Cascade synthesis of 2-pyridones using acrylamides and ketones (original) (raw)

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Cite this: RSC Adv., 2014, 4, 44141

Received 3rd July 2014
Accepted 8th September 2014
DOI: 10.1039/c4ra06619 g10.1039 / \mathrm{c} 4 \mathrm{ra} 06619 \mathrm{~g}
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Cascade synthesis of 2-pyridones using acrylamides and ketones †\dagger

Sunil K. Rai, Shaziya Khanam, Ranjana S. Khanna and Ashish K. Tewari*
Microwave assisted non-catalytic condensation of 2-cyanoacetamide with aromatic aldehydes, and enolate mediated Michael-type addition to acrylamide followed by oxidative cyclization, produce 2pyridones in good to excellent yield. Unsymmetrical ketones produce two regioisomeric enolates, therefore thermodynamic and kinetic products of butan-2-one and pentan-2-one have been isolated and fully characterized.

Introduction

The literature reveals that 2-pyridones are found in a wide range of compounds including natural products (I-III) and medicines 1(IV){ }^{1}(\mathbf{I V}) (Fig. 1) having a broad spectrum of biological activity like vasodilatory, antimalarial, antiasthma, antiepilepsy, antidiabetic, antimicrobial, antioxidant and antiviral activity etc. 2{ }^{2} Due to dual properties of an aromatic ring and an amide or tautomerism between 2-pyridones and 2-hydroxypyridines, these compounds may act as simple models for investigating the mechanisms of some enzymatic reactions or for identifying the behavior of nucleic acids bases in connection with mutation because of base mispairing. 3{ }^{3} Recent studies have shown the usefulness of 2-pyridones as intermolecular connectors between building blocks in material science. 4{ }^{4}

The development of their efficient synthesis is an important target in current organic synthesis. 5{ }^{5} So far, various methods have been developed for the synthesis of substituted 2-pyridone. 6{ }^{6} Among them, the methods employing α,β\alpha, \beta-unsaturated ketones and 2 -substituted acetamides as substrates have been
img-0.jpeg

Fig. 1 Representatives of 2-pyridone alkaloids.

Department of Chemistry (Center of Advanced Study), Faculty of Science, Banaras Hindu University, Varanasi 221005, India. E-mail: tashish2002@yahoo.com
†\dagger Electronic supplementary information (ESI) available: Experimental section, copies of 1H{ }^{1} \mathrm{H} and 13C{ }^{13} \mathrm{C} NMR spectra and single crystal X-ray structure data. CCDC 981162-981164 and 981430. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra06619 g10.1039 / \mathrm{c} 4 \mathrm{ra} 06619 \mathrm{~g}
employed in the preparation of a variety of 2-pyridones. 7{ }^{7} These transformations involve 1,4-addition of the acetamides to α,β\alpha, \beta unsaturated ketones followed by cyclization and elimination (Scheme 1 previous work). In these reactions instead of oxidation, elimination was facilitated under harsh conditions. Elimination of alkane from position 4 has also been reported under basic condition (at pH 8.0 ) at ambient temperature. 8{ }^{8} This technique intrigued our mind to synthesized the 2-pyridones by retaining the cyano group at position 3 and eliminating the hydrogen from position 4.

From the year 2000 to 2013 repetition of papers have been published by taking α,β\alpha, \beta-unsaturated ketones and 2 -substituted acetamides. 7{ }^{7} The drawback of these reactions was the requirement of high temperature and use of toxic solvents. To make this synthesis facile at room temperature, we used acrylamides as precursor instead of α,β\alpha, \beta-unsaturated ketones. Precursor molecules are synthesized by Knoevenagel condensation of aromatic aldehydes with 2-cyanoacetamide in absence of catalyst. 9{ }^{9} However, Knoevenagel condensation has been reported in presence of catalyst, variety of solvent and by use of microwave. 10{ }^{10} In 2-cyanoacetamide, presence of amide group receives our attention and we tried to condense aromatic aldehydes with
img-1.jpeg

Scheme 1 Methods for the synthesis of 2-pyridones.

