Lithium bromide, an inexpensive and efficient catalyst for opening of epoxide rings by amines at room temperature under solvent-free condition

Article abstract of DOI:10.1002/ejoc.200400253

Lithium bromide has been found to be an inexpensive and efficient catalyst for the opening of epoxide rings by amines, and this provides an environmentally friendly method for the synthesis of β-amino alcohols. Aromatic and aliphatic amines react with cycloalkene oxides to exclusively form trans-2-(aryl/alkylamino)cycloalkanols in high yields. A 98-100% selectivity in favour of nucleophilic attack at the benzylic carbon atom of styrene oxide is observed with aromatic amines. However, aliphatic amines exhibit a marginal preference for the reaction at the terminal carbon atom of the epoxide ring in styrene oxide. Non-styrenoidal, unsymmetrical alkene oxides undergo selective nucleophilic attack at the sterically less hindered carbon atom by aniline. The chelation effect of the Li⁺ ion enables selective opening of the epoxide ring in 3-phenoxypropylene oxide in the presence of styrene oxide. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejoc.200400253

SHORT COMMUNICATION
Lithium Bromide, an Inexpensive and Efficient Catalyst for Opening of
Epoxide Rings by Amines at Room Temperature under Solvent-Free Condition
Asit K. Chakraborti,*[a] Santosh Rudrawar,[a] and Atul Kondaskar[a]
Keywords: Lithium bromide / Catalyst / Epoxide / Amines / β-Amino alcohols / Selectivity
Lithium bromide has been found to be an inexpensive and
efficient catalyst for the opening of epoxide rings by amines,
and this provides an environmentally friendly method for the
synthesis of β-amino alcohols. Aromatic and aliphatic amines
react with cycloalkene oxides to exclusively form trans-2-
(aryl/alkylamino)cycloalkanols in high yields. A 98−100% se-
lectivity in favour of nucleophilic attack at the benzylic car-
bon atom of styrene oxide is observed with aromatic amines.
However, aliphatic amines exhibit a marginal preference for
the reaction at the terminal carbon atom of the epoxide ring
in styrene oxide. Non-styrenoidal, unsymmetrical alkene ox-
ides undergo selective nucleophilic attack at the sterically
less hindered carbon atom by aniline. The chelation effect of
the Li+ ion enables selective opening of the epoxide ring in
3-phenoxypropylene oxide in the presence of styrene oxide.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
β-Amino alcohols are versatile synthons for a wide range cases, these methods are applicable to aromatic amines
of biologically active natural and synthetic products,[1] un- only.[21] Therefore, considering the potential industrial ap-
natural amino acids[2] and chiral auxiliaries.[3] The classical plications of the epoxide ring opening reaction by amines,
synthesis of β-amino alcohols by nucleophilic opening of the development of a cheap and efficient catalyst is in
the epoxide ring involves treatment of the epoxides with high demand.
amines while heating.[4,5] However, the method does not
We have recently found that the strong oxophilicity of
work so well with poor nucleophilic amines and also poses Liϩ activates oxygen-containing electrophiles for nucleo-
problems when sensitive epoxides are used due to the poten- philic attack.[22,23] Herein, we report that lithium bromide
tial side reactions at high temperatures and the need for efficiently catalyzes the opening of epoxide rings by amines
excess amine. This led to the necessary use of catalysts to at room temperature under solvent-free conditions to afford
activate the epoxide ring for nucleophilic cleavage. There- a cost-effective and environmentally friendly process for the
fore, continuous efforts have been made to develop various synthesis of β-amino alcohols.
catalysts such as alumina,[6] metal amides and triflamide,[7]
To find out the best reaction conditions, cyclohexene ox-
alkali metal perchlorates and tetrafluoroborate,[8] metal tri- ide (1) (2.5 mmol) was taken as a symmetrically substituted
flates,[9] metal alkoxides,[10] zirconium sulfophenyl phos- epoxide and treated with various amines 2 (2.5 mmol) in
phonate,[11] metal halides,[12] silica gel under high press- the presence of LiBr (Scheme 1).
