Toward stereocontrolled, chemoenzymatic synthesis of unnatural peptides

An efficient, chemoenzymatic method for the multicomponent synthesis of unnatural tripeptides is presented. Development of a previously described procedure combines the diversity offered by multicomponent reactions with the selectivity of biocatalysts and allows the convenient introduction of varied amino acid moieties into the tripeptide scaffold, with control of the stereochemistry. Additionally, it allows the introduction of a methyl group to the amide nitrogen, leading to derivatives of N-methylated amino acids.     

A new and general method for the synthesis of tripeptide aldehydes based on the multi-component Ugi reaction

Tripeptide aldehydes, such as Z-Leu-Leu-Leu-H (MG-132), are an important class of compounds due to their biological activity. A new, general method for the synthesis of tripeptide aldehydes based on the multi-component Ugi reaction was developed. A careful choice of isocyanides makes it possible to obtain tripeptide precursors whose functionalization led to target structures. This method can be used for the preparation of tripeptide aldehydes with non-natural amino acid side chains.     

Synthesis of carborane analogues of γ-aminobutanoic acid

General method for preparation of high yields of novel N-protected carborane amino acid derivatives, 3-acylamino-1-carboxymethyl-2-R-o-carboranes (R = H, Me, Ph), is reported. The synthesis starts from readily available 3-amino-o-carboranes and includes the protection of amino group, introduction of carboxymethyl function to the carbon atom of polyhedron via the metallation of carborane CH bond with sodium amide in liquid ammonia, and treatment of corresponding sodium carboranes with sodium bromoacetate. Deprotection of N-acylated carborane amino acids is studied in acidic media. Depending on the procedure employed, closo- or nido-carborane amino acids were obtained.     

Selective palladium-catalyzed amination of the heterocyclic core of variolins

A new and selective palladium-catalyzed amination of the pyrido[3′,3′:4,5]pyrrolo[1,2-c]pyrimidine nucleus, the heterocyclic core of the variolin alkaloids, is described. The method allows the introduction of amino and aryl- and alkylamino substituents on the C9 position in advanced precursors of variolin B and deoxyvariolin.     

The first example of linear peptides containing a N-trifluoroethylated backbone amide linkage and the surprising solution dynamics observed by F NMR

The α-amino group of (l)phenylalanine methyl ester was trifluoroethylated using (2,2,2-trifluoroethyl)phenyliodonium N,N-bis(trifluoromethylsulfonyl)imide. A dipeptide Gly(l)Phe containing a trifluoroethylated peptide bond was synthesized by removing the α-amino proton of N^α-trifluoroethyl (l)phenylalanine methyl ester followed by coupling with N^α-phthaloyl glycine acid fluoride. The dipeptide was further coupled with (l)leucine methyl ester under conventional carboxyl activation conditions to provide two diastereomers of the tripeptide Gly(d,l)Phe(l)Leu. The solution dynamic behavior of the tripeptide was investigated as a function of solvents, by NOESY and variable temperature (VT) ¹⁹F NMR experiments.     

Stereoselective synthesis of 4-amino-3-hydroxy-2-methylpentanoic acids: stereochemistry of the amino acid occurring in the marine toxin janolusimide

Four diastereomers of 4-amino-3-hydroxy-2-methylpentanoic acid, an amino acid constituent of the hexapeptide portion of the antitumor antibiotic bleomycin A₂, have been stereoselectively synthesized by crotylboration of N-Boc-l-alaninal. The synthesis allowed the assignment of the stereochemistry as 2R,3S,4S to the 4-amino-3-hydroxy-2-methylpentanoic acid occurring in the tripeptide marine toxin janolusimide.     

Application of log D for the prediction of hydrophobicity in the advanced Marfey's method

In the advanced Marfey's method, the resolution between the diastereomers derivatized with 1-fluoro-2,4-dinitrophenyl-5-l-leucinamide (l-FDLA) and 1-fluoro-2,4-dinitrophenyl-5-d-leucinamide (d-FDLA) is reflected by the difference of hydrophobicity of the two functional groups at the asymmetric carbon. However, no effective method has been developed for the estimation of hydrophobicity so far. For this purpose, we introduced log D from the ACD Labs LogD and applied it to relatively simple primary amines, amino acids and secondary alcohols in the present study. It was found that the difference of the retention times (Δt_R) correlated with that of log D (Δlog D) for both diastereomers based on the obtained experimental results. Based on these results, the following procedure was proposed for the non-empirical determination of the absolute configuration of primary amines including amino acids and secondary alcohols: (1) estimate the hydrophobicity by the calculation of log D for the two substituent groups at the asymmetric carbon, (2) locate the trans-type arrangement of the two more hydrophobic substituents in the l-DLA derivative and judge the asymmetric carbon to be R or S in the trans-type that is eluted first, (3) derivatize the desired compound with l- or d-FDLA and analyze by LC/MS, and (4) compare the elution order with the prospective one and determine the absolute configuration at the asymmetric carbon. Furthermore, log D could also be used to predict the retention times of unavailable amino acids and small peptides, indicating that the combination of the advanced Marfey's method with log D would provide more reliable structural information on a mixture composed of amino acids and small peptides. The developed method is being applied to more complicated compounds.     

