2-Oxopropanal, Hydroxy-2-propanone, and 1-Pyrroline - Important Intermediates in the Generation of the Roast-Smelling Food Flavor Compounds 2-Acetyl-1-pyrroline and 2-Acetyltetrahydropyridine

On the basis of labeling experiments with [¹³C]₆-glucose/unlabeled proline as well as quantitative data obtained in model studies using stable isotope dilution assays, 1-pyrroline and hydroxy-2-propanone were identified as effective intermediates in generating the roast-smelling food odorant 2-acetyltetrahydropyridine (ATHP; two tautomers). Synthesis of the key precursor compound, 2-(1-hydroxy-2-oxo-propyl)pyrrolidine, and studies on its degradation confirmed the important role of this intermediate in ATHP formation. Boiling of the intermediate for 30 min in aqueous solution generated >30% of ATHP on a molar basis. 1-Pyrroline and 2-oxopropanal were confirmed as important intermediates in the generation of the further roast food odorant, 2-acetyl-1-pyrroline (AP). On the basis of results of labeling experiments with [¹³C]₆-glucose/unlabeled proline, two different mechanisms could be proposed. One leads to AP via 2-(1,2-dioxopropyl)pyrrolidine as the precursor with elimination of the aldehyde group in 2-oxopropanal as carbon dioxide. The other one suggests elimination of carbon-2 of the pyrroline ring. The latter mechanism was further established by a result showing that from the reaction of 2-methyl- 1-pyrroline with 2-oxopropanal AP was also generated.