Diastereoselectivity-switchable and highly enantioselective 1,3-dipolar cycloaddition of nitrones to alkylidene malonates

Trisoxazoline 1/Co(ClO4)(2).6H(2)O catalyzed the 1,3-cycloaddition between nitrones 3 with alkylidene malonates 2 at 0 degreesC to give the isoxazolidines with both high enantioselectivity and high exo selectivity. However, when the temperature was lowered from 0 to 40 degreesC, the same cycloaddition afforded endo isomers as the major products with good to high enantioselectivity. A mechanism is provided.