New facile synthesis of 3,5-dihydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones by an iminophosphorane-mediated annulation

Article abstract of DOI:10.1002/ejoc.200600201

Iminophosphorane 1 reacted with an aromatic isocyanate to unexpectedly give a mixture of carbodiimides 2, 3 and 4 through both the normal and the abnormal aza-Wittig reactions. 3-Aminoimidazolone 10 was obtained from the reaction of hydrazine hydrate with carbodiimide 2. Reaction of 10 with triphenyphosphane, hexachloroethane and triethylamine produced iminophosphorane 11 in good yield. A tandem aza-Wittig reaction of iminophosphorane 11 with isocyanate or CS ₂ generated 3,5-dihydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones 13 or 15 in satisfactory yield. Carbodiimides 18, obtained from normal aza-Wittig reactions of vinyl iminophosphorane 17 with aromatic isocyanates, reacted with hydrazine to give 2-arylamino-3-amino-4H-imidazol-4-ones 20. One-pot reactions of 2-arylamino-3-amino-4H-imidazol-4-ones 20 with isocyanates (or acyl chlorides), triphenyphosphane, hexachloroethane and triethylamine produced 3,5-dihydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones 22 (or 23) in good yields. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200600201

FULL PAPER
DOI: 10.1002/ejoc.200600201
New Facile Synthesis of 3,5-Dihydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones by
an Iminophosphorane-Mediated Annulation
Ju-Zhen Yuan,[a] Bo-Qiao Fu,[a] Ming-Wu Ding,*[a] and Guang-Fu Yang*[a]
Keywords: 3,5-Dihydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-one / Iminophosphorane / Carbodiimide / Abnormal aza-Wittig
reaction / Synthetic methods / Annulation
Iminophosphorane 1 reacted with an aromatic isocyanate to
unexpectedly give a mixture of carbodiimides 2, 3 and 4
through both the normal and the abnormal aza-Wittig reac-
tions. 3-Aminoimidazolone 10 was obtained from the reac-
tion of hydrazine hydrate with carbodiimide 2. Reaction of
10 with triphenyphosphane, hexachloroethane and triethyl-
amine produced iminophosphorane 11 in good yield. A tan-
18, obtained from normal aza-Wittig reactions of vinyl imino-
phosphorane 17 with aromatic isocyanates, reacted with hy-
drazine to give 2-arylamino-3-amino-4H-imidazol-4-ones 20.
One-pot reactions of 2-arylamino-3-amino-4H-imidazol-4-
ones 20 with isocyanates (or acyl chlorides), triphenyphos-
phane, hexachloroethane and triethylamine produced 3,5-di-
hydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones 22 (or 23) in
dem aza-Wittig reaction of iminophosphorane 11 with isocya- good yields.
nate or CS2 generated 3,5-dihydro-6H-imidazo[1,2-b]-1,2,4-
triazol-6-ones 13 or 15 in satisfactory yield. Carbodiimides
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
activities.[15–21] Here we wish to report a new facile synthesis
of previously unreported 3,5-dihydro-6H-imidazo[1,2-b]-
1,2,4-triazol-6-ones.
Introduction
Many 4H-imidazol-4-ones have shown biological and
pharmaceutical activities.[1–3] Some of them exhibited good
antibacterial, antifungal and angiotensin II antagonistical
activities,[4–6] whereas others appear in a variety of biolo-
gically active molecules, particularly in some alkloids in
which a common structural unit is a derivatized 2-amino-
4H-imidazol-4-one moiety.[7,8] The introduction of a tri-
azole ring to the imidazolone system is expected to influ-
ence the biological activities significantly. However, useful
methods for the synthesis of 3,5-dihydro-6H-imidazo[1,2-
b]-1,2,4-triazol-6-ones are rare,[9] probably because of the
fact that these fused heterocycles, which contain a bridge-
head nitrogen atom, are not easily accessible by routine syn-
thetic methods. The aza-Wittig reactions of iminophos-
phoranes have received increased attention in view of their
utility in the synthesis of nitrogen heterocyclic com-
pounds.[10–14] Annelation of ring systems with N-heterocy-
cles by means of an aza-Wittig reaction has been widely
utilized because of the availability of functionalized imino-
phosphoranes. Recently we have been interested in the syn-
thesis of quinazolinones, thienopyrimidinones, imid-
azolones and imidazothiadiazolones by way of the aza-Wit-
tig reaction, with the aim of evaluating their fungicidal
Results and Discussion
It was reported that iminophosphorane 1 reacted with an
aromatic isocyanate to give only carbodiimide 2 by the aza-
Wittig reaction.[22] However, we observed that this reaction
unexpectedly gave a mixture of carbodiimides 2, 3 and 4
(Scheme 1). A GC–MS analysis of the reaction mixture re-
vealed the proportions of carbodiimides 2, 3 and 4 as 45%,
32% and 23%, respectively. Carbodiimides 3 and 4 were
probably formed by abnormal aza-Wittig reactions.[23–25]
[a] Key Laboratory of Pesticide & Chemical Biology, Ministry of
Education, College of Chemistry, Central China Normal Uni-
versity,
Scheme 1. Reaction conditions: (i) PhNCO, CH2Cl2, room temp.,
2 h.
Wuhan, 430079, P. R. China
The proposed mechanism of the abnormal aza-Wittig re-
action, in contrast to the normal aza-Wittig reaction, is out-
E-mail: mwding@mail.ccnu.edu.cn
gfyang@mail.ccnu.edu.cn
4170
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Synthesis of Dihydroimidazotriazolones by Iminophosphorane-Mediated Annulation
FULL PAPER
Scheme 2. Proposed mechanism of the abnormal aza-Wittig reaction leading to two different carbodiimides versus that of the normal
aza-Wittig reaction.
lined in Scheme 2. Both betaines 5 and 6 can be formed
upon treatment of iminophosphorane 1 with phenyl isocya-
nate. Breakdown of betaine 5 involving loss of tri-
phenylphosphane oxide results in carbodiimide 2 as the
normal aza-Wittig product. In contrast, betaine 6 can lead
to isocyanate group 7 as the abnormal aza-Wittig product
with the loss of iminophosphorane 8. Further reaction of
iminophosphorane 1 with 7 or iminophosphorane 8 with
PhNCO produced carbodiimide 4 or 3. The above abnor-
mal aza-Wittig reaction may be a result of the steric hin-
drance of iminophosphorane 1. Considering betaine struc-
tures 5 and 6, it is understandable that steric hindrance be-
Scheme 3. Reaction conditions: (i) NH2NH2, CH3CN, room temp.,
tween bulky dimethyl groups and the phenylimine group in
2 h; (ii) Ph3P, C2Cl6, CH3CN, NEt3, room temp., 4–6 h.
betaine 5 can lead to the preferred formation of betaine 6.
