The application of diastereoselective free radical reactions in carbohydrate chemistry

This thesis focuses essentially on the application of diastereoselective radical reactions to the preparation of O- and C-glycosidic linkages. As such, the introduction surveys both the general area of glycoside synthesis and diastereoselective radical reactions and in particular, their use in carbohydrate chemistry. Methyl [Phenyl 4,5,7-tri-O-benzyl-3-deoxy-2-sulphonyl-β-D-arabino-2-heptulopyranoside]onate (sulphone ester) was prepared from tri-O-benzyl-D-glucal according to a method previously developed in the laboratory. The highly diastereoselective preparation of 6-C-glycosides was achieved by reductive desulphonylation of the sulphone ester with lithium naphthalenide followed by quenching with an alkyl halide and ultimately by Barton reductive decarboxylation. Diastereoselectivities in excess of 95:5 were routinely observed. Thermal elimination of phenylsulphinic acid from the sulphone ester gave the corresponding 1-carbomethoxy glycal. cw-Hydroxylation with osmium tetroxide gave exclusively the gluco-isomer. Acetonation, saponification and reductive decarboxylation gave l,2-O-isopropylidene-3,4,6-tri-O-benzyl-β-D-gluco-pyranose as a single anomer and the first example of a 1,2-trans-isopropylidene derivative of a pyranose sugar. Other β-O-gluco-pyranosides were prepared as single anomers by related process. The extension of the methodology to the preparation of β-O-manno-pyranosides was investigated. The extrapolation of the method to the preparation of furanosidic linkages was investigated. 2,3:4,6-Di-O-isopropylidene-2-keto-L-gulonic acid was chosen as starting material and methods were developed for its selective deprotection and protection. The stereoselectivity of the radical decarboxylation step was investigated. Finally, in the context of the C-glycoside synthesis, 6-trimethylsilylethoxymethyl chloride (SEM-Cl) was developed as a practical equivalent to formaldehyde in low temperature, non-aqueous aldol type reactions

Sticking under wet conditions: the remarkable attachment abilities of the torrent frog, staurois guttatus

Tree frogs climb smooth surfaces utilising capillary forces arising from an air-fluid interface around their toe pads, whereas torrent frogs are able to climb in wet environments near waterfalls where the integrity of the meniscus is at risk. This study compares the adhesive capabilities of a torrent frog to a tree frog, investigating possible adaptations for adhesion under wet conditions. We challenged both frog species to cling to a platform which could be tilted from the horizontal to an upside-down orientation, testing the frogs on different levels of roughness and water flow. On dry, smooth surfaces, both frog species stayed attached to overhanging slopes equally well. In contrast, under both low and high flow rate conditions, the torrent frogs performed significantly better, even adhering under conditions where their toe pads were submerged in water, abolishing the meniscus that underlies capillarity. Using a transparent platform where areas of contact are illuminated, we measured the contact area of frogs during platform rotation under dry conditions. Both frog species not only used the contact area of their pads to adhere, but also large parts of their belly and thigh skin. In the tree frogs, the belly and thighs often detached on steeper slopes, whereas the torrent frogs increased the use of these areas as the slope angle increased. Probing small areas of the different skin parts with a force transducer revealed that forces declined significantly in wet conditions, with only minor differences between the frog species. The superior abilities of the torrent frogs were thus due to the large contact area they used on steep, overhanging surfaces. SEM images revealed slightly elongated cells in the periphery of the toe pads in the torrent frogs, with straightened channels in between them which could facilitate drainage of excess fluid underneath the pad

Casuarina equisetifolia cone as sustainable adsorbent for removal of Malachite green dye from aqueous solution using batch experiment method

Various methods have been devised in order to treat the wastewaters before they are disposed into water bodies such as the rivers and the seas. One of the methods used is adsorption as it has the advantages of cheap, environmentally friendly and wide range of materials can be used. In this study Casuarina equisetifolia cone (CEC) was used to remove malachite green (MG) from aqueous solution and the adsorption process was investigated through contact time, pH, temperature, ionic strength and dye concentration. The experimental results indicated that CEC can operate well in high ionic strength solution and was not significantly affected by temperature. The adsorption process followed the pseudo-2nd order and can be described using the Langmuir model where the maximum adsorption capacity was determined as 58 mg g-1

Nutritional attributes of Hemiparasitic mistletoe Scurrula ferruginea in Brunei Darussalam

Three different associations of Scurrula ferruginea parasites on three different hosts, namely Tabebuia pallida, Acacia holosericea and Acacia auriculiformis were collected from the Brunei-Muara District, Brunei Darussalam. Moisture content and chemical analyses (ash content, total carbohydrate content, crude protein, proline and mineral content composition) were determined to explain the host-parasite physiological biochemistry. Scurrula ferruginea contained relatively higher moisture content (47 – 65%) and ash content (2.1-2.5%, dry basis) than the hosts (0.7-1.4%, dry basis). High nutrient and moisture contents in Scurrula ferruginea make it more preferred food source than its hosts for generalist herbivores in a given community. The mistletoe exhibited differential storage profile of total carbohydrate (1.9-6.4%, dry basis) and total nitrogen (1.2 – 3.0%, dry-basis) when compared to hosts (total carbohydrate 2.3 - 3.0 % dry basis; total nitrogen 1.6 - 2.1%). Meanwhile the proline content (24.9-56.0 mg/kg, dry basis) were found in Scurrula ferruginea. Among all the minerals analysed, potassium is the most abundant mineral present in all mistletoe-host associations. Data indicated that certain host desired solutes are preferentially absorbed and stored in mistletoe. </jats:p

Existence and Uniqueness of the Exponentially Stable Limit Cycle for a Class of Nonlinear Systems via Time-Domain Approach with Differential Inequality

The concept of the exponentially stable limit cycle (ESLC) is introduced, and the ESLC phenomenon for a class of nonlinear systems is explored. Based on time-domain approach with differential inequality, the existence and uniqueness of the ESLC for such nonlinear systems can be guaranteed. Besides, the period of oscillation, the amplitude of oscillation, and guaranteed convergence rate can be accurately estimated. Finally, two numerical simulations are provided to illustrate the feasibility and effectiveness of the obtained result