THE REGIOSELECTIVITY OF NITRONE AND NITRILE OXIDE CYCLOADDITIONS TO ALKYLIDENECYCLOPROPANES

The 1,3-dipolar cycloaddition of nitrile oxides and a nitrone to alkylidencyclopropanes 1-5, used as model for methylenecyclopropanes substituted with aryl group (1), electron-releasing groups (2 and 3) and electron-attracting groups (4 and 5) is reported. The cycloaddition to alkylidenecyclopropanes 1-3 gives prevalently or exclusively adducts bearing the cyclopropane ring on the C4 position of the isoxazoline (isoxazol idine) ring, whereas methoxycarbonyl substituted methylenecyclopropanes 4 and 5 give adducts with the opposite regiochemistry. The high regioselectivity observed in the case of dialkyl substituted methylenecyclopropanes raises the question of the role played by the cyclopropylidene ring in these cycloadditions. A FMO approach based both on semiempirical and ab initio calculations is unable to explain this "cyclopropylidene effect".