One-pot synthesis of multifunctionalized cyclopropanes

Article abstract of DOI:10.1016/j.tet.2014.02.024

A facile one-step synthetic protocol toward multifunctionalized cyclopropanes 4 is developed from substituted chalcones 1 and sulfones 2 in good yields via a [2C+1C] annulation.

Full text of DOI:10.1016/j.tet.2014.02.024

Tetrahedron 70 (2014) 2257e2263
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
One-pot synthesis of multifunctionalized cyclopropanes
*
Meng-Yang Chang , Yi-Chia Chen, Chieh-Kai Chan
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile one-step synthetic protocol toward multifunctionalized cyclopropanes 4 is developed from
substituted chalcones 1 and sulfones 2 in good yields via a [2Cþ1C] annulation.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 21 November 2013
Received in revised form 5 February 2014
Accepted 13 February 2014
Available online 18 February 2014
Keywords:
Cyclopropanes
Sulfones
Chalcones
Benzaldehydes
Ketones
Annulation
1. Introduction
A cyclopropane unit is a three-membered ring with a small and
highly strained motif that forms the core structure of a large family
of natural products and many pharmacological agents with im-
portant biological activities and interesting structural properties.1
The formation of a cyclopropane unit, based upon the activation
of electron-deficient olefins, has been accomplished using transi-
tion metal promoted routes.2 Compared to literature reports, the
Michael-initiated ring-closure procedures, using
a
-diazonium,3
sulfur ylide,4 -pyridinium,5 -halo,6 or -arsonium7 carbonyls
a
a
a
(one-carbon synthon), are attractive alternatives to control the
specific stereochemistry for the synthesis of multifunctionalized
cyclopropanes. To continue our recent investigation of the Clai-
seneSchmidt condensation for the synthesis of multifunctionalized
molecules (e.g., cyclohexanes, m-terphenyls, benzo[g]indazoles,
azahomoisotwistanes, and oxtanes),8 a stereochemical, high-yield,
easy-operation route for NaH-mediated synthesis of 1,2,3-
trisubstituted cyclopropanes was studied next (Fig. 1).
Fig. 1. Synthetic routes of cyclopropanes.
condensation of substituted methyl ketones with arylaldehydes
under a methanolic refluxing solution.8 Next, synthesis of benzyl
sulfones
2 was achieved from nucleophilic substitution of
substituted benzylic bromide with sodium sulfinates (3, RSO2Na) in
good yields. In an attempt to develop a practical protocol of mul-
tifunctional cyclopropane 4a with the adjacent cis-diphenyl sub-
stituents, a NaH-mediated one-pot [2Cþ1C] synthetic route of the
starting chalcone 1a (R1¼R2¼Ph) with benzyl sulfone 2a in
refluxing THF provided cyclopropane 4a at an 80% yield, as shown
in Scheme 1. For Michael-initiated ring-closure approaches, this
present protocol is a novel route to achieve the formation of
a three-membered ring via a one-carbon synthon with a sulfonyl
group.
2. Results and discussion
To initiate the synthetic work, several chalcones 1 were pre-
pared in nearly quantitative yields according to our recent litera-
ture
methods
from
NaOH-mediated
ClaiseneSchmidt
* Corresponding author. Tel.: þ886 7 3121101x2220; e-mail addresses: mychang@
kmu.edu.tw, mychang624@yahoo.com.tw (M.-Y. Chang).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.02.024

2258
M.-Y. Chang et al. / Tetrahedron 70 (2014) 2257e2263
Table 2
Synthesis of multisubstituted cyclopropanes 4a
O
H
O
H
O
O
H
R
1
R
NaH, THF
reflux
R
S
S
H
Na
+
+
R
1
R
2
R
2
O
O
H
R
H
R
3
3
1
2
4
Entry
1
Chalcones 1aex
Sulfones 2aev
Cyclopropanes 4aey/yield (%)
Scheme 1. One-pot route of cyclopropanes 4a.
To change the bases with different equivalents in various sol-
vents from 25 ꢀC to refluxing temperature, we found different
yields of cyclopropane 4a obtained by the one-pot reaction of
model chalcone 1a with benzyl sulfone 2a (see Table 1). By
adjusting the equivalents of NaH, reaction time, temperature, and
concentration (for entries 1e7), the isolated total yield of cyclo-
propane 4a was noticeably enhanced (see entry 4). After screening
the base-mediated reaction conditions (entries 8e13), we found
that NaH provided higher yields than other bases (t-BuOK, DBU,
DMAP, MeONa). Furthermore, when the base and solvent were
replaced with MeONa/MeOH, the isolated total yield of cyclopro-
pane 4a was decreased to 27% (for entry 13). According to the
experimental results, we envisioned that the combination of NaH
(4 equiv)/THF (15 mL) is an optimal reaction condition for ele-
vating the total yields of cyclopropane 4a under boiling THF
conditions for 3 h.
2
3
4
5
Table 1
Reactions of chalcone 1a and benzyl sulfone 2aa
6
Entry
Base (equiv), solvent (mL), temp (ꢀC), time (h)
4a yield (%)
1
2
3
4
5
6
7
8
NaH (2.5), THF (15), 25, 3
NRb
52c
72
80
75
78
76
78
70
NaH (2.5), THF (15), reflux, 3
NaH (2.5), THF (15), reflux, 10
NaH (4), THF (15), reflux, 3
NaH (4), THF (15), reflux, 10
NaH (4), THF (30), reflux, 3
NaH (10), THF (15), reflux, 3
t-BuOK (4), THF (15), reflux, 3
t-BuOK (4), THF (15), reflux, 10
t-BuOK (10), THF (15), reflux, 3
DBU (4), THF (15), reflux, 3
DMAP (4), THF (15), reflux, 3
MeONa (4), MeOH (15), reflux, 3
7
9
8
10
11
12
13
72
45c
37c
27d
a
The reactions were run on a 0.5 mmol scale with chalcone 1a and benzyl sulfone
9
2a.
b
NR is no reaction.
c
The starting materials 1a and 2a were recovery (for entry 2, 25%; entry 11, 38%;
entry 12, 45%).
d
The starting materials 1a and 2a (w28%) were recovery and unknown mixture
10
11
12
13
was isolated (w34%).
