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The reactions of a TTQ model compound [1, 3-methyl-4-(3'-methylindol-2'-yl)indole-6,7-dione] with several amines have been investigated in organic media to obtain mechanistic information on the action of quinoprotein methylamine and aromatic amine dehydrogenases. It has been found that compound 1 acts as an efficient catalyst for the autorecycling oxidation of benzylamine by molecular oxygen in CH(3)OH. In order to evaluate the oxidation mechanism of amines by 1, the product analyses and kinetic studies have been carried out under anaerobic conditions. In the first stage of the reaction of 1 with amines, 1 is converted into an iminoquinone-type adduct (so-called substrateimine), which was isolated and characterized by using cyclopropylamine as a substrate. The observed NOE of the isolated product indicates clearly that the addition position of the amine is C-6 of the quinone. The molecular orbital calculations suggest that the thermodynamic stability of the carbinolamine intermediate is a major factor to determine such regioselectivity; the C-6 carbinolamine is more stable than the C-7 counterpart by 2.9 kcal/mol. The reactivity of several primary amines and the electronic effect of the p-substituents of benzylamine derivatives in the iminoquinone formation suggest that the addition step of the amine to the quinone is rate-determining. When amines having an acidic alpha-proton such as benzylamine derivatives are employed as substrates, formation of the iminoquinone adduct was followed by rearrangement to the productimine. The kinetic analysis has revealed that this rearrangement consists of noncatalyzed and general base-catalyzed processes. Large kinetic isotope effects of 7.8 and 9.2 were observed for both the noncatalyzed and general base-catalyzed processes, respectively, since these steps involve a proton abstraction from the alpha-position of the substrate. In the reaction with benzhydrylamine, the product imine was isolated quantitatively and well characterized by several spectroscopic data. In the case of benzylamine, the product imine is further converted into the aminophenol derivative by the imine exchange reaction with excess benzylamine. These results indicate clearly that the amine oxidation by compound 1 proceeds via a transamination mechanism as suggested for the enzymatic oxidation of amines by TTQ cofactor.

3,4-Disubstituted 6,7-indolequinones [1,3-dimethyl-4-(3‘-methylindol-2‘-yl)indole-6,7-dione (2), 3-methyl-4-phenylindole-6,7-dione (3), and 3,4-dimethyl-6,7-dione (4)] and a 3,7-disubstituted 4,5-indolequinone [3,7-dimethylindole-4,5-dione (5)] have been synthesized as models for the novel organic cofactor TTQ of bacterial amine dehydrogenases. The substituent and structural effects on the physicochemical properties of the quinones have been investigated in detail by comparing the spectroscopic data (UV−vis, IR, 1H- and 13C-NMR), pK a values of the pyrrole proton, and the two-electron redox potentials with those of model compound 1 [3-methyl-4-(3‘-methylindol-2‘-yl)indole-6,7-dione] previously reported (ref ). Reactivity of each quinone in the transamination process [iminoquinone formation (k 1), rearrangement to product−imine (k 2), and aminophenol formation (k 3)] has been investigated kinetically, revealing that the substituent and structural effects on the amine-oxidation reaction are not so significant. In the aerobic catalytic oxidation of benzylamine, however, the aromatic substituents on the quinone ring play an important role to protect the quinone from the deactivation process of a Michael-type addition by the amine, making it act as an efficient turnover catalyst.

Dimethyl 9,10-dihydro-9,10-dioxobenzo[/]quinoline-2,4-dicarboxylate (4) was synthesized, and its physical and chemical properties were compared to those of the trimethyl ester of PQQ (PQQTME,2) and the 1-methyl derivative (3). The synthesis of 4 was accomplished by a Doebner-von Miller-type annulation between 3-amino-2-naphthol and dimethyl 2-oxoglutaconate and a subsequent oxidation with Fremy's salt. The electronic effect of the pyrrole nucleus of coenzyme PQQ (1) was examined by comparing the reactivity of 4 to that of 2 and 3 in the acetone adduct formation reaction, the redox reaction with phenylhydrazine, and the aerobic autorecycling oxidation of benzylamine. The significant role of the pyrrole nucleus in conducting the intramolecular general base catalysis in the amine oxidation is discussed.

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