Asymmetric michael addition reactions between 3-substituted benzofuran-2(3H)-ones and 1,1-bis(phenylsulfonyl)ethylene catalyzed by bifunctional catalysts containing tertiary amine and thiourea groups

Article abstract of DOI:10.1002/ejoc.201101765

Highly enantioselective catalytic conjugate additions of 3-substituted benzofuran-2(3H)-ones to 1,1-bis(phenylsulfonyl)ethylene in the presence of catalysts based on Cinchona alkaloids and containing tertiary amine and thiourea groups have been developed.

Full text of DOI:10.1002/ejoc.201101765

FULL PAPER
DOI: 10.1002/ejoc.201101765
Asymmetric Michael Addition Reactions between 3-Substituted
Benzofuran-2(3H)-ones and 1,1-Bis(phenylsulfonyl)ethylene Catalyzed by
Bifunctional Catalysts Containing Tertiary Amine and Thiourea Groups
Xin Li,*[a] Yue-Yan Zhang,[a] Xiao-Song Xue,[a] Jia-Lu Jin,[a] Bo-Xuan Tan,[a] Cong Liu,[a]
Nan Dong,[a] and Jin-Pei Cheng*[a]
Keywords: Organocatalysis / Asymmetric catalysis / Michael addition / Hydrogen bonds
Highly enantioselective catalytic conjugate additions of 3-
substituted benzofuran-2(3H)-ones to 1,1-bis(phenylsulf-
onyl)ethylene in the presence of catalysts based on Cinchona
alkaloids and containing tertiary amine and thiourea groups
have been developed. Good to excellent stereoselectivities
(up to 99% ee) could be achieved. An interesting effect of
substituent positions on stereoselectivities was observed, and
a theoretical study to explain this phenomenon was under-
taken.
Benzofuran-2(3H)-ones and their derivatives have re-
ceived extensive attention because building blocks based on
compounds of this type with a quaternary chiral center at
the 3-position are a prominent feature in a number of natu-
ral products.[5] Furthermore, the corresponding natural
products exhibit biological activities ranging from antioxi-
dant properties to applicability in anticancer therapy and
have been viewed as potential medicines.[6] In this context,
considerable efforts have in recent years focused on the total
synthesis of the corresponding chiral benzofuran-2(3H)-
one-type compounds.[7] However, enantioselective synthesis
Introduction
Over the past decade, organocatalysis exploiting the use
of asymmetric hydrogen bonding,[1] especially by chiral bi-
functional catalysts containing combinations of thiourea/
urea and tertiary amine groups, has emerged as an area of
intense interest in asymmetric synthesis owing to its opera-
tional simplicity and the mild reaction conditions re-
quired.[2] In catalysts of this class, the thiourea/urea and
tertiary amino groups function cooperatively as hydrogen-
bond donors and acceptors, respectively; this enables the
simultaneous activation of a nucleophile and an electrophile
in a suitable reaction direction, thereby leading to desirable
stereochemical yields. Of the many asymmetric reactions
promoted by bifunctional systems based on tertiary amine
and thiourea/urea groups that have been developed,
Michael addition reactions dominate markedly.[3] Several
impressive Michael acceptors, such as nitro olefins, chalc-
ones, MVK, α,β-unsaturated imides and imines, etc., which
can be activated through the double hydrogen-bonding in-
teraction of the N–H of thiourea/urea to fulfill specific roles
in efficient enantiocontrol, have been successfully applied.
Corresponding applications of compounds of the vinyl sulf-
one type, especially 1,1-bis(phenylsulfonyl)ethylene, which
has four S=O functionalities, however, are comparatively
limited.[4] The development of new and efficient asymmetric
organocatalytic conjugate addition reactions in which 1,1-
bis(phenylsulfonyl)ethylene is used as a Michael acceptor is
therefore still of significant importance.
[a] The State Key Laboratory of Elemento-Organic Chemistry,
Department of Chemistry, Nankai University,
Tianjin 300071, P. R. China
Fax: +86-22-23499147
Scheme 1. Strategy for Michael addition of 3-substituted benzo-
furan-2(3H)-ones to 1,1-bis(phenylsulfonyl)ethylene catalyzed by a
bifunctional tertiary amine/thiourea system.
E-mail: xin_li@nankai.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101765.
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Eur. J. Org. Chem. 2012, 1774–1782

Asymmetric Michael Additions of 3-Substituted Benzofuran-2(3H)-ones
of such significant chiral benzofuran-2(3H)-ones remains a
Results and Discussion
considerable challenge. As a result, only a few examples of
organocatalytic methodologies for the construction of chi-
ral quaternary centers in benzofuran-2(3H)-ones
(Scheme 1) are to be found in the literature.[8] Our group
reported enantioselective conjugate addition reactions of
benzofuran-2(3H)-ones to several α,β-unsaturated carbonyl
compounds in the presence of chiral thioureas as cata-
lysts.[8a–8c] Melchiorre devised three distinct amino-catalytic
cascade reactions leading to enantiomerically pure spirocy-
clic benzofuranones.[8e] Ma and co-workers described
highly enantioselective aminations of benzofuranones cata-
lyzed by a binol-derived P-spiro quaternary phosphonium
salt.[8f] Very recently, our group achieved highly diastereo-
and enantioselective asymmetric allylic alkylation reactions
with prochiral 3-substituted benzofuran-2(3H)-ones and
Morita–Baylis–Hillman (MBH) carbonate in the presence
of a chiral biscinchona alkaloid catalyst.[8d]
We initiated our investigations with 3-phenylbenzofuran-
2(3H)-one (1a, Table 1) and 1,1-bis(phenylsulfonyl)ethylene
(2) in the presence of Takemoto’s catalyst 4a (10 mol-%)
in CH2Cl2 at room temperature. As expected, the Michael
product 3a was obtained almost quantitatively, but the
enantioselectivity was poor (Entry 1 in Table 1). The four
other widely used bifunctional catalysts 4b–4e (Figure 1),
with tertiary amine and thiourea/urea groups and different
chiral scaffolds, were then screened in the model reaction.
