Structural Reassignment and Absolute Stereochemistry of Madurastatin C1 (MBJ-0034) and the Related Aziridine Siderophores: Madurastatins A1, B1, and MBJ-0035

Article abstract of DOI:10.1021/acs.jnatprod.7b00082

The madurastatins are pentapeptide siderophores originally described as containing an unusual salicylate-capped N-terminal aziridine ring. Isolation of madurastatin C1 (1) (also designated MBJ-0034), from Actinomadura sp. DEM31376 (itself isolated from a

Full text of DOI:10.1021/acs.jnatprod.7b00082

Article
pubs.acs.org/jnp
Structural Reassignment and Absolute Stereochemistry of
Madurastatin C1 (MBJ-0034) and the Related Aziridine Siderophores:
Madurastatins A1, B1, and MBJ-0035
†
‡
†
†
†
Andrew R. Tyler, Hamed Mosaei, Stephanie Morton, Paul G. Waddell, Corinne Wills,
†
§
⊥
∥,‡
∥
‡
William McFarlane, Joe Gray, Michael Goodfellow, Jeff Errington,
and Michael J. Hall*
Nick Allenby, Nikolay Zenkin,
,†
†
⊥
School of Chemistry and School of Biology, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K.
‡
§
Centre for Bacterial Cell Biology and Pinnacle Laboratory, Institute for Cell and Molecular Biosciences, Newcastle University,
Newcastle Upon Tyne, NE2 4AX, U.K.
∥
Demuris Limited, Newcastle Biomedicine Bio-Incubators, Framlington Place, Newcastle upon Tyne, NE2 4HH, U.K.
*
S Supporting Information
ABSTRACT: The madurastatins are pentapeptide side-
rophores originally described as containing an unusual
salicylate-capped N-terminal aziridine ring. Isolation of
madurastatin C1 (1) (also designated MBJ-0034), from
Actinomadura sp. DEM31376 (itself isolated from a deep sea
sediment), prompted structural reevaluation of the madur-
astatin siderophores, in line with the recent work of Thorson
and Shaaban. NMR spectroscopy in combination with partial
synthesis allowed confirmation of the structure of madurastatin
C1 (1) as containing an N-terminal 2-(2-hydroxyphenyl)-
oxazoline in place of the originally postulated aziridine, while
absolute stereochemistry was determined via Harada’s
advanced Marfey’s method. Therefore, this work further
supports Thorson and Shaaban’s proposed structural revision of the madurastatin class of siderophores (madurastatins A1
(
2), B1 (3), C1 (1), and MBJ-0036 (4)) as N-terminal 2-(2-hydroxyphenyl)oxazolines.
iderophores are key components of the bacterial
endogenous secondary metabolome, facilitating intra-
investigations we compared isolated madurastatin C1 (1) to
both aziridine- and 2-(2-hydroxyphenyl)oxazoline-containing
synthetic analogues, as well as examined the absolute
stereochemistry of madurastatin C1 (1) via a Marfey’s analysis.
S
3
+
cellular uptake of Fe and other essential metals from the
1
surrounding environment. Pathogenic bacteria also employ
siderophores to sequester metals from the host organism, thus
2
playing an important role in virulence. The genus Actino-
RESULTS AND DISCUSSION
3
■
madura, from the family Thermomonosporaceae, contains both
Madurastatin C1 (1) was first isolated from the fermentation
broth of Actinomadura sp. DSMZ 13491 by Sosio et al. in
environmental (predominately soil dwelling) and opportunisti-
cally pathogenic species and are a source of a number of
5b
4
5
2012 and again by Shin-ya et al. in 2014 (as MBJ-0034, along
bioactive secondary metabolites including the madurastatin
and maduraferrin siderophores.
5
c
6
with the related siderophore MBJ-0035 (4)) and was
originally assigned as a linear pentapeptide containing a
salicylate-capped N-terminal aziridine ring by spectroscopic
analysis and analogy with the previously reported madurastatins
A1 (2) and B1 (3) (Figure 1).
