Applying an aza-wittig reaction for the synthesis of novel thieno[3′,2′:5,6] pyrido[4,3-d]pyrimidinone derivatives

Article abstract of DOI:10.1080/10426500802101109

A series of new 2-substituted tetrahydrobenzo[4′,5′] thieno[3′,2′:5,6]pyrido[4,3-d] pyrimidin-4(3H)-ones 5 has been designed and synthesized via an aza-Wittig reaction. Iminophosphorane 3a or iminophosphorane 3b reacted with 4-Cl-phenyl(or 4-F-phenyl) iso

Full text of DOI:10.1080/10426500802101109

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Applying an Aza-Wittig
Reaction for the Synthesis
of Novel Thieno[3′,2′:5,6]
Pyrido[4,3-d]pyrimidinone
Derivatives
Jian-Chao Liu a b & Hong-Wu He a
a Key Laboratory of Pesticide & Chemical Biology,
Ministry of Education, College of Chemistry , Central
China Normal University , Wuhan , P. R. China
b College of Chemistry and Chemical Engineering ,
Central South University , Changsha , P. R. China
Published online: 19 Dec 2008.
To cite this article: Jian-Chao Liu & Hong-Wu He (2008) Applying an Aza-Wittig
Reaction for the Synthesis of Novel Thieno[3′,2′:5,6] Pyrido[4,3-d]pyrimidinone
Derivatives, Phosphorus, Sulfur, and Silicon and the Related Elements, 184:1, 234-241,
DOI: 10.1080/10426500802101109
To link to this article: http://dx.doi.org/10.1080/10426500802101109
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Phosphorus, Sulfur, and Silicon, 184:234–241, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802101109
Applying an Aza-Wittig Reaction for the Synthesis
of Novel Thieno[3ꢀ,2ꢀ:5,6] Pyrido[4,3-d]pyrimidinone
Derivatives
Jian-Chao Liu1,2 and Hong-Wu He1
1Key Laboratory of Pesticide & Chemical Biology, Ministry of Education,
College of Chemistry, Central China Normal University,
Wuhan, P. R. China
2College of Chemistry and Chemical Engineering, Central South
University, Changsha, P. R. China
A series of new 2-substituted tetrahydrobenzo[4ꢀ,5ꢀ]thieno[3ꢀ,2ꢀ:5,6]pyrido[4,3-d]
pyrimidin-4(3H)-ones 5 has been designed and synthesized via an aza-Wittig reac-
tion. Iminophosphorane 3a or iminophosphorane 3b reacted with 4-Cl-phenyl(or
4-F-phenyl) isocyanate to give carbodiimide 4a or carbodiimide 4b, which were fur-
ther treated with phenols to cyclize to give compounds 5 in presence of a catalytic
amount of K2CO3. The structures of compound 5 have been confirmed by 1H NMR,
EI-MS, IR spectroscopy, and elemental analyses.
Keywords Aza-Wittig reaction; carbodiimide; iminophosphorane; pyrido[4,3-
d]pyrimidin-4(3H)-ones
INTRODUCTION
Thienopyridines are of chemical and pharmacological interest due
to their structures being similar to quinolines and isoquinolines,
two important heterocycles in many alkaloids.1,2 The derivatives of
pyridopyrimidine have also attracted the interest of pharmaceutical
companies recently. This is due in part to the wide range of biological
activities associated with this structure. For example, some related
4-(phenylamino)pyrido[d]pyrimidines have been reported as selective
inhibitors of tyrosine phosphorylation by epidermal growth factor
Received 6 March 2008; accepted 3 April 2008.
We gratefully acknowledge financial support of this work by the Post Doctoral Science
Foundation of Central South University and the National Natural Science Foundation
of China (No. 20372023 ).
Address correspondence to Jian-Chao Liu, Key Laboratory of Pesticide & Chemical
Biology, Ministry of Education, College of Chemistry, Central China Normal University,
Wuhan, 430079, P. R. China. E-mail: journal@mail.ccnu.edu.cn
234

