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The sesquiterpene lactone isoconfertiflorin, C17H22O5, 1, crystallizes in trigonal space group P31 with a = 10.4498(5), c = 12.7516(8) Å, V = 1205.9(2) Å3, and Z = 3. It differs from confertiflorin in having an endocyclic C=C bond in the lactone ring, which carries an exocyclic methyl group. The seven-membered ring adopts an approximate twist-chair conformation with C10 lying in the local two-fold axis, and asymmetry parameter ΔC2 = 5.8°. The cyclopentanone ring has the half-chair conformation with C4 on the local two-fold axis, and ΔC2 = 0.9°. The lactone ring is almost planar with maximum deviation 0.014(2) Å. Both methyl groups on the 7-membered ring are β–oriented. The acetyl group at C8 has an α orientation.