Regiospecific synthesis of arenofurans via cascade reactions of arenols with Morita-Baylis-Hillman acetates of nitroalkenes and total synthesis of isoparvifuran

Article abstract of DOI:10.1016/j.tet.2013.04.023

A cascade process involving an S_N2′ reaction and an intramolecular oxa-Michael addition has been developed by treating Morita-Baylis-Hillman acetates of nitroalkenes with arenols, such as β-naphthols, α-naphthols, and substituted phenols under basic conditions. The products, arenofurans, are formed as single regioisomers in good to excellent yield in most cases. The methodology has been successfully employed for the total synthesis of an anti fungal agent isoparvifuran.

Full text of DOI:10.1016/j.tet.2013.04.023

Tetrahedron 69 (2013) 4964e4972
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Regiospecific synthesis of arenofurans via cascade reactions of
arenols with MoritaeBayliseHillman acetates of nitroalkenes and
total synthesis of isoparvifuran
,
Tarun Kumar a, Shaikh M. Mobin b, Irishi N.N. Namboothiri a
*
a Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400 076, India
b Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Indore, India
a r t i c l e i n f o
a b s t r a c t
A cascade process involving an SN20 reaction and an intramolecular oxa-Michael addition has been de-
Article history:
Received 2 March 2013
Received in revised form 1 April 2013
Accepted 4 April 2013
veloped by treating MoritaeBayliseHillman acetates of nitroalkenes with arenols, such as
b-naphthols,
a-naphthols, and substituted phenols under basic conditions. The products, arenofurans, are formed as
single regioisomers in good to excellent yield in most cases. The methodology has been successfully
employed for the total synthesis of an anti fungal agent isoparvifuran.
Ó 2013 Elsevier Ltd. All rights reserved.
Available online 10 April 2013
Keywords:
Arenofurans
Isoparvifuran
Nitroalkenes
MoritaeBayliseHillman acetates
Cascade reaction
1. Introduction
electrophiles in the formation of furans 3, pyrans 4, and imidazo-
pyridines 1 (Scheme 1).16 Chen et al. have also reported trans-
Furan is a prominent heterocycle in synthesis and in the bi-
ological domain.1 Functionalized and fused furans exhibit a multi-
tude of biological activities.2 Besides the well-established methods,
such as PaaleKnorr synthesis3 and FeisteBenary synthesis,4 many
transition metal-mediated strategies5 for the synthesis of furans
have appeared in the recent literature. Furans fused to other aro-
matic rings, in particular, are part of numerous natural products and
play a pivotal role in medicinal chemistry.1,2 Arenofurans have also
been employed as fluorescent probes and photosensitizers.6 Classical
methods for the synthesis of fused furans, particularly arenofurans,
such as GarsteSpencer annulation of cycloalkanones,7 Moor-
eeDanheiser rearrangement of cyclobutenones,8 NickleCasiraghi
annulation of Li phenolates with allyl halide,9 Perkin rearrangement
of coumarins10 and RappeStoermer annulation of salicyl aldehydes
formation of 6 to 3 and 4 as well as to cyclopentene derivatives 8.17
The strategy involves a cascade Michael-SN20-intramolecular Mi-
chael sequence that took place under simple and mild conditions.
We envisaged that reaction of MBH acetate 6 with phenol 9 in
a similar fashion would lead to synthetically and biologically useful
highly substituted benzofurans 10e12 in a regioselective manner.18
2. Results and discussion
At the outset, acetate 6a was treated with b-naphthol 9a in the
presence of various amine bases (Table 1, entries 1e7). The re-
actions carried out using DABCO as base in THF, DCM, and toluene
and other amine bases, such as DBU, Et3N, and imidazole in toluene
provided the product 10a only in low yield (entries 1e6). However,
diisopropylamine did improve the yield to afford naphthofuran 10a
in 78% yield (entry 7). In principle, amine bases, such as DBU (pKa
12.0), Et3N, and diisopropylamine (pKa w11.0) should be capable of
providing the naphthoxide ion via deprotonation of 9a (pKa w9.5)
although imidazole (pKa 7.0) and DABCO (pKa 8.7), which are much
weaker could be less effective. However, the nucleophilic amine
bases are also likely to add to the MBH acetate 6a in a Michael
fashion leading to side reactions, the prospects of further im-
provement in the yield using amine bases appeared remote.
with a
-haloketones11 found only limited applicability.12 Many of the
recent miscellaneous methods,13,14 including metal-mediated ones,13
are much more efficient, but are too substrate specific and often
require multistep reaction sequences.
Recently, we have reported the possible application of acetates
of MoritaeBayliseHillman adducts of nitroalkenes 615 as bi-
* Corresponding author. E-mail addresses: irishi@iitb.ac.in, inamboothiri@
gmail.com (I.N.N. Namboothiri).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.04.023

T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
4965
Scheme 1. Cascade reactions of MBH acetate 6.
Table 1
Screening of basesa
Entry
Base (equiv)
Solvent
Time (h)
% Yieldb
1
2
3
4
5
6
7
8
DABCO (1.0)
DABCO (1.0)
DABCO (1.0)
DBU (1.0)
THF
DCM
65
50
45
24
48
48
80
96
56
50
2
25c
35c
35c
22c
41c
Tracec
78
69
89
89
93
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Et3N (1.0)
Imidazole (1.0)
(i-Pr)2NH (1.0)
LiOH (1.0)
K2CO3 (1.0)
K2CO3 (1.5)
K2CO3 (2.0)
Cs2CO3 (2.0)
d
9
10
11
12
13
5
48
93
d
-
Bold entry indicates best condition.
a
Reactions were carried out with 0.2 mmol of 9a and 0.26 mmol (1.3 equiv) of 6a.
Isolated yield after silica gel column chromatography.
Complete conversion of 6a with considerable decomposition was observed in these cases.
No reaction.
b
c
d
Therefore, we screened LiOH and other alkali metal carbonates
10), remarkable rate acceleration and further rise in the yield was
observed with 2 equiv of K2CO3 (2 h, 93%, entry 11). However,
a longer reaction time was required for getting comparable yields
with 2 equiv of Cs2CO3 (5 h, 93%, entry 12). After confirming that
base was necessary for the progress of the reaction (entry 13), we
proceeded with screening other substrates for the synthesis of
various arenofurans (Tables 2e4).
(entries 8e12). While LiOH (pKa 15.7) was less effective (69% yield,
entry 8), a dramatic rise in the yield to 89% was observed when
K2CO3 (pKa 10.3) was employed (entry 9). This is attributable to the
tight ion pair formed by Liþ with the naphthoxide ion thus reducing
its nucleophilicity. Although a marginal rate acceleration was ob-
served with 1.5 equiv of K2CO3 with no change in the yield (entry

4966
T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
Table 2
Synthesis of naphthofurans 10 via addition of b
-naphthols 9aeb to MBH acetates 6a
Entry
6, R
9
Time (h)
10
% Yieldb
1
2
3
4
5
6
7
8
6a, 2-furyl
6b, 2-thienyl
6c, Ph
6d, 4-MePh
6e, 4-MeOPh
6f, 3,4-(MeO)2Ph
6g, 2,4-(MeO)2Ph
6h, 4-ClPh
9a
9a
9a
9a
9a
9a
9a
9a
9a
9a
9a
9b
2
1
2
1
7
1
1.2
2
3
10a
10b
10c
10d
10e
10fc
10g
10h
10i
93
98
92
91
95
95
87
91
86
81
83
94
9
6i, 2-NO2Ph
10
11
12
6j, 1-naphthyl
6k, PhCH]CH
6a, 2-furyl
1.5
2
5
10j
10k
10l
a
Reactions were carried out with 0.3 mmol of 9aeb, 0.4 mmol of 6, and 0.6 mmol of K2CO3.
Isolated yield after silica gel column chromatography.
Confirmed by X-ray analysis.
b
c
Table 3
Synthesis of naphthofurans 11 via addition of
a
-naphthol 9ced to MBH acetates 6a
Entry
6, Ar
9
Time (h)
11
% Yieldb
1
2
3
4
5
6
7
6a, 2-furyl
6b, 2-thienyl
6c, Ph
6d, 4-MePh
6e, 4-MeOPh
9c
9c
9c
9c
9c
4
3.5
5
4
4
11a
11b
11c
11d
11e
11f
77
90
79
78
90
97
65
6f, 3,4-(OMe)2Ph
6a, 2-furyl
9c
9d
1
3
11g
a
Reactions were carried out with 0.3 mmol of 9ced, 0.4 mmol of 6, and 0.6 mmol of K2CO3.
Isolated yield after silica gel column chromatography.
b
Table 4
Scope of phenols 9a
Entry
9
R1
R2
R3
Time (h)
13
% Yieldb
1
2
3
4
9e
9f
9g
9h
H
OMe
OMe
eOCH2Oe
H
OMe
OMe
H
H
OMe
H
15
9
10
1.5
13a
13b
13c
13d
22c
72
73
82
a
Reactions were carried out with 0.3 mmol of 9, 0.4 mmol of 6a, and 0.6 mmol of K2CO3.
Isolated yield after silica gel column chromatography.
