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

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 tri-peptidomimetics incorporating Xaa-Pro-type FADIs is challenging due to the high susceptibility to racemization of the -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 -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.