Penta‐2,4‐dien‐1‐ones by Formal [3+2] Cycloaddition–Rearrangement of Electron‐Deficient Diethyl 2‐(Dicyanomethylene)malonate with Alk­ynes
The reactions of diethyl 2-(dicyanomethylene)malonate, an electron-deficient alkene, with donor-activated and unactivated alkynes have been investigated. Moderately electron-rich and unactivated alkynes undergo efficient formal [3+2] cycloaddition–rearrangement cascades to provide the corresponding penta-2,4-dien-1-one adducts in yields of up to 84 %. The structures of the solid dienone products were confirmed by X-ray diffraction analysis. The buta-1,3-diene moieties in the dienones do not adopt planar s-trans conformations but rather nonplanar geometries in which the two olefinic bonds are nearly orthogonal to each other. This transformation proceeds with excellent regioselectivity and with a wide range of alkynes without the need for a catalyst. Under the optimized reaction conditions, no competition with the [2+2] cycloaddition–retroelectrocyclization reaction (CA–RE) was observed.