A facile aerobic copper-catalyzed α-oxygenation of aryl thioacetamides: An efficient access to α-keto aryl thioamides

Article abstract of DOI:10.1055/s-2008-1042925

Copper(II) efficiently catalyzes the aerobic oxidation of aryl thioacetamides into the corresponding α-keto aryl thioamides in moderate to high yields in the presence of K₂CO₃ under O ₂ atmosphere. This protocol is simple, clean, and generates water as the only byproduct. A mechanism is proposed for the reaction course. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1042925

892
LETTER
A Facile Aerobic Copper-Catalyzed a-Oxygenation of Aryl Thioacetamides:
An Efficient Access to a-Keto Aryl Thioamides
Access to
a
-K
i
r
l
Thioam
o
ides uz Matloubi Moghaddam,* Zohreh Mirjafary, Hamdollah Saeidian, Marjan Jebeli Javan
Laboratory of Organic Synthesis & Natural Products, Department of Chemistry, Sharif University of Technology,
P. O. Box 11155-9516, Tehran, Iran
E-mail: matloubi@sharif.edu
Received 17 December 2007
The reaction of (N,N-dimethylthiocarbomyl) lithium (A)
Abstract: Copper(II) efficiently catalyzes the aerobic oxidation of
aryl thioacetamides into the corresponding a-keto aryl thioamides
in moderate to high yields in the presence of K2CO3 under O2 atmo-
sphere. This protocol is simple, clean, and generates water as the
only byproduct. A mechanism is proposed for the reaction course.
with methyl benzoate, afforded a-keto aryl thioamides
(Scheme 2).7 Reagent A has been generated by Seebach at
–100 °C by deprotonation of dimethylthioformamide with
LDA. It is evident that the need for the development of
new and flexible protocols is required to access a-keto
aryl thioamides without using toxic reagents and avoiding
extreme conditions.
Key words: CuCl2·2H2O, oxygenation, a-keto aryl thioamides,
thioacetomorpholides
O
The development of improved oxidation reactions is an
area of great current interest owing to their almost ubiqui-
tous nature in both academic and industrial laboratories.1
Transition-metal-catalyzed oxidation of organic com-
pounds with atom-efficient oxidants such as O2 or H2O2 is
rapidly gaining importance as a viable alternative to the
environmentally hazardous metal-promoted stoichiomet-
ric oxidations.2 Traditionally, most oxidations are accom-
plished by at least stoichiometric amounts of often toxic
oxidants, especially chromium(VI) reagents.3 As a conse-
quence, different catalytic methods using small amounts
of metallic derivatives and clean oxidants have been de-
veloped.
S
PhCO2Me
NMe2
Ph
Li
NMe2
S
A
Scheme 2
Aryl thioacetamides have been used as useful synthons in
the synthesis of heterocycles.8 The availability of such
aryl thioamides in our laboratory9 provided a unique op-
portunity for examining further their synthetic utility. In
continuation of our research in this area,10 herein we re-
port a convenient preparation of a-keto aryl thioamides
from readily available aryl thioamides in good to excellent
yields. To the best of our knowledge, this is the first dem-
onstration of the oxidation of aryl thioacetamides.
Copper(II) salt catalysis of oxidation and oxygenation re-
actions is of recognized importance for carrying out selec-
tive transformation.4 The oxidation of hydroxyl and
methylene groups to the corresponding carbonyl moieties
remains one of the most fundamental reactions in organic
synthesis. Benzylic methylene groups of aromatic alde-
hydes and ketones have been oxidized by copper(II) to the
corresponding carbonyl function,5 but there is no report
about oxidation of aryl thioacetamides which leads to the
synthesis of a-keto aryl thioamide derivatives. Limited at-
tention has been given to the synthesis of this framework.
Addition of H2S onto acylcyanides according to Scheme 1
gave corresponding N-unsubstituted a-keto aryl thio-
amides.6 However, the synthesis of 2-oxothioamides is re-
stricted to the N-unsubstituted derivatives.