Applying the Aza-Wittig Reaction
235
receptor (EGFR) and become an important class of potential anticancer
drugs.3,4 In this article, we are interested in the synthesis of new
pyridine derivatives that contain the thienopyridine ring and the
pyridopyrimidine ring.
The aza-Wittig reactions of iminophosphoranes have received in-
creasing attention in view of their utility in the synthesis of nitro-
gen heterocyclic compounds.5,6 Recently we have become interested
in the synthesis of pyrazolopyrimidinones and thienopyrimidinones
from various iminophosphoranes, with the aim of evaluating their
biological activities.7 Here we wish to report further a facile syn-
thesis of tetrahydrobenzo[4ꢀ,5ꢀ]thieno[3ꢀ,2ꢀ:5,6]pyrido[4,3-d]pyrimidin-
4(3H)-one derivatives 5, in which are contained the thienopyridine ring
and the pyridopyrimidine ring, from easily accessible iminophospho-
rane 3.
RESULTS AND DISCUSSION
The tetrahydrobenzo[4,5]thieno[2,3-b]pyridine 2a (or 2b), easily ob-
tained from tetrahydrobenzo[b]thiophene 1, methyl acetoacetate (or
ethyl acetoacetate), and stannic chloride, was converted to iminophos-
phorane 3a (or 3b) via reaction with triphenylphosphine, hex-
achloroethane, and Et3N (see Scheme 1). The yield of 2a (38%) is less
than the yield of 2b (64%), but the yield of 3a (90%) is almost equal to
that of 3b (91%).
Iminophosphorane 3a (or iminophosphorane 3b) reacted with 4-
Cl-phenyl(or 4-F-phenyl) isocyanate to give carbodiimide 4. Even
in refluxing toluene and by heating, 4 was not allowed to react
NH2
CN
COOR
CH3
Ph3P
CH3COCH2COOR
SnCl4
C2Cl6/Et3N
S
NH2
S
N
1
2
OR1
Ar
O
NAr
C
N
N
N
PPh3
N
N
N
N
COOR
COOR
HOR1
base
ArNCO
S
CH3
S
CH3
S
CH3
5
4
3
1
2,3: R=CH (a), C H (b); 5: Ar=4-Cl-Ph, 4-F-Ph; R =Ph, 4-Cl-Ph, etc.
3
2 5
SCHEME 1

236
J.-C. Liu and H.-W. He
TABLE I Physical Constants of Compound 5
Compd.
R1
Ar
Color
m.p./◦C aYield /% bYield/%
5a
5b
5c
5d
5e
5f
5g
5h
5i
4-CH3Ph
4-ClPh
4-ClPh White crystals 280∼282
4-ClPh White crystals 288∼289
4-ClPh White crystals 272∼275
4-ClPh Yellow crystals 220∼221
4-ClPh White crystals 290∼292
4-ClPh White crystals 283∼285
4-ClPh White crystals 300∼301
4-ClPh White crystals 261∼262
4-ClPh White crystals 264∼266
4-ClPh White crystals 252∼254
4-ClPh White crystals 239∼240
76
50
64
62
85
43
68
68
51
78
86
63
65
39
37
58
70
62
57
59
69
31
63
71
74
30
57
66
52
69
75
49
64
24
31
64
58
44
34
52
Ph
4-NO2Ph
2,4-Cl2Ph
2-ClPh
4-BrPh
2,4-F2Ph
3-FPh
5j
5k
5l
2-Cl-4-FPh
3-Cl-4-FPh
2-Cl-5-CH3Ph 4-ClPh White crystals 276∼279
4-Cl-3-CH3Ph 4-ClPh White crystals 260∼261
5m
5n
5o
5p
5q
5r
5s
5t
3,5-F2Ph
3-MePh
4-NO2Ph
2,4-Cl2Ph
4-MePh
4-ClPh White crystals 283∼284
4-ClPh Yellow crystals 284∼285
4-FPh White crystals 219∼221
4-FPh White crystals 248∼249
4-FPh White crystals 300∼301
2-Cl-5-CH3Ph 4-FPh White crystals 265∼266
4-Cl-3-CH3Ph 4-FPh White crystals 221∼223
aThe yields of 5 from 3a.
bThe yields of 5 from 3b.
with phenols to produce 2-aryloxy(ethoxy)-8,9,10,11-tetrahydrobenzo
[4ꢀ,5ꢀ]thieno[3ꢀ,2ꢀ:5,6]pyrido[4,3-d]pyrimidin-4(3H)-one 5. However,
when carried out in the presence of catalytic K2CO3, the reaction took
place to give 5 in good yields under the condition of heating (see Table I).
Irrespective of whether the substitutes on the phenols were electron-
withdrawing or electron-releasing groups, the cyclization was carried
out smoothly. The yields of 5 from 3 a are a bit more than those from
3b (see Table I).
All the products
5 were purified by recrystallization from
dichloromethane and petroleum ether. The results are listed in Ta-
ble I. The structures of 8,9,10,11-tetrahydrobenzo[4ꢀ,5ꢀ]thieno[3ꢀ,2ꢀ:5,6]
pyrido[4,3-d]pyrimidin-4(3H)-one 5 was confirmed by 1H NMR, IR, MS,
and elementary analyses(see Tables II–IV). For example, the IR spectra
of 5a revealed C O absorption band at 1701 cm−1 and 3124 cm−1 due
to Ph-H group. The 1H NMR spectral data of 5a show the signal of CH3
(CH3 of pyridyl) at 3.02 ppm as singlet and signals of cyclohexenyl CH2
at 1.59, 1.80, 2.44, and 2.79 ppm. 2.39 ppm is the signal of CH3 of the
benzene ring. The other signals appeared at 6.99∼7.57 (m, 8H, Ar-H).

