[I-].CS[C:4]1[S:5][CH2:6][C@@H:7]2[CH2:16][C:15]3[CH:14]=[CH:13][CH:12]=[CH:11][C:10]=3[CH2:9][N+:8]=12.[NH2:17][C:18]1[S:19][C:20]([CH2:23][C:24]([O:26][CH2:27][CH3:28])=[O:25])=[CH:21][N:22]=1>N1C=CC=CC=1>[CH2:27]([O:26][C:24]([CH2:23][C:20]1[S:19][C:18]([N:17]=[C:4]2[N:8]3[CH2:9][C:10]4[CH:11]=[CH:12][CH:13]=[CH:14][C:15]=4[CH2:16][C@H:7]3[CH2:6][S:5]2)=[N:22][CH:21]=1)=[O:25])[CH3:28](S)-3-Methylthio-1,5,10,10a-tetrahydrothiazolo-[3,4-b]isoquinolinium iodide (10 g) is added to a solution of ethyl (2-aminothiazol-5-yl)acetate (5.1 g) in pyridine (150 cc). After 48 hours at a temperature of the order of 20° C., the solution is concentrated to dryness under reduced pressure (25 mm Hg; 3.3 kPa). The residue is dissolved in a mixture of methylene chloride (200 cc) and N sodium hydroxide solution (100 cc). The organic phase is decanted off, washed with water (3×50 cc), dried over sodium sulphate and then concentrated to dryness under reduced pressure (25 mm Hg; 3.3 kPa) at 40° C. The residue is crystallised by dissolving it in boiling ethanol (100 cc) and cooling to 0° C. The resulting crystals are filtered off, washed with ethanol (30 cc) and diethyl ether (20 cc) and dried at 20° C. under reduced pressure (0.1 mm Hg; 0.013 kPa) to yield (S)-3-[(5-ethoxycarbonylmethylthiazol-2-yl)imino-]-1,5,10,10a-tetrahydrothiazolo[3,4-b]isoquinoline (7.9 g) m.p.=132° C.
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