Total synthesis and reassignment of the structures of the antimicrobial lipodepsipeptides circulocin γ and δ

Article abstract of DOI:10.1021/acs.joc.5b00349

The structures of the naturally occurring antimicrobial lipodepsipeptides circulocin γ and circulocin δ have been reported to comprise a common cyclic depsipeptide core attached to 3-hydroxy,ω-guanidino fatty acid chains differing in length by two methyle

Full text of DOI:10.1021/acs.joc.5b00349

Article
pubs.acs.org/joc
Total Synthesis and Reassignment of the Structures of the
Antimicrobial Lipodepsipeptides Circulocin γ and δ
James R. Cochrane, Claudia J. Exner, and Katrina A. Jolliffe*
School of Chemistry, The University of Sydney, Sydney 2006, New South Wales, Australia
S
* Supporting Information
ABSTRACT: The structures of the naturally occurring anti-
microbial lipodepsipeptides circulocin γ and circulocin δ have
been reported to comprise a common cyclic depsipeptide core
attached to 3-hydroxy,ω-guanidino fatty acid chains differing in
length by two methylene units, but analysis of the reported data
suggested that the originally reported structures had incorrect side
chain lengths. The total synthesis of both side chain epimers of the
originally reported structure of circulocin γ bearing a 19-
guanidino-3-hydroxynonadecanoyl (GHND) side chain has been
accomplished using a modular approach involving synthesis of the
cyclic depsipeptide and side chain fragments followed by a late
stage coupling reaction. This revealed that the originally reported structure for circulocin γ bearing the GHND side chain is
incorrect and that this structure is actually that of circulocin δ. It has also enabled the absolute configuration of the side chain
hydroxy group of the natural product to be assigned as (R). Subsequent synthesis of the analogue bearing a 17-guanidino-3-(R)-
hydroxyheptadecanoyl (GHHD) side chain provided confirmation that this revised structure is that of circulocin γ.
functionality of the GHPD side chain is crucial for activity.14
A
INTRODUCTION
■
recent total synthesis of fusaricidin A (LI-F04a) 1 enabled the
assignment of the absolute stereochemistry of the stereogenic
center at C3 of the GHPD side chain as (R)-configured15 and
an analogue lacking the hydroxy group at this position had
significantly reduced antifungal activity.16
The rapid increase in multidrug resistant bacterial and fungal
infections in recent years has led to an urgent need for the
development of new antimicrobial agents.1−5 Naturally
occurring cyclic lipo(depsi)peptides are promising lead
structures for antimicrobial drug discovery, with a number of
compounds from this class such as daptomycin already in
clinical use. Strains of Paenibacillus and Bacillus are known
producers of bioactive cyclic lipopeptides, and a number of
antimicrobial compound classes including the iturins,6
surfactins7,8 and fusaricidins (or LI-F class)9,10 have been
isolated from these microorganisms.11 These compounds are
often produced as mixtures of closely related structures, and
different strains of microorganism can sometimes produce
structures of high similarity. For example, the fusaricidin class of
cyclic lipodespipeptides, which contain a cyclic depsihexapep-
tide core with a 15-guanidino-3-hydroxypentadecanoyl
(GHPD) side chain attached to the cycle via the nitrogen
atom of an L-threonine residue, are obtained as a mixture of at
least 12 compounds from the L-1129 strain of Paenibacillus
polymyxa9,10 as well as from other strains of Paenibacillus.12,13
The components of this mixture differ only in the compositions
of three of the amino acids (L-Thr, D-allo-Thr and D-Ala are
conserved) in the cyclic depsipeptide core, while the GHPD
side chain remains constant throughout the series. The
promising antimicrobial activity displayed by the fusaricidin
cyclic peptides against both fungi and bacteria (including some
drug resistant strains) has led to the synthesis and evaluation of
the antimicrobial activity of a number of analogues of these
compounds, leading to the conclusion that the guanidino
The GHDP side chain of the fusaricidin peptides is unique,
with most other classes of antimicrobial cyclic lipodepsipeptides
bearing fatty acid side chains terminating in methyl groups.
However, the structures of the circulocins (2−5), a series of
four cyclic lipodepsipeptides isolated from the J2154 strain of
Bacillus circulans, are reported to contain similar guanidino-
terminated fatty acid side chains with a C3-hydroxy group,
albeit of different lengths to those reported for the fusaricidin
series. (Figure 1).17 The circulocins exhibit antibiotic activity
against clinically relevant species of Gram-positive bacteria,
including piperacillin-resistant staphylococci and vancomycin-
resistant enterococci, as well as antifungal activity against
Candida albicans. Of the four peptides isolated, circulocins α
and β 2 and 3 are reported to have cyclic depsipentapeptide
cores, while circulocins γ and δ are reportedly cyclic
depsihexapeptides (4 and 5) with striking similarities to the
fusaricidin class (Figure 1). In particular, circulocins γ and δ
have a cyclic hexadepsipeptide core that is almost identical to
that of fusaricidin A, with the only differences being
substitution of D-Val and L-Val residues by D-Leu and L-Ile,
respectively.
Received: February 13, 2015
Published: March 31, 2015
© 2015 American Chemical Society
4491
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
Figure 1. Structure of fusaricidin A and the reported17 and revised structures of circulocins α−δ.
Scheme 1. Retrosynthesis
The structures proposed by He et al.17 for circulocins γ 4 and
δ 5 differ only in the lengths of the guanidino-terminated side
chains, with that in circulocin γ 4 reported to be a 19-
guanidino-3-hydroxynonadecanoic acid (GHND) side chain,
while that in circulocin δ 5 is reported to be a 21-guanidino-3-
hydroxy-heneicosanoic acid (GHHC) side chain (Figure 1).
However, close inspection of the data reported by He et al. for
circulocins γ and δ suggests that the reported side chain lengths
are incorrect. Notably, the molecular formulas for the proposed
structures (C47H86N10O11 for circulocin γ and C49H90N10O11
for circulocin δ) do not match the molecular formulas provided
by high resolution mass spectrometry (m/z = 939.6243,
corresponding to MH+ = C45H83N10O11 and m/z = 967.6556,
corresponding to MH+ = C47H87N10O11 for circulocins γ 4 and
δ 5, respectively). The molecular mass of the proposed
structures differ from those obtained by high resolution mass
spectrometry by the mass of two methylene units (C2H4) in
each case. We postulated that the most likely reason for this
difference was a mis-assignment of the number of methylene
units in the alkyl side chains and that the correct structures of
circulocins γ (6) and δ (4) bear 17-guanidino-3-hydroxy-
heptadecanoic acid (GHHD) and 19-guanidino-3-hydroxyno-
nadecanoic acid (GHND) side chains, respectively.
4492
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
To clarify the structures of circulocins γ and δ we undertook
the total synthesis of the revised structures 4 and 6.
Additionally, in order to confirm the absolute stereochemistry
of the stereogenic center at C3 of the side chains and determine
whether it was identical to that observed for fusaricidin A 1,14
we prepared both side chain epimers of the revised structure of
circulocin δ 4.
Scheme 2. Synthesis of Cyclic Depsipeptide 7
RESULTS AND DISCUSSION
■
Given that both 4 and 6 contain an identical cyclic depsipeptide
core, we chose to employ a convergent synthetic strategy,
similar to that used previously in our total synthesis of 1,15 to
provide ready access to compounds of varying side chain length
and to both side chain epimers of 4 in an efficient manner
(Scheme 1). This route allows the cyclic peptide core 7 and the
required GHND and GHHD side chains (8 and 9, respectively)
to be synthesized independently and combined in late stage
coupling reactions. We envisaged that the protected GHND
and GHHD side chains 8 and 9 should be readily accessible via
alkylation of the chiral epoxides (R)-10 and (S)-10 with alkynes
11 and 12, respectively.
In order to prepare the cyclic depsipeptide core of circulocins
γ and δ using our previously optimized macrolactonization
approach,15,16 we first prepared the linear hexapeptide 13 by
standard solid phase peptide synthesis protocols using 2-
chlorotrityl chloride resin as the solid support, PyBOP/Hunig’s
̈
base as the coupling agent and an Fmoc/t-Bu protecting group
strategy (Scheme 2). The N-terminal threonine residue was
incorporated as the Cbz-protected derivative to allow later
removal of this protecting group while leaving the side chain
protecting groups intact. Cyclization of 13 was then performed
using the modified Yamaguchi lactonization conditions18
described in our previous publication, which were optimized
to mimimize epimerization of the C-terminal residue during the
macrocyclization reaction to prepare the core of fusaricidin
A.15,16 This gave the desired cyclic peptide 7 in a 38% isolated
yield after purification by HPLC, but the formation of a
significant quantity (12%) of the C-terminal epimerized cyclic
peptide epi-7 was also observed (Scheme 2). In efforts to
improve the yield of the desired cyclic product, we also
attempted the cyclization of 13 using 2-methyl-6-nitrobenzoic
anhydride (MNBA),19 however this did not lead to improved
yields or reduced quantities of the epimer (7 and epi-7 were
obtained in 34 and 14% yields, respectively). Alternative
approaches to the cyclic product 7, involving macrolactamiza-
tions at the D-Asn-D-Ala; D-Leu-L-Ile and L-Thr-D-Leu junctions
were also explored, however analysis of the crude reaction
mixtures indicated only low yields of the required cyclic
product and the presence of diasteromeric mixtures, so these
were not investigated further. It is interesting to note that the
seemingly minor changes to the linear peptide structure
resulting from the replacement of the two Val residues present
in the fusaricidin A core by the D-Leu and L-Ile residues
required for the circulocin core had a significant effect on the
isolated yield and amount of epimerization occurring in the
cyclization reaction. This highlights the impact of linear peptide
sequence in determining the success of the cyclization reaction,
with small changes significantly affecting the reaction outcome.
