New photochromic diarylethenes with a pyrimidine moiety

Article abstract of DOI:10.1016/j.tetlet.2012.11.141

A novel class of photochromic diarylethene derivatives based on the hybrid skeleton of six-membered pyrimidine and five-membered thiophene moieties has been firstly synthesized. The substituent effects on their properties, including photochromism, fatigue

Full text of DOI:10.1016/j.tetlet.2012.11.141

Accepted Manuscript
New photochromic diarylethenes with a pyrimidine moiety
Hongliang Liu, Shouzhi Pu, Gang Liu, Bing Chen
PII:
S0040-4039(12)02104-1
DOI:
http://dx.doi.org/10.1016/j.tetlet.2012.11.141
TETL 42237
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
2 October 2012
28 November 2012
30 November 2012
Please cite this article as: Liu, H., Pu, S., Liu, G., Chen, B., New photochromic diarylethenes with a pyrimidine
moiety, Tetrahedron Letters (2012), doi: http://dx.doi.org/10.1016/j.tetlet.2012.11.141
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New photochromic diarylethenes with a
pyrimidine moiety
Hongliang Liu, Shouzhi Pu*, Gang Liu, Bing Chen
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang
330013, PR China

Tetrahedron Letters
1
TETRAHEDRON
LETTERS
Pergamon
New photochromic diarylethenes with a pyrimidine moiety
Hongliang Liu, Shouzhi Pu*, Gang Liu, Bing Chen
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China
Abstract—A novel class of photochromic diarylethene derivatives based on the hybrid skeleton of six-membered
pyrimidine and five-membered thiophene moieties has been firstly synthesized. The substituent effects on their
properties, including photochromism, fatigue resistance, and fluorescence, have been systematically investigated. All
these diarylethenes showed significant photochromism and notable fluorescence switching properties in both solution
and poly(methylmethacrylate) films. The electron-donating substituent enhanced their cyclization quantum yield,
fatigue resistance, and fluorescence quantum yield, whereas electron-withdrawing group exhibited contrary effects.
During the past few decades, various types of photochromic
molecules,1 such as stilbene,2 azobenzenes,3 furylfulgides,4
spirooxazines,5 and diarylethenes,6 have been developed.
These photochromic compounds have been applied as
optical memories and switches.7 Among them, diarylethene
derivatives with heterocyclic aryl rings are one of the most
promising candidates for practical applications because of
their excellent thermal stability, notable fatigue resistance,
and high photo reactivity in solid state.8
derivatives with two phenyl/naphthyl groups have been
reported.15 The majority of these derivatives are thermally
reversible and have poor photochromic activity. In previous
publications,13,16 we reported a new class of diarylethenes
with both five- and six-membered aryl rings, which showed
new characteristics different from the ones with five-
membered aryl rings. All of these compounds exhibited
excellent photochromic and fluorescence switching
properties in both solution and poly (methylmethacrylate)
(PMMA) films. These diarylethenes with a six-membered
aryl moiety are potential candidates for optoelectronic
applications.
Generally, the aryl moieties and functional substituents
have important effects on the photochromic properties of
diarylethenes. For example, diarylethenes with thiophene or
benzothiophene moieties show excellent thermal stability
and fatigue resistance,8a,9 and the diarylethenes with indole
rings can act like effective fluorescent photoswitches.10
Yamaguchi and Irie11 reported that bulky alkyl chains at 2-
positions of benzothiophene and benzofuran rings improved
the cyclization quantum yield. Morimitsu et al.12 and Pu et
al.13 revealed that bulky alkoxy substituents at the reactive
carbons decreased the thermal stability and fatigue
resistance. So far, the hexatriene backbones of the reported
diarylethene systems are mainly confined to the five-
membered heteroaryl rings.14 In the cases of six-membered
heteroaryl rings, only a few symmetrical diarylethene
To date, the six-membered aryl rings used in known
diarylethenes were only limited to benzene and
pyridine.13,16 To the best of our knowledge, photochromic
diarylethenes based on a hybrid structure of pyrimidine and
thiophene have not hitherto been reported. In this Letter, we
designed and synthesized a new class of diarylethenes (1o–
3o) with both pyrimidine and thiophene, and systematically
investigated their properties. All of these diarylethenes
showed good photochromism in solution, PMMA
amorphous films, and crystalline phase. The photochromic
scheme of diarylethenes 1o–3o is shown in Scheme 1.
