An Efficient Sonochemical Synthesis of Novel Schiff's Bases, Thiazolidine, and Pyrazolidine Incorporating 1,8-Naphthyridine Moiety and Their Cytotoxic Activity against HePG2 Cell Lines

Novel Schiff's bases 4a–e, 5a, 5b, and 6, thiazolidine 7a–d, and pyrazolidine 8 have been synthesized using the versatile synthon 4-hydroxy-2,7-dimethyl-1,8-naphthyridine 1. Reactions carried out under ultrasound irradiation showed higher rates and yields than those done under silent conditions. The newly synthesized compounds were evaluated for HepG2 cell growth inhibition. The results obtained revealed that the tested compounds possess inhibitory effect on the growth of HepG2 liver cancer cells. The results were compared to doxorubicin as a reference drug (IC50: 0.04). Compounds 4a and 7b showed the highest inhibition activity against the HepG2 cell line (IC50: 0.047 and 0.041 µM, resp.) among all the tested compounds.

The application of ultrasound in synthetic organic chemistry became crucial. Sonochemistry is a new trend in organic chemistry, offering a versatile pathway for a wide variety of syntheses. A large number of organic reactions can be carried out under ultrasonic irradiation in high yields, short time, and mild conditions [15][16][17][18][19][20]. We are motivated by the aforementioned findings, our ongoing endeavors in the development of convenient synthetic approaches for the construction of biologically active heterocycles, and the growing interest in sonochemistry [21]. Our strategy is to develop a facile sonochemical synthesis and high yield procedure to prepare some novel 1,8-naphthyridine-4-oxyacetohydrazide Schiff 's bases and 1,8-naphthyridine-4-oxyacetamide incorporated into thiazolidine and pyrazolidine moieties and the investigation of their biological activities in suppressing the growth of HepG2 liver cancer cells. synthesized by the reaction of 4-hydroxy-2,7-dimethyl-1,8-naphthyridine 1 [22] with ethyl bromoacetate in the presence of anhydrous potassium carbonate [23] in absolute ethanol under reflux to give the nonisolated ester 2. The ester 2 reacted directly with hydrazine hydrate in refluxing ethanol which afforded the acid hydrazide 3 (Scheme 1). The structure of the acid hydrazide was established on elemental analysis and from spectral data. The IR spectra revealed the two absorption bands at 3338 and 3193 cm −1 which correspond to -NHNH 2 and a band at 1671 cm −1 due to amidic carbonyl group. Its 1 H NMR spectrum showed two D 2 O exchangeable signals due to NH 2 and NH protons at 3.99 and 9.28, respectively. Two singlets due to 2 methyl groups at 2.44 and 2.61 were observed. A broad singlet is present at 5.20 for methylene protons in addition to the 3 aromatic protons of naphthyridine at 6.18, 7.20, and 8.46. The hydrazide obtained 3 was then condensed with different aromatic aldehydes, ketones, and isatine, in absolute ethanol under ultrasound irradiation at 60-65 ∘ C, and produced the corresponding Schiff 's bases 4a-e, 5a, 5b, and 6, respectively (Scheme 1).

