(αE)-α-[(3,4-Dihydroxy-5-nitrophenyl)methylene]-β-oxo-1-piperidinepropanenitrile is an impurity of Entacapone (E558500), a peripherally acting inhibitor of catechol-O-methyl transferase (COMT), an enzyme involved in the metabolism of catecholamine neurotransmitters and related drugs. Antiparkinsonian.