Steric inhibition to cyclization of β-keto amides to indeno[1,2,3-de]quinolinones and related compounds

The preferred conformation of N-ethyl-2,2,2′-trichlorobenzoylacetanilide (1f) was deduced from ¹H NMR data and served to account for the failure of 1f to cyclize in H₂SO₄ in terms of a steric hindrance to attainment of the requisite conformation for ring closure. Consistent with this view was the observation that pentachlorodihydroquinolinone 7a with H₂SO₄ underwent ring opening to give as chief product alkene 8, and only a minor amount of indenoquinolinone 2. Substrate 7a was formed, in preference to amide 11, from difluorooxyborane 6a and SO₂Cl₂, on allowing HCl to escape from the reaction mixture.