Results and Discussion
In the 1 H NMR spectra of Schiff's bases 4a-e, the disappearance of the broad singlet band at 3.99 which corresponds to NH 2 protons and an additional set of signals assigned to the -N=CH-group in the range 7.95-8.12 were observed. This observation confirmed the condensation between the amino group of the hydrazide and the carbonyl compounds. The structure of Schiff 's bases derived from aromatic ketones 5a, b was established on the basis of its elemental analysis and spectral data. The 1 H NMR spectrum of 5a revealed a new singlet signal for an extra CH 3 group at 2.17 beside the 3 protons of the thiophene ring at 7.06, 7.32, and 7.37. Moreover, an interesting observation appeared in the IR spectra of Schiff 's base derived from the isatin 6, where broad absorption bands shown at 3214-3454 cm −1 were attributed to the contribution of enolic OH and NH groups. This observation is consistent with similar reported compounds containing the isatin moiety [24]. The enolic character in this compound was further confirmed by the 1 H NMR at 600 MHz. Four signals centered in the range of 10.8 to 12.27 were assigned to amidic iminol structures [25] (Scheme 2 and Figure 2).
To find the specific effect of ultrasound on this reaction, all previously mentioned reactions were carried out under the same conditions in the absence of ultrasound irradiation ( Table 1). The data cited in Table 1 show that the reaction Treatment of 4a-d with thioglycolic acid in acetic acid under "silent" conditions resulted in cyclocondensation giving the corresponding thiazolidinone derivatives 7a-d. Upon repeating the reaction using ultrasonic irradiation instead of the classical method, the formation of the desired product in a shorter time (as examined by TLC) without an improvement in yield was observed. However, a catalyzed ultrasound irradiation process using molecular sieve (4Å) resulted in a good yield from 7a-d in an even shorter time. The structures of the compounds 7a-d were established on the basis of their elemental analysis, IR, 1 H NMR, 13 C NMR, and mass spectral data. Compounds 7a-d may be formulated as the oxo-form 7 I a-d or its tautomeric enol-form 7 II a-d (Scheme 3).
IR spectra of the isolated products revealed the predominance of the enol-form 7 II a-d due to the existence of strong absorption peaks in the region of ] = 3360-3380 cm −1 which corresponds to cyclic enol, while the amidic carbonyl absorption appeared at ] = 1680 cm −1 . The 1 H NMR of compound 7a showed a new singlet signal at 2.07 due to an acetyl group, two doublet signals at 3.71 and 3.90 for CH 2 -in thiazolidine, and CH-thiazolidine appeared at 8.29. The time of the reaction and the product yields are cited in Table 2, which also shows that the catalyzed ultrasound technique reduced the time of the reactions from several hours to minutes and improved the product yields from 40-49% (under conventional conditions) to 89-93%. Treatment of Schiff 's base 4b with bromoacetyl bromide in ethanol and the presence of MgO as a solid base catalyst under ultrasonic irradiation afforded only one isolable product (as examined by TLC) identified as pyrazolidin-3-one core structure 8 in 96% yield within 10 minutes. The same reaction was carried out in the absence of ultrasonic irradiation and gave just 45% The Scientific World Journal Figure 2 yield, in a much longer reaction time (72 h). The structure of 8 was established on the basis of its elemental analysis and spectral data. For example, its mass spectrum revealed a molecular ion peak at m/z 485 and at 487 for M + +2 and; its 1 H NMR revealed two doublet signals due to the pyrazolidine 2-CH at 2.99 and 3.32. In general, the improvement induced by ultrasound in the abovementioned reaction is based on the well-established cavitation theory [26]. The formation of Schiff 's bases follows a false sonochemistry type according to the sonochemical reactions classification of Luche [27,28]. The cavitation effect provides the mechanical energy for all subsequent chemical reactions, including bond scission induced by viscous frictional forces. In the present study, the substantial improvement induced by ultrasound irradiation in the reactions involving the formation of thiazolidine and pyrazolidine was assisted by the presence of solid catalysts. The pronounced enhancement of the ultrasound effect in the presence of solid catalysts is mainly due to cavitation in the liquid-solid system [29,30]. The cavitation occurred in the liquid near the solid surface of the catalyst, resulting in a cavity collapse that generates high-speed jets of liquid, which hit the surface 4 The Scientific World Journal of the catalyst with tremendous force. This process could generate more reaction active sites at the catalyst's surface, which led to a pronounced increase in the reaction rate and the production of a high percentage yield in short reaction times. Table 3. It is clear from the data cited in Table 3 that the tested compounds exhibit a moderate to strong growth inhibition activity on the tested cell line between 0.041 and 0.094 M concentrations in comparison to the known anticancer drug doxorubicin (DOX.). The cytotoxic activity of the selected derivatives on liver HepG2 cell lines, in comparison to the traditional anticancer drug DOX, revealed that compounds 4a and 7b were the most active and induced a marked growth inhibition against HepG2 when compared to DOX (4a and 7b IC 50 equal 0.047 and 0.041 M, resp., whereas DOX was 0.04 M).

