Facile structural elucidation of imidazoles and oxazoles based on NMR spectroscopy and quantum mechanical calculations

The reaction of 1,2,3-tricarbonyl derivatives with hexamethylenetetramine and ammonium acetate in acetic acid provides an unambiguous approach to the synthesis of imidazoles, whereas the Bredereck reaction of α-haloketones in formamide, yields both imidazoles and oxazoles. Herein we describe a facile methodology for distinguishing between these heterocyclic compounds based on a combination of NMR spectroscopy and quantum mechanical calculations. In the NMR data the oxazole C-2 has a chemical shift of ca. 150 ppm whereas in the imidazoles it is found at ca. 135 ppm, with a ¹J_C-H of ca. 250 Hz for the oxazoles and ca. 210 Hz for the imidazoles. ¹J_C-H values can be easily obtained from a gated-decoupled ¹³C spectrum, and even more trivially, from the separation of the H-2 ¹³C satellites in the ¹H spectra. Additionally, the computed NMR data, obtained from density functional theory, are found to be in good agreement with the experimental data and serve as valuable tools in identifying the products of the Bredereck reaction.