Enantioselective Grignard addition to nitroolefin

Article abstract of DOI:10.1016/j.tetlet.2013.04.100

A novel synthesis of naproxen utilizing nitroaldol, 1,4-addition and Nef reaction is demonstrated where the desired S-enantiomer is achieved by resolution. Further to this first enantioselective 1,4-addition of Grignard reagent to nitroolefin is demonstrated to synthesize derivatives of naproxen.

Full text of DOI:10.1016/j.tetlet.2013.04.100

Accepted Manuscript
Enantioselective Grignard addition to nitroolefin
Prashanth Reddy, Rakeshwar Bandichhor
PII:
S0040-4039(13)00706-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.04.100
TETL 42868
Reference:
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Tetrahedron Letters
Please cite this article as: Reddy, P., Bandichhor, R., Enantioselective Grignard addition to nitroolefin, Tetrahedron
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Graphical Abstract
Enantioselective Grignard addition to nitroolefin
Prashanth Reddy.G,1,2Rakeshwar Bandichhor1*

3
Tetrahedron Letters
journal homepage: www.elsevier.com
Enantioselective Grignard addition to nitroolefin
Prashanth Reddy.G,1,2 Rakeshwar Bandichhor1*
1 API R&D, Integrated Product Development, Innovation Plaza, Dr. Reddy’s Laboratories Ltd, Bachupally, Qutubullapur, Hyderabad-500072, A.P., India
2Center for Environment, Institute of Science and Technology, Jawaharlal Nehru Technological University, Kukatpally, Hyderabad 500072, A.P., India
*Corresponding author Tel.: +91 4044346117; fax: +91 40446285; e-mail: rakeshwarb@drreddys.com
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A novel synthesis of naproxen utilizing nitro aldol, 1,4-addition and Nef reaction is
demonstrated where the desired S-enantiomer is achieved by resolution. Further to this
first enantioselective 1,4-addition of Grignard reagent to nitroolefin is demonstrated to
synthesize derivatives of naproxen.
Available online
Keywords:
Nitroaldol
2009 Elsevier Ltd. All rights reserved.
1,4-Addition
Nef reaction
Anti-inflammatory drug
Naproxen
Naproxen 1 as shown in Figure 1 is a nonsteroidal anti-
inflammatory drug (NASAID)1 meant for the treatment
of pain, fever and inflammation. Although, there are
myriad approaches2 that have been developed for 1,
majority of them useprocess shown in Scheme 1,3that
involves overall five tedious steps. In particular,
synthesis of 1starts with 2-acetyl-6-methoxy
naphthalene 7, which upon Darzen condensation in the
presence of potassium secondary butoxide with
isopropylester of chloroacetic acid followed by
hydrolysis and decarboxylation afforded 2-(6-
methoxynaphthalen-2-yl)propanal 5. Thereafter, the
condensation of 5 with hydroxyl amine in presence of
sulfuric acid followed by dehydration and hydrolysis
with caustic lye afforded the racemic naproxen 2 via 4
and 3.
to nitroolefin. Interestingly, enantioselective 1,4-
addition of organometallic reagents to nitroolefin or
enones is indeed one of the most elegant methods for
the synthesis of enantiomerically pure substituted
nitroalkanes
or
saturated
ketones/aldehydes.
Nevertheless, there has been excellent research work
published in this context.4-6
In order to use reactive Grignard reagents in these
reactions, essentially a stoichiometric amount of a
chiral additive is required to obtain a significant level
of enantioselectivity.7 Catalytic method for the
asymmetric 1,2-addition of Grignard reagents has
recently been reported.8 However to the best of our
knowledge
a catalytic enantioselective Grignard
reagent addition to nitroolefin in 1,4-fashion remains
unprecedented.
Herein, we disclose novel and short formal synthesis
for naproxen 1featuring nitroaldol, dehydration, 1,4-
addition and Nef reaction. Further to this first
enantioselective 1,4-addition of Grignard reagents to
,-unsaturated nitro derivatives, catalyzed by a Cu(I)
and chiral ligand derived species.