ure,[13] montmorillonite
K 10 under microwave ir-
radiation,[14] hexafluoro-2-propanol (HFIP) under re-
flux,[15] ionic liquids,[16] nBu3P,[17] Yb(OTf)3 in supercritical
CO2 under high pressure at 55 °C,[18] and silica gel.[19] How-
ever, some of these methodologies require long reaction
times, elevated temperatures, high pressure, air- and moist-
ure-sensitive catalysts, stoichiometric amounts of costly
catalysts, special apparatus etc. The rearrangement of the
starting epoxide to allylic alcohols[20] and potential hazards
in handling pyrophoric/moisture-sensitive reagents in the
preparation of the catalyst are also notable disadvantages
of some of the reported methodologies Further, in most
Scheme 1. LiBr-catalyzed epoxide ring opening of 1 with various
amines
[a]
Use of 5 mol % of LiBr caused the reaction to go to
completion at room temperature under neat conditions in
1Ϫ5 h (GCMS) (Table 1). No significant amount of the am-
ino alcohol 3a was formed (GCMS) from 1 and 2a in the
Department of Medicinal Chemistry, National Institute of
Pharmaceutical Education and Research (NIPER),
Sector 67, S. A. S. Nagar, Punjab 160 062, India
Fax: (internat.) ϩ 91-172-2214692
E-mail: akchakraborti@niper.ac.in
Eur. J. Org. Chem. 2004, 3597Ϫ3600
DOI: 10.1002/ejoc.200400253
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3597

A. K. Chakraborti, S. Rudrawar, A. Kondaskar
SHORT COMMUNICATION
absence of LiBr; the epoxide and the amine remained un-
changed. This establishes the need to use LiBr to promote
the epoxide opening reaction. Excellent results were ob-
tained with various aromatic amines such as aniline (2a), 4-
methylaniline (2b) and 4-chloroaniline (2c): 90, 93, and 88%
yields, respectively. The reaction worked well with aliphatic
amines such as benzylamine (2d), pyrrolidine (2e) and pip-
eridine (2f) resulting in the formation of the corresponding
amino alcohols in 88, 100, and 85% yields, respectively.[21]
In each case the resultant 2-(aryl/alkylamino)cyclohexanols
were found to be the trans diastereoisomer 3 on the basis
of NMR spectroscopy.[8,9e,12h]
Table 2. Reactions of 4 with various amines catalyzed by LiBr;
styrene oxide (4) (2.5 mmol) was treated with the amine 2
(2.5 mmol) in the presence of LiBr (5 mol %) at room temperature
under neat conditions for 5 h
Table 1. Reactions of 1 with various amines catalyzed by LiBr;
cyclohexene oxide (1) (2.5 mmol) was treated with the amine
(2.5 mmol) in the presence of LiBr (5 mol %) at room temperature
under neat conditions
[a]
[b]
Isolated yields of the corresponding amino alcohols.
Deter-
mined by GCMS (Entries 1Ϫ3) and 1H NMR spectroscopy
(Entries 4Ϫ6).
tion with 2a eluted with different retention time, and the
regioselectivity was determined by GCMS. In the MS, the
regioisomer 5a exhibits a daughter ion [Mϩ Ϫ 31] due to
the loss of CH2OH, and the diagnostic feature in the mass
spectra of 7a is the ion peak [Mϩ Ϫ 106], which is assigned
to the loss of PhCHO. The reactions with 2b and 2c af-
forded a single amino alcohol (GCMS), and the absence of
an ion peak [Mϩ Ϫ 106] and the presence of the daughter
ion [Mϩ Ϫ 31] as the base peak confirms the formation of
the regioisomer 5 on each occasion.
In the case of the aliphatic amines, the regioisomers could
not be separated by column chromatography and eluted
simultaneously in the GCMS (capillary column). However,
the presence of ion peaks [Mϩ Ϫ 31] and [Mϩ Ϫ 106] indi-
cates the formation of 5 and 6, respectively. The regioselec-
tivity was determined by 1H NMR spectroscopy on the
basis of the integral values of the benzylic methine and
methylene proton signals corresponding to 5 and 6, which
appear at δ ഠ 3.9 and 4.7 ppm, respectively.[24]
[a] The 1H and 13C NMR data confirm the trans stereochemistry of
the product. [b] Isolated yields of the corresponding amino alcohol.
To evaluate the regioselectivity, styrene oxide (4) was
chosen as a representative unsymmetrical epoxide and
treated with various amines (Scheme 2) under the catalytic
influence of LiBr (Table 2).
