The smallest organocatalyst in highly enantioselective direct aldol reaction in wet solvent-free conditions

Article abstract of DOI:10.1039/c4ra02690j

The catalytic efficacy of the smallest organocatalyst, l-proline hydrazide, prepared from a cheaply available natural amino acid, such as l-proline, was studied for the direct asymmetric aldol reaction of various ketones with aromatic aldehydes at room temperature in the presence of several acid additives. A loading of 10 mol% of catalyst 1 and p-toluenesulphonic acid as an additive was employed in this reaction, and good yields (up to 99%) with high anti/syn diastereoselectivities (up to 95 : 5) and enantioselectivities (up to >99.9%) were achieved in aqueous media. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4ra02690j

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The smallest organocatalyst in highly
enantioselective direct aldol reaction in wet
Cite this: RSC Adv., 2014, 4, 24311
solvent-free conditions†
a
b
a
Received 27th March 2014
Accepted 15th May 2014
*
Sudipto Bhowmick, Sunita S. Kunte and Kartick C. Bhowmick
DOI: 10.1039/c4ra02690j
www.rsc.org/advances
The catalytic efficacy of the smallest organocatalyst, L-proline hydra- proline-based organocatalysts by primarily modifying their
zide, prepared from a cheaply available natural amino acid, such as basic skeletons in three ways.8–10 One modication was aimed at
L-proline, was studied for the direct asymmetric aldol reaction of the carboxylic-acid functionality,3c,8 another was performed on
various ketones with aromatic aldehydes at room temperature in the the hydroxyl group of 4-hydroxy-L-proline,6a,9 whereas the third
presence of several acid additives. A loading of 10 mol% of catalyst 1 was performed on both groups simultaneously.10 Suitably
and p-toluenesulphonic acid as an additive was employed in this substituted molecules from L-proline and 4-hydroxy-L-proline
reaction, and good yields (up to 99%) with high anti/syn diaster- have been found to act as efficient chiral catalysts for different
eoselectivities (up to 95 : 5) and enantioselectivities (up to >99.9%) asymmetric carbon–carbon bond-forming reactions in aqueous
were achieved in aqueous media.
media.2b The modied catalysts were mostly bulky molecules,
which were conjugates of two or more molecular units such as
4-hydroxy-L-proline or L-proline or its derivatives.2b,8–10 It is well
known that in organic solvents, the smallest organocatalyst,
L-proline, requires high loadings (ꢀ30 mol%) and does not
catalyze the aldol reaction in water.11a Some of the examples are
present in the literature where aldol reactions were performed
under wet solvent-free or solvent-free conditions with moderate
to high asymmetric induction.8w,9f,11b,c In addition, there are few
examples where small modications on the L-proline structure
generated organocatalysts, which showed poor to moderate
catalytic activity in aqueous media.8b,u,v Simple amino acids
could also provide abysmally low yields of aldol product in
water, unless derivatized with a suitable hydrophobic group.3f In
fact, some efficient organocatalysts derived from L-proline
have been reported with double hydrogen-bonding ability in
aqueous media but these catalysts were structurally not very
small.3e,6b,8q,s,12 Now, the question is why would L-proline itself or
a very negligibly modied L-proline-based organocatalyst not be
able to impart good stereoselectivity in aqueous media? Is it
because of any lack of suitable placement of more than one
hydrogen-bonding unit in the catalyst structure? Therefore, we
are interested in identifying an organocatalyst that is structur-
ally very small and the missing link between the L-proline and
its longer derivatives in an aqueous environment. Herein, we
present the smallest chiral organocatalyst, L-proline hydrazide
1, for direct aldol reactions in aqueous environment. Such a
small molecule containing a simple hydrazide unit has never
been reported as an asymmetric organocatalyst for any carbon–
carbon bond-forming reaction in wet solvent-free conditions.
