C-C (alkynylation) vs C-O (ether) bond formation under Pd/C-Cu catalysis: Synthesis and pharmacological evaluation of 4-alkynylthieno[2,3-d]pyrimidines

Article abstract of DOI:10.3762/bjoc.7.44

The Pd/C-CuI-PPh₃ catalytic system facilitated C-C bond formation between 4-chlorothieno[2,3-d]pyrimidines and terminal alkynes in methanol with high selectivity without generating any significant side products arising from C-O bond formation b

Full text of DOI:10.3762/bjoc.7.44

C–C (alkynylation) vs C–O (ether) bond formation
under Pd/C–Cu catalysis: synthesis and
pharmacological evaluation of
4-alkynylthieno[2,3-d]pyrimidines
Dhilli Rao Gorja1,2, K. Shiva Kumar1, K. Mukkanti2 and Manojit Pal*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 338–345.
1Institute of Life Sciences, University of Hyderabad Campus,
Gachibowli, Hyderabad-500 046, India and 2Chemistry Division,
Institute of Science and Technology, JNT University, Kukatpally,
Hyderabad 500 072, India
doi:10.3762/bjoc.7.44
Received: 05 December 2010
Accepted: 08 March 2011
Published: 21 March 2011
Email:
Manojit Pal* - manojitpal@rediffmail.com
Associate Editor: J. Aubé
* Corresponding author
© 2011 Gorja et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
catalysis; C–C bond; copper; palladium; thieno[2,3-d]pyrimidine
Abstract
The Pd/C–CuI–PPh3 catalytic system facilitated C–C bond formation between 4-chlorothieno[2,3-d]pyrimidines and terminal
alkynes in methanol with high selectivity without generating any significant side products arising from C–O bond formation
between the chloro compounds and methanol. A variety of novel 4-alkynylthieno[2,3- d]pyrimidines were prepared via alkynyl-
ation of 4-chlorothieno[2,3-d]pyrimidines in good to excellent yields. Some of the compounds synthesized were tested for cyto-
toxic activity in vitro.
Introduction
Alkynyl substituted pyrimidines are of considerable pharmaco- with a thiophene ring might afford compounds, i.e., alkynyl
logical interest because of their notable biological activities [1], substituted thienopyrimidines of potential pharmacological
in particular, adenosine kinase inhibitory activity in the treat- interest. Notably, 6-ethynylthieno[3,2-d]- and 6-ethynyl-
ment of pain and inflammatory diseases [2] and thymidylate thieno[2,3-d]pyrimidin-4-aniline derivatives were found to be
synthase inhibitory properties in cancer therapy [3]. On the potent inhibitors of ErbB family receptor tyrosine kinases
other hand, the thiophene moiety is a common feature in many (EGFR, ErbB-2) and the proliferation of tumor cells that highly
bioactive agents and drugs [4] and is considered as a bioisostere express these kinases [6]. In continuation of our research
of the benzene ring [5]. Thus, one can anticipate that combining program into new drug discovery, we became interested in the
the pyrimidine ring of an alkynyl substituted pyrimidine moiety generation of a small-molecule library A (Figure 1) based on
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Beilstein J. Org. Chem. 2011, 7, 338–345.
Figure 1: Diversity-based thieno[2,3-d]pyrimidine scaffold [7].
thieno[2,3-d]pyrimidine for in-house pharmacological evalua-
tion. Accordingly, we recently reported the synthesis of
4-(hetero)aryl substituted thieno[2,3-d]pyrimidines B [7]. As a
further extension of this research and in view of possible phar-
macological value of compounds containing alkyne, thiophene
and pyrimidine moieties, we now wish to report the synthesis
and in vitro cytotoxicity of novel 4-alkynylthieno[2,3-d]pyrim-
idines C (Figure 1). These derivatives are attractive due to the
synthetic potential of C-4 alkynyl fragments for further use in
library construction.
Scheme 1: Pd/C-mediated synthesis of 4-alkynyl-substituted
thieno[2,3-d]pyrimidines.