———
Keyword: Diarylethene; Photochromism; Pyrimidine moiety; Optical characteristic.
Corresponding author. Tel./Fax: +86-791-3831996; E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).

2
Tetrahedron Letters
alternating irradiation with UV light and visible light of
F
F
F
F
F
F
F
F
appropriate wavelength in both solution and solid media.
The absorption spectral changes of diarylethene 1 upon
photochromic reactions are shown in Figure 1. In hexane,
the absorption maximum of 1o was observed at 290 nm (ε,
3.92 × 104 L mol-1 cm-1) due to π→π* transition.19 Upon
irradiation with 297 nm light, a new visible absorption band
centered at 533 nm (ε, 1.95 × 104 L mol-1 cm-1) emerged,
while the original peak at 290 nm decreased with color
change from colorless to red (Figure 2A), which could be
ascribed to the formation of closed-ring isomer 1c.
Reversely, the red solution of 1c could be completely
bleached upon irradiation with visible light (λ > 450 nm). In
photostationary state, the isosbestic point of 1 was observed
at 320 nm, which was in agreement with the reversible two-
component photochromic reaction scheme.20 Similar to 1,
diarylethenes 2 and 3 also exhibited evident photochromism
by photoirradiation. The absorption maxima were observed
at 522 nm for 2c and 518 nm for 3c. The color changes of 2
and 3 by photoirradiation in hexane are also showed in
Figure 2A. In photostationary state, the isosbestic points of
2 and 3 were observed at 310 and 318 nm, respectively. In
addition, the photoconversion ratios of 1–3 were obtained
by HPLC analysis in photostationary state, with the value of
80% for 1, 72% for 2, and 84% for 3 (Table 1).
UV
Vis
F
F
F
F
N
N
O
O
S
O
S
N
N
O
R
R
OCH3
H
1o: R =
2o: R =
3o: R = CF3
OCH3
H
1c: R =
2c: R =
3c: R = CF3
Scheme 1. Photochromism of diarylethenes 1o–3o.
F
F
Br
F
F
Br
B(OH)2
R
Br
n-BuLi/THF, 195 K
F
F
S
F
C5F8
Pd(PPh3)4
Na2CO3,aq.
S
S
R
4
OCH3
H
CF3
5a: R =
5b: R =
5c: R =
OCH3
H
6a: R =
6b: R =
6c: R =
R
CF3
Br
O
F
F
F
F
N
N
F
F
OCH3
H
1o: R =
2o: R =
3o: R = CF3
O
N
O
O
195 K
n-BuLi/THF,
S
N
R
Scheme 2. Synthetic route for diarylethenes 1o–3o.
The synthetic route for 1o–3o is shown in Scheme 2. Suzuki
coupling of bromobenzene derivatives with thiophene
boronic acid (4)17 afforded compounds 5a–c. Lithiation and
coupling with perfluorocyclopentene gave mono-substituted
6a–c. Lithiation of 5-bromo-2,4-dimethoxypyrimidine and
coupling with 6a–c afforded the unsymmetrical
diarylethenes 1o–3o, respectively. The structures of 1o–3o
were confirmed by elemental analysis, NMR, IR, and MS.18
The PMMA films of 1o–3o were prepared by dissolving 10
mg of diarylethene sample and 100 mg of PMMA in
chloroform (1 mL) with the aid of ultrasound, and the
homogeneous solution was spin-coated on
a quartz
Figure 2. The color changes of diarylethenes 1–3 by photoirradiation in
different media at room temperature: (A) in hexane, (B) in PMMA films,
and (C) in crystalline phase.
substrate (20 × 20 × 1 mm3) with a rotating speed at 1500
rpm. The films were dried in air and kept in darkness at
room temperature.
