Regiospecific normal Diels-Alder reaction of trans -1,2- biscoumarinylethenes

Article abstract of DOI:10.1055/s-0031-1290454

The reaction of different 7,8-substituted coumarin 4-acetic acids with 7-diethylaminocoumarin-3-carbaldehyde in the presence of piperidine in methanol gives 1,2-biscoumarinylethenes. These compounds undergo regiospecific Diels-Alder reactions at their electron-rich diene components CCCC with electron-deficient dienophiles. The feasibility of normal electron-demand Diels-Alder reactions could be explained on the basis of the HOMO-LUMO gap. All the compounds are new and are intensely colored. Georg Thieme Verlag Stuttgart ? New York.

Full text of DOI:10.1055/s-0031-1290454

LETTER
▌2189
lRetteergiospecific Normal Diels–Alder Reaction of trans-1,2-Biscoumarinylethenes
Diels–Alder Reaction of trans-1,2-Biscoumarinylethenes
Kailas K. Sanap, Shriniwas D. Samant*
Department of Chemistry, Institute of Chemical Technology, N. Parekh Marg, Matunga, Mumbai 400 019, India
Fax +91(22)33611020; E-mail: samantsd@yahoo.com; E-mail: sd.samant@ictmumbai.edu.in
Received: 07.05.2012; Accepted after revision: 27.06.2012
Alder reaction gives 3,4-annulated coumarins.11 Couma-
Abstract: The reaction of different 7,8-substituted coumarin 4-acetic
rin-containing electron-deficient diene at the 3-position
acids with 7-diethylaminocoumarin-3-carbaldehyde in the presence
undergoes an inverse-electron-demand Diels–Alder reac-
tion with electron-rich dienophiles and gives 3,4-annulat-
of piperidine in methanol gives 1,2-biscoumarinylethenes. These
compounds undergo regiospecific Diels–Alder reactions at their
electron-rich diene components C3–C4–C10–C9, with electron-defi- ed coumarins.12
cient dienophiles. The feasibility of normal electron-demand Diels–
As a part of our ongoing interest in biscoumarinylethenes
Alder reactions could be explained on the basis of the HOMO–
LUMO gap. All the compounds are new and are intensely colored.
and their applications in cycloaddition reactions to build
polycyclic compounds, we herein report the regiospecific
Diels–Alder reactions of (E)-1-(7-diethylaminocoumarin-
3-yl)-2-(7,8-substituted coumarin-4-yl)ethenes 3a–h. We
thought 3 to be interesting substrates for the Diels–Alder
reaction as it contains two distinct dienes, C4–C3–C9–C10
and C3–C4–C10–C9, with different orbital characteristics
and different electron demands; possibly providing the re-
giospecificity in the reaction.
Key words: coumarins, cycloaddition reaction, Diels–Alder reac-
tion, 3,4-annulated coumarin, regiospecific reaction
Coumarins (2H-1-benzopyran-2-ones) and polycyclic
compounds containing coumarin moiety occur in many
plants1 and have important applications in biology. They
form a group of more than 40 drugs, which are used in
medicine and have diverse biological activities, viz. anti-
coagulant, antifungal, hypertensive, CNS depressant, an-
tihelmintic, hypnotic, antitumor agents, and HIV protease
inhibition.2 Coumarin compounds are used as additives in
foods, perfumes, cosmetics, pharmaceuticals, cigarettes,
and alcoholic beverages.3 They find application in fluo-
rescent dyes, as they are effective fluorophores, character-
ized by high fluorescence quantum yields.4 Undeniably,
they constitute the largest class of fluorescent dyes5 and
are widely used as emission layers in organic light-emit-
ting diodes (OLED),6 optical brighteners,7 and nonlinear
optical chromophores.8
7-Diethylaminocoumarin-3-carbaldehyde (1) was synthe-
sized from 4-(diethylamino)salicylaldehyde (4) in two
steps according to a literature procedure (Scheme 1).13
Coumarin-4-acetic acids 2 were prepared by the conden-
sation of phenols 10 with acetone dicarboxylic acid (9),
which in turn was prepared in situ by reacting citric acid
(8) with concentrated sulfuric acid (Scheme 2).14
Coumarin-4-acetic acids 2 are known to react with aro-
matic aldehydes in the presence of piperidine to form 4-
styrylcoumarins.15 Taking inspiration from this, reaction
of 2a with 1 was carried out in the presence of piperidine
in methanol at ambient temperature when (E)-1-(7-dieth-
ylaminocoumarin-3-yl)-2-(7-methyl coumarin-4-yl)ethane
(3a) was obtained (Scheme 3). Different biscoumarinyle-
thenes were thus prepared using optimized conditions
(Table 1). Compounds 3c and 3e were methylated using
DMS and K2CO3 by the standard procedure to obtain 3g
and 3h, respectively.
One of the sites of coumarins that attracts the attention of
chemists is the 3–4 double bond. It shows the properties
of an olefinic double bond and undergoes addition reac-
tions.9 Due to activation by the adjacent carbonyl group, it
also functions as a dienophile.10 Coumarins containing a
vinyl substituent at the 3- and styryl substituent at the 4-
position behave as dienes, and the corresponding Diels–
O
COOEt
CHO
OH
OEt
OEt
EtOH, reflux
piperidine
+
Et2N
O
O
Et2N
O
6
1
4
O
5
H
DMF
POCl3, 60 °C
HCl–H2O (1:1)
100 °C
6
Et2N
O
O
Et2N
O
O
7
Scheme 1 Synthesis of diethylaminocoumarin-3-carbaldehyde
SYNLETT 2012, 23, 2189–2194
Advanced online publication: 21.08.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290454; Art ID: ST-2012-B0397-L
© Georg Thieme Verlag Stuttgart · New York

