Regioselective synthesis of aurone derivatives via PBu3-catalyzed cyclization of 2-alkynoylphenols

Article abstract of DOI:10.1002/cjoc.201400597

Aurone derivatives were synthesized in good to high yields by PBu₃-catalyzed intramolecular 5-exo cyclization of 2-alkynoylphenols. The reaction proceeds in high regioselectivity without forming 6-endo cyclization products.

Full text of DOI:10.1002/cjoc.201400597

FULL PAPER
DOI: 10.1002/cjoc.201400597
Regioselective Synthesis of Aurone Derivatives via
PBu3-Catalyzed Cyclization of 2-Alkynoylphenols
Changqing Liu,a,b Zhannan Zhang,a Jitan Zhang,a Xing Liu,a and Meihua Xie*,a
a Key Laboratory of Functional Molecular Solids (Ministry of Education), Anhui Key Laboratory of Molecular
Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000,
China
b Department of Chemistry, Bengbu Medical College, Bengbu, Anhui 233030, China
Aurone derivatives were synthesized in good to high yields by PBu3-catalyzed intramolecular 5-exo cyclization
of 2-alkynoylphenols. The reaction proceeds in high regioselectivity without forming 6-endo cyclization products.
Keywords 2-alkynoylphenol, PBu3, intramolecular cyclization, aurone
Introduction
Due to the electron-withdrawing effect of carbonyl
group, acetylenic ketones have been widely used as
electron-deficient alkynes in organic synthesis.[11] Re-
cently, we reported the synthesis of polysubstituted al-
kenes from acetylenic ketones.[12] In continuation of our
research interest in this field, we wish to report herein a
facile synthesis of aurone derivatives by PBu3-catalyzed
cyclization of 2-alkynoylphenols via the intramolecular
addition of acetylenic ketones.
Flavonoids are a large class of plant natural products
and exhibit multiple biological activities.[1] Aurones,
2-benzylidenzofuran-3(2H)-ones, constitute a minor
subclass of flavonoids and are responsible for the pig-
mentation of the flowers and fruits.[2] Aurones also ex-
hibit a broad variety of biological activities such as an-
tifeedant activity, antibacterial activity or antifungal
activity[3] and have been used as anticancer agents,
anti-inflammatory agents or antioxidants.[4] Therefore,
the syntheses of aurone derivatives are well established
and keep being developed.[5]
Results and Discussion
Initially, the intramolecular addition reaction of
2-alkynoylphenol 1a catalyzed by various bases was
investigated. The results are summarized in Table 1. It
was found that no reaction happened when 2-alkynoyl-
phenol 1a was dissolved in CH2Cl2 and stirred at room
temperature for 24 h (Table 1, entry 1). 5-Exo cycliza-
tion product 2a was isolated in 66% yield and trace
6-endo cyclization product 3a was formed when 5
mol% PPh3 was added as catalyst (Table 1, entry 4).
Gratifyingly, 2a was isolated in 95% yield with high
regioselectivity when the reaction was catalyzed by 5
mol% PBu3 (Table 1, entry 5). Et3N, pyridine,
tri-t-butylphosphine (TTBuP) or Bu3PO can not be used
as catalyst to promote the reaction (Table 1, entries 2, 3,
6 and 7). When the PBu3-catalyzed cyclization reaction
of 1a was carried out in solvents other than CH2Cl2, 2a
was isolated in inferior yields (Table 1, entries 8－11).
An increase in the amount of PBu3 has no apparent ef-
fect on the yield while a lower yield of 2a was obtained
when a reduced amount of PBu3 was used (Table 1, en-
tries 13 and 14). Therefore, we used 5 mol% PBu3 in
CH2Cl2 at room temperature as the general reaction
The popular method to synthesis aurone derivatives
is based on the condensation of benzofuran-3(2H)-ones
with benzaldehydes.[6] However, the aldol-like reaction
requires the synthesis of benzofuran-3(2H)-ones which
is usually carried out under harsh reaction conditions
and yields are modest. Oxidative cyclisation of 2'-hy-
droxychalcones are also reported to prepare aurones.[7]
Li and Pale groups reported the regioselective synthesis
of aurones respectively from metal-catalyzed cycliza-
tion of 2-(1-hydroxyprop-2-ynyl)phenols.[8] These
methods, however, usually cannot completely prevent
the formation of flavones (6-endo cyclisation product)
and metal catalyst such as gold or silver is needed.
