Elansolid A, a Unique Macrolide Antibiotic from Chitinophaga sancti Isolated as Two Stable Atropisomers

Article abstract of DOI:10.1002/anie.201005226

Inflexibility: Elansolid-A was isolated from Chitinophaga sancti (Flexibacter sancti ) as the two separable isomers A1 and A2 (1 and 1*, respectively). Molecular modeling revealed that they are stable atropisomers which are "frozen" by 14 inflexible bonds

Full text of DOI:10.1002/anie.201005226

Communications
DOI: 10.1002/anie.201005226
Structure Elucidation
Elansolid A, a Unique Macrolide Antibiotic from Chitinophaga sancti
Isolated as Two Stable Atropisomers**
Heinrich Steinmetz, Klaus Gerth, Rolf Jansen, Nadin Schlꢀger, Richard Dehn, Silke Reinecke,
Andreas Kirschning,* and Rolf Mꢁller*
In memory of Jꢁrgen Wehland
Bacterial species of the genera Flexibacter and Chitinophaga
are known to produce biologically active peptides of rele-
vance to anti-infective research because of their interesting
mechanisms of action.[1] For instance, the formadicins, mono-
cyclic b-lactam antibiotics from Flexibacter alginoliquefaciens,
act selectively against pseudomonads and have proven to be
hydrolysis-resistant against various types of b-lactamases.[2]
The anti-MRSA[3] dipeptides TAN-1057A–D isolated from
Flexibacter sp.[4] were shown to inhibit peptide elongation
during the bacterial translation.[5]
elansolids are the first polyketide-derived macrolides from
the genus Chitinophaga.[8]
Early work on Flexibacter strains by Steinmetz, Gerth,
and Hꢀfle resulted in the isolation of a group of novel
metabolites named elansolids. The planar structure of the
major component was elucidated by spectroscopic methods,
degradation by cross-methathesis with ethylene, and biosyn-
thetic reasoning as elansolid A1 (1).[6] Later, in the course of
our biological screening of extracts from non-myxobacterial
gliding bacteria we re-investigated in depth the family of
elansolids produced by Flexibacter sancti, a species recently
reclassified as Chitinophaga sancti (comb. nov.).[7] The
The basic structure of the elansolids is illustrated with
variant A1 (1). The HR-ESI mass spectrum of the molecular
ion cluster [M+H]+ combined with the 13C and 1H NMR data
in [D6]DMSO (Table S1 in the Supporting Information)
indicate the elemental composition C37H48O6. The 13C NMR
spectrum shows signals for all carbon atoms, and the HMQC
spectrum provides the correlations to their directly bound
protons, leaving four exchangeable protons.
1
The structural units derived from H,1H coupling were
[*] H. Steinmetz, Dr. K. Gerth, Dr. R. Jansen, S. Reinecke,
Prof. Dr. R. Mꢀller
interconnected utilizing relevant correlations in the HMBC
spectrum as shown in Figure 1. Thus, the largest unit A is
linked with the double bond of unit B. Following the HMBC
correlations of the carboxy C1 atom (dC = 165.7 ppm) with 3-
H and 25-H, the carboxy group is used to close a macro-
lactone ring. Its presence is also supported by the diagnostic
downfield shift of the oxymethine 25-H signal appearing at
dH = 5.93 ppm, which is enhanced by the aromatic residue C
Mikrobielle Wirkstoffe, Helmholtz Zentrum fꢀr Infektionsforschung
Inhoffenstrasse 7, 38124 Braunschweig (Germany)
Fax: (+49)531-6181-9499
E-mail: rom@helmholtz-hzi.de
Prof. Dr. R. Mꢀller
Helmholtz Institut fꢀr Pharmazeutische Forschung des Saarlandes
Universitꢁt des Saarlandes
Postfach 151150, 66041 Saarbrꢀcken (Germany)
N. Schlꢁger, Dr. R. Dehn, Prof. Dr. A. Kirschning
Institut fꢀr Organische Chemie und Biomolekulares
Wirkstoffzentrum (BMWZ), Leibniz Universitꢁt Hannover
Schneiderberg 1B, 30167 Hannover (Germany)
Fax: (+49)511-762-3011
E-mail: andreas.kirschning@oci.uni-hannover.de
[**] R.D. thanks the Fonds der chemischen Industrie for a scholarship.
