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

Abstract

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 mm 2(91) and mm 3(92) force fields of equilibrium structures for 1 , 2 are also given. According to all calculations the [2.2.2] frame structure of (1 R ,4 R )-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 mm 2(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 ( 3 J HH ) of dilactones 1 , 2 are also consistent with a single conformer in solution.