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 pK_b of the base used for N^α-deprotection has been studied. A glycosylated pentapeptide as synthesized using 50%morpholine, 10%, piperidine or 2% DBU, respectively, in DMF for deprotection. The dehydropentapeptide N^α-.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.