With the cyclic core of circulocins γ and δ in hand, we turned
our attention to the synthesis of the GHND and GHHD side
chains. We focused initially on the synthesis of both
enantiomers of side chain 8, which would provide access to
our revised structure of circulocin δ, since this would allow us
to confirm both the validity of our structural revisions (given
that this is identical to the structure of circulocin γ originally
proposed by He et al.17) and the absolute stereochemistry of
the C3 alcohol in 4.
To enable the assignment of the stereochemistry of the C3
alcohol in circulocin δ 4, both enantiomers of the side chain 8
were required. It was envisioned that this could be achieved by
BF3·OEt2 promoted alkylation of both enantiomers of the
epoxide 10 with the alkyne 11 using similar methods to those
employed previously in our synthesis of fusaricidin A.15 In
order to prepare the alkyne chain of the required length, we
commenced with propargyl alcohol 14 as outlined in Scheme 3.
The dianion of 14 was prepared upon treatment with 2 equiv of
n-BuLi in a solution of THF and HMPA. This was then treated
with lauryl bromide to give the corresponding 1-hydroxy 2-
alkyne 1520 in 70% yield. Treatment of this acetylenic alcohol
with 6.5 equiv of potassium 3-aminopropylamide (KAPA) in 3-
aminopropylamine in the acetylenic zipper reaction21 gave the
required terminal alkyne 16 in 80% yield and the resulting
primary alcohol was then protected as a TBDPS ether to give
11 in 83% yield. The terminal alkyne 11 was then reacted with
both enantiomers of epoxide 10 and the resulting secondary
alcohols were used to synthesize the required pair of
enantiomeric side chains following similar procedures to
those developed previously for the synthesis of the 15-
4493
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
Scheme 3. Synthesis of Both Enantiomers of the GHND Side Chain 8
guanidino-3-hydroxypentadecanoyl (GHPD) side chain.15
Scheme 4. Synthesis of (R)-9
Briefly, BF3·OEt2 promoted alkylation of terminal alkyne 11
with the chiral epoxide (S)-10 gave the secondary alcohol (R)-
17 in 63% yield. The protection of the secondary alcohol as a
MOM ether followed by silyl deprotection proceeded smoothly
to give the primary alcohol (R)-18 in 73% yield. The primary
alcohol was then converted to the protected guanidine (R)-19
in 78% yield upon treatment with di-tert-butoxycarbonylguani-
dine under Mitsonobu conditions.22 Concomitant reduction of
the internal alkyne and hydrogenolysis of the benzyl ether using
a palladium hydroxide on charcoal catalyst were followed by
oxidation of the resulting primary alcohol using ruthenium
tetroxide23 to give the carboxylic acid (R)-8 in 43% yield over
the two steps. The (S)-enantiomer was synthesized in a similar
manner, commencing with the opposite enantiomer of the
chiral epoxide, (R)-10, to give (S)-8 in 18% overall yield.
Synthesis of the (R)-(GHHD) side chain was performed
using the same procedures but starting from the reaction of
propargyl alcohol 14 with 1-bromodecane to give alkyne 21,
which was subsequently isomerized and protected to give the
terminal alkyne 12.20 Reaction of 12 with epoxyalcohol (S)-10
and subsequent manipulation as described above for the longer
side chain gave [(R)-9] in 24% yield over the 6 steps (Scheme
4).
In order to complete the total synthesis of both side chain
epimers of the revised structure of circulocin δ 4, the N-
terminal Cbz protecting group was removed from the L-Thr
residue of the cyclic peptide core 7 by hydrogenolysis with an
excess of Pd/C catalyst in THF. This reaction was sluggish with
some over-reduced side products evident, as previously
observed for deprotection of the core of fusaricidin A,15 but
under these conditions the free amine 22 was obtained in 59%
yield after RP-HPLC purification (Scheme 5). Amine 22 was
then condensed with the side chain fragment (R)-8 using
solution phase peptide coupling conditions employing an
yield over the two steps after RP-HPLC purification (Scheme
5). The side chain (S)-epimer [(S)-4] was prepared in a similar
manner from 7, but without purification of the intermediates.
Using this method (S)-4 was obtained in 3.2% yield over the
three steps from 7 (Scheme 5). This yield was significantly
lower than when the product was isolated after each step
(14%), predominantly because purification of the final product
was difficult. However, sufficient material to allow the side
chain absolute stereochemistry to be established was obtained.
Similarly, 22 was also condensed with the side chain fragment
(R)-9, followed by global protecting group removal to give (R)-
6 in 22% overall yield.
The experimentally obtained 1H NMR and 13C NMR spectra
of both compounds (R)-4 and (R)-6 were very similar to each
other and to those reported for both circulocins δ and γ by He
et al.,17 suggesting that the original error in the determination
of the structures of circulocins δ and γ was not in the
excess of the coupling reagent HATU and Hunig’s base in
̈
DMF, followed by global removal of the acid labile protecting
groups with 90:5:5 TFA:CH2Cl2:H2O to give (R)-4 in 24%
4494
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
Scheme 5. Synthesis of Final Compounds
assignment of the peptide core, as if this was the case the
spectra of the synthesized compounds would be significantly
different to that reported for the natural products. Compounds
(R)-4 and (S)-4 were synthesized with a 19-guanidino-3-
hydroxy-nonadecanoic acid side chain, originally assigned to
circulocin γ. The fact that the molecular formulas for these
compounds, as determined by HRMS in the original paper
(Table 1), were the same as that for the proposed structure of
natural product (Table S2, Supporting Information). However,
close inspection indicated that while the 13C NMR spectrum of
(R)-4 had a maximum Δδ of 0.3 ppm compared to the data for
the natural product, that of (S)-4 had larger discrepancies with
the greatest difference being observed for the carbon at the
stereogenic center of the side chain (Δδ of 0.9 ppm). As was
the case with fusaricidin A,15 the 1H NMR spectrum of the (R)-
isomer was almost identical to the data for the natural product,
with a maximum Δδ of 0.02 ppm, while the 1H NMR spectrum
of the (S)-isomer was slightly inconsistent with that of the
natural product, with a maximum Δδ of 0.08 ppm (Table S1,
Supporting Information). The most obvious differences
between the two spectra are in the chemical shifts of the
diastereotopic protons attributable to the prochiral carbon C2
(Figure 2), together with the coupling constants between the
proton at C3 and the diastereotopic protons on C2. In (R)-4
the C2 protons are observed as two doublets of doublets at 2.45
and 2.32 ppm, respectively. This is identical to the reported
chemical shifts for these protons in the natural product. For
(S)-4 the first of these doublets of doublets was observed at
2.43 ppm, but the second was observed at 2.24 ppm, 0.06 ppm
upfield of the reported chemical shift. There was also a
difference in the coupling constants between the proton at C3
and the diastereotopic protons on C2. In (R)-4 the coupling
constants between these protons are 5.4 and 6.4 Hz. This is
very similar to the coupling constants reported for the natural
product (4.9 and 6.4 Hz for these protons). In the case of (S)-4
the coupling constants are 9.0 and 3.7 Hz respectively, which is
Table 1. HRMS Data for Originally Reported17 and
Synthetic Material
originally reported [M + H]+ synthetic material [M + H]+
circulocin γ
circulocin δ
939.6243
967.6556
939.6240
967.6572
circulocin δ together with a comparison of all data for our
synthetic 4 with that reported by He et al. (see below) indicate
that in the original report the length of the alkyl chains of
circulocin δ and circulocin γ were mis-assigned. We therefore
propose that circulocin γ actually has a C17 (GHHD) side
chain and circulocin δ a C19 (GHND) side chain.
To confirm the revised structure of circulocin δ, the physical
and spectroscopic data for (R)-4 and (S)-4 were compared to
the data reported for the natural product (see Supporting
Information for tables showing comparative data). The 13C
NMR spectra of the synthetic compounds were very similar and
a close to match to the data reported in the literature for the
Figure 2. Revised structures of circulocin γ and δ with side chain carbons numbered.
4495
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
significantly different to the data reported for the natural
product. There was also a difference in the observed chemical
shift of the diastereotopic protons on C4 between (R)-4 and
(S)-4. In (R)-4 this multiplet is observed at 1.37 ppm, in the
natural product this resonance is reported at 1.36 ppm, while in
(S)-4 it was observed 0.08 ppm downfield at 1.44 ppm. Finally,
comparison of the optical rotations of the synthetic material
with the (R)-configured side chain (+10°) and that of the (S)-
configured side chain (+2°) with those reported for the natural
product (+12.9°)17 indicated that the natural product is most
likely to bear a side chain with (R)-stereochemistry at C3.
