Synthesis and μ-opioid receptor affinity of a new series of nitro substituted 3,8-diazabicyclo[3.2.1]octane derivatives

Article abstract of DOI:10.1016/S0014-827X(98)00065-2

A new series of analogues (1c-j; 2c-i) of the previously reported analgesic 3,8-diazabicyclo[3.2.1]octanes (1a,b; 2a,b) was synthesized and tested for their affinity towards μ-opioid receptors. Modifications were introduced either at the cinnamyl or the acyl side chains. The majority of the new compounds, with the exception of 1c,j and 2c, showed K(i) values better or comparable with those of the models.

Full text of DOI:10.1016/S0014-827X(98)00065-2

Il Farmaco 53 (1998) 557±562
Synthesis and m-opioid receptor af®nity of a new series of nitro substituted
3,8-diazabicyclo[3.2.1]octane derivatives
a
D. Barlocco , G. Cignarella , P. Vianello , S. Villa , G.A. Pinna , P. Fadda , W. Fratta
a,
a
a
b
c
c
*
aIstituto Chimica Farmaceutico e Tossicologico, Viale Abruzzi 42, 20131 Milan, Italy
bIstituto Chimica Farmaceutica, Via Muroni 21, 07100 Sassari, Italy
cDipartimento di Neuroscienze, Via Porcell 4, 09100 Cagliari, Italy
Received 4 July 1998; accepted 1 September 1998
Abstract
A new series of analogues (1c±j; 2c±i) of the previously reported analgesic 3,8-diazabicyclo[3.2.l]octanes (1a,b; 2a,b) was synthesized
and tested for their af®nity towards m-opioid receptors. Modi®cations were introduced either at the cinnamyl or the acyl side chains. The
majority of the new compounds, with the exception of 1c,j and 2c, showed Ki values better or comparable with those of the models. q 1998
Elsevier Science S.A. All rights reserved.
Keywords: Opioid receptors; Analgesic activity; Diazabicyclooctane
1. Introduction
1a,b and 2a,b for their af®nity towards m-opioid receptors.
In particular, the following modi®cations were considered:
The development of potent opioid agonist and antagonist
with high speci®city for each of the three major receptor
types (m, d, k) continues to be of great concern in opioid
pharmacology.
1. shift of the nitro group to different positions of the phenyl
ring (1c,d; 2c,d);
2. insertion of a chlorine atom adjacent the nitro group (1e±
g; 2e±g);
3. elongation of the acyl chain up to 5 atoms (1h±j; 2h,i).
In previous papers [1,2] we described the 3-(cinnamyl)-8-
propionyl-3,8-diazabicyclo[3.2.1]octane (1a) as a potent, m-
selective opioid agonist (Ki ˆ 55:2 nM) showing signi®cant
analgesic activity (ED50 ˆ 1:1 mg/kg). Inversion of the
substituent at the N3 and N8 gave a compound 2a still
provided with high m-af®nity and selectivity (Ki ˆ 160
nM), though with lower analgesic potency (ED50 ˆ 16:0
mg/kg). Insertion of a para-nitro group on the phenyl ring
of both 1a and 2a led to compounds 1b and 2b, respectively,
which, besides in vitro higher m/d selectivity with respect to
morphine, showed in vivo better analgesic properties and
minor side-effects [3]. It is interesting to note that contrary
to the unsubstituted 1a and 2a, inversion of 1b to give 2b,
increased both m af®nity and analgesic potency. A possible
explanation for the remarkable activity of these compounds
was based on the existence of an additional H-bond between
one of the nitro oxygens and a complementary site on the
receptor [4].
This paper describes the synthesis of compounds 1 and 2
as well as their binding af®nity towards m-receptors.
To further explore the role of the nitro group, we have
now synthesized and tested several analogues (1c±j; 2c±i) of
* Corresponding author. Fax: +39-02-29514197.
0014-827X/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved.
