(2S,4E)-4-(2-oxoethylidene)-2,3-dihydro-1H-pyridine-2,6-dicarboxylic acid (2S,4E)-4-(2-ketoethylidene)-2,3-dihydro-1H-pyridine-2,6-dicarboxylic acid 18766-66-0 C08538 Betalanic acid Betalamic acid