Two tandem sequential reactions catalyzed by [Au], N-heterocyclic carbenes (NHC) and organic bases

Article abstract of DOI:10.1016/j.tet.2011.05.135

Herein we describe two coupling reactions of aldehydes and activated alkynes to give chalcones or alkenyl esters depending on the base used. Chalcones were obtained by activation of alkynes by Au^I and of aldehydes by NHC in presence of triethyl

Full text of DOI:10.1016/j.tet.2011.05.135

Tetrahedron 67 (2011) 5784e5788
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Two tandem sequential reactions catalyzed by [Au], N-heterocyclic carbenes
(NHC) and organic bases
*
Mauro F. A. Adamo , Gea Bellini, Surisetti Suresh
Centre for Synthesis and Chemical Biology (CSCB), Department of Pharmaceutical and Medicinal Chemistry, Royal College of Surgeons in Ireland, 123 St. Stephen’s Green,
Dublin 2, Dublin, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein we describe two coupling reactions of aldehydes and activated alkynes to give chalcones or
alkenyl esters depending on the base used. Chalcones were obtained by activation of alkynes by AuI and
of aldehydes by NHC in presence of triethylamine. Alkenyl esters were obtained via generation of gold
nanoparticles, aerial oxidation of aldehydes to carboxylates and their subsequent nucleophilic addition to
the alkyne functionality.
Received 1 February 2011
Received in revised form 11 May 2011
Accepted 31 May 2011
Available online 13 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
O
O
R2
R3
R1
R3
Chalcones represent a synthetically versatile class of organic
molecules possessing interesting biological properties. Biological
activities comprise antioxidants,1 anti-inflammatory,2 anti-cancer,3
cytotoxic,4 hypoglycaemic5 and antimitotic activities.6 Moreover,
chalcones can be found as intermediates in the synthesis of flavo-
R1
H
R2
1
2
3
Fig. 1. Disconnection proposed for the preparation of chalcones 1.
noids.7,8 The
a,b-unsaturated moiety of chalcones allowed several
transformations, including epoxidations,9,10 Michael additions,11,12
cyclocondensation13 and synthesis of pyrazoline through a modi-
fied reaction of the ‘Huisgen Zwitterion’.14 Although chalcones
could be prepared via the classic ClaiseneSchmidt condensation,
over the last 20 years a number of synthetic methodologies have
been developed, including Suzuki15 and JuliaeKocienski olefina-
tion.16 Our interest in NHC chemistry prompted us to investigate
a modification of the classic Stetter reaction,17 in order to develop
a new synthesis of chalcone derivatives 1 (Fig. 1). It was reasoned
that, while the NHCs chemistry would activate an aldehyde 2
resulting in the generation of a nucleophile,17 a concomitant gold-
catalyzed activation of an electrophilic component, for example, an
a nucleophilic attack. Based on these concepts, a huge variety of
gold-catalyzed reactions have been developed and various exam-
ples are reported in literature.23e28
A tandem reaction involving aldehydes and alkynes was there-
fore planned according to the following principles (Scheme 1): a.
NHC 4 reacts with aldehyde 2 to give a Breslow intermediate 5,
which acts as a nucleophile (Nu);29e35 b. AuI coordinates alkyne 3
generating a more electrophilic species, which could be then
attacked by nucleophile 5.18e22
2. Results and discussion
alkyne 3, could be exploited to build an a,b-unsaturated moiety in
a tandem NHC/Au catalyzed process.
To test our hypothesis, we carried out a simple experiment
heating diphenylacetylene 8 and benzaldehyde 2a under the catal-
ysis of 1,3-bis-(2,4,6-trimethylphenyl)imidazolium chloride (IMes)
6 and AuIII using triethylamine as the base (Table 1, entries 1e3). In
these experiments, IMes 6, aldehyde 2a and triethylamine were
allowed to react in order to engage the NHC in a Breslow in-
termediate and avoiding formation of an NHC gold complex.22
Under these conditions, no formation of chalcone 1a was ob-
served and starting materials 8 and 2a were recovered after 24 h.
