Abstract: A phenylalanine derivative having the formula (I): ##STR1## where R.sup.1 and R.sup.2 are independently hydrogen provided that both R.sup.1 and R.sup.2 are not hydrogen at the same time;C.sub.1 -C.sub.8 alkyl which may be substituted with hydroxy, hydroxycarbonyl, C.sub.1 -C.sub.4 alkoxycarbonyl, C.sub.1 -C.sub.4 alkylmercapto, C.sub.1 -C.sub.4 alkoxy, carbamoyl, sulfamoyl, pyridyl, or phenyl which may further be substituted with nitro, C.sub.1 -C.sub.4 alkoxy, or halogen;C.sub.6 -C.sub.8 cycloalkyl which may be substituted with hydroxy, C.sub.1 -C.sub.4 alkoxy, hydroxylcarbonyl, C.sub.1 -C.sub.4 alkoxycarbonyl, or C.sub.1 -C.sub.4 alkyl;phenyl which may be substituted with halogen, nitro, trifluoromethyl, C.sub.1 -C.sub.4 alkoxy C.sub.1 C.sub.4 alkylmercapto, C.sub.1 -C.sub.4 alkylcarbonyl, phenylcarbonyl, hydroxycarbonyl, C.sub.1 -C.sub.4 alkoxycarbonyl, carbamoyl, sulfamoyl, amidino, pyridylcarbonyl, or C.sub.1 -C.sub.6 alkyl which may further be substituted with C.sub.1 -C.sub.