Synthesis of 3-aryl-3,4-dihydroquinazolin-2(1H)-ones with the aid of low-valent titanium reagent (TiCl4-Sm)

Article abstract of DOI:10.3184/030823407X248306

A short and facile synthesis of a series of 3-aryl-3,4-dihydroquinazolin- 2(1H)-ones was accomplished in good yields via the novel reductive cyclisation of 2-nitrobenzylamines with bis(trichloromethyl)carbonate (triphosgene) promoted by TiCl₄/S

Full text of DOI:10.3184/030823407X248306

JOURNAL OF CHEMICAL RESEARCH 2007
SEPTEMBER, 545–547
RESEARCH PAPER 545
Synthesis of 3-aryl-3,4-dihydroquinazolin-2(1H ꢀ) -ones with the aid of low-
valent titanium reagent (TiCl –Sm)
4
a,b,c
b
b
Daqing Shi
*, Guolan Dou and Zheng-Yi Li
a
College of Chemistry and Chemical Engineering, Key Laboratory of Organic Synthesis of Jiangsu Province, Suzhou University,
Suzhou 215123, P. R.. China
bCollege of Chemistry and Chemical Engineering, Xuzhou Normal University, Xuzhou 221116, P. R. China
cThe Key Laboratory of Biotechnology on Medical Plants of Jiangsu Province, Xuzhou 221116, P. R. China
A short and facile synthesis of a series of 3-aryl-3,4-dihydroquinazolin-2(1H)-ones was accomplished in good
yields via the novel reductive cyclisation of 2-nitrobenzylamines with bis(trichloromethyl)carbonate (triphosgene)
1
promoted by TiCl /Sm system. The structures of the products were characterised by IR, H NMR and elemental
4
analysis and the structure of 3a was confirmed by X-ray diffraction analysis.
Keywords: 3-aryl-3,4-dihydroquinazolin-2(1H)-one, low-valent titanium reagent, triphosgene, 2-nitrobenzylamine
Low-valent titanium reagents have an exceedingly high ability
to promote reductive coupling of carbonyl compounds and
X
X
Ar
N
H
NO2
Ar
N
TiCl
THF, reflux
-Sm
4
+ CO(OCCl3)2
1
are attracting increasing interest in organic synthesis. Many
N
H
O
2
other functional groups can also be coupled. Recently, we
have reported the low-valent titanium-induced intermolecular
reductive coupling reaction of carboxylic derivatives with
aromatic ketones,3 the intramolecular reductive coupling
1
2
3
Scheme 1
4
reaction of 4,4-dicyano-1,3-diarylbutan-1-one, the cyclo-
dimerisation of α,β-unsaturated ketones5 and the intra-
6
molecular reductive coupling reaction of ketomalononitriles.
Quinazolinone derivatives are interesting because
of their diverse range of biological activities such as
anticancer, diuretic, anti-inflammatory, anticonvulsant, and
antihypertensive agents.7 Recently, these compounds have
+
2+
8
also been reported as Na /Ca exchanger inhibitor. The
main synthetic approaches to such compounds consist of the
9
condensation of imidates with anthranilic acid, the reaction
of anthranilamides with aldehydes, Pd-catalysed cyclisation
10
11
reaction of aryl-benzyl ureas and the aza-Wittig reactions of
α–azido-substituted aromatic imides.12 Although some of the
methods provide useful strategies for the synthesis of these
bicyclic compounds, they suffer major drawbacks, such as
involving a multistep synthesis, producing low yields, and
requiring expensive catalyst, and are, thus, less desirable
commerically.
Fig. 1 Molecular structure of 3a showing the atom numbering
scheme.
Table 1 The synthesis of 3-aryl-3,4-dihydroquinazolin-2(1H)-
ones promoted by TiCl4/Sm
Our interest in recent years has been focused on the
application of the low-valent titanium reagent in organic
synthesis. We have previously reported the synthesis of
Entry
X
Ar
Yield/%
Mp/°C
3a
3b
3c
H
4-CH3C6H4
4-CH OC H
96
82
93
95
94
92
94
93
89
86
90
91
94
89
222–224
248–249
282–284
203–205
185–187
174–175
166–167
218–219
225–227
206–207
214–216
198–200
157–159
185–187
13
14
H
bioactive molecules such as indoles, 2-aminoquinolines,
3
6
4
1
5
16
H
4-BrC6H4
4-FC H4
quinazolin-4(3H)-ones, imidazo[1,2-c]quinazolines and
3
3
d
e
H
pyrroles17 with the aid of this reagent. Here we wish to
6
H
3-Cl-4-CH C H
3 6
3
3
describe a method induced by the TiCl /Sm system for the
4
3f
H
3-Cl-4-FC H3
6
preparation of 3-aryl-3,4-dihydroquinazolin-2(1H)-ones using
3g
3h
H
3-CH C H
3
6
4
2
-nitrobenzylamines as the starting materials.
