Angiotensin-converting enzyme inhibitors. Mercaptan, carboxyalkyl dipeptide, and phosphinic acid inhibitors incorporating 4-substituted prolines

A Thiol-Free Route to Alkyl Aryl Thioethers

Most existing methods for the synthesis of alkyl aryl thioethers require the use of mercaptans as the starting materials, which comes with practical limitations. Reactions of diaryliodonium salts with xanthate salts, easily prepared from the corresponding alcohols and CS2, under the developed conditions represent an operationally simple, thiol-free method for the synthesis of these valuable compounds. The protocol features high functional group tolerance and can be applied to the late-stage C-H functionalization and for the introduction of a CD3S group.

Synthetic Routes to Approved Drugs Containing a Spirocycle

The use of spirocycles in drug discovery and medicinal chemistry has been booming in the last two decades. This has clearly translated into the landscape of approved drugs. Among two dozen clinically used medicines containing a spirocycle, 50% have been approved in the 21st century. The present review focuses on the notable synthetic routes to such drugs invented in industry and academia, and is intended to serve as a useful reference source of synthetic as well as general drug information for researchers engaging in the design of new spirocyclic scaffolds for medicinal use or embarking upon analog syntheses inspired by the existing approved drugs.

Phosphinic Peptides as Tool Compounds for the Study of Pharmacologically Relevant Zn-Metalloproteases

Phosphinic peptides constitute an important class of bioactive compounds that have found a wide range of applications in the field of biology and pharmacology of Zn-metalloproteases, the largest family of proteases in humans. They are designed to mimic the structure of natural substrates during their proteolysis, thus acting as mechanism-based, transition state analogue inhibitors. A combination of electrostatic interactions between the phosphinic acid group and the Zn cation as well as optimal noncovalent enzyme-ligand interactions can result in both high binding affinity for the desired target and selectivity against other proteases. Due to these unique properties, phosphinic peptides have been mainly employed as tool compounds for (a) the purposes of rational drug design by serving as ligands in X-ray crystal structures of target enzymes and allowing the identification of crucial interactions that govern optimal molecular recognition, and (b) the delineation of biological pathways where Zn-metalloproteases are key regulators. For the latter objective, inhibitors of the phosphinopeptidic type have been used either unmodified or after being transformed to probes of various types, thus expanding the arsenal of functional tools available to researchers. The aim of this review is to summarize all recent research achievements in which phosphinic peptides have played a central role as tool compounds in the understanding of the mechanism and biological functions of Zn-metalloproteases in both health and disease.

Metal-Binding Q-Proline Macrocycles

We introduce the efficient Fmoc-SPPS and peptoid synthesis of Q-proline-based, metal-binding macrocycles (QPMs), which bind metal cations and display nine functional groups. Metal-free QPMs are disordered, evidenced by NMR and a crystal structure of QPM-3 obtained through racemic crystallization. Upon addition of metal cations, QPMs adopt ordered structures. Notably, the addition of a second functional group at the hydantoin amide position (R₂) converts the proline ring from Cγ-endo to Cγ-exo, due to steric interactions.

Engineering endogenous l-proline biosynthetic pathway to boost trans-4-hydroxy-l-proline production in Escherichia coli

Non-proteinogenic trans-4-hydroxy-l-proline (t4HYP), a crucial naturally occurred amino acid, is present in most organisms. t4HYP is a regio- and stereo-selectively hydroxylated product of l-proline and a valuable building block for pharmaceutically important intermediates/ingredients synthesis. Microbial production of t4HYP has aroused extensive investigations because of its low-cost and environmentally benign features. Herein, we reported metabolic engineering of endogenous l-proline biosynthetic pathway to enhance t4HYP production in trace l-proline-producing Escherichia coli BL21(DE3) (21-S0). The genes responsible for by-product formation from l-proline, pyruvate, acetyl-CoA, and isocitrate in the biosynthetic network of 21-S0 were knocked out to channel the metabolic flux towards l-proline biosynthesis. PdhR was knocked out to remove its negative regulation and aceK was deleted to ensure isocitrate dehydrogenase's activity and to increase NADPH/NADP⁺ level. The other genes for l-proline biosynthesis were enhanced by integration of strong promoters and 5'-untranslated regions. The resulting engineered E. coli strains 21-S1 ∼ 21-S9 harboring a codon-optimized proline 4-hydroxylase-encoding gene (P4H) were grown and fermented. A titer of 4.82 g/L of t4HYP production in 21-S6 overexpressing P4H was obtained at conical flask level, comparing with the starting 21-S0 (26 mg/L). The present work paves an efficient metabolic engineering way for higher t4HYP production in E. coli.

