The thermal decomposition of 4-acyl substituted furan-2,3-diones (1a, b) leads to the alpha-oxo ketenes (2a, b) as an intermediate which undergoes nucleophilic addition reactions with amides. Some of alpha-N-acyl-oxo-amide derivatives (4a-h) obtained show keto-enol tautomerism. The reaction mechanism of ketene (R1) with formamide (R2) are calculated by AMI method. Transition states (TS1 and TS2) were further confirmed by vibrational analysis and characterized by the corresponding imaginary vibration modes and frequencies. According to calculations, intermediate (IN) is in the planar zwitterion structure. The transition structure (TS2) has almost a planar structure. Analysis of the molecular orbitals indicates that the reaction is best described as pseudopericyclic.