2-Alkynoylphenols have been widely used to prepare
flavone derivatives by 6-endo cyclization.[9] For exam-
ple, Doi and co-workers synthesized flavones regiose-
lectively from 2-alkynoylphenols.[9a,9b] On the contrary,
synthesis of aurone derivatives from 2-alkynoylphenol
by 5-exo-digonal cyclization is limited because 6-endo-
digonal cyclization is generally a favorable process that
has been reported as a Baldwin’s rule in 1976.[9a,10]
*
E-mail: xiemh@mail.ahnu.edu.cn; Tel.: 0086-0553-3869310; Fax: 0086-0553-3883517
Received September 2, 2014; accepted October 12, 2014; published online November 7, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc. 201400597 or from the author.
Chin. J. Chem. 2014, 32, 1233—1237
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Liu et al.
Table 2 PBu3-catalyzed cyclization of 2-alkynoylphenols
FULL PAPER
conditions in the subsequent studies.
O
O
Table 1 Investigation of cyclization of 2-alkynoylphenol 1aa
R1
R1
H
PBu3 (5 mol%)
CH2Cl2, r.t.
O
Ar
O
O
Ar
OH
R2
R2
conditions
+
1
2
O
OH
En-
try
Substrate 1
(R1, R2, Ar)
Yield/
%
OCH3
O
Product 2
1a
OCH3
2a
O
H, H,
p-CH3OC6H4
(1a)
O
1
95
O
OCH3
OCH3
(2a)
3a
T/ Time/ Ratio of Yieldc/
O
Entry Solvent
Catalyst
℃
r.t.
r.t.
h
2a/3ab
%
NR
NR
NR
66
H, H,
p-CH3C6H4
(1b)
O
1
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
THF
NONE
NEt3
24
24
24
－
2
3
4
93
92
80
－
CH3
(2b)
3
Pyridine r.t.
－
4
PPh3
PBu3
Bu3PO
TTBuP
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
40
24 ＞98∶2
O
5
1
10
24
1
＞98∶2
－
95
H, H, C6H5
O
6
NR
NR
89
(1c)
(2c)
7
－
8
＞98∶2
＞98∶2
86∶14
68∶32
＞98∶2
＞98∶2
＞98∶2
O
9
1
86
H, H, p-FC6H4
(1d)
O
10
11
12
13d
14e
CH3CN
DMSO
CH2Cl2
CH2Cl2
CH2Cl2
1
79
1
61
F
(2d)
1
87
r.t.
r.t.
1
92
O
1
90
H, CH3O,
p-CH3OC6H4
(1e)
a Reaction conditions: 1a (0.5 mmol) and 5 mol% of catalyst in 2
mL of solvent. b The ratio of 2a and 3a was determined by crude
1H NMR. c Isolated yield. d 10 mol% PBu3 was used. e 2 mol%
PBu3 was used.
O
5
6
7
87
81
75
OCH3
OCH3
(2e)
O
O
With the optimal reaction conditions in hand, the
scope and generality of the intramolecular cyclization
reaction of 2-alkynoylphenol was investigated. The re-
sults are compiled in Table 2. Table 2 shows that the
cyclization reactions of various 2-alkynoylphenol pos-
sessing either a methoxy, fluoro or t-Bu group on the
left benzene ring or methyl, fluoro or methoxy group on
the right benzene ring proceed smoothly. The corre-
sponding aurone derivatives were obtained in good to
excellent yield. The products 2 were obtained in 81%－
95% yields when electron-donating groups such as
methyl (Table 1, entry 2), methoxy (Table 1, entries 1, 5,
6) or t-Bu (Table 1, entries 11, 12) attached to the left or
right benzene ring, while the yields of 2 are lower (62%
－83%) when the substrates contain fluoro either in the
left or right benzene ring (Table 1, entries 4, 7－10, 13).
