Cycloaddition of 2-acylmethyl-1H-benzimidazoles to 4-arylidene-1,3-oxazol- 5-ones

Article abstract of DOI:10.1007/s10593-007-0135-5

Cycloaddition of 2-phenacyl-1H-benzimidazole to 2-phenyl-or 2-methyl-4-arylidene-1,3-oxazol-5-ones occurs regioselectively to form the previously unknown N-(3-aryl-4-benzoyl-1-oxo-1,2,3,4-tetrahydropyrido[1,2-a] benzimidazol-2-yl)benz-and-acetamides. The analogous cycloaddition of the 2-acetonyl-1H-benzimidazole is complicated by prototropic isomerization and leads to the corresponding 1,2,3,5-tetrahydropyrido[1,2-a]benzimidazole.

Full text of DOI:10.1007/s10593-007-0135-5

Chemistry of Heterocyclic Compounds, Vol. 43, No. 7, 2007
CYCLOADDITION OF 2-ACYLMETHYL-
1H-BENZIMIDAZOLES TO 4-ARYLIDENE-
1,3-OXAZOL-5-ONES
I. B. Dzvinchuk and M. O. Lozinskii
Cycloaddition of 2-phenacyl-1H-benzimidazole to 2-phenyl- or 2-methyl-4-arylidene-1,3-oxazol-5-ones
occurs regioselectively to form the previously unknown N-(3-aryl-4-benzoyl-1-oxo-1,2,3,4-tetra-
hydropyrido[1,2-a]benzimidazol-2-yl)benz- and -acetamides. The analogous cycloaddition of the
2-acetonyl-1H-benzimidazole is complicated by prototropic isomerization and leads to the correspon-
ding 1,2,3,5-tetrahydropyrido[1,2-a]benzimidazole.
Keywords: azlactones, benzimidazoles, pyrido[1,2-a]benzimidazoles, [3+3]cycloaddition, isomerization,
regioselectivity.
Pyrido[1,2-a]benzimidazoles possess a broad spectrum of biological activity [1-6], have fluorescence
properties [7], and are a component of light sensitive materials [8, 9]. We hoped to prepare previously
unavailable derivatives on the basis of the reaction of 2-acylmethyl-1H-benzimidazoles of type 1 with
4-arylidene-1,3-oxazol-5-ones (azlactones) of type 2 and 3 for the following reasons. 2-Aminobenzimidazole
reacts regioselectively with azlactones to give pyrimido[1,2-a]benzimidazoles, in such a way that the exocyclic
amino group is acylated as a result of opening of the oxazole ring and the endocyclic by addition to the arylidene
fragment [10]. It is advisable to use compounds of type 1 as the 1,3-dinucleophile component. In the first place
2-phenacyl-1H-bezimidazole 1a is acylated by carboxylic acid anhydrides at the active methylene group [11]. In
the second, azlactones are analogs of cyclic anhydrides which react with active methylene compounds to form
C-acylation products [12].
We have found that the reaction of compound 1 with azlactones 2a-f, 3a-c indeed occurs efficiently via
the cycloaddition of a 1,3-dinucleophile to a 1,3-dielectrophile. However, the products of the expected structure
4 or any kind of prototropic isomers were not formed. A contrary scheme of regioselective construction or the
ring is achieved via acylation at a benzimidazole nitrogen atom (possible by the intermediate formation of
compound 5) and Michael addition of the methylene group to the arylidene fragment to give the previously
unknown N-(3-aryl-4-benzoyl-1-oxo-1,2,3,4-tetrahydropyridino[1,2-a]benzimidazol-2-yl)benz- and -acetamides
6a-f, 7a-c.
The reaction was carried out by refluxing in pyridine and is complete after 1 h. The product yields were
70-95%.
__________________________________________________________________________________________
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Kiev 02094; e-mail:
Rostov@bpci.kiev.ua. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 78, pp. 1008-1014, July,
2007. Original article submitted March 18, 2006.
850
0009-3122/07/4307-0850©2007 Springer Science+Business Media, Inc.

