Synthesis and pharmacological activity of 1-N-and 10-n-substituted 1(10),2,3,4-tetrahydropyrimido-[1,2-a]benzimidazoles

Full text of DOI:10.1023/A:1021888520874

Pharmaceutical Chemistry Journal
Vol. 36, No. 9, 2002
SYNTHESIS AND PHARMACOLOGICAL ACTIVITY OF 1-N-
AND 10-N-SUBSTITUTED 1(10),2,3,4-TETRAHYDROPYRIMIDO-
[1,2-a]BENZIMIDAZOLES
V. A. Anisimova,1 M. M. Osipova,1 A. A. Spasov,2 A. F. Turchaeva,2 G. P. Dudchenko,2
N. P. Larionov,2 and S. G. Kovalev2
Translated from Khimiko-Farmatsevticheskii Zhurnal, Vol. 36, No. 9, pp. 11 – 16, September, 2002.
Original article submitted July 24, 2001.
In continuation of our previous search [1 – 4] for new bi-
ologically active substances among tricyclic systems repre-
senting benzimidazole derivatives, we have synthesized a se-
ries of new N-substituted 1(10),2,3,4-tetrahydropyrimido-
[1,2-a]- benzimidazoles and studied their pharmacological
properties.
can be obtained using a method proposed in our previous
study [6] for the synthesis of 10-benzyl- and 10-phenyl-sub-
stituted tricyclic derivatives, which is based on the
intramolecular cyclization of 2-chloropropylaminobenz- imi-
dazoles (VIIa – VIId) prepared from readily accessible
benzimidazole-2-sulfonic acids (Va – Vd) [7].
N
N
N
· nHX
N
R
N
Ià – Ii
R1 = H
SO3H
N
R
NH(CH2)3OH
N
R
Và –Vd
VIà –VId
N
N
N
Ia –Id
· HCl
· nHX
R = H
NHR1
NH(CH2)3Cl
N
N
N
N
R1
R
R
IIa – IIe
VIIà –VIId
IIIa – IIIg, IVa – IVe
R1 = H (IIIa – IIIg), R = H (IVa – IVe);
R, R1 = CH3 (Ia – IVa), R = C3H7 (Ib, IIIb), C4H9 (Ic, IIIc);
V – VII: R = CH3 (a), C3H7 (b), C4H9 (c), CH2C6H5 (d).
R, R1 = CH2C6H5 (Id, IIId, IIb, IVb), CH2CH2N(C2H5)2 (Ie, If, IIc, IIIe,
IVc), CH2CH2H(CH2)5 (Ig, IId, IIIf, IVd), CH2CH2N(CH2CH2)2O (Ih, Ii,
IIe, IIIg, IVe);
The IR spectra of hydroxypropylamines VIa – VId dis-
play
a
broad absorption band in the region of
3025 – 3110 cm – 1 (OH), a broad band at 3200 – 3290 cm – 1
(NH), and a characteristic absorption band at 1660 cm – 1
(C=N). In the spectra of bases of chloropropylamines VII,
the absorption band of hydroxy groups vanishes, while the
n = 1 (Ia – Id, IIa, IIb), 2 (Ie – Ii, IIc – IIe);
X = Cl (Ia, Ib, Id, Ie, Ih, IIa, IIb, IId, IIe), 1/2SO4 (If, IIc), Br (Ic, Ig, Ii).
Earlier, Kochergin [5] proposed a method for the synthe-
sis of 2,3,4,10-tetrahydropyrimido[1,2-a]benzimidazoles
based on the interaction of 1-ethyl-2-aminobenzimidazoles
with 1,3-bromochloropropane in toluene. However, this
pathway is ineffective in obtaining compounds Ia – Id be-
cause half of the initial amine IIIa – IIId is spent for binding
a hydrogen halide evolved during the reaction, so that the tar-
get product yield does not exceed 45 – 50%. Better results
band of NH groups shifts to 3290 – 3320 cm – 1
.
The subsequent thermal cyclization of the bases of
chloropropylamines VIIa – VIId readily proceeds in the ab-
sence of solvent or in an inert solvent (such as benzene, chlo-
roform, acetonitrile, toluene) with a high yield of the target
products.
