Facile and selective metalation of 7-dehydrocholesterol (pro-vitamin D3)

Successive treatment of 7-dehydrocholesterol (I) with Me3SiCH2K in THF, dry ice, mineral acid, and diazomethane gives a regioisomerically pure, homoconjugated ester II as a diastereomeric mixt., the minor component of which is thermodynamically more stable than the conjugated isomer III. [on SciFinder (R)]