Stereoselective benzylic hydroxylation of 2-substituted indanes using toluene dioxygenase as biocatalyst
Indane, 1A, and a series of 2-substituted indane substrates, 1B–1D, 1G, 1I–1L, were found to undergo benzylic monohydroxylation catalysed by toluene dioxygenase, present in the intact cells of Pseudomonas putida UV 4, to yield enantiopure cis-indan-1-ols, 2A–2D, 2G, 2I–2L of the same absolute configuration at C-1 as major bioproducts. Enantiopure trans-indan-1-ols 6B, 6C, and 6G were also obtained as minor metabolites. Evidence of further sequential benzylic hydroxylation (bis-hydroxylation) was found only with substrates 2A, 1C, 1D and 1L to yield the corresponding enantiopure trans-1,3-diols, 3A, 3C, 3D and 3L. Minor enzyme-catalysed processes also observed include benzylic alcohol oxidation to ketones (4A, 5A, 4B, 4L, 5L), ketone reduction to benzylic alcohol 6A, ester hydrolysis to indan-2-ol 1B, and cis-dihydroxylation of indan-1-ol 6A to triol 7. The enantiopurities and absolute configurations of bioproducts have been determined using MTPA ester formation, circular dichroism spectroscopy and stereochemical correlation methods.
The contribution of asymmetric oxidation and kinetic resolution to the production of bioproducts of high ee (>98%), and the metabolic sequence involved in their biotransformation by P. putida UV4 is discussed. Enantiocomplementarity was found during the benzylic hydroxylation of indan-2-ol 1B, using toluene dioxygenase and naphthalene dioxygenase, when both single enantiomers of the metabolites 2B, 4B and 6B of opposite configurations were obtained.