Racemization-free peptide bond formation via 2-nitrobenzensulfonyl strategy for diastereoselective synthesis of (Z)-fluoroalkene-type peptidomimetics

Abstract

The Xaa-Pro-type (Z)-fluoroalkene dipeptide isostere (FADI) serves as a versatile surrogate for peptide bonds, effectively restricting cis–trans isomerization of the prolyl amide bond and offering advantages in the development of conformationally constrained peptide analogues. However, the diastereoselective synthesis of tripeptidomimetics incorporating Xaa-Pro-type FADIs is challenging due to the high susceptibility to racemization of the α-stereogenic center during peptide bond formation. Here, we introduce a racemization- and epimerization-free coupling strategy for the stereoselective synthesis of fluoroalkene-type peptidomimetics by reacting Xaa-Pro-type FADIs with amino acid benzyl esters or peptides. This approach leverages the unique properties of the 2-nitrobenzenesulfonyl (Ns) group as an N-terminal protecting group, which promotes sulfonamide anion formation, effectively suppressing α-deprotonation and thereby preventing racemization or epimerization. Our findings highlight the pivotal role of the N-Ns group in peptide synthesis and provide a robust platform for expanding the utility of FADIs in peptidomimetic designing.