An unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic acid in the synthesis of new dibenzyltetrahydroquinoxalinediones

Article abstract of DOI:10.1016/j.tet.2014.04.077

Chemical and electrochemical oxidation of different catechols were carried out in the presence of N,N′-dibenzylethylenediamine (DBEDA) in a phosphate buffer/acetonitrile solution for the synthesis of different new dibenzyltetrahydroquinoxalinedione derivatives. The oxidation of catechol (1a), 2,3-dihydroxybenzoic acid (1e), and 3,4-dihydroxybenzoic acid (1d) led to the same product, probably due to the decarboxylation reaction of intermediates. An oxidative decarboxylation reaction of 3,4-dihydroxybenzoic acid (1d) has been reported before, while an unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic acid (1e) in the presence of DBEDA is reported for the first time.

Full text of DOI:10.1016/j.tet.2014.04.077

Accepted Manuscript
An unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic acid in the
synthesis of new dibenzyltetrahydroquinoxalinediones
Davood Habibi, Davood Nematollahi, Sahereh Meshkinghalam, Fahimeh Varmaghani
PII:
S0040-4020(14)00611-5
10.1016/j.tet.2014.04.077
TET 25527
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 27 January 2014
Revised Date: 9 April 2014
Accepted Date: 25 April 2014
Please cite this article as: Habibi D, Nematollahi D, Meshkinghalam S, Varmaghani F, An
unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic acid in the synthesis of new
dibenzyltetrahydroquinoxalinediones, Tetrahedron (2014), doi: 10.1016/j.tet.2014.04.077.
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ACCEPTED MANUSCRIPT
An unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic
acid in the synthesis of new dibenzyltetrahydroquinoxalinediones
Davood Habibi*, Davood Nematollahi, Sahereh Meshkinghalam and Fahimeh Varmaghani
Department of Organic Chemistry, Faculty of Chemistry, Bu Ali Sina University, Hamedan, 6517838683, Iran,
Email: davood.habibi@gmail.com
Abstract: Chemical and electrochemical oxidation of different catechols were caried out in the presence of
N,N'-dibenzylethylenediamine (DBEDA) in a phosphate buffer/acetonitrile solution for the synthesis of different
new dibenzyltetrahydroquinoxalinedione derivatives. The oxidation of catechol (1a), 2,3-dihydroxybenzoic acid
(1e) and 3,4-dihydroxybenzoic acid (1d) led to the same product, probably due to the decarboxylation reaction
of intermediates. An oxidative decarboxylation reaction of 3,4-dihydroxybenzoic acid (1d) has been reported
before, while an unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic acid (1e) in the
presence of DBEDA is reported for the first time.
Keywords: Catechols; N,N’-Dibenzylethylenediamine; Dibenzyltetrahydroquinoxalinediones; Chemical and
electrochemical synthesis; K3Fe(CN)6
1. Introduction
A vast number of quinones with great structural divergence are provided by nature which some of them play
a major role in the redox electron-transport chains of living systems.1-4 Large libraries of quinones, substituted
quinones, bisquinones and polyquinones were synthesized during the last century and their properties studied.5
The aminoquinone derivatives are obtained from the reactions between quinones and the ordinary amines
(primary and secondary). When the steric hindrance is absent, two molecules of the amine add to the quinone,
while only one molecule will usually be added to non-hindered quinones.6
Quinoxalinediones have been used extensively as pharmaceutical agents for treatment of different diseases.7-
10
Also, they have affinities to the quisqualate receptors and are suitable as pharmaceutical agents for treatment
of diseases of the central nervous system.11 In addition, they can be used for treatment of neurological and
psychiatric disorders that are triggered by the over stimulation of the AMPA receptors.12
In continuation of our studies on the synthesis of nitrogen- and oxygen-containing heterocycles and
application of different catalysts in the organic syntheses,13-16 we would like to report the synthesis of the new
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dibenzylquinoxalinedione derivatives from the reaction of various catechols with DBEDA as a nucleophile by
both chemical and electrochemical methods (Scheme 1).