The reaction of 4 with aromatic amines afforded the
product obtained from nucleophilic attack at the benzylic
carbon atom as the major regioisomer (Table 2, Entries
1Ϫ3). Aliphatic amines showed a marginal preference for
nucleophilic attack at the terminal carbon atom (Table 2,
Entries 4Ϫ6). The regioselective formation of 5 in the reac-
tion with aromatic amines may be accounted for by the fact
that the phenyl group in 4 induces a carbocationic character
at the benzylic carbon atom in the LiBr complex of 4 due
to the resonance effect. Thus, aromatic amines react selec-
tively at the benzylic carbon atom of 4 due to their lower
nucleophilicity. The relatively better nucleophilicity of ali-
phatic amines favours an SN2 process, thus influencing the
nucleophilic attack at the terminal carbon atom of 4. There-
Scheme 2. Regioselectivity in the LiBr-catalyzed epoxide ring open-
ing of 4 with various amines
The reaction with aromatic amines 2aϪ2c and aliphatic fore, the competing electronic effect of the phenyl group in
amines such as pyrrolidine (2e), piperidine (2f) and morph- 4 and the increased nucleophilic property of the aliphatic
oline (2g) afforded 92Ϫ100% yields of the corresponding amines, relative to the aromatic amine, result in overall
amino alcohols. The regioisomers obtained from the reac- poor regioselectivity.
3598
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Eur. J. Org. Chem. 2004, 3597Ϫ3600

Lithium Bromide, a Catalyst for Opening of Epoxide Rings by Amines
SHORT COMMUNICATION
To evaluate the generality, various epoxides were treated
with 2a in the presence of LiBr (Table 3).
The relative coordinating ability of different epoxides
with the Liϩ ion should enable the selective opening of the
epoxide ring during intermolecular competition. Thus, a
mixture of 3-phenoxypropylene oxide (2.5 mmol) and styr-
ene oxide (2.5 mmol) was treated with aniline (2.5 mmol) in
the presence of LiBr (5 mol %) at room temperature for 5 h
in the absence of solvent (Scheme 3).
Table 3. Reaction of various epoxides with 2a catalyzed by LiBr;
the epoxide (2.5 mmol) was treated with 2a (2.5 mmol) in the pres-
ence of LiBr (5 mol %) at room temperature under nitrogen in the
absence of solvent for 5 h
Scheme 3. Selectivity of epoxide ring opening by aniline during
intermolecular competition catalyzed by LiBr
The exclusive formation of the amino alcohol from nucle-
ophilic cleavage of the epoxide ring of 3-phenoxypropylene
oxide took place, and no significant amount of the amino
alcohols resulting from nucleophilic attack by aniline on
styrene oxide was detected (GCMS). The selective opening
of the epoxide ring in 3-phenoxypropylene oxide may be
explained by the chelating ability of the oxygen atoms of
the phenoxy group and the epoxide oxygen atom of 3-
phenoxypropylene oxide with the Liϩ ion, which leads to a
favourable five-membered transition state. The absence of
such a chelation effect with styrene oxide results in a less
favourable transition state, and selective activation of the
epoxide ring in 3-phenoxypropylene oxide takes place.
In conclusion, this study demonstrates that LiBr is a
highly efficient catalyst for the opening of epoxides with
amines. The mild reaction conditions, applicability with
various epoxides and amines (aromatic and aliphatic) and
chelation effect leading to selective epoxide activation, offer
specific advantages. With increasing environmental con-
cerns,[26] the use of an inexpensive, easy to handle, non-
toxic catalyst and solvent-free reaction conditions should
fulfil the ‘‘triple bottom line’’[21] philosophy of green chem-
istry, and thus the present method is ‘‘environmentally fri-
endly’’ and potentially useful for industrial applications.
[a]
1
[b]
Determined by GCMS and H/13C NMR spectroscopy.
Iso-
lated yields of the corresponding amino alcohol.
Quantitative formation of the corresponding amino al-
cohols demonstrated the efficiency of LiBr. The reaction
with cyclopentene oxide resulted in the exclusive formation
of the trans diastereoisomer (Table 3, Entry 1).[15] In the
cases of unsymmetrical epoxides such as methyloxirane, gly-
cidyl phenyl ether and tert-butyl glycidyl ether (Table 3, En-
tries 2Ϫ4), complete selectivity for nucleophilic attack at
the less hindered carbon atom of the epoxide was observed,
which demonstrates the generality of regioselectivity for
non-styrenoidal oxides. These observations suggest that the
reversal of the regioselectivity (nucleophilic attack at the
more hindered benzylic carbon atom) during the reaction
of 4 with 2a and other aromatic amines was controlled by
the electronic factor of the phenyl group in 4.