A number of methods have been reported for the synthesis of to a known method reported earlier [7]. The other chloro com-
alkynyl substituted pyrimidines and most of which involve the pound 4-chloro-6,7,8,9-tetrahydro-5H-cyclohepta[4,5]thieno-
use of Sonogashira coupling of halopyrimidines with terminal [2,3-d]pyrimidine (1d) was prepared from cycloheptanone and
alkynes [1-3] (for a review see [8]). While alkynylation of the ethyl cyanoacetate following a similar procedure as shown in
thiophene ring of thienopyrimidines under Sonogashira condi- Scheme 2.
tions [9] has previously been reported [6], a similar coupling
reaction on the pyrimidine ring of thieno[2,3-d]pyrimidines is Condensation of resulting amino ester, i.e., ethyl 2-amino-
uncommon in the literature [10]. The use of Pd/C–CuI–PPh3 as 5,6,7,8-tetrahydro-4H-cyclohepta[b]thiophene-3-carboxylate (4)
a less expensive catalyst system for efficient Sonogashira with formamide gave 6,7,8,9-tetrahydro-5H-cyclohepta-
coupling has been explored earlier. Due to our continuing [4,5]thieno[2,3-d]pyrimidin-4-one (5) which on treatment with
interest in Pd/C-mediated alkynylation of aryl and heteroaryl POCl3 under refluxing conditions provided the desired 4-chloro
halides we decided to investigate the Pd/C-based methodology derivative 1d. All the terminal alkynes used were commercially
for the synthesis of our target compounds, i.e., 4-alkynyl- available. Initially, we chose to examine the coupling reaction
thieno[2,3-d]pyrimidines as shown in Scheme 1.
of 4-chloro-5,6,7,8-tetrahydrobenzothieno[2,3-d]pyrimidine
(1a) with phenylacetylene (2a) in the presence of 10% Pd/C
(0.023 mmol), PPh3 (0.17 mmol), CuI (0.04 mmol), and tri-
Results and Discussion
The key starting materials, i.e., 4-chlorothieno[2,3-d]pyrim- ethylamine (2.67 mmol) in various solvents. The results of this
idines 1a–c required for our synthesis were prepared according study are summarized in Table 1. The reaction was initially
Scheme 2: Preparation of 4-chloro-6,7,8,9-tetrahydro-5H-cyclohepta[4,5]thieno[2,3-d]pyrimidine 1d.
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Beilstein J. Org. Chem. 2011, 7, 338–345.
The use of another palladium catalyst (PPh3)2PdCl2 was also
examined and found to be effective (Table 1, entry 7).
However, we preferred to use Pd/C because it is less expensive,
easy to handle and separable from the product and has the
potential for recyclability, for a review see [13]. To assess the
recyclability of the recovered Pd/C-catalyst in the present
coupling reaction, the reaction mixture of 1a and 2a (Table 1,
entry 2) was allowed to cool to room temperature and filtered.
The residue was washed with MeOH, acetone, and DCM. After
drying under vacuum the recovered catalyst was used directly in
the reaction of 1a with 2a in the presence of PPh3, CuI, and
Et3N: A conversion of 85% was observed confirming the re-
cyclability of the recovered Pd/C-catalyst.
Table 1: Effect of solvent on the coupling reaction of 4-chloro-5,6,7,8-
tetrahydrobenzothieno[2,3-d]pyrimidine (1a) with phenylacetylene
(2a).a
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
MeOH
MeOH
THF
5
67
10
14
12
10
10
10
90
80
MeCN
1,4-dioxane
DMF
57 (80c)
59 (80c)
49 (75c)
83d
MeOH
aAll reactions were carried out by using 1 (0.89 mmol), 2a (1.33 mmol),
10% Pd/C (0.023 mmol), PPh3 (0.17 mmol), CuI (0.04 mmol), and
Et3N (2.67 mmol) at 60–65 °C under a nitrogen atmosphere. bIsolated
yields. cThe reaction was carried out at 80 °C. d(PPh3)2PdCl2–CuI was
used instead of 10% Pd/C–PPh3–CuI.
Scheme 3: Reactivity of 4-chlorothieno[2,3-d]pyrimidines 1 towards
terminal alkynes and MeOH under Pd/C–Cu catalysis.
carried out in MeOH for 5 h and the desired product, i.e.,
4-(phenylethynyl)-5,6,7,8-tetrahydrobenzothieno[2,3-d]pyrimi-
dine (3a) was isolated in 67% yield (Table 1, entry 1). The yield Having established the optimum reaction conditions for the
of 3a however was significantly improved when the reaction preparation of 3a we decided to test the generality and scope of
time was increased to 10 h (Table 1, entry 2). Thus the reaction this protocol further. Thus, a variety of commercially available
proceeded well in MeOH to give the expected product via a terminal alkynes was employed under the reaction conditions
C–C bond forming reaction (Scheme 3, path a) and no side pro- indicated in entry 2 of Table 1 and the results are summarized in
duct as a result of C–O bond formation [11] due to participa- Table 2.
