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Influence of chirality of the preceding acyl moiety on the cis/trans ratio of the proline peptide bond
Breznik, Matej, farmacevt ...
We report that the cis/trans ratio of the proline peptide bond can be strongly influenced by the chirality of the acyl residue preceding proline. Acyl moieties derived from ...(2S)-2,6-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid (8) and (2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoic acid (5) in acyl-Pro molecules influence isomerization of the proline peptide bondconstraining the Ž dihedral angle to the trans orientation. Structures of benzyl (2S)-1-Šš(2S)-2,6-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-ylđcarbony lĐ-2 -pyrrolidinecarboxylate (3) derived from 2D 1H-NMR conformational analysis andcrystallographic data exhibit only the trans conformation of proline peptide bond. On the other hand the diastereomer 4 which contains an (R) acyl moiety exhibits two sets of signals in 1H NMR spectra. The signals were assigned to trans (72%) and cis (28%) conformers. Crystallographic analysis of4 showed that only the cis conformation is present in the crystalline state.The 1H-NMR chemical shift pattern of three sets of signals observed in 2, was observed also in benzyl (2S)-1-š(2RžS)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoy lđ-2 -pyrrolidinecarboxylate. (R)-Carboxylic acid 5 after coupling with (S)-ProOBn,yielded benzyl (2S)-1-š(2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoylđ -2-p yrrolidinecarboxylate (6), which in DMSO-d6 exhibited only the trans conformation of the proline peptide bond. These results suggest that in these particular cases acyl-Pro peptide bond isomerization is strongly influenced bythe stereochemistry of the acyl residue preceding proline. (2S)-2,6-Dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid (8) and (2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoic acid (5) are promising chiral peptidomimetic building blocks that can be used as acyl moieties to force the proline peptide bond into the trans conformation in a variety of acyl-Pro molecules.
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