Isolation and total synthesis of icumazoles and noricumazoles-antifungal antibiotics and cation-channel blockers from Sorangium cellulosum

Article abstract of DOI:10.1002/anie.201106435

Joint efforts: The antifungal icumazole A, the potassium-ion-channel inhibitor noricumazole A, and their glycosylated derivatives are new, structurally related secondary metabolites isolated from Sorangium cellulosum. Their structures have been fully assi

Full text of DOI:10.1002/anie.201106435

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Angewandte
Communications
DOI: 10.1002/anie.201106435
Natural Products
Isolation and Total Synthesis of Icumazoles and Noricumazoles—
Antifungal Antibiotics and Cation-Channel Blockers from Sorangium
cellulosum**
Jenny Barbier, Rolf Jansen, Herbert Irschik, Stefan Benson, Klaus Gerth, Bettina Bçhlendorf,
Gerhard Hçfle, Hans Reichenbach, Jens Wegner, Carsten Zeilinger, Andreas Kirschning,* and
Rolf Mꢀller*
Myxobacteria are a potent source of secondary metabolites[1]
of which the genus Sorangium has yielded the largest
number.[2] In the course of our ongoing bioactivity-guided
screening program, an unknown antifungal spectrum of
activity was detected in the extract of S. cellulosum,
So ce701, and led to the isolation of the unique isochroma-
none- and oxazole-containing icumazoles (1a–c).[3] Later the
same characteristic structural element was rediscovered in the
noricumazoles (2a,b).
Here, we provide a concise report on the icumazoles (1a–
c) and noricumazoles (2a–c) which covers their isolation and
structure elucidation as well as studies on their ion-channel-
inhibitory properties. We approached these goals utilizing a
broad range of strategies based on various spectroscopic
methods and the chemical determination of relative and
absolute configurations by derivatization, degradation, and
fragment synthesis, and unequivocally by the total synthesis of
noricumazole A (2a). In addition, we provide evidence for
the ion-channel-inhibitory effect of noricumazole A (2a)
utilizing a new screening concept for oligomeric ion channels.
For the production of icumazoles (1a–c) the producer was
cultivated at 308C for 14 days in a 100 L fermentor, and
icumazoles were isolated as described in the Supporting
Information. Icumazoles (1a–c) are easily recognized in
UPLC-DAD-UVanalyses (ultra-performance liquid chroma-
tography with diode array UV detection) by their UV spectra,
especially by the intense main absorption at 302 nm with side
bands at 290 and 316 nm, which are characteristic for
conjugated short polyenes. In the high-resolution ESIMS,
the signal for the molecular ion M+ of the peak eluting at Rt =
15.37 min indicated the molecular formula C33H45NO7 for
icumazole A (1a), which was fully compatible with the NMR
data (Table S1 in the Supporting Information). The 33 carbon
signals in the 13C NMR spectrum were classified according to
the DEPT spectrum, and three heteroatom-bound exchange-
able protons were calculated, which were not visible in the
1
1H NMR spectra in [D4]MeOH. All protons in the H NMR
spectra could be unambiguously assigned to their correspond-
ing carbon atoms from their correlations in the 1H,13C HMQC
spectrum. The correlations in the 1H,1H COSY spectrum
provided four major domains (A–D; Figure 1). The remaining
unsaturated methine singlet (H19, dH/C = 7.80/138.0 ppm) was
recognized as part of a heteroaromatic ring because of its
1
large JHC coupling constant of 209 Hz. Along with the
quaternary carbon atoms C18 and C20 next to one nitrogen
and one oxygen atom, this methine C19 forms an oxazole
[*] Dr. R. Jansen,[+] Dr. H. Irschik, Dr. K. Gerth, B. Bçhlendorf,[$]
Prof. Dr. G. Hçfle, Prof. Dr. H. Reichenbach, Prof. Dr. R. Mꢀller
Helmholtz-Zentrum fꢀr Infektionsforschung (HZI)
Arbeitsgruppe Mikrobielle Wirkstoffe
Inhoffenstrasse 7, 38124 Braunschweig (Germany)
E-mail: rom@mx.uni-saarland.de
J. Barbier,[+] Dr. S. Benson, J. Wegner, Prof. Dr. A. Kirschning
Institut fꢀr Organische Chemie und Zentrum fꢀr Biomolekulare
Wirkstoffe (BMWZ)
Leibniz-Universitꢁt Hannover
Schneiderberg 1B, 30167 Hannover (Germany)
E-mail: andreas.kirschning@oci.uni-hannover.de
Dr. C. Zeilinger
Institut fꢀr Biophysik, Leibniz-Universitꢁt Hannover
Herrenhꢁuser Strasse 2, 30419 Hannover (Germany)
[$] Current address: DSM Nutritional Products Ltd.
