Background

Microwave irradiation (MW) has emerged as a powerful technique offering simple, clean, fast, efficient, and economical method for the synthesis of a large number of biologically active molecules [1–6]. The application of microwave irradiation in organic synthesis has been the focus of considerable attention in recent years and is becoming an increasingly popular technology [7–14].

The carbonyl group at position 2 of N-acetylisatin behaves as an amide carbonyl group which is opposed to the conventional amide carbonyl functionality in isatin [15]. Because of this behavior, N-acetylisatin is more susceptible to nucleophilic attack via ring-opening in the presence of nucleophiles, such as amines and alcohols. This ring-opening reaction led to numerous synthetic methods for preparation of α-ketoamides [16–22], which are of interest in organic and medicinal chemistry [23–27].

In continuation of our earlier work [11, 12], we present herein the reaction of N-acetyl- and N-propionylisatin in methanol with different amines and diamines under microwave irradiation to afford α-ketoester, series of α-ketoamide and bis-(α-ketoamide) derivatives in excellent yields and purities.

Results and discussion

Isatin and its derivatives undergo nucleophilic attack at the carbonyl group at position 2 and/or 3. The chemoselectivity of these reactions depends on the nature of the substituent attached to the isatin nucleus, on the nature of the nucleophiles, and the nature of the group attached to the nitrogen atom of the isatin nucleus [15]. N-acetylisatin (1) undergoes ring-opening reaction with primary amines and alcohols as reported in the literature (Scheme 1) [28].

Scheme 1
scheme 1

General mechanism for the reaction of N -acylisatin with amine or alcohol.

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N-Acetylisatin (1) and N-propionylisatin (2) were initially prepared by reaction of isatin with acetic anhydride or propanoic anhydride respectively (Scheme 2), using the same reported conventional conditions [29]. Compound 1 and 2 were also prepared by microwave irradiation for 10 min at 100°C/400 W using a multimode reactor (Synthos 3000 Aton Paar, GmbH, 1400 W maximum magnetron) (Scheme 2). The microwave irradiation afforded the product in less reaction time, higher yield and purity than conventional heating.

Scheme 2
scheme 2

Synthesis of N -acylisatin.

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N-Acetylisatin, 1 and N-propionylisatin, 2 were reacted with different secondary amines 3a-c at room temperature in acetonitrile as a solvent to afford α-ketoamide 4a-c and 5a-c in yield 64-72% (Scheme 3). The microwave irradiation of 1 and 2 with different amines 3a-c afforded the final products 4a-c and 5a-c in higher yields and purities than conventional method (Experimental section). The structures of the prepared compounds were confirmed by IR, NMR (1H-NMR and 13C-NMR) and elemental analysis.

Scheme 3
scheme 3

Reaction of N -acylisatin with secondary amines.

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As prototype the IR spectrum of 4a showed four characteristic peaks at 3399, 1691, 1630, 1605 cm−1, corresponding to the NH, α-ketoamide (COCONH) and two CONH, respectively. The 1H-NMR of 4a agreed well with the structure, showing resonance peaks located at δ 1.55 (m, 2H, CH2-CH 2 -CH2, piperidine moiety), 1.70 (m, 4H, CH 2 -CH2-CH2, piperidine moiety), 2.25 (s, 3H, COCH 3 ), 3.30 and 3.69 (two m, 4H, CH 2 -N-CH2, piperidine moiety), 7.12 (t, 1H, ArH), 7.65 (m, 2H, ArH), 8.88 (d, 1H, ArH), 11.30 (s, 1H, NH, D2O exchangeable). The 13C-NMR of 4a also confirmed the structure, showing the characteristic signals at δ 164.5, 169.6 and 196.3 ppm, related to the two carbonyl of the amide group and one related to the α-ketoamide group respectively along with the rest of the expected carbon signals of the compound. X-Ray single crystal structure determination of compound 4a and 5a also confirmed the structure of the products (Figure 1a and b). Crystal data and structure refinement details for compound 4a and 5a are shown in Table 1.

