Design, synthesis, and structure-activity relationship studies of novel 1-[(1-acyl-4-piperidyl)methyl]-1H-2-methylimidazo[4,5-c]pyridine derivatives as potent, orally active platelet-activating factor antagonists

Article abstract of DOI:10.1021/jm950555i

Replacement of the polar head of our previous series of 1-acyl-4-[(2-methyl-3-pyridyl)-cyanomethyl]piperazines with a 2-methylimidazo[4,5-c]pyridine group has led to the identification of a new series of 1-[(1-acyl-4-piperidyl)methyl]-1H-2-methylimidazo[4,5-c]pyridine derivatives as potent, orally active platelet-activating factor (PAF) antagonists. On the basis of the general structure - activity relationship trends found for the acyl substituent in our earlier series, five groups of compounds were tested, diaryl- or alkylarylpropanoyl derivatives, their 3-hydroxy-substituted analogues, and urea, carbamate and amino acid derivatives. The optimal compound 19 (UR-12670), bearing the 3,3-diphenylpropanoyl moiety, exhibited very high in vitro and in vivo potency (IC₅₀ = 0.0076 μM for the in vitro PAF-induced platelet aggregation assay, ID₅₀ = 0.0086 mg/kg for the in vivo PAF-induced hypotension test in normotensive rats, and ID₅₀ = 0.092 mg/kg po and 0.0008 mg/kg iv for the PAF-induced mortality test in mice). Compound 19 also showed long duration of activity. It gave 100% protection against PAF-induced mortality in mice 7 h after iv administration of a single dose of 1 mg/kg and also provided 100% inhibition of PAF-induced aggregation in dog whole blood 6 h after iv administration of the same dose. The lead structure 19 has been selected for in-depth pharmacological evaluation.

Full text of DOI:10.1021/jm950555i

J . Med. Chem. 1996, 39, 487-493
487
Design , Syn th esis, a n d Str u ctu r e-Activity Rela tion sh ip Stu d ies of Novel
1-[(1-Acyl-4-p ip er id yl)m eth yl]-1H-2-m eth ylim id a zo[4,5-c]p yr id in e Der iva tives a s
P oten t, Or a lly Active P la telet-Activa tin g F a ctor An ta gon ists
Elena Carceller,* Manuel Merlos, Marta Giral, Dolors Balsa, J ulia´n Garc´ıa-Rafanell, and J avier Forn
Research Center, J . Uriach & C´ıa. S.A., Dega` Bah´ı 59-67, 08026 Barcelona, Spain
Received J uly 25, 1995X
Replacement of the polar head of our previous series of 1-acyl-4-[(2-methyl-3-pyridyl)-
cyanomethyl]piperazines with a 2-methylimidazo[4,5-c]pyridine group has led to the identifica-
tion of a new series of 1-[(1-acyl-4-piperidyl)methyl]-1H-2-methylimidazo[4,5-c]pyridine de-
rivatives as potent, orally active platelet-activating factor (PAF) antagonists. On the basis of
the general structure-activity relationship trends found for the acyl substituent in our earlier
series, five groups of compounds were tested, diaryl- or alkylarylpropanoyl derivatives, their
3-hydroxy-substituted analogues, and urea, carbamate and amino acid derivatives. The optimal
compound 19 (UR-12670), bearing the 3,3-diphenylpropanoyl moiety, exhibited very high in
vitro and in vivo potency (IC50 ) 0.0076 µM for the in vitro PAF-induced platelet aggregation
assay, ID50 ) 0.0086 mg/kg for the in vivo PAF-induced hypotension test in normotensive rats,
and ID50 ) 0.092 mg/kg po and 0.0008 mg/kg iv for the PAF-induced mortality test in mice).
Compound 19 also showed long duration of activity. It gave 100% protection against PAF-
induced mortality in mice 7 h after iv administration of a single dose of 1 mg/kg and also
provided 100% inhibition of PAF-induced aggregation in dog whole blood 6 h after iv
administration of the same dose. The lead structure 19 has been selected for in-depth
pharmacological evaluation.
In tr od u ction
coordination centers of both molecules superimposed.
In order to test our hypothesis, compounds 4-7, with
the heterocyclic head linked directly to the piperidine
ring or through a methylene bridge, were prepared (see
Table 1). The activities of the longer compounds 4 and
5 were very close to those of their analogues 1-3, and
preliminary chemical stability studies indicated that the
new skeleton was free of hydrolysis problems. With this
result on hand, we proceeded to the fine optimization
of the acyl radical following the trends found in previous
series. The present report provides an account of the
synthesis and pharmacological evaluation of 1-[(1-acyl-
4-piperidyl)methyl]-1H-2-methylimidazo[4,5-c]pyri-
dine derivatives, from which a new lead, 19, has been
selected.