As for the condensation of amines IIIe – IIIg with
bromochloropropane, the reaction is facilitated due to the
ability of dialkylaminoalkyl groups to accept the evolved hy-
drogen halides. The target tricyclic derivatives Ie – Ii were
obtained with high yields and isolated in the form
1
Research Institute of Physical and Organic Chemistry, Rostov State Uni-
versity, Rostov-on-Don, Russia.
Volgograd State Medical Academy, Volgograd, Russia.
2
468
0091-150X/02/3609-0468$27.00 © 2002 Plenum Publishing Corporation

Synthesis of 1-N- and 10-N-Substituted 1(10),2,3,4-Tetrahydropyrimido-[1,2-a]benzimidazoles
469
dihydrohalides. By using a high-boiling solvent (xylene) in-
stead of toluene and 1,3-dibromopropane instead of bromo-
chloropropane, the reaction time can be reduced to 1 – 1.5 h.
Isomeric pyrimidobenzimidazoles IIa – IIe were also ob-
tained by the method of cyclocondensation, proceeding from
2-(R1-amino)benzimidazoles IV. The initial 2-methylamino
(IVa) and 2-benzylamino (IVb) benzimidazoles were ob-
tained from benzimidazole-2-sulfonic acid [8, p. 132] by a
method described in [9]. In the reaction with 2-dialkylamino
ethylamines, substitution of the sulfonic group takes place at
a higher temperature and is accompanied by side reactions
and resinification, which leads to considerable difficulties in
obtaining pure samples of amines IVc – IVe. Taking into ac-
count that these compounds cannot be synthesized from
2-chlorobenzimidazole and can be obtained only with a
small yield from 2-methylsulfonyl benzimidazole [10, 11],
we developed a method based on the easy nucleophilic sub-
stitution of chlorine in 2-(2-chloroethylamino)benzimidazole
[2]. The reaction proceeds very smoothly for 3 – 4 days at
25 – 35°C. An increase in the temperature leads to a decrease
in the yield of target amines as a result of the parallel
intramolecular cyclization of 2-(2-chloroethylamino)benz-
imidazole into 1H-2,3-dihydroimidazo[1,2-a]-benzimidazole
[8, p. 124].
creasing the target product yield. For this reason,
pyrimidobenzimidazoles II were obtained with relatively low
yields even upon prolonged (12 – 20 h) boiling of the reac-
tion mass (Table 1).
EXPERIMENTAL CHEMICAL PART
The IR absorption spectra were measured on a Specord
75-IR spectrophotometer (Germany) using samples prepared
1
as nujol mulls. The H NMR spectra were recorded on a
Unity-300 spectrometer operating at a working frequency of
300 MHz (Table 2). The course of the reactions was moni-
tored and the purity of the reaction products was checked by
TLC on Al2O3 plates eluted in chloroform and developed by
exposure to iodine vapor in a wet chamber. The yields and
physicochemical characteristics of the synthesized com-
pounds and intermediate products are presented in Tables 1
and 3. The data of elemental analyses (C, H, N, Hal) agree
with the results of analytical calculations using the empirical
formulas.
1-R-2-(3-Hydroxypropylamino)benzimidazoles (VIa –
VId). A mixture of 10 mmole of 1-R-benzimidazole-
2-sulfonic acid (V) and 1.6 ml (20 mmole) of
3-amino-1-propanol was carefully heated with stirring for
1.5 – 2.5 h at 140 – 145°C (for compounds Va and Vb),
125 – 130°C (Vc), or 145 – 150°C (Vd) so as to avoid
resinification of the reaction mass. Then the reaction mass
was cooled to 60 – 70°C and treated with 15 – 20 ml of wa-
ter. After keeping for 2 h at 5 – 8°C, the precipitate of
aminoalcohol VI was separated by filtration, washed with
water, dried in air, and recrystallized.
