One-pot Vilsmeier reagent-mediated multicomponent reaction: A direct synthesis of oxazolones and Erlenmeyer azlactones from carboxylic acids

Abdulhamid Fadavi; Maaroof Zarei

doi:10.1016/j.crci.2017.12.001

One-pot Vilsmeier reagent-mediated multicomponent reaction: A direct synthesis of oxazolones and Erlenmeyer azlactones from carboxylic acids

Abdulhamid Fadavi ¹ ; Maaroof Zarei ²

¹ Department of Chemistry, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran
² Department of Chemistry, Faculty of Sciences, University of Hormozgan, Bandar Abbas 71961, Iran

Comptes Rendus. Chimie, Volume 21 (2018) no. 1, pp. 9-13.

Abstract

N-Acylation of glycine with carboxylic acids was carried out by using the Vilsmeier reagent. The resulting N-acyl-α-amino acids were subsequently cyclodehydrated into oxazol-5-ones via the Vilsmeier reagent. Finally, treatment of oxazol-5-ones with aldehydes in the presence of the Vilsmeier reagent delivered Erlenmeyer azlactones. By combining these steps, using the Vilsmeier reagent allowed direct one-pot conversion of carboxylic acids into oxazol-5-ones and direct one-pot conversion of carboxylic acids into Erlenmeyer azlactones in the presence of aldehydes. These Vilsmeier reagent-mediated multicomponent reactions proceeded smoothly in reasonable chemical yields at room temperature. The chemical structure of the title compounds was confirmed by spectral data.

Metadata

Received: 2017-11-03
Accepted: 2017-12-04
Published online: 2018-01-01

DOI: 10.1016/j.crci.2017.12.001

Keywords: Oxazol-5-ones, Erlenmeyer azlactones, Vilsmeier reagent, Cyclodehydration, One-pot synthesis

Author's affiliations:

Abdulhamid Fadavi ¹; Maaroof Zarei ²

¹ Department of Chemistry, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran
² Department of Chemistry, Faculty of Sciences, University of Hormozgan, Bandar Abbas 71961, Iran

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     author = {Abdulhamid Fadavi and Maaroof Zarei},
     title = {One-pot {Vilsmeier} reagent-mediated multicomponent reaction: {A} direct synthesis of oxazolones and {Erlenmeyer} azlactones from carboxylic acids},
     journal = {Comptes Rendus. Chimie},
     pages = {9--13},
     publisher = {Elsevier},
     volume = {21},
     number = {1},
     year = {2018},
     doi = {10.1016/j.crci.2017.12.001},
     language = {en},
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Abdulhamid Fadavi; Maaroof Zarei. One-pot Vilsmeier reagent-mediated multicomponent reaction: A direct synthesis of oxazolones and Erlenmeyer azlactones from carboxylic acids. Comptes Rendus. Chimie, Volume 21 (2018) no. 1, pp. 9-13. doi : 10.1016/j.crci.2017.12.001. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2017.12.001/

Original version of the full text

1 Introduction

Oxazol-5-ones (oxazolones) 1 and 4-arylmethyleneoxazolones (Erlenmeyer azlactones) 2 are versatile organic compounds with applications in different fields of research [1].

Oxazolone derivatives possess important biological activities [1,2]. Some prepared oxazolones showed inhibition of tyrosinase [3] and Human NK1 receptor antagonist [4]. Also, they are used as antioxidant and anticorrosive additives for lubricant oils [5]. In addition, oxazolones are used as versatile building blocks in synthetic organic chemistry for the synthesis of several heterocycles and biologically active molecules [6].

According to their importance, many efforts made for the synthesis of oxazolones and azlactones have been reported [7]. The general method for the preparation of oxazolones involves the classical cyclization of an N-acyl-α-amino acid with acetic anhydride under reflux conditions, where N-acyl-α-amino acids are prepared separately from acyl chlorides and glycine [8].

In a pioneering work, Erlenmeyer described the synthesis of azlactone by condensation of benzaldehyde with N-acetylglycine in the presence of acetic anhydride and sodium acetate [9]. To avoid the acidic and harsh conditions using acetic anhydride [10], other dehydrating agents such as carbodiimides [11], ethyl chloroformate [12], and arylsulfonyl chloride [13] have been reported.

Some drawbacks in these methods are multistep reactions from carboxylic acid to oxazolones, the need for low or high temperatures, lower yield of the products, harsh conditions, unnecessary waste, and chromatographic separations of products.

