SYNTHESIS OF NOVEL FLAVONES AND ISOFLAVONES This thesis is submitted in fulfilment of the degree of Doctor of Philosophy By RUTH VANDANA DEVAKARAM Supervisors: A/Prof. Naresh Kumar Prof. David StC. Black School of Chemistry The University of New South Wales Kensington, Australia February 2011 CERTIFICATE OF ORIGINALITY ‘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 …………………………………………….............. ii COPYRIGHT STATEMENT ‘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 part in the University libraries in all forms of media, now or hereafter known, subject to the provisions of the Copyright Act 1968. I retain all proprietary 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 abstract of my thesis in Dissertations Abstract International (this is applicable to doctoral theses only). 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.’ Signed …………………………………………….............. Date …………………………………………….............. AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ Signed …………………………………………….............. Date …………………………………………….............. iii ABSTRACT The primary aim of this project is to synthesize new heterocyclic compounds derived from flavones and isoflavones and thereby, to investigate various methodologies for their synthesis. The biflavonoid, dependensin was a major focus of our study since the benzopyrano[4,3- b]benzopyran ring system present in the natural product was unique and hence, desirable for biological applications. We targeted the acid-catalyzed reactions of 5-methoxy-, 6-methoxy- and 7-methoxyflavenes and found the dimerization to be dependent on the position of the methoxy substituent in the flavene ring. A series of benzopyrano[4,3-b]benzopyrans was generated from the acid-catalyzed reactions of 7-methoxyflavenes and 5,7,8-trimethoxyflavenes. Interestingly, the acid-catalyzed reactions attempted on 5-methoxy- and 6-methoxyflavenes were found to undergo dimerization differently to yield a novel range of biflavonoids containing the tetrahydrochromeno[2,3-b]chromene ring system. A plausible mechanism for the observed rearrangement has also been proposed. The effect of various acids used as catalysts on the dimerization reactions has been evaluated and discussed in this thesis. Similarly, the acid-catalyzed dimerization reactions on isoflavanols and their corresponding isoflavenes were studied in depth and dimerization was found to be dependent on the presence of electron rich substituents in ring B of the isoflavanols and also in performing the reaction in very cold conditions. A probable rationale to this rearrangement has been described. The widespread applications of Mannich bases in medicinal chemistry are noteworthy. Hence, another part of the project was to study the Mannich reactions on flavones using various primary and secondary amines, aminals and amino acids and to probe into the regioselectivity and chemoselectivity of the products obtained. The key point of interest to note was the site of activation in the flavone ring, which was C5 in 6-hydroxyflavones and C8 in 7-hydroxyflavones. The primary amines yielded benzoxazines whereas the secondary amines, aminals and amino acids yielded simple Mannich bases. Further, the benzoxazines were found to undergo cleavage to generate the corresponding simple Mannich bases. Our research group has previously worked on the synthesis of a series of 4-arylflavans and 4- arylisoflavans, but however, there are no reports on the synthesis of 4-arylazaflavans in the literature. Hence, 4-arylazaflavans were synthesized via the acid-catalyzed reactions of iv azaflavanols with nucleophiles such as naphthol and dimethoxyphenol, whereas 4- heteroarylazaflavans were obtained using heterocycles such as furan, indoles and isoflavenes. Attempts were made to synthesize the corresponding azaflavenes with the primary aim on subjecting them to acid-catalyzed dimerization reactions similar to the flavenes. However, the dominant product isolated was the corresponding quinoline in all cases. Several experiments were conducted in order to oxidize the 4-arylazaflavans to the corresponding quinolines. The optimum conditions were achieved with the use of iodine as catalyst. This introduces a new strategy to the synthesis of 2,4-disubstituted quinolines. v ACKNOWLEDGEMENTS