Asymmetric Total Synthesis of the Naturally Occurring Antibiotic Anthracimycin

Article abstract of DOI:10.1021/acs.orglett.0c01913

The first total synthesis of the potent antibiotic anthracimycin was achieved in 20 steps. The synthesis features an intramolecular Diels-Alder reaction to forge the trans-decalin moiety, and an unprecedented aldol reaction using a complex β-ketoester to provide the tricarbonyl motif. A Stork-Zhao olefination and Grubbs ring closing metathesis delivered the E/Z-diene and forged the macrocycle. The C2 configuration was set with a base-mediated epimerization, providing access to (-)-anthracimycin.

Full text of DOI:10.1021/acs.orglett.0c01913

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Letter
Asymmetric Total Synthesis of the Naturally Occurring Antibiotic
Anthracimycin
Emma K. Davison, Jared L. Freeman, Wanli Zhang, William M. Wuest, Daniel P. Furkert,
and Margaret A. Brimble*
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ABSTRACT: The first total synthesis of the potent antibiotic anthracimycin was achieved in 20
steps. The synthesis features an intramolecular Diels−Alder reaction to forge the trans-decalin
moiety, and an unprecedented aldol reaction using a complex β-ketoester to provide the tricarbonyl
motif. A Stork−Zhao olefination and Grubbs ring closing metathesis delivered the E/Z-diene and
forged the macrocycle. The C2 configuration was set with a base-mediated epimerization, providing
access to (−)-anthracimycin.
ethicillin resistant Staphylococcus aureus (MRSA)
Mcontinues to present high infection and mortality rates
globally.1−3 Vancomycin has been a frontline antibiotic for
treatment of MRSA infections for many years; however,
vancomycin resistant Staphylococcus aureus (VRSA) and
vancomycin-intermediate Staphylococcus aureus (VISA) in-
fections are now relatively common.1,2 Secondary antibiotic
treatments are also demonstrating reduced efficacy due to the
emergence of resistance, meaning that the discovery of
antibiotics capable of tackling MRSA by novel mechanisms
of action is a global health priority.1,2 The lack of effective
treatments, the high rate of resistance, and the diminished
pipeline of novel antibiotics in development make MRSA a
global health issue of the highest order.2,3
In 2013, anthracimycin ((−)-1) was isolated by bioassay-
guided fractionation of the marine sediment derived
Streptomyces sp. CNH365, collected off the coast of Santa
Barbara, USA.4 (−)-Anthracimycin ((−)-1) exhibited potent
in vitro antibacterial activity against several MRSA strains
(MIC 0.03−0.0625 μg/mL) alongside Bacillus anthracis, a
major bioterrorism agent (UM23C1-1, MIC 0.031 μg/mL),
and M. tuberculosis (H37Ra, MIC 1−2 μg/mL).4−6 Anthraci-
mycin ((−)-1) displayed minimal toxicity to human cells (IC50
70 mg/L) and demonstrated protection against MRSA-
induced mortality in mice following a single dose of 1 mg/
kg.5 Anthracimycin has also been found to suppress mTOR
signaling and to be an effective inhibitor of hepatocellular
carcinoma proliferation.7 Anthracimycin ((−)-1) inhibits DNA
and RNA synthesis through a process which does not involve
DNA intercalation, though its exact mechanism of action has
yet to be revealed.5 However, the recent isolation of 2-epi-
anthracimycin alongside (−)-1 revealed that both natural
products displayed cytotoxic activity against Jurkat cell lines
through a mechanism involving G1 phase cell cycle arrest.8
The potent in vitro and in vivo antibacterial activity of (−)-1,
combined with its novel mechanism of action, make
anthracimycin an ideal lead compound for development of
novel antibiotics to combat MRSA.
