Preparation of 8-hydroxyquinoline derivatives as potential antibiotics against Staphylococcus aureus

Article abstract of DOI:10.1016/j.bmcl.2013.10.072

This work describes the preparation of quinoline compounds as possible anti-bacterial agents. The synthesized quinoline derivatives show anti-bacterial activity towards Staphylococcus aureus. It is interesting to observe that the synthetic 5,7-dibromo-2-methylquinolin-8-ol (4) shows a similar minimum inhibitory concentration of 6.25 μg/mL as compared to that of methicillin (3.125 μg/mL) against Staphylococcus aureus.

Full text of DOI:10.1016/j.bmcl.2013.10.072

Bioorganic & Medicinal Chemistry Letters 24 (2014) 367–370
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Preparation of 8-hydroxyquinoline derivatives as potential
antibiotics against Staphylococcus aureus
b
c
d
c
Kim-Hung Lam a, , Roberto Gambari , Kenneth Ka-Ho Lee , Yi-Xin Chen , Stanton Hon-Lung Kok ,
⇑
Raymond Siu-Ming Wong c, Fung-Yi Lau c, Chor-Hing Cheng a, Wai-Yeung Wong e, Zhao-Xiang Bian f,
Albert Sun-Chi Chan e,f, Johnny Cheuk-On Tang a, , Chung-Hin Chui
a,c,f,
⇑
⇑
a State Key Laboratory of Chirosciences, Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong, China
b Centre of Biotechnology, Department of Life Sciences and Biotechnology, The University of Ferrara, Ferrara, Italy
c School of Biomedical Sciences and Department of Medicine and Therapeutics, The Chinese University of Hong Kong, Hong Kong, China
d Department of Chemistry, Guangzhou University, Guangzhou, People’s Republic of China
e Institute of Creativity, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China
f Clinical Division, School of Chinese Medicine, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
This work describes the preparation of quinoline compounds as possible anti-bacterial agents. The syn-
thesized quinoline derivatives show anti-bacterial activity towards Staphylococcus aureus. It is interesting
to observe that the synthetic 5,7-dibromo-2-methylquinolin-8-ol (4) shows a similar minimum inhibi-
Received 30 August 2013
Revised 28 October 2013
Accepted 31 October 2013
Available online 8 November 2013
tory concentration of 6.25
l
g/mL as compared to that of methicillin (3.125
lg/mL) against Staphylococcus
aureus.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Antibacterial agents
Antibiotics
Quinolines
Staphylococcus aureus
There is an increasing search for new antibiotics due to the rise
of antibiotic-resistant microorganisms in society.1–4 Quinine, chlo-
roquine and amodiaquine were well known quinoline antimalari-
als.5 Mitscher and co-workers found out that quaternary
alkaloids isolated from Ptelea trifoliate showed antimicrobial activ-
ity towards Staphylococcus aureus and Mycobacterium tuberculosis.6
Fournet reported that the presence of 2-substituted quinoline
alkaloids, such as 2-propylquinoline and chimanine D, in the plant
Galipea longiflora demonstrated activity against Leishmania ama-
zonensis.7 Heteroaromatic compounds containing quinoline moiety
bearing various biological and pharmacological activities, such as
antituberculosis,8 antimalarial,9 anti-inflammatory,10 antican-
cer,11–15 antibiotic,16 antihypertensive,17 antiHIV18,19 as HIV
integrase inhibitors (e.g., FZ41),20,21 herbicidal,22 as well as anti-
bacterial23 and antifungal properties.24,25
Figure 1. Quinoline derivatives for anti-bacterial study.
disease.26 In addition, the reported derivatives also possess biolog-
ical activities on the proliferation of rat mesenchymal stem cells
(rMSCs).27,28 In recent years, we reported the anti-tumor activities
of quinoline type compounds.12–14 In particular, we found out that
8-hydroxyquinoline derivatives are effective for anti-tumor
agents.12–14 Among the selected compounds, 8-hydroxy-2-quino-
linecarbaldehyde showed the best in vitro cytotoxicity against
the human cancer cell lines, including MDA231, T-47D, Hs578t,
SaoS2, K562, SKHep1, Hep3B and normal fibroblast NIH3T3 with
Musiol and co-workers25 reported that the derivatives of quin-
olin-8-ol are important for the antifungal activity. In addition,
quinolin-8-ol derivatives were also reported and studied for the
treatment of neurodegenerative diseases such as Alzheimer’s
MTS50 range of 6.25–12.5 lg/mL.
