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

X-ray structural analyses of two α,γ-diketo esters and one α,γ-diketo acid, which are amino acid antagonists, have shown them all to be present in the solid state in the α,β-unsaturated γ-keto enolic form. The structures of the esters β-acetyl-pyruvic acid methyl ester (1) and 2-oxo-cyclopentyl-glyoxylic acid ethyl ester (3) are stabilized by intra-molecular 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 γ-keto oxygen and the acid proton is observed. The bond lengths and angles in 1 indicate a significant contribution from the mesomeric β,γ-unsaturated enol form. For comparison purposes the structure of the γ-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 α,γ-diketo acid and an α,γ-diketo ester are reported.