyl-2H-pyran-2-one (2.46)

The title compound was synthesised from compound 2.45 (103 mg) and 4-iodopyridine (51 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 40 % EtOAc/petroleum spirit, provided a viscous yellow oil (67 mg, 64 %).

1H NMR (400 MHz, CDCl3) δ 2.68 (t, J = 6.8 Hz, 2H), 2.95 (t, J = 6.9 Hz, 2H), 3.62 (s, 2H), 3.8 (s, 2H), 6.37 (d, J = 1.7 Hz, 1H), 6.71 (d, J = 1.6 Hz, 1H), 7.18 (d, J = 6.1 Hz, 2H), 7.26-7.54 (m, 10H),

8.49 (d, J = 6.1 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 18.9, 52.8, 55.3, 58.8, 79.6, 93.9, 103.6, 109.1, 125.9, 126.8, 127.6, 128.7, 128.8, 129.3, 130.8, 132.0, 135.9, 138.5, 149.8, 155.3, 163.2, 163.5; FTIR (cm-1) 3027, 2822, 2224, 1709 (C=O), 1636, 1590, 1543; HRMS (ESI) m/z calc.

C28H25N2O2 [M+H]+ 421.1911, found 421.1905.

6-((Benzyl(4-(4-methoxyphenyl)but-3-yn-1-yl)amino)methyl)-4-phenyl-2H-pyran-2-one (2.47)

The title compound was synthesised from compound 2.45 (103 mg) and 4-iodoanisole (59 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 10 % EtOAc/petroleum spirit, provided a viscous yellow oil (57 mg, 51 %).

1H NMR (400 MHz, CDCl3) δ 2.65 (t, J = 6.9 Hz, 2H), 2.92 (t, J = 6.9 Hz, 2H), 3.63 (s, 2H), 3.80 (s, 3H), 3.83 (s, 2H), 6.36 (d, J = 1.7 Hz, 1H), 6.76-6.80 (m, 3H, CH), 7.24-7.37 (m, 12H); 13C NMR

(100 MHz, CDCl3) δ 18.7, 53.2, 55.3, 55.4, 58.7, 81.7, 86.7, 103.5, 108.9, 114.0, 115.9, 126.8, 127.5, 128.6, 128.8, 129.2, 130.7, 133.0, 135.9, 138.7, 155.4, 159.3, 163.3, 163.8; FTIR (cm-1)

106 John Reed - January 2017 Chapter 5: Experimental

3055, 2931, 2834, 1711 (C=O), 1636, 1544, 1507; HRMS (ESI) m/z calc. C30H28NO3 [M+H]+ 450.2064, found 450.2055.

General procedure for the intramolecular diene-regenerative Diels-Alder reaction between alkynes and 2-pyrones

In a sealed microwave vial, a solution of alkynyl-pyrone (40 µmol), BHT (120 μmol), and Proton Sponge® (12 μmol) in ortho-dichlorobenzene (2 mL, 20 mM) was heated to 250 oC via microwave irradiation and held at this temperature for 4 h. After cooling to room temperature, the reaction mixture was eluted through a plug of silica gel eluting first with n-hexane to remove the solvent, then with ethyl acetate to elute the crude product. This was further purified by flash chromatography.

2-Benzyl-7-methyl-5-(pyridin-4-yl)-1,2,3,4-tetrahydroisoquinoline (2.27)

The title compound was synthesised from compound 2.23 (14 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a viscous yellow oil (9.2 mg, 73 %).

1H NMR (400 MHz, CDCl3) δ 2.31 (s, 3H), 2.64-2.72 (m, 4H), 3.65-3.69 (m, 4H), 6.87-6.89 (m, 2H),

7.21-7.40 (m, 7H), 8.60 (d, J = 6.0 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 21.0, 28.2, 50.9, 56.6, 63.0, 124.3, 127.3, 127.7, 128.1, 128.5, 128.8, 129.2, 135.6, 135.7, 138.3, 139.1, 149.67, 149.72;

FTIR (cm-1) 3024, 2917, 2796, 1939, 1647, 1594; HRMS (ESI) m/z calc. C22H23N2 [M+H]+ 315.1856, found 315.1846.

John Reed - January 2017 107 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Methyl 4-(2-benzyl-7-methyl-1,2,3,4-tetrahydroisoquinolin-5-yl)benzoate (2.30)

The title compound was synthesised from compound 2.29 (17 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous yellow oil (10 mg, 71 %).

1H NMR (600 MHz, CDCl3) δ 2.31 (s, 3H), 2.63-2.70 (m, 4H), 3.66-3.69 (m, 4H), 3.94 (s, 3H), 6.86

(s, 1H), 6.89 (s, 1H), 7.26-7.41 (m, 7H), 8.05 (d, J = 8.3 Hz, 2H); 13C NMR (150 MHz, CDCl3) δ 21.0, 28.2, 29.8, 45.4, 51.0, 52.2, 56.6, 62.9, 127.2, 127.3, 128.4, 128.8, 129.5, 129.28, 129.32, 129.5, 135.4, 140.9, 146.6, 167.2; FTIR (cm-1) 2923, 2852, 1722 (C=O), 1607; HRMS (ESI) m/z calc. C25H26NO2 [M+H]+ 372.1958, found 372.1957.

2-Benzyl-5-(4-methoxyphenyl)-7-methyl-1,2,3,4-tetrahydroisoquinoline (2.33)

The title compound was synthesised from compound 2.32 (15 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous yellow oil (10 mg, 74 %).

1H NMR (400 MHz, CDCl3) δ 2.30 (s, 3H), 2.65-2.72 (m, 4H), 3.65-3.68 (m, 4H), 3.84 (s, 3H), 6.81 (s, 1H), 6.89 (d, 1H), 6.90-6.94 (m, 2H), 7.20-7.25 (m, 2H), 7.26-7.41 (m, 5H); 13C NMR (100 MHz,

CDCl3) δ 20.9, 28.3, 51.1, 55.3, 56.6, 62.9, 113.5, 126.2, 127.1, 128.3, 128.7, 129.1, 129.2, 130.2, 134.0, 135.0, 135.1, 138.4, 141.4, 158.5; FTIR (cm-1) 3028, 2928, 2834, 1646, 1605; HRMS (ESI) m/z calc. C24H26NO [M+H]+ 344.2009, found 344.2005.

108 John Reed - January 2017 Chapter 5: Experimental

2-Benzyl-7-methyl-5-(naphthalene-1-yl)-1,2,3,4-tetrahydroisoquinoline (2.52)

The title compound was synthesised from compound 2.50 (16 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous yellow oil (12 mg, 83 %).

1H NMR (600 MHz, CDCl3) δ 2.36 (s, 3H), 2.36-2.75 (m, 4H), 3.60-3.76 (m, 4H), 6.95 (s, 1H), 6.98

(d, 1H), 7.26-7.56 (m, 10H), 7.86-7.92 (m, 2H); 13C NMR (150 MHz, CDCl3) δ 21.1, 27.4, 51.0, 56.7, 62.9, 125.5, 125.8, 126.1, 126.3, 126.7, 126.8, 127.2, 127.5, 128.3, 128.4, 129.15, 129.21, 130.4, 132.1, 133.6, 134.9, 138.4, 139.5, 140.1; FTIR (cm-1) 3076, 2973, 2928, 1528; HRMS (ESI) m/z calc. C27H26N [M+H]+ 364.2060, found 364.2051.

2-Benzyl-5-(2,6-dimethoxyphenyl)-7-methyl-1,2,3,4-tetrahydroisoquinoline (2.54)

The title compound was synthesised from compound 2.53 (17 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, provided a viscous yellow oil (11 mg, 74 %).

1H NMR (400 MHz, CDCl3) δ 2.29 (s, 3H), 2.46 (t, J = 5.9 Hz, 2H), 2.66 (t, J = 6.0 Hz), 3.67 (s, 4H), 3.71 (s, 6H), 6.62 (s, 1H), 6.64 (s, 1H), 6.81 (d, J = 8.4 Hz, 2H), 7.25-7.41 (m, 6H); 13C NMR (100

MHz, CDCl3) δ 21.2, 26.8, 51.0, 56.0, 56.6, 62.8, 104.0, 118.6, 126.7, 127.1, 128.3, 128.7, 129.2, 129.4, 131.0, 134.1, 134.4, 134.5, 138.7, 157.9; FTIR (cm-1) 2928, 1646, 1577; HRMS (ESI) m/z calc. C25H28NO2 [M+H]+ 374.2115, found 374.2110.

John Reed - January 2017 109 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

2,5-Bis(4-methoxyphenyl)-7-methyl-1,2,3,4-tetrahydroisoquinoline (2.36)

The title compound was synthesised from compound 2.34 (16 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a viscous yellow oil (14 mg, 97 %).

1H NMR (600 MHz, CDCl3) δ 2.40 (s, 3H), 2.88 (t, J = 5.6 Hz, 2H), 3.37 (t, J = 5.8 Hz, 2H), 3.79 (s, 3H), 3.85 (s, 3H), 4.34 (s, 2H), 6.88 (m, 2H), 6.94-6.98 (m, 6H), 7.24-7.27 (m, 2H); 13C NMR (150

MHz, CDCl3) δ 21.1, 28.2, 48.7, 53.1, 55.4, 55.8, 113.7, 114.7, 117.9, 126.2, 128.9, 129.6, 130.4, 133.8, 135.0, 135.4, 141.5, 145.5, 153.5, 158.8; FTIR (cm-1) 2954, 1655, 1607, 1510, 1464; HRMS

(ESI) m/z calc. C24H26NO2 [M+H]+ 360.1958, found 360.1957.

2-(Furan-2-ylmethyl)-5-(4-methoxyphenyl)-7-methyl-1,2,3,4-tetrahydroisoquinoline (2.37)

The title compound was synthesised from compound 2.35 (15 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a viscous yellow oil (5.9 mg, 44 %).

