Divergent, stereoselective access to heterocyclic α,α-quaternary- and β(2,3,3)-amino acid derivatives from a N-Pmp-protected Orn-derived β-lactam

A suitably protected Orn-derived (3S,4S)-β-lactam was used as common intermediate in the synthesis of conformationally constrained (3S,4S)-2-oxoazepane α,α- and (2S,3S)-2-oxopiperidine-β(2,3,3)-amino acid derivatives. Compared to alternative procedures using an N-p-methoxybenzyl group at the 2-azetidinone, the incorporation of a p-methoxyphenyl moiety is crucial for the excellent stereochemical outcomes in the preparation of these heterocyclic amino acids. Chemoselective 7- or 6-exo-trig cyclization was achieved through alternative sequences of Pmp-deprotection/Boc-activation, followed by inter- and intramolecular β-lactam ring opening, respectively.

Experimental and theoretical studies on the rearrangement of 2-oxoazepane α,α-amino acids into 2'-oxopiperidine β(2,3,3) -amino acids: an example of intramolecular catalysis

Enantiopure β-amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4-carboxy-2-oxoazepane α,α-amino acids to lead to 2'-oxopiperidine-containing β(2,3,3) -amino acids, upon basic or acid hydrolysis of the 2-oxoazepane α,α-amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six-membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4-carboxylic acid substituted 2-oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2-oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis.

De novo designed library of linear helical peptides: an exploratory tool in the discovery of protein-protein interaction modulators

Protein-protein interactions (PPIs) have emerged as important targets for pharmaceutical intervention because of their essential role in numerous physiological and pathological processes, but screening efforts using small-molecules have led to very low hit rates. Linear peptides could represent a quick and effective approach to discover initial PPI hits, particularly if they have inherent ability to adopt specific peptide secondary structures. Here, we address this hypothesis through a linear helical peptide library, composed of four sublibraries, which was designed by theoretical predictions of helicity (Agadir software). The 13-mer peptides of this collection fixes either a combination of three aromatic or two aromatic and one aliphatic residues on one face of the helix (Ac-SSEEX(5)ARNX(9)AAX(12)N-NH2), since these are structural features quite common at PPIs interfaces. The 81 designed peptides were conveniently synthesized by parallel solid-phase methodologies, and the tendency of some representative library components to adopt the intended secondary structure was corroborated through CD and NMR experiments. As proof of concept in the search for PPI modulators, the usefulness of this library was verified on the widely studied p53-MDM2 interaction and on the communication between VEGF and its receptor Flt-1, two PPIs for which a hydrophobic α-helix is essential for the interaction. We have demonstrated here that, in both cases, selected peptides from the library, containing the right hydrophobic sequence of the hot-spot in one of the protein partners, are able to interact with the complementary protein. Moreover, we have discover some new, quite potent inhibitors of the VEGF-Flt-1 interaction, just by replacing one of the aromatic residues of the initial F(5)Y(9)Y(12) peptide by W, in agreement with previous results on related antiangiogenic peptides. Finally, the HTS evaluation of the full collection on thermoTRPs has led to a few antagonists of TRPV1 and TRPA1 channels, which open new avenues on the way to innovative modulators of these channels.

Highly functionalized 1,2-diamino compounds through reductive amination of amino acid-derived β-keto esters

1,2-Diamine derivatives are valuable building blocks to heterocyclic compounds and important precursors of biologically relevant compounds. In this respect, amino acid-derived β–keto esters are a suitable starting point for the synthesis of β,γ–diamino ester derivatives through a two-step reductive amination procedure with either simple amines or α–amino esters. AcOH and NaBH₃CN are the additive and reducing agents of choice. The stereoselectivity of the reaction is still an issue, due to the slow imine-enamine equilibria through which the reaction occurs, affording mixtures of diastereoisomers that can be chromatographically separated. Transformation of the β,γ–diamino esters into pyrrolidinone derivatives allows the configuration assignment of the linear compounds, and constitutes an example of their potential application in the generation of molecular diversity.

Design, synthesis and biological evaluation of new peptide-based ureas and thioureas as potential antagonists of the thrombin receptor PAR1

By applying a diversity oriented synthesis strategy for the search of new antagonists of the thrombin receptor PAR1, a series of peptide-based ureas and thioureas, including analogues of the PAR1 reference antagonist RWJ-58259, has been designed and synthesized. The general synthetic scheme involves reduction of basic amino acid-derived amino nitriles by hydrogen transfer from hydrazine monohydrate in the presence of Raney Ni, followed by reaction with diverse isocyanates and isothiocyanates, and protecting group removal. All new compounds have been evaluated as inhibitors of human platelet aggregation induced by the PAR1 agonist SFLLRN. Some protected peptide-based ureas displayed significant antagonist activity.

