Crystal structure of 7β-hydroxyroyleanone isolated from Taxodium ascendens (B.)

The title compound, 7β-hydroxyroyleanone, an abietane-type diterpene, was isolated from Taxodium ascendens (B.).


Chemical context
Taxodium ascendens Brongn belongs to the Taxodiaceae species, which is native to the south-east of North America and has spread widely over southern China (Si et al., 2001). Previous chemical studies of Taxodium ascendens (B.) have described many diterpenes, such as 6,7-dehydroroyleanone, salvinolone and xanthoperol (Kusumoto et al., 2009;Gonzalez, 2015), and the diterpenoids have attracted much attention in recent years because of their diverse biological properties (Burmistrova et al., 2013;Tanaka, 2001), such as antibacterial (Yang et al., 2001), antioxidant (Kolak et al., 2009), antifungal (Topçu & Gö ren, 2007) and anticholinesterase activities (Topçu et al., 2013). A detailed phytochemical investigation of a petroleum ether extract of the pollen of Taxodium ascendens Brongn has been carried out and a series of diterpenoids have been isolated, including the title compound, 7-hydroxyroyleanone. Herein, we present the crystal structure of 7hydroxyroyleanone carried out in order to establish unambiguously the stereochemical features of this natural product.
The crystal structure of the 10R stereoisomer of the title compound, isolated from the roots of Premna obtusifolia (Verbenaceae), has been reported twice (see x4, Database survey). It crystallized in the chiral space group P2 1 2 1 2 1 , and the absolute structure was determined as (4bS,8aS,10R) by resonant scattering using Cu K radiation (Razak et al., 2010). Comparing the two compounds indicates that the configuration of the three stereocentres in the title compound are (4bS,8aS,10S).

Supramolecular features
In the crystal, two strong O-HÁ Á ÁO hydrogen bonds, namely O2-H2AÁ Á ÁO3 i and O4-H4Á Á ÁO1 ii , both approximately running along the b axis, are formed via the hydroxy group and the carbonyl groups ( Fig. 2 and Table 1). Furthermore, a weak C11-H11Á Á ÁO1 ii hydrogen bond occurs from a ring C atom to a carbonyl group, also running along the b-axis direction. These interactions result in the formation of chains propagating along the b-axis direction ( Fig. 2 and Table 1).

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.27, last update February 2017; Groom et al., 2016) for the octahydrophenanthrene-1,4-dione skeleton revealed 14 entries. These include two reports of a compound similar to the title compound, but with no hydroxy group in position 10, i.e. CSD refcodes HACGUN (Eugster et al., 1993) and HACGUN01 (Fun et al., 2011), and two reports of the stereoisomer of the title compound, with the hydroxy group in position 10 having an R configuration, i.e. QICLIX (Chen et al., 2000) and QICLIX01 (Razak et al., 2010).  Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; y À 1; z; (ii) x; y þ 1; z.

Figure 1
The molecular structure of the title compound, with the atom labelling and 50% probability displacement ellipsoids. Intramolecular O-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines (see Table 1).

Figure 2
A view along the c axis of the crystal packing of the title compound, with hydrogen bonds shown as dashed lines. Only H atoms involved in these interactions have been included.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms were positioned with idealized geometry and refined using a riding model, with O-H = 0.82 Å and C-H = 0.94-0.98 Å , and with U iso (H) = 1.5U eq (O,C) for hydroxy and methyl groups, and 1.2U eq (C) for other H atoms.

Special details
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.