Biocatalytic and chemical routes to all the stereoisomers of methionine and ethionine sulfoxides

Abstract 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 ( S S S C ) diastereomers can be obtained from l -series substrates by crystallisation of the biotransformation extract, and the corresponding ( S S R C ) products obtained from d -series substrates by chromatography of the biotransformation extract. Hydrogen peroxide-catalysed oxidation of the N -phthaloyl derivatives of d - and l -methionine and of d - and l -ethionine gives diastereomeric mixtures from which the ( S S S C ) and ( R S R C ) diastereomers can be obtained by crystallisation, and the ( S S R C ) and ( R S S C ) diastereomers obtained by chromatography. N- Cbz- and N - t -Boc methionines are also converted to sulfoxides with predominant ( S S ) configuration by both B. bassiana and B. caledonica , but the isolated yields and d.e. of products were generally lower than those obtained from the N- phthaloyl substrates. Removal of the N- phthaloyl group from diastereomerically pure methionine and ethionine sulfoxides gave the corresponding amino acid sulfoxides in high yield; removal of N- Cbz and N - t -Boc groups from protected methionine sulfoxides was also achieved without loss of configuration at sulfur.