Benzamide bioisosteres incorporating dihydroheteroazole substructures: EPC synthesis and SAR leading to a selective dopamine D4 receptor partial agonist (FAUC 179)

Article abstract of DOI:10.1016/S0960-894X(01)00484-X

Conformationally restricted benzamide bioisosteres were investigated when the chiral phenyldihydroimidazole derivative 4e (FAUC 179) showed strong and highly selective dopamine D4 receptor binding (K_ihigh = 0.95 nM). Mitogenesis experiments indicated partial agonist properties (42%). EPC syntheses of the target compounds of type 4 were performed starting from α-amino acids.

Full text of DOI:10.1016/S0960-894X(01)00484-X

Bioorganic & Medicinal Chemistry Letters 11 (2001) 2533–2536
Benzamide Bioisosteres Incorporating Dihydroheteroazole
Substructures: EPC Synthesis and SAR Leading to a Selective
Dopamine D4Receptor Partial Agonist (FAUC 179)
Jurgen Einsiedel, Harald Hubner and Peter Gmeiner*
Department of Medicinal Chemistry, Emil Fischer Center, Friedrich-Alexander University, Schuhstraße 19, D-91052 Erlangen, Germany
Received 28 March 2001; revised 3July 2001; accepted 10 July 2001
Abstract—Conformationally restricted benzamide bioisosteres were investigated when the chiral phenyldihydroimidazole derivative
4e (FAUC 179) showed strong and highly selective dopamine D4 receptor binding (Kihigh=0.95 nM). Mitogenesis experiments
indicated partial agonist properties (42%). EPC syntheses of the target compounds of type 4 were performed starting from a-amino
acids. # 2001 Elsevier Science Ltd. All rights reserved.
Besides psychotic, addictive and personality disorders,
the dopamine D4 receptor DRD4 gene1 has been impli-
cated with ADHD (attention deficit hyperactivity dis-
orders).2 A genetic association between a 50 120-bp
tandem duplication polymorphism and an exon 348-bp
variable number of tandem repeats with ADHD leads
to the hypothesis that selective dopamine D4 receptor
agonists or partial agonists might be beneficial for an
effective pharmacotherapy.3
were reported.4 Furthermore, ligand efficacy was
described for a few N-arylpiperazines including PD
168077 (1),5 FAUC 299 (2),6 NGD 94-1 (3)7 as well as
L-745,8708 and U-101,958.9 As a complement to our
previous studies on heterocyclic benzamide bioiso-
steres,10 we herein report EPC syntheses and biological
investigations of dihydroheteroazoles of type 4 designed
by a structural hybridization of the lead compounds 1–
3. For studying the influence of chirality on the receptor
binding profiles, we decided to prepare the target pro-
ducts optically pure starting from the a-amino acids
serine, threonine and cysteine in natural and unnatural
absolute configuration.
The preparation of the phenyloxazolines 4a–d and the
phenylthiazoline 4f was envisioned by building up the
basic scaffolds from the corresponding a-amino esters
and suitable activated benzoic acid derivatives. Phe-
nylpiperazine should be introduced by reduction of the
ester functionality, activation and subsequent SN2-dis-
placement. For the synthesis of the highly polar and
basic imidazoline derivative 4e, we decided to perform
the cyclization as the last reaction step after preparing
an appropriate 1,2-diamine building block.
During the last years, a number of SAR studies giving
insights into the molecular properties that are respon-
sible for dopamine D4 receptor affinity and selectivity
In detail, (R)-serine methyl ester hydrochloride (5a)11
and (2R,3S)-threonine methyl ester hydrochloride (5b)12
were cyclized with methyl benzimidate13 to give the
oxazoline derivatives 6a14,15 and 6b, respectively
(Scheme 1).16,17 In order to obtain high yields, it was
advantageous to perform the reactions in MeOH. Low
*Corresponding author. Tel.: +49-9131-852-9383; fax: +49-9131-
852-2585; e-mail: gmeiner@pharmazie.uni-erlangen.de
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00484-X

