OxymaPure/DIC: An efficient reagent for the synthesis of a novel series of 4-[2-(2-acetylaminophenyl)-2-oxo-acetylamino] benzoyl amino acid ester derivatives

Article abstract of DOI:10.3390/molecules181214747

OxymaPure (ethyl 2-cyano-2-(hydroxyimino)acetate) was tested as an additive for use in the carbodiimide (DIC) approach for the synthesis of a novel series of α-ketoamide derivatives (4-[2-(2-acetylaminophenyl)-2-oxo-acetylamino] benzoyl amino acid ester derivatives). OxymaPure showed clear superiority to HOBt/DIC or carbodiimide alone in terms of purity and yield. The title compounds were synthesized via the ring opening of N-acylisatin. First, N-acetylisatin was reacted with 4-aminobenzoic acid under conventional heating as well as microwave irradiation to afford 4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzoic acid. This α-ketoamide was coupled to different amino acid esters using OxymaPure/DIC as a coupling reagent to afford 4-[2-(2-acetylaminophenyl)-2- oxoacetylamino] benzoyl amino acid ester derivatives in excellent yield and purity. The synthesized compounds were characterized using FT-IR, NMR, and elemental analysis.

Full text of DOI:10.3390/molecules181214747

Molecules 2013, 18, 14747-14759; doi:10.3390/molecules181214747
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
OxymaPure/DIC: An Efficient Reagent for the Synthesis of a
Novel Series of 4-[2-(2-Acetylaminophenyl)-2-oxo-acetylamino]
Benzoyl Amino Acid Ester Derivatives
Ayman El-Faham 1,2,*, Zainab Al Marhoon 1, Ahmed Abdel-Megeed 3,4 and
Fernando Albericio 5,6,7,8,
*
1
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451,
Saudi Arabia; E-Mail: zanzaki20042000@yahoo.com
2
3
4
5
6
7
8
Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, Ibrahimia,
Alexandria 12321, Egypt
Department of Botany and Microbiology, College of Science, King Saud University, P.O. Box 2455,
Riyadh 11451, Saudi Arabia; E-Mail: aamahmoud@ksu.edu.sa
Department of Plant Protection, Faculty of Agriculture, Saba Basha, Alexandria University,
Alexandria 12321, Egypt
Institute for Research in Biomedicine (IRB), Barcelona Science Park, Baldiri Reixac 10,
Barcelona 08028, Spain
CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine,
Barcelona Science Park, Baldiri Reixac 10-12, Barcelona 08028, Spain
Department of Organic Chemistry, University of Barcelona, Martí i Franqués 1-11,
Barcelona 08028, Spain
School of Chemistry & Physics, University of KwaZulu-Natal, Durban 4001, South Africa
* Authors to whom correspondence should be addressed; E-Mails: aymanel_faham@hotmail.com or
aelfaham@ksu.edu.sa (A.E.-F.); albericio@irbbarcelona.org (F.A.); Tel.: +96-611-467-3195 (A.E.-F.);
Fax: +96-611-467-5992 (A.E.-F.); +34-93-403-7126 (F.A.).
Received: 24 September 2013; in revised form: 15 November 2013 / Accepted: 20 November 2013 /
Published: 28 November 2013
Abstract: OxymaPure (ethyl 2-cyano-2-(hydroxyimino)acetate) was tested as an
additive for use in the carbodiimide (DIC) approach for the synthesis of a novel series of
α-ketoamide derivatives (4-[2-(2-acetylaminophenyl)-2-oxo-acetylamino]benzoyl amino
acid ester derivatives). OxymaPure showed clear superiority to HOBt/DIC or carbodiimide
alone in terms of purity and yield. The title compounds were synthesized via the ring
opening of N-acylisatin. First, N-acetylisatin was reacted with 4-aminobenzoic acid under

Molecules 2013, 18
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conventional heating as well as microwave irradiation to afford 4-(2-(2-acetamidophenyl)-
2-oxoacetamido)benzoic acid. This α-ketoamide was coupled to different amino acid esters
using OxymaPure/DIC as a coupling reagent to afford 4-[2-(2-acetylaminophenyl)-2-oxo-
acetylamino]benzoyl amino acid ester derivatives in excellent yield and purity. The
synthesized compounds were characterized using FT-IR, NMR, and elemental analysis.
