Highly enantioselective pictet-spengler reaction catalyzed by SPINOL-phosphoric acids

Article abstract of DOI:10.1002/chem.201103207

Chiral SPINOL-phosphoric acids are highly enantioselective catalysts for the asymmetric Pictet-Spengler reaction of N_b-α-naphthylmethyl tryptamines with a series of aliphatic and aromatic aldehydes, affording optically active tetrahydro-β-carbolines in excellent yields and ee values. The current protocol has been applied in the asymmetric total synthesis of (-)-harmicine. Copyright

Full text of DOI:10.1002/chem.201103207

DOI: 10.1002/chem.201103207
Highly Enantioselective Pictet–Spengler Reaction Catalyzed by SPINOL-
Phosphoric Acids
Dan Huang, Fangxi Xu, Xufeng Lin,* and Yanguang Wang*[a]
Chiral tetrahydro-b-carbolines represent an important
class of heterocycles in organic chemistry. They are structur-
al units in a variety of natural products and important phar-
maceuticals (Figure 1)[1] and useful intermediates for con-
ther applications with substrates, such as Nb-sulfenyliminium
ions or Nb-benzyl tryptamine, were carried out by Hiemstra
et al.[3f,g] and a N-acyliminium ion cyclization cascade was
developed by Dixon et al.[3h] More recently, a BINOL-phos-
phoric acid catalyzed Pictet–Spengler reaction has also been
employed in the natural product synthesis of (ꢀ)-arbori-
AHCTUNGTRENNUNG
cine[3i] as well as novel spiroindolinone compounds.[3j] De-
spite these elegant examples, general and efficient strategies
for catalytic enantioselective protocols that tolerate a broad
substrate scope with high enantioselectivity are in great
demand.
Recently, we designed and synthesized a novel class of
chiral SPINOL-phosphoric acids as Brønsted acids, which
possess geometrically different and more rigid chiral param-
eters compared to the classic BINOL-phosphoric acids.[5] In
our initial experiments, we found that these chiral SPINOL-
phosphoric acids nicely promote the asymmetric Friedel–
Crafts reaction of various indoles with a range of imines,
giving comparable results to BINOL-phosphoric acids. List
et al. independently reported a highly enantioselective kinet-
ic resolution of homoaldols through a transacetalization re-
action by using this novel class of chiral phosphoric acids.[6a]
Lately, they reported a SPINOL-phosphoric acid catalyzed
asymmetric Fischer indolization.[6b] Hu et al. also demon-
strated that SPINOL-phosphoric acids give excellent enan-
tioselectivities in the reaction of indoles with aldimines or
b,g-unsaturated-a-ketoesters.[7] More recently, Zhou et al.
Figure 1. Alkaloids containing a tetrahydro-b-carboline unit.
structing several drugs, such as tadalafil.[2] Consequently, ef-
ficient methods for the preparation of optically pure tetrahy-
dro-b-carbolines are in great demand. Whereas diastereo-
specific Pictet–Spengler reactions are well-known,
approaches to tetrahydro-b-carbolines by using catalytic
enantioselective variants, which provide the optically active
tetrahydro-b-carbolines in a straightforward manner, have
been a long-standing challenge.[3]
Jacobsen et al. reported the first organocatalytic asymmet-
ric Pictet–Spengler reaction of N-acyliminium ions by using
a chiral thiourea catalyst (limited to aliphatic aldehydes, 5–
10 mol% catalyst, 65–81% yield, and 85–93% ee).[3c] Re-
cently, they also reported that chiral thiourea derivatives in
combination with benzoic acid co-catalyze the asymmetric
Pictet–Spengler reaction of electronically and sterically di-
verse imines, providing unprotected tetrahydro-b-carbolines
in high yields and ee values.[3d] Since the pioneering work by
Akiyama and Terada, BINOL-phosphoric acids have found
widespread application as metal-free catalysts.[4] The first ex-
ample of a BINOL-phosphoric acid catalyzed Pictet–Spen-
gler reaction was reported by List et al., using a carboxy-
substituted tryptophan as the key indole substrate
(20 mol% catalyst, 40–96% yield, and 62–96% ee).[3e] Fur-
ꢀ
reported an asymmetric N H insertion reaction, coopera-
tively catalyzed by Rhodium and SPINOL-phosphoric
acids.[8] Herein, we would like to report the first example of
a
SPINOL-phosphoric acid catalyzed enantioselective
Pictet–Spengler reaction.
