Copper(I)-Catalyzed 3-Position Methylation of Coumarins by Using Di-tert-butyl Peroxide as the Methylation Reagents

Article abstract of DOI:10.1002/cjoc.201500836

The copper-catalyzed methylation of coumarin by using di-tert-butyl peroxide (DTBP) has been described. The reaction provides direct access to a wide range of 3-methylcoumarins in moderate to good yields. In this procedure, it is noteworthy that DTBP was employed not only as the oxidant, but also as the methyl source. The copper-catalyzed methylation of coumarin by using di-tert-butyl peroxide (DTBP) has been described. The reaction provides direct access to a wide range of 3-methylcoumarins in moderate to good yields. In this procedure, it is noteworthy that DTBP was employed not only as the oxidant, but also as the methyl source.

Full text of DOI:10.1002/cjoc.201500836

FULL PAPER
DOI: 10.1002/cjoc.201500836
Copper(I)-Catalyzed 3-Position Methylation of Coumarins by
Using Di-tert-butyl Peroxide as the Methylation Reagents
Huan Zhuang, Runsheng Zeng,* and Jianping Zou*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and
Materials Science, Soochow University, Suzhou, Jiangsu 215123, China
The copper-catalyzed methylation of coumarin by using di-tert-butyl peroxide (DTBP) has been described. The
reaction provides direct access to a wide range of 3-methylcoumarins in moderate to good yields. In this procedure,
it is noteworthy that DTBP was employed not only as the oxidant, but also as the methyl source.
Keywords CuCl, di-tert-butyl peroxide, methylation, coumarin
Introduction
3-methylcoumarin (3a) in modiate yield at 140 ℃ (Ta-
ble 1, Entry 1). Followed by screening different Cu salt
Coumarins are an important class of natural products
that display a diverse range of biological activities and
they are also widely used in organic materials due to
their good optical properties. Recently, simple cou-
marins have been used successfully in transition-metal-
catalyzed direct C－H bond activation reactions with
different nucleophiles.
Metal-catalyzed methylation reactions have been
such as CuI, CuO, Cu(OAc)
2 2
•H O (Table 1, Entries 2－
[1]
4
), the results showed that CuCl works well for the re-
action. Reaction time screening showed that 10 h was
the best (Entries 6－10). Solvent screening showed that
MeCN, DMSO, DMF and THF were not effective at all
[
2]
(
Entries 13－16).
[3]
A control experiment revealed that 18% of product
was detected in the absence of any catalyst (Table 1,
Entries 5).
[
4]
developed for the magical methyl effect. In 2008, Li
[
5]
and co-workers firstly reported palladium-catalyzed
methylation of arenes by using peroxides. In the past
Other than the above-mentioned factors, the effects
of peroxides were also investigated (Table 1, Entries 9,
11, 12), and the optimal reaction conditions were deter-
mined to be DTBP as peroxides and CuCl as catalyst in
chlorobenzene at 140 ℃ for 10 h under air.
To explore the scope of the copper-catalyzed methy-
lation of coumarin, we examined a variety of coumarins
including electron-withdrawing group and electron-do-
nating group (Table 2). It was found that coumarins
bearing electron-donating groups like methyl and
methoxy on the phenyl ring afforded the expected
3-methylcoumarins in 50%－58% yields (Table 2, En-
tries 1－10). On the contrary, coumarins bearing elec-
decade, organic peroxides were widely used in Csp -H
bond activation processes, especially in the cross-
3
[
6]
dehyodrgenative coupling reactions. In these reactions,
organic peroxides were usually used as H-acceptors to
activate C－H bonds. Thus metal-catalyzed cross-
dehydrogenative coupling/cyclization of activated al-
kenes with methylic C－H bonds for the synthesis of
[7]
heterocycles has also been developed. Recently, cop-
per-catalyzed N-methylation and O-methylation have
[
8]
been reported by using peroxides.
In our previous studies, we reported the direct tri-
fluoromethylation of coumarins. Herein we report a
novel, selective and efficient method for 3-position me-
thylation of coumarins by using peroxides.
