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Synthetic Studies of the Yunnaneic Acids Daniel R. Griffith Submitted In Synthetic Studies of the Yunnaneic Acids Daniel R. Griffith Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2013 © 2013 Daniel R. Griffith All rights reserved ABSTRACT Synthetic Studies of the Yunnaneic Acids Daniel R. Griffith Chapter 1. Introduction. Caffeic acid and its metabolites are widely distributed throughout the plant kingdom. Nature exploits the reactivity of the catechol and unsaturated acid functionalities of multiple caffeic acid units to create a diverse library of molecules, many of which are biologically active. Like other plant-based polyphenols, this biological activity often manifests as anti-oxidant activity. The reason for such activity usually lies in the catechol group common to all of these natural products. Biosynthetic pathways to these compounds from caffeic acid, which itself arises via the shikimic acid pathway, are discussed. The proposed biosynthesis of the yunnaneic acids is also discussed, and a possible alternative is put forward, informed by the recent isolation of rufescenolide. Selected total syntheses of some of these caffeic acid metabolites are described, providing a backdrop for a discussion of the unique synthetic challenges presented by the yunnaneic acids. Chapter 2. Synthetic Studies of Yunnaneic Acids C and D The total syntheses of yunnaneic acids C and D were completed using a lead-mediated oxidative dearomatization/Diels–Alder reaction cascade to forge the bicyclo[2.2.2]octene core of the molecules in one step from simple precursors. The route featuring this key step was developed after a first-generation route using hypervalent iodine to effect dearomatization had been explored and was found to be fraught with inefficiencies. The tricyclic key intermediate resulting from the lead-mediated cascade sequence was also exploited to complete the total synthesis of rufescenolide. Chapter 3. Synthetic Studies of Yunnaneic Acids A and B Model studies towards the dimeric yunnaneic acids A and B were undertaken, which resulted in the characterization of several novel dimeric compounds whose connectivity and/or stereochemistry did not correspond to that found in the natural products. Although the total synthesis of yunnaneic acids A and B could not be realized, a selective pseudodimerization between model versions of yunnaneic acids C and D was accomplished, with no competing homodimerization, by exposing the two dimerization partners to hexafluoroisopropanol, a protic, non-nucleophilic solvent. TABLE OF CONTENTS Chapter 1. Introduction ................................................................................................................1 1.1 Caffeic Acid Metabolites ...................................................................................................2 1.2 Biological Significance of Plant-Based Polyphenols and General Mechanism of Action ..................................................................................................................................5 1.3 Biosynthesis of Caffeic Acid and Some of its Derivatives ..............................................8 1.3.1 Caffeic Acid ....................................................................................................................8 1.3.2 Rosmarinic Acid .............................................................................................................9 1.3.3 Podophyllotoxin ............................................................................................................11 1.3.4 Lithospermic Acid ........................................................................................................11 1.3.5 Helicterin Natural Products ...........................................................................................12 1.3.6 Yunnaneic Acids ...........................................................................................................16 1.4 Synthetic Studies of Caffeic Acid Metabolites–Selected Examples .............................22 1.4.1 Podophyllotoxin ............................................................................................................22 1.4.2 Lithospermic Acid ........................................................................................................24 1.4.3 Helicterin Natural Products ...........................................................................................26 1.5 Yunnaneic Acids: A Unique Set of Synthetic Challenges ............................................28 1.6 References .........................................................................................................................30 Chapter 2. Synthetic Stuides of Yunnaneic Acids C and D .....................................................34 2.1 Early Synthetic Explorations ..........................................................................................35 2.2 Second Generation Synthesis of Model Yunnaneic Acid C and Successful Approach to Model Yunnaneic Acid D ............................................................................................45 i 2.3 Total Synthesis of Rufescenolide ....................................................................................53 2.4 Total Synthesis of Yunnaneic Acids C and D ................................................................55 2.5 Conclusion ........................................................................................................................62 2.6 References .........................................................................................................................65 2.7 Experimental Section .......................................................................................................68 Chapter 3. Synthetic Stuides of Yunnaneic Acids A and B ...................................................186 3.1 Dimerization Strategy and Literature Precedent .......................................................187 3.2 Dimerization Studies Towards Yunnaneic Acid A .....................................................194 3.3 Dimerization Studies Towards Yunnaneic Acid B .....................................................199 3.4 Conclusion ......................................................................................................................200 3.5 References .......................................................................................................................202 3.6 Experimental Section .....................................................................................................204 ii ACKNOWLEDGMENTS First and foremost, I would like to acknowledge Prof. Scott Snyder for being such a supportive mentor. Total synthesis can be a real battle, especially when working on a project alone. Whenever I thought that I was not equal to the task, Scott always responded with encouragement and with total belief in my abilities as a chemist. Scott also provided me with total freedom to pursue the ideas that excited me the most, which helped immeasurably in my growth as a scientist. After five years as a member of the Snyder lab, I can confidently say that pursuing a Ph.D. in chemistry was the best decision that I ever made, and that could not have been the case without a great mentor to guide those five years. I would like to thank my family–Mom, Dad, and Allison–for their love and encouragement. I could not have made it to this point without their support. My parents’ continual assertion that “everything will work out” became my mantra throughout graduate school. I am also grateful to Allison for deciding to come to Columbia for her Ph.D. studies. Having a supportive family member so nearby the past few years has enhanced my graduate school experience tremendously. I have also had the privilege of seeing her scientific development up close, which has made me one proud older brother. I would like to thank Dr. Lorenzo Botta for his help in making the synthesis of yunnaneic acid C a reality and his diligent efforts in the dimerization studies towards yunnaneic acid B. After three years of struggling on my own, it was a tremendous relief to have his help on this project. I could not have had a nicer guy to work with, and he made the difficulties associated with this project much more bearable. I would also like to thank Tyler St. Denis for being such an enthusiastic undergraduate and for his work on rufescenolide. Tyler was always eager to listen to my half-baked ideas and then do the necessary literature searching to make them work in the iii lab. I would also like to acknowledge Dr. Ferenc Kontes for his help in the early stages of the project and, most importantly, for training me when I started in the group and serving as an invaluable source of advice throughout graduate school. I would also like to acknowledge the members of the Snyder group for being such an awesome group of co-workers. I could not have put in the commitment to research that I did without such a great environment in which to do chemistry. In particular, I would like to acknowledge the following individuals: Myles Smith (for being a great hood-mate and sharing his enthusiasm for chemistry along with his encyclopedic knowledge of the literature); Dr. Adel ElSohly (for being essentially a second advisor; I look forward to reuniting in Berkeley); Trevor Sherwood (a true brother-in-arms; BMS is a lucky company); Dr. Daniel Treitler and Nathan Wright (for being great company during the early mornings in lab and for being
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