A novel and efficient route towards alpha-GalNAc-Ser and alpha-GalNAc-Thr building blocks for glycopeptide synthesis

Michael addition of serine and threonine derivatives 4a-4c to 3,4,6-tri-O-b enzyl-2-nitro-D-galactal (1) afforded the corresponding 2-deoxy-2-nitro-alp ha-D-galactopyranosides 5a-5c in good yield and stereoselectivity. 2-deoxy- 2-nitroglycosides 5a and 5b were reduced to the 2-acetamido compounds by pl atinized Raney nickel T4. Manipulation of the protecting groups afforded kn own N-Fmoc-O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-alpha-D-galactopyranos yl)-L-serine (8a) and -threonine (8b), valuable building blocks for O-glyco peptide synthesis.