Stereochemistry and Tautomerism of Amino Acid Antagonists: a,/-Diketo Acids and Esters and their Cyclisation Products

X-ray structural analyses of two a,y-diketo esters and one a,y-diketo acid, which are amino acid antagonists, have shown them all to be present in the solid state in the a,ßunsaturated y-keto enolic form. The structures of the esters /3-acetyl-pyruvic acid methyl ester (1) and 2-oxo-cyclopentyl-glyoxylic acid ethyl ester (3) are stabilized by intramolecular O---H(-O) hydrogen bonds involving the enol proton. In the case of camphor oxalic acid (4) an intramolecular O -H-O hydrogen bond between the y-keto oxygen and the acid proton is observed. The bond lengths and angles in 1 indicate a significant contribution from the mesomeric ß,y-unsaturated enol form. For comparison purposes the structure of the y-enol methyl ether of 1, (4-methoxy-2-oxo-pentene)carboxylic acid methyl ester (2) has also been determined. The X-ray structures of the cyclization products of respectively an a,y-diketo acid and an a,y-diketo ester are reported.