Palladium(II)-Catalyzed Asymmetric 1,3-Dipolar Cycloaddition of Nitrones to 3-Alkenoyl-1,3-oxazolidin-2-ones

Chiral phosphinepalladium(II)-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrones to α,β-unsaturated carboxylic acid derivatives has been investigated. In the presence of a catalytic amount of [Pd(NCMe) 2 {( S )-tolbinap}](BF 4 ) 2 [TolBINAP = 2,2‘-bis(di- p -tolylphosphino)-1,1‘-binaphthyl], the reaction of 3-alkenoyl-1,3-oxazolidin-2-ones as dipolarophiles and N-substituted N -benzylidenenitrones has been successfully performed to give isoxazolidine derivatives in high yields with high enantioselectivities. For example, 3-((2,5-dimethyl-3-phenylisoxazolidin-4-yl)carbonyl)-1,3-oxazolidin-2-one was obtained from the reaction of N -benzylidenemethylamine N -oxide and 3-crotonoyl-1,3-oxazolidin-2-one in 89% yield with 60% endo selectivity and 91% ee of the endo isomer. The cycloaddition of N -benzylidenebenzylamine N -oxide and 3-crotonoyl-1,3-oxazolidin-2-one afforded 3-((2-benzyl-5-methyl-3-phenylisoxazolidin-4-yl)carbonyl)-1,3-oxazolidin-2-one in 94% yield with 93% endo selectivity and 89% ee of the endo isomer. Remarkably, the endo/exo selectivity of the products depended on the N-substituent group of the nitrones. These selectivities were explained using molecular modeling.