Enantioselective Pd-catalyzed hydrogenation of tetrasubstituted olefins of cyclic β-(arylsulfonamido)acrylates

Article abstract of DOI:10.1016/j.tetlet.2012.03.035

Asymmetric hydrogenation of tetrasubstituted olefins of cyclic β-(arylsulfonamido)acrylates produces the corresponding cyclic β-(arylsulfonamido)propionates using Pd(OCOCF₃) ₂/diphosphine complexes as catalysts in the presence of TFA

Full text of DOI:10.1016/j.tetlet.2012.03.035

Tetrahedron Letters 53 (2012) 2560–2563
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Enantioselective Pd-catalyzed hydrogenation of tetrasubstituted olefins
of cyclic b-(arylsulfonamido)acrylates
Chang-Bin Yu a, Kai Gao a, Qing-An Chen a, Mu-Wang Chen a, Yong-Gui Zhou a,b,
⇑
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Asymmetric hydrogenation of tetrasubstituted olefins of cyclic b-(arylsulfonamido)acrylates produces
the corresponding cyclic b-(arylsulfonamido)propionates using Pd(OCOCF3)2/diphosphine complexes as
catalysts in the presence of TFA with up to 96% ee.
Received 27 January 2012
Revised 1 March 2012
Accepted 9 March 2012
Available online 15 March 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Tetrasubstituted olefins
Cyclic b-(arylsulfonamido)acrylates
Asymmetric hydrogenation
Palladium complex
Enantiomerically pure b-amino acids and their derivatives are
important chiral building blocks for the synthesis of many natural
products and biologically active compounds.1 They are also key
structural elements of b-peptides, b-lactams.2,3 Among them, cyclic
b-(arylsulfonamido)propionate acid derivatives drew extreme
attention of researchers due to their wide chemical and biological
activities.4 In the past few decades, many methods have been suc-
cessfully developed to these b-amino acid deriatives.5 However,
despite the importance of b-amino acid derivatives, few researches
have been done on the synthesis of chiral cyclic b-(arylsulfonamido)
propionates.
Previous work:
O
O
O
S
O
O
S
O
H+
S
"Pd"
H2
NH
NH
*
N
R
R
R
Enesulfonamides
N-Sulfonylimines
This work:
RO2C
NSO2Ar
Dynamic Kinetic Resolution
RO2C
NHSO2Ar
H+
+
1
RO2C
NHSO2Ar
RO2C
NSO2Ar
According to retrosynthetic analysis of chiral b-(arylsulfonami-
do)propionates, asymmetric hydrogenation of tetrasubstituted ole-
fins of the corresponding cyclic b-(arylsulfonamido)acrylates is one
of the most atom economic and efficient approaches.
"Pd"
H2
2
Scheme 1. Synthesis of chiral cyclic b-amino acids.
Although some examples on the asymmetric hydrogenation of
tetrasubstituted unfunctionalized and functionalized olefins have
been successfully reported recently,6 exploring new catalyst system
to asymmetric hydrogenation of tetrasubstituted olefins of cyclic
b-(arylsulfonamido)acrylates is highly desirable. Recently, the
chiral palladium complexes have been successfully applied to asym-
metric hydrogenation of imines, ketones, simple indoles and
pyrroles by us7 and other groups.8 Very recently, we reported the
asymmetric hydrogenation of enesulfonamides.9 The mechanism
study showed that the hydrogenation was conducted via Brønsted
acid catalyzed tautomerization of enesulfonamides to N-sulfonyli-
mine intermediates (Scheme 1). Considering that the cyclic
b-(arylsulfonamido)acrylates 1 can be readily transformed into
the corresponding sulfonylimine intermediates through isomeriza-
tion in the presence of Brønsted acid (Scheme 1). We envision that
the active imine intermediates should be easily hydrogenated with
a proper catalytic system. In this Letter, asymmetric hydrogenation
of cyclic b-(arylsulfonamido)acrylates is successfully developed
using Pd(OCOCF3)2/DuanPhos complex as catalysts in the presence
of TFA with up to 96% ee.
Cyclic b-(arylsulfonamido)acrylates 1 can be readily synthe-
sized from the corresponding cyclic b-ketoester and substituted
arylsulfonamides by slightly modified literature procedures in
moderate to good yields (Scheme 2).10
Our initial study began with 1i as the model substrate and
Pd(OCOCF3)2/(R,Sp)-JosiPhos as the catalyst on the basis of our
⇑
Corresponding author. Tel./fax: +86 411 84379220.
