Accelerated Synthesis of 4-Arylideneisoxazolones in Glycerol Medium

1. Introduction

Among N, O-heterocyclic compounds, five-membered rings such as isoxazoles have received special attention due to their potential applications in synthetic chemistry, the pharmaceutical industry, and biological sciences [1]. Among the isoxazole family, arylideneisoxazol-5(4H)-ones and their derivatives have garnered considerable attention owing to their potential as antimycobacterial [2], antifungal [3], anticancer [4,5], antibacterial [6], enzyme inhibitor [7], antimicrobial, antidiabetic, antiandrogen [8], and antioxidant [9] agents. In addition to the aforementioned characteristics, arylideneisoxazole-5(4H)-one derivatives are used as anti-corrosion reagents [10]. The structures of some important arylidene isoxazol-5(4H)-ones are depicted in Figure 1. The five-membered heterocyclic scaffold is an important intermediate for the construction of other heterocyclic products and organic molecules, including polysubstituted pyridines [11], spirocyclic azepino[4,3,2-cd]indole derivatives [12], cyclopentenyl spiroisoxazolones [13], fully substituted pyrrole-2-carboxylic acids [14], imidazoles [15], 4-aryl-3-[(E)-(hydroxyimino)(aryl)methyl]-4,6,7,8-tetrahydroquinoline-2,5(1H,3H)-diones [16], spiro[isoxazole-pyrazoloquinoline]s [17], spirocarbocycles [18], and others [19]. Consequently, in the research community of chemistry and pharmaceuticals, synthesizing arylideneisoxazol-5(4H)-ones has garnered considerable attention. A feasible and widely accepted strategy involves a three-component process that has been catalyzed under various conditions with a variety of catalysts, including water extract of orange fruit peel ash (WEOFPA) in glycerol [4], glutamic acid catalyst in glycerol [5], Cu@Ag−CeO₂/chitosan nanocomposite [20], yttrium (III)-MMZ [21], Na₂S₂O₃ [22], Na₂SO₃ [23], g-C₃N₄·OH nanocomposite [24], sulfamic acid [25], 1-methyl-3-carboxymethylimidazolium chloride [26], lemon juice [27], FeS_1-x-Fe₃O₄/C nanocatalyst [28], sodium benzoate [29], NaOAc [30], MMWCNT-PEG-PIP [31], sodium lauryl sulfate [32], amine-functionalized cellulose [33], SiO₂ NPs [34], urea [35], triphenylphosphine [36], DMAP [37], ruthenium-hydride complex [38], ferrite nanoparticles [39], CeO₂ nanoparticles [40], aqueous extract of Areca nut [41], theophylline hydrogen sulfate [42], CuNPs@N-GQDs@APTES) [43], guanidine hydrochloride [44], and malic acid [45]. Other catalysts for synthesizing arylideneisoxazolones can be found in a recently published review [46].

Propane-1,2,3-triol (glycerol or glycerin, MW = 92.09 g/mol, d = 1.258 g/mL) is the simplest trihydric alcohol and was used as a vital chemical with numerous applications in modern energy, environmental technologies, and the synthesis of value-added products [47]. Glycerol, also termed “organic water,” is readily available, cheap, polar, non-toxic, biodegradable, and easily forms strong networks of hydrogen bonds [48]. Glycerol is found in the “generally recognized as safe” list made by the U.S. Food and Drug Administration [49]. Glycerin is employed in cakes, beverages, pharmaceuticals, skin and hair products, the production of shampoos, soaps, toothpastes, esters, polyethers, detergents, cellophane, and some alkyd resins [50,51]. As compared with the conventional organic solvents, propane-1,2,3-triol reveals many advantages, including low toxicity, non-volatility, inflammability, and low production cost. This polyhydroxy alcohol, as a classic example of a “green solvent,” improves the reaction selectivity and increases the reaction rate [49]. Glycerol not only acts as a phase transfer catalyst but also as a recyclable solvent [52]. This biocompatible polyalcohol was used as an eco-friendliness reaction medium or additive to synthesizing different heterocyclic compounds such as 4-(1H-pyrazol-4-yl)-polyhydroquinolines [49], pyrido[2,3-d]pyrimidines [53], 1,4-dihydropyridines [54,55], functionalized thioamides and 4H-thiopyrans [56], imidazo[2,1-b]thiazoles [57], thiazole Betti bases [58], 2-amino-3-phenylsulfonyl-4H-benzochromens [59], 3-benzoxazol-2-yl-chromen-2-ones [60], 2-amino-4,8-dihydropyrano[3,2-b]pyran-3-carbonitriles [61], 2,3-dihydroquinazolin-4(1H)-ones [62], 6-amino-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles [63,64], 1,4-disubstituted 1,2,3-triazoles [65], 5′-thioxospiro[indoline-3,3′-[1,2,4]triazolidin]-2-ones [66], α-aminophosphonates [67], nitrones [68], 2-arylimidazo[1,2-a]pyrimidine-3-carbaldehydes [69], and others [70].