Cl
Cl
Cl
Cl
Cl
Cl
Cl
4-CH3C6H4
3
3
3
3
3
i
4-BrC H4
When 2-nitrobenzylamines 1 and triphosgene 2 were treated
6
j
4-ClC H4
6
with low-valent titanium prepared from titanium tetrachloride
and samarium powder in anhydrous THF, the reductive
cyclisation products 3-aryl-3,4-dihydroquinazolin-2(1H)-ones
k
l
m
3-Cl-4-CH C H
3
4
6
3-CH C H
3
6
3-CH OC H
3
6
4
3
were obtained (Scheme 1).
3n
2-ClC H4
6
Table 1 summarises our results on the intermolecular
reductive cyclisation of 2-nitrobenzyl- amines and triphosgene.
All substrates were cyclised in good yields to afford only one
product 3. The reductive products of nitro compounds were
not detected. The chloro, bromo and fluoro groups of the
substrates could not be reduced under the reaction conditions.
As shown in Table 1, this protocol could be applied not only
to 3-aryl substituted with electron-donating groups, but also
to 3-aryl substituted with electron-withdrawing groups.
The structures of compounds 3 were characterised by IR,
1
H NMR and elemental analysis. The structure of 3a was
1
8
further confirmed by X-ray diffraction (Fig. 1).
In summary, series of 3-aryl-3,4-dihydroquinazo-
a
lin-2(1H)-ones were synthesised via reductive cyclisation
of 2-nitrobenzylamines with triphosgen induced by the
TiCl /Sm. The advantages of our new method are the easily
4
accessible starting materials, convenient manipulation and
high yields.
*
Correspondent.
PAPER: 07/4724

5
46 JOURNAL OF CHEMICAL RESEARCH 2007
Experimental
found C 75.54, H 5.87, N 11.91.
-Chloro-3-(4-methylphenyl)-3,4-dihydroquinazolin-2(1H)-one
6
THF was distilled from sodium-benzophenone immediately
prior to use. All reactions were conducted under N2 atmosphere.
Melting points are uncorrected. IR spectra were recorded on Tensor
-1
(
3h): IR: ν/cm 3195, 3045, 2924, 1670, 1599, 1492, 1467, 1444,
1
1
399, 1304, 1275, 1205, 1084, 927, 878, 819, 747, 718, 668; H
NMR (DMSO-d6): δ 2.31 (3H, s, CH3), 4.78 (2H, s, CH2), 6.87
1H, d, J = 8.4 Hz, ArH), 7.17–7.20 (2H, m, ArH), 7.23–7.24 (2H,
-1 1
2
7 spectrometer in KBr with absorptions in cm . H NMR spectra
(
were determined on Bruker DPX 400 MHz spectrometer in DMSO-
d6 solution. J values are in Hz. Chemical shifts are expressed in ppm
downfield from internal standard TMS. Microanalyses were carried
out on Perkin-Elmer 2400 II instruments. X-ray diffraction was
recorded on a Siemens P4 diffractometer.
m, ArH), 7.25–7.28 (2H, m, ArH), 9.66 (1H, s, NH); Anal. calcd for
C H ClN O: C 66.06, H 4.80, N 10.27; found C 66.19, H 4.74, N
1
5
13
2
1
0.41.