Chloromethylenedimethylammonium chloride (Vilsmeier reagent) 3 is easily prepared by the reaction of DMF and oxalyl chloride or thionyl chloride in dry dichloromethane [14]. This reagent has been identified as a formylating agent [15], a coupling reagent in the synthesis of esters [16], amides [17], acid chlorides [18], β-sultams [19], β-lactams [20], diacylhydrazines [21], 1,3,4-oxadiazoles [22], and 1,3,4-thiadiazoles [23].

To the best of our knowledge, the Vilsmeier reagent has not been used for the synthesis of oxazolones. Hence, in this article, we discussed the application of the Vilsmeier reagent for the conversion of carboxylic acid to oxazolones and Erlenmeyer azlactones.

2 Results and discussion

2.1 Synthesis

Hippuric acid 4a used as a starting material was simply prepared from benzoic acid and glycine in the presence of the Vilsmeier reagent. The prepared hippuric acid 4a was stirred with the Vilsmeier reagent in the presence of triethylamine in DMF to yield 2-phenyloxazol-5-one 5a in 66% yield. The progress of the reaction was monitored by thin-layer chromatography (TLC) until all hippuric acid 4a was consumed in the reaction. The reaction mixture was then quenched with water and ethyl acetate. The organic layer was washed with saturated sodium bicarbonate and dried over Na₂SO₄. After removal of the solvent and recrystallization from ethanol, 2-phenyloxazol-5-one 5a was obtained as pale yellow crystals.

To optimize the reaction conditions for the synthesis of 2-phenyloxazol-5-one 5a from hippuric acid 4a, various solvents and different molar amounts of the Vilsmeier reagent as well as temperature were investigated (Table 1). It was found that the yield of 5a in chloroform was higher than in other solvents. In addition, the best temperature for this reaction was room temperature giving highest yield of the desired product. As it is shown in Table 1, an equimolar amount of the Vilsmeier reagent was sufficient for completion of the reaction.

Entry	Solvent	Temperature	Vilsmeier reagent (mmol)	Yield (%)
1	DMF	rt	1.0	66
2	CH₂Cl₂	rt	1.0	76
3	Benzene	rt	1.0	34
4	CH₃CN	rt	1.0	58
5	CHCl₃	rt	1.0	81
6	DMSO	rt	1.0	61
7	CHCl₃	rt	0.8	55
8	CHCl₃	rt	1.3	76
8	CHCl₃	0 °C	1.0	–^a
9	CHCl₃	61 °C	1.0	70

^a Trace amount of the product was observed on TLC monitoring but it was not isolated.

Treatment of N-acyl-α-amino acids 4a–l with the Vilsmeier reagent and triethylamine in chloroform at room temperature gave oxazol-5-ones 5a–l with good to excellent yield after crystallization from ethanol (Scheme 1, Table 2). The structures of all compounds 5a–l were confirmed by elemental analysis and spectroscopic methods (IR, ¹H NMR, and ¹³C NMR).

Scheme 1
Synthesis of oxazol-5-ones 5a–l from N-acyl-α-amino acids 4a–l.

Entry	R	RCONHCH₂CO₂H	Isolated yield (%)	Oxazol-5-one	Isolated yield (%)	Isolated yield (%) (one-pot synthesis)
1	Ph	4a	85	5a	81	86
2	4-ClC₆H₄	4b	83	5b	88	89
3	4-MeOC₆H₄	4c	75	5c	78	84
4	4-NO₂C₆H₄	4d	90	5d	86	91
5	4-MeC₆H₄	4e	83	5e	82	80
6	C₆H₅CH₂	4f	79	5f	72	78
7	4-(Me₂N)C₆H₄	4g	74	5g	77	73
8	4-Pyridyl	4h	72	5h	70	79
9	2-MeOC₆H₄	4i	81	5i	86	88
10	3-ClC₆H₄	4j	84	5j	85	90
11	2-Naphthyl	4k	89	5k	87	92
12	CH₃CHCH	4l	74	5l	79	76

Analysis of the IR spectrum of oxazol-5-ones 5a–l showed two characteristic signals at about 1650 and 1800 cm⁻¹, which were ascribed to the presence of CN and CO bonds, respectively. Of course, the NH, CO amide, OH, and CO carboxylic acid peaks of N-acyl-α-amino acids 4a–l had vanished. Furthermore, the ¹H NMR and ¹³C NMR spectra showed characteristic CH₂, CN, and CO peaks.