Given the similarities between the structures of circulocin γ
and δ, and the trend for lipodepsipeptides bearing a C3
hydroxyl group to have the R-configuration at this stereo-
center,11 only a single isomer of our revised structure for
circulocin γ was prepared, in which the C3 hydroxy group was
R-configured. The HRMS data for the synthetic material was
found to be a close match to that originally reported for the
natural product (Table 1) confirming the molecular formula of
this compound as C45H83N10O11. The optical rotation of the
synthetic (R)-6 (+8.2, c 0.08, MeOH) is close to that reported
for the isolated natural product (+12.3, c 0.31, MeOH). The 1H
NMR and 13C NMR data obtained for the synthetic (R)-6 also
match well with the data reported for the natural product, with
EXPERIMENTAL SECTION
■
General Experimental Methods. Materials, solvents, instrumen-
tation, and general methods were essentially as described in previous
publications from our laboratory.15,16 1H Nuclear magnetic resonance
(NMR) spectra were recorded at a frequency of 300 or 400 MHz. 13C
Nuclear magnetic resonance spectra were recorded at a frequency of
100 or 150 MHz. Low resolution mass spectra (MS) were recorded on
an ion trap mass spectrometer (ESI). High resolution mass spectra
(HRMS) were recorded on a 7.0 T Fourier transform ion cyclotron
resonance mass spectrometer (FTICR) with an analytical electrospray
source. Infrared spectra (IR) were obtained with an FT-IR
spectrometer with ATR unit. Flash column chromatography was
carried out using Kieselgel 60 silica gel (SiO2, 0.04−0.065 μm) with
the indicated solvents. Analytical, preparative and semipreparative
reverse phase HPLC (RP-HPLC) was performed using a multisolvent
delivery system and pump with a 2998 photodiode array detector or a
programmable wavelength detector operating at 254 and 214 nm.
Analytical HPLC employed a Sunfire C18 column (2.1 × 150 mm
column, 5 μm particle size, flow rate of 0.8 mL min−1). Preparative RP-
HPLC employed a Sunfire Prep C18 OBD column (19 × 150 mm, 5
μm particle size, flow rate 7 mL min−1). Semipreparative RP-HPLC
employed a Sunfire C18 column (10 × 250 mm, 5 μm particle size,
flow rate 4 mL min−1). The mobile phase consisted of eluents A (0.1%
TFA in water) and B (0.1% TFA in acetonitrile). For all HPLC runs a
gradient of 30−100% B over 40 min was used, unless otherwise stated.
Most reagents were commercially available reagent grade chemicals
and were used without further purification. CH2Cl2, methanol and
triethylamine were distilled from calcium hydride, THF was distilled
from sodium and benzophenone before use. DMF was obtained as
peptide synthesis grade and stored over 4 Å molecular sieves. All
reactions were carried out under an atmosphere of nitrogen unless
otherwise stated. Compounds 12, 15, 16 and 21 were prepared using
similar methods to those reported by Breit et al.20
General Procedure 1. Resin Loading. 2-Chlorotrityl chloride
resin (0.50 g, 1.4 mmol/g) was swollen with CH2Cl2 (5 mL) for 30
min. To this was added a solution of Fmoc-amino acid (0.88 mmol) in
4:1 CH2Cl2:DMF (5 mL). The mixture was treated with
diisopropylethylamine (0.67 mL, 3.5 mmol) and left to stir overnight.
The resin was then collected by filtration and washed with CH2Cl2 (3
× 20 mL), 17:2:1 CH2Cl2:methanol:diisopropylethylamine (3 × 20
mL) followed by CH2Cl2 (5 × 20 mL). The resin was then dried
under reduced pressure. The loading was determined by deprotection
of the resin with 10% piperidine in DMF (2 × 5 mL for 3 min) and
then measuring the absorbance of the piperidine-fulvene adduct at λ =
301 nm.
General Procedure 2. Iterative Peptide Assembly. The desired
peptide was synthesized by coupling amino acids manually in
polypropylene syringes with sintered discs (Torviq). The following
sequence of steps was used: deprotection, washing, coupling, and
washing. Deprotection: The resin was treated with 10% piperidine in
DMF (2 × 5 mL for 3 min). Resin loading was determined after each
coupling step by measuring the absorbance of the piperidine-fulvene
adduct at λ = 301 nm. Washing: The resin was sequentially washed
with DMF (5 × 5 mL), CH2Cl2 (5 × 5 mL) and DMF (5 × 5 mL).
Coupling: One of two coupling methods was employed. Method (A): A
solution of the Fmoc-amino acid (4 equiv), PyBOP (4 equiv) and
diisopropylethylamine (8 equiv) was dissolved in DMF (1 mL) and
added to the resin. The resin was then shaken on an orbital shaker
(175 rpm) for 2 h. For the amino acids Fmoc-D-Leu-OH, Fmoc-L-Ile-
OH and Cbz-L-Thr-OH a double coupling method was performed
where the solution was discarded and a further preactivated solution
was added prior to the washing step. Method (B): Fmoc-D-allo-
Thr(tert-butyl)-OH was coupled by adding preactivated acid (1.5
equiv), PyBOP (2 equiv) and diisopropylethylamine (2 equiv) to the
resin and then agitating on an orbital shaker (175 rpm) for 16 h.
General Procedure 3. Epoxide Coupling. To a cooled (−78 °C)
solution of alkyne (1.5 equiv) dissolved in dry THF (10 mL), was
added n-BuLi (2.0 M in hexanes; 1.5 equiv; 7.0 mmol) under an
atmosphere of argon. This solution was stirred for 0.5 h, before boron
1
a maximum Δδ of 0.03 ppm in the H NMR data. Similarly
there is a maximum deviation of only 0.2 ppm in the 13C NMR
spectrum. The coupling constants of the critical C2 protons on
the alkyl chain of the synthetic material are also a good match
to those of the natural product (4.9 compared to 5.0 Hz and 6.2
compared to 6.5 Hz). Analysis of the data indicates that for
both compounds 4 and 6, the C3 R-isomer corresponds to the
natural product. The fact that in both the circulocin and
fusaricidin compounds the C3 alcohol is (R)-configured
indicates a common biosynthetic pathway and that the other
closely related peptides that have been isolated are most likely
to also have the (R)-configuration.
CONCLUSIONS
■
In summary, we have completed the first reported total
syntheses of circulocin δ 4 and its C3 epimer (S)-4, together
with the (R)-isomer of circulocin γ 6. The total syntheses of 4
and 6 have enabled us to revise the structures of these
compounds. We have found that both circulocin γ and
circulocin δ have a different length alkyl chain than was
originally reported by He et al.12 We argue that, based on our
data together with that reported originally, circulocin δ has a
C19 alkyl chain, rather than the C21 chain reported. This was
1
confirmed by comparison of the reported HRMS, H and 13C
NMR data for circulocin δ with that of the synthesized
compounds (R)-4 and (S)-4. Similarly based upon the available
experimental information we propose that circulocin γ 6 has a
C17 alkyl chain, rather than the C19 chain reported. In
addition, the synthesis of both side chain epimers of the revised
structure of circulocin δ has also allowed the complete
stereochemical assignment of these natural products, confirm-
ing the absolute configuration of the stereogenic center at C3 as
(R). For both fusaricidin A and circulocin δ, subtle yet
significant differences can be observed in the 1H NMR and 13
C
NMR spectra of the (R)- and the (S)-isomers. These may be
useful in the future for assigning the stereochemistry in related
peptides and similar systems, for example heptadepsin, a cyclic
depsipeptide that was recently isolated from Paenibacillus sp.
BML771−113F9 by Umezawa and co-workers.24
4496
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
trifluoride diethyl etherate complex (1.5 equiv) was added. The
epoxide (1 equiv) dissolved in dry THF (10 mL), was cooled to −78
°C and then added dropwise to the alkyne solution. This mixture was
stirred for 2 h at −78 °C before it was warmed to room temperature
and quenched with satd. NH4Cl (50 mL). This solution was extracted
with EtOAc (3 × 50 mL). The combined organic extracts were washed
with brine (50 mL), dried (Na2SO4), and evaporated. Chromatog-
raphy on silica gel (5:1 hexanes: Et2O), afforded the alcohol as a
colorless oil.
General Procedure 4. MOM Protection. The alcohol (1 equiv)
was dissolved in THF (10 mL) and diisopropylethylamine (20 equiv)
was added slowly under an atmosphere of argon. The solution was
then cooled to 0 °C and chloromethyl methyl ether (10 equiv) was
added dropwise. The solution was stirred for 16 h. The reaction
mixture was then quenched with water (100 mL) and extracted with
Et2O (3 × 100 mL). The combined organic extracts were then washed
with brine (100 mL), dried (Na2SO4) and the solvent evaporated.
Chromatography on silica gel (4:1 hexanes:EtOAc), afforded the
protected alcohol as a colorless oil.
General Procedure 5. Silyl Deprotection. The protected alcohol
(1 equiv) was dissolved in dry THF (25 mL). To this solution was
added tetrabutylammonium fluoride (1 M solution in THF, 2 equiv).
This solution was stirred for 16 h, before being concentrated under
reduced pressure. Chromatography on silica gel (2:1 hexanes:Et2O),
afforded the deprotected alcohol, as a colorless oil.
General Procedure 6. Guanidinylation. The alcohol (1 equiv)
was dissolved in dry THF (10 mL) and added to a solution of 1,3-
bis(tert-butoxycarbonyl)guanidine (2 equiv) and triphenylphosphine
(2 equiv) in dry THF (10 mL) under an atmosphere of argon. This
solution was then cooled to 0 °C and diisopropyl azodicarboxylate (2
equiv) was added. The solution was then stirred at room temperature
for 16 h. The solvent was removed under reduced pressure and the
residue subjected to flash chromatography (4:1 hexanes:EtOAc) to
give the product as a colorless oil.
General Procedure 7. Hydrogenolysis. The alkyne (1 equiv) was
dissolved in EtOAc (10 mL) and 10% Pd(OH)2/C (10% w/w; 50 mg)
catalyst was added. The reaction flask was evacuated and purged with
H2 three times and the solution was stirred under an atmosphere of H2
for 16 h. The solution was then filtered through a pad of Celite and the
solvent removed under reduced pressure. The residue was purified by
flash chromatography (4:1 hexanes:EtOAc). This yielded the desired
alcohol as a colorless oil.