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J. Einsiedel et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2533–2536
temperature LiAlH4 reduction afforded the hydroxy-
methyl derivatives 7a and 7b, which could be activated
to furnish the mesylates 8a,b in over 90% yield.
Nucleophilic displacement with phenylpiperazine resul-
ted in formation of the target compounds 4a,b. Com-
parison of the optical rotation data of the intermediates
7a,b with those reported in the literature for ent7a,b did
not give any evidence for partial racemization.16,18,19
For the threonine derivative 4b, formation of a cis-dia-
stereomer as a side product could not be observed in the
1H NMR spectra, indicating configurational integrity.
The 5-bromo-2-methoxy-phenyl analogues were synthe-
sized by N-acylation of the serine and threonine ester
hydrochlorides 5a,b with 5-bromo-2-methoxy-benzoyl
chloride 12 (available by SOCl2-activation of the corre-
sponding benzoic acid 1120) to give the N-acylamino
acids 14a,b. Subsequent intramolecular O-alkylation
resulted in formation of the oxazoline derivatives 15a,b.
For the N-acylthreonine derivative 14b, SOCl2 induced
cyclization occurred under complete inversion when the
cis-oxazoline 15b was obtained in 78% yield. On the
other hand, activation of the N-acylserine derivative 14a
with SOCl2 gave disappointing results concerning yield
and purity of the oxazoline 15a. However, smooth and
high yielding ring closure could be performed by appli-
cation of the Burgess reagent.21 Chemoselective reduc-
tion of the carboxylic esters 15a,b furnished the primary
alcohols 16a,b that were converted into the mesylates
17a,b and subsequently reacted with phenylpiperazine
to give the target compounds 4c,d. The cis-disposition
between the hydroxymethyl and the methyl group in the
positions 4 and 5 of the oxazoline 16b was proved by
NOE experiments when irradiation of H-4 showed a
strong positive enhancement at H-5 and vice versa.
The thiazoline 4f was synthesized starting from the
unnatural (S)-cysteine ethyl ester hydrochloride (18).22
In detail, coupling with methyl benzimidate afforded the
cyclization product 1923 that could be transformed che-
moselectively into the alcohol 20 when low temperature
LiAlH4 reduction was employed.24 The hydroxymethyl
group was activated to give the mesylate 21 and subse-
quently subjected to phenylpiperazine. Because of the
high costs of (S)-cysteine, we established an alternative
route starting from the readily available oxazoline 4a.
Thus, thiolysis of 4a with NEt3/H2S/MeOH afforded
the thioamide 22 that was cyclized to give thiazoline 4f
by intramolecular S-alkylation induced by Burgess
reagent.25
The oxazoline derivative 4a should also be exploited for
the synthesis of the chiral imidazoline derivative 4e. Our
initial attempts involving reductive ring opening by
LiAlH4, N-protection, O-activation and subsequent
nucleophilic amination failed. When using N,N-dibenzyl
protection, we observed rearrangement via an azir-
idinium intermediate and low yield of the cyclization
precursor. On the other hand, reduction of the nucleo-
philicity by N-benzyl-N-Cbz protection precluded the
formation of aziridinium intermediates and facilitated a
Scheme 1. (a) Methyl benzimidate, MeOH, refl, 2 h (6a: 82%, 6b: 80%); (b) LiAlH4, Et2O, ꢀ30 ꢁC, 15 min (7a: 85%, 7b: 67%, 16a: 45%, 16b: 53 %,
20: 82%); (c) MesCl, NEt3, THF, ꢀ23 ꢁC, 30 min (8a, 8b: 100%, 17a,b: 82%, 21: 96%); (d) phenylpiperazine, DMF, 70 ꢁC, 4a,b: 12 h, 4c,d: 24 h, 4f:
6 h (4a: 82%, 4b: 62%, 4c: 64%, 4d: 38%, 4f: 40%); (e) TMS-N3, t-BuOH, 120 ꢁC, 36 h (86%); (f) LiAlH4, THF, 0 ꢁC–refl (crude: 100%); (g) H2,
Pd(OH)2/C, RT, 8 h (92%); (h) methyl benzimidate–HCl, MeOH, refl, 45 min (68%); (i) SOCl2, toluene, CHCl3, cat DMF, 60 ꢁC, 15 h (crude); (j) (1)
NEt3, CHCl3, ꢀ5 ꢁC, 5 min; (2) NEt3, 12, 0 ꢁC–rt, 15 min (14a: 94%, 14b: 78%); (k) (1) SOCl2, ꢀ5 ꢁC, 12 h; (2) CHCl3, aq Na2CO3, 0 ꢁC (78%); (l)
MeO2CNSO2NEt3 (Burgess reagent), THF, 70 ꢁC, 45 min (70%); (m) NEt3, MeOH, H2S, rt, 45 min (100%); (n) MeO2CNSO2NEt3, THF, rt, 45 min
(47%); (o) methyl benzimidate, EtOH, rt, 45 min (91%).

