Towards novel 1,2,4,5-Tetrazine mediated peptide macrocyclisations

Interest in linear peptides and peptidomimetics experienced rapid growth within the pharmaceutical industry due to their ability to disrupt protein-protein interactions; and their ability to modulate protein function by allosteric regulation and competitive binding. However, many linear peptides that show therapeutic potential cannot be readily developed into bioavailable pharmaceuticals due to their poor lipophilicity and protease stability; and their unpredictable secondary structures. Macrocyclic peptides show increased lipophilicity and protease stability; and their constrained secondary structures often lead to improved target binding affinity and selectivity. Consequently, there has been a resurgence of interest within the pharmaceutical industry towards peptide based therapeutics and increased research towards novel peptide macrocyclisation strategies. Progress towards a novel solid-phase peptide macrocyclisation strategy based on the inverse electron demand Diels-Alder reaction of 1,2,4,5-tetrazines is reported in this thesis. The solid-phase oxidation activation peptide macrocyclisation strategy uses the in situ oxidation of a dihydro-1,2,4,5-tetrazine to trigger the inverse electron demand Diels-Alder reaction of a 1,2,4,5,-tetrazine and therefore peptide macrocyclisation. Also, the solid-phase oxidation activation peptide macrocyclisation strategy enables selective late stage incorporation of the dihydro-1,2,4,5-tetrazine moiety onto the peptide backbone in the presence of a dienophile. A successful proof of concept in situ oxidation and inverse electron demand Diels-Alder reaction of a dihydro-1,2,4,5- tetrazine was achieved. However, the poor stability of a strained dienophile towards an oxidant was also highlighted. To overcome this problem a library of alternative oxidants and a library of unstrained dienophiles were screened to successfully optimise the proof of concept in situ oxidation and inverse electron demand Diels-Alder reaction of a dihydro-1,2,4,5-tetrazine. The optimised in situ oxidation and inverse electron demand Diels-Alder reaction of a dihydro-1,2,4,5-tetrazine was successfully transferred onto solid-phase using resin-bound dienophiles. However, attempts to synthesise resin-bound dihydro-1,2,4,5-tetrazine derivatised amino acids were unsuccessful. Therefore, an alternative dihydro-1,2,4,5-tetrazine was selected for future development of the solid-phase oxidation activation peptide macrocyclisation strategy