We thank W. Kessler and his colleagues for fermentations, F. Sasse
for the determination of the cytotoxicity, and C. Kakoschke, B.
Jaschok-Kentner, and E. Hofer for expert assistance with the NMR
experiments.
Supporting information for this article (tables of NMR data, a table
comparing the biological activity of elansolid A1 (1) and A2 (1*),
tables comparing the observed and calculated ROESY data and
vicinal coupling constants, and a full description of experimental
details and chemical data of all compounds) is available on the
WWW under http://dx.doi.org/10.1002/anie.201005226.
Figure 1. Elansolid A1/A2 (1/1*) and selected correlations from the 2D
NMR spectra of 1.
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appended to C25. The p-hydroxyphenyl residue was eluci-
dated from the double intensity of the methine signals 27-H
and 28-H, the HMBC correlations, and the 13C chemical shifts,
especially of C29 (dC = 156.2 ppm). The phenolic proton gives
rise to a singlet at dH = 9.33 ppm, whose position was assigned
from the nuclear Overhauser effect (NOE) with 28-H in the
ROESY and NOESY spectra.
NOEs with 24-H, 16-H, and 3-H as well as the expected NOEs
with 25-H and 28-H. Consequently, the lactone must be
located at C25 as shown in Figure 2. The dihedral angle
f24H,25H of 878 is in good agreement with the small vicinal
3
coupling constant J24,25 of 2.7 Hz. Thus, according to the
NMR evidence, the relative stereochemistry of the core of
elansolid A1 (1) is 16R*,19R*,20R*,23R*,24R*,25R*.[9]
The HMBC spectrum also reveals that the quaternary
carbon C22 (dC = 37.2 ppm) correlates with a pair of geminal
methyl groups C34/C35 (dH = 1.21 and 0.92 ppm), and that the
tertiary carbon atom C20 (dC = 74.3 ppm) is connected to the
remaining methyl group C33 (dH = 1.03 ppm) and the hydroxy
group 20-OH (dH = 4.48 ppm) (Figure 1). Both quaternary
carbons and their methyl substituents are adjacent to the
methylene group C21 (dC = 59.7 ppm) as judged by their
HMBC correlations with the pair of geminal protons 21-Ha
and 21-Hb (dH = 1.69 and 1.55 ppm). Additional HMBC
correlations observed for C23 and C19 allow the connection
of the remaining bonds to give the tetrahydroindane core of
elansolid A1 (1). The configuration of the double bonds of the
Z,E,Z-triene in 1 was deduced from vicinal coupling constants
and supported by appropriate ROESY data, while the E
configuration at the D[4,5] bond was determined from the
ROESY correlations of 3-H, 5-H, and the methyl group C30.
Within the tetrahydroindane system the vicinal coupling
constants acquired in [D6]DMSO at 608C unambiguously
indicate the trans configuration for 19-H, 23-H, and 24-H (J =
11.3–12 Hz) and a gauche orientation of 24-H and 16-H (J =
3.8 Hz). These findings were supported by NOEs between 24-
H and 19-H and 16-H (Figure 2, left). Further ROESY
Since the triene unit between C16 and C9 is expected to
have a nearly planar conformation, the relative stereochem-
istry of the tetrahydroindane core of 1 can be extended from
C16 to C9 in the second stereodomain: the coupling constant
3J9,10 of 8.8 Hz indicates a transoid arrangement of protons 9-
H and 10-H, which is supported by a very strong NOE
between 9-H and 12-H. As the tetrahydroindane–triene and
the diene–lactone are rigid moieties, the entire lactone ring
must be highly strained, leaving no vacant inner space for
either the 9-OH or the methyl group. Consequently, the 9-OH
group should be exo oriented. Based on these considerations
we conclude that the relative configuration at C9 is also R*. A
3
small coupling constant of about 4 Hz for J8,9 and a large
3
coupling constant of 8 Hz for J7,8 (Table S1) indicate gauche
and nearly staggered relative orientations (Figure 2, right),
respectively, which also suggests R* configurations of the
stereogenic centers C7 and C8.
