Conformation by NMR of two tetralin-based receptor ligands

Article abstract of DOI:10.1016/S0014-827X(03)00064-8

The conformation in solution of 1-phenyl-3-propionamido-1,2,3,4-tetrahydronaphthalene and 1-phenyl-3-(N,N-dimethylamino)-1,2,3,4-tetrahydronaphthalene has been determined by a combination of nuclear magnetic resonance measurements and molecular mechanics

Full text of DOI:10.1016/S0014-827X(03)00064-8

Il Farmaco 58 (2003) 469ꢀ476
/
www.elsevier.com/locate/farmac
Conformation by NMR of two tetralin-based receptor ligands
Giuseppe Gatti *, Giovanni Piersanti, Gilberto Spadoni
Istituto di Chimica Farmaceutica, Universit a` degli Studi di Urbino, Piazza Rinascimento 6, I-61029 Urbino, Italy
Received 28 November 2002; accepted 20 January 2003
Abstract
The conformation in solution of 1-phenyl-3-propionamido-1,2,3,4-tetrahydronaphthalene and 1-phenyl-3-(N,N-dimethylamino)-
,2,3,4-tetrahydronaphthalene has been determined by a combination of nuclear magnetic resonance measurements and molecular
1
mechanics calculations. The results indicate that in the cis isomers the cyclohexene ring is in a locked conformation and the trans
isomers correspond to a mixture of the two inverted half chairs. Moreover, the data allowed the identification of the two purposely-
synthesized geometrical isomers of 1-phenyl-3-propionamidotetralin. Binding studies on melatonin receptor subtypes showed that
the (9
subtype than the (9
#
/
)-cis-1-phenyl-3-propionamido-1,2,3,4-tetrahydronaphthalene has higher affinity and selectivity ratio toward the MT
)-trans-isomer.
2003 Editions scientifiques et m e´ dicales Elsevier SAS. All rights reserved.
2
/
´
Keywords: Conformational analysis; Tetralin synthesis; NMR spectroscopy; Molecular mechanics; Melatonin
1
. Introduction
tions can be used for describe the conformation of the
molecules in vacuo and in the solid state, respectively.
However, a spectroscopic method like nuclear magnetic
resonance can give information relative to the solution
state, which might be more appropriate for studies of
biological activity. Recently, the tetralin skeleton has
been successfully used as a rigid template for the
synthesis of non-indolic melatonin-like agents [2]. Mel-
atonin (N-acetyl-5-methoxytryptamine), a neurohor-
mone mainly secreted by the pineal gland during the
dark period, is an indole derivative with a flexible
ethylamido chain attached at the C3 position. In
mammals, melatonin modulates a variety of cellular,
neuroendocrine and physiological processes through
activation of at least two high-affinity G-protein
coupled receptors, named MT and MT [6,7]. Recent
A frequently encountered problem in drug design is to
decide which conformation the drug molecule adopts at
the binding site, i.e. the bioactive conformation. This is
particularly true in the case of simple flexible molecules,
such as arylethylamines, having a considerable rota-
tional freedom. A method commonly used for reduce
the flexibility has been the incorporation of the ethyla-
mine fragment into a bicycle system. For instance,
tetralins with appropriate substitution patterns have
been used as convenient scaffolds for the preparation of
molecules which act as ligands of pharmacological
receptors [1ꢀ5]. The conformational analysis of the
/
tetralin skeleton is a suitable starting point for the study
of the pharmacological activity of these receptor ligands.
The cyclohexene ring present in tetralin can in principle
adopt half-chair conformation, with the substituent in
position three being defined as pseudoequatorial or
pseudoaxial and the substituent in position one being
defined as equatorial or axial. Molecular mechanics
calculations and crystallographic structure determina-
1
2
studies have identified some compounds that are
selective for melatonin MT receptor binding [1,8,9]],
2
suggesting some differences in the structure, or flex-
ibility, of the MT2 receptor compared to the MT1
receptor. The most interesting compound, 1-phenyl-3-
propionamido-1,2,3,4-tetrahydro-naphthalene [1], be-
longs to the tetralin series; however, despite the fact
that it is a powerful and selective MT2 melatonin
receptor ligand, to our knowledge it has not been
reported for it any cis/trans characterization.
