Access to unnatural glycosyl amino acid building blocks via a one-pot Ritter reaction

α-D-Galacto-2-deoxy-oct-3-ulopyranosonic acids, α-D-gluco-2- deoxy-oct-3-ulopyranosonic acids and α-L-galacto-2,8-dideoxy-oct-3- ulopyranosonic acids can be converted into unnatural glycosyl amino acids via a one-pot intramolecular Ritter reaction. Initially, a ketopyranoside-based acid condenses under Lewis acid-promoted conditions with a nitrile (benzonitrile or acetonitrile) and a partially protected diamino ester (Boc-DAB-Ot-Bu, Boc-Orn-Ot-Bu) to form unnatural glycosyl amino esters. The resulting glycosyl amino esters are useful building blocks for solid-phase glycopeptide synthesis. For example, the glycosyl amino acid derived by condensation of α-D-galacto-2-deoxy-oct-3-ulopyranosonic acid with benzonitrile and DAB was used to replace serine in the potent opioid peptide sequence H ₂N-Tyr-D-Thr-Gly-Phe-Leu-Ser-CONH₂.