Synthesis and biological evaluation of 1,2-disubstituted 4-quinolone analogues of Pseudonocardia sp. Natural products

Article abstract of DOI:10.3762/bjoc.14.245

A series of analogues of Pseudonocardia sp. natural products were synthesized, which have been reported to possess potent antibacterial activity against Helicobacter pylori and induce growth defects in Escherichia coli and Staphylococcus aureus. Taking inspiration from a methodology used in our total synthesis of natural products, we applied this methodology to access analogues possessing bulky N-substituents, traditionally considered to be challenging scaffolds. Screening of the library provided valuable insights into the structure-activity relationship of the bacterial growth defects, and suggested that selectivity between bacterial species should be attainable. Furthermore, a structurally related series of analogues was observed to inhibit production of the virulence factor pyocyanin in the human pathogen Pseudomonas aeruginosa, which may be a result of their similarity to the Pseudomonas quinolone signal (PQS) quorum sensing autoinducer. This provided new insights regarding the effect of N-substitution in PQS analogues, which has been hitherto underexplored.

Full text of DOI:10.3762/bjoc.14.245

Synthesis and biological evaluation of 1,2-disubstituted
4-quinolone analogues of Pseudonocardia sp.
natural products
Stephen M. Geddis1, Teodora Coroama1, Suzanne Forrest2, James T. Hodgkinson*3,
Martin Welch*2 and David R. Spring*1
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2680–2688.
1Department of Chemistry, University of Cambridge, Lensfield Road,
Cambridge, CB2 1EW, UK, 2Department of Biochemistry, University
of Cambridge, 80 Tennis Road, Cambridge, CB2 1GA, UK and
3Leicester Institute of Structural and Chemical Biology, and
Department of Chemistry, University of Leicester, George Porter
Building, University Road, Leicester, LE1 7RH, UK
doi:10.3762/bjoc.14.245
Received: 31 July 2018
Accepted: 10 October 2018
Published: 19 October 2018
This article is part of the thematic issue "Antibacterials, bacterial small
molecule interactions and quorum sensing".
Email:
James T. Hodgkinson* - jthodgkinson@leicester.ac.uk;
Martin Welch* - mw240@cam.ac.uk; David R. Spring* -
spring@ch.cam.ac.uk
Associate Editor: I. R. Baxendale
© 2018 Geddis et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
antibiotics; cross-coupling; heterocycles; quorum-sensing;
structure–activity relationships
Abstract
A series of analogues of Pseudonocardia sp. natural products were synthesized, which have been reported to possess potent anti-
bacterial activity against Helicobacter pylori and induce growth defects in Escherichia coli and Staphylococcus aureus. Taking
inspiration from a methodology used in our total synthesis of natural products, we applied this methodology to access analogues
possessing bulky N-substituents, traditionally considered to be challenging scaffolds. Screening of the library provided valuable
insights into the structure–activity relationship of the bacterial growth defects, and suggested that selectivity between bacterial
species should be attainable. Furthermore, a structurally related series of analogues was observed to inhibit production of the viru-
lence factor pyocyanin in the human pathogen Pseudomonas aeruginosa, which may be a result of their similarity to the Pseudomo-
nas quinolone signal (PQS) quorum sensing autoinducer. This provided new insights regarding the effect of N-substitution in PQS
analogues, which has been hitherto underexplored.
Introduction
The quinolone core has long been implemented in structures Structural optimisation of quinolones possessing intriguing
possessing formidable activity in a broad range of fields, includ- properties can lead to the discovery of important drug classes,
ing antibiotics, bacterial signalling and iron metabolism [1]. as demonstrated by the fluoroquinolone antibiotics, which were
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inspired by the observation of an antibacterial quinolone side- and biofilm formation when most impactful to the host organ-
product generated during the synthesis of the antimalarial ism [5]. This process is mediated by signalling molecules such
chloroquine [2].
as PQS, and natural product structures 1–8 analogous to PQS
may provide interspecies QS-modulator chemical probes. It has
Given this high potential for the discovery of useful chemical been proposed that such a strategy may perturb bacterial viru-
entities, we have recently been engaged in research regarding a lence and pathogenicity associated with QS, thus conferring a
family of quinolone natural products which are produced by the therapeutic benefit, without applying a selection pressure for
actinomycete Pseudonocardia sp. CL38489, and were first iso- resistance [6]. Whilst recent experiments suggest that resis-
lated by Dekker et al. (1–8, Figure 1) [3]. The authors noted the tance may still emerge, it has been proposed that this develop-
potent antibacterial activity of these compounds against Heli- ment should be limited under certain conditions [7].
