Copper-mediated N-arylation of electron-deficient pyrroles and indoles

Article abstract of DOI:10.1139/V06-187

We have found that it is possible to N-arylate electron-deficient pyrroles and indoles having no carbonyl group a to the nitrogen ring in good to excellent yields under the Chan and Lam conditions using N-Ethyldiisopropylamine (DIEA) base as a substitute for triethylamine (TEA) and pyridine.

Full text of DOI:10.1139/V06-187

42
Copper-mediated N-arylation of electron-deficient
pyrroles and indoles
Henri Bekolo1
Abstract: We have found that it is possible to N-arylate electron-deficient pyrroles and indoles having no carbonyl
group α to the nitrogen ring in good to excellent yields under the Chan and Lam conditions using N-
Ethyldiisopropylamine (DIEA) base as a substitute for triethylamine (TEA) and pyridine.
Key words: cupric acetate, arylboronic acid, N-arylation, pyrroles, indoles.
Résumé : Nous avons trouvé qu’il est possible de faire la N-arylation des pyrroles et des indoles pauvres en électrons
ne possèdant pas de groupement carbonyle en position α de l’atome d’azote du noyau pyrrolique, avec de bons rende-
ments, dans les conditions de Chan et Lam en remplaçant la triéthylamine (TEA) et la pyridine par la N-éthyldiisopro-
pylamine (DIEA).
Mots-clés : acétate cuprique, acide arylboronique, N-arylation, pyrroles, indoles.
Bekolo 46
Introduction
Considering the successful N-arylation of amides, imides,
sulfonamides, and ureas (1), Mederski et al. (5) hypothe-
sized that the presence of a carbonyl group at the C-2 posi-
tion of pyrrole and indole derivatives activates the
heterocyclic ring nitrogen, thereby contributing to the posi-
tive outcome of the N-arylation process. They demonstrated
the concept by N-arylation of the electron-deficient ethyl
pyrrole-2-carboxylate in 50% yield and of a series of 2-
carboxyindoles in 21% to 50% yields.
Under similar conditions, excellent N-arylation yields of
electron-deficient pyrroles were reported by Srirangam and
co-workers (6). However, these remarkable yields were re-
stricted to pyrroles having a carbonyl group at either the 2 or
the 5 position. Ethyl pyrrole-3-glyoxalate, the only reported
example of stoichiometric copper-mediated N-arylation of
an electron-deficient pyrrole lacking a carbonyl group α to
the ring nitrogen, exhibited very poor reactivity, yielding
traces of expected products in three runs out of four and
with only 25% of the expected product. According to the au-
thors, the excellent yields with α-deactivated pyrroles “might
in part be attributed to the chelation of carbonyl oxygen to
the pyrrole nitrogen-Cu complex”.
The C–N bond forming reaction using stoichiometric (1–
8) or catalytic (9–14) amounts of copper salt, a boronic acid,
a nucleophile, and a base is a fast growing methodology that
was pioneered by Chan and co-workers in its stoichiometric
version (1, 2). Although various substrates have been effi-
ciently N-arylated (15), the reaction outcome is somewhat
unpredictable (8). To account for the successful N-arylation
of electron-deficient pyrroles and indoles, it has been hy-
pothesized that the presence of a carbonyl group α to the
pyrrole ring nitrogen promotes the reaction and is, in part, a
key to a successful reaction (5,6). Herein, we report good to
excellent N-arylation yields of some commercially available
electron-deficient pyrroles (Table 1, 1 and 2) and indoles
(Table 2, 5 and 6) having no carbonyl group α to the pyrrole
ring nitrogen.
