Three-component Barbier allylation, Friedel-Crafts alkylation and intramolecular hydroalkoxylation in an ionic liquid for the direct synthesis of 4-arylchromans

Article abstract of DOI:10.1055/s-2007-980353

4-Arylchromans can be synthesized directly using a one-pot Barbier allylation, Friedel-Crafts alkylation and intramolecular hydroalkoxylation of aromatic aldehydes, allylbromides and phenols in an ionic liquid (BPyX-SnCl ₂·2H₂O). The

Full text of DOI:10.1055/s-2007-980353

LETTER
1357
Three-Component Barbier Allylation, Friedel–Crafts Alkylation and Intra-
molecular Hydroalkoxylation in an Ionic Liquid for the Direct Synthesis of
4-Arylchromans
D
irec
t
Synthesis
o
i
f
4-Ary
a
lchromansn-Liang Zhao, Li Liu,* Yong-Jun Chen, Dong Wang*
Beijing National Laboratory for Molecular Sciences (BNLMS), Laboratory for Chemical Biology, Institute of Chemistry, Chinese Acade-
my of Sciences, Beijing 100080, P. R. of China
Fax +86(10)62554449; E-mail: lliu@iccas.ac.cn, dwang210@iccas.ac.cn
Received 26 December 2006
chromans (5) with the aid of an intramolecular cyclization
Abstract: 4-Arylchromans can be synthesized directly using a one-
reaction. The precursor 4 can be synthesized through the
pot Barbier allylation, Friedel–Crafts alkylation and intramolecular
Friedel–Crafts alkylation of phenols with allyl benzyl
hydroalkoxylation of aromatic aldehydes, allylbromides and
alcohol derivatives,7 which in turn can be prepared by
phenols in an ionic liquid (BPyX–SnCl2·2H2O). The intramolecular
hydroalkoxylation of 4-aryl-4-(2-hydroxylphenyl)but-1-enes can Barbier allylation of carbonyl compounds.8
be promoted using the Lewis acid ZnCl2 in an ionic liquid.
Combining sequences of individual transformations into a
one-pot process to reduce synthetic steps and enhance
Key words: Barbier, Friedel–Crafts, hydroalkoxylation, one-pot
process, 4-arylchroman, ionic liquid
synthetic efficiency is challenging in the synthesis of
heterocyclics.9 If the formation of intermediate (4) and
final product (5) can be combined into a one-pot tandem
3,4-Dihydro-2H-benzopyran (chroman) is an important
structural unit in many natural compounds with biological
and pharmaceutical activities.1 Among them, 4-arylchro-
mans have also received considerable attention due to the
interest in their biological activities, including inhibition
of prostaglandin synthesis,2 anti-estrogenic and anti-fer-
tility,3 a1-adrenoreceptor subtypes,4 polymerase b-inhibi-
tors and COX-2 inhibitors.5 A lot of synthetic methods
have been developed for the construction of the chroman
unit. One of the most convenient and efficient methods is
through intramolecular cyclization of an appropriate sub-
strate.6
process then the direct synthesis of 4-arylchromans from
simple and readily available starting materials can be
achieved.
Recently, we reported that Barbier–Prins cyclization reac-
tions can be carried out in a one-pot manner in an ionic
liquid to accomplish a direct synthesis of tetrahydropyran
compounds.10 Continuing our interest in one-pot multi-
component reactions in ionic liquids, we report herein a
one-pot Barbier allylation, Friedel–Crafts alkylation and
intramolecular hydroalkoxylation of allylbromides, aro-
matic aldehydes and phenols promoted by the ionic liquid
BPyX–SnCl2·2H2O for the direct synthesis of 4-aryl-
chromans (Scheme 1).
In terms of retrosynthetic analysis, 4-aryl-4-(2¢-hydroxyl-
phenyl)but-1-ene (4) is a favorable precursor to 4-aryl-
R
OH
R'
OM
ArOH
O
R
R'
R
Friedel–Crafts reaction
hydroalkoxylation
5
4
Barbier reaction
CHO
+
Br
R
Barbier/Friedel–Crafts/hydroalkoxylation in a one-pot process
Scheme 1
SYNLETT 2007, No. 9, pp 1357–1364
0
1.
