Biomimetic Formation of 2‐Tropolones by Dioxygenase‐Catalysed Ring Expansion of Substituted 2,4‐Cyclohexadienones

Abstract

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.