Synthesis of imidazoles from ketimines using tosylmethyl isocyanide (TosMIC) catalyzed by bismuth triflate

Article abstract of DOI:10.2174/157017809788681275

The preparation of a series of 1, 5-disubstituted-4-methyl imidazoles is reported, based on an unprecedented reaction between an acetimine and TosMIC in the presence of tert-butylamine and catalytic amounts of bismuth(III) triflate. The mechanism is belie

Full text of DOI:10.2174/157017809788681275

354
Letters in Organic Chemistry, 2009, 6, 354-358
Synthesis of Imidazoles from Ketimines Using Tosylmethyl Isocyanide
(TosMIC) Catalyzed by Bismuth Triflate
Mokhtar Fodili1, Bellara Nedjar-Kolli*,1, Bernard Garrigues2, Christian Lherbet3 and
Pascal Hoffmann*,3
1Laboratoire de Chimie Organique Appliquée, Faculté de Chimie, Université Houari Boumédiéne, Alger, Algérie
2Laboratoire d’Hétérochimie Fondamentale et Appliquée, UMR-CNRS 5069, Université Paul Sabatier, Toulouse,
France
3Laboratoire de Synthèse et Physico-Chimie de Molécules d’Intérêt Biologique, UMR-CNRS 5068, Université Paul Sa-
batier, Toulouse, France
Received January 12, 2009: Revised April 20, 2009: Accepted April 20, 2009
Abstract: The preparation of a series of 1,5-disubstituted-4-methyl imidazoles is reported, based on an unprecedented re-
action between an acetimine and TosMIC in the presence of tert-butylamine and catalytic amounts of bismuth(III) triflate.
The mechanism is believed to involve formation of an imidazoline intermediate, followed by the methyl group migration,
and subsequent aromatization to the imidazole ring.
Imidazoles are a class of heterocycles incorporated into
many important biological molecules such as histidine,
adenine, guanine or vitamin B12 [1], and that also occur in a
number of pharmaceuticals, fungicides or herbicides [2].
Consequently many synthetic approaches have been devel-
oped for the construction of this heterocyclic ring system.
Among these, a variety of substituted imidazoles were
prepared from various imino compounds using the versatile
reagent tosylmethyl isocyanide (TosMIC) [3]. This approach
leads to 1,5-disubstituted imidazoles when unsubstituted
TosMIC is used, or to 1,4,5-trisubstituted imidazoles, which
are otherwise not easily accessible, when ꢀ-monosubstituted
group occurred on the imidazoline intermediate, enabling the
aromatization to imidazole with concomitant loss of the
tosylsulphinic group. To our surprise, such a migration and
consequently the formation of 4-methylimidazole com-
pounds (3) were observed when acetimine derivatives (2),
prepared from dehydroacetic acid (1), were allowed to react
with TosMIC in the presence of catalytic amounts of bis-
muth triflate (Scheme 1).
Given the important role that dehydroacetic acid (DHA)
derivatives play in the preparation of organic compounds of
biological relevance [5], or as ligand for metal complexation
H3C
R
HO
O
O
HO
N
O
HO
N
R
NH2
TosMIC
CH3
CH3
N
R
H3C
O
H3C
O
H3C
O
O
1
2
3
Scheme 1.
TosMIC derivatives are used [4]. The mechanism by which
the imidazole ring forms involves cycloaddition of TosMIC
to an aldimine (R’CH=NR) carbon-nitrogen double bond,
followed by cyclization and proton rearrangement to produce
an imidazoline intermediate, which then aromatizes to give
an imidazole ring with loss of tosylsulphinic acid. This
mechanism is unlikely to apply to ketimines (R’R’’C=NHR),
but could become a possibility if migration of the R’ or R’’
[6], we became interested in synthesizing DHA-imidazole
conjugates, and envisaged the use of TosMIC for their prepa-
ration. On the other hand, we recently found that the reaction
of aldimines with TosMIC for the preparation of imidazole
compounds was considerably accelerated by the presence of
catalytic amounts of bismuth(III) chloride or triflate [7].
Indeed, it is known that ꢀ-isocyano carbanion species are
thermolabile and decompose easily in solution. It was
assumed that the presence of metal ions stabilized the inter-
mediate carbanion arising after ꢀ-deprotonation of the metal-
lated TosMIC by a base through formation of a complex
such as I (Scheme 2), as previously reported with silver [8]
or platinum [9].