With the experimental results in hand, one-pot preparation of
multisubstituted cyclopropanes 4 was further examined. Changing
R1 and R2 substituents of chalcones 1aen and R and R3 substituents
of sulfones 2aeh, 21 cyclopropanes 4aeu with cis-diaryl and aroyl
groups were isolated with good yields by one-pot methodology
(entries 1e22); they are summarized in Table 2. The formation of
cyclopropanes
4 was confirmed through spectral analysis,
including 1H and 13C NMR and HRMS spectra. The structural
frameworks of compounds 4d, 4l, 4r, and 4s were determined by
single-crystal X-ray crystallography, as shown in Fig. 2.9 But, when
the R3 group was changed to a 4-nitrophenyl or 2,6-difluorophenyl
aryl group, a complex unknown mixture was observed (entries
23e24). Alternatively, when the R1 group was an n-octyl or

M.-Y. Chang et al. / Tetrahedron 70 (2014) 2257e2263
2259
Table 2 (continued )
Entry
Chalcones 1aex
Sulfones 2aev
Cyclopropanes 4aey/yield (%)
14
15
16
17
18
19
Fig. 2. X-ray structure of compound 4r.
n-dodecyl aliphatic substituent, the starting materials 1a and 2g, h
were recovered as major products (entries 25e26). No compounds
4vey were isolated. Compared with the isolated yields of products
4aeu, it was found that different kinds of substituents (e.g.,
electron-withdrawing oxygen-containing groups, heterocyclic 2-
thiophene group, aliphatic tert-butyl group) did not obviously af-
fect the product yields. For the transecisetrans geometric con-
formationally restricted character of skeleton 4, three contiguous
stereogenic centers of a three-membered ring exhibited good ori-
entation among arylcarbonyl or tert-butylcarbonyl and two aryl
groups via a cyclopropanation reaction of chalcone 1aen with
sulfones 2aeh.
20
21
A possible explanation for the one-pot (C2þC1) route is shown
in Scheme 2. Intermediate A of sodium a-carbanion was first gen-
erated via NaH-mediated deprotonation of skeleton 2 in refluxing
THF. Under thermodynamic conditions, both possible conforma-
tions of intermediates B1 and B2 should be generated from the
conjugated addition of intermediate A. Generating intermediate B1
should not be preferred since the relative orientation between the
R2 aryl group and the sulfonyl group was cis-configured with
stronger repulsion. At another intermediate B2 stage, gauche-con-
figuration between the R2 aryl group and the sulfonyl group easily
triggered the occurrence of an intramolecular SN2 ring-closure, and
the three-membered ring could be cyclized for the formation of
a cyclopropane skeleton. After the removal of sulfonic sodium salt
(RSO2Na), intermediate C1, with the cis-diaryl configuration, should
be afforded. Because RSO2Na could be isolated by the extraction
22
23
24
25
26
aFor the best one-pot reaction conditions: substituted chalcones 1aev (0.5 mmol),
sulfones 2 (0.5 mmol), NaH (60%, 80 mg, 2.0 mmol), THF (15 mL), reflux, 3 h.
bUnknown mixture was isolated.
cNo reaction and the starting materials 1a and 2g, h were recovery.
Scheme 2. A possible mechanism to skeleton 4.

2260
M.-Y. Chang et al. / Tetrahedron 70 (2014) 2257e2263
process (from the aqueous layer), the reaction pathway from in-
termediate B2 to C was proposed (Scheme 3).
Finnigan/Thermo Quest MAT 95XL. X-ray crystal structures were
obtained with an Enraf-Nonius FR-590 diffractometer (CAD4,
Kappa CCD). Elemental analyses were carried out with Heraeus
Vario III-NCSH, Heraeus CHN-OS-Rapid Analyzer or Elementar
Vario EL III.
4.2. A representative synthetic procedure of skeleton 4 is as
follows
Sodium hydride (NaH, 60% in oil, 80 mg, 2.0 mmol) was added to
a solution of sulfones 3 (0.5 mmol) in THF (8 mL). A solution of
skeleton 2 (0.5 mmol) in the THF (7 mL) was added to the reaction
mixture at rt. The reaction mixture was stirred at reflux for 3 h. The
reaction mixture was cooled to rt. Water (1 mL) was added to the
reaction mixture at 0 ꢀC. The solvent was concentrated under re-
duced pressure. The residue was diluted with water (10 mL) and the
mixture was extracted with EtOAc (3ꢁ20 mL). The combined or-
ganic layers were washed with brine, dried, filtered, and evapo-
rated to afford crude product. Purification on silica gel (hexanes/
EtOAc¼10/1e6/1) afforded skeleton 4.
Scheme 3. Reduction of compound 4a.
To extend this application of cyclopropanyl compound 4a, re-
duction and hydrogenation were further studied. Under a standard
NaBH4-mediated reduction condition, compound 5 was isolated at
a 90% yield and no ring-opening product was observed. When
hydrogenation of compound 4a was treated with hydrogen in the
presence of catalytic palladium, the sole compound 6 was isolated
at an 82% yield. The cyclopropanyl motif was opened due to the ring
strain. The present route was a high-yield for preparing 1,3,4-
triphenylbutan-1-ol via hydrogenation. Changing the catalyst
from Pd/C to PtO2, the covalent single bond between two phenyl
groups was cleaved to form the skeleton of compound 7 at an 85%
yield. Three reduction conditions caused different compounds 5e7
regioselectively.
4.2.1. Compound (4a). Yield¼80% (119 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C22H19O 299.1436, found 299.1439; 1H NMR
(400 MHz, CDCl3): d 8.15e8.13 (m, 2H), 7.64e7.61 (m,1H), 7.56e7.52
(m, 2H), 7.21e7.14 (m, 6H), 7.06e7.02 (m, 4H), 3.58 (t, J¼5.2 Hz, 1H),
3.35 (d, J¼5.2 Hz, 2H); 13C NMR (100 MHz, CDCl3):
d 198.46, 137.66,
136.08, 133.08 (2ꢁ), 128.89 (4ꢁ), 128.67 (2ꢁ), 128.11 (2ꢁ), 127.99
(4ꢁ), 126.49 (2ꢁ), 36.27 (2ꢁ), 32.30.
