Crystal structure and identification of resonance forms of diethyl 2-(3-oxoiso-1,3-di­hydro­benzo­furan-1-yl­idene)malonate

The reaction of diethyl malonate with phthaloyl chloride in aceto-nitrile in the presence of tri-ethyl-amine and magnesium chloride results in the formation of the title compound, diethyl 2-(3-oxo-1,3-di-hydro-2-benzo-furan-1-yl-idene)propane-dioate, C15H14O6. One of the ester groups of the diethyl malonate fragment is almost coplanar with the isobenzo-furan unit, while the plane of the other group is perpendicular to it [dihedral angles = 5.45 (3) and 83.30 (3)°, respectively]. The C-C and C-O distances both in the heterocyclic furan ring and the diethyl malonate fragment are indicative of the dipolar delocalization occurring within the isobenzo-furan unit. This delocalization is likely to be responsible for the unusual inter-molecular O⋯O contact [2.756 (2) Å], established between the O atom of the furan ring and the carbonyl O atom of the diethyl malonate fragment. In the crystal, weak C-H⋯O inter-actions are observed, which link the mol-ecules into [100] chains.