Total synthesis of 1-deoxy-7,8a-di-epi-castanospermine and formal synthesis of pumiliotoxin-251D

A concise and efficient synthesis of (6R,7S,8R,8aS)-6,7,8-trihydroxyindolizidine (1-deoxy-7,8a-di-epi-castanospermine) 2 is described. The synthesis employs cross metathesis in building the key intermediate 9 and is used effectively in constructing indolizidine skeleton for the total synthesis of 1-deoxy-7,8a-di-epi-castanospermine and also for the bicyclic framework of pumiliotoxin 251D 12, 13. The indolizidine skeleton is achieved in one pot sequence of transformations such as deprotection of Cbz group, reduction of double bond, and cyclization. The configurational and conformational structures of compound 10 are unambiguously confirmed by X-ray analysis.