New N-halosuccinimide-mediated reactions for the synthesis of pyridines

Article abstract of DOI:10.1055/s-2004-820019

5-Bromo-2,6-dialkylpyridine-4-carboxylates are generated in excellent yield by the Michael addition of enaminoesters and ethynyl ketones followed by bromocyclization using N-bromosuccinimide within 1 hour at 0°C. Treatment of the same aminopentadienone intermediates with N-iodosuccinimide facilitates a low temperature cyclodehydration under very mild conditions to give 2,3,6-trisubstituted pyridines with total regiocontrol.

Full text of DOI:10.1055/s-2004-820019

LETTER
811
New N-Halosuccinimide-Mediated Reactions for the Synthesis of Pyridines
N
M
ew
N
-Halosuccinimi
a
de-Mediate
r
R
eacti
k
C
is of
P
yridines . Bagley,* Christian Glover, Eleanor A. Merritt, Xin Xiong
School of Chemistry, Cardiff University, PO Box 912, Cardiff, CF10 3TB, UK
Fax +44(29)20874030; E-mail: Bagleymc@cf.ac.uk
Received 20 October 2003
introduce diversity in the target heterocycle accessible
from these precursors, we set out to investigate the bro-
mocyclization of aminodienone intermediates 3 generated
in the traditional Bohlmann–Rahtz reaction.8
Abstract: 5-Bromo-2,6-dialkylpyridine-4-carboxylates are gener-
ated in excellent yield by the Michael addition of enaminoesters and
ethynyl ketones followed by bromocyclization using N-bromo-
succinimide within 1 hour at 0 °C. Treatment of the same amino-
pentadienone intermediates with N-iodosuccinimide facilitates a
low temperature cyclodehydration under very mild conditions to
give 2,3,6-trisubstituted pyridines with total regiocontrol.
Dechoux reported recently that when d-dienaminoesters 6
were treated with N-bromosuccinimide (NBS) under neu-
tral conditions, 1,2,3,5-tetrasubstituted pyrroles 7 were
Key words: pyridines, b-enaminoesters, bromocyclization, Bohl-
mann–Rahtz
9
formed, whereas under basic conditions treatment with
N-halosuccinimides generated the corresponding 3-halo-
1
0
2
-1H-pyridones 8 in reasonable yield (Scheme 2). Based
upon this precedent, the NBS-mediated bromocyclization
of Bohlmann–Rahtz intermediates, generated by a
Michael addition, under neutral conditions was expected
to provide a two-step route to pyrrole heterocycles 5
The development of new routes to polysubstituted py-
ridines, with diversity in the nature of substituents, contin-
ues to attract considerable attention for application in
heterocyclic chemistry, natural product synthesis and me-
dicinal chemistry. We have developed new heteroannula-
tion methods for the synthesis of 2,3,6-trisubstituted and
(
Scheme 1).
CO Me
2
O
2
,3,4,6-tetrasubstuted pyridines 4 using either the acid
1
–3
catalyzed Bohlmann–Rahtz reaction
irradiation of enaminoesters 1 and alkynones 2
or microwave
R3
1
4
R HN
R2
(
Scheme 1) and applied these reactions in the synthesis of
fused heterocycles and multiple-component reactions.7
5
,6
6
NXS/MeOH
MeONa
NBS/CH2Cl2
(5–95%)
In order to expand the versatility of these processes, and
(37–83%)
O
O
R3
R2
X
R3
R3
O
+
R4
CO Me
2
R2
N
R
N
H
O
R6
R2
NH2
1
7
8
1
2
9
10
EtOH
Scheme 2 Dechoux’s synthesis of pyrroles 7 and pyridinones 8.
O
In order to facilitate the initial Michael addition, enamines
R6
3
1
a–d were reacted with alkynones 2a–f in ethanol at 50
Bohlmann-Rahtz
intermediate 3
R3
8
R4
NH2
°C to give the aminodienones 3a–g after purification on
silica (Scheme 3). The course of reaction was followed by
TLC analysis, in order to minimize cyclodehydration of
dienones 3 to pyridines 4. The optimum conditions11 in-
volved stirring the enamine 1 and alkynone 2 in EtOH at
R2
ref 1–8
NBS
R4
R3
R4
5
0 °C for 1–18 hours to give the Michael addition product
R3
R2
R6
3
a–g, for the most part, in good yield (Table 1).
