Expeditious approach to coumarins via pechmann reaction catalyzed by molecular iodine or AgOTf

Article abstract of DOI:10.1080/00397910600773692

An efficient and facile route for the synthesis of coumarins via the Pechmann reaction catalyzed by molecular iodine or AgOTf was described. Copyright Taylor & Francis Group, LLC.

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Expeditious Approach to Coumarins via
Pechmann Reaction Catalyzed by Molecular
Iodine or AgOTf
Jie Wu a , Tianning Diao a , Wei Sun a & Yizhe Li a
a Department of Chemistry, Fudan University, Shanghai, China
Published online: 16 Feb 2007.
To cite this article: Jie Wu , Tianning Diao , Wei Sun & Yizhe Li (2006): Expeditious Approach to Coumarins
via Pechmann Reaction Catalyzed by Molecular Iodine or AgOTf, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 36:20, 2949-2956
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Synthetic Communicationsw, 36: 2949–2956, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600773692
Expeditious Approach to Coumarins via
Pechmann Reaction Catalyzed by Molecular
Iodine or AgOTf
Jie Wu, Tianning Diao, Wei Sun, and Yizhe Li
Department of Chemistry, Fudan University, Shanghai, China
Abstract: An efficient and facile route for the synthesis of coumarins via the Pechmann
reaction catalyzed by molecular iodine or AgOTf was described.
Keywords: AgOTf, coumarin, iodine, Pechmann reaction
The prominence of coumarin in natural products and biologically active
molecules has encouraged considerable efforts to synthesize it. As a privileged
scaffold, substituted coumarin shows interesting biological properties,
especially for anti-HIV and antibiotic activities.[1] Our continued interest to
build a coumarin-based combinatorial library led us to develop efficient
methods for the synthesis of diversified coumarin molecules.[2]
Coumarins have been synthesized by several routes, including
Pechmann,[3] Perkin,[4] Knoevenagel,[5] Reformatsky,[6] and Wittig
reactions[7] and by flash vacuum pyrolysis.[8] Among these, the Pechmann
reaction is the most widely used method, because the reaction involves the
use of simple starting materials, that is, phenols and b-keto esters, in the
presence of acidic condensing agents. Various acids have been used to carry
out this reaction.[9] However, many of these procedures suffered from harsh
reaction conditions, difficulties in workup, and the use of stoichiometric
and/or toxic, relatively expensive reagents. Because coumarin derivatives
are increasingly useful and important in pharmaceuticals and industry, the
development of simple, environmentally benign, low-cost protocols is still
desirable.
Received in R.O.C. December 26, 2005
Address correspondence to Jie Wu, Department of Chemistry, Fudan University,
220 Handan Road, Shanghai 200433, China. E-mail: jie_wu@fudan.edu.cn
2949

2950
J. Wu et al.
Recently, the use of molecular iodine has received considerable
attention as an inexpensive, nontoxic, readily available catalyst for
various organic transformations, which affords the corresponding products
in excellent yields with high selectivity.[10] The mild Lewis acidity associ-
ated with iodine expanded its use in organic synthesis to several organic
transformations, using stoichiometric levels to catalytic amounts. Owing
to numerous advantages associated with this ecofriendly element, iodine
has been explored as a powerful catalyst for various organic transform-
ations.[10] Recently, we found that it was also efficient as a catalyst in
the synthesis of coumarins via the Pechmann reaction, which is disclosed
herein.
An initial study was performed by treatment of resorcinol 1a with ethyl
acetoacetate 2a in CH2Cl2 in the presence of a catalytic amount of I2
(5 mol%) at room temperature. To our delight, we observed the formation
of 7-hydroxy-4-methyl-coumarin 3a after 24 h (76% yield). When the
reaction temperature was elevated to 408C, complete conversion and 80%
isolated yield was obtained after 16 h (Scheme 1).
Further studies established that 1 mol% of catalyst was also efficient in
this reaction, although prolonged reaction time was needed (48 h, 81%
yield). Moreover, it is noteworthy that this reaction could be run in air
without loss of efficiency. Among the solvents screened, (THF, MeCN,
MeOH, toluene) toluene was demonstrated as the best. Further study for
temperature showed that the reaction worked most efficiently at 808C,
and it went to completion in 3 h in toluene (with 85% isolated yield). To
demonstrate the generality of this method, we next investigated the scope
of this reaction under the optimized conditions (toluene, 5 mol% of
iodine, 808C), and the results are summarized in Table 1. As shown in
Table 1, this method is equally effective for both phenols and b-keto
esters. Various substituted phenols 1 reacted smoothly with b-keto esters
2 to produce a range of coumarin derivatives. Generally, reactions were
performed at 808C for 3–12 h irrespective of various range of substitutions.
Complete conversion and good to excellent isolated yields were observed
for all substrates employed. And in some cases, high catalyst loading had
to be employed to obtain respective yields. For example, 50 mol% of
iodine had to be utilized when resorcinol 1a or benzene-1,2,3-triol 1b
reacted with ethyl trifluoroacetoacetate 2b. On the other hand, because of
Scheme 1. Reaction of phenol 1a with b-ketoester 2a catalyzed by molecular iodine.