As part of a program to identify new natural products from
7
actinobacteria, we have investigated the secondary metabolite
production of a marine isolate, Actinomadura sp. DEM31376.
Along with the known cyclic heptapeptide RNAP inhibitor
8
5b
In our hands, madurastatin C1 (1) was reisolated from
GE23077, we isolated the siderophore madurastatin C1 (1)
5
c
Actinomadura sp. DEM31376, a strain originally isolated from
(
also designated MBJ-0034). Spectroscopic analysis of
10
deep sea marine sediment (Canary Basin, Atlantic Ocean).
isolated 1 was however not consistent with the originally
reported structure. Recent work by Thorson and Shaaban
proposed a structural revision for the madurastatins as
containing a 2-(2-hydroxyphenyl)oxazoline moiety, based on
an analysis of the NMR data pertaining to both related bacterial
On the basis of 16S rRNA analysis, strain DEM31376 was
recovered in the genus Actinomadura, forming a subgroup with
Received: January 26, 2017
9
siderophores and synthetic intermediates. Thus, as part of our
©
XXXX American Chemical Society and
American Society of Pharmacognosy
A
DOI: 10.1021/acs.jnatprod.7b00082
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
Fe3+ and Ga , respectively. Interestingly chelation with
3+
Fe(acac) gave rise to an additional compound (663.2 m/z),
3
demonstrating that hydrolyzed 1 is also a competent ligand for
3+
Fe . Furthermore, 1 showed growth reduction against Bacillus
subtilis (disk diffusion assay) presumably through an inhibition
of Fe3+ uptake.
The NMR data of 1 did however highlight a number of
discrepancies with the originally proposed structure. In
particular the 13C NMR shifts for the α- and β-carbons of the
serine residue are observed at 67 and 69 ppm, highly atypical of
1
3
an aryloyl-substituted aziridine ring (shift range 25−45 ppm).
Thorson and Shaaban recently proposed that the madurastatin
siderophores contain an N-terminal 2-(2-hydroxyphenyl)-
oxazoline ring, a bidentate coordination moiety present in a
Figure 1. Previously proposed structures of the “aziridine”-containing
madurastatin siderophores.
9
number of other mixed ligand siderophores (e.g., acinetobac-
1
4
15
16
17
tin, the amamistatins, the amychelins, brasilibactin A,
the type strains A. mexicana, A. citrea, and A. scrupli. Although
closely related to both A. mexicana and A. citrea (99.4% 16S
rRNA gene similarity), taxonomic analysis suggests that strain
DEM31376 may form the nucleus of a novel species within the
18
19
gobichelins A and B, nocardichelins A and B, transvalencin
2
0
21
Z, and the spoxazomicins A−C ), in which the correspond-
ing 13C shifts typically occur from 65 to 72 ppm (Table 1).
To test this hypothesis, we synthesized the salicylate-
containing fragment of 1 as both an aziridine 5 and as an
oxazoline 6 for comparison. N-Trityl-L-serine methyl ester 7
was activated with mesyl chloride followed by base-catalyzed
intramolecular ring closure to give methyl (S)-1-tritylaziridine-
11
genus Actinomadura (Figure 2).
Strain DEM31376 was cultivated in ISP2 media in a 20 L
bioreactor. Absorption of the fermentation broth onto
Amberlite XAD16N beads (elution MeOH), solid-phase
capture on C8 SPE cartridges (elution with 50−100%
MeOH), and finally reversed-phase column chromatography
22
2-carboxylate 8, confirmed by single-crystal X-ray analysis.
Trityl deprotection with TFA/Et SiH in DCM followed by in
23
(
gradient elution, H O to MeOH, Biotage Isolera SNAP C )
3
2
18
12
+
provided 238 mg of 1. ESIHRMS showed an [M + H] ion at
92.2732 m/z, confirming the molecular formula
C H O N ). MS/MS gave fragments at 462, 434, 403, 349,
situ reaction with 2-(benzyloxy)benzoyl chloride gave the
desired aziridine-containing fragment 5 (Scheme 1).
