(11R)-13-Dimethyl-ammonio-11,13-dihydro-4,5-epoxy-costunolide semifumarate.

Crystals of the title salt, C(17)H(28)NO(3) (+)·C(4)H(3)O(4) (-), were obtained by reacting parthenolide with dimethyl-amine followed by conversion of the amine adduct into a water-soluble fumarate salt. Subsequent crystallization of the fumarate salt from water afforded colorless ortho-rhom-bic crystals. The amine addition is highly stereospecific yielding exclusively a single diastereomer with R-configuration at the newly formed C-11 chiral carbon. In the crystal, intermolecular O-H⋯O and N-H⋯O hydrogen bonds help to establish the packing.