Helenanolide-type sesquiterpene lactones. Part 2. The molecular conformations of arnifolin and some related helenanolides as determined by X-ray crystallographic and NMR spectroscopic analyses

Abstract

In continuation of our studies on the molecular geometry of helenanolide-type sesquiterpene lactones (T.J. Schmidt, J. Mol. Struct., 385 (1996)), the present study is directed towards the conformation of arnifolin 1 as well as those of 11α,13-dihydroarnifolin 2 and two 4H-derivatives, 2-deacetyl-6- O-tigloylchamissonolide 3 and 2-deacetyl-4- O-tigloylchamissonolide 4. It can be shown by X-ray crystallographic analyses that in 1 and 2 the seven-membered ring adopts the same twist chair conformation with the pseudodiad passing through C-7 (TC7) whereas the 4H-derivatives both adopt a twist boat conformation with the pseudo symmetry axis passing through C-10 (TB10). These conformational preferences can be shown to occur also in solution by evaluation of the 1H-NMR coupling constants and 2D-NOESY spectra. Possible reasons for these differences in molecular geometry are discussed briefly against the background of molecular mechanics calculations and previous findings reported for similar pseudoguaianolides.