Figure1. Naproxen framework
In our endeavor, we attempted to develop an
enantioselective synthesis of 1. In this effort, we end
up unearthing first enantioselective Grignard addition
The synthesis commences with the reaction of 6-
methoxy-2-napthaldehyde 11 with nitromethane in

4
presence of base in acetic acid (in situ salt) at 90-100
°C for 10-12h followed by distillation of solvents
under reduced pressure at <80 °C and isolation from
water at pH 7. Screening of the bases (insitu salts)
suggests that the ammonium acetate9is the choice of
reagent for nitroaldol followed by dehydration to
afford intermediate 10.
Scheme 2.Novel synthetic approach for 1
Screening of NaNO2 equivalence at 90-100 °C
suggests that the 3 equiv NaNO2 (Table 1; entry 2) is
the optimal condition for affording the racemic
naproxen 2.
Table1. Nef reaction conditions
NaNO2
equiv.
%Yield of
S. No.
Temp °C
Time(h)
2
1
2
3
4
5
3
3
3
4
6
70
24
24
24
24
24
72
78
78
76
75
100
120
100
100
After completing the above sequence, we were
intrigued by the potential of a transformation switch
strategy involving the Grignard reagent of 2-methoxy-
6-bromo naphthalene 12 and its 1,4-addition onto the
1-nitroprop-1-ene to afford the intermediate 9 as shown
in Scheme 3.
Scheme 3. Alternate approach to intermediate 9
In this approach, the1-nitroprop-1-ene (precaution:
unsafe to handle; dry distillation should be avoided)
was added to the Grignard reagent of 2-methoxy-6-
bromo naphthalene 12in THFat 0- 5 °C for about 30-60
min that afforded 1,4-addition product, 9in excellent
yield.
Having gained intellectual control over reaction
sequence, we decided to develop enantioselective
version of 1,4-addition. In our endeavor, we screened
various catalysts by involving ligands (L1-L3) and
Cu(I) combinations to achieve enantioselectivity.
Scheme 1. Precedented synthesis for 1
The above isolated nitroolefin 10 was treated with 3M
methyl magnesium chloride in THF at 0-5 °C in
toluene for about 30-60 min. followed by quenching
with ammonium chloride solution at 0-5 °C to obtain
1,4-addition product nitroalkane intermediate 9.
Thereafter, nitroalkane intermediate 9 was subjected to
Nef reaction conditions by employing sodium nitrite in
acetic acid and DMSO for 24 h at 40-50 °C. After the
completion of the reaction (TLC), the reaction mass
was quenched into 10% hydrochloric acid, stirred for
about 15 min and theproduct was extracted into
toluene. The mixture of the organic layer and water,
stirred about 15 min. at 25 to 35°C, adjusted pH to 13-
14 with caustic lye, separated the toluene and washed
with toluene to remove the impurities followed by
extraction into toluene at pH 3-4. Distillation of
toluene afforded racemic naproxen 2.
Figure 2. Structure of ligands

5
Table 2.1,4-Addition by using ligand L1, L2andL3
mol% of CuTC or CuI or Zn(OTf)2as a metal source,
1.2 eq. of methyl magnesium halide in THF or toluene
or dichloromethane and Michael acceptor 10 to obtain
9 as shown in Table 2 (entries 1 and 15). No
selectivity was observed in any of the experiments
conducted. Undeterred with the results, we continued
our efforts to understand the ligand and catalyst
quantity.In a second set of experiments, we employed
20 mol% of (S, S)-isopropyl bisoxazoline (L1)& (R)-
BINAP (L2) ligands in combination with 15 mol% of
CuTC or CuI in THF, 1.2 eq. methyl magnesium halide
in THF, Michael accepter 10 to obtain 9 as shown in
Table 2 (entries 16 and 19). We did not observe
anyselectivity in all the experiments but wecontinued
our efforts also to understand the steric effect of
Grignard reagents and solvents.In a third set of
experiments ethyl, isopropyl and tert-butyl Grignard
reagents were screened in tBuOMe (MTBE) or THF or
CH2Cl2solvent by using ligand 20 mol% of (S,S)-
isopropyl bisoxazoline L1& (R)-BINAP L2 ligands in
combination with 15 mol% of CuTC. Eventually, we
observed slight improvement in selectivity as shown in
Table 2 (entries 23 and 24).Moreover, third set of
experiments revealed that there is steric effect arising
from the combination of the reagent and solvent.
