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Methionine and cysteine derivatives were oxidized asymmetrically by chlorine dioxide to sulfinyl derivatives.

Biotransformations of the N-phthaloyl derivatives of D- and L-methionine and of D- and L-ethionine by Beauveria bassiana ATCC 7159 or Beauveria caledonica ATCC 64970 produce the corresponding (S(s)) sulfoxides in good yield and diastereomeric excess. Pure

Biocatalytic or chemical oxidations can be used in a complementary manner for the preparation of all four diastereomers of methionine sulfoxide with high diastereomeric purity in overall isolated yields of 20-55% from methionine. The N-phthaloyl derivatives of L- and D-methionine were selectively oxidised to the (S(s)S(c)) and (S(s)R(c)) sulfoxides respectively by biotransformation using the fungus Beauveria bassiana ATCC 7159. Hydrogen peroxide oxidation of the same materials gave mixtures from which the (S(s)S(c)) and (R(s)R(c)) isomers can be readily isolated by crystallisation. Chromatography of the residual material then afforded the (R(s)S(c)) and (S(s)R(c)) isomers.

Tetrabromoaurate(III) is an efficient catalyst for the oxidation of sulfides to sulfoxides by nitric acid in a biphasic system (nitromethane/water).The system is selective and can be applied to the oxidation of any type of dialkyl and alkyl aryl sulfide and also of diaryl sulfides activated by electron-releasing substituents.The nature of the active species has been investigated in relation to the mechanistic aspects.