* Corresponding author.
E-mail address: g.gatti@uniurb.it (G. Gatti).
´
0
014-827X/03/$ - see front matter # 2003 Editions scientifiques et m e´ dicales Elsevier SAS. All rights reserved.
doi:10.1016/S0014-827X(03)00064-8

470
G. Gatti et al. / Il Farmaco 58 (2003) 469ꢀ476
/
This short communication reports on the NMR
FT-48 spectrometer. Elemental analyses for C, H and N
were performed on a Carlo Erba analyzer and are within
determination of the conformation of both cis and trans
diastereomers of 1-phenyl-3-propionamido-1,2,3,4-tet-
rahydro-naphthalene (11, 13) and of 1-phenyl-3-(N,N-
dimethylamino)-1,2,3,4-tetrahydronaphthalene (10, 12).
The conformational analysis has been carried out on the
racemic mixture of the enantiomeric pair of each
diastereoisomer, exploiting the property that the NMR
spectrum is invariant to reflection.
94% of the theoretical value.
/
3.2.1. 1,4-Diphenyl-1-buten-3-one (1)
Compound 1 was prepared as described in the
literature [15]. M.p. 69ꢀ
/
73 8C (lit. [10] 69ꢀ
/
73 8C); IR
(cm ) (CCl solution): 3061, 1720; MS (m/z): 222, 131;
ꢂ1
4
1
H NMR: 7.75ꢀ
/
7.25 (m, 10H), 6.82 (s, 1H), 6.78 (s, 1H),
An additional purpose of the present paper was the
synthesis, the identification of the individual cis and
trans isomers (11, 13) and the characterization of their
pharmacological profile. The latter information might
be used for future studies, aimed to characterize the
melatonin binding site.
3.97 (s, 2H).
3.2.2. 1-Phenyl-3-oxo-1,2,3,4-tetrahydronaphthalene (2)
Compound 2 was prepared as described previously by
-
1
Wyrick et al. [5]. Gum, MS (m/z): 222, 131; IR (cm )
1
(CCl solution): 3065, 1720; H NMR: 7.5ꢀ
/
7.0 (m, 9H),
4
4.5 (t, 1H), 3.65 (dd, 2H,), 2.78 (m, 2H).
2
. Chemistry
)-Cis and (9)-trans-1-phenyl-3-aminotetralins (8
and 9) were prepared following the synthetic pathways
3.2.3. (9
tetrahydronaphthalene (3)
Compound 3 was prepared as described previously
[5]. White solid, m.p. 110ꢀ112 8C (lit. [5] 112ꢀ115 8C);
MS (m/z): 224, 206; IR (cm ) (CCl solution) 3623,
/
)-cis-1-Phenyl-3-hydroxy-1,2,3,4-
(
9
/
/
/
/
ꢂ1
outlined in Scheme 1 and according to previously
4
1
described methods [5,10]. (9
phenyl-3-dimethylaminotetralins (10, 12) and (9
and (9)-trans-1-phenyl-3-propionamidotetralins (11,
3) were obtained by methylation of the corresponding
primary ammines (8, 9) with the EschweilerꢀClark
/
)-Cis and (9
/
)-trans-1-
3057, 2966, 1613; H NMR 7.4ꢀ7.0 (m, 8H), 6.77 (d,
/
/)-cis-
1H), 4.18 (m, 2H), 3.22 (dd, 1H), 2.93 (dd, 1H), 2.4 (m,
1H), 1.9 (m, 1H).