It is noteworthy that only the normal aza-Wittig reaction
product was obtained when similar but less sterically hin-
dered iminoposphoranes [such as RCH(COOEt)N=PPh3]
were used.[26,27]
Carbodiimide 2 was isolated from the reaction mixture
by column chromatography and was allowed to react with
hydrazine hydrate to give 3-aminoimidazolone 10. The for-
mation of 10 can be rationalized in terms of an initial nucle-
ophilic addition of hydrazine to give guanidine intermediate
9, which cyclizes across the strong nucleophilic hydrazine
group rather than the phenylamine group to give 10. Com-
pound 10 was easily converted to novel functionalized imi-
nophosphorane 11 by its reaction with triphenylphosphane,
hexachloroethane and triethylamine in good yield (83%,
Scheme 3).
When solutions of iminophosphorane 11 in dry dichloro-
Scheme 4. Reaction conditions: (i) Ar1NCO, CH2Cl2, room temp.,
1–2 h.
methane were treated with an aromatic isocyanate at room
temperature, 2-arylamino-6H-imidazo[1,2-b]-1,2,4-triazol-
6-ones 13 were isolated as crystalline solids in good yields
(81–91%, Scheme 4). Presumably, the conversion of 11 into
Iminophosphorane 11 reacted with CS2 in refluxing
13 involves an initial aza-Wittig reaction between imino- dichloromethane to give 2-thioxo-6H-imidazo[1,2-b]-1,2,4-
phosphorane 11 and the isocyanate to give carbodiimide 12 triazol-6-one (15) in high yield (95%, Scheme 5). The for-
as a highly reactive intermediate, which easily undergoes mation of 15 can be viewed as an initial aza-Wittig reaction
ring closure across the arylamino group to give 13.
between iminophosphorane 11 and CS2, which affords in-
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J.-Z. Yuan, B.-Q. Fu, M.-W. Ding, G.-F. Yang
FULL PAPER
termediate isothiocyanate 14 that undergoes cyclization to
give 15. S-Alkylation of 15 with an alkyl halide in the pres-
ence of solid potassium carbonate gave 2-alkylthio-6H-im-
idazo[1,2-b]-1,2,4-triazol-6-ones 16 in satisfactory yields.
Scheme 6. Reaction conditions: (i) Ar1NCO, CH2Cl2, room temp.,
4–6 h; (ii) NH2NH2, EtOH, room temp., 10 min; (iii) Ph3P, C2Cl6,
CH3CN, NEt3, room temp., 4–6 h.
Scheme 5. Reaction conditions: (i) CS2, CH2Cl2, 40 °C, 3 h; (ii)
RX, CH3CN, K2CO3(s), room temp. or 50–60 °C, 1–4 h.
In order to avoid the side reactions, a one-pot reaction
When vinyl iminophosphorane 17 was used, only car- was carried out at room temperature. When triethylamine
bodiimide 18 was produced by the normal aza-Wittig reac- was added to a mixture of 3-aminoimidazol-4-ones 20, aro-
tion. Carbodiimides 18 were allowed to react with hydra- matic isocyanates, triphenyphosphane and hexachloroeth-
zine to selectively give 2-arylamino-3-aminoimidazol-4-ones ane in CH2Cl2, the colour of the reaction mixture quickly
20 in good yields. Compounds 20 should be easily con- turned red, and 2-arylamino-6H-imidazo[1,2-b]-1,2,4-tri-
verted to iminophosphoranes 21 through the reaction with azol-6-ones 22 were obtained in good overall yields (71–
triphenylphosphane, hexachloroethane and triethylamine; 90%) after 1 h reaction. In these cases, iminophosphoranes
however, when triethylamine was added to the mixture of 21, generated in situ from 20, reacted immediately with aro-
20, triphenylphosphane and hexachloroethane in CH2Cl2 at matic isocyanates to give 22 in good yields (Scheme 7). This
room temperature, the colour of the reaction mixture one-pot reaction method was also successful when acyl
quickly turned red, becoming dark gradually, and imino- chlorides were utilized instead of isocyanates. Thus, 6H-im-
phosphoranes 21 were not obtained because of side reac- idazo[1,2-b]-1,2,4-triazol-6-ones 23 were obtained in 57–
tions (Scheme 6).
80% yield from 2-arylamino-3-aminoimidazol-4-ones 20,
Scheme 7. (i) Ph3P, C2Cl6, CH2Cl2, NEt3, room temp.; (ii) Ar2NCO, 1–2 h; (iii) RCOCl, 2–3 h. (iv) CS2, 2 h.
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Synthesis of Dihydroimidazotriazolones by Iminophosphorane-Mediated Annulation
FULL PAPER
2125 (N=C=N), 1734 (C=O), 1596, 1151 cm–1. MS: m/z (%) = 231
(28) [M]+, 158 (100), 117 (83), 76 (31). C13H16N2O2 (232.3): calcd.
C 67.22, H 6.94, N 12.06; found C 67.48, H 6.79, N 12.12.
acyl chlorides, triphenylphosphane and hexachloroethane
in the presence of triethylamine in dichloromethane at room
temperature. Attempts to prepare 2-mercapto-6H-imid-
1
azo[1,2-b]-1,2,4-triazol-6-ones 24 by the one-pot reaction of Carbodiimide 3: Light yellow oil: H NMR (CDCl3, 400 MHz): δ
= 7.14–7.19 (m, 6 H, Ar–H), 7.31–7.35 (m, 4 H, Ar–H) ppm. 13C
2-arylamino-3-aminoimidazol-4-ones 20 with carbon disul-
fide, triphenyphosphane, hexachloroethane and triethyl-
NMR (CDCl3, 100 MHz): δ = 124.1 (4 C), 125.5 (2 C), 129.4 (4
C), 135.2, 138.3 (2 C) ppm. IR (KBr): ν = 2137 (N=C=N), 1590,
˜
amine were unsuccessful. Although iminophosphorane 21
might be produced in this case, the reaction between 21 and
CS2 is very slow at room temperature, and the colour of the
reaction mixture quickly turned red, becoming dark grad-
ually because of side reactions.
