A New Approach to the Synthesis of 2-Methyl-4H-furo[3,2-c]benzopyran-4-ones and 2H,5H-Pyrano[3,2-c]benzopyran-5-ones
17 September 2002 (online)
The reaction of various 4-hydroxy-2H-benzopyran-2-ones 1 with allyl bromide and potassium carbonate in acetone yielded the corresponding 4-allyloxy derivatives 2. Claisen rearrangement of 2 gave 3-allyl-4-hydroxy-2H-benzopyran-2-ones 3, which were reacted with sodium hydroxide to give the sodium benzopyranolate salt 4. Oxidative cyclization of 4 with equimolar quantities of dichlorobis(benzonitrile)palladium gave a 1:1 mixture of 2-methyl-4H-furo[3,2-c]benzopyran-4-ones 5 and 2H,5H-pyrano[3,2-c]-benzopyran-5-ones 6, in 88-90% combined yield.