An Insight into Fluorinated Imines and Hydrazones as Antibacterial Agents

Abstract

Fluorinated imines (Schiff bases) and fluorinated hydrazones are of particular interest in medicinal chemistry due to their potential usefulness in treating opportunistic strains of bacteria that are resistant to commonly used antibacterial agents. The present review paper is focused on these fluorinated molecules revealing strong, moderate or weak in vitro antibacterial activities, which have been reported in the scientific papers during the last fifteen years. Fluorinated building blocks and reaction conditions used for the synthesis of imines and hydrazones are mentioned. The structural modifications, which have an influence on the antibacterial activity in all the reported classes of fluorinated small molecules, are highlighted, focusing mainly on the importance of specific substitutions. Advanced research techniques and innovations for the synthesis, design and development of fluorinated imines and hydrazones are also summarized.

1. Introduction

There is still a continuous search for novel antibacterial agents due to the increasing number of isolated microbial strains resistant to clinically approved antibacterial agents. Both imines (Schiff bases) and hydrazones—containing a double bond between carbon and nitrogen—are of particular interest to medicinal chemists due to their potential usefulness in treating opportunistic pathogens. These azomethine molecules can be synthesized by condensing carbonyl compounds with various nitrogen-nucleophilic compounds containing the terminal amino group. Aldimine-type Schiff bases—holding a secondary azomethine (CH=N) group, and ketimine-type Schiff bases—having the characteristic imine (C=N) group, can be obtained by nucleophilic attack of the primary amine on the electrophilic carbonyl function of an aldehyde or ketone, respectively, under strictly established reaction conditions. Hugo Schiff was the first research worker who performed successfully the synthesis of aldimines and ketimines, followed by a concomitant elimination of water (as the result of dehydration of hemiaminal) under azeotropic distillation [1]. However, hydrazones can be obtained via the nucleophilic attack of substituted hydrazines or hydrazides on the carbonyl function of aldehyde or ketone as an electrophilic center. Hydrazones can be divided into hydrazine–hydrazones containing the -NH-N=CH- formation and hydrazide–hydrazones bearing the -CONH-N=CH- moiety. Both mechanism of imine and hydrazone formation is based on the attachment of the amine nitrogen to the carbon of the carbonyl group and the elimination of a water molecule from an intermediate hemiaminal (carbinolamine) formed [1,2]. Due to the fact that the hydrazone group is less reactive compared to the imine group, hydrazones are generally more hydrolytically stable [3]. Notwithstanding, the stability of different hydrazones may vary because it depends on the nature of the substituents and the presence of adjacent functional groups.

Chakraborti et al. [2] have proved by extensive experimentation that in the case of highly nucleophilic (primary amines, phenylhydrazines) and electrophilic (aldehydes, ketones) functionalized reactants, the formation of Schiff bases or phenylhydrazones proceeds easily without the use of any catalyst. On the other hand, the same group of researchers have confirmed experimentally that in the case of electronic and/or steric effects of substituents, that might decrease the nucleophilicity and electrophilicity of starting reactants, an appropriate catalyst assistance is needed. In this case, the condensation process leading to the targeted imines or phenylhydrazones had to be catalyzed by Brönsted–Lowry or Lewis acids to facilitate an amine or phenylhydrazine nucleophilic attack on the activated carbonyl group of an aldehyde or a ketone and the subsequent elimination of water [2].

The imine or hydrazone moiety occurs in the structure of various pharmacologically active molecules. Antibacterials of the Schiff base- or hydrazone-type, such as nitrofurantoin, nifurtoinol, nifurzide, nifuroxazide, furazolidone, nitrofurazone and thioacetazone, are commonly used in human medicine (Table 1) [4]. These pharmaceuticals act at low doses and do not produce resistant strains. Nitrofurantoin and nifurtoinol are approved as antibacterial agents in the treatment of urinary tract infections. Nifurzide, nifuroxazide and furazolidone belong to antibacterial agents that are used in the treatment of gastrointestinal tract infections, such as acute and chronic diarrhea of bacterial origin. Nitrofurazone is used topically as an antiseptic for the eyes, ears, skin and mucous membranes of the throat and vagina. In addition, nitrofurazone and furazolidone reveal antiprotozoal—against G. lamblia—activities. Terizidone and thioacetazone are antimycobacterial agents. Thioacetazone—a prodrug activated by the bacterial monooxygenase to an active drug—is employed in the treatment of tuberculosis in combination with more effective antimycobacterial agents. Although displaying a weak activity against M. tuberculosis, this drug is currently recommended for preventing resistance to potent antituberculostatic agents such as isoniazid and rifampicin [4]. Ftivazide and verazide—which are hydrazones related to isoniazid—have previously been used in medicine as antitubercular agents with prolonged release and lower toxicity than the parent drug [5,6]. However, ambazone is used to treat bacterial infections of the throat and mouth in humans, revealing the bacteriostatic action on S. pyogenes, S. pneumoniae and S. viridans [4]. Furonazide has been used in human and veterinary medicine as an antitubercular agent, long-acting and less toxic than isoniazid [7] (Table 1).

Table 1.

Antibacterial drugs of the imine- or hydrazone-type used in medicine currently and in the past.

Structural Formula	International Name	IUPAC Name
	Nitrofurantoin	1-[(E)-(5-Nitrofuran-2-yl)methylideneamino]imidazolidine-2,4-dione
	Nifurtoinol	3-(Hydroxymethyl)-1-[(E)-(5-nitrofuran-2-yl)methylideneamino]imidazolidine-2,4-dione
	Nifurzide	5-Nitro-N-[(E)-[(E)-3-(5-nitrofuran-2-yl)prop-2-enylidene]amino]thiophene-2-carboxamide
	Nifuroxazide	4-Hydroxy-N-[(E)-(5-nitrofuran-2-yl)methylideneamino]benzamide
	Furazolidone	3-[(5-Nitrofuran-2-yl)methylideneamino]-1,3-oxazolidin-2-one
	Nitrofurazone	[(E)-(5-Nitrofuran-2-yl)methylideneamino]urea
	Terizidone	4-[[4-[(3-Oxo-1,2-oxazolidin-4-yl)iminomethyl]phenyl]methylideneamino]-1,2-oxazolidin-3-one
	Thioacetazone	N-[4-[(E)-(Carbamothioylhydrazinylidene)methyl]phenyl]acetamide
	Ftivazide	N-[(E)-(4-Hydroxy-3-methoxyphenyl)methylideneamino]pyridine-4-carboxamide
	Verazide	N-[(E)-(3,4-Dimethoxyphenyl)methylideneamino]pyridine-4-carboxamide
	Ambazone	[4-[2-(Diaminomethylidene)hydrazinyl]phenyl]iminothiourea
	Furonazide	N-[(E)-1-(Furan-2-yl)ethylideneamino]pyridine-4-carboxamide

Aldimine- and ketimine-type Schiff bases have been reported to play an important function in biochemical processes as the intermediates in various enzymatic reactions [8,9]. The human rhodopsin (visual purple found in rod cells of the retina) is an aldimine-type Schiff base, which is essential in the photoreception mechanism [8]. In this holoprotein, the opsin apoprotein and the chromophore 11-cis-retinal are linked via a protonated azomethine bond. Aldimine-type Schiff bases of pyridoxal phosphate coenzyme play a role as transporting agents in the biochemical pathways of important amino acids. In turn, the ketimine-type Schiff base of dihydroxyacetone phosphate is involved in the metabolism of carbohydrates [9].

A number of fluorinated imines and hydrazones, derived from various nucleophilic amines and their derivatives, diamines, hydrazines, hydrazides, dihydrazides and electrophilic carbonyl molecules (such as aldehydes and ketones), have been synthesized and their diversified structures have been extensively antibacterially investigated—over the last fifteen years—to develop more effective and selective antibacterial agents. Considering the importance of fluorine-containing drugs/drug candidates in current medicinal chemistry [10,11,12,13,14], this review paper is focused on fluorinated imines and hydrazones revealing antibacterial action in vitro, and on structural modifications that affect the activity in each set of diversified fluorinated molecules. The azomethine or imine group in fluorinated molecules appears to be crucial for their antibacterial activity, while the substitution with fluorine atom(s) may improve their metabolic stability and permeation through biomembranes [11]. The general synthesis approaches (these straightforward as well as those more advanced), reaction conditions, yields as well as antibacterial activities of molecules from particular classes of fluorinated Schiff bases and hydrazones are mentioned in this review. This paper presents the usefulness of nucleophilic and electrophilic fluorinated building blocks and catalysts that can be successfully used in the synthesis of fluorinated imines and hydrazones. In addition, this review gives an overview of some useful advanced research techniques and innovations for the design and development of highly selective and non-toxic fluorinated molecules regarded as possible inhibitors of the Escherichia coli β-ketoacyl-acyl carrier protein synthase III (ecKAS III).

2. Fluorinated Aldimine-Type Schiff Bases

Raache et al. [15] have reported the synthesis, structural investigations and preliminary antibacterial activity studies of (E)-1-phenyl-N-(2,3,5,6-tetrafluoropyridin-4-yl)methanimine (1) (Figure 1). The synthesis of this aldimine was achieved successfully by reacting equimolar ratios of 4-amino-2,3,5,6-tetrafluoropyridine and benzaldehyde in tetrahydrofuran containing an ethanolic solution of potassium hydroxide for 72 h at ambient temperature.

Figure 1.

The structure of fluorinated aldimine 1.

The antibacterial activity of aldimine 1 has been evaluated in the disc diffusion assay using both Gram-positive (S. aureus ATCC 6538, E. faecium ATCC 19434, S. agalactiae) and Gram-negative (E. coli ATCC 8739, S. typhimurium ATCC 14028) bacterial strains. Ampicillin—a broad-spectrum aminopenicillin—was used as a standard antibiotic. Tetrafluorinated aldimine 1 at the highest concentration tested (983.5 µM) was shown to reveal moderate activity against both Gram-negative bacteria of clinical interest, considering its zone inhibition sizes in relation to that of ampicillin [15].

Avila-Sorrosa et al. [16] have described a straightforward synthesis route, determination of the structure (including that in the solid state) and preliminary antibacterial evaluation of fluorinated aldimine-type Schiff bases 2–4 (Figure 2). The synthesis of these aldimines was performed by reacting almost equimolar amounts of 3,5-difluoroaniline, 3-(trifluoromethyl)aniline or 3,5-bis(trifluoromethyl)aniline with 3-hydroxybenzaldehyde. The condensation process was carried out in dichloromethane at ambient temperature for 48 h, and the removal of water from an intermediate hemiaminal was facilitated by the use of activated molecular sieves.

Figure 2.

Structures of fluorinated aldimines 2–4.

Avila-Sorrosa research group has employed S. aureus ATCC 25922 and B. subtilis ATCC 9372 as Gram-positive bacilli, whereas E. coli ATCC 25923 and K. pneumoniae ATCC 700603 as Gram-negative bacilli of clinical interest to assess the antibacterial activity of fluorinated aldimines 2–4 in the disc diffusion assay. Ampicillin was used as a standard antibiotic to confirm the susceptibility of all pathogenic strains of bacteria. Results of this antimicrobial test revealed that both Gram-positive and Gram-negative bacterial strains are susceptible to all the aldimines fluorinated in the meta positions/position (2–4), whose activities are comparable to that of ampicillin. The conducted studies gave convincing proof that the substitution at meta positions/position of the phenyl moiety by two fluorine atoms, two trifluoromethyl groups or one trifluoromethyl group is preferred for the antibacterial activity in this class of small molecules [16].

Cheng et al. [17] have reported the synthesis and results of in vitro antibacterial studies for fluorinated aldimine-type Schiff base 5, i.e., N-{3-[(E)-(5-fluoro-2-hydroxybenzylidene)amino]propyl}-2-hydroxybenzamide (Figure 3). The synthesis of this aldimine was carried out by condensing N-(3-aminopropyl)-2-hydroxybenzamide and 5-fluorosalicylaldehyde in methanol—as the reaction medium—at 50 °C for 3 h.

Figure 3.

The structure of fluorinated aldimine 5.

The authors have employed clinical isolates of two Gram-positive (S. aureus ATCC 6538, B. subtilis ATCC 6633) and two Gram-negative (P. aeruginosa ATCC 13525, E. coli ATCC 35218) bacterial strains to determine MIC values of Schiff base 5 in the assay MTT-based. An aminoglycoside antibiotic—kanamycin B—was used as a positive control, to confirm the susceptibility of all bacterial strains recruited. Fluorinated aldimine 5 was found to be antibacterially active, revealing significant potencies against P. aeruginosa, S. aureus, E. coli and moderate activity against B. subtilis (Table 2). Simultaneously, its MIC value against P. aeruginosa proved to be 1.3-fold lower than that of kanamycin B. In addition, this molecule was disclosed as a highly potent inhibitor (having the half-maximal inhibition constant (IC₅₀) of 5.6 µM) of the ecKAS III [17]. This suggests that the enzymatic inhibition mechanism is responsible for its potent antibacterial activity.

Table 2.

Antibacterial activities of fluorinated aldimines 5 and 6.

Bacterial Strain	MIC (µM) *
	5	6	Kanamycin B
E. coli ATCC 35218	9.9	18.9	6.5
P. aeruginosa ATCC 13525	4.9	151.4	6.5
B. subtilis ATCC 6633	79.0	9.5	3.2
S. aureus ATCC 6538	4.9	37.8	3.2

* MIC values provided in the original papers [17,18] in µg mL⁻¹ were converted into molar concentrations.

Employing a similar concept, Cheng et al. [18] have synthesized, confirmed the structure and then antibacterially studied fluorinated Schiff base 6, i.e., N-{2-[(E)-(5-fluoro-2-hydroxybenzylidene)amino]propyl}-2-hydroxy-4-methylbenzamide (Figure 4). This aldimine was afforded as the final product of the reaction between N-(2-aminopropyl)-2-hydroxy-4-methylbenzamide and 5-fluorosalicylaldehyde. The synthesis of this molecule was performed successfully by boiling the stoichiometric ratios of the functionalized reactants in methanol at 50 °C for 3 h.

Figure 4.

The structure of fluorinated aldimine 6.

Cheng et al. have screened fluorinated Schiff base 6 against two Gram-positive (S. aureus ATCC 6538, B. subtilis ATCC 6633) and two Gram-negative (P. aeruginosa ATCC 13525, E. coli ATCC 35218) bacteria of clinical interest in the assay MTT-based. As a positive control, kanamycin B was used. Fluorinated aldimine 6 has been shown to possess remarkable activities against B. subtilis, E. coli, S. aureus and moderate activity against P. aeruginosa (Table 2). Furthermore, this compound was reported to reveal the IC₅₀ of 17.1 µM when tested in the target enzymatic assay for its inhibitory activity against the ecKAS III [18].

Shanmugam et al. [19] have designed, synthesized and confirmed the structure (with the use of spectroscopic techniques) of a number of aldimine-type Schiff bases (7–13) (Figure 5). The synthesis of these fluorinated aldimines was carried out by stirring (at an ambient temperature for 1 h) and then refluxing (at 50 °C for 4–6 h) the stoichiometric ratios of primary (aromatic, aliphatic, aromatic-aliphatic, heterocyclic or heterocyclic-aliphatic) amines with meta-fluorosalicylaldehyde, in methanol, under assistance of an efficient catalyst (i.e., 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and potassium hydroxide).

Figure 5.

Structures of fluorinated aldimines 7–13.

All the obtained aldimine-type Schiff bases (7–13) have been subjected to the assay based on two-fold serial dilutions for estimating their MIC values on Gram-positive (S. aureus ATCC 25930, B. subtilis ATCC 530) as well as Gram-negative (K. pneumoniae ATCC 700603, S. typhi ATCC 25021, P. aeruginosa ATCC 27853, E. coli ATCC 26032) bacteria of clinical interest. As a positive control, an aminoglycoside antibiotic—streptomycin—was employed to confirm the susceptibility of all bacterial strains recruited. The vast majority of fluorinated aldimines have been reported to reveal moderate antibacterial activities in these studies (Table 3). Schiff base 10 (containing the benzo[d]thiazol-2-yl moiety with an electron-donating ethoxy group) was disclosed to be more potent against most of the selected bacteria than remaining fluorinated molecules, revealing strong antibacterial effects against B. subtilis, K. pneumoniae, P. aeruginosa, S. typhi and S. aureus (MIC values superior to that of streptomycin). Simultaneously, aldimine 13 (bearing the 1H-indol-3-ylethyl formation) proved to be 1.9-fold more active against S. aureus than a standard drug. Schiff base 9 (containing the benzyl moiety) exhibited the highest activities against E. coli (comparable to that of streptomycin) and B. subtilis. Aldimine 12 (bearing the 2-thiazolyl moiety) revealed comparable MIC values against E. coli and P. aeruginosa to that of streptomycin. However, Schiff base 7 (containing the naphthyl moiety) was reported to possess the best activity against S. typhi (similar to that of a standard drug) and E. coli. In turn, two aldimines 8 (bearing the 1,3-dihydroxy-2-methylpropan-2-yl moiety) and 11 (containing the benzyloxypyridin-2-yl moiety) were disclosed to be the least antibacterially active molecules [19].

