Investigations on sterols: I. A new synthesis of 7‐dehydrocholesterol (provitamin D)

A new synthesis of 7‐dehydrocholesterol (provitamin D3) from cholesterol is described, which gives a higher yield than the well known Windaus synthesis. The synthesis is carried out via a 7‐bromocholesteryl ester, from which hydrogen bromide is eliminated to give an ester of 7‐dehydrocholesterol. Careful hydrolysis of the impure 7‐bromocholesteryl benzoate affords either 7‐α‐ or 7‐β‐hydroxycholesteryl‐3 benzoate depending on the reagent. The two epimeric 7‐bromocholesteryl benzoates are prepared from the hydroxy‐compounds by interaction with phosphorus tribromide; the two hydroxy‐compounds are regenerated by hydrolysis.