A preparation of N-Fmoc-N-methyl-α-amino acids and N-nosyl-N-methyl-α-amino acids

A convenient route for the synthesis of lipophilic N-Fmoc-N-methyl-α-amino acids and N-nosyl-N-methyl-α-amino acids, interesting building blocks to be used for the preparation of N-methylated peptides, is presented. Both nosyl- and Fmoc-protected monomers are accessible, so these compounds can be used in solution as well as in solid phase peptide synthesis. The methodology is based on the use of benzhydryl group to protect temporarily the carboxyl function of N-nosyl-α-amino acids and on the subsequent methylation of the N-nosyl-α-amino acid benzhydryl esters with diazomethane. The benzhydryl esters offer several beneficial features such as simple preparation, stability to methylation and selective deprotection under mild conditions. The overall procedure is highly efficient in that the adopted conditions keep the chiral integrity of amino acid precursors and the process does not require chromatographic purification of the methylated products.     

Synthesis of biheterocyclicα-amino acids

Summary We report here the synthesis of biheterocyclic-amino acids by 1,3 dipolar cycloaddition of acetylenic compounds on-azido-amino esters.     

Alicyclic β-amino acids in Medicinal Chemistry

Summary. The structural element of alicyclic β-amino acids shows some remarkable biological effects: For some 5- and 6-membered β-amino acids a unique anti fungal activity has been observed, 7-membered β-amino acid derivatives have been investigated for neurological disorders. The application of 5-, 6- and 7-membered alicyclic β-amino acids in Medicinal Chemistry will be reported.     

Syntheses ofγ-fluoro-α-amino acids

Summary Methods for the synthesis of racemic and optically active title compounds are presented. Key step of these four-step procedures is the alkylation with 1-bromo-2-fluoroalkanes of glycine-ester-derived imines in anhydrous medium using lithium diisopropylamide as a base at low temperature or phase transfer catalyzed alkylation with 50% NaOH and triethylbenzylammoniumchloride as the phase transfer catalyst, respectively. Subsequent three-step deprotection gave the free acids in 13–33% overall yield. Deracemization of-fluoro--aminobutyric acid methyl and ethyl esters with-chymotrypsin was shown to give the (–)-enantiomers of the esters and (+)--fluoro--aminobutyric acid in >98% ee, while from thetert-butylester the opposite stereochemical result was observed giving the (–)-acid with 88% ee. Optically active-fluoro--amino acids were synthesized alternatively by phase transfer catalysis with N-benzyl-cinchonium chloride or using an auxiliary-directed asymmetric alkylation of the imine derived from (R)-(+)-camphor or (R)-(+)-2-hydroxypinan-3-one. These processes gave different enantiomers of-fluoro--aminobutyric acid via a monomeric lithium enolate in the first or a dimeric lithium enolate in the second case, respectively. The enantiomeric excess can be improved by lithium/magnesium exchange.     

How to build optically activeα-amino acids

Summary Various methodologies published in the literature dealing with-amino carboxylic acid asymmetric synthesis are presented in a digest form. In each case, only some recent or most typical works are mentioned.     

