**1. Introduction**

254 Antimicrobial Agents

White, T.J., Bruns, T.D., Lee, S. and Taylor, J.W. (1990). Amplification and direct sequencing

(Lauraceae). *Phytochemistry*. Vol. 66, pp. 2363-2367.

White). Academic Press, New York: 315-322.

from *Curvularia* sp., an endophytic fungus associated with *Ocotea corymbosa* 

of fungal ribosomal RNA genes for phylogenetics. In: *PCR Protocols: A Guide to Methods and Applications* (eds. M.A. Innis, D.H. Gelfand, J.S. Sninsky and T.J.

> After the concept of selective toxicity in chemotherapy was introduced at the beginning of the 20th century, (Ehrlich, 1913), classes of substances with antibacterial properties, produced by microorganisms or created through synthesis were obtained. After the discovery of penicillin, the first antibiotic introduced in clinical use in man in 1940s, a large number of different types of antibiotics were produced. Antibiotics such as beta-lactams, macrolides, aminoglycozides and tetracyclines were discovered and introduced during an extremely short period. These were obtained either by isolation from fungi or by chemically modification of the naturally isolated substrates. These dominated the antimicrobial industry, while synthetically obtained substances only played a minor role. (Chu & Fernandes, 1991)

> In 1962, G. Y. Lesher and his collaborators introduced the first quinolone derivative, nalidixic acid **(**1-ethyl-1,4-dihydro-7-methyl-4-oxo-1,8-naphthyridine-3-carboxilyc acid), (1, Lesher et al. 1962) which had moderate activity against gram-negative organisms and was used for treating urinary tract infections. In the following years, a large gamma of derivatives from common elements were synthesized, which could be grouped by: cinoline (cinoxacin**),** pyrido-pyrimidine (pipemidic acid; piromidic acid), naphthyridine (nalidixic acid) and quinolones (oxolinic acid, miloxacin , tioxacin, etc.). These derivatives, with differentiated structures, have 2 common pharmacological properties, which allowed them to be classified as first generation biologically active derivatives with quinolone structure. The two common characteristics for first generation quinolones are:


Quinolones: Synthesis and Antibacterial Activity 257

F N HN OCH3

F

HN F

CH3

F N HN F

and some similar bacteria effects: photo toxicity, neurotoxicity, cartilage toxicity.

F N

COOH

CH3

Sparfloxacin

CH3

Temafloxacin

Clinafloxacin

O

OCH3 N

F

<sup>N</sup> HN H

infections and on patients with immune deficiencies.

H S S

H2N

CH3

N

Gatifloxacin

N

COOH

COOH

BAY y 3118

O

NH2

N

F

The four generations have the following common aspects: an identical mechanism of action by inhibition the A subunit of DNA-gyrasa, an exclusively chromosomal bacteria resistance

> Cl N

Until now a large number of antibacterial substances belonging to the above mentioned class have been used in medicine. Quinolones are used when treating infections of the urinary tract, the respiratory tract, intestinal infections, ear/nose/throat infections, STD's, soft tissue and skin infections, meningitis caused by gram negative and Staphilococci bacteria, liver and bile infections, septicemia and endocarditis, prophylaxis and surgical

O

O

O

COOH

COOH

N

N

N N

O

F

O

N

S

COOH

O

O

CH3

Grepafloxacin

F N HN CH3

F N <sup>N</sup> <sup>O</sup> CH3 CH3

> F N

Levofloxacin

H2N

Sitafloxacin

O

Cl N

F

<sup>N</sup> HN H

H S S

N F

<sup>F</sup> H2N Cl

COOH

<sup>R</sup> <sup>S</sup>

Tosufloxacin

COOH

COOH

COOH

F

N

Balofloxacin

COOH

OCH3

O

O

F

N

F

<sup>N</sup> OCH3

Gemifloxacin

Pazufloxacin

N N

O

F

N

<sup>O</sup> CH3

O

COOH

Fig. 3. Third- generation quinolones.

COOH

Moxifloxacin

Fig. 4. Fourth- generation quinolones.

F

N N

NH CH3

H2N

F

Trovafloxacin

H

H2N H F N

NH2

COOH

The success of first generation quinolones spurred the research in this area, which led to the obtainment through synthesis, after 1980, of a new series of compounds with stronger antibacterial properties and a broader spectrum of antibacterial activity which included gram positive and gram negative organisms, and which where defined by their ability to be applied on all localized infections. Koga and his collaborators introduced Norfloxacin into clinical use in 1980, the first quinolone with a fluorine atom substituted at the C-6 position and a piperazine C-7. Norfloxacin (Koga et al. 1980) was the first quinolone with increased antimicrobial activity, acting on a large spectrum of gram positive and gram negative microorganisms, including Pseudomonas aeruginosa.

Fig. 2. Second-generation quinolones.

