José I. Miranda

Asymmetric synthesis of α-keto β-lactams via [2+2] cycloaddition reaction : a concise approach to optically active α-hydroxy β-lactams and β-alkyl(aryl)isoserines

Abstract The cycloaddition reaction of the Evans-Sjogren ketenes to imines, followed by α-hydroxylation of the resulting cycloadducts provides an efficient general asymmetric synthesis of α-keto β-lactams and derivatives.

Cyclic RGD β-Lactam Peptidomimetics Induce Differential Gene Expression in Human Endothelial Cells

Angiogenesis is a fundamental step in the transition of solid tumors from a dormant state to a malign one. Many of the low-molecular-weight anti-angiogenic drug candidates mimic the short peptide epitope Arg-Gly-Asp (RGD), disrupting the extracellular matrix/integrin adhesion and, ultimately, leading to tumor cell apoptosis. In contrast to the detailed structural information available for the extracellular adhesion inhibition of endothelial cells through the recognition of integrins (typically aVb3) by RGD peptidomimetics, [2] most aspects of possible intracellular angiogenic gene regulation caused by peptidomimetics remain unexplored. Importantly, dysfunction of this signaling system is suspected to be behind the resistance phenomena developed in anti-angiogenic therapies. Inside the endothelial cell, dozens of proteins mediate or control the signaling pathways of angiogenesis after integrin activation, but only a couple of kinases (JNK, ERK) and transcription factors (NFkB, FoxO) are able to promote gene regulation (Figure 1). In addition, large environmental ligands, such as vascular endothelial growth factors (VEGFs) or peptide hormones, are required to elicit proangiogenic gene regulation. In this context, we set out to design alternative lowmolecular-weight RGD probes for interaction with aVb3 integrin and gene regulation in HUVECs. Ideally, these peptidomimetics should: 1) contain a minimal scaffold to prevent undesired scaffold/integrin interactions, 2) have a uniform and predictable scaffold conformation, 3) bear a maximum of recognition groups, including hydrophobic or hydrophilic ones, and 4) permit the deletion of selected residues from the RGD triad without global shape change. We selected five-membered and four-membered small cyclic peptides for development. The residual flexibility of these cyclopeptides can be further constrained by incorporating lactam bridges between neighboring amino acids to stabilize protein secondary structure motifs characterized by combinations of b-turns and/or g-turns. Several lactam pseudopeptides resulting from such an extension of Freidinger’s concept (Scheme 1; 1!2) have been explored with the goals of mimicking receptor-bound conformations of bioactive peptides and of providing pharmacophore information for nonpeptide drug design. However, despite its apparent simplicity, this design is not always reliable for the precise positioning of a maximum number of recognition groups around the pseudopeptide cyclic core. Because the interresidual lactam bridge created by modification of the side chain (R) shares recognition and constraint functions, the design of mimetic libraries becomes difficult and non-general, when synthetically achievable. An alternative way to constrain cyclic peptides is based on the incorporation of a d-amino acid and an N-alkyl-amino acid into the macrocycle, as illustrated by the remarkable aVb3 antagonist cilengitide (3, cyclo-[Arg-Gly-Asp-d-Phe-N(Me)Val]) developed by Kessler et al. Here we report an alternative, more versatile solution to the above design problem, by introducing a-amino-b-lactam scaffolds originating from aCH/NH proton mimicry (Scheme 2; 1! 4). This “b-lactam scaffold-assisted design” (b-LSAD) approach is based on the separation of recognition and constraint groups, and has previously proven efficient for promotion of unusually stable type-II (II’) b-turn peptidomimetics from extended open native peptides. Accordingly, the straight application of the b-LSAD principle to RGD cyclic mimetics led us to the b-lactam pseudopeptides 4a–d, which fulfilled several structural requirements to trigger quite different signaling interactions with aVb3 integrin. The hydrophobic benzyl group in the d-Phe residue of cilengitide (3), for instance, was replaced by the strongly hydrophilic 1,2-dihydroxyethyl moiety in the mimetic 4a or by the poorly hydrophobic methyl group in compound 4b. Conversely, the hydrophobic R–R substituents Figure 1. Signaling pathways initiated by integrins affecting the gene regulation of angiogenesis. ERK=extracellular-signal-regulated kinase, FAK= focal adhesion kinase, FoxO= forkhead box O, JNK=c-Jun N-terminal kinase, NFkB=nuclear factor k of B-cells.

Conformation and Chiral Effects in α,β,α-Tripeptides

Short α,β,α-tripeptides comprising a central chiral trisubstituted β(2,2,3)*-amino acid residue form unusual γ-turns and δ-turns in CDCl(3) and DMSO-d(6) solutions but do not form β-turns. Thermal coefficients of backbone amide protons, 2D-NMR spectra, and molecular modeling revealed that these motifs were strongly dependent on the configuration (chiral effect) of the central β-amino acid residue within the triad. Accordingly, SSS tripeptides adopted an intraresidual γ-turn like (C6) arrangement in the central β-amino acid, whereas SRS diastereomers preferred an extended δ-turn (C9) conformation. A different SRS-stabilizing bias was observed in the crystal structures of the same compounds, which shared the extended δ-turn (C9) found in solution, but incorporated an additional extended β-turn (C11) to form an overlapped double turn motif.

The underlying mechanisms for self-healing of poly(disulfide)s.

