Peptide coupling reagent frequently used to synthesize amide bonds. Exists as a mixture with its cyclic tautomer. Prepn of parent and derivatives: J. C. Sheehan et al., J. Org. Chem. 26, 2525 (1961) DOI. Tautomerism studies: T. Tenforde et al., ibid. 37, 3372 (1972) DOI; and reaction mechanism studies: A. Williams, I. T. Ibrahim, J. Am. Chem. Soc. 103, 7090 (1981) DOI. Spectrophotometric determn: K. Seno et al., Anal. Sci. 24, 505 (2008) DOI PubMed. Synthetic applications: M. K. Dhaon et al., J. Org. Chem. 47, 1962 (1982) DOI; G.-J. Ho et al., ibid. 60, 3569 (1995) DOI; D. S. Bose, K. S. Sunder, Synth. Commun. 29, 4235 (1999) DOI; H. Sai et al., Synthesis 2003, 201 DOI. Cross-linking applications: J. M. Lee et al., J. Mater. Sci. Mater. Med. 7, 531 (1996) DOI; H. M. Powell, S. T. Boyce, Biomaterials 27, 5821 (2006) DOI PubMed. Brief description and review of synthetic uses: R. S. Pottorf, P. Szeto in Encyclopedia of Reagents for Organic Synthesis 4, L. A. Paquette, Ed. (John Wiley & Sons, New York, 1995) pp 2430-2432.