Conformational Properties of Aromatic Amides Bearing Imidazole Ring and Acid-Induced Trans-Cis Amide Switching.
The Journal of organic chemistry(2022)
摘要
Aromatic amides bearing secondary amide bond exist in trans conformation both in the crystal and in solution, whereas the conformation of the N-methylated derivatives is cis in the crystal and predominantly cis in various solvents. The cis conformational preference of N-alkylated benzanilide provides access to aromatic foldamers such as oligo(N-alkyl-p-benzamide)s, which adopt dynamic helical structures. Here, the conformational properties of imidazole-substituted amide in the crystal and in solution were examined. Imidazole-substituted amides 2a and 4a existed mainly in the cis conformation in solution. The ratio of the cis conformer of N-methyl-N-(1-methyl-1H-imidazol-4-yl)benzamide (4a) was smaller than that of N,1-dimethyl-N-phenyl-1H-imidazole-2-carboxamide (2a) or N-methylbenzanilide, but the introduction of a substituent strongly affected the conformer ratio. Compounds 6a and 7a bearing an electron-withdrawing group on the imidazole ring existed predominantly in trans form. On the other hand, the introduction of an electron-withdrawing group on the phenyl ring or a bulky substituent on the amide nitrogen of 4a increased the ratio of cis conformer. Further, the major conformer of N-alkylated N-imidazolylamides was switched from cis to trans by the addition of acid. These results suggest that imidazole-substituted amides might be applicable as conformational switches in aromatic foldamers to enable environment-dependent structural change.
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关键词
aromatic amides,imidazole bearing,acid-induced
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