Copper-catalyzed one-pot synthesis of functionalized 1,4-dihydroazete derivatives from sulfonyl azides, terminal alkynes, and tetramethylguanidine

Article abstract of DOI:10.1055/s-0032-1317075

Ketenimine intermediates generated by the addition of copper acetylides to sulfonyl azides are trapped by tetramethylguanidine to afford 1,4-dihydro-N,N-dimethyl-3-aryl(alkyl)-4-aryl(alkyl)sulfoniminoazet-2-amine derivatives in moderate to good yields. Ge

Full text of DOI:10.1055/s-0032-1317075

LETTER
▌2215
lCettoerpper-Catalyzed One-Pot Synthesis of Functionalized 1,4-Dihydroazete
Derivatives from Sulfonyl Azides, Terminal Alkynes, and Tetramethylguanidine
Functionalized 1,4-Dihydroazete Derivatives
Issa Yavari,* Manijeh Nematpour
Department of Chemistry, Tarbiat Modares University, PO Box 14115-175, Tehran, Iran
Fax +98(21)82883455; E-mail: yavarisa@modares.ac.ir
Received: 20.05.2012; Accepted after revision: 18.07.2012
rived from monocyclic azet have been exploited for their
Abstract: Ketenimine intermediates generated by the addition of
utility as versatile synthetic intermediates.9–12
copper acetylides to sulfonyl azides are trapped by tetramethyl-
guanidine to afford 1,4-dihydro-N,N-dimethyl-3-aryl(alkyl)-4-
aryl(alkyl)sulfoniminoazet-2-amine derivatives in moderate to
good yields.
The ketenimine intermediates generated in the copper-
catalyzed azide–alkyne cycloaddition reaction can be
trapped by various nucleophiles.13–15 In this way, skele-
tons of various heterocycles were successfully synthe-
sized.16–20 Applying this strategy, we used
tetramethylguanidine to trap the in situ generated keten-
imines and obtained 1,4-dihydro-N,N-dimethyl-3-aryl-
(alkyl)-4-aryl(alkyl)sulfoniminoazet-2-amine derivatives
(Table 1). Herein, we report the details of this work.21
Key words: 1,4-dihydroazete, copper iodide, tetramethyl-
guanidine, sulfonyl azide, terminal alkyne, ketenimine
Four-membered nitrogen heterocycles possess a wide
range of biological activities and are used as antibiotic and
antibacterial agents.1 These strained ring systems have
found increasing applications as intermediates in the syn-
thesis of pharmaceuticals.2,3 Among them, the 2-azetine
series, especially azetinones, has gained much attention.4,5
It has been reported that monocyclic azet systems are re-
markably active, not only as antibacterials, but also as
inhibitors of cytomegalovirus protease,6 cholesterol ab-
sorption,7 and human tryptase.8 Moreover, scaffolds de-
First, we studied the reaction between the ketenimine gen-
erated from phenylacetylene (1a) and p-toluensulfonyl
azide (2a) with tetramethylguanidine (3). This reaction
proceeded smoothly at room temperature and afforded
1,4-dihydro-N,N-dimethyl-3-phenyl-4-tosyliminoazet-2-
amine (4a) in 80% yield. This result prompted us to opti-
mize the reaction conditions for the synthesis of other
derivatives (Table 1).
