Diethyl oxomalonate is the diethyl ester of mesoxalic acid (ketomalonic acid), the simplest oxodicarboxylic acid and thus the first member (n = 0) of a homologous series HOOC–CO–(CH2)n–COOH with the higher homologues oxalacetic acid (n = 1), α-ketoglutaric acid (n = 2) and α-ketoadipic acid (n = 3) (the latter a metabolite of the amino acid lysine). Diethyl oxomalonate reacts because of its highly polarized keto group as electrophile in addition reactions and is a highly active reactant in pericyclic reactions such as the Diels-Alder reactions, cycloadditions or ene reactions. At humid air, mesoxalic acid diethyl ester reacts with water to give diethyl mesoxalate hydrate and the green-yellow oil are spontaneously converted to white crystals.