Benzothiazin (III) was prepared by condensation of 2-aminothiophenol (I) with ethyl 4-bromoacetoacetate (II). Subsequent reduction of (III) with NaBH3CN provided (IV), which was brominated using N-bromosuccinimide to afford (V). This was coupled with ethyl oxalyl chloride to give amide (VI). Nitration of (VI) was then performed with HNO3 in H2SO4 at -10 C. Further reduction of the nitro group of (VII) with concomitant cyclization produced the tricyclic compound (VIII). After resolution by chiral preparative HPLC, the required (R)-enantiomer was hydrolyzed with NaOH to afford carboxylic acid (IX). This compound was finally isolated as the sodium salt upon treatment with aqueous NaHCO3, followed by lyophilization.