To our delight, all of the chiral bifunctional hydrogen-
bonding catalysts 4b–4e exhibited high catalytic activities,
with catalyst 4d found to give the optimal enantioselectivity
(99% yield and 49% ee, Entry 4 in Table 1). Moreover, cata-
lysts 4f and 4g of the Cinchona alkaloid type were also ex-
amined, although the corresponding catalytic results were
disappointing (Entries 6–7 in Table 1).
In continuation of our recent research program involving
a new strategy for the synthesis of chiral 3,3Ј-disubstituted
benzofuran-2(3H)-ones with the aid of organocataly-
sis[8a–8d] and based on our experience with use of bifunc-
tional catalysts containing tertiary amine and thiourea
groups,[8a–8c,9] here we report highly enantioselective
Michael additions of 3-substituted benzofuran-2(3H)-ones
to 1,1-bis(phenylsulfonyl)ethylene promoted by bifunctional
tertiary amine/thiourea catalyst systems (Figure 1). As a re-
sult, a number of 3-substituted benzofuran-2(3H)-ones were
obtained by this synthetic strategy. A DFT theoretical cal-
culation was also included in this study with the aim of
explaining the observed effects of the positions of the sub-
stituents on stereoselectivity.
Table 1. Optimization of reaction conditions.[a]
Entry Cat.
Solvent
Time [h] Yield[b] [%] ee[c] [%]
1
2
3
4
5
6
7
8
4a
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
Cl(CH2)2Cl
toluene
benzene
xylene
2
2
2
2
2
2
2
2
2
2
2
2
2
24
48
48
95
96
90
99
87
90
91
98
95
93
96
95
90
97
96
90
6
1
4
4b
4c
4d
49
–26
–6
–4
47
38
29
20
21
16
70
82
59
4e
4f
4g
4d
9
4d
10
11
12
13
14
15
16
4d
4d
4d
4d
THF
4d[d]
4d[e]
4d[f]
CH2Cl2
CH2Cl2
CH2Cl2
[a] The reactions were carried out on a 0.1 mmol scale in 1 mL
solvent at room temperature and with a 1a/2 molar ratio of 1.3:1.
[b] Isolated yields. [c] Determined by chiral HPLC. [d] The reaction
was carried out on a 0.1 mmol scale in CH2Cl2 (1 mL) at –40 °C
and with a 1a/2 molar ratio of 1.3:1. [e] The reaction was carried
out on a 0.1 mmol scale in CH2Cl2 (1 mL) at –80 °C and with a
1a/2 molar ratio of 1.3:1. [f] The reaction was carried out on a
0.1 mmol scale in CH2Cl2 (1 mL) with molecular sieves (4 Å) at
–80 °C and with a 1a/2 molar ratio of 1.3:1.
The solvent optimization process found that the initially
used CH2Cl2 was the most suitable solvent for enantio-
selectivity (Entries 8–13 in Table 1). Further improvement
could be achieved by lowering the reaction temperature
(Entries 14–15 in Table 2), but addition of molecular sieves
(4 Å) to the reaction mixture decreased the enantio-
selectivity (Entry 16 in Table 2). Collectively, the best result
Figure 1. Catalysts examined.
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X. Li, J.-P. Cheng et al.
FULL PAPER
with respect to yield and enantioselectivity was obtained by
performing the reaction in the presence of 4d (10 mol-%)
at –80 °C in CH2Cl2. Under these conditions, the reaction
provided the desired product with 96% yield and 82% ee.
Table 2. Substrate scope.[a]
Entry R1
R2
Time [h]
Yield[b] [%]
ee[c] [%]
Figure 2. Results of Michael additions of 7(6)-substituted 3-phenyl-
benzofuran-2(3H)-ones and 1,1-bis(phenylsulfonyl)ethylene under
optimized conditions after 24 h.
1
2
3
H
MeO
Et
H
H
H
H
H
H
H
H
Cl
Cl
Cl
48
44
36
40
40
24
40
20
32
40
48
3a: 95
3b: 99
3c: 90
3d: 99
3e: 99
3f: 98
3g: 98
3h: 92
3i: 99
3j: 99
3k: 95
82
71
83
4
5
6
7
Me
tBu
Ph
Br
71(99)[d]
75
85
50
3-Benzylbenzofuran-2(3H)-one and 3-methylbenzofuran-
2(3H)-one were also examined as nucleophiles.[10] The cor-
responding products 3r and 3s were obtained in very good
yields with moderate enantioselectivities (Figure 3). In ad-
dition, the Michael reaction between 3-phenylbenzofuran-
2(3H)-one and (vinylsulfonyl)benzene, which has only one
activating substituent, was also attempted.[11] Not unexpec-
tedly, the reactivity and enantioselectivity were both less
than in the case of the Michael addition between 3-phenyl-
benzofuran-2(3H)-one and 1,1-bis(phenylsulfonyl)ethylene.
Only a 48% yield of the desired product 3t with a 24% ee
were obtained under the optimized conditions at room tem-
perature after 7 d (Figure 3).
8
9
I
H
76(95)[d]
45
10
11
Me
Et
77
64
[a] The reactions were carried out on a 0.1 mmol scale in CH2Cl2
(1 mL) at –80 °C and with a 1/2 molar ratio of 1.3:1. [b] Isolated
yields. [c] Determined by chiral HPLC. [d] Data in parentheses
correspond to single recrystallizations.
With the optimal conditions in hand, the substrate scope
was next explored. Firstly, eight 5-substituted 3-phenyl-
benzofuran-2(3H)-ones were examined. As shown in
Table 2 (Entries 1–8), the reactions were shown to work well
with a range of 5-substituted 3-phenylbenzofuran-2(3H)-
ones bearing either electronic-withdrawing or -donating
groups to give the desired products with very good yields
(90–99%) and moderate to very good enantioselectivities
(50–85% ee). Next, the differently substituted 3-arylbenzo-
furan-2(3H)-ones 1i–1k were examined in the same system.