However, there is no major impact of L1 and L2
ligands. Optimistically, we started exploring the
possibility of improving the selectivity with different
class of ligands e.g. chiral ferrocenyl diphosphine (L3).
Thus, in the fourth set of experiments we involved
ligand L3 and keeping the reaction conditions almost
similar to those in the third set.Here excellent
selectivity was observed for the compound 15with
moderate yield as shown in Table 2 (entry 32).
er (R/S)
(HPLC)
Yield
(%)
S.No.
Catalyst
R1/X
Temp.°C
Solvent
Product
1
CuTC/L1
CuTC/L1
CuI/L1
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Me/Cl
Et/Cl
-60
-20
-60
-20
-60
-20
-60
-20
-60
-20
-60
-20
-70
-20
-35
-60
-60
-60
-60
-40
-40
-40
-40
-70
-70
-70
-70
-70
-70
-70
-70
-70
-70
-70
-70
-70
-70
-70
-70
THF
THF
50/50
49/51
75
75
70
70
70
70
75
75
70
70
70
70
70
70
72
72
70
74
75
70
69
67
68
69
65
66
60
68
64
64
62
62
62
60
58
62
64
65
63
9
9
2
3
THF
45/55
9
4
CuI/L1
THF
49/51
9
5
Zn(OTf)2/L1
Zn(OTf)2/L1
CuTC/L2
CuTC/L2
CuI/L2
THF
50/50
9
6
THF
50/50
9
7
THF
49/51
9
8
THF
50/50
9
9
THF
49/51
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
CuI/L2
THF
50/50
9
Zn(OTf)2/L2
Zn(OTf)2/L2
CuTC/L1
CuTC/L1
CuTC/L1
CuTC/L1
CuTC/L2
CuI/L1
THF
49/51
9
THF
50/50
9
THF
49/51
9
DCM
Toluene
THF
49/51
9
48/52
9
48.5/51.5
48/52
9
THF
9
THF
47/53
9
CuI/L2
THF
48/52
9
CuTC/L1
CuTC/ L1
CuTC/ L1
CuTC/ L1
CuTC/ L1
CuTC/ L1
CuTC/ L1
CuTC/L2
CuI/L3
MTBE
MTBE
MTBE
MTBE
MTBE
THF
48.5/51.5
49/51
9
13
14
15
15
15
15
15
9
iPr/Br
tBu/Cl
tBu/Cl
tBu/Cl
tBu/Cl
tBu/Cl
Me/Cl
Et/Cl
49/51
72/28
76.5/23.5
52/48
DCM
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
DCM
THF
53/47
The catalyst precursor CuI along with L1 or L2 was
experimentally compared to other commonly applied
Cu(I) salt e.g. CuTCand it turned out to be superior in
comparison to other salts (vide infra). The influence of
the solvent on the selectivity of the 1,4 addition was
studied with the Cu(I)/L3catalyst. This revealed that
the ethereal solvents performed better in terms of
stereoselectivity. Less bulky solvent furnished the 1,4-
addition product with poor selectivity whereas
sterically more bulky ethers e.g. MTBE provided the
best enantioselectivities (Table 2; entries 23, 24, 31,
&32). Other solvents such as THF and CH2Cl2 led to
almost racemic products (Table 2; entries 25, 26, 35 &
36). Eventually, MTBE as a solvent was found to be a
choice for further studies. Importantly, with branched-
57/43
49.2/50.8
49/51
CuI/L3
13
14
15
15
16
17
15
15
15
15
15
CuI/L3
iPr/Br
tBu/Cl
tBu/Cl
Ph/Cl
49.1/50.9
61/39
CuTC/L3
CuI/L3
98.5/1.5
50.5/49.4
49.4/50.5
70/30
CuTC/L3
CuTC/L3
CuI/L3
Benzyl/Cl
tBu/Cl
tBu/Cl
tBu/Cl
tBu/Cl
tBu/Cl
CuI/L3
75/25
CuI/L3
MTBE
MTBE
MTBE
97.9/2.1
97.9/2.1
97.8/2.2
CuI/L3
CuI/L3
HPLC: Agilent, model no.: 1260, Chiralcell OJ 250 X 4.6, 5 µm, n-Hexane, Ethanol & Acetic
acid=6:4:0.1/9:1:0.001, 0.9 mL/ min, 240 nm.
t
chain and bulky Grignard reagents such as BuMgCl a
dramatic improvement in the enantioselectivity to an er
of 98.5/1.5 was observed (Table 2; entry 32).
