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

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 ).