Synthesis of (22)-, (22)-22-Fluoro-, and 22,22-Difluoro-25-hydroxyvitamin D and Effects of Side-Chain Fluorination on Biological Activity and CYP24A1-Dependent Metabolism.

Three novel analogues of C22-fluoro-25-hydroxyvitamin D (-) were synthesized and evaluated to investigate the effects of side-chain fluorination on biological activity and metabolism of vitamin D. These novel analogues were constructed by convergent synthesis applying the Wittig-Horner coupling reaction between CD-ring ketones (,,) and A-ring phosphine oxide (). The introduction of C22-fluoro units was achieved by stereoselective deoxy-fluorination for synthesizing and or two-step cationic fluorination for . The absolute configuration of the C22-fluoro-8-oxo-CD-ring () was confirmed by X-ray crystallographic structure determination. The basic biological activity of the side-chain fluorinated analogues, including compounds (-), was evaluated. Generally, osteocalcin promoter transactivation activity decreased in the order of C24-fluoro, C23-fluoro, and C22-fluoro analogues. In addition, the metabolic stability of C22-fluoro-25-hydroxyvitamin D (-) against hCYP24A1 metabolism was also evaluated. 22,22-Difluoro-25(OH)D () was more stable against hCYP24A1 metabolism compared with its non-fluorinated counterpart 25-hydroxyvitamin D (), but fluorination at the C22 position had little effect on the metabolic stability compared with C24- and C23-fluoro analogues. Our research clarified that side-chain fluorination in vitamin D markedly changes CYP24A1 metabolic stability depending on the fluorinating position.