Chan and Lam conditions for N-arylation of NH-
containing heterocycles include a nucleophile, stoichiometric
anhyd. Cu(OAc)2, a boronic acid, and pyridine or Et3N as a
base in dichloromethane (DCM) at RT. Examples in the lit-
erature include the N-arylation of azoles (2), purines (3, 4),
2-pyridones and 3-pyridazinones (5). Closely related to this
work is the N-arylation of indoles and pyrroles. Pyrrole it-
self has been N-arylated using catalytic copper salt in 89%
yield (9), whereas, under stoichiometric Cu(OAc)2 condi-
tions, Lam et al. (2) and Srirangam and co-workers (6) re-
ported less than 3% and 0% yields, respectively. In Lam et
al. (2), indole itself was also reported to have been N-
arylated in less than 3% yield.
Although reasonable, the above hypothesis suggests the
necessity of having a carbonyl group α to the pyrrole ring ni-
trogen to achieve synthetically useful yields of stoichiometric
copper-mediated N-arylation of electron-deficient pyrroles
and indoles. This prompted us to investigate the reactivity of
electron-deficient pyrroles 1 and 2, similar to ethyl pyrrole-
3-glyoxalate, and indoles 5 and 6.
Results and discussion
Received 16 August 2006. Accepted 7 December 2007.
Published on the NRC Research Press Web site at
http://canjchem.nrc.ca on 26 January 2007.
To our knowledge, DIEA has not been reported in experi-
mental screenings as an optimal base in the Cu-mediated
arylation of heteroatoms with arylboronic acids. DIEA was
tested in the synthesis of the N-arylpyrrole 1a (Table 1),
which we isolated in 70% yield. TEA and pyridine did not
H. Bekolo. Laboratoire d’Ingénierie Moléculaire et Biochimie
Pharmacologique – 1, Boulevard Arago, 57078 Metz, France.
1Corresponding author (e-mail: hbekolo@hotmail.com).
Can. J. Chem. 85: 42–46 (2007)
doi:10.1139/V06-187
© 2007 NRC Canada

Bekolo
43
Table 1. Cu(OAc)2-mediated N-arylation of 1 and 2 with ArB(OH)2.
Time
(days)
Product
Recovered
Product
Time
(days)
Product
yield (%)a
pyrrole 1 (%)a
Productb
yield (%)a
3
3
3
5
5
1a R = H
70
25c
0d
70
56
50
47
12
45
69
0
18
23
38
2a R = H
70
9
1b R = o-Me
1c R = p-t-Bu
1d R = p-OEt
1e R = p-Cl
2b R = o-Me
2c R = p-t-Bu
2d R = p-OEt
2e R = p-Cl
70
60e
60
43
7
14
14
6
14
5
aIsolated yields.
bRun on the 0.5 mmol scale in DMF. For 2b, DCM (2 mL) was used; for 2e, DCM was used.
cEt3N was used instead of DIEA.
dPyridine was used instead of DIEA.
eThe reaction is cleaner in DCM.
efficiently promote the formation of 1a. With this result, we
decided to test the nonnucleophilic base DIEA with a range
of N-arylpyrroles (Table 1). The expected products were iso-
lated, along with the remaining starting pyrroles, phenolic
by-products (up to 19%) that were already observed by Lam
et al. (2), and symmetrical biaryls (up to 16%) that were also
reported as side products by Lam et al. (14). These biaryls,
characterized as dimers from arylboronic acids, most likely
arose from the well-established Cu(OAc)2-mediated oxida-
tive dimerization of arylboronic acids (16).
Trisubstituted pyrrole 3 and tetrasubstituted pyrrole 4
(Fig. 1) were reacted with phenylboronic acid under our
standard reaction conditions and found to be poor coupling
partners. The reaction mixture with pyrrole 3 turned in-
tensely red over time. After a 10-day reaction, 20% of the
starting pyrrole 3 was recovered and 7% of the expected
product 3a (Fig. 1) was isolated (17), along with unidenti-
fied reddish by-products. Based on TLC analysis, pyrrole 4
did not react at all, even after a longer reaction time of
14 days.
