AlCl3 Catalyzed Oxo-Diels-Alder and Wittig-Type Reaction of 2,3-Dimethyl-1,3-Butadiene with Acyl Phosphonates

Article abstract of DOI:10.1080/10426507.2014.884089

Different aliphatic and aromatic acyl phosphonates were synthesized using Arbuzov reaction and subjected to Oxo-Diels-Alder (ODA) reaction with 2,3-dimethyl-1,3-butadiene at-78°C. AlCl3 was found the best catalyst to obtain the desired ODA products from a

Full text of DOI:10.1080/10426507.2014.884089

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Phosphorus, Sulfur, and Silicon and the
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AlCl Catalyzed Oxo-Diels-Alder
3
and Wittig-Type Reaction of 2,3-
Dimethyl-1,3-Butadiene With Acyl
Phosphonates
ab
a
S. Nadeem & Ayhan S. Demir
a
Department of Chemistry, Middle East Technical University, 06531
Ankara, Turkey
b
International Center for Chemical and Biological Sciences, HEJ
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Research Institute of Chemistry, University of Karachi, Karachi
75270, Pakistan
Accepted author version posted online: 30 Apr 2014.Published
online: 21 Oct 2014.
To cite this article: S. Nadeem & Ayhan S. Demir (2014) AlCl Catalyzed Oxo-Diels-Alder and Wittig-
3
Type Reaction of 2,3-Dimethyl-1,3-Butadiene With Acyl Phosphonates, Phosphorus, Sulfur, and Silicon
and the Related Elements, 189:11, 1603-1610, DOI: 10.1080/10426507.2014.884089
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Phosphorus, Sulfur, and Silicon, 189:1603–1610, 2014
Copyright ꢀ
Taylor & Francis Group, LLC
C
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2014.884089
AlCl CATALYZED OXO-DIELS-ALDER AND WITTIG-TYPE
3
REACTION OF 2,3-DIMETHYL-1,3-BUTADIENE WITH ACYL
PHOSPHONATES
1
,2
1
S. Nadeem and Ayhan S. Demir
1
Department of Chemistry, Middle East Technical University, 06531
Ankara, Turkey
International Center for Chemical and Biological Sciences, HEJ Research
2
Institute of Chemistry, University of Karachi, Karachi 75270, Pakistan
GRAPHICAL ABSTRACT
Abstract Different aliphatic and aromatic acyl phosphonates were synthesized using Arbuzov
reaction and subjected to Oxo-Diels-Alder (ODA) reaction with 2,3-dimethyl-1,3-butadiene
◦
at −78 C. AlCl3 was found the best catalyst to obtain the desired ODA products from acyl
◦
phosphonates in only 30 min at low temperature, as compared to 100 C—previously reported.
The same reaction was repeated at room temperature but, instead of ODA, Wittig-type product
was obtained. All the products were characterized by NMR spectroscopy and mass spectrometry.
1
31
13 31
H, P and C, P couplings were observed in the NMR spectra.
Keywords Oxo-Diels-Alder; Arbuzov reaction; Wittig reaction; acyl phosphonates
INTRODUCTION
Acyl phosphonates (ACPS) are potent acyl anion precursors. They generate acyl anion
equivalents in the presence of cyanide anion via phosphonate-phosphate rearrangement.
These anions readily attack activated ketones, which result in the formation of acyloin-
1
type coupling products. ACPS are useful reagents in cross-benzoin reactions with certain
advantages: they are easy to prepare from carboxylic acids or aldehydes and are stable
2
–5
and reactive under a variety of conditions. The formation of acyl anions from ACPS is
6
very similar to that from acyl silanes. ACPS are normally synthesized through Arbuzov
7
,8
reaction. In a typical reaction, acyl chlorides are reacted with trialkyl phosphites, which
Received 9 April 2013; accepted 9 November 2013.
Address correspondence to S. Nadeem, Department of Chemistry, Middle East Technical University, 06531
Ankara, Turkey. E-mail: said81nadeem@yahoo.com
1
603

1604
S. NADEEM AND A. S. DEMIR
Table 1 Substituents R used in the reaction with diene 1 and yields of the products obtained
R
Product (3)
Yield (%)
70
3
a
3
3
3
3
b
73
58
61
84
c
d
e
produce ACPS as the main product, while alkyl chlorides are formed as the side product.
ACPS are mostly greenish liquids, which are distilled at reduced pressure with the help of
high-vacuum pumps connected to a glass assembly. Our group is already involved in ACPS
1
1,12
chemistry
and successfully employed acyl phosphonates in Hetero-Diels-Alder (HDA)
reactions as dienophile partner.