R2
N
Ethynyl ketones, such as 2a–d, were usually successful in
this reaction (Scheme 3, Table 1, entries 1–4), but trans-
formations involving hexynone 2e, phenylbutynone 2f or
aminocrotonitrile 1d failed to give appreciable yields of
the product, principally due to spontaneous cyclodehydra-
tion under the reaction conditions to give the correspond-
ing pyridine 4.
N
R6
H
O
4
5
Scheme 1 Proposed route for pyrrole 5 synthesis.
SYNLETT 2004, No. 5, pp 0811–0814
Advanced online publication: 24.02.2004
0
2.
0
4.
2
0
0
4
DOI: 10.1055/s-2004-820019; Art ID: D25803ST.pdf
©
Georg Thieme Verlag Stuttgart · New York

8
12
M. C. Bagley et al.
LETTER
9
R3
R2
former (Scheme 4). Facile deprotonation, rather than
intramolecular nucleophilic substitution, prevents the
formation of pyrrole 4a and gives instead the (4E)-hexa-
dienone intermediate 10a, thus avoiding the need for dou-
ble bond isomerization. This will undergo spontaneous
cyclodehydration under mild reaction conditions to give
pyridine 11a. We would expect that the cyclization of
bromodienone 10a to pyridine 11a would be much more
facile than the corresponding cyclization of d-dienami-
noesters 6 to give pyridinones 8 and so proceeds under
milder reaction conditions in the absence of sodium
methoxide base.10 This method provides 5-substituted
pyridines possessing latent functionality suitable for
subsequent elaboration, a substitution pattern hitherto un-
available by traditional Bohlmann–Rahtz methodology.
O
EtOH
R4
+
R6
50 °C
0–95%)
NH2
(
2
3
4
6
1
R , R
2 R , R
a Me, CO Et;
a H, Ph; b H, 4-MeOC6H4;
c H, 4-ClC6H4; d SiMe3, Me;
e Et, Me; f Ph, Me.
2
b Ph, CO Et;
2
t
c Me, CO2 Bu;
d Me, CN.
2
3
6
3
R , R , R
O
a Me, CO Et, Ph;
2
R6
b Me, CO Et, 4-MeOC H ;
2
6 4
R3
c Me, CO2Et, 4-ClC6H4;
d Me, CO2Et, Me;
e Ph, CO2Et, Ph;
R2
NH2
f Ph, CO Et, Me;
2
t
g Me, CO Bu, Me.
2
O
O
H
Scheme 3 Synthesis of Bohlmann–Rahtz intermediates 3a–g.
Table 1 Michael Addition of Enamines 1 and Alkynones 2
Br
R6
R6
NBS
RO2C
R2
RO2C
EtOH
or CH2Cl2)
(
NH2
R2
NH2+
Entry
Enamine Alkynone Product
Time (h) Yield (%)
3
a–g
9a–g
1
2
3
4
5
6
7
1a
1a
1a
1a
1a
1b
1b
1c
2a
2b
2c
3a
3b
3c
3d
–
1
85
95
86
82
6
O
6
Br
RO C
Br
R6
R6
2
–
H O
2
E
2d
2e,f
2a
2d
2d
6
R2
N
(83–98%)
RO C
0a
2
6
R2
10a–g
NH2
3e
3f
7
68
23a
68
1
1a–g
3
Scheme 4 Proposed mechanistic course of the bromocyclization.
8
3g
18
In order to explore the scope of this new bromination-
heteroannulation procedure, a range of different amino-
dienones 3a–g was treated with NBS at 0 °C for between
30 and 60 minutes. It was found that reactions conducted
a
Significant cyclodehydration to pyridine 4 accompanied the
reaction.
2
3
The bromocyclization of aminodienone 3a (R = Me, R =
CO Et, R = Ph) with NBS was investigated in CH Cl at
0
prisingly, no pyrrole 5 could be isolated from the reaction
mixture and instead facile bromination-cyclodehydration
occurred to give the bromopyridine 11a (Scheme 4) in
6
(
12
6
in EtOH, as opposed to CH Cl , were much more effi-
2 2
2
2
2
9
cient, reducing formation of the corresponding pyridine 4
°C according to the reported conditions. However, sur-
as a reaction byproduct, to give the 5-bromopyridines
3–19
1
1a–g1
in excellent yield (Scheme 4, Table 2).