Earlier work in our laboratory led to the discovery of
(pyridylcyanomethyl)piperazines and -piperidines of
general formula I and II as a new class of potent
platelet-activating factor (PAF) antagonists and to the
selection of UR-12460 (1) and UR-12519 (2) for develop-
ment (see Table 1).1 These compounds, which have
demonstrated to be fairly stable as solids, were found
to suffer a relatively rapid hydrolysis of the (cyano-
methyl)amino group in aqueous solution. This fact
together with a short duration of action in vivo led us
to discontinue them and initiate a search for a surrogate
polar head within the same skeleton.
A structural feature of many PAF antagonists is the
presence of an sp2 nitrogen at a given distance from and
orientation to an amide or other isosteric groups.2 We
demonstrated that piperazinic derivatives I and their
carba analogues II featured essentially the same activity
in in vitro and iv in vivo tests (see 1 vs 3 in Table 1),
showing that the piperazinic nitrogen serves merely as
a framework.1 The presence of the cyano group, on the
other hand, was essential for activity, suggesting that
in addition to the requisite named above another feature
may be important in the interaction of this type of
compound with the PAF receptor, namely, a third
coordination center close to the pyridine sp2 nitrogen.
Inspection of the molecular model for the imidazo[4,5-
c]pyridine moiety, which is present in different PAF
antagonists,3 indicates that the two more distant nitro-
gens might also fulfill the latter requisite. Accordingly,
structures like III resulting from the replacement of the
(cyanomethyl)pyridyl moiety of II with 2-methylimidazo-
[4,5-c]pyridine appear to be interesting candidates. This
hypothesis is illustrated in Figure 1, which shows the
MM2-optimized modeled structures4 for (cyanomethyl)-
pyridine II (R ) CH3) and the proposed imidazopyridine
series III (R ) CH3) overlaid with the three indicated
F igu r e 1. Overlay of (cyanomethyl)pyridine II (R ) Me) and
imidazopyridine III (R ) Me).
X Abstract published in Advance ACS Abstracts, December 15, 1995.
0022-2623/96/1839-0487$12.00/0 © 1996 American Chemical Society

488 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 2
Carceller et al.
Ta ble 1. Comparison of (Cyanomethyl)pyridines with Imidazopyridines
a
b
Concentration required to inhibit PAF-induced maximum aggregation by 50%. Parentheses contain 95% confidence limits. Dose
required to reduce the lowering of the arterial blood pressure caused by PAF by 50%. Parentheses contain 95% confidence limits. c Dose
d
required to inhibit PAF-induced mortality by 50% (intravenous route). Dose required to inhibit PAF-induced mortality by 50% (oral
administration). e Compounds were purified by chromatography and triturated with ether. f Satisfactory elemental analyses ((0.4%) were
obtained for C, H, and N for all compounds except for the following products (compd, element, calcd (found value)): 7 N, 15.93 (15.34).
Sch em e 1a
a
(a) BOC2O, CHCl3, room temperature, 18 h; (b) 4-chloro-3-nitropyridine, Et3N, CH3Cl, reflux for 18 h, 64% (two steps); (c) H2, 10%
Pd/C, MeOH, 18 h; (d) Na2S2O4, pyridine/H2O, room temperature, 18 h; (e) ethyl acetimidate hydrochloride, EtOH, reflux, 18 h, 62% (two
steps); (f) 6.5 N HClg/dioxane, MeOH, room temperature, 18 h, 78%; (g) R1COOH, DCC, HOBT, DMF, room temperature, 18 h; (h)
Ph(CH3)2CCH2SO2Cl, Et3N, CHCl3, room temperature, 18 h; (i) R2COOH, (PhO)3, Et3N, benzene, 90 °C, 2 h and then 8 was added, 90 °C,
18 h; (j) R2NHCOOPh, pyr, 130 °C, 18 h; (k) Ph(C3H4)CH2OCOOPh, pyr, 130 °C, 18 h.
Ch em istr y
lowed by alkylation of the primary amine with 4-chloro-
3-nitropyridine5 gave nitropyridine 10. This compound
was reduced with Na2S2O4 in pyridine/H2O6 or alter-
natively hydrogenated over 10% Pd/C to diamine 11,
which was cyclized to the imidazopyridine 12 using
All the (piperidylmethyl)imidazopyridines were pre-
pared from a common intermediate, amine 8, which was
synthesized according to Scheme 1. BOC-protection of
the secondary amine of 4-(aminomethyl)piperidine fol-

2-Methylimidazo[4,5-c]pyridines as PAF Antagonists
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 2 489
ethyl acetimidate hydrochloride. Deprotection of the
secondary amine of 12 afforded amine 8.
26-30) except in the oral test, where only cyclopropyl
derivative 29 showed acceptable activity. It was inter-
esting to note that the enantiomer of 27, 28, was
significally less active. Taking into account that we
found activity in urea derivatives, we tried compounds
with the urea function as the linker between the
piperidine and the leucine ethyl ester, moiety present
in BB-882 (see compound 31), in comparison with ethyl
phenylglycines 33 and 34 and the racemic carba ana-
logue 32. Good results were obtained for 31, 33, and
its ether analogue 35 in the first three screening tests,
but only ether 35 showed modest activity in the oral
test. R-Amino amide 36, which had the advantage of
lacking chiral centers, showed diminished activity that
again increased in the presence of a 2-methoxy sub-
stituent in the aromatic ring (see compound 37); how-
ever, this compound proved orally inactive.
