Cerium(III) chloride-catalyzed one-pot synthesis of tetrahydrobenzo[b] pyrans

Article abstract of DOI:10.1080/00397910802378399

Efficient one-pot synthesis of 4H-benzo[b]pyrans via a three-component cyclocondensation of aryl aldehydes, malononitrile, and dimedone has been reported using CeCl₃·7H₃O as catalyst. Copyright Taylor & Francis Group, LLC.

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Cerium(III) Chloride–Catalyzed
One-Pot Synthesis of
Tetrahydrobenzo[b]pyrans
Gowravaram Sabitha a , K. Arundhathi a , K.
Sudhakar a , B. S. Sastry b & J. S. Yadav a
a Organic Division I, Indian Institute of Chemical
Technology , Hyderabad, India
b University College of Pharmaceutical Sciences,
Andhra University , Vizag, India
Published online: 08 Jan 2009.
To cite this article: Gowravaram Sabitha , K. Arundhathi , K. Sudhakar , B. S.
Sastry & J. S. Yadav (2009) Cerium(III) Chloride–Catalyzed One-Pot Synthesis of
Tetrahydrobenzo[b]pyrans, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 39:3, 433-442, DOI:
10.1080/00397910802378399
To link to this article: http://dx.doi.org/10.1080/00397910802378399
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Synthetic Communications1, 39: 433–442, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802378399
Cerium(III) Chloride–Catalyzed One-Pot
Synthesis of Tetrahydrobenzo[b]pyrans
Gowravaram Sabitha,1 K. Arundhathi,1 K. Sudhakar,1 B. S. Sastry,2
and J. S. Yadav1
1Organic Division I, Indian Institute of Chemical Technology, Hyderabad, India
2University College of Pharmaceutical Sciences, Andhra University, Vizag, India
Abstract: Efficient one-pot synthesis of 4H-benzo[b]pyrans via a three-
component cyclocondensation of aryl aldehydes, malononitrile, and dimedone
has been reported using CeCl3Á7H2O as catalyst.
Keywords: Benzo[b]pyrans, cerium(III) chloride, MCRs, one pot
INTRODUCTION
Multicomponent reactions (MCRs) have attracted considerable interest
because of their exceptional synthetic and practical efficiency.[1] MCRs
involve three or more starting materials reacting in a single flask to form
a new product, where basically all the atoms contribute to the newly
formed product. One example of an MCR is a three-component, one-
pot synthesis of tetrahydrobenzo[b]pyrans. Many natural products
containing benzopyran core structure display a wide array of pharmaco-
logical activities such as antibacterial, antitumor, antiallergic, anticoagu-
lant, spasmolytic, diuretic, and potassium channel activators.[2] Pyrans
and their derivatives are also key synthons[3] in the fields of pharmaceu-
ticals, cosmetics, and perfumes. Some 2-amino-4H-pyrans are employed
as photoactive materials.[4] The biological importance of this group of
compounds has evoked widespread interest in the development of new
methodologies. Some of the reported procedures[5] for the synthesis of
Received May 1, 2008.
Address correspondence to Gowravaram Sabitha, Organic Division I,
Indian Institute of Chemical Technology, Hyderabad 500 007, India. E-mail:
gowravaramsr@yahoo.com
433