R&D Human Nutrition and Health, Basel (Switzerland)
[+] These authors contributed equally to this work.
Figure 1. Icumazoles A–B2 (1a–c), noricumazoles A–C (2a–c), and
acetonides 3 and 4 with relevant 13C NMR data and determination of
relative stereochemistry (stereochemistry shown as elucidated in this
paper; numbering does not relate to IUPAC numbering; for clarity
reasons the depicted numbering was chosen; A–D refer to NMR
domains).
[**] This work was funded by the Fonds der Chemischen Industrie
(scholarship for J.W.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106435.
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1
ring, which links units C and D as indicated by their H,13C
judged from the 13C NMR data and the relevant 1H,1H
coupling constants (J22,23 = 10.3 Hz and J23,24 = 10.1 Hz for 3;
18,19 = 10.4 Hz and J19,20 = 10.0 Hz for 4[8]). Next, noricuma-
zol A (2) was transformed into trisacetonide 5 and fragments
6 and 7 by a set of standard transformations (Scheme 1).
Derivative 5 confirmed the relative configuration at C18–C20
HMBC correlations (Table S1 with figure). HMBC correla-
tions between oxymethine C11 and the methoxy group C33
established the ether group at position C11 in unit C. Based
on the HMBC correlations of structural elements A, B, and C
and of the remaining quaternary carbon atoms, the isochro-
manone system was established. Although direct evidence for
a ring closure between C1 and C9 could not be drawn from
the HMBC spectrum, only a lactone ring can account for the
low-field shift of H9 (d = 4.83) and for the chemical shift of C1
(d = 172.1). The Z,Z,E configuration of the triene unit in 1a
was derived from the vicinal coupling constants J12,13 and
J
J
14,15 = 11–12 Hz and J16,17 = 15.2 Hz. A ROESY correlation
between H17 and H19 showed the cisoidal stereochemical
relation of the oxazole methine C19 and the triene methine
C17.
Additionally, two more-polar icumazoles were recognized
during analyses of extracts along with considerable amounts
of carolacton.[4] The elemental composition C38H53NO11 of
icumazole B1 (1b) (Rt = 14.0 min) accounts for an additional
C5H8O4 unit, which suggested the presence of a pentosyl
Scheme 1. Preparation of acetonide 5 and relevant 13C NMR data and
preparation of fragments 6 and 7 from noricumazole A (2a). Reagents
and conditions: a) 2a, LiBH4, THF, 08C, 10 min; b) 2,2-dimethoxy-
propane, pTsOH, RT, 10 h, 18% (for two steps); c) 2a, NMO,
K2OsO4·2H2O, tBuOH, H2O, 08C to RT, 4 h, 45%; d) 1. NaIO4, THF,
1.5 h; 2. NaBH4, CH2Cl2/MeOH (3:1), RT, 1 h, 32% of 6, 98% of 7.
NMO=4-methylmorpholine N-oxide, Ts=tosyl.
1
residue. Based on a comparison of H and 13C NMR data of
1b (Table S2 in the Supporting Information) with the
literature values, we propose an a-arabino-furanosyl resi-
due.[5] Details on the structure elucidation of 1b and 1c are
discussed in the Supporting Information.
In contrast to icumazoles (1a–c), the characteristic
oxazole-triene chromophore (290, 302, 316 nm) was missing
in the UV spectrum of noricumazole A (2a). The elemental
composition C28H39NO7 was determined by high-resolution
ESIMS from the [M+H]+ ion cluster at m/z 502.2797 (calcd.
