Site‐Directed Asymmetric Quaternization of a Peptide Backbone at a C‐Terminal Azlactone

Dual‐purpose activation : Peptide C‐terminal azlactones I undergo stereoselective alkylation with high efficiency by the use of a newly devised chiral tetraaminophosphonium salt as a phase‐transfer catalyst, and the alkylated azlactone products II can be employed directly for peptide ligation (see scheme, LG=leaving group). In this way, a wide range of chiral quaternary α‐amino acid residues can be incorporated at specific sites of a peptide strand.