Copper catalyzed room temperature lactonization of aromatic C-H bond: A novel and efficient approach for the synthesis of dibenzopyranones

Article abstract of DOI:10.1039/c4ra13665a

We have developed a novel and efficient methodology for the intramolecular aryl C-H oxidative lactonization of 2-arylbenzaldehyde using a low-cost CuCl catalyst and TBHP as the oxidant at room temperature. We applied the methodology to the synthesis of a series of dibenzopyranones.

Full text of DOI:10.1039/c4ra13665a

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Table of content
Copper catalyzed room temperature lactonization of aromatic C-H bond: A
novel and efficient approach for the synthesis of dibenzopyranones
Raju Singha, Shubhendu Dhara, Munmun Ghosh and Jayanta K. Ray*
A novel and efficient methodology has been developed for the intramolecular aryl C-H oxidative lactonization of 2-
arylbenzaldehyde using the cheap CuCl catalyst and TBHP as the oxidant at room temperature.
O
O
tBuOOH (6 equiv.)
H
O
Ar1
Ar1
H
CuCl (5 mol%), DMSO
r.t., 4h
Ar2
Ar2
mild reaction condition
18 examples
up to 87 % yield

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Journal Name
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DOI: 10.1039/C4RA13665A
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Copper catalyzed room temperature lactonization of aromatic C-H
bond: A novel and efficient approach for the synthesis of
dibenzopyranones
Raju Singhaa, Shubhendu Dharaa, Munmun Ghosha and Jayanta K. Ray*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Dibenzopyranone is an interesting class of lactones found as the
core structure of many natural products such as autumnariol
(Fig.1, 1), alternariol, altenuisol, autumnariniol, and
40 graphislactones (Fig. 1, 2) and bioactive compounds.3
Dibenzopyranone scaffolds have been used as intermediates in
the synthesis of many pharmaceutically interesting compounds
We have developed a novel and efficient methodology for the
intramolecular aryl C-H oxidative lactonization of 2-
arylbenzaldehyde using the cheap CuCl catalyst and TBHP as
10 the oxidant at room temperature. We have applied the
methodology for the synthesis of
dibenzopyranones.
a
series of
such
as
progesterone,4
androgen
receptor
ligands,5
glucocorticoids6 and endothelial cell proliferation inhibitors.7
45 Dibenzopyranones are also found in various naturally occurring
food sources such as citrus fruits, herbs and vegetables.8
Furthermore such lactones are also present in human metabolites
such as urolithins . Urolithins A-C (Fig. 1, 3a-c) are produced by
in vitro fermentation of punicalagins.9
Transition metal catalyzed un-activated aromatic C-H bond
functionalization is a very powerful synthetic strategy for the
15 synthesis of complex molecules from simple precursors.1 Of
these reactions, the oxidative lactonization of aromatic C-H bond
gives an access for the synthesis of important lactone containing
molecules. Very recently a number of methods have been
reported in literature for the sp2C-H lactonization with carboxylic
20 acid group in presence of expensive transition metal catalysts
along with various oxidants and additives at elevated
temperature.2 Thus, the development of a newer methodology for
the oxidative C-H bond lactonization using a cheaper catalyst and
under mild reaction conditions especially at room temperature is
25 highly desirable.
OR1
O
OH
O
50
O
R1
R2
O
O
O
OR2
MeO
OH
Me
Me
OMe
OH
R1, R2 = H, Me
55
3a R1 = OH, R2 = H, Urolithin A
3b R1 = R2 = H, Urolithin B
3c R1 = R2 = OH, Urolithin C
1
2
Herein, we are reporting a methodology for the synthesis of
dibenzopyranones through copper catalyzed intra-molecular aryl
C-H oxidative lactonization of 2-arylbenzaldehydes at room
temperature (Scheme 1).