Experimental Section
Typical Procedure. 2-(Phenylamino)cyclohexanol: LiBr (10 mg, 5
mol %) was added to a magnetically stirred mixture of 1 (0.25 mL,
2.5 mmol) and 2a (0.225 mL, 2.5 mmol) at room temperature under
nitrogen. After completion of the reaction (1 h, GCMS), the reac-
The reaction of epichlorohydrin (Table 3, Entry 5) exem-
plified the excellent chemoselectivity that led to quantitative
formation of the amino alcohol, corresponding to nucleo-
philic attack at the terminal carbon atom of the epoxide tion mixture was diluted with water (10 mL) and extracted with
Et2O (2 ϫ 15 mL). The combined ethereal extracts were dried
(Na2SO4) and concentrated under vacuum to afford 2-(phenylami-
no)cyclohexanol (3a) (0.428 g, 90%). IR (neat): ν˜ ϭ 3354, 2931,
ring. No product arising from nucleophilic displacement of
the chlorine atom could be detected through GCMS analy-
sis of the reaction mixture. The high oxophilicity of Liϩ
entails a strong coordination between Liϩ and the alkoxide
generated after nucleophilic attack on the LiBr-complexed
epoxide, thus prohibiting concommitant elimination of the
chloride anion. The quantitative formation of the desired
2858, 1601, 1500, 1448, 1319, 1067 cmϪ1 1H NMR (300 MHz,
.
CDCl3): δ ϭ 1.03Ϫ1.42 (m, 4 H), 1.72Ϫ1.78 (m, 2 H), 2.9 (m, D2O
exchangeable, 2 H), 2.10Ϫ2.16 (m, 2 H), 3.13 (ddd, J ϭ 3.9, 10.0,
10.1 Hz, 1 H), 3.33 (ddd, J ϭ 4.2, 10.4, 10.5 Hz, 1 H), 6.7Ϫ7.2 (m,
5 H) ppm. 13C NMR (300 MHz, CDCl3): δ ϭ 24.27, 25.02, 31.62,
33.15, 60.17, 74.55, 114.40, 118.38, 129.34, 147.81 ppm. EIMS:
m/z ϭ 191 [Mϩ]. Identical (1H and 13C NMR and MS) with an
product relative to 81 and 69% yields obtained during the
[12h]
ZrCl4
and silica-gel[19] catalyzed reactions, respectively,
establish that this newly developed catalyst is more effective. authentic sample.[12h] This general procedure was applied for other
Eur. J. Org. Chem. 2004, 3597Ϫ3600
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A. K. Chakraborti, S. Rudrawar, A. Kondaskar
SHORT COMMUNICATION
fluoroacetate): S. Rampalli, S. S. Chaudhari, K. G. Akaman-
chi, Synthesis 2000, 78Ϫ80.
M. Curini, F. Epifano, M. C. Marcotullio, O. Rosati, Eur. J.
Org. Chem. 2001, 4149Ϫ4152.
reactions. On each occasion, the product was identified by compar-
ing the spectroscopic data (IR, NMR, MS) with those reported in
the literature.
[11]
[12] [12a]
CoCl2: J. Iqbal, A. Pandey, Tetrahedron Lett. 1990, 31,
[12b]
575Ϫ576.
Prakash, Synthesis. 2000, 1817Ϫ1818. [12c] SmCl3[:1X2d.]-L. Fu, S.-
H. Wu, Synth. Commun. 1997, 27, 1677Ϫ1684. CeCl3: L.
TaCl5: S. Chandrasekhar, T. Ramchander, S. J.
[1] [1a]
D. R. Gehlert, D. J. Goldstein, P. A. Hipskind, Ann. Rep.
[1b]
R. Reddy, M. A. Reddy, N. Bhanumathi, K. R. Rao, Synthesis
Med. Chem. 1999, 201Ϫ210.
Chem. Int. Ed. 1999, 38, 1931Ϫ1934.
E. J. Corey, F. Zhang, Angew.
2001, 831Ϫ832. [12e] BiCl3: T. Ollevier, G. Lavie-Compin, Tetra-
[1c]
J. De Cree, H. Geu-
hedron Lett. 2002, 43, 7891Ϫ7893. [12f] ZnCl2: L. D. Pachon, P.