We started the screening of the catalytic system by
using
6
mol% of (S,S)-isopropyl bisoxazoline
(L1)and(R)-BINAP (L2) ligands in combination with 5

6
Considering the optimized conditions, establishing the
reproducibility of this transformation was achieved by
successfully conducting three consecutive batches
(Table 2; entries 37, 38& 39).
the alkylcopper species with the C−C double bond of
the activated olefin can result in the formation of the
heterobimetallic complex. Subsequent alkyl transfer
from the copper species to the substrate can generate
the magnesium nitronate. The enantioselectivity of the
overall transformation is presumed to be determined by
the alkyl transfer in a preferential manner. The
resulting chiral magnesium nitronatecan finally
decompose to afford 1,4-addition product.
Assuming the heterobimetallic species and significant
steric hindrance involved in this transformation we
propose a transition state where an addition of R2
group occurred anti to the plane in which hydrogen is
placed affording R configured product. The transition
state plausibly indicates that apart from chiral ligand
(L3), the size of the substituents (R1) in nitroolefin
substrate in combination with the incoming group (R2)
imparts in this stereoselective transformation as shown
in Figure 4.
Substrate scope was also explored. We employed
various
olefinsderived
frombenzaldehyde,
4-
ethoxybenzaldehyde, 4-methoxybenzaldehyde, 4-
chlorobenzaldehyde,
4-flourobenzaldehyde
and
napthaldehyde in the1,4-addition oftBuMgCl and tert-
pentylMgCl as Grignard reagent in MTBE. Some of
the substrates have rendered excellent selectivities
(Table 3; entries 3,5,7 and 10).
Table3. Screening of various nitro olefins and tertiary
Grignard’sby using ligand L3 in MTBE
S.No
.
er (R/S)
(HPLC)
%
(Yield)
Catalyst
R1/R2/X
°C
Product
1
2
CuI/L3
CuI/L3
Ph/tBu/Cl
-70
-70
60/40
77/23
60
65
18
19
4-EtO-Ph/tBu/Cl
3
4
5
6
7
CuI/L3
CuI/L3
CuI/L3
CuI/L3
CuI/L3
CuI/L3
CuI/L3
CuI/L3
4-F-Ph/tBu/Cl
4-MeO-Ph/tBu/Cl
4-Cl-Ph/tBu/Cl
Np/ tBu/Cl
-70
-70
-70
-70
-70
-70
-70
-70
96.5/3.5
71.4/28.6
99/1
84.3/15.7
99/1
87.8/12.2
73/27
96.2/3.8
60
57
55
58
48
50
52
45
20
21
22
23
24
25
26
27
6Mn/tPentyl/Cl
Np/ tPentyl /Cl
8
9
4-MeO-Ph/ tPentyl /Cl
10
4-F-Ph/ tPentyl /Cl
HPLC: Agilent, model no.: 1260, Chiralcell OJ 250 X 4.6, 5 µm, n-Hexane, Ethanol & Acetic
Figure 4. Proposed transition state
acid=9:1:0.001, 0.9/9.5:0.5:0.001; 0.9 mL/ min, 240 nm.
As evident in the various reactions, the chiral
ferrocenyl diphosphine ligand L3 afforded best
enantioselectivity, which can be envisaged by
considering the proposed catalytic cycle outlined in
Figure 3.
The working hypothesis may involve the transfer of an
alkyl ligand from the organometallic reagent to the
Cu(I).
Resulting nitroalkanes(15 and 20) were converted to
corresponding carboxylic acids 28and29by using Nef
reaction conditions(Figure 5) in good yields (70% &
64%) as well as with acceptable enantioselectivities (ee
95.8%, & 93%).
Figure 5. Structure of acid derivatives 28 and 29
Table4. Conversion of nitro alkanes 15 and 20 to
carboxylic acid derivatives 28and29 by using Nef
reaction conditions
er (R/S)
(HPLC)
S.No.