Keywords: N-acetylisatin; 4-aminobenzoic acid; amino acid esters; DIC; OxymaPure;
α-ketoamide
1. Introduction
α-Ketoamides are compounds of interest in organic chemistry and are present in many active
pharmaceutical compounds [1–6]. Parallel with the application of the α-ketoamide moiety in medicinal
chemistry, numerous synthetic methods have been described [7–21]. Several authors have
demonstrated that the synthesis of the α-ketoamide fragment can be achieved by ring opening of
N-acetylisatin (1) by the attack of an amine at C2-carbonyl group of N-acetylisatin (Scheme 1) [22–27].
Recently, Cheah et al. [28,29] reported the reaction of N-acetylisatin with L-α-amino acid esters as a
novel class of N-glyoxylamide peptide mimics.
Scheme 1. General mechanism for the reaction of N-acylisatin (1) with amines.
Here we present the use of OxymaPure/DIC as a coupling reagent for the synthesis of a novel class
of α-ketoamide derivatives (4-[2-(2-acetylaminophenyl)-2-oxo-acetylamino]benzoyl amino acid esters).
2. Results and Discussion
1-Acetylindoline-2,3-dione (N-acetylisatin, 1) was initially prepared by reaction of isatin with
acetic anhydride, using conventional heating under the same conditions as those described in the
literature [30,31]. However, we demonstrate that the use of a microwave irradiation, using a
multimode reactor (Synthos 3000, Anton Paar GmbH, Graz, Austria, 1,400 W maximum magnetron,
method B; Experimental section), renders 1 from isatin and acetic anhydride (Scheme 2) in excellent
yield in less reaction time and higher purity than the conventional method, as observed from spectral
data. This observation is consistent with data in the literature [30].
N-Acetylisatin (1) was then reacted with the poor nucleophile, 4-aminobenzoic acid (2) using
conventional heating for 1 h in methanol as a solvent to afford the product 3 (Scheme 2). The IR and
NMR spectral analysis of the product revealed that the reaction proceeded through the ring opening to

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afford the α-ketoamide derivative 3 and not the Schiff base derivative, in contrast to what was reported
in the reaction of bromo-N-acetylisatin with aminobenzoic acid. [32,33] The IR spectra of 3 showed
three characteristic peaks at 3270, 1679, and 1601 cm−1, corresponding to the COOH, the
1
α-ketoamide (COCONH), and NHCOCH3, respectively. The H-NMR of 3 agreed well with the
structure, showing eight distinct resonance peaks located at δ 1.99 (s, 3H, COCH3), 7.27–7.730
(m, 2H, Ar), 7.62–7.68 (m, 2H, Ar), 7.90 (d, 2H, Ar), 7.90 (d, 2H, Ar), 10.55 (s, 1H, NH), 10.99
(s, 1H, NH), and 13.00 (brs, 1H, COOH). These peaks were assigned to the acetyl group, aromatic
proton (isatin), aromatic proton (4-aminobenzoic acid), two NHs, and COOH respectively. The 13C-NMR
of 3 also confirmed the structure, showing the characteristic signals at δ 161.0, 166.1, 168.2, and 188.1
corresponding to the two amide groups, one carbonyl of the carboxyl group, and the α-ketoamide
group respectively, along with the rest of the expected carbon signals of the compound.
Scheme 2. Synthesis and reaction of N-acylisatin (1) with 4-aminobenzoic acid (2).
The same reaction was repeated under microwave irradiation using a multimode reactor (Anton
Paar GmbH Synthos 3000, 1,400 W maximum magnetron, method B in the Experimental section) to
afford product 3 (Scheme 2) in less reaction time and high purity as shown by its spectral data. The IR
and NMR spectra of 3 proved its structure and were in agreement with the product obtained by
conventional heating.
Recently, OxymaPure (ethyl 2-cyano-2-(hydroxyimino)acetate, Scheme 3) was used as an additive
for peptide synthesis in combination with carbodiimides [34]. It displayed an appropriate balance of
availability and ease of handling. In addition it was safer than HOBt and showed clear superiority in
terms of coupling efficiency [34–42].
Here we tested OxymaPure as an additive for the coupling of compound 3 to H-Ala-OMe·HCl. In a
general experiment, 3 was preactivated with OxymaPure/DIC for 5 min in DMF to generate the
corresponding active ester (Scheme 3), which reacted directly with H-Ala-OMe·HCl in the presence of
1 equiv. of DIEA at 0 °C for 1 h and then at r.t overnight. After workup and removal of the solvent,
product 4a was obtained as a white solid in 88% yield (Scheme 3).
