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Method Development for the Stereoselective Synthesis of Medium-Sized Cyclic Ethers and Application to Natural Product Synthesis: Part I

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Method Development for the Stereoselective Synthesis of Medium-Sized Cyclic Ethers and Application to Natural Product Synthesis: Part I Method Development for the Stereoselective Synthesis of Medium-Sized Cyclic Ethers and Application to Natural Product Synthesis: Part I. Organocatalytic Oxa-Conjugate Addition for α,α´-trans-Oxepanes Part II. Gold(I)-Catalyzed Alkoxylation for α,α´-cis - Oxocenes Part III. Studies toward the Synthesis of (+)-Intricenyne by Megan L. Lanier Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Jiyong Hong, Supervisor ___________________________ Ross A. Widenhoefer ___________________________ Qiu Wang ___________________________ Katherine J. Franz Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2015 ABSTRACT Method Development for the Stereoselective Synthesis of Medium-Sized Cyclic Ethers and Application to Natural Product Synthesis: Part I. Organocatalytic Oxa-Conjugate Addition for α,α´-trans-Oxepanes Part II. Gold(I)-Catalyzed Alkoxylation for α,α´-cis - Oxocenes Part III. Studies toward the Synthesis of (+)-Intricenyne by Megan L. Lanier Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Jiyong Hong, Supervisor ___________________________ Ross A. Widenhoefer ___________________________ Qiu Wang ___________________________ Katherine J. Franz An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2015 Copyright by Megan L. Lanier 2015 Abstract Medium-sized cyclic ethers are challenging synthetic targets due to enthalpic and entropic barriers. Methods for the stereoselective synthesis of α,α΄-disubstituted medium- sized cyclic ethers began to appear with the discovery of naturally-occurring, ladder- shaped polycyclic ethers, such as brevetoxin B, and monocyclic ethers, such as (+)- laurencin. Despite the progress made in this field, limitations remain including competing formation of smaller ring sizes and scarcity of catalytic methods. Our aim has been to develop stereoselective syntheses for 7- and 8-membered cyclic ethers which have potential for application in natural product synthesis. The C–O bond disconnection was selected for the methods described within because cyclization and stereoinduction could be achieved simultaneously. In the case of 7-membered cyclic ethers, an organocatalytic oxa-conjugate addition reaction promoted by the gem -disubstituent (Thorpe−Ingold) effect has been developed to stereoselectively provide α,α′-trans -oxepanes. A gold(I)-catalyzed alkoxylation reaction has also allowed access to α,α′-cis-oxocenes. This method has been probed for feasibility in the stereoselective synthesis of (+)-intricenyne, an 8-membered cyclic ether belonging to the C 15 nonterpenoid acetogenin natural product class. These methods have the potential to become general and efficient routes to highly functionalized oxepanes and oxocenes. iv Dedication To my parents, Andrew D. Lanier and Cecilia A. Lanier, for their constant support and encouragement. To Ryan L. Hanna for waiting for me to come home to you. To Professor Debra D Dolliver and Professor David Norwod for mentoring me. To my family and friends, for bringing meaning to my life. v Contents Abstract ......................................................................................................................................... iv List of Tables ................................................................................................................................. ix List of Figures ................................................................................................................................ x List of Schemes .......................................................................................................................... xiii List of Abbreviations ............................................................................................................... xvii Acknowledgements ................................................................................................................... xxi 1. Introduction........................................................................................................................... 1 1.1 Natural Product Synthesis .............................................................................................. 1 1.2 Significance of Medium-Sized Cyclic Ether Synthesis ................................................ 3 1.3 Challenges of Medium-Sized Cyclic Ether Synthesis .................................................. 7 1.4 General Approaches to the Synthesis of Medium-Sized Cyclic Ethers .................... 8 1.5 Numbering System for Entire Dissertation .................................................................. 9 1.6 Summary .......................................................................................................................... 10 2. Stereoselective Synthesis of α,α´-trans -Oxepanes via an Organocatalytic Oxa- Conjugate Addition Promoted by the gem -Disubstituent Effect .......................................... 11 2.1 Background ..................................................................................................................... 11 2.1.1 Stereoselective Synthesis of α,α´-cis -Oxepanes ..................................................... 15 2.1.2 Stereoselective Synthesis of α,α´-trans -Oxepanes ................................................. 22 2.1.3 Oxa–Conjugate Addition as a Method for Oxacycle Synthesis .......................... 30 2.1.4 Organocatalytic Activation of α, β-Unsaturated Aldehydes ................................ 35 2.1.5 Previous Studies ........................................................................................................ 41 vi 2.1.6 Summary of Background ......................................................................................... 46 2.2 Results and Discussion .................................................................................................. 47 2.2.1 Optimization of Organocatalyzed Oxa-Conjugate Addition Conditions ......... 47 2.2.2 Development of the Tandem Organocatalytic Oxa-Conjugate Addition/α- Oxidation Reaction ............................................................................................................. 61 2.3 Attempts to Extend Methodology to the Preparation of Eight-Membered Cyclic Ethers ...................................................................................................................................... 66 2.4 Summary .......................................................................................................................... 76 2.5 Experimental Section ..................................................................................................... 77 3. Stereoselective Synthesis of α,α´-cis -Oxocenes via Gold(I)-Catalyzed Alkoxylation . 123 3.1 Background ................................................................................................................... 123 3.1.1 Stereoselective Synthesis of α,α´-Disubstituted Oxocenes ................................ 125 3.1.2 Gold(I)-Catalyzed Alkoxylation as a Method for Oxacycle Synthesis ............ 142 3.1.3 Summary of Background ....................................................................................... 147 3.2 Results and Discussion ................................................................................................ 148 3.2.3 Preliminary Studies ................................................................................................. 148 3.2.3 Optimization of the Gold(I)-Catalyzed Alkoxylation Reaction for α,α´- Disubstituted-Oxocenes .................................................................................................. 161 3.2.4 Determination of the Relative Configuration of α,α΄-Disubstituted Oxocene 3.101 .................................................................................................................................... 166 3.3 Summary ........................................................................................................................ 171 3.4 Experimental Section ................................................................................................... 171 4. Studies toward a Stereoselective Total Synthesis of (+)-Intricenyne ............................. 178 vii 4.1 Background ................................................................................................................... 180 4.1.1 Isolation and Structural Elucidation of (+)-Intricenyne ..................................... 180 4.1.2 Similar Oxocene Natural Products ....................................................................... 181 4.1.2.1 Total Syntheses of (+)-Laurencin.................................................................... 185 4.1.2.2 Background ....................................................................................................... 186 4.1.2.3 Previous Syntheses of Laurencin ................................................................... 187 4.1.3 Summary of Background ....................................................................................... 220 4.2 Results and Discussion ................................................................................................ 221 4.2.1 Retrosynthetic Analysis of (+)-Intricenyne .........................................................
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