Table 3.

Antibacterial activities of fluorinated aldimines 7–13.

Bacterial Strain	MIC (µM) *
	7	8	9	10	11	12	13	STM
K. pneumoniae ATCC 700603	188.5	110.0	109.1	39.5	155.1	112.5	708.4	86.0
S. typhi ATCC 25021	94.2	880.2	218.1	79.0	620.5	112.5	708.4	86.0
S. aureus ATCC 25930	753.9	880.2	218.1	79.0	620.5	899.9	44.3	86.0
B. subtilis ATCC 530	753.9	na	54.5	19.8	155.1	112.5	88.6	21.5
P. aeruginosa ATCC 27853	753.9	na	872.4	39.5	620.5	56.2	177.1	43.0
E. coli ATCC 26032	47.1	220.0	27.3	158.1	155.1	28.1	na	21.5

* MIC values provided in the original paper [19] in µg mL⁻¹ were converted into molar concentrations. STM—streptomycin; na—not active.

Shi et al. [20] have reported the general synthesis route and results of the in vitro antibacterial examination for a series of fluorinated aldimine-type Schiff bases (14–27) (Figure 6). The synthesis of the above-mentioned fluorinated aldimines was performed successfully by condensing the stoichiometric ratios of differently substituted primary amines and 5-fluorosalicylaldehyde, in a methanolic medium, at ambient temperature, without any catalyst assistance.

Figure 6.

Structures of fluorinated aldimines 14–27.

All the synthesized aldimines 14–27 were subjected to the two-fold serial dilution assay in order to determine their in vitro abilities to inhibit the growth of four clinical isolates of bacterial Gram-positive (B. subtilis, S. aureus) and Gram-negative (E. coli, P. fluorescence) strains. As a positive control, the antibiotic kanamycin—possessing a broad-spectrum of antibacterial activity—was employed to confirm the susceptibility of all bacterial strains recruited. MIC values of Schiff bases 14–27 (Table 4) were established in the assay MTT-based. Aldimine 19 (bearing the 4-fluorophenol moiety linked via an azomethine linkage in ortho position to the 4-hydroxyphenylethyl moiety) was reported to be the most potent, revealing significant activities against E. coli, S. aureus, P. fluorescence and B. subtilis. Noteworthy is that its activity against E. coli was comparable to that of kanamycin, while against S. aureus and P. fluorescence it was 1.9-fold lower than that of a standard drug. Shi et al. have found that the replacement of a hydrophilic hydroxy group of the phenyl moiety in the most active structure (19) by electron-donating alkyl groups results in a remarkable decrease in antibacterial activity which can be seen in the case of two structures containing the methyl (20) or isopropyl (21) group (with MIC values ranging from 218.1 to >436.2 µM). It has been reported that among all fluorinated aldimines with additional hydrophobic electron-withdrawing halogen substituents attached to the phenyl moiety (22–26), two structures: ortho-fluoro- and ortho-chlorosubstituted (25 and 26)—revealing MIC values ranging from 25.0 to 53.6 µM—are more active than their para-fluoro- and para-chlorosubstituted counterparts (22 and 23)—revealing MIC values ranging from 50.1 to 200.3 µM. Furthermore, the antibacterial effects of all the compounds bearing a fluoro substitution (22 and 25) were found to be superior to those containing a chloro substitution (23 and 26). On the other hand, it has been proved that the substitution by cyclopentyl, cyclohexyl and cyclohexylmethyl did not affect the activity since it resulted in synthetic fluorinated aldimines 14, 15 and 16 possessing comparable antibacterial activities (MIC values ranging from 26.6 to 120.6 µM). In turn, the substitution by morpholinoethyl or piperazinoethyl moiety was not favorable for the antibacterial effect as is clearly seen for fluorinated aldimines 17 and 18 with MIC values ranging from 49.7 to 199.0 µM. The most antibacterially active molecule—aldimine 19—when tested by Shi and co-workers in the target enzymatic assay, was identified to be a potent inhibitor (IC₅₀ = 2.7 µM) of the ecKAS III, playing a significant role in fatty acid synthesis pathway in bacteria. Therefore, their results have proved that the presence of an electron-withdrawing and lipophilic fluorine atom at the phenol moiety attached to an azomethine bridge is preferred for the antibacterial activity as well as inhibitory potency towards the ecKAS III. The authors have carried out ligand-docking studies and have shown the most likely binding conformation of compound 19 at the active site of the crystal structure of ecKAS III, suggesting that the enzymatic inhibition mechanism is responsible for its potent antibacterial activity [20].

Table 4.

Antibacterial activities of fluorinated aldimines 14–27.

Bacterial Strain	MIC (µM) *
	14	15	16	17	18	19	20	21	22	23	24	25	26	27	KNM
E. coli	60.3	28.2	26.6	99.1	49.7	6.0	218.1	>388.6	53.6	50.1	85.0	26.8	25.0	231.3	6.5
P. fluorescence	60.3	56.5	26.6	99.1	99.5	12.1	218.1	388.6	53.6	100.1	85.0	26.8	50.1	462.5	6.5
B. subtilis	120.6	113.0	53.1	198.2	199.0	24.1	>436.2	>388.6	107.2	200.3	170.0	53.6	50.1	462.5	3.2
S. aureus	120.6	56.5	106.2	198.2	99.5	6.0	436.2	>388.6	53.6	100.1	170.0	26.8	50.1	462.5	3.2

* MIC values provided in the original paper [20] in µg mL⁻¹ were converted into molar concentrations. KNM—kanamycin.

Xu et al. [21] have synthesized and conducted antimicrobial studies on fluorinated aldimine-type Schiff bases (28–31) (Figure 7). The synthesis of these aldimines was performed by condensing the stoichiometric ratios of various primary amines (i.e., 4-fluorobenzylamine, 4-fluoroaniline, 2-fluoroaniline or 2,4-difluoroaniline) with 2-hydroxy-3,5-diiodobenzaldehyde, in ethanol as the reaction medium without any catalytic assistance.

Figure 7.

Structures of fluorinated aldimines 28–31.

The authors have used clinical isolates of three Gram-positive (B. subtilis, S. aureus, S. faecalis) and three Gram-negative (P. aeruginosa, E. coli, E. cloacae) bacterial strains as well as penicillin (benzylpenicillin) and kanamycin as antibacterial agents for comparison purposes. MIC values of Schiff bases 28–31 were established in the assay MTT-based. The majority of aldimines revealed strong antibacterial activities against all recruited bacteria (Table 5). MIC values of molecules 29–31 against E. cloacae, 28–29, 31 against E. coli, 28 against S. faecalis and 31 against B. subtilis proved to be lower or comparable to that of standard drugs. Schiff base 31—bearing the 2,4-difluorophenyl moiety–has been disclosed to be the most potent among the screened compounds [21].

Table 5.

Antibacterial activities of fluorinated aldimines 28–31.

Bacterial Strain	MIC (µM)
	28	29	30	31	Penicillin	Kanamycin
B. subtilis	26.1	6.7	13.3	3.1	3.6	0.8
S. aureus	13.1	26.8	26.8	12.9	3.6	4.6
S. faecalis	6.6	13.4	107.1	25.8	3.6	6.4
P. aeruginosa	26.4	26.8	107.1	25.8	18.7	6.4
E. coli	3.3	3.3	53.6	6.5	18.7	6.4
E. cloacae	13.1	3.3	3.3	3.3	9.3	3.2

Khungar et al. [22] have synthesized and confirmed the structure of fluorinated Schiff base 32, i.e., 1-[3-(4-{(E)-[(4-fluorophenyl)imino]methyl}-3-hydroxyphenoxy)propyl]-3-methyl-1H-imidazol-3-ium bromide (Figure 8). This aldimine was synthesized by reacting 4-fluoroaniline with the suitable ionic liquid salicylaldehyde derivative (i.e., 1-[3-(4-formyl-3-hydroxyphenoxy)propyl]-3-methyl-1H-imidazol-3-ium bromide) at a molar ratio of 4:3, in boiling ethanol for 4 h.

Figure 8.

The structure of fluorinated aldimine 32.

Schiff base 32 was screened for the in vitro ability to inhibit the growth of six bacterial strains in the assay based on two-fold serial dilutions. For this, two Gram-positive (B. cereus MTCC 430, S. aureus MTCC 96) and four Gram-negative (E. coli MTCC 1652, K. pneumoniae MTCC 432, S. typhimurium MTCC 98, P. putida MTCC 102) bacteria were selected. Unfortunately, this fluorinated aldimine was proven to be weak antibacterially active (revealing no inhibitory effects against all pathogenic bacterial strains recruited—MIC values above 294.7 µM) [22].

Mandewale et al. [23] have reported the procedure for synthesis, and they have carried out the antimycobacterial evaluation of aldimine-type Schiff bases 33–37 (Figure 9). These molecules were obtained by reacting equimolar quantities of 4-fluoroaniline, 2-fluoro-3-chloroaniline, 2-(trifluoromethyl)aniline, 3-(trifluoromethyl)aniline or 4-amino-2-(trifluoromethyl)benzonitrile with 6-fluoro-2-hydroxyquinoline-3-carboxaldehyde, under reflux for 0.5 h in ethanol without any catalyst assistance.

Figure 9.

Structures of fluorinated aldimines 33–37.

Activities of Schiff bases 33–37 against M. tuberculosis H₃₇Rv strain were established in the in vitro microplate Alamar Blue assay. Pyrazinamide (a pyrazine derivative), ciprofloxacin (a fluoroquinolone) and streptomycin (an aminoglycoside antibiotic) were used as reference drugs for comparison purposes. The designed aldimines 33–37 can be considered promising antimycobacterial agents as they revealed MIC values lower or comparable to that of recruited antimycobacterial agents (Table 6). The most active against M. tuberculosis H37Rv proved to be three molecules: 4-{(E)-[(6-fluoro-2-hydroxyquinolin-3-yl)methylidene]amino}-2-(trifluoromethyl)benzonitrile (37), 6-fluoro-3-{(E)-[(4-fluorophenyl)imino]methyl}quinolin-2-ol (33) and 6-fluoro-3-[(E)-{[2-(trifluoromethyl)phenyl]imino}methyl]quinolin-2-ol (35) [23].

Table 6.

Antimycobacterial activities of fluorinated aldimines 33–37.

Bacterial Strain	MIC (µM) *
	33	34	35	36	37	PZA	CPFX	STM
M. tuberculosis H37Rv	5.6	39.2	9.3	18.7	4.5	25.4	9.4	10.8

* MIC values provided in the original paper [23] in µg mL⁻¹ were converted into molar concentrations. PZA—pyrazinamide, CPFX—ciprofloxacin, STM—streptomycin.

İskeleli et al. [24] have reported the synthesis and structural characterization of 4-[(3-fluoro-4-hydroxy-5-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one (38) (Figure 10). This fluorinated aldimine was obtained by refluxing equimolar ratios of 4-amino-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one and 3-fluoro-4-hydroxy-5-methoxybenzaldehyde for 3 h in ethanol as the reaction medium.

Figure 10.

The structure of fluorinated aldimine 38.

The aldimine-type Schiff base 38 was subjected to the bioassay based on two-fold serial dilutions to determine its MIC values against some strains of bacteria: methicillin-sensitive S. aureus ATCC 25923, methicillin-resistant S. aureus ATCC 43300, S. pneumoniae ATTC 49619, E. faecalis ATTC 29212, E. coli ATCC 25922, K. pneumoniae ATCC 700603, P. aeruginosa ATCC 27853, S. maltophiliae ATCC 17666, H. influenzae ATCC 40247, E. casseliflavus ATCC 700327 and Salmonella spp. An aminoglycoside antibiotic—amikacin—was recruited as a standard antibacterial agent. Fluorinated aldimine 38 proved to be antibacterially active against three (S. pneumoniae, H. influenzae and E. faecalis) out of all eleven bacteria selected. In addition, its activity against S. pneumoniae and H. influenzae was higher than that of amikacin (Table 7) [24].

Table 7.

Antibacterial activities of fluorinated aldimine 38.

Bacterial Strain	MIC (µM) *
	38	Amikacin
S. pneumoniae ATCC 700603	140.7	170.8
H. influenzae ATCC 40247	70.4	85.4
E. faecalis ATTC 29212	70.4	42.7

* MIC values provided in the original paper [24] in µg mL⁻¹ were converted into molar concentrations.

Prakash and Raja [25] have synthesized and confirmed the structure and then conducted antimicrobial studies on fluorinated aldimine-type Schiff bases 39–50 (Figure 11) which may be regarded as novel hybrids with fluorinated quinolone ciprofloxacin. All these fluorinated structures may also be considered important Mannich bases due to the presence of the piperazin-1-ylmethyl moiety at the N1 of the indolin-2-one template. The synthesis of these aldimines was carried out by reacting equimolar ratios of 7-{4-(3-[4-aminophenylimino]-5-fluoro-2-oxoindolin-1-yl)methyl)piperazin-1-yl)}-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid with benzaldehyde, variously substituted benzaldehydes or cinnamylaldehyde, in refluxing ethanol for 8 h, containing a small amount of glacial acetic acid as an efficient catalyst.

Figure 11.

Structures of fluorinated aldimines 39–50.

To screen the antibacterial activities of particular fluorinated hybrids 39–50, that differ in electron-donating and electron-withdrawing substituent/substituents on the phenyl moiety attached to an azomethine function, three Gram-positive (S. aureus ATCC 9144, S. epidermidis ATCC 155, M. luteus ATCC 4698) and three Gram-negative (E. coli ATCC 25922, P. aeruginosa ATCC 2853, K. pneumoniae ATCC 11298) bacterial strains of clinical interest were recruited. To confirm the susceptibility of all bacterial strains a broad-spectrum antibacterial agent—ciprofloxacin—from the class of fluoroquinolones was used. Prakash and Raja have determined the MICs of all the synthesized fluorinated aldimines in the agar streak dilution assay. The majority of them revealed strong to moderate antibacterial activities (Table 8). Compounds containing electron-donating groups (41, 42, 44–47, 49) were found to be more active than those bearing electron-withdrawing groups (40, 43, 48). Therefore, the authors have suggested that electron-donating groups are preferred for the antibacterial activities of fluorinated aldimines. Schiff base 47—containing the 3-methoxy-4-hydroxyphenyl moiety—has been disclosed to be the most potent against K. pneumoniae. Its activity against this bacterial strain was 2.2-fold superior to that of ciprofloxacin. Additionally, its MIC values for S. epidermidis, M. luteus, S. aureus and E. coli were comparable to those of a standard drug. Aldimine 41—with the para-hydroxyphenyl moiety—has been reported to demonstrate the highest activity towards S. aureus, P. aeruginosa and E. coli with MIC values 4.2-, 2.1- and 2.1-fold lower, respectively, than those of ciprofloxacin. In addition, its activity against S. epidermidis and M. luteus proved to be similar to that of a standard drug. Schiff base 46—bearing the 3,4,5-trimethoxyphenyl moiety—has been identified to be the most active against S. epidermidis and M. luteus with MIC values 2.4-fold superior than those of ciprofloxacin. Moreover, its activity against S. aureus, P. aeruginosa and E. coli was higher or comparable to that of a standard drug. Aldimine 49—containing the para-dimethylaminophenyl moiety—has been reported to be the most active against P. aeruginosa and M. luteus. Its activity against these two bacterial strains was 2-fold superior to that of ciprofloxacin. Additionally, its MIC values for S. aureus, E. coli and S. epidermidis were lower or comparable to those of a standard drug. However, Schiff base 42—bearing the para-methoxyphenyl moiety—has been disclosed to exhibit 2.2-fold higher activity against S. aureus and E. coli when compared to ciprofloxacin [25].

Table 8.

Antibacterial activities of fluorinated aldimines 39–50.

Bacterial Strain	MIC (µM) *
	39	40	41	42	43	44	45	46	47	48	49	50	CPFX
S. aureus ATCC 9144	91.0	43.3	11.1	21.8	85.4	88.9	44.6	20.1	42.6	na	20.4	87.6	47.1
S. epidermidis ATCC 155	45.5	86.6	22.2	43.6	na	44.4	22.3	10.0	21.3	170.8	20.4	43.8	23.6
M. luteus ATCC 4698	na	43.3	22.2	43.6	170.8	44.4	na	10.0	21.3	85.4	10.2	na	23.6
E. coli ATCC 25922	91.0	43.3	22.2	21.8	na	44.4	44.6	40.2	42.6	na	40.8	na	47.1
P. aeruginosa ATCC 2853	89.1	86.6	11.1	43.6	85.4	88.9	44.6	20.1	na	na	10.2	87.6	23.6
K. pneumoniae ATCC 11298	na	21.6	22.2	43.6	85.4	44.4	89.1	20.1	5.3	85.4	20.4	175.3	11.8

* MIC values provided in the original paper [25] in µg mL⁻¹ were converted into molar concentrations. CPFX—ciprofloxacin; na—not active.

