3.1. Structure
Hydrazide–hydrazones are a numerous group of hybrid molecules which can connect diametrically different alkyl or aryl fragments through an unsymmetrical amide–imine bridge -C(=O)-NH-N=CH-, which builds the heterogeneity of this group of compounds (Figure 29). Its name unsurprisingly comes from the dual possibility of reading the order of atoms in a moiety. If the nomenclature begins with the carbonyl group, it can be said that researchers are dealing with monosubstituted carbonyl hydrazides, while if researchers regard the imine bond as the beginning, the—monosubstituted—hydrazone clearly appears.
Figure 8. The basic structure of a hydrazide–hydrazone, where R and R1 are either alkyl or an aryl groups.
The hydrazide–hydrazone moiety can be identified via spectroscopic methods. The IR spectrum shows signals at around 3050 cm
−1, 1650 cm
−1 and 1550 cm
−1, corresponding to the -NH-, C=O and C=N group, respectively. Two singlets appear in the
1H NMR spectrum, one in the range of δ 8–9 ppm and the other in the range of δ 10–13 ppm, signals corresponding to the -CH= and -NH- group, respectively. In the
13C NMR spectrum, there are signals of carbon atoms from the CH= and C=O group in the range of δ 145–160 ppm and δ 160–170 ppm, respectively [
133].
3.2. Importance
Hydrazide–hydrazones are synthesized in search of effective antibacterial and antifungal agents—especially due to the growing problem of antibiotic resistance [
134]. In many cases, it is the presence of electron-withdrawing aromatic ring substituents that is essential for the antimicrobial activity of hydrazide–hydrazones. Studies on the biological activity of, among others, the following compound (
Figure 30) provide an example of the mentioned relationship. The lowest MIC values (μg/mL) are shown for the chlorine substituent in position 2, against
E. coli,
S. aureus and
B. subtilis, 0.31, 0.62 and 0.31, respectively. They were compared with the MIC values of ciprofloxacin (an organic antimicrobial compound inhibiting bacterial DNA topoisomerase) of 0.01, 0.15 and 0.12, respectively. On the other hand, the highest efficacy among the tested derivatives against
C. albicans was recorded for the -NO
2 substituent in position 2 (MIC = 0.31 μg/mL). The value was compared with clotrimazole (an organic compound with antifungal activity that inhibits the biosynthesis of sterols that build fungal cell membranes), for which the MIC value is 0.10 μg/mL [
135].
Figure 9. Hydrazide–hydrazone derivative of biphenyl-4-carboxylic acid, where R = NO
2, -Cl or -Br [
135].
The following compounds (
Figure 10) were tested against streptomycin and against
B. subtilis,
K. pneumoniae and
E. coli and showed lower MIC values for the chloro, fluoro and
para substituents. Moreover, lower values were also recorded for the above-mentioned derivatives and for the derivative with the chlorine atom in the “
ortho” position against fluconazole and against
A. flavus,
A. niger,
C. albicans and
Candida6 [
136].
Figure 10. Structure of the 2r, 4c-diaryl-3-azabicyclo [3.3.1] nonan-9-one-4-methyl-1,2,3-thiadiazole-5-carbonyl hydrazide–hydrazone [
136].
There have also been reports in the literature on the anti-cancer properties of hydrazide–hydrazones, e.g., in the case of colorectal cancer acting by increasing the permeability of the outer mitochondrial membranes of neoplastic cells. The cytotoxic activity of the hydrazide–hydrazone derivative of 2,6-difluorobenzoic acid and 6-bromoindole (
Figure 11) was confirmed against the HTC-116, DLD-1 and SW-620 cell lines, while its lack—against healthy fibroblasts of the L929 cell line [
77].
Figure 11. Derivative of 2,6-difluorobenzoic acid and 6-bromoindole—hydrazide–hydrazone with anti-cancer properties [
77].