Cyclic amino acid linkers stabilizing key loops of brain derived neurotrophic factor

Based on β-turn-like BDNF loops 2 and 4, involved in receptor interaction, cyclic peptide replicas were designed, synthesized and tested. In addition to the native turn residues, the cyclic peptides include a linker unit between the N- and C-termini, selected by molecular modeling among various non-proteinogenic cyclic amino acids. NMR conformational studies showed that most of the cyclic peptides were able to adopt turn-like structures. Several of the analogues displayed significant inhibition of the BDNF-induced TrkB receptor phosphorylation, and hence could be useful templates for developing improved antagonists for this receptor.

Synthesis and SAR studies on azetidine-containing dipeptides as HCMV inhibitors

SAR studies on an azetidine-containing dipeptide prototype inhibitor of HCMV are described. Three series of structurally modified analogues, involving substitutions at the N- and C-terminus, and at the C-terminal side-chain were synthesized and evaluated for antiviral activity. Aliphatic or no substituents at the C-carboxamide group, an aliphatic C-terminal side-chain, as well as a benzyloxycarbonyl moiety at the N-terminus were absolute requirements for anti-HCMV activity. The conformational restriction induced by the 2-azetidine residue into the dipeptide derivatives, identified by ¹H NMR as a γ-type reverse turn, seems to have influence on the activity of these molecules.

Thrombin-activated receptors: promising targets for cancer therapy?

In addition to the key role of thrombin in blood coagulation, this multifunctional serine protease activates platelets and regulates the behavior of other cells through G-protein coupled protease activated receptors (PARs). PAR-1 is the principal thrombin-activated receptor involved in platelet aggregation and in endothelial and tumor cell proliferation. PAR-1 is overexpressed in invasive and metastatic tumors and the expression levels directly correlate with the degree of invasiveness of the cancer. In an attempt to give some insight into the perspectives of targeting PAR-1 in cancer and angiogenesis, this review provides an overview on the thrombin/PAR-1 interaction, receptor activation, signaling, desensitization and dysregulation mechanisms in relation to these diseases. A central aspect of this review is that directed to summarize the approaches that have been followed to the search of PAR-1 antagonists, illustrating with some significant examples. Attention is called to the scarce data concerning the effects of these antagonists on anticancer assay models.

Strategies for design of non peptide CCK1R agonist/antagonist ligands

This review mainly covers last five year literature on CCK1R agonists and antagonists. These CCK1R ligands have been found following the two usual and complementary strategies for drug discovery: rational design based on structure activity relationships on the CCK-7 and CCK-4 peptide sequences of the endogenous ligands and random screening of diverse compounds, followed by hit optimization. The first group includes: chimeric bifunctional opioid/CCK peptides, designed as opioid agonists with balanced CCK1R/CCK2R antagonist activity for the treatment of neuropathic pain, antagonist and agonist dipeptoids, and 1,3-dioxoperhydropyrido[1,2-c]pyrimidine- and anthranilic acid-based antagonists. Among the ligands derived from random screening, a few new 1,4-benzodiazepine-, 1,5-benzodiazepine-, and five member ring heterocycle-based CCK1R ligands have been reported. Finally, taking into account the importance of receptor mapping studies for ligand optimization and future precise de novo receptor structure-based design of new selective and more effective ligands, the most significant conclusions of these studies have also been reviewed.

Synthesis of Indole Alkaloid Analogues: Novel Domino Stereoselective Electrophile Addition−Cyclizations of Tryptophan-Derived α-Amino Nitriles

The synthesis of new indole alkaloid analogues, containing a 1,2,4,5,10b,10c-hexahydropyrrolo[1',2',3':1,9a,9]imidazo[1,2-a]indole skeleton, via highly stereoselective novel domino cyclative halogenation or prenylation reactions of tryptophan-derived alpha-amino nitriles, is described.

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.

The neuroprotective activity of GPE tripeptide analogues does not correlate with glutamate receptor binding affinity

The influence of several modifications on the GPE tripeptide structure upon the binding to GluRs and on their neuroprotective effects has been studied. The results indicated that the prevention of neuronal death showed by GPE and some analogues is not directly related to their affinity at glutamate receptors.