434
G. Sabitha et al.
Scheme 1.
tetrahydrobenzopyrans use DMF/acetic acid as solvents, which make the
workup tedious and result in poor yields of products. In spite of merits
involved in the use of reactants in solid or molten state,[6] they have their
own disadvantages such as very high temperatures, long reaction times,
and two-step process.[5b,7] Other reports for the synthesis of benzo[b]pyr-
ans include use of microwave irradiation,[8] and a recent report utilizes
proline[9] as the catalyst. In recent years, cerium(III) chloride has emerged
as a potential green Lewis acid. Thus, we considered cerium(III) chloride
as cheap, nontoxic and water-tolerant reagent,[10] which impelled us to
use CeCl3 as an ideal Lewis acid[11] for effecting the one-pot, three-
component cyclocondensation reaction.
In continuation of our interest in the study of MCRs,[12] herein we
describe a mild, convenient, and simple procedure for effecting the one-
pot, three-component reaction of aryl aldehydes, malononitrile, and
dimedone for the preparation of 4H-benzo[b]pyrans in aq. ethanol using
a CeCl3ꢀ7H2O catalyst (Scheme 1).
EXPERIMENTAL
A solution of equimolar amounts of benzaldehyde 1a, malononitrile 2,
and dimedone 3 in aqueous ethanol was heated in the presence of a cata-
lytic amount of cerium(III) chloride for 1.5 h to afford tetrahydroben-
zo[b]pyran 4a[13] in 90% yield after recrystallization from ethanol. The
structure of 4a was confirmed by spectral data as well as by comparison
with an authentic sample. Encouraged by this finding, various other aro-
matic aldehydes 1b-m were treated with malononitrile 2 and dimedone 3
under similar conditions to afford the corresponding tetrahydrobenzo[b]-
pyrans 4b-m in quantitative yields, and the results are given in Table 1.
However, the electronic nature of the aryl substituents in the aldehydes
did not show strong effects in terms of yields. Thus, the electron-
withdrawing substituted aromatic aldehydes and the aromatic aldehydes
bearing electron-donating groups on the aromatic ring underwent clean
conversion under the reaction conditions to produce the corresponding

Tetrahydrobenzo[b]pyran Synthesis
435
Table 1. Cerium(III) chloride–catalyzed synthesis of tetrahydrobenzo[b]pyrans
Time (h) Yield (%) Mp (ꢀC)
Entry
Aldehyde
Product
a
1.5
90
83
227–229
199–201
b
2
c
2
80
210–212
d
e
1
1
93
90
175–177
222–224
f
2
77
171–173
(Continued)

436
G. Sabitha et al.
Table 1. Continued
Entry
Aldehyde
Product
Time (h) Yield (%) Mp (ꢀC)
g
2.5
70
200–202
h
2
70
235–237
i
1.5
83
226–228
j
1.5
86
206–208
k
1
94
190–193
(Continued)

Tetrahydrobenzo[b]pyran Synthesis
Table 1. Continued
437
Entry
Aldehyde
Product
Time (h) Yield (%) Mp (ꢀC)
l
2
75
240–242
m
2
76
220–223
1
All products were characterized by IR, MS, and H NMR spectral data.
Isolated yields after recrystallization.
benzopyran products in good yields. The reaction is equally good with
heteroaromatic aldehyde (Table 1, entry m), resulting in good yields of
benzo[b]pyran. The present reaction conditions are very simple, and the
products precipitated out from the reaction mixture. Analytically pure
samples were prepared by recrystallization from ethanol.
In conclusion, we have demonstrated a simple, efficient, and con-
venient synthesis of 4H-benzo[b]pyran derivtatives by a one-pot, three-
component condensation reaction using CeCl3 as catalyst. The method
offers several advantages such as mild conditions, high yields of products,
operational simplicity, and the use of an environmentally friendly green
Lewis acid catalyst.
Typical Procedure
A mixture of benzaldehyde 1a (1 mmol), malononitrile 2 (1 mmol), dime-
done 3 (1 mmol), and a catalytic amount of CeCl3Á7H2O (10 mol%) in
aqueous ethanol (ethanol/water 4:2) was heated. The progress of the
reaction was monitored by thin-layer chromatography (TLC). After com-
plete conversion, the solid precipitated out was collected by filtration,

