X-RAY STRUCTURE-ANALYSIS AND NMR-STUDY OF SESQUITERPENE LACTONE HIRSUTOLIDE

Hirsutolide, the C(8)-lactonized sesquiterpene of the heliangolide typ e has been studied by X-ray and NMR methods. In crystal the ten-member ed ring conformation closely resembles the conformation observed in C( 6)-lactonized heliangolides. This conformation, which can be described as a boat-chair is contrasted with the chair-chair conformation found in the crystals of other C(8)-lactonized heliangolide, scorpioidine. Broad signals in H-1 and C-13 NMR spectra indicated the conformation e quilibrium in solution which was confirmed by observation of signals o f two conformers (ca 3 : 1) in low temperature H-1 NMR spectra. The ma jor conformer is the one present in the crystal structure of hirsutoli de and the minor conformer corresponds very likely to the one observed in the crystal structure of scorpioidine.