0
6.
2
0
0
7
Advanced online publication: 23.05.2007
DOI: 10.1055/s-2007-980353; Art ID: W25906ST
© Georg Thieme Verlag Stuttgart · New York

1358
X.-L. Zhao et al.
LETTER
The SnCl2-mediated Barbier-type reaction of allylhalides phenyl ring, the hydroxyl-directed ortho-selectivity in the
with carbonyl compounds is an efficient method in the Friedel–Crafts alkylation was still predominate, giving
preparation of homoallylic alcohols, although it requires ortho-hydroxyl-substituted product 4b as the sole regio-
additional catalyst in most cases. Among the catalysts isomer in 64% yield despite the methoxy group being a
suitable for this type of reaction, quaternary ammonium strong para- and ortho-directing group.
salts are most convenient.11 As reported, mixing quaterna-
Recently, studies on the addition of an O–H bond to an
ry ammonium salts, BPyCl or BPyI, with SnCl2·2H2O
alkene forming a C–O bond (hydroalkoxylation) have
(1:2) under solvent-free conditions produced complexes
progressed greatly. Intramolecular hydroalkoxylation is
an ideal method to use in the synthesis of 4-arylchromans
from 4-aryl-4-(2¢-hydroxylphenyl)-but-1-enes (for exam-
ple 4a–b). Various catalysts are suitable for hydroalkoxy-
10a or 10b (Figure 1),12 which were in liquid state at room
temperature (so-called room-temperature ionic liquids). It
is expected that the ionic liquids (10a and 10b) can serve
as both a functionalized reagent with Lewis acidity and as
lation reactions, including protonic acids such as triflic
reaction media in the three-component reaction of allyl-
acid (TfOH),14 Lewis acids such as AlCl3,15 FeCl3,16
bromide, aromatic aldehydes and phenols.13
Cu(OTf)2,17 Sn(OTf)418 and transition-metal reagents,19 as
well as N-bromosuccimide (NBS).20 More recently, Li21
and Youn22 reported respectively, that chroman units can
⋅SnCl2⋅2H2O
be constructed through AgOTf/AuCl3- or AgOTf-cata-
+
x–
N
lyzed intramolecular hydroalkoxylation. As reported,14
Bu
hydroalkoxylation reactions depend strongly on the reac-
tivity of the double bond involved in the reaction, which
10a, X = Cl
10b, X = I
decrease in the following order, trisubstituted > gem-di-
substituted > monosubstituted. It was found that
Yb(OTf)3, Sc(OTf)3, AgOTf, AgOTf–RuCl3, p-toluene-
sulfonic acid and HCl had no catalytic ability in the intra-
molecular cyclization of 4a in both CH2Cl2 and ionic
liquid. This was attributed to the low reactivity of 4a,
which possesses an isolated, monosubstituted terminal
double bond. Although TfOH can catalyze the intra-
molecular hydroalkoxylation of 4a in CH2Cl2 to give
cyclization product 5a, no chroman product was obtained
under ionic liquid conditions in the presence of catalytic
TfOH. After screening, Lewis acid ZnCl2 was found to be
a good catalyst for the hydroalkoxylation reaction of 4a in
10a, giving the product 5a in 82% yield (Scheme 2, path
B¢). To our delight, after the initial reaction of 1b, 2a and
3a in 10a at room temperature for 24 hours, ZnCl2 was
added to the reaction and the subsequent hydroalkoxyla-
tion proceeded smoothly at 70 °C to directly afford
product 5a in 64% yield (Scheme 2, path A).
