Structural characterization of two novel potential anticholinesterasic agents

Article abstract of DOI:10.1016/S0022-2860(03)00367-3

Two novel compounds with possible anticholinesterase activity have been synthesized containing a carbamate and a dimethylamine group in 1,2-positions of a cyclohexane ring (cis and trans isomers). Conformer populations were established by a combination of NMR ¹H coupling constant analysis and DFT (B3LYP/6-311 + G(d,p)) calculations. ¹³C chemical shifts were calculated in order to confirm signal attributions. The cis isomer adopts a conformation in which the carbamate group lies at the axial position (>99%), whereas the trans isomer adopts a diequatorial arrangement (98%). These preferences have been explained in terms of syn-1,3-diaxial interactions of the individual groups.

Full text of DOI:10.1016/S0022-2860(03)00367-3

Journal of Molecular Structure 657 (2003) 191–198
www.elsevier.com/locate/molstruc
Structural characterization of two novel potential
anticholinesterasic agents
a
Paulo R. Oliveira , Franciele Wiectzycosky , Ernani A. Basso , Regina
b
b,
*
c
A.C. Gon c¸ alves , Rodrigo M. Pontes
b
a
Instituto de Qu ´ı mica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, S a˜ o Paulo, Brazil
b
Departamento de Qu ´ı mica, Universidade Estadual de Maring a´ , Av. Colombo, 5790, 87020-900, Maring a´ -PR, Brazil
c
Departamento de Farm a´ cia e Farmacologia, Universidade Estadual de Maring a´ , Av. Colombo, 5790, 87020-900, Maring a´ -PR, Brazil
Received 25 April 2003; revised 30 May 2003; accepted 30 May 2003
Abstract
Two novel compounds with possible anticholinesterase activity have been synthesized containing a carbamate and a
dimethylamine group in 1,2-positions of a cyclohexane ring (cis and trans isomers). Conformer populations were established by
1
13
a combination of NMR H coupling constant analysis and DFT (B3LYP/6-311 þ G(d,p)) calculations. C chemical shifts were
calculated in order to confirm signal attributions. The cis isomer adopts a conformation in which the carbamate group lies at the
axial position (.99%), whereas the trans isomer adopts a diequatorial arrangement (98%). These preferences have been
explained in terms of syn-1,3-diaxial interactions of the individual groups.
q 2003 Elsevier B.V. All rights reserved.
Keywords: Anticholinesterasic; Synthesis; Conformational analysis; Nuclear magnetic resonance; Theoretical calculations
1
. Introduction
obtaining more potent, selective and safe AChE
inhibitors [1–6]. Two structural features are common
to most of the molecules classified as anticholines-
terasic agents, viz. an alkylammonium group and an
Anticholinesterasic compounds are drugs that
inhibit the action of the acetycholinesterase enzyme
AChE), and are important in the treatment of illness
such as myasthenia gravis and Alzheimer’s disease
1–3]. The role of an anticholinesterase inhibitor is to
˚
oxygen atom 4.4–5.9 A apart. In some cases, the
(
alkylammonium group may be replaced by a tertiary
amine nitrogen, which can be positively charged at
physiological pH [2].
[
bind to AChE, reducing the enzyme degradation of
the neurotransmitter acetylcholine (ACh).
Our laboratory has synthesized two potential
antycholinesterasic agents, namely cis-2-N,N-
Several compounds have been developed with this
end, and ongoing research has been aimed at
0
0
dimethylaminecyclohexyl 1-N ,N -dimethylcarbamate
1; Fig. 1a) and trans-2-N,N-dimethylaminecyclo-
(
0
0
hexyl 1-N ,N -dimethylcarbamate (2; Fig. 1b). Each
of them has two possible conformations generated
by the interconversion of the cyclohexane ring.
*
Corresponding author. Tel.: þ55-44-261-4332; fax: þ55-44-
63-5784.
E-mail address: eabasso@uem.br (E.A. Basso).
2
0022-2860/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-2860(03)00367-3

192
P.R. Oliveira et al. / Journal of Molecular Structure 657 (2003) 191–198
Fig. 1. Conformational equilibrium for: (a) 1; and (b) 2.
Pharmacological studies for these compounds are
being conducted and the results will be published
elsewhere. The main purpose of this article is to give a
description of their molecular structure, including a
study of the conformational preference by means of
nuclear magnetic resonance (NMR) spectroscopy and
theoretical calculations. The knowledge of structural
features is of fundamental importance in the under-
standing of the biological behavior.
2. Experimental
2.1. Compound preparations
Compounds 1 and 2 were synthesized following
the route in Fig. 2. The individual steps are well
described in the literature. Cyclohexanone (3) was
reacted with bromine to obtain 2-bromocyclohexa-
none (4) [7], which was placed in an autoclave with
Fig. 2. Synthetic route for obtaining of compounds 1 and 2.

