On the hydrolysis of diethyl 2-(perfluorophenyl)malonate

Article abstract of DOI:10.3762/BJOC.16.153

Diethyl 2-(perfluorophenyl)malonate was synthesized in 47% isolated yield by the reaction of sodium diethyl malonate and hexafluorobenzene. The resulting compound was considered as a starting material for synthesizing 2-(perfluorophenyl)malonic acid by hydrolysis. It was found that the desired 2-(perfluorophenyl)malonic acid could not be obtained from this ester by hydrolysis, neither under basic nor under acidic conditions. Nevertheless, hydrolysis of the ester with a mixture of HBr and AcOH gave 2-(perfluorophenyl)acetic acid in a good preparative yield of 63%. A significant advantage of this new approach to 2-(perfluoro-phenyl)acetic acid is that handling toxic substances such as cyanides and perfluorinated benzyl halides is avoided.

Full text of DOI:10.3762/BJOC.16.153

On the hydrolysis of diethyl 2-(perfluorophenyl)malonate
Ilya V. Taydakov*1,2 and Mikhail A. Kiskin3
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 1863–1868.
1P.N. Lebedev Physical Institute of the Russian Academy of
doi:10.3762/bjoc.16.153
Sciences. Leninskiy prospect, 53, Moscow, GSP-1, 119991, Russian
Federation, 2G.V. Plekhanov Russian University of Economics,
Stremyanny per. 36, Moscow, 117997, Russian Federation and 3N.S.
Kurnakov Institute of General and Inorganic Chemistry, Russian
Academy of Sciences, Leninskiy prospect, 31, Moscow, GSP-1,
Received: 19 May 2020
Accepted: 13 July 2020
Published: 28 July 2020
119991, Russian Federation
Associate Editor: B. Stoltz
Email:
© 2020 Taydakov and Kiskin; licensee Beilstein-Institut.
License and terms: see end of document.
Ilya V. Taydakov* - taydakov@gmail.com
*
Corresponding author
Keywords:
decarboxylation; fluorinated aromatic compounds; hydrolysis of
esters; 2-(perfluorophenyl)acetic acid; 2-(perfluorophenyl)malonic acid
Abstract
Diethyl 2-(perfluorophenyl)malonate was synthesized in 47% isolated yield by the reaction of sodium diethyl malonate and hexa-
fluorobenzene. The resulting compound was considered as a starting material for synthesizing 2-(perfluorophenyl)malonic acid by
hydrolysis. It was found that the desired 2-(perfluorophenyl)malonic acid could not be obtained from this ester by hydrolysis,
neither under basic nor under acidic conditions. Nevertheless, hydrolysis of the ester with a mixture of HBr and AcOH gave
2
-(perfluorophenyl)acetic acid in a good preparative yield of 63%. A significant advantage of this new approach to 2-(perfluoro-
phenyl)acetic acid is that handling toxic substances such as cyanides and perfluorinated benzyl halides is avoided.
Introduction
2
-Phenylmalonic acid (1) and its esters are useful and versatile other properties can be modified in a broad range by fluori-
intermediates in the synthesis of many practically important nation (directly or indirectly) of initial organic molecules. In the
compounds, e.g., pharmacologically active substances [1-5], in ongoing project, we are interested in synthesizing 2-(per-
asymmetric synthesis and catalysis [6-8], as precursors of fluorophenyl)malonic acid (2, Figure 1) as a new ligand for the
heterocyclic compounds [9-11], as ligands in coordination preparation of 3d and 4f heterometallic coordination com-
chemistry [12-16] and for other applications [17,18]. Incorpora- pounds.
tion of fluorine atoms into an organic molecule is known to be
one of the most powerful tools for fine tuning of chemical and Results and Discussion
physical properties [19,20]. For example, thermal stability, To our surprise, 2-(perfluorophenyl)malonic acid (2) was not
solubility, metabolic and oxidative stability, lipophilicity, mem- described in the literature to date. The only reference to this
brane permeability, complexing ability, acid-base, and many compound actually concerned the formation of the anion from
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Beilstein J. Org. Chem. 2020, 16, 1863–1868.
fied by Vlasov et al. [26], then NaH was replaced [27] by an-
hydrous K2CO3 and the reaction temperature was decreased
[
28] to 60 °C (Scheme 2). Although the latter method claimed
to give diethyl 2-(perfluorophenyl)malonate (3) in 92% yield, it
can hardly be scaled up due to the utilization of gradient column
chromatography for the separation of the desired product. We
obtained diethyl 2-(perfluorophenyl)malonate (3) in 47% isolat-
ed yield by a modified method [26]. The product was isolated
on 150 mmol scale by simple vacuum distillation.
