Cyclic dipeptides as building blocks for combinatorial libraries. Part 2: Synthesis of bifunctional diketopiperazines

High-temperature Raman spectra of dipeptide α-L-aspartyl-L-alanine crystal

Crystals of dipeptide α-L-aspartyl-L-alanine (α-Asp-Ala), C₇H₁₂N₂O₅, were studied under high-temperature conditions through vibrational spectroscopy (IR and Raman) and thermal analysis (Differential Scanning Calorimetry - DSC). From the analysis of the results, it is possible to conclude that: (i) the studied material undergoes a reversible order-disorder phase transition at 373 K on heating, where several changes were observed in the vibrational spectra, especially with vibrational modes of the units that participate directly of the hydrogen bonds; (ii) the phase transition undergone by the α-Asp-Ala crystal (about 373 K) involves changes in hydrogen bonds, possibly the rupture of at least one of them, and change in the conformation of the molecules in the unit cell.

Co-solvent polarity tuned thermochromic nanotubes of cyclic dipeptide-polydiacetylene supramolecular system

The cooperative non-covalent interactions arising from structurally integrated multiple molecules have emerged as a powerful tool for the creation of functional supramolecular structures. Herein, we constructed cyclic dipeptide (CDP)–polydiacetylene (PDA) conjugate (CDP–DA) by introducing cyclo(l-Phe-l-Lys) to the linear 10,12-pentacosadiynoic acid. Owing to extensive hydrogen bonding characteristics, together with structural chirality of cyclo(l-Phe-l-Lys) and strong π–π stacking diacetylenic template, CDP–DA generated supramolecular nanotubes. The structural visualization using scanning and transmission electron microscopy revealed chloroform/methanol co-solvent polarity tuned morphological transformation of intrinsic lamellar assemblies into nanotubes comprising single-wall and multi-wall structure. The mechanistic understanding by X-ray diffraction patterns confirms bilayer organization in lamellar structure, which forms nanotubes via a gradual lamellar curling-to-scrolling process. The supramolecular CDP–DA nanotubes are transformed into the rigid covalently cross-linked blue-phase polydiacetylene (CDP–PDA) by UV irradiation. Very interestingly, the blue-phase nanotubes display reversible thermochromic changing temperature up to 150 °C with excellent repeatability over a dozen thermal cycles. This work provides an efficient strategy for precise morphological control and aiding the perspective for development in nanostructures for functional devices.

Co-solvent controlled fabrication of thermo-responsive chromogenic nanotubes of a cyclic dipeptide–polydiacetylene supramolecular system.

Solid-phase synthesis of diverse spiroisoxazolinodiketopiperazines

A convenient, efficient protocol to prepare diverse spiroisoxazolino-diketopiperazines via a parallel solid-supported synthesis was developed. The key steps are (1) a coupling reaction of an amino acid; (2) tosylation with concomitant β-elimination to form an α, β-unsaturated ester; (3) a 1,3-dipolar cycloaddition with an oxime to form isoxazoline rings; and (4) cyclic cleavage to release the product from the resin. All reaction steps and workup procedures were modified to allow the use of automated or semiautomated equipment. A 100-member demonstration library with two diversity sites was prepared in good purity and acceptable overall yields.

Peptides by Extension at the N- or C-terminii of Lysine

Dipeptides 3a-g, (3a + 3a'), (3d + 3d'), (3l + 3l')a and tripeptides 6a-e, (6b + 6b'), (6e + 6e') incorporating Z(epsilon)-Lys were prepared in high yields (70-95%) and enantiopurity (> or =97%) in partially aqueous acetonitrile solution by coupling using (i) Z(epsilon)-Lys with N-(Z- and Fmoc-aminoacyl)benzotriazoles 1a-g, (ii) Z(epsilon)-Lys with N-Z-dipeptidoylbenzotriazoles 5a-c, and (iii) N-Fmoc(alpha)-Z(epsilon)-l-Lys-Bt 1h and amino acids 2a,c-e. Unnatural dipeptides 3h-j, (3h + 3h') and tripeptides 6f were similarly prepared from Z(alpha)-Lys. Retention of chirality was demonstrated by parallel experiments involving l-Ala, dl-Ala, l-Met, and dl-Met by NMR and HPLC analysis.

The synthesis of alternative diketopiperazines as potential RGD mimetics

Alternative RGD mimetics-with the exception of glycine-c(Arg-Asp) 1, c(Arg-Glu) 2 and c[Arg-Asp(Phe-OH)] 3 were synthesized. The DKPs were prepared on solid phase with orthogonal protection allowing further derivatization in solution. During solution phase cyclization in NH(3)/methanol, the side chain benzyl ester group of H-Arg(Tos)-Asp(OBzl)-OMe and H-Arg(Tos)-Glu(OBzl)-OMe suffer transesterification, while beta-t-butyl or beta-cyclohexyl esters are stable under the same conditions. In spite of the simple structure, all compounds bind selectively to the alpha(v)beta(3) integrin receptor, 3 showing the highest affinity with an IC(50) value of 0.74 microM value. On the other hand only 3 binds with measurable activity to the alpha(IIb)beta(3) receptor (IC(50) 159 microM). The binding affinities seem to be in accordance with the distances between the arginine guanidino and the aspartic acid carboxyl group in extended conformation determined by semiempirical geometry optimization.

The cyclization of peptides and depsipeptides

Constricting the peptide backbone into a more defined conformational form through cyclization is an activity evolved in nature and in synthetic work, the latter straddling only the most recent decades. The resulting conformational constraints increase the probability of an optimum response with bio-receptors. The purpose of this review is to highlight developments that have proved to be reasonably efficient in the macrocyclization of linear precursors into cyclic peptides and depsipeptides.