In PMMA films, 1–3 also showed similar photochromism
to those in hexane (Figure 2B). The absorption maxima of
the closed-ring isomers 1c–3c in PMMA films are at longer
wavelengths than those in hexane. The redshift values of
absorption maxima of the closed-ring isomers are 3 nm for
1c, 15 nm for 2c, and 10 nm for 3c, which may be ascribed
to the polar effect of polymer matrix and the stabilization of
molecular arrangement in solid medium.21 It was noted that
1o–3o showed obvious photochromism in crystalline phase
(Figure 2C), different from the reported diarylethenes with
a benzene moiety, which showed no photochromism in
crystalline phase.13a Compounds 1–3 with a pyrimidine
were found to be the second class of diarylethenes with a
six-membered aryl ring, which exhibited photochromic
activity in crystalline phase.16b
0.8
0.6
0.4
0.2
0.0
Vis
UV
UV
Vis
300
400
500
600
Wavelength/nm
Figure 1. Absorption spectral changes of diarylethene
1
by
photoirradiation in hexane (2.0 × 10-5 mol L-1) at room temperature.
The cyclization and cycloreversion quantum yields of 1–3
were measured in hexane at room temperature. The results
listed in Table 1 suggested that altering the substituents at
para-position of the terminal benzene had significant
effects on the photochromic features of 1–3, including the
Diarylethenes 1–3 showed evident photochromism and
could be toggled between the colorless open-ring isomers
(1o–3o) and the colored closed-ring isomers (1c–3c) by

Tetrahedron Letters
3
absorption maxima, molar absorption coefficients, and
quantum yields. When the hydrogen atom was replaced
with a methoxy group at the para-position of phenyl ring,
the molar absorption coefficients and cyclization quantum
yields increased, but the cycloreversion quantum yields
decreased. However, the molar absorption coefficients of
the closed-ring isomers and the quantum yields did not
change much, when the hydrogen atom was replaced with a
trifluoromethyl group. Furthermore, no matter replacing the
hydrogen atom at the para-position of the terminal benzene
with an electron-donating methoxy group (1) or with an
electron-withdrawing trifluoromethyl group (3), the molar
absorption coefficients of the open-ring isomers clearly
decreased. As a result, the unsubstituted parent diarylethene
2 has the largest molar absorption coefficient among the
open-ring isomers and the smallest molar absorption
coefficient among the closed-ring isomers. The absorption
maxima of 1–3 in hexane showed remarkable blueshift with
increase of electron-withdrawing ability. In addition, all
absorption maxima of the closed-ring isomers 1c–3c were
observed between 518–533 nm, shorter than the
diarylethenes with benzene, pyridine, pyrazole or thiophene
moieties.13a,16b,22 However, their absorption maxima were
longer than those of diarylethenes with thiazole or
naphthalene moieties.14c,16a The results suggested that the
pyrimidine can be effective to shift the absorption
maximum of diarylethene to a shorter wavelength direction,
but its action is slightly less than that of the thiazole or
naphthalene.
ultimately results in the thermal instability of the closed-
ring forms of 1–3.23
The fatigue resistance of 1–3 was tested in both hexane and
PMMA films by alternative irradiation with UV and visible
light in air at room temperature (Figure 3). In hexane, the
coloration and decoloration cycles of 1–3 were repeated 20
times with 21% degradation of 1c, 74% of 2c, and 87% of
3c, which may result from the formation of epoxides.24
Their fatigue resistance in PMMA films is much better.
After 50 repeated cycles, the degradation percentages of 1–
3 in PMMA films were 45% for 1c, 53% for 2c, and 59%
for 3c. This improvement may result from suppression of
O2 diffusion in solid medium.8g The fatigue resistance of the
diarylethene with an electron-donating methoxy group (1)
was better than that of diarylethene with an electron-
withdrawing trifluoromethyl group (3) in both hexane and a
PMMA film, which is in accordance with the results
reported for diarylethenes with a pyridine,16b but contrary to
the results reported for diarylethenes with a naphthalene.16a
Compared to the diarylethenes with a naphthalene or
pyridine moiety,16 1–3 showed weaker fatigue resistance.
But its fatigue resistance was stronger than that of the
diarylethenes with a benzene moiety. 13b
1.0
1.0
A
B
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
0
A
0
A/
A
A/
1o
2o
3o
1o
2o
3o
Table 1. Absorption spectral properties of diarylethenes 1–3 in hexane (2.0
× 10-5 mol L-1) and in PMMA films (10%, w/w) at room temperature.