Figure 1
figure 1

ORTEP representation of the crystal structure of 4a and 5a. (a) Ortep of N-(2-(2-oxo-2-(piperidin-1-yl)acetyl)phenyl)acetamide 4a CCDC 943007. (b) Ortep of N-(2-(2-oxo-2-(piperidin-1-yl)acetyl)phenyl)propionamide 5a CCDC 945756.

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Table 1 Crystal data and structure refinement details for compounds 4a and 5a
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Alternatively, the N-acylisatin 1 and 2 were dissolved in dry methanol and then irradiated under microwave irradiation for 3–5 min followed by cooling to afford the products 9a and 9b in yield 95% and 91%, respectively (Scheme 4). A plausible mechanism involves nucleophilic attack by methanol at C2 carbonyl carbon of N-acetylisatin and subsequent ring opening to generate the α-keto ester (Scheme 4).

Scheme 4
scheme 4

Reaction mechanism of N -acylisatin with methanol.

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The IR spectrum of 9a showed three carbonyl groups at 1747 (CO-ester), 1696 (α-CO), and 1657 (CONH) cm−1. The 1HNMR spectrum for 9a showed singlet peaks at δ 2.25 and 3.99 ppm related to (NHCOCH 3 ) and (COOCH 3 ) respectively. The 13CNMR of 9a showed a characteristic peaks for CO-ester and α-CO- at δ 169.6 and 190.3 ppm respectively.

The X-ray crystallographic structure for 9a (Figure 2, Table 2) also confirmed its structure. Crystal data for compound 9a are shown in Table 2. In the crystal, the molecules are linked via intermolecular N—H----O and C—H-----O hydrogen bonds.

Figure 2
figure 2

Ortep of methyl 2-(2-acetamidophenyl)-2-oxoacetate 9a CCDC No. 942706.

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Table 2 Crystal data and structure refinement details for compound 9a
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Reaction of α-phenylglycoxyl methyl ester derivatives 9a and 9b with piperidine and morpholine under the conventional heating (2 h) and microwave irradiation (2–5 min/80°C/200 W) afforded the same product 4a-b and 5a-b, respectively as obtained from the direct aminolysis of N-acylisatin 1 and 2 (Scheme 4).

Reaction of N-acetylisatin, 1 and N-propionylisatin, 2 with different diamines 6a-e in ratio (2:1) at room temperature for 12 h in acetonitrile as solvent afforded bis-(α-ketoamide) 7a-e 8a and 8e in yields of 68-73% (Scheme 5). The microwave irradiation of 1 and 2 with diamines afforded the products 7a-e, 8a and 8e in higher yield and purity (Experimental section). The structures of the prepared compounds were confirmed by IR, NMR (1H-NMR and 13C-NMR) and elemental analysis

Scheme 5
scheme 5

Reaction of N -acylisatin with diamines.

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Experimental section

Materials and methods

The solvents used were of HPLC reagent grade. Melting points were determined with a Mel-Temp apparatus. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on Nicolet 560. Nuclear magnetic resonance spectra (1H NMR and 13C NMR spectra) were recorded on a JOEL 400 MHz spectrometer with chemical shift values reported in δ units (ppm) relative to an internal standard. X-Ray data collection was carried out on Bruker SMART APEX II CCD diffractometer, cell refinement: SAINT; data reduction: SAINT; program used to solve structure: SHELXS; program used to refine structure: SHELXL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL [30] and PLATON [31]. The microwave irradiation employed a multimode reactor (Synthos 3000, Aton Paar GmbH, and 1400 W maximum magnetron). Elemental analyses were performed on Perkin-Elmer 2400 elemental analyzer, and the values found were within ±0.3% of the theoretical values. Follow-up of the reactions and checks of the purity of the compounds was done by TLC on silica gel-protected aluminum sheets (Type 60 GF254, Merck). The compounds were named using Chem. Draw Ultra version 11, Cambridge soft Corporation.