Pechmann Reaction Catalyzed by Molecular Iodine or AgOTf
2951
Table 1. Reaction of phenol 1 with b-ketoester 2 catalyzed by molecular iodine
Yield
(%)a
Entry
1
Substrate 1
Substrate 2
Product
85
2b
99
51
89
3
4
5b
82
6
7
70
88
(continued)

2952
J. Wu et al.
Table 1. Continued
Yield
(%)a
Entry
8
Substrate 1
Substrate 2
Product
76
aIsolated yield based on phenol.
b50 mol% of I2 was employed.
the growing concern about the influence of the organic solvent on the
environment as well as on the human body, organic reactions without use
of conventional organic solvents have attracted the attention of synthetic
organic chemists.[11,12] We found that under solvent-free conditions, the
reaction of resorcinol 1a with ethyl acetoacetate 2a also proceeded
smoothly to afford the corresponding product 3a at 608C in 4 h, although
the yield is slightly lower (80%).
We also screened a series of metal salts, which are easily available for the
reaction shown in Scheme 1 under solvent-free conditions at 608C. Among the
various metal salts studied for this transformation, AgOTf was found also
effective for the reaction. The scope and generality of this process is illustrated
with respect to various phenols 1 and b-keto esters 2, and the results are
summarized in the Table 2. Similar yields were obtained using 10 mol% of
AgOTf at 608C (3–10 h).
In a typical experimental procedure, the catalyst (method A: iodine,
5 mol%; method B: AgOTf, 10 mol%) was added to a mixture of resorcinol
1a (0.50 mmol) and ethyl acetoacetate 2a (0.60 mmol, 1.2 equiv.) under an
air atmosphere (method A: toluene, 808C; method B: solvent free, 608C),
and the mixture was stirred for a period of time (3–12 h). Following
completion of the reaction as monitored by thin-layer chromatography
(TLC), the reaction mixture was cooled, diluted with ethyl acetate (10 mL),
washed with water (5.0 mL), and dried over Na2SO4. The solvent was
removed under vacuum, and the residue was purified by flash chromatography
to give the corresponding products 3a.
In conclusion, we describe an efficient route for the synthesis of
coumarins utilizing molecular iodine or silver triflate as a novel catalyst via
the Pechmann reaction. This method not only provides a good complement
to coumarin synthesis via the Pechmann reaction but also avoids the use of
hazardous acids and harsh reaction conditions. The advantages of this
method include the use of inexpensive reagents and catalyst under mild
conditions and operational ease.

Pechmann Reaction Catalyzed by Molecular Iodine or AgOTf
2953
Table 2. Reaction of phenol 1 with b-ketoester 2 catalyzed by AgOTf
Yield
(%)a
Entry
1
Substrate 1
Substrate 2
Product
85
2
3
60
65
4
5
95
68
aIsolated yield based on phenol.
EXPERIMENTAL
All reactions were performed in test tubes under air atmosphere at room
temperature. Flash column chromatography was performed as described by
Still et al.[13] using silica gel (60-A pore size, 32–63 mm, standard grade,
˚
Sorbent Technologies). Analytical TLC was performed using glass plates
precoated with 0.25-mm, 230–400 mesh silica gel impregnated with a

