Replacing the cyclohexene-linker of FR181157 leading to novel IP receptor agonists: Orally active prostacyclin mimetics. Part 6

Article abstract of DOI:10.1016/j.bmcl.2006.06.076

The synthesis and biological activity of novel derivatives of our previously reported IP receptor agonist FR181157 is described. SAR studies to replace the cyclohexene-linker of FR181157 led to the discovery of compound 1i (FR207845) as a potent non-prost

Full text of DOI:10.1016/j.bmcl.2006.06.076

Bioorganic & Medicinal Chemistry Letters 16 (2006) 4861–4864
Replacing the cyclohexene-linker of FR181157 leading to novel IP
receptor agonists: Orally active prostacyclin mimetics. Part 6
Akira Tanaka,a,* Kouji Hattori,a Kiyoshi Taniguchi,a Osamu Okitsu,a Seiichiro Tabuchi,a
Mie Nishio,b Yasunori Nagakura,b Noriaki Maeda,b Hidetsugu Muraic and Jiro Sekid
aChemistry Research Laboratories, Astellas Pharma Inc., 2-1-6 Kashima, Yodogawa-ku, Osaka 532-8514, Japan
bPharmacology Research Laboratories, Astellas Pharma Inc., 2-1-6 Kashima, Yodogawa-ku, Osaka 532-8514, Japan
cAnalysis and Pharmacokinetics Research Laboratories, Astellas Pharma Inc., 2-1-6 Kashima,
Yodogawa-ku, Osaka 532-8514, Japan
dDrug Safety Research Laboratories, Astellas Pharma Inc., 2-1-6 Kashima, Yodogawa-ku, Osaka 532-8514, Japan
Received 10 May 2006; revised 15 June 2006; accepted 17 June 2006
Available online 11 July 2006
Abstract—The synthesis and biological activity of novel derivatives of our previously reported IP receptor agonist FR181157 is
described. SAR studies to replace the cyclohexene-linker of FR181157 led to the discovery of compound 1i (FR207845) as a potent
non-prostanoid PGI2 mimetic with good oral bioavailability.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Prostacyclin (PGI2) primarily derived from vascular
endothelium is one of the metabolites of arachidonic
acid and has an important role as an inhibitor of platelet
aggregation and as a potent vasodilator.1 Although
these pharmacological properties are considered to be
clinically useful, the therapeutic application of PGI2 is
limited due to its inherent instability.1,2 Since the report
that octimibate, known as an inhibitor of acyl-CoA:cho-
lesterol O-acyltransferase (ACAT), acts as a non-prosta-
noid IP receptor agonist appeared in 1990,3 intensive
Figure 1. Conversion of the cyclic linker of FR181157.
studies to identify new non-prostanoid PGI2 mimetics
with chemical and metabolic stability have been per-
formed.4–7 These investigations led us to a research pro-
gram directed at the development of a new class of
prostacyclin analogues, and we have investigated and
reported several novel series of non-prostanoid PGI2
mimetic.8 From these series, FR181157 which has a
cyclohexene core structure with a chiral center was iden-
tified as a potent orally active non-prostanoid IP recep-
tor agonist (Fig. 1).8b However, a metabolism study
revealed the existence of epoxides as active metabolites
exhibiting 5- to 10-fold more potent human platelet
aggregation inhibitory activity than the parent com-
pound, and in a rat hepatic injury model FR181157
showed unpredictable potent activity derived from its
complicated metabolism.8e Although the stability and
toxicity of epoxides of FR181157 have not been fully
investigated, in general, some kinds of epoxides are reac-
tive with biogenic substances and sometimes induce
unpredictable toxicological effects such as hepatotoxici-
ty and genotoxicity.9 Therefore, we aimed to investigate
new linker structures without a double bond to prevent
production of epoxides as active metabolites with likely
reactivity. In this letter, we wish to disclose the synthesis
and biological activities of FR181157-related derivatives
prepared by replacing the cyclohexene-linker as novel
PGI2 mimetics.
Keywords: Prostacyclin; PGI2; IP receptor; Agonist.
*
The synthetic route to compounds 1a and 1c–i newly
prepared in this letter is illustrated in Schemes 1–4.
