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A new strategy for the biocatalytic resolution of (R,S)-phenylglycinonitrile, a crucial intermediate in the antibiotic industry, has been developed. While former techniques exploit nitrilases or combinations of nitrile hydratases and amidases, manipulating with nitrile functionality, the current approach is based on a highly efficient and enantioselective acylation of the α-amino group with phenylacetic acid catalyzed by a well known enzyme, penicillin acylase from E. coli, in slightly acidic aqueous medium. It is shown that since the condensation product is poorly soluble, removal of (S)-phenylglycinonitrile from the reaction sphere is almost complete and irreversible, favoring kinetics of the process and making high conversion possible. The proposed approach is characterized by high space-time yield and extends the scope of enzymatic synthesis in aqueous medium.

(S)-cyclohexylglycine (1) is prepared in high yield by hydrogenation of (S)-phenylglycine (2) using rhodium on carbon as the catalyst.

Enantiomerically pure 4-methylthio- and 4-methylsulfonyl-substituted (2S,3R)-3-phenylserines are prepared by enzymatic resolution of the corresponding racemic threo amides using the amidase from Ochrobactrum anthropi NCIMB 40321. The unwanted (2R,3S) enantiomers of the amides are separated from the enantiopure amino acids and easily racemized after Schiff base formation with the corresponding 4-(methylthio)- and 4-(methylsulfonyl)benzaldehyde. The racemization can be combined with the preparation of the racemic amides by aldol reaction under crystallization conditions to yield only the threo isomers. Enantiopure (2S,3R)-3-[4-(methylthio)phenyl]serine and (2S,3R)-3-[4-(methylsulfonyl)phenyl]serine are thus obtained in 78% and 62% overall yields starting from the corresponding substituted benzaldehydes. (2S,3R)-3-[4-(Methylthio)phenyl]serine is reduced to (1R,2R)-2-amino-1-[4-(methylthio)phenyl]propane-1,3-diol with NaBH4/H2SO4 and can be used for the synthesis of thiamphenicol and florfenicol.

Our model of the active site of the sweet taste receptor is shown to be consistent with the aspartame analogues in which the L-Phe 2 residue is replaced by L-(αMe)Phg, L-(αMe)Phe or L-(αMe)Hph. The analogues containing either the first or the third C α -methylated, phenyl-containing residue in the second position of the dipeptide were synthesized and found to be approximately as sweet as aspartame itselfand its L-(αMe)Phe 2 analogue.

Enantiomerically pure 2,2-disubstituted 2-amino-ethanols are prepared in 65 – 99% yield by reduction of a , a -disubstituted amino acid amides using liquid sodium metal in refluxing 1-propanol.

Terminally blocked homotri- and homotetra-peptides from (αMe)Leu, a chiral Cα-methylated, γ-branched α-amino acid, have been prepared by solution methods and fully characterized. The molecular and crystal structures of pBrBz-[D-(αMe)Leu]3-OH monohydrate and pBrBz-[D-(αMe)-Leu]4-OBut(where pBrBz indicates p-bromobenzoyl) were determined by X-ray diffraction. The tripeptide carboxylic acid adopts a type-III β-turn conformation followed by an uncommon oxyanalogue of a type-III β-turn, the latter being stabilized by a 1â†�4 CO ⋯ H–O intramolecular H-bond. The three independent molecules in the asymmetric unit of the tetrapeptide ester are folded in a regular right-handed 310-helix. All (αMe)Leu residues exhibit φ, Ψ torsion angles in the helical region of the conformational map. These results indicate that: (i) the (αMe)Leu residue is an effective β-turn and helix promoter and (ii) the relationship between (αMe)Leu chirality and turn and helix handedness is the same as that shown by the γ-branched (αMe)Phe residue, but it is opposite to that characteristic of isovaline (Iva), with a linear side chain, the β-branched (αMe)Val residue and protein amino acids (including Leu).

