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 de nsity functional theoretical levels with the 6-31G basis set have bee n used to obtain the optimized structures of 2,5-dioxabicyclo[2.2.2]oc tane-3,6-dione (1) and its 1,4-dimethyl derivative (2) which reproduce the available experimental (X-ray analysis) data for 2 reasonably wel l. Molecular mechanics calculations with MM2(91) and MM3(92) force fie lds of equilibrium structures for 1, 2 are also given. According to al l 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. Investigatio n 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 stabilizat ion of the single form of dilactones 1, 2 are considered. Experimental coupling constants ((3)J(HH)) Of dilactones 1, 2 are also consistent with a single conformer in solution.