Toxin to medicine and bioisosterism in drug development: a study of the discovery and development of ACE inhibitors from snake venom

The advent of the angiotensin-converting enzyme (ACE) inhibitors is a landmark in drug discovery and a breakthrough in the management of hypertension. Their clinical introduction has led to appreciable increase in the lifespan of hypertensive patients. And their development initiated a new era of structure-based or rational drug design that has subsequently been applied successfully for development of drugs for many other disorders. This paper presents an account of the discovery, design and development of ACE inhibitors from an academic perspective and possibly, as a guide to future research. The paper highlights the milestones and recounts the challenges encountered and the strategies applied in the search for ACE inhibitors. This exposition also expounds some of the concepts and intricacies of drug discovery, design and development. Keywords: drug development, ACE inhibitors, snake venom peptide, bioisosterism, antihypertensive agents

Continuous Flow Synthesis of ACE Inhibitors From N-Substituted l-Alanine Derivatives

A strategy for the continuous flow synthesis of angiotensin converting enzyme (ACE) inhibitors is described. An optimization effort guided by in situ IR analysis resulted in a general amide coupling approach facilitated by N-carboxyanhydride (NCA) activation that was further characterized by reaction kinetics analysis in batch. The three-step continuous process was demonstrated by synthesizing 8 different ACE inhibitors in up to 88 % yield with throughputs in the range of ≈0.5 g h^-1 , all while avoiding both isolation of reactive intermediates and process intensive reaction conditions. The process was further developed by preparing enalapril, a World Health Organization (WHO) essential medicine, in an industrially relevant flow platform that scaled throughput to ≈1 g h^-1 .

Potential chemical transformation of phosphinic acid derivatives and their applications in the synthesis of drugs

The chemical transformation of phosphinic acid is a well-considered mature area of research on account of the historical significant reactions such as Kabachnik-Fields, Mannich, Arbuzov, Michaelis-Becker, etc. Considerable advances have been made over last years especially in metal-catalyzed, free-radical processes and asymmetric synthesis using catalytic enantioselective. As a result, the aim of this synopsis is to make the reader familiar with advances in the approaches of phosphinic acids toward the synthesis of highly functionalized and valuable buildings blocks. Another purpose of this survey is to provide the current status of the applications of phosphinic acids in the synthesis of drugs.

4-Vinylproline

Enantiomerically pure 4-vinylproline (Vyp) was synthesized by a five-step approach from N-(Boc)iodo-alanine (2) featuring copper-catalyzed S_N2' substitution of the corresponding zincate onto ( Z)-1,4-dichlorobut-2-ene to prepare methyl 2- N-(Boc)amino-4-(chloromethyl)hexenoate (3). Intra- and intermolecular displacement of the chloride provided respectively Vyp and methyl 2- N-(Boc)amino-4-(azidomethyl)hexenoate (7) suitable for the synthesis of constrained peptide analogs.

Approaches towards the development of chimeric DPP4/ACE inhibitors for treating metabolic syndrome

Designing drug candidates exhibiting polypharmacology is one of the strategies adopted by medicinal chemists to address multifactorial diseases. Metabolic disease is one such multifactorial disorder characterized by hyperglycaemia, hypertension and dyslipidaemia among others. In this paper we report a new class of molecular framework combining the pharmacophoric features of DPP4 inhibitors with those of ACE inhibitors to afford potent dual inhibitors of DPP4 and ACE.

