25S‑Adamantyl-23-yne-26,27-dinor-1α,25-dihydroxyvitamin D₃: Synthesis, Tissue Selective Biological Activities, and X‑ray Crystal Structural Analysis of Its Vitamin D Receptor Complex

Both 25R- and 25S-25-adamantyl-23-yne-26,27-dinor-1α,25-dihydroxy­vitamin D₃ (4a and 4b) were stereoselectively synthesized by a Pd(0)-catalyzed ring closure and Suzuki–Miyaura coupling between enol-triflate 7 and alkenyl-boronic ester 8. The 25S isomer (4b) showed high vitamin D receptor (VDR) affinity (50% of that of the natural hormone 1α,25-dihydroxy­vitamin D₃, 1) and transactivation potency (kidney HEK293, 90%). In endogenous gene expression, it showed high cell-type selectivity for kidney cells (HEK293, CYP24A1 160% of 1), bone cells (MG63, osteocalcin 64%), and monocytes (U937, CAMP 96%) over intestine (SW480, CYP24A1 8%) and skin (HaCaT, CYP24A1 7%) cells. The X-ray crystal structural analysis of 4b in complex with rat VDR-ligand binding domain (LBD) showed the highest Cα positional shift from the 1/VDR-LBD complex at helix 11. Helix 11 of the 4b and 1 VDR-LBD complexes also showed significant differences in surface properties. These results suggest that 4b should be examined further as another candidate for a mild preventive osteoporosis agent.