Synthesis and Structure-Activity Relationship of Di-(3,8-diazabicyclo[3.2.1]octane) Diquaternary Ammonium Salts as Unique Analgesics

Article abstract of DOI:10.1002/ardp.200300749

Based on the structure characteristics of the lead compounds, 1,1′2-octanedioyl-4,4′-dimethyl-4,4′-dibenzyl dipiperazinium dibromide (2) and 3,8-disubstituted-3,8-diazabicyclo[3.2.1]octanes (DBO), di-(3,8-diazabicyclo[3.2.1]octane) diquaternary ammonium salts 3 a-c were designed and synthesized through a highly practical procedure. Target compounds 3 a-c and the hydrochloride salts of their precursors 10 a-c were evaluated for their in vivo analgesic and sedative activities. Interestingly, the introduction of an endoethylenic bridge in the piperazine of lead compound 2 causes loss of the analgesic activity and increases the toxicity dramatically. This result shows that the flexible conformation of piperazine in compound 2 is favorable for interaction with the receptor, and the quaternization of compounds 10 a-c is the main reason for the toxicity increase.

Full text of DOI:10.1002/ardp.200300749

510 Liu et al.
Arch. Pharm. Pharm. Med. Chem. 2003, 336, 510–513
Hong Liu,
Synthesis and Structure-Activity Relationship of
Di-(3,8-diazabicyclo[3.2.1]octane) Diquaternary
Ammonium Salts as Unique Analgesics
Tie-Ming Cheng,
Hong-Mei Zhang,
Run-Tao Li
School of Pharmaceutical
Sciences,
Peking University, Beijing,
P. R . C h i n a
Based on the structure characteristics of the lead compounds, 1,1Ј-octanedioyl-
4,4Ј-dimethyl-4,4Ј-dibenzyl dipiperazinium dibromide (2) and 3,8-disubstituted-3,8-
diazabicyclo[3.2.1]octanes (DBO), di-(3,8-diazabicyclo[3.2.1]octane) diquaternary
ammonium salts 3 a–c were designed and synthesized through a highly practical
procedure.Target compounds 3 a–c and the hydrochloride salts of their precursors
10 a–c were evaluated for their in vivo analgesic and sedative activities.Interesting-
ly, the introduction of an endoethylenic bridge in the piperazine of lead compound 2
causes loss of the analgesic activity and increases the toxicity dramatically.This re-
sult shows that the flexible conformation of piperazine in compound 2 is favorable for
interaction with the receptor, and the quaternization of compounds 10 a–c is the
main reason for the toxicity increase.
Keywords: Di-(3,8-diazabicyclo[3.2.1]octane) diquaternary ammonium salts;
Analgesic activities; Structure-activity relationship
Received: November 25, 2002; Accepted: May 9, 2003 [FP749]
DOI 10.1002/ardp.200300749
3,8-diazabicyclo[3.2.1]octane (DBO) is the flexibility-
limited piperazine with an endoethylenic bridge. On the
Introduction
The development of new analgesics with high efficiency
and safety profiles still remains a challenge in pharma-
ceutical research [1]. In our previous papers, several
classes of piperazinium salts with analgesic activity
were reported [2–4].Among them, the class 1 shows the
remarkable analgesic activity and compound 2 is the
most potent although the exact mechanisms of their bio-
activities are still unclear.
basis of the structure characteristics of compounds 2
and DBO, a new kind of flexibility-limited diquaternary di-
piperazinium salts, namely di-(3,8-diazabicyclo[3.2.1]-
octane) diquaternary ammonium salts 3a-c were de-
signed and synthesized.We expect to explore the struc-
ture-activity relationships further and hope to find more
potent analgesic compounds.
Recently, Cignarella et al. reported that 3,8-disubstitut-
ed-3,8-diazabicyclo[3.2.1]octanes had high affinity and
selectivity for the δ-opioid receptors with potent analge-
sic activity [5]. From the structural point of view,
Chemistry
As shown in Scheme 1, target compounds 3a-c were
prepared in a highly practical fashion. Diethyl meso-2,5-
dibromo-hexyldicarboxylate 4 was obtained from hexan-
dioic acid through a one-step acylation, bromination,
and esterification.According to the literature method [6],
the product 4 was a mixture of meso and racemic iso-
Correspondence: Run-Tao Li, School of Pharmaceutical
Sciences, Peking University, Beijing, 100083, P. R. China.
