Conformational Studies of Symmetric Diesters

Conformational studies of symmetric diesters, dimethyl maleate, dimethyl 5,6-isopropylidenedioxybicyclo[2.2.1]hepta-2-ene- 2,3-dicarboxylate, and dimethyl succinate have been performed. It was found that the conformation wherein one of the carbonyl oxygens interacts with the carbon of the other carbonyl is the most stable in all the cases. In particular, in dimethyl maleate and dimethyl 5,6- isopropylidenedioxybicyclo[2.2.1]hepta-2-ene-2,3-dicarboxylate, it was found that the structures wherein one of the carbonyl groups is oriented near-perpendicular with respect to the C=C bond and the other carbonyl group is oriented near-parallel with respect to the C=C bond have the lowest energies, showing good agreement with the X-ray crystal structures. Bonding interaction between the carbonyl oxygen and the other carbonyl carbon was also found in all these structures. These results suggest the existence of the n->π* interaction between the two carbonyl groups and are expected to provide insight into the non-covalent interaction between the two carbonyl groups in these symmetric diesters.