cobacter pylori, which is responsible for the generation of nu-
merous digestive disorders [4]. Furthermore, with the presence We wished to investigate the potential of 1–8 to modulate QS in
of a lipophilic chain in the 2-position, there is a structural P. aeruginosa, however, the compounds are available in only
resemblance to the Pseudomonas quinolone signal (PQS), and trace amounts from natural sources [3], and so we embarked on
its biosynthetic precursor 2-heptyl-4(1H)-quinolone (HHQ), the total synthesis of the compounds. We first developed a
which are vital to the cooperative behaviour of the human strategy which constructed natural products 1–4 by uniting the
pathogen Pseudomonas aeruginosa via quorum sensing (QS). quinolone cores with the side chain by means of an sp2–sp3
This is a means by which bacteria alter their phenotype in Suzuki–Miyaura coupling reaction [8]. Whilst these com-
response to changes in population density, regulating virulence pounds unfortunately provided no modulation of PQS quorum
Figure 1: A family of quinolone natural products 1–8, which were first isolated from Pseudonocardia sp. CL38489 by Dekker et al. [3], shown along-
side the structure of the Pseudomonas quinolone signal (PQS), a key autoinducer used in quorum sensing signalling by the human pathogen
Pseudomonas aeruginosa, and 2-heptyl-4(1H)-quinolone (HHQ), the biosynthetic precursor of PQS.
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screening (as determined using a heterologous Escherichia coli products 1–4 in E. coli and S. aureus. Furthermore, exploration
reporter system [9]), an intriguing effect upon the growth of of any effect on QS of the analogues 14 would be valuable, as
E. coli and Staphylococcus aureus was noted, which showed an to our knowledge studies on the SAR of PQS analogues have
extended lag phase in response to the compounds (except 4, not yet thoroughly assessed substitution at the 1-position of the
which was inactive towards E. coli. It should be noted that in quinolone system [13]. This is perhaps as a result of direct
this previous publication, the graphical data for compounds 3 alkylation at this position being very challenging, with low
and 4 was erroneously switched). It is tentatively proposed that yields and poor O- vs N-selectivity being encountered, particu-
this is as a result of disruption of electron transport, as the com- larly with a sterically demanding substituent present in the
pounds bear resemblance to the menaquinones which are used 2-position [14,15].
by bacteria for this purpose [10]. Following on from this, we
recently reported a divergent strategy which granted access to As a measure of modulation of QS in P. aeruginosa, it was
remainder of the natural products 5–8, alongside offering more desired to measure the amount of pyocyanin produced by bacte-
efficient synthesis of 1 and 4 [11]. Allylic alcohols 5 and 6 were rial cultures after treatment with the compounds. This virulence
accessed from a mutual precursor (constructed using methodol- factor is known to be under the regulation of PQS signalling
ogy adapted from that reported by Bernini et al. [12]) using an system, and is capable of disrupting many important biochem-
acid-catalysed transposition, whilst 4, 7 and 8 were derivatised ical processes [16]. This leads to numerous deleterious effects
from 1.
on human cells, including inhibited respiration and ciliary
action [17]. These effects allow pyocyanin to play a critical role
In this current work, we turn to the further elucidation of the bi- in infection; indeed, mutant P. aeruginosa strains which are
ological activity of this class of compounds. In order to gain ad- unable to produce pyocyanin have been shown to be unsuccess-
ditional insight into the associated structure–activity relation- ful in infecting the lungs of mice [18]. Being able to prevent the
ships (SAR), it was desired to generate analogues of the natural production of pyocyanin could therefore be of great therapeutic
products. The chemistry developed towards the allylic alcohols benefit.
