Phosphine-Catalyzed Synthesis of Highly Functionalized Coumarins

2-Styrenyl allenoates are converted into cyclopentene-fused dihydrocoumarins through phosphine-catalyzed regio- and diastereoselective [3
+ 2] cycloadditions. Remarkably, changing the solvent from THF to benzene promotes the conversion of the 2-(2-nitrostyrenyl) allenoate into
a tricyclic nitronate through a previously undocumented mode of phosphine catalysis. This nitronate was subjected to efficient face-, regio-,
and exo-selective 1,3-dipolar cycloadditions to provide tetracyclic coumarin derivatives.