Three-center intramolecular hydrogen bonding in oxamide derivatives. NMR and X-ray diffraction study

This contribution describes the synthesis and structural investigation of the symmetric and non-symmetric oxamides N,N′-bis(2-hydroxyphenyl)oxamide 1, N,N′-bis(5-tert-butyl-2-hydroxyphenyl)-oxamide 2, N,N′-bis(3,5-dimethyl-2-hydroxyphenyl)oxamide 3, N,N′-bis(2-hydroxybenzyl)oxamide 4, N,N′-diphenethyloxamide 5, N-(2-hydroxyphenyl)-N′-(2-methoxyphenyl)oxamide 6, N-(2-hydroxy-phenyl)-N′-phenethyloxamide 7, (1S,2R)-(-)-N-(2-hydroxyphenylcarbamoylcarbonyl)norephedrine 8, (1R,2S)-(-)-N-(2-hydroxyphenylcarbamoylcarbonyl) 9, ethyl N-(2-hydroxyphenyl)oxalamate 10 and ethyl N-(2-methoxyphenyl)oxalamate 11. The structures were established by ¹H, ¹³C, ¹⁵N and variable temperature NMR spectroscopy. Compounds 1-4 and 6-11 are stabilized by intramolecular three-center hydrogen bonding between the amide proton and two oxygen atoms. The ¹H NMR Δδ/ΔT value of the amide proton correlates with the ¹⁵N NMR chemical shift. The X-ray diffraction molecular structures of 1 and 11 showed a planar conformation with trans configuration in the solid state, corresponding to the preferred conformation found in solution.