Reactions of salicylaldehydes with activated terminal alkynes in aqueous media: Synthesis of 3-substituted 4-hydroxy chromenes as potential cytotoxic agents

Article abstract of DOI:10.1039/c4ra03882g

Terminal alkynes containing ester, amide or ketone moieties have been reacted with salicylaldehydes in the presence of DABCO affording a direct and single-step method for the regioselective synthesis of 3-substituted 4-hydroxy-4H-chromenes in aqueous 1,4-dioxane. A number of novel chromene derivatives were prepared in good yields using this methodology some of which showed cytotoxic properties when tested against cancer cells.

Full text of DOI:10.1039/c4ra03882g

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Reactions of salicylaldehydes with activated
terminal alkynes in aqueous media: synthesis of 3-
substituted 4-hydroxy chromenes as potential
cytotoxic agents†
Cite this: RSC Adv., 2014, 4, 24870
Received 28th April 2014
Accepted 28th May 2014
Shambhu Nath Singh,ab Ramu Bopanni,a Sarva Jayaprakash,a
DOI: 10.1039/c4ra03882g
d
e
d
K. Venkateshwara Reddy,bc Mohd Ashraf Ashfaq, K. Shiva Kumar and Manojit Pal
*
www.rsc.org/advances
Terminal alkynes containing ester, amide or ketone moieties have environmental friendly but also enhance the reaction rate and
been reacted with salicylaldehydes in the presence of DABCO selectivity on many occasions.11 We therefore aimed to
affording a direct and single-step method for the regioselective explore the use of aqueous media in the synthesis of our target
synthesis of 3-substituted 4-hydroxy-4H-chromenes in aqueous 1,4- chromene derivatives
B (Fig. 1). Among the several
dioxane. A number of novel chromene derivatives were prepared in approaches reported for the construction of 4H-chromene
good yields using this methodology some of which showed cytotoxic ring two major strategies involving the use of (i) transition
properties when tested against cancer cells.
metal catalyzed reactions12 and (ii) organocatalysis found
wide applications. While the metal catalyzed reactions have
broader functional group tolerability and wider substrate
scope the use of toxic metal catalysts oen causes major
concern. Organocatalyzed reactions on the other hand are
carried out using sub-stiochiometric amounts of organic
compounds and are free from the use of any inorganic
materials or enzymes (e.g. biocatalysis). Organocatalysis
therefore provide better alternatives to both metal and bio-
Chromenes (2H-chromenes and 4H-chromenes) constitute an
important class of oxygen heterocycles and are found in many
natural products.1 They also exhibit a range of pharmacological
properties such as anti-HIV,2 antitumor,3 antibacterial/antimi-
crobial,4 fungicidal,5 and insecticidal activities.6 Additionally,
this class of compounds have been well explored for the iden-
tication of potential anticancer agents.7,8 For example the
chromene derivative A (Fig. 1) showed encouraging activities
when tested against melanoma, prostate and glioma cancer cell
lines.9 Indeed, one of them showed strong cytotoxicity in
cellular assays. Prompted by these observations and due to our
continuing interest in the novel and potential cytotoxic agents10
we decided to evaluate a library of small molecules based on
structurally similar but simplied chromene template B (Fig. 1)
for their growth inhibition potential against cancer cells.
It is well known that the use of aqueous media in organic
reactions not only make the process economic and
catalysis and emerged as
a powerful tool in organic
synthesis.13 Accordingly, 4-tosylamino-4H-chromene deriva-
tives (A and B, Scheme 1) have been synthesized by Shi et al.
under the conditions of organocatalysis e.g. via DABCO or
phosphine-catalyzed reactions of salicyl N-tosylimines with
acetylenic (e.g. but-3-yn-2-one and methyl propiolate)14a or
allenic compounds (allenic esters and ketones).14b Notably, it
was observed by the same group that though the amine-
catalyzed reaction between ethyl-2-butynoate and salicyl N-
tosylimine did not afford the corresponding chromenes (C,
Scheme 1) in satisfactory yield, the reaction of diethyl acety-
lenedicarboxylate with salicyl N-tosylimines or salicylalde-
hydes proceeded smoothly (Scheme 2).14c The reaction was
aCustom Pharmaceuticals Services, Dr. Reddy's Laboratories Ltd, Bollaram Road,
Miyapur, Hyderabad 500049, India
bDepartment of Chemistry, JNTUH College of Engineering, JNT University, Kukatpally,
Hyderabad 500085, India
cCMR Engineering College Affiliated to JNTU, Medchal Road, Kandlaykoya,
Hyderabad, Andhra Pradesh 501401, India
dOrganic and Medicinal Chemistry Department, Dr. Reddy's Institute of Life Sciences,
University of Hyderabad Campus, Gachibowli, Hyderabad 500046, India. E-mail:
manojitpal@rediffmail.com
eDepartment of Chemistry, Osmania University, Hyderabad 500007, India
† Electronic supplementary information (ESI) available: Experimental procedures,
copies of the 1H and 13C NMR spectra, HRMS, and method for cytotoxicity assay.