2190
K. K. Sanap, S. D. Samant
LETTER
R2
O
R1
HO
OH
O
COOH
HOOC
COOH
COOH
concd H2SO4
70 °C, 30 min
10
0 °C to r.t.
O
HO
COOH
8
R1
O
9
R2
2
Scheme 2 Synthesis of coumarin-4-acetic acids
R1
R2
O
O
O
OH
O
piperidine
H
+
O
MeOH
r.t.
Et2N
O
O
R1
O
R2
2a–f
1
Et2N
O
O
3a–f
Scheme 3 Synthesis of biscoumarinylethenes
Compound 3a was reacted with electron-deficient dieno- δ = 7.93 ppm, which is close to the position where it ap-
philes – tetracyanoethylene (11), N-phenylmaleimide peared in the 1H NMR spectrum of 3a. On the other hand
(12), maleic anhydride (13), and diethyl acetylene dicar- the peak due to C3H disappeared. This indicated that the
boxylate (14) (Table 2). Tetracyanoethylene (11) was Diels–Alder reaction of 3a took place on the C3–C4–C10–
1
used with a specific aim to investigate regiospecificity in C9 diene system. The H NMR spectrum of 16a did not
the reaction of 3a. Due to 1,2-tetracyanosubstitution in the show the presence of any olefinic hydrogen; instead it
product, the structure could be deciphered easily.
showed methylene and methine protons (i.e., C10H and
C9H) at δ = 3.59 and 4.29, respectively. It meant that after
the Diels–Alder reaction a 1,3-prototropic shift took place
to form the more stable adduct 16a instead of 15a, and
such a 1,3-prototropic shift is known.16
Table 1 Reaction of 1 with 2a–fa
Entry
R1
R2
Product Time (h)b Yield of
3
3 (%)c
It can be seen that 3 can be represented by many charge-
separated resonance structures (Figure 1). The diethyl-
amino group in the donor ring and the carbonyl group in
the acceptor ring form two termini of an extended conju-
gation, which incorporates both the diene systems. Reso-
nance structures I–IV well represent this situation, and
structure V show that the positive charge is located at C4′.
This indicates that the diene system C3–C4–C10–C9 is elec-
tron rich as compared to diene system C4′–C3′–C9–C10 and
hence the former is susceptible to undergo the normal
electron-demand Diels–Alder reaction.
1
2
3
4
5
6
7
8
Me
H
3a17
3b
3c
19
18
21
24
28
16
–
64
65
61
67
61
71
–
H
H
OH
H
NHCO2Et
OH
H
3d
3e
OH
H
Cl
3f
OMe
OMe
H
3g
3h
OMe
–
–
The HOMO and LUMO energies of 3 and 11 were com-
puted at Accelrys Material Studio using the Dmol3 pro-
gramme (MS 5.2, Table 3). It was found that the energy
gap between the HOMO of 3 and the LUMO of 11 is
smaller than that between the LUMO of 3 and HOMO of
11. Hence, it was concluded that the reaction proceeded
by the normal electron-demand Diels–Alder pathway.
a Reaction conditions: molar ratio of 1/2 = 1:1, piperidine (1 equiv),
MeOH.
b Time for total consumption of 1.
c Isolated yield.
Compound 16a was analyzed by 1H NMR spectroscopy;
C4′H and C3H protons in 3a were decisive in locating the
placement of dienophile in 16a. In the 1H NMR spectrum
of 3a C4′H and C3H appeared at δ = 7.79 and 6.49 ppm,
respectively. In the 1H NMR of 16a, the C4′H appeared at
To widen the scope of reaction, dienes 3a–h were allowed
to react with 11 (Scheme 4, Table 4). Good yield of the
products were obtained in all the cases. The reaction of 3e
and 3h took place rapidly to furnish 16e and 16h in 73%
Synlett 2012, 23, 2189–2194
© Georg Thieme Verlag Stuttgart · New York

