Assignment of configuration in a series of dioxolanone-type secondary metabolites from Guignardia bidwellii - A comparison of VCD and ECD spectroscopy

Article abstract of DOI:10.1002/ejoc.201300530

The absolute configurations of a series of phytotoxic dioxolanone-type secondary metabolites isolated from culture filtrates of the grape black rot fungus Guignardia bidwellii were determined by vibrational circular dichroism (VCD spectroscopy in the mid-IR frequency range. Comparison of the recorded data with DFT calculations showed good agreement between experiment and theory for VCD, whereas electronic circular dichroism (ECD data for the compounds matched poorly with the predicted spectra obtained by time-dependent DFT (TDDFT calculations at the same level of theory. The absolute configurations of a series of dioxolanone-type secondary metabolites from Guignardia bidwellii were determined by vibrational circular dichroism (VCD); electronic circular dichroism (ECD gave ambiguous results. Copyright

Full text of DOI:10.1002/ejoc.201300530

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FULL PAPER
Fungal Phytotoxic Metabolites
L. Andernach, L. P. Sandjo,
J. C. Liermann, I. Buckel, E. Thines,
T. Opatz* .......................................... 1–7
Assignment of Configuration in a Series of
Dioxolanone-Type Secondary Metabolites
from Guignardia bidwellii – A Comparison
of VCD and ECD Spectroscopy
The absolute configurations of a series of
dioxolanone-type secondary metabolites
from Guignardia bidwellii were determined
by vibrational circular dichroism (VCD);
electronic circular dichroism (ECD) gave
ambiguous results.
Keywords: Natural products / Configur-
ation determination / Circular dichroism /
Density functional calculations /
Conformation analysis
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T. Opatz et al.
FULL PAPER
DOI: 10.1002/ejoc.201300530
Assignment of Configuration in a Series of Dioxolanone-Type Secondary
Metabolites from Guignardia bidwellii – A Comparison of VCD and ECD
Spectroscopy
Lars Andernach,[a] Louis P. Sandjo,[a] Johannes C. Liermann,[a] Iris Buckel,[b]
Eckhard Thines,[b] and Till Opatz*[a]
Keywords: Natural products / Configuration determination / Circular dichroism / Density functional calculations /
Conformation analysis
The absolute configurations of a series of phytotoxic dioxol- ment between experiment and theory for VCD, whereas
anone-type secondary metabolites isolated from culture fil-
trates of the grape black rot fungus Guignardia bidwellii
were determined by vibrational circular dichroism (VCD)
spectroscopy in the mid-IR frequency range. Comparison of
the recorded data with DFT calculations showed good agree-
electronic circular dichroism (ECD) data for the compounds
matched poorly with the predicted spectra obtained by time-
dependent DFT (TDDFT) calculations at the same level of
theory.
ultraviolet region of the electromagnetic spectrum and re-
quires only small amounts of sample.
Introduction
In contrast, VCD observes vibrational transitions in the
infrared region and is much less sensitive than ECD. How-
ever, the use of low-dead-volume sample cells permits milli-
gram quantities of natural products to be analyzed by VCD
spectroscopy in a measurement time of a few hours. Here
we report on a comparison of the two spectroscopic meth-
ods for the determination of the ACs of a series of natural
dioxolanones.
Nature still represents a rich source of new chemical
structures. In particular, the kingdom of fungi is known to
provide secondary metabolites of high structural diversity
and potent biological activity. For complete structural char-
acterization of a chiral natural product, the determination
of its absolute configuration (AC) is required, and might
also provide insights into its biogenetic origin as well as into
its interactions with potential molecular targets. Moreover,
knowledge about the stereochemistry is a prerequisite for
synthetic approaches to the correct enantiomer.
The first, and still widely applied, method for AC assign-
ment without prior derivatization is through anomalous
dispersion in X-ray crystallography, introduced by Johannes
Martin Bijvoet around 1950.[1] However, single crystals of
sufficient quality and the presence of heavy atoms (e.g., S,
P, Cl, Br, Rb) within the unit cell are generally required to
allow reliable determination of the Flack parameter, which
indicates the correctness of the assumed AC. Whereas pre-
diction of the easily measurable optical rotation is still diffi-
cult and afflicted with large errors,[2–8] other chiroptical
methods such as ECD provide more dependable data.[9] The
latter technique is based on electronic transitions in the
Results and Discussion
The fungus Guignardia bidwellii is the causal agent of
grape black rot, a devastating disease that threatens viney-
ards in, for example, the Moselle and Nahe region and the
Middle Rhine valley in Germany. We have recently eluci-
dated the structures of eight dioxolanone-type secondary
metabolites isolated from culture filtrates of G. bidwel-
lii.[10,11] These compounds have in common a core structure
similar to that of the previously known metabolite[12] guig-
nardic acid (1, Figure 1) and differ only in the substituents
at C-2 and the carboxylate function.