Durmuş et al. [26] have disclosed a synthesis scheme and preliminary results of the antibacterial evaluation of fluorinated dimeric disulfide Schiff base (51), i.e., (Z,Z)-N,N′-(disulfanediyldibenzene-2,1-diyl)bis[1-(2-fluorophenyl)methanimine] (Figure 12). The synthesis of this molecule was prepared by condensing 2,2′-disulfanediyldianiline with ortho-fluorobenzaldehyde (in molar ratios 1:2), in ethanol containing cerium oxide nanoparticles as an efficient catalyst, according to the general procedure reported earlier [27].

Figure 12.

The structure of fluorinated aldimine 51.

Durmuş research group have employed three Gram-negative bacilli such as K. pneumoniae, E. coli and A. baumannii, and one Gram-positive strain of S. aureus (all bacteria isolated from the hospitalized patients) to assess the antibacterial activities of fluorinated aldimine 51 in the disc diffusion assay. A third-generation cephalosporin, i.e., cefotaxime, and a broad-spectrum aminopenicillin, i.e., amoxicillin (in combination with an irreversible β-lactamase inhibitor—clavulanic acid), were used as antibacterial agents for comparison purposes. This fluorinated Schiff base was reported to reveal superior—to that of cefotaxime—activities against all human pathogenic bacterial strains, and comparable—to that of amoxicillin/clavulanic acid—effects against S. aureus and K. pneumoniae. The authors suggested that the attendance of two electron-withdrawing fluoro groups in an ortho position of both phenyl moieties as well as a very important structural feature, i.e., the reductive disulfide bridge, are necessary for the antibacterial activity of this fluorinated aldimine [26].

Oboňová et al. [28] have designed and synthesized (E,E)-N,N′-cyclohexane-1,2-diylbis[1-(4-fluorophenyl)methanimine (52) (Figure 13) by reacting 1,2-cyclohexanediamine with para-fluorobenzaldehyde (in molar ratios 1:2) in methanol at room temperature for 2 h and allowing the reaction mixture to successful crystallization for several days. This synthesis was relatively straightforward and proceeded without any catalyst assistance. The structure of this fluorinated aldimine-type bis-Schiff base was determined on the basis of spectroscopic and X-ray diffraction data [28].

Figure 13.

The structure of fluorinated aldimine 52.

The aldimine-type Schiff base 52 has been tested against a Gram-negative bacterial strain of E. coli CNCTC 377/79 and a Gram-positive bacterial strain of S. aureus CNCTC Mau 29/58 in the broth dilution assay. The most potent antibacterial agent among fluoroquinolones—ciprofloxacin—was chosen as a positive control. Fluorinated aldimine 52 revealed the same MIC value of 5706.3 µM against both bacteria. The authors suggested that the weak antibacterial activity of this compound is due to its rigid scaffold. Such a rigid structure containing two double CH=N bonds was most likely not flexible enough to enable interactions with the active sites of enzymes [28].

Zhang et al. [29] have reported the synthesis scheme, structure determination and results of in vitro antibacterial studies for two fluorinated aldimine-type Schiff bases—53 and 54 (Figure 14)—containing in their structures the moiety of 1-phenyl-3-phenylthiourea linked via an azomethine bridge to the 4-fluorophenyl or 2-fluorophenyl moiety, respectively. The synthesis of these aldimines was achieved successfully by condensing the stoichiometric ratios of 1-(4-aminophenyl)-3-phenylthiourea and 4-fluorobenzaldehyde or 2-fluorobenzaldehyde for 3–4 h at 80 °C in toluene containing PTSA as an efficient catalyst.

Figure 14.

Structures of fluorinated aldimines 53 and 54.

Zhang and co-workers have employed S. aureus ATCC 6538 and B. subtilis ATCC 6633 as Gram-positive bacterial strains, whereas E. coli ATCC 35218 and P. aeruginosa ATCC 13525 as Gram-negative bacterial strains to assess the antibacterial activities of fluorinated aldimines 53 and 54 in the two-fold serial dilution assay. As a positive control, two antibiotics such as penicillin G (benzylpenicillin) and kanamycin B (an aminoglycoside antibiotic), were employed in order to confirm the susceptibility of all bacterial strains recruited. The aldimine-type Schiff base 53, having the fluoro group at para position of the phenyl moiety, was reported to be moderately active against all pathogens selected (Table 9). This molecule revealed the highest activity against S. aureus, although its potency was found to be 3.8- and 11.2-fold lower than that of penicillin G and kanamycin B. However, the ortho-fluoro substituted aldimine 54 proved to be antibacterially inactive even at a concentration of 286.2 µM. Thus, the results of these antibacterial studies clearly indicated that the substitution by a fluorine atom at the para position of the phenyl moiety is more favorable for the antibacterial effect [29].

Table 9.

Antibacterial activities of fluorinated aldimines 53 and 54.

Bacterial Strain	MIC (µM) *
	53	54	Kanamycin B	Penicillin G
E. coli ATCC 35218	143.1	>286.2	3.2	9.4
P. aeruginosa ATCC 13525	143.1	>286.2	6.4	18.7
B. subtilis ATCC 6633	143.1	>286.2	0.8	4.7
S. aureus ATCC 6538	71.5	>286.2	6.4	18.7

* MIC values provided in the original paper [29] in µg mL⁻¹ were converted into molar concentrations.

Aggarwal et al. [30] have synthesized, confirmed the structure and then performed antimicrobial examination on two fluorinated aldimine-type Schiff bases—structures 55 and 56 (Figure 15)—which have in their molecular framework the privileged 4H-1,2,4-triazole-5-thiol scaffold linked via an azomethine bridge to the aromatic benzene ring bearing the 4-fluoro or 4-trifluoromethyl group, respectively. Molecules 55 and 56 were prepared by reacting 3-(4-amino-5-sulfanyl-4H-1,2,4-triazol-3-yl)-1-ethyl-7-methyl-1,8-naphthyridin-4(1H)-one with a molar excess of 4-fluorobenzaldehyde or 4-trifluoromethylbenzaldehyde, respectively, in boiling dioxane, with efficient catalytic assistance of a small amount of concentrated sulfuric acid.

Figure 15.

Structures of fluorinated aldimines 55 and 56.

Aldimine-type Schiff bases 55 and 56 were tested in the assay based on two-fold serial dilutions to estimate their in vitro abilities to inhibit the growth of two Gram-positive (S. aureus ATCC 2937, B. subtilis ATCC 12711) and three Gram-negative (E. coli ATCC 8739, K. pneumoniae ATCC 31488, P. aeruginosa ATCC 9027) bacterial strains of clinical interest. As a positive control, an aminoglycoside antibiotic streptomycin and a fluoroquinolone ciprofloxacin were employed to confirm the susceptibility of all bacterial strains used. The relatively high MIC values of fluorinated aldimines 55 and 56 against the majority of bacterial strains were reported in the studies of Aggarwal and co-workers (Table 10), confirming a low susceptibility of most pathogenic bacteria to both compounds. Notwithstanding, aldimine 55, with the fluoro substitution in para position of the phenyl moiety, proved to be 3.5-fold more active against P. aeruginosa than that containing the para-trifluoromethyl substitution (56), revealing a MIC value of 39.2 µM. Simultaneously, its activity was found to be 5.7- and 10.3-fold lower than that of streptomycin and ciprofloxacin, respectively. In turn, the fluorinated aldimine 56 was found to be 2.2-fold more active than 55 against K. pneumoniae [30].

Table 10.

Antibacterial activities of fluorinated aldimines 55 and 56.

Bacterial Strain	MIC (µM) *
	55	56	Streptomycin	Ciprofloxacin
S. aureus ATCC 2937	306.0	545.3	25.8	30.2
B. subtilis ATCC 12711	154.2	137.4	8.6	37.7
E. coli ATCC 8739	612.1	545.3	22.4	37.7
K. pneumoniae ATCC 31488	612.1	272.7	3.4	7.5
P. aeruginosa ATCC 9027	39.2	137.4	6.9	3.8

* MIC values provided in the original paper [30] in µg mL⁻¹ were converted into molar concentrations.

Malladi et al. [31] have synthesized, confirmed the structure and investigated the antibacterial activities of fluorinated aldimine-type Schiff bases 57–59 (Figure 16). These hybrid molecules contain in their molecular framework the privileged 4H-1,2,4-triazole-3-thiol template linked via an azomethine bridge to the pyrrazole scaffold bearing the 4-fluorophenyl at C3. The synthesis of these fluorinated aldimines was achieved successfully by condensing equimolar ratios of 4-amino-4H-1,2,4-triazole-3-thiol (unsubstituted or substituted by one alkyl group such as the ethyl or propyl at position 5) with 3-(4-fluorophenyl)-1H-pyrrazole-4-carboxaldehyde, in a two-component solvent medium (containing ethanol and dioxane), under reflux with the catalytic assistance of a small amount of concentrated sulfuric acid.

Figure 16.

Structures of fluorinated aldimines 57–59.

The authors have used clinical isolates of two Gram-positive (S. aureus, B. subtilis) and two Gram-negative (E. coli, P. aeruginosa) bacterial strains as well as antibiotic ceftriaxone from third-generation cephalosporins as a positive control to assess the antibacterial activities of aldimines 57–59. All fluorinated Schiff bases were reported to reveal significant antibacterial potencies against bacterial strains of S. aureus, B. subtilis, E. coli and P. aeruginosa with MIC values ranging from 5.1 to 43.4 µM (Table 11) when tested in the assay based on two-fold serial dilutions. Fluorinated aldimine 57 (with the ethyl substitution at position 5 of 1,2,4-triazole ring) has been identified as the most effective against all the selected bacteria. Nevertheless, its MIC values against S. aureus, B. subtilis, E. coli and P. aeruginosa proved to be about 1.8-fold higher than that of ceftriaxone [31].

Table 11.

Antibacterial activities of fluorinated aldimines 57–59.

Bacterial Strain	MIC (µM) *
	57	58	59	Ceftriaxone
S. aureus	9.9	10.8	20.5	5.6
B. subtilis	5.1	10.8	41.0	2.8
E. coli	5.1	43.4	41.0	2.8
P. aeruginosa	5.1	21.7	20.5	2.8

* MIC values provided in the original paper [31] in µg mL⁻¹ were converted into molar concentrations.

Zhang et al. [32] have designed and synthesized three fluorinated aldimine-type Schiff bases (structures 60–62) (Figure 17) bearing the 5-(2-pyrazinyl)-4H-1,2,4-triazole-3-thiol template linked via an azomethine bridge to the benzene ring containing a fluorine atom in various positions. The synthesis of compounds 60–62 was performed by reacting the starting 4-amino-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol with para-fluorobenzaldehyde, ortho-fluorobenzaldehyde or meta-fluorobenzaldehyde, respectively, in an ethanolic medium containing a small amount of acetic acid as an efficient catalyst.

Figure 17.

Structures of fluorinated aldimines 60–62.

To evaluate antibacterial activities of Schiff bases 60–62, the authors used three Gram-positive (S. aureus, B. subtilis, B. amyloliquefaciens) and two Gram-negative (E. coli, P. aeruginosa) bacterial strains of clinical interest as well as an antibiotic kanamycin B. MIC values of three fluorinated aldimines—established in the assay based on two-fold serial dilutions—ranged from 83.2 to above 166.5 µM (Table 12). All Schiff bases revealed the highest—although lower than that of kanamycin B—activity against E. coli. Zhang and co-workers have disclosed that the ortho-fluorophenyl group in this class of aldimines is a preferred substituent. Schiff base 61 with that substituent proved to be more active against P. aeruginosa than its para- and meta-fluorinated counterparts (60 and 62). The para-and ortho-fluorinated aldimine structures (60 and 61) were also reported to be more active against B. subtilis than their meta-fluorinated congener (62). In turn, all fluorinated Schiff bases (60–62) were found to be equally effective against S. aureus and B. amyloliquefaciens [32].

Table 12.

Antibacterial activities of fluorinated aldimines 60–62.

Bacterial Strain	MIC (µM) *
	60	61	62	Kanamycin B
E. coli	83.2	83.2	83.2	6.5
P. aeruginosa	>166.5	166.5	>166.5	3.2
S. aureus	166.5	166.5	166.5	3.2
B. subtilis	166.5	166.5	>166.5	6.5
B. amyloliquefaciens	166.5	166.5	166.5	3.2

* MIC values provided in the original paper [32] in µg mL⁻¹ were converted into molar concentrations.

Alshammari et al. [33] have synthesized and antibacterially screened fluorinated aldimine-type Schiff bases 63–67 (Figure 18). These compounds were obtained by condensing equimolar ratios of 4-amino-3-sulfanyl-6-(trifluoromethyl)-1,2,4-triazin-5(4H)-one with various aromatic aldehydes (i.e., 4-fluorobenzaldehyde, 4-chlorobenzaldehyde, 4-bromobenzaldehyde, 4-nitrobenzaldehyde or 4-(trifluoromethyl)benzaldehyde) in boiling ethanol containing a catalytic amount of sulfuric acid.

Figure 18.

Structures of fluorinated aldimines 63–67.

Aldimines 63–67 have been evaluated for their antibacterial activity against two Gram-negative (E. coli ATCC 25955, S. typhi) and two Gram-positive (S. aureus NRRL B-767, B. subtilis ATCC 6633) bacterial strains. Ciprofloxacin (a fluoroquinolone) was used as a standard drug. Among all fluorinated Schiff bases, the para-fluorophenyl-substituted aldimine (63) was found to be the most potent against E.coli, S. aureus and B. subtilis (Table 13) [33].

Table 13.

Antibacterial activities of fluorinated aldimines 63–67.

Bacterial Strain	MIC (µM) *
	63	64	65	66	67	CPFX
E. coli ATCC 25955	12.3	nd	329.7	nd	339.4	1.2
S. typhi	196.4	93.4	41.2	90.5	nd	nd
S. aureus NRRL B-767	12.3	93.4	82.4	181.0	84.9	3.8
B. subtilis ATCC 6633	24.5	93.4	164.8	90.5	84.9	9.8

* MIC values provided in the original paper [33] in µg mL⁻¹ were converted into molar concentrations. CPFX—ciprofloxacin; nd—not determined.

3. Fluorinated Ketimine-Type Schiff Bases

Chai et al. [34] have synthesized, confirmed the structure and then carried out antimicrobial studies on a series of ketimine-type Schiff bases (68–79) (Figure 19) related to gatifloxacin—a drug that belongs to the family of fluorinated quinolones. They may be regarded as novel imine hybrids with gatifloxacin. Ketimines 68–75 and 76–79 were obtained by refluxing for 3–4 h variable substituted amine hydrochlorides (in a molar excess) with 1-cyclopropyl-6-fluoro-8-methoxy-7-[4-(2-oxopropyl)-3-methylpiperazin-1-yl]-4-oxo-1,4-dihydroquinoline-3-carboxylic acid or 1-cyclopropyl-6-fluoro-8-methoxy-7-[4-(3-oxobutyl)-3-methylpiperazin-1-yl]-4-oxo-1,4-dihydroquinoline-3-carboxylic acid, respectively, in methanol containing an aqueous solution of sodium bicarbonate.

Figure 19.

Structures of fluorinated ketimines 68–79.