237

238
J.-C. Liu and H.-W. He
TABLE III 1H NMR Spectral Data of Compounds 5
Compd.
1H NMR ( ppm, CDCl3 , TMS, 400 MHz)
5a
1.60∼1.80(m, 4H, 2CH2), 2.39 (s, 3H, CH3 of phenyl), 2.44∼2.79 (m, 4H,
2CH2), 3.02 (s, 3H, CH3 of pyridyl), 6.99∼7.57 (m, 8H, Ar-H).
1.59∼1.81 (m, 4H, 2CH2), 2.45∼ 2.81 (m, 4H, 2CH2), 3.03 (s, 3H, CH3),
7.08∼7.58 (m, 8H, Ar-H).
5b
5c
5d
5e
5f
1.56∼1.77 (m, 4H, 2CH2), 2.40∼2.78 (m, 4H, 2CH2), 3.03 (s, 3H, CH3),
7.13∼7.58 (m, 9H, Ar-H).
1.56∼1.78 (m, 4H, 2CH2), 2.41∼2.80 (m, 4H, 2CH2), 3.04 (s, 3H, CH3),
7.27∼8.35 (m, 8H, Ar-H).
1.61∼1.81 (m, 4H, 2CH2), 2.34∼2.80 (m, 4H, 2CH2), 3.03 (s, 3H, CH3),
7.15∼7.58 (m, 7H, Ar-H).
1.95∼2.01 (m, 4H, 2CH2), 2.49∼2.81 (m, 4H, 2CH2), 3.01 (s, 3H, CH3),
7.09∼7.57 (m, 8H, Ar-H).
5g
5h
5i
1.62∼1.80 (m, 4H,2CH2), 2.42∼2.80 (m, 4H, 2CH2), 3.03 (s, 3H, CH3),
7.02∼7.58 (m, 8H, Ar-H).
1.60∼1.79 (m, 4H, 2CH2), 2.41∼2.80 (m, 4H, 2CH2), 3.04 (s, 3H, CH3),
6.95∼7.58 (m, 7H, Ar-H).
1.63∼1.81 (m, 4H, 2CH2), 2.50∼2.82 (m, 4H, 2CH2), 3.05 (s, 3H, CH3),
6.94∼7.58 (m, 8H, Ar-H).
5j
1.59∼1.79 (m, 4H, 2CH2), 2.36∼2.82 (m, 4H, 2CH2), 3.04 (s, 3H, CH3),
1.09∼7.59 (m, 7H, Ar-H)
5k
5l
1.69∼1.82 (m, 4H, 2CH2), 2.54∼2.83 (m, 4H, 2CH2), 3.03 (s, 3H, CH3),
7.01∼7.58 (m, 7H, Ar-H).
1.57∼1.80 (m, 4H, 2CH2), 2.37 (s, 3H, CH3 of phenyl), 2.37∼2.81 (m, 4H,
2CH2), 3.04 (s, 3H, CH3 of pyridyl), 7.06∼7.58 (m, 7H, Ar-H).
1.65∼1.83 (m, 4H, 2CH2), 2.47 (s, 3H, CH3 of phenyl), 2.46∼2.82 (m, 4H,
2CH2), 3.05 (s, 3H, CH3 of pyridyl), 6.91∼7.58 (m, 7H, Ar-H).
1.95∼2.02 (m, 4H, 2CH2), 2.91∼3.08 (m, 4H, 2CH2), 3.00 (s, 3H, CH3),
7.24∼7.52 (m, 7H, Ar-H).
5m
5n
5o
5p
5q
5r
5s
5t
1.59∼1.80 (m, 4H, 2CH2), 2.39 (s, 3H, CH3 of phenyl), 2.74∼2.97 (m, 4H,
2CH2), 3.00 (s, 3H, CH3 of pyridyl), 7.09∼7.54 (m, 8H, Ar-H).
1.57∼1.80 (m, 4H, 2CH2), 2.42∼2.81 (m, 4H, 2CH2), 3.06 (s, 3H, CH3),
7.27∼8.35 (m, 8H, Ar-H).
1.62∼1.81 (m, 4H, 2CH2), 2.34∼2.80 (m, 4H, 2CH2), 3.03 (s, 3H, CH3),
7.16∼7.53 (m, 7H, Ar-H).
1.59∼1.80 (s, 4H, 2CH2), 2.39 (s, 3H, CH3), 2.44∼2.79 (m, 4H, 2CH2),
3.02 (s, 3H, CH3), 6.99∼7.57 (m, 8H, Ar-H).
1.57∼1.78 (m, 4H, 2CH2), 2.38 (s, 3H, CH3 of phenyl), 2.38∼2.79 (m, 4H,
2CH2), 3.03 (s, 3H, CH3 of pyridyl), 7.05∼7.48 (m, 7H, Ar-H).
1.65∼1.81 (m, 4H, 2CH2), 2.47 (s, 3H, CH3 of phenyl), 2.50∼2.80 (m, 4H,
2CH2), 3.03 (s, 3H, CH3 of pyridyl), 6.91∼7.40 (m, 7H, Ar-H).
The MS spectrum of 5a shows an obvious molecule ion peak at m/z 488
with 100% abundance. The structure of 5a was also established on the
basis of elemental analysis data: Anal. Calcd. (%) for C27H22ClN3O2S:
C, 66.45; H, 4.54; N, 8.61. Found: C, 66.67; H, 4.41; N, 8.72.

