Lewis acid-promoted stereoselective Diels-Alder cycloadditions of captodative olefins acetylvinyl carboxylates and NMR structural study of their cyclopentadiene adducts

A study of Lewis acid-promoted Diels-Alder cycloadditions of the captodative olefins 1-acetyl vinyl carboxylates 1 with cyclopentadiene is described. Catalyst, temperature and solvent were the assessed variables, the exo/endo ratio being more significantly modified by the first one. Znh and TiCl4 showed the most remarkable effect with olefin la, giving very high and opposite stereoselectivity, since exo isomer 3 and endo 4 were the major adducts, respectively. The steric effect of the carboxylate substituent of 1 could participate in controlling the stereoselectivity. Structural characterization of the adducts was made by NMR and X-ray analysis. Electronic and anisotropic effects are probably involved in unusual proton chemical shifts of the norbornene structure of the adducts.