5 and 6, outlined in Scheme 1, seemed ideal to this end. A range
of alkynes 10 could undergo Sonogashira coupling with the Results and Discussion
commercially available acid chloride 9. The resultant ynones 11 In the implementation of the strategy outlined in Scheme 1,
could then undergo conjugate addition with primary amines 12, alkynes 10a and 10b were first subjected to Sonogashira cou-
which following metal-catalysed cyclisation would give 1,2- pling with commercially available acid chloride 9 according to
disubstituted quinolones 14.
the previously reported conditions (Scheme 2) [12]. These
alkynes were chosen so as to allow access to valuable SAR data
Upon the successful synthesis of analogues of the form 14, bio- regarding the side chain of the natural products 1–8. Commer-
logical evaluation of these and the natural products 1, 4 and 5–7 cially available alkyne 10a would ultimately lead to a simple
(synthesised during our previous study [11]) would then be saturated side chain of the same length as that observed natu-
possible. In particular, it was desired to further probe the rally, whilst 10b (itself synthesised according to a literature pro-
intriguing growth defects which had been observed for natural cedure [19]) would provide analogues possessing a truncated
Scheme 1: Proposed use of the chemistry developed towards the total synthesis of 5 and 6 for generation of natural product analogues. Modular
coupling of alkynes 10 and amines 12 with commercially available acid chloride 9 was proposed to give 1,2-disubstituted quinolones 14.
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Scheme 2: Sonagashira coupling of alkynes 10a and 10b with commercially available acid chloride 9 to give ynones 11a and 11b.
prenyl-type substituent. In the event, ynone 11a was obtained These ynones were then subjected to a conjugate addition with
with good yield, however, a poorer yield resulted for 11b, an assortment of primary amines 12a–f (Scheme 3). The reac-
which was attributed to difficulties in obtaining its precursor tions proceeded with excellent yield in all cases, with aliphatic
10b with high purity which stemmed from its volatility.
and aromatic moieties well tolerated. Given the high volatility
Scheme 3: Conjugate addition of primary amines 12a–f with ynones 11a and 11b. aFollowing concentration in vacuo, further purification using silica
gel flash chromatography was required.
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of most of the amine starting materials, the products 13 were in amine-type products 14ab and 14bb. Given the likely
general analytically pure following concentration in vacuo of hydrolytic instability of synthetic precursors possessing an en-
the reaction mixture. However, use of higher boiling-point amine moiety, these compounds would likely be highly chal-
amines necessitated purification by flash column chromatogra- lenging to synthesise by other means. However, it is proposed
phy, which may account for the slightly lower yields in these that the involvement of the nitrogen lone-pair in the aromaticity
cases (13ad and 13ae).
of the quinolone system attenuates the susceptibility of 14ab
and 14bb towards hydrolysis.
With the compounds 13 now in hand, their cyclisation to the
desired analogues 14 was explored. However, whilst the condi- Intriguingly, the employment of an excess of DMEDA in the
tions which had proved successful in the total synthesis of Cu-catalysed cyclisation of 13bb generated 14bg as a side-
natural products 5 and 6 proved satisfactory in most cases, some product, which represents another interesting analogue for bio-
optimisation was required for substrates possessing unsaturated logical study (Scheme 5). This may putatively result from the
functionality attached to the amine (Table 1). When the palla- displacement of the allylamine in 13bb by the DMEDA ligand,
dium-catalysed conditions were employed [20], a complex mix- followed by heterocyclisation with concomitant N–>N’ methyl
ture resulted, from which no product could be obtained transfer.
(Table 1, entry 1). Meanwhile, use of base-induced SNAr-type
conditions allowed a small amount of product to be isolated With the library of natural products and analogues in hand, our
(Table 1, entry 2) [21], but copper-catalysed conditions offered attention turned to their biological activity. It was desired to
a higher yield (Table 1, entry 3) [12]. This behaviour stands in further explore the growth defects which had been previously
contrast to that noted for substrates bearing an alkyl substituent noted for natural products 1–4 against E. coli and S. aureus, and
in our previous study, for which these copper-catalysed condi- so these species were grown in the presence of the compounds.
tions resulted in dimerization [11].
The results for E. coli ESS are shown in Figure 2, split into the
natural product series, the series of analogues with a saturated
Following the discovery of this substrate-dependent dichotomy side chain, and the truncated series of analogues. As can be seen
with respect to optimal reaction conditions, the entire library of in Figure 2A, natural product 1 resulted in slowed bacterial
compounds was successfully cyclised. Whilst the yields ranged growth whilst 4 elicited no such effect, consistent with our
from low to moderate (Scheme 4), sufficient quantities were ob- previous observations (although the later recovery in popula-
tained to facilitate biological screening. It appeared that bulkier tion in the presence of 4 was less pronounced in the present
N-substituents (e.g., 14ae) resulted in lower yields than less case) [8]. Meanwhile, 5 appeared to show a moderate growth-
bulky derivatives (e.g., 14bf), underlining the importance of slowing effect, which when compared to the stronger effect pre-
steric factors during cyclisation. Interestingly, the allyl-substi- viously observed for 3, demonstrates that oxidation of the
tuted substrates 13ab and 13bb underwent an isomerisation geranyl side chain is deleterious to the biological effect under
under the reaction conditions, with the double bond moving into investigation. The regioisomeric 6 showed a very small effect,
conjugation with the amine to give inseparable mixtures of en- further showing the lack of tolerance of the effect towards side-
Table 1: Optimisation of conditions for the cyclisation of 13ad to natural product analogue 14ad.