Input
°C
% (Yield)
Product
Figure 3. Proposed C-C bond formation triggered by copper
catalyzed conjugate addition of Grignard reagent
-77.5
-35
28
29
1
15
20
25
25
97.9/2.1
96.5/3.5
70
64
2
HPLC: Agilent, model no.: 1260, (S,S) whelk-01 250 X 4.6 mm (5μ); n-Hexane, Ethanol & Acetic
acid=80:20:0.5.
Subsequent complexation of the –MgBr byproduct
with the nitro groupand formation of the π−complex of

7
3. (a) Kou, F.; Yang, G.; Li, Y. Yiyao Gongye1986, 17, 83-84. (b)
Gaddameedhi, P. R.; Mukkanti, K.; Mohanty, S.; Bandichhor, R.
Chem. Bio. Inter. 2012, 2, 48-51.
In conclusion, we demonstrated the first
enantioselective 1,4-addition of Grignard reagent to
nitroolefin and novel racemic synthesis of naproxen
(which was further resolved into the corresponding
enantiomers) by employing nitro aldol followed by
1,4-addition and Nef reaction.
4. (a) Rossiter, B. E.; Swingle, N. M. Chem. Rev.1992, 92, 771-
806; (b) Alexakis, A. In Transition Metal Catalysed Reactions;
Murahashi, S.-I., Davies, S. G., Eds.; IUPAC Blackwell Science:
Oxford, UK, 1999; p 303; (c) Tomioka, K.; Nagaoka, Y. In
ComprehensiveAsymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: New York, 2000; p 1105; (d) Sibi,
M. P.; Manyem, S. Tetrahedron2000, 56, 8033-8061; (e) Krause,
N.; Hoffmann-Röder, A. Synthesis2001, 171-196; (f) Alexakis, A.
In Methodologiesin Asymmetric Catalysis; Malhotra, S.V., Ed.
American Chemical Society Symponium Series 880; Washington,
DC, 2004. Chapter 4, pp 43 – 59; (g) Krause, N. Modern
Organocopper Chemistry; Wiley-VCH: Weinheim, 2002; (h)
Alexakis, A.; Benhaim, C. Eur. J. Org. Chem.2002, 3211-3236; (i)
Woodward, S. Chem. Soc. Rev.2000, 29, 393; (j) Alexakis, A.;
Bäckvall, J. E.; Krause, N.; Pàmies, O.; Diéguez, M. Chem.
Rev.2008, 108, 2796 - 2823; (k) Haratyunyan, S. R.; den Hartog,
T.; Geurts, K.; Minnaard, A. J.; Feringa, B. L. Chem. Rev.2008,
108, 2824-2852.
Acknowledgement
We thank the management of R&D-CTO & IPDO, Dr.
Reddy’s Laboratories Ltd. for supporting this work.
Cooperation from the colleagues working in analytical
research and development division is highly
appreciated.
Supplementary Material
5. (a) Alexakis, A.; Vastra, J.; Burton, J.; Mangeney, P.
Tetrahedron: Asymmetry1997, 8, 3193; (b) Soai, K.; Hayasaka, T.;
Ugajin, S. J. Chem. Soc., Chem. Commun.1989, 516, (c) Polet, D.,
Alexakis, A. Tetrahedron Lett. 2005, 46, 1529–1532; (d) Eva, R.;
Oscar, P.; Montserrat, D.; Stephane, R.; Alexander, A.
Tetrahedron: Asymmetry2009, 20, 2167-2172; (e) Hojae, C.;
Zihao, H.; Iwao, O. Org.Lett.2004, 6, 2689-2691.
Experimental
procedures
and
compound
characterization data are described in supplementary
material.
DRL-IPD Communication number: IPDO IPM –
00282
6. (a) Hu, X.; Chen, H.; Zhang, X. Angew. Chem., Int. Ed.1999, 38,
3518; (b) Feringa, B. L. Acc. Chem. Res.2000, 33, 346; (c) Escher,
I. H.; Pfaltz, A. Tetrahedron2000, 56, 2879; (d) Yan, M.; Zhou, Z.-
Y.; Chan, A. S. C. Chem. Commun. 2000, 115; (e) Borner, C.;
Dennis, M. R.; Sinn, E.; Woodward, S. Eur. J. Org. Chem.2001,
2435; (f) Watanabe, T.; Knoepfel, T. F.; Carreira, E. M. Org.
