Ergosterol acetate peroxide

Scheme X.5. The mechanism proposed for the peroxidation of ergosterol acetate by HiOr-FcCb in pyridine,...

Sensitizer of Photooxygenation. Barton showed that oxygen, in the presence of trityl fluoroborate and ordinary light, adds to cisoid dienes at —78 °C in very high yields. For example, the peroxide of ergosterol acetate is formed in quantitative yields under these conditions (eq 10), which have been used also for photocycloreversions of cyclobutanes. ... 

The hydroxy-group of cholesterol is protected either as the acetate or benzoate. Bromination in the allylic position by Karl Zieglers method using N-hro-mosuccinimide or l,3-dibromo-5,5-dimethylhydantoin, then dehydrobromina-tion with 2,4,6-collidine and hydrolysis gives 7-dehydrocholesterol. Photolysis of 7-dehydrocholesterol and ergosterol, respectively, with a mercury vapour lamp in an inert solvent (e.g. peroxide-free diethyl ether, methanol, cyclohexane... 

---