Copper(II) triflate-sodium dodecyl sulfate catalyzed preparation of 1,2-diphenyl-2,3-dihydro-4-pyridones in aqueous acidic medium

Article abstract of DOI:10.1055/s-0031-1291040

The reactions between N-benzylideneanilines and Danishefsky's diene proceed smoothly in acidic aqueous medium in the presence of a catalytic amount of copper(II) triflate-sodium dodecyl sulfate [Cu(OTf)₂-SDS] to afford the corresponding 1,2-diphenyl-2,3-dihydro-4-pyridones in excellent yields of 84-95%. The aqueous solution containing the catalyst is recovered and reused without any loss in efficiency. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0031-1291040

SPECIAL TOPIC
▌2181
sCpeociapl tpopeicr(II) Triflate–Sodium Dodecyl Sulfate Catalyzed Preparation of
1,2-Diphenyl-2,3-dihydro-4-pyridones in Aqueous Acidic Medium
Preparation of 1,2-Diphenyl-2,3-dihydro-4-pyridones
Daniela Lanari,a,b Oriana Piermatti,a Ferdinando Pizzo,*a Luigi Vaccaro*a
a
Laboratory of Green Synthetic Organic Chemistry, CEMIN – Dipartimento di Chimica, Università di Perugia, via Elce di Sotto, 8,
06123 Perugia, Italy
Fax +39(075)5855560; E-mail: luigi@unipg.it; E-mail: pizzo@unipg.it
b
Dipartimento di Chimica e Tecnologia del Farmaco, Università di Perugia, Via del Liceo, 06123 Perugia, Italy
Received: 19.03.2012; Accepted: 22.03.2012
The reaction between N-benzylideneanilines 1 and
Danishefsky’s diene (2) is classified as an aza-Diels–
Alder cycloaddition reaction (Scheme 1). Due to its im-
portance, this particular transformation has been studied
Abstract: The reactions between N-benzylideneanilines and
Danishefsky’s diene proceed smoothly in acidic aqueous medium in
the presence of a catalytic amount of copper(II) triflate–sodium do-
decyl sulfate [Cu(OTf)2–SDS] to afford the corresponding 1,2-di-
phenyl-2,3-dihydro-4-pyridones in excellent yields of 84–95%. The by employing a variety of experimental conditions includ-
aqueous solution containing the catalyst is recovered and reused
without any loss in efficiency.
ing the use of acids to promote the reaction in different or-
ganic media,6 under solvent-free conditions,7 and also in
water.8
Key words: aza-Diels–Alder reaction, Lewis acid, aqueous medi-
um, pyridones, copper(II) triflate
R1
R1
OMe
N
N
H2O
The 2,3-dihydro-4-pyridone moiety is a versatile building
block in organic synthesis as the presence of both enone
and amine functionalities allows it to be used either as a
donor or acceptor partner in fundamental organic transfor-
mations.1 The stereochemical features of this structure are
also interesting: in order to minimize the allylic 1,3-strain,
the C-2 substituent is forced to adopt an axial position re-
sulting in the molecule having a distorted shape.2 Such de-
formation is responsible for the high selectivity observed
in 1,2- and 1,4-additions and consequent alkylation at the
C-3 position. These features make 2,3-dihydro-4-pyri-
dones suitable precursors for the synthesis of alkaloids
and aza-sugars.3
+
O
Me3SiO
R2
R2
1a–i
2
3a–i
a
c
e
g
b
d
f
h
i
R1
R2
4-MeO 4-MeO
4-Br
H
4-O2N
H
H
H
H
H
H
H
H
4-Br
4-MeO
4-MeO
4-O2N 2-HO
Scheme 1 Synthesis of 1,2-diphenyl-2,3-dihydro-4-pyridones 3a–i
via the reaction of N-benzylideneanilines 1a–i and Danishefsky’s di-
ene (2)
Water has proven to be an efficient and green solvent for
various organic transformations, and often has successful-
ly enabled processes that cannot be performed in organic
media.4 In fact, thanks to its peculiar properties and struc-
ture (e.g., pH control, H-bonding ability), water may rep-
resent an ideal reaction medium for achieving the
optimum efficiency in many synthetic processes.
