De Novo Asymmetric Syntheses of d-, l- and 8-epi-Swainsonine.

A highly enantioselective and stereocontrolled approach to d-, l- and 8-epi-d-swainsonine has been developed from achiral furan and γ-butyrolactone. A one-pot hydrogenolysis of both azide and benzyl ether followed by an intramolecular reductive amination has been employed as key step to establish the indolizidine ring system. The absolute stereochemistry was installed by the Noyori reduction and the relative stereochemistry by the use of several highly diastereoselective palladium catalyzed glycosylation, Luche reduction, dihydroxylation and palladium catalyzed azide allylation reactions. This practical approach provide multigram quantities of indolizidine natural product d-swainsonine in 13 steps and 25% overall yield.