For the HTC-116 cell line, the pathway of programmed cell death has been established, starting with the inhibition of Bcl2 proteins, which regulate this process and act in an anti-apoptotic manner, and, consequently, regulate the release of pro-apoptotic cytochrome c. Caspases-9 and -3, which regulate the production of reactive oxygen species, take over further control of the process [
77]. Cytochrome c binds Apaf-1, which is the first factor activating the apoptotic protease-activating factor 1. The formation of this complex (apoptosome) leads to the activation of procaspase-9 to the initiator caspase-9, which in turn activates the executive caspases, caspase-3 and caspase-7, of which caspase-3 is the master caspase and caspase-7 is the helper caspase. Caspase-9 begins the secretion of reactive oxygen species that have a proteolytic effect on the cell subjected to apoptosis, and caspase-3 acts as an inhibitor, ending the process [
137].
Hydrazide–hydrazones are also used as antidiabetic agents in type II diabetes [
138,
139]. They play the role of non-competitive antagonists of the glucagon receptor—thus, they inhibit the glucagon-induced processes: glycogenolysis and gluconeogenesis, which in turn leads to a reduction in blood sugar levels.
A very interesting biological activity of the hydrazide–hydrazone of lactic acid was described by Noshiranzadeh et al. [
140]. They synthesized a number of this type of lactic acid derivatives, two of which (
Figure 12) showed particular activity against the selected bacterial strains (Minimum Inhibitory Concentration MIC = 64–128 µg/mL), which turned out to be lower than the reference gentamicin.
Figure 12. New hydrazide–hydrazones of lactic acid with antibacterial activity.
Olayinka et al. synthesized several 2-propylquinoline-4-carboxylic acid hydrazide–hydrazones [
141]. The compound shown in the
Figure 13 showed the highest activity against six strains of bacteria.
Figure 13. Quinoline derivative with significant antibacterial properties.
The authors showed that the presence of an electron-donating substituent in position 4 and an electron-withdrawing substituent in position 2 is of key importance for the activity of the derivatives obtained.
Indole-2-one was used by Salem et al. to obtain a series of hydrazide–hydrazones, of which the compound in
Figure 14 had the most interesting properties [
142].
Figure 14. Indol-2-one derivative with antibacterial activity.
For the above compound, they performed an activity assay against DNA gyrase isolated from S. aureus, which showed that its half-maximal inhibitory concentration was IC50 = 19.32 ± 0.99 µM, while for the reference ciprofloxacin, IC50 = 26.43 ± 0.64 µM.
Of the
N-substituted indole derivatives synthesized by Tiwari et al., the compound shown in
Figure 15 showed the highest activity against Gram-positive bacteria [
143]. In the case of
E. coli MTCC 433 and
B. subtilis MTCC 1427, it showed higher activity than the reference Chloramphenicol.
Figure 15. N-substituted indole derivative with antibacterial properties.
El-Etrawa et al. prepared a number of 2-thiouracil derivatives, of which the following compound (
Figure 16) turned out to be the most active against
E. coli,
P. aeruginosa and
S. aureus [
144].
Figure 16. N-(2-Thiouracil-5-oyl)hydrazone derivative with antibacterial activity.
Recently, Paruch et al. synthesized 1,2,3-thiadiazole hydrazide–hydrazones, of which the compound shown in
Figure 17 showed the highest activity [
145].
Figure 17. 4-Methyl-1,2,3-thiadiazole-carboxylic acid hydrazide derivative active against a panel of bacterial strains.
This compound showed activity against almost all tested bacterial strains, and in the case of S. epidermidis ATCC 12228 and M. luteus ATCC 10240, even two and eight times higher, respectively, than the reference nitrofurantoin.
In 2020, the results of the synthesis and biological tests of eugenol hydrazide–hydrazones were published [
146]. The compound in the
Figure 18 showed the highest activity against
M. tuberculosis H37Rv.
Figure 18. Pyrazine derivative with antitubercular properties.