Copper(II)-(S)-bisoxazoline and zinc(II)-(S)-bisoxazoline catalysed asymmetric 1,3-dipolar cycloaddition reactions of nitrones with electron-deficient alkenes

A transition-metal-catalysed regio-, diastereo- and enantioselective 1,3-dipolar cycloaddition reaction between nitrones and electron-deficient alkenes has been developed employing 25 mol% of chiral copper(II)- or zinc(II)-(S)-bisoxazolines as catalysts. In the presence of powdered 4 Å molecular sieves and copper(II)-(S)-bisoxazoline as catalyst, the nitrones smoothly react with alkenes at −60°C to give isoxazolidines in good yields and diastereoselectivity and with high enantioselectivities of up to 62% ee. By crystallisation of the isoxazolidines from MeOH an optical purity of >97% ee is obtained. The influence of the metal salts, temperature, molecular sieves, catalyst amount and solvents on the reaction course was discussed.