0
5
10
15
20
0
10
20
30
40
50
c
o,max /nma (ε/L mol-1 cm-1
)
λ
c,max /nmb (ε/L mol-1 cm-1
)
Φ
Reapeat cycles
Reapeat cycles
λ
PR/%d
Compound
Hexane
PMMA
Hexane
PMMA
Figure 3. Fatigue resistance of 1–3 in hexane and PMMA films in air
atmosphere at room temperature: (A) in hexane, (B) in PMMA films.
Initial absorbance of the sample was fixed at 1.0.
Φ
Φ
c-o
o-c
1
2
3
290(3.92×104)
285(4.02×104)
291(3.46×104)
291
287
294
533(1.95×104)
522(1.00×104)
518(1.01×104)
536
537
528
0.51
0.17
0.15
0.10
0.26
0.29
80
72
84
1500
5000
4000
3000
2000
1000
0
3o
A
B
3o
1200
900
600
300
0
2o
a The maximum absorption peak of the open-ring isomers; b The maximum
absorption peak of the closed-ring isomers; c Quantum yields of cyclization
(Φo–c) and cycloreversion (Φc–o). Photoconversion ratios of 1–3 in the
2o
1o
d
1o
photostationary state.
The thermal stability of the open-ring and closed-ring
isomers of 1–3 was tested in hexane at both room
temperature and 341 K. After the solutions of 1o–3o were
stored in the dark at room temperature and exposed to air
for more than 10 days, no changes in the UV/Vis spectra
were observed. But 1c–3c returned to 1o–3o, when exposed
to air in the dark for only 8 h, 6.5 h and 4 h, respectively. At
341 K, the red color of diarylethenes 1c–3c faded
completely after 100 s, 80 s and 50 s, respectively. These
results suggested that the thermal stability of diarylethenes
with a pyrimidine are different from that of diarylethenes
with five-membered rings.14c,d The weak thermal stability of
1c–3c was expected due to the higher aromaticity of six-
membered pyrimidine, compared to five-membered
heteroaryl rings. The greatly enhanced ground-state energy
difference between the open-ring and closed-ring forms
350
400
450
500
550
350
450
550
400Wavelength/nm 500
Wavelength/nm
Figure 4. Emission spectra of 1o–3o in hexane (1.0 × 10-4 mol L-1) and in
PMMA films (10%, w/w) at room temperature: (A) in hexane and (B) in
PMMA films.
The fluorescence spectra of 1o–3o in both hexane (1.0 × 10-
mol L-1) and PMMA films (10%, w/w) were measured at
4
room temperature. As shown in Figure 4, the emission
peaks were observed at 449 nm (λex, 327 nm) for 1o, 427
nm (λex, 319 nm) for 2o, and 413 nm(λex, 317 nm) for 3o
in hexane, and were observed at 469 nm (λex, 319 nm) for
1o, 442 nm (λex, 300 nm) for 2o, and 418 nm (λex, 291 nm)
for 3o in PMMA films. Compared to those in hexane, the
emission peaks of 1o–3o showed a bathochromic shift in
PMMA films, with the value of 20 nm for 1, 15 nm for 2,
and 5 nm for 3. Compared to the unsubstituted 2o, the

4
Tetrahedron Letters
electron-donating methoxy group of 1o shifted the emission
peak to a longer wavelength and notably decreased the
emission intensity. However, the trifluoromethyl group of
3o shifted the emission peak to a shorter wavelength and
greatly increased the emission intensity. The result
suggested that different substituents attached at the para-
position of the terminal benzene ring had significant effects
on both wavelength and intensity of the emission peaks.
This result is consistent with that of diarylethenes with a
pyridine.16b The fluorescence quantum yields of 1o–3o were
determined to be 0.018, 0.0065, and 0.0047, using
anthracene as reference. The electron-donating methoxy
substituent of 1o effectively enhanced the fluorescence
quantum yield. This result is different from those of the
diarylethenes with a naphthalene or pyridine moiety, whose
fluorescence quantum yields improved with the increase of
electron-withdrawing ability of the substituents.16
central cyclopentene ring participated in the photoinduced
cyclization reaction in solution, PMMA films, and even
crystalline phase. In addition, different substituents at the
para-position of the terminal benzene ring showed
significant effects on the optical properties and fatigue
resistance of the diarylethenes. The pyrimidine moiety
rendered this new type of diarylethenes distinctive
photochromic features. The results are valuable for design
and synthesis of novel photoactive diarylethene derivatives
for practical applications in optoelectronic devices.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (20962008, 21162011), the Project of
Jiangxi
Academic
and
Technological
leader
(2009DD00100), the Project of Jiangxi Advantage Sci-
Techn Innovative Team (20113BCB24023), the Project of
Jiangxi Youth Scientist, and the Project of the Science
Funds of Jiangxi Education Office (GJJ10241, GJJ1067).