1H NMR (600 MHz, CDCl3) δ 2.29 (s, 3H), 2.68-2.73 (m, 4H), 3.69 (s, 2H), 3.71 (s, 2H), 3.85 (s, 3H), 6.26 (d, J = 3.1 Hz, 1H), 6.33 (m, 1H), 6.82 (s, 1H), 6.87 (s, 1H), 6.92 (m, 2H), 7.20 (m, 2H),

7.39 (m, 1H); 13C NMR (150 MHz, CDCl3) δ 21.1, 28.2, 51.0, 54.7, 55.4, 56.1, 108.9, 110.2, 113.6, 126.3, 128.8, 129.1, 130.3, 134.1, 134.8, 135.1, 141.5, 142.4, 151.9, 158.6; FTIR (cm-1) 3041,

2931, 1607, 1553; HRMS (ESI) m/z calc. C22H24NO2 [M+H]+ 334.1802, found 334.1793.

110 John Reed - January 2017 Chapter 5: Experimental

2-Cyclopentyl-7-methyl-5-(pyridin-4-yl)-1,2,3,4-tetrahydroisoquinoline (2.39)

The title compound was synthesised from compound 2.38 (13 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a viscous yellow oil (9 mg, 77 %).

1H NMR (600 MHz, CDCl3) δ 1.51-1.74 (m, 6H), 1.93-1.99 (m, 2H) 2.32 (s, 3H), 2.66-2.70 (m, 5H), 3.72 (s, 2H), 6.86 (s, 1H), 6.92 (s, 1H), 7.22-7.26 (m, 2H), 8.60-8.62 (m, 2H); 13C NMR (150 MHz,

CDCl3) δ 21.0, 24.4, 28.2, 30.9, 50.0, 55.9, 67.1, 124.3, 127.8, 128.0, 128.7, 132.3, 135.5, 135.7,

139.0, 149.7; FTIR (cm-1) 2950, 2865, 1643, 1595; HRMS (ESI) m/z calc. C20H25N2 [M+H]+ 293.2012, found 293.2008.

2-Benzyl-5-(4-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline (2.43)

The title compound was synthesised from compound 2.42 (13 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 10 % EtOAc/petroleum spirit, provided a viscous yellow oil (10 mg, 76 %).

1H NMR (400 MHz, CDCl3) δ 2.66 (t, J = 5.8 Hz, 2H), 2.76 (t, J = 5.8 Hz, 2H), 3.68 (s, 2H), 3.71 (s, 2H), 3.84 (s, 3H), 6.93 (d, J = 8.8 Hz, 2H), 6.98 (d, J = 7.6 Hz, 1H), 7.05 (d, J = 7.5 Hz, 1H), 7.16,

(t, J = 7.6 Hz, 1H), 7.23 (d, J = 8.8 Hz, 2H), 7.26-7.42 (m, 5H); 13C NMR (100 MHz, CDCl3) δ 28.8, 51.1, 55.4, 56.8, 63.0, 113.6, 125.7, 125.8, 127.3, 127.9, 128.4, 129.3, 130.3, 132.4, 134.0, 135.3,

138.5, 141.7, 158.7; FTIR (cm-1) 3026, 2927, 2833, 1648, 1608; HRMS (ESI) m/z calc. C23H24NO [M+H]+ 330.1852, found 330.1847.

John Reed - January 2017 111 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

2-Benzyl-7-phenyl-5-(pyridin-4-yl)-1,2,3,4-tetrahydroisoquinoline (2.48)

The title compound was synthesised from compound 2.46 (13 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a viscous yellow oil (14 mg, 93 %).

1H NMR (400 MHz, CDCl3) δ 2.71-2.80 (m, 4H), 3.72 (s, 2H), 3.78 (s, 2H), 7.28-7.44 (m, 12H),

7.55-7.58 (m, 2H), 8.64-8.67 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 28.4, 50.9, 56.8, 63.0, 124.3, 125.7, 126.1, 127.1, 127.4, 127.5, 128.5, 128.9, 129.2, 131.0, 136.4, 138.3, 139.1, 140.5, 149.5,

149.9; FTIR (cm-1) 3026, 2919, 2800, 1594; HRMS (ESI) m/z calc. C27H24N2 [M+H]+ 377.2012, found 377.2007.

2-Benzyl-5-(40methoxyphenyl)-7-phenyl-1,2,3,4-tetrahydroisoquinoline (2.49)

The title compound was synthesised from compound 2.47 (13 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, provided a viscous yellow oil (11 mg, 68 %).

1H NMR (400 MHz, CDCl3) δ 2.70 (t, J = 5.7 Hz, 2H), 2.80 (t, J = 5.7 Hz, 2H), 3.71 (s, 2H), 3.77 (s, 2H), 3.85 (s, 3H), 6.95 (d, J = 4.4 Hz, 2H), 7.22-7.43 (m, 12H), 7.55-7.59 (m, 2H); 13C NMR (150

MHz, CDCl3) δ 28.6, 29.8, 31.8, 51.1, 55.5, 56.9, 63.0, 113.7, 116.1, 123.7, 124.2, 124.4, 126.8, 127.1, 127.2, 127.3, 128.5, 128.8, 129.3, 130.4, 131.7, 133.9, 135.8, 138.4, 138.7, 141.0, 142.1,

158.8; FTIR (cm-1) 3028, 2951, 1606, 1510; HRMS (ESI) m/z calc. C29H27NO [M+H]+ 406.2165, found 406.2163.

112 John Reed - January 2017 Chapter 5: Experimental

4-Methoxy-6-methyl-2H-pyran-2-one (2.56)

A mixture of 4-hydroxy-6-methyl-2H-pyran-2-one (1.00 g, 7.93 mmol), potassium carbonate (1.32 g, 9.55 mmol) and trimethylphosphate (1.94 mL, 16.6 mmol) was heated to 140 oC and stirred vigorously for 1 h. The mixture was cooled to room temperature, diluted with H2O (20 mL) and extracted with EtOAc (3 x 20 mL). The combined organic layers were washed with brine (30 mL), dried over Na2SO4 and concentrated in vacuo to give a yellow solid. The crude product was recrystallized from EtOAc/n-hexane to give the title compound as yellow crystals (933 mg, 84 %). All spectroscopic data were in agreement with those previously reported.4

Mp: 87-88 oC

1H NMR (300 MHz, CDCl3) δ 2.20 (s, 3H), 3.78 (s, 3H), 5.40 (d, J = 2.2 Hz, 1H), 5.77 (q, J = 0.9

Hz, J = 2.2 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 19.9, 55.9, 87.5, 100.5, 162.2, 165.1, 171.4.

3-Bromo-6-(bromomethyl)-4-methoxy-2H-pyran-2-one (2.57)

To a solution of 4-methoxy-6-methyl-2H-pyran-2-one (2.56) (500 mg, 3.57 mmol) in benzene (50 mL) was added in succession N-bromosuccinimide (1.46 g, 8.21 mmol) and azobisisobutyronitrile (58 mg, 357 μmol) at room temperature. The reaction mixture was then stirred and irradiated with visible light from a 7 W fluorescent lamp for 10 hours. The solvent was removed and the resultant residue partitioned between EtOAc and saturated aqueous NH4Cl. The organic layer was washed with brine, dried over Na2SO4 and concentrated in vacuo to give a pale yellow solid. The crude product was recrystallized from CH2Cl2 to give off white crystals (647 mg, 61 %). All spectroscopic data were in agreement with those previously reported.5

Mp: 162-163 oC

1H NMR (300 MHz, CDCl3) δ 4.02 (s, 3H), 4.18 (s, 2H), 6.37 (s, 1H); 13C NMR (75 MHz, CDCl3) δ 26.4, 57.7, 91.4, 95.1, 96.8, 159.2, 166.1.

John Reed - January 2017 113 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

6-((Benzyl(but-3-yn-1-yl)amino)methyl)-3-bromo-4-methoxy-2H-pyran-2-one (2.58)

To a solution of 3-bromo-6-(bromomethyl)-4-methoxy-2H-pyran-2-one (2.57) (500 mg, 1.68 mmol) in ethanol (17 mL) was added in succession K2CO3 (348 mg, 2.52 mmol), NaI (25 mg, 168 μmol) and N-benzylbut-3-yn-1-amine (2.18a) (321 mg, 2.01 mmol) at room temperature. The reaction mixture was heated to 40 oC and stirred for 22 h. After cooling to room temperature, the reaction mixture was diluted with saturated aqueous NaHCO3 solution (30 mL) and extracted with EtOAc (3 x 20 mL). The combined organic layers were washed with brine, dried over Na2SO4 and concentrated in vacuo. Purification of the crude product by flash chromatography, eluting with 70 % EtOAc/petroleum spirit, provided a viscous yellow oil (468 mg, 74 %).

1H NMR (600 MHz, CDCl3) δ 2.01 (t, J = 2.7 Hz, 1H), 2.43 (dt, J = 2.6 Hz, J = 6.8 Hz, 2H), 2.83 (t, J = 6.8 Hz, 2H), 3.52 (d, J = 1.0 Hz, 2H), 3.74 (s, 2H), 3.97 (s, 3H), 6.55 (t, J = 1.0 Hz, 1H), 6.27-

6.35 (m, 5H); 13C NMR (150 MHz, CDCl3) δ 17.8, 53.2, 55.1, 57.5, 58.7, 69.7, 82.8, 89.1, 94.8, 127.7, 128.7, 128.8, 138.1, 160.7, 164.7, 167.2; FTIR (cm-1) 3007, 1720 (C=O); HRMS (ESI) m/z calc. C18H19BrNO3 [M+H]+ 376.0543, found 376.0540.

