Catalytic Asymmetric 1,3‐Dipolar Cycloaddition of Nitrones to Alkylidene Malonates: Highly Enantioselective Synthesis of Multisubstituted Isoxazolidines
All under control! A catalytic asymmetric 1,3‐dipolar cycloaddition reaction of nitrones to alkylidene malonates, catalyzed by chiral N,N′‐dioxide–Ni(ClO4)2⋅6 H2O complexes, has been developed with excellent yields, diastereo‐, and enantioselectivities (see scheme, R1=aryl, R2=alkyl, R3, R4=Ph). In addition, a possible transition state has also been proposed to elucidate the high level of enantio‐
6H(2)O catalyzed the 1,3-cycloaddition between nitrones 3 with alkylidene malonates 2 at 0 degrees C to give the isoxazolidines with both high enantioselectivity and high exo selectivity. However, when the temperature was lowered from 0 to -40 degrees C, the same cycloaddition afforded endo isomers as the major products with good to high enantioselectivity. A mechanism is provided.