Selective Crystallization of an Allylic 7α-Bromide: A Facile Synthesis of (1α,3β)-3-Hydroxycholeste-5,7-diene-1,25-diol Diacetate

An efficient synthesis of (3β)-7-dehydro-1α,25-diacetoxycholesterol ( 1 ) is described. When acetonitrile was used as the crystallizing solvent, only the 7α-bromide 6a was isolated and bromide 6a was prepared in a 69% yield starting from 3β-TBDMS-1α,25-diacetoxycholesterol ( 5 ). Dehydrobromination with sym -collidine led to the regioselective formation of only the 5,7-diene 7 . After deprotection with concentrated hydrochloric acid, the diene diacetate 1 was obtained in an overall yield of 44% without chromatographic separations. When tetrabutylammonium fluoride trihydrate was used as the base, the 7β-fluoride 10 was isolated.