Table 2 Bromocyclization of Aminodienones 3a–g with NBS
3% yield along with a trace of trisubstituted pyridine 4a
R = H) that could be separated easily from the main Entry Diene Product R2
4
R
R6
Ph
Yield (%)
product by washing with dilute acid as a consequence of
the reduced basicity of the 5-bromopyridine.
1
2
3
4
5
6
3a
3b
3c
3d
3e
3f
11a13
Me
Me
Me
Me
Ph
Et
Et
Et
Et
Et
Et
t-Bu
92
88
89
11b14
4-MeOC H
Considering the difficulty experienced in previous studies
to facilitate the acid-catalyzed cyclodehydration of Bohl-
mann–Rahtz intermediates, which proceeds under tra-
ditional uncatalyzed conditions often at temperatures in
excess of 120 °C (and for certain substrates requires tem-
peratures in the region of 200 °C), this finding seemed
startling since the reaction was complete after only 15
minutes at 0 °C. The course of the reaction was rational-
ized by considering initial regioselective addition to give
s-cis bromide 9a, in equilibrium with the s-trans con-
6
4
11c15
4-ClC H
6
4
1
–3
11d16
Me
Ph
96
11e17
83a
5
,8
11f18
11g19
Ph
Me
Me
>98
97
7
3g
Me
a
Reaction was run at –10 °C to prevent formation of pyridine 4e.
Synlett 2004, No. 5, 811–814 © Thieme Stuttgart · New York

LETTER
New N-Halosuccinimide-Mediated Reactions for the Synthesis of Pyridines
813
With the scope and versatility of the method established, synthesis of 2,3,6-trisubstituted pyridines 4 in excellent
it was decided to investigate other N-halosuccinimides in yield and with total regiocontrol.
the halogenation/heteroannulation process. The use of N-
In summary, N-halosuccinimide-mediated processes pro-
chlorosuccinimide in EtOH at 0 °C led only to a complex
vide complementary and efficient routes to either 2,3,5,6-
mixture of products. However, when aminodienones
tetrasubstituted or 2,3,6-trisubstituted pyridines under
3
a–d, 3f or 3g were treated with N-iodosuccinimide (NIS)
mild conditions. The reaction of Bohlmann–Rahtz inter-
mediates, generated by Michael addition of an enami-
noester and ethynyl ketone, with N-bromosuccinimide
provides facile access to 5-bromopyridines, whereas treat-
ment of the same aminodienone intermediates with N-io-
dosuccinimide facilitates the rapid cyclodehydration of
these substrates at a, comparatively, low temperature.
in EtOH at 0 °C none of the corresponding 5-iodo-
pyridines were produced and instead an extremely facile
2
0
cyclodehydration of the normally stable dienes 3 oc-
3
curred to give the 2,3,6-trisubstituted pyridines 4
(
2
1
Scheme 5, Table 3). Furthermore, the use of a catalytic
quantity of NIS (20 mol%) did not adversely affect the
course or efficiency of reaction, generating pyridine 4a in
quantitative yield (entry 2).
Acknowledgment
O
We thank Pfizer Ltd (Summer Scholarship Award to E. A. Merritt),
the Great Britain-China Educational Trust and Henry Lester Trust
Ltd. (award to X. Xiong) for their generous support and the
E.P.S.R.C. Mass Spectrometry Service, Swansea for high resolu-
tion spectra.
R6
RO2C
R2
NIS, EtOH
°C
(84–98%)
RO2C
0
N
R6
R2
NH2
3a–d,f,g
4a–d,f,g
References
Scheme 5 NIS-mediated cyclodehydration of aminodienones 3.
Table 3 Cyclodehydration of Aminodienones 3 with NIS
(
(
1) Bagley, M. C.; Dale, J. W.; Bower, J. Synlett 2001, 1149.
2) Bagley, M. C.; Dale, J. W.; Hughes, D. D.; Ohnesorge, M.;
Phillips, N. G.; Bower, J. Synlett 2001, 1523.
3) Bagley, M. C.; Brace, C.; Dale, J. W.; Ohnesorge, M.;
Phillips, N. G.; Xiong, X.; Bower, J. J. Chem. Soc., Perkin
Trans. 1 2002, 1663.
(
Entry Diene
Product R2
R
R6
Ph
Ph
Yield%
>98
1
3a
3a
3b
3c
3d
3d
3d
3f
4a
4a
4b
4c
4d
4d
4d
4f
Me
Et
Et
Et
Et
Et
Et
Et
Et
tBu
(
(
(
4) Bagley, M. C.; Lunn, R.; Xiong, X. Tetrahedron Lett. 2002,
43, 8331.