We then performed an in depth screening including variation of
temperature, reactants, solvent, base and reaction time. Results
collected with activated alkynes, such as arylpropynoic acid esters
In the last decade there has been a fast growing interest in gold
catalysis. Gold is present in three different oxidative states: Au0, AuI
and AuIII, where the catalytic active species is finally AuI or
AuIII.18e22 Alkynes are the most common substrates used in the gold
catalysis as AuI can activate the
p-system by coordination, allowing
* Corresponding author. Tel.: þ353 1 4022208; fax: þ353 1 4022168; e-mail
address: madamo@rcsi.ie (M.F.A. Adamo).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.05.135

M.F.A. Adamo et al. / Tetrahedron 67 (2011) 5784e5788
5785
however an identified dark solid was found in suspension after
few minutes. The formation of the dark solid was established an
essential requisite for the occurrence of compound 10c. An ex-
planation of this result may involve the formation of Au0 nano-
particles, which arose from reduction of AuI by amines.38,39 The
gold nanoparticles isolated as a powder. In turn, the gold nano-
particles catalyzed the aerobic oxidation of aldehydes to carbox-
ylates. Finally, carboxylates added to alkyne 9 to give compound
10c as observed by Liang.40 We have briefly investigated the ox-
idation of aromatic aldehydes to carboxylic acids under the con-
ditions described in Table 3. This study revealed that aromatic
aldehydes, for example, benzaldehyde 2a, could be efficiently
converted in less than 16 h to benzoic acid, which were isolated
in good to excellent yields.
The structure of alkenyl ester 10c was confirmed by comparison
of its analytical data with the correlated dimethyl ester 11, for
which X-ray diffraction was obtained (Fig. 2).41 The validity of this
new protocol was shown by reacting aldehydes 2aee under similar
conditions to afford alkenyl esters 10aee (Table 3) in good isolated
yields.
O
R1
H
2
R'
N
X
R2
R3
[AuI]
[AuI]
X = N, S
4
3
R'
N
R'
N
OH
R1
OH
R1
X
X
5
R2
R3
Nucleophile
O
Electrophile
R1
R3
R2
1
Scheme 1. Tandem sequential reaction to prepare chalcones 1.
Table 1
Identification of suitable conditions for the NHC/Aux-catalyzed preparation of chalcones 1
N
N
N
N
Cl
NHC catalyst (0.1 equiv.)
Base (0.3 equiv.)
O
H
R
R
6
(1.0 equiv.)
R
R
AuI or AuIII (0.1 equiv.)
Solvent, r.t., 16 h
Ph
Cl
O
2a
8 R= Ph
9 R= CO2Et
1a R= Ph
7
1b R= CO2Et
Entry
NHC
Au
Alkyne
Base
Solvent
Product
Yielda (%)
1
2
3
4
5
6
7
6
6
6
7
7
7
7
AuIII
AuIII
AuIII
AuI
8
8
8
9
9
9
9
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
DCM
Toluene
Et3N
THF
THF
THF
1a
1a
1a
1b
1b
1b
1b
d
d
d
38
5
AuIII
d
d
AuI
DCM
70
a
Isolated yields after column chromatography on silica gel.
arylpropynoic acid esters, arylpropynenitriles or diarylpropynones
gave disappointing results, however, when the more reactive but-
2-ynedioic acid diethyl ester 9 was used as the electrophilic com-
ponent, AuIII was switched to AuI and THF was used as solvent,
chalcone 1b was isolated in 38% yield (Table 1, entry 4). The reaction
was further optimised by replacing IMes 6 with 1,3-diisopropyl
imidazolinium chloride 7 and changing THF with DCM (Table 1,
entry 7).