502.2799). The NMR data of the isochromanone and oxazole
units in 2a were identical to those measured for icumazole A
(1a) (Table S4 in the Supporting Information). However, the
lower mass of metabolite 2a (by 66 Da, which corresponds to
C5H6) relative to the mass of 1a is associated with the
replacement of the methoxyl by a hydroxy group and the loss
of both Z double bonds. According to the ROESY correlation
H13/H15, the cisoidal conformation between H13 at the last E
double bond and the oxazole 15H remains similar to that in
1a–c. In analogy to the icumazoles, two more-polar deriva-
tives were isolated in addition to noricumazole A (2a) (Rt =
12.6 min). Noricumazole B (2b) (Rt = 11.4 min) was identi-
fied as the 18O-a-arabino-furanoside related to icumazole 1b
(Table S5) and noricumazole C (2c) (Rt = 10.9 min) is a 11O-
b-gluco-pyranoside (for details see the Supporting Informa-
tion and Table S6).[6]
of noricumazole A (2a) and established the anti orientation
of the diol at C9 and C11. Then, the fragments 6, its
diastereomer epi-6, and 7 were independently synthesized
and compared with the authentic samples collected from the
oxidative fragmentation. The syntheses established the abso-
lute configuration of all stereogenic centers including the
isolated one at C25.
The phenolic precursor 9 was first modified by three
successive organometallic transformations introducing the
three C substituents (Scheme 2). The sp3–sp2 cross-coupling
reaction with (S)-iodide 8[9] required lithiation of the bromide
and worked best with MOM protection of the phenol group.
After substantial optimization, the iron-catalyzed version[10,11]
of the Kumada reaction[12] gave the cross-coupling product in
excellent yield. The second ortho position was functionalized
again by lithiation followed by treatment with tert-butyl
isocyanate. The resulting amide then served to direct another
ortho lithiation and alkylation employing both enantiomeric
oxiranes 12a,b[13,14] which independently yielded adducts 10a
and 10b, respectively. d-Lactones 6 and epi-6 were formed
after removal of all O-protecting groups, and the resulting
phenols 11a and 11b were cyclized under acidic conditions.
Importantly, the C24 protons differed for epimers 6 and epi-6
Neither icumazole A (1a) nor noricumazole A (2a)
provided suitable crystals for X-ray crystallographic analysis.
Therefore, we had to elucidate the stereochemistry of all
stereogenic centers by a chemical approach that focused on
noricumazole A (2a), which is chemically more stable than
triene 1a.
1
in the H NMR spectrum and thus were used as a diagnostic
criterion for elucidating the relative stereochemistry between
C25 and C9.[15] Comparison (NMR and CD spectroscopy; see
Figure S1) with fragment 6 obtained from the oxidative
fragmentation of noricumazole A (2a) revealed the identity
of natural and synthetically derived 6 and established the
absolute configuration of the stereogenic centers at C25 and
C9 to be S. Consequently, the configuration at C11 was
assigned to be R with reference to acetonide 5. If one assumes
stereochemical identity of icumazole A (1a) and noricuma-
We used acetonides
3 (1a, 2,2-dimethoxypropane,
pTsOH, RT, 60 min, 73%) and 4 (2a, 2,2-dimethoxypropane,
pTsOH, RT, 15 min, 67%) to determine the relative config-
uration of the 1,3-diol moiety by Rychnovskyꢀs method
(Figure 1).[7] The relative stereochemistries at C22–C24 in 3
and C18–C20 in 4 were both determined to be anti,anti as
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cyclization and aromatization. Finally, the ester 16 was
transformed into the desired triol 7, and its relative config-
uration and identity was proven by comparison of the NMR
data with that of the the degradation fragment 7 (Figure S2a
in the Supporting Information). The absolute configurations
(18S,19R,20R) were further substantiated by identical optical
20
rotations (natural fragment: ½aꢀD ¼ꢁ22.5, c = 0.2, MeOH);
20
synthetic fragment: ½aꢀD ¼ꢁ20.8, c = 0.2, MeOH) and CD
spectroscopy (see Figure S2 in the Supporting Information).