2a
Me
Me
Me
Me
Me
Me
Ph
>98
5) Bagley, M. C.; Hughes, D. D.; Lloyd, R.; Powers, V. E. C.
Tetrahedron Lett. 2001, 42, 6585.
6) Bagley, M. C.; Hughes, D. D. Synlett 2002, 1332.
3
4
4-MeOC H >98
6
4
4-ClC H
97
66
(7) Bagley, M. C.; Dale, J. W.; Bower, J. Chem. Commun. 2002,
682.
8) Bohlmann, F.; Rahtz, D. Chem. Ber. 1957, 90, 2265.
(9) (a) Agami, C.; Dechoux, L.; Hebbe, S. Synlett 2001, 1440.
b) Agami, C.; Dechoux, L.; Hamon, L.; Hebbe, S. Synthesis
6
4
1
5
Me
Me
Me
Me
Me
(
6b
71
(
7c
2003, 859.
84
(
(
10) Agami, C.; Dechoux, L.; Hebbe, S.; Moulinas, J. Synlett
2002, 79.
8
>98
>98
11) General Procedure for Michael Addition of Enamines 1
and Alkynones 2. A solution of enamine 1 (0.36 mmol, 1
equiv) and alkynone (0.56 mmol, 1.5 equiv) in EtOH (5 mL)
was stirred at 50 °C for 1–7 h, cooled and evaporated in
vacuo. Purification by flash chromatography on silica gel,
eluting with EtOAc–light petroleum gave dienone 3.
12) General Procedure for the Bromocyclization of
Aminodienones 3 using NBS. A solution of aminodienone
9b
3g
4g
Me
a
A catalytic (20 mol%) quantity of NIS was used.
Reactions were run in the presence of NaHCO3.
Reaction was run over the course of 4 h rather than 1 h.
b
c
(
(
It was proposed that the Lewis acidity of NIS was respon-
sible for this remarkably facile cyclodehydration, that tra-
ditionally requires temperatures well in excess of 120 °C,
and this was supported by further experimentation. Re-
peating the process in the absence of NIS returned unre-
acted starting material 3 only. When the NIS was purified
by recrystallization prior to use, or employed in the pres-
3
(0.28 mmol, 1 equiv) and N-bromosuccinimide (0.34
mmol, 1.2 equiv) in EtOH (5 mL) was stirred at 0 °C for 1 h
and evaporated in vacuo. Purification by column
chromatography on silica gave bromopyridine 11a–g.
13) Ethyl 5-bromo-2-methyl-6-phenylpyridine-3-carboxylate
(11a). Mp 88–89 °C (aq MeOH). HRMS: m/z [MH] calcd for
7
9
+
C H BrNO : 320.0286; found [MH ]: 320.0286. IR
1
5
14
2
(
1
KBr): 2974, 2925, 1730, 1571, 1432, 1286, 1259, 1090,
016, 928, 835, 780, 694 cm . H NMR (400 MHz, CDCl ):
ence of NaHCO (to remove HI present or generated
3
–
1 1
throughout the course of reaction), this did not adversely
affect the yield of pyridine 4 and in some instances im-
proved the reaction efficiency. The use of NIS in EtOH
represents a new mild method for the low temperature cy-
clodehydration of Bohlmann–Rahtz intermediates for the
3
d = 8.41 (s, 1 H, 4-H), 7.64 (m, 2 H, o-PhH), 7.41 (m, 3 H,
m,p-PhH), 4.34 (q, J = 7.1 Hz, 2 H, CH ), 2.77 (s, 3 H, 2-
2
13
Me), 1.36 (t, J = 7.1 Hz, 3 H, Me). C NMR (100 MHz,
CDCl ) d = 165.6 (C), 160.2 (C), 158.8 (C), 143.8 (CH),
3
139.3 (C), 129.8 (CH), 129.7 (CH), 128.5 (CH), 125.4 (C),
Synlett 2004, No. 5, 811–814 © Thieme Stuttgart · New York

8
14
M. C. Bagley et al.
LETTER
1
16.6 (C), 62.0 (CH ), 25.0 (Me), 14.7 (Me). MS (APcI):
(17) Ethyl 5-bromo-2,6-diphenylpyridine-3-carboxylate (11e).
