Novel Synthesis of Aurones by 2-PyONa-catalyzed Regioselective Cyclization of o-(Alkynon-1-yl)phenols

Full text of DOI:10.1002/bkcs.11447

Note
DOI: 10.1002/bkcs.11447
BULLETIN OF THE
J. I. Lee
KOREAN CHEMICAL SOCIETY
Novel Synthesis of Aurones by 2-PyONa-catalyzed Regioselective
Cyclization of o-(Alkynon-1-yl)phenols
*
Jae In Lee
Department of Chemistry, College of Natural Science, Duksung Women’s University, Seoul 01369, South
Korea. *E-mail: jilee@duksung.ac.kr
Received December 13, 2017, Accepted March 9, 2018
Keywords: Aurones, o-(Alkynon-1-yl)phenols, Condensation, Cyclization
Aurones, 2-benzylidenebenzofuran-3(2H)-ones, as struc-
tural isomers of flavones are distributed in plants and con-
tribute to yellow coloration of flowers and fruits.1 They
5-exo cyclization of o-(alkynon-1-yl)phenols afforded aur-
ones together with trace amounts of flavones.
To date, several synthetic methods for aurones have been
reported, but some suffer from the use of excess reagent,
harsh conditions, and tedious separation. The intramolecu-
lar cyclization of o-(alkynon-1-yl)phenols is a very conve-
nient method for the synthesis of aurones, but their reports
are rare. In this paper, we report that aurones can be novely
synthesized by regioselective cyclization of o-(alkynon-1-
yl)phenols using 0.1 equiv of sodium 2-pyridyloxide (2-
PyONa) as a base in high yields.
possess
diverse
biological
activities,2
including
antioxidant,3 anticancer,4 and antimalarial properties.5 They
are also described as phytoalexins against infections of
plants6 and used for the treatment of Alzheimer’s disease7
and as inhibitors of hepatitis C virus.8
The most common methods for the synthesis of aurones
are the condensation of benzofuran-3(2H)-ones with aryl
aldehydes and the oxidative cyclization of 20-hydroxychal-
cones.9
The
intramolecular
cyclization
of
2-
N-Methoxy-N-methyl o-hydroxybenzamides (2a–e) as
precursors of o-(alkynon-1-yl)phenols (3) were easily pre-
hydroxyphenacyl chlorides with bases such as NaOAc7 and
NaOMe10 in refluxing CH3OH or 2-phenoxyacetic acids
with polyphosphoric acid11 at 80ꢀC provided benzofuran-3
(2H)-ones. These intermediates were then condensed with
aryl aldehydes using basic reagents such as KOH7,10a and
pared by treating
a
mixture of methyl o-
hydroxybenzamides (1a–e) and Me(OMe)NH2Cl with
3 equiv of isopropylmagnesium chloride (Scheme 1). The
slow addition of isopropylmagnesium chloride to a slurry
solution of 1 and Me(OMe)NH2Cl in THF at −10ꢀC pro-
duced the corresponding phenoxymagnesium chlorides and
Me(OMe)NMgCl. The acyl substitution of methoxy group
in 1 by Me(OMe)NMgCl proceeded rapidly within 0.5 h to
afford 2 in 64–87% yields after the acidic work-up and
chromatographic separation (2a: 82%, 2b: 64%, 2c: 75%,
2d: 87%, 2e: 81%). The synthesis of 3 was efficiently
accomplished by the reaction of pretreated 2 with lithium
diisopropylamide (LDA) and arylethynyllithiums. The reac-
tion seemed to proceed via the intermediacy of 5-membered
chelates between lithium atom and two oxygen atoms of
carbonyl/methoxy groups in 2, which were converted to
3 by quenching with 1 N HCl solution. The kinds of
electron-donating or electron-withdrawing groups in both
phenyl rings were compatible for the synthesis of 3. After
the acidic work-up and chromatographic separation, 3 was
obtained in 71–90% yields.