In conclusion, we have demonstrated that the steric imi-
nophosphorane 1 reacts with phenyl isocyanate in normal
and abnormal aza-Wittig reactions. A new facile synthesis
of 2-substituted 3,5-dihydro-6H-imidazo[1,2-b]-1,2,4-tri-
azol-6-ones by way of the aza-Wittig reactions has also
been developed. Because of the mild one-pot reaction con-
ditions, good yields, easily accessible starting materials and
straightforward product isolation, this method has the po-
tential in the synthesis of many biologically and pharma-
ceutically active imidazotriazolone derivatives.
1203 cm–1. MS: m/z (%) = 194 (100) [M]+, 90 (14), 76 (27).
C13H10N2 (194.2): calcd. C 80.39, H 5.19, N 14.42; found C 80.31,
H 5.33, N 14.25.
1
Carbodiimide 4: Light yellow oil. H NMR (CDCl3, 400 MHz): δ
= 1.28 (t, J = 7.2 Hz, 6 H, 2CH3), 1.54 (s, 12 H, 4CH3), 4.20 (q, J
= 7.2 Hz, 4 H, 2CH2) ppm. 13C NMR (CDCl3, 100 MHz): δ = 14.0
(2 C), 27.4 (4 C), 60.6 (2 C), 61.4 (2 C), 137.8, 173.5 (2 C=O) ppm.
IR (KBr): ν = 2146 (N=C=N), 1743 (C=O), 1385, 1151 cm–1. MS:
˜
m/z (%) = 270 (4) [M]+, 242 (8), 227 (9), 197 (86), 168 (81), 114
(83), 55 (100). C13H22N2O4 (270.3): calcd. C 57.76, H 8.20, N 10.36;
found C 57.68, H 8.23, N 10.18.
3-Amino-5,5-dimethyl-2-phenylamino-4H-imidazol-4-one (10): To
the solution of carbodiimide 2 (1.16 g, 5 mmol) prepared above in
CH3CN (15 mL) was added hydrazine hydrate (0.29 g, 5 mmol,
85%) in CH3CN (5 mL). The mixture was stirred for 1 h at room
temperature, and the solvent was removed under reduced pressure.
The residue was recrystallized from ether and petroleum ether (1:2)
1
Experimental Section
to give 10. White solid (yield 0.92 g, 84%). M.p.: 114–116 °C. H
NMR (CDCl3, 400 MHz): δ = 1.45 (s, 6 H, 2 CH3), 4.19 (s, 2 H,
NH2), 7.06 (s, 1 H, N–H), 7.33–7.67 (m, 5 H, Ar–H) ppm. MS:
m/z (%) = 218 (92) [M]+, 202 (29), 190 (22), 175 (100), 118 (44), 76
(28). C11H14N4O (218.3): calcd. C 60.53, H 6.47, N 25.67; found C
60.72, H 6.40, N 25.53.
General Remarks: Melting points are uncorrected. MS were mea-
sured with a Finnigan Trace MS spectrometer. IR were recorded
with a PE-983 infrared spectrometer as KBr pellets. NMR were
recorded in CDCl3 or [D6]DMSO with a Varian Mercury 400 spec-
trometer and resonances relative to TMS. Elementary analyses
were taken with a Vario EL III elementary analysis instrument.
N-(5,5-Dimethyl-2-phenylamino-4H-imidazol-4-on-3-yl)iminotri-
phenylphosphorane (11): To a mixture of 10 (1.74 g, 8 mmol), PPh3
(3.14 g, 12 mmol) and C2Cl6 (2.84 g, 12 mmol) in dry CH3CN
(20 mL) was added dropwise NEt3 (2.42 g, 24 mmol) at room tem-
perature. After stirring for 5 h, the precipitate was filtered, washed
quickly with cold water and dried to give iminophosphorane 11.
White solid (yield 3.17 g, 83 %). M.p.: 224–226 °C. 1H NMR
(CDCl3, 400 MHz): δ = 1.10 (s, 6 H, 2 CH3), 7.02 (s, 1 H, N–H),
7.27–7.85 (m, 20 H, Ar–H) ppm. MS: m/z (%) = 478 (91) [M]+, 463
(31), 435 (19), 319 (38), 262 (100), 183 (62), 76 (53). C29H27N4OP
(478.5): calcd. C 72.79, H 5.69, N 11.71; found C 72.86, H 5.45, N
11.95.
N-(1-Ethoxycarbonyl-1-methylethyl)iminotriphenylphosphorane (1):
Triphenylphosphane (2.62 g, 10 mmol) in dry CH3CN (10 mL) was
added dropwise to a solution of ethyl 2-azido-2-methylpropionate
(1.89 g, 12 mmol) in dry CH3CN (20 mL) at 60 °C under an argon
atmosphere. After the reaction mixture was stirred for 2 h at 80 °C,
the solvent was removed under reduced pressure, and the residue
was then recrystallized from petroleum ether to give iminophos-
phorane 1. White solid (yield 3.41 g, 87%). M.p.: 55–57 °C. 1H
NMR (CDCl3, 400 MHz): δ = 1.04 (t, J = 7.2 Hz, 3 H, CH3), 1.38
(s, 6 H, 2CH3), 3.69 (q, J = 7.2 Hz, 2 H, CH2), 7.38–7.75 (m, 15
H, Ar–H) ppm. MS: m/z (%) = 391 (78) [M]+, 375 (99), 258 (90),
179 (92), 105 (100), 91 (29). C24H26NO2P (391.45): calcd. C 73.64,
H 6.69, N 3.58; found C 73.58, H 6.60, N 3.67.
2-Amino-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones (13): To a solution
of iminophosphorane 11 (0.48 g, 1 mmol) in dry CH2Cl2 (10 mL)
was added isocyanate (1 mmol) at room temperature. After stirring
for 1–2 h, the solvent was removed under reduced pressure and
the residue was recrystallized from dichloromethane and petroleum
ether to give 13. Isocyanate used for 13a: phenyl isocyanate; 13b:
4-chlorophenyl isocyanate; 13c: 2-chlorophenyl isocyanate; 13d: 3-
methylphenyl isocyanate; 13e: n-butyl isocyanate.