Synthesis of optically active lipidicα-amino acids and lipidic 2-amino alcohols

Summary Lipidic-amino acids (LAAs) are a class of compounds combining structural features of amino acids with those of fatty acids. They are non-natural-amino acids with saturated or unsaturated long aliphatic side chains. Synthetic approaches to optically active LAAs and lipidic 2-amino alcohols (LAALs) are summarized in this review. A general approach to enantioselective synthesis of saturated LAAs is based on the oxidative cleavage of 3-amino -1,2-diols obtained by the regioselective opening of enantiomerically enriched long chain 2,3-epoxy alcohols. Unsaturated LAAs are prepared in their enantiomeric forms by Wittig reactionvia methyl (S)-2di-tert-butoxycarbonylamino-5-oxo-pentanoate. This key intermediate aldehyde is obtained by selective reduction of dimethyl N,N-di-Boc glutamate with DIBAL. (R) or (S) LAALs may be prepared starting from D-mannitol or L-serine. LAAs are converted into LAALs by chemoselective reduction of their fluorides using sodium borohydride with retention of optical purity. Replacement of the hydroxyl group of LAALs by the azido group, followed by selective reduction leads to unsaturated optically active lipidic 1,2-diamines.Abbreviations Bn benzyl - Boc tert-butoxycarbonyl - DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone - DET diethyl tartrate - DIBAL diisobutyl aluminum hydride - DMAP 4-dimethylaminopyridine - DMF N,N-dimethylformamide - DMSO dimethyl sulfoxide - EDC N-ethyl-N-(3-dimethylaminopropyl)carbodiimide - Et₃N triethylamine - HMPA hexamethylphosphoramide - HOBt 1-hydroxybenzotriazole - KN(TMS)₂ potassium bis(trimethylsilyl)-amide - LAA lipidic-amino acid - LAAAl lipidic 2-amino alcohol - LDA lipidic 1,2-diamine - LP lipidic peptide - MPM-Cl p-methoxybenzyl chloride - MsCl methanesulphonyl chloride - MTPA -methoxy--(trifluoromethyl)phenylaccitc - PLA₂ phospholipase A₂ - TBIIP tert-butyl hydroperoxide - THF tetrahydrofuran - TMSCl trimethylsilyl chloride - Tr trityl - Z benzyloxycarbonyl     

Synthesis of cyclic RGD-peptides containing β-amino acids

Summary The solid phase synthesis of cyclic RGD-peptides containing β-amino acids according to two different protocols is described. The second strategy allows multiple or combinatorial syntheses of this type of cyclic peptides, because it enables backbone cyclization while the RGD-peptide is still bound to the resin. The newly synthesized RGD-peptides were characterized by MALDI-TOF MS and NMR and their physiological activity was determined by aggregometry.     

Synthesis of medicinally useful lipidicα-amino acids, 2-amino alcohols and diamines

Summary The lipidic-amino acids (LAAs) are non-natural-amino acids with saturated or unsaturated long aliphatic side chains. LAAs and their derivatives (lipid mimetics) together with the lipidic peptides represent a class of compounds which combine structural features of lipids with those of amino acids and peptides. Racemic LAAs may be prepared by classical methods and resolved by chemical or enzymatic methods. LAA amides and esters with saturated or unsaturated long chain amines and alcohols respectively, as well as lipidic dipeptide derivatives inhibit both pancreatic and human platelet phospholipase A₂. Lipophilic peptide derivatives are inhibitors of human neutrophil elastase. LAAs and their oligomers have been used as drug delivery system. A Lipid-Core-Peptide system has been designed and used as a combined adjuvant-carrier-vaccine system. A variety of lipid mimetics such as lipidic 2-amino alcohols, lipidic 1,2- and 1,3-diamines have been prepared based upon LAAs. Some of them are potent inhibitors of phospholipase A₂. A general approach to enantioselective synthesis of LAAs and lipid mimetics is based on the oxidative cleavage of 3-amino-1,2-diols obtained by the regioselective opening of enantiomerically enriched long chain 2,3-epoxy alcohols.Abbreviations Boc tert-butoxycarbonyl - BSA bovine serum albumin - CD circular dichroism - DET diethyl tartrate - DIBAL diisobutyl aluminum hydride - DMF N,N-dimethylformammide - HMPA hexamethylphosphoramide - HNE human neutophil elastase - LAA lipidic amino acid - LAAL lipidic amino alcohol - LH-RH luteinizing hormone-releasing hormone - LCP lipid-core-peptide - LDA lipidic diamine - LP lipidic peptide - MAP multiple antigenic peptide - PLA₂ phospholipase A₂ - TBHP tert-butyl hydroperoxide - THF tetrahydrofuran - TRH thyrotropin-releasing hormone - Z benzyloxycarbonyl     

Synthesis of some racemicγ-fluoro-α-amino acids

Summary Versatile three-step procedures for syntheses of seven racemi-fluoro-a-amino acids are described. Alkylation oftert-butyl N-(diphenylmethylene) glycinate with 1-bromo-2-fluoroalkanes gave N-protected aminoacid esters both in anhydrous medium using lithium-diisopropylamide as base at low temperature or in a two phase system of 50% aqueous sodium hydroxide and methylene chloride with triethylbenzylammonium chloride as the phase transfer catalyst at room temperature. Subsequent two-step deprotection with citric acid and hydrochloric acid gave the title compounds in 13–33% overall yields.Dedicated to Professor Dr.mult., Dr.h.c. Alois Haas on the occasion of his 65th birthday     