Amide derivatives 13-15, 17-25, 32, 36, and 37 were
obtained by reacting 8 with the corresponding carboxylic
acid (R1COOH) via DCC coupling in the presence of
HOBT. Ureas 26-29, 31, and 33-35 were obtained by
two methods: either by reaction of 8 with the isocyanate
generated through treatment of the corresponding R2-
COOH with diphenyl phosphorazidate or by coupling 8
with the appropiate phenyl carbamate (R2NHCOOPh)
in pyridine heating at reflux. Similar conditions were
used for the reaction of 8 with Ph(C3H4)CH2OCOPh to
give 30. Sulfonamide 16 was prepared by reaction of 9
with Ph(CH3)2CH2SO2Cl.7 The carboxylic acids are
commercially available or were prepared by known
methods.8 Compounds 6 and 7 were obtained from 1-(4-
piperidyl)-1H-2-methylimidazo[4,5-c]pyridine. This in-
termediate was synthesized as described for amine 8
starting from 1-(tert-butoxycarbonyl)-4-aminopiperidine.
To facilitate the final selection of a lead structure from
the most interesting compounds that have emerged from
this work (15, 19, 24, 25, 27, and 37), duration of
activity studies in two animal species were carried out
(see Table 3). All the new compounds showed better
results than our previous leader 1 showing that the
isosteric replacement of the (cyanomethyl)pyridine moi-
ety with 2-methylimidazo[4,5-c]pyridine produced a
substantial increase in the duration of activity. Com-
pound 19 showed the longest duration of activity. It
gave 100% protection against PAF-induced mortality in
mice 7 h after iv administration of a single dose of 1
mg/kg and also provided 100% inhibition of PAF-
induced aggregation in dog whole blood12 6 h after iv
administration of the same dose.
Resu lts a n d Discu ssion
Several different assays were used to evaluate the
potency of the PAF antagonists described here: the in
vitro PAF-induced platelet aggregation assay,9 the in
vivo PAF-induced hypotension test in normotensive
rats,10 and the PAF-induced mortality test in mice11 (po
and iv administration). Along with the standard refer-
ence compound WEB-2086, we used BB-882, also an
imidazopyridine derivative and one of the most ad-
vanced PAF antagonists presently in clinical trials. On
the basis of the general structure-activity relationship
(SAR) trends found for the acyl substituent in our
earlier series, five groups of compounds were tested,
diaryl- or alkylarylpropanoyl derivatives 13-22, their
3-hydroxy-substituted analogues 23-25, the urea and
carbamate derivatives 26-30, and amino acid deriva-
tives 31-37 (Table 2). Diphenyl- or alkylphenylpro-
panoyl derivatives 13-15 and 19 gave potent activities
in the three tests involving the in vitro assay and iv in
vivo tests, 3,3-diphenylpropanoyl derivative 19 being the
most active. As we found that unsubstituted 3-phenyl-
propanoyl derivative 17 showed a significant loss of
activity, we infer that branched substitution on the acyl
moiety seems to be important for activity. Introduction
of a methoxy group at the 2-position of the aromatic
ring, as in compound 18, also enhanced potency. Con-
sidering unsaturated derivatives 20 and 21, we found
that the ethyl derivative 21 was more favored and
comparable to its ether analogue 22. While few com-
pounds (13-15 and 21) showed modest activities in the
oral test, 3,3-diphenylpropanoyl derivative 19 was found
to show the highest potency comparable to reference
compound BB-882. Interestingly, replacing the amide
group in compound 15 with a sulfonamide moiety
(compound 16) was detrimental to oral activity.
In conclusion, we have found that replacement of the
(cyanomethyl)pyridine moiety present in our previous
series with 2-methylimidazo[4,5-c]pyridine results in
compounds of greater PAF antagonist activity. Among
the different types of acyl radicals tested, the 3,3-
diphenylpropanoyl substituent of 19 was found to confer
the highest potency and the longest duration of activity.
The lead structure 19 has been selected for in-depth
pharmacological evaluation.
Exp er im en ta l Section
Ch em istr y. Melting points were determined with a Mettler
FP 80 central processor melting-point apparatus and are
uncorrected. 1H NMR (80 MHz) and 13C NMR (20.1 MHz)
spectra were recorded on a Bru¨cker AC80 spectrometer, and
1H NMR (250 MHz) spectra were recorded on an AVANCE
DPX spectrometer and are reported in ppm on the δ scale from
the indicated reference. Mass spectra were measured on an
HP-5988 quadrupole mass spectrometer. Combustion analy-
ses were performed with a Carlo Erba 1106 analyzer. Liquid
chromatography was performed with a forced flow (flash
chromatography) of the indicated solvent system on SDS silica
gel chromagel 60 a C.C. (230-400 mesh). Unless otherwise
specified, all nonaqueous reactions were conducted under a
rigorously dried argon atmosphere, using oven-dried glass-
ware. C-18-PAF acether was synthesized from (S)-batyl
alcohol13 following a published procedure.14
1-(ter t-Bu toxycar bon yl)-4-(am in om eth yl)piper idin e (9).