N-(1-Ethoxycarbonyl-1-methylethyl)-N-phenylcarbodiimide (2): To a
solution of iminophosphorane 1 (1.96 g, 5 mmol) in dry dichloro-
methane (15 mL) was added phenyl isocyanate (0.60 g, 5 mmol) un-
der nitrogen at room temperature. After the reaction mixture was
stirred for 2 h at room temperature, the solvent was removed under
reduced pressure and ether/petroleum ether (1:2, 20 mL) was added
to precipitate triphenylphosphane oxide. The precipitate was fil-
tered, and the solvent was removed to give a mixture of carbodi-
imides 2 (45%), 3 (32%) and 4 (23%), which was analyzed by GC–
MS. Carbodiimides 2, 3, and 4 were isolated from the reaction
mixture by column chromatography on silica gel in 40%, 24% and
15% yield, respectively.
3,5-Dihydro-5,5-dimethyl-3-phenyl-2-phenylamino-6H-imidazo[1,2-
b]-1,2,4-triazol-6-one (13a): White solid (yield 0.27 g, 85%). M.p.:
186–187 °C. 1H NMR (CDCl3, 400 MHz): δ = 1.48 (s, 6 H, 2 CH3),
6.38 (s, 1 H, N–H), 7.10–7.67 (m, 10 H, Ar–H) ppm. 13C NMR
(CDCl3, 100 MHz): δ = 24.7 (2 C), 75.5, 118.8, 123.8, 126.5, 128.4
(2 C), 129.2, 131.0, 132.0 (2 C), 151.8, 154.3, 174.8 (C=O) ppm.
MS: m/z (%) = 319 (11) [M]+, 291 (55), 277 (100), 250 (48), 198
(15), 76 (49). C18H17N5O (319.4): calcd. C 67.70, H 5.37, N 21.93;
found C 67.57, H 5.24, N 21.98.
1
Carbodiimide 2: Light yellow oil. H NMR (CDCl3, 400 MHz): δ
= 1.27 (t, J = 7.2 Hz, 3 H, CH3), 1.58 (s, 6 H, 2CH3), 4.22 (q, J =
7.2 Hz, 2 H, CH2), 7.16–7.30 (m, 5 H, Ar–H) ppm. 13C NMR
(CDCl3, 100 MHz): δ = 13.9, 27.7 (2 C), 61.4, 61.8, 123.7 (2 C), 2-(4-Chlorophenylamino)-3,5-dihydro-5,5-dimethyl-3-phenyl-6H-
124.8, 129.1 (2 C), 136.7, 139.6, 173.4 (C=O) ppm. IR (KBr): ν = imidazo[1,2-b]-1,2,4-triazol-6-one (13b): White solid (yield 0.32 g,
˜
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J.-Z. Yuan, B.-Q. Fu, M.-W. Ding, G.-F. Yang
91%). M.p.: 252–253 °C. 1H NMR (CDCl3, 400 MHz): δ = 1.50 (s, (89), 217 (67), 183 (88), 77 (100). C14H16N4OS (288.4): calcd. C
FULL PAPER
6 H, 2 CH3), 6.19 (s, 1 H, N–H), 7.27–7.68 (m, 9 H, Ar–H) ppm.
13C NMR (CDCl3, 100 MHz): δ = 24.6 (2 C), 75.6, 120.0 (2 C),
126.6 (2 C), 129.0, 129.3 (2 C), 130.0, 130.2, 131.1 (2 C), 135.3,
151.5, 154.1, 174.7 (C=O) ppm. MS: m/z (%) = 353 (8) [M]+, 325
(85), 283 (100), 234 (48), 76 (61). C18H16ClN5O (353.8): calcd. C
61.11, H 4.56, N 19.79; found C 61.18, H 4.47, N 19.71.
58.31, H 5.59, N 19.43; found C 58.15, H 5.45, N 19.47.
3,5-Dihydro-5,5-dimethyl-3-phenyl-2-propylthio-6H-imidazo[1,2-b]-
1,2,4-triazol-6-one (16b): White solid (yield 0.27 g, 88 %). M.p.:
1
111–112 °C. H NMR (CDCl3, 400 MHz): δ = 1.04 (t, J = 7.2 Hz,
3 H, CH3), 1.50 (s, 6 H, 2 CH3), 1.78–1.84 (m, 2 H, CH2), 3.20 (t,
J = 7.2 Hz, 2 H, SCH2), 7.46–7.54 (m, 5 H, Ar–H) ppm. 13C NMR
(CDCl3, 100 MHz): δ = 13.2, 22.1, 24.6 (2 C), 33.5, 76.4, 125.7 (2
C), 129.3, 130.1 (2 C), 130.4, 156.0, 159.4, 174.4 (C=O) ppm. MS:
2-(2-Chlorophenylamino)-3,5-dihydro-5,5-dimethyl-3-phenyl-6H-
imidazo[1,2-b]-1,2,4-triazol-6-one (13c): White solid (yield 0.29 g,
83%). M.p.: 253–254 °C. 1H NMR (CDCl3, 400 MHz): δ = 1.51 (s, m/z (%) = 302 (17) [M]+, 273 (53), 231 (100), 189 (31), 76 (61).
6 H, 2 CH3), 7.00–7.69 (m, 9 H, Ar–H), 8.41 (s, 1 H, N–H) ppm.
13C NMR (CDCl3, 100 MHz): δ = 24.6 (2 C), 75.7, 119.7, 121.6,
124.0, 126.2 (2 C), 128.3, 128.9, 129.9, 130.2, 131.0 (2 C), 133.4,
151.1, 153.9, 174.8 (C=O) ppm. MS: m/z (%) = 353 (23) [M]+, 325
(100), 284 (67), 76 (21). C18H16ClN5O (353.8): calcd. C 61.11, H
4.56, N 19.79; found C 61.25, H 4.74, N 19.95.
C15H18N4OS (302.4): calcd. C 59.58, H 6.00, N 18.53; found C
59.64, H 5.86, N 18.44.
