Arenes large-scale oxidations

What is remarkable, however, is the stereochemical influence of a 13-hydroxyl group, p-hydroxycarbocations such as 31 are formed not only from arene oxide as precursors but from arene dihydrodiols. As shown for the parent benzene dihydrodiols in Scheme 23, arene dihydrodiols exist as cis-and /ra/rv-isomers. The m-isomers are obtained as products of the action on the aromatic molecule of dioxygenase enzymes and have been prepared on a large scale by fermentation.92 The trans-isomers are normally accessible by straightforward synthesis, for example, from the arene oxide. Both isomers undergo acid-catalyzed dehydration to the parent aromatic molecule, as is also shown in Scheme 23. It is clear that their reactions should involve a common carbocation intermediate,163 164 and in so far as there is little difference in the stabilities of the isomers,165 their difference in reactivities might have been expected to be small. 

The oxidation of phenols to catechols or hydroquinones by tyrosinase enzymes has been developed for biocatalysis. For example, the ortho-hydroxylation of L-tyrosine 162 (and also substituted variants) to give l-DOPA 163 has been extensively studied due to the importance of l-DOPA in the treatment of Parkinson s disease [92, 93]. An arene hydroxy lating enzyme having a broad substrate scope is 2-hydroxybiphenyl 3-monooxygenase from Pseudomonas azelaica, which is able to oxidize many ortho-substituted phenols 68 to the corresponding catechols 127 [94], as shown in Scheme 32.19. A notable example of an industrial biocatalytic arene hydroxylation that has been employed on very large scale (lOOm fermentation) is the pora-hydroxylation of R)-2-phenoxypropionic acid 164 by whole cells of Beauveria bassiana Lu 700 to give (R)-2-(4-hydroxyphenoxy)propionic acid 165, an important intermediate in herbicide manufacture [95]. 

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