4.2.2. Compound (4b). Yield¼72% (140 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C25H25O4 389.1753, found 389.1755; 1H NMR
(400 MHz, CDCl3):
d 8.11e8.09 (m, 2H), 7.63e7.59 (m, 1H),
7.54e7.50 (m, 2H), 7.25e7.16 (m, 3H), 7.13e7.11 (m, 2H), 6.12 (s, 2H),
3.77 (s, 3H), 3.60 (s, 6H), 3.45 (t, J¼5.2 Hz, 1H), 3.33 (dd, J¼5.2,
9.6 Hz, 1H), 3.21 (dd, J¼5.2, 9.6 Hz, 1H); 13C NMR (100 MHz, CDCl3):
3. Conclusion
d
198.19, 152.59 (2ꢁ), 137.56, 136.09, 133.11 (2ꢁ), 132.05, 129.08
(2ꢁ), 128.67 (2ꢁ), 128.14 (2ꢁ), 128.08 (2ꢁ), 126.63, 105.98 (2ꢁ),
60.76, 55.82 (2ꢁ), 36.14, 35.90, 33.20.
In summary, we have successfully presented a novel, one-pot
[C2þC1] methodology for synthesizing a series of cyclopropanes
4 via two important steps: (1) conjugated addition of chalcones 1
4.2.3. Compound (4c). Yield¼70% (130 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C24H21O4 373.1440, found 373.1448; 1H NMR
with the
a-carbanion intermediate A of sulfones 2 and (2) re-
moval of RSO2Na/intramolecular SN2 cyclization of possible in-
termediate B2. The structural skeletons of key products were
confirmed by X-ray crystal analysis. The one-pot transition
metal-free synthetic approach begins with simple starting ma-
terials and reagents, and provides a potential protocol for the
synthetic research and biological activities of cyclopropanes.
Further investigation regarding one-pot cascade synthesis of
multifunctionalized arenes will be conducted and published in
due course.
(400 MHz, CDCl3):
d
8.11 (d, J¼8.8 Hz, 2H), 7.22e7.13 (m, 3H),
7.05e7.03 (m, 2H), 7.00 (d, J¼8.8 Hz, 2H), 6.63 (d, J¼8.0 Hz,1H), 6.56
(dd, J¼1.6, 8.0 Hz, 1H), 6.49 (d, J¼1.6 Hz, 1H), 5.86 (d, J¼1.6 Hz, 1H),
5.85 (d, J¼1.6 Hz, 1H), 3.88 (s, 3H), 3.44 (t, J¼5.2 Hz, 1H), 3.25e3.21
(m, 2H); 13C NMR (100 MHz, CDCl3):
d 196.61, 163.52, 147.24, 146.07,
136.29, 130.59, 130.32 (2ꢁ), 129.93, 128.66 (2ꢁ), 127.97 (2ꢁ),
126.38, 122.28, 113.80 (2ꢁ), 109.32, 107.81, 100.77, 55.40, 35.69,
35.60, 32.01.
4.2.4. Compound (4d). Yield¼70% (117 mg); Colorless solid;
mp¼108e110 ꢀC (recrystallized from hexanes and EtOAc); HRMS
(ESI, Mþþ1) calcd for C21H19O2S 335.1106, found 335.1112; 1H NMR
4. Experimental section
4.1. General
(400 MHz, CDCl3):
d
8.13 (d, J¼8.8 Hz, 2H), 7.27e7.21 (m, 3H),
7.19e7.16 (m, 2H), 7.05e7.03 (m, 1H), 7.02 (d, J¼8.8 Hz, 2H), 6.82
(dd, J¼3.6, 5.2 Hz, 1H), 6.67 (dd, J¼1.2, 3.6 Hz, 1H), 3.89 (s, 3H), 3.52
(t, J¼5.2 Hz, 1H), 3.38 (dd, J¼5.2, 9.2 Hz, 1H), 3.32 (dd, J¼5.2, 9.2 Hz,
All other reagents and solvents were obtained from commer-
cial sources and used without further purification. Reactions were
routinely carried out under an atmosphere of dry air or nitrogen
with magnetic stirring. Products in organic solvents were dried
with anhydrous MgSO4 before concentration in vacuo. Melting
points were determined with a SMP3 melting apparatus. 1H and
13C NMR spectra were recorded on a Varian INOVA-400 spec-
trometer operating at 200/400 and at 100 MHz, respectively.
1H); 13C NMR (100 MHz, CDCl3):
d 195.88, 163.61, 139.94, 135.70,
130.46, 130.40 (2ꢁ), 128.90 (2ꢁ), 128.00 (2ꢁ), 126.73, 126.45,
126.04, 123.92, 113.84 (2ꢁ), 55.42, 35.70, 33.25, 30.59; Anal. Calcd
for C21H18O2S: C, 75.42; H, 5.42. Found: C, 75.68; H, 5.53. Single-
crystal X-ray diagram: crystal of compound 4d was grown
by slow diffusion of EtOAc into a solution of compound 4d in CH2Cl2
to yield colorless prisms. The compound crystallizes in the
Chemical shifts (
coupling constants (J) are given in Hertz. High resolution mass
spectra (HRMS) were measured with mass spectrometer
d) are reported in parts per million (ppm) and the
ꢀ
triclinic crystal system, space group Pꢂ1, a¼8.1733(2) A,
3
ꢀ
ꢀ
ꢀ
a
b¼10.1489(2) A, c¼11.3877(3) A, V¼831.60(3) A , Z¼2,

M.-Y. Chang et al. / Tetrahedron 70 (2014) 2257e2263
2261
dcalcd¼1.336 g/cm3, F(000)¼352, 2
(all data) R1¼0.0658, wR2¼0.1614.
q
range 1.93e26.40ꢀ, R indices
(100 MHz, CDCl3):
d
197.58, 139.84, 135.63, 134.87, 132.54, 129.85,
129.67, 128.99 (2ꢁ), 128.65, 128.57, 128.46, 128.13 (2ꢁ), 127.81,
126.91, 126.88, 126.56, 126.24, 124.12, 123.93, 36.24, 33.87, 31.24;
Anal. Calcd for C24H18OS: C, 81.32; H, 5.12. Found: C, 81.63; H, 5.28.