General Procedure 8. Oxidation. Sodium periodate (2.5 equiv)
was added to a solution of the alcohol (1 equiv) in dichloromethane
(2.0 mL), acetonitrile (2.0 mL) and water (3.2 mL). After 5 min
ruthenium trichloride (10 mol %) was added and the mixture stirred
for 2 h. The solvent was removed under reduced pressure and the
residue purified by flash chromatography on silica gel (9:1
CHCl3:MeOH) to afford the acid as a colorless oil.
67.8, 67.3, 62.6, 60.3, 60.2, 53.1, 51.8, 50.0, 41.4, 39.2, 36.9, 28.5, 26.4,
25.9, 23.6, 21.8, 20.1, 18.8, 18.1, 15.8, 11.2, (2 signals obscured or
overlapping); MS (ESI) m/z 1064 [(M + H)+ 100%], HRMS (ESI,
MNa+) Calcd for C58H77N7O12Na, 1086.5522, found 1086.5532; IR
(CH2Cl2) 3311, 2972, 1713, 1651, 1519 cm−1.
Cyclic Depsipeptide (7). The peptide 13 (0.18 g, 0.16 mmol) was
dissolved in dry toluene (40 mL) and added dropwise over 16 h to a
solution of triethylamine (33 μL, 0.25 mmol), 2,4,6 trichlorobenzoyl
chloride (39 μL, 0.25 mmol) and DMAP (80 mg, 0.66 mmol) in
toluene (170 mL, final concentration of acid 0.001 M) under an
atmosphere of argon. After the addition, the solution was stirred for 72
h and then the toluene was removed under reduced pressure. The
crude product was purified by flash chromatography (9:1
CHCl3:MeOH) to give 7 as a colorless solid (66 mg, 38%); mp 260
°C (decomp.); [α]20 +15 (c 0.1, MeOH); 1H NMR (400 MHz,
D
DMSO-d6) δ 8.66 (1H, br s), 8.38 (4H, m), 7.68 (1H, br s), 7.35−7.17
(21H, m), 5.24 (1H, dq, J = 6.6, 2.0 Hz), 5.10 (1H, d, J = 12.7 Hz),
5.04 (1H, d, J = 12.7 Hz), 4.70 (1H, m), 4.40 (1H, dd, J = 7.6, 7.6),
4.23 (2H, m), 4.18 (1H, dd, J = 9.0, 2.0 Hz), 4.11 (1H, dd, J = 6.6,
6.6), 3.94 (1H, m), 3.11 (1H, dd, J = 15.0, 6.3 Hz), 2.50 (1H, m), 1.78
(3H, m),1.60−1.55 (2H, m), 1.40 (5H, m), 1.15 (1H, d, J = 6.0 Hz),
1.11 (12H, m, H3), 1.05 (1H, d, J = 6.1 Hz), 0.84−0.78 (12H, m); 13C
NMR (100 MHz, DMSO-d6) δ 172.8, 171.0, 170.4, 170.3, 170.0,
169.6, 168.1, 156.4, 145.2, 137.0, 128.6, 128.3, 128.2, 127.8, 127.4,
126.3, 73.6, 70.2, 69.5, 65.8, 65.8, 59.7, 59.4, 56.4, 50.4, 50.2, 47.8,
42.4, 37.7, 36.9, 28.1, 24.1, 23.9, 22.5, 21.7, 19.0, 17.4, 16.4, 15.4, 11.0;,
(2 signals obscured or overlapping); MS (ESI) m/z 1046 [(M + H)+
100%], HRMS (ESI, MNa+) Calcd for C58H75N7O11Na, 1068.5417,
found 1068.5420; IR (CH2Cl2) 3288, 2973, 1736, 1665, 1635, 1523
cm−1.
1-[(tert-Butyldiphenylsilyl)oxy]pentadec-15-yne (11). Penta-
dec-14-yn-1-ol (16) (1.7 g, 7.6 mmol) was dissolved in DMF (20 mL).
To this solution was added tert-butyldiphenylsilyl chloride (2.3 mL, 9.1
mmol) and imidazole (1.3 g, 19 mmol). The solution was then stirred
for 2 h before being quenched with satd. NH4Cl (100 mL) and
extracted with Et2O (3 × 100 mL). The combined organic fractions
were then washed with brine (100 mL) and dried (Na2SO4). The
solvent was removed under reduced pressure and the residue
chromatographed on silica gel (9:1 hexanes:Et2O), to give 11 as a
1
colorless oil (1.3 g, 83%): H NMR (400 MHz, CDCl3) δ 7.68 (4H,
dd, J = 8.0, 1.7 Hz), 7.44−7.36 (6H, m), 3.66 (2H, t, J = 6.6 Hz), 2.19
(2H, dt, J = 7.1, 2.7 Hz), 1.94 (1H, t, J = 2.7 Hz), 1.60−1.50 (6H, m),
1.41−1.26 (16H, m), 1.06 (9 H, s); 13C NMR (100 MHz, CDCl3) δ
135.6, 134.2, 129.5, 127.6, 84.8, 68.0, 64.0, 32.6, 29.6, 29.5, 29.4, 29.1,
28.8, 28.5, 26.9, 25.8, 19.2, 18.4, (3 signals overlapping or obscured);
MS (APCI) m/z 463 [(M + H)+, 100%], 371 (60), 417 (50) ; IR
(CH2Cl2) 2927, 2855 cm−1.
(R)-1-(Benzyloxy)-19-[(tert-butyldiphenylsilyl)oxy]nonadec-
5-yn-3-ol [(R)-17]. Treatment of alkyne 11 (3.1 g, 6.7 mmol), with
epoxide (S)-10 (0.80 g, 4.5 mmol) according to general procedure 3
gave the alcohol (R)-17 (1.8 g, 63%) as a colorless oil; [α]20D −1.7 (c
Cbz-L-Thr-D-Leu-L-Ile-Val-D-allo-Thr(tert-butyl)-D-Asn(Trt)-D-
Ala-OH (13). The hexapeptide 13 was synthesized following the
general procedures for solid phase peptide synthesis (procedures 1 and
2). The peptide was cleaved from the resin by agitating with (4:1 v/v
CH2Cl2:1,1,1,3,3,3-hexafluoro-2-propanol) (3 × 5 mL) for 20 min.
The solvent was removed under reduced pressure and the residue
azeotropically distilled with toluene (3 × 30 mL). The residue was
then purified by flash chromatography (4:1 chloroform:methanol).
This afforded 13 as an amorphous colorless solid (0.18 g, 82%, based
1
0.39, CHCl3); H NMR (400 MHz, CDCl3) δ 7.68 (4H, dd, J = 8.0,
1.7 Hz), 7.43−7.27 (11H, m), 4.52, (2H, s), 3.92 (1H, m), 3.73−3.62
(2H, m), 3.65 (2H, t, J = 6.4), 2.36 (2H, dt, J = 6.4, 2.5 Hz), 2.14 (2H,
tt, J = 7.4, 2.5 Hz), 1.94−1.78 (2H, m), 1.56 (4H, m), 1.52−1.44 (2H,
m), 1.34 (2H, m), 1.25 (14H, m), 1.04 (9H, s), (OH not observed);
13C NMR (100 MHz, CDCl3) δ 138.0, 135.6, 134.2, 129.5, 128.4,
127.7, 127.7, 127.5, 82.9, 76.1, 73.3, 69.7, 68.6, 64.0, 35.5, 32.6, 29.6(4)
(4C), 29.6(0), 29.4, 29.2, 29.0, 28.9, 27.6, 26.9 (3C), 25.8, 19.2, 18.8;
MS (ESI) m/z 664 [(M + Na)+, 100%]; HRMS (ESI, MNa+) Calcd
for C42H60O3SiNa, 663.4204, found 663.4204; IR (CH2Cl2) 3410,
2925, 2853 cm−1.
on 0.23 mmol scale); mp 167−170 °C (decomp.); [α]20 +21.7 (c
D
1
0.41, MeOH); H NMR (400 MHz, CD3OD) δ 8.63 (1H, s), 8.23
(1H, d, J = 8.3 Hz), 7.95 (1H, d, J = 6.5 Hz), 7.37−7.19 (20H, m),
5.13 (1H, d, J = 12.5 Hz), 5.09 (1H, d, J = 12.5 Hz), 4.66 (1H, dd, J =
8.6, 5.1 Hz), 4.54 (1H, dd, J = 8.3, 4.5 Hz), 4.44 (1H, m), 4.33 (1H, q,
J = 7.3 Hz), 4.24 (1H, dq, J = 6.6, 4.5 Hz), 4.18−4.07 (3H, m), 2.92
(1H, dd, J = 15.7, 5.1 Hz), 2.85 (1H, dd, J = 15.7, 8.3 Hz), 1.88 (1H,
m), 1.60 (5H, m), 1.36 (3H, d, J = 7.3 Hz), 1.19 (3H, d, J = 6.6 Hz),
1.17 (9H, m), 1.03 (3H, d, J = 6.6 Hz), 0.95 (3H, d, J = 6.9 Hz), 0.92−
0.85 (9H, m), (6 NH and OH protons not observed); 13C NMR (100
MHz, CDCl3) δ 176.5, 175.0, 174.9, 173.8, 172.5, 172.1, 171.7, 158.5,
145.9, 138.1, 130.1, 129.5, 129.1, 128.9, 128.7, 127.8, 75.5, 71.8, 68.6,
(S)-1-(Benzyloxy)-19-[(tert-butyldiphenylsilyl)oxy]nonadec-
5-yn-3-ol [(S)-17]. Treatment of alkyne 11 (2.6 g, 5.7 mmol) with
epoxide (S)-10 (0.63 g, 3.5 mmol) according to general procedure 3
afforded the alcohol (S)-17 (1.4 g, 68%) as a colorless oil; [α]20
D
+1.18 (c 1.0, CHCl3). Other characterization data were identical to
those reported for compound (S)-17.
