Synthesis and in vitro antibacterial activities of 7-(3-aminopyrrolo[3,4-c]pyrazol-5(2H,4H,6H)-yl)quinolone derivatives

Article abstract of DOI:10.1016/j.cclet.2010.06.011

A series of novel 7-(3-aminopyrrolo[3,4-c]pyrazol-5(2. H,4. H,6. H)-yl)quinolone derivatives were designed, synthesized and evaluated for in vitro antibacterial activities. Compounds 6g, 7g and 7h with the potencies similar to those of gemifloxacin, moxif

Full text of DOI:10.1016/j.cclet.2010.06.011

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 1141–1144
www.elsevier.com/locate/cclet
Synthesis and in vitro antibacterial activities of
7-(3-aminopyrrolo[3,4-c]pyrazol-5(2H,4H,6H)-yl)quinolone
derivatives
Xin Guo a,b, Yi Liang Li a,c, Yu Fei Liu d, Hui Yuan Guo a, Yu Cheng Wang a,
*
a Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050, China
b College of Pharmaceutical Sciences, Southwest University, Chongqing 400715, China
c Tianjin Institute of Pharmaceutical Research, Tianjin Key Laboratory of Molecular Design & Drug Discovery, Tianjin 300193, China
d Xuzhou Medical College, Xuzhou 221004, China
Received 25 February 2010
Abstract
A series of novel 7-(3-aminopyrrolo[3,4-c]pyrazol-5(2H,4H,6H)-yl)quinolone derivatives were designed, synthesized and
evaluated for in vitro antibacterial activities. Compounds 6g, 7g and 7h with the potencies similar to those of gemifloxacin,
moxifloxacin, gatifloxacin and levofloxacin against Gram-positive organisms, worth further investigation.
# 2010 Yu Cheng Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Fluoroquinolone; Synthesis; Antibacterial activities
Since the discovery of norfloxacin, most of the quinolone antibacterial research has been focused on the basic group
at the C-7 position to produce new potent quinolones, namely, ciprofloxacin, ofloxacin, lomefloxacin, fleroxacin, and
sparfloxacin, all of which contain a piperazine derivative at the C-7 position [1–3]. Warner–Lambert reported that this
piperazine structure has been successfully replaced with two appropriate mimics, 3-aminopyrrolidine and 3-
(aminomethyl)pyrrolidine, tosufloxacin and clinafloxacin are two representative quinolones containing an
aminopyrrolidine residue [4–6]. Moreover, gemifloxacin and moxifloxacin (Fig. 1), two important quinolones,
also contain a pyrrolidine residue. Inspired by these previous research, we designed a series of 7-(3-aminopyrrolo[3,4-
c]pyrazol-5(2H,4H,6H)-yl)quinolone derivatives. In this paper, we report the synthesis and in vitro antibacterial
activities of these new quinolone derivatives.
Synthetic pathways toward side chains 2 and 4 and novel fluoroquinolone derivatives 6a–g and 7a–h are depicted in
Scheme 1. 2,4,5,6-Tetrahydropyrrolo[3,4-c]pyrazol-3-amine hydrochloride 2 was prepared by tert-butyl 3-cyano-4-
oxopyrrolidine-1-carboxylate 1 with hydrazine hydrochloride in ethanol. Another side chain was prepared by tert-
butyl 3-cyano-4-oxopyrrolidine-1-carboxylate 1 with methylhydrazine in ethanol, and then compound 3 was
subjected to deprotection by pumping hydrogen chloride gas, to provide 2,4,5,6-tetrahydro-2-methylpyrrolo[3,4-
* Corresponding author.
E-mail address: wyc9999@gmail.com (Y.C. Wang).
1001-8417/$ – see front matter # 2010 Yu Cheng Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.06.011

