13-Methyl-substituted des-C,D analogs of (20S)-1alpha,25-dihydroxy-2-methylene-19-norvitamin D3 (2MD): synthesis and biological evaluation.

ABSTRACT

Analogs of (20S)-1alpha,25-dihydroxy-2-methylene-19-norvitamin D(3) (2, 2MD), substituted at C-13 but lacking both C and D rings, were prepared in convergent syntheses, starting with the chiral ester 14 and the phosphine oxide 29. Two of the synthesized vitamins (11 and 32) were analogs in which the 13-methyl group constituted a substituent of an unsaturated fragment, that is, C(13)-C(17) double bond, whereas in the two other cases (12 and 13), the methyl group belonged to a ternary carbon stereogenic center. The aim of these studies was to further explore extensive modifications in the 'upper' part of the vitamin D skeleton in the hope of finding biologically active analogs of potential therapeutic value. The commercial (R)-(-)-methyl-3-hydroxy-2-methylpropionate (14) was converted in six steps to alcohol 18, the vitamin D side chain fragment. Its subsequent three-step transformation led to aldehyde 20 which was subjected to the Still-Gennari HWE reaction with anion derived from ester 21. The obtained alpha,beta-unsaturated esters 22 and 23 served as convenient starting compounds to the syntheses of the corresponding chiral acyclic aldehydes, beta,gamma-unsaturated (28) and saturated (39 and 40), required for the final Wittig-Horner coupling with the anion of the phosphine oxide 29. After hydroxyl deprotection, the synthesized vitamin D analogs 11-13 and 32 were purified and biologically tested. Only the (13R,20S)-analog 12 retained substantial, although 30 times lower than 1alpha,25-(OH)(2)D(3), binding ability to the full-length rat recombinant vitamin D receptor (VDR). This analog was also very effective in differentiation of HL-60 cells, and it exerted significant transcriptional activity (2 times and 15 times less potent, respectively, as compared to the native hormone). The in vivo tests showed that all synthesized vitamin D analogs were devoid of calcemic activity.