It should be noted that the starting compound 1a exists in solutions in equilibrium with an enamino
ketone tautomeric form [13] but the obtained products 6 and 7 do not show a similar tendency to isomerize to
form 8.
O
O
R
Ph
O
Ph
N
N
N
N
O
N
+
H
O
CHAr
1a
Ar
NHCOR
2a-f, 3a-c
Py,
120°C,
1 h
4
O
O
Ph
Ar
N
Ph
Ar
N
N
O
N
NHCOR
O
NHCOR
6a-f, 7a-c
5
O
H
N
Ph
Ar
N
O
NHCOR
8
2, 6 R = Ph; 3, 7 R = Me; 2, 3, 6, 7 a Ar = Ph, b Ar = 4-MeOC6H4, c Ar = 4-ClC6H4,
2, 6 d Ar = 4-O2NC6H4, e Ar = 3-Py, f Ar = 4-Py
On the other hand, the reaction of 2-acetonyl-1H-benzimidazole 1b with the azlactones 2a and 3a occurs
by the route revealed for compound 1a but is complicated by a prototropic isomerization. The reaction can be
carried out in pyridine or in chlorobenzene but the latter solvent is more convenient to use in the separation of
the products from the reaction mixtures. Cycloaddition of the azlactone 2a gave the expected compound 9 and its
enamino ketone tautomer 10. The cycloaddition of azlactone 3a gave only the isomerization product 11.
O
O
H
2a
Me
Ph
N
N
O
Me
Ph
N
+
ClPh,
135°C,
2 h
N
O
NHCOPh
NHCOPh
9
10
O
O
H
Me
N
Me
Ph
N
3a
ClPh, 135°C,
N
H
N
O
2 h
1b
NHCOMe
11
851

Compounds 9-11 are fully stable in DMSO-d6 solution for 3 h at 20-25ºC. Evidently the shift of the
proton between the carbon and nitrogen atoms in positions 4 and 5 is hindered and is possible only at higher
temperatures. Such a prototropic feature is likely due to the limit to conformational mobility of the structural
fragment C(4)–C(4a)–N(5), the central atom of which is situated at the junction of two condensed rings. This also
does not exclude the contribution of steric hindrance arising between the substituent in positions 2, 3, and 4.
TABLE 1. Characteristics of Compounds Synthesized
Found, %
——————
Calculated, %
Com-
pound
Empirical
formula
mp, °С
Yield, %
95
С
Н
N
6a
6b
6c
6d
6e
6f
C31H23N3O3
C32H25N3O4
C31H22ClN3O3
C31H22N4O5
C30H22N4O3
C30H22N4O3
C26H21N3O3
C27H23N3O4
С26H20ClN3O3
C26H21N3O3
C26H21N3O3
C21H19N3O3
76.52
76.69
74.46
74.55
71.58
71.61
70.02
70.18
73.89
74.06
74.03
74.06
73.62
73.74
71.48
71.51
68.15
68.20
73.59
73.74
73.57
73.74
69.63
69.79
4.48
4.77
4.78
4.89
4.32
4.26
4.29
4.18
4.43
4.56
4.49
4.56
5.16
5.00
5.28
5.11
4.28
4.40
5.06
5.00
5.05
5.00
5.17
5.30
8.48
8.65
8.03
8.15
8.09
8.08
10.39
10.56
11.35
11.52
11.34
11.52
9.79
9.92
9.17
9.27
9.09
9.18
9.81
9.92
9.77
9.92
11.53
11.63
293-294.5
316-317.5
302.5-304
306-307.5