Synthesis of the oxazoline-containing fragment 6 involved
reaction of L-serine methyl ester 9 with 2-(benzyloxy)benzoyl
chloride to give amide 10. Attempted oxazoline formation with
thionyl chloride gave only low yields of 6 alongside an
unwanted chlorinated derivative, 11. Treatment of 10 with
diethylaminosulfur trifluoride (DAST) provided the desired
oxazoline 6 in 79% yield. Confirmation of the structure of 6
5
(
26
37
9
7
5b
and 318 m/z in line with a previous analysis, while HPLC-
ESIMS showed the presence of a related molecule at 610 m/z,
1
3
formed via hydrolysis of the parent ion. C NMR revealed 26
carbon signals, six corresponding to an ortho-disubstituted
benzene and six to carbonyl-like carbons, three of which were
shown to be primary amides though HMBC correlations with
exchangeable protons at 8.52, 8.11, and 7.93 ppm. Thus, the
remaining two degrees of unsaturation are due to the presence
of a further two ring systems. H NMR, COSY, and N− H
HSQC revealed five amino acid spin systems, namely, glycine,
β-alanine, a modified serine, and two modified ornithines.
24
2
5
2
2
was given by single-crystal X-ray analysis, including
comparison to the closely related X-ray structure of
1
15
1
21a
spoxazomicin C (Scheme 2).
Comparison of the H and 13C NMR spectra of 1, 5, and 6
1
confirmed the presence of an oxazoline in the natural product.
Both the C NMR shifts of the ring carbons (C-2 and C-3)
and carbonyl-like carbon (C-1) and the H NMR shifts for H ,
H , and H of 1 align well with those of synthetic oxazoline 6.
1
5
13
Indirect measurement of the natural abundance N spectra by
1
15
1
H− N HMBC showed the presence of seven nitrogen atoms
a
in four different chemical shift ranges, indicative of three
amides, two hydroxamic acids, one amine, and an unusual
imine-like nitrogen. The role of 1 as a hexadentate siderophore
b
c
In addition, coupling constant analysis of 6 shows J , Jtrans, and
cis
26
Jgem of 10.5, 7.6, and 8.8 Hz, respectively, corresponding
closely to those observed in 1 (note that in 5 Jcis = 5.3, Jtrans
3.1, and Jgem = 1.3 Hz, demonstrating the small geminal
was confirmed through chelation with both Fe(acac) and
=
3
Ga(acac) , HPLC-ESIMS showing signals at 645.2 and 658.2
3
27
m/z corresponding to the protonated 1:1 complexes of 1 with
coupling characteristic of an aziridine). Analysis of the
Figure 2. (a) Scanning electron micrograph of strain DEM31376 showing spiral chains of warty ornamented spores following growth on ISP3 agar;
bar indicates 2 μm. (b) Neighbor-joining tree based on nearly complete 16S rRNA gene sequences showing relationships between strain DEM31376
and closely related type strains of Actinomadura species. Asterisks indicate branches also recovered using maximum likelihood and maximum
parsimony tree making methods. Numbers at nodes indicate bootstrap values based on neighbor-joining analysis of 1000 resampled data sets.
B
DOI: 10.1021/acs.jnatprod.7b00082
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Journal of Natural Products
Article
Table 1. Revised Structure and NMR Data for Madurastatin
Scheme 2. Synthesis of the Oxazoline-Containing Fragment
6
a
C1 (DMSO-d , 298 K, 700 MHz)
6
c
position
δC, type
δH (J in Hz)
δN
1
2
3
4
5
6
7
8
9
159.1, C
116.6, CH
134.1, CH
119.1, CH
128.1, CH
109.9, C
7.01, dd (8.3, 1.1)
7.47, ddd (8.7, 7.3, 1.8)
6.95, td (7.5, 1.1)
7.65, dd (7.8, 1.8)
a
Conditions: (i) 2-(benzyloxy)benzoyl chloride, Et N, −78 °C to RT,
3
1
8 h, 95%. (ii) DCM, DAST, −78 °C to RT, 2 h, 79%.
