Furo[3,4-b]benzofurans: Synthesis and Reactions.

Starting with coumarin (1) furo[3,4-b]benzofuran 4 was synthesized using the Hamaguchi–Ibata methodology. Intermolecular Diels–Alder reactions provide compounds 5–8. In a [4 + 3] cycloaddition reaction with oxyallyl compound 9 was obtained. The C-annulated furans 16a (in situ) and 16b have been prepared by a similar methodology starting with 10a,b. In an intramolecular Diels–Alder reaction compounds 17a,b were obtained. Computational studies concerning the reactivity of furo[3,4-b]benzofurans in inter- and intramolecular Diels–Alder reactions using both semiempirical (AM1, PM3) and density functional theoretical methods (B3LYP/6-31G*) are reported.