The pseudoguaianolide isoconfertiflorin

The sesquiterpene lactone isoconfertiflorin, C17H22O5, 1, crystallizes in t rigonal space group P3(1) with a = 10.4498(5), c = 12.7516(8) Angstrom, V = 1205.9(2) Angstrom(3), and Z = 3. It differs from confertiflorin in having an endocyclic C=C bond in the lactone ring, which carries an exocyclic met hyl group. The seven-membered ring adopts an approximate twist-chair confor mation with C10 lying in the local two-fold axis, and asymmetry parameter D elta C-2 = 5.8 degrees. The cyclopentanone ring has the half-chair conforma tion with C4 on the local two-fold axis, and Delta C-2 = 0.9 degrees. The l actone ring is almost planar with maximum deviation 0.014(2) Angstrom. Both methyl groups on the 7-membered ring are beta-oriented. The acetyl group a t C8 has an alpha orientation.