A New synthetic protocol for the preparation of carbodiimides using a hypervalent iodine(III) reagent

Article abstract of DOI:10.1055/s-0030-1258414

A new, simple, and efficient preparation of symmetrical and unsymmetrical carbodiimides from the corresponding thioureas via dehydrosulfurization using a hypervalent iodine(III) reagent is described. The oxidation afforded carbodiimides in excellent yields and high selectivity. A possible mechanism for the transformation is proposed. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1258414

PAPER
711
A New Synthetic Protocol for the Preparation of Carbodiimides Using a
Hypervalent Iodine(III) Reagent
Preparation of
C
a
h
rbodiimidesenjie Zhu, Dan Xu, Yunyang Wei*
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. of China
Fax +86(25)84317078; E-mail: ywei@mail.njust.edu.cn
Received 15 November 2010; revised 20 December 2010
synthetic transformations. Recently, several highly effi-
cient systems for the N-acylation of 1,3-disubstituted
thioureas8 and the construction of heterocycles via de-
sulfurization mediated by hypervalent iodine reagents9
Abstract: A new, simple, and efficient preparation of symmetrical
and unsymmetrical carbodiimides from the corresponding thioureas
via dehydrosulfurization using a hypervalent iodine(III) reagent is
described. The oxidation afforded carbodiimides in excellent yields
and high selectivity. A possible mechanism for the transformation have also been reported by Patel et al.
is proposed.
In continuation of our efforts to develop new applications
for hypervalent iodine compounds,10 here, we would like
to report a facile procedure for the preparation of symmet-
rical and unsymmetrical carbodiimides, including dialkyl-
carbodiimides, from the corresponding thioureas with the
hypervalent iodine(III) reagent [hydroxy(tosyloxy)io-
do]benzene [HTIB, PhI(OH)OTs, Koser’s reagent]
(Scheme 1).
Key words: desulfurization, hypervalent iodine, oxidations, carbo-
diimides, catalysis
Carbodiimides rank as one of the most important classes
of reagents in organic synthesis due to their accessibility,
versatile chemical properties, and utility in coupling reac-
tions. They have also found several applications, such as
the synthesis of heterocycles, permease inhibitors, and
polymer stabilizers.1 Numerous methods via dehydrosul-
furization of thioureas,2 dehydration of ureas,3 and ther-
mal coupling of isocyanates,4 have been traditionally
employed to synthesize carbodiimides. The direct con-
densation of an amine and an isonitrile using a palladium
catalyst has also been explored.5 However, these methods
often have several drawbacks from a practical point of
view such as the use of expensive reagents, halogenated
solvents, and the requirement for specialized reaction con-
ditions. Recently, Patel and co-workers reported a ‘green-
er’ synthetic protocol for the preparation of carbodiimides
from thioureas using iodine and triethylamine.6 The pro-
cedure works well with arylthioureas, but transformation
of 1,3-dialkylthioureas, such as 1,3-dicyclohexylthiourea,
to the corresponding carbodiimides was not successful.