800
A
B
1500
UV Vis
UV Vis
600
1200
400
900
600
200
References and notes
300
0
400
450
500
550
600
400
450
500
550
600
Wavelength/nm
Wavelength/nm
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Figure 5. Emission intensity changes of diarylethenes 1 in hexane (1.0 ×
10-4 mol L-1) and in a PMMA film (10%, w/w) by photoirradiation at room
temperature: (A) in hexane (λex = 327 nm) and (B) in a PMMA film (λex
=
319 nm).
For most of the reported diarylethenes,17b,22c,25 their open-
ring isomers exhibit fluorescence, while their closed-ring
isomers are non-fluorescent. Consequently, they can act like
a fluorescent switch upon changing from open-ring isomers
to closed-ring isomers by photoirradiation. Similarly, 1–3
exhibited notable fluorescence switching properties during
photoisomerization. The changes in fluorescence of 1 in
both hexane and a PMMA film are shown in Figure 5.
Upon irradiation with 297 nm UV light, the sample reached
the photostationary state, in which its emission intensity
was quenched to ca. 17% in hexane and 66% in a PMMA
film. Back irradiation with visible light regenerated the
open-ring isomer 1o and restored the original emission
spectra. In the photostationary state, the emission intensity
was quenched to ca. 25% for 2 and 18% for 3 in solution,
and ca. 61% for 2 and 70% for 3 in PMMA films. As a
result, modulation of the fluorescence of 1–3 in hexane was
much bigger than that in PMMA films. This result is
contrary to that of the diarylethenes with a pyridine
moiety,16b whose fluorescence modulation efficiency in
hexane was much lower than that in PMMA films. Among
all known diarylethenes with
a six-membered aryl
rings,13b,16 1–3 showed the biggest fluorescence modulation
efficiency in solution, suggesting that this class of
diarylethenes may have potential applications in
photoswitchable device materials.27
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for the first time. The pyrimidine attached directly to the

Tetrahedron Letters
5
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406–407 K; Calcd for C23H15F9N2O2S (%): Calcd C, 49.83;
H, 2.73; N, 5.05. Found C, 49.95; H, 2.77; N, 5.07; 1H NMR
(400 MHz, CDCl3, ppm): δ 2.06 (s, 3H, -CH3), 3.73 (s, 3H, -
OCH3), 4.01 (s, 3H, -OCH3), 7.27 (s, 1H, thiophene-H), 7.63
(s, 4H, benzene-H), 8.34 (s, 1H, pyrimidine-H); 13CNMR
(100 MHz, CDCl3, TMS): δ 14.22, 54.20, 55.29, 103.90,
123.98, 124.0, 126.04, 126.44, 129.70, 136.66, 140.30,
141.04, 159.10, 166.17, 168.00; IR (KBr, ν, cm-1): 519, 561,
600, 640, 704, 737, 797, 826, 854, 893, 984, 1015, 1067,
1128, 1177, 1198, 1242, 1271, 1298, 1323, 1377, 1410,
1476, 1493, 1555, 1591, 1651, 2937, 2963, 2993, 3416,
3470; LRMS, ESI+ m/z 555.1 (MH+, [C23H16F9N2O2S]+
requires 555.1).
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Imamoto, Y.; Kamikubo, H.; Kataoka, M.; Kawai, T. Chem.
Mater. 2007, 19, 3479–3483.
