Simple synthesis, structure and ab initio study of 1,4-benzodiazepine-2,5- diones

Article abstract of DOI:10.1016/j.molstruc.2003.12.024

A simple procedure for the synthesis of pyrido[2,1-c][1,4] benzodiazepine-6,12-dione (1) and 1,4-benzodiazepine-2,5-diones (2a-2d), using microwave irradiation and/or conventional heating is reported. The configuration of 1 was determined by single-crysta

Full text of DOI:10.1016/j.molstruc.2003.12.024

Journal of Molecular Structure 692 (2004) 37–42
www.elsevier.com/locate/molstruc
Simple synthesis, structure and ab initio study
of 1,4-benzodiazepine-2,5-diones
a,
a
a
b
*
¨
Khosrow Jadidi , Reza Aryan , Morteza Mehrdad , Thomas Lugger ,
F. Ekkehardt Hahnb, Seik Weng Ngc
aDepartment of Chemistry, Shahid Beheshti University, Evin, 19834 Tehran, Iran
b
¨ ¨ ¨ ¨
Anorganisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Wilhelm-Klemm-Straße 8, D-48149 Munster, Germany
cDepartment of Chemistry, University of Malaya, 50603 Kualalumpour, Malaysia
Received 22 September 2003; revised 21 November 2003; accepted 2 December 2003
Abstract
A simple procedure for the synthesis of pyrido[2,1-c][1,4] benzodiazepine-6,12-dione (1) and 1,4-benzodiazepine-2,5-diones (2a–2d),
using microwave irradiation and/or conventional heating is reported. The configuration of 1 was determined by single-crystal X-ray
diffraction. A detailed ab initio B3LYP/6-31G* calculation of structural parameters and substituent effects on ring inversion barriers ðDG#Þ
and also free energy differences ðDG0Þ for benzodiazepines are reported.
q 2004 Elsevier B.V. All rights reserved.
Keywords: 1,4-Benzodiazepine-2,5-dione; B3LYP/6-31G* calculation; Pyrido[2,1-c][1,4] benzodiazepine-6,12-dione; X-ray structure; Microwave irradiation
1. Introduction
on ring inversion and barriers ðDG#Þ and also free energy
differences ðDG0Þ between the participants in the diaster-
eomeric and enantiomeric conformational equilibrium
(M Y P) of benzodiazepines 2a, 2b, 2e, 3a, 3b (Fig. 1).
The literature survey reveals an important and wide
range of valuable pharmacological properties for the
1,4-benzodiazepine-2,5-diones (BZDs), especially antitu-
mor, anti AIDS, antihypertensive, anti-inflammatory,
analgesic agents, muscle relaxant and central nervous
system (CNS) depressant properties [1–7].
2. Results and discussion
We have found that condensation of pipecolic acid with
isatoic anhydride in DMSO or 2-methoxy ethanol results in
the rapid formation of (1) in quantitative yield when
subjected to microwave irradiation (Table 1).
Several methods have been reported for the synthesis of
BZDs. However, these methods suffer from lengthy
procedures and/or low yields and references related to
synthesis of some of those are reported in the literature in
the form of patent [7–17].
Examination of this reaction in the same solvent under
conventional thermal heating for 1.5 h produced product 1
as well as microwave-induced condition.
To the best of our knowledge, little attention has been
given to the conformational properties of BZDs [18] and
also no DG0 values for these systems have been published.
Herein, we wish to report a convenient facile and efficient
method for the synthesis of compounds 1, 2a–2d by
condensation of isatoic anhydride with corresponding
a-amino acids with spectroscopic exact data (Scheme 1).
Then we investigate detailed ab initio B3LYP/6-31G*
calculation of structural parameters and substituent effects
Acyclic a-amino acids reacted to isatoic anhydride in
DMSO or acetic acid under conventional conditions to give
the products (2a–2e) in excellent yield. Interestingly, when
these reactions were carried out under microwave
irradiation, the products were obtained in low yields
(Table 1). We have tested these reactions under microwave
activation employing DMF, DMSO, DMAC, 2-methoxy
ethanol and mixed them as solvents.