Figure 1: Phenylmalonic acids.
diethyl 2-(perfluorophenyl)malonate (3) with excess NaH [21]. Hydrolysis of diethyl 2-(perfluorophenyl)malonate (3) unex-
At the same time, diethyl 2-(perfluorophenyl)malonate (3) is a pectedly turned out to be quite challenging. Unsubstituted
readily accessible compound. Diethyl 2-phenylmalonate (4) is diethyl or dimethyl 2-phenylmalonate can be readily hydro-
usually obtained (Scheme 1) by condensation of ethyl phenyl- lyzed under basic conditions by heating with aqueous or mixed
acetate (5) and diethyl carbonate under basic conditions [22] water–EtOH solutions [29] or in biphasic water–Et2O mixtures
because of the low reactivity of bromobenzene in noncatalytic [30] under reflux conditions.
nucleophilic reactions with sodium salts of diethyl malonate (6)
[
23,24].
An exhaustive analysis of the literature revealed that virtually
the same conditions were suitable for the hydrolysis of diethyl
In contrast with bromobenzene, hexafluorobenzene (7) is suffi- 2‐(2,6‐difluoro‐4‐methoxyphenyl)malonate (8) [31], dimethyl
ciently reactive in nucleophilic substitution reactions, and thus 2-(2-fluorophenyl)-2-methylmalonate (9) [32], diethyl 2-(3-
the synthesis of 2-(perfluorophenyl)malonate (3) is straightfor- fluorophenyl)-2-methylmalonate (10) [33], and diethyl 2-(4-
ward. A synthesis of diethyl 2-(perfluorophenyl)malonate (3) fluorophenyl)malonate (11) [6,32] (Figure 2) with minor varia-
was first described in patent literature [25]. The target com- tions in the alkali concentration and temperature. No nucleo-
pound was obtained by the reaction of diethyl malonate, NaH philic substitution of activated fluorine atoms or other side reac-
and C6F6 in DMF for 5 h under reflux. This method was modi- tions were observed under the conditions mentioned above.
Scheme 1: Synthesis of diethyl 2-phenylmalonate (4).
Scheme 2: Synthesis of diethyl 2-(perfluorophenyl)malonate (3).
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Beilstein J. Org. Chem. 2020, 16, 1863–1868.
Figure 2: Esters of fluorine-substituted 2-phenylmalonic acids.
In preliminary experiments, we tested the mildest system – induce a reaction. Different reaction conditions and bases were
biphasic 10% KOH aqueous solution–Et2O at reflux tempera- used, and the results are summarized in Table 1.
ture (35 °C) and vigorous agitation for the hydrolysis of diethyl
2
-(perfluorophenyl)malonate (Scheme 3)
In general, one can conclude that under mild reaction condi-
tions (room temperature, biphase mixtures) the starting ester
remained intact, while under drastic conditions (high concentra-
tion of alkali, homogeneous solutions, elevated temperatures),
decomposition and/or decarboxylation occurred. However, it is
possible that decarboxylation took place during the isolation of
the free acid. In all cases, no desired malonic acid 2 was isolat-
ed from the reaction mixtures; the main part of the original ma-
terial was recovered. In some experiments, variable amounts of
2
-(perfluorophenyl)acetic acid (12) were obtained after acidifi-
cation of the basic solution. Moreover, noticeable decomposi-
tion of 3 was observed along with the formation of acid 12. The
nature of these byproducts was not analyzed. Probably, they are
Scheme 3: Hydrolysis of diethyl 2-(perfluorophenyl)malonate (3).
No reaction occurred under these conditions after 5 h of formed by nucleophilic substitution of fluorine atoms in the
refluxing. Increasing the alkali concentration up to 20% did not perfluorophenyl moiety.
Table 1: Hydrolysis of 3 under basic conditions.