Old Molecules for New Receptors: Trp(Nps) Dipeptide Derivatives as Vanilloid TRPV1 Channel Blockers

The transient receptor potential vanilloid member 1 (TRPV1), an integrator of multiple pain-producing stimuli, is regarded nowadays as an important biological target for the discovery of novel analgesics. Here, we describe the first experimental evidence for the behavior of an old family of analgesic dipeptides, namely Xaa-Trp(Nps) and Trp(Nps)-Xaa (Xaa=Lys, Arg) derivatives, as potent TRPV1 channel blockers. We also report the synthesis and biological investigation of a series of new conformationally restricted Trp(Nps)-dipeptide derivatives with improved TRPV1/NMDA selectivity. Compound 15 b, which incorporates an N-terminal 2S-azetidine-derived Arg residue, was the most selective compound in this series. Collectively, a new family of TRPV1 channel blockers emerged from our results, although further modifications are required to fine-tune the potency/selectivity/toxicity balance.

New Gly-Pro-Glu (GPE) analogues: expedite solid-phase synthesis and biological activity

A suitable solid-phase approach, based on Fmoc/(t)Bu methodology and on the use of 2-chlorotrityl resin, allowed a rapid and efficient preparation of new GPE analogues. Most of the synthesized tripeptides displayed glutamate receptor binding affinity comparable to that of GPE, but only a few derivatives showed significant neuroprotective activity.

Conformationally Constrained CCK4 Analogues Incorporating IBTM and BTD β-Turn Mimetics

To test whether a turnlike arrangement is involved in the bioactive conformation of CCK4 analogues upon CCK1 receptor recognition, we describe the preparation of two series of CCK4 derivatives, in which the central dipeptide Met-Asp has been replaced by recognized beta-turn mimetics {(2S,5S,11bR)- and (2R,5R,11bS)-2-amino-5-carboxy-3-oxo-2,3,5,6,11,11b-hexahydro-1H-indolizino[8,7-b]indole (IBTM) and beta-turn dipeptide, 2-oxo-7-thio-1-azabicyclo[4.3.0]nonane (BTD)}. The incorporation of the indolizinoindole IBTM type II beta-turn mimetic is preferred over its type II' counterpart for efficient CCK1 receptor recognition, while BTD derivatives were completely inactive. The structure-conformation-activity relationship study in the IBTM series has shown some essential requirement of these CCK4 derivatives to favorably interact with CCK1 receptors: (a) the adoption of turnlike conformations, (b) the presence of an L-Phe residue and a C-terminal carboxamide moiety, and (c) the indole ring of the IBTM skeleton. Moreover, the existence of pi-pi interactions between the phenyl ring of d-Phe residues and the indole ring of IBTM framework is detrimental for binding affinity. A series of potent and selective CCK1 receptor antagonists, exemplified by compounds 8a and 8b, emerges among these IBTM-containing derivatives.

Novel Domino Cyclization of Tryptophan-Derived Amino Nitriles: Scope and Stereoselectivity

[reaction: see text] The scope and stereoselectivity of the acid-promoted cyclization of new tryptophan-based alpha-amino nitriles derived from either ketones or aldehydes to novel hexahydropyrrolo[1',2',3':1,9a,9]imidazo[1,2-a]indoles is described. This cyclization involves the generation of two or three stereogenic centers. The time and stereoselectivity of this reaction mostly depended on both the steric volume of the substituents at the amino nitrile and its stereochemistry. Unhindered amino nitriles gave exclusively 2-exo-isomers, while hindered amino nitriles, which required higher reaction times, provided also these isomers under kinetic control. Under thermodynamic control, the 2-endo-isomer was the main reaction product, except for the benzaldehyde-derived alpha-amino nitriles, where a favorable electronic interaction between the phenyl and methoxycarbonyl groups in a relative cis-disposition might be responsible of the formation of the 2-exo-isomer as the only cyclization product.