The molecular structure of compound 2b was con-
H, CH3O, C6H5
(1f)
OCH3
(2f)
O
H, CH3O,
p-FC6H4
(1g)
O
OCH3
F
(2g)
O
F
F, H,
p-CH3OC6H4
(1h)
O
8
83
OCH3
(2h)
1234
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Chin. J. Chem. 2014, 32, 1233—1237

Regioselective Synthesis of Aurone Derivatives via PBu3-Catalyzed Cyclization of 2-Alkynoylphenols
cyclization to form vinyl carbanion B, which was
Continued
probably arising from the kinetically controlled nucleo-
philic attach at the α-carbonyl carbon. The vinyl car-
banion B was rapidly protonated by the protonated
phosphines H-P＋Bu3 to give 2.
En-
try
Substrate 1
Yield/
%
Product 2
(R1, R2, Ar)
O
F
F
F, H, C6H5
9
76
62
Scheme 1 A plausible mechanism for the formation of aurones
O
(1i)
O
O
(2i)
(2j)
R1
R1
O
O
Bu3P
Ar
Ar
O
OH
R2
2
F, H, p-FC6H4
(1j)
R2
10
H-P+Bu3
O
O
R1
F
R1
-
Ar
O
O
Ar
O-
O
R2
R2
A
B
t-Bu, t-Bu,
11 p-CH3OC6H4
(1k)
93
OCH
Conclusions
3 (2k
In conclusion, we have demonstrated a convenient
and transition-metal-free procedure for the regioselec-
tive syntheses of aurone derivatives in good to high
yields from the PBu3-catalyzed intramolecular cycliza-
tion of 2-alkynoylphenol. The method induces exclusive
formation of 5-exo cyclization without formation of the
6-endo cyclized product.
)
O
t-Bu, t-Bu, C6H5
12
90
79
O
(1l)
(2l)
O
O
t-Bu, t-Bu,
Experimental
13
p-FC6H4
(1m)
All solid products were recrystallized from ethyl
acetate and hexane, and the melting points are uncor-
rected. All reactions were carried out under an air at-
F
(2m)
1
mosphere. H NMR and 13C NMR spectra were re-
corded on a BRUKER AVANCE-300 in CDCl3 with
TMS as the internal standard. The starting material
2-alkynoylphenols were prepared according to previ-
ously reported procedure.[9b]
firmed by X-ray diffraction analysis (Figure 1). Figure 1
shows that 5-exo cyclization of 2-alkynoylphenol was
induced by PBu3 to afford aurone derivatives and the
reaction proceeds with Z-isomer selectivity.
Typical procedure for the synthesis of aurone de-
rivatives 2
5 mol% PBu3 was added to a solution of
2-alkynoylphenol 1 (0.5 mmol) in CH2Cl2 (2 mL) and
the reaction mixture was stirred at room temperature.
After the completion of the reaction (monitored by
TLC), the reaction mixture was diluted with water and
extracted with ethyl acetate (5 mL×3). The combined
organics were dried over anhydrous Na2SO4 and the
filtrate was concentrated under reduced pressure. The
crude product was purified by column chromatography
on silica gel (ethyl acetate/petroleum ether＝1/20, V/V)
to afford product 2.
(Z)-2-(4-Methoxybenzylidene)benzofuran-3(2H)-
one (2a) Yellow solid; m.p. 135－136 ℃; 1H NMR
(300 MHz, CDCl3) δ: 7.90 (d, J＝8.6 Hz, 2H), 7.81 (d,
J＝7.4 Hz, 1H), 7.67－7.62 (m, 1H), 7.33 (d, J＝8.2 Hz,
1H), 7.27－7.19 (m, 1H), 6.99 (d, J＝8.6 Hz, 2H), 6.89
Figure 1 The molecular structure of compound 2b.
Based on the reported literature[9b,13] and the above
experimental results, a plausible mechanism for the
formation of aurone derivatives was depicated in
Scheme 1. First, the PBu3 acted as a base to deprotonate
the phenolic hydroxyl group to generate the intermedi-
ate A and the resulting phenoxide A employed 5-exo
Chin. J. Chem. 2014, 32, 1233—1237
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1235

Liu et al.
