Carboxylates react rapidly and smoothly with 3-unsubstitutedisoxazoliumsalts under very mild conditions to yield enol esters. In this paper we report the application of this reaction as the carboxyl-activating step in a simple and practical synthesis of peptides. The utility of the specific peptide forming reagent, N-ethyl-5-phenylisoxazolium-3′-sulfonate, is described in some detail.
An unconventional approach to peptidecyclization involving the use of acyl ammonium species was developed. Rapid and epimerization/dimerization-free cyclization of synthetically challenging peptides was possible, including a difficult cyclization reaction involving N-methyl amide bond formation. The approach is characterized by ease of purification of the products, high productivity, and high reaction