Cycloaddition reactions of 5,6-dihydro-1,3,2-oxazine 3-oxide and conformational analysis of the resultant bicyclic isoxazolidines

The rate constants for several addition reactions of a heterocyclic nitrone, 5,6-dihydro-1,3,2-oxazine 3-oxide (7), have been determined by a ¹H NMR technique. The heterocyclic nitrone is found to be as reactive as its carbocyclic counterparts. The nitrone underwent regio- and stereo-selective cycloaddition reaction with several alkenes to afford bicyclic isoxazolidines efficiently. The NMR studies showed that the isoxazolidines prefer the conformer with cis ring fusion having an equatorially oriented nitrogen lone pair capable of manifesting an endo anomeric effect.