Conclusion
A class of novel Schiff 's bases, thiazolidine and pyrazolidine, incorporated into 1,8-naphthyridine nucleus under both sonication and classical conditions were synthesized successfully. Ultrasonic irradiation resulted in pronounced improvements in both rates and yield of reactions. The use of solid catalysts enhances the efficacy of sonication and leads to the formation of high percentage yields in shorter reaction times. The cytotoxicity screening of some selected new compounds revealed that the selected compounds showed reasonable antitumor activity against the HepG2 cancer cell line in The Scientific World Journal   Reactions. An equimolar mixture of 3 (0.5 gm, 0.002 moL) and the appropriate aromatic aldehyde (0.002 moL) in 10 mL absolute ethanol was stirred under reflux for a suitable time (until the disappearance of starting materials as examined by TLC). The reaction mixture was concentrated and cooled and the crude product, so-formed, was collected by filtration and recrystallized from ethanol to give the title compound 4a-e.

Method B: Sonicated Reactions.
To a solution of 2-(2,7-dimethyl-1,8-naphthyridin-4-yloxy) acetohydrazide 3 (0.002 moL) in ethanol (10 mL) and appropriate aromatic aldehyde (0.002 moL) in a 100 mL Erlenmeyer flask. The mixture was subjected to ultrasound irradiation at 60-65 ∘ C for suitable time (cf. Table 1) until the starting material was no longer detectable by TLC. The precipitate formed was filtered off and recrystallized from ethanol to produce the corresponding Schiff 's bases 4a-e.

Method A: Silent
Reactions. An equimolar mixture of (3) (0.004 moL) and the appropriate aromatic heterocyclic ketones, namely, a 2-acetylthiophene and 2-acetylfuran (0.004 moL) in 20 mL absolute ethanol, were stirred under reflux for 6 h. The reaction mixture was concentrated and cooled and the formed precipitate was recrystallized from ethanol/petroleum ether to give the title product 5a, 5b.

Method B: Sonicated Reactions.
To a solution of 2-(2,7-dimethyl-1,8-naphthyridin-4-yloxy) acetohydrazide 3 (0.004 moL) in ethanol (20 mL) in a 100 mL Erlenmeyer flask, an appropriate aromatic heterocyclic ketone (0.004 moL) was added. The mixture was subjected to ultrasound irradiation for a suitable time (cf. Table 1) until the starting materials were no longer detectable by TLC. The reaction was kept at temperature 60-65 ∘ C which was attained by addition or removal of water in ultrasonic bath, (the temperature inside the reaction vessel was 60 ∘ C). The precipitate formed was filtered off and recrystallized from ethanol/pet. ether to afford the corresponding Schiff 's bases 5a, 5b.  (6) 4.1.2.3.1. Method A: Silent Reactions. A mixture of isatin (0.01 moL) and acid hydrazide 3 (0.01 moL) in ethanol, acidified with 4 drops of glacial acetic acid, was refluxed for 1 h. The reaction mixture was then concentrated, cooled, and filtered. The filtered precipitate was washed thoroughly with ether and recrystallized from dilute ethanol to give 6 as brown crystals.

Method B: Sonicated Reactions.
A solution (0.01 moL) of isatin and acid hydrazide 3 (0.01 moL) in ethanol, acidified with 4 drops of glacial acetic acid in a 100 mL Erlenmeyer flask, was subjected to ultrasound irradiation for 10 minutes. The reaction was kept at a temperature of 60-65 ∘ C which was attained by addition or removal of water in the ultrasonic bath (the temperature inside the reaction vessel was 60 ∘ C). The filtered precipitate was washed thoroughly with ether and recrystallized from dilute ethanol to produce the corresponding Schiff 's bases 6 as brown crystals; m.p.   Table 2). The reaction mixture was cooled at r.t., neutralized with ammonia and the precipitate formed was collected, filtered, and crystallized from ethanol.