438
G. Sabitha et al.
washed with cold ethanol, and dried. Further purification was
accomplished by recrystallization from ethanol to afford pure 4a (90%
yield).
Spectral Data
Compound 4a
Mp 227–229 ꢁC; IR (KBr): 3390, 3215, 3032, 2961, 2199, 1681, 1590,
1440 cmꢂ1 1H NMR (400 MHz, DMSO): 0.95 (s, 3H, CH3), 1.02
;
(s, 3H, CH3), 2.10 (d, J ¼ 16.0 Hz, 1H, C8-H), 2.23 (d, J ¼ 16.0 Hz, 1H,
C8-H), 2.42–2.56 (m, 2H, C6-H), 4.16 (s, 1H, C4-H), 6.52 (s, 2H, NH2),
7.12–7.4 (m, 5H, ArH); ESI MS: m=z 294 (Mþ).
Compound 4b
Mp 199–201 ꢁC; IR (KBr): 3370, 3182, 2960, 1684, 1655, 1592, 1502,
1
1456 cmꢂ1; H NMR (400 MHz CDCl3): 0.94 (s, 3H, CH3), 1.04 (s, 3H,
CH3), 2.08 (d, J ¼ 16.0 Hz, 1H, C8-H), 2.23 (d, J ¼ 16.0 Hz, 1H, C8-H),
2.46–2.52 (m, 2H, C6-H), 3.72 (s, 3H, CH3), 4.2 (s, 1H, C4-H), 6.72 (s,
2H, NH2), 6.95 (d, J ¼ 8.0 Hz, 2H, ArH), 7.03 (d, J ¼ 8.0 Hz, 2H,
ArH); ESI MS: m=z 324 Mþ þ 23).
Compound 4c
Mp 210–212 ꢁC; IR (KBr): 3420, 3331, 2950, 2190, 1670, 1640, 1601,
1
1510, 1450, 1410 cmꢂ1; H NMR (400 MHz CDCl3): 0.96 (s, 3H, CH3),
1.02 (s, 3H, CH3), 2.12 (d, J ¼ 16.0 Hz, 1H, C8-H), 2.21 (d, J ¼ 16.0 Hz,
1H, C8-H), 2.27 (s, 3H, CH3), 2.43–2.55 (m, 2H, C6-H), 4.17 (s, 1H,
C4-H), 6.9 (s, 2H, NH2), 7.01 (d, J ¼ 8.0 Hz, 2H, ArH ), 7.08 (d,
J ¼ 8.0 Hz, 2H, ArH); ESI MS: m=z 308 (Mþ).
Compound 4d
ꢁ
1
Mp 175–178 C; IR (KBr): 3401, 3305, 3192, 2200, 1692, 1601 cmꢂ1; H
NMR (400 MHz, CDCl3): 1.03 (s, 3H, CH3), 1.12(s, 3H, CH3), 2.25 (s,
2H, C8-H), 2.46–2.50 (m, 2H, C6-H), 4.51 (s, 1H, C4-H), 6.12 (s, 2H,
NH2), 7.31–7.43(d, J ¼ 8.0 Hz, 2H, ArH), 8.19–8.30 (d, J ¼ 8.0 Hz, 2H,
ArH); ESI MS: m=z 339 (Mþ þ 23).

Tetrahydrobenzo[b]pyran Synthesis
439
Compound 4e
ꢁ
1
Mp 222–224 C; IR (KBr): 3450, 3312, 3003, 2200, 1690, 1605 cmꢂ1; H
NMR (300 MHz, DMSO): 0.98 (s, 3H, CH3), 1.04 (s, 3H, CH3), 2.12
(m, 2H, C8-H), 2.43 (s, 2H, C6-H), 4.63 (s, 1H, C4-H), 6.42 (s, 2H,
NH2), 7.41 (m, 4H, ArH); ESI MS: m=z 339 (Mþ).
Compound 4f
Mp 171–173 ꢁC; IR (KBr): 3300, 3030, 2990, 2241, 1650, 1621,
1
1520 cmꢂ1; H NMR (400 MHz, DMSO): 1.01 (s, 3H, CH3), 1.08 (s,
3H, CH3), 2.10 (s, 2H, C8-H), 2.35 (s, 2H, C6-H), 3.75 (s, 6H, OCH3),
4.43 (s, 1H, C4-H), 6.4 (s, 2H, NH2), 6.82 (m, 4H, ArH); ESI MS: m=z
(%) 354 (Mþ þ 23).
Compound 4g
ꢁ
1
Mp 200–202 C; IR (KBr): 3250, 3040, 2990, 2240, 1650, 1603 cmꢂ1; H
NMR (300 MHz, DMSO): 1.02 (s, 3H, CH3), 1.06 (s, 3H, CH3), 2.21
(s, 2H, C8-H), 2.53 (s, 2H, C6-H), 3.76 (s, 6H, OCH3), 3.85 (3H,
OCH3), 4.6 (s, 1H, C4-H), 6.5 (s, 2H, NH2), 7.3 (m, 3H, ArH); ESI
MS: m=z 384 (Mþ þ 23).
Compound 4h
ꢁ
1
Mp 235–237 C; IR (KBr): 3310, 3254, 2890, 2192, 1670, 1596 cmꢂ1; H
NMR (300 MHz, DMSO): 1.02 (s, 3H, CH3), 1.10 (s, 3H, CH3), 2.2–
2.24 (m, 2H, C8-H), 2.40 (s, 2H, C6-H), 4.21 (s, 1H, C4-H), 6.3 (s, 2H,
NH2), 6.59–6.78 (m, 3H, ArH), 6.85 (s, 1H, OH); ESI MS: m=z 340
(Mþ þ 23).
Compound 4i
Mp 220–222 ꢁC; IR (KBr): 3421, 3330, 2950, 2180, 1670, 1640, 1601 cmꢂ1
;
1H NMR (400 MHz, DMSO): 0.98 (s, 3H, CH3), 1.03 (s, 3H, CH3), 2.10
(d, J ¼ 16.0 Hz, 1H, C8-H), 2.2 (d, J ¼ 16.0 Hz, 1H, C8-H), 2.43–2.55
(m, 2H, C6-H), 4.32 (s, 1H, C4-H), 6.4 (s, 2H, NH2), 7.01 (d,
J ¼ 8.0 Hz, 2H, ArH), 7.18 (d, J ¼ 8.0 Hz, 2H, ArH); ESI MS: m=z 384
(Mþ þ 23).

