2,5-Dioxybicyclo[2.2.2]octane-3,6-diones. A conformational study by ab initio molecular orbital methods and molecular mechanics calculations

Ab initio molecular orbital calculations at the RHF, MP2 and hybrid density functional theoretical levels with the 6-31G^* basis set have been used to obtain the optimized structures of 2,5-dioxabicyclo[2.2.2]octane-3,6-dione (1) and its 1,4-dimethyl derivative (2) which reproduce the available experimental (X-ray analysis) data for 2 reasonably well. Molecular mechanics calculations with mm2(91) and mm3(92) force fields of equilibrium structures for 1, 2 are also given. According to all calculations the [2.2.2] frame structure of (1R,4R)-dilactones 1 and 2 corresponds to a single synchro( +, +, + )-twist-form (A) which is in agreement with the experimental crystal structure for 2. Investigation of the torsional energy surface of 1 by the mm2(91) method does not suggest the existence of any other structure than the synchro( +, +, + )-twist-A-conformer. The factors that determine the relative stabilization of the single form of dilactones 1, 2 are considered. Experimental coupling constants (³J_HH) of dilactones 1, 2 are also consistent with a single conformer in solution.