The complete assignment of the relative and absolute
configurations of 1 was based on the seco acid derivatives 2
and 3 isolated from fermentation extracts (Scheme 1).
Scheme 1. Preparation of acetonide 4 and selected ROESY correla-
tions.: a) CH2N2, EtOAc, RT, 71%; b) dimethoxypropane, PPTS, 18%.
PPTS=pyridinium p-toluenesulfonate.
Figure 2. Partial view of a model of elansolid A1 (1) with selected
nuclear Overhauser effects (left) and proposed stereogenic centers at
C7–C9 in the lactone ring of 1 (right) with the relative stereochemistry
R*. C gray, H white, O black.
Derivative 2 may be an artifact resulting from a reaction
with methanol during workup. As judged by HPLC/HR-ESI-
MS elansolid B2 (2) has the elemental composition of 1 plus
CH3OH, that is, C38H52O7. Compared to the NMR data of
elansolid 1 the 1H NMR spectrum of 2 in [D6]acetone
(Table S2) reveals an additional methoxy group at d =
3.1 ppm and the 25-H doublet is shifted upfield by 1.3 ppm
to d = 4.64 ppm. However, the doublet still shows the small
vicinal coupling J24,25 of 2.6 Hz, which is diagnostic for the
25R* configuration of elansolid A1 (1) (Table S5).
correlations allowed identification of the relative positions of
the substituents in the cyclopentane moiety: 1) 23-H shows
NOEs with methyl groups C35 and C33 and both have NOEs
with 21-Hb; 2) on the reverse face 19-H shows NOEs with 20-
OH and the methyl group C34, while both have NOEs with
21-Ha. The strongest NOE was observed between 23-H and
15-H.
Additionally, the position of the lactone proton 25-H was
determined from the observation of NOEs owing to the close
proximity of both methyl groups C34 and C35. On the reverse
face, the indistinguishable pair of aromatic protons 27-H show
The connection between the methoxy group and C25 was
assigned from the NOE correlation with 25-H. Additional
ROESY correlations supported the assigned 25R* stereo-
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Communications
chemistry, as the methoxy group shows correlations with 27-H
and the methyl group C35. On the other hand, 25-H displays a
strong ROESY correlation with the methyl group C34, while
the aromatic protons 27-H show correlations with 16-H and
24-H.
Finally, we derivatized the 25-hydroxy variant elanso-
lid B1 (3) (Scheme 1) which was also found in the crude
extract of strain GBF13 and which was characterized in a
similar manner to that described for B2 (2). The spectroscopi-
cally derived relationship between the stereocenters at C7,
C8, and C9 (see above) was chemically determined by
applying Rychnovskyꢁs acetonide method as shown in
Scheme 1.[10] The 13C NMR spectrum of acetonide 4 shows
characteristic shifts for the acetonide methyl groups at dC =
26.1 and 25.0 ppm and for the quarternary acetonide carbon
at dC = 101.0 ppm. These data suggest a 1,3-trans relationship
of the stereocenters at C7 and C9 (see Figure 2, right). Since
the dioxolane ring in 4 adopts a flexible twist conformation,
unequivocal interpretation of the ROESY experiments is not
possible. However, the observed correlations, for example
between the C39 methyl group and 7-H, the C39 and the C31
methyl groups, and the C38 methyl group and 9-H, fully
concur with the relative R* stereochemistry of C7–C9. With
this analysis in hand we initiated a synthetic program for
finally proving the relative as well as the absolute stereo-
chemistry at C7–C9 of 1. For that purpose, elansolid B2 (2)
was fragmented by treatment with ethylene in the presence of
the second-generation Grubbs–Hoveyda catalyst to yield the
two metathesis fragments 5 and 6.