The molecular and crystal structures of one derivative and three homopeptides (from the di-to the tetrapeptide level) of the chiral, Cα, α-disubstituted glycine Cα-methyl, Cα-benzylglycine [(αMe)Phe], have been determined by x-ray diffraction. The derivative is mClAc-D-(αMe)Phe-OH, and the peptides are pBrBz-[D-(αMe)Phe]2-NHMe, pBrBz-[D-(αMe)Phe]3-OH hemihydrate, and pBrBz-[D-(αMe)Phe]4-OtBu sesquihydrate. All (αMe)Phe residues prefer ϕ,ψ torsion angles in the helical region of the conformational map. The dipeptide methylamide and the tripeptide carboxylic acid adopt a β-turn conformation with a 1 4 CO…HN intramolecular H bond. The structure of the tripeptide carboxylic acid is further stabilized by a 1 4 CO…HO intramolecular H bond, forming an “oxy-analogue” of a β-turn. The tetrapeptide ester is folded in a regular (incipient) 310-helix. In general, the relationship between (αMe)Phe chirality and helix screw sense is opposite to that exhibited by protein amino acids. A comparison is made with the conclusions extracted from published work on homopeptides from other Cα-methylated α-amino acids. © 1993 John Wiley & Sons, Inc.

The scope and limitations of the enzymatic resolution of α,α-disubstituted α-amino acid amides by an amino acid amidase from Mycobacterium neoaurum and of the corresponding ethyl esteis with Pig liver estetase (PLE) have been studied. Moderate enantiomeric excesses were obtained with PLE, with only a narrow substrate specificity. Mycobacterium neoaurum on the contrary yields a broad range of S-α,α-disubstituted α-amino acids 1 and the corresponding R-amides 2.

Terminally blocked homodi- and homotripeptides from (αMe)Val, a C α,α -disubstituted glycine, were prepared by solution methods and fully characterized. The preferred conformation in CHCl 3 solution was assessed by FTIR and 1 H NMR as a function of concentration and addition of perturbing agents. The molecular and crystal structures of the dipeptide and tripeptide amides, Z-(D-(αMe)Val] n -NHiPr (n= 2, 3) were also determined by X-ray diffraction. While the dipeptide amide adopts a type-III' β-turn conformation stabilized by a 1←4 C=O...H-N intramolecular H bond, the tripeptide amide is folded in an incipient left-handed 3 10 -helix

α,α-Disubstituted α-amino acid amides were prepared in 17–86% chemical yield by the phase-transfer catalyzed alkylation of N-benzylidene α-H amino acid amides, followed by weak acidic hydrolysis of the Schiff bases.

The scope and limitations of the enzymatic resolution of α,α-disubstituted α-amino acid amides by an amino acid amidase from Mycobacterium neoaurum and of the corresponding ethyl esteis with Pig liver estetase (PLE) have been studied. Moderate enantiomeric excesses were obtained with PLE, with only a narrow substrate specificity. Mycobacterium neoaurum on the contrary yields a broad range of S-α,α-disubstituted α-amino acids 1 and the corresponding R-amides 2.

(S)-cyclohexylglycine (1) is prepared in high yield by hydrogenation of (S)-phenylglycine (2) using rhodium on carbon as the catalyst.

A short and expedient synthetic route to optically active, saturated and γ,δ-unsaturated α-amino acids is reported. The key step is a BF 3 .OEt 2 -mediated reaction of allylsilanes with N-(alkoxycarbonyl)-α-methoxyglycinamides 11-15, leading to the corresponding γ,δ-unsaturated α-aminocarboxamides. The genuine S N 1-character of this process with iminium ion 6 as intermediate is proven in the case of the glycine ester 10

α,α-Disubstituted α-amino acid amides were prepared in 17–86% chemical yield by the phase-transfer catalyzed alkylation of N-benzylidene α-H amino acid amides, followed by weak acidic hydrolysis of the Schiff bases.

Several ω-alkenyl dichloroacetates cyclize in moderate to excellent yields to 3,5-dichloro-oxocon-2-ones (8-membered ring lactones) when heated in benzene at 80–180 °C in the presence of a catalytic amount of the cuprous chloride 2,2′-bipyridine complex.