LETTER
Diels–Alder Reaction of trans-1,2-Biscoumarinylethenes
Table 3 HOMO and LUMO Energies for 3a
2191
and 83% yield, respectively, while 3d and 3f were found
to be less reactive.
Table 2 Diels–Alder Reaction of 3a with Different Dienophilesa,b
3
HOMO (eV) LUMO (eV) DieneHOMO
–
DienophileHOMO –
dienophileLUMO dieneLUMO
Entry Dienophile Product
Time Yield
(h)c (%)d
3a –4.862
3b –4.887
–3.008
–3.127
–3.013
–2.969
–3.086
–3.262
–3.039
–3.012
0.761
0.736
0.763
0.773
0.713
0.628
0.766
0.734
5.308
5.189
5.303
5.347
5.23
(λmax, nm
)
1
2
4
6
6
85
79
77
81
Me
NC
CN
CN
3c
–4.860
NC
3d –4.850
O
11
3e
3f
–4.910
–4.995
O
5.054
5.277
5.304
CN
CN
3g –4.857
3h –4.889
NC
O
CN
Et2N
O
16a (400)
a Tetracyanoethylene (11): HOMO: –8.316 eV, LUMO: –5.623 eV.
Me
O
2
components. Reaction of trans-1,2-biscoumarinylethenes
with electron-deficient dienophiles took place regiospe-
cifically on the electron-rich diene component, that is, C3–
C4–C10–C9.
N
Ph
O
O
O
O
12
Table 4 Diels–Alder Reaction of 3 with 11 in Dioxanea,b
N
Et2N
O
O
O
O
Entry Substrate Product (λmax,nm
)
Time Yield
(h)c (%)d
O
Ph
3
17 (380)
Me
O
1
3a
2
85
Me
3
O
O
O
O
O
O
O
CN
13
CN
NC
O
O
CN
Et2N
O
Et2N
O
O
16a18 (400)
18 (382)
Me
4e
CO2Et
2
3b
2
81
O
O
CO2Et
O
CN
O
14
CN
NC
O
CN
CO2Et
Et2N
O
CO2Et
Et2N
O
16b (401)
OH
3
3c
2.25 80
19 (380)
a Reaction conditions: molar ratio of 3a/dienophile = 1:2.
b Reflux in dioxane.
O
c Time for the total consumption of 3a.
d Isolated yield.
O
e Reflux in o-dichlorobenzene. For UV: DMSO, 10–3 M (10 ppm),
28–30 °C.
CN
CN
NC
O
CN
Et2N
O
In conclusion, condensation of coumarin-4-acetic acids
with diethylaminocoumarin-3-carbaldehyde gave trans-
1,2-biscoumarinylethenes which have two different diene
16c (398)
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2189–2194