[9]
tron-withdrawing groups like Cl and NO
ring gave the products in good yields (Table 2, Entries
1－13). In addition, 4-methyl coumarins also gave the
2
on the phenyl
1
products in good yield (Table 2, Entries 14－16).
The plausible mechanism for the CuCl-catalyzed
methylation reaction of coumarin (1) with DTBP (2a)
was proposed in Scheme 1. DTBP produced the tert-
butoxy radical, which converted to the methyl radical by
releasing 1 equiv. of acetone. Then, methyl radical de-
rived from 2a selectively added to the 3-position of 1 to
form intermediate radical 4, which was oxidized by
Results and Discussion
When the model reaction of coumarin (1a) with
di-tert-butyl peroxide (DTBP) (2a) was performed at
1
00 ℃ in chlorobenzene in the presence of CuCl, no
reaction took place (Table 1, Entry 17). However, the
reaction proceeded to afford the desired product
*
E-mail: zengrunsheng@suda.edu.cn; jpzou@suda.edu.cn
Received November 29, 2015; accepted January 11, 2016; published online XXXX, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500836 or from the author.
Chin. J. Chem. 2016, XX, 1—5
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Zhuang, Zeng & Zou
FULL PAPER
a
a
Table 1 Optimization of the reaction conditions
Table 2 Reactions of coumarins (1a) and DTBP (2a)
b
Entry Catalyst
Solvent Peroxide Time/hYield /%
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
CuCl
CuI
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
MeCN
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBPB
TBHP
DTBP
24
24
24
24
24
1
51
46
Cu(OAc)2
CuO
50
47
－
18
1
2
b
Entry
R
R
Product
3a
Yield /%
c
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
N.D.
20
1
2
3
4
5
6
7
8
9
H
H
53
50
52
56
58
55
53
51
50
58
66
64
68
65
62
68
60
4
6-Me
7-Me
6-MeO
7-MeO
H
3b
3c
8
43
H
10
12
10
10
10
10
10
10
10
10
53
H
3d
3e
0
1
2
3
4
5
6
7
8
51
H
48
7-CH COO
H
3f
c
c
c
c
c
3
N.D.
N.D.
N.D.
N.D.
N.D.
8-Me
H
3g
6,8-di Me
5,8-di Me
7,8-di Me
6-Cl
H
3h
3i
DMSO DTBP
H
DMF
THF
PhCl
PhCl
DTBP
DTBP
DTBP
DTBP
1
0
1
1
3
4
5
H
3j
1
H
3k
3l
e
f
d
N.R.
32
2
7-Cl
H
1
a
1
1
1
6-NO2
H
3m
3n
3o
Reaction conditions: Coumarin (1a) (1.0 mmol), catalyst (10
6-Me
4-Me
4-Me
4-Me
4-MeO
mol%), and peroxide (2) (4.0 mmol, 4 equiv.) (DTBP ＝
di-tert-butyl peroxide, TBPB＝tert-butyl peroxybenzoate, TBHP
＝
5,7-di-MeO
tert-butyl hydroperoxide) in PhCl (3 mL) at 140 ℃ for 10 h
16
7-CH
3
COO
3p
3q
b
c
d
under air. Isolated yield. N.D. means not detected. N.R.
means no reaction. At 100 ℃. At 120 ℃.
17
H
e
f
a
Reaction conditions: Coumarin (1) (1.0 mmol), CuCl (10 mol%),
and peroxide (2a) (4.0 mmol, 4 equiv.) in PhCl (3 mL) at 140 ℃
b
Cu(II) to form carbocation 5. Finally deprotonation of 5
gave the product 3.
for 10 h under air. Isolated yield.
Finally, the control experiment was performed
Scheme 2). When the radical-trapping reagent 2,2,6,6-
this methylation involves free radical intermediates.