Base
Solvent(s)
Time, h
Temperature, °C
Yield of compounda
2
12
KOH (5 equiv)
KOH (5 equiv)
NaOH (3 equiv)
NaOH (3 equiv)
NaOH (3 equiv)
NaOH (3 equiv)
LiOH (2 equiv)
LiOH (2 equiv)
LiOH (2 equiv)
10% H2O + Et2O
20% H2O + Et2O
15% H2O
5
reflux (35)
n.d.b
n.d.
n.d.
n.d.
n.d.
12c
n.d.
15c
n.d.
11c
17c
5
reflux (35)
15
5
20
n.d.b
n.d.
15% H2O
80
15% H2O + EtOH (1:2 v/v)
15% H2O + EtOH (1:2 v/v)
dioxane
15
5
20
n.d.b
n.d.b
n.d.b
n.d.b
n.d.
reflux (80)
15
15
5
20
20
80
dioxane–H2O (10%)
dioxane–H2O (10%)
an.d. – not detected by NMR, see Supporting Information File 1; bthe starting material (ester 3) was recovered; cisolated yield;
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Beilstein J. Org. Chem. 2020, 16, 1863–1868.
Acid 12 is fairly soluble in water and the separation of reaction decarboxylation of the desired 2-(perfluorophenyl)malonic acid.
products is cumbersome (see Supporting Information File 1). To prove this hypothesis, 2-(perfluorophenyl)malonate (3) was
The structure of acid 12 in solid form was studied by single heated under reflux conditions with an excess of 48% aqueous
crystal X-ray diffraction experiments (Figure 3). The structural HBr solution according to the method described in [36], but
parameters were deposited at CCDC (deposit No. 1993963, see only traces of 2-(perfluorophenyl)acetic acid (12) were isolated
Supporting Information File 1 for details).
after 16 h of reflux along with the unchanged original ester. In
addition, considerable darkening of the biphase reaction mix-
ture was observed. To overcome the problem of miscibility of
the ester and acid, AcOH was used as a co-solvent. After some
optimization of the reaction conditions, it was found that best
results were achieved if a 1:5 v/v mixture of 48% HBr and
glacial AcOH was used. The starting ester is fully soluble in this
mixture, and the reaction occurs in a homogeneous solution
(
Scheme 4).
Figure 3: Molecular structure of 2-(perfluorophenyl)acetic acid (12).
Since basic conditions seem to be unsuitable for the hydrolysis
of diethyl 2-(perfluorophenyl)malonate, acid-catalyzed reac-
tions were also tested. Trifluoroacetic acid (TFA) is a common
reagent for the catalytic cleavage of tert-butyl esters, but in the
stoichiometric quantity it is also suitable for the cleavage of
nonvolatile methyl or ethyl esters due to transesterification
Scheme 4: Formation of 2-(perfluorophenyl)acetic acid (12).
[
34,35]. The removal of highly volatile methyl or ethyl tri-
fluoroacetates from the reaction mixture is the driving force of Unfortunately, in all experiments hydrolysis was accompanied
this process. Unfortunately, 2-(perfluorophenyl)malonate does by complete decarboxylation, and only 2-(perfluoro-
not react with excess TFA (up to 10 equivalents) even under phenyl)acetic acid (12) was separated from the reaction mix-
reflux conditions, but in one experiment a catalytic amount of tures. The results of the acidic hydrolysis are summarized in
concentrated H2SO4 was added, and then coincidentally the Table 2. It seems that 2-(perfluorophenyl)malonic acid (2) is
reaction mixture was slightly overheated (to 100 °C), which led unexpectedly thermally unstable, because as a rule, decarboxyl-
to the evaporation of the entire acid. The remaining solid was ation of other phenylmalonic acids with strong electron-
identified as 2-(perfluorophenyl)acetic acid (12). We believe withdrawing groups required much higher temperatures
that 2-(perfluorophenyl)acetic acid might be formed by thermal (160–200 °C) or the presence of a catalyst [31,37].
Table 2: Hydrolysis of 3 under acidic conditions.
Acid
Solvent(s)
Time, h
Temperature, °C
Yield of compounda
2
12
TFA (5 equiv)
TFA (10 equiv)
TFA (5 equiv)
TFA (5 equiv)
TFA (5 equiv)
HBr (25 equiv)
HBr (25 equiv)
HBr (6 equiv)
CH2Cl2
15
15
6
20
n.d.b
n.d.b
n.d.b
n.d.b
n.d.
n.d.
n.d.
trace
trace
35c
neat
20
neat
reflux (75)
neat + 1 drop of conc. H2SO4
neat + 1 drop of conc. H2SO4
48% H2O
10
5
reflux (75)
reflux (75) + overheat (100)
15
15
10
100
n.d.b
n.d.b
n.d.
