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All possible A-ring diastereomers of 2-methyl-1alpha,25-dihydroxyvitamin D(3) (2) and 20-epi-2-methyl-1alpha,25-dihydroxyvitamin D(3) (3) were synthesized 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-hydroxy-2-methyl-propionate, in good yields. X-ray crystallographic analysis of 2alpha-methyl-1alpha,25-dihydroxyvitamin D(3) (2b) and conformational analysis of the A-ring of 2alpha-methyl-(2b) and 2beta-methyl-1alpha,25-dihydroxyvitamin D(3) (2f) were carried out, and the results are described. All A-ring diastereomers (2 and 3), thus synthesized, were biologically evaluated both in vitro and in vivo. The biologic potency was highly dependent on the stereochemistry of the A-ring substituents. In particular, 2b showed 4-fold higher vitamin D receptor [VDR] binding activity than the natural hormone, and its 20-epimer (3b) exhibited exceptionally high activity, 12-fold more potent in VDR binding, 7-fold in calcium mobilization, and 590-fold in induction of human promyelocytic leukemia (HL-60) cell differentiation as compared with the natural hormone. Further, the 20-epi-2beta-Me-1beta, 3alpha(OH)(2) isomer (3g) had significant biologic potencies compared to the natural hormone despite having 1beta-OH configuration. The transcriptional activities on human osteocalcin gene promoter, including VDRE in transfected mammalian cells, were also evaluated. Finally, there was a clear contrast between the effects of the 2-methyl group on the HL-60 cell differentiation- and apoptosis-inducing activities of 2 and 3.

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