Efficient synthesis of new tetracyclic benzofuro[3,2-d]-imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones

Article abstract of DOI:10.1002/cjoc.201090072

The aza-Wittig reaction of iminophosphorane (1) with aromaic isocyanates gave carbodiimides (2), which were allowed to react further with α-amino ester in the presence of a catalytic amount of sodium ethoxide to give selectively new tetracyclic benzofuro[3,2-d]imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones (5) in good yields. X-ray structure analysis of 5i verified the proposed structure and the reaction selectivity.

Full text of DOI:10.1002/cjoc.201090072

FULL PAPER
Efficient Synthesis of New Tetracyclic Benzofuro[3,2-d]-
imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones
Hu, Yanggena,b(胡扬根)
b Department of Pharmacy, Taihe Hospital of Yunyang Medical College and Institute of Medicine Chemistry,
Yunyang Medical College, Shiyan, Hubei 442000, China
Liu, Mina(刘敏)
Ding, Mingwu*,a(丁明武)
a Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Central China Normal University,
Wuhan, Hubei 430079, China
The aza-Wittig reaction of iminophosphorane (1) with aromaic isocyanates gave carbodiimides (2), which were
allowed to react further with α-amino ester in the presence of a catalytic amount of sodium ethoxide to give selec-
tively new tetracyclic benzofuro[3,2-d]imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones (5) in good yields. X-ray
structure analysis of 5i verified the proposed structure and the reaction selectivity.
Keywords benzofuro[3,2-d]imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-dione, iminophosphorane, aza-Wittig reaction,
isocyanate, carbodiimide
Introduction
via aza-Wittig reaction of β-ethoxycarbonyl-imino-
phosphorane (1) with aromatic isocyanate and subsequent
reaction with various nucleophiles under mild condi-
tions.19 However, the reaction of α-amino ester with
β-ethoxycarbonyl-carbodiimide was not investigated. As
a continuation of our research for new biologically active
heterocycles, we herein wish to report further an efficient
synthesis of the previously unreported benzofuro-
[3,2-d]imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones via
the iminophosphorane (1).
Benzofuropyrimidinones are important heterocycles
bearing remarkable biological activities. Some of them
have shown good analgesic, anti-inflammatory and an-
timicrobial activities,1-3 whereas others exhibit good
anticoccidial and blood sugar-lowering activities.4,5 On
the other hand, heterocycles containing an imidazolone
nucleus also exhibit various biological activities. Sev-
eral of them have shown good antibacterial, antifungal
activities or been used as leukotriene B4 receptor an-
tagonist and potassium channel openers.6-10 The intro-
duction of an imidazolone ring to the benzofuro-
[3,2-d]pyrimidin-4(3H)-one system is expected to in-
fluence the biological activities significantly. However,
this tetracyclic system has been much less investigated1
and there is no report on synthesis of benzofuro[3,2-d]-
imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones, probably
due to the fact that the tetracyclic system is not easily
accessible by routine synthetic methods.
The aza-Wittig reactions of iminophosphoranes have
received increased attention in view of their utility in the
synthesis of nitrogen heterocyclic compounds.11-14 An-
nelation of ring systems with N-heterocycles by means
of an aza-Wittig reaction has been widely utilized be-
cause of the availability of functionalized iminophos-
phoranes. Recently we have been interested in the syn-
thesis of fused pyrimidinones and imidazolones via
aza-Wittig reaction, with the aim of evaluating their
fungicidal activities.15-21 We also reported an efficient
preparation of benzofuro[3,2-d]pyrimidin-4(3H)-ones
Experimental
General procedure
Melting points were determined using a Beijing
Taike X-4 model apparatus and are uncorrected. MS
data were measured on a Finnigan Trace MS instrument.
IR spectra were recorded on a PE-983 infrared spec-
－
1
trometer as KBr pellets with absorption in cm . NMR
spectra were recorded in CDCl3 or DMSO-d6 on a Var-
ian Mercury 400 spectrometer and resonances are given
in δ relative to TMS. Elemental analyses were taken on
a Vario EL III elemental analysis instrument. The X-ray
diffraction data were collected on a Bruker SMART
AXS CCD diffractometer.