263.5-265
296-294.5
290-291.5
260-261.5
270-271.5
285-286.5
217.5-219
236-237
85
89
85
70
84
90
85
88
48
26
62
7a
7b
7c
9
10
11
1
TABLE 2. IR and H NMR Spectroscopic Characteristics of Compounds
Synthesized
Com- IR spectrum, сm-1
pound (C=S, С=О N-H)
1Н NMR spectrum, δ, ppm (J, Hz)
1
2
3
6a
1615, 1660,
1680, 1740,
3349, 3405
4.48 (1H, m, H-3); 5.70 (1H, br. m, H-2); 6.35 (1H, d, J = 11.7, H-4);
7.09 (1H, t, J = 7.2, 3-C6H5: H-4); 7.20 (2H, t, J = 7.2, 3-C6H5: H-3,5);
7.39-7.50 (6H, m, H-7,8 + 3-C6H5: H-2,6 + NCOC6H5: H-3,5);
7.50-7.57 (3H, m, 4-COC6H5: H-3,5 + NCOC6H5: H-4);
7.65-7.67 (4H, m, H-6 + 4-COC6H5: H-4 + NCOC6H5: H-2,6);
7.98 (2H, d, J = 7.5, 4-COC6H5: H-2,4); 8.21 (1H, d, J = 7.5, H-9);
8.94 (1H, br. m, NHCO, hindered exchange with D2O)
6b
6c
1610, 1645,
1680, 1735,
3340
3.60 (3H, s, OCH3); 4.40 (1H, m, H-3); 5.73 (1H, br. m, H-2);
6.29 (1H, d, J = 12.0, H-4); 6.76 and 7.29 (2 + 2H, two d, J = 8.4,
C6H4OCH3); 7.40-7.48 (4H, m, H-7,8 + NCOC6H5: H-3,5);
7.51-7.59 (3H, m, 4-COC6H5: H-3,5 + NCOC6H5: H-4);
7.65-7.69 (4H, m, H-6 + 4-COC6H5: H-4 + NCOC6H5: H-2,6);
7.99 (2H, d, J = 7.2, 4-COC6H5: H-2,6); 8.20 (1H, d, J = 7.5, H-9);
8.91 (1H, br. m, NHCO)
1620, 1650,
1680, 1740,
3340
4.47 (1H, m, H-3); 5.71 (1H, br. m, H-2); 6.35 (1H, d, J = 12.0, H-4);
7.29 (2Н, d, J = 8.4, 4-ClC6H4: H-2,6); 7.39-7.48 (6H, m,
H-7,8 + NCOC6H5: H-3,5 + 4-ClC6H4: H-4,5); 7.52-7.60 (3H, m,
4-COC6H5: H-3,5 + NCOC6H5: H-4); 7.66-7.73 (4H, m,
H-6 + 4-COC6H5: H-4 + NCOC6H5: H-2,6); 7.99 (2H, d, J = 7.5,
4-COC6H5: H-2,6); 8.21 (1H, d, J = 7.2, H-9); 8.97 (1H, br. m, NHCO)
852

TABLE 2. (continued)
1
2
3
6d
1615, 1660,
1690, 1755,
3320
4.65 (1H, m, H-3); 5.80 (1H, br. m, H-2); 6.46 (1H, d, J = 12.6, H-4);
7.40-7.51 (4H, m, H-7,8 + NCOC6H5: H-3,5);
7.55-7.60 (3H, m, 4-COC6H5: H-3,5 + NCOC6H5: H-4); 7.68 (6H, m,
H-6 + 4-COC6H5: H-4 + NCOC6H5: H-2,6 + 4-NO2C6H4: H-2,6);
8.00 (2H, d, J = 7.2, 4-COC6H5: H-2,6); 8.12 (2H, d, J = 8.4,
4-NO2C6H4: H-3,5); 8.21 (1H, d, J = 7.2, H-9); 9.02 (1H, br. m, NHCO)
6e
1645, 1680,
1725, 3360
4.50 (1H, m, H-3); 5.82 (1H, br. m, H-2); 6.42 (1H, d, J = 12.0, H-4);
7.24-7.28 (1H, m, C5H4N: H-5); 7.39-7.50 (4H, m, H-7,8 + NCOC6H5:
H-3,5); 7.52-7.60 (3H, m, 4-COC6H5: H-3,5 + NCOC6H5: H-4);
7.66-7.72 (4H, m, H-6 + 4-COC6H5: H-4 + NCOC6H5: H-2,6);
7.87 (1H, d, J = 7.8, C5H4N: H-4); 8.01 (2H, d, J = 7.5, COC6H5:
H-2,6); 8.22 (1H, d, J = 7.2, H-9); 8.32 (1H, d, J = 4.5, C5H4N: H-6);