Anthracimycin ((−)-1) is a 14-membered macrolide, whose
structure was determined using a combination of NMR and X-
ray crystallography.4 Anthracimycin ((−)-1) possesses remark-
able structural similarities to the antimalarial natural product
chlorotonil A (2), which bears an additional C8 methyl group
and C4 gem-dichloride moiety and exhibits the opposite
configuration at all chiral centers that are common to
anthracimycin (Figure 1).9,10 Chlorotonil A (2) was
synthesized in 2008 by Rahn and Kalesse,11 and despite the
structural similarities between natural products (−)-1 and 2,
anthracimycin has not yet succumbed to total synthesis.
Encouraged by our previous successful endo-selective intra-
molecular Diels−Alder (IMDA) route to the trans-decalin
Figure 1. Structures of anthracimycin ((−)-1) and chlorotonil A
(2).4,9
Received: June 7, 2020
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framework of anthracimycin ((−)-1),12 we sought to complete
the first synthesis of (−)-1. Completion of the total synthesis
would also enable access to analogues of (−)-1, thereby
facilitating structure−activity relationship studies of this
important antibiotic.
Scheme 2. Gram Scale IMDA and PMB Deprotection
Sequence
The Wittig−IMDA sequence used in our previously
reported route to trans-decalin fragment 3a presented a
major bottleneck for our intended total synthesis, since 3a/b
was obtained in only 9% yield over three steps from alcohol 4,
and as a 4:1 mixture of diastereomers (3a:3b) (Scheme 1,
Scheme 1. Previously Reported Wittig−IMDA Sequence to
trans-Decalin Fragment 3a12
postulated that this diastereomer results from cycloaddition
of Z-5 proceeding via a similar mechanism to 3a (endo, C2 Si-
face addition) affording cis-fused decalin 3c.13,14 It remains
unclear why the IMDA reaction of Z-5 was not observed
originally. While diastereomers 3a−c were only partially
separable, it was found that following p-methoxybenzyl
(PMB) deprotection of the mixture, desired alcohol 8a could
be separated from diastereomers 8b and 8c, thus providing a
pure sample with the same stereochemical configuration as
anthracimycin ((−)-1).
With gram scale access to alcohol 8a established, attention
was turned to its elaboration to the macrocycle of
anthracimycin ((−)-1). It was envisaged that the tricarbonyl
moiety could be forged using an aldol reaction or Claisen
condensation with β-ketoester 9, while the E,Z-diene motif
could feasibly be installed using a Grubbs ring closing
metathesis (RCM) (Scheme 3). As the C2 methyl group of
(−)-1 was expected to be readily epimerizable due to the
different conformational stabilities of the C2 epimers, the
configuration at this position would be set following
construction of the macrocycle.11 The requisite diene 10 or
11 could be installed by Stille coupling of tributyl(vinyl)tin
with Z-vinyl iodide 12 or 13 which in turn could be obtained
via a Stork−Zhao olefination of aldehyde 14 or 15. Access to
14 or 15 could feasibly be achieved following auxiliary cleavage
and functional group manipulations of alcohol 8a.
red).12 To obtain sufficient material to pursue construction of
the macrocycle, and complete the synthesis of (−)-1,
optimization of the Wittig−IMDA sequence was imperative.
As such, Wittig reaction conditions were screened (Table S1,
Supporting Information (SI)), and while the stereoselectivity
of the reaction could not be improved, it was found that
heating the reaction to reflux in the presence of lithium
chloride provided 5 in 81% yield (6:5, E/Z). Pleasingly, these
conditions were found to be amenable to gram scale [1.7 g
(5.15 mmol) of alcohol 4], yielding 5 in 62% yield over two
steps (1:1, E/Z) (Scheme 1, blue). This material was then
treated with diethylaluminum chloride (as opposed to
dimethylaluminum chloride as used previously, owing to
availability of the reagents), to provide 1.0 g of adduct 3
(61% combined yield) (Scheme 2). This gram-scale IMDA
reaction of 5 yielded a partially separable mixture of three
diastereomers; 3a, 3b, and 3c (7:1:7). Adducts 3a and 3b were
previously characterized and proposed to result from endo
cycloaddition of E-5 to the C2 Si-face and C2 Re-face of the
dienophile moiety of 5, respectively.12
Given that our planned construction of the macrocycle
hinged on the coupling of β-ketoester 9 to a decalin containing
fragment, it was first deemed prudent to explore its
accessibility.
