Reaction of benzothiazole with dimethyl acetylenedicarboxylate in aqueous methanol gives irons-dimethyl 4formyl-2,3-dihydro-l,4-benzothiazine-2,3-dicarboxylate in high yield, and a mechanism is proposed for this novel transformation. The scope and limitations of the process have been defined by examination of the reactions of a wide range of substituted benzothiazoles with dimethyl acetylenedicarboxylate in aqueous methanol; the results are fully consistent with the postulated mechanism.

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Bei der Umsetzung der ggf. substituierten Benzothiazole (I) mit Acetylendicarbonsäuredimethylester (II) erhält man in Wasser/Methanol als Lösungsmittel über diskutierte Zwischenstufen wie (III)‐(V) in hohen Ausbeuten die N‐Formyl‐benzothiazin‐Derivate (VI).