SYNTHESIS OF A C-14',C-15' DIHYDRO DERIVATIVE OF THE SOUTH HEXACYCLIC STEROID UNIT OF CEPHALOSTATIN-1 .1. REGIOSPECIFIC RH[II]-MEDIATED INTERMOLECULAR OXYGEN ALKYLATION OF A PRIMARY NEOPENTYL ALCOHOL

Hecogenin acetate 5 was converted to an intermediate suitable for the construction of a C-14',C-15' dihydro derivative of the ''South'' hexacyclic spiroketal of cephalostatin 11. The key transformations include: (i) proximal functionalization of the C-18 methyl group in 17 via hypoiodite homolysis; (ii) Rhodium [II] catalyzed intermolecular oxygen alkylation of primary neopentyl hydroxyl group in 20 and (iii) intramolecular Wadsworth-Emmons reaction to provide 23, the ester precursor of aldehyde 4.