E-mail address: ygzhou@dicp.ac.cn (Y.-G. Zhou).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.035

C.-B. Yu et al. / Tetrahedron Letters 53 (2012) 2560–2563
2561
Table 1
RO2C
O
RO2C
NHSO2Ar
Toluene, TsOH
Reflux
The effect of additives on the reactivity and enantioselectivitya
+
ArSO2NH2
5 equiv.
( )n
EtO2C
NHTs
EtO2C
NHTs
*
( )n
1
Pd(OCOCF3)2/(R,Sp)-JosiPhos
2 (600 psi), 70 oC, TFE, additive
*
H
Scheme 2. Synthesis of b-(arylsulfonamido)acrylates 1.
1i
2i
Entry
Additive
—
Convb (%)
eec (%)
1
2
57
70
64
63
previous successful hydrogenation of activated enesulfonamides.9
However, low conversion (57%) and moderate 64% ee value were
observed (Table 1, entry 1). To enhance the activity and enantiose-
lectivity, a number of Brønsted acids were tested, the results are
L
-Tartaric acid
3
68
63
D
-Tartaric acid
4
5
6
7
8
Benzoic acid
4-Nitrobenzoic acid
Phthalic acid
Trifluoro-acetic acid
2,3,4,5,6-Pentafluorobenzoic acid
2-Hydroxy-3,5-dinitrobenzoic acid
Morpholine-TFA
80
61
68
>95
28
77
68
76
81
82
81
77
summarized in Table 1. When 1 equiv of L-tartaric acid was added,
the conversion was improved and with the almost same enantiose-
lectivity (63% ee, Table 1, entry 2). The chirality of the additive had
no influence on the hydrogenation (Table 1, entries 2 vs 3). Other
Brønsted acids also were screened, giving incomplete conversions
but still with moderate enantioselectivities (Table 1, entries 4–6).
When strong acid CF3CO2H (TFA) was added, full convention and
9
10
>95
27
a
Conditions: 0.125 mmol of 1i, Pd(OCOCF3)2 (0.0025 mmol), (R,Sp)-JosPhos
(0.003 mmol), H2 (600 psi), additive (100 mol %), 3 mL of TFE, 16 h, 70 °C.
Determined by 1H NMR on the crude mixture.
b
c
Determined by HPLC.
Table 2
Ligand screening for the Pd-catalyzed asymmetric hydrogenation of 1ia
EtO2C
NHTs
EtO2C
NHTs
*
Pd(OCOCF3)2/Ligand
H2 (600 psi), 70 oC, TFE, TFA
*
1i
2i
Entry
Ligand
Convb (%)
eec (%)
1
2
3
4
5
6
7
8
9
10
11
12e
13f
14g
L1
L2
L3
L4
L5
L6
L7
L8
>95
>95
64
31
60
29
71
62
47
81
78
75
68
80
79
58
77
89
91
90
90
92
L9
>95
71
L10
L11
L11
L11
L11
O
97d
92d
95d
92d
O
Pt-Bu2
CH3
PCy2
CH3
Ph2P
Ph2P
O
O
O
O
PPh2
PPh2
PPh2
PPh2
Fe
Fe
H
H
O
O
L1: (R)-(S)-PPF-Pt-Bu2 L2: (R,Sp)-JosiPhos
L3: (R)-SynPhos
L4: (S)-SegPhos
P
P
PPh2
PPh2
O
O
PPh2 MeO
PPh2 MeO
PPh2
PPh2
L5
L6
L7: (S)-MeO-Biphep L8
: (R,R)-Me-DuPhos
: (S)-H8-BINAP
: (R)-C4-TunePhos
t-Bu
i-Pr
P
t-Bu
P
P
P
H
P
i-Pr
i-Pr
H
H
P
H
t-Bu
t-Bu
i-Pr
L10: (S)-BINAPINE
L11
: (1R,1'R,2S,2'S)-DuanPhos
L9
: (S,S)-i-Pr-DuPhos
a
The reaction was carried out with 1i (0.125 mmol), Pd(OCOCF3)2 (0.0025 mmol), ligand (0.003 mmol), H2 (600 psi), TFA (100 mol %), and 3 mL TFE under directed
condition for 16 h.
b
Determined by 1H NMR on the crude mixture.
Determined by HPLC.
Isolated yield.
H2 (800 psi).
H2 (400 psi).
