Received 7 May 2012
Cinerin C: a macrophyllin-type bicyclo[3.2.1]octane neolignan from Pleurothyrium cinereum (Lauraceae)
aLaboratorio de Bioorgánica, Departamento de Química, Facultad de Ciencias Básicas, Universidad Militar Nueva Granada, AA 49300, Colombia,bLaboratorio de Productos Naturales Vegetales, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, AA 14490, Colombia, and cInstitut für Chemie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany
The structure of naturally-occurring cinerin C [systematic name: (7S,8R,3'R,4'S,5'R)-8'-4'-hydroxy-5,5',3'-trimethoxy-3,4-methylenedioxy-2',3',4',5'-tetrahydro-2'-oxo-7.3',8.5'-neolignan], isolated from the ethanol extract of leaves of Pleurothyrium cinereum (Lauraceae), has previously been established by NMR and HRMS spectroscopy, and its absolute configuration established by circular dichroism measurements. For the first time, its crystal strucure has now been established by single-crystal X-ray analysis, as the monohydrate, C22H26O7·H2O. The bicyclooctane moiety comprises fused cyclopentane and cyclohexenone rings which are almost coplanar. An intermolecular O-HO hydrogen bond links the 4'-OH and 5'-OCH3 groups along the c axis.
The title compound, (I), is a 7.3',8.5'-connected bicyclo[3.2.1]octane neolignan related to the macrophyllin type (Sefkow, 2003). This fascinating class of neolignans has attracted significant attention because of its chemistry, configuration and biological activities, and its occurrence is limited to certain plants belonging to the Magnoliaceae, Piperaceae and Lauraceae families (Coy-Barrera et al., 2009). Although the macrophyllin-type neolignans are less common than the guianin-type, the macrophyllin-type bicyclooctanes have exhibited better platelet-activating factor antagonism (anti-PAF activity) (Wang et al., 2002; Coy-Barrera et al., 2009). Indeed, the title compound, cinerin C, was found to be the most potent PAF antagonist (IC50 = 1.1 µM) among 26 evaluated neolignans (Coy et al., 2009a).
As part of our research on Lauraceaous neolignans, cinerin C was first isolated from the chloroform-soluble fraction obtained from an ethanol extract of leaves of Pleurothyrium cinereum (Lauraceae family; Coy et al., 2009b), a tree important to the Awá tribe in Colombia (Coy & Cuca, 2008). The structure was previously determined by NMR spectroscopy and high-resolution mass spectrometry (HRMS), establishing the bicyclo[3.2.1]octane group as well as the 7S,8R,3'R,4'S,5'R absolute configuration [by circular dichroism (CD) measurements; Coy et al., 2009b]. In order to grow suitable crystals of (I), different crystallization methods were investigated, including: (a) recrystallization from various solvents and solvent mixtures by heating and subsequent slow cooling; (b) overlaying a solution of the compound with an antisolvent; and (c) diffusion of an antisolvent into a solution of the compound via the gas phase, in a closed chamber. Procedures (a) and (b) failed to afford any suitable crystals, and only procedure (c), using n-hexane as antisolvent, was successful. Because there are no heavy atoms in the structure with a sufficiently strong anomalous scattering effect, a reliable crystallographic determination of the absolute configuration cannot be assured. Therefore, the enantiomer shown, (I), was chosen to be compatible with the configuration established by the CD measurements.
So far, only three crystal structures of macrophyllin-type neolignans have been reported: the oxidation product from macrophyllin B, (II) (Nectandra spp. Lauraceae; Rodrigues et al., 1984), piperulin A, (III) [Piper puberulum (Piperaceae); Zhang et al., 1995, 1996], and kadsurenin C, (IV) [Piper kadsura (Piperaceae); Jiang et al., 2003]. The first two neolignans [which differ structurally from (I) by a methoxy group at C3' and a veratryl group instead of a 5-methoxypiperonyl group, as well as by an acetyl group at C4' in (III)] have similar relative configurations to that of (I), since both molecules have an aryl-C7/methyl-C8 trans relationship, an endo-methyl group at C8 (which is easily determined from a 13C NMR chemical shift at ca 13 p.p.m.), and the 4'-OAc or 4'-OH group, respectively, oriented towards the aryl group. In addition, (I) and (IV) showed negative Cotton effects at 246 and 330 nm by CD measurements (Han et al., 1992), which means that both molecules have identical absolute configurations. On the other hand, although (II) exhibits a mutually trans arrangement of the aryl-C7 and methyl-C8 bonds, there are two significant differences: the OH group is located at C2' and the methyl group at C8 has an exo orientation (13C NMR chemical shift at ca 16 p.p.m.; Guilhon et al., 1992). However, the space group for the closely related molecules (III) and (IV) is P212121 (orthorhombic), while (I) crystallizes in the hexagonal space group P61. This suggests that the 3'-methoxy group significantly affects the symmetry of the crystal structure, possibly through the presence of an intermolecular O-HO hydrogen bond between the 4'-OH and 3'-OCH3 groups [O5-H5O6i = 2.947 (3) Å; symmetry code: (i) -x + 1, -y, z + ]. These hydrogen bonds link the molecules into chains which run parallel to the crystallographic c axis (Fig. 2).