P.R. Oliveira et al. / Journal of Molecular Structure 657 (2003) 191–198
193
Table 1
solvents were obtained commercially and used as
received.
Boiling points and yieldings for the compounds involved in the
synthetic route of 1 and 2
2.3. Calculations
Compound
Boiling point (8C)/P (Torr)
Yield%
1
3
4
5
6
,2
,7
101–102/1.5
154/760
57
–
The GAUSSIAN 98 package [11] was used to carry
out all the calculation. DFT was applied with Becke’s
three-parameter hybrid method [12] and correlation
functional of Lee, Yang and Parr (B3LYP) [13], in
conjunction with the 6-311 þ G(d,p) basis set for
geometry optimization and 6-31G(d,p) for shielding
tensors calculations. Geometries were optimized and
confirmed as minima by frequencies calculations, and
zero-point energies were scaled by 0.9806 [14]. The
optimized structures were submitted to magnetic
properties calculations using the GIAO method [15].
Comparison with experiment ðdÞ was accomplished
by calculating the absolute shielding for tetramethyl-
silane (TMS) and using the following equation
52–53/1.0
53–54/1.2
55–57/1.6
56
53
77
N,N-dimethylamine solution to give 2-N,N-dimethy-
laminecyclohexanone (5) [8]. Reduction of 5 with
lithium aluminum hydride [9] gave a mixture of cis
and trans (25:75) 2-N,N-dimethylaminecyclohexanol
(
6 and 7, respectively) that were subsequently reacted
with N,N-dimethylcarbamyl chloride to obtain the
target compounds [10]. The cis and trans isomers
were isolated from each other through elution of the
reaction products in a silica column (acetone/ethanol
dðCiÞ ¼ sTMSðCÞ 2 sðCiÞ
7:3). The data regarding the synthetic route are
summarized in Table 1.
where sTMSðCÞ is the TMS absolute shielding and
sðC Þ is the absolute shielding for nucleus i:
i
2
.2. NMR measurements
3. Results and discussion
NMR spectra were acquired on a Varian Gemini
000/300 spectrometer. CDCl3 solutions were
3.1. Assignments and conformational preference
by H spectra
1
2
employed to obtain the spectra used for signal
attribution. Samples were prepared by placing ca.
Signal assignments were accomplished by taking
1
into account H coupling constants, when possible,
5
5
0 mg of the compound in 0.5 ml of the solvent in a
1
3
mm o.d. NMR tube. Typical conditions for
C
and combining these data with DEPT and HETCOR
experiments (Tables 2 and 3). Additionally, we have
spectra were sweep width of 18 kHz, pulse angle of
08, 256 scan and 1 s delay time. Spectra were stored
1
3
3
evaluated C chemical shifts through DFT-B3LYP/
6-31G(d,p) calculations. The calculated chemical
shifts are, in most of the cases, larger than the
observed ones, which may be due the fact that
1
in 32 K data blocks. For H spectra, sweep width was
typically 4000 Hz, pulse angle of 458, 62 scans, 1 s
delay time and 16 K of data points. Deuterated
Table 2
1
H NMR chemical shifts for compounds 1 and 2
a
a
a
a
a
a
a
a
H-1
H-2
H-3(eq)
H-3(ax)
H-4(eq)
H-4(ax)
H-5(eq)
H-5(ax)
H-6(eq)
H-6(ax)
H-7/H-8 H-10 H-11
1
2
5.30 2.16 1.78
4.77 2.50 1.82
1.68
1.34
1.82
1.72
1.32
1.20
1.47
1.66
1.47
1.28
1.93
2.03
1.38
1.28
2.33
2.35
2.93
2.92
2.95
2.92
Spectrum acquired in CDCl
3
solution; in ppm downfield from TMS.
a
eq and ax refers to equatorial and axial hydrogens, respectively, in the cyclohexane ring.

