posted on 2015-10-23, 00:00authored byLouis-Félix Nothias-Scaglia, Jean-François Gallard, Vincent Dumontet, Fanny Roussi, Jean Costa, Bogdan I. Iorga, Julien Paolini, Marc Litaudon
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 (1H and 13C chemical shifts and 1H–1H
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
(3JHH) 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.