Synthesis of Fused Coumarins via Cycloaddition Reactions

Significance

Intramolecular phosphine-catalyzed allene/alkene [3+2] cycloadditions of allenoates B and D to produce substituted coumarin adducts C and E, respectively, is reported. A single diastereo­isomer of C was obtained in good to excellent yield. Both EWG and EDG were compatible with the reaction conditions. Interestingly, nitrostyrenyl D gave [3+2] annulation product E in THF using PPh₃ in moderate yield with a minor amount of F while D follows a [4+2] annulation pattern to form the nitronate F as major product under benzene/P(4-FC₆H₄)₃ conditions. The ni­tronate F was further utilized in the 1,3-dipolar cycloaddition with alkenes to afford isomeric tetracyclic coumarin derivatives G in excellent yields.