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

N,NЈ-Diacyl-N,NЈ-dialkoxyhydrazines are HERON amides that exhibit theoretical and physical properties of bisheteroatom-substituted amides. Amide nitrogens are pyramidal and they adopt a preferential conformation which permits an anomeric interaction in which one nitrogen overlaps strongly with the adjacent N-O σ* orbital. A crystal structure of N,NЈ-di(p-chlorobenzoyl)-N,NЈ-diethoxyhydrazine 4d has been obtained which confirms these properties in the solid state. Infrared data for ten such hydrazines indicate unusually high carbonyl stretch frequencies in accord with pyramidality at nitrogen. Diastereotopic resonances and dynamic 1 H NMR studies indicate both a significant N-N rotational barrier of between 65 and 72 kJ mol Ϫ1 , which is consistent with a strong anomeric interaction, as well as a much smaller than usual amide rotation barrier of 54 kJ mol Ϫ1 , a direct consequence of pyramidality at nitrogen.