Retro-Aldol and Redox Reactions of Amadori Compounds:  Mechanistic Studies with Variously Labeled <scp>d</scp>-[¹³C]Glucose

Abstract

Oxidation−reduction reactions necessary to justify many of the products observed in Maillard model systems are usually attributed to molecular oxygen and the so-called reductons. The proline specific 1-(1‘-pyrrolidinyl)-2-propanone and 1-(1‘-pyrrolidinyl)-2-butanone are such compounds that require reduction steps to justify their formation. Experimental evidence using glucose separately labeled at 13 C1, 13 C2, 13 C3, 13 C4, 13 C5, and 13 C6 indicates that 1-(1‘-pyrrolidinyl)-2-propanone is formed by two related pathways, initiated by a retro-aldol cleavage of proline Amadori compound at C3−C4, and 1-(1‘-pyrrolidinyl)-2-butanone is formed by three pathways, one initiated by a retro-aldol reaction at C2−C3 of the 1-(prolino)-1-deoxy-4-hexosulose (an isomer of Amadori product formed by carbonyl migration) and two others by similar retro-aldol reactions at C4−C5 from both 3-deoxyglucosone and 1-(prolino)-1,4-dideoxy-2,3-hexodiulose. All of the proposed mechanisms require reduction steps for the formation of the target compounds. Model studies have indicated that reductions in Maillard systems can be effected by three pathways: through hydride transfer from formic acid; through cyclic dimerization of α-hydroxy carbonyl compounds followed by electrocyclic ring opening to produce oxidation/reduction products; and by disproportionation of enediols with α-dicarbonyl compounds through double proton transfer. Keywords: Amadori; decomposition mechanisms; oxidation; reduction; Maillard reaction; 13 C-labeled glucoses; proline; morpholine