Recently we have developed an electrochemical method for oligosaccharide synthesis based on the automated electrochemical assembly (AEA) of carbohydrate building blocks. This method enables us to access fully protected oligosaccharides rapidly in a preparative scale; however, the assembly of building blocks is one of the steps of oligosaccharide synthesis. In order to increase efficiency of the entire process, it is important to introduce a strategy which facilitates separation and purification of oligosaccharides thus-obtained.

Separation tags are small molecules which are covalently attached to the substrates to make the separation step easier and faster. There are several tags including a fluorous tag, an ionic liquid tag, and others which have already been introduced into oligosaccharide synthesis. In this study, we introduced an ionic liquid tag to the building block for electrochemical glycosylation, because we presume that an ionic liquid tag works not only as a separation tag but also as an electrolyte and a reservoir of the counter anion for the reactive intermediate of glycosylation. Here we report the development of a novel ionic liquid tag and its application for electrochemical glycosylation.

The developed benzyl ether-type ionic liquid tags were prepared from the corresponding precursors that can be prepared in a single step by the reaction of tertiary amines and introduced to a thioglycoside building block also in a single step with high yields. Electrochemical glycosylation of the thioglycoside equipped with the ionic liquid tag affords the corresponding disaccharides which can be purified easily by washing with water and organic solvent sequentially. In the presence of palladium carbon under the hydrogen gas atmosphere, removal of the tag and benzyl protecting groups of hydroxyl groups can be performed to obtain the fully deprotected disaccharides.