Isobenzofurane, a Transient Intermediate

A study of Diels–Alder reactions of tetraphenylcyclopentadienone (II) coupled with development of two improved procedures for the generation of benzyne, one of which affords a practical route to the reactive dienophile 1,4-dihydronaphthalene-1,4-endo-oxide (Ia), led to an investigation of the reaction of Ia and II. At 80° these reactants combine readily to give the adduct IIIa, which is cleaved on pyrolysis to isobenzofurane VIIIa and 1,2,3,4-tetraphenylbenzene (IX). The transient existence of isobenzofurane (VIIIa) was established by trapping it with the 1,4-endo-oxide Ia and isolation of the adducts X and XI. In confirmation of the interpretation, X and XI were obtained by independent synthesis from the adduct Va from Ia and α-pyrone and thermal decomposition of Va in the presence of Ia as trapping agent. Parallel experiments with 1,4-dimethyl-1,4-dihydronaphthalene-1,4-endo-oxide, as well as mixed additions, supported the interpretations advanced. Evidence is presented to show that isobenzofurane (VIIIa) invariably adds exo to the bicyclic dienophile.