AbstractThe absolute configurations of (+)‐cantharic acid, a chiral derivative of the achiral insect defensive substance cantharidin (1), and of palasonin, a lower homologue of1occurring in the plantButea frondosa, were shown to be represented by formulas2and3, respectively (scheme 1). These results were obtained by application of theHoreaumethod (table 1) on the (+)‐cantharic acid derivatives (+)‐5, (−)‐7, and (−)11(scheme 2), and the palasonin derivatives (+)‐29and (+)‐30(scheme 4), as well as by comparison of the chiroptical properties (tables 2and3) of a number of derivatives, prepared from either cantharic acid or palasonin. – Attempts to incorporate various radioactively labelled precursors into palasonin (3), using young and adult plants, have been so