A5-30-Hydroxysteroids

Rosenfield RL, Rich BH, Wolfsdorf JI, Cassorla F, Parks JS, Bongiovanni AM, Wu CH, Shack-leton CH (1980) Pubertal presentation of congenital A5-3/ -hydroxysteroid dehydrogenase deficiency. J Clin Endocrinol Metab 51 345-353... 

Use of diphenyl phosphite for etherification is limited to A5-3-hydroxysteroids (a homoallylic cation is probably involved in this case). Thus cholesterol is converted into 3/)-phenoxy-A5-cholestene in 50% yield by treatment with diphenyl phosphite in the presence of an acid catalyst. 

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