Acid-Catalyzed Aldol Condensation of Pyruvate to Form Parapyruvate and Enol-Lactone Derivative

This shows a mechanism for the acid-catalyzed dimerization of pyruvate, and the dimer, which is parapyruvate, can undergo a reversible cyclization to a lactone that then tautomerizes to the enol lactone form that's the predominant tautomer. The structures of the lactone, which is supposedly the predominant species that's present initially and forms within 20 minutes or so, and parapyruvate, which predominates subsequently, at least under the conditions used by the authors, are shown here [Montgomery & Webb, 1956: (http://www.jbc.org/cgi/reprint/221/1/359.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/13345826)].