INITIAL STEPS OF THE METAL-CATALYZED DEGRADATION OF L-DEHYDROASCORBICACID IN ACIDIC AQUEOUS-SOLUTIONS

The initial steps of the degradation of L-dehydroascorbic acid (L-DHA) in acidic aqueous solutions and the catalytic effect of different tra nsition metal ions on this reaction were studied. The main product was identified as 3,6-furanosido-2,3-hexodiulosonic acid-2-hydrate (compo und I) by GLC-MS and C-13-NMR, formed by lactone hydrolysis and hydrat ion of the carbonyl group in the C-2 position of L-dehydroascorbic aci d. In addition, number of other compounds were detected; they are form ed from compound I by simple enolisation, lactonisation, hydration and dehydration reactions as well as by cleavage and formation of cyclic half acetal bonds. The chemical structures of these compounds were ten tatively deduced by the mass spectra of their TMS derivatives and a re action scheme for their formation is proposed. The velocity and the di rection of the reactions were found to be strongly influenced by the p resence of catalytic amounts of different transition metal ions. It wa s concluded that in acidic medium, in contrast to the situation in neu tral and alkaline solutions, the opening of the lactone ring of L-DHA is, to a certain degree, a reversible reaction.