Organo-cation Catalyzed Site-selective Sequential Alkylation of Dimeric 2-Oxindoles: Concise Catalytic Approach to (+)-Calycanthidine and (+)-Idiospermuline

Abstract

Expeditious approach to the naturally occurring bis(cyclotryptamine) alkaloids sharing vicinal all-carbon quaternary stereocenters connected through an elongated and labile C-3a–C-3a′ σ-bond, has been realized via the development of an organo-cation catalyzed site-selective, enantio- and diastereo-selective sequential alkylation of bis-oxindole with bromo ethylacetate in the presence of Ooi catalyst (C5). This strategy provides (+)-11 with excellent ee (up to 94% ee) and dr (up to 20:1), enroute to a concise total synthesis of naturally occurring (+)-calycanthidine (7b) and a formal total synthesis of (+)-idiospermuline (8).