The reaction of 3 with H-Ala-OMe was then repeated using various coupling reagents (Table 1).
The best results were obtained using OxymaPure/DIC; OxymaPure showed clear superiority to
HOBt/DIC and carbodiimide alone in terms of yield and purity. DCC consistently showed some
impurities from dicyclohexylurea (DCU) as observed from NMR spectra.

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Scheme 3. Synthesis 4-[2-(2-acetylaminophenyl)-2-oxo-acetylamino]benzoyl amino acid
ester derivatives 4a–i.
Table 1. Reaction of 3 with H-Ala-OMe.HCl using various coupling conditions.
Coupling Condition
Yield (%)
Mp (°C)
174–176
168–172
172–175
170–174
170–173
168–173
DIC/Oxyma
DCC/Oxyma *
DIC/HOBt
DCC/HOBt *
DIC
88
82
72
70
60
60
DCC *
* NMR showed impurities corresponding to the dicyclohexylurea byproduct.
The IR spectrum of 4a showed four characteristic peaks at 3288, 1741, 1672, and 1607 cm−1,
corresponding to the NH, CO-ester, α-ketoamide and CO-amide, respectively. The 1H-NMR showed a
doublet peak at δ 1.41, which was related to the CH3 for the alanine unit, two singlet peaks at δ 2.00
and 3.65 for the acetyl and the methyl ester, and three singlet peaks at δ 8.73, 10.55, 10.92 for three
NHs, respectively. The 13C-NMR also confirmed the structure of 4a, showing signals at δ 17.3
(CHCH3), 24.2 (NCOCH3), 49.9 (COOCH3), 52.1 (CH-NH), 162.3, 166.2, and 169.5 (for three CONHs),
173.8 (COOCH3), and 190.0 (COCO), along with the remaining carbon residues related to 4a.
Several amino acid esters were prepared following the reported method [43,44] and coupled with 3
using OxymaPure/DIC under the same conditions used for the coupling of H-Ala-OMe. This approach
afforded products 4a–i in excellent yield and purity (Scheme 3, Table 2). The structures of all the
compounds synthesized were confirmed by IR, NMR (1H-NMR and 13C-NMR) and elemental analysis.

Molecules 2013, 18
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Table 2. Yield (%), Mp (°C), and Elemental analysis of 4a–i.
Elemental Analysis Calcd. (Found)
Compd. No Yield (%)
Mp (°C)
C
H
N
4a
4b
4c
4d
4e
4f
88
81
83
76
82
88
86
83
78
174–176
178–180
168–170
216–218
154–156
154–156
118–120
180–182
238–240
61.31 (61.60)
62.86 (63.02)
66.52 (66.67)
62.11 (62.38)
66.52 (66.33)
61.31 (61.09)
62.11 (62.37)
63.56 (63.38)
63.85 (64.06)
5.14 (5.34)
5.73 (5.96)
5.17 (5.24)
5.45 (5.66)
5.17 (5.26)
5.14 (5.23)
5.45 (5.67)
6.00 (6.13)
5.58 (5.65)
10.21 (10.00)
9.56 (9.581)
8.62 (8.88)
9.88 (10.07)
8.62( 8.90)
10.21 (10.48)
9.88 (10.04)
9.27 (9.53)
9.31 (9.04)
4g
4h
4i
3. Experimental
3.1. General
The solvents used were of HPLC reagent grade. Melting points were determined with a Mel-Temp
apparatus and are uncorrected. Fourier transform infrared spectroscopy (FTIR) spectra was recorded
13
on Nicolet 560 spectrometer. Nuclear magnetic resonance spectra (1H-NMR and C-NMR spectra)
were recorded on a JOEL 400 MHz spectrometer with chemical shift values reported in δ units (ppm)
relative to an internal standard. The microwave irradiation used a multimode reactor (Synthos 3000,
Anton Paar GmbH, and 1,400 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 were done by TLC on
silica gel-protected aluminum sheets (Type 60 GF254, Merck) and the spots were detected by exposure
to a UV-lamp at λ 254 nm for a few seconds. The compounds were named using ChemDraw Ultra
version 11, Cambridge Soft Corporation (Cambridge, MA, USA).
3.2. Synthesis of 1-Acetyl-1H-indole-2,3-dione (N-acetylisatin) (1)
Conventional method (A): A mixture of isatin (0.01 M) and acetic anhydride (5 mL) was refluxed
for 5 h. After cooling to r. t., it was left to stand overnight. The precipitate was collected, washed with
96% ethanol and air-dried.
