Three mechanistic probes were used to investigate whether the Criegee rearrangement step of catechol 1,2-dioxygenase (CatA) from Acinetobacter sp. proceeds via a direct 1,2-acyl migration, via homolytic O-O cleavage, or via a benzene oxide-oxepin rearrangement. Incubation of CatA with 3-chloroperoxybenzoic acid led to the formation of a 9:1 mixture of 2-chlorophenol and 3-chlorophenol, via a mechanism involving O-O homolytic cleavage. Incubation of CatA with 2-hydroperoxy-2-methylcyclohexanone led to formation of 5,6-diketoheptan-1-ol, also consistent with an O-O homolytic cleavage mechanism, and not consistent with a direct 1,2-acyl migration. No reaction product was isolated from incubation of CatA with 6-hydroxymethyl-6-methylcyclohexa-2,4-dienone, an analogue that is able to undergo the benzene oxide-oxepin rearrangement, but not able to undergo O-O homolytic cleavage. In contrast, incubation of extradiol dioxygenase MhpB from Escherichia coli with 6-hydroxymethyl-6-methylcyclohexa-2,4-dienone led to the formation of a 2-tropolone ring expansion product, consistent with a direct 1,2-alkenyl migration for extradiol cleavage. Taken together, the results imply different mechanisms for the Criegee rearrangement steps of intradiol and extradiol catechol dioxygenases: a direct 1,2-alkenyl migration for extradiol cleavage and an O-O homolytic cleavage mechanism for intradiol cleavage.

Substituted 2-tropolone natural products are found in plants and fungi. Their biosynthesis is thought to occur by ring expansion from a cyclohexadienone precursor, but this reaction has not previously been demonstrated experimentally. Treatment of 6-hydroxy-6-hydroxymethylcyclohexa-2,4-dienone with the non-haem iron(II)-dependent extradiol catechol dioxygenase MhpB from Escherichia coli results in the formation of the 2-tropolone ring-expansion product through a pinacol-type rearrangement. Three further substituted cyclohexa-2,4-dienone analogues were prepared, and treatment of each analogue was found to give the substituted 2-tropolone ring-expansion product. This ring expansion could also be effected nonenzymatically by treatment with 1,4,7-triazacyclononane and FeCl(2). This is a novel transformation for non-haem iron-dependent enzymes, and this is the first experimental demonstration of the proposed ring-expansion reaction in tropolone biosynthesis.