Synthesis of 3-halo-analogues of HHQ, subsequent cross-coupling and first crystal structure of Pseudomonas quinolone signal (PQS)

Article abstract of DOI:10.1016/j.tetlet.2010.09.013

2-Aryl- and 2-alkyl-quinolin-4-ones and their N-substituted derivatives have several important biological functions such as the Pseudomonas quinolone signal (PQS) molecule participation in quorum sensing. Herein, we report the synthesis of its biological precursor, 2-heptyl-4-hydroxy-quinoline (HHQ) and possible isosteres of PQS; the C-3 Cl, Br and I analogues. N-Methylation of the iodide was also feasible and the usefulness of this compound showcased in Pd-catalysed cross-coupling reactions, thus allowing access to a diverse set of biologically important molecules. The first crystal structure of PQS is also included.

Full text of DOI:10.1016/j.tetlet.2010.09.013

Tetrahedron Letters 51 (2010) 5919–5921
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of 3-halo-analogues of HHQ, subsequent cross-coupling and first
crystal structure of Pseudomonas quinolone signal (PQS)
Gerard P. McGlacken a, , Christina M. McSweeney a, Timothy O’Brien b, Simon E. Lawrence a,
⇑
Curtis J. Elcoate a, F. Jerry Reen c, Fergal O’Gara c
a Department of Chemistry and Analytical and Biological Research Facility, University College Cork, Ireland
b School of Pharmacy, University College Cork, Ireland
c BIOMERIT Research Centre, Department of Microbiology, University College Cork, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2010
Revised 16 August 2010
Accepted 3 September 2010
Available online 15 September 2010
2-Aryl- and 2-alkyl-quinolin-4-ones and their N-substituted derivatives have several important biologi-
cal functions such as the Pseudomonas quinolone signal (PQS) molecule participation in quorum sensing.
Herein, we report the synthesis of its biological precursor, 2-heptyl-4-hydroxy-quinoline (HHQ) and pos-
sible isosteres of PQS; the C-3 Cl, Br and I analogues. N-Methylation of the iodide was also feasible and the
usefulness of this compound showcased in Pd-catalysed cross-coupling reactions, thus allowing access to
a diverse set of biologically important molecules. The first crystal structure of PQS is also included.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
PQS
HHQ
Quorum signaling
Quinolone
Pd-catalysis
Quinolones are best known as broad-spectrum antibacterial
agents,1 for example, fluoroquinolone sales accounted for 18% of
the antibacterial market in 2006.2 An attractive feature of these
molecules is their ability to kill bacteria very rapidly; an ability
not widely attributable to other antibacterial agents. The related
2-aryl and 2-alkylquinolin-4-ones have recently received consider-
able attention due to their more wide ranging pharmacological
applications. For example, 2-arylquinolin-4-one derivatives also
exhibit anti-bacterial3 and anti-tumour properties.4 N-Substituted
2-arylquinoline derivatives can act as anti-malarial agents,
immunostimulants and non-nucleoside HIV-1 inhibitors.5 2-Hep-
tyl-4-hydroxyquinoline N-oxide (HHQNO) is effective against
Staphylococcus aureus.6 2-Heptyl-3-hydroxy-4-quinolone,7 other-
wise known as the Pseudomonas quinolone signal (PQS, Fig. 1),
has emerged as a key regulator of bacterial cooperative behaviour
known as quorum sensing in the antibiotic resistant human path-
ogen Pseudomonas aeruginosa.8 Derived from its biological precur-
sor, 2-heptyl-4-quinolone (HHQ), PQS has a vast and varied array
of biological functions9 influencing iron homeostasis,10 vesicle for-
mation,11 secondary metabolite production and biofilm forma-
tion.12 P. aeruginosa PQS signaling is highly responsive to
environmental and host-specific cues, including Mg2+ and the CF
therapeutic colistin.13 Recent evidence has revealed that PQS is
O
O
OH
6
N
H
N
H
6
HHQ
PQS
Figure 1. HHQ and PQS structures.
capable of modulating immune responses and human T-cell
proliferation.14
Our interest is twofold; firstly, in the synthesis of 3-haloquino-
lin-4-ones as analogues of PQS. These substrates will facilitate
mechanistic studies into PQS signaling in virulent Pseudomonas
populations with important clinical applications. Secondly, to
explore if a new N-methyl version can be used in palladium
cross-coupling reactions, thus providing access to an array of
new biologically important quinolones. Importantly, the 2-heptyl
chain is essential for certain biological functions such as the
stimulation of outer vesicle formation in P. aeruginosa11 and thus
synthetic procedures on compounds bearing this bulky and hydro-
phobic substituent are important. There are no reports of haloge-
nation or subsequent cross-coupling of HHQ. From a synthetic
viewpoint, the presence of the long hydrophobic chain represents
a challenge due to low solubility and the obvious steric hindrance.
⇑
Corresponding author. Fax: +353 021 427 4097.
E-mail address: g.mcglacken@ucc.ie (G.P. McGlacken).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.013