*Address correspondence to these authors at the Laboratoire de Chimie
Organique Appliquée, Faculté de Chimie, Université Houari Boumédiéne,
Alger, Algérie; E-mail: bellara_kollidz@yahoo.fr
Laboratoire de Synthèse et Physico-Chimie de Molécules d’Intérêt Bi-
ologique, UMR-CNRS 5068, Université Paul Sabatier, Toulouse, France;
E-mail: hoffmann@cict.fr
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

Synthesis of Imidazoles from Ketimines Using Tosylmethyl Isocyanide
Letters in Organic Chemistry, 2009, Vol. 6, No. 5
355
EXPERIMENTAL
CN
H
SO2Tol
H
General
"M"
base
M
CN
SO2Tol
Important Note
H
TosMIC
("M" = metal ion)
I
It should be noted that problems of reproducibility for the
preparation of compound 3 were occasionally encountered in
our laboratory. To overcome these problems, attention
should be paid to the purity of starting materials (in particu-
lar TosMIC reagent, obtained from Aldrich) and solvents.
Scheme 2.
Therefore, we successfully prepared a series of imida-
zoles from aldimines and TosMIC in the presence of cata-
lytic quantities of bismuth triflate [7]. Moreover, unlike other
Lewis acids such as AlCl3, ZnCl2 or SnCl4, bismuth(III) salts
have been proved to be stable in protic solvents like metha-
nol, which is often used as solvent in TosMIC-induced reac-
tions. It should be noted that the catalytic properties of
Bi(III) compounds as Lewis acids are also known in a num-
ber of reactions like Michael reactions [10], acylation reac-
tions [11], oxidation of ꢀ-ketols [12] or epoxides [13], and
Diels-Alder reactions [14].
All chemicals were obtained from Aldrich or Acros
Organics. Nuclear magnetic resonance spectra were recorded
on a Bruker AC 300 spectrometer (1H and 13C NMR). Fol-
lowing abbreviations were used: s, singlet; d, doublet; t, trip-
let; q, quartet; m, multiplet. Mass spectra were recorded on a
Nermag R10-10C mass spectrometer using ionization energy
of 70 eV, and IR spectra on a Perkin-Elmer 883 spectropho-
tometer. Melting points were determined on a Büchi-512.
Ketimines 2c [15], 2f [16] and 2g [17] were synthesized
as already published. Ketimines 2a, 2b, 2d and 2e were pre-
pared by Schiff-base condensation of dehydroacetic acid
(0.01 mol) with a suitable amine (0.01 mol) in ethanol (40
mL) for 6 h at room temperature. The mixture was then con-
centrated to about 10 mL under reduced pressure, and the
expected ketimine 2 precipitated and was collected by filtra-
tion. 2a: Yield, 60 %. mp, 125 C. H NMR (CDCl3) ꢁ
(ppm), 2.10 (s, 3H, CH3), 2.70 (s, 3H, CH3), 4.60 (d, 2H, J =
5 Hz, CH2), 5.61 (s, 1H, CH=C), 6.30 (d, 2H, J = 2 Hz, H-
arom), 7.35 (t, 1H, J = 2 Hz, H-arom), 14.28 (s, 1H, OH).
2b: Yield, 68 %. mp, 120 C. H NMR (CDCl3) ꢁ (ppm),
2.18 (s, 3H, CH3), 2.75 (s, 3H, CH3), 4.84 (d, 2H, J = 5 Hz,
CH2), 5.70 (s, 1H, CH=C), 7.05 (d, 2H, J = 2 Hz, H-arom),
7.31 (t, 1H, J = 2 Hz, H-arom), 14.30 (s, 1H, OH). Ketimine
2d: Yield, 75 %. mp, 160 C. H NMR (CDCl3) ꢁ (ppm),
2.15 (s, 3H, CH3), 2.68 (s, 3H, CH3), 4.65 (d, 2H, J = 5 Hz,
CH2), 5.68 (s, 1H, CH=C), 7.20-7.50 (m, 4H, H-arom), 14.35
With these results in mind, we envisaged to investigate
whether ketimines 2 would also react with TosMIC under
our conditions to form the imidazole ring system. Compound
2 was first prepared in ethanol by Schiff-base condensation
of dehydroacetic acid (1) with a series of primary amino
compounds and the reactivity towards TosMIC was then
studied. The reactions were carried out in methanol, and the
best results were obtained when TosMIC was reacted in the
presence of an excess of tert-butylamine (5 eq) and a cata-
lytic amount of bismuth triflate (0.1 eq) at room temperature
for 48 hours. Workup and chromatography provided, as
major compounds, imidazoles 3 in 60-73 % yield (Table 1).