The role of the pH of the aqueous medium9 on the effi-
ciency of such transformations and on the use of Lewis
acid–surfactant–combined catalysts such as copper(II) tri-
flate–sodium dodecyl sulfate [Cu(OTf)2–SDS] has not
been investigated.
Taking into account our previous experience on Lewis ca-
talysis in aqueous media4e,f,10 we have investigated
the preparation of 1,2-diphenyl-2,3-dihydro-4-pyridones
3a–i via the reactions of N-benzylideneanilines 1a–i and
Danishefsky’s diene (2) in water, catalyzed by Lewis
acids under neutral (pH 7.0) and acidic (pH 4.0) condi-
tions (Scheme 1).
On the other hand, Lewis acid–surfactant–combined cata-
lysts (LASCs) are able to promote several organic trans-
formations; these catalytic systems furnish a hydrophobic
molecular environment that favors the approach of reac-
tants. Lewis acid–surfactant–combined catalysts have
been successfully employed in Diels–Alder, Mannich and
allylation reactions, as well as in nucleophilic ring-
opening reactions of epoxides and aldol additions.5
The aza-Diels–Alder reaction between N-benzylideneani-
line (1a) and Danishefsky’s diene (2) was chosen as a
model system. Equimolar amounts of substrates were
used and the reactions were performed at pH 7.0 or pH
4.0, at a temperature of 30 °C. The results obtained after a
reaction time of 10 minutes are summarized in Table 1.
SYNTHESIS 2012, 44, 2181–2184
Advanced online publication: 25.05.2012
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0031-1291040; Art ID: SS-2012-C0282-ST
© Georg Thieme Verlag Stuttgart · New York

2182
D. Lanari et al.
SPECIAL TOPIC
In aqueous medium under neutral conditions the reaction yl sulfate system (1 mol%) was employed. A quantitative
between 1a and diene 2 led to formation of only traces of yield of 2,3-dihydro-4-pyridone 3a was obtained in 10
the reaction product 3a (<2%) being detected (Table 1, en- minutes (Table 1, entry 16).
try 1). In the presence of sodium dodecyl sulfate (SDS) (1
mol%), after 10 minutes, the conversion of 1a into 2,3-di-
hydro-4-pyridone 3a was good, but still not complete
(Table 1, entry 2).
The experimental procedure was extremely simple and
straightforward: copper(II) triflate, sodium dodecyl sul-
fate, N-benzylideneaniline (1a) and diene 2 were added to
the aqueous medium while the pH was kept constant. Af-
ter completion of the reaction, pyridone 3a was extracted
with ethyl acetate and purified by column chromatogra-
phy.
Table 1 Optimization of the Conditions for the Aza-Diels–Alder Re-
action between N-Benzylideneaniline (1a) and Diene 2
The role of the counter ion in the copper(II) triflate–sodi-
OMe
um dodecyl sulfate catalytic system plays a crucial part in
N
H2O
N
+
the outcome of the reaction. In fact, when copper(II) chlo-
ride (CuCl2) or copper(II) nitrate [Cu(NO3)2] were em-
ployed as catalysts the reaction between 1a and 2 did not
go to completion, and together with product 3a, enone 4a
was also detected (Figure 1). The formation of adduct 4a
suggested a Mannich-type addition mechanism instead of
a concerted [4+2] cycloaddition pathway.
