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

Subsequent treatment of 7-dehydrocholesterol with trimethylsilylmethylpotassium in tetrahydrofuran, dry ice, mineral acid and diazomethane gives a regioisomerically pure, homoconjugated ester as a diastereomeric mixture, the minor component of which turns out to be thermodynamically more stable than a conjugated isomer.