Total Synthesis of Natural Product Pterocarpans Useful As Selective Estrogen Receptor Modulators
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A Dissertation entitled Total Synthesis of Natural Product Pterocarpans Useful as Selective Estrogen Receptor Modulators by Neha Malik Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Medicinal Chemistry ___________________________________________ Paul W. Erhardt, Ph.D., Committee Chair ___________________________________________ L. M. V. Tillekeratne, Ph.D., Committee Member ___________________________________________ Ronald E. Viola, Ph.D., Committee Member ___________________________________________ Amanda Bryant-Friedrich, Ph.D., Committee Member ___________________________________________ Jeffrey G. Sarver, Ph.D., Committee Member ___________________________________________ Patricia R. Komuniecki, Ph.D., Dean College of Graduate Studies The University of Toledo December 2013 Copyright 2013, Neha Malik This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of Total Synthesis of Natural Product Pterocarpans Useful as Selective Estrogen Receptor Modulators by Neha Malik Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Medicinal Chemistry The University of Toledo December 2013 Breast cancer is one of the leading causes of cancer deaths in women worldwide. Inhibition of estrogen receptors is an important strategy for the treatment of estrogen dependent breast cancer. Estrogen mimicking plant compounds called ‘phytoestrogens,’ can bind to estrogen receptors and exert estrogenic effects. They also serve to block the effects of the normal estrogens present in humans. Thus, these phytoestrogens have the ability to act as natural, selective estrogen receptor modulators or SERMs. Soy glyceollins (GLYs) have been recently identified as estrogen mimics or phytoestrogens and represent novel natural SERMs. The GLYs are 6a-hydroxypterocarpans that are elicited as phytoalexins or plant defensive agents when soybean plants or seeds are stressed. They are produced in trace quantities as a mixture of three isomers glyceollin I, II and III. Despite their promising therapeutic potential, complete pharmacological characterization of such 6a-hydroxypterocarpans remains hampered by limited access to these systems either from natural or synthetic sources. Most notable is the lack of methodology to establish the 6a-hydroxy group for which only a few practical routes have been made available by reported syntheses. The most common route deploys a iii Sharpless dihydroxylation that requires osmium tetroxide to produce a benzopyran-diol that can subsequently be closed to the benzofuran cis-fused system. A drawback for the latter, however, becomes the toxicity associated with osmium tetroxide that diminishes enthusiasm for its use on large scale. This dissertation involves the development of novel approaches for the synthesis of 6a-hydroxypterocarpans which do not require osmium tetroxide. These routes are centered on an intramolecular benzoin condensation to install the key 6a-hydroxy center present in these systems. The practicality of this chemical route has been demonstrated by the novel synthesis of glycinol (GLO) in its racemic form. GLO is the key biosynthetic precursor to the GLYs and has interesting biological properties which are similar to the parent GLYs. This phytoalexin is known for its estrogen-like effects and hence it’s potential to be developed as a SERM for use during hormone replacement therapy. A one-pot method was developed that facilitated intramolecular cyclization leading to assembly of the benzofuran and direct production of GLO. The chemistry developed to obtain racemic GLO was then utilized for the total syntheses of glyceollin II and III in their racemic forms. This project also involves medicinal chemistry efforts directed toward further defining the unique SERM activity observed for this novel family of phytoalexin natural products. In that regard, molecular modeling and receptor docking studies were performed to further define the interactions of these molecules with their cellular targets, namely, the estrogen receptors. Lastly, because a new chemical route was developed, certain key synthetic steps have been devised. These steps have general applicability as new methods that are useful for future process chemistry initiatives that may be undertaken within the iv context of this interesting class of novel SERM compounds and 6a-hydroxypterocarpans in general. v Acknowledgements I would like to express my gratitude to all the people who have contributed toward getting me to this pedestal. I feel blessed to have had these people in my life that have played instrumental roles in my professional and personal development. Firstly, I would like to express my deepest appreciation for my mentor and advisor, Dr. Paul Erhardt. He has been a pillar of support for me throughout the course of my Ph.D. at the University of Toledo. He is a great scientist with unlimited zeal and a wonderful human being. I truly feel honored to have worked under him and could not have asked for a better mentor. I would next like to thank Dr. Tillekeratne, Dr. Viola, Dr. Bryant-Friedrich and Dr. Sarver for serving on my dissertation committee. Their guidance, encouragement and valuable insights over these years have immensely helped me in thinking critically and being a better scientist. I specially thank all CD3 lab members, former and present, especially Mike Reese, Rahul, Amarjit, Mohammad, Mike Robinson, Pravin, Rachael, Chris, Jill and Nicole for being extremely wonderful co-workers. I would like to acknowledge and thank Dr. Yong- Wah Kim with all the help he has given me with the NMR instrumentation over the past five years. Last but not least, my sincere and most heartfelt thanks to my friends and family members especially my parents B.K. Malik and Poonam Malik, my sister Radhika and my brother-in-law Omnath for their constant motivation and unwavering support. vi Contents Abstract ......................................................................................................................iii Acknowledgments......................................................................................................vi Contents .....................................................................................................................vii List of Tables .............................................................................................................x List of Figures ............................................................................................................xi List of Schemes ..........................................................................................................xiv List of Abbreviations ................................................................................................xvi List of Spectra and Chromatograms ..........................................................................xx 1 Introduction .............................................................................................................1 1.1 Outline and Aim of Dissertation ..............................................................1 1.2 Breast Cancer ..........................................................................................2 1.3 Role of Estrogens and Estrogen Receptors in Breast Cancer ..................2 1.4 Selective Estrogen Receptor Modulators (SERMs) ................................5 1.5 Phytoestrogens .........................................................................................8 1.6 Isoflavonoids ...........................................................................................11 1.6.1 Pterocarpans ..............................................................................12 1.6.1.1 6a-Hydroxypterocarpans ............................................12 1.6.2 Phytoalexins .............................................................................13 1.6.3 Glyceollins (GLYs) ...................................................................14 vii 1.7 Biological Testing of Compounds ...........................................................16 2 Results and Discussion - Chemical Syntheses ........................................................17 2.1 Synthesis of 6a-Hydroxypterocarpans .....................................................17 2.1.1 Model Chemistry - Synthesis of Glycinol (GLO) ....................18 2.1.1.1 Biomimetic Synthesis ................................................19 2.1.1.2 Non-Biomimetic Route ..............................................23 2.1.1.3 Synthesis via Intramolecular Benzoin Condensation..............................................................23 2.2 Synthesis of Glyceollin II (GLY II) .........................................................39 2.2.1 Retrosynthetic Strategy ..............................................................39 2.2.2 Early Efforts and Results ...........................................................41 2.2.3 Completion of Synthesis ............................................................42 2.3 Synthesis of Glyceollin III (GLY III) ......................................................50 2.3.1 Initial Retrosynthetic Strategy ...................................................50 2.3.2 Later Synthetic Efforts ...............................................................53 2.3.3 Future Directions to Synthesis ...................................................58