PREPARATION AND STEREOSELECTIVITY OF 1,3-DIPOLAR CYCLOADDITION OF C-GLYCOSYL NITRONES TO N-ARYLMALEIMIDES

The cycloaddition of 3'-hydroxyglycosyl-N-methylnitrone (1) to N-arylmaleimides gave the syn isoxazolidines 6, whereas 3'-acetoxyglycosyl-N-methylnitrone (2) afforded the anti isoxazolidines 8 and 10. The formation of 6 was rationalized by an exo attack, stereoelectronically preferred through the hydrogen bond between the pentose hydroxyl group and one of the carbonyl groups of N-arylmaleimide. The sterically preferred endo attack avoiding the repulsions between N-arylmaleimide and sugar moiety was proposed for addition of 2. The structure and steric configuration of the products have been assigned on the basis of H-1 and C-13-NMR spectroscopy, mainly by nuclear Overhauser effect difference spectroscopy. AM1 calculations of the nitrones and MM2 calculations of the adducts were performed.