Synthesis of 24,24-Difluoro-1,3-<i>cis</i>-25-dihydroxy-19-norvitamin D₃ Derivatives and Evaluation of Their Vitamin D Receptor-Binding Affinity

Abstract

Fig. 1; also known as calcitriol) is the active hormonal form of vitamin D 3 (1); it regulates calcium and phosphorus homeostasis and exhibits potent antiproliferative activity.1-4) Its activity is regulated by the cytochrome oxidase P450 family member CYP24A1, which oxidizes 2 at the C24 and C23 positions to afford biologically inactive, side-chain-truncated, water-soluble end products.5-7) However, the metabolic stability of 2 can be increased by substitution of side-chain hydrogen with fluorine, which is very similar in size to hydrogen, but has different chemical bonding properties. The highly electron-withdrawing nature of fluorine means that the C-F bond has a marked ionic character, and this blocks the oxidation by CYP24A1. Indeed, 24,24-difluorinated 1α,25-dihydroxyvitamin D 3 showed high metabolic stability and almost 100% absorption in rats, although this enhanced metabolic stability did not yield increased biological activity.8) Many fluorinated derivatives of 2 have already been reported. Among them, falecalcitriol [9][10][11][12] is used to treat hypercalcemia, osteomalacia, and rickets, and is more efficient than calcitriol (2). Recently, DeLuca and colleagues reported 24,24-difluoro-1α,25-(OH) 2 -19-norvitamin D 3 (4), which showed stronger vitamin D receptor (VDR) binding affinity than compound 2 and promoted bone formation more efficiently.13) Thus, although 24-difluoro substitution has little effect on the biological potency of the natural hormone (2), it does alter the biological activity profile of 1α,25-(OH) 2 -19-norvitamin D 3 (3).With this background, we decided to extend our previously reported synthetic studies of 1,3-cis-25-dihydroxy-19-norvitamin D 3 (5a, b) 14) to obtain the 24-difluoro series (6a, b), to investigate the effects of fluoro substitution at the side chain on the biological activities of those ligands. Here, we describe the synthesis of 24,24-difluoro-1β,3β,25-dihydroxy-19-norvitamin D 3 (6a) and 24,24-difluoro-1α,3α,25-dihydroxy-19-norvitamin D 3 (6b), and a comparison of their VDR-binding affinity with that of compounds 5a and b. RESULTS AND DISCUSSIONIn our previous study, we synthesized 1β,3β,25-dihydroxy-19-norvitamin D 3 (5a) and 1α,3α,25-dihydroxy-19-norvitamin D 3 (5b) by means of the Julia-Kocienski coupling with a 1,3-cis type A-ring synthon, followed by separation of the isomers 5a and b.14) Here we adopted similar methodology, using ketone 11 and sulfone 10. The sulfone 10 was synthesized from the Grundmann's ketone 7, which was reported by DeLuca and colleagues 13,15) (Chart 1). Compound 7 was subjected to Horner-Wittig olefination using triethyl phosphonoacetate in the presence of sodium hydride in tetrahydrofuran (THF) to give the α,β-unsaturated ester 8, and the ester was then reduced with diisobutylaluminium hydride (DIBAL)-H in toluene to give the allylic alcohol 9 in 68% yield (2 steps). The allylic alcohol 9 was subjected to the Mitsunobu reaction using 2-mercaptobenzothiazole in the presence of disopropyl azodicarboxylate (DIAD) and triphenylphosphi...

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

“…Figure 25 (2015-2017) [337][338][339][340][341][342][343][344][345][346][347][348][349][350][351]. 1α,25-Dihydroxy-21-(3-hydroxy-3-methyl-1-methylenebutyl)vitamin D 3 (1428, UV1) [337] presented potent antitumoral effects over a wide panel of tumor cell lines without inducing hypercalcemia or toxicity in vivo.…”

Synthetic Chemical Probes That Dissect Vitamin D Activities

“…In our effort to delineate VD 3 activity, we initially searched for VD 3 analogs that activate VDR but lack SREBP inhibitory activity. An in-house library of 250 vitamin D congeners − was screened for their ability to inhibit the activity of an SREBP-responsive luciferase reporter, in which expression of luciferase is controlled by three SREBP binding sites. We found that a series of vitamin D analogs, bearing alkyltriazole and alkyltetrazole substituents at C2 ( 4 – 9 ) (Figure ), displayed limited SREBP inhibitory activities (Figure A).…”

Julia‐Kocienski Olefination in the Synthesis of Trisubstituted Alkenes: Recent Progress

“…However, biological assays of the corresponding triols 268 have shown poor binding to VDR. Nagasawa et al [109] also reported the synthesis of gem-24-difluoro-19-norvitamin D 3 derivatives of the same family (268, A = F), in order to evaluate their VDR binding affinity. In this case, the reaction between BT-sulfone 265 b and ketone 266 b with the aid of LiHMDS provided an equimolecular mixture of diastereomeric olefins 267 e in 92 % yield.…”

“…However, biological assays of the corresponding triols 268 have shown poor binding to VDR. Nagasawa et al [109] . also reported the synthesis of gem‐24‐difluoro‐19‐norvitamin D 3 derivatives of the same family ( 268 , A=F), in order to evaluate their VDR binding affinity.…”