1[(ig._1)TD$FIG] 142
X. Guo et al. / Chinese Chemical Letters 21 (2010) 1141–1144
Fig. 1. Structures of four quinolone derivatives.
[(Schme_1)TD$FIG]
Scheme 1. Reagents and conditions: (i) NH2NH2ÁHCl, C2H5OH, r.t., 12 h, 50.6%; (ii) NH2NHCH3, C2H5OH, r.t., 12 h, 23.0%; (iii) MeOH, HCl
(gas), r.t., 1 h; (iv) a: CH3CN, Et3N, 25–60 8C, 1–48 h; b: 5% NaOH/H2O, 40 8C, 0.5–2 h; c: 2 mol/L HCl, r.t., 17.9–91.3%.
c]pyrazol-3-amine hydrochloride 4. Finally, the target compounds 6a–g and 7a–h were obtained by SN reaction of
boric chelating compounds 5 with 2 or 4, and then hydrolysis of chelating groups [7–9].
The novel fluoroquinolones 6a–g and 7a-h were evaluated for their in vitro antibacterial activity against
representative Gram-negative and Gram-positive strains using standard techniques. Minimum inhibitory
concentration (MIC) is defined as the concentration of the compound required to give complete inhibition of
bacterial growth and MICs of the synthesized compounds along with the standard drugs gemifloxacin (GM),
moxifloxacin (MX), gatifloxacin (GT) and levofloxacin (LV) for comparison are reported in Table 1 [10].
All 15 compounds display generally rather weak potency against the tested Gram-negative strains, but most of them
exhibit good potency in inhibiting the growth of Staphylococcus aureus including MRSA and Staphylococcus
epidermidis including MRSE (MIC: 0.125–8 mg/mL). In particular, the most active compound 7g is 2–32-fold more
potent than MX, GM, GT and LV against Streptococcus pneumoniae 08-3, Klebsiella pneumoniae 09-23 and
Pseudomonas aeruginosa ATCC27853, 4–32-fold more potent than MX, GM and LV against K. pneumoniae 09-21,
and more active than or comparable to the four reference drugs against P. aeruginosa 09-32.
Generally, the activity of the quinolone nuclei in this study is in the order 1-(2-fluoroethyl)-8-fluoro-quinolone > 1-
cyclopropyl-8-methoxylquinolone > 1-cyclopropyl-8-fluoroquinolone ꢀ 1-cyclopropyl-8-
difluoromethoxylquinolone > 1-cyclopropyl-1,8-naphthyridone ꢀ 1-(2,4-difluorophenyl)-1,8-naphthyridine ꢀ 1-
ethyl-8-fluoroquinolone for fluoroquinolones containing 3-aminopyrrolo[3,4-c]pyrazol-5(2H,4H,6H)-yl substitution
at C-7 position; 1-cyclopropyl-8-fluoroquinolone > 1-cyclopropyl-8-difluoromethoxylquinolone > 1-cyclopropyl-
1,8-naphthyridone ꢀ 1-(2,4-difluorophenyl)-1,8-naphthyridine ꢀ 1-ethyl-8-fluoroquinolone > 1-(2-fluoroethyl)-8-
fluoroquinolone ꢀ 1-cyclopropyl-8-fluoroquinolone for fluoroquinolones containing 3-amino-2-methyl-pyrrolo[3,4-
c]pyrazol-5(2H,4H,6H)-yl substitution at C-7 position. In addition, fluoroquinolones containing 3-aminopyrrolo[3,4-
c]pyrazol-5(2H,4H,6H)-yl substitution at C-7 position appear to be less than corresponding 2-methyl analogs.