Preparation
of
benzofuro[3,2-d]imidazo[1,2-a]-
pyrimidine-2,5-(1H,3H)-diones (5a— 5k)
To a solution of iminophosphorane (1)19 (0.93 g, 2
mmol) in dry methylene dichloride (15 mL) was added
*
E-mail: mwding@mail.ccnu.edu.cn; Tel.: 0086-027-63158845; Fax: 0086-027-67862041
Received May 15, 2009; revised Augest 26, 2009; accepted October 10, 2009.
Project supported by the National Natural Science Foundation of China (No. 20772041), the Key Project of Chinese Ministry of Education (No.
107082), the Key Project of Hubei Provincial Department of Education (No. D200724001) and the Science Innovation Team Research Project of
Yunyang Medical College (No. 2008 CXG01).
Chin. J. Chem. 2010, 28, 309— 312
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
309

Hu, Liu & Ding
FULL PAPER
－1
aromatic isocyanate (2 mmol) under nitrogen in 0—5
℃. After the reaction mixture stood for 6—8 h in 0—5
℃, the solvent was removed under reduced pressure and
ether/petroleum ether (V∶V＝1∶2, 20 mL) was added
to precipitate triphenylphosphine oxide. Removal of the
solvent gave carbodiimide (2), which was directly used
without further purification. A mixture of α-amino acid
ester hydrochloride (2 mmol) and triethylamine (0.61 g,
4 mmol) in acetonitrile (10 mL) was stirred for 10 min
and filtered. Then the filtrate was added to the solution
of carbodiimide (2) prepared above in dry methylene
dichloride (10 mL) at room temperature. After being
stirred for 0.5 h, the solution was concentrated and an-
hydrous EtOH (10 mL) with several drops of EtONa in
EtOH was added. The mixture was stirred for 4—6 h at
room temperature and concentrated under reduced pres-
sure, and the residual was recrystallized from methylene
dichloride/petroleum ether to give benzofuro[3,2-d]-
imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones (5a—5k).
1-Phenylbenzofuro[3,2-d]imidazo[1,2-a]pyrimidine-
2,5-(1H,3H)-dione (5a) White solid, m.p.＞300 ℃;
1H NMR (DMSO-d6, 400 MHz) δ: 7.90—7.41 (m, 9H,
Ar-H), 4.79 (s, 2H, CH2); IR (KBr) ν: 1754 (C＝O),
1713 (C＋＝O), 1546, 1347, 1189 cm ; MS m/z (%): 359
(29) [M ], 316 (38), 287 (23), 277 (100), 260 (25), 130
(33), 101 (16). Anal. calcd for C21H17N3O3: C 70.18, H
4.77, N 11.69; found C 70.03, H 4.71, N 11.83.
3-Methyl-1-phenylbenzofuro[3,2-d]imidazo[1,2-a]-
pyrimidine-2,5-(1H,3H)-dione (5e) White solid, m.p.
290—291 ℃; 1H NMR (DMSO-d6, 400 MHz) δ: 7.90—
7.41 (m, 9H, Ar-H), 5.07 (q, J＝7.2 Hz, 1H, CH), 1.80
(d, J＝7.2 Hz, 3H, CH3); IR (KBr) ν: 1764 (C＝O),
－1
1715 (C＝O), 1548, 1341, 1172 cm ; MS m/z (%): 331
(100) [M＋], 302 (30), 287 (51), 275 (51), 130 (40), 101
(25), 91 (13). Anal. calcd for C19H13N3O3: C 68.88, H
3.95, N 12.68; found C 68.83, H 3.70, N 12.84.
1-(3-Tolyl)benzofuro[3,2-d]imidazo[1,2-a]pyrimi-
dine-2,5-(1H,3H)-dione (5f) White solid, m.p.＞300
1
℃; H NMR (DMSO-d6, 400 MHz) δ: 7.91—7.33 (m,
8H, Ar-H), 4.78 (s, 2H, CH2), 2.42 (s, 3H, CH3); MS m/z
(%): 331 (100) [M＋], 302 (30), 287 (51), 275 (51), 130
(40), 101 (25), 91 (13). Anal. calcd for C19H13N3O3: C
68.88, H 3.95, N 12.68; found C 68.61, H 3.89, N 12.73.
