Synthesis and biological activity of two C-7 methyl analogues of vitamin D.

Two novel vitamin D analogues of the hormone 1α,25-(OH)2D3 modified at C-7, namely, 7-methyl-1α,25-(OH)2D3 (12) and 7-methyl-1α,25-(OH)2-19-nor-D3 (26), were synthesized and biologically evaluated to gain further insights into the structure-function relationships of vitamin D. Key steps in the synthesis of 12 include the functionalization at C-7 by an efficient regioselective hydrostannylation of an allene precursor, and the construction of the triene framework by a palladium-catalyzed intramolecular cyclization-Suzuki-Miyaura coupling cascade. Since the calcitriol analogue 12 was prone to conversion into its previtamin D form by thermal equilibration, the corresponding 19-nor-compound 26 was also synthesized. The diene moiety of compound 26 was constructed by a modified Julia coupling. UV data as well as X-ray analysis indicate that introduction of the methyl group at C-7 results in a significant deviation from planarity of the 5,7-diene moiety. The new vitamin D analogues 12 and 26 retained good VDR binding ability.