Recently, self-healing polymers based on disulfide compounds have gained attention due to the versatile chemistry of disulfide bonds and easy implementation into polymeric materials. However, the underlying mechanisms of disulfide exchange which induce the self-healing effect in poly(disulfide)s remain unclear. In this work, we elucidate the process of disulfide exchange using a variety of spectroscopic techniques. Comparing a model exchange reaction of 4-aminophenyl disulfide and diphenyl disulfide with modified reactions in the presence of additional radical traps or radical sources confirmed that the exchange reaction between disulfide compounds occurred via a radical-mediated mechanism. Furthermore, when investigating the effect of catalysts on the model exchange reaction, it could be concluded that catalysts enhance the disulfide exchange reaction through the formation of S-based anions in addition to the radical-mediated mechanism.

Zwitterionic polymerization of glycidyl monomers to cyclic polyethers with B(C6F5)3

A new method of generating cyclic polyethers is reported. Glycidyl monomers react with B(C6F5)3 to generate cyclic polyethers under anhydrous conditions. In the presence of water, linear chains are formed. A zwitterionic ring-opening polymerization mechanism is postulated based on experimental evidence and DFT calculations. The obtained cyclic polyethers can be considered a new family of crown ethers, where peripheral functional groups such as phenyls, fluorinated aliphatic chains or hydroxyls decorate the rings.

Mechanistic Insights on the Magnesium(II) Ion-Activated Reduction of Methyl Benzoylformate with Chelated NADH Peptide β-Lactam Models

Mechanistic details of the Mg(2+) ion-activated enantioselective reduction of methyl benzoylformate have been investigated at a B3LYP/6-31G* theory level, using peptide NADH models 1 rigidified with a beta-lactam ring. Computation of the reaction pathway revealed important structural differences between the intermediate NADH/Mg(2+)/ArCOCO(2)R ternary complexes 3 and the corresponding transition states leading to enantiomeric methyl mandelates. Thus, ternary complexes showed the dihydronicotinamide moiety placed quasiequatorial to a seven-membered chelation pseudoplane including the two amide carbonyls and the Mg(2+) cation, whereas productive transition states were strongly deformed with the dihydronicotinamide group oriented quasiaxial to the chelation pseudoplane. This chelation model was further applied to acyclic nonrigidified NADH models and, based on the fluxional mobility of the peptide chain bonds, experimental enantioselectivities were correctly predicted. Parallel experiments were also conducted in deuterated acetonitrile, using NMR techniques, to study the structure of the binary complexes 2 (NADH/Mg(2+)) and ternary complexes 3 (NADH/Mg(2+)/PhCOCO(2)Me). Finally, owing to the incorporation of two diastereotopic trimethylsilyl NMR-tags in the beta-lactam-NADH peptidomimetics, a nonproductive ternary complex predicted by calculations could be observed and its structure characterized on the basis of ROESY experiments and molecular modeling.

A concise synthesis of α-amino acid N-carboxy anhydrides of (2S,3S)-β-substituted serines

A general access to 3-hydroxy-4-(1-hydroxyalkyl)--lactams with a C4-C1 relative configuration unlike is provided. The subsequent oxidative ring expansion of the corresponding 3-hydroxy -lactam smoothly affords an -amino acid N-carboxy anhydride (NCA) formally derived from (2S,3S)--substituted serine, which upon sequential peptide coupling furnishes the tripeptide segment 2, present in lysobactin.

Diastereoselective [2 + 2] cycloaddition of dichloroketene with α-oxyaldehydes and α-amino aldehydes

A new route to natural products containing 1,2-diol and 1,2-amino alcohol subunits based on the [2 + 2] cycloaddition of dichloroketene to α-oxyaldehydes and α-amino aldehydes is demonstrated.

Simple access to the nonproteinogenic peptide fragments of lysobactin from azetidin-2-one frameworks

A convenient route to the β-hydroxy α-aminoacid-derived tripeptides found in the macrocyclic peptide lactone antibiotic lysobactin from azetidin-2-one frameworks is provided for the first time.

Chirality-driven folding of short β-lactam pseudopeptides.

Novel enantiopure pseudopeptide models containing a central -(β-lactam)-(Aa)- scaffold characterized by the combined presence of an α-alkyl-α-amino-β-lactam (i+1) residue and a α-substituted (i + 2) amino acid have been readily synthesized from α-alkyl serines. The conformational analysis of such β-lactam pseudopeptides conducted in CDCl(3) and DMSO-d(6) solutions using 1D- and 2D-NMR techniques revealed an equilibrium between β-II turn and γ-turn conformers, which was ultimately modulated by the relative configuration of the -(β-lactam)-(Aa)- residues. Long-range chiral effects on the α-lactam pseudopeptide conformers were also found when two (i) and (i + 3) chiral residues were attached to the termini of a central -(β-lactam)-(Aib)- segment. In such mimetics, heterochiral (i) and (i + 3) residues reinforced a β-II turn conformer, whereas homochiral corner residues stabilized an overlapped β-II/ β-I double turn motif. No β-hairpin nucleation was observed in any instance. In good agreement with the conformers found in solution, β-turned and open structures were also characterized by X-ray crystallography. Relative stabilities of the different conformers were estimated computationally at a B3LYP/6-31++G** calculation level, and finally, a conformation equilibrium model based on steric inter-residual interactions around the -(β-lactam)-(i + 2)- segment was proposed to account for the observed chiral effects.