The [L-(αMe) Phe] 2 -analogue of aspartame was synthesized and analyzed by X-ray diffraction. This compound is as sweet as aspartame itself but far more stable at pH=4. Several N-protected analogues were synthesized. The N-formylcarbamoyl [L-(αMe Phe)] 2 -aspartame analogue is also sweet. The compounds fit well within the sweet perception model as developed by Temussi, Toniolo and coworkers.

When the Pd(0)-catalyzed hydrostannolysis of the N-allyloxycarbonyl group is carried out in the presence of an activated carbonyl compound, a coupling product is formed. This new method appears to be applicable to the synthesis of a wide range of amides and peptides.

Two routes are presented for the conversion of optically active α-amino acid amides into the title compounds. One route(route A) features the formation of the Schiff's base 6 which is subsequently oxidized to the corresponding oxaziridines 7 . Route B is characterized by the formation of an imidazolin 11 which is hydroxylated to compound 12 . Alcoholysis of 7 and 12 in the presence of hydroxylamine hydrochloride yields the title compounds in overall yields ringing from 65 to 85%(route A) and from 14 to 21%(route B).

A new strategy for the biocatalytic resolution of (R,S)-phenylglycinonitrile, a crucial intermediate in the antibiotic industry, has been developed. While former techniques exploit nitrilases or combinations of nitrile hydratases and amidases, manipulating with nitrile functionality, the current approach is based on a highly efficient and enantioselective acylation of the α-amino group with phenylacetic acid catalyzed by a well known enzyme, penicillin acylase from E. coli, in slightly acidic aqueous medium. It is shown that since the condensation product is poorly soluble, removal of (S)-phenylglycinonitrile from the reaction sphere is almost complete and irreversible, favoring kinetics of the process and making high conversion possible. The proposed approach is characterized by high space-time yield and extends the scope of enzymatic synthesis in aqueous medium.

[reaction: see text]. Diastereoselective Strecker reactions based on (R)-phenylglycine amide as chiral auxiliary are reported. The Strecker reaction is accompanied by an in situ crystallization-induced asymmetric transformation, whereby one diastereomer selectively precipitates and can be isolated in 76-93% yield and dr > 99/1. The diastereomerically pure alpha-amino nitrile obtained from pivaldehyde (R1 = t-Bu, R2 = H) was converted in three steps to (S)-tert-leucine in 73% yield and >98% ee.

The crystal-state preferred conformations of two tripeptides, one tetrapeptide, and one pentapeptide, each containing a single residue of the chiral, C alpha, alpha-disubstituted glycine C alpha-methyl, C alpha-benzylglycine [(alpha Me)Phe], have been determined by X-ray diffraction. The tripeptides are Z-L-(alpha Me)Phe-(Aib)2-OH dihydrate and Z-Aib-D-(alpha Me)Phe-Aib-OtBu, the tetrapeptide is Z-(Aib)2-D-(alpha Me)Phe-Aib-OtBu, and the pentapeptide is pBrBz-(Aib)2-DL-(alpha Me)Phe-(Aib)2-OtBu. While the two tripeptides are folded in a beta-bend conformation, two such conformations are consecutively formed by the tetrapeptide. The pentapeptide adopts a regular 3(10)-helix promoted by three consecutive beta-bends. This study confirms the strong propensity of short peptides containing C alpha-methylated alpha-aminoacids to fold into beta-bends and 3(10)-helical structures. Since Aib is achiral, the handedness of the observed bends and helices is dictated by the presence of the (alpha Me)Phe residue. In general, we have found that the relationship between (alpha Me)Phe chirality and helix handedness is opposite to that exhibited by protein aminoacids. A comparison with the preferred conformation of other extensively investigated C alpha-methylated aminoacids is made.