Microwave method (B): A multimode reactor (Anton Paar GmbH Synthos 3000, 1,400 W maximum
magnetron) was used. The initial step was conducted with a 2-Teflon vessels rotor (MF 100). Isatin
(5 mmol) was suspended in acetic anhydride (10 mL) and the reaction was processed by heating the
vessels for 5 min. at 80 °C and holding it at the same temperature for 5 min (under 0.2/s bar pressure,
400 W). Cooling was accomplished by a fan (for 5 min) and the desired product was obtained as a
yellow needle in excellent yield without further recrystallization. The spectral data were in accordance
with the data reported in the literature [30].
1-Acetyl-1H-indole-2,3-dione (N-acetylisatin) (1) The product was obtained as green crystals, mp:
146–148 °C; yields: 79% (method A); 95% (method B) (lit. [23] 141 °C, yield 97%; lit. [30] 137–139 °C,

Molecules 2013, 18
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1
yield 70%). H-NMR (CDCl3) δ (ppm): 2.69 (s, 3H, COCH3), 7.31 (t, J = 8.04 Hz, 1H), 7.69 (t,
J = 7.32 Hz,1H), 7.73 (d, J = 7.32 Hz, 1H), 8.36 (d, J = 8.08 Hz,1H); 13C-NMR (CDCl3) δ (ppm): 26.5
(COCH3), 118.3, 119.2, 125.3, 126.2, 139.0, 148.6, 158.0 (CO), 169.8 (CO), 180.2 (CO).
3.3. Synthesis of 4-(2-(2-Acetamidophenyl)-2-oxoacetamido)benzoic Acid (3)
Conventional method (A): A mixture of N-Acetyl isatin (0.01 M) and 4-amino-benzoic acid (0.01 M)
in absolute methanol (20 mL) was refluxed for 1 h in the presence of 2–3 drops of glacial acetic acid.
After cooling, was filtered and recrystallized from ethanol to afford the product in 96% yield.
Microwave method (B): A multimode reactor (Synthos 3000 Aton Paar, GmbH, 1400 W maximum
magnetron) was used. The initial step was conducted with 2-Teflon vessels rotor (MF 100) that allows
the reaction to be processed under the same conditions. N-acetylisatin and 4-aminobenzoic acid were
mixed in methanol as a solvent in the presence or absence of glacial acetic acid (2–3 drops).
The individual vessels were placed in the corresponding rotor, and finally the rotor was closed with a
protective hood. The vessels were heated for 2 min. at 80 °C and held at the same temperature for
another 2 min (~2 bar pressure, 400 W). Cooling was accomplished by a fan (5 min). The final product
was washed with cold methanol, and then dried under vacuum to afford the product in a pure state as
observed from spectral analysis.
The product was obtained as a pale yellow powder, mp: 238–240 °C; yield (57% method A); (86%
1
method B). H-NMR (DMSO-d6) δ (ppm): 1.99 (s, 3H, COCH3), 7.25–7.30 (m, 2H), 7.62–7.68
(m, 2H), 7.90 (d, J = 8.79 Hz, 2H), 7.95 (d, J = 8.43 Hz, 2H), 10.55(s, 1H, NH), 10.99 (s, 1H, NH),
13.00 (brs, 1H, COOH); 13C-NMR (DMSO-d6) δ (ppm): 22.8 (NHCOCH3), 118.7, 121.0, 123.1, 124.3,
125.4, 129.6, 130.3, 132.9, 136.8, 141.2, 161.00 (CONH), 166.1(CONH), 168.2(COOH), 188.1
(COCO). IR (cm−1): 3270 (COOH), 1679, 1601 (C=O). Anal. Calcd. for C17H14N2O5: C, 62.57; H,
4.32; N, 8.59. Found: C, 62.80; H, 4.21; N, 8.33.
3.4. General Method for the Synthesis of Amino Acid Esters
Thionyl chloride (10 mL) was slowly added to a cold suspension solution of the appropriate amino
acid (50 mmol) in methanol (50 mL) at 0 °C. The reaction mixture was stirred for 8–10 h and then
concentrated on a rotary evaporator. The white precipitate formed was washed with anhydrous ether
and then dried under vacuum. All data agreed with the reported data [43,44].