(R)-1-(Benzyloxy)-19-[(tert-butyldiphenylsilyl)oxy]-3-(me-
thoxymethoxy)-pentadec-5-yne [(R)-23]. Treatment of alcohol
4497
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
(R)-17 (1.2 g, 2.1 mmol) according to general procedure 4 gave
26.6, 25.2; MS (ESI) m/z 602 [(M + H)+ 100%], 502 (50); HRMS
(ESI, MH+) Calcd for C32H64N3O7, 602.4739, found 602.4731; IR
(CH2Cl2) 3386, 2926, 2855, 1713, 1641, 1610, 1510 cm−1.
1
alcohol (R)-23 (1.1 g, 86%); [α]20 −15 (c 0.2, CHCl3); H NMR
D
(400 MHz, CDCl3) δ 7.67 (4H, dd, J = 8.0, 1.7), 7.43−7.27 (11H, m),
4.71 (1H, d, J = 6.9 Hz), 4.64 (1H, d, J = 6.9 Hz), 4.50 (2H, s), 3.84
(1H, m), 3.66 (2H, t, J = 6.6 Hz), 3.67−3.54 (2H, m), 3.37 (3H, s),
2.44 (2H, m), 2.14 (2H, tt, J = 7.0, 2.5 Hz), 2.04−1.85 (2H, m), 1.59−
1.44 (4H, m), 1.34 (4H, m), 1.25 (14H, s), 1.05 (9H, s); 13C NMR
(100 MHz, CDCl3) δ 138.5, 135.6, 134.2, 129.5, 128.3, 127.7, 127.7,
127.5, 96.0, 82.3, 76.1, 73.6, 72.9, 66.8, 64.0, 55.5, 34.4, 32.6, 29.7,
29.6(4) (4 C), 29.6(0), 29.4, 29.2, 29.0. 28.9, 26.9 (3 C), 25.8, 19.2,
18.8; MS (ESI) m/z 708 [(M + Na)+ 100%]; HRMS (ESI, MNa+)
Calcd for C44H64O4SiNa, 707.4466, found 707.4455; IR (CH2Cl2)
2927, 2854 cm−1. Anal. Calcd for C44H64O4Si: C, 77.1; H, 9.4. Found:
C, 77.4; H, 9.6.
(S)-19-{[N,N′-Bis(tert-butoxycarbonyl)]guanidino}3-(methox-
ymethoxy)-nonadecan-1-ol [(S)-24]. Treatment of alkyne (S)-19
(0.69 g, 1.0 mmol) according to general procedure 7 gave the desired
alcohol (S)-24 (0.51 g, 85%); [α]20 +15.0 (c 0.40, CHCl3). Other
D
characterization data were identical to those reported for compound
(R)-24.
(R)-19-{[N,N′-Bis(tert-butoxycarbonyl)]guanidino}3-(methox-
ymethylenoxy)-nonadecanoic acid [(R)-8]. Treatment of the
alcohol (R)-24 (0.24 g, 0.4 mmol) according to general procedure 8
1
gave the acid (R)-8 (0.15 g, 60%); [α]20 −2.6 (c 2.0, CHCl3); H
D
NMR (400 MHz, CDCl3) δ 9.17 (3H, s br), 4.66 (2H, dd, J = 13.7, 7.0
Hz), 3.97 (1H, m), 3.86 (2H, t, J = 7.3 Hz), 3.35 (3H, s), 2.58 (1H,
dd, J = 15.7, 7.1 Hz), 2.50 (1H, dd, J = 15.7, 5.4 Hz), 1.66−1.52 (4H,
m), 1.50 (9H, s), 1.47 (9H, s), 1.28−1.24 (26H, m); 13C NMR (100
MHz, CDCl3) δ 176.5, 163.6, 160.5, 155.2, 95.9, 83.4, 78.7, 74.5, 55.5,
44.7, 40.0, 34.7, 29.6(1), 29.6(0), 29.5(3) (7 C), 29.5(1), 29.2, 28.7,
28.3 (3 C), 28.0 (3 C), 26.6, 25.1; MS (ESI) m/z 616 [(M + H)+
100%]; HRMS (ESI, MH+) Calcd for C32H62N3O8, 616.4531, found
616.4534; IR (CH2Cl2) 3379, 2978, 2925, 2854, 1712, 1639, 1610,
1510 cm−1.
(R)-19-(Benzyloxy)-13-(methoxymethoxy)-pentadec-14-yn-
1-ol [(R)-18]. Treatment of (R)-23 (1.1 g, 1.8 mmol) according to
general procedure 5 gave the alcohol (R)-18 (0.68 g, 85%); [α]20
−18.4 (c 1.16, CHCl3); H NMR (400 MHz, CDCl3) δ 7.34−7.26
D
1
(5H, m), 4.71 (1H, d, J = 7.0 Hz), 4.64 (1H, d, J = 7.0 Hz), 4.50 (2H,
s), 3.83 (1H, m), 3.65−3.57 (4H, m), 3.37 (3H, s), 2.43 (2H, m), 2.13
(2H, tt, J = 7.1, 2.5 Hz), 2.04−1.85 (2H, m), 1.60−1.40 (6H, m),
1.40−1.26 (16H, m), (OH not observed), 13C NMR (100 MHz,
CDCl3) δ 138.5, 128.3, 127.6, 127.5, 95.9, 82.3, 76.1, 73.6, 72.9, 66.7,
63.1, 55.5, 34.4, 32.8, 29.5(9), 29.5 (7) (3 C), 29.5(2), 29.4, 29.1, 29.0,
28.9, 25.7, 25.1, 18.8; MS (ESI) m/z 469 [(M + Na)+ 100%], HRMS
(ESI, MNa+) Calcd for C28H46O4Na, 469.3288, found 469.3284; IR
(CH2Cl2) 3410, 2925, 2853 cm−1.
(S)-19-{[N,N′-Bis(tert-butoxycarbonyl)]guanidino}3-(methox-
ymethylenoxy)-nonadecanoic acid [(S)-8]. Treatment of the
alcohol (S)-24 (0.50 g, 0.90 mmol) according to general procedure
8 gave the acid (S)-8 (0.30 g, 54%); [α]20 +1.5 (c 0.40, CHCl3).
D
(S)-1-(Benzyloxy)-19-[(tert-butyldiphenylsilyl)oxy]-3-(me-
thoxymethoxy)-pentadec-5-yne [(S)-23]. Treatment of alcohol
(S)-17 (1.2 g, 2.1 mmol) according to general procedure 4 gave
Other characterization data were identical to those reported for
compound (R)-8.
Circulocin δ [(R)-4]. Cyclic peptide 7 (30 mg, 29 μmol) was
dissolved in 10 mL of dry THF and Pd/C catalyst (10% w/w, 0.1 g)
was added. The reaction flask was evacuated and purged with H2 three
times and the solution was left stirring under an atmosphere of H2 for
24 h. More Pd/C catalyst (10% w/w, 50 mg) was then added and the
solution was left stirring under an atmosphere of H2 for another 24 h.
The solution was then filtered through a pad of Celite and the solvent
removed under reduced pressure. The residue was purified by
preparative RP-HPLC using the standard conditions. The amine 22
was isolated after lyophilization as an amorphous white solid (15 mg,
59%). MS (ESI) m/z 912 [(M + H)+ 100%], HRMS (ESI, MNa+)
Calcd For C50H69N7O9Na, 934.5049, found 934.5064.