/
1
/
3.2.4. (9)-trans-1-Phenyl-3-hydroxy-1,2,3,4-
/
method [11], and by acylation with propionic anhydride,
respectively.
tetrahydronaphthalene (5)
Compound 5 was prepared as described previously
[
(
5]. White solid, m.p. 56ꢀ58 8C (lit. [5] as a gum); MS
/
ꢂ1
m/z): 224, 206; IR (cm ) (CCl solution) 3623, 3057,
4
1
3
. Experimental
2966, 1613; H NMR: 7.4ꢀ
t, 1H), 4.3 (m, 1H), 3.28 (dd, 1H), 2.87 (dd, 1H), 2.3 (m,
2H).
/
7.0 (m, 8H), 6.85 (d, 1H), 4.4
(
3
.1. Radioligand binding assays
The binding affinity of compounds 11 and 13 was
3.2.5. (9)-cis-1-Phenyl-3-amino-1,2,3,4-
tetrahydronaphthalene (8)
Compound 8 was prepared as previously described
/
1
25
determined using 2-[ I]-iodomelatonin (100 pM) as the
labelled ligand in competition experiments on cloned
human MT and MT receptor subtypes expressed in
[5]. M.p. 79ꢀ
/
81 8C (lit. [5] as a gum); MS (m/z): 206
1
2
ꢁ
ꢂ1
NIH3T3 rat fibroblast cells [12,13].
Their intrinsic activity was evaluated on [ S]-guano-
(M ꢂ17) 179; IR (cm ) (nujol): 3364, 3287, 3060,
1
/
3
5
2936, 2860, 1613; H NMR (CD COCD ): 7.4ꢀ
/
7.0 (m,
2.85
3
3
3
5
sine-5?-O-(3-thiotriphosphate), ([ S]GTPgS), binding in
NIH3T3 rat fibroblast cells, transfected with human
MT and MT receptors [13,14].
8H), 6.93 (d, 1H), 4.15 (m, 1H), 3.9 (bs, 2H), 3.10ꢀ
(m, 3H), 2.4 (m, 1H), 1.8 (m, 1H).
/
1
2
3
.2.6. (9
tetrahydronaphthalene (9)
Compound 9 was prepared as described previously
/
)-trans-1-Phenyl-3-amino-1,2,3,4-
3
.2. Chemistry
ꢂ1
Melting points were determined on a B u¨ chi SMP-510
capillary melting point apparatus and are uncorrected.
[5]. M.p. 57ꢀ
2936, 2860 1613; MS (m/z): 223 (M ), 206; H NMR
(CD COCD ): 7.35ꢀ6.9 (m, 9H), 4.35 (m, 1H), 3.10 (m,
1H,), 3.5 (bs, 2H), 2.57ꢀ2.25 (m, 2H), 2.15 (m, 2H).
/
59 8C; IR (cm ) (nujol) 3364, 3287, 3060,
ꢁ
1
1
H-NMR spectra were recorded on a Bruker AC 200
/
3
3
spectrometer using CDCl as solvent unless otherwise
3
/
noted. Chemical shifts (d scale) are reported in parts per
million (ppm), coupling constants (J values) are given in
hertz (Hz). EI MS spectra (70 eV) were taken on a
Fisons Trio 1000. Only molecular ions (M ) and base
peaks are given. Infrared spectra were taken on a Bruker
3.2.7. (9)-cis-1-Phenyl-3-(N,N-dimethylamino)-
/
1,2,3,4-tetrahydronaphthalene (10)
The procedure of Eschweiler and Clark [11] was used.
ꢁ
ꢂ1
M.p. 232ꢀ234 8C; IR (cm ) (nujol) 3090, 2926, 2880,
/

G. Gatti et al. / Il Farmaco 58 (2003) 469ꢀ
/476
471
Scheme 1. Reagents: (a) PhCHO, KOH, 18 h, 60 8C; (b) polyphosphoric acid, xilene, reflux; (c) NaBH4, MeOH; (d) diethyl azodicarboxylate,
P, PhCOOH; (e) NaOH; (f) TsCl, Py; (g) NaN3, DMF/H O; (h) H , 10% Pd/C; (i) HCHO, HCOOH; (j) (C CO) O Et N, toluene, 24 h.