6-((Benzyl(4-(trimethylsilyl)but-3-yn-1-yl)amino)methyl)-3-bromo-4-methoxy-2H-pyran-2- one (2.59)

A flame dried 2-neck round bottom flask was charged with freshly distilled diisopropylamine (41 μL, 292 μmol) and THF (2 mL). After cooling the solution to -78 oC, n-butyllithium (1.32 M in hexane, 201 μL, 266 μmol) was added dropwise. This solution was stirred for 30 minutes at -78 oC. This solution was then added dropwise via cannula transfer to a pre-cooled (-78 oC) solution of 6- ((benzyl(but-3-yn-1-yl)amino)methyl)-3-bromo-4-methoxy-2H-pyran-2-one (2.58) (100 mg, 266 μmol) in THF (5 mL). After stirring for 1 h, chlorotrimethylsilane (41 μL, 319 μmol) was added dropwise. The reaction mixture was left in the cooling bath, which was allowed to slowly warm to room temperature. After 10 h, the reaction was quenched with saturated aqueous NaHCO3 solution (20 mL). The product was extracted with EtOAc (3 x 10 mL) and the combined organic layers were washed with brine, dried over Na2SO4 and concentrated in vacuo. Purification of the crude product

114 John Reed - January 2017 Chapter 5: Experimental by flash chromatography, eluting with 50 % EtOAc/petroleum spirit, provided a pale yellow oil (52 mg, 44 %).

1H NMR (400 MHz, CDCl3) δ 0.14 (s, 9H), 2.45 (t, J = 7.0 Hz, 2H), 2.83 (t, J = 7.0 Hz, 2H), 3.54 (d, J = 0.8 Hz, 2H), 3.75 (s, 2H), 3.95 (s, 3H), 6.43 (t, J = 0.8 Hz, 1H), 7.27-7.36 (m, 5H); 13C NMR

(100 MHz, CDCl3) δ 0.20, 19.2, 25.3, 53.4, 55.2, 57.5, 59.0, 86.3, 89.1, 94.7, 105.0, 127.7, 128.7, 138.4, 160.7, 164.8, 167.0; FTIR (cm-1) 3027, 2953, 2826, 2170, 1712 (C=O); HRMS (ESI) m/z calc. C21H26BrNNaO3Si [M+Na]+ 470.0763, found 470.0703.

2-Benzyl-6-bromo-7-methoxy-5-(trimethylsilyl)-1,2,3,4-tetrahydroisoquinoline (2.55)

The title compound was synthesised from compound (2.59) according to the general procedure for the diene-regenerative Diels-Alder reaction (p. 107). Purification by flash chromatography, eluting with 30 % EtOAc/petroleum spirit, gave a yellow oil (5.8 mg, 36 %).

1H NMR (400 MHz, CDCl3) δ 0.49 (s, 9H), 2.68 (t, J = 5.9 Hz, 2H), 2.96 (t, J = 5.9 Hz, 2H), 3.58 (s,

2H), 3.65 (s, 2H), 3.81 (s, 3H), 6.51 (s, 1H), 7.27-7.42 (m, 5H); 13C NMR (100 MHz, CDCl3) δ 4.19, 31.8, 51.4, 56.4, 57.3, 63.0, 111.2, 119.2, 127.3, 128.5, 129.3, 134.0, 134.7, 138.3, 141.4, 153.3;

FTIR (cm-1) 3048, 2941, 2803, 1576; HRMS (ESI) m/z calc. C20H27BrNOSi [M+H]+ 404.1040, found 404.1041.

5.3 Experiments Described in Chapter 3 Ethyl (Z)-3-iodoacrylate (3.19)

A solution of ethyl propiolate (5.00 g, 51.0 mmol), sodium iodide (9.17 g, 61.2 mmol) in acetic acid (35 mL) was heated at reflux for 2 h. After cooling to room temperature, the reaction mixture was diluted with diethyl ether (70 mL) and washed with an aqueous NaOH solution (50 mL, 1 M). The aqueous layer was extracted with diethyl ether (3 x 30 mL) and the combined organic layers were sequentially washed with a saturated aqueous NaHCO3 solution (50 mL), an aqueous Na2S2O3 solution (50 mL, 2 M), and brine, before being dried over MgSO4 and concentrated in vacuo to provide a yellow liquid (11.5 g, quant.). All spectroscopic data were in agreement with those reported in the literature.6

John Reed - January 2017 115 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

1H NMR (300 MHz, CDCl3) δ 1.32 (t, J = 7.1 Hz, 3H), 4.25 (q, J = 7.1 Hz, 2H), 6.89 (d, J = 8.9 Hz,

1H), 7.43 (d, J = 8.9 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 14.2, 62.8, 94.6, 129.9, 164.6.

Ethyl (Z)-undeca-2-en-4,10-diynoate (3.20)

A flame dried Young’s tube under an atmosphere of N2 was charged with CuI (13.5 mg, 70.6 μmol) and Pd(PPh3)2Cl2 (99.2 mg, 141 μmol). 1,7-Octadiyne (1.50 g, 14.1 mmol), iodide 3.19 (3.19 g,

14.1 mmol) and Et3N (15 mL) were then added under a positive flow of N2. The reaction mixture was degassed by three freeze-pump-thaw cycles before being heated to 50 oC and stirred for 16 h. After cooling to room temperature, the reaction was quenched with a saturated aqueous NH4Cl solution (15 mL). The product was extracted with ethyl acetate (3 x 20 mL) before being washed with brine (30 mL), dried over Na2SO4, and concentrated in vacuo. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a pale yellow oil (2.53 g, 88 %).

1H NMR (400 MHz, CDCl3) δ 1.30 (t, J = 7.1 Hz, 3H), 1.64-1.76 (m, 2H), 1.95 (t, J = 2.7 Hz, 1 H), 2.23 (td, J = 2.7 Hz, J = 6.8 Hz, 2 H), 2.48 (td, J = 2.4 Hz, J = 6.6 Hz, 2 H), 4.22 (q, J = 7.1 Hz, 2

H), 6.03 (d, J = 11.4 Hz, 1H), 6.13 (dt, J = 2.4 Hz, J = 11.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 14.1, 16.3, 18.1, 24.7, 26.5, 62.1, 68.1, 78.4, 83.1, 93.2, 125.9, 129.0, 166.5; FTIR (cm-1) 2214,

1731 (C=O); HRMS (ESI) m/z calc. C13H17O2 [M+H]+ 205.1223, found 205.1227.

6-(Hex-5-yn-1-yl)-2H-pyran-2-one (3.18)

Using Negishi’s ZnBr2 protocol:

An aqueous solution of LiOH (1 M, 36 mL) was slowly added to a solution of ester 3.20 (2.50 g, 12.2 mmol) in MeOH:THF (1:1, 72 mL) at room temperature and stirred for 10 h. The reaction mixture was carefully acidified to pH ~3-4 with an aqueous HCl solution (1 M). The product was extracted with ethyl acetate (5 x 75 mL) and washed with brine (50 mL), dried over Na2SO4 and concentrated in vacuo. The crude acid thus obtained was immediately diluted in THF (245 mL). To this solution was added a solution of freshly sublimed ZnBr2 in THF (1 M, 2.44 mL) at room temperature followed by stirring for 48 h. The THF was removed in vacuo and the crude product

116 John Reed - January 2017 Chapter 5: Experimental purified by flash chromatography, eluting with 15 % ethyl acetate/petroleum spirit, giving a pale yellow oil (2.03 g, 94 %).

Using Schreiber’s gold(I) catalysed procedure:

To a solution of freshly distilled propiolic acid (70.0 mg, 1.00 mmol) and 1,7-octadiyine (531 mg,

5.00 mmol) in CH2Cl2 (4.4 mL) were added sequentially (PPh3)AuCl (24.7 mg, 50.0 μmol) and AgOTf (12.8 mg, 50.0 μmol) at room temperature. The reaction mixture was stirred at room temperature for 16 h. The CH2Cl2 was removed in vacuo and the crude product purified by flash chromatography, eluting with 15 % ethyl acetate/petroleum spirit, giving a pale yellow oil (100 mg, 57 %).

1H NMR (300 MHz, CDCl3) δ 1.52-1.63 (m, 2H), 1.73-1.85 (m, 2H), 1.95 (t, J = 2.7 Hz, 1H), 2.22 (td, J = 2.7 Hz, J = 7.0 Hz, 2H), 2.51 (t, J = 7.6 Hz, 2H), 5.98 (dd, J = 0.7 Hz, J = 6.6 Hz, 1H), 6.15

(d, J = 9.4 Hz, 1H), 7.25 (dd, J = 6.6 Hz, J = 9.4 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 18.2, 26.0, 27.7, 33.4, 68.9, 83.9, 102.9, 113.4, 143.8, 162.9, 166.2; FTIR (cm-1) 2937, 2871, 1707 (C=O),

1631; HRMS (ESI) m/z calc. C11H13O2 [M+H]+ 177.0910, found 177.0911.

Oct-1-en-7-yne (3.24)

To a solution of 5-hexen-1-ol (5.00 g, 49.9 mmol) in CH2Cl2 (240 mL) at 0 oC were added 4- toluenesulfonyl chloride (11.5 g, 60.4 mmol), 4-N,N-dimethylaminopyridine (671 mg, 5.49 mmol) and Et3N (15.4 g, 152 mmol). The reaction was allowed to warm to room temperature and stirred for 16 h. The reaction was quenched with an aqueous HCl solution (1 M, 200 mL) and the product extracted with CH2Cl2 (3 x 100 mL). The combined organic layers were washed with a saturated aqueous NaHCO3 solution (100 mL) and brine (100 mL) before being dried over Na2SO4 and concentrated in vacuo to give a yellow liquid that was immediately diluted in DMSO (450 mL) and cooled to 0 oC. To this solution was added lithium acetylide, ethylenediamine complex (6.89 g, 74.9 mmol) in portions over 20 minutes, after which it was allowed to warm to room temperature and stirred for 16 h. The reaction mixture was poured onto a stirred mixture of brine/pentane (1:1, 1.00 L) at 0 oC. After stirring for 5 minutes, the layers were allowed to separate. The aqueous layer was extracted with pentane (3 x 200 mL) and the combined organic layers were washed with brine (300 mL), dried over Na2SO4 and concentrated in vacuo to give a pale yellow oil (5.13 g, 95 %). All spectroscopic data were in agreement with those reported in the literature.7

John Reed - January 2017 117 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

1H NMR (300 MHz, CDCl3) δ 1.45-1.58 (m, 4H), 1.94 (t, J = 2.7 Hz, 1H), 203-2.11 (m, 2H), 2.15- 2.23 (m, 2H), 4.92-5.05 (m, 2H), 5.81 (ddt, J = 6.7 Hz, J = 10.2 Hz, J = 17.3 Hz, 1H); 13C NMR (75

MHz, CDCl3) δ 14.2, 18.4, 22.5, 28.0, 33.3, 68.3, 114.8, 138.7.