Scheme 2. Preparation of the eastern fragment 6. Reagents and
conditions: a) Et3N, (cy)2BOTf, CH2Cl2, ꢁ788C to 08C, 18 h, 79%;
b) TESOTf, 2,6-lutidine, ꢁ788C, 70 min, 78%; c) DIBAL-H, CH2Cl2,
ꢁ788C to ꢁ508C, 6 h, 81%; d) Dess–Martin periodinane, CH2Cl2,
NaHCO3, RT, 1.5 h; e) vinylmagnesium bromide, THF, ꢁ788C, 1.5 h,
(4,5-anti/4,5-syn=2:1), 78% for 2 steps; f) TBAF·3H2O, THF, 08C, 1 h,
86% for 4,5-anti, 84% for 4,5-syn; g) 2,2-dimethoxypropane, PPTS,
CH2Cl2, RT, 1 h, 83% for 4,5-anti and for 84% 4,5-syn); h) TESOTf, 2,6-
lutidine, CH2Cl2, ꢁ788C, 40 min, 81%; i) DDQ, CH2Cl2/buffer (pH 7),
08C, 2.5 h, 74%; j) Dess–Martin periodinane, CH2Cl2, NaHCO3, RT,
18 h; k) (carbethoxyethylidene)triphenylphosphorane, CHCl3, RT, 18 h,
74% for 2 steps; l) DIBAL-H, CH2Cl2, ꢁ788C, 1 h, 83%; m) Dess–
Martin periodinane, CH2Cl2, NaHCO3, RT, 18 h; n) (carbethoxymethyl-
ene)triphenylphosphorane, toluene, 608C, 5 d, 57% for 2 steps; o) 1m
LiOH, THF, MeOH, RT, 22 h, 54%. Cy=cyclohexyl, DDQ=2,3-
dichloro-5,6-dicyano-1,4-benzoquinone, DIBAL-H=diisobutylaluminum
hydride, Mes=2,4,6-trimethylphenyl, PMB=para-methoxybenzyl,
TBAF=tetrabutylammonium fluoride, TES=triethylsilyl, Tf=trifluoro-
methansulfonyl.
As outlined in Scheme 2 the total synthesis of enantio-
merically enriched carboxylic acid 6 was achieved, and the
product was compared with the fragment 6 obtained from the
natural product. As the absolute configuration was unknown
at this stage of the project, we arbitrarily decided to prepare
the all-R isomer. The synthesis of 6 commenced with an anti-
selective Masamune aldol reaction between the chiral ester 7
and the known aldehyde 8[11] to yield the 2,3-anti product 9.[12]
After protection of the hydroxy group and cleavage of the
auxiliary, the primary alcohol was oxidized to the correspond-
ing aldehyde. This was alkylated using vinylmagnesium
bromide to afford the allylic alcohols 10 and 11 in a
diastereomeric ratio of 2:1. The relationship between the
new hydroxy group at C5 and the fixed stereogenic center at
C3 was assigned after formation of acetonides 12 and 13,
respectively, and their analysis using Rychnovskyꢁs method.
Functional-group manipulation and two successive Wittig
reactions resulted in the complete assembly of the carbon
skeleton. Finally, removal of all protecting groups and
saponification of the ethyl ester in one step yielded the C1–
C11 fragment 6 of the elansolids. Comparison of the optical
rotations of the authentic sample obtained from elansolid B2
Figure 3. Absolute configuration of elansolid A1 (1).
In the course of a supplementary production of 1 an
additional compound 1* was identified in the analytical
HPLC of the culture extract at 7.6 min with UV and MS data
similar to that of elansolid A1 (1). Elansolid A2 (1*) was
isolated by preparative reversed-phase (RP) HPLC. Analysis
of the NMR data in [D6]DMSO at room temperature and at
708C (Table S4) unexpectedly resulted in the same structural
formula. HPLC analyses revealed a slow interconversion of
1* into 1 ([D6]DMSO, RT, 55%, 6 d) while their chemical
ring-opening lead to the same product, elansolid B (2)
20
(2) {½aꢀD ¼ + 32.5 (c = 0.12, MeOH)} with the synthetic sample
20
6 {½aꢀD ¼ + 24.8 (c = 0.40, MeOH)} as well as their identical
NMR spectra established simultaneously the relative and the
absolute all-R configuration of the three stereogenic centers
C7–C9. Since the relative configuration of elansolid A1 (1)
had already been determined (see above), these results also
established the six remaining stereogenic centers around the
tetrahydroindane moiety and at C25 (Figure 3).
