Configuration of the endocyclic double-bond and conformation of the germacranolide dilactones, isabelin, isoisabelin, and related germacranolide monolactones, as studied by nuclear Overhauser effect

Abstract

The conformations of the two conformational isomers of isabelin (1), a germacranolide dilactone isolated from Ambrosia psilostachya DC. (Compositae), have been determined in chloroform solution by use of the intramolecular nuclear Overhauser effect; and further, the configuration of the endocyclic double-bond in its ten-membered ring has been confirmed. It has been concluded that the major and minor isomers of isabelin mainly adopt the conformations (A) and (B), respectively, which have a trans-orientation of the endocyclic double-bond. Since isabelin has been correlated with artemisiifolin (3), salonitenolide (4), and cnicin (5), the results obtained here have also established a trans configuration for these three germacranolide monolactones. In addition, isoisabelin (2), a geometrical isomer of isabelin, has been examined to determine the configuration of its C(9)–C(10) double-bond and its conformation.