(
tetramethylpiperidine N-oxide (TEMPO) was added to
coumarins (1a) and DTBP (2a), the reaction was com-
Conclusions
pletely suppressed. In addition, the TEMPO-CH
A was detected by LC-MS, which could indicate that
3
adduct
In conclusion, a novel method for 3-position meth-
ylation of coumarins was developed through the radical
Scheme 1 Plausible mechanism for reactions of coumarins (1) and DTBP (2a) catalyzed by Cu(I)
Cu(II)
O O
+
Cu(I)
O
CH3
R2
O
O
2
a
R1
O
1
R2
O
R2
+
R2
H
Cu(II)
CH3
H+
H
CH3
-
R1
R1
R1
CH3
O
O
O
O
O
Cu(I)
3
5
4
2
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© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2016, XX, 1—5

Copper(I)-Catalyzed 3-Position Methylation of Coumarins
＋
[M＋1] : 175; found 175.
Scheme 2 Radical capture experiment
[11]
3
,7-Dimethyl-2H-chromen-one (3c)
Column
chromatography on silica gel (ethyl acetate/petroleum
ether 1∶20) afforded the title product in 52% isolated
1
yield as a white solid; H NMR (400 MHz, CDCl
3
) δ:
7
.50 (s, 1H), 7.31 (d, J＝7.9 Hz, 1H), 7.13 (s, 1H), 7.08
13
(
(
d, J＝7.9 Hz, 1H), 2.45 (s, 3H), 2.21 (s, 3H); C NMR
100 MHz, CDCl ) δ: 163.3, 154.1, 142.3, 140.0, 127.4,
3
1
26.2, 125.3, 117.9, 117.4, 22.4, 17.9. MS calcd for
＋
C
11
H
10
O
2
[M＋1] : 175; found 175.
[
11]
6
-Methoxy-3-methyl-2H-chromen-2-one (3d)
Column chromatography on silica gel (ethyl acetate/
reaction of coumarins with DTBP catalyzed by CuCl.
This procedure tolerated a series of functional groups,
avoided the use of poisonous methyl iodide and pro-
vided a facile and straightforward way for the synthesis
of 3-methylcoumarins.
petroleum ether 1∶20) afforded the title product in
1
5
6% isolated yield as a white solid; H NMR (400 MHz,
CDCl
3
) δ: 7.46 (s, 1H), 7.23 (d, J＝9.0 Hz, 1H), 7.03
(
(
dd, J＝9.0, 2.9 Hz, 1H), 6.85 (d, J＝2.9 Hz, 1H), 3.84
s, 3H), 2.21 (s, 3H); C NMR (100 MHz, CDCl ) δ:
3
13
1
63.1, 156.7, 148.4, 139.8, 126.9, 120.7, 118.7, 118.1,
Experimental
＋
110.0, 56.5, 18.0. MS calcd for C11
H
10
O
3
[M＋1] : 191;
General
found 191.
[
12]
7
-Methoxy-3-methyl-2H-chromen-2-one
(3e)
Column chromatography was performed with silica
Column chromatography on silica gel (ethyl acetate/
petroleum ether 1∶20) afforded the title product in
gel (300－400 mesh) and analytical TLC on silica 60-F
1
13
2
54. H NMR and C NMR spectra were determined in
1
5
8% isolated yield as a white solid; H NMR (400 MHz,
CDCl on an Inova-400 MHz spectrometer and chemi-
3
CDCl
.83 (m, 2H), 3.88 (s, 3H), 2.20 (s, 3H); C NMR (100
MHz, CDCl ) δ: 163.4, 162.5, 155.7, 140.1, 128.6,
23.0, 114.0, 113.2, 101.3, 56.5, 17.8. MS calcd for
3
) δ: 7.47 (s, 1H), 7.33 (d, J＝8.6 Hz, 1H), 6.86－
cal shifts (δ) reported relative to internal TMS. HRMS
was recorded on a Micromass OA-TOF instrument. All
reagents were used directly as obtained commercially.
13
6
3
1
＋
General procedure for the synthesis of 3-amino-4-
cyano-substituted isoquinoline-1(2H)-ones (3a－3z)
C
11
H
10
O
3
[M＋1] : 191; found 191.
[
13]
3
-Methyl-2-oxo-2H-chromen-7-yl acetate (3f)
A glassware with a magnetic stirring bar was
charged with coumarins 1 (1.0 mmol), CuCl (10 mol%),
DTBP 2 (4 mmol, 4 equiv.) and PhCl (3 mL). The reac-
tion mixture was stirred at 140 ℃ for 10 h under air.