8.53 (1H, s, C5H4N: H-2); 9.00 (1H, br. m, NHCO)
6f
1655, 1680,
1745, 3290,
3490
4.50 (1H, m, H-3); 5.77 (1H, br. m, H-2); 6.39 (1H, d, J = 11.7, H-4);
7.38-7.48 (6H, m, H-7,8 + NCOC6H5: H-3,5 + C5H4N: H-2,6);
7.52-7.60 (3H, m, 4-COC6H5: H-3,5 + NCOC6H5: H-4);
7.66-7.73 (4H, m, H-6 + 4-COC6H5: H-4 + NCOC6H5: H-2,6);
8.01 (2H, d, J = 7.5, COC6H5: H-2,6); 8.21 (1H, d, J = 7.2, H-9);
8.42 (2H, d, J = 5.4, C5H4N: H-3,5); 8.99 (1H, br. m, NHCO)
7a
1610, 1675,
1690, 1745,
3315
1.72 (3H, s, CH3); 4.29 (1H, m, H-3); 5.33 (1H, br. m, H-2);
6.22 (1H, d, J = 12.3, H-4); 7.14 (1H, t, J = 7.2, 3-C6H5: H-4);
7.23 (2H, t, J = 7.2, 3-C6H5: H-3,5); 7.32 (2H, d, J = 7.2, 3-C6H5:
H-2,6); 7.36-7.47 (2H, m, H-7,8); 7.53 (2H, t, J = 7.8, COC6H5: H-3,5);
7.63-7.68 (2H, m, H-6 + 4-COC6H5: H-4); 7.94 (2H, d, J = 7.5,
4-COC6H5: H-2,4); 8.19 (1H, d, J = 7.5, H-9);
8.35 (1H, br. d, J = 6.9, NHCO, hindered exchange with D2O)
7b
1620, 1670,
1685, 1740,
3310
1.73 (3H, s, CH3); 3.65 (3H, s, CH3O); 4.22 (1H, m, H-3);
5.26 (1H, br. m, H-2); 6.18 (1H, d, J = 12.3, H-4);
6.79 and 7.23 (2 + 2H, two d, J = 8.4, C6H4OCH3);
7.36-7.46 (2H, m, H-7,8); 7.54 (2H, t, J = 7.5, COC6H5: H-3,5);
7.62-7.69 (2H, m, H-6 + 4-COC6H5: H-4); 7.95 (2H, d, J = 7.8,
4-COC6H5: H-2,6); 8.18 (1H, d, J = 7.8, H-9); 8.34 (1H, br. m, NHCO)
7c
1620, 1670,
1685, 1740,
3320
1.74 (3H, s, CH3); 4.30 (1H, m, H-3); 5.32 (1H, br. m, H-2);
6.22 (1H, d, J = 12.3, H-4); 7.32 and 7.36 (2 + 2H, two d, J = 7.8,
C6H4Cl); 7.37-7.48 (2H, m, H-7,8); 7.56 (2H, t, J = 7.5, COC6H5:
H-3,5); 7.63-7.71 (2H, m, H-6 + 4-COC6H5: H-4);
7.97 (2H, d, J = 7.8, 4-COC6H5: H-2,6); 8.19 (1H, d, J = 7.8, H-9);
8.39 (1H, br. d, J = 7.8, NHCO)
9
1620, 1670,
1740, 3420
2.22 (3H, s, CH3); 4.26 (1H, dd, J = 12.3, H-3); 5.15 (1H, d,
J = 12.6, H-2); 5.49 (1H, m, H-4); 7.23 (1H, t, J = 7.5, 3-C6H5: H-4);
7.32 (2H, t, J = 7.5, 3-C6H5: H-3,5); 7.38 (2H, d, J = 7.8, 3-C6H5:
H-2,6); 7.43-7.49 (4H, m, H-7,8 + COC6H5: H-3,5); 7.52 (1H, t,
J = 7.8, COC6H5: H-4); 7.63 (2H, d, J = 7.2, COC6H5: H-2,6);
7.77 (1H, m, H-6); 8.19 (1H, m, H-9);
8.90 (1H, br. d, J = 7.5, NHCO, hindered exchange with D2O)
10
11
1630, 1655,
1680, 1740,
3285, 3450
1.92 (3H, s, CH3); 4.51 (1H, d, J = 7.2, H-3); 5.78 (1H, dd, J = 7.2,
H-2, changes to a doublet on treatment with D2O, J = 7.5);
7.03-7.05 (2H, m, H-7,8); 7.20-7.30 (4H, 3-C6H5: H-2,3,5,6);
7.33 (1H, t, J = 7.8, 3-C6H5: H-4); 7.46 (2H, t, J = 7.5, H-3,5);
7.54-7.57 (2H, m, H-6 + COC6H5: H-4); 7.83 (2H, d, J = 7.2, COC6H5:
H-2,6); 7.99 (1H, d, J = 7.8, H-9);
8.35 (1H, d, J = 7.5, NHCO, disappears on treatment with D2O);
12.47 (1Н, s, H-5, disappears on treatment with D2O)
1630, 1660,
1740, 3270
1.89 (3H, s, 4-COCH3); 1.95 (3H, s, NCOCH3);
7.39 (1H, d, J = 7.2, H-3); 5.50 (1H, dd, J = 7.5, H-2,
changes to a doublet on treatment with D2O, J = 7.2);
6.99-7.02 (2H, m, H-7,8); 7.19-7.34 (5H, m, C6H5);
7.55 (1H, d, J = 7.8, H-6); 7.96 (1H, d, J = 7.8, H-9);
8.03 (1H, d, J = 7.2, NHCO, disappears on treatment with D2O);
12.43 (1H, s, H-5, disappears on treatment with D2O)
853

The composition and structure of the synthesised compounds were confirmed by elemental analysis
(Table 1) and by IR and 1H NMR spectroscopic data (Table 2).
The position of the oxo group vicinal to the benzimidazole nitrogen atom in compounds 6, 7, 9-11 is
evident from the spectroscopic data. The IR spectra show the presence of a carbonyl group absorption band at
1725-1755 cm-1 which is at higher frequency than the remaining νC=O bands (1660-1690 cm-1) and is
characteristic of the compound (see [13]) in which the unshared electron pair of the amide nitrogen atom takes
part in the formation of the aromatic conjugated system. In the 1H NMR spectra the H-9 proton is found at lower
field (7.96-8.21 ppm) than the remaining aromatic protons since it is deshielded by the sterically nearby carbonyl
group.
The presence of a partially hydrogenated pyridine ring in the 4-benzoyl-substituted compounds 6, 7 is
1
confirmed in the H NMR spectra by the appearance of the spin spin interaction in the RCONH–CH(C=O)–
CH(Ar)–CH(C=O) fragment. The 4-H proton is seen as a clear doublet and the amino group proton as a
broadened signal, the doublet nature of which is not clearly seen. The protons corresponding to positions 2 and 3
are seen to higher fields as broadened signals. Such a spectroscopic picture can be explained by restricted
conformational interconversion in the tetrahydropyridine ring. On the other hand, in the 4-acetyl analog 9 the
multiplicity of the proton signals for position 3 and the amino group are distinctly seen. Evidently in this the
conformational mobility of the ring is higher and this can be explained by the increase tendency of the
compound to undergo conversion to the prototropic isomer 10. A deuterium exchange of the amide group proton
in compounds 6, 7, 9 is hindered, the intensity of the signal being decreased by 10-20% after 12 h.
