Absolute configuration of natural cyclohexene oxide by time dependent density functional theory calculation of the rotation: the absolute configuration of (-)sphaeropsidone and (-)episphaeropsidone revised

The optical rotatory power of some natural cyclohexene oxides, such as (+)-chaloxone, I, (+)-epiepoformin, II, (+)-epoformin, III, (+)-epoxidone, V, (-)-sphaeropsidone, VI, (-)-episphaeropsidone, VII, and the synthetic compd. (+)-epitheobroxide, IV, has been calcd. by means of the TDDFT/B3LYP method using the 6-31G(d) and aug-cc-pVDZ basis sets, both in the gas phase and in soln. by means of the polarizable continuum model. For compds. I and II, which possess high (about 300 units) optical rotations, gas-phase calcns. with the smaller basis set are able to reproduce the exptl. values both in sign and order of magnitude. By contrast, a larger basis set is required to satisfactorily simulate the OR values of III and IV, which show smaller (about 100 units or less) rotations. The inclusion of the solvent effects is different for different compds.; for I and II, it leads to a better agreement between expt. and prediction, while for III and IV, the presence of hydrogen bonding groups makes the application of continuum solvation models less satisfactory. For the flexible system V, the abs. configuration could not be detd. using gas-phase calcns. and the smaller basis set, but both inclusion of solvent and larger basis set effects are compulsory. It is noteworthy that calcns. both in the gas phase and in the solvent lead to a pos. rotatory power for the levorotatory natural compds. VI and VII if the abs. configurations reported in the literature are employed to do the theor. prediction. This strongly indicates that the ACs previously assigned to these compds. in the literature are not correct and that the TDDFT prediction of OR values has become by now a practicable tool for AC assignments.