Synthesis and physico-chemical properties of nitrocaffeic acids

The synthesis and spectroscopic properties of the three isomers of nitrocaf feic acid are described. The three pK(a) s of each isomer were measured by UV-visible spectroscopy. The comparison of the UV-visible spectra of nitroc affeic acids and those obtained from the reaction of caffeic acid with reac tive nitrogen species led to the conclusion that the nitration of caffeic a cid with acidic nitrite does not significantly occur and confirmed the abse nce of nitration when caffeic acid reacts with peroxynitrite. Attempts to o btain free radical species from nitrocaffeic acids by classical methods sho wed a different reactivity to that of nitroaromatics and catechols. Nitroca ffeic acids do not autoxidize under aqueous basic conditions and are insens itive to t-BuOK or O-2(-.) (two reactants known for their capabilities to o xidize catechols and reduce nitroaromatics). Nitroaromatic anion radicals m ay be obtained using sodium borohydride as reductant and are particularly s table under an uncontrolled atmosphere. Copyright (C) 2000 John Wiley & Son s, Ltd.