To a cooled (0 °C) solution of 4-(aminomethyl)piperidine (150
g, 1.3 mol) in CHCl3 (800 mL) was added a solution of BOC2O
(147 g, 0.65 mol) in CHCl3 (500 mL), and the mixture was
stirred for 18 h. After adding H2O (400 mL), the organic phase
was separated, dried (Na2SO4), and evaporated. The resulting
material was used directly in the next step: 1H NMR (80 MHz,
CDCl3) δ (TMS) 4.11 (br d, J ) 13.4 Hz, 2H), 2.69 (m, 2H),
2.58 (d, J ) 5.5 Hz, 2H), 1.45 (s, 9H), 1.8-0.8 (m, 7H).
We next introduced a polar group with the ability to
establish hydrogen bonding in the 3,3-diaryl- or 3,3-
arylalkylpropanoyl chain, a strategy that had increased
bioavailability in previous series. Among compounds
23-25, which carry a hydroxy group on position 3, 24
and 25 were only slightly less active than 19 in the in
vitro and iv in vivo tests and compounds 23 and 25
proved to be 3 times more active than either 19 or BB-
882 in the oral test. Replacing the amide with urea or
carbamate was in general well tolerated (compounds

490 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 2
Ta ble 2. PAF Antagonist Activities of Compounds 13-37
Carceller et al.

2-Methylimidazo[4,5-c]pyridines as PAF Antagonists
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 2 491
Ta ble 2 (Continued)
a-d
See corresponding footnotes to Table 1. e A, EtOAc/hexane; B, EtOAc/MeOH; C, EtOAc/ether; D, EtOAc. All other compounds were
purified by chromatography and triturated with ether. f Satisfactory elemental analyses ((0.4%) were obtained for C, H, and N for all
compounds except for the following products (compd, element, calcd (found value)): 17 N, 14.90 (15.79); 31, N, 16.32 (15.64).
Ta ble 3. Duration of Activity of Compounds 15, 19, 24, 25, 27,
and 37a
1-[[1-(ter t-Bu toxyca r bon yl)-4-p ip er id yl]m eth yl]-1H-2-
m eth ylim id a zo[4,5-c]p yr id in e (12). A solution of crude
product 11 (0.24 mol) and ethyl acetimidate hydrochloride
(60.3 g, 0.488 mol) in EtOH (930 mL) was refluxed for 18 h.
More ethyl acetimidate hydrochloride (30.15 g, 0.24 mol) was
added, and the mixture was heated for an additional 18 h. The
solvent was removed in vacuo and the residue partitioned
between 2 N NaOH and EtOAc. The organic phase was dried
(Na2SO4) and concentrated to an oil that was purified by
chromatography on silica gel (CHCl3:MeOH, 4%) to give a
creamy solid (49 g, 62% for two steps): 1H NMR (80 MHz,
CDCl3) δ (TMS) 8.90 (s, 1H), 8.30 (d, J ) 5.5 Hz, 1H), 7.13 (d,
J ) 5.5 Hz, 1H), 4.10 (br d, J ) 13.4 Hz, 2H), 3.90 (d, J ) 7.3
Hz, 2H), 2.55 (s, 3H), 2.54 (br t, J ) 12.7 Hz, 2H), 1.46 (s,
9H), 2.2-1.0 (m, 5H).
PAF-induced
mortality in mice,
inh (%) at 1 mg/kg iv
PAF-induced platelet
aggregation,
inh (%) at 1 mg/kg iv
compd
15
19
24
3 h
5 h
7 h
4 h
6 h
88
100
80
50
100
60
100
100
59
100
100
43
100
25
90
70
27
37
1
56
90
15
27
22
70
100
100
39
100
100
77
87
30
55
100
WEB-2086
BB-882
31
56
55
100
95
1-(4-P ip er id ylm eth yl)-1H-2-m eth ylim id a zo[4,5-c]p yr i-
d in e (8). To a cooled solution (0 °C) of 12 (45.1 g, 0.136 mol)
in MeOH (470 mL) was added dropwise 6.2 N HCl(g)/dioxane
solution (93 mL). After the addition was completed, the cooling
bath was removed and the mixture was stirred at room
temperature for 18 h. It was then evaporated and the residue
partitioned between cooled 2 N NaOH solution and CHCl3. The
aqueous phase was reextracted twice with CHCl3, and the
combined organic extracts were dried (Na2SO4) and evaporated
to give a creamy solid (24.3 g, 78%): mp 128-129 °C; 1H NMR
(80 MHz, CDCl3) δ (TMS) 8.96 (s, 1H), 8.35 (d, J ) 5.5 Hz,
1H), 7.20 (d, J ) 5.5 Hz, 1H), 3.95 (d, J ) 7.3 Hz, 2H), 3.06
(br d, J ) 12.0 Hz, 2H), 2.61 (s, 3H), 2.51 (br t, J ) 12.7 Hz,
2H), 2.2-1.0 (m, 6H). Anal. (C13H18N4‚1/4H2O).
a
Percent protection of PAF-induced mortality in mice and
percent inhibition of PAF-induced platelet aggregation in dog
whole blood.