2-cyanoacetamide in absence of catalyst in water at elevated temperature ( 4−9 h4-9 \mathrm{~h} ) and under microwave ( 60 W,≤2 min60 \mathrm{~W}, \leq 2 \mathrm{~min} ). To our great delight, smooth reactions occurred for all aromatic aldehydes and delivered the products 1-8 in good to excellent yields. Herein, we have synthesized various substituted 2-pyridones through cascade reactions carrying out at ambient temperature in ethanol using sodium ethoxide as base, within 2 to 6 hours.

Results and discussion

Acrylamides were synthesized by condensation of different aldehydes on 2-cyanoacetamide by thermal heating or microwave application in water as shown in Scheme 2 and Table 1. Products obtained in all acrylamides are highly stereoselective, with an EE-geometry (for detail descriptions see ref. 10b). Both electron-rich and electron-deficient aldehydes gave high yields of products. Sterically hindered aldehydes substituted at orthoposition gave relatively lower yields than that of meta- and parasubstituted isomers.

Acrylamides obtained in Scheme 2 were reacted with enolates to produce corresponding 2-pyridones in good to excellent yield. Optimization of the reaction conditions for 6-methyl-2-oxo-4-phenyl-1,2-dihydropyridine-3-carbonitrile (1a), was performed in DMF, DMSO and ethanol with sodium methoxide, sodium ethoxide and potassium tt-butoxide at ambient temperature as shown in Table 2. The combination of sodium ethoxide with ethanol produces products in higher yield in lesser time.

After establishment of efficient reaction condition for (1a), we explored the substrate scope of the reaction (Scheme 3) as summarized in Table 3. It was observed that product yield is lower with acetone than that of aromatic ketones. The product yield was not affected much by substituents at para-position of aromatic ring in acrylamides. While substituents at meta-
img-2.jpeg

Scheme 2 Synthesis of acrylamide by Knoevenagel condensation.

Table 1 Thermal and microwave-assisted synthesis of acrylamide

Compound R1\mathrm{R}_{1} Time Yield (%)
Thermal (h) MW (s) Thermal MW
1 C6H5\mathrm{C}_{6} \mathrm{H}_{5} 4 20 98 99
2 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 6 30 92 98
3 3,4−CH3OC6H53,4-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{5} 7.5 50 91 98
4 4−CH3OC6H44-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{4} 7 40 93 97
5 4−CH3C6H44-\mathrm{CH}_{3} \mathrm{C}_{6} \mathrm{H}_{4} 5 25 94 99
6 3−NO2C6H43-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} 8 60 85 96
7 2−NO2C6H42-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} 9 70 75 92
8 4−NO2C6H44-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} 8.5 65 80 95

Table 2 Solvent and base screening for the synthesis of compound 1a
img-3.jpeg

Entry Base Solvent Time (h) Yield a(%)^{\mathrm{a}}(\%)
1 C2H5ONa\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONa} DMF 24 0
2 CH3ONa\mathrm{CH}_{3} \mathrm{ONa} DMF 24 0
3 tt-BuOK DMF 24 0
4 C2H5ONa\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONa} DMSO 24 0
5 CH3ONa\mathrm{CH}_{3} \mathrm{ONa} DMSO 24 0
6 tt-BuOK DMSO 24 0
7 C2H5ONa\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONa} C3H5OH\mathrm{C}_{3} \mathrm{H}_{5} \mathrm{OH} 2 100
8 CH3ONa\mathrm{CH}_{3} \mathrm{ONa} C3H5OH\mathrm{C}_{3} \mathrm{H}_{5} \mathrm{OH} 10 25
9 tt-BuOK C3H5OH\mathrm{C}_{3} \mathrm{H}_{5} \mathrm{OH} 12 35

a{ }^{a} Yield has been reported with respect to starting material as observed in 1H{ }^{1} \mathrm{H} NMR spectra. Product isolated via entry 7 showed no peak for starting in 1H{ }^{1} \mathrm{H} NMR spectra, which indicates the 100%100 \% conversion of starting to product.
img-4.jpeg

Scheme 3 Synthesis of 2-pyridone by acrylamide and ketone.
position tend to decrease the yield, perhaps due to the steric hindrance near β\beta-position of acrylamides (compounds 3a-c and 6a−c6 \mathbf{a}-\mathbf{c} ). The effect of steric hindrance was observed significantly in compound 7 a that is ortho-substituted.

