Selective O-acylation of unprotected N-benzylserine methyl ester and O,N-acyl transfer in the formation of cyclo[Asp-Ser] diketopiperazines

Cyclic isoDGR and RGD peptidomimetics containing bifunctional diketopiperazine scaffolds are integrin antagonists

The cyclo[DKP-isoDGR] peptidomimetics 2-5, containing bifunctional diketopiperazine (DKP) scaffolds that differ in the configuration of the two DKP stereocenters and in the substitution at the DKP nitrogen atoms, were prepared and examined in vitro in competitive binding assays with purified αv β3 and αv β5 integrin receptors. IC50 values ranged from low nanomolar (ligand 3) to submicromolar with αv β3 integrin. The biological activities of ligands cyclo[DKP3-RGD] 1 and cyclo[DKP3-isoDGR] 3, bearing the same bifunctional DKP scaffold and showing similar αV β3 integrin binding values, were compared in terms of their cellular effects in human U373 glioblastoma cells. Compounds 1 and 3 displayed overlapping inhibitory effects on the FAK/Akt integrin activated transduction pathway and on integrin-mediated cell infiltration processes, and qualify therefore, despite the different RGD and isoDGR sequences, as integrin antagonists. Both compounds induced apoptosis in glioma cells after 72 hour treatment.

Synthesis and conformational studies of a stable peptidomimetic β-hairpin based on a bifunctional diketopiperazine turn inducer

A new β-hairpin mimic foldamer based on the assembly of a reverse turn inducer, a peptidomimetic strand, and a tetrapeptide sequence was prepared, and its conformation in solution was assessed by NMR and computational investigations.

Transition-metal clusters as catalysts for chemoselective transesterification of alcohols in the presence of amines

Acylation is one of the most abundant organic transformations of alcohols (esterification) and amines (amidation). Because of the greater nucleophilicity of the amino group compared to the hydroxyl group and the stability of amides compared to esters, N-acylation occurs predominantly in organic synthetic reactions. We reported that the μ-oxo-tetranuclear zinc cluster Zn4(OCOCF3)6O efficiently catalyzes highly chemoselective acylation of hydroxyl groups in the presence of primary and secondary alkyl amino groups to afford the corresponding esters in high yields. Not only zinc carboxylate complexes but also various carboxylate complexes of first-row late transition metals, such as Mn, Fe, Co, and Cu, become catalysts for such the hydroxy group-selective acylation in the presence of amines. Among these carboxylate compounds, we found that the combination of an octanuclear cobalt carboxylate cluster [Co4(OCOR)6O]2 (R = CF3, CH3, and tBu) with nitrogen-containing ligands such as 2,2′-bipyridine show sufficient catalytic activity toward O-selective transesterification. Notably, an alkoxide-bridged dinuclear complex, Co2(OCOtBu)2(bpy)2(μ2-OCH2-C6H4-4-CH3)2, was successfully isolated as a key intermediate that proceeds with Michaelis–Menten behavior through an ordered ternary complex mechanism similar to dinuclear metallo-enzymes, suggesting that the formation of alkoxides, followed by coordination of the ester, is responsible for the unique O-selective acylation.

Synthesis and biological evaluation (in vitro and in vivo) of cyclic arginine-glycine-aspartate (RGD) peptidomimetic-paclitaxel conjugates targeting integrin αVβ3

A small library of integrin ligand-paclitaxel conjugates 10-13 was synthesized with the aim of using the tumor-homing cyclo[DKP-RGD] peptidomimetics for site-directed delivery of the cytotoxic drug. All the paclitaxel-RGD constructs 10-13 inhibited biotinylated vitronectin binding to the purified αVβ3 integrin receptor at low nanomolar concentration and showed in vitro cytotoxic activity against a panel of human tumor cell lines similar to that of paclitaxel. Among the cell lines, the cisplatin-resistant IGROV-1/Pt1 cells expressed high levels of integrin αVβ3, making them attractive to be tested in in vivo models. cyclo[DKP-f3-RGD]-PTX 11 displayed sufficient stability in physiological solution and in both human and murine plasma to be a good candidate for in vivo testing. In tumor-targeting experiments against the IGROV-1/Pt1 human ovarian carcinoma xenotransplanted in nude mice, compound 11 exhibited a superior activity compared with paclitaxel, despite the lower (about half) molar dosage used.

Cyclic RGD peptidomimetics containing bifunctional diketopiperazine scaffolds as new potent integrin ligands

The synthesis of eight bifunctional diketopiperazine (DKP) scaffolds is described; these were formally derived from 2,3-diaminopropionic acid and aspartic acid (DKP-1-DKP-7) or glutamic acid (DKP-8) and feature an amine and a carboxylic acid functional group. The scaffolds differ in the configuration at the two stereocenters and the substitution at the diketopiperazinic nitrogen atoms. The bifunctional diketopiperazines were introduced into eight cyclic peptidomimetics containing the Arg-Gly-Asp (RGD) sequence. The resulting RGD peptidomimetics were screened for their ability to inhibit biotinylated vitronectin binding to the purified integrins α(v)β(3) and α(v)β(5), which are involved in tumor angiogenesis. Nanomolar IC(50) values were obtained for the RGD peptidomimetics derived from trans DKP scaffolds (DKP-2-DKP-8). Conformational studies of the cyclic RGD peptidomimetics by (1)H NMR spectroscopy experiments (VT-NMR and NOESY spectroscopy) in aqueous solution and Monte Carlo/Stochastic Dynamics (MC/SD) simulations revealed that the highest affinity ligands display well-defined preferred conformations featuring intramolecular hydrogen-bonded turn motifs and an extended arrangement of the RGD sequence [Cβ(Arg)-Cβ(Asp) average distance ≥8.8 Å]. Docking studies were performed, starting from the representative conformations obtained from the MC/SD simulations and taking as a reference model the crystal structure of the extracellular segment of integrin α(v)β(3) complexed with the cyclic pentapeptide, Cilengitide. The highest affinity ligands produced top-ranked poses conserving all the important interactions of the X-ray complex.