Uncyclized oxa-Michael product 13a0 was isolated in 45% yield.
b
c

T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
4967
MBH acetates 6 in which R is heteroaryl, aryl, including fused
aryl, and styrenyl, have been treated with -naphthol 9a under our
optimized conditions (Table 2). No appreciable substituent effect on
the rate of reaction or yield was observed in these cases. For in-
dearomatization to acetate 6 and concomitant SN20 substitution of
acetate to form intermediate I. Intramolecular oxa-Michael addi-
tion via rearomatization to the newly formed acrylic ester moiety in
I in a 5-exo-trig fashion to form the dihydrofuran intermediate II
followed by base mediated elimination of HNO2 from II affords
arenofuran 13. The mass spectrometric analysis of the crude re-
action mixture for 10b (Table 2, entry 2) after 20 and 40 min con-
firmed the intermediacy of I/II with a characteristic peak appearing
at m/z 383 (Mþ), 406 (MNaþ), and 422 (MKþ).19
A practical application of our methodology for the first total syn-
thesis of an antifungal agent isoparvifuran 17, which is isolated from
the heartwood of Dalbergia parviflora,20 is outlined in Scheme 3. It
begins with the MBH reaction of commercially available nitrostyrene
14 followed by acetylation of the resulting MBH alcohol to provide
acetate 6c in 84% yield. K2CO3 mediated addition of phenol 9i to ac-
etate 6c under our optimized conditions afforded benzofuran 13e in
76% yield. Alkaline hydrolysis of the ester group in 13e proceeded
smoothly to provide acid 15 in 93% yield, which was subjected to
CaO/NaOHmediated decarboxylationinDMAunderrefluxconditions
to give benzylated isoparvifuran 16 in 80% yield. Debenzylation by
hydrogenolysis took place in quantitative yield to deliver the natural
product isoparvifuran 17 in an overall yield of 47% for six steps.
b
stance, acetates 6aeb with heteroaryl groups
b to the nitro group
afforded furans 10aeb in excellent yield (93 and 98%, respectively,
entries 1e2). Furans 10ceg were also obtained in excellent yield
(87e95%) from acetates 6ceg with phenyl as well as weakly and
strongly electron donating aryl groups (entries 3e7). Weakly and
strongly deactivating substituents on the aromatic ring of acetates,
e.g., 6hei, also did not have any adverse effect on the rate of re-
action or yield of the products 10hei (91 and 86%, respectively,
entries 8e9). While the yield was marginally lower in the case of
a fused aromatic substituent on the acetate, e.g., 6j (81%, entry 10),
acetate 6k with a styrenyl substituent furnished the product 10k in
excellent yield (83%, entry 11). A substituted b-naphthol 9b, though
required slightly longer duration, also reacted with acetate 6a to
furnish furan 10l in 94% yield (entry 12).
The structure and regiochemistry of naphthofurans 10 were
confirmed by detailed analysis of their spectral data. For instance,
an AB quartet in the aromatic region (
d 7.60e7.80) with a J value of
8.8e9.1 Hz for the protons in all the naphtho[2,1-b]furans 10
g, d
was indicative of the fusion of furan ring with the naphthalene in
an angular fashion. This was further unambiguously established
by single crystal X-ray analysis of a representative structure 10f
(see Table 1 and the Experimental section).
3. Conclusions
The synthesis of naphtho[2,1-b]furans, naphtho[1,2-b]furans,
and benzofurans has been carried out via a one-pot cascade process.
The steps involved are a base mediated carba-Michael addition of
arenols to the MBH acetates of nitroalkenes followed by substitution
of acetate in an overall SN20 reaction and then an intramolecular
oxa-Michael addition.21 The reaction is highly regiospecific in that
Having synthesized a variety of naphtho[2,1-b]furans 10 in
excellent yield by treating acetates 6aek with
(Table 2), we desired to investigate the reactivity of acetates 6 with
-naphthols 9ced under our optimized conditions in anticipation
b-naphthols 9aeb
a
that the products would be regioisomeric to furans 10 (Table 3).
Synthesis of regioisomers of naphthofurans, viz. naphtho[2,1-b]fu-
rans 10 and naphtho[1,2-b]furans 11, from the same acetates 6 just by
switching the commercially available naphthols appeared very at-
tractive. Thus, when selected acetates 6aef were treated with
b-naphthols react with MBH acetates to afford naphtho[2,1-b]furans
whereas -naphthols furnish naphtho[1,2-b]furans. A practical ap-
a
plication of our methodology was demonstrated by synthesizing the
antifungal agent isoparvifuran in six simple steps starting from
commercially available nitrostyrene in an overall yield of 47%.
a
-naphthol 9c under our optimized conditions, naphtho[1,2-b]furans
11 were formed in good to excellent yield (Table 3, entries 1e7).
Unlike in the reaction of acetates 6 with -naphthol 9a where no
b
4. Experimental section
4.1. General
appreciable effect of substituent on the aryl group was observed
(Table 2), strongly electron donating aryl groups in the acetates 6
seemed to favor the formation of furans 11 in greater yield as com-
pared to those with weak electron donating ability (Table 3). Thus
the yields of furans 11a and 11ced were in the range of 77e79% in
the case of acetates 6a and 6ced, respectively (entries 1 and 3e4).
On the other hand, furans 11b and 11eef were formed in 90e97%
yield from their corresponding acetates 6b and 6eef (entries 2 and
5e6). Reaction of 6a with methoxynaphthol 9d was also facile,
though the yield of furan 11g was relatively moderate (65%, entry 7).
Further scope of our methodology was demonstrated by treat-
ing a representative acetate 6a with substituted phenols 9eeh
(Table 4). Interestingly, the major product in the reaction of the
parent phenol 9e with acetate 6a was not the desired furan 13a, but
the oxa-Michael adduct 13a0, which was formed in 45% yield (entry
1). This is attributable to the poor nucleophilicity of the ortho-
carbon of phenoxide in the absence of electron donating sub-
stituents. On the other hand, phenols with strongly electron do-
nating alkoxy groups 9feh reacted well with 6a and delivered
furans 13bed in good to excellent yields (72e82%, entries 2e4).
As for the structure and regiochemistry of naphthofurans 11 and
13, while the possibility of regioisomers did not arise in naphtho
[1,2-b]furans 11, and benzofurans 13a and 13c, the regiochemistry
of benzofurans 13b and 13dee (13e: vide infra) is evident from
The melting points recorded are uncorrected. NMR spectra
(1H, 1H decoupled 13C) were recorded with TMS as the internal
standard. The coupling constants (J values) are given in Hertz. High
resolution mass spectra were recorded under ESI Q-TOF conditions.
X-ray data were collected on a diffractometer equipped with
graphite monochromated Mo K
a radiation. The structure was
solved by direct methods shelxs97 and refined by full-matrix least
squares against F2 using shelxl97 software.
4.2. General procedure for the synthesis of arenofurans 10, 11,
and 13
To a stirred solution of naphthol/phenol 9 (0.3 mmol) and K2CO3
(0.6 mmol, 2 equiv) in toluene (2 mL) was added MBH acetate 6
(0.4 mmol, 1.3 equiv) at room temperature. After completion of the
reaction (monitored by TLC), the solvent was evaporated invacuo and
the crude residue was purified by silica gel column chromatography
by gradient elution with ethyl acetate/petroleum ether (1:99 to 7:93).
4.2.1. Ethyl 2-(1-(furan-2-yl)naphtho[2,1-b]furan-2-yl)acetate (10a).
Colorless liquid; yield 89 mg, 93%; Rf 0.60 (20% ethyl acetate/petro-
leum ether); nmax (liquid film) 3020 (s), 2929 (w), 1736 (s), 1264 (m),
appearance of two singlets in 1H NMR in the range of
d 6.90e7.30.
The proposed mechanism taking phenol 9e as the representa-
tive arenol is shown in Scheme 2. It involves deprotonation of
phenol 9e by K2CO3 followed conjugate addition of phenoxide via
1217 (vs), 759 (vs) cmꢀ1
; dH (400 MHz, CDCl3) 1.28 (t, J 7.1 Hz, 3H), 3.9
(s, 2H), 4.22 (q, J 7.1 Hz, 2H), 6.62e6.65 (m, 2H), 7.44e7.50 (m, 2H)
7.71 (d, J 1.0 Hz, 1H), 7.72 (ABq, J 8.9 Hz, 2H), 7.92e7.97 (m, 2H); dC

4968
T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
Scheme 2. Proposed mechanism for the formation of arenofurans 13.
Scheme 3. Synthesis of isoparvifuran 17.
(100 MHz, CDCl3) 14.3, 33.7, 61.7, 111.1, 111.5, 112.1, 112.4, 121.9, 123.8,
124.7, 126.2, 126.5, 127.9, 129.0, 131.0, 143.2, 146.2, 149.2, 152.3, 169.1;
MS (ESI) m/z (rel intensity) 321 (MHþ, 100), 248 (10), 247 (45); HRMS
(ESI) m/z found 321.1121 for C20H17O4 (MHþ), requires 321.1127.
7.36e7.42 (m, 1H), 7.48e7.56 (m, 5H), 7.71 (ABq, J 8.9 Hz, 2H), 7.76
(d, J 8.3 Hz, 1H), 7.92 (d, J 8.0 Hz, 1H); dC (100 MHz, CDCl3) 14.3, 33.1,
61.5, 121.8, 122.1, 123.2, 124.4, 125.6, 126.0, 128.1, 128.2, 128.4, 128.9,
129.0, 130.6, 130.9, 133.4, 147.0, 152.0, 169.4; MS (ESI) m/z (rel in-
tensity) 331 (MHþ, 100), 321 (50), 308 (8), 257 (17), 247 (8); HRMS
(ESI) m/z found 331.1337 for C22H19O3 (MHþ), requires 331.1334.
