The reaction of cysteine with 1,4-benzoquinone: a revision

Under carefully controlled conditions, the reaction of L-cysteine with 1,4-benzoquinone leads to a mixt. of 1,4-benzothiazine oligomers, arising by decarboxylation and subsequent polymn. of the cyclic quinonimine I, previously reported as the reaction product. When the reaction was stopped in the early stages by addn. of NaBH4, work up to the mixt. gave, besides the reduced dimer, the dihydrobenzothiazine II, consistent with the intermediacy of the 3-unsubstituted benzothiazine III.