Figure 1 Structure of some natural products and bioactive compounds
Literature reports
60 In literature, several methods are available for the synthesis of
dibenzopyranones. Most of the popular methods involve: 1)
Baeyer-Villiger oxidation of fluorenone (Scheme 2, path a);10 2)
Lewis acid or metal mediated lactonization of ester and methoxy
groups (path b);11 3) lactonization of 2-halobiarylcarboxylic acid
65 derivative (Scheme 2, path c);12 4) cross coupling of 2-
halobenzaldehyde and o-hydroxyarylboronic acid and followed
by lactonization (path d);13 5) cross coupling of aryl o-
halobenzoate (path e);14 and 6) C-H activation/carbonylation of 2-
arylphenol (path f).15 All the present methods in spite of their
70 individual advantages suffers from some drawbacks such as, (a)
multistep procedures, (b) extremely low (-78 °C) or high
temperature reaction conditions, (c) use of readily unavailable
starting materials, (d) requirement of pre-functionalization of the
O
30
H
O
Pd or Cu
CO2H
Present work
H
oxidant, heat
O
O
CuCl
CHO
TBHP, r.t.
35
Scheme 1: Literature reports and present work
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starting materials, and (e) the use of poisonous CO gas. To 55 5p). In presence of the electron withdrawing group on either of
overcome all of the above drawbacks a mild, efficient and room
temperature reaction procedure starting from readily
synthesizable starting materials is highly desirable. Herein we are
the aryl rings, biaryl substrates gave lower yield (Table 2, entries
5c, 5i-k) while the electron donating groups on aryl rings gave
higher yield (Table 2, entries 5d-e, 5l, 5p-r). The yield was lower
for the entries 5f and 5g due to the presence of steric repulsion
60 between methoxy group and peri hydrogen of the adjacent phenyl
ring. The substrate 2-(m-tolyl)benzaldehyde gave two possible
pyranone derivatives in 3:2 ratio (entry 2n). The substituents at
ortho-position of Ar2 ring were unfavourable for the product
formation (entry 5m, 5o). Finally the methyl ethers of natural
65 product Urolithins A-C were obtained from the corresponding
suitably substituted starting materials in excellent yields due to
the presence of electron donating methoxy group (Table 2, entries
5p-r).
5
reporting a CuCl catalyzed reaction for the synthesis of
dibenzopyranones at room temperature.
O
CO2Me
HO
10
15
20
H2O2, AcOH
a
MeO
f
BBr3
O
CO, Pd, Cu
b
O
O
CO2H
H
This work
Pd, Cu
Table 1: Screening of the reaction conditions
c
O
X
X = I, Br, Cl, NO2, H
d
CHO
Cu cat, solvent, TBHP
Room Temperature
Pd
70
O
Pd
e
O
CHO HO
O
5a
TBHP (equiv.) time (h) yielda
4a
75 Entry solvent
X
(HO)2B
X
X = I, Br
X = I, Br
catalyst
Scheme 2: Literature reports and present work
1
2
3
4
5
6
7
8
9
CH3CN
CH3CN
CH3CN
DMF
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuI
2
4
6
6
6
6
6
6
6
6
6
6
0
6
24
24
8
33
54
25 Our investigation was started by the reaction 2-
phenylbenzaldehyde in presence of CuBr catalyst and TBHP (2
equiv.) as oxidant in acetonitrile solvent at room temperature and
it gave the desired product dibenzopyranone in 33% yield along
with the unconsumed substrate. Then we varied the amount of
30 TBHP, catalyst and solvent to get the optimized reaction
condition and the results are shown in Table 1.
68
24
24
4
0
80
DMA
trace
74b
66
19c
trace
78d
70e
38
DMSO
DCM
4
H2O
24
24
4
DMSO
DMSO
DMSO
85 10
11
CuCl
CuCl
At first, we had increased the amount of oxidant TBHP to 4
equivalents and the yield was increased to 54% along with the
unconsumed substrate. The yield was improved to 68% on further
35 increasing the amount of TBHP to 6 equivalents and the substrate
also vanished within 8h. Then we had varied the nature of solvent
(Table 1, entries 3-8) and the DMSO was come out as the best
solvent. Then we had used different copper catalysts and we
found that CuCl catalyst gave the highest yield of 78 % (Table 1,
40 entry 10). The Cu(II)acetate also promoted the reaction but gave
the lower yield. The reaction did not occurred in absence of either
the catalyst or TBHP (entries 13 and 14).
