Development of a New Family of Conformationally Restricted Peptides as Potent Nucleators of β-Turns. Design, Synthesis, Structure, and Biological Evaluation of a β-Lactam Peptide Analogue of Melanostatin

Stapling Amantadine to Melanostatin Neuropeptide: Discovery of Potent Positive Allosteric Modulators of the D2 Receptors

This work describes the synthesis and pharmacological and toxicological evaluation of melanostatin (MIF-1) bioconjugates with amantadine (Am) via a peptide linkage. The data from the functional assays at human dopamine D2 receptors (hD2R) showed that bioconjugates 1 (EC50 = 26.39 ± 3.37 nM) and 2 (EC50 = 17.82 ± 4.24 nM) promote a 3.3- and 4.9-fold increase of dopamine potency, respectively, at 0.01 nM, with no effect on the efficacy (Emax = 100%). In this assay, MIF-1 was only active at the highest concentration tested (EC50 = 23.64 ± 6.73 nM, at 1 nM). Cytotoxicity assays in differentiated SH-SY5Y cells showed that both MIF-1 (94.09 ± 5.75%, p < 0.05) and carbamate derivative 2 (89.73 ± 4.95%, p < 0.0001) exhibited mild but statistical significant toxicity (assessed through the MTT reduction assay) at 200 μM, while conjugate 1 was found nontoxic at this concentration.

β-Turn Induction by a Diastereopure Azepane-Derived Quaternary Amino Acid

β-Turns are one of the most common secondary structures found in proteins. In the interest of developing novel β-turn inducers, a diastereopure azepane-derived quaternary amino acid has been incorporated into a library of simplified tetrapeptide models in order to assess the effect of the azepane position and peptide sequence on the stabilization of β-turns. The conformational analysis of these peptides by molecular modeling, NMR spectroscopy, and X-ray crystallography showed that this azepane amino acid is an effective β-turn inducer when incorporated at the i + 1 position. Moreover, the analysis of the supramolecular self-assembly of one of the β-turn-containing peptide models in the solid state reveals that it forms a supramolecular helical arrangement while maintaining the β-turn structure. The results here presented provide the basis for the use of this azepane quaternary amino acid as a strong β-turn inducer in the search for novel peptide-based bioactive molecules, catalysts, and biomaterials.

Asymmetric Synthesis of Functionalizable Type II β-Turn-Inducing α-Amino Acid Building Blocks

Peptidomimetics are emerging as a promising class of potent and selective therapeutics. Among the current approaches to these compounds, the utilization of constrained lactams is a key element in enforcing the active peptide conformation, and the development of efficient and stereocontrolled methods for generating such lactam building blocks is an important objective. Current methods typically rely on the elaboration of existing α-amino acids, and in so doing, the side chain is sacrificed during the ring-forming process. We report a new asymmetric approach to lactam-constrained α-amino acid building blocks bearing a range of polar and hydrophobic side chains. The chemistry is amenable to rapidly generating di- and tripeptides, and the potential for these lactams to stabilize type II β-turns is demonstrated in the synthesis of the melanocyte-inhibiting factor peptidomimetic.

Reunderstanding the photoinduced charge transfer process of ammonium polyoxomolybdate

Protonation of 4-phenylpyridine can realize photoinduced electron transfer from the carboxyl group to 4-phenylpyridine and photochromism of ammonium polyoxomolybdate without proton transfer.

Efficient synthesis of conformationally constrained, amino-triazoloazepinone-containing di- and tripeptides via a one-pot Ugi-Huisgen tandem reaction

Herein we describe a catalyst-free procedure employing an Ugi-4CR between a β-azido-α-amino acid, propargylamine, an isocyanide and an aldehyde, followed by a thermal azide-alkyne Huisgen cycloaddition to generate a 16-member library of amino-triazoloazepinone-bearing di- and tripeptides with up to four points of diversification and high atom economy.

Conformational restriction: an effective tactic in 'follow-on'-based drug discovery

The conformational restriction (rigidification) of a flexible ligand has often been a commonly used strategy in drug design, as it can minimize the entropic loss associated with the ligand adopting a preferred conformation for binding, which leads to enhanced potency for a given physiological target, improved selectivity for isoforms and reduced the possibility of drug metabolism. Therefore, the application of conformational restriction strategy is a core aspect of drug discovery and development that is widely practiced by medicinal chemists either deliberately or subliminally. The present review will highlight current representative examples and a brief overview on the rational design of conformationally restricted agents as well as discuss its advantages over the flexible counterparts.

Solid-phase synthesis of tetrahydropyridazinedione-constrained peptides

The design and solid-phase synthesis of tetrahydropyridazine-3,6-dione (Tpd) peptidomimetics derived from backbone-aminated peptides is reported. The described protocol features the synthesis of chiral α-hydrazino acids suitable for chemoselective incorporation into growing peptide chains. Acid-catalyzed cyclization to form the Tpd ring during cleavage affords the target peptidomimetics in good yield and purity. The scope of Tpd incorporation is demonstrated through the synthesis of constrained peptides featuring nucleophilic/electrophilic side chains and sterically encumbered α-substituted hydrazino acid residues.

