(1S,2R,3R,8R,10S)-3-Chloro-2,8-dihy­droxy-3,7-dimethyl-11-methyl­idene-13-oxabicyclo­[8.3.0]tridec-6-en-12-one

The title compound, C(15)H(21)ClO(4), was synthesized from 9α-hy­droxy­parthenolide (9α-hy­droxy-4,8-dimethyl-12-methylen-3,14-dioxatricyclo­[9.3.0.0(2,4)]tetra­dec-7-en-13-one), which was isolated from the chloro­form extract of the aerial parts of Anvillea radiata. The mol­ecule is built up from fused five- and ten-membered rings. The five-membered lactone ring has an envelope conformation with the flap atom, C(H)-C-C(H), displaced by 0.2325 (15) Å from the mean plane through the remaining four atoms, whereas the ten-membered ring displays an approximate chair–chair conformation. The dihedral angle between the two rings is 66.4 (2)°. In the crystal, mol­ecules are linked into chains propagating along the a axis by O—H⋯O hydrogen bonds.