Development of Novel Acyl Thiol-ene Mediated Peptide Ligation Strategies

Abstract

This thesis, entitled Development of Novel Acyl Thiol-ene Mediated Peptide Ligation Strategies is composed of six chapters. Chapter 1 provides an introductory overview of the fields of amide synthesis and peptide ligation. Particular emphasis is given to the emergence of Native Chemical Ligation and the extended methodologies developed to further the reach of this powerful synthetic technique. The limitations of these approaches, with regards to thioester formation via thioesterification, are also described. In addition, Chapter 1 details the application of thiol-ene chemistries to the synthesis and derivatisation of biomolecules, with specific attention given to the extent of the reaction within the field of peptide chemistry. The use of the acyl thiol-ene reaction to synthesise thioesters in organic syntheses is also detailed, along with the established chemical methods to access peptide thioacids. Chapter 1 is concluded with a brief description of the work described in this thesis and the overarching aims of this work.

Chapter 2 details efforts to develop acyl thiol-ene mediated ligation strategies via the synthesis and investigation of auxiliary bearing dipeptide systems. Unsaturated auxiliaries affixed to amino acid side-chains or α-amino groups of native peptides would permit the rapid formation of peptidic thioesters via acyl thiol-ene addition of a peptide thioacid component. S-to-N acyl transfer of the resulting thioester and subsequent auxiliary cleavage would furnish native amide products. Unfortunately, none of the auxiliary systems investigated possessed the necessary balance of synthetic accessibility, acyl transfer reliability and reaction stability to prompt their insertion into larger systems.

Chapter 3 describes the synthesis of derivatives of unsaturated unnatural amino acids vinylglycine and L-3,4-didehydrovaline inspired by literature precedent. Significant synthetic optimisation was required in the case of L-3,4-didehydrovaline. Nonetheless, routes to access gram quantities of N-tert-butyloxycarbonyl protected vinylglycine and L-3,4-didehydrovaline are detailed. The synthesis of unsaturated dipeptides from these precursors and investigation of their reactivity towards acyl thiol-ene addition with a range of amino acid thioacids is outlined. S-to-N acyl transfer and subsequent derivatisation of thioesters to native tripeptide products is also demonstrated.

Chapter 4 details efforts to synthesise two fragments of the therapeutically valuable peptide human parathyroid hormone. The synthesis of unsaturated decapeptides bearing vinylglycine and L-3,4-didehydrovaline functionalities via solid phase peptide synthesis is described. Efforts to synthesise peptide thioacids through literature protocols are also detailed.

Chapter 5 concludes the work in this thesis and briefly outlines possible future work.

Chapter 6 describes the general experimental procedures employed in the course of this work and characterisation data of the compounds synthesised during the preparation of this thesis.