[C:1]1([C:7]2[S:8][C:9]3[CH2:15][CH2:14][C:13]4=[C:16]([C:23]([O:25]CC)=[O:24])[S:17][C:18]([S:19][CH2:20][CH2:21][CH3:22])=[C:12]4[C:10]=3[N:11]=2)[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1.C(O)C.[OH-].[Na+].Cl>O.O1CCCC1>[C:1]1([C:7]2[S:8][C:9]3[CH2:15][CH2:14][C:13]4=[C:16]([C:23]([OH:25])=[O:24])[S:17][C:18]([S:19][CH2:20][CH2:21][CH3:22])=[C:12]4[C:10]=3[N:11]=2)[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1Ethyl 4,5-dihydro-2-phenyl-8-propylthiothieno[3,4-e]benzothiazole-6-carboxylate (3.62 g) was suspended in a mixed solvent of ethanol (25 ml) and tetrahydrofuran (15 ml); a 2 N aqueous solution of sodium hydroxide (13.3 ml) was added at 50° C. After the reaction mixture was stirred at 50° C. for 3 hours, the solution obtained was poured over water (400 ml), acidified with concentrated hydrochloric acid, and extracted with ethyl acetate-tetrahydrofuran (3:1, v/v). The organic layer was washed with water and dried (MgSO4), after which the solvent was distilled off to yield 4,5-dihydro-2-phenyl-8-propylthiothieno[3,4-e]benzothiazole-6-carboxylic acid (3.33 g, 98.5%), which was then recrystallized from chloroform-methanol. Light-yellow prisms. Melting point 236-237° C. (decomposed).
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