In the 1,2,3,5-tetrahydropyrido[1,2-a]benzimidazoles 10 and 11 the proton signals are characteristic: H-5
is seen as a singlet with a chemical shift typical of a benzimidazole (see [13]) and H-3 and the amide amino
group as a doublet due to interaction with the H-2 proton (the latter being observed as a double doublet). When
treated with deuterium oxide the signals for both protons bonded to nitrogen atoms disappear and the H-2 signal
becomes a doublet.
In the molecules of the tetrahydro compounds 6, 7, 9-11 the H-2,3 and H-4 protons occupy
predominantly axial positions as indicated by their high mutual spin spin coupling values (7.2-12.6 Hz).
Hence cycloaddition of 2-acylmethyl-1H-benzimidazoles to 4-arylidene-1,3-oxadiazol-5-ones provides
an efficient method for synthesizing N-(3-aryl-4-acyl-1-oxo-1,2,3,4-tetrahydropyridino[1,2-a]benzimidazol-
2-yl)benz- and acetamides which, in specific cases, tend to undergo prototropic isomerization to give the rather
stable 1,2,3,5-tetrahydropyrido[1,2-a]benzimidazoles.
EXPERIMENTAL
Monitoring of the reaction course and the purity of the compounds prepared was carried out by TLC on
Silufol UC-254 plates in the solvent system benzene-ethanol (9: 1) and revealed using UV light. IR Spectra were
1
recorded on a UR-20 instrument for KBr tablets. The H NMR spectra were taken on a Varian VXR-300 (300
MHz) spectrometer using DMSO-d6 solvent and TMS internal standard. All of the compounds were dried for 5 h
at 110ºC before elemental and spectroscopic investigation.
N-(4-Benzoyl-1-oxo-3-phenyl-1,2,3,4-tetrahydropyrido[1,2-a]benzimidazol-2yl)benzamide (6a). A
mixture of compound 1a (0.518 g, 2.2 mmol), azlactone 2a (0.498 g, 2 mmol), and anhydrous pyridine (2 ml)
was held on an oil bath at 120ºC for 1 h. The crystallized reaction mixture was diluted with glacial acetic acid
(2 ml) and stirred to a homogenous state. After cooling, the precipitate was filtered off and washed with
2 propanol. The product was obtained in an analytically pure state after drying.
Compounds 6b-f, 7a-c were obtained similarly from compound 1a and the corresponding azlactones
2b-f, 3a-c. For the separation of compound 6e the reaction mixture was mixed with water (2 ml) instead of acetic
acid and for 6f with 2-propanol (2 ml).
854

N-(4-Acetyl-1-oxo-3-phenyl-1,2,3,4-tetrahydropyrido[1,2-a]benzimidazol-2-yl)benzamide (9) and
N-(4-Acetyl-1-oxo-3-phenyl-1,2,3,5-tetrahydropyrido[1,2-a]benzimidazol-2-yl)benzamide (10). A mixture
of compound 1b (0.174 g, 1 mmol), azlactone 2a (0.249 g, 1 mmol), and chlorobenzene (1 ml) was heated on an
oil bath at 135ºC for 2 h. The reaction mixture was diluted with chlorobenzene (1 ml) and filtered hot. The solid
produced was washed with hot chlorobenzene (1 ml) and the filtrate was allowed to cool slowly. The precipitate
was recrystallized from a mixture of pyridine and water (1:1) to give compound 9 (0.205 g). A second precipitate
was formed on cooling the chlorobenzene solution and this was filtered off and washed with 2-propanol to give
analytically pure compound 10 (0.110 g) after drying.
N-(4-Acetyl-1-oxo-3-phenyl-1,2,3,5-tetrahydropyrido[1,2-a]benzimidazol-2-yl)acetamide (11). A
mixture of compound 1b (0.174 g, 1 mmol), azlactone 2a (0.206 g, 1.1 mmol), and chlorobenzene (1 ml) was
heated on an oil bath at 135ºC for 2 h. The hot solution was diluted with 2-propanol (1 ml), stirred until
crystallization began, and left to cool slowly. The precipitate was filtered off, washed with 2-propanol, and dried
to give analytically pure compound 11 (0.213 g).
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