30 °C, 10 min
O
Me3SiO
1a
3a
2
Entry
pH
Catalyst
Conv. (%)a
1
2
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
4.0
4.0
4.0
4.0
4.0
4.0
–
<2
65
85
20
15
80
30
25
90
75
40
60
90
90
97
99
SDS
OMe
HN
OMe
HN
NO2
3
Cu(OTf)2
CoCl2
4
O
O
5
NiCl2
6
Zn(OTf)2
CoCl2–SDS
NiCl2–SDS
Cu(OTf)2–SDS
Zn(OTf)2–SDS
–
4a
4i
7
Figure 1 Intermediates suggesting a Mannich-type addition mecha-
nism
8
9
As we were interested in developing a simple and sustain-
able procedure to access the 1,2-diphenyl-2,3-dihydro-4-
pyridone moiety, we next investigated if the aqueous me-
dium and the catalytic system employed could be recycled
and reused. A test reaction between N-benzylideneaniline
(1a) and diene 2 was performed, and following comple-
tion, the aqueous solution containing the catalyst was re-
covered and reused twice more to promote the same aza-
Diels–Alder reaction with no loss of activity being ob-
served. Recycling of the catalyst can be repeated with
similar results being obtained with complete recovery of
the aqueous layer. Such results are important, making our
protocol more convenient from an environmental point of
view with respect to other processes showing similar
chemical efficiency.
10
11
12
13
14
15
16
SDS
Zn(OTf)2
Zn(OTf)2–SDS
Cu(OTf)2
Cu(OTf)2–SDS
a Conversion determined by 1H NMR spectroscopy; the remaining
material was unreacted 1a.
Poor levels of conversion were obtained when cobalt(II)
chloride (CoCl2) and nickel(II) chloride (NiCl2) (both 1
mol%) were employed as catalysts (Table 1, entries 4, 5,
7 and 8). On the other hand, copper(II) triflate [Cu(OTf)2]
and zinc(II) triflate [Zn(OTf)2] proved to be better cata-
lysts and enhanced the reactivity of the aza-Diels–Alder
reaction, especially when combined with sodium dodecyl
sulfate (Table 1, entries 3, 6, 9 and 10).
The importance of the aqueous medium was stressed by
comparing the reactivity of the model reaction between 1a
and diene 2 in different reaction media employing cop-
per(II) triflate–sodium dodecyl sulfate as the catalytic sys-
tem. In acetonitrile and dichloromethane good
conversions were achieved, however a mixture of the de-
sired product 3a and Mannich adduct 4a was formed. Un-
der solvent-free conditions, slightly inferior results were
obtained. Furthermore, using the aqueous medium makes
recovery and reuse of the catalytic system more straight-
forward.
To further explore the utility of these catalytic systems the
reaction between 1a and diene 2 was also run under acidic
conditions (pH 4.0). The optimum reaction conditions
were achieved when the copper(II) triflate–sodium dodec-
Synthesis 2012, 44, 2181–2184
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
Preparation of 1,2-Diphenyl-2,3-dihydro-4-pyridones
2183
hyde and 4-nitroaniline at 120 °C for 12 hours. Pyridones 3a–g are
known compounds, however 3f is not fully described in the litera-
ture. Mannich adduct 4i and pyridones 3h and 3i are new com-
pounds. Adduct 4a was not characterized as it was only detected in
the reaction forming product 3a. Column chromatography was per-
formed using Merck Kieselgel 60 silica gel (230–400 mesh). IR
spectra were recorded on a Perkin-Elmer RXI FT-IR spectropho-
tometer using CHCl3 as the solvent. 1H (400 MHz) and 13C (100.6
MHz) NMR spectra were recorded on a Bruker DRX 400 Avance
spectrometer. Samples were dissolved in the appropriate deuterated
solvent with the residual solvent peak as the internal standard, or
TMS in the case of CDCl3. Chemical shifts are reported in ppm and
coupling constants in Hz. GC–MS (EI) analyses were carried out
using an Agilent HP6890N mass-selective detector equipped with
an electron impact ionizer at 70 eV. Elemental analyses were re-
corded using a Fisons EA1008 CHNS analyzer.
Finally, the optimized conditions (Table 1, entry 16) were
employed for the preparation of a range of 1,2-diphenyl-
2,3-dihydro-4-pyridones 3a–i starting from N-benzyli-
deneanilines 1a–i and diene 2, as reported in Table 2.