The dramatic difference in reactivity between pyrroles 1
and 3 or 2 and 4 with phenylboronic acid may be accounted
for by the methyl group steric hindrance at the α positions of
3 and 4 as copper-catalyzed coupling reactions are well-
known to be extremely sensitive to steric effects in the nitrogen
nucleophile. Pyrrole itself was reacted with phenylboronic
acid in the dark for three days. The expected N-
phenylpyrrole was isolated in 4% yield (9). We did not have
an electron-rich pyrrole to test.
In this work, Cu(OAc)2 is suspected to oxidize pyrrole an-
ions as dark reddish unidentified by-products formed
throughout the preparation of N-arylpyrroles. This hypothe-
sis is supported by the fact that Cu(OAc)2 is known to oxi-
dize lithium enolates at temperatures as low as –78 °C (18).
To the best of our knowledge, there has been no report on
the N-arylation under the Chan and Lam conditions of
electron-deficient indoles having no carbonyl group at the C-
2 position. Under the Chan and Lam conditions using DIEA,
we have found that 3-acetylindole 5 and 5-nitroindole 6
lacking C-2 carbonyl groups undergo efficient N-arylations
(Table 2).
When reacted with phenylboronic acid in the presence of
DIEA, Et3N, and pyridine, 3-acetylindole 5 exhibited similar
reactivity as 3-acetylpyrrole 1. That is, the use of DIEA as a
base for the preparation of N-arylindole 5a led to the isola-
tion of 5a in high yield (85%) after a two-day reaction
(Table 2), whereas, the use of Et3N for the same reaction re-
sulted in low isolated yield of 5a (35%) even after a longer
reaction time of four days. The use of pyridine resulted in no
formation of the expected product 5a, as with a base-free re-
action. For the most part, the balance of the material was
likely the remaining 3-acetylindole 5, as judged by TLC
analysis. The use of DIEA was then extended to the synthe-
sis of other N-arylindoles (Table 2), and we were pleased to
find that the reactions proceeded uneventfully in high iso-
lated yields. The 3-acetylindole series afforded higher prod-
uct yields. Low yield (45%) of the expected product was
observed for the synthesis of N-arylindole 6d, in the case of
which a thick mixture formed within 2 h under our standard
reaction conditions that had to be diluted with an additional
3 mL of DCM. Extending the reaction time from 6 to
10 days did not improve the yield.
The modest reactivity of 4-chlorophenylboronic acid in
both pyrrole and indole series reflects the known coupling
partners dependency of the Cu–ArB(OH)2 N-arylation reac-
tions (1,8).
The electron-rich 5-methoxyindole was allowed to react
with phenylboronic acid to assess its reactivity under our re-
© 2007 NRC Canada

44
Can. J. Chem. Vol. 85, 2007
Table 2. Cu(OAc)2–ArB(OH)2 N-arylation of indoles 5 and 6.
aIsolated yields.
bDMF was used instead of DCM.
cTEA was used instead of DIEA
dPyridine was used instead of DIEA
eDCM (5 mL) was used for 1 mmol of 6.
fThe starting material 6 was recovered in 30% yield.
action conditions. After reacting for a week, the expected 5-
methoxy-1-phenylindole was obtained in 7% isolated yield.
The starting 5-methoxyindole was recovered in 70% yield.
Under the same reaction conditions, the parent indole gave
N-phenylindole (19) in 16% isolated yield, which is note-
worthy. Indole was recovered in 76%.
This efficient room temperature N-arylation of 3-
acetylindole and 5-nitroindole complements the N-arylation
methodologies for existing indoles that rely on the use of
aryl halides as coupling partners at high temperature (19–34)
and those that rely on the use of aryl bismuths (35) or
arylboronic acids (5) as coupling partners at room tempera-
ture.
Conclusion
In conclusion, the reactivity of pyrroles 1 and 2, indoles 5
and 6, demonstrates that it is possible to achieve syntheti-
cally useful N-arylation yields of electron-deficient pyrroles
and indoles having no carbonyl group α to the pyrrole ring
nitrogen, under the Chan and Lam conditions using DIEA as
a base.
