Toluene → Ethyl 5-(dimethylamino)-2-[(dimethylamino)methylene]-4-(4-fluorophenyl)-3-oxopent-4-enoate

A mixed solution of ethyl 4-(4-fluorophenyl)-3-oxobutanoate (0.95 g), N,N-dimethylformamide dimethylacetal (1.59 g) and toluene (7 ml) was stirred at 100° C. to 105° C. for 14 hours while methanol was distilled off. After cooling to room temperature, the residue was concentrated under reduced pressure. Further, 7 ml of toluene was added and the residue was concentrated to one-third of its volume under reduced pressure. Toluene (3.5 ml) was added to give a solution of ethyl 5-(dimethylamino)-2-[(dimethylamino)methylene]-4-(4-fluorophenyl)-3-oxopent-4-enoate. This solution was cooled to 0° C. to 5° C., trifluoroethyl amine (0.43 ml) and a 4.3 M solution of hydrogen chloride in ethyl acetate (1.57 ml) were added, and the mixture was stirred at 100° C. to 105° C. for three hours. After cooling to room temperature, the layers were separated by adding water, and the organic layer was concentrated to 3.5 ml. Toluene (1.4 ml) was added, followed by stirring at room temperature. The resulting solid was collected by filtration to give 1.07 g of the title compound as a solid.