Halo Substituent Effects on Intramolecular Cycloadditions Involving Furanyl Amides

Intramolecular Diels−Alder reactions involving a series of N-alkenyl-substituted furanyl amides were
investigated. Stable functionalized oxanorbornenes were formed in high yield upon heating at 80−110
°C. The cycloaddition reactions include several bromo-substituted furanyl amides, and these systems
were found to proceed at a much faster rate and in higher yield than without substitution. This effect was
observed by incorporating a halogen in the 3- or 5-position of the furan ring and appears to be general.
The origin of increased cycloaddition rates for halo-substituted furans has been investigated with quantum
mechanical calculations. The success of these reactions is attributed to increases in reaction exothermicities;
this both decreases activation enthalpies and increases barriers to retrocycloadditions. Halogen substitution
on furan increases reactant energy and stabilizes the product, which is attributed to the preference of
electronegative halogens to be attached to a more highly alkylated and therefore more electropositive
framework.