Finally, the structure of noricumazole A (2a) was
unequivocally confirmed by total synthesis (Scheme 4). This
was achieved by fusing vinyl iodide 20 with aldehyde 19
Scheme 2. Preparation of western fragments 6 and epi-6. Reagents and
conditions: a) 1. (S)-8, Mg, THF, D, 1 h; 2. 9, HMTA, TMEDA, [Fe-
(acac)3], 08C, 1.5 h, 98%; b) 1. nBuLi, TMEDA, Et2O, ꢁ308C, 2 h, then
3 h at ꢁ58C; 2. tBuNCO (10 equiv), RT, 14 h, 94%; c) nBuLi (3 equiv),
TMEDA (3 equiv), Et2O, ꢁ788C to ꢁ408C, 2 h, then addition of (S)-12
(2 equiv), ꢁ788C to ꢁ408C, 14 h, 10a 73% (99% based on recovered
starting material), likewise using 12b yields 10b 81%; d) HClconc
,
EtOH, 508C, 2.5 h, 11a 89%, 11b 94%; e) pTsOH, toluene, D, 30 min,
6 89%, epi-6 95%. acac=acetylacetonate, HMTA=hexamethylphos-
phorus triamide, MOM=methoxymethyl, TMEDA=tetramethyl-
ethylene diamine.
zole A (2a) one can conclude that the additional stereogenic
center at C8 in 1c is R.
Scheme 4. Completion of the total synthesis of noricumazole A (2a).
Reagents and conditions: a) Dess–Martin periodinane, NaHCO3,
CH2Cl2, 08C, 45 min, 85%; b) DIBAL-H, CH2Cl2, ꢁ788C, 1 h, quant.;
c) K2CO3, Ohira–Bestmann reagent, MeOH, 08C to RT, 86%;
d) Schwartz reagent, THF, 08C, 1 h, then NIS, ꢁ788C, 25 min, 94%;
e) tBuLi, 20, Et2O, ꢁ788C, 1 h, then Me2Zn, ꢁ788C, 15 min, then 19,
Et2O, ꢁ788C, 2.5 h, 74%, d.r.=1:1 (21/11-epi-21); f) 11-epi-21, PPh3,
p-nitrobenzoic acid, DEAD, THF, 08C to RT, 3 h, 90%; g) NaOH, H2O,
THF/MeOH (2:1), 08C, 12 h, 92%; h) 21, BF3·Et2O, CH3CN, 08C, 12 h,
90%. DEAD=diethyl azodicarboxylate, NIS=N-iodosuccinimide,
TBS=tert-butyldimethylsilyl.
The synthesis of the stereotriade in fragment 7 relied on
the Masamune anti aldol reaction[16] and the Nagao acetate
aldol reaction[17] (Scheme 3) starting from propionate 13 and
propanal. The Nagao aldol reaction with aldehyde 14 using
thioxothiazolidine 17 furnished carboxylic acid 15 after O-
silylation and hydrolytic removal of the chiral auxiliary. This
was coupled with serine methyl ester 18 and transformed to
methyl ester 16 in good overall yield by DAST-mediated
(obtained from the western fragment 6 under strictly acid-
and water-free conditions) via the corresponding vinyl zinc
intermediate, which was generated by transmetalation of the
lithiated vinyl iodide using Me2Zn. It yielded the desired
product 21 and its epimer 11-epi-21 (d.r. = 1:1).[18] Methyl
ester 16 was elaborated to the vinyl iodide 20 by reduction to
the corresponding aldehyde followed by Seyferth–Gilbert
homologation[19] using Bestmannꢀs reagent.[20] The resulting
alkyne was subjected to a syn-hydrozirconation[21] followed by
iodination. After chromatographic separation of the two
diastereomers, the undesired isomer 11-epi-21 was directly
converted into 21 by Mitsunobu inversion[22] which was
combined with ester hydrolysis. This sequence improved the
yield of the desired coupling product 21 to 68%. Completion
of the synthesis was achieved by Lewis acid mediated TBS
removal[23] to furnish noricumazole A (2a) in 15% overall
yield over 15 steps (longest linear sequence) (including
Mitsunobu inversion: 17 steps and 26% overall yield).