tion of MeOH (Scheme 3, path b) was detected in the reaction
mixture. The use of MeOH as a nucleophile in Pd-catalyzed As evident from Table 2, the reaction proceeded well with both
reactions has been well documented in the literature, see for aliphatic (Table 2, entries 2–6 and 8–14) and aromatic alkynes
example [12]. Moreover, when the addition of alkyne was (Table 2, entry 1 and 7). The chloro compounds containing a
omitted without changing the other reaction conditions, com- five (1b), a six (1a) or a seven membered cycloalkane ring (1d)
pound 1a reacted with MeOH to give the corresponding ether, or without a ring (1c) were found to be equally effective under
4-methoxy-5,6,7,8-tetrahydrobenzothieno[2,3-d]pyrimidine, the reaction conditions employed. All the reactions were gener-
albeit in low yield. The use of other solvents such as THF, ally complete within 10 h irrespective of the nature of
acetonitrile, 1,4-dioxane and DMF (Table 1, entries 3-6) also substituents present in the terminal alkynes 2 to afford the
gave good yields of product but MeOH was found to be supe- desired products 3a–n in good to excellent yields.
rior. Moreover, the reaction temperature (and duration in some
cases) was higher, i.e., 80 °C in case of acetonitrile, 1,4-dioxane A plausible mechanism for the Pd/C–Cu mediated alkynylation
and DMF compared to 60–65 °C in case of MeOH. While tri- of 4-chlorothieno[2,3-d]pyrimidines 1 is shown in Scheme 4.
ethylamine was used as a base in all these cases, the use of a The alkynylation proceeds via generation of an active Pd(0)
secondary amine, e.g., pyrrolidine was also examined. A side species in situ that undergoes oxidative addition with 1 to give
product was observed in this case due to the C–N bond forming the organo-Pd(II) species E. The active Pd(0) species is gener-
reaction between 1a and pyrrolidine and was identified as 4-(N- ated from the minor portion of the bound palladium (Pd/C) via a
pyrrolidinyl)-5,6,7,8-tetrahydrobenzothieno[2,3-d]pyrimidine. Pd leaching process in the solution [13]. The leached Pd then
340

Beilstein J. Org. Chem. 2011, 7, 338–345.
Table 2: Pd/C-mediated synthesis of 4-alkynylthieno[2,3-d]pyrimidine in methanol.a
Entry
4-Chlorothieno[2,3-d]pyrimidine
R–≡ (R=)
Productsb
Yieldc (%)
1
–C6H5 (2a)
90
1a
1a
3a
2
–CH2OH (2b)
88
3b
3
1a
–CH2CH2OH (2c)
85
3c
4
1a
–(CH2)2CH3 (2d)
75
3d
5
1a
–C(CH3)2OH (2e)
77
3e
6
–C(CH3)2OH (2e)
80
1b
3f
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Beilstein J. Org. Chem. 2011, 7, 338–345.
Table 2: Pd/C-mediated synthesis of 4-alkynylthieno[2,3-d]pyrimidine in methanol.a (continued)
7
–C6H5 (2a)
90
1c
1c
3g
8
–CH2OH (2b)
85
3h
9
1c
–(CH2)2CH3 (2d)
79
3i
10
11
12
–CH2OH (2b)
87
1d
1d
3j
–CH2CH2OH (2c)
88
3k
1d
–C(CH3)3 (2f)
85
3l
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Beilstein J. Org. Chem. 2011, 7, 338–345.
Table 2: Pd/C-mediated synthesis of 4-alkynylthieno[2,3-d]pyrimidine in methanol.a (continued)
13
1d
–C(CH3)2OH (2e)
85
3m
14
1d
–C(CH2)3CN (2g)
80
3n
aAll the reactions were carried out with 1 (0.89 mmol), 2 (1.33 mmol), 10% Pd/C (0.023 mmol), PPh3 (0.17 mmol), CuI (0.04 mmol), and Et3N
(2.67 mmol) in MeOH (5.0 mL) at 60–65 °C for 10–12 h. bIdentified by 1H NMR, IR, and MS. cIsolated yields.