Applying the Aza-Wittig Reaction
239
TABLE IV The EI-Mass Spectra of Compound 5
Compd.
MS (EI, m/z, %)
5a
5b
5c
5d
5e
5f
5g
5h
5i
488(M+ 100), 473(18), 396(8), 380(10).
509(23), 508(M+ 100), 495(11), 479(12), 396(10), 380(14).
475(44), 474(M+ 100), 458(22), 445(15).
520(49), 519(M+ 100), 518(80), 504(10), 491(16) , 396(29), 380(19).
543(M+ 100) , 543(90), 529(10), 396(16), 380(18), 354(15).
508(M+ 5), 400(36), 396(100), 368(11), 216(26).
552(M+ 100), 539(14), 526(11) .
511(35), 510(M+ 100) , 495(13), 481(18), 396(12), 380(26).
493(39), 492(M+ 100), 477(18), 463(20), 396(13), 380(18).
528(64), 527(M+ 35), 526(100), 510(12), 497(13), 396(12), 380(20).
5j
5k
5l
528(62), 527(M+ 43), 526(100), 512(12), 510(19), 499(15), 497(30), 380(13).
523(27), 522(M+ 100), 506(11), 488(12), 396(14), 380(28).
523(49), 522(M+ 100), 508(12), 506(19), 493(14), 396(17), 380(28).
510(M+ 12), 398(100), 397(99), 393(11), 369(14), 272(35), 215(55).
488(M+ 100), 473(18), 397(58), 380(10).
5m
5n
5o
5p
5q
5r
5s
5t
502(M+ 100), 488(19), 475(26), 380(13), 364(13).
527(21) , 526(M+ 100), 510(11), 497(10), 380(9).
473(33), 472(M+ 100), 381(98), 353(38), 215(13).
507(49), 506(M+ 100), 505(86), 490(20), 470(15), 380(38), 360(10), 333(11).
507(49), 506(M+ 100), 505(99), 490(24), 382(30), 380(42), 364(54), 334(13).
EXPERIMENTAL
Melting points were determined with a WRS-1B digital melting point
apparatus and were uncorrected. EI-MS spectra were measured on
a Finnigan Trace Mass Spectrometer. IR spectra were recorded on a
Shimadzu IR-408 Infrared Spectrometer. 1H NMR spectra were taken
on a Varian XL-300 Spectrometer. Elementary analyses were recorded
on a Varian EL III elementary analysis instrument. All of the solvents
and materials were reagent grade and purified as required.
Preparation of Thienopyridine Derivatives 28
2-Amino-thienonitrile 1 (1.78 g, 10 mmol) and SnCl4 (2.3 mL, 20 mmol)
were added to a stirred solution of methyl acetoacetate (1.18 g, 10
mmol) in dry toluene (20 mL). The reaction mixture was stirred at
room temperature for 1 h and then heated under reflux for 5 h. The
mixture was added to a saturated aqueous solution of Na2CO3 (60 mL,
pH = 10∼10.5). The suspension was extracted with ethyl acetate(3 ×
50 mL). The combined extracts were dried (Na2SO4) and concentrated
under reduced pressure to give the thienopyridine derivative 2a in 38%.

240