1-(4-Chlorophenyl)-3-(i-propyl)benzofuro[3,2-d]-
imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-dione
(5g)
1
White solid, m.p. 266—267 ℃; H NMR (DMSO-d6,
400 MHz) δ: 7.91—7.43 (m, 8H, Ar-H), 5.01 (d, J＝3.2
Hz, 1H, CH), 3.03—2.99 (m, 1H, CH), 1.26 (d, J＝7.2
Hz, 3H, CH3), 0.90 (d, J＝7.2 Hz, 3H, CH3); IR (KBr) ν:
1757 (C＝O), 1722 (C＝O), 1600, 1498, 1368, 1188
－1
1706 (C＝O), 1568, 1379, 1048 cm ; MS m/z (%): 317
(M＋, 100), 288 (58), 260 (59), 185 (12), 130 (62), 101
(32), 77 (12). Anal. calcd for C18H11N3O3: C 68.14, H
3.49, N 13.24; found C 68.10, H 3.52, N 13.19.
－1
＋
3-Hydroxymethyl-1-phenylbenzofuro[3,2-d]imid-
azo[1,2-a]pyrimidine-2,5-(1H,3H)-dione (5b) White
cm ; MS m/z (%): 393 (23) [M ], 350 (24), 310 (100),
287 (12), 130 (29), 101 (21). Anal. calcd for
C21H16ClN3O3: C 64.05, H 4.09, N 10.67; found C 63.99,
H 4.02, N 10.75.
1
solid, m.p. 255—256 ℃; H NMR (DMSO-d6, 400
MHz) δ: 7.90—7.42 (m, 9H, Ar-H), 5.50 (t, J＝6.0 Hz,
1H, CH), 5.15 (s, 1H, OH), 4.54—4.49 (m, 1H, CHHO),
4.05—4.00 (m, 1H, CHHO); IR (KBr) ν: 3419 (O—H),
1-(4-Fluorophenyl)-3-(i-propyl)benzofuro[3,2-d]-
imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-dione
(5h)
－1
1
1765 (C＝O), 1705 (C＝O), 1599, 1350, 1211 cm ;
White solid, m.p. 216—218 ℃; H NMR (CDCl3, 400
MHz) δ: 7.95—7.25 (m, 8H, Ar-H), 4.95 (d, J＝3.6 Hz,
1H, CH), 3.29—3.21 (m, 1H, CH), 1.38 (d, J＝7.2 Hz,
3H, CH3), 0.94 (d, J＝7.2 Hz, 3H, CH3); MS m/z (%):
377 (25) [M＋], 335 (24), 295 (100), 279 (20), 130 (26),
101 (18), 83 (14). Anal. calcd for C21H16FN3O3: C 66.84,
H 4.27, N 11.14; found C 66.79, H 4.22, N 11.05.
MS m/z (%): 347 (17) [M＋], 329 (99), 317 (60), 288
(41), 260 (88), 130 (100), 101 (47), 77 (20). Anal. calcd
for C19H13N3O4: C 65.70, H 3.77, N 12.10; found C
65.93, H 3.62, N 12.05.
3-Ethoxycarbonylmethyl-1-phenylbenzofuro[3,2-
d]imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-dione (5c)
1
White solid, m.p. 183—184 ℃; H NMR (CDCl3, 400
3-Ethoxycarbonylmethyl-1-(4-fluorophenyl)benzo-
furo[3,2-d]imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-dio-
MHz) δ: 7.94—7.34 (m, 9H, Ar-H), 5.02 (t, J＝5.6 Hz,
1H, CH), 4.10—4.03 (m, 3H, OCH2 and CHHCOO),
3.38—3.33 (m, 1H, CHHCOO), 1.18 (t, J＝7.2 Hz, 3H,
CH3); 13C NMR (100 MHz, CDCl3) δ: 170.1, 169.1,
157.0, 151.2, 151.1, 143.9, 135.7, 131.2, 129.8, 129.3,
129.1, 127.0, 123.8, 122.3, 121.8, 112.8, 61.4, 55.8,
32.2, 13.9; IR (KBr) ν: 1764 (C＝O), 17－27 (C＝O),
1
ne (5i) White solid, m.p. 204—206 ℃; H NMR
(CDCl3, 400 MHz) δ: 7.94—7.25 (m, 8H, Ar-H), 5.02 (t,
J＝3.6 Hz, 1H, CH), 4.10—4.04 (m, 3H, OCH2 and
CHHCO2), 3.35 (dd, J1＝18.0 Hz, J2＝3.2 Hz, 1H,
CHHCO2), 1.19 (t, J＝7.2 Hz, 3H, CH3); 13C NMR (100
MHz, CDCl3) δ: 170.2, 169.2, 163.8, 161.3, 157.0, 151.2,
151.1, 143.8, 135.7, 129.8, 129.1, 129.0, 127.1, 123.8,
122.3, 121.7, 116.5, 116.3, 112.8, 61.4, 55.8, 32.2, 13.9;
IR (KBr) ν: 1772 (C－＝O), 1745 (C＝O), 1712 (C＝O),
1
1705 (C＝O), 16＋04, 1352, 1209, 1129 cm ; MS m/z
(%): 403 (38) [M ], 357 (15), 329 (100), 275 (15), 260
(27), 130 (16). Anal. calcd for C22H17N3O5: C 65.50, H
4.25, N 10.42; found C 65.43, H 4.23, N 10.18.