Chai et al. have selected Gram-positive (S. aureus ATCC 25923, methicillin-resistant S. aureus 08-1, methicillin-sensitive S. aureus 08-1, methicillin-resistant S. epidermidis 09-4, methicillin-sensitive S. epidermidis 09-3, methicillin-sensitive S. epidermidis 09-6, S. pneumoniae 08-2, S. pneumoniae 08-4, E. faecium 08-2, E. faecium 08-7, E. faecalis 08-10, E. faecalis 08-12) and Gram-negative (E. coli ATCC 25922, E. coli 08-21, E. coli 08-22, K. pneumoniae 09-22, K. pneumoniae 09-23, P. aeruginosa ATCC 27853, P. aeruginosa 09-32, P. aeruginosa 09-33, P. aeruginosa 09-34) bacterial strains of clinical interest that are susceptible or resistant to commonly used antibacterial agents to study antibacterial activities of all the synthesized fluorinated ketimines (68–79) in the assay based on two-fold serial dilutions. Gatifloxacin and levofloxacin (belonging to fluorinated quinolones) were employed as drugs for comparison purposes. It has been disclosed that the substitution at the C7 is a decisive factor for antibacterial activity in this class of compounds and that the vast majority of fluorinated ketimine structures exhibit high antibacterial potencies (Table 14). Fluorinated ketimine-type Schiff base 79 was identified as a possible antibacterial agent with a broad spectrum of activity. This molecule proved to be the most potent among all fluorinated ketimines, revealing MIC values ranging from 0.1 µM to 1.9 µM. In addition, its antibacterial activity against all the recruited strains was found to be superior to that of gatifloxacin and/or levofloxacin. Schiff base 73 also proved to be more active against the vast majority of bacteria than standard drugs. S. aureus, S. epidermidis and their methicillin-resistant strains were reported to be the most susceptible to fluorinated ketimines 69–71, 73, 74 and 79, revealing MIC values ranging from 0.1 µM to 0.6 µM. In turn, some Gram-negative strains of bacteria were found to be the most susceptible to compounds 71–73, 78 and 79. In addition, in cytotoxicity studies reported by the authors, gatifloxacin-derived Schiff base 71 was found to be the least toxic (IC₅₀ = 1450.4 µM) among all other ketimines (having IC₅₀ values ranging from 21.2 to 933.5 µM) towards mammalian Vero cells of the epithelial origin, indicating its high selectivity for bacterial cells [34].

Table 14.

Antibacterial activities of fluorinated ketimines 68–79.

Bacterial Strain	MIC (µM) *
	68	69	70	71	72	73	74	75	76	77	78	79	LVFX	GTFX
S. aureus ATCC25923	1.1	0.3	0.3	0.5	3.6	0.2	0.6	4.3	1.1	131.0	1.0	0.1	0.7	0.3
MRSA 08-1	>278.0	0.5	0.5	0.5	58.1	0.2	>286.7	69.5	>269.7	32.8	4.0	0.5	0.2	0.3
MSSA 08-1	0.5	0.5	0.5	0.5	3.6	0.2	0.6	69.5	4.2	262.0	2.0	0.2	0.7	0.2
MRSE 09-4	1.1	0.5	0.5	0.5	3.6	0.5	0.6	0.5	4.2	1.0	2.0	0.1	1.4	0.7
MSSE 09-3	1.1	0.5	0.5	0.5	0.5	0.5	0.6	0.5	1.1	1.0	0.5	0.1	0.2	0.7
MSSE 09-6	8.7	0.5	0.5	0.5	0.5	0.5	0.6	0.5	2.1	1.0	0.5	0.5	5.5	21.3
S. pneumoniae 08-2	139.0	>269.7	>269.7	16.4	58.1	0.5	286.7	69.5	>269.7	262.0	15.9	0.9	44.3	10.7
S. pneumoniae 08-4	4.3	4.2	4.2	0.5	7.3	0.5	0.6	8.7	>269.7	>262.0	4.0	0.5	11.1	0.7
E. faecium 08-2	>278.0	>269.7	>269.7	32.8	116.2	7.7	>286.7	139.0	>269.7	>262.0	15.9	0.9	11.1	5.3
E. faecium 08-7	>278.0	>269.7	>269.7	32.8	116.2	7.7	>286.7	139.0	>269.7	>262.0	15.9	0.9	22.1	10.7
E. faecalis 08-10	>278.0	>269.7	>269.7	32.8	232.5	7.7	>286.7	>278.0	>269.7	>262.0	15.9	0.9	22.1	10.7
E. faecalis 08-12	>278.0	>269.7	>269.7	32.8	232.5	7.7	>286.7	>278.0	>269.7	>262.0	15.9	1.9	22.1	5.3
E. coli ATCC25922	>278.0	269.7	269.7	16.4	29.1	1.0	286.7	34.7	269.7	>262.0	4.0	0.5	5.5	5.3
E. coli 08-21	>278.0	>269.7	>269.7	32.8	232.5	7.7	>286.7	139.0	>269.7	>262.0	15.9	1.9	22.1	10.7
E. coli 08-22	>278.0	>269.7	>269.7	32.8	116.2	0.5	>286.7	139.0	>269.7	>262.0	8.0	1.9	11.1	10.7
K. pneumoniae 09-22	>278.0	>269.7	>269.7	0.5	29.1	0.5	>286.7	139.0	>269.7	262.0	2.0	0.9	44.3	0.3
K. pneumoniae 09-23	>278.0	>269.7	>269.7	8.2	58.1	0.2	>286.7	17.4	>269.7	>262.0	4.0	1.9	22.1	5.3
P. aeruginosa ATCC 27853	>278.0	>269.7	>269.7	8.2	7.3	3.8	71.7	17.4	269.7	>262.0	8.0	0.9	11.1	2.7
P. aeruginosa 09-32	>278.0	>269.7	>269.7	0.5	7.3	0.5	71.7	8.7	269.7	262.0	0.5	0.9	44.3	21.3
P. aeruginosa 09-33	>278.0	>269.7	>269.7	131.3	232.5	7.7	>286.7	>278.0	>269.7	>262.0	31.8	0.9	22.1	10.7
P. aeruginosa 09-34	>278.0	>269.7	>269.7	4.1	58.1	0.5	>286.7	17.4	>269.7	>262.0	4.0	0.5	2.8	2.7

* MIC values provided in the original paper [34] in µg mL⁻¹ were converted into molar concentrations. MRSA—methicillin-resistant S. aureus, MSSA—methicillin-sensitive S. aureus, MRSE—methicillin-resistant S. epidermidis, MSSE—methicillin-sensitive S. epidermidis; LVFX—levofloxacin, GTFX—gatifloxacin.

Malhotra et al. [35] have reported the synthesis and results of the antimicrobial examination for a series of fluorinated ketimine-type Schiff bases 80–82 (Figure 20) bearing the privileged scaffold of 2,3-dihydro-1,3,4-oxadiazole. The synthesis of the above ketimines was carried out by reacting the stoichiometric ratios of the primary aromatic amine, such as ortho-fluoroaniline, meta-fluoroaniline or para-fluoroaniline, and ketone, i.e., 1-[5-(biphenyl-4-yl)-2-(2-hydroxyphenyl)-1,3,4-oxadiazol-3(2H)-yl]ethanone, for 9–11 h in refluxing anhydrous ethanol, containing a small amount of glacial acetic acid as an efficient catalyst.

Figure 20.

Structures of fluorinated ketimines 80–82.

The authors have recruited two Gram-positive (B. subtilis MTCC 96, S. aureus MTCC 121) and two Gram-negative (P. aeruginosa MTCC 2453, E. coli MTCC 40) bacterial strains of clinical interest to investigate antibacterial activities of ketimines 80–82 in the assay based on two-fold serial dilutions. Ciprofloxacin was used as an antibacterial agent. The MIC and MBC (minimum bactericidal concentration) values of Schiff bases 80–82—which differ in the position of fluorine substitution (ortho, meta and para) at the phenyl moiety—against all the recruited bacterial strains are listed in Table 15. Results revealed that fluorinated ketimines possess MIC values ranging from 27.7 to 110.7 µM and MBC values ranging from 55.4 to 221.4 µM. It has been shown that structure 80, bearing the ortho-fluorophenyl moiety, reveals the highest antibacterial activities, although lower than those of ciprofloxacin. Furthermore, it has been established that molecule with the para-fluorophenyl moiety (82) is more antibacterially active than that with the meta-fluorophenyl moiety (81) [35].

Table 15.

Antibacterial activities of fluorinated ketimines 80–82.

Bacterial Strain	80		81		82		Ciprofloxacin
	MIC (µM) *	MBC (µM) *	MIC (µM) *	MBC (µM) *	MIC (µM) *	MBC (µM) *	MIC (µM) *	MBC (µM) *
B. subtilis MTCC 96	55.4	110.7	110.7	221.4	27.7	55.4	18.1	37.7
S. aureus MTCC 121	27.7	110.7	55.4	110.7	27.7	55.4	18.1	37.7
P. aeruginosa MTCC 2453	55.4	110.7	27.7	55.4	110.7	221.4	18.1	37.7
E. coli MTCC 40	27.7	55.4	55.4	110.7	55.4	110.7	18.1	37.7

* MIC and MBC values provided in the original paper [35] in µg mL⁻¹ were converted into molar concentrations.

Haj Mohammad Ebrahim Tehrani et al. [36] have described the antibacterial evaluation of fluorinated ketimine-type Schiff bases 83–88 (Figure 21) obtained by a condensation/dehydration reaction of 1-aminohydantoin, semicarbazide or thiosemicarbazide and variously substituted isatines (stoichiometric amounts). The synthetic process has been carried out in refluxing ethanol for 5 h, with the assistance of small amount of glacial acetic acid as an efficient catalyst. The obtained fluorinated compounds possess in their molecular framework the privileged 2-oxo-1,2-dihydro-3H-indole template linked via an azomethine bridge to the important pharmacophoric moieties.

Figure 21.

Structures of fluorinated ketimines 83–88.

The authors have recruited three Gram-positive (S. aureus ATCC 25923, methicillin-resistant S. aureus ATCC 43300, E. faecalis ATCC 29212) and two Gram-negative (P. aeruginosa ATCC 27853, E. coli ATCC 25922) bacterial strains of clinical interest to investigate antibacterial activities of fluorinated ketimines 83–88 in the assay based on two-fold serial dilutions. As a positive control, a broad-spectrum antibiotic—amikacin—was included to confirm the susceptibility of all bacterial strains used. Haj Mohammad Ebrahim Tehrani et al. have reported that the antibacterial activity of the synthesized small molecules is dependent on their lipophilicity, and the most active compounds reveal Clog p values ranging from 0.72 to 2.27. It has been disclosed that enhanced antibacterial activity is achieved by introducing at N1 the phenylmethyl moiety fluorinated in ortho, meta or para position. Three 1,2-dihydro-3H-indolin-2-one-hydantoin hybrids (83, 84 and 85) have been identified to be the most promising molecules that might find utility in the future as possible antibacterial agents after further lead optimization studies. Their MIC values against E. coli, S. aureus and methicillin-resistant S. aureus were 1.5 or 3.0-fold lower than those of amikacin (Table 16). In turn, it has been proved that replacing the fluorinated phenylmethyl moiety with a hydrogen atom and introducing a fluorine atom into the position C5 of 1,2-dihydro-3H-indolin-2-one template is disadvantageous, leading to a less antibacterially active fluorinated molecule 86. The authors have also disclosed that fluorinated isatin-based thiosemicarbazone (88) is more active than fluorinated isatin-based semicarbazone (87) [36].

Table 16.

Antibacterial activities of fluorinated ketimines 83–88.

Bacterial Strain	MIC (µM) *
	83	84	85	86	87	88	Amikacin
P. aeruginosa ATCC 27853	35.5	35.5	35.5	95.3	56.3	26.2	27.3
E. coli ATCC 25922	17.7	35.5	17.7	47.7	>225.0	209.9	27.3
E. faecalisATCC 29212	35.5	35.5	35.5	>190.7	112.5	209.9	21.3
S. aureus ATCC 25923	17.7	17.7	17.7	47.7	>225.0	104.9	52.9
MRSA ATCC 43300	17.7	17.7	17.7	95.3	>225.0	104.9	52.9

* MIC values provided in the original paper [36] in µg mL⁻¹ were converted into molar concentrations. MRSA—methicillin-resistant S. aureus.

Hassan et al. [37] have described the general synthetic approach leading to thiosemicarbazones with the privileged template of 2-oxo-1,2-dihydro-3H-indole. The synthesis of these fluorinated ketimine-type Schiff bases (89–91) (Figure 22) was carried out by condensing equimolar ratios of thiosemicarbazide with 1-(2-fluorobenzyl)-1H-indole-2,3-dione, 1-(3-fluorobenzyl)-1H-indole-2,3-dione or 1-(4-fluorobenzyl)-1H-indole-2,3-dione, respectively, for 6 h in refluxing ethanol containing a catalytic amount of glacial acetic acid.

Figure 22.

Structures of fluorinated ketimines 89–91.

A number of bacterial strains of clinical interest (S. aureus PTCC 1337, S. epidermidis PTCC 1435, B. cereus PTCC 1015, E. coli PTCC 1330, P. aeruginosa PTCC 1310, E. faecalis PTCC 13294, methicillin-resistant S. aureus, Salmonella spp.) were used in the microbroth assay based on two-fold serial dilutions to investigate antibacterial activities (expressed as MICs and MBCs—Table 17) of ketimines 89–91. As positive controls, two broad-spectrum antibiotics, such as amikacin (a semisynthetic kanamycin derivative) and teicoplanin (a bactericidal glycopeptide), were employed to confirm the susceptibility of all bacterial strains recruited. Hassan et al. have reported that all fluorinated ketimine-type Schiff bases (89–91), bearing an electron-withdrawing and lipophilic fluoro group in ortho, meta or para position of the phenylmethyl moiety, are able to inhibit the growth of all recruited bacterial strains (including opportunistic strains of Salmonella spp. and P. aeruginosa) at MICs ranging from 152.3 µM to 243.6 µM. In addition, these fluorinated thiosemicarbazones have been shown to be bactericidal against most bacterial strains at a concentration of 243.6 µM. Interestingly, all the compounds—although different in having the fluorine atom in ortho, meta or para position at the benzyl moiety—showed the same antibacterial activities against Gram-positive and Gram-negative bacterial strains, suggesting that this activity is due to electron-withdrawing properties of the fluorine atom, and not its position [37].

Table 17.

Antibacterial activities of fluorinated ketimines 89–91.

Bacterial Strain	89		90		91		Amikacin		Teicoplanin
	MIC (µM) *	MBC (µM) *	MIC (µM) *	MBC (µM) *	MIC (µM) *	MBC (µM) *	MIC (µM) *	MBC (µM) *	MIC (µM) *	MBC (µM) *
S. aureus PTCC 1337	213.2	243.6	213.2	243.6	213.2	243.6	52.9	52.9	4.2	4.2
S. epidermidis PTCC 1435	213.2	na	213.2	na	213.2	na	52.9	52.9	4.2	4.2
E. coli PTCC 1330	243.6	243.6	243.6	243.6	243.6	243.6	52.9	52.9	nd	nd
Salmonella spp.	152.3	243.6	152.3	243.6	152.3	243.6	25.6	25.6	nd	nd
B. cereus PTCC 1015	243.6	243.6	243.6	243.6	243.6	243.6	52.9	52.9	nd	nd
E. faecalis PTCC 13294	213.2	na	213.2	na	213.2	na	25.6	na	1.92	1.92
P. aeruginosa PTCC 1310	213.2	243.6	213.2	243.6	213.2	243.6	25.6	25.6	nd	nd
MRSA	213.2	243.6	213.2	243.6	213.2	243.6	52.9	52.9	16.5	16.5

* MIC and MBC values provided in the original paper [37] in µg mL⁻¹ were converted into molar concentrations. MRSA—methicillin-resistant S. aureus; na—not active, nd—not determined.

4. Fluorinated Hydrazine-Hydrazones

Shirinzadeh et al. [38] have published a report in which they performed the antibacterial evaluation of six fluorinated hydrazine-hydrazones with the indole scaffold (92–97) (Figure 23). These compounds were obtained by condensing 1-methylindole-3-carboxaldehyde with a small molar excess of para-fluorophenylhydrazine, meta-fluorophenylhydrazine, ortho-fluorophenylhydrazine, 2,4-difluorophenylhydrazine, 2,5-difluorophenylhydrazine or 3,5-difluorophenylhydrazine, in ethanol containing sodium acetate as a catalyst [39].

Figure 23.

Structures of fluorinated hydrazine-hydrazones 92–97.

The authors have used four Gram-positive (S. aureus ATCC 25923, methicillin-resistant S. aureus ATCC 43300, methicillin-resistant S. aureus isolate, B. subtilis ATCC 6633) and one Gram-negative (E. coli 23556) bacterial strains to determine antibacterial activities of the designed hydrazones 92–97. For this, the biological assay based on two-fold serial dilutions was performed, in which sultamicillin, ampicillin (aminopenicillins) and ciprofloxacin (a fluoroquinolone) were used as positive controls. Among all the molecules tested only two fluorinated hydrazones: 3-{(E)-[2-(2,4-difluorophenyl)hydrazinylidene]methyl}-1-methyl-1H-indole (95) and 3-{(E)-[2-(4-fluorophenyl)hydrazinylidene]methyl}-1-methyl-1H-indole (92) proved to be more active than ampicillin against B. subtilis (Table 18) [38].

Table 18.

Antibacterial activities of fluorinated hydrazine-hydrazones 92–97.