4.2.2. Ethyl
2-(1-(thiophen-2-yl)naphtho[2,1-b]furan-2-yl)acetate
(10b). Brown liquid; yield 99 mg, 98%; Rf 0.70 (20% ethyl acetate/
petroleum ether); nmax (liquid film) 3021 (s), 2985 (m), 2874 (m),
4.2.4. Ethyl 2-(1-p-tolylnaphtho[2,1-b]furan-2-yl)acetate (10d).
Brown solid; yield 94 mg, 91%; mp 94e96 ꢁC; Rf 0.63 (20% ethyl
acetate/petroleum ether); nmax (KBr) 3020 (m), 2925 (w), 1735 (m),
1729 (vs), 1374 (s), 1248 (vs), 1046 (s), 758 (s), 704 (m) cmꢀ1
; dH
(400 MHz, CDCl3) 1.28 (t, J 7.1 Hz, 3H), 3.86 (s, 2H), 4.22 (q, J 7.1 Hz,
2H), 7.24e7.26 (m, 2H), 7.38e7.42 (m, 1H), 7.42e7.48 (m, 1H), 7.55
(d, J 4.8 Hz, 1H), 7.72 (ABq, J 8.9 Hz, 2H), 7.89e7.95 (m, 2H); dC
(100 MHz, CDCl3) 14.3, 33.2, 61.6, 112.4, 114.4, 122.4, 123.1, 124.6,
125.9, 126.3, 127.3, 127.7, 128.0, 129.0, 129.2, 131.0, 133.3, 149.0,
151.9, 169.1; MS (ESI) m/z (rel intensity) 337 (MHþ, 100), 321 (30),
297 (5), 263 (18); HRMS (ESI) m/z found 337.0898 for C20H17O3S
(MHþ), requires 337.0898.
1216 (s), 759 (vs), 669 (m) cmꢀ1
; dH (400 MHz, CDCl3) 1.26 (t, J 7.1 Hz,
3H), 2.49 (s, 3H), 3.76 (s, 2H), 4.18 (q, J 7.1 Hz, 2H), 7.28e7.34 (m, 3H),
7.38e7.44 (m, 3H), 7.69 (ABq, J 8.9 Hz, 2H), 7.80 (d, J 8.3 Hz, 1H), 7.92
(d, J 8.1 Hz,1H); dC (100 MHz, CDCl3) 14.3, 21.6, 33.2, 61.5,112.5,121.7,
122.2, 123.3, 124.3,125.6,126.0, 128.9, 129.0, 129.6, 130.2, 130.4, 130.9,
137.9, 147.0, 152.0, 169.5; MS (ESI) m/z (rel intensity) 345 (MHþ, 100),
343 (27), 271 (36), 215 (10), 214 (49); HRMS (ESI) m/z found 345.1494
for C23H21O3 (MHþ), requires 345.1491.
4.2.3. Ethyl 2-(1-phenylnaphtho[2,1-b]furan-2-yl)acetate (10c).
Colorless solid; yield 91 mg, 92%; mp 93e96 ꢁC; Rf 0.73 (20% ethyl
acetate/petroleum ether); nmax (KBr) 3021 (m), 2929 (w), 1736 (s),
4.2.5. Ethyl 2-(1-(4-methoxyphenyl)naphtho[2,1-b]furan-2-yl)ace-
tate (10e). Colorless solid; yield 102 mg, 95%; mp 65e67 ꢁC; Rf 0.70
(20% ethyl acetate/petroleum ether); nmax (KBr) 2979 (w), 2928 (w),
1738 (s), 1602 (w), 1511 (m), 1393 (w), 1286 (m), 1247 (s), 1178 (m),
1217 (s), 759 (vs), 704 (m) cmꢀ1
;
dH (400 MHz, CDCl3) 1.26 (t, J
7.1 Hz, 3H), 3.77 (s, 2H), 4.19 (q, J 7.1 Hz, 2H), 7.26e7.33 (m, 1H),

T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
4969
1033 (m), 805 (m) cmꢀ1
;
dH (400 MHz, CDCl3) 1.27 (t, J 7.0 Hz, 3H),
8.9 Hz, 2H), 7.76e7.79 (m, 1H), 7.92 (d, J 8.0 Hz, 1H), 8.19 (dd, J 8.0,
1.3 Hz, 1H); Benzylic CH2 appears as ABq presumably due to atro-
pisomerism; dC (100 MHz, CDCl3) 14.2, 33.3, 61.7, 112.5, 117.0, 122.3,
122.6, 124.5, 125.0, 126.0, 126.4, 127.9, 128.2, 129.1, 129.9, 130.9,
133.4, 133.7, 146.9, 150.0, 152.0, 168.7; MS (ESI) m/z (rel intensity)
398 (MNaþ, 100), 376 (MHþ, 85), 302 (35), 263 (15), 262 (79); HRMS
(ESI) m/z found 376.1189 for C22H18NO5 (MHþ), requires 376.1185.
3.77 (s, 2H), 3.92 (s, 3H), 4.20 (q, J 7.0 Hz, 2H), 7.07 (d, J 8.6 Hz, 2H),
7.30e7.36 (m, 1H), 7.38e7.44 (m, 1H), 7.47 (d, J 8.6 Hz, 2H), 7.70
(ABq, J 8.8 Hz, 2H), 7.81 (d, J 8.0 Hz, 1H), 7.92 (d, J 7.8 Hz, 1H); dC
(100 MHz, CDCl3) 14.3, 33.2, 55.5, 61.5, 112.5, 114.3, 121.4, 122.3,
123.3, 124.3, 125.4, 125.6, 126.0, 128.3, 129.0, 130.9, 131.7, 147.0,
151.9, 159.6, 169.5; MS (ESI) m/z (rel intensity) 361 (MHþ, 100), 360
(Mþ, 5), 297 (6), 287 (12); HRMS (ESI) m/z found 361.1447 for
C23H21O4 (MHþ), requires 361.1440.
4.2.10. Ethyl 2-(1-(naphthalen-1-yl)naphtho[2,1-b]furan-2-yl)ace-
tate (10j). Light yellow solid; yield 92 mg, 81%; mp 80e82 ꢁC; Rf
0.60 (20% ethyl acetate/petroleum ether); nmax (KBr) 3057 (w), 1739
(vs), 1623 (w), 1384 (w), 1265 (m), 1190 (s), 1156 (m), 1029 (m), 804
4.2.6. Ethyl
2-(1-(3,4-dimethoxyphenyl)naphtho[2,1-b]furan-2-yl)
acetate (10f). Colorless solid; yield 111 mg, 95%; mp 123e125 ꢁC; Rf
0.4 (20% ethyl acetate/petroleum ether); nmax (KBr) 3021 (s), 1735
(s), 780 (m) cmꢀ1
; dH (400 MHz, CDCl3) 1.16 (t, J 7.1 Hz, 3H), 3.69
(s), 1250 (s), 1217 (s), 1098 (m), 758 (vs), 669 (w) cmꢀ1
;
dH
(ABq, J 16.6 Hz, 2H), 4.09 (two overlapped q, J 7.1 Hz, 2H), 7.02e7.07
(m, 1H), 7.11 (d, J 8.4 Hz, 1H), 7.29e7.33 (m, 2H), 7.48e7.54 (m, 1H),
7.59e7.64 (m, 2H), 7.66e7.68 (m, 1H), 7.76 (ABq, J 9.1 Hz, 2H), 7.89
(d, J 8.1 Hz, 1H), 7.97 (d, J 8.2 Hz, 1H), 8.01e8.04 (m, 1H); dC
(100 MHz, CDCl3) 14.2, 33.3, 61.5, 112.6, 119.5, 123.2, 123.3, 124.4,
125.8, 125.9, 126.2, 126.3, 126.4, 126.7, 128.1, 128.5, 128.7, 128.8,
129.0, 130.8, 130.9, 133.1, 133.9, 147.8, 152.2, 169.3; MS (ESI) m/z
(rel intensity) 381 (MHþ, 100), 380 (Mþ, 5), 307 (14), 253 (5); HRMS
(ESI) m/z found 381.1497 for C26H21O3 (MHþ), requires 381.1491.
(400 MHz, CDCl3) 1.26 (t, J 7.1 Hz, 3H), 3.77 (s, 2H), 3.86 (s, 3H), 3.98
(s, 3H), 4.18 (q, J 7.1 Hz, 2H), 7.06 (ABq, J 7.1 Hz, 2H, the lower half
further split into d, J 1.6 Hz), 7.07e7.09 (d, J 1.6 Hz, 1H), 7.29e7.33
(m, 1H), 7.37e7.42 (m, 1H), 7.69 (ABq, J 8.9 Hz, 2H), 7.84 (d, J 8.3 Hz,
1H), 7.90 (d, J 8.0 Hz, 1H); dC (100 MHz, CDCl3) 14.3, 33.2, 56.0, 61.5,
111.4,112.5,113.6,121.6,122.1,122.8,123.3,124.4,125.6,126.0,128.2,
129.0, 130.9, 147.0, 148.9, 149.0, 151.9, 169.5; MS (ESI) m/z (rel in-
tensity) 391 (MHþ, 100), 385 (8), 317 (9), 79 (11); HRMS (ESI) m/z
found 391.1541 for C24H23O5 (MHþ), requires 391.1545; selected X-
ray data (CCDC 930323): C48H44O10, M¼780.83, Monoclinic, space
4.2.11. (E)-Ethyl 2-(1-styrylnaphtho[2,1-b]furan-2-yl)acetate (10k).