10b
11
Ba(OH)2 or acidic reagents such as Al2O3 and ethano-
lic HCl12 to afford aurones. The condensation of 20-
hydroxyacetophenones with aryl aldehydes in ethanolic
KOH afforded 20-hydroxychalcones, which were dehydro-
genatively cyclized to give aurones by transition metals
13
such as 2 equiv of Hg(OAc)2 in HOAc or DMSO and
14
CuBr2 in DMF at reflux. 20-Hydroxychalcones were also
cyclized by 3 equiv of Tl(NO3)3 to give aurones after treat-
ment with aq HCl for 10 h at 65ꢀC.15
The gold(I) chloride16 or silver17 complex-catalyzed
cyclization of 2-(1-hydroxy-3-arylprop-2-ynyl)phenols,
prepared by the addition of 2 equiv of lithium arylacety-
lides to salicylaldehydes, in the presence of K2CO3 or i-
Pr2NEt and subsequent oxidation of the cyclized products
with MnO2 afforded aurones. Alternatively, 2-(1-hydroxy-
3-arylprop-2-ynyl)phenols were directly cyclized using
2 equiv of AgNO3/K2CO3 in refluxing toluene to give aur-
ones.18 The intramolecular cyclization of o-(alkynon-1-yl)
phenols is also useful for the synthesis of aurones. How-
ever, the cyclization of o-(alkynon-1-yl)phenols using
K2CO3 or NaOEt was problematic because it provided a
mixture of aurones and flavones by 5-exo and 6-endo
attack, respectively.19 Recently, the use of Lewis bases
To investigate the optimum conditions for the 5-exo
cyclization of 3, the effect of bases and solvents was exam-
ined for the reaction of 1-(2-hydroxyphenyl)-3-phenyl-2-
propyn-1-one (3af) (Table 1). The cyclization of 3af with
0.1 equiv of bases such as 2-PyOLi, 2-PyONa, and 2-PyOK
in THF afforded aurone (6af, Rf 0.56 in 30% EtOAc/n-hex-
ane) in 68, 93, and 57% yields, respectively, after
65ꢀC/10 h, rt/6 h, and rt/1 h, respectively, together with
20
21
such as a catalytic PBu3 and 3 equiv of Cs2CO3 on the
Bull. Korean Chem. Soc. 2018
© 2018 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
1

Note
BULLETIN OF THE
ISSN (Print) 0253-2964 | (Online) 1229-5949
KOREAN CHEMICAL SOCIETY
R1
R1
R1
R6
R5
R2
R3
OH
OMe
R2
R3
OH
R2
R3
OH
O
1)
2)
b
a
Me
N
R4
Li
R4
OMe
R5
R6
O
O
c
1a-e
2a-e
3
R1=R2=R3=H (a); R1=OMe, R2=R3=H (b);
R2=OMe, R1=R3=H (c); R3=Cl, R1=R2=H (d);
R3=Me, R1=R2=H (e)
R4=R5=R6=H (f); R4=Br, R5=R6=H (g); R4=OMe, R5=R6=H (h);
R5=Cl, R4=R6=H (i); R6=Br, R4=R5=H (j); R6=Me, R4=R5=H (k);
6
4
5
3-Th
Li (m)
R =OMe, R =R =H (l);
R6
R6
5' 4'
B
R1
R6
R5
R1
R1
3'
R5
R2
R3
O
O
6'
Na
R5
2-PyOH
d
1
O
R2
R3
7
R2
2'
R4
O
1'
6
R4
A
R4
2
Na
R3
5
3
4
O
O
4
5
6
Scheme 1. Reagents and conditions: (a) Me(OMe)NH2Cl, 3 equiv of i-PrMgCl, THF, −10ꢀC, 0.5 h; 0.5 N HCl; (b) LDA, THF, 0ꢀC,
10 min; (c) THF, −10ꢀC, 20 min; 1 N HCl; (d) 0.1 equiv of 2-PyONa, THF, rt, 1–12 h; 50ꢀC, 3–7 h for 5bf, 5ck, 5dh.
