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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.

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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 Cs 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).

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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.

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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 Å3. The previously reported triclinic (P-1) form [Raston & Scott (2000). Green Chem., 2, 49–52] has mol­ecular volume 309.6 Å3 at the same temperature. All three mol­ecules deviate significantly and systematically from the putative Cs symmetry (δr.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.

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The title compound, C15H13N5, crystallizes with two independent mol­ecules in the asymmetric unit. The mol­ecular conformations are stabilized by C—H...N contacts forming S(6) ring motifs. In the crystal, pairs of mol­ecules are connected into R22(12) dimers by N—H...N hydrogen bonds. C—H...π inter­actions and π–π stacking inter­actions [centroid–centroid distances = 3.6085 (8), 3.6657 (8), 3.4745 (8) and 3.5059 (8) Å] also also observed.

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In the title compound, C10H9N5, the phenyl ring is twisted with respect to the pyrazole ring, forming a dihedral angle of 24.00 (6)°. In the crystal, mol­ecules are linked by N—H...N hydrogen bonds into chains running parallel to [010] containing alternating R22(6) and R22(12) rings. Further inter­actions are found in the crystal, viz. N—H...π(phen­yl) inter­actions and weak face-to-face π–π stacking inter­actions [centroid–centroid distance = 3.8890 (6) Å] between the centroids of the pyrazole and phenyl rings are observed.

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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.

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The title compound, C13H15ClN2O6, was synthesized by hypochlorous acid-mediated chlorination of N-acetyl-3-nitro-L-tyrosine ethyl ester. The OH group forms an intra­molecular O—H...O hydrogen bond to the nitro group and the N—H group forms an inter­molecular N—H...O hydrogen bonds to an amide O atom, linking the mol­ecules into chains along [100]. The crystal studied was a non-merohedral twin, with a 0.907 (4):0.093 (4) domain ratio.
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