2-Butylthio-3,5-dihydro-5,5-dimethyl-3-phenyl-2-butylthio-6H-imid-
azo[1,2-b]-1,2,4-triazol-6-one (16c): White solid (yield 0.26 g, 82%).
M.p.: 114–116 °C. 1H NMR (CDCl3, 400 MHz): δ = 0.92 (t, J =
7.6 Hz, 3 H, CH3), 1.51 (s, 6 H, 2 CH3), 1.40–1.79 (m, 4 H,
CH2CH2), 3.23 (t, J = 7.2 Hz, 2 H, SCH2), 7.44–7.56 (m, 5 H, Ar–
H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 13.5, 21.8, 24.6 (2 C),
3,5-Dihydro-5,5-dimethyl-2-(3-methylphenylamino)-3-phenyl-6H-
imidazo[1,2-b]-1,2,4-triazol-6-one (13d): White solid (yield 0.29 g,
87%). M.p.: 250–251 °C. 1H NMR (CDCl3, 400 MHz): δ = 1.48 (s, 31.2, 32.3, 76.3, 125.7 (2 C), 129.4, 130.1 (2 C), 130.3, 156.0, 159.5,
6 H, 2 CH3), 2.34 (s, 3 H, CH3), 6.08 (s, 1 H, N–H), 6.91–7.65 (m,
174.3 (C=O) ppm. MS: m/z (%) = 316 (10) [M]+, 292 (31), 230
9 H, Ar–H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 21.5, 24.7 (2 (100), 182 (57), 76 (34). C16H20N4OS (316.4): calcd. C 60.73, H
C), 75.5, 115.9, 119.2, 124.8, 126.6 (2 C), 129.2, 130.2 (2 C), 131.1
(2 C), 136.5, 139.4, 151.1, 153.9, 174.8 (C=O) ppm. MS: m/z (%) =
333 (9) [M]+, 305 (100), 264 (78), 214 (21). C19H19N5O (333.4):
calcd. C 68.45, H 5.74, N 21.01; found C 68.53, H 5.57, N 21.25.
6.37, N 17.71; found C 60.62, H 6.34, N 17.76.
3,5-Dihydro-5,5-dimethyl-2-ethoxycarbonylmethylthio-3-phenyl-6H-
imidazo[1,2-b]-1,2,4-triazol-6-one (16d): White solid (yield 0.29 g,
85%). M.p.: 109–110 °C. 1H NMR (CDCl3, 400 MHz): δ = 1.32 (t,
J = 7.2 Hz, 3 H, CH3), 1.48 (s, 6 H, 2CH3), 4.04 (s, 2 H, SCH2),
4.26 (q, J = 7.2 Hz, 2 H, OCH2), 7.46–7.54 (m, 5 H, Ar-H) ppm.
2-(n-Butylamino)-3,5-dihydro-5,5-dimethyl-3-phenyl-6H-imidazo-
[1,2-b]-1,2,4-triazol-6-one (13e): White solid (yield 0.24 g, 81%).
M.p.: 217–218 °C. 1H NMR (CDCl3, 400 MHz): δ = 0.93 (t, J = 13C NMR (CDCl3, 100 MHz): δ = 14.0, 24.9 (2), 34.6, 61.9, 76.4,
7.2 Hz, 3 H, CH3), 1.28–1.64 (m, 4 H, CH2CH2), 1.45 (s, 6 H, 2 125.5 (2), 129.5, 130.1 (2), 130.4, 158.6, 159.4, 174.4 (C=O) ppm.
CH3), 3.33–3.38 (m, 2 H, NCH2), 4.13 (s, 1 H, N–H), 7.45–7.59
MS: m/z (%) = 346 (12) [M]+, 318 (100), 294 (19), 245 (26).
(m, 5 H, Ar–H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 13.6, 19.9, C16H18N4O3S (346.4): calcd. C 55.48, H 5.24, N 16.17; found C
24.6 (2 C), 31.1, 42.5, 75.2, 125.9 (2 C), 129.4, 130.7 (3 C), 155.1, 55.64, H 5.36, N 16.01.
155.4, 174.5 (C=O) ppm. MS: m/z (%) = 299 (99) [M]+, 271 (100),
2-Arylamino-3-amino-4H-imidazol-4-ones 20: To a solution of vi-
256 (45), 214 (87), 159 (41). C16H21N5O (299.4): calcd. C 64.19, H
nyliminophosphorane 17[21] (2.25 g, 5 mmol) in dry dichlorometh-
7.07, N 23.39; found C 64.13, H 7.17, N 23.21.
ane (15 mL) was added aromatic isocyanate (5 mmol) under nitro-
2-Thioxo-6H-imidazo[1,2-b]-1,2,4-triazol-6-one (15): To a solution
of iminophosphorane 11 (4.78 g, 10 mmol) in dry dichloromethane
(15 mL) was added excess CS2 (10 mL) under nitrogen at room
temperature. After the reaction mixture was heated at reflux for
3 h, the precipitated solid was collected by filtration to give 15.
gen at room temperature. After the reaction mixture was stirred for
4–6 h, the solvent was removed under reduced pressure, and ether/
petroleum ether (1:2, 20 mL) was added to precipitate tri-
phenylphosphane oxide. The precipitate was filtered, and the sol-
vent was removed to give carbodiimides 18, which were used di-
White solid (yield 2.47 g, 95 %). M.p.: 240–241 °C. 1H NMR rectly without further purification. To the solution of 18 prepared
(CDCl3/TFA, 400 MHz): δ = 1.72 (s, 6 H, 2 CH3), 7.44–7.74 (m, 5
H, Ar–H) ppm. MS: m/z (%) = 260 (4) [M]+, 230 (100), 189 (21),
140 (28), 77 (22). C12H12N4OS (260.3): calcd. C 55.37, H 4.65, N
21.52; found C 55.18, H 4.69, N 21.65.
above in EtOH (15 mL) was added hydrazine hydrate (0.35 g,
6 mmol, 85%) in EtOH (5 mL). The mixture was stirred for 10 min
at room temperature and filtered to give 2-arylamino-3-amino-4H-
imidazol-4-ones 20. Isocyanate used for: 20a: phenyl isocyanate;
20b: 4-chlorophenyl isocyanate.