Ph
Ph
Ph
Ph
R1
R1
Electrochemical
or Chemical
R2
OH
OH
O
O
N
N
NH
NH
+
-6H+, -6e-
1a-e
6a-c
1a: R1 = H, R2 = H
6a: R1 = H
1b: R1 = CH3, R2 = H
6b: R1 = CH3
1c: R1 = OCH3, R2 = H
1d: R1 = H, R2 = COOH
1e: R1 = COOH, R2 = H
6c: R1 = OCH3
Scheme 1. Synthesis of different dibenzyltetrahydroquinoxalinedione derivatives
Also, we have shown three pathways for the synthesis of 1,4-dibenzyl-1,2,3,4-tetrahydroquinoxaline-6,7-
dione (6a). The results of these studies as well as the full characterization of the obtained products are presented
and discussed.
2. Results and Discussion
2.1. Electrochemical oxidation of catechol (1a), 3-methylcatechol (1b) and 3-methoxycatechol (1c) in the
presence of DBEDA:
A cyclic voltammetry (CV) of 1a (1.0 mM) in a mixture of phosphate buffer solution (PBS) (pH = 7.0,
0.15 M) and CH3CN (70:30 v/v) shows one anodic peak (A1) at 0.27 V and the corresponding cathodic peak
(C1) at 0.01 V versus SCE which correspond to the transformation of 1a to -benzoquinone (2a) and vice versa
c =
o
within a quasi-reversible two-electron process (Figure 1, curve a).13 The peak–current ratio (IpC1/IpA1) is near
unity, which can be considered as a criterion for the stability of 2a produced at the surface of the electrode under
the experimental conditions. This indicates that any hydroxylation17 or dimerization18 reactions are too slow to
be observed on the time scale of CV.13 The oxidation of 1a in the presence of DBEDA (1.0 mM) was studied in
some details. Figure 1, II shows the first CV obtained for 1a (1.0 mM) in the presence of DBEDA. The CV
exhibits a new cathodic peak C0 with
with an p value of − 0.31 V versus SCE. These new redox peaks (A0 and C0) are related to the electrooxidation
of intermediate 5a to -benzoquinone 6a and vice-versa. In this Figure, curve b is the voltammogram of
Ep = − 0.34 V versus SCE. In the second cycle, a new peak (A0) appears
E
p
DBEDA which shows the nucleophile is not electroacive in the studied potential range.
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Figure 1. (I): CVs of 1.0 mM (a): 1a and (b): DBEDA, (II) first and second cycle CVs of 1a (1.0 mM) in the presence of DBEDA (1.0 mM)
at a glassy carbon electrode in a mixture of PBS (pH = 7, c = 0.15 M) and CH3CN (70:30 v/v). Scan rate: 50 mVs−1, T = 25 ± 1 °C.
Furthermore, it is seen that proportional to the augmentation of the potential sweep rate and in parallel with
decreasing in the height of C0, the height of C1 increases (Figure 2). The subsequent chemical reactions between
o-benzoquinone (2a) and DBEDA is supported by the following evidence:
•
•
•
Appearance of an anodic peak A0 and its cathodic counterpart (C0) in less positive potentials that show
the formation of an electroactive species;
Decreasing of IpC1 during the reverse scan. This could be indicative of the fact that electrochemically
generated o-benzoquinone (2a) is partially removed by the chemical reaction with DBEDA;
Variation of peak current ratios (IpA1 /IpC1) and (IpC0/IpC1) versus scan rate for a mixture of 1a and
DBEDA which appear as a decrease in the IpA1/IpC1 and IpC0/IpC1 ratios at higher scan rates.
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Figure 2. Typical CVs of 1a (1.0 mM) in the presence of DBEDA (1.0 mM) at various scan rates. Scan rates from a to d are: 25, 50, 100
and 250 mVs−1, respectively at a glassy carbon electrode in a mixture of PBS (pH = 7, c = 0.15 M) and CH3CN (70:30 v/v). Inset: Variation
of peak current ratios (IpA1 /IpC1) and (IpC0/IpC1) versus scan rate.
Constant-current coulometry was performed in a mixture of PBS (pH = 7, c = 0.15 M) and CH3CN (70/30
v/v) containing 1a (0.25 mmol) and DBEDA (0.25 mmol) in an undivided cell under a low constant current
density (1 mA/cm2). Monitoring of the electrolysis progress was carried out by CV (Figure 3). It is shown that
proportional to the advancement of coulometry and in parallels with decreasing the height of the anodic peak
A1, the heights of A0 and C0 increase. A characteristic feature of electrolysis is that a low-current density is
required.
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I
II
Figure 3. (I) CVs of 1a (0.25 mmol) in the presence of DBEDA (0.25 mmol) in a mixture of PBS (pH = 7, c = 0.15 M) and CH3CN (70:30
v/v) at a glassy carbon electrode during constant-current coulometry. Scan rate 50 mVs−1. (II) Variation of peak current IpA1 vs. charge
consumed during constant-current coulometry.