J.-C. Liu and H.-W. He
White crystals, M.p.: 177∼179◦C. Anal. Calcd. (%) for C14H16N2O2S: C
60.85, H 5.84, N 10.14; ound C 60.59, H 5.51, N 9.95. 1H NMR (CDCl3,
400 Hz): δ = 1.87∼1.93 (m, 4H, CH2CH2 ), 2.69 (s, 3H, CH3 of pyridyl),
2.81∼3.02 (m, 4H, 2CH2), 3.92 (s, 3H, OCH3), 6.63 (s, 2H, NH2) ppm.
Following this procedure, with ethyl acetoacetate (1.30 g, 10 mmol)
instead of methyl acetoacetate (1.18 g, 10 mmol), the compound 2b
was obtained in 64%.White crystals, M.p.: 137∼138◦C. Anal. Calcd.
(%) for C15H18N2O2S: C 62.04, H 6.25, N 9.65; Found C 62.53, H 6.31,
N 9.95. 1H NMR (CDCl3, 400 Hz): δ = 1.42(t, J= 7.2 Hz, 3H, CH3),
1.87∼1.94 (m, 4H, CH2CH2 ), 2.73 (s, 3H, CH3 of pyridyl), 2.80∼3.01
(m, 4H, 2CH2), 4.39 (q, J= 7.2 Hz, 2H, OCH2), 6.60 (s, 2H, NH2) ppm.
Preparation of Iminophosphorane 39,10
A solution of thienopyridine derivative 2a (1.09 g, 4 mmol) in CH3CN
(15mL) was added to triphenylphosphine (1.31 g, 5 mmol) and C2Cl6
(1.19 g, 5 mmol). The mixture was treated with triethylamine (8.0 mL),
then stirred for 18∼24 h at 0◦C. The solution was condensed, and the
residue was recrystallized from CH3CH2OH to give iminophosphorane
3a in yield 91%. M.p.: 211∼212◦C. Anal. Calcd. (%) for C33H31N2O2PS:
C 71.98, H 5.67, N 5.09; Found C 71.69, H 5.90, N 5.28. 1H NMR
(CDCl3, 400 MHz): δ = 1.63∼1.66 (m, 4H, CH2CH2 ), 2.39 (s, 3H, CH3
of pyridyl), 2.53∼2.68 (m, 4H, 2CH2), 3.02 (s, 3H, OCH3), 7.43∼7.62
(m, 15H, Ar-H) ppm.
Following this procedure with 2b (1.16 g, 4 mmol) instead of
2a (1.09 g, 4 mmol), the compound 3b was obtained in yield 90%.
M.p.:224∼225◦C. Anal. Calcd. (%) for C32H29N2O2PS: C 71.62, H 5.45,
1
N 5.22; Found C 71.90, H 5.28, N 5.49. H NMR(CDCl3, 400 MHz):
δ = 0.99 (t, J= 7.2 Hz, 3H, CH3), 1.40∼1.64 (m, 4H, CH2CH2 ), 2.41
(s, 3H, CH3 of pyridyl), 2.53∼2.67 (m, 4H, 2CH2), 3.38 (q, J= 7.2 Hz,
2H, OCH2), 7.44∼7.62 (m, 15H, Ar-H) ppm.
General Procedure for the Preparation of Compound 5
Method A
To a solution of iminophosphorane 3a(0.53 g, 1 mmol) in an-
hydro CH2Cl2 (10 mL), aromatic isocyanate (1.1 mmol) under N2
at room temperature was added. After the reaction mixture was
left unstirred for 30–40 min, the solvent was removed under re-
duced pressure, and Et2O/petroleum ether was added to precipi-
tate triphenylphosphine oxide. Removal of the solvent gave carbodi-
imides 4, which were used directly without further purification. To