Phone: +86 10 82801504, Fax: +86 10 6234-6154, e-mail:
lirt@mail.bjmu.edu.cn.
© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arch. Pharm. Pharm. Med. Chem. 2003, 336, 510–513 Di-(3,8-diazabicyclo[3.2.1]octane) Diquaternary Ammonium Salts as Analgesics 511
Scheme 1. Synthesis of target compounds 3 a–c.
mers, however, only the meso isomer could be cy-
clized with a primary amine in the next step [7]. Fortu-
nately, we found that most of the racemic isomers of 4
could be transformed into the meso isomer of 4 with
92 % yield through a few cycles of crystallization and
filtration.
Pharmacological results and discussion
The preliminary analgesic activities and the toxicity of
compounds 3 a–c and the hydrochloride salts of
10 a–c were tested by our reported method [3]. The
results are listed in Table 1.
As shown in Table 1, the introduction of the endoethyl-
enic bridge in the piperazine of lead compound 2
causes the loss of analgesic activity and increases the
toxicity dramatically. This result indicates that the flexi-
ble conformation of piperazine in compound 2 is favo-
rable to increase the analgesic activity. Comparing the
toxicity of compounds 3a-c and 10a-c clearly demon-
strates that the quaternization of compounds 10 a–c is
the main reason for the toxicity increase.
The most important steps in the synthetic route were
the cyclizations leading to 5 and 8. The cis-diethyl N-
benzyl pyrrolidine 2,5-dicarboxylate 5 was constructed
according to Blackman’s method [7]. The intermediate
8 was prepared from 5 according to the literature de-
scribed for the preparation of the 8-methyl analogue of
8 [8]. Reduction of 8 with LiAlH4 gave 8-benzyl-3,8-
diazabicyclo[3.2.1]octane 9. The reaction of two equiv-
alents of 9 with one equivalent of the corresponding
diacyl chloride in the presence of triethylamine yielded
the key intermediates 10 a–c. Then, 10 a–c were
quaternized with methyl bromide in acetone at 0 °C to
give the target compounds 3 a–c.
Acknowledgment
This research was supported by a fund of the National
Science Foundation of China (NSFC 29972005).
© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

512 Liu et al.
Arch. Pharm. Pharm. Med. Chem. 2003, 336, 510–513
cis-1-Benzylpyrrolidine-2,5-dicarboxylic acid diethyl ester 5
Table 1. The analgesic activities of compounds 3 a–c and
10 a–c × 2 HCl.
To a solution of 4 (10 g, 0.028 mol) in toluene (50 mL) benzyl
amine (10 mL, 0.0916 mol) was dropped, and then refluxed for
24 h.The mixture was cooled to room temperature and diluted
with ether. The benzyl ammonium bromide was filtered and
washed with ether.The filtrate was concentrated, and the resi-
due was purified by column chromatography (Petrolum ether/
ether 3:1) to provide 5.8 g of 5 as a yellow oil (65 %), 5 · HCl,
mp. 123–125 °C (lit. 123–125 °C [7]).
Compd.
n
Dose
(mg/kg sc)
Analgesic
activity (%)
Rate of death
(%)
2[2]
3 a
100
20
0
80.3
100
0
cis-N-Benzylpyrrolidine-2,5-dicarboxylic acid 6
2
4
6
20
10
1
–
–
–0.5
100
100
0
A mixture of 5 (2 g, 8 mmol) and concentrated HCl (20 mL) was
refluxed for 2 h. After standing overnight at room temperature,
the precipitate was collected by filtration and recrystallized
from water to give compound 6 as white needle crystal (1.2 g,
74 %), mp: 254–258 °C.
