A gene cluster encoding the biosynthesis of the fungal tropolone stipitatic acid was discovered in Talaromyces stipitatus ( Penicillium stipitatum ) and investigated by targeted gene knockout. A minimum of three genes are required to form the tropolone nucleus: tropA encodes a nonreducing polyketide synthase which releases 3-methylorcinaldehyde; tropB encodes a FAD-dependent monooxygenase which dearomatizes 3-methylorcinaldehyde via hydroxylation at C-3; and tropC encodes a non-heme Fe(II)-dependent dioxygenase which catalyzes the oxidative ring expansion to the tropolone nucleus via hydroxylation of the 3-methyl group. The tropA gene was characterized by heterologous expression in Aspergillus oryzae , whereas tropB and tropC were successfully expressed in Escherichia coli and the purified TropB and TropC proteins converted 3-methylorcinaldehyde to a tropolone in vitro. Finally, knockout of the tropD gene, encoding a cytochrome P450 monooxygenase, indicated its place as the next gene in the pathway, probably responsible for hydroxylation of the 6-methyl group. Comparison of the T. stipitatus tropolone biosynthetic cluster with other known gene clusters allows clarification of important steps during the biosynthesis of other fungal compounds including the xenovulenes, citrinin, sepedonin, sclerotiorin, and asperfuranone.

A new biosynthetic pathway to the sorbicillinoid natural products is proposed based on the observation of oxidative dearomatisation of dihydrosorbicillin 10b.

A series of directed knockout experiments, combined with an in vitro assay of pathway components, has elucidated for the first time the chemical steps involved in the biosynthesis of the tropolone class of fungal maleic anhydrides. The pathway involves the stepwise oxidation of aldehyde and methyl carbon atoms to form a 1,2-dicarboxylate. A hydrolase-catalyzed interconversion of this and the corresponding maleic anhydride, followed by decarboxylation of the diacid leads to the pathway's final product of stipitatic acid.

A series of directed knockout experiments, combined with an in vitro assay of pathway components, has elucidated for the first time the chemical steps involved in the biosynthesis of the tropolone class of fungal maleic anhydrides. The pathway involves the stepwise oxidation of aldehyde and methyl carbon atoms to form a 1,2-dicarboxylate. A hydrolasecatalyzed interconversion of this and the corresponding maleic anhydride, followed by decarboxylation of the diacid leads to the pathways final product of stipitatic acid.

ABSTRACT. The ground state neutral geometries of some natural tropolones, i.e. stipitatonic acid (AF1), stipitalide (AF2), stipitaldehydic acid (AF3) and methyl stipitate (AF4) have been optimized by using Density Functional Theory (DFT) at B3LYP/6-31G*, B3LYP/6-31G**, B3LYP/6-31+G* and B3LYP/6-31+G** levels of theory. The excited state geometries of AF1-AF4 were optimized by adopting the Time Dependent Density Functional Theory (TDDFT) at the same levels of theory. The frequencies and cation species of AF1-AF4 were also computed at all the above mentioned levels of theory. We shed light on the electro-optical and molecular properties, e.g. energy gaps, highest occupied molecular orbitals, lowest unoccupied molecular orbitals, absorption wavelengths, electronegativity (χ), hardness (η), electrophilicity (ω), softness (S), electrophilicity index (ωi) and the radical scavenging activity (RSA). Hydrogen atom transfer (HAT) and one-electron transfer mechanisms have been discussed to shed light on the RSA. The smallest ionization potential and bond dissociation energy of AF4 are revealing that this compound would have more RSA than those of other counterparts.