A view of the solid-state conformation of (I), with the atom-numbering scheme, is provided in Fig. 1. The bond lengths agree in general with expected values (Zhang et al., 1995; Jiang et al., 2003), except for the elongated C7-C3' bond of 1.585 (3) Å, which is due to repulsion between the aromatic ring at atom C7 and atoms O5 and O6. The molecule of (I) can be subdivided into three parts, namely a 5-methoxypiperonyl group, a bicyclooctane group and an allyl group. The 5-methoxypiperonyl group has a methoxy group at C5 nearly coplanar with the benzene ring, with a C11-O3-C5-C6 torsion angle of 3.5 (5)°, and, as expected, the methylenedioxy group is twisted slightly out of the plane of the benzene ring, having torsion angles of C10-O2-C4-C3 = 6.6 (3)° and C10-O1-C3-C4 = -6.9 (3)°. In addition, the benzene ring is oriented through the C1-C7 bond by a gauche conformation with respect to atoms C7 and C8 of the bicyclooctane group, with torsion angles of C8-C7-C1-C6 = 60.4 (3)° and C3'-C7-C1-C2 = 118.1 (3)°.
The conformation of the bicyclooctane group is characterized by two fragments: a cyclohexenone fragment (comprising atoms C1', C2', C3', C4', C5' and C6') and a cyclopentane fragment (comprising atoms C3', C4', C5', C8 and C7), fused along C3'-C4'-C5'. The first fragment adopts an envelope conformation, where atoms C1'/C2'/C3'/C5'/C6' lie in a plane [the maximum deviation from the least-squares plane is 0.046 (2) Å for atom C2'], and atom C4' lies 0.885 (2) Å out of that plane. An approximate C2 symmetry axis passing through atom C7 and the mid-point of the C5'-C4' bond relates the cyclopentane ring torsion angles, indicating a half-chair form. Likewise, the cyclohexenone ring has an approximate mirror plane of symmetry passing through atoms C4' and C1', with two adjacent torsion angles [Cl'-C2'-C3'-C4' = 45.0 (3)° and C1'-C6'-C5'-C4' = -31.7 (3) °] indicating that the ring has a half-boat (1,2-diplanar) form. The dihedral angle between the C1'/C2'/C3'/C4'and C1'/C6'/C5'/C4' planes is 43.6 (2)°. The olefin from the allyl fragment is disordered over two sites, which lie on the same side as the methyl group at C8, with a C1C7' distance of 5.817 (5) Å. Finally, the methoxy groups at C5' and C3' have a pseudo-1,3-diequatorial orientation, with an O4O6 distance of 4.866 (3) Å.
| || Figure 1 |
The molecular structure of (I), showing the atom-labelling scheme. One component of the disordered vinyl group has been omitted for clarity. Displacement ellipsoids are drawn at the 30% probability level for the non-H atoms.
| || Figure 2 |
The intermolecular hydrogen bonds (dashed lines) in the crystal structure of (I), running along the crystallographic c axis. With the exception of atom H5, all H atoms have been omitted for clarity. [Symmetry code: (i) -x + 1, -y, z + .]
Plant material was collected from P. cinereum in the indigenous reservation Awá at Alto Albí, Tumaco County, Department of Narinho, Colombia, in November 2005 and identified by biologist Ayda Lucia Patino. A voucher specimen (No. COL518334) was deposited at the Herbario Nacional Colombiano of the Universidad Nacional de Colombia in Bogota. Detailed extraction and isolation processes and spectroscopic features for (I) have been reported previously (Coy et al., 2009b). The isolated compound was crystallized by diffusion of n-hexane antisolvent vapour into a solution of the compound in ethyl acetate. Elemental analysis: C 65.7, H 6.3, O 27.9%; C22H26O7 requires: C 65.7, H 6.5, O 27.8%.
