To a solution of O-methoxymethyl-N-methyl-N-(2-nitrophenylthio)-β-benzoyloxytyrosine methyl ester (threo isomer) (4.94 g) in dichloromethane (50 ml) were added thiophenol (4.8 ml) and trifluoroacetic acid (2.5 ml) at 0° C. After stirring for 30 minutes, sodium bicarbonate aqueous solution was added to the mixture. The organic layer was washed with sodium bicarbonate aqueous solution and brine. After evaporating, the residue was put on a column of silica gel (MERCK 7734, 100 g) and eluted with 5% methanol in chloroform to give O-methoxymethyl-N-methyl-β-benzoyloxytyrosine methyl ester (threo isomer) (0.32 g).
