Catalytic enantioselective cycloaddition and ene reactions

Abstract

Discovery of truly efficient methods of obtaining chiral substances is a substantial challenge for synthetic chemists. Among the several methods to obtain chiral substances, the use of chiral catalysts in organic syntheses has recieved considerable attention recently. Among the several catalysts reported, we tried to investigate the application of two types of catalysts, $C_2$-symmetric bis(oxazoline)-Lewis acid catalysts and BINOL derived Brønsted acid catalysts. α’-Phosphoric enone and α’-sulfonyl enone were chosen as bidentate substrates for $C_2$-symmetric bis(oxazoline)-Lewis acid catalysts. This catalyst-substrate system gave excellent results in 1,3-dipolar cycloaddition reaction of nitrones. When α’-phosphoric enone was used as a bidentate substrate, the endo/exo ratio of resulting isoxazolidine product was over 99 : 1 and the enantioselectivity was up to 99\%. We also investigated the possibility of using this catalyst-substrate system for other 1,3-dipolar cycloaddition reactions. But no meaningful results were obtained. Finally, the stereoselectivity of the Diels-Alder reaction using α’-phosphoric enone and α’-sulfonyl enone as bidentate substrates was investigated. Interestingly, each substrate gave high stereoselectivity for Diels-Alder reaction with cyclopentadiene. In addition, the possibility for using BINOL derived Brønsted acid catalyst as a catalyst of ene-reaction was investigated and we found some N-triflyl phosphoramide derivatives of BINOL as good catalysts for the ene reaction between α-methyl styrene and ethyl glyoxalate (up to 93\% ee). But, the scope of the catalyst was too narrow to be a useful reaction.