S
R1
R2
PhI(OH)OTs, Et3N
R1
N C N
R2
N
N
H
H
R1 = aryl, alkyl; R2 = aryl, alkyl
Scheme 1 Preparation of carbodiimides with HTIB and triethyl-
amine
Recently, Patel and co-workers found that the treatment of
1,3-disubstituted thioureas with (diacetoxyiodo)benzene
gave N-acylated ureas via carbodiimides as intermedi-
ates.8 We thought that it might be possible to stop the re-
action at the carbodiimide stage in the absence of an
external nucleophile. The initial experiments were carried
out using 1,3-diphenylthiourea as the model substrate
(Table 1). [Hydroxy(tosyloxy)iodo]benzene (HTIB) was
Hypervalent iodine(III) reagents have drawn considerable tested in this reaction, which is usually used as an efficient
attention as mild and highly chemoselective oxidizing re- reagent for the direct a-tosyloxylation of ketones,11 and is
agents for various organic transformations.7 As a result of
a precursor for the formation of alkynyl(phenyl)iodonium
their nontoxic nature, affordability, and safety profile, hy- tosylates.12 Fortunately, when 1,3-diphenylthiourea was
pervalent iodine(III) reagents are now popular reagents treated with HTIB (1.0 equiv)/triethylamine (2.0 equiv) in
for the formation of carbon–carbon, carbon–heteroatom, acetonitrile at room temperature, diphenylcarbodiimide
and heteroatom–heteroatom bonds. The activation of car- was obtained in 56% yield (entry 1). In the control exper-
bon–hydrogen bonds, rearrangements, and fragmenta- iments there was no product formation in the absence of
tions can be also induced by these reagents. Therefore, HTIB or triethylamine (entries 2 and 3). Encouraged by
hypervalent iodine compounds have high potential for the these results, a range of bases such as pyridine, imidazole,
improvement of known reactions, not only from an envi- and
1-butyl-3-methylimidazolium
hydroxide
ronmental point of view; they are also potentially interest- ([bmim][OH]) were used as the catalyst in the reaction,
ing reagents for the development of completely new however, they were all unsatisfactory, except for pyridine
which showed some activity (entries 4–6). An investiga-
tion of the effect of the solvent on the reaction showed that
SYNTHESIS 2011, No. 5, pp 0711–0714
Advanced online publication: 17.01.2011
DOI: 10.1055/s-0030-1258414; Art ID: F20410SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
1
1
ethyl acetate was the most efficient solvent giving the
highest yield (entries 7–12). To optimize the reaction con-
ditions, the role of triethylamine was studied and it was

712
C. Zhu et al.
PAPER
found that two equivalents of triethylamine were essential the HTIB and triethylamine system. As can be seen in
for this reaction. The use of less than two equivalents of Table 2, most thioureas 1 underwent smooth transforma-
triethylamine led to a drastic decrease in the yield of tion to afford the corresponding carbodiimides 2 in mod-
diphenylcarbodiimide (entries 13–15). With regard to the erate to excellent yields. Good yields were obtained with
amount of HTIB, the yield of diphenylcarbodiimide 1,3-diarylthioureas (entries 1–4 and 7–9). For symmetri-
reached a maximum when 1.2 equivalents of HTIB were cal 1,3-diarylthioureas 1a–d, the electronic properties of
used (entry 16).
the substituents in the aromatic ring had a relatively minor
influence on the rate of the reaction. Strong electron-
donating groups, such as the methoxy group, improved
the reaction rate (entry 4). Electron-withdrawing groups,
such as the chloro group, lowered the reaction rate slightly
(entry 2). The yields of dialkylcarbodiimides 2, such as di-
cyclohexylcarbodiimide (DCC, 2e) and diisopropylcarbo-
diimide (DIC, 2f), obtained from the corresponding 1,3-
dialkylthioureas under the same conditions are also quite
high (entries 5 and 6); dicyclohexylcarbodiimide (2e) and
diisopropylcarbodiimide (2f) are widely used in organic
reactions as dehydrating agents. The fact that the transfor-
mation of 1,3-dialkylthioureas to the corresponding car-
bodiimides is much more difficult than the corresponding
reaction with 1,3-diarylthioureas, means the results ob-
tained with this procedure are very satisfactory. Based on
the mechanism proposed by Patel et al,6 1,3-dialkylthio-
ureas with a higher pKa cannot undergo this transforma-
tion as they are not sufficiently acidic for a NH proton to
be abstracted by triethylamine. However, in the present
system the attachment of hypervalent iodine to sulfur
Table 1 Optimization Studies for the Conversion of 1,3-Diphe-
nylthiourea into Diphenylcarbodiimide
S
HTIB, base
Ph
Ph
Ph
N C N Ph
N
N
solvent, temp
H
H
Entry Oxidant
(equiv)
Base
(equiv)
Solvent
Temp Yielda
(°C) (%)
1
2
3
4
5
6b
7
8
9
HTIB (1.0)
Et3N (2.0)
MeCN
MeCN
MeCN
MeCN
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0
56
–
HTIB (1.0)
–
–
Et3N (2.0)
–
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.0)
HTIB (1.2)
HTIB (1.4)
HTIB (1.2)
HTIB (1.2)
pyridine (2.0)
43
17
–
imidazole (2.0) MeCN
[Bmim][OH]
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (1.5)
Et3N (1.0)
Et3N (2.5)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
–
CHCl3
CH2Cl2
MeOH
toluene
THF
51
36
–
Table 2 Preparation of Carbodiimides from Thioureasa
S
10
<10
<10
67
33
15
66
77
74
81
85
R1
R2
R1
N
C
N
R2
N
N
H
H
11
12
13
14
15
16
17
18
19c
1
2
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
Entry Substrate
R1
Time (h) Productb Yieldc (%)
R2
1
Ph
Ph
5
6
5
4
8
8
6
5
5
8
8
2a
2b
2c
2d
2e
2f
85
73
84
90
67
71
63
88
89
80
71
2d 4-ClC6H4
4-ClC6H4
4-MeC6H4
4-MeOC6H4
Cy
3
4
5
6
7
8
9
4-MeC6H4
4-MeOC6H4
Cy
i-Pr
Ph
0
i-Pr
a Yields were determined by GC-MS.
