A Direct Method for the Construction of Benzene Rings

Reaction of 1,4-diacetoxybutadiene with dienophiles at elevated temperatures yields, in many cases, aromatic products resulting from elimination of acetic acid from the first-formed Diels-Alder adducts. This reaction provides a direct synthesis of benzene rings, especially useful in the synthesis of unsymmetrical biphenyls. Dienophiles used successfully include quinones, β-nitrostyrenes, chalcones, and acetylenes. The reaction of acetylenic dienophiles with 1-acetoxybutadiene also affords aromatic products. The adduct of 1,4-diacetoxy-butadiene and benzonorbornadiene did not aromatize below temperatures of 450°, and then only with rearrangement, to yield dibenzocycloheptatriene.