Table 1 Synthesis of Compounds 4a–i
NH
R1
O
O
R2
R1
N
S
H
R2
Me2N
NMe2
1
S
N
CuI (10 mol%), Et3N
THF, r.t. 12 h
O
+
C
O
NH
3
O
O
Me2N
R1
4
S
R2
N3
2
8
Entry
1, 2, 4
R1
R2
Yield of 4 (%)
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
Ph
Ph
Ph
4-Tol
Ph
80
82
74
67
60
57
62
57
53
Me
n-Pr
n-Pr
n-Pr
n-Bu
n-Bu
n-Bu
4-Tol
Ph
Me
g
h
i
4-Tol
Ph
Me
SYNLETT 2012, 23, 2215–2218
Advanced online publication: 17.08.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1317075; Art ID: ST-2012-D0430-L
© Georg Thieme Verlag Stuttgart · New York

2216
I. Yavari, M. Nematpour
LETTER
R1
Cu
SO2R2
N2
R1
N
CuI
2
H+
– N2
1
R1
Cu
N
N
SO2R2
Cu
N
5
6
7
–
R1
R1
NSO2R2
H
C
NSO2R2
Me
3
– NHMe2
intramolecular
cyclization
C
C
NSO2R2
HN
4
Me2N
Me2N
R1
NH
N
+
Me2N
Me
8
10
9
Scheme 1
Several catalysts such as CuI, CuBr, CuCl, Cu2O, and iminoazet-2-amine derivatives. The present method may
copper powder were tested with CuI and CuBr giving the be considered as a practical route for the synthesis of func-
best results. Among several solvents screened, tetrahydro- tionalized 4-iminosulfonyl-N,N-dimethyl-1,4-dihydroaz-
furan (THF) was the best. Thus, the optimized reaction et-2-amine, which may be converted to azetin-2-one
conditions used were 10 mol% of CuI, 1 mmol of the derivatives.
alkyne, 1.2 mmol of sulfonyl azide, and 1 mmol of tetra-
methylguanidine in THF at room temperature.
References and Notes
Phenylacetylene readily participates in the coupling reac-
tion to furnish the corresponding 1,4-dihydro-N,N-
dimethyl-3-aryl-4-aryl(alkyl)sulfoniminoazet-2-amine
(1) Alcaide, B.; Almendros, P. In Progress in Heterocyclic
Chemistry; Vol. 17; Gribble, G. W.; Joule, J. A., Eds.;
Elsevier: Oxford, 2005, 64–83.
derivatives in good yields (Table 1). Aliphatic acetylenes
served as low-yielding substrates compared to phenyl-
acetylene. Aromatic and aliphatic sulfonyl azides reacted
efficiently, and the corresponding products were obtained
in good yields.
(2) Parrick, J.; Mehta, L. K. In Comprehensive Heterocyclic
Chemistry II; Padwa, A., Ed.; Elsevier: Oxford, 1996, 659–
720.
(3) Pinder, A. R. Nat. Prod. Rep. 1992, 9, 491.
(4) Furin, G. G.; Lopyrev, V. A.; Zhuzhgov, E. I.; Protsuk, N. I.
Russ. J. Org. Chem. 2000, 36, 99.
(5) Zanatta, N.; Barichello, R.; Bonacorso, H. G.; Martins, M.
A. P. Synthesis 1999, 765.
(6) Yoakim, C.; Ogilvie, W. W.; Cameron, D. R.; Chabot, C.;
Guse, I.; Hache, B.; Naud, J.; O’Meara, J. A.; Plante, R.;
Deziel, R. J. Med. Chem. 1998, 41, 2882.
1
Structures of compounds 4a–i were assigned by IR, H
1
NMR, 13C NMR and mass spectral data. The H NMR
spectrum of 4a exhibited three singlets for methyl (δ =
2.38 ppm), dimethyl amino (δ = 2.80 ppm), and NH (δ =
3.94 ppm, exchanged with D2O) protons, along with char-
acteristic multiplets for the aryl groups. The 13C NMR
spectrum of 4a exhibits 13 signals in agreement with the
proposed structure. The mass spectrum of 4a displayed
the molecular ion peak at m/z = 341. The NMR spectra of
compounds 4b–i are similar to those of 4a, except for the
substituents, which showed characteristic signals in the
appropriate regions of the spectra.
(7) Kvaerno, L.; Werder, M.; Hauser, H.; Carreira, E. M. J. Med.
Chem. 2005, 48, 6035.
(8) Bisacchi, G. S.; Slusarchyk, W. A.; Bolton, S. A.; Hartl, K.
S.; Jacobs, G.; Mathur, A.; Meng, W.; Ogletree, M. L.; Pi,
Z.; Sutton, J. C.; Treuner, U.; Zahler, R.; Zhao, G.; Seiler, S.
M. Bioorg. Med. Chem. Lett. 2004, 14, 2227.
(9) Ma, Y.; Qian, C. T. Tetrahedron Lett. 2000, 41, 945.
(10) Burtosolo, A. C. B.; Correa, C. R. D. Tetrahedron Lett.
2006, 47, 6377.