PII S0014-827X(98)00065-2

558
D. Barlocco et al. / Il Farmaco 53 (1998) 557±562
Scheme 1. (a) RCOCl/toluene/NEt3/D; (b) H2, 10% Pd±C/EtOH; (c) Ar±CHyCHCH2Cl (6)/K2CO3/acetone; (d) O[COOC(CH3)3]2, CHCl3; (e) (CH3)3CCOCl/
CH2Cl2/NEt3/D; (f) 1508C/3 h; (g) Et2O/H1.
2. Chemistry
so obtained 4 were then treated with equimolar amounts of
the required aralkenyl chloride (6) and potassium carbonate
in re¯uxing acetone for 6 h. The inorganic salts were ®ltered
off, the ®ltrate evaporated and the residue puri®ed by silica
gel chromatography to give the desired 1c±j. The 3-acyl-8-
aralkenyl isomers (2c±i; 8-DBO) were prepared in the same
way starting from the appropriate 3-acyl-3,8-diazabicy-
clo[3.2.1]octane (5), which could either directly be obtained
from the corresponding 4 by shifting the acyl chain to the 3-
position [7] (compounds 5a,b) or from the intermediate 3
The 3-aralkenyl-8-acyl derivatives (1c±j; 3-DBO) were
prepared starting from the known 3-benzyl-3,8-diazabicy-
clo[3.2.1]octane (3) [5], following a previously described
procedure [6], as shown in Scheme 1. Accordingly, a mixture
of 3 and equimolar amounts of the required acyl chloride and
triethylamine in toluene was re¯uxed for 2 h to give the
benzyl derivative of the corresponding 4, easily deprotected
by catalytic hydrogenation in the presence of 10% Pd±C. The

D. Barlocco et al. / Il Farmaco 53 (1998) 557±562
559
Table 1
Physicochemical properties of compounds 1c±j; 2c±i
Compound
R
R0
Et
% Yield
51
M.p. (8C)
Formula
1H NMR d (ppm)
1c
2-NO2
220±223a,b
C18H23N3O3
1.2 (t, 3H); 1.8±2.1 (m, 4H); 2.2±
2.5 (m, 4H); 2.7±2.9 (m, 2H);
3.1±3.3 (m, 2H); 4.1±4.2 (m,
1H); 4.6±4.8 (m, 1H); 6.1±6.3
(m, 1H); 7.0 (d, 1H); 7.3±7.5 (m,
1H); 7.5±7.7 (m, 2H); 7.9 (d, 1H)
1.1 (t, 3H); 2.0±2.4 (m, 6H); 3.0±
3.1 (m, 2H); 3.9±4.0 (m, 2H);
4.5±4.6 (m, 2H); 6.7±6.8 (m, 1H)
7.0 (d, 1H); 7.8 (t, 1H); 8.0 (d,
1H); 8.2 (dd, 1H); 8.4 (s, 1H)
1.1 (t, 3H); 1.5±2.0 (m, 2H); 2.9
(d, 1H); 3.8 (s, 2H); 4.2 (d, 1H);
7.1 (s, 1H); 7.3±7.8 (m, 4H)
1.2 (t, 3H); 1.9±2.6 (m, 8H); 2.7
(t, 3H); 3.1 (d, 1H); 4.1 (s, 1H);
4.6; (s, 1H); 6.5 (d, 1H); 7.4 (d,
2H); 7.9 (s, 1H);
1d
3-NO2
Et
45
168±170a
C18H23N3O3
1e
1f
2-NO2,5-Cl
3-NO2,4-Cl
Et
Et
20
45