Since Nb-protected tetrahydro-b-carbolines are key inter-
mediates for the synthesis of biologically active pyrroloqui-
nolones (e.g., tadalafil) through Winterfeldt oxidation, we
carried out the organocatalytic Pictet–Spengler reaction
starting directly from Nb-protected tryptamine. Thus, we se-
lected the reaction between Nb-a-naphthylmethyl trypt-
AHCTUNGTREGaNNNU mine (3a) and p-bromobenzaldehyde (4a) as a model reac-
tion, using 2 mol% of catalyst in benzene at 308C in the
presence of powdered 4 ꢀ molecular sieves. First, we inves-
tigated the influence of the catalyst and a series of chiral
phosphoric acids was screened. As shown in Table 1, the
sterically congested SPINOL-phosphoric acids were found
to be crucial for a high activity and enantioselectivity. Inter-
estingly, catalyst 1 f, which gave excellent enantioselectivities
[a] D. Huang, F. Xu, Prof. Dr. X. Lin, Prof. Dr. Y. Wang
Department of Chemistry, Zhejiang University
Hangzhou 310027 (P. R. China)
Fax : (+86)571-87951512
E-mail: lxfok@zju.edu.cn
orgwyg@zju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103207.
3148
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3148 – 3152

COMMUNICATION
Table 1. Catalyst screening.[a]
Table 2. Screening of the protecting group of tryptamine 3.[a]
Entry
1
Trypt-
amine
PG
t
Product
Yield
[%][b]
ee
[h]
[%][c]
3a
24
5a
96
98
Entry
Catalyst
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
2a
2b
48
16
24
24
8
24
24
24
72
84
80
79
99
96
70
56
87
85
78
87
96
98
79
44
2
3
4
3b
3c
3d
72
24
24
5b
5c
5d
35
98
93
91
90
90
[a] Reaction conditions: catalyst (2 mol%), 3a (0.1mmol), 4a (0.3mmol),
molecular sieves (4ꢀ, 0.15 g), benzene (1mL), 30 8C. [b] Yield of the iso-
lated, pure product. [c] The enantiomeric excess was determined by
chiral HPLC analysis.
5
6
7
3e
3 f
3g
24
24
24
5e
5 f
5g
90
90
92
93
G
(32)[d]
in our previous report,[5] also was the best catalyst for the
present reaction, affording tetrahydro-b-carboline 5a in
96% yield with 98% ee (Table 1, entry 6). However,
BINOL-phosphoric acids 2a and b gave low yields with
poor enantioselectivities in this model reaction (Table 1, en-
tries 7 and 8). These results demonstrate that SPINOL-phos-
phoric acids are highly efficient chiral organocatalysts,
giving higher activities and enantioselectivities than the clas-
sic BINOL-phosphoric acids in some cases.
63
95
[a] Reaction conditions: catalyst 1 f (2 mol%),
3 (0.1mmol), 4a
(0.3mmol), molecular sieves (4ꢀ, 0.15 g), benzene (1mL), 30 8C.
[b] Yield of the isolated, pure product. [c] The enantiomeric excess was
determined by chiral HPLC analysis. [d] 2b (2 mol%) was used.
Table 3. Optimization of the reaction conditions.[a]
We envisioned that the yields and the ee values of tetrahy-
dro-b-carbolines may be remarkably affected by the type of
the Nb-protecting group of the tryptamine. Thus, a variety of
Nb-protected tryptamines 3 were screened in the reaction
with p-bromobenzaldehyde (4a) using 2 mol% of SPINOL-
phosphoric acid 1 f as catalyst, and the results are summar-
ized in Table 2. High yields were obtained, except when Nb-
2-MeO-benzyl tryptamine (3b) and Nb-1-anthracen-9-yl-
methyl tryptamine (3g) were used (Table 2, enties 2 and 7).