© 2007 NRC Canada

Bekolo
45
Fig. 1. Pyrroles 3, 3a, and 4.
Pyrrole 1c
1
FC: DCM 100%. H NMR δ: 7.63 (m, 1H), 7.45 (d, 2H,
J = 8.55 Hz), 7.30 (d, 2H, J = 9.15 Hz), 7.00 (m, 1H), 6.74
(m, 1H), 2.44 (s, 3H), 1.37 (s, 9H). 13C NMR δ: 193.4,
150.1, 137.1, 127.3, 126.5, 126.0, 124.1, 121.2, 120.6,
114.8, 110.4, 34.4, 31.2, 27.1.
Pyrrole 2c
1
FC: DCM 100%. H NMR δ: 7.49 (d, 2H, J = 8.55 Hz),
7.24 (d, 2H, J = 8.52 Hz), 6.78 (d, 1H, J = 3.05 Hz), 6.67 (d,
1H, J = 3.05 Hz), 2.77 (t, 2H, J = 6.10 Hz), 2.51 (t, J =
6.4 Hz), 2.17–2.00 (m, 2H), 1.36 (s, 9H). 13C NMR δ: 194.5,
150.8, 143.4, 135.9, 126.8, 124.7, 123.8, 123.1, 106.1, 37.5,
34.6, 31.5, 23.9, 22.5.
Experimental
Chemicals were purchased from commercial sources and
used as received. The H and 13C NMR spectra were re-
1
corded on an AC Brucker 250 MHz spectrometer using
CDCl3 as solvent. The chemical shifts (δ) are reported in
ppm. Melting points were determined on a Stuart Scientific
SMP 3 capillary melting point apparatus and are uncor-
rected.
Pyrrole 1d
1
FC: DCM 100%. H NMR δ: 7.55 (m, 1H), 7.25 (m, 2H),
6.90 (m, 3H), 6.70 (m, 1H), 3.97 (q, 2H, J = 4.87 Hz), 2.41
(s, 3H), 1.40 (t, 3H, J = 4.85 Hz). 13C NMR δ: 193.3, 157.8,
132.8, 127.0, 122.4, 115.1, 110.1, 63.7, 27.0, 14.6.
Typical procedure
Pyrrole 2d
In a sealed (glass cap and parafilm) oven-dried 25 mL
flask, a mixture of pyrrole or indole (1.0 mmol), ArB(OH)2
(2.5 mmol), anhyd. Cu(OAc)2 (2.5 mmol), DIEA (99%,
2.5 mmol) in dry DCM (2 mL) was stirred at RT for the in-
dicated time. DCM was removed under reduced pressure.
Water (10 mL) and HCCl3 (10 mL) were added. The aque-
ous layer was extracted with HCCl3 (2 × 10 mL). The
concentrated organic layer was purified by flash chromatog-
raphy (FC) on silica gel.
FC: DCM 100%. 1H NMR δ: 7.20 (d-like, 2H, J =
7.02 Hz), 6.98 (d-like, 2H, J = 6.71 Hz), 6.74 (d, 1H, J =
3.05 Hz), 6.65 (d, 1H, J = 3.05 Hz), 4.06 (q, 2H, J =
6.72 Hz), 2.71 (t, 2H, J = 6.10 Hz), 2.50 (t, 2H, J =
5.50 Hz), 2.20–2.05 (m, 2H), 1.45 (t, 3H, J = 6.72 Hz). 13C
NMR δ: 194.6, 158.4, 143.7, 131.4, 126.1, 121.3, 115.0,
105.9, 63.8, 37.7, 23.6, 22.8, 14.8.
Pyrrole 1e
Please see ref. 19 for analytical data on 6a (FC: DCM–
cyclohexane, 3:7).
FC: DCM 100%. 1H NMR δ: 7.61 (m, 1H), 7.50–7.25 (m,
4H), 6.99 (m, 1H), 6.75 (m, 1H), 2.44 (s, 3H). 13C NMR δ:
193.4, 138.2, 132.5, 129.8, 127.8, 123.8, 122.1, 121.1,
110.9, 27.1.