General method for synthesis of α-ketoamide and bis-(α-ketoamide) derivatives

Method A (Conventional Procedure)

To a solution of N-acetylisatin 1 or N-propionylisatin 2 (10 mmol) in acetonitrile (20 mL), secondary amine 3a-c or diamine 6a-e in ratio (1:1 and 2:1, respectively) was added at room temperature. The reaction mixture was stirred at room temperature for 12 h. On the next day, the solvent was removed under vacuum and the crude product was recrystallized from dichloromethane and hexane (1:2). In the case of diamine, the precipitate was filtered, washed with acetonitrile (5 mL), and dried under vacuum to afford the pure product.

Method B (Microwave-Irradiation)

The reaction was performed using a multimode reactor (Synthos 3000 Aton Paar, GmbH, 1400 W maximum magnetron). The initial step was conducted with 4-Teflon vessels rotor (MF 100) that allows the reactions to be processed 4 at a time under the same conditions. In each vessel N-acylisatin mixed with different amine or diamine in small amount of acetonitrile (2–5 mL). The individual vessels were purged with nitrogen gas for 2 min and then placed in the corresponding rotor. The vessels were heated for 3 min at 80°C and held at the same temperature for a further 2 min at 200 W. Cooling was accomplished by a fan (5 min); the final product was precipitated after cooling, filtered, dried under vacuum, and then recrystallized from dichloromethane-hexane (1:2).

N-[2-(2-Oxo-2-(piperidin-1-yl)acetyl)phenyl]acetamide 4a (1H NMR and 13C NMR attached as a supporting information; Additional file 1)

Light brown crystals; mp 132-133°C; yield (65% method A, Lit. [17] mp 130-132 °C); (91% method B); IR (KBr, cm−1): 3399 (NH), 1691 (α-COCON), 1630 (CO-amide), 1605 (CO-amide). 1H-NMR (CDCl3): δ = 1.55 (m, 2H, CH2), 1.70 (m, 4H, 2CH2), 2.25 (s, 3H, COCH3), 3.30 (m, 2H, CH2), 3.69 (m, 2H, CH2), 7.12 (t, J = 8.08 Hz, 1H, ArH), 7.65 (m, 2H, ArH), 8.88 (d, J = 8.80 Hz, 1H, ArH), 11.30 (s, 1H, NH, D2O exchangeable). 13C-NMR (CDCl3): δ = 24.3, 25.5, 25.7, 26.2, 42.3, 47.2, 117.9, 120.7, 122.8, 133.6, 136.9, 142.5, 164.5 (CO-amide), 169.6 (CO-amide), 196.2 (α-COCON). Anal. Calcd for C15H18N2O3: C, 65.68; H, 6.61; N, 10.21. Found: C, 65.56; H, 6.67; N, 10.33.

N-[2-(2-Morpholino-2-oxoacetyl)phenyl]acetamide 4b (1H NMR and 13C NMR attached as a supporting information; Additional file 1)

Light brown crystals; mp 111-113°C; yield (69% method A); (93% method B); IR (KBr, cm−1): 3399 (NH), 1693 (α-CO-CON), 1635 (CO-amide), 1608 (CO-amide). 1H-NMR (CDCl3): δ = 2.24 (s, 3H, COCH3), 3.34 (t, J = 5.12 Hz, 2H, CH2), 3.65 (t, J = 4.40 Hz, 2H, CH2), 3.77 (m, 4H, 2CH2), 7.13 (t, J = 7.36 Hz, 1H, ArH), 7.67 (m, 2H, ArH), 8.78 (d, J = 8.04 Hz, 1H, ArH), 11.21 (s, 1H, NH, D2O exchangeable) ppm; 13C-NMR (CDCl3): δ = 25.7, 41.3, 46.4, 66.7, 117.8, 120.8, 122.9, 133.53, 137.2, 142.6, 164.6 (CO-amide), 169.6 (CO-amide), 195.4 (α-COCON) ppm. Anal. Calcd for C14H16N2O4: C, 60.86; H, 5.84; N, 10.14. Found: C, 60.78; H, 5.77; N, 10.31.