´
kens, J. Leempoels, H. Verhaegen, Drug Dev. Res. 1986, 8,
[1d]
Gamez, J. J. M. van Bassel, J. Reedijk, Tetrahedron Lett. 2003,
44, 6025Ϫ6027.
109Ϫ117.
J. G. Smith, Synthesis 1984, 629Ϫ656.
[12g]
[2] [2a]
[2b]
VCl3: G. Sabitha, G. S. K. K. Reddy, K.
P. O ЈBrien, Angew. Chem. Int. Ed. 1999, 38, 326Ϫ329.
[12h]
B. Reddy, J. S. Yadav, Synthesis 2003, 2298Ϫ2300.
ZrCl4:
G. Li, H.-T. Chang, K. B. Sharpless, Angew. Chem. Int. Ed.
Engl. 1996, 35, 451Ϫ454.
D. J. Ager, I. Prakash, D. R. Schaad, Chem. Rev. 1996, 96,
A. K. Chakraborti, A. Kondaskar, Tetrahedron Lett. 2003,
[3]
44, 8315Ϫ8319.
[13]
H. Kotsuki, T. Shimanouchi, M. Teraguchi, M. Kataoka, A.
Tatsukawa, H. Nishizawa, Chem. Lett. 1994, 2159Ϫ2162.
835Ϫ876.
[4] [4a]
[4b]
S. C. Bergmeier, Tetrahedron 2000, 56, 2561Ϫ2576.
M. Hodgson, A. R. Gibbs, G. P. Lee, Tetrahedron 1996, 52,
14361Ϫ14384.
437Ϫ475.
D.
[14] [14a] R. Gupta, S. Paul, A. K. Gupta, P. L. Kachroo, A. Dandia,
[14b]
[4c]
Ind. J. Chem. 1997, 36B, 281Ϫ283.
M. M. Mojtahedi, M.
R. M. Hanson, Chem. Rev. 1991, 91,
O. Mitsunobu, in Comprehensive Organic Syn-
[4d]
R. Saidi, M. Bolourtchian, J. Chem. Res. (S) 1999, 128Ϫ129.
U. Das, B. Crousse, V. Kesavan, D. Bonnet-Delpon, J.-P. Begue,
J. Org. Chem. 2000, 65, 6749Ϫ6751.
[15]
[16]
´
´
thesis (Ed.: E. Winterfeldt), Pergamon Press, New York, 1996,
vol. 6, part 1.3.4.1. [4e]A. S. Rao, S. K. Paknikar, J. G. Kirtane,
Tetrahedron 1983, 39, 2323Ϫ2367.
J. S. Yadav, B. V. S. Reddy, A. K. Basak, A. V. Narsaiah, Tetra-
hedron Lett. 2003, 44, 1047Ϫ1050.
[5] [5a]
P. A. Crooks, R. Szyudler, Chem. Ind. (London) 1973,
[17]
[18]
[19]
R.-H. Fan, X.-L. Hou, J. Org. Chem. 2003, 68, 726Ϫ730.
M. Shi, Y. Chen, J. Fluorine Chem. 2003, 122, 219Ϫ227.
A. K. Chakraborti, S. Rudrawar, A. Kondaskar, Org. Biomol.
Chem. 2004, 2, 1277Ϫ1280.
1111Ϫ1112. [5b] J. A. Deyrup, C. L. Moyer, J. Org. Chem. 1969,
[5c]
34, 175Ϫ179.
M. Freifelder, G. R. Stone, J. Org. Chem.
1961, 26, 1477Ϫ1480. [5d] M. Mousseron, J. Jullien, Y. Jolchine,
Bull. Soc. Chim. Fr. 1952, 757Ϫ766. [5e] R. E. Lutz, J. A. Freek,
R. S. Murphy, J. Am. Chem. Soc. 1948, 70, 2015Ϫ2023.
Alumina: G. H. Posner, D. Z. Rogers, J. Am. Chem. Soc. 1977,
99, 8208Ϫ8214.
[20]
Metal alkylamides induce the rearrangement of epoxides to al-
[20a]
lylic alcohols.
C. E. Harris, G. B. Fisher, D. Beardsley, L.
[6]
Lee, C. T. Goralsky, L. W. Nicholson, B. Singaram, J. Org.
[20b]
Chem. 1994, 59, 7746Ϫ7751.
J. Yamada, Y. Yomoto, Y.
[7] [7a]
Et2AlNHR: L. E. Overman, L. A. Flippin, Tetrahedron
[20c]
Yamamoto, Tetrahedron Lett. 1989, 30, 4255Ϫ4258.
