3-Methyl-ideneoxolane-2,5-dione.

The title compound (itaconic anhydride), C5H4O3, consists of a five-membered carbon-oxygen ring in a flat envelope conformation (the unsubstituted C atom being the flap) with three exocyclic double bonds to two O atoms and one C atom. In contrast to the bond lengths, which are very similar to those in itaconic acid in its pure form or in adducts with other mol-ecules, the bond angles differ significantly because of the effect of ring closure giving rise to strong distortions at the C atoms involved in the exocyclic double bonds. In the crystal, C-H⋯O inter-actions link the mol-ecules, forming an extended three-dimensional network.