J. Einsiedel et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2533–2536
2535
Table 1. Receptor binding data [Ki values (nM) based on the means of 2–4 experiments each performed in triplicate]
Compd
-X-
R0
R00
D1
D2long
1700
D2short
D3D4
4a
H
H
OMe
OMe
H
H
Br
Br
9700
4000
9000
2000
1100
330
900
110
3300
690
5200
1000
340
410
1600
390
ent4a
4c
ent4c
690
2200
400
4b
ent4b
H
H
H
H
20000
2000
3200
820
1900
320
2700
710
700
440
4d
ent4d
OMe
OMe
Br
Br
3500
480
3700
21,000
2400
18,000
2500
10,000
290
3300
4e
ent4e
H
H
H
H
12,000
nd
10,000
14,000
11,000
10,000
7100
9000
0.95/51
72
4f
ent4f
H
H
H
H
16,000
4500
1500
1100
1100
720
1200
1200
1800
1200
quinpirole
nd
64/31,000
52/4000
24/420
1.8/53
smooth amination by NaN3. However, we clearly
observed complete racemization which is obviously due
to the formation of an azetidinium intermediate via the
nucleophilic piperazine nitrogen. Inspired by a recently
reported amination sequence giving access to neur-
aminidase inhibitors,26 we tried to initiate a nucleophilic
ring opening of 4a by TMS-N3. In practice, utilization
of t-butanol as a solvent afforded an 86% yield of the
azide 9. Subsequently, it was possible to reduce both the
amide and the azide function in one step to give the
benzylamine derivative 10. Hydrogenolytic debenzyla-
tion afforded the 1,2-diamine 13 in 92% yield. Deriva-
tization of both amino functions with (R)-phenylethyl
isocyanate and subsequent HPLC-analysis indicated a
satisfactory configurational integrity of the synthesis.27
The final product 4e28 was obtained by cyclization of
the diamine 13 with methyl benzimidate hydrochlo-
ride.29
Table 2. Agonist effects of the imidazoline 4e, quinpirole and cloza-
pine at the D4.2 receptor investigated by measuring the stimulation of
mitogenesis
Test compounds
4e
Quinpirole
Clozapine
Agonist effect (%)a
EC50 (nM)b
42
31
100
9.5
0
nd
aRate of incorporation of [3H]thymidine (in %) relative to the max-
imal effect of the full agonist quinpirole (100%); the results are the
means of quadruplicates of 10–14 experiments.
bEC50 values derived from the mean curves of 10–14 experiments; nd,
not determined.
the (R)-enantiomer 4e (FAUC 179) showed high D4
affinity. Careful analysis of the D4 binding experiments
employing a large number of test concentrations
showed a biphasic curve (nH=0.55). The calculation
provided a Ki of 0.95 nM for the high affinity binding
site and 51 nM for the low affinity binding site. These
data are comparable to the D4 binding properties of the
unselective dopamine receptor agonist quinpirole.
However, FAUC 179 showed high selectivity over D1,
D2long, D2short and D3. Emloying porcine brain homo-
genates and the radioligands [3H]8-OH-DPAT and
[3H]ketanserine, substantial selectivity over 5-HT1A
(Ki=45 nM) and 5-HT2 (Ki=870 nM), respectively, was
observed.
The enantiomers ent4a–f were prepared analogously
starting from the respective natural a-amino acids.
The test compounds ent4a–f and the dopamine receptor
agonist quinpirole were evaluated in vitro for their
abilities to displace [3H]spiperone from the cloned
30
human dopamine receptors D2long, D2short
,
D331 and
D4.432 being stably expressed in CHO cells (see Table
1).33 D1 affinity was determined by employing bovine
striatal membrane preparations and the D1 selective
antagonist [3H]SCH 23390.33 The oxazolines ent4a–d
and the thiazolines ent4f showed only moderate affinity
in the radioligand binding experiments. However, the Ki
value of the (S)-imidazoline ent4e (72 nM) indicated
substantial D4 affinity. It turned out that receptor
binding was strongly dependent on the spatial orienta-
tion of the phenylpiperazinylmethyl side chain. Thus,
To investigate the intrinsic effect of FAUC 179 (4e), an
in vitro functional assay measuring the [3H]thymidine
uptake in growing CHO cells stably expressing the
dopamine D4.2 receptor was performed6 when a 42%
stimulation of mitogenesis (compared to the full agonist
effect of quinpirole and the D4 antagonist clozapine)
was determined (EC50=31 nM) (Table 2).34

