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Further studies on the synthesis of 24(S),25-epoxycholesterol. A new, efficient preparation of desmosterol

Spencer, Thomas A. and Li, Dansu and Russel, Jonathon S. and Tomkinson, Nicholas C. O. and Willson, Timothy M. (2000) Further studies on the synthesis of 24(S),25-epoxycholesterol. A new, efficient preparation of desmosterol. Journal of Organic Chemistry, 65 (7). pp. 1919-1923. ISSN 0022-3263

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Abstract

Efforts to improve the synthesis of 24(S),25-epoxycholesterol from stigmasterol have included identification of 6α-hydroxy-i-steroid (I) (R1 = OH, R2 = αOH) as a byproduct from the ozonolysis of steroid (II) and an attempt to effect conversion of sulfone I (R1 = SO2Ph, R2 = βOMe) (III) to diol (IV) via Payne rearrangement and nucleophilic trapping of (S)-2-hydroxymethyl-3,3-dimethylepoxide, which led instead to (V) (R3 = α or β OH) (97% yield). A more efficient synthesis of 24(S),25-epoxycholesterol was achieved via coupling of cuprate I (R1 = CuCNLi, R2 = βOMe) (VI) with allylic acetate H2C=CHCMe2OAc to give 73% of I (R1 = CH2CH=CMe2, R2 = βOMe) (VII), in the most efficient conversion yet of a C22 intermediate to desmosterol or its acetate.