Research in the field of derivatives with a quinolone structure have lead to new compounds obtained recently, which have been classified as third and fourth generation systemic quinolones, largely effective against Staphilococcus aureus. Their large antibacterial spectrum includes anaerobes, Chlamydia and Mycoplasma. (Brighty & Gootz, 2000)

N N

Piromidic Acid

The success of first generation quinolones spurred the research in this area, which led to the obtainment through synthesis, after 1980, of a new series of compounds with stronger antibacterial properties and a broader spectrum of antibacterial activity which included gram positive and gram negative organisms, and which where defined by their ability to be applied on all localized infections. Koga and his collaborators introduced Norfloxacin into clinical use in 1980, the first quinolone with a fluorine atom substituted at the C-6 position and a piperazine C-7. Norfloxacin (Koga et al. 1980) was the first quinolone with increased antimicrobial activity, acting on a large spectrum of gram positive and gram negative

N

C2H5

Pefloxacin

O

F N

N

N

<sup>O</sup> CH3

Ofloxacin

Research in the field of derivatives with a quinolone structure have lead to new compounds obtained recently, which have been classified as third and fourth generation systemic quinolones, largely effective against Staphilococcus aureus. Their large antibacterial

spectrum includes anaerobes, Chlamydia and Mycoplasma. (Brighty & Gootz, 2000)

O

COOH

O F COOH

<sup>N</sup> CH3

N HN F C2H5 CH3 Lomefloxacin

> F N

<sup>N</sup> CH3

COOH

O

C2H5

COOH

N

N

HN

N N

Pipemidic Acid

O

C2H5

N

N

Amifloxacin

N

S

Rufloxacin

O

COOH

O F COOH

N

N <sup>N</sup> NHCH3 CH3

> F N

<sup>N</sup> CH3

Ciprofloxacin

HN

O F COOH

COOH

N

N

N

Fig. 1. First-generation quinolones.

N

C2H5

Norfloxacin

N

Fleroxacin

Enoxacin

Fig. 2. Second-generation quinolones.

N <sup>N</sup> <sup>F</sup> CH2CH2F CH3

N N

N HN C2H5

O F COOH

O F COOH

O

F

N

HN

COOH

Miloxacine

O

O

OCH3

O

COOH

microorganisms, including Pseudomonas aeruginosa.

The four generations have the following common aspects: an identical mechanism of action by inhibition the A subunit of DNA-gyrasa, an exclusively chromosomal bacteria resistance and some similar bacteria effects: photo toxicity, neurotoxicity, cartilage toxicity.

Fig. 4. Fourth- generation quinolones.

Until now a large number of antibacterial substances belonging to the above mentioned class have been used in medicine. Quinolones are used when treating infections of the urinary tract, the respiratory tract, intestinal infections, ear/nose/throat infections, STD's, soft tissue and skin infections, meningitis caused by gram negative and Staphilococci bacteria, liver and bile infections, septicemia and endocarditis, prophylaxis and surgical infections and on patients with immune deficiencies.

Quinolones: Synthesis and Antibacterial Activity 259

Quinolone derivatives are an important class of antibacterial agents with wide action. Basic structure of these compounds (Figure 6) is a bicyclic structure, which contains a ring of type *A* 4-piridinona combined with aromatic or heteroaromatic ring *B.* The ring type *A* 4 piridinona is a ring with absolute necessity: an unsaturation in position 2-3, a free acid

**2. Quinolones: Structural features and method of synthesis** 

function in position 3 and a substituent at nitrogen in position 1.

Z

6 7 8

Y

B A

5 4

X N

1 2 3

CO2H

O

R

The studies on quinolones indicated that in order for the compound to have antibacterial activity, the N-1 position requires a substituent. Many quinolones contain in N-position : ethyl (norfloxacin, pefloxacin, lomefloxacin) , fluoroethyl (fleroxacin), vinyl, clloroethyl, trifluoroethyl, aminoethyl,, cyclopropyl (ciprofloxacin), *t*-butyl, bicyclopentyl,*p*-

Quinolones contain at C-2 hydrogen (R2=H). The replacement at hydrogen has generally proven to be disadvantageous. However, some compounds containing a suitable C-1, C-2 ringhave recently been shown to possess biological activity. (Figure7 -Segawa 1992) (Figure

X N

<sup>3</sup> <sup>5</sup> <sup>4</sup> <sup>6</sup>

R5

R6

R7

7 8 O

R1

1 2 CO2H

R2

**2.1 Structural features** 

Fig. 6. Basic structure of quinolones.

fluorophenyl,2,4-difluorophenyl (Scott 1997)

Fig. 7. Basic structure of bicyclic quinolones.

**2.1.1 Bicyclic quinolones** 

*Position 1* 

*Position 2* 

8 – Scott 1997)

The mechanism of action of quinolone antibacterial agents involves the inhibition of DNA gyrase (a bacterial topoisomerase II) resulting in a rapid bactericidal effect.

The antibacterial activity of quinolones (measured in terms of MIC), however, is the result of the combination of bacterial cell penetration and DNA gyrase inhibitory activity. The antibacterial activity of quinolones depends not only on the bicyclic heteroaromatic system combining the 1,4-dihydro-4-pyridine-3-carboxylic acid moiety and an aromatic ring, but also on the nature of the peripheral substituents and their spatial relationships. These substituents exert their influence on bacterial activity by providing additional affinity for bacterial enzimes, enhancing cell penetration or altering the pharmacokinetics.

The research for an ideal quinolone continues worldwide. Such a quinolone must be biologically active on a large spectrum of gram positive and gram negative bacteria, aerobes and anaerobes and mycobacteria, must have as few side effects as possible, excellent solubility in water and oral bioavailability.

In figure 5, the most common synthesized chemical variations obtained during the research for new quinolones with antibacterial activity, are visible.

Fig. 5. Structural variations of the most recent quinolones.
