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

Abstract

The BF 3-catalyzed cyclization of 3-acetyl-1-aryl-2-pentene-1,4-diones 1a– e in the presence of water in boiling tetrahydrofuran gave bis(3-acetyl-5-aryl-2-furyl)methanes 2a– e in 26–79% yields along with a small amount of 3-acetyl-5-aryl-2-methylfurans 3a– e . The exact structure of 2a was determined by X-ray crystallography. The use of a half volume of the solvent for the reaction of 1a resulted in the formation of 2,4-bis(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-phenylfuran ( 4 ) together with 2a and 3a . A similar reaction of 1a was carried out in the presence of 3-acetyl-5-(4-methylphenyl)-2-methylfuran ( 3d ) to afford 4-(3-acetyl-5-phenyl-2-furfuryl)-3-acetyl-5-(4-methylphenyl)-2-methylfuran ( 5 ) in 49% yield. The BF 3-catalyzed reaction of 1a with 2,4-pentanedione in dry tetrahydrofuran at 23°C gave 3-(3-acetyl-5-phenyl-2-furfuryl)-4-hydroxy-3-penten-2-one ( 6a ) and 3-(3-acetyl-2-methyl-4-phenyl-5-furyl)-4-hydroxy-3-penten-2-one ( 7a ) in 66 and 24% yields, respectively. The product distribution depended on the reaction temperature. A similar reaction of 1b– e also yielded the corresponding trisubstituted furans 6b– e and tetrasubstituted furans 7b– e in good yields. These results suggested the presence of the furfuryl carbocation intermediate A during the reaction. The one-pot synthesis of 6a and 7a was also achieved by a similar reaction using phenylglyoxal. The deoxygenation of 1a with triphenylphosphine gave 3a in 88% yield, while 1a was treated with concentrated hydrochloric acid to yield 3-acetyl-2-chloromethyl-5-phenylfuran ( 8 ) which was quantitatively transformed in ethanol into 3-acetyl-2-ethoxymethyl-5-phenylfuran ( 9 ) and in water into 3-acetyl-5-phenylfurfuryl alcohol ( 10 ), respectively. In addition, the Diels–Alder reaction of cyclopantadiene with 1a gave the corresponding [4+2] cycloaddition products 11 and 12 .