Table 2 Copper(II) Triflate–Sodium Dodecyl Sulfate Catalyzed
Preparation of Pyridones 3a–i in Aqueous Medium (pH 4.0) at 30 °C
Entry Imine R1
R2
Time
(min)
Product Yield
(%)a
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
H
H
10
60
3a
3b
3c
3d
3e
3f
95
H
4-MeO
H
88
4-MeO
4-MeO
H
60
85
4-MeO
4-Br
H
60
94b
85
1,2-Diphenyl-2,3-dihydropyridin-4(1H)-ones 3a–h; General
Procedure
To a screw-capped vial equipped with a magnetic stir bar, H2O (2
mL), Cu(OTf)2 (1 mol%), SDS (1 mol%), N-benzylideneaniline 1a–
h (1.0 × 10–3 mol) and Danishefsky’s diene (2) (1.0–3.0 × 10–3 mol)
were added consecutively at 30 °C. The mixture was maintained at
pH 4.0 by addition of HCl (1 M). After completion of the reaction,
the mixture was extracted with EtOAc (2 × 3 mL), and the com-
bined organic phase dried over Na2SO4 and evaporated. The crude
residue was purified by column chromatography (hexane–EtOAc,
8:2) to yield pure pyridone 3a–h. The reaction between imine 1i and
diene 2 performed under these conditions gave the corresponding
Mannich adduct 4i (0.307 g, 0.94 × 10–3 mol) in 94% yield as a yel-
low oil.
30
4-Br
H
120
60
84
1g
1h
1i
4-O2N
2-HO
H
3g
3h
3id
95c
95c
89c
H
60
4-O2N
180
a Yield of isolated product.
b Three equivalents of diene 2 were used.
c Two equivalents of diene 2 were used.
d Obtained after acidic treatment of adduct 4i.
1-(4-Bromophenyl)-2-phenyl-2,3-dihydropyridin-4(1H)-one
(3f)
Yield: 0.275 g, 0.84 × 10–3 mol (84%); colorless oil.
The copper(II) triflate–sodium dodecyl sulfate protocol
used to synthesize the pyridone 3a worked well for deriv-
atives 3b–h, which were obtained in excellent 84–95%
yields (Table 2, entries 2–8). In the case of N-benzyli-
deneanilines, 1d, 1g, 1h and 1i, greater than stoichiomet-
ric amounts (2–3 equivalents) of diene 2 were necessary
to allow the aza-Diels–Alder reaction to go to completion
(Table 2, entries 4, 7, 8 and 9). The reaction between im-
ine 1i and diene 2 yielded the Mannich adduct 4i (Figure
1) instead of the expected product 3i (Table 2, entry 9),
probably due to the presence of the electron-withdrawing
nitro group on the aniline fragment. Nevertheless, the de-
sired 2,3-dihydro-4-pyridone 3i was obtained through
acidic treatment of adduct 4i [THF–1 M HCl (1:1), 1 M,
30 °C, 14 h, see experimental section].
IR (CHCl3): 1648, 1578, 1493, 1322 cm–1.
1H NMR (400 MHz, CDCl3): δ = 2.78 (dd, J = 2.5, 16.4 Hz, 1 H),
3.27 (dd, J = 7.1, 16.4 Hz, 1 H), 5.23 (d, J = 4.2 Hz, 1 H), 5.29 (d,
J = 7.8 Hz, 1 H), 6.88 (d, J = 8.4 Hz, 2 H), 7.22–7.38 (m, 7 H), 7.61
(d, J = 7.9 Hz, 1 H).
13C NMR (100.6 MHz, CDCl3): δ = 43.2, 61.3, 103.2, 116.9, 119.8
(2 C), 125.8 (2 C), 127.7, 128.8 (2 C), 132.2 (2 C), 137.2, 143.4,
147.4, 189.8.
