Unexpected formation of dienes in the Diels-Alder reaction of exocyclic 1-bromobutadienes of polycyclic hydrocarbons

Polycyclic dienes having an exocyclic 1-bromobutadiene moiety react with dienophiles and fullerene-C-60 to afford exclusively dienes via a cycloaddition-elimination mechanism. Neither the primary adducts nor the double addition products derived from a second cycloaddition of the dienophile to the diene could be detected. In one case only, i.e. with 4-phenyl-1,2,4-triazoline-3,5-dione, was the double addition product formed. Contrary to expectations, X-ray diffractometric analysis shows that this adduct is formed following a contrasteric approach.