Efficient regio- and stereo-selective cleavage of aziridines and epoxides using an ionic liquid as reagent and reaction medium

Article abstract of DOI:10.1139/V07-040

Ionic liquids, containing a variety of functionalities such as halo, azido, and thiocyano, efficiently cleave aziridines and epoxides to the corresponding products in high yields. The cleavages are regio- and stereo-selective. The reactions are complete i

Full text of DOI:10.1139/V07-040

366
Efficient regio- and stereo-selective cleavage of
aziridines and epoxides using an ionic liquid as
reagent and reaction medium
Brindaban C. Ranu, Laksmikanta Adak, and Subhash Banerjee
Abstract: Ionic liquids, containing a variety of functionalities such as halo, azido, and thiocyano, efficiently cleave
aziridines and epoxides to the corresponding products in high yields. The cleavages are regio- and stereo-selective. The
reactions are complete in 1 h at 60 °C and do not require any other catalyst or organic solvent. Thus, a convenient
synthetic route to 1,2-haloamines, 1,2-azidoamines, 1,2-thiocyanoamines, 1,2-azidoalcohols, and 1,2-thiocyanoalcohols
is developed.
Key words: aziridine, epoxide, ionic liquid, cleavage, regioselectivity, stereoselectivity
Résumé : Des liquides ioniques contenant diverses fonctions, telles halogéno, azido ou thiocyano, permettent de cliver
efficacement les aziridines et les époxydes pour fournir les produits correspondants avec des rendements élevés. Les
clivages sont régio- et stéréosélectifs. Les réactions sont complètes en moins d’une heure, à 60 °C, et elles ne nécessi-
tent pas l’utilisation d’autres catalyseurs ou solvants organiques. On a donc développé une voie de synthèse commode
pouvant conduire aux 1,2-halogénoamines, 1,2-azidoamines, 1,2-thiocyanoamines, 1,2-azidoalcools et 1,2-
thiocyanoalcools.
Mots clés : aziridine, époxyde, liquide ionique, clivage, régiosélectivité et stéréosélectivité.
[Traduit par la Rédaction] Ranu et al. 371
Introduction
Scheme 1.
Ionic liquids are attracting increasing attention in organic
synthesis because of their unique advantages as reaction me-
dia (1), compared with organic solvents, and efficient pro-
moters for a variety of reactions (2). However, the potential
of ionic liquids as reagents has been less explored, and as a
part of our activities in this area (3) we initiated an investi-
gation to explore the utility of ionic liquids as efficient re-
agents (3a) for useful transformations.
n
Aziridines (4) and epoxides (5) are very useful synthetic
intermediates. Because of their ring strain and high reactiv-
ity, their reactions with various nucleophiles lead to regio-
and stereo-selective synthesis of various important synthons.
Considerable progress towards the development of several
procedures (6) has been achieved in the nucloephilic ring-
opening reactions of aziridines and epoxides. The
nucleophilic reagents involved in these procedures include
tributylphosphine in H2O (6a), CeCl3 in CH3CN (6b),
NaN3–CeCl3 in CH3CN (6c), Me2SBr2 in CH3CN (6d),
LiClO4–NaX (where X = N3, CN) in CH3CN (6e), KSCN–
sulfated zirconia in CH3CN (6f), zinc halides in CH2Cl2
(6g), KSCN–LiClO4 in CH3CN (6h), KSCN–β-cyclodextrin
in H2O (6i), TBAX–β-cyclodextrin in H2O (6j), Bu4NF–
Me3SiX in THF (6k), TMSN3–Sn(OTf)2 in benzene (6l),
InX3 in CH3CN (6m), NaN3 or KCN–SiO2 in H2O (6n),
TMSN3 in THF (6o), NaN3 in ionic liquid H2O (6p), acti-
vated DMF complexes (6q), NaX–Ce(OTf)4 in micelle (6r),
fluoride anion (6s), and aminosaccharides (6t) in the ionic
liquid N-methylpyridinium tosylate, among others. However,
most of these procedures involve a nucleophilic reagent
(usually metal salts), an acid catalyst, and organic solvents
like CH3CN and CH2Cl2. We report here a new strategy for
the cleavage of aziridines and epoxides with a variety of
nucleophiles using a core ionic liquid, 1-(1-carboxy-ethyl)-
3-methyl-3H-imidazolium derivative [PnMIm]X [where X =
N3, SCN, Cl, Br, I] without any additional reagent, catalyst,
or organic solvent (Scheme 1).
Received 5 March 2007. Accepted 23 April 2007. Published
on the NRC Research Press Web site at http://canjchem.nrc.ca
on 18 April 2007.
Results and discussion
B.C. Ranu,1 L. Adak, and S. Banerjee. Department of
Organic Chemistry, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata 700 032, India.
The general experimental procedure is very simple and
convenient. A mixture of aziridine (or epoxide) and ionic
liquid [PnMIm]X was heated at 60 °C for a certain period of
time, as required to complete the reaction (TLC). The usual
1Corresponding author (e-mail: ocbcr@iacs.res.in).
Can. J. Chem. 85: 366–371 (2007)
doi:10.1139/V07-040
© 2007 NRC Canada

Ranu et al.
367
Table 1. Ring opening of aziridines by ionic liquids [PnMlm]X.
work-up and purification by column chromatography pro-
vided the product.
NMR spectra of the corresponding amines (Table 1, entries
3–7). The disubstituted aziridines also furnished trans-
functionalized amines (Table 1, entries 16–23).
Several structurally diverse aziridines underwent cleav-
ages by this procedure using a core ionic liquid [PnMIm]X
(where X = Cl, Br, I, N3, SCN) to provide the corresponding
2-chloro-, bromo-, iodo-, azido-, and thiocyano-amines. The
results were summarized in Table 1. Both alkyl- and aryl-
substituted aziridines participated in this reaction. The alkyl-
substituted aziridines underwent nucleophilic attack at the
less hindered (substituted) carbon atom (Table 1, entries 1
and 2), whereas in aryl-substituted aziridines nucleophiles
attacked at the benzylic positions (Table 1, entries 8–23) be-
cause of their electronic factor. The cyclohexene azide pro-
The epoxides also underwent cleavages by this ionic liq-
uid, producing 2-azido-, nitro-, and thiocyano-alcohols under
this procedure. The cleavages by halo-bearing ionic liquids
were reported earlier (3a). The reactions of alkyl- and aryl-
substituted as well as cyclohexene epoxides proceeded
smoothly. The results are reported in Table 2. The epoxide
of an α,β-unsaturated ketone underwent nucleophilic attack
at the α-carbon atom followed by subsequent dehydration to
furnish the corresponding α-azido α,β-unsaturated ketone
(Table 2, entry 9) under the reaction conditions.
1
vided trans isomers by this cleavage, as indicated by the H
In general, the reactions were very clean, fast, and high-
© 2007 NRC Canada

368
Can. J. Chem. Vol. 85, 2007
Table 1 (concluded).
1
aYields of pure isolated products characterized by IR, H NMR, and 13C NMR spectroscopic data.
yielding. The cleavages of both aziridines and epoxides by
this reagent were highly regioselective, affording a single
product. The stereochemistry of products was determined as
trans by the coupling constant (J value) of the hydrogens at
the cleaved carbon atoms. It has been observed that
nucleophilic attack occurred at the less-substituted carbon
atoms of the alkyl-substituted terminal aziridines and
epoxides, whereas aryl-substituted substrates underwent at-
tack at the benzylic position, being dictated by the electronic
factor. It may be anticipated that these cleavage reactions
such as Cl, OMe, and OPh, remained unaffected during this
reaction. The ionic liquids were easily prepared by substitu-
tion reaction of the core imidazolium bromide with the
sodio-salt of the corresponding anion.
The products of this cleavage reaction, 2-azidoalcohols, 2-
thiocyanoalcohol, 2-azido amines, 2-haloamines, and 2-
thiocyanoamines are very useful intermediates and have
wide applications in organic synthesis (7, 8).
Conclusion
2
proceed through the usual SN path as outlined in our earlier
paper on epoxide cleavage (3a). Several functional groups,
In conclusion, the present procedure provides a novel and
© 2007 NRC Canada

Ranu et al.
369
Table 2. Ring opening of epoxides by ionic liquids [PnMlm]X.
1
aYields of pure isolated products characterized by IR, H NMR, and 13C NMR spectroscopic data.
general route to the cleavage of epoxides and aziridines by a
variety of nucleophiles carried out by an inexpensive and
easily accessible ionic liquid [PnMIm]X, which itself works
as reagent, catalyst, and reaction medium. The other signifi-
cant advantages offered by this method are operational sim-
plicity, considerably fast reaction time (30–60 min), high
isolated yields of products, general applicability to a wide
variety of substrates, excellent regio- and stereo-selectivity,
thus providing a better and practical alternative to the exist-
ing procedures (6). Moreover, this demonstrates the poten-
tial of ionic liquids as efficient reagents and shows promise
for further useful applications.
in CDCl3 and d6-DMSO solution at 300 and 75 MHz, re-
spectively (Bruker 300 DPX instrument).
General experimental procedure for the preparation of
ionic liquid [PnMIm]x
2-Bromopropionic acid (3.06 g, 20 mmol) was added
dropwise to
a
cooled (0–5 °C) solution of N-
methylimidazole (1.64 g, 20 mmol) in dry acetonitrile
(10 mL). The reaction mixture was then stirred for 12 h at
room temperature (RT). The crude product, obtained by
evaporation of solvent, was washed with Et2O (2 × 5 mL)
and dried under vacuum to provide the bromo ionic liquid,
[PnMIm]Br, ready to be used for cleavage reactions.
The iodo and chloro analogues were prepared from the
corresponding 2-iodopropionic acid and 2-chloropropionic
acid following the same procedure.
Experimental
General
1-(1-Carboxy-ethyl)-3-methyl-3H-imidazol-1-ium bromide
[PnMIm]Br
IR spectra were taken as thin films for liquid compounds
and as KBr pellets for solids using a Shimadzu 8300 FTIR
Viscous liquid. IR (neat, cm–1) ν: 3389, 3001, 2945, 1745,
1
1
spectrophotometer. H and 13C NMR spectra were recorded
1579, 1454, 1176, 1091. H NMR (DMSO-d6 + CDCl3) δ:
© 2007 NRC Canada

370
Can. J. Chem. Vol. 85, 2007
1.73 (d, J = 7.4 Hz, 3H), 3.92 (s, 3H), 5.48 (quartet, J =
7.35 Hz, 1H), 7.49 (s, 1H), 7.65 (s, 1H), 9.25 (s, 1H), 9.60
(broad, 1H). 13C NMR (DMSO-d6 + CDCl3) δ: 15.8, 34.0,
55.4, 117.9, 121.4, 133.8, 168.5. HRMS calcd. for [M + H]+
– Br: 156.0899; found: 155.0284. HRMS calcd. for [M +
Na]+ – Br: 178.0719; found: 177.0060. Anal. calcd. for
C7H11BrN2O2: C 35.76, H 4.72, N 11.92; found: C 35.65, H
4.62, N 11.83.
calcd. for [M + H]+ – N3: 156.0899; found: 155.0016.
HRMS calcd. for [M + Na]+ – N3: 178.0719; found:
176.9774. Anal. calcd. for C7H11N5O2: C 42.64, H 5.62, N
35.51; found: C 42.54, H 5.51, N 35.40.
General experimental procedure for the ring opening of
aziridines. Representative procedure for the ring
opening of 2-hexyl-1-(toluene-4-sulfonyl)-aziridine with
[PnMIm]Cl (Table 1, entry 1)
1-(1-Carboxy-ethyl)-3-methyl-3H-imidazol-1-ium iodide
[PnMIm]I
A
mixture of 2-hexyl-1-(toluene-4-sulfonyl)-aziridine
(281 mg, 1 mmol) and [PnMIm]Cl (225 mg, 1.3 mmol) was
heated at 60 °C with stirring for 50 min (TLC). The reaction
mixture was quenched with saturated brine and extracted
with ether (3 × 10 mL). Evaporation of ether left the crude
product, which was purified by column chromatography
over silica gel (hexanes–Et2O 50:50) to provide the pure N-
(1-chloromethyl-heptyl)-4-methyl-benzenesulfonamide as a
colourless liquid (285 mg, 90%), whose spectroscopic data
(IR, 1H, and 13C NMR) are in good agreement with those re-
ported (6q).
Viscous liquid. IR (neat, cm–1) ν: 3417, 3107, 2984, 1715,
1620, 1454, 1393, 1281, 1177. 1H NMR (DMSO-d6
+
CDCl3) δ: 1.84 (d, J = 7.1 Hz, 3H), 3.95 (s, 3H), 5.20 (q, J =
7.42 Hz, 1H), 7.31 (s, 1H), 7.45 (s, 1H), 7.68 (s, 1H), 9.57
(broad, 1H). 13C NMR (DMSO-d6 + CDCl3) δ: 16.8, 34.1,
57.7, 119.7, 120.3, 134.5, 169.8. HRMS calcd. for [M + H]+ –
I: 156.0899; found: 155.0193. HRMS calcd. for [M + Na]+ –
I: 178.0719; found: 177.0168. Anal. calcd. for C7H11IN2O2:
C 29.81, H 3.93, N 9.93; found: C 29.70, H 3.81, N 9.79.
This procedure was followed for the nucleophilic cleavage
of all aziridines listed in Table 1. The known compounds
(referenced in Table 1) were identified by the comparison of
their spectroscopic data with those reported. The unknown
compounds (Table 1, entries 15 and 17) were properly char-
acterized by its spectroscopic data and elemental analysis.
1-(1-Carboxy-ethyl)-3-methyl-3H-imidazol-1-ium chloride
[PnMIm]Cl
Viscous liquid. IR (neat, cm–1) ν: 3391, 3105, 2951, 2897,
1
2750, 1740, 1580, 1427, 1176. H NMR δ: 1.78 (d, J =
7.2 Hz, 3H), 4.04 (s, 3H), 5.25 (q, J = 7.41 Hz, 1H), 7.35 (s,
1H), 7.48 (s, 1H), 8.79 (s, 1H), 10.04 (broad, 1H). 13C NMR
δ: 16.1, 33.5, 55.9, 121.1, 122.2, 135.9, 167.2. HRMS calcd.
for [M + H]+ – Cl: 156.0899; found: 155.0481. HRMS
calcd. for [M + Na]+ – Cl: 178.0719; found: 177.0191. Anal.
calcd. for C7H11ClN2O2: C 44.10, H 5.82, N 14.70; found: C
43.93, H 5.71, N 14.56.
N-[2-Chloro-2-(4-methoxy-phenyl)-ethyl]-4-methyl-
benzenesulf onamide (Table 1, entry 15)
Colourless viscous liquid. IR (neat, cm–1) ν: 3493, 3282,
1
2922, 1612, 1514, 1454, 1326, 1250, 1159. H NMR δ: 2.38
(s, 3H), 2.97–3.24 (m, 2H), 3.76 (s, 3H), 4.74 (dd, J1 =
3.72 Hz, J2 = 8.64 Hz, 1H), 5.10 (t, J = 4.75 Hz, 1H), 6.85
(d, J = 8.64 Hz, 2H), 7.19 (d, J = 8.64 Hz, 2H), 7.29 (d, J =
7.98 Hz, 2H), 7.72 (d, J = 8.34 Hz, 2H). 13C NMR δ: 21.9,
50.6, 55.7, 72.8, 114.4 (2C), 127.5 (2C), 127.5 (2C), 130.1
(2C), 133.3, 137.1, 143.9, 159.9. Anal. calcd. for
C16H18ClNO3S: C 56.55, H 5.34, N 4.12; found: C 56.48, H
5.25, N 4.01.
The bromo ionic liquid was then used for the preparation
of other derivatives (X = N3, SCN) by treatment with NaN3
(or NaSCN) (30 mmol) in acetonitrile (5 mL) under stirring
for 24 h at RT. The precipitated NaBr was filtered and the
filtrate was evaporated under vacuum to give the crude prod-
uct, which was washed with Et2O (2 × 5 mL) and thor-
oughly dried under vacuum to furnish the pure ionic liquid
characterized by spectroscopic data provided in the follow-
ing sections.
N-(2-Iodo-1-methyl-2-phenyl-ethyl)-4-methyl-benzene
sulfonamide (Table 1, entry 17)
Colourless viscous liquid. IR (neat, cm–1) ν: 3500, 3273,
1-(1-Carboxy-ethyl)-3-methyl-3H-imidazol-1-ium
thiocyanate
1
1596, 1450, 1326, 1161, 1091. H NMR δ: 1.13 (d, J =
Yellow viscous liquid. IR (neat, cm–1) ν: 3420, 3147,
6.54 Hz, 3H), 2.42 (s, 3H), 3.57–3.64 (m, 1H), 4.93 (d, J =
8.37 Hz, 1H), 5.05 (d, J = 5.73 Hz, 1H), 7.15–7.33 (m, 7H),
7.73 (d, J = 8.16 Hz, 2H). 13C NMR δ: 19.9, 21.5, 38.5,
56.1, 127.0 (2C), 128.5 (2C), 129.0, 129.3 (2C), 129.7 (2C),
139.0, 139.3, 143.6. Anal. calcd. for C16H18INO2S: C 46.27,
H 4.37, N 3.37; found: C 46.16, H 4.29, N 3.28.
2058, 1693, 1614, 1394, 1172. 1H NMR (DMSO-d6
+
CDCl3) δ: 2.20 (d, J = 7.2 Hz, 3H), 4.36 (s, 3H), 5.56 (quar-
tet, J = 7.29 Hz, 1H), 7.82 (s, 1H), 7.91 (s, 1H), 9.01 (broad,
1H), 9.12 (s, 1H). 13C NMR (DMSO-d6 + CDCl3) δ: 16.8,
33.6, 57.6, 120.6, 120.9, 134.4, 170.1. HRMS calcd. for
[M + H]+ – SCN: 156.0899; found: 154.9482. HRMS calcd.
for [M + Na]+ – SCN: 178.0719; found: 176.9132. Anal.
calcd. for C8H11N3O2S: C 45.06, H 5.20, N 19.70; found: C
44.95, H 5.30, N 19.58.
General experimental procedure for the ring opening of
epoxides. Representative procedure for the ring opening
of 2-hexyl-oxirane by [PnMIm]N3 (Table 2, entry 1)
A mixture of 2-hexyl-oxirane (128 mg, 1 mmol) and
[PnMIm]N3 (256 mg, 1.3 mmol) was heated at 60 °C with
stirring for 45 min (TLC). The reaction mixture was
quenched with saturated brine and extracted with ether (3 ×
10 mL). Evaporation of ether left the crude product, which
was purified by column chromatography over silica gel
(hexane–Et2O 80:20) to provide the pure 1-azido-octan-2-ol
1-(1-Carboxy-ethyl)-3-methyl-3H-imidazol-1-ium azide
Colourless viscous liquid. IR (neat, cm–1) ν: 3419, 3099,
2104, 1738, 1614, 1583, 1394, 1176. 1H NMR δ: 2.19 (d, J =
7.0 Hz, 3H), 4.36 (s, 3H), 5.57 (quartet, J = 7.21 Hz, 1H),
7.74 (s, 1H), 7.84 (s, 1H), 7.98 (broad, 1H), 9.0 (s, 1H). 13C
NMR δ: 17.5, 34.9, 58.3, 120.8, 121.4, 135.2, 170.8. HRMS
© 2007 NRC Canada

Ranu et al.
371
6. (a) R.-H. Fan and X.-L. Hou. J. Org. Chem. 68, 726 (2003);
(b) G. Sabitha, R.S. Babu, M. Rajkumar, Ch. S. Reddy, and
J.S. Yadav. Tetrahedron Lett. 42, 3955 (2001); (c) G. Sabitha,
R.S. Babu, M. Rajkumar, and J.S. Yadav. Org. Lett. 4, 343
(2002); (d) B. Das, M. Krishnaiah, and K. Venkateswarlu. Tet-
rahedron Lett. 47, 4457 (2006); (e) J.S. Yadav, B.V.S. Reddy,
G. Parimala, and P.V. Reddy. Synthesis, 2383 (2002); (f) B.
Das, R. Ramu, B. Ravikanth, and K.R. Reddy. Tetrahedron
Lett. 47, 779 (2006); (g) M.K. Ghorai, K. Das, A. Kumar, and
K. Ghosh. Tetrahedron Lett. 46, 4103 (2005); (h) J.S. Yadav,
B.V.S. Reddy, B. Jyothirmai, and M.S.R. Murty. Tetrahedron
Lett. 46, 6385 (2005); (i) M.S. Reddy, M. Narender, Y.V.D.
Nageswar, and K.R. Rao. Tetrahedron Lett. 46, 6437 2005); (j)
M. Narender, K. Surendra, N.S. Krishnaveni, M.S. Reddy, and
K.R. Rao. Tetrahedron Lett. 45, 7995 (2004); (k) J. Wu, X.-L.
Hou, and L.-X. Dai. J. Org. Chem. 65, 1344 (2000); (l) M.
Chandrasekhar, G. Sekar, and V.K. Singh. Tetrahedron Lett.
41, 10079 (2000); (m) J.S. Yadav, B.V.S. Reddy, and G.M.
Kumar. Synlett, 1417 (2001); (n) S. Minakata, D. Kano, Y.
Oderaotoshi, and M. Komatsu. Angew. Chem. Int. Ed. 43, 79
(2004); (o) H. Konno, E. Toshiro, and N. Hinoda. Synthesis,
2161 (2003); (p) J.S. Yadav, B.V.S. Reddy, B. Jyothirmai, and
M.S.R. Murty. Tetrahedron Lett. 46, 6559 (2005); (q) M.K.
Pandey, A. Bisai, and V.K. Singh. Tetrahedron Lett. 45, 9661
(2004); (r) N. Iranpoor, H. Firouzabadi, and M. Shekarize.
Org. Biomol. Chem. 1, 724 (2003); (s) J. Kroutil and K.
Jenistova. Collect. Czech. Chem. Commun. 70, 2075 (2005);
(t) J. Kroutil and M. Budesinsky. Carbohydr. Res. 342, 147
(2007).
as a colourless liquid (162 mg, 95%). The product was iden-
1
tified by comparison of its IR, H, and 13C NMR spectro-
scopic data with those previously reported (6p).
This procedure was followed for the nucleophilic cleavage
of all oxiranes listed in Table 2. The products were all
known compounds (referenced in Table 2) and were identi-
fied by comparison of their spectroscopic data with those re-
ported.
Acknowledgements
This investigation has enjoyed financial support from the
Council of Scientific and Industrial Research, New Delhi
(CSIR) [Grant No. 01(1936)/04]. LA and SB thank CSIR for
their fellowships.
References
1. (a) T. Welton. Chem. Rev. 99, 2071 (1999); (b) P.
Wasserscheid and W. Keim. Angew. Chem. Int. Ed. 39, 3773
(2000); (c) J.S. Wilkes. Green. Chem. 4, 73 (2002); (d) H.M.
Zerth, N.M. Leonard, and R.S. Mohan. Org. Lett. 5, 55 (2003);
(e) A. Kumar and S.S. Pawar. J. Org. Chem. 69, 1419 (2004);
(f) D.-G. Gu, S.-J. Ji, Z.-Q. Jiang, M.-F. Zhou, and T.-P. Loh.
Synlett, 959 (2005).
2. (a) J.A. Boon, J.A. Levinsky, J.I. Pflug, and J.S. Wilkes. J.
Org. Chem. 51, 480 (1986); (b) J.R. Harjani, S.J. Nara, and
M.M. Salunkhe. Tetrahedron Lett. 43, 1127 (2002); (c) V.V.
Namboodiri and R.S. Varma. Chem. Commun. (Cambridge),
342 (2002); (d) W. Sun, C.-G. Xia, and H.-W. Wang. Tetrahe-
dron Lett. 44, 2409 (2003); (e) K. Qiao and C. Yakoyama.
Chem. Lett. 33, 472 (2004); (f) T. Akaiyama, A. Suzuki, and
K. Fuchibe. Synlett, 1024 (2005).
3. (a) B.C. Ranu and S. Banerjee. J. Org. Chem. 70, 4517 (2005);
(b) B.C. Ranu, S. Banerjee, and R. Jana. Tetrahedron, 63, 776
(2007) and refs. cited therein.
4. (a) J.E.G. Kemp. In Comprehensive organic synthesis. Vol. 7.
Edited by B.M Trost and I. Fleming. Pergamon, Oxford, UK.
1991. pp. 469–513; (b) H.M.I. Osborn and J. Sweeney. Tetra-
hedron: Asymmetry, 8, 1693 (1997); (c) R.S. Coleman.
Synlett, 1031 (1998).
7. (a) J. Schubert, R. Schwesinger, and H. Prinzbach. Angew.
Chem. Int. Ed. Engl. 23, 167 (1984); (b) G.A. Jacobs, J.A.
Tino, and R. Zahler. Tetrahedron Lett. 30, 6955 (1989);
(c) E.F.V. Scriven and K. Turnbull. Chem. Rev. 88, 297 (1988).
8. (a) M.T. Reets, R. Jaeger, R. Drewlies, and M. Hubel. Angew.
Chem. Int. Ed. Engl. 30, 103 (1991); (b) D. Lucet, T. Le Gall,
and C. Mioskowski. Angew. Chem. Int. Ed. 37, 2850 (1998).
9. G.D.K. Kumar and S. Baskaran. Synlett, 1719 (2004).
10. J. Wu, X. Sun, and H.-G. Xia. Eur. J. Org. Chem. 4769 (2005).
11. V.V. Thakur, S.K. Talluri, and A. Sudalai. Org. Lett. 5, 861
(2003).
12. H. Sharghi, M.A. Nasseri, and K. Niknam. J. Org. Chem. 66,
7287 (2001).
13. G.D.K. Kumar and S. Baskaran. J. Org. Chem. 70, 4520
(2005).
5. (a) R.E. Parker and N.S. Isaacs. Chem. Rev. 59, 737 (1959);
(b) C. Bonini and G. Righi. Synthesis, 225 (1994); (c) M.
Shimizu, A. Yoshida, and T. Fujisawa. Synlett, 204 (1992);
(d) J.U.G. Smith. Synthesis, 629 (1984).
14. M. Mizuno and T. Shioiri. Tetrahedron Lett. 40, 7105 (1999).
© 2007 NRC Canada