New C15-substituted active vitamin D3

C15-Substituted 1α,25-dihydroxyvitamin D(3) analogs were synthesized for the first time to investigate the effects of the modified CD-ring on biological activity concerning the agonistic positioning of helix-3 and helix-12 of the vitamin D receptor (VDR). X-ray cocrystallographic analysis proved that 0.6 ? shifts of the CD-ring and shrinking of the side chain were necessary to maintain the position of the 25-hydroxy group for proper interaction with helix-12. The 15-hydroxy-16-ene derivative showed higher binding affinity for hVDR than the natural hormone.