-Chloro-3-(4-bromophenyl)-3,4-dihydroquinazolin-2(1H)-one
6
-1
(3i): IR: ν/cm 3203, 3087, 2936, 1687, 1600, 1492, 1467, 1438,
1
391, 1297, 1272, 1218, 1204, 1072, 1008, 927, 826, 806, 732, 709;
General procedure for the synthesis of 3-aryl-3,4-dihydroquinazolin-
1
H NMR (DMSO-d ): δ 4.82 (2H, s, CH ), 6.88 (1H, d, J = 8.4 Hz,
6
2
2
(1H)-ones
ArH), 7.25–7.28 (2H, m, ArH), 7.35 (2H, d, J = 8.4 Hz, ArH),
.57 (2H, d, J = 8.4 Hz, ArH), 9.79 (1H, s, NH); Anal. calcd for
TiCl (1.1 ml, 10 mmol) was added dropwise using a syringe to a
4
7
stirred suspension of Sm powder (1.5 g, 10 mmol) in freshly distilled
anhydrous THF (20 ml) at r.t. under a dry N2 atmosphere. After
completion of the addition, the mixture was refluxed for 2 h. The
suspension of the low-valent titanium reagent formed was cooled to
r.t. and a solution of 2-nitrobenzylamines (2 mmol) and triphosgene
C H BrClN O: C 49.81, H 2.99, N 8.30; found C 49.93, H 2.95,
14 10
2
N 8.38.
6
-Chloro-3-(4-chlorophenyl)-3,4-dihydroquinazolin-2(1H)-one
-1
(3j): IR: ν/cm 3222, 3095, 2938, 1674, 1593, 1489, 1457, 1442,
1
394, 1307, 1273, 1211, 1165, 1093, 1015, 927, 853, 824, 748, 716;
(
3 mmol) in THF (10 ml) was added dropwise. The reaction mixture
1
H NMR (DMSO-d ): δ 4.82 (2H, s, CH ), 6.88 (1H, d, J = 8.4 Hz,
was then refluxed for 0.5 h under N . After this period, the TLC
6
2
2
ArH), 7.24–7.28 (2H, m, ArH), 7.40 (2H, d, J = 8.4 Hz, ArH),
.42 (2H, d, J = 8.4 Hz, ArH), 9.79 (1H, s, NH); Anal. calcd for
analysis of the mixture showed the reaction to be completed. The
reaction mixture was quenched with 5% HCl (50 ml) and extracted
with ClCH CH Cl (3 × 50 ml). The combined extracts were
7
C H Cl N O: C 57.36, H 3.44, N 9.56; found C 57.48, H 3.39, N
14 10
2
2
2
2
9
.63.
washed with water (3 × 50 ml) and dried over anhydrous Na SO .
2
4
6
-Chloro-3-(3-chloro-4-methylphenyl)-3,4-dihydroquinazolin-
After evaporation of the solvent under reduced pressure, the crude
-1
2
(1H)-one (3k): IR: ν/cm 3200, 3057, 2930, 1675, 1598, 1506,
product was purified by recrystallisation from 95% ethanol.
3
-(4-Methylphenyl)-3,4-dihydroquinazolin-2(1H)-one(3a):IR:ν/cm-
1
1463, 1389, 1304, 1209, 1160, 1081, 1047, 927, 879, 825, 781, 747,
1
7
03, 667; H NMR (DMSO-d ): δ 2.32 (3H, s, CH ), 4.80 (2H, s,
3
214, 3069, 2913, 1692, 1606, 1512, 1423, 1306, 1253, 1217,
6
3
1
CH ), 6.87 (1H, d, J = 8.0 Hz, ArH), 7.22–7.28 (3H, m, ArH), 7.34
1161, 1111, 1035, 1020, 1003, 974, 931, 859, 808, 784; H NMR
2
(
1H, d, J = 7.6 Hz, ArH), 7.46 (1H, s, ArH), 9.78 (1H, s, NH); Anal.
(
DMSO-d ): δ 2.31 (3H, s, CH ), 4.77 (2H, s, CH ), 6.86–6.93 (2H,
6
3
2
calcd for C H Cl N O: C 58.65, H 3.94, N 9.12; found C 58.61, H
m, ArH), 7.15–7.26 (6H, m, ArH), 9.53 (1H, s, NH); Anal. calcd for
C H N O: C 75.61, H 5.92, N 11.76; found C 75.72, H 5.84, N
15 12
2
2
3.89, N 9.27.