Given the synthetic significance of isoxazolone-containing compounds, the development of a green, efficient, less energy-consuming, faster, more economical, and sustainable method is of interest to many researchers in the field of synthetic organic chemistry. Therefore, it was decided to investigate the synthesis of several isoxazol-5(4H)-one derivatives (Scheme 1) based on our laboratory resources.

2. Materials and Methods

2.1. General

All the essential chemicals and solvents were obtained from commercial sources and used as received. All solvents were distilled before use. The reactions were monitored by thin-layer chromatography (TLC) analysis using pre-coated aluminum sheets (Silica gel G 60 F₂₅₄, Merck, Germany). TLC was visualized by ultraviolet (UV) irradiation, exposure to iodine vapor, and spray reagent. The melting points were recorded using an electrothermal 9100 melting point apparatus and were uncorrected. ¹H- and ¹³C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer using CDCl₃ or DMSO-d₆ as solvent. Splitting patterns are designated as follows: s, singlet; d, doublet; dd, doublet of doublets; and m, multiplet. Chemical shift values are specified in parts per million (ppm). Coupling constant values are reported in hertz (Hz).

2.2. General Procedure for the Synthesis of Target Heterocyclic Compounds (4a-p)

Ethyl acetoacetate (1a, 1 mmol) or ethyl 3-oxo-3-phenylpropanoate (1b, 1 mmol), hydroxylamine hydrochloride (2, 1 mmol), and aryl/heteroaryl aldehydes (3a-3k, 1 mmol) were poured into a round-bottom flask equipped with a magnetic stirring bar, and glycerol (41.0 mmol, 3.0 mL) was added. The reaction was stirred at 60 °C for appropriate times. After the reaction was completed (monitored by TLC analysis), the reaction mixture was cooled to room temperature, and the solvent was removed via filtration. The precipitate of the desired compounds remained on the filter paper. After recrystallization of the precipitates in ethanol, pure heterocyclic products (4a-4p) were obtained.

2.2.1. 4-(4-Hydroxy-3-Methoxybenzylidene)-3-Methylisoxazol-5(4H)-one (4a)

¹H NMR (DMSO-d₆, 400 MHz): δ 2.25 (s, 3H, CH₃), 3.85 (s, 3H, OCH₃), 6.96 (d, J = 8.4 Hz, 1H, Ar-H), 7.78 (s, 1H, H-vinyl), 7.91 (dd, J = 2.0, 8.8 Hz, 1H, Ar-H), 8.52 (d, J = 2.0, Hz, 1H, Ar-H), 10.77 (s, 1H, OH); ¹³C NMR (DMSO-d₆, 100 MHz): δ 11.8 (CH₃), 56.0 (3H, OCH₃), 114.2, 116.3, 117.2, 125.5, 132.1, 147.9, 152.3, 154.3, 172.7 (C=N), 169.5 (C=O).

2.2.2. 4-(4-Hydroxybenzylidene)-3-Phenylisoxazol-5(4H)-one (4m)

¹H NMR (DMSO-d₆, 400 MHz): δ 6.95 (d, J = 9.2 Hz, 2H, Ar-H), 7.59-7.69 (m, 6H, Ar-H, H-vinyl), 8.45 (d, J = 8.8 Hz, 2H, Ar-H), 11.16 (s, 1H, OH); ¹³C NMR (DMSO-d₆, 100 MHz): δ 113.2, 116.6, 124.9, 127.9, 129.3, 129.4, 129.7, 131.3, 138.5, 153.8, 164.8, 169.4.