Polymerization on the Rocks: Negatively-Charged α-amino acids

Oligomers of the negatively-charged amino acids, glutamic acid, aspartic acid, and O-phospho-L-serine are adsorbed by hydroxylapatite and illite with affinities that increase with oligomer length. In the case of oligo-glutamic acids adsorbed on hydroxylapatite, addition of an extra residue results in an approximately four-fold increase in the strength of adsorption. Oligomers much longer than the 7-mer are retained tenaciously by the mineral. Repeated incubation of short oligo-glutamic acids adsorbed on hydroxylapatite or illite with activated monomer leads to the accumulation of oligomers at least 45 units long. The corresponding reactions of aspartic acid and O-phospho-L-serine on hydroxylapatite are less effective in generating long oligomers, while illite fails to accumulate substantial amounts of long oligomers of aspartic acid or of O-phospho-L-serine.     

Synthesis of γ-amino acid analogues from natural α-amino acids by a radical pathway

Summary. New γ-amino esters and amides were prepared by a radical 1,4-addition of carbon radicals to acrylic derivatives. α-Amino acids derivatives holding chiral auxiliaries as radical precursors and different chiral olefins were used and chiral induction on the C-γ center was discussed.     

Bicyclic γ-amino acids as inhibitors of γ-aminobutyrate aminotransferase

The γ-aminobutyrate (GABA)-degradative enzyme GABA aminotransferase (GABA-AT) is regarded as an attractive target to control GABA levels in the central nervous system: this has important implications in the treatment of several neurological disorders and drug dependencies. We have investigated the ability of newly synthesized compounds to act as GABA-AT inhibitors. These compounds have a unique bicyclic structure: the carbocyclic ring bears the GABA skeleton, while the fused 3-Br-isoxazoline ring contains an electrophilic warhead susceptible of nucleophilic attack by an active site residue of the target enzyme. Out of the four compounds tested, only the one named (+)-3 was found to significantly inhibit mammalian GABA-AT in vitro. Docking studies, performed on the available structures of GABA-AT, support the experimental findings: out of the four tested compounds, only (+)-3 suitably orients the electrophilic 3-Br-isoxazoline warhead towards the active site nucleophilic residue Lys329, thereby explaining the irreversible inhibition of GABA-AT observed experimentally.     

Ammonia lyases and aminomutases as biocatalysts for the synthesis of α-amino and β-amino acids

Ammonia lyases catalyse the reversible addition of ammonia to cinnamic acid (1: R=H) and p-hydroxycinnamic (1: R=OH) to generate L-phenylalanine (2: R=H) and L-tyrosine (2: R=OH) respectively (Figure 1a). Both phenylalanine ammonia lyase (PAL) and tyrosine ammonia lyase (TAL) are widely distributed in plants, fungi and prokaryotes. Recently there has been interest in the use of these enzymes for the synthesis of a broader range of L-arylalanines. Aminomutases catalyse a related reaction, namely the interconversion of α-amino acids to β-amino acids (Figure 1b). In the case of L-phenylalanine, this reaction is catalysed by phenylalanine aminomutase (PAM) and proceeds stereospecifically via the intermediate cinnamic acid to generate β-Phe 3. Ammonia lyases and aminomutases are related in sequence and structure and share the same active site cofactor 4-methylideneimidazole-5-one (MIO). There is currently interest in the possibility of using these biocatalysts to prepare a wide range of enantiomerically pure l-configured α-amino and β-amino acids. Recent reviews have focused on the mechanism of these MIO containing enzymes. The aim of this review is to review recent progress in the application of ammonia lyase and aminomutase enzymes to prepare enantiomerically pure α-amino and β-amino acids.     

