Routes to F3-etidronic acid (1-hydroxy-2,2,2 trifluoroethylidene-bisphosphonic acid), molecular structure of the tris(trimethylsilyl)ester, antimineralization, and antiresorption effects of the disodium salt

Abstract

Geminal bisphosphonates R~R2C(PO3H2)2, such as etidronate (R 1 = Me, R 2= OH), pamidronate (R l = C H z C H z N H 2 , R 2 = OH), clodrohate (R 1 = R 2 = C1), and alendronate (R 1 = C H z ( C H 2 ) z N H 2 , R 2 = OH), characterized by a P-C-P type structure are used clinically as antiresorption [1] and anticalcification [2] agents. The introduction of fluorine or perfluoroalkyl groups into molecules is a versatile tool for modifying their physicochemical properties and physiological behavior [3, 4] by increasing lipophilicity [3] and, due to the strong electron-withdrawing inductive effect of fluorine, influencing the reactivity of neighboring groups, for example, acidity [4]. Recently monoand difluoromethylene bisphosphonates have been shown to be potential candidates for bisphosphate analogues inhibiting bone lysis ([5, 6] and references cited in [6]). Since a correlation between increasing acidity and improved antiresorption activity of bisphosphonates has been suggested [7], we considered it of interest to synthesize and to examine a perfluorinated analogue of clinically used bisphosphonate. In this paper we describe the synthesis and evaluation of the trifluoro derivative of etidronate. Trimethylsilylated phosphites are wellknown precursors for phosphonates and phosphonic acids [8-13]. Trifluoroacetyl and acetyl chloride were reported to react with tris(trimethylsilyl)phosphite [9] to yield the corresponding a-ketophosphonates [14-16] (for a recent comprehensive review on acylphosphonates a-ketophosphonates see [16]). We were encouraged to reinvestigate this reaction as a synthetic route to our target compound, trifluoro analogue of l-hydroxyethane-l,l-bisphosphonate [17]. The reaction of trifluoroacetyl chloride and tris(trimethylsilyl) phosphite (1) at 7 0 ° C furnished chlorotrimethylsilane and 1-trimethylsiloxy-2,2,2-trifluoroethane-tetrakis(trimethylsilyl) bisphosphonate (3) [F3 pentakis(trimethylsilyl) etidronate] (Fig. 1), as a water-sensitive, colorless, low melting point solid. The expected a-ketophosphonate [141 CF3C(O)P(O)(OSiM%) 2 (2) was not observed but probably formed via intermediate A, regardless of the starting materials' stoichiometry (Fig. 1). However, 2-(trifluoroacetyloxy)pyridine (TFAP) [18] reacted with phosphite 1 and furnished monophosphonate 4, which was characterized by nuclear magnetic resonance (NMR) spectroscopy. Presumably the reaction proceeded via the 1,3 dipolar intermediate B, which underwent a 1,4-trimethylsilyl group. Warming up to ambient temperature, compound 4 decomposed to give the a-ketophosphonate 2 and 2-trimethylsiloxypyridine (Fig. 1). Further addition of phosphite 1 to compound 2 again gave bisphosphonate 3, via intermediate C. A stabilizing effect for compound 4 could possibly be a Lewis acid base interaction of the phosphonate phosphorus and the pyridine nitrogen atom since the analogous reaction carried out with phenyltrifluoroacetate directly gave bisphosphonate 3 exclusively. The bisphosphonate was then easily converted using a methanol/ water mixture to form the waxy free acid 5 (F3 etidronic acid), for which two pKa values (1.55 and 5.56) were measured. No thermal rearrangement of the PC(CF3)(OH)P skeleton to give a POCH(CFflP framework was observed [17]. To obtain more information about the molecular structure of compounds 3 or 5 we tried unsuccessfully to grow single crystals for X-ray diffraction; however, after partial hydrolysis appropriate crystals of l-hydroxy-2,2,2-trifluoroethylidene-tris(trimethylsilyl)bisphosphonate (6) were obtained. Molecules of compound 6 were found to arrange pairwise in the unit cell taking advantage of two P = O. • .HO-P (251.5 pro) and two P = O . . . H O C (264.1pm) hydrogen bridges (Fig. 1). The molecular structure showed a slightly distorted tetrahedral geometry at carbon C1, P1, and P2 (Fig. 2) presumably caused by the involvement of the oxygen intermolecular hydrogen bonding comparable to other bisphosphonate structures [19, 20]. The P1-C1-P2 angle was found to be 110.3 ° correlating [20] to the relatively long P1-C1 and P2-C1 distances (185.6 pro); for the mono hydrate of eftdronic acid the corresponding parameters were 115.1° and 183.2 and 184.0 pm [19]. The calcification of bioprosthetic tissue implanted subdermally in rats is a wellestablished model for the study of ectopic calcification and for the evaluation of drugs potentially affecting it [21-26]. It has been shown [23-25] that there is a good correlation between anticalcification effect of bisphosphonates in vivo and antimineralization effect in vitro, at least in part as a consequence of their action on crystal growth [23-25]. In addition, the subdermal model of bioprosthetic tissue calcification is not mediated by osteogenic cellular factors [24, 27]. Thus, novel bisphosphonates can be screened conveniently by examining their effect