Synthesis of 23,23-Difluoro-24-nor- and 24′,24′-Difluoro-24-Homovitamin D3 Analogues and Unexpected Structure-Activity Relationships

We synthesized two vitamin D3 analogues, 3 and 4, which have a shortened or elongated fluoro-side-chain based on 24,24-difluoro-25-hydroxyvitamin D3 (5) using an efficient convergent approach and studied their preliminary biological activity. Both analogues exhibited greater resistance to CYP24A1-mediated metabolism than the natural 25-hydroxyvitamin D3 (6), although their stability was lower than that of 5. Analogue 3 showed an approximately 100-fold lower human vitamin D receptor (hVDR)-binding affinity compared with 5 and 6. Despite this marked reduction in VDR-binding affinity, it demonstrated an approximately 1.5-fold increase in VDR-ligand binding domain (LBD) transcriptional activation of the natural ligand 6. In contrast, analogue 4 displayed moderate VDR-binding affinity and VDR-LBD transactivation compared with 5 and 6. We found that compound 3 is a unique vitamin D analogue with a fluorinated and shortened side-chain, exhibiting low binding affinity for hVDR but potent transcriptional activity through VDR-LBD with its long half-life; thus, 3 may serve as a basic structural skeleton for advancing medicinal chemistry and drug discovery.