Synthesis of Isomeric Analogues of Coenzyme Pyrroloquinoline Quinone (PQQ)

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Abstract
Three isomeric analogues 2-4 of the redox-active coenzyme 4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylic acid (1, PQQ, methoxatin) were synthesized from methoxynitroanilines 8, 9 and 10a, respectively. Reaction of the diazonium salts of each of the starting compounds with ethyl α-methylacetoacetate gave the corresponding substituted phenylhydrazones of ethyl pyruvate. These intermediates underwent acid-catalyzed Fischer indolization and gave the esters 13, 17 and 25, respectively. Reduction to the corresponding aminoindoles with hydrogen over Pd/C, followed by Doebner-von Miller quinoline synthesis with dimethyl trans-2-ketoglutaconate, oxidation of the intermediate methoxy compound to the o-quinone, and hydrolysis of triester products gave 2, 3, and 4, respectively. These isomers will serve as authentic examples to define their possible formation in nature and will also serve as isosteric probes to define the binding of PQQ at active sites in PQQ-requiring quinoproteins.

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