Novel side chain analogs of 1α,25-dihydroxyvitamin D3:: design and synthesis of the 21,24-methano derivatives

The syntheses of the new 21,24-methane derivatives of 1 alpha ,25-dihydroxyvitamin D-3 [viz. 1(S),3(R)-dihydroxy-17(R)-(1',4'-cis-(4'-(1'hydroxy-1'- hydroxy-1'-methylethyl)-cyclo-hexyl))-9, 10-seco-androsta-5(Z),7(E), 10(19)-triene (MC 2108) and its(1',4'-trans)-isomer (MC 2110)] are described. The key step is the establishment, by Diels-Alder reaction on a CD-ring side chain diene intermediate prepared from vitamin D-2, of a 1,4-disubstituted cyclohexene moiety in the side chain. Hydrogenation to a 1:1 mixture of cis and trans cyclohexane derivatives and separation of the two series at a stage prior to the standard Horner-Wittig coupling with the (Hoffmann-La Roche) ring-A building block were other important steps in the syntheses of the target analogs. The relative configurations of intermediates were assigned by NMR spectroscopy. MC 2108 and MC 2110 are of interest as conformationally locked side chain derivatives to probe the receptor interactions of not only the parent vitamin D hormone but also its biologically active symmetrical 'double side chain' analog [21-(3'-hydroxy-3'methylbutyl)-9, 10-seco-cholesta-5(Z),7(E), 10(19)-triene-1 (S),3(R),25-triol (MC 2100)], 'both' side chains of which can formally be traced out in the new analogs. The preferred conformations, inferred from an analysis of C-13-NMR characteristics, notably the chemical shift of C-17 in a series of analogs, to have the tertiary alcohol (1'-hydroxy-1'-methylethyl) substituent equatorial on the cyclohexane chair, are confirmed by molecular modeling. (C) 2001 Elsevier Science Inc. All rights reserved.