On the regioselectivity in nitrone cycloadditions to γ-oxo α,β- unsaturated esters

The 1,3-dipolar cycloadditions of the cyclic nitrones 1 and 2 to several γ-oxo α,β-unsaturated esters, 5-10, are reported. A strong predominance of the regioisomers with the oxygen atom of the dipole attached to the β-ester position is observed. This high regioselectivity is attributed to steric factors. The reduction of the carbonyl group of some of the major cycloadducts is a good yielding procedure for the preparation of some hydroxylic derivatives that are not formed or obtained only as minor stereoisomers in the cycloadditions of the same nitrones to the corresponding γ-hydroxy α,β-unsaturated esters.