Transformation of oxazol-5-ones and aldehydes to Erlenmeyer azlactones in chloroform has been previously reported [8d]. Then, oxazol-5-one 5a was converted to Erlenmeyer azlactone 6a in the presence of benzaldehyde and of the Vilsmeier reagent in 84% yield (Scheme 2).

Scheme 2
Transformation of oxazol-5-ones to Erlenmeyer azlactones.

These results prompted us to develop an easy method for the one-pot synthesis of oxazol-5-ones and azlactones from carboxylic acids. Reaction of benzoic acid (1.0 mmol) with glycine (1.0 mmol) in the presence of the Vilsmeier reagent 3, triethylamine, and benzaldehyde (1.0 mmol) in dry chloroform at room temperature for 3 h allowed direct conversion into the crude azlactone 6a. The mixture was washed with saturated NaHCO₃ solution. Crystallization from 70% ethanol gave pure azlactone 5a as a yellow solid. In the absence of benzaldehyde, oxazol-5-one 5a was obtained. Then, using similar procedures, various pure oxazol-5-ones 5a–l and azlactones 6a–m were prepared directly from carboxylic acids (Scheme 3, Tables 2 and 3).

Scheme 3
One-pot synthesis of oxazol-5-ones and azlactones from carboxylic acids.

Entry	R¹	R²	Product	Isolated yield (%)
1	Ph	Ph	6a	92
2	Ph	4-ClC₆H₄	6b	94
3	Ph	4-MeOC₆H₄	6c	87
4	Ph	CH₃	6d	69
5	4-MeOC₆H₄	2-Furyl	6e	82
6	4-ClC₆H₄	4-MeOC₆H₄	6f	88
7	4-NO₂C₆H₄	4-NO₂C₆H₄	6g	85
8	C₆H₅CH₂	4-MeC₆H₄	6h	90
9	4-(Me₂N)C₆H₄	Ph	6i	87
10	4-(Me₂N)C₆H₄	4-ClC₆H₄	6j	94
11	4-Pyridyl	4-MeOC₆H₄	6k	66
12	2-Naphthyl	4-MeC₆H₄	6l	85
13	CH₃CHCH	Ph	6m	78

The synthesized compounds were characterized by IR, ¹H NMR, ¹³C NMR, and elemental analysis.

The method was clean and convenient giving moderate to excellent isolated yields. In addition, the byproducts (DMF and salts) could be easily removed from the reaction mixture by aqueous work up.

3 Conclusion

To sum up, we have demonstrated the effectiveness and efficiency of the Vilsmeier reagent for the synthesis of oxazol-5-ones (oxazolones) and 4-arylmethyleneoxazolones (Erlenmeyer azlactones) proceeding via the cyclization of N-acyl-α-amino acids. Ready availability, high reactivity, and low cost make the Vilsmeier reagent a versatile and useful reagent for these reactions. In addition, we have developed facile and efficient methods for the synthesis of aliphatic, (hetero)-aromatic oxazol-5-ones, and 4-arylmethyleneoxazolones directly from carboxylic acids in a one-pot procedure. This methodology offers the advantage of mild reaction condition, good to excellent yield of products, and simplicity of work. Pure products were obtained by simple aqueous work up and crystallization.

4 Experimental section

4.1 Chemical reagents

All needed chemicals were purchased from Merck, Fluka, and Acros chemical companies. IR spectra were run on a Shimadzu FT-IR 8300 spectrophotometer. ¹H NMR and ¹³C NMR spectra were recorded in DMSO-d₆ and CDCl₃ using a Bruker Avance DPX instrument (¹H NMR 250 MHz, ¹³C NMR 62.9 MHz). Chemical shifts were reported in ppm (δ) downfield from tetramethylsilane. All of the coupling constants (J) are in Hertz. The mass spectra were recorded on a Shimadzu GC-MS QP 1000 EX instrument. Elemental analyses were run on a Thermo Finnigan Flash EA-1112 series. Melting points were determined in open capillaries with Buchi 510 melting point apparatus. TLC was carried out on silica gel 254 analytical sheets obtained from Fluka.

4.2 General procedure for the conversion of N-acyl-α-amino acids 4a–l to oxazol-5-ones 5a–l

A solution of N-acyl-α-amino acids 4a–l (1.0 mmol), Vilsmeier's reagent 3 (1.0 mmol), and Et₃N (2.0 mmol) in dry chloroform (15 mL) was stirred at room temperature for 1.5 h. Water (15 mL) was added and the organic layer was separated. The organic layer was washed with brine (20 mL), dried (Na₂SO₄), filtered, and the solvent was removed under reduced pressure to give the crude products. The crude residues were purified by crystallization from 95% ethanol. Spectral data for 5a–f and 5i–j have been previously reported [24].

