Trimerisation of indole through laccase catalysis

Indole is cleanly bioconverted into a trimer upon laccase action. The formed compound has been identified as 2,2-bis(3′-indolyl)indoxyl (1) by X-ray diffraction study. Optimisation of the reaction through the use of dioxygen overpressure and TEMPO as mediator allowed a yield of more than 50% of isolated product for 1. 2,2-Bis(3′-indolyl)indoxyl is a natural compound that has been isolated from bacterial sources. However, this is the first report on a clearly identified enzyme that could be involved in its (bio)synthesis. A mechanism, based on the initial formation of a transient indole hydroperoxide, is proposed to account for the laccase catalysed synthesis of 1.