Scheme 3. Synthesis of the eastern fragment 7. Reagents and condi-
tions: a) c-HexBOTf, NEt3, propanal, CH2Cl2, ꢁ788C, 2 h, 91%,
d.r.>20:1; b) TBSOTf, 2,6-lutidine, CH2Cl2, ꢁ788C to RT, 50 min,
quant.; c) DIBAL-H, CH2Cl2, ꢁ788C, 30 min, 91%; d) Dess–Martin
periodinane, CH2Cl2, RT, 1 h; e) 17, TiCl4, iPr2EtN, ꢁ408C to ꢁ788C,
1.5 h; f) TBSOTf, 2,6-lutidine, CH2Cl2, ꢁ788C, 30 min, 91% over three
steps; g) 1m LiOH, 30% H2O2, THF/H2O 4:1, 08C to RT, 12 h, 99%;
h) 18, HOBt, TBTU, iPr2EtN, CH2Cl2, RT, 12 h, 97%; i) DAST, CH2Cl2,
ꢁ788C, 3 h, 78% (88% based on recovered starting material); j) DBU,
BrCCl3, CH2Cl2, 08C to RT, 6 h, 85%; k) DIBAL-H (2 equiv), CH2Cl2,
ꢁ788C, 1 h, quant; l) BF3OEt2, CH3CN, 08C, 7 h, 72%. DAST=diethyl-
aminosulfur trifluoride, DBU=1,8-diazabicyclo[5.4.0]undec-7-ene,
Hex=n-hexyl, HOBt=hydroxybenzotriazole, TBSOTf=t-butyldimethyl-
silyl triflate, TBTU=O-benzotriazol-yl-N,N,N’,N’-tetramethyluronium
tetrafluoroborate.
The physical and analytical data of the product obtained
by total synthesis were identical with those recorded for
authentic noricumazole A (2a),[24] thus establishing the abso-
lute configurations of all stereogenic centers of noricumazole
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(2a) as depicted in Figure 1. Based on chemical and
biosynthetic considerations it can be expected that these
assignments match the stereochemistry of the icumazoles
(1a–c).
chloride is 85 mm and for nickel chloride 190 mm (see
Figure S6C).
These results indicate that noricumazole A (2a) stabilizes
the architecture of KcsA and inhibits the transport of
potassium cations through the ion channel. The latter result
was further confirmed in electrophysiological measurements
because noricumazol A (2a) completely inhibits single-chan-
nel activity at 1 mm concentration (Figure S7 in the Supporting
Information).[32] These values are in the same range as those
reported for other specific ion-channel inhibitors like the
scorpion toxin charybdotoxin, which exerts a 10- to 100-fold
higher affinity to human potassium channels.[33]
In conclusion, icumazole A (1a) and noricumazole A (2a)
along with two glycosylated congeners are new oxazole- and
isochromanone-containing metabolites from different S. cel-
lulosum strains. The aglycons in 1a and 2a differ only by the
presence of a methoxy or a hydroxy group at C11 and in the
length of the central triene or monoene units. The absolute
configuration of noricumazole A (2a) was unequivocally
elucidated by total synthesis of noricumazole A (2a).
Assessment of the biological properties of icumazole A
(1a) revealed activity (minimum inhibitory concentration
(MIC) in mgmLꢁ1) against various fungi, for example, Mucor
hiemalis (1.25), Botrytis cinerea (0.04), Pythium debaryanum
(0.3), Sclerotina sclerotiorum (1.25) (selected from Table S7 in
the Supporting Information), and the yeast-like Rhodotorula
glutinis (0.3). Experiments with R. glutinis, which grows as
single cells, showed that 1a inhibited growth but did not act
fungicidal even at tenfold concentration of the MIC (Fig-
ure S3 in the Supporting Information).[25] However, DNA,
RNA, and protein syntheses were blocked immediately after
addition of icumazole A (1a), probably because of inhibition
of a basic metabolic process. Consequently, the influence of
1a on the oxidation of NADH in cell-free preparations of
mitochondria from R. glutinis was studied; it was found that
NADH oxidation was inhibited with a maximal inhibition of
60–70% at a concentration of about 5 ngmLꢁ1 (Figure S4 in
the Supporting Information). Similar behavior had been
observed with myxothiazol acting on membranes of Para-
coccus denitrificans.[25b] In principle, complete inhibition did
not occur because several redundant fungal pathways for the
oxidation of NADH can exist.[26]
Our initial experiments suggest that icumazole A (1a)
targets the respiratory chain while noricumazole A (2a)
exerts potassium-ion-channel inhibitory activity which may
be related to its stabilizing property of the KcsA architecture.