Colorless solid; yield 88 mg, 83%; mp 103e105 ꢁC; Rf 0.56 (20% ethyl
acetate/petroleum ether); nmax (KBr) 3020 (m), 2959 (m), 2929 (m),
ꢀ
ꢀ
ꢀ
group P21/n, a¼7.9520(4) A, b¼8.7362(4) A, c¼29.4465(16) A,
3
ꢀ
3
ꢁ
a
¼90ꢁ,
b
¼96.652(5)ꢁ,
g
¼90 , V¼2031.88(18) A , Dc¼1.276 Mg/m ,
Z¼2, F(000)¼824,
l
¼0.71073 A,
m
¼0.089 mmꢀ1
,
total/unique
K,
1735 (s), 1265 (s), 1217 (s), 759 (vs) cmꢀ1
; dH (400 MHz, CDCl3) 1.32
ꢀ
reflections¼23,711/6796
[R(int)¼0.0365],
T¼150(2)
q
(t, J 7.1 Hz, 3H), 4.00 (s, 2H), 4.26 (q, J 7.1 Hz, 2H), 6.97 (d, J 16.1 Hz,
1H), 7.34e7.38 (m, 1H), 7.43e7.58 (m, 4H), 7.54 (d, J 16.1 Hz, 1H),
7.61e7.63 (m, 2H), 7.69 (ABq, J 8.9 Hz, 2H), 7.95 (d, J 7.8 Hz, 1H), 8.37
(d, J 8.3 Hz, 1H); dC (100 MHz, CDCl3) 14.4, 33.8, 61.7, 112.5, 119.6,
119.8, 122.0, 123.7, 124.5, 125.8, 126.5, 126.8, 128.2, 128.6, 129.0,
129.1, 131.0, 134.4, 137.2, 146.6, 152.0, 169.5; MS (ESI) m/z (rel in-
tensity) 357 (MHþ, 100), 283 (22), 279 (11), 167 (8), 149 (8); HRMS
(ESI) m/z found 357.1496 for C24H21O3 (MHþ), requires 357.1491.
range¼3.13e32.50ꢁ, final R [I>2
(I)]: R1¼0.0777, wR2¼0.2152, R
s
(all data): R1¼0.1312, wR2¼0.2579.
4.2.7. Ethyl
2-(1-(2,4-dimethoxyphenyl)naphtho[2,1-b]furan-2-yl)
acetate (10g). Colorless liquid; yield 102 mg, 87%; Rf 0.46 (20% ethyl
acetate/petroleum ether); nmax (liquid film) 2936 (w), 1738 (vs),
1603 (vs), 1579 (s), 1507 (m), 1464 (m), 1304 (m), 1209 (s), 1158 (m),
1031 (s), 805 (m) cmꢀ1
; dH (400 MHz, CDCl3) 1.25 (t, J 7.1 Hz, 3H),
3.66 (s, 3H), 3.82 (ABq, J 16.6 Hz, 2H), 3.92 (s, 3H), 4.17, 4.18 (two
overlapped q, J 7.1 Hz, 2H), 6.62e6.65 (m, 2H), 7.28e7.40 (m, 3H),
7.68 (ABq, J 9.1 Hz, 3H), 7.89 (d, J 8.1 Hz, 1H); benzylic CH2 appears
as ABq presumably due to atropisomerism; dC (100 MHz, CDCl3)
14.3, 33.3, 55.6, 61.4, 99.1, 104.6, 112.6, 114.4, 117.5, 122.8, 123.2,
124.1, 125.2, 125.8, 128.6, 128.7, 130.8, 132.8, 147.2, 152.0, 159.0,
161.4, 169.6; MS (ESI) m/z (rel intensity) 391 (MHþ, 100), 317 (12),
252 (15), 73 (19); HRMS (ESI) m/z found 391.1549 for C24H23O5
(MHþ), requires 391.1545.
4.2.12. Ethyl
2-(7-bromo-1-(furan-2-yl)naphtho[2,1-b]furan-2-yl)
acetate (10l). Yellow solid; yield 112 mg, 94%; mp 79e81 ꢁC; Rf 0.63
(20% ethyl acetate/petroleum ether); nmax (KBr) 3020 (s), 2928 (w),
2854 (w), 1734 (s), 1504 (w), 1422 (w), 1216 (vs), 1021 (m), 929 (w),
759 (vs) cmꢀ1
; dH (400 MHz, CDCl3) 1.27 (t, J 7.1 Hz, 3H), 3.90 (s, 2H),
4.21 (q, J 7.1 Hz, 2H), 6.61e6.63 (m, 2H), 7.05 (dd, J 8.8, 2.0 Hz, 1H),
7.64 (ABq, J 9.0 Hz, 2H), 7.69 (dd, J 1.7, 1.0 Hz, 1H), 7.80 (d, J 8.8 Hz,
1H), 8.05 (d, J 2.0 Hz, 1H); dC (100 MHz, CDCl3) 14.3, 33.7, 61.8, 111.2,
111.6,112.0,113.4,118.4,122.0,125.2,125.6,126.3,129.7,130.9,132.3,
143.3, 145.7, 149.7, 152.2, 168.9; MS (ESI) m/z (rel intensity) 401
([MHþ2]þ, 100), 399 (MHþ, 98), 391 (95), 326 (22), 317 (40); HRMS
(ESI) m/z found 399.0220 for C20H16BrO4 (MHþ), requires 399.0232.
4.2.8. Ethyl 2-(1-(4-chlorophenyl)naphtho[2,1-b]furan-2-yl)acetate
(10h). Colorless solid; yield 99 mg, 91%; mp 73e75 ꢁC; Rf 0.63 (20%
ethyl acetate/petroleum ether); nmax (KBr) 3054 (w), 2984 (w), 1738
(s), 1491 (w), 1393 (w), 1265 (m), 807 (m), 740 (s), 704 (m) cmꢀ1
;
dH
4.2.13. Ethyl2-(3-(furan-2-yl)naphtho[1,2-b]furan-2-yl)acetate (11a).
Colorless solid; yield 74 mg, 77%; mp 60e63 ꢁC; Rf 0.70 (20% ethyl
acetate/petroleum ether); nmax (KBr) 2917 (w), 2852 (w), 1736 (vs),
1645 (w), 1461 (w), 1381 (m), 1265 (m), 1178 (m), 1029 (m), 810 (m),
(400 MHz, CDCl3) 1.27 (t, J 7.1 Hz, 3H), 3.75 (s, 2H), 4.19 (q, J 7.1 Hz,
2H), 7.32e7.36 (m, 1H), 7.40e7.44 (m, 1H), 7.48e7.53 (m, 4H), 7.71
(ABq, J 8.9 Hz, 4H), 7.73 (d, J 7.9 Hz, 1H), 7.93 (d, J 8.0 Hz, 1 Hz); dC
(100 MHz, CDCl3) 14.3, 33.1, 61.6, 112.5, 120.7, 121.8, 123.1, 124.5,
125.9, 126.2, 128.1, 129.1, 129.2, 130.9, 131.9, 132.0, 134.3, 147.1, 152.1,
169.3; MS (ESI) m/z (rel intensity) 365 (MHþ, 100), 345 (60), 321 (6),
297 (9), 291 (12), 271 (8); HRMS (ESI) m/z found 365.0953 for
C22H18O3Cl (MHþ), requires 365.0944.
748 (s) cmꢀ1
; dH (400 MHz, CDCl3) 1.28 (t, J 7.0 Hz, 3H), 4.23 (s, 2H),
4.25 (q, J 7.0 Hz, 2H), 6.58 (d, J 1.5 Hz, 1H), 6.72 (d, J 3.1 Hz, 1H),
7.49e7.55 (m, 1H), 7.58e7.62 (m, 2H), 7.72 (d, J 8.8 Hz, 1H), 7.92 (d, J
8.8 Hz, 2H), 7.95 (d, J 9.6 Hz, 1H), 8.33 (d, J 8.2 Hz, 1H); dC (100 MHz,
CDCl3)14.3, 34.6, 61.6,107.3,111.4,111.8,119.1,120.3,121.2,121.8,123.9,
125.5, 126.5, 128.4, 131.6, 142.0, 146.3, 147.6, 150.0, 169.3; MS (ESI) m/z
(rel intensity) 321 (MHþ, 100), 247 (11), 208 (6), 158 (5); HRMS (ESI)
m/z found 321.1139 for C20H16O4 (MHþ), requires 321.1127.