(11) George, L.; Bernhardt, P. V.; Netsch, K. P.; Wentrup, C.
Org. Biomol. Chem. 2004, 2, 3518.
(12) Finnerty, J. J.; Wentrup, C. J. Org. Chem. 2004, 69, 1909.
(13) Wang, J.; Wang, J.; Zhu, Y.; Lu, P.; Wang, Y. Chem.
Commun. 2011, 47, 3275.
(14) Whiting, M.; Fokin, V. V. Angew. Chem. Int. Ed. 2006, 45,
3157.
(15) Xu, X.; Cheng, D.; Li, J.; Guo, H.; Yan, J. Org. Lett. 2007,
9, 1585.
(16) Cui, S. L.; Lin, X. F.; Wang, Y. G. Org. Lett. 2006, 8, 4517.
(17) Cui, S. L.; Wang, J.; Wang, Y. G. Tetrahedron 2008, 64,
487.
(18) Yao, W.; Pan, L.; Zhang, Y.; Wang, G.; Wang, X.; Ma, C.
Angew. Chem. Int. Ed. 2010, 49, 9210.
A plausible mechanism for the formation of compounds 4
is given in Scheme 1. The yellow copper acetylide 5,
formed from 1 and CuI, undergoes a 1,3-dipolar cyclo-
addition reaction with sulfonyl azide 2, to generate the tri-
azole derivative 6.22 This intermediate undergoes ring
opening to afford the α-diazoimino species 7, which is
converted to ketenimine derivative 8 by loss of nitrogen
gas and protonation.23–25 Ketenimine intermediate 8 is
trapped by 1,1,3,3-tetramethylguanidine to yield the
zwitterionic adduct 9, which undergoes an intramolecular
cyclization reaction and subsequent loss of dimethyl-
amine to generate product 4.
(19) Yavari, I.; Nematpour, M.; Yavari, S.; Sadeghizadeh, F.
Tetrahedron Lett. 2012, 53, 1889.
(20) Yavari, I.; Nematpour, M.; Ghazanfarpour-Darjani, M.
Tetrahedron Lett. 2012, 53, 942.
In summary, ketenimine intermediates generated by the
addition of copper acetylides to sulfonyl azides are
trapped by 1,1,3,3-tetramethylguanidine to yield 1,4-di-
hydro-N,N-dimethyl-3-aryl(alkyl)-4-aryl(alkyl)sulfon-
Synlett 2012, 23, 2215–2218
© Georg Thieme Verlag Stuttgart · New York

LETTER
Functionalized 1,4-Dihydroazete Derivatives
2217
(21) General Procedure for the Synthesis of Compounds 4
To a mixture of sulfonyl azide 2, (1.2 mmol), alkyne 1
(1 mmol), and CuI (0.1 mmol), Et3N (1 mmol) in THF (2
mL) was slowly added tetramethylguanidine (1 mmol). The
mixture was stirred at r.t. After completion of the reaction
[about 12 h; TLC (EtOAc–hexane = 1:5) monitoring], the
mixture was diluted with CH2Cl2 (2 mL) and aq NH4Cl
solution (3 mL), stirred for 30 min, and the layers were
separated. The aqueous layer was extracted with CH2Cl2
(3 × 3 mL), and the combined organic fractions were dried
(Na2SO4) and concentrated under reduced pressure. The
residue was purified by flash column chromatography [silica
gel (230–400 mesh; Merck), hexane–EtOAc = 5:1] to give
the product.
91 (55), 77 (44), 43 (22). Anal. Calcd (%) for C15H21N3O2S
(307.01): C, 58.61; H, 6.89; N, 13.67. Found: C, 58.89; H,
6.94; N, 13.54.