Ethyl (Z)-undeca-2,10-dien-4-ynoate (3.25)

A flame dried Young’s tube under an atmosphere of N2 was charged with CuI (13.2 mg, 69.3 μmol) and Pd(PPh3)2Cl2 (97.3 mg, 139 μmol). Alkyne 3.24 (1.50 g, 13.9 mmol), iodide 3.19 (3.13 g, 13.9 mmol) and Et3N (15 mL) were then added under a positive flow of N2. The reaction mixture was degassed by three freeze-pump-thaw cycles before being heated to 50 oC and stirred for 16 h. After cooling to room temperature, the reaction was quenched with a saturated aqueous NH4Cl solution (15 mL). The product was extracted with ethyl acetate (3 x 20 mL) before being washed with brine

(30 mL), dried over Na2SO4, and concentrated in vacuo. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a pale yellow oil (2.31 g, 81 %).

1H NMR (300 MHz, CDCl3) δ 1.30 (t, J = 7.1 Hz, 3H), 1.47-1.66 (m, 4H), 2.04-2.12 (m, 2H), 2.45 (td, J = 2.3 Hz, J = 6.8 Hz, 2H), 4.22 (q, J = 3.6 Hz, 2H), 4.92-5.05 (m, 2H), 5.73-5.88 (m, 1H), 6.02

(d, J = 11.5 Hz, 1H), 6.11-6.17 (m, 1H); 13C NMR (75 MHz, CDCl3) δ 14.1, 20.1, 28.0, 28.2, 33.4, 62.4, 104.3, 114.8, 124.3, 127.1, 138.7; FTIR (cm-1) 2208, 1728 (C=O); HRMS (ESI) m/z calc.

C13H19O2 [M+H]+ 207.1380, found 207.1375.

6-(Hex-5-en-1-yl)-2H-pyran-2-one (3.22)

Using Negishi’s ZnBr2 protocol:

An aqueous solution of LiOH (1 M, 29 mL) was slowly added to a solution of ester 3.25 (2.00 g, 9.70 mmol) in MeOH:THF (1:1, 58 mL) at room temperature and stirred for 10 h. The reaction mixture was carefully acidified to pH ~3-4 with an aqueous HCl solution (1 M). The product was extracted with ethyl acetate (5 x 50 mL) and washed with brine (30 mL), dried over Na2SO4 and concentrated in vacuo. The crude acid thus obtained was immediately diluted in THF (200 mL). To this solution was added a solution of freshly sublimed ZnBr2 in THF (1 M, 1.96 mL) at room temperature followed by stirring for 48 h. The THF was removed in vacuo and the crude product

118 John Reed - January 2017 Chapter 5: Experimental purified by flash chromatography, eluting with 15 % ethyl acetate/petroleum spirit, giving a pale yellow oil (1.76 g, 88 %).

Using Schreiber’s gold(I) catalysed procedure:

To a solution of freshly distilled propiolic acid (70.0 mg, 1.00 mmol) and alkyne 3.24 (541 mg, 5.00 mmol) in CH2Cl2 (4.4 mL) were added sequentially (PPh3)AuCl (24.7 mg, 50.0 μmol) and AgOTf (12.8 mg, 50.0 μmol) at room temperature. The reaction mixture was stirred at room temperature for 16 h. The CH2Cl2 was removed in vacuo and the crude product purified by flash chromatography, eluting with 15 % ethyl acetate/petroleum spirit, giving a pale yellow oil (81 mg, 45 %).

1H NMR (300 MHz, CDCl3) δ 1.33-1.52 (m, 2H), 1.63-1.73 (m, 2H), 2.04-2.11 (m, 2H), 2.48 (t, J = 7.6 Hz, 2H), 4.81-5.12 (m, 2H), 5.74-5.86 (m, 1H), 5.96 (d, J = 6.5 Hz, 1H), 6.15 (d, J = 9.3 Hz,

1H), 7.25 (dd, J = 6.5 Hz, J = 9.3 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 26.5, 28.3, 33.5, 33.8, 102.8, 113.3, 115.0, 138.4, 143.8 163.1, 166.7; FTIR (cm-1) 1729 (C=O); HRMS (ESI) m/z calc.

C11H15O2 [M+H]+ 179.1067, found 179.1068.

Ethyl (Z)-3-iodobut-2-enoate (3.29)

A solution of ethyl 2-butynoate (2.00 g, 17.8 mmol), sodium iodide (3.21 g, 21.4 mmol) in acetic acid (12 mL) was heated at reflux for 2 h. After cooling to room temperature, the reaction mixture was diluted with diethyl ether (70 mL) and washed with an aqueous NaOH solution (50 mL, 1 M). The aqueous layer was extracted with diethyl ether (3 x 30 mL) and the combined organic layers were sequentially washed with a saturated aqueous NaHCO3 solution (50 mL), an aqueous

Na2S2O3 solution (50 mL, 2 M), and brine, before being dried over MgSO4 and concentrated in vacuo to provide a yellow liquid (4.28 g, quant.). All spectroscopic data were in agreement with those reported in the literature.8

1H NMR (300 MHz, CDCl3) δ 1.24 (t, J = 7.1 Hz, 3H), 2.67 (d, J = 1.4 Hz, 3H), 4.16 (q, J = 7.1 Hz,

2H), 6.23 (q, J = 1.4 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 14.2, 36.5, 60.4, 113.2, 125.6, 164.2.

John Reed - January 2017 119 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Ethyl (Z)-methylundeca-2,10-dien-4-ynoate (3.30)

A flame dried Young’s tube under an atmosphere of N2 was charged with CuI (10.6 mg, 55.5 μmol) and Pd(PPh3)2Cl2 (77.9 mg, 111 μmol). Alkyne 3.24 (1.20 g, 11.1 mmol), iodide 3.29 (2.66 g, 11.1 mmol) and Et3N (10 mL) were then added under a positive flow of N2. The reaction mixture was degassed by three freeze-pump-thaw cycles before being heated to 50 oC and stirred for 16 h. After cooling to room temperature, the reaction was quenched with a saturated aqueous NH4Cl solution (10 mL). The product was extracted with ethyl acetate (3 x 20 mL) before being washed with brine

(20 mL), dried over Na2SO4, and concentrated in vacuo. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a pale yellow oil (2.27 g, 93 %).

1H NMR (300 MHz, CDCl3) δ 1.28 (t, J = 7.1 Hz, 3H), 1.48-1.67 (m, 4H), 2.01 (d, J = 1.4 Hz, 3H), 2.04-2.12 (m, 2H), 2.45 (t, J = 6.9 Hz, 2H), 4.18 (q, J = 7.2 Hz, 2H), 4.92-5.05 (m, 2H), 5.74-5.88

(m, 1H), 5.92 (q, J = 1.4 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 14.4, 20.0, 26.0, 28.0, 28.2, 33.4, 60.0, 80.0, 102.9, 114.7, 123.5, 135.8, 138.7, 165.4; FTIR (cm-1) 2977, 2932, 2221, 1721 (C=O),

1619; HRMS (ESI) m/z calc. C14H20NaO2 [M+H]+ 243.1361, found 243.1345.

6-(Hex-5-en-1-yl)-4-methyl-2H-pyran-2-one (3.31)

An aqueous solution of LiOH (1 M, 27 mL) was slowly added to a solution of ester 3.30 (2.00 g, 9.08 mmol) in MeOH:THF (1:1, 54 mL) at room temperature and stirred for 10 h. The reaction mixture was carefully acidified to pH ~3-4 with an aqueous HCl solution (1 M). The product was extracted with ethyl acetate (5 x 40 mL) and washed with brine (20 mL), dried over Na2SO4 and concentrated in vacuo. The crude acid thus obtained was immediately diluted in THF (180 mL). To this solution was added a solution of freshly sublimed ZnBr2 in THF (1 M, 1.81 mL) at room temperature followed by stirring for 48 h. The THF was removed in vacuo and the crude product purified by flash chromatography, eluting with 15 % ethyl acetate/petroleum spirit, giving a pale yellow oil (1.23 g, 76 %).

1H NMR (300 MHz, CDCl3) δ 1.36-1.44 (m, 2H), 1.57-1.67 (m, 2H), 1.99-2.06 (m, 2H), 2.08 (d, J = 1.1 Hz, 3H), 2.41 (t, J = 7.6 Hz, 2H), 4.88-5.00 (m, 2H), 5.66-5.78 (m, 1H), 5.80 (d, 1.1 Hz, 1H),

120 John Reed - January 2017 Chapter 5: Experimental

5.89 (s, 1H); 13C NMR (75 MHz, CDCl3) δ 21.4, 26.3, 28.2, 33.3, 33.5, 105.8, 110.6, 114.9, 138.3, 156.3, 163.3, 164.7; FTIR (cm-1) 3073, 2928, 1728 (C=O), 1642, 1561; HRMS (ESI) m/z calc.

C12H17O2 [M+H]+ 193.1223, found 193.1220.