Combination of molecular modeling, site-directed mutagenesis, and SAR studies to delineate the binding site of pyridopyrimidine antagonists on the human CCK1 receptor

A rational combination of site-directed mutagenesis studies, structure-activity relationships, and dynamic-based docking of pyridopyrimidine-derived CCK1R antagonists into a refined three-dimensional model of the CCK1R allowed us to identify the receptor residues and the ligand functional groups implicated in the molecular recognition process. Our results provided unambiguous evidence that the binding site of these antagonists is overlapping that of the C-terminal tetrapeptide of CCK. In particular, Asn333 and Arg336 residues of the CCK1R are essential for high-affinity binding of these ligands. Moreover, the 2-aryl group in the pyridopyrimidine derivatives shares the same binding pocket as the C-terminal Phe side chain of CCK. Our [pyridopyrimidine.CCK1R] complex model is consistent with previous suggestions concerning the molecular basis that governs functional activity and provides useful considerations about the high CCK1 versus CCK2 selectivity of our derivatives and could contribute to fine-tune the rational design of new molecules with optimized properties.

From 1-acyl-beta-lactam human cytomegalovirus protease inhibitors to 1-benzyloxycarbonylazetidines with improved antiviral activity. A straightforward approach to convert covalent to noncovalent inhibitors

Starting from the structure of known beta-lactam covalent human cytomegalovirus (HCMV) protease inhibitors and from the knowledge of the residues implicated in the active site of this enzyme, we designed a series of phenylalanine-derived 2-azetidinones bearing a 4-carboxylate moiety that could be apt for additional interactions with the guanidine group of the Arg165/Arg166 residues of the viral protease. Some compounds within this series showed anti-HCMV activity at 10-50 muM, but rather high toxicity. The presence of aromatic 1-acyl groups and a certain hydrophobic character in the region of the 4-carboxylate were stringent requirements for anti-HCMV activity. To go a step ahead into the search for effective HCMV medicines, we then envisaged a series of noncovalent inhibitors by simple deletion of the carbonyl group in the beta-lactam derivatives to provide the corresponding azetidines. This led to low micromolar inhibitors of HCMV replication, with 17 and 27 being particularly promising lead compounds for further investigation, although their toxicity still needs to be lowered.

Analogues of the neuroprotective tripeptide Gly-Pro-Glu (GPE): synthesis and structure–activity relationships

A series of GPE analogues, including modifications at the Pro and/or Glu residues, was prepared and evaluated for their NMDA binding and neuroprotective effects. Main results suggest that the pyrrolidine ring puckering of the Pro residue plays a key role in the biological responses, while the preference for cis or trans rotamers around the Gly-Pro peptide bond is not important.

Molecular Diversity via Amino Acid Derived α-Amino Nitriles: Synthesis of Spirocyclic 2,6-Dioxopiperazine Derivatives

[reaction: see text] Chiral spirocyclic 2,6-dioxopiperazines were synthesized from amino acid derived alpha-quaternary alpha-amino nitriles via H(2)SO(4)-promoted cyano hydration, followed by base-mediated cyclization and N-alkylation. This methodology, requiring the previous preparation of the amino nitrile by a modified Strecker reaction, was applied to Phe, Trp, Pro, Asp, Glu, and Ser derivatives. In the case of the Trp-derived amino nitrile the major product of the treatment with H(2)SO(4) was not the expected carboxamide, but a new tetracyclic indoline derivative containing the novel heterocyclic system hexahydropyrrolo[1',2',3':1,9a,9]imidazo[1,2-a]indole, as a result of a domino tautomerization. The treatment of this indoline derivative with refluxing 1 N HCl led to a Trp-derived 2,6-dioxopiperazine. The 2,6-dioxopiperazine ring opened under the reaction conditions of methyl ester saponification, giving N-(carboxyalkyl)amino acid derivatives. Therefore, the synthesis of 2,6-dioxopiperazines containing free carboxylic acids from the respective methyl esters required transesterification to benzyl esters, followed by hydrogenolysis.

Synthesis and anti-HCMV activity of 1-acyl-beta-lactams and 1-acylazetidines derived from phenylalanine

Different Phe-derived 1-acyl-beta-lactams, analogous to a series of 2-azetidinones acting as HCMV serine protease inhibitors, were synthesized. Some of these compounds were modest inhibitors of the HCMV replication. Interestingly, removal of the carbonyl group of the beta-lactam ring, most likely acting as the serine trap, resulted in an azetidine derivative with anti-HCMV activity comparable to that of the reference compound ganciclovir.

