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New Methods for the Synthesis of All-Carbon Quaternary Centers Via the Reactions Of New Methods for the Synthesis of All-Carbon Quaternary Centers via the Reactions of N-Vinyl Nitrones and Phenyl Hydrazines with Ketenes Ryan Edward Michael B.S., Chemistry, University of Central Florida, 2007 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Chemistry and Biochemistry 2013 i This thesis entitled: New Methods for the Synthesis of All-Carbon Quaternary Centers via the Reactions of N-Vinyl Nitrones and Phenyl Hydrazines with Ketenes Written by Ryan Edward Michael Had been approved for the Department of Chemistry and Biochemistry Tarek Sammakia Xiang Wang Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. ii Michael, Ryan Edward (Ph.D., Chemistry) New Methods for the Synthesis of All-Carbon Quaternary Centers via the Reactions of N-Vinyl Nitrones and Phenyl Hydrazines with Ketenes Thesis directed by Professor Tarek Sammakia This dissertation describes the development of new cascade reactions for the synthesis of quaternary carbons bearing additional proximal hindrance. The general reaction design principles are described which include the in-situ generation of a vicinal di-π system capable of undergoing a charge-accelerated [3,3]-sigmatropic rearrangement facilitated by the cleavage of a weak heteroatom-heteroatom bond. Also described is a practical route for the synthesis of N- vinyl nitrones that utilizes a 1,4-conjugate elimination as the key step. The reaction of an N- vinyl nitrone and a disubstituted ketene to form all-carbon quaternary centers is also detailed. Mechanistic studies provide evidence that the reaction proceeds by a pericyclic cascade reaction involving an initial [3 + 2] dipolar cycloaddition followed by two consecutive [3,3]-sigmatropic rearrangements. An asymmetric version of this reaction based on a chiral auxiliary approach is also described. Finally, a new method for the synthesis of oxindoles containing a quaternary carbon at the 3-position is described. This novel method proceeds via the copper triflate- mediated reaction of a phenyl-hydrazine and a ketene. iii Acknowledgements I would like to start by thanking the people who made the Sammakia lab such a wondrous place to work these last few years. Carolynn, I will always miss your nail biting stories. Will, you always know how to cheer someone up. Jeffrey, you’ve shown me how to lead by example. Mai, you just made things happen. Katelyn, you rekindled my interest in Disney movies. And honorary member Price, I will always regret that Thanksgiving I missed out on. In all seriousness though, I could not have made it without you. Thanks. I would also like to thank Professor Tarek Sammakia, who without his support none of this would have ever happened. Tarek has taught me not only chemistry, but also how to think about problems, and most importantly, how to communicate. Thank you Tarek. Most of all I would like to thank my beautiful wife Maureen. Maureen’s patience and support have made all this possible. And lastly, my little Lily, who brings more joy into my life than I thought possible. iv Contents Abstract .......................................................................................................................................... iii Acknowledgements ........................................................................................................................ iv Table of Contents .............................................................................................................................v List of Tables ............................................................................................................................... viii List of Figures ................................................................................................................................ ix List of Schemes ................................................................................................................................x 1 The Claisen and Related Rearrangements ...........................................................................1 1.1 Introduction ..........................................................................................................................1 1.2 The Claisen Rearrangement .................................................................................................1 1.3 Variations of the Claisen Rearrangement ............................................................................4 1.4 Charge Accelerated Variants .............................................................................................11 1.5 Stereochemical Considerations ..........................................................................................19 1.6 General Considerations in Reaction Design ......................................................................22 1.7 References ..........................................................................................................................25 2 The Synthesis of N-Vinyl Nitrones ......................................................................................31 2.1 Previous Syntheses of N-Vinyl Nitrones ...........................................................................31 2.2 The Difficulties with Traditional Methods for the Synthesis of N-Vinyl Nitrones ...........38 2.3 N-Vinyl Nitrones via a 1,4-Conjugate Elimination of α-Chloro-aldehydes ......................39 v 2.4 Application of a 1,4-Conjugate Elimination for the Synthesis of Ketone Derived N-Vinyl Nitrones ..............................................................................................................................46 2.5 The Synthesis of Tri-Substituted N-Vinyl Nitrones ...........................................................58 2.6 Concluding Remarks ..........................................................................................................60 2.7 Experimental ......................................................................................................................61 2.8 References ..........................................................................................................................87 3 The Reaction of N-Vinyl Nitrones and Ketenes .................................................................93 3.1 General Reaction Considerations .......................................................................................93 3.2 Initial Studies of the Reaction of N-Vinyl Nitrones and Ketenes ......................................94 3.3 Mechanistic Studies .........................................................................................................106 3.4 The Development of an Asymmetric N-Vinyl Nitrone/Ketene Pericyclic Reaction Cascade ............................................................................................................................114 3.5 Concluding Remarks ........................................................................................................122 3.6 Experimental ....................................................................................................................123 3.7 References ........................................................................................................................145 4 The Reaction of Phenyl-Hydrazines and Ketenes ............................................................149 4.1 Introduction ......................................................................................................................149 4.2 The Reaction of Phenyl-Hydrazines and Ketenes for the Synthesis of 3,3-Disubstituted Oxindoles .........................................................................................................................152 4.3 Investigations of an Enantioselective Phenyl-Hydrazine/Ketene Reaction .....................164 4.4 Concluding Remarks ........................................................................................................167 4.5 Experimental ....................................................................................................................167 4.6 References ........................................................................................................................171 vi Bibliography .............................................................................................................................175 vii List of Tables Table 2.1 Optimization of the Conditions for the 1,4-Conjugate Elimination of HCl .................42 Table 2.2 The Condensation of α-Chloro Aldehydes With Hydroxylamines ..............................43 Table 2.3 Scope of t-BuOK Induced 1,4-Elimination of HCl from α-Chloro Nitrones ...............45 Table 2.4 Synthesis of α-Substituted Nitrones .............................................................................49 Table 2.5 Optimization of 1,4-Conjugate Elimination Conditions for Ketone Derived Nitrones ........................................................................................................................50 Table 2.6 Scope of the α-Phenoxyketone/ α-acetoxyketone Hydroxylamine Condensation .......53 Table 2.7 Scope of the 1,4-Conjugate Elimination for the Synthesis of Ketone Derived N-Vinyl Nitrones ........................................................................................................................56
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