One-Pot Three-Component Synthesis of 7H-Phenaleno[1,2-b]furan-7-ones

Significance

Reported is a synthesis of 7H-­phenaleno[1,2-b]furan-7-one derivatives 4 by a one-pot three-component condensation of commercially available 3-hydroxy-1H-phenalene-1-one (1) with various aldehydes 2 and isocyanides 3. The mechanism proposed involves a Knoevenagel-type reaction of 1 with an aldehyde followed by ­either a [4+1] cycloaddition or a Michael addition of isocyanide. A subsequent double bond isomerization leads to the target structures in good to excellent preparative yields. However, under the conditions used, the formation of 5 by treatment of 1 with 2 is not observed, but independently prepared 5 undergoes reaction with isocyanide 3, smoothly resulting in the formation of 4. These experiments allow the conclusion that isocyanide plays a significant catalytic role in the Knoevenagel condensation.