A SELECTIVE DEGRADATION OF THE EXOMETHYLENE GROUP OF VITAMIN-D - A SIMPLE ENTRY TO 10-OXO-19-NORCHOLECALCIFEROL AND 10-OXO-19-NORERGOCALCIFEROL

Nitrile oxides, generated in situ from nitroalkanes, add exclusively t o the exomethylene group of (Z)- and (E)-vitamin D3 acetates (1a, 12a) and D2 acetates (1b, 12b). The so generated spirocyclic isoxazoline d erivatives are transformed through a MO(CO)6-induced fragmentation int o (5Z,7E)- and 3-acetoxy-19-nor-9,10-secocholesta-5,7-dien-10-one (20a , 22a) as well as (5Z,7E,22E)- and (5E,7E,22E)-. cetoxy-19-nor-9,10-se coergosta-5,7,22-trien-10-one (19a, 21a). The discribed method allows the synthesis of 19-nor-10-oxo vitamin D derivatives without partial i somerisation of the DELTA5-double bond.