合成路线1

Synthesis of the cholestane aglycone (XXXII): The condensation of the commercial dehydroisoandrosterone (XVIII) with ethyl triphenylphosphonium bromide (XIX) and potassium tert-butoxide in refluxing THF gives the ethylidene derivative (XX), which is silylated at the 3-OH group with Tbdps-Cl and imidazole to yield the silyl ether (XXI). The reaction of (XXI) with paraformaldehyde in the presence of catalytic BF3/Et2O affords the desired homoallylic alcohol (XXII) stereoselectively. Oxidation of alcohol (XXII) with DMP in dichloromethane provides the corresponding aldehyde (XXIII), which when treated with the Grignard reagent (XXIV) of the 1-bromo-3-methyl butane in ether furnishes the adduct (XXV). The oxidation of the secondary alcohol of (XXV) with PDC in DMF/dichloromethane gives the expected ketone (XXVI), which is protected as the spiroketal (XXVIII) by reaction with ethyleneglycol (XXVII), TsOH and triethyl orthoformate. The Tbdps protecting group of (XXVIII) was now converted into the Tbdms group by desilylation with TBAF and resilylation with Tbdms-Cl and imidazole to give (XXIX). The oxidation of (XXIX) with OsO4 and pyridine in ether yields the 16alpha,17alpha-diol (XXX), which is oxidized with oxalyl chloride to afford the ketone (XXXI). The reduction of (XXXI) with NaBH4 and CeCl3 in THF provides the target 16beta,17alpha-diol (XXXII).