The cyclocondensation of amines IV proceeds less
smoothly than the analogous process with compounds III,
since the bulky substituent in the amino group creates steric
hindrances to annelation of the pyrimidine ring. Attempts at
conducting the process in various high-boiling solvents
(chlorobenzene, DMF, DMSO, excess halogenopropane) led
to partial resinification of the reaction mixture rather than in-
TABLE 1. Yields and Physicochemical Characteristics of N-Substituted 1(10),2,3,4-Tetrahydropyrimido[1,2-a]benzimidazoles I and II
Yield, %
M.p. (decomp.), °C
(solvent for crystallization)
Compound
Empirical formula
Base
Salt *
Ia
257 – 258 (EtOH–Et2O)
85
87
90
95
86
…
82
92
…
44
43
42
48
43
88
86
76
98
92
79
98
94
95
92
93
86
88
87
C11H13N3 × HCl
Ib
Ic
³ 230 (i-PrOH)
C13H17N3 × HCl
209 – 210 (i-PrOH–Et2O)
266 – 266.5 (EtOH–H2O)
78 – 80** (i-PrOH–MeCO)
120** (i-PrOH–Et2O)
274 – 275 (EtOH)
C14H19N3 × HBr
Id
Ie
C17H17N3 × HCl
C16H24N4 × 2HCl × 2H2O
C16H24N4 × H2SO4 × 3H2O
C17H24N4 × 2HBr
If
Ig
Ih
Ii
277 – 279 (i-PrOH)
307 – 308 (EtOH)
C16H22N4O × 2HCl
C16H22N4O × 2HBr
C11H13N3 × HCl
IIa
IIb
IIc
IId
IIe
240 – 241 (i-PrOH)
219 – 220 (EtOH)
126 – 128** (CH3CN)
188 – 190 (i-PrOH)
284 – 285*** (EtOH–Et2O)
C17H17N3 × HCl × 2H2O
C16H24N3 × H2SO4 × 3H2O
C17H24N4 × 2HCl × 3H2O
C16H22N4O × 2HCl × 2H2O
*
**
Salt yield calculated for base yield.
Highly hygroscopic salts (crystal hydrates melt without decomposition).
Premelting at 190°C.
***

470
V. A. Anisimova et al.
tricyclic bases converted into salts Ia – Id by conventional
methods.
Upon overheating of the reaction mass and/or increasing
the duration of heating, the TLC patterns may exhibit addi-
tional spots corresponding to tricyclic bases of compounds
Ia – Id.
10-Dialkylaminoethyl-2,3,4,10-tetrahydropyrimido[1,
2-a]benzimidazole salts (Ie – Ii). A mixture of 10 mmole of
amine IIIe – IIIg with 10 mmole of dihalogenpropane in
10 ml of anhydrous xylene was boiled for 1.5 – 5 h. This was
accompanied by dissolution of the initial amine, after which
a dense oil began to form and precipitate on the flask bottom,
showing partial crystallization on boiling. Upon termination
of the reaction according to TLC data, (Rf , ~ 0.2 for the ini-
tial amine and ~0.45 for the final tricycle), the mixture was
cooled, diluted with 50 ml of water, and treated in a shaker
until complete dissolution of the precipitated pyrimi-
do[1,2-a]benzimidazole salt. The aqueous layer was sepa-
rated, alkalized to pH 9 with a 22% aqueous ammonia solu-
tion, and extracted with chloroform (4 ´ 15 ml). The com-
bined chloroform extracts were evaporated to a final volume
of 8 – 10 ml and passed through a Schott funnel with g-Al2O3
(eluted with CHCl3) in order to purify the product base from
resins. Then the chloroform was evaporated and the residual
slightly yellowish oil (well soluble in water and many or-
ganic solvents) dried in a vacuum desiccator over KOH. The
dehydrated product was dissolved in 20 ml of anhydrous
ether or acetone and the solution was acidified by adding
concentrated HBr, an ether solution of HCl, or H2SO4. The
precipitated crystalline salt Ie was separated by filtration,
washed with anhydrous ether, placed into a vacuum desicca-
1-R-2-(3-Chloropropylamino)benzimidazole hydro-
chlorides (VIIa – VIId). To a suspension of 10 mmole of
aminoalcohol VI in 15 – 20 ml of anhydrous CHCl3 was
slowly added with stirring 1 ml (14 mmole) of thionyl chlo-
ride, which is accompanied by heating of the mixture and
dissolution of the initial residue. The solution was stirred at
room temperature for 5 – 7 h until completion of the reaction
according to TLC data: Rf, 0.15 (compounds VI), ~0.9 (VII).
Then the solvent (chloroform) was evaporated and the resi-
due treated with petroleum ether. The precipitated hydro-
chloride VII was separated by filtration and washed with pe-
troleum ether.
