Into a 500 ml reactor thoroughly purged with nitrogen, 16.50 ml of an ether solution containing 49.60 mmol of ethylmagnesium bromide and 10 ml of THF were introduced. To the reactor, a solution containing 12.31 g (47.20 mmol) of 2-(1-adamantyl)-4-methoxyphenol diluted with 50 ml of a THF was dropwise added at 0° C. over a period of 15 minutes. Then, 150 ml of toluene was added, and the system was heated to 95° C. to distill off about 40 ml of a mixed solution of ether and THF, whereby an opaque white slurry was obtained. After the slurry was cooled to 20° C., 3.43 g (114.30 mmol) of paraformaldehyde and 9.40 ml (67.50 mmol) of triethylamine were added, followed by stirring at 90° C. for 30 minutes. The reaction solution was allowed to cool to room temperature and then quenched with 32 ml of 18% hydrochloric acid. The organic layer was washed with 100 ml of water, 50 ml of a sodium hydrogencarbonate aqueous solution and 100 ml of a sodium chloride aqueous solution, and then concentrated. The resulting solid was purified by silica gel column chromatography to obtain 4.95 g (yield: 37%) of 3-(1-adamantyl)-5-methoxysalicylaldehyde.
