Design, new synthesis, and calcilytic activity of substituted 3H-pyrimidin-4-ones

Article abstract of DOI:10.1016/j.bmcl.2005.03.054

Design, new synthesis, structure-activity relationship studies and calcium receptor antagonist (calcilytic) properties of novel 3H-pyrimidin-4-ones are described.

Full text of DOI:10.1016/j.bmcl.2005.03.054

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2537–2540
Design, new synthesis, and calcilytic activity of
substituted 3H-pyrimidin-4-ones
Irina Shcherbakova,a,* Guangfei Huang,b Otto J. Geoffroy,b Satheesh K. Nair,a
Krzysztof Swierczek,a Manuel F. Balandrin,a John Fox,a William L. Heatona and
Rebecca L. Conklina
aDrug Discovery, NPS Pharmaceuticals, Inc., 383 Colorow Drive, Salt Lake City, UT 84108, USA
bAlchem Laboratories, Inc., 13305 Rachel Blvd., Alachua, FL 32615, USA
Received 7 February 2005; revised 11 March 2005; accepted 15 March 2005
Available online 12 April 2005
Abstract—Design, new synthesis, structure–activity relationship studies and calcium receptor antagonist (calcilytic) properties of
novel 3H-pyrimidin-4-ones are described.
Ó 2005 Elsevier Ltd. All rights reserved.
A new anabolic therapy for osteoporosis that results in
the growth of structurally normal new bone involves
daily injections of parathyroid hormone (PTH).1 An
alternative therapeutic approach, which might over-
come the systemic administration of costly peptides, is
to stimulate the secretion of endogenous PTH from
the parathyroid glands that is regulated by the calcium
receptor (CaR).2,3 Compounds that block CaR activity
(calcilytics) stimulate secretion of PTH.4,5 Orally active
small molecule calcilytics increase the level of circulat-
ing PTH and stimulate new bone formation in ovariec-
tomized rats as was demonstrated for the first reported
small molecule calcilytic, NPS 21434 (Fig. 1). This
implicates calcilytics as a viable bone-building treat-
ment for osteoporosis. There are a limited number of
known chemotypes as calcium receptor antagonists, and
those include N1-arylsulfonyl-N2-(1-(1-naphthyl)ethyl)-
1,2-diamino-cyclohexanes,6,7 4-aryl-1H-quinazolin-2-ones,8,9
2-benzylpyrrolidine-substituted
aryloxypropanols,10
which are structurally similar to NPS 2143 (Fig. 1),
and trisubstituted heteroaromatic derivatives.11 Re-
cently, we have described structure–activity relationship
(SAR) studies around a new calcilytic hit, NPS 53574
that led to the discovery of novel potent CaR antagonists,
3H-quinazolin-4-ones 1 (Fig. 1).12–14 Initial SAR of the
quinazolinone series 1 rapidly revealed the key binding
elements required to achieve a high level of potency and
selectivity at CaR, and the 2-(2-hydroxy-phenyl) and
3-phenethyl substituents as essential for calcilytic acti-
vity.12,14 Based on the SAR studies for compounds 1,
design of the 3H-pyrimidin-4-one template 2 (Fig. 1)
was proposed as a series with a potentially improved
O
O
O
C
CN
OH
C
C
H
N
N
N
R2
R1
O
Cl
R3
B
N
B
R3
R1
N
H
N
A
A
N
H
A
O
O
O
NPS 2143
(IC50 = 0.043 µM)
1
NPS 53574
(IC50 = 3.5 µM)
2
(IC50 from 1.6 µM to 0.19 µM)
Figure 1.
Keywords: 3H-Pyrimidin-4-ones; Calcium receptor antagonists; Calcilytics; Osteoporosis.
*
Corresponding author. Tel.: +1 801 583 4939; fax: +1 801 583 4961; e-mail: ishcherbakova@npsp.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.03.054