A solution of the acid (R)-8 (18.2 mg, 30 μmol), HATU (13.5 mg,
36 μmol) and diisopropylethylamine (10 μL, 59 μmol) in DMF (0.5
mL) was added to a solution of amine 22 (9 mg, 10 μmol) in DMF
(0.5 mL). The mixture was shaken on an orbital shaker (175 rpm) for
16 h. The crude reaction mixture was then purified by RP-HPLC using
the standard conditions. The protected compound 30 was then treated
with a solution of TFA:H2O:CH2Cl2 (90:5:5) for 3 h. The solvent was
removed under reduced pressure and the residue azeotropically
distilled with toluene (3 × 30 mL). The residue was then purified by
semipreparative RP-HPLC (0−50% B over 40 min). This gave [(R)-4]
after lyophilization as a colorless solid (2.3 mg, 24%); mp 217−220 °C
(decomp.); [α]20D +10 (c 0.1, MeOH); 1H NMR (300 MHz, DMSO-
d6) δ 8.38 (1H, br m), 8.37 (1H, br m), 8.09 (1H, d, J = 7.0 Hz), 8.06
(1H, d, J = 8.5 Hz), 7.46 (1H, br m), 7.45 (1H, d, J = 8.0 Hz), 7.42
(1H, br s), 7.35 (1H, d, J = 7.3 Hz), 7.30−6.80 (3H, br m), 7.00 (1H,
br s), 5.30 (1H, dq, J = 6.5, 2.5 Hz), 5.02 (1H, d, J = 5.0 Hz), 4.93
(1H, d, J = 4.0 Hz), 4.66 (1H, dd, J = 7.7, 7.8 Hz), 4.39 (1H, dd, J =
8.5, 2.2 Hz), 4.28 (1H, m), 4.20 (1H, m), 3.97 (1H, m), 3.92 (2H, m),
3.81 (1H, m), 3.07 (1H, dd, J = 7.0, 7.0 Hz), 3.05 (1H, dd, J = 7.0, 7.0
Hz), 2.78 (1H, dd, J = 15, 7.3 Hz), 2.59 (1H, dd, J = 15, 6.2), 2.45
(1H, dd, J = 13.2, 5.4), 2.32 (1H, dd, J = 13.2, 6.4 Hz), 1.77 (1H, m),
1.43 (1H, m), 1.42 (2H, m), 1.40 (1H, m), 1. 39 (2H, m), 1.37 (2H,
m), 1.25 (26H, m), 1.18 (4H, m), 1.12 (3H, m), 1.10 (3H, m), 0.84
(6H, d, J = 6.5 Hz), 0.81 (3H, d, J = 7.0 Hz), 0.79 (3H, t, J = 7.3 Hz);
13C NMR (150 MHz, DMSO-d6) δ 172.7, 172.4, 171.5, 171.4, 170.6,
170.2, 169.7, 167.9, 156.5, 70.3, 67.5, 65.4, 60.1, 56.6, 56.6, 50.3, 49.9,
47.6, 42.7, 42.3, 40.5, 36.4, 36.4, 35.8, 28.9, 28.5, 28.3, 25.9, 25.4, 24.0,
23.9, 22.6, 22.2, 19.3, 17.0, 16.0, 15.3, 10.6; MS (ESI) m/z 968 [(M +
alcohol (S)-23 (1.1 g, 86%); [α]20 +11.5 (c 1.0, CHCl3). Other
D
characterization data were identical to those reported for compound
(R)-23.
(S)-19-(Benzyloxy)-13-(methoxymethoxy)-nonadec-14-yn-1-
ol [(S)-18]. Treatment of (S)-23 (0.88 g, 1.3 mmol) according to
general procedure 5 gave (S)-18 (0.60 g, 98%); [α]20 +15.0 (c 1.4,
D
CHCl3). Other characterization data were identical to those reported
for compound (R)-18.
(R)-1-(Benzyloxy)-19-{[N,N′-bis(tert-butoxycarbonyl)]-
guanidino}-3-(methoxymethoxy)-nonadec-5-yne [(R)-19].
Treatment of alcohol (R)-18 (0.82 g, 1.84 mmol) according to
general procedure 6 gave (R)-19 (0.98 g, 78%); [α]20 −13.2 (c 1.0,
D
1
CHCl3); H NMR (400 MHz, CDCl3) δ 9.31 (2H, s br), 7.34−7.26
(5H), 4.71 (1H, d, J = 6.6 Hz), 4.64 (1H, d, J = 6.6 Hz), 4.50 (2H, s
br), 3.90−3.80 (3H, m), 3.64−3.55 (2H, m), 3.37 (3H, s), 2.43 (2H,
dt, J = 6.0, 2.5 Hz), 2.13 (2H, tt, J = 7.0, 2.2 Hz), 2.03−1.84 (2H, m),
1.58−1.42 (22H, m), 1.36−1.25 (18H, m); 13C NMR (100 MHz,
CDCl3) δ 164.0, 160.8, 155.2, 138.4, 128.3, 127.7, 127.5, 95.9, 83.4,
82.3, 78.6, 76.1, 73.5, 72.9, 66.7, 55.5, 44.7, 34.3, 29.7 (4 C), 29.6, 29.3,
29.2, 29.0, 28.9, 28.8, 28.3 (3 C), 28.0 (3 C), 26.7, 25.1, 18.8; MS
(ESI) m/z 711 [(M + Na)+ 100%], 688 (50); HRMS (ESI, MH+)
Calcd for C39H66N3O7, 688.4895, found 688.4886; IR (CHCl3) 3393,
2923, 2858, 1713, 1645, 1611, 1511 cm−1. Anal. Calcd for
C39H65N3O7: C, 68.1; H, 9.5, N, 6.1. Found: C, 68.1; H, 9.7, N, 6.1.
(S)-1-(Benzyloxy)-19-{[N,N′-bis(tert-butoxycarbonyl)]-
guanidino}-3-(methoxymethoxy)-nonadec-5-yne [(S)-19].
Treatment of alcohol (S)-18 (0.60 g, 1.3 mmol) according to general
procedure 6 gave (S)-19 (0.69 g, 77%); [α]20D +11.0 (c 0.42, CHCl3).
Other characterization data were identical to those reported for
compound (R)-19.
(R)-19-{[N,N′-Bis(tert-butoxycarbonyl)]guanidino}3-(methox-
ymethoxy)-nonadecan-1-ol [(R)-24]. Treatment of alkyne (R)-19
(0.93 g, 1.2 mmol) according to general procedure 7 gave the desired
alcohol (R)-24 (0.52 g, 71%); [α]20D −24.0 (c 0.4, CHCl3); 1H NMR
(400 MHz, CDCl3) δ 9.30 (2H, s br), 4.67 (1H, d, J = 6.9 Hz), 4.63
(1H, d, J = 6.9 Hz), 3.86 (2H, m, CH2), 3.81−3.66 (3H, m), 3.38 (3H,
s), 2.52 (1H, s br), 1.83−1.62 (2H, m), 1.56−1.51 (4H, m), 1.50 (9H,
s), 1.47 (9H, s), 1.27−1.23 (26H, m); 13C NMR (100 MHz, CDCl3) δ
163.9, 160.7, 155.1, 95.8, 83.3, 78.6, 76.4, 59.8, 55.7, 44.6, 36.6, 34.6,
29.8, 29.7 (4 C), 29.6 (4 C), 29.5, 29.2, 28.7, 28.3 (3 C), 28.0 (3 C),
4498
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
H)+ 100%], HRMS (ESI, MH+) Calcd for C47H87N10O11, 967.6550,
found 967.6572.
(R)-1-Benzyloxy-17-(tert-butyldiphenylsiloxy)-3-(methoxy-
methoxy)-heptadec-5-yne [(R)-26]. Treatment of alcohol (R)-25
(0.54 g, 0.88 mmol) according to general procedure 4 gave protected
epi-Circulocin δ [(S)-4]. Cyclic peptide 7 (44 mg, 42 μmol) was
dissolved in 20 mL of dry THF and Pd/C catalyst (10% w/w, 0.10 g)
was added. The reaction flask was evacuated and purged with H2 three
times and the solution was left stirring under an atmosphere of H2 for
24 h. More Pd/C catalyst (10% w/w, 50 mg) was then added and the
solution was left stirring under an atmosphere of H2 for another 24 h.
The solution was then filtered through a pad of Celite and the solvent
removed under reduced pressure. The amine was then used without
further purification
1
alcohol (R)-26 (0.52, 90%): [α]20 −13 (c 0.11, CHCl3); H NMR
D
(500 MHz, CDCl3) δ 7.69 (4H, d, J = 6.5 Hz), 7.43−7.28 (11H, m),
4.72 (1H, d, J = 6.5 Hz), 4.66 (1H, d, J = 6.5 Hz), 4.52 (2H, s), 3.85
(1H, m), 3.67 (2H, t, J = 6.5 Hz), 3.64−3.57 (2H, m), 3.38 (3H, s),
2.45 (2H, m), 2.15 (2H, m), 2.07−1.98 (1H, m), 1.95−1.88 (1H, m),
1.58 (1H, m), 1.49 (2H, m), 1.36−1.32 (5H, m), 1.26 (10H, s), 1.07
(9H, s); 13C NMR (125 MHz, CDCl3) δ 138.6, 135.7, 134.3, 129.6,
128.5, 127.8, 127.8, 127.7, 96.1, 82.4, 76.2, 73.7, 73.1, 66.9, 64.1, 55.6,
34.5, 32.7, 29.74, 29.72, 29.68, 29.5, 29.3, 29.2, 29.1, 27.0, 25.9, 25.3,
19.4, 19.0; MS (APCI) m/z 674 [(M + NH4)+ 100%]; HRMS (APCI,
A solution of the acid (S)-8 (34 mg, 55 μmol), HATU (25 mg, 66
μmol) and diisopropylethylamine (20 μL, 0.11 mmol) in DMF (0.5
mL) was added to a solution of amine 22 (16.8 mg, 18.4 μmol) in
DMF (0.5 mL). The mixture was shaken on an orbital shaker (175
rpm) for 16 h. The crude reaction mixture was then treated with a
solution of TFA:H2O:CH2Cl2 (90:5:5) for 3 h. The solvent was
removed under reduced pressure and the residue azeotropically
distilled with toluene (3 × 30 mL). The residue was then purified by
semipreparative RP-HPLC (0−50% B over 40 min). This gave the
(S)-isomer of circulocin δ (S)-4 after lyophilization as a colorless solid
+
MNH4 ) calc. for C42H64NO4Si 674.4599, found 674.4595; IR (thin
film) 3060, 2930, 2855, 1465, 1455, 1425 cm−1.