165.9, C
208.2
literature pertaining to the madurastatin siderophores A1 and
MBJ-0036 reveals that the H and C NMR data of these
67.4, CH
69.4, CH2
5.01, dd (10.4, 7.7)
4.65, dd (10.5, 8.4)
1
13
1
0
4
.52, t (8.0)
compounds also closely match those of oxazoline 6, suggesting
that, in agreement with Thorson and Shaaban, these structures
9
1
1
1
1
2
3
170.2, C
42.2, CH2
168.4, C
8.52, t (5.9)
109.2
112.5
175.9
should also be similarly revised as containing an N-terminal 2-
(2-hydroxyphenyl)oxazoline (Table 2).
3.75, dd (16.5, 6.1)
3
.67, dd (16.5, 5.7)
Table 2. H and 13C NMR Comparison of 1 with Synthetic
1
14
15
16
17
18
19
20
21
22
7.93, t (5.8 Hz)
3.28−3.22, m
2.53−2.51, m
Compounds 5 and 6
34.6, CH2
32.0, CH2
170.9, C
b
47.0, CH2
22.8, CH2
30.2, CH
3.51−3.42, m
1.63−1.53, m
1.50−1.45, m
position
, δC
1
5
6
1
2
3
165.9
67.4
69.4
178.1
33.1
37.5
168.1
69.3
70.8
1
.43−1.38, m
, δC
2
2
2
2
2
2
2
3
4
5
6
7
8
9
63.7, CH
2.87, t (6.6)
, δC
29.1
H , δ (J in Hz) 5.05 dd (10.7,
3.23 dd (5.3, 3.1) 4.97 dd (10.5,
7.6)
34.2, CH3
173.5, C
2.21, s
a
H
8
.1)
H , δ (J in Hz) 4.69 dd (10.7,
2.63 dd (5.3, 1.3) 4.62 dd (10.5,
8.8)
b
H
8.11, d (8.3)
118.8
8
.5)
49.4, CH
27.8, CH2
4.32, ddd (10.9, 8.2, 5.3)
b
H , δ (J in Hz) 4.61 t (8.3)
2.62 dd (3.1, 1.3) 4.67 dd (8.8, 7.6)
c
H
b
1.96−1.82, m
a
b
7
00 MHz, CD OD. Signal is an apparent triplet due to similar values
3
1
.67 qd (12.5, 4.3)
of Jtrans and Jgem
.
b
30
31
32
33
20.4, CH2
51.2, CH2
1.96−1.82, m
3.51−3.42, m
b
171.8
Finally Harada’s advanced Marfey’s method was employed to
165.0, C
determine the absolute stereochemistry of madurastatin C1
28
29
a
HMBC performed at 10 Hz. Signals overlapping. c15N shifts
measured indirectly by H− N HMBC performed at both 8 and 12
Hz, referenced against liquid NH .
b
(
1). Hydrolysis of 1 with concentrated HI and derivatiza-
1
15
tion with Nα-(5-fluoro-2,4-dinitrophenyl)-L-leucinamide was
followed by analysis by HPLC-ESIMS. Comparison to
preprepared amino acid standards revealed the presence of D-
serine, L-N-methylornithine, and L-ornithine, allowing absolute
stereochemical determination of all three stereocenters (Figure
3
Scheme 1. Formation of the Aziridine-Containing Fragment
a
5
3
).
In conclusion, we have confirmed Thorson and Shaaban’s
revised structure of madurastatin C1 (1) as an oxazoline-
a
Conditions: (i) MsCl, Et N, DCM, 0 °C to RT, 18 h, then Et N,
3
3
THF, reflux, 16 h, 70%. (ii) Et SiH, Et N, DCM, 0 °C to RT, 5 h. (iii)
3
3
2
-(benzyloxy)benzoyl chloride, Et N, −78 °C to RT, 48 h, 35% over 2
3
steps.
