Unexpected phenomena in glycosylations of acceptors with L‐idose configuration

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Abstract
For the synthesis of Galß(1 → 4)IdoA disaccharides 1, several glycosylations of L‐iduronic acid derivatives 5 and 13 and of L‐idopyranose derivative 21 were performed with differently protected D‐galactopyranosyl donors under various conditions. It was found that these coupling reactions mainly led to the formation of the undesired α‐interglycosidic linkage, even when the donor contained a participating group at C‐2. On the other hand, coupling of the corresponding D‐glucopyranosyl donor 19 with 13 led to a much higher β/α ratio (2/1). By changing the conformation of the acceptor into the 1,6‐anhydro‐L‐idopyranose acceptor 23, 90% of the desired β‐coupled disaccharide was obtained in the glycosylation with galactopyranosyl donor 12. Finally it was found that the α/β ratio of the condensation with 25, promoted by an insoluble silver salt, was influenced in an unusual way by the substituents at the galactosyl bromide.

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