Pyrrole 1a
1
FC: DCM–cyclohexane 1:1. Pale yellow oil. H NMR δ:
7.66 (t-like, 1H, J = 1.84 Hz), 7.55–7.20 (m, 5H), 7.04 (m,
1H), 6.81–6.71 (m, 1H), 2.46 (s, 3H). 13C NMR δ: 193.2,
139.4, 130.0, 129.1, 127.4, 120.7, 6.9.
Pyrrole 2e
1
FC: DCM 100%. H NMR δ: 7.55–7.40 (m, 2H), 7.32–
7.20 (m, 2H), 6.77 (d, 1H, J = 3.05 Hz), 6.70 (d, 1H, J =
3.65 Hz), 2.75 (t, 2H, J = 6.10 Hz), 2.52 (t, 2H, J =
6.40 Hz), 2.20–2.05 (m, 2H). 13C NMR δ: 194.4, 143.0,
137.0, 133.5, 129.5, 125.0, 122.8, 121.9, 106.6, 37.6, 23.8,
22.8.
Pyrrole 2a
FC: DCM 100%. Beige solid, mp 95 to 96 °C. 1H NMR δ:
7.60–7.2 (m, 5H), 6.80 (d, 1H, J = 3.05 Hz), 6.69 (d, 1H, J =
3.05 Hz), 2.77 (t, 2H, J = 6.10 Hz), 2.51 (t, 2H, J =
6.40 Hz), 2.20–2.00 (m, 2H). 13C NMR δ: 194.5, 143.3,
138.5, 129.3, 127.6, 124.6, 124.7, 123.0, 106.2, 37.6, 23.8,
22.9.
Indole 5a
1
FC: DCM 100%. Beige solid, mp 145 °C. H NMR δ:
8.52–8.45 (m, 1H), 7.93 (s, 1H), 7.65–7.42 (m, 6H), 7.40–
7.20 (m, 2H), 2.56 (s, 3H). 13C NMR δ: 193.2, 138.2, 136.8,
134.6, 129.8, 129.7, 127.9, 126.4, 124.8, 124.7, 123.8,
122.9, 122.6, 118.4, 110.7, 27.5.
Pyrrole 1b
1
FC: DCM 100%. H NMR δ: 7.40–7.10 (m, 5H), 6.73 (s-
like, 2H), 2.43 (s, 3H), 2.19 (s, 3H). 13C NMR δ: 193.4,
139.1, 131.1, 128.5, 128.4, 126.6, 126.5, 126.2, 123.7,
113.4, 109.2, 27.0, 17.5.
Indole 5b
1
FC: DCM 100%. Yellow solid, mp 117 to 118 °C. H
NMR δ: 8.50–8.40 (m, 1H), 7.81 (s, 1H), 7.55–7.15 (m, 6H),
7.02 (d, 1H, J = 7.30 Hz), 2.56 (s, 3H), 2.08 (s, 3H). 13C
NMR δ: 193.2, 137.8, 136.7, 135.5, 135.1, 131.3, 129.1,
127.8, 127.0, 125.6, 123.6, 122.7, 122.4, 110.7, 27.5, 17.3.
Pyrrole 2b
1
FC: DCM 100%. H NMR δ: 7.40–7.10 (m, 4H), 6.70 (d,
1H, J = 3.05 Hz), 6.62 (d, 1H, J = 3.05 Hz), 2.70–2.30 (m,
4H), 2.20–2.00 (m, 5H). 13C NMR δ: 194.4, 144.2, 137.3,
135.2, 130.9, 128.9, 127.3, 126.7, 120.7, 105.6, 37.7, 23.7,
21.9, 17.1.