Selected organophosphorus compounds with biological activity. Applications in medicine

The purpose of this article is to provide an overview of the latest applications of organophosphorus compounds (OPs) that exhibit biological activity.

Copper-catalyzed O-arylation of N-protected 1,2-aminoalcohols using functionalized trivalent organobismuth reagents

The O-arylation of 1,2-aminoalcohols using functionalized triarylbismuth reagents is reported.

d-Glucose based syntheses of β-hydroxy derivatives of l-glutamic acid, l-glutamine, l-proline and a dihydroxy pyrrolidine alkaloid

The β-hydroxy derivatives of l-glutamic acid, l-glutamine and l-proline, useful for peptide/protein studies, were synthesized starting from d-glucose.

Discovery of potent and specific dihydroisoxazole inhibitors of human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme that catalyzes the posttranslational modification of glutamine residues on protein or peptide substrates. A growing body of literature has implicated aberrantly regulated activity of TG2 in the pathogenesis of various human inflammatory, fibrotic, and other diseases. Taken together with the fact that TG2 knockout mice are developmentally and reproductively normal, there is growing interest in the potential use of TG2 inhibitors in the treatment of these conditions. Targeted-covalent inhibitors based on the weakly electrophilic 3-bromo-4,5-dihydroisoxazole (DHI) scaffold have been widely used to study TG2 biology and are well tolerated in vivo, but these compounds have only modest potency, and their selectivity toward other transglutaminase homologues is largely unknown. In the present work, we first profiled the selectivity of existing inhibitors against the most pertinent TG isoforms (TG1, TG3, and FXIIIa). Significant cross-reactivity of these small molecules with TG1 was observed. Structure–activity and −selectivity analyses led to the identification of modifications that improved potency and isoform selectivity. Preliminary pharmacokinetic analysis of the most promising analogues was also undertaken. Our new data provides a clear basis for the rational selection of dihydroisoxazole inhibitors as tools for in vivo biological investigation.

Proline editing: a general and practical approach to the synthesis of functionally and structurally diverse peptides. Analysis of steric versus stereoelectronic effects of 4-substituted prolines on conformation within peptides

Functionalized proline residues have diverse applications. Herein we describe a practical approach, proline editing, for the synthesis of peptides with stereospecifically modified proline residues. Peptides are synthesized by standard solid-phase peptide synthesis to incorporate Fmoc-hydroxyproline (4R-Hyp). In an automated manner, the Hyp hydroxyl is protected and the remainder of the peptide synthesized. After peptide synthesis, the Hyp protecting group is orthogonally removed and Hyp selectively modified to generate substituted proline amino acids, with the peptide main chain functioning to "protect" the proline amino and carboxyl groups. In a model tetrapeptide (Ac-TYPN-NH2), 4R-Hyp was stereospecifically converted to 122 different 4-substituted prolyl amino acids, with 4R or 4S stereochemistry, via Mitsunobu, oxidation, reduction, acylation, and substitution reactions. 4-Substituted prolines synthesized via proline editing include incorporated structured amino acid mimetics (Cys, Asp/Glu, Phe, Lys, Arg, pSer/pThr), recognition motifs (biotin, RGD), electron-withdrawing groups to induce stereoelectronic effects (fluoro, nitrobenzoate), handles for heteronuclear NMR ((19)F:fluoro; pentafluorophenyl or perfluoro-tert-butyl ether; 4,4-difluoro; (77)SePh) and other spectroscopies (fluorescence, IR: cyanophenyl ether), leaving groups (sulfonate, halide, NHS, bromoacetate), and other reactive handles (amine, thiol, thioester, ketone, hydroxylamine, maleimide, acrylate, azide, alkene, alkyne, aryl halide, tetrazine, 1,2-aminothiol). Proline editing provides access to these proline derivatives with no solution-phase synthesis. All peptides were analyzed by NMR to identify stereoelectronic and steric effects on conformation. Proline derivatives were synthesized to permit bioorthogonal conjugation reactions, including azide-alkyne, tetrazine-trans-cyclooctene, oxime, reductive amination, native chemical ligation, Suzuki, Sonogashira, cross-metathesis, and Diels-Alder reactions. These proline derivatives allowed three parallel bioorthogonal reactions to be conducted in one solution.

