1H-indol-3-yl glycosides are powerful tools for histochemical detection of glycosidase activity. Enzymatic hydrolysis of the glycosidic linkage releases free 1H-indol-3-ol, which is rapidly oxidized to an indigo-type dye. This method allows fast and easy in vivo screening without isolation or purication of enzymes, as well as rapid tests of multiple biocatalysts at the same time, for example, in microwell plates or blue-white screening. Unfortunately, the synthesis of the corresponding glycosides proved to be difficult. Previously, the most common synthetic pathway was by use of sodium hydrox-ide in acetone for glycosylation employing the respective N-acetylated 1H-indol-3-ol as an acceptor. Due to low nucleophilicity of the indole hydroxy function, and side reactions, synthesis as well as isolation of the product are challenging. Glycosidation of α-acetobromoglucose with indoxylic acid methyl ester followed by ester cleavage and decarboxylation yielded indicane in approximately 50% yield. Problems associated with this approach are side reactions at the high temperature (160°C) required for decarboxylation, and the glycoside needs to be deprotected before decarboxylation.