Here, we report the synthesis and characterization of eight quin-
oline compounds(Fig. 1). Their potential anti-bacterial effect against
Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) were
screened. In addition, their possible cellular cytotoxicity towards
⇑
Corresponding authors.
E-mail addresses: kim.hung.lam@polyu.edu.hk (K.-H. Lam), dr-johnny.tang@
polyu.edu.hk (J.C.-O. Tang), chchui@graduate.hku.hk (C.-H. Chui).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.10.072

368
K.-H. Lam et al. / Bioorg. Med. Chem. Lett. 24 (2014) 367–370
Table 1
Minimum inhibitory concentration of nine quinolines towards S. aureus and E. coli
MIC (lg/mL)
S. aureus
E. coli
2a
50
50
>50
N
CHO
CHO
OH
2b
>50
>50
>50
N+
H
Cl-
OH
3a
>50
N
OCH3
Scheme 1. Reagents: (a) SeO2, dioxane/H2O, reflux; (b) HCl (g), DCM; (c) RX (X = Cl
or Br), K2CO3, DMF; (d) Br2 (l), MeOH.
3b
3c
>50
>50
N
OCH2CH3
the human skin keratinocytes HaCaT and normal esophageal cells
NE3 were performed.29–31 Of which, compound 4, (5,7-dibromo-2-
methylquinolin-8-ol) showed an attractive anti-S. aureus activity
with a mild cellular cytotoxicity in vitro.
>50
>50
>50
N
OCH2CH2CH2CH3
Both research groups of Fournet7 and Musiol25 has mentioned
that the 2-substitution on the quinoline moiety is important to ac-
count for the biological activity. As a starting point, we selected
several commercial available 2-substituted quinolines, such as 2-
methylquinoline, 2-phenylquinoline, 2-chloroquinoline, 2-quino-
linecarboxylic acid and 2-quinolinecarbonitrile. However, they
did not exhibit significant activity against S. aureus and E. coli. As
an extension of our previous work, we explore the anti-bacterial
application of our quinoline derivatives. Several commercial avail-
able hydroxyquinoline compounds, such as 4-hydroxyquinoline, 6-
hydroxyquinoline, 7-hydroxyquinoline and 8-hydroxyquinoline
with hydroxyl group at position 4, 6, 7 and 8 were selected for
our initialize study; however, only 8-hydroxylquinoline show
anti-microbial activity. From our previous reported work,14 we
demonstrated that the hydroxyl group at 8-position of quinoline
moiety is important for the significant biological activity. In addi-
tion, our screening test found out that the commercial available
8-hydroxyquinaldine (1) is effective for S. aureus and E. coli inhibi-
tion. Therefore, 8-hydroxyquinaldine (1) was employed as a start-
ing chemical for this project. Scheme 1 shows the synthesis of
quinoline derivatives.13,14,32,33 The 8-hydroxy-2-quinolinecarbal-
dehyde (2a) which bears a carboxyaldehyde group on position 2
together with a hydroxyl group on position 8 and its corresponding
salt 2-formyl-8-hydroxyquinolinium chloride (2b) exhibited rela-
tive low inhibitory effect on S. aureus and E. coli. The 8-alkoxy-
substituted quinaldine, such as 8-methoxy-2-methylquinoline
(3a), 8-ethoxy-2-methylquinoline (3b), 8-butoxy-2-methylquino-
line (3c), 8-(2-(piperidin-1-yl)ethoxy)-2-methylquinoline (3d), as
well as the 8-(benzyloxy)-2-methylquinoline (3e) showed a fur-
ther decrease in activity. The results may suggest that the hydroxyl
group is important for anti-bacterial activity. In our earlier find-
ings, we found out that the presence of bromine on the 2,6-dimeth-
oxylpyridine ring core is important to enhance the biological
activity.15 Herein, we will introduced the bromine group to the
quinoline moiety and studied the corresponding anti-bacterial
activity of 5,7-dibromo-2-methylquinolin-8-ol (4).