Corresponding author. Tel.: +81 6 6390 5497; fax: +81 6 6304
5414; e-mail: akira-tanaka@jp.astellas.com
0960-894X/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.06.076

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A. Tanaka et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4861–4864
Scheme 1. Reagents: (a) benzoin, DMAP, EDC, CH2Cl2 (61%); (b) AcONH4, AcOH (78%); (c) i—Mg, THF; ii—3-benzyloxybenzaldehyde (two
steps 54%); (d) H2, 10% Pd/C, EtOAc, MeOH, HCl (28%); (e) methyl bromoacetate, K2CO3, DMF (98%); (f) 1 N NaOH, DME (90%).
benzaldehyde derivative, deprotection–dehydroxylation,
and introduction of an ethyl acetate moiety, followed by
hydrolysis (Scheme 1). Synthesis of 1c–d with a cyclo-
hexane-linker was accomplished by hydrogenation of
optically active ester 6,8b an intermediate for the synthe-
sis of FR181157, followed by separation of the isomers
and hydrolysis (Scheme 2). Optically active cyclopen-
tane compounds 1e–f were prepared from optically pure
acetate 710 as shown in Scheme 3. Treatment of 7 with 3-
methoxybenzyl Grignard reagent in the presence of CuI
was accompanied by a reduction in optical purity, and
was followed by hydrolysis, co-recrystallization with
(+)-a-phenylethylamine from Et2O to enhance the opti-
cal purity, and treatment with 1 N HCl gave carboxylic
acid 8 whose optical purity was determined to be >99%
ee after transformation to a diphenyloxazole derivative
9. Subsequent hydrogenation, separation of the isomers,
demethylation, and introduction of an ester moiety,
Scheme 2. Reagents: (a) i—H2, 10% Pd/C, EtOAc; ii—SiO2 column
separation (43% for 7a; 38% for 7b); (b) 1 N NaOH, THF, EtOH (87%
for 1c; 72% for 1d).
Compound 1b was already reported in our previous
paper.8b Compound 1a having a benzene-linker was pre-
pared from 2-bromobenzoic acid by construction of the
diphenyloxazole ring, metalation, treatment with a
Scheme 3. Reagents: (a) 3-methoxybenzylchloride, Mg, CuI, THF (quant., 77% ee); (b) 1 N NaOH, EtOH, dioxane (71%); (c) i—(+)-a-
phenylethylamine, Et2O; ii—recrystallization from EtOAc–hexane (2·) (67%); (d) 1 N HCl, EtOAc (quant.); (e) SOCl2, CH2Cl2; (f) benzoin,
pyridine, CH2Cl2; (g) AcONH4, AcOH (three steps 80%, >99% ee); (h) i—H2, 10% Pd/C, EtOH; ii—SiO2 column separation (50% for 10a; 42% for
10b); (i) BBr3, CH2Cl2; (j) ethyl bromoacetate, K2CO3, DMF (two steps 85% from 10a; 85% for 10b); (k) 1 N NaOH, EtOH (86% for 1e; 86% for 1f).
Scheme 4. Reagents: (a) benzoin, DMAP, EDC, CH2Cl2 (quant. from 12a; 96% from 12b); (b) AcONH4, AcOH (80% for 13a; 94% for 13b); (c) H2,
10% Pd/C, MeOH (98% from 13a; 98% from 13b); (d) ethyl 3-(bromomethyl)phenoxyacetate, K2CO3, DMF (95% for 14a; 44% for 14b); (e) 1 N
NaOH, EtOH (96% for 1g; 87% for 1h); (f) i—neutralized with 1 N HCl, then 4 N HCl in EtOAc, Et2O; ii—recrystallization from MeOH–Et2O (69%
from 1g).

A. Tanaka et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4861–4864
4863
followed by hydrolysis, provided the target molecules.
