Furan Approach to Vitamin D Analogues. Synthesis of the A-Ring of Calcitriol and 1α-Hydroxy-3-deoxyvitamin D3

The A-rings of calcitriol (1α,25-dihydroxyvitamin D3) and 1α-hydroxy-3-deoxyvitamin D3 were synthesized using the furan approach. The critical steps in the synthesis of the A-ring of calcitriol involved an asymmetric carbonyl-ene reaction of 3-methylene-2,3-dihydrofuran with 3-(tert-butyldimethylsiloxy)propanal, a diastereoselective Friedel−Crafts hydroxyalkylation, an oxidation of the 2,3-disubstituted furan to give a γ-hydroxybutenolide, and a Peterson olefination. The A-ring (Z)-dienol of calcitriol was synthesized in 12 steps from 3-(tert-butyldimethylsiloxy)propanal in 17% yield.