Development of peptidomimetic ligands of Pro-Leu-Gly-NH(2) as allosteric modulators of the dopamine D(2) receptor

A variety of stable, small-molecule peptidomimetic ligands have been developed to elucidate the mechanism by which the neuropeptide Pro-Leu-Gly-NH₂ (PLG) modulates dopaminergic neurotransmission. Photoaffinity labeling ligands based upon PLG peptidomimetics have been used to establish that PLG binds to the D₂ dopamine receptor at a site that is different from the orthosteric site, thus making PLG and its peptidomimetics allosteric modulators of the dopamine receptor. Through the design, synthesis and pharmacological evaluation of conformationally constrained peptidomimetics containing lactam, bicyclic, and spiro-bicyclic scaffolds, support was provided for the hypothesis that the bioactive conformation of PLG is a type II β-turn. In addition, studies with peptidomimetics designed to mimic either a type VI β-turn or polyproline II helix conformation yielded molecules that were able to modulate dopamine receptors because of their ability to place the carboxamide NH₂ pharmacophore in the same topological space as that seen in the type II β-turn. Extensive studies with the spiro-bicyclic PLG peptidomimetics also established that both positive and negative modes of modulation were possible for the same series of peptidomimetics simply as a result of minor differences in the stereochemistry about the bridgehead carbon within the scaffold. This information was used to transform existing positive modulators into negative modulators, which demonstrated that small structural changes in the spiro-bicyclic dopamine receptor modulators are capable of causing major changes in the modulatory activity of PLG peptidomimetics.

Intramolecular hydroamination of dithioketene acetals: an easy route to cyclic amino acid derivatives

Catalytic intramolecular hydroamination of dithioketene acetals was developed for the synthesis of cyclic amino acid derivatives. Triggered by the addition of a catalytical amount of n-BuLi, the reaction proceeds to give proline and pipecolic acid derivatives in excellent yields and diastereoselectivity.

Design, synthesis and evaluation of β-lactam antigenic peptide hybrids; unusual opening of the β-lactam ring in acidic media

β-Lactam peptides were envisioned as conformational constraints in antigenic peptides (APs). Three different β-lactam tripeptides of varying flexibility were prepared in solution and incorporated in place of the central part of the altered melanoma associated antigenic peptide Leu(27)-Melan-A(26-35) using solid phase synthesis techniques. Upon TFA cleavage from the solid support, an unexpected opening of the β-lactam ring occurred with conservation of the amide bond. After adaptation of the solid phase synthesis strategy, β-lactam peptides were successfully obtained and both opened and closed forms were evaluated for their capacity to bind to the antigen-presenting class-I MHC HLA-A2 protein system. None of the closed β-lactam peptides bound to HLA-A2, but their opened variants were shown to be moderate to good HLA-A2 ligands, one of them being even capable of stimulating a Melan-A-specific T cell line.

Intramolecular anodic olefin coupling reactions and the synthesis of cyclic amines

Anodic olefin coupling reactions using a tosylamine trapping group have been studied. The cyclizations are favored by the use of a less-polar radical cation and more basic reaction conditions. The most significant factor for obtaining good yields of cyclic product is the use of the more basic reaction conditions. However, a number of factors including the nature of both the solvent and the electrolyte used can influence the yield of the cyclizations. The cyclizations allow for the rapid synthesis of both substituted proline and pipecolic acid type derivatives.

Mechanistic insights on the magnesium(II) ion-activated reduction of methyl benzoylformate with chelated NADH peptide beta-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 mild and efficient synthesis of monofluorinated α-lactam pseudopeptides via a novel dehydrofluorination of Ugi products

Six novel monofluorinated α-lactam pseudopeptide derivatives were synthesized via Ugi reaction using gem-difluoromethylene-containing isocyanide as a key component, followed by dehydrofluorination of Ugi products in one pot without additional base.

4-(9-Anthr-yl)-1-(3-bromo-phen-yl)spiro-[azetidine-3,9'-xanthen]-2-one

In the title mol-ecule, C(35)H(22)BrNO(2), the four-membered ring of the β-lactam unit is nearly planar [maximum deviation = 0.003 (3) Å] and makes dihedral angles of 87.07 (15), 59.80 (16) and 20.81 (19)°, respectively, with the xanthene system, the anthracene system and the bromo-substituted benzene ring. The mol-ecular conformation is stabilized by weak intra-molecular C-H⋯O and C-H⋯N hydrogen bonds. The crystal structure features weak C-H⋯π inter-actions.