Figure 3. Revised structure of madurastatin C1 (1) including absolute
stereochemistry.
C
DOI: 10.1021/acs.jnatprod.7b00082
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Journal of Natural Products
Article
containing mixed ligand N-terminal 2-(2-hydroxyphenyl)-
oxazoline hexadentate siderophore and have determined its
absolute stereochemistry. On the basis of NMR analysis, similar
structural revisions should also be applied across the
madurastatin family of natural products including madurastatins
267−269. (c) Kawahara, T.; Itoh, M.; Izumikawa, M.; Sakata, N.;
Tsuchida, T.; Shin-ya, K. J. Antibiot. 2014, 67, 577−580.
(
6) Keller-Schierlein, W.; Hagmann, L.; Zah
Chim. Acta 1988, 71, 1528−1540.
7) Baksh, A.; Kepplinger, B.; Isah, H. A.; Probert, M. R.; Clegg, W.;
̈
ner, H.; Huhn, W. Helv.
(
5
,9
Wills, C.; Goodfellow, M.; Errington, J.; Allenby, N.; Hall, M. J. Nat.
Prod. Res. 2017, 10.1080/14786419.2016.1263854.
A1, B1, and MBJ-0035.
(
8) (a) Marazzi, A.; Kurz, M.; Stefanelli, S.; Colombo, L. J. Antibiot.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnat-
prod.7b00082.
■
2005, 58, 260−267. (b) Mariani, R.; Granata, G.; Maffioli, S. I.; Serina,
S.; Brunati, C.; Sosio, M.; Marazzi, A.; Vannini, A.; Patel, D.; White, R.;
Ciabatti, R. Bioorg. Med. Chem. Lett. 2005, 15, 3748−3752. (c) Zhang,
Y.; Degen, D.; Ho, M. X.; Sineva, E.; Ebright, K. Y.; Ebright, Y. W.;
Mekler, V.; Vahedian-Movahed, H.; Feng, Y.; Yin, R.; Tuske, S.;
Irschik, H.; Jansen, R.; Maffioli, S.; Donadio, S.; Arnold, E.; Ebright, R.
H. eLife 2014, 3, e02450.
*
S
Characterization details for DEM31376, isolation proce-
dures and analysis (including 1D and 2D NMR spectra)
for madurastatin C1 (1), experimental details for the
synthesis and characterization of compounds 5, 6, 8, and
(9) Shaaban, K. A.; Saunders, M. A.; Zhang, Y.; Tran, T.; Elshahawi,
S. I.; Ponomareva, L. V.; Wang, X.; Zhang, J.; Copley, G. C.; Sunkara,
M.; Kharel, M. K.; Morris, A. J.; Hower, J. C.; Tremblay, M. S.;
Prendergast, M. A.; Thorson, J. S. J. Nat. Prod. 2017, 80, 2−11.
10, crystal data for 6, 8, and 10 (PDF)
(
10) Maldonado, L. A.; Stach, J. E.; Pathom-aree, W.; Ward, A. C.;
Bull, A. T.; Goodfellow, M. Antonie van Leeuwenhoek 2005, 87, 11−18.
11) The 16s rRNA sequence for Actinomadura sp. DEM31376 was
deposited with the NCBI, GenBank accession number KY512569.
12) Production of madurastatin C1 from Actinomadura sp.
AUTHOR INFORMATION
■
(
Corresponding Author
Tel (M. J. Hall): +44 (0) 191 208 7321. Fax: +44 (0) 191 208
929. E-mail: michael.hall@newcastle.ac.uk.
*
(
6
DEM31376 was estimated at 19 mg/L (bioreactor, ISP2 media), in
comparison to the reported 65 mg/L from Actinomadura sp. DSMZ
13491 (shake flask, AFT media) [ref 5b] and 32 mg/L from
Streptosporangium sp. 32552 (shake flasks, custom media) [ref 5c].