3. Conclusion
In conclusion, we have shown a conceptually new coupling of
aldehydes and electrophilic alkenes. The process relied on gener-
ation of a nucleophile by addition of NHC to aldehydes and on
enhancement of alkyne electrophilicity furnished by AuI. A modi-
fied procedure of this reaction, e.g., replacement of triethylamine
with DBU, allowed the formation of alkenyl esters in good isolated
yields.
Notably, the reaction did not proceed without AuI (Table 1, entry
6) or without NHC or base. Importantly, studies by Nair36 and Shi37
have established that triethylamine did not catalyse the addition of
electrophilic alkynes to aldehydes under similar conditions, vali-
dating the catalytic cycle proposed for the formation of compound
1c. Other aromatic aldehydes were also submitted to reaction with
alkyne 9. Aldehydes 2bee containing an electron withdrawing
group reacted well under the optimised conditions giving chal-
cones 1cef in good yields (Table 2). Conversely, aldehydes holding
electron donating groups reacted sluggishly furnishing the pinacol
as the main product.
4. Experimental section
4.1. General experimental
Melting points were determined using a Stuart scientific
melting point apparatus and are uncorrected. Infrared spectra
(IR) were recorded as KBr discs using a Bruker Tensor27 FT-IR
instrument. Absorption maximum (nmax) was reported in wave
numbers (cmꢀ1) and only selected peaks are reported. NMR ex-
periments were performed on a Bruker Avance 400 instrument
and samples were obtained in CDCl3 (referenced to 7.26 ppm for
1H and 77.0 for 13C) and in DMSO-d6 (referenced to 2.52 and
During this study, it was noted that replacement of NEt3
with DBU lead to formation of unexpected alkenyl ester 10c
(Table 3). This reaction was set up as a homogeneous mixture,

5786
M.F.A. Adamo et al. / Tetrahedron 67 (2011) 5784e5788
Table 3
Isolated yields for alkenyl esters 10aee, 11
(0.1 equiv.)
DBU (0.3 equiv.)
CO2Et 9 (1 equiv.)
Table 2
Isolated yields for chalcones 1bef
7
H
CO2Et
O
1)
7 (0.1 equiv.)
H
CO2Et
O
O
NEt3 (0.3 equiv.)
DCM, 60 °C, 2 h
EtO2C
EtO2C
EtO2C
R
O
O
AuCl (0.1 equiv.)
R
R
EtO2C
CO2Et
AuCl (0.1 equiv.)
DCM, r.t., 16 h
9 (1.0 equiv.)
2)
1b-f
DCM, r.t., 16 h
2b-e
R
2a-e
10a-e, 11
Entry
1
R
Product
Yielda (%)
70
Entry
R
Product
Yielda (%)
H
CO2Et
H
CO2Et
H
2a
EtO2C
O
EtO2C
O
H
2a
1
40
45
O
1b
10a
10b
H
EtO2C
CO2Et
H
CO2Et
O
Cl
2b
Cl
EtO2C
2
68
70
O
Cl
2b
O
2
1c
Cl
H
CO Et
2
NO2
2c
EtO C
2
NO
2
3
4
H
CO2Et
O
O
EtO2C
1d
O
NO2
2c
3
62
H
CO2Et
NO2
CN
2d
10c
EtO2C
CN
80
49
O
O
1e
H
CO2Et
O
EtO2C
H
CO2Et
CN
2d
O
O
O
4
67
EtO2C
COOCH3
2e
5
CN
10d
1f
a
H
CO2Et
O
Isolated yields after column chromatography on silica gel.