15
14
2
1
6-Chloro-3-(3-methylphenyl)-3,4-dihydroquinazolin-2(1H)-one
(3l): IR: ν/cm- 3204, 3049, 2916, 1684, 1602, 1473, 1430, 1394,
1299, 1272, 1207, 1162, 1082, 1030, 927, 870, 814, 779, 699;
1
ν/cm 3210, 3071, 2917, 2838, 1666, 1604, 1507, 1475, 1426, 1322,
1
1
1298, 1242, 1184, 1168, 1029, 859, 844, 811, 782, 756; H NMR
H NMR (DMSO-d6): δ 2.35 (3H, s, CH3), 4.79 (2H, s, CH2), 6.88
(
DMSO-d ): δ 3.77 (3H, s, CH O), 4.75 (2H, s, CH ), 6.85–6.91 (2H,
(1H, d, J = 8.4 Hz, ArH), 7.04 (1H, d, J = 7.6 Hz, ArH), 7.12–7.19
(2H, m, ArH), 7.23–7.29 (3H, m, ArH), 9.69 (1H, s, NH); Anal. calcd
for C15H13ClN2O: C 66.06, H 4.80, N 10.27; found C 66.14, H 4.76,
N 10.35.
6
3
2
m, ArH), 6.94 (2H, d, J = 8.8 Hz, ArH), 7.14–7.21 (2H, m, ArH), 7.28
(
2H, d, J = 8.8 Hz,ArH), 9.49 (1H, s, NH);Anal. calcd for C H N O :
15 14 2 2
C 70.85, H 5.55, N 11.02; found C 70.97, H 5.39, N 11.16.
1
3
-(4-Bromophenyl)-3,4-dihydroquinazolin-2(1H)-one(3c):IR:ν/cm-
6-Chloro-3-(3-methoxyphenyl)-3,4-dihydroquinazolin-2(1H)-one
(3m): IR: ν/cm- 3197, 3047, 2920, 1668, 1602, 1494, 1396, 1319,
1304, 1254, 1212, 1164, 1150, 1086, 1037, 1015, 922, 867, 845, 818,
1
3
1
203, 3067, 2916, 1667, 1603, 1587, 1574, 1489, 1463, 1427, 1398,
1
319, 1301, 1267, 1224, 1071, 1009, 833, 798, 756, 728; H NMR
1
(DMSO-d ): δ 4.81 (2H, s, CH ), 6.88 (1H, d, J = 7.6 Hz, ArH),
743, 698; H NMR (DMSO-d6): δ 3.76 (3H, s, CH3O), 4.81 (2H, s,
6
2
6
.93 (1H, t, J = 7.2 Hz, ArH), 7.16–7.21 (2H, m, ArH), 7.36 (2H, d,
CH2), 6.81 (1H, d, J = 8.0 Hz, ArH), 6.88 (1H, d, J = 8.4 Hz, ArH),
6.93–6.95 (2H, m, ArH), 7.24–7.31 (3H, m, ArH), 9.71 (1H, s, NH);
Anal. calcd for C H ClN O : C 62.40, H 4.54, N 9.70; found C
J = 8.4 Hz, ArH), 7.56 (2H, d, J = 8.4 Hz, ArH), 9.68 (1H, s, NH);
Anal. calcd for C H BrN O: C 55.47, H 3.66, N 9.24; found C
5
14
11
2
15 13
2
2
5.54, H 3.63, N 9.18.
-(4-Fluorophenyl)-3,4-dihydroquinazolin-2(1H)-one (3d): IR:
62.53, H 4.47, N 9.85.
3
6-Chloro-3-(2-chlorophenyl)-3,4-dihydroquinazolin-2(1H)-one
-1
-1
ν/cm 3207, 3066, 2911, 1670, 1598, 1509, 1470, 1440, 1426, 1411,
(3n): IR: ν/cm 3196, 3053, 2928, 1670, 1587, 1482, 1442, 1398,
1
1
1
320, 1296, 1263, 1215, 1155, 840, 793, 750; H NMR (DMSO-d ):
1305, 1271, 1206, 1128, 1088, 1065, 927, 818, 744, 725, 668; H
6
δ 4.80 (2H, s, CH ), 6.87 (1H, d, J = 8.0 Hz, ArH), 6.91–6.94 (1H, m,
NMR (DMSO-d ): δ 4.61 (1H, d, J = 14.4 Hz, CH), 4.80 (1H, d, J
2
6
ArH), 7.15–7.24 (4H, m, ArH), 7.40–7.43 (2H, m, ArH), 9.60 (1H, s,
NH); Anal. calcd for C H FN O: C 69.41, H 4.58, N 11.56; found C
= 14.4 Hz, CH), 6.87 (1H, d, J = 8.4 Hz, ArH), 7.24–7.26 (2H, m,
ArH), 7.36-7.46 (2H,m, ArH), 7.54–7.59 (2H, m, ArH), 9.73 (1H, s,
NH); Anal. calcd for C H Cl N O: C 57.36, H 3.44, N 9.56; found
1
4
11
2
6
9.57, H 4.53, N 11.64.
