Phosphine-catalyzed [4 + 2] annulation of γ-benzyl allenoates: facile synthesis of benzothieno[3,2-b]pyran derivatives.

An efficient tris(4-methoxyphenyl)phosphine-catalyzed domino reaction between γ-benzyl allenoates and ethyl (Z)-2-(3-oxobenzo[b]thiophen-2(3H)-ylidene)acetate has been developed, which produces a series of 2H-benzo[4,5]thieno[3,2-b]pyran derivatives in high yields. The substrate scope includes both electron-withdrawing (e.g., halogen) and electron-donating (e.g., methoxy) groups on both the benzothiophene and allenoate moieties. The reaction can also be performed on the gram scale with good yield (e.g., 77%). In this reaction, γ-substituted allenoate acts as a two-carbon synthon, in a manner rarely reported in the literature to date.