Enantioselective Total Synthesis of Cotylenin A.

A convergent enantioselective total synthesis of cotylenin A is described. The A-ring fragment, prepared via the catalytic asymmetric intramolecular cyclopropanation developed in our laboratory, and the C-ring fragment, prepared from a known chiral compound via a modified acyl radical cyclization, were successfully assembled by the Utimoto coupling reaction. The formidable carbocyclic eight-membered ring of cotylenin A was efficiently constructed by a palladium-mediated cyclization. All the hydroxy groups in the scaffold were stereoselectively introduced, and a modified reducing reagent, MeNBH(OCPr), has been developed. The sugar moiety fragment was prepared via three consecutive carbon-oxygen bond-forming reactions, and the glycosylation was accomplished using Wan's protocol.