To a solution of 1-[4-nitro-2-(3,3,5,5-tetramethylcyclohex-1-enyl)phenyl]piperazine (10 mg, 0.029 mmol) produced in Example (88c) in tetrahydrofuran (1 mL) were added butyraldehyde (3.2 mg, 0.044 mmol), sodium triacetoxyborohydride (12 mg, 0.058 mmol) and acetic acid (2 mg, 0.029 mmol) in that order, followed by stirring for 3 hours at room temperature. Water was added to the reaction mixture, extraction was performed with ethyl acetate, and the organic layer was concentrated. The resultant residue was purified by NH silica gel column chromatography (ethyl acetate/heptane) to give 1-butyl-4-[4-nitro-2-(3,3,5,5-tetramethylcyclohex-1-enyl)phenyl]piperazine.