Optically active 1g and 1h were prepared from Z-pro-
tected D- or L-proline (12), respectively. Construction
of the diphenyloxazole ring, deprotection, alkylation
with benzylbromide derivative, followed by hydrolysis,
afforded the object compounds.
was considered that the sp2 carbon atom, to which the
left part benzyl moiety is attached, may cause an unfa-
vorable effect on the interaction of agonists with the
receptor. To investigate the importance of the double
bond in cyclohexene of FR181157, we synthesized and
evaluated cyclohexane analogues 1c–d. These were only
twofold less active than the parent compound, and no
effect of cis/trans stereochemistry on the human platelet
aggregation inhibitory activity was observed. The
retained activity of these two compounds led us to inves-
tigation of ring size, and accordingly, cyclopentane ana-
logues 1e and 1f were prepared and exhibited slightly
more potent activity than FR181157. Looking at the
lipophilicity, the ClogP value of natural prostacyclin
is 2.33.11 On the other hand, that of FR181157 calculat-
ed as a free form is quite high (6.88), and such a high
lipophilic compound does not meet Lipinski’s ‘rule of
five,’ a well-known method to predict drug-likeness.12
From this point of view, cyclopentane analogues 1e
and 1f are still lipophilic, and besides have two chiral
centers requiring a more complicated synthetic route
than FR181157. Therefore, we planned to introduce a
nitrogen atom as a functionality to reduce the lipophil-
icity and simultaneously delete one of two chiral centers.
In general, proline is a well-known useful chiral synthon,
because it is relatively inexpensive and both D- and L-en-
antiomers with high optical purity are easily available.
Thus, both enantiomers 1g and 1h were readily synthe-
sized from D- and L-proline and were assessed. As a
result, 1h showed potent activity comparable to
FR181157, and 1g was slightly more potent. The stereo-
chemistry of these two compounds gave a great effect on
rat platelet aggregation. Compound 1g retained the
activity, however, its enantiomer 1h resulted in complete
loss of activity for rat platelets. The species difference of
1g (15-fold) was smaller than FR181157 (20-fold), and
also much smaller than previously reported our another
type of diphenylcarbamate compound FK-788 (80-
fold).8d It is well known that this class of PGI2 mimetics
has a species difference.3b,8c–e Based on the results dis-
cussed above, compound 1g, easily synthesized from
D-proline, with a relatively small species difference and
lower ClogP (4.85, calculated as a free form), was
selected for further evaluation.
The compounds prepared were evaluated for their abil-
ity to inhibit aggregation of ADP-induced human and
rat platelets in platelet-rich plasma as PGI2 receptor
agonistic activity, and is expressed in Table 1 as the
nanomolar concentration of a compound required to
inhibit 50% of the aggregation (IC50).
Replacement of the cyclohexene-linker of FR181157 by
benzene to delete an isolated double bond and simulta-
neously a chiral center resulted in compound 1a with re-
duced activity. In our previous paper,8b compound 1b
was reported to show a similar tendency. Therefore, it
Table 1. Biological activity of prepared compounds
Compound
Cyclic linker
ADP-induced platelets
aggregation inhibitory
activitya IC50 (nM)
Humanb
Ratc
1a
930
ND
1b
533
115
114
53
ND
1c
ND
1d
ND
1e
ND
PGI2 receptor binding was examined by the conventional
ligand binding assay based on the displacement of [3H]-
iloprost from the cloned human PGI2 receptor (IP).13 1g
exhibited high binding affinity for the IP receptor with a
Ki value of 76 nM, and was comparable to that of
60 nM shown for FR181157. A pharmacokinetic (PK)
study with 1i (a crystalline hydrochloride salt of 1g)14 in
fasted rats (n = 3) revealed its good oral bioavailability
(F = 41%) which is comparable to that of FR181157
(F = 50%), although Cmax and AUC were relatively lower
(1i: Cmax = 15.5 1.3 ng/mL, AUC = 81.7 1.8 ng h/mL
at 1.0 mg/kg po; FR181157: Cmax = 16.4 0.88 ng/mL,
AUC = 147.0 14.3 ng h/mL at 0.32 mg/kg po).
1f
39
ND
1g
48
700
1h
58
>10000
1200
FR181157
60
In summary, we have prepared novel analogues of
FR181157 replacing the cyclohexene-linker and assessed
them as IP receptor agonists. Amongst them, the unique
zwitter compound 1g with the large benefit of synthetic
ND denotes not determined.
a Values are the average of two experiments.
b Evaluated at a concentration of 2.5 lM ADP.
c Evaluated at a concentration of 2.0 lM ADP.

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A. Tanaka et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4861–4864
K. S.; Zavoico, G. B.; Seiler, S. M. J. Med. Chem. 1993,
36, 3884.
accessibility, relatively small species difference, and low-
er ClogP was identified as a potent non-prostanoid
PGI2 mimetic designed not to produce reactive metabo-
lites such as epoxide. A PK study of 1i (FR207845), a
hydrochloride salt of 1g, revealed its good oral bioavail-
ability in rats.
