Synthesis of new 26,27-difluoro- and 26,26,27,27-tetrafluoro-25-hydroxyvitamin D 3 : Effects of terminal fluorine atoms on biological activity and half-life.

As an extension of our research on providing a chemical library of side-chain fluorinated vitamin D analogues, we newly designed and synthesized 26,27-difluoro-25-hydroxyvitamin D (1) and 26,26,27,27-tetrafluoro-25-hydroxyvitamin D (2) using a convergent method applying the Wittig-Horner coupling reaction between CD-ring ketones (13,14) and A-ring phosphine oxide (5).The basic biological activities of analogues, 1, 2, and 26,26,26,27,27,27-hexafluoro-25-hydroxyvitamin D [HF-25(OH)D] were examined. Although the tetrafluorinated new compound 2 exhibited higher binding affinity for vitamin D receptor (VDR) and resistance to CYP24A1-dependent metabolism compared with the difluorinated 1 and its non-fluorinated counterpart 25-hydroxyvitamin D [25(OH)D], HF-25(OH)D showed the highest activity among these compounds. Osteocalcin promoter transactivation activity of these fluorinated analogues was tested, and it decreased in the order of HF-25(OH)D, 2, 1, and 25(OH)D in which HF-25(OH)D showed 19-times greater activity than the natural 25(OH)D.