Synthesis, Conformational Analysis, and Cytotoxicity of Conformationally Constrained Aplidine and Tamandarin A Analogues Incorporating a Spirolactam β-Turn Mimetic

With the aim of studying the contribution of the beta II turn conformation at the side chain of didemnins to the bioactive conformation responsible for their antitumoral activity, conformationally restricted analogues of aplidine and tamandarin A, where the side chain dipeptide Pro8-N-Me-d-Leu7 is replaced with the spirolactam beta II turn mimetic (5R)-7-[(1R)-1-carbonyl-3-methylbutyl]-6-oxo-1,7-diazaspiro[4.4]nonane, were prepared. Additionally, restricted analogues, where the aplidine (pyruvyl9) or tamandarin A [(S)-Lac9] acyl groups are replaced with the isobutyryl, Boc, and 2-methylacryloyl groups, were also prepared. These structural modifications were detrimental to cytotoxic activity, leading to a decrease of 1-2 orders of magnitude with respect to that exhibited by aplidine and tamandarin A. The conformational analysis of one of these spirolactam aplidine analogues, by NMR and molecular modeling methods, showed that the conformational restriction caused by the spirolactam does not produce significant changes in the overall conformation of aplidine, apart from preferentially stabilizing the trans rotamer at the pyruvyl9-spirolactam amide bond, whereas in aplidine both cis and trans rotamers at the pyruvyl9-Pro8 amide bond are more or less equally stabilized. These results seem to indicate a preference for the cis form at that amide bond in the bioactive conformation of aplidine. The significant influence of this cis/trans isomerism upon the cytotoxicity suggests a possible participation of a peptidylprolyl cis/trans isomerase in the mechanism of action of aplidine.

5-(Tryptophylamino)-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-Based Cholecystokinin Receptor Antagonists: Reversal of CCK1 Receptor Subtype Selectivity toward CCK2 Receptors

With the aim of reversing selectivity or antagonist/agonist functionality in the 5-(tryptophylamino)-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-derived potent and highly selective CCK(1) antagonists, a series of 4-benzyl and 4-methyl derivatives have been synthesized. Whereas the introduction of the benzyl group led, in all cases, to complete loss of the binding affinity, the incorporation of the methyl group gave a different result depending on the stereochemistry of the 1,3-dioxoperhydropyrido[1,2-c]pyrimidine scaffold. Thus, the introduction of the methyl group into the (4aS,5R)-diastereoisomers, giving a (4S)-configuration, produced a 3-fold increase in the CCK(1) binding potency and selectivity. However, the same structural manipulation in the opposite (4aR,5S)-stereochemistry, leading to a (4R,4aR,5S)-configuration, produced reversal of the selectivity for CCK(1) to the CCK(2) receptors. The replacement of the Boc group at the tryptophan moiety by a 2-adamantyloxycarbonyl group also contributed to that reversal. The resulting compounds displayed moderate CCK(2) antagonist activity in rat and human receptors, and a very small partial agonist effect on the production of inositol phosphate in COS-7 cells transfected with the wild-type human CCK(2) receptor.

Pharmacological Study of IQM-97,423, a Potent and Selective CCK1 Receptor Antagonist with Protective Effect in Experimental Acute Pancreatitis

The pharmacological profile of the new CCK1 receptor antagonist IQM-97,423, (4aS,5R)-2-benzyl-5-(tert-butylaminocarbonyl-tryptophyl)amino-1,3-dioxoperhydropyrido-[1,2-c]pyrimidine, was examined in in vitro and in vivo studies and compared with typical CCK1 antagonists such as devazepide and lorglumide. IQM-97,423 showed a high affinity at [3H]-pCCK8-labeled rat pancreatic CCK1 receptors, and was virtually devoid of affinity at brain CCK2 receptors. IQM-97,423 antagonized CCK8S-stimulated alpha-amylase release from rat pancreatic acini with a potency similar to devazepide and much higher than lorglumide. In the guinea pig isolated longitudinal muscle-myenteric plexus preparation, IQM-97,423 produced a full antagonism of the contractile response elicited by CCK8S and a weaker effect on the contraction elicited by CCK4, suggesting a selective antagonism at CCK1 receptors. The protective effect of IQM-97,423 and devazepide was tested in two models of acute pancreatitis in rats, induced by injection of cerulein or by combined bile and pancreatic duct obstruction. The new compound fully prevented the cerulein-induced increase in plasma pancreatic enzymes and in pancreas weight with a potency similar to devazepide. In common bile-pancreatic duct ligature-induced acute pancreatitis, IQM-97,423 partially prevented, like devazepide, the increase in plasma pancreatic enzyme activity and in pancreas weight. Consequently, the pyridopyrimidine derivative IQM-97,423 is a potent and highly selective CCK1 receptor antagonist with preventive effects in two experimental models of acute pancreatitis and a potential therapeutic interest.