The structure of pyrido [2,1-c][1,4]benzodiazepine-6,12-
dione (1) has been established on the basis of spectral data
*
Corresponding author. Tel.: þ98-21-240-1765; fax: þ98-21-240-3041.
E-mail address: k-jadidi@cc.sbu.ac.ir (K. Jadidi).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2003.12.024

38
K. Jadidi et al. / Journal of Molecular Structure 692 (2004) 37–42
The calculated relative energy values in Table 4 show that
there is a small energy difference between conformers with
pseudoaxial and pseudoequatorial. It means that substitutent
at C3-substituent does not have an inherent preference for
above two position. Although no experimental data are
available, the calculated DG# is consistent with the NMR
data, which indicate that presence of a single C3-substitution
1,4-benzodiazepine-2,5-dione isomer in solution from 230
to 100 8C, consequently DG# for the ring inversion is small.
Barrier height ðDG#Þ for the conversion of 2M to 2P and 3M
to 3P is calculated to be 6–9 and 13 kcal/mol, respectively
(Tables 4 and 5). These results show that the nature of C3 and
C4 substituents have only a small effect onthe energybarrier of
two rapid inter converting enantiomeric conformational states
(Fig. 1). However, replacing the N hydrogen atom with a
methyl group (compound 3) has significant effect on the
inversion barrier and shape of TS. Corresponding to these
results, it seems that locating a methyl group on N 2 1 atom
will increase the steric hindrance between the positions 1 and 9
via a particular interaction called peri interaction.
Scheme 1.
and X-ray diffraction measurements, which have been shown
the configuration of seven-membered ring is boat-shaped and
six membered ring is chair-shaped (Fig. 2).
The proposed structures for benzodiazepine-2, 5-diones
2a–2d are in agreement with the analytical and spectral data
(Section 3).
Variable temperature 1H and 13C NMR spectrum of these
compounds (2a–2d) in the 230 to 100 8C range (Fig. 1) did
not show the presence of two conformers.
According to the X-ray data, to determine which method
and level of calculation is more proper for predicting a more
accurate parameter of title compounds, full optimization of
the geometry 2b was carried out by using ab initio hartree–
fock (HF) and density functional (DFT) methods at different
levels.
Comparison of f7123 and f2345 torsional angles of
compounds listed in Tables 4 and 5 clearly show only the
backbone of the transition structure of molecules 2a is flat,
while the TSs of other molecules are forced to adopt a
nonplanar shape because of the presence of bulky groups,
especially in the N1 position. In our opinion, in the case of
compound 2a the possibility of maximum pp–pp overlap in
TS may serve to lower the energy barrier for interconversion
of the isomers in contrast to the others (Tables 4 and 5).
As can be deduced from Table 4, upon going from the
equatorial to axial conformation in compound 2e, the u123
bond angle is expanded by about 78 and the f3456 torsional
angles are decreased by 238. These observations clearly
indicate that the seven-membered ring is less puckered when
the tert-butyl group is in the axial position.
The differences between calculated average bond dis-
tances and bond angles that are obtained from X-ray study of
compound 1 (Standard Deviation), reveal the fact that the
B3LYP/6-31G* level of theory with the lowest SD value of
0.010 and 0.301 (bond distance and bond angle, respectively)
is able to predict geometrical parameters, total energy and
unknown behavior of 1,4-benzodiazepine 2,4-diones in detail
(Tables 2 and 3).
Selected geometrical parameters, total energies and
relative energy (Erel) values of the participants in the
diastereomer and enantiomer conformational equilibrium
(P Y M) of compound 2a, 2b, 2e and 3a, 3b are calculated at
B3LYP/6-31G(d) levels of theory and results are summarized
in Tables 4 and 5.