4.2.5. Compound (4e). Yield¼73% (130 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C24H21O3 357.1491, found 357.1495; 1H NMR
(400 MHz, CDCl3):
d
8.05 (d, J¼8.4 Hz, 2H), 7.33 (d, J¼8.4 Hz, 2H),
4.2.11. Compound (4k). Yield¼86% (138 mg); Colorless oil;
mp¼77e79 ꢀC (recrystallized from hexanes and EtOAc); HRMS (ESI,
Mþþ1) calcd for C21H23O3 323.1647, found 323.1651; 1H NMR
7.23e7.15 (m, 3H), 7.08e7.05 (m, 2H), 6.65 (d, J¼8.4 Hz, 1H), 6.58
(dd, J¼2.0, 8.4 Hz, 1H), 6.52 (d, J¼2.0 Hz, 1H), 5.85 (d, J¼1.6 Hz, 1H),
5.85 (d, J¼1.6 Hz, 1H), 3.51 (t, J¼5.2 Hz, 1H), 3.28e3.26 (m, 2H), 2.46
(400 MHz, CDCl3):
d 7.18e7.13 (m, 3H), 6.97e6.95 (m, 2H), 6.60
(s, 3H); 13C NMR (100 MHz, CDCl3):
d
197.79, 147.21, 146.05, 143.81,
(d, J¼8.0 Hz, 1H), 6.47 (dd, J¼1.6, 8.0 Hz, 1H), 6.41 (d, J¼1.6 Hz, 1H),
136.16, 135.04, 129.79, 129.27 (2ꢁ), 128.62 (2ꢁ), 128.14 (2ꢁ), 127.93
5.86 (s, 2H), 2.95 (br s, 3H), 1.30 (s, 9H); 13C NMR (100 MHz, CDCl3):
(2ꢁ),126.37,122.24,109.28,107.76,100.73, 35.91, 35.82, 32.17, 21.54.
d
213.20, 147.25, 146.06, 136.20, 129.92, 128.61 (2ꢁ), 127.99 (2ꢁ),
126.38, 122.16, 109.25, 107.84, 100.80, 44.19, 35.56, 35.47, 30.84,
26.26 (3ꢁ); Anal. Calcd for C21H22O3: C, 78.23; H, 6.88. Found: C,
78.34; H, 7.01.
4.2.6. Compound (4f). Yield¼72% (115 mg); Colorless oil;
mp¼96e97 ꢀC (recrystallized from hexanes and EtOAc); HRMS (ESI,
Mþþ1) calcd for C21H19OS 319.1157, found 319.1162; 1H NMR
(400 MHz, CDCl3):
d
8.02 (d, J¼8.0 Hz, 2H), 7.33 (d, J¼8.0 Hz, 2H),
4.2.12. Compound (4l). Yield¼82% (134 mg); Colorless solid;
mp¼107e109 ꢀC (recrystallized from hexanes and EtOAc); HRMS
(ESI, Mþþ1) calcd for C24H25O 329.1905, found 329.1906; 1H NMR
7.29e7.14 (m, 5H), 7.04 (dd, J¼1.2, 4.8 Hz, 1H), 6.82 (dd, J¼3.6,
4.8 Hz, 1H), 6.66 (dd, J¼1.2, 3.6 Hz, 1H), 3.53 (t, J¼5.2 Hz, 1H), 3.38
(dd, J¼5.2, 9.2 Hz, 1H), 3.32 (dd, J¼5.2, 9.2 Hz, 1H), 2.45 (s, 3H); 13
C
(400 MHz, CDCl3):
d
7.74 (dd, J¼2.4, 7.2 Hz, 1H), 7.68 (dd, J¼2.4,
NMR (100 MHz, CDCl3):
d
197.22, 144.06, 139.89, 135.66, 135.00,
7.2 Hz, 1H), 7.61 (d, J¼7.2 Hz, 1H), 7.47 (br s, 1H), 7.45e7.39 (m, 2H),
7.15e7.10 (m, 3H), 7.04e7.00 (m, 3H), 3.23e3.13 (m, 3H), 1.37
129.40 (2ꢁ), 128.93 (2ꢁ), 128.29 (2ꢁ), 128.05 (2ꢁ), 126.81, 126.48,
126.11, 124.01, 35.99, 33.54, 30.89, 21.66; Anal. Calcd for C21H18OS:
C, 79.21; H, 5.70. Found: C, 79.50; H, 5.94.
(s, 9H); 13C NMR (100 MHz, CDCl3):
d 213.23, 136.13, 133.94, 133.06,
132.09, 128.83 (2ꢁ), 128.01 (2ꢁ), 127.51, 127.48, 127.45, 127.37,
126.98, 126.42, 125.92, 125.45, 44.26, 35.77 (2ꢁ), 30.91, 26.90 (3ꢁ);
Anal. Calcd for C24H24O: C, 87.76; H, 7.37. Found: C, 87.60; H, 7.66.
Single-crystal X-ray diagram: crystal of compound 4l was grown by
slow diffusion of EtOAc into a solution of compound 4l in CH2Cl2 to
yield colorless prisms. The compound crystallizes in the ortho-
4.2.7. Compound (4g). Yield¼78% (133 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C24H23O2 343.1698, found 343.1697; 1H NMR
(400 MHz, CDCl3):
d
8.01 (d, J¼8.0 Hz, 2H), 7.30 (d, J¼8.0 Hz, 2H),
7.20e7.14 (m, 3H), 7.08 (d, J¼8.4 Hz, 1H), 7.06e7.04 (m, 2H), 6.67
(dd, J¼2.4, 8.4 Hz, 1H), 6.63 (dd, J¼0.8, 7.6 Hz, 1H), 6.50 (t, J¼2.0 Hz,
1H), 3.62 (s, 3H), 3.51 (t, J¼5.2 Hz, 1H), 3.30 (dd, J¼5.2, 9.2 Hz, 1H),
3.25 (dd, J¼5.2, 9.2 Hz, 1H), 2.43 (s, 3H); 13C NMR (100 MHz, CDCl3):
ꢀ
rhombic crystal system, space group P 21 21 21, a¼5.7470(3) A,
3
ꢀ
ꢀ
ꢀ
b¼15.8344(8) A, c¼19.9110(10) A, V¼1811.91(16) A , Z¼4,
dcalcd¼1.204 g/cm3, F(000)¼704, 2
q
range 1.64e26.37ꢀ, R indices
d
197.93, 159.17, 143.90, 137.90, 136.21, 135.13, 129.88, 129.35 (2ꢁ),
(all data) R1¼0.0353, wR2¼0.0765.
128.93 (2ꢁ), 128.24 (2ꢁ), 128.02 (2ꢁ), 126.48, 121.42, 114.41, 112.07,
54.99, 36.04, 35.97, 32.25, 21.63.