N-[2-(2-(4-Methylpiperazin-1-yl)-2-oxoacetyl)phenyl]acetamide 4c (1H NMR and 13C NMR attached as a supporting information; Additional file 1)

Light brown crystals; mp 113-115°C; yield (64% method A); (89% method B); IR (KBr, cm−1): 3396 (NH), 1692 (α-CO-CON), 1633 (CO-amide), 1608 (CO-amide). 1H-NMR (CDCl3): δ 2.23 (s, 3H, COCH3), 2.30 (s, 3H, N-CH3), 2.35 (t, J = 5.01 Hz, 2H, CH2), 2.49 (t, J = 5.12 Hz, 2H, CH2), 3.33 (t, J = 5.12 Hz, 2H, CH2), 3.77 (t, J = 5.12 Hz, 2H, CH2), 7.12 (t, J = 7.32 Hz, 1H, ArH), 7.63 (m, 2H, ArH), 8.77 (d, J = 8.80 Hz, 1H, ArH), 11.24 (s, 1H, NH, D2O exchangeable) ppm; 13C-NMR (CDCl3): δ = 25.6, 41.3, 46.0, 46.1, 54.5, 54.9, 117.9, 120.7, 122.8, 133.6, 137.1, 142.5, 164.4 (CO-amide), 169.8 (CO-amide), 195.7 (α-COCON). Anal. Calcd for C15H19N3O3: C, 62.27; H, 6.62; N, 14.52. Found: C, 62.38; H, 6.71; N, 14.69.

N-[2-(2-Oxo-2-(piperidin-1-yl)acetyl)phenyl]propionamide 5a (1H NMR and 13C NMR attached as a supporting information; Additional file 2)

Light brown crystals; mp 102-103°C; yield (70% method A); (95% method B); IR (KBr, cm−1): 3396 (NH), 1695 (α-CO-CON), 1630 (CO-amide), 1608 (CO-amide). 1H-NMR (CDCl3): δ = 1.26 (t, J = 7.32 Hz, 3H, CH2CH3), 1.53 (m, 2H, CH2), 1.68 (m, 4H, 2CH2), 2.49 (q, J = 7.36 Hz, 2H, CH2CH3), 3.26 (t, J = 5.12 Hz, 2H, CH2), 3.68 (m, 2H, CH2), 7.12 (t, J = 8.08 Hz, 1H, ArH), 7.64 (m, 2H, ArH), 8.81 (d, J = 8.80 Hz, 1H, ArH), 11.31 (s, 1H, NH, D2O exchangeable) ppm; 13C-NMR (CDCl3): δ = 9.5, 24.4, 25.4, 25.7, 26.2, 31.8, 42.3, 47.2, 120.7, 118.0, 122.7, 133.5, 136.9, 142.6, 164.5 (CO-amide), 173.4 (CO-amide), 196.2 (α-COCON) ppm. Anal. Calcd for C16H20N2O3: C, 66.65; H, 6.99; N, 9.72. Found: C, 66.38; H, 6.81; N, 9.98.

N-[2-(2-Morpholino-2-oxoacetyl)phenyl]propionamide 5b (1H NMR and 13C NMR attached as a supporting information; Additional file 2)

Light brown crystals; mp 112-112°C; yield (69% method A); (93% method B); IR (KBr, cm−1): 3394 (NH), 1691 (α-CO), 1632 (CO-amide), 1610 (CO-amide). 1H-NMR (CDCl3): δ = 1.29 (t, J = 7.32 Hz, 3H, CH2CH3), 2.48 (q, J = 7.36Hz, 2H, CH2CH3), 3.36 (t, J = 5.12 Hz, 2H, CH2), 3.66 (t, J = 5.12 Hz, 2H, CH2), 3.78 (brs, 4H, 2CH2), 7.13 (t, J = 7.13 Hz, 1H, ArH), 7.67 (m, 2H, ArH), 8.83 (d, J = 8.80 Hz, 1H, ArH), 11.24 (s, 1H, NH, D2O exchangeable) ppm; 13C-NMR (CDCl3): δ = 9.5, 31.8, 41.7, 46.4, 66.7, 117.8, 120.9, 122.7, 133.5, 137.2, 142.8, 164.6 (CO-amide), 173.4 (CO-amide), 195.4 (α-COCON) ppm. Anal. Calcd for C15H18N2O4: C, 62.06; H, 6.25; N, 9.65. Found: C, 62.18; H, 6.17; N, 9.89.