2.2.3. 4-(3-Ethoxy-4-hydroxybenzylidene)-3-phenylisoxazol-5(4H)-one (4o)

¹H NMR (DMSO-d₆, 400 MHz): δ 1.39 (t, J = 6.8 Hz, 3H, CH₃), 4.35 (q, J = 6.8 Hz, 2H, OCH₂), 6.96 (d, J = 8.4 Hz, 1H, Ar-H), 7.62-7.68 (m, 6H, Ar-H), 7.89 (dd, J = 1.6, 7.6 Hz, 1H, Ar-H), 8.50 (s, 1H, H-vinyl), 10.85 (s, 1H, OH); ¹³C NMR (DMSO-d₆, 100 MHz): δ 15.0 (CH₃), 64.4 (OCH₂), 113.0, 116.3, 118.5, 125.4, 127.9, 129.3, 129.7, 131.2, 132.6, 147.1, 154.1, 155.0, 164.9 (C=N), 169.6 (C=O).

3. Results and Discussion

The synthesis of desired heterocyclic compounds was accomplished by the synthetic route illustrated in Scheme 1. To this purpose, the reaction conditions should be optimized using a model reaction. The reaction of ethyl acetoacetate (1a), hydroxylamine hydrochloride (2), and vanillin (3a) was selected as a model reaction (Scheme 2).

The results of the optimization of the reaction conditions under solvent-free and various solvents can be seen in Table 1. At first, the reaction was studied in a solvent-free environment without any additives or catalysts. At room temperature (RT), the yield of the isoxazolone product (4a) was moderate after 60 min (Table 1, Entry 1). Running the model reaction in water (H₂O) as a green solvent led to 55% isolated yield after 60 min (Table 1, Entry 2). Performing the same reaction at different temperature conditions, including refluxing H₂O, did not lead to satisfactory results (one result is shown in row 3 of Table 1). Changing the solvent from water to ethanol (EtOH) under reflux conditions did not significantly improve the reaction yield (Table 1, Entry 4). After the previous unattractive results, it was decided to check the reaction in other organic solvents such as ethyl acetate (EtOAc), chloroform (CHCl₃), n-hexane, dichloromethane (CH₂Cl₂), and dioxane in the hope that better results might be obtained (Table 1, Entries 5-9). Performing the reactions at different temperatures and boiling points of solvents resulted in disappointing results. That’s why we decided to use a beautiful organic water, namely glycerol, to implement the model reaction. Therefore, the model reaction was explored in the presence of glycerol with different mol%, and an increase in the reaction yields and a decrease in the reaction times were seen in different volumes of glycerol (Table 2, Entries 1-6). The cyclocondensation reaction using 0.6 mL (8.2 mmol) of glycerol leads to the formation of the product 4a in 75% yield after 32 min (Table 2, Entry 1). At RT, when the amounts of glycerol were increased from 0.6 mL (8.2 mmol) to 1.2 (16.4), 1.8 (24.6), 2.4 (32.8), and 3.0 mL (41.0 mmol), the heterocyclic product 4a was formed in good to excellent yield (Table 2, Entries 2-5). The reaction was also tested using 3.6 mL (49.2 mmol) glycerol, but found that the target product was formed in 90% yield (Table 2, Entry 6). So, 3.0 mL (41.0 mmol) glycerol is the ideal medium for the model reaction. To study the impact of temperature on the reaction yields and reaction times, more experiments were carried out in 3.0 mL (41.0 mmol) glycerol at different temperatures (30-80 °C) (Table 2, Entries 7-17). An improvement in yields and reaction times was observed. Raising the reaction temperature from 25 °C to 60 °C significantly decreased the reaction time and also increased the yield of the reaction. Based on the results of the optimization, performing the cyclocondensation reaction in glycerol at 60 °C resulted in a rapid synthesis of the heterocyclic product 4a in a higher isolated reaction yield (Table 2, Entry 13). Increasing the reaction temperature above 60 °C did not lead to notable improvements in the reaction times and the yields (Table 2, Entries 14-17). Accordingly, the best reaction results were obtained at 60 °C in glycerol (Table 2, Entry 13; the optimized conditions).

