5-Amino-3-ethoxymethylisoquinoline → (S)-3-[(3-Ethoxymethylisoquinol-5-yl)imino]-1,5,10,10a-tetrahydrothiazolo[3,4-b]isoquinoline

(S)-3-Methylthio-1,5,10,10a-tetrahydrothiazolo[3,4-b]isoquinolinium iodide (7.2 g) is added to a solution of 5-amino-3-ethoxymethylisoquinoline (4.1 g) in pyridine (150 cc). The suspension is stirred for 24 hours at a temperature of about 20° C. The insoluble material gradually dissolves. The solution is concentrated to dryness under reduced pressure (20 mm Hg; 2.7 kPa) at 60° C. The residue is dissolved in a mixture of methylene chloride (100 cc) and N sodium hydroxide solution (100 cc). The organic phase is decanted, washed with water (50 cc), dried over magnesium sulphate and filtered and the filtrate is evaporated to dryness at 40° C. under reduced pressure (20 mm Hg; 2.7 kPa). The residue is recrystallised from isopropyl alcohol (80 cc). (S)-3-[(3-Ethoxymethylisoquinol-5-yl)imino]-1,5,10,10a-tetrahydrothiazolo[3,4-b]isoquinoline (5.2 g) is thus obtained in the form of a pale yellow solid melting at 88° C.