4.2.13. Compound (4m). Yield¼87% (124 mg); Colorless solid;
mp¼93e95 ꢀC (recrystallized from hexanes and EtOAc); HRMS
(ESI, Mþþ1) calcd for C18H21OS 285.13131, found 285.1316; 1H NMR
4.2.8. Compound (4h). Yield¼80% (173 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C26H25O6 433.1651, found 433.1735; 1H NMR
(400 MHz, CDCl3):
d 7.23e7.16 (m, 3H), 7.08e7.05 (m, 2H), 7.01
(400 MHz, CDCl3):
d
7.34 (s, 2H), 7.20e7.11 (m, 3H), 7.06e6.99(m, 2H),
(dd, J¼1.2, 5.2 Hz, 1H), 6.78 (dd, J¼3.2, 5.2 Hz, 1H), 6.58 (dd, J¼1.2,
6.61(d, J¼8.0Hz,1H), 6.54(dd,J¼1.6, 8.0Hz,1H),6.47(d, J¼1.6Hz,1H),
5.83 (d, J¼1.6 Hz, 1H), 5.82 (d, J¼1.6 Hz, 1H), 3.92 (s, 3H), 3.89 (s, 6H),
3.38 (t, J¼5.2 Hz,1H), 3.25e3.16 (m, 2H); 13C NMR (100 MHz, CDCl3):
3.2 Hz, 1H), 3.10e3.01 (m, 3H), 1.31 (s, 9H); 13C NMR (100 MHz,
CDCl3):
d
212.58, 139.88, 135.69, 128.81 (2ꢁ), 128.03 (2ꢁ), 126.74,
126.47, 125.92, 123.91, 44.24, 35.55, 32.11, 30.56, 26.19 (3ꢁ); Anal.
d
196.81, 152.99 (2ꢁ), 147.19, 146.06, 142.57, 135.97, 132.69, 129.59,
Calcd for C18H20OS: C, 76.01; H, 7.09. Found: C, 76.35; H, 7.32.
128.49 (2ꢁ), 127.93 (2ꢁ), 126.39, 122.10, 109.16, 107.73, 105.62 (2ꢁ),
100.71, 60.76, 56.14 (2ꢁ), 35.70, 35.49, 32.39.
4.2.14. Compound (4n). Yield¼82% (126 mg); Colorless solid;
mp¼52e54 ꢀC (recrystallized from hexanes and EtOAc); HRMS (ESI,
Mþþ1) calcd for C21H25O2 309.1855, found 309.1856; 1H NMR
4.2.9. Compound (4i). Yield¼72% (141 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C27H21O3 393.1491, found 393.1495; 1H NMR
(400 MHz, CDCl3):
d 7.17e7.10 (m, 3H), 6.96e6.93 (m, 2H), 6.87
(400 MHz, CDCl3):
d
8.65 (s, 1H), 8.17 (dd, J¼1.6, 8.4 Hz, 1H), 8.02 (d,
(d, J¼8.8 Hz, 2H), 6.69 (d, J¼8.8 Hz, 2H), 3.72 (s, 3H), 3.00e2.95 (m,
J¼8.0 Hz,1H), 7.95 (d, J¼8.4 Hz,1H), 7.91 (d, J¼8.0 Hz,1H), 7.64e7.56
(m, 2H), 7.27e7.17 (m, 3H), 7.13e7.10 (m, 2H), 6.68 (d, J¼8.0 Hz, 1H),
6.62 (dd, J¼1.6, 8.0 Hz, 1H), 6.56 (d, J¼1.6 Hz, 1H), 5.88 (d, J¼1.6 Hz,
1H), 5.87 (d, J¼1.6 Hz, 1H), 3.67 (t, J¼5.2 Hz, 1H), 3.39e3.32 (m, 2H);
3H), 1.31 (s, 9H); 13C NMR (100 MHz, CDCl3):
d
213.38, 158.09,
136.42, 129.87 (2ꢁ), 128.73 (2ꢁ), 128.18, 127.93 (2ꢁ), 126.27, 113.42
(2ꢁ), 55.09, 44.19, 35.50, 35.13, 30.92, 26.29 (3ꢁ); Anal. Calcd for
C21H24O2: C, 81.78; H, 7.84. Found: C, 82.10; H, 7.63.
13C NMR (100 MHz, CDCl3):
d 198.16, 147.28, 146.15, 136.14, 135.51,
134.90, 132.45, 129.78, 129.62, 129.55, 128.71 (2ꢁ), 128.52, 128.41,
128.03 (2ꢁ), 127.71, 126.75, 126.49, 123.88, 122.33, 109.34, 107.85,
100.80, 36.29, 36.17, 32.53.
4.2.15. Compound (4o). Yield¼71% (116 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C23H21O2 329.1542, found 329.1546; 1H NMR
(400 MHz, CDCl3):
d
8.11 (d, J¼8.4 Hz, 2H), 7.63e7.59 (m, 1H),
7.54e7.51 (m, 2H), 7.21e7.15 (m, 3H), 7.04e7.02 (m, 2H), 6.95
(d, J¼8.8 Hz, 2H), 6.72 (d, J¼8.4 Hz, 2H), 3.74 (s, 3H), 3.49 (t,
J¼4.8 Hz, 1H), 3.28 (d, J¼4.8 Hz, 2H); 13C NMR (100 MHz, CDCl3):
4.2.10. Compound (4j). Yield¼70% (124 mg); Colorless solid;
mp¼114e116 ꢀC (recrystallized from hexanes and EtOAc); HRMS
(ESI, Mþþ1) calcd for C24H19OS 355.1157, found 355.1162; 1H NMR
d
198.57, 158.21, 137.75, 136.32, 133.03, 129.99 (2ꢁ), 128.84 (2ꢁ),
(400 MHz, CDCl3):
d
8.64 (s,1H), 8.14 (dd, J¼1.6, 8.4 Hz,1H), 8.03 (d,
128.67 (2ꢁ), 128.10 (2ꢁ), 128.06, 128.00 (2ꢁ), 126.42, 113.48 (2ꢁ),
J¼8.0 Hz,1H), 7.96 (d, J¼8.4 Hz,1H), 7.90 (d, J¼8.0 Hz,1H), 7.65e7.55
(m, 2H), 7.28e7.18 (m, 5H), 7.06 (dd, J¼1.2, 5.2 Hz, 1H), 6.84 (dd,
J¼3.6, 5.2 Hz, 1H), 6.70 (d, J¼3.6 Hz, 1H), 3.69 (t, J¼5.2 Hz, 1H), 3.45
(dd, J¼5.2, 9.6 Hz, 1H), 3.40 (dd, J¼5.2, 9.6 Hz, 1H); 13C NMR
55.12, 36.17, 35.80, 32.65.