Table 2. Optimization of the cyclocondensation reaction using glycerol.^1.

Entry	Glycerol/mL (mmol)	Temp./°C	Time/min.	Isolated yields for 4a /%
1	0.6 (8.2)	RT	32	75
2	1.2 (16.4)	RT	27	80
3	1.8 (24.6)	RT	23	85
4	2.4 (32.8)	RT	17	89
5	3.0 (41.0)	RT	12	90
6	3.6 (49.2)	RT	15	90
7	3.0 (41.0)	30	10	92
8	3.0 (41.0)	35	9	92
9	3.0 (41.0)	40	8	93
10	3.0 (41.0)	45	7	93
11	3.0 (41.0)	50	6	93
12	3.0 (41.0)	55	5	94
132	3.0 (41.0)	60	3	96
14	3.0 (41.0)	65	3	93
15	3.0 (41.0)	70	3	90
16	3.0 (41.0)	75	3	90
17	3.0 (41.0)	80	3	90

¹ Reaction performed using ethyl acetoacetate (1a, 1 mmol), hydroxylamine hydrochloride (2, 1 mmol), vanillin (3a, 1 mmol), and glycerol (x mL, y mmol), stirred at various temperatures. ² Optimized conditions.

Table 2. Optimization of the cyclocondensation reaction using glycerol.^1.

Entry	Glycerol/mL (mmol)	Temp./°C	Time/min.	Isolated yields for 4a /%
1	0.6 (8.2)	RT	32	75
2	1.2 (16.4)	RT	27	80
3	1.8 (24.6)	RT	23	85
4	2.4 (32.8)	RT	17	89
5	3.0 (41.0)	RT	12	90
6	3.6 (49.2)	RT	15	90
7	3.0 (41.0)	30	10	92
8	3.0 (41.0)	35	9	92
9	3.0 (41.0)	40	8	93
10	3.0 (41.0)	45	7	93
11	3.0 (41.0)	50	6	93
12	3.0 (41.0)	55	5	94
132	3.0 (41.0)	60	3	96
14	3.0 (41.0)	65	3	93
15	3.0 (41.0)	70	3	90
16	3.0 (41.0)	75	3	90
17	3.0 (41.0)	80	3	90

¹ Reaction performed using ethyl acetoacetate (1a, 1 mmol), hydroxylamine hydrochloride (2, 1 mmol), vanillin (3a, 1 mmol), and glycerol (x mL, y mmol), stirred at various temperatures. ² Optimized conditions.

After optimization of the reaction conditions, to evaluate the generality of the process, the reaction between 1a, 2, and substituted benzaldehydes (3a-3k) and heteroaryl aldehydes was studied in glycerol at 60 °C (Table 3, Entries 2-10). Aryl aldehydes containing electron-donating functional groups like OH, OCH₃, and OC₂H₅, the process was completed, and the target products (4a-4i) were formed in excellent isolated yield after 3-10 min. Under the optimized conditions, the process using substituted benzaldehydes carrying electron-withdrawing substituents, including NO₂ or Cl, did not obtain satisfactory results. Interestingly, the condensation of thiophene-2-carbaldehyde and 1H-indole-3-carbaldehyde with 1a and 2 led to the construction of the heterocyclic molecules (4j and 4k) in excellent isolated yields (Table 3, Entries 10 and 11). The utility of this sustainable cyclocndensation was explored with ethyl 3-oxo-3-phenylpropanoate substrate (1b) instead of 1a. The yields remained high (78-92%), and the reactions remained fast (7-12 min), confirming that our method works well for both types of starting materials. Under the optimized conditions, the reactions yielded the heterocyclic compounds (4l-4p) in high isolated yields and shorter reaction times (Table 3, Entries 12-16).

Table 2. Green synthesis of arylideneisoxazol-4-ones (4a-4p) in glycerol medium at 60 °C.^1.

