Susceptibility of glycans to β-elimination in Fmoc-based O-glycopeptide synthesis

order to investigate the possible extent of β‐elimination occuring in Fmoc‐based continuous‐flow solid‐phase glycopeptide synthesis, the influence of the pKb of the base used for Nα‐deprotection has been studied. A glycosylated pentapeptide as synthesized using 50%morpholine, 10%, piperidine or 2% DBU, respectively, in DMF for deprotection. The dehydropentapeptide Nα‐.Ac‐Thr‐Thr‐δAba‐Val‐Thr‐NH2, which would be formed in the case of β‐elimination, was prepared independently and used as a control in HPLC analysis; however, this product was not formed under any of the deprotection conditions applied. Furthermore, a 23 amino acid long glycopeptide from human intestinal mucin was prepared using 2% DBU as a base for Fmoc cleavage, and similarly no β‐elimination was observed. The glycopeptide products were subjected to a prolonged treatment with sodium hydroxide in methanol/water without significant formation of byproducts, and the pure glycopeptides were isolated and characterized by 1H‐NMR spectroscopy.