0.027; wR factor = 0.068; data-to-parameter ratio = 12.7. The title structure (systematic name: 9-hydroxymethyl-3,6dimethyl-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) A ˚. Related literature For phytochemical reports of the title compound, see: Bohlmann et al. (1981); Djordjevic et al. (2004); Sarg et al. (1982); Song et al. (1995). The crystal structures of several related compounds have been reported: 8--hydroxyachillin (Campos et al., 1989); matricarin (Parvez et al., 2002); lactucin (Ruban et al., 1978); lactucopicrin (Ren et al., 2003); absolute configuration of sesquiterpene lactones Fischer et al. (1979). For analysis of Bijvoet pairs, see: Hooft et al. (2008)
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