Synthesis and in vitro antibacterial activities of 7-(4-alkoxyimino-3-hydroxypiperidin-1-yl)quinolone derivatives

Article abstract of DOI:10.1016/j.cclet.2009.07.011

A series of novel 7-(4-alkoxyimino-3-hydroxypiperidin-1-yl)quinolone derivatives were designed, synthesized and evaluated for in vitro antibacterial activities. Compounds 8f, 8g, 8i and 8j with the potencies similar to or better than those of levofloxacin and IMB against Staphylococcus aureus and Staphylococcus epidermidis, worth further investigation.

Full text of DOI:10.1016/j.cclet.2009.07.011

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 55–58
www.elsevier.com/locate/cclet
Synthesis and in vitro antibacterial activities of
7-(4-alkoxyimino-3-hydroxypiperidin-1-yl)quinolone derivatives
*
Ju Xian Wang, Qiang Guo, Yun Chai, Lian Shun Feng, Hui Yuan Guo, Ming Liang Liu
Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050, China
Received 4 June 2009
Abstract
A series of novel 7-(4-alkoxyimino-3-hydroxypiperidin-1-yl)quinolone derivatives were designed, synthesized and evaluated
for in vitro antibacterial activities. Compounds 8f, 8g, 8i and 8j with the potencies similar to or better than those of levofloxacin and
IMB against Staphylococcus aureus and Staphylococcus epidermidis, worth further investigation.
# 2009 Ming Liang Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Fluoroquinolone; Synthesis; Antibacterial activities
Fluoroquinolone as chemotherapeutic drugs for the treatment of various bacterial infections in both community and
hospital settings have attracted great attention because of their outstanding potency and steady safety [1]. However,
most of the fluoroquinolones currently on the market or under development have only moderate activities against many
Gram-positive cocci including staphylococci and streptococci, which has not only limited their use in infections
caused by these organisms, but is also believed to be one of the reasons for the rapidly developing fluoroquinolone
resistance. Therefore, recent efforts have been directed toward the synthesis of new fluoroquinolones that can provide
improved Gram-positive antibacterial activities, while retaining the good Gram-negative activities of early
fluoroquinolones, such as ciprofloxacin and ofloxacin [2].
Since the discovery of norfloxacin in the early 1980s, most of the research concerning these drugs has been focused
on the basic group at the C-7 position which greatly influences their potency, spectrum, and safety [3]. Generally, 5- or
6-membered nitrogenous heterocycles as side chains such as piperazine, pyrolidine and piperidine ring, have been
proven to be the optimal substituents for chemical modification [4]. Of the three, piperidinyl analogs are the least
studied [5].
Recently, as part of an ongoing program to find potent and broad-spectrum antibacterial agents that display strong
Gram-positive activities, we also have focused on introducing new functional groups to the piperidine ring.
Interestingly, IMB (1, Fig. 1), a new 8-methoxylfluoroquinolone incorporating a 3-amino-4-methoxyimino-piperidine
at C-7 position, shows excellent in vitro and in vivo antibacterial activities [6]. In addition, nadifloxacin (2, Fig. 1)
which incorporates a 4-hydroxypiperidine at C-7 exhibits good antibacterial activities to Gram-positive, Gram-
negative and anaerobic bacteria [7].
* Corresponding author.
E-mail address: lmllyx@yahoo.com.cn (M.L. Liu).
1001-8417/$ – see front matter # 2009 Ming Liang Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.07.011

