The present invention provides phenylalanine derivatives that inhibit SH2 domain binding with a phosphoprotein. These derivatives include compounds of the formula: W—Y-(AA)n-Z wherein n is 0 to 15; Y is a phenylalanyl radical having a phenyl ring, an amine end, and a carboxyl end, the phenyl ring having one or more substituents, e.g., hydroxyl, carboxyl, formyl, carboxyalkyl, carboxyalkyloxy, dicarboxyalkyl, dicarboxyalkyloxy, dicarboxyhaloalkyl, dicarboxyhaloalkyloxy, and phosphonoalkyl, or phosphonohaloalkyl; W is a moiety attached to the nitrogen of Y and is, e.g., alkylcarbonyl, oxalyl, alkylaminooxalyl, arylaminooxalyl, arylalkylaminooxalyl, or alkoxyoxalyl; AA is an amino acid, the amine end of which is attached to the carboxyl end of Y; and Z is an arylalkylamino or arylheterocyclyl alkylamino; or a salt thereof; with the proviso that W is not arylalkylamino when the phenyl ring of phenylalanyl contains a phosphonoalkyl or phosphonohaloalkyl substituent at a position para to the alkylamido group and the ortho and meta positions are unsubstituted. The present invention further provides precursors suitable for preparing the phenylalanine derivatives and a method for the preparation of the precursors. The present invention further provides conjugates comprising a precursor and a conjugant that are covalently linked. These conjugates have biological and/or pharmacological properties.