24
24
24
24
12
DMSO Cu(OAc)2,2 H2O
13
DMSO
DMSO
CuCl
------
0
14
trace
90
aIsolated yield. About 6 % of fluorenone was formed along with
c
the normal product. About 27% of 2-phenylbenzoic acid was
isolated. dOptimized reaction condition: the substrate 4a (0.25
b
mmol), CuCl (5 mol%), DMSO (2 mL), TBHP (6 equiv.) stirred
e
95 at room temperature for 4h. The catalyst 2 mol% of CuCl was
used.
Thus the optimized reaction conditions are the substrate (0.25
mmol), CuCl catalyst (5 mol %), DMSO solvent (2 mL), TBHP
45 (6 equiv.) stirred at room temperature for 4h. After getting the
optimized reaction conditions we applied it on different substrates
to examine the scope of this methodology and the results are
summarized in Table 2. The substrate 2-arylbenzaldehydes (4)
were synthesized by the Suzuki-Miyaura cross coupling of 2-
50 bromobenzaldehydes and arylboronic acid derivatives. We
applied the optimized reaction condition on the substrates (4)
bearing a range of electron withdrawing and donating group on
both of the aryl rings and we got the corresponding
dibenzopyranones in good to excellent yields (Table 2, entries 5a-
After getting success in the synthesis of
a series of
dibenzopyranones, the methodology was applied on fused o-
phenylnaphthaldehydes and the results are given in Table 3.
100 Under the optimized reaction condition, the substrate o-
phenylnaphthaldehydes took longer reaction time for completion
of the reaction and the product naphthochromens were formed
along with a significant amount of fluorenone derivatives.
Finally, the yield of the reaction was lower in comparison to the
105 benzochromen systems.
2
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Table 2: Investigation of the substrates scope a,b,c
55 After the successful application of this methodology on different
o-arylbenzaldehyde derivatives, we thought that the methodology
could also be applicable on 2-arylbenzylalcohol or benzoic acid
derivatives and for that we had applied the methodology on 2-
phenylbenzylalcohol and 2-phenylbenzoic acid. The substrate 2-
60 phenylbenzylalcohol promoted the reaction successfully and gave
the product dibenzopyranone along with the unreacted
intermediate 2-phenylbenzaldehyde under the optimized reaction
condition (Table 4). However, the 2-phenylbenzoic acid gave the
product dibenzopyranone in trace amount even after 48 h.
O
CHO
CuCl (5 mol%), DMSO
O
Ar1
Ar1
TBHP (6 equiv.)
r.t., 4h
Ar2
Ar2
5
10
15
20
25
30
4
5
O
O
O
O
MeO
O
F
O
O
O
Me
65 Table 4: Investigation on the scope of the reactiona
5a, Y = 78%
5b, Y = 77%
5d, Y = 84%
5c, Y = 63%
O
O
O
O
MeO
MeO
O
O
MeO
MeO
O
CHO
OH
CuCl, TBHP
O
MeO
+
OMe
5f, Y = 60%
DMSO, r.t., 12h
OMe
5e, Y = 82%
7
5g, Y = 63%
O
O
70
O
O
5a, Y = 56%
4a, Y = 12%
O
O
F
O
O
O
O
O
O
Me
Cl
OH
CuCl, TBHP
5h, Y = 76%
OMe
5i, Y = 68%
Me
5j, Y = 72%
5k, Y = 70%
DMSO, r.t., 48 h
75
O
O
O
O
O
O
8
MeO
O
O
trace amount
5a
+
Me
aReaction conditions: 2-arylbenzaldehyde (0.25 mmol), CuCl (5
Cl
Me
5m, Y = 0%
5l, Y = 80%
5n, Y = 75% (3:2)
80 mol%), DMSO (2 mL), TBHP (6 equiv.) stirred at room
temperature for the required time.
