Synthesis of urazole-containing peptides. We began with the development of a general procedure to prepare urazole-containing peptides 14 (Figure ). Despite the utility of urazole-containing peptides in labeling and protein bioconjugation applications, strategies for their assembly are underdeveloped. Our initial approach evaluated a method reported by Börner and coworkers wherein a N4-substituted urazole can be built from an N-terminal amine of a solid supported peptide. 53 Peptides on solid phase (12) react in two steps to form semicarbazide-peptides (13). Upon cleavage from resin, semicarbazide-peptides (13b) react with K2CO3 in refluxing ethanol to selectively provide urazole-peptides (14) and the conditions avoid hydantoin-peptide (15) formation in sequences bearing acyclic residues at position-2.