Stereoselective Ribosylation of Amino Acids

The glycosylation properties of ribofuranosyl N-phenyltrifluoroacetimidates toward carboxamide side chains of asparagine and glutamine were investigated. Conditions were found that promote nearly exclusive formation of the alpha-anomerically configured N-glycosides. The strategy allows for the synthesis of Fmoc-amino acids suitably modified for the preparation of ADP-ribosylated peptides. Furthermore, ribosylation of serine with these donors proved to be completely alpha-selective, and for the first time, alpha-ribosylated glutamic and aspartic acid, the naturally occurring sites for poly-ADP-ribosylation, were synthesized.