[4 pi+2 pi]cycloaddition reactions of o-benzoquinones with symmetrical 6,6-dialkyl and cycloalkylfulvenes: Formation of bicyclo[2.2.2]octene diones and cyclopenta[b][1,4]benzodioxins. Some novel observations

3,5-Di-tert-butyl-o-benzoquinone on reaction with 6,6-dimethylfulvene furnished the bicycle [2.2.2] adduct 5 whereas the 6,6-tetramethylenefulvene afforded a novel product 11 which arises by the hetero Diels-Alder reaction of the quinone with the isomerized fulvene. Analogous reaction of 6,6-pentamethylenefulvene gave rise to the benzodioxin 14 and the bicycloadduct 15; 6,6-Hexamethylenefulvene afforded only the bicyclo[2.2.2] adduct 16. Similar reactivity is observed in the case of 4-tert-butyl-o-benzoquinone.