Figure 1
Considering the regioselectivity of the Friedel–Crafts
alkylation, the para-substituted phenols (3a–b) were
selected as one of the three components for the synthesis
of chroman compounds. At first, the three-component
reaction of allylbromide 2a, p-methylbenzaldehyde (1a)
and p-methylphenol (3a) (R = Me) was attempted in ionic
liquid 10a. There was no organic solvent and the reaction
was carried out at room temperature for 24 hours. The
Barbier allylation/Friedel–Crafts alkylation product, 4-
(4¢-methylphenyl)-4-(2¢-hydroxyl-4¢-methylphenyl)but-
1-ene (4a) was obtained in 76% yield (Scheme 2, path B).
The Friedel–Crafts alkylation occurred at the ortho-posi-
tion of the phenolic hydroxyl group to give exclusively
ortho-isomers. However, no product of the subsequent
intramolecular cyclization was detected. For 3b (R =
OMe) bearing both methoxy and hydroxyl groups in the
R
CHO
OH
1) 10a, r.t.
O
Br
+
+
2) 10a, ZnCl2, 70 °C
R
(path A)
3a, R = Me
3b, R = OMe
5a, R = Me
5b, R = OMe
1a
2a
R
OH
ZnCl2
10a, 70 °C
(path B')
10a, r.t.
(path B)
4a, R = Me
4b, R = OMe
Scheme 2
Synlett 2007, No. 9, 1357–1364 © Thieme Stuttgart · New York

LETTER
Direct Synthesis of 4-Arylchromans
1359
Using the optimized experimental procedure, various and then ZnCl2 (1.2 equiv) was added and the reaction was
phenolic compounds (3a–d) were reacted with 2a and stirred at 70 °C for six hours, giving 4-arylchromanes
aromatic aldehydes (1a–b) in ionic liquid for the direct (5a–g) in good yields (54–64%) (Table 1).
synthesis of 4-arylchromans. The mixture of three sub-
strates was stirred in 10a at room temperature for 24 hours
Table 1 Three-Component Reactions of Aldehydes, Allyl Bromide, and Phenols in Ionic Liquid 10aa
OH
O
R'
1) 10a, r.t.
CHO
Br
+
+
2) ZnCl2, 10a, 70 °C
R'
R
R
1a,b
2a
3a–d
5a–g
Entry
Aldehyde
Phenol
Product
Yield (%)b
OH
CHO
O
1
64
3a
1a
1a
5a
MeO
OH
O
2
58
MeO
3b
5b
OH
O
3
1a
63
3c
5c
OH
O
4
5
1a
57
3d
5d
CHO
O
3a
64
MeO
1b
MeO
5e
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1360
X.-L. Zhao et al.
LETTER
Table 1 Three-Component Reactions of Aldehydes, Allyl Bromide, and Phenols in Ionic Liquid 10aa (continued)
OH
O
R'
1) 10a, r.t.
CHO
Br
+
+
2) ZnCl2, 10a, 70 °C
R'
R
R
1a,b
2a
3a–d
5a–g
Entry
Aldehyde
Phenol
Product
Yield (%)b
O
6
1b
3c
54
MeO
5f
O
7
1b
3d
63
MeO
5g
a (i) 24 h, r.t.; (ii) ZnCl2, 70 °C, 6 h.
b Isolated yield.
The use of 2-hydroxylbenzaldehydes (salicylaldehydes, oxylation in ionic liquid, giving the cyclization products,
6a–c) extended the scope of the substrates suitable for the 4-arylchromans 8a–f (Table 2).23 Even anisole (7) can be
three-component reaction. Phenols without a substituent used as a substrate in the three-component reaction to af-
at the para-position of the phenolic hydroxyl group ford the 4-arylchroman product 8g in 58% yield (Table 2,
(3e–h) are also suitable substrates for the three-compo- entry 7). The products (5a–g and 8a–g) are mixtures of a
nent reaction in ionic liquid 10a. Salicylaldehydes not pair of diastereomers with diastereomeric ratios (dr) in the
only participated in the Barbier/Friedel–Crafts reaction, range of 1:1 to 1:1.2, determined by 1H NMR spectra.
but the hydroxyl group in their molecules was also
involved in ZnCl2 mediated intramolecular hydroalk-
Table 2 One-Pot Reactions of Aldehydes 6a–c, Allyl Bromide, and Phenols in Ionic Liquid 10aa
OH
1) 10a, r.t.
