REACTION OF DIENOPHILES WITH A 5,6-BISMETHYLENENORBORN-2-EN-7-ONE - THE SYNTHESIS OF A STABLE NORBORNA-2,5-DIEN-7-ONE

Reaction of hemicyclone (10) with (E)-1,4-dibromobut-2-ene (9) under f orcing conditions yielded a 1 : 1 adduct (11) which served as the prec ursor for the 5,6-bismethylenenorborn-2-en-7-one (12) through fluoride -promoted dehydrobromination. The conjugated diene group in (12) enter s into Diels-Alder reactions with reactive dienophiles to produce inte rmediate norbornadienones en route to aromatic products. It has been p ossible to identify the norborna-2,5-dien-7-one (16) resulting from th e 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 select ivity in its reaction with the transient Dewar benzene (24). The cyclo butamaleimide moiety in (24) is screened on each face, yet facial disc rimination in its reaction with (12) is high (7:1). Thermolysis of the 5,6-bismethylenenorborn-2-en-7-one (12) yields the substituted benzoc yclobutene (14), a reaction which may involve the o-xylylene (13) as i ntermediate.