2 CRYSTALLINE FORMS OF SESQUITERPENE LACTONE HELENALIN - X-RAY AND NMR-STUDY

Helenalin, a naturally occurring sesquiterpene lactone of the pseudogu aianolide type, has been reported to exist in two polymorphic forms, b oth crystallizing in the monoclinic space group P2(1). In this paper w e report the X-ray structure of the third room temperature polymorph, crystallizing in the orthorhombic space group P2(1)2(1)2(1) The isolat ed molecules in all three polymorphs exhibit nearly the same molecular conformation, so the three crystalline modifications differ mainly in the mode the molecules pack in the solid state. Tao main hydrogen bon d accepters, the oxygen atom of the cyclopentenone ring and the oxygen atom of the lactone carbonyl group, are competing in the hydrogen bon d formation with the secondary hydroxyl group. Having an access to two polymorphs which differ in the hydrogen bond accepters, we have perfo rmed the solid state C-13 NMR measurements using a cross-polarization magic angle spinning technique. By comparison of the spectra taken in non-polar solvent with the spectra taken in the solid state we were ab le to demonstrate the shielding effects resulting from the intermolecu lar hydrogen bonding in crystals. The fast conformational equilibrium between two conformers TC(10) and TC(7) was suggested from NMR study o f helenalin in solution.