We have synthesized and fully characterized the hypersweet super-aspartame analogue p CN-C6H4-NHCO-L-Asp-L-(αMe)Phe-OME 1; the [D-(αMe)Phe]3-analogue of the formyl-methionyl tripeptide chemoattractant HCO-L-Met-L-Leu--D-(αMe)Phe-OMe 2, the first D-chemotactic peptide being found more active than its L-diastereomer; and the model pentapeptide p BrBz-D-(αMe)Phe-(Aib)2-D-(αMe)Phe-Aib-O t Bu 3. The preferred conformation of the three peptides, as determined by X-ray diffraction analyses, is discussed in terms of the proposed receptor models for sweet perception [peptide 1] and neutrophil chemotaxis [peptide 2], and as a promising candidate for molecular recognition studies [peptide 3].

The conformationally restricted tripeptides HCO-L-Met-L-Phe-OMe [where Xxx is L (D)-Iva, -(αMe)Val or -(αMe)Phe] were synthesized and tested for their β-glucosaminidase release activity. The [L-(αMe)Val]2- and [L-(αMe)Leu]2- analogues turned out to be the most active compounds examined. The positive effects induced by increasing side-chain hydrophobicity and by the L-chirality of the Xxx residue on peptide-receptor interactions are discussed.

Recent progress in the chemo-enzymatic production of amino acids is reviewed. Both recently developed commercial processes and potentially important new developments are discussed. Emphasis is placed on the use of acylases, aminopeptidases and hydantoinases. The discovery of d-specific enzymes in combination with racemases is an exciting and promising new area. Also, a goal-orientated approach towards the selective generation of these novel enzyme activities using in vivo protein engineering techniques is highlighted. The interest in dipeptide sweeteners has triggered a major research effort towards the production of l-phenylalanine and d-alanine. A number of methods for the production of these amino acids is briefly discussed. Finally, chemo-enzymatic methods for the synthesis of enantiomerically pure α-alkyl-α-amino acids are reviewed.

o-Phthalaldehyde in combination with N-acetyl- l -cysteine is a useful derivatization reagent for the optical resolution of enantiomeric α-H-α-amino acids, α-alkyl-α-amino acids and the corresponding acid amides. By using reversed-phase high-performance liquid chromatography with a mobile phase containing copper(II) acetate and l -proline, the diastereomeric derivatives of the α-amino compounds can be separated under isocratic conditions. The rate of reaction of α-alkyl-α-amino compounds with o-phthalaldehyde-N-acetyl- l -cysteine can be increased by selectively increasing the amount of o-phthalaldehyde in the reaction mixture. When the derivatization parameters were controlled automatically, the derivatization process showed good reproducibility and the method was found to be suitable for quantitative measurements. The method was applied to monitor the enantiomeric purity of α-H-α-amino acids and α-alkyl-α-amino acids obtained by enantioselective hydrolysis of the corresponding acid amides using an aminopeptidase.

Both enantiomers of homomethionine have been obtained very readily by either of two newly developed resolution methods. The absolute configuration of homomethionine has been established with the aid of circular dichroic (CD) measurements.

The X-ray diffraction structure analysis of a novel peptide oxazol-5(4H)-one with a chiral carbon atom in the heterocyclic moiety has been performed for the Z-L-Pro-L-(αMe) Phe-OH oxazolone.

The synthesis and characterization of the two diastereomeric dipeptides Ac-L-(αMe)Phe-L-Lys-OH (5) and Ac-D-(αMe)Phe-L-Lys-OH (6), conformationally restricted analogues of ‘anti-aspartame-type’ sweetners, are described. Both compounds are tasteless. The X-ray diffraction structure of 6, also reported in this paper, in conjuction with the sweet perception model, as developed by Temussi, Toniolo and co-workers, is used to explain the lack of sweet taste of these compounds. The two diastereomeric synthetic intermediates Ac-L-(αMe)Phe-L-Lys(Z)-OBut(9) and Ac-D-(αMe)Phe-L-Lys(Z)-OBut(10) have also been characterized by X-ray diffraction.

Determination de l'exces enantiomere par reaction d'α-alkyl α-aminoacides avec le chlorure de l'acide chloro-2 propionique