206±208a
225±226a
C18H22ClN3O3
C18H22ClN3O3
1g
1h
2-Cl,5-NO2
Et
56
50
210±212a
C18H22ClN3O3
1.1 (t, 3H); 1.6±2.0 (m, 6H); 2.3±
2.4 (m, 2H); 2.9 (d, 1H); 3.2±3.5
(m, 4H); 4.2 (d, 1H); 6.4 (dt, 1H);
6.9 (d, 1H); 7.5 (d, 1H); 8.0 (dd,
1H); 8.4 (d, 1H)
4-NO2
n-prop
82±83
C19H25N3O3
1.0 (t, 3H); 1.1±2.4 (m, 10H);
2.6±2.7 (m, 2H); 3.1 (d, 2H);
4.0±4.1 (m, 1H); 4.5±4.6 (m,
1H); 6.3±6.7 (m, 2H); 7.3±7.4
(m, 2H); 8.1±8.2 (m, 2H)
1i
1j
2c
4-NO2
4-NO2
2-NO2
t-but
n-pent
Et
45
37
48
128±29
76±78
C20H27N303
C20H27N3O3
C18H23N3O3
1.3 (s, 9H); 1.7±1.8 (m, 4H); 2.3
(d, 2H); 2.8 (d, 2H); 3.2 (d, 2H);
4.6 (s, 2H); 6.3±6.4 (m, 1H); 6.6
(d, 1H); 7.5 (d, 2H); 8.1 (d, 2H)
1.2 (t, 3H); 1.2±1.3 (m, 14H);
2.6±2.7 (m, 2H); 3.1 (d, 2H); 4.1
(s, 1H); 4.6 (s, 1H); 6.5 (d, 1H);
7.4 (d, 2H); 7.9 (s, 1H)
220±221a
1.1 (t, 3H);1.8±2.6 (m, 6H); 3.6±
3.8 (m, 4H); 4.0±4.1 (m, 2H);
4.3±4.6 (m, 2H); 6.5±6.7 (m,
1H); 7.1±7.4 (m, 1H); 7.4±7.7
(m, 3H); 8.0 (d, 1H); 13.1 (br s,
1H)
2d
2e
3-NO2
Et
Et
38
20
170±175a
200±204a
C18H23N3O3
1.0 (t, 3H); 1.6±1.9 (m, 2H); 2.2±
2.3 (m, 4H); 3.4 (d, 1H); 4.1 (br s,
2H); 4.4 (d, 1H); 6.9±7.0 (m,
1H); 7.1 (d, 1H); 7.8 (t, 1H); 8.0
(d, 1H); 8.2 (dd, 1H); 8.4 (s, 1H)
1.1 (t, 3H); 1.6±1.8 (m, 6H); 2.9
(d, 1H); 3.1±3.5 (m, 4H); 4.2 (d,
1H) 6.3 (dt, 1H); 7.0 (d, 1H); 7.4
(dd, 1H); 7.6 (d, 1H); 7.9 (d, 1H)
(continued)
2-NO2,5-C1
C18H22ClN3O3

560
D. Barlocco et al. / Il Farmaco 53 (1998) 557±562
Table 1 (continued)
Compound
R
R0
Et
% Yield
35
M.p. (8C)
Formula
1H NMR d (ppm)
2f
3-NO2,4-Cl
210±11a
C18H22ClN3O3
1.1 (t, 3H); 1.6±1.7 (m, 2H); 1.8±
2.0 (m, 2H); 2.2±2.4 (m, 2H);
2.8±3.2 (m, 3H); 3.3±3.5 (m,
4H); 4.2 (d, 1H); 6.3±6.4 (m,
1H); 6.5 (d, 1H) 7.5 (s, 2H); 7.8
(s, 1H)
2g
2-C1,5-NO2
Et
56
180±184a
C18H22ClN3O3
1.5 (t, 3H); 1.6±2.0 (m, 6H); 2.3±
2.4 (m, 2H); 2.9 (d, 1H); 3.2±3.5
(m, 4H); 4.2 (d, 1H); 6.4 (dt, 1H);
6.9 (d, 1H); 7.5 (d, 1H); 8.0 (dd,
1H); 8.4 (d, 1H)
2h
2i
4-NO2
4-NO2
n-prop
t-But
45
40
94±96
C19H25N3O3
1.0 (t, 3H); 1.5±2.4 (m, 8H); 2.9±
3.4 (m, 7H); 4.1±4.2 (m, 1H);
6.4±6.7 (m, 2H); 7.5 (d, 2H); 8.2
(d, 2H)
142±43
C20H27N3O3
1.3 (s, 9H); 1.6±1.9 (m, 4H); 3.1±
3.2 (m, 6H); 4.0±4.1 (m, 2H);
6.5±6.6 (m, 2H); 7.3 (d, 2H); 8.2
(d, 2H)
a As the hydrochloride.