As shown in Table 2 (Entries 9–11), the desired Michael
products 3i–3k were obtained with excellent yields (95–
99%) and moderate to good enantioselectivities (45–
77% ee). It is worth noting that the stereoselectivities of the
Michael products can be easily improved by single
recrystallizations (Entries 4 and 8 in Table 2).
On further investigation of the substrate scope, we found
an interesting effect of substituents’ positions on stereose-
lectivity. Higher selectivity was obtained when 6-methyl-3-
phenylbenzofuran-2(3H)-one was selected as Michael do-
nor (Figure 2, 3l), whereas excellent enantioselectivity was
obtained with a methyl substituent at the 7-position of
benzofuran-2(3H)-one (Figure 2, 99% ee for 3m). It is clear
that the stereoselectivity of the reaction is sensitive to the
substitution pattern on the benzofuran ring. Further expan-
sion of the scope of 7-substituted 3-phenylbenzofuran-
2(3H)-ones as Michael donors validated this observation,
with the corresponding Michael products 3n–3q being ob-
tained in 93–96% yields with 90–97% ee values (Figure 2).
Figure 3. Results of Michael addition reactions between 3-alkyl-
benzofuran-2(3H)-ones and 1,1-bis(phenylsulfonyl)ethylene and
between 3-phenylbenzofuran-2(3H)-one and (vinylsulfonyl)benz-
ene.
We obtained the X-ray crystal structure of product 3h
(Figure 4),[12] which established the absolute configuration
for 3h. The absolute configurations of other products can
therefore be inferred by analogy.
Figure 4. X-ray crystal structure of 3h.
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Asymmetric Michael Additions of 3-Substituted Benzofuran-2(3H)-ones
To investigate the large-scale synthetic potential of this
Michael strategy, 3-phenylbenzofuran-2(3H)-one (1a,
4 mmol, 0.84 g) and 1,1-bis(phenylsulfonyl)ethylene (2,
3 mmol, 0.93 g) were treated under the optimized condi-
tions and the corresponding product was obtained without
any decrease in yield (1.49 g, 96% yield) or selectivities
(82% ee, Figure 5). In addition, further derivation of the
Michael product was also attempted.[7a,8f] We try to convert
3a into
a chiral 3-alkylbenzofuranone product re-
ductively,[4h] but no desired product was isolated. The reac-
tion did not proceed cleanly under Lu’s conditions,[4h] un-
der which a similar Michael product of oxindole and 1,1-
bis(phenylsulfonyl)ethylene was successfully converted into
a useful chiral 3-alkyl oxindole. We hypothesized that the
lactone structure of the benzofuranone-type compound was
unstable under the corresponding reductive conditions.
Figure 6. The interaction of 1,1-bis(phenylsulfonyl)ethylene with
the catalyst in TS3a and TS3m.
Figure 5. Reaction carried out on a large scale.
With the aim of explaining the origin of the observed
stereoselectivity and why the presence of a methyl group
at the 7-position in 3-phenylbenzofuran-2(3H)-one leads to
significantly enhanced stereoselectivity, we carried out de-
tailed computational studies.[13]
According to the dual activation model proposed as a
result of previous study,[2g–2i,16] the two substrates involved
in the reaction are activated simultaneously by the bifunc-
tional thiourea catalyst as shown in Scheme 1. Unlike in the
proposed transition-state model of Michael additions of 3-
substituted oxindoles and 1,1-bis(phenylsulfonyl)ethylene
studied by Lu,[4h] 1,1-bis(phenylsulfonyl)ethylene was as-
sumed to interact with the thiourea moiety through H-
bonds, enhancing in this way the electrophilic character of
the reacting carbon center (Figure 6). It is worth noting that
the two phenylsulfonyl groups in the electrophile are very
important for the reactivity and enantioselectivity, through
the formation of multiple H-bonds between 2 and the thio-
urea functional group of 4d. This can also be shown by the
poor result of the catalyzed Michael addition between 3-
phenylbenzofuran-2(3H)-one and the 1,1-bis(phenylsulf-
onyl)ethylene counterpart in which the electrophile has only
one phenylsulfonyl group (see 3t in Figure 3). On the other
hand, the enolic forms of the benzofuran-2(3H)-ones are
assumed to interact with the tertiary amine group, with
Figure 7. The interaction of benzofuranones with the catalyst in
TS3a and TS3m.
Transition-state structures based on the above model for
subsequent deprotonation resulting in highly nucleophilic the C–C bond-forming step in the reactions between 3-sub-
enolate species (Figure 7). In this model, the C–C bond for- stituted benzofuran-2(3H)-ones (1a and 1m) and 1,1-bis-
mation step thus takes place through the formation of a (phenylsulfonyl)ethylene are given in Figure 8. Accordingly,
multiple H-bonded complex and the enantioselectivity of TS3a(s) is 0.8 kcalmol–1 higher in free energy than TS3a(r),
the reaction is governed by the binding mode of both sub- giving a predicted 73% ee at –80 °C. This result is close to
strates to the bifunctional thiourea catalyst 4d.
the experimentally observed 82% ee for the Michael prod-
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X. Li, J.-P. Cheng et al.
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uct 3a. It is clear to see that the R stereoisomer was the 2(3H)-one compounds containing all-carbon-substituted
main product of our asymmetric Michael addition, which quaternary stereocenters. This methodology substantially
is consistent with the experimental results. Favorable elec- broadens benzofuranone chemistry and offers highly func-
trostatic interaction between the partial positive hydrogen tionalized chiral products. Furthermore, a computational
atoms on the methylene group and the oxygen atom of the study directed towards explaining the origin of the observed
furan ring (δ+N–C–H···O) would contribute to stabilization stereoselectivity and why the presence of a methyl group at
of TS3a(r), which might account for the origin of the ob- the 7-position in a 3-phenylbenzofuran-2(3H)-one system
served stereoselectivity.[17]
leads to significantly enhanced stereoselectivity was also
carried out. More endeavors demonstrating further devel-
opments and reactions of the Michael products are in pro-
gress in our laboratory.
