Facile construction of a furan-fused methano[11]annulenone skeleton by successive aldol condensations from 3,4-furandicarbaldehydes

Aldol condensation of furan-3,4-dicarbaldehyde with pentanedial in acetic acid gave 6H-cyclohepta[c]furan-5,7-dicarbaldehyde in a good yield. Then, successive condensation of 6H-cyclohepta[c]furan-5,7-dicarbaldehyde with dimethyl 1,3-acetonedicarboxylate under azeotropic conditions gave a diester derivative of furan-fused methano[ll]annulenone also in a good yield. The hydrolysis of the diester groups and subsequent decarboxylation provided a non-substituted furan-fused methano[ll]annulenone. A similar sequence starting from 2,5-dimethylfuran-3,4-dicarbaldehyde gave its dimethyl derivative. Cycloaddition and protonation of these furan-fused methano[ll]annulenones were also studied.