440
G. Sabitha et al.
Compound 4j
Mp 206–208 ꢁC; IR (KBr): 3370, 3180, 2950, 2185, 1660, 1590, 1490,
1
1380 cmꢂ1; H NMR (400 MHz CDCl3): 0.96 (s, 3H, CH3), 1.05 (s, 3H,
CH3), 2.1 (d, J ¼ 16.0 Hz, 1H, C8-H), 2.26 (d, J ¼ 16.0 Hz, 1H, C8-H),
2.43–2.52 (m, 2H, C6-H), 4.21 (s, 1H, C4-H), 6.5 (s, 2H, NH2), 7.12 (d,
J ¼ 7.2 Hz, 2H, ArH), 7.31 (d, J ¼ 7.2 Hz, 2H, ArH); ESI MS: m=z 329
(Mþ þ 23).
Compound 4k
Mp 190–192 ꢁC; IR (KBr): 3360, 3176, 2960, 2192, 1675, 1628, 1590,
1
1406, 1367 cmꢂ1; H NMR (400 MHz CDCl3): 0.92 (s, 3H, CH3), 1.04
(s, 3H, CH3), 2.09 (d, J ¼ 16.0 Hz, 1H, C8-H), 2.24 (d, J ¼ 16.0 Hz, 1H,
C8-H), 2.47–2.56 (m, 2H, C6-H), 4.16 (s, 1H, C4-H), 6.61 (s, 2H, NH2),
7.08–7.22 (m, 4H, ArH); ESI MS: m=z (%) 313 (Mþ).
Compound 4l
Mp 240–242 ꢁC; IR (KBr): 3420, 3321, 2950, 2180, 1680, 1640, 1602 cmꢂ1
;
1H NMR (400 MHz, DMSO): 0.95 (s, 3H, CH3), 1.06 (s, 3H, CH3), 1.24
(s, 6H, CH3), 1.36 (s, 3H, CH3), 2.20 (s, 2H, C8-H), 2.51 (s, 2H, C6-H),
4.6 (s, 1H, C4-H), 6.51 (s, 2H, NH2), 6.95 (d, J ¼ 8.0 Hz, 2H, ArH),
7.31 (d, J ¼ 8.0 Hz, 2H, ArH); ESI MS: m=z (%) 338 (Mþ þ 23).
Compound 4m
Mp 220–222 ꢁC; IR (KBr): 3400, 3301, 3040, 2993, 2240, 1675, 1601 cmꢂ1
;
1H NMR (300 MHz, DMSO): 1.02 (s, 3H, CH3), 1.09 (s, 3H, CH3), 2.23
(s, 2H, C8-H), 2.42 (s, 2H, C6-H), 4.43 (s, 1H, C4-H), 6.42 (s, 2H, NH2),
6.95–7.37 (m, 3H, ArH); ESI MS: m=z 284 (Mþ þ 23).
ACKNOWLEDGMENT
K. S. thanks UGC, New Delhi, for the award of a fellowship.
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