H2 (200 psi).
c
d
e
f
g

2562
C.-B. Yu et al. / Tetrahedron Letters 53 (2012) 2560–2563
Table 3
NHTs
EtO2C
NHTs
∗
EtO2C
Pd-catalyzed asymmetric hydrogenation of 1a
∗
∗
∗
RO2C
NHSO2Ar
RO2C
NHSO2Ar
Bn
Pd(OCOCF3)2/L11
2q
2p
H
2 (200 psi), 70 oC, TFE, TFA
Conv. >95%, d.r. = 68:32
Ee. = 57% and 62%
Conv. >95%, d.r. = 1:1
1
2
Ee. = 96% and 89%
Entry
Rof 1
Ar of 1
Yieldb (%)
eec (%)
Scheme 4. Asymmetric hydrogenation of 1p and 1q.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15e
Me
Et
n-Bu
n-Hex
Bn
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-ClC6H4
84 (2a)
92 (2b)
93 (2c)
89 (2d)
93 (2e)
91 (2f)
90 (2g)
92 (2h)
92 (2i)
90 (2j)
91 (2k)
93 (2l)
92 (2m)
93 (2n)
87 (2o)
91 (+)
93 (+)
90 (+)
92 (+)
89 (+)
86 (+)
92 (+)
91 (+)
92 (1S,2R)d
90 (+)
89 (+)
91 (+)
87 (+)
91 (+)
89 (+)
O
S
O
S
O
O
HN
HO
EtO2C
HN
LiAlH4, THF
RT
C6H5CH2CH2
i-Pr
Me
Et
n-Bu
n-Hex
Bn
2o
3
(1S,2R)-
Cl
Yield: 82%, Ee: 89%
Cl
Ee: 89%
Scheme 5. Synthesis of drug intermediate 3.
C6H5CH2CH2
i-Pr
Et
14%
EtO2C
71%
D
TFA, TFE, D2
NHTs
a
The reaction was carried out with 1 (0.125 mmol), Pd(OCOCF3)2 (0.0025 mmol),
ligand (0.003 mmol), TFA (0.125 mmol), and 3 mL TFE under directed condition for
16 h.
EtO2C
NHTs
α
β
(1R,1'R,2S,2'S)-DuanPhos
b
Pd(OCOCF3)2
Isolated yield.
c
7%
D
1i
ee was determined by HPLC.
The absolute configuration of 2i was determined by X-ray analysis. Other
products’ configurations were assigned by analogy to 2i.
85% D
EtO2C
d
NHTs
TFA, d3-TFE, H2
e
4 mol % catalyst was used.
Scheme 6. Isotopic labeling experiments using D2 and d3-TFE.
The hydrogen pressure had no apparent effect on enantioselectivity
(Table 2, entries 12 and 13). When the H2 pressure was lowered to
200 psi, the highest ee value (92%, Table 2, entry 14) was obtained.
Under the optimal reaction conditions, a wide variety of cyclic
b-(arylsulfonamido)acrylates 1 were explored to examine the reac-
tion scope. As shown in Table 3, the substrates with steric differ-
ence in esters resulted in excellent enantioselectivities (86–93%
ee, entries 1–15, Table 3). The phenyl 4-methylphenyl or 4-chloro-
phenyl sulfonamide group substituted cyclic b-(arylsulfonamido)
acrylates 1 can be successfully hydrogenated, giving the corre-
sponding cyclic b-(arylsulfonamido)propionates in 87–92% ee (en-
tries 8–15, Table 3).
The absolute configuration of hydrogenation product 2i was
ambiguously assigned to be (1S,2R) (Scheme 3) by X-ray crystallo-
graphic analysis after recrystallization from CH2Cl2/hexane.
In addition to five-membered cyclic b-(arylsulfonamido)acry-
lates, the hydrogenation of the six-membered cyclic b-(tosylamido)
acrylates 1p and a cyclic ethyl 2-benzyl-3-(tosylamido)but-2-eno-
ate 1q was also studied. Under the above optimal conditions, mod-
erate to excellent ees but poor diastereoselectivities were obtained
(Scheme 4).
Scheme 3. The absolute configuration of 2i.
To further demonstrate the value of this asymmetric hydroge-
nation of tetrasubstituted olefins of cyclic b-(arylsulfonami-
do)acrylates, we focused upon the development of a facile and
expeditious route to key the intermediate 3 of the potential drugs
to Alzheimer’s disease developed by GSK company.4d As can be
seen in Scheme 5, hydrogenation product 2o was reduced with
LiAlH4 in THF to afford the key intermediate 3 in 82% yield without
the loss of the optical purity.