General procedure for the olefin cross-metathesis reactions of pyrones 3.22 and 3.31 with terminal alkenes

R1 R1 O O Grubbs Gen II. o O CuI, Et2O, 35 C O R R To a solution of pyrone 3.22 or 3.31 (1 eq.) and terminal alkene (3 eq.) in diethyl ether (to give a pyrone concentration of 100 mM) was added Grubbs 2nd generation catalyst (0.02 eq.) and CuI (0.03 eq.) under a positive flow of nitrogen. The reaction mixture was degassed via 3 freeze-pump- thaw cycles before being heated to 35 oC. After 10, the reaction mixture was cooled to room temperature and the ether was removed in vacuo to give the crude product.

Ethyl (E)-7-(2-oxo-2H-pyran-6-yl)hept-2-enoate (3.21)

The title compound was synthesised from pyrone 3.22 (500 mg) and ethyl acrylate (845 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a colourless oil (618 mg, 88 %).

1H NMR (600 MHz, CDCl3) δ 1.28 (t, J = 7.2 Hz, 3H), 1.49-1.54 (m, 2H), 1.67-1.72 (m, 2H), 2.23 (qd, J = 1.5 Hz, J = 7.2 Hz, 2H), 2.49 (t, J = 7.6 Hz, 2H), 4.18 (q, J = 7.1 Hz, 2H), 5.81 (dt, J = 1.6 Hz, J = 15.7 Hz, 1H), 5.96 (dd, J = 0.8 Hz, J = 6.6 Hz, 1H), 6.15 (dd, J = 0.8 Hz, J = 9.4 Hz, 1H), 6.92 (dt, J = 7.0 Hz, J = 15.6 Hz, 1H), 7.25 (dd, J = 6.5 Hz, J = 9.4 Hz, 1H); 13C NMR (150 MHz,

CDCl3) δ 14.4, 26.5, 37.9, 33.7, 60.4, 102.9, 113.5, 122.0, 143.8, 148.3, 162.9, 166.1, 166.7; FTIR

(cm-1) 2976, 2934, 2868, 1769, 1703 (C=O), 1650; HRMS (ESI) m/z calc. C14H19O4 [M+H]+ 251.1278, found 251.1269.

John Reed - January 2017 121 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

(E)-6-(6-Phenylhex-5-en-1-yl)-2H-pyran-2-one (3.23)

The title compound was synthesised from pyrone 3.22 (40 mg) and styrene (70 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, provided a colourless oil (36 mg, 63 %).

1H NMR (400 MHz, CDCl3) δ 1.49-1.56 (m, 2H), 1.69-1.77 (m, 2H), 2.25 (q, J = 7.1 Hz, 2H), 2.52 (t, J = 7.6 Hz, 2H), 5.97 (d, J = 6.5 Hz, 1H), 6.14-6.22 (m, 2H), 6.39 (d, J = 15.8 Hz, 1H), 7.19-7.35

(m, 6H); 13C NMR (100 MHz, CDCl3) δ 26.6, 28.8, 32.7, 33.8, 102.8, 113.3, 126.1, 127.1, 128.6, 130.2, 130.5, 137.8, 143.8, 163.0, 166.6; FTIR (cm-1) 3056, 2933, 2862, 1718, 1629; HRMS (ESI) m/z calc. C17H19O2 [M+H]+ 255.1380, found 255.1388.

(E)-6-(6-(p-Tolyl)hex-5-en-1-yl)-2H-pyran-2-one (3.26)

The title compound was synthesised from pyrone 3.22 (40 mg) and 4-methylstyrene (80 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, provided a colourless oil (49 mg, 81 %).

1H NMR (400 MHz, CDCl3) δ 1.49-1.56 (m, 2H), 1.64-1.76 (m, 2H), 2.23 (qd, J = 1.2 Hz, J = 7.2 Hz, 2H), 2.32 (s, 3H), 2.50 (q, J = 7.8 Hz, 2H), 5.96 (d, J = 6.5 Hz, 1H), 6.09-6.17 (m, 2H), 6.35 (d,

J = 15.8 Hz, 1H), 7.09 (d, J = 7.9 Hz, 2H), 7.21-7.25 (m, 3H); 13C NMR (100 MHz, CDCl3) δ 21.2, 26.5, 28.9, 32.7, 33.8, 102.8, 113.3, 115.0, 126.0, 129.1, 129.3, 130.3, 135.0, 136.8, 138.3, 143.8,

163.0, 166.6; FTIR (cm-1) 3023, 2914, 1715 (C=O), 1647; HRMS (ESI) m/z calc. C18H21O2 [M+H]+ 269.1536, found 269.1542.

122 John Reed - January 2017 Chapter 5: Experimental

(E)-6-(6-(4-Methoxyphenyl)hex-5-en-1-yl)-2H-pyran-2-one (3.27)

The title compound was synthesised from pyrone 3.22 (40 mg) and 4-methoxystyrene (90 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, provided a colourless oil (47 mg, 74 %).

1H NMR (400 MHz, CDCl3) δ 1.47-1.55 (m, 2H), 1.67-1.76 (m, 2H), 2.21 (q, J = 6.9 Hz, 2H), 2.50 (t, J = 7.6 Hz, 2H), 3.79 (s, 3H), 5.96 (d, J = 6.6 Hz, 1H), 6.04 (dt, J = 7.0 Hz, J = 15.8 Hz, 1H), 6.14 (d, J = 9.4 Hz, 1H), 6.32 (d, J = 15.8 Hz, 1H), 6.83 (d, J = 8.7 Hz, 2H), 7.20-7.26 (m, 3H); 13C NMR

(100 MHz, CDCl3) δ 26.5, 28.9, 32.7, 33.8, 55.4, 102.8, 113.3, 114.0, 127.1, 128.0, 129.8, 130.6, 143.8, 158.8, 163.0, 166.6; FTIR (cm-1) 2929, 2854, 1722 (C=O), 1631, 1555; HRMS (ESI) m/z calc. C18H21O3 [M+H]+ 285.1485, found 285.1479.

(E)-6-(Dec-5-en-1-yl)-2H-pyran-2-one (3.28)

The title compound was synthesised from pyrone 3.22 (40 mg) and 1-hexene (57 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 10 % EtOAc/petroleum spirit, provided a colourless oil (42 mg, 80 %).

1H NMR (400 MHz, CDCl3) δ 0.88 (t, J = 7.1 Hz, 3H), 1.25-1.45 (m, 6H), 1.60-1.76 (m, 2H), 1.94- 2.08 (m, 4H), 2.48 (t, J = 7.6 Hz, 2H), 5.30-5.45 (m, 2H), 5.96 (d, J = 6.5 Hz, 1H), 6.14 (d, J = 9.4

Hz, 1H), 7.24 (dd, J = 6.6 Hz, J = 9.3 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 14.1, 22.3, 26.5, 29.0, 31.9, 32.3, 32.4, 33.8, 102.7, 113.3, 129.5, 131.3, 143.8, 163.0, 166.8; FTIR (cm-1) 2948, 2837,

1718 (C=O), 1655; HRMS (ESI) m/z calc. C15H23O2 [M+H]+ 235.1693, found 235.1699.

John Reed - January 2017 123 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Ethyl (E)-7-(4-methyl-2-oxo-2H-pyran-6-yl)hept-2-enoate (3.32)

The title compound was synthesised from pyrone 3.31 (50 mg) and ethyl acrylate (78 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided a colourless oil (57 mg, 83 %).

1H NMR (400 MHz, CDCl3) δ 1.28 (t, J = 7.2 Hz, 3H), 1.46-1.54 (m, 2H), 1.64-1.72 (m, 2H), 2.11 (d, J = 1.2 Hz, 3H), 2.22 (qd, J = 1.5 Hz, J = 7.2 Hz, 2H), 2.46 (t, J = 7.5 Hz, 2H), 4.18 (q, J = 7.2 Hz, 2H), 5.81 (dt, J = 1.6 Hz, J = 15.7 Hz, 1H), 5.82 (s, 1H), 5.94 (s, 1H), 6.92 (dt, J = 7.0 Hz, J =

15.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 14.4, 21.6, 26.5, 27.5, 31.9, 33.5, 60.4, 106.0, 110.9, 122.0, 148.4, 156.2, 163.3, 164.4, 166.7; FTIR (cm-1) 2979, 2933, 2861, 1758 (C=O), 1711 (C=O),

1643; HRMS (ESI) m/z calc. C15H21O4 [M+H]+ 265.1434, found 265.1434.

(E)-4-Methyl-6-(6-phenylhex-5-en-1-yl)-2H-pyran-2-one (3.33)

The title compound was synthesised from pyrone 3.31 (50 mg) and styrene (81 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, provided a colourless oil (41 mg, 59 %).

1H NMR (300 MHz, CDCl3) δ 1.47-1.56 (m, 2H), 1.69-1.80 (m, 2H), 2.11 (d, J = 1.1 Hz, 3H), 2.20- 2.28 (m, 2H), 2.45-2.54 (m, 2H), 5.83 (s, 1H), 5.95 (s, 1H), 6.13-6.25 (m, 1H), 6.39 (d, J = 15.8 Hz,

1H), 7.20-7.35 (m, 5H); 13C NMR (75 MHz, CDCl3) δ 21.6, 26.6, 28.9, 32.8, 33.7, 105.9, 110.8, 126.1, 127.1, 128.6, 130.3, 130.5, 137.8, 156.3, 164.8; FTIR (cm-1) 2930, 2860, 1699 (C=O); HRMS

(ESI) m/z calc. C18H21O2 [M+H]+ 269.1536, found 269.1526.

124 John Reed - January 2017 Chapter 5: Experimental

(E)-4-Methyl-6-(6-(p-tolyl)hex-5-en-1-yl)-2H-pyran-2-one (3.34)

O Me The title compound was synthesised from pyrone 3.31 (50 mg) and 4-methylstyrene (92 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, provided a colourless oil (31 mg, 42 %).