Asymmetric organocatalysis, a metal-free catalysis, is currently a
widely used environmentally benign catalytic methodology in
asymmetric organic synthesis.1 The past two decades have wit-
nessed an explosive growth in the eld of asymmetric organo-
catalysis, in which water has been used as a solvent or co-
solvent.2,3 The pioneering observations reported by Breslow
et al. and Grieco et al. on the Diels–Alder reaction in the early
1980s revealed the new concept regarding the improved effect of
water on reaction rates and regioselectivities compared with
organic solvents.4 The use of water as a reaction media in
organocatalyzed asymmetric organic transformations has many
advantages because it is abundant, safe, and environmentally
benign.3b,5 Moreover, it can signicantly inuence the course of
a reaction via increased hydrophobic–hydrophobic interaction
and hydrogen-bond formation between the reactants and water
molecules, resulting in high reactivity and stereoselectivity.2c,6
Since Breslow's observation, existence of such interactions and
their effects are now a well-documented fact in aqueous-phase
reactions irrespective of “in water” or “on water” concept in the
background.2c,7 In this direction, massive efforts have been
concentrated on the development of L-proline- and 4-hydroxy-L-
aDivision of Organic Synthesis, Department of Chemistry, Visva-Bharati University,
Bolpur, West Bengal-731 235, India. E-mail: kartickc.bhowmick@visva-bharati.ac.in
bDivision of Organic Chemistry, Analytical Section, National Chemical Laboratory,
Pune-411 008, India
† Electronic supplementary information (ESI) available: Experimental procedure,
NMR, spectra, and HPLC traces. See DOI: 10.1039/c4ra02690j
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The idea of selecting such a system is to achieve a small but absence of water, did not improve enantiomeric excess (Table 1,
strong hydrogen-bond-forming pocket in the catalyst structure. entry 1). This indicates that an appropriate amount of water is
Our presumption has been proven to be true; catalyst 1 provided necessary for the formation of suitable micelles, in which
aldol products during the reaction between aromatic aldehydes stereo-controlled hydrophobic interactions and hydrogen
and ketones with good yields (up to 99%), high anti/syn bonding between the organic reactants can occur to obtain
diastereoselectivities (up to 95 : 5) and enantioselectivities enhanced stereoselectivity. We also observed that when the
(up to >99.9%) in wet solvent-free conditions.
reaction was carried out with 10 mol% of catalyst 1 in 0.01 ml of
The L-proline hydrazide 1 was synthesized from readily water in the absence of 4-nitrobenzoic acid, the enantiose-
available L-proline using an inexpensive reagent such as lectivity dropped from 93% to 77% (Table 1, entries 2 and 6).
hydrazine in two simple steps aer slight modication of the Thus, it is clear from the aforementioned experiments that an
reported method (Scheme 1).13 For the esterication step, acetyl appropriate volume of water and the presence of a suitable acid
chloride was used instead of thionyl chloride and a better additive are essential for obtaining optimum results for a
overall yield (77%) was obtained using our modied method.
particular organocatalyst. Previously reported studies also
Aldol reactions were systematically carried out using support the fact that water volume and acid additives can
4-nitrobenzaldehyde and cyclohexanone as substrates to opti- signicantly inuence the yield as well as the stereoselectivity in
mize different parameters such as the volume of water, catalyst organocatalyzed synthesis in aqueous media.2c,8l,w,x In this case,
loading, and the effect of acid additives for this small organo- the optimum volume of water was determined to be 0.01 ml
catalyst 1. It is reported that for aldol reactions performed with catalyst 1 for the aldol reaction between cyclohexanone and
under wet solvent-free conditions, the reactivity and stereo- 4-nitrobenzaldehyde at room temperature in the presence of
selectivities are signicantly inuenced because of such factors 4-nitrobenzoic acid as an acid additive (Table 1, entry 2).