1
4
10
2
2
3
-(3-Chloro-4-methylphenyl)-3,4-dihydroquinazolin-2(1H)-one
C 57.53, H 3.39, N 9.65.
-1
(3e): IR: ν/cm 3211, 3064, 2992, 2918, 1672, 1604, 1566, 1485,
1
8
473, 1419, 1382, 1307, 1261, 1216, 1161, 1045, 1016, 994, 863,
We are grateful to the “Surpassing Project” Foundation of
Jiangsu Province and the Foundation of the Key Laboratory
of Biotechnology on Medical Plants of Jiangsu Province for
financial support.
1
17, 735, 664; H NMR (DMSO-d ): δ 2.33 (3H, s, CH ), 4.81 (2H,
6
3
s, CH ), 6.88 (1H, d, J = 7.6 Hz, ArH), 6.91–6.95 (1H, m, ArH),
2
7
.16–7.21 (2H, m, ArH), 7.27 (1H, d, J = 8.0 Hz, ArH), 7.35 (1H, d,
J = 8.4 Hz, ArH), 7.47 (1H, s, ArH), 9.64 (1H, s, NH); Anal. calcd
for C H ClN O: C 66.06, H 4.80, N 10.27; found C 66.19, H 4.85,
15
13
2
N 10.12.
-(3-Chloro-4-fluorophenyl)-3,4-dihydroquinazolin-2(1H)-one
Received 2 July 2007; accepted 17 September 2007
Paper 07/4724 doi: 10.3184/030823407X248306
3
-1
(3f): IR: ν/cm 3206, 3068, 2990, 2921, 1694, 1604, 1505, 1470,
1
8
445, 1425, 1404, 1312, 1257, 1240, 1218, 1157, 1059, 1014, 857,
1
10, 794, 738, 716; H NMR (DMSO-d ): δ 4.83 (2H, s, CH ), 6.88
6
2
(
1H, d, J = 8.0 Hz, ArH), 6.92–6.96 (1H, m, ArH), 7.16–7.26 (2H, m,
ArH), 7.40–7.46 (2H, m, ArH), 7.65–7.67 (1H, m, ArH), 9.69 (1H,
s, NH); Anal. calcd for C H ClFN O: C 60.77, H 3.64, N 10.12;
1
4
10
2
found C 60.83, H 3.58, N 10.20.
-(3-Methylphenyl)-3,4-dihydroquinazolin-2(1H)-one(3g):IR:ν/cm-
1
3
3
1
200, 3060, 2916, 1675, 1602, 1475, 1441, 1423, 1321, 1299, 1273,
1
214, 1161, 1088, 1035, 1002, 859, 808, 780, 759, 740, 702; H NMR
(DMSO-d ): δ 2.32 (3H, s, CH ), 4.79 (2H, s, CH ), 6.87–6.95 (2H,
6 3 2
m, ArH), 7.03 (1H, d, J = 7.2 Hz, ArH), 7.15–7.29 (5H, m, ArH), 9.55
1H, s, NH),; Anal. calcd for C H N O: C 75.61, H 5.92, N 11.76;
(
1
5
14
2
PAPER: 07/4724

JOURNAL OF CHEMICAL RESEARCH 2007 547
5
6
7
L.H. Zhou, D.Q. Shi, G.Y. Dai and W.X. Chen, Tetrahedron Lett., 1997,
8, 2729.
D.Q. Shi, L.C. Rong, C.L. Shi, Q.Y. Zhuang, X.S. Wang, S.J. Tu and
H.W. Hu, Synthesis, 2005, 717.
(a) J.H. Chan, J.S. Hong, L.F. Kuyper, M.L. Jones, D.P. Baccanari,
R.L. Tansik, C.M. Boytos, S.K. Rudolph and A.D. Brown, J. Heterocycl.