Bacterial Strain	MIC (µM) *
	92	93	94	95	96	97	SLT	AMP	CPFX
S. aureus ATCC 25923	93.5	374.1	374.1	350.5	350.5	21.9	1.3	4.5	0.6
MRSA ATCC 43300	na	187.1	374.1	350.5	87.6	350.5	35.8	nd	nd
MRSA isolate	187.1	374.1	374.1	87.6	350.5	21.9	nd	nd	nd
E. coli ATCC 23556	374.1	374.1	374.1	175.3	175.3	175.3	42.0	nd	0.3
B. subtilis ATCC 6633	93.5	187.1	374.1	87.6	175.3	175.3	1.3	143.1	0.3

* MIC values provided in the original paper [38] in µg mL⁻¹ were converted into molar concentrations. MRSA—methicillin-resistant S. aureus; SLT—sultamicillin, AMP—ampicillin, CPFX—ciprofloxacin; na—not active, nd—not determined.

Maddila et al. [40] have synthesized and antibacterially investigated fluorinated hydrazine-hydrazone 98 (Figure 24) bearing the privileged pyrido[2,3-d]pyrimidin-4(3H)-one template. The synthesis of this hydrazone was carried out in a straightforward manner, by reacting heterocyclic hydrazine (i.e., 5-amino-6-(1,3-benzothiazol-2-yl)-7-(4-chlorophenyl)-2-hydrazinylpyrido[2,3-d]pyrimidin-4(3H)-one) with 4-fluorobenzaldehyde in molar ratios 1:3, at ambient temperature for 10 h without any catalyst assistance, employing N,N-dimethylformamide as the reaction medium.

Figure 24.

The structure of fluorinated hydrazine-hydrazone 98.

The authors have recruited two Gram-positive (S. aureus, S. pyogenes) and three Gram-negative (E. coli, K. pneumoniae, P. aeruginosa) bacterial strains of clinical interest to determine the antibacterial activities of hydrazone 98. For this, the biological assay based on two-fold serial dilutions was carried out. In turn, ciprofloxacin was used as a positive control. Fluorinated hydrazone 98 was reported to reveal remarkable potencies against all Gram-positive as well as Gram-negative bacteria (Table 19). Its activity against S. aureus and K. pneumoniae was 3.3-fold superior to that of ciprofloxacin, while against S. pyogenes, E. coli and P. aeruginosa it was 1.6-fold better when compared to this standard drug. Therefore, this molecule was proposed as a possible antibacterial agent [40].

Table 19.

Antibacterial activities of fluorinated hydrazine-hydrazone 98.

Bacterial Strain	MIC (µM) *
	98	Ciprofloxacin
S. aureus	23.1	75.4
S. pyogenes	23.1	37.7
E. coli	46.1	75.4
K. pneumoniae	46.1	150.9
P. aeruginosa	46.1	75.4

* MIC values provided in the original paper [40] in µg mL⁻¹ were converted into molar concentrations.

Hamurcu et al. [41]—by reacting equimolar ratios of 3,5-di-tert-butyl-2-hydroxybenzaldehyde and (pentafluorophenyl)hydrazine in ethanol at room temperature in the presence of catalytic amount of magnesium sulfate—have synthesized new fluorinated hydrazine-hydrazone, i.e., 3,5-di-tert-butyl-6-[2-(pentafluorophenyl)hydrazinylidene]metyl}phenol as a mixture of E:Z enantiomers (99 E and 99 Z) (Figure 25). Based on integral intensities of signals corresponding to the proton of the OH group in 500 MHz PMR spectrum, the E:Z enantiomer ratios in solution were established as 88:12. Additionally, the synthesized molecule was characterized by FTIR, ¹³C NMR, ¹⁹F NMR and X-ray diffraction data [41].

Figure 25.

Structures of fluorinated hydrazine-hydrazone enantiomers 99.

The authors’ original paper has included information that this molecule was tested against pathogenic bacterial strains of E. coli and S. aureus in the broth microdilution method. However, there is a lack of a full description of these bacterial strains. Compound 99 revealed only weak antibacterial activity towards E. coli and S. aureus as its determined MIC values against these bacterial strains were found to be higher than 603.3 µM [41].

Dommati et al. [42] have synthesized fluorinated hydrazine-hydrazone 100 (Figure 26), by condensing equimolar ratios of 2-{2-[(4-fluorophenyl)sulfanyl]ethoxy}-5-[(E)-hydrazinylidenemethyl]-3-methoxybenzonitrile and 2,5-difluorobenzaldehyde in ethanol under reflux for 1 h without any catalyst assistance. The authors have recorded duplication of signals in the 400 MHz ¹H NMR spectrum for this compound, giving proof that this hydrazone exists as a mixture of anti- and syn-periplanar conformers.

Figure 26.

The structure of fluorinated hydrazine-hydrazone 100.

The antibacterial activity of hydrazone 100 towards Gram-negative (E. coli MTCC 2692, P. aeruginosa MTCC 2453) and Gram-positive (S. aureus MTCC 902, B. subtilis MTCC 441) bacteria has been determined in the disc-diffusion method, employing streptomycin (an aminoglycoside antibiotic) as a standard drug. Unfortunately, this fluorinated hydrazone was capable of revealing only moderate antibacterial activity against bacterial strains recruited when compared to that of streptomycin [42].

Celik et al. [43] have reported the antibacterial studies of four fluorinated hydrazine-hydrazones (101–104) (Figure 27) that have been previously synthesized. Compounds 101 and 102 were obtained by condensing quinoline-2-carbaldehyde with a small molar excess of 2-fluorophenylhydrazine or 4-fluorophenylhydrazine in boiling ethanol for 8 h [44], whereas molecules 103 and 104 were synthesized by reacting quinoline-2-carbaldehyde with a molar excess of 2,4-difluorophenylhydrazine or 2,5-difluorophenylhydrazine in refluxing ethanol containing sodium acetate as an efficient catalyst [45].

Figure 27.

Structures of fluorinated hydrazine-hydrazones 101–104.

The authors have employed S. aureus ATCC 29213, S. aureus isolate, E. faecalis ATCC 29212 and E. faecalis isolate as Gram-positive bacteria and E. coli ATCC 25922, E. coli isolate, P. aeruginosa ATCC 27853 and P. aeruginosa isolate as Gram-negative bacteria to investigate the antibacterial activity of fluorinated hydrazones 101–104 in the microbroth assay based on two-fold serial dilutions. Ampicillin (an aminopenicillin), vancomycin (a glycopeptide), gentamycin (an aminoglycoside), ciprofloxacin (a fluoroquinolone) and cefotaxime (a second-generation cephalosporin) were used as positive controls to confirm the susceptibility of all bacterial strains recruited. Among all the screened hydrazones, compound 104—bearing the 2,5-difluorophenyl substitution—was found to be the most active, revealing a MIC value against E. faecalis ATCC 29212 that was superior or comparable to most antibiotics recruited (Table 20) [43].

Table 20.

Antibacterial activities of fluorinated hydrazine-hydrazones 101–104.

Bacterial Strain	MIC (µM) *
	101	102	103	104	AMP	VAN	GEN	CPFX	CTX
S. aureus ATCC 29213	241.3	241.3	225.9	225.9	1.43	0.3	0.5	1.5	2.2
S. aureus isolate	120.6	120.6	113.0	113.0	>45.8	1.4	>33.5	>48.3	>35.1
E. faecalis ATCC 29212	60.3	60.3	56.5	7.1	5.7	1.4	8.4	6.1	8.8
E. faecalis isolate	60.3	60.3	56.5	56.5	>45.8	>5.5	>16.8	>12.1	>17.6
E. coli ATCC 25922	120.6	120.6	113.0	56.5	22.9	nd	1.0	0.05	0.3
E. coli isolate	120.6	120.6	113.0	28.2	>45.8	nd	>16.8	>6.1	>17.6
P. aeruginosa ATCC 27853	60.3	120.6	113.0	113.0	nd	nd	1.0	0.4	17.6
P. aeruginosa isolate	120.6	120.6	113.0	113.0	nd	nd	>16.8	>6.1	nd

* MIC values provided in the original paper [43] in µg mL⁻¹ were converted into molar concentrations. AMP—ampicillin; VAN—vancomycin; GEN—gentamicin; CPFX—ciprofloxacin; CTX—cefotaxime; nd—not determined.

5. Fluorinated Hydrazide-Hydrazones

Popiołek et al. [46] have synthesized fluorinated hydrazide-hydrazones 105–107 (Figure 28) by reacting 5-nitrofuran-2-carboxylic acid hydrazide with a small molar excess of ortho-fluorobenzaldehyde, meta-fluorobenzaldehyde or para-fluorobenzaldehyde, respectively, in ethanol for 2 h without any catalyst assistance.

Figure 28.

Structures of fluorinated hydrazide-hydrazones 105–107.

Seven Gram-positive (S. aureus ATCC 25923, S. aureus ATCC 6538, S. aureus ATCC 43300, S. epidermidis ATCC 12228, M. luteus ATCC 10240, B. subtilis ATCC 6633, B. cereus ATCC 10876) and six Gram-negative (B. bronchiseptica ATCC 4617, K. pneumoniae ATCC 13883, P. mirabilis ATCC 12453, S. typhimurium ATCC 14028, E. coli ATCC 25922, P. aeruginosa ATCC 9027) bacterial strains have been recruited by the authors to determine—in the assay based on two-fold serial dilutions—antibacterial activities of the synthesized hydrazones 105–107. Four antimicrobial agents—such as ciprofloxacin (a fluoroquinolone), cefuroxime (a second-generation cephalosporin), ampicillin (an aminopenicillin) and nitrofurantoin (a derivative of 5-nitrofurfural)—were used as standard drugs. All the designed fluorinated hydrazones proved to be more active against Gram-positive S. aureus ATCC 6538, S. epidermidis ATCC 12228 and B. subtilis ATCC 6633 than nitrofurantoin, while against B. subtilis ATCC 6633 also than cefuroxime and ampicillin. Additionally, compounds 105 and 106 revealed better activity against S. aureus ATCC 43300 than that of nitrofurantoin (Table 21) [46].

Table 21.

Antibacterial activities of fluorinated hydrazide-hydrazones 105–107.

Bacterial Strain	MIC (µM) *
	105	106	107	CPFX	NFN	CXM	AMP
S. aureus ATCC 25923	225.5	112.7	225.5	1.5	65.6	1.2	nd
S. aureus ATCC 6538	56.4	56.4	14.1	0.7	65.6	2.3	nd
S. aureus ATCC 43300	28.2	28.2	56.4	0.7	32.8	nd	nd
S. epidermidis ATCC 12228	14.1	7.0	1.7	0.4	16.4	0.6	nd
M. luteus ATCC 10240	450.9	1803.7	901.9	3.0	262.4	2.3	nd
B. subtilis ATCC 6633	7.0	7.0	14.1	0.1	16.4	36.8	178.9
B. cereus ATCC 10876	225.5	225.5	112.7	0.2	32.8	73.6	nd
B. bronchiseptica ATCC 4617	3607.4	3607.4	na	3.0	524.9	nd	nd
K. pneumoniae ATCC 13883	na	na	901.9	0.4	65.6	nd	nd
P. mirabilis ATCC 12453	na	na	901.9	0.1	262.4	nd	nd
S. typhimurium ATCC 14028	na	na	901.9	0.2	131.2	nd	nd
E. coli ATCC 25922	na	na	450.9	0.01	32.8	nd	nd
P. aeruginosa ATCC 9027	na	na	na	1.5	na	nd	nd

* MIC values provided in the original paper [46] in µg mL⁻¹ were converted into molar concentrations. CPFX—ciprofloxacin, NFN—nitrofurantoin, CXM—cefuroxime, AMP—ampicillin; na—not active, nd—not determined.

Li et al. [47] have reported the design, synthesis scheme and results of in vitro antibacterial studies for a series of fluorinated hydrazide-hydrazones (structures 108–111) (Figure 29). They may also be regarded as derivatives of the drug secnidazole, revealing antibacterial and antiprotozoal activities and belonging to the common family of 5-nitroimidazoles. The synthesis of these fluorinated small molecules was performed by reacting stoichiometric ratios of the starting 2-(2-methyl-5-nitro-1H-imidazol-1-yl)acetohydrazide with benzaldehyde fluorinated in different position/positions, in methanol (as the reaction medium) without any catalytic assistance, on ice bath for 3–6 h.

Figure 29.

Structures of fluorinated hydrazide-hydrazones 108–111.

Two Gram-positive (S. aureus ATCC 6538, B. subtilis ATCC 530) and two Gram-negative (E. coli ATCC 25922, P. aeruginosa ATCC 27853) bacterial strains of clinical interest have been selected in order to determine—in the assay based on two-fold serial dilutions—antibacterial activities of all fluorinated hydrazones (108–111). An aminoglycoside antibiotic, kanamycin B, was employed as a positive control to confirm the susceptibility of all bacterial strains. Hydrazone 111, containing the 2,4-difluoro substitution at the phenyl moiety, was reported to reveal the highest activity against S. aureus. However, comparing the results of preliminary antibacterial screenings carried out on particular hydrazones (108, 109 and 110) with the substitution by one fluoro group at the phenyl moiety in para, meta and ortho positions, respectively, it is clearly seen that a meta-fluoro substitution in this class of molecules is necessary for the antibacterial activity against all bacterial strains. It has been confirmed for fluorinated hydrazone 109, which was reported to be the most active against S. aureus and B. subtilis (Table 22). Even the least active fluorinated hydrazones (108 and 110) were reported by Li and co-workers to reveal the IC₅₀ values of 58.3 and 47.5 µM, respectively, when tested as ligands in the target enzymatic assay for their inhibitory activities against the ecKAS III [47].

Table 22.

Antibacterial activities of fluorinated hydrazide-hydrazones 108–111.

Bacterial Strain	MIC (µM) *
	108	109	110	111	Kanamycin B
E. coli ATCC 25922	>327.6	164.0	>327.6	154.7	6.5
P. aeruginosa ATCC 27853	>327.6	164.0	>327.6	154.7	6.5
B. subtilis ATCC 530	>327.6	81.9	>327.6	154.7	3.2
S. aureus ATCC 6538	>327.6	81.9	>327.6	77.3	3.2

* MIC values provided in the original paper [47] in µg mL⁻¹ were converted into molar concentrations.

Kumar et al. [48] have prepared fluorinated hydrazide-hydrazones 112 and 113 (Figure 30) containing the privileged scaffold of 1H-benzo[d]imidazole via the condensation reaction of 1-propyl-2-(2,4-dichlorophenyl)-1H-benzo[d]imidazol-5-ylcarbohydrazide with 2-fluorobenzaldehyde or 4-fluorobenzaldehyde, respectively, in ethanol, in the presence of small amount of glacial acetic acid as an efficient catalyst.

Figure 30.

Structures of fluorinated hydrazide-hydrazones 112 and 113.

Two Gram-positive (S. aureus MTCC 3160, B. subtilis MTCC 441) and two Gram-negative (E. coli MTCC 4351, K. pneumoniae MTCC 3384) bacterial strains of clinical interest have been recruited to determine antibacterial activities of hydrazones 112 and 113 in the bioassay based on two-fold serial dilutions. Ampicillin has been used as a standard antibacterial agent. Both fluorinated compounds were capable of revealing lower activities against S. aureus, B. subtilis, E. coli and K. pneumoniae than ampicillin (Table 23). The most antibacterially active was found to be hydrazone 113 with a fluorine atom at position 4 of the phenyl moiety which proved to be two-fold more active against S. aureus and K. pneumoniae than hydrazone 112 with a fluorine atom at position 2, suggesting that in this case fluorine substitution in the para position of the phenyl is preferred [48].

Table 23.

Antibacterial activities of fluorinated hydrazide-hydrazones 112 and 113.

Bacterial Strain	MIC (µM) *
	112	113	Ampicillin
S. aureus MTCC 3160	26.6	13.3	4.5
B. subtilis MTCC 441	13.3	13.3	4.5
E. coli MTCC 4351	13.3	13.3	4.5
K. pneumoniae MTCC 3384	26.6	13.3	8.9

* MIC values provided in the original paper [48] in µg mL⁻¹ were converted into molar concentrations.

Yadav et al. [49] have designed and synthesized fluorinated hydrazone 114, i.e., 2-(1H-benzimidazol-2-ylsulfanyl)-N′-[(E)-(4-fluorophenyl)methylidene]acetohydrazide (Figure 31), by refluxing equimolar ratios of 2-(1H-benzimidazol-2-ylsulfanyl)acetohydrazide with 4-fluorobenzaldehyde in ethanol, in the presence of small amount of glacial acetic acid as a catalyst.

Figure 31.

The structure of fluorinated hydrazide-hydrazone 114.

