p,p'-Dimethoxydibenzoylmethane 1 combines with oxalyl chloride 2 to yield novel furan-2,3-dione 3. Thermal transformation is supposed to occur by the ring opening of the 3 followed by the intermediate p,p'-dimethoxydibenzoylketene IN1 cyclization resulting in the gamma -pyrone IN3. The reaction of 3 with Schiff bases under thermolysis conditions leads to 1,3 oxazinones 5,6. The reaction pathways include formal [4+2] cycloaddition processes, obviously, Electronic structures of reactants, transition states, intermediates and final products of the reaction are calculated by the AM1 method. Transition states was further confirmed by vibrational analysis (computation of force constants analytically) and characterised by the corresponding imaginary vibration modes and frequencies.