Chem., 1997, 34, 145; (b) S.L. Gackenheimer, J.M. Schaus and
D.R. Gehlert, J. Pharmacol. Exp. Ther., 1996, 732, 113; (c) R.Q. Dempcy
and E.B. Skibo, Biochemistry, 1991, 30, 8480; (d) S.F. Campbell and
M. Davey. J. Drug Design Delivery, 1986, 83; (e) J. Imagawa and
K. Sakai, Eur. J. Pharmacol., 1986, 131, 257.
H. Hasegawa, M. Muraoka, K. Matsui and A. Kojima, Bioorg. Med.
Chem. Lett., 2003, 13, 3471.
(a)W. Ried andA. Sinharray, Chem. Ber., 1963, 96, 3306; (b) L.F. Hennequin,
F.T. Boyle, J.M. Wardleworth, P.R. Marsham, R. Kimbell and
A.L. Juckman, J. Med. Chem., 1996, 39, 9; (c) D.J. Connolly and
P.J. Guiry, Synlett, 2001, 1707.
16 D.Q. Shi, J.X. Wang, C.L. Shi, L.C. Rong, Q.Y. Zhuang and H.W. Hu,
3
Synlett, 2004, 1098.
1
1
7
8
D.Q. Shi, C.L. Shi, X.S. Wang, Q.Y. Zhuang, S.J. Tu and H.W. Hu,
Synlett, 2004, 2239.
Crystal suitable for X-ray analysis was obtained by slow evaporation of
an EtOH solution of 3a. C15H14N2O, M = 238.28, triclinic, space group
P-1,a=6.344(1),b=9.504(1),c=10.726(2)Å,α=78.92(1)°,β=83.04(1)°,
3
-3
γ = 78.91(1) °, V = 620.38(18) Å , Z = 2, ρcalcd = 1.276 g·cm , F(000) =
-1
252, μ(MoKα) = 0.082 mm , colourless block crystals, crystal size 0.60
mm × 0.50 mm × 0.20 mm. Intensity data were collected at 296K on a
Siemens P4 diffractometer with graphite-monochromated M K radiation
8
9
o
α
(
l = 0.71073 Å): 2311 independent reflections were collected using w scan
mode in the range of 1.94° to 25.49°, of which 1395 intensity data with
I>2σ (I)] were observed. The corrections for Lp factors were applied.
[
The structure was solved by direct methods and expanded using Fourier
techniques. Non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were introduced in the F value calculation but fixed during
the structure refinement. A full matrix least-squares refinement gave
1
0
D.J. Connoly, D. Cusack, T.P. O’ Sullivan and P.J. Guiry, Tetrahedron,
2
005, 61, 10153.
(a) A. Schlapbach, R. Heng and F.D. Padova, Bioorg. Med. Chem. Lett.,
004, 14, 357; (b) R. Ferraccioli and D. Carenzi, Synthesis, 2003, 1383.
(a) H. Takeuchi, S. Haguvara and S. Eguchi, Tetrahedron, 1989, 45, 6375;
b) H. Takeuchi, S. Haguvara and S. Eguchi, J. Org. Chem., 1991, 56,
11
2
2
2
2
1 2
final R = 0.0452 and wR = 0.1146, with w = 1/[σ (Fo ) + (0.0668P) ]
1
2
2
2
where P = (Fo + 2Fc )/3, S = 0.957. The maximum peak in the final
difference Fourier map was 0.150 e/Å and the minimum peak was
0.126 e/Å . All calculations were performed using TEXSAN program
(
1
3
3
535; (c) P. Molina, M. Alajarin and A. Vidal, Tetrahedron Lett., 1988, 29,
849.
3
-
package. Full crystallographic data has been deposited at the Cambridge
Crystallographic Data Centre, deposition number CCDC 659078. Copies
of the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB2 1E2, UK (fax: ( + 44)1223-336-033;
E-mail: deposit@ccdc.cam.ac.uk).
1
3
J. Li, D.Q. Shi and W.X. Chen, Heterocycles, 1997, 45, 2381.
L.H. Zhou, S.J. Tu, D.Q. Shi and G.Y. Dai, J. Chem. Res., Synop.,1998,
1
4
3
98
1
5
D.Q. Shi, L.C. Rong, J.X. Wang, Q.Y. Zhuang, X.S. Wang and H.W. Hu,
Tetrahedron Lett., 2003, 44, 3199.
PAPER: 07/4724