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At least 37 records · Page 2Linked to original sources
PubMed1995

Conformational studies of heterochiral peptides with diastereoisomeric residues: crystal and molecular structures of linear dipeptides derived from leucine, isoleucine, and allo-isoleucine.

The x-ray diffraction analyses of three N- and C-terminally blocked L,D dipeptides, namely t-Boc-D-Leu-L-Leu-OMe (1), t-Boc-L-Ile-D-aIle-OMe (2), and t-Boc-D-aIle-L-Ile-OMe (3) containing enantiomeric or diastereomeric amino acid residues have been carried out. The structures were determined by direct methods and refined anisotropically to final Rf actors of 0.077, 0.058, and 0.072 for (1), (2), and (3), respectively. Peptides 1-3 all assume a similar U-shaped structure with phi and psi torsion angles corresponding to one of the possible calculated minimum energy regions (regions E and G for L residues, and F*, D* and H* for D residues). The peptide backbones of 1-3 are almost superimposable [provided that the appropriate inversion of the chiral centers of (2) is made]. Side-chain conformations of Leu residues in peptide (1) are g- (tg-) for the L-Leu residue and the mirrored g+ (tg+) for the D-Leu residue; however, in peptides (2) and (3) the conformations of the isoconfigurational side chains of the Ile or allo-Ile residues are (g-t) t and (tg+) t for the L-Ile and the D-allo-Ile moieties, respectively. In all cases, these conformations correspond to the more populated conformers of beta-branched residues statistically found in crystal structures of small peptides. The results seem to indicate that, at least in short peptides with enantiomeric or diastereoisomeric residues, the change in chirality in the main-chain atoms perturbs the backbone conformation to a lesser extent and the side chain conformation to a greater extent.

Crystallography, X-Ray↗
PubMed2000

Memory of chirality in diastereoselective alpha-alkylation of isoleucine and allo-isoleucine derivatives.

[reaction: see text] alpha-Methylation of 3 gave 5 as a major product whereas 4 gave 6 predominantly, although both 3 and 4 have an (S)-chiral center at C(3). This indicates that chirality at C(2) in 3 and 4 was memorized in the corresponding intermediate enolates and the induced chirality made a major contribution in the stereochemical course of the reaction, while chirality at the adjacent chiral center C(3) had little effect.

Alkylation↗