Synthesis of Naphthalenes Using Acid-Catalyzed Ring-Opening and Recyclization of 3-Acetyl-5,5-diaryl-2-methyl-4,5-dihydrofurans. Isolation of Intermediates.

3-Acetyl-5,5-diaryl-2-methyl-4,5-dihydrofurans were heated in concentrated hydrochloric acid to give the 4-aryl-1-methylnaphthalenes in high yields. The same reaction was carried out in hydrochloric acid diluted with acetonitrile to give the 5,5-diaryl-4-penten-2-ones (87–96%), while treatment of the dihydrofurans with p-toluenesulfonic acid in acetonitrile afforded the 3-(2,2-diarylethenyl)-4-hydroxy-3-penten-2-ones (73–91%) which were transformed in diluted hydrochloric acid into the 5,5-diaryl-4-penten-2-ones. It was demonstrated that the 3- and 4-penten-2-ones were intermediates of the 4-aryl-1-methylnaphthalenes since the 3- and 4-penten-2-ones were easily converted into the corresponding naphthalenes in concentrated hydrochloric acid. The UV irradiation of the dihydrofurans in the presence of hydrochloric acid quantitatively gave the 2-acetyl-4-aryl-1-methylnaphthalenes (94–97%) via the same 3-penten-2-one intermediates.