See DOI: 10.1039/c4ra03882g
Fig. 1 Known chromene derivative A and the designed template B
(EWG ¼ electron withdrawing group).
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Initially, the reaction of commercially available 2-hydroxy-
benzaldehyde (1a) with ethyl propiolate (2a) was used to
establish the optimum reaction conditions and the results are
summarized in Table 1. The reaction was performed using the
catalytic amount of DABCO in water at room temperature. The
reaction was completed within 24 h affording the desired ethyl
4-hydroxy-4H-chromene-3-carboxylate (3a) in 65% yield (entry 1,
Table 1). The product yield was increased (85%) considerably
when 1 : 1 aqueous 1,4-dioxane was used (entry 2, Table 1). The
use of other bases like DBU, Et3N and K2CO3 was examined but
found to be less effective (entries 3–5, Table 1). Indeed, the use
of K2CO3 was explored in a range of solvents (entries 6–8, Table
1) but no or poor yield of 3a was observed in these cases.
Notably, a mixture of E/Z-isomers of ethyl 3-(2-formylphenoxy)-
acrylate was isolated as side products in >50% yield in some of
these cases (entries 3–6, Table 1). These side products were
thought to be formed due to an oxa-Michael addition of 1a with
2a suggesting that this might be a key step in the present
synthesis of 3a. Overall, the best result was obtained using
DABCO as a catalyst and 1 : 1 aqueous 1,4-dioxane as a solvent
at room temperature. We therefore used this reaction condi-
tions for further studies.
Scheme
derivatives.
1 Previous synthesis of 4-tosylamino-4H-chromene
With the optimized reaction conditions in hand we then
decided to expand the scope and generality of the present
synthesis of 3-substituted 4-hydroxy chromenes. Thus a range of
salicylaldehydes (1) were reacted with terminal alkynes (2)
containing various carbonyl functionalities (Table 2). Substitu-
ents such as chloro and methoxy on the benzene ring of 1 were
well tolerated irrespective of their position. The terminal
alkynes employed may contain an ester or ketone or N-(un)
substituted amide (e.g. CONH2, CONHMe, or CONHEt) moiety.
The reaction proceeded well in all these cases affording the
desired 3-substituted 4-hydroxy chromene 3 in good yields. The
reaction was found to be highly regioselective as the formation
Scheme 2 Previous synthesis of 4-tosylamino/4-hydroxy-4H-chro-
mene derivatives.
thought to be facilitated by the presence of electron-with-
drawing ester group in place of electron-donating methyl
group in ethyl 2-butynoate.14c These observations prompted
Zhang and Cao et al.15 to explore the use of alkyne reactants
containing highly electronegative polyuoroalkyl groups.
Thus, peruoroalkyl containing substituted 4H-chromenes
were prepared via the Et3N-catalyzed reaction of salicyl N-
tosylimines or salicylaldehydes with methyl 2-per-
uoroalkynoates. Interestingly, while the reaction of internal
alkynes (containing electron withdrawing groups at both the
sp-carbons) with salicylaldehydes leading to the 2,3-disub-
stituted 4-hydroxy-4H-chromenes16 has been explored the
reaction of activated terminal alkynes with salicylaldehydes
has not been studied earlier. Additionally, this strategy would
potentially afford the 3-substituted 4-hydroxy chromene
derivatives in a single step. Herein we report our preliminary
results of this study (Scheme 3) and to the best of our
knowledge this is the rst direct synthesis of 4-hydroxy
chromenes possessing substituents only at C-3 position.
Table 1 Effect of reaction conditions on the reaction of 1a with 2aa
Entry Solvent
Catalyst Time (h) Temp (ꢀC) %Yieldb
1
2
3
4
5
6
7
8
H2O
DABCO 24
rt
rt
rt
rt
65
85
40d
20d
25d
10d
0
1,4-Dioxane–H2Oc DABCO 12
1,4-Dioxane–H2Oc DBU
1,4-Dioxane–H2Oc Et3N
1,4-Dioxane–H2Oc K2CO3
12
12
24
12
12
12
rt
THF–H2Oc
MeCN
THF
K2CO3
K2CO3
K2CO3
65
80
65
0
a
Reactions were carried out using salicylaldehyde (1) (1.0 mmol), alkyne
(2) (1.0 mmol), and a base (0.50 mmol) in a solvent (2.0 mL). b Isolated
yield. Ratio
formylphenoxy)acrylate was isolated as side products in >50% yield. rt
c
d
¼
1 : 1.
A mixture of E/Z-isomers of ethyl 3-(2-
Scheme 3 The reaction of salicylaldehydes (1) with activated terminal
alkynes (2) leading to 3-substituted 4-hydroxy chromene derivatives.
¼ room temp.