10c
48% H2O + AcOH (1:2 v/v)
48% H2O + AcOH (1:5 v/v)
20
n.d.
63c
reflux (120)
an.d. – not detected by NMR, see Supporting Information File 1; bthe starting material (ester 3) was recovered; cisolated yield;
1866

Beilstein J. Org. Chem. 2020, 16, 1863–1868.
However, acid-catalyzed hydrolysis of diethyl 2-(perfluoro- ORCID® iDs
phenyl)malonate (3) can be used as a convenient, inexpensive
Ilya V. Taydakov - https://orcid.org/0000-0001-9860-4141
and simple multigram approach to 2-(perfluoro-phenyl)acetic
acid (12). This new method is shorter and much safer than the References
classical one based on the hydrolysis of 2-(perfluoro-
1. Bui, M.; Hao, X.; Shin, Y.; Cardozo, M.; He, X.; Henne, K.;
Suchomel, J.; McCarter, J.; McGee, L. R.; San Miguel, T.;
phenyl)acetonitrile [38,39], because utilization of both toxic
Medina, J. C.; Mohn, D.; Tran, T.; Wannberg, S.; Wong, J.; Wong, S.;
1
-(bromomethyl)-2,3,4,5,6-pentafluorobenzene and alkali metal
Zalameda, L.; Metz, D.; Cushing, T. D. Bioorg. Med. Chem. Lett. 2015,
cyanides is avoided.
25, 1104–1109. doi:10.1016/j.bmcl.2015.01.001
2.
Ng, P. S.; Manjunatha, U. H.; Rao, S. P. S.; Camacho, L. R.; Ma, N. L.;
Herve, M.; Noble, C. G.; Goh, A.; Peukert, S.; Diagana, T. T.;
Smith, P. W.; Kondreddi, R. R. Eur. J. Med. Chem. 2015, 106,
Conclusion
To summarize, we have extensively investigated the reactivity
of diethyl 2-(perfluorophenyl)malonate (3) towards hydrolysis
in acidic and basic media. It was revealed, that the ester is
fairly stable in basic and acidic solutions at ambient tempera-
tures and decompose to a mixture of products (2-(per-
fluorophenyl)acetic acid (12) was identified as a major product)
at harsh basic conditions. Vigorous hydrolysis by a mixture
of aqueous HBr and AcOH at reflux temperature led to the
formation of 2-(perfluorophenyl)acetic acid (12) as single
product in good preparative yield. Evidently, ethyl
1
44–156. doi:10.1016/j.ejmech.2015.10.008
3
.
.
Koryakina, I.; McArthur, J.; Randall, S.; Draelos, M. M.; Musiol, E. M.;
Muddiman, D. C.; Weber, T.; Williams, G. J. ACS Chem. Biol. 2013, 8,
200–208. doi:10.1021/cb3003489
4
Zhang, W.; Holyoke, C. W., Jr.; Barry, J.; Cordova, D.; Leighty, R. M.;
Tong, M.-H. T.; Hughes, K. A.; Lahm, G. P.; Pahutski, T. F.; Xu, M.;
Briddell, T. A.; McCann, S. F.; Henry, Y. T.; Chen, Y.
Bioorg. Med. Chem. Lett. 2017, 27, 911–917.
doi:10.1016/j.bmcl.2017.01.002
5.
Rivkin, A.; Kim, Y. R.; Goulet, M. T.; Bays, N.; Hill, A. D.; Kariv, I.;
Krauss, S.; Ginanni, N.; Strack, P. R.; Kohl, N. E.; Chung, C. C.;
Varnerin, J. P.; Goudreau, P. N.; Chang, A.; Tota, M. R.; Munoz, B.
Bioorg. Med. Chem. Lett. 2006, 16, 4620–4623.