ORCID
Jeff Errington: 0000-0002-6977-9388
Michael J. Hall: 0000-0001-6475-9161
Notes
(
13) Fong, C. F.; Grant, H. G. Aust. J. Chem. 1981, 34, 2307−2312.
(14) (a) Yamamoto, S.; Okujo, N.; Sakakibara, Y. Arch. Microbiol.
The authors declare no competing financial interest.
1
994, 162, 249−254. (b) Shapiro, J. A.; Wencewicz, T. A. ACS Infect.
Dis. 2016, 2, 157−168.
ACKNOWLEDGMENTS
The authors thank Newcastle University for funding including
Ph.D. studentships (A.T. and S.M.), the Wellcome Trust
102851) and Leverhulme Trust (PLP-2014-229) for funding
N.Z.), the EPSRC UK National Mass Spectrometry Facility at
Swansea University, and EPSRC for X-ray crystallography
facilities (EP/F03637X/1).
■
(15) (a) Suenaga, K.; Kokubo, S.; Shinohara, C.; Tsuji, T.; Uemura,
D. Tetrahedron Lett. 1999, 40, 1945−1948. (b) Kokubo, S.; Suenaga,
K.; Shinohara, C.; Tsuji, T.; Uemura, D. Tetrahedron 2000, 56, 6435−
6440.
(
(
(16) Seyedsayamdost, M. R.; Traxler, M. F.; Zheng, S.-L.; Kolter, R.;
Clardy, J. J. Am. Chem. Soc. 2011, 133, 11434−11437.
(17) Tsuda, M.; Yamakawa, M.; Oka, S.; Tanaka, Y.; Hoshino, Y.;
Mikami, Y.; Sato, A.; Fujiwara, H.; Ohizumi, Y.; Kobayashi, J. J. Nat.
Prod. 2005, 68, 462−464.
(
REFERENCES
■
18) Chen, Y.; Unger, M.; Ntai, I.; McClure, R. A.; Albright, J. C.;
(
1) Hider, R. C.; Konga, X. Nat. Prod. Rep. 2010, 27, 637−657.
2) Miethke, M.; Marahiel, M. A. Microbiol. Mol. Biol. Rev. 2007, 71,
13−451.
3) Trujillo, M. E.; Goodfellow, M. Actinomadura. Bergey’s Manual of
Systematics of Archaea and Bacteria; Lechevalier and Lechevalier, 400
emend. Kroppenstedt, Stackebrandt and Goodfellow, 156, John Wiley
Sons, Inc., in association with Bergey’s Manual Trust, 2015; pp 1−
Thomson, R. J.; Kelleher, N. L. MedChemComm 2013, 4, 233−238.
19) Schneider, K.; Rose, I.; Vikineswary, S.; Jones, A. L.;
Goodfellow, M.; Nicholson, G.; Beil, W.; Sussmuth, R. D.; Fiedler,
H.-P. J. Nat. Prod. 2007, 70, 932−935.
20) Mukai, A.; Fukai, T.; Matsumoto, Y.; Ishikawa, J.; Hoshino, Y.;
Yazawa, K.; Harada, K.; Mikami, Y. J. Antibiot. 2006, 59, 366−369.
21) (a) Inahashi, Y.; Iwatsuki, M.; Ishiyama, A.; Namatame, M.;
(
(
4
(
̈
AL
(
&
(
3
2.
Nishihara-Tsukashima, A.; Matsumoto, A.; Hirose, T.; Sunazuka, T.;
Yamada, H.; Otoguro, K.; Takahashi, Y.; Omura, S.; Shiomi, K. J.
Antibiot. 2011, 64, 303−7. (b) Zhang, J.; Hughes, R. R.; Saunders, M.
A.; Elshahawi, S. I.; Ponomareva, L. V.; Zhang, Y.; Winchester, S. R.;
Scott, S. A.; Sunkara, M.; Morris, A. J.; Prendergast, M. A.; Shaaban, K.