EtO2C
O
COOCH3
2e
3.35 ppm for 1H and 40.0 for 13C). Coupling constants (J) are in
hertz. Multiplicities are reported as follows: s, singlet, d, doublet,
dd, doublets of doublets, t, triplet, q, quartet, m, multiplet, c,
complex, and br, broad. High resolution mass spectra were
obtained on a Waters Micro mass LCT and low resolution mass
spectra were recorded on Waters Micro mass Quattro LCeMS
spectrometers at 70 eV. Tetrahydrofuran was freshly distilled
over sodium benzophenone prior to use according to standard
procedure. All other reagents and solvents were used as pur-
chased from Aldrich. Reactions were checked for completion
by TLC (EM Science, silica gel 60 F254). Flash chromatography
was performed using silica gel 60 (0.040e0.063 mm,
230e400 mesh).
5
43
O
O
10e
H
COOCH3
H3COOC
O
O
NO2
2c
6
58b
NO2
11
a
Isolated yields after column chromatography on silica gel.
Performed by using but-2-ynedioic acid dimethyl ester.
b
4.2.1. 2-Benzoyl-but-2-enedioic acid diethyl ester 1b. Yellow liquid
(193 mg, 70%). Rf¼0.60 (petroleum ether/EtOAc 8:2 v/v); nmax (KBr)/
cmꢀ1 3125, 2978, 1729, 1687, 1520; dH (400 MHz, CDCl3) 7.82e7.37
(m, 5H, Ar), 6.99 (s, 1H, CH), 4.17 (q, J¼7.2, 2H, eOCH2CH3), 3.97 (q,
J¼7.2, 2H, eOCH2CH3), 1.12 (t, J¼7.2, 3H, eOCH2CH3), 0.99 (t, J¼7.2,
3H, eOCH2CH3); dc (100.6 MHz, CDCl3) 192.3, 163.8, 163.2, 142.2,
135.2, 133.8, 130.6, 128.8, 128.2, 62.5, 61.7, 13.9, 13.6; m/z: 276 (45%,
Mꢀ); HRMS: m/z found [Mꢀ] 276.0914, C15H16O5 requires 276.0998.
4.2. General procedure for the preparation of chalcones 1bef
1,3-Diisopropyl imidazolium chloride 7 (19 mg, 0.1 mmol,
0.1 equiv), triethylamine (42 mL, 0.3 mmol, 0.3 equiv) and aldehyde 2
(1 mmol, 1 equiv) were dissolved in DCM (4 mL) and the reaction
mixture was stirred for 2 h at 60 ꢁC. Diethyl acetylene dicarboxylate 9
(170 mL,1 mmol,1 equiv) and AuCl (24 mg, 0.1 mmol, 0.1 equiv) were
then added and the reaction stirred at room temperature for further
16 h. Then the solvent was evaporated and the residue purified by
flash chromatography on silica gel eluting with petroleum ether and
4% EtOAc to give the title compounds 1bef as yellow liquids.
4.2.2. 2-(4-Chloro-benzoyl)-but-2-enedioic acid diethyl ester
1c. Yellow liquid (212 mg, 68%). Rf¼0.65 (petroleum ether/EtOAc
8:2 v/v); nmax (KBr)/cmꢀ1 3130, 2989, 1728, 1680, 1520; dH

M.F.A. Adamo et al. / Tetrahedron 67 (2011) 5784e5788
5787
Fig. 2. Planar and crystal structure of compound 11.
(400 MHz, CDCl3) 7.76e7.35 (m, 4H, Ar), 6.98 (s, 1H, CH), 4.16 (q,
J¼7.2, 2H, eOCH2CH3), 4.00 (q, J¼7.2, 2H, eOCH2CH3), 1.12 (t, J¼7.2,
3H, eOCH2CH3), 1.02 (t, J¼7.2, 3H, eOCH2CH3); dc (100.6 MHz,
CDCl3) 191.1, 163.7, 162.9, 144.8, 140.1, 134.2, 130.8, 130.0, 129.1, 62.5,
61.8, 13.9, 13.7; m/z: 310 (20%, Mꢀ); HRMS: m/z found [Mꢀ]
310.0621, C15H15ClO5 calculated 310.0608.
the reaction mixture stirred for 2 h at room temperature. Diethyl
acetylene dicarboxylate 9 (170 L, 1 mmol, 1 equiv.) and AuCl
m
(24 mg, 0.1 mmol, 0.1 equiv.) were then added and the reaction was
allowed to stir at room temperature for 16 h. The reaction mixture
was concentrated in vacuo and the residue obtained purified by
flash chromatography on silica gel eluting with petroleum ether
and 4% EtOAc to give compound 10.