N-[2-(2-(4-Methylpiperazin-1-yl)-2-oxoacetyl)phenyl]propionamide 5c (1H NMR and 13C NMR attached as a supporting information; Additional file 2)

Light brown crystals; mp 90-92°C; yield (72% method A); (89% method B); IR (KBr, cm−1): 3366 (NH), 1710 (α-CO), 1644 (CO-amide), 1581 (CO-amide). 1H-NMR (CDCl3): δ = 1.26 (s, 3H, CH2CH3), 2.30 (s, 3H, N-CH3), 2.35 (t, J = 5.12 Hz, 2H, CH2), 2.51 (m, 4H, CH2CH3, CH2), 3.35 (t, J = 4.80 Hz, 2H, CH2), 3.77 (t, J = 5.16 Hz, 2H, CH2), 7.12 (t, J = 8.08 Hz, 1H, ArH), 7.63 (m, 2H, ArH), 8.81 (d, J = 8.80 Hz, 1H, ArH), 11.27 (s, 1H, NH, D2O exchangeable) ppm; 13C-NMR (CDCl3): δ = 9.5, 31.8, 41.3, 46.0, 46.4, 54.5, 54.9, 117.9, 120.8, 122.7, 133.6, 137.1, 142.7, 164.5 (CO-amide), 173.4 (CO-amide), 195.7 (α-COCON) ppm. Anal. Calcd for C16H21N3O3: C, 63.35; H, 6.98; N, 13.85; Found: C, 63.21; H, 6.87; N, 14.03.

N, N'-(Ethane-1,2-diyl)bis[2-(2-acetamidophenyl)-2-oxoacetamide] 7a (1H NMR and 13C NMR attached as a supporting information; Additional file 3)

White solid; mp 220-221°C (dec.); yield (72% method A); (92% method B); IR (KBr, cm−1): 3351 (NH), 1685 (α-CO), 1653 (CO-amide), 1587 (CO-amide). 1H-NMR (DMSO-d6): δ = 2.06 (s, 6H,2 COCH3), 3.36 (m, 4H, 2CH2), 7.20 (t, J = 7.32 Hz, 2H, ArH), 7.65 (t, J = 8.08 Hz, 2H, 2 NH), 7.67(d, J = 8.08 Hz, 2H, 2ArH), 7.81 (d, J = 8.08 Hz, 2H, ArH), 8.84 (d, 2H, ArH), 11.55 (s, 2H, 2NH) ppm; 13C-NMR (DMSO-d6): δ = 24.5, 39.9, 122.1, 124.0, 124.8, 134.4, 138.8, 164.2 (CO-amide), 169.3 (CO-amide), 191.2 (α-COCON) ppm. Anal. Calcd for C22H22N4O6: C, 60.27; H, 5.06; N, 12.78; Found: C, 60.38; H, 5.11; N, 12.69.

