Systematic studies on synthesis, structural elucidation, and biological evaluation of A-ring diastereomers of 2-methyl-1 alpha,25-dihydroxyvitamin D-3 and 20-epi-2-methyl-1 alpha,25-dihydroxyvitamin D-3

All possible A-ring diastereomers of 2-methyl-1 alpha ,25-dihydroxyvitamin D-3 (2) and 20-epi-2-methyl-1 alpha ,25-dihydroxyvitamin D-3 (3) were synth esized by palladium-catalyzed coupling reaction of A-ring 'enyne' synthons with CD-ring portions. The A-ring synthons were rationally synthesized via a novel and practical route, starting with methyl (R)-(+)- and (S)-(-)-3-hy droxy-2-methyl-propionate, in good yields. X-ray crystallographic analysis of 2 alpha -methyl-1 alpha ,25-dihydroxyvitamin D-3 (2b) and conformational analysis of the A-ring of 2 alpha -methyl-(2b) and 2 beta -methyl-14 alpha ,25-dihydroxyvitamin D-3 (2F) were carried out, and the results are descri bed. All A-ring diastereomers (2 and 3), thus synthesized, were biologicall y evaluated both in vitro and in vivo. The biologic potency was highly depe ndent on the stereochemistry of the A-ring substituents. In particular, 2b showed 4-fold higher vitamin D receptor [VDR] binding activity than the nat ural hormone, and its 20-epimer (3b) exhibited exceptionally high activity, 12-fold more potent in VDR binding, 7-fold in calcium mobilization, and 59 0-fold in induction of human promyelocytic leukemia (HL-60) cell differenti ation as compared with the natural hormone. Further, the 20-epi-2P-Me-1 bet a, 3 alpha (OH)(2) isomer (3g) had significant biologic potencies compared to the natural hormone despite having 1 beta -OH configuration. The transcr iptional activities on human osteocalcin gene promoter, including VDRE in t ransfected mammalian cells, were also evaluated. Finally, there was a clear contrast between the effects of the 2-methyl group on the HL-60 cell diffe rentiation- and apoptosis-inducing activities of 2 and 3. (C) 2001 Elsevier Science Inc. All rights reserved.