Synthesis and application of bifunctional hypervalent iodine reagents for peptide modification.

Protein-protein interactions (PPIs) are responsible for regulating many biological processes in human bodies. PPIs often have a large binding site and often interact through an alpha helical segment. Therefore, the inhibition of PPIs can be difficult using small molecules. In contrast, short helical peptides have the potential to inhibit protein interactions. Nevertheless, isolated helical sequences lack the necessary structural rigidity, binding affinity and cell permeability. Peptide stapling - a covalent linking of two amino acid side chains, can be used to improve these properties of helices. In addition, the linker itself can be involved in binding to a protein of interest. Therefore, easy access to a wide range of structurally varied stapled peptides is crucial for the development of efficient inhibitors of PPIs. Further linker diversity can be achieved via post-stapling modifications. Hypervalent iodine reagents (HIR), particularly, EthynylBenziodoXolone (EBX) derivatives, are known for their exceptional reactivity and selectivity towards thiols, leading to thioalkynes or S-vinylbenziodoxolones (S-VBX). Therefore, various EBXs have been applied for the functionalisation of Cys residues in peptides, proteins and even in living cells. The first goal of this thesis was to develop a novel stapling method using EBX reagents. Design and synthesis of bifunctional hypervalent iodine reagents, bearing an activated ester, led to an efficient Cys-Lys stapling methodology. EBX reagents provided unique and structurally diverse linkers bearing a thioalkyne moiety. Post-stapling modifications of the stapled products were achieved via an amidation of an activated ester, or via Ru-catalysed azide-thioalkyne cycloaddition (RuAtAC) with the alkyne group present on the linker. Stapling of a peptide, derived from p53, showed a significant increase in helicity and binding affinity to MDM2, which is a protein overexpressed in tumours. Next, the development of proteolysis targeting chimeric (PROTAC) degraders was done by G. Menoud as part of his Master thesis under the supervision of Prof. Fierz`s group, Prof. Waser and myself. The stapling methodology and Ru "click" cycloaddition were used to develop a degrader of Death Domain-associated Protein (DAXX).The second objective was to design novel bifunctional hypervalent iodine reagents to expand the linker variability. In this context, work was done to develop EBX reagents bearing alkyl linkers between the hypervalent iodine and activated ester moieties. Reagent with a butyl carbon chain led to Cys-Lys stapling bearing a S-VBX linker in moderate yields. Additionally, a synthesis of more rigid alkyl derivatives was investigated by Jonas Dechent as part of his Master thesis under the supervision of Prof. Waser and myself. 1,3-disubstituted bicyclo[1.1.1]-pentanes (BCP)-based EBX reagents were synthesised and applied to yield S-VBX modified peptide products.Finally, an alternative approach was used to develop novel and stable bifunctional reagents by introducing the additional electrophilic moiety on the core of the hypervalent iodine reagent, instead of on the alkyne. This approach was used to synthesise HIRs bearing sulphur (VI) fluoride exchange (SuFEx) groups, such as sulfonyl fluoride (SF) and fluorosulphates (FS). The reagents efficiently yielded S-VBX functionalised peptides, bearing SF or FS groups.