N, N'-(Butane-1,4-diyl)bis[2-(2-acetamidophenyl)-2-oxoacetamide] 7b (1H NMR and 13C NMR attached as a supporting information; Additional file 3)

White solid; mp 205-207°C (dec.); yield (70% method A); (90% method B); IR (KBr, cm−1): 3258 (NH), 1698 (α-CO), 1663 (CO-amide), 1602 (CO-amide). 1H-NMR (DMSO-d6): δ 1.55 (m, 4H, 2CH2), 2.05 (s, 6H, 2COCH3), 3.23 (m, 4H, 2 CH2), 7.23 (t, J = 8.04 Hz, 2H, ArH), 7.64 (m, 4H, 2NH, 2ArH), 7.89 (d, J = 8.08 Hz, 2H, ArH), 8.75 (t, J = 5.16 Hz, 2H, ArH), 11.60 (s, 2H, 2NH) ppm; 13C-NMR (DMSO-d6): δ = 24.6, 26.8, 39.6, 121.9, 124.0, 124.2, 132.3, 134.7, 139.2, 164.3 (CO-amide), 169.4 (CO-amide), 192.4 (α-COCON) ppm. Anal. Calcd for C24H26N4O6: C, 61.79; H, 5.62; N, 12.01. Found: C, 61.87; H, 5.71; N, 12.19.

N, N'-(Hexane-1,6-diyl)bis[2-(2-acetamidophenyl)-2-oxoacetamide] 7c (1H NMR and 13C NMR attached as a supporting information; Additional file 3)

White solid; mp 176-178°C (dec.); yield (72% method A); (94% method B); IR (KBr, cm−1): 3257 (NH), 1698 (α-CO), 1662 (CO-amide), 1608 (CO-amide). 1H-NMR (DMSO-d6): δ = 1.33 (m, 4H, 2CH2), 1.51 (m, 4H, 2CH2), 2.07 (s, 6H, 2 COCH3), 3.21 (m, 4H, 2 CH2), 7.23 (t, J = 7.32 Hz, 2H, ArH), 7.62 (m, 4H, 2NH, 2ArH), 7.91 (d, J = 8.08 Hz, 2H, ArH), 8.73 (t, J = 5.88 Hz, 2H, ArH), 10.91 (s, 2H, 2NH) ppm; 13C-NMR (DMSO-d6): δ = 24.7, 26.6, 29.6, 39.1, 121.9, 124.0, 124.1, 132.3, 134.7, 139.3, 164.3 (CO-amide), 169.3 (CO-amide), 192.5 (α-COCON) ppm. Anal. Calcd for C26H30N4O6: C, 63.15; H, 6.11; N, 11.33. Found: C, 63.27; H, 5.98; N, 11.49.

1, 1'-(Piperazine-1,4-diyl)bis[2-(2-acetamidophenyl)ethane-1,2-dione] 7d (1H NMR and 13C NMR attached as a supporting information; Additional file 4)

White solid; mp 118-120°C (dec.); yield (69% method A); (93% method B); IR (KBr, cm−1): 3364 (NH), 1712 (α-CO), 1644 (CO-amide), 1580 (CO-amide). 1H-NMR (DMSO-d6): δ = 2.13 (s, 3H, COCH3), 2.18 (s, 3H, COCH3), 3.41 (m, 2H, CH2), 3.55 (m, 2H, CH2), 3.64 (m, 2H, CH2), 3.77 (m, 2H, CH2), 7.29 (m, 2H, ArH), 7.75 (m, 4H, ArH), 8.17 (d, J = 8.08 Hz, 1H, ArH), 8.31 (d, J = 8.08 Hz, 1H, ArH), 10.73 (s, 1H, NH), 10.83 (s, 1H, NH) ppm; 13C-NMR (DMSO-d6): δ = 25.1, 25.2, 41.1, 41.4, 45.4, 46.0, 121.8, 121.9, 122.1, 124.2, 124.2, 124.4, 133.4, 133.7, 136.5, 136.7, 140.6, 141.0, 164.6 (CO-amide), 164.7 (CO-amide), 169.9 (CO-amide), 193.4 (α-COCON), 193.9 (C = O) ppm. Anal. Calcd for C24H24N4O6: C, 62.06; H, 5.21; N, 12.06. Found: C, 62.31; H, 5.11; N, 12.29.