2-Alkylthio-6H-imidazo[1,2-b]-1,2,4-triazol-6-one (16): A mixture of
15 (0.26 g, 1 mmol), alkyl halide (1.2 mmol) and solid potassium
carbonate (0.28 g, 2 mmol) in CH3CN (10 mL) was stirred for 1–
4 h at room temperature or 50–60 °C and filtered; the filtrate was
condensed, and the residue was recrystallized from dichlorometh-
ane/petroleum ether to give 16. Alkyl halide and reaction condi-
tions used for: 16a: ethyl iodide, room temp.; 16b: n-propyl bro-
mide, 50 °C; 16c: n-butyl bromide, 60 °C; 16d: ethyl bromoacetate,
room temp.
3-Amino-3,5-dihydro-2-phenylamino-5-phenylmethylene-4H-
imidazol-4-one (20a): Yellow solid (yield 1.06 g, 76%). M.p.: 191–
1
193 °C. H NMR (CDCl3, 400 MHz): δ = 4.23 (s, 2 H, NH2), 6.88
(s, 1 H, =CH), 7.14–8.14 (m, 11 H, Ar–H and N–H) ppm. IR
(KBr): ν = 3340, 3322 (N–H), 1702 (C=O), 1660 (C=C), 1599,
˜
1156 cm–1. MS: m/z (%) = 278 (98) [M]+, 263 (19), 249 (22), 219
(37), 116 (100). C16H14N4O (278.3): calcd. C 69.05, H 5.07, N
20.13; found C 69.30, H 5.01, N 20.26.
3,5-Dihydro-5,5-dimethyl-2-ethylthio-3-phenyl-6H-imidazo[1,2-b]-
3-Amino-2-(4-chlorophenylamino)-3,5-dihydro-5-phenylmethylene-
1,2,4-triazol-6-one (16a): White solid (yield 0.24 g, 83 %). M.p.:
4H-imidazol-4-one (20b): Yellow solid (yield 1.16 g, 74 %). M.p.:
114–115 °C. H NMR (CDCl3, 400 MHz): δ = 1.45 (t, J = 7.2 Hz, 245–246 °C. 1H NMR ([D6]DMSO, 400 MHz): δ = 5.34 (s, 2 H,
1
3 H, CH3), 1.50 (s, 6 H, 2 CH3), 3.24 (q, J = 7.2 Hz, 2 H, SCH2),
NH2), 6.66 (s, 1 H, =CH), 7.30–8.14 (m, 9 H, Ar–H), 9.63 (s, 1 H,
7.44–7.55 (m, 5 H, Ar–H) ppm. 13C NMR (CDCl3, 100 MHz): δ = N–H) ppm. IR (KBr): ν = 3330, 3302 (N–H), 1728 (C=O), 1660
˜
14.1, 24.6 (2 C), 26.1, 76.3, 125.7 (2 C), 129.4, 130.1 (2 C), 130.3, (C=C), 1596, 1154 cm–1. MS: m/z (%) = 314 (30), 312 (88) [M]+,
155.9, 159.2, 174.3 (C=O) ppm. MS: m/z (%) = 288 (14) [M]+, 260
297 (21), 253 (18), 218 (13), 168 (68), 116 (100). C16H13ClN4O
4174
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Synthesis of Dihydroimidazotriazolones by Iminophosphorane-Mediated Annulation
FULL PAPER
(312.8): calcd. C 61.45, H 4.19, N 17.91; found C 61.69, H 4.06, N
17.94.
204 (61), 116 (100). C23H16ClN5O (413.9): calcd. C 66.75, H 3.90,
N 16.92; found C 66.71, H 3.98, N 16.75.
3-(4-Chlorophenyl)-3,5-dihydro-2-(3-methylphenylamino)-5-phenyl-
2-Arylamino-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones 22: To a mix-
ture of imidazolone 20 (3 mmol), Ar2NCO (3 mmol), PPh3 (1.05 g,
4 mmol) and C2Cl6 (0.95 g, 4 mmol) in dry CH2Cl2 (15 mL) was
added dropwise NEt3 (0.81 g, 8 mmol) at room temperature. The
colour of the reaction mixture quickly turned red. After the solu-
tion was stirred for 1–2 h the solvent was removed under reduced
pressure, and the residue was recrystallized from EtOH to give 2-
arylamino-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones 22. Starting ma-
terial and isocyanate used for: 22a: 20a and phenyl isocyanate; 22b:
20a and 4-chlorophenyl isocyanate; 22c: 20a and 4-methylphenyl
isocyanate; 22d: 20a and 3-methylphenyl isocyanate; 22e: 20b and
phenyl isocyanate; 22f: 20b and 3-methylphenyl isocyanate; 22g:
20b and 4-methylphenyl isocyanate; 22h: 20b and 4-chlorophenyl
isocyanate.
methylene-6H-imidazo[1,2-b]-1,2,4-triazol-6-one (22f): Yellow crys-
1
tals (yield 1.10 g, 86 %). M.p.: Ͼ300 °C. H NMR ([D6]DMSO,
400 MHz): δ = 2.30 (s, 3 H, CH3), 6.88–8.04 (m, 14 H, Ar–H and
=CH), 8.93 (s, 1 H, N–H) ppm. IR (KBr): ν = 3406 (N–H), 1714
˜
(C=O), 1661 (C=C), 1589, 1095 cm–1. MS: m/z (%) = 429 (28), 427
(82) [M]+, 399 (71), 370 (39), 232 (75), 116 (100). C24H18ClN5O
(427.9): calcd. C 67.37, H 4.24, N 16.37; found C 67.45, H 4.17, N
16.20.
3-(4-Chlorophenyl)-3,5-dihydro-2-(4-methylphenylamino)-5-phenyl-
methylene-6H-imidazo[1,2-b]-1,2,4-triazol-6-one (22g): Yellow crys-
1
tals (yield 0.94 g, 73 %). M.p.: Ͼ300 °C. H NMR ([D6]DMSO,
400 MHz): δ = 2.27 (s, 3 H, CH3), 6.88 (s, 1 H, =CH), 7.12–8.02
(m, 13 H, Ar–H), 8.91 (s, 1 H, N–H) ppm. IR (KBr): ν = 3437 (N–
˜
H), 1699 (C=O), 1658 (C=C), 1591, 1093 cm–1. MS: m/z (%) = 429
(10), 427 (27) [M]+, 399 (28), 370 (13), 116 (100). C24H18ClN5O
(427.9): calcd. C 67.37, H 4.24, N 16.37; found C 67.30, H 4.11, N
16.44.
