The improved synthesis of two methyl glycosides, <I>viz</I> methyl 4-<I>O</I>-methyl-α-D-glucopyranoside and methyl 4-<I>O</I>-(α-D-glucopyranosyl)-α-D-glucopyranoside (methyl α-maltoside) from readily available starting materials is described for use as model compounds for oxidative studies of the industrially important polysaccharide amylose. Attempts to acetylate methyl-α-D-glucoside regioselectivity proved fruitless, whilst regioselective pivaloylation was more successful, albeit in yields of poor synthetic utility. In a variation to this approach, the glucoside, is synthesized <I>via</I> selective tri-<I>O</I>-benzoylation of methyl-α-D-glucoside with the regiospecificity of the reaction being a function of the relative strengths of intramolecular hydrogen bound rings, which are governed by the conditions under which the reaction is performed. In the second step, methylation of the free hydroxy group can only be achieved under pressure with silver oxide as catalyst. The target compound is obtained in the final step by conventional cleavage of the ester functions. Initial attempts to synthesise methyl-α-maltoside were convergent, being conducted with variants of the Koenigs-Knorr methodology and using reactions of previously prepared haloglucosides with variously protected 4-hydroxymethyl-α-D-glucosides. No combination of catalysts could be found to activate either of the substrates, or in an alternative strategy, mediate the reaction between a halomaltoside and an alkoxy nucleophile. The target maltoside is formed eventually <I>via</I> the selective anomeric deprotection of per-<I>O</I>-acetylated maltose, followed by stereospecific methylation in a manner similar to that described previously. It is found that in this penultimate step an intramolecular silver alkoxy - esteric salt inhibits mutarotation, thus allowing the predominant formation of the α-anomer. Further deacetylation gives rise to methyl-α-maltoside in 63% yield and in only three steps.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:651371 |
| Date | January 1998 |
| Creators | Gebbie, Stuart J. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13881 |
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