Rapid synthesis and antimicrobial activity of novel 4-oxazolidinone heterocycles

Article abstract of DOI:10.1016/j.bmcl.2015.06.003

The synoxazolidinone family of marine natural products bear an unusual 4-oxazolidinone heterocyclic core and promising antimicrobial activity against several strains of pathogenic bacteria. As part of our research program directed at the synthesis and che

Full text of DOI:10.1016/j.bmcl.2015.06.003

Bioorganic & Medicinal Chemistry Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Rapid synthesis and antimicrobial activity of novel 4-oxazolidinone
heterocycles
Nataliia V. Shymanska a, Il Hwan An a, Sebastián Guevara-Zuluaga a,b, Joshua G. Pierce a,
⇑
a Department of Chemistry, North Carolina State University, Raleigh, NC 27695, USA
b Department of Chemistry, University of Puerto Rico, Río Piedras Campus, San Juan, PR 00931, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synoxazolidinone family of marine natural products bear an unusual 4-oxazolidinone heterocyclic
core and promising antimicrobial activity against several strains of pathogenic bacteria. As part of our
research program directed at the synthesis and chemical biology of this family of natural products we
have developed a one-step method for the generation of variously substituted 4-oxazolidinone scaffolds
from readily available materials. These studies revealed the importance of an electron deficient aromatic
ring for antimicrobial activity and serve as the basis for future SAR studies around the 4-oxazolidinone
core.
Received 1 May 2015
Revised 28 May 2015
Accepted 2 June 2015
Available online xxxx
Keywords:
Antibiotic
Heterocycle
Oxazolidinone
Natural products
Ó 2015 Elsevier Ltd. All rights reserved.
The emergence of bacterial pathogens resistant to commonly
employed antibiotics represents a significant threat to human
and animal health.1 Numerous approaches to combat resistant
bacterial infections exist; however, the development of antibiotics
bearing novel scaffolds and mechanisms of action is of significant
interest.2 Natural products have served as a fruitful source of
antimicrobial agents and continue to produce exciting lead
molecules.3 The synoxazolidinone family of marine natural prod-
ucts, recently isolated from Synoicum Pulmonaria (ascidian found
off the Norwegian coast), displays promising antimicrobial activity
as well as a unique 4-oxazolidinone heterocyclic core (Fig. 1).4
4-Oxazolidinone heterocycles have received little attention from
the chemical and medicinal community, likely due to their rarity
in nature and perceived chemical instability.5 From a synthetic
standpoint, methods to prepare 4-oxazolidinones bearing exo-
cyclic alkenes are limited and rely on multi-step protocols that
are intolerant of diverse functional groups, thereby inhibiting rapid
probe generation and structure-activity relationship (SAR)
studies.6
Figure 1. Synoxazolidinone family of marine natural products.
O
O
O
acylation/
cyclization
dehydration/
cyclization
Ph
NH2
Ph
Cl
Ph
O
O
N
R
3
6
O
+
+
R
O
N
C4H9
5
C4H9
C4H9
a) our previous work
Figure 2. Approaches to 4-oxazolidinones developed in our laboratory.
H
H
4
7
b) this work
As part of our program directed toward the discovery of novel
scaffolds with antimicrobial activity we have explored several
approaches for the preparation of 4-oxazolidinone heterocycles
(Fig. 2). During our initial efforts targeted at the natural products
themselves, we developed an imine acylation approach to generate
4-oxazolidinones bearing the chloride and guanidine present in 1,
along with a small set of analogs to define an initial SAR for the
synoxazolidinone family of natural products (Fig. 2a).4d During
these efforts we also optimized an acid-promoted dehydration/
cyclization approach to 4-oxazolidinones that couples
a
phenylpyruvic amide (6) with an aldehyde (7) in one step (Fig. 2b).
Our development of this method and antimicrobial evaluation of
the resulting 4-oxazolidinone products is presented herein.
⇑
Corresponding author.
E-mail address: jgpierce@ncsu.edu (J.G. Pierce).
http://dx.doi.org/10.1016/j.bmcl.2015.06.003
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Shymanska, N. V.; et al. Bioorg. Med. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.bmcl.2015.06.003

