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

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 .