The First Convergent Synthesis of 23,23-Difluoro-25-hydroxyvitamin D3 and Its 24-Hydroxy Derivatives: Preliminary Assessment of Biological Activities

Synthetic Studies on Vitamin D Derivatives with Diverse but Selective Biological Activities

2α-Functionalization of 1α,25-dihydroxyvitamin D3 (active vitamin D3) A-ring enhances binding affinity for the vitamin D receptor (VDR) and prolongs the half-life in target cells due to gaining resistance to CYP24A1-dependant metabolism. The wide variety of modified A-ring precursor enynes for Trost coupling with CD-ring bromoolefin were synthesized from d-glucose. The A-ring modification provided potent, selective biological activities without calcemic side-effects in vivo; for example, 2α-(3-hydroxypropyl)-19-nor-1α,25-dihydroxyvitamin D3 (MART-10) exhibits potent antitumor activity (0.3µg/kg/d, twice/week for 3 weeks) in nude mice inoculated with BxpC-3 cancer cells, 2α-[2-(tetrazol-2-yl)ethyl]-1α,25-dihydroxyvitamin D3 (AH-1) shows better bone-forming effects (0.02µg/kg/d, 5d/week for 4 weeks) in ovariectomized (OVX) rats as an osteoporosis model than natural active vitamin D3, and NS-74c exhibits potent VDR-antagonistic activity (IC50 7.4pM) in HL-60 culture cells. The A-ring modification was also applicable to the synthesis of stable 14-epi-19-nortachysterols, and their novel VDR binding mode was confirmed by X-ray co-crystallographic analysis. 25-Hydroxyvitamin D3 has two independent target molecules: VDR and a sterol regulatory element-binding protein (SREBP)/SREBP cleavage-activating protein (SCAP) complex, and 25-hydroxyvitamin D3 shows SREBP/SCAP inhibitory activity. The VDR-silent vitamin D analog KK-052 with selective SREBP/SCAP inhibitory activity in vivo was developed. A chemical library of side-chain fluorinated vitamin D analogs is currently under construction, and some analogs have shown potent anti-inflammatory activity and therapeutic effects on psoriasis model mice.

Efficient Stereo-Selective Fluorination on Vitamin D3 Side-Chain Using Electrophilic Fluorination

Our research regarding side-chain fluorinated vitamin D3 analogues has explored a series of efficient fluorination methods. In this study, a new electrophilic stereo-selective fluorination methodology at C24 and C22 positions of the vitamin D3 side-chain was developed using N-fluorobenzenesulfonimide (NFSI) and CD-ring imides with an Evans chiral auxiliary (26,27,30).

Synthesis of (22R)-, (22S)-22-Fluoro-, and 22,22-Difluoro-25-hydroxyvitamin D3 and Effects of Side-Chain Fluorination on Biological Activity and CYP24A1-Dependent Metabolism

Three novel analogues of C22-fluoro-25-hydroxyvitamin D3 (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 D3 (5-7) against hCYP24A1 metabolism was also evaluated. 22,22-Difluoro-25(OH)D3 (7) was more stable against hCYP24A1 metabolism compared with its non-fluorinated counterpart 25-hydroxyvitamin D3 (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.

Synthesis of New 26,27-Difluoro- and 26,26,27,27-Tetrafluoro-25-hydroxyvitamin D3: 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 D3 analogues, we newly designed and synthesized 26,27-difluoro-25-hydroxyvitamin D3 (1) and 26,26,27,27-tetrafluoro-25-hydroxyvitamin D3 (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 D3 [HF-25(OH)D3] 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 D3 [25(OH)D3], HF-25(OH)D3 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)D3, 2, 1, and 25(OH)D3 in which HF-25(OH)D3 showed 19-times greater activity than the natural 25(OH)D3.