This fact may be attributed to the capability of the
benzodiazepine ring to adjust the interaction in the axial
conformer between C3-substitution and p-electron density of
the molecule.
Fig. 1. Conformational interconversion of molecules of 2,3.

K. Jadidi et al. / Journal of Molecular Structure 692 (2004) 37–42
39
Table 1
a Heraus CHN-O Rapid analyzer. IR spectra were obtained
with Shimadzu IR-470 spectrometer. 1H and 13C NMR
Products of the reaction of a-aminoacids with isatoic anhydride
spectra were recorded on
AVANCHE spectrometer.
a BRUKER DRX-500
Product
Conventional heating
Irradiation conditions
t=h
Yield (%)
t=m
Yield (%)
Microwave irradiations were carried out in a National
oven, model 5250 at 2450 MHz (800 W, 100% power).
Isatoic anhydride and a-amino acids were obtained from
Sigma-Aldrich Company and were used without further
purification.
1
2
98
92
90
95
90
85
3
4
3
5
3
3
98
20
25
18
15
20
2a
2b
2c
2d
2e
2.5
2
3
2
2
3.1. Computational methods
The ab initio calculations were performed with the
GAUSSIAN-98 program package [18] on a PENTIUM 4
computer. The conformers were optimized using Beck’s
three-parameter hybrid method with the Lee, Yang, and Parr
correlation functional (B3LYP) [19–22]. The basis set, 6-
31G* was used in the present calculations [23,24].
3.2. X-ray crystallography
Colorless crystal of compound 1 was obtained by slow
evaporation of n-hexane-ethanol (1:4). The room tempera-
ture diffraction measurements were carried with a colorless
0.14 £ 0.14 £ 0.06 mm3 plate of crystal compound on a
Bruker AXS area-detector diffractometer. The crystallo-
graphic data for the compound is listed in Table 6. Positional
parameters and equivalent isotropic displacement par-
ameters are listed in Table 7.
Fig. 2. Molecular structure and atom numbering.
These assignments imply that the effect steric groups at
the C3 are more important that those about the other center
in shape of the benzodiazepine ring.
The ORTEP view of compound with atomic numbering
is shown in Fig. 2. The structure was solved by using
SHELXS 97. The structure refinement and data reduction was
carried out with SAINI (Bruker 1997). The nonhydrogen
atoms were refined anisotropically and all the hydrogen
atoms were located from subsequent difference fourier maps
and refine to a final R value of 0.045.
3. Experimental
Melting points were measured on a Mettler FP5. Mass
spectra were recorded on a Shimadzu QP100EX mass
spectrometer operating at an ionization potential of 70 eV.
Elemental analysis for C, H, and N were performed using
Table 2
The differences between calculated average bond distances and bond angles with X-ray data
6-31G
6-31G*
6-31G**
6-31 þ G
6-31 þ G*
6-31 þ G**
6-311G*
6-311G**
˚
Bond length (A)
0.020
0.399
0.008
0.141
0.010
3.700
0.021
0.585
0.010
0.301
0.010
0.280
0.008
0.140
0.008
0.144
Bond angle (degrees)
Table 3
Calculated and experimental selected geometrical parameters of compound 2b (equatorial)
˚
Bond lengths (A)
Bond angles (degrees)
r2–12
r6–7
r7–8
u789
u123
u712
Exp
1.226
1.221
1.398
1.412
1.390
1.408
120.800
121.249
114.200
117.778
127.000
131.700
(B3LYP)6-31G*

40
K. Jadidi et al. / Journal of Molecular Structure 692 (2004) 37–42
Table 4
