Direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone: synthesis of 3,4-dihydrocoumarins

Article abstract of DOI:10.1016/j.tetlet.2009.10.112

The direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone is reported. In the presence of BF₃·OEt₂, the reaction of 3-arylpropionic acids with PIFA or Oxone proceeded smoothly at 30 °C to give 3,4-dihydrocoumarins

Full text of DOI:10.1016/j.tetlet.2009.10.112

Tetrahedron Letters 51 (2010) 192–196
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone:
synthesis of 3,4-dihydrocoumarins
*
Yonghong Gu , Kun Xue
Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2009
Revised 23 October 2009
Accepted 27 October 2009
Available online 30 October 2009
The direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone is reported. In the presence
of BF3ꢀOEt2, the reaction of 3-arylpropionic acids with PIFA or Oxone proceeded smoothly at 30 °C to give
3,4-dihydrocoumarins in good to excellent yields.
Ó 2009 Elsevier Ltd. All rights reserved.
Coumarin and its derivatives are one of the most privileged
structural motifs frequently found in natural products and phar-
maceuticals.1 As an important class of coumarins, 3,4-dihydro-
coumarins are widely distributed in nature2 and exhibit some
interesting biological activities, such as anti-herpetic, anti-inflam-
matory, anti-oxidative, anti-aging, and anti-cancer activities.3
Therefore, development of efficient methods for the synthesis of
dihydrocoumarins have attracted great interest in recent years.4
The conventional methods for the syntheses of dihydrocoumarins
include (1) the transition-metal-catalyzed hydrogenation of cou-
marins;5 (2) Lewis or protonic acid-mediated hydroarylation of
cinnamic acids with phenols;6 (3) the transition-metal-catalyzed
intramolecular electrophilic hydroarylation of alkene substrates;7
(4) Lewis acid-mediated reaction of highly activated phenols with
acrylonitriles,8 5-alkylidene Meldrum’s acids9 and hydroxyketene-
S,S-acetals;10 (5) the Baeyer–Villiger oxidation of 1-indanones
formed in situ by the Friedel–Crafts cyclization of 3-arylpropionic
acids.11 However, many of these methods suffer disadvantages
from using a large excess of expensive transition-metals, such as
Pb(OAc)2,7f–h Ru(I),4c Ru(III),7e Y(OTf)3,6a Yb(OTf)3,9 Cr(CO)5,4e and
harsh conditions. To achieve the environmental demands for
‘green’ chemical procedures, we were prompted to explore the
feasibility of dihydrocoumarin synthesis by transition-metal-free
oxidative C–O formation reaction from the corresponding carbox-
ylic acids.
phenol14 or phenol ether derivatives15 to spiro dienone lactones
using hypervalent iodine(III) reagents (Scheme 1). This method in-
volves oxidation of phenols or phenol ethers to reactive electro-
philic intermediates which are then trapped by carboxylic acid
moiety. However, there is no report for the oxidative cyclization
of electron-neutral or electron-deficient 3-arylylpropionic acids.
We envisioned that oxidation of 3-arylylpropionic acids with
hypervalent iodine reagents would generate radical cation inter-
mediates, which could be attacked by carboxylic acid moiety to
provide bicyclolactones. Herein, we disclose our investigation for
the oxidative cyclization of 3-arylpropionic acids to 3,4-
dihydrocoumarins using hypervalent iodine reagents as oxidants.
The initial attempts of the cyclization of 3-phenylpropionic acid
(1a) with phenyliodine(III) diacetate (PIDA) or phenyliodine(III)
Kita's work:
OR1
OR1
O
R2
R2
R2
PIFA or
PIDA
OH
OH
O
O
O
R1 = H, Me, Bn
O
R2 = H, Me, OMe, OBn
Recently, utilization of hypervalent iodine compounds in oxida-
tive reactions has received great attention for their low toxicity
compared with heavy metal oxidants, mild reaction conditions,
special reactivity, and easy handling.12 Several novel methods have
been developed for the syntheses of lactones by intramolecular
C–O bond formation using hypervalent iodine reagents.13 Kita
and co-workers reported the cyclization of para-substituted
This work:
R1
R1
R1
OH
O
OH
O
O
O
R1= electron-neutral or electron-withdrawing groups
Scheme 1. Cyclization of para-substituted phenol or phenol ether derivatives.
* Corresponding author. Tel.: +86 551 3602470; fax: +86 551 3601592.
E-mail address: ygu01@ustc.edu.cn (Y. Gu).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.10.112

