Synthesis and evaluation of furo[3,4-d]pyrimidinones as selective α(1a)-adrenergic receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(99)00653-8

Furo[3,4-d]pyrimidinones were found to be metabolites of dihydropyrimidinones such as 1a-b that are subtype-selective antagonists of the α(1a)-adrenergic receptor. A versatile synthesis that provides access to furo[3,4-d]pyrimidinones in high yield and in enantiomerically pure forms is described along with structure-activity relationships in the series.

Full text of DOI:10.1016/S0960-894X(99)00653-8

Bioorganic & Medicinal Chemistry Letters 10 (2000) 175±178
Synthesis and Evaluation of Furo[3,4-d]pyrimidinones as Selective
ꢀ1a-Adrenergic Receptor Antagonists
Bharat Lagu, a,* Dake Tian, a George Chiu, a Dhanapalan Nagarathnam, a
James Fang, a Quanrong Shen, a Carlos Forray, b Richard W. Ransom, c
Raymond S. L. Chang, c Kamlesh P. Vyas, d Kanyin Zhang d
and Charles Gluchowski a
aDepartment of Chemistry, Synaptic Pharmaceutical Corporation, 215 College Road, Paramus, NJ 07652, USA
bDepartment of Pharmacology, Synaptic Pharmaceutical Corporation, 215 College Road, Paramus, NJ 07652, USA
cDepartment of Pharmacology, Merck & Co., Inc., West Point, PA 19486, USA
dDepartment of Drug Metabolism, Merck & Co., Inc., West Point, PA 19486, USA
Received 2 September 1999; accepted 19 November 1999
AbstractÐFuro[3,4-d]pyrimidinones were found to be metabolites of dihydropyrimidinones such as 1a±b that are subtype-selective
antagonists of the a1a-adrenergic receptor. A versatile synthesis that provides access to furo[3,4-d]pyrimidinones in high yield and in
enantiomerically pure forms is described along with structure±activity relationships in the series. # 2000 Elsevier Science Ltd. All
rights reserved.
Alpha-1-adrenergic receptor antagonists are used for
the treatment of benign prostatic hyperplasia (BPH), a
urological disorder prevalent in the elderly male popu-
lation that results in the obstruction of urine ¯ow.1,2 We
have recently reported that dihydropyrimidinones such
as 1a±b are a1a selective antagonists with a potential for
the treatment of BPH with fewer side e€ects than the
nonselective a1 adrenergic antagonists.3 These com-
pounds exhibit good binding anity (<1 nM) and
excellent selectivity (>300-fold) for the a1a receptor
over a1b and a1d subtypes. The in vitro metabolism stu-
dies4 revealed that the dihydropyrimidinones 1a±b give
rise to furo[3,4-d]pyrimidinone (2) as one of the metabo-
lites, presumably via intermediate 1c. A synthetic route
was needed to con®rm the structural identity of 2 and to
determine whether it contributes to the in vivo activity of
the parent compounds as an active metabolite. The syn-
thesis and structure±activity relationships (SAR) of furo-
[3,4-d]pyrimidinones such as 2 are presented in this letter.
to obtain benzylidene 4 which was then reacted with O-
methylisourea in presence of NaHCO3 to obtain dihy-
dropyrimidine 5 in good yield.5 An analogous con-
densation of tetronic acid (7) with aromatic aldehydes,
however, required strongly acidic conditions (conc HCl
as solvent).6 Furthermore, benzylidene 8 failed to give
the desired furo[3,4-d]pyrimidinone under conditions
used to obtain 5 (Scheme 1, eq (2)).
An alternate synthesis of furo[3,4-d]pyrimidinones 9,
has been reported, albeit in low yields (4±18%).7 We
therefore developed a novel synthetic route for the
synthesis of the target compounds in high yields by
modi®cation of a previously reported synthesis (Scheme
2).8,9 Thus intermediate 6 was obtained from the dihy-
dropyrimidine 5 (as a racemic mixture or pure enantio-
mers) by known procedures3,5 and was treated with
bromine in CHCl3 at 0 ꢀC to obtain bromide 10 in good
yield. Compound 10 was heated (neat) in an oil bath at
130 ꢀC for 5 h, cooled and washed with chloroform to
obtain the furo[3,4-d]pyrimidinone 11 in quantitative
yield.10 Intermediate 11, without puri®cation, was treated
with several primary amines to yield the desired com-
pounds represented by general structure 12.
The initial attempts to synthesize furo[3,4-d]pyr-
imidinones were modeled on the route used for the
synthesis of 1 (Scheme 1, eq. (1)), where an appropriate
b-ketoester was condensed with an aromatic aldehyde
Some noteworthy features of this route include: (1) the
presence of p-nitrophenyloxycarbonyl group on the N-1
*Corresponding author. Tel.: +1-201-261-1331 ext. 644; fax: +1-201-
261-0623.
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(99)00653-8

