[C:1]1([C:7]2[NH:12][C:11]3[CH:13]=[CH:14][CH:15]=[CH:16][C:10]=3S[C:8]=2[C:17]#[N:18])[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1.C(O)(=O)C.[Mn]([O-])(=O)(=O)=O.[K+].[S:29]([O-:33])(O)(=O)=[O:30].[Na+]>O>[C:1]1([C:7]2[NH:12][C:11]3[CH:10]=[CH:16][CH:15]=[CH:14][C:13]=3[S:29](=[O:33])(=[O:30])[C:8]=2[C:17]#[N:18])[CH:2]=[CH:3][CH:4]=[CH:5][CH:6]=1A mixture of 3-phenyl-4H-1,4-benzothiazine-2-carbonitrile (0.10 g, prepared according to Liso et al., J. Heterocycl. Chem. 18, 279 (1981), Liso et al., Synthesis 1983, 755) and acetic acid (6 ml) was added to a stirred solution of potassium permanganate (88 mg) in water (10 ml) at ambient temperature. After 2 h, 3 h and 18 h additional amounts (25 mg, 40 mg and 40 mg) of potassium permanganate were added. Then a 40% solution of sodium hydrogensulfate (200 μl) was added. After stirring for ½ h the mixture was filtered through celite. The filtrate was extracted with ethyl acetate (2×25 ml). The combined extracts were dried over sodium sulfate, filtered and concentrated in vacuo to give a solid. Trituration with ethyl acetate (2 ml), filtration and drying gave the title compound as light yellow crystals. Yield 34 mg (38%); mp 286-289° C.; 1H-nmr(DMSO) δ (ppm): 12.21 (br, 1H), 8.07-7.97 (m, 1H), 7.84-7.5 (m, 8H); IR(KBr) ν (cm−1): 3298 (NH), 2198 (C≡N), 1598, 1554, 1514, 1476, 1433, 1301, 1171, 1154, 1136; MS m/e (rel. intensity): 282 (25 %, M+), 250 (4%), 218 (100%), 190 (15%), 115 (10%), 77 (10%).
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