Lett.2003, 5, 4557; (g) Rimkus, A.; Sewald, N. Synthesis 2004,
135; (h) Hua, Z.; Vassar, V. C.; Choi, H.; Ojima, I. Proc. Natl.
Acad. Sci. 2004, 101, 5411.
7. von dem Bussche-Huennefeld, J. L.; Seebach, D.
Tetrahedron1992, 48, 5719.
8. Madduri, A. V. R.; Minnaard, A. J.; Harutyunyan, S. R. Chem.
Com.2012, 48, 1478-1480.
The chiral HPLC chromatograms were recorded on
Agilent Model No.: 1260 by Chiralcell OJ 250 X 4.6, 5
µm, mobile phase: n-Hexane, Ethanol & acetic acid
(6:4:0.001)/ (9:1:0.001)/(9.5:0.5:0.001).
References and Notes
1. http://www.rxlist.com/cgi/generic/naproxsod.htm.
2. (a) Harrison, I. T.; Lewis, B.; Nelson, P.; Rooks, W.;
Roszkowski, A.; Tomolonis, A.; Fried, J. H. J. Med. Chem. 1970,
13, 203; (b) Holton, P. G. U.S. 4,515,811, May 7, 1985; (c)
Arnold, R. A.; Matthews, G. J. Ger. 2,805,488, Aug 17, 1978; (d)
Crosby, J. Tetrahedron 1991, 47, 4789; (e) Noyori, R. Asymmetric
Catalysis in Organic Synthesis; Wiley: New York, 1994; (f)
Seyden-Penne, J. Chiral Auxiliaries and Ligands in Asymmetric
Synthesis; Wiley: New York, 1995; (g) Sonawane, H. R.; Bellur, N.
S.; Ahuja, J. R.; Kulkarni, D. G. Tetrahedron:Asymmetry 1992, 3,
163; (h) Giordano, C.; Castaldi, G.; Cavicchioli, S.; Villa, M.
Tetrahedron 1989, 45, 4243; (i) Ohta, T.; Takaya, H.; Kitamura,
M.; Nagai, K.; Noyori, R. J. Org. Chem. 1987, 52, 3174; (j)
Wagenknecht, J. H. U.S. 4,601,797, July 22, 1986. (k) Stille, J. K.;
Su, H.; Brechot, P.; Parinello, G.; Hegedus, L. S. Organometallics
1991, 10, 1183; (l) Babin, J. E.; Whiteker, G. T. WO 9303,839,
March 4, 1993. (m) Hiyama, T.; Wakasa, N.; Kusumoto, T. Synlett
1991, 569; (n) Alper, H.; Hamel, N. J. Am. Chem. Soc. 1990, 112,
2803; (o) Alper, H. Pct Int. Appl. WO 91 03,452, March 21, 1991;
(p) RajanBabu, T. V.; Casalnuovo, A. L. J. Am. Chem. Soc. 1992,
114, 6265; (q) Casalnuovo, A. L.; RajanBabu, T. V. U.S.
5,175,335, Dec 29, 1992; (r) Jacques, J.; Collet, A.; Wilen, S. H.
Enantiomers, Racemates, andResolutions; Wiley: New York, 1981;
(s) Galletti, P.; Pori, M.; Giacomini, D. Synlett2010, 17, 2644-
2648; (t) Shiina, I.; Nakata, K.; Ono, K.; Yu-suke Onda, Y-s.;
Itagaki, M. J.Am. Chem. Soc.2010, 132, 11629-11641; (u) Thalen,
L. K.; Sumic, A.; Bogar, K.; Norinder, J.; Persson, A.K.A;
Backvall, J.-E. J.Org. Chem.2010, 75, 6842-6847.
9. (a) Bandichhor, R.; Lowell, A. N.; Kozlowski, M. C. J. Org.
Chem.2011, 76, 6475-6487; (b) Sheng-Nan, L.; Lan-Ting, X.; Yue,
C.; Ju-Lian, L.; Ling, H. Lett.Org.Chem.2011, 8, 416-422.
10. Palais, L; Babel, L.; Quintard, A.; Belot, S.; Alexakis, A. Org.
Lett.2010, 12, 1988-1991.