1
Methyl 1-aminocyclopentane carboxylate HCl. White powder, mp: 204–206 °C, yield 87%. H-NMR
(DMSO-d6) δ (ppm): 1.62–1.72 (m, 2H, CH2CH2CH2), 1.81–1.91 (m, 2H, HNCH2CH2CH2), 1.94–1.96
(m, 2H, CH2CH2CO), 2.07–2.14 (m, 2H, CH2CH2NH), 3.73 (s, 3H, COOCH3), 8.81(brs, 2H, NH2).
13C-NMR (DMSO-d6) δ (ppm): 24.3, 35.2, 52.4 (COOCH3), 63.4, 171.8(COOCH3).
Methyl 3-aminopropionate HCl. White powder, mp: 90–92 °C, yield 85%. 1H-NMR (DMSO-d6) δ (ppm):
2.72 (t, J = 7.35 Hz, 2H, NH2CH2CH2CO), 2.98 (m, 2H, NH2CH2CH2CO), 3.62 (s, 3H, COOCH3),
8.22 (brs, 2H, NH2).

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1
Methyl 4-aminobutanoate HCl. White powder, mp: 104–106 °C, yield 87%. H-NMR (DMSO-d6) δ
(ppm): 1.80 (m, 2H, NH2CH2CH2CH2CO), 2.43 (t, J = 7.0 Hz, 2H, NH2CH2CH2CH2CO), 2.78
(m, 2H, NH2CH2CH2CH2CO), 3.59 (s, 3H, COOCH3), 8.16 (brs, 2H, NH2).
Methyl 6-aminohexanoate HCl. White powder, mp: 104–106 °C, yield 90%. 1H-NMR
(DMSO-d6) δ (ppm): 1.26–1.31 (m, 2H, NH2CH2CH2CH2CH2CH2CO), 1.49–1.57 (m, 4H,
NH2CH2CH2CH2CH2CH2CO), 2.29 (t, J = 7.30 Hz, 2H, NH2CH2CH2CH2CH2CH2CO), 2.71 (m, 2H,
NH2CH2CH2CH2CH2CH2CO), 3.57 (s, 3H, COOCH3), 8.08 (brs, 2H, NH2).
1
Dimethyl 2-aminosuccinate HCl. White powder, mp: 110–112 °C, yield 82%. H-NMR (DMSO-d6)
δ (ppm): 3.03 (q, J = 4.0 Hz, 2H, COCH2CHNH2), 3.64 (s, 3H, COOCH3), 3.72 (s, 3H, COOCH3),
4.33 (m, 1H, CH2CHNH2), 8.75 (brs, 2H, NH2).
3.5. General Procedure for the Synthesis of 4a–i
Acid 3 (1 mmol), Oxyma (1 mmol), and DIC (1 mmol) were mixed in DMF (5 mL) at 0 °C. The
reaction mixture was stirred for 5 min at 0 °C to preactivate the acid and generate the active ester, and
then DIEA (1 mmol) followed by amino acid ester (1 mmol) were added. The reaction mixture was
stirred at 0 °C for 1 h and at room temperature overnight. The mixture was then diluted with ethyl
acetate (50 mL) and then extracted with 1 N HCl (2 × 10 mL), 10% NaHCO3 (2 × 10 mL), and
saturated NaCl (2 × 10 mL). The organic phase was dried over anhydrous MgSO4, filtered, and the
solvent was removed under vacuum. The residue was recrystallized from dichloromethane-hexane to
afford the pure product.
Methyl 2-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)propanoate (4a). White powder, mp:
174–176 °C, yield 88%. 1H-NMR (DMSO-d6) δ (ppm): 1.41 (d, J = 7.32 Hz, 3H, CHCH3), 2.00 (s, 3H,
COCH3), 3.65 (s, 3H, COOCH3), 4.48 (m, 1H, NHCHCH3), 7.45 (t, J = 7.2 Hz, 1H), 7.64 (d, J = 4.4 Hz,
1H), 7.68 (d, J = 7.36 Hz, 2H), 7.89 (m, 4H, Ar), 8.73(s, 1H, NH), 10.55(s, 1H, NH), 10.92 (s, 1H,
NH); 13C-NMR (DMSO-d6) δ (ppm): 17.3 (NHCHCH3), 24.2 (COCH3), 49.9 (COOCH3), 52.1
(NHCHCH3), 120.0, 122.4, 124.4, 126.4, 128.9, 129.0, 131.6, 134.21, 139.4, 142.2, 162.3 (CONH),
166.2 (CONH), 169.5 (CONH), 173.8 (COOCH3), 190.0 (COCO). IR (cm−1): 3288, 3124 (NH), 1741,
1672, 1607, 1536 (C=O). Anal. Cacld for C21H21N3O6: C, 61.31; H, 5.14; N, 10.21. Found: C, 61.60;
H, 5.34; N, 10.00.
