Tandem cyclopropanation/ring closing metathesis, enyne metathesis: Developments and application towards natural product synthesis
Peppers, Brian Patrick
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Chapter one. Tandem cyclopropanation/ring-closing metathesis of dienynes . Certain dienynes give cyclorearrangement by tandem cyclopropanation/ring-closing alkene metathesis, triggered by either a ruthenium carbene or noncarbene ruthenium (II) precatalyst. The process represents a variation of enyne metathesis where presumed cyclopropyl carbene intermediates undergo a consecutive ring-closing metathesis. A mechanistic proposal is offered and investigated by comparing reaction results with substrate design. Sequential use of catalysts provided a tandem ring-closing enyne/alkene metathesis product. Chapter Two. Functional group scope in the methylene-free, tandem enyne metathesis . An optimized procedure for methylene-free metathesis has been identified. These procedures allow the successful transformation of a diverse group of functional group rich terminal and internal alkynes to 1,3-cyclohexadienes. The Grubbs second-generation catalyst was found to mediate this reaction in increased efficiency over the faster propagating phosphine-free catalysts. Chapter Three. Studies directed towards the synthesis of the scabrosins: Validation of a tandem enyne metathesis approach . A synthetic approach to the scabrosin family of antibiotics featuring a ruthenium carbene catalyzed tandem metathesis as a key step is described. The chiral propargyl amino acid is furnished through enantioselective phase transfer propargylation. The complementary methods, previously developed in our lab, of methylene-free and 1,5-hexadiene-alkyne metatheses are compared. The indoline heterocycles are formed using a two-step chloro acetoxylation (Bäckvall reaction) with subsequent nucleophilic attack by an amine nucleophile. The indoline subunits were joined and cyclized to furnish the core diketopiperazine ring. The stereochemical assignment of intermediates is also discussed.
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