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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.

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