Reactivity on Photooxidation of Selected Five-Membered Heterocyclic Flavor Compounds

Abstract

Conjugated five-membered heterocyclic compounds including 2-ethylfuran, 2-furfural, 2-acetylfuran, 5-(hydroxymethyl)-2-furfural (HMF), 2-ethylthiophene, 2-formylthiophene, 2-acetylthiophene, 2-ethylpyrrole, 2-formylpyrrole, 2-acetylpyrrole, 2,4,5-trimethyloxazole, and 2,4,5-trimethylthiazole in absolute alcohol containing chlorophylls were bubbled with oxygen under light for 1 h. The reactivity of these heterocyclic compounds with singlet oxygen was studied using a high-performance liquid chromatographic method, in which the percentage of substrate remaining was determined. The photooxidation reactivity of these selected five-membered flavor compounds was as follows: 2-ethylpyrrole > 2-ethylfuran > 2-ethylthiophene; 2,4,5-trimethyloxazole > 2,4,5-trimethylthiazole; 2-ethylfuran > HMF > 2-acetylfuran > 2-furfural; 2-ethylpyrrole > 2-acetylpyrrole > 2-formylpyrrole. The electron density at C-1 and C-4 may explain the photooxidation reactivity of these five-membered heterocyclic compounds. Keywords: Five-membered heterocyclic compounds; singlet oxygen; photooxidation reactivity; chlorophyll