The reaction can be also conducted by heating the reac-
tion solution to boiling, under which conditions the process
terminates within 1.5 – 2 h. However, this may be accompa-
nied by darkening of the reaction mass.
10-Alkyl-2,3,4,10-tetrahydropyrimido[1,2-a]benzimi
dazole salts (Ia – Id). A mixture of 10 mmole of hydrochlo-
ride VII with 20 mmole of triethylamine in anhydrous ben-
zene or toluene (15 – 20 ml) was boiled until completion of
the cyclization reaction (0.5 – 2.5 h). The precipitated tri-
ethylamine hydrochloride was separated by filtration. The
filtrate was evaporated to dryness and the residues of
TABLE 2. Parameters of the 1H NMR Spectra of Tetrahydropyrimido[1,2-a]benzimidazoles I and II
Compound
Ia
Solvent
DMSO-d6
Proton chemical shift, d, ppm
2.14 (2H, q, 3-H2), 3.52 (2H, t, I 4-H2), 3.17 (3H, s, CH3), 4.11 (2H, t, I 2-H2), 7.29 – 7.35 (2H, m), 7.49 – 7.56
(2H, m, Harom), 10.45 (1H, bs, N+H).
Ib
DMSO-d6 – CCl4
DMSO-d6
DMSO-d6 – CCl4
CDCl3
1.0 (3H, t, CH3), 1.75 (2H, m, CH2CH6), 2.20 (2H, q, 3-H2), 3.56 (2H, t, 4-H2), 4.15 (2H, t, I 2H2). 4.30 (2H, t,
NCH2C2H5), 7.22 – 7.30 (2H, m), 7.42 – 7.55 (2H, m, Harom), 10.65 (1H, s, N+H).
Ic
0.95 (3H, t, CH3), 1.42 (2H, m, CH2CH6), 1.77 (2H, m, NCH2CH2), 2.25 (2H, q, 2-H2), 3.63 (2H, t, I 4-H2), 4.17
(2H, t, 2-H2), 4.38 (2H, t, NCH2CH2), 7.25 – 7.30 (2H, m), 7.48 – 7.58 (2H, m, Harom), 10.2 (1H, s, N+H).
Id
2.22 (2H, q, 3-H2), 3.62 (2H, t, 1 4-H2), 4.17 (2H, t, 2-H2), 5.75 (2H, s, HCH2), 7.18 – 7.54 (9H, m, Harom), 11.55
(1H, s, N+H).
Ie*
Ig*
Ih*
Ia
1.0 (6H, t, 2CH3), 1.91 (2H, q, 3-H2), 2.6 (4H, q, CH2N(CH2)2), 2.72 (2H, t, CH2N(CH2)2), 3.51 (2H, t,
4-H2), 3.74 (2H, t, 2-H2), 3.86 (2H, t, NCH2CH2), 6.68 – 6.97 (4H, m, Harom).
CDCl3
1.4 (2H, q, CH2(CH2)2), 1.55 (4H, m, CH2(CH2)2), 1.93 (2H, q, 3-H2), 2.45 (2M, m, N(CH2)2), 2.6 (2H, t,
CH2N(CH2)5), 3.54 (2H, t, 4-H2), 3.78 (2H, t, 2H2), 3.92 (2H, t, NCH2CH2), 6.7 – 6.98 (4H, m, Harom).
CDCl3
1.92 (2H, q, 3-H2), 2.5 (4H, t, N(CH2)2), 2.64 (2H, t, CH2N), 3.5 (2H, t, 4-H2), 3.65 (4H, t, (CH2)2O),
3.76 (2H, t, 2H2), 3.9 (2H, t, NCH2CH2), 6.7 – 6.95 (4H, m, Harom).
DMSO-d6
DMSO-d6
CDCl3
2.22 (2H, q, 3-H2), 3.28 (3H, s, CH3), 3.53 (2H, t, 4-H2), 4.05 (2H, t, 2-H2), 7.25 – 7.30 (2H, m), 7.42 – 7.48 (2H,
m, Harom), 12.6 – 14.4 (1H, broad signal, N+H).
IIb
IIc*
IId*
IIe*
2.19 (2H, q, 3-H2), 3.51 (2H, t, 4-H2), 4.08 (2H, t, 2-H2), 4.88 (2H, s, NCH2), 7.22 – 7.54 (9H, m, Harom), 13.8
(1H, bs, N+H).
