Expected and unexpected reactions of 1,3-benzothiazine derivatives, I. Ring transformation of β-lactam-condensed 1,3-benzothiazines into 4,5-dihydro-1,4-benzothiazepines and indolo-1,4-benzothiazepines

The ring-enlargement reactions of monochloro-β-lactam-fused 2-aryl-1,3-benzothiazines revealed that the reactions of ortho-nitro aryl-substituted derivatives with sodium methoxide in methanol provided two products, depending on the amount of the base. With 2 equiv of reagent, the expected 1,4-benzothiazepines were obtained. Somewhat surprisingly, treatment with a large excess of sodium methoxide led to the formation of indolo-1,4-benzothiazepines via a novel rearrangement. The structures of the new ring systems were determined by means of X-ray crystallography and NMR spectroscopy.