4-[[[1-(ter t-Bu toxycar bon yl)-4-piper idyl]m eth yl]am in o]-
3-n itr op yr id in e (10). To a mixture of POCl3 (120 mL) and
PCl5 (112 g) heated at 65 °C was added slowly 4-hydroxy-3-
nitropyridine (75 g, 0.535 mol). This mixture was stirred at
130 °C for 18 h. The solution was reduced in vacuo to an oil
that was redisolved in CHCl3 (700 mL). After cooling to 0 °C,
a solution of 9 (0.65 mol) and Et3N (110 mL) in CHCl3 (500
mL) was added and the mixture heated at reflux for 18 h. It
was then evaporated, and the residue was partitioned between
1 N NaOH and EtOAc. The aqueous phase was reextracted
twice with EtOAc, and the combined organic extracts were
dried (Na2SO4) and concentrated to a total volume of 400 mL.
After cooling (-20 °C) overnight, a yellow solid was collected
and dried (115 g, 64% for two steps): mp 131-138 °C;1H NMR
(80 MHz, CDCl3) δ (TMS) 9.20 (s, 1H), 8.30 (d, J ) 5.5 Hz,
1H), 8.19 (m, 1H), 6.72 (d, J ) 5.5 Hz, 1H), 4.18 (br d, J )
13.4 Hz, 2H), 3.26 (t, J ) 5.9 Hz, 2H), 2.72 (br t, J ) 12.7 Hz,
2H), 1.46 (s, 9H), 1.8-0.8 (m, 5H). Anal. (C16H24N4O4) C, H,
N.
1-[[1-[(2-Meth yl-2-p h en ylp r op yl)su lfon yl]-4-p ip er id yl]-
m eth yl]-1H-2-m eth ylim id a zo[4,5-c]p yr id in e (16). To a
solution of amine 8 (1 g, 4 mmol) and Et3N (0.6 mL) in CHCl3
(20 mL) was added 2-methyl-2-phenylpropanesulfonyl chlo-
ride7 (2.32 g, 10 mmol), and the mixture was stirred at room
temperature overnight. The resulting solution was diluted
with CHCl3, washed with 0.5 N NaOH, dried, and concen-
trated. The residue was purified by chromatography on silica
gel (CHCl3:MeOH, 5%) to afford a white solid which was
1
recrystallized from EtOAc (0.4 g, 25%): mp 165-166 °C; H
NMR (80 MHz, CDCl3) δ (TMS) 8.98 (s, 1H), 8.38 (d, J ) 5.5
Hz, 1H), 7.32 (m, 5H), 7.17 (d, J ) 5.5 Hz, 1H), 3.94 (d, J )
6.9 Hz, 2H), 3.65 (br d, J ) 12.0 Hz, 2H), 3.16 (s, 2H), 2.60 (s,
3H), 2.18 (br t, J ) 12.0 Hz, 2H), 1.58 (s, 6H), 1.48 (m, 5H).
Anal. (C23H30N4O2S) C, H, N.
1-[[1-(3,3-Dip h en ylp r op a n oyl)-4-p ip er id yl]m eth yl]-1H-
2-m eth ylim id a zo[4,5-c]p yr id in e (19). To a cooled solution
(0 °C) of 8 (19.9 g, 0.083 mol), 3,3-diphenylpropanoic acid (18.7
g, 0.083 mol), and HOBT (11.18 g, 0.083 mol) in DMF (300
mL) was added DCC (17.2 g, 0.083 mol). After the solution
was stirred at room temperature for 18 h, the solid material
was separated and the filtrate concentrated. The residue was
partitioned between NaHCO3 solution and CHCl3. The organic
phase was dried (Na2SO4) and concentrated to an oil that was
purified by chromatography on silica gel (CHCl3:MeOH, 4%)
to give a white solid (35 g). Recrystallization from EtOAc
afforded a white solid (23.4 g, 68%): mp 207-208 °C; 1H NMR
3-Am in o-4-[[[1-(ter t-bu toxycar bon yl)-4-piper idyl]m eth -
yl]a m in o]p yr id in e (11). A mixture of 10 (26.2 g, 0.077 mol)
and 10% Pd/C (3.83 g) in MeOH (500 mL) was hydrogenated
at atmospheric pressure for 18 h. The insoluble material was
removed by filtration and the filtrate evaporated to give 22.9
g (96%) of an oil: 1H NMR (80 MHz, CDCl3) δ (TMS) 7.99 (d,
J ) 5.5 Hz, 1H), 7.94 (s, 1H), 6.56 (d, J ) 5.5 Hz, 1H), 4.20 (br
d, J ) 13.4 Hz, 2H), 3.19 (m, 5H), 2.81 (br t, J ) 12.0 Hz, 2H),
1.58 (s, 9H), 2.1-1 (m, 5H).