In the absence of any catalyst, imidazole 3a was obtained but
with a dramatically reduced yield (7 %). Relatively poor
yields were also obtained when potassium carbonate was
used as base, and bismuth(III) chloride as catalyst. As all
attempts to crystallize imidazole compound 3 for X-ray
structural determination failed, assignments of its structures
were based on microanalytical and spectral data that all con-
firmed imidazole ring formation. The 13C NMR spectra
display a signal atꢁ ~ 135 ppm attributed to the tertiary
carbon in the imidazole ring at position 2, and a signal at
ꢁ ~ 12 ppm corresponding to the methyl group at position 4
(ꢁ = 30 ppm in compounds 2). Moreover, all compounds
°
1
°
1
°
1
°
1
(s, 1H, OH). Ketimine 2e: Yield, 72 %. mp, 158 C. H
NMR (CDCl3) ꢁ (ppm), 2.15 (s, 3H, CH3), 2.65 (s, 3H, CH3),
4.70 (d, 2H, J = 5 Hz, CH2), 5.69 (s, 1H, CH=C), 7.10-7.50
(m, 4H, H-arom), 14.38 (s, 1H, OH).
General Procedure for the Preparation of Imidazole
Compound 3
1
displayed H NMR signals at ꢁ ~ 2.40 ppm (3H) and 8.20
A solution of ketimine 2 (10 mmol), tosylmethylisocya-
nide (1.95 g, 10 mmol), bismuth triflate (66 mg, 1 mmol)
and tert-butylamine (3.65 g, 50 mmol) in 40 mL methanol
was stirred at room temperature for 48 hours and purified by
column chromatography on silica gel using a mixture of
ethyl acetate and methanol (9:1) as eluent.
ppm (1H) assigned to the methyl group and the H2 proton of
the imidazole ring, respectively. The actual ring-forming
intermediate was not identified, but it is likely to be the
anticipated imidazoline-carbocation intermediate without
suitable hydrogens next to the positively charged carbon,
thus leading to migration of the methyl group to the adjacent
position, and subsequent aromatization (Scheme 3). This
intramolecular rearrangement would explain the formation of
4-methyl imidazole compound 3 in a straightforward
manner.
3-(1-((Furan-2-yl)methyl)-4-methyl-1H-imidazol-5-yl)-4-
hydroxy-6-methyl-2H-pyran-2-one (3a)
Yield, 60 %. mp, 179-181 °C. 1H NMR (CDCl3) ꢁ (ppm),
2.10 (s, 3H, CH3), 2.24 (s, 3H, CH3), 5.16 (s, 2H, CH2), 6.23
(s, 1H, CH=C), 6.37 (dd, 1H, J = 2.1 and 3.0 Hz, H-furan),
6.49 (d, 1H, J = 3.1 Hz, H-furan), 7.40 (d, 1H, J = 2.1 Hz),
8.30 (s, 1H, CH=N), 15.40 (s, 1H, OH). 13C NMR (CDCl3) ꢁ
(ppm), 12.4 (CH3), 19.1 (CH3), 44.3, 97.7, 106.4, 108.3,
111.1, 125.8, 135.6, 141.8, 142.2, 152.3, 154.3, 163.1, 183.2.
IR (KBr) ꢂ (cm-1), 1640, 3300. MS/EI, m/z 286 (M+). Anal.
In summary, we present here a novel application of the
versatile reagent TosMIC that expands its utility to the
synthesis of 1,5-disubstituted-4-methyl imidazoles from
dehydroacetic-substituted acetimines. The extension of this
work to other ketimines and the use of other Lewis acid
catalysts are currently being tested to improve the generality
of the method.

356 Letters in Organic Chemistry, 2009, Vol. 6, No. 5
Fodili et al.
