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Approaches Towards the Synthesis of Saxitoxin Alkaloids

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Approaches Towards the Synthesis of Saxitoxin Alkaloids DISSERTATION APPROACHES TOWARDS THE SYNTHESIS OF SAXITOXIN ALKALOIDS Submitted by Aaron Daniel Pearson Department of Chemistry In partial fulfillment of the requirements For the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Summer 2013 Doctoral Committee: Advisor: Robert M. Williams John L. Wood Alan J. Kennan Ellen R. Fisher Robert W. Woody ABSTRACT APPROACHES TOWARDS THE SYNTHESIS OF SAXITOXIN ANALOGS Zetekitoxin AB is a toxin isolated from the Panamanian golden frog (Atelopus zeteki). The structure and activity of zetekitoxin AB was a mystery for 30 years until 2004 when it was elucidated by Yamashita and coworkers1. It was found to be a potent analog of Saxitoxin, a marine neurotoxin. Saxitoxin is a sodium channel blocker and has been used extensively as a research probe. Zetekitoxin AB shows an affinity profile similar to saxitoxin, but is considerably more potent. Due to the endangerment of the Panamanian golden frog there is no source of zetekitoxin AB, preventing further studies. Presented herein is a concise synthesis of 4,5-epi-11-hydroxy-saxitoxinol, which utilizes D-ribose to direct an asymmetric Mannich reaction. This approach allows many modes of reactivity, which can be used to potentially access various analogs of saxitoxin with novel bioactivity. ii ACKNOWLEDGEMENTS The work contained herein would not have been possible without the support of my colleagues, family and friends. First and foremost, I would like to thank Professor Robert Williams for his continued support and for allowing me to explore my own ideas. This has allowed me to develop independence and problem solving skills that will be with me the rest of my life. Thanks to all the Williams group members, past and present, for creative and constructive chemistry discussions. In particular I want to give a special thanks to Dr. Paul Schuber and Phil Bass whose friendship helped me to survive the Colorado State University chemistry PhD program. I would also like to thank Chris Rithner for teaching me so much about NMR spectroscopy and for helping me figure out the difficult structures I encountered during the course of my project. Most of all, I would like to thank my parents, Dennis and Terri, and my brother Matthew for always encouraging me and allowing me to pursue my interest in chemistry starting at the young age of 15. Without their continuous support I would have never made it thus far. iii TABLE OF CONTENTS CHAPTER 1: INTRODUCTION ................................................................................................... 1 1.1 NEUROTOXINS ................................................................................................................ 1 1.1.1 Ion Channels ........................................................................................................ 1 1.1.2 Neurotoxins Affecting Ion Channels ................................................................... 1 1.1.3 Therapeutics Targeting Ion Channels .................................................................. 2 1.2 SAXITOXIN AND ANALOGS ............................................................................................. 3 1.2.1 Isolation and Structure Determination ................................................................ 3 1.2.2 Zetekitoxin AB .................................................................................................... 5 1.2.3 Toxicity ............................................................................................................... 6 1.2.4 Human Impact and Control ................................................................................. 7 1.3 SAXITOXIN BIOGENESIS .................................................................................................. 7 1.3.1 Feeding Experiments ........................................................................................... 8 1.3.2 Isolation and Characterization of the Saxitoxin Gene Cluster .......................... 13 1.4 PREVIOUS SYNTHETIC ACHIEVEMENTS ......................................................................... 15 1.4.1 Kishi .................................................................................................................. 15 1.4.2 Jacobi ................................................................................................................. 17 1.4.3 Du Bois .............................................................................................................. 19 1.4.4 Nagasawa ........................................................................................................... 24 1.4.5 Nishikawa .......................................................................................................... 27 1.4.6 Looper ............................................................................................................... 28 1.5 CONCLUSION AND GOALS ............................................................................................. 30 CHAPTER 2: SYNTHESIS OF SAXITOXIN ANALOGS ........................................................ 31 iv 2.1 DEVELOPMENT OF THE MANNICH REACTION ................................................................ 32 2.1.1 Precedence ......................................................................................................... 32 2.1.2 Initial Attempts .................................................................................................. 34 2.1.3 Masked Imines as Reactive Mannich Precursors .............................................. 35 2.1.4 Successful Mannich Reaction ........................................................................... 37 2.1.5 Useful Mannich Substrate ................................................................................. 39 2.1.6 Scale-up of Mannich Substrate .......................................................................... 41 2.2 BIS-GUANIDINYLATION APPROACH .............................................................................. 42 2.2.1 Retrosynthesis ................................................................................................... 42 2.2.2 Formation of diamine and installation of guanidines ........................................ 43 2.2.3 Debenzylation .................................................................................................... 44 2.2.4 Reduction of Lactol and Oxidation of Alcohol ................................................. 46 2.2.5 Deprotection of Guanidines ............................................................................... 50 2.2.6 Differentiated guanidines .................................................................................. 52 2.2.7 Late-Stage Guanidine installation ..................................................................... 63 2.3 REDUCTIVE AMINATION APPROACH ............................................................................. 72 2.3.1 Oxazolidinone Formation .................................................................................. 72 2.3.2 Initial Reductive Amination Attempts .............................................................. 74 2.3.3 Reductive Amination: A Stepwise Approach ................................................... 75 2.3.4 Formation of the 9-Member Ring ..................................................................... 76 2.3.5 Reductive Amination ......................................................................................... 80 2.3.6 Isolation and Characterization of Polar Compounds ......................................... 91 2.3.7 Progress towards β-Hydroxysaxitoxinol ........................................................... 93 v 2.4 SYNTHETIC SUMMARY AND CONCLUSION .................................................................. 106 CHAPTER 3: EXPERIMENTAL SECTION ............................................................................ 108 3.1 GENERAL CONSIDERATIONS ....................................................................................... 108 3.2 EXPERIMENTAL PROCEDURES ..................................................................................... 110 REFERENCES ............................................................................................................................ 227 APPENDIX 1: X-RAY DATA ................................................................................................... 232 COMPOUND 211 ................................................................................................................ 232 LIST OF ABBREVIATIONS ..................................................................................................... 242 vi Chapter 1: Introduction 1.1 Neurotoxins Neurotoxins represent one of the many types of toxins, which are classified by their biological target. The common classifications of toxins are: cytotoxins, which affect cells; hepatotoxins, which affect the liver; and neurotoxins, which affect the nervous system. The classes of toxins themselves are divided into different groups based on their mode of action. Neurotoxins, for example, can exhibit toxicity by ion channel disruption, altering the blood brain barrier, synaptic vesicle release inhibition and cytoskeleton disruption. 1.1.1 Ion Channels The basis of nerve transmissions is the action potential, which is the buildup and fall of an electrical membrane potential. This potential is controlled by the flow of ions through membrane-bound proteins known as ion channels. There two main classes of ion channels: voltage-gated and ligand-gated.
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