Experimental Section
General Information: Commercial reagents were used as received,
unless otherwise stated. 1H and 13C NMR were recorded with a
Bruker DPX 400 spectrometer. Chemical shifts are reported in ppm
from tetramethylsilane with the solvent resonance as the internal
standard. The following abbreviations were used to designate chem-
ical shift mutiplicities: s = singlet, d = doublet, t = triplet, q =
quartet, h = heptet, m = multiplet, br. = broad. All first-order
splitting patterns were assigned on the basis of the appearance of
the multiplet. Splitting patterns that could not be easily interpreted
are designated as multiplet (m) or broad (br). Mass spectra were
obtained with an electron ionization (ESI) mass spectrometer. Cat-
alysts 4a,[2g] 4b,[9a] 4c,[2h] 4d,[2k] and 4e[2l] were prepared by the lit-
erature procedure. Catalysts 4f and 4g were commercially available
reagents.
General Experimental Michael Addition Procedure: The bifunc-
tional thiourea catalyst (0.01 mmol) was added at –80 °C to a
stirred solution of
a
3-substituted benzofuran-2(3H)-one
(0.13 mmol) and 1,1-bis(phenylsulfonyl)ethylene (0.1 mmol) in dry
CH2Cl2 (1 mL). After the reaction had gone to completion, the
reaction solution was concentrated in vacuo and the crude residue
was purified by flash chromatography to afford the product.
Figure 8. Computed transition-state structures and relative free en-
ergies (kcalmol–1) of reactions between 3-substituted benzofuran-
2(3H)-ones (1a and 1m) and 1,1-bis(phenylsulfonyl)ethylene.
On the other hand, the computed free energy difference
between the TS3m(r) and TS3m(s) was found to be as high
as 4.9 kcalmol–1, giving a predicted Ͼ99% ee at –80 °C,
which is in good agreement with the experimental observa-
tion that the presence of a methyl group at the 7-position
Compound 3a: The Michael product was synthesized by the Gene-
ral Procedure as a white solid in 95% overall yield; m.p. 173–
175 °C. [α]2D0 = –50.1 (c = 1.0, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 7.83 (dd, J = 21.84, 7.80 Hz, 4 H), 7.70–7.68 (m, 2
H), 7.57–7.48 (m, 5 H), 7.35–7.25 (m, 8 H), 4.54 (t, J = 4.49 Hz, 1
in the 3-phenylbenzofuran-2(3H)-one system leads to sig- H), 3.34 (d, J = 4.60 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 176.9, 154.2, 139.8, 137.5, 136.1, 134.9, 130.5, 129.6, 129.2,
nificantly enhanced stereoselectivity. On careful examina-
tion of the transition state structures, we found that the
distance between the methyl group and the F atom in
TS3m(s) (Figure 4) is only 2.83 Å, which is shorter than the
sum of their van der Waals radii. The steric repulsion be-
tween the methyl group and the CF3 group would thus de-
stabilize the TS3m(s), leading to the enhanced stereoselecti-
vity.
128.5, 127.6, 126.4, 126.1, 124.7, 80.4, 54.1, 32.8 ppm. IR (KBr):
ν = 1799, 1456, 1323, 1124, 827 cm–1. HRMS (ESI+): calcd. for
˜
[C28H22O6S2Na]+ 541.0750; found 541.0747. The enantiomeric ex-
cess was determined by HPLC with an OD-H column at 210 nm
(propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR = 35.5 min (minor),
41.9 min (major).
Compound 3b: The Michael product was synthesized by the Gene-
ral Procedure as a colorless oil in 99% overall yield. [α]2D0 = –49.9
(c = 1.0, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.96 (d, J =
7.37 Hz, 2 H), 7.83 (d, J = 7.54 Hz, 2 H), 7.77–7.55 (m, 7 H), 7.36–
7.21 (m, 5 H), 7.04 (dd, J = 8.66, 2.70 Hz, 1 H), 6.98 (d, J =
2.60 Hz, 1 H), 4.57 (q, J = 3.31 Hz, 1 H), 3.90 (s, 3 H), 3.40–3.29
(m, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.8, 156.9,
147.7, 139.6, 137.7, 135.8, 134.7, 130.7, 129.6, 129.5, 129.2, 129.1,
128.9, 128.5, 128.3, 128.2, 126.4, 126.2, 126.1, 121.5, 115.5, 112.6,
Conclusions
We have developed conjugate additions between benzo-
furan-2(3H)-ones and 1,1-bis(phenylsulfonyl)ethylene that
proceed with high yields and enantioselectivities when cata-
lyzed by tertiary amine/thiourea systems based on Cinchona
alkaloids. The reaction scope is substantial, and a number
of 3-substituted benzofuran-2(3H)-ones have been success-
111.7, 111.4, 80.4, 56.1, 49.3, 32.4 ppm. IR (KBr): ν = 1789, 1489,
˜
1446, 1288, 1125, 1074, 825 cm–1. HRMS (ESI+): calcd. for
[C29H24O7S2Na]+ 571.0856; found 571.0854. The enantiomeric ex-
fully applied, giving multifunctional chiral benzofuran- cess was determined by HPLC with an OD-H column at 210 nm
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Asymmetric Michael Additions of 3-Substituted Benzofuran-2(3H)-ones
(propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR = 37.0 min (minor),
40.4 min (major).
7.80–7.72 (m, 2 H), 7.67–7.60 (m, 5 H), 7.49 (d, J = 1.73 Hz, 1 H),
7.36–7.35 (m, 3 H), 7.29–7.27 (m, 2 H), 7.19 (d, J = 8.59 Hz, 1 H),
4.47 (dd, J = 7.31, 3.03 Hz, 1 H), 3.42–3.27 (m, 2 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 175.7, 152.8, 138.9, 137.8, 135.3,
135.2, 134.8, 133.5, 131.0, 129.9, 129.5, 129.3, 129.2, 128.8, 128.6,
Compound 3c: The Michael product was synthesized by the General
Procedure as a white solid in 78% overall yield; m.p. 181–183 °C.
