Mechanistic study of the aldol condensation occurring in the alkaline solution of 3-hydroxy 2-oxo propranoate (β hydroxypyruvate)

With 3-bromo-2-oxopropanoate (β bromopyruvate) and its ethyl ester, the ionisation of the gemdiol of the hydrated form BrCH₂-C(OH)₂-COOR 1 initiates the elimination of bromide anion yielding 3-hydroxy-2-oxopropanoate 2 (β-hydroxypyruvate). The mechanism of the reaction was investigated essentially by polarography in aqueous solution.In neutral (and acid) media, the polarographic behaviour of 2 resembled that of other α-ketoacids: reduction at the mercury electrode yielded glycerate.In alkaline media, there was evidence of the carbanion enolate ^-O—CHC(OH)—COO^-. 3'. The overall rate constant was determined according to a kinetic law of the typer:v = k.¦2¦.¦OH^-¦; found k = 1,56 min^-1 in NaOH 0.5 N at 25°.In the pH range 10.5 to 11.5,3' existed in minor amounts and initiated a slow aldol condensation with the tautomer 3-oxo-2-hydroxypropanoate 4 (tartronate Semialdehyde) according to a kinetic law of the type v = k?.¦2¦².¦OH^-¦found k? = 211.mol^-1 min ^-1 at 25° , at pH 11.0.The aldol product was isolated as a sodium sail and its structure established by ¹³C NMR.