(C
6
H
5
)
3
2
2
2
H
5
2
3
ꢁ
1
1
595, 1224; MS (m/z): 251 (M ) 178; H NMR 7.4ꢀ
/7.0
3.2.9. (9)-cis-1-Phenyl-3-propionamido-1,2,3,4-
/
(
2
m, 8H), 6.77 (d, 1H), 4.10 (dd, 1H), 3.15ꢀ2.85 (m, 3H),
.45 (s, 6H) 2.37 (m,1H), 1.76 (ddd,1H).
/
tetrahydronaphthalene (11)
Propionic anhydride (0.115 ml, 0.9 mmol) was added
to a cooled and stirred mixture of (9
aminotetralin (8) (200 mg, 0.9 mmol) and triethylamine
0.15 ml, 1.07 mmol) in toluene (10 ml) under argon.
/
)-cis-phenyl-3-
(
3
.2.8. (9
/
)-trans-1-Phenyl-3-(N,N-dimethylamino)-
The mixture was slowly warmed to room temperature
and stirred for 24 h. Triethylamine HCl was removed by
filtration and washed with toluene (10 ml). The filtrate
was concentrated under reduced pressure to give a solid
residue which was purified by flash cromatography
1
,2,3,4-tetrahydronaphthalene (12)
The procedure of Eschweiler and Clark [11] was used.
ꢁ
ꢂ1
M.p. 60ꢀ
/
62 8C; MS (m/z): 251 (M ) 178; IR (cm
)
7.0 (m,
1
(
nujol) 3090, 2926, 2880, 1595; H NMR 7.4ꢀ
/
8
1
H), 6.95 (d,1H), 4.38 (t, 1H), 3.06 (dd, 1H,), 2.87 (dd,
H), 2.65 (m,1H), 2.3 (s, 6H), 2.15 (m, 2H).
using cyclohexaneꢀethyl acetate 7/3 as eluent. Crystal-
/

472
G. Gatti et al. / Il Farmaco 58 (2003) 469ꢀ476
/
lization from acetone/hexane gave 88 mg (35%) of the
desired compound; m.p. 170ꢀ172 8C; MS (m/z): 279
M ) 206; IR (cm ) (nujol): 3289, 3089, 3024, 2938,
4. Results and discussion
/
ꢁ
ꢂ1
(
The affinities of (9
/
)-cis- and (9)-trans-1-phenyl-3-
/
1
1
670; H NMR 7.4ꢀ7.0 (m, 8H), 6.90 (d, 1H), 5.4
/
propionamidotetralin (11, 13) for both MT and MT
1
2
(
(
(
bd,1H), 4.39 (m,1H), 4.25 (dd, 1H), 3.22 (dd, 1H,), 2.80
dd, 1H), 2.42 (m,1H), 2.15 (q, 2H), 1.8 (ddd,1H), 1.15
t, 3H).
human melatonin receptors are shown in Table 1.
Concerning the structural aspects of tetralins 10ꢀ13,
the problem has been approached with the following
/
1
procedure. The H NMR spectra of the cyclohexene
moiety have been analyzed for connectivity by standard
decoupling technique. Then the first-order parameters,
chemical shifts and interproton coupling constants,
obtained by inspection, have been refined by simulation
of the spectrum and by fitting to the observed frequen-
cies and intensities. A representative example of the
quality of the fitting is reproduced in Fig. 1. The results
of the treatment are collected in Table 2. Here the
labelling a and b of the methylene resonances refers to
the position in the spectrum, meaning high and low
frequency, respectively. The interpretation of the spectra
in terms of configuration and conformation has then
been achieved with the following procedure. The inter-
proton vicinal coupling constants of the cyclohexene
ring have been correlated with the respective torsion
angles by using the empirical relationship proposed by
Altona and coworkers [16]. For this purpose the torsion
angles were estimated by modeling the molecules in
vacuo with the MM2 force field of Burkert and Allinger
3
tetrahydronaphthalene (13)
Compound 13 was prepared as described above
starting from trans-4-phenyl-2-aminotetralin (9). M.p.