In continuation to the exploration of substrate scope, we used unsymmetrical aliphatic ketones (butan-2-one and pentan-2-one). Unsymmetrical ketones produce two products i.e. thermodynamic (A) and kinetic (B) products (Scheme 4). Generically it has been seen that alpha hydrogen deprotonated to form the kinetic enolate is less hindered, and therefore deprotonated more quickly. 11{ }^{11} However, under thermodynamic conditions equilibrium is established between the ketone and the two possible enolates. In Scheme 41d,2d41 \mathbf{d}, 2 \mathbf{d} and 5d\mathbf{5 d} are thermodynamic products while 1e,2e1 \mathbf{e}, 2 \mathbf{e} and 5e5 \mathbf{e} are kinetic products of butan-2-one. At ambient temperature butan-2-one form thermodynamic and kinetic products in ∼90:10\sim 90: 10 ratio. Similarly, 1f, 2 f and 4 d are thermodynamic products while 1 g,2 g1 \mathrm{~g}, 2 \mathrm{~g} and 4e4 \mathrm{e} are kinetic products of pentan-2-one but in this case thermodynamic and kinetic product ratio is ∼30:70\sim 30: 70. This inversion of product ratio can be easily expected due to sterically hindered more substituted enolate of pentan-2-one.

Single crystal X-ray quality crystals of compound 1d, 1e, 1g and 2d were grown in ethanol by slow evaporation at room temperature. ORTEP diagram of compound 1d, 1e, 1 g and 2d has been shown in Fig. 2. Compound 1g shows two polymorphs

Table 3 Synthesis of 2-pyridones by acrylamides and ketones

Compound R1\mathrm{R}_{1} R2\mathrm{R}_{2} Yield a(%)^{a}(\%)
1a C6H5\mathrm{C}_{6} \mathrm{H}_{5} CH3\mathrm{CH}_{3} 85
1b C6H5\mathrm{C}_{6} \mathrm{H}_{5} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 96
1c C6H5\mathrm{C}_{6} \mathrm{H}_{5} 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 95
2a 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} CH3\mathrm{CH}_{3} 83
2b 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 94
2c 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 97
3a 3,4−CH3OC6H33,4-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{3} CH3\mathrm{CH}_{3} 81
3b 3,4−CH3OC6H33,4-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{3} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 91
3c 3,4−CH3OC6H33,4-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{3} 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 94
4a 4−CH3OC6H34-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{3} CH3\mathrm{CH}_{3} 78
4b 4−CH3OC6H44-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{4} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 85
4c 4−CH3OC6H44-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{4} 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 91
5a 4−CH3C6H44-\mathrm{CH}_{3} \mathrm{C}_{6} \mathrm{H}_{4} CH3\mathrm{CH}_{3} 80
5b 4−CH3C6H44-\mathrm{CH}_{3} \mathrm{C}_{6} \mathrm{H}_{4} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 90
5c 4−CH3C6H44-\mathrm{CH}_{3} \mathrm{C}_{6} \mathrm{H}_{4} 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 88
6a 3−NO2C6H43-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} CH3\mathrm{CH}_{3} 75
6b 3−NO2C6H43-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 84
6c 3−NO2C6H43-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} 4−ClC6H44-\mathrm{ClC}_{6} \mathrm{H}_{4} 86
7a 2−NO2C6H42-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 60
8a 4−NO2C6H44-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} C6H5\mathrm{C}_{6} \mathrm{H}_{5} 87

a{ }^{a} Yield has been reported for isolated crude product.
img-5.jpeg

Scheme 4 Synthesis of 2-pyridones by acrylamides and unsymmetrical ketones.
in asymmetric unit cell while 2d shows four polymorphs in asymmetric unit cell.