“Dipeptoids”: From the Chemical Structure of the Endogenous Peptide to the Design of Peptidomimetics

The present review details the rational multi-step process followed for the discovery of a family of non-peptide CCK receptor ligands ("dipeptoids"), starting from the structure of the endogenous peptide, CCK(8). Emphasis will be made on the N- and C-terminal modifications, on the singular effects of the stereochemical changes and the incorporation of conformational constraints into the structure of "dipeptoids", and on the modifications directed to improve the pharmacological profile of these compounds to afford valuable clinical candidates.

Unprecedented Stereospecific Synthesis of a Novel Tetracyclic Ring System, a Hybrid of Tetrahydropyrrolo[2,3-b]indole and Tetrahydroimidazo[1,2-a]indole, via a Domino Reaction upon a Tryptophan-Derived Amino Nitrile

Compounds containing a novel tetracyclic ring system, a hybrid of tetrahydropyrrolo[2,3-b]indole and tetrahydroimidazo[1,2-a]indole, are synthesized via an acid-mediated stereospecific domino tautomerization of a tryptophan-derived alpha-amino nitrile. Characterization of these new compounds and preliminary studies on the reactivity of the tetracyclic heterocyclic system are reported. [reaction: see text]

Expedient one-pot synthesis of novel chiral 2-substituted 5-phenyl-1,4-benzodiazepine scaffolds from amino acid-derived amino nitriles

An efficient and stereocontrolled synthesis of phenylalanine- and tryptophan-derived 5-phenyl-1,4-benzodiazepines is described. This new methodology involves, as a key step, the synthesis of 5-phenyl-2,3-dihydro-1H-1,4-benzodiazepines by a one-pot cyano reduction and reductive cyclization of the appropriate amino nitrile, which were obtained via a modified Strecker reaction of N-protected alpha-amino aldehydes with 2-aminobenzophenone and trimethylsilyl cyanide. The subsequent reduction of these 2,3-dihydro-1H-1,4-benzodiazepines, followed by regioselective alkylation or acylation at position 4, led to 2,4-disubstituted-5-phenyl-2,3,4,5-tetrahydro-1H-1,4-benzodiazepine.

General approach for the stereocontrolled construction of the beta-lactam ring in amino acid-derived 4-alkyl-4-carboxy-2-azetidinones

The first general approach toward the asymmetric synthesis of 4-alkyl-4-carboxy-2-azetidinones derived from amino acids is described. The stereoselective construction of the beta-lactam ring was achieved through base-mediated intramolecular cyclization of the corresponding N(alpha)-chloroacetyl derivatives bearing (+)- or (-)-10-(N,N-dicyclohexylsulfamoyl)isoborneol as chiral auxiliary (ee up to 82%).

2-Oxopiperazine-based gamma-turn conformationally constrained peptides: synthesis of CCK-4 analogues

2-Oxopiperazine derivatives 1 have been designed as mimetics of gamma-turn conformationally constrained tripeptides. The synthetic pathway devised for the preparation of both epimers of 1 at C(5) involves a reductive amination of cyanomethyleneamino pseudopeptides with amino acid derivatives, followed by regiospecific lactamization of the resulting C-backbone branched pseudopeptides. The versatility of this methodology is illustrated in the synthesis of analogues of the tetrapeptides Boc-[Nle(31)]-CCK-4 and Boc-[Lys(o-tolylaminocarbonyl)(31)]-CCK-4. The introduction of the new conformational restriction into these Boc-CCK-4 analogues led to a loss of 2 or 3 orders of magnitude in the affinity at CCK receptors. These results suggest the absence of a gamma-turn in the bioactive conformation of the C-terminal tripeptide of CCK-4.

Effects of the incorporation of IBTM beta-turn mimetics into the dipeptoid CCK(1) receptor agonist PD 170292

Replacement of the 2-Adoc-D-alphaMeTrp residue in the non-selective CCK(1) receptor agonist PD 170292 by the Z-(2R,5R,11bS)-IBTM skeleton, able to fix a type II beta-turn-like conformation, led to a conformationally restricted dipeptoid analogue, namely 3a, which exhibited a notable increase in the CCK(1) selectivity and antagonist properties.