CHO
Br
+
O
R
+
2) ZnCl2, 10a, 80 °C
R'
OH
6a–c
R
R'
2a
3
8a–f
Entry
Aldehyde
Phenol
Product
Yield (%)b
CHO
OH
O
1
70
OH
3e
6a
HO
8a
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LETTER
Direct Synthesis of 4-Arylchromans
1361
Table 2 One-Pot Reactions of Aldehydes 6a–c, Allyl Bromide, and Phenols in Ionic Liquid 10aa (continued)
OH
1) 10a, r.t.
CHO
Br
+
O
R
+
2) ZnCl2, 10a, 80 °C
R'
OH
6a–c
R
R'
2a
3
8a–f
Entry
Aldehyde
Phenol
Product
Yield (%)b
OH
O
2
6a
64
3f
HO
8b
OH
O
3
6a
62
3g
HO
8c
OH
O
MeO
4
6a
64
OMe
3h
HO
8d
F
F
CHO
OH
O
5
3e
53
6b
HO
8e
Br
Br
CHO
O
6
3e
64
OH
6c
HO
8f
OMe
O
7
6a
58
7
MeO
8g
a Reaction conditions: (i) 10 h, r.t.; (ii) 70 °C, 6 h.
b Isolated yield.
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1362
X.-L. Zhao et al.
LETTER
OH
10b, 40 °C
CHO
OH
Cl
+
O
R
+
R'
R
R'
HO
6a,b
2b
3e–h
9a–e
Scheme 3
OSnCl3
Ar
ArCHO
Br
SnCl3
+
BPyCl/SnCl2
12
11
R
R
R
HO
ZnCl2
ClZnO
Ar
HO
Ar
Ar
13
4
14
HCl
R
R
O
ClZnO
Ar
Ar
5
15
Scheme 4
When 2-methylallylchloride (2b) was employed in the re- is easily eliminated to form the benzyl carbocation 13. In
action of 6a (R = H) with 3e (R¢ = H) in 10a at 70 °C, no the presence of an aromatic carbon nucleophile (phenolic
product was detected and only starting materials were re- compound), the Friedel–Crafts alkylation of 13 takes
covered. This was attributed to the low reactivity of allyl- place, giving product 4. At that time, if ZnCl2 is added, the
chloride. Changing 10a to the iodo analogue 10b, allowed elimination of HCl and formation of an O–Zn bond occur
the three-component reaction to proceed smoothly at to generate intermediates 14 and 15, which subsequently
40 °C giving the product of the Barbier/Friedel–Crafts/ undergo intramolecular hydroalkoxylation to form cy-
hydroalkoxylation reaction, 2,2-dimethyl-4-(4¢-hydroxyl- clization product 5. As indicated above, HCl did not cata-
phenyl)chroman (9a) in 66% yield (Scheme 3).24 It was lyze the intramolecular hydroalkoxylation of 4a in ionic
assumed that the transformation of chloride to iodine in liquid 10a in the absence of ZnCl2. BPyCl–ZnCl2 does not
allylchloride 2b could have taken place. The involved promote the three-component Barbier, Friedel–Crafts
intramolecular hydroalkoxylation did not need additional alkylation reaction either, but is able to promote the in-
Lewis acid catalyst ZnCl2, probably due to the relatively tramolecular hydroalkoxylation of 4a at 70 °C, affording
high reactivity of the gem-disubstituted double bond for the product 5a in 82% yield. It can be assumed that SnCl2
hydroalkoxylation. The experimental results are listed in and ZnCl2 play different roles in the one-pot process.
Table 3. For anisole 7, the chroman product 9f was also
obtained in 54% yield (Table 3, entry 6).