A method for the separation of peptides and α-amino acids

1. Peptides and alpha-amino acids, occurring in mixtures from various sources, can be separated into one fraction containing the amino acids and several peptide fractions. This is achieved by chelation of the mixture with Cu(2+) ions and subsequent chromatography of these chelates over the acetate form of diethylaminoethylcellulose or triethylaminoethylcellulose. 2. The amino acid fraction is obtained by elution with 0.01m-collidine-acetate buffer, pH8.0. 3. Peptide fractions are eluted with 0.01m-collidine-acetate buffer, pH4.5, 0.17n-acetic acid and 0.1n-hydrochloric acid respectively. 4. With the exception of aspartic acid and glutamic acid, which are partly found in the acidic peptide fraction, the amino acids are completely separated from the peptides. 5. Contamination of the acidic peptide fraction with glutamic acid and aspartic acid can be largely avoided by previous addition of an excess of arginine. 6. Copper is removed from the eluates by extraction with 8-hydroxyquinoline in chloroform.     

β,γ-Alkynylα-amino acids: a synthetic challenge

Summary Ethynyl glycine is a naturally occurring unusual-amino acid. Its known chemical and biological properties are summarized in the first part of this review. The second part is an overview on racemic syntheses of ethynyl glycine and other,-alkynyl-amino acid derivatives, including patent data. These small polyfunctional compounds revealed as being very labile and the synthesis of mainly fully or partially protected forms seemed to have been actually performed. The last part deals with the approaches to the enantioselective synthesis of,-alkynyl-amino acids derivatives, and details the only satisfactory strategy that has led to optically active,-alkynyl-amino acids derivatives up to now.     

Pro-apoptotic activity of lipidic α-amino acids isolated from Protopalythoa variabilis

Lipidic α-amino acids (LAAs) have been described as non-natural amino acids with long saturated or unsaturated aliphatic chains. In the continuing prospect to discover anticancer agents from marine sources, we have obtained a mixture of two cytotoxic LAAs (1a and 1b) from the zoanthid Protopalythoa variabilis. The anti-proliferative potential of 14 synthetic LAAs and 1a/1b were evaluated on four tumor cell lines (HCT-8, SF-295, MDA-MB-435, and HL-60). Five of the synthetic LAAs showed high percentage of tumor cell inhibition, while 1a/1b completely inhibited tumor cell growth. Additionally, apoptotic effects of 1a/1b were studied on HL-60 cell line. 1a/1b-treated cells showed apoptosis morphology, loss of mitochondrial potential, and DNA fragmentation.     

Helix induction potential of N-terminal α-methyl, α-amino acids

Summary A series of longer analogues of the C-peptide of RNAse A has been synthesized with the aim of assessing the helix induction potential in water of α-methyl, α-amino acids at the N-terminus of the chain. The circular dichroism data indicate that one isovaline residue is effective in increasing the helix content of the 13-residue peptide by about 7%.     

Transport of imino acids and non-α-amino acids across the brush-border membrane of the rabbit ileum

Summary The transport of -alanine and MeAIB and their effects as inhibitors of the transport of alanine, leucine and lysine across the brush-border membrane of the intact epithelium from the rabbit's distal ileum has been examined. Two separate transport systems have been characterized: 1) A sodium-dependent, -alanine-accepting system, which is a high-affinity transport system for -amino-monocarboxylic acids (neutral a.a.) and for cationic a.a., accepts non--amino acids as well as non--imino acids, is moderately stereospecific, and for which the affinity of a neutral a.a. is greatly reduced by N-methylation. 2) A sodium-dependent transport system for imino acids, which is inaccessible to cationic amino acids and non--amino acids but accepts cyclic, non--imino acids, is moderately stereospecific, and for which neutral a.a. have much lower affinities than their N-methylated derivatives. On the basis of the observations of this and the preceding paper five transport systems for amino acids are ascribed to the rabbit ileum. Some discrepancies between the present results and those obtained with brush-border membrane microvesicles from the rabbit small intestine are discussed.     

Synthesis of conformationally constrained hydroxy-α-amino acids by intramolecular conjugate addition

Summary. An efficient and easily applicable method for the synthesis of a variety of hydroxy-α-amino acids analogues of serine and phenylalanine has been established. The method involves the stereoselective intramolecular conjugate addition of the benzamide group to cyclohexenone promoted by Lewis acid. Subsequent transformations of functional groups pro-vide the conformationally constrained 2-hydroxy- and 2,4-dihydroxy-6-phenylcyclohexane-α-amino acids. Received February 5, 1999, Accepted May 16, 1999