Me
O
O
O
O
O
O
MeO
MeO
Scheme 3: Plausible reaction mechanism
O
O
MeO
OMe
t-BuOOH
t-BuO
[CuII]
t-BuOOH
+
OH
OH
MeO
OMe
5p, Y = 82%
OMe
5r, Y = 83%
[CuI]
5o, Y = 0%
5q, Y = 87%
O
OBu-t
H2O t-BuOO
+
35 aReaction conditions: 2-arylbenzaldehyde (0.25 mmol), CuCl (5
mol%), DMSO (2 mL), TBHP (6 equiv.) stirred at room
temperature for 4h.bIsolated yield.c5n isomers as an NMR ratio.
+
O
85
O
O
t-BuOH
t-BuO
H
B
t-BuOO
O
Table 3: Extension of the reaction scope for the synthesis of
naphthochromensa,b
A
4a
t-BuOOH
-
C
by product observed
O
O
O
40
45
50
t-BuO
-[CuI]
OBu-t
O
[CuII] +
O
CHO
O
O
O H
CuCl, TBHP
O
[CuI]
-t-BuOH
5a
DMSO, r.t., 12h
E
4s
D
5s
(5 : 6)
B
6a
90
30+37=67
H2O
OH
O
by product detected
O
CuCl, TBHP
O
8
CHO
O
DMSO, r.t., 12h
(7 : 5)
25 + 35=60
We observed that the substrate 4a under the optimized reaction
conditions gave the product 5a in 22% yield only in presence of
5t
6b
4t
95 radical scavenger TEMPO (2.0 equiv.).
aReaction conditions: 2-arylbenzaldehyde (0.25 mmol), CuCl (5
mol%), DMSO (2 mL), TBHP (6 equiv.) stirred at room
temperature for 12 h.bIsolated yield.
On the basis of our experimental results and literature reports,16
the tentative mechanism of the reaction is described in Scheme 3.
At first, in the presence of CuCl catalyst the TBHP decomposes
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to generate tert-butoxyl and tert-butylperoxy radicals. Then the
tert-butoxyl radical abstracts the aldehyde proton from 4a and
gives the radical intermediate A. Then the intermediate A either
combines with tert-butylperoxy radical to give the perester
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decomposes to form the intermediate D and tert-butoxyl radical.
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55
5
60
10 give the radical intermediate E which forms the product 5a after
abstraction of a proton by tert-butoxyl radical. The intermediate
D gave 2-phenylbenzoic acid when the reaction was performed in
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65
In conclusion, we have developed
a novel and efficient
15 methodology for the synthesis of dibenzopyranones involving the
CuCl catalyzed intramolecular oxidative lactonization of aromatic
C-H bond with aldehyde group in 2-arylbenzaldehyde
derivatives. We have successfully applied our methodology for
the synthesis of methyl ether of natural product Urolithins A-C.
20 Finally we believe that the methodology will be widely exploited
by synthetic organic chemists because of low cost CuCl catalyst,
ambient reaction temperature, easily synthesizable starting
materials, scalable17 and finally the good to excellent yields of the
reaction. The mechanistic details and the application of this
25 methodology for the synthesis other benzochromen containing
natural products are under progress in our laboratory.
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Acknowledgement:
We gratefully acknowledge DST for providing funds and R.S.
thanks CSIR New Delhi for the fellowship.
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Occurrence, Chemistry and Biochemistry; John Wiley & Sons:
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30 Notes and references
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a Department of Chemistry, Indian Institute of Technology, Kharagpur
721302, India; Fax: 91-3222 282252; Tel: +91 3222 283326; E-mail:
jkray@chem.iitkgp.ernet.in
† Electronic Supplementary Information (ESI) available: [details of any
35 supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
85
‡ Footnotes should appear here. These might include comments relevant
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17. A 10 mmol biphenyl-2-carbadehyde under the optimized
reaction conditions afforded 71% yield.
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5