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

✍ Scribed by Marco Avenati; Pierre-Alain Carrupt; Daniel Quarroz; Pierre Vogel

• Publisher: John Wiley and Sons
• Year: 1982
• Tongue: German
• Weight: 865 KB
• Volume: 65
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19820650118

✦ Synopsis

Abstract

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.