Synthesis of vinca alkaloids and related compounds. Part XCIII. Skeletalrearrangement of cyclovinblastine derivatives: Formation of a novel bisindole system

Bisindole alkaloids of the vinblastine (VLB) type can be oxidized to give a Psi-aspidosperma-aspidosperma type skeleton via 3'-7'-transannular cycliza tion. Acid catalysis triggers an aspidospermane-->eburnane skeletal rearran gement of these cyclic derivatives, thus giving a novel bisindole system wi th a Psi-eburnea-aspidosperma type skeleton. A previously unexplored aspect of this transformation is the observed retention or inversion at C(16') de pending on the starting C(16') configuration. The present paper gives a det ailed account of the synthetic aspect of this work together with preliminar y NMR and CD results concerning the epimerization at C(16').