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Table
2 DABCO-mediated synthesis of 3-substituted 4-hydroxy
chromene (3)a
Salicyldehydes
Alkynes
Entry (R1, R2, R3 ¼) 1 (X ¼) 2
Time (h) Product 3 Yieldb (%)
Scheme 4 The proposed reaction mechanism.
1
2
3
4
5
H, H, H
1a
OMe, H, H
1b
H, H, OMe
1c
H, OMe, H
1d
1a
OEt
2a
2a
12
10
12
12
10
3a
3b
3c
3d
3e
85
84
72
68
82
polar protic solvent hence the intramolecular aldol reaction (a
reversible step) is favoured in aqueous 1,4-dioxane. Addition-
ally, the last proton transfer step is also expected to be faster in
aqueous 1,4-dioxane. Overall, a combined effect of all these
factors seemed to help the reaction proceed well in aqueous 1,4-
dioxane. While the reaction also proceeded in pure water (entry
1, Table 1) the poor solubility of participating reactants in pure
water perhaps affected the reaction thereby decreasing the
product yield. When E-2 failed to undergo further trans-
formation into E-3 it was simply protonated to give a mixture of
E/Z-isomers of ethyl 3-(2-formylphenoxy)acrylate as observed
during optimization step (entries 3–6, Table 1).
2a
2a
OMe
2b
2b
2b
2a
6
7
8
1b
1c
Cl, H, H
1e
1a
12
12
12
3f
3g
3h
81
69
82
9
Me
2c
8
3i
75
Having synthesized a range of 4-hydroxy chromenes these
compounds were tested for their anti-proliferative properties
against oral (CAL 27) and breast (MD-AMB-231) cancer cell lines
at 10 mM using a sulphorhodamine B assay. Gemcitabine, was
used as a reference compound in this assay.17 The results of
some of these compounds found to be active especially against
breast cancer cell lines are presented in Table 3. While the
compound 3e showed growth inhibition against oral cancer
cells (Table 3), most of the other compounds, for example, 3c,
3d, 3e, 3h and 3i were found to be effective against breast cancer
and 3c and 3d being the best among them (Table 3). Notably,
none of these compounds showed signicant effects on non-
cancer cells e.g. HEK 293T [Human Embryonic Kidney 293 cells]
when tested at 10 mM indicating their selectivity towards cancer
cells. Since breast cancer accounts for 23% of all cancers in
women worldwide, hence identication of appropriate agents to
10
11
12
1b
1e
1a
2c
2c
NH2
2d
2d
2d
NHMe
2e
8
10
12
3j
3k
3l
78
72
82
13
14
15
1b
1c
1a
12
15
10
3m
3n
3o
80
70
77
16
1b
2e
12
3p
78
a
Reactions were carried out using salicylaldehyde (1) (1.0 mmol), alkyne
(2) (1.0 mmol), and DABCO (0.50 mmol) in 1,4-dioxane (1.0 mL) and
water (1.0 mL) at room temperature. b Isolated yield.
of other regioisomer i.e. 2-substituted 4-hydroxy chromene was
not detected in the reaction mixture. This was supported by the
1HNMR spectra of 3 where the C-2 proton appeared as a singlet
in the region 7.6–7.9d. In the case of other regioisomer the C-3
proton was expected to appear as a doublet (due to coupling
with the C-4 proton) at a lower d value. Nevertheless, all the
compounds synthesized were characterized by spectral (NMR,
IR, MS and HRMS) data.
Table 3 The % of growth inhibition of cancer cells by compound 3 at
10 mM
Based on the earlier reports14,15 and the results of Table 1, a
plausible reaction mechanism for the present synthesis of 4-
hydroxy chromenes is shown in Scheme 4. The reaction involves
three major steps e.g. (i) deprotonation, (ii) oxa-Michael addi-
tion, (iii) intramolecular aldol reaction and nally (iv) proton-
ation. Thus, the salicylaldehyde (1) undergoes deprotonation in
the presence of DABCO to generate the anion E-1 and the
conjugate acid DABCOH+. An oxa-Michael addition of E-1 with
the terminal alkyne 2 affords the intermediate E-2 which on
intramolecular aldol type reaction and subsequent protonation
affords the product 3 with the regeneration of the free base
DABCO. Since the ionic intermediate E-3 is stabilized better in a
% inhibitiona
Compounds (3)
CAL 27 (oral cancer)
MDA-MB231 (breast cancer)
3a
3b
3c
3d
3e
3h
3i
32.68
10.26
6.48
28.11
38.48
48.64
48.51
46.72
42.78
45.73
36.62
4.98
46.20
32.63
11.54
0.97
3m
a
Average of at least three determinations.
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ght against breast cancer is an important goal and the
compounds presented here therefore may have medicinal value.
In conclusion, we have demonstrated for the rst time that
appropriately activated terminal alkynes can be reacted with
salicylaldehyde to give 3-substituted 4-hydroxy chromenes in a
regioselective manner. This reaction can be performed in the
presence of DABCO in an aqueous media and a range of
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the analytical group of DRL for support.
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