N
3d
3e
>50
>50
O
N
N
OBn
Br
4
6.25
>50
Br
N
OH
MIC of methicillin towards S. aureus was 3.125
E. coli was 7.5 g/mL. Three independent experiments were performed and similar
results were obtained.
lg/mL while ampicillin towards
l
to be 50
was found to be 6.25
that of methicillin, which was found to be 3.125
remaining quinolines, their MIC values were found to be greater
l
g/mL which were considerably high while that of 4
g/mL. This MIC value was comparable to
g/mL. For the
l
l
than 50 lg/mL. After determining the MIC values, 2a, 2b and 4
were further selected for zone of inhibition assays using S. aureus.35
Figure 2 shows that the zone of inhibition (radius) from 2a was
greater than that of 2b when they were loaded at 50
4, on the other hand, showed a better anti-S. aureus activity. When
4 was loaded at 15 g, the zone of inhibition was comparable to
that of 2 which was at 50 g.
The possible cellular cytotoxicity of compounds 2a, 2b and 4
were tested using the human skin keratinocyte HaCaT and normal
esophageal cells NE3 by the sulforhodamine B assay36 (Fig. 3). After
24 h of incubation, all the three compounds had an average 50%
lg. Compound
l
l
inhibitory concentration between 18 and 22 lg/mL (Table 2).
Here we describe the preparation of quinoline compounds and
their potential anti-bacterial activity were screened using gram po-
sitiveS. aureus andgram negative E. coli. None ofthem hadsignificant
inhibitory effect on E. coli. On the other hand, three of them showed
anti-bacterialeffect againstS. aureus. For2a and2b, weobservedthat
We have determined the minimum inhibitory concentration
(MIC) of these quinolines against both S. aureus and E. coli respec-
tively after 24 h of incubation.34 As shown in Table 1, all the quin-
olines were found to be inactive towards the gram negative E. coli
(MIC >50 lg/mL). When considering the gram positive S. aureus, it
their MIC values were considerably high (50
lg/mL). However, we
is found that 2a, 2b and 4 could inhibit the bacterial growth to a
found that the synthetic 5,7-dibromo-2-methylquinolin-8-ol
different extent. The MIC values of 2a and 2b were determined

K.-H. Lam et al. / Bioorg. Med. Chem. Lett. 24 (2014) 367–370
369
(4) showed a similar minimum inhibitory concentration of 6.25
lg/
mL as compared to that of methicillin(3.125 g/mL) against S. aureus
l
and simultaneously exhibited a mild cytotoxicity towards both hu-
man skin keratinocytes HaCaT and human esophageal NE3 cells with
an average 50% inhibitory concentration between 18 and 22 lg/mL.
Further chemical modification would be our next step to improve
their anti-bacterial activity and simultaneously reduce their cellular
toxicity in order to discover effective quinoline compounds as novel
effective antibiotics.
Acknowledgments
Dr. C.-H. Chui acknowledges the start-up research fund (38-40-
116) from Hong Kong Baptist University. The authors also thank
the financial support from Professor K.K.-H. Lee (Chinese Univer-
sity of Hong Kong), Professor X.-Z. Bian (03-16-176, Hong Kong
Baptist University) and a research grant from Research Grant Com-
mittee, Innovation Technology Commission and Hong Kong Poly-
technic University. Dr. J.C.-O. Tang acknowledges the internal
grant from Hong Kong Polytechnic University (G-YM35) and Dr.
P. Chan who provided the compounds for the biological
experiments.
Figure 2. Zone of inhibition study. (a) 2a at 50
lg/mL; (b) 2b at 50 lg/mL; (c)
methicillin at 5 g/mL and (d) 4 at 15 g/mL. Three independent experiments were
l
l
conducted and representative results are shown. The zone of clearance was
measured as the radius (mean SD from three results) where no bacterial growth
could be found.
References and notes
1. National Nosocomial Infections Surveillance (NNIS) System. Am. J. Infect.
Control 2004, 32, 470.
2. Goossens, H. Chemotherapy 2005, 51, 177.
3. Styers, D.; Sheehan, D. J.; Hogan, P.; Sahm, D. F. Ann. Clin. Microbiol. Antimicrob.
2006, 5, 1.
4. Moellering, R. C.; Graybill, J. R.; McGowan, J. E.; Corey, L. Am. J. Infect. Control
2007, 35, S1.