Alternatively, 11 was prepared as follows. To a solution of
10 (82.2 g, 0.244 mol) in pyridine (400 mL) was added a
solution of Na2S2O4 (170 g, 0.976 mol) in H2O (500 mL). The
mixture was stirred at room temperature for 24 h and then
partitioned between EtOAc (800 mL) and 5 N NaOH (450 mL).
The organic phase was separated, dried (MgSO4), and evapo-
rated to give a yellow solid (101 g), which still contained
pyridine.

492 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 2
Carceller et al.
(250 MHz, CDCl3) δ (TMS) 8.90 (s, 1H), 8.45 (d, J ) 5.5 Hz,
1H), 7.23 (m, 11H), 4.67 (t, J ) 7.4 Hz, 1H), 4.65 (m, 1H), 3.85
(t, J ) 7.0 Hz, 2H), 3.83 (m, 1H), 3.07 (q of d, J ) 13, 8.1 Hz,
2H), 2.75 (br t, J ) 11.8 Hz, 1H), 2.62 (s, 3H), 2.37 (br t, J )
11.8 Hz, 1H), 1.98 (m, 1H), 1.58 (br d, J ) 11.8 Hz, 1H), 1.41
(br d, J ) 11.8 Hz, 1H), 1.08 (q of d, J ) 11, 0.4 Hz, 1H), 0.66
(q of d, J ) 11, 0.4 Hz, 1H); 13C NMR (20.15 MHz, CDCl3) δ
(TMS) 169.25, 153.17, 143.78, 143.37, 140.85, 140.71, 139.84,
138.97, 127.93, 127.58, 127.19, 125.87, 104.74, 48.62, 47.11,
44.90, 40.88, 38.00, 36.32, 29.66, 29.03, 13.37. Anal.
(C28H30N4O) C, H, N.
1-[[1-[[(1-P h en ylcyclop r op yl)a m in o]ca r bon yl]-4-p ip er -
id yl]m et h yl]-1H -2-m et h ylim id a zo[4,5-c]p yr id in e (29).
Diphenyl phosphorazidate (2.14 mL, 0.01 mol) was added
dropwise to a solution of 1-phenyl-1-cyclopropanecarboxylic
acid (1.6 g, 0.01 mol) and Et3N (1.14 mL) in benzene (40 mL).
The mixture was heated at 90 °C for 2 h. Amine 8 (1.6 g,
0.0068 mol) was then added, and the mixture was heated at
90 °C for 18 h more. After cooling the mixture was partitioned
between EtOAc and 1 N NaOH solution. The aqueous phase
was reextracted twice with EtOAc, and the combined organic
phases were dried (Na2SO4) and evaporated. The crude
product was purified by chromatography on silica gel (CHCl3:
MeOH, 10%) to give a white solid (1.2 g, 46%): mp 227-228
°C; 1H NMR (80 MHz, CDCl3) δ (TMS) 8.98 (s, 1H), 8.38 (d, J
) 5.5 Hz, 1H), 7.20 (m, 6H), 5.44 (s, 1H), 3.97 (d, J ) 7.3 Hz,
2H), 3.95 (m, 2H), 2.62 (s, 3H), 2.60 (br t, J ) 12.7 Hz, 2H),
2.2-1.0 (m, 5H), 1.22 (s, 4H). Anal (C23H27N5O‚1/4H2O) C, H,
N.
1-[[1-[[(1-P h en y-1-cyclop r op yl)m et h oxy]ca r b on yl]-4-
piper idyl]m eth yl]-1H-2-m eth ylim idazo[4,5-c]pyr idin e (30).
Amine 8 (0.72 g, 0.0031 mol) was added to a solution of phenyl
[(1-phenyl-1-cyclopropyl)methyl]carbonate (prepared from re-
action of (1-phenyl-1-cyclopropyl)methanol with phenyl chlo-
roformate; 1.1 g, 0.0041 mol) in pyridine (30 mL). The mixture
was refluxed for 18 h. After removal of the solvent in vacuo,
the residue was partitioned between CHCl3 and 1 N NaOH.
The aqueous phase was reextracted twice with CHCl3, and the
combined organic phases were dried (Na2SO4) and evaporated.