4.2.9. Ethyl
2-(1-(2-nitrophenyl)naphtho[2,1-b]furan-2-yl)acetate
(10i). Brown solid; yield 97 mg, 86%; mp 105e107 ꢁC; Rf 0.43 (20%
ethyl acetate/petroleum ether); nmax (KBr) 3055 (w), 2986 (w),
2927 (w), 1736 (m), 1530 (m), 1351 (w), 1266 (s), 1028 (w), 740 (s)
4.2.14. Ethyl 2-(3-(thiophen-2-yl)naphtho[1,2-b]furan-2-yl)acetate
(11b). Yellow liquid; yield 90 mg, 90%; Rf 0.67 (20% ethyl acetate/
petroleum ether); nmax (liquid film) 3063 (w), 2981 (m), 2928 (m),
1736 (vs), 1383 (m), 1370 (m), 1188 (s), 1027 (m), 810 (m), 753 (s),
cmꢀ1
; dH (400 MHz, CDCl3) 1.22 (t, J 7.1 Hz, 3H), 3.69 (ABq, J 16.4 Hz,
2H), 4.10e4.18 (m, 2H), 7.25e7.31 (m, 2H), 7.39 (ddd, J 8.1, 6.2,
1.9 Hz, 1H), 7.62 (dd, J 7.4, 1.5 Hz, 1H), 7.68e7.73 (m, 1H), 7.72 (ABq, J

4970
T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
699 (m) cmꢀ1
;
dH (400 MHz, CDCl3) 1.27 (t, J 7.1 Hz, 3H), 4.07 (s, 2H),
0.56 (20% ethyl acetate/petroleum ether); nmax (KBr) 2928 (w), 1738
4.23 (q, J 7.1 Hz, 2H), 7.17e7.19 (m, 1H), 7.31e7.32 (m, 1H), 7.41e7.42
(m, 1H), 7.47e7.50 (m, 1H), 7.56e7.60 (m, 1H), 7.75 (ABq, J 8.6 Hz,
2H), 7.92 (d, J 8.1 Hz, 1H), 8.31 (d, J 8.2 Hz, 1H); dC (100 MHz, CDCl3)
14.3, 34.0, 61.7, 114.9, 118.7, 120.3, 121.2, 123.4, 123.9, 125.5, 125.7,
126.4, 126.6, 127.9, 128.5, 131.7, 133.2, 146.7, 150.0, 169.2; MS (ESI)
m/z (rel intensity) 337 (MHþ, 89), 318 (30), 317 (100), 298 (23), 253
(20), 233 (16), 214 (16); HRMS (ESI) m/z found 337.0887 for
C20H17O3S (MHþ), requires 337.0898.
(vs), 1645 (m), 1589 (m), 1381 (m), 1225 (m), 1159 (m), 763 (s), 736
(s) cmꢀ1
; dH (400 MHz, CDCl3) 1.26 (t, J 7.1 Hz, 3H), 4.06 (s, 3H), 4.14
(s, 2H), 4.21 (q, J 7.1 Hz, 2H), 6.57 (dd, J 3.2, 1.8 Hz, 1H), 6.66 (d, J
3.2 Hz, 1H), 7.18 (s, 1H), 7.47e7.51 (m, 1H), 7.58e7.62 (m, 2H), 8.23
(d, J 8.2 Hz, 1H), 8.31 (d, J 8.4 Hz, 1H); dC (100 MHz, CDCl3) 14.3, 34.7,
56.1, 61.6, 96.5, 107.1, 111.4, 112.1, 120.0, 121.3, 121.6, 123.2, 124.2,
124.9, 127.2, 142.0, 145.0, 146.3, 147.8, 152.6, 169.3; MS (ESI) m/z (rel
intensity) 351 (MHþ, 100), 350 (Mþ, 4), 277 (3); HRMS (ESI) m/z
found 351.1236 for C21H19O5 (MHþ), requires 351.1232.
4.2.15. Ethyl 2-(3-phenylnaphtho[1,2-b]furan-2-yl)acetate (11c).
Yellow liquid; yield 78 mg, 79%; Rf 0.73 (20% ethyl acetate/petro-
leum ether); nmax (liquid film) 3059 (w), 2981 (w), 2929 (w), 1740
(vs), 1381 (m), 1258 (m), 1190 (m), 1028 (m), 812 (m), 748 (s), 701
4.2.20. Ethyl 2-(3-(furan-2-yl)-6,7-dihydrobenzofuran-2-yl)acetate
(13a). Colorless liquid; yield 17 mg, 22%; Rf 0.63 (20% ethyl acetate/
petroleum ether); nmax (liquid film) 3020 (s), 2927 (w), 1735 (m),
(m) cmꢀ1
;
dH (400 MHz, CDCl3) 1.30 (t, J 7.1 Hz, 3H), 3.98 (s, 2H),
1265 (m), 1216 (vs), 1026 (w), 910 (vs), 755 (vs) cmꢀ1
; dH (400 MHz,
4.24 (q, J 7.1 Hz, 2H), 7.41e7.48 (m, 1H), 7.50e7.55 (m, 3H),
7.58e7.66 (m, 3H), 7.67e7.69 (unresolved m, 2H), 7.94 (d, J 8.1 Hz,
1H), 8.34 (d, J 8.2 Hz, 1H); dC (100 MHz, CDCl3) 14.4, 33.7, 61.6, 118.7,
120.3, 121.2, 121.4, 123.7, 123.8, 125.3, 126.5, 127.7, 128.5, 129.1,
129.2, 131.7, 132.2, 146.2, 150.1, 169.6; MS (ESI) m/z (rel intensity)
331 (MHþ, 100), 307 (5), 297 (5), 258 (5), 257 (16); HRMS (ESI) m/z
found 331.1337 for C22H19O3 (MHþ), requires 331.1334.
CDCl3) 1.26 (t, J 7.1 Hz, 3H), 4.11 (s, 2H), 4.21 (q, J 7.1 Hz, 2H), 6.54
(dd, J 3.3, 1.6 Hz, 1H), 6.67 (d, J 3.3 Hz, 1H), 7.28e7.35 (m, 2H),
7.47e7.49 (m, 1H), 7.54 (d, J 1.6 Hz, 1H), 7.82e7.85 (m, 1H); dC
(100 MHz, CDCl3) 14.3, 34.6, 61.6, 107.3, 110.7, 111.4, 111.5, 120.8,
123.3, 124.8, 126.3, 142.0, 147.1, 147.4, 154.5, 169.1; MS (ESI) m/z (rel
intensity) 271 (MHþ, 100), 225 (6),198 (8), 197 (65); HRMS (ESI) m/z
found 271.0971 for C16H15O4 (MHþ), requires 271.0970.
4.2.16. Ethyl 2-(3-p-tolylnaphtho[1,2-b]furan-2-yl)acetate (11d).
Yellow solid; yield 80 mg, 78%; mp 71e73 ꢁC; Rf 0.66 (20% ethyl
acetate/petroleum ether); nmax (KBr) 3022 (m), 2983 (m), 2926 (m),
1737 (s), 1513 (w), 1382 (m), 1258 (m), 1216 (s), 812 (m), 758 (vs)
4.2.21. Ethyl 4-(furan-2-yl)-3-nitro-4-phenoxybut-2-enoate
(13a0). Yellow solid; yield 43 mg, 45%; mp 76e78 ꢁC; Rf 0.4 (20%
ethyl acetate/petroleum ether); nmax (KBr) 2973 (w), 1763 (s), 1649
(m), 1519 (m), 1494 (m), 1319 (vs),1215 (vs),1061 (m), 1025 (m), 756
cmꢀ1
;
dH (400 MHz, CDCl3) 1.29 (t, J 7.1 Hz, 3H), 2.44 (s, 3H), 3.96 (s,
(m) cmꢀ1
; dH (400 MHz, CDCl3) 1.25 (t, J 7.1 Hz, 3H), 4.29 (ABqq, J
2H), 4.23 (q, J 7.0 Hz, 2H), 7.40 (ABq, J 7.7 Hz, 4H), 7.47e7.49 (m, 1H),
7.58e7.62 (m, 1H), 7.64e7.70 (m, 2H), 7.93 (d, J 8.0 Hz, 1H), 8.33 (d, J
8.1 Hz, 1H); dC (100 MHz, CDCl3) 14.3, 21.5, 33.7, 61.6, 118.7, 120.3,
121.1, 121.4, 123.6, 123.9, 125.3, 126.4, 128.4, 129.1, 129.2, 129.8,
131.6, 137.5, 146.0, 150.0, 169.6; MS (ESI) m/z (rel intensity) 345
(MHþ, 100), 343 (19), 322 (14), 321 (60), 271 (23), 247 (19); HRMS
(ESI) m/z found 345.1501 for C23H21O3 (MHþ), requires 345.1491.
17.8, 7.1 Hz, 2H), 6.59 (s, 1H), 6.60 (dd, J 3.6, 1.8 Hz, 1H), 6.96e7.20
(m, 3H), 7.04 (d, J 3.6 Hz, 1H), 7.22e7.26 (m, 2H), 7.61 (d, J 1.8 Hz,
1H), 8.02 (s, 1H); dC (100 MHz, CDCl3) 14.2, 62.5, 72.4, 113.8, 116.8,
122.8, 123.5, 125.0, 129.6, 142.1, 146.3, 148.1, 157.6, 167.8; MS (ESI)
m/z (rel intensity) 341 (MHNaþ, 21), 340 (MNaþ, 100), 337 (22), 335
(18), 272 (22), 244 (20), 225 (24), 224 (31); HRMS (ESI) m/z found
341.0871for C16H16O6Na (MHNaþ), requires 341.0875.
