Pyrazolopyridines with potent activity against herpesviruses: Effects of C5 substituents on antiviral activity

Article abstract of DOI:10.1016/j.bmcl.2007.11.120

Synthesis of a series of 5-substituted as well as 5,7-disubstituted 3-[2-(cyclopentylamino)-4-pyrimidinyl]-2-phenylpyrazolo[1,5-a]pyridin-7-amines with potent activity against herpes simplex viruses is described. Synthetic approaches allowing for variation of the substitution pattern are outlined and resulting changes in antiviral activity are highlighted. Several compounds with in vitro antiviral activity similar to or better than acyclovir are described.

Full text of DOI:10.1016/j.bmcl.2007.11.120

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1157–1161
Pyrazolopyridines with potent activity against herpesviruses:
Effects of C5 substituents on antiviral activity
Kristjan S. Gudmundsson,* Brian A. Johns and Scott H. Allen
Department of Medicinal Chemistry, Infectious Diseases Center of Excellence for Drug Discovery,
GlaxoSmithKline Research & Development, Five Moore Drive, Research Triangle Park, NC 27709-3398, USA
Received 26 September 2007; revised 28 November 2007; accepted 30 November 2007
Available online 4 December 2007
Abstract—Synthesis of a series of 5-substituted as well as 5,7-disubstituted 3-[2-(cyclopentylamino)-4-pyrimidinyl]-2-phenylpyrazolo
[1,5-a]pyridin-7-amines with potent activity against herpes simplex viruses is described. Synthetic approaches allowing for variation
of the substitution pattern are outlined and resulting changes in antiviral activity are highlighted. Several compounds with in vitro
antiviral activity similar to or better than acyclovir are described.
Ó 2007 Elsevier Ltd. All rights reserved.
The herpesvirus family contains eight known human
viruses, amongst them herpes simplex virus 1 (HSV-1)
and herpes simplex virus 2 (HSV-2).1 HSV-1 and
HSV-2 cause mucocutaneous infections, resulting in
cold sores (HSV-1) and genital lesions (HSV-2), respec-
tively. Much research has been focused on HSV-1 and
HSV-2 as these viruses have a high incidence rate
(ꢀ1.6 million new cases of HSV-2 predicted per year
in the US) and a high prevalence.2
limited number of potential new therapies compelled
our efforts to fully explore the pyrazolopyridine scaffold.
In previous reports, we have disclosed optimization of
substituents on the 2-phenyl moiety,11b optimization of
the C7 pyrazolopyridine substituent11d and 6,7-disusbsti-
tuted pyrazolopyridines11c as well as optimization of the
C3 heterocycle.11e Substitution of the remaining 4 and 5
positions on the pyrazolopyridine wase the next area of
our focus. Due to the tight SAR requirements of the C3
pyrimidine11e and the likelihood of significant interaction
with substituents at the 4 position, we elected to focus our
efforts on C5 and combinations with the C7 position.
Additionally, the resonance electronic similarity of the 5
position to that of the 7 position with respect to the core
heterocycle electron density made examination of the 5
position of significant interest. Herein, we describe the
synthesis and antiviral activity of 5-substituted pyrazolo-
pyridines along with 5,7-disubstituted pyrazolopyridines.
Previous antiviral research on herpes simplex viruses has
primarily focused on the development of nucleoside ana-
logs,3 such as acyclovir (Zovirax),4 valacyclovir,5 famci-
clovir,6 and penciclovir. Recently, immunomodulators
(imiquimod and resiquimod),7 nonnucleoside viral poly-
merase inhibitors (4-hydroxyquinoline-3-carboxam-
ides),8 and viral helicase inhibitors (thiazolylphenyl and
thiazolylamide)9 have received considerable attention.
Though numerous strategies and considerable effort
have been spent in the search for the next generation
antiherpetic therapy, it has proved difficult to outper-
form acyclovir.10
The 5-chloropyrazolopyridine 6 was prepared in three
steps from commercially available 4-chloro-2-picoline
7
1
HN
X
N
6
5
We have recently described a series of pyrazolo[1,5-a]pyr-
idines, such as 1, which show potent and selective inhibi-
tion of HSV-1 and HSV-2 (Fig. 1).11 The medical
significance of herpesvirus infection combined with the
2
N
N
N
N
F
3
4
1
N
N
N
H
N
N
H
Keywords: Pyrazolopyridine; HSV; Antiviral; Herpes viruses; SAR.
*
Corresponding author. Tel.: +1 919 483 6006; fax: +1 919 483
6053; e-mail: brian.a.johns@gsk.com
Fig. 1. Pyrazolopyridines.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.11.120

