• IEDDA reaction
• Diels Alder reaction
• dehydroaminoacids
• benzyl protection
• antibiotic resistance
• amide coupling reaction
• siderophore
• siderophore-antibiotic conjugate

In this study, the synthesis of siderophore-thiostrepton conjugates was investigated.
In order to bypass antibiotic resistance, science started to focus attention on peptide antibiotics, like the ribosomally synthesised and post-translational modified peptides (RiPPs). Thiostrepton is a peptide antibiotic, active against Gram-positive bacteria, that belongs to a class of RiPPs called thiopeptides, containing dehydroaminoacids. Among these, the dehydroalanine residue (Dha) is interesting because of its electrophilic properties that allow different reactions in a site-selective way. For instance, Dha that occurs in thiostrepton can undergo a Diels-Alder reaction without causing the antibiotic to lose its antimicrobial activity and a IEDDA ‘click’ reaction in order to introduce a tetrazine.
Tetrazine is a suitable tool for the attachment of targeting groups to enhance the antimicrobial activity of thiostrepton and to improve its pharmacological properties. An example of such a targeting group are siderophores, a class of low molecular weight iron chelators that bacteria synthesise and excrete under low iron conditions. The iron-siderophore complex, formed outside the cell, is actively transferred inside bacteria where the iron is released. This mechanism has inspired the strategy to synthesise siderophore-thiostrepton conjugates that work as ‘Trojan horse’ for bacteria. During iron uptake the antibiotic is accumulated inside the cell where its antimicrobial action takes place. This strategy could enable thiostrepton to broaden its antimicrobial activity allowing the antibiotic to cross the outer cell membrane of Gram-negative bacteria, to reach its intracellular target and to effectively inhibit cell growth.
In this project the synthesis of two different siderophores with a 1,2,4,5-tetrazine moiety is studied. Siderophore 5, characterized by two benzyl protected catecholate groups, is successfully obtained. However, the deprotection to provide free catechols, very important for the interaction between siderophore and iron, failed. Siderophore 12 with two catecholate groups and an extra hydroxamic acid group, which is essential for a higher binding affinity with iron, was also investigated. Unfortunately, the synthesis could not be completed because of the failure of amide coupling reaction, although significant synthetic intermediates were actually obtained.