(3S,11Z)-14,16-Dihydroxy-3-methyl-3,4,5,6,9,10-hexahydro-1H-2-benzoxacyclotetradecine-1,7(8H)-dione (cis-zearalenone): a redetermination

The title compound, also known as cis-zearalenone (cis-ZEN), C18H22O5, has already been reported elsewhere [Griffin et al. (1981 ▶). ACA Ser. 29, 35], but no atomic coordinates are publicly available. The molecule is of interest with respect to its toxicity. In the crystal, intramolecular O—H⋯O hydrogen bonds stabilize the molecular conformation, while intermolecular O—H⋯O hydrogen bonds link the molecules to form infinite chains along the [110] and [1-10] directions. The absolute configuration has been assigned by reference to an unchanging chiral centre in the synthetic procedure.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BG2443).
In chemical terms, zearalenone belongs to the group of resorcyclic acid lactones. Due to the ethylenic double bond between C 11 and C 12 in the lactone ring ZEN can exist in two stereoisomeric forms: cis and trans. From mycelia of the fungus F. graminearum only trans-ZEN could be isolated and its structure was elucidated using classical chemical, NMR and mass spectrometric analysis (Urry et al. 1966). This finding, which was confirmed also by other studies, led to the assumption that in the ZEN production by the fungi an isomer specific biosynthetic pathway is involved. According to IUPAC the name zearalenone is a synonym only for the pure (3S,11E)-14,16-dihydroxy-3-methyl-3,4,5,6,9,10-hexahydro-1H-2-benzoxacyclotetradecine-1,7(8H)-dione (= trans-ZEN, CAS: 17924-92-4) and describes not the isomeric mixture of cis-and trans-ZEN. Therefore, worldwide all established maximum levels for ZEN in food and feed apply only to trans-ZEN. However, the absorption of (ultraviolet) light induces isomerization from trans-to the more stable cis-ZEN, so that at presence of any ZEN contamination both isomers can occur. Only very little is known about the occurrence, fate and risks associated with cis-ZEN entering the food chain. This causes a major problem for the official control of foodstuffs and consumer protection. Most of the various analytical methods for the determination of ZEN in food and feed, including the official methods (e.g., ASU (german: "Amtliche Sammlung von Untersuchungsverfahren") according to paragraph 64 of the LFGB (german: "Lebensmittel-, Bedarfsgegenstände-und Futtermittelgesetzbuch")) are not able to distinguish between the two ZEN isomers. Hence, depending on the chromatographic separation this could potentially lead to "false positive" or "false negative" results and therefore to enormous public health or economic consequences. The compound crystallizes in the monoclinic space group P2 1 .
The molecular structure of the compound had already been reported elsewhere (Griffin et al., 1981; CCDC code: ZEARLN) but no atomic coordinates were made publicly available at the time, for what the present redetermination was attempted.
The atom-labeling scheme is shown in Fig. 1. The absolute configuration could not be defined confidently based on the single-crystal diffraction data. It should be noted that a light induced cis-/trans-isomerization of pure (stereochemical defined) trans-ZEN proceeds under retention of the stereochemical sense at C 3 . The isomeric purity of the title compound 2).
The purity of the isolated white powder (yield: 16 mg (64%)) was determined to be ≥95% by analytical HPLC-FLD. In addition, 1 H-NMR and HPLC-MS/MS have also been used to identify cis-ZEN and to evaluate its purity. For structural identification colorless crystals were grown by slow solvent evaporation in absence of light at ambient temperature as detailed below. In a 1.5 ml HPLC glass vial 5.0 mg (18.5 µmol) of purified cis-ZEN were weighed in and dissolved in 0.5 ml dichloromethane. Afterwards, n-hexane (1.0 ml) was added to the incipient precipitation point and then the solution was set aside at room temperature for 72 h in the dark to evaporate slowly. The title compound crystallized as colorless plates.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.