194
P.R. Oliveira et al. / Journal of Molecular Structure 657 (2003) 191–198
Table 3
13
C Experimental and calculated (in parenthesis) chemical shifts for compounds 1 and 2
a
C-1
C-2
C-3
C-4
C-5
C-6
C-7/C-8
C-9
C-10
C-11
1
2
70.01
65.81
25.65
25.38
20.29
30.46
42.64
156.26
36.00
36.41
(
71.44)
73.79
74.02)
(70.49)
66.06
(31.18)
26.00
(30.30)
24.71
(25.95)
24.10
(35.16)
32.19
(45.64)
41.34
(149.75)
156.56
(37.56)
36.04
(38.41)
36.45
(
(72.07)
(26.01)
(30.20)
(29.80)
(37.29)
(43.09)
(149.50)
(36.99)
(37.87)
Computed at B3LYP/6-31G(d,p); spectra acquired in CDCl
3
solution; in ppm downfield from TMS.
a
These carbons appear as a single peak and the calculated values were obtained by averaging over the values of each carbon.
calculations do not take into account solvent effects.
Nevertheless, the relative positions are sufficient to
help in the assignments of some dubious signals. This
is the case, for instance, of nucleus 10 and 11 (Fig. 1).
The attribution of these peaks could be performed
through ASIS (aromatic solvent induced shift)
couplings (with H-6 and H-2) plus any long-range
couplings. The value so obtained is 13 Hz, in
accordance to what is expected by taking into account
the individual splittings resulting from an equatorial
position. If H-1 were in an axial arrangement, the total
coupling ðJ Þ; i.e. the value obtained by measuring
T
1
experiments on the H spectra [16,17], but this
requires that the peaks present a reasonable separ-
ation, which is not the case. In this way, we have
attributed C-10 and C-11 with the help of the
calculated values. In compound 1, H-10 and H-11
show a slightly separation (0.02 ppm), but the
HETCOR spectra was not enough resolved to allow
a direct correlation with C-10 and C-11. Therefore, we
have attributed H-10 and H-11 by admitting that they
1
3
follow the same pattern of the C spectra.
Conformational preference may be assessed by
analyzing the information contained in the coupling
patterns of the cyclohexane hydrogens [10,18].
Unfortunately, most of these signals are involved in
second order spectra, which makes difficult to identify
the individual couplings. However, the proton close to
the oxygen atom (H-1) appears at sufficient high field
to allow an individual analysis. Fig. 3 depicts this
signal for 1 and 2. Notice the difference in shape;
while the couplings are clear for 2, for 1, it is not
possible to identify any splitting pattern.
It is a well-known fact that proton vicinal
couplings depend markedly on the dihedral angle
ðvÞ between the two C–H bonds [19]. In this way, for
v close to 1808, the coupling constant presents a value
of about 10 Hz, while for v close to 608, its value is
approximately 5 Hz. In the case of 1, it is not possible
to identify any individual coupling for H-1, so we
have measured the W value, i.e. the signal width at
half height, which measures the sum of vicinal
1
Fig. 3. H spectrum for the H-1 region (in CDCl
3
): (a) 1; (b) 2.