.2.10. (9)-trans-1-Phenyl-3-propionamido-1,2,3,4-
/
ꢁ
ꢂ1
1
20ꢀ
/
122 8C; MS (m/z): 279 (M ), 206; IR (cm
)
7.0 (m,
1
(
nujol) 3289, 3024, 2938, 1686; H NMR 7.4ꢀ
/
8
1
3
H), 6.90 (d, 1H), 5.5 (bd, 1H), 4.38 (m, 1H), 4.20 (t,
H), 3.36 (dd, 1H), 2.75 (dd, 1H), 2.25 (m, 4H), 1.15 (t,
H).
3
.3. Spectroscopy
NMR spectra of samples 10ꢀ13 were measured at 600
/
MHz on a Bruker Avance spectrometer, at the probe-
head temperature. Samples were dissolved in 0.01 M
concentration in deuterated chloroform containing
TMS as internal reference. Spectra were processed off-
line by using the WINNMR Bruker software for personal
computers. Digital resolution of the experimental spec-
tra was 0.05 Hz. Simulations and iterations were
performed by using the WINDAISY Bruker software for
personal computers. R-factors better than 2% were
considered acceptable.
[
17], assuming that solvent effects could be neglected.
For the cis isomers the observed trend in the coupling
constants resulted very close to that expected for the half
chair conformation, with the phenyl substituent in
position 1-pseudoequatorial, and the nitrogen atom in
position 3-equatorial, as shown in Fig. 2. Slight adjust-
ments of some of the torsion angles predicted by the
MM2 model produced the coupling constants reported
in Table 3 (where the labeling a or b of the hydrogen
atoms refers to their cis or trans position, respectively,
relative to the nitrogen substituent). In addition, this
assignment allowed identifying the geometrical config-
Molecular mechanics calculations were performed
with the MM2 force field by using the HYPERCHEM
software for personal computers.
Table 1
Binding affinity and intrinsic activity of 11 and 13 for the human
a
MT
1
and MT
2
melatonin receptors
Human MT
uration of propionamidoꢀ
from the dimethylaminoꢀtetralin case, was not de-
scribed in the literature.
/
tetralin, which, differently
/
Compound
1
Human MT
pK`ı
2
b
pK`ı
IAr
IAr
Concerning the two trans isomers, the experimental
values of the interproton vicinal coupling constants of
the cyclohexene ring could be interpreted as arising from
a mixture of two half chair conformations I (with the
phenyl substituent being pseudoaxial and the NHCOEt
substituent equatorial) and II (with phenyl pseudoequa-
torial and NHCOEt axial), inverting rapidly on the
NMR time scale, as shown in Fig. 2. The average
coupling constants were best calculated with a molar
ratio I/II equal to 50/50 for the case of the
Melatonin
9.63
6.98
6.62
1
9.43
10.80
8.45
1
c
(
(
9
/
)-11
0.07
0.05
0.37
0.08
9)-13
/
a
i
pK values were calculated from IC50 values obtained from
competition curves by the method of Cheng and Prusoff [19] and are
the means of at least three independent determinations performed in
duplicate.
b
The relative intrinsic activity values (IAr) were obtained by
dividing the maximal analogue-induced G-protein activation by that
of melatonin.
c
125
propionamidoꢀ
dimethylaminoꢀ
In all cases the agreement with the experimental values,
/tetralin
and
75/25
for
the
pIC50 value. Compound 11 competed with 2-[ I]-iodomelatonin
binding in NIH3T3MT2 membranes with a slope of ꢂ0.58 (signifi-
cantly different from unity), thus not allowing K calculation.
/
tetralin and are reported in Table 3.