b [Bmim][OH] (5 mL) was used in the reaction.
c HTIB was added portionwise.
4-ClC6H4
4-MeC6H4
4-MeOC6H4
Cy
2g
2h
2i
Ph
Ph
In comparison with results obtained at room temperature,
the reaction performed under ice-cold conditions gave
better a yield and fewer side products (entry 18). Further-
more, portionwise addition of HTIB over a period of 30
minutes furnished better yields compared to the addition
in one portion (entry 19).
10 Ph
11e,f Ph
2j
Bu
2k
a Reaction conditions: thiourea (1.0 mmol), HTIB (1.2 mmol), Et3N
(2.0 mmol), EtOAc (10 mL), 0 °C.
b Products were confirmed by 1H NMR, MS, and IR.
c Yields of isolated products unless otherwise noted.
d The reaction was started at 0 °C and then gradually warmed to r.t.
e The product was not separated due to its instability.
f Yield was determined by GC-MS.
In order to evaluate the versatility of this novel system, we
synthesized a series of symmetrical and unsymmetrical
thioureas 1a–k, which were subjected to treatment with
Synthesis 2011, No. 5, 711–714 © Thieme Stuttgart · New York

PAPER
Preparation of Carbodiimides
713
Ph
I
OTs
OH
Ph
S
I
OH
A
PhI + S + Et3NH+ –OH
S
(I)
R1
R2
R1
R2
R1
N C N
R2
N
N
H
N
H
N
H
Et3NH+ –OTs
(II)
Et3N
Et3N
Ph
I
OH
H
PhI + S + H2O
S
R1
R2
N
N
Scheme 2 A plausible reaction pathway
detector. GC-MS spectra were recorded on a Thermo Trace DSQ
GC-MS spectrometer using a TRB-5MS (30 m × 0.25 mm × 0.25
mm) column. MS spectra were recorded on Thermal Finnigan TSQ
Quantum ultra AM spectrometer using a TRB-5MS (30 m × 0.25
mm × 0.25 mm) column. IR spectra were recorded on a Shimadzu
spectrophotometer using KBr discs. Melting points were deter-
mined on a Yamato melting point apparatus Model MP-21. The re-
action progress was monitored by TLC (silica gel, polygrams SIL
G/UV 254 plates). Column chromatography was performed using
Silicycle (40–60 mm) silica gel. All products are known compounds
and they were identified by comparison of their physical and spectra
data with those reported in the literature.2d,6
makes it a much better leaving group, leading to facile
deprotonation by triethylamine. Based on this reasoning,
we believe that an amine with a lower pKa should yield the
carbodiimide faster because of its easy deprotonation.
This was further supported by comparing the reaction rate
of 1,3-diphenylthiourea (pKa of aniline is 4.63) and 1,3-di-
cyclohexylthiourea (pKa of cyclohexylamine is 10.66)
(entries 1 and 5).