Calculated parameters for various conformers of compounds 2a, 2b, 2e
P (axial)
M (equatorial)
TS (P Y M)p
2a
2b
2e
2a
2b
2e
2a
2b
2e
Bond lengths
˚
(A)
r12
r23
r34
r45
1.381
1.524
1.452
1.366
1.381
1.527
1.461
1.372
1.379
1.528
1.462
1.374
1.381
1.524
1.452
1.366
1.380
1.532
1.461
1.369
1.382
1.537
1.462
1.368
1.371
1.516
1.451
1.372
1.373
1.523
1.460
1.370
1.385
1.523
1.463
1.380
Bond angles
(degrees)
u123
115.6
117.8
120.5
115.6
111.2
125.2
116.3
114.9
113.5
124.3
123.4
130.6
120.3
124.3
123.0
u234
u345
111.2
125.2
116.3
111.9
128.9
119.1
112.9
132.8
120.4
109.3
126.4
117.5
107.5
126.5
117.3
120.6
139.7
122.9
117.3
136.5
122.0
u456
Dihedral angles
(degrees)a
f1234
f2345
f3456
f4567
f7123
57.5
50.8
70.7
19.8
25.0
9.1
39.9
62.3
22.5
19.0
11.3
57.7
75.1
14.6
30.2
9.9
59.7
76.8
15.8
31.2
7.5
63.4
78.5
14.5
32.9
4.3
0.10
0.10
0.10
0.10
0.20
1.9
3.5
5.8
2.2
3.1
10.0
32.6
36.8
3.2
75.1
14.6
30.2
9.9
37.8
Energiesb
Etot
Erel
2381261.42
0
2405912.97
2.29
2479862.71
3.36
2381261.42
0
2405915.26
0
2479866.07
0
2381254.71
6.71
2405907.33
7.93
2479856.97
9.10
c
a
b
c
The energy surface for interconversion of conformations of 2a–2f was investigated in detail by changing f1234
Etot and Erel, in units of kcal mol21
Relative energy with respect to the most stable conformation.
.
.
3.3. General procedure
Table 5
Calculated parameters for various conformers of compounds 3a,b
3.3.1. Microwave
P (or M)
TS (P M)*
A mixture of isatoic anhydride (10 mmol) a-amino acid
(10 mmol), and 5 ml DMSO was placed in a tall beaker. The
beaker, covered with a watch glass, was irradiated at 600 W
for 5 min in the microwave oven. The mixture was diluted
with 20 ml water and extracted three times with 30 ml of
toluene. The toluene was dried over anhydrous magnesium
sulfate and resultant solution was concentrated under
reduced pressure and crystals deposited by slow
evaporation.
3a
3b
3a
3b
˚
Bond lengths (A)
r12
r23
r34
r45
1.384
1.526
1.452
1.366
1.380
1.524
1.458
1.372
1.387
1.526
1.449
1.374
1.366
1.516
1.457
1.379
Bond angles (degrees)
u123
u234
u345
u456
115.6
111.3
125.2
116.3
114.1
111.7
122.3
117.9
124.3
123.4
130.6
120.3
123.9
126.1
132.5
123.1
3.3.2. Thermal
Dihedral angles (degrees)a
A: A mixture of isatoic anhydride (20 mmol), a-amino
acid (20 mmol), and 20 ml DMSO were heated under reflux
for 1.5 h. The mixture was diluted with 80 ml of water and
extracted three times with (3 £ 10 ml) toluene. The toluene
was dried over anhydrous magnesium sulfate and resultant
solution was concentrated under reduced pressure and
crystals deposited by slow evaporation.
f1234
f2345
f3456
f4567
f7123
61.5
72.1
4.5
38.4
11.2
63.1
76.6
10.6
36.6
8.7
26.7
40.7
42.0
18.8
28.5
4.9
12.0
21.8
11.9
5.8
Energiesb
Etot
2405908.50
0
2406038.19
0
2405895.49
13.01
2406028.97
9.22
c
Erel
B: A mixture of isatoic anhydride (20 mmol), a-amino
acid (20 mmol), and 20 ml glacial acetic acid (20 ml) were
heated under reflux for 1.5 h. The solvent was removed
under reduced pressure. The residue was washed with 10%
a
The energy surface for interconversion of conformations of 2a–2f was
investigated in detail by changing f1234
:
b
Etot and Erel, in units of kcal mol21
.
c
Relative energy with respect to the most stable conformation.