2538
I. Shcherbakova et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2537–2540
pharmacological profile. Pharmacological relevance of
3H-pyrimidin-4-ones is well recognized; examples
include angiotensin II receptor antagonists,15 PPAR
agonists16 and the atypical antipsychotic drug
risperidone.17
this method. There are a limited number of routes to
synthesize 2,3,5,6-substituted pyrimidinones, and those
include (i) alkylation of 3H-pyrimidinones unsubstituted
at N-3,21,22 (ii) condensation of substituted amidines of
type 3 (Scheme 1) with substituted malonyl chlo-
rides,23,24 and (iii) dehydration of an enediamide.25
None of these approaches is general and each of them
is limited by the nature of substituents in the pyrimidi-
none ring, and/or by the formation of byproducts,
lengthy reaction time and low yields. Also, the target
compounds 2 (Fig. 1) should contain a bulky functional
group, the 2-(2-hydroxy-phenyl) substituent that limited
the application of known methods. In search of a gen-
eral approach to the target compounds 2, we developed
a novel synthesis of 2,3,5,6-substituted 3H-pyrimidin-4-
ones via the key intermediates, eneaminoamides 418
(Scheme 2). The subsequent acylation of compounds 4
with acetyl salicyl chloride produced the enediamides
5. We optimized the conditions of cyclization of the ene-
diamides 5, and the final pyrimidinones 2d–o were
formed in moderate to good yields (Scheme 1), whereas
the previously described procedure did not exceed a 12%
yield.25 Investigation on the scope of our pyrimidinone
synthesis methodology, including introduction of steri-
cally hindered substituents at N-3 and/or at position 5
of the heterocyclic ring, will be reported elsewhere.
While our patent on synthesis of 2,3,5,6-substituted
3H-pyrimidin-4-ones was pending,18 a practical synthe-
sis of 3-substituted 3H-pyrimidinones via intermediate
We now report the design, new synthesis, and initial
SAR studies for substituted 3H-pyrimidin-4-ones 2 that
led to the generation of novel series of potent CaR
antagonists.18,19 During exploration of the SAR of the
quinazolinone series 1,12 we found limited opportunity
to modify substituents in rings B and C (Fig. 1) and re-
tain or improve the activity profile. Equally important,
the quinazolinone compounds 1 had relatively poor solu-
bility (observed in handling but not quantitatively
measured). Therefore, we turned to B ring modifications
as we sought a means to optimize potency and solubi-
lity. Modifications in ring B were proposed by replacing
a p-function of the annelated benzene ring in com-
pounds 1 with one or two lipophilic fragments, the alkyl
substituents that would result in pyrimidinone com-
pounds 2 (Fig. 1). The synthesis of pyrimidinones 2a–c
unsubstituted at position 5 involved condensation of
the amidines 3a–c with 3-oxo-butyric acid methyl ester
similar to the procedure described by Tice and Bryman20
(Scheme 1).
However, 2-alkyl-3-oxo-butyric acid esters failed to pro-
duce 5,6-dialkyl substituted 3H-pyrimidin-4-ones 2 by
R3
O
O
O
NH2
NH
OMe
N
NH
N
O
a. AcCl/HCl
b. NaHCO3
R3
+
MeOH
R3
N
N
xylenes
4 h, reflux
22-35%
H
OH
MeOH, NaHCO3
4 h, rt
OH
OH
HO
2a-c
3a-c
Scheme 1. Compounds 2 and 3: (a) R3 = H; (b) R3 = 2-F; (c) R3 = 3-F.
R3
O
O
d
O
O
O
O
c
e
O
R1
O
O
O
b
a
O
O
O
R1
O
R1
OH
R1
N
H
R1
Cl
OEt
OEt
O
R2
R2
R2
R2
R2
R3
R3
g
NH2
O
O
OAc
f
R3
h
R1
N
H
R1
N
H
O
NH
O
R2
R2
R1
N
H
R2
4
O
N
5
R2
R1
R3
N
35-70%
HO
2d-o
Scheme 2. Reagents and conditions: (a) HOCH2CH2OH (1.05 equiv), p-TosOHÆH2O (cat), anhydrous toluene, reflux; (b) KOH, dioxane–H2O,
35 °C, 12 h; (c) oxalyl chloride (2.3 equiv) in CH2Cl2, 0 °C; 2 h, rt; (d) (R3)C6H4CH2CH2NH2, pyridine, 0 °C; rt, 12 h; (e) acetone–H2O, p-
TosOHÆH2O (cat), 95 °C, 3 h; (f) Et2O, NH3, AlCl3 (anhydrous, 1 equiv); (g) 2-AcOC6H4COCl (1 equiv), pyridine, reflux, 4 h; (h) EtOH–H2O, KOH
(1.5 equiv), reflux, 12 h. Compounds 2d–o: for R1, R2, and R3, see Table 1.