Indole 6b
1
FC: cyclohexane–DCM 6:4. H NMR δ: 8.65 (d, 1H, J =
1.83 Hz), 8.10–8.00 (m, 1H), 7.50–7.20 (m, 5H), 7.04 (d,
© 2007 NRC Canada

46
Can. J. Chem. Vol. 85, 2007
5. W.W.K.R. Mederski, M. Lefort, M. Germann, and D. Kux.
Tetrahedron, 55, 12757 (1999).
6. S. Yu, J. Saenz, and J.K. Srirangam. J. Org. Chem. 67, 1699
(2002).
1H, J = 9.15 Hz), 6.85 (d, 1H, J = 3.05 Hz), 2.04 (s, 3H).
13C NMR δ: 141.8, 139.5, 136.8, 135.4, 131.8, 131.3, 129.0,
127.7, 127.0, 118.0, 117.6, 110.3, 104.7, 17.3.
7. P.Y.S. Lam, D. Bonne, G. Vincent, C.G. Clark, and A.P.
Indole 5c
Combs. Tetrahedron Lett. 44, 1691 (2003).
8. D.J. Cundy and S.S. Forsyth. Tetrahedron Lett. 39, 7979
(1998).
9. T.D. Quach and R.A. Batey. Org. Lett. 5, 4397 (2003).
10. J.J. Strousse, M. Jeselnik, F. Tapaha, C.B. Jonsson, W.B.
Parker, and J.B. Arterburn. Tetrahedron Lett. 46, 5699 (2005).
11. J.P. Collman and M. Zhong. Org. Lett. 2, 1233 (2000).
12. J.P. Collman, M. Zhong, L. Zeng, and S. Costanzo. J. Org.
Chem. 66, 1528 (2001).
1
FC: DCM–cyclohexane 2:8 then 1:1. H NMR δ: 8.49 (d,
1H, J = 7.25 Hz), 7.93 (s, 1H), 7.65–7.20 (m, 7H), 2.58 (s,
3H), 1.46 (s, 9H). 13C NMR δ: 193.4, 151.3, 137.2, 135.7,
134.9, 126.8, 124.5, 123.8, 123.0, 122.7, 118.4, 115.0,
111.0, 34.8, 27.6, 26.9.
Indole 6c
1
FC: DCM–cyclohexane 2:8. H NMR δ: 8.63 (d, 1H, J =
13. J.P. Collman, M. Zhong, C. Zhang, and S. Costanzo. J. Org.
2.25 Hz), 8.20–8.00 (m, 1H), 7.70–7.30 (m, 6H), 6.83 (d,
1H, J = 3.50 Hz), 1.40 (s, 9H). 13C NMR δ: 131.4, 126.8,
124.2, 118.2, 117.7, 110.6, 105.3, 31.3.
Chem. 66, 7892 (2001).
14. P.Y.S. Lam, G. Vincent, C.G. Clark, S. Deudon, and P.K.
Jadhav. Tetrahedron Lett. 42, 3415 (2001).
15. D.M.T. Chan and P.Y.S. Lam. In Boronic acids in organic syn-
thesis and chemical biology. Edited by D.G. Hall. Wiley-VCH,
Weinheim, Germany. 2005. p. 205.
16. A.S. Demir, O. Reis, and M. Emrullahoglu. J. Org. Chem. 68,
10130 (2003).
Indole 5d
1
FC: DCM–cyclohexane 1:1. H NMR δ: 8.50–8.40 (m,
1H), 7.89 (s, 1H), 7.60–7.20 (m, 7H), 2.60 (s, 3H). 13C
NMR δ: 134.3, 130.1, 126.1, 124.1, 123.2, 122.9, 110.6,
27.7.
17. A.S. Demir, I.M. Akhmedov, and Ö. Sesenoglu. Tetrahedron,
58, 9793 (2002).
18. T. Kawabata, T. Minami, and T. Hiyama. J. Org. Chem. 57,
1864 (1992).