4.2.3. 2-(4-Nitro-benzoyl)-but-2-enedioic acid diethyl ester 1d. Yellow
liquid (224 mg, 70%); Rf¼0.6 (petroleum ether/EtOAc 8:2 v/v); nmax
(KBr)/cmꢀ1 3113, 2929, 1742, 1694, 1531; dH (400 MHz, CDCl3)
8.27e7.97 (m, 4H, Ar), 7.05 (s, 1H, CH), 4.19 (q, J¼7.2, 2H, eOCH2CH3),
4.03 (q, J¼7.2, 2H, eOCH2CH3), 1.15 (t, J¼7.2, 3H, eOCH2CH3), 1.07 (t,
J¼7.2, 3H, eOCH2CH3); dc (100.6 MHz, CDCl3) 191.0,163.8,162.6,150.6,
144.5,140.1,131.6,129.6,124.1, 62.8, 62.1,14.0,13.8;m/z:320(26%,Mꢀ);
HRMS: m/z found [Mꢀ] 320.0810, C15H15NO7, calculated 321.0849.
4.3.1. 2-Benzoyloxy-but-2-enedioic acid diethyl ester 10a. Yellow
liquid (110 mg, 40%), Rf¼0.55 (petroleum ether/EtOAc 8:2 v/v); nmax
(KBr)/cmꢀ1 1736; dH (400 MHz, DMSO-d6) 8.08e7.61 (m, 5H, Ar),
6.79 (s, 1H, CH), 4.27 (q, J¼6.8, 2H, eOCH2CH3), 4.11 (q, J¼7.2, 2H,
eOCH2CH3), 1.24 (t, J¼7.0, 3H, eOCH2CH3), 1.09 (t, J¼7.2, 3H,
eOCH2CH3); dc (100.6 MHz, CDCl3) 163.7, 162.8, 161.1, 146.7, 134.0,
130.3, 128.6, 128.1, 117.4, 62.6, 61.1, 14.0, 13.9; m/z: 292 (80%, Mþ).
HRMS: m/z found [Mþ] 292.0914, C15H16O6 requires 292.0947.
4.2.4. 2-(4-Cyano-benzoyl)-but-2-enedioicaciddiethylester1e. Yellow
liquid (240 mg, 80%); Rf¼0.55 (petroleum ether/EtOAc 8:2 v/v); nmax
(KBr)/cmꢀ1 3130, 2969, 1710, 1690, 1528; dH (400 MHz, CDCl3)
7.92e7.89 (m, 4H, Ar), 7.03 (s, 1H, CH), 4.18 (q, J¼7.2, 2H, eOCH2CH3),
4.02 (q, J¼7.2, 2H, eOCH2CH3), 1.14 (t, J¼7.2, 3H, eOCH2CH3), 1.06 (t,
J¼7.2, 3H, eOCH2CH3); dc (100.6 MHz, CDCl3) 191.1,163.7,162.6,144.4,
138.6, 132.6, 131.5, 128.9, 117.8, 116.8, 62.7, 62.0, 13.9, 13.7; m/z:
301(36%, Mꢀ); HRMS: m/z found [Mþ] 301.0941, C16H15NO5 calcu-
lated 301.0950.