The Rint value is relatively high because of the overall poor quality of the data, which, in turn, is the result of limited crystal quality. Friedel opposites were merged because of the absence of significant anomalous scattering effects. The crystal structure displays channels (ca 3.7 Å in diameter) containing water molecules. In spite of several attempts, the electron density in this area could not be resolved satisfactorily. Therefore, the contribution of the disordered solvent species was subtracted from the structure-factor calculations using the SQUEEZE module of PLATON (Spek, 2009). The electron count of 71 per unit cell corresponds to approximately one water molecule per asymmetric unit. Although solvent atoms were not included in the refinement model, they are included in the calculation of the formula weight, density, and F(000).
The H atoms of the methyl groups and of the allyl group were placed geometrically and refined as riding, with C-H = 0.96 (methyl) or 0.93 Å (allyl) and Uiso(H) = 1.2Ueq(C). All other H atoms were located from a difference Fourier map and their positions were refined, with displacement parameters constrained to Uiso(H) = 1.2Ueq(C). For atom H5, the isotropic displacement parameter was also allowed to refine. The H atoms attached to C7 were modelled as disordered over two orientations, with the four C7-H and two HH distances restrained to common refined values with an s.u. of 0.02 Å.
Data collection: X-AREA (Stoe & Cie, 2004); cell refinement: X-AREA; data reduction: X-RED (Stoe & Cie, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997), based on ORTEPIII (Burnett & Johnson, 1996), and DIAMOND (Brandenburg, 2007); software used to prepare material for publication: SHELXL97.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: BM3119 ). Services for accessing these data are described at the back of the journal.
The authors thank the University of Potsdam and the Deutscher Akademischer Austauschdienst (DAAD) for a fellowship.
Brandenburg, K. (2007). DIAMOND. Crystal Impact GbR, Bonn, Germany.
Burnett, M. N. & Johnson, C. K. (1996). ORTEPIII. Report ORNL-6895. Oak Ridge National Laboratory, Tennessee, USA.
Coy, B. E. D. & Cuca, S. L. E. (2008). Biochem. Syst. Ecol. 36, 674-677.
Coy, B. E. D., Cuca, S. L. E. & Sefkow, M. (2009a). Bioorg. Med. Chem. Lett. 19, 6922-6925.
Coy, B. E. D., Cuca, S. L. E. & Sefkow, M. (2009b). J. Nat. Prod. 72, 1245-1248.
Coy-Barrera, E. D., Cuca-Suarez, L. E. & Sefkow, M. (2009). Phytochemistry, 70, 1309-1314.
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.
Guilhon, G. M. S. P., Conserva, L. M., Maia, J. G. S., Yoshida, M. & Gottlieb, O. R. (1992). Phytochemistry, 31, 2847-2850.
Han, G. Q., Dai, P., Xu, L., Wang, S. C. & Zheng, Q. T. (1992). Chin. Chem. Lett. 3, 521-524.
Jiang, R.-W., Mak, T. C. W. & Fung, K.-P. (2003). J. Mol. Struct. 654, 177-182.
Rodrigues, M. M., Fernandes, J. B., Braz Filho, R., Yoshida, M. & Gottlieb, O. R. (1984). Phytochemistry, 23, 667-669.
Sefkow, M. (2003). Synthesis, pp. 2595-2625.
Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.
Spek, A. L. (2009). Acta Cryst. D65, 148-155.
Stoe & Cie (2004). X-AREA and X-RED. Stoe & Cie, Darmstadt, Germany.
Wang, M., Wu, A., Pan, X. & Yang, H. (2002). J. Org. Chem. 67, 5405-5407.
Zhang, S.-X., Chen, K., Lui, X.-J., Zhang, D.-C., Tao-Wiedmann, T.-W., Leu, S.-L., McPhail, A. T. & Lee, K.-H. (1995). J. Nat. Prod. 58, 540-547.
Zhang, S.-X., Chen, K., Lui, X.-J., Zhang, D.-C., Tao-Wiedmann, T.-W., Leu, S.-L., McPhail, A. T. & Lee, K.-H. (1996). J. Nat. Prod. 59, 1002.