Synthesis of (22 R )-, (22 S )-22-Fluoro-, and 22,22-Difluoro-25-hydroxyvitamin D 3 and Effects of Side-Chain Fluorination on Biological Activity and CYP24A1-Dependent Metabolism.

Three novel analogues of C22-fluoro-25-hydroxyvitamin D 3 ( 5 - 7 ) 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 ( 41 , 42 , 44 ) and A-ring phosphine oxide ( 11 ). The introduction of C22-fluoro units was achieved by stereoselective deoxy-fluorination for synthesizing 5 and 6 or two-step cationic fluorination for 7 . The absolute configuration of the C22-fluoro-8-oxo-CD-ring ( 39 ) was confirmed by X-ray crystallographic structure determination. The basic biological activity of the side-chain fluorinated analogues, including compounds ( 5 - 7 ), 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 3 ( 5 - 7 ) against hCYP24A1 metabolism was also evaluated. 22,22-Difluoro-25(OH)D 3 ( 7 ) was more stable against hCYP24A1 metabolism compared with its non-fluorinated counterpart 25-hydroxyvitamin D 3 ( 1 ), 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.