＋
1
1584, 1352, 1186 cm ; MS m/z (%): 421 (10) [M ], 347
(70), 294 (21), 279 (59), 222 (13), 130 (100), 101 (80), 95
(27). Anal. calcd for C22H16FN3O5: C 62.71, H 3.83, N
9.97; found: C 62.62, H 3.95, N 10.05.
1-Phenyl-3-(i-propyl)benzofuro[3,2-d]imidazo[1,2-
a]pyrimidine-2,5-(1H,3H)-dione (5d) White solid,
1
m.p. 266—267 ℃; H NMR (DMSO-d6, 400 MHz) δ:
7.90—7.43 (m, 9H, Ar-H), 5.02 (d, J＝3.2 Hz, 1H, CH),
3.04—2.98 (m, 1H, CH), 1.27 (t, J＝7.2 Hz, 3H, CH3),
0.91 (t, J＝7.2 Hz, 3H, CH3); IR (KBr) ν: 1762 (C＝O),
1-(4-Fluorophenyl)-3-methylbenzofuro[3,2-d]imi-
dazo[1,2-a]pyrimidine-2,5-(1H,3H)-dione (5j) White
solid, m.p. 230—232 ℃; 1H NMR (CDCl3, 400 MHz) δ:
310
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Chin. J. Chem. 2010, 28, 309— 312

Synthesis of Tetracyclic Benzofuro[3,2-d]imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-diones
7.90—7.42 (m, 8H, Ar-H), 5.07 (q, J＝7.2 Hz, 1H, CH),
1.80 (d, J＝7.2 Hz, 3H, CH3); IR (KBr) ν: 1760 (C＝O),
Scheme 1 Preparation of compounds 5
－1
1720 (C＝O), 1590, 1352, 1192 cm ; MS m/z (%): 349
(100) [M＋], 320 (60), 295 (26), 279 (54), 130 (54), 101
(28), 95 (14). Anal. calcd for C19H12FN3O3: C 65.33, H
3.46, N 12.03; found C 65.28, H 3.39, N 11.90.
3-(s-Butyl)-1-(4-fluorophenyl)-benzofuro[3,2-d]-
imidazo[1,2-a]pyrimidine-2,5-(1H,3H)-dione
(5k)
1
White solid, m.p. 224—226 ℃; H NMR (CDCl3, 400
MHz) δ: 7.94—7.25 (m, 8H, Ar-H), 5.04 (d, J＝3.2 Hz,
1H, CH), 3.03—2.98 (m, 1H, CH), 1.93—1.69 (m, 2H,
CH2), 1.11 (t, J＝7.6 Hz, 3H, CH3), 0.89 (d, J＝6.8 Hz,
3H, CH3); IR (KBr) ν: 1760 (C＝O), 1718 (C＝O),
＋
－1
1578, 1352, 1129 cm ; MS m/z (%): 391 (17) [M ],
335 (100), 295 (88), 279 (45), 222 (9), 130 (12). Anal.
calcd for C22H18FN3O3: C 67.51, H 4.64, N 10.74; found
C 67.41, H 4.60, N 10.65.