Methyl 2-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)-3-methylbutanoate (4b). White powder,
1
mp: 178–180 °C, yield 81%. H-NMR (CDCl3) δ (ppm): 1.00 (t, J = 8.60 Hz, 6H, CH (CH3)2), 1.59
(m, 1H, CHCH(CH3)2), 2.27 (s, 3H, COCH3), 3.78 (s, 3H, COOCH3), 4.78 (m, 1H, CHCHNH), 6.61
(d, J = 8.08 Hz, 1H), 7.17 (t, J = 8.08 Hz, 1H), 7.64 (t, J = 8.08 Hz, 1H), 7.78 (d, J = 8.08 Hz, 1H),
7.86 (d, J = 8.08 Hz, 2H), 8.50 (d, J = 8.08 Hz, 1H) 8.64 (d, J = 8.08 Hz, 1H), 8.99 (s, 1H, NH), 10.79
(s, 1H, NH); 13C-NMR (CDCl3) δ (ppm): 18.1, 19.1(CHCH(CH3)2), 25.0 (COCH3), 31.0 (CHCH(CH3)2),
52.4 (COOCH3),57.6 (CHCH(CH3)2), 119.4, 119.8, 120.9, 122.8, 128.4, 130.6, 134.3, 136.8, 140.0,
142.1,160.6 (CONH), 166.5 (CONH), 169.4 (CONH), 172.8 (COOCH3), 190.8 (COCO). IR (cm−1):

Molecules 2013, 18
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3293 (NH), 1747, 1679, 1634, 1608, 1526 (C=O). Anal. Cacld. for C23H25N3O63: C, 62.86; H, 5.73; N,
9.56. Found: C, 63.02; H, 5.96; N, 9.81.
Methyl 2-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)-3-phenylpropanoate (4c). White
1
powder, mp: 168–170 °C, yield 83%. H-NMR (CDCl3) δ (ppm): 2.24 (s, 3H, COCH3), 3.25 (m, 2H,
CHCH2C6H5), 3.80 (s, 3H, COOCH3), 5.08 (m, 1H, CHCH2C6H5), 6.56 (d, J = 7.36 Hz, 1H), 7.12 (d,
J = 6.6 Hz, 1H), 7.15–7.29 (m, 5H, CHCH2C6H5), 7.65 (t, J = 6. 6 Hz, 1H), 7.76 (m, 4H, NHC6H4CO),
13
8.47 (d, J = 8.08 Hz, 1H), 8.63 (d, J = 8.08 Hz, 1H), 9.01(s, 1H, NH), 10.79 (s, 1H, NH); C-NMR
(CDCl3) δ (ppm): 25.0 (COCH3), 38.0 (CHCH2C6H5), 49.2 (COOCH3), 52.6 (CHCH2C6H5), 118.8,
119.8, 121.0, 122.8, 127.3, 128.39, 128.7, 129.4, 130.5, 134.4, 135.9, 136.9, 139.9, 142.1, 160.3
(CONH), 166.0 (CONH), 169.4 (CONH), 172.1 (COOCH3), 190.6 (COCO). IR (cm−1): 3250, 3116
(NH), 1750, 1679, 1635, 1608, 1523 (C=O). Anal. Calcd for C27H25N3O6: C, 66.52; H, 5.17; N, 8.62.
Found: C, 66.67; H, 5.24; N, 8.88.
Methyl 2-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)-2-methylpropanoate (4d). White
1
powder, mp: 212–218 °C, yield 76%. H-NMR (DMSO-d6) δ (ppm): 1.46 (s, 6H, HNC(CH3)2), 1.99
(s, 3H, COCH3), 3.58 (s, 3H, COOCH3), 7.63–7.87 (m, 8H, NHC6H4COCONHC6H4CO), 8.54 (s, 1H,
13
NH), 10.56 (s, 1H, NH), 10.90 (s, 1H, NH); C-NMR (DMSO-d6) δ (ppm): 25.6 (COCH3), 40.5
(HNC(CH3)2), 52.4 (COOCH3), 56.1 HNC(CH3)2, 119.8, 121.3, 124.5, 126.4, 129.0, 130.0, 131.5,
134.2, 138.1, 142.20,158.0 (CONH), 165.9 (CONH), 169.5(CONH), 175.1 (COOCH3), 189.5
(COCO). IR (cm−1): 3428 (NH), 1749, 1680, 1637, 1525 (C=O). Anal. Calcd. for C22H23N3O6: C,
62.11; H, 5.45; N,9.88. Found: C, 62.38; H, 5.66; N, 10.07.
