Synthesis of 7,9-didecarboxymethoxatin (4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2-carboxylic acid) and comparison of its chemical properties with those of methoxatin and analogous o-quinones. Model studies directed toward the action of PQQ requiring bacterial oxidoreductases and mammalian plasma amine oxidase