High ee values were obtained with various Nb-protected
tryptamines 3a–g, and Nb-a-naphthylmethyl tryptamine (3a)
gave the best enantioselectivity (Table 2, entry 1). In the
case of Nb-benzyl tryptamine (3 f), the classic BINOL-phos-
phoric acid 2b gave poor activity and enantioselectivity
(Table 2, entry 6). It is worth noting that the enantioselectiv-
ity could not be improved when the bulkier Nb-1-anthracen-
9-ylmethyl tryptamine substrate (3g) was used (Table 2,
entry 7).
Entry
Solvent
T [8C]
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
benzene
toluene
xylene
CH2Cl2
ClCH2CH2Cl
THF
benzene
benzene
benzene
30
30
30
30
30
30
23
40
30
24
24
24
8
20
16
48
12
42
96
80
79
95
94
94
81
98
92
98
97
94
91
92
94
95
95
96
8
9[d]
[a] Reaction conditions: catalyst 1 f (2 mol%), 3a (0.1mmol), 4a
(0.3mmol), molecular sieves (4ꢀ, 0.15 g), benzene (1mL), 30 8C.
[b] Yield of the isolated, pure product. [c] The enantiomeric excess was
determined by chiral HPLC analysis. [d] 1 f (1 mol%) was used.
and enantioselectivities, and benzene gave the best result
(Table 3, entry 1). It was also found that increasing or lower-
ing the temperature slightly reduced the enantioselectivity
(Table 3, entries 7 and 8). Moreover, a decrease of the cata-
We also examined different solvents, such as toluene,
xylene, CH2Cl2, ClCH2CH2Cl, and THF (Table 3). In all
cases, the reactions proceeded with good to excellent yields
Chem. Eur. J. 2012, 18, 3148 – 3152
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3149

Y. Wang et al.
lyst loading (from 2 to 1 mol%) resulted in a slight drop of
both the yield and the enantioselectivity (Table 3, entry 9).
With the optimized reaction conditions identified, we
evaluated the substrate scope of the SPINOL-phosphoric
acid catalyzed enantioselective Pictet–Spengler reaction of
Nb-a-naphthylmethyl tryptamine with aldehydes. The repre-
sentative results are summarized in Table 4. All reactions
Scheme 1. Pictet–Spengler reaction of tryptamine 3j with aldehyde 4a.
Table 4. Scope of the reaction.[a]
mized conditions. A remarkable decrease in enantioselectiv-
ity was observed, although an excellent yield was obtained
ꢀ
(Scheme 1). This investigation indicates that the Na H bond
of the tryptamine enhances the enantioselectivity. We be-
lieve that the bifunctional nature of the chiral phosphoric
acid concurrently activates both the nucleophilic group and
the electrophilic iminium intermediate, generated in situ
through hydrogen-bonding interactions in the Pictet–Spen-
gler reaction.
To extend the synthetic value of our approach, the depro-
tection of the tetrahydro-b-carboline products in a conven-
ient way would be of great value (Scheme 2). To our delight,
the a-naphthylmethyl group of 5u was successfully removed
Entry Trypt- R2 (4)
t
Product Yield ee
amine
[h]
[%][b] [%][c]
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3h
3i
p-BrC6H4 (4a)
m-BrC6H4 (4b)
m-ClC6H4 (4c)
m-FC6H4 (4d)
p-NO2C6H4 (4e)
m-NO2C6H4 (4 f)
3,5-(CF3)2C6H3 (4g)
Ph (4h)
p-MeOC6H4 (4i)
piperonyl (4j)
dihydrobenzofuryl (4k) 48 5q
furyl (4l)
Et (4m)
n-pentyl (4n)
iPr (4o)
24 5a
12 5h
12 5i
12 5j
12 5k
12 5l
16 5m
30 5n
40 5o
44 5p
96
94
99
97
99
99
98
90
94
95
91
92
76
98
96
99
96
91
98
95
97
97
96
94
90
97
95
92
93
98
90
91
97
98
95
95
9[d]
10[d]
11[d]
12
13
14
15
16
17
18
10 5r
4
5
5
6
3
5s
5t
5u
5v
5w
Cy (4p)
iPr (4o)
iPr (4o)
12 5x
[a] Reaction conditions: 1 f (2 mol%), 3 (0.1mmol), 4 (0.3mmol), molec-
ular sieves (4ꢀ, 0.15 g), benzene (1 mL), 30 8C. [b] Yield of the isolated,
pure product. [c] The enantiomeric excess was determined by chiral
HPLC analysis. [d] 1e (2 mol%) was used instead of 1 f.
