Asymmetric 1,3-dipolar cycloaddition of optically active trifluoromethylated alpha,beta-unsaturated aryl sulfones with nitrones: The use of o-dialkylaminoethyl chiral auxiliaries

Optically active trifluoromethylated alpha,beta-unsaturated aryl sulfones 8a-c, which have a chiral N,N-dialkylaminoethyl group on the ortho position, were synthesized from (S)-1-phenylethylamine 2 and ethyl trifluoroacetate, Asymmetric 1,3-dipolar cycloaddition of sulfones 8a-c with some selected nitrones 9a-c gave the corresponding isoxazolidines 10a-c, 11a-c and 12-15 regio-(> 98%) and diastereo-selectively (36-56% de) in 58-80% yields, The absolute configurations of the cycloadducts were assigned on the basis of X-ray crystallographic analysis of the adduct 10a and by the H-1 NMR spectra, The obtained facial selectivity was rationalized by comparison of four possible stable conformers of compound 8a based on AM1 calculations.