Intramolecular 1,5- versus 1,6-hydrogen abstraction reaction promoted by alkoxyl radicals in pyranose and furanose models

The primary alkoxyl radical generated by reaction of 1-(2-hydroxyethyl)-glycosides with (diacetoxyiodo)benzene (DIB) and iodine can undergo regioselective intramolecular hydrogen abstraction (IHA) reactions to furnish four different dioxabicyclic systems derived from carbohydrates. The results strongly suggest that the regiocontrol and feasibility of the cyclisation are dependant not only on geometric and stereoelectronic factors, but also on polar and thermochemical factors. The correct selection of the substituents at the precursors can favour the 1,6-IHA against the 1,5-IHA pathway.

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