0.97 (6H, t, 2CH3), 2.16 (2H, q, 3-H2), 2.55 (4H, q, N(CH2)2), 2.72 (2H, t, CH2N), 3.47 (2H, t, 4-H2),
3.66 (2H, t, 2-H2), 3.89 (2H, t, NCH2), 6.95 – 7.38 (4H, m, Harom).
CDCl3
1.44 (6H, m, (CH2)3), 2.15 (2H, q, 3-H2), 2.42 (4H, m, N(CH2)2), 2.63 (2H, t, CH2N), 3.43 (2H, t, 4-H2),
3.68 (2H, t, 2-H2), 3.88 (2H, t, NCH2), 6.94 – 7.34 (4H, m, Harom).
CDCl3
2.16 (2H, q, 3-H2), 2.46 (4H, m, N(CH2)2), 2.63 (2H, t, CH2N), 3.43 (2H, t, 4-H2), 3.65 (6H, m, (CH2)2O,
2-H2), 3.88 (2H, t, NCH2), 6.96 – 7.35 (4H, m, Harom).
* Compound studied in the base form (because the salts appear as crystal hydrates).

Synthesis of 1-N- and 10-N-Substituted 1(10),2,3,4-Tetrahydropyrimido-[1,2-a]benzimidazoles
471
tor with P2O5 and dried to constant weight; the precipitates of
salts Ig – Ii were dried in a thermal box at 105 – 110°C.
Hydrochlorides and hydrobromides can be also obtained
by bubbling anhydrous gaseous HCl and HBr through the so-
lutions of tricyclic bases in dry benzene, toluene, or ether. All
target salts are highly hygroscopic and form stable crystal
hydrates, which do not release water when dry-treated under
the conditions indicated above.
2-(2-Dialkylaminoethyl)benzimidazoles (IVc – IVe).
A mixture of 10 mmole of hydrochloride VII with 30 mmole
of the corresponding secondary amine (diethylamine, piperi-
dine, morpholine) in anhydrous benzene was stirred at
25 – 35°C until completion of the reaction. The precipitated
secondary amine hydrochloride was separated by filtration
and washed by benzene. The filtrate was evaporated to dry-
ness and the residual salt IVc – IVe recrystallized from ap-
propriate solvent.
The acute hypotensive effect was studied in experiments
on white rats weighing 160 – 220 g narcotized with
ethaminal (pentobarbital) sodium (50 mg/kg, i.p.). The
change in arterial pressure 1 h after drug injection was mea-
sured by a mercury manometer in the carotid artery. The
hypotensive activity was evaluated by determining the effec-
tive dose (ED50) producing a 20% decrease in the arterial
pressure within 30 – 60 min after intraperitoneal injection.
The reference drugs were dibazole and apressin.
The influence of the synthesized compounds on the gas-
tric secretion was judged by determining the change in the
volume of gastric juice and in the concentration of hydro-
chloric acid and pepsin upon a 7-h ligation of the pyloric part
of the stomach [15] in white mongrel rats weighing
210 – 240 g. The test compounds and the reference drug
(cimetidine) were introduced, in the form of suspensions in
1% carboxymethylcellulose solution, by intraduodenal injec-
tions in a concentration of 30 mg/kg. The concentration of
hydrochloric acid was determined by titration to pH 9.0 with
0.1 N KOH solution, while the activity of pepsin was evalu-
ated as described in [16] using albumin as the substrate. Gas-
tric juice production was characterized as percentage change
relative to control.
1-R1-1,2,3,4-Tetrahydropyrimido[1,2-a]benzimidazole
salts (IIa – IIe). Compounds IIa – IIe were obtained, simi-
larly to isomers Id – Ii, by boiling amines IVa – IVe with
1,3-dihalogenpropane in anhydrous xylene for 12 – 16 h.
The target salts IIc – IIe are hygroscopic and their bases ap-
pear as dense oils, which rapidly darken on storage.