2
N. V. Shymanska et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
Table 1
Table 3
Coupling of phenylpyruvic acid
cleavage
8
to generate
9
and subsequent acid promoted
Acid concentration screen for 4-oxazolidinone formation
Entry
Conditionsa
Yield (%)b
NRc
NRc
NRc
NRc
43
1
2
3
4
5
6
7
TFA-DCM (or THF), 1:1, rt
p-TsOH, PhMe, rt
CAN, MeCN-H2O, 9:1, rt
BF3 Et2O or BF3 AcOH, DCM
p-TsOH, PhMe, 85 °C
TFA-THF, 2:1, 70 °C
57
TFA-THF, 1:1, 70 °C
65
a DCC = dicyclohexyl carbodiimide; HOBt = hydroxy benzotriazole; DMBA = 2,4-
dimethoxybenzyl amine; DMB = 2,4-dimethoxybenzyl; TFA = trifluoroacetic acid;
DCM = dichloromethane; THF = tetrahydrofuran; p-TsOH = p-toluene sulfonic acid;
CAN = ceric ammonium nitrate.
b Isolated yields.
a Isolated yields.
c NR = no reaction.
b Incomplete conversion of 6.
c Significant impurities present.
d 0.4 mmol scale.
Box in this table signifies the optimized conditions.
e Slow conversion.
Box in this table signifies the optimized conditions.
Table 2
Substrate scope for amide coupling/DMB cleavage protocol
evaluated 2-step protocols for the preparation of 6 (Table 1).
Coupling of acid 8 with 2,4-dimethoxybenzylamine employing
DCC/HOBt provided the secondary amide 9 in 72% yield. With 9
in hand we conducted acid-promoted cleavage of the DMB group
and found TFA at elevated temperature (70 °C, 1 h) provided
efficient conversion to the targeted primary amide 6 (entry 7,
Table 1).8 Although we found other nucleophilic amines to also
function well in the coupling reaction, this 2-step protocol afforded
the most consistent yields on scale and we were able to apply this
approach to a series of substituted aromatic derivatives bearing
both electron rich and electron poor rings (12a–i, Table 2).
With scalable and rapid access to a wide range of substituted
amides in hand we explored the acid catalyzed dehydration/cycliza-
tion approach to prepare 4-oxazolidinones (Fig. 2b). At the outset of
this work it was unclear whether such reaction conditions would
a Isolated yield.
Table 4
Substrate scope for dehydration/cyclization reaction
To begin our development of the dehydration/cyclization
approach to 5 we first required access to phenylpyruvic amide 6.
Although the synthesis of many
a-keto amides is readily accom-
plished through standard coupling protocols and nitrile hydrolysis,
there are no reports for the preparation of highly enolizable pri-
mary phenylpyruvic amides such as 6 from their corresponding
acid precursors.4d,7 We desired an approach that converted
phenylpyruvic acids to their primary amide derivatives in one
step, and therefore explored the activation of enolizable
acids and coupling with ammonia or ammonia surrogates.
Disappointingly, all attempts to prepare via this strategy
a-keto
6
provided unacceptably low yields due to the reactivity of the
enol tautomer of the activated acid predominating under the
reaction conditions, leading to dimeric and polymeric material
and low yields of desired amide 6 (0–36% yield on 50 mg scale;
<10% yield on gram scale). It became clear that
a more
nucleophilic amine (an easily revealed primary amide synthon)
was required for the desired coupling reaction and therefore we
a Isolated yield.
Please cite this article in press as: Shymanska, N. V.; et al. Bioorg. Med. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.bmcl.2015.06.003