Moreover, five of the new metabolites, as well as guig-
nardic acid itself, have been demonstrated to display phyto-
toxic activity against the leaves of Vitis vinifera, with phen-
guignardic acid (2, Figure 1) being most potent. The AC of
1 was determined by total synthesis and comparison of its
optical rotation and ECD spectra with those of the natural
product, but ECD spectroscopy of 2 gave inverted bands
relative to 1. The presence, however, of an additional phenyl
ring in 2 required comparison of its ECD spectrum with
[a] Institute of Organic Chemistry, University of Mainz,
Duesbergweg 10–14, 55128 Mainz, Germany
E-mail: opatz@uni-mainz.de
Homepage: http://www.chemie.uni-mainz.de/OC/AK-Opatz/
[b] Institute of Biotechnology and Drug Research, University of
Mainz,
Duesbergweg 10–14, 55128 Mainz, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300530.
2
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Guignardia bidwellii Dioxolanone Secondary Metabolites
Finally, TDDFT excited state calculations [TD-B3LYP/
6-311G(d,p)] for all conformers less than 2.0 kcalmol–1
above the lowest-energy conformer were performed and
combined to provide the UV and ECD spectra as Boltz-
mann-weighted averages. Unfortunately, though, the calcu-
lated ECD spectrum of 2 did not match the experimental
data, although the calculated UV spectrum showed a rea-
sonable agreement with the recorded absorption bands
(Figure 2).
Figure 2. ECD spectra (top) and UV spectra (bottom) of phenguig-
nardic acid (2).
Figure 1. Structures of the dioxolanone-type metabolites from
Guignardia bidwellii.
Whereas ECD is suited for detecting long-range interac-
tions of chromophores and is the method of choice for the
determination of axial chirality (e.g., in biaryls[22–26]), VCD
data obtained by TDDFT calculations. For this purpose, operates on the smaller-length scale of bond vibrations and
conformation analysis was carried out with the aid of the should be more appropriate for analysis of central chiral-
MMFF[13] force field (Spartan ’10).[14] All obtained con- ity.[8,27,28] We therefore recorded the VCD spectrum of 2
formers were subjected to geometry optimization [B3LYP/ (CDCl3 solution) in a low-dead-volume BaF2 sample cell.
6-31G(d,p),[15–18] Gaussian09[19]], and conformers less than Comparison with
a
simulated spectrum [B3LYP/6-
10.0 kcalmol–1 above the lowest-energy conformer were re- 311G(d,p)] with IEFPCM solvation (CHCl3, conformer se-
optimized at the B3LYP/6-311G(d,p)[20] level of theory. lection as described for ECD simulation) showed a better
During this operation and the subsequent TDDFT calcula- agreement of theory and experiment. The numbers of con-
tion, solvation was treated with the integral equation for- formers examined for each compound are given in Table 1.
malism polarizable continuum model (IEFPCM)[21] with For each compound, two different orientations of the ester/
the default values for acetonitrile.
carboxylic acid moiety (C2–C6 bond) together with up to
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three staggered orientations of the C2–C7 bond were found
In the VCD spectrum of 2, the signs of the four major
in silico. As would be expected, the presence of side chains experimental bands at 1286 cm–1 [ν(C4–C5) + ν(C1Ј–C8)
with three rotatable bonds in compounds 4, 5, and 7 led to + δtwist(H–C7–H)], 1251 cm–1 [ν(C2–C7–C1ЈЈ)], 1207 cm–1
significant increases in the number of conformers.
[δ(C5–C4=C8–H)], and 1175 cm–1 [νs(O–C2–O) + δ(O–H)]
are correctly predicted for the S enantiomer, but the rota-
tional strengths at 1251 cm–1 and 1207 cm–1 are not (Fig-
ure 3).
Table 1. Numbers of conformers examined for each compound.
One reason for this could be the known aggregation of
carboxylic acids in nonpolar solvents, which is difficult to
represent in silico.[29,30] The considerable deviation of the
frequency of the C=O stretch of the COOH group
(1741 cm–1 and 1808 cm–1 for experiment and theory,
respectively), with all other bands being close to the pre-
Solvent
Calculated
conformers
Conformers within
2 kcalmol–1 window
2
CHCl3
MeCN
CHCl3
MeCN
CCl4
MeCN
CCl4
MeCN
CCl4
MeCN
CCl4
7
7
5
7
7
4
2
3
3
5
4
4
110
110
30
30
13
13
93
93
7
37
35
17
15
7
4
5
5
6
6
7
7
44
46
6
7
2
7
MeCN
CCl4
MeCN
CCl4
10
10
11
11
7
3
3
MeCN
2
Scheme 1. Esterification of 2 and 3 with diazomethane.
Figure 3. VCD spectra (top) and IR spectra (bottom) of phenguig-
nardic acid (2).
Figure 4. VCD spectra (top) and IR spectra (bottom) of phenguig-
nardic acid methyl ester (10).
