Synthesis of cyclically constrained sugar derived α/β- and α/γ-peptides.

A general approach to enantiopure conformationally constrained sugar derived α/β- and α/γ-peptides has been established. Five-membered ring α/β-peptides were synthesized via formyl C-glycofuranosides, easy available from hexose-derived azido-2-equatorial-OH-glycopyranosides by DAST-promoted ring contraction. By means of a regioselective oxidation with TEMPO at C-6 of hexose-derived 3-azido glycopyranosides as the key step, two- and three-residue α/γ-peptides having a six-membered ring were obtained in good yields and under very simple experimental conditions.