3 b
3 c
20
10
1
–
–
2.5
100
50
0
cis-N-Benzylpyrrolidine 2-benzyl amide 5-carboxylic acid 7
6 (2 g, 9.1 mmol) was added in acetic anhydride (30 mL) and
heated at 100 °C until the solution was clear. After removal of
acetic anhydride under vacuum, the residue was dissolved in
benzene (30 mL), and then NH3 · H2O (37 %, 0.64 mL) was add-
ed drop-wise.The resulting mixture was stirred at room temper-
ature for 2 h.The solvent was removed, and the residue was co-
evaporated twice with anhydrous EtOH to yield a yellow solid
compound 7 (1.7 g, 78 %) that was used in the next step without
further purification.
20
10
1
–
–
–0.8
100
50
0
10 a
10 b
10 c
2
4
6
20
20
20
–2.9
–3.6
–5.0
0
0
0
8-Benzyl-3,8-diazabicyclo[3.2.1]octane-2,4-dione 8
A solution of 7 (1.66 g, 6.3 mmol) in acetic anhydride (30 mL)
was heated at 100 °C for 0.5 h. Acetic anhydride was evaporat-
ed under vacuum, and the residue was purified by column chro-
matography (Petrolum ether/acetone 6:1) to give 680 mg of
compound 8 as a white solid (47.2 %). 1H NMR (CDCl3): 1.94
(dd, J = 8.4 and 4.2 Hz, 2 H), 2.34–2.43 (m, 2 H), 3.75–3.78 (m,
2 H, H-1, H-5), 3.81 (s, 2 H, CH2Ph), 7.26–7.38 (m, 5 H, ArH),
7.68 (br, 1 H, NH); 13C NMR (75 MHz, CDCl3): 26.68, 53.43,
63.01 (CH2Ph), 128.64, 128.74, 127.82, 136.64, 173.44 (2 ×
CO); Anal. C13H14O2N2 (C, H, N).
Experimental
Chemistry
Melting points were determined on X4 microscope and are un-
corrected. 1H NMR and 13C NMR spectra were recorded on a
VXR300 (300 MHz) instrument (Varian, Palo Alto, CA, USA) in
CDCl3 or DMSO-d6. Elemental analyses were performed on a
PE-2400 Analyzer (PE Company, USA).Thin layer chromatog-
raphy was carried out on pre-coated GF254 silica gel plates
(Qingdao Haiyang Chemical Co. Ltd., Qingdao, China). MS
were carried out on a LDI1700 instrument (Linear Scientific Co.
Ltd. USA).
8-Benzyl-3,8-diazabicyclo[3.2.1]octane 9
To a mixture of LiAlH4 (2.4 g, 70.6 mmol) in anhydrous THF
(50 mL) the solution of 8 (3.6 g, 15.9 mmol) in anhydrous THF
(30 mL) was added.The resulting mixture was stirred under re-
flux for 5 hr.Water was dropped until the H2 production stopped.
The reaction suspension was filtered and washed with THF.
The solvent was removed from the filtrate, and the residue was
purified by column chromatography (CHCl3/MeOH 6:1) to give
9 (2.6 g, 82.3 %) as colorless syrup. 1H NMR (CDCl3) 2.23 (m,
4 H, H6, H7), 3.07–3.10 (m, 2 H), 3.24–3.27 (m, 4 H), 3.54 (s,
2 H, CH2Ph), 7.26–7.35 (m, 5 H, ArH).
meso-2,5-Dibromohexanedioic acid diethyl ester 4
The mixture of hexanedioic acid (219 g, 1.5 mol) and sulfinyl
chloride (330 ml) was heated at 50–60 °C for 3 h, then excess
sulfinyl chloride was evaporated under vacuum.To the residue
bromine (174 mL, 3.4 mol) was added drop-wise at 80–90 °C
for 5 h and stirred for further 2 h to yield 2,5-dibromo-hexanedi-
oyl dichloride. The 2,5-dibromohexanedioyl dichloride was
added drop-wise to the anhydrous ethanol while stirring and
cooling in an ice bath. The mixture sat overnight at room tem-
perature and was poured into ice water (2 L).The organic layer
was separated, and the water phase was extracted with CHCl3
(3 × 500 mL).The combined organic phases were washed with
2 % NaHSO3, 3 % Na2CO3, and water successively and dried
over anhydrous K2CO3.The solution was concentrated to give a
red oil product that was a mixture of (meso)- and (racemic)-2,5-
dibromo-hexanedioic acid diethyl ester.This oil mixture was left
at room temperature until crystals formed.The crystals were fil-
tered and washed with MeOH to give meso-2, 5-dibromohex-
anedioic acid diethyl ester 4.The filtrate was further concentrat-
ed and left to rest again to yield more product 4. After a few cy-
cles of crystallization, a total of 459 g product 4 was obtained in
92 % yield, mp 65–68 °C (lit. mp 65–66 °C [7]).
