Phosphinic peptides

The synthesis uses Evans chemistry to introduce enantioselectively an azide group, which is then reduced to the a-amino group (Scheme 16). Note that in all cases, the phosphinyl group is protected as its sulfide during the synthesis to avoid complication (oxidation) observed with the phosphine. The phosphine sulfide is compatible with Fmoc- and Boc-peptide synthesis conditions. The free phosphine can later be regenerated by treating the sulfide with Raney nickel in MeOH (as shown in Scheme 15) or by methylation followed by treatment with HMPTJ50 This reduction was even applied successfully to sulfide-protected phosphine peptides still attached to the solid support.151 ... 

Peptidomimetics in which one amide bond is replaced by a phosphinic acid (R-P(0H)(=0)-R phosphinic peptides ) are of interest as potential protease inhibitors [17-19]. These compounds have been prepared either from orthogonally protected phosphorus-containing monomers [17,18,20], or by forming the phosphorus-containing fragments on solid phase, as sketched in Figure 11.4 [19,21], Phosphinic acids have been prepared on solid phase mainly by reaction of carbon electrophiles with monoalkylphosphinates. As carbon electrophiles, acrylates, aldehydes, reactive alkyl halides, or a, 3-unsaturated ketones can be used. 

Vincent, B., Jiracek, J., Noble, F., et al. (1997) Effect of a novel selective and potent phosphinic peptide inhibitor of endopeptidase 3.4.24.16 on neurotensin-induced analgesia and neuronal inactivation. Br. J. Pharmacol. 121,705-710. 

The solution-phase synthesis and resolution of new phosphinopeptidic building blocks containing a triple bond and their involvement in 1,3-dipolar cycloaddition with a variety of in j// -prepared nitrile oxides allowed the diastereoselective preparation of a novel class of isoxazole-containing phosphinic peptides 619. Inhibition assays of some of these peptides revealed their behavior as very potent inhibitors of metalloproteases, outmatching previously reported phosphinic peptides in terms of potency <2003CEJ2079>. 

An optimised solid-phase method for the generation of diverse a-amino-alkyl or -aryl phosphonates derived from peptides and polymer-assisted solution-phase parallel synthesis of dipeptide p-nitroanilides and dipeptide diphenyl phosphonates have been reported. A modular method for the construction of polypeptides containing the Phe-Arg phosphinic acid isostere has been described. A novel methodology for the solid-phase synthesis of phosphinic peptides has been developed in which the phosphorus-carbon bond was formed... 

Buchardt, J. Schiodt, C.B. Krog-Jensen, C. Delaisse, J-M. Foged, N.T. Meldal, M. Solid Phase Combinatorial Library of Phosphinic Peptides for Discovery of Matrix Metalloproteinase Inhibitors, J. Comb. Chem. 2,624-638 (2000). 

Phosphinic Peptides as Potent Inhibitors of Zinc-Metalloproteases... 

Since fosinoprilat development, phosphinic peptide chemistry on solid-support has been exploited to allow easier identification of potent inhibitors for various zinc-metalloproteases. In this review, the synthetic routes to obtain the desired phosphinic peptides are reported, as well as several examples showing that this class of compounds behaves as extremely potent inhibitors toward different subfamilies of zinc-metalloproteases. Moreover, the development of these compounds has stimulated efforts to obtain crystal structures of these inhibitors in interaction with their targets, leading currently to about 111 crystal structures in the protein data bank. Analysis of these structures illustrates how phosphinic peptides, as transition-state analogues, have provided important clues on the active site residues involved in catalytic processes, leading to a global view on the catalytic mechanisms of this class of enzymes. Finally, recent use of phosphinic peptide inhibitors for in vivo studies is discussed. 

Fig. 2 (a) Schematic representation of the zinc-protease active site with subsite (S) nomenclature. X features a zinc-chelating group such as thiolate, carboxylate or hydroxamate. (b) Generic structure of a phosphinic peptide covering the S2 to S2 subsites of the active site... 

These remarks have historically justified the development of the chemistry of phosphinic peptides as stable compounds and the ability to inhibit zinc-proteases potently [16]. Indeed, it has been found that the replacement of an NH by a CH2... 

Synthetic Chemistry of Phosphinic Peptides 3.1 General Considerations... 

From a synthetic point of view, phosphinic peptides are densely functionalized structures with increasing degree of complexity, depending on the presence of additional functional groups on their side chains. Therefore, the synthesis of such molecules has been a research area of intense activity in both industry and academia for almost 30 years now. Among the several excellent reviews in the literature concerning phosphinic peptides, the prototype work of Yiotakis [19] and the more recent article by Mucha [20] have attempted a systematic approach to the synthetic aspects of these structures. The discussion that follows aims at a concise description of the recent developments in the field and focuses on all current findings and future perspectives. 

Fig. 4 Schematic representation of s)mthetic challenges in phosphinic peptides...

The synthesis of aminophosphinic acids with subsequent formation of the second P-C bond via an NP-i-C strategy has also been investigated as a shorter and faster alternative toward phosphinic peptides. The research group of Haemers coupled the addition of BTSP to tritylimines with a subsequent Michael addition of acrylates by activating in situ intermediate A-Trt-A-TMS-protected silyl aminopho-sphinates with BSA and adding acrylates to the resulting phosphonite 14... 

Scheme 7 Application of NP+C strategy to the solid phase synthesis of phosphinic peptides by (a) Buchardt et al. [45] and (b) Manzenrieder et al. [47] (Nem IV-ethyl morpholine, SPPS solid phase peptide synthesis, DBU l,8-diazabicycloundec-7-ene, NMP lV-methyl-2-pyrrolidone, TFE trifluoroethanol, DCM dichloromethane)...

The synthesis of phosphinic peptides by a reverse sequence of P-C bond formation events (N+PC approach) is a less frequently applied strategy which may offer important diversification possibilities. In particular, an amidoalkylation condensation reaction between amides, aldehydes, and alkylphosphinic acids (the three-component Kabachnik-Fields reaction) affords in a single step the main pseudopeptidic backbone, thus facilitating fast screening of the nature of Pi position. In 1996, Chen and Coward observed that a mixture of benzyl carbamates, aldehydes, and alkylphosphinic acid 23 in AcCl can lead to Cbz-protected phosphinic pseudodipeptides 24 (Scheme 10a) [53]. This method was adjusted by Matziari et al. to the synthesis of Fmoc-protected phosphinic building blocks 25 and peptides thereof (Scheme 10b) [54]. 

However, none of these protocols have ever been used in synthesis of phosphinic peptide libraries by automated synthetic methods. 

Thermolysin (TEN EC 3.4.24.28), a thermostable bacterial protease isolated from Bacillus thermoproteolyticus, has been studied as the prototype of zinc-metallopeptidases at a time where no crystal structure was available for this class of proteases [122]. Crystallographic analysis of a number of TLN/inhibitor complexes has allowed an understanding of the binding mode of these inhibitors and allowed the mechanism of action of this protease to be determined [122]. These seminal studies have greatly inspired the development of NEP inhibitors, given the close stmctural relationship between TEN and NEP [123]. To examine further the structural relationships between these two peptidases, various phosphinic peptides were prepared. One of these compounds (58, Table 1) exhibits a Ki value of 26 nM toward thermolysin and 22 nM toward NEP [124]. 

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