Synthesis of S-glycosyl amino acids and S-glycopeptides via photoinduced click thiol-ene coupling

We report on the photoinduced addition of glycosyl thiols to alkenyl glycines (thiol-ene coupling) to give S-glycosyl amino acids in high yields (53-90%). One of the amino acids thus prepared was used in the construction of a tripeptide via solution phase chemistry. Moreover, a one-pot two-step approach to an S-glycopeptide was developed using glutathione (GSH) as model starting material. This approach involved GSH S-alkenylation followed by photoinduced hydrothiolation by a glycosyl thiol.     

An efficient enzymatic preparation of rhinovirus protease inhibitor intermediates

The development of an efficient route for the preparation of (2S)-2-[3-{[(5-methylisoxazol-3-yl)carbonyl]amino}-2-oxopyridin-1(2H)-yl]pent-4-ynoic acid (4), a key intermediate in the synthesis of a human rhinovirus (HRV) protease inhibitor, is presented. In the presence of 40% acetonitrile, the alkaline protease from Bacillus lentus can catalyze the kinetic resolution of racemic ester 7 to afford (S)-acid 4 in 49% chemical yield/per cycle with 98% ee and >98% HPLC purity. The (R)-ester can then be readily recycled via a DBU catalyzed epimerization. The enzymatic preparation described here is superior to the existing chemical resolution route, exhibiting lower costs as well as higher yields, enantioselectivity, and substrate loads. In addition, this protease displays broad substrate specificity toward this class of compounds and can be easily extended to the preparation of other tripeptide mimetics of rhinovirus protease inhibitors.     

Scale-up synthesis of vinyl polymer-grafted nano-sized silica by radical polymerization of vinyl monomers initiated by surface initiating groups in the solvent-free dry-system

For the purpose of the prevention of the environmental pollution and the simplification of reaction process, the scale-up radical graft polymerization of vinyl monomers onto nano-sized silica surface initiated by azo groups and peroxycarbonate groups previously introduced onto the surface in the solvent-free dry-system was investigated. The introduction of azo groups onto the silica surface was achieved by the reaction of surface amino groups with 4,4′-azobis(4-cyanopentanoic acid chloride). On the other hand, the introduction of peroxycarbonate groups onto the silica surface was achieved by Michael addition of surface amino groups to t-butylperoxy-2-methacryloyloxyethylcarbonate. The graft polymerization of vinyl monomers onto the surface was successfully achieved by splaying monomers to nano-sized silica having azo and peroxycarbonate groups in solvent-free dry-system. It is interesting to note that the formation of ungrafted polymer was depressed in comparison with graft polymerization in solution: the grafting efficiency was 90-95%. In addition, in the solvent-free dry-system, the grafting of copolymer having pendant peroxycarbonate groups onto the nano-sized silica surface and the radical postgraft polymerization of styrene initiated by the pendant initiating groups of the grafted copolymer chain on the silica surface was investigated.     

An improved large scale procedure for the preparation of N-Cbz amino acids

A simple and scalable method for the preparation of N-Cbz protected amino acids is presented which uses a mixture of aqueous sodium carbonate and sodium bicarbonate to maintain the appropriate pH during the addition of benzyl chloroformate. The method has been extended to other N-protections and is amenable to large scale preparation of an intermediate toward Zofenopril, an ACE inhibitor.     

tert-Butyl esters of tripeptides based on Pro-Phe as organocatalysts for the asymmetric aldol reaction in aqueous or organic medium

The enantioselective aldol reaction between ketones and aldehydes constitutes one the most common reaction models for the evaluation of novel organocatalysts. The last few years, it has been shown that the organocatalytic aldol reaction can be performed in water. A family of tripeptides consisting of proline, phenylalanine, and tert-butyl esters of amino acids was successfully employed in this asymmetric transformation. The products of the reaction between various ketones and aldehydes were obtained in high yields (up to 99%) with excellent diastereo- (up to 97:3 dr) and enantioselectivities (up to 99% ee). The C-terminal amino acid determines the ability of the tripeptide (Pro-Phe-AA-O^tBu) to act efficiently in aqueous or organic medium.     

