Intramolecular carbonyl-carbonyl interaction as the directing motif of the opposed conformational preferences among [4 + 2] cycloadducts of 3-ethoxycarbonyl- and 3-acetyl-coumarins with 2,3-dimethyl-1,3-butadiene

In this work, the molecular structures of the Diels-Alder adducts: ethyl (6aR/S,10aR/S)-6a,7,10,10a-tetrahydro-8,9-dimethyl-6-oxodibenzo[b,d] pyran-6a-carboxylate C₁₈H₂₀O₄ 1 and (6aR/S,10aR/S)-6a-acetyl-6a,7,10,10a-tetrahydro-8,9-dimethyl-6-oxodibenzo[b,d] pyran C₁₇H₁₈O₃ 2; and the epoxide of this last (6aR/S,7aR/S,8aS/R,9aR/S)-6a-acetyl-6a,7,7a,8a,9,9a-hexahydro-7a, 8a-dimethyl-6-oxo-6H-5,8-dioxacyclopropa[b]phenanthrene C₁₇H ₁₈O₄ 3, are comparatively analyzed. Compound 1 is triclinic, space group P-1 with a = 7.7529(6) Å, b = 9.8625(8) Å, c = 11.3103(9) Å, α = 109.387(9)°, β = 95.484(1)°, γ = 99.753(1)°, Z = 2. Compound 2 is monoclinic, space group Cc, a = 7.7285(7) Å, b = 16.6813(15) Å, c = 11.3213(10) Å, β = 92.470(2)°, Z = 4. Compound 3 is monoclinic, space group P2 ₁ /c a = 11.2036(10), b = 15.8326(14), c = 8.3182(7), β = 90.600(2)°, Z = 4. The molecular structures of compounds 1-3 show conformational differences between lactone and 6a-acyl carbonyls. CO•••CO dipolar interactions stabilize the less favoured syn conformation in 2-3, conclusions are supported on theoretical calculations. The molecular structure of compound 3 demonstrates that epoxidation of 2 leads to the stereo-selective addition of the oxygen atom. Graphical Abstract: Conformation directed by carbonyl interactions] - >[Figure not available: see fulltext.]