A new and versatile class of cyclic diazodicarboxamides that reacts efficiently and selectively with phenols and the phenolic side chain of tyrosine through an ene-like reaction is reported. This mild aqueous tyrosine ligation reaction works over a broad pH range and expands the repertoire of aqueous chemistries available for small molecule, peptide, and protein modification. The tyrosine ligation reactions are shown to be compatible with the labeling of native enzymes and antibodies in a buffered aqueous solution. This reaction provides a novel synthetic approach to bispecific antibodies. We believe this reaction will find broad utility in peptide and protein chemistry and in the chemistry of phenol-containing compounds.