18. Data for 1o: Mp 367–368 K; Calcd for C23H18F6N2O3S (%):
Calcd C, 53.49; H, 3.51; N, 5.42. Found C, 53.53; H, 3.54; N,
1
5.47; H NMR (400 MHz, CDCl3, ppm): δ 2.01 (s, 3 H, -
CH3), 3.74 (s, 3H, -OCH3), 3.84 (s, 3H, -OCH3), 4.01 (s, 3H,
-OCH3), 6.91 (d, 2H, J = 8.0 Hz, benzene-H), 7.06 (s, 1H,
thiophene-H), 7.45 (d, 2H, J = 8.0 Hz, benzene-H), 8.33 (s,
1H, pyrimidine-H); 13C NMR(100 MHz, CDCl3, ppm): δ
14.08, 54.16, 55.21, 55.30, 104.13, 114.36, 121.38, 125.91,
126.16, 126.84, 138.58, 142.05, 159.04, 159.47, 166.05,
168.03; IR(KBr, ν, cm-1): 507, 544, 650, 739, 758, 799, 822,
841, 891, 986, 1003, 1034, 1074, 1123, 1198, 1257, 1298,
1337, 1371, 1408, 1476, 1518, 1549, 1595, 1647, 2843,
2941, 2964, 3014, 3412, 3688; LRMS, ESI+ m/z 517.1 (MH+,
[C23H19F6N2O3S]+ requires 517.1); Data for 2o: Mp 358-359
K; Calcd for C22H16F6N2O2S (%): Calcd C, 54.32; H, 3.32;
21. Kasatani, K.; Kambe, S.; Irie, M. J. Photochem. Photobiol. A
1999, 122, 11–15.
22. (a)Pu, S. Z.; Yang, T. S.; Xu, J. K.; Chen, B. Tetrahedron Lett.
2006, 47, 6473–6477; (b) Tian, H.; Chen, B. Z.; Tu, H. Y.;
Müllen, K. Adv. Mater. 2002, 14, 918–923; (c) Feng, Y. L.;
Yan, Y. L.; Wang, S.; Zhu, W. H.; Qian, S. X.; Tian, H. J.
Mater. Chem. 2006, 16, 3685–3692.
23. (a) Yang, Y. H.; Xie, Y. S.; Zhang, Q.; Nakatani, K.; Tian, H.;
Zhu, W. H. Chem. Eur. J. 2012, 18, 11685–11694; (b) Zhu,
W. H.; Yang, Y. H.; Métivier, R.; Zhang, Q.; Guillot, R.;
Xie, Y. S.; Tian, H.; Nakatani, K. Angew. Chem. Int. Ed.
2011, 50, 10986–10990.
1
N, 5.76. Found C, 54.43; H, 3.37; N, 5.81; H NMR (400
MHz, CDCl3, ppm): δ 2.04 (s, 3H, -CH3), 3.74 (s, 3H, -
OCH3), 4.01 (s, 3H, -OCH3), 7.19 (s, 1H, thiophene-H), 7.29
(t, 1H, J = 7.6 Hz, benzene-H), 7.39 (t, 2H, J = 7.6 Hz,
benzene-H) 7.53 (d, 2H, J = 7.6 Hz, benzene-H), 8.35 (s, 1H,
pyrimidine-H); 13C NMR(100 MHz, CDCl3, ppm): δ 14.22,
54.23, 55.30, 104.10, 122.47, 125.53, 126.05, 127.88,
129.00, 133.31, 142.09, 146.34, 159.04, 166.04, 167.98; IR
(KBr, ν, cm-1): 469, 544, 567, 596, 654, 692, 762, 799, 827,
845, 856, 893, 934, 984, 1003, 1074, 1134, 1194, 1248,
1296, 1335, 1375, 1410, 1479, 1545, 1599, 1649, 1599,
1649, 2868, 2966, 3020, 3306, 3689; LRMS, ESI+ m/z 487.1
(MH+, [C22H17F6N2O2S]+ requires 487.1); Data for 3o: Mp
24. Higashiguchi, K.; Matsuda, K.; Yamada, T.; Kawai, T.; Irie,
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25. Giordano, L.; Jovin, T. M.; Irie, M.; Jares-Erijman, E. A. J.
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26. Chen, B. Z.; Wang, M. Z.; Wu, Y. Q.; Tian, H. Chem.
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27. (a) Chen, B. Z.; Wang, M. Z.; Wu, Y. Q.; Tian, H. Chem.
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Yuan, M. J.; Yin, X. D.; Zuo, Z. C.; Ouyang, C. B.; Liu, H.
B.; Li, Y. L.; Zhu, D. B. Tetrahedron Lett. 2009 50, 1588–
1592.