Methyl 4-hydroxy-2-propyl-2H-1,2-benzothiazine-3-carboxylate 1,1-dioxide

In the title compound, C13H15NO5S, the thiazine ring adopts a distorted half-chair conformation. The enolic H atom is involved in an intramolecular O—H⋯O hydrogen bond, forming a six-membered ring. In the crystal, molecules are linked through weak intermolecular C—H⋯O hydrogen bonds, resulting in zigzag chains lying along the c axis.

In the title compound, C 13 H 15 NO 5 S, the thiazine ring adopts a distorted half-chair conformation. The enolic H atom is involved in an intramolecular O-HÁ Á ÁO hydrogen bond, forming a six-membered ring. In the crystal, molecules are linked through weak intermolecular C-HÁ Á ÁO hydrogen bonds, resulting in zigzag chains lying along the c axis.

Experimental
In the molecule of the title compound ( Fig. 1), the thiazine ring exhibits a distorted half-chair conformation with S1/C1/ C6/C7 atoms lying in a plane and N1 showing significant departure from the plane due to its pyramidal geometry projecting the propyl group approximately perpendicular to the ring. Like other 1,2-benzothiazine 1,1-dioxide derivatives (Fabiola et al., 1998;Zia-ur-Rehman et al., 2007), the enolic hydrogen on O3 is involved in intramolecular hydrogen bonding (Table1).
Also, C7-C8 bond length [1.346 (3) Å] (very close to normal C-C bond 1.36 Å) indicates a partial double-bond character indicating the dominance of enolic form in the molecule. The C1-S1 bond distance [1.759 (3) Å] is as expected for typical C(sp2)-S bond (1.751 Å). Each molecule is linked to neighbouring molecules via weaker C-H···O=S interactions giving rise to zigzag chains along the c axis (Fig. 2).

Experimental
Propyl iodide (5.10 g, 30.0 mmol) was added drop wise to the mixture of methyl 4-hydroxy-2H-1,2-benzothiazine-3carboxylate-1,1-dioxide (3.83 g, 15.0 mmol), anhydrous potassium carbonate (1.68 g, 30.0 mmol) and dimethylformamide (20.0 ml) in a round bottom flask. Contents were stirred at room temperature for 7 h under nitrogen atmosphere and poured over ice cooled water (300 ml) resulting in an immediate formation of a white solid, which was filtered and washed with cold water. Crystallization from methanol yielded pure compound.

Refinement
All hydrogen atoms were identified in the difference map and subsequently fixed in ideal positions and treated as riding on their parent atoms. In the case of the methyl and hydroxyl H atoms the torsion angles were freely refined. The following distances were used: C-H = 0.98 Å for methyl, C-H = 0.95 Å for aromatic and O-H = 0.84 Å for hydroxyl. U iso (H) was set to 1.2U eq of the parent atoms or 1.5U eq for methyl groups.