as increased hydrophobic interactions between the substrate
Aer optimizing the volume of water, we investigated the
molecules as well as hydrogen-bond formation between the amount of catalyst loading. With an increase or decrease in the
reactants and water molecules in the transition state.2c In our catalyst loading from 10 mol%, enantioselectivity dropped from
experiments, it has been observed that there was gradual dete- 93% to 86% and 48%, respectively, at room temperature for the
rioration in the enantioselectivities in the aldol products from same aldol reaction (Table 1, entries 7–9). It is notable that with
93% to 38%, with an increase in the amount of water from a gradual increase in catalyst loading from 1 mol% to 20 mol%,
0.01 ml to 1 ml in the presence of 4-nitrobenzoic acid as an acid the yield of aldol product jumped from 81% to 98%, whereas
additive (Table 1, entry 2–5). The same reaction, even in the the reaction time was reduced from 96 h to only 20 h (Table 1). A
blank experiment was carried out in the absence of the catalyst
1 and we observed that there was no product formation even
aer 5 days (Table 1, entry 10).
Once the volume of water (0.01 ml) and catalyst loading
(10 mol%) was optimized, we began screening various acid
additives to determine the most appropriate additive for our
catalyst 1 in the same aldol reaction between p-nitrobenzaldehyde
Scheme 1
Table 1 Effect of water volume and catalyst loading in aldol reaction between cyclohexanone and 4-nitrobenzaldehyde catalyzed by 1
Entry
Catalyst 1 (mol%)
H2O (ml)
Time (h)
Yielda (%)
anti/synb
eec
1
2
3
4
10
10
10
10
10
10
1
5
20
—
—
0.01
0.1
0.5
1
0.01
0.01
0.01
0.01
0.01
22
24
48
72
96
20
96
72
20
—
95
95
90
96
90
96
81
86
98
—
78/22
86/14
87/13
72/28
65/35
81/19
72/28
69/31
83/17
—
50
93
86
70
38
77
48
54
86
—
5
6d
7
8
9
10
a
Isolated yield aer purication by column chromatography. b Diastereomer ratios (anti/syn) were determined by 1H NMR spectrum of the crude
product mixture. c Determined by chiral HPLC analysis. In the absence of 4-nitrobenzoic acid additive.
d
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and cyclohexanone. A broad spectrum of acid additives with a optimized conditions (Table 2). For substrates containing an
long range of pKa, starting from simple aromatic acids such as electron-withdrawing group, such as p/m-nitrobenzaldehydes,
benzoic acid, 4-nitrobenzoic acid, picric acid to p-toluenesul- and an electron-donating group, such as p-chlorobenzaldehyde
phonic acid, were chosen for the optimization study (see ESI, and o-methoxybenzaldehyde, responded well in the presence of
Table 2†). The effect of long-chain fatty acids such as stearic and catalyst 1 as far as the enantioselectivity of the aldol product is
oleic were also tested in this reaction. Although the reactions were concerned. Perhaps some of the substrates provided very poor
comparatively faster in the presence of these fatty acids, the enantioselectivity but the entire results are extremely signicant
enantioselectivities were not very impressive. At the outset, PTSA since catalyst 1 is very small and does not possess signicant
has been determined to be the best acid additive for the hydrazide bulk around the catalytic site. This preliminary observation has
1-catalyzed aldol reaction in water at room temperature with 99% been recorded in Table 2. We have also observed an extremely
yield and 97% enantiomeric excess (see ESI, entry 1, Table 2†). In fast reaction when acetone and cyclopentanone were the aldol
the aforementioned experiments, although a non-linear relation- donors in the presence of catalyst 1. The reactions were
ship (specically a non-monotonic relationship) between the pKa completed within 6–8 h with impressive enantioselectivity
and the obtained ees have been observed, we were observed that (Table 2, entries 14 and 15). These results are highly encour-
the strongest acid additive, such as PTSA, among the screened aging as far as such a small-sized organocatalyst is concerned in
acids provided the best results in terms of yield (99%), anti : syn aqueous media.