The authors have recruited three reference bacterial strains (E. coli MTCC 1652, B. subtilis MTCC 2063 and S. aureus MTCC 2901) and one reference strain of M. tuberculosis H₃₇Rv to determine antibacterial activities of fluorinated hydrazone 114. Standard drugs—cefadroxil (a first-generation cephalosporin) and streptomycin (an aminoglycoside antibiotic)—have been used. This compound proved to be 9-fold more potent than cefadroxil against E. coli, B. subtilis and S. aureus. On the other hand, hydrazone 114 was capable of revealing 2.1-fold lower antitubercular activity than streptomycin against M. tuberculosis (Table 24) [49].

Table 24.

Antibacterial activities of fluorinated hydrazide-hydrazone 114.

Bacterial Strain	MIC (µM) *
	114	Cefadroxil	Streptomycin
E. coli MTCC 1652	0.04	0.35	nd
B. subtilis MTCC 2063	0.04	0.35	nd
S. aureus MTCC 2901	0.04	0.35	nd
M. tuberculosis H37Rv	45.7	nd	21.5

* MIC values for M. tuberculosis H₃₇Rv provided in the original paper [49] in µg mL⁻¹ were converted into molar concentrations. nd—not determined.

Manikandan et al. [50] have synthesized N′-[(Z)-(4-fluorophenyl)methylidene]benzohydrazide 115 (Figure 32), by stirring equimolar ratios of benzohydrazide with 4-fluorophenylbenzaldehyde, at ambient temperature for 0.5 h, in anhydrous ethanol, in the presence of sodium hydroxide.

Figure 32.

The structure of fluorinated hydrazide-hydrazone 115.

Ince et al. [51] have conducted antibacterial studies on fluorinated hydrazide-hydrazones 116–122 derived from para-hydroxybenzoic acid hydrazide (Figure 33). These compounds were synthesized by condensing equimolar quantities of substituted aromatic aldehydes (i.e., 4-fluoro-3-(trifluoromethyl)benzaldehyde, 2-(trifluoromethoxy)benzaldehyde, 3-(trifluoromethoxy)benzaldehyde, 4-fluoro-3-methoxybenzaldehyde, 4-(trifluoromethoxy)benzaldehyde, 3,5-bis(trifluoromethoxy)benzaldehyde or 4-fluoro-3-phenoxybenzaldehyde) with this hydrazide in ethanol containing a catalytic amount of glacial acetic acid [52].

Figure 33.

Structures of fluorinated hydrazide-hydrazones 116–122.

All fluorinated hydrazones (116–122) have been screened in the assay MTT-based for their antibacterial activity against S. aureus (ATCC 29213, as well as the clinical isolate) and E. coli (ATCC 25922, as well as the clinical isolate). For comparison purposes, ampicillin, gentamicin and vancomycin were used as standard antibiotics. Among the studied molecules, only hydrazone 121 revealed significant inhibition of S. aureus ATCC 29213 strain, and its activity was comparable (MIC = 5.3 µM) to that of ampicillin (MIC = 5.1 µM). MIC values of the remaining compounds against bacterial strains recruited ranged from 197.4 to 888.1 µM [51]. Based on this, it can be assumed that the substitution of the phenyl moiety with two meta-trifluoromethyl groups in 121 was responsible for the potent activity of this promising antibacterial molecule candidate.

Wang et al. [53] have designed, synthesized and investigated fluorinated hydrazide-hydrazones 123–125 (Figure 34) derived from vanillic acid carbohydrazide. The synthesis of the above-mentioned hydrazones was successfully performed by condensing equimolar ratios of the starting 4-hydroxy-3-methoxybenzohydrazide with para-fluorobenzaldehyde, meta-fluorobenzaldehyde or ortho-fluorobenzaldehyde, in ethanol containing a small amount of glacial acetic acid as a catalyst.

Figure 34.

Structures of fluorinated hydrazide-hydrazones 123–125.

Wang et al. have recruited two Gram-positive (S. aureus ATTC 6538, B. subtilis ATCC 530) and two Gram-negative (E. coli ATCC 25922, P. aeruginosa ATCC 27853) strains of bacteria to study antibacterial activities of fluorinated hydrazones 123–125 in the assay based on two-fold serial dilutions. Kanamycin B has been mentioned to be a positive control. Results of these studies revealed that the substitution by a fluorine atom in the meta position of the phenyl moiety in this class of compounds is the most profitable for the antibacterial activity. Therefore, fluorinated hydrazone 124, revealing MIC values ranging from 43.4 to 86.7 µM (Table 25), proved to be distinctly more active than its para and ortho counterparts. Furthermore, it was confirmed that the substitution by a fluorine atom in the para position (structure 123) is more profitable than the substitution by a fluorine atom in the ortho position (molecule 125). Two bacterial strains such as P. aeruginosa and B. subtilis were found to be more susceptible to compound 123 and less susceptible to molecule 125 [53].

Table 25.

Antibacterial activities of fluorinated hydrazide-hydrazones 123–125.

Bacterial Strain	MIC (µM) *
	123	124	125	Kanamycin B
E. coli ATCC 25922	173.5	86.7	86.7	3.2
P. aeruginosa ATCC 27853	86.7	43.4	173.5	3.2
S. aureus ATCC 6538	173.5	86.7	173.5	6.5
B. subtilis ATCC 530	86.7	86.7	173.5	6.5

* MIC values provided in the original paper [53] in µg mL⁻¹ were converted into molar concentrations.

Rambabu et al. [54] have synthesized fluorinated hydrazide-hydrazones 126 and 127 (Figure 35) by condensing equimolar quantities of 2-hydroxy-6-pentadecylbenzoylhydrazide with 4-fluorobenzaldehyde or 4-(trifluoromethoxy)benzaldehyde, respectively, in ethanol under reflux for 0.5 h.

Figure 35.

Structures of fluorinated hydrazide-hydrazones 126 and 127.

The antibacterial activity of hydrazones 126 and 127 towards Gram-negative (P. aeruginosa MTCC 424, E. coli MTCC 443) and Gram-positive (S. aureus MTCC 96, S. pyogenes MTCC 442) bacterial strains has been established at concentrations of 53.4, 106.7, 213.4 and 533.5 μM for 126 and at concentrations of 46.8, 93.5, 187.0 and 467.6 μM for 127, in the disc-diffusion method, employing ampicillin as a standard antibiotic. Hydrazone 127—with the para-(trifluoromethoxy)phenyl group—was found to be more antibacterially active than 126—with the para-fluorophenyl moiety. The activity of both compounds at the highest concentration proved to be similar to that of ampicillin at a concentration of 715.5 μM [54].

Kratky et al. [55] have designed and synthesized fluorinated hydrazide-hydrazones 128–134 (Figure 36), by condensing 4-(trifluoromethyl)benzohydrazide with 4-chlorobenzaldehyde, 3-chlorobenzaldehyde, 4-hydroxybenzaldehyde, 3-hydroxybenzaldehyde, 2-hydroxybenzaldehyde, 2-hydroxy-5-chlorobenzaldehyde or 4-nitrobenzaldehyde, respectively. Moreover, the authors have obtained fluorinated hydrazide-hydrazones 135–138 (Figure 36) by reacting 4-(trifluoromethyl)benzohydrazide in a slight molar excess with propan-2-one, cyclopentanone, cyclohexanone or camphor, respectively, in methanol under reflux for 2 h, using a catalytic amount of concentrated sulfuric acid.

Figure 36.

Structures of fluorinated hydrazide-hydrazones 128–138.

All these hydrazones (128–138) have been screened for their antimycobacterial activity against clinical isolates of M. tuberculosis 331/88, M. avium 330/88, M. kansasii 235/80 and M. kansasii 6509/96. Additionally, hydrazones 128–134 have been tested for their antibacterial activity against some Gram-positive (S. aureus CCM 4516/08, methicillin-resistant S. aureus H 5996/08, S. epidermidis H 6966/08, E. faecalis J 14365/08) and Gram-negative (E. coli CCM 4517, K. pneumoniae D 11750/08, K. pneumoniae J 14368/08, P. aeruginosa CCM 1961) strains. Isoniazid (an antimycobacterial agent) and bacitracin (a cyclic peptide antibiotic) were used as standard drugs. The majority of fluorinated hydrazones were found to be more active against M. kansasii 235/80 (128–134) and M. avium 330/88 (128, 129, 131–133) than isoniazid, and also against E. coli (128, 130, 132, 133) than bacitracin (Table 26 and Table 27). Exclusively hydrazone 138 (i.e., 4-(trifluoromethyl)-N′-[(2E)-3,7,7-trimethylbicyclo[2.2.1]hept-2-ylidene]benzohydrazide) showed significant activity against M. tuberculosis 331/88, although its potency was found to be 8-fold (after 14 days) and 4-fold (after 21 days) lower than that of isoniazid (Table 26). In turn, compound 133, containing the 2-hydroxy-5-chlorophenyl moiety, proved to be the most active molecule against all the recruited Gram-positive bacteria, showing clearly better MIC values (2–3.9 μM) than those of bacitracin (7.8–62.5 μM) (Table 27) [55]. Therefore, special attention should be paid to this molecule as a possible antibacterial agent.

Table 26.

Antimycobacterial activities of fluorinated hydrazide-hydrazones 128–138.

Bacterial Strain	Time	MIC (µM)
		128	129	130	131	132	133	134	135	136	137	138	INH
M. tuberculosis 331/88	14 d	16	>125	125	250	125	62.5	125	>125	500	250	4	0.5
	21 d	16	>125	125	250	250	62.5	>250	>125	500	500	4	1
M. avium 330/88	14 d	>125	>125	250	250	125	62.5	>250	>125	>125	>125	>250	>250
	21 d	>125	>125	500	250	250	125	>250	>125	>125	>125	>250	>250
M. kansasii 235/80	7 d	16	>125	62.5	62.5	250	62.5	32	125	500	500	>250	>250
	14 d	16	>125	125	125	250	125	62.5	>125	>1000	>1000	>250	>250
	21 d	16	>125	125	250	250	125	62.5	>125	>1000	>1000	>250	>250
M. kansasii 6509/96	7 d	16	>125	250	250	250	62.5	250	>125	500	500	>250	8
	14 d	16	>125	500	500	250	125	>250	>125	1000	1000	>250	8
	21 d	16	>125	500	500	250	125	>250	>125	1000	1000	>250	8

INH—isoniazid.

Table 27.

Antibacterial activities of fluorinated hydrazide-hydrazones 128, 130, 132 and 133.

Bacterial Strain	Time	MIC (µM)
		128	130	132	133	BAC
S. aureus CCM 4516/08	24 h	62.5	250	500	2	7.8
	48 h	62.5	250	>500	2	15.6
MRSA H 5996/08	24 h	62.5	250	500	2	15.6
	48 h	62.5	250	>500	2	15.6
S. epidermidis H 6966/08	24 h	31.2	125	250	3.9	15.6
	48 h	62.5	125	500	3.9	31.2
E. faecalis J 14365/08	24 h	62.5	>250	250	2	15.6
	48 h	>125	>250	250	3.9	62.5
E. coli CCM 4517	24 h	>125	>250	500	250	>500
	48 h	>125	>250	500	250	>500

MRSA—methicillin-resistant S. aureus; BAC—bacitracin. None of the tested compounds and bacitracin were active against K. pneumoniae D 11750/08, K. pneumoniae J 14368/08 and P. aeruginosa CCM 1961.

Coelho et al. [56] have synthesized three fluorinated hydrazide-hydrazones, i.e., N′-[(E)-(2-fluorophenyl)methylidene]pyridine-4-carbohydrazide (139), N′-[(E)-(3-fluorophenyl)methylidene]pyridine-4-carbohydrazide (140) and N′-[(E)-(4-fluorophenyl)methylidene]pyridine-4-carbohydrazide (141) (Figure 37), by reacting pyridine-4-carbohydrazide (i.e., isoniazid) with 2-fluorobenzaldehyde, 3-fluorobenzaldehyde or 4-fluorobenzaldehyde, respectively.

Figure 37.

Structures of fluorinated hydrazide-hydrazones 139–141.

Fluorinated hydrazones 139–141 have been screened for their activity against clinical isolates of M. tuberculosis, such as isoniazid-susceptible M. tuberculosis RG500 and isoniazid-resistant M. tuberculosis RGH102, M. tuberculosis RGH103 and M. tuberculosis RGH113. All the compounds were capable of revealing significant activity—although lower than that of isoniazid—against M. tuberculosis RG500. Hydrazone 140, bearing the meta-fluorophenyl moiety, proved to be the most active among these compounds against M. tuberculosis RGH103 and M. tuberculosis RGH113 (Table 28) [56].

Table 28.

Antimycobacterial activities of fluorinated hydrazide-hydrazones 139–141.

Bacterial Strain	MIC (µM)
	139	140	141	Isoniazid
M. tuberculosis RG500	2.1	4.1	4.1	1.1
M. tuberculosis RGH102	>72.9	>72.9	>72.9	>72.9
M. tuberculosis RGH103	>72.9	8.2	>72.9	>72.9
M. tuberculosis RGH113	>72.9	8.2	>72.9	>72.9

Habala et al. [57] have synthesized and confirmed the structure (both in the solution and solid state) and studied antibacterial activities of fluorinated hydrazide-hydrazones 142–146 (Figure 38) derived from an antitubercular agent isoniazid (i.e., pyridine-4-carbohydrazide). The synthesis of these hydrazones was accomplished by refluxing for 80 min in a two-component methanol-chloroform solution with equimolar ratios of pyridine-4-carbohydrazide and the suitable fluorinated benzaldehyde, i.e., 4-(trifluoromethyl)benzaldehyde, 2-(trifluoromethyl)benzaldehyde, 4-fluorobenzaldehyde, 5-fluoro-2-hydroxybenzaldehyde or 3-fluoro-2-hydroxybenzaldehyde.

Figure 38.

Structures of fluorinated hydrazide-hydrazones 142–146.

The authors have used Gram-positive S. aureus CNCTC Mau 82/78 and Gram-negative E. coli CNCTC 327/73 to study their vulnerability to fluorinated hydrazones 142–146, to which antibacterial activities were determined in the assay based on two-fold serial dilutions. Ciprofloxacin was employed as an antibacterial agent. Results have revealed that the synthesized compounds possess very weak activities against S. aureus and E. coli (Table 29). In addition, their activities against the above bacterial strains were found to be distinctly lower than that of ciprofloxacin. Hydrazone 142, containing a para-trifluoromethyl group at the phenyl moiety, was reported to be 2-fold more active against E. coli than its counterpart 143, bearing an ortho-trifluoromethyl group at the phenyl moiety. In turn, hydrazone 145, containing 5-fluoro-2-hydroxy substitutions at the phenyl moiety, was disclosed to be 16-fold more active against E. coli than its counterpart 146, bearing 3-fluoro-2-hydroxy substitutions in the same moiety. In addition, all hydrazones were found to the distinctly less active (ICs₅₀ > 500 µM) than acetohydroxamic acid (IC₅₀ = 185 µM) when tested as urease inhibitors [57]. Considering the fact that all these fluorinated hydrazones (142–146) were obtained from isoniazid effective against human tuberculosis, further studies with the use of Mycobacterium tuberculosis H₃₇Rv and its resistant strains to antitubercular agents are needed to confirm or rule out their anticipated antituberculosis activity.

Table 29.

Antibacterial activities of fluorinated hydrazide-hydrazones 142–146.

Bacterial Strain	MIC (µM) *
	142	143	144	145	146	Ciprofloxacin
S. aureus CNCTC Mau 82/78	37,500	37,500	>100,000	>100,000	50,000	0.7
E. coli CNCTC 327/73	9370	18,760	25,000	1550	25,000	<0.3

* MIC values provided in the original paper [57] in mM were converted into µM concentrations.

Ozkay et al. [58] have obtained 4-(1H-benzimidazol-2-yl)-N′-[(E)-(4-fluorophenyl)methylidene]benzohydrazide (147) and 4-(1H-benzimidazol-2-yl)-N′-{(E)-[4-(trifluoromethyl)phenyl]methylidene}benzohydrazide (148) (Figure 39) by condensing equimolar ratios of 4-(1H-benzimidazole-2-yl)benzoic acid hydrazide with 4-fluorobenzaldehyde or 4-(trifluoromethyl)benzaldehyde, respectively, in n-butanol under reflux for 3 h, with a small amount of glacial acetic acid as an efficient catalyst.

Figure 39.

Structures of fluorinated hydrazide-hydrazones 147 and 148.

Fluorinated hydrazones 147 and 148 have been evaluated for their activity against four Gram-positive strains of bacteria, such as L. monocytogenes, S. aureus ATCC 25923, E. faecalis ATCC 29212, B. subtilis and six Gram-negative strains of bacteria, such as E. coli ATCC 35218, E. coli ATCC 25922, P. vulgaris NRRL B-123, S. typhimurium NRRL B-4420, K. pneumoniae ATCC 13883, P. aeruginosa ATCC 27853. Chloramphenicol (a derivative of propandiol) has been used as a standard antibiotic. Both fluorinated compounds revealed better activities against Gram-negative P. vulgaris, S. typhimurium and P. aeruginosa than those of chloramphenicol. Additionally, hydrazone 148, containing the para-trifluoromethyl moiety, proved to be more active against Gram-positive E. faecalis than this standard drug (Table 30) [58].

