Synthesis and Biological Evaluation of Trehalose Glycolipids

Abstract

Numerous α,α-trehalose diesters have been isolated from bacteria such as Mycobacteria and Corynebacteria, and more recently from Caenorhabditis elegans dauer larvae. Although these glycolipids are thought to confer protection to the bacteria and larvae against harsh environmental conditions, it is the biological activities of these compounds, including anti-tumour and adjuvant activities, which have been of major interest to scientists over recent years. In this thesis, three different aspects relating to the synthesis and testing of defined trehalose glycolipids will be presented. First, the synthesis of a variety of fatty acid trehalose diesters (TDEs) with varying lipid lengths was performed and the ability of these glycolipids to activate macrophages was studied. Two different synthetic strategies were employed to attain the TDEs of interest and it was observed that lipid lengths of more than 18 carbons were required for macrophage activation. Furthermore, the C22 fatty acid trehalose monoester (TME) and the C26 TME were also synthesised and interestingly they both showed macrophage activation abilities, with subsequent studies indicating that like TDEs, the TMEs were also ligands for mincle, a C-type lectin found on macrophages. This is the first time that TMEs have been tested for their ability to activate macrophages via Mincle. The cytotoxicity of these compounds and subsequent anti-tumour activity of a few selected compounds were also studied and although the TDEs and TMEs did not exhibit any significant cytotoxicity, in in vivo models the C10 TDE and C22 TDE both showed anti-tumour activity. This depicts that the mechanism for anti-tumour activity of these compounds is not due to cytotoxicity but due to as yet unidentified pathway. Methodology that can be applied to the synthesis of more complex trehalose glycolipids, such as trehalose dicorynomycolates (TDCMs, isolated from Corynebacteria) and trehalose dimycolates (TDMs, isolated from Mycobacteria) was also explored. One of the key steps frequently used in the synthesis of these glycolipids is the Fráter-Seebach alkylation. To improve the efficacy of this methodology allylic iodides, rather than alkyl iodides were used for the -alkylation of β-hydroxy esters. Our results showed that for all substrates studied, the yield of the α-alkylation was greatly improved when the allylic, rather that the alkyl halide was used. The use of this methodology in the synthesis of trehalose monocorynomycolate (TMCM) was also investigated. The third aspect of this thesis focuses on the use of Affinity Based Proteome Profiling (AfBPP) for elucidating the receptors that TDMs bind to upon interacting with host cell. AfBPP focuses on using small molecules which mimic the natural substrate for a particular protein and through the use of ‘trap’ and ‘tag’ groups on the molecule the identity of the protein/receptors can be determined. The synthesis of a TDM probe containing a benzophenone ‘trap’ group and an alkyne ‘tag’ group will be discussed.