Received: September 11, 2011
Published online: December 23, 2011
In contrast, noricumazole A (2a) does not exert marked
antibiotic activity. Surprisingly, we found that noricumazo-
le A (2a) is able to stabilize the tetrameric form of the
potassium channel (KcsA) in a temperature-dependent
manner. About half of all tetrameric KcsA ion channels
dissociate into their monomers at 828C.[27] In contrast, in the
presence of noricumazole A (2a) KcsA tetramers and even
oligomers are formed at lower temperatures (50–808C) and
remain stable up to 908C as judged by SDS-PAGE (see
Figure S5 in the Supporting Information). To the best of our
knowledge, this is the first example in which a natural product
induces such a stabilization of the KcsA architecture. It is only
known that sodium chloride has a similar effect, and in
contrast potassium chloride stabilizes the KcsA channel.[28,29]
To connect this unusual activity of noricumazole A (2a) to
a biological function, a fluorescence assay was established
that relies on a potassium sensing fluorescence dye (PBFI)
used to detect potassium transport in reconstituted biological
systems.[30] Liposomes were charged with PBFI and the
purified ion channel KcsA was integrated into the artificial
membrane, thus forming channelsomes. In the presence of
potassium chloride (1m added to the assay to yield 0.5m final
concentration) potassium ions are transported inside the
channelsomes and the fluorescence signal was monitored (see
Figure S6A in the Supporting Information). In the absence of
KcsA, control liposomes showed only a weak response to the
presence of potassium ions (see Figure S6A). Addition of
noricumazole A (2a) to the channelsomes led to a reduction
of the fluorescence signal and this effect was dose dependent
(see Figure S6A).[31] The concentration-dependent inhibitory
effect (IC50) of noricumazole A (2a) on the ion channel KcsA
was determined to be 4 mm, while the IC50 value for sodium
Keywords: antibiotics · ion-channel inhibitors · myxobacteria ·
natural products · structure elucidation · total synthesis
.
[1] S. C. Wenzel, R. Mꢁller, Curr. Opin. Drug Discovery Dev. 2009,
12, 220 – 230.
[2] K. Gerth, S. Pradella, O. Perlowa, S. Beyer, R. Mꢁller, J.
Biotechnol. 2003, 106, 233 – 253.
[3] H. Reichenbach, G. Hçfle, B. Bçhlendorf, H. Irschik, Ger. Pat 4
305 486A1, 1994; CA 1994, 121, 3000667v.
[4] R. Jansen, H. Irschik, V. Huch, D. Schummer, H. Steinmetz, M.
Bock, T. Schmidt, A. Kirschning, R. Mꢁller, Eur. J. Org. Chem.
2010, 1284 – 1289.
[5] a) S. Ek, H. Kartimo, S. Mattila, A. Tolonen, J. Agric. Food
Chem. 2006, 54, 9834 – 9842; b) R. C. Beier, B. P. Mundy, J.
Carbohydr. Chem. 1984, 3, 253 – 266.
[6] G. J. Gerwig, J. P. Kamerling, J. F. G. Vliegenthart, Carbohydr.
Res. 1978, 62, 349 – 357.
[7] R. D. Rychnovsky, B. N. Rogers, T. I. Richardson, Acc. Chem.
Res. 1983, 31, 9 – 17.
[8] The absolute configuration of the carbinol at C11 in 4 could not
be determined by the Mosher ester method because epimeriza-
tion occurred under the esterification conditions: H. Dale, S.
Mosher, J. Org. Chem. 1973, 38, 2143 – 2147.
[9] Iodide 7 was prepared from the corresponding alcohol: V.