Table 30.

Antibacterial activities of fluorinated hydrazide-hydrazones 147 and 148.

Bacterial Strain	MIC (µM) *
	147	148	Chloramphenicol
L. monocytogenes	558.1	489.8	154.7
S. aureus ATCC 25923	139.5	61.2	38.7
E. faecalis ATCC 29212	69.8	30.6	38.7
B. subtilis	69.8	122.4	38.7
E. coli ATCC 35218	69.8	61.2	38.7
E. coli ATCC 25922	139.5	244.9	38.7
P. vulgaris NRRL B-123	139.5	122.4	154.7
S. typhimurium NRRL B-4420	17.4	30.6	38.7
K. pneumoniae ATCC 13883	69.8	61.2	38.7
P. aeruginosa ATCC 27853	139.5	122.4	154.7

* MIC values provided in the original paper [58] in µg mL⁻¹ were converted into molar concentrations.

Abdelrahman et al. [59] have synthesized heterocyclic hydrazide-hydrazone, i.e., 6-chloro-N′-[(E)-(4-fluorophenyl)methylidene]-4-oxo-1,4-dihydroquinoline-3-carbohydrazide (149) (Figure 40) by condensing equimolar ratios of 6-chloro-4-oxo-1,4-dihydroquinoline-3-carbohydrazide and 4-fluorobenzaldehyde in N,N-dimethylformamide under reflux for 4 h.

Figure 40.

The structure of fluorinated hydrazide-hydrazone 149.

The fluorinated hydrazone 149 has been tested for its activity against S. pneumoniae RCMB 010010, S. aureus RCMB 010028, P. aeruginosa RCMB 010043 and E. coli RCMB 010052 in the two-fold dilution assay. Ampicillin and ciprofloxacin have been employed as standard drugs. Hydrazone 149 was able to reveal higher activity against Gram-positive S. pneumoniae and S. aureus. Nevertheless, its activity towards these bacterial strains was unfortunately about 16- and 32-fold lower than that of ampicillin (Table 31) [59].

Table 31.

Antibacterial activities of fluorinated hydrazide-hydrazone 149.

Bacterial Strain	MIC (µM) *
	149	Ampicillin	Ciprofloxacin
S. pneumoniae RCMB 010010	45.5	2.8	nd
S. aureus RCMB 010028	90.9	2.8	nd
P. aeruginosa RCMB 010043	na	nd	5.9
E. coli RCMB 010052	181.8	nd	3.0

* MIC values provided in the original paper [59] in µg mL⁻¹ were converted into molar concentrations. na—not active, nd—not determined.

Allaka et al. [60] have designed and synthesized fourteen fluorinated hydrazide-hydrazones 150–163 (Figure 41), by stirring for 2.5–4 h in ethanol at ambient temperature 1-ethyl-6-fluoro-7-(4-methylpiperazin-1-yl)-4-oxo-1,4-dihydroquinoline-3-carbohydrazide with a small molar excess of 2,6-dichlorobenzaldehyde, 4-nitrobenzaldehyde, 3,4,5-trimethoxybenzaldehyde, 5-bromo-2-hydroxybenzadehyde, 4-hydroxybenzaldehyde, 2,5-dimethoxybenzaldehyde, 3-hydroxybenzaldehyde, 4-methylbenzaldehyde, benzaldehyde, 3-methoxy-4-hydroxybenzaldehyde, 3-nitrobenzaldehyde, 2-chlorobenzaldehyde, 4-fluorobenzaldehyde or 3-(N,N-dimethylamino)benzaldehyde, respectively. Moreover, the same compounds have been prepared under microwave irradiation for 1–3 min.

Figure 41.

Structures of fluorinated hydrazide-hydrazones 150–163.

All these hydrazones (150–163) have been initially tested for their ability to inhibit the growth of M. smegmatis. Exclusively active compounds—that were capable of showing at least 30% inhibition of the growth of this bacterium—have been subjected to further evaluation of their MICs. Finally, the MIC values of molecules 150, 152, 156, and 157 have been determined and compared to those of commonly used antitubercular agents such as rifampicin and isoniazid. The activity of the most potent compounds bearing the 2,6-dichlorophenyl or meta-hydroxyphenyl moiety (150 and 156) proved to be more than twice less active than isoniazid (Table 32) [60].

Table 32.

Antimycobacterial activities of fluorinated hydrazide-hydrazones 150, 152, 156 and 157.

Bacterial Strain	MIC (µM)
	150	152	156	157	Rifampicin	Isoniazid
M. smegmatis	27.0	81.2	27.2	42.5	2.15	12.02

Rasras et al. [61] have obtained steroidal hydrazide-hydrazone, i.e., (3α,5β,7α,12α)-3,7,12-trihydroxy-N-[(1E)-4-fluorophenylmethylene]cholan-24-hydrazide (164) (Figure 42) by heating cholinic acid hydrazide with 4-fluorobenzaldehyde in anhydrous ethanol for 10 h.

Figure 42.

The structure of fluorinated hydrazide-hydrazone 164.

The activity of fluorinated hydrazone 164, expressed as MIC values, has been evaluated against E. coli, P. aeruginosa, E. aerogenes (Gram-negative bacteria) and S. aureus, E. faecalis, B. megaterium (Gram-positive bacteria) in the two-fold serial dilution assay and compared to that of cefaclor and cefixime from a second- and third-generation cephalosporins, respectively. This hydrazone was active against Gram-positive S. aureus, E. faecalis and B. megaterium. Simultaneously, this molecule was capable of revealing higher activity against S. aureus than cefaclor and cefixime as well as against B. megaterium than cefaclor. In turn, hydrazone 164 was found to be inactive against all the recruited Gram-negative bacterial strains (Table 33) [61].

Table 33.

Antibacterial activities of fluorinated hydrazide-hydrazone 164.

Bacterial Strain	MIC (µM) *
	164	Cefaclor	Cefixime
E. coli	na	na	4.3
P. aeruginosa	na	na	43.1
E. aerogenes	na	20.8	68.9
S. aureus	59.1	82.9	68.9
E. faecalis	118.2	82.9	68.9
B. megaterium	59.1	82.9	na

* MIC values provided in the original paper [61] in µg mL⁻¹ were converted into molar concentrations. na—not active.

Aouad [62] has reported the synthesis and biological evaluation of fluorinated bis-hydrazones (165–180) (Figure 43). The synthesis of these hydrazide-hydrazones was performed by refluxing 1-(R-phenyl)-1H-1,2,3-triazole-4,5-dicarbohydrazide with benzaldehyde and its derivatives, such as 4-fluorobenzaldehyde, 4-methoxybenzaldehyde, or 4-nitrobenzaldehyde, for 2 h in ethanol with catalytic assistance of small amount of hydrochloric acid.

Figure 43.

Structures of fluorinated bis-hydrazide-hydrazones 165–180.

Standard pathogenic strains of Gram-positive (S. aureus RCMB 010025, S. pneumoniae RCMB 010010, B. subtilis RCMB 010067) and Gram-negative (P. aeruginosa RCMB 010043, K. pneumoniae RCMB 010058, E. coli RCMB 010052) clinical isolates have been included to assess antibacterial activities of hydrazones 165–180 in the assay based on two-fold serial dilutions. As a positive control, a broad-spectrum fluoroquinolone ciprofloxacin was used to confirm the susceptibility of all bacteria recruited. Fluorinated bis-hydrazone structures 165, 167, 169, 171, 175, 177–180 have been reported as the most active molecules, revealing remarkable activities (with MIC values ranging from 6.0 to 28.6 µM) against all bacterial strains (Table 34). Their efficacy towards S. pneumoniae, S. aureus and P. aeruginosa was often higher or comparable to that of ciprofloxacin. The author proved that enhanced antibacterial potency was achieved by introducing two preferred para-fluorophenyl or para-nitrophenyl moieties. He suggested that an azomethine bridge as well as the 1,2,3-triazole scaffold are necessary for the antibacterial activity in this series of molecules [62].

Table 34.

Antibacterial activities of fluorinated bis-hydrazide-hydrazones 165–180.

Bacterial Strain	MIC (µM) *
	165	166	167	168	169	170	171	172	173	174	175	176	177	178	179	180	CPFX
S. pneumoniae	8.1	31.0	14.7	29.4	8.1	31.0	14.7	35.1	15.8	30.2	28.6	34.1	13.0	25.0	11.9	27.5	≤15.1
B. subtilis	8.1	15.5	14.7	29.4	8.1	31.0	14.7	35.1	31.7	30.2	14.3	34.1	13.0	25.0	6.0	13.8	≤3.0
S. aureus	8.1	15.5	7.3	14.7	16.3	15.5	7.3	17.6	15.8	30.2	14.3	34.1	6.5	12.5	6.0	13.8	≤15.1
P. aeruginosa	16.3	31.0	14.7	29.4	16.3	31.0	14.7	35.1	31.7	30.2	14.3	34.1	13.0	12.5	11.9	13.8	≤15.1
E. coli	8.1	15.5	7.3	14.7	8.1	15.5	14.7	17.6	31.7	59.0	28.6	66.6	13.0	25.0	11.9	27.5	≤3.0
K. pneumoniae	16.3	31.0	7.3	29.4	16.3	31.0	7.3	35.1	15.8	59.0	14.3	66.6	25.9	25.0	11.9	27.5	≤3.0

* MIC values provided in the original paper [62] in µg mL⁻¹ were converted into molar concentrations. CPFX—ciprofloxacin.

Rezki et al. [63] have reported the synthesis, structural characterization and antibacterial evaluation of fluorinated bis-hydrazide-hydrazones 181 and 182 (Figure 44). The synthesis of hydrazones 181 and 182 was carried out by refluxing 4,4′-(1,3,4-thiadiazole-2,5-diyldisulfanediyl)dibutanehydrazide with 4-fluorobenzaldehyde or 4-trifluoromethylbenzaldehyde, respectively, in ethanol for 4–6 h with the use of catalytic assistance of small amount of hydrochloric acid.

Figure 44.

Structures of fluorinated bis-hydrazide-hydrazones 181 and 182.

Rezki and co-workers have recruited Gram-positive (S. pneumoniae RCMB 010010, B. subtilis RCMB 010067, S. aureus RCMB 010025) and Gram-negative (P. aeruginosa RCMB 010043, K. pneumoniae RCMB 010058, E. coli RCMB 010052) bacterial strains of clinical interest to assess antibacterial activities of fluorinated hydrazones 181 and 182 in the assay based on two-fold serial dilutions. As a positive control, a broad-spectrum fluoroquinolone ciprofloxacin was used to confirm the susceptibility of all microorganisms recruited. Both fluorinated bis-hydrazide-hydrazones have been disclosed to possess remarkable antibacterial effects against all pathogenic microorganisms with MIC values ranging from 7.1 µM to 24.2 µM (Table 35). In addition, it is clearly seen that three bacterial strains: S. aureus, E. coli and B. subtilis are more susceptible to bis-hydrazone 181, containing two para-fluorophenyl moieties. Simultaneously, the activity of both fluorinated bis-hydrazones against S. pneumoniae and P. aeruginosa as well as the efficacy of compound 181 towards S. aureus was better or similar to that of ciprofloxacin [63].

Table 35.

Antibacterial activities of fluorinated bis-hydrazide-hydrazones 181 and 182.

Bacterial Strain	MIC (µM) *
	181	182	Ciprofloxacin
S. pneumoniae RCMB 010010	14.2	12.1	≤15.1
B. subtilis RCMB 010067	7.1	12.1	≤3.0
S. aureus RCMB 010025	7.1	24.2	≤15.1
P. aeruginosa RCMB 010043	14.2	12.1	≤15.1
E. coli RCMB 010052	7.1	24.2	≤3.0
K. pneumoniae RCMB 010058	14.2	12.1	≤3.0

* MIC values provided in the original paper [63] in µg mL⁻¹ were converted into molar concentrations.

Morjan et al. [64] have synthesized fluorinated hydrazide-hydrazone 183, i.e., N′-[(2Z)-1,1,1-trifluoropropan-2-ylidene]pyridine-3-carbohydrazide (Figure 45) by refluxing in ethanol pyridine-3-carbohydrazide with 1,1,1-trifluoropropan-2-one.

Figure 45.

The structure of fluorinated hydrazide-hydrazone 183.

The activity of fluorinated hydrazone 183 against P. aeruginosa, K. pneumoniae and S. aureus has been established in the two-fold dilution assay. The tested molecule revealed remarkable potency against P. aeruginosa. Unfortunately, its antibacterial activity has not been compared to any known antibacterial agent (Table 36) [64].

Table 36.

Antibacterial activities of fluorinated hydrazide-hydrazone 183.

Bacterial Strain	MIC (µM) *
	183
P. aeruginosa	1.0
K. pneumoniae	60.6
S. aureus	30.4

* MIC values provided in the original paper [64] in µg mL⁻¹ were converted into molar concentrations.

Sankar and Pandiarajan [65] have carried out a study on fluorinated hydrazone 184 (Figure 46) incorporating in the molecular framework the pharmacophoric isonicotinic acid hydrazide-hydrazone moiety. This moiety is also present in antimycobacterial hydrazones of aromatic aldehydes obtained from isoniazid, such as ftivazid, verazid and furilazon, which have been used in clinical practice as antitubercular agents that were less toxic than isoniazid. The synthesis of this hydrazone was accomplished successfully by reacting 2,4-bis(4-fluorophenyl)-3-azabicyclo[3.3.1]nonan-9-one with an excess of isonicotinic acid hydrazide for 2–3 h in refluxing two-component methanol-chloroform solution (1:1), containing a small amount of acetic acid as an efficient catalyst.

Figure 46.

The structure of fluorinated hydrazide-hydrazone 184.

Two clinically important strains of M. tuberculosis, i.e., M. tuberculosis H₃₇Rv ATCC 27294 and resistant to isoniazid M. tuberculosis, have been selected by the authors to evaluate antimycobacterial activities (expressed as the percentage of reduction in the Related Lights Units—RLU) of fluorinated hydrazone 184 at two concentrations (2.24 and 4.48 µM) in the luciferase reporter phage assay. In addition, two Gram-positive (S. aureus NCIM 2492, B. subtilis NCIM 2439) and three Gram-negative (E. coli NCIM 2345, P. aeruginosa NCIM 2035, K. pneumoniae) bacteria have been recruited to study antibacterial activities of this molecule. Isoniazid, penicillin G and streptomycin were used as standard antibacterial agents. Fluorinated structure 184 was proposed as a potential antimycobacterial agent, showing at concentrations of 2.24 and 4.48 µM very good in vitro potency against M. tuberculosis H₃₇Rv (76.63 and 86.12% reduction in RLU, respectively) and M. tuberculosis strain resistant to isoniazid (67.63 and 75.08% reduction in RLU, respectively). Moreover, this hydrazone was found to be active against B. subtilis and S. aureus with a MIC value of 112.0 µM, and was able to completely inhibit the growth of E. coli and P. aeruginosa at a MIC value of 224.0 µM (Table 37). In addition, its activity against S. aureus was only 1.3-fold lower than that of streptomycin, and against B. subtilis—1.5-fold lower than that of penicillin. G. Sankar and Pandiarajan have suggested that the release of the active hydrazide structure of isoniazid via hydrolysis of an azomethine bond and the presence (at both phenyl moieties) of two fluorine atoms capable of forming the strong hydrogen bond, are responsible for the promising antitubercular action of hydrazone 184 [65].

Table 37.

Antibacterial activities of fluorinated hydrazide-hydrazone 184.

Bacterial Strain	MIC (µM) *
	184	Penicillin G	Streptomycin
B. subtilis	112.0	74.8	21.5
K. pneumoniae	>448.0	37.4	86.0
E. coli	224.0	149.5	21.5
S. aureus	112.0	37.4	86.0
P. aeruginosa	224.0	149.5	21.5

* MIC values provided in the original paper [65] in µg mL⁻¹ were converted into molar concentrations.

Xaiver et al. [66] have synthesized fluorinated hydrazide-hydrazone, i.e., 4-amino-N′-[2r,4c-bis(4-fluorophenyl)]-3-azabicyclo[3.3.1]non-9-ylidene)benzohydrazide (185) (Figure 47) by condensing 4-aminobenzoic acid hydrazide (in a molar excess) with 2,4-difluorophenyl-3-azabicyclo[3.3.1]nonan-9-one in methanol/chloroform (1:1 v/v) under reflux for 2–4 h.