/
i

G. Gatti et al. / Il Farmaco 58 (2003) 469ꢀ
/476
473
3
Fig. 1. Observed (upper trace) and calculated (lower trace) spectrum of H of trans-1-phenyl-3-propionamidotetralin.
graphically summarized in Fig. 3, is of the order usually
expected for such kind of treatment.
protons. However, MM2 models show that in each
individual form I and II the two interproton distances
˚
are extremely different, 2.5 and 4.3 A, and this fact rules
As a check of the assumption of a conformational
equilibrium between forms I and II, it was measured the
out the presence of a single conformer, which indeed
should show cross peaks with different intensity. In-
stead, the 50/50 mixture hypothesis agrees with the
observed equal cross peaks, resulting from the averaging
of internuclear distances. An additional information
contained in the ROESY spectrum is the lack of any
2
D-ROESY spectrum of the trans-1-phenyl-3-propio-
namido-1,2,3,4-tetrahydronaphthalene. The cross peak
observed between protons 4b and 2a and the cross peak
observed between 4a and 2b, having the same intensity,
must be accounted for by a proximity of the involved
Fig. 2. Cyclohexene ring conformation of tetralins (only one enantiomer shown).

474
G. Gatti et al. / Il Farmaco 58 (2003) 469ꢀ476
/
dipolar coupling between protons in position 1 and 4:
this observation shows that the high energy boat form is
not detectable by NMR in solution.
cyclohexene ring appears to be in a locked conforma-
tion, but not in the trans case, which resulted as a
mixture of ring conformations. Moreover, the composi-
tion of the mixture resulted to depend on the nature of
the ring substituent.
In order to reduce the rate of conformational inter-
conversion and thus ascertain the presence of the
conformational equilibrium, proton spectra were mea-
sured at low temperatures (consistently with the freezing
point of the solvent). Indeed, a considerable broadening
of the signals was observed, however, it appears that the
energy barrier to interconversion is not enough to reach
the coalescence temperature.
Concerning the conformation across the CHꢀ
/NH
bond in the propionamidoꢀtetralins, the same predo-
/
minant anti orientation of the two hydrogen atoms was
observed for both cis and trans isomers, showing that
this local conformation is not related to the cyclohexene
ring arrangement.
Moreover, an evidence of the conformation across the
Finally, the above synthesis and analysis allowed, for
the first time, the unambiguous identification of the
geometrical isomers of 1-phenyl-3-propionamidotetra-
lin. Moreover, the observed higher binding affinity for
CHꢀ
/
NH bond in the propionamidoꢀtetralin could be
/
obtained from the measured value of the corresponding
coupling constant. In the approximation of three rota-
tional isomers, one with anti and two with gauche
arrangement of the involved protons, this parameter is
the molar average of the coupling constants of the
MT receptor of the cis isomer relative to the trans can
2
Table 2
1
H NMR spectrum parameters of tetralins
individual rotamers, Jꢃ
/xJanti
ꢁ
/
(1ꢂ
/
x)Jgauche, where x
is the molar fraction of the anti form and Janti and
Jgauche are the coupling constants of the corresponding
rotamers. The latter parameters were estimated by a
Karplus type relationship [18] and this allowed to
evaluate the molar fraction x. The result is 75% of the
anti form and 25% of the gauche forms.
Finally, concerning the binding to melatonin recep-
tors, the conformation of the cis-isomer of 1-phenyl-3-
propionamido-1,2,3,4-tetrahydro-naphthalene appears
to be more suitable for the interaction with the MT2
Chemical shifts Coupling constants (Hz)
ppm from inter-
nal TMS
(9)-cis-1-Phenyl-3-propionamido-tetralin
/
d (NH)ꢀ
/
J (1,2a)ꢀ
/
5.739
/
0.001
J (3,4a)ꢀ
/
5.2190.001
/
5
.4134
d (3)ꢀ
d (1)ꢀ
/
4.3799
J (1,2b)ꢀ
J (2a,2b)ꢀ
.001
3.2278 J (2a,3)ꢀ
2.7891 J (2b,3)ꢀ
2.4213 J (2a,4a)ꢀ
1.7850 J (NH,3)ꢀ
/
11.569
/
0.002 J (3,4b)ꢀ
J (4a,4b)ꢀ
0.001
/
10.899
/
0.001
/4.2475
/
ꢂ
/
12.469
/
/
ꢂ
/
15.799
/
0
d (4a)ꢀ
d (4b)ꢀ
d (2a)ꢀ
d (2b)ꢀ
/
/
3.199
11.559
2.1390.001
8.0490.002
/
0.001
/
/
/0.002
receptor. Indeed, the two racemates (9
/
)-cis-11 and (9)-
/
/
/
/
trans-13 showed comparable MT binding affinity, but
1
/
/
/
the cis isomer has a considerably higher MT affinity
2
(
9
/
)-trans-1-Phenyl-3-propionamido-tetralin
d (NH)ꢀ J (1,2a)ꢀ6.1990.001
5.3963
d (3)ꢀ
d (1)ꢀ
leading to a better MT /MT selectivity ratio than the
1
2
/
/
/
J (3,4a)ꢀ
/
5.0790.001
/
trans form.