2
-(perfluorophenyl)malonate (3) is not suitable for the prepara-
tion of 2-(perfluorophenyl)malonic acid (2), due to its thermal
instability and strong tendency to decarboxylation. We
believed, that di-tert-butyl 2-(perfluorophenyl)malonate or
dibenzyl 2-(perfluorophenyl)malonate, which are cleaving
under very mild conditions are better precursors, but these
esters are expensive, hardly accessible and can barely be used
for large-scale preparation of 2-(perfluorophenyl)malonic acid
doi:10.1016/j.bmcl.2006.06.014
6
.
.
Nascimento de Oliveira, M.; Arseniyadis, S.; Cossy, J. Chem. – Eur. J.
2018, 24, 4810–4814. doi:10.1002/chem.201800641
7
Ariyarathna, Y.; Tunge, J. A. Org. Biomol. Chem. 2014, 12, 8386–8389.
doi:10.1039/c4ob01752h
8. Reddy Chidipudi, S.; Burns, D. J.; Khan, I.; Lam, H. W.
Angew. Chem., Int. Ed. 2015, 54, 13975–13979.
doi:10.1002/anie.201507029
(
12).
9
1
1
.
Wang, Y.; Zhao, F.; Zhou, Y.; Chi, Y.; Wang, Z.; Zhang, W.-X.; Xi, Z.
Chem. – Eur. J. 2013, 19, 10643–10654. doi:10.1002/chem.201301633
0.Petina, O.; Yakovlev, I.; Geffken, D. Synthesis 2013, 45, 803–809.
doi:10.1055/s-0032-1316851
Supporting Information
1.Elagawany, M.; Ibrahim, M. A.; Panda, S. S. Tetrahedron Lett. 2016,
57, 4910–4913. doi:10.1016/j.tetlet.2016.09.070
Supporting Information File 1
Detailed information about experimental procedures, X-ray
diffraction experiments for compound 12 and
characterization data for compounds 3 and 12.
12.Zhu, M.; Cui, X.; Zhang, S.; Liu, L.; Han, Z.; Gao, E. J. Inorg. Biochem.
016, 157, 34–45. doi:10.1016/j.jinorgbio.2016.01.016
2
1
1
1
3.Li, Z.-Y.; Xu, Y.-L.; Zhang, X.-F.; Zhai, B.; Zhang, F.-L.; Zhang, J.-J.;
Zhang, C.; Li, S.-Z.; Cao, G.-X. Dalton Trans. 2017, 46, 16485–16492.
doi:10.1039/c7dt03562d
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-153-S1.pdf]
4.Carter, A.; Mason, A.; Baker, M. A.; Bettler, D. G.; Changas, A.;
McMillen, C. D.; Tapu, D. Organometallics 2017, 36, 1867–1872.
doi:10.1021/acs.organomet.7b00206
5.Pasán, J.; Lago, A. B.; Cañadillas-Delgado, L.; Fabelo, Ó.;
Ruiz-Pérez, C.; Lloret, F.; Julve, M. Polyhedron 2019, 170, 217–222.
doi:10.1016/j.poly.2019.05.045
Acknowledgements
Single crystal X-ray diffraction analysis was performed using
the equipment at the Centre for Collective Use of the Kurnakov
Institute RAS, which operates with the support of the state as-
signment of the Kurnakov Institute RAS in the field of funda-
mental scientific research.
16.Pasán, J.; Sanchiz, J.; Fabelo, Ó.; Cañadillas-Delgado, L.; Déniz, M.;
Díaz-Gallifa, P.; Martínez-Benito, C.; Lloret, F.; Julve, M.;
Ruiz-Pérez, C. CrystEngComm 2014, 16, 8106–8118.
doi:10.1039/c4ce00834k
17.Chichetti, S. M.; Ahearn, S. P.; Adams, B.; Rivkin, A. Tetrahedron Lett.
2007, 48, 8250–8252. doi:10.1016/j.tetlet.2007.08.134
Funding
18.Imao, D.; Itoi, A.; Yamazaki, A.; Shirakura, M.; Ohtoshi, R.; Ogata, K.;
Ohmori, Y.; Ohta, T.; Ito, Y. J. Org. Chem. 2007, 72, 1652–1658.
doi:10.1021/jo0621569
This work was financially supported by the Russian Science
Foundation (Project №19-13-00272).
1867

Beilstein J. Org. Chem. 2020, 16, 1863–1868.
