Deacetylation of Nα-methylated glycopeptides reveals that aza-enolates provide protection against β-elimination of carbohydrates O-linked to serine

Three glycopeptides Ac-Ala-Ser[β-Gal(OAc)4]-Phe-NH2, Ac-Ala-Nα-Me-Ser[β-Gal(OAc)4]-Phe-NH2 and Ac-Ala-Ser[β-Gal(OAc)4]-Nα-Me-Phe-NH2 have been prepared and treated with base in order to remove the O-acetyl protective groups. The glycopeptide which carried the N-methyl group on the glycosylated serine, was substantially more susceptible to β-elimination than the two others. This reveals that formation of an aza-enolate from the amide bond to a glycosylated serine provides protection against β-elimination under basic conditions.