GC–MS: m/z (%) = 329 (100) [81Br, M]+, 328 (26), 327 (100) [79Br,
M]+, 252 (72), 250 (73), 225 (97), 224 (20), 223 (99), 197 (22), 195
(22), 184 (26), 182 (26), 157 (32), 155 (31), 116 (89), 103 (21), 89
(23), 76 (21).
Anal. Calcd for C17H14BrNO: C, 62.21; H, 4.30; N, 4.27. Found: C,
62.29; H, 4.34; N, 4.12.
2-(2-Hydroxyphenyl)-1-phenyl-2,3-dihydropyridin-4(1H)-one
(3h)
In conclusion, the highly efficient synthesis of 1,2-diphe-
nyl-2,3-dihydro-4-pyridones via the aza-Diels–Alder re-
action between a range of N-benzylideneanilines 1 and
Danishefsky’s diene (2) using copper(II) triflate–sodium
dodecyl sulfate as the catalytic system in acidic water (pH
4.0) has been described. The aqueous solution containing
the catalyst could be recovered and reused without any
loss in efficiency. Hence, the reported procedure can be
considered a chemically and environmentally efficient
strategy for accessing the 2,3-dihydro-4-pyridone moiety.
Yield: 0.252 g, 0.95 × 10–3 mol (95%); colorless oil.
IR (CHCl3): 2254, 1643, 1771, 908 cm–1.
1H NMR (400 MHz, CDCl3): δ = 2.78 (d, J = 16.5 Hz, 1 H), 3.35
(dd, J = 7.4, 16.7 Hz, 1 H), 5.34 (d, J = 4.2 Hz, 1 H), 5.56 (d, J = 4.3
Hz, 1 H), 6.96–7.49 (m, 10 H), 7.56 (d, J = 7.7 Hz, 1 H).
13C NMR (100.6 MHz, CDCl3): δ = 43.0, 54.7, 101.4, 117.6, 118.5
(2 C), 119.9, 126.6 (2 C), 127.5, 129.1 (2 C), 130.4, 148.4, 149.2,
158.3, 189.7.
Anal. Calcd for C17H15NO2: C, 76.96; H, 5.70; N, 5.28. Found: C,
76.88; H, 5.75; N, 5.31.
All chemicals were purchased from commercial suppliers and were
used without any further purification. N-Benzylideneaniline (1a) is
commercially available whereas substrates 1b–h were prepared by
mixing the corresponding aldehydes and anilines for 1–3 hours at
60 °C. Imine 1i was synthesized by heating a mixture of benzalde-
(1E)-1-Methoxy-5-[(4-nitrophenyl)amino]-5-phenypent-1-en-3-
one (4i)
Yield: 0.307 g, 0.94 × 10–3 mol (94%); yellow oil.
IR (CHCl3): 1600, 1326, 1113, 910 cm–1.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2181–2184

2184
D. Lanari et al.
SPECIAL TOPIC
1H NMR (400 MHz, CDCl3): δ = 2.65 (dd, J = 1.6, 16.5 Hz, 1 H),
3.51 (dd, J = 7.6, 16.8 Hz, 1 H), 3.66 (s, 3 H), 4.77–5.03 (m, 1 H),
5.50 (d, J = 18.3 Hz, 1 H), 5.80 (d, J = 4.1 Hz, 1 H), 6.48 (d, J = 16.5
Hz, 2 H), 7.19–7.39 (m, 5 H), 7.51 (d, J = 17.8 Hz, 1 H), 8.09 (d,
J = 15.9 Hz, 2 H).
13C NMR (100.6 MHz, CDCl3): δ = 49.6, 53.4, 57.7, 103.3, 122.0
(2 C), 126.2 (2 C), 127.1 (2 C), 127.4, 129.8 (2 C), 138.2, 143.5,
152.3, 159.9, 199.4.
1993, 115, 8851. (d) Comins, D. L.; Hong, H. J. Org. Chem.
1993, 58, 5035. (e) Comins, D. L.; Radi Benjelloun, N.