Controlled-potential coulometry also performed in an undivided cell in the above mentioned condition. It is
shown that proportional to the advancement of coulometry, anodic peak A1 and its counterpart (C1) decrease and
disappear when the charge consumption was about 6e− per molecule of 1a. These observations allow us to
propose the ECECE pathway for the electrooxidation of 1a in the presence of DBEDA (Scheme 2).
It should be noted that the electrooxidation of 1b or 1c in the presence of DBEDA occurs in a similar
manner to that of 1a.
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R
R
O
OH
OH
R = H
R = CH3 1b-6b
R = OCH3 1c-6c
1a-6a
E (1)
ꢀ2eꢀ, ꢀ2H+
O
1a-c
2a-c
Ph
Ph
R
NH
NH
N
O
O
O
C (2)
+
NH
OH
R
Ph
Ph
Ph
Ph
2a-c
N
OH
N
O
(3)
E (4)
C (5)
NH
OH
NH
OH
H
R
R
R
R
Ph
Ph
3a-c
Ph
Ph
NH
N
O
O
NH
N
OH
OH
ꢀ2eꢀ, ꢀ2H+
4a-c
3a-c
Ph
Ph
Ph
Ph
R
R
N
N
OH
O
NH
N
O
O
4a-c
Ph
Ph
Ph
Ph
R
R
H
N
N
OH
OH
NH
OH
O
(6)
N
5a-c
Ph
Ph
Ph
Ph
R
R
N
N
O
O
N
N
OH
E (7)
ꢀ2eꢀ, ꢀ2H+
OH
6a-c
5a-c
Ph
Ph
Scheme 2. Proposed mechanism for the electrochemical oxidation of 1a-c in the presence of DBEDA.
Formation of 2a–c was followed by a 1,4-Michael addition of DBEDA to the quinone to produce the adducts
3a–c. These adducts then undergo the abstraction of a second pair of electrons leading to o-benzoquinone 4a–c.
Intramolecular addition produces catechol derivatives 5a–c and further oxidation of these compounds led to
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formation of the final product 6a–c. Oxidation of intermediates 3a–c and 5a–c is easier than oxidation of the
parent starting molecules 1a–c by virtue of the presence of the electron rich amino groups on the quinone ring.
Besides, it is possible that the oxidation of 3a–c and 5a–c take place through a solution electron transfer reaction
(Scheme 3).13
Ph
Ph
R
R
N
OH
OH
N
O
O
O
O
OH
OH
+
+
NH
NH
R
3a-c
R
4a-c
Ph
Ph
Ph
Ph
2a-c
1a-c
R
R
O
O
N
N
O
N
N
OH
OH
OH
OH
+
+
O
R
5a-c
R
Ph
Ph
Ph
2a-c
1a-c
6a-c
Ph
Ph
Ph
Ph
N
N
O
O
N
N
OH
OH
N
O
O
N
OH
OH
+
+
NH
R
4a-c
NH
R
R
5a-c
R
3a-c
Ph
Ph
Ph
6a-c
Scheme 3. Solution electron transfer reaction.
According to our results, the anodic peaks of the voltammograms presented in Figure 1 (A1 and A0) are
related to the oxidation of 1a and 1,4-dibenzyl-1,2,3,4-tetrahydroquinoxaline-6,7-diol (5a) to o-benzoquinone
(2a) and 1,4-dibenzyl-1,2,3,4-tetrahydroquinoxaline-6,7-dione (6a), respectively. Obviously, the cathodic peaks
C1 and C0 shown in Figure 1 can also correspond to the reduction of 2a and 6a, respectively. Due to the fast
intramolecular Michael addition reaction (Scheme 2, Eq. 4), the anodic and cathodic counterpart peak for the
oxidation of 3a–c were not observed.
2.2. Electrochemical oxidation of 3,4-dihydroxybenzoic acid (1d) in the presence of DBEDA
In order to study the effect of the presence of a group in a reactive site of catechol ring, the electrochemical
oxidation of 1d has been studied in the presence of DBEDA. A first and second cycle CVs of 1d (1.0 mM) in
the presence of DBEDA (1.0 mM) in a mixture of PBS (pH = 7, c = 0.15 M) and CH3CN (70:30 v/v) is shown
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in Figure 4. The CVs exhibit two cathodic peaks C1 and C0 with two anodic counterparts A1 and A0,
respectively. Also, other voltammetry and coulometry data are as same as the previous cases.
