Thermal Decomposition of Caffeic Acid in Model Systems: Identification of Novel Tetraoxygenated Phenylindan Isomers and Their Stability in Aqueous Solution

Caffeic acid subjected to mild pyrolysis (225−226 °C) under vacuum resulted in rapid decarboxylation and the formation of simple catechol monomers as well as more complex cyclocondensed dimers and polymers. This reaction yielded the same spectrum of products as did acid-catalyzed cyclization of caffeic acid. The major pyrolysis products were identified by reversed-phase HPLC and LC−tandem mass spectrometry. Two novel compounds, identified by MS, 1 H NMR, and 13 C NMR as 1,3- cis - and 1,3- trans -tetraoxygenated phenylindans, were present as major products in both the caffeic acid pyrolysate and the acid-treated sample. The stability and reactivity of the pyrolysis products in weakly buffered aqueous solutions were determined concomitantly by measuring hydrogen peroxide generation and by monitoring the concentration of the individual components by reversed-phase HPLC. Such model studies may provide information pertaining to reaction mechanisms and the nature of the compounds involved in hydrogen peroxide formation in coffee.