Synthesis and biological evaluations of A-ring isomers of 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3 Original Research Article

The activated vitamin D3 derivative 26,27-F6-1α,25(OH)2D3 (2a), its three A-ring diastereomers (2b, 2c, 2d), and 5,6-trans isomer (2e) were prepared. Two analogues (2b, 2c) of these isomers were synthesized by a palladium catalyzed coupling reaction using vinyl bromide 5 and enynes (6a, 6b), which were derived from readily commercially available 2S-(+)-glycidyl p-toluenesulfonate 7, as a common starting material. Competitive vitamin D receptor (VDR) binding affinities of these diastereomers of 2a were evaluated. Interestingly, the stereochemical effects at C-1,3 of 2a were considerably more moderate than those of 1α,25(OH)2D3 (1). In particular, isomerization at the 5,6-double bond of 2a only slightly reduced VDR affinity, whereas 5,6-trans-1α,25(OH)2D3 had a significantly lower binding affinity than 1.