Oxidative decarboxylation of 3,4-dihydroxymandelic acid to 3,4-dihydroxybenzaldehyde: electrochemical and HPLC analysis of the reaction mechanism.

Cyclic voltammetric and chronoamperometric data are consistent with a process in which 3,4-dihydroxymandelic acid (DOMA) is oxidized initially in a two-electron step to its corresponding o-benzoquinone. This species is unstable and undergoes the rate-determining loss of CO2 (k = 1.6 s-1 at pH 6 and 25 degrees C) to give an unobserved p-benzoquinone methide intermediate that rapidly isomerizes to 3,4-dihydroxybenzaldehyde (DOBAL), DOBAL is also electroactive at the applied potential and is oxidized in a two-electron step to 4-formyl-1,2-benzoquinone. Subsequent reactions of 4-formyl-1,2-benzoquinone include the oxidation of unreacted DOMA and the hydration of its aldehyde functional group. Oxidation of DOMA directly to its p-benzoquinone methide apparently does not occur. Derivatives of mandelic acid (e.g., 4-hydroxymandelic acid) that are expected to give only their corresponding p-benzoquinone methides upon oxidation afford redox behavior that differs distinctly from that for DOMA.

Electrochemical oxidation of N-acyldopamines and regioselective reactions of their quinones with N-acetylcysteine and thiourea.

The metabolism of catechols often involves their oxidation to quinones and subsequent nucleophilic addition reactions with sulfur-containing compounds. Adducts formed during these reactions may play important roles in many biological systems. We have studied the electrochemical oxidation of N-acetyldopamine (NADA) and N-beta-alanyldopamine (NBAD) in the presence of two sulfur-centered nucleophiles, N-acetylcysteine (NACySH) and thiourea (TU), and have characterized the adducts and reaction pathways. NADA and NBAD react similarly, but their adducts with NACySH and TU were formed regioselectively. NACySH yields mainly 5-adducts and TU only 6-adducts. The NACySH adducts are oxidized more easily than the parent N-acyldopamine, and their oxidations are chemically reversible. However, the TU adducts are more difficult to oxidize, and their oxidation products undergo further chemical reactions. An intramolecular base catalysis mechanism for adduct formation with NACySH is proposed, which facilitates removal of the proton from the sulfhydryl group of NACySH and directs formation of the 5-adduct via a 1,6-Michael addition reaction. The absence of a proton on the thioureylene sulfur atom leads to formation of the 6-thioureylene adduct via a 1,4-Michael addition reaction of TU. This mechanism is consistent with the formation of other sulfur-centered adducts of catechols previously reported in the literature.

On-column reduction of catecholamine quinones in stainless steel columns during liquid chromatography.

The chromatographic behavior of quinones derived from the oxidation of dopamine and N-acetyldopamine has been studied using liquid chromatography (LC) with both a diode array detector and an electrochemical detector that has parallel dual working electrodes. When stainless steel columns are used, an anodic peak for the oxidation of the catecholamine is observed at the same retention time as a cathodic peak for the reduction of the catecholamine quinone. In addition, the anodic peak exhibits a tail that extends to a second anodic peak for the catecholamine. The latter peak occurs at the normal retention time of the catecholamine. The origin of this phenomenon has been studied and metallic iron in the stainless steel components of the LC system has been found to reduce the quinones to their corresponding catecholamines. The simultaneous appearance of a cathodic peak for the reduction of catecholamine quinone and an anodic peak for the oxidation of the corresponding catecholamine occurs when metallic iron in the exit frit reduces some of the quinones as the latter exits the column. This phenomenon is designated as the "concurrent anodic-cathodic response." It is also observed for quinones of of 3,4-dihydroxybenzoic acid and probably occurs with o- or p-quinones of other dihydroxyphenyl compounds. The use of nonferrous components in LC systems is recommended to eliminate possible on-column reduction of quinones.

Characterization of products from the reactions of N-acetyldopamine quinone with N-acetylhistidine.

When insects harden or sclerotize their exoskeletons, quinones of N-acetylated catecholamines such as N-acetyldopamine (NADA) undergo nucleophilic addition reactions with amino acids such as histidine in cuticular proteins. To determine the products that might form when this type of reaction occurs during cuticle sclerotization, the reactions between electrochemically prepared NADA quinone and N-acetylistidine (NAcH), a protein model nucleophile, have been investigated at pH 7. Two major products, 6-[N-(N-acetylhistidyl)]-N-acetyldopamine and 2-[N-(N-acetylhistidyl)]-N-acetyldopamine, were purified by semipreparative reversed-phase liquid chromatography and identified by mass spectrometry and nuclear magnetic resonance spectroscopy. The relative molar ratio of the C(6) mono-addition adduct to the C(2) mono-addition adduct is 87:13. UV/vis spectroscopic analysis shows that both products have an absorption maximum at 284 nm. Cyclic voltammetry shows that these adducts are oxidized less readily than NADA.