Y. Gu, K. Xue / Tetrahedron Letters 51 (2010) 192–196
193
Table 1
respectively (entries 10 and 11). Notably, when the substrate con-
tains the hydroxyl group at the b-position of the carboxylic acid,
the reaction gave the dehydrated product 2l in moderate yield
(entry 12).
Optimization of reaction conditions using PIDA or PIFA as an oxidanta
O
Oxidant(1.5 equiv)
OH
To make this reaction more synthetically applicable, we
extended our research to seek low-cost and ‘greener’ oxidants. Per-
oxides as inexpensive, environmentally benign oxidants are exten-
sively used in modern organic synthesis. Due to their prominent
properties, several peroxides were explored in the oxidative cycli-
zation of 3-phenylpropionic acid (1a) (Table 3). It was found that
peroxides, such as m-CPBA, H2O2, and t-BuOOH failed to provide
any desired product 2a in this reaction (entries 1–3). To our de-
light, both K2S2O8 and Oxone were effective to this transformation.
In the presence of 2.0 equiv oxidants and 1.5 equiv of BF3ꢀOEt2 in
TFA at 30 °C, the reactions provided 2a in 42% and 61% yields,
respectively (entries 4 and 5). In addition, Lewis acid BF3ꢀOEt2 is
also essential in this transformation (entries 5 and 6). This result
indicated that BF3ꢀOEt2 probably activated the epoxide intermedi-
ate 8 formed in the reaction and facilitated this transformation
(Scheme 2). Careful examination of the solvent showed TFA is
the ideal solvent (entries 7–9). Therefore, using Oxone19 as an oxi-
dant and BF3ꢀOEt2 as an additive in TFA were chosen as desired
reaction conditions.
Additive(1.5 equiv)
O
O
solvent, 30 oC, 12 h
2a
1a
Oxidant
Entry
Additive
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
PIDA or PIFA
PIDA or PIFA
PIDA or PIFA
PIDA or PIFA
PIFA
None
None
None
None
CH2Cl2
CH3CN
AcOH
CF3CH2OH
TFA
NDc
ND
ND
<5
17
<5
None
PIDA
None
TFA
PIFA
PIFA
BF3ꢀOEt2
TFA
TFA/CH2Cl2
78
BF3ꢀOEt2
62d
a
Unless otherwise specified, all the reactions were carried out in the presence of
0.2 mmol of 1a, 0.3 mmol of oxidant, and 0.3 mmol of additive in 1 mL of solvent at
30 °C for 12 h.
b
Isolated yield after column chromatography.
No desired product was detected by TLC analysis.
0.2 mL of TFA and 1 mL of CH2Cl2 were used.
c
d
In the presence of 2.0 equiv Oxone and 1.5 equiv BF3ꢀOEt2, the
reaction of acids 1a–l in TFA proceeded smoothly to provide cy-
clized products 2a–l in 16–71% yields (Table 2). This reaction pro-
ceeded generally comparable or moderately lower yields compared
with PIFA-mediated reaction. This transformation could be applied
to the substrates containing various functional groups, including
methyl, phenyl, halides, and methoxy groups (entries 2–11). Nota-
bly, in some cases, Oxone showed the improved functional group
tolerance relative to PIFA. For example, the cyclization of methyl
ether 1i provided 16% yield of 2i, the analogous reaction failed to
yield 2i with PIFA as an oxidant (entry 9). However, when the sub-
strate contains an electron-withdrawing group, (e.g., in 3-(4-carbo-
methoxyphenyl)propionic acid (1h)), the reaction was not
successful (entry 8).
bis(trifluoroacetate) (PIFA) in CH2Cl2, CH3CN and AcOH did not give
any 3,4-dihydrocoumarin (2a) at all, and no conversion of 1a was
observed (Table 1, entries 1–3). When CF3CH2OH was used as a sol-
vent, trace amount of 2a was obtained under otherwise identical
conditions (entry 4). To our pleasure, when TFA was employed as
a solvent, cyclization of 1a with PIFA provided the product 2a in
17% yield (entry 5). While the analogous reaction gave only trace