According to the obtained electrochemical data in accompany with the spectroscopic data of final product,
an ECECE mechanism is proposed with a decarboxylation reaction for the electrooxidation of 1d in the
presence of DBEDA.
Figure 4. First and second cycle CVs of 1d (left, 1.0 mM) and 1e (right, 1.0 mM) in the presence of DBEDA (1.0 mM) at a glassy carbon
electrode in a mixture of PBS (pH = 7, c = 0.15 M) and CH3CN (70:30 v/v). Scan rate: 50 mVs−1; T = 25 ± 1 °C.
2.3. Electrochemical oxidation of 2,3-dihydroxybenzoic acid (1e) in the presence of DBEDA
The CVs of 1e in the presence of DBEDA was shown in Figure 4. Comparison of the first and second cycle
CVs with that of other studied catechol derivatives represent that the electrochemical oxidation of 1e in the
presence of DBEDA behaves in the same manner by appearing the new redox peaks (A0 and C0).
Also, the spectroscopic data indicates that the electrochemically generated o-quinones 2a, 2d and 2e
participate in the Michael addition reaction with DBEDA via the ECECE mechanism to produce 6a; the
decarboxylation reaction occurs during the oxidation of 2,3-dihydroxybenzoic acid (1e) in the presence of
DBEDA (Scheme 4.).
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O
OH
(1)
(2)
2e
3e
1e
ꢀ2eꢀ ꢀ2H+
O
OH
ꢀ
ꢀ
CO2
CO2
Ph
Ph
NH
NH
O
O
NH
N
O
+
OH
ꢀ
ꢀ
CO2
CO2
Ph
Ph
Ph
Ph
NH
N
O
NH
N
OH
OH
(3)
(4)
4e
5e
OH
H
ꢀ
ꢀ
CO2
CO2
Ph
Ph
Ph
Ph
NH
N
OH
OH
NH
N
O
O
ꢀ
ꢀ2eꢀ ꢀ2H+
ꢀ
CO2
CO2
Ph
Ph
Ph
Ph
NH
N
O
O
N
N
OH
O
6e
7e
(5)
(6)
ꢀ
CO2
ꢀ
CO2
Ph
Ph
Ph
Ph
H
N
N
OH
O
N
N
OH
OH
ꢀ
ꢀ
CO2
CO2
Ph
Ph
Ph
Ph
N
N
OH
OH
N
N
OH
OH
H+
(7)
(8)
ꢀCO2
C
O
ꢀO
Ph
Ph
Ph
Ph
N
N
OH
OH
N
N
O
O
6a
ꢀ2eꢀ ꢀ2H+
Ph
Ph
Scheme 4. Proposed mechanism for the oxidation of 1e in the presence of DBEDA.
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As shown in Scheme 5, three pathways can be presented for the synthesis of 6a. Unlike 3,4-dihydroxy-
benzoic acid (1d), the decarboxylation reaction of 2,3-dihydroxybenzoic acid (1e) in the presence of nucleophile
has not been reported so far.
2.4. Chemical oxidation of catechol derivatives (1a-e) in the presence of DBEDA
In chemical oxidation, a suitable oxidizing agent is one that only oxidizes catechols and intermediates
without any side effects on DBEDA. Potassium ferricyanide is a stable, easily handled and commercially
available oxidizing agent. The oxidation potential of potassium ferricyanide is close to catechols. When
potassium ferricyanide was treated with catechols (1a-e) in the presence of DBEDA, dibenzyltetrahydro-
quinoxalinediones (6a-c) were obtained in good yields.
Conclusion
Different catechols (1a-e) were oxidized in the mixture of PBS and CH3CN solution to their o-
benzoquinones by chemical or electrochemical methods and attacked by DBEDA to form the corresponding
dibenzyl- tetrahydroquinoxalinedione derivatives (6a-c). The overall reaction mechanisms for anodic oxidation
of these derivatives in the presence of DBEDA as a nucleophile are presented in Schemes 2 and 4. An
unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic acid (1e) in the presence of DBEDA
was shown in Scheme 4. The paper introduced three pathways for the synthesis of 1,2,3,4-tetrahydro-
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quinoxaline-6,7-dione (6a). In addition, from the green chemistry point of view, application of the electro-
synthetic method has some important advantages. Clean synthesis, use of electricity as an alternative source of
energy instead of oxidative reagents, one-step reaction, working in room temperature, technical feasibility and
high atom economy are of prominent green advantages. Both chemical and electrochemical methods give the
same products. While the chemical synthesis is faster, the electrochemical synthesis provides higher yields.
