Synthesis of Unnatural Alfa-N-linked Glycopeptides with Potential Antifreeze Activity

The synthesis of neo-glycoconjugates has been gaining much attention in recent years due to the relevance of glycopeptides and glycoproteins in many biological processes.[i] Our group has been actively dedicating its efforts to the synthesis of α-N-linked glycosylamides and glycopeptides.[ii] α-N-linked glycopeptides are unnatural molecules, since they display an α linkage between the peptide side chain and the sugar moiety, unlike natural glycopeptides which connect the peptide to the glycan through a β-N-glycosidic bond. This novel type of glycosylation of peptides could introduce modifications that can mimic and/or interfere with molecular recognition events.[iii] Direct glycosylation of peptide chains is not viable for the synthesis of molecules with α-N-linked configuration, since the corresponding α-glycosyl amines isomerise to the β-anomers. Only very recently Nα-Fmoc-protected glycosyl amino acids have been efficiently and stereoselectively synthesized and linearly incorporated into a peptide sequence.[iii] In the present paper these novel building blocks have been employed for the synthesis of complex structures that resemble antifreeze glycopeptides (Figure 1).[iv] These sequences were prepared using solid phase synthesis with Fmoc protocol, experimenting with different conditions and also using microwave assisted solid phase synthesis, in an effort to enhance the reactivity of our unnatural building block. The α-N-linked glycopeptides were obtained with modest yields, their secondary structure was assessed by circular dichroism and their antifreeze properties were evaluated in the group of Prof. Robert N. Ben. Despite the fact that our compounds do not show significant antifreeze activity, this work constitutes the first attempt towards the synthesis of complex α-N-linked glycopeptides and has been useful to understand the behaviour, sometimes unexpected, of these molecules, in terms of reactivity and stability. [i] D. P. Gamblin, E. M. Scanlan, B. G. Davis, Chem. Rev. 2009, 109, 131-163. [ii] a) F. Nisic, A. Bernardi, Carbohydr. Res. 2011,346, 465-71. b) C. Colombo, A. Bernardi,Eur. J. Org. Chem. 2011, 3911–3919. [iii] F. Marcelo, F. J. Cañada, S. André, C. Colombo, F. Doro, H. J. Gabius, A. Bernardi, J. Jiménez-Barbero. Organic & Biomolecular Chemistry 2012, DOI: 10.1039/C2OB07135E. [iv] a) Garner, J.; Harding, M. M. ChemBioChem 2010, 11 2489-2498. b) Leclere, M; Kwok, K. B.; Luke K. W.; Allan, D. S.; Ben, R. N. Bioconjugate Chem. 2011, 22, 1804-1810.