After cooling to room temperature, the reaction mixture
was diluted with water and extracted with ethyl acetate.
The organic layer was dried over anhydrous sodium
sulfate and evaporated under reduced pressure. The
crude product was purified by chromatography on silica
gel using ethyl acetate/petroleum ether as eluent to af-
ford 3-methylcoumarins 3.
Column chromatography on silica gel (ethyl acetate/
petroleum ether 1∶16 to 1∶4) afforded the title prod-
1
uct in 55% isolated yield as a white solid; H NMR (400
MHz, CDCl
3
) δ: 7.51 (s, 1H), 7.43 (d, J＝8.4 Hz, 1H),
7.10 (s, 1H), 7.04－7.02 (m, 1H), 2.34 (s, 3H), 2.22 (s,
1
3
3H); C NMR (100 MHz, CDCl
3
) δ: 169.6, 162.6,
154.5, 152.8, 139.4, 128.4, 126.0, 118.2, 110.7, 21.9,
＋
17.9. MS calcd for C12
10 4
H O [M＋1] : 219; found 219.
[
14]
3,8-Dimethyl-2H-chromen-one (3g)
Column
chromatography on silica gel (ethyl acetate/petroleum
ether 1∶32 to 1∶20) afforded the title product in 53%
[
10]
1
3
-Methyl-2H-chromen-2-one (3a)
Column
isolated yield as a white solid; H NMR (400 MHz,
chromatography on silica gel (ethyl acetate/petroleum
CDCl
3
) δ: 7.52 (s, 1H), 7.32 (d, J＝7.3 Hz, 1H), 7.27 (d,
ether 1∶20) afforded the title product in 53% isolated
J＝7.7 Hz, 1H), 7.18－7.14 (m, 1H), 2.47 (s, 3H), 2.23
1
13
yield as a white solid; H NMR (400 MHz, CDCl
3
) δ:
(s, 3H); C NMR (100 MHz, CDCl
3
) δ: 163.2, 152.4,
7
1
.52 (s, 1H), 7.48－7.41 (m, 2H), 7.30 (d, J＝8.3 Hz,
140.4, 132.6, 126.6, 126.1, 125.4, 124.6, 120.1, 17.9,
＋
13
H), 7.27－7.23 (m, 1H), 2.22 (s, 3H); C NMR (100
) δ: 162.9, 153.9, 140.0, 131.2, 127.7,
26.5, 125.0, 120.3, 117.1, 17.9. MS calcd for C10
10 2
16.2. MS calcd for C11H O [M＋1] : 175; found 175.
MHz, CDCl
1
3
3,6,8-Trimethyl-2H-chromen-2-one (3h) Column
chromatography on silica gel (ethyl acetate/petroleum
ether 1∶32 to 1∶20) afforded the title product in 51%
8 2
H O
＋
[
M＋1] : 161; found 161.
[11]
1
3
,6-Dimethyl-2H-chromen-2-one (3b)
Column
isolated yield as a white solid; H NMR (400 MHz,
chromatography on silica gel (ethyl acetate/petroleum
CDCl
(s, 3H), 2.37 (s, 3H), 2.22 (s, 3H); C NMR (100 MHz,
CDCl ) δ: 163.4, 150.5, 140.4, 134.1, 133.7, 126.2,
126.0, 125.2, 119.8, 21.5, 17.9, 16.1. HRMS calcd for
3
) δ: 7.45 (s, 1H), 7.14 (s, 1H), 7.04 (s, 1H), 2.43
1
3
ether 1∶20) afforded the title product in 50% isolated
1
yield as a white solid; H NMR (400 MHz, CDCl
3
) δ:
3
7
.45 (s, 1H), 7.26 (dd, J＝8.5, 1.7 Hz, 1H), 7.20－7.18
13
＋
(
m, 1H), 2.39 (s, 3H), 2.20 (s, 3H); C NMR (100 MHz,
CDCl ) δ: 163.2, 152.1, 139.9, 134.6, 132.1, 127.5,
26.3, 120.0, 116.8, 21.5, 17.9. MS calcd for C11
12 12 2
C H O [M＋Na] : 211.0732; found 211.2225.