Also, the M+ peaks of nitrogen- or oxygen-containing compounds usually show some extra mass units which
are more than the exact molecular masses. It usually depends on the number of the nitrogen or oxygen in the
molecule and is related to protonation of the quinonic moiety or/and amino group.6
3. EXPERIMENTAL
3.1. Apparatus and reagents
CV and controlled-potential coulometry were performed using an Auto lab model PGSTAT 20
potentiostat/galvanostat. A glassy carbon electrode, 1.8 mm2 areas was used as working electrode and platinum
wire was used as the counter electrode. The potential of working electrode was measured versus SCE (all
electrodes from Azar Electrode). Macroscale electrolyses were carried out in an undivided cell using the
assembly of four graphite rods (0.6 × 6 cm, total area 31 cm2 and a large platinum gauze cylinder (25 cm2 area)
constituted the counter electrode.
The peak current ratios (IpC1/IpA1) were determined using the following equation:19
Ipc/Ipa = (Ipc)0/Ipa + 0.485(Isp)0/Ipa + 0.086
(Ipc)0 and (Isp)0 are cathodic peak current and switching potential current respect to the zero current,
respectively. Ipc and Ipa have their usual meanings.
3.2. Chemical synthesis of 6a-c
3.2.1. Chemical synthesis of 1,4-dibenzyl-1,2,3,4-tetrahydroquinoxaline-6,7-dione (6a)
To a stirred PBS (80 mL, pH = 7, c = 0.15) and CH3CN (70:30 v/v), were added K3Fe(CN)6 (6.0 mM) and
DBEDA (1.0 mM). A solution of 1a (1.0 mM) dissolved in PBS/CH3CN (70:30, 10 mL) was dropwise added
during the period of 15 min which the solution became dark and precipitated. At the end of reaction, the product
was extracted with CH2Cl2 (2 × 25 mL) and the solvent evaporated. The product was purified by
recrystallization from n-hexane/acetone mixture (20:80) and characterized by IR, 1H NMR, 13C NMR and MS.
The residue was not dissolved in any solvent, which could be attributed to the oligomers or polymers of
catechols. The same method as above was applied for the synthesis of 6b-c.
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3.3. Electrochemical synthesis of dibenzyltetrahydroquinoxalinediones (6a–c)
A mixture of PBS (80 mL, pH = 7.0, c = 0.15 M) and CH3CN (70/30 v/v) containing 1.0 mmol of catechols
(1a–e) and DBEDA (1.0 mmol) was electrolyzed at 25 °C under constant-current density of 1 mA/cm2. Progress
the reaction was followed by TLC. The process was interrupted during the electrolysis and the graphite anode
was washed in acetone in order to reactivate it. At the end of electrolysis (~ 20 h), the product was extracted
with CH2Cl2 (2 × 25 mL) and the solvent evaporated. The product was purified by recrystallization from n-
hexane/acetone mixture (20:80) and characterized by IR, 1H NMR, 13C NMR and MS.
3.4. Characterisation of products (6a-c)
3.4.1 1,4-Dibenzyl-1,2,3,4-tetrahydroquinoxaline-6,7-dione (6a)
Isolated yields 98 % (electrochemical) and 36 % (chemical synthesis); IR(KBr) νmax = 3050 (=C-H), 3025 (-C-
H), 1597 (C=O), 1494, 1451 (CH2 bending), 1415, 1391, 1361, 1299, 1244 (C=O bending), 1191, 1130, 1090,
1
1076, 1027, 960, 916 (C-N), 886, 841, 786, 747, 736, 702 and 597 cm-1; H NMR (90 MHz, CDCl3): δ = 3.48
(s, 4H), 4.42 (s, 4H methylene), 5.44 (s, 2H quinone) and 7.24 (s, 10H benzyl); 13C NMR (22.5 MHz, CDC13): δ
= 46.0 (2 C of CH2 in piperazine ring), 55.2 (2 C of benzyl CH2), 98.6 (2 C of CH of quinone ring), 126.1, 127.2
and 128.2 (10 C of CH of phenyl aromatic rings), 133.2 (2 C of phenyl aromatic rings), 148.7 (2 C of fused
rings) and 178.0 (2 C of carbonyl groups); MS: m/z (relative intensity): 346 [M+2H] (100), 329 (64), 251 (100),
163 (95), 91 (97), 65 (96).6
It should be noted that the chemical and electrochemical oxidation of 1a, 1d and 1e in the presence of
DBEDA led to the same product (6a).
