Dopamine/serotonin receptor ligands. 9.1 oxygen-containing midsized heterocyclic ring systems and nonrigidized analogues. A step toward dopamine D5 receptor selectivity

Article abstract of DOI:10.1021/jm049720x

Eleven-membered heterocycles (dibenz[g,j]-1-oxa-4-azacycloundecenes) and open-chain analogues were synthesized and investigated for affinities to human dopamine receptor subtypes. The moderately rigidized rings displayed nanomolar and subnanomolar K_i values at D₁-like receptors with a significant D₁ to D₂ and a slight D₅ to D ₁ selectivity. The open-chain analogues showed lower affinities but significant D₁ to D₂ selectivities. Compound 3 (K _i(D₅) = 0.57 nmol) showed antagonistic or inverse agonistic binding characteristics in a functional Ca assay.

Full text of DOI:10.1021/jm049720x

J . Med. Chem. 2004, 47, 4155-4158
4155
Dop a m in e/Ser oton in Recep tor Liga n d s.
9.1 Oxygen -Con ta in in g Mid sized
Heter ocyclic Rin g System s a n d
Non r igid ized An a logu es. A Step tow a r d
Dop a m in e D5 Recep tor Selectivity
F igu r e 1. Condensed midsize heterocyclic ring systems with
antidopaminergic activity.
Thomas W. Wittig, Michael Decker, and
J ochen Lehmann*
Sch em e 1. Synthesis of 3a
Institut fu¨r Pharmazie, Lehrstuhl fu¨r Pharmazeutische/
Medizinische Chemie, Friedrich-Schiller-Universita¨t J ena,
Philosophenweg 14, D-07743 J ena, Germany
Received April 13, 2004
Abstr a ct: Eleven-membered heterocycles (dibenz[g,j]-1-oxa-
4-azacycloundecenes) and open-chain analogues were synthe-
sized and investigated for affinities to human dopamine
receptor subtypes. The moderately rigidized rings displayed
nanomolar and subnanomolar Ki values at D1-like receptors
with a significant D1 to D2 and a slight D5 to D1 selectivity.
The open-chain analogues showed lower affinities but signifi-
cant D1 to D2 selectivities. Compound 3 (Ki(D5) ) 0.57 nmol)
showed antagonistic or inverse agonistic binding characteris-
tics in a functional Ca assay.
Dopamine receptor mediated neurotransmission plays
a key role in psychiatric, motoric, and endocrinological
disorders. While a limited number of D1-like receptor
ligands such as the antagonist LE 300 (1)1-3 are known,
none of them show a significant binding selectivity to
the D1 or the D5 receptor. Hence, knowledge about the
physiological impact of activation or inhibition of these
single receptors, especially the D5 receptor, is limited
or not existent.4
Originating from lead LE 300 (1) (Figure 1), our
interests are focused on dibenzo derivatives. We syn-
thesized compounds with an enlarged central ring
system (Figure 1, Scheme 1) and an isosteric replace-
ment of the methylene group in position 5 against an
oxygen atom (therefore resulting in resorcine mono- and
diethers), leading to a lower degree of rigidization and
a different electronic situation in the central ring system
as well as induction of a different electrostatic field in
the respective benzene moiety (Figure 1). To investigate
the effect of rigidization on the binding affinities of these
substances, we also synthesized (2-benzylphenoxy)alkyl-
amines (5-14) representing open-chain analogues of the
11-membered heterocycles varying in their substitution
patterns at the benzyl C atom and at the N atom
(Scheme 2).
The dibenz[g,j]-1-oxa-4-azacycloundecenes were syn-
thesized as 3-monomethoxylated (2) and 3-monohy-
droxylated (3) compounds. 2-[3-(Benzyloxy)phenoxy]-
ethylamine (4) was obtained by monobenzylation of
resorcine,5 reaction with 2-methyl-2-oxazoline to the
acetamide, and hydrolysis in ethanolic 6.3 M NaOH
solution (superior to phosphoric acid, which led to
debenzylation).6
a
Reagents and conditions: (i) 120 °C, 8 h; (ii) CHCl3, pyridine,
room temp, 1 h; (iii) POCl3, CH3CN, reflux, 6 h; (iv) ethanolic KOH,
room temp, 16 h; (v) POCl3, 60 °C, 1 h; (vi) NaBH4, ice-cooling,
MeOH, 1 h; (vii) MeI, toluene, 90 °C, 16 h; (viii) Na0, liquid NH3,
-40 °C, 7 min, termination with NH4Cl.
but using a benzyl group to protect the phenolic func-
tion, which could be removed in the final reaction step
under Birch conditions together with the cleavage of the
central C-N bond (Scheme 1). The synthesis of the
nonrigidized analogue 5 could be carried out by per-
forming a Mitsunobu etherification of 2-benzoylphenol
with N-(2-hydroxyethyl)pyrrolidine yielding 5 (2-ben-
zylphenol yielded 9 in a one-step procedure). A two-step
procedure via phenoxyalkylbromide intermediates and
subsequent substitution reactions with different cyclic
and noncyclic alkylamines yielded 7-13 (Scheme 2).
Compounds 7-11 were reported in the literature7 as
potential antihistaminergic agents but were synthesized
in a different, less efficient and less comfortable way.
Benzophenone 5 was further reduced to the alcohol
6 by using lithium aluminum hydride. Additionally to
Compound 3 was obtained following a similar syn-
thetic protocol as described for the 3-methoxylated 26
* To whom correspondence should be addressed. Phone: +49-3641-
949800. Fax: +49-3641-949802. E-mail: j.lehmann@uni-jena.de.
10.1021/jm049720x CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/16/2004

