Main observation and conclusionrnSelenoesters are useful substitutes for traditional thioesters in protein ligation chemistry due to their high reactivity in the trans-thio/selenoesterification reaction.However,existing synthetic routes to access peptide selenoester require a selenoesterifica-tion reaction between a selenide and a protected peptide with a free carboxylate at the C-terminus.Herein,we introduce an efficient method to convert peptide acyl hydrazide,a convenient thioester surrogate,into the desired selenoester for peptide ligation.Our methodology can be applied to fully de-protected peptides with various C-terminal amino acid residues in high yield without racemi-zation.We believe that this method provides a useful alternative to access peptide C-terminal selenoesters for protein ligation.