amount of 2a with PIDA as an oxidant (entry 6). Notably, activation
of PIFA with Lewis acid BF3ꢀOEt2 improved the reaction dramati-
cally (entry 7).16 In addition, when CH2Cl2 (5:1 CH2Cl2/TFA) was
used as co-solvent, the reaction afforded the product 2a in a
slightly lower yield (entry 8). As a result, when the reaction was
carried out in the presence of 1.5 equiv PIFA and 1.5 equiv BF3ꢀOEt2
in TFA at 30 °C, the best result was achieved.
With optimized conditions in hand, next we examined the
reaction scope and the results are summarized in Table 2. This
reaction was compatible with a variety of functionalities, includ-
ing methyl, phenyl, fluoride, bromide, iodide, and ester groups.
The reaction of 3-(4-methylphenyl)propionic acid (1b) with PIFA
provided the cyclized product 2b in 80% yield (entry 2). When
the methyl group was introduced to the ortho position of phenyl
ring, the reaction gave a slightly lower yield of 2c, probably due
to steric effect (entry 3). When the methyl group was introduced
to the meta position of phenyl ring, two products 2da and 2db
were obtained in 82% yield as a 2:1 mixture with a less sterically
hindered product 2da as the major (entry 4).17 When the para
position of phenyl ring was substituted with halogens (F, Br, I),
the reaction generally proceeded well to give corresponding prod-
ucts 2e–g in 54–79% yields (entries 5–7). It is noteworthy that
when the substrate with an electron-withdrawing group, ester
group at the para position of phenyl ring was employed, the reac-
tion gave the cyclized product 2h in a slightly lower yield for a
longer reaction time (entry 8). Nevertheless, when the electron-
donating group, methoxy group, was introduced to the para posi-
tion of phenyl ring, the reaction failed to yield any desired prod-
uct 2i; instead, a complex mixture was obtained (entry 9).18 To
expand the substrate scope, we examined the substitution effect
on the chain of 3-arylpropionic acid. It was found that when
the substrates with methyl or phenyl group at the b-position of
the carboxylic acid were employed, the reaction proceeded
smoothly to give the products 2j and 2k in 90% and 75% yields,
PIFA or Oxone (1.5-2.0 equiv)
.
BF3 OEt2 (1.5 equiv)
ð1Þ
O
O
TFA, 30 oC, 24 h
O
2a
3
It should be noteworthy that the oxidation of 1-indanone (3)
with PIFA or Oxone in the presence of BF3ꢀOEt2 in TFA did not
provide 3,4-dihydrocoumarin (2a) (Eq. 1). Therefore, the forma-
tion of 1-indanone from the 3-phenylpropionic acid with TFA fol-
lowed by the oxidation to dihydrocoumarin should be excluded.
Two plausible path ways are proposed according to the literature
examples and our above experimental results (Scheme 2).15c,20
First, the one electron oxidation of aromatic ring of 1 by PIFA
forms the radical cation 4. Then intramolecular nucleophilic at-
tack by the carboxylic acid moiety on radical cation yields the
bicyclolactone cation 5 (path a) or spiro lactone cation 6 (path
b); rearrangement of the carboxylic unit of 6 leads to intermedi-
ate 5, the deprotonation of 5 generates dihydrocoumarin 2. Addi-
tionally, the reaction of phenol derivatives (R1 = OH) with PIFA
and BF3ꢀOEt2 at ꢁ78 °C afforded the spiro dienone lactone 7 in
57% yield. This result suggests that the reaction may proceed
via the rearrangement pathway. Furthermore, we also proposed
the mechanism of the oxidative cyclization by Oxone.21 Oxidation
of 1 by Oxone forms the epoxide 8, followed by intramolecular
nucleophilic attack of epoxide with carboxylic acid moiety to pro-
vide the intermediate 9. Last dehydration of 9 furnishes dihydro-
coumarin 2.