The antagonist action of calcium was studied on isolated
portal vein of rat [17] placed in a chamber with
Krebs – Henseleit buffer medium permanently aerated by
bubbling with a gas mixture of 95% oxygen and 5% carbon
dioxide. The isolated vessel contractions were studied in an
isometric regime under a 1-g load, measured with the aid of a
6MX2B mechanotron tube, and recorded using an H338-4N
fast-response recorder [18]. The calcium channels were acti-
vated by adding calcium chloride to a final concentration of
60 mM. The calcium channel activity was evaluated by mea-
EXPERIMENTAL PHARMACOLOGICAL PART
The spasmolytic effect was studied as described in [12]
using an isolated section of the rat small intestine kept at
37°C in an aerated Tyrodes solution. The muscle contrac-
tions were detected with the aid of an isometric device and
recorded with a chart recorder. The spasm was induced by a
barium chloride solution (2 ´ 10 – 4 g/ml). The spasmolytic
activity was expressed in terms of EC50, representing the ef-
fective concentration (mole/liter) suppressing the barium
chloride spasm by half. The reference drugs were dibazole
and papaverine.
TABLE 3. Yields and Physicochemical Characteristics of
Benzimidazoles IV, VI, and VII – Intermediate Products in the Syn-
thesis of Tetrahydropyrimido[1,2-a]benzimidazoles
The effect of the synthesized compounds on the aggrega-
tion of rabbit thrombocytes was studied in vitro [13]. The
thrombocyte aggregation process was induced by ADP
(5 mM). The antiaggregant activity was evaluated by deter-
mining the percentage decrease in the degree of aggregation
relative to control. The most active compounds were charac-
terized by the effective concentration (EC50) inhibiting the
aggregation process by half. The reference compound was
acetylsalicylic acid.
The action of drugs upon the blood glucose level was
studied as described in [14] on white mongrel rats weighing
170 – 210 g. The synthesized compounds and a reference
drug (chlorpropamide) were intraperitoneally injected in a
dose of 50 mg/kg to animals in the test group deprived of
food for 18 h. The content of glucose was determined in
blood samples taken from jugular veins before and after (2
and 4 h) drug injection. The analyses were performed using a
special glucose test kit (Lachema). The dynamics of glucose
level in the rat blood was expressed as percentage variation
(D, %) relative to the initial level.
Compound
M.p.*, °C
Yield, %
Empirical formula
IVc
121 – 122**
131 – 132**
142 – 143**
149 – 150
114 – 115
109 – 110
166 – 167
179 – 180
198 – 200
159 – 160
191 – 192
92.3
93.7
95.2
83.9
84.2
91.4
94.6
96.5
97.0
98.2
99.2
C13H20N4
C14H20N4
IVd
IVe
C13H18N4O
VIa
C11H15N3O
VIb
VIc
C13H19N3O
C14H21N3O
VId
VIIa
VIIb
VIIc
VIId
C17H19N3O
C11H14ClN3 × HCl
C13H18ClN3 × HCl
C14H20ClN3 × HCl
C17H18ClN3 × HCl
*
Compounds IVc – IVe and VIa – VIc were purified by
recrystallization from acetonitrile, and compound IVd from ethyl
acetate.
**
Melts with decomposition.

472
V. A. Anisimova et al.
TABLE 4. Pharmacological
Properties
of
N-Substituted
TABLE 5. Effect of Compounds Ic, Id, and Ih on the Calcium
1(10),2,3,4-Tetrahydropyrimido[1,2-a]benzimidazoles
Channels of Smooth-Muscle Cells in Rat Portal Vein
Spasmolytic Antiaggregant Hypotensive
Compound
EC50, M
Acute toxicity
LD50,
activity
activity
activity
3.1 ´ 10 – 5
1.5 ´ 10 – 5
> 1.0 ´ 10 – 4
1.8 ´ 10 – 5
1.8 ´ 10 – 6
Compound
Ic
mg/kg
EC50, M
EC50, mg/kg ED50, mg/kg
Id
> 1 ´ 10 – 3
> 1 ´ 10 – 3
Ia
…
…
8.0
…
Ih
Ib
3.2
38.4
62.0
37.0
76.5
…
Cinnarizine
Diltiazem
> 1 ´ 10 – 5 5.49 ´ 10 – 4
8.7 ´ 10 – 6 4.57 ´ 10 – 4
7 ´ 10 – 5 3.98 ´ 10 – 4
Ic
5.1
Id
> 20.0
9.2
Ie
4.22 ´ 10 – 4
If
…
2.9
7.2 ´ 10 – 5
> 1 ´ 10 – 3
> 1 ´ 10 – 3
> 1 ´ 10 – 3
5.31 ´ 10 – 4
> 1 ´ 10 – 3
8.6 ´ 10 – 4
1.9 ´ 10 – 4
2.82 ´ 10 – 4
…
Ig
13.1
5.3
105.0
187.5
…
RESULTS AND DISCUSSION
Ih
…
…
…
…
…
…
…
Ii
20.0
7.2
It was established that the studied substances possess
hypotensive properties. The maximum effect was observed
for compounds containing dialkylaminoethyl substituents at
the first (IIc, IIe) and tenth (If, Ih) nitrogen atom: these com-
pounds were superior to dibazole and approached apressin in
hypotensive action (Table 4).
