Unexpected formation of complex bridged tetrazoles via intramolecular 1,3-dipolar cycloaddition of 1,2-O-cyanoalkylidene derivatives of 3-azido-3-deoxy-D-allose

An unexpected and interesting intramolecular side reaction occurred during the attempted synthesis of glycosyl cyanides upon treatment of 1-O-acetyl-3-azido-3-deoxyallose derivatives with TMSCN and different Lewis acids. Exo-1,2-O-cyanoalkylidene derivatives formed by neighboring group participation and attack of cyanide underwent, after Lewis-acid mediated isomerization to the endo-isomer, intramolecular azide-cyanide cycloaddition leading to the formation of tetrazoles embedded in bridged tetracyclic ring systems. The efficiency of cycloaddition is dependent on the ring structure of the sugar (pyranose or furanose). Of the studied molecules, 3-azido-1,2-O-cyanoethylidene-3-deoxy-allopyranose provides the most suitable scaffold for intramolecular [2+3] cycloaddition under exceptionally mild conditions. Our results highlight the capability of carbohydrates to act as scaffolds for the precise positioning of functional groups productive for a specific chemical reaction.