Catalytic Asymmetric Synthesis of Either Enantiomer of the Calabar Alkaloids Physostigmine and Physovenine

Abstract

A potentially versatile asymmetric route to hexahydropyrrolo[2,3-b]indoles having carbon substituents at C-3a (Scheme 1) is demonstrated through enantioselective total syntheses of the Calabar alkaloids (−)-physostigmine (2), (−)-physovenine (10), and their enantiomers. The synthesis of enantiopure (−)-physostigmine proceeds from commercially available 2-butyn-1-ol (11) and N-methyl-p-anisidine (15) in 15−20% overall yield by way of eight isolated and purified intermediates. The central step is catalytic asymmetric Heck cyclization of (Z)-2-methyl-2-butenanilide 17 to form oxindole aldehyde (S)-19 in 84% yield and 95% ee.