4.2.1 2-(4-(Dimethylamino)phenyl)oxazol-5(4H)-one (5g)

Yellow solid, mp 129–131 °C. IR (KBr) cm⁻¹: 1802 (CO), 1631 (CN). ¹H NMR (CDCl₃) δ 3.17 (2 Me, s, 6H), 4.35 (CH₂, s, 2H), 6.84 (ArH, d, 2H, J = 7.6), 7.47 (ArH, d, 2H, J = 7.6). ¹³C NMR (CDCl₃) δ 44.8 (Me), 57.6 (CH₂), 109.4, 125.2, 129.9, 154.6 (aromatic carbons), 164.5 (CN), 176.2 (CO). Anal. Calcd for C₁₁H₁₂N₂O₂: C, 64.69; H, 5.92; N, 13.72. Found: C, 64.81; H, 6.08; N, 13.66.

4.2.2 2-(Pyridin-4-yl)oxazol-5(4H)-one (5h)

Pale-yellow solid, mp 113–115 °C. IR (KBr) cm⁻¹: 1811 (CO), 1634 (CN). ¹H NMR (CDCl₃) δ 4.18 (CH₂, s, 2H), 7.33 (ArH, d, 2H, J = 7.9), 8.46 (ArH, d, 2H, J = 7.9). ¹³C NMR (CDCl₃) δ 55.9 (CH₂), 119.7, 134.1, 151.5 (aromatic carbons), 167.1 (CN), 178.4 (CO). Anal. Calcd for C₈H₆N₂O₂: C, 59.26; H, 3.73; N, 17.28. Found: C, 59.35; H, 3.84; N, 17.31.

4.2.3 2-(Naphthalen-2-yl)oxazol-5(4H)-one (5k)

Orange solid, mp 154–156 °C. IR (KBr) cm⁻¹: 1805 (CO), 1639 (CN). ¹H NMR (CDCl₃) δ 4.53 (CH₂, s, 2H), 7.26–7.29 (ArH, m, 2H), 7.60–7.65 (ArH, m, 3H), 7.72–7.75 (ArH, m, 2H). ¹³C NMR (CDCl₃) δ 53.7 (CH₂), 108.4, 114.7, 119.4, 121.2, 125.9, 127.5, 128.3, 131.7, 138.6, 144.9 (aromatic carbons), 166.2 (CN), 174.5 (CO). Anal. Calcd for C₁₃H₉NO₂: C, 73.92; H, 4.30; N, 6.63. Found: C, 74.02; H, 4.44; N, 6.55.

4.2.4 2-(Prop-1-en-1-yl)oxazol-5(4H)-one (5l)

Colorless viscous oil. IR (KBr) cm⁻¹: 1808 (CO), 1626 (CN). ¹H NMR (CDCl₃) δ 1.39 (Me, m, 3H), 5.26 (vinylic H, d, 1H, J = 15.7), 5.94 (vinylic H, m, 1H). ¹³C NMR (CDCl₃) δ 20.6 (Me), 56.1 (CH₂), 116.8, 138.5 (CC), 159.4 (CN), 177.1 (CO). Anal. Calcd for C₆H₇NO₂: C, 57.59; H, 5.64; N, 11.19. Found: C, 57.51; H, 5.71; N, 11.16.

4.3 General procedure for one-pot synthesis of oxazol-5-ones 5a–l from carboxylic acids

Glycine (1.0 mmol) was added to a solution of carboxylic acid (1.0 mmol), Vilsmeier's reagent (2.0 mmol), and Et₃N (4.0 mmol) in dry chloroform (15 mL) at room temperature. The mixture was stirred at room temperature 2 h. Saturated NaHCO₃ (20 mL) was added and the organic layer was separated. It was washed with brine (20 mL), dried (Na₂SO₄), filtered, and the solvent was removed under reduced pressure to give the crude products. The crude residues were purified by crystallization from 95% ethanol.