N. V. Shymanska et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
3
Table 5
Compounds 13–22 were screened for their ability to inhibit the
growth of three pathogenic bacterial strains and compared with
natural products 1 and 2 (Table 5).4d The synthesized compounds
were designed to probe the impact of the benzylidene fragment
of the synoxazolidinones on antimicrobial activity. Overall,
electron rich aryl groups (13–15, 21) possessed low antimicrobial
activity against Gram-positive Staphylococcus aureus and MRSA
and no activity at the concentrations tested against Gram-
negative Acinetobacter baumannii. Conversely, electron deficient
aromatic rings (18, 22) were significantly more potent with the
4-CF3 (18) displaying the lowest MIC of the series.
Trifluoromethyl substituted oxazolidinone 18 has subsequently
inspired natural product analogs with improved activity and
simplified synthetic access.4d
Antimicrobial activity of 4-oxazolidinones
In conclusion, we have developed a one step protocol for the
preparation of 4-oxazolidinone heterocycles from
and aldehydes. In the course of this work we have also prepared
primary -keto amides bearing enolizable protons via a 2-step
amine coupling/deprotection approach. These synthetic efforts
have led to a series of 4-oxazolidinone products that allowed for
systematic evaluation of the aromatic ring’s impact in the antimi-
crobial activity of the synoxazolidinones. Further exploration into
the SAR of this family of marine natural products and efforts to
uncover the mechanism of action of our most potent compounds
is underway and will be reported in due course.
a-keto amides
a ATCC 29213.
a
b ATCC 33591.
c ATCC 19606.
Box in this table signifies the most potent analog.
provide 4-oxazolidinones via the desired O-addition or would
instead provide 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones via a
competing C-addition process.9 We selected to focus our efforts on
trifluoroacetic acid for the acid promoter since it was readily remov-
able from our reaction products and proved compatible with the
amide starting materials (Table 3). Screening of acid concentration
and reactant ratios revealed that 2:1 TFA/THF (v/v) and 2 equiv of
aldehyde provided the most efficient conversion, yielding 57% of
13 after purification on silica gel (entry 10, Table 3). We found that
there was an ideal range for the acid/reactant ratio and reactions
that were too dilute or too concentrated were significantly messier,
likely due to the slow initial dehydration reaction and additional
decomposition pathways of the starting material or 4-oxazolidinone
product if the reaction is allowed to proceed for extended periods. It
is important to note that the mass recovery observed in these reac-
tions is high (>80%) and a significant loss of material arises during
the purification process (SiO2).
Acknowledgments
S.G.Z. was a REU student at NC State and financial support from
the Department of Chemistry is acknowledged. JGP thanks North
Carolina State University for start-up funds and the NC State
Department of Chemistry for generous support. We also thank
Prof. Roger Linington and Dr. Weng Ruh Wong (UCSC) for initial
help with antimicrobial assays and helpful discussions. Mass spec-
troscopy data was obtained at the NC State University Mass
Spectroscopy Facility.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.06.
003.
We then applied the acid promoted dehydration/cyclization
reaction to the previously prepared
a-keto amides (12a–i,
Table 2) and hexanal to define the functional group tolerance of
the transformation as well as to prepare a series of compounds
to explore the SAR of the left hand fragment of the synoxazolidi-
nones. Utilizing the optimized conditions (entry 8, Table 3) a num-
ber of electron rich and electron poor arylpyruvic amides were
converted to their corresponding 4-oxazolidinone products in
moderate to good yields (Table 4). In all cases the 4-oxazolidinone
was the only significant product observed in the crude reaction
mixtures and no trace of the C-addition regioisomer was identified.
The acid promoted dehydration/cyclization method is straight-
forward for the preparation of 4-oxazolidiones derived from
unsubstituted aliphatic aldehydes; however, it is not compatible
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with more sterically hindered aldehydes such as a-chloro aldehy-
des, or functionalized aldehydes such as those bearing guanidine
groups. Although these limitations have not allowed this approach
to be extended to the synoxazolidinone family of natural products,
this acid catalyzed method provides an attractive one-step synthe-
sis of analog structures. At the outset of our efforts there was con-
cern regarding the stability of the 4-oxazolidinone heterocycles for
use as chemical probes and medicinal lead structures; however,
the 4-oxazolidinone products have proven bench stable for months
and can be purified by reverse phase chromatography (0.1% TFA).
Further, no significant decomposition was observed in biologically
relevant aqueous buffers for extended periods of time.
Please cite this article in press as: Shymanska, N. V.; et al. Bioorg. Med. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.bmcl.2015.06.003