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 capto dative olefins 1-acetylvinyl carboxylates 1 with cyclopentadiene is de scribed. Catalyst, temperature and solvent were the assessed variables , the exo/endo ratio being more significantly modified by the first on e. ZnI2 and TiCl4 showed the most remarkable effect with olefin la, gi ving very high and opposite stereoselectivity, since exo isomer 3 and endo 4 were the major adducts, respectively. The stet ic effect of the carboxylate substituent of 1 could participate in controlling the: st ereoselectivity. Structural characterization of the adducts was made b y NMR and X-ray analysis, Electronic and anisotropic effects are proba bly involved in unusual proton chemical shifts of the norbornene struc ture of the adducts.