K. Jadidi et al. / Journal of Molecular Structure 692 (2004) 37–42
41
3
3
Table 6
(m, 1 H), 2.79 (d.t. J1 ¼ 3:5 Hz, J2 ¼ 13:1 Hz,1H), 4.12
(q, 1H), 4.3 0 (m, 1H), 7.06 (d, 1H, Ar–H), 7.19 (t, 1 H,
Ar–H), 7.47 (d. of t., 1 H, Ar–H), 7.69 (d, 1 H, Ar–H), 10.3
5 (s. 1H, N–H); 13C NMR (DMSO-d6) d (ppm) 19.31,
22.84, 23.31, 40.06, 51.01, 120.95, l24.35, 127.60, 130.91,
132.24, 137.32, 168.15, 171.52; MS: m=e ¼ 230;
Crystal data and structure refinement
Empirical formula
Formula weight
Temperature
C13 H14 N2 O2
230.26
293(2) K
˚
Wavelength
0.71073 A
Triclinic
P-1
Crystal system
Space group
2a: White powder; m.p. 320 8C (Lit 325 8C); IR (KBr):
1
nmax=cm 1660, 1686; H NMR (DMSO-d6): d (ppm) 3.60
˚
Unit cell dimensions
a ¼ 5.1058(4) A, a ¼ 100.636(1)8
(d, 3J ¼ 5:6 Hz, 2H, CH2), 7.11 (d, 1 H, Ar–H), 7.22 (t, 1 H,
Ar–H), 7.51 (t, 1 H, Ar–H), 7.75 (d, 1 H, Ar–H), 8.54
(s, 1H, N–H), 10.3 5 (s, 1H, Ar–H); 13C NMR (DMSO-d6):
3 ppm 45.32 (CH2), 121.78, 124.75, 131.66, 133.12,
138.01,168.95, 172.00; MS: m=e ¼ 176;
˚
b ¼ 8.3897(6) A, b ¼ 95.707(2)8
˚
c ¼ 12.903(1) A, g ¼ 94.024(2)8
3
˚
Volume
538.32(7) A
Z
2
Density (calculated)
Absorption coefficient
Fð000Þ
1.421 Mg/m3
0.10 mm21
2b: White powder; m.p. 332 8C (Lit 335 8C); IR (KBr):
244
0.14 £ 0.14 £ 0.06 mm3
1
Crystal size
nmax=cm 1664, 1693; H NMR (DMSO-d6): d (ppm) 1.21
(d, 3J ¼ 6:7 Hz, 3H, CH3), 3.79 (m, 1H, CH), 7.08 (d, 111,
Ar–H), 7.19 (t, 1H, Ar–H), 7.49 (t, 1H, Ar–H), 7.72 (d, 1H,
Ar–H), 8.39 (d, 11-1, N–H), 10.34 (s, 1H, N–H); 13C NMR
Theta range for data
collection
2.5–29.18
Index ranges
26 # h # 6, 211 # k # 11, 217 # l # 17
5900
Reflections collected
Independent reflections
(DMSO-d6):
d (ppm) 14.24 (CH3), 47.75 (CH),
2846 [R(int) ¼ 0.018]
Completeness to u ¼ 29:18 99.3%
121.34,124.30, 126.70, 130.87, 132.62, 137.22, 168.17,
172.66; MS: m=e ¼ 190;
Absorption correction
Refinement method
None
Full-matrix least-squares on F2
2c: White crystalline powder; m.p. 246–248 8C
(Lit 250 8C); IR (KBr): nmax=cm 1675, 1693; 1H NMR
Data/restraints/parameters 2846/0/210
Goodness-of-fit on F2
1.03
3
(DMSO-d6): d (ppm) 0.75 (d, J ¼ 6:5 Hz, 3H,CH3), 0.83
