A COMPUTATIONAL STUDY OF THE THERMAL CYCLOREVERSION OF 2,2,6-TRIMETHYL-4H-1,3-DIOXIN-4-ONE AND A RELATED SPECIES - RETRO-DIELS-ALDER REACTION OR CONCERTED NUCLEOPHILIC-ATTACK

Calculations up to the MP2/6-31G//HF/3-21G level have been carried ou t to study the thermal cycloreversion of 2,2,6-trimethyl-4H-1,3-dioxin -4-one (1) and 2,2,4-trimethyl-6H-1,3-oxazin-6-one (3). At this level of calculation, the enthalpy of activation for the thermal cyclorevers ion of dioxinone 1 was found to be 31.3 kcal/mol with zero-point vibra tional energy correction. The experimental value for the reaction in s olution is 30.4 kcal/mol. The enthalpy of activation for the as of yet unsynthesized oxazinone 3 was found to be 44.2 kcal/mol with zero-poi nt correction. Contrasting with the standard view that the thermal cyc loreversion of dioxinone 1 is a retro-Diels-Alder reaction is the stru ctural information from the HF/3-21G-optimized transition state 5 whic h shows the plane of the acetone dienophile tilted roughly 90 degrees to the plane of the acylketene diene.