4.2.17. Ethyl 2-(3-(4-methoxyphenyl)naphtho[1,2-b]furan-2-yl)ace-
tate (11e). Colorless solid; yield 97 mg, 90%; mp 86e88 ꢁC; Rf 0.56
(20% ethyl acetate/petroleum ether); nmax (KBr) 2932 (m), 2835 (w),
1736 (s), 1604 (w), 1512 (m), 1380 (m), 1289 (m), 1251 (s), 1179 (s),
4.2.22. Ethyl 2-(3-(furan-2-yl)-4,5-dimethoxybenzofuran-2-yl)ace-
tate (13b). Colorless solid; yield 73 mg, 72%; mp 85e87 ꢁC; Rf 0.59
(20% ethyl acetate/petroleum ether); nmax (KBr) 2927 (m), 1737 (m),
1489 (m),1325 (w),1219 (m),1031 (m), 758 (vs) cmꢀ1
; dH (400 MHz,
1035 (m), 812 (m), 753 (m) cmꢀ1
;
dH (400 MHz, CDCl3) 1.29 (t, J
CDCl3) 1.25 (t, J 7.1 Hz, 3H), 3.92 (s, 3H), 3.96 (s, 3H), 4.03 (s, 2H),
4.20 (q, J 7.1 Hz, 2H), 6.54 (dd, J 3.2, 1.6 Hz, 1H), 6.60 (d, J 3.2 Hz, 1H),
7.03 (s, 1H), 7.25 (s, 1H), 7.54 (d, J 1.6 Hz, 1H); dC (100 MHz, CDCl3)
14.3, 34.5, 56.4, 56.7, 61.6, 95.4, 102.3, 107.0, 110.8, 111.4, 118.2, 141.9,
146.0, 146.9, 147.7, 148.3, 149.2, 169.3; MS (ESI) m/z (rel intensity)
331 (MHþ, 100), 330 (Mþ, 5), 257 (13); HRMS (ESI) m/z found
331.1170 for C18H19O6 (MHþ), requires 331.1182.
7.1 Hz, 3H), 3.88 (s, 3H), 3.95 (s, 2H), 4.24 (q, J 7.1 Hz, 2H), 7.06 (dt, J
8.8, 2.1 Hz, 2H), 7.47e7.52 (m, 1H), 7.53 (dt, J 8.8, 2.1 Hz, 2H),
7.57e7.63 (m, 1H), 7.66 (ABq, J 8.5 Hz, 2H), 7.93 (d, J 8.1 Hz, 1H), 8.33
(d, J 8.2 Hz, 1H); dC (100 MHz, CDCl3) 14.3, 33.7, 55.5, 61.6, 114.5,
118.7, 120.3, 120.8, 121.4, 123.5, 123.9, 124.4, 125.2, 126.4, 128.4,
130.4, 131.6, 145.8, 150.0, 159.3, 169.7; MS (ESI) m/z (rel intensity)
361 (MHþ, 100), 301 (7), 298 (14), 297 (42), 287 (17); HRMS (ESI) m/
z found 361.1445 for C23H21O4 (MHþ), requires 361.1440.
4.2.23. Ethyl 2-(3-(furan-2-yl)-4,5,6-trimethoxybenzofuran-2-yl)ac-
etate (13c). Colorless solid; yield 78 mg, 73%; mp 79e81 ꢁC; Rf 0.65
(20% ethyl acetate/petroleum ether); nmax (KBr) 2936 (m), 1739 (vs),
4.2.18. Ethyl 2-(3-(3,4-dimethoxyphenyl)naphtho[1,2-b]furan-2-yl)
acetate (11f). Colorless solid; yield 113 mg, 97%; mp 82e84 ꢁC; Rf
0.40 (20% ethyl acetate/petroleum ether); nmax (KBr) 2928 (s), 2855
(m), 1738 (vs), 1515 (s), 1464 (m), 1381 (m), 1261 (vs), 1028 (s), 754
1621 (m), 1469 (s), 1155 (m), 1053 (m), 1020 (w), 757 (s) cmꢀ1
; dH
(400 MHz, CDCl3) 1.26 (t, J 7.1 Hz, 3H), 3.83 (s, 3H), 3.88 (s, 3H), 3.89
(s, 3H), 4.00 (s, 2H), 4.19 (q, J 7.1 Hz, 2H), 6.50 (dd, J 3.3, 1.8 Hz, 1H),
6.82 (s, 1H), 6.84 (dd, J 3.3, 0.7 Hz, 1H), 7.50 (dd, J 1.8, 0.7 Hz, 1H); dC
(100 MHz, CDCl3) 14.3, 34.4, 56.4, 61.5, 61.8, 91.5, 110.0, 110.3, 111.4,
113.1, 139.3, 142.0, 146.9, 151.4, 152.5, 169.5; MS (ESI) m/z (rel in-
tensity) 361 (MHþ, 100), 360 (Mþ, 11), 297 (4), 287 (9); HRMS (ESI)
m/z found 361.1281 for C19H21O7 (MHþ), requires 361.1287.
(m) cmꢀ1
; dH (400 MHz, CDCl3) 1.29 (t, J 7.1 Hz, 3H), 3.94 (s, 3H),
3.96 (s, 3H), 3.97 (s, 2H), 4.24 (q, J 7.1 Hz, 2H), 7.02 (d, J 8.0 Hz, 1H),
7.14e7.17 (m, 2H), 7.47e7.52 (m, 1H), 7.57e7.62 (m, 1H), 7.67 (ABq, J
8.7 Hz, 2H), 7.93 (d, J 8.1 Hz, 1H), 8.33 (d, J 8.2 Hz, 1H); dC (100 MHz,
CDCl3) 14.3, 33.8, 56.1, 61.6, 111.7, 112.4, 118.6, 120.3, 121.0, 121.4,
121.5, 123.6, 123.8, 124.7, 125.3, 126.5, 128.4, 131.6, 145.8, 148.7,
149.3, 150.0, 169.7; MS (ESI) m/z (rel intensity) 391 (MHþ, 100), 390
(Mþ, 6), 317 (6), 297 (13), 235 (5); HRMS (ESI) m/z found 391.1561
for C24H23O5 (MHþ), requires 391.1545.
4.2.24. Ethyl 2-(8-(furan-2-yl)benzofuro[5,4-d][1,3]dioxol-7-yl)ace-
tate (13d). Colorless solid; yield 77 mg, 82%; mp 67e69 ꢁC; Rf 0.56
(20% ethyl acetate/petroleum ether); nmax (KBr) 2932 (w), 1739 (vs),
1621 (w), 1463 (m), 1370 (w), 1324 (m), 1196 (m), 1155 (s), 1035 (s),
4.2.19. Ethyl 2-(3-(furan-2-yl)-5-methoxynaphtho[1,2-b]furan-2-yl)
acetate (11g). Colorless solid; yield 65 mg, 65%; mp 70e72 ꢁC; Rf
738 (m) cmꢀ1
;
dH (400 MHz, CDCl3) 1.25 (t, J 7.0 Hz, 3H), 4.02 (s, 2H),
4.20 (q, J 7.0 Hz, 2H), 5.97 (s, 2H), 6.50e6.52 (unresolved m, 1H),

T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
4971
6.56 (d, J 2.7 Hz, 1H), 6.95 (s, 1H), 7.20 (s, 1H), 7.51 (unresolved m,
1H); dC (100 MHz, CDCl3) 14.3, 34.4, 61.5, 93.5, 99.4, 101.5, 106.9,
111.0, 111.3, 119.4, 141.8, 144.9, 146.3, 146.4, 147.4, 149.6, 169.2; MS
(ESI) m/z (rel intensity) 315 (MHþ, 100), 314 (Mþ, 7), 247 (4), 241
(17); HRMS (ESI) m/z found 315.0879 for C17H15O6 (MHþ), requires
315.0869.
After stirring for 1.5 h at room temperature under H2 (balloon) at-
mosphere, the reaction mixture was filtered through a Celite pad and
concentrated in vacuo to afford pure product 17. White solid; yield
59 mg, 99%; mp 118e120 ꢁC (lit20a 121e122 ꢁC); nmax (KBr) 3433 (br
m), 2917 (w), 2849 (w), 1619 (s), 1485 (vs), 1439 (m), 1362 (m), 1312
(vs), 1266 (m), 1198 (m), 1156 (m), 1026 (m), 761 (vs), 704 (m) cmꢀ1
;
dH (400 MHz, CDCl3) 2.52 (s, 3H), 3.93 (s, 3H), 5.61 (br s, 1H), 7.02
(s, 1H), 7.15 (s, 1H), 7.35e7.38 (m, 1H), 7.47e7.52 (m, 4H); dC
(100 MHz, CDCl3) 13.0, 56.5, 94.5, 103.7, 116.9, 121.5, 126.9, 128.8,
133.1, 142.6, 144.7, 148.1, 150.4; MS (ESI) m/z (rel intensity) 277
(MNaþ, 45), 255 (MHþ, 100), 227 (4), 223 (8); HRMS (ESI) m/z found
255.1018 for C16H15O3 (MHþ), requires 255.1021. Consistent with
literature.20a
4.2.25. Ethyl 2-(5-(benzyloxy)-6-methoxy-3-phenylbenzofuran-2-yl)
acetate (13e). Light yellow solid; yield 45 mg, 76%; mp 102e104 ꢁC;
Rf 0.46 (20% ethyl acetate/petroleum ether); nmax (KBr) 3062 (w),
2981 (w), 2938 (w), 1737 (vs), 1619 (m), 1489 (vs), 1452 (s), 1327
(m), 1266 (m), 1191 (m), 1164 (m), 737 (vs), 703 (m) cmꢀ1
; dH
(400 MHz, CDCl3) 1.28 (t, J 7.1 Hz, 3H), 3.82 (s, 2H), 3.93 (s, 3H), 4.21
(q, J 7.1 Hz, 2H), 5.12 (s, 2H), 7.08 (s, 1H), 7.09 (s, 1H), 7.29e7.34 (m,
1H), 7.35e7.41 (3H), 7.43e7.50 (m, 6H); dC (100 MHz, CDCl3) 14.3,
33.6, 56.6, 61.6, 72.3, 95.9, 105.3, 120.0, 120.2, 127.6, 127.8, 128.0,
128.6, 128.9, 129.1, 132.3, 137.4, 145.7, 149.1, 149.6, 169.6; MS (ESI)
m/z (rel intensity) 439 (MNaþ, 80), 417 (MHþ, 100), 401 (7), 335 (9),
158 (5); HRMS (ESI) m/z found 417.1712 for C26H25O5 (MHþ),
requires 417.1702.