Scheme 4: Plausible mechanism of Pd/C-mediated alkynylation of 4-chlorothieno[2,3-d]pyrimidines 1.
becomes an active species by interacting with phosphine formation of C–C bond via (i) trans organometallation with
ligands. Thus, a soluble Pd(0)–PPh3 complex is the active copper acetylide generated in situ from CuI and the terminal
species that actually catalyzes the C–C bond forming reaction in alkyne followed by (ii) reductive elimination of Pd(0) to afford
solution. The catalytic cycle therefore works in solution rather 4-alkynylthieno[2,3-d]pyrimidine (3). A C–O bond forming
than on the surface, and at the end of the reaction, re-precipita- reaction between 1 and MeOH (Scheme 2, path b) was not ob-
tion of Pd occurs on the surface of the charcoal. Once gener- served perhaps due to the higher reactivity of copper acetylide
ated, the organo-Pd(II) species E then facilitates the stepwise over MeOH (even although present in excess) towards E.
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Beilstein J. Org. Chem. 2011, 7, 338–345.
their anticancer properties by inhibiting a key enzyme, i.e.,
thymidylate synthase (TS) which is essential for cellular growth
[14]. A possible explanation for observed cytotoxic activities of
present series of compounds 3 therefore could be their potential
inhibition of TS [15]. The CMe2OH group next to the alkynyl
moiety may facilitate the binding of the TS enzyme through its
sulfhydryl (–SH) moiety with compounds 3f and 3g thereby
generating the corresponding drug–enzyme allene intermedi-
ates (Figure 2). Nevertheless, the present study indicated that
4-alkynylthieno[2,3-d]pyrimidine can be used as a template for
the identification of novel and potential anticancer agents.
Table 3: Cytotoxic activity of 4-alkynylthieno[2,3-d]pyrimidines (3)
against chronic myelogenous leukemia (CML) cell line.
% of cell death at two concentrations
Compound
10 µM
20 µM
3a
20
10
25
28
30
32
24
5.4
nil
26
24
26
36
54
60
26
10
nil
3b
3c
3d
3f
3g
3h
Conclusion
In conclusion, the present study demonstrates the first efficient
synthesis 4-alkynylthieno[2,3-d]pyrimidines in good to excel-
lent yields. The combination of Pd/C–CuI–PPh3 proved to be an
10% DMSO
Untreated
We have shown that an alkynyl moiety can be introduced effi- efficient and selective catalytic system for the C–C bond forma-
ciently at the C-4 position of a thieno[2,3-d]pyrimidine ring via tion between 4-chlorothieno[2,3-d]pyrimidine and terminal
C–C coupling under Pd/C–Cu catalysis. To the best of our alkynes in methanol providing a general and practical method
knowledge, only two examples of similar coupling have been for the preparation of these novel compounds. The reaction
reported using the system (PPh3)2PdCl2/CuI as catalyst in the proceeds well with both hydrophobic and hydrophilic terminal
presence of Et3N [10]. The alkynyl substituent of compound 3 alkynes without generating any significant side products arising
could be utilized for further structural elaboration leading to the from C–O bond formation or dimerization of the terminal
functionalized derivatives of thieno[2,3-d]pyrimidine prepara- alkynes. The reaction does not involve the use of expensive
tion which may be difficult to access by other methods. Some of catalysts or reagents and is easy to perform. Some of the com-
the 4-alkynyl-derivatives synthesized were screened for their pounds synthesized were tested for cytotoxic activities in vitro.
cytotoxic activity against chronic myelogenous leukemia The methodology is amenable to the diversity-oriented syn-
(CML) cell line in vitro. The percentage of cell death measured thesis of thieno[2,3-d]pyrimidine derivatives of potential phar-
for each compound at two different concentrations, i.e., 10 and macological significance and therefore may find use in organic
20 µM is shown in Table 3. As evident from Table 3, most of and medicinal chemistry.
the compounds showed moderate to low activity against CML
under the in vitro condition employed. The compounds 3f and
Supporting Information
3g however, showed reasonable activity when tested at 10 µM
which doubled at 20 µM. Comparing the structural features of
Supporting Information features details on experimental
compounds 3f and 3g with the other compounds tested, it may
procedures and spectral data as well as NMR spectra of
be noted that the presence of CMe2OH moiety played an impor-
compounds 3a–n.
tant role in the cytotoxic activities of these compounds. It is
Supporting Information File 1
known that alkynyl substituted pyrimidine derivatives exhibit
Experimental procedures and spectral data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-44-S1.pdf]
Supporting Information File 2
NMR spectra of compounds 3a–n.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-44-S2.pdf]
Acknowledgements
Figure 2: Possible interactions of compounds 3f and 3g with TS
enzyme.
The authors sincerely thank the management of Institute of Life
Sciences for continuous encouragement and support.
344

Beilstein J. Org. Chem. 2011, 7, 338–345.
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