4.4 General procedure for one-pot synthesis of azlactones 6a–m from carboxylic acids

The corresponding aldehyde (1.0 mmol) was added to a solution of the corresponding carboxylic acid (1.0 mmol), Vilsmeier's reagent (3.0 mmol), and Et₃N (5.0 mmol) in dry chloroform (20 mL) at room temperature. After 2 h, saturated NaHCO₃ (20 mL) was added and the organic layer was separated. It was washed with brine (20 mL), dried (Na₂SO₄), filtered, and the solvent was removed under reduced pressure to give the crude products. The crude residues were purified by crystallization from 70% ethanol. Spectral data for 6a–g, 6i, and 6m have been previously reported [8d,25].

4.4.1 2-Benzyl-4-(4-methoxybenzylidene)oxazol-5(4H)-one (6h)

Orange powder, mp 173–175 °C. IR (KBr) cm⁻¹: 1631 (CN), 1793 (CO). ¹H NMR (CDCl₃) δ 3.11 (CH₂, s, 2H), 3.57 (OMe, s, 3H), 7.06 (ArH, d, 2H, J = 7.9), 7.28–7.32 (ArH, m, 5H), 7.73 (ArH, s, 1H), 7.87 (ArH, d, 2H, J = 7.9). ¹³C NMR (CDCl₃) δ 33.1 (CH₂), 55.3 (OMe), 112.6, 114.9, 119.0, 122.4, 123.0, 124.7, 130.1, 138.3, 143.4, 154.8 (CC, aromatic carbons), 162.8 (CN), 166.4 (CO). Anal. Calcd for C₁₈H₁₅NO₃: C, 73.71; H, 5.15; N, 4.78. Found: C, 73.81; H, 5.29; N, 4.83.

4.4.2 4-(4-Chlorobenzylidene)-2-(4-(dimethylamino)phenyl)-oxazol-5(4H)-one (6j)

Bright yellow solid, mp 186–188 °C. IR (KBr) cm⁻¹: 1637 (CN), 1801 (CO). ¹H NMR (CDCl₃) δ 3.06 (Me, s, 3H), 6.91 (ArH, d, 2H, J = 8.1), 7.33–7.39 (ArH, m, 4H), 7.77–7.81 (ArH, m, 3H). ¹³C NMR (CDCl₃) δ 42.5 (Me), 111.2, 115.7, 119.3, 124.4, 126.1, 127.5, 129.8, 133.6, 145.3, 151.7 (CC, aromatic carbons), 161.3 (CN), 165.8 (CO). Anal. Calcd for C₁₈H₁₅ClN₂O₂: C, 66.16; H, 4.63; N, 8.57. Found: C, 66.11; H, 4.71; N, 8.54.

4.4.3 4-(4-Methoxybenzylidene)-2-(pyridin-4-yl)oxazol-5(4H)-one (6k)

Orange solid, mp 174–176 °C. IR (KBr) cm⁻¹: 1630 (CN), 1791 (CO). ¹H NMR (CDCl₃) δ 3.69 (OMe, s, 3H), 7.17 (ArH, d, 2H, J = 7.7), 7.49 (ArH, d, 2H, J = 7.7), 7.75–7.81 (ArH, m, 3H), 8.30 (ArH, d, 2H, J = 7.6). ¹³C NMR (CDCl₃) δ 56.5 (OMe), 110.6, 114.1, 117.8, 122.6, 127.3, 132.4, 133.9, 147.5, 155.3 (CC, aromatic carbons), 162.6 (CN), 167.3 (CO). Anal. Calcd for C₁₆H₁₂N₂O₃: C, 68.56; H, 4.32; N, 9.99. Found: C, 68.64; H, 4.43; N, 10.05.

4.4.4 4-Benzylidene-2-(naphthalen-2-yl)oxazol-5(4H)-one (6l)

Orange solid, mp 195–197 °C. IR (KBr) cm⁻¹: 1636 (CN), 1797 (CO). ¹H NMR (CDCl₃) δ 7.29–7.37 (ArH, m, 5H), 7.44–7.47 (ArH, m, 2H), 7.71–7.76 (ArH, m, 4H), 7.98–8.01 (ArH, m, 2H). ¹³C NMR (CDCl₃) δ 107.4, 111.8, 120.6, 123.1, 125.8, 126.1, 126.5, 127.9, 128.5, 132.6, 132.9, 134.7, 138.4, 142.0, 146.2, 147.9 (CC, aromatic carbons), 164.3 (CN), 166.1 (CO). Anal. Calcd for C₂₀H₁₃NO₂: C, 80.25; H, 4.38; N, 4.68. Found: C, 80.36; H, 4.53; N, 4.60.

Acknowledgments

Support for this work by the Marvdasht Branch of Islamic Azad University is gratefully acknowledged.

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