Synthesis of methyl (20R,22E)- and (20S,22E)-3-oxochola-1,4,22-trien-24-oate
Methyl (22E)-3-oxochola-1,4,22-trien-24-oate (4;C25H34O3) is a naturally occurring steroid with unknown configuration at C(20). Starting from the (20S)-3-oxo-23,24-dinorchol-4-en-22-al (1a), we prepared both diastereoisomeric methyl esters 4a and 4b by a three-step procedure (Scheme). In the case of 4b, the initial epimerization of aldehyde 1a was followed by completion of the sequence and then separation via fractional crystallization to afford pure (20R)-methyl ester 4a and its (20S)-diastereomer 4b. Only the analytical data of the (20S)-compound 4b were in good agreement with those reported for the natural product.
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