entry
conditions
result
1
2
3
Cs2CO3, Pd2dba3, P(2-furyl)3, toluene, 100 °C, 24 h [20]
KOt-Bu, dioxane, 90 °C, 24 h [21]
complex mixturea
13%b
CuI, DMEDA, NaOt-Bu, DMSO, 80 °C, 2 h [12]
22%b
aAs determined by LCMS and 1H NMR analysis of the crude reaction product. bIsolated yield.
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Scheme 4: Cyclisation of precursors 13 to natural product analogues 14 using palladium- or copper-catalysed conditions. Yields quoted are isolated.
a13ab used as starting material, E/Z ratio = 29:71 based on 1H NMR data. b13bb used as starting material, E/Z ratio = 15:85 based on 1H NMR data.
Scheme 5: Use of an excess of DMEDA in the Cu-catalysed cyclisation of 13bb resulted in the generation of 14bg as a side-product,
alongside 14bb.
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Meanwhile, Figure 3 shows the data for S. aureus 25923, which
is split into the same three series as before. Considering the
natural products (Figure 3A), we see that both 1 and 4 resulted
in a slowing of growth consistent with that reported previously,
although the effect for 1 was less pronounced in the present
case, operating for only the first three hours [8]. Whilst most of
the other natural products appeared to show only slight effects,
moderate activity was observed for 7, which stands in an inter-
esting contrast to the inactivity of this compound against E. coli.
This implies that the structural requirements for optimal activi-
ty differ between the species, a conclusion which is further
bolstered by the results for the saturated series of analogues
shown in Figure 3B. Whilst these compounds were completely
inactive against E. coli, in this case a strong effect was ob-
served, which for 14aa, 14ab and 14ac was comparable to the
Figure 2: Growth of E. coli ESS with time in the presence of 200 μM of
each compound. A) Natural product series. B) Saturated analogue
series. C) Truncated analogue series. Neg = negative control (DMSO
blank), Pos = positive control (gentamicin). Experiments were carried
out in triplicate, with the exception of 7, for which there was insufficient
material for repeats. Error bars refer to standard deviation.
chain oxidation. Finally, neither replacement of the N-Me of 4
with the methylthiomethylene substituent of 7, nor epoxidation
of 4’s side chain to give 8, offered any improvement in the bio-
logical activity. The result for 8 is particularly intriguing, as this
natural product was noted to have the strongest effect upon the
growth of H. pylori in the study by Dekker et al., which may
imply that these compounds are acting through different mecha-
nisms upon each bacterial species. Next, considering Figure 2B
and C, we see that none of the analogues were capable of
affecting the growth of E. coli, further demonstrating the impor-
tance of the geranyl side for biological activity against this
species. This observation is particularly striking for analogue
14bf, which possesses an identical quinolone core structure to
natural product 3, which was previously shown to be active.
Figure 3: Growth of S. aureus 25923 with time in the presence of
200 μM of each compound. A) Natural product series. B) Saturated an-
alogue series. C) Truncated analogue series. Neg = negative control
(DMSO blank), Pos = positive control (gentamicin). Experiments were
carried out in triplicate, with the exception of 7, for which there was
insufficient material for repeats. Error bars refer to standard deviation.
