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

The Xaa-Pro-type (<i>Z</i>)-fluoroalkene dipeptide isostere (FADI) serves as a versatile surrogate for peptide bonds, effectively restricting <i>cis</i>-<i>trans</i> 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 <i>N</i>-Ns group in peptide synthesis and provide a robust platform for expanding the utility of FADIs in peptidomimetic designing.