Stereochemically Controlled Acetalizations in the Ten-Membered Ring of (Z)- and (E)-5,10-Seco-Steroidal Ketones

Sa？etak (Z)-3~-Acetoxy-5,10-seco-cholest-l(10)-en-5-one2a reacts with m-chloroperbenzoic acid to give the cis-(lS,10R)-epoxide 4, which, under acid-catalyzed conditions, undergoes stereospecific intramolecular cyclization to afford the (lS,5S,10S)-acetal 5. On the other hand, hydrolysis of the osmate ester of (E)-3~-acetoxy- -5,10-seco-cholest-1(10)-en-5-one 3a (with aqueous hydrochloric acid in methanol solution) produces two stereochemically different acetal derivatives, i. e. the (1R,5R,10R)-product7 and (1S,5S,10S)- -isomer 5, in ca. 49% and 28°/1 yields, respectively. The stereochemical course of these transformations is discussed in terms of the most stable ground-state conformations of the starting (Z)- and (E)-seco-ketones (2a and 3a) in solution