Applying the Aza-Wittig Reaction
241
the solution of 4 prepared above in CH3CN (15 mL) was added
phenol (1.1 mmol) and catalytic K2CO3. The mixture was stirred
for 12 h at 80◦C, the solution was condensed, and the residue
was recrystallized from CH3CN to give 2-alkoxyl(aryloxyl)-3-(4-Cl-
phenyl or 4-F-phenyl)-5-methyl-8,9,10,11-tetrahydrobenzo[4ꢀ,5ꢀ]thieno
[3ꢀ,2ꢀ:5,6]pyrido[4,3-d]pyrimidin-4(3H)-ones 5.
Method B
Following this general procedure with iminophosphorane 3b
(0.55 g, 1 mmol) instead of iminophosphorane 3a (0.53 g, 1 mmol),
the compounds 2-alkoxyl(aryloxyl)-3-(4-Cl-phenyl or 4-F-phenyl)-5-
methyl-8,9,10,11-tetrahydrobenzo[4ꢀ,5ꢀ]thieno[3ꢀ,2ꢀ:5,6]pyrido[4,3-d]-
pyrimidin-4(3H)-ones 5 were also obtained.
REFERENCES
[1] G. Amaury and L. Jean-Francois, Synthesis, 12, 1935–1937 (2004).
[2] F. Al-Omran, M. M. A. Khalik, H. Al-Awadhi, and M. H. Elnagdi, Tetrahedron, 52,
11915–11928 (1996).
[3] G. W. Rewcastle, B. D. Palmer, A. M. Thompson, and A. J. Bridges, J. Med. Chem.,
39, 1823–1835 (1996).
[4] J. B. Smail, B. D. Palmer, and G. W. Rewcastle, J. Med. Chem., 42, 1803–1815 (1999).
[5] D. Vazquez Vlarelle, C. Peinador Veira, and J. M. Quintela Lopez, Tetrahedron, 60,
275–283 (2004).
[6] T. Okawa and S. Eguchi, Tetrahedron, 54, 5853–5868 (1998).
[7] D. W. Ding, S. J. Yang, and J. Zhu, Synthesis, 1, 75–79 (2004).
[8] A. C. Veronese, R. Callegari, and C. F. Morelli, Tetrahedron, 51, 12277–12284 (1995).
[9] M. W. Ding, S. Z. Xu, and J. F. Zhao, J. Org. Chem., 69, 8366–8371 (2004).
[10] H. Wamhoff, S. Herrmann, and S. Stolbern, Tetrahedron, 49, 581–594 (1993).