Synthesis of oxa-aza spirobicycles by intramolecular hydrogen atom transfer promoted by N-radicals in carbohydrate systems

Abstract

The nitrogen-centred radical generated by reaction of an N-phosphoramidate or N-cyanamide, attached to a tri- or tetramethylene tether extended from the C-1 of a carbohydrate, with (diacetoxyiodo)benzene (DIB) and iodine can undergo a regio- and stereoselective intramolecular hydrogen atom transfer (HAT) reaction to furnish four different oxa-azaspirobicyclic systems: 1-oxa-6-azaspiro[4.4]nonane, 1-oxa-6-azaspiro[4.5]decane, 6-oxa-1-azaspiro[4.5]decane and 1-oxa-7-azaspiro[5.5]undecane. A tandem 1,5- or 1,6-HAT-oxidation-nucleophilic cyclisation mechanism is proposed.