Mechanism of aromatic hydroxylation in fungi. Evidence for the formation of arene oxides

Abstract

Aromatic and aliphatic hydroxylations as well as U-demethylations occurred during metabolism of aromatic substrates by a range of fungi. para-Hydroxylation of monosubstituted aryl rings by fungi proceeded with migration and retention of deuterium label (NIH shift) and appeared to involve onlyarene 3.4-oxide intermediates, in common with plant and animal metabolism. The possibility of contributions from arene 1.2-or 2.3-oxide intermediates during orrho-hydroxylation is examined in the light of the results of oxygen-18 incorporation, the lack of deuterium-hydrogen primary kinetic isotope effects, and the wide range of deuterium migration and retention values. From these last experimental data no evidence for an arene oxide isomerase enzyme w a s found. istry, Queen'

Enzyme-catalysed synthesis and reactions of benzene oxide/oxepine derivatives of methyl benzoates

“…ortho-(5, R = Me, 11, 65% D) and para-substituted phenols (5 , R = OMe, 49, 71% D, Scheme 1). 5 These observations were consistent with the involvement of fungal monooxygenase (MO) enzymes and the intermediacy of arene oxides/oxepines 2/2 -4/4 , (Scheme 1). The benzene oxides (2-4) were assumed to be the initial metabolites existing in a state of rapid equilibration with the corresponding oxepine valence tautomers (2 -4 ) at ambient temperature.…”

“…Our previous studies of the fungal enzyme-catalysed aromatic hydroxylation of 2-D-and 4-D-labelled toluenes 1 (R = Me, X or Y = D) and anisoles 1 (R = OMe, X or Y = D) showed the characteristic migration and partial retention of the D label on a neighbouring carbon atom associated with the NIH Shift in the Scheme 1 Monooxygenase (MO)-catalysed epoxidation of substituted benzenes (1) to form 1,2-(2), 2,3-(3) and 3,4-benzene oxides (4) and the corresponding oxepines (2 , 3 , 4 ), and acid-catalysed rearrangement to yield the corresponding ortho-(5) and para-phenols (5 ).…”

“…It is probable that the biotransformation of methyl benzoates 1a-k, to the corresponding benzylic alcohols 7a-k, was initiated via enzymecatalysed hydrolysis to give the corresponding carboxylic acids, as previously reported for methyl benzoate substrate 1a in growing cultures of the fungi R. stolonifer and R. arrhizus. 5,6 In microbial systems, the reduction of carboxylic acids to the corresponding aldehydes and benzylic alcohols is well precedented (Scheme 4).…”

Biotransformation Reactions by Rhizopus spp.

“…Generally, ortho-hydroxylation was found to be preferred over para-hydroxylation [35]. Similar results of predominantly ortho-hydroxylation were observed for the incubation of chloroand bromobenzene with Rhizopus [36].…”

“…The hydroxylation of arenes proceeds via the intermediacy of arene oxides [35]. The pathway involves rearrangement of the arene oxide to the keto tautomer, which enolizes to the phenol.…”