Crystal and molecular structure of parthenolide [4,5-epoxygermacra-1(10),11(13)-dien-12,6-olactone]

The crystal structure of the title compound has been determined from single-crystal X-ray data by direct methods. Crystals are orthorhombic, a= 11.929(1), b= 12.267(2), c= 18.761 (2)Å, space group P212121, Z= 8. i.e. two independent molecules per asymmetric unit. Full-matrix least-squares refinement based on 2 330 independent reflections has reached R 0.047. The double bond C(1)–C(10) and the equivalent double bond C(4)–C(5) are both trans, both methyl groups are β, and the ten-membered ring has a conformation typical of several germacranolides.