The germacrolide-class sesquiterpene lactone, 1,10-epoxyparthenolide, C15H20O4,1, crystallizes with two independent molecules in monoclinic space groupP21witha=10.6845(5),b=9.0763(4),c=15.4326(7) Å, β=105.887(4)°,V=1439.4(3) Å3,Z=4.R=0.037 for 2425 observed data. Its 11βH, 13-dihydro-derivative 1,10-epoxydihydroparthenolide, C15H22O4,2, crystallizes in orthorhombic space groupP212121witha=7.6414(10),b=12.559(2),c=14.6821(14) Å,V=1409.0(3) Å3Z=4.R=0.031 for 1555 observed data. The corresponding unexposidized compound, 11βH,13-dihydrocostunolide, C15H22O2,3, crystallizes with three independent molecules in orthorhombic space groupP212121witha=7.3576(5),b=23.505(3),c=24.185(2) Å,V=4182(1) Å3,Z=12.R=0.070 for 2767 observed data. In all, the 10-membered rings adopt approximate chair-chair conformations. In all, the double bonds or epoxidized double bonds are E, both methyl groups on the 10-ring are β, and the α-methylene-γ-lactone (or α-methyl-γ-lactone) istrans-fused at C6 and C7 with H6 β and H7 α. In the dihydro compounds, t
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