1H NMR (400 MHz, CDCl3) δ 1.43-1.52 (m, 2H), 1.61-1.70 (m, 2H), 2.08 (d, J = 1.0 Hz, 3H), 2.18- 2.24 (m, 2H), 2.32 (s, 3H), 2.51 (t, J = 7.6 Hz, 2H), 5.88 (s, 1H), 5.97 (s, 1H), 6.01 (dt, J = 7.0 Hz, J = 15.7 Hz, 1H), 6.88 (d, J = 15.8 Hz, 1H), 7.05 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.6 Hz, 2H); 13C

NMR (100 MHz, CDCl3) δ 21.6, 26.5, 29.0, 32.7, 33.7, 55.4, 105.9, 110.8, 114.1, 127.2, 128.1, 129.8, 130.6, 156.3, 158.9, 163.4, 164.9; FTIR (cm-1) 2919, 1717 (C=O), 1641; HRMS (ESI) m/z calc. C19H23O2 [M+H]+ 283.1693, found 283.1691.

(E)-4-Methyl-6-(6-(4-methoxyphenyl)hex-5-en-1-yl)-2H-pyran-2-one (3.35)

The title compound was synthesised from pyrone 3.31 (50 mg) and 4-methoxystyrene (105 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, provided a colourless oil (70 mg, 90 %).

1H NMR (400 MHz, CDCl3) δ 1.47-1.55 (m, 2H), 1.66-1.76 (m, 2H), 2.11 (d, J = 1.0 Hz, 3H), 2.18- 2.24 (m, 2H), 2.48 (t, J = 7.6 Hz, 2H), 3.80 (s, 3H), 5.83 (s, 1H), 5.94 (s, 1H), 6.04 (dt, J = 7.0 Hz, J = 15.7 Hz, 1H), 6.33 (d, J = 15.8 Hz, 1H), 6.83 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.6 Hz, 2H); 13C

NMR (100 MHz, CDCl3) δ 21.6, 26.5, 29.0, 32.7, 33.7, 55.4, 105.9, 110.8, 114.1, 127.2, 128.1, 129.8, 130.6, 156.3, 158.9, 163.4, 164.9; FTIR (cm-1) 2930, 2857, 1726 (C=O), 1643; HRMS (ESI) m/z calc. C19H23O3 [M+H]+ 299.1642, found 299.1640.

(E)-4-Methyl-6-(dec-5-en-1-yl)-2H-pyran-2-one (3.36)

John Reed - January 2017 125 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

The title compound was synthesised from pyrone 3.31 (50 mg) and 1-hexene (66 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 10 % EtOAc/petroleum spirit, provided a colourless oil (54 mg, 84 %).

1H NMR (400 MHz, CDCl3) δ 0.86-0.92 (m, 3H), 1.25-1.34 (m, 4H), 1.35-1.43 (m, 2H), 1.60-1.75 (m, 2H), 1.94-2.03 (m, 4H), 2.11 (d, J = 1.0 Hz, 3H), 2.44 (t, J = 7.6 Hz, 2H), 5.30-5.45 (m, 2H),

5.82 (s, 1H), 5.94 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 14.1, 21.6, 22.3, 26.5, 29.1, 31.9, 32.3, 32.4, 33.6, 105.8, 110.8, 129.5, 131.2, 156.3, 163.5, 165.0; FTIR (cm-1) 2952, 2923, 2855, 1721

(C=O), 1643, 1560; HRMS (ESI) m/z calc. C16H25O2 [M+H]+ 249.1849, found 249.1860.

General procedure for the intramolecular diene-regenerative Diels-Alder reactions of 2- pyrones with functionalised alkenes

A solution of pyrone (1 eq.), BHT (1 eq.) and Proton Sponge® (0.3 eq.) in xylenes (to give a pyrone concentration of 20 mM) was heated at reflux for 48 h. The xylenes were removed in vacuo to give the crude product.

Ethyl (1S*,8aR*)-1,5,6,7,8,8a-hexahydronaphthalene-1-carboxylate (3.41)

The title compound was synthesised from pyrone 3.21 (86 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 3 % EtOAc/petroleum spirit, provided a colourless oil (68 mg, 96 %).

1H NMR (400 MHz, CDCl3) δ 1.19-1.24 (m, 1H), 1.28 (t, J = 7.1 Hz, 3H), 1.32-1.36 (m, 1H), 1.38- 1.47 (m, 1H), 1.74-1.85 (m, 2H), 1.85-1.92 (m, 1H), 2.03-2.12 (m, 1H), 2.29-2.36 (m, 1H), 2.69- 2.78 (m, 1H), 3.02 (dt, J = 2.8 Hz, J = 12.7 Hz, 1H), 4.19 (qd, J = 1.5 Hz, J = 7.1 Hz, 2H), 5.50-5.57

(m, 2H), 5.83-5.88 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 14.4, 26.3, 27.8, 34.6, 35.9, 38.0, 48.5,

126 John Reed - January 2017 Chapter 5: Experimental

60.9, 115.9, 120.1, 124.4, 141.7, 174.8; FTIR (cm-1) 2929, 2855, 1728 (C=O), 1639; HRMS (ESI) m/z calc. C13H19O2 [M+H]+ 207.1380, found 207.1384.

(4aR*,5S*)-5-Phenyl-1,2,3,4,4a,5-hexahydronaphthalene (3.42)

The title compound was synthesised from pyrone 3.23 (25 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 3 % EtOAc/petroleum spirit, provided a colourless oil (13 mg, 63 %).

1H NMR (600 MHz, CDCl3) δ 0.85-0.90 (m, 1H), 1.29-1.34 (m, 1H), 1.35-1.42 (m, 1H), 1.50-1.56 (m, 1H), 1.69-1.64 (m, 1H), 1.71-1.79 (m, 2H), 1.97-2.03 (m, 1H), 2.12-2.21 (m, 1H), 3.21-3.24 (m, 1H), 5.50-5.54 (m, 1H), 5.62-5.64 (m, 1H), 5.84-5.89 (m, 1H), 7.19-7.26 (m, 5H); 13C NMR (150

MHz, CDCl3) δ 23.1, 36.5, 34.7, 35.5, 44.5, 49.3, 116.5, 121.7, 125.5, 128.0, 128.4, 131.5, 141.3,

144.4; FTIR (cm-1) 3055, 1938; HRMS (ESI) m/z calc. C16H18 [M]+ 210.1409, found 210.1418.

(4aR*,5S*)-5-(p-Tolyl)-1,2,3,4,4a,5-hexahydronaphthalene (3.43)

The title compound was synthesised from pyrone 3.26 (30 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 3 % EtOAc/petroleum spirit, provided a colourless oil (18 mg, 72 %).

1H NMR (400 MHz, CDCl3) δ 1.21-1.31 (m, 2H), 1.34-1.42 (m, 2H), 1.64-1.82 (m, 4H), 1.96-2.04 (m, 1H), 2.34 (s, 3H), 3.20 (dt, J = 2.7 Hz, J = 12.8 Hz, 1H), 5.49-5.54 (m, 1H), 5.62-5.67 (m, 1H),

5.84-5.88 (m, 1H), 7.07-7.15 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 21.2, 26.5, 28.0, 34.8, 35.6, 44.4, 48.9, 116.5, 122.9, 127.7, 128.3, 129.2, 135.8, 141.4, 144.1; FTIR (cm-1) 3044, 2981, 2203,

1510; HRMS (ESI) m/z calc. C17H20 [M]+ 224.1565, found 224.1560.

John Reed - January 2017 127 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

(4aR*,5S*)-5-(4-Methoxyphenyl)-1,2,3,4,4a,5-hexahydronaphthalene (3.44)

The title compound was synthesised from pyrone 3.27 (30 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 5 % EtOAc/petroleum spirit, provided a colourless oil (19 mg, 75 %).

1H NMR (400 MHz, CDCl3) δ 1.22-1.31 (m, 2H), 1.32-1.42 (m, 2H), 1.77-1.82 (m, 2H), 1.96-2.03 (m, 1H), 2.25-2.37 (m, 2H), 3.18 (dt, J = 2.6 Hz, J = 12.6 Hz, 1H), 3.79 (s, 3H), 5.49-5.52 (m, 1H), 5.61-5.65 (m, 1H), 5.83-5.88 (m, 1H), 6.83-6.86 (m, 2H), 7.13-7.17 (m, 2H); 13C NMR (100 MHz,

CDCl3) δ 26.5, 28.0, 34.7, 35.6, 44.5, 48.4, 66.4, 113.9, 116.5, 122.9, 127.8, 129.3, 139.3, 141.5,

158.2; FTIR (cm-1) 3096, 2955, 1602, 1498; HRMS (ESI) m/z calc. C17H21O [M+H]+ 241.1587, found 241.1598.

(4aR*,5S*)-5-Butyl-1,2,3,4,4a,5-hexahydronaphthalene (3.45)

The title compound was synthesised from pyrone 3.28 (35 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 3 % EtOAc/petroleum spirit, provided a colourless oil (24 mg, 84 %).

1H NMR (400 MHz, CDCl3) δ 0.91-0.95 (m, 3H), 1.45-1.67 (m, 4H), 1.70-1.79 (m, 2H), 1.93-2.04 (m, 4H), 2.05-2.14 (m, 2H), 2.24-2.33 (m, 1H), 2.53-2.59 (m, 1H), 2.69-2.78 (m, 1H), 2.89 (dt, J = 7.5 Hz, J = 20.4 Hz, 1H), 5.49-5.56 (m, 1H), 5.63-5.67 (m, 1H), 5.70-5.75 (m, 1H); 13C NMR (100

MHz, CDCl3) δ 14.3, 27.0, 27.5, 29.2, 29.5, 30.2, 35.4, 35.6, 40.9, 42.4, 115.6, 125.3, 134.4, 142.6;

FTIR (cm-1) 2947, 2155, 1497, 1485; HRMS (ESI) m/z calc. C14H22 [M]+ 190.1722, found 190.1722.

