Dioxygenase-catalysed mono-, di- and tri-oxygenation of dialkyl sulfides and thioacetals: chemoenzymatic synthesis of enantiopure cis-diol sulfoxides

Abstract

Toluene dioxygenase (TDO)-catalysed monooxygenation of methylsulfanylmethyl phenyl sulfide 1 and methylsulfanylmethyl 2-pyridyl sulfide 4, using whole cells of Pseudomonas putida UV4, occurred exclusively at the alkyl aryl sulfur centre to yield the alkyl aryl sulfoxides 2 and 5 respectively. These sulfoxides, accompanied by the dialkyl sulfoxides 3 and 6, were also obtained from naphthalene dioxygenase (NDO)-catalysed sulfoxidation of thioacetals 1 and 4 using intact cells of P. putida NCIMB 8859. Enzymatic oxidation of methyl benzyl sulfide 7, 2-phenyl-1,3-dithiane 19, and 2-phenyl-1,3-dithiolane 23, using TDO, gave the corresponding dialkyl sulfoxides 8, 20 and 24 as minor bioproducts. TDO-catalysed dioxygenation of the alkyl benzyl sulfides 7, 15 and 17 and the thioacetals 19 and 23, with P. putida UV4, yielded the corresponding enantiopure cis-dihydrodiols 9, 16, 18, 21 and 25 as major metabolites and cis-dihydrodiol sulfoxides 14, 22 and 26 as minor metabolites, resulting from a tandem trioxygenation of substrates 7, 19 and 23 respectively. Chemical oxidation, of the enantiopure cis-dihydrodiol sulfides 9, 16, 18 and 21 with dimethyldioxirane (DMD), gave separable mixtures of the corresponding pairs of cis-dihydrodiol sulfoxide diastereoisomers 14 and 27, 28 and 29, 30 and 31, 22 and 32. While dialkyl sulfoxide bioproducts 3, 6, 20 and 24 were of variable enantiopurity (27–≥98% ee), alkyl aryl monosulfoxides 2 and 5, cis-dihydrodiols 9, 16, 18, 21 and 25 and cis-dihydrodiol sulfoxide bioproducts 14, 22 and 26 were all single enantiomers (≥98% ee). The absolute configurations of the products, obtained from enzyme-catalysed (TDO and NDO) and chemical (DMD) oxidation methods, were determined by stereochemical correlation, circular dichroism, and X-ray crystallographic methods.