Collective formal synthesis of (+/-)-rhynchophylline and homologues

摘要

A collective formal synthesis approach to bioactive oxindole alkaloids, including (+/-)-rhynchophylline, (+/-)-isorhynchophylline, (+/-)-mitraphylline, (+/-)-formosanine, (+/-)-isomitraphylline, and (+/-)-isoformosanine, is completed in a protecting-group free manner. Besides multigram-scaled operations, the notable feature of the synthesis is the application of two one-pot, sequential transformations. Namely, two key tetracyclic intermediates pyridinium salt 9 and monoester 14 were prepared by a one-pot N-alkylation/cross-dehydrogenative coupling sequence and a one-pot Michael/Karpocho sequence with minimal purification, respectively.