Syntheses of Pyrroloquinoline Quinone Derivatives: Model Compounds of a Novel Coenzyme PQQ (Methoxatin)

Pyrroloquinoline quinone derivatives were synthesized as model compounds of a novel coenzyme PQQ. The 2-mono- and 2,9-dicarboxylic acid derivatives 1b and 1d and the 1-methylated derivative If were synthesized relatively easily from the intermediates 5 and 4 of the PQQ-synthesis. The 2, 7-dicarboxylic acid derivative 1e was also synthesized via a combination of the Japp-Klingemann reaction and Fischer indolization.