Acknowledgements
I.N.N.N. thanks DST India for financial assistance. T.K. thanks
CSIR India for a senior research fellowship.
Supplementary data
4.2.26. 2-(5-(Benzyloxy)-6-methoxy-3-phenylbenzofuran-2-yl)acetic
acid (15). To a stirred solution of benzofuran 13e (0.28 mmol,
117 mg) in EtOH/H2O (9:1) was added KOH (32 mg, 0.56 mmol) at
room temperature. The stirring was continued till the starting
material was fully consumed (3e4 h). Then Amberlyst 15 acid resin
(56 mg) was added and the reaction mixture was stirred for
10e15 min in order to neutralize the excess KOH. The resin was
filtered through a pad of Celite and the solvent was evaporated in
vacuo. The acid isolated was sufficiently pure for further reaction.
Colorless solid; yield 101 mg, 93%; mp 135e137 ꢁC; Rf 0.13 (20%
ethyl acetate/petroleum ether); nmax (KBr) 3060 (w), 2931 (m),
2588 (br w),1718 (s),1620 (m),1557 (w),1489 (s),1451 (s),1313 (m),
Copies of NMR spectra for all the new/relevant compounds.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2013.04.023.
References and notes
1. Selected books: (a) Keay, B. A.; Dibble, P. W. In Comprehensive Heterocyclic
Chemistry II; Elsevier: Oxford, UK, 1997; Vol. 2, p 395; (b) Donnelly, D. M. X.;
Meegan, M. J. In Comprehensive Heterocyclic Chemistry II; Katritzky, A. R., Rees,
C. W., Eds.; Pergamon: Oxford, UK, 1984; Vol. 4, p 657; Selected reviews: (c)
Brown, R. C. D. Angew. Chem., Int. Ed. 2005, 44, 850; (d) Hou, X. L.; Cheung, H. Y.;
Hon, T. Y.; Kwan, P. L.; Lo, T. H.; Tong, S. Y.; Wong, H. N. C. Tetrahedron 1998, 54,
1955; (e) Kirsch, S. F. Org. Biomol. Chem. 2006, 4, 2076.
1163 (m), 1123 (m), 1021 (s), 744 (s), 703 (s) cmꢀ1
; dH (400 MHz,
2. Anticancer: (a) Rao, R. R.; Chaturvedi, V.; Babu, K. S.; Reddy, P. P.; Rao, V. R. S.;
Sreekanth, P.; Sreedhar, A. S.; Rao, J. M. Med. Chem. Res. 2012, 21, 634 Tubulin
polymerization inhibitor: (b) Kerr, D. J.; Hamel, E.; Jung, M. K.; Flynn, B. L. Bi-
oorg. Med. Chem. 2007, 15, 3290 Anti-microbial: (c) Khan, M. W.; Alam, M. J.;
Rashid, M. A.; Chowdhury, R. Bioorg. Med. Chem. 2005, 13, 4796; (d) Zanatta, N.;
Alves, S. H.; Coelho, H. S.; Borchhardt, D. M.; Machado, P.; Flores, K. M.; Silva, F.
M.; Spader, T. B.; Santurio, J. M.; Bonacorso, H. G.; Martins, M. A. P. Bioorg. Med.
Chem. 2007, 15, 1947.
3. (a) Paal, C. Chem. Ber. 1885, 18, 367; (b) Knorr, L. Chem. Ber. 1885, 18, 299; (c)
Chen, J.; Wu, H.; Zheng, Z.; Jin, C.; Zhang, X.; Su, W. Tetrahedron Lett. 2006, 47,
5383; (d) Ravindran, G.; Muthusubramanian, S.; Selvaraj, S.; Perumal, S. ARKI-
VOC 2007, 23; (e) Minetto, G.; Raveglia, L. F.; Sega, A.; Taddei, M. Eur. J. Org.
Chem. 2005, 5277; (f) Yavari, I.; Abbasinejad, M. A.; Alizadeh, A. Tetrahedron
Lett. 2002, 43, 4503.
CDCl3) 3.85 (s, 2H), 3.92 (s, 3H), 5.12 (s, 2H), 7.06 (s, 1H), 7.07 (s, 1H),
7.28e7.33 (m, 1H), 7.34e7.41 (m, 3H), 7.41e7.49 (m, 6H); dC
(100 MHz, CDCl3) 33.2, 56.5, 72.2, 95.9, 105.3, 120.1, 120.4, 127.7,
127.8, 128.0, 128.7, 128.9, 129.2, 132.0, 137.3, 144.8, 145.8, 149.2,
149.7, 175.5; MS (ESI) m/z (rel intensity) 389 (MHþ, 100), 367 (31),
359 (52), 345 (23), 343 (26), 320 (10), 267 (7); HRMS (ESI) m/z
found 389.1406 for C24H21O5 (MHþ), requires 389.1389.
4.2.27. 5-(Benzyloxy)-6-methoxy-2-methyl-3-phenylbenzofuran
(16). To a stirred solution of benzofuran carboxylic acid 15 (78 mg,
0.2 mmol) in dimethylacetamide (5 mL) were added CaO/NaOH
(9:1, 74 mg) and the resulting mixture was refluxed for 4 h. The
reaction mixture was cooled to ambient temperature and diluted
with ether. The organic phase was subsequently washed with water
(3ꢂ5 mL) and dried over anhyd Na2SO4. The combined organic
layers were concentrated in vacuo and the residue was purified by
silica gel column chromatography (1% EtOAc/petroleum ether).
White solid; yield 55 mg, 80%; mp 131e133 ꢁC; Rf 0.60 (20% ethyl
acetate/petroleum ether); nmax (KBr) 3063 (w), 2921 (s), 2850 (m),
1634 (m), 1486 (vs), 1455 (vs), 1310 (m), 1265 (vs), 1204 (s), 1185 (s),
4. (a) Feist, F. Chem. Ber. 1902, 35, 1537; (b) Benary, E. Chem. Ber. 1911, 44, 489; (c)
Mross, G.; Holtz, E.; Langer, P. J. Org. Chem. 2006, 71, 8045; (d) Ayyagari, N.; Jose,
D.; Mobin, S. M.; Namboothiri, I. N. N. Tetrahedron Lett. 2011, 52, 258.
5. (a) Hashmi, A. S. K.; Schwarz, L.; Choi, J.-H.; Frost, T. M. Angew. Chem., Int. Ed.
2000, 39, 2285; (b) Ma, S.; Zhang, J.; Lu, L. Chem.dEur. J. 2003, 9, 2447; (c)
Marshall, J. A.; Bartley, G. S. J. Org. Chem. 1994, 59, 7169; (d) Dudnik, A. S.;
Gevorgyan, V. Angew. Chem., Int. Ed. 2007, 46, 5195; (e) Aurrecoechea, J. M.;
Durana, A.; Perez, E. J. Org. Chem. 2008, 73, 3650; (f) Hashmi, A. S. K.; Haeffner,
T.; Rudolph, M.; Rominger, F. Eur. J. Org. Chem. 2011, 4, 667; (g) Hashmi, A. S. K.
€
Angew. Chem., Int. Ed. 1995, 34, 1581; (h) Hashmi, A. S. K.; Ruppert, T. L.; Knofel,
T.; Bats, J. W. J. Org. Chem. 1997, 62, 7295.
6. (a) Fluorescent probe: Aziz, M. A.; Auping, J. V.; Meador, M. A. J. Org. Chem. 1995,
60, 1303; (b) Photosensitizer: Frederiksen, P. K.; Jorgensen, M.; Ogilby, P. R. J.
Am. Chem. Soc. 2001, 123, 1215.
1164 (vs), 1091 (s), 806 (m), 738 (vs), 699 (m) cmꢀ1
; dH (400 MHz,
CDCl3) 2.51 (s, 3H), 3.94 (s, 3H), 5.13 (s, 2H), 7.06 (s, 1H), 7.09 (s, 1H),
7.30e7.35 (m, 1H), 7.36e7.44 (m, 5H), 7.45e7.50 (m, 4H); dC
(100 MHz, CDCl3) 13.0, 56.6, 72.3, 95.8, 105.1, 116.9, 120.7, 127.0,
127.8, 128.0, 128.6, 128.8, 128.9, 133.2, 137.5, 145.5, 148.4, 149.1,
150.4; MS (ESI) m/z (rel intensity) 345 (MHþ, 100), 331 (71), 317 (8),
295 (8), 276 (18), 262 (13), 254 (8), 236 (8), 91 (8); HRMS (ESI) m/z
found 345.1483 for C23H21O3 (MHþ), requires 345.1491.