flavone (Rf 0.31 in 30% EtOAc/n-hexane) in 21, 3, and
34% yields, respectively. When DME, CH3OH, and
CH3CN were employed as solvents using 0.1 equiv of 2-
PyONa, 6af was obtained in 89, 48, and 43% yields,
respectively, after 1, 1, and 2 h, respectively, at room tem-
perature. Thus, the highest 5-exo cyclization of 3 was
accomplished with 0.1 equiv of 2-PyONa, which might
provide the sodium phenoxides (4) having optimized nucle-
ophilicity in less polar THF solvent. The reaction seemed
to proceed by the abstraction of hydroxyl proton in 3 by 2-
PyONa as a base to produce the corresponding sodium
phenoxides (4), which underwent 5-exo cyclization through
kinetic control to afford vinyl carbanion intermediates (5).
Compound 5 was rapidly protonated by 2-PyOH as a pro-
ton source to give aurones (6).
be more stable than (E)-form, avoiding the repulsion
between B-ring and carbonyl group in 6.22 (Z) Assignment
of C C in aurones was based on the H chemical shift of
1
the vinylic proton. For example, the chemical shift of the
vinylic Cβ proton in (Z)-aurone (6af) was observed as δ
6.90 ppm, which was consistent with the previous
results.16,20a This observation could be explained by the
diamagnetic shielding of the vinylic proton by carbonyl
group and thus the chemical shift of the vinylic proton in
(Z)-isomer was found at a lower value than that in (E)-iso-
mer. In cases of (Z)-aurones with 20-substituents such as
methoxy (5ah, 5dh) and bromo (5eg), the chemical shifts
of the vinylic proton were observed as δ 7.49 (Ref. 3—
7.49, Ref. 16—7.48), 7.50, and 7.59 ppm, respectively,
and the downfield shift might be attributed to anomeric
“ortho effect”. The cyclization of o-(alkynon-1-yl)phenols
using 0.1 equiv of 2-PyONa exclusively afforded aurones
by highly regioselective 5-exo attack and the 6-endo
cyclized flavones were not obtained to isolable amounts in
most cases. As shown in Table 2, various aurones were
synthesized from methyl o-hydroxybenzoates in high over-
all yields (47–68%). The conversion of 3 to 6 worked
well with both electron-donating methoxy or methyl group
and electron-withdrawing chloro group on the A-ring. o-
(Alkynon-1-yl)phenols possessing methoxy, methyl,
bromo, and chloro group on the B-ring were also cyclized
as well under the same conditions. Furthermore, the
cyclization of 1-(2-hydroxy-5-methylphenyl)-3-(3-thienyl)-
2-propyn-1-one proceeded well to give (Z)-2-(3-thienyl-
methylene)-5-methylbenzofuran-3(2H)-one (6em) in 93%
yield.
The configuration of C C double bond in synthesized
aurones was appeared as (Z)-form, which was expected to
Table 1. The effect of bases and solvents for the cyclization of 1-
(2-hydroxyphenyl)-3-phenyl-2-propyn-1-one (3af).a
Yields, %
Bases
Solvents
temp, oC; time, h
Aurone
Flavone
None
2-PyOLi
2-PyONa
THF
THF
THF
DME
CH3OH
CH3CN
THF
65; 18
65; 10
rt; 6
rt; 1
rt; 1
0
68
93
89
48
43
57
0
21
3
7
43
50
34
rt; 2
rt; 1
2-PyOK
a
0.1 equiv of bases was used.
