A novel and practical route to A-ring enyne synthon for 1α,25-dihydroxyvitamin D3 analogs: Synthesis of A-ring diastereomers of 1α,25-dihydroxyvitamin D3 and 2-methyl-1,25-dihydroxyvitamin D3
A novel and practical route to the A-ring enyne synthon (2), which can be versatile for a variety of A-ring analogs of 1 alpha,25-dihydroxyvitamin D3 (1), was developed. This novel method led to an improved synthesis of the A-ring diastereomers of 1, the compounds 13-15, and synthesis of the new analogs, 2-methyl-1,25-dihydroxyvitamin D3 (4) with its all possible diastereomers. The biological evaluation
Synthesis, Biological Evaluation, and Conformational Analysis of A-Ring Diastereomers of 2-Methyl-1,25-dihydroxyvitamin D<sub>3</sub> and Their 20-Epimers: Unique Activity Profiles Depending on the Stereochemistry of the A-Ring and at C-20
All eight; possible A-ring diastereomers of 2-methyl-1,25-dihydroxyvitamin D-3 (2) and 2-methyl-20-epi-1,25-dihydroxyvitamin D-3 (3) were convergently synthesized. The A-ring enyne synthons 19 were synthesized starting with methyl(S)-(+)- or (R)-(-)-3-hydroxy-2-methylpropionate (8). This was converted to the alcohol 14 as a 1:1 epimeric mixture in several steps. After having been separated by column chromatography, each isomer led to the requisite A-ring enyne synthons 19 again as 1:1 mixtures at C-1. Coupling of the resulting A-ring enynes 20a-h with the CD-ring portions 5a,b in the presence of a Pd catalyst afforded the 2-methyl analogues 2a-h and 3a-h in good yield. In this way, all possible A-ring diastereomers were synthesized. The synthesized analogues were biologically evaluated both in vitro and in vivo. The potency was highly dependent on the stereochemistry of each isomer. In particular, the alpha alpha beta -isomer 2g exhibited 4-fold higher potency than 1 alpha ,25-dihydroxyvitamin D-3 (1) both in bovine thymus VDR binding and in elevation of rat serum calcium concentration and was twice as potent, as the parent compound in HL-60 cell differentiation. Furthermore, its 20-epimer, that is, 20epi-alpha alpha beta 3g, exhibited exceptionally high activities: 12-fold higher in VDR binding affinity, 7-fold higher in calcium mobilization, and 590-fold higher in HL-60 cell differentiation, as compared to 1 alpha ,25-dihydroxyvitamin D3 (1). Accordingly, the double modification of 2-methyl substitution and 20-epimerization resulted in unique activity profiles. Conformational analysis of the A-ring by H-1 NMR and an X-ray crystallographic analysis of the alpha alpha beta -isomer 2g are also described.