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positive gentamicin control over the timescale of concern. PQS ligand. This observation is highly intriguing, as HHQ
These results show the high efficacy of the saturated side chain (Figure 1), which differs from these analogues only by the lack
against S. aureus, however, considering the data for 14ad and of an N-substituent and a slightly shortened side chain, is
14ae, we can observe that adding bulky aromatic moieties to the known to activate P. aeruginosa QS (although it is incapable of
N-substituent results in reduced activity, with smaller alkyl inducing full pyocyanin expression) [23]. It would therefore
groups in this position instead being optimal. Finally, the trun- appear that this inhibitory activity is likely due to the N-substi-
cated series of analogues appeared to show only small effects tution, an avenue which has been to our knowledge underex-
upon the growth of S. aureus (Figure 3C).
plored in the SAR analysis of PQS and HHQ. In particular, ana-
logues possessing smaller N-substituents (14aa, 14ab and 14ac)
Attention then turned to the ability of the compounds to modu- appeared to elicit a stronger effect than those possessing larger
late P. aeruginosa PA01 QS, as measured by the production of aromatic moieties (14ad and 14ae).
the virulence factor pyocyanin. Bacterial cultures were grown
for eight hours in the presence of each compound, followed by Conclusion
extraction of the pyocyanin under acidic conditions [22]. This We have reported the synthesis of structural analogues of a
was then quantified by measurement of the OD520, which corre- family of 4-quinolone Pseudonocardia sp. natural products,
sponds to absorption by the toxin. The results are shown in which encompassed variation of both the side chain and N-sub-
Figure 4, normalised by the bacterial density as measured by stituent. This represented an extension of the chemistry which
OD600 (no significant effect on the overall growth of the we employed towards the natural products, utilising sequential
bacteria was observed for any of the compounds, see Support- Sonogashira coupling, high-yielding conjugate addition, and
ing Information File 1 for details). The most promising results metal-catalysed cyclisation. A dichotomy in the optimal condi-
were replicated to ensure validity (due to the large amount of tions for the final step was discovered, depending on the nature
chemicals required for the assay, it was not practical to perform of the N-substituent. These analogues were then tested, together
this for every compound). Of the natural products, only 4 with a number of previously synthesised natural products, for
seemed to show attenuation of pyocyanin production relative to their ability to bring about an intriguing “growth-slowing”
the negative control. Meanwhile, whilst the truncated series of effect towards certain species of bacteria. Whilst the presence of
analogues appeared to lack activity, a marked reduction in a geranyl side chain was shown to be vital for strong activity in
pyocyanin production was noted for the compounds possessing E. coli, analogues possessing a saturated side chain exhibited
a linear octanyl side chain. We speculate that this is due to marked inhibition of S. aureus growth. This intriguing result
the similarity to the heptanyl chain present in the native implies that slightly different mechanisms may be at work in
each case, and suggests that it may be possible to attain selec-
tive therapeutic treatment of a specific species. Furthermore, the
saturated series of analogues were demonstrated to inhibit the
production of pyocyanin by P. aeruginosa, a virulence factor
known to be under QS regulation, providing valuable new SAR
insights regarding N-substitution of PQS and HHQ analogues.
Supporting Information
Supporting Information File 1
Experimental procedures and analytical data.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-245-S1.pdf]
Acknowledgements
SF was supported by a BBSRC studentship. DRS acknowl-
Figure 4: OD520 (absorption corresponding to pyocyanin) normalised
by the culture population (measured by OD600) for cultures of
edges support from the Engineering and Physical Sciences
Research Council (EP/P020291/1) and Royal Society (Wolfson
Research Merit Award). Data accessibility: all data supporting
this study are provided as Supporting Information accompa-
nying this paper.
P. aeruginosa PA01 grown in the presence of concentrations of
200 μM of natural products and analogues after 8 hours. WT = wild
type, no treatment added. DMSO = treated with DMSO blank. The ex-
periment for 7 was not performed due to lack of material. The most
promising results were replicated once to ensure validity, as shown by
error bars.
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23.Xiao, G.; Déziel, E.; He, J.; Lépine, F.; Lesic, B.; Castonguay, M.-H.;
Milot, S.; Tampakaki, A. P.; Stachel, S. E.; Rahme, L. G. Mol. Microbiol.
2006, 62, 1689–1699. doi:10.1111/j.1365-2958.2006.05462.x
ORCID® iDs
Suzanne Forrest - http://orcid.org/0000-0002-9605-7962
Martin Welch - http://orcid.org/0000-0003-3646-1733
David R. Spring - http://orcid.org/0000-0001-7355-2824
License and Terms
References
1. Heeb, S.; Fletcher, M. P.; Chhabra, S. R.; Diggle, S. P.; Williams, P.;
Cámara, M. FEMS Microbiol. Rev. 2011, 35, 247–274.
doi:10.1111/j.1574-6976.2010.00247.x
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
2. Bisacchi, G. S. J. Med. Chem. 2015, 58, 4874–4882.
doi:10.1021/jm501881c
3. Dekker, K. A.; Inagaki, T.; Gootz, T. D.; Huang, L. H.; Kojima, Y.;
Kohlbrenner, W. E.; Matsunaga, Y.; McGuirk, P. R.; Nomura, E.;
Sakakibara, T.; Sakemi, S.; Suzuki, Y.; Yamauchi, Y.; Kojima, N.