P.R. Oliveira et al. / Journal of Molecular Structure 657 (2003) 191–198
195
Table 4
˚
Selected structural parameters for the conformers of compounds 1 and 2 (bond lengths in A, bond angles in 8, dipole moments in debyes and
energies in hartrees)
1
ae
1ea
2ee
2aa
Bond lengths
rðC –OÞ
1.450
1.219
1.359
1.366
1.456
1.455
1.473
1.460
1.464
1.546
1.530
1.540
1.532
1.537
1.532
1.443
1.219
1.357
1.366
1.456
1.454
1.471
1.456
1.455
1.555
1.528
1.556
1.534
1.537
1.533
1.445
1.219
1.359
1.365
1.455
1.454
1.469
1.459
1.455
1.540
1.530
1.548
1.534
1.534
1.533
1.457
1.218
1.361
1.366
1.456
1.455
1.479
1.460
1.464
1.550
1.528
1.541
1.534
1.536
1.543
1
rðC yOÞ
9
rðC –OÞ
9
rðC –NÞ
9
rðN–C
rðN–C
Þ
Þ
1
0
1
1
rðC –NÞ
2
rðC –NÞ
7
rðC –NÞ
8
rðC –C Þ
1
2
rðC –C Þ
1
6
rðC –C Þ
2
3
rðC –C Þ
5
6
rðC –C Þ
3
4
rðC –C Þ
4
5
Bond angles
/
/
/
/
/
/
/
/
/
/
(C
1
–O–C
9
)
117.7
117.0
117.8
116.9
(O–C
(O–C
9
9
–N)
yO)
111.7
123.9
123.8
118.7
108.8
108.1
111.1
114.2
112.2
111.9
123.5
124.0
118.8
112.2
108.2
114.4
112.7
118.7
117.7
123.9
124.5
119.1
109.2
107.1
111.8
116.1
113.5
111.8
123.6
124.3
119.0
108.6
107.6
109.2
114.5
111.8
(C
(C
9
–N–C10
–N–C11
)
9
)
(O–C
(O–C
1
1
–C
–C
2
)
)
6
(C
1
(C
2
(C
2
2
–C –N)
–N–C
–N–C
7
)
)
8
Dihedral angles
fðC –O–C yOÞ
5.1
25.1
174.4
4.3
28.4
177.8
20.5
21.2
2178.7
23.5
1
9
fðC –N–C yOÞ
2173.3
25.7
1
0
9
fðC –N–C yOÞ
1
1
9
fðO–C –C –NÞ
261.7
168.2
235.3
152.1
275.7
2169.3
178.3
57.5
2168.4
2163.4
72.1
1
2
fðC –C –N–C Þ
2155.2
73.4
1
2
7
fðC –C –N–C Þ
266.1
64.7
1
2
8
fðO–C –C –C Þ
2173.1
175.1
66.6
1
2
3
fðO–C –C –C Þ
264.8
266.6
1
6
5
m
2.44
2.20
2.31
2.36
E
2692.580026
0.323663
2692.573382
0.323511
2692.582630
0.323218
2692.579147
0.323622
a
ZPE
Computed at the B3LYP/6-311 þ G(d,p).
a
Zero-point energy correction scaled by 0.9806.
the distance between the tops of the extreme peaks,
should be close to 20 Hz.
about 13 Hz if H-1 were in the equatorial position
(conformation 1ea, Fig. 1), but the observed value
is 17.55 Hz, in accordance to what is expected by
analyzing the couplings resulting from an axial
position. Therefore, we conclude that 1ae is
Another evidence of the conformational preference
of 1 can be obtained by considering the H-2 signal.
The total coupling constant should present a value of

196
P.R. Oliveira et al. / Journal of Molecular Structure 657 (2003) 191–198
the predominant conformer of 1. Theoretical calcu-
lations corroborate with this preference, as will be
discussed in Section 3.3.
Table 5
21
Relative energies (in kcal mol ) and mole fractions ðnÞ for the
conformers of compounds 1 and 2
a
E1ea 2E1ae E2aa 2E2ee n1ae
a
A similar analysis may be conducted for
compound 2. The H-1 signal (Fig. 3b) presents a
well-resolved shape and, in addition to W; we were
able to measure the individual couplings and JT:
The values so obtained are W ¼ 27:5 Hz,
Level
n2ee
HF/STO-3G
3.71
5.24
4.28
4.19
2.41
1.95
2.98
2.54
2.44
0.998 0.983
0.999 0.964
0.999 0.994
0.999 0.986
0.999 0.984
HF/3-21G
HF/6-31G(d,p)
B3LYP/6-31G(d,p)
JðH-1 ; H-2 Þ ¼ 9:75 Hz,
JðH-1 ; H-6 Þ ¼
T
ðaxÞ
ðaxÞ
ðaxÞ
ðaxÞ
B3LYP/6-311 þ G(d,p) 4.07
9
:75 Hz, JðH-1 ; H-6 Þ ¼ 4:5 Hz, J ¼ 24 Hz
ðaxÞ
the estimated value is 25 Hz). These values indicate
ðeqÞ
a
Including ZPE corrections.
(
that H-1 makes two axial coupling, which is
possible only in the case of conformation 2ee.
Similarly, the H-2 peak presents a value of 24.75 Hz
values were obtained by approximating the Gibbs
0
for J ; for an estimation of 25 Hz.
T
energy difference ðDG Þ by the enthalpy difference at
0
0
0
K ðDH Þ—that includes electronic, nuclear repul-
3.2. Theoretical analysis of the conformational
equilibrium
sion and zero-point energies—and using van’t Hoff
equation
0
DG ¼ 2RT ln K
Table 4 presents the geometrical parameters for 1
and 2, and Fig. 4 shows the optimized structures. In
where K ð¼ n =n Þ is the equilibrium constant, R
ax eq
21
21
Table 5 we have listed the relative energies and
calculated populations ðn Þ for each conformer. These
( ¼ 1.98722 cal K mol ) is the gas constant and T
(set as 298.15 K) is the absolute temperature.
For comparison, conformer populations were
determined at five different levels of theory. All of
them agree that 1ae and 2ee represent almost all the
compound population, even when small basis set are
used. These preferences may be explained in terms of
steric repulsion, as follows.
i
Fig. 5 depicts the repulsion between the
individual substituents and the hydrogens at 1 and
3
arrangements (syn-1,3-diaxial interactions). When
the substituents adopt equatorial positions, these
repulsions are avoided. Guided by this principle,
we can understand the conformational preference in
2
by noting that in the conformer 2aa, the two
substituents occupy axial positions, i.e. syn-1,3-
diaxial interactions predominate. On the contrary,
in 2ee, both substituents occupy equatorial
positions, and, consequently, steric repulsion is
minimized.
Compound 1 requires some more considerations.
In this case, both conformers possess an axial and an
equatorial group, and we are lead to ask why 1ea is
so unstable relative to 1ae. To answer this question,
we must consider the conformational equilibrium of
the individual substituents in the cyclohexane ring.
The experimental proportions of the equatorial
Fig. 4. Optimized structures for the conformers of compounds 1
and 2.

