Reversal of Diastereoselectivity of Nitrile Oxide 1,3-Dipolar Cycloadditions by Mg(II). Acceleration of Cycloaddition by Microwave Irradiation

1,3-Dipolar cycloadditions of mesitonitrile oxide to Baylis–Hillman adducts (β-hydroxy-α-methylene esters) proceed regioselectively in good yields. Addition of Grignard reagent reverses the diastereoselectivity of the cycloaddition. Microwave irradiation strongly accelerates the reaction with only a small effect on its diastereoisomeric excess.