4156 J ournal of Medicinal Chemistry, 2004, Vol. 47, No. 17
Sch em e 2. Synthesis of Nonrigidized Analoguesa
Letters
a
Reagents and conditions: (i) diisopropylazodicarboxylate (DIAD), triphenylphosphine (TPP), dry THF, 0 °C, then room temp, 48 h;
(ii) LiAlH4, dry ether, ice-cooling, then room temp, 2 h; (iii) K2CO3, toluene, reflux, 16 h; (iv) pyrrolidine, K2CO3, toluene, reflux, 16 h.
Sch em e 3. Synthesis of Anilino Derivative 15a
At least two independent experiments were carried out
in triplicate each. Ki values were calculated from IC50
values by applying the equation of Cheng and Prusoff9
and are given in nanomolar units (Table 1).
The oxaazacycloundecenes 2 and 3 show nanomolar
affinities at the hD1 receptor subtype with a 10-fold
higher affinity of the 3-hydroxylated compound. Fur-
thermore, 3 binds with subnanomolar affinity to the hD5
receptor. The D5/D1 selectivity ratio (1:6) is moderate.
The 3-methoxylated derivative 2 on the other hand
shows lower affinities but a D5/D1 affinity ratio of 1:20
(Table 1).
Both oxygen-containing midsize heterocycles display
weaker affinities at the D2-like receptor family (D2L, D3,
D
a
Reagents and conditions: (i) dry ether, ice-cooling, then room
temp, 2 h; (ii) K2CO3, toluene, reflux, 16 h; (iii) LiAlH4, dry THF,
ice-cooling, then reflux, 3 h.
4.4) in the range 10-7-10-6 M for their Ki values.
Concerning D5 over D2L binding selectivity, the hy-
droxylated 3 shows a significant ratio of almost 1:500,
while the methoxylated derivative 2 still reaches a ratio
of 1:88. While affinities to the D2 and D4 receptor
subtype are very similar, the binding affinity to the D3
receptor turned out to be the lowest with both com-
pounds, showing only micromolar Ki values. For com-
parison, the well-established antipsychotic haloperidol
shows nanomolar affinities to all hD receptor subtypes
with around 10 times higher affinities to the D2-like
family: hD1, 82 nM; hD2, 2.2; hD3, 7.8; hD4.2, 7.3; hD5,
58 nM (representative values).10 Compared to the
heterocycles, all of the nonrigidized analogues 5-15 are
compounds with lower affinities to all of the binding
sites.
the (2-benzylphenoxy)ethylamines, a (2-benzylphenoxy)-
propylamine containing a pyrrolidine ring (14) was
synthesized to evaluate the influence of side chain
elongation on the binding characteristics of these com-
pounds (Scheme 2). Finally an anilino compound of 1-[2-
(2-benzylphenoxy)ethyl]pyrrolidine (15) was synthe-
sized, resulting in a compound in which the phenolic
oxygen is replaced by a secondary amine function
(Scheme 3). Synthesis of 15 was performed starting from
2-benzylaniline and 2-bromoacetyl bromide yielding the
bromoacetamide, followed by a substitution reaction
with pyrrolidine and reduction of the amide function to
the secondary amine (Scheme 3).
Compounds 2, 3, and 5-15 were screened for their
binding affinities to human cloned dopamine receptor
subtypes by in vitro radioligand binding studies follow-
ing a protocol so far previously described8 but in 96-
well format.
D1, D2L, D3, D4.4, and D5 receptors were stably
expressed in HEK 293 or CHO cells. [3H]SCH 23390 and
[3H]spiperone were used as radioligands for experiments
at the D1-like and D2-like receptor family, respectively.
Incubations at 27 °C were terminated after 90 min by
rapid filtration with a Perkin-Elmer Mach III harvester.
The Ki values range from 10-8 to 10-6 M for D1 and
D5 and from 10-6 to 10-5 M for D2L, D3, and D4.4
.
Generally, these open-chain analogues show consis-
tently higher D1 over D2 binding affinities with the
exception of 8.
The D1 receptor affinity of the 2-benzylphenoxyeth-
ylamines reaches a maximum with 10, carrying a
piperidine ring as the alkylamino moiety, while D1/D2
selectivity reaches its maximum with 11, carrying a
morpholine ring. Interestingly enough, affinity to the