1
[α]2D0 = –51.8 (c = 1.0, CHCl3). H NMR (400 MHz, CDCl3): δ =
126.2, 117.2, 113.8, 80.4, 54.5, 32.4 ppm. IR (KBr): ν = 1799, 1467,
˜
7.92 (d, J = 7.44 Hz, 2 H), 7.82 (d, J = 7.44 Hz, 2 H), 7.73 (dd, J
= 17.31, 7.46 Hz, 2 H), 7.62–7.54 (m, 4 H), 7.37–7.28 (m, 6 H),
7.23–7.21 (m, 2 H), 4.58 (dd, J = 7.32, 3.01 Hz, 1 H), 3.42–3.30
(m, 2 H), 2.78 (q, J = 7.51 Hz, 2 H), 1.35 (t, J = 7.41 Hz, 3 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 176.8, 152.1, 140.9, 139.9, 137.7,
135.8, 134.9, 134.7, 130.9, 130.7, 129.8, 129.4, 129.1, 129.0, 128.4,
127.3, 126.4, 125.3, 111.8, 80.4, 54.4, 32.5, 28.7, 16.1 ppm. IR
1446, 1317, 1230, 1153, 1132, 995, 817 cm–1. HRMS (ESI+): calcd.
for [C28H21BrO6S2Na]+ 618.9855; found 618.9850. The enantio-
meric excess was determined by HPLC with an OD-H column at
210 nm (propan-2-ol/hexane 1:19), 1.0 mLmin–1; tR = 46.5 min
(minor), 57.1 min (major).
Compound 3h: The Michael product was synthesized by the Gene-
ral Procedure as a white solid in 92% overall yield; m.p. 179–
181 °C. [α]2D0 = –16.2 (c = 1.0, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 8.02 (d, J = 7.50 Hz, 2 H), 7.88 (d, J = 7.43 Hz, 2 H),
7.81 (dd, J = 8.36, 1.77 Hz, 1 H), 7.77–7.59 (m, 7 H), 7.34–7.24
(m, 5 H), 7.06 (d, J = 8.52 Hz, 1 H), 4.44 (dd, J = 7.35, 3.01 Hz,
1 H), 3.40–3.23 (m, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
175.6, 153.6, 139.5, 139.0, 137.8, 135.2, 134.8, 134.3, 131.0, 130.3,
129.5, 129.4, 129.3, 129.2, 128.8, 126.2, 114.3, 80.4, 54.3, 32.2 ppm.
(KBr): ν = 1791, 1456, 1338, 1286, 1132, 1078, 827 cm–1. HRMS
˜
(ESI+): calcd. for [C30H26O6S2Na]+ 569.1063; found 569.1065. The
enantiomeric excess was determined by HPLC with an OD-H col-
umn at 210 nm (propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR
=
23.5 min (minor), 26.8 min (major).
Compound 3d: The Michael product was synthesized by the Gene-
ral Procedure as a white solid in 99% overall yield; m.p. 182–
185 °C. [α]2D0 = –45.1 (c = 1.0, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 7.94 (d, J = 7.45 Hz, 2 H), 7.84 (d, J = 7.46 Hz, 2 H),
7.73 (dd, J = 17.35, 7.41 Hz, 2 H), 7.63–7.56 (m, 4 H), 7.37–7.28
(m, 6 H), 7.20–7.15 (m, 2 H), 4.58 (dd, J = 7.32, 3.01 Hz, 1 H),
3.40–3.29 (m, 2 H), 2.49 (s, 3 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 176.8, 151.9, 139.9, 137.8, 135.8, 134.9, 134.7, 134.3,
130.9, 130.7, 129.5, 129.1, 129.0, 128.4, 127.4, 126.4, 126.1, 111.7,
IR (KBr): ν = 1807, 1446, 1317, 1228, 1134, 995, 818 cm–1. HRMS
˜
(ESI+): calcd. for [C28H21IO6S2Na]+ 666.9716; found 666.9724. The
enantiomeric excess was determined by HPLC with an OD-H col-
umn at 210 nm (propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR
=
28.6 min (minor), 34.1 min (major).
Compound 3i: The Michael product was synthesized by the General
Procedure as a colorless oil in 99% overall yield. [α]2D0 = –41.5 (c =
1.0, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.85–7.79 (m, 4 H),
7.71 (dd, J = 14.67, 7.22 Hz, 2 H), 7.59–7.50 (m, 5 H), 7.39–7.21
(m, 7 H), 4.51 (t, J = 5.31 Hz, 1 H), 3.28 (d, J = 5.19 Hz, 2 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 176.1, 153.9, 138.1, 137.4, 135.9,
135.0, 134.8, 134.7, 130.7, 130.5, 129.5, 129.3, 129.2, 129.1, 127.9,
80.5, 54.3, 32.6, 21.3 ppm. IR (KBr): ν = 1792, 1485, 1446, 1331,
˜
1153, 1080, 1002, 819 cm–1. HRMS (ESI+): calcd. for
[C29H24O6S2Na]+ 555.0907; found 555.0908. The enantiomeric ex-
cess was determined by HPLC with an OD-H column at 210 nm
(propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR = 26.3 min (minor),
33.2 min (major).
127.1, 125.9, 124.8, 112.2, 80.2, 53.7, 32.8 ppm. IR (KBr): ν = 1789,
˜
Compound 3e: The Michael product was synthesized by the General
1489, 1456, 1334, 1319, 1134, 1024, 802 cm–1. HRMS (ESI+): calcd.
for [C28H21ClO6S2Na]+ 575.0360; found 575.0364. The enantio-
meric excess was determined by HPLC with an OD-H column at
210 nm (propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR = 33.6 min
(minor), 39.7 min (major).