Martischonok, G. G. Melikyan, M. Alfred, O. Vostrowsly, H. J.
Bestmann, Synthesis 1991, 560 – 564.
[10] G. Cahiez, V. Habiak, C. Duplais, A. Moyeux, Angew. Chem.
2007, 119, 4442 – 4444; Angew. Chem. Int. Ed. 2007, 46, 4364 –
4366.
[11] W. M. Czaplik, M. Mayer, M. A. von Wangelin, Angew. Chem.
2009, 121, 616 – 620; Angew. Chem. Int. Ed. 2009, 48, 607 – 610.
[12] Review: A. Fꢁrstner, Angew. Chem. 2009, 121, 1390 – 1393;
Angew. Chem. Int. Ed. 2009, 48, 1364 – 1367.
Angew. Chem. Int. Ed. 2012, 51, 1256 –1260
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1259

.
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Communications
[13] J. A. Frick, J. B. Klassen, A. Bathe, J. M. Abramson, H. Rapo-
port, Synthesis 1992, 621 – 623.
[25] A similar effect had been observed with the antifungal
myxothiazol: a) G. Thierbach, H. Reichenbach, Antimicrob.
Agents Chemother. 1981, 19, 504 – 507; b) G. Thierbach, H.
Reichenbach, Arch. Microbiol. 1983, 134, 104 – 107.
[26] B. Maresca, A. M. Lambowitz, G. S. Kobayashi, G. Medoff, J.
Bacteriol. 1979, 138, 647 – 649.
[14] K. Mori, M. Ikunaka, Tetrahedron 1984, 40, 3471 – 3479.
[15] In addition, the two epimers showed different optical rotations:
20
20
6 ½aꢀD ¼ꢁ33.0 (c = 0.1 gcmꢁ3, CHCl3) and epi-6 ½aꢀD ¼ + 15.2 (c =
0.5, CHCl3).
[16] T. Inoue, J.-F. Liu, D. C. Buske, A. Abiko, J. Org. Chem. 2002, 67,
5250 – 5256.
[17] Other asymmetric variants with chiral ligands failed as a result of
poor diastereocontrol (d.r. not better than 1:1.5) (also see the
Supporting Information).
[18] Y. Nagao, Y. Hagiwara, T. Kumagai, M. Ochiai, J. Org. Chem.
1986, 51, 2391 – 2393.
[19] J. C. Gilbert, U. Weeresooriya, J. Org. Chem. 1979, 44, 4997 –
4998.
[27] D. M. Cortes, E. Perozo, Biochemistry 1997, 36, 10343 – 10352.
[28] F. I. Valiyaveetil, Y. Zhou, R. MacKinnon, Biochemistry 2002,
41, 10771 – 10777.
[29] M. L. Renart, F. N. Barrera, M. L. Molina, J. A. Encinar, J. A.
Poveda, A. M. Fernꢂndez, J. Gꢃmez, J. M. Gonzꢂlez-Ros, J. Biol.
Chem. 2006, 281, 29905 – 29915.
[30] P. Jezek, F. Mahdi, K. D. Garlid, J. Biol. Chem. 1990, 265, 10522 –
10526.
[31] The ion-channel-activity of icumazole A (1a) could not be tested
because of its low chemical stability and partial degradation in
the assays.
[20] S. Mꢁller, B. Liepold, G. J. Roth, H. J. Bestmann, Synlett 1996,
521 – 522.
[21] D. W. Hart, J. Schwartz, J. Am. Chem. Soc. 1974, 96, 8115 – 8116.
[22] O. Mitsunobu, Synthesis 1981, 1 – 28.
[23] S. A. King, B. Pipik, A. S. Thompson, Tetrahedron Lett. 1995, 36,
4563 – 4566.
[24] For details, including NMR spectra of mixtures of synthetic and
natural 2a, refer to the Supporting Information.
[32] C. Zeilinger, M. Steffens, H.-A. Kolb, Biochim. Biophys. Acta
Biomembr. 2005, 1720, 35 – 43.
[33] Other established channel blockers like TBA, TEA, and Ba2+
require higher concentrations than noricumazole A (2a) for
inhibiting the KcsA channel.
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