Synthesis of 2-methyl-1-[(5-methylfuran-2-yl)methyl]- and 2-methyl-1-[(5-methylpyrrol-2-yl)methyl]-1H-benzimidazoles

Article abstract of DOI:10.1007/s10593-013-1374-2

A method for the synthesis of 2-methyl-1-[(5-methylfuran-2-yl)methyl]-1H- benzimidazoles based on the intramolecular cyclization of vicinal N-[(5-methylfuran-2-yl)methyl]aminoanilides has been developed. A study was carried out on the protolytic opening o

Full text of DOI:10.1007/s10593-013-1374-2

Chemistry of Heterocyclic Compounds, Vol. 49, No. 9, December, 2013 (Russian Original Vol. 49, No. 9, September, 2013)
SYNTHESIS OF 2-METHYL-1-[(5-METHYLFURAN-2-YL)-
METHYL]- AND 2-METHYL-1-[(5-METHYLPYRROL-2-YL)-
METHYL]-1H-BENZIMIDAZOLES
T. A. Stroganova1*, V. M. Red'kin1, G. A. Kovalenko1,
V. K. Vasilin1, and G. D. Krapivin1
A method for the synthesis of 2-methyl-1-[(5-methylfuran-2-yl)methyl]-1H-benzimidazoles based on the
intramolecular cyclization of vicinal N-[(5-methylfuran-2-yl)methyl]aminoanilides has been developed.
A study was carried out on the protolytic opening of the furan ring leading to the formation of a
diketone fragment, which was then used for the formation of N-substituted pyrrole ring by the Paal–
Knorr method. The effect of the nature of the amine on the cyclization was demonstrated.
Keywords: 1,2-disubstituted benzimidazoles, furan, N-substituted pyrroles, cyclization, ring opening.
Benzimidazole derivatives have long held interest in medicinal chemistry. Some of these compounds
display antihistaminic [1] and antibacterial activity [1, 2] and may act as selective antagonists of the
neuropeptide YY1 receptor [3] and hCRTh2 receptors as well as factor Xa inhibitor [5]. An important feature of
benzimidazole derivatives is their pronounced activity toward the human immunodeficiency virus (HIV),
viruses responsible for herpes and influenza, the human cytomegalovirus [1, 6-12], and virus which causes
hepatitis B [13]. Some benzimidazoles display cytostatic, local anesthetic, hypotensive, and antipyretic activity
[1, 14].
1,2-Disubstituted benzimidazoles are quite significant in this compound group. The presently available
methods for the synthesis of these compounds fall into the following groups: 1) N-alkylation of the prepared
2-substituted benzimidazole system in the presence of strong base [8, 9, 15, 16], 2) N-alkylation of ortho-
nitroanilides with the subsequent reductive cyclization [9, 17], and 3) cyclocondensation of N-substituted ortho-
aminoanilides [18]. 1,2-Disubstituted benzimidazoles may also be obtained by the condensation of N-substituted
ortho-phenylenediamines with aldehydes in the presence of various acid catalysts [19-27].
We became interested in benzimidazoles containing a furfuryl fragment at position 1. Many methods
have been reported for the synthesis of 1-(furan-2-yl)methylbenzimidazoles: 1) reaction of furfural with ortho-
phenylenediamine using various catalysts [28-36], 2) reaction of furfural with ortho-nitroaniline [37], and
3) N-alkylation of the prepared benzimidazole system with furfuryl alcohol [38].
However, the furylbenzimidazoles described in the literature do not include compounds with the
5-methylfurfuryl fragment at position 1. Interest in the synthesis of such structures is related to the ability of
_______
*To whom correspondence should be addressed, e-mail: tatka_s@mail.ru.
1Kuban State Technological University, 2 Moskovskaya St., Krasnodar 350072, Russia.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1356-1366, September, 2013.
Original article submitted June 22, 2013; revision submitted July 14, 2013.
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0009-3122/13/4909-1264©2013 Springer Science+Business Media New York

dialkylfuran to undergo recyclization to give other heterocycles [39, 40], which allows the introduction of
heterocyclic fragments at position 1 of the benzimidazole system. The introduction of such fragments at this site
is difficult by other methods.
In our opinion, the most suitable method for the synthesis of 1-(5-methylfurfuryl)benzimidazoles is the
acid-catalyzed intramolecular cyclization of vicinal N-furfurylaminoanilides similar to the approach reported by
Beaulieu et al. [27]. In the present work, results are given on the synthesis of 2-methyl-1-[(5-methylfuran-2-yl)-
methyl]-1H-benzimidazoles and a study of the acid-catalyzed opening of the furan fragment and its cyclization
to give a pyrrole ring.