ratio (93 : 7) and enantioselectivity (97%). This result indicates the
The most probable transition state for the hydrazide
requirement of a strong acid additive for achieving the best results 1-catalyzed aldol reaction has been depicted in Fig. 1 similar to
for the present catalyst 1. Nevertheless, presently, we are unsure of the one in the previously reported.8l,w,14,15 In the T.S. 1, we have
the existence of any additional fundamental effects of acid addi- considered the anti-enamine at the below and the aldehyde
tives exist in the solution in addition to the established facts such acceptor in the upper face where the –CO–NH–NH2 moiety of
as reactivity enhancement through hydrogen bonding and the catalyst 1 is present, and a strong hydrogen bonding effec-
maintaining the pH of the reaction media.
tively results in a stable and favorable transition state, leading
Finally, we have screened many substituted aromatic alde- to the formation of the corresponding anti-enantiomer as a
hydes for aldol reactions between various cyclic aliphatic major enantiomer. Whereas in the case of another T.S. 2 for the
ketones, such as cyclopentanone, cyclohexanone, and open- formation of minor anti-enantiomer described by Noto et al.,
chain ketones, such as acetone, to investigate the general considering the syn-enamine at the below and aldehyde at the
efficacy of this small hydrazide catalyst 1 in water under above with an aromatic group toward the hydrophilic region, is
Table 2 Substrate scope was investigated under optimal conditions and organocatalyst 1 catalyzed direct aldol reactions in the presence of
water
Entry (product no.)
R3CHO
Time (h)
Yielda (%)
anti/synb
eec (%) (anti)
1 (2a)
2 (2b)
3 (2c)
4 (2d)
5 (2e)
6 (2f)
7 (2g)
8 (2h)
9 (2i)
10 (2j)
11 (2k)
12 (2l)
13 (2m)
14 (3a)
15 (3b)
p-NO2-C6H4-CHO
m-NO2-C6H4-CHO
o-NO2-C6H4-CHO
o-F-C6H4-CHO
72
36
36
48
36
72
48
36
48
48
60
72
36
8
99
87
85
86
87
86
83
85
87
87
84
85
96
91
96
93/7
91/9
89/11
88/12
73/27
92/8
97
98
88
68
32
89
64
46
91
49
44
61
80
98
>99.9
m-F-C6H4-CHO
o-MeO-C6H4-CHO
o-Cl-C6H4-CHO
m-Cl-C6H4-CHO
p-Cl-C6H4-CHO
p-Me-C6H4-CHO
1-Naphthaldehyde
2-Naphthaldehyde
p-CF3-C6H4-CHO
p-NO2-C6H4-CHO
p-NO2-C6H4-CHO
95/5
85/15
89/11
84/16
82/18
86/14
93/7
—
94/6
6
a
Isolated yield aer purication by column chromatography. b Diastereomer ratios (anti/syn) were determined by 1H NMR spectrum of the crude
product mixture. c Determined by chiral HPLC analysis.
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Simple Amino Acids and Short-Chain Peptides as Efficient
Metal-Free Catalysis in Asymmetric Synthesis, Wiley-VCH,
Weinheim,
2003,
pp.
178–186;
(b)
Asymmetric
Organocatalysis: From Biomimetic Concepts to Applications in
¨
Asymmetric Synthesis, ed. A. Berkessel and H. GrOger,
Wiley-VCH, Weinheim, 2005; (c) B. List, Chem. Rev., 2007,
107, 5413–5883.
´
´
´
2 (a) J. G. Hernandez, V. Garcıa-Lopez and E. Juaristi,
Tetrahedron, 2012, 68, 92–97; (b) For recent discussions on
proline
and
proline-derived
organocatalysts,
see:
H. Yamamoto and B. List, Synlett, 2011, 461–512; (c)
R. N. Butler and A. G. Coyne, Chem. Rev., 2010, 110, 6302–
6337.
3 (a) S. Bhowmick and K. C. Bhowmick, Tetrahedron:
Fig. 1 Proposed transition state model for major and minor anti-
enantiomers.
