Synthesis of polyfunctionalized furans from 3-acetyl-1-aryl-2-pentene-1,4-diones

The BF3-catalyzed cyclization of 3-acetyl-1-aryl-2-pentene-1,4-diones in the presence of water in boiling tetrahydrofuran gave bis(3-acetyl-5-aryl-2-furyl)methanes in 26–79% yields along with a small amount of 3-acetyl-5-aryl-2-methylfurans . The exact structure of was determined by X-ray crystallography. The use of a half volume of the solvent for the reaction of resulted in the formation of 2,4-bis(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-phenylfuran () together with and . A similar reaction of was carried out in the presence of 3-acetyl-5-(4-methylphenyl)-2-methylfuran () to afford 4-(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-(4-methylphenyl)-2-methylfuran () in 49% yield. The BF3-catalyzed reaction of with 2,4-pentanedione in dry tetrahydrofuran at 23°C gave 3-(3-acetyl-5-phenyl-2-furfuryl)-4-hydroxy-3-penten-2-one () and 3-(3-acetyl-2-methyl-4-phenyl-5-furyl)-4-hydroxy-3-penten-2-one () in 66 and 24% yields, respectively. The product distribution depended on the reaction temperature. A similar reaction of also yielded the corresponding trisubstituted furans and tetrasubstituted furans in good yields. These results suggested the presence of the furfuryl carbocation intermediate during the reaction. The one-pot synthesis of and was also achieved by a similar reaction using phenylglyoxal. The deoxygenation of with triphenylphosphine gave in 88% yield, while was treated with concentrated hydrochloric acid to yield 3-acetyl-2-chloromethyl-5-phenylfuran () which was quantitatively transformed in ethanol into 3-acetyl-2-ethoxymethyl-5-phenylfuran () and in water into 3-acetyl-5-phenylfurfuryl alcohol (), respectively. In addition, the Diels–Alder reaction of cyclopantadiene with gave the corresponding [4+2] cycloaddition products and .