81% ee value were obtained (Table 1, entry 7). Next, some strong
Brønsted acids were also tested (Table 1, entries 8 and 9). Morpho-
line-TFA salt was also tested; only 27% conversion and 77% ee
value were obtained. Eventually, TFA was found to be the optimal
choice.
Subsequently, some commercially available bisphosphine li-
gands were examined under the above optimal conditions
(Table 2). Initially, ferrocenyl ligands L1, L2 gave full conversion
and moderate enantioselectivity (Table 2, entries 1 and 2). Lower
enantioselectivities were also observed with axial chiral ligands
(Table 2, entries 3–7) and ligands L8, L9 (Table 2, entries 8 and 9).
When (S)-BINAPINE was employed, the enantioselectivity was
increased to 89% ee (Table 2, entry 10). It is noteworthy that
(1R,10R,2S,20S)-DuanPhos, developed by Zhang group,11 showed
slightly higher enantioselectivity and activity (Table 2, entry 11).
To investigate the process of the reaction, two isotopic labeling
experiments were carried out. When 1i was subjected to hydrogena-
tion with D2, the deuterium atoms were incorporated to the b-posi-
tion with 71% and
a-position with 14%, respectively (Scheme 6).
When the hydrogenation was carried out in d3-TFE (Scheme 6), 1H
NMR analysis of the hydrogenated product showed that the deute-
rium atoms were taken up to the
a-position with 85% incorporation
and b-position with 7%, respectively. The result suggested that the

C.-B. Yu et al. / Tetrahedron Letters 53 (2012) 2560–2563
2563
Wang, W.-H.; Condon, J. S.; Lin, Y.-I.; Skotnicki, J. S.; Park, K.
US20060211730A1, 2006.; (d) King, D.; Meng, Z.-X.; McDonald, I. M.; Olson,
R. E.; Macor, J. E. US20100240708A1, 2010.; (e) Robert, G. I.; Mary, S. R.;
Raymond, F.; John, T. M.; Andrew, C. J.; Andrew, S.; Robert, T. A.; Jurgen, V. H. R.
WO2010032009A1, 2010.
hydrogenation of 1i was mainly proceeded via N-sulfonylimine
intermediates in the presence of acids, and the tautomerization pro-
cess of enesulfamide to N-sulfonylimine intermediates was faster
than the hydrogenation, which is in fact a dynamic kinetic resolution
process.
In conclusion, we have developed an efficient and highly enantio-
selective Pd-catalyzed hydrogenation of tetrasubstituted olefins of
cyclic b-(arylsulfonamido)acrylates with TFE as the solvent in the
presence of TFA, giving the chiral cyclic arylsulfonamido substituted
b-amino acid derivatives with up to 96% ee. Further exploring the
applications of this method in various asymmetric syntheses of bio-
logically active compounds is currently underway.
5. (a)Enantioselective Synthesis of b-Amino Acids; Juaristi, E. C., Soloshonok, V. A.,
Eds., 2nd ed.; Wiley-VCH Ltd: New York, 2005; (b) Frackenpohl, J.; Arvidsson, P.
I.; Schreiber, J. V.; Seebach, D. Chembiochem 2001, 2, 445; (c) Wu, J.; Hou, X.-L.;
Dai, L.-X. J. Org. Chem. 2001, 65, 1344; (d) Winkler, M.; Martínková, L.; Knall, A.
C.; Krahulec, S.; Klempier, N. Tetrahedron 2005, 61, 4249; (e) Preiml, M.;
Hillmayer, K.; Klempier, N. Tetrahedron Lett. 2003, 44, 5057; (f) Liu, M.; Sibi, M.