The following reaction sequence was used to obtain 1-[(5-methylfuran-2-yl)methyl]benzimidazoles 6
from the corresponding ortho-nitroanilines 1.
O
NH2
NO2
NHAc
NH2
NHAc
NO2
Ac2O
Me
N2H4
O
PhH, H+
, 4–8 h
Ni (Raney)
R
R
R
3a–c
1a–c
2a–c
NHAc
NHAc
NaBH4, EtOH
HCl, EtOH
dioxane
50–55°C
1–3 h
60°C
7–12 h
R
N
R
N
H
O
O
Me
Me
4a–c
5a–c
3
4
3a
N
N
N
5
6
2
O
Me
O
Me
O
HCl, EtOH
, 6–20 h
1
N
7a
7
Me
R
R
6a–c
7a–c
Me
a R = H, b R = Mе, c R = ОMе
Azomethines 4a-c, obtained by the condensation of ortho-aminoanilides 3a-c with 5-methylfurfural,
smoothly undergo reduction of the C=N bond by the action of NaBH4 to give N-furfurylamines 5a-c in yields
1
from 67 to 77% (Table 1). The H NMR spectra of amines 5a-c show signals for the protons of the methylene
group adjacent to the amino group as doublets with coupling constants 5.0-5.1 Hz and the signal for the amino
group proton as a triplet with the same coupling constant at  5.27-5.41 ppm (Table 2). The signal for the proton
of the amide group is found at 8.97-9.14 ppm. The 13C NMR spectra show a characteristic signal for the carbon
atom of the amide carbonyl group at 168.6-168.8 ppm (Table 2).
The intramolecular cyclization of amines 5a-c leading to benzimidazoles proceeds at 60°C in ethanol
solution saturated with dry gaseous hydrogen chloride (10% concentration). The yields of 1-furfuryl-
benzimidazoles 6a-c were 84-89% (Table 1). In the contrast to aminoanilides 5a-c, the IR spectra of
furfurylbenzimidazoles 6a-c (Table 2) lack absorption bands for the stretching vibrations of the secondary
amino group at 3369-3372 cm-1, amide group at 3223-3240 cm-1 (NH) and 1641-1649 cm-1 (C=O). Evidence for
the cyclization lies in the disappearance of the signals for the protons of the amide and amino groups in the
1H NMR spectra and change in the multiplicity of the signals of the methylene group protons, which appear as a
singlet with 2H intensity at 5.31-5.37 ppm, and the lack of a signal for the carbon atom of the amide carbonyl
group in the 13C NMR spectra of benzimidazoles 6a-c (Table 2).
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TABLE 1. Physicochemical Characteristics of Compounds 5-7 a-c and 9a-g
Found, %
——————
Calculated, %
Com-
pound
Empirical
formula
Mp, °С
Yield, %
C
H
N
5a
5b
5c
6a
6b
6c
7a
7b
7c
9a
9b
9c
9d
9e
9f
C14H16N2O2
68.85
68.83
69.69
69.74
65.62
65.68
74.36
74.31
75.01
74.97
70.24
70.29
68.78
68.83
69.79
69.74
65.61
65.68
75.24
75.21
71.56
71.62
76.55
76.49
75.59
75.65
73.04
73.11
76.00
75.92
80.15
80.21
6.54
6.60
7.00
7.02
6.53
6.61
6.29
6.24
6.66
6.71
6.24
6.29
6.57
6.60
6.98
7.02
6.63
6.61
6.59
6.63
6.33
6.31
6.66
6.71
6.08
6.05
6.43
6.41
6.35
6.37
7.07
7.04
11.41
11.47
10.76
10.84
10.13
10.21
12.29
12.38
11.59
11.66
11.01
10.93
11.53
11.47
10.80
10.84
10.15
10.21
13.11
13.16
12.46
12.53
12.12
12.16
12.53
12.60
11.70
11.63
17.62
17.71
12.69
12.76
91-92
97-99
67
73
77
84
88
89
59
67
64
61
64
71
72
63
83
73
C15H18N2O2
C15H18N2O3
C14H14N2O
C15H16N2O
C15H16N2O2
C14H16N2O2
C15H18N2O2
C15H18N2O3
C20H21N3O
C20H21N3O2
C22H23N3O
C21H20FN3
C22H23N3O2
C20H20N4
101-102
93-94
94-95
97-98
65-67
103-104
141-142
113-114
135-136
127-128
69-70
87-88
112-113
85-86
9g
C22H23N3
Protolytic opening of the furan ring to give the corresponding diketones 7a-c occurs upon heating of
furfurylbenzimidazoles 6a-c in 20% ethanolic hydrogen chloride solution at reflux for 6-20 h (Tables 1 and 2).
