Ketone-phenol reactions and the promotion of aromatizations by food phenolics

Abstract

Heating of either 3,5-heptadien-2-one or 2,6-heptanedione in the presence of ammonia produced 2,6-dimethylpyridine, and also 3-methylcyclohex-2-en-1-one for the second ketone. When phenolics were present, inhibition of pyridine formation was only observed in mixtures of 3,5-heptadien-2-one and resorcinol. This inhibition was due to the formation of ketone-resorcinol adducts, which were isolated and identified by nuclear magnetic resonance (NMR) and mass spectrometry (MS) as 2,4-dimethyl-5,6-dihydro-4H-2,6-methanobenzo[d][1,3]dioxocin-9-ol and 1-(7-hydroxy-4-methylchroman-2-yl)propan-2-one. The other assayed phenolics increased pyridine formation. This increase was mainly observed in the presence of oxygen, at slightly basic pH values, depended on time, temperature, and the phenolic concentration, and had an activation energy of 56.8 kJ/mol for the formation of 2,6-dimethylpyridine from 2,6-heptanedione in the presence of orcinol. This increase was a consequence of the promotion by phenolics of a required aromatization step in the pyridine formation pathway. This phenolic function needs to be considered when phenolics are added to food products.