Entry	Structure of isoxazolone products (4a-4p)	Yield (%)/time (min)	Melting points observed/reported [ref.]
1		96/3	214-216/215-216 [33]
2		90/4	210-212/211-212 [33]
3		84/10	198-200/200-202 [33]
4		85/5	200-202/202-204 [33]
5		89/5	222-225/225-227 [33]
6		95/7	138-140/135-137 [33]
7		85/5	184-186/182-184 [36]
8		93/4	124-126/127-128 [33]
9		75/10	214-216 [33]
10		90/7	142-143/146-148 [33]
11		85/8	238-240/235-238 [45]
12		90/7	215-218/212-214 [37]
13		85/10	208-210/207-210 [34]
14		79/12	203-205/201-202 [34]
15		92/10	152-156/150-152 [37]
16		78/8	215-217/214-216 [37]

¹ Reaction performed using dicarbonyl compounds (1a-1b, 1 mmol), hydroxylamine hydrochloride (2, 1 mmol), various aldehydes (4a-4k, 1 mmol), and glycerol 3.0 mL, 41.0 mmol) stirred at 60 °C.

Table 2. Green synthesis of arylideneisoxazol-4-ones (4a-4p) in glycerol medium at 60 °C.^1.

Entry	Structure of isoxazolone products (4a-4p)	Yield (%)/time (min)	Melting points observed/reported [ref.]
1		96/3	214-216/215-216 [33]
2		90/4	210-212/211-212 [33]
3		84/10	198-200/200-202 [33]
4		85/5	200-202/202-204 [33]
5		89/5	222-225/225-227 [33]
6		95/7	138-140/135-137 [33]
7		85/5	184-186/182-184 [36]
8		93/4	124-126/127-128 [33]
9		75/10	214-216 [33]
10		90/7	142-143/146-148 [33]
11		85/8	238-240/235-238 [45]
12		90/7	215-218/212-214 [37]
13		85/10	208-210/207-210 [34]
14		79/12	203-205/201-202 [34]
15		92/10	152-156/150-152 [37]
16		78/8	215-217/214-216 [37]

¹ Reaction performed using dicarbonyl compounds (1a-1b, 1 mmol), hydroxylamine hydrochloride (2, 1 mmol), various aldehydes (4a-4k, 1 mmol), and glycerol 3.0 mL, 41.0 mmol) stirred at 60 °C.

The structure of the target compounds was confirmed using spectral data. For example, the ¹H NMR spectrum of 4o in DMSO-d₆ solvent showed the signals at δ 1.39 (t, 3H, CH₃ of ethoxy), 4.35 (q, 2H, CH₂ of ethoxy), 10.85 (s, 1H, proton of hydroxy group), 6.96 (d, 1H, proton ortho to OH), 7.89 (dd, 1H, proton meta to OH), 8.50 (s, 1H, proton vinyl group between two isoxazolone and aryl ring), 7.89 (d, 1H, proton ortho to ethoxy group), and 7.62-7.68 (m, 6H, phenyl group). Moreover, the ¹³C NMR spectrum of this compound in CDCl₃ displayed four distinct signals belonging to CH₃, OCH₂, C=O, and C=N, at δ 15.0, 64.4, 169.6, and 164.9 ppm, respectively. The signals of the other carbons resonated at δ 155.0-113.0 ppm (12 distinct signals).

To highlight the merits of the present glycerol-promoted protocol, we compared our optimized conditions with previously reported catalytic systems for the synthesis of 4-arylideneisoxazol-5(4H)-ones. Table 3 summarizes the key features of several representative methods from recent literature. As can be seen from Table 3, the present method offers several distinct advantages over previously reported protocols:

a) Reaction Time: The reaction completes within 3 minutes, which is significantly faster than all other methods listed (ranging from 40 to 180 minutes).

b) Reaction Temperature: The optimized temperature of 60 °C is milder than most reported methods (typically 70-85 °C). Lower energy consumption is an important aspect of green chemistry.

c) Yield: The isolated yield of 96% is among the highest reported for this model substrate, demonstrating the excellent efficiency of this protocol.

d) Operational Simplicity: The one-pot, three-component reaction requires no special apparatus, no inert atmosphere, and no chromatographic separation. The workup involves only cooling, filtration, and recrystallization from ethanol.

e) Green Chemistry Aspects: Glycerol is a non-toxic, biodegradable, renewable, and non-volatile solvent. The method uses glycerol as both the solvent and promoter without any additional catalyst synthesis.

Table 3. Comparison of different catalytic systems for the synthesis of 4a.