We would like to note that the opening of the furan ring and formation of a diketone is an equilibrium process.
This behavior was already reported in our previous study on the recyclization of furylmethane derivatives of
3-aminothienyl[2,3-b]pyridine [41]. The desired diketones 7a-c were isolated from the reaction mixture by
column chromatography.
The IR spectra of diketones 7a-c contain strong carbonyl group absorption bands at 1697-1705 cm-1
(Table 2), while the 13C NMR spectra of these compounds show signals for the carbon atoms of the two ketone
groups at 203.1-203.8 and 206.4-207.2 ppm. The 1H NMR spectra lack signals for furan ring protons but show a
multiplet with 4H intensity at 2.73-2.84 ppm, corresponding to the signal of the two methylene group protons of
the COCH2CH2CO fragment.
Diketones 7a-c are convenient precursors for the Paal–Knorr pyrrole synthesis. However, attempts to
carry out this reaction under the normal conditions entailing heating in acetic acid at reflux were unsuccessful,
leading to decomposition of the starting compounds and the formation of unseparable mixture of products.
The reaction of diketones 7a-c with primary aromatic and aliphatic amines was carried out by refluxing
in toluene in the presence of Ti(OPr-i)4 and triethylamine to give smooth closure of the pyrrole ring (Table 3).
The yields of the desired 1-pyrrolylmethylbenzimidazoles 9a-g were from 61 to 83% (Tables 1 and 2).
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1267

1268

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TABLE 3. Conditions for the Cyclization of Diketones 7a-c and Yields of
Compounds 9a-g
Yield of
compound 9,
%
Yield of
compound 9,
%
Reaction
time, h
Reaction
time, h
Diketone 7 Amine 8
Diketone 7 Amine 8
3
3
61 (9a)
64 (9b)
71 (9c)
72 (9d)
63 (9e)
12
9
83 (9f)
7b
7c
7b
7b
7c
8a
8a
8b
8c
8b
7b
7b
7a
7c
7b
8e
8d
8f
73 (9g)
6
24
22
12
–
–
–
10
7
8f
8g
Table 3 shows that the reaction time depends largely on the nature of the amine reagent. Thus, the
reaction with furfurylamine proceeds in 3 h, while for aromatic amines the reaction time is from 6 to 12 hours.
We should note that formation of a pyrrole ring does not occur at all when employing 4-nitroaniline (8f) and an
aliphatic amine with a bulky substituent, namely, tert-butylamine (8g).
N
Me
Ti(OPr-i)4
N
R1NH2
R
7a–c
+
Et3N, PhMe
8a–g
, 2–12 h
N
R1
Me
9a–g
8 a R1 = furfuryl, b R1 = 4-MeOC6H4, c R1 = 4-FC6H4, d R1 = 4-MeC6H4,
e R1 = 3-pyridyl, f R1 = 4-O2NC6H4, g R1 = t-Bu; 9 a R = Me, R1 = furfuryl; b R = MeO,
R1 = furfuryl; c R = Me, R1 = 4-MeOC6H4; d R = Me, R1 = 4-FC6H4; e R = MeO,
R1 = 4-MeOC6H4; f R = Me, R1 = 3-pyridyl; g R = Me, R1 = 4-MeC6H4
Evidence for the formation of a pyrrole ring in products 9a-g is found in the disappearance of the
carbonyl group absorption bands in the IR spectra and of the signals of the methylene group protons in the
COCH2CH2CO fragment in the 1H NMR spectra (Table 2). At the same time, the 1H NMR spectra show signals
for the -protons of the pyrrole ring at 5.23-6.30 ppm.
In conclusion, we have developed a method for the synthesis of new benzimidazole derivatives
containing a (5-methylfuran-2-yl)methyl fragment at position 1, studied the protolytic opening of the furan ring,
and found conditions for the subsequent cyclization of the diketones to provide pyrrole derivatives. Success of
the pyrrole ring formation was found to depend on the nature of the amine used.