Final R indices ½I . 2sðIÞꢀ R1 ¼ 0.045, wR2 ¼ 0.119
3
3
R indices (all data)
Largest diff. peak and
hole
R1 ¼ 0.060, wR2 ¼ 0.128
(d, J ¼ 6:5 Hz, 3H,CH3), 1.54 (t, J ¼ 6:6 Hz, 2H, CH2),
23
˚
0.26 and 20.23 e A
3
1.69 (m, 1H, CH), 3.58 (q, J ¼ 6:4 Hz, 1H, CH), 7.08
(d, 1H, Ar–H),7.20 (t, 1H, Ar–H), 7.49 (t, 1H, Ar–H), 7.73
(d, 1H, Ar–H), 8.42 (d, 1H, N–H),10.35 (s, 1H, N–H); 13C
NMR (DMSO-d6): d (ppm) 22.01 (CH3),23.29 (CH3),
24.34 (CH2), 36.63 (CH), 50.74 (CH), 121.35, 124.36,
126.73, 130.83, 132,67, 137.21, 168.21, 172.12; MS:
m/z ¼ 232;
sodium bicarbonate solution followed by water and then
recrystallized from appropriate solvents.
1: White crystalline powder; m.p. 226–228 8C (Lit
1
229 8C); IR (KBr): nmax=cm 1672, 1695; H NMR (DMSO-
d6): d (ppm) 1.46 (m, 1H), 1.59 (m, 2H) 1.76 (m, 2H), 2.02
2d: White powder; m.p. 245 8C (Lit 250 8C); IR (KBr):
n
max=cm 1671,1691, 1H NMR (DMSO-d6): d (ppm) 2.83 (dd,
3J1 ¼ 4:9 Hz, J2 ¼ 14:1 Hz, 1H), 3.11 (dd, J1 ¼ 9:4 Hz,
3J2 ¼ 14:1 Hz, 1H), 3.87 (m, 1H, CH), 7.08 (d, 1H, Ar–H),
7.18 (q, 2H, Ar–H), 7.24 (t, 2H, Ar–H), 7.30 (d, 2H,
Ar–H), 7.49 (t, 1H, Ar–H), 7.65 (d, 1H, Ar–H), 8.50 (d, 1H,
N–H), 10.40 (s. 1 H, N–H); 13C NMR (DMSO-d6): d (ppm)
39.48, 48.65, 110.71, 115.80, 121.43, 123.98, 126.81,
128.64, 129.39, 129.8,130.83, 137.40, 141.865, 147.55,
160.33, 1171.75; MS: m=e ¼ 266:
3
3
Table 7
Atomic coordinates ( £ 104) and equivalent isotropic displacement
2
3
˚
parameters (A £ 10 ) for pyrido[2,1-c][1,4]benzodiazepine-6,12-dione (1)
x
y
z
U(eq)
O(1)
O(2)
N(1)
N(4)
C(2)
6546(2)
0.9859(2)
7652(2)
6687(2)
8062(2)
6103(2)
6054(2)
4739(2)
5633(2)
4604(3)
2648(3)
1693(3)
2761(3)
8534(3)
8174(3)
8214(3)
6019(3)
6682(1)
3345(1)
5587(1)
4319(1)
4086(1)
3421(1)
5877(2)
6625(1)
6545(1)
7513(2)
8496(2)
8546(1)
7640(2)
3717(2)
1896(2)
1038(2)
1581(2)
6076(1)
8814(1)
9000(1)
6680(1)
8490(1)
7516(1)
6740(1)
7680(1)
8727(1)
9557(1)
9359(1)
8329(1)
7501(1)
5930(1)
5562(1)
6498(1)
7150(1)
49(1)
44(1)
35(1)
32(1)
31(1)
29(1)
34(1)
34(1)
32(1)
40(1)
46(1)
46(1)
41(1)
41(1)
47(1)
42(1)
37(1)
C(3)
References
C(5)
C(6)
[1] G.A. Archer, L.H. Sternbaeh, Chem. Rev. 68 (1968) 747.
