The chemistry of the compositae. Part XXXI. Absolute configuration of the sesquiterpene lactones centaurepensin (chlorohyssopifolin A), acroptilin (chlorohyssopifolin C), and repin

Abstract

Direct comparison of the sesquiterpene lactones chlorohyssopifolin A (6c), from Centaurea hyssopifolia, and centaurepensin, from Centaurea repens showed them to be identical. The former was converted, by dehalogenation (zinc–copper couple) and subsequent saponification, into the deacyldihydrocyanaropicrin (10a), a derivative of cyanaropicrin (11j), the absolute stereochemistry of which for the centres of asymmetry C-1 (α-H), C-5 (α-H), C-6 (β-H), and C-7 (α-H) has been rigorously established. This stereochemistry is the same as has been found to date in all the guaianolides for which absolute configurations are known, and is the opposite of that previously assigned to centaurepensin. Correlation between structure (6c) and the lactones acroptilin and repin, reported to have structures (4c and d)(C-1 β-H), suggests that in these substances the fusion of rings A and B is also cis C-1 α-H and not trans.