Tetrahedron Lett. 1994, 35, 829. (f) Comins, D. L.; Huang,
S.; McArdle, C. L.; Ingalls, C. L. Org. Lett. 2001, 3, 469.
(g) Tsukanov, S. V.; Comins, D. L. Angew. Chem. Int. Ed.
2011, 50, 8626.
(4) (a) Paradowska, J.; Stodulski, M.; Mlynarski, J. Angew.
Chem. Int. Ed. 2009, 48, 4288. (b) Gruttadauria, M.;
Giacalone, F.; Noto, R. Adv. Synth. Catal. 2009, 351, 33.
(c) Kobayashi, S. Pure Appl. Chem. 2007, 79, 235.
(d) Organic Synthesis in Water; Lindström, U. M., Ed.;
Blackwell: Oxford, 2007. (e) Bonollo, S.; Lanari, D.; Pizzo,
F.; Vaccaro, L. Eur. J. Org. Chem. 2011, 2587. (f) Bonollo,
S.; Lanari, D.; Pizzo, F.; Vaccaro, L. Org. Lett. 2011, 13,
2150.
(5) (a) Manabe, K.; Mori, Y.; Wakabayashi, T.; Nagayama, S.;
Kobayashi, S. J. Am. Chem. Soc. 2000, 122, 7202.
(b) Azoulay, S.; Manabe, K.; Kobayashi, S. Org. Lett. 2005,
7, 4593. (c) Ogawa, C.; Azoulay, S.; Kobayashi, S.
Heterocycles 2005, 66, 201. (d) Boudou, M.; Ogawa, C.;
Kobayashi, S. Adv. Synth. Catal. 2006, 348, 2585.
(e) Ogawa, C.; Wang, N.; Boudou, M.; Azoulay, S.;
Manabe, K.; Kobayashi, S. Heterocycles 2007, 72, 589.
(6) (a) Kobayashi, S.; Ishitani, H. Chem. Rev. 1999, 99, 1069.
(b) Jorgensen, K. A. Angew. Chem. Int. Ed. 2000, 39, 3558.
(c) Laurent-Robert, H.; Garrigueus, B.; Dubac, J. Synlett
2000, 1160. (d) Buonora, J.-C.; Olsen, O. T. Tetrahedron
2001, 57, 6069. (e) Collin, J.; Jaber, N.; Lannou, M. I.
Tetrahedron Lett. 2001, 42, 7405. (f) Yuan, Y.; Li, X.; Ding,
K. Org. Lett. 2002, 4, 3309. (g) Bari, L. D. A.; Guillarme, S.;
Hermitage, S.; Howard, J. A. K.; Yay, D.; Pescitelli, G.;
Whiting, A.; Yufit, D. S. Synlett 2004, 708. (h) Cheng, K.;
Lin, L.; Chen, S.; Feng, X. Tetrahedron 2005, 61, 9594.
(i) Park, Y.; Park, E.; Jung, H.; Lee, Y.-J.; Jew, S.-S.; Park,
H.-G. Tetrahedron 2011, 67, 1166.
Anal. Calcd for C18H18N2O4: C, 66.25; H, 5.56; N, 8.58. Found: C,
66.32; H, 5.53; N, 8.54.
1-(4-Nitrophenyl)-2-phenyl-2,3-dihydropyridin-4(1H)-one (3i)
To a screw-capped vial equipped with a magnetic stir bar were add-
ed a solution of 4i (163.17 mg, 5.0 × 10–4 mol) in THF (1 mL) fol-
lowed by HCl (1 M, 1 mL). The resulting mixture was stirred at
30 °C for 14 h and then extracted with EtOAc (2 × 3 mL). The com-
bined organic phase was dried over Na2SO4 and the solvent evapo-
rated to give pyridone 3i.
Yield: 0.279 g, 0.95 × 10–3 mol (95%); colorless oil.
IR (CHCl3): 1735, 1650, 1493, 1214 cm–1.