4.2.16. Compound (4p). Yield¼80% (143 mg); Colorless solid;
mp¼109e111 ꢀC (recrystallized from hexanes and EtOAc); HRMS

2262
M.-Y. Chang et al. / Tetrahedron 70 (2014) 2257e2263
(ESI, Mþþ1) calcd for C25H27O2 359.2011, found 359.2013; 1H NMR
129.31 (2ꢁ), 128.85, 128.18 (2ꢁ), 122.25, 113.48 (2ꢁ), 109.29, 107.83,
(400 MHz, CDCl3):
d
7.75 (dd, J¼2.4, 7.2 Hz, 1H), 7.69 (dd, J¼2.4,
100.80, 55.09, 35.76, 35.48, 32.54, 21.59.
7.2 Hz, 1H), 7.62 (d, J¼8.8 Hz, 1H), 7.47 (br s, 1H), 7.45e7.38 (m, 2H),
7.02 (dd, J¼2.0, 8.8 Hz, 1H), 6.92 (d, J¼8.4 Hz, 2H), 6.67 (d, J¼8.8 Hz,
2H), 3.70 (s, 3H), 3.19e3.08 (m, 3H),1.36 (s, 9H); 13C NMR (100 MHz,
4.2.21. Compound (4u). Yield¼70% (125 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C24H23O3 359.1647, found 359.1656; 1H NMR
CDCl3):
d
213.30, 158.12, 134.16, 133.07, 132.07, 129.90 (2ꢁ), 128.09,
(400 MHz, CDCl3):
d
8.10 (d, J¼7.6 Hz, 2H), 7.63e7.59 (m, 1H),
127.51, 127.47, 127.40, 127.30, 126.95, 125.90, 125.42, 113.48 (2ꢁ),
55.05, 44.22, 35.66, 35.27, 31.18, 26.30 (3ꢁ); Anal. Calcd for
7.54e7.50 (m, 2H), 7.23e7.16 (m, 3H), 7.09 (d, J¼7.6 Hz, 2H), 6.25
(t, J¼2.0 Hz,1H), 6.15 (d, J¼2.0 Hz, 2H), 3.61 (s, 6H), 3.51 (d, J¼5.2 Hz,
1H), 3.33 (dd, J¼5.2, 9.6 Hz, 1H), 3.23 (dd, J¼5.2, 9.6 Hz, 1H); 13C
C
25H26O2: C, 83.76; H, 7.31. Found: C, 83.95; H, 7.48.
NMR (100 MHz, CDCl3):
d
198.37, 160.29 (2ꢁ), 138.61, 137.63, 136.10,
4.2.17. Compound (4q). Yield¼82% (144 mg); Colorless solid;
mp¼105e107 ꢀC (recrystallized from hexanes and EtOAc); HRMS
(ESI, Mþþ1) calcd for C22H25O4 353.1753, found 353.1756; 1H NMR
133.11, 128.93 (2ꢁ), 128.68 (2ꢁ), 128.12 (2ꢁ), 128.09 (2ꢁ), 126.57,
107.02 (2ꢁ), 98.76, 55.13 (2ꢁ), 36.29, 36.18, 32.53.
(400 MHz, CDCl3):
d
6.79 (d, J¼8.8 Hz, 2H), 6.62 (d, J¼8.8 Hz, 2H),
4.2.22. Compound (5). Sodium borohydride (75 mg, 2.0 mmol)
was added to a solution of compound 4a (150 mg, 0.5 mmol) in
the co-solvent of THF and MeOH (10 mL, v/v¼1/1). The reaction
mixture was stirred at rt for 3 h. Water (1 mL) was added to the
reaction mixture at rt. The solvent was concentrated under re-
duced pressure. The residue was diluted with water (10 mL) and
the mixture was extracted with EtOAc (3ꢁ20 mL). The combined
organic layers were washed with brine, dried, filtered, and
evaporated to afford crude product. Purification on silica gel
(hexanes/EtOAc¼10/1e6/1) afforded compound 5. Yield¼90%
(135 mg); Colorless solid; mp¼80e82 ꢀC (recrystallized from
6.53 (d, J¼8.0 Hz, 1H), 6.38 (dd, J¼1.6, 8.0 Hz, 1H), 6.32 (d, J¼1.6 Hz,
1H), 5.78 (s, 2H), 3.65 (s, 3H), 2.83e2.81 (m, 3H), 1.21 (s, 9H); 13C
NMR (100 MHz, CDCl3):
d 213.27, 158.10, 147.23, 146.01, 130.13,
129.65 (2ꢁ), 128.17, 122.10, 113.47 (2ꢁ), 109.20, 107.83, 100.79,
55.09, 44.15, 35.31, 35.03, 31.09, 26.26 (3ꢁ); Anal. Calcd for
C
22H24O4: C, 74.98; H, 6.86. Found: C, 75.27; H, 7.20.
