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General procedure for the synthesis of 4-piperidin-1-yl-benzaldehyde from hexahydropyridine and p-fluorobenzaldehyde: A mixture of p-fluorobenzaldehyde (25.0 g, 0.200 mol), hexahydropyridine (0.300 mol), and anhydrous potassium carbonate (40.0 g) was mixed in DMF (300 mL) and a catalytic amount of Aliquat 336 reagent was added. The reaction mixture was refluxed at 100 °C for 24 hours. Upon completion of the reaction, the mixture was concentrated under reduced pressure and cooled to room temperature. Subsequently, the mixture was poured into ice water and allowed to stand overnight. The solid product formed was filtered, washed with water and recrystallized with methanol to afford the target compound 4-(piperidin-1-yl)benzaldehyde. The compound was obtained in 98% yield as yellow crystals with a melting point of 64°C, which is in agreement with literature reports [47,48].