Dimethyl 2-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)succinate (4e). White powder, mp:
1
154–156 °C, yield 82%. H-NMR (DMSO-d6) δ (ppm): 1.99 (s, 3H, COCH3), 2.85–2.98 (m, 2H,
CHCH2COOCH3), 3.62 (s, 3H, COOCH3), 3.65 (s, 3H, COOCH3), 4.83 (m, 1H, HNCHCH2), 7.28–7.87
(m, 8H, NHC6H4COCONH C6H4CO), 8.86 (d, 1H, NH), 10.54 (s, 1H, NH), 10.93 (s, 1H, NH);
13C-NMR (DMSO-d6) δ (ppm): 23.9 (COCH3), 36.0 (NHCHCH2CO), 49.8 (NHCHCH2CO), 52.3
(COOCH3), 52.8 (COOCH3), 119.9, 122.4, 124.4, 125.7, 128.9, 129.5, 131.55, 134.2, 138.1, 141.5,
162.3 (CONH), 166.2 (CONH), 169.5 (CONH), 171.1(COOCH3), 171.9 (COOCH3), 189.5 (COCO).
IR (cm−1): 3295 (NH), 1748, 1667, 1608, 1526 (C=O). Anal. Calcd. for C27H25N3O6: C, 66.52; H, 5.17;
N, 8.62. Found: C, 66.33; H, 5.26; N, 8.90.
Methyl 3-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)propanoate (4f). White powder, mp:
154–156 °C, yield 88%. 1H-NMR (DMSO-d6) δ (ppm): 1.99 (s, 3H, COCH3), 2.60 (t, J = 6.60 Hz, 2H,
NHCH2CH2CO), 3.49 (q, 2H, NHCH2CH2CO), 3.61 (s, 3H, COOCH3), 7.29–7.85 (m, 8H,
13
NHC6H4COCONH C6H4CO), 8.51 (s, 1H, NH), 10.55 (s, 1H, NH), 10.90 (s, 1H, NH); C-NMR
(DMSO-d6) δ (ppm): 23.9 (COCH3), 34.1 (NHCH2CH2CO), 36.1(NHCH2CH2CO), 52.0 (COOCH3),
119.9, 122.4, 124.4, 125.6, 128.6, 130.4, 131.6, 134.3, 138.2, 141.1, 162.4 (CONH), 166.3 (CONH),
169.5 (CONH), 172.4 (COOCH3), 189.6 (COCO). IR (cm−1): 3295 (NH), 1742, 1666, 1635, 1608,
1527 (C=O). Anal. Calcd. for C21H21N3O6: C, 61.31; H, 5.14; N, 10.21. Found: C, 61.09; H, 5.23; N, 10.48.

Molecules 2013, 18
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Methyl 4-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)butanoate (4g). White powder, mp:
118–120 °C, yield 86%. 1H-NMR (DMSO-d6) δ (ppm): 1.77 (m, 2H, NHCH2CH2CH2CO), 1.99 (s, 3H,
COCH3), 2.37 (t, J = 7.32 Hz, 2H, NHCH2CH2CH2CO), 3.27 (q, 2H, NHCH2CH2CH2CO), 3.58
(s, 3H, COOCH3), 7.28 (t, J = 7.3, 2.2 Hz, 1H), 7.64–7.85 (m, 7H, Ar), 8.42 (brs, 1H, NH), 10.55
(s, 1H, NH), 10.89 (s, 1H, NH); 13C-NMR (DMSO-d6) δ (ppm): 23.9 (NHCH2CH2CH2CO), 24.2
(COCH3), 31.4 (NHCH2CH2CH2CO), 41.6 (NHCH2CH2CH2CO), 51.8 (COOCH3), 119.9, 122.4,
124.4, 125.6, 128.6, 130.7, 131.6, 134.3, 138.7, 141.0, 162.3 (CONH), 166.2 (CONH), 169.5 (CONH),
173.7 (COOCH3), 189.7 (COCO). IR (cm−1): 3343 (NH), 1738, 1661, 1630, 1600, 1527 (C=O). Anal.
Calcd. for C22H23N3O6: C, 62.11; H, 5.45; N, 9.88. Found: C, 61.37; H, 5.67; N, 10.04.