AlCl3 CATALYZED OXO-DIELS-ALDER AND WITTIG-TYPE REACTION
1605
Our group already reported the thermally activated HDA reaction between
benzoylphosphonate and 1-methoxy-3-trimethylsilyloxy-1,3-butadiene (Danishefsky’s
7
◦
diene). The reactants were mixed and heated up to 60 C, but no HDA product was
observed. The desired glycosyl-type phosphonates were obtained in 89% yield when the
◦
temperature was increased to 100 C. Here, we report the Oxo-Diels-Alder (ODA) reaction
of different ACPS with 2,3-dimethyl-1,3-butadiene at low temperatures in the presence of
AlCl3 as catalyst. Thus, we were able to perform ODA reaction of ACPS as dienophile
and 2,3-dimethyl-1,3-butadiene as diene at mild conditions, i.e., at low temperature. It
is well established that at low temperature, chemical reactions show high selectivity.
ACPS were also reacted with 2,3-dimethyl-1,3-butadiene in the presence of AlCl3 at room
temperature in dichloromethane. Interestingly, Wittig-type products were obtained instead
of ODA-type products. The oxygen atom of the carbonyl group was replaced by a CH2
moiety. We propose that the CH2 moiety originates from dichloromethane (solvent), but
to investigate our assumption, a separated project has been started.
RSULTS AND DISCUSSION
Different acyl phosphonates were synthesized according to literature procedures
utilizing Arbuzov reaction. In this reaction, the phosphorus atom of the phosphite molecule
attacks the acyl carbon atom with chloride acting as a leaving group. The chloride anion
subsequently attacks the methyl carbon atom, which results in the formation of methyl
◦
chloride. Due to the low boiling point of methyl chloride (−23.8 C), it is a gas at room
temperature and develops pressure in the vessel during the reaction. To avoid explosion in
this reaction, continues argon supply and an exhaust line is necessary. The formation of
ACPS 3 can be observed from its color change from colorless to greenish.
Oxo-Diels-Alder Reactions
The acyl phosphonates 1 were subjected to ODA reactions with 2,3-dimethyl-1,3-
butadiene (2). In a typical reaction, the phosphonate 1 was transferred into a dry flask filled
with argon and sealed with a rubber septum. Dichloromethane was added with a syringe
◦
and the temperature of the solution was lowered to −78 C using dry ice in acetone in a
thermostat. Dry aluminum chloride was added to the flask while flushing argon. Butadiene
2
was added at the same temperature dropwise with continuous stirring. After the addition
was completed, the reaction mixture was stirred for half an hour and allowed to warm
to room temperature. The reaction mixture was poured into distilled water and stirred.
The crude product 3 was extracted with diethyl ether. The ODA products showed special
behavior on the TLC. After developing, TLC was dipped into 4-methoxybenzaldehyde and
heated with a heat gun—dark blue spot appeared on TLC plate.
AlCl3 was used to increase the polarity of the carbonyl double bond. The benzoyl
phosphonates 1 were allowed to react with 2 in dichloromethane as shown in Scheme 1.
The crude product was distilled in high vacuum and the pure product was investigated
1
by H NMR spectroscopy to confirm the formation of the ODA product.
Wittig-Type Reaction
The same reactants were used and same reaction conditions were applied in this
reaction except that the reaction was performed at room temperature. In a typical reaction,

1606
S. NADEEM AND A. S. DEMIR
Scheme 1 ODA of phosphonates with 2,3-dimethyl-1,3-butadiene
phosphonate 1 (R Ph) and 2,3-dimethyl-1,3-butadiene 2 were added to the reaction tube.
AlCl3 was added to the reaction mixture under argon (Scheme 2). After 1 h stirring, the
reaction mixture was poured into dilute HCl and stirred. The organic phase was extracted
with diethyl ether and purified chromatographically. The main product obtained was sub-
jected to NMR analysis. ODA product was expected but no NMR signals for the methyl and
1
CH2 groups were observed. Instead, H NMR signals for two protons were observed in
the form of doublet of doublets with high P,H coupling constants of 22.1 and 45.7 Hz. The
doublet of each proton was further split into a doublet with coupling constant of 1.3 Hz. So
obviously, a Wittig-type reaction occurred and the oxygen atom of the carbonyl group has
been exchanged with by CH2 moiety as shown in Scheme 2.
Scheme 2 Wittig-type reaction of phosphonate with 2,3-dimethyl-1,3-butadiene
Most probably, the CH2 group originates from dichloromethane used in the reaction
as solvent. A separate project has been started to put light on the mechanism of the reaction.
If we would be able to prove that it will be a cheap and easy reagent to replace Wittig reagent.
CONCLUSION
Oxo-Diels-Alder reaction was performed on different acyl phosphonates with 2,3-
◦
dimethyl-1,3-butadiene at −78 C. This temperature used is much lower as compared to that
◦
of 100 C reported previously. AlCl3 was used as a catalyst in ODA reaction. When the same
reaction was performed at room temperature, instead of ODA product a Wittig-type product
was obtained, where the oxygen atom of the carbonyl group is replaced by CH2 moiety.
We propose that the CH2 moiety originates from the solvent dichloromethane—similar to
Friedel-Craft reaction. Studies are underway to prove our proposed mechanism. This will

