Stereoselective Synthesis of Four Calcitriol Lactone Diastereomers at C23 and C25.

(23 S,25 R)-Calcitriol lactone is a major metabolite of vitamin D3, but its synthesis has been far less well investigated than that of 1α,25(OH)2 vitamin D3, the active form of vitamin D3, even though the lactone is present at a significant level in serum. This paper describes stereoselective syntheses of natural calcitriol lactone and its diastereomers at C23 and C25. This work features (i) the diastereoselective Reformatsky-type crotylation of aldehyde 25 in the presence of chiral ligand L2 to construct the stereochemistry at C23 and (ii) the diastereoselective epoxidation of homoallylic-allylic alcohol 31 to control the stereochemistry at C25. These key reactions allowed us to synthesize CD-ring synthon 30 with all four stereoisomers, and these were further converted into calcitriol lactones 3a-3d by reaction with ene-yne-type A-rings 33 in the presence of a palladium (0) catalyst.