Bull. Korean Chem. Soc. 2018
© 2018 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
2

BULLETIN OF THE
Note
KOREAN CHEMICAL SOCIETY
Table 2. Synthesis of 3 and aurones 6 from methyl o-hydroxybenzoates 1.a
Entry
Chemical name
R1
R2
R3
R4
R5
R6
Isolated yields of 3 and 6 (%)b
af
ah
ai
al
bf
ci
ck
dh
dj
eg
em
Aurone
H
H
H
H
OMe
H
H
H
H
H
H
H
H
H
H
OMe
OMe
H
H
H
H
H
H
H
H
H
H
Cl
Cl
Me
Me
H
OMe
H
H
H
H
H
OMe
H
Br
—
H
H
Cl
H
H
Cl
H
H
H
H
3-Th
H
H
H
OMe
H
H
Me
H
Br
H
87; 93 (66)
90; 92 (68)
86; 95 (67)
82; 84 (56)
85; 88c(48)
71; 94 (50)
76; 82d(47)
80; 90 (63)
74; 88 (57)
81; 97 (64)
75; 93 (56)
20-Methoxyaurone
30-Chloroaurone
40-Methoxyaurone
7-Methoxyaurone
30-Chloro-6-methoxyaurone
6-Methoxy-40-methylaurone
5-Chloro-20-methoxyaurone
40-Bromo-5-chloroaurone
20-Bromo-5-methylaurone
2-(3-Thienylmethylene)-5-
methylbenzofuran-3(2H)-one
H
H
—
a
The conversion of 3 to 6 was carried out using 0.1 equiv of 2-PyONa in THF.
b
c
d
The numbers in parentheses indicate the overall yields from methyl o-hydroxybenzoates 1.
8-Methoxyflavone was obtained in 5% yield.
7-Methoxy-40-methylflavone was obtained in 13% yield.
Experimental
113.0; FT-IR (KBr) 1703 (C O), 1655 (C C) cm−1; Ms
m/z (%) 222 (M+, 88), 221 (100).
Preparation of 1-(2-Hydroxyphenyl)-3-phenyl-2-propyn-
1-one (3af). To a pretreated solution of N-methoxy-N-
methyl o-hydroxybenzamide (2a, 725 mg, 4.0 mmol) with
LDA (2.0 M in THF, 2.0 mL, 4.0 mmol) in THF (12 mL)
was added a solution of phenylethynyllithium, which was
generated from phenylacetylene (490 mg, 4.8 mmol) and
methyllithium (1.5 M in Et2O, 3.2 mL, 4.8 mmol), at −10ꢀC
under argon atmosphere. After being stirred for 20 min, the
mixture was quenched with 1 N HCl solution, and the sol-
vents were evaporated in vacuo. The mixture was diluted
with 0.1 N HCl solution (40 mL) and extracted with methy-
lene chloride (3✕20 mL). The organic phases were dried
over anhydrous MgSO4 and filtered. The filtrate was concen-
trated and purified by column chromatography on silica gel
using 30% EtOAc/n-hexane to give 3af (773 mg, 87%). mp
63-64ꢀC; 1H NMR (300 MHz, CDCl3) δ 11.76 (s, 1H), 8.13
(dd, J = 8.3, 1.7 Hz, 1H), 7.67–7.72 (m, 2H), 7.48–7.56 (m,
2H), 7.41–7.47 (m, 2H), 6.96–7.03 (m, 2H); 13C NMR
(75 MHz, CDCl3) δ 182.3, 162.8, 137.1, 133.1, 133.0,
131.2, 128.8, 120.8, 119.7, 119.4, 118.2, 96.0, 85.7; FT-IR
(KBr) 2207 (C C), 1624 (C O) cm−1; Ms m/z (%)
222 (M+, 100), 194 (61).