´
Asymmetry, 2011, 22, 1945–1979; (b) J. G. Hernandez and
E. Juaristi, Chem. Commun., 2012, 48, 5396–5409; (c)
Y. Qiao, Q. Chen, S. Lin, B. Ni and A. D. Headley, J. Org.
Chem., 2013, 78, 2693–2697; (d) Z.-B. Xie, N. Wang,
G.-F. Jiang and X.-Q. Yu, Tetrahedron Lett., 2013, 54, 945–
948; (e) H.-W. Zhao, Y.-Y. Yue, H.-L. Li, X.-Q. Song,
Z.-H. Sheng, Z. Yang, W. Meng and Z. Yang, Synlett, 2013,
24, 2160–2164; (f) X. Wu, Z. Jiang, H.-M. Shen and Y. Lu,
Adv. Synth. Catal., 2007, 349, 812–816.
4 (a) D. C. Rideout and R. Breslow, J. Am. Chem. Soc., 1980, 102,
7816–7817; (b) Organic Synthesis in Water, ed. P. A. Grieco,
Blackie Academic and Professional, London, 1998.
5 (a) U. Scheffler and R. Mahrwald, Chem.–Eur. J., 2013, 19,
14346–14396; (b) M. Raj and V. K. Singh, Chem. Commun.,
2009, 6687–6703; (c) M. Gruttadauria, F. Giacalone and
R. Noto, Adv. Synth. Catal., 2009, 351, 33–57; (d)
J. Paradowska, M. Stodulski and J. Mlynarski, Angew.
Chem., Int. Ed., 2009, 48, 4288–4297.
6 (a) Y.-J. An, Y.-X. Zhang, Y. Wu, Z.-M. Liu, C. Pi and J.-C. Tao,
Tetrahedron: Asymmetry, 2010, 21, 688–694; (b) S. Saha and
J. N. Moorthy, Tetrahedron Lett., 2010, 51, 912–916; (c) For
theoretical study see: Y. Jung and R. A. Marcus, J. Am.
Chem. Soc., 2007, 129, 5492–5502; (d) H. Y. Bae, S. Some,
J. S. Oh, Y. S. Lee and C. E. Song, Chem. Commun., 2011,
47, 9621–9623.
7 (a) D. G. Blackmond, A. Armstrong, V. Coombe and A. Wells,
Angew. Chem., Int. Ed., 2007, 46, 3798–3800; (b) Y. Hayashi,
Angew. Chem., Int. Ed., 2006, 45, 8103–8104.
8 (a) T. He, K. Li, M.-Y. Wu, M.-B. Wu, N. Wang, L. Pu and
X.-Q. Yu, Tetrahedron, 2013, 69, 5136–5143; (b) S. S. Chimni
and D. Mahajan, Tetrahedron: Asymmetry, 2006, 17, 2108–
2119; (c) Y. Wu, Y. Zhang, M. Yu, G. Zhao and S. Wang,
Org. Lett., 2006, 8, 4417–4420; (d) S. Guizzetti, M. Benaglia,
L. Raimondi and G. Celentano, Org. Lett., 2007, 9, 1247–
1250; (e) V. Maya, M. Raj and V. K. Singh, Org. Lett., 2007,
9, 2593–2595; (f) W.-P. Huang, J.-R. Chen, X.-Y. Li, Y.-J. Cao
and W.-J. Xiao, Can. J. Chem., 2007, 85, 208–213; (g)
D. Gryko and W. J. Saletra, Org. Biomol. Chem., 2007, 5,
2148–2153; (h) C. Wang, Y. Jiang, X.-X. Zhang, Y. Huang,
B.-G. Li and G.-L. Zhang, Tetrahedron Lett., 2007, 48, 4281–
4285; (i) L. Zu, H. Xie, H. Li, J. Wang and W. Wang, Org.