5. Kaur, K.; Jain, M.; Reddy, R. P.; Jain, R. Eur. J. Med. Chem. 2010, 45, 3245.
6. Mitscher, L. A.; Bathala, M. S.; Clark, G. W.; Beal, J. L. Lioydia 1975, 38, 109.
7. Fournet, A.; Barrios, A. A.; Munoz, V.; Hocquemiller, R.; Cave, A.; Bruneton, J.
Chemotherapy 1993, 37, 859.
8. Lilienkampf, A.; Mao, J.; Wan, B.; Wang, Y.; Franzblau, S. G.; Kozikowski, A. P. J.
Med. Chem. 2009, 52, 2109.
9. Nasveld, P.; Kitchener, S. Trans. R. Soc. Trop. Med. Hyg. 2005, 99, 2.
10. Leatham, P. A.; Bird, H. A.; Wright, V.; Seymour, D.; Gordon, A. Eur. J. Rheumatol.
Inflamm. 1983, 6, 209.
11. Denny, W. A.; Wilson, W. R.; Ware, D. C.; Atwell, G. J.; Milbank, J. B.; Stevenson,
R. J. U.S. Patent 7,064,117, 2006.
12. Chan, A. S. C.; Tang, J. C. O.; Lam, K. H.; Chui, C. H.; Kok, S. H. L.; Chan, S. H.;
Cheung, F.; Ferrara, R. G.; Cheng, C. H. U.S. 2009/0054482, 2009.
13. Tang, J. C. O.; Chan, A. S. C.Lam, K. H.; Chan, S. H. U.S. 2012/0165370A1, 2012.
14. Chan, S. H.; Chui, C. H.; Chan, S. W.; Kok, S. H. L.; Chan, D.; Tsoi, M. Y. T.; Leung,
P. H. M.; Lam, A. K. Y.; Chan, A. S. C.; Lam, K. H.; Tang, J. C. O. ACS Med. Chem.
Lett. 2013, 4, 170.
15. Lam, K. H.; Gambari, R.; Yuen, M. C. W.; Kan, C. W.; Chan, P.; Xu, L. J.; Tang, W.
J.; Chui, C. H.; Cheng, Y. M.; Wong, S. M.; Lau, F. Y.; Tong, S. W.; Chan, K. W.; Lai,
B. S.; Kok, S. H. L.; Cheng, C. H.; Chan, A. S. C.; Tang, J. C. O. Bioorg. Med. Chem.
Lett. 2009, 2266.
16. Mahamoud, A.; Chevalier, J.; Davin-Regli, A.; Barbe, J.; Pages, J.-M. Curr. Drug
Targ. 2006, 7, 843.
17. Muruganantham, N.; Sivakumar, R.; Anbalagan, N.; Gunasekaran, V.; Leonard, J.
T. Biol. Pharm. Bull. 2004, 27, 1683.
18. Maguire, M. P.; Sheets, K. R.; McVety, K.; Spada, A. P.; Zilberstein, A. J. Med.
Chem. 1994, 37, 2129.
19. Wilson, W. D.; Zhao, M.; Patterson, S. E.; Wydra, R. L.; Janda, L.; Strekowski, L.
Med. Chem. Res. 1992, 2, 102.
20. Polanski, J.; Niedbala, H.; Musiol, R.; Podeszwa, B.; Tabak, D.; Palka, A.; Mencel,
A.; Finster, J.; Mouscadet, J. F.; Le Bret, M. Lett. Drug Des. Discov. 2006, 3, 175.
21. Polanski, J.; Niedbala, H.; Musiol, R.; Podeszwa, B.; Tabak, D.; Palka, A.; Mencel,
A.; Mouscadet, J.-F.; Le Bret, M. Lett. Drug Des. Discov. 2007, 4, 99.
22. Musiol, R.; Tabak, D.; Niedbala, D. H.; Podeszwa, B.; Jampilek, J.; Kralova, K.;
Dohnal, J.; Finster, J.; Mencel, A.; Polanski, J. Bioorg. Med. Chem. 2008, 16, 4490.