1,4-Dihydro-N,N-dimethyl-3-propyl-4-
benzenesulfoniminoazet-2-amine (4e)
Yellow oil; yield 0.17 g (60%). IR (KBr): νmax = 3455, 2925,
1513, 1269, 1142, 1087 cm–1. 1H NMR (500 MHz, CDCl3):
δ = 0.87 (3 H, t, 3J = 7.0 Hz, Me), 1.57–1.60 (2 H, m, CH2),
2.51 (2 H, t, 3J = 7.0 Hz, CH2), 2.89 (6 H, s, NMe2), 3.93 (1
H, s, NH), 7.39 (1 H, t, 3J = 8.0 Hz, Ar), 7.43 (2 H, t, 3J = 8.0
Hz, Ar), 7.91 (2 H, d, 3J = 8.0 Hz, Ar). 13C NMR (125.7
MHz, CDCl3): δ = 13.9 (Me), 21.1 (CH2), 22.8 (CH2), 40.5
(NMe2), 86.1 (C), 124.3 (CH), 125.6 (2 CH), 128.2 (2 CH),
135.7 (C), 162.9 (C), 172.4 (C). MS (EI): m/z (%) = 293 (3)
[M+], 249 (13), 223 (8), 141 (100), 111 (18), 77 (34), 43 (35).
Anal. Calcd (%) for C14H19N3O2S (293.00): C, 57.31; H,
6.53; N, 14.32. C, 57.16; H, 6.48; N, 14.21.
1,4-Dihydro-N,N-dimethyl-3-phenyl-4-tosyliminoazet-2-
amine (4a)
Yellow oil; yield 0.27 g (80%). IR (KBr): νmax = 3438, 2923,
1511, 1396, 1270, 1139, 1083 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 2.38 (3 H, s, Me), 2.80 (6 H, s, NMe2), 3.94
(1 H, s, NH), 7.18 (2 H, t, 3J = 7.5 Hz, Ar), 7.24 (1 H, t, 3J =
7.5 Hz, Ar), 7.25–7.28 (4 H, m, Ph), 7.76 (2 H, d, 3J = 7.9
Hz, Ar). 13C NMR (125.7 MHz, CDCl3): δ = 29.7 (Me), 40.5
(NMe2), 88.3 (C), 125.7 (CH), 126.4 (2 CH), 128.1 (2 CH),
128.7 (2 CH), 129.6 (2 CH), 137.5 (C), 141.1 (C), 142.3 (C),
166.1 (C), 169.2 (C). MS (EI): m/z (%) = 341 (2) [M+], 297
(8), 271 (12), 155 (100), 145 (22), 91 (70), 77 (54). Anal.
Calcd (%) for C18H19N3O2S (341.00): C, 63.32; H, 5.61; N,
12.31. Found: C, 63.79; H, 5.54; N, 12.38.
1,4-Dihydro-N,N-dimethyl-3-propyl-4-
methanesulfoniminoazet-2-amine (4f)
Yellow oil; yield 0.13 g (57%). IR (KBr): νmax = 3436, 1518,
1269, 1142, 1086 cm–1. 1H NMR (500 MHz, CDCl3): δ =
0.86 (3 H, t, 3J = 6.9 Hz, Me), 1.57–1.61 (2 H, m, CH2), 2.50
(2 H, t, 3J = 6.9 Hz, CH2), 2.76 (3 H, s, Me), 2.90 (6 H, s,
NMe2) 3.92 (1 H, s, NH). 13C NMR (125.7 MHz, CDCl3): δ
= 13.8 (Me), 19.1 (CH2), 22.6 (CH2), 29.6 (Me), 40.5
(NMe2), 86.7 (C), 162.9 (C), 172.5 (C). MS (EI): m/z
(%) = 231 (2) [M+], 187 (11), 161 (18), 119 (41), 79 (100),
43 (25). Anal. Calcd (%) for C9H17N3O2S (231.02): C, 46.73;
H, 7.41; N, 18.17. Found: C, 47.02; H, 7.34; N, 18.08.
1,4-Dihydro-N,N-dimethyl-3-butyl-4-tosyliminoazet-2-
amine (4g)
Yellow oil; yield 0.20 g (62%). IR (KBr): νmax = 3452, 2933,
1520, 1401, 1270, 1118 cm–1. 1H NMR (500 MHz, CDCl3):
δ = 0.85 (3 H, t, 3J = 6.9 Hz, Me), 1.23–1.27 (2 H, m, CH2),
1.59–1.62 (2 H, m, CH2), 2.40 (3 H, s, Me), 2.50 (2 H, t,
3J = 7.0 Hz, CH2), 2.85 (6 H, s, NMe2), 3.97 (1 H, s, NH),
7.38 (2 H, d, 3J = 8.0 Hz, Ar), 7.92 (2 H, d, 3J = 8.0 Hz, Ar).