/
4.3287
J (1,2b)ꢀ
J (2a,2b)ꢀ
.001
3.2614 J (2a,3)ꢀ
2.6424 J (2b,3)ꢀ
2.1960 J (NH,3)ꢀ
1.9670
/
7.649
/
0.001
J (3,4b)ꢀ
J (4a,4b)ꢀ
0.001
/
6.379
/
0.001
/
4.1358
/
ꢂ/
13.129
/
/
ꢂ
/
16.609
/
0
5
. Conclusion
In summary we have shown that a complete analysis
d (4a)ꢀ
d (4b)ꢀ
d (2a)ꢀ
d (2b)ꢀ
/
/
7.929
2.979
7.799
/
0.001
0.001
0.003
/
/
/
/
/
/
/
of the NMR spectrum of tetralins, combined with force
field calculations, describes the conformation in solution
of the 1,3-disubstituted tetralins. It results that the cis
isomer corresponds to the cyclohexene ring conforma-
tion in which the phenyl group in position one is
pseudoequatorial and the amido or the dimethylamino
group in three is equatorial. However, the trans isomer
is a mixture of the two conformers I and II with the
inverted half chair cyclohexene ring. The preference of
the dimethylamino group for the equatorial orientation
appears to be larger than that of the propionamido
group, and this observation could be ascribed to a
(9
/
)-cis-1-Phenyl-3-(N,N-dimethylamino)tetralin
d (1)ꢀ4.0357 J (1,2a)ꢀ5.4590.001 J (3,4a)ꢀ
d (4a)ꢀ3.0195 J (1,2b)ꢀ12.3290.002 J (3,4b)ꢀ
d (4b)ꢀ2.9202 J (2a,3)ꢀ2.8190.002 J (4a,4b)ꢀ
.002
/
/
/
/
4.889
11.789
15.679
/
0.003
/
/
/
/
/0.003
/
/
/
/
ꢁ/
/
0
d (3)ꢀ
d (2a)ꢀ
/
2.8709
2.3227 J (2a,2b)ꢀ
.001
1.7060 J (2a,4a)ꢀ
J (2b,3)ꢀ
/
11.7890.001
/
/
/
ꢂ
/
12.479
/
0
d (2b)ꢀ
/
/
2.179
/
0.002
(9
/
)-trans-1-Phenyl-3-(N,N-dimethylamino)tetralin
d (1)ꢀ4.3847 J (1,2a)ꢀ4.7790.001 J (3,4a)ꢀ
d (4a)ꢀ3.0423 J (1,2b)ꢀ5.8590.001 J (3,4b)ꢀ
d (4b)ꢀ2.8762 J (2a,2b)ꢀ 12.909 J (4a,4b)ꢀ
.001 0.001
J (2a,3)ꢀ
2.1577 J (2b,3)ꢀ
2.1308
/
/
/
/
4.959
9.629
16.259
/
0.001
/
/
/
/
/0.001
/
/
ꢂ/
/
/
ꢂ/
/
strong steric 1ꢀ3 interaction of the dimethylamino
/
0
group with the pseudoaxial H in form II.