6. (a) Hamanaka, N.; Takahashi, K.; Nagao, Y.; Torisu, K.;
Tokumoto, H.; Kondo, K. Bioorg. Med. Chem. Lett. 1995,
5, 1065; (b) Hamanaka, N.; Takahashi, K.; Nagao, Y.;
Torisu, K.; Takada, H.; Tokumoto, H.; Kondo, K.
Bioorg. Med. Chem. Lett. 1995, 5, 1071; (c) Hamanaka,
N.; Takahashi, K.; Nagao, Y.; Torisu, K.; Tokumoto, H.;
Kondo, K. Bioorg. Med. Chem. Lett. 1995, 5, 1077; (d)
Hamanaka, N.; Takahashi, K.; Nagao, Y.; Torisu, K.;
Tokumoto, H.; Kondo, K. Bioorg. Med. Chem. Lett. 1995,
5, 1083; (e) Nagao, Y.; Takahashi, K.; Torisu, K.; Kondo,
K.; Hamanaka, N. Heterocycles 1996, 42, 517.
Acknowledgment
We especially thank Dr. David Barrett for his critical
reading of this manuscript.
7. For reviews of non-prostanoid PGI2 mimetics, see (a)
Wise, H.; Jones, R. L. Trends Pharmacol. Sci. 1996, 17, 17;
(b) Meanwell, N. A.; Romine, J. L.; Seiler, S. M. Drugs
Future 1994, 19, 361.
References and notes
8. (a) Tsubaki, K.; Taniguchi, K.; Tabuchi, S.; Okitsu, O.;
Hattori, K.; Seki, J.; Sakane, K.; Tanaka, H. Bioorg. Med.
Chem. Lett. 2000, 10, 2787; (b) Hattori, K.; Tabuchi, S.;
Okitsu, O.; Taniguchi, K. Bioorg. Med. Chem. Lett. 2003,
13, 4277; (c) Hattori, K.; Okitsu, O.; Tabuchi, S.;
Taniguchi, K.; Nishio, M.; Koyama, S.; Seki, J.; Sakane,
K. Bioorg. Med. Chem. Lett. 2005, 15, 3279; (d) Hattori,
K.; Tanaka, A.; Okitsu, O.; Tabuchi, S.; Taniguchi, K.;
Nishio, M.; Koyama, S.; Higaki, M.; Seki, J.; Sakane, K.
Bioorg. Med. Chem. Lett. 2005, 15, 3091; (e) Hattori, K.;
Takamura, F.; Tanaka, A.; Takasugi, H.; Taniguchi, K.;
Nishio, M.; Koyama, S.; Seki, J.; Sakane, K. Bioorg. Med.
Chem. Lett. 2005, 15, 3284.
9. (a) Raney, K. D.; Shimada, T.; Kim, D.-H.; Groopman, J.
D.; Harris, T. M.; Guengerich, F. P. Chem. Res. Toxicol.
1992, 5, 202; (b) Guengerich, F. P. Chem. Res. Toxicol.
2001, 14, 611.
10. Takano, S.; Yamada, O.; Iida, H.; Ogasawara, K.
Synthesis 1994, 592.
1. (a) Moncada, S.; Gryglewski, R.; Bunting, S.; Vane, J. R.
Nature 1976, 263, 663; (b) Therapeutic Applications of
Prostaglandins; Vane, J., O’Grady, J., Eds.; Edward
Arnold: London, 1993; (c) Dusting, G. J.; MacDonald,
P. S. Pharmacol. Ther. 1990, 48, 323.
˚
2. Bergman, N.-A.; Jansson, M.; Chiang, Y.; Kresge, A. J.
J. Org. Chem. 1988, 53, 2544.
3. (a) Seiler, S.; Brassard, C. L.; Arnold, A. J.; Meanwell, N.
A.; Fleming, J. S.; Keely, S. L., Jr. J. Pharmacol. Exp.
Ther. 1990, 255, 1021; (b) Merritt, J. E.; Hallam, T. J.;
Brown, A. M.; Boyfield, I.; Cooper, D. G.; Hickey, D. M.
