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 1H chemical shifts for the olefinic protons, originally at C3 and C4 of sorbic acid, are to be found at δ 5.7–5.9 and 6.3–6.5, respectively in the adduct. Those at C2, C5 and C6 are at δ 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.