Dioxygenase-catalysed oxidation of disubstituted benzene substrates: benzylic monohydroxylation versus aryl cis-dihydroxylation and the meta effect

Abstract

Biotransformations of a series of ortho-, meta- and para-substituted ethylbenzene and propylbenzene substrates have been carried out, using Pseudomonas putida UV4, a source of toluene dioxygenase (TDO). The ortho- and para-substituted alkylbenzene substrates yielded, exclusively, the corresponding enantiopure cis-dihydrodiols of the same absolute configuration. However, the meta isomers, generally, gave benzylic alcohol bioproducts, in addition to the cis-dihydrodiols (the meta effect). The benzylic alcohols were of identical (R) absolute configuration but enantiomeric excess values were variable. The similar (2R) absolute configurations of the cis-dihydrodiols are consistent with both the ethyl and propyl groups having dominant stereodirecting effects over the other substituents. The model used earlier, to predict the regio- and stereo-chemistry of cis-dihydrodiol bioproducts derived from substituted benzene substrates has been refined, to take account of non-symmetric substituents like ethyl or propyl groups. The formation of benzylic hydroxylation products, from meta-substituted benzene substrates, without further cis-dihydroxylation to yield triols provides a further example of the meta effect during toluene dioxygenase-catalysed oxidations.

Enzyme‐Catalysed Synthesis and Absolute Configuration Assignments of cis‐Dihydrodiol Metabolites from 1,4‐Disubstituted Benzenes

“…Thus, SMe (u 0.60) [16] CH=CH 2 (u 0.60), [16] Et (u 0.56) [18] and nPr (u 0.68) [18] groups having smaller u values were found to have a stronger stereodirecting effect than the substituents having larger u values (e.g. CF 3 , I) studied earlier.…”

“…[15] This model has recently been updated to also allow predictions for substrates bearing substituents which are non-spherically symmetrical and whose size will be conformationally dependent but which have a dominant stereodirecting effect, for example, SMe, [16] CH=CH 2 , [17] Et, [18] Pr. [18] With increasing use now being made of disubstituted benzene cis-dihydrodiols as chiral precursors in synthesis, [19][20][21][22][23][24][25][26][27] the development of more convenient and rigorous methods for stereochemical assignment is very important. The simplest literature method used for the direct determination of enantiopurity of benzene cis-dihydrodiols is chiral stationary phase HPLC (CSPHPLC) at ambient temperature.…”

Oxidative biotransformations of phenol substrates catalysed by toluene dioxygenase: A molecular docking study

“…It is also reasonable to presume that the polar part of the TDO binding pocket, involving His-311 and Gln-215, is responsible for the reported inhibitory effect of an alkylgroup in the meta-position (termed the 'meta-effect') of non-phenolic substrates (e.g. m-halotoluenes) leading to benzylic monohydroxylation rather than cis-dihydroxylation [16]. The predicted binding interactions of phenols with His-311 and Gln-215 may also apply to other types of substrates capable of hydrogen bonding, e.g.…”

“…Conversely, the TDO-catalysed cisdihydroxylation was inhibited by the presence of an alkyl group at the meta-position of non-phenolic disubstituted benzene substrates, e.g. m-chlorotoluene [16].…”

“…Based on earlier studies of non-phenolic 1,4-disubstituted benzene substrates [16], 4-substituted phenols, e.g. 7a, might be expected to undergo TDO-catalysed cis-dihydroxylation, to yield conjugated cis-dihydrodiols, e.g.…”