Highlighting the possible secondary interactions in the role of balhimycin and its analogues for enantiorecognition in capillary electrophoresis

It is believed that the enantiorecognition mechanism based on macrocyclic antibiotics involves multimodal interactions via hydrogen bonding, π–π interaction, steric hindrance, hydrophobic interaction and so on. A variety of enantiomeric N-benzoylated amino acids were separated using balhimycin (A) or its analogues bromobalhimycin (B) and dechlorobalhimycin (C) as chiral mobile phase additive using a CE method, which combined the partial filling technique with the dynamic coating technique and the co-EOF electrophoresis technique. The enantioresolution and the migration time were highly relevant to the structure of analytes, especially to the substitutions on the N-tagged benzoyl moiety of the amino acids. A steric effect and π–π interaction based mechanism is proposed in order to explain some observed enantioresolution differences between positional isomers. Notably dechlorobalhimycin exhibited the best enantioresolution for several N-benzoylated derivatives of leucine, which was rarely observed for N-dansylated amino acid derivatives. The hydrophobicity difference of the aglycone pocket among three chiral selectors was assumed to account for this behaviour.     

Benzylic-acetoxylation of alkylbenzenes with PhI(OAc)2 in the presence of catalytic amounts of TsNH2 and I2

Treatment of alkylbenzenes with (diacetoxyiodo)benzene in the presence of catalytic amounts of p-toluenesulfonamide or p-nitrobenzenesulfonamide, and molecular iodine in 1,2-dichloroethane at 60 °C gave the corresponding (α-acetoxy)alkylbenzenes in good to moderate yields. The present reaction is a simple method for the introduction of an acetoxy group to the benzylic position of alkylbenzenes.     

Synthesis of the ‘southern’ tripeptide of Cyclomarins A and C having novel anti-tuberculocidal mode of action

The synthesis of the ‘southern’ tripeptide of Cyclomarins A and C has been described which includes two unusual amino acids. The unusual amino acids have been synthesized and coupled with l-alanine to obtain the tripeptide.     

Stereoselective total synthesis of the E-isomer of putative lucentamycin A

A synthesis of the E-isomer of the proposed structure of the novel tripeptide, lucentamycin A, was performed in an attempt to define the correct stereochemistry of this natural product. The synthetic route developed employs a stereoselective Rh-catalyzed reductive cyclization process to generate the key pyrrolidine residue in the target and a stereospecific inversion of the Z-olefin geometry to form desired E-isomer. Subsequent amide coupling reactions afforded the desired E-isomer of putative lucentamycin A. A comparison of the NMR data of synthetic E-1a with that of the naturally occurring lucentamycin A demonstrated that they are not identical substances and the E-1a was found to display no anti-proliferative activity on the colon cancer cell line HCT-116 in contrast to natural lucentamycin A.     

An effective C-C double bond formation via Cu(I)-catalyzed dehydrogenation

A dehydrogenation method using Cu(I) and either N,N-di-tert-butylthiadiaziridine 1,1-dioxide or N,N-di-tert-butyldiaziridinone is reported. The dehydrogenation allows a facile introduction of C-C double bond(s) into various carbocycles and heterocycles such as oxazolines and thiazolines.     

Differential pulse polarographic determination of the n-nitroso derivative of a tripeptide in pharmaceutical dosage forms

A method is described for determining a tripeptide (pareptide) in tablets and capsules, which is based on the pulse polarographic measurement of itsN-nitroso derivative. The electrochemical behavior of the derivative was found to be similar to that of other N-nitrosamines with regard to the effects of temperature, pH, and mercury pressure. The reduction process is irreversible and complicated by adsorption. Optimum conditions for preparation of the N-nitroso derivative, which appear to be atypical, are presented. Spectroscopic data from i.r., m.s., n.m.r. and u.v. analyses suggest that the derivative differs from the parent compound only in that it has a single N-nitroso group on the proline segment of the tripeptide chain. Replicate assays on tablets containing 2 mg of pareptide gave excellent precision with a coefficient of variation of 0.37 %. Comparison of results obtained on capsules containing 2 mg of pareptide and stored at high temperatures showed good correlation between the polarographic and an h.p.l.c. method. Results obtained with a number of possible decomposition products indicate that the polarographic method is specific for pareptide.     

Synthesis of a N-glycan nonasaccharide of the bisecting type with additional core-fucose

A chemical synthesis was developed for N-glycans substituted with core-fucose and an additional bisecting GlcNAc (LEC10 motif). The synthesis was conducted using a suitably functionalized N-glycan pentasaccharide assembled from modular building blocks. Selective introduction of the bisecting GlcNAc residue was followed by attachment of the α1,6-arm and core-fucose. After introduction of an aminohexanoyl spacer the nonasaccharide was completely deprotected providing a substrate for enzymatic elongation and conjugation to proteins for further biological studies.     

Synthesis of triazole cages containing C3-symmetric α-cyclic tripeptide scaffold

Two water-soluble C₃-symmetric 1,2,3-triazole cages containing α-cyclic tripeptide were efficiently synthesized. The key steps include a click reaction to incorporate l-glutamic or l-aspartic acids to a tripodal linker and a unique one-pot cyclotrimerization.