b See Ref. [1].
(compound 5c), according to standard procedures (see
Scheme 1). Finally, the required aralkenyl chlorides 6 were
synthesized according to reported methods [8,9].
mol) in acetone (20 ml) was re¯uxed for 6 h. The inorganic
salts were ®ltered off, the solvent evaporated and the residue
puri®ed by silica gel chromatography (eluent CH2Cl2/EtAc
1:1) to give the desired 1c±j. When stable, the correspond-
ing hydrochloride was prepared. See Table 1 for data.
3. Binding studies
4.3. 3-Acyl-8-aralkenyl-3,8-diazabicyclo[3.2.1]octanes
(2c±i): general method
Compounds 1c±j; 2c±i together with 1a,b and 2a,b were
submitted to binding studies on mouse brain homogenates in
the presence of [3H]DAMGO [(D-Ala2,N-Me-Phe4,Gly-
ol5)-enkephalin] as m-selective ligand. Morphine was used
as the reference compound.
Compounds were prepared as above reported for 1, start-
ing from the required 5. See Table 1 for data.
4.4. 8-Acyl-3,8-diazabicyclo[3.2.1]octanes (4a±d): general
method
4. Experimental
(a) A mixture of 3-benzyl-3,8-diazabicyclo[3.2.1]octane
(0.01 mol) (3) [5], TEA (0.01 mol) and the required acyl
chloride (0.01 mol) in toluene (20 ml) was re¯uxed for 2 h.
After cooling, the salts were ®ltered off, the solvent evapo-
rated and the residue puri®ed by silica gel chromatography,
eluting with cyclohexane/ethylacetate 1:1.
(b) A mixture of the appropriate above described 3-
benzyl derivative of 4 (0.01 mol), and 10% Pd±C (40 mg)
in ethanol (10 ml) was hydrogenated at room temperature.
The catalyst was ®ltered off and the solvent evaporated to
give the desired 4, which were used without further puri®-
cation.
4.1. Chemistry
Melting points were determined on a BuÈchi 510 capillary
melting points apparatus and are uncorrected. Analyses
indicated by the symbols were within ^ 0.4 of the theore-
1
tical values. H NMR spectra were recorded on a BruÈker
AC200 spectrometer; chemical shifts are reported as d
(ppm) relative to tetramethylsilane as internal standard.
CDCl3 was used as the solvent, unless otherwise noted.
TLC on silica gel plates was used to check product purity.
Silica gel 60 (Merck; 70±230 mesh) was used for column
chromatography. The structures of all compounds were
consistent with their analytical and spectroscopic data.
1. 4a: see Ref. [6].
1
2. 4b: yield 65%, H NMR d: 1.0 (t, 3H); 1.6±1.8 (m, 2H);
1.8±2.1 (m, 4H); 2.2±2.5 (m, 4H); 2.7±2.8 (m, 1H); 2.8±
3.0 (m, 1H); 4.0 (s, 1H); 4.7 (s, 1H).
4.2. 3-Aralkenyl-8-acyl-3-8-diazabicyclo[3.2.1]octanes
(1c±j): general method
1
3. 4c: yield 70%, m:p: ˆ 103±1048C, H NMR d: 1.3 (s,
A mixture of the appropriate 8-acyl derivative (4; 0.01
mol), the required halide (6; 0.01 mol) and K2CO3 (0.01
9H); 1.9±2.2 (m, 7H); 2.8±3.0 (m, 4H).