P. Tetrahedron 2002, 58, 7991; (g) Perlmutter, P.; Rose, M.; Vounatsos, F. Eur. J.
Org. Chem. 2003, 756; (h) Fustero, S.; Sánchez-Roselló, M.; Sanz-Cervera, J. F.;
Aceña, J. L.; Pozo, C. D.; Fernández, B.; Bartolomé, A.; Asensio, A. Org. Lett. 2006,
8, 4633; (i) Davis, F. A.; Theddu, N. J. Org. Chem. 2010, 75, 3814; (j) Eniko, F.;
Ferenc, F. Chem. Eur. J. 2007, 13, 6397; (k) Yamazaki, T.; Zhu, Y. F.; Probstl, A.;
Chadha, R. K.; Goodman, M. J. Org. Chem. 1991, 56, 6644; (l) LePlae, P. R.;
Umezawa, N.; Lee, H. S.; Gellman, S. H. J. Org. Chem. 2001, 66, 5629; (m)
Aggarwal, V.; Roseblade, S.; Alexander, R. Org. Biomol. Chem. 2003, 1, 684; (n)
Aggarwal, V. K.; Roseblade, S. J.; Barrell, J. K.; Alexander, R. Org. Lett. 2002, 4,
1227; (o) Frró, E.; Fulöp, F. Org. Lett. 2003, 5, 1209.
6. (a) Dupau, P.; Bruneau, C.; Dixneuf, P. H. Adv. Synth. Catal. 2001, 343, 331; (b)
Zhang, Z.-G.; Zhu, G.-X.; Jiang, Q.-Z.; Xiao, D.-M.; Zhang, X. J. Org. Chem. 1999,
64, 1774; (c) Burk, M. J.; Bedingfield, K. M.; Kiesman, W. F.; Allen, J. G.
Tetrahedron Lett. 1999, 40, 3093; (d) Sawamura, M.; Kuwano, R.; Ito, Y. J. Am.
Chem. Soc. 1995, 117, 9602; (e) Burk, M. J.; Gross, M. F.; Marinez, J. P. J. Am.
Chem. Soc. 1995, 117, 9375; (f) Dobbs, D. A.; Vanhessche, K. P. M.; Brazi, E.;
Rautenstrauch, V.; Lenoir, J. Y.; Genét, J. P.; Wiles, J.; Bergens, J. W. Angew.
Chem., Int. Ed. 2000, 39, 1992; (g) Tang, W.-J.; Wu, S.-L.; Zhang, X. J. Am. Chem.
Soc. 2003, 125, 9570; (h) Schrems, M. G.; Neumann, E.; Pfaltz, A. Heterocycles
2008, 76, 771; (i) Wüstenberg, B.; Pfaltz, A. Adv. Synth. Catal. 2008, 350, 174; (j)
Troutman, M. V.; Appella, D. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121,
4916; (k) Schrems, M. G.; Neumann, E.; Pfaltz, A. Angew. Chem., Int. Ed. 2007, 46,
8274.
Acknowledgments
We are grateful to financial support from National Natural
Science Foundation of China (21125208 & 21032003), and National
Basic Research Program of China (2010CB833300).
Supplementary data
Supplementary data associated with this article can be found, in
theonline version, at http://dx.doi.org/10.1016/j.tetlet.2012.03.035.
References and notes
7. (a) Wang, Y.-Q.; Lu, S.-M.; Zhou, Y.-G. Org. Lett. 2005, 7, 3235; (b) Wang, Y.-Q.;
Zhou, Y.-G. Synlett 2006, 1189; (c) Wang, Y.-Q.; Yu, C.-B.; Wang, D.-W.; Wang,
X.-B.; Zhou, Y.-G. Org. Lett. 2008, 10, 2071; (d) Yu, C.-B.; Wang, D.-W.; Zhou, Y.-
G. J. Org. Chem. 2009, 74, 5633; (e) Chen, M.-W.; Duan, Y.; Chen, Q.-A.; Wang,
D.-S.; Yu, C.-B.; Zhou, Y.-G. Org. Lett. 2010, 12, 5075; (f) Wang, D.-S.; Chen, Q.-A.;
Li, W.; Yu, C.-B.; Zhou, Y.-G.; Zhang, X. J. Am. Chem. Soc. 2010, 132, 8909; (g)
Wang, D.-S.; Tang, J.; Zhou, Y.-G.; Chen, M.-W.; Yu, C.-B.; Duan, Y.; Jiang, G. F.
Chem. Sci. 2011, 2, 803; (h) Duan, Y.; Chen, M.-W.; Ye, Z.-S.; Wang, D.-S.; Chen,
Q.-A.; Zhou, Y.-G. Chem. Eur. J. 2011, 17, 7193; (i) Wng, D.-S.; Ye, Z.-S.; Chen, Q.-
A.; Zhou, Y.-G.; Yu, C.-B.; Fan, H.-J.; Duan, Y. J. Am. Chem. Soc. 2011, 133, 8866.