A.; Thorson, J. S. J. Nat. Prod. 2017, 80, 12−18.
(
4) (a) Gerber, N. N. Tetrahedron Lett. 1970, 11, 809−812. (b) Zein,
N.; Solomon, W.; Colson, K. L.; Schroeder, D. R. Biochemistry 1995,
3
4, 11591−11597. (c) Simmons, L.; Kaufmann, K.; Garcia, R.; Schwar
G.; Huch, V.; Muller, R. Bioorg. Med. Chem. 2011, 19, 6570−6575.
d) Wyche, T. P.; Piotrowski, J. S.; Hou, Y.; Braun, D.; Deshpande, R.;
̈
,
̈
(
McIlwain, S.; Ong, I. M.; Myers, C. L.; Guzei, I. A.; Westler, W. M.;
Andes, D. R.; Bugni, T. S. Angew. Chem., Int. Ed. 2014, 53, 11583−
1586. (e) Shaaban, K. A.; Elshahawi, S. I.; Wang, X.; Horn, J.; Kharel,
M. K.; Leggas, M.; Thorson, J. S. J. Nat. Prod. 2015, 78, 1723−1729.
f) Shin, B.; Kim, B.-Y.; Cho, E.; Oh, K.-B.; Shin, J.; Goodfellow, M.;
Oh, D.-C. J. Nat. Prod. 2016, 79, 1886−1890. (g) Kimura, T.; Iwatsuki,
M.; Asami, Y.; Ishiyama, A.; Hokari, R.; Otoguro, K.; Matsumoto, A.;
Sato, N.; Shiomi, K.; Takahashi, Y.; O
Antibiot. 2016, 69, 818−824. (h) Kodani, S.; Komaki, H.; Ishimura, S.;
Hemmi, H.; Ohnishi-Kameyama, M. J. Ind. Microbiol. Biotechnol. 2016,
3, 1159−1165.
5) (a) Harada, K.; Tomita, K.; Fujii, K.; Masuda, K.; Mikami, Y.;
Yazawa, K.; Komaki, H. J. Antibiot. 2004, 57, 125−135. (b) Mazzei, E.;
Iorio, M.; Maffioli, S. I.; Sosio, M.; Donadio, S. J. Antibiot. 2012, 65,
(
22) CCDC 1525889−1525891 contain the supplementary crystallo-
graphic data for compounds 5, 6, and 10. These data can be obtained
free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/structures.
1
(
(23) Vedejs, E.; Klapars, A.; Warner, D. L.; Weiss, A. H. J. Org. Chem.
2001, 66, 7542−7546.
(24) Hu, J.; Miller, M. J. J. Am. Chem. Soc. 1997, 119, 3462−3468.
(25) Phillips, A. J.; Uto, Y.; Wipf, P.; Reno, M. J.; Williams, D. R. Org.
Lett. 2000, 2, 1165−1168.
̅
mura, S.; Nakashima, T. J.
4
(
(26) Wohl, R. A.; Cannie, J. J. Org. Chem. 1973, 38, 1787−1790.
(27) Lwowsky, W. In Comprehensive Heterocyclic Chemistry; Katritzky,
A. R.; Rees, C. W., Eds.; Pergamon Press: Oxford, 1984; Vol. 7,
Chapter 5.
D
DOI: 10.1021/acs.jnatprod.7b00082
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
(
28) Harada, K.; Fujii, K.; Hayashi, K.; Suzuki, M.; Ikai, Y.; Oka, H.
Tetrahedron Lett. 1996, 37, 3001−3004.
29) (a) Stephan, H.; Freund, S.; Meyer, J.-M.; Winkelmann, G.;
(
Jung, G. Liebigs Ann. Chem. 1993, 1993, 43−48. (b) Kreutzer, M. F.;
Kage, H.; Nett, M. J. Am. Chem. Soc. 2012, 134, 5415−5422.
E
DOI: 10.1021/acs.jnatprod.7b00082
J. Nat. Prod. XXXX, XXX, XXX−XXX