176
B. Lagu et al. / Bioorg. Med. Chem. Lett. 10 (2000) 175±178
Scheme 1. (a) ArCHO, PhH, >90%; (b) O-methylisourea, NaHCO3, DMF, 60±70%; (c) p-nitrophenylchloroformate, CH2Cl2, DMAP, 90%; (d)
ArCHO, concd HCl, 40±58%.
Scheme 2. Br2, CHCl3; (b) heat, 130 ꢀC; (c) RNH2, THF or CH2Cl2, 60±80% yield from 6.
position enables the introduction of a variety of side
chains from a common intermediate, (2) the route pro-
vides access to enantiomerically pure furo[3,4-d]pyr-
imidinones, (3) the synthetic sequence is concise, and (4)
the compounds are obtained in high yield.
and 18 with the desired binding and selectivity pro®les.
The structure±activity relationship for furo[3,4-d]pyr-
imidinones parallels that observed for compounds such
as 1a±b as previously described.3 Compound 16, which
contains a 3,4-di¯uorophenyl substituent, shows a par-
ticularly noteworthy binding and selectivity pro®le and
as such was further characterized.
The generality of the synthetic route is highlighted by
the preparation of compounds 13±22 shown in Tables 1
and 2. A number of compounds were screened against
recombinant human a1 receptors in binding assays using
[3H]-prazosin as the radioligand and the results are
shown in Tables 1 and 2. Compound 13, with an elec-
tron-donating substituent on the C-6 phenyl ring, shows
lower binding anity and selectivity while compounds 14
and 15 containing electron-withdrawing substituents
exhibit good anity and high selectivity for the recombi-
nant a1a receptor. The three-carbon tether is the optimal
length for the linker resulting in compounds 16, 17,
Both enantiomers of compound 16 were synthesized
starting from the pure enantiomers of compound 5.3
The (+)-enantiomer showed better binding anity and
selectivity for the cloned human a1a receptor than the
( )-enantiomer or the racemate.11 Encouraged by the
binding anity of (+)-16, we decided to explore the
SAR of analogues of (+)-16, in which the 4-methoxy-
carbonyl-4-phenyl piperidine moiety was replaced with
the preferred substituted piperidines12 or piperazines13
as shown in Table 2.

B. Lagu et al. / Bioorg. Med. Chem. Lett. 10 (2000) 175±178
Table 1. Structure±activity relationship for furo[3,4-d]pyrimidinones
177
prostate tissue (Kb=0.28 nM) compared to rat aorta
(Kb=540 nM). The selectivity observed in the functional
assay is in good agreement with the selectivity for the
cloned human a1a receptor over a1d receptor in the
binding assays and can be regarded as a potential mea-
sure of uroselectivity. In the anesthetized rats, (+)-16
potently antagonized A-6160316 induced contraction of
the rat prostate (AD50=67 mg/kg) over a 2 h period.
These results suggest that compounds such as (+)-16
may not only be active metabolites, but may also be an
intrinsically interesting series of a1 antagonists in their
own right. Compound (+)-16 shows a short plasma
half-life (ꢁ1.5 h) in rats and dogs presumably due to the
other metabolic pathways such as N-dealkylation as
observed for compounds that are structurally similar to
1a±b.3,12
Ki (nM)a,b
Compd.
Ar
n
a1a
a1b
a1d
13
14
15
16
17
18
Piperonyl
4-NO2-Ph
1
1
1
1
0
2
1
1
35
390
130
300
330
1200
340
500
250
1900
420
340
430
1300
420
230
130
1.1
0.9
0.9
62
15
0.4
1.6
3,4-Benzofurazan
3,4-di-F-Ph
3,4-di-F-Ph
3,4-di-F-Ph
3,4-di-F-Ph
3,4-di-F-Ph
(+)-16
)-16
(
In summary, a versatile, high yielding synthesis of
furo[3,4-d]pyrimidinones has been developed that pro-
vides access to the desired compounds in enantiomeri-
cally pure forms. Select compounds show good binding
anity and selectivity for the a1a receptor. Compounds
such as (+)-16, show good uroselectivity in the func-
tional assays as well and therefore, may be useful as a1a
selective antagonists for treatment of BPH.
aKi values obtained by displacement of [3H]-prazosin from recombi-
nant human receptors.
bAll Ki values are‹5% S.E. or less for n>2. In cases where n=2, both
Ki values are within 2-fold of each other and the values shown are the
average of the two experiments.
Table 2. Side chain modi®cations
Acknowledgements
We thank Mr. Yong Zheng for the technical assistance
in cell culture and membrane preparation and Mr.
Boshan Li and Mr. Vincent Jorgensen for the radio-
ligand displacement assays.
Ki (nM)a
References and Notes
Compound
R
a1a
a1b
a1d
(+)-16
(+)-19
20
(+)-21
(+)-22
CO2Me
CN
Ph
CONH2
NO2
0.4
5.8
9.1
2.3
0.6
500
970
120
350
210
230
1100
130
690
180
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4. The in vitro metabolism was studied using suspensions of
freshly isolated hepatocytes and microsomal preparations
from rat, dog and humans. The compounds were incubated
with liver microsomal protein and NADPH for 60 min. Fol-
lowing centrifugation, the supernatant was dried, recon-
stituted with 30% acetonitrile and analyzed by electrospray
LC±MS/MS.
aPlease see footnote for Table 1.
Substitution of the ester functionality with a cyano
group gave compound (+)-19 that maintained the
selectivity against the other a1 subtypes (>166-fold),
but showed lower binding anity for the a1a receptor
(5.8 nM cf. 0.4 nM for (+)-16). Replacement of the
methoxycarbonyl group by a phenyl substituent gave
compound 20, which showed a substantial decrease in
binding anity as well as selectivity for the a1a receptor.
Compounds (+)-21 and (+)-22 containing substituted
phenyl-piperazine moieties13 in place of the piperidine
moieties exhibit comparable binding anities (2.3 and
0.6 nM, respectively) for the a1a receptor with good
subtype-selectivity (>100-fold).
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Compound (+)-16 was evaluated in a number of in
vitro and in vivo assays.14 Compound (+)-16 shows
greater than 1500-fold selectivity over a2 adrenoceptors
and the rat L-type calcium channel.15 In the functional
studies, (+)-16 shows good potency and selectivity to
antagonize phenylephrine induced contraction of rat