2954
J. Wu et al.
fluorescent indicator (254 nm). TLC plates were visualized by exposure to
ultraviolet light. Organic solutions were concentrated on Bu¨chi R-200
rotary evaporators at ꢀ20 Torr (house vacuum) at 25–358C. Commercial
reagents and solvents were used as received. Proton nuclear magnetic
resonance (1H NMR) spectra were recorded with Varian 400 Inova
spectrometers and are recorded in parts per million (PPM) from internal
tetramethylsilane on the d scale and are referenced from the residual
protium in the NMR solvent (CHCl3: d 7.27, DMSO-d6: d 2.50). Data is
reported as follows: chemical shift [multiplicity (s ¼ singlet, d ¼ doublet,
q ¼ quartet, m ¼ multiplet), coupling constant(s) in Hertz, integration,
assignment].
General Procedure
Catalyst (method A: iodine, 5 mol%; method B: AgOTf, 10 mol%) was added
into a mixture of phenol 1 (0.50 mmol) and b-keto esters 2 (0.60 mmol, 1.2
equiv.) under an air atmosphere (method A: toluene, 808C; method B:
solvent free, 608C), and the mixture was stirred for a period of time
(3–12 h). Following completion of the reaction as monitored by TLC, the
reaction mixture was cooled, diluted with ethyl acetate (10 mL), washed
with water (5.0 mL), and dried over Na2SO4. The solvent was removed
under vacuum, and the residue was purified by flash chromatography to
give the corresponding products 3. (All the products are known compounds.
The characterizations of these compounds are identical with the literature
reports.[9]
)
Data
7-Hydroxy-4-methyl-2H-chromen-2-one 3a:[9a] 1H NMR (400 MHz, CDCl3):
d (ppm) 2.41 (s, 3H), 6.15 (s, 1H), 6.86 (d, J ¼ 8.0 Hz, 1H), 7.00 (s, 1H), 7.05
(s, 1H), 7.50 (d, J ¼ 8.8 Hz, 1H).
7-Hydroxy-4-(trifluoromethyl)-2H-chromen-2-one 3b:[9a] 1H NMR (400 MHz,
CDCl3): d (ppm) 6.70 (s, 1H), 6.78 (s, 1H), 6.87 (s, 1H), 7.08 (s, 1H), 7.50
(d, J ¼ 8.8 Hz, 1H).
4-(Chloromethyl)-7-hydroxy-2H-chromen-2-one 3c:[9a] 1H NMR (400 MHz,
CDCl3): d (ppm) 5.00 (s, 2H), 6.41 (s, 1H), 6.75 (s, 1H), 6.85 (s, 1H), 7.66
(d, J ¼ 8.8 Hz, 1H), 10.66 (s, 1H).
7,8-Dihydroxy-4-methyl-2H-chromen-2-one 3d:[9a] 1H NMR (400 MHz, CDCl3):
d (ppm) 2.37 (s, 3H), 6.10 (s, 1H), 6.80 (d, J ¼ 8.8 Hz, 1H), 7.10 (d, J ¼ 8.0 Hz,
1H), 9.30 (br, 1H), 10.04 (br, 1H).

Pechmann Reaction Catalyzed by Molecular Iodine or AgOTf
2955
7,8-Dihydroxy-4-(trifluoromethyl)-2H-chromen-2-one 3e:[9a] 1H NMR
(400 MHz, CDCl3): d (ppm) 6.70 (s, 1H), 6.92 (d, J ¼ 8.8 Hz, 1H), 7.04
(s, 1H), 9.40–10.50 (br, 2H).
5,7-Dihydroxy-4-methyl-2H-chromen-2-one 3f:[9a] 1H NMR (400 MHz,
DMSO-d6): d (ppm) 2.44 (s, 3H), 3.90–4.30 (br, s, 2H), 5.90 (s, 1H), 6.25
(d, J ¼ 1.8 Hz, 1H), 6.35 (d, J ¼ 1.8 Hz, 1H).
7-Hydroxy-4,5-dimethyl-2H-chromen-2-one 3 g:[9a] 1H NMR (400 MHz,
CDCl3): d (ppm) 2.24 (s, 3H), 2.50 (s, 3H), 3.34 (s, 1H), 6.00 (s, 1H), 6.54
(s, 1H), 6.57 (s, 1H).
4-Methyl-2H-benzo[h]chromen-2-one 3 h:[9a] 1H NMR (400 MHz, CDCl3):
d (ppm) 2.49 (s, 3H), 6.47 (s, 1H), 7.67–7.82 (m, 4H), 7.95–8.15 (m, 1H),
8.32–8.35 (m, 1H).
ACKNOWLEDGMENT
Financial support from the National Natural Science Foundation of China (No.
20502004), Ministry of Education of China, and the Science and Technology
Commission of Shanghai Municipality, is gratefully acknowledged.
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