4.3.2. 1,2-Di(ethoxycarbonyl)vinyl 4-chlorobenzoate 10b. Yellow
liquid (148 mg, 45%); Rf¼0.55 (petroleum ether/EtOAc 8:2 v/v); nmax
(KBr)/cmꢀ11741; dH (400 MHz, DMSO-d6) 8.00e7.70 (m, 4H, Ar), 6.81
(s, 1H, CH), 4.27 (q, J¼7.0, 2H, eOCH2CH3), 4.11 (q, J¼7.0, 2H,
eOCH2CH3), 1.24 (t, J¼7.0, 3H, eOCH2CH3), 1.10 (t, J¼7.0, 3H,
eOCH2CH3); dc (100.6 MHz, DMSO-d6) 163.5, 160.6, 145.7, 139.8,
131.5, 130.3, 128.9, 128.6, 117.8, 61.8, 60.1, 14.0, 13.5; m/z: 292 (65%,
Mþ). HRMS: m/z found [Mþ] 326.0517, C15H15ClO6 requires 326.0557.
4.2.5. 2-(4-Methoxycarbonyl-benzoyl)-but-2-enedioic acid diethyl
ester 1f. Yellow liquid (164 mg, 49%); Rf¼0.52 (petroleum ether/
EtOAc 8:2 v/v); nmax (KBr)/cmꢀ1 3122, 2919, 1722, 1695, 1598; dH
(400 MHz, CDCl3) 8.08e7.87 (m, 4H, Ar), 7.02 (s, 1H, CH), 4.18 (q,
J¼7.2, 2H, eOCH2CH3), 4.00 (q, J¼7.2, 2H, eOCH2CH3), 3.89 (s, 3H,
OCH3), 1.13 (t, J¼7.2, 3H, eOCH2CH3), 1.02 (t, J¼7.2, 3H, eOCH2CH3);
dc (100.6 MHz, CDCl3) 191.8, 166.1, 163.7, 162.9, 144.8, 138.8, 134.4,
131.1, 130.0, 128.5, 62.6, 61.9, 52.5, 13.9, 13.7; m/z: 334 (24%, Mꢀ).
HRMS: m/z found [Mþ] 334.1040, C17H18O7 calculated 334.1053.
4.3.3. 1,2-Di(ethoxycarbonyl)vinyl 4-nitrobenzoate 10c. Yellow liq-
uid (209 mg, 62%); Rf¼0.52 (petroleum ether/EtOAc 8:2 v/v); nmax
(KBr)/cmꢀ1 1739; dH (400 MHz, DMSO-d6) 8.45e8.31 (m, 4H, Ar),
6.86 (s, 1H, CH), 4.29 (q, J¼7.2, 2H, eOCH2CH3), 4.12 (q, J¼7.2, 2H,
eOCH2CH3), 1.25 (t, J¼7.0, 3H, eOCH2CH3), 1.11 (t, J¼7.0, 3H,
eOCH2CH3); dc (100.6 MHz, CDCl3) 162.6, 162.0, 160.8, 151.1, 146.4,
133.7, 131.6, 123.8, 118.0, 62.9, 61.4, 14.1, 14.0; m/z: 337 (90%, Mþ).
HRMS: m/z found [Mþ] 337.0762, C15H15NO8 requires 337.0798.
4.3.4. 1,2-Di(ethoxycarbonyl)vinyl 4-cyanobenzoate 10d. Yellow
liquid (212 mg, 67%); Rf¼0.5 (petroleum ether/EtOAc 8:2 v/v); nmax
(KBr)/cmꢀ1 1729; dH (400 MHz, DMSO-d6) 8.23e8.10 (m, 4H, Ar),
6.84 (s, 1H, CH), 4.28 (q, J¼7.2, 2H, eOCH2CH3), 4.12 (q, J¼7.2, 2H,
eOCH2CH3), 1.24 (t, J¼7.0, 3H, eOCH2CH3), 1.10 (t, J¼7.0, 3H,
eOCH2CH3); dc (100.6 MHz, DMSO-d6) 162.1, 162.0, 160.3, 145.4,
133.2, 131.2, 130.49, 117.9, 117.8, 116.7, 62.6, 61.1, 13.7, 13.7; m/z: 337
4.3. General procedure for the preparation of alkenoic acid
esters 10aee
To a solution of 1,3-diisopropyl imidazolium chloride 7 (19 mg,
0.1 mmol, 0.1 equiv.) in DCM (4 mL), were sequentially added DBU
(45 mL, 0.3 mmol, 0.3 equiv.) and aldehyde 2 (0.1 mL, 1 mmol) and

5788
M.F.A. Adamo et al. / Tetrahedron 67 (2011) 5784e5788
(90%, Mþ). HRMS: m/z found [Mþ] 317.0863, C16H15NO6 requires
317.0899.
