Conformations of germacra-1(10),4-dien-6,12-olides and -8,12-olides. A comparison of x-ray diffraction, NMR, and molecular mechanics derived conformations

Abstract

as slightly yellow crystals, mp 343-345 °C.X-ray Crystallography. Crystals were colorless prisms and preliminary X-ray investigation indicated a monoclinic unit cell.The crystal used for data collection was approximately 0.3 X 0.15 X 0.1 mm.3 With 15 reflections, measured at ±0 angles between 20°and 30°, least-squares refinement produced a cell with dimensions a = 11.982 (1) Á, b = 16.383 (1) Á, c = 12.509 (1) Á, ß = 116.71 (1)°(assumed wavelength for Cu : 1.5418 Á). The space group was uniquely determined as P21/o. Assuming four molecules of colchicine, the calculated crystal density of 1.209 g cm™3 was reasonable. The phase problem was solved with MITHRIL.40 The distribution of intensities was centric, and all heavy atoms of the molecule were visible in the E-map. The structure was refined by standard methods using the programs of XRAY7241 (isotropic followed by anisotropic refinement of (40) Gilmore, C. J. MITHRIL. A computer program for the automatic solution of crystal structures from X-ray data,

A Highly Substituted Germacranolide from Leontodon cichoraceus

“…The 13C NM R data (Table I) are in good agreem ent with those reported for other 8-acyl-15-hydroxysalonitenolide derivatives (B arrero et al, 1989;Neves et al, 1999). Figure 2 and suggest that the predom inant conformation of 2 in solution is the UU-conform ation sensu Watson and Kashyap (1986). This conform ation was also suggested for the related compound onopordopicrin (Lonergan et al, 1992).…”

NMR and x‐ray conformational study of artemisiifolin and three other related germacranolides

“…Four conformations, UU, UD, DU and DD, where U (up) and D (down) refer to the orientation of the C-10 methyl and C-4 substituient moieties on the ring, were considered in accord with the usual exchange models established for trans,transgermacradienolides. 9 Examination of the models suggests that the 1A-1B, 1A-1D, 1B-1C and 1C-1D interconversions (Fig. 1) are relatively easy through a low-energy exchange barrier.…”

Germacrone Derivatives as new Insecticidal and Acaricidal Compounds: A Structure-Activity Relationship

“…The three sets of signals appear better resolved when the NMR spectrum was performed at −20 °C (Figure 2, see also Supplementary Material). The ability of the ten-membered ring of the germacradiene species to undergo conformational changes is well known, and has been the subject of multiple computational studies [14,15,16,17,18,19]. This conformational flexibility plays an important role in their biosynthetic transformation to other sesquiterpene skeletons [20,21], and in the reactivity of these compounds, since the spatial disposition of the different conformers determines the outcome of the corresponding reactions [22,23].…”

“…The 8 main conformers found for the 7 S -epimer are represented in Figure 3. These conformers are described as up–up (UU), up–down (UD), down–up (DU) and down–down (DD), depending upon the orientation of the C14 and C15 methyl groups, in accordance with the terminology introduced by Watson and Kashyap [19].…”