Cycloaddition reactions on activated exo-glycals

Cycloaddition reactions of activated exo-glycals and nitrones proceeded only under microwave activation, with excellent facial selectivities on furanoglycosylidenes and good stereocontrol on the nitrone producing only two diastereomeric spiroisoxazolidines. α/β-Spiro sugar-isoxazolidines are obtained from pyrano exo-glycals. The cycloaddition reaction with nitrile oxide proceeds at room temperature and gives open-chain isoxazoles due to facile β-elimination of the sugar ring oxygen on the intermediate isoxazoline ring system. All the heterocycles obtained this way can be regarded as nucleoside analogues.