Preparation of 2,3-dimethylene-2,3-dihydrofurans by the flash vacuum pyrolysis of substituted furylmethyl esters

Pyrolysis of 3-methylfurfuryl benzoate gives a 21% yield of 3-methyl-4-methylenecyclobutenone and a 24% yield of 4H,5H,9H,10H-cycloocta]1,2-b:6,5-b']difuran (8), the head-to-head, [4 + 4] dimer of 2,3-dimethylene-2,3-dihydrofuran (6). A similar pyrolysis of 2-methyl-3-furylmethyl benzoate (10) gives 8 in 51% yield. Low-temperature 1H and 13C NMR spectral studies show that 6 is the intermediate in the formation of 8. Compound 6 reacts with methyl acrylate to form a mixture of the isomeric Diels-Alder adducts. Pyrolysis of 2-methyl-3-furylmethyl-α,α-d2 benzoate (10-d2) gives 8-d4 via the intermediacy of 6-d2. Pyrolysis of 2,4-dimethyl-3-furylmethyl benzoate gives a 43% yield of 3,6-dimethyl-4H,5H,9H,10H-cycloocta[1,2-b:6,5-b'] difuran (13), the head-to-head, [4 + 4] dimer of 4-methyl-2,3-dimethylene-2,3-dihydrofuran (14). Low-temperature 1H and 13C NMR studies show that 14 is the intermediate in the formation of 13. Compound 14 can be trapped with methyl acrylate to form a 3.1 to 1 ratio of the Diels-Alder adducts 15 and 16. The structure proof of 15 and 16 involves the conversion of 15 to the commercially available, natural product menthofuran and 16 to isomenthofuran, which is synthesized by an independent route