Oxidative Fragmentations of the 5 α ‐ and 5 β ‐Hydroxy‐ B ‐norcholestan‐ 3 β ‐yl Acetates

Oxidations of 5 α ‐hydroxy‐ B ‐norcholestan‐3 β ‐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 (5 R ,10 R )‐ and (5 S ,10 S )‐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 5 β ‐hydroxy‐ B ‐norcholestan‐3 β ‐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 5 α ‐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 5 β ,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 β ‐oriented epoxy bridge, i.e. the (10 R )‐enol ether 20 and (5 S ,10 R )‐acetal 5‐acetate 21 ( Scheme 8 ). Possible mechanistic interpretations of the above transformations are discussed.