FULL PAPER
(s, 1H), 3.87 (s, 3H); 13C NMR (75 MHz, CDCl3) δ:
184.6, 165.8, 161.0, 145.9, 136.6, 133.5, 125.0, 124.5,
123.3, 121.9, 114.5, 113.4, 112.9, 55.4; IR (KBr) ν:
1697, 1651, 1595, 1473, 883, 748, 696 cm−1; HRMS (EI)
calcd for C16H12O3 (M＋) 252.0786, found 252.0782.
(Z)-2-(4-Methylbenzylidene)benzofuran-3(2H)-
J＝8.4 Hz), 127.5, 123.0, 121.9, 117.6, 115.2 (d, J＝
21.8 Hz), 114.8, 111.4; IR (KBr) ν: 1714, 1654, 1598,
1440, 900, 833, 74 2 cm−1; HRMS m/z (ESI) calcd for
C16H12FO3 (M＋H＋) 271.0770, found 271.0765.
(Z)-2-(4-Methoxybenzylidene)-5-fluorobenzofu-
ran-3(2H)-one (2h) Yellow solid; m.p. 152－153 ℃;
1H NMR (CDCl3, 300 MHz) δ: 7.89 (d, J＝8.7 Hz, 2H),
7.48－7.26 (m, 3H), 7.00 (d, J＝8.7 Hz, 2H), 6.91 (s,
1H), 3.88 (s, 3H); 13C NMR (CDCl3, 75 MHz) δ: 182.8,
160.8, 160.3, 157.7 (d, J＝242.9 Hz), 145.4, 132.6,
123.8, 122.9 (d, J＝25.8 Hz), 121.6 (d, J＝7.7 Hz),
113.5, 113.3, 113.0 (d, J＝7.7 Hz), 109.0 (d, J＝24.1
Hz), 54.4; IR (KBr) ν: 1697, 1649, 1597, 1454, 817, 792
cm−1; HRMS m/z (ESI) calcd for C16H12FO3 (M＋H＋)
271.0770, found 271.0765.
1
one (2b) Yellow solid; m.p. 93－94 ℃; H NMR
(300 MHz, CDCl3) δ: 7.83 (d, J＝7.1 Hz, 3H), 7.68－
7.63 (m, 1H), 7.35－7.22 (m, 4H), 6.90 (s, 1H), 2.41 (s,
3H); 13C NMR (75 MHz, CDCl3) δ: 184.6, 165.9, 146.4,
140.4, 136.7, 131.5, 129.7, 129.5, 124.5, 123.3, 121.7,
113.3, 112.9, 21,6; IR (KBr) ν: 1710, 1655, 1596, 1465,
883, 746, 690 cm−1; HRMS (EI) calcd for C16H12O2
(M＋) 236.0837, found 236.0831.
(Z)-2-Benzylidenebenzofuran-3(2H)-one
(2c)
1
Yellow solid; m.p. 100－101 ℃; H NMR (300 MHz,
CDCl3) δ: 7.93 (d, J＝7.2 Hz, 2H), 7.82 (d, J＝7.4 Hz,
1H), 7.69－7.64 (m, 1H), 7.49－7.33 (m, 4H), 7.26－
7.20 (m, 1H), 6.91 (s, 1H); 13C NMR (75 MHz, CDCl3)
δ: 183.7, 165.1, 145.8, 135.9, 131.3, 130.5, 128.9, 127.9,
123.6, 122.4, 120.6, 112.0, 111.9; IR (KBr) ν: 1712,
1658, 1598, 1450, 883, 746, 686 cm−1; HRMS (EI)
calcd for C15H10O2 (M＋) 222.0681, found 222.0673.
(Z)-2-(4-Fluorobenzylidene)benzofuran-3(2H)-
(Z)-2-Benzylidene-5-fluorobenzofuran-3(2H)-one
(2i) Yellow solid; m.p. 119 －120 ℃ ; 1H NMR
(CDCl3, 300 MHz) δ: 7.92 (d, J＝6.9 Hz, 2H), 7.48－
7.33 (m, 6H), 6.92 (s, 1H); 13C NMR (CDCl3, 75 MHz)
δ: 183.1, 161.1, 157.8 (d, J＝242.9 Hz), 146.4, 131.0,
130.6, 129.2, 127.9, 123.2 (d, J＝25.7 Hz), 121.3 (d,
J＝8.0 Hz), 113.1 (d, J＝7.9 Hz), 112.9, 109.1 (d, J＝
24.1 Hz); IR (KBr) ν: 1708, 1651, 1604, 1485, 904, 813,
777 cm−1; HRMS m/z (ESI) calcd for C15H10FO2 (M＋
H＋) 241.0665, found 241.0659.
