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This thesis concerns the construction of 2,3-dihydro-4r7-pyran-4-one and 3(2//)-furanone ring systems by mercury(II)-catalysed reactions, or by more conventional cyclisations, and the potential of such reactions for the synthesis of natural products. Chapter one provides a literature survey of reactions permitting the construction of 2,3-dihydro-4/7-pyran-4-one and 3(2//)-furanone ring systems background literature to relevant natural products is also provided, such as polyether antibiotics, carbohydrates and antitumer agents. Chapter two describes the application of mercury(II)-catalysed cyclisations of dihydroxylated ynones to give 3(2//)-furanones this was achieved by the dihydroxylation of enynones using Sharpless's asymmetric dihydroxylation conditions followed by treatment with a mecury(II) catalyst. The scope and limitations of this method were investigated and shown in several cases to give good yields and high enantiomeric excess. In cases where there was an electron-donating group on the alkyne functionality, such as an ethoxy group, the cyclisation occurred spontaneously during the dihydroxylation step. This methodology was applied for the synthesis of a natural product to confirm the selectivity of the cyclisation step. In chapter three the reactions developed in chapter two formed the basis of a proposed route to the natural product (-)-Pestalotin, a gibberellins synergist. Thus, the Sharpless asymmetric dihydroxylation method was applied to (3,y-unsaturated ketones to give a main intermediate for the synthesis. Chapter four details different approaches to NK10958P, a plant growth regulator. The synthesis of two main fragments was achieved and the coupling of these fragments by syn-selective aldol addition is expected to furnish NK10958 P and its methyl analogue, pironetin, which has been reported to have good cytotoxic and immunosuppressive activity. Full experimental details follow chapter 2-4 and reference sections are provided at the end of each chapter.

Asymmetric catalysis is an ideal method for synthesising optically active compounds. A small amount of a chiral catalyst produces chiral materials. To achieve maximum chiral induction, efficient catalytic systems must be created. Enantioselective addition of organometallic reagents to aldehydes affords optically active secondary alcohols. This is one of the most important and fundamental asymmetric reactions. The optically active alcohols are components of naturally occurring compounds. The secondary alcohols are obtained by stereoselective alkylation of aldehydes using organometallic reagents. The most common ligands that are used in the alkylation of benzaldehyde in the presence of dialkylzinc are amino alcohols. The term "nonlinear effects" was first used in 1986. There are two types of NLE, a positive nonlinear effect (positive NLE) and a negative nonlinear effect (negative NLE). A positive NLE operates if eeprod is higher than eeaux whereas a negative NLE operates if eeprod is lower than eeaux• To gain a better understanding of the behaviour of catalysts during the course of a reaction that leads to NLE, mathematical models were developed. Only the ML2 and the reservoir effect are discussed in this project. One of the most notable examples of NLEs is, Noyori's example using DAIB in the ethylation of benzaldehyde. MIB was used as a starting point to verify the reported NLE and prove the reliability of our results. Following this, a set of oxazolines were tested for NLE. These ligands have proven to be successful in the I alkylation of aromatic aldehydes. We applied them to the ethylation of benzaldehyde I and assessed their catalytic activity; whether they exhibit NLE or not. Computational studies were carried out on the set of oxazolines that express NLE in hope to aid us in constructing a model to predict NLE.

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