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Angew. Chem. Int. Ed. 2011, 50, 532 –536

[MeOH/H2O (8:2), 0.1m NaOH 1%, RT]. We thus concluded
that both compounds most likely represent atropisomers.
To unravel the conformational details of the atropisomers,
their NMR data were compared more closely and molecular
modeling was carried out. As several signals of interest of
both elansolids are very broad in [D6]DMSO, NMR spectra of
1 and 1* were measured in [D6]acetone (Table S5) at room
temperature as well as at 250 K.
Molecular modeling started from the absolute all-R
configuration of elansolid A1 (1). The dihedral angles from
5-H to 10-H were varied without any constraints using the
“Conformational Search” module and finally the semiempir-
ical PM3 method in HyperChem Version 8.5.[13] Solvent
effects were not calculated, because the conformations are
“frozen” by the ring strain and the stereochemistry of the four
stereogenic centers in the lactone ring. The two structures
representing the lowest local minima (Figure 4)[14] showed the
expected high degree of similarity of their common structural
units, that is, the tetrahydroindane system, the diene lactone
unit, the triene unit, and the positions of the C8 and C9
substituents.
coupling constants between 5-H, 6-Ha/b, and 7-H also support
this analysis.
Similarly, vicinal coupling constants for dihedral angles of
the models were compared with the observed values
(Tables S7a and S7b).[15] Both models account for the small
vicinal coupling constants of 2–4 Hz observed between 8-H
and 9-H in 1 and 1* with dihedral angles of 788 and 718,
respectively. The relative position of the aromatic and the
lactone rings at the tetrahydroindane moiety is also found to
be similar in 1 and 1*, as another conspicuous small vicinal
coupling constant of 1–3 Hz between 24H and 25H is
consistent with dihedral angles of 878 and 778, respectively.
The main conformational consequences of this atropiso-
merism are circled in Figure 4. In elansolid A1 (1) the
methylene protons at C6 are directed to the outside of the
lactone ring and the secondary alcohol C7 is “folded in” the
lactone ring. This situation is reversed in elansolid A2 (1*),
where the model shows the methylene group C6 “folded in”
while the hydroxy group at C7 is directed outwards. Con-
sequently, the 7-OH and 9-OH groups assume a cisoidal
relation in 1*, while a nearly orthogonal relation of the
hydroxy groups is fixed in 1, where 7-OH is directed to the
front of the figure.
The intensities of ROESY correlations for selected atom
distances in the atropisomers were also calculated and
compared with the observed intensities (Tables S8a and
S8b).[16] The three most intense nuclear Overhauser effects
of the two elansolids were accounted for by atom distances
between 1.8 and 1.9 ꢂ for the proton pairs H-15/H-23, H-9/H-
12, and H-13/H-16. Further comparison of selected distances
and NOEs show a good agreement between the models and
the ROESY NMR data. The anticipated ring strain stabilizing
the isomers was indicated by diene and triene bonds deviating
from planarity by 10.68 to 16.88 for dihedral angles f1-O,2-H, f11-
und f13-H,14-H in the models of 1 and 1* (Table S9,
H,12-H
Figure 4).
The characteristic differences between conformations 1
and 1* clearly rationalize the different physicochemical
behavior, that is, retention times, which depend on the
distribution of polarity on the surface of a molecule. Similarly,
different biological effects, which are mainly controlled by
hydrogen bonding between hydroxy groups of small mole-
cules and their biological target, would be expected for the
two stable conformational isomers. In fact, the two atropiso-
meric elansolids differ in their biological activity. While
elansolid A2 (1*) shows antibiotic activity against Gram-
positive bacteria (Table S6) in the range between 0.2 and
64 mgmLꢁ1, elansolid A1 (1) is only weakly active. Similarly,
no cytotoxicity was observed with L929 mouse fibroblast cells
for atropisomer A1 (1) up to 40 mgmLꢁ1 while the conformer
elansolid A2 (1*) showed an IC50 value of 12 mgmLꢁ1.