[2] J.T. Sharp, in: A.R. Katritzky, C.W. Rees, W. Lwowski (Eds.),
Comprehensive Heterocyclic Chemistry, vol. 7, Pergamon Press, New
York, 1984, p. 593.
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
[3] H. Tucker, D.J. Lecount, in: A. R. Katritzky, E.F. Scriven (Eds.),
Comprehensive Heterocyclic Chemistry, vol. 9, Pergamon Press, New
York, 1996, p. 151.
[4] N.S. Cho, Y. Song, C. Parkanyi, J. Heterocycl. Chem. 26 (1989)
1807.
[5] P.G. Baraldi, G. Balboni, B. Cacciari, A. Guiotto, S. Manfredini, R.
Romagnoli, G. Spalluto, D.E. Thurston, P.W. Howard, N. Bianchi, C.
Rutigliano, C. Mischiati, R. Gambari, J. Med. Chem. 42 (1999) 5131.
UðeqÞ is defined as one-third of the trace of the orthogonalized Uij
tensor.

42
K. Jadidi et al. / Journal of Molecular Structure 692 (2004) 37–42
[18] P.K. Bridson, H.A. Kurtz, F. Sayyarpour, J. Mol. Struct. (Theochem.)
[6] G.B. Jones, C.L. Davey, T.C. Jenkins, A. Kamal, G.G. Kneale, S.
199 (1989) 175.
Neidle, G.D. Webster, D.E. Thurston, Anti-cancer Drug Des. 5
(1990) 249.
[19] M.J. Fisch, G.W. Trucks, H.B. Schelegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, V.G. Zakrzewski, A.J. Montgomery Jr., R.E.
Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels,
K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,
R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J.
Ochterski, G.A. Peterson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K.
Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski,
J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-
Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres,
C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUS-
SIAN98, Revision A6, Guassian, Inc., Pittsburgh, PA, 1998.
[20] A.D. Becke, Phys. Rev. A 38 (1988) 3098.
[7] G. Mohiuddin, P.S. Keddy, K. Ahmed, C.V. Ratnam, Heterocycles 24
(1986) 3489.
[8] P.M. Carabates, US Patent 3,732,212, 1973; Chem. Abstr.
(1973) p42570.
[9] P.M. Carabateas, US Patent 3,860,600, 1975; Chem. Abstr., 83
(1975) p58892.
[10] W.B. Wright, US Patent 3947408, 1976; Chem. Abstr. 85
(1976) 46771.
[11] M. Mori, M. Kiura, Y. Uozumi, Y. Ban, Tetrahedron Lett. 26
(1985) 5947.
[12] D.E. Thurston, D.S. Bose, Chem. Rev. 94 (1994) 433.
[13] D.H. Kim, J. Heterocycl. Chem. (1975) 1323.
[14] M. Akssira, H. Kasmi, A. Dohdouh, M. Boumzebra, Tetrahedron Lett.
33 (1992) 1887.
[21] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[22] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[23] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157
(1989) 200.
[15] T.A. Keating, R.W. Armstrong, J. Org. Chem. 61 (1996) 8935.
[16] G. Mohiuddin, P.S.N. Neddy, K. Ahmed, C.V. Ratanam, Indian
J. Chem. 24B (1985) 905.
[17] M. Akssira, M. Boumzebra, H. Kasmi, A. Dahdouh, Tetrahedron 50
(1994) 9051.
[24] W.J. Hehre, L. Radom, P.V.R. Schleyer, J.A. Pople, Ab Initio
Molecular Orbital Theory, Wiley, New York, 1986.