Prepd by hydrolyzing sodium diethyloxalacetate with concd HCl: C. Heidelberger, R. B. Hurlbert, J. Am. Chem. Soc. 72, 4704 (1950); C. Heidelberger, Biochem. Prep. 3, 59 (1953). Structural studies of the enol form: D. H. Flint et al., J. Org. Chem. 57, 7270 (1992).