P.R. Oliveira et al. / Journal of Molecular Structure 657 (2003) 191–198
197
Table 6
Distances between the amine (or ammonium) nitrogen (Nam) and
˚
the ester (Oester) or carbonyl (Ocarb) oxygen (in A)
rðNam· · ·Oester
Þ
rðNam· · ·OcarbÞ
Acetylcholine
3.737
2.908
2.829
2.877
3.727
5.140
3.657
3.699
3.502
4.275
1
1
2
2
ae
ea
ee
aa
Fig. 5. Steric interactions between the studied substituents and the
hydrogens at 1 and 3 arrangements.
Computed at the B3LYP/6-311 þ G(d,p)
conformer in these equilibriums are 80% [10,20] and
9
compounds 1 and 2 with those obtained directly
from density functional calculation for ACh. In
Table 6 we list the distances so obtained after
geometry optimization for ACh at the same level of
theory used for 1 and 2. Fig. 6 depicts the optimized
structure for ACh.
3% [21] for –O(CyO)N(CH3)2 and –N(CH3)2,
respectively. From this, it is clear that the individual
behavior prevails when the two groups are combined.
Inspection of Fig. 5 clearly shows that the carbamate
group has a larger volume, but the 1,3-diaxial
interactions involve mainly the ‘ester’ oxygen,
since the other atoms are relatively distant from the
ring. On the other hand, the entire amine group is
close enough for the cyclohexane ring to cause a
greater repulsion.
According to Table 6, the predominant confor-
mers, i.e. 1ae and 1ea, have expressively different
atom distances relative to ACh, which, in principle,
suggests an absence of activity for these compounds.
Notice, however, the distances for the 2aa conformer.
In this case, rðN · · ·O Þ is very close to that in
am
ester
3
.3. Comparison with acetylcholine
ACh, which could be used, in principle, to classify this
conformer as a muscarinic agent. Nevertheless, 2aa is
practically absent in the conformational equilibrium
of 2 (the energy difference between 2ee and 2aa is
ACh is one of the benchmark molecules used for
assessing antycholinesterase activity. The interaction
of a drug with the enzyme active site depends
crucially on the distance between the amine nitrogen
and the carbamate oxygens. Based on such dis-
tances, the actions of ACh have been divided into
two categories, nicotinic and muscarinic [2], by
analogy with the effects of nicotine and muscarine.
Beers and Reich [2] have proposed that the specific
binding of nicotinic agents to their binding sites
depends on Coulombic interactions involving
the amine nitrogen, and on a hydrogen bond formed
2
1
2.44 kcal mol , see Table 5). The energy of binding
with an acceptor group disposed at approximately
˚
5
muscarinic agents, this distance is approximately
.9 A from the nitrogen ðrðN · · ·O ÞÞ: In
am
carb
˚
4
.4 A ðrðN · · ·O ÞÞ: Actually, the distances used
am
ester
by the authors were defined as the distance from the
nitrogen atom to a point on the van der Waals
surface of the electron donor oxygen. Space-fill
models were used for that study. For the present
investigation, we opt for comparing the distances in
Fig. 6. Structural representation (top) and optimized structure
(bottom) for acetylcholine.

