New Techniques of Structure Elucidation for Sesquiterpenes
277
Several reviews have covered articles on vibrational circular dichroism of organic
compounds up to 2015 [60–65]. Therefore, this section includes the relevant work
on sesquiterpenes generated from 2015 to May 2020.
The absolute configuration of inuloxin A (141), a phytotoxin produced by the plant
species Inula viscosa, and seiricardine A (142), a phytotoxin isolated from the fungus
Seiridium cardinale, was determined by chiroptical experimental measurements in
comparison with theoretical calculations [66]. Vibrational CD spectroscopy was
essential in this stereochemical study that was conducted in combination with optical
rotatory dispersion and electronic circular dichroism. The conformational search of
141 was accomplished at the molecular mechanics level to give 15 conformers that
were optimized at the DFT/B3LYP/6-31G and then at the DFT/B3LYP/TZVP levels
of theory providing the three most populated conformers. Vibrational CD calculations were performed also using DFT methods at the M06-2X/TZVP level of theory
with implicit solvation models IEF-PCM in chloroform and acetonitrile. The study
of seiricardine A (142) was carried out on the 4-bromobenzoate derivative 143,
which contains a convenient chromophore for the ECD spectrum, higher ORD and
VCD values, and less intermolecular interactions. The absolute configuration of (+)inuloxin A (141) was established as 7R,8R,10S, while that of (–)-seiricardine A (142)
and its derivative 143 was assigned as (1S,2R,3aS,4S,5R,7aS) [66].
O
O
O
OH
RO
141 (inuloxin A)
142 R = H (seiricardine A)
143 R = 4-BrPhCO
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
2
3
4 5
6
7
8 9
10
11
12
13
3a
7a
The absolute configuration of the sesquiterpene lactone (–)-centratherin (144)
was determined by a theoretical and experimental study that included VCD, optical
rotatory dispersion, and electronic circular dichroism [67]. The VCD study of this
furanoheliangolide was conducted according to a protocol that comprised a conformational search, carried out with the MMFF94s force field, followed by DFT geometry optimization and spectral calculation achieved at the B3LYP/aug-cc-pVDZ
level of theory. The vibrational and electronic dissymmetry factors were used to
evaluate the results and validate the absolute configuration of (–)-centratherin as
(6R,7R,8S,10R,2
Z)-144 [67].
O
O
O
O
HO
O
O
O
O
O
OH
144 ((—)-(6 R,7R,8S,10R,2Z)-centratherin)
145 (farinosin)
1
2
3
4 5
6
7
8
9
10
11
12
13
14
15
277
Several reviews have covered articles on vibrational circular dichroism of organic
compounds up to 2015 [60–65]. Therefore, this section includes the relevant work
on sesquiterpenes generated from 2015 to May 2020.
The absolute configuration of inuloxin A (141), a phytotoxin produced by the plant
species Inula viscosa, and seiricardine A (142), a phytotoxin isolated from the fungus
Seiridium cardinale, was determined by chiroptical experimental measurements in
comparison with theoretical calculations [66]. Vibrational CD spectroscopy was
essential in this stereochemical study that was conducted in combination with optical
rotatory dispersion and electronic circular dichroism. The conformational search of
141 was accomplished at the molecular mechanics level to give 15 conformers that
were optimized at the DFT/B3LYP/6-31G and then at the DFT/B3LYP/TZVP levels
of theory providing the three most populated conformers. Vibrational CD calculations were performed also using DFT methods at the M06-2X/TZVP level of theory
with implicit solvation models IEF-PCM in chloroform and acetonitrile. The study
of seiricardine A (142) was carried out on the 4-bromobenzoate derivative 143,
which contains a convenient chromophore for the ECD spectrum, higher ORD and
VCD values, and less intermolecular interactions. The absolute configuration of (+)inuloxin A (141) was established as 7R,8R,10S, while that of (–)-seiricardine A (142)
and its derivative 143 was assigned as (1S,2R,3aS,4S,5R,7aS) [66].
O
O
O
OH
RO
141 (inuloxin A)
142 R = H (seiricardine A)
143 R = 4-BrPhCO
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
2
3
4 5
6
7
8 9
10
11
12
13
3a
7a
The absolute configuration of the sesquiterpene lactone (–)-centratherin (144)
was determined by a theoretical and experimental study that included VCD, optical
rotatory dispersion, and electronic circular dichroism [67]. The VCD study of this
furanoheliangolide was conducted according to a protocol that comprised a conformational search, carried out with the MMFF94s force field, followed by DFT geometry optimization and spectral calculation achieved at the B3LYP/aug-cc-pVDZ
level of theory. The vibrational and electronic dissymmetry factors were used to
evaluate the results and validate the absolute configuration of (–)-centratherin as
(6R,7R,8S,10R,2
Z)-144 [67].
O
O
O
O
HO
O
O
O
O
O
OH
144 ((—)-(6 R,7R,8S,10R,2Z)-centratherin)
145 (farinosin)
1
2
3
4 5
6
7
8
9
10
11
12
13
14
15