Radical reactions involving cyclodextrins and model sugars

Time-resolved EPR/laser flash photolysis studies of aqueous solutions of D-glucose, alpha-methyl-D-glucose, maltose and alpha-, beta- and gamma-cyclodextrin (CD) with acetone, pyruvic acid (PA), methylethylketone (MEK) and levulinic acid (LA) showed that the excited triplet state of these ketones could be used to selectively produce different polarized sugar radicals depending on the substrate used. For both D-glucose and maltose the C1 carbon centered radical was observed to be most intense and thought to be a consequence of the anomeric effect which favors binding of the ketone to the hydroxyl group situated on this carbon. For alpha-methyl-D-glucose the C7, C6 and C2 centered radicals were most intense and their selective formation is attributed to affinity effects. The following CD radicals were identified. From alpha-CD; C3 and C5 centered radicals. From beta-CD; C3, C5 and C1. From gamma-CD; C1 and C6. A C5 carbon centered radical formed from glucopyranoside ring-opening of the C1 radical was also observed for beta-CD and gamma-CD. Changes in the intensity of the spectra of the radicals observed between alpha-, beta- and gamma-CD were explained to arise from the extent ketone binding to the cyclodextrin cavity.