6ah, 6al, 6bf. Known compounds.3,16,20
(Z)-30-Chloroaurone (6ai). mp 108-109ꢀC; 1H NMR
(300 MHz, CDCl3) δ 7.94 (s, 1H), 7.80 (d, J = 7.6 Hz,
1H), 7.64–7.75 (m, 2H), 7.33–7.39 (m, 3H), 7.20–7.26 (m,
1H), 6.79 (s, 1H); 13C NMR (75 MHz, CDCl3) δ 184.7,
166.2, 147.3, 137.2, 134.8, 134.0, 130.9, 130.1, 129.8,
129.6, 124.8, 123.8, 121.4, 113.1, 111.2; FT-IR (KBr)
1707 (C O), 1653 (C C) cm−1; Ms m/z (%) 258 (M++2,
28), 257 (51), 256 (M+, 81), 255 (100), 221 (87).
1
(Z)-30-Chloro-6-methoxyaurone (6ci). mp 151-153ꢀC; H
NMR (300 MHz, CDCl3) δ 7.93 (s, 1H), 7.64–7.72 (m,
2H), 7.31–7.38 (m, 2H), 6.80 (d, J = 2.0 Hz, 1H), 6.76
(dd, J = 8.5, 2.1 Hz, 1H), 6.72 (s, 1H), 3.94 (s, 3H); 13C
NMR (75 MHz, CDCl3) δ 182.8, 168.7, 167.7, 148.3,
134.8, 134.2, 130.7, 130.0, 129.5, 129.4, 125.9, 114.6,
112.5, 110.0, 96.7, 56.1; FT-IR (KBr) 1701 (C O), 1657
(C C) cm−1; Ms m/z (%) 288 (M++2, 24), 287 (42),
286 (M+, 63), 285 (100).
(Z)-6-Methoxy-40-methylaurone (6ck). mp 159-160ꢀC;
1H NMR (300 MHz, CDCl3) δ 7.78 (d, J = 8.1 Hz, 2H),
7.70 (d, J = 8.3 Hz, 1H), 7.25 (d, J = 8.3 Hz, 2H), 6.80 (s,
1H), 6.73–6.78 (m, 1H), 6.72 (d, J = 2.1 Hz, 1H), 3.92 (s,
3H), 2.40 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 183.1,
168.5, 167.3, 147.4, 140.2, 131.4, 129.7 (overlapped),
125.8, 115.0, 112.2, 112.1, 96.6, 56.0, 21.6; FT-IR (KBr)
1693 (C O), 1652 (C C) cm−1; Ms m/z (%) 266 (M+, 84),
265 (100), 251 (76).
Preparation of (Z)-Aurone (6af). To a solution of 3af
(667 mg, 3.0 mmol) in THF (9 mL) was added sodium 2-
pyridyloxide (35 mg, 0.3 mmol) and stirred for 6 h at room
temperature. After evaporation of THF, the mixture was
directly subjected to silica gel column chromatography
using 30% EtOAc/n-hexane to give 6af (620 mg, 93%) as
1
(Z)-5-Chloro-20-methoxyaurone (6dh). mp 206-208ꢀC; H
1
a yellow solid. mp 110-111ꢀC (Ref. 13a, 108-109ꢀC); H
NMR (300 MHz, CDCl3) δ 8.26 (d, J = 7.8 Hz, 1H), 7.76 (d,
J = 2.3 Hz, 1H), 7.58 (dd, J = 8.7, 2.3 Hz, 1H), 7.50 (s, 1H),
7.35–7.43 (m, 1H), 7.27 (d, J = 8.7 Hz, 1H), 7.02–7.11 (m,
1H), 6.33 (d, J = 8.3 Hz, 1H), 3.90 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 183.4, 164.1, 159,0, 147.0, 136.3, 132.1,
131.9, 129.0, 124.2, 123.1, 121.0, 120.9, 114.2, 110.8, 108.4,
NMR (300 MHz, CDCl3) δ 7.93 (d, J = 7.7 Hz, 2H), 7.81
(d, J = 7.7 Hz, 1H), 7.60–7.69 (m, 1H), 7.37–7.50 (m, 3H),
7.34 (d, J = 8.4 Hz, 1H), 7.18–7.25 (m, 1H), 6.90 (s, 1H);
13C NMR (75 MHz, CDCl3) δ 184.9, 166.2, 146.9, 137.0,
132.3, 131.6, 130.0, 128.9, 124.7, 123.5, 121.6, 113.1,
Bull. Korean Chem. Soc. 2018
© 2018 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
3

BULLETIN OF THE
Note
KOREAN CHEMICAL SOCIETY
55.6; FT-IR (KBr) 1702 (C O), 1642 (C C) cm−1; Ms m/z
(%) 288 (M++2, 6), 286 (M+, 18), 257 (37), 255 (100).
