Cycloaddition reactions of conjugated dienoic carboxylic acids and esters with N-substituted maleimides and Schiff bases

Sorbic acid and its esters were investigated as potential dienes in Diels-Alder reactions with N-substituted maleimides. The purpose was to characterise such Diels-Alder adducts by means of NMR, and to provide reference compounds to identify the formation of related products in model food systems. These reactions occur both in aqueous and non-aqueous systems and it is possible that the C=N bond of Schiff bases could act as a dienophile towards sorbic acid. In general, the H-1 chemical shifts for the olefinic protons, originally at C-3 and C-4 of sorbic acid, are to be found at delta 5.7-5.9 and 6.3-6.5, respectively in the adduct. Those at C-2, C-5 and C-6 are at delta 2.4-2.6, 3.4-4.2 and 1.4-1.5, respectively. All these values are characteristically different from the corresponding proton chemical shifts in sorbic acid. (C) 1997 Elsevier Science Ltd.