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

In order to investigate the possible extent of beta-elimination occurring in Fmoc-based continuous-flow solid-phase glycopeptide synthesis, the influence of the pKb of the base used for N alpha-deprotection has been studied. A glycosylated pentapeptide was synthesized using 50% morpholine, 10% piperidine or 2% DBU, respectively, in DMF for deprotection. The dehydropentapeptide N alpha-Ac-Thr-Thr-delta Aba-Val-Thr-NH2, which would be formed in the case of beta-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 beta-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.