Significant spasmolytic properties were exhibited by
compounds Ic – Ie and Ig. The most active compound (Id)
was superior to dibazole and comparable with papaverine
(Table 4).
The synthesized tetrahydropyrimido[1,2-a]benzimida-
zole derivatives suppress hyperpotassium contractility in the
rat portal vein, but are significantly inferior to diltiazem with
respect to the calcium antagonist activity. However, the most
active compound (Id) somewhat exceeds cinnarizine in this
respect (Table 5).
It was found that 10-N-substituted pyrimido[1,2-a]-
benzimidazoles Ia, Ib, and Ig – Ii do not influence ADP-in-
duced thrombocyte aggregation (Table 4), while the other
compounds (Ic – If, IIa, IIc – IIe) produce a significant
antiaggregant effect exceeding that of acetylsalicylic acid by
a factor of 10 and above.
IIa
87.4
101.2
176.6
141.7
143.1
310.0
…
IIb
3.8
IIc
1.2
IId
13.2
1.3
IIe
8.2 ´ 10 – 5
8.6 ´ 10 – 6
Dibazole
Papaverine
22.1
…
…
Acetylsali-
cylic acid
4.4 ´ 10 – 3
…
…
…
131.0
80.0
Apressin
…
4.0
suring the anisometric tension developed by the isolated ves-
sel in response to the KCl solution added. The test com-
pounds were introduced, in gradually increasing concentra-
tions, into the Krebs – Henseleit solution 30 min before the
induction of hyperpotassium contractility. The ability of the
test substances to block the calcium channels was judged by
the decrease in the level of sample response manifested in
the plot of contractility versus drug dose. The reference
drugs were cinnarizine and diltiazem.
Both 1-N- (IIa, IId) and 10-N-substituted (Ic, Id, Ih)
tetrahydropyrimido[1,2-a]benzimidazoles exhibit hypogly-
cemic activity comparable to that of chlorpropamide (Ta-
ble 6). However, there is a difference between these deriva-
tives with respect to both the rate of development and the
magnitude of the hypoglycemic effect. While the
1-N-substituted tetrahydropyrimido[1,2-a]benzimidazoles
induce a fast and short-time change in the glucose level, the
action of 10-N-substituted derivatives gradually increases to
become reliably distinguished by the end of the experiment
(4 h). The most active and fast hypoglycemic tetrahydropyri-
mido[1,2-a]benzimidazole is compound IId containing a
piperidinoethyl substituent at the first nitrogen atom.
In contrast to the previously reported antisecretor activity
of 9-N-substituted imidazo[1,2-a]benzimidazole derivatives
[23, 24], our tetrahydropyrimido[1,2-a]benzimidazoles with
analogous substituents virtually did not change the volume
of gastric juice and the hydrochloric acid concentration.
The anticalmodulin activity of the synthesized com-
pounds was tested in vitro by monitoring the effect of
calmodulin on cAMP phosphodiesterase activity. The experi-
ments were performed with calmodulin isolated from bovine
brain [19] and partly purified calcium-dependent cAMP
phosphodiesterase [20]. The phosphodiesterase activity was
determined as described in [21] and expressed as (nM
cAMP)/(mg protein) per minute. The calmodulin action was
evaluated in terms of the phosphodiesterase activation factor,
equal to the ratio of the calmodulin-dependent and basal ac-
tivities. The test compounds were introduced into the incuba-
tion medium in a concentration of 100 mM.