X. Guo et al. / Chinese Chemical Letters 21 (2010) 1141–1144
1143
Table 1
In vitro antibacterial activities of target compounds 6a–g and 7a–h (MIC: mg/mL).
Compd. Organism
S.a.1
2
S.a.2
16
S.a.3
>128
32
S.a.4
S.a.5
S.e.1
S.e.2
8
S.e.3
8
S.e.4
8
S.e.5
8
S.p.1
S.p.2 S.p.3
E.fa.1 E.fa.2
6a
6b
6c
8
4
8
8
8
8
8
8
8
>128 16
32
16
32
>128
>128
>128
>128
>128
128
2
8
4
4
32
4
32
4
32
128
8
8
8
32
32
32
>128 >128
6d
6e
0.25
0.25
0.25
0.25
0.25
0.25
>128 0.25
>128 0.25
>128 0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
1
0.25
2
0.25
0.25
0.25
32
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
2
128
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
>128 0.25
>128 16
>128
>128
64
>128
>128
64
6f
32
1
0.25
0.25
0.25
0.25
0.25
8
0.25
2
6g
7a
7b
7c
0.125 0.25
32
>128 0.25
>128
2
0.25
0.25
0.25
0.25
32
0.25
0.25
8
>128
>128
>128
>128
>128
>128
>128
128
>128 0.25
4
4
4
0.125 0.25
8
0.25
1
0.125 0.25
0.125 16
0.125 >128 16
16
7d
7e
0.5
0.5
1
>128
2
4
2
1
2
1
2
1
1
>128
64
128
64
64
1
8
>128 >128
0.5
2
2
2
2
0.25
64
0.5
0.25
1
0.25
4
128
64
8
7f
0.125 0.25
0.125 0.25
0.25
0.25
0.125 0.125 0.125 0.25
0.5
0.5
0.5
0.25
0.5
0.125
128
7g
7h
MX
GM
GT
LV
0.125 0.25
0.5
0.125 0.25
0.25
0.25
0.5
0.25
0.25
0.25
0.125
0.25
0.25
2
>128
>128
2
0.25
0.25
0.125 0.25
0.125 0.25
0.06
>128
1
0.125 0.25
0.125 0.25
0.125 0.125 0.06
0.25
0.125 0.125 0.06
0.125 0.06
0.03
0.125 0.25
0.25 0.125
0.25
0.06
0.06
0.125 0.25
16
8
8
16
8
16
0.06
0.125 0.25
0.06
0.25
0.25
0.25
0.25
4
4
4
0.125 0.125 0.25
0.125 0.125 0.25
3
2
8
8
Compd. Organism
E.fm.1 E.fm.2 E.c.1
E.c.2
E.c.3
E.c.4
E.c.5
K.p.1 K.p.2 K.p.3 P.a.1
P.a.2
P.a.3
P.a.4
P.a.5
6a
6b
6c
>128
>128
>128
>128
>128
>128
64
>128
>128
32
>128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128
>128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128
>128 >128 >128 >128 >128 >128 >128
>128 >128 >128 >128 >128 >128 128
8
>128 >128 >128 >128 >128
6d
6e
>128
>128
>128
64
>128 128
>128 16
128
16
128
128
>128
128
128
128
>128 >128 >128 >128 >128 16
>128 >128 >128 >128 >128 64
>128 >128 32
>128 >128 64
6f
>128
64
8
8
0.25
8
6g
7a
7b
7c
>128 >128 >128 >128 64
128
128
64
16
>128
>128
>128
>128
128
>128
>128
>128
>128
128
>128 >128 >128 >128 >128 >128 >128 >128 128
>128 >128 >128 >128
16 >128 >128 16
>128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128 >128
128
128
>128 >128 >128 >128 >128 >128 >128 128
7d
7e
>128 >128 >128 >128 >128 128
4
128
4
128
64
128
128
8
4
128
32
32
32
32
0.5
8
8
>128 >128 >128 128
>128 >128 >128 128
>128 >128 >128 >128 0.5
4
8
7f
64
128
32
128
128
2
64
4
64
4
32
8
7g
7h
MX
GM
GT
LV
>128
>128
2
>128
>128
2
>128 0.5
1
0.25
4
64
2
>128 >128 >128 >128 >128 >128 >128
4
>128 128
4
2
2
2
2
2
2
1
8
1
2
0.25
0.5
16
16
2
2
4
4
0.25
0.5
1
0.5
0.5
0.5
0.5
2
16
4
16
8
16
8
16
2
32
1
8
2
2
4
2
0.125 0.06
16 16
2
2
4
16
2
16
8
16
16
16
4
2
S.a.1, Staphylococcus aureus ATCC259223; S.a.2, methicillin-resistant Staphylococcus aureus 08-1; S.a.3, methicillin-resistant Staphylococcus
aureus 08-2; S.a.4, methicillin-sensitive Staphylococcus aureus 08-1; S.a.5, methicillin-sensitive Staphylococcus aureus 08-2; S.e.1, methicillin-
resistant Staphylococcus epidermidis 09-2; S.e.2, methicillin-resistant Staphylococcus epidermidis 09-3; S.e.3, methicillin-resistant Staphylococcus
epidermidis 09-4; S.e.4, methicillin-sensitive Staphylococcus epidermidis 09-3; S.e.5, methicillin-sensitive Staphylococcus epidermidis 09-6; S.p.1,
Streptococcus pneumoniae 08-2; S.p.2, Streptococcus pneumoniae 08-3; S.p.3, Streptococcus pneumoniae 08-4; E.fa.1, Enterococcus faecalis 08-
10; E.fa.2, Enterococcus faecalis 08-12; E.fm.1, Enterococcus faecium 08-2; E.fm.2, Enterococcus faecium 08-7; E.c.1, Escherichia coli ATCC
25922; E.c.2, Escherichia coli 08-21; E.c.3, Escherichia coli 08-22; E.c.4, Escherichia coli 08-23; E.c.5, Escherichia coli 08-24; K.p.1, Klebsiella
pneumoniae 09-21; K.p.2, Klebsiella pneumoniae 09-22; K.p.3, Klebsiella pneumoniae 09-23; P.a.1, Pseudomonas aeruginosa ATCC27853; P.a.2,
Pseudomonas aeruginosa 09-32; P.a.3, Pseudomonas aeruginosa 09-33; P.a.4, Pseudomonas aeruginosa 09-34; P.a.5, Pseudomonas aeruginosa
09-35.