128 John Reed - January 2017 Chapter 5: Experimental

Ethyl (1S*,8aR*)-3-methyl-1,5,6,7,8,8a-hexahydronaphthalene-1-carboxylate (3.46)

The title compound was synthesised from pyrone 3.32 (30 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 5 % EtOAc/petroleum spirit, provided a colourless oil (18 mg, 72 %).

1H NMR (400 MHz, CDCl3) δ 1.17-1.22 (m, 1H), 1.28 (t, J = 7.1 Hz, 3H), 1.34-1.47 (m, 2H), 1.73 (s, 3H), 1.76-1.84 (m, 2H), 1.87-1.94 (m, 1H), 2.05-2.14 (m, 1H), 2.29-2.36 (m, 1H), 2.63-2.74 (m, 1H), 2.98-3.03 (m, 1H), 4.19 (qd, J = 1.1 Hz, J = 7.1 Hz, 2H), 5.23 (s, 1H), 5.45 (d, J = 0.9 Hz, 1H);

13C NMR (100 MHz, CDCl3) δ 14.4, 21.6, 26.3, 27.4, 34.5, 35.7, 37.8, 49.0, 60.8, 120.0, 120.7,

132.0, 141.6, 175.2; FTIR (cm-1) 2980, 2874, 1720 (C=O); HRMS (ESI) m/z calc. C14H21O2 [M+H]+ 221.1536, found 221.1528.

(4aR*,5S*)-7-Methyl-5-phenyl-1,2,3,4,4a,5-hexahydronaphthalene (3.47)

The title compound was synthesised from pyrone 3.33 (32 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 3 % EtOAc/petroleum spirit, provided a colourless oil (19 mg, 71 %).

1H NMR (400 MHz, CDCl3) δ 1.05-1.10 (m, 1H), 1.14-1.21 (m, 1H), 1.65-1.72 (m, 2H), 1.75 (s, 3H), 1.76-1.80 (m, 1H), 1.99-2.06 (m, 1H), 2.22-2.39 (m, 2H), 2.45-2.53 (m, 1H), 3.19 (dt, J = 2.5 Hz, J

= 12.7 Hz, 1H), 5.22 (s, 1H), 5.53 (s, 1H), 7.26-7.31 (m, 5H); 13C NMR (100 MHz, CDCl3) δ22.8, 26.5, 27.2, 27.9, 35.3, 44.0, 49.9, 120.6, 122.1, 126.2, 128.4, 128.5, 130.3, 149.3, 149.6; FTIR (cm-

1) 3028, 2241, 1597, 1436; HRMS (ESI) m/z calc. C17H20 [M]+ 224.1565, found 224.1560.

John Reed - January 2017 129 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

(4aR*,5S*)-7-Methyl-5-(p-tolyl)-1,2,3,4,4a,5-hexahydronaphthalene (3.48)

The title compound was synthesised from pyrone 3.34 (21 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 3 % EtOAc/petroleum spirit, provided a colourless oil (14 mg, 79 %).

1H NMR (400 MHz, CDCl3) δ 1.33-1.37 (m, 1H), 1.76-1.82 (m, 1H), 1.79 (t, J = 2.1 Hz, 3H), 1.99- 2.05 (m, 2H), 2.22-2.30 (m, 2H), 2.34 (s, 3H), 2.37-2.42, (m, 2H), 2.48-2.53 (m, 1H), 3.26 (dd, J = 1.7 Hz, J = 16.5 Hz, 1H), 5.26 (s, 1H), 5.68 (d, J = 1.8 Hz, 1H), 7.12-7.14 (m, 2H), 7.17-7.19 (m,

2H); 13C NMR (100 MHz, CDCl3) δ 21.2, 21.4 25.9, 27.8, 34.1, 41.2, 48.5, 51.6, 115.0, 122.7, 127.7,

129.1, 129.8, 134.3, 142.1, 143.1; FTIR (cm-1) 2964, 2198, 1505; HRMS (ESI) m/z calc. C18H22 [M]+ 238.1722, found 238.1726.

(4aR*,5S*)-5-(4-Methoxyphenyl)-7-methyl-1,2,3,4,4a,5-hexahydronaphthalene (3.49)

The title compound was synthesised from pyrone 3.35 (30 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 5 % EtOAc/petroleum spirit, provided a colourless oil (22 mg, 86 %).

1H NMR (600 MHz, CDCl3) δ 1.17-1.39 (m, 4H), 1.74 (s, 3H), 1.75-1.81 (m, 2H), 1.98-2.07 (m, 1H), 2.17-2.24 (m, 1H), 2.32-2-37 (m, 1H), 3.14 (dt, J = 2.53 Hz, J = 12.5 Hz, 1H), 3.79 (s, 3H), 5.20 (s,

1H), 5.52 (d, J = 1.5 Hz, 1H), 6.81-6.86 (m, 2H), 7.11-7.16 (m, 2H); 13C NMR (150 MHz, CDCl3) δ 21.6, 26.5, 27.9, 34.6, 35.3, 44.2, 48.9, 55.4, 113.8, 125.5, 129.3, 130.1, 139.8, 141.5, 150.9, 158.1;

FTIR (cm-1) 3023, 2913, 1484; HRMS (ESI) m/z calc. C18H23O [M+H]+ 255.1743, found 255.1743.

130 John Reed - January 2017 Chapter 5: Experimental

(4aR*,5S*)-5-Butyl-7-methyl-1,2,3,4,4a,5-hexahydronaphthalene (3.50)

The title compound was synthesised from pyrone 3.36 (30 mg) according to the general procedure described above. Purification of the crude product by flash chromatography, eluting with 3 % EtOAc/petroleum spirit, provided a colourless oil (22 mg, 89 %).

1H NMR (400 MHz, CDCl3) δ 0.83-0.95 (m, 3H), 1.32-1.49 (m, 4H), 1.59-1.70 (m, 4H), 1.72-1.80 (m, 2H), 1.82-1.91 (m, 2H), 1.96-2.04 (m, 2H), 2.26 (s, 3H), 2.31-2.35 (m, 1H), 2.79 (m, 1H), 5.22

(s, 1H), 5.55 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 14.3, 19.9, 21.7, 27.0, 28.7, 29.8, 30.7, 33.6, 36.3, 41.3, 41.9, 122.5, 125.9, 129.7, 136.6; FTIR (cm-1) 2986, 2195, 1489; HRMS (ESI) m/z calc.

C15H24 [M]+ 204.1878, found 204.1887.

Ethyl (3aR*,5S*)-1,3-dioxo-1,3a,4,5,5a,6,7,8,9,9b-decahydro-3H-4,9a-ethenonaphtho[1,2- c]furan-5-carboxylate (3.51c)

Maleic anhydride (19.0 mg, 194 μmol) was added in one portion to a solution of diene 3.41 (20 mg, 97.0 μmol) in xylenes (5 mL) and this solution was heated at reflux for 16 hours. The xylenes were removed in vacuo and the crude product was purified by flash chromatography, eluting with 15 % EtOAc/petroleum spirit, to provide a colourless oil (28 mg, 95 %).

1H NMR (600 MHz, CDCl3) δ 0.83 (qd, J = 3.7 Hz, J = 13.0 Hz, 1H), 1.23-1.27 (m, 1H), 1.29 (t, J = 7.1 Hz, 3H), 1.36 (qt, J = 3.3 Hz, J = 13.2 Hz, 1H), 1.51 (td, J = 3.7 Hz, J = 13.5 Hz, 1H), 1.64-1.71 (m, 2H), 1.75-1.80 (m, 2H), 1.95 (dd, J = 2.6 Hz, J = 5.4 Hz, 1H), 2.49 (d, J = 13.8 Hz, 1H), 2.84 (d, J = 8.9 Hz, 1H), 3.34-3.37 (m, 1H), 3.48 (dd, J = 3.2 Hz, J = 8.8 Hz, 1H), 4.18 (q, J = 7.1 Hz,

2H), 6.12 (d, J = 8.3 Hz, 1H), 6.34 (dd, J = 6.7 Hz, J = 8.1 Hz, 1H); 13C NMR (150 MHz, CDCl3) δ 14.3, 22.7, 25.6, 32.5, 33.2, 34.4, 41.7, 42.4, 42.6, 48.8, 50.1, 61.4, 131.4, 136.0, 170.7, 172.9, 173.9; FTIR (cm-1) 2925, 2854, 1858, 1835, 1773 (C=O), 1718 (C=O); HRMS (ESI) m/z calc.

C17H20NaO5 [M+Na]+ 327.1208, found 327.1203.

John Reed - January 2017 131 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

Ethyl (1aR*,4S*,4aR*,8aS*)-1a,4,4a,5,7,8-hexahydro-6H-naphtho[1,8a-b]oxirene-4- carboxylate (3.52a)

EtO C 2 H

O m-CPBA (17.6 mg, 102 μmol) was added to a solution of diene 3.41 (20 mg, 97.0 μmol) in CH2Cl2 (600 μL) at 0 oC and stirred at this temperature for 6 hours. Sodium bicarbonate (20 mg, 238 μL) was added and the reaction mixture was allowed to warm to room temperature while being stirred. The solids were then removed by filtration and the filtrate concentrated in vacuo. Purification of the crude product by flash chromatography, eluting with 20 % EtOAc/petroleum spirit, provided the title compound as a colourless oil (18 mg, 84 %).

1H NMR (400 MHz, CDCl3) δ 1.27 (t, J = 7.1 Hz, 3H), 1.34 (m, 1H), 1.38-1.44 (m, 2H), 1.60-1.68 (m, 1H), 1.74-1.81 (m, 1H), 1.85-1.98 (m, 4H), 2.81 (ddd, J = 2.2 Hz, J = 3.2 Hz, J = 10.3 Hz, 1H), 3.10 (dd, J = 1.6 Hz, J = 4.1 Hz, 1H), 4.17 (q, J = 7.1 Hz, 2H), 5.77 (dt, J = 1.9 Hz, J = 9.8 Hz, 1H),

6.04 (ddd, J = 3.3 Hz, J = 4.1 Hz, J = 9.8 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 14.4, 23.9, 25.9, 31.0, 33.8, 36.3, 46.2, 54.5, 61.0, 62.6, 124.9, 130.3, 174.2; FTIR (cm-1) 2978, 2928, 2854, 1729

(C=O); HRMS (ESI) m/z calc. C13H19O3 [M+H]+ 223.1329, found 223.1329.