23. Palit, P.; Paira, P.; Hazra, A.; Banerjee, S.; Das Gupta, A.; Dastidar, S. G.; Mondal,
N. B. Eur. J. Med. Chem. 2009, 44, 845.
Figure 3. Representative cytotoxic effects of 4 and doxorubicin towards human
skin keratinocytes HaCaT (a–c) and human normal esophageal cells NE3 (d–f). (a)
and (d) were untreated control; (b) and (e) were 25
lg/mL of 4; (c) was 4 lg/mL of
doxorubicin and (f) was 2 g/mL of doxorubicin. Certain degree of cell death
l
morphology included cell shrinkage could be observed in (b) and (e) while
significant cell death could be identified in (c) and (f).
Table 2
IC50 (lg/mL) of 2a, 2b and 4 on human skin keratinocytes HaCaT and human
esophageal NE3 cells
24. Musiol, R.; Jampilek, J.; Buchta, V.; Niedbala, H.; Podeszwa, B.; Palka, A.;
Majerz-Maniecka, K.; Oleksyn, B.; Polanski, J. Bioorg. Med. Chem. 2006, 14, 3592.
25. Musiol, R.; Jampilek, J.; Kralova, K.; Richardson, D. R.; Kalinowski, D. S.;
Podeszwa, B.; Finster, J.; Niedbala, H.; Palka, A.; Polanski, J. Bioorg. Med. Chem.
2007, 15, 1280.
2a
2b
4
Doxorubicin
HaCaT
NE3
19.15 1.26
16.06 0.92
22.53 0.97
18.14 1.03
22.47 1.44
18.62 0.84
3.26 0.53
1.89 0.44
26. Barnham, K.; Gautier, E.; Kok, G.; Krippner G. PCT Patent WO/2004/007461,
2004.
27. Zeng, H. P.; Wang, T. T.; Ouyang, X. H.; Zhou, Y. D.; Jing, H. L.; Yuan, G. Z.; Chen,
D. F.; Du, S. H.; Li, H.; Zhou, J. H. Bioorg. Med. Chem. 2006, 14, 5446.
Doxorubicin was used as positive control. Three independent experiments were
conducted. Each experiment was performed in triplicate and their mean values
were calculated. Results are shown as mean SD from the three experiments.

370
K.-H. Lam et al. / Bioorg. Med. Chem. Lett. 24 (2014) 367–370
28. Katarzyna, M.-M.; Musiol, R.; Agnieszka, S.-S.; Dominik, T.; Pawel, M.; Barbara,
J.; Oleksyn, J.; Polanski, J. Bioorg. Med. Chem. 2011, 19, 1606.
29. Lam, P. L.; Lee, K. K. H.; Kok, S. H. L.; Cheng, G. Y. M.; Tao, X. M.; Hau, D. K. P.;
Yuen, M. C. W.; Lam, K. H.; Gambari, R.; Chui, C. H.; Wong, R. S. M. Soft Matter
2012, 8, 5027.
30. Lam, P. L.; Gambari, R.; Yip, J.; Yuen, M. C. W.; Lam, K. H.; Wong, R. S. M.; Wang,
X. W.; Tang, J. C. O.; Kok, S. H. L.; Chui, C. H. Bioorg. Med. Chem. Lett. 2012, 22,
468.
1H, J = 8.5 Hz), 7.79 (s, 1H), 8.26 (d, 1H, J = 8.5 Hz); 13C NMR (125 MHz, CDCl3):
d 25.41, 104.24, 110.65, 124.61, 125.48, 133.30, 136.65, 138.64, 149.77, 159.47;
HRMS (ESI): Calcd for C10H8NOBr2 [M+H]+, 315.8973; found 315.8981; Melting
Point = 126.9–128.5 °C; Yield = 64%.
34. Determination of MICs. The concentration of S. aureus and E. coli were adjusted
to 1 Â 106 cell/mL. The MIC values of synthesized compounds were determined
by the LB broth dilution method. Various concentrations of compounds were
added from a starting concentration of 50 lg/mL (stocks were prepared as
31. Lam, P. L.; Yuen, M. C. W.; Kan, C. W.; Wong, R. S. M.; Cheng, G. Y. M.; Lam, K.
H.; Gambari, R.; Kok, S. H. L.; Chui, C. H. Aust. J. Chem. 2012, 65, 72.
