N-(2,6-Dichlorobenzoyl)-4-(2,6-dimethoxyphenyl)-L-phenylalanine methyl ester (1.59 g) was dissolved in THF (4 mL) under N2 then 70% HNO3 (4 mL) was added and the mixture was stirred at 50° C. overnight. The mixture was diluted with AcOEt (150 mL) and washed with water (100 mL). The organic layer was dried (MgSO4), filtered and evaporated. The residue was dissolved in anhydrous MeOH (100 mL) and dry HCl gas was bubbled through the mixture at 0° C. for a few minutes. The mixture was stirred at room temperature overnight, concentrated, taken up with AcOEt and washed with 1N HCl, satd. NaHCO3 and brine. The organic layer was dried (MgSO4), filtered and evaporated. The crude product was purified by flash column chromatography (silica gel; eluent: hexanes and AcOEt) to give N-(2,6-dichlorobenzoyl)-4-(2,6-dimethoxy-3-nitrophenyl)-L-phenylalanine methyl ester (1.1 g).
