Intramolecular Diels–Alder additions to 2-benzopyran-3-ones; anti-selectivity induced by the phenylsulfonyl group1

Abstract

Intramolecular Diels–Alder additions of the 2-benzopyran-3-ones 2d, 2e and 2f with an E-SO₂Ph substituent on the dienophile (X = SO₂Ph in 2) show greatly enhanced exo-addition of the tether than is shown in the absence of the E-SO₂Ph group (X = H in 2). For 2e and 2f, exo-chain addition becomes preferred, and for 2d, endo-chain addition much less preferred, than in related cases with X = H. An E-CO₂Me group on the dienophile is also effective in enhancing exo-chain addition, but less effective than an E-SO₂Ph group. The adducts 4e and 4f undergo reductive elimination (5% Na–Hg) to give the diterpene related products 32 and 33 respectively.