Acid- and base-induced conformational alterations of N-aryl-N-troponyl amides

Synthesis and Conformational Analysis of N-Aromatic Acetamides Bearing Thiophene: Effect of Intramolecular Chalcogen-Chalcogen Interaction on Amide Conformational Stability

The conformations of aromatic amides bearing an N-(2-thienyl) or N-(3-thienyl) group were investigated in solution and in the crystal state. NMR spectral data indicate that the conformational preferences of these amides in solution are dependent not only on the relative π-electron densities of the N-aromatic moieties, but also on the three-dimensional relationship between carbonyl oxygen and the N-aromatic moieties. A comparison of the conformational preferences of N-(2-thienyl)amides and N-(3-thienyl)amides revealed that the Z-conformers of N-(2-thienyl)acetamides are stabilized by 1,5-type intramolecular S···O═C interactions between amide carbonyl and thiophene sulfur. The crystal structures of these compounds were similar to the solution structures. The stabilization energy due to 1,5-type intramolecular S···O═C interaction in N-aryl-N-(2-thienyl)acetamides and N-methyl-N-(2-thienyl)acetamide was estimated to be ca. 0.74 and 0.93 kcal/mol, respectively.

N-Ethynylation of Anilides Decreases the Double-Bond Character of Amide Bond while Retaining trans-Conformation and Planarity

Activated amide bonds have been attracting intense attention; however, most of the studied moieties have twisted amide character. To add a new strategy to activate amide bonds while maintaining its planarity, we envisioned the introduction of an alkynyl group on the amide nitrogen to disrupt amide resonance by nN→C_sp conjugation. In this context, the conformations and properties of N-ethynyl-substituted aromatic amides were investigated by DFT calculations, crystallography, and NMR spectroscopic analysis. In contrast to the cis conformational preference of N-ethyl- and vinyl-substituted acetanilides, N-ethynyl-substituted acetanilide favors the trans conformation in the crystal and in solution. It also has a decreased double bond character of the C(O)-N bond, without twisting of the amide. N-Ethynyl-substituted acetanilides undergo selective C(O)-N bond or N-C(sp) bond cleavage reactions and have potential applications as activated amides for coupling reactions or easily cleavable tethers.