Procedure as a white solid in 99% overall yield; m.p. 201–204 °C.
[α]2D0 = –37.8 (c = 1.0, CHCl3). H NMR (400 MHz, CDCl3): δ =
1
7.82 (d, J = 7.40 Hz, 2 H), 7.85 (d, J = 7.46 Hz, 2 H), 7.69 (dd, J
= 17.30, 7.40 Hz, 2 H), 7.57–7.49 (m, 6 H), 7.33–7.21 (m, 6 H),
4.54 (dd, J = 7.30, 3.01 Hz, 1 H), 3.41–3.29 (m, 2 H), 1.40 (s, 9
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.8, 151.9, 148.1,
139.9, 137.5, 135.9, 134.9, 134.7, 130.5, 129.4, 129.1, 128.4, 127.4,
126.7, 126.5, 123.2, 111.3, 80.3, 54.6, 34.9, 32.3, 31.7 ppm. IR
Compound 3j: The Michael product was synthesized by the General
Procedure as a white solid in 99% overall yield; m.p. 180–182 °C.
[α]2D0 = –37.3 (c = 1.0, CHCl3). H NMR (400 MHz, CDCl3): δ =
1
(KBr): ν = 1809, 1489, 1446, 1327, 1176, 1155, 1068, 821 cm–1.
˜
7.92 (d, J = 7.47 Hz, 2 H), 7.83 (d, J = 7.53 Hz, 2 H), 7.74 (dd, J
= 16.33, 7.71 Hz, 2 H), 7.62–7.56 (m, 4 H), 7.34–7.12 (m, 7 H),
4.56 (t, J = 5.00 Hz, 1 H), 3.29 (d, J = 5.02 Hz, 2 H), 2.49 (s, 3
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.4, 151.8, 138.3,
137.7, 135.8, 135.0, 134.8, 134.6, 134.5, 131.1, 130.7, 129.4, 139.3,
129.2, 129.1, 127.9, 127.0, 126.0, 111.9, 80.3, 53.8, 32.7, 21.3 ppm.
HRMS (ESI+): calcd. for [C32H30O6S2Na]+ 597.1376; found
597.1369. The enantiomeric excess was determined by HPLC with
an OD-H column at 210 nm (propan-2-ol/hexane 1:19),
1.0 mLmin–1; tR = 31.7 min (major), 35.6 min (minor).
Compound 3f: The Michael product was synthesized by the General
IR (KBr): ν = 1791, 1506, 1417, 1338, 1132, 823 cm–1. HRMS
˜
Procedure as a white solid in 98% overall yield; m.p. 192–194 °C.
(ESI+): calcd. for [C29H23ClO6S2Na]+ 589.0517; found 589.0509.
The enantiomeric excess was determined by HPLC with an OD-
[α]2D0 = –43.0 (c = 1.0, CHCl3). H NMR (400 MHz, CDCl3): δ =
1
8.04 (d, J = 7.46 Hz, 2 H), 7.78–7.29 (m, 21 H), 4.51 (dd, J =
4.50, 2.27 Hz, 1 H), 3.50–3.32 (m, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 176.5, 153.4, 139.8, 139.5, 138.2, 137.7, 135.5, 135.1,
134.6, 130.9, 129.3, 129.2, 129.1, 128.6, 127.9, 127.8, 127.1, 126.5,
H column at 210 nm (propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR
=
26.3 min (minor), 33.9 min (major).
Compound 3k: The Michael product was synthesized by the Gene-
ral Procedure as a colorless oil in 95% overall yield. [α]2D0 = –79.4
(c = 1.0, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.85 (d, J =
7.51 Hz, 2 H), 7.78 (d, J = 7.52 Hz, 2 H), 7.70 (dd, J = 16.82,
7.78 Hz, 2 H), 7.58–7.52 (m, 4 H), 7.35–7.17 (m, 7 H), 4.54 (t, J =
5.00 Hz, 1 H), 3.28 (d, J = 4.92 Hz, 2 H), 2.76 (q, J = 7.49 Hz, 2
H), 1.32 (t, J = 7.68 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 176.4, 152.0, 141.1, 138.3, 137.6, 135.9, 134.9, 134.7, 134.6,
130.6, 129.9, 129.4, 129.3, 129.2, 129.1, 127.9, 126.9, 125.1, 111.9,
124.7, 112.3, 80.5, 54.5, 32.4 ppm. IR (KBr): ν = 1801, 1473, 1319,
˜
1263, 1078, 837 cm–1. HRMS (ESI+): calcd. for [C34H26O6S2Na]+
617.1063; found 617.1060. The enantiomeric excess was determined
by HPLC with an OD-H column at 210 nm (propan-2-ol/hexane
1:19), 1.0 mLmin–1; tR = 48.2 min (minor), 51.5 min (major).
Compound 3g: The Michael product was synthesized by the Gene-
ral Procedure as a white solid in 98% overall yield; m.p. 173–
176 °C. [α]2D0 = –19.9 (c = 1.0, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 8.04 (d, J = 7.57 Hz, 2 H), 7.90 (d, J = 7.46 Hz, 2 H),
80.2, 53.9, 32.6, 28.7, 16.0 ppm. IR (KBr): ν = 1816, 1489, 1448,
˜
Eur. J. Org. Chem. 2012, 1774–1782
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1779

X. Li, J.-P. Cheng et al.
1309, 1159, 1078, 827 cm–1. HRMS (ESI+): calcd. for (c = 0.5, CHCl3). H NMR (400 MHz, CDCl3): δ = 7.83–7.66 (m,
FULL PAPER
1
[C30H25ClO6S2Na]+ 603.0673; found 603.0680. The enantiomeric
excess was determined by HPLC with an OD-H column at 210 nm
(propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR = 22.1 min (minor),
27.4 min (major).