To demonstrate the scope and efficiency of the present
method, the HTIB and triethylamine system was then ex-
tended to unsymmetrical thioureas. As shown in Table 2,
unsymmetrical thioureas 1g–k can be converted into the
corresponding carbodiimides 2 in excellent yields (entries
7–11). In particular, unsymmetrical thioureas with an N-
aryl group and an N¢-alkyl were also converted into the
corresponding carbodiimides in good yields under the
same conditions (entries 10 and 11).
Carbodiimides 2; General Procedure
To a stirred and ice-cooled soln of thiourea (1 mmol) in EtOAc (10
mL) was added Et3N (0.20 g, 2 mmol). To this was added HTIB
(0.46 g, 1.2 mmol) portionwise over a period of 30 min. The mixture
was stirred for several hours while checking the reaction progress
by GC or TLC. After completion, the precipitated sulfur (mp 117–
119 °C) was filtered, and the soln was concentrated under vacuum
and purified by column chromatography (petroleum ether–EtOAc,
10:1) to afford the analytically pure product, which was identified
by 1H NMR, IR, and MS.
Based on these results, a plausible reaction pathway for
this reaction is depicted in Scheme 2. The sulfur atom of
thiourea attacks the thiophilic iodine of HTIB displacing
OTs, giving intermediate A. Reductive b-elimination of
the l3-iodane intermediate A with the expulsion of sulfur
leads to the formation of the final product, the carbodiim-
ide. The formation of iodobenzene and the precipitation of
elemental sulfur support the proposed mechanism. Anoth-
er route (II) via intramolecular dehydration is also possi-
ble, but the mechanism via route (I) is more likely since
the reaction requires two equivalents of triethylamine;
diphenylcarbodiimide was formed in only 15% yield
when 1,3-diphenylthiourea was reacted with one equiva-
lent of triethylamine (entry 14).
Diphenylcarbodiimide (2a)
Colorless liquid.
IR (KBr): 2139 cm–1.
1H NMR (500 MHz, CDCl3): d = 7.23 (m, 6 H), 7.37 (m, 4 H).
Bis(4-chlorophenyl)carbodiimide (2b)
White solid; mp 54–56 °C.
IR (KBr): 2137 cm–1.
1H NMR (500 MHz, CDCl3): d = 7.12 (d, J = 8.5 Hz, 4 H), 7.32 (d,
J = 8.5 Hz, 4 H).
Bis(4-tolyl)carbodiimide (2c)
Colorless liquid.
IR (KBr): 2137 cm–1.
1H NMR (500 MHz, CDCl3): d = 2.37 (s, 6 H, 2 CH3), 7.11 (d,
J = 8.5 Hz, 4 H), 7.16 (d, J = 8.5 Hz, 4 H).
In summary, the simple, mild, and highly efficient system
of HTIB and triethylamine has been developed, which
allows the preparation of carbodiimides from the corre-
sponding thioureas in excellent yields and high selectivi-
ty. It is noteworthy to mention that dialkylcarbodiimides
could also be obtained with this system.
Bis(4-methoxyphenyl)carbodiimide (2d)
Colorless liquid.
IR (KBr): 2127 cm–1.
1H NMR (500 MHz, CDCl3): d = 3.82 (s, 6 H, 2 OCH3), 6.88 (d,
J = 9.0 Hz, 4 H), 7.13 (d, J = 9.0 Hz, 4 H).
All chemicals (AR grade) were obtained from commercial sources
and used without further purification. Thioureas were synthesized
by literature procedures.13 1H NMR spectra were obtained with
TMS as internal standard in CDCl3 using a Bruker DRX 500 (500
MHz) spectrometer. Gas chromatography (GC) analysis was per-
formed on an Agilent GC-6820 chromatograph with a HP-Innowax
capillary column (30 m × 0.32 mm × 0.5 mm) and a flame ionization
Dicyclohexylcarbodiimide (2e)
White solid; mp 34–35 °C.
Synthesis 2011, No. 5, 711–714 © Thieme Stuttgart · New York

714
C. Zhu et al.
PAPER
IR (KBr): 2122 cm–1.
1H NMR (500 MHz, CDCl3): d = 1.29–1.35 (m, 10 H), 1.57–1.63
(m, 2 H), 1.74–1.77 (m, 4 H), 1.92–1.94 (m, 4 H), 3.21 (m, 2 H).
