STRUCTURE AND CHEMISTRY OF ISABELIN - A NEW GERMACRANOLIDE DILACTONE FROM AMBROSIA PSILOSTACHYA DC (COMPOSITAE)

Details are given for the isolation, structure determination, chemistry of isabelin (1), a new germacranolide dilactone from Ambrosia psilostachya. NMR studies at different temperatures established that isabelin exists in solution at room temperature in a 10:7 ratio of two conformers. The dihydro and hexahydro derivatives of isabelin were key compounds for the structure elucidation since these derivatives were correlated by IR with known substances. In the course of the structure analysis, a number of other derivatives and transformation products were prepared and characterized; these included tetrahydroisabelin, mono- and dipyrazolines, Michael adducts, isoisabelin, bromodihydroisoisabelin and two ketocarboxylic acid methyl esters. A separate proof for the structure of isabelin was provided by its synthesis from cnicin (5), a germacranolide from Centaurea species. © 1969.