[CH3:1][O:2][C:3]1[CH:8]=[CH:7][C:6]([NH2:9])=[CH:5][CH:4]=1.[CH3:10][C:11](O)([C:13](=O)[CH3:14])[CH3:12].C(O)(=O)C.C1(C)C=CC=CC=1>O>[CH3:14][C:13]1[C:11]([CH3:12])([CH3:10])[C:5]2[CH:4]=[C:3]([O:2][CH3:1])[CH:8]=[CH:7][C:6]=2[N:9]=1246 parts of p-anisidine, 204 parts of 2-methyl-2-hydroxy-butan-3-one and 10 parts of acetic acid in 650 parts of toluene are heated, with removal of water from the system, until no more water separates off. The solvent is then distilled off and the residue is heated for 5 hours at 220° C. After cooling to 160° C., 10 parts of concentrated sulfuric acid are added; the mixture is kept at 160° C. for 3 hours, whilst distilling off the water formed. Distillation under reduced pressure (21 mbar) gives 318.7 parts of 5-methoxy-2,3,3-trimethyl-indolenine (melting point 53°-54° C.), corresponding to 84.3% of theory.
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