B.; Jaxa-Chamiec, A. A.; Kaumann, A. J.; Keen, M.;
Kelly, E.; Kozlowski, U.; Lynham, J. A.; Moores, K. E.;
Murray, K. J.; MacDermot, J.; Rink, T. J. Br.
J. Pharmacol. 1991, 102, 251.
4. (a) Muir, G.; Jones, R. L.; Will, S. G.; Winwick, T.;
Peesapati, V.; Wilson, N. H.; Griffiths, N.; Nicholson, W.
V.; Taylor, P.; Sawyer, L.; Blake, A. J. Eur. J. Med. Chem.
1993, 28, 609; (b) Armstrong, R. A.; Jones, R. L.;
MacDermot, J.; Wilson, N. H. Br. J. Pharmacol. 1986,
87, 543.
5. (a) Meanwell, N. A.; Rosenfeld, M. J.; Wright, J. J. K.;
Brassard, C. L.; Buchanan, J. O.; Federici, M. E.;
Fleming, J. S.; Seiler, S. M. J. Med. Chem. 1992, 35,
389; (b) Meanwell, N. A.; Rosenfeld, M. J.; Trehan, A. K.;
Wright, J. J. K.; Brassard, C. L.; Buchanan, J. O.;
Federici, M. E.; Fleming, J. S.; Gamberdella, M.; Zavoico,
G. B.; Seiler, S. M. J. Med. Chem. 1992, 35, 3483; (c)
Meanwell, N. A.; Rosenfeld, M. J.; Trehan, A. K.;
Romine, J. L.; Wright, J. J. K.; Brassard, C. L.; Buchanan,
J. O.; Federici, M. E.; Fleming, J. S.; Gamberdella, M.;
Zavoico, G. B.; Seiler, S. M. J. Med. Chem. 1992, 35, 3498;
(d) Meanwell, N. A.; Rosenfeld, M. J.; Wright, J. J. K.;
Brassard, C. L.; Buchanan, J. O.; Federici, M. E.;
Fleming, J. S.; Gamberdella, M.; Hartl, K. S.; Zavoico,
G. B.; Seiler, S. M. J. Med. Chem. 1993, 36, 3871; (e)
Meanwell, N. A.; Romine, J. L.; Rosenfeld, M. J.; Martin,
S. W.; Trehan, A. K.; Wright, J. J. K.; Malley, M. F.;
Gougoutas, J. Z.; Brassard, C. L.; Buchanan, J. O.;
Federici, M. E.; Fleming, J. S.; Gamberdella, M.; Hartl,
11. ClogP values are calculated using CLOGP program
Biobyte ver. 4.1 (BioByte Corp.).
12. (a) Lipinski, C. A.; Lombardo, F.; Dominy, B. W.;
Feeney, P. J. Adv. Drug Delivery Rev. 1997, 23, 3; (b)
Clark, D. E.; Pickett, S. D. Drug Discovery Today 2000, 5,
49.
`
13. (a) Abramovitz, M.; Adam, M.; Boie, Y.; Carriere, M.-C.;
Denis, D.; Godbout, C.; Lamontagne, S.; Rochette, C.;
Sawyer, N.; Tremblay, N. M.; Belley, M.; Gallant, M.;
Dufresne, C.; Gareau, Y.; Ruel, R.; Juteau, H.; Labelle,
M.; Ouimet, N.; Metters, K. M. Biochim. Biophys. Acta
2000, 1483, 285; (b) Negishi, M.; Sugimoto, Y.; Ichikawa,
A. Biochim. Biophys. Acta 1995, 1259, 109.
14. Physical data for FR207845 (1i): mp 119–121 ꢁC;
22
½aꢀD +9.2ꢁ (c 0.93, MeOH); 1H NMR (200 MHz,
DMSO-d6): d 2.00–2.30 (2H, m), 2.30–2.70 (2H, m),
3.30–3.70 (2H, m), 4.40–4.70 (4H, m), 4.85–5.10 (1H, m),
6.80–7.70 (14H, m), 11.34 (1H, br), 13.01 (1H, br); IR
(KBr) 2952, 2536, 1720, 1597, 1498, 1448, 1387 cmꢁ1
;
APCI-MS m/z 455 (M+HꢁHCl)+. Anal. Calcd for
C28H27ClN2O4: C, 68.50; H, 5.54; N, 5.71. Found: C,
68.14; H, 5.43; N, 5.86.