Access to phosphorus-containing cyclopenta[b]benzofuranols via a serendipitous cascade reaction promoted by a bifunctional phosphonium salt

In the current work, we serendipitously discovered a cascade reaction of readily available flavonoid derivatives with phosphine oxides by harnessing a bifunctional phosphonium salt catalyst, affording a variety of architecturally complex cyclopenta[b]benzofuranols with phosphorus-containing units in good yields. Mechanistic investigations revealed that this cascade process involved a cumulative diene as the key intermediate. Furthermore, an initial study of an asymmetric version of this reaction was conducted, in which the chiral product was generated in moderate yield with moderate enantioselectivity.