New Techniques of Structure Elucidation for Sesquiterpenes
289
OH
OH
OH
O
O
OAc
OH
OH
O
O
OH
OH
O
O
O
OH
O
O
O
O
O
OH
O
O
209
1
1
2
0
1
2
212
213
3.2.2 The Exciton Chirality Method and Molecular Modeling
This section considers those examples that have used the exciton chirality method
[95, 96] in combination with molecular modeling of sesquiterpenes using timedependent DFT calculations. It should be mentioned that the specific alignment of
the chromophores involved in the transitions can be estimated and visualized in the
DFT optimized molecular models for each calculated structure in a highly efficient
manner.
A phytochemical study of the rhizomes of Acorus tatarinowii, a Chinese medicinal plant, gave four novel acorane sesquiterpenes (214–217) together with two other
new sesquiterpenes. Their structures were elucidated by extensive NMR spectroscopic analysis together with X-ray crystallography, while their absolute configurations were determined by a combined method that included calculation of the
Flack parameter for the single-crystal X-ray analysis of 214 and the exciton chirality
method applied to acotatarone B (217). The ECD spectrum showed a split curve with
a negative band at 221 nm and a positive band at 246 nm associated with the exciton
coupling between the two α,β-unsaturated carbonyl chromophores, indicating their
absolute configuration of the spiro carbon at C-5 to be (R). The concordance between
the experimental ECD spectrum of 217 with that calculated at the DFT B3LYP/631G(d) level of theory in methanol confirmed the absolute configuration of this
compound as (1S,5R) [97]. It is important to remark that ECD calculated spectrum
arise from Boltzmann averaging of individual curves of each low energy conformer.
The anti-Alzheimer effects of all compounds were estimated by means of acetylcholinesterase and β-site amyloid precursor protein cleaving enzyme 1 inhibition
assays [97].
289
OH
OH
OH
O
O
OAc
OH
OH
O
O
OH
OH
O
O
O
OH
O
O
O
O
O
OH
O
O
209
1
1
2
0
1
2
212
213
3.2.2 The Exciton Chirality Method and Molecular Modeling
This section considers those examples that have used the exciton chirality method
[95, 96] in combination with molecular modeling of sesquiterpenes using timedependent DFT calculations. It should be mentioned that the specific alignment of
the chromophores involved in the transitions can be estimated and visualized in the
DFT optimized molecular models for each calculated structure in a highly efficient
manner.
A phytochemical study of the rhizomes of Acorus tatarinowii, a Chinese medicinal plant, gave four novel acorane sesquiterpenes (214–217) together with two other
new sesquiterpenes. Their structures were elucidated by extensive NMR spectroscopic analysis together with X-ray crystallography, while their absolute configurations were determined by a combined method that included calculation of the
Flack parameter for the single-crystal X-ray analysis of 214 and the exciton chirality
method applied to acotatarone B (217). The ECD spectrum showed a split curve with
a negative band at 221 nm and a positive band at 246 nm associated with the exciton
coupling between the two α,β-unsaturated carbonyl chromophores, indicating their
absolute configuration of the spiro carbon at C-5 to be (R). The concordance between
the experimental ECD spectrum of 217 with that calculated at the DFT B3LYP/631G(d) level of theory in methanol confirmed the absolute configuration of this
compound as (1S,5R) [97]. It is important to remark that ECD calculated spectrum
arise from Boltzmann averaging of individual curves of each low energy conformer.
The anti-Alzheimer effects of all compounds were estimated by means of acetylcholinesterase and β-site amyloid precursor protein cleaving enzyme 1 inhibition
assays [97].