1H NMR (400 MHz, CDCl3): δ = 2.78 (dd, J = 1.8, 16.4 Hz, 1 H),
3.36 (dd, J = 7.6, 16.5 Hz, 1 H), 5.39 (d, J = 4.3 Hz, 1 H), 5.46 (d,
J = 7.7 Hz, 1 H), 7.09 (d, J = 7.7 Hz, 2 H), 7.22–7.37 (m, 5 H), 7.76
(d, J = 8.4 Hz, 1 H), 8.18 (d, J = 7.9 Hz, 2 H).
13C NMR (100.6 MHz, CDCl3): δ = 43.4, 62.3, 101.5, 105.6 (2 C),
118.2 (2 C), 126.2 (2 C), 127.8, 128.7 (2 C), 143.0 (2 C), 152.8,
158.4, 189.8.
Anal. Calcd for C17H14N2O3: C, 69.38; H, 4.79; N, 9.52. Found: C,
69.34; H, 4.83; N, 9.52.
Acknowledgment
We gratefully acknowledge the Ministero dell’Istruzione, dell’Uni-
versità e della Ricerca (MIUR) within the projects PRIN 2008 and
‘Firb-Futuro in Ricerca’, and the Università degli Studi di Perugia
for financial support.
(7) Bellezza, F.; Cipiciani, A.; Costantino, U.; Fringuelli, F.;
Orrù, M.; Piermatti, O.; Pizzo, F. Catal. Today 2010, 152,
61.
(8) (a) Manabe, K.; Mori, Y.; Kagoshima, H. Tetrahedron Lett.
1999, 40, 7831. (b) Akiyama, T.; Takaya, J.; Kagoshima, H.
Tetrahedron Lett. 1999, 40, 7831. (c) Manabe, K.; Mori, Y.;
Kobayashi, S. Tetrahedron 2001, 57, 2537. (d) Loncaric, C.;
Manabe, K.; Kobayashi, S. Chem. Commun. 2003, 574.
(e) Loncaric, C.; Manabe, K.; Kobayashi, S. Adv. Synth.
Catal. 2003, 345, 475.
(9) (a) Li, C.-J.; Chen, L. Chem. Soc. Rev. 2006, 35, 68.
(b) Bonollo, S.; Fringuelli, F.; Pizzo, F.; Vaccaro, L. Green
Chem. 2006, 8, 960. (c) Bonollo, S.; Fringuelli, F.; Pizzo, F.;
Vaccaro, L. Synlett 2007, 2683. (d) Bonollo, S.; Fringuelli,
F.; Pizzo, F.; Vaccaro, L. Synlett 2008, 1574.
References
(1) (a) Comins, D. L.; Brown, J. D. Tetrahedron Lett. 1986, 27,
4549. (b) Comins, D. L.; Hong, H.; Salvador, J. M. J. Org.
Chem. 1991, 56, 7197. (c) Comins, D. L.; Killpack, M. O.
J. Am. Chem. Soc. 1992, 114, 10972. (d) Creighton, C. J.;
Zapf, C. W.; Bu, J. H.; Goodman, M. Org. Lett. 1999, 1,
1407. (e) Dong, D.; Bi, X.; Liu, Q.; Cong, F. Chem.
Commun. 2005, 3580.
(2) (a) Hoffman, R. W. Chem. Rev. 1989, 89, 1841. (b) Johnson,
F. Chem. Rev. 1968, 68, 375.
(3) (a) Comins, D. L.; Dehghani, A. Tetrahedron Lett. 1991, 32,
5697. (b) Comins, D. L.; Al-Awar, R. S. J. Org. Chem. 1992,
57, 4098. (c) Comins, D. L.; Hong, H. J. Am. Chem. Soc.
(10) (a) Fringuelli, F.; Pizzo, F.; Tortoioli, S.; Vaccaro, L. Org.
Lett. 2005, 7, 4411. (b) Bonollo, S.; Fringuelli, F.; Pizzo, F.;
Vaccaro, L. Synlett 2007, 2683.
Synthesis 2012, 44, 2181–2184
© Georg Thieme Verlag Stuttgart · New York