Stereospecific synthesis of 1,2-dioxolanes by alkoxy radical β-fragmentation of steroidal cyclic peroxyhemiacetals

A new one-pot synthesis of 3,5-substituted 1,2-dioxolanes from six-membered cyclic peroxyhemiacetal derivatives of hexahydrobenzo[c][1,2]dioxin-8a-ol is described. A number of steroidal peroxyhemiacetals, (2S,2'S,3R)-(11), (2S,2'R,3R/S)-(13), (2R,2'R,3R/S)-2-(2'-hydroperoxypropyl-3'-phenylsulfonyl)-5 alpha-cholestan-3-one 2',3-peroxyhemiacetal (17), and (2S,5R)-2-hydroperoxy-3-phenylsulfonyl-4-nor-3,4-secocholestan-5-one 2,5-peroxyhemiacetal (24), have been prepared in order to test the scope and diastereoselectivity of the present methodology. These substrates were irradiated with visible light in the presence of (diacetoxyiodo)benzene and iodine to generate the corresponding alkoxy radical, which undergoes regioselective beta-fission yielding a carbon radical, that further reacts in a stereoselective manner with an iodine atom to give a 10-membered peroxylactone. This iodide intermediate is intramolecularly substituted by the peroxide anion generated by intermolecular nucleophilic attack of a hydroxide or methoxide anion to-the acyl carbon with concomitant carbon-oxygen bond cleavage. This operationally simple reaction permitted us to synthesize methyl (2S,2'S)-(33), (2S,2'R)-(34), (2R,2'R)-1-[5'-[(phenylsulfonyl)methyl]-[1',2']dioxolan-3'yl]-2-nor-2,3-secocholestan-3-oate (36) and methyl (1'R,1'''R,3'aS,3 " R,4'S,5'S,5 " S, 7'aR)-3-[5'-[5 "-(phenylsulfonyl)methyl]-3 "-methyl[1 ",2 "]dioxolan-3 "-yl]-1'-(1''',5'''-dimethylhexyl)-7'a-methyloctahydroinden-4'-yl]propanoate (37) in good yields.