The Grignard reaction of 2-trifluoromethyl-6-fluorothioxanthone (I) with allylmagnesium bromide (II) in ether gives 2-trifluoromethyl-6-fluoro-9-(2-propenylidene)thioxanthene (III), which is then condensed with 4-(2-hydroxyethyl)piperidine (IV) at 90 C.