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X. Gong et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4944–4947
methyl-1-piperazinylsulfonyl)phenyl]pyrimid-4(3H)-one.2
According to our previous study, modifying the sulfamide group
may improve the selectivity over PDE6 of PDE5 inhibitors using sil-
denafil as a starting template.6
We fixed the 5-bromo-6-isopropyl-2-(2-propoxyphenyl)pyrim-
idin-4(3H)-one and focused on exploration of the effect of substit-
uents at the 50-position of the phenyl ring to obtain both potent
and selective PDE5 inhibitors. The solved crystal structures of
PDE5 catalytic domain in complex with the inhibitors also indicate
that there might be a great tolerance for the substitution at this po-
sition.7–10 We therefore synthesized three types of inhibitors bear-
ing various series of substitutions and evaluated their bioactivities.
Compounds 6a–g were synthesized as shown in Scheme 3. The
key intermediate 4, which was prepared following the procedure
we previously reported,2 was treated with chlorosulfonic acid,
and then followed by the corresponding amine moiety, resulting
in the compounds 6a–g. The intermediate 7, prepared through bro-
mation of 4 in acetic acid at 60 °C,11 was coupled with vinyl n-butyl
ether by Pd(OAc)2-catalyzed coupling reaction in the presence of
bis(diphenylphosphino)butane and triethylamine, and then the re-
sulted products were treated with hydrochloric acid (10%) to ob-
tain 8.12 The solution of 8 and bromine in acetic acid was stirred
for 3 h at 30 °C, giving the intermediate 9.13 Compounds 10a and
10b were obtained by the reaction of 9 with N-methylpiperazine
or morpholine.14
The intermediate 4 was nitrated and hydrogenated to give 12.15
Compound 13a was obtained by the sulfonylation of 12 employing
methanesulfonyl chloride.11
The alcoholic solution of 12 was treated with triethylamine and
phenylisothiocyanate to give 13b.16 Compounds 13c and 13d were
formed through the glucosylation and mannosylation of the amine
group in 12, respectively (Scheme 4).17
The inhibition activities of those substituted compounds to
PDE5 were summarized in Table 2. Although the value of IC50 var-
ied in a wide range from 1.7 to 29.3 nM, there is at least one potent
inhibitor (IC50 <10 nM) for each type of substituents at the 50-posi-
tion of the phenyl ring. This result is consistent with the previous
suggestion that there is tolerance for such substitution. Two com-
pounds, 6e and 13a, were chosen to test the selectivity of PDE5
over PDE6, and compound 13a showed an excellent selectivity
(PDE6/5 = 941).
The crystal structures of the catalytic domain of PDE5 in com-
plex with compounds 2e and 10a were determined by soaking
the compounds into the apo crystals of the protein (Fig. 1). The pro-
tocol for solving the 3D structures of PDE5/2e and PDE5/10a com-
plexes was the same as that described in our previous
publications.2,5 The complete statistics, as well as the quality of
the two solved structures, are shown in Supplementary data (Sup-
plementary Table S1). The binding position of the two compounds
inside the catalytic domain of PDE5 is very similar to that of silde-
nafil as well as our previously reported PDE5 inhibitors. Briefly, the
protein–ligand interactions include the conserved bidentate
hydrogen bonds (H-bonds) between the pyrimidinone rings and
the side chain of Q817, a H-bond between the carbonyl oxygen
of the pyrimidinone ring and the side chain of Q775 via a water
molecule, a face-to-face
p–p stacking interactions between the
pyrimidinone ring and the phenyl ring of F820, and hydrophobic
interactions between the compounds and other residues nearby.
Moreover, a halogen bond between the bromo atom substituted
at the 5-position of the pyrimidinone ring and the hydroxyl oxygen
of the residue Y612 which simultaneously H-bonds to D764
through a water molecule is formed (Fig. 1C). For the compound
2e, its long alkyl chain substituted at the 6-position of the pyrimid-
inone ring fits into the cavity toward the hydrophobic residue L725
as shown in Figure 1A. Hydrophobic interactions are formed be-
tween the alkyl chain and residues such as L725. In the complex
Figure 1. Crystal structures of PDE5/2e and PDE5/10a complexes. (A, B) Molecular
surface representation of the binding pocket of compounds 2e (A) and 10a (B) in the
catalytic domain of PDE5. (C) The H-bonding and hydrophobic interactions between
the compound 10a and the residues of PDE5.
of PDE5/10a, there are no strong interaction observed between
the substituent at 50-position of the phenyl ring of 10a and the
Gong, Xudong
