Crystal structures and properties of mutagenic 'N'-acyloxy-'N'-alkoxyamides - 'most pyramidal' acyclic amides

X-Ray data for two N-acyloxy-N-alkoxyamides, a class of direct-acting mutagens, indicate extreme pyramidalisationat the amide nitrogen in keeping with spectroscopic and theoretically determined properties of amides with bisoxosubstitutionat nitrogen. The combined electronegativity of two oxygens leads to average angles at nitrogen of 107.8and 108.1° and |χN| of 66° and 65°. The sp³ nature of nitrogen results in negligible amide resonance as evidencedby long N–C(O) bonds, high IR carbonyl stretch frequencies, carbonyl ¹³C NMR data and very low amideisomerisation barriers. In addition, conformations in the solid state support a strong n₀–σ*[NOAc] anomeric interactionas predicted by molecular orbital theory. HF/6-31G* calculations on formamide, N-methoxyformamide andN-formyloxy-N-methoxyformamide support these findings.