1
2
2
2
2
9.Richardson, P. Expert Opin. Drug Discovery 2016, 11, 983–999.
doi:10.1080/17460441.2016.1223037
License and Terms
0.Gerstenberger, M. R. C.; Haas, A. Angew. Chem., Int. Ed. Engl. 1981,
2
0, 647–667. doi:10.1002/anie.198106471
1.Vlasov, V. M.; Krivousova, E. D.; Yakobson, G. G. Zh. Org. Khim.
970, 6, 758–767.
2.Wallingford, V. H.; Homeyer, A. H.; Jones, D. M. J. Am. Chem. Soc.
941, 63, 2056–2059. doi:10.1021/ja01853a008
This is an Open Access article under the terms of the
Creative Commons Attribution License
1
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
1
3.Semmes, J. G.; Bevans, S. L.; Mullins, C. H.; Shaughnessy, K. H.
Tetrahedron Lett. 2015, 56, 3447–3450.
doi:10.1016/j.tetlet.2015.01.072
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
2
4.Aramendıa, M. A.; Borau, V.; Jiménez, C.; Marinas, J. M.; Ruiz, J. R.;
Urbano, F. J. Tetrahedron Lett. 2002, 43, 2847–2849.
doi:10.1016/s0040-4039(02)00314-3
(https://www.beilstein-journals.org/bjoc)
2
5.Hull, R. New pentafluorobenzene derivatives and processes for their
manufacture. G.B. Patent 901880, July 25, 1962.
Chem. Abstr. 1963, 58, 8714.
The definitive version of this article is the electronic one
which can be found at:
2
6.Vlasov, V. M.; Yakobson, G. G. Zh. Obshch. Khim. 1969, 39, 785–787.
7.Pees, K. J. Pentafluorophenylazolopyrimidines. U.S. Patent
US5,817,663, Oct 6, 1998.
doi:10.3762/bjoc.16.153
2
Chem. Abstr. 1999, 131, 257580.
2
2
3
8.Senaweera, S. M.; Weaver, J. D. J. Org. Chem. 2014, 79,
1
0466–10476. doi:10.1021/jo502075p
9.Fatiha, M.; Touati, A.; Rahal, S.; Moulay, S. Asian J. Chem. 2011, 2,
61–967.
9
0.Capaccio, V.; Sicignano, M.; Rodríguez, R. I.; Della Sala, G.;
Alemán, J. Org. Lett. 2020, 22, 219–223.
doi:10.1021/acs.orglett.9b04195
3
1.Woo, L. W. L.; Wood, P. M.; Bubert, C.; Thomas, M. P.; Purohit, A.;
Potter, B. V. L. ChemMedChem 2013, 8, 779–799.
doi:10.1002/cmdc.201300015
32.Miyamoto, K.; Tsuchiya, S.; Ohta, H. J. Fluorine Chem. 1992, 59,
225–232. doi:10.1016/s0022-1139(00)82414-8
3
3.Leonard, K. A. Heteroaryl linked quinolinyl modulators of RORyt. U.S.
Patent US2014107097, April 17, 2014.
Chem. Abstr. 2014, 160, 606703.
3
3
3
4.Todo, Y.; Nitta, J.; Miyajima, M.; Fukuoka, Y.; Yamashiro, Y.;
Nishida, N.; Saikawa, I.; Narita, H. Chem. Pharm. Bull. 1994, 42,
2063–2070. doi:10.1248/cpb.42.2063
5.Hutchinson, J. H. Autotaxin inhibitor compounds. WO Patent
WO2015048301, April 2, 2015.
Chem. Abstr. 2014, 162, 501145.
6.Kuchař, M.; Brunová, B.; Rejholec, V.; Roubal, Z.; Němeček, O.
Collect. Czech. Chem. Commun. 1981, 46, 1173–1187.
doi:10.1135/cccc19811173
3
3
3
7.Toussaint, O.; Capdevielle, P.; Maumy, M. Synthesis 1986,
1029–1031. doi:10.1055/s-1986-31861
8.Barbour, A. K.; Buxton, M. W.; Coe, P. L.; Stephens, R.; Tatlow, J. C.
J. Chem. Soc. 1961, 808–817. doi:10.1039/jr9610000808
9.Birchall, J. M.; Haszeldine, R. N. J. Chem. Soc. 1961, 3719.
doi:10.1039/jr9610003719
1868