Can N-acylazetones ever be obtained? The reaction between di-t-butoxyethyne and benzoyl isocyanate leading to 2-phenyl-4,5-di-t-butoxy-1,3-oxazin-6-one

Abstract

Di-t-butoxyethyne (14) reacts with benzoyl isocyanate (13) affording 2-phenyl-4,5-di-t-butoxy-1,3-oxazin-6-one (15) and N-benzoyl-4-hydroxy-3,5,6-tri-t-butoxy-2-pyridone (16). The crystal structure of (15) has been determined by X-ray diffraction. The formation of (15) and (16) is explained in terms of the intermediacy of N-benzoyl-3,4-di-t-butoxyazet-2(1H)-one (8) in equilibrium with the corresponding iminoketene (17). The equilibria between some model azet-2-ones (20)–(23) and N-formylazet-2-ones (24)–(27) and the corresponding iminoketenes (28)–(35), as well as the conversion of the N-formyliminoketenes (32)–(35) into the 1,3-oxazin-6-ones (40)–(43), have been studied by the MNDO procedure. The results of this study suggest the impossibility of preparing N-acylazet-2-ones by [2 + 2] cycloadditions of acetylenes with acyl isocyanates.