Table 1. Synthesis of Substituted DHA-Imidazole Compounds
H3C
HO
CH3
HO
N
TosMIC, tBuNH2
Bi(OTf)3
N
R
N
R
CH3OH
H3C
O
O
H3C
O
O
2
3
Substrate (2)
Product (3) Yielda
Entry
m.p (°C)
H3C
HO
O
N
HO
O
CH3
N
3a
N
1
180
H3C
H3C
H3C
O
O
H3C
O
O
60 %
H3C
HO
O
N
HO
O
CH3
N
3b
N
2
192
O
S
H3C
O
S
70 %
H3C
HO
O
N
HO
O
CH3
N
3c
N
3
220
O
H3C
O
65 %
H3C
HO
N
HO
CH3
3d
N
N
4
225
H3C
O
O
H3C
O
O
Cl
Cl
68 %
H3C
HO
O
N
HO
O
CH3
N
3e
N
5
219
H3C
O
H3C
O
Br
Br
73 %

Synthesis of Imidazoles from Ketimines Using Tosylmethyl Isocyanide
Letters in Organic Chemistry, 2009, Vol. 6, No. 5
357
(Table 1). Contd…..
Substrate (2)
Entry
Product (3) Yielda
H3C
m.p (°C)
HO
N
HO
O
CH3
3f
N
N
6
163
H3C
O
O
CH3
H3C
O
63 %
CH3
N
HO
O
HO
CH3
3g
N
N
7
212
H3C
O
H3C
O
O
72 %
aIsolated yield after chromatography.
Tos
H
CH3
N
N
O
TosMIC
N
C
R2 NH2
R1
CH3
+
N
R1
CH3
R2
R1
R2
CH3
CH3
H
H
N
N
- Tos-
R1
R1
R1
N
N
N
CH3
R2
R2
R2
Scheme 3.
calcd for C15H14N2O4: C, 62.93; H, 4.93; N, 9.78. Found: C,
62.89; H, 5.01; N, 9.73.
C17H16N2O3: C, 68.91; H, 5.44; N, 9.45. Found: C, 68.87; H,
5.41; N, 9.42.
4-Hydroxy-6-methyl-3-(4-methyl-1-((thiophen-2-
yl)methyl)-1H-imidazol-5-yl)-2H-pyran-2-one (3b)
3-(1-(4-Chlorobenzyl)-4-methyl-1H-imidazol-5-yl)-4-
hydroxy-6-methyl-2H-pyran-2-one (3d)
Yield, 70 %, mp, 191-193 °C. 1H NMR (CDCl3) ꢀ
(ppm), 2.13 (s, 3H, CH3), 2.30 (s, 3H, CH3), 5.37 (s, 2H,
CH2), 6.28 (s, 1H, CH=C), 7.09 (d, 1H, J = 2.1 Hz, H-thio),
6.68 (dd, 1H, J = 2.0 and 3.1 Hz, H-thio), 6.72 (d, 1H, J =
3.1 Hz, H-thio), 8.23 (s, 1H, CH=N), 15.45 (s, 1H, OH). 13C
NMR (CDCl3) ꢀ (ppm), 12.2 (CH3), 19.8 (CH3), 44.7, 97.9,
107.8, 123.8, 125.7, 126.1, 127.2, 135.7, 137.7, 141.4, 153.9,
163.2, 184.1. IR (KBr) ꢁ (cm-1), 1635, 3310. MS/EI, m/z 302
(M+). Anal. calcd for C15H14N2O3S: C, 59.59; H, 4.67; N,
9.27. Found: C, 59.64; H, 4.71; N, 9.33.
Yield, 68 %. mp.224-226°C. 1H NMR (CDCl3) ꢀ (ppm),
2.13 (s, 3H, CH3), 2.40 (s, 3H, CH3), 5.20 (s, 2H, CH2), 6.25
(s, 1H, CH=C), 7.11-7.35 (m, 4H, H-arom), 8.23 (s, 1H,
CH=N) 15.45 (s, 1H, OH). 13C NMR (CDCl3) ꢀ (ppm), 12.2
(CH3), 19.2 (CH3), 46.7, 97.3, 108.2, 126.7, 126.9, 129.3,
131.4, 134.5, 135.8, 144.1, 153.1, 163.4, 184.7. IR (KBr) ꢁ
(cm-1), 1645, 3315. MS/EI, m/z 330 (M+). Anal. calcd for
C17H15ClN2O3: C, 61.73; H, 4.57; N, 8.47. Found: C, 61.69;
H, 4.54; N, 8.45.
