Stereoselective synthesis of beta-D-glycopyranosyl-L-serinate or -threoninate derivatives with an unusual migration

Glycosylation of α-amino acids by sugar acetate donors with InBr3. Minimally competent Lewis acids

A simplified method for the preparation of Fmoc-serine and Fmoc-threonine glycosides for use in O-linked glycopeptide synthesis is described. Lewis acids promote glycoside formation, but also promote undesired reactions of the glycoside products. Use of 'minimally competent' Lewis acids such as InBr(3) promotes the desired activation catalytically, and with greatly reduced side products from sugar peracetates.

Conformational consequences of protein glycosylation: preparation of O-mannosyl serine and threonine building blocks, and their incorporation into glycopeptide sequences derived from alpha-dystroglycan

With the goal to investigate the structural impact of O-mannosyl glycosylation on alpha-dystroglycan, a glycoprotein that has an important role in the extracellular organization of muscle, glycopeptides derived from its mucin-like sequence have been prepared by solid-phase peptide synthesis. Two approaches have been explored to obtain needed mannosylated serine and threonine building blocks. With the alpha-carboxyl group unprotected, and with tetraaceto-1-fluoro-alpha-D-mannose as the sugar donor, the desired alpha-O-mannosyl-Fmoc-Ser/Thr formed, along with mannosyl ester isomers and the species with mannose attached to both hydroxyl and carboxyl functions. Relevant mechanistic questions and stability issues were elucidated. Alternatively, building blocks were made with the alpha-carboxyl protected/activated as the pentafluorophenyl (Pfp) ester. Glycopeptides synthesized herein contained 5-9 residues, and featured one, two, and four consecutive Ser and/or Thr residues O-glycosylated with mannose. Circular dichroism (CD) spectra for Man-containing glycopeptides recorded in water show them to be not well ordered. For one of the alpha-dystroglycan-derived sequences, the comparative conformational consequences of glycosylation by either Man or GalNAc have been examined by CD and NMR, with both methods showing a more organized structure when GalNAc is present.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Stereoselective synthesis of O-serinyl/threoninyl-2-acetamido-2-deoxy-alpha- or beta-glycosides

General glycosidation methodology has been developed which can selectively provide 2-acetamido-2-deoxy-alpha- or beta-glycosides of beta-hydroxy-alpha-amino acid derivatives [glucopyranoside-(8, 43), galactopyranoside- (9, 13), mannopyranoside- (10), lactoside analogs (11, 38) and 3-O-beta-galactopyranosyl-mannopyranoside (12)] stereoselectively in excellent yield from the highly nucleophilic alpha-imino esters (Schiff bases) of L-serine and L-threonine. Various glycosides were converted via their amino and acetamido derivatives to Fmoc-protected serinyl- or threoninyl-glycosides (24-28, 37, 41, 46) which are all suitable building blocks for the solid-phase synthesis of O-glycopeptides. Complete 1H- and 13C-NMR data are provided for all compounds.