SYNTHESIS OF (-)-SLAFRAMINE AND RELATED INDOLIZIDINES

An enantioselective synthesis of the indolizidine alkaloid (-)-slaframine 1 is reported. Reductive double cyclization of the azido epoxy tosylate 48 afforded the indolizidine 52, which was converted to (-)-slaframine in two steps. The cyclization substrate 48 was prepared in optically pure form from L-glutamic acid. A similar sequence starting with the epoxide 49 allowed the synthesis of (-)-1,8a-diepislaframine 56. Other routes to slaframine were investigated, often using an intramolecular cycloaddition of an azide with an alkene as a key step. Although these routes did not produce slaframine, they illustrated novel and efficient methods for the assembly of the indolizidine skeleton. Cyclization of the azidodiene 20 afforded the indolizidine 21 in one step as a single diastereomer, presumably a result of a chairlike transition state in the initial dipolar cycloaddition. Desulfurization and deprotection produced (-)-8a-epidesacetoxyslaframine 27. Cyclopropylimine rearrangement of 30 gave the indolizidine 31, which was also converted into (-)-8a-epidesacetoxyslaframine 27. Dipolar cycloaddition of 38 gave the 1-pyrroline 39, which was converted to the indolizidine 40 in one operation using Evans' double alkylation of the 1-metalloenamine derivative of 40. Attempted oxidation of 40 to the ketone 41 was unsuccessful, precluding a reductive amination approach to slaframine.