Ajugatakasins A and B, new diterpenoids from Ajuga decumbens, and feeding stimulative activity of related neoclerodane analogs toward the turnip sawfly

Adults of the turnip sawfly, Athalia rosae ruficornis, are strongly attracted to the leaves of Ajuga decumbens (Labiatae). Specific feeding stimulants of the sawfly were examined in the leaf surface extracts of A. decumbens. Among seven neoclerodane diterpenoids isolated from the extracts, two new analogs, ajugatakasins A and B, were characterized as 6 alpha,18-diacetoxy-1 beta,12-ditigloyloxy-4 alpha,17-epoxyneoclerod-13-en-15,16-olide and its 1 beta,12-di-(2-methylbutanoyl)-oxy analog, respectively. Among these analogs, clerodendrin D, a very minor constituent which possesses a tetrahydrofurofuran moiety, was identified as the feeding stimulant for A. rosae ruficornis, while the other analogs with an alpha,beta-unsaturated gamma-lactone moiety were found to be completely inactive in the sawfly feeding test.