3,5-Dihydro-3-phenyl-2-phenylamino-5-phenylmethylene-6H-
imidazo[1,2-b]-1,2,4-triazol-6-one (22a): Yellow crystals (yield
0.81 g, 71%). M.p.: 241–242 °C. 1H NMR (CDCl3, 400 MHz): δ =
6.13 (s, 1 H, N–H), 7.09–8.03 (m, 16 H, Ar–H and =CH) ppm. 13
C
NMR (CDCl3, 100 MHz): δ = 119.2, 120.2, 122.2, 123.6, 124.2,
128.1 (2 C), 128.7 (3 C), 129.2 (3 C), 130.3 (2 C), 130.9, 131.2 (2
3-(4-Chlorophenyl)-2-(4-chlorophenylamino)-3,5-dihydro-5-phenyl-
methylene-6H-imidazo[1,2-b]-1,2,4-triazol-6-one (22h): Yellow crys-
C), 132.4 (2 C), 143.8, 153.6, 159.8 (C=O) ppm. IR (KBr): ν = 3358
1
˜
tals (yield 1.21 g, 90 %). M.p.: Ͼ300 °C. H NMR ([D6]DMSO,
(N–H), 1713 (C=O), 1658 (C=C), 1580, 1100 cm–1. MS: m/z (%) =
379 (1) [M]+, 351 (3), 233 (7), 116 (85), 77 (100). C23H17N5O
(379.4): calcd. C 72.81, H 4.52, N 18.46; found C 72.89, H 4.44, N
18.61.
400 MHz): δ = 6.90 (s, 1 H, =CH), 7.36–8.04 (m, 13 H, Ar–H),
9.17 (s, 1 H, N–H) ppm. IR (KBr): ν = 3255 (N–H), 1702 (C=O),
˜
1659 (C=C), 1587, 1092 cm–1. MS: m/z (%) = 447 (75) [M]+, 419
(68), 390 (17), 232 (64), 116 (100). C23H15Cl2N5O (448.3): calcd. C
61.62, H 3.37, N 15.62; found C 61.48, H 3.21, N 15.85.
2-(4-Chlorophenylamino)-3,5-dihydro-3-phenyl-5-phenylmethylene-
6H-imidazo[1,2-b]-1,2,4-triazol-6-one (22b): Yellow crystals (yield
1.03 g, 83%). M.p.: Ͼ300 °C. H NMR ([D6]DMSO, 400 MHz): δ
= 6.89 (s, 1 H, =CH), 7.35–8.04 (m, 14 H, Ar–H), 9.18 (s, 1 H, N–
2-Substituted 6H-Imidazo[1,2-b]-1,2,4-triazol-6-ones 23: To a mix-
ture of imidazolone 20 (3 mmol), RCOCl (3 mmol), PPh3 (1.05 g,
4 mmol) and C2Cl6 (0.95 g, 4 mmol) in dry CH2Cl2 (15 mL) was
added dropwise NEt3 (1.11 g, 11 mmol) at room temperature. The
colour of the reaction mixture turned red. After the solution was
stirred for 2–3 h the solvent was removed under reduced pressure,
and the residue was recrystallized from EtOH to give 6H-imid-
azo[1,2-b]-1,2,4-triazol-6-ones 23. Starting material and acyl chlo-
ride used for: 23a: 20a and benzoyl chloride; 23b: 20a and acetyl
chloride; 23c: 20b and benzoyl chloride; 23d: 20b and acetyl chlo-
ride.
1
H) ppm. IR (KBr): ν = 3355 (N–H), 1708 (C=O), 111 (C–H), 222
˜
(N–H), 1661 (C=C), 1580, 1109 cm–1. MS: m/z (%) = 415 (12), 413
(32) [M]+, 385 (38), 356 (7), 233 (14), 152 (45), 116 (100).
C23H16ClN5O (413.9): calcd. C 66.75, H 3.90, N 16.92; found C
66.70, H 3.81, N 16.95.
3,5-Dihydro-2-(4-methylphenylamino)-3-phenyl-5-phenylmethylene-
6H-imidazo[1,2-b]-1,2,4-triazol-6-one (22c): Yellow crystals (yield
0.93 g, 79%). M.p.: 270–271 °C. 1H NMR (CDCl3, 400 MHz): δ =
2.31 (s, 3 H, CH3), 6.08 (s, 1 H, N–H), 7.11–8.02 (m, 15 H, Ar–H
and =CH) ppm. 13C NMR (CDCl3, 100 MHz): δ = 20.9, 119.0,
120.2, 122.3, 123.6, 124.1, 128.0 (2 C), 128.9 (3 C), 129.5 (2 C),
130.4 (2 C), 130.9, 131.4 (2 C), 132.3 (2 C), 143.8, 153.8, 159.8
3,5-Dihydro-2,3-diphenyl-5-phenylmethylene-6H-imidazo[1,2-b]-
1,2,4-triazol-6-one (23a): Yellow crystals (yield 0.86 g, 79%). M.p.:
247–249 °C. 1H NMR (CDCl3, 400 MHz): δ = 7.17 (s, 1 H, =CH),
7.34–8.08 (m, 15 H, Ar–H) ppm. 13C NMR (CDCl3, 100 MHz): δ
= 123.0, 124.9, 127.3 (2 C), 128.6 (4 C), 128.7 (4 C), 129.4, 129.5,
129.8 (2 C), 131.4 (2 C), 132.4, 143.3, 155.4, 160.6 (C=O) ppm. IR
(C=O) ppm. IR (KBr): ν = 3362 (N–H), 1711 (C=O), 1659 (C=C),
˜
1582, 1104 cm–1. MS: m/z (%) = 393 (4) [M]+, 365 (10), 336 (5),
233 (8), 116 (100). C24H19N5O (393.4): calcd. C 73.27, H 4.87, N
17.80; found C 73.46, H 4.95, N 17.55.
(KBr): ν = 1739 (C=O), 1652 (C=C), 1581, 1095 cm–1. MS: m/z (%)
˜
= 364 (42) [M]+, 335 (45), 232 (11), 116 (31). C23H16N4O (364.4):
calcd. C 75.81, H 4.43, N 15.37; found C 75.64, H 4.57, N 15.21.