7. (a) Garst, M. E.; Spencer, T. A. J. Am. Chem. Soc. 1973, 95, 250; (b) Garst, M. E.;
Spencer, T. A. J. Org. Chem. 1983, 48, 2442; (c) Song, Z. Z.; Ho, M. S.; Wong, H. N.
C. J. Org. Chem. 1994, 59, 3917; (d) Kurosu, M.; Marcin, L. R.; Kishi, Y. Tetrahedron
€
Lett. 1998, 39, 8929; (e) Katritzky, A. R.; Hur, D.; Kirichenko, K.; Ji, Y.; Steel, P. J.
ARKIVOC 2004, 109; (f) Kurosu, M.; Kishi, Y. J. Synth. Org. Chem., Jpn. 2004, 1205.
8. (a) Danheiser, R. L.; Gee, S. K. J. Org. Chem. 1984, 49, 1672; (b) Karlsson, J. O.;
Nguyen, N. V.; Foland, L. D.; Moore, H. W. J. Am. Chem. Soc. 1985, 107, 3392; (c)
Harrowven, D. C.; Pascoe, D. D.; Guy, I. L. Angew. Chem., Int. Ed. 2007, 46, 425; (d)
Mak, X. Y.; Crombie, A. L.; Danheiser, R. L. J. Org. Chem. 2011, 76, 1852.
9. (a) Nickl, J. Chem. Ber. 1958, 91, 553; (b) Bolzoni, L.; Casiraghi, G.; Casnati, G.;
Sartori, G. Angew. Chem., Int. Ed. 1978, 17, 684; (c) Choi, H. D.; Seo, P. J.; Son, B.
W.; Kang, B. W. Arch. Pharm. Res. 2004, 27, 19; (d) Liang, Y.; Tao, L. M.; Zhang, Y.
H.; Li, J. H. Synthesis 2008, 3988; (e) Yadav, A. K.; Singh, B. K.; Singh, N.; Tripathi,
R. P. Tetrahedron Lett. 2007, 48, 6628.
4.2.28. 6-Methoxy-2-methyl-3-phenylbenzofuran-5-ol (Isoparvifuran)
(17).20a To a stirred solution of decarboxylated benzofuran 16
(80 mg, 0.23 mmol) in dry THF (2 mL) was added 10% Pd/C (10 mg).

4972
T. Kumar et al. / Tetrahedron 69 (2013) 4964e4972
10. (a) Perkin, W. H. J. Chem. Soc. 1867, 20, 568; (b) Perkin, W. H. J. Chem. Soc. 1871,
24, 37; (c) Bowden, K.; Battah, S. J. Chem. Soc., Perkin Trans. 2 1998, 1603; (d)
Review: Johnson, I. R. Org. React. 1942, 1, 210.
11. (a) Rapp, E. Gazz. Chim. Ital. 1895, 25, 285; (b) Stoermer, R. Justus Liebigs Ann.
Chem. 1900, 312, 237; (c) Rao, M. L. N.; Awasthi, D. K.; Banerjee, D. Tetrahedron
Lett. 2007, 48, 431; (d) De Luca, L.; Nieddu, G.; Porcheddu, A.; Giacomelli, G.
Curr. Med. Chem. 2009, 16, 1; (e) Wang, Z.; Gu, J.; Jing, H.; Liang, Y. Synth.
Commun. 2009, 39, 4079.
Verkade, J. G.; Nagarajan, M.; Kisanga, P. B. Org. Lett. 2000, 2, 2409; (g) Park, K.
K.; Jeong, J. Tetrahedron 2005, 61, 545; (h) Kraus, G. A.; Schroeder, J. D. Synlett
2005, 2504; (i) From captodative olefins: Cruz, M. C.; Tamariz, J. Tetrahedron
Lett. 2004, 45, 2377; (j) Via intramolecular photocyclization: Park, K. K.; Seo, H.;
Kim, J. G.; Suh, I. H. Tetrahedron Lett. 2000, 41, 1393.
15. (a) Deb, I.; Dadwal, M.; Mobin, S. M.; Namboothiri, I. N. N. Org. Lett. 2006, 8,
1201; (b) Deb, I.; Shanbhag, P.; Mobin, S. M.; Namboothiri, I. N. N. Eur. J. Org.
Chem. 2009, 4091.
12. Hassner, A.; Namboothiri, I. Organic Syntheses Based on Name Reactions, 3rd ed.;
A Practical Guide to 750 Transformations Elsevier: Oxford, UK, 2012.
13. Transition metal catalyzed, review: (a) Zeni, G.; Larock, R. C. Chem. Rev. 2004,
104, 2285 Selected articles, oxidative cyclization: (b) Youn, S. W.; Eom, J. I. Org.
Lett. 2005, 6, 3355; (c) Satoh, T.; Tsuda, T.; Kushino, Y.; Miura, M.; Nomura, M. J.
Org. Chem. 1996, 61, 6476 Via propargylation of indoles and phenols: (d) Liu, Z.;
Lui, L.; Shafiq, Z.; Wu, Y. C.; Wang, D.; Chen, Y. J. Synthesis 2007, 13, 1961 Via
RCM: (e) van Otterlo, W. A. L.; Morgans, G. L.; Madeley, L. G.; Kuzvidza, S.;
Moleele, S. S.; Thornton, N.; de Koning, C. B. Tetrahedron 2005, 61, 7746 Via
intramolecular Heck reaction: (f) Xie, X.; Chen, B.; Lu, J.; Han, J.; She, X.; Pan, X.
Tetrahedron Lett. 2004, 45, 6235 From 3-silyloxy-1,5-enynes: (g) Hashmi, A. S.
K.; Yang, W.; Rominger, F. Angew. Chem., Int. Ed. 2011, 50, 5762; (h) Hashmi, A. S.
K.; Yang, W.; Rominger, F. Chem.dEur. J. 2012, 18, 6576.
16. (a) Furans and pyrans: Nair, D. K.; Mobin, S. M.; Namboothiri, I. N. N. Tetrahe-
dron Lett. 2012, 53, 3349; (b) Imidazopyridines: Nair, D. K.; Mobin, S. M.;
Namboothiri, I. N. N. Org. Lett. 2012, 14, 4580.
17. (a) Furans and pyrans: Huang, W. Y.; Chen, Y. C.; Chen, K. Chem.dAsian J. 2012,
7, 688; (b) Cyclopentenes: Yeh, L. F.; Anwar, S.; Chen, K. Tetrahedron 2012, 68,
7317.
18. MBH reaction, original reports: (a) Morita, K.; Suzuki, Z.; Hirose, H. Bull. Chem. Soc.
Jpn. 1968, 41, 2815; (b) Baylis, A. B.; Hillman, M. E. D. Ger. Offen. 1972, DE 2155113.
Selected recent reviews: (c) Basavaiah, D.; Veeraraghavaiah, G. Chem. Soc. Rev.
2012, 41, 68; (d) Shi, M.; Wang, F.; Zhao, M. X.; Wei, Y. The Chemistry of the
MoritaeBayliseHillman Reaction; Royal Society of Chemistry: 2011; (e) Declerck,
V.; Martinez, J.; Lamaty, F. Chem. Rev. 2009, 109, 1; (f) Singh, V.; Batra, S. Tetra-
hedron 2008, 64, 4511; (g) Kaur, K.; Namboothiri, I. N. N. Chimia 2012, 66, 913.
19. We thank one of the reviewers for suggesting this experiment.
20. Isolation and characterization: (a) Muangnoicharoen, N.; Frahm, A. W. Phyto-
chemistry 1981, 20, 291; (b) Donnelly, D. M. X.; Criodain, T. O.; O’Sollivan, M. J.
C. S. J. Chem. Soc., Chem. Commun. 1981, 1254 Partial synthesis; (c) Walia, S.;
Kulshrestha, S. K.; Mukerjee, S. K. Tetrahedron 1986, 42, 4817.
14. Other methods: (a) Via cyclofragmentation: Nicolaou, K. C.; Snyder, S. A.; Bigot,
A.; Pfefferkorn, J. A. Angew. Chem., Int. Ed. 2000, 39, 1093; (b) From
yphenyl ketones: Katritzky, A. R.; Ji, Y.; Fang, Y.; Prakash, I. J. Org. Chem. 2001,
66, 5613; (c) From naphthol/phenol and -haloketone: Jorgensen, M.; Krebs, F.
a-hydrox-
a
C.; Bechgaard, K. J. Org. Chem. 2000, 65, 8783; (d) Habermann, J.; Ley, S. V.;
Smits, R. J. Chem. Soc., Perkin Trans. 1 1999, 2421; (e) From N-(phenoxymethyl)-
benzotriazoles: Katritzky, A. R.; Serdyuk, L.; Xie, L. J. Chem. Soc., Perkin Trans. 1
1998, 1059; (f) From o-alkoxybenzoylarene derivatives: Kraus, G. A.; Zhang, N.;
21. While proceeding with the submission of this manuscript, a report on similar
transformation using Cs2CO3 in DCM has appeared: Anwar, S.; Huang, W.-Y.;
Chen, C.-H.; Cheng, Y.-S.; Chen, K. Chem.dEur. J. 2013, 19, 4344.