5920
G. P. McGlacken et al. / Tetrahedron Letters 51 (2010) 5919–5921
O
O
O
O
O
O
OH
O
O
O
O
MeOH
Cl
MeO
py, CH2Cl2, rt
1 (99%)
O
2 (93%)
NH2
Ph
O
O
O
O
HN
CHO
6
OH
H2O2
Ph2O
HMTA
MeO
6
EtOH, NaOH
CF3COOH
N
H
N
H
N
H
6
6
3 (60%)
PQS (29%)
>10 g HHQ (33% over 2 steps)
Scheme 1. Synthesis of HHQ and PQS.
Our initial goal was the synthesis of multi-gram quantities of the
useful precursor HHQ and to investigate if HHQ could be haloge-
nated at the 3-position. A modified route was designed. Initially
Meldrum’s acid (0.14 mol) was reacted with octanoyl chloride giv-
ing compound 1 followed by boiling in MeOH affording b-ketoester
2 in excellent yield (Scheme 1).15 Formation of the enamine by
reaction with aniline using Dean–Stark apparatus occurred with
>98% conversion (1H NMR) over 16 h.16 Conrad–Limpach cyclisa-
tion occurred best using a method described by Bangdiwala and
Desai.17 An alternative cyclisation method reported by Woschek
et al. failed to give any product in our hands.18 As quantities of
PQS were also required for biological testing, we carried out our
synthesis based on conditions described by Pesci et al.7
O
O
X
(a), (b) or (c)/(d)
N
H
N
H
6
6
4 X=Cl, 5 X=Br, 6 X=I
O
I
(e)
N
Me
6
7
The Duff formylation reaction proved problematic. In fact no
isolable aldehyde could be obtained using the experimental condi-
tions reported. We found using two equivalents of hexamine
(HMTA) crucial to obtain a decent yield of aldehyde 3. Oxidation
of precursor 3 proceeded with moderate yield to give PQS as de-
scribed.7 For the first time X-ray crystallographic data were ob-
tained for PQS (Fig. 2).19 Interesting dimeric H-bonding indicates
the potential for similar interactions in biological systems.
Chlorination of HHQ occurred smoothly using sodium dichloroi-
socyanurate.20 Bromination also proceeded in reasonable yield
with either pyridinium tribromide (PTB) or Br2. Iodination with I2
in basic THF afforded 6.21 The anticipated low reactivity associated
with the sterically demanding neighbouring alkyl chain never
materialised in these reactions. Furthermore, iodide 6 could be eas-
ily methylated under standard conditions affording 7 in 67% yield
(Scheme 2).22 To our delight, Pd-cross-coupling reactions could be
carried out on 7. Using Pd(PPh3)4 as catalyst a phenyl group could
be introduced at the 3-position.23
Scheme 2. Reagents and conditions: (a) C3Cl2N3NaO3, 2 M NaOH, MeOH, H2O, 59%
(b) PTB, AcOH, 68% (c) Br2, 1 I2 crystal, AcOH, 44% (d) I2, Na2CO3, THF, 48% (e) NaH,
DMF, MeI.
After heating at 130 °C for 30 min, palladium black was seen to
precipitate and the reaction was stopped. The coupled product 8
was isolated in 50% yield.24 An alkenyl group was also introduced
using the Mizoroki–Heck reaction, iodide 7 and styrene giving al-
kene 9. Two catalytic systems were tried over 16 h at 100 °C using
Pd2(dba)3.dba and Pd(PPh3)4 with NMR analysis indicating conver-
sions of ca. 15% and 30%, respectively. Using Pd(PPh3)4 at 120 °C
only improved the conversion to 60% with an isolated yield of
52% (Scheme 3). No further optimisation was carried out.
In conclusion, we have described the synthesis of >10 g
quantities of HHQ, its halogenation at the 3-position, subsequent
O
O
N
Me
6
I
(a) or (b)
8
9
N
Me
O
6
N
Me
6
Scheme 3. Reagents and conditions: (a) PhB(OH)2, Pd(PPh3)4, DMF, 2 M Na2CO3,
50% (b) styrene, Pd(PPh3)4, NMP, Et3N, 52%.
Figure 2. X-ray crystal structure of PQS depicting dimeric H-bonding.19

