Phosphine-catalyzed dearomative (3 + 2) annulation of 2-nitrobenzofurans and nitrobenzothiophenes with allenoates.

An efficient Ph2PMe-catalyzed dearomative (3 + 2) annulation of 2-nitrobenzofurans, 2-nitrobenzothiophenes, and 3-nitrobenzothiophenes with allenoates has been developed. With the developed protocol, a series of structurally important cyclopenta[b]benzofurans and cyclopenta[b]benzothiophenes were obtained in good to excellent yields (up to 98%) under mild conditions. In addition, preparative-scale experiments and transformations were conducted to exemplify the synthetic utility. The asymmetric version of this dearomative (3 + 2) annulation reaction was tentatively investigated by using chiral phosphine catalysts.