HRMS: m/z [M] calcd for C H NO Br: 381.0364; found
2
8
1
+
79
+
m/z (%) = 322 (36) {M[ Br]H }, 320 (37) {M[ Br]H },
42 (9), 93 (23), 79 (100).
2
0
16
2
+
2
[M ]: 381.0360. IR (KBr): 2976, 1732, 1558, 1426, 1372,
–
1 1
(
14) Ethyl 5-bromo-2-methyl-6-(4-methoxyphenyl)pyridine-3-
1244, 1114, 1088, 1017, 922, 772, 687 cm . H NMR (400
carboxylate (11b). Mp 90–91 °C (MeOH). HRMS: m/z
MHz, CDCl ) d = 8.33 (s, 1 H, 4-H), 7.71 (m, 2 H, 6-(o-
3
+
[
MH] calcd for C H BrNO : 350.0392; found [MH ]:
PhH)], 7.50 (m. 2 H, 2-(o-PhH)], 7.43–7.33 (m, 6 H, m,p-
1
6
16
3
3
50.0393. IR (KBr): 2963, 1724, 1609, 1570, 1512, 1433,
PhH), 4.12 (q, J = 7.1 Hz, 2 H, CH ), 1.02 (t, J = 7.1 Hz, 3
2
–
1
1
13
1257, 1178, 1087, 1027, 803, 701 cm . H NMR (400 MHz,
H, CH Me). C NMR (100 MHz, CDCl ) d = 166.7 (C),
2
3
CDCl ): d = 8.38 (s, 1 H, 4-H), 7.66 (d, J = 8.8Hz, 2 H, 2¢-H,
159.2 (C), 157.2 (C), 143.0 (CH), 139.2 (C), 138.8 (C),
129.6 (CH), 129.3 (CH), 128.9 (CH), 128.7 (CH), 128.1
3
6
¢-H), 6.91 (d, J = 8.8 Hz, 2 H, 3¢-H, 5¢-H), 4.34 (q, J = 7.1
Hz, 2 H, CH Me), 3.79 (s, 3 H, OMe), 2.75 (s, 3 H, Me), 1.34
(CH), 128.0 (CH), 126.5 (C), 117.3 (C), 61.8 (CH ), 13.7
2
2
1
3
+
(
t, J = 7.1 Hz, 3 H, CH Me). C NMR (100 MHz, CDCl ):
(Me). MS (APcI): m/z (%) = 384 (100) {MH }, 382 (80)
2
3
+
d = 165.7 (C), 160.8 (C), 159.6 (C), 158.7 (C), 143.8 (CH),
[MH ], 304 (15).
131.6 (C), 131.4 (CH), 124.8 (C), 116.2 (C), 113.8 (CH),
(18) Ethyl 5-bromo-6-methyl-2-phenylpyridine-3-carboxylate
+
6
1.9 (CH ), 55.8 (Me), 25.0 (Me), 14.7 (Me). MS (APcI):
(11f). HRMS: m/z [MH ] calcd for C H NO Br: 320.0281;
2
15 14
2
8
1
+
79
+
m/z (%) = 352 (32) {M[ Br]H }, 350 (50) {M[ Br]H }.
found: 320.0281. IR (nujol): 2918, 2853, 1725, 1572, 1462,
–
1 1
(
15) Ethyl 5-bromo-2-methyl-6-(4-chlorophenyl)pyridine-3-
1377, 1295, 1242, 1112, 1061, 1027, 771, 722, 696 cm . H
carboxylate (11c). Mp 106–108 °C (aq EtOH). HRMS: m/z
NMR (400 MHz, CDCl ): d = 8.18 (s, 1 H, 4-H), 7.50–7.34
3
+
[
MH] calcd for C H BrClNO : 353.9896; found [MH ]:
(m, 5 H, PhH), 4.05 (q, J = 7.1 Hz, 2 H, CH ), 2.68 (s, 3 H,
1
5
13
2
2
1
3
3
53.9896. IR (KBr): 2973, 1730, 1595, 1570, 1534, 1492,
Me), 1.00 (t, J = 7.1 Hz, 3 H, CH Me). C NMR (100 MHz,
2
–
1 1
1432, 1259, 1089, 1016, 928, 835, 780 cm . H NMR (400
CDCl ): d = 166.6 (C), 159.6 (C), 157.2 (C), 141.3 (CH),
3
MHz, CDCl ): d = 8.40 (s, 1 H, 4-H), 7.61 (d, J = 8.6 Hz, 2
139.5 (C), 128.7 (CH), 128.5 (CH), 128.2 (CH), 125.8 (C),
3
H, 2¢-H, 6¢-H), 7.37 (d, J = 8.6 Hz, 2 H, 3¢,5¢-H), 4.34 (q, J =
119.3 (C), 61.7 (CH ), 25.3 (Me), 13.7 (Me). MS (APcI):
2
+
7
3
1
1
2
{
{
.1 Hz, 2 H, CH Me), 2.75 (s, 3 H, Me), 1.35 (t, J = 7.1 Hz,
m/z (%) = 322 (100) [MH ], 320 (98).