1
one (2d) Yellow solid; m.p. 146－147 ℃; H NMR
(CDCl3, 300 MHz) δ: 7.93－7.81 (m, 3H), 7.69－7.67
(m, 1H), 7.35－7.33 (m, 1H), 7.26－7.16 (m, 3H), 6.87
(s, 1H); 13C NMR (CDCl3, 75 MHz) δ: 183.6, 165.0,
162.4 (d, J＝250.9 Hz), 145.4, 135.9, 132.5 (d, J＝8.4
Hz), 127.5 (d, J＝3.3 Hz), 123.7, 122.5, 120.6, 115.1 (d,
J＝21.7 Hz), 111.9, 110.8; IR (KBr) ν: 1714, 1658,
1598, 1473, 881, 750, 696 cm−1; HRMS m/z (ESI) calcd
for C15H10FO2 (M＋H＋) 241.0665, found 241.0659.
(Z)-2-(4-Methoxybenzylidene)-7-methoxybenzo-
furan-3(2H)-one (2e) Yellow solid; m.p. 136 －
137 ℃; 1H NMR (CDCl3, 300 MHz) δ: 7.92 (d, J＝8.7
Hz, 2H), 7.40－7.37 (m, 1H), 7.17－7.10 (m, 2H), 7.00
(d, J＝8.7 Hz, 2H), 6.92 (s, 1H), 4.03 (s, 3H), 3.87 (s,
3H); 13C NMR (CDCl3, 75 MHz) δ: 183.6, 160.1, 154.5,
144.9, 144.8, 132.6, 124.0, 122.7, 122.3, 117.2, 114.7,
113.5, 112.9, 55.3, 54.3; IR (KBr) ν: 1693, 1643, 1598,
1438, 896, 833, 754 cm−1; HRMS m/z (ESI) calcd for
C17H15O4 (M＋H＋) 283.0970, found 283.0965.
(Z)-2-(4-Fluorobenzylidene)-5-fluorobenzofuran-
3(2H)-one (2j) Yellow solid; m.p. 132－133 ℃; H
1
NMR (CDCl3, 300 MHz) δ: 7.92 (dd, J＝8.5, 5.6 Hz,
2H), 7.48－7.29 (m, 3H), 7.16 (t, J＝8.6 Hz, 2H), 6.88
(s, 1H); 13C NMR (CDCl3, 75 MHz) δ: 184.0, 163.6 (d,
J＝242.6 Hz), 161.9, 158.8 (d, J＝242.3 Hz), 147.0,
133.7 (d, J＝8.3 Hz), 128.3 (d, J＝2.9 Hz), 124.4 (d,
J＝25.5 Hz), 122.2 (d, J＝8.9 Hz), 116.2 (d, J＝21.8
Hz), 114.1 (d, J＝8.0 Hz), 112.7, 110.2 (d, J＝23.9 Hz);
IR (KBr) ν: 1710, 1658, 1597, 1485, 827, 798 cm−1;
HRMS m/z (ESI) calcd for C15H9F2O2 (M ＋ H ＋ )
259.0570, found 259.0565.
(Z)-2-(4-Methoxybenzylidene)-5,7-di-tert-butyl-
benzofuran-3(2H)-one (2k) Yellow solid; m.p. 166－
167 ℃; 1H NMR (CDCl3, 300 MHz) δ: 7.91 (d, J＝8.7
Hz, 2H), 7.66－7.62 (m, 2H), 7.00 (d, J＝8.7 Hz, 2H),
6.87 (s, 1H), 3.88 (s, 3H), 1.56 (s, 9H), 1.36 (s, 9H); 13
C
NMR (CDCl3, 75 MHz) δ: 184.5, 161.7, 159.9, 145.3,
144.7, 133.8, 132.0, 130.2, 124.2, 120.7, 117.2, 113.5,
111.6, 54.4, 33.8, 33.3, 30.5, 28.6; IR (KBr) ν: 1691,
1639, 1600, 1462, 829 cm−1; HRMS m/z (ESI) calcd for
C24H29O3 (M＋H＋) 365.2117, found 365.2111.