3-(1-(4-Bromobenzyl)-4-methyl-1H-imidazol-5-yl)-4-
hydroxy-6-methyl-2H-pyran-2-one (3e)
3-(1-Benzyl-4-methyl-1H-imidazol-5-yl)-4-hydroxy-6-
methyl-2H-pyran-2-one (3c)
Yield, 73 %. mp, 218-220 °C. 1H NMR (CDCl3) ꢀ
(ppm), 2.15 (s, 3H, CH3), 2.42 (s, 3H, CH3), 5.22 (s, 2H,
CH2), 6.28 (s, 1H, CH=C), 7.14-7.40 (m, 4H, H-arom), 8.24
(s, 1H, CH=N) 15.40 (s, 1H, OH). 13C NMR (CDCl3) ꢀ
(ppm), 12.3 (CH3), 19.9 (CH3), 46.8, 97.1, 106.7, 120.6,
121.4, 127.1, 131.7, 135.8, 136.4, 144.8, 152.9, 163.2, 185.7.
IR (KBr) ꢁ (cm-1), 1650, 3315. MS/EI, m/z 375 (M+). Anal.
calcd for C17H15BrN2O3: C, 54.42; H, 4.03; N, 7.47. Found:
C, 54.39; H, 4.01; N, 7.45.
Yield, 65 %, mp, 219-221 °C. 1H NMR (CDCl3) ꢀ (ppm),
2.12 (s, 3H, CH3), 2.39 (s, 3H, CH3), 5.23 (s, 2H, CH2), 6.22
(s, 1H, CH=C), 7.15-7.38 (m, 5H, H-arom), 8.25 (s, 1H,
CH=N), 15.30 (s, 1H, OH). 13C NMR (CDCl3) ꢀ (ppm), 12.4
(CH3), 19.4 (CH3), 43.9, 98.0, 108.7, 125.2, 126.8, 127.8,
129.3, 135.3, 136.4, 142.9, 153.3, 163.0, 183.8. IR (KBr) ꢁ
(cm-1), 1640, 3320. MS/EI, m/z 296 (M+). Anal. calcd for

358 Letters in Organic Chemistry, 2009, Vol. 6, No. 5
Fodili et al.
[2]
[3]
De Luca, L. Curr. Med. Chem., 2006, 13, 1 and references cited
therein.
For reviews on TosMIC chemistry, see: (a) Lambert, C. J. Prakt.
Chem., 1998, 340, 483. (b) van Leusen, D.; van Leusen, A.M. Org.
React., 2001, 57, 417. (c) Tandon, V.; Rai, S. Sulfur Rep., 2003, 24,
307.
(a) ten Have, R.; Huisman, M.; Meetsma, A.; van Leusen, A.M.
Tetrahedron, 1997, 53, 11355. (b) Sisko J.; Kassick, A.J.; Mellin-
ger, M.; Filan, J.J.; Allen, A.; Olsen, M.A. J. Org. Chem., 2000, 65,
1516. (c) Chen, B.-C.; Zhao, R.; Bednarz, M.S.; Wang, B.;
Sundeen, J.E.; Barrish, J.C. J. Org. Chem., 2004, 69, 977. (d)
Samanta, S.K.; Kylanlahti, I.; Yli-Kauhaluoma, J. Bioorg. Med.
Chem. Lett., 2005, 15, 3717. (e) Bellina, F.; Cauteruccio, S.; Rossi,
R. Tetrahedron, 2007, 63, 4571.
Somogyi, L. Liebigs Ann., 1995, 721. (b) Kumar, D.; Singh, S.P.;
Martinez, A.; Fruchier, A.; Elguero, J.; Martinez-Ripoll, M.;
Carrio, J.S.; Virgili, A. Tetrahedron, 1995, 51, 4891. (c) El Kihel,
A.; Benchidmi, M.; Essassi, E.; Bauchat, P.; Danion-Bougot, R.
Synth. Commun., 1999, 29, 2435. (d) Sato, M.; Oda, T.; Iwamoto,
K.; Fujii, S. Heterocycles, 2003, 60, 899.
Liu, S.; Rettig, S.J.; Orvig, C. Inorg. Chem., 1991, 30, 4915. (b)
Cindric, M.; Vrdoljak, V.; Novak, T.K.; Curic, M.; Brbot-
Saranovic, A.; Kamenar, B. J. Mol. Struct., 2004, 701, 111.