N, N'-(Cyclohexane-1,4-diyl)bis(2-(2-acetamidophenyl)-2-oxoacetamide] 7e (1H NMR and 13C NMR attached as a supporting information; Additional file 4)

Off White solid; mp 249-250°C; yield (68% method A); (89% method B); IR (KBr, cm−1): 3269 (NH), 1686 (α-CO), 1645 (CO-amide), 1588 (CO-amide). 1H-NMR (DMSO-d6): δ = 1.45 (m, 4H, CH2), 1.89 (m, 4H, CH2), 2.08 (s, 6H, 2 COCH3), 3.67 (m, 2H, CH), 7.26 (t, J = 7.36 Hz, 2H, 2ArH), 7.63 (m, 4H, 2NH, 2ArH), 7.97 (d, J = 8.08 Hz, 2H, ArH), 8.69 (d, J = 8.08 Hz, 2H, 2ArH), 10.64 (s, 2H, 2NH) ppm; 13C-NMR (DMSO-d6): δ = 24.7, 24.8, 31.1, 47.8, 121.8, 121.9, 123.7, 124.1, 132.3, 132.5, 134.7, 135.1, 139.5, 163.8 (CO-amide), 169.4 (CO-amide), 192.7 (α-COCON) ppm. Anal. Calcd for C26H28N4O6: C, 63.40; H, 5.73; N, 11.38. Found: C, 63.28; H, 5.81; N, 11.49.

N, N'-(Ethane-1,2-diyl)bis[2-(2-propionamidophenyl)-2-oxoacetamide] 8a (1H NMR and 13C NMR attached as a supporting information; Additional file 5)

White solid; mp 204-206°C (dec.); yield (70% method A); (91% method B); IR (KBr, cm−1): 3291 (NH), 1697 (α-CO), 1667 (CO-amide), 1607 (CO-amide). 1H-NMR (DMSO-d6): δ = 1.09 (t, J = 7.32 Hz, 6H, 2CH2CH3), 2.37 (q, J = 7.33 Hz, 4H, 2CH2CH3), 3.38 (m, 4H, 2CH2), 7.21 (t, J = 8.04 Hz, 2H, ArH), 7.62 (t, J = 7.32 Hz, 2H, 2NH), 7.69 (d, J = 7.32 Hz, 2H, ArH), 7.92 (d, J = 8.80 Hz, 2H, ArH), 8.88 (brs, 2H, ArH), 10.60 (s, 2H, 2NH) ppm; 13C-NMR (DMSO-d6): δ = 9.9, 30.4, 39.4, 121.8, 123.8, 124.0, 132.5, 134.7, 139.2, 164.3 (CO-amide), 172.9 (CO-amide), 191.9 (α-COCON) ppm. Anal. Calcd for C24H26N4O6: C, 61.79; H, 5.62; N, 12.01; O, 20.58. Found: C, 62.00; H, 5.51; N, 12.39.

1, 1'-(Piperazine-1,4-diyl)bis[2-(2-propionamidophenyl)ethane-1,2-dione] 8d (1H NMR and 13C NMR attached as a supporting information; Additional file 5)

White solid; mp 221-223°C; yield (72% method A); (93% method B); IR (KBr, cm−1): 3318 (NH), 1701 (α-CO), 1644 (CO-amide), 1581 (CO-amide). 1H-NMR (DMSO-d6): δ = 1.13 (m, 4H, 2CH2CH3), 2.48 (m, 4H, 2CH2CH3), 3.40 (m, 2H, CH2), 3.55 (m, 2H, CH2), 3.64 (m, 2H, CH2), 3.78 (m, 2H, CH2), 7.29 (m, 2H, ArH), 7.75 (m, 4H, ArH), 8.30 (d, J = 8.08 Hz, 1H, ArH), 8.44 (d, J = 8.04 Hz, 1H, ArH), 10.80 (s, 1H, NH), 10.90 (s, 1H, NH) ppm; 13C-NMR (DMSO-d6): δ = 9.8, 9.8, 30.8, 30.9, 41.1, 41.4, 45.4, 46.0, 120.4, 121.5, 121.7, 124.0, 124.1, 133.7, 134.0, 136.7, 136.9, 141.1, 141.5, 164.7 (CO-amide), 164.7 (CO-amide), 173.3, 193.9 (α-COCON), 194.3 (α-COCON) ppm. Anal. Calcd for C26H28N4O6; C, 63.40: H, 5.73; N, 11.38. Found: C, 63.31; H, 5.81; N, 11.19.