3.4.2 1,4-Dibenzyl-5-methyl-1,2,3,4-tetrahydroquinoxaline-6,7-dione (6b)
Isolated yields 98 % (electrochemical) and 42 % (chemical synthesis); IR(KBr): νmax = 3026 (=C-H), 2879 (-C-
H), 1600 (C=O), 1543, 1494, 1476, 1449 (CH2 bending), 1349, 1320, 1234 (C=O bending), 1155, 1095, 1074,
1
1026, 950, 916 (C-N), 823, 759, 729 and 696 cm-1; H NMR (90 MHz, CDC13): δ = 1.90 (s, 3H methyl), 3.33
(s, 4H methylene), 4.52 (s, 4H, benzylic CH2), 5.37 (s, 1H quinone) and 7.26 (s, 10H benzene ring); 13C NMR
(22.5 MHz, CDC13): δ = 13.2 (C of methyl), 46.1 (2 C of CH2 in piperazine ring), 55.7 (C of benzyl CH2, near
to methyl), 58.0 (C of benzyl CH2, far to methyl), 95.5 (ipso carbon in quinone ring), 111.6 (C of CH in quinone
ring), 126.6, 127.7 and 128.8 (10 C of CHs of phenyl aromatic rings), 134.2 (C of phenyl aromatic ring, near to
methyl), 136.0 (C of phenyl aromatic ring, far to methyl), 149.5 (C of fused ring, near to methyl), 154.5 (C of
fused ring, far to methyl), 177.5 (C of carbonyl group, near to methyl) and 180.8 (C of carbonyl group, far to
methyl); MS: m/z (relative intensity): 343 [M+2H] (50), 269 (100), 177 (98), 91 (97).6
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3.4.3 1,4-Dibenzyl-5-methoxy-1,2,3,4-tetrahydroquinoxaline-6,7-dione (6c)
Isolated yields 66 % (electrochemical) and 28 % (chemical synthesis); IR(KBr): νmax = 3062 (=C-H), 3027,
2982 (-C-H), 2973, 1711 (C=O), 1601, 1528, 1495, 1476, 1450 (CH2 bending), 1422, 1361, 1234 (C=O
bending), 1156, 1088, 1027 (C-O-C), 952 (C-N), 848, 727, 693 and 590 cm-1; 1H NMR (90 MHz, CDC13): δ =
3.52 (s, 3H methoxy), 4.45 (s, 2H methylene), 4.82 (s, 2H methylene), 5.32 (s, 1H quinone) and 7.23 (s, 10H
benzyl); 13C NMR (22.5 MHz, CDCl3): δ = 46.8 (C of methoxy), 55.9 (2 C of CH2 in piperazine ring) , 57.2 (C
of benzyl CH2, far to methoxy), 59.6 (C of benzyl CH2, near to methoxy), 95.1 (C of CH in quinone ring),
126.1, 126.4, 126.8, 127.2, 128.1 and 128.3 (10 C of CHs of phenyl aromatic rings), 133.7 (ipso carbon in
quinone ring), 136.2 (2 C of phenyl aromatic rings), 139.5 (C of fused ring, near to methoxy), 152.0 (C of fused
ring, far to methoxy), 174.8 (C of carbonyl group, near to methoxy) and 176.4 (C of carbonyl group, far to
methoxy); MS: m/z (relative intensity): 376 [M+2H] (100), 359 (100), 285 (100), 267 (66), 251 (50), 221 (33),
191 (100), 91 (100), 65 (100).6
Acknowledgments
The authors gratefully acknowledge the financial supports from the Bu-Ali Sina University, Hamedan
6517838683, Iran.
References
1. Thompson, R. H. Naturally Occurring Quinones; 4th ed.; Academic: London, 1997.
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Graphical Abstract
An unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic
acid in the synthesis of new dibenzyltetrahydroquinoxalinediones
Davood Habibi*, Davood Nematollahi, Sahereh Meshkinghalam and Fahimeh Varmaghani
Department of Organic Chemistry, Faculty of Chemistry, Bu Ali Sina University, Hamedan, 6517838683, Iran,
Email: davood.habibi@gmail.com
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