Sonicated Reaction.
A solution of Schiff 's bases 4a-d (0.001 moL) in 8 mL glacial acetic acid and (0.4 mL, 0.001 moL) thioglycolic acid in a 50 mL Erlenmeyer flask was subjected to ultrasound irradiation for a suitable time (cf. Table 2) until the starting material was no longer detectable by TLC. The reaction was kept at 70-75 ∘ C which was attained by addition or removal of water in the ultrasonic bath (the temperature inside the reaction vessel was 70 ∘ C). The reaction mixture was cooled to r.t. and neutralized with ammonia and the precipitate formed was collected, filtered, and crystallized from ethanol.

Catalyzed Sonicated Reaction. To a solution of
Schiff 's bases 4a-d (0.001 moL) in 8 mL glacial acetic acid and thioglycolic acid (0.4 mL and 0.001 moL) in a 50 mL Erlenmeyer flask, a molecular sieve (1 g, 4Å) was added. The mixture was subjected to ultrasound irradiation for a suitable time (cf. Table 2) until the starting material was no longer detectable by TLC. The reactants were kept at 70-75 ∘ C which was attained by addition or removal of water in the ultrasonic bath (the temperature inside the reaction vessel was 70 ∘ C). The reaction mixture was decanted, cooled, and neutralized 8 The Scientific World Journal with ammonia. The formed precipitate was collected, filtered, and crystallized from ethanol.
The synthesized compounds (7a-d) with their physical data are listed below.

Sonicated Reaction.
A solution of 2-(2,7-dimethyl-1,8-naphthyridin-4-yloxy)-N -(4-methoxybenzylidene) acetohydrazide 4b (0.36 gm, 0.001 moL), (0.09 mL, 0.001 moL) bromoacetyl bromide in 10 mL absolute ethanol, and MgO (0.5 gm) in a 50 mL Erlenmeyer flask was subjected to ultrasound irradiation for 10 min until the starting material was no longer detectable by TLC. The reaction was kept at 70-75 ∘ C which was attained by addition or removal of water in the ultrasonic bath (the temperature inside the reaction vessel was 70 ∘ C). The reaction mixture was cooled at r.t.; the precipitate formed was filtered off, washed with ethanol, and recrystallized from ethanol/DMF to produce compound 8 in a yield of 96%  4a, 4b, 4c, 4d, 7a, 7b, 7c, and 7d) were subjected to a screening system for evaluation of their antitumor activity against the liver HepG2 cancer cell line in comparison to the known anticancer drug DOX. The potential cytotoxicity of the selected 1,8-naphthyridine derivatives was tested using the method of Skehan et al. [31] as follows: cells were plated in a 96-multiwell plate (10 4 cells/well) for 24 h before treatment with the compounds to allow attachment of the cells to the The Scientific World Journal 9 wall of the plate. Different concentrations of the compound under test (5, 12.5, 25, and 50 g/mL) were added to the cell monolayer. Triplicate wells were prepared for each individual dose. Monolayer cells were incubated with the compounds for 48 h at 37 ∘ C and in an atmosphere of 5% CO 2 . Cultures were fixed with trichloroacetic acid and stained for 30 min with 0.4% (wt/vol.) sulforhodamine B (SRB) dissolved in 1% acetic acid. Unbound dye was removed by four washes with 1% acetic acid, and protein-bound dye was extracted with 10 M unbuffered tris base [tris(hydroxymethyl)aminomethane] for determination of optical density in a computer-interfaced, 96-well microtiter plate reader. The SRB assay results were linear with the number of cells and with values for cellular protein measured by both the Lowry and Bradford assays at densities ranging from sparse subconfluence to multilayered supraconfluence. The signal-to-noise ratio at 564 nm was approximately 1.5 with 1,000 cells per well. The relation between the surviving fraction and drug concentration is plotted to get the survival curve of both cancer cell lines after treatment with the specified compound.