The Guaianolide 11βH,13-Dihydromicheliolide

The lactone ring of the guaianolide-class sesquiterpene lactone [3S-(3α,3aα,9α,9aα,9bβ)]-3a,4,5,7,8,9,9a,9b-octahydro-9-hydroxy- 3,6,9-trimethylazuleno[4,5-b]furan-2(3H)-one, C₁₅H₂₂O₃, is trans-fused to the seven-membered ring. Two molecules are present in the asymmetric unit, and they differ only slightly in conformation. The seven-membered ring is in the chair conformation, with the local mirror bisecting the double bond. The lactone is in the envelope conformation, while the other five-membered ring adopts the half chair. The two independent molecules form a hydrogen-bonded dimer, with OO distances 2.907 (3) and 2.966 (2) Å.