Methyl 6-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)hexanoate (4h). White powder,
mp: 180–182 °C, yield 83%. 1H-NMR (DMSO-d6)
δ
(ppm): 1.32–1.36 (m, 2H,
NHCH2CH2CH2CH2CH2CO), 1.49–1.56 (m, 4H, NHCH2CH2CH2CH2CH2CO), 2.00 (s, 3H, COCH3),
2.31 (t, J = 7.32 Hz, 2H, NHCH2CH2CH2CH2CH2CO), 3.25 (t, J = 5.88 Hz, 2H, NHCH2CH2CH2 CH2
CH2CO), 3.63 (s, 3H, COOCH3), 7.28 (t, J = 5.2 Hz, 1H), 7.64–7.85 (m, 7H, Ar), 8.38 (brs, 1H, NH),
10.55 (s, 1H, NH), 10.88 (s, 1H, NH); 13C-NMR (DMSO-d6) δ (ppm): 24.76 (COCH3), 26.5
(NHCH2CH2CH2CH2CH2CO), 29.4 (NHCH2CH2CH2CH2CH2CO, 33.8 (NHCH2CH2CH2CH2CH2CO),
41.5 (NHCH2CH2CH2CH2CH2CO), 51.7 (COOCH3), 119.9, 122.4, 124.4, 126.5, 128.5, 130.8, 131.6,
134.3, 138.2, 140.9, 162.3 (CONH), 166.0 (CONH), 169.5 (CONH), 173.9 (COOCH3), 189.7(COCO).
IR (cm−1): 3290 (NH), 1738, 1664, 1637, 1608, 1528 (C=O). Anal. Calcd. for C24H27N3O6: C, 63.56;
H, 6.00; N, 9.27. Found: C, 63.38; H, 6.13; N, 9.53.
Methyl 1-(4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzamido)cyclopentanecarboxylate (4i). White
1
powder, mp: 238–240 °C, yield 78%. H-NMR (DMSO-d6) δ (ppm): 1.69–1.71 (m, 4H,
CH2CH2CH2CH2), 1.99 (s, 3H, COCH3), 2.00–2.13 (m, 4H, CH2CH2CH2CH2), 3.58 (s, 3H, COOCH3),
7.28 (t, J = 6.16 Hz, 1H,), 7.63–7.87 (m, 7H, Ar), 8.50(s, 1H, NH), 10.55(s, 1H, NH), 10.90 (s, 1H, NH);
13C-NMR (DMSO-d6) δ (ppm): 24.1 (CH2CH2CH2CH2), 24.7 (COCH3), 39.8(CH2CH2CH2CH2), 52.5
(COOCH3), 66.2 (C), 119.8, 122.5, 124.5, 125.9, 129.0, 130.0, 131.5, 134.2, 138.0, 141.3, 162.2
(CONH), 166.4 (CONH), 169.5 (CONH), 174.9 (COOCH3), 189.5 (COCO). IR (cm−1): 3291, 3114
(NH), 1746, 1679, 1633, 1607, 1524 (C=O). Anal. Calcd. for C24H25N3O6 (451.47): C, 63.85; H, 5.58;
N, 9.31. Found: C, 64.06; H, 5.65; N, 9.04.
4. Conclusions
In conclusion, we have demonstrated that reaction of N-acetylisatin (1) with 4-aminobenzoic
acid (2), either using conventional heating or microwave irradiation affords the α-ketoamide
4-(2-(2-acetamidophenyl)-2-oxoacetamido)benzoic acid (3) in good yield. All the spectral data proved
the ring opening structure in which the 4-aminobenzoic acid attacks C2 rather than C3. Reaction of the
4-aminobenzoic acid derivative 3 with different amino acid esters using Oxyma/DIC afforded a novel
series of 4-[2-(2-acetylaminophenyl)-2-oxo-acetylamino]benzoyl amino acid esters 4a–i. OxymaPure/DIC
showed clear superiority to HOBt/DIC and carbodiimide alone in terms of yield and purity.

Molecules 2013, 18
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Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/18/12/14747/s1.
Acknowledgments
The authors thank the Deanship of Scientific Research at King Saud University for funding this
work through research group “RGP-VPP-234”. The work in Barcelona was partially supported by
CICYT (CTQ2012-30930) and the Generalitat de Catalunya (2009SGR 1024).
Conflicts of interest
The authors declare no conflict of interest.
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Sample Availability: Samples of compounds are available from authors.
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