Improved synthesis of lysine- and arginine-derived Amadori and Heyns products and in vitro measurement of their angiotensin I-converting enzyme inhibitory activity

The L-lysine- and L-arginine-derived Amadori and Heyns products consisting of N-(1-deoxy-d-fructos-1-yl)amino acid and N-(2-deoxy-d-glucos-2-yl)amino acid were prepared by reaction of d-fructose and d-glucose with l-lysine hydrochloride and l-arginine hydrochloride using commercial zinc powder as deprotonating reagent and also as catalyst precursor in a simple synthetic route in high yield. These compounds were screened for angiotensin I-converting enzyme (ACE) inhibitory activity using a high-throughput colorimetric assay (utilizing porcine kidney ACE). The IC(50) values fall in the range of 1030-1175 μM, with N(α)-(1-deoxy-d-fructos-1-yl)arginine showing the best IC(50) value (1030 ± 38 μM). This study demonstrates an improved synthetic method for simple Amadori and Heyns products and their moderate ACE inhibitor activity.

Antioxidant activity of peptide-based angiotensin converting enzyme inhibitors

Angiotensin converting enzyme (ACE) inhibitors are important for the treatment of hypertension as they can decrease the formation of vasopressor hormone angiotensin II (Ang II) and elevate the levels of vasodilating hormone bradykinin. It is observed that bradykinin contains a Ser-Pro-Phe motif near the site of hydrolysis. The selenium analogues of captopril represent a novel class of ACE inhibitors as they also exhibit significant antioxidant activity. In this study, several di- and tripeptides containing selenocysteine and cysteine residues at the N-terminal were synthesized. Hydrolysis of angiotensin I (Ang I) to Ang II by ACE was studied in the presence of these peptides. It is observed that the introduction of L-Phe to Sec-Pro and Cys-Pro peptides significantly increases the ACE inhibitory activity. On the other hand, the introduction of L-Val or L-Ala decreases the inhibitory potency of the parent compounds. The presence of an L-Pro moiety in captopril analogues appears to be important for ACE inhibition as the replacement of L-Pro by L-piperidine 2-carboxylic acid decreases the ACE inhibition. The synthetic peptides were also tested for their ability to scavenge peroxynitrite (PN) and to exhibit glutathione peroxidase (GPx)-like activity. All the selenium-containing peptides exhibited good PN-scavenging and GPx activities.

Introduction of C(5/6) side chains onto 2-azabicyclo[2.1.1]hexanes via a 6-anti-bromo-5-anti-hydroxy derivative

Oxidation of the title bromoalcohol provided the strained ketone, 5-bromo-6-oxo-2-azabicyclo[2.1.1]hexane. Additions of nucleophiles to either this or the debrominated ketone have been used to introduce 5(6)-syn-alkyl and aryl groups, 5(6)-alkylidene linkages, and 5(6)-anti-alkyl and acyl substituents. Facial selectivity is for additions to the 6-bromo-5-ketone and 5-alkylidene azabicycles to occur from the face syn to the nitrogen atom. The bromine atom of the title alcohol has also been replaced by a 6-anti-(1-hydroxyethyl) substituent using a directed radical addition process. The stereoselective functionalization reactions expand the range of available methano-bridged pyrrolidines.

Synthesis, characterization and antioxidant activity of angiotensin converting enzyme inhibitors

Angiotensin converting enzyme (ACE) catalyzes the conversion of angiotensin I (Ang I) to angiotensin II (Ang II). ACE also cleaves the terminal dipeptide of vasodilating hormone bradykinin (a nonapeptide) to inactivate this hormone. Therefore, inhibition of ACE is generally used as one of the methods for the treatment of hypertension. 'Oxidative stress' is another disease state caused by an imbalance in the production of oxidants and antioxidants. A number of studies suggest that hypertension and oxidative stress are interdependent. Therefore, ACE inhibitors having antioxidant property are considered beneficial for the treatment of hypertension. As selenium compounds are known to exhibit better antioxidant behavior than their sulfur analogues, we have synthesized a number of selenium analogues of captopril, an ACE inhibitor used as an antihypertensive drug. The selenium analogues of captopril not only inhibit ACE activity but also effectively scavenge peroxynitrite, a strong oxidant found in vivo.