Lett., 2008, 10, 1211–1214; (j) D. Alma¸si, D. A. Alonso,
unfavorable due to the hydrophobic–hydrophilic interaction.14
The aromatic moiety will always remain toward the hydro-
phobic region for a better hydrophobic–hydrophobic interac-
tion in an aqueous environment. The syn-enantiomers are also
unfavourable for the similar reason reported in the literature.14
In summary, we have found a catalyst 1, which is the smallest
organocatalytic system reported thus far in the literature for an
aqueous-phase asymmetric aldol reaction. The attachment of a
hydrazide unit in the L-proline structure has been proven to be
very effective for the reactivity and enantioselectivity in the aldol
reaction. The existence of a suitable hydrogen-bonding pocket
(two hydrogen bonds within a short space) in the catalyst
structure is considered to be the probable reason for this
impressive result. It should be noted that the L-proline hydra-
zide 1 is the smallest and, most importantly, the best among the
small organocatalysts in an aqueous environment reported thus
far in the literature.3f,8b,u,v Some substrates screened in the
presence of this small catalyst 1 provided enantioselectivity
higher than 90%, which is an undoubtedly impressive result so
far as the least bulk around the active site of the catalyst is
concerned in aqueous media. Additional structural modica-
tion on catalyst 1 considering the steric and electronic effect is
under way in our laboratory to thoroughly understand the
detailed catalytic activity in an aqueous-phase aldol reaction.
The present work will guide us in the future to identify the
minimum bulk (steric environment) as well as the hydrogen-
bonding unit (electronic environment) required in the organo-
catalyst structure in an aqueous environment.
Acknowledgements
This work has been funded by Department of Science and Tech-
nology, Government of India [Grant no. SR/FT/CS-013/2009]. S.B.
acknowledges support from the University Grants Commission,
Government of India, through a research fellowship.
References
¨
1 (a) H. GrOger, J. Wilken and A. Berkessel, in Organic
´
Synthesis Highlights V, ed. H.-G. Schmalz and T. Wirth,
A.-N. Balaguer and C. Najera, Adv. Synth. Catal., 2009, 351,
24314 | RSC Adv., 2014, 4, 24311–24315
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
1123–1131; (k) B. Wang, G.-H. Chen, L.-Y. Liu, W.-X. Chang
and J. Li, Adv. Synth. Catal., 2009, 351, 2441–2448; (l)
N. Mase, N. Noshiro, A. Mokuya and K. Takabe, Adv. Synth.
Angew. Chem., Int. Ed., 2013, 52, 2906–2910; (f) Y. Hayashi,
T. Sumiya, J. Takahashi, H. Gotoh, T. Urushima and
M. Shoji, Angew. Chem., Int. Ed., 2006, 45, 958–961.
Catal., 2009, 351, 2791–2796; (m) B. Tan, D. Zhu, L. Zhang, 10 (a) M. Gruttadauria, F. Giacalone, A. M. Marculescu and
P. J. Chua, X. Zeng and G. Zhong, Chem.–Eur. J., 2010, 16,
3842–3848; (n) L. Qin, L. Zhang, Q. Jin, J. Zhang, B. Han
and M. Liu, Angew. Chem., Int. Ed., 2013, 52, 7761–7765; (o)
S. Bayat, B. A. Tejo, A. B. Salleh, E. Abdmalek, Y. M. Normi
and M. B. A. Rahman, Chirality, 2013, 25, 726–734; (p)
A. Patti and S. Pedotti, Eur. J. Org. Chem., 2014, 624–630;
R. Noto, Adv. Synth. Catal., 2008, 350, 1397–1405; (b)
J.-F. Zhao, L. He, J. Jiang, L.-F. Cun and L.-Z. Gong,
Tetrahedron Lett., 2008, 49, 3372–3375; (c) A. Monge-
¨
´
Marcet, X. Cattoen, D. A. Alonso, C. Najera, M. W. C. Man
and R. Pleixats, Green Chem., 2012, 14, 1601–1610; (d)
T. E. Kristensen, K. Vestli, M. G. Jakobsen, F. K. Hansen
and T. Hansen, J. Org. Chem., 2010, 75, 1620–1629; (e)