AlCl3 CATALYZED OXO-DIELS-ALDER AND WITTIG-TYPE REACTION
1607
open new ways to perform Wittig reaction at mild conditions, using less expense reagents
and a cheap catalyst instead of Wittig reagent.
EXPERIMENTAL
All the reactions were performed according to standard methods with clean glassware
and dry solvents. Trimethyl phosphite, AlCl3, and all solvents used were purchased from
Merck. Compounds were purified by column chromatography using flash silica. All acyl
phosphonates were synthesized using Arbuzov reaction according to the literature proce-
8
dure. Freshly distilled solvents were used in all reactions. Dry AlCl3 was handled in a
dry box. Reactions were performed under argon. Characterization of the pure products was
done by different techniques including EIMS (JEOL JMS600), NMR (Bruker Avance 400,
Avance AV 300), and TLC.
Synthesis of Phosphonates (1): General Procedure
In a typical reaction, 1.24 g trimethyl phosphite (10 mmol) was added dropwise to
◦
1
.4 g benzoyl chloride (10 mmol) at 0 C—under argon, no solvent and stirred constantly
for 30 min. The flask was sealed with a rubber septum. As a byproduct, methyl chloride
gas) is produced during the reaction that develops pressure. An injection needle was
(
put throughout the septum to release that pressure. Color of reaction mixture changed to
green which is the indication of phosphonate formation. 30 min later, the reaction mixture
was allowed to reach room temperature. The crude liquid was subjected to high vacuum
distillation. Until the green product condensed, predistillate—white in color and contained
mostly remaining benzoyl chloride—were collected and discarded. The green product was
◦
collected till the temperature was constant at 100 C. The green compound was subjected
NMR, which showed clear spectrum without any impurity peaks. Different aromatic and
aliphatic acyl phosphonates were synthesized by this way.
Oxo-Diels-Alder Reaction: General Procedure
A model reaction procedure for ODA is given below:
Dimethyl (4,5-dimethyl-2-phenyl-3,6-dihydro-2H-pyran-2-yl)
phosphonate (3a)
To an oven-dried Schlenk tube equipped with a dried magnet stick, filled with argon,
and sealed with a rubber septum, 1-mL-dried DCM was added by a syringe followed by
the addition of 150 mg (0.7 mmol) of benzoyl phosphonate. 3.52 mg (1.1 equiv.) of diene
◦
2
(
was added by a syringe and the reaction mixture was stirred for 5 min at −78 C. 111 mg
1.2 equiv.) of AlCl3 was added while passing argon at the same temperature. The tube was
sealed again under argon and after stirring for 10 min, the reaction mixture was warmed
◦
to 0 C (ice bath) and stirred. After 1 h, the reaction mixture was poured into 20 mL of
distilled water and vigorously stirred. The crude product 5 was extracted with 3 × 10 mL
of diethyl ether. The organic layer was dried with Na2SO4 and the solvent was evaporated.
On TLC, the product showed bluish color when dipped in 4-methoxybenzaldehyde and
heated. The product was purified by column chromatography using 30% ethyl acetate in
hexane as eluent. The solvent was evaporated and the compound was dried in high vacuum.

