Revised efficient and reproducible synthesis of an Fmoc-protected Tn antigen

Abstract

In 2021, our team reported a concise synthesis of Thomsen-Nouveau (Tn) antigen, a tumour-associated O-linked mucin glycopeptide. We have since realized that our characterization of the reported glycoside was mistaken. Instead of the intended ether-bonded α anomer, the β-anomer containing an ester glycosidic bond was formed using palladium catalysis and characterised incorrectly as the spectra are remarkably similar. We demonstrated this conclusively be repeating Danishefky's synthesis, with some required modifications of the protocol, of Fmoc-Tn for confirmation. The error is too significant for a correction as it does affect the conclusions of the work, and the original paper has been retracted, though its work is included in this manuscript. In this replacement we report a successful synthesis of the Tn antigen using adjusted glycosylation conditions: a different glycosyl acceptor, N-Fmoc serine benzyl ester, using TMSOTf as the catalyst. This remains, to the best of our knowledge, the shortest Tn antigen synthesis reported from galactose, although it provides the Tn antigen now. Furthermore, this route gives ready access to an essential Tn antigen building block that can be used for large-scale solid phase peptide synthesis.