194
Y. Gu, K. Xue / Tetrahedron Letters 51 (2010) 192–196
Table 2
Reaction of 3-arylpropionic acid with PIFA or Oxonea
R2
O
R2
O
PIFA (1.5 equiv) or
Oxone (2.0 equiv)
OH
R1
R1
.
BF3 OEt2 (1.5 equiv)
O
1
TFA, 30 oC, 12-24 h
2
Entry
1
Substrate
Product
Time (h)
12
Yieldb (%) PIFA/Oxone
78/61
O
OH
O
O
O
2a
1a
OH
O
2
3
12
16
80/61
63/44
O
2b
1b
1c
O
OH
O
O
O
2c
OH
O
O
2da
4
12
82/61(2:1)
1d
O
O
2db
O
OH
F
Br
I
O
O
5
6
16
12
54/49
79/58
2e
F
1e
1f
O
OH
O
O
2f
Br
O
OH
O
O
O
7
8
9
12
24
12
65/56
I
2g
1g
O
OH
O
MeO2C
2h
50/NDc
MeO2C
1h
O
OH
O
MeO
O
ND/16
MeO
2i
1i

Y. Gu, K. Xue / Tetrahedron Letters 51 (2010) 192–196
195
Table 2 (continued)
Entry
Substrate
Product
Time (h)
12
Yieldb (%) PIFA/Oxone
O
OH
10
11
90/71
O
O
2j
1j
Ph
Ph
O
O
OH
OH
12
16
75/66
48/51
O
O
O
O
1k
2k
2l
OH
12
1l
a
Reactions were carried out with 0.2 mmol of 1, 0.3 mmol of PIFA or 0.4 mmol of Oxone, 0.3 mmol of BF3ꢀOEt2 in 1 mL of TFA at 30 °C.
Isolated yield after column chromatography.
No desired product was detected by TLC analysis.
b
c
Table 3
In conclusion, we developed a novel method for the synthesis of
3,4-dihydrocoumarin through direct oxidative cyclization of 3-aryl-
propionicacids using PIFA or Oxone as an oxidant. The syntheticutil-
ity of these reactions is enhanced by using Oxone as an inexpensive,
safe, and environmentally benign oxidant. Ongoing work to expand
the substrate scope and apply this reaction to the synthesis of more
complex products is underway.
Optimization of reaction conditions using peroxide as an oxidanta
O
Oxidant(2.0 equiv)
OH
Additive(1.5 equiv)
solvent, 30 oC, 12 h
O
2a
O
1a
Entry
Oxidant
Additive
Solvent
Yieldb (%)
Acknowledgments
1
2
3
4
5
6
7
8
9
m-CPBA
H2O2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
None
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
TFA
TFA
TFA
TFA
TFA
TFA
CH2Cl2
CF3CH2OH
AcOH
NDc
ND
<5
42
61
t-BuOOH
K2S2O8
Oxone
Oxone
Oxone
Oxone
Oxone
We thank the financial support for this work from the National
Natural Science Foundation of China (No. 20602033) and National
Basic Research Program of China (2006CB922001).
35
ND
ND
ND
Supplementary data
a
Unless otherwise specified, all the reactions were carried out in the presence of
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.10.112.
0.2 mmol of 1a, 0.4 mmol of oxidant, and 0.3 mmol of additive in 1 mL of solvent at
30 °C for 12 h.
References and notes
O
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R1
O
O
(R1 = OH)
57%
5
2
OCOCF3
OH
.
BF3 OEt2
Oxone
O
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BF3 OEt2
HO
1
O
O
HO
O
O
H
8
9
OCOCF3
Scheme 2. Proposed mechanism of lactonization of 3-arylpropionic acid.

196
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