S. V. Kochetkov, A. S. Kucherenko and S. G. Zlotin, Eur.
J. Org. Chem., 2011, 6128–6133; (f) J. P. Delaney and
L. C. Henderson, Adv. Synth. Catal., 2012, 354, 197–204; (g)
Z.-H. Tzeng, H.-Y. Chen, R. J. Reddy, C.-T. Huang and
K. Chen, Tetrahedron, 2009, 65, 2879–2888.
´
´
(q) R. Pedrosa, J. M. Andres, R. Manzano, D. Roman and
´
S. Tellez, Org. Biomol. Chem., 2011, 9, 935–940; (r)
A. J. Pearson and S. Panda, Org. Lett., 2011, 13, 5548–5551;
¨
¨
(s) G. Tang, U. Gun and H.-J. Altenbach, Tetrahedron, 2012,
68, 10230–10235; (t) S. Paladhi, J. Das, P. K. Mishra and
J. Dash, Adv. Synth. Catal., 2013, 355, 274–280; (u)
S. Aratake, T. Itoh, T. Okano, T. Usui, M. Shoji and 11 (a) N. Mase and C. F. Barbas III, Org. Biomol. Chem., 2010, 8,
´
Y. Hayashi, Chem. Commun., 2007, 2524–2526; (v)
A. Bellomo, R. Daniellou and D. Plusquellec, Green Chem.,
2012, 14, 281–284; (w) N. Mase, Y. Nakai, N. Ohara,
H. Yoda, K. Takabe, F. Tanaka and C. F. Barbas III, J. Am.
4043–4050; (b) G. Guillena, M. C. Hita, C. Najera and
S. F. Vio'zquez, J. Org. Chem., 2008, 73, 5933–5943; (c)
J. G. Hernandez and E. Juaristi, J. Org. Chem., 2011, 76,
´
1464–1467.
ˆ
Chem. Soc., 2006, 128, 734–735; (x) G. Angelici, R. J. Correa, 12 (a) J. Li, G. Yang, Y. Qin, X. Yang and Y. Cui, Tetrahedron:
S. J. Garden and C. Tomasini, Tetrahedron Lett., 2009, 50,
814–817.
9 (a) Q. Zhao, Y.-h. Lam, M. Kheirabadi, C. Xu, K. N. Houk and
Asymmetry, 2011, 22, 613–618; (b) J. P. Delaney,
H. L. Brozinski and L. C. Henderson, Org. Biomol. Chem.,
2013, 11, 2951–2960.
C. E. Schafmeister, J. Org. Chem., 2012, 77, 4784–4792; (b) 13 (a) V. Gauchot and A. R. Schmitzer, J. Org. Chem., 2012, 77,
L. Zhong, Q. Gao, J. Gao, J. Xiao and C. Li, J. Catal., 2007,
250, 360–364; (c) Y.-Q. Fu, Y.-J. An, W.-M. Liu, Z.-C. Li,
4917–4923; (b) A. Kudelko, W. Zielinski and K. Ejsmont,
Tetrahedron, 2011, 67, 7838–7845.
G. Zhang and J.-C. Tao, Catal. Lett., 2008, 124, 397–404; (d) 14 F. Giacalone, M. Gruttadauria, P. Lo Meo, S. Riela and
S. Aratake, T. Itoh, T. Okano, N. Nagae, T. Sumiya, M. Shoji R. Noto, Adv. Synth. Catal., 2008, 350, 2747–2760.
and Y. Hayashi, Chem.–Eur. J., 2007, 13, 10246–10256; (e) 15 Y.-N. Jia, F.-C. Wu, X. Ma, G.-J. Zhu and C.-S. Da, Tetrahedron
E. Huerta, P. J. M. Stals, E. W. Meijer and A. R. A. Palmans,
Lett., 2009, 50, 3059–3062.
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