The crystal structure of a p-bromobenzoyl derivative, C27H3306Br, of the diterpene e-caesalpin has been determined by the heavy-atom method using three-dimensional X-ray data. The absolute configuration was established from the anomalous scattering of the bromine atom. The three six-membered rings A, B and C are fused to form a trans-anti-trans system. Rings A and B both occur in the chair form while the unsaturated ring C adopts a distorted half-chair conformation. An intramolecular hydrogen bond (2.649/~) occurs between the two 1,3-diaxial hydroxyl groups. The crystals are monoclinic, space group P21 with two molecules in a unit cell of dimensions a = 6.563, b = 12.999, c = 14.809/~; B=94.50 °. The structure has been refined by least-squares methods to a final residual (R) of 12.3% for 3115 reflexions.
The synthetic acid oxalate, CzzH30OsN.C2HO4, m.p. 189-192°C, crystallizes in the monoclinic space group C2/c, with eight molecules in a unit cell of dimensions a=23.972 (1), b= 10.346 (2), c= 18"656 (I) /~; fl=93.16 (2) °. The densities are: Dx= 1.373, Din: 1.372 g.cm -a. The structure was determined by the symbolic addition method from data collected at room temperature on a four-circle diffractometer using the 0-20 scan technique. It was refined by the block-diagonal least-squares method to R = 4.28 % for 3115 observed reflexions. Ring A occurs in the chair conformation, and the C(1)-methoxyl is in the equatorial position, cis to the nitrogen bridge. This finding makes it necessary to revise the configurations of the corresponding methoxyl groups in delphinine and aconitine. Pairs of cations are hydrogenbonded to pairs of oxalate anions, a centre of symmetry being located between each pair. Each cation is linked to two anions by one normal and two bifurcated hydrogen bonds.
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