New Techniques of Structure Elucidation for Sesquiterpenes
255
of NMR spectrometers to unprecedented levels of quantitation and accuracy are also
included.
2.1 Calculation of Chemical Shifts
A remarkable example of the application of these NMR-DFT methodologies
was described for the structural determination of a triquinane derivative isolated
from Pulicaria vulgaris [1]. The structure of this sesquiterpenoid was deduced as
presilphiperfolane-7α,8α-diol (1) by combining solvent-induced removal of chemical shift degeneracy and computational DFT-gauge including atomic orbital (GIAO)
prediction of NMR spectra with the analysis of
1 H NMR splitting patterns obtained by
the full spin simulation using the MestReNova program [2]. The chemical shifts and
coupling constants of presilphiperfolane-7α,8α-diol (1) and seven other geometryoptimized diastereomers were calculated at the DFT B3LYP 6-311++G(d,p) level of
theory and the solvent effects were modeled by the polarizable continuum model selfconsistent reaction field (PCM-SCRF) protocol. The values of correlation coefficients
and root mean-square errors between calculated and experimental data supported 1 as
the correct structure [1]. All DFT calculations were accomplished using the Gaussian
09 program package [3].
Altogether nine guaianolides, some of them with anti-inflammatory properties,
were isolated from the aerial parts of Ormenis mixta collected from Constantine,
Algeria [4]. Of this group of compounds, the stereochemical assignments of 2–4
were based mainly on a combined quantum mechanical/NMR approach by comparison of the experimental
13 C/
1 H NMR chemical shifts and J H-H coupling constants
with the predicted values. The calculations for each compound involved the conformational search of all possible diastereoisomers of 2–4 followed by optimization of
their geometries and calculation of the
13 C and
1 H NMR chemical shifts for every
selected conformer using the MPW1PW91 functional and 6-31G(d,p) basis set in
methanol also using the Gaussian 09 program [3]. Calculations of the weighted averaged chemical shifts were carried out taking into account the Boltzmann distributions
according to the conformational energies and considering the calculated chemical
shifts of tetramethylsilane as the reference compound [4].
255
of NMR spectrometers to unprecedented levels of quantitation and accuracy are also
included.
2.1 Calculation of Chemical Shifts
A remarkable example of the application of these NMR-DFT methodologies
was described for the structural determination of a triquinane derivative isolated
from Pulicaria vulgaris [1]. The structure of this sesquiterpenoid was deduced as
presilphiperfolane-7α,8α-diol (1) by combining solvent-induced removal of chemical shift degeneracy and computational DFT-gauge including atomic orbital (GIAO)
prediction of NMR spectra with the analysis of
1 H NMR splitting patterns obtained by
the full spin simulation using the MestReNova program [2]. The chemical shifts and
coupling constants of presilphiperfolane-7α,8α-diol (1) and seven other geometryoptimized diastereomers were calculated at the DFT B3LYP 6-311++G(d,p) level of
theory and the solvent effects were modeled by the polarizable continuum model selfconsistent reaction field (PCM-SCRF) protocol. The values of correlation coefficients
and root mean-square errors between calculated and experimental data supported 1 as
the correct structure [1]. All DFT calculations were accomplished using the Gaussian
09 program package [3].
Altogether nine guaianolides, some of them with anti-inflammatory properties,
were isolated from the aerial parts of Ormenis mixta collected from Constantine,
Algeria [4]. Of this group of compounds, the stereochemical assignments of 2–4
were based mainly on a combined quantum mechanical/NMR approach by comparison of the experimental
13 C/
1 H NMR chemical shifts and J H-H coupling constants
with the predicted values. The calculations for each compound involved the conformational search of all possible diastereoisomers of 2–4 followed by optimization of
their geometries and calculation of the
13 C and
1 H NMR chemical shifts for every
selected conformer using the MPW1PW91 functional and 6-31G(d,p) basis set in
methanol also using the Gaussian 09 program [3]. Calculations of the weighted averaged chemical shifts were carried out taking into account the Boltzmann distributions
according to the conformational energies and considering the calculated chemical
shifts of tetramethylsilane as the reference compound [4].