2
1
3
H, CH Me). C NMR (100 MHz, CDCl ) d = 165.1 (C),
(19) tert-Butyl 5-bromo-2,6-dimethylpyridine-3-carboxylate
2
3
+
58.5 (C), 158.4 (C), 143.5 (CH), 137.2 (C), 135.4 (C),
30.8 (CH), 128.3 (CH), 125.3 (C), 116.0 (C), 61.7 (CH2),
4.5 (Me), 14.3 (Me). MS (APcI): m/z (%) = 358 (16)
(11g). HRMS: m/z [M ] calcd for C H NO Br: 285.0359;
1
2
16
2
found: 285.0359. IR (nujol): 2922, 2852, 1726, 1579, 1462,
–
1 1
1377, 1277, 1167, 1095, 970, 848, 782, 722 cm . H NMR
8
1
37
+
+
M[ Br Cl]H }, 356 (100) [MH ], 358 (71)
(400 MHz, CDCl ): d = 8.11 (s, 1 H, 4-H), 2.81 (s, 3 H, Me),
3
7
9
35
+
13
M[ Br Cl]H }, 117 (38), 71 (30).
2.62 (s, 3 H, Me), 1.55 (s, 9 H, CMe3). C NMR (100 MHz,
(
16) Ethyl 5-bromo-2,6-dimethylpyridine-3-carboxylate (11d).
CDCl ): d = 177.3 (C), 165.1 (C), 159.9 (C), 157.9 (C), 142.0
3
Mp 32.1–32.2 °C. Anal. Calcd for C H BrNO : C, 46.5; H,
(CH), 124.5 (C), 118.1 (C), 24.9 (Me), 24.1 (Me), 14.3 (Me).
1
0
12
2
+
4
.7; N, 5.4. Found: C, 46.5; H, 5.0; N, 5.4. HRMS: m/z [MH]
MS (APcI): m/z (%) = 288 (100) [MH ], 286 (94).
7
9
+
calcd for C H BrNO : 258.0129; found [MH ]:
(20) For example, stirring a solution of aminodienone 3d in EtOH
at 0 °C for 1 h results in the return of unreacted starting
material and no cyclodehydration to pyridine 4d.
10
12
2
2
58.0124. IR (film): 2984, 1725, 1576, 1542, 1436, 1392,
–
1 1
1366, 1267, 1232, 1100, 1025, 970, 780, 680 cm . H NMR
(
400 MHz, CDCl ): d = 8.23 (s, 1 H, 4-H), 4.30 (q, J = 7.1
(21) General Procedure for the Cyclodehydration of
Aminodienones 3 using NIS. A solution of aminodienone 3
(0.2 mmol, 1 equiv) and N-iodosuccinimide (0.25 mmol, 1.2
equiv) in EtOH (4 mL) was stirred at 0 °C for 1 h and eva-
porated in vacuo. Purification by flash chromatography on
silica, eluting with EtOAc–light petroleum, gave pyridine 4.
3
Hz, 2 H, CH ), 2.69 (s, 3 H, 6-Me), 2.60 (s, 3 H, 2-Me), 1.33
(
1
1
2
1
3
t, J = 7.1 Hz, 3 H, Me); C NMR (100 MHz, CDCl ): d =
65.6 (C), 160.3 (C), 158.4 (C), 142.0 (CH), 124.7 (C),
3
18.3 (C), 61.8 (CH ), 25.4 (Me), 24.7 (Me), 14.6 (Me). MS
2
8
1
+
(APcI): m/z (%) = 260 (100) {M[ Br]H }, 258 (95)
7
9
+
{
M[ Br]H }.
Synlett 2004, No. 5, 811–814 © Thieme Stuttgart · New York