Wilhelmus Hubertus Joseph Boesten

Enzymatic resolution of α,α-disubstituted α-amino acid esters and amides

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.

Aspartame dipeptide analogues: effect of number of side-chain methylene group spacers and Cα-methylation in the second position

Abstract 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.

Bioactive and model peptides characterized by the helicogenic (αMe)Phe residue

Abstract 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].

New aspartame-like sweeteners containing L-(αMe)Phe

Abstract 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.

Oxidative preparation of optically active n-hydroxy-α-amino acid amides

Abstract 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 Schiffs 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).

Determination of the enantiomers of α-H-α-amino acids, α-alkyl-α-amino acids and the corresponding acid amides by high-performance liquid chromatography

Abstract 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.

Chemo-enzymatic synthesis of amino acids and derivatives

A short overview is given of new developments in the chemo-enzymatic synthesis of amino acids. Amino acid resolution procedures based on stereoselective enzymatic hydrolysis of amino acid amides have been developed. Two enzymes, an aminopeptidase from Pseudomonas putida and an amidase from Hycobacterium neoaurum have been purified to homogeneity. In addition, methods for the stereoselective synthesis of a-alkyl-substituted hydroxy acids are given.

Resolution of (RS)-phenylglycinonitrile by penicillin acylase-catalyzed acylation in aqueous medium

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.

Synthesis of medium-sized lactones by copper(I) catalyzed atom transfer cyclization

Abstract 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.

Position of side-chain branching and handedness of turns and helices of homopeptides from chiral Cα-methylated amino acids. Crystal-state structural analysis of (αMe)Leu trimer and tetramer

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).