α,ω-Di(8-benzyl-3,8-diazabicyclo[3.2.1]octane-3)-α,ω-dione 10
(general method)
A mixture of α,ω-dicarboxylic acid (1 mmol) and sulfonyl chlo-
ride (5 mL) was stirred at 50 °C until the emission of hydrogen
chloride ceased. Excess of sulfonyl chloride was removed un-
der vacuum, and the residue was dissolved in dichloromethane
(5 mL). This solution was dropped into the mixture of 9
(2.2 mmol) and triethylamine (3.2 mmol, 0.47 mL) in dichlo-
romethane (5 mL) under stirring in an ice bath. The reaction
mixture was stirred at room temperature for 1h, and then dilut-
ed with dichloromethane (10 mL). The organic phase was
washed with saturated NaHCO3, water, and saturated NaCl so-
lution successively. After drying over anhydrous MgSO4, and
© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arch. Pharm. Pharm. Med. Chem. 2003, 336, 510–513 Di-(3,8-diazabicyclo[3.2.1]octane) Diquaternary Ammonium Salts as Analgesics 513
3 c
removal of the solvent, the crude products were purified by col-
umn chromatography (CHCl3/CH3COCH3 4:1) to give com-
pound 10.
65.8 %; white solid; mp 166–168 °C; 1H NMR (DMSO-d6)
1.01–1.03 (m, 5 H), 1.31–1.52 (m, 7 H), 1.79–2.27 (m, 8 H),
2.49 (m, 3 H), 2.77 (br, 3 H), 3.08 (br, 2 H), 3.69–4.51 (m, 11 H),
4.51 (br, 1 H), 4.94 (br, 2 H), 7.51–7.54 (m, 10 H, ArH);13C NMR
(75 MHz, DMSO-d6) 23.99, 24.24, 24.35, 24.44, 25.51, 28.67,
32.34, 42.81, 43.14, 45.47, 45.76, 46.27, 57.13, 62.01, 63.23,
65.44, 65.94, 66.52, 66.87, 127.83, 129.07, 129.19, 129.36,
129.97, 130.40, 132.13, 133.01, 173.34; Anal. C36H52O2N4Br2 ·
3.2 H2O (C, H, N).
10 a
72.7 %; colorless syrup; 1H NMR (CDCl3) 1.53–1.55 (m, 4 H),
2.08 (br, 4 H), 2.44–2.64 (m, 4 H), 2.83 (d, 2 H, J = 12.3 Hz, H-
4a, H-4aЈ), 3.11 (br, 4 H), 3.25–3.29 (m, 2 H), 3.44 (s, 4 H, 2 ×
CH2Ph), 3.44–3.48 (m, 2 H), 4.05 (d, 2 H, J = 12.3 Hz, H-4e, H-
4eЈ), 7.18–7.31 (m, 10 H, ArH); Anal. C30H38O2N4 (C, H, N).
