CO2-induced amidobromination of olefins with bromamine-T

Article abstract of DOI:10.1039/b923253m

The carbon dioxide (CO₂)-induced amidobromination of olefins with bromamine-T is described. The method can be used in reactions with a wide range of olefins, both aromatic and aliphatic, as well as electron-rich and deficient olefins, leading to the regioselective formation of amidobrominated compounds.

Full text of DOI:10.1039/b923253m

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www.rsc.org/obc | Organic & Biomolecular Chemistry
CO2-induced amidobromination of olefins with bromamine-T†
Junpei Hayakawa, Mitsuhiro Kuzuhara and Satoshi Minakata*
Received 5th November 2009, Accepted 22nd December 2009
First published as an Advance Article on the web 22nd January 2010
DOI: 10.1039/b923253m
The carbon dioxide (CO2)-induced amidobromination of olefins with bromamine-T is described. The
method can be used in reactions with a wide range of olefins, both aromatic and aliphatic, as well as
electron-rich and deficient olefins, leading to the regioselective formation of amidobrominated
compounds.
can be readily prepared from chloramine-T, Br2, and aqueous
NaOH.8
Introduction
Vicinal haloamine derivatives are valuable synthetic intermediates
for use in constructing a variety of functional materials and biolog-
ically active compounds.1 The direct functionalization of carbon–
carbon double bonds with amine and halogen groups is a signif-
icant approach to useful building blocks for organic synthesis.2
Among the halogens, the bromine atom is a versatile substituent
that can be used in organic transformations. In this context,
since the amidobromination of olefins constitutes a significant
and potentially useful transformation, many systems employing
brominated amide derivatives,3 combinations of amides (sulfon-
amides or carboxamides) and N-bromosuccinimides (NBS)4 or N-
bromo-N-sodio-p-toluenesulfonamide (bromamine-T)5 have been
developed.
In our continuing efforts to develop new methods for the
synthesis of N-heterocycles, we previously reported on the utility of
nitrogen–halogen bonds bearing electron-withdrawing groups on
the nitrogen.6 As a representative reagent containing a N–Cl bond,
N-chloro-N-sodio-p-toluenesulfonamide (chloramine-T: CT) was
utilized in various transformations, and a unique CO2-induced
vicinal amidochlorination was reported.7 This amidochlorination
could lead to the development of a metal-free process for this type
of synthesis. However, the functionalization of olefins using CT
was restricted to relatively electron-rich olefins.
Results and discussion
As described in our previous report,7 the nitrogen–chlorine bond
of CT regioselectively added to the carbon–carbon double bond of
styrene under an atmosphere of carbon dioxide to give exclusively
a-chlorinated compound 2a (X = Cl) (Table 1, entries 1–3).
When BT was employed in the reaction instead of CT, a-
brominated compound 2a (X = Br) was obtained selectively under
an atmospheric pressure of CO2 (entry 4). When the pressure
of CO2 was increased, the reverse regioselectivity was observed,
giving b-brominated compound 1a (X = Br) in good yields (entries
5–8). Although the reason for this phenomenon is unclear (vide
infra), it is interesting to note that regioselectivity is dependent on
the specific halogen species in the haloamine salt.
p-Substituted styrene derivatives were subjected to the amido-
bromination, in an attempt to determine the active species for
the reaction. The reaction of p-nitrostyrene with BT under CO2
afforded predominantly the a-brominated sulfonamide 2b, but
the regioselectivity was the reverse of that observed for styrene
(Table 2, entry 1). Electron-donating groups at the para-position
of styrene induced the selective formation of b-brominated com-
pounds (1c–e) and the reaction was complete in less time (entries
2–4). These results suggest that the reaction proceeds through a
cationic active species, such as a cyclic bromonium intermediate.
The amidobromination reaction was applied to disubstituted
aromatic and aliphatic olefins, and an enol ether (Table 3).
When trans-b-methylstyrene was treated with BT under 30 atm of
CO2, the a-brominated product 3 was stereo- and regioselectively
obtained as the sole product in good yield. Although a b-
brominated amide was mainly produced in the case of styrene,
in the presence of a b-substituent attached in a trans orientation,
the regioselectivity was reversed (entry 1). BT added to indene
to give the b-brominated amide 4 with the same regioselectivity
that was observed for styrene (entry 2). Terminal, internal and
cyclic aliphatic olefins were also converted to amidobrominated
compounds (entries 3–6). It is especially noteworthy that the
stereospecific addition of BT to trans- and cis-3-hexene suggests
that a cyclic cationic intermediate is likely involved in the reaction.
An electron-rich olefin, butyl vinyl ether, was found to be a
good substrate for the reaction, resulting in the formation of the
expected terminal bromide 9 in good yield (entry 7).
From these points of view, we report herein on the use of
bromamine-T (BT) in the CO2-induced amidobromination of
olefins. BT showed a different regioselectivity in amidohalogena-
tion reactions, compared with that of CT, and could function
successfully in the amidobromination of not only aromatic and
aliphatic olefins, but also electron-rich and deficient olefins
(Scheme 1). Although BT is not commercially available, it
Scheme 1
Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan.
E-mail: minakata@chem.eng.osaka-u.ac.jp; Fax: +81 6-6879-7402
† Electronic supplementary information (ESI) available: NMR spectra and
mass spectrometry results. See DOI: 10.1039/b923253m
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Table 1 Amidohalogenation of styrene under CO2 (CT vs. BT)
Entry
X
CO2 (atm)
Yield (%)
1a : 2a
1
Cl
Cl
Cl
Br
Br
Br
Br
Br
1
9
0 : 100
0 : 100
0 : 100
15 : 85
75 : 25
97 : 3
2a
3a
4
10
30
1
10
30
30
30
80
63
46
72
73
81
80
5
6
7b
8c
95 : 5
97 : 3
a Reaction time: 6 h. b Reaction time: 24 h. c Addition of MS4A, reaction time: 3 h.
Table 2 Addition of BT to styrene derivatives under CO2
Entry
Ar
Time/h
Yield (%)
1 : 2
1
2
3
4
4-NO2C6H4
4-ClC6H4
4-MeC6H4
4-MeOC6H4
12
4.5
3
88 (1b + 2b)
88 (1c + 2c)
86 (1d)
15 : 85
84 : 16
100 : 0
100 : 0
1
90 (1e)
Although the reaction of CT with electron-deficient olefins
in the presence of CO2 did not proceed at all, BT was found
to react with olefins (Table 4). All the reactions proceeded
under an atmospheric pressure of CO2. Even though the use of
one equivalent of BT yielded the desired products, the use of
two equivalents resulted in improved yields. In the case of the
amidobromination of chalcone, the position b to carbonyl group
was regioselectively brominated (entry 1). The amidobromination
of electron-deficient olefins conjugated with aromatic rings has
been extensively investigated,2–5 but no examples of the amidohalo-
genation of terminal electron-deficient olefins have been reported.9
Using the present method, the addition reaction proceeded,
giving a-brominated compounds (entries 2–5). Among these, the
amidobromination of an a,b-unsaturated amide proceeded with
relatively good product yield (entry 5).
Scheme 2
leading to the formation of the enantiopure (S)-aziridine 17. The
purity of aziridine 17 was confirmed by chiral HPLC analysis10 and
S configuration was determined by optical rotation measurements,
which were in agreement with previously reported data.11 This
chiral aziridine should be a good precursor of amino acid
derivatives through a ring-opening reaction.12 From the absolute
configuration of aziridine 17, the stereocenter, a to the carbonyl
group, in the major diastereomer of 16 would be predicted to have
an R configuration.
Since the present reaction proceeded under an atmosphere of
CO2, the reaction of BT with CO2 was monitored by H NMR
(Fig. 1). Signals corresponding to the tosyl group were shifted
downfield by the addition of CO2, indicating that CO2 is inserted
into the to N–Na bond of BT.
This result (Table 4, entry 5) prompted us to investigate
diastereoselective amidobromination of an olefin bearing a
chiral auxiliary (Evans’ oxazolidinone) (Scheme 2).
A 2-
acryloyloxazolidinone without a stereocentre was first employed
in the reaction. The desired amidobromination proceeded, even
under an atmospheric pressure of CO2, but the efficiency of the
reaction was greatly improved when a CO2 pressure of 30 atm was
used. A phenyl-substituted chiral olefin also reacted, affording
the a-bromo-b-amino product in good diastereoselectivity. The
diastereomers were readily separated and the major isomer
was treated with 4-di(methylamino)pyridine (DMAP) in MeOH,
1
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a
a
Table 3 Addition of BT to olefins under CO2
Table 4 Addition of BT to electron-deficient olefins under CO2
Entry
1b
Olefin
Adduct
Yield (%)
84
Entry
1
Olefin
Adduct
Yield (%)
53 (10)
2
3
4
5
48 (11)
41 (12)
24 (13)
61 (14)
2b
3
62
45c
53
4d
5
60
a Reaction conditions: olefin (0.3 mmol), BT (0.6 mmol), CO2 (1 atm),
MeCN (1.5 cm3), rt, 24 h.
6
51
83
7e
a Reaction conditions: olefin (0.5 mmol), BT (0.6 mmol), CO2 (30 atm),
MS4A (180 mg), PhH (1.5 mL), rt, 3 h. b 9 h, without MS4A. c Regioisomer
ratio: 84 : 16 (major isomer is depicted). d 9 h. e Olefin (0.5 mmol), BT
(0.3 mmol), THF (1.5 cm3).
Scheme 3
regioselectivity in the reaction of BT with styrene, but the reaction
of CT led to the production of an a-chlorinated sulfonamide
regardless of the pressure. In the case of CT, nitrogen nucleophiles
(carbamate anion or CT) would attack the less hindered carbon of
the cyclic chloronium intermediate. An increase of CO2 pressure
might increase the polarity of solvent, which would polarize the
benzylic carbon–Br bond in contrast to that of the C–Cl bond.
This phenomena would induce the formation of the b-brominated
sulfonamide (Scheme 4).
Fig. 1 1H NMR spectra of BT (A) and BT with CO2 (B).
Scheme 4
We conclude that the active species 18 induces a cyclic
bromonium intermediate on reaction with an olefin, as shown in
Scheme 3. This conclusion is supported by the results shown in
Table 2.
Conclusions
The case of CT would also be predicted to proceed via the same
pathway, because the measurement of 1H NMR of CT with CO2
showed the same results as that of BT. The results in Table 1
can be explained as follows. The pressure of CO2 affects the
In summary, the use of a unique combination of BT and CO2
permitted the successful amidobromination of various olefins.
When BT is used, the regioselectivity of the reaction is different
from that with CT in the amidohalogenation of some olefins.
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The use of CT under CO2 does not lead to the desired addition
reaction in the case of terminal electron-deficient olefins, but the
present method using BT readily gave amidobrominated products.
The method provides a clear demonstration of producing an
enantiopure aziridine via diastereoselective amidobromination.
Typical procedure for the amidobromination of elecron-
deficient olefins with chloramine-T under CO2: methyl acrylate
(86 mg, 0.3 mmol) was added to a suspension of bromamine-T
(108 mg, 0.6 mmol) in MeCN (10 cm3) at room temperature under
an atmosphere of CO2. After stirring for 24 h, aqueous Na2S2O3
(0.5 M, 5 cm3) was added to the reaction mixture and the solution
was extracted with CH2Cl2 (20 cm3 ¥ 5). The combined organic
extracts were dried over Na2SO4 and concentrated to give the crude
product. Purification by flash column chromatography (silica gel;
ethyl acetate in hexane) gave 11 (48 mg, 48%).
Experimental
General
Melting points were determined on a Yanaco melting point
apparatus and are uncorrected. Infrared spectra were obtained
Spectroscopic data
on a JASCO FT/IR-410 infrared spectrophotometer. 1H and 13
C
Bromamine-T·1.5 H2O8. Pale yellow crystal; mp. 202–205 ◦C;
1H NMR (270 MHz, CD3CN) d 2.35 (s, 3H), 7.21 (d, 2H, J = 8.0
Hz), 7.63 (d, 2H, J = 8.0 Hz); 13C NMR (68 MHz, CD3CN) d 21.3,
128.1, 129.3, 140.8, 142.9; Anal. Calcd for C7H10BrNNaO3.5S: C,
28.11; H, 3.37; N, 4.68. Found: C, 28.08; H, 3.26; N, 4.71.
NMR spectra were recorded on a JEOL FT-NMR JNM EX
270 spectrometer (1H NMR, 270 MHz; 13C NMR, 68 MHz)
using tetramethylsilane as an internal standard. Mass spectra
were obtained on a JEOL JMS-DX303HF mass spectrometer.
Elemental analyses were performed at the Analytical Center,
Faculty of Engineering, Osaka University. Optical rotations were
measured on a JASCO DIP-1000 digital polarimeter using a
10 cm microcell at ambient temperature. Products were purified
by chromatography on silica gel BW-300 (Fuji Silysia Chemical
Co.). Analytical thin-layer chromatography was performed on
precoated silica gel glass plates (silica gel 60 F254, 0.25 mm
thickness) (Merck Co.). Compounds were visualized by UV light
or treatment with an ethanolic solution of phosphomolybdic
acid followed by heating. Since the minor regioisomers 2a, 2b
and 2c could not be isolated in pure form, the structures were
determined from selected spectral data, mainly 1H NMR and
mass measurements, in a form of a mixture with the corresponding
major regioisomers.
2-Bromo-1-phenyl-1-(p-toluenesulfonamido)ethane (1a)4a. Col-
◦
orless solid (136 mg, 77%); mp. 167–169 C; IR (KBr, nmax/cm-1
3259, 1313, 1161; 1H NMR (270 MHz, CDCl3) d 2.42 (s, 3H), 3.54-
3.65 (m, 2H), 4.56 (td, 1H, J = 5.9, 5.9 Hz), 5.13 (br d, 1H, J =
5.9 Hz, D2O exchangeable), 7.10-7.26 (m, 7H), 7.63 (d, 2H, J = 8.4
Hz); 13C NMR (68 MHz, CDCl3) d 21.7, 36.3, 57.5, 127.0, 128.0,
128.6, 129.4, 134.0, 136.0, 136.5, 143.6; MS (CI, isobutane): m/z
(relative intensity, %) 354 ([M + H]+, 99), 356 ([M + 2 + H]+, 100),
274 ([M-Br]+, 65); HRMS (CI): m/z Calcd for C15H17BrNO2S (M +
H) 354.0163, found 354.0169; Anal. Calc. for C15H16BrNO2S: C,
50.9; H, 4.6; N, 3.95. Found: C, 50.6; H, 4.2; N, 3.95.
1a and 1-bromo-1-phenyl-2-(p-toluenesulfonamido)ethane (2a)4a.
1
Obtained as an inseparable mixture: colorless solid; H NMR
Experimental procedure
(270 MHz, CDCl3) d 2.40 (s, 3H, 1a), 2.45 (s, 3H, 2a), 3.48-3.66
(m, 4H, 2H of 1a and 2 H of 2a), 4.57 (td, 1H, J = 5.9, 5.9 Hz, 1a),
4.69 (br t, 1H, J = 7.3 Hz, D2O exchangeable, 2a), 4.90 (dd, 1H,
J = 7.4, 7.4 Hz, 2a), 5.14 (br d, 1H, J = 6.5 Hz, D2O exchangeable,
1a), 7.10-7.13 (m, 2H, 1a), 7.19-7.34 (m, 12H, 5H of 1a and 7H of
Recrystallized chloramine-T·3H2O (9 g) was dissolved in water
(130 cm3) and liquid bromine (2 cm3) was added dropwise from a
burette with constant stirring of the solution. The golden yellow
precipitate of dibromamine-T was thoroughly washed with water,
filtered under suction and dried in a vacuum desiccator for 24 h to
give dibromamine-T (10.3 g, 98%). Dibromamine-T (10.3 g) was
dissolved, in small lots at a time and with stirring, in an aqueous
solution of sodium hydroxide (4.32 g) in water (20 cm3), and the
solution was cooled in ice. Pale yellow crystals of bromamine-T
separated out. They were filtered off under suction, washed quickly
with the minimum quantity of water and dried over phosphorus
pentoxide, to give bromamine-T·1.5H2O (7.2 g, 75%).
2a), 7.62 (d, 2H, J = 8.4 Hz, 1a), 7.73 (d, 2H, J = 8.4 Hz, 2a); 13
C
NMR (68 MHz, CDCl3) (1a + 2a) d 21.5, 21.6, 36.6, 50.0, 52.6,
58.1, 126.6, 126.9, 127.0, 127.5, 128.1, 128.5, 128.8, 129.0, 129.4,
129.7, 136.7, 137.5, 138.0, 143.4, 143.7; MS (CI, isobutane): m/z
(relative intensity, %) 1a: 354 ([M + H]+, 96), 356 ([M + 2 + H]+,
100), 274 ([M - Br]+, 48) 2a: 354 ([M + H]+, 19), 356 ([M + 2 +
H]+, 22), 274 ([M - Br]+, 100).
Typical procedure for the amidobromination of styrene deriva-
tives and aliphatic olefins with chloramine-T under CO2: a
magnetic stirring bar, bromamine-T (180 mg, 0.6 mmol), MS4A
(180 mg), p-methoxystyrene (134 mg, 0.5 mmol), and benzene
(1.5 cm3) were placed in a 50 cm3 stainless steel autoclave lined
with a glass liner. The autoclave was closed, purged three times
with carbon dioxide, pressurized with 30 atm of CO2 and then
stirred at room temperature for 1 h. After discharging the excess
CO2, aqueous Na2S2O3 (0.5 M, 5 cm3) was added to the reaction
mixture. The solution was then extracted with CH2Cl2 (20 cm3 ¥
5). The combined organic extracts were dried over Na2SO4 and
concentrated to give the crude product. Purification by flash
column chromatography (silica gel; ethyl acetate in hexane) gave
1e (173 mg, 90%).
2-Bromo-1-(4-nitrophenyl)-1-(p-toluenesulfonamido)ethane (1b)
and
1-bromo-1-(4-nitrophenyl)-2-(p-toluenesulfonamido)ethane
(2b). Obtained as an inseparable mixture: brown oil (176 mg,
88%); 1H NMR (270 MHz, CDCl3) d 2.41 (s, 3H, 1b), 2.45 (s, 3H,
2b), 3.52-3.60 (m, 4H, 2H of 1b and 2H of 2b), 4.57 (td, 1H, J =
5.7, 5.7 Hz, 1b), 4.94 (br t, 1H, J = 7.0 Hz, D2O exchangeable,
2b), 5.01 (dd, 1H, J = 7.2, 7.2 Hz, 2b), 5.38 (br d, 1H, J = 7.0 Hz,
D2O exchangeable, 1b), 7.21-7.37 (m, 6H, 4H of 1b and 2H of
2b), 7.49 (d, 2H, J = 8.9 Hz, 2b), 7.62 (d, 2H, J = 8.4 Hz, 1b),
7.70 (d, 2H, J = 8.4 Hz, 2b), 8.11 (d, 2H, J = 8.4 Hz, 1b), 8.17
(d, 2H, J = 8.4 Hz, 2b); 13C NMR (68 MHz, CDCl3) (1b + 2b) d
21.6, 21.6, 35.9, 49.7, 49.9, 57.2, 123.6, 123.9, 126.8, 127.0, 127.7,
128.8, 129.6, 129.8, 136.3, 136.4, 144.0, 144.1, 144.7, 145.1, 147.4,
147.7; MS (CI, isobutane): m/z (relative intensity, %) 1b: 399
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([M + H]+, 30), 401 ([M + 2 + H]+, 37), 319 ([M - Br]+, 25) 2b:
399 ([M + H]+, 75), 401 ([M + 2 + H]+, 89), 319 ([M - Br]+, 100).
Hz), 5.06 (d, 1H, J = 3.8 Hz), 7.25-7.31 (m, 7H), 7.72 (d, 2H, J =
8.4 Hz); 13C NMR (68 MHz, CDCl3) d 17.1, 21.6, 55.1, 61.7, 126.9,
128.1, 128.2, 128.3, 129.7, 137.5, 137.6, 143.5; MS (CI, isobutane):
m/z (relative intensity, %) 368 ([M + H]+, 25), 370 ([M + 2 + H]+,
26), 288 ([M - Br]+, 100); HRMS (CI, isobutane): m/z Calc. for
C16H19BrNO2S (M+H) 368.0320, Found 368.0314.
2-Bromo-1-(4-chlorophenyl)-1-(p-toluenesulfonamido)ethane
(1c)4b. Colorless solid (123 mg, 76%); mp. 147–148 ◦C; IR (KBr,
n
max/cm-1 3240, 1321, 1166; 1H NMR (270 MHz, CDCl3) d 2.42 (s,
3H), 3.54 (d, 2H, J = 5.9 Hz), 4.56 (td, 1H, J = 5.9, 6.2 Hz), 5.11
(br d, J = 6.2 Hz, 1H, D2O exchangeable), 7.06 (d, 2H, J = 8.1 Hz),
7.20-7.26 (m, 4H), 7.61 (d, 2H, J = 8.4 Hz); 13C NMR (68 MHz,
CDCl3) d 21.7, 36.3, 57.5, 127.0, 128.0, 128.6, 129.4, 134.0, 136.0,
136.5, 143.6; MS (CI, isobutane): m/z (relative intensity, %) 388
([M + H]+, 72), 390 ([M + 2 + H]+, 100), 308 ([M - Br]+, 73);
HRMS (CI, isobutane): m/z Calc. for C15H16BrClNO2S (M + H)
387.9975, Found 387.9765; Anal. Calc. for C15H15BrClNO2S: C,
46.35; H, 3.9; N, 3.6. Found: C, 46.1; H, 3.8; N, 3.6.
( )-trans-2-Bromo-1-(p-toluenesulfonamido)indane (4)4a. Col-
orless solid (113 mg, 62%); dec. 169–170 ◦C; IR (KBr, nmax/cm-1
3249, 1331, 1153; H NMR (270 MHz, CDCl3) d 2.47 (s, 3H),
1
3.15-3.23 (dd, 1H, J = 16.7, 5.4 Hz), 3.54-3.62 (dd, 1H, J = 16.6,
6.8 Hz), 4.32 (q, 1H, J = 5.7 Hz), 4.80 (d, 1H, J = 7.8 Hz), 4.86-
4.94 (m, 1H), 7.09 (d, 1H, J = 8.1 Hz), 7.14-7.37 (m, 5H), 7.85 (d,
2H, J = 8.4 Hz); 13C NMR (68 MHz, CDCl3) d 21.7, 41.1, 51.8,
67.1, 124.5, 124.8, 127.3, 127.8, 129.2, 129.7, 137.1, 139.1, 140.1,
143.8; MS (CI, isobutane): m/z (relative intensity, %) 366 ([M +
H]+, 96), 368 ([M + 2 + H]+, 100), 286 ([M - Br]+, 49); HRMS
(CI, isobutane): m/z Calc. for C16H17BrNO2S (M + H) 366.0163,
Found 366.0158.
1c and 1-bromo-1-(4-chlorophenyl)-2-(p-toluenesulfonamido)-
ethane (2c)4b. Obtained as an inseparable mixture: colorless solid;
1H NMR (270 MHz, CDCl3) d 2.41 (s, 3H, 1c), 2.45 (s, 3H, 2c),
3.50-3.57 (m, 4H, 2H of 1c and 2H of 2c), 4.56 (td, 1H, J = 5.9,
5.9 Hz, 1c), 4.87-4.92 (m, 2H, 2c) 5.33 (br d, 1H, J = 6.48, 1c),
7.06 (m, 4H, 2H of 1c and 2H of 2c), 7.18-7.59 (m, 8H, 4H of
1c and 4H of 2c), 7.61 (d, 2H, J = 8.4 Hz, 1c), 7.71 (d, 2H, J =
8.6 Hz, 2c); 13C NMR (68 MHz, CDCl3) d 21.6, 36.4, 50.0, 51.5,
57.4, 126.9, 127.1, 128.0, 128.7, 128.9, 129.0, 129.5, 129.8, 134.1,
134.8, 136.0, 136.6, 136.7, 143.7, 143.8; MS (CI, isobutane): m/z
(relative intensity, %) 1c: 388 ([M + H]+, 72), 390 ([M + 2 + H]+,
100), 308 ([M - Br]+, 32).
2-Bromo-1-(p-toluenesulfonamido)octane (major regioisomer of
5). Colorless oil (68 mg, 38%); IR (neat, cm-1) 3440, 1328, 1159;
1H NMR (270 MHz, CDCl3) d 0.88 (t, 3H, J = 6.6 Hz), 1.24-1.46
(m, 8H), 1.76 (dt, 2H, J = 7.8, 7.8 Hz), 2.44 (s, 3H), 3.11-3.21
(m, 1H), 3.33-3.42 (m, 1H), 3.92-4.02 (m, 1H), 4.87 (br t, 1H, J =
7.3 Hz, D2O exchangeable), 7.32 (d, 2H, J = 8.4 Hz), 7.75 (d,
2H, J = 8.4 Hz); 13C NMR (68 MHz, CDCl3) d 14.1, 21.6, 22.6,
27.3, 28.5, 31.6, 36.1, 49.6, 55.7, 126.9, 129.7, 136.7, 143.6; MS
(CI, isobutane): m/z (relative intensity,%) 362 ([M + H]+, 95), 364
([M + 2 + H]+, 100), 282 ([M - Br]+, 69); HRMS (CI, isobutane):
m/z Calc. for C15H25BrNO2S (M + H) 362.0791, Found 362.0783;
Anal. Calc. for C15H24BrNO2S: C, 49.75; H, 6.7; N, 3.9. Found: C,
49.7; H, 6.5; N, 4.0.
2-Bromo-1-(4-methylphenyl)-1-(p-toluenesulfonamido)ethane
(1d). Colorless solid (145 mg, 86%); mp. 142–143 ◦C; IR (KBr,
n
max/cm-1 3242, 1321, 1167; 1H NMR (400 MHz, CDCl3) d 2.30 (s,
3H), 2.41 (s, 3H), 3.54-3.63 (m, 2H), 4.48-4.53 (m, 1H), 5.09 (br d,
1H, J = 4.3 Hz, D2O exchangeable), 7.00 (d, 2H, J = 8.4 Hz), 7.06
(d, 2H, J = 8.0 Hz), 7.22 (d, 2H, J = 8.0 Hz), 7.63 (d, 2H, J = 8.4
Hz); 13C NMR (100 MHz, CDCl3) d 21.2, 21.7, 36.8, 57.8, 126.4,
127.1, 129.2, 129.3, 134.4, 136.6, 138.0, 143.4; MS (CI, isobutane):
m/z (relative intensity, %) 368 ([M + H]+, 64), 370 ([M + 2 + H]+,
66), 288 ([M - Br]+, 73); HRMS (CI, isobutane): m/z Calc. for
C16H19BrNO2S (M + H) 368.0321, Found 368.0327; Anal. Calc.
for C16H18BrNO2S: C, 52.2; H, 4.9; N, 3.8. Found: C, 51.8; H, 4.6;
N, 3.9.
1-Bromo-2-(p-toluenesulfonamido)octane (minor regioisomer of
5)13. Colorless oil (13 mg, 7%); IR (neat, nmax/cm-1 3440, 1329,
1
1159; H NMR (270 MHz, CDCl3) d 0.85 (t, 3 H, J = 6.9 Hz),
1.14-1.25 (m, 8 H), 1.44-1.52 (m, 2 H), 2.44 (s, 3 H), 3.29-3.43 (m,
3 H), 4.65 (br d, 1H, J = 8.4 Hz, D2O exchangeable), 7.31 (d, 2
H, J = 8.4 Hz), 7.76 (d, 2 H, J = 8.4 Hz); 13C NMR (68 MHz,
CDCl3) d 14.1, 21.6, 22.5, 25.3, 28.7, 31.6, 33.5, 38.4, 53.1, 126.9,
129.6, 137.6, 143.5; MS (CI, isobutane): m/z (relative intensity,
%) 362 ([M + H]+, 98), 364 ([M + 2 + H]+, 100), 282 ([M - Br]+,
45); HRMS (CI, isobutane): m/z Calc. for C15H25BrNO2S (M +
H) 362.0791, Found 362.0786.
2-Bromo-1-(4-methoxyphenyl)-1-(p-toluenesulfonamido)ethane
(1e). White solid (173 mg, 90%); mp. 124–126 ◦C; IR (KBr,
1
n
max/cm-1 3273, 1325, 1252, 1165; H NMR (400 MHz, CDCl3)
d 2.41 (s, 3H), 3.52-3.61 (m, 2H), 3.77 (s, 3H), 4.50 (td, 1H, J =
6.0, 6.0 Hz), 5.12 (br d, 1H, J = 4.1 Hz, D2O exchangeable), 6.76
(d, 2H, J = 8.8 Hz), 7.03 (d, 2H, J = 8.8 Hz), 7.20 (d, 2H, J =
8.0 Hz), 7.63 (d, 2H, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3)
d 21.6, 36.7, 55.2, 57.8, 113.8, 127.0, 127.7, 129.3, 129.5, 136.7,
143.2, 159.1; MS (FAB, NBA and NaI): m/z (relative intensity,
%) 406 ([M + Na]+, 98), 408 ([M + 2 + Na]+, 100), 326 ([M - Br +
Na]+, 32); HRMS (CI, isobutane): m/z Calc. for C16H19BrNO2S
(M) 383.0191, Found 383.0199; Anal. Calc. for C16H18BrNO3S:
C, 50.00; H, 4.72; N, 3.65. Found: C, 49.91; H, 4.61; N, 3.62.
3-Bromo-4-(p-toluenesulfonamido)hexane (6). Colorless solid
◦
(89 mg, 53%); dec. 114–115 C; IR (KBr, nmax/cm-1 3273, 1325,
1171; 1H NMR (400 MHz, CDCl3) d 0.81 (t, 3H, J = 7.4 Hz), 0.95
(t, 3H, J = 7.2 Hz), 1.41-1.58 (m, 2H), 1.71-1.80 (m, 2H), 2.44
(s, 3H), 3.13-3.19 (m, 1H), 3.81-3.85 (m, 1H), 4.71 (br d, 1H, J =
6.5 Hz, D2O exchangeable), 7.31 (d, 2H, J = 8.2 Hz), 7.76 (d, 2H,
J = 8.2 Hz); 13C NMR (100 MHz, CDCl3) d 10.3, 12.9, 21.7, 23.2,
29.4, 59.0, 65.2, 126.8, 129.5, 137.9, 143.3; MS (CI, isobutane):
m/z (relative intensity, %) 334 ([M + H]+, 96), 336 ([M + 2 + H]+,
100), 254 ([M - Br]+, 33); HRMS (CI, isobutane): m/z Calcd for
C13H21BrNO2S (M + H) 334.0477, found 334.0485; Anal. Calc.
for C13H20BrNO2S: C, 46.7; H, 6.0; N, 4.2. Found: C, 46.4; H, 5.9;
N, 4.2.
( )-trans-1-Bromo-1-phenyl-2-(p-toluenesulfonamido)propane
◦
(3)4a. Colorless oil (84%); mp. 123–124 C; IR (KBr, nmax/cm-1
3274, 1331, 1161; 1H NMR (270 MHz, CDCl3) d 1.10 (d, 3H, J =
5.4 Hz), 2.42 (s, 3H), 3.54-3.62 (m, 1H), 4.99 (br d, 1H, J = 8.9
1428 | Org. Biomol. Chem., 2010, 8, 1424–1430
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3-Bromo-4-(p-toluenesulfonamido)hexane (7). Colorless solid
2-Bromo-3-oxo-1-(p-toluenesulfonamido)butane (12)14. Color-
less oil (39 mg, 41%); IR (neat, nmax/cm-1 3290, 1716, 1329, 1159;
1H NMR (270 MHz, CDCl3) d 2.37 (s, 3H), 2.44 (s, 3 H), 3.29-
3.51 (m, 2H), 4.45 (dd, 1H, J = 5.7, 5.4 H), 5.12 (br t, 1H, J =
6.5 Hz, D2O exchangeable), 7.33 (d, 2H, J = 8.4 Hz), 7.73 (d,
2H, J = 8.4 Hz); 13C NMR (68 MHz, CDCl3) d 21.6, 27.7, 44.6,
48.6, 126.9, 129.8, 136.5, 143.8, 201.1; MS (CI, isobutane): m/z
(relative intensity, %) 320 ([M + H]+, 97), 322 ([M + 2 + H]+,
100), 242 ([M - Br]+, 48); HRMS (CI, isobutane): m/z Calc. for
C11H15BrNO3S (M + H) 319.9956, Found 319.9965; Anal. Calc.
for C11H14BrNO4S: C, 41.3; H, 4.4; N, 4.4. Found: C, 41.2; H, 4.2;
N, 4.3.
◦
(100 mg, 60%); dec. 99–101 C; IR (KBr, nmax/cm-1 3261, 1331,
1163; 1H NMR (400 MHz, CDCl3) d 0.74 (t, 3H, J = 7.6 Hz), 0.97
(t, 3H, J = 7.2 Hz), 1.38-1.49 (m, 1H), 1.53-1.66 (m, 1H), 1.70-
1.87 (m, 2H), 2.43 (s, 3H), 3.24-3.30 (m, 1H), 3.97-4.01 (m, 1H),
4.58 (br d, 1H, J = 6.5 Hz, D2O exchangeable), 7.30 (d, 2H, J =
8.2 Hz), 7.76 (d, 2H, J = 8.2 Hz); 13C NMR (100 MHz, CDCl3)
d 10.6, 12.8, 21.7, 27.9, 29.3, 36.5, 58.9, 62.5, 126.7, 129.5, 138.3,
143.2; MS (CI, isobutane): m/z (relative intensity, %) 334 ([M +
H]+, 99), 336 ([M + 2 + H]+, 100), 254 ([M - Br]+, 35); HRMS
(CI, isobutane): m/z Calc. for C13H21BrNO2S (M + H) 334.0477,
Found 334.0475; Anal. Calc. for C13H20BrNO2S: C, 46.7; H, 6.0;
N, 4.2. Found: C, 46.45; H, 5.7; N, 4.2.
2-Bromo-2-cyano-1-(p-toluenesulfonamido)butane (13)14. Col-
orless crystal (22 mg, 24%); mp. 117–119 ◦C; IR (KBr, nmax/cm-1
( )-trans-2-Bromo-1-(p-toluenesulfonam◦ide)cyclohexane (8)4a.
Colorless solid (85 mg, 51%); mp. 65–66 C; IR (KBr, nmax/cm-1
3317, 2254, 1336, 1155; H NMR (270 MHz, CDCl3) d 2.45 (s,
1
1
3245, 1380, 1188; H NMR (270 MHz, CDCl3) d 1.26 (m, 3H),
3H), 3.41-3.60 (m, 2H), 4.44 (dd, 1H, J = 7.2, 7.2 Hz), 5.50 (br
t, 1H, J = 6.5 Hz, D2O exchangeable), 7.35 (d, 2H, J = 8.4 Hz),
7.77 (d, 2H, J = 8.4 Hz); 13C NMR (68 MHz, CDCl3) d 21.7,
26.5, 47.0, 115.5, 126.9, 130.0, 136.1, 144.4; MS (CI, isobutane):
m/z (relative intensity, %) 303 ([M + H]+, 98), 305 ([M + 2 + H]+,
100), 225 ([M - Br]+, 31); HRMS (CI, isobutane): m/z Calc. for
C10H12BrN2O2S (M + H) 302.9803, Found 302.9796; Anal. Calc.
for C10H11BrN2O2S: C, 39.6; H, 3.7; N, 9.2. Found: C, 39.8; H, 3.5;
N, 9.0.
1.65-1.86 (m, 3H), 2.05-2.27 (m, 2H), 2.43 (s, 3H), 3.11-3.21 (m,
1H), 3.80-3.89 (m, 1H), 4.89 (br d, 1H, J = 5.1 Hz), 7.31 (d, 2H,
J = 8.4 Hz), 7.78 (d, 2H, J = 8.4 Hz); 13C NMR (68 MHz, CDCl3)
d 21.5, 23.5, 25.5, 33.0, 35.9, 55.1, 58.7, 127.2, 129.4, 136.7, 143.3;
MS (CI, isobutane): m/z (relative intensity, %) 332 ([M + H]+,
100), 334 ([M + 2 + H]+, 99), 252 ([M - Br]+, 37); HRMS (EI):
m/z Calc. for C13H18BrNO2S (M) 331.0242, Found 331.0240.
2-Bromo-2-n-butoxy-1-(p-toluenesulfonamido)ethane (9). Col-
orless oil (87 mg, 83%); IR (neat, nmax/cm-1 3277, 1335, 1159; 1H
NMR (270 MHz, CDCl3) d 0.85 (t, J = 7.2 Hz, 3H), 1.18-1.31
(m, 2H), 1.37-1.47 (m, 2H), 2.43 (s, 3H), 3.23-3.53 (m, 4H), 4.73
(ddd, 1 H, J = 9.7, 4.3, 3.5 Hz), 5.22 (br d, 1H, J = 9.5 Hz, D2O
exchangeable), 7.31 (d, 2H, J = 8.4 Hz), 7.77 (d, 2H, J = 8.4);
13C NMR (68 MHz, CDCl3) d 13.9, 19.2, 21.6, 31.2, 35.1, 68.3,
82.2, 126.7, 129.6, 137.9, 143.7; MS (FAB): m/z (relative intensity,
2-Bromo-N,N-dimethyl-3-(p-toluenesulfonamido)propanamide
(14)14. Colorless oil (64 mg, 61%); IR (neat, nmax/cm-1 3228, 1647,
1
1331, 1159; H NMR (270 MHz, CDCl3) d 2.44 (s, 3H), 2.98 (s,
3H), 3.05 (s, 3H), 3.38-3.59 (m, 2H), 4.52 (dd, 1H, J = 9.2, 5.1
Hz), 5.23 (br t, 1H, J = 5.1 Hz, D2O exchangeable), 7.32 (d, 2H,
J = 8.4 Hz), 7.74 (d, 2H, J = 8.4 Hz); 13C NMR (68 MHz, CDCl3)
d 21.6, 36.2, 37.5, 39.6, 45.9, 126.8, 129.7, 136.8, 143.5, 167.1; MS
(CI, isobutane): m/z (relative intensity, %) 349 ([M + H]+, 96), 351
([M + 2 + H]+, 100), 269 ([M - Br]+, 31); HRMS (CI, isobutane):
m/z Calc. for C12H18BrN2O3S (M + H) 349.0222, Found 349.0233.
n
%) 350 ([M + H]+, 4), 352 ([M + 2 + H]+, 4), 276 ([M - BuO]+,
n
61), 278 ([M + 2 - BuO]+, 62); HRMS (FAB): m/z Calc. for
C13H21BrNO2S (M + H) 350.0426, Found 350.0418; Anal. Calc.
for C13H20BrNO3S: C, 44.6; H, 5.8; N, 4.0. Found: C, 44.6; H, 5.6;
N, 3.9.
3-[{1-Bromo-2-(p-toluenesulfonamido)ethyl}carbonyl]-2-oxa-
◦
zolidinone (15). Colorless solid (84 mg, 72%); dec. 132–133 C;
( )-trans-3-Phenyl-3-Bromo-2-(p-toluenesulfonamido)propio-
IR (KBr, nmax/cm-1 3250, 1765, 1701, 1336, 1157; 1H NMR
(270 MHz, CDCl3) d 2.44 (s, 3H), 3.45-3.66 (m, 2H), 3.97-4.12
(m, 2H), 4.47 (t, 2H, J = 8.1 Hz), 5.11 (br t, 1H, J = 6.8 Hz, D2O
exchangeable), 5.41 (dd, 1H, J = 8.6, 5.4 Hz), 7.33 (d, 2H, J = 8.4
Hz), 7.74 (d, 2H, J = 8.4 Hz); 13C NMR (68 MHz, CDCl3) d 21.5,
39.9, 42.6, 45.0, 62.2, 126.8, 129.7, 136.4, 143.7, 152.7, 167.6; MS
(CI, isobutane): m/z (relative intensity, %) 391 ([M + H]+, 41), 393
([M + 2 + H]+, 43), 311 ([M - Br]+, 100); HRMS (CI, isobutane):
m/z Calc. for C13H16BrN2O5S (M + H) 390.9964, Found 390.9968.
Anal. Calc. for C11H14BrNO4S: C, 39.9; H, 3.9; N, 7.2. Found: C,
39.8; H, 3.7; N, 7.1.
◦
phenone (10)4a. Colorless solid (73 mg, 53%); mp. 123–124 C;
IR (KBr, nmax/cm-1 3426, 1681, 1340, 1161; 1H NMR (270 MHz,
CDCl3) d 2.26 (s, 3H), 5.11 (d, 1H, J = 7.0 Hz), 5.48-5.50 (m,
2H), 7.00 (d, 2H, J = 7.8 Hz), 7.25-7.60 (m, 10H), 7.77 (d, 2H,
J = 7.3 Hz); 13C NMR (68 MHz, CDCl3) d 21.5, 51.6, 60.9, 127.0,
128.4, 128.5, 128.6, 128.7, 128.9, 129.3, 134.1, 134.9, 136.2, 136.4,
143.5; MS (CI): m/z (relative intensity, %) 458 ([M + H]+, 57), 460
([M + 2 + H]+, 58), 378 ([M - Br]+, 100); HRMS (CI): m/z Calc.
for C22H21BrNO3S (M + H) 458.0426, Found 458.0419.
Methyl 2-bromo-3-(p-toluenesulfonamido)propanoate (11)14.
Colorless oil (48 mg, 48%); IR (neat, nmax/cm-1 3291, 1741, 1331,
1159; 1H NMR (270 MHz, CDCl3) d 2.44 (s, 3 H), 3.34-3.57 (m, 2
H), 3.78 (s, 3 H), 4.36 (dd, 1 H, J = 8.2, 6.1 Hz), 5.29 (br t, 1H, D2O
exchangeable), 7.33 (d, 2 H, J = 8.4 Hz), 7.74 (d, 2 H, J = 8.4 Hz);
13C NMR (68 MHz, CDCl3) d 21.6, 41.7, 45.8, 53.4, 126.9, 129.8,
136.5, 143.8, 168.9; MS (CI, isobutane): m/z (relative intensity,
%) 336 ([M + H]+, 95), 338 ([M + 2 + H]+, 100), 256 ([M - Br]+,
16); HRMS (CI, isobutane): m/z Calc. for C11H15BrNO4S (M +
H) 335.9905, Found 335.9917; Anal. Calc. for C11H14BrNO4S: C,
39.3; H, 4.2; N, 4.2. Found: C, 39.15; H, 4.0; N, 4.1.
3-[{(1R)-1-Bromo-2-(p-toluenesulfonamido)ethyl}carbonyl]-4-
phenyl-(4R)-2-oxazolidinone (major diastereoisomer of 16).
◦
White solid (63 mg, 45%); dec. 181–183 C; IR (KBr, nmax/cm-1
1
3305, 1767, 1705, 1335, 1159; H NMR (270 MHz, CDCl3) d
2.43 (s, 3H), 3.38-3.56 (m, 2H), 4.28 (dd, 1H, J = 8.9, 4.6 Hz),
4.76 (dd, 1H, J = 8.9, 8.9 Hz), 5.06 (br t, 1H, J = 6.8 Hz, D2O
exchangeable), 5.41 (dd, 1H, J = 8.9, 4.6 Hz), 5.63 (dd, 1H, J =
8.4, 5.9 Hz), 7.29-7.40 (m, 7H), 7.72 (d, 2H, J = 8.4 Hz); 13C NMR
(68 MHz, CDCl3) d 21.5, 40.4, 44.8, 57.6, 70.1, 125.4, 126.7, 128.8,
129.1, 129.7, 136.4, 137.4, 143.7, 152.0, 167.0; MS (CI, isobutane):
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m/z (relative intensity, %) 467 ([M + H]+, 30), 469 ([M + 2 + H]+,
32), 387 ([M - Br]+, 100); HRMS (CI, isobutane): m/z Calc. for
C19H20BrN2O5S (M + H) 467.0277, Found 467.0274; Anal. Calc.
for C19H19BrNO2S: C, 48.8; H, 4.1; N, 6.0. Found: C, 49.0; H, 4.0;
N, 5.9.
