Organo-cation-catalyzed site-selective sequential alkylation of dimeric 2-oxindoles: a concise catalytic approach to (+)-calycanthidine and (+)-idiospermuline

Abstract

An expeditious approach for the synthesis of 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, enantioselective and diastereoselective sequential alkylation of bis-oxindole with bromoethyl acetate in the presence of the Ooi catalyst (C5). This strategy provides (+)-11 with excellent ee (up to 94% ee) and dr (up to 20 : 1), en route to a concise total synthesis of naturally occurring (+)-calycanthidine (7b) and a formal total synthesis of (+)-idiospermuline (8).