Fortunately, removal of the fluorenylmethyloxycarbonyl (Fmoc) protecting group from the secondary amine could be remedied by treatment of the product with Fmoc-OSu in the presence of sodium bicarbonate in order to protect the amino functionality again. After protection of the glycosyl OH groups with acetyl groups using acetic anhydride and pyridine, the desired mono-di-and triarabinosylated hydroxyproline building blocks Fmoc-Hyp[(2,3,5-OAc)Ara]1-OH 14, Fmoc-Hyp[(2,3,5-OAc)Ara]2-OH 15, and Fmoc-Hyp[(2,3,5-OAc)Ara]3-OH 16, were obtained in yields of 66%, 59%, and 58%, respectively. Flash column chromatography purification of the arabinosylated building blocks resulted in higher yields of pure compound, when compared to previous reported procedure involving reverse-phase HPLC purification. Scheme 2. Synthesis of Fmoc-and Ac-protected mono-, di-, and triarabinosylated hydroxyproline a With these Fmoc/Ac-protected arabinosylated hydroxyproline building blocks in hand, SPPS of the Brassica PSY1 peptide was commenced (Scheme 3). Starting with Fmoc-Ala bound to a Wang resin (loading: 0.68 mmol/g), the peptide chain was elongated with Fmoc-Pro-OH. For coupling of the arabinosylated hydroxyproline (14, 15, or 16) monomers we used the coupling reagents 2-(1H-benzotriazole-1-yl)-1,1,3,3tetramethyl-uronium hexafluorophosphate (HBTU) and 1-hydroxy-benzotriazole (HOBt). Introduction of arabinosylated Fmoc-Hyp 14-16 to the dipeptide was monitored by ninhydrin/chloranil tests and high-resolution mass spectrometry (HRMS) after cleavage of small aliquots of resins. As we used a slight excess of resin-bound amine groups with respect to the arabinosylated building blocks, remaining amine-groups were permanently acetylated used Ac2O and pyridine. After this, Fmoc-Val-OH coupling was conducted manually in order to monitor coupling this beta-substituted amino acid to the different sterically hindered glycosylated tripeptides. Once formation of tetrapeptides 17-19 was confirmed, subsequent chain elongation was conducted using an automated peptide synthesizer. After acidolytic cleavage from the resin, the crude (glyco)peptides were purified using reversed-phase preparative-HPLC, yielding analytically pure Np-and Ac-protected (glyco)peptides 20-23 (see ESI).