Lewis Acid‐Catalysed Anomerisation and Rearrangement of Alkyl D‐Glycopyranosides During Acetalisation with Methyl Pyruvate: How to Utilise it for the Preparation of 1‐(Carboxyethylidene)glycopyranosyl Donors

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Abstract
The preparation of 4,6‐O‐(1‐methoxycarbonylethylidene)‐D‐glycopyranosides 5 starting from 2,3‐di‐O‐benzoyl‐D‐glycopyranosides 1 and their 4,6‐bis‐O‐trimethylsilyl ethers 2 using methyl 2,2‐bis(phenylthio)propionate (4) and methyl pyruvate (3), respectively, under Lewis acid catalysis conditions is described. In the D‐gluco series anomerisation of alkyl β‐D‐glucopyranosides is observed as a side reaction, giving complex mixtures and low yields of the title compounds whereas alkyl‐α‐D‐glucopyranosides and alkyl or phenyl 1‐thio‐β‐D‐glucopyranosides react without anomerisation. The reactions are accompanied by isomerisation of the initially formed diastereomer (R)‐5 having an equatorial methoxycarbonyl group to the thermodynamically favoured diastereomer (S)‐5 having the methoxycarbonyl group in an axial position. In the D‐galacto series similar side reactions are observed accompanied by rearrangement to furanosides 9 of alkyl and phenyl β‐D‐galactopyranosides. The results are used for efficient syntheses of two protected disaccharides 8 and 11 representing structural fragments of the polysaccharide repeating units of Mycobacteria and Rhizobia via pyruvate acetal‐containing glucosyl and galactosyl donors 7 and 10.