Abstract
There are two independent molecules in the asymmetric unit of the title compound, C13H13NO7S, which have almost identical geometries. The thiazine ring adopts a sofa conformation in both molecules and the molecular conformations are stabilized by intramolecular O-H center dot center dot center dot O hydrogen bonds. Intermolecular C-H center dot center dot center dot O hydrogen bonds stabilize the crystal packing.