EXPERIMENTAL
The IR spectra were recorded on a Perkin-Elmer Spectrum Two spectrometer with a NPVO annex. The
13
1Н and С NMR spectra were recored on a Agilent 400-MR spectrometer (400 and 100 MHz, respectively) in
DMSO-d6 with ТMS as internal standard. The mass spectra were recorded on a Kratos MS-30 mass
spectrometer, electron impact ionization (70 eV). Elemental analysis was carried out on a Flash EA 1112 CHN
analyzer. Melting points were determined on a Stuart SMP 30 apparatus and were not corrected. TLC was
carried out on Silufol UV-254 and Sorbfil (JSC "Sorbpolymer") plates, visualization was by iodine or bromine
vapor. For column chromatography, silica gel KSK (JSC "Sorbpolymer", 50-100 μ) was used.
Aminoanilides 3a-c were obtained by consecutive acylation of o-nitroanilines 1a-c using acetic
anhydride by a method described by Tietze and Eicher [42] and reduction of the nitro group in the resultant
amides 2a-c using Raney nickel in the presence of NH2NH2·H2O according to the procedure described in our
previous work [43].
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Compounds 4a-c (General Method). Ion exchange resin Amberlist 15 (1 g) was added to a solution of
о-aminoanilide 3a-c (50 mmol) and 5-methylfurfurol (5 ml, 50 mmol) in benzene (150 ml). The obtained
mixture was refluxed with azeotropic removal of water for 4-8 h. After cooling, the resin was filtered off, and
the filtrate was evaporated to dryness in vacuo. The obtained residue was recrystallized from EtOH.
N-(2-{[(5-Methylfuran-2-yl)methylidene]amino}phenyl)acetamide (4a). Yield 75%, light-yellow
crystals, mp 94-95°С. IR spectrum, ν, cm-1: 3248, 3105, 1678, 1622, 1549, 1522, 1443, 1367, 1301, 1278, 1027,
1
754. Н NMR spectrum, δ, ppm (J, Hz): 2.09 (3H, s, CH3); 2.36 (3H, s, CH3); 6.36 (1H, d, J = 3.3, H-4 Fur);
7.05-7.08 (1H, m, H Ph); 7.13 (1H, d, J = 3.3, H-3 Fur); 7.14-7.16 (1H, m, H Ph); 7.18-7.23 (1Н, m, H Ph);
8.08 (1H, d, J = 7.8, H Ph); 8.32 (1H, s, CH=N); 9.31 (1H, s, NH). 13С NMR spectrum, δ, ppm: 14.1; 24.5;
109.8; 118.0; 120.2; 121.4; 124.5; 126.7; 133.3; 141.1; 148.1; 151.2; 157.0; 168.6. Found, %: С 69.35; Н 5.77;
N 11.59. C14H14N2O2. Calculated, %: С 69.41; Н 5.82; N 11.56.
N-(4-Methyl-2-{[(5-methylfuran-2-yl)methylidene]amino}phenyl)acetamide (4b). Yield 77%, light-
yellow crystals, mp 153-154°С. IR spectrum, ν, cm-1: 3245, 3110, 1677, 1625, 1541, 1519, 1444, 1363, 1302,
1264, 1114, 1028, 959, 797. 1Н NMR spectrum, δ, ppm (J, Hz): 2.06 (3H, s, CH3); 2.24 (3H, s, CH3); 2.38 (3H,
s, CH3); 6.41 (1H, d, J = 3.0, H-4 Fur); 6.75 (1H, d, J = 8.6, H Ph); 6.82 (1H, s, H Ph); 7.05 (1H, d, J = 3.0, H-3
Fur); 7.88 (1H, d, J = 8.6, H Ph); 8.36 (1H, s, CH=N); 9.00 (1H, s, NH). 13С NMR spectrum, δ, ppm: 14.1; 24.3;
26.8; 109.7; 111.9; 120.1; 122.6; 123.5; 126.7; 143.2; 148.6; 151.2; 156.4; 157.0; 168.5. Found, %: С 70.22; Н
6.24; N 11.00. C15H16N2O2. Calculated, %: С 70.29; Н 6.29; N 10.93.