3,5-Dihydro-2-(3-methylphenylamino)-3-phenyl-5-phenylmethylene-
6H-imidazo[1,2-b]-1,2,4-triazol-6-one (22d): Yellow crystals (yield
0.99 g, 84%). M.p.: 240–241 °C. 1H NMR (CDCl3, 400 MHz): δ =
2.36 (s, 3 H, CH3), 6.06 (s, 1 H, N–H), 6.92–8.03 (m, 15 H, Ar–H
3,5-Dihydro-2-methyl-3-phenyl-5-phenylmethylene-6H-imidazo-
[1,2-b]-1,2,4-triazol-6-one (23b): Yellow crystals (yield 0.57 g, 57%).
1
M.p.: 243–244 °C. H NMR (CDCl3, 400 MHz): δ = 2.35 (s, 3 H,
CH3), 7.13 (s, 1 H, =CH), 7.29–8.04 (m, 10 H, Ar–H) ppm. 13C
NMR (CDCl3, 100 MHz): δ = 12.2, 122.6, 126.7 (2 C), 128.6 (2 C),
129.3, 129.6, 130.0 (2 C), 131.2, 131.3 (2 C), 131.6, 134.2, 143.5,
and =CH) ppm. IR (KBr): ν = 3391 (N–H), 1704 (C=O), 1658
˜
(C=C), 1581, 1107 cm–1. MS: m/z (%) = 393 (96) [M]+, 364 (89),
336 (29), 233 (63), 116 (100). C24H19N5O (393.4): calcd. C 73.27,
H 4.87, N 17.80; found C 73.32, H 4.79, N 17.83.
155.3, 160.3 (C=O) ppm. IR (KBr): ν = 1728 (C=O), 1657 (C=C),
˜
1580, 1102 cm–1. MS: m/z (%) = 302 (100) [M]+, 273 (80), 232 (29),
205 (8), 117 (41). C18H14N4O (302.3): calcd. C 71.51, H 4.67, N
18.53; found C 71.68, H 4.72, N 18.47.
3-(4-Chlorophenyl)-3,5-dihydro-2-phenylamino-5-phenylmethylene-
6H-imidazo[1,2-b]-1,2,4-triazol-6-one (22e): Yellow crystals (yield
1.00 g, 81%). M.p.: Ͼ300 °C. 1H NMR (CDCl3, 400 MHz): δ =
6.14 (s, 1 H, N–H), 7.12–8.02 (m, 15 H, Ar–H and =CH) ppm. IR
3-(4-Chlorophenyl)-3,5-dihydro-2-phenyl-5-phenylmethylene-6H-
(KBr): ν = 3305 (N–H), 1710 (C=O), 1655 (C=C), 1589, 1101 cm–1.
imidazo[1,2-b]-1,2,4-triazol-6-one (23c): Yellow crystals (yield
˜
MS: m/z (%) = 415 (28), 413 (83) [M]+, 385 (70), 356 (30), 232 (78), 0.96 g, 80%). M.p.: Ͼ300 °C. H NMR ([D6]DMSO, 400 MHz): δ
1
Eur. J. Org. Chem. 2006, 4170–4176
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4175

J.-Z. Yuan, B.-Q. Fu, M.-W. Ding, G.-F. Yang
FULL PAPER
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1998, 54, 8687–8690.
[9] P. J. Garratt, S. N. Thorn, R. Wrigglesworth, Tetrahedron 1993,
49, 165–176.
[10] F. Palacios, C. Alonso, M. Rodriguez, E. Martinez de Mari-
gorta, G. Rubiales, Eur. J. Org. Chem. 2005, 1795–1804.
[11] P. N. D. Singh, R. F. Klima, S. Muthukrishnan, R. S. Murthy,
J. Sankaranarayanan, H. M. Stahlecker, B. Patel, A. D. Gud-
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= 7.03 (s, 1 H, =CH), 7.39–8.11 (m, 14 H, Ar–H) ppm. IR (KBr):
ν = 1723 (C=O), 1653 (C=C), 1587, 1092 cm–1. MS: m/z (%) =
˜
400 (30), 398 (88) [M]+, 370 (68), 335 (12), 266 (70), 151 (100).
C23H15ClN4O (398.9): calcd. C 69.26, H 3.79, N 14.05; found C
69.32, H 3.64, N 14.11.
3-(4-Chlorophenyl)-3,5-Dihydro-2-methyl-5-phenylmethylene-6H-
imidazo[1,2-b]-1,2,4-triazol-6-one (23d): Yellow crystals (yield
0.71 g, 72%). M.p.: 242–244 °C. 1H NMR ([D6]DMSO, 400 MHz):
δ = 2.50 (s, 3 H, CH3), 6.96 (s, 1 H, =CH), 7.33–8.07 (m, 9 H, Ar–
H) ppm. IR (KBr): ν = 1725 (C=O), 1655 (C=C), 1584, 1094 cm–1.
˜
[13] E. Bellur, H. Goerls, P. Langer, J. Org. Chem. 2005, 70, 4751–
MS: m/z (%) = 338 (5), 336 (14) [M]+, 308 (23), 266 (19), 232 (8),
151 (51), 116 (100). C18H13ClN4O (336.8): calcd. C 64.20, H 3.89,
N 16.64; found C 64.28, H 3.96, N 16.47.
4761.
[14] S. Brase, C. Gil, K. Knepper, V. Zimmermann, Angew. Chem.
Int. Ed. 2005, 44, 5188–5240.
[15] M. W. Ding, S. Z. Xu, J. F. Zhao, J. Org. Chem. 2004, 69, 8366–
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Acknowledgments
[16] M. W. Ding, Y. F. Chen, N. Y. Huang, Eur. J. Org. Chem. 2004,
3872–3878.
[17] J. F. Zhao, C. Xie, S. Z. Xu, M. W. Ding, W. J. Xiao, Org. Bi-
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[18] M. W. Ding, S. J. Yang, J. Zhu, Synthesis 2004, 75–79.
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34, 1022–1023.
We gratefully acknowledge financial support of this work by the
Natural Science Foundation of Hubei Province for outstanding
youth (2006ABB016) and the Natural Science Foundation of China
(20102001).
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Received: March 7, 2006
Published Online: July 19, 2006
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