6 H), 7.57–7.51 (m, 4 H), 7.37–7.25 (m, 7 H), 7.13 (dd, J = 7.37,
1.24 Hz, 1 H), 4.58 (dd, J = 3.20, 3.41 Hz, 1 H), 3.40–3.27 (m, 2
H), 3.03–3.00 (m, 1 H), 1.79–1.60 (m, 2 H), 1.30 (d, J = 6.99 Hz,
3 H), 0.85 (td, J = 4.85, 2.78 Hz, 3 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 176.6, 152.0, 151.9, 140.2, 137.5, 136.4, 134.8, 134.7,
132.2, 130.4, 129.6, 129.1, 128.9, 128.4, 128.3, 127.1, 126.4, 126.3,
124.6, 123.2, 123.1, 80.5, 54.3, 35.6, 32.9, 29.7, 29.5, 20.2, 19.8,
Compound 3l: The Michael product was synthesized by the General
Procedure as a white solid in 99% overall yield; m.p. 193–195 °C.
[α]2D0 = –46.7 (c = 0.9, CHCl3). H NMR (400 MHz, CDCl3): δ =
1
12.1 ppm. IR (KBr): ν = 1803, 1446, 1334, 1153, 1078, 885 cm–1.
˜
7.89 (d, J = 7.50 Hz, 2 H), 7.81 (d, J = 7.59 Hz, 2 H), 7.70 (dd, J
= 16.62, 7.71 Hz, 2 H), 7.59–7.52 (m, 4 H), 7.29–7.10 (m, 8 H),
4.55 (t, J = 5.36 Hz, 1 H), 3.31 (d, J = 4.90 Hz, 2 H), 2.48 (s, 3
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.7, 154.1, 141.2,
140.0, 137.7, 136.0, 134.9, 134.7, 130.6, 129.5, 129.1, 129.0, 128.4,
126.4, 125.7, 125.3, 124.3, 112.5, 80.5, 54.1, 32.8, 21.8 ppm. IR
HRMS (ESI+): calcd. for [C32H30O6S2Na]+ 597.1376; found
597.1377. The enantiomeric excess was determined by HPLC with
an OD-H column at 210 nm (propan-2-ol/hexane 1:9),
1.0 mLmin–1; tR = 12.3 min (minor), 17.1 min (major).
Compound 3q: The Michael product was synthesized by the Gene-
ral Procedure as a white solid in 95% overall yield; m.p. 203–
206 °C. [α]2D0 = –40.3 (c = 0.4, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 7.83–7.64 (m, 6 H), 7.57–7.48 (m, 6 H), 7.30–7.20 (m,
6 H), 4.58 (dd, J = 3.17, 3.35 Hz, 1 H), 3.42–3.28 (m, 2 H), 1.40
(s, 9 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.7, 151.9,
148.1, 140.0, 137.7, 136.1, 134.8, 134.6, 130.4, 129.4, 129.1, 129.0,
128.4, 127.3, 126.8, 126.4, 123.3, 111.2, 80.4, 54.6, 34.9, 32.4,
(KBr): ν = 1801, 1489, 1446, 1338, 1068, 947, 825 cm–1. HRMS
˜
(ESI+): calcd. for [C29H24O6S2Na]+ 555.0907; found 555.0913. The
enantiomeric excess was determined by HPLC with an OD-H col-
umn at 210 nm (propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR
=
29.9 min (minor), 34.5 min (major).
Compound 3m: The Michael product was synthesized by the Gene-
ral Procedure as a white solid in 96% overall yield; m.p. 198–
200 °C. [α]2D0 = –51.8 (c = 1.0, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 7.82 (d, J = 7.74 Hz, 2 H), 7.74 (d, J = 7.71 Hz, 2 H),
7.65–7.44 (m, 6 H), 7.22–7.09 (m, 8 H), 4.46 (t, J = 5.36 Hz, 1 H),
3.25 (d, J = 4.88 Hz, 2 H), 2.31 (s, 3 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 175.6, 151.5, 138.9, 136.7, 134.9, 133.9, 133.7, 130.8,
129.6, 128.5, 128.1, 128.0, 127.4, 125.9, 125.4, 123.3, 1221.1, 121.4,
31.7 ppm. IR (KBr): ν = 1807, 1489, 1446, 1155, 1078, 995,
˜
822 cm–1. HRMS (ESI+): calcd. for [C32H30O6S2Na]+ 597.1376;
found 597.1382. The enantiomeric excess was determined by HPLC
with an AD-H column at 210 nm (propan-2-ol/hexane 1:9),
1.0 mLmin–1; tR = 16.6 min (minor), 20.3 min (major).
Compound 3r: The Michael product was synthesized by the General
Procedure as a colorless oil in 95% overall yield. [α]2D0 = –6.1 (c =
0.5, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.89–7.81 (m, 4 H),
7.71–7.66 (m, 2 H), 7.57–7.51 (m, 4 H), 7.34–7.09 (m, 6 H), 6.96
(d, J = 7.92 Hz, 1 H), 6.79 (d, J = 7.16 Hz, 2 H), 4.74 (s, 1 H),
3.22–2.90 (m, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 177.5,
153.4, 137.6, 136.2, 134.9, 134.7, 133.6, 130.4, 130.2, 129.9, 129.5,
129.2, 129.1, 128.1, 127.5, 127.4, 124.5, 124.2, 111.5, 79.3, 52.0,
79.5, 53.6, 31.6, 14.2 ppm. IR (KBr): ν = 1799, 1446, 1330, 1178,
˜
1043, 999 cm–1. HRMS (ESI+): calcd. for [C29H24O6S2Na]+
555.0907; found 555.0916. The enantiomeric excess was determined
by HPLC with an OD-H column at 210 nm (propan-2-ol/hexane
1:9), 1.0 mLmin–1; tR = 18.7 min (minor), 23.9 min (major).