13C NMR (125.7 MHz, CDCl3): δ = 13.9 (Me), 19.4 (CH2),
22.4 (CH2), 27.3 (CH2), 30.4 (Me), 40.3 (NMe2), 86.6 (C),
125.6 (2 CH), 128.2 (2 CH), 137.7 (C), 141.2 (C), 166.0 (C),
172.5 (C). MS (EI): m/z (%) = 321 (3) [M+], 277 (16), 251
(13), 155 (100), 125 (20), 91 (51), 77 (41), 57 (52). Anal.
Calcd (%) for C16H23N3O2S (321.00): C, 59.78; H, 7.21; N,
13.07. Found: C, 59.51; H, 7.27; N, 13.18.
1,4-Dihydro-N,N-dimethyl-3-phenyl-4-
benzenesulfoniminoazet-2-amine (4b)
Yellow oil; yield 0.27 g (82%). IR (KBr): νmax = 3437, 2929,
1520, 1400, 1273, 1142, 1087 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 2.85 (6 H, s, NMe2), 3.98 (1 H, s, NH), 7.15
(2 H, t, 3J = 7.5 Hz, Ar), 7.20 (1 H, t, 3J = 7.5 Hz, Ar), 7.23
(2 H, d, 3J = 7.5 Hz, Ar), 7.25–7.29 (3 H, m, Ph), 7.78 (2 H,
d, 3J = 7.9 Hz, Ar). 13C NMR (125.7 MHz, CDCl3): δ = 40.6
(NMe2), 88.5 (C), 125.6 (CH), 126.5 (2 CH), 128.3 (2 CH),
128.8 (2 CH), 129.7 (2 CH), 137.5 (CH), 140.2 (C), 142.5
(C), 165.7 (C), 170.8 (C). MS (EI): m/z (%) = 327 (3) [M+],
283 (12), 257 (17), 145 (31), 141 (100), 77 (54). Anal. Calcd
(%) for C17H17N3O2S (327.05): C, 62.36; H, 5.23; N, 12.83.
Found: C, 62.06; H, 5.18; N, 12.75.
1,4-Dihydro-N,N-dimethyl-3-phenyl-4-
methanesulfoniminoazet-2-amine (4c)
Yellow oil; yield 0.20 g (74%). IR (KBr): νmax = 3435, 2925,
1531, 1401, 1271, 1120 cm–1. 1H NMR (500 MHz, CDCl3):
δ = 2.81 (3 H, s, Me), 2.93 (6 H, s, NMe2), 4.01 (1 H, s, NH),
7.26 (2 H, t, 3J = 7.4 Hz, Ar), 7.30 (1 H, t, 3J = 7.4 Hz, Ar),
7.42 (2 H, d, 3J = 7.4 Hz, Ar). 13C NMR (125.7 MHz,
CDCl3): δ = 29.2 (Me), 40.4 (NMe2), 87.2 (C), 124.4 (CH),
125.7 (2 CH), 126.4 (2 CH), 140.9 (C), 162.3 (C), 172.5 (C).
MS (EI): m/z (%) = 265 (2) [M+], 221 (10), 195 (14), 145
(30), 79 (100), 77 (44). Anal. Calcd (%) for C12H15N3O2S
(265.00): C, 54.32; H, 5.70; N, 15.06. Found: C, 54.66; H,
5.64; N, 15.16.
1,4-Dihydro-N,N-dimethyl-3-butyl-4-
benzenesulfoniminoazet-2-amine (4h)
Yellow oil; yield 0.17 g (57%). IR (KBr): νmax = 3413, 2926,
1517, 1401, 1268, 1141, 1086 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 0.84 (3 H, t, 3J = 6.9 Hz, Me), 1.24–1.28 (2 H,
m, CH2), 1.59–1.62 (2 H, m, CH2), 2.49 (2 H, t, 3J = 7.0 Hz,
CH2), 2.92 (6 H, s, NMe2), 3.98 (1 H, s, NH), 7.26 (1 H, t,
3J = 7.9 Hz, Ar), 7.39 (2 H, t, 3J = 7.9 Hz, Ar), 7.80 (2 H, d,
3J = 7.9 Hz, Ar). 13C NMR (125.7 MHz, CDCl3): δ = 14.1
(Me), 20.0 (CH2), 22.5 (CH2), 27.3 (CH2), 40.5 (NMe2), 86.4
(C), 125.6 (CH), 128.7 (2 CH), 129.6 (2 CH), 138.6 (C),
167.3 (C), 172.5 (C). MS (EI): m/z (%) = 307 (3) [M+], 263
(12), 237 (10), 141 (100), 125 (29), 77 (35), 57 (50). Anal.
