Sesquiterpenoids — XVI: Conformations of cycloheptane and α-methylene-γ-lactone rings in sesquiterpenoids: X-ray determination of the constitution,absolute stereochemistry,and conformation of euparotin bromoacetate

The molecular structure of euparotin, a highly oxygenated sesquiterpene of the guaianolide type, has been determined by crystal-structure analysis of euparotin bromoacetate. The bromo-derivative crystallizes from benzene-petrol as a benzene solvate, C₂₂H₂₅O₈Br·$\text{1}\text{2}$C₆H₆, crystals of which are monoclinic, of space group C2, with a = 34·85, b = 7·04, c = 10·90 Å, β = 106°35′, and Z = 4. The atomic co-ordinates were determined by Fourier and least-squares calculations which employed 1947 photographic |F₀| values and converged at R = 12·8% with isotropic thermal parameters for the C and O atoms and anisotropic parameters for the Br atom. The absolute configuration was established by the anomalous-dispersion effect. The cycloheptane ring of the sesquiterpenoid has a conformation which is closer to a twist chair (C₂) than a chair (C_s) form, and a survey shows that this is also true for other perhydroazulene sesquiterpenoids. In the α-methylene-γ-lactone of euparotin and several other sesquiterpenoids the CCCO torsion angle has the same sign as the C(α)C(β)C(γ)-O torsion angle, establishing the basis for Stöcklin et al's correlation between the position and stereochemistry of lactone fusion and the sign of the Cotton effect of the n → π* transition of the CCCO chromophore.