SYNTHETIC METHODS FOR ESTER BOND FORMATION AND CONFORMATIONAL ANALYSIS OF ESTER-CONTAINING CARBOHYDRATES

This dissertation encompasses work related to synthetic methods for the formation of ester linkages in organic compounds, as well as the investigation of the conformational influence of the ester functional group on the flexibility of inter-saccharide linkages, specifically, and the solution phase structure of ester-containing carbohydrate derivatives, in general. Stereoselective reactions are an important part of the field of asymmetric synthesis and an understanding of their underlying mechanistic principles is essential for rational method development. Here, the exploration of a diastereoselective O-acylation reaction on a trans-2-substituted cyclohexanol scaffold is presented, along with possible reasons for the observed reversal of stereoselectivity dependent on the presence or absence of an achiral amine catalyst. In particular, this work establishes a structureactivity relationship with regard to the trans-2-substituent and its role as a chiral auxiliary in the reversal of diastereoselectivity. In the second part, the synthesis of various ester-linked carbohydrate derivatives, and their conformational analysis is presented. Using multidimensional NMR experiments and computational methods, the compounds’ solution-phase structures were established and the effect of the ester functional group on the molecules’ flexibility and three-dimensional (3D) structure was investigated and compared to ether or glycosidic linkages. To aid in this, a novel Karplus equation for the C(sp2)OCH angle in esterlinked carbohydrates was developed on the basis of a model ester-linked carbohydrate. This equation describes the sinusoidal relationship between the C(sp2)OCH dihedral angle and the corresponding 3JCH coupling constant that can be determined from a JHMBC NMR experiment. The insights from this research will be useful in describing the 3D structure of naturally occurring and lab-made ester-linked derivatives of carbohydrates, as well as guiding the de novo-design of carbohydrate based compounds with specific shape constraints for its use as enzyme inhibitors or similar targets. In addition, the above project led to the development of a methodology for the synthesis of symmetrical ester molecules from primary alcohols using a mild oxidative esterification reaction, which proceeds in hydrous solvents using a nitrosyl radical catalyst. The reaction could be performed with a variety of alcohols and the resulting compounds are of interest in the fragrance and flavor industries

Laser Induced Plasmas: A Light source for Biological Spectroscopy

The work presented in this thesis investigates the viability using a laser induced plasmas as a high intensity, UV/VIS/IR benchtop light source for biological spectroscopic applications. It is well known that plasmas make excellent light sources due to their broadband emission throughout the UV/VIS/IR. A detailed account is given of the construction and testing of a prototype benchtop spectrometer that utilised a laser induced plasma as its light source. The parts to build this spectrometer had a total cost of £6480.49, a competitive price for an instrument capable of measuring absorbance/turbidity spectra in the wavelength range of 380 to 700 nm at a rate of up to 5 times a second and to a wavelength resolution of approximately 0.2 nm. The production of shorter wavelength is very possible using this technology but would require a much more expensive, higher repetition rate laser, which was beyond the scope of this project. A review of the physical factors influencing the emission of light from a plasma is first presented. Several noble gases were tested to optimise the intensity and short wavelength output of the plasma. The results showed that argon had the highest emission intensity light for reasonable cost. The prototype instrument can be run in two different modes, static mode and dynamic mode. In static mode, the spectrometer can determine the absorbance spectrum of samples that do not change over time such as coloured dyes. In dynamic mode that monitors the absorbance of samples whose absorbance/turbidity varies with time. The accuracy of the prototype plasma spectrometer was tested by determining the absorption spectrum of holmium oxide in perchloric acid, a standard solution often employed for spectrometer calibration and testing. The prototype spectrometer was then successfully employed to investigate the effect of heparin, in the presence of the cations Fe2+, Zn2+ and Cu2+, on the aggregation rate of the protein human lysozyme, over a range of pHs. The aggregation of lysozyme was an interesting system to monitor, as it has been proposed as a model for the formation of the fibrils found in amyloidal plaques

The mechanism of racemisation of 5-substituted hydantoins in aqueous solution

This thesis describes our studies of the racemisation of substituted hydantoins and is divided in six chapters. Chapter 1 presents the concepts of chirality and racemisation and its implications in drug development. The literature on the stereolability of 5-substituted hydantoins is summarised. Chapter 2 describes detailed kinetic and mechanistic studies of the racemisation of (5)-5-benzylhydantoin and (5)-3-iV-methyl-5-benzylhydantoin, aimed at establishing the mechanistic aspects of racemisation of these molecules. Kinetics of H/D exchange and racemisation, kinetic isotope effects, and solvent kinetic isotope effects all favour the SeI mechanism of racemisation as opposed to the Se2 process proposed by others. Chapter 3 discusses the effects of structural modifications on the stereolability of a series of model 5-substituted hydantoins with improved water solubility as compared to (iS)-5-benzylhydantoin and (5)-3-A-methyl-5-benzylhydantoin. Hydantoins containing a protonated amino or an ammonium group showed increased stereolability. This finding was attributed to intramolecular facilitation of racemisation by the positive charge. The primary and solvent kinetic isotope effects on the racemisation of two model 5-substituted hydantoins were determined and again supported an SeI mechanism of racemisation. Chapter 4 deals with solvent effects on racemisation and H/D exchange of a series of 5-benzylhydantoins. DMSO added to phosphate buffers showed a marked rate-increasing effect for all of the substrates under study. Co-added 2-propanol and dioxane showed a rate-decreasing effect on neutral hydantoins and a rate-increasing effect on a cationic hydantoin. Solvent effects on the basicity of anionic catalysts and phenomena of preferential solvation were proposed as important factors affecting the rate constants in mixed media. Chapter 5 reports the preliminary results of exploratory experiments aimed at assessing potentials and limitations of VCD and IR spectroscopy for kinetic and mechanistic studies of racemisation. Finally, Chapter 6 summarises our findings and presents recommendation for future work