[
15]
3,5,8-Trimethyl-2H-chromen-2-one (3i)
umn chromatography on silica gel (ethyl acetate/petro-
Col-
3
1
H O
10 2
Chin. J. Chem. 2016, XX, 1—5
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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3

Zhuang, Zeng & Zou
FULL PAPER
leum ether 1∶32 to 1∶20) afforded the title product in
(400 MHz, CDCl
3
) δ: 6.44 (d, J＝2.5 Hz, 1H), 6.32 (d,
1
5
0% isolated yield as a white solid; H NMR (400 MHz,
J＝2.5 Hz, 1H), 3.86 (d, J＝8.3 Hz, 6H), 2.56 (s, 3H),
13
CDCl
3
) δ: 7.70 (s, 1H), 7.18 (d, J＝7.6 Hz, 1H), 6.97 (d,
2.17 (s, 3H); C NMR (100 MHz, CDCl
3
) δ: 163.0,
J＝7.6 Hz, 1H), 2.47 (s, 3H), 2.41 (s, 3H), 2.24 (s, 3H);
162.3, 159.4, 155.8, 149.3, 118.5, 106.3, 96.4, 93.8,
1
3
C NMR (100 MHz, CDCl
3
) δ: 163.0, 152.7, 137.3,
＋56.5, 56.4, 20.3, 13.8. MS calcd for C10
: 221; found 221.
H
7
ClO
2
[M＋1]
1
1
33.1, 132.2, 125.7, 125.3, 124.2, 118.7, 18.9, 18.1,
＋
6.1. MS calcd for C12
H
12
O
2
[M＋1] : 189; found 189.
3,4-Dimethyl-2-oxo-2H-chromen-7-yl
acetate
[
17]
3
,7,8-Trimethyl-2H-chromen-2-one (3j) Column
(3p)
Column chromatography on silica gel (ethyl
chromatography on silica gel (ethyl acetate/petroleum
acetate/petroleum ether 1∶16 to 1∶4) afforded the title
product in 68% isolated yield as a white solid; H NMR
1
ether 1∶32 to 1∶20) afforded the title product in 58%
1
isolated yield as a white solid; H NMR (400 MHz,
(400 MHz, CDCl
3
) δ: 7.61 (d, J＝8.6 Hz, 1H), 7.10－
CDCl
3
) δ: 7.47 (s, 1H), 7.15 (d, J＝7.9 Hz, 1H), 7.06 (d,
7.05 (m, 2H), 2.41 (s, 3H), 2.35 (s, 3H), 2.23 (s, 3H);
1
3
J＝7.9 Hz, 1H), 2.38 (d, J＝3.8 Hz, 6H), 2.21 (s, 3H);
3
C NMR (100 MHz, CDCl ) δ: 169.6, 162.5, 153.3,
1
3
C NMR (100 MHz, CDCl
3
) δ: 163.5, 152.4, 140.8,
152.7, 146.3, 125.9, 122.5, 119.2, 118.6, 110.8, 21.0,
＋
1
1
40.6, 126.4, 124.9, 124.7, 124.5, 118.1, 21.0, 17.7,
2.2. HRMS calcd for C12H O [M＋Na] : 211.0732;
12 2
15.9, 14.2. MS calcd for C13
233.
12 4
H O [M＋1] : 233; found
＋
found 211.2225.
-Chloro-3-methyl-2H-chromen-2-one
4-Methoxy-3-methyl-2H-chromen-2-one (3q)
Column chromatography on silica gel (ethyl acetate/
[
11]
6
(3k)
petroleum ether 1∶20) afforded the title product in
Column chromatography on silica gel (ethyl acetate/
petroleum ether 1∶20) afforded the title product in
1
5
6% isolated yield as a white solid; H NMR (400 MHz,
1
CDCl
3
) δ: 7.72 (dd, J＝7.9, 1.5 Hz, 1H), 7.53－7.49 (m,
6
6% isolated yield as a white solid; H NMR (400 MHz,
1
4
H), 7.34 (dd, J＝8,3, 0.7 Hz, 1H), 7.33－7.28 (m, 1H),
.02 (s, 3H), 2.20 (s, 3H); C NMR (100 MHz, CDCl )
3
CDCl ) δ: 7.45 (s, 1H), 7.42－7.40 (m, 2H), 7.28－7.24
3
13
13
(
m, 1H), 2.24 (s, 3H); C NMR (100 MHz, CDCl
3
) δ:
δ: 165.0, 164.4, 153.1, 132.0, 124.8, 123.7, 118.0, 117.4,
1
62.3, 152.3, 138.7, 131.1, 130.2, 128.0, 127.0, 121.＋4,
＋
10 3
113.2, 62.0, 11.4. MS calcd for C11H O [M＋1] : 191;
118.6, 21.0, 18.0. MS calcd for C10
H
7
ClO
2
[M＋1] :
found 191.