Benyaqad, F.; Oussaid, A.; Fodili, M.; Oussaid, B.; Garrigues, B. J.
Mar. Chim. Heterocycl., 2003, 2, 1.
Grigg, R.; Lansdell, M.I.; Thornton-Pett, M. Tetrahedron, 1999,
55, 2025.
Motoyama, Y.; Kawakami, H.; Shimozono, K.; Aoki, K.;
Nishiyama, H. Organometallics, 2002, 21, 3408.
Le Roux, C.; Gaspard-Iloughmane, H.; Dubac, J.; Jaud, J.;
Vignaux, P. J. Org. Chem., 1993, 58, 1835.
Le Roux, C.; Mandrou, S.; Dubac, J. J. Org. Chem., 1996, 61,
6835.
3-(1-Ethyl-4-methyl-1H-imidazol-5-yl)-4-hydroxy-6-
methyl-2H-pyran-2-one (3f)
Yield, 63 %. mp, 162-164 °C. 1H NMR (CDCl3) ꢀ
(ppm), 1.52 (t, 3H, J = 7.1 Hz, CH3), 2.13 (s, 3H, CH3), 2.38
(s, 3H, CH3), 3.97 (q, 2H, J = 7.1 Hz, CH2), 6.28 (s, 1H,
CH=C), 8.24 (s, 1H, CH=N) 15.40 (s, 1H, OH). 13C NMR
(CDCl3) ꢀ (ppm), 12.0 (CH3), 17.5 (CH3), 19.3 (CH3), 35.4,
98.3, 107.7, 126.6, 135.7, 144.6, 153.2, 163.2, 184.7. IR (K-
Br) ꢁ (cm-1), 1650, 3320. MS/EI, m/z 234 (M+). Anal. calcd
for C12H14N2O3: C, 61.53; H, 6.02; N, 11.96. Found: C,
61.48; H, 5.58; N, 11.82.
[4]
[5]
[6]
4-Hydroxy-6-methyl-3-(4-methyl-1-phenyl-1H-imidazol-5-
yl)-2H-pyran-2-one (3g)
Yield, 72 %. mp, 211-213 °C. 1H NMR (CDCl3) ꢀ
(ppm), 2.15 (s, 3H, CH3), 2.40 (s, 3H, CH3), 6.28 (s, 1H,
CH=C), 8.24 (s, 1H, CH=N), 6.89-7.30 (m, 5H, H-arom),
15.40 (s, 1H, OH). 13C NMR (CDCl3) ꢀ (ppm), 12.3 (CH3),
20.2 (CH3), 96.2, 108.3, 122.3, 125.4, 128.1, 130.3, 135.0,
137.5, 146.7, 152.7, 163.0, 185.6. IR (KBr) ꢁ (cm-1), 1650,
3315. MS/EI, m/z 282 (M+). Anal. calcd for C16H14N2O3: C,
68.08; H, 5.00; N, 9.92. Found: C, 67.97; H, 4.93; N, 9.83.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
ACKNOWLEDGEMENT
The authors acknowledge the “Comité Mixte
d’Evaluation et de Prospective de la Coopération Inter-
Universitaire Algéro-Française” (CMEP), the “Ministère de
l’Enseignement Supérieur et de la Recherche Scientifique”
and the French Embassy at Algiers for financial support.
Le Boisselier, V.; Coin, C.; Postel, M.; Dunach, E. Tetrahedron
Lett., 1995, 51, 4991.
Le Boisselier, V.; Dunach, E.; Postel, M. J. Organomet. Chem.,
1994, 482, 119.
Garrigues, B.; Gonzaga, F.; Robert, H.; Dubac, J. J. Org. Chem.,
1997, 62, 4880.
Schut, R.N.; Strycker, W.G.; Liu, T.M.H. J. Org. Chem., 1963, 28,
3046.
REFERENCES
[16]
[17]
Garratt, S. J. Org. Chem., 1963, 28, 1886.
Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc.,
2000, 122, 10405.
[1]
Grimmett, M.R. In Comprehensive Heterocyclic Chemistry II,
Katritzky, A.R.; Rees, C.W.; Scriven, E.F.V.; Eds.; Pergamon
Press: Oxford, 1996; Vol. 3, pp. 77-220.