Crandall, M. Apparu, Org. React. 1983, 29, 345Ϫ443.
Whitesel, S. W. Felman, J. Org. Chem. 1980, 45, 755Ϫ756.
J. K.
Lett. 1981, 22, 196Ϫ198. [7b] Silicon amides: A. Papini, I. Ricci,
M. Taddei, G. Seconi, P. Dembach, J. Chem. Soc., Perkin
[20d]
J.
[20e]
[7c]
Trans. 1 1984, 2261Ϫ2265.
R2NMgBr: M. C. Carre, J. P.
C. L. Kissel, B. Rickborn, J. Org. Chem. 1972, 37, 2060Ϫ2063.
The reported catalysts such as CoCl2, Cu(OTf)2, HFIP, zir-
conium sulfophenyl phosphonate, TaCl5, CeCl3/NaI, VCl3, and
BiCl3 are not effective for reactions with aliphatic amines.
Houmounou, P. Caubere, Tetrahedron Lett. 1985, 26,
[21]
3107Ϫ3110. [7d] RЈ3PbNR2: J.-I. Yamada, M. Yumoto, Y. Yam-
[7e]
amoto, Tetrahedron Lett. 1989, 30, 4255Ϫ4258.
Y. Yamamoto, N. Asao, M. Meguro, N. Tsukade, H. Nemoto,
Cu amide:
[22]
[23]
[24]
´
A. Basak (nee Nandi), M. K. Nayak, A. K. Chakraborti, Tetra-
N. Adayari, J. G. Wilson, H. Nakamura, J. Chem. Soc., Chem.
hedron Lett. 1998, 39, 4883Ϫ4886.
A. K. Chakraborti, A. Basak (born Nandi), V. Grover, J. Org.
Chem. 1999, 64, 8014Ϫ8017.
S. A. Anderson, J. T. Ayers, K. M. Ito, F. DeVries, D. Menden-
hall, B. C. Vanderplas, Tetrahedron: Asymmetry 1999, 10,
2655Ϫ2663.
D. E. McClure, B. H. Arison, J. J. Baldwin, J. Am. Chem. Soc.
1979, 101, 3666Ϫ3668.
[7f]
Commun. 1993, 1201Ϫ1203.
LiNTf2: J. Cossy, V. Bellosta,
C. Hamoir, J.-R. Desmurs, Tetrahedron Lett. 2002, 43,
7083Ϫ7086.
[8]
M. Chini, P. Crotti, F. Macchia, Tetrahedron Lett. 1990, 31,
4661Ϫ4664.
[9] [9a]
[25]
[26]
Ph4SbOTf: M. Fujiwara, M. Imada, A. Baba, H. Matsuda,
[9b]
Tetrahedron Lett. 1989, 30, 739Ϫ742.
La(OTf)3: M. Chini,
P. Crotti, L. Favero, F. Machhia, M. Pineschi, Tetrahedron Lett.
R. L. Garrett, in Designing Safer Chemicals (Eds.: R. L. Gar-
rett, S. C. De Vilto), American Chemical Society Symposium
Series 640, Washington DC, 1996, chapter 1.
J. Elkington, http://www.sustainability.co.uk/sustainability.htm
1994, 35, 433Ϫ436. [9c] Yb(OTf)3: M. Meguro, N. Asao, Y. Ya-
[9d]
mamoto, J. Chem. Soc., Perkin Trans. 1 1994, 2597Ϫ2601.
[27]
LiOTf: J. Auge, F. Leroy, Tetrahedron Lett. 1996, 37,
7715Ϫ7716. [9e] Cu(OTf)2 and Sn(OTf)2: G. Sekar, V. K. Singh,
J. Org. Chem. 1999, 64, 287Ϫ289.
[28] [28a]
´
A. Solladie-Cavello, M. Bencheqroun, J. Org. Chem. 1992,
[28b]
57, 5831Ϫ5834.
J. M. Klunder, S. Y. Ko, K. B. Sharpless,
[10] [10a]
Ti(OiPr)4: S. Sagava, H. Abe, Y. Hase, T. Inaba, J. Org.
J. Org. Chem. 1986, 51, 3710Ϫ3712.
Chem. 1999, 64, 4962. [10b] DIPAT (diisopropoxyaluminium tri-
Received April 15, 2004
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