(Z)-40-Bromo-5-chloroaurone (6dj). mp 195-197ꢀC; 1H
NMR (300 MHz, CDCl3) δ 7.72–7.80 (m, 3H), 7.55–7.64
(m, 3H), 7.29 (dd, J = 8.7, 0.8 Hz, 1H), 6.82 (s, 1H); 13C
NMR (75 MHz, CDCl3) δ 183.3, 164.2, 147.2, 136.8,
132.9, 132.3, 130.9, 129.4, 124.8, 124.3, 122.7, 114.3,
112.7; FT-IR (KBr) 1710 (C O), 1648 (C C) cm−1; Ms
m/z (%) 338 (M++4, 26), 336 (M++2, 100), 334 (M+, 78),
310 (12), 308 (49), 306 (36), 257 (14), 255 (42).
4. S. Demirayak, L. Yurttas, N. Gundogdu-Karaburun,
A. C. Karaburun, I. Kayagil, J. Enzyme Inhib. Med. Chem.
2015, 30, 816.
5. N. Adhikari, A. K. Halder, C. Mondal, T. Jha, Med. Chem.
Res. 2013, 22, 6029.
6. A. Boumendjel, Curr. Med. Chem. 2003, 10, 2621.
7. Y. Li, X. Qiang, L. Luo, Y. Li, G. Xiao, Z. Tan, Y. Deng,
Bioorg. Med. Chem. 2016, 24, 2342.
8. A. Meguellati, A. Ahmed-Belkacem, W. Yi, R. Haudecoeur,
M. Crouillere, R. Brillet, J.-M. Pawlotsky, A. Boumendjel,
M. Peuchmaur, Eur. J. Med. Chem. 2014, 80, 579.
9. For reviews, see: (a) I. B. Masesane, Int. J. Chem. Stud. 2015,
3, 53. (b) S. V. Jagtap, A. A. Khan, Int. J. Pure App. Biosci.
2016, 4, 137.
10. (a) A. Boumendjel, C. Beney, N. Deka, A.-M. Mariotte,
M. A. Lawson, D. Trompier, H. Baubichon-Cortay,
A. D. Pietro, Chem. Pharm. Bull. 2002, 50, 854.
(b) S. Kumar, Green Chem. Lett. Rev. 2014, 7, 95.
11. N. J. Lawrence, D. Rennison, A. T. McGown, J. A. Hadfield,
Bioorg. Med. Chem. Lett. 2003, 13, 3759.
1
(Z)-20-Bromo-5-methylaurone (6eg). mp 152-153ꢀC; H
NMR (300 MHz, CDCl3) δ 8.32 (d, J = 7.9 Hz, 1H), 7.65
(d, J = 8.0 Hz, 1H), 7.59 (s, 1H), 7.36–7.48 (m, 2H),
7.14–7.28 (m, 3H), 2.40 (s, 3H); 13C NMR (75 MHz,
CDCl3) δ 184.7, 164.7, 148.0, 138.2, 133.5, 133.4, 132.4,
132.1, 130.7, 127.7, 126.5, 124.5, 121.4, 112.5, 110.4,
20.8; FT-IR (KBr) 1704 (C O), 1651 (C C) cm−1; Ms m/
z(%) 316 (M++2, 8), 314 (M+, 8), 235 (100).