I consider this page as the most important one in my entire thesis as I take time to thank all who have contributed in making this thesis successful. First and foremost, I thank God Almighty for bringing me to this planet, Australia to do my doctorate. He has been so faithful to me, in providing all my needs and also helped me every step of the way till the very end. As I look back to the past three and half years, I should say that the path has not been smooth, but surely God’s grace has been sufficient for me and His presence so dearly felt to comfort, strengthen and guide me all the way through. Next, I would like to express my gratitude to my supervisors, A/Prof. Naresh Kumar and Prof. David Black, for giving me the opportunity to work on flavonoid chemistry. I thank them for their valuable guidance and help throughout the course of the project. I wish to thank all the faculty members in the School of Chemistry, especially the professional staff namely, Jim Hook, Hilda Stender, Donald Thomas, Adelle Moore, Don Craig, Mohan Bhadbhade, Barry Ward, Ian Aldred, Joseph Antoon, Toby Jackson, Ken McGuffin, Jodee Anning, Rick Chan, Nick Robert and Anne Ayres for their timely help. I am also grateful for the help received from Thanh, Michael, Sharif, Berta, Peta, Nancy and Sveto in the teaching labs. I am thankful to the past and present members of the Kumar and Black group. I owe my thanks to Dr. Abel Salek for his help with the dimerization reactions of isoflavanols and isoflavenes. I wish to acknowledge Chin Fei Chee who worked with me from October to December 2008 on mannich reactions of flavones. I am extremely grateful to my parents, who have been so dear to me. I owe my thanks to them for their love, prayers, constant support and moral encouragement throughout the course of my work. I also thank all my relatives and friends for their well wishes for my success. I would also like to mention the Christian groups that I was part of, in Sydney who have always been around at all times to share my joys and my sorrows, and have made my stay in Sydney a pleasant and memorable one. I am thankful to the members of Randwick Presbyterian Church for their warmth and care. I am also grateful to the fellowship I had with my fellow Indians in vi FUESIA (Friends of UESI in Australia). I thank all at FOCUS (Fellowship of Overseas Christian University Students), especially the Friday night bible studies, which were so refreshing. I will be failing in my duty if I do not acknowledge my friends in UESI (Union of Evangelical Students in India), members of AMC (Annanagar Methodist Church), Chennai and my colleagues at Orchid Research Laboratories Limited, Chennai. UIPA (University International Postgraduate Award) from UNSW during the three and half years is greatly acknowledged. I thank the School of Chemistry for the award of the supplementary Postgraduate Teaching Fellowship. Finally, I thank one and all, who have helped me with this thesis. vii TABLE OF CONTENTS Certificate of originality ii Copyright and authenticity statement iii Abstract iv Acknowledgements vi Table of contents viii Abbreviations xiii Publications and presentations xv CHAPTER 1: INTRODUCTION 1.1. General introduction .2 1.2. Phytoestrogens 2 1.3. Classification of phytoestrogens 3 1.4. Mechanism of action of phytoestrogens 5 1.4.1. Inhibition of aromatase 6 1.4.2. Mechanism of action via the estrogen receptor 9 1.5. Chemistry of the flavonoids 10 1.6. Medicinal uses of flavonoids and isoflavonoids 12 1.7. Biflavonoids 13 1.8. Biisoflavonoids 17 1.9. Limitations 18 1.10 Aims of the present work 18 viii CHAPTER 2: ACID-CATALYZED DIMERIZATION OF FLAVENES: SYNTHESIS OF BENZOPYRANO[4,3-b]BENZOPYRANS 2.1. Introduction 21 2.1.1. Known synthetic methodologies 21 2.2. Dependensin 23 2.3. Synthesis of 7-methoxyflavenes 25 2.4. Acid-catalyzed reactions on 7-methoxyflavenes 29 2.4.1. Mechanism for the acid-catalyzed rearrangement 32 2.5. Synthesis of 5,7,8-trimethoxyflavenes 33 2.6. Acid-catalyzed reactions on 5,7,8-trimethoxyflavenes 35 2.6.1. Biological activity of dependensin analogues 38 2.7. Synthesis and acid-catalyzed reaction of 4c,5,7-trimethoxyflavene 39 2.8. Conclusion 41 CHAPTER 3: ACID-CATALYZED DIMERIZATION OF FLAVENES: SYNTHESIS OF TETRAHYDROCHROMENO[2,3-b]CHROMENES 3.1.