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A mixture of 1.4 g. of 2-methylhexanoic acid (10.8 mmol.) and 3 ml. of thionyl chloride (41.1 mmol.) in 20 ml. of Skellysolve B was refluxed on a steam bath for five hours. The mixture was then concentrated under reduced pressure and the residue azeotroped twice with 30 ml. of cyclohexane to yield a colorless oil. The oil was then added dropwise to a cold, stirred suspension of 1.2 g. of 7-chloro-4-(4-chlorophenyl)-2-(hydroxymethyl)-5H-pyrido[3,4-b][1,4]benzothiazin-3(2H)-one (3.1 mmol.) in 20 ml. of pyridine. After stirring for 35 minutes, the reaction mixture was poured onto ice-water and allowed to stand for 16 hours at room temperature, resulting in formation of a yellow solid, which was collected, washed with water, air-dried, then purified using flash chromatography on silica gel, employing a mixture of 2% ethyl acetate: 98% methylene chloride as the eluent. Fractions containing the product were combined and triturated with ether to yield 7-chloro-4-(4-chlorophenyl)-2-[(2-methyl-1-oxohexyloxy)methyl]-5H-pyrido[3,4-b][1,4]benzothiazin-3(2H)-one as a yellow crystalline solid (1.17 g; 76% yield) having the structural formula, ##STR19## a melting point in the range of 140°-146° C. and the following elemental analysis:

1. additionA mixture of 1.4 g
2. concentrationThe mixture was then concentrated under reduced pressure
3. customthe residue azeotroped twice with 30 ml
4. customof cyclohexane to yield a colorless oil
5. additionThe oil was then added dropwise to a cold
6. additionsuspension of 1.2 g
7. stirringAfter stirring for 35 minutes
8. additionthe reaction mixture was poured onto ice-water
9. customresulting in formation of a yellow solid, which
10. customwas collected
11. washwashed with water
12. customair-dried
13. custompurified
14. additiona mixture of 2% ethyl acetate
15. additionFractions containing the product
16. customtriturated with ether