Methyl 4-hy-droxy-2-meth-oxy-carbonyl-methyl-1,1-dioxo-1,2-dihydro-1λ,2-benzothia-zine-3-carboxyl-ate.

There are two independent mol-ecules in the asymmetric unit of the title compound, C(13)H(13)NO(7)S, which have almost identical geometries. The thia-zine ring adopts a sofa conformation in both mol-ecules and the mol-ecular conformations are stabilized by intramolecular O-H⋯O hydrogen bonds. Inter-molecular C-H⋯O hydrogen bonds stabilize the crystal packing.