1
95; found 195.
-Chloro-3-methyl-2H-chromen-2-one
7
(3l)
Column chromatography on silica gel (ethyl acetate/
Acknowledgement
petroleum ether 1∶32 to 1∶20) afforded the title
1
We thank the National Natural Science Foundation
of China (Nos. 20772088, 21172163) and a project
funded by the Priority Academic Program Development
of Jiangsu Higher Education Institutions for funding this
work.
product in 64% isolated yield as a white solid; H NMR
(
7
3
400 MHz, CDCl ) δ: 7.48 (s, 3H), 7.36－7.28 (m, 2H),
.23－7.21 (m, 1H), 2.21 (s, 3H); C NMR (100 MHz,
13
CDCl
3
) δ: 162.2, 154.1, 139.2, 136.9, 128.5, 126.6,
ClO [M
1
＋
25.6, 118.8, 117.4, 17.9. MS calcd for C10
1] : 195; found 195.
H
7
2
＋
References
3-Methyl-6-nitro-2H-chromen-2-one (3m) Col-
umn chromatography on silica gel (ethyl acetate/petro-
[1] (a) Li, B.; Pai, R.; Di, M.; Aiello, D.; Barnes, M. H.; Butler, M. M.;
leum ether 1∶16 to 1∶8) afforded the title product in
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1
2
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8% isolated yield as a white solid; H NMR (400 MHz,
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3
) δ: 8.38 (d, J＝2.5 Hz, 1H), 8.35 (dd, J＝9.0, 2.6
Hz, 1H), 7.62 (s, 1H), 7.46 (d, J＝9.0 Hz, 1H), 2.30 (s,
1
3
3
1
H); C NMR (100 MHz, CDCl
3
) δ: 161.3, 157.5,
38.6, 129.3, 126.1, 123.6, 121.4, 120.4, 118.4, 18.1.
2
013, 56, 5288; (e) Chen, Y.; Wang, S.; Xu, X.; Liu, X.; Yu, M.;
＋
MS calcd for C10
H
7
NO
4
[M＋1] : 206; found 206.
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[
16]
3
,4,6-Trimethyl-2H-chromen-2-one (3n)
[
Column chromatography on silica gel (ethyl acetate/
petroleum ether 1∶20) afforded the title product in
1
6
5% isolated yield as a white solid; H NMR (400 MHz,
2
010, 11, 247; (c) Sashidhara, K. V.; Kumar, M.; Khedgikar, V.;
CDCl
3
) δ: 7.37 (s, 1H), 7.27－7.25 (m, 1H), 7.19 (d,
Kushwaha, P.; Modukuri, R. K.; Kumar, A.; Gautam, J.; Singh, D.;
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1
3
C NMR (100 MHz, CDCl
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34.3, 132.1, 125.0, 122.9, 121.0, 117.2, 21.9, 15.8,
4.2. HRMS calcd for C12H O [M＋Na] : 211.0727;
12 2
＋
found 211.0825.
5
3o)
,7-Dimethoxy-3,4,-dimethyl-2H-chromen-2-one
[
17]
(
Column chromatography on silica gel (ethyl
acetate/petroleum ether 1∶16 to 1∶8) afforded the title
product in 62% isolated yield as a white solid; H NMR
4
9, 10935.
1
[4] (a) Liu, Y.-Y.; Yang, X.-H.; Huang, X.-C.; Wei, W.-T.; Song, R.-J.;
4
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