Solvent-dependent equilibria of folded-in and folded-out
conformations have been observed in, for example, the 14-
membered macrolide antibiotics erythromycin and oleandro-
mycin, and the 16-membered macrolide tylosin.[17] The
elansolids A1 (1) and A2 (1*) differ from these examples as
the rigidity of their macrocyclic rings sustains the atropiso-
merism more firmly.[18] Similar to elansolids A1 (1) and A2
(1*) atropisomerism of small rings lacking steric repulsion has
Figure 4. a) Model of elansolid A1 (1); b) model of elansolid A2 (1*).
C gray, H white, to the pyrrole groups O black.
The NMR chemical shifts (d) within the bicyclic tetrahy-
droindane core and the diene lactone unit of elansolids 1 and
1* are nearly identical, and differences in chemical shifts but
still identical coupling constants are observed within the
triene unit. The major difference between the conformation-
related NMR data of 1 and 1* and between the two models
are apparent in the lactone ring segment between methylene
C6 and methine C8. While the model in Figure 4a shows a
staggered arrangement of 7-H and 8-H (f7H,8H = 1498) which
corresponds well with the coupling J7,8 of 9.4 Hz observed for
elansolid A1 (1) (Table S8a), the model in Figure 4b, featur-
ing a torsion angle f7H,8H of 618, is compatible only with the
coupling J7,8 of 4.1 Hz of elansolid A2 (1*) (Table S8b). The
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Communications
[6] G. Hꢀfle, GBF-internal Annual Report, unpublished.
[7] P. Kꢃmpfer, C. C. Young, K. R. Sridhar, A. B. Arun, W. A. Lai,
F. T. Shen, P. D. Rekha, Int. J. Syst. Evol. Microbiol. 2006, 56,
2223 – 2228.
[8] K. Gerth, H. Steinmetz, G. Hꢀfle, EP 2 093 212A1, 2009.
[9] “R*” denotes relative R stereochemistry.
[10] a) S. D. Rychnovsky, D. J. Skalitzky, Tetrahedron Lett. 1990, 31,
945 – 948; b) S. D. Rychnovsky, B. N. Rogers, T. I. Richardson,
Acc. Chem. Res. 1998, 31, 9 – 17; c) D. A. Evans, D. L. Rieger,
J. R. Gage, Tetrahedron Lett. 1990, 49, 7099 – 7100.
[11] F. M. Cordero, F. Pisaneschi, M. Gensini, A. Goti, A. Brandi,
Eur. J. Org. Chem. 2002, 12, 1941 – 1951.
[12] T. Inoue, J. Liu, D. C. Buske, A. Abiko, J. Org. Chem. 2002, 67,
5250 – 5256; A. Abiko, J. Liu, S. Masamune, J. Am. Chem. Soc.
1997, 119, 2586 – 2587.
been observed, for example, in the case of abyssomycin C and
its synthetic counterpart atrop-abyssomicin C.[19] Our future
work is directed towards deciphering the biosynthesis of this
structurally unique class of macrolactones.
Received: August 20, 2010
Published online: December 8, 2010
Keywords: antibiotics · atropisomers · Chitinophaga ·
.
molecular modeling · structure elucidation
[1] SQ 28332, 28502, 28503: a) R. Cooper, K. Bush, P. A. Principe,
W. H. Trejo, J. S. Wels, R. B. Sykes, J. Antibiot. 1983, 36, 1252 –
1257; b) R. Cooper, J. Antibiot. 1983, 36, 125—1262; c) P. D.
Singh, J. H. Johnson, P. C. Ward, J. S. Wells, W. H. Trejo, R. B.
Sykes, J. Antibiot. 1983, 36, 1245 – 1251. FR901451: T. Fujita, H.
