Advanced Structural Determination of Diterpene Esters Using Molecular Modeling and NMR Spectroscopy

International audience; Three new jatrophane esters (1−3) were isolated from Euphorbia amygdaloides ssp. semiperfoliata, including an unprecedented macrocyclic jatrophane ester bearing a hemiketal substructure, named jatrohemiketal (3). The chemical structures of compounds 1−3 and their relative configurations were determined by spectroscopic analysis. The absolute configuration of compound 3 was determined unambiguously through an original strategy combining NMR spectroscopy and molecular modeling. Conformational search calculations were performed for the four possible diastereomers 3a−3d differing in their C-6 and C-9 stereocenters, and the lowest energy conformer was used as input structure for geometry optimization. The prediction of NMR parameters (1 H and 13 C chemical shifts and 1 H− 1 H coupling constants) by density functional theory (DFT) calculations allowed identifying the most plausible diastereomer. Finally, the stereostructure of 3 was solved by comparison of the structural features obtained by molecular modeling for 3a−3d with NMR-derived data (the values of dihedral angles deduced from the vicinal proton−proton coupling constants (3 J HH) and interproton distances determined by ROESY). The methodology described herein provides an efficient way to solve or confirm structural elucidation of new macrocyclic diterpene esters, in particular when no crystal structure is available. L ower diterpenoids isolated from Euphorbia species represent a unique group of highly structurally diverse compounds endowed with potent biological activity such as antiviral activities 1 and modulation of PKC and P-glycoprotein activities. 2,3 More than 650 diterpenoids based on 20 types of carbon skeletons have been characterized from this genus so far. 3 The planar structures and the relative configurations of these skeletons have been usually solved by interpretation of NMR spectroscopic data 4−11 or obtained by X-ray crystallographic analysis. 12−18 Conformational search and geometry optimization have also been used to support their structural elucidation based on NMR spectroscopy. 19−30 The absolute configurations of these skeletons have been established using X-ray diffraction analysis 6,17,31,32 or electronic circular dichroism (ECD) data analysis 27−29 or deduced from the NMR data of Mosher esters. 6 Moreover, taking into account that all macrocyclic diterpene esters of the lathyrane 5,16,33 and jatrophane types, 16,17,33 and most of their biogenetically related skeletons, 2,3 possess a constant trans-ring junction with a 4α-proton and 15β-oxygenated functionality, the relative configuration of these diterpenoids can be confidently upgraded to the absolute one. 23,30,34 When no crystal structure is available, the three-dimensional structures of these natural products can be established through the interpretation of NOESY or ROESY experiments along with the analysis of vicinal proton−proton coupling constants 3 J HH , observed in their 1 H NMR spectra. 35 However, based on NMR spectroscopic analysis, the determination of the relative configuration of some new diterpene scaffolds can be challenging. For example, the stereostructure of the tetracyclic pepluane isolated by Jakupovic et al. was misassigned. 21 Despite the authors having performed a molecular modeling analysis, the full stereochemistry of the compound could not be determined. The 3D structure of pepluane was established finally via single-crystal X-ray diffraction study. 36 A conforma-tional study showed that pepluane possesses a relatively rare ring D conformation, which probably led to the misinterpretation of the NMR data. 36 The challenging 3D determination of some jatrophane esters may also be exemplified by a study by Hohmann and associates, in which the relative configuration of esulatin C could not be assigned. 34 The high flexibility of the macrocyclic part of this compound was one of the explanations made. Several studies have shown that jatrophane esters can adopt different conformations depending on their esterification pattern. 37−40 The full structural determination of natural products can be strengthened confidently by molecular modeling and density