Beetles Do It Differently: Two Stereodivergent Cyclisation Modes in Iridoid‐Producing Leaf‐Beetle Larvae

Larvae of the Chrysomelina species Phaedon cochleariae , Hydrothassa marginella , Phratora vulgatissima , Gastrophysa viridula , Gastrophysa atrocyanea , Gastrophysa cyanea and Gastrophysa polygoni produce the iridoid chrysomelidial ( 1 ) to defend themselves against predators. Feeding experiments with a deuterated precursor ([ 2 H 5 ]8‐hydroxygeraniol 9 ) and in vitro isotope exchange experiments with defensive secretion in 2 H 2 O revealed differences in the cyclisation of the ultimate precursor 8‐oxogeranial ( 8 ) to 1 , between members of the genus Gastrophysa and all other species. In P. cochleariae , H. marginella and P. vulgatissima 1 is most likely produced by a Rauhut–Currier‐type cyclisation via a “transoid dienamine”, with loss of a single deuterium atom from C(4) of the precursor. In contrast, members of the genus Gastrophysa cyclise 8 via a “cisoid dienamine” intermediate, with exchange of all three deuterium atoms from the methyl group at C(3). To study whether the different cyclisation modes influence the stereochemistry of 1 , the absolute configuration of 1 of the larvae was determined by GC‐MS on a chiral column. In accordance with literature (J. Meinwald, T. H. Jones, J. Am. Chem. Soc. 1978 , 100 , 1883 and N. Shimizu, R. Yakumaru, T. Sakata, S. Shimano, Y. Kuwahara, J. Chem. Ecol. 2012 , 38 , 29), we found (5 S ,8 S )‐chrysomelidial ( 1 ) in H. marginella and P. vulgatissima , but P. cochleariae and all investigated members of the genus Gastrophysa synthesise (5 R ,8 R )‐chrysomelidial ( 1 ).