4.2.18. Compound (4r). Yield¼71% (164 mg); Colorless solid;
mp¼164e166 ꢀC (recrystallized from hexanes and EtOAc); HRMS
(ESI, Mþþ1) calcd for C27H27O7 463.1757, found 463.1759; 1H NMR
(400 MHz, CDCl3):
d
7.33 (s, 2H), 6.94 (d, J¼8.8 Hz, 2H), 6.72
hexanes and EtOAc); HRMS (ESI, Mþþ1) calcd for C22H21
O
(d, J¼8.8 Hz, 2H), 6.62 (d, J¼8.0 Hz, 1H), 6.53 (dd, J¼2.0, 8.0 Hz, 1H),
6.47 (d, J¼2.0 Hz, 1H), 5.86 (s, 2H), 3.92 (s, 3H), 3.90 (s, 6H), 3.73
(s, 3H), 3.28 (t, J¼5.2 Hz, 1H), 3.17 (dd, J¼5.2, 9.6 Hz, 1H), 3.13 (dd,
301.1592, found 301.1598; 1H NMR (400 MHz, CDCl3):
d 7.54e7.52
(m, 2H), 7.42e7.38 (m, 2H), 7.36e7.31 (m, 1H), 7.13e7.04 (m, 6H),
6.93e6.91 (m, 2H), 6.85e6.83 (m, 2H), 4.67 (d, J¼6.8 Hz, 1H), 2.68
(dd, J¼5.6, 9.6 Hz, 1H), 2.60 (dd, J¼6.0, 9.6 Hz, 1H), 2.19 (dd, J¼5.6,
J¼5.2, 9.6 Hz, 1H); 13C NMR (100 MHz, CDCl3):
d 197.03, 158.19,
153.08 (2ꢁ), 147.28, 146.10, 142.66, 132.88, 129.92, 129.66 (2ꢁ),
127.99, 122.15, 113.51 (2ꢁ), 109.21, 107.84, 105.72 (2ꢁ), 100.80,
60.87, 56.28 (2ꢁ), 55.06, 35.51, 35.35, 32.77; Anal. Calcd for
6.0 Hz, 1H), 2.18 (br s, 1H); 13C NMR (100 MHz, CDCl3):
d 143.34,
137.42, 137.10, 128.99 (2ꢁ), 128.90 (2ꢁ), 128.61 (2ꢁ), 127.80,
127.77 (2ꢁ), 127.75 (2ꢁ), 126.02 (2ꢁ), 125.83 (2ꢁ), 76.45, 33.03,
29.32, 28.97.
C
27H26O7: C, 70.12; H, 5.67. Found: C, 70.39; H, 5.90. Single-crystal
X-ray diagram: crystal of compound 4r was grown by slow diffu-
sion of EtOAc into a solution of compound 4r in CH2Cl2 to yield
colorless prisms. The compound crystallizes in the triclinic crystal
4.2.23. Compound (6). Palladium on activated carbon (10%,
20 mg) was added to a solution of compound 4a (150 mg,
0.5 mmol) in EtOAc (20 mL) at rt. Then hydrogen was bubbled
into the mixture for 10 min, and the mixture was stirred at rt for
20 h. The reaction mixture was filtered and evaporated to yield
crude product. Purification on silica gel (hexanes/EtOAc¼6/1)
afforded compound 6. Yield¼82% (124 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C22H23O 303.1749, found 303.1752; 1H NMR
ꢀ
ꢀ
system, space group Pꢂ1, a¼9.8563(4) A, b¼10.3346(4) A,
3
c¼11.6530(5) A, V¼1135.98(8) A , Z¼2, dcalcd¼1.352 g/cm3, F(000)¼
ꢀ
ꢀ
488,
2
q
range 1.83e26.38ꢀ,
R
indices (all data) R1¼0.0713,
wR2¼0.1724.
4.2.19. Compound (4s). Yield¼80% (126 mg); Colorless solid;
mp¼75e76 ꢀC (recrystallized from hexanes and EtOAc); HRMS (ESI,
Mþþ1) calcd for C19H23O2S 315.1419, found 315.1425; 1H NMR
(400 MHz, CDCl3):
d
7.38e7.09 (m, 15H), 4.72 (d, J¼3.6 Hz, 1H),
2.76 (dd, J¼8.8, 14.0 Hz, 1H), 2.70e2.61 (m, 2H), 2.56 (dd, J¼6.4,
(400 MHz, CDCl3):
d
7.02e7.00 (m, 1H), 6.99 (d, J¼8.4 Hz, 2H), 6.79
14.0 Hz, 1H), 2.41e2.33 (m, 1H), 1.79 (br s, 1H); 13C NMR
(dd, J¼3.6, 4.8 Hz, 1H), 6.75 (d, J¼8.4 Hz, 2H), 6.58 (dd, J¼1.2, 3.6 Hz,
(100 MHz, CDCl3):
d
143.44, 140.88, 140.83, 129.18 (2ꢁ), 129.07
1H), 3.75 (s, 3H), 3.07e2.93 (m, 3H), 1.30 (s, 9H); 13C NMR
(2ꢁ), 128.33 (2ꢁ), 128.24 (2ꢁ), 128.19 (2ꢁ), 127.15, 126.01 (2ꢁ),
(100 MHz, CDCl3):
d
219.61, 158.41, 140.10, 129.87 (2ꢁ), 127.63,
125.86, 125.79, 73.60, 49.42, 35.96, 33.99.
126.44, 125.79, 123.80, 113.48 (2ꢁ), 55.10, 44.17, 34.98, 32.47, 30.49,
26.17 (3ꢁ); Anal. Calcd for C19H22O2S: C, 72.57; H, 7.05. Found: C,
72.81; H, 7.27. Single-crystal X-ray diagram: crystal of compound 4s
was grown by slow diffusion of EtOAc into a solution of compound
4s in CH2Cl2 to yield colorless prisms. The compound crystallizes in
4.2.24. Compound (7).10 Platinum oxide (10%, 20 mg) was added to
a solution of compound 4a (150 mg, 0.5 mmol) in EtOAc (20 mL) at
rt. Then hydrogen was bubbled into the mixture for 10 min, and the
mixture was stirred at rt for 20 h. The reaction mixture was filtered
and evaporated to yield crude product. Purification on silica gel
(hexanes/EtOAc¼6/1) afforded compound 7. Yield¼85% (128 mg);
Colorless solid; mp¼75e77 ꢀC (recrystallized from hexanes and
EtOAc); HRMS (ESI, Mþþ1) calcd for C22H21O 301.1592, found
the monoclinic crystal system, space group
P 1 21/c 1,
3
ꢀ
ꢀ
ꢀ
ꢀ
a¼9.8895(15) A, b¼5.6828(9) A, c¼30.188(5) A, V¼1677.7(4) A ,
Z¼4, dcalcd¼1.245 g/cm3, F(000)¼672, 2
q
range 1.36e26.47ꢀ, R in-
dices (all data) R1¼0.0756, wR2¼0.0918.