Entry	Catalyst / [ref.]	Solvent	Temp. (°C)	Time (min)	Yield (%)
1	Na₂S₂O₃ [22]	EtOH	80	40	85
2	Na₂SO₃ [23]	H2O	70	120	88
3	Sulfamic acid [25]	H2O	80	60	92
4	Lemon juice [27]	H2O	RT	180	88
5	SiO₂ NPs [34]	H2O	RT	60	90
6	Urea [35]	H2O	RT	45	85
7	Malic acid [45]	H2O	50	45	90
8	Glycerol (This work)	Glycerol	60	3	96

Table 3. Comparison of different catalytic systems for the synthesis of 4a.

Entry	Catalyst / [ref.]	Solvent	Temp. (°C)	Time (min)	Yield (%)
1	Na₂S₂O₃ [22]	EtOH	80	40	85
2	Na₂SO₃ [23]	H2O	70	120	88
3	Sulfamic acid [25]	H2O	80	60	92
4	Lemon juice [27]	H2O	RT	180	88
5	SiO₂ NPs [34]	H2O	RT	60	90
6	Urea [35]	H2O	RT	45	85
7	Malic acid [45]	H2O	50	45	90
8	Glycerol (This work)	Glycerol	60	3	96

Some sustainable chemistry metrics were evaluated for the formation of compound 4f (Scheme 3). In this case, some parameters such as the environmental factor (E-factor), atom economy (AE), atom efficiency (A. Ef.), the reaction mass efficiency (RME, Curzons definition), optimum efficiency (OE), carbon efficiency (CE), greener atomic level (GAL), and the process mass intensity (PMI) were calculated as follows (1-7 equations) [36].

$$Atom economy={\displaystyle \frac{Molecular mass of \mathbf{4}\mathit{a} product}{Molecular mass of reactants}}\times 100$$

(1)

$$E-factor={\displaystyle \frac{g waste}{g product}}$$

(2)

$$Atom efficiency={\displaystyle \frac{mass of \mathbf{4}\mathit{a} product}{mass of product+byproducts}}\times 100$$

(3)

$$RME={\displaystyle \frac{mass of the product}{the mass of the reactants}}\times 100$$

(4)

$$OE={\displaystyle \frac{RME}{AE}}\times 100$$

(5)

$$CE={\displaystyle \frac{carbon in product}{total carbon in reactants}}\times 100$$

(6)

$$PMI={\displaystyle \frac{mass of all materials used}{mass of product}}$$

(7)

In view of sustainable chemistry, this reaction has green aspects. Therefore, from the perspective of sustainable chemistry and considering the green metric results, implementing this green reaction for the synthesis of isoxazoles is of great importance (Scheme 3).

Scheme 3. Some green metrics for the synthesis of 4f.

Scheme 3. Some green metrics for the synthesis of 4f.

Based on reports in the literature [53,54,55,62,63,64,69], a reaction mechanism for the synthesis of target heterocycles 4a-4p, promoted by glycerol, is depicted in Scheme 4. At first, ketoesters 1a-1b are protonated by glycerol via hydrogen bonding (H-binding). The nucleophilic attack of hydroxylamine (2) on the activated carbonyl group of 1a-1b led to the formation of the oxime intermediate (A). In the next step, cyclization of the oxime intermediate, along with the elimination of ethanol, led to the formation of isoxazolone B. Isoxazolone B can be enolized using glycerol. The aryl/heteroaryl aldehydes 3 were also activated through H-bonding with glycerol. Activated aldehydes (D) undergo Knoevenagel condensation with enolized isoxazolone (C), resulting in intermediates E. Finally, after elimination of water, the condensed desired heterocyclic products (4a-4p) were obtained (Scheme 4).

4. Conclusions

In summary, glycerol was introduced as a green and effective reaction medium for synthesizing arylideneisoxazol-5-ones via a three-component process. The results have demonstrated that this process enables easy, rapid access to isoxazolone-containing heterocycles in relatively high yields. This method highlights the simplicity and sustainability of the preparation of isoxazole-5-one derivatives. This protocol accommodates many different aldehydes (especially those with electron-donating groups) and two β-ketoesters, delivering desired compounds in short reaction times.