CH2=C(COOH)CH2.COOH

from The Collaborative International Dictionary of English v.0.48

itaconic acid \it`a*con"ic ac"id\, n. (Chem.)
   The unsaturated dicarboxylic acid {CH2=C(COOH)CH2.COOH}, also
   called {methylenesuccynic acid} and {propylene dicarboxylic
   acid}.
   [PJC]