1608
S. NADEEM AND A. S. DEMIR
1
45 mg of the product was obtained as slight yellow viscous liquid corresponding to 70%
1
yield. EI-MS: m/z = 296.3. H NMR (400 MHz, CDCl3): δ = 7.52 (m, 2 H), 7.37 (m, 2H),
7
.3 (m, 1H), 4.0 (d, J = 16.0 Hz, 1H), 3.87 (d, J = 16.0 Hz, 1H), 3.69 (d, J = 8.0 Hz, 3H,
OCH3), 3.61 (d, J = 8.0 Hz, 3H, OCH3), 2.98 (t, J = 16.0 Hz, 1H), 2.70 (d, J = 16.0 Hz,
13
1
1
1
H), 1.70 (s, 3H, CH3), 1.35 (s, 3H, CH3). C NMR (100 MHz, CDCl3): δ = 135.9, 129.9,
28.2 (d, JPC = 3 Hz), 127.9 (d, JPC = 2.9 Hz), 127.8 (d, JPC = 4.4 Hz), 123.6 (d, JPC =
.5 Hz), 121.0 (d, JPC = 10.9 Hz), 64.6 (d, JPC = 10.9 Hz), 54.1 (d, JPC = 6.5 Hz), 53.9
(d, JPC = 7.3 Hz), 32.7, 18.7, 13.8.
Dimethyl (2-(4-chlorophenyl)-4,5-dimethyl-3,6-dihydro-2H-pyran-2-yl)
phosphonate (3b)
1
39 mg (70%) of 3b was obtained as slight yellow viscous liquid. EI-MS: m/z =
1
3
4
3
(
1
2
30.08. H NMR (400 MHz, CDCl3): δ = 7.37 (dd, J = 12.0 Hz, J = 4.0 Hz, 2H), 7.27
3
3
d, J = 8.0 Hz, 2H), 3.93 (d, J = 16.0 Hz, 1H), 3.74 (d, J = 16.0 Hz, 1H), 3.64 (dd, JPH =
3
0.6 Hz, 3H, OCH3), 3.60 (d, JPH = 10.3 Hz, 3H, OCH3), 2.89 (t, JPH = 16.1 Hz, 1H),
13
.55 (d, J = 16.0 Hz, 1H), 1.62 (s, 3H, CH3), 1.28 (s, 3H, CH3). C NMR (100 MHz,
CDCl3): δ = 134.7, 133.9 (d, JPC = 3.6 Hz), 131.3, 129.3 (d, JPC = 5.1 Hz), 128.4 (d, JPC =
2
(
.9 Hz), 123.7 (d, JPC = 1.5 Hz), 120.9 (d, JPC = 10.9 Hz), 64.6 (d, JPC = 10.9 Hz), 54.2
d, JPC = 6.6 Hz), 54.0 (d, JPC = 7.3 Hz), 32.7, 18.7, 13.7.
Dimethyl (2-(4-methoxyphenyl)-4,5-dimethyl-3,6-dihydro-2H-pyran-2-yl)
phosphonate (3c)
1
16 mg (58%) of 3c was obtained as slight yellow viscous liquid. EI-MS: m/z =
1
3
4
3
(
1
26.13. H NMR (400 MHz, CDCl3): δ = 7.35 (dd, J = 8.8 Hz, J = 2.3 Hz, 2H), 6.84
3
d, J = 9.0 Hz, 2H), 3.89 (d, J = 15.6 Hz, 1H), 3.74 (s, 3H, OCH3), 3.73 (d, J = 16.0 Hz,
3
3
H), 3.61 (d, JPH = 10.3 Hz, 3H, OCH3), 3.57 (d, JPH = 10.3 Hz, 3H, OCH3), 2.88
(
t, JPH = 15.0 Hz, 1H), 2.57 (d, J = 16.8 Hz, 1H), 1.63 (s, 3H, CH3), 1.29 (s, 3H, CH3).
1
3
C NMR (100 MHz, CDCl3): δ = 159.3, 149.6, 134.5, 129.5, 121.2 (d, JPC = 10.9 Hz),
1
5
23.7, 120.9 (d, JPC = 10.9 Hz), 113.7 (d, JPC = 2.9 Hz), 64.4 (d, JPC = 10.9 Hz), 55.2,
4.1 (d, JPC = 7.3 Hz), 53.8 (d, JPC = 7.3 Hz), 32.6, 18.6, 13.8.
Dimethyl (2-(4-fluorophenyl)-4,5-dimethyl-3,6-dihydro-2H-pyran-2-yl)
phosphonate (3d)
1
24 mg (61%) of 3d was obtained as slight yellow viscous liquid. EI-MS: m/z =
1
3
1
1
14.11; H NMR (400 MHz, CDCl3): δ = 7.4 (m, 2H), 6.98 (m, 2H), 3.92 (d, J = 15.6 Hz,
3
3
H), 3.73 (d, J = 16.0 Hz, 1H), 3.63 (d, JPH = 10.2 Hz, 3H, OCH3), 3.59 (d, JPH =
0.2 Hz, 3H, OCH3), 2.90 (t, JPH = 15.6 Hz, 1H), 2.57 (d, J = 16.0 Hz, 1H), 1.63 (s, 3H
13
CH3), 1.29 (s, 3H CH3). C NMR (100 MHz, CDCl3): δ = 163.7 (d, JPC = 3.7 Hz), 161.2
13
19
(
2
(
d, JPC = 3.7 Hz), 132.4 (d, JPC = 5.0 Hz), 129.7 (d, JCF = 4.4 Hz, C, F), 123.7 (d, JPC =
.2 Hz), 120.9 (d, JPC = 10.9 Hz), 113.7 (d, JFC = 2.9 Hz), 64.6 (d, JPC = 13.4 Hz), 54.1
d, JPC = 7.3 Hz), 53.9 (d, JPC = 7.3 Hz), 32.8, 18.6, 13.7.