178
B. Lagu et al. / Bioorg. Med. Chem. Lett. 10 (2000) 175±178
10. Furo[3,4-d]pyrimidinone 11 (Ar=3,4-di¯uorophenyl): 1H
NMR (DMSO-d6) d 4.94 (br s, 2 H), 6.08 (s, 1 H), 7.20±7.43
(m, 5 H), 8.35 (d, J=10.2 Hz, 2 H).
Bendesky, R.; Harrell, C. M.; Vyas, K.; Zhang, K.; Gilbert, J.;
Pettibone, D. J.; Kling, P.; Patane, M. A.; Bock, M. G.; Frei-
dinger, R. M.; Gluchowski, C. J. Med. Chem 1999, 42, 4778.
13. Lagu, B.; Tian, D.; Nagarathnam, D.; Marzabadi, M. R.;
Wong, W. C.; Miao, S. W.; Zhang, F.; Sun, W.; Chiu, G.;
Fang, J.; Zhang, J.; Forray, C.; Chang, R. S. L.; Ransom, R.
W.; Chen, T. B.; O'Malley, S.; Kling, P.; Zhang, K.; Vyas, K.;
Gluchowski, C. J. Med. Chem. 1999, 42, 4794.
14. For a detailed description of protocols for binding and
functional assays, see ref 3.
15. Most of the furo[3,4-d]pyrimidinones described in ref 7 are
Ca2+ channel modulators.
1
11. Compound (+)-16: H NMR (CDCl3) d 1.62±1.73 (m, 2
H), 1.91±1.98 (m, 2 H), 2.17±2.36 (m, 2 H), 2.40 (br t, J=6.6
Hz, 2 H), 2.52±2.57 (m, 2 H), 2.76±2.93 (m, 2 H), 3.20±3.40
(m, 2 H), 3.61 (s, 3 H), 4.69 (s, 2 H), 6.30 (br s, 1 H), 6.38 (s, 1
H), 7.04±7.35 (m, 8 H), 9.09 (br t, 1 H). HCl salt. m.p. 142±
145 ꢀC; [a]d +128ꢀ (c 0.52, CHCl3). Anal. calcd for C29H31
N4O6F2Cl.0.23 CHCl3: C, 55.55; H, 4.98; N, 8.87. Found: C,
55.25; H, 5.03; N, 8.52.
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Lagu, B.; Wong, W. C.; Chiu, G.; Tyagarajan, S.; Miao, S.
W.; Zhang, F.; Sun, W.; Tian, D.; Shen, Q.; Zhang, J.; Wetzel,
J. M.; Forray, C.; Chang, R. S. L.; Broten, T. P.; Schorn, T.
W.; Chen, T. B.; O'Malley, S.; Ransom, R. W.; Schneck, K.;
16. Knepper, S. M.; Buckner, S. A.; Brune, M. E.; DeBernar-
dis, J. F.; Meyer, M. D.; Hancock, A. A. J. Pharmacol. Exp.
Ther. 1995, 274, 97.