Synthesis of methyl 2-(2-acetamidophenyl)-2-oxoacetate (9a) and methyl 2-oxo-2-(2-propionamidophenyl)acetate (9b) using microwave irradiation

N-Acylisatin 1 or 2 (2 mmol) was dissolved in methanol (10 mL) and the reaction was microwave irradiated using a multimode reactor (Synthos 3000 Aton Paar, GmbH, 1400 W maximum magnetron) for 3 min at 80°C and hold at the same temperature for 2 min at 200 W. Cooling was accomplished by a fan (5 min) and the desired product was obtained after cooling with an excellent yield without further recrystallization.

Methyl 2-(2-acetamidophenyl)-2-oxoacetate 9a (1H NMR and 13C NMR attached as a supporting information; Additional file 6)

The product was obtained as yellow needles from methanol; mp 104-105°C; yield 91%. IR (KBr, cm−1): 3220.99 (NH), 1746.58 (CO-ester), 1696.22 (α-CO), 1656.79 (CO-amide). 1H NMR (CDCl3): δ = 2.25 (s, 3H, COCH3), 3.98 (s, 3H, COOCH3), 7.13 (t, J = 7.36 Hz, 1H, Ar), 7.62-7.68 (m, 2H, Ar), 8.78(d, J = 8.08 Hz, 1H, Ar), 11.06 (s, 1H, NH) ppm. 13C NMR (CDCl3): δ = 25.6, 53.1, 117.1, 120.8, 122.6, 133.6, 137.3, 142.8, 142.8, 164.0 (CO-amide), 169.6 (CO-ester), 190.3 (α-COCON) ppm. Anal. Calcd for C11H11NO4: C 59.73, H 5.01, N 6.33. Found C 60.00, H 5.18, N 6.44.

Methyl 2-(2-propionamidophenyl)-2-oxo-acetate 9b (1H NMR and 13C NMR attached as a supporting information; Additional file 7)

The product was obtained as yellow needles from methanol; mp 68-70°C; yield 89%. IR (KBr, cm−1): 3220.99 (NH), 1746.58 (CO-ester), 1696.22 (α-CO), 1656.79 (CONH). 1H NMR (CDCl3): δ 1.28 (t, J = 7.32 Hz, 3H, CH2CH3), 2.48 (q, J = 7.32 Hz, 2H, COCH2CH3), 3.98 (s, 3H, COOCH3), 7.13 (t, J = 8.04 Hz, 1H, Ar), 7.63-7.68 (m, 2H, Ar), 8.82 (d, J = 8.08 Hz, 1H, Ar), 11.10 (s, 1H, NH) ppm; 13C NMR (CDCl3): δ = 9.5, 31.8, 53.1, 117.1, 120.8, 122.5, 133.7, 137.3, 142.9, 164.0 (CO-amide), 173.4 (CO-ester), 190.4 (α-COCON) ppm. Anal. Calcd for C12H13NO4: C, 61.27; H, 5.57; N, 5.95. Found C 61.05, H 5.71, N 6.04.

Conclusions

In conclusion, we have demonstrated that microwave irradiation could be employed efficiently for the synthesis of biologically important α-ketoamide and bis-(α-ketoamide) derivatives. The microwave irradiation showed more advantageous over the classical method with regard to reaction time, solvent quantity, and product yield in almost every case. Reaction of N-acylisatin with methanol under microwave irradiation afforded the α-phenylglyoxyl methyl ester derivatives with an excellent yield and purity. Aminolysis of phenylglyoxyl ester derivatives with piperidine and morpholine under microwave irradiation afford the same α-ketoamide derivatives as obtained from direct aminolysis of N-acylisatin with amine.