(3) (a) Stevens, C. L.; Singhal, G. H.; Ash, A. B. J. Org. Chem.
1967, 32, 2895. (b) Hessel, E. T.; Jones, W. D.
Organometallics 1992, 11, 1496. (c) Schlama, T.;
Gouverneur, V.; Mioskowski, C. Tetrahedron Lett. 1996,
37, 7047. (d) Fell, J. B.; Coppola, G. M. Synth. Commun.
1995, 25, 43. (e) Zhang, M.; Vedantham, P.; Flynn, D. L.;
Hanson, P. R. J. Org. Chem. 2004, 69, 8340.
Diisopropylcarbodiimide (2f)
Colorless liquid.
IR (KBr): 2116 cm–1.
1H NMR (500 MHz, CDCl3): d = 1.14 (m, 12 H, 4 CH3), 3.48 (m, 2
H).
(4) (a) Deeming, A. J.; Hardcastle, K.; Fuchita, Y.; Henrick, K.;
McPartlin, M. J. Chem. Soc., Dalton Trans. 1986, 2259.
(b) Bryan, J. C.; Rheingold, A. L.; Geib, S. J.; Mayer, J. M.
J. Am. Chem. Soc. 1987, 109, 2825. (c) Tang, J.; Mohan, T.;
Verkade, J. G. J. Org. Chem. 1994, 59, 4931. (d) Barbaro,
G.; Battaglia, A.; Giorgianni, P.; Guerrini, A.; Scconi, G. J.
Org. Chem. 1995, 60, 6032. (e) Rahman, A. K. F.; Nicholas,
K. M. Tetrahedron Lett. 2007, 48, 6002.
(4-Chlorophenyl)phenylcarbodiimide (2g)
Colorless liquid.
IR (KBr): 2135 cm–1.
1H NMR (500 MHz, CDCl3): d = 7.18 (m, 5 H), 7.34 (m, 4 H).
(5) (a) Ito, Y.; Hirao, T.; Saegusa, T. J. Org. Chem. 1975, 40,
2981. (b) Pri-Bar, I.; Schwartz, J. Chem. Commun. 1997,
347.
Phenyl(4-tolyl)carbodiimide (2h)
Colorless liquid.
IR (KBr): 2129 cm–1.
(6) Ali, A. R.; Ghosh, H.; Patel, B. K. Tetrahedron Lett. 2010,
51, 1019.
(7) (a) Varvoglis, A. Hypervalent Iodine in Organic Synthesis;
Academic Press: London, 1997. (b) Hypervalent Iodine
Chemistry; Wirth, T., Ed.; Springer: Berlin, 2003.
(c) Stang, P. J.; Zhdankin, V. V. Chem. Rev. 1996, 96, 1123.
(d) Varvoglis, A. Tetrahedron 1997, 53, 1179. (e) Wirth,
T.; Hirt, U. H. Synthesis 1999, 1271. (f) Zhdankin, V. V.;
Stang, P. J. Chem. Rev. 2002, 102, 2523. (g) Tohma, H.;
Kita, Y. Adv. Synth. Catal. 2004, 346, 111. (h) Koser, G. F.
Adv. Heterocycl. Chem. 2004, 86, 225. (i) Moriarty, R. M.
J. Org. Chem. 2005, 70, 2893. (j) Wirth, T. Angew. Chem.
Int. Ed. 2005, 44, 3656. (k) Ladziata, U.; Zhdankin, V. V.
ARKIVOC 2006, (ix), 26. (l) Ochiai, M. Chem. Rec. 2007, 7,
12. (m) Kita, Y.; Fujioka, H. Pure Appl. Chem. 2007, 79,
701. (n) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2008, 108,
5299. (o) Ochiai, M.; Miyamoto, K. Eur. J. Org. Chem.
2008, 4229. (p) Zhdankin, V. V. ARKIVOC 2009, (i), 1.
(q) Dohi, T.; Kita, Y. Chem. Commun. 2009, 2073.
(r) Uyanik, M.; Ishihara, K. Chem. Commun. 2009, 2086.
(s) Uyanik, M.; Okamoto, H.; Yasui, T.; Ishihara, K. Science
2010, 328, 1376.
1H NMR (500 MHz, CDCl3): d = 2.38 (s, 3 H, CH3), 7.19 (m, 7 H),
7.37 (m, 2 H).
(4-Methoxyphenyl)phenylcarbodiimide (2i)
Colorless liquid.
IR (KBr): 2131 cm–1.
1H NMR (500 MHz, CDCl3): d = 3.82 (s, 3 H, OCH3), 6.89 (m, 2
H), 7.19 (m, 5 H), 7.36 (m, 2 H).
Cyclohexyl(phenyl)carbodiimide (2j)
Colorless liquid.
IR (KBr): 2129 cm–1.
1H NMR (500 MHz, CDCl3): d = 1.35–2.06 (m, 10 H), 3.50 (m, 1
H), 7.12 (m, 3 H), 7.31 (m, 2 H).
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.
(8) Singh, C. B.; Ghosh, H.; Murru, S.; Patel, B. K. J. Org.
Chem. 2008, 73, 2924.
(9) Ghosh, H.; Yella, R.; Nath, J.; Patel, B. K. Eur. J. Org.
Chem. 2008, 6189.
(10) (a) Zhu, C.; Ji, L.; Wei, Y. Synthesis 2010, 3121. (b) Zhu,
C.; Ji, L.; Wei, Y. Catal. Commun. 2010, 11, 1017. (c)Zhu,
C.; Sun, C.; Wei, Y. Synthesis 2010, 4235. (d) Zhu, C.; Ji,
L.; Zhang, Q.; Wei, Y. Can. J. Chem. 2010, 88, 362.
(e) Zhu, C.; Ji, L.; Wei, Y. Monatsh. Chem. 2010, 141, 327.
(f) Zhu, C.; Wei, Y.; Ji, L. Synth. Commun. 2010, 40, 2057.
(11) (a) Yamamoto, Y.; Togo, H. Synlett 2006, 798. (b) Akiike,
J.; Yamamoto, Y.; Togo, H. Synlett 2007, 2168.
(c) Yamamoto, Y.; Kawano, Y.; Toy, P. H.; Togo, H.
Tetrahedron 2007, 63, 4680.
Acknowledgment
We are grateful to Nanjing University of Science and Technology
for financial support.
References
(1) (a) Khorana, H. G. Chem. Rev. 1953, 53, 145. (b) Kurzer,
F.; Douraghi-Zadeh, K. Chem. Rev. 1967, 67, 107.
(c) Williams, A.; Ibrahim, I. T. Chem. Rev. 1981, 81, 589.
(d) Mikolajczyk, M.; Kielbasinski, P. Tetrahedron 1981, 37,
233.
(2) (a) Ulrich, H.; Sayigh, A. A. R. Angew. Chem., Int. Ed. Engl.
1966, 5, 704. (b) Kim, S.; Yi, K. Y. Tetrahedron Lett. 1986,
27, 1925. (c) Kim, S.; Yi, K. Y. J. Org. Chem. 1986, 51,
2613. (d) Isobe, T.; Ishikawa, T. J. Org. Chem. 1999, 64,
6984.
(12) (a) Stang, P. J. Angew. Chem., Int. Ed. Engl. 1992, 31, 274.
(b) Zhdankin, V. V.; Stang, P. J. Tetrahedron 1998, 54,
10927.
(13) (a) Ramadas, K.; Srinivasan, N.; Janarthanan, N.
Tetrahedron Lett. 1993, 34, 6447. (b) Novák, L.; Hanania,
M.; Kovács, P.; Kovács, C. E.; Kolonits, P.; Szántay, C.
Synth. Commun. 1999, 29, 1757. (c) Bollini, M.; Casal, J. J.;
Alvarez, D. E.; Boiani, L.; González, M.; Cerecetto, H.;
Bruno, A. M. Bioorg. Med. Chem. 2009, 17, 1437.
Synthesis 2011, No. 5, 711–714 © Thieme Stuttgart · New York