G. P. McGlacken et al. / Tetrahedron Letters 51 (2010) 5919–5921
5921
14. (a) Kim, K.; Kim, Y. U.; Koh, B. H.; Hwang, S. S.; Kim, S.-H.; Lépine, F.; Cho, Y.-H.;
Lee, G. R. Immunology 2010, 129, 578; (b) Hooi, D. S. W.; Bycroft, B. W.;
Chhabra, S. R.; Williams, P.; Pritchard, D. I. Infect. Immun. 2004, 72, 6463.
15. Kocien´ ski, P. J.; Pelotier, B.; Pons, J.-M.; Prideaux, H. J. Chem. Soc. 1998, 1373.
16. Lokot, I. P.; Pashkovsky, F. S.; Lakhvich, F. A. Tetrahedron 1999, 55, 4783.
17. Bangdiwala, B. P.; Desai, C. M. J. Indian Chem. Soc. 1953, 30, 655.
18. Woschek, A.; Mahout, M.; Mereiter, K.; Hammerschmidt, F. Synthesis 2007,
1517.
N-methylation and finally Pd-cross coupling of 3-iodo-HHQ. The
crystal structure of the prominent biological agent PQS is also de-
scribed. These compounds and analogues are currently undergoing
full biological evaluation, which will be reported in due course.
Acknowledgements
19. Notable features include a dimeric structure with two moderate strength
hydroxy-carbonyl intermolecular H-bonds with a discrete amino carbonyl H-
bond capping the dimer. A second crystallographically different dimer was also
observed (omitted in diagram for clarity). The data has been deposited at the
CCDC 780780.
The authors thank Science Foundation Ireland (G.P.M., C.M.S.
Grant No. 09/RFP/CHS2353, S.E.L., C.J.E. Grant No. 07/SRC/B1158
and 05/PICA/B802/EC07) for funding, Mary Ellen Buckley for earlier
work on the synthesis of HHQ, J. B. Milbank for helpful discussion
and Johnson-Matthey for the gift of transition metal catalysts.
F.OG. thanks the European Commission (MTKD-CT-2006-042062;
O36314), SFI (SFI 04/BR/B0597; 07/IN.1/B948; 08/RFP/GEN1295;
09/RFP/BMT2350), the Department of Agriculture and Food (DAF
RSF 06 321; DAF RSF 06 377; FIRM 08/RDC/629), Irish Research
Council for Science, Engineering and Technology (05/EDIV/
FP107), the Health Research Board (RP/2006/271; RP/2007/290;
HRA/2009/146), the Environmental Protection Agency (EPA2006-
PhD-S-21; EPA2008-PhD-S-2), the Marine Institute (Beaufort
award) and the Higher Education Authority of Ireland (PRTLI3).
20. Staskun, B. J. Org. Chem. 1988, 53, 5287.
21. Example of halogenation: 3-Iodo-2-heptylquinolin-4(1H)-one: A mixture of
HHQ (0.2 g, 0.823 mmol), iodine (0.418 g, 1.646 mmol) and Na2CO3 (0.131 g,
1.235 mmol) in THF (4 mL) was stirred at rt for 18 h. The mixture was
quenched with Na2S2O3 (0.613 g, 3 equiv) and the precipitate was collected by
filtration before washing with ice-cold H2O (50 mL). Recrystallisation was
carried out (EtOH) affording 6 (146 mg) in 48% yield. Mp: 241–243 °C. IR mmax
(KBr): 3210, 3060, 2923, 2851, 2360 1628, 1578, 1555, 1497, 1473, 1435 cmÀ1
;
1H NMR (400 MHz CD3SOCD3) d: 0.86 (3H, t, J 8.5), 1.27–1.42 (8H, m), 1.68 (2H,
m), 2.91 (2H, t, J 9.9), 7.33–7.38 (1H, m), 7.58 (1H, d, J 10.1), 7.65–7.7 (1H, m),
8.07 (1H, d, J 8.7), 12.08 (1H, br s); 13C NMR (400 MHz CD3SOCD3) d: 13.9, 22.0,
27.9, 28.3, 28.6, 31.1, 38.7, 85.7, 117.8, 120.6, 123.8, 125.5, 131.9, 139.0, 154.6,
173.2. Exact mass calcd for C16H21INO (M+H)+, 370.0668. Found 370.0656.
22. N-Methylation: 2-Heptyl-3-iodo-1-methylquinolin-4(1H)-one:
suspension of 6 (120 mg, 0.446 mmol) in dry DMF (3 mL) was treated with
NaH (60% dispersion, 1.5 equiv) at room temperature under nitrogen
A
stirred
a
atmosphere then stirred at 40 °C for 5 h. The mixture was treated with
iodomethane (1.5 equiv, 69 mg) and stirred for 12 h at 40 °C. The mixture was
quenched with cold H2O. The product was extracted with CHCl3, washed with
brine and dried (MgSO4). The solvent was evaporated and the product was
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1.34 (6H, m), 1.53 (2H, m), 1.68 (2H, m), 3.22 (2H, t, J 7.9), 3.89 (3H, s), 7.36 (1H,
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1498 cmÀ1 1H NMR (400 MHz CDCl3) d: 0.85 (3H, t, J 7.2), 1.19 (8H, m), 1.55
;
(2H, m), 2.63 (2H, t, J 8.2), 3.83 (3H, s), 7.20–7.45 (6H, m), 7.55 (1H, d, J 8.6),
7.65–7.75 (1H, m), 8.5 (1H, dd, J 1.4, 8); 13C NMR (400 MHz CDCl3) d: 14.0, 22.6,
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