Reaction of Dienophiles With a 5,6-Bismethylenenorborn-2-en-7-one: the Synthesis of a Stable Norborna-2,5-dien-7-one

Abstract

Reaction of hemicyclone (10) with (E)-1,4-dibromobut-2-ene (9) under forcing conditions yielded a 1:1 adduct (11) which served as the precursor for the 5,6-bismethylenenorborn-2-en-7-one (12) through fluoride-promoted dehydrobromination. The conjugated diene group in (12) enters into Diels-Alder reactions with reactive dienophiles to produce intermediate norbornadienones en route to aromatic products. It has been possible to identify the norborna-2,5-dien-7-one (16) resulting from the reaction of (12) with N- tolyltriazoline dione (15), thereby offering authentic evidence for the rare norborna-2,5-dien-7-one ring system. In addition, (12) has been used as a reagent to evaluate facial selectivity in its reaction with the transient Dewar benzene (24). The cyclobutamaleimide moiety in (24) is screened on each face, yet facial discrimination in its reaction with (12) is high (7: 1). Thermolysis of the 5,6-bismethylenenorborn-2-en-7-one (12) yields the substituted benzocyclobutene (14), a reaction which may involve the o-xylylene (13) as intermediate.