d (3)ꢀ
d (2a)ꢀ
d (2b)ꢀ
/
2.6843
/
2.909
10.209
/
0.001
0.001
1
/
/
/
The data show that the reduction in flexibility
attainable through the use of a bicycle substructure is
easily achieved in the case of cis isomer, where the
/

G. Gatti et al. / Il Farmaco 58 (2003) 469ꢀ
/476
475
Table 3
Calculated and observed vicinal coupling constants of cyclohexene ring in tetralins
Hydrogen atoms
Torsion angle (8)
J coupling constant
Calculated Observed
J
calc
ꢂ
/
J
obs
(
1
1
2
2
3
3
9
/
)-cis 1-Phenyl-3-propionamido-tetralin, 1ceqꢀ
/
3eq form
12
5.3
ꢀ
ꢀ/
/
2a
169
47
11.6
5.7
ꢁ0.4
/
2b
ꢂ
/
0.4
0.2
0.6
0.7
0.2
aꢀ
bꢀ
/
3
3
175
68
11.8
2.6
11.6
3.2
ꢁ
/
/
ꢂ
/
ꢀ
ꢀ/
/
4a
175
51
11.6
5.0
10.9
5.2
ꢁ
/
4b
ꢂ
/
(
1
1
2
2
3
3
9
/
)-cis 1-Phenyl-3-(N,N-dimethylamino)tetralin, 1ceqꢀ3eq form
/
ꢀ
ꢀ/
/
2a
172
49
12.2
4.9
12.3
5.5
ꢂ
/
0.1
2b
ꢂ
/
0.6
aꢀ
bꢀ
/
3
3
177
64
11.8
3.0
11.8
2.8
0
/
ꢂ
/
0.2
0.6
0.4
ꢀ
ꢀ/
/
4a
175
55
11.2
5.3
11.8
4.9
ꢂ
/
4b
ꢁ/
Hydrogen atoms
Torsion angle (8)
J coupling constant
J ꢂ/J
calc obs
Calculated
Observed
I
II
I
II
Mixture
(
1
1
2
2
3
3
9)-trans 1-Phenyl-3-(N,N-dimethylamino)tetralin
/
ꢀ
ꢀ/
aꢀ
bꢀ
ꢀ/
ꢀ/
/
2a
48
62
50
167
56
4.9
2.6
4.8
4.9
5.0
9.9
3.0
9.2
5.4
4.8
5.8
ꢁ0.1
/
2b
11.9
4.0
2.6
3.0
3.8
ꢂ
/
0.8
0.3
0.1
0.5
0.4
/
3
3
178
64
11.8
3.1
10.2
3.0
ꢂ
/
/
60
63
ꢂ
/
4a
163
45
10.8
5.9
9.7
5.0
ꢂ
/
4b
50
ꢁ
/
(
1
1
2
2
3
3
9)-trans 1-Phenyl-3-propionamido-tetralin
/
ꢀ
ꢀ/
aꢀ
bꢀ
ꢀ/
/
2a
45
67
50
167
56
5.4
2.1
4.8
11.9
4.0
5.2
7.0
7.9
2.6
6.9
4.3
6.2
7.6
7.9
3.0
6.4
5.1
ꢂ
/
1.0
2b
ꢂ
/
0.6
/
3
3
175
68
11.8
2.6
0
/
60
67
2.6
2.5
ꢂ
/
0.4
0.5
0.8
4a
169
52
11.3
4.8
ꢁ
/
ꢀ/4b
50
3.8
ꢂ/
Fig. 3. Calculated versus observed vicinal proton-proton coupling constants of cyclohexene ring.

476
G. Gatti et al. / Il Farmaco 58 (2003) 469ꢀ476
/
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provide further insights for the characterization of the
melatonin binding sites.
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[
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Acknowledgements
We thank Dr. Marazzi of Biosearch, Milan, for the
measurements of the NMR spectra, Dr. V. Lucini and
Dr. R. Nonno of University of Milan, for binding
studies. This work has been supported by a grant from
the University of Urbino.
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