(R)-17-(Benzyloxy)-15-(methoxymethoxy)-heptadec-12-yn-
1-ol [(R)-27]. Treatment of protected alcohol (R)-26 (0.35 g, 0.53
mmol) according to general procedure 5 gave alcohol (R)-27 (0.18 g,
1
83%); [α]20 −15 (c 0.1, CHCl3); H NMR (500 MHz, CDCl3) δ
D
7.33 (4H, m), 7.27 (1H, m), 4.71 (1H, d, J = 6.9 Hz), 4.64 (1H, d, J =
6.9 Hz), 4.51 (1H, d, J = 11.9 Hz), 3.83 (1H, m), 3.62 (2H, t, J = 6.6
Hz), 3.61−3.56 (2H, m), 3.36 (3H, s), 2.45−2.41 (2H, m), 2.15−2.11
(2H, m), 2.03−1.95 (1H, m), 1.92−1.85 (1H, m), 1.55 (2H, m),
1.49−1.42 (3H, m), 1.34−1.26 (15H, m).13C NMR (125 MHz,
CDCl3) δ 138.6, 128.5, 127.8, 127.7, 96.1, 82.4, 76.2, 73.7, 73.1, 66.9,
63.2, 55.7, 34.5, 32.9, 29.7, 29.6(5), 29.6(3), 29.5, 29.3, 29.1, 29.0,
25.9, 25.3, 18.9; MS (ESI) m/z 441 [(M + Na)+ 100%]; HRMS (ESI,
MNa+) Calcd for C26H42NaO4 441.2975, found 441.2975; IR (thin
film) 3300, 3060, 2925, 2855, 1455 cm−1.
(1.3 mg, 3.2%); mp 216−219 °C (decomp.); [α]20 +2.0 (c 0.1,
D
MeOH); 1H NMR (300 MHz, DMSO-d6) δ 8.43 (1H, d, J = 7.4 Hz),
8.39 (1H, d, J = 4.2 Hz), 8.07 (1H, d, J = 8.3 Hz), 8.02 (1H, d, J = 7.0
Hz), 7.47 (2H, br m), 7.45 (1H, br s), 7.30−6.80 (3H, br s), 7.28 (1H,
d, J = 8.2), 7.00 (1H, br s), 5.29 (1H, dq, J = 2.0, 6.7 Hz), 4.93 (1H, br
s), 4.81 (1H, d, J = 4.7 Hz), 4.70 (1H, m), 4.36 (1H, dd, J = 8.2, 2.0
Hz), 4.30 (1H, m), 4.13 (1H, t, J = 7.0 Hz), 3.95 (2H, m), 3.91 (1H,
m), 3.79 (1H, m), 3.10 (1H, d, J = 6.6 Hz), 3.05 (1H, d, J = 6.6 Hz),
2.85 (1H, dd, J = 15.0, 6.6 Hz), 2.57 (1H, dd, J = 15.0, 7.2 Hz), 2.43
(1H, dd, J = 13.1, 9.0 Hz), 2.24 (1H, dd, J = 13.1, 3.7 Hz), 1.75 (1H,
m), 1.44 (4H, m), 1.24 (30H, m), 1.19 (4H, d, J = 7.0 Hz), 1.17 (3H,
d, J = 6.6 Hz), 1.09 (3H, d, J = 5.5 Hz), 0.87−0.78 (12H, m); 13C
NMR (100 MHz, DMSO-d6) δ 173.0, 172.3, 172.2, 171.1, 170.4,
170.6, 169.7, 168.1, 156.6, 70.1, 68.4, 65.4, 59.7, 56.9, 56.8, 50.3, 49.7,
47.7, 43.0, 42.2, 40.5, 36.5, 36.4, 35.4, 28.5−29.0 (11 C), 28.2, 25.8,
25.6, 23.7, 23.6, 22.6, 21.9, 18.9, 16.8, 15.8, 15.0, 10.3; MS (ESI) m/z
968 [(M + H)+ 100%], HRMS (ESI, MNH+) Calcd for C47H87N10O11,
967.6550, found 967.6563.
(R)-1-(Benzyloxy)-17-{[N,N′-bis(tert-butoxycarbonyl)]-
guanidino}-3-(methoxymethoxy)-heptadec-5-yne [(R)-28].
Treatment of alcohol (R)-27 (0.17 g, 0.42 mmol) according to
general procedure gave (R)-28 (0.22 g, 81%); [α]20 −13 (c 0.15,
D
CHCl3); 1H NMR (400 MHz, CDCl3) δ 9.36 (1H, br s), 9.28 (1H, br
s), 7.35 (4H, m), 7.28 (1H, m), 4.72 (1H, d, J = 6.9 Hz), 4.65 (1H, d, J
= 6.9 Hz), 4.51 (2H, s), 3.90 (2H, dd, J = 7.6, 7.4 Hz), 3.85 (1H, m),
3.65−3.56 (2H, m), 3.38 (3H, s), 2.45 (2H, m), 2.15 (2H, m), 2.05−
1.97 (1H, m), 1.94−1.86 (1H, m), 1.59−1.43 (4H, m), 1.53 (9H, s)
1.51 (9H, s), 1.40−1.32 (2H, m), 1.28 (12H, s). 13C NMR (100 MHz,
CDCl3) δ 164.1, 160.9, 155.3, 138.6, 128.4, 127.7, 127.6, 96.0, 83.5,
82.4, 78.7, 76.2, 73.6, 73.0, 66.8, 55.6, 44.8, 34.5, 29.7, 29.4, 29.3, 29.1,
29.0, 28.9, 28.8, 28.4, 28.1, 26.8, 25.3, 18.9 (1 signal obscured or
overlapping); MS (ESI) m/z 660 [(M + H)+ 100%]; HRMS (ESI,
MH+) Calcd for C37H62N3O7 660.4582, found 660.4580; IR (thin
film) 3380, 3060, 2975, 2925, 2855, 1710, 1640, 1610, 1505, 1455
cm−1.
1-[(tert-Butyldiphenylsilyl)oxy]tridec-12-yne (12). Tridec-12-
yn-1-ol (21) (1.6 g, 8.2 mmol) was dissolved in DMF (20 mL). To
this solution was added tert-butyldiphenylsilyl chloride (2.5 mL, 9.6
mmol) and imidazole (1.4 g, 20 mmol). The solution was then stirred
for 2 h before being quenched with satd. NH4Cl (100 mL) and
extracted with Et2O (3 × 100 mL). The combined organic fractions
were then washed with brine (100 mL) and dried (Na2SO4). The
solvent was removed under reduced pressure and the residue
chromatographed on silica gel (9:1 hexanes:Et2O), to give 12 as a
(R)-17-{[N,N′-Bis(tert-butoxycarbonyl)]guanidino}3-(methox-
ymethoxy)-heptadecan-1-ol [(R)-29]. Treatment of alkyne (R)-28
(0.11 g, 0.17 mmol) according to general procedure 7 gave the desired
1
1
alcohol (R)-29 (0.079 g, 81%); [α]20 −19 (c 3.2, CHCl3); H NMR
colorless oil (3.6 g, quant). H NMR (300 MHz, CDCl3) δ 7.70 (d,
D
4H, J = 7.5 Hz), 7.46−7.37 (m, 6H), 3.68 (t, 2H, J = 6.5 Hz), 2.20 (dt,
2H, J = 6.9, 2.5 Hz), 1.95 (t, 1H, J = 2.5 Hz), 1.63−1.50 (m, 4H),
1.49−1.33 (m, 4H), 1.29 (s,10H), 1.08 (s, 9H). 13C NMR (75 MHz,
CDCl3) δ 135.7, 134.3, 129.6, 127.7, 84.9, 68.2, 64.2, 32.8, 29.7, 29.6,
29.5, 29.3, 28.9, 28.7, 25.9, 27.0, 19.4, 18.5.
(500 MHz, CDCl3) δ 9.25 (2H, br s), 4.67 (1H, d, J = 6.6 Hz), 4.63
(1H, d, J = 6.5 Hz), 3.86 (2H, m), 3.77−3.67 (3H, m), 3.38 (3H, s),
2.41 (1H, br s), 1.78 (1H, m), 1.66 (1H, m), 1.59−1.46 (4H, m), 1.49
(9H, s), 1.47 (9H, s), 1.23 (22H, m); 13C NMR (125 MHz, CDCl3) δ
164.1, 160.9, 155.3, 96.0, 83.5, 78.8, 76.6, 60.0, 55.8, 44.8, 36.8, 34.7,
29.9 (4 C), 29.8 (3 C), 29.7, 29.4, 28.9, 28.4, 28.1, 26.8, 25.4; MS
(ESI) m/z 596 [(M + Na)+ 100%]; HRMS (ESI, MNa+) Calcd for
C30H59N3NaO7 596.4245, found 596.4242; IR (thin film) 3380, 2975,
2925, 2855, 1710, 1610, 1510, 1455 cm−1.
(R)-1-(Benzyloxy)-17-[(tert-butyldiphenylsilyl)oxy]heptadec-
5-yn-3-ol [(R)-25]. Treatment of alkyne 12 (0.73 g, 0.17 mmol), with
epoxide (S)-10 (0.24 g, 0.11 mmol) according to general procedure 3
gave the alcohol (R)-25 (0.55 g, 80%); [α]20D −1.0 (c 0.1, CHCl3); 1H
NMR (500 MHz, CDCl3) δ 7.69 (4H, dd, J = 6.5, 1.5 Hz), 7.43−7.28
(11H, m), 4.54 (2H, s), 3.94 (1H, m), 3.76−3.73 (1H, m), 3.67 (3H,
m, 3H), 2.95 (1H, br s), 2.38 (2H, d, J = 4.5 Hz), 2.16 (2H, J = 7.0
Hz), 1.92−1.89 (1H, m), 1.87−1.81 (1H, m), 1.57 (2H, m), 1.49 (2H,
m), 1.36 (4H, m), 1.26 (10H, m), 1.06 (9H, s); 13C NMR (125 MHz,
CDCl3) δ 138.1, 135.7, 134.3, 129.6, 128.6, 127.9, 127.8, 127.7, 83.0,
76.3, 73.4, 69.9, 68.8, 64.1, 35.6, 32.7, 29.7(4), 29.7(0), 29.6(7), 29.5,
29.3, 29.2, 29.1, 27.7, 27.0, 25.9, 19.3, 18.9; MS (ESI) m/z 635 [(M +
Na)+ 100%]; HRMS (ESI, MNa+) Calcd for C40H56O3NaSi 635.3891,
found 635.3884; IR (thin film) 3450, 3075, 3050, 2925, 2855, 1470,
1425 cm−1.