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J. Einsiedel et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2533–2536
In conclusion, SAR investigations on conformationally
restricted benzamide bioisosteres led to the highly
selective dopamine D4 receptor partial agonist 4e
(FAUC 179) incorporating a chiral imidazoline sub-
structure. The biological activity was found to be
strongly enantiospecific. Replacement of the imidazo-
line moiety by oxazoline or thiazoline was not success-
ful. This leads to the assumption that the NH function is
essential for D4 receptor recognition of the investigated
family of compounds.
13. Wheeler Am. Chem. J. 1895, 17, 358. Release of the free
imidate base according to: Glickman, S. A.; Cope, A. C. J.
Am. Chem. Soc. 1945, 67, 1017.
14. Elliot, D. F. J. Chem. Soc. 1949, 589.
15. Tkaczuk, P.; Thornton, E. R. J. Org. Chem. 1981, 46,
4393.
16. Leveque, H.; Malhiac, C.; Speybrouck, D.; Combret, Y.;
Guerpin, V.; Combret, J.-C. Bull. Soc. Chim. Fr. 1995, 132,
801.
17. Seebach, D.; Aebi, J. D.; Gander-Coquoz, M. Helv. Chim.
Acta 1987, 70, 1194.
18. ent7a: a2D0=+84.5ꢁ (c 1.68, CHCl3); for comparison:
Blake, A. J.; Boyd, E. C.; Gould, R. O.; Paton, R. M. J. Chem.
Soc. Perkin Trans. 1 1994, 2841: a2D0=+81ꢁ (c 1.0, CHCl3).
19. ent7b: a2D0=+77.8ꢁ (c 1.43, CHCl3); for comparison: Ste-
vens, C. L.; Gillis, T. G.; Haskell, T. H. J. Am. Chem. Soc.
1959, 81, 1435: a2D0=+79.2ꢁ (c 1.63, CHCl3).
20. Laatsch, H.; Pudleiner, H. Liebigs Ann. Chem. 1989, 863.
21. (a) Atkins, G. M.; Burgess, E. M. J. Am. Chem. Soc. 1968,
90, 4744. (b) Wipf, P.; Miller, C. P. Tetrahedron Lett. 1992, 33,
907.
22. 18 was synthesized according to: Trentmann, W.; Mehler,
T.; Martens, J. Tetrahedron: Asymmetry 1997, 12, 2033.
23. Crawhall, J. C.; Elliott, D. F.; Hooper, K. C. J. Chem.
Soc. 1956, 338, 588.
24. ent20: a2D0=+111.6ꢁ (c 1.0, CHCl3); for comparison: Ish-
izuka, N.; Sato, T.; Makisumi, Y. Chem. Pharm. Bull. 1990,
38, 1396: a2D0=+108.0ꢁ (c 1.712, CHCl3).
Acknowledgements
The Fonds der Chemischen Industrie is acknowledged
for financial support. We thank Dr. J.-C. Schwartz and
Dr. P. Sokoloff (INSERM, Paris), Dr. H. H. M. Van
Tol (Clarke Institute of Psychiatry, Toronto) and Dr. J.
Shine (The Garvan Institute of Medical Research, Syd-
ney) for providing D3, D4.4 and D2 receptor-expressing
cell lines. Dr. R. Huff (Pharmacia & Upjohn, Inc.,
Kalamazoo, MI, USA) is acknowledged for providing a
D4-expressing cell line employed for mitogenesis. Spe-
cial thanks are due to Dr. R. Waibel for helpful discus-
sions and to Mrs. H. Szczepanek, Mrs. P. Schmitt, Mrs.
P. Hubner and Mrs. I. Torres-Berger for skillful techni-
cal assistance.
25. Wipf, P.; Miller, C. P.; Venkatraman, S.; Fritch, P. C.
Tetrahedron Lett. 1995, 36, 6395.
26. Chandler, M.; Bamford, M. J.; Conroy, R.; Lamont, B.;
Patel, B.; Patel, V. K.; Seeples, I. P.; Storer, R.; Weir, N. G.;
Wright, M.; Williamson, C. J. Chem. Soc., Perkin Trans. 1
1995, 1173.
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