1144
X. Guo et al. / Chinese Chemical Letters 21 (2010) 1141–1144
In summary, we report herein the synthesis of a series of novel 7-(3-amino-(2-methyl-)pyrrolo[3,4-c]-pyrazol-
5(2H,4H,6H)-yl)fluoroquinolone derivatives. The antibacterial activity of the newly synthesized compounds were
evaluated and correlated with their physicochemical properties. Results reveal that most of the target compounds have
good activity S. aureus including MRSA and S. epidermidis including MRSE. However, all of them display generally
rather weak potency against the tested Gram-negative strains.
Acknowledgment
This work was supported by the National Major Science and Technology Project of China (‘‘Innovation and
Development of New Drugs’’, No. 2009ZX09301-003).
References
[1] J.M. Domagala, Antimicrob. Chemother. 33 (1994) 685.
[2] D.C. Hooper, J.S. Wolfson, N. Engl. J. Med. 32 (1991) 384.
[3] D.W. Chu, P.B. Fernandes, Adv. Drug Res. 21 (1991) 39.
[4] J.M. Domagala, C.L. Heifetz, T.F. Mich, et al. J. Med. Chem. 29 (1986) 445.
[5] J.P. Sanchez, J.M. Domagala, S.E. Hagen, et al. J. Med. Chem. 31 (1988) 983.
[6] D.T.W. Chu, P.B. Fernandes, A.K. Caiborne, et al. J. Med. Chem. 29 (1986) 2363.
[7] N. Takagi, H. Fubasami, H. Matukubo, EP 464823 (1992).
[8] Synthesis of 1-cyclopropyl-6-fluoro-7-(3-aminopyrrolo[3,4-c]pyrazol-5(2H,4H,6H)-yl)-1,4-dihydro-4-oxo-1,8-naphthy-ridine-3-carboxylic
acid (6a): a mixture of 2 (0.38 g, 1.9 mmol), 5 (0.53 g, 1.3 mmol), triethylamine (1.0 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 5 with 2 mol/L HCl. The precipitate was collected by suction to give off-white amorphous
product 6a (0.32 g, 66.5%).
[9] Typical data of the synthetic compounds: 6f 1H NMR (400 MHz, DMSO-d6): d 1.00–1.18 (m, 4H), 4.05–4.09 (m, 1H), 4.57–4.60 (m, 4H), 6.88
(t, 1H), 8.63 (s, 1H), 7.87 (d, 1H), 8.75 (s, 1H); 13C NMR (400 MHz, DMSO-d6): d 9.3, 40.8, 49.5, 49.8, 107.2, 108.8, 109.1, 114.7, 117.3,
118.2, 119.9, 130.7, 135.4, 142.2, 151.2, 152.4, 154.9, 165.3, 175.7; HRMS-ESI m/z: 458.10521 (calcd. for C19H16F3N5O4Na [M+Na]+). 7f 1H
NMR (400 MHz, DMSO-d6): d 1.00–1.18 (m, 4H), 3.52 (s, 3H), 4.05–4.09 (m, 1H), 4.55–4.73 (m, 4H), 6.90 (t, 1H), 7.89 (d, 1H), 8.75 (s, 1H);
13C NMR (400 MHz, DMSO-d6): d 9.3, 36.4, 40.8, 48.8, 50.6, 105.6, 107.3, 108.9, 114.7, 117.3, 118.6, 119.9, 131.0, 135.3, 137.7, 144.0,
147.4, 151.3, 165.3, 175.7; HRMS-ESI m/z: 450.13891 (calcd. for C20H19F3N5O4 [M+H]+). 7g 1H NMR (400 MHz, CDCl3): d 1.02–1.25 (m,
4H), 3.66 (s, 3H), 3.72 (s, 3H), 4.03–4.07 (m, 1H), 4.63–4.79 (m, 4H), 7.88 (d, 1H), 8.81 (s, 1H); 13C NMR (400 MHz, DMSO-d6): d 8.9, 40.9,
48.5, 50.1, 61.4, 106.0, 106.1, 106.4, 119.0, 134.2, 136.0, 136.1, 143.7, 144.5, 147.4, 150.3, 153.1, 155.6, 165.6, 176.0; HRMS-ESI m/z:
414.15776 (calcd. for C20H21FN5O4 [M+H]+). 7h 1H NMR (400 MHz, CDCl3): d 1.45–1.47 (m, 3H), 3.50 (s, 3H), 4.32–4.35 (m, 1H), 4.52–
4.91 (m, 6H), 7.59 (d, 1H), 8.93 (s, 1H); 13C NMR (400 MHz, DMSO-d6): d 17.8, 36.3, 49.0, 50.6, 54.8, 67.9, 103.5, 106.2, 117.5, 124.9, 130.0,
137.6, 144.6, 146.1, 147.3, 152.5, 154.9, 166.1, 176.0; HRMS-ESI m/z: 400.14211 (calcd. for C19H19FN5O4 [M+H]+).
[10] 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.