11β,13-Dihydrolactucin-8-O-acetate hemihydrate

The title structure (systematic name: 9-hydroxymethyl-3,6-dimethyl-3-methylene-2,7-dioxo-3,3a,4,5,9a,9b-hexahydroazuleno[4,5-b]furan-4-yl acetate hemihydrate), C17H20O6·0.5H2O, from Lactuca floridana, has two independent sesquiterpene lactone molecules in the asymmetric unit. Both have their seven-membered rings in the chair conformation. In the crystal, the OH groups and the water molecule form classical O—H⋯O hydrogen bonds with O⋯O distances in the range 2.6750 (17)–2.8160 (18) Å.


S1. Comment
Hairy root cultures of blue-flowered lettice, Lactuca floridana, tribe Lactuceae (Asteraceae) are useful for the study of the biosynthesis of guaianolide-type sesquiterpene lactones (Song et al., 1995). The title guaianolide was isolated from L.
floridana and crystallized as the hemihydrate.
The structures of both independent molecules are shown in Fig. 1. The conformations of (1) and (1 A) are very similar.
Hydrogen bonding involves the OH groups, the water molecule, and the acetate CO group, and forms double-strand chains along [0 1 0]. In each chain, the alternation of hydrogen bonds is (O6A-H···O6-H···H 2 O···) n .
The absolute configuration was determined by refinement of the Flack (1983) parameter, based on resonant scattering of the light atoms. It agrees with that of lactucin (Ruban et al., 1978) and with the accepted configuration of sesquiterpene lactones from higher plants (Fischer et al., 1979). Analysis of the Bijvoet pairs using the method of Hooft et al. (2008) yielded y = 0.03 (4) for this structure, confirming the absolute configuration.

S2. Experimental
Isolation of the title compound from Lactuca floridana has been described (Bohlmann et al., 1981;Song et al., 1995). Crystals were grown by evaporation from ethyl acetate.

S3. Refinement
H atoms on C were placed in idealized positions with C-H distances 0.95 -1.00 Å and thereafter treated as riding.
Coordinates for the H atoms on O were refined. U iso for H was assigned as 1.2 times U eq of the attached atoms (1.5 for methyl and OH). A torsional parameter was refined for each methyl group.  Ellipsoids at the 50% probability level, with H atoms having arbitrary radius. Both molecules are shown in the same orientation.   (10) 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.