Scheme 2. Synthesis of terahydro-ß-carboline derivate 6.
proceeded in good to excellent yields and with generally ex-
cellent enantioselectivities. It is worth noting that tetrahy-
dro-b-carboline 5q can be employed as a useful intermedi-
ate for the synthesis of PDE5 inhibitor tadalafil.[2] Furfural-
dehyde also gave an exciting result (Table 4, entry 12). Im-
portantly, a series of aliphatic aldehydes was examined and
showed good tolerance (Table 4, entries 13–16, 76–99%
yield, 90–98% ee). We also examined different tryptamines
3h and i under the optimized conditions, which provided
high yields and excellent ee values (Table 4, entries 17 and
18).
The absolute configuration of tetrahydro-b-carbolines 5a–
x was correlated to the absolute configuration of 5u, which
was confirmed to be S by comparing the optical rotation of
its Nb-deprotected product with the literature (see the Sup-
porting Information for details).
by using 10 mol% Pd(OH)2 (10 wt%)/C as catalyst with a
balloon of H2 under mild reaction conditions. After treate-
ment with (Boc)2O, Nb-Boc-tetrahydro-b-carboline derivate
6, which could be employed in Winterfeldt oxidation to
obtain (ꢀ)-quinolactacin B,[9] was obtained in 96% yield
with 97% ee over two steps.
This current protocol was also applied to the asymmetric
total synthesis of (ꢀ)-harmicine (Scheme 3).[10] Initially,
compound 5z was readily prepared in 96% yield by treat-
ment of tryptamine 3a with aldehyde 4q under the opti-
mized reaction conditions. Subsequent deprotection of the
TBDPS group afforded compound 7 in 93% yield with 93%
ee, which was further deprotected under 1 atm H2 to gener-
ate 8 in 94% yield. Finally, cyclization through Mitsunobu
reaction led to the desired (ꢀ)-harmicine in 85% yield with
93% ee, and the total yield was 71% over four steps.
In conclusion, we have developed an efficient SPINOL-
phosphoric acid catalyzed asymmetric Pictet–Spengler reac-
Moreover, tryptamine 3j was examined as a substrate for
the asymmetric Pictet–Spengler reaction under the opti-
3150
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3148 – 3152

Enantioselective Pictet–Spengler Reaction
COMMUNICATION
[3] For reviews, see: a) M. Lorenz,
M. L. Van Linn, J. M. Cook, Curr.
Org. Synth. 2010, 7, 189–223;
b) J. Stçckigt, A. P. Antonchick,
F. Wu, H. Waldmann, Angew.
Chem. 2011, 123, 8692–8719;
Angew. Chem. Int. Ed. 2011, 50,
8538–8564; for selected examples
of asymmetric catalytic Pictet–
Spengler reactions, see: c) M. S.
Taylor, E. N. Jacobsen, J. Am.
Chem. Soc. 2004, 126, 10558–
10559; d) R. S. Klausen, E. N. Ja-
cobsen, Org. Lett. 2009, 11, 887–
890; e) J. Seayad, A. M. Seayad,
Scheme 3. Total synthesis of (ꢀ)-harmicine. TBDPS=tert-butyldiphenylsilyl, TBAF=tetra-n-butylammonium
fluoride, DEAD=diethyl azodicarboxylate.
B. List, J. Am. Chem. Soc. 2006,
128, 1086–1087; f) M. J. Wanner,
R. N. S.
van der Haas,
K. R.
de Cuba, J. H. van Maarseveen,
H. Hiemstra, Angew. Chem. 2007, 119, 7629–7631; Angew. Chem.
Int. Ed. 2007, 46, 7485–7487; g) N. V. Sewgobind, M. J. Wanner, S.