Ethyl (1S*,4S*,4aR*,8aR*)-4-4a-dihydroxy-1,4,4a,5,6,7,8,8a-octahydronaphthalene-1- carboxylate (3.53)

To a solution of diene 3.41 (1.1 mg, 5.33 μmol) in methanol (500 μL) at 0 oC was added an aqueous solution of KMnO4 (10 mM, 533 μL, 5.33 μmol) followed by an aqueous sodium hydroxide solution (4.0 mM, 67 μL, 0.27 μmol). After stirring for 10 minutes at 0 oC, the reaction mixture was diluted with brine (5 mL) and extracted with ethyl acetate (3 x 5 mL). The combined organic extracts were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by flash chromatography, eluting with 40 % EtOAc/petroleum spirit, to give the title compound as a colourless oil (1.2 mg, 94 %).

1H NMR (600 MHz, CDCl3) δ 1.27 (t, J = 7.2 Hz, 3H), 1.28-1.36 (m, 2H), 1.43 (qd, J = 3.6 Hz, J = 12.8 Hz, 1H), 1.57-1.67 (m, 3H), 1.71-1.75 (m, 1H), 1.93 (qd, J = 4.1 Hz, J = 10.2 Hz, 1H), 2.03 (d, J = 8.2 Hz, 1H), 2.10 (dq, J = 2.7 Hz, J = 13.8 Hz, 1H), 2.98 (dq, J = 2.9 Hz, J = 10.1 Hz, 1H), 3.71 (bs, 1H), 3.97-4.00 (m, 1H), 4.17 (qd, J = 1.2 Hz, J = 7.2 Hz, 2H), 5.61-5.65 (m, 1H), 5.68-5.71 (m,

132 John Reed - January 2017 Chapter 5: Experimental

1H); 13C NMR (150 MHz, CDCl3) δ 14.4, 21.3, 25.5, 26.5, 35.3, 41.0, 46.6, 61.0, 70.9, 72.8, 126.5,

130.5, 174.0; FTIR (cm-1) 3465 (O-H), 2929, 2855, 1729 (C=O); HRMS (ESI) m/z calc. C13H20NaO4 [M+Na]+ 263.1259, found 263.1252.

Ethyl (1R*,2S*,4aR*,8aR*)-1,5,6,7,8,8a-hexahydro-2H-2,4a-epidioxynaphthalene-1- carboxylate (3.55a) and ethyl (1R*,2R*,4aS*,8aR*)-1,5,6,7,8,8a-hexahydro-2H-2,4a- epidioxynaphthalene-1-carboxylate (3.55b)

To a solution of diene 3.41 (25 mg, 121 μmol) in CH2Cl2 (1.00 mL) was added Rose Bengal (1.27 mg, 1.21 μmol) at room temperature. Gaseous oxygen was bubbled through the reaction vessel and slow, constant rate. The reaction was irradiated with visible light from a 7 W fluorescent lamp for 36 h, after which time TLC analysis indicated the consumption of all starting material, and formation of two major products and one minor product. The reaction mixture was diluted with a saturated aqueous solution of NaHCO3 (5 mL), and extracted with dichloromethane (3 x 10 mL).

The combined organic extracts were washed with brine, dried over Na2SO4, and concentrated in vacuo. The products were separated by gradient flash chromatography, eluting with 10 % ethyl acetate/petroleum spirit to 50 % ethyl acetate/petroleum spirit.

Compound 3.55a was isolated as a pale orange oil (10 mg, 35 %).

1H NMR (600 MHz, CDCl3) δ 1.25 (t, J = 7.2 Hz, 3H), 1.30-1.36 (m, 1H), 1.54-1.58 (m, 1H), 1.65- 1.73 (m, 3H), 1.76-1.81 (m, 1H), 1.89 (dt, J = 4.6 Hz, J = 12.1 Hz, 1H), 1.95-1.99 (m, 1H), 2.05- 2.10 (m, 1H), 2.73 (t, J = 4.2 Hz, 1H), 4.12 (q, J = 7.2 Hz, 2H), 4.89 (m, 1H), 6.35 (dd, J = 1.2 Hz,

J = 8.2 Hz, 1H), 6.54 (dd, J = 5.9 Hz, J = 8.2 Hz, 1H); 13C NMR (150 MHz, CDCl3) δ 14.3, 20.8, 25.3, 29.6, 32.1, 38.3, 48.4, 61.1, 71.3, 75.5, 129.9, 137.9, 172.0; FTIR (cm-1) 2977, 2931, 2855,

1727 (C=O), 1686, 1444; HRMS (ESI) m/z calc. C13H18NaO4 [M+Na]+ 261.1103, found 261.1097.

Compound 3.55b was isolated as a pale orange oil (8 mg, 28 %).

1H NMR (600 MHz, CDCl3) δ 0.80 (qd, J = 3.65 Hz, J = 13.0 Hz, 1H), 1.29 (t, J = 7.15 Hz, 3H), 1.38-1.42 (m, 1H), 1.46-1.52 (m, 2H), 1.67-1.70 (m, 1H), 1.80-1.83 (m, 1H), 1.83-1.86 (m, 1H), 1.90-1.94 (m, 1H), 1.98 (dd, J = 1.10 Hz, J = 5.25 Hz, 1H), 2.47 (dt, J = 4.76 Hz, J = 12.9 Hz, 1H), 4.23 (qd, J = 2.51 Hz, J = 7.15 Hz, 2H), 5.00 (dt, J = 1.23 Hz, J = 6.24 Hz, 1H), 6.45 (d, J = 8.24

Hz, 1H), 6.70 (dd, J = 6.30 Hz, J = 8.22 Hz, 1H); 13C NMR (150 MHz, CDCl3) δ 14.4, 22.9, 25.3,

John Reed - January 2017 133 Application of the Diels-Alder Reaction to the Synthesis of Natural Product-like Libraries

30.9, 33.2, 38.7, 47.6, 61.4, 73.2, 77.9, 130.9, 135.4, 172.1; FTIR (cm-1) 2978, 2933, 2858, 1732,

1612, 1449; HRMS (ESI) m/z calc. C13H18NaO4 [M+H]+ 261.1103, found 261.1097.

Ethyl (1S*,4aS*,8aS*)-decahydronaphthalene-1-carboxylate (3.56)

A solution of diene 3.41 (206 mg, 1.00 mmol) in methanol (10 mL) was hydrogenated using the H- Cube® Continuous-flow hydrogenation reactor over a ruthenium on carbon (5 %) catalyst. The reaction temperature was set to 60 oC, with hydrogen pressure maintained at 50 mbar and a flow rate of 0.5 mL/min. After the reagent solution had been fully consumed, the flow reactor was flushed with another 50 mL of methanol and collected in the same flask. The methanol was then removed in vacuo. Purification of the crude product by flash chromatography, eluting with 2 % ethyl acetate/petroleum spirit afforded the title compound as a colourless oil (210 mg, 100 %).

1H NMR (400 MHz, CDCl3) δ 0.88-0.93 (m, 1H), 0.93-0.96 (m, 1H), 0.99-1.07 (m, 2H), 1.17-1.23 (m, 2H), 1.25 (t, J = 7.1 Hz, 3H), 1.25-1.27 (m, 1H), 1.31 (dt, J = 3.6 Hz, J = 12.8 Hz, 1H), 1.49 (td, J = 3.6 Hz, J = 12.8 Hz, 1H), 1.53-1.63 (m, 3H), 1.65-1.72 (m, 2H), 1.76 (dt, J = 3.2 Hz, J = 12.8 Hz, 1H), 1.82-1.88 (m, 1H), 2.01 (ddd, J = 3.5 Hz, J = 10.8 Hz, J = 12.1 Hz, 1H), 4.12 (qd, J = 1.3

Hz, J = 7.2 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 14.5, 25.5, 26.5, 26.6, 30.4, 31.5, 33.6, 34.2,

42.1, 44.7, 60.0, 176.4; FTIR (cm-1) 2967, 1719 (C=O); HRMS (ESI) m/z calc. C13H23O2 [M+H]+ 211.1693, found 211.1689.

134 John Reed - January 2017 Chapter 5: Experimental

5.4 Chapter 5 References (1) Liu, Z.; Yao, Y.; Kogiso, M.; Zheng, B.; Deng, L.; Qiu, J. J.; Dong, S.; Lv, H.; Gallo, J. M.; Li, X.-N.; Song, Y. 2014. (2) Wickel, S. M.; Citron, C. A.; Dickschat, J. S. European J. Org. Chem. 2013, 2013 (14), 2906–2913. (3) Iyer, S.; Liebeskind, L. S. J. Am. Chem. Soc. 1987, 109 (9), 2759–2770. (4) Hansen, C. A.; Frost, J. W. J. Am. Chem. Soc. 2002, 124 (21), 5926-5927. (5) Bacardit, R.; Moreno-Mañas, M.; Pleixats, R. J. Heterocycl. Chem. 1982, 19 (1), 157–160. (6) Chong, E.; Blum, S. A. J. Am. Chem. Soc. 2015, 137 (32), 10144-10147. (7) Pandey, G.; Soma Sekhar, B. B. V. Tetrahedron 1995, 51 (5), 1483–1494. (8) Huang, Y.; Fañanás-Mastral, M.; Minnaard, A. J.; Feringa, B. L.; Jie, M. S. F. L. K.; Pasha, M. K. Chem. Commun. 2013, 49 (32), 3309.

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136 John Reed - January 2017