J. Antibiot. 1998, 51, 145–152. doi:10.7164/antibiotics.51.145
4. Rho, T. C.; Bae, E.-A.; Kim, D.-H.; Oh, W. K.; Kim, B. Y.; Ahn, J. S.;
Lee, H. S. Biol. Pharm. Bull. 1999, 22, 1141–1143.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(https://www.beilstein-journals.org/bjoc)
doi:10.1248/bpb.22.1141
5. Whiteley, M.; Diggle, S. P.; Greenberg, E. P. Nature 2017, 551,
313–320. doi:10.1038/nature24624
The definitive version of this article is the electronic one
which can be found at:
6. Clatworthy, A. E.; Pierson, E.; Hung, D. T. Nat. Chem. Biol. 2007, 3,
541–548. doi:10.1038/nchembio.2007.24
doi:10.3762/bjoc.14.245
7. Allen, R. C.; Popat, R.; Diggle, S. P.; Brown, S. P. Nat. Rev. Microbiol.
2014, 12, 300–308. doi:10.1038/nrmicro3232
8. Salvaggio, F.; Hodgkinson, J. T.; Carro, L.; Geddis, S. M.;
Galloway, W. R. J. D.; Welch, M.; Spring, D. R. Eur. J. Org. Chem.
2016, 434–437. doi:10.1002/ejoc.201501400
9. Cugini, C.; Calfee, M. W.; Farrow, J. M., III; Morales, D. K.;
Pesci, E. C.; Hogan, D. A. Mol. Microbiol. 2007, 65, 896–906.
doi:10.1111/j.1365-2958.2007.05840.x
10.Kurosu, M.; Begari, E. Molecules 2010, 15, 1531–1553.
doi:10.3390/molecules15031531
11.Geddis, S. M.; Carro, L.; Hodgkinson, J. T.; Spring, D. R.
Eur. J. Org. Chem. 2016, 5799–5802. doi:10.1002/ejoc.201601195
12.Bernini, R.; Cacchi, S.; Fabrizi, G.; Sferrazza, A. Synthesis 2009,
1209–1219. doi:10.1055/s-0028-1087990
13.Ó Muimhneacháin, E.; Reen, F. J.; O’Gara, F.; McGlacken, G. P.
Org. Biomol. Chem. 2018, 16, 169–179. doi:10.1039/C7OB02395B
14.Abe, H.; Kawada, M.; Inoue, H.; Ohba, S.-i.; Nomoto, A.; Watanabe, T.;
Shibasaki, M. Org. Lett. 2013, 15, 2124–2127. doi:10.1021/ol400587a
15.Mehra, M. K.; Tantak, M. P.; Arun, V.; Kumar, I.; Kumar, D.
Org. Biomol. Chem. 2017, 15, 4956–4961. doi:10.1039/C7OB00940B
16.Lau, G. W.; Hassett, D. J.; Ran, H.; Kong, F. Trends Mol. Med. 2004,
10, 599–606. doi:10.1016/j.molmed.2004.10.002
17.Sorensen, R. U.; Klinger, J. D. Basic Research and Clinical Aspects of
Pseudomonas Aeruginosa. International Symposium on Pseudomonas
Aeruginosa; 1987; Vol. 39, pp 113–124.
18.Lau, G. W.; Ran, H.; Kong, F.; Hassett, D. J.; Mavrodi, D.
Infect. Immun. 2004, 72, 4275–4278.
doi:10.1128/IAI.72.7.4275-4278.2004
19.Smith, W. N.; Beumel, O. F., Jr. Synthesis 1974, 441–443.
doi:10.1055/s-1974-23341
20.Wolfe, J. P.; Rennels, R. A.; Buchwald, S. L. Tetrahedron 1996, 52,
7525–7546. doi:10.1016/0040-4020(96)00266-9
21.Janni, M.; Arora, S.; Peruncheralathan, S. Org. Biomol. Chem. 2016,
14, 8781–8788. doi:10.1039/C6OB01568A
22.Frank, L. H.; DeMoss, R. D. J. Bacteriol. 1959, 77, 776–782.
2688