Ingemann, R. de Gelder, J. H. van Maarseveen, H. Hiemstra, J. Org.
Chem. 2008, 73, 6405–6408; h) M. E. Muratore, C. A. Holloway,
A. W. Pilling, R. I. Storer, G. Trevitt, D. J. Dixon, J. Am. Chem. Soc.
2009, 131, 10796–10797; i) M. J. Wanner, R. N. A. Boots, B. Eradus,
R. de Gelder, J. H. van Maarseveen, H. Hiemstra, Org. Lett. 2009,
11, 2579–2581; j) S. Duce, F. Pesciaioli, L. Gramigna, L. Bernardi,
A. Mazzanti, A. Ricci, G. Bartoli, G. Bencivenni, Adv. Synth. Catal.
2011, 353, 860–864.
tion of Nb-a-naphthylmethyl tryptamines with a wide range
of aliphatic and aromatic aldehydes, affording a series of
highly enantioenriched tetrahydro-b-carboline derivatives in
excellent yields and ee values. In addition, the utility of this
method has been demonstrated in the preparation of the
key intermediate of (ꢀ)-quinolactacin B and in the asym-
metric total synthesis of (ꢀ)-harmicine. Applications of the
current protocol in the total synthesis of other natural prod-
ucts are underway.
[4] For pioneering works, see: a) T. Akiyama, J. Itoh, K. Yokota, K. Fu-
chibe, Angew. Chem. 2004, 116, 1592–1594; Angew. Chem. Int. Ed.
2004, 43, 1566–1568; b) D. Uraguchi, M. Terada, J. Am. Chem. Soc.
2004, 126, 5356–5357; for VAPOL-derived phosphoric acids, see:
c) G. B. Rowland, H. Zhang, E. B. Rowland, S. Chennamadhavuni,
Y. Wang, J. C. Antilla, J. Am. Chem. Soc. 2005, 127, 15696–15697;
for TADDOL-derived phosphoric acids, see: d) T. Akiyama, Y.
Saitoh, H. Morita, K. Fuchibe, Adv. Synth. Catal. 2005, 347, 1523–
1526 for recent reviews on chiral Brønsted acid and hydrogen-bond-
donor catalysis, see: e) M. S. Taylor, E. N. Jacobsen, Angew. Chem.
2006, 118, 1550–1573; Angew. Chem. Int. Ed. 2006, 45, 1520–1543;
f) T. Akiyama, J. Itoh, K. Fuchibe, Adv. Synth. Catal. 2006, 348,
999–1010; g) A. G. Doyle, E. N. Jacobsen, Chem. Rev. 2007, 107,
5713–5743; h) T. Akiyama, Chem. Rev. 2007, 107, 5744–5758; i) M.
Terada, Chem. Commun. 2008, 4097–4112; j) X. H. Yu, W. Wang,
Chem. Asian J. 2008, 3, 516–532; k) M. Terada, Synthesis 2010,
1929–1982; l) A. Zamfir, S. Schenker, M. Freund, S. B. Tsogoeva,
Org. Biomol. Chem. 2010, 8, 5262–5276; m) M. Rueping, A. Kuen-
kel, I. Atodiresei, Chem. Soc. Rev. 2011, 40, 4539–4549; n) H. Ya-
mamoto, N. Payette in Hydrogen Bonding in Organic Synthesis
(Ed.: P. M. Pihko), WileyVCH, Weinheim, 2009, Chapter 5, pp. 73–
140; o) D. Kampen, C. M. Reisinger, B. List in Topics in Current
Chemistry, Vol. 291: Asymmetric Organocatalysis (Ed.: B. List),
Springer, Berlin, 2010, pp. 395–456.
Experimental Section
General procedure for the enantioselective Pictet–Spengler reaction: A
mixture of Nb-a-naphthylmethyl tryptamine 3 (0.1 mmol), catalyst 1 f
(0.002 mmol), and 4 ꢀ molecular sieves (0.15 g, powdered) in benzene
(1 mL) was stirred for 5 min at room temperature under a nitrogen at-
mosphere. Subsequently, aldehyde 4 (0.3 mmol) was added, and the mix-
ture was stirred at 308C for the appropriate time. The reaction was moni-
tored by TLC. After the reaction was complete, the reaction mixture was
directly purified by flash column chromatography (eluent: ethyl acetate/
petroleum ether 1:15!1:8 with 2% TEA) to give the desired product 5.