1158
K. S. Gudmundsson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1157–1161
(2) and ethyl 4-fluorobenzoate (3) as outlined in Scheme
F
F
CO2Et
Cl
N
Cl
1, using the same approach as we previously reported
for preparation of 7-chloropyrazolopyridine.11 Formy-
lation of 6 under Vilsmeier–Haack conditions gave an
excellent yield of aldehyde 7. This aldehyde was treated
with the commercially available ethynyl Grignard re-
agent at low temperature to give the propargyl alcohol.
This alcohol was easily oxidized to the ketone 8 using
MnO2. Treatment of the alkynyl ketone 8 with cyclo-
pentylguanidine resulted in formation of the desired 5-
chloro substituted pyrazolopyridine 9. Previously, we
have shown that a 7-chloro substituent on pyrazolopyri-
dine can be nucleophilically displaced by amines (e.g.,
formation of 1 from the corresponding 7-chloropyrazol-
opyridine). In the case of the 5-chloropyrazolopyridine 9
this was not the case. Several attempts at thermally dis-
placing the 5-chloro substituent of 9 with cyclopentyl-
amine at elevated temperature failed, but the 5
position could be coupled with cyclopentylamine using
Buchwald amination conditions (Pd(OAc)2, rac-BINAP,
Cs2CO3)12 to give 10. Thus, the 5-chloropyrazolopyri-
dine 9 behaves quite differently toward nucleophilic sub-
stitution than the analogous 7-chloropyrazolopyridine.
c, d
3
X
N
N
a
N
R
2
F
6
4 X = O
5 X = NOH
b
Cl
F
6 R = H
7 R = CHO
e
N
N
N
N
i
O
h
N
F
f, g
Cl
NH
9
Cl
8
N
j
N
H2N
N
H
H
N
N
N
N
N
N
F
F
N
NH2
H
N
10
11
N
H
N
N
H
Scheme 1. Reagents and conditions: (a) LiN (TMS)2, THF, 0–20 °C,
15 h, 99%; (b) hydroxylamine HCl, NaOH, MeOH, reflux, 2 h, 84%; (c)
(CF3CO)2O, Et3N, DME, 0 °C–rt; (d) FeCl2, 1,2-dimethoxyethane,
75 °C, 15 h, 57% for 2 steps; (e) POCl3 (1.5–2 equiv), DMF, rt, 12 h
(85%), (f) ethynyl magnesium bromide (2.4 equiv), THF, À78 °C to
0 °C, 2 h; (g) MnO2 (excess), CH2Cl2, 1 h (62%, 2 steps); (h) cyclopentyl
guanidine HCl (2 equiv), K2CO3 (2 equiv), DMF, 80 °C, 12 h (74%); (i)
Pd(OAc)2 (0.2 equiv), BINAP (0.3 equiv), Cs2CO3 (1.5 equiv), cyclo-
pentylamine (neat), 80 °C, 24 h (70%); (j) benzophenone imine
(3 equiv), Pd2dba3 (0.1 equiv), BINAP (0.3 equiv), NaOtBu (3 equiv),
toluene, 100 °C, 3 h; 4 N HCl, THF, 0 °C, 30 min (57% for 2 steps).
The 5-amine derivative 11 was prepared from 9 by treat-
ment with benzophenone imine, Pd2dba3, and BINAP in
the presence of base to give the imine that was subse-
quently hydrolyzed in aqueous acid to give 11.
The 5-cyclopentylamine derivative 10 showed similar
anti-HSV activity as the corresponding 7