Oxidative fragmentations of the 5α- and 5β-hydroxy-B-norcholestan-3β-yl acetates

Oxidations of 5alpha-hydroxy-B-norcholestan-3beta-yl acetate (8) with Pb(OAc)(4) under thermal or photolytic conditions or in the presence of iodine afforded only complex mixtures of compounds. However, the HgO/I-2 version of the hypoiodite reaction gave as the primary products the stereoisomeric (Z)- and (E)-1(10)unsaturated 5,10-seco B-nor-derivatives 10 and 11, and the stereoisomeric (5R,10R)- and (5S,10S)-acetals 14 and 15 (Scheme 4). Further reaction of these compounds under conditions of their formation afforded, in addition, the A-nor 1,5-cyclization products 13 and 16 (from 10) and 12 (from 11) (see also Scheme 6) and the 6-iodo-5,6-secolactones 17 and 19 (from 14 and 15, resp.) and 4-iodo-4,5-secolactone 18 (from 15) (see also Scheme 7). Oxidations of 5beta-hydroxy-B-norcholestan-3beta-yl acetate (9) with both hypoiodite-forming reagents (Pb(OAc)(4)/I-2 and HgO/I-2) proceeded similarly to the HgO/I-2 reaction of the corresponding 5alpha-hydroxy analogue 8. Photolytic Pb(OAc)4 oxidation of 9 afforded, in addition to the (Z)- and (E)-5,10-seco 1(10)unsaturated ketones 10 and 11, their isomeric 5,10-seco 10(19)-unsaturated ketone 22, the acetal 5-acetate 21, and 5beta,19-epoxy derivative 23 (Scheme 9). Exceptionally, in the thermal Pb(OAc)4 oxidation of 9, the 5,10-seco ketones 10, 11, and 22 were not formed, the only reaction being the stereoselective formation of the 5,10-ethers with the beta-oriented epoxy bridge, i.e. the (10R)-enol ether 20 and (5S,10R)-acetal 5-acetate 21 (Scheme 8). Possible mechanistic interpretations of the above transformations are discussed.