N-(4-Methoxy-2-{[(5-methylfuran-2-yl)methylidene]amino}phenyl)acetamide (4c). Yield 75%,
light-yellow crystals, mp 178-179°С. IR spectrum, ν, cm-1: 3242, 3107, 1678, 1623, 1574, 1521, 1443, 1355,
1
1302, 1242, 1153, 1121, 1025, 948, 799. Н NMR spectrum, δ, ppm (J, Hz): 2.03 (3H, s, CH3); 2.39 (3H, s,
CH3); 3.75 (3H, s, ОCH3); 6.36 (1H, d, J = 3.3, Н-4 Fur); 6.73 (1H, d, J = 8.5, H Ph); 6.78 (1H, s, H Ph); 7.12
(1H, d, J = 3.3, Н-3 Fur); 7.83 (1H, d, J = 8.5, H Ph); 8.34 (1H, s, CH=N); 8.99 (1H, s, NH). 13С NMR
spectrum, δ, ppm: 14.1; 24.3; 55.8; 103.6; 109.8; 112.0; 120.1; 123.6; 126.3; 143.2; 148.6; 151.2; 156.6; 156.9;
168.3. Found, %: С 66.09; Н 5.89; N 10.22. C15H16N2O3. Calculated, %: С 66.16; Н 5.92; N 10.29.
Synthesis of N-2-(4-R-2-{[(5-Methylfuran-2-yl)methyl]amino}phenyl)acetamides 5a-c by
Reduction of Azomethines 4a-c (General Method). NaBH4 (4.44 g, 120 mmol) was added portionwise to a
solution of compound 4a-c (40 mmol) in a mixture of dioxane (70 ml) and EtOH (20 ml). The reaction mixture
was stirred at 50-55°С until the complete dissolution of the starting azomethine (1-3 h). The cooled reaction
mixture was poured into ice water (300 ml) and stirred until crystalline precipitate was formed. The precipitate
was filtered off, washed with water, dried, and recrystallized from EtOAc–petroleum ether to give amines 5a-c
as white powders.
Synthesis of 6-R-2-Methyl-1-[(5-methylfuran-2-yl)methyl]benzimidazoles 6a-c (General Method). A
10% solution of dry HCl in MeOH (20 ml) was added to a solution of compound 5a-c (25 mmol) in EtOH (70 ml).
The obtained mixture was maintained for 7-12 h at 60°С until complete disappearance of the starting amine 5a-c
as determined by TLC (eluent EtOAc). The cooled reaction mixture was poured into cold water (200 ml) and
neutralized by addition of NaHCO3 to рН 6-7. The presipitate formed was filtered off, washed with water, dried,
and recrystallized from EtOAc–petroleum ether to give benzimidazoles 6a-c as white powders
Synthesis of 1-(6-R-2-Methyl-1Н-benzimidazol-1-yl)hexane-2,5-diones 7a-c (General Method). A
solution of compound 6a-c (20 mmol) in a mixture of EtOH (20 ml) and 20% solution of dry HCl in EtOH
(35 ml) was heated at reflux for 6-20 h. The cooled reaction mixture was poured into cold water (200 ml) and
neutralized by addition of NaHCO3 to рН 6-7. The presipitate formed was filtered off, washed with water, dried,
and subjected to column chromatography (eluent PhH–2-PrOH, 8:3) to give diketones 7a-c as white powders.
Synthesis of 6-R-2-Methyl-1-{[1-R1-5-methylpyrrol-2-yl]methyl}-1H-benzimidazoles 9a-g (General
Method). Ti(OPr-i)4 (2.46 ml, 2.1 mmol) was added dropwise to a solution of diketone 7a-c (2.0 mmol), amine
8a-g (2.1 mmol), and freshly distilled Et3N (1.08 ml, 2.1 mmol) in PhMe (20 ml). The obtained mixture was
heated at reflux for 3-12 h until the dissapearance of diketone 7a-c as determined by TLC
1271

(eluent EtOAc). After the completion of the reaction, the reaction mixture was cooled, 10% NaOH solution
(30 ml) was added and filtrated at reduced pressure. The organic layer of the filtrate was separated, and the
aqueous layer was extracted with PhMe (2×10 ml). The combined organic phase was dried over Na2SO4 and
evaporated in vacuo to 1/4 of the original volume. Petroleum ether was added to the residue, and the mixture
was left to crystallize. The crystals were filtered off, dried, and recrystallized form EtOAc–petroleum ether, to
give products 9a-g as white powders.
This research was carried out in the framework of a state project of the Ministry of Education and
Science of the Russian Federation (contract 3.1578.2011) and with the support of the Ministry of Education and
Science of the Russian Federation (contract 14.V37.21.0829).
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