Ando and H. Terauchi, Chem. Pharm. Bull., 1967, 15, 1193; (c) F. A.
Daniher and P. E. Butler, J. Org. Chem., 1968, 33, 4336; (d) H. Terauchi,
S. Takemura and Y. Ueno, Chem. Pharm. Bull., 1975, 23, 640; (e) T.
Tsuritani, H. Shinokubo and K. Oshima, J. Org. Chem., 2003, 68, 3246;
(f) X.-L. Wu and G.-W. Wang, J. Org. Chem., 2007, 72, 9398.
3 (a) M. S. Kharasch and H. M. Priestley, J. Am. Chem. Soc., 1939, 61,
3425; (b) A. Zwierzak and K. Osowska-Pacewicka, Angew. Chem., Int.
Ed. Engl., 1976, 15, 302; (c) S. Zawadzki and A. Zwierzak, Tetrahedron,
1981, 37, 2675; (d) K. Osowska-Pacewocka and A. Zwierzak, Tetra-
hedron, 1985, 41, 4717; (e) A. Klepacz and A. Zwierzak, Tetrahedron
3-[{(1S)-1-Bromo-2-(p-toluenesulfonamido)ethyl}carbonyl]-4-
phenyl-(4R)-2-oxazolidinone (minor diastereoisomer of 16). Col-
orless oil (7 mg, 5%); IR (neat, nmax/cm-1 3305, 1767, 1707, 1334,
1159; 1H NMR (270 MHz, CDCl3) d 2.43 (s, 3 H), 3.38-3.56 (m,
2 H), 4.25 (dd, 1 H, J = 8.6, 3.5 Hz), 4.76 (dd, 1H, J = 8.6, 8.6
Hz), 4.76 (br, 1H, D2O exchangeable), 5.39 (dd, 1H, J = 8.6, 3.5
Hz), 5.62 (dd, 1H, J = 8.0, 5.8 Hz), 7.26-7.44 (m, 7H), 7.67 (d,
2H, J = 8.1); 13C NMR (68 MHz, CDCl3) d 21.5, 40.1, 45.0, 58.0,
70.1, 125.5, 126.7, 128.9, 129.2, 129.7, 136.4, 137.8, 143.7, 152.0,
167.4; MS (CI, isobutane): m/z (relative intensity, %) 467 ([M +
H]+, 21), 469 ([M + 2 + H]+, 23), 387 ([M - Br]+, 100); HRMS
(CI, isobutane): m/z Calc. for C19H20BrN2O5S (M + H) 467.0277,
Found 467.0279.
´
Lett., 2001, 42, 4539; (f) A. Sliwin´ska and A. Zwierzak, Tetrahedron,
´
2003, 59, 5927; (g) A. Sliwin´ska and A. Zwierzak, Tetrahedron Lett.,
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Lee and J. Yoon, J. Org. Chem., 2003, 68, 9140; (i) Y.-Y. Yeung, X. Gao
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5 J.-J. Xia, X.-L. Wu and G.-W. Wang, ARKIVOC, 2009, (xvi),
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6 S. Minakata, Acc. Chem. Res., 2009, 42, 1172.
7 S. Minakata, Y. Yoneda, Y. Oderaotoshi and M. Komatsu, Org. Lett.,
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8 C. G. R. Nair and P. Indrasenan, Talanta, 1976, 23, 239.
9 D. Chen, C. Timmons, S. Chao and G. Li, Eur. J. Org. Chem., 2004,
3097.
(2S)-Methyl N-p-toluenesulfonylaziridine-2-carboxylate (17)11.
1
Brown oil; H NMR (270 MHz, CDCl3) d 2.46 (s, 3 H), 2.56
(d, 1H, J = 4.1 Hz), 2.77 (d, 1H, J = 4.1 Hz), 3.38-3.59 (dd, 2H,
J = 7.0, 4.1 Hz), 3.74 (s, 3H), 7.34-7.37 (d, 2H), 7.84 (d, 2 H, J =
8.4 Hz); 13C NMR (68 MHz, CDCl3) d 21.9, 32.2, 35.8, 53.1, 128.2,
128.9, 133.9, 145.3, 167.3; [a]2D3 -20.4 (c 0.764, in CHCl3); HPLC
(Daicel Chiralcel IA, hexane–2-propanol, 95 : 5, 0.5 mL min-1, 254
nm), tS = 35.5 min (for racemic 17, tR = 34.8 min, tS = 35.7 min.)
10 See the ESI†.
11 L. Marzorati, G. C. Barazzone, M. A. B. Filho, B. Wladislaw and C. D.
Vitta, Tetrahedron Lett., 2007, 48, 6509.
12 (a) K. Sato and A. P. Kozikowski, Tetrahedron Lett., 1989, 30, 4073;
(b) J. E. Baldwin, R. M. Adlington, I. A. O’Neil, C. Schofield, A. C.
Spivey and J. B. Sweeney, J. Chem. Soc., Chem. Commun., 1989, 1852;
(c) J. E. Baldwin, A. C. Spivey, C. J. Schofield and J. B. Sweeney,
Tetrahedron, 1993, 49, 6.
Notes and references
1 J. E. G. Kemp in Comprehensive Organic Synthesis, ed. B. M. Trost and
I. Fleming, Pergamon, Oxford, 1991, Vol. 7, pp. 469–513.
2 (a) For well documented reviews, see: G. Li, S. R. S. S. Kotti and C.
Timmons, Eur. J. Org. Chem., 2007, 2745; (b) Y. Ueno, S. Takemura, Y.
13 S. Minakata, Y. Okada, Y. Oderaotoshi and M. Komatsu, Org. Lett.,
2005, 7, 3509.
14 A. M. M. Antunes, V. D. B. Bonifacio, S. C. C. Nascimento, A. M.
Lobo, P. S. Branco and S. Prabhakar, Tetrahedron, 2007, 63, 7009.
1430 | Org. Biomol. Chem., 2010, 8, 1424–1430
This journal is
The Royal Society of Chemistry 2010
©