In summary, the Barbier allylation, Friedel–Crafts alkyla-
tion, and intramolecular hydroalkoxylation can be com-
The mechanism for the three-component reaction is pro- bined into a one-pot process promoted by a Lewis acid
posed in Scheme 4. First, a Barbier reaction of allybro- ionic liquid. SnCl2 and ZnCl2 played different roles in the
mide with SnCl2 in the presence of pyridine salt produces one-pot process. This method provides a practical and
the allyltin intermediate 11,25 which subsequently under- convenient synthesis of 4-arylchromans starting from
goes reaction with the aldehyde to generate reactive inter- simple and commercially available materials.
mediate 12. In the reaction media (10a), the OSnCl3 group
Synlett 2007, No. 9, 1357–1364 © Thieme Stuttgart · New York

LETTER
Direct Synthesis of 4-Arylchromans
1363
Table 3 One-Pot Reactions of Aldehydes 6a–c, Alkenyl Chloride 2b and Phenols in Ionic Liquid 10b
Entry
Aldehyde
Phenols
Product
Yield (%)a
CHO
OH
O
O
O
1
66
OH
3e
6a
6a
HO
9a
OH
2
3
4
5
62
60
60
52
3f
HO
9b
OH
6a
3g
HO
9c
OH
O
MeO
6a
OMe
3h
3e
HO
9d
F
F
CHO
OH
O
6b
HO
9e
OMe
O
6
6a
54
7
MeO
9f
a Isolated yield.
Acknowledgment
References and Notes
We thank the National Natural Foundation of China and The Chine-
se Academy of Sciences for financial support.
(1) (a) Browers, W. S.; Ohta, T.; Cleere, J. S.; Marsella, P. A.
Science 1976, 193, 542. (b) Ellis, G. P.; Lockhart, I. M.
Chromanes and Tocopherols; Wiley-Interscience: New
York, 1981.
(2) Ogundaini, A.; Farah, M.; Perera, P. J. Nat. Prod. 1996, 59,
587.
Synlett 2007, No. 9, 1357–1364 © Thieme Stuttgart · New York

1364
X.-L. Zhao et al.
LETTER
(3) Hajela, K.; Pandey, J.; Dwivedy, A.; Dhar, J. D.; Sarkhel, S.;
Maulik, P. R.; Velumurugan, D. Bioorg. Med. Chem. 1999,
7, 2083.
(4) Quaglia, W.; Pigini, M.; Piergentili, A.; Giannella, M.;
Gentilli, F.; Marrucci, G.; Carrieri, A.; Carotti, A.; Pogges,
E.; Leonardi, A.; Melchiorre, C. J. Med. Chem. 2002, 45,
1633.
(16) Morimoto, T.; Takaki, K. Angew. Chem. Int. Ed. 2006, 45,
2930.
(17) Talyor, J. G.; Whittall, N.; Hii, K. K. Chem. Commun. 2005,
5103.
(18) Coulombel Favier, I.; Dunach, E. Chem. Commun. 2005,
2286.
(19) For examples, see: (a) Bienayme, H.; Ancel, J.-E.; Meilland,
P.; Simonato, J.-P. Tetrahedron Lett. 2000, 41, 3339.
(b) Qian, H.; Han, X.; Widenhoefer, R. A. J. Am. Chem. Soc.
2004, 126, 9536. (c) Oe, Y.; Ito, Y. Chem. Commun. 2004,
1620. (d) Yang, C.-G.; He, Ch. J. Am. Chem. Soc. 2005, 127,
6966. (e) Gligorich, K. M.; Schultz, M. J.; Sigman, M. S.
J. Am. Chem. Soc. 2006, 128, 2794.
(5) (a) Deng, J.-Z.; Starck, S. R.; Li, S.; Hecht, S. M. J. Nat.
Prod. 2005, 68, 1625. (b) Maloney, D. J.; Starck, S. R.; Gao,
Z.; Hechit, S. M. J. Am. Chem. Soc. 2005, 127, 4140.
(c) Sawadjoon, S.; Kittikara, K.; Vichai, V.; Tanticharoen,
M.; Thebta-Ranonth, Y. J. Org. Chem. 2002, 67, 5470.
