An Experimental and Computational Investigation of the Diels−Alder Cycloadditions of Halogen-Substituted 2(H)-Pyran-2-ones

Diels−Alder reactions of 3- and 5-halo-subsituted 2(H)-pyran-2-ones with both electron-rich and electron-deficient dienophiles afford stable and readily isolable bridged bicyclic lactone cycloadducts. These cycloadditions proceed with excellent regioselectivity and very good stereoselectivity. In contrast, Diels−Alder reactions of 4-halo-subsituted 2(H)-pyran-2-ones afford cycloadducts which are very prone to loss of bridging CO₂ and the subsequent formation of barrelenes ([2.2.2]cyclooctenes). Furthermore, these cycloadditions proceed with only moderate regio- and stereoselectivity. For both series of the 3- and 5-halo-subsituted 2(H)-pyran-2-ones and 4-halo-subsituted 2(H)-pyran-2-ones, the reactivity patterns do not significantly change between the halogens. The regio- and stereochemical preferences of the cycloadditions of halo-substituted 2(H)-pyran-2-ones are investigated computationally. Calculations were carried out on the transition states leading to the four possible regio- and stereoisomeric cycloadducts by using density functional theory (B3LYP/6-31G*). These studies allow prediction of the regio- and stereoselectivity in these reactions which are broadly in line with experimental observations.