(1R,4R,5aS,7S,9aS)-7,9a-Dimethyl-6-methyl­ene-3-oxo-1,3,4,5,5a,6,7,8,9,9a-deca­hydro­naphtho­[1,2-c]furan-1,4-diyl diacetate

The title compound, C₁₉H₂₄O₆, is a sesquiterpene lactone isolated from the Kenyan plant Warburgia ugandensis. Ring A adopts a chair conformation, ring B is in a C₂ twist conformation and the lactone ring is nearly planar with maximum deviation 0.007 (1) Å. The reported absolute configuration is based on that of the similar compound bromo-parasiticolide A and is supported by analysis of Bijvoet differences from light atoms in Mo Kα radiation.

4-Nitro-1-[(trimethyl­sil­yl)ethyn­yl]benzene: low-temperature polymorph at 100 K

The title compound, C₁₁H₁₃NO₂Si, is a low-temperature form of the previously reported room-temperature structure [Garcia et al. (1998). Acta Cryst. C54, 489-491]. At 298 K, the material crystallizes in the space group Pnma and occupies a crystallographic mirror plane, but at 100 K the space group changes to P2₁2₁2₁, the volume decreases by 5% and the mol­ecule distorts. The greatest mol­ecular distortions from C_s symmetry are rotations of the trimethyl­silyl and nitro groups by 10.56 (8) and 11.47 (9)°, respectively, to the benzene mean plane. At low temperature, the crystal also becomes an inversion twin, the refined ratio of the twin components being 0.35 (15):0.65 (15).

Monoclinic polymorph of 2,5-dide­oxy-2,5-epithio-1,3:4,6-bis-O-[(R)-phenyl­methyl­ene]-L-iditol

The title compound C₂₀H₂₀O₄S, is polymorphic. In the tetra­gonal form, the mol­ecule lies on a crystallographic twofold axis, while the monoclinic form has only approximate C₂ mol­ecular symmetry. The greatest excursion from C₂ symmetry is in the orientation of the two phenyl rings; at 100 K, one of the rings is rotated -37.2 (3)° and the other by 46.9 (3)° from their symmetric (tetra­gonal) positions. There are only minor differences in the three-ring nucleus; the best mol­ecular fit of the tetra­gonal and monoclinic forms, both at 100 K and excluding phenyl rings and H atoms, shows an r.m.s. deviation of 0.066 Å. Both forms have the same absolute configuration.

Ortho­rhom­bic polymorph of (2E)-2-(2,3-dihydro-1H-inden-1-yl­idene)-2,3-di­hydro-1H-inden-1-one

The title compound, C₁₈H₁₄O, is polymorphic at 123 K. The ortho­rhom­bic form reported herein has two independent mol­ecules in the asymmetric unit, with mol­ecular volume 313.5 ų. The previously reported triclinic (P-1) form [Raston & Scott (2000). Green Chem., 2, 49–52] has mol­ecular volume 309.6 ų at the same temperature. All three mol­ecules deviate significantly and systematically from the putative C_s symmetry (δ_r.m.s. = 0.0265, 0.0256, 0.0497 Å). Comparison of the two molecules in the orthorhombic polymorph shows that 16 of the 19 equivalent pairs of framework atoms have a mirror-image pattern of deviations (above/below plane), suggesting that the two are quasi-enanti­omorphs. The pattern of deviations in the triclinic form is nearly the same (13 of 19 atom pairs) as the ortho­rhom­bic form.

1,2-Bis{2-[2-(trimethyl­sil­yl)ethyn­yl]phen­yl}ethane-1,2-dione

The title compound, C₂₄H₂₆O₂Si₂, has C₂ crystallographic symmetry. The dihedral angle between the aromatic rings is 84.5 (2)°. The acetyl­ene group is slightly non-linear, with angles at the acetyl­ene C atoms of 175.7 (2) and 177.0 (2)°. In the crystal structure, only van de Waals interactions occur.