(R)-17-{[N,N′-Bis(tert-butoxycarbonyl)]guanidino}-3-(me-
thoxymethoxy)-heptadecanoic acid ([(R)-9]. Treatment of the
alcohol (R)-29 (0.065 g, 0.11 mmol) according to general procedure 8
1
gave the acid (R)-9 (0.040 g, 60%); [α]20 −2.4 (c 0.43, CHCl3); H
D
NMR (500 MHz, CDCl3) δ 9.30 (2H, br s), 4.70 (2H, d, J = 7.0 Hz),
3.98 (1H, m), 3.86 (2H, dd, J = 7.5, 7.5 Hz), 3.37 (3H, s), 3.00 (1H, br
s), 2.61−2.51 (2H, m), 1.64−1.10 (4H, m), 1.52 (9H, s), 1.49 (9H, s),
1.25 (22H, m); 13C NMR (125 MHz, CDCl3) δ 175.5, 164.1, 160.9,
155.4, 96.1, 83.6, 78.9, 74.8, 55.8, 44.9, 40.1, 34.9, 29.8 (2 C), 29.7 (2
C), 29.7(5), 29.7(4), 29.7(2), 29.6(7), 29.5, 28.9, 28.5, 28.2, 26.9,
25.3; MS (ESI) m/z 588 [(M + H)+ 100%]; HRMS (ESI, MH+) Calcd
4499
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500

The Journal of Organic Chemistry
Article
for C30H58N3O8 588.4218, found 588.4219; IR (thin film) 3380, 2975,
2925, 1790, 1710, 1610, 1510, 1455 cm−1.
(10) Kuroda, J.; Fukai, T.; Konishi, M.; Uno, J.; Kurusu, K.; Nomura,
T. Heterocycles 2000, 53, 1533.
Circulocin γ [(R)-6]. A solution of the acid (R)-9 (9.5 mg, 16
μmol), HATU (7.5 mg, 20 μmol) and diisopropylethylamine (10 μL,
59 μmol) in DMF (0.5 mL) was added to a solution of amine 22 (10
mg, 11 μmol) in DMF (0.5 mL). The mixture was shaken on an
orbital shaker (175 rpm) for 16 h. The crude reaction mixture was
then purified by column chromatography (CH2Cl2/MeOH 99:1) to
give the coupled product 31 (12.0 mg, 72%). The protected
compound 31 was then treated with a solution of TFA:H2O:CH2Cl2
(90:5:5) for 3 h. The solvent was removed under reduced pressure and
the residue azeotropically distilled with toluene (3 × 30 mL). The
residue was then purified by semipreparative RP-HPLC (0−50% B
(11) Cochrane, S. A.; Vederas, J. C. Med. Res. Rev. 2014,
DOI: 10.1002/med.21321.
(12) Li, J.; Jensen, S. E. Chem. Biol. 2008, 15, 118.
(13) Raza, W.; Yang, X.; Wu, H.; Wang, Y.; Xu, Y.; Shen, Q. Eur. J.
Plant. Pathol. 2009, 125, 471.
(14) Bionda, N.; Stawikowski, M.; Stawikowska, R.; Cudic, M.;
́
Lopez-Vallejo, F.; Treitl, D.; Medina-Franco, J.; Cudic, P.
ChemMedChem 2012, 7, 871.
(15) Cochrane, J. R.; McErlean, C. S. P.; Jolliffe, K. A. Org. Lett. 2010,
12, 3394.
(16) Cochrane, J. R.; Yoon, D. H.; McErlean, C. S. P.; Jolliffe, K. A.
Beilstein J. Org. Chem. 2012, 8, 1344.
(17) He, H.; Shen, B.; Korshalla, J.; Carter, G. T. Tetrahedron 2001,
57, 1189.
(18) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989.
(19) Shiina, I.; Kubota, M.; Ibuka, R. Tetrahedron Lett. 2002, 43,
over 40 min). This gave [(R)-6] after lyophilization as a colorless solid
1
(3.5 mg, 31% from 9). [α]20 +8.2 (c 0.08, MeOH); H NMR (500
D
MHz, DMSO) δ 8.50 (1H, br s), 8.39 (1H, m), 8.13 (1H, m), 8.13−
8.08 (1H, br s), 7.49 (1H, d, J = 9.2 Hz), 7.47−7.39 (5H, m), 7.02
(1H, s), 5.30 (1H, m), 4.64 (1H, m), 4.41 (1H, d, J = 8.3 Hz), 4.31
(1H, m), 4.20 (1H, m), 3.95 (3H, m), 3.81 (1H, m), 3.06 (2H, m),
2.76 (1H, dd, J = 14.5, 5.6 Hz), 2.61 (1H, dd, J = 14.5, 6.6 Hz), 2.47
(1H, dd, J = 13.3, 4.7 Hz), 2.31 (1H, dd, J = 13.6, 6.5 Hz), 1.77 (1H,
m), 1.49−1.43 (1H, m), 1.47−1.40 (2H, m), 1.40 (1H, m), 1.38 (4H,
m), 1.32 (2H, m), 1.23 (22H, s), 1.18−1.12 (1H, m), 1.19 (3H, d, J =
7.0 Hz), 1.13 (3H, d, J = 6.2 Hz) 1.10 (3H, d, J = 5.0 Hz), 0.83−0.81
(12H, m); 13C NMR (HMQC-/HMBC-NMR) (500 MHz, DMSO) δ
172.9, 171.6, 171.5, 170.4, 170.1, 169.5, 167.8, 70.3, 67.5, 65.4, 60.1,
56.7, 56.5, 50.3, 50.1, 47.6, 42.6, 42.1, 40.4, 36.5, 35.6, 28.6, 25.8, 28.3,
24.1, 23.9, 22.3, 19.2, 16.7, 15.8, 15.2, 10.6; the guanidinyl carbon
signal could not be observed using the applied parameters; MS (ESI)
m/z 940 [(M + H)+ 100%]; HRMS (ESI, MH+) Calcd for
C45H83N10O11 939.6237, found 939.6240 [M + H+]; IR (thin film)
3310, 2930, 2855, 1670, 1635, 1545, 1450 cm−1.
7535.
(20) Lumbroso, A.; Abermil, N.; Breit, B. Chem. Sci. 2012, 3, 789.
(21) Brown, C. A.; Yamashita, A. J. Am. Chem. Soc. 1975, 97, 891.
(22) Dodd, D. S.; Kozikowski, A. P. Tetrahedron Lett. 1994, 35, 977.
(23) Joo, J. E.; Pham, V. T.; Tian, Y. S.; Chung, Y. S.; Oh, C. Y.; Lee,
K. Y.; Ham, W. H. Org. Biomol. Chem. 2008, 6, 1498.
(24) Ohno, O.; Ikeda, Y.; Sawa, R.; Igarashi, M.; Kinoshita, N.;
Suzuki, Y.; Miyake, K.; Umezawa, K. Chem. Biol. 2004, 11, 1059.
ASSOCIATED CONTENT
* Supporting Information
■
S
Comparison tables listing NMR data for synthesized com-
pounds versus data reported for the isolated natural products;
1H and 13C NMR spectra for all new compounds; HPLC traces
for final products. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kate.jolliffe@sydney.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the University of Sydney for funding and the award
of a postgraduate scholarship to J.R.C.
■
REFERENCES
■
(1) Park, B. J.; Wannemuehler, K. A.; Marston, B. J.; Govender, N.;
Pappas, P. G.; Chiller, T. M. AIDS 2009, 23, 525.
(2) Butts, A.; Krysan, D. J. PLoS Pathog. 2012, 8, e1002870.
(3) Carlet, J.; Jarlier, V.; Harbarth, S.; Voss, A.; Goossens, H.; Pittet,
D. Antimicrob. Resist. Infect. Control 2012, 1, 1.
(4) Rodloff, C.; Koch, D.; Fau-Schaumann, R.; Schaumann, R. Eur. J.
Med. Res. 2011, 16, 187.
(5) Roemer, T.; Xu, D.; Singh, S. B.; Parish, C. A.; Harris, G.; Wang,
H.; Davies, J. E.; Bills, G. F. Chem. Biol. 2011, 18, 148.
(6) Maget-Dana, R.; Peypoux, F. Toxicology 1994, 87, 151.
(7) D’Auria, L.; Deleu, M.; Dufour, S.; Mingeot-Leclercq, M. P.;
Tyteca, D. Biochim. Biophys. Acta, Biomembr. 2013, 1828, 2064.
(8) Peypoux, F.; Bonmatin, J. M.; Wallach, J. Appl. Microbiol.
Biotechnol. 1999, 51, 553.
(9) Kajimura, Y.; Kaneda, M. J. Antibiot. 1996, 49, 129.
4500
DOI: 10.1021/acs.joc.5b00349
J. Org. Chem. 2015, 80, 4491−4500