1
4. 4d: yield 65%, oil, H NMR d: 1.0 (t, 3H); 1.3±1.4 (m,

D. Barlocco et al. / Il Farmaco 53 (1998) 557±562
561
1
1. 6e yield 74%, H NMR d: 4.2 (d, 2H); 6.3±6.4 (m, 1H);
7.2 (d, 1H); 7.4 (dd, 1H); 7.6 (d, 1H); 8.0 (d, 1H).
6H); 1.5±1.6 (br s, 1H, exch with D2O); 1.9±2.0 (m, 4H);
2.2-2.3 (m, 4H); 2.8-2.9 (m, 2H); 4.1 (br s, 1H); 4.7 (br s,
1H).
1
2. 6f: yield 55%, H NMR d: 4.4 (d, 2H); 6.3-6.4 (m, 1H);
6.6 (d, 1H); 7.5 (s, 2H) 7.9 (s, 1H).
1
3. 6g: yield 73%, H NMR d: 4.3 (d, 2H); 6.4-6.5 (m, 1H);
7.1 (d, 1H); 7.5 (d, 1H); 8.1 (dd. 1H); 8.41 (d, 1H).
4.5. 3-Propionyl-3,8-diazabicyclo[3.2.1]octane (5a)
The compound was prepared by heating 4a at about 1508C
for 3 h, according to a previously reported method [7]. The
homologue 5b was obtained from 4b by the same procedure.
4.8. Binding studies
Male Sprague±Dawley rats (Charles River, Italy) weight-
ing 180±200 g were used. Rat brain membrane binding
studies were carried out as described by Gillan and Kosterlitz
with slight modi®cations [10]. Whole brain minus cerebel-
lum was homogenized with Polytron in 50 vols. (w/v) of 50
mM Tris±HCl (pH 7.7), centrifuged at 48 000 £ g for 20 min
at 48C, resuspended in 50 vols. of the same buffer and incu-
bated at 378C for 45 min. After centrifugation at 48 000 £ g
for 20 min at 48C, the ®nal pellet was resuspended in the same
buffer to a ®nal concentration of 0.8±1.0 mg protein/ml.
[3H]DAMGO (2 nM) (New England Nuclear, Germany)
was used to label m-receptors. Membrane suspensions were
incubated with the ligand at 08C for 60 min in the presence or
the absence of 10 m-molar naloxone. Final protein concen-
trations were determined by the method of Lowry et al. [11].
Ki values were cal-culated with the LIGAND program [12],
from displace-ment curves of each compound at a concentra-
tion range between 10210 M and 1024 M. Values are the mean
from two assays.
4.6. 3-t-Butyl-3,8-diazabicyclo[3.2.1]octane (5c)
(a) To a solution of 3 [5] (0.01 mol) in anhydrous chloro-
form (20 ml) di-tert-butyldicarbonate (3 g; 0.01 mol) in anhy-
drous chloroform (20 ml) was added dropwise under nitrogen
and the mixture stirred at room temperature overnight. After
evaporation of the solvent, the residue (7) was dissolved in
ethanol (40 ml) and hydrogenated at room temperature in the
presence of 10% Pd±C (0.04 g). The catalyst was ®ltered off,
the solvent evaporated and the thus obtained 8-Boc-diazabi-
cyclooctane (8) dissolved in toluene and treated with
trimethylacetyl chloride as reported for the corresponding
4. Finally, the protecting group was removed by treatment
with a solution of hydrochloric acid in diethyl ether. The
solution was then made alkaline by 20% NaOH, extracted
with diethyl ether (2 £ 25 ml), the solvent dried (Na2SO4) and
1
evaporated to give 5c. Yield 60%. H NMR d: 1.3 (s, 9H);
1.5±1.8 (m, 4H); 2.1±2.4 (m, 2H); 2.7 (d, 1H); 3.2 (d, 1H);
3.4±3.6 (m, 2H); 4.2 (d, 1H).