I. Shcherbakova et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2537–2540
2539
enamide esters, which are structurally similar to enedi-
amides 5, was published.26 This method, however, fails
to produce 3H-pyrimidin-4-ones with bulky alkyl sub-
stituents, such as isopropyl or tert-butyl, at N-3 and/or
at positions 5.
C-5 and C-6 was well tolerated but did not lead to a sig-
nificant potency enhancement (compounds 2l–o). Also,
in the pyrimidinone series 2, fluoro substitution in the
3-phenethyl fragment did not influence potency to the
same extent as in the quinazolinone series 1.12,14 Overall,
the pyrimidinone compounds 2d–o demonstrated a
noticeable potency improvement in comparison with
the parent series, quinazolinone compounds 114 (Fig.
1). In addition, the pyrimidinones 2a–o possessed good
aqueous solubility (>10 lmol/L) in the physiological
pH range. This might be due to the amphoteric nature
of the 2-(2-hydroxy-phenyl) substituted pyrimidinones
2 that involve contributions to aqueous solubility from
the ionizable OH group at pH >7, and from the proton-
ated basic N-1 atom at pH <7.
Similarly to quinazolinones 1,14 the pyrimidinone com-
pounds 2a–o were evaluated in vitro for their calcilytic
activity.27
The data in Table 1 summarize an initial SAR around
the 5,6-substitution in the compounds 2a–o. The pyri-
midinones 2a–c unsubstituted at position 5 were 10-fold
less potent than the parent quinazolinones 1.14
5,6-Dimethyl substituted compounds 2d,e showed mark-
edly increased activity in comparison with 5-unsubsti-
tuted pyrimidinones 2a–c. It is apparent that the alkyl
substituent at position 5 of the pyrimidinone ring in
compounds 2 is particularly important for calcilytic
activity in this series. Introduction of the trifluoromethyl
group at position 6 (compound 2f) resulted in reduced
potency. Thus, an additional electron-withdrawing effect
at position 6 at the pyrimidinone ring was not beneficial
for calcilytic activity. Starting with the increased po-
tency of 5-ethyl-6-methyl compounds 2g,h (Table 1),
the alkyl substitution at position 5 was arbitrarily se-
lected for exploration of structural modifications in the
pyrimidinone ring. It was found that introduction of a
propyl group at C-5 decreased potency slightly (com-
pound 2i) comparing to the 5-ethyl derivative 2g,
whereas an isopropyl group at C-5 and 3-fluoro substi-
tuent at 3-phenethyl fragment (compound 2j) offered
the most attractive package of potency and potential
metabolic stability of fragments other than the 2-(2-hydr-
oxy-phenyl) group. Cycloalkyl substitution involving
In vivo, the pyrimidinones 2 with IC50 values <0.2 lM
were more effective at lower doses than the quinazoli-
none calcilytics 112,14 at transiently increasing plasma
PTH levels following bolus iv injection in rats. Com-
pound 2h (1 or 3 lmol/kg) or vehicle was administered
by intravenous injection over about 15 s to normal con-
scious male Sprague–Dawley rats with chronic indwell-
ing arterial and venous catheters. Arterial blood
samples were collected immediately before, and at 1, 5,
10, and 30 min after the start of the injection for mea-
surement of the levels of PTH and Ca2+ in plasma.
PTH was measured using a specific rat PTH(1-84) ELI-
SA (Immutopics, San Clemente, CA). Injection of com-
pound 2h induced a rapid, but transient dose-related
increase in plasma PTH levels that were maximal at
1 min after the injection. Plasma PTH levels had re-
turned to pre-dose levels by 10 min (Fig. 2). Similarly
to in vivo effect of the quinazolinone calcilytics,14 there
were no changes in plasma Ca2+ levels, probably be-
cause PTH levels were elevated for such a short time
and/or because the study was only of 30 min duration.
In summary, the 3H-pyrimidin-4-ones represent a new
and highly potent class of calcium receptor antagonists
with an attractive pharmacological profile. Optimization
of this calcilytic series to orally bioavailable compounds
with balanced pharmacokinetic and pharmacodynamic
properties is in progress.
Table 1. In vitro calcilytic activity of substituted 3H-pyrimidin-4-ones
O
R2
N
R3
R1
N
HO
2a-o
R1
R2
R3
IC50, lM
Vehicle
3 µmol/kg
1 µmol/kg
Compds
600
400
200
0
a
b
c
d
e
f
Me
Me
Me
Me
Me
CF3
Me
Me
Me
Me
Me
H
H
1.9
1.6
H
2-F
3-F
H
H
Me
1.1
0.2
Me
Me
3-F
H
0.17
0.8
g
h
i
Et
Et
H
0.1
2-F
3-F
3-F
3-F
H
0.097
0.14
0.095
0.12
0.14
0.16
0.2
Pr
Pri
Cyclopropyl
j
k
l
0
10
20
30
–(CH2)4–
Time, min
m
n
o
–(CH2)4–
–(CH2)3–
–(CH2)3–
3-F
H
Figure 2. Effect of bolus iv injection of compound 2h on plasma PTH
levels in normal rats.
3-F
0.2

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16. Madhavan, G. R.; Chakrabarti, R.; Vikramadithyan, R.
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