Figure 47.

The structure of fluorinated hydrazide-hydrazone 185.

The fluorinated hydrazone 185 has been tested against S. typhimurium MTCC 98, E. coli MTCC 443, V. cholerae, S. typhi MTCC 531, P. aeruginosa MTCC 741, K. pneumoniae MTCC 2272, B. subtilis MTCC 121 and S. aureus MTCC 96 in the two-fold serial dilution assay. Its antibacterial activity expressed as MIC values has been compared to that of streptomycin (Table 38). This hydrazone proved to be the most active against B. subtilis, although its potency against this bacterial strain was found to be 2.5-fold lower than that of streptomycin. Additionally, the activity of this molecule against V. cholerae was only 1.2-fold weaker than that of the standard drug [66].

Table 38.

Antibacterial activities of fluorinated hydrazide-hydrazone 185.

Bacterial Strain	MIC (µM) *
	185	Streptomycin
S. typhimurium MTCC 98	217.2	43.0
E. coli MTCC 443	217.2	86.0
V. cholerae	108.6	86.0
S. typhi MTCC 531	217.2	43.0
P. aeruginosa MTCC 741	434.3	86.0
K. pneumoniae MTCC 2272	108.6	34.4
B. subtilis MTCC 121	54.3	21.5
S. aureus MTCC 96	108.6	43.0

* MIC values provided in the original paper [66] in µg mL⁻¹ were converted into molar concentrations.

Kodisundaram et al. [67] have obtained fluorinated hydrazide-hydrazone 186 (Figure 48), by refluxing 4-methyl-1,2,3-thiadiazole-5-carboxylic acid hydrazide (in a molar excess) with 2,4-difluorophenyl-3-azabicyclo[3.3.1]nonan-9-one, in a methanol/chloroform mixture (1:1 v/v), for 3–4 h, in the presence of catalytic amount of acetic acid.

Figure 48.

The structure of fluorinated hydrazide-hydrazone 186.

The authors have screened the antibacterial activities of hydrazone 186 against two Gram-positive (B. subtilis, S. aureus) and three Gram-negative (K. pneumoniae, E. coli, P. aeruginosa) strains in the two-fold serial dilution assay, and compared its activities to those of streptomycin. This fluorinated hydrazone proved to be 1.6-fold more potent towards B. subtilis, K. pneumoniae and E. coli than the standard drug (Table 39) [67].

Table 39.

Antibacterial activities of fluorinated hydrazide-hydrazone 186.

Bacterial Strain	MIC (µM) *
	186	Streptomycin
B. subtilis	13.4	21.5
S. aureus	53.5	21.5
K. pneumoniae	13.4	21.5
E. coli	26.8	43.0
P. aeruginosa	53.5	21.5

* MIC values provided in the original paper [67] in µg mL⁻¹ were converted into molar concentrations.

Kaki et al. [68] have synthesized two fluorinated hydrazide-hydrazones, i.e., 2-(2,3-dihydro-1-benzofuran-5-yl)-N′-[(1E)-1-(4-fluoro-2-hydroxyphenyl)ethylidene]acetohydrazide (187) and 2-(2,3-dihydro-1-benzofuran-5-yl)-N′-[(1E)-1-(4-fluorophenyl)ethylidene]acetohydrazide (188) (Figure 49) by refluxing 2-(2,3-dihydro-1-benzofuran-5-yl)acetohydrazide (in a molar excess) with 1-(4-fluoro-2-hydroxyphenyl)ethanone or 1-(4-fluorophenyl)ethanone, respectively, in ethanol for 8 h in the presence of glacial acetic acid as an efficient catalyst.

Figure 49.

Structures of fluorinated hydrazide-hydrazones 187 and 188.

These fluorinated hydrazones (187 and 188) have been screened for their activity against E. coli MTCC 443 and P. aeruginosa MTCC 424 (Gram-negative bacteria) as well as S. aureus MTCC 96 and S. pyogenes MTCC 442 (Gram-positive bacteria) in the disc-diffusion assay, using as a standard antibiotic ampicillin at a concentration of 715.5 μΜ. Both compounds—187 at a concentration of 761.4 μΜ and 188 at a concentration of 800.4 μΜ—were capable of revealing remarkable antibacterial activity against all the recruited bacterial strains, as their zones of inhibition were comparable to those of ampicillin [68].

Skrickus et al. [69] have synthesized bis-hydrazide-hydrazone 189 (Figure 50) by condensing 3,3′-[disulfanediylbis(benzene-2,1-diylimino)]dipropanoic acid hydrazide with 4-fluorobenzaldehyde (in molar ratios 1:2.5) in boiling isopropanol for 2–3 h.

Figure 50.

The structure of fluorinated hydrazide-hydrazone 189.

Fluorinated hydrazone 189 has been screened for its activity against S. aureus ATCC 9144, L. monocytogenes ATCC 35152, E. coli ATCC 13076 and S. enterica ATCC 8739 in the microbroth assay based on two-fold serial dilutions, using a first-generation cephalosporin—cefazolin––as a standard drug. This compound was found to be antibacterially active, revealing significant potency against L. monocytogenes and moderate activities against the remaining recruited bacterial strains. Simultaneously, its MIC value against L. monocytogenes proved to be slightly lower than that of a standard drug [69].

Haj Mohammad Ebrahim Tehrani et al. [36] have described the antibacterial evaluation of fluorinated hydrazide-hydrazones 190–191 (Figure 51) obtained from stoichiometric ratios of benzohydrazide or isonicotinic acid hydrazide and 5-fluoro-1H-indole-2,3-dione, in refluxing ethanol for 4–6 h, with assistance of small amount of glacial acetic acid as an efficient catalyst. The authors have reported that microwave irradiation at 110 °C for 5 min was an alternative method of their synthesis.

Figure 51.

Structures of fluorinated hydrazide-hydrazones 190–191.

Antibacterial screening of hydrazide-hydrazones 190–191 has been performed in the assay based on two-fold serial dilutions against three Gram-positive (S. aureus ATCC 25923, methicillin-resistant S. aureus ATCC 43300, E. faecalis ATCC 29212) and two Gram-negative (P. aeruginosa ATCC 27853, E. coli ATCC 25922) bacterial strains of clinical interest. Amikacin (an aminoglycoside antibiotic) was included as a positive control. The results of the study have confirmed that fluorinated hydrazone related to benzoic acid hydrazide (190), revealing a broader spectrum of antibacterial activity, is distinctly more active than fluorinated hydrazone related to isonicotinic acid hydrazide (191) (Table 40) [36].

Table 40.

Antibacterial activities of fluorinated hydrazide-hydrazones 190–191.

Bacterial Strain	MIC (µM) *
	190	191	Amikacin
P. aeruginosa ATCC 27853	88.3	>175.9	27.3
E. coli ATCC 25922	44.1	88.0	27.3
E. faecalisATCC 29212	88.3	>175.9	21.3
S. aureus ATCC 25923	44.1	>175.9	52.9
MRSA ATCC 43300	44.1	>175.9	52.9

* MIC values provided in the original paper [36] in µg mL⁻¹ were converted into molar concentrations. MRSA—methicillin-resistant S. aureus.

6. Advanced Research Techniques for the Synthesis, Design and Development of Fluorinated Imines and Hydrazones of Pharmacological Importance

Some advanced research techniques and innovations have been developed and reported during the past fifteen years for the synthesis, design and development of fluorinated Schiff bases and fluorinated hydrazones. Cerium oxide nanoparticles have been used by Durmuş et al. [26,27] as an efficient and eco-friendly catalyst in the synthesis of fluorinated dimeric aldimine, containing the disulfide bridge of biochemical interest. In this case, the authors have reported the enhanced reaction rates and yields by comparing the results with the conventional method of synthesis without the use of any catalyst. Microwave irradiation, providing a much greater efficiency of energy transfer, has been reported by Singh et al. [70] in the microwave-assisted synthesis of fluorinated ketimines. The conversion of substrates (alkyl amines and 5-fluoro-2-hydroxyacetophenone) to imines in quantitative yields ranging from 89 to 98% and very short reaction times (1–5 min) were the main advantages of this greener technique compared to the conventional synthesis method. An innovative procedure for the synthesis of pharmaceutically important building blocks, i.e., (S)-N-tert-butanesulfinyl-aldimines bearing heavily fluorinated alkyl groups, has been developed by Xie et al. [71]. The authors reported that the treatment of starting substrates (i.e., (S)-tert-butylsulfinamide and fluorine-containing aldehydes which form very stable gem-aminoalcohols) in dichloromethane, with two-step use of two dehydrating agents (magnesium sulfate and activated molecular sieves, 4 Å), gives the desired results, as the S-enantiomers of chiral imines are formed in above 80% yields and there is no need for their further purification by distillation. Activated molecular sieves made from 4 Å zeolite powder have been employed by Avila-Sorrosa et al. [16] in a good-yielding method for the synthesis of fluorinated aldimines in order to facilitate the removal of water from intermediate hemiaminal.

The advanced research (identifying a suitable target—ecKAS-CoA complex structure, screening of the designed ligand in the enzymatic assay, structure-guided drug design, docking into the Escherichia coli active site—ecKAS—and finding the most likely binding conformation of the ligand) has been reported by Shi et al. [20] and Cheng et al. [17] leading to the development of two novel highly potent inhibitors (aldimine-type Schiff bases 5 and 19; Table 41) of the Escherichia coli β-ketoacyl-acyl carrier protein synthase III (ecKAS III). They may find application in the near future as antibacterial agents belonging to the important fluorinated imines and showing high selectivity against the bacterial target. This is a significant finding because the ecKAS III is an example of recently discovered enzymes that may prove useful as antibacterial agent targets. This enzyme is involved in the biosynthesis of fatty acids in bacteria, and there is no homologous enzyme in humans. Therefore, small molecules inhibiting ecKAS III catalyzed reaction should be highly selective and non-toxic antibacterial agents.

Table 41.

The half maximal inhibition constant (IC₅₀) against the ecKAS III for thirteen fluorinated molecules studied.

	5	6	14	15	16	18	19	22	23	25	26	108	110
IC50 (µM) *	5.6	17.1	18.4	7.4	6.8	48.5	2.7	28.6	42.5	8.6	11.7	58.3	47.5
Reference	[17]	[18]	[20]	[20]	[20]	[20]	[20]	[20]	[20]	[20]	[20]	[47]	[47]

* MIC values provided in the original papers [17,18] in µg mL⁻¹ were converted into molar concentrations. ecKAS III—the Escherichia coli β-ketoacyl-acyl carrier protein synthase III.

7. Concluding Remarks

The present review paper is exclusively focused on aldimine- and ketimine-type fluorinated Schiff bases, and fluorinated hydrazones, showing strong, moderate or weak in vitro antibacterial activities, that have been reported in the scientific literature over the last fifteen years. As mentioned in this article, some of these small molecules revealed not only promising antibacterial activity but also a potent inhibitory effect against ecKAS when tested in the target enzymatic assay. Receptor-ligand modeling studies have enabled researchers to develop more selective fluorinated aldimines (5 and 19) that may be suitable for future use as potential antibacterial agents. These structures can be extremely useful in the rational design of highly selective antibacterial agents. In addition, all antibacterially active fluorinated imines and hydrazones collected from the last fifteen years can serve as lead structures with a documented activity profile. They are an excellent material for further development and ongoing studies on the implementation of potential antibacterial drugs by medicinal chemists. All the collected small molecular weight structures (<900 daltons) can serve for further fruitful modification in the ongoing search for new antibacterially active fluorinated imines and hydrazones, which would be promising for more advanced development.

In this review, we have presented detailed information on the sensitivity of various bacterial strains (including drug-resistant ones) to fluorinated imines and hydrazones. We hope that MIC values taken from the literature, converted to molar concentrations and compared to MICs of reference drugs, should help medicinal chemists in designing more active fluorinated antibacterial agents. We have shown several cases in which the antimicrobial activity of fluorinated imines or hydrazones was improved in relation to clinically approved antibacterial agents. This was particularly evident in the case of fluorinated aldimines 33 and 37, fluorinated ketimine 79, fluorinated hydrazine-hydrazone 98 and fluorinated hydrazide-hydrazones 114, 133, 165, 167, 169, 179 and 181. The most potent hydrazide-hydrazones have been obtained from heterocyclic hydrazides, 4-trifluoromethylated benzoic acid hydrazide or heterocyclic bis-hydrazides. Hence, just fluorinated imines and hydrazones—presented in this review—with antibacterial activity superior to that of commonly used antibacterials seem to have potential usefulness to be applied in the future as pharmaceutics. Nevertheless, further in vivo studies, followed by pharmacokinetic and clinical tests are needed to determine the therapeutic efficacy of these fluorinated molecules, as well as whether they have a clear advantage over the clinically useful pharmaceutical(s). However, in the case of some isoniazid-derived fluorinated hydrazide-hydrazones that have not yet been tested for their antimycobacterial activity, the initial screening against M. tuberculosis H₃₇Rv susceptible to the primary antitubercular agents (e.g., isoniazid, rifampicin, streptomycin and ethambutol) is necessary to confirm or exclude their anticipated antitubercular activity.

Abbreviations

A. baumannii	Acinetobacter baumannii
AMP	Ampicillin
BAC	Bacitracin
B. amyloliquefaciens	Bacillus amyloliquefaciens
B. cereus	Bacillus cereus
B. megaterium	Bacillus megaterium
B. subtilis	Bacillus subtilis
B. bronchiseptica	Bordetella bronchiseptica
CPFX	Ciprofloxacin
CTX	Cefotaxime
CXM	Cefuroxime
E. aerogenes	Enterobacter aerogenes
E. cloacae	Enterobacter cloacae
E. casseliflavus	Enterococcus casseliflavus
E. faecalis	Enterococcus faecalis
E. faecium	Enterococcus faecium
ecKAS III	Escherichia coli β-ketoacyl-acyl carrier protein synthase III
E. coli	Escherichia coli
G. lamblia	Giardia lamblia
GEN	Gentamicin
GTFX	Gatifloxacin
H. influenzae	Haemophilus influenzae
IC50	Half maximal inhibitory concentration
INH	Isoniazid
IUPAC	International Union of Pure and Applied Chemistry
KNM	Kanamycin
K. oxytoca	Klebsiella oxytoca
K. pneumoniae	Klebsiella pneumoniae
L. monocytogenes	Listeria monocytogenes
LVFX	Levofloxacin
MBC	Minimum bactericidal concentration
MIC	Minimum inhibitory concentration
MRSA	Methicillin-resistant Staphylococcus aureus
MRSE	Methicillin-resistant Staphylococcus epidermidis
MSSA	Methicillin-sensitive Staphylococcus aureus
MSSE	Methicillin-sensitive Staphylococcus epidermidis
MTT	3′-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide
M. luteus	Micrococcus luteus
M. avium	Mycobacterium avium
M. kansasii	Mycobacterium kansasii
M. smegmatis	Mycobacterium smegmatis
M. tuberculosis	Mycobacterium tuberculosis
na	Not active
nd	Not determined
NFN	Nitrofurantoin
PZA	Pyrazinamide
P. mirabilis	Proteus mirabilis
P. vulgaris	Proteus vulgaris
P. aeruginosa	Pseudomonas aeruginosa
P. fluorescence	Pseudomonas fluorescence
P. putida	Pseudomonas putida
PTSA	p-toluenosulfonic acid
RLU	Related Lights Units
S. enterica	Salmonell enterica
S. typhi	Salmonella typhi
S. typhimurium	Salmonella typhimurium
S. aureus	Staphylococcus aureus
S. epidermidis	Staphylococcus epidermidis
S. maltophiliae	Stenotrophomonas maltophiliae
S. agalactiae	Streptococcus agalactiae
S. faecalis	Streptococcus faecalis
S. pneumoniae	Streptococcus pneumoniae
S. pyogenes	Streptococcus pyogenes
S. viridans	Streptococcus viridans
STM	Streptomycin
SLT	Sultamicillin
VAN	Vancomycin
V. cholerae	Vibrio cholerae
V. parahaemolyticus	Vibrio parahaemolyticus

Author Contributions

Conceptualization, M.S. and K.S.; project administration, M.S. and K.S.; literature search, M.S., A.W. and K.S.; data collection, M.S., A.W. and K.S.; formal analysis, M.S., A.W. and K.S.; writing—original draft preparation, M.S., A.W. and K.S.; writing—review and editing, M.S. and K.S.; funding acquisition, M.S. and K.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest. Author Agata Wilk was employed by the company Green Lanes Proteins Sp. z o.o. The company had no role in the design of the study; in the collection, analyses, and interpretation of data; in the writing of the manuscript, and in the decision to publish the results. The remaining authors declare that the article was written without any commercial and financial relationships that could be construed as a potential conflict of interest.

Funding Statement

This research received no external funding.