4.7. General method for the synthesis of the nitro, chloro-
substituted aralkenyl chloride (6e±g)
5. Results and discussion
As shown in Table 2, shifting the nitro group from the para
to meta position of the cinnamyl chain led to derivatives still
comparable with the models both in the normal 3-DBO (1d,
Ki ˆ 10 versus 33 nM for 1b) and in the reverted 8-DBO
series (2d, Ki ˆ 9:5 versus 5.1 nM for 2b). On the contrary,
the presence of an ortho-nitro group led to a relevant loss of
(a) To a suspension of NaH (1 g; 80% dispersion in
mineral oil) in toluene (15 ml), triethyl phosphonoacetate
(7.2 g; 0.03 mol) was added dropwise under a nitrogen
stream at such a rate to keep the temperature below 408C.
The mixture was stirred for 1 h at room temperature, added
of a solution of the required substituted benzaldehyde (0.03
mol) in anhydrous toluene (20 ml) and stirred for further 2 h.
Water (20 ml) was added under cooling and the aqueous
layer was extracted with CH2Cl2. After evaporation of the
solvent, the residue was puri®ed by silica gel chromatogra-
phy, eluting with CH2Cl2/MeOH 9:1.
Table 2
Inhibition constants of morphine and compounds 1a±j, 2a±i towards m-
opioid receptors
Compound
3H-DAMGOa
Compound
3H-DAMGOa
Ki (nM)
Ki (nM)
(b) To an ice-cooled solution of the above described ester
(0.015 mol) in anhydrous toluene (70 ml) a 1 M solution of
diisobutylaluminum hydride (DIBALH) in toluene (3.4 ml)
was added dropwise under a nitrogen stream. The mixture
was stirred for 1 h at 58C, cautiously added to a saturated
solution of K1/Na1 tartrate and then stirred at room
temperature for 18 h. The aqueous layer was repeatedly
extracted with diethyl ether and dried over sodium sulfate.
After evaporation of the solvent, the thus obtained alcohol
and freshly distilled thionyl chloride (30 ml) were re¯uxed
for 30 min. After usual work-up, the residue was puri®ed by
silica gel chromatography (eluent cyclohexane/ethylacetate
7:3).
1ab
1bb
1c
1d
1e
1f
55
33
2ab
160
5.1
325
9.5
112
14
2bb
2c
1750
10
2d
2e
30
48
2f
2g
1g
1h
1i
55
21
87
18
2h
2i
Morphine
22
13
1j
360
2.8
a Ki values were calculated with the LIGAND program [12], based on a kd
value of 1 nM for 3H-DAMGO. Values are the mean from two experiments.
b See Ref. [1].

562
D. Barlocco et al. / Il Farmaco 53 (1998) 557±562
[2] L. Toma, G. Cignarella, D. Barlocco, F. Ronchetti, Tetrahedron 48
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af®nity, though by different degree in the two series (1c,
Ki ˆ 1750; 2c, Ki ˆ 325). However, it should be noted
that insertion of a chlorine atom para to the nitro group of
1c gave a compound (1e) whose potency (Ki ˆ 30 nM) was
fully comparable to that of 1b. On the contrary, in the meta-
nitro derivatives the presence of a chlorine in different posi-
tions of the phenyl ring, either left the af®nity almost
unchanged (compound 2f, Ki ˆ 14 nM) or slightly decreased
(compounds 1f,g; 2g). Finally, modi®cation of the acyl
substituent was well tolerated in both series up to 4 carbon
atom chains, either linear or branched (compounds 1h,i; 2h).
Further elongation brought about a remarkable loss of activ-
ity (compound 1j, Ki ˆ 360 nM).
[3] P. Fadda, D. Barlocco, S. Tronci, G. Cignarella, W. Fratta, Nauyn±
Schmiedeberg's Arch. Pharmacol. 356 (1997) 596±602.
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