Compound 3n: The Michael product was synthesized by the Gene-
ral Procedure as a colorless oil in 90% overall yield. [α]2D0 = –105.8
(c = 0.5, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.83 (t, J =
7.06 Hz, 4 H), 7.69 (t, J = 7.38 Hz, 2 H), 7.57–7.53 (m, 4 H), 7.35–
7.25 (m, 7 H), 7.16 (d, J = 7.17 Hz, 1 H), 4.56 (t, J = 4.96 Hz, 1
H), 3.33 (d, J = 5.19 Hz, 2 H), 2.77 (q, J = 7.55 Hz, 2 H), 1.28 (t,
J = 7.13 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.7,
152.2, 140.1, 137.6, 136.2, 134.8, 134.7, 130.5, 130.1, 129.6, 129.1,
129.0, 128.7, 128.4, 126.9, 126.4, 124.5, 123.2, 80.5, 54.5, 32.8, 22.9,
45.6, 30.8 ppm. IR (KBr): ν = 1803, 1462, 1448, 1317, 1151, 1126,
˜
1061, 883, 613 cm–1. HRMS (ESI+): calcd. for [C29H24O6S2Na]+
555.0907; found 555.0901. The enantiomeric excess was determined
by HPLC with an OD-H column at 210 nm (propan-2-ol/hexane
1:9), 1.0 mLmin–1; tR = 45.4 min (minor), 50.4 min (major).
Compound 3s: The Michael product was synthesized by the General
Procedure as a white solid in 99% overall yield; m.p. 189–191 °C.
[α]2D0 = 7.6 (c = 1.0, CHCl3). 1H NMR (400 MHz, CDCl3): δ =
7.88–7.81 (m, 4 H), 7.64–7.59 (m, 2 H), 7.57–7.51 (m, 4 H), 7.40
(7, J = 7.40 Hz, 1 H), 7.19–7.12 (m, 3 H), 4.72 (s, 1 H), 2.90–2.77
(m, 2 H), 1.51 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
178.8, 152.8, 137.8, 136.2, 134.9, 134.6, 130.4, 130.3, 129.7, 129.5,
14.1 ppm. IR (KBr): ν = 1809, 1448, 1325, 1151, 1080, 881 cm–1.
˜
HRMS (ESI+): calcd. for [C30H26O6S2Na]+ 569.1063; found
569.1067. The enantiomeric excess was determined by HPLC with
an OD-H column at 210 nm (propan-2-ol/hexane 1:9),
1.0 mLmin–1; tR = 16.4 min (minor), 21.5 min (major).
Compound 3o: The Michael product was synthesized by the Gene-
ral Procedure as a white solid in 96% overall yield; m.p. 181–
183 °C. [α]2D0 = –77.2 (c = 0.5, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 7.76–7.70 (m, 4 H), 7.64–7.59 (m, 2 H), 7.50–7.45 (m,
4 H), 7.33–7.18 (m, 7 H), 7.06 (d, J = 7.28 Hz, 1 H), 4.50 (dd, J =
3.22, 3.37 Hz, 1 H), 3.31–3.16 (m, 3 H), 1.24 (d, J = 6.90 Hz, 6
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.7, 151.7, 140.2,
137.4, 136.3, 134.8, 134.7, 133.2, 130.4, 129.7, 129.2, 129.1, 129.0,
128.4, 127.6, 126.9, 126.4, 124.7, 123.2, 80.4, 54.3, 32.9, 28.4, 22.6,
129.1, 124.6, 123.5, 111.6, 79.1, 45.4, 31.8, 25.9 ppm. IR (KBr): ν
˜
= 1801, 1618, 1466, 1447, 1331, 1163, 1045, 731, 617 cm–1. HRMS
(ESI+): calcd. for [C23H20O6S2Na]+ 479.0599; found 479.0599. The
enantiomeric excess was determined by HPLC with an OD-H col-
umn at 210 nm (propan-2-ol/hexane 1:9), 1.0 mLmin–1; tR
=
35.6 min (minor), 40.8 min (major).
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, NMR spectra, and HPLC data for all
the new compounds, X-ray crystallography of 3h and calculation
details.
22.1 ppm. IR (KBr): ν = 1803, 1446, 1334, 1155, 1074, 877 cm–1.
˜
HRMS (ESI+): calcd. for [C31H28O6S2Na]+ 583.1220; found
583.1221. The enantiomeric excess was determined by HPLC with
an OD-H column at 210 nm (propan-2-ol/hexane 1:9),
1.0 mLmin–1; tR = 14.2 min (minor), 19.5 min (major).
Acknowledgments
Compound 3p: The Michael product was synthesized by the Gene-
ral Procedure as a colorless oil in 93% overall yield. [α]2D0 = –159.0
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (grant numbers 20902091, 21172112,
1780
www.eurjoc.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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The reaction was carried out on a 0.1 mmol scale catalyzed by
4d (10 mol-%) in CH2Cl2 (500 μL) at room temperature with
a benzofuranone/1,1-bis(phenylsulfonyl)ethylene molar ratio of
1.3:1.
The reaction was carried out on a 0.1 mmol scale catalyzed by
4d (10 mol-%) in CH2Cl2 (500 μL) at room temperature with
a benzofuranone/(vinylsulfonyl)benzene molar ratio of 1.3:1.
Product 3t is a known compound.[8a]
CCDC-850047 contains the supplementary crystallographic
data for compound 3h. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. For details of the crys-
tallographic data of 3h see also Table S1 in the Supporting In-
formation.
All calculations were performed at the B3LYP/6–
311++G(d,p)//B3LYP/3–21G level by use of the Gaussian 03
suite of programs.[14] All the bond lengths are in Ångstroms
[Å], and energies in kcalmol–1. Structures were generated by
use of CYLview (see ref.[15]).
20832004, and 21172118) and the National Basic Research Pro-
gram of China (973 Program, grant numbers 2010CB833300 and
2012CB821600). X. L. also thanks the Fundamental Research
Funds for the Central Universities for support.
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X. Li, J.-P. Cheng et al.
FULL PAPER
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Received: December 8, 2011
Published Online: February 6, 2012
1782
www.eurjoc.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1774–1782