The guaianolide sesquiterpene pyrethroidinin (1) and the corresponding ketone parishin A (2) were isolated from Tanacetum densum subsp, amani (Asteraceae). The absolute stereochemistry of 1 was established as (1R, 3R, 6S, 7S, 10R) on the basis of the differential shielding in the H-1 NMR spectra of the (-)- and (+)-O-methylmandelate esters. The structure, including absolute stereochemistry, of 2 was confirmed by oxidation of 1 with Jones' reagent.
