STRUCTURAL ELUCIDATION AND CHEMISTRY OF A NOVEL FAMILY OF BIOACTIVE SESQUITERPENES - HELIANNUOLS

From the moderately polar active fractions of leaf aqueous extract of Helianthus annuus L. var. SH-222 and VYP, we have isolated three new s esquiterpenes which contain a previously unknown skeleton, heliannuols B-D (2-4). The structural elucidation of heliannuols was based on ext ensive spectral studies, including H-1-H-1 COSY, H-1-C-13 HETCOR, and NOE difference experiments, X-ray diffraction analysis of 4, and chemi cal correlation between 1, 2, and 4. A biosynthetic pathway that invol ves an oxirane ring opening is proposed for heliannuols A (1) and D (4 ), while a phenonium ion intermediate is proposed for heliannuol C (3) . The oxirane ring opening and the formation of this intermediate have been evaluated using the semiempirical method PM3. Allelopathic activ ity bioassays of compound 1-4 suggest that those members of a new clas s of bioactive sesquiterpenes may be involved in the cultivar sunflowe r defense against dicotyledon species.