A mixture of 5-[4′-(bromomethyl)biphenyl-2-yl]-1-(p-methoxybenzyl)-1H-tetrazole (compound 5a, 9.25 g, 21.2 mmol), acetonitrile (107 ml), methyl 2-(tert-butoxycarbonylamino)-3-nitrobenzoate (compound 19a, 6.43 g, 21.7 mmol) and potassium carbonate (3.00 g, 21.7 mmol) was heated under reflux for 6 hr under a nitrogen atmosphere. The reaction was monitored by TLC (eluent: toluene/ethyl acetate (4:1)). The reaction mixture was cooled, and filtered, and the cake was washed with acetonitrile (35 mL). The filtrate was concentrated under reduced pressure at 35° C. or lower to give a crude product. The crude product was purified by silica gel column chromatography (eluent: toluene/ethyl acetate (100:0-4:1)), and the fraction was concentrated. To the residue was added chloroform (about 30 ml), and the mixture was concentrated. The operations were repeated four times. The residue was dried under reduced pressure in a bath at 40° C. to give methyl 2-(N-tert-butoxycarbonyl-N-{(2′-[1-(p-methoxybenzyl)-1H-tetrazol-5-yl]biphenyl-4-yl)methyl}amino)-3-nitrobenzoate (compound 20a) as a yellowish brown amorphous solid (9.83 g (71.1%)).
