C.Examples of particularly suitable Lewis acids for this are boron trifluoride and its etherate, aluminum trichloride, phosphorus trichloride and boron tribromide.Preferred solvents are ethers such as tetrahydrofuran, halogenated hydrocarbons such as chloroform and hydrocarbons such as toluene and corresponding mixtures.The subsequent reaction of the N-(3-hydroxyphenyl)tetrahydroindazole IIb with