8. (a) Abe, H.; Amii, H.; Uneyama, K. Org. Lett. 2001, 3, 313; (b) Nanayakkara, P.;
Alper, H. Chem. Commun. 2003, 2384; (c) Suzuki, A.; Mae, M.; Amii, H.;
Uneyama, K. J. Org. Chem. 2004, 69, 5132; (d) Yang, Q.; Shang, G.; Gao, W.-Z.;
Deng, J.-G.; Zhang, X. Angew. Chem., Int. Ed. 2006, 45, 3832; (e) Rubio-Pérez, L.;
Pérez-Flores, F. J.; Sharma, P.; Velasco, L.; Cabrera, A. Org. Lett. 2009, 11, 265; (f)
Goulioukina, N. S.; Bondarenko, G. N.; Bogdanov, A. V.; Gavrilov, K. N.;
Beletskaya, I. P. Eur. J. Org. Chem. 2009, 510.
1. (a) Gademann, K.; Hintermann, T.; Schreiber, J. V. Curr. Med. Chem. 1999, 6, 905;
(b) Gelman, S. H. Acc. Chem. Res. 1998, 31, 173; (c) Seebach, D.; Abele, S.;
Gademann, K.; Guichard, G.; Hintermann, T.; Jaun, B.; Matthews, J. L.; Schreiber,
J. V. Helv. Chim. Acta 1998, 81, 932; (d) Fulop, F. Chem. Rev. 2001, 101, 2181; (e)
Cheng, R. P.; Gellman, S. H.; DeGrado, W. F. Chem. Rev. 2001, 101, 3219; (f)
Spatola, A. F. Chemistry and biochemistry of amino acids. In Peptides and
Proteins; Weinstein, B., Ed.; Marcel Dekker: New York, 1983; (g) Hawkins, J. M.;
Lewis, T. A. J. Org. Chem. 1994, 59, 649; (h) Seebach, D.; Matthews, J. L. Chem.
ˇ ˇ
ˇ
Commun. 1997, 2015; (i) Štefane, B.; Brozic, P.; Vehovc, M.; Rizner, T. L.; Gobec,
S. Eur. J. Med. Chem. 2009, 44, 2563; (j) Lengyel, G. A.; Frank, R. C.; Horne, W. S. J.
Am. Chem. Soc. 2011, 133, 4246.
2. (a) Porter, E. A.; Wang, X.; Lee, H.-S.; Weisblum, B.; Gellman, S. H. Nature 2000,
404, 565; (b) Wang, X.; Espinosa, J. F.; Gellman, S. H. J. Am. Chem. Soc. 2000, 122,
4821; (c) Appella, D. H.; Christianson, L. A.; Klein, D. A.; Richards, M. R.; Powell,
D. R.; Gellman, S. H. J. Am. Chem. Soc. 1999, 121, 7574.
3. (a) Konishi, M.; Nishio, M.; Saitoh, T.; Miyaki, T.; Oki, T.; Kawaguchi, H. J.
Antibiot. 1989, 42, 1749; (b) Oki, T.; Hirano, M.; Tomatsu, K.; Numata, K.; Kamei,
H. J. Antibiot. 1989, 42, 1756; (c) Werder, M.; Hauser, H.; Abele, S.; Seebach, D.
Helv. Chim. Acta 1999, 82, 1774; (d) Liu, D.; DeGrado, W. F. J. Am. Chem. Soc.
2001, 123, 7553; (e) Hamuro, Y.; Schneider, J. P.; DeGrado, W. F. J. Am. Chem.
Soc. 1999, 121, 12200.
9. Yu, C.-B.; Gao, K.; Wang, D.-S.; Shi, L.; Zhou, Y.-G. Chem. Commun. 2011, 47,
5052.
10. (a) Christoffers, J.; Röbler, U.; Werner, T. Eur. J. Org. Chem. 2000, 701; (b) Geng,
H.-L.; Zhang, W.-C.; Chen, J.; Hou, G.-H.; Zhou, L.; Zou, Y.-P.; Wu, W.-J.; Zhang,
X. Angew. Chem., Int. Ed. 2009, 48, 6052.
4. (a) Becker, D. P.; Husa, R. K.; Moormann, A. E.; Villamil, C. I.; Flynn, D. L.
Tetrahedron 1999, 55, 11787; (b) Levin J. I.; Chen, J. M.; Zask, A.
US006326516B1, 2001.; (c) Levin, J. I.; Li, Z.; Diamantidis, G.; Lovering, F. E.;
11. Liu, D.; Zhang, X. Eur. J. Org. Chem. 2005, 646.