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Guignardia bidwellii Dioxolanone Secondary Metabolites
dicted frequencies, points in this direction. However,
The same trends were observed for the alaguignardic acid
measurements in [D6]DMSO did not improve this (3)/alaguignardic acid methyl ester (11) pair (Scheme 1).
situation. The agreement of the VCD spectra is again improved
If aggregation or interactions of the free carboxylic acid in the absence of hydrogen bonds (Figure 6). In this case,
with discrete solvent molecules are responsible for the ob- simulated and measured ECD spectra showed appreciable
served discrepancies, esterification should improve the qual- agreement for the S enantiomer. The absence of an ad-
ity of the spectral simulations. Compound 2 was therefore ditional aromatic ring in 3 and 11 makes their ECD spectra
transformed into its methyl ester (10, Scheme 1) by treat- similar to that of guignardic acid (1) and alleviates the band
ment with diazomethane. Indeed, the agreement between inversion mentioned earlier (Figure 7).
the predicted VCD spectrum and the experimental data was
much better than for 2 itself (Figure 4), and this is paral-
leled by the corresponding IR spectra. As expected, no sig-
nificant deviation between calculated and measured absorp-
tion frequencies, including both carbonyl stretches, was ob-
served. As in the case of 2, the signs of the six major VCD
bands at 1765 cm–1 [ν(C6=O)], 1286 cm–1 [νas(C2–C6–O)],
1250 cm–1 [ν(C4–C5)
+ ν(C1Ј–C8) + νas(C2–C6–O)],
1202 cm–1 [νs(C2–C7–C1ЈЈ)], 1172 cm–1 [δ(C5–C4=C8–H)],
and 1060 cm–1 [νs(C2–C6–O)] are correctly predicted.
Moreover, only one of these (1250 cm–1) shows a significant
underestimation of its rotational strength by DFT. On the
other hand, esterification did not improve the consistency
of simulated and measured ECD data for 10 (Figure 5).
Figure 6. VCD spectra of alaguignardic acid (3, top) and its methyl
ester (11, bottom).
The naturally occurring esters 4–7 had highly similar
VCD spectra (Figure 8), each with a characteristic triplet
consisting of a positive band around 1282 cm–1 [νas(C2–C6–
O)] and two negative bands around 1247 cm–1 [ν(C4–C5) +
ν(C1Ј–C8)] and 1170 cm–1 [ν(C4–C5) + ν(C1Ј–C8)]. This
conserved pattern is predicted with high accuracy, with the
exceptions of the rotational strengths of the lowest-fre-
quency bands in compounds 5 and 7, which are overesti-
mated by DFT. These results clearly indicate that all investi-
gated members of this compound class have the S configu-
ration. ECD spectra of 4–6 show a reasonable agreement
between theory and experiment, whereas for guignardi-
anone D (7) the benzyl substituent on the acetal carbon
leads to inversion of the signs of the ECD bands, which is
not reproduced by DFT calculations in this case (see the
Supporting Information).
Figure 5. ECD spectra (top) and UV spectra (bottom) of phenguig-
nardic acid methyl ester (10).
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Figure 7. ECD spectra of alaguignardic acid (3, top) and its methyl
ester (11, bottom).
Conclusions
Comparison of simulated and measured ECD and VCD
spectra of the natural products 2–7 and of the semisynthetic
derivatives 10 and 11 shows the superiority of VCD over
ECD for the determination of these compounds’ absolute
configurations. This might be due to the central chirality
being better reflected in the vibrational transitions involved
in VCD spectroscopy. Whereas ECD is sensitive to the exact
Figure 8. VCD spectra of compounds 4–7.
relative arrangement of the chromophores (unsaturated sys- 1.0 mm pathlength at 20.0 °C. All spectra in the IR and UV range
were solvent-subtracted.
tems), VCD is more rugged and has better predictivity for
nonrigid molecules at the same level of theory.
Vibrational Spectroscopy: The IR (16 scans, 4000–900 cm–1) and
VCD (260 min accumulation time, 1800–900 cm–1) spectra were re-
corded in CCl4 solution (all esters) and CDCl3 solution (both acids)
with concentrations between 0.1 and 0.2 molL–1.
Experimental Section
Electronic Spectroscopy: The UV and ECD spectra were recorded
in acetonitrile with concentrations between 0.3 and 1.5 mmolL–1,
a scanning speed of 20 nmmin–1, and two repetitions from 170 nm
to 400 nm.
General Experimental Procedures: Optical rotations were measured
with a Perkin–Elmer 241 polarimeter at 546 nm and 578 nm and
extrapolated to 589 nm.[31] IR and VCD spectra were recorded with
a Bruker Tensor 27 IR spectrometer fitted with a PMA 50 module
containing one photoelastic modulator, with 4 cm–1 resolution, and
the instrument was optimized at 1400 cm–1 in a 100 μm pathlength
cell with BaF2 windows. The UV and ECD spectra were recorded
with a Jasco J-815 CD spectrometer in a quartz cuvette with
Computational Methods: All calculations were performed on stan-
dard desktop computers with Intel Core i7 CPUs and use of Spar-
tan ’10 and Gaussian09 Rev A.02. The calculations were started
with a thorough conformer search by use of the MMFF force field
and the algorithm to analyze conformer distributions as im-
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spectra for compounds 10 and 11.
Acknowledgments
This work was supported by the Rhineland-Palatinate Center for
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as Malte Brutschy (Mainz) and Professor Siegfried R. Waldvogel
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Received: April 11, 2013
Published Online: ᭿
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