Regioselectivity of the Diels‐Alder Additions of 2‐Substituted 5,6 Dimethylidenenorbornanes and‐bicyclo[2.2.2]octanes

The cycloadditions of methyl propynoate and methyl vinyl ketone to 5,6‐dimethylidene‐2‐norbornanone ( 6 ) are para ′‐regioselective “Para” ( p ) designs in this paper the 4, 9‐disubstituted tricclo[6.2.1.0 2, 7 ] undecane‐ and 4, 9‐disubsstituted tricycle[6.2.20 2,7 ] dodecane derivatives, “meta” ( m ) design the corresponding 4, 10‐disubstituted compounds. . A smaller para ‐regioselectivity is observed for the addition of methyl propynoate to 5,6‐dimethylidene‐2bicyclo[2.2.2]octanone ( 10 ). No regioselectivity is observed with 5,6‐dimethylidene‐2 exo ‐norbornyl alcohol ( 3 ), acetate ( 5 ) and 5,6‐dimethylidene‐2 exo ‐bicyclo[2.2.2]octanol ( 9 ). PMO arguments based on the shape of the HOMO's and subHOMO's of the dienes allow to rationalize these observations. Unpredictable para ′‐ or ‘ meta ’regioselectivities are found for the Diels,‐Alder additions of 5,6‐dimethylidene‐2endo‐norbornyl alcohol ( 2 ), acetate ( 4 ) and 5,6‐dimethylidene‐2 endo ‐bicyclo[2.2.2]octanol ( 8 ). The carbonyl group of β,γ unsaturated ketones such as 6 and 10 can act as an electron donating homoconjugated substituent. The n(CO) ↔ σ[C(1), C(2)] ↔ π[C(5), C(6)] hyperconjugative interaction can override the usual electron‐withdrawing effect of this function.