[NH:1]1[C:11]2[C:6](=[CH:7][CH:8]=[CH:9][CH:10]=2)[C:4](=O)[C:2]1=[O:3].[CH3:12][N:13]=[C:14]=[O:15].C[NH:17][C:18]([N:20]1C2C(=CC=CC=2)C(=O)C1=O)=[O:19].Br.C(SC(=N)N)C>O1CCCC1.C(N(CC)CC)C>[CH3:12][N:13]1[C:4]2([C:2](=[O:3])[NH:20][C:18](=[O:19])[NH:17]2)[C:6]2[C:11](=[CH:10][CH:9]=[CH:8][CH:7]=2)[NH:1][C:14]1=[O:15]Isatin (294.3 g) is suspended in tetrahydrofuran (3 liter) and thereto are added with stirring triethylamine (279 ml) and methyl isocyanate (129.8 ml) at 15° C. The mixture is stirred at 20°-25° C. for 2.5 hours (1-methylcarbamoylisatin is produced in the mixture). To the reaction mixture are added 2-ethylisothiourea hydrobromide (444.2 g) and triethylamine (55.8 ml), and the mixture is refluxed for 2 hours. After cooling, the solvent is distilled off under reduced pressure. To the residue [i.e. 3-methyl-4-hydroxy-4-(2-ethylisothioureido)carbonyl-2-oxo1,2,3,4-tetrahydroquinazoline] is added 10% hydrochloric acid (4 liter) and the mixture is stirred at 70° C. for 4 hours. After cooling, the precipitates are taken by filtration, washed with water, methanol (1 liter) and chloroform (1 liter) in this order and then dried to give 3-methyl-spiro[1,2,3,4-tetrahydroquinazoline-4,4'-imidazolidine]-2,2',5'-trione (369.2 g, yield 75 %).
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