198
P.R. Oliveira et al. / Journal of Molecular Structure 657 (2003) 191–198
of a series of AChE inhibitors was found to be
[6] M.F. Braga, E.G. Rowan, A.L. Harvey, W.C. Bowman, Br.
J. Anaesth. 70 (1993) 405.
2
1
essentially constant, viz. 4.2 kcal mol [22]; taking
this fact into account, we see that the binding of 2aa
with AChE could be able to displace the equilibrium
between 2ee and 2aa in the direction of the latter.
Finally, bond rotations or ring torsions could
[
[
7] J. Allinger, N.L. Allinger, Tetrahedron 2 (1958) 64.
8] M. Mousseron, J. Jullian, Y. Jolchine, Bull. Soc. Chem. Fr. 19
(
1952) 757.
[
9] C. Bernard, M.T. Maurette, A. Latters, Bull. Soc. Chem. Fr. 1/
2 (1976) 145.
recover the differences in distances (that are about
˚
[10] E.A. Basso, P.R. Oliveira, J. Caetano, I.T.A. Schuquel, J. Braz.
Chem. Soc. 12 (2001) 215.
0
.8 A for the nitrogen and ester oxygen) between ACh
[
11] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.
Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. 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. Petersson, P.Y. Ayala,
Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck,
K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz,
A.G. Baboul, 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, M. Challa-
combe, 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, GAUSSIAN 98 (Revision A.9), Gaussian, Inc., Pittsburgh
PA, 1998.
and the compounds under study. How effectively the
binding process can accomplish this, experiments will
show.
Acknowledgements
This research was supported by Funda c¸ a˜ o Arau-
c a´ ria (grant number: 325) and Funda c¸ a˜ o de Amparo a`
Pesquisa do Estado de S a˜ o Paulo (FAPESP; grant
number: 07692-5). CAPES (Coordena c¸ a˜ o de Aperfei-
c¸ oamento de Pessoal de N ´ı vel Superior) is acknowl-
edged for scholarships to P.R.O., F.W. and R.M.P.
The authors also thanks to CENAPAD-SP (Centro
Nacional de Processamento de Alto Desempenho de
S a˜ o Paulo) for the computer facilities.
[
[
[
[
12] A.D. Becke, J. Chem. Phys. 98 (1993) 1372.
13] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
14] A.P. Scott, L. Radon, J. Phys. Chem. 100 (1996) 16502.
15] J.R. Cheeseman, G.W. Trucks, T.A. Keith, M.J. Frish, J. Chem.
Phys. 104 (1996) 5497.
[
[
[
16] B.B. Wayland, R.S. Drago, H.F. Henneike, J. Am. Chem. Soc.
88 (1966) 2455.
References
17] C.F. Tormena, R. Rittner, R.J. Abraham, E.A. Basso, R.M.
Pontes, J. Chem. Soc. Perkin Trans. 2 (2000) 2054.
18] E.A. Basso, C. Kaiser, R. Rittner, J.B. Lambert, J. Org. Chem.
[1] S. Roman, N.M. Vivas, A. Badia, M.V. Clos, Neurosci. Lett.
325 (2002) 103.
5
8 (1993) 7865.
[
[
[
2] W.H. Beers, E. Reich, Nature 228 (1970) 917.
[
[
19] M. Karplus, J. Am. Chem. Soc. 85 (1963) 2870.
20] E.A. Basso, R.M. Pontes, J. Mol. Struct. (THEOCHEM) 594
(2002) 199.
3] W.C. Bowman, Anesth. Analg. 59 (1980) 935.
4] E. Narimatsu, Y. Nakayama, S. Sumita, H. Iwasaki, N.
Fujimura, K. Satoh, A. Namiki, Acta Anaesth. Scand. 43
(
1999) 196.
[21] H. Booth, M.L. Josefowicz, J. Chem. Soc. Perkin Trans. 2
(1976) 895.
[
5] J.B. Kay, J.B. Robinson, B. Cox, D. Polkonjak, J. Pharm.
Pharmacol. 22 (1970) 214.
[22] B. Belleau, J.L. Lavoie, Can. J. Biochem. 46 (1968) 1397.