:For a synthetic analogue see also 3,5-Difluoro-4-hydroxybenzylidene imidazolinone.
Mechanistically, the process involves base-mediated cyclization followed by dehydration and oxidation. In the reaction of 7a to 8 involves the formation of an enamine from the imine, while in the reaction of 7b to 9 a proton is abstracted. The formed HBI fluorophore is highlighted in green.
The reactions are catalyzed by residues Glu222 and Arg96. An analogous mechanism is also possible with threonine in place of Ser65.