Calcd (%) for C15H21N3O2S (307.00): C, 58.61; H, 6.89; N,
13.67. Found: C, 58.86; H, 6.74; N, 13.55.
1,4-Dihydro-N,N-dimethyl-3-propyl-4-tosyliminoazet-2-
amine (4d)
Yellow oil; yield 0.20 g (67%). IR (KBr): νmax = 3436, 2924,
1510, 1397, 1271, 1138, 1084 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 0.87 (3 H, t, 3J = 6.9 Hz, Me), 1.57–1.62 (2 H,
m, CH2), 2.38 (3 H, s, Me), 2.52 (2 H, t, 3J = 7.0 Hz, CH2),
2.91 (6 H, s, NMe2), 3.94 (1 H, s, NH), 7.24 (2 H, d, 3J =
8.0 Hz, Ar), 7.81 (2 H, d, 3J = 8.0 Hz, Ar). 13C NMR (125.7
MHz, CDCl3): δ = 13.8 (Me), 21.4 (CH2), 22.7 (CH2), 29.7
(Me), 40.5 (NMe2), 86.1 (C), 126.5 (2 CH), 128.4 (2 CH),
139.0 (C), 142.0 (C), 166.6 (C), 172.5 (C). MS (EI): m/z
(%) = 307 (2) [M+], 263 (14), 197 (14), 155 (100), 111 (28),
1,4-Dihydro-N,N-dimethyl-3-butyl-4-
methanesulfoniminoazet-2-amine (4i)
Yellow oil; yield 0.13 g (53%). IR (KBr): νmax = 3513, 2934,
1521, 1403, 1270, 1118, 1042 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 0.85 (3 H, t, 3J = 6.9 Hz, Me), 1.21–1.29 (2 H,
m, CH2), 1.62–1.66 (2 H, m, CH2), 2.47 (2 H, t, 3J = 7.0 Hz,
CH2), 2.82 (3 H, s, Me), 2.90 (6 H, s, NMe2), 3.96 (1 H, s,
NH). 13C NMR (125.7 MHz, CDCl3): δ = 14.0 (Me), 19.8
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2215–2218

2218
I. Yavari, M. Nematpour
LETTER
formation of the desired 1-sulfonyltriazoles: Yoo, E. J.;
Ahlquist, M.; Kim, S. H.; Bae, I.; Fokin, V. V.; Sharpless, K.
B.; Chang, S. Angew. Chem. Int. Ed. 2007, 46, 1730.
(23) Cassidy, M. P.; Raushel, J.; Fokin, V. V. Angew. Chem. Int.
Ed. 2006, 45, 3154.
(CH2), 22.4 (CH2), 27.3 (CH2), 30.7 (Me), 40.4 (NMe2), 86.2
(C), 166.2 (C), 172.6 (C). MS (EI): m/z (%) = 245 (2) [M+],
201 (12), 175 (10), 125 (39), 79 (100), 57 (46). Anal. Calcd
(%) for C10H19N3O2S (245.00): C, 48.95; H, 7.81; N, 17.13.
Found: C, 48.76; H, 7.75; N, 17.04
(22) Recently, Sharpless and co-workers established anhydrous
conditions with CuI in CHCl3/2,6-lutidine at 0 °C to prevent
intermediate 6 from decomposing and provide selective
(24) Lu, P.; Wang, Y. Synlett 2010, 165.
(25) Yoo, E. J.; Ahlquist, M.; Bae, I.; Sharpless, K. B.; Fokin, V.
V.; Chang, S. J. Org. Chem. 2008, 73, 5520.
Synlett 2012, 23, 2215–2218
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