32. Preparation of quinoline derivatives (2–4). All reactions were monitored by thin
layer chromatography (TLC) on Merck aluminum-coated plates of silica gel
60 F 254 and observed under ultra-violet light (k = 254 nm). NMR spectra were
recorded on a Varian Oxford AS 500 MHz Fourier transform spectrometer using
CDCl3 as solvent unless otherwise specified. Residual protic solvents in CDCl3
(dH 7.26 ppm, s; dC 77.7 ppm, t) was used as internal reference for 1H NMR and
13C NMR spectra. Coupling constants were recorded in hertz (Hz) and
multiplicities were recorded as the following abbreviations: s, singlet; br s,
broad singlet; d, doublet; t, triplet; q, quartet; m, multiplet. Mass analyses
were performed utilizing a Micromass Q-Tof-2 spectrometer. As reported in
previous work,14 8-hydroxyl-2-methylquinoline (2a) was prepared by
oxidation of 8-hydroxyquinaldine (1) with selenium dioxide in dioxane/
water mixture under reflux overnight; and its corresponding salt, 2-formyl-8-
hydroxyquinolinium chloride (2b), was then prepared by bubbling the in-situ
generated HCl gas into dichloromethane solution of 8-hydroxyl-2-
methylquinoline (2a). 8-Alkoxyquinoline derivatives (3a–e) were prepared
by simple substitution of 8-hydroxyquinaldine (1) with alkyl halide under
basic condition. 5,7-dibromo-2-methylquinolin-8-ol (4) was synthesized by
dropwisely addition of bromine in methanol solution to 8-hydroxyquinaldine
(1).13,14 Unless otherwise specified, the reaction was generally run at room
temperature and monitored by TLC until the reaction was completed. The
crude products were extracted by using either dichloromethane or ethyl
acetate; and purified by silica gel column chromatography using ethyl acetate/
hexane mixture as eluting solvent.
50 mg/mL in dimethylsulfoxide, DMSO) and they were diluted serially in the
96 wells microtitre plate. Methicillin was used as a positive control for S. aureus
(started at 50
(started at 30
l
g/mL) and ampicillin was used as positive control for E. coli
l
g/mL) while 0.1% DMSO was used as a vehicle control. They
were then incubated at 37 °C for 24 h. The minimum concentrations of
compounds that induced a complete growth inhibition will be recorded as MIC
values.
35. Zone of inhibition study. S. aureus was used to study the anti-bacterial activity of
compounds 2a, 2b and 4 against its grown in culture. S. aureus was diluted
with broth medium and plated on the nutrient agar plate and the holes were
created on the agar using a sterile transfer pipette. The tested compounds and
methicillin (as positive control) were placed in the holes of agar. The plates
were then incubated at 37 °C for 24 h and the inhibition zones (mm, in terms of
radius) of bacteria on the agar plates were recorded.
36. Cytotoxicity assay. Human cells including HaCaT skin keratinocytes and normal
esophageal cells NE3 were removed from 75 mL sterile cell culture flasks with
trypsin and neutralized with fetal bovine serum. After washing with phosphate
buffered saline and centrifugation, skin cells were re-suspended in complete
cell culture medium at a concentration of approximately 1 Â 105 cells/mL and
counted manually using a haematocytometer under an inverted microscope.
Human cells seeded in the 96 wells microtitre plates for 24 h were prepared for
the screening of our compounds. Compounds 2a, 2b and 4 with the starting
concentration of 50 lg/mL and 4 lg/mL doxorubicin (as positive control) were
incubated with cells for a further period of 24 h. 0.1% DMSO was used as
vehicle control. The cytotoxicity assessment was conducted by sulforhodamine
B protein staining method. Skin cells were fixed with trichloroacetic acid,
washed with deionized water and stained with sulforhodamine B. Afterwards,
cells were washed again with acetic acid. Photomicrographs of skin
keratinocytes were captured and optical absorptions were then measured at
575 nm after dissolving in Tris-base buffer using a a Perkin Elmer Victor V
multiwell plate reader.
33. Preparation of 5,7-dibromo-2-methylquinolin-8-ol (4). 2-Methyl-8-quinolinol
(1.6 g, 10 mmol) was dissolved in 150 mL MeOH. 1 mL bromine liquid in MeOH
was added into the solution dropwise. When the reaction completed, Na2SO3
was added and the product was extracted with DCM to give a crude product.
The crude product was purified by silica gel column chromatography to give a
pure product as a pink solid. 1H NMR (500 MHz, CDCl3): d 2.75 (s, 3H), 7.39 (d,