Leng, Linda Lim Biaw; (1991) The application of diastereoselective free radical reactions in carbohydrate chemistry. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

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.

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