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J.X. Wang et al. / Chinese Chemical Letters 21 (2010) 55–58
Fig. 1. Chemical structures of IMB and Nadifloxacin.
Inspired by those previous research results [6,7], we considered replacement of the 3-aminopiperidinyl with the
bioisosteric 3-hydroxylpiperidinyl moiety. In this paper, we report the synthesis and in vitro antibacterial activities of a
series of 7-(4-alkoxyimino-3-hydroxypiperidin-1-yl)quinolone derivatives.
Synthetic pathways toward substituted piperidines 6a–c and novel fluoroquinolone derivatives 8a–j are depicted in
Scheme 1. N-Boc-4, 4-dimethoxy-3-hydroxypiperidine (4) was prepared by the oxidation of N-Boc-4-piperidone (3)
with PhI(OAc)2 in the presence of KOH in MeOH [8]. And then compound 4 was subjected to deprotection via
treatment with 6 mol/L HCl, to provide 3-hydroxypiperidin-4-one hydrochloride (5), on which the oxime function
group was introduced via condensation with hydroxylamine, methoxylamine or ethoxylamine to yield 6a, 6b or 6c,
respectively. Finally, the target compounds 8a–j were obtained by SN reaction of boric chelating compounds 7 with 6,
and then hydrolysis of chelating groups [9,10]. In addition, as expected, the oxime geometry of compound 6a has the E
configuration confirmed by single crystal X-ray diffraction analysis [11].
The target compounds 8a–j were evaluated for their in vitro antibacterial activities against representative Gram-
positive and Gram-negative organisms (Table 1) using standard techniques [12]. The minimum inhibitory
concentration (MIC) values were compared with those of levofloxacin (LVFX) and IMB.
The in vitro activities of compounds 8f, 8g, 8i and 8j against Staphylococcus aureus and Staphylococcus
epidermidis were comparable to or better than both LVFX and IMB. In particular, activities of compound 8i (MIC:
0.06 mg/mL) were 4-fold and 8–16-fold times more potent than those of LVFX (MIC: 0.25 mg/mL) and IMB (MIC:
0.5–1 mg/mL) against the two strains, respectively, while the remaining compounds generally showed inferior
activities against all the tested strains.
These data suggest that replacement of the amino group in 7-(4-alkoxyimino-3-amino)-8-methoxy-
fluoroquinolone with a hydroxyl group could markedly improve the activities against staphylococci, but generally
decrease the activities against the other strains. The relative contribution of the substituents at C-7 to antibacterial
activities was as follows: 4-ethyloxime-3-hydroxypiperidine ꢀ 4-methyloxime-3-hydroxypiperidine > 4-oxime-3-
hydroxypiperidine.
Scheme 1. Reagents and conditions: (i) PhI(OAc)2, MeOH, KOH, 0–5 8C, 12 h, 57.9%; (ii) EtOH, 6 mol/L HCl, r.t., 1 h; (iii) RONH2ÁHCl, H2O,
NaHCO3, 25–55 8C, 2 h, 59.1–86.3% (from 4); (iv) a: CH3CN, Et3N, 25–50 8C, 1–14 h; b: 5% NaOH/H2O, 40 8C, 0.5–2 h; c: 2 mol/L HCl, r.t.,
40.2–65.6%.

