Trichodermaerin: a diterpene lactone from Trichoderma asperellum

The title compound, C20H28O3, known as ‘trichodermaerin’ [systematic name: (4E)-4,9,15,16,16-pentamethyl-6-oxatetracyclo[10.3.1.01,10.05,9]hexadec-4-ene-7,13-dione], is a diterpene lactone which was isolated from Trichoderma asperellum. The structure has a tetracycic 6–5–7–5 ring system, with the cyclohexanone ring adopting a twisted half-chair conformation and the cyclopentane ring adopting a half-chair conformation, whereas the cycloheptene and tetrahydrofurananone rings are in chair and envelope (with the methyl-substituted C atom as the flap) conformations, respectively. The three-dimensional architecture is stabilized by C—H⋯O interactions.


Comment
Trichoderma genus is accepted as a superior biocontrol agent of plant pathogens (Hajieghrari et al., 2008;Kumar et al., 2012). Secondary metabolites from Trichoderma fungi have been reported to inhibit the phytopathogenic growth against Colletotrichum gloeosporioides (De los Santos-Villalobos et al., 2011), Pythium irregular, Sclerotinia sclerotiorum, Rhizoctonia solani (Vinale, 2009) and Sclerotium rolfsii (Evidente et al., 2006). Our study on the chemical constituents and bioactive compounds from Trichoderma asperellum stain F009, collected from soils in Suphan Buri province (Thailand), has led us to the isolation of the title diterpene lactone (I) which is known as "Trichodermaerin". The title compound was briefly reported together with Trichoderma erinaceum (Xie et al., 2013). Our antifungal assay revealed that at 200 ppm of the extract had 76.5% growth inhibition against Colletotrichum gloeosporioides. Herein we report the crystal structure of (I).

Experimental
Trichoderma asperellum stain F009 was inoculated in potato dextrose broth (PDB) for 27 days at room temperature. The broth culture (18 L) was extracted with ethyl acetate to obtain a crude ethyl acetate extract (1.956 g) as a brown viscous liquid. The crude extract was submitted to purification by column chromatography on silica gel with solvent mixtures of increasing polarity (hexane to CH 3 OH) to give fourteen fractions (F1-F14). Further separation of the subfraction F4 (122.4 mg) on silica gel column chromatography eluted with 20% ethyl acetate-hexane afforded compound (I) (5.6 mg).
Colorless block-shaped single crystals of (I) suitable for X-ray structure determination were recrystallized from ethyl acetate by the slow evaporation of the solvent at room temperature after several days, Mp. 484.05-484.95 K.

Refinement
All H atoms were placed in calculated positions with d(C-H) = 1.00 Å for CH, 0.99 Å for CH 2 and 0.98 Å for CH 3 atoms. The U iso values were constrained to be 1.5U eq of the carrier atom for methyl H atoms and 1.2U eq for the remaining H atoms. A rotating group model was used for the methyl groups. A total of 2218 Friedel pairs were merged before final refinement as there is no large anomalous dispersion for the determination of the absolute configuration.

Figure 1
The molecular structure of the title compound, showing 40% probability displacement ellipsoids.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.16 e Å −3 Special details Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K. 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.