Indole 6d
1
FC: DCM–cyclohexane 2:8. H NMR δ: 8.63 (m, 1H),
19. J.C. Antilla, A. Klapars, and S.L. Buchwald. J. Am. Chem.
8.20–8.05 (m, 1H), 7.60–7.35 (m, 6H), 6.87 (d, 1H, J =
3.00 Hz). 13C NMR δ: 131.0, 130.2, 125.9, 118.3, 118.1,
110.3, 106.0.
Soc. 124, 11684 (2002).
20. J. Lindley. Tetrahedron, 40, 1433 (1984).
21. M.A. Khan and J.B. Polya. J. Chem. Soc. C, 85 (1970).
22. C.A. Harbert, J.J. Plattner, W.M. Welch, A. Weissman, and
B.K. Koe. J. Med. Chem. 23, 635 (1980).
23. H. Lexy and T. Kauffmann. Chem. Ber. 113, 2755 (1980).
24. P.C. Unangst, D.T. Connor, S.R. Stabler, and R.J. Weikert. J.
Heterocycl. Chem. 24, 811 (1987).
5-Methoxy-1-phenylindole
FC: DCM–cyclohexane 3:7. H NMR δ: 7.58–7.43 (m,
5H), 7.40–7.28 (m, 2H), 7.16 (d, 1H, J = 2.5 Hz), 6.94–6.84
(dd, 1H, J = 10 Hz, J = 2.5 Hz), 6.65–6.55 (m, 1H), 3.90 (s,
3H). 13C NMR δ: 154.6, 140.0, 131.1, 129.8, 129.7, 128.4,
126.3, 124.1, 123.3, 118.9, 112.5, 111.4, 103.3, 102.7, 55.9.
1
25. Y. Kato, M.M. Conn, and J. Rebek, Jr. J. Am. Chem. Soc. 116,
3279 (1994).
26. Y. Murakami, T. Watanabe, T. Hagiwara, Y. Akiyama, and H.
Ishii. Chem. Pharm. Bull. 43, 1281 (1995).
27. G. Mann, J.F. Hartwig, M.S. Driver, and C. Fernandez-Rivas.
J. Chem. Soc. 120, 827 (1998).
28. J.F. Hartwig, M. Kawatsura, S.I. Hauck, K.H. Shaughnessy,
and L.M. Alcazar-Roman. J. Org. Chem. 64, 5575 (1999).
29. D.W. Old, M.C. Harris, and S.L. Buchwald. Org. Lett. 2, 1403
(2000).
Acknowledgments
HB thanks Professor G. Kirsch for providing space and
chemicals. The financial support from Le Ministère de la
Recherche-France for this work is acknowledged.
30. M. Watanabe, M. Nishiyama, T. Yamamoto, and Y. Koie. Tet-
References
rahedron Lett. 41, 481 (2000).
1. D.M.T. Chan, K.L. Monaco, R.P. Wan, and M.P. Winters. Tet-
rahedron Lett. 39, 2933 (1998).
31. G.A. Grasa, M.S. Viciu, J. Huang, and S.P. Nolan. J. Org.
Chem. 66, 7729 (2001).
2. P.Y.S. Lam, C.G. Clark, S. Saubern, J. Adams, M.P. Winters,
D.M.T. Chan, and A. Combs. Tetrahedron Lett. 39, 2941
(1998).
32. W.J. Smith, III and J.S. Sawyer. Tetrahedron Lett. 37, 299
(1996).
33. S. Maiorana, C. Baldoli, P. Del Buttero, M. DI Ciolo, and A.
3. A.K. Bakkestuen and L-L. Gundersen. Tetrahedron Lett. 44,
3359 (2003).
4. Q. Dai, C. Ran, and R.G. Harvey. Tetrahedron, 62, 1764
(2006).
Papagni. Synthesis, 735 (1998).
34. W.J. Smith, III and J.S. Sawyer. Heterocycles, 51, 157 (1999).
35. D.H.R. Bartona, J.-P. Fineta, and J. Khamsia. Tetrahedron Lett.
29, 1115 (1988).
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