(6) For example, see: (a) Larock, R. C.; Yang, H.; Pace, P.
Tetrahedron Lett. 1998, 39, 237. (b) Trost, B. M.; Asakawa,
N. Synthesis 1999, 1491. (c) Nicolaou, K. C.; Pfefferkorn, J.
A.; Barluenga, S.; Mitchell, H. J.; Roecker, A. J.; Cao, G.-Q.
J. Am. Chem. Soc. 2000, 122, 9968. (d) Barluenga, J.;
Trincado, M.; Rubio, E.; Gonzalez, J. M. J. Am. Chem. Soc.
2004, 126, 3416. (e) Merchaet, A.; Delbeke, P.; Daloze, D.;
Dive, G. Tetrahedron Lett. 2004, 45, 4697. (f) Trost, B. M.;
Shen, H. C.; Surivet, J.-P. J. Am. Chem. Soc. 2004, 126,
12565. (g) Vyvyan, J. R.; Oaksmith, J. M.; Parks, B. W.;
Peterson, E. M. Tetrahedron Lett. 2005, 46, 2457.
(7) (a) Shiina, I.; Suzuki, M.; Yokoyama, K. Tetrahedron Lett.
2002, 43, 6395. (b) Shiina, I.; Suzuki, M.; Yokoyama, K.
Tetrahedron Lett. 2004, 45, 965. (c) Mertines, K.; Iovel, I.;
Kischel, J.; Zapf, A.; Beller, M. Angew. Chem. Int. Ed. 2005,
44, 238.
(20) Talluri, S. K.; Sudalai, A. Org. Lett. 2005, 7, 855.
(21) Nguyen, R.-V.; Yao, X.; Li, Ch.-J. Org. Lett. 2006, 8, 2397.
(22) Youn, S. W.; Eom, J. I. J. Org. Chem. 2006, 71, 6705.
(23) 2-Methyl-4-(4¢-hydroxylphenyl)chroman (8a): A mixture of
salicylaldehyde 6a (61 mg, 0.5 mmol), allyl bromide (2a)
(120 mg, 1 mmol) and phenol 3a (69 mg, 0.75 mmol) in
ionic liquid (10a) derived from BPyCl–SnCl2·2H2O was
stirred at ambient temperature for 10 h. ZnCl2 (102 mg, 0.75
mmol) was added, followed by stirring at 70 °C for 5 h. The
reaction mixture was extracted with Et2O. The combined
Et2O extracts were washed with aqueous HCl (2 M), and
then dried over Na2SO4. The solvent was removed under
vacuum and the crude product was purified by flash column
chromatography on silica gel (EtOAc–PE, 1:30) to afford 8a
as a colorless oil (84 mg, 70%); FTIR (film): 3394, 2971,
1649, 1612, 1581, 1514, 1483, 1455, 1232 cm–1; 1H NMR
(300 MHz, CDCl3): d = 7.10–7.30 (m, 6 H), 6.72–7.07 (m, 2
H), 4.12–4.34 (m, 2 H), 1.91–1.26 (m, 2 H), 1.38, 1.48 (2 ×
d, J = 6.9, 6.2 Hz, 3 H); 13C NMR (75 MHz, CDCl3):
d = 154.9, 153.9 (153.6)*, 138.6, 136.7, 130.5, 129.4, 129.3,
127.2 (127.6), 125.7, 120.0, 116.2 (116.5), 114.8 (115.2),
67.2 (72.2), 41.8 (38.8), 39.7 (37.5), 20.7 (21.1); (* data in
parentheses represents diastereomeric peaks); HRMS (EI):
m/z calcd for C16H16O2: 240.1150; found: 240.1149.
