Highly Selective Diels-Alder Cycloadditions of Captodative Dienophiles 1-Acetylvinyl Arenecarboxylates to Unsymmetrically Substituted Butadienes

Thermal Diels-Alder cycloadditions of captodative olefins 1-acetylvinyl arenecarboxylates, CH₂—C(COC-H₃)OCOAr, la (Ar = C₆H₄pNO₂), lb (Ar = a-naphthyl), and lc (Ar = β-naphthyl), with isoprene (2) were shown to be regioselective. This regioselectivity was greatly improved by using Lewis acids catalysis (ZnCl₂, BF₃.Et₂O), the para adduct being the main isomer. The addition of dienophile la to 1-substituted dienes 3, 4, and 6 and 1.3- disubstituted butadiene 5 was highly regioselective too, and the ortho isomer was the only observed adduct. Stereoselectivity of these reactions was examined, and it was determined for all of these dienes, including the 1.4- diacetoxybutadiene (7), that the endo stereoisomer was obtained in a high proportion (>80%). The structure of major adducts 8a, 18a, 20a, 22a, 26a, and 29a was established by ^lH and ¹³C NMR spectroscopy. Regioselectivity of these cycloadditions has been rationalized in terms of the FMO theory by MO calculations of dienophiles 1, using MINDO/3 and ab initio methods. It is suggested that secondary orbital interactions might be responsible for the observed endo selectivity.