Peptide synthesis with benzisoxazolium salts—I

The general features and limitations of 2-ethylbenzisoxazolium fluoroborate as a peptide coupling reagent are described. Strategies for effecting improvements through benzo substitution led to the preparation and study of the 5-nitro, 7-hydroxy, 5-nitro-7-hydroxy, and 4,6-dihydroxy-2-ethylbenzisoxazolium cations. The general properties of these and of several related systems are outlined.