Synthesis of new captodative alkenes: Alkyl 2-aroyloxy acrylates - Structure, and reactivity in Diels-Alder cycloadditions

Novel captodative alkenes, namely the methyl and ethyl esters of 2-aroyloxyacrylic acids, 2 and 3, have been prepared. Their reactivity and selectivity have been evaluated in Diels-Alder cycloadditions with unsymmetrical dienes, which generate the corresponding adducts with high regioselectivity. No significant stereoselectivity was observed in the reaction with cyclopentadiene (9), although Lewis acid catalysis improved the exo/endo isomeric ratio. Structural and electron spectroscopic studies of these alkenes are supported by MO calculations. FMO theory accounts for the regioselectivity observed with isoprene (7), and the reactivity seen in Diels-Alder additions correlates with the stabilization of the relevant vacant π* MO in these alkenes, which is mainly due to the electron-withdrawing group.