Investigation of Protecting Group for Sialic Acid Carboxy Moiety Toward Sialylglycopeptide Synthesis by the TFA-labile Protection Strategy

The sialylation using sialyl donor with 2,6-dimethylbenzyl (DMBn) ester at C-1 carboxy group rapidly proceeded with good α-selectivity to yield sialyl-N-acetylgalactosaminylated 9-fluorenylmethoxycarbonyl (Fmoc) serine unit. The unit was used for the solid-phase peptide synthesis (SPPS) of the sialyl glycopeptide. The DMBn group was removed during the final deprotection by the trifluoroacetic acid (TFA) treatment keeping the sialyl bond intact and the desired sialyl glycopeptide was successfully obtained. This protecting group strategy provided easier access to sialyl glycoamino acids for sialyl glycopeptides synthesis.