Known β-ketoester 9 was found to be inaccessible via
transesterification of ethyl 2-methylacetoacetate with commer-
cially available (R)-3-buten-2-ol using Gilbert and Kelly’s
procedure.15 It was suspected that the volatility of (R)-3-
In the present work, as an additional observation to our
previously reported study, Z-5 was completely consumed, and
an additional diastereomer, 3c, of the Diels−Alder adduct was
observed. The configuration of this new diastereomer 3c could
not be determined unequivocally by 2D NMR spectroscopy
due to unresolved bridgehead protons. However, it was
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material 18. Meanwhile, treatment of 18 with diisobutylalu-
minum hydride (DIBAL-H) failed to provide direct access to
an aldehyde, instead yielding an intractable mixture of
products. Carboxylic acid and Weinreb amide derivatives
were also inaccessible under numerous conditions; however,
ethanethiolate was pleasingly found to mediate auxiliary
cleavage of 18, providing thioester 19 in good yield.17 Stille
coupling of 19 with tributyl(vinyl)tin proceeded smoothly,
affording diene 20 in excellent yield. Diene 20 was then
converted to methyl ester 10 using silver trifluoroacetate;18
however, treatment of methyl ester 10 with the dianion of 9
(generated either using NaH and nBuLi, or lithium
diisopropylamide) resulted only in returned starting material
10 under a range of conditions.
Scheme 3. Retrosynthetic Analysis of the Macrocycle of
(−)-1
It was conceived that a more electrophilic coupling partner,
such as aldehyde 11, could enable union of these two
fragments. Fukuyama reduction17 of thioester 20 gave rise to a
complex, inseparable mixture, presumably resulting from
undesired reduction of the diene moiety. Treatment of
thioester 20 with DIBAL-H also failed to directly provide
aldehyde 11, although the corresponding alcohol was delivered
as the major product, with trace formation of desired aldehyde
11. This was deemed inconsequential however, since treatment
of the crude mixture with DMP pleasingly afforded desired
aldehyde 11 in 75% yield over two steps. Aldol reaction of
aldehyde 11 with the dianion of 9 resulted in consumption of
the starting materials; however, NMR and TLC analysis of the
crude material were complicated by the presumed presence of
four diastereomeric aldol products. Pleasingly, immediate
treatment of this crude mixture of diastereomers with DMP
afforded tricarbonyl 21 as a 1:1 mixture of C2 epimers, albeit
in moderate yield (30% over two steps). Most examples of
aldol reactions with dienolates in the academic literature
buten-2-ol was impeding smooth transesterification under the
high temperatures required; thus, an alternative method to
obtain 9 was sought. Ethyl 2-methylacetoacetate was therefore
hydrolyzed to the crude carboxylic acid, which was then
directly coupled with (R)-3-buten-2-ol at room temperature to
provide ester 9 in moderate yield (see SI for further details).
With β-ketoester 9 in hand, attention turned to elaboration
of alcohol 8a. Oxidation of 8a with Dess-Martin periodinane
(DMP) afforded aldehyde 16 (Scheme 4). Pleasingly, Stork−
Zhao olefination of 16 with iodomethylphosphonium salt 1716
at −78 °C yielded the desired Z-vinyl iodide 18 in 63% yield.
Direct cleavage of the auxiliary of 18 to an ester or aldehyde
moiety was then attempted, however, the desired methyl ester
could not be obtained using dimethyl carbonate and sodium
methoxide under several conditions, returning only starting
Scheme 4. Synthesis of Anthracimycin ((−)-1) from Alcohol 8a
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employ simple β-ketoesters, such as ethyl 2-methylacetoacetate
and ethyl acetoacetate, or their methyl or tert-butyl ester
counterparts.19−23 To the best of our knowledge, this is the
sole example of an aldol reaction using a β-ketoester with a
more complex C1 substituent in peer-reviewed literature.
While attempts to improve the yield of 21 were disappointing,
sufficient material was nevertheless obtained to attempt the
final metathesis.
