13-Methyl-substituted des-C,D analogs of (20 S)-1α,25-dihydroxy-2-methylene-19-norvitamin D 3 (2MD): Synthesis and biological evaluation

Abstract

Analogs of (20 S)-1α,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 α,β-unsaturated esters 22 and 23 served as convenient starting compounds to the syntheses of the corresponding chiral acyclic aldehydes, β,γ-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 (13 R,20 S)-analog 12 retained substantial, although 30 times lower than 1α,25-(OH) 2D 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.