On the Lewis acid-induced [1,3]-dipolar cycloaddition of allylic and homoallylic alcohols to N-methyl-C-phenyl nitrone

Both the rate and stereoselectivity of [1,3]-dipolar cycloadditions between monosubstituted allylic and homoallylic alcohols 7, 17, 25, 28 and 34 and the benzaldehyde-derived nitrone 8 are increased enormously when the reactions are carried out in the presence of one equivalent of magnesium bromide-diethyl ether. Stereoselectivities in the range of 5 : 1 up to > 95 : 5 are achieved in 2-5 hours at 80 degrees C, in contrast to purely thermal reactions which are almost stereorandom and which require over 48 hours at this temperature. The major products 9, 18, 26, 29 and 35 are endo adducts with the opposite trans-stereochemistry across the isoxazolidine ring relative to the exo adducts 42 obtained from similar catalysed reactions using C-acyl nitrones 4. Models which account for this difference are proposed.