The aberrance of the 4S diastereomer of 4-hydroxyproline

Prolyl 4-hydroxylases install a hydroxyl group in the 4R configuration on the gamma-carbon atom of certain (2S)-proline (Pro) residues in tropocollagen, elastin, and other proteins to form (2S,4R)-4-hydroxyproline (Hyp). The gauche effect arising from this prevalent post-translational modification enforces a C(gamma)-exo ring pucker and stabilizes the collagen triple helix. The Hyp diastereomer (2S,4S)-4-hydroxyproline (hyp) has not been observed in a protein, despite the ability of electronegative 4S substituents to enforce the more common C(gamma)-endo ring pucker of Pro. Here, we use density functional theory, spectroscopy, crystallography, and calorimetry to explore the consequences of hyp incorporation on protein stability using a collagen model system. We find that the 4S-hydroxylation of Pro to form hyp does indeed enforce a C(gamma)-endo ring pucker but a transannular hydrogen bond between the hydroxyl moiety and the carbonyl of hyp distorts the main-chain torsion angles that typically accompany a C(gamma)-endo ring pucker. This same transannular hydrogen bond enhances an n-->pi* interaction that stabilizes the trans conformation of the peptide bond preceding hyp, endowing hyp with the unusual combination of a C(gamma)-endo ring pucker and high trans/cis ratio. O-Methylation of hyp to form (2S,4S)-4-methoxyproline (mop) eliminates the transannular hydrogen bond and restores a prototypical C(gamma)-endo pucker. mop residues endow the collagen triple helix with much more conformational stability than do hyp residues. These findings highlight the critical importance of the configuration of the hydroxyl group installed on C(gamma) of proline residues.

Molecular tools for cell and systems biology

The sequencing of the genomes of key organisms and the subsequent identification of genes merely leads us to the next real challenge in modern biology-revealing the precise functions of these genes. Further, detailed knowledge of how the products of these genes behave in space and time is required, including their interactions with other molecules. In order to tackle these considerable tasks, a large and continuously expanding toolbox is required to probe the functions of proteins on a cellular level. Here, the currently available tools are described and future developments are projected. There is no doubt that only the close interplay between the life science disciplines in addition to advances in engineering will be able to meet the challenge.

Design and synthesis of long-acting inhibitors of dipeptidyl peptidase IV

A series of (4-substituted prolyl)prolinenitriles were synthesized and evaluated as inhibitors of dipeptidylpeptidase IV (DPP-IV). Among those tested, the 4beta-[4-(hydroxyphenyl)prolyl]prolinenitriles showed a potent inhibitory activity with a long duration of action. Metabolic formation of the corresponding phenol glucuronates was found to contribute to their long duration of action. The activity profiles of the synthesized compounds are reported and structure-activity relationships are also presented.

Divergent Chemical Synthesis of Prolines Bearing Fluorinated One-Carbon Units at the 4-Position via Nucleophilic 5-Endo-Trig Cyclizations

N-[3-(Trifluoromethyl)homoallyl]sulfonamides, prepared via ring opening of (S)-glycidyl ethers or 2-aryloxiranes with 1-(trifluoromethyl)vinyllithium, underwent intramolecular addition or S(N)2'-type reaction in the normally disfavored 5-endo-trig fashion, leading to 2-substituted 4-(trifluoromethyl)- or 4-(difluoromethylene)pyrrolidines. Both alpha- and beta-face-selective hydrogenation of the 4-difluoromethylene group afforded syn- and anti-4-(difluoromethyl)pyrrolidines, respectively. These sequences, followed by the oxidation of a 2-hydroxymethyl or 2-aryl group, successfully provided prolines with a trifluoromethyl, difluoromethylene, or difluoromethyl group at the 4-position, including optically active prolines.