The crude product was purified by chromatography on silica
gel (CHCl3:MeOH:NH3, 60:2:0.2) to give a white solid (0.3 g,
25%): mp 138-140 °C; 1H NMR (80 MHz, CDCl3) δ (TMS)
8.97 (s, 1H), 8.37 (d, J ) 5.5 Hz, 1H), 7.27 (s, 5H), 7.19 (d, J
) 5.5 Hz, 1H), 4.16 (s, 2H), 4.15 (br d, J ) 13.6 Hz, 2H), 3.94
(d, J ) 7.21 Hz, 2H), 2.60 (s, 3H), 2.60 (m, 2H), 2.3-0.8 (m,
5H), 0.92 (s, 4H). Anal. (C24H28N4O2‚1/2H2O) C, H, N.
Biologica l Meth od s: In h ibition of P la telet Aggr ega -
tion in Vitr o. Platelet aggregation studies were done by the
method of Born.9 Blood was collected in 3.8% sodium citrate
(1 vol/9 vol of blood) by cardiac puncture from male New
Zealand rabbits (2-2.5 kg body weight). Platelet-rich plasma
(PRP) was prepared by centrifuging the blood at 250g for 10
min at 4 °C. The PRP was diluted with platelet-poor plasma
obtained by further centrifuging at 3000g for 10 min. The
platelet number was adjusted to 3.5 × 105 cells/mm3. Platelet
aggregation was induced by C-18-PAF (1.5 × 10-8 M) and
measured with a dual-channel aggregometer Chrono-log 560
instrument. Activity is expressed as the IC50 value, i.e., the
concentration required to inhibit platelet aggregatory response
by 50%. The values shown in the tables were calculated by
linear regression from a single experimental curve with no less
than four data points, each point being the mean of the
percentage inhibition at a given concentration obtained from
one to three independent experiments.
of test compound required to inhibit hypotension by 50%, or
as percentage inhibition at a given dose of test compound. The
ID50 values were calculated by linear regression from a single
experimental curve with no less than four points, each point
being the mean of the percentage inhibition at a given dose
obtained from two or more independent experiments.
In h ibition of P AF -In d u ced Mor ta lity in Mice.11 Groups
of 10 male Swiss mice weighing 22-26 g were used; 100 µg/
kg C-18-PAF plus 1 mg/kg propanolol was administered
through a lateral tail vein 5 min after iv or 60 min after po
administration of the test compounds (10 mL/kg iv or 20 mL/
kg po) or vehicle in control groups (saline iv or 1% Tween 80
po). The animals were observed 2 h after the PAF injection.
Following this protocol, we obtained a consistent mortality of
70-100% in the control group. Percentage inhibition of
mortality due to treatment in comparison with the control
group was calculated. Results are given as ID50 values, i.e.,
the dose required to inhibit PAF-induced mortality by 50%.
The results were calculated by linear regression from a single
experimental curve with at least four data points. For
duration of activity studies, test compounds were given at a
single dose of 1 mg/kg iv at time 0. C-18-PAF plus propanolol
was administered at different times (10-40 mice/time), and
percentage inhibition at each time was recorded.
In h ibition of P AF -In d u ced P la telet Aggr ega tion in
Dog Wh ole Blood . Blood was collected by venipuncture from
male Beagle dogs (8-12 kg body weight) before and 4 and 6 h
after the administration of a single dose of 1 mg/kg iv of the
test compounds; 3.8% sodium citrate (1 vol/9 vol of blood) was
used as the anticoagulant. Blood was further diluted 1:1 (v/
v) with saline. Platelet aggregation was induced by C-18-PAF
(7.5 × 10-9 M) and measured by an impedance method12 with
a dual-channel aggregometer Chrono-log 560 instrument.
Percentage inhibition of platelet aggregation at 4 and 6 h was
calculated by comparison with the platelet response in the
same animal before the administration of the test compound.
Results are the mean percentage inhibition obtained from one
to three dogs.
Sta tistics. Statistical analyses of pharmacological data
were done with a standard pharmacology program.15
Ack n ow led gm en t. This work was done with the
support of the Plan de Fomento de la Investigacio´n en
la Industria Farmace´utica, from the Ministerio de
Industria y Energ´ıa (Exp. 47/87). We thank J ordi
Belloc, Pere J osep J ime´nez, Nu´ria Recasens, J ordi
Salas, Consol Ferreri, Guadalupe Mart´ınez, Rosa Oliva,
Adoracio´n Rodriguez, and Alejandro Moliner for their
excellent technical assistance.
Refer en ces
(1) (a) Carceller, E.; Almansa, C.; Merlos, M.; Giral, M.; Bartrol´ı,
J .; Garc´ıa-Rafanell, J .; Forn, J . (Pyridylcyanomethyl)piperazines
as Orally Active PAF-Antagonists. J . Med. Chem. 1992, 35,
4118-4134. (b) Carceller, E.; Merlos, M.; Giral, M.; Almansa,
C.; Bartrol´ı, J .; Garc´ıa-Rafanell, J .; Forn, J . Synthesis and
Structure Activity Relationships of 1-Acyl-4-((2-methyl-3-pyr-
idyl)cyanomethyl)piperazines as PAF-Antagonists. J . Med. Chem.