J.X. Wang et al. / Chinese Chemical Letters 21 (2010) 55–58
57
Table 1
In vitro antibacterial activities of target compounds 8a–j (MIC: mg/mL).
Organism
Compounds
8a
8b
8c
8d
8e
8f
0.125
8g
8h
8i
0.06
8j
0.25
LVFX
IMB
S.a.
S.e.
S.p.
S.py.
E.f.
E.c.
K.p.
P.a.
S.s.
S.f.
1
1
1
1
2
2
1
1
0.5
1
0.125
4
0.25
0.25
2
1
0.125
16
0.125
4
64
128
32
2
0.06
4
0.25
8
0.5
1
8
8
32
16
8
32
32
4
128
128
4
8
4
1
1
1
8
1
1
16
2
16
2
16
2
32
16
2
8
0.5
1
0.5
1
1
0.125
2
2
1
0.5
4
8
1
0.06
0.5
0.25
0.5
1
2
2
1
8
1
1
2
8
2
4
1
1
0.25
2
16
2
8
16
4
32
16
16
16
2
64
4
16
16
2
2
8
1
1
0.25
8
8
4
2
4
4
S.a., Staphylococcus aureus ATCC29213; S.e., Staphylococcus epidermidis ATCC12228; S.p., Streptococcus pneumoniae 97100; S.py., Strepto-
coccus pyogenes 9619; E.f., Enterococcus faecalis ATCC29212; E.c., Escherichia coli ATCC 25922; K.p., Klebsiella pneumoniae 7; P.a.,
Pseudomonas aeruginosa 17; S.s., Shigella sonnei 51592; S.f., Shigella flexneri 06-3.
In conclusion, a set of 4-alkoxyimino-3-hydroxypiperindines were prepared and coupled with a variety of
quinolone nuclei to produce a series of novel fluoroquinolone derivatives. Compounds 8f, 8g, 8i and 8j with the
potencies similar to or better than those of LVFX and IMB against S. aureus and S. epidermidis, worth further
investigation.
Acknowledgment
This work was supported by the IMB Research Foundation (No. IMBF 20060404).
References
[1] D.C. Hooper, J.S. Wolfson, N. Engl. J. Med. 32 (1991) 384.
[2] B.K. Srivastava, M. Solanki, B. Mishra, et al. Bioorg. Med. Chem. Lett. 17 (2007) 1924.
[3] H. Koga, A. Itoh, S. Murayama, et al. J. Med. Chem. 23 (1980) 1358.
[4] J.M.J. Domagala, Antimicrob. Chemother. 33 (1994) 685.
[5] Z. Dang, Y.S. Yang, R.Y. Ji, et al. Bioorg. Med. Chem. Lett. 17 (2007) 4523.
[6] X.Y. Wang, Q. Guo, Y.C. Wang, et al. Acta Pharm. Sin. 43 (2008) 819.
[7] S.N. Ishijima, I. Kurokawa, H.J. Nakaya, Infect. Chemother. 8 (2002) 187.
[8] R.M. Moriarty, H. Hu, S.C. Gupta, Tetrahedron Lett. 22 (1981) 1283.
[9] N. Takagi, H. Fubasami, H. Matukubo, EP 464823 (1992).
[10] Synthesis of 1-cyclopropyl-6-fluoro-7-(4-ethyloximino-3-hydroxypiperidine-1-yl)-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid
(8a): a mixture of 6c (0.24 g, 1.2 mmol), 7 (0.41 g, 1 mmol), triethylamine (2.0 mL) and anhydrous acetonitrile (10 mL) was stirred at
room temperature for 0.5 h, and then concentrated in vacuo. To the residue was added 5% NaOH solution (6.0 mL), the reaction mixture was
stirred at 40 8C for 0.5 h and then adjusted to pH 2 with 2 mol/L HCl. The precipitate was collected by suction to give off-white amorphous
product 8a (0.32 g, 79.2%). mp: 215–218 8C. 1H NMR (400 MHz, CDCl3): d 1.07–1.11 (m, 2H), 1.25–1.31(m, 5H), 2.48–2.55 (m, 1H), 3.11–
3.17 (m, 1H), 3.61–3.78 (m, 3H), 4.10–4.23 (m, 3H), 4.39–4.42 (m, 1H), 4.47–4.53 (m, 1H), 8.09 (d, 1H, J = 12.8 Hz), 8.71 (s, 1H), 14.86 (br,
1H); HRMS-ESI m/z: 405.15742 (calcd. for C19H22FN4O5 [M+H]+). 8b mp: 210–212 8C. 1H NMR (400 MHz, CDCl3): d 1.07–1.10 (2H, m),
1.25–1.32 (2H, m), 2.45–2.52 (1H, m), 3.09–3.15 (1H, m), 3.61–3.77 (3H, m), 3.92 (3H, s), 4.18–4.22 (m, 1H), 4.39–4.42 (m, 1H), 4.49–4.53
(m, 1H), 8.10 (d, 1H, J = 12.8 Hz), 8.95 (s, 1H); HRMS-ESI m/z: 413.12219 (calcd. for C18H19FNaN4O5, [M+Na]+). 8c mp: 234–236 8C. 1H
NMR (400 MHz, CDCl3): d 1.06–1.10 (m, 2H), 1.13–1.21 (m, 2H), 2.53–2.61 (m, 1H), 2.97–3.01 (m, 1H), 3.15–3.16 (m, 1H), 3.62–3.74 (m,
2H), 4.20 (br, 1H), 4.39–4.43 (m, 1H), 4.55–4.59 (m, 1H), 5.42 (1H, s), 8.04 (d, 1H, J = 13.6 Hz), 8.60 (s, 1H), 10.76 (s, 1H), 15.26 (s, 1H).
HRMS-ESI m/z: 377.12944 (calcd. for C17H18FN4O5, [M+H]+). 8d mp: 204–206 8C. 1H NMR (400 MHz, CDCl3): d 2.08–2.25 (m, 1H), 2.99–
3.05 (m, 2H), 3.44–3.48 (m, 1H), 3.87–3.90 (m, 1H), 4.02–4.26 (m, 2H), 5.35–5.39 (m, 1H), 7.34–7.66 (m, 2H), 7.78–7.87 (m, 1H), 8.12 (d, 1H,
J = 13.2 Hz), 10.75 (s, 1H), 15.03 (br, 1H). HRMS-ESI m/z: 449.10888 (calcd. for C20H16F3N4O5, [M+H]+). 8e mp: 210–212 8C. 1H NMR
(400 MHz, CDCl3): d 1.26 (t, 3H, J = 7.2 Hz), 1.56–1.57 (m, 1H), 2.26–2.34 (m, 1H), 2.88–2.94 (m, 2H), 3.39–3.86 (m, 2H), 4.12 (q, 2H,
J = 7.2 Hz), 4.19–4.23 (m, 1H), 7.09–7.39 (m, 3H), 8.16 (d, 1H, J = 12.8 Hz), 8.69 (s, 1H), 14.66(br, 1H). HRMS-ESI m/z: 499.12000 (calcd.
for C22H19F3NaN4O5, [M+Na]+). 8f mp: 215–216 8C. 1H NMR (400 MHz, CDCl3): d 1.29 (t, 3H, J = 7.2 Hz), 1.62–1.63 (m, 3H), 2.44–2.54
(m, 1H), 3.09–3.12 (m, 1H), 3.18–3.46 (m, 4H), 3.74–3.77 (m, 1H), 4.16 (q, 2H, J = 7.2 Hz), 4.33–4.38 (m, 2H), 4.46–4.49 (m, 2H), 7.78 (d,

