Intramolecular Diels–Alder additions to 2-benzopyran-3-ones; anti-selectivity induced by the phenylsulfonyl group

Abstract

The 2-benzopyran-3-ones 7a and 7c undergo intramolecular Diels–Alder addition, via preferred endo-addition of the connecting chain, whereas for 7d and 7e(X = SO₂Ph), exo-addition of the chain is preferred; the main adducts from the latter additions (9d and 9e), give the diterpene-related products 12(R = H, Y = OMe) and 12(R = Me, Y = OMe) upon treatment with sodium amalgam.