A New Synthesis of 24-Fluoro-1.ALPHA.,25-dihydroxyvitamin D2 and Its 24-Epimer, and Determination of the C-24 Configuration.

A new synthesis of 24-fluoro-1α, 25-dihydroxyvitamin D2 (4a) and its 24-epimer (4b) is described. Starting with 1α, 3β-bis[(tert-butyldimethylsilyl)oxy]-24-norchol-5, 7-dien-23-al (5), a mixture of 4a and 4b was obtained in 3% overall yield in 6 steps. Reversed-phase HPLC cleanly separated the mixture into the two C-24 epimers. The X-ray crystallographic analysis of the 4-phenyl-1, 2, 4-triazoline-3, 5-dione (PTAD) adduct 11b, which was derived from the ester 6, unambiguously determined the configuration at C-24 of this compound. Based on the X-ray analysis, the configuration at C-24 of 4a and 4b was unequivocally determined.