301.1589; 1H NMR (400 MHz, CDCl3):
d 7.35e7.13 (m, 15H),
4.2.20. Compound (4t). Yield¼76% (147 mg); Colorless oil; HRMS
(ESI, Mþþ1) calcd for C25H23O4 387.1596, found 387.1599; 1H NMR
4.03e3.99 (m, 1H), 3.14 (dd, J¼8.0, 14.4 Hz, 2H), 2.77 (dd, J¼6.8,
14.4 Hz, 2H).
(400 MHz, CDCl3):
d
8.00 (d, J¼8.4 Hz, 2H), 7.31 (d, J¼8.4 Hz, 2H),
6.95 (d, J¼8.8 Hz, 2H), 6.73 (d, J¼8.8 Hz, 2H), 6.64 (d, J¼8.0 Hz, 1H),
6.54 (dd, J¼2.0, 8.0 Hz, 1H), 6.48 (d, J¼2.0 Hz, 1H), 5.87 (s, 2H), 3.75
(s, 3H), 3.28 (t, J¼5.2 Hz, 1H), 3.20 (dd, J¼5.2, 9.6 Hz, 1H), 3.16
(dd, J¼5.2, 9.6 Hz, 1H), 2.44 (s, 3H); 13C NMR (100 MHz, CDCl3):
Acknowledgements
The authors would like to thank the National Science Council of
the Republic of China for its financial support (NSC 102-2113-M-
037-005-MY2).
d
197.97, 158.15, 147.25, 146.07, 143.83, 135.17, 130.10, 129.75 (2ꢁ),

M.-Y. Chang et al. / Tetrahedron 70 (2014) 2257e2263
2263
X.; Chi, Y. R. Org. Lett. 2011, 13, 5366; (i) Gao, L.; Hwang, G.-S.; Ryu, D. H. J. Am.
Chem. Soc. 2011, 133, 20708.
Supplementary data
3. (a) Aggarwal, V. K.; Smith, H. W.; Jones, R. V. H.; Fieldhouse, R. Chem. Commun.
1997, 1785; (b) Aggarwal, V. K.; Smith, H. W.; Hynd, G.; Jones, R. V. H.; Field-
house, R.; Spey, S. E. J. Chem. Soc., Perkin Trans. 1 2000, 3267; (c) Aggarwal, V. K.;
Alonso, E.; Fang, G.; Ferrara, M.; Hynd, G.; Porcelloni, M. Angew. Chem., Int. Ed.
2001, 40, 1433.
Scanned photocopies of 1H and 13C NMR spectral data were
supported. Supplementary data related to this article can be found
at http://dx.doi.org/10.1016/j.tet.2014.02.024.
4. Hartikka, A.; Arvidsson, P. I. J. Org. Chem. 2007, 72, 5874.
5. (a) Fu, Q.; Yan, C.-G. Tetrahedron 2013, 69, 5841 Ferrary, T.; David, E.; Milanole,
G.; Besset, T.; Jubault, P.; Pannecuucke, X. Org. Lett. 2013, 15, 5598.
6. Xie, H.; Zu, L.; Li, H.; Wang, J.; Wang, W. J. Am. Chem. Soc. 2007, 129, 10886.
7. Zhao, Y.-H.; Zhao, G.; Cao, W.-G. Tetrahedron: Asymmetry 2007, 18, 2462.
8. (a) Chang, M.-Y.; Wu, M.-H. Tetrahedron 2012, 68, 9616; (b) Chang, M.-Y.; Chan,
C.-K.; Lin, S.-Y.; Wu, M.-H. Tetrahedron 2013, 69, 9616; (c) Chang, M.-Y.; Tai, H.-
Y.; Chen, Y.-L.; Hsu, R.-T. Tetrahedron 2012, 68, 7941; (d) Chang, M.-Y.; Wu, M.-
H.; Tai, H.-Y. Org. Lett. 2012, 14, 3936; (e) Chang, M.-Y.; Tsai, C.-Y.; Wu, M.-H.
Tetrahedron 2013, 69, 6364.
9. CCDC 968619 (4d), 966502 (4l), 966501 (4r), and 970284 (4s) contain the
supplementary crystallographic data for this paper. This data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: 44-1223-336033; e-mail:
deposit@ccdc.cam.ac.uk).
References and notes
1. For reviews, see: (a) Donaldson, W. A. Tetrahedron 2001, 57, 8589; (b) Lebel, H.;
Marcoux, J.-F.; Molinaro, C.; Charette, A. B. Chem. Rev. 2003, 103, 977; (c) Pie-
truszka, J. Chem. Rev. 2003, 103, 1051; (d) Reissig, H.-U.; Zimmer, R. Chem. Rev.
2003, 103, 1151; (e) Wessjohann, L. A.; Brandt, W. Chem. Rev. 2003, 103, 1625; (f)
Brackmann, F.; de Meijere, A. Chem. Rev. 2007, 107, 4493; (g) Pellissier, H. Tet-
rahedron 2008, 64, 7041; (h) Chen, D. Y.-K.; Pouwer, R. H.; Richard, J.-A. Chem.
Soc. Rev. 2012, 41, 4631; (i) Shi, M.; Lu, J.-M.; Wei, Y.; Shao, L.-X. Acc. Chem. Res.
2012, 45, 641; (j) Shi, M.; Shao, L.-X.; Lu, J.-M.; Wei, Y.; Mizuno, K.; Maeda, H.
Chem. Rev. 2010, 110, 5883.
2. For selected examples, see: (a) Sohn, S. S.; Bode, J. W. Angew. Chem., Int. Ed.
2006, 45, 6021; (b) Kakei, H.; Sone, T.; Sohtome, Y.; Matsunaga, S.; Shibasaki, M.
J. Am. Chem. Soc. 2007, 129, 13410; (c) Bode, J. W.; Sohn, S. S. J. Am. Chem. Soc.
2007, 129, 13798; (d) Zhu, S.; Perman, J. A.; Zhang, X. P. Angew. Chem., Int. Ed.
2008, 47, 8460; (e) Sherrill, W. M.; Rubin, M. J. Am. Chem. Soc. 2008, 130, 13804;
(f) Li, G.-Q.; Dai, L.-X.; You, S.-L. Org. Lett. 2009, 11, 1623; (g) Phan, D. H. T.; Kou,
K. G. M.; Dong, V. M. J. Am. Chem. Soc. 2010, 132, 16354; (h) Lv, H.; Mo, J.; Fang,
10. (a) Diez-Barra, E.; de la Hoz, A.; Loupy, A.; Martinez-Gonzalez, A.; Martinez-
Merino, V.; Merino, S.; Paugam, R.; Sanchez-Verdu, P.; Sansoulet, J.; Torres, J.
Tetrahedron 1997, 53, 3659; (b) Armesto, D.; Horspool, W. M.; Martin, J. A. F.;
Perez-Ossorio, R. Tetrahedron Lett. 1985, 26, 5217.