Synthesis and Applications of Ynimines and Enantioselective Total Synthesis of Artatrovirenol A

The first chapter of this thesis describe the development of a general synthesis of ynimines, an under-exploited motif in organic chemistry. In the presence of an inexpensive copper catalyst and 2,2'-biquinoline, reaction of easily accessible O-acyloximes with commercially available terminal alkynes affords ynimines in good to excellent yields. Oxidative addition of the N-O bond to copper is the key to the success of this protocol. Exploiting the unique reactivity of ynimine, either as a transient intermediate or an isolated species has been the focus of chapters 2-5. Chapter 2 deals with the development of an unprecedented [5+1] heteroannulation reaction of O-acyloximes with terminal alkynes exploiting the in situ generated ynimines for the synthesis of medicinally important heterocycles such as isoquinolines, azaindoles, azabenzofurans, azabenzothiophenes and carbolines. The third chapter details the synthesis of di- to penta-substituted pyridines. This transformation relied on the formation of ynimines, which, without isolation, underwent a mild acid-catalyzed reaction involving ketenimine formation, 6-pi-electrocyclization and aromatization to afford pyridines. The wide scope showed excellent functional-group compatibility and even penta-alkyl substituted pyridines could be accessed. The first total synthesis of anibamine B, an indolizinium alkaloid with potent antiplasmodial activity, was accomplished featuring this reaction as a key step. The fourth chapter focus on the synthesis of polysubstituted 1-pyrrolines. Ynimines, generated in situ, underwent a highly stereoselective Alder-ene reaction to afford 1-pyrrolines with concurrent generation of a quaternary carbon and a 2-azadiene function. Mechanistic studies indicate a concerted mechanism. The so-obtained heterocycles undergo, under mild acidic conditions, a regioselective Wagner-Meerwein rearrangement to provide 2-pyrroles in excellent yields. 2-Pyrroles are also accessible in a one-pot fashion without isolation of the 1-pyrrolines.The fifth chapter details the Bergman cyclization of ynimines. The in situ formed C,N-dialkynyl imines were converted to stereodefined alkynylacrylonitriles via a Bergman cyclization/retro-Bergman reaction sequence. The reaction formed the products in good to excellent yields. Substrates were designed to trap the putative intermediate, i.e., 2,5-didehydropyridine diradical, without success. The second part of this thesis, presented in the chapter 6, describes the enantioselective total synthesis of artatrovirenol A, a structurally unusual cage-like sesquiterpenoid isolated in 2020. The synthesis features the following key steps; a) cationic chiral oxazaborolidinium catalyzed Diels-Alder reaction between isoprene and ethyl (E)-5-((tert-butyldimethylsilyl)oxy)-4-oxopent-2-enoate for the rapid synthesis of an enantioenriched 10-carbon bicyclic lactone; b) union of two enantioenriched fragments by a diastereoselective Mukaiyama-Michael addition for the convergent assembly of an intermediate with all 15 carbons of the natural product; c) intramolecular de Mayo [2+2] cycloaddition/retro-aldol sequence transforming a bicyclic compound to a tetracyclic one with concomitant generation of a five and a seven-membered ring; d) Lewis acid-triggered intramolecular ring opening of epoxide generating the norbornane substructure; e) Chugaev elimination converting the norbornane to the more strained norbornene.