Synthetic approaches to naturally occurring strigolactones and selected analogues

Strigolactones (SLs) are carotenoid-derived plant hormones, that have garnered great interest from the scientific community for their wide range of biological activities promising many agricultural applications. Due to their structural complexity and the importance of stereochemistry to their biological activity, synthetic routes to natural SLs are often lengthy and cumbersome. As a result, investigation into synthetic methodologies that allow access to the natural isomers of SLs is of great importance. This thesis sets out to provide a review of current existing methodologies for the synthesis of natural SLs as well as develop novel synthetic routes to SLs. In the first chapter this thesis will discuss the current existing methodologies for the synthesis of the natural SLs strigol and orobanchol, as well as a synthetic analogue GR24. In the second chapter this thesis will discuss an enantioselective approach to the natural SL strigol by employing a Noyori asymmetric transfer hydrogenation to install a chiral centre. Strigol 1a was produced through existing and novel chemistry both racemically and asymmetrically. The application of a variety of novel methods to access the intermediate 65 were met with mixed success. Overall, strigol was prepared racemically in eleven steps (3% yield) and was synthesized enantioselectively in twelve steps (0.34% yield). In the third chapter this thesis will discuss a similar methodology for obtaining the natural SL orobanchol. This synthesis constitutes the first asymmetric preparation of orobanchol since its structural revision in 2011. Racemic orobanchol 2a was obtained in a 0.67% yield over 4 steps from the ABC system 65. Enantiopure orobanchol (‚Äö-)-2a was obtained in a 0.12% yield over ten steps from the ABC intermediate 65. The latter constitutes the first asymmetric synthesis of orobanchol since its structural revision by Ueno and co-workers in 2011. In the fourth and final chapter this thesis will discuss the synthesis of selected synthetic analogues employing the Noyori asymmetric transfer hydrogenation.