X-ray crystal structure analysis for compound (5i)
formula C22H16FN3O5, colorless crystal. The crystal is
of triclinic, space group P1 with a＝5.7933(9) Å, b＝
9.1494(14) Å, c＝10.2380(16) Å, β＝96.327(2)°, V＝
477.97(13) Å3, Z＝1, Dc＝1.464 g/cm3, F(000)＝218,
－1
µ＝0.122 mm , R＝0.0342 and wR＝0.0957 for 1941
observed reflections with I＞2σ(I0). Crystallographic
data (excluding structure factors) for the structure of 5i
reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication no. CCDC-714882. Copies of the data
can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK [fax.: (inter-
nat.) 00 44 1223/336-033; e-mail: deposit@ccdc.cam.
ac.uk].
Table 1 Preparation of compounds 5a— 5k from iminophos-
phorane 1
Compd.
5a
Ar
Ph
R
H
Yielda/%
81
5b
5c
Ph
CH2OH
CH2COOEt
i-Pr
76
Ph
86
5d
5e
Ph
79
Ph
Me
83
Results and discussion
5f
3-CH3C6H4
4-ClC6H4
4-FC6H4
4-FC6H4
4-FC6H4
4-FC6H4
H
80
Iminophosphorane (1) was reacted with aromatic
isocyanates to give carbodiimides (2), which were al-
lowed to react with α-amino ester at room temperature
in the presence of a catalytic amount of sodium ethoxide
to give benzofuro[3,2-d]imidazo[1,2-a]pyrimidine-2,5-
(1H,3H)-diones (5) selectively. A variety of α-amino
esters and isocyanates could be used for this synthetic
strategy and the products 5 were obtained in good yields
(Scheme 1). Presumably the reaction of carbodiimides 2
with α-amino ester should afford primarily guanidine
intermediates of type 3. From these intermediates 3, the
formation of two cyclized products imidazolone 4 (via
path a), and benzofuro[3,2-d]pyrimidin-4(3H)-one (6)
(via path b) could in principle take place. The formation
of 5 might probably be due to a consecutive cyclization
of 3 to imidazolone intermediate 4 and further base
catalytic cyclization between the imidazolone ring NH
and ethoxylcarbonyl. The result illustrated also that the
imidazolone (4) was more easily produced from inter-
mediate 3 than benzofuro[3,2-d]pyrimidin-4(3H)-one
(6).
5g
i-Pr
81
5h
5i
i-Pr
78
CH2COOEt
Me
85
5j
79
5k
s-Bu
74
a Isolated yields based on iminophosphorane 1.
their spectrum data. For example, the 1H NMR spectrum
of 5e shows the signal of CH at δ 5.07 as quartet and
signal of CH3 at δ 1.80 as doublet. The signals attribut-
able to the Ar-Hs are found in δ 7.90—7.41 as mutiplets.
The IR spectra of 5e revea－led two C＝O absorption
1
bands at 1764 and 1715 cm due to the imidazolone
and pyrimidinone carbonyl groups, respectively. The
mass spectrum of 5e shows a strong molecular ion peak
at m/z 331 with 100% abundance. Furthermore, a single
crystal of 5i was obtained from a CH2Cl2 solution of 5i.
X-ray structure analysis verified again the proposed
structure, and showed that all ring atoms in the tetra-
cyclic moiety were nearly coplanar and the fluorophenyl
ring was twisted with respect to the pyrimidinone ring
system by 62.9(2)°. The C atoms of the ethyl side chain
The structure of benzofuro[3,2-d]imidazo[1,2-a]-
pyrimidine-2,5-(1H,3H)-diones (5) was confirmed by
Chin. J. Chem. 2010, 28, 309— 312
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
311

Hu, Liu & Ding
FULL PAPER
were disordered over two positions, with site occupancy
factors 0.60 and 0.40, and were refined using a split
model.
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Figure 1 ORTEP diagram of the crystal structure of tetracyclic
compound 5i (Drawn at the 50% thermal ellipsoids).
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In conclusion, we have developed a new efficient
synthesis for benzofuro[3,2-d]imidazo[1,2-a]pyrimidine-
2,5-(1H,3H)-diones via an iminophosphorane. This
method utilizes easily accessible starting materials and
allows mild one-pot reaction conditions, straightforward
product isolation and good yields.
15 Huang, N. Y.; Nie, Y. B.; Ding, M. W. Synlett 2009, 611.
16 Huang, N. Y.; Liang, Y. J.; Ding, M. W.; Fu, L. W.; He, H. W.
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18 Liu, M. G.; Hu, Y. G.; Ding, M. W. Tetrahedron 2008, 64,
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Chin. J. Chem. 2010, 28, 309— 312