5. Satyanarayana, M.; Tiwari, P.; Tripathi, B. K.; Srivastava, A. K.; Pratap, R. Bioorg.
Med. Chem. 2004, 12, 883.
6. Yang, J. W.; Hechavarria Fonseca, M. T.; Vignola, N.; List, B. Angew. Chem., Int. Ed.
2005, 44, 108.
7. Prilezhaeva, E. N. Russ. Chem. Rev. 2000, 69, 367.
8. Bellingham, R.; Jarowicki, K.; Kocienski, P. V. M. Synthesis 1996, 285.
4.3.5. 1,2-Di(ethoxycarbonyl)vinyl methyl terephthalate 10e. Yellow
liquid (158 mg, 43%); Rf¼0.5 (petroleum ether/EtOAc 8:2 v/v); nmax
(KBr)/cmꢀ1 1732; dH (400 MHz, DMSO-d6) 8.19e8.18 (m, 4H, Ar),
6.84 (s, 1H, CH), 4.28 (q, J¼7.2, 2H, eCH2CH3), 4.11 (q, J¼7.2, 2H,
eOCH2CH3), 3.92 (s, 3H, OCH3) 1.24 (t, J¼7.0, 3H, eOCH2CH3), 1.09
(t, J¼7.0, 3H, eOCH2CH3); dc (100.6 MHz, DMSO-d6) 165.3, 162.6,
162.2, 160.4, 145.5, 134.6, 131.2, 130.3, 129.9, 129.8, 117.8, 62.6, 61.1,
52.7, 52.6, 13.7, 13.7; m/z: 350 (50%, Mþ). HRMS: m/z found [Mþ]
350.1010, C17H18O8 requires 350.1003.
ꢀ
ꢀ
ꢀ
9. Bako, T.; Bako, P.; Keglevich, G.; Bombicz, P.; Kubinyi, M.; Pal, K.; Bodor, S.;
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Mako, A.; Toke, L. Tetrahedron: Asymmetry 2004, 15, 1589.
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15, 3751.
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1997, 62, 6210.
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4.3.6. 1,2-Di(methoxycarbonyl)vinyl 4-nitrobenzoate 11. Colourless
solid, mp 105e110 ꢁC (180 mg, 58%); Rf¼0.45 (petroleum ether/
EtOAc 8:2 v/v); nmax (KBr)/cmꢀ11729,1525; dH (400 MHz, DMSO-d6)
8.45e8.43 (m, 4H, Ar), 6.90 (s, 1H, CH), 3.83 (s, 3H, OCH3), 3.68
(s, 3H, OCH3); dc (100.6 MHz, DMSO-d6) 162.6, 161.7, 160.8, 151.0,
145.4, 132.5, 131.4, 124.3, 117.7, 53.6, 52.3. m/z: 309 (55%, Mþ).
HRMS: m/z found [Mþ] 309.0463, C13H11NO8 requires 309.0485.
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Acknowledgements
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We acknowledge the PTRLI cycle III and IRCSET for a grant to
MFAA. We also like to thank SFI grant RFP2007 CHEF839 for mu-
nificent support and precious advice.
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Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2011.05.135.
ꢀ
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