摘要:
Chiral synthesis of the A-ring synthon 3 of 1alpha,25-dihydroxyvitamin D3 (1a) based on palladium-catalyzed cyclization of 8-bromo-2,8-nonadienoates is described. Reaction of (E)-4b and (Z)-4d in the presence of Pd(OAc)2, PPh3, and K2CO3 gave (E)-3b and (Z)-3d, respectively. Optically active 4d was prepared from 24 by asymmetric aldol reaction using 31, which was cyclized to 3d. With further reactions, 1alpha,25-dihydroxyvitamin D3 (1a) was obtained.