Asymmetric strecker synthesis of alpha-amino acids via a crystallization-induced asymmetric transformation using (R)-phenylglycine amide as chiral auxiliary.

[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.

New developments in the synthesis of natural and unnatural amino acids.

Amino acids play an important role in biochemistry and chemistry. They are the building blocks of proteins and play an essential role in the regulation of the metabolism of living organisms. In general, it can be stated that microbial processes (fermentation) are the industrial production methods of choice for large-scale production of naturally occurring proteinogenic L-alpha-H-amino acids, while for the production of synthetic D- and/or L-alpha-H-amino acids, several other methods are highly competitive. At DSM, several routes, i.e., (chemoenzymatic) synthesis, towards L-alpha-H and D-alpha-H-amino acids have been elaborated since the midseventies. A general process for the synthesis of natural as well as synthetic optically pure amino acids has been developed, using an enzymatic kinetic resolution step on racemic amino acid amides as the key step. In this case, both enantiomers of the alpha-H-amino acids are prepared in one single step. This process has been commercialized since 1988. More recent developments using L- or D-amino peptidases in combination with amino acid amide racemases and an asymmetric transformation concept are discussed.

Reverse relationship between alpha-carbon chirality and helix handedness in (alpha Me)Phe peptides.

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.

Onset of the fully extended conformation in (alpha Me)Leu derivatives and short peptides.

The X-ray diffraction crystal structures of the (alpha Me)Leu derivative mClAc-D-(alpha Me) Leu-OH and the terminally protected tripeptide Z-D-(alpha Me) Leu-(L-Ala)2-OMe show the onset of the fully extended (C5) conformation for the (alpha Me) Leu residue in both independent molecules in the asymmetric unit of the former compound and in two out of the four independent molecules in the asymmetric unit of the latter compound. In addition, conformational analysis in CDCl3 solution (using FT-infra-red absorption and 1H nuclear magnetic resonance) revealed the occurrence of a significant population of fully extended conformers throughout the entire sequence of the (alpha Me) Leu homochiral homopeptides pBrBz-[D-(alpha Me) Leu]n-OtBu (from monomer to tetramer). Taken together, these results represent a clear indication that this peptide secondary structure, uncommon for protein amino acids and other C alpha-methylated chiral residues, is not a rare observation in (alpha Me) Leu derivatives and short peptides.

Peptides from chiral C alpha,alpha-disubstituted glycines. Synthesis and characterization, conformational energy computations and solution conformational analysis of C alpha-methyl, C alpha-isopropylglycine [(alpha Me)Val] derivatives and model peptides.

Conformational energy computations on Ac-L-(alpha Me)Val-NHMe indicate that turns and right-handed helical structures are particularly stable conformations for this chiral C alpha-methyl, C alpha-alkylglycyl residue. We have synthesized and characterized a variety of L-(alpha Me)Val derivatives and peptides (to the pentamer level). The results of the solution conformational analysis, performed using infrared absorption, 1H nuclear magnetic resonance, and circular dichroism, are in general agreement with those obtained from the theoretical investigation, in the sense that the L-(alpha Me)Val residue turns out to be a strong beta-turn and right-handed helix former. A comparison is also made with the conclusions extracted from published work on peptides rich in other C alpha-methyl, C alpha-alkylglycyl residues.

A conformation-activity study of the [L-(alpha Me)Phe]3 analog of the formyl methionyl tripeptide chemoattractant.

CHO-L-Met-L-Leu-L-(alpha Me)Phe-OMe, an analog of the formyl methionyl tripeptide chemoattractant CHO-L-Met-L-Leu-L-Phe-OH, was synthesized by solution methods and fully characterized. This compound was prepared to determine the effect of the incorporation of a conformationally restricted C alpha,alpha-disubstituted alpha-amino acid residue at position 3 on the relation of three-dimensional structure to biological activity. The peptide was tested for its ability to induce granule enzyme secretion from rabbit peritoneal polymorphonuclear leukocytes. In parallel, a conformational analysis was performed in the crystal state by x-ray diffraction and in organic solution by infrared absorption and 1H nuclear magnetic resonance. These biological and conformational data are discussed in relation to those of the prototype tripeptide and its methyl ester.

Peptides from chiral C alpha,alpha-disubstituted glycines. Crystallographic characterization of conformation of C alpha-methyl, C alpha-isopropylglycine [(alpha Me)Val] in simple derivatives and model peptides.

The molecular and crystal structures of one derivative and three model peptides (to the pentapeptide level) of the chiral C alpha,alpha-disubstituted glycine C alpha-methyl, C alpha-isopropylglycine [(alpha Me)Val] have been determined by X-ray diffraction. The derivative is mClAc-L-(alpha Me)Val-OH, and the peptides are Z-L-(alpha Me)Val-(L-Ala)2-OMe monohydrate, Z-Aib-L-(alpha Me)Val-(Aib)2-OtBu, and Ac-(Aib)2-L-(alpha Me)Val-(Aib)2OtBu acetonitrile solvate. The tripeptide adopts a type-I beta-turn conformation stabilized by a 1----4N--H...O = C intramolecular H-bond. The tetra- and pentapeptides are folded in regular right-handed 3(10)-helices. All four L-(alpha Me)Val residues prefer phi, psi angles in the right-handed helical region of the conformational map. The results indicate that: (i) the (alpha Me)Val residue is a strong type-I/III beta-turn and helix former, and (ii) the relationship between (alpha Me)Val chirality and helix screw sense is the same as that of C alpha-monosubstituted protein amino-acids. The implications for the use of the (alpha Me)Val residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.

New developments in the chemo-enzymatic production of amino acids.

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 alpha-alkyl-alpha-amino acids are reviewed.

Synthesis, characterization, and solution conformational analysis of C alpha-methyl-, C alpha-benzylglycine [(alpha Me)Phe] model peptides.

We have synthesized, by solution methods, and fully characterized a variety of (alpha Me)Phe derivatives and model peptides (to the pentapeptide level). The results of the solution conformational analysis, performed by using infrared absorption and 1H nuclear magnetic resonance, support the view that the (alpha Me)Phe residue is a stronger beta-turn and helix promoter than the unmethylated Phe analog. A comparison is also made with the conclusions extracted from published work on peptides rich in other C alpha-alkylglycyl residues.

Synthesis and conformational analysis of (alpha Me)Leu/Aib model peptides.

We have synthesized by solution methods and fully characterized a variety of (alpha Me)Leu/Aib model peptides to the octapeptide level. A solution conformational analysis was performed by using infrared absorption. 1H nuclear magnetic resonance, and circular dichroism. The crystal-state structures of Z-D-(alpha Me)Leu-(Aib)2-OtBu, pBrBz-(Aib)2-D-(alpha Me)Leu-(Aib)2-OtBu, and Ac-(Aib)2-D-(alpha Me)Leu-(Aib)2-OtBu monohydrate were solved by x-ray diffraction. The results indicate that the (alpha Me)Leu residue may be easily incorporated into beta-bends and 3(10)-helical structures, and suggest that this residue tends to induce a helix handedness opposite to that promoted by its unmethylated counterpart (Leu) of the same optical configuration.