2192
K. K. Sanap, S. D. Samant
LETTER
Table 4 Diels–Alder Reaction of 3 with 11 in Dioxanea,b (continued)
Acknowledgment
K.K.S. is thankful to the CSIR, New Delhi, for a fellowship.
Entry Substrate Product (λmax,nm
)
Time Yield
(h)c (%)d
3
NHCO2Et
O
Supporting Information for this article is available online at
4
5
6
3d
3e
3f
2.5
1.5
2.5
74
73
76
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
m
t
iornat
References and Notes
O
(1) (a) Geissman, T. A. The Chemistry of Flavonoid
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Flavonoids: The Advances in Research since 1980;
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Medicinal Plants, Vol. 3; Rastogi, R. P., Ed.; Central Drug
Research Institute and Publications and Information
Directorate: Lucknow/New Delhi, 1993, 273.
CN
CN
NC
O
CN
Et2N
O
16d (400)
OH
OH
O
(3) Izquierdo, M. E. F.; Granados, J. Q.; Mir, V. M.; Martinez,
M. C. L. Food Chem. 2000, 70, 251.
(4) Kuznetsova, N. A.; Kaliya, O. L. Usp. Khim. 1992, 61,
1243.
O
CN
CN
(5) (a) Jones, G.; Jackson, W. R.; Choi, C.; Bergmark, W. R.
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Martínez, F. J.; González, J.; García-Báez, E. V.; Cruz, A.;
Padilla-Martínez, I. I. Molecules 2010, 15, 1513.
(g) Amantini, D.; Fringuelli, F.; Pizzo, F. J. Org. Chem.
2002, 67, 7238.
NC
O
CN
Et2N
O
16e (399)
Cl
O
O
CN
CN
NC
O
CN
Et2N
O
O
O
16f (402)
OMe
7
3g
2.25 81
O
O
CN
CN
NC
O
CN
Et2N
16g (401)
OMe
8
3h
1.5
83
OMe
O
(11) (a) Yasuda, M.; Kishi, T.; Goto, C.; Satoda, H.;
Nakabayashi, K.; Minami, T.; Shima, K. Tetrahedron Lett.
1992, 33, 6465. (b) Minami, T.; Matsumoto, Y.; Nakamura,
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77, 1828. (d) Mustafa, A.; Kamel, M.; Allam, M. A. J. Am.
Chem. Soc. 1956, 78, 4692. (e) El-Shafei, A.; Fadda, A. A.;
Abdel-Gawad, I. I.; Youssif, E. H. E. Synth. Commun. 2009,
39, 2954.
O
CN
CN
NC
O
CN
Et2N
16h (402)
a Reaction conditions: molar ratio of 3/11 = 1:2.
b Reflux temperature.
c Time for total consumption of 3.
(12) (a) Bodwell, G. J.; Pi, Z.; Pottie, I. R. Synlett 1999, 477.
(b) Pottie, I. R.; Nandaluru, P. R.; Bodwell, G. J. Synlett
d Isolated yield. For UV: DMSO, 10–3M (10 ppm), 28–30 °C.
Synlett 2012, 23, 2189–2194
© Georg Thieme Verlag Stuttgart · New York

