Allenes. Part 48. A new general method for the synthesis of dihydro-4H-1,4-thiazines

Abstract

The base-catalysed reaction between allenic nitriles and aminoethanethiol, cystamine, cysteine, cysteine methyl ester, cystine or cystine dimethyl ester in refluxing ethanol, with passage of oxygen through the reaction mixture, gives 80–90% yieldsof3-alkyl-5,6-dihydro-4H-1,4-thiazine-2-carbonitriles and chiral 5-carboxy and 5-methoxycarbonyl analogues. Detailed investigation and spectroscopic studies have elucidated the seven-step mechanism of this one-pot reaction. Under similar conditions, 2-aminothiophenol failed to give the corresponding 1,4-benzothiazines. Alkaline hydrogen peroxide oxidised the dihydrothiazine to the S-oxide quantitatively.