(Z)-2-(3-Thienylmethylene)-5-methylbenzofuran-3(2H)-o
12. S. Y. Shin, M. C. Shin, J.-S. Shin, K.-T. Lee, Y. S. Lee,
Bioorg. Med. Chem. Lett. 2011, 21, 4520.
1
ne (6em). mp 126-128ꢀC; H NMR (300 MHz, CDCl3) δ
7.30 (d, J = 2.4 Hz, 1H), 7.61 (d, J = 5.0 Hz, 1H), 7.56 (s,
1H), 7.44 (d, J = 8.4 Hz, 1H), 7.33 (dd, J = 5.0, 2.9 Hz,
1H), 7.19 (d, J = 8.4 Hz, 1H), 6.94 (s, 1H), 2.39 (s, 3H);
13C NMR (75 MHz, CDCl3) δ 184.8, 164.3, 146.6, 137.9,
133.9, 133.1, 130.3, 129.3, 126.3, 124.2, 121.9, 112.5,
106.9, 20.8; FT-IR (KBr) 1700 (C O), 1648 (C C) cm−1;
Ms m/z (%) 242 (M+, 100), 241 (77), 134 (67).
13. (a) H. Sekizaki, Bull. Chem. Soc. Jpn. 1988, 61, 1407.
(b) T. Narsinghani, M. C. Sharma, S. Bhargav, Med. Chem.
Res. 2013, 22, 4059.
14. K. L. Ameta, N. S. Rathore, B. Kumar, E. S. Malaga,
M. Verastegui, R. H. Gilman, B. L. Verma, J. Org. Chem.
2012, 2, 295.
15. K. Thakkar, M. Cushman, J. Org. Chem. 1995, 60, 6499.
16. H. Harkat, A. Blanc, J.-M. Weibel, P. Pale, J. Org. Chem.
2008, 73, 1620.
17. (a) M. Yu, R. Skouta, L. Zhou, H. Jiang, X. Yao, C.-
J. Li, J. Org. Chem. 2009, 74, 3378. (b) M. Yu, M. Lin,
C. Han, L. Zhu, C.-J. Li, X. Yao, Tetrahedron Lett. 2010,
51, 6722.
Acknowledgments. This research was supported by the
Duksung
Women’s
University
Research
Grants
3000002784 (2017).
References
18. S. Li, F. Jin, M. Viji, H. Jo, J. Sim, H. S. Kim, H. Lee, J.-
K. Jung, Tetrahedron Lett. 2017, 58, 1417.
19. H. Garcia, S. Iborra, J. Primo, J. Org. Chem. 1986,
51, 4432.
1. C. Ameta, A. K. Pathak, K. L. Ameta, In Natural Heterocy-
cles, K. L. Ameta, S. Chanthai Eds., Nova Science Publishers,
New York, 2015, p. 107.
20. (a) C. Liu, Z. Zhang, J. Zhang, X. Liu, M. Xie, Chin.
J. Chem. 2014, 32, 1233. (b) K. Saito, M. Yoshida, Chem.
Lett. 2015, 44, 141.
21. C. Taylor, Y. Bolshan, Tetrahedron Lett. 2015, 56, 4392.
22. A. Rahman, M. I. Choudhary, S. Hayat, A. M. Khan,
A. Ahmed, Chem. Pharm. Bull. 2001, 49, 105.
2. For reviews, see: (a) C. Zwergel, F. Gaascht, S. Valente,
M. Diederich, D. Bagrel, G. Kirsch, Nat. Prod. Commun.
2012, 7, 389. (b) A. R. Mahesh, S. Y. Mshelia, V. J. Samuel,
World J. Pharm. Pharm. Sci. 2016, 5, 668.
3. A. Detsi, M. Majdalani, C. A. Kontogiorgis, D. Hadjipavlou-
Litina, P. Kefalas, Bioorg. Med. Chem. 2009, 17, 8073.
Bull. Korean Chem. Soc. 2018
© 2018 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
4