Acknowledgements
We thank the National Natural Science Foundation of China (No.
21032005) and the Fundamental Research Funds of Zhejiang University.
[5] F. X. Xu, D. Huang, C. Han, W. Shen, X. F. Lin, Y. G. Wang, J. Org.
Chem. 2010, 75, 8677–8680.
ˇ
´
[6] a) I. Coric, S. Mꢃller, B. List, J. Am. Chem. Soc. 2010, 132, 17370–
17373; b) S. Mꢃller, M. J. Webber, B. List, J. Am. Chem. Soc. 2011,
133, 18534–18537.
Keywords: phosphoric
acids
·
heterocycles
·
organocatalysis · Pictet–Spengler reaction · tryptamines
[7] C. H. Xing, Y. X. Liao, J. Ng, Q. S. Hu, J. Org. Chem. 2011, 76,
4125–4131.
[8] B. Xu, S. F. Zhu, X. L. Xie, J. J. Shen, Q. L. Zhou, Angew. Chem.
2011, 123, 11685–11688; Angew. Chem. Int. Ed. 2011, 50, 11483–
11486.
[9] a) N. Kakinuma, H. Iwai, S. Takahashi, K. Hamano, T. Yanagisawa,
K. Nagai, K. Tanaka, K. Suzuki, F. Kirikae, T. Kirikae, A. Nakaga-
wa, J. Antibiot. 2000, 53, 1247–1251; b) S. Takahashi, N. Kakinuma,
H. Iwai, T. Yanagisawa, K. Nagai, K. Suzuki, T. Tokunaga, A. Naka-
gawa, J. Antibiot. 2000, 53, 1252–1256; c) X. Q. Zhang, W. Q. Jiang,
Z. H. Sui, J. Org. Chem. 2003, 68, 4523–4526.
[1] a) E. D. Cox, J. M. Cook, Chem. Rev. 1995, 95, 1797–1842; b) M.
Somei, F. Yamada, Nat. Prod. Rep. 2004, 21, 278–311; c) T. Kawasa-
ki, K. Higuchi, Nat. Prod. Rep. 2005, 22, 761–793; d) S. E.
O’Connor, J. J. Maresh, Nat. Prod. Rep. 2006, 23, 532–547; e) Y.-C.
Shen, C.-Y. Chen, P.-W. Hsieh, C.-Y. Duh, Y.-M. Lin, C.-L. Ko,
Chem. Pharm. Bull. 2005, 53, 32–36.
[2] A. Daugan, P. Grondin, C. Ruault, A.-C. Le Monnier de Gouvill, H.
Coste, J. Kirilovsky, F. Hyafil, R. Labaudiniꢂre, J. Med. Chem. 2003,
46, 4525–4532.
Chem. Eur. J. 2012, 18, 3148 – 3152
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3151

Y. Wang et al.
[10] a) T.-S. Kam, K.-M. Sim, Phytochemistry 1998, 47, 145–147; for ex-
amples of the total synthesis of harmicine, see: b) T. Itoh, M. Miya-
zaki, K. Nagata, M. Yokoya, S. Nakamura, A. Ohsawa, Heterocycles
2002, 58, 115–118; c) J. Szawkało, S. J. Czarnocki, A. Zawadzka, K.
Wojtasiewicz, A. Leniewski, J. K. Maurin, Z. Czarnocki, J. Drabo-
wicz, Tetrahedron: Asymmetry 2007, 18, 406–413; d) I. T. Raheem,
P. S. Thiara, E. A. Peterson, E. N. Jacobsen, J. Am. Chem. Soc. 2007,
129, 13404–13405; e) W. A. da Silva, M. T. Rodrigues, Jr., N. Shan-
karaiah, R. B. Ferreira, C. K. Z. Andrade, R. A. Pilli, L. S. Santos,
Org. Lett. 2009, 11, 3238–3241.
Received: October 11, 2011
Published online: February 22, 2012
3152
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3148 – 3152