(Z)-2-Benzylidene-7-methoxybenzofuran-3(2H)-
1
one (2f) Yellow solid; m.p. 143－144 ℃; H NMR
(CDCl3, 300 MHz) δ: 7.95 (d, J＝6.7 Hz, 2H), 7.48－
7.39 (m, 4H), 7.17 (s, 2H), 6.93 (s, 1H), 4.04 (s, 3H);
13C NMR (CDCl3, 75 MHz) δ: 183.8, 154.8, 145.8,
144.9, 131.2, 130.6, 128.9, 127.9, 122.9, 122.0, 117.7,
114.8, 112.5, 55.3; IR (KBr) ν: 1703, 1651, 1600, 1438,
904, 754, 688 cm−1; HRMS m/z (EI) calcd for C16H12O3
(M＋) 252.0786, found 252.0777.
(Z)-2-(4-Fluorobenzylidene)-7-methoxybenzofu-
ran-3(2H)-one (2g) Yellow solid; m.p. 148－149 ℃;
1H NMR (CDCl3, 300 MHz) δ: 7.98－7.93 (m, 2H),
7.41－7.38 (m, 1H), 7.20－7.13 (m, 4H ), 6.89 (s, 1H),
4.04 (s, 3H); 13C NMR (CDCl3, 75 MHz) δ: 183.8,
162.5 (d, J＝251.1 Hz), 154.7, 145.4, 144.9, 132.7 (d,
(Z)-2-Benzylidene-5,7-di-tert-butylbenzofuran-
1
3(2H)-one (2l) Yellow solid; m.p. 76－77 ℃; H
NMR (300 MHz, CDCl3) δ: 7.95 (d, J＝6.8 Hz, 2H),
7.66 (d, J＝6.8 Hz, 2H), 7.47－7.43 (m, 3H), 6.88 (s,
1H), 1.56 (s, 9H), 1.36 (s, 9H); 13C NMR (75 MHz,
CDCl3) δ: 184.8, 162.0, 145.8, 145.6, 134.0, 131.6,
130.6, 130.2, 128.7, 128.0, 120.5, 117.3, 111.2, 33.9,
33.4, 30.5, 28.6; IR (KBr) ν: 1705, 1651, 1593, 1446,
902, 785 cm−1; HRMS m/z (EI) calcd for C23H26O2 (M＋),
334.1933, found 334.1932.
1236
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 1233—1237

Regioselective Synthesis of Aurone Derivatives via PBu3-Catalyzed Cyclization of 2-Alkynoylphenols
Shadnia, H.; Amini, M.; Samadi, N.; Faramarzi, M. A.; Shafiee, A.
Bioorg. Med. Chem. Lett. 2007, 17, 6354.
(Z)-2-(4-Fluorobenzylidene)-5,7-di-tert-butylben-
zofuran-3(2H)-one (2m) Yellow solid; m.p. 138－
139 ℃; 1H NMR (CDCl3, 300 MHz) δ: 8.01－7.92 (m,
2H), 7.66 (d, J＝4.7 Hz, 2H), 7.19－7.14 (m, 2H), 6.85
(s, 1H) 1.55 (s, 9H), 1.36 (s, 9H); 13C NMR (CDCl3, 75
MHz) δ: 184.6, 162.3 (d, J＝250.4 Hz), 161.9, 145.7,
145.5, 133.9, 132.1 (d, J＝8.3 Hz), 130.6, 127.8 (d, J＝
3.0 Hz), 120.4, 117.3, 115.2 (d, J＝21.7 Hz), 110.0,
33.8, 33.3, 30.5, 28.6; IR (KBr) ν: 1699, 1651, 1593,
1487, 823, 781 cm−1; HRMS m/z (ESI) calcd for
C23H26FO2 (M＋H＋) 353.1917, found 353.1911.
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Acknowledgement
The authors are grateful to the financial support from
the National Natural Science Foundation of China (No.
21272004).
Supporting Information
The cif file of compound 2b is available. Single
crystal data for compound 2b (CCDC 1022096) has
been deposited in the Cambridge Crystallographic Data
Center.
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