Abstract
Lactones and esters are important flavour and aroma constituents in many natural products; they are pervasive molecules in nature. The lactones are relatively weak flavor chemicals that occur in products such as butter, coconut, raspberry, strawberry, tea, apricot, beer, cheddar cheese, peach, pineapple and Rum. This thesis demonstrates the scope and limitation of the lanthanum bis(trimethylsilyl)amide [La{N(SiMe3)2}3] as a homogenous Tishchenko catalyst. The study provides a series of examples of the success of the catalyst in various Tishchenko esterification reactions. A series of substituted aromatic aldehydes were used successfully in the homogenous Tishchenko reaction. These aldehydes included in particular a series of oxygen protected compounds illustrating the compatibility of the catalyst with a variety of oxygen containing functional groups. The homo-esterification and cross-esterification studies carried out, showed furthermore that the esterification of para-substituted benzaldehydes depended dramatically on the electronic properties of the substituents. Cross-esterification studies using two different benzaldehydes with different electronic properties has shown that mixed esters can be obtained with some selectivity. The aldehydes with electron withdrawing substituents act preferentially as a hydride source. Conclusions regarding the mechanism of the Tishchenko reaction were drawn Apart from the electronic effect, the steric hindrance was found to play a role in the esterification process. A series of intramolecular Tishchenko reactions were investigated. In particular eight membered lactones were obtained in good yields. The final part of the study involved the attempted natural product synthesis of the Sennosides. In this part of the work a successful synthesis of the sennoside C aglycon in a few synthetic steps was achieved. Chrysophanol was also synthesised from aloin A in three synthetic steps. All compounds synthesised are valuable intermediates in organic synthesis or for the fine chemicals industry.