Synthesis of the South Unit of Cephalostatin. 7. Total Syntheses of (+)-Cephalostatin 7, (+)-Cephalostatin 12, and (+)-Ritterazine K¹

Abstract

Transformation of alcohol 4 to α-azidoketone 6, a hexacyclic steroid bearing the requisite functionality and spiroketal stereochemistry of the symchiral South portion of cephalostatin 7 (10) is described. Reaction of a 1:1 mixture of α-azidoketones 5 and 6 with sodium hydrogen telluride is followed by cleavage of the protecting groups cephalostatin 12 (9), cephalostatin 7 (10), and ritterazine K (11).