Formation of pyridinium betaines by reaction of hexoses with primary amines

The reaction of glucose or fructose with primary amines and the thermal degradation of Amadori products of glucose leads to the formation of 6-formyl-1-alkylpyridinium-3-olates (5), of which 6-formyl-1-propylpyridinium-3-olate was isolated from a reaction mixture. A reference substance can be prepared in a two-step procedure from 5-(hydroxymethyl)-2-furaldehyde. Comparatively high yields of these betaines in relation to other UV-absorbing products are obtained under typical food processing and physiological conditions. With primary amines, the betaines readily form a Schiff base that undergoes Strecker-type degradation when heated in aqueous solution.