The synthesized compounds were also characterized by
the acute daily toxicity determined upon intraperitoneal in-
jections to white mongrel mice weighing 17 – 21 g and ex-
pressed by a graphically determined LD50 value. The experi-
mental data were statistically processed in terms of the Stu-
dent t-criterion [22].

Synthesis of 1-N- and 10-N-Substituted 1(10),2,3,4-Tetrahydropyrimido-[1,2-a]benzimidazoles
473
TABLE 6. Hypoglycemic Effect of N-Substituted 1(10)-Tetra-
TABLE 7. Effect of Compounds Ic and Id on the Basal and
hydropyrimido[1,2-a]benzimidazoles in Intact Rats
Calmodulin-Dependent Activity of cAMP Phosphodiesterase (PDE)
Glucose level variation D (% of initial)
Specific PDE activity (nM
cAMP)/(mg protein) min
Concentra-
tion,
PDE acti-
vation
*
for time after drug injection
Compound
K0.5
,
Compound
mM;
mM
factor**
2 h
4 h
Calmodulin-
Basal
dependent
Ia
– 5.0 ± 1.7
– 7.4 ± 2.8
– 4.5 ± 2.5
– 5.1 ± 2.3
– 3.5 ± 2.7
– 7.8 ± 2.3
– 7.0 ± 3.8
– 4.1 ± 1.1
– 12.1 ± 3.1*
– 0.8 ± 0.8
– 5.3 ± 1.4
– 16.3 ± 4.1*
– 3.0 ± 1.7
– 10.6 ± 2.9*
– 6.3 ± 4.3
– 5.5 ± 3.5
Control
–
–
6.05
5.80
8
0.28 ± 0.03 1.69 ± 0.07
0.28 ± 0.04 1.63 ± 0.28
0.19 ± 0.05 1.52 ± 0.24
Ib
Ic
Id
> 1000
40
100
100
Ic
– 10.8 ± 2.9*
– 13.1 ± 2.4*
– 9.9 ± 1.5*
– 9.7 ± 2.0*
– 10.5 ± 3.9*
– 7.1 ± 1.0*
– 5.7 ± 2.1
Id
Ie
*
Half-maximum displacement of a diS-C3-(5¢) fluorescent dye
from calcium-dependent calmodulin complex.
Ig
**
Ratio of the calmodulin-dependent and basal activities.
Ih
Ii
IIa
7. A. V. El’tsov and K. M. Krivozheiko, Zh. Org. Khim., 2, 189
(1966).
8. A. F. Pozharskii, V. A. Anisimova, and E. B. Tsupak, A Practi-
cal Guide in the Chemistry of Heterocycles [in Russian], Izdat.
Rostov. Univ., Rostov-on-Don (1988).
IIb
– 2.8 ± 1.5
IIc
– 8.0 ± 1.3*
– 1.9 ± 0.9
IId
IIe
– 5.0 ± 2.6
9. A. M. Simonov and V. N. Komissarov, Khim. Geterotsikl.
Soedin., No. 6, 826 – 828 (1976).
Chlorpropamide
– 12.2 ± 2.4*
*
10. S. M. Deshpande and K. Nain, Bull. Chem. Soc. Jpn., 40(4), 998
(1967), Chem. Abstr., 67, 43740s (1967).
Differences from control are reliable for p < 0.05.
11. E. Hoggarth, J. Chem. Soc., 3311 (1949).
12. Ya. I. Khodzhai, G. V. Obolentseva, and A. P. Prokopenko,
Farmakol. Toksikol., 29, 156 – 160 (1966).
13. G. V. R. Born, Nature, 194, 927 – 929 (1962).
14. G. Holland, D. Jaeger, R. Wagner, et al., J. Med. Pharm. Chem.,
3, 99 (1961).
The synthesized tetrahydropyrimido[1,2-a]benzimida-
zoles influenced neither the basal activity of cAMP
phosphodiesterase nor the calmodulin-stimulated activity of
this enzyme. A small tendency for interaction with calmo-
dulin was observed only for compound Id (Table 7).
15. Y. Shay, S. A. Komarov, S. S. Fels, et al., Gastroenterology, 5,
43 – 61 (1945).
16. M. L. Anson, J. Gen. Phisiol., 22, 79 – 89 (1938).
17. K. Golenhofen, N. Hermstein, and E. Lammel, Microvasc. Res.,
5, 73 – 80 (1973).
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