Analgesic activity
10 b
Acetic acid writhing test was used on outbred kuming mice
(Department of Animals, Peking University, China). Groups of
three mice (body weight 20–30 g) of both sexes, were given a
dose of a test compound.Thirty minutes later the animals were
injected intraperitoneally with 0.20 mL/mouse of 0.70 % acetic
acid solution.The number of writhing was recorded for 5 min af-
ter the injection of acetic acid. Each compound was tested in
2 ~ 3 groups, and 3 ~ 4 groups are treated as control one in
every batch.The percentage of inhibition was calculated as fol-
lows:
62.6 %; colorless syrup; 1H NMR (CDCl3): 1.49–1.57 (m, 8 H),
1.89–1.93 (m, 4 H), 2.20–2.34 (m, 4 H), 2.80 (d, 2 H, J = 12.3
Hz, H-4a, H-4aЈ), 3.11 (br, 4 H), 3.22–3.36 (m, 4 H), 3.44 (s, 4 H,
2 × CH2Ph), 4.06 (d, 2 H, J = 12.6 Hz, H-4e, H-4eЈ), 7.15–7.31
(m, 10 H, ArH);13C NMR (75 MHz, CDCl3):24.81, 24.99, 25.10,
33.16, 48.26, 51.85, 56.81, 58.04, 58.26, 126.91, 128.16,
128.51, 138.89, 172.89 (2 × CO); Anal. C32H42O2N4 (C, H, N).
10 c
78.4 %; colorless syrup; 1H NMR (CDCl3) 1.26 (br, 4 H), 1.48–
1.54 (m, 8 H), 1.89–1.92 (m, 4 H), 2.08–2.27 (m, 4 H), 2.80 (d,
2 H, J = 12.3 Hz, H-4a, H-4aЈ), 3.10–3.22 (m, 4 H), 3.25–3.39
(m, 4 H), 3.44 (s, 4 H, 2 × CH2Ph), 4.07 (d, 2 H, J = 12.3 Hz, H-
4e, H-4eЈ), 7.15–7.31 (m, 10 H, ArH); 13C NMR (75 MHz,
CDCl3) 24.99, 25.11, 29.17, 33.35, 48.25, 51.92, 56.84, 58.09,
58.31, 126.94, 128.18, 128.54, 138.92, 173.25; Anal.
C34H46O2N4 · 0.8 H2O (C, H, N).
% inhibition = 100 – (A/B × 100)
where A = incidence of writhing in the treated group and B = in-
cidence of writhing in the control group, occurring from the 5th to
10th min after administration of the noxious agents.
α,ω-Di(8-benzyl-8-methyl-3,8-diazabicyclo[3.2.1]octane-
3)-α,ω-dione dibromide 3 (general method)
To a solution of 10 (0.2 mmol) in acetone (2 mL) a ca.30 % solu-
tion of methyl bromide in acetone (5 mL) was added at 0 °C.The
mixture was allowed to stand below 0 °C for several weeks until
TLC indicated most of the starting material was consumed.The
solvent was removed, and the residue was recrystallized with
acetone/methanol to give compounds 3.
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3 a
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50.4 % yield; white solid; mp 192–194 °C; 1H-NMR (DMSO-d6):
1.76 (m, 2 H), 2.07–2.27 (m, 11 H), 2.78 (br, 5 H), 3.10 (s, 2 H),
3.72–4.24 (m, 11 H), 4.52 (br, 1 H), 4.96 (br, 2 H), 7.52–7.55 (m,
10 H, ArH); 13C NMR (75 MHz, DMSO-d6): 23.93, 24.34, 27.53,
42.79, 43.18, 45.38, 45.69, 46.36, 57.19, 63.31, 65.62, 65.95,
66.65, 66.94, 127.78, 129.08, 129.21, 129.32, 129.98, 130.42,
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3 b
89.1 %; white solid; mp 172–174 °C; 1H NMR (DMSO-d6) 1.45
(m, 4 H), 1.55–2.78 (m, 8 H), 2.49–2.56 (m, 3 H), 2.79 (br, 3 H),
3.10–3.15 (m, 3 H), 3.26–3.33 (m, 3 H), 3.71–4.26 (m, 11 H),
4.56 (br, 1 H), 4.99 (br, 2 H), 7.51–7.54 (m, 10 H, ArH);13C NMR
(75 MHz, DMSO-d6) 23.99, 24.12, 24.37, 24.46, 30.72, 32.28,
42.85, 43.15, 45.49, 45.75, 46.22, 48.57, 57.02, 63.12, 65.38,
65.94, 66.49, 66.79, 127.87, 129.03, 129.15, 129.42, 129.91,
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© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim