After a solution of 1,4-dibenzyl-2-(ethoxycarbonyl)piperazine (6.76 g) in methylene chloride (250 ml) was cooled to −78° C., diisobutylaluminum hydride (a 1.0 mol/l hexane solution, 39.90 ml) was added dropwise and the mixture was stirred at −78° C. for 2 hours. A saturated aqueous solution of ammonium chloride and methylene chloride were added to the reaction mixture. The water layer was extracted three times. The organic layers were combined, washed with distilled water and dried over anhydrous sodium sulfate. The residue obtained by distilling off the solvent under reduced pressure was provided for the subsequent reaction without purification. After cooling a tetrahydrofuran solution (150 ml) of methyltriphenylphosphonium iodide (8.07 g) to −78° C., n-butyl lithium (a 1.52 mole hexane solution, 13.14 ml) was added dropwise and the mixture was stirred at −78° C. for 2 hours. A solution of the residue, which residue had been obtained above, in tetrahydrofuran was then added. The reaction mixture was heated from −78° C. to 0° C. while stirring for 4 hours and then, a saturated aqueous solution of ammonium chloride was added to terminate the reaction. Diethyl ether was added and the water layer was extracted three times. The organic layers were combined, washed with saturated aqueous NaCl solution and dried over anhydrous magnesium sulfate. The residue obtained by distilling off the solvent under reduced pressure was subjected to chromatography on a silica gel column (methanol:methylene chloride=1:50), whereby the title compound (3.22 g) was obtained as a pale yellow oil.