AlCl3 CATALYZED OXO-DIELS-ALDER AND WITTIG-TYPE REACTION
1609
Dimethyl (4,5-dimethyl-2-(4-(trifluoromethyl)phenyl)-3,6-dihydro-2H-
pyran-2-yl)phosphona-te (3e)
1
63 mg (84%) of 3e was obtained as slight yellow viscous liquid. EI-MS: m/z =
1
3
2
64.11; H NMR (400 MHz, CDCl3): δ = 8.21 (d, J = 8.0 Hz, 2H), 7.73 (d, J = 8.3 Hz,
3
H), 4.05 (d, J = 15.8 Hz, 1H), 3.84 (d, J = 16.0 Hz, 1H), 3.75 (d, JPH = 10.3 Hz, 3H,
OCH3), 3.69 (d, JPH = 10.3 Hz, 3H, OCH3), 3.01 (t, JPH = 13.8 Hz, 1H), 2.69 (d, J =
13
1
1
(
(
7.0 Hz, 1H), 1.71 (s, 3H, CH3), 1.37 (s, 3H, CH3). C NMR (100 MHz, CDCl3): δ =
42.0, 132.8, 132.7 (d, JPC = 9.5 Hz), 129.7 (d, J = 4.4 Hz), 123.7 (d, JPC = .2 Hz), 120.9
d, JPC = 10.9 Hz), 115.7 (d, JPC = 22.0 Hz), 115.5, 68.6, 63.6 (d, JPC = 13.0 Hz), 53.3
d, JPC = 7.6 Hz), 53.0 (d, JPC = 7.5 Hz), 31.7, 19.3, 13.9,
Wittig-Type Reaction
Dimethyl (1-phenylvinyl)phosphonate (4). To an oven-dried Schlenk tube
equipped with a dried magnet stick, filled with argon, and sealed with a rubber sep-
tum, 1 mL of dried dichloromethane was added with a syringe. 150 mg (0.7 mmol) of
phosphonate 3 and 87 μL (1.1 equiv.) of butadiene 2 were added to the reaction tube
and the mixture was stirred for 5 min at room temperature. 111 mg (1.2 equiv.) of AlCl3
was added while passing argon. The reaction tube was sealed again with a rubber septum
while flushing argon. The reaction mixture was stirred for 1 h and then was poured in
20 mL of 0.5 M HCl solution and stirred vigorously. The crude product was extracted
with 3 × 10 mL of diethyl ether. The organic layer was washed with 0.1 M NaOH
solution. The organic layer was dried with Na2SO4 and the solvent was evaporated by
rotary evaporator. The product was purified by column chromatography using 5% ethyl
acetate in hexane as eluent. 97 mg (65%) of 4 was obtained as colorless liquid. EI-
1
3
4
MS: m/z = 212.13. H NMR (400 MHz, CDCl3): δ = 7.47 (dd, J = 8.5 Hz, J =
3
4
3
1
.5 Hz, 2H) 7.34 (dd, J = 8.5 Hz, J = 1.5 Hz, 2H), 6.34 (dd, J = 22.1 Hz, J =
3
1
.26 Hz, 1H, vinyl-H anti to P), 6.17 (dd, J = 45.7 Hz, J = 1.26 Hz, 1H, vinyl-H syn
3
13
to P), 3.76 (d, JPH = 11.0 Hz, 6H, OCH3), 3.74 (s, 3H, OCH3). C NMR (400 MHz,
CDCl3): δ = 138.6, 136.8, 135.0 (d, JPC = 46.7 Hz), 134.5 (d, JPC = 5.8 Hz), 132.5 (d,
JPC = 32.1 Hz), 128.8, 128.7, 52.8 (d, JPC = 23.4 Hz, P OCH3).
ACKNOWLEDGMENTS
The authors are thankful to the TUBITAK (The Scientific and Technological Research
Council of Turkey) for financial support (Program# 2216) and Middle East Technical
University, Ankara, Turkey.
SUPPLEMENTAL MATERIAL
Supplementary data for this article can be accessed on the publisher’s website,
www.tandfonline.com/gpss
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