Redetermination and absolute configuration of atalaphylline

The title acridone alkaloid [systematic name: 1,3,5-trihydroxy-2,4-bis(3-methylbut-2-enyl)acridin-9(10H)-one], C23H25NO4, has previously been reported as crystallizing in the chiral orthorhombic space group P212121 [Chantrapromma et al. (2010 ▶). Acta Cryst. E66, o81–o82] but the absolute configuration could not be determined from data collected with Mo radiation. The absolute configuration has now been determined by refinement of the Flack parameter with data collected using Cu radiation. All features of the molecule and its crystal packing are similar to those previously described.

The title acridone alkaloid [systematic name: 1,3,5-trihydroxy-2,4-bis(3-methylbut-2-enyl)acridin-9(10H)-one], C 23 H 25 NO 4 , has previously been reported as crystallizing in the chiral orthorhombic space group P2 1 2 1 2 1 [Chantrapromma et al. (2010). Acta Cryst. E66, o81-o82] but the absolute configuration could not be determined from data collected with Mo radiation. The absolute configuration has now been determined by refinement of the Flack parameter with data collected using Cu radiation. All features of the molecule and its crystal packing are similar to those previously described.

Related literature
For details of acridone alkaloids see: Basu & Basa (1972). For the previous structure determination, see: Chantrapromma et al. (2010). For hydrogen-bond motifs, see Bernstein et al. (1995). For bond-length data, see: Allen et al. (1987). For the stability of the temperature controller used in the data collection, see Cosier & Glazer, (1986 Table 1 Hydrogen-bond geometry (Å , ). Redetermination and absolute configuration of atalaphylline H.-K. Fun, C. S. Yeap and S. Chantrapromma

Comment
The title acridone alkaloid (I) known as atalaphylline (Basu & Basa, 1972), was isolated from the roots of Atalantia monophylla Corrêa, a mangrove plant which was collected from Trang province in the southern part of Thailand. Although (I) has been previously reported (Chantrapromma et al., 2010), the absolute configuration could not be determined due to insufficient anomalous dispersion from the light atoms using the data set collected with Mo radiation. The data of the same sample was recollected using Cu radiation with our newly-installed Bruker Apex-Duo CCD diffractometer and the absolute configulation was determined by making use of the large anomalous scattering of Cu Kα X-radiation with the Flack parameter being refined to 0.05 (13). We report herein the crystal structure of (I) with data collected using Cu radiation. Fig. 1 shows the molecular structure of (I), bond lengths and angles are closely similar to those previously described (Chantrapromma et al., 2010). (I) is chiral even though it has no chiral center because its mirror image cannot be superposed onto itself. This is due to the arrangements of the two 3-methylbut-2-enyl side-chains at atoms C1 and C12. (I) crystallized as a single enantiomer in chiral orthorhombic P2 1 2 1 2 1 space group. The current structure determination represents a significant improvement compared with the structure determined from the data taken with Mo radiation and it confirmed the absolute conformation of the side-chains for (I). To be precise the two 3-methyl-2-enyl groups at C1 and C12 are attached in such a way that these two side-chains are below the acridone molecular plane indicating the (-)-anticlinal conformation with the torsion angles C2-C1-C19-C20 and C13-C12-C14-C15 are -102.65 (13) and -119.77 (33)°, respectively. interaction and a π-π interaction with a Cg 1 ···Cg 2 distance of 3.7643 (7) Å (symmetry code: -1+x, y, z); Cg 1 and Cg 2 are the centroids of C3-C5/C10-C11/N1 and C5-C10 rings, respectively . These differences are due to the fact that all the hydrogen atoms are refined freely whereas in previous report by Chantrapromma et al. (2010), the hydrogen atoms were positioned geometrically and allowed to ride on their parent atoms.

Experimental
The compound was isolated and crystal grown as reported by Chantrapromma et al. (2010).

sup-2 Figures
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (14) 0.1596 (9) 0.071 (6)