Letters
J ournal of Medicinal Chemistry, 2004, Vol. 47, No. 17 4157
Ta ble 1. Ki Values of Dibenz[g,j]-1-oxa-4-azycycloundecenes and Open-Chain Analogues at Dopamine Receptor Subtypes
Ki (nM)a
compd
R
D1
D2L
D3
D4.4
D5
2
3
OCH3
OH
35.5 ( 11.7
3.2 ( 0.8
158 ( 38
274 ( 0.5
1601 ( 306
1384 ( 454
546 ( 110
375 ( 106
1.8 ( 0.2
0.57 ( 0.1
Ki (nM)a
compd
X
n
R1
NR2
D1
D2L
D3
D4.4
D5
5
6
7
8
9
10
11
12
13
14
15
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
3
2
dO
-OH
H
H
H
H
H
H
H
pyrrolidinyl
pyrrolidinyl
N(CH3)2
N(C2H5)2
pyrrolidinyl
piperidinyl
morpholinyl
2-methylmorpholinyl
phenylpiperazinyl
pyrrolidinyl
pyrrolidinyl
1651 ( 408
2621 ( 1121
343 ( 29
1959 ( 140
379 ( 157
33.7 ( 0.4
201 ( 98
5317 ( 1038
2408 ( 1139
>10000
>10000
nd
nd
414 ( 18
nd
951 ( 334
43 ( 5
366 ( 8
nd
620 ( 255
113 ( 26
nd
6280 ( 1621
2740 ( 415
2024 ( 7
>10000
>10000
>10000
2765 ( 469
>10000
1428 ( 198
4476 ( 2496
6484 ( 462
>10000
>10000
1261 ( 84
>10000
4667 ( 993
1818 ( 156
7804 ( 2731
>10000
1015 ( 516
>10000
>10000
4188 ( 302
1006 ( 342
>10000
1676 ( 429
1480 ( 197
113 ( 3
>10000
O
O
NH
2860 ( 1094
2923 ( 1448
4654 ( 988
H
H
>10000
a
Ki values are the mean of at least two experiments ( SEM (performed in triplicate).
D1 receptor subtype and thereby D1/D2 selectivity are
dramatically lowered by introduction of a methyl group
next to the oxygen in the morpholine ring. By compari-
son of 9 and 14, side chain elongation to a phenoxypro-
pyl derivative leads to higher affinity to the D1, D2, and
D5 receptors. The isosteric replacement of the phenolic
oxygen atom by a secondary amine function leads to
dramatic loss of affinity with Ki > 10000 nM, only at
the D3 subtype showing micromolar affinity.
Compound 3 was investigated for its functional
behavior at the hD1 receptor in an intracellular Ca2+
assay developed in our group.11 HEK293 cells stably
expressing the hD1 receptor were loaded with a fluo-
rescent dye (Oregon Green), and after preincubation
with rising concentrations of 3 an agonist (SKF 38393)
was injected and fluorescence was measured with a
NOVOSTAR microplate reader. In this assay, 3 was able
to suppress agonist-induced Ca2+ influx with rising
concentrations, indicating an antagonistic or inverse
agonistic behavior at the D1 receptor subtype (Figure
2). From the inhibition curve, a Ki value could also be
defined (12.3 ( 1.3 nM), which is slightly higher than
that obtained by radioligand binding studies (3.2 ( 0.8
nM).
Compound 2 revealed high binding affinities to D1-
like receptors in the nanomolar range and represents
the first step to a structurally new class of dopaminergic
ligands with binding selectivity to the hD5 receptor
subtype, thus giving chances for evaluating the proper-
ties of D5 receptors in the brain. The 3-hydroxylated
oxaazacycloundecene derivative 3 binds with even higher
affinity to the D5 receptor, reaching a subnanomolar Ki
value, but lower selectivity to the D5 receptor.
F igu r e 2. Suppression of agonist-induced receptor-mediated
Ca2+ influx by rising concentrations of 3.
interesting candidate for further pharmacological char-
acterization as a potential neuroleptic drug candidate
with a novel receptor binding profile, especially concern-
ing the D2 receptor, which is responsible for extrapyr-
amidal motoric effects.
The investigated open-chain analogues represent
dopamine receptor ligands with lower affinities but
remarkable selectivities at dopamine receptor subtypes.
Almost all derivatives showed significant D1 binding
affinitiy over D2 binding affinity. The phenolic oxygen
atom in the compounds presented here seems to play
an essential role for receptor binding because isosteric
replacement against a secondary amine function and
thereby a switch from a proton acceptor to a proton
donor moiety result in a nearly complete loss of binding
potential to dopamine receptors except for the D3
subtype. This finding helps in the design of new
compounds with stronger proton acceptor properties,
which is on the way.
Compound 3 was identified as an inhibitory ligand
at the hD1 receptor subtype. Hence, it could be an

4158 J ournal of Medicinal Chemistry, 2004, Vol. 47, No. 17
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Ack n ow led gm en t. Appreciation is expressed to
Ba¨rbel Schmalwasser, Petra Wiecha, and Heidi Traber
for skillful technical assistance. Financial support for
M.D. by the Fonds der Chemischen Industrie (FCI) is
gratefully acknowledged.
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