The specific mechanistic details have been outlined in Scheme 5. We may propose the initial formation of adduct between anion of ketone and acrylamide in step-1. This addition generates anion at α\alpha-position and may form intermediate1. Presence of nitrile group at α\alpha-position of acrylamide stabilizes the anionic intermediate-1. In step-2, base-promoted cyclization occurs to produce intermediate-2 followed by dehydration (step3) which generates intermediate-3. Subsequently, this species would be converted to oxidized product-1 via single electron transfer (SET) and formal loss of a hydrogen atom from the
img-6.jpeg

Fig. 2 ORTEP diagram of compound 1d, 1e, 1g and 2d.
img-7.jpeg

Scheme 5 Plausible mechanism showing different steps and intermediates.
intervening radical. 12{ }^{12} The anionic intermediate-3 was confirmed by isolation of protonated product-2 ( 1h\mathbf{1 h} and 1i\mathbf{1 i} in Experimental section) from incomplete reaction mixture in trace amount.

The last two steps (step 3 and 4) in the mechanism are comparatively faster and cannot decide the rate of reaction. However, addition and cyclization (step 1 and 2 respectively) is comparatively slower and can be investigated through UVspectroscopy experiment as shown in Fig. 3 (see Table 4 for λmax \lambda_{\text {max }} ). Initially the individual absorption spectrum of acetophenone (λmax⁡=279 nm)\left(\lambda_{\max }=279 \mathrm{~nm}\right) and acrylamide 1(λmax⁡=301 nm)\mathbf{1}\left(\lambda_{\max }=301 \mathrm{~nm}\right) was recorded in the same concentration range as mentioned in Fig. 3. Two minutes stirred solution of acetophenone in sodium ethoxide-ethanol solution showed hypsochromic shift and λmax⁡\lambda_{\max } appear at 241 nm . After addition of acrylamide, spectrum was recorded at 1 and 2 minutes intervals but no absorption band was observed for π−π∗\pi-\pi^{*} of 2-pyridones. 13{ }^{13} This indicates the immediate consumption of acrylamide and acetophenone to

img-8.jpeg

Fig. 3 The UV absorption spectra for in situ generation of compound 1b diluted in ethanol. Dilution is of ≈150×10−4 moldm−3\approx 150 \times 10^{-4} \mathrm{~mol} \mathrm{dm}^{-3} range.

Table 4λmax 4 \lambda_{\text {max }} values for starting materials and reaction mixture at subsequent time gaps

Compound name λmax⁡(nm)\lambda_{\max }(\mathrm{nm})
Acetophenone 279
Acrylamide (1) 301
Enolate 241
Reaction mixture after 15 min 360
Reaction mixture after 30 min 356
Reaction mixture after 50 min 359
Reaction mixture after 60 min 359
Reaction mixture after 70 min 362
Reaction mixture after 100 min 359
Reaction mixture after 150 min 360

form saturated acyclic adduct. Consequently, we can say that the addition is so fast to observe in minutes. The first significant spectrum of reaction mixture was observed in 15 minutes stirred solution which showed λmax \lambda_{\text {max }} at 360 nm . Spectrum recorded at subsequent time gaps showed continuous increased absorption intensity. This indicates that the addition is faster while cyclization is slower and thus later is rate determining step in the reaction.

Conclusions

In summary, the present work showed new route for the efficient synthesis of substituted 2-pyridones in milder condition at ambient temperature in good to excellent yield. This route has shown the advantage of oxidative aromatization over elimination of nitrile group (in previous works). We not only explored the substrate scope by using symmetrical and unsymmetrical ketones, but also rationalized the thermodynamic and kinetic products from unsymmetrical ketones. Since, 2-pyridone core containing nitrile group at position 3 is ubiquitous in biologically active molecules, dye stuffs and natural products, this route is likely to find widespread use in the synthesis of relevant compounds.

Acknowledgements

AKT acknowledge DST India grant no. SR/S1/OC-42/2012 for financial assistance. Department of Chemistry, Faculty of Science, BHU, Varanasi, India is acknowledged for departmental facilities. SKR also acknowledges to CSIR-New Delhi for SRF.

Notes and references

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