1993, 36, 2984-2997.
(2) Hodgkin, E. E.; Miller, A.; Whittaker, M. A Partial Pharma-
cophore for the Platelet Activating Factor (PAF). Bioorg. Med.
Chem. Lett. 1992, 2, 597-602.
(3) Whittaker, M. PAF Receptor Antagonists: Recent Advances.
Curr. Opin. Ther. Pat. 1992, 2, 583-623.
(4) The MM2 minimizations were conducted on an Apple MacIntosh
workstation using the Chem-3D-Plus program (Cambridge Sci-
entific Computing, Inc.), and N-acetylpiperidine torsional pa-
rameters were taken from Schnur, D. M.; Yuh, Y. H.; Dalton,
D. R. A. Molecular Mechanics Study of Amide Conformations.
J . Org. Chem. 1989, 54, 3779-3785.
(5) Nielsen, O. B. T.; Nielsen, C. K.; Feit, P. W. Aminobenzoic Acid
Diuretics. 5. 3-Amino-4-arylmethyl-5-sulfamylbenzoic Acid De-
rivatives and Related compounds. J . Med. Chem. 1973, 16,
1170-1177.
(6) Houston, D. M.; Dolence, E. K.; Keller, B. T.; Patel-Thmbre, U.;
Borchardt, R. T. Potential Inhibitors of S-Adenosylmethionine-
Dependent Methyltransferases. 8. Molecular Dissections of
Carbocyclic 3-Deazaadenosine as Inhibitors of S-Adenosylho-
mocysteine Hydrolase. J . Med. Chem. 1985, 28, 467-471.
In h ibition of P AF -In d u ced Hyp oten sion in Nor m o-
ten sive Ra ts.10,1 Male Sprague-Dawley rats, weighing 180-
220 g, were anesthetized with sodium pentobarbital (50 mg/
kg ip). Blood pressure was recorded from the left carotid
artery using a Statham pressure transducer coupled to a
Beckman R611 recorder. Right and left femoral veins were
catheterized to inject the test compound and PAF (0.5 µg/kg).
Test compounds were administered by intravenous injection
(1 mL/kg, dissolved in saline) 3 min before PAF injection.
Blood pressure was monitored, and percentage inhibition of
PAF-induced hypotension with respect to controls was calcu-
lated. The results are expressed as ID50 values, i.e., the dose

2-Methylimidazo[4,5-c]pyridines as PAF Antagonists
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 2 493
(7) Quast, H.; Kees, F. Alkansulfonyl-chloride aus Alkyllithium und
sulfurylchloride. (Alkanesulfenyl chloride from Alkyllithium and
sulfuryl chloride.) Synthesis 1974, 7, 489-490.
(8) (a) Pecherer, B.; Sunbury, R.; Randall, L.; Brossi, A. Synthesis
and Biological Properties of 1-Dimethylamino-3-methyl-3-(3-
hydroxyphenyl)butane, a Potential Analgesic. J . Med. Chem.
1968, 11, 340-342. (b) Prout, F. S.; Hartman, R. J .; Huang, P.
Y.; Korpics, C. J .; Tichelaar, G. R. 3-Benzyl-3-methylpentanoic
Acid. Organic Syntheses; Wiley: New York, 1963; Collect. Vol.
IV, pp 93-98.
(9) Born, G. V. R. Aggregation of Blood Platelets by Adenosine
Diphosphate and its reversal. Nature (London) 1962, 194, 927-
929.
(10) Baranes, J .; Hellegouarch, A.; Le Hegarat, M.; Viossat, I.;
Auguet, M.; Chabrier, P.; Braquet, F.; Braquet, P. The Effects
of PAF-acether on the Cardiovascular System and their Inhibi-
tion by a New Highly Specific PAF-acether Receptor Antagonist
BN 52021. Pharmacol. Res. Commun. 1986, 18, 717-737.
(11) Carlson, R. P.; O'Neill-Davis, L.; Chang, J . Pharmacologic
modulation of PAF-induced mortality in mice. Agents Actions
1987, 21, 379-381.
(12) Cardinal, D. C.; Flower, R. J . The Electronic Aggregometer: A
Novel Device for Assessing Platelet Behavior in Blood. J .
Pharmacol. Methods 1980, 3, 135-158.
(13) Golding, B.; Ioannou, P. Rapid Synthesis of 3-O-Benzyl-sn-
glycerol and 2-O-Benzylglycerol. Synthesis 1977, 423-424.
(14) Wissner, A.; Kohler, C. A.; Golstein, B. M. Analogues of Platelet
Activating Factor 5. Multiple Oxygen Substitution of the Alkoxy
Chain. J . Med. Chem. 1986, 29, 1315-1319.
(15) Tallarida, R. J .; Murray, R. B. Procedure 8: Graded Dose-
Response. Manual of Pharmacologic Calculations; Springer-
Verlag: New York, 1981; pp 14-19.
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