To find the optimal conditions, the oxidation of phenyl
thioacetomorpholide (1a) in the presence of catalyst and
base was chosen as a model reaction. A mixture of phenyl
thioacetomorpholide (1 mmol), catalyst, base (1 mmol),
and solvent (3 mL) was stirred under various reaction con-
ditions (Table 1). Our first experiment showed that the
presence of CuCl2 along with K2CO3 is required to
achieve the oxidation of thioamides and no reaction was
observed when the oxidation was performed with CuCl2
alone (Table 1, entries 1, 2). The Cu(OAc)2 was also ex-
amined as catalyst for this reaction, however, it was less
effective compared to CuCl2 and afforded a-keto aryl
thioamides in 80% yield (Table 1, entry 4). We then con-
tinued to optimize the model reaction by considering the
efficiency of polar and nonpolar solvents. A polar solvent
such as DMF was much better than a nonpolar solvent
(Table 1, entries 3, 5, 6). The effect of temperature was
also studied by carrying out the model reaction in the pres-
ence of CuCl2 at room temperature, 50 °C, and 80 °C. It
was observed that the yield was increased as the reaction
temperature was raised (Table 1, entries 3, 7, 8). Finally,
we found that 1 mol% of CuCl2 could effectively catalyze
O
O
H2S
NH2
R
Et3N
R
CN
S
Scheme 1
SYNLETT 2008, No. 6, pp 0892–0896
x
x.
x
x.
2
0
0
8
Advanced online publication: 11.03.2008
DOI: 10.1055/s-2008-1042925; Art ID: D40307ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Access to a-Keto Aryl Thioamides
893
Table 1 Oxidation of Phenyl Thioacetomorpholide Catalyzed by
Different Catalytic Systems
the reaction for synthesis of the desired product. Using
more than 1 mol% CuCl2 has little effect on the yield of
the reaction (Table 1, entries 3, 12). The reaction time is
reduced by seven times upon changing the atmosphere
from air to oxygen. Considering the reaction time, amount
of catalyst and the yield, the optimized conditions are:
K2CO3 (1 equiv), CuCl2 (1 mol%), aryl thioacetamide (1
equiv) in DMF (3 mL) at 80 °C under oxygen atmosphere
for 3 hours. To evaluate the scope of the reaction, the ox-
idation of some aryl thioacetamide derivatives was stud-
ied under these optimized conditions.
O
O
O
catalyst
N
N
base, solvent
S
S
1a
2a
Entry Catalyst
(mol%)
Solvent Base
Time Temp Yield
(h)
(°C) (%)a
1
2
3
4
5
6
7
8
9
none
DMF
DMF
DMF
K2CO3
20
80
80
80
80
65
80
25
50
80
80
80
80
80
trace
The structures of the products were confirmed by GC-MS,
1H NMR, 13C NMR analysis,12 and finally the compounds
were transformed into corresponding known aryl glyoxal-
ic acids by our reported procedure and compared with au-
thentic samples (Scheme 3).9b
CuCl2 (10)
CuCl2 (10)
No base 20
no reaction
K2CO3
K2CO3
K2CO3
20
20
20
20
20
20
88
Cu(OAc)2 (10) DMF
80
CuCl2 (10)
CuCl2 (10)
CuCl2 (10)
CuCl2 (10)
CuCl2 (10)
THF
trace
no reaction
trace
40
O
O
O
toluene K2CO3
OH
NaOH (5% aq)
N
DMF
DMF
DMF
DMF
DMF
DMF
K2CO3
K2CO3
reflux, 30 min
or MW, 1 min
O
S
2a
3a
Cs2CO3 20
90
Scheme 3
10 CuCl2 (10)
11 CuCl2 (10)
12 CuCl2 (1)
13b CuCl2 (1)
a Isolated yield.
K2CO3
K2CO3
K2CO3
5
10
20
3
trace
46
As shown in Table 2, all the substrates consistently under-
went oxidation selectively to the corresponding a-keto
aryl thioamides in high yields. This reaction was not lim-
ited to simple benzene-containing aromatics; the pyri-
dine-containing substrate also afforded a-keto aryl
thioamide in good yield (Table 2, entry 7). In the case of
phenyl dithioacetomorpholide, this conversion was ac-
companied by C–C bond cleavage, yielding 4-(2-mor-
pholino-2-thioxoacetyl)benzoic acid (Table 2, entry 10).
85
DMF K2CO3
94
b Reaction carried out under O2 atmosphere.
Table 2 Oxidation of Various Aryl Thioamides Catalyzed by CuCl2 (1 mol%) under O2 Atmosphere
O
O
O
CuCl2⋅2H2O (1 mol%)
N
N
Ar
Ar
K2CO3, DMF, O2
80 °C, 3 h
S
S
1a–j
2a–j
Entry
1
Thioacetomorpholide 1 Ar
Product
Mp (°C)
109–111
Yield (%)a
O
O
N
1a
1b
Ph
94
S
2a
O
O
N
2
4-MeC6H4
100–102
83
S
Me
2b
Synlett 2008, No. 6, 892–896 © Thieme Stuttgart · New York

894
F. M. Moghaddam et al.
LETTER
Table 2 Oxidation of Various Aryl Thioamides Catalyzed by CuCl2 (1 mol%) under O2 Atmosphere (continued)
O
O
O
CuCl2⋅2H2O (1 mol%)
N
N
Ar
Ar
K2CO3, DMF, O2
80 °C, 3 h
S
S
1a–j
2a–j
Entry
3
Thioacetomorpholide 1 Ar
Product
Mp (°C)
122–124
Yield (%)a
O
O
N
1c
4-MeOC6H4
80
S
MeO
2c
O
O
N
4
5
6
1d
4-BrC6H4
144–145
153–155
160–162
85
83
87
S
Br
2d
O
O
O
O
N
N
1e
1f
4-ClC6H4
S
S
Cl
2e
4-biphenyl
Ph
2f
O
O
N
7
8
9
1g
1h
1i
4-pyridyl
82–83
90
75
80
N
S
2g
O
O
N
a-naphthyl
b-naphthyl
118–120
156–158
S
2h
O
O
N
S
2i
O
O
N
10
1j
4-phenylacetothiomorpholide
131–133
69
HO
S
O
2j
a Isolated yield.
Our observation of the C–C bond cleavage is consistent more strongly basic than ours (Et3N/pyridine vs. K2CO3).
with a previous finding of Sayre et al.5 for the ketones pro- It should also be mentioned that when the reaction time
moted by Cu(II), though their reaction conditions were was extended to 30 hours under air atmosphere, desulfur-
Synlett 2008, No. 6, 892–896 © Thieme Stuttgart · New York

LETTER
Access to a-Keto Aryl Thioamides
895
ization of the a-keto aryl thioamide was observed and the erates Cu(II). a-Hydroperoxide then undergoes competi-
corresponding a-keto aryl amides were obtained as the ex- tive dehydration to the a-keto aryl thioamide. The use of
clusive product (Scheme 4).12
catalytic amount of the Cu(II) indicates that the Cu(I) to
Cu(II) regeneration by O2 occurs during the reaction. It
should be noted that when 1,4-hydroquinone (0.1 equiv)
was used in the model reaction, the product was obtained
in lower yield (62% vs. 94%), which is evidence for a
free-radical pathway for the reaction.11
O
O
O
CuCl2⋅2H2O
N
N
K2CO3, DMF
O
S
30 h
1a
In conclusion, we have reported an efficient procedure for
the synthesis of a-keto aryl thioamides using CuCl2·2H2O
as a nontoxic and inexpensive heterogeneous catalyst with
easy workup, which make it a useful and attractive strate-
gy for the synthesis of a-keto aryl thioamides. These com-
pounds have the potential to be converted into aryl
glyoxalic acids.
Scheme 4
To examine the chemoselectivity of the reaction we also
tested the a-oxygenation of the phenyl acetomorpholide
as well as phenyl acetic acid under optimized conditions:
no traces of the desired products were observed
(Scheme 5).
General Procedure for the Synthesis of a-Keto Aryl Thioamides
The aryl thioamides were prepared according to previously our re-
ported procedure.9 To a stirred suspension of CuCl2·2H2O (1 mol%)
in DMF (3 mL) were added aryl thioamide (1 mmol) and K2CO3 (1
mmol). The reaction mixture was stirred at 80 °C for 3 h under an
O2 atmosphere. The progress of the reaction was monitored by TLC.
After completion of the reaction, the mixture was poured into ice-
cold H2O and stirred for 10 min, which resulted in precipitation of
the desired a-keto aryl thioamides. The precipitated solid was fil-
tered, washed with PE, and dried. The structure of the products was
confirmed by analytical, GC-MS, 1H NMR, and 13C NMR data.12
O
N
CuCl2·2H2O
O
or
no reaction
K2CO3, DMF
OH
O
Scheme 5
The detailed mechanism of reaction is not clear at this
stage. A possible reaction mechanism to account for the
formation of substituted a-keto aryl thioamides using cat-
alytic amount of CuCl2 and K2CO3 is proposed in
Scheme 6. The first step is a base-promoted thioenoliza-
tion then tautomerization of I gives II and Cu(I).5 In the
presence of molecular O2, oxygen incorporation occurs
and provides a a-hydroperoxide equivalent III and regen-
Acknowledgment
We would like to acknowledge the Islamic Development Bank
(IDB) for granting the loan in 1993 for purchasing a 500 MHZ Bru-
ker NMR spectrometer.
References and Notes
(1) Backvall, J. E. Modern Oxidation Methods; Wiley-VCH:
Weinheim, 2004.
(2) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation
of Organic Compounds; Academic Press: New York, 1991.
(b) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879.
(3) Muzart, J. Chem. Rev. 1992, 92, 113.
(4) (a) Pelletier, G.; Powell, D. A. Org. Lett. 2006, 8, 6031.
(b) Tokunaga, M.; Shirogane, Y.; Aoyama, H.; Obora, Y.;
Tsuji, Y. J. Organomet. Chem. 2005, 690, 5378. (c) Ajjou,
A. N.; Ferguson, G. Tetrahedron Lett. 2006, 47, 3719.
(d) Menini, L.; Gusevskaya, E. V. Appl. Catal., A: General
2006, 309, 122. (e) Sherman, E. S.; Chemler, S. R.; Tan, T.
B.; Gerlits, O. Org. Lett. 2004, 6, 1573.
(5) (a) Sayre, L. M.; Jin, S. J. J. Org. Chem. 1984, 49, 3498.
(b) Jin, S. J.; Arora, P. K.; Sayre, L. M. J. Org. Chem. 1990,
55, 3011.
O
N
Ar
S
I
CuII
–
HCO3
K2CO3, CuII
O
O
N
N
Ar
Ar
CuII
S
S
CuI
1
II
Ar
O
(6) Asinger, F.; Gentz, F. Angew. Chem., Int. Ed. Engl. 1963, 2,
397.
H+
O2
N
H2O
O
O
(7) Creary, X.; Zhu, C. J. Am. Chem. Soc. 1995, 117, 5859.
(8) Jagodzinski, S. T. Chem. Rev. 2003, 103, 197.
(9) (a) Moghaddam, F. M.; Hojabri, L.; Dohendou, M. Synth.
Commun. 2003, 33, 4279. (b) Moghaddam, F. M.;
Ghaffarzadeh, M. Synth. Commun. 2001, 31, 317.
(c) Moghaddam, F. M.; Ghaffarzadeh, M.; Dekamin, M. G.
J. Chem. Res., Synop. 2000, 228.
S
CuII
O
O
III
N
Ar
S
2
Scheme 6
Synlett 2008, No. 6, 892–896 © Thieme Stuttgart · New York

896
F. M. Moghaddam et al.
LETTER
(10) (a) Moghaddam, F. M.; Saeidian, H.; Mirjafary, Z.; Sadeghi,
A. Lett. Org. Chem. 2007, 4, 576. (b) Moghaddam, F. M.;
Saeidian, H.; Mirjafary, Z.; Taheri, S. J. Sulfur Chem. 2006,
27, 545. (c) Moghaddam, F. M.; Zali Boinee, H. Synlett
2005, 1612. (d) Moghaddam, F. M.; Zali Boinee, H.
Tetrahedron 2004, 60, 6085. (e) Moghaddam, F. M.;
Zali Boinee, H. Tetrahedron Lett. 2003, 44, 6253.
(f) Saeidian, H.; Sadeghi, A.; Mirjafary, Z.; Moghaddam, F.
M. Synth. Commun. 2008, in press.
(11) Zaborovskiy, A. B.; Lutsyk, D. S.; Prystansky, R. E.;
Kopylets, V. I.; Timokhin, V. I.; Chatgilialoglu, C.
J. Organomet. Chem. 2004, 689, 2912.
C12H12ClNO2S: C, 53.43; H, 4.48; N, 5.19. Found: C, 53.44;
H, 4.44; N, 5.08.
Compound 2f: 1H NMR (500 MHz, CDC13): d = 8.00 (d,
J = 6.81 Hz, 2 H), 7.65 (d, J = 8.38 Hz, 2 H), 7.56 (d,
J = 7.20 Hz, 2 H), 7.42 (t, J = 6.53 Hz, 2 H), 7.37 (t, J = 5.28
Hz, 1 H), 4.29 (t, J = 4.93 Hz, 2 H), 3.86 (t, J = 4.93 Hz, 2
H), 3.66 (t, J = 4.71 Hz, 2 H), 3.58 (t, J = 4.63 Hz, 2 H) ppm.
13C NMR (125 MHz, CDC13): d = 196.20, 187.97, 147.64,
139.98, 132.42, 130.87, 129.48, 129.02, 128.04, 127.76,
66.99, 66.63, 52.40, 47.59 ppm.
Compound 2g: 1H NMR (500 MHz, CDC13): d = 8.81 (d,
J = 4.68 Hz, 2 H), 7.79 (d, J = 4.79 Hz, 2 H), 4.27 (t, J = 4.33
Hz, 2 H), 3.88 (t, J = 4.40 Hz, 2 H), 3.69 (t, J = 4.23 Hz, 2
H), 3.57 (t, J = 4.41 Hz, 2 H) ppm. 13C NMR (125 MHz,
CDC13): d = 193.71, 185.27, 150.79, 140.28, 123.01, 66.90,
66.73, 52.41, 47.69 ppm.
(12) Spectral Data of the Products
Compound 2a: 1H NMR (500 MHz, CDC13): d = 7.93 (d,
J = 7.18 Hz, 2 H), 7.55 (t, J = 7.41 Hz, 1 H), 7.43 (t, J = 7.76
Hz, 2 H), 4.27 (t, J = 4.94 Hz, 2 H), 3.84 (t, J = 4.95 Hz, 2
H), 3.63 (t, J = 4.77 Hz, 2 H), 3.53 (t, J = 4.76 Hz, 2 H) ppm.
13C NMR (125 MHz, CDC13): d = 196.16, 188.31, 134.86,
133.70, 130.27, 129.38, 66.93, 66.81, 52.35, 47.54 ppm. MS
(EI): m/z (%) = 235 (73) [M+], 177 (30), 130 (100), 105 (53),
86 (71), 77 (46). Anal Calcd for C12H13NO2S: C, 61.25; H,
5.57; N, 5.95. Found: 61.59; H, 5.72; N, 5.94.
Compound 2h: 1H NMR (500 MHz, CDC13): d = 9.21 (d,
J = 8.70 Hz, 1 H), 8.07 (d, J = 8.15 Hz, 1 H), 8.02 (d,
J = 7.10 Hz, 1 H), 7.90 (d, J = 8.15 Hz, 1 H), 7.69 (t, J = 7.40
Hz, 1 H), 7.58 (t, J = 7.50 Hz, 1 H), 7.50 (t, J = 7.72 Hz, 1
H), 4.37 (t, J = 4.86 Hz, 2 H), 3.42 (t, J = 4.87 Hz, 2 H),
3.71–3.69 (m, 4 H) ppm. 13C NMR (125 MHz, CDC13): d =
197.38, 189.76, 135.85, 134.62, 133.27, 131.93, 129.84,
129.55, 129.14, 127.37, 126.48, 124.66, 66.91, 66.79, 52.47,
47.88 ppm.
Compound 2b: 1H NMR (500 MHz, CDC13): d = 7.82 (d,
J = 8.21 Hz, 2 H), 7.22 (t, J = 8.11 Hz, 2 H), 4.26 (t, J = 4.95
Hz, 2 H), 3.83 (t, J = 4.95 Hz, 2 H), 3.62 (t, J = 4.77 Hz, 2
H), 3.53 (t, J = 4.71 Hz, 2 H), 2.36 (s, 3 H) ppm. 13C NMR
(125 MHz, CDC13): d = 196.47, 188.30, 146.13, 131.19,
130.40, 130.10, 66.93, 66.81, 52.32, 47.51, 22.29 ppm.
Compound 2c: 1H NMR (500 MHz, CDC13): d = 7.88 (d,
J = 8.80 Hz, 2 H), 6.87 (d, J = 8.82 Hz, 2 H), 4.24 (t, J = 4.23
Hz, 2 H), 3.81 (t, J = 4.63 Hz, 2 H), 3.80 (s, 3 H), 3.60 (t,
J = 4.02 Hz, 2 H), 3.51 (s, 2 H) ppm. 13C NMR (125 MHz,
CDC13): d = 196.54, 188.31, 165.07, 132.73, 126.46,
114.73, 66.95, 66.79, 56.10, 52.33, 47.57 ppm.
Compound 2i: 1H NMR (500 MHz, CDC13): d = 8.54 (s, 1
H), 7.97 (d, J = 8.52 Hz, 1 H), 7.90 (d, J = 8.15 Hz, 1 H),
7.86 (d, J = 8.62 Hz, 1 H), 7.82 (d, J = 8.09 Hz, 1 H), 7.57 (t,
J = 7.47 Hz, 1 H), 7.50 (t, J = 7.53 Hz, 1 H), 4.32 (t, J = 4.89
Hz, 2 H), 3.87 (t, J = 4.90 Hz, 2 H), 3.63 (t, J = 4.54 Hz, 2
H), 3.57 (t, J = 4.28 Hz, 2 H) ppm. 13C NMR (125 MHz,
CDC13): d = 196.25, 188.46, 136.61, 132.94, 132.85,
131.04, 130.27, 129.78, 129.39, 128.36, 127.60, 124.76,
66.98, 66.85, 52.44, 47.67 ppm.
Compound 2d: 1H NMR (500 MHz, CDC13): d = 7.79 (d,
J = 8.40 Hz, 2 H), 7.56 (d, J = 8.40 Hz, 2 H), 4.24 (t, J = 4.87
Hz, 2 H), 3.80 (t, J = 4.90 Hz, 2 H), 3.63 (t, J = 4.75 Hz, 2
H), 3.52 (t, J = 4.65 Hz, 2 H) ppm. 13C NMR (125 MHz,
CDC13): d = 195.49, 185.86, 141.22, 132.30, 131.49,
129.55, 66.90, 66.72, 52.33, 47.58 ppm.
Compound 2j: 1H NMR (500 MHz, CDC13): d = 10.11 (s, 1
H), 8.15 (d, J = 8.28 Hz, 2 H), 7.99 (d, J = 8.25 Hz, 2 H),
4.34 (t, J = 4.93 Hz, 2 H), 3.92 (t, J = 4.95 Hz, 2 H), 3.71 (t,
J = 5.23 Hz, 2 H), 3.62 (t, J = 5.15 Hz, 2 H) ppm. 13C NMR
(125 MHz, CDC13): d = 194.98, 191.65, 186.43, 140.14,
138.34, 130.74, 130.30, 66.94, 66.79, 52.44, 47.69 ppm.
Compound 2k: 1H NMR (500 MHz, CDC13): d = 7.88 (d,
J = 7.19 Hz, 2 H), 7.60 (t, J = 6.90 Hz, 1 H), 7.45 (t, J = 7.78
Hz, 2 H), 3.75–3.71 (m, 4 H), 3.59 (t, J = 2.82 Hz, 2 H), 3.31
(t, J = 4.82 Hz, 2 H) ppm. 13C NMR (125 MHz, CDC13):
d = 191.56, 165.89, 135.37, 133.48, 130.10, 129.53, 67.16,
67.09, 46.70, 42.06 ppm.
Compound 2e: 1H NMR (500 MHz, CDC13): d = 7.94 (d,
J = 8.56 Hz, 2 H), 7.46 (d, J = 8.52 Hz, 2 H), 4.31 (t, J = 4.95
Hz, 2 H), 3.89 (t, J = 4.95 Hz, 2 H), 3.69 (t, J = 4.82 Hz, 2
H), 3.58 (t, J = 4.82 Hz, 2 H) ppm. 13C NMR (125 MHz,
CDC13): d = 195.54, 186.76, 141.38, 132.34, 131.57,
129.69, 66.92, 66.75, 52.35, 47.61 ppm. Anal. Calcd for
Synlett 2008, No. 6, 892–896 © Thieme Stuttgart · New York