Stereoselective introduction of hydroxy groups into the cholesterol side chain. Preparation of (24R)- and (24S)-24,25-dihydroxy- and (25R)- and (25S)-25,26-dihydroxyvitamin D3 by asymmetric synthesis

Abstract

24,25-Epoxy-26-hydroxy-3β-tetrahydropyranyloxycholest-5-enes (7a) and (8a), prepared by asymmetric epoxidation of the allylic alcohol (4), and 24-hydroxy-3β-tetrahydropyranyloxycholesta-5,25-dienes (11) and (12), synthesized by asymmetric reduction of the enone (6), were stereoselectively converted into 25,26- and 24,25-dihydroxycholesterol derivatives, which could be transformed into 25,26- and 24,25-dihydroxyvitamin D₃. The highly stereoselective epoxide cleavage of 26-benzoyloxy-24,25-epoxides (7b), (8b), (25), and (26) was found to proceed with retention at C-24.