Hatanaka, K. Hayashi, N. Shigematsu, S. Takase, M. Okamoto,
M. Okuhara, K. Shimatani, A. Satoh, J. Antibiot. 1994, 47, 1359 –
1364. FR901451: T. Fujita, H. Hatanaka, K. Hayashi, N.
Shigematsu, S. Takase, M. Okamoto, M. Okuhara, K. Shimatani,
A. Satoh, J. Antibiot. 1994, 47, 1359 – 1364. YM-47141 and YM-
47142: d) K. Yasumuro, Y. Suzuki, M. Shibazaki, K. Teramura,
K. Abe, M. Orita, J. Antibiot. 1995, 48, 1425 – 1429; e) M. Orita,
K. Yasumuro, K. Kokubo, M. Shimizu, K. Abe, T. Tokunaga, H.
Kaniwa, J. Antibiot. 1995, 48, 1430 – 1434. Topostin: f) K. Suzuki,
H. Yamaguchi, S. Miyazaki, K. Nagai, S.-I. Watanabe, T. Saito, K.
Ishii, M. Hanada, T. Sekine, Y. Ikegami, T. Andoh, J. Antibiot.
1990, 43, 154 – 157; g) Y. Ikegami, N. Takeuchi, M. Hanada, Y.
Hasegawa, K. Ishii, T. Andoh, T. Sato, K. Suzuki, H. Yamaguchi,
S. Miyazaki, K. Nagai, S.-J. Watanabe, T. Saito, J. Antibiot. 1990,
43, 158 – 162.
[13] The conformational search option in HyperChem aims to find
the conformations that constitute local energy minima. It does so
by randomly initializing torsional angles and performing sub-
sequent MM + optimizations, looking for unique conformations
that are local energy minima.
[14] Calculated energies: (1): PM3: E = ꢁ9368.31 kcalmolꢁ1; MM + :
44.456 kcalmolꢁ1; (1*): PM3: E = ꢁ9370.85 kcalmolꢁ1; MM + :
44.410 kcalmolꢁ1. Color figures are presented in the Supporting
Information.
[15] C. A. G. Haasnoot, F. A. A. M. DeLeeuw, C. Altona, Tetrahe-
dron 1980, 36, 2783 – 2792 (http://www.stenutz.eu/conf/haasnoot.
php).
[16] D. A. Evans, M. J. Bodkin, S. R. Baker, G. J. Sharman, Magn.
Reson. Chem. 2007, 45, 595 – 600 (Janocchio—a Java applet for
viewing 3D structures and calculating NMR couplings and
NOEs: http://janocchio.sourceforge.net/).
[17] a) W. E. Steinmetz, J. D. Sadowsky, J. S. Rice, J. J. Roberts, Y. K.
Bui, Magn. Reson. Chem. 2001, 39, 163 – 172; b) P. Novak, Z. B.
Tomisic, P. Tepes, G. Lazarevski, J. Plavec, G. Turkalj, Org.
Biomol. Chem. 2005, 3, 39 – 47; c) P. M. Ivanov, J. Mol. Struct.
2002, 606, 217 – 229.
[18] G. Bringmann, A. J. Price Mortimer, P. A. Keller, M. J. Gresser,
J. Garner, M. Breuning, Angew. Chem. 2005, 117, 5518 – 5563;
Angew. Chem. Int. Ed. 2005, 44, 5384 – 5427.
[2] N. Katayama, Y. Nozaki, K. Okonogi, H. Ono, S. Harada, H.
Okazaki, J. Antibiot. 1985, 38, 1117 – 1125; T. Hida, S. Tsubotani,
N. Katayama, H. Okazaki, S. Harada, J. Antibiot. 1985, 38, 1128 –
1140.
[3] MRSA = methicillin-resistant Staphylococcus aureus.
[4] N. Katayama, S. Fukusumi, Y. Funabashi, T. Iwahi, H. J. Ono, J.
Antibiot. 1993, 46, 606 – 613.
[19] K. C. Nicolaou, S. T. Harrison, J. Am. Chem. Soc. 2007, 129, 429 –
440, and references therein.
[5] N. Bꢀddeker, G. Bahador, C. Gibbs, E. Mabery, J. Wolf, L. Xu, J.
Watson, RNA 2002, 8, 1120 – 1128.
536
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