Cephalostatin synthesis. 13. Synthesis of the North 1 unit of the cephalostatin family from hecogenin acetate

Hecogenin acetate (1) was converted to North I azidoketone 5 involving several key transformations: (1) conversion of cyclic sulfate 33b to allylic alcohol 40 via Reich iodoso olefination; (2) E-ring annulation via intermolecular oxygen alkylation of highly functionalized secondary alcohol 40 using: rhodium-catalyzed decomposition of an alpha-diazophosphonoacetate to provide alpha-alkoxyphosphonoacetate 52, with subsequent intramolecular Wadsworth-Emmons reaction to provide alkoxydihydrofuran 53; and (3) establishment of the : C20 stereochemistry by chromium(II) reduction of tertiary bromide 86 to a 9:1 mixture of diastereomeric spiroketals 90 alpha/90 beta, separated as silyl ethers 91 alpha/91 beta. Conversion of 91 alpha to alpha-azidoketone 5 was uneventful.