(24) 2,2-Dimethyl-4-(4¢-hydroxylphenyl)chroman (9a): A
mixture of salicylaldehyde 6a (61 mg, 0.5 mmol), 2-methyl-
allyl chloride (2b) (82 mg, 1.0 mmol) and phenol 3e (69 mg,
0.75 mmol) in ionic liquid (10b) derived from BPyI–
SnCl2·2H2O was stirred at 40 °C for 24 h. The reaction
mixture was extracted with Et2O. The combined Et2O phase
was washed with aqueous HCl (2 M), then dried over
Na2SO4. The solvent was removed under vacuum and the
crude product was purified by flash column chromatography
on silica gel (EtOAc–PE, 1:30) to afford 9a as a colorless oil
(83 mg, 66%); FTIR (film): 3394, 2925, 1612, 1571, 1514,
1486, 1449, 1368, 1253, 1124 cm–1; 1H NMR (300 MHz
CDCl3): d = 7.01–7.16 (m, 3 H), 6.69–6.88 (m, 5 H), 5.63 (br
s, 1 H), 4.01 (dd, J = 7.2, 4.8 Hz, 1 H), 2.08–2.33 (m, 2 H),
1.43 (s, 3 H), 1.36 (s, 3 H); 13C NMR (75 MHz, CDCl3):
d = 154.6, 154.2, 137.1, 129.9, 127.7, 125.0, 119.9, 117.2,
115.5, 74.8, 43.6, 39.1, 30.0, 24.3; HRMS (EI): m/z calcd for
C17H18O2: 254.1307; found: 254.1308.
(8) For a review on the Barbier reaction, see: Yamamoto, Y.;
Asao, N. Chem. Rev. 1993, 93, 2207.
(9) (a) Multicomponent Reactions; Zhu, J.; Bienayme, H., Eds.;
Wiley-VCH: Weinheim, 2005. (b) Tietze, L. F. Chem. Rev.
1996, 96, 115.
(10) Zhao, X. L.; Liu, L.; Chen, Y. J.; Wang, D. Tetrahedron
2006, 62, 7113.
(11) (a) Masuyama, Y.; Kishida, M.; Kurusu, Y. Tetrahedron
Lett. 1996, 37, 7103. (b) Masuyama, Y.; Ito, A.; Kurusu, Y.
Chem. Commun. 1998, 315. (c) Masuyama, Y.; Ito, T.;
Tachi, K.; Ito, A.; Kurusu, Y. Chem. Commun. 1999, 1261.
(d) Ito, A.; Kishida, M.; Kurusu, Y.; Masuyama, Y. J. Org.
Chem. 2000, 65, 494.
(12) Wasserscheid, P.; Waffenschmidt, H. J. Mol. Catal. A:
Chem. 2000, 164, 61.
(13) For examples of three-component reactions in ionic liquid,
see: (a) Aza-Diels–Alder reaction: Yadav, J. S.; Reddy, B.
V. S.; Reddy, J. S. S.; Rao, R. S. Tetrahedron 2003, 59,
1599. (b) Coupling reaction of aldehyde, alkyne and amine:
Li, Z.; Wei, C.; Chen, L.; Varma, R. S.; Li, C.-J. Tetrahedron
Lett. 2004, 45, 2443. (c) Mannich reaction: Kabalka, G. W.;
Venkataiah, B.; Dong, G. Tetrahedron Lett. 2004, 45, 729.
(d) Asymmetric Mannich reaction: Chen, S.-L.; Ji, S.-J.;
Loh, T.-P. Tetrahedron Lett. 2003, 44, 2405. (e) Biginelli
reaction: Peng, J.; Deng, Y. Tetrahedron Lett. 2001, 42,
5917.
(14) (a) Coulombel, L.; Dunach, E. Green Chem. 2004, 6, 499.
(b) Rosenfeld, D. C.; Shekhar, S.; Takemiya, A.;
Utsunomiya, M.; Hartwig, J. F. Org. Lett. 2006, 8, 4179.
(15) Bolzoni, L.; Casiraghi, G.; Casnati, G.; Sartori, G. Angew.
Chem., Int. Ed. Engl. 1978, 17, 684.
(25) Ito, A.; Kishida, M.; Kurusu, Y.; Masuyama, Y. J. Org.
Chem. 2000, 65, 494.
Synlett 2007, No. 9, 1357–1364 © Thieme Stuttgart · New York