LETTER
Diels–Alder Reaction of trans-1,2-Biscoumarinylethenes
2193
R1
R1
R1
R2
R2
R2
7
8
6
5
1
O
O
O
4
10
3'
–
2
O
O
O
–
3
4'
5'
8'
9
–
6'
+
+
+
2'
7'
Et2N
O
O
Et2N
O
O
Et2N
O
O
1'
I
II
III
R1
R1
R2
O
R2
O
O–
O–
+
+
Et2N
O
O
Et2N
O
O
IV
V
Figure 1 Resonance structures of 3
R1
R1
R1
R2
3
R2
R2
2
1
4
O
O5
O
1,3 prototropic
shift
NC
NC
CN
CN
H
10
9
6
O
O
O
4'
5'
8'
7
CN
CN
11
8
6'
CN
CN
dioxane
reflux
NC
O
NC
O
CN
CN
7'
Et2N
O
O
O
O
Et2N
Et2N
1'
3
15
16a–h
Scheme 4 Synthesis of tetrahydro-6H-dibenzo[b,d]pyran-6-one derivatives by the Diels–Alder reaction of 3 and 11
2011, 2245. (c) Pottie, I. R.; Nandaluru, P. R.; Benoit, W. L.;
Miller, D. O.; Dawe, L. N.; Bodwell, G. J. J. Org. Chem.
2011, 76, 9015. (d) Nandaluru, P. R.; Bodwell, G. J. Org.
Lett. 2012, 14, 310.
chromatography on silica gel using toluene–EtOAc (70:30,
v/v); orange solid; 64% yield; mp 231–233 °C. IR (neat): ν
= 1698, 1609, 1575, 1513, 1352, 1252, 1192, 1132 cm–1. 1H
NMR (500 MHz, CDCl3): δ = 1.25 (t, 6 H, 2 × CH3, J = 7.5
Hz), 2.46 (s, 3 H, CH3), 3.47 (q, 4 H, 2 × CH2, J = 7.5 Hz),
6.49 (s, 1 H, C3H), 6.51 (d, 1 H, C8′H, J = 2.0 Hz), 6.65 (dd,
1 H, C6′H, J = 2.0, 9.0 Hz), 7.13–7.15 (m, 2 H, C5′H and
C8H), 7.20 (d, 1 H, C10H, J = 15.5 Hz), 7.37 (d, 1 H, C6H,
J = 8.5 Hz), 7.77 (d, 1 H, C5H, J = 8.5 Hz), 7.79 (s, 1 H,
C4′H), 7.99 (d, 1 H, C9H, J = 15.5 Hz). MS: m/z = 402.57
[M + 1]. Anal. Calcd (%) for C25H23NO4 (401.45): C, 74.79;
H, 5.77; N, 3.49. Found: C, 74.58; H, 5.88; N, 3.52.
(13) Wu, J.-S.; Liu, W.-M.; Zhuang, X.-Q.; Wang, F.; Wang,
P.-F.; Tao, S.-L.; Zhang, X.-H.; Wu, S.; Lee, S.-T. Org. Lett.
2007, 9, 33.
(14) Laskowski, S. C.; Clinton, R. O. J. Am. Chem. Soc. 1950, 72,
3987.
(15) Soliman, A. Y.; El-Kafrawy, A. F.; Mohamed, F. K.; Baker,
H. M.; Abdel-Gawad, A. M. Indian J. Chem., Sect. B: Org.
Chem. Incl. Med. Chem. 1991, 30, 477.
(16) Kudale, A. A.; Kendall, J.; Miller, D. O.; Collins, J. L.;
Bodwell, G. J. J. Org. Chem. 2008, 73, 8437.
(17) Experimental Procedure for 3a
(18) Experimental Procedure for 16a
Compound 3a (0.401 g, 1 mmol) and 11 (0.256 g, 2 mmol)
were refluxed in dioxane (6 mL) for 2 h. After complete
consumption of 3a, the solution was cooled to r.t. and was
evaporated to obtain a sticky mass which was purified by
column chromatography on silica gel using toluene–EtOAc
(80:20, v/v); buff solid; 85% yield; mp 269–270 °C. IR
(neat): δ = 2975, 1702, 1594, 1519, 1251, 1128 cm–1. 1H
NMR (300 MHz, CDCl3): δ = 1.23 (t, 6 H, 2 × CH3, J = 6.9
Hz), 2.50 (s, 3 H, CH3), 3.50 (q, 4 H, 2 × CH2, J = 6.9 Hz),
3.59 (d, 2 H, C10H, J = 7.2 Hz), 4.29 (t, 1 H, C9H, J = 7.2
Hz), 6.44 (d, 1 H, C8′H, J = 2.1 Hz), 6.64 (dd, 1 H, C6′H,
Compound 2a (0.218 g, 1 mmol) and piperidine (0.085 g,
1 mmol) were stirred in MeOH (6 mL) for 15 min and 1
(0.245 g, 1 mmol) was added slowly at r.t. After complete
consumption of 1, the precipitated solid was collected by
filtration and washed with a small quantity of cold MeOH
followed by H2O to remove traces of piperidine. Solid 3a
was washed with EtOAc. The second crop of 3a was
obtained from the filtrate; the filtrate was evaporated to
obtain a sticky mass which was purified by column
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2189–2194

2194
K. K. Sanap, S. D. Samant
LETTER
CN), 109.8 (C6′), 111.6 (C4a′), 114.6 (C6a), 117.7 (C4),
124.8 (C2), 127.2 (C1), 130.1 (C5′), 142.8 (C4′), 147.4 (C3),
152.0 (C7′), 152.3 (C10a), 152.9 (C4a′), 156.5 (C8a′), 157.1
(C2′), 161.5 (C6). MS: m/z = 530.46 [M + 1]. Anal. Calcd
(%) for C31H23N5O4 (529.55): C, 70.31; H, 4.38; N, 13.23.
Found: C, 70.48; H, 4.47; N, 13.19.
J = 2.1, 8.7 Hz), 7.22 (s, 1 H, C8H), 7.24 (d, 1 H, C6H, J =
7.8 Hz), 7.38 (d, 1 H, C5′H, J = 8.7 Hz), 7.58 (d, 1 H, C5H,
J = 8.1 Hz), 7.93 (s, 1 H, C4′H). 13C NMR (300 MHz,
CDCl3): δ = 12.4 (2 × CH3), 22.0 (ArCH3), 28.3 (C10), 36.9
(C9), 42.3 (C8), 45.1 (2 × CH2), 46.5 (C7), 97.0 (C8′), 107.5
(C3′), 108.9 (CN), 109.2 (CN), 109.4 (C10b), 109.7 (2 ×
Synlett 2012, 23, 2189–2194
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