Jared L. Freeman − School of Chemical Sciences, University of
Auckland, Auckland 1010, New Zealand; The Maurice Wilkins
Centre for Molecular Biodiscovery, The University of Auckland,
Auckland 1142, New Zealand
Wanli Zhang − Department of Chemistry, Emory University,
Atlanta, Georgia 30322, United States
William M. Wuest − Department of Chemistry, Emory
University, Atlanta, Georgia 30322, United States; Emory
Antibiotic Resistance Center, Emory School of Medicine, Atlanta,
Georgia 30322, United States
Daniel P. Furkert − School of Chemical Sciences, University of
Auckland, Auckland 1010, New Zealand; The Maurice Wilkins
Centre for Molecular Biodiscovery, The University of Auckland,
Auckland 1142, New Zealand; orcid.org/0000-0001-6286-
9105
Treatment of triketone 21 with Grubbs I catalyst in
dichloromethane under reflux resulted in rapid consumption
of the starting material. NMR analysis of the crude material
indicated the presence of anthracimycin ((−)-1) as a 1:1
mixture with its C2 epimer (see SI for details). Pleasingly,
subsequent treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) effected epimerization of 2-epi-1. Additionally, it was
found this could be conducted as a one-pot procedure,
whereby addition of DBU to the metathesis reaction mixture
following consumption of 21 provided anthracimycin ((−)-1)
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01913
1
in 37% yield as a single diastereomer. Comparison of the H
Notes
and 13C NMR data of synthetic (−)-1 showed excellent
agreement with those reported for authentic anthracimycin
((−)-1) (Tables S2 and S3, SI).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors wish to acknowledge funding support from the
Royal Society of New Zealand Te Aparangi for a Rutherford
■
It is worth noting that 2-epi-1 was stable in organic solvents
and to simple aqueous workup under pH neutral conditions.
As Tomoda and co-workers reportedly detected 2-epi-1
following extraction (without altering the pH) of the
Streptomyces sp. culture broth with organic solvents,8 we do
not have any reason to believe 2-epi-1 is an artifact of
accidental epimerization of (−)-1 during the isolation process.
To conclude, the first asymmetric total synthesis of
anthracimycin ((−)-1) was achieved in 20 steps, using an
unprecedented aldol reaction with a complex C1 substituted β-
ketoester to furnish the tricarbonyl fragment, and a one-pot
Grubbs RCM/base-mediated epimerization to forge the
macrocycle and set the C2 configuration. Studies of the
antibacterial activity and mechanism of action of (−)-1 are
ongoing and will be disclosed in due course.
̅
Foundation Postdoctoral Fellowship (E.K.D.), the Kate Edger
Educational Charitable Trust for a Post-Doctoral Research
Award (E.K.D.), and the University of Auckland for the award
of a Doctoral Scholarship (J.L.F.). We also wish to thank the
Maurice Wilkins Centre for Molecular Biodiscovery and the
New Zealand Ministry of Business, Innovation and Employ-
ment (5000449) for financial support. W.M.W. acknowledges
the National Institute of General Medical Sciences
(GM119426) for generous funding.
REFERENCES
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ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01913.
Detailed experimental procedures, full spectroscopic
data for all new compounds and NMR comparison
tables for authentic and synthetic anthracimycin (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Margaret A. Brimble − School of Chemical Sciences and School
of Biological Sciences, University of Auckland, Auckland 1010,
New Zealand; The Maurice Wilkins Centre for Molecular
Biodiscovery, The University of Auckland, Auckland 1142, New
Zealand; orcid.org/0000-0002-7086-4096;
̈
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Ed. 2008, 47, 600−602.
Email: m.brimble@auckland.ac.nz
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̈
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Emma K. Davison − School of Chemical Sciences and School of
Biological Sciences, University of Auckland, Auckland 1010,
New Zealand; The Maurice Wilkins Centre for Molecular
Biodiscovery, The University of Auckland, Auckland 1142, New
Zealand
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2019, 6, 2954−2963.
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