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J.X. Wang et al. / Chinese Chemical Letters 21 (2010) 55–58
1H, J = 12.8 Hz), 8.63 (s, 1H), 14.81(br, 1H). HRMS-ESI m/z: 420.15453 (calcd. for C20H23FN3O6, [M+H]+). 8g mp: 200–202 8C. 1H NMR
(400 MHz, DMSO-d6): d 1.42–1.46 (m, 3H), 2.32 (br, 1H), 3.40–3.48 (m, 3H), 3.79 (s, 3H), 4.26–4.39 (m, 2H), 4.52–4.59 (m, 2H), 4.86–4.91
(m, 2H), 5.19 (br, 1H), 7.56 (d, 1H, J = 12.8 Hz), 7.98 (s, 1H), 15.22 (br, 1H). HRMS-ESI m/z: 406.14144 (calcd. for C19H21FN3O6, [M+H]+).
8h mp: 176–178 8C. 1H NMR (400 MHz, DMSO-d6): d 1.41–1.44 (m, 3H), 1.92–1.97 (m, 1H), 2.16–2.23 (m, 1H), 2.52–2.54 (m, 1H), 3.45–
3.49 (m, 1H), 3.67–3.69 (m, 1H), 4.01–4.07 (m, 1H), 4.14–4.32 (m, 2H), 4.50–4.54(m, 1H), 4.84–4.87 (m, 1H), 7.53 (d, 1H, J = 12.8 Hz), 8.87
(s, 1H), 15.39 (br, 1H). HRMS-ESI m/z: 392.12581 (calcd. for C18H19FN3O6, [M+H]+). 8i mp: 208–211 8C. 1H NMR (400 MHz, CDCl3): d
0.98–1.02 (m, 2H), 1.20–1.26 (m, 2H), 1.30 (t, 3H, J = 6.8 Hz), 2.37–2.45 (m, 1H), 3.19–3.35 (m, 3H), 3.49–3.56 (m, 1H), 3.76 (s, 3H), 3.87–
3.91 (m, 1H), 4.00–4.03 (m, 1H), 4.17 (q, 2H, J = 6.8 Hz), 4.40–4.43 (m, 1H), 7.91 (d, 1H, J = 11.6 Hz), 8.83 (s, 1H). HRMS-ESI m/z:
434.17274 (calcd. for C21H25FN3O6, [M+H]+). 8j mp: 194–196 8C. 1H NMR (400 MHz, DMSO-d6): d 1.02–1.15 (m, 4H), 2.66–2.86 (m, 2H),
3.45–3.57 (m, 2H), 3.70–3.79 (m, 2H), 3.86 (s, 3H), 4.08–4.17 (m, 2H), 5.21 (br, 1H), 7.79 (d, 1H, J = 12.8 Hz), 8.64 (s, 1H), 15.12 (br, 1H).
HRMS-ESI m/z: 406.14144 (calcd. for C19H21FN3O6, [M+H]+).
[11] X-ray data for 6a: C5H11N2O2Cl from ethanol and ethyl acetate solution, F.W. 165.60, triclinic, 0.20 mm  0.40 mm  0.60 mm, P-1,
3
˚
˚
˚
˚
a = 6.270(1) A, b = 6.738(1) A, c = 9.568(1) A, a = 107.11(1)8, b = 101.06(1)8, g = 96.01(1)8, V = 373.5(2) A , radiation = Mo Ka
˚
(l = 0.71073 A), Z = 2, Rf = 0.0494, Rw = 0.1413.
[12] MICs were determined as described by the NCCLS (see National Committee for Clinical Laboratory Standards. 2001. Performance standards
for antimicrobial susceptibility testing: 11th informational supplement. Vol. 21, M100-S11. National Committee for Clinical Laboratory
Standards, Wayne, PA). The MIC was defined as the lowest concentration of each compound resulting in inhibition of visible growth of bacteria
after incubation at 37 8C for 18–24 h.