Synthesis and evaluation of novel pyridine based PLG tripeptidomimetics

Analogues of the pyridine based PLG (Pro-Leu-Gly-NH(2)) peptidomimetic were synthesized and evaluated as dopamine modulating agents. Modifications in the position corresponding to the leucine side chain in PLG afforded derivatives , and , substituted with H, Me and Bn instead of the isobutyl group, respectively. Changes in the proline residue produced derivative , substituted with a symmetrical piperidine ring instead of the pyrrolidine ring and , in which the pyrrolidine ring is connected to the pyridine ring via a hydroxymethyl group instead of a keto function. The peptidomimetics were tested for their ability to enhance the maximal effect of N-propylapomorphine (NPA) at dopamine D2 receptors in the functional cell-based R-SAT assay. Compounds , , and , produced a statistically significant increase in the maximal NPA response at 10 nM (117 +/- 6%, 118 +/- 6%, and 116 +/- 3%, respectively), which is similar to the effect of PLG in this assay, whereas was able to potentiate the response to a similar extent at 1 nM concentration (115 +/- 5%). All derivatives produced a bell-shaped dose-response curve and none of the compounds were active at the D2 receptor alone, which indicates that the mechanism behind the activity of both the pyridine based mimetics and PLG is the same. Interestingly, l-Pro-d-Leu-Gly-NH(2) was found to be more potent than PLG and produced a 119 +/- 1% increase in the NPA response at 1 nM.

Exceptional stereoselectivity in the synthesis of 1,3,4-trisubstituted 4-carboxy beta-lactam derivatives from amino acids

[reaction: see text] The base-promoted cyclization of optically pure N-(p-methoxybenzyl)-N-(2-chloro)propionyl amino acid derivatives resulted in a diastereo- and enantioselective approach to valuable 1,3,4,4-tetrasubstituted beta-lactams. The stereochemical outcome of the reaction is exclusively governed by the configuration of the N-(2-chloro)propionyl moiety.

Functional characterization of the modified melanocortin peptides responsible for ligand selectivity at the human melanocortin receptors

The melanocortin system plays an important role in energy homeostasis as well as skin pigmentation, steroidogenesis and exocrine gland function. In this study, we examined eight Ac-His-Phe-Arg-Trp-NH(2) tetrapeptides that were modified at the Phe position and pharmacologically characterized their activities at the human MCR wild-types and their mutants. Our results indicate that at the hMC1R, all D stereochemical modified residues at the Phe position of peptides increase cAMP production in a dose-dependent manner. At the hMC3R, the DPhe peptide dose dependently increases cAMP production but all other three tetrapeptides were not. At the hMC4R, both the DPhe and DNal(1') peptides induce cAMP production. However, both DTyr and DNal(2') were not able to induce cAMP production. Further studies indicated that at the hMC1R M128L mutant receptor, the all D-configured tetrapeptides reduce their potencies as compared to that of hMC1R wild-type. However, at the hMC3R and hMC4R L165M and L133M mutant receptors, the DNal(2') and DTyr tetrapeptides possess agonist activity. These findings indicate that DPhe in tetrapeptide plays an important role in ligand selectivity and specific residue TM3 of the melanocortin receptors is crucial for ligand selectivity.

Synthesis of Enantiopure Pyrrolidine-Derived Peptidomimetics and Oligo-β-peptides via Nucleophilic Ring-Opening of β-Lactams

The synthesis of the two enantiomers of pyrrolidine-derived spiro beta-lactams by resolution with D- and L-Boc phenylalanine is described. The potential of these optically active spiro beta-lactams on the synthesis of peptidomimetics as analogues of melanostatin is evaluated. Theoretical studies of several models, at the Becke3LYP/6-31+G* level of theory, together with previous experimental evidences from our group, gathered by NMR, allow us to design structures that can efficiently mimic some biologically active peptide-type molecules. On the other hand, the spiro beta-lactams have shown their utility in the preparation of beta-peptides. As an example, a homo-tetra-beta-peptide was synthesized. This research will continue in the future in order to obtain higher peptides with potential biological activity.

Studies on Stereoselective [2+2] Cycloadditions between N,N-Dialkylhydrazones and Ketenes

Staudinger-like cycloadditions between chiral, non-racemic N,N-dialkylhydrazones 1 and functionalized ketenes constitute an efficient methodology for the stereoselective construction of the beta-lactam ring. The potential for fine tuning of the dialkylamino auxiliary structure, the availability of a high-yielding deprotection method for the release of the free azetidinones, and the high thermal and chemical stability of hydrazones as N-dialkylamino imines are highlighted as the key elements for the success of the strategy. This last aspect is of particular importance concerning generality: even hydrazones from easily enolizable aldehydes or from formaldehyde reacted to afford the corresponding cycloadducts with high chemical and stereochemical yields. The syntheses of the beta-amino-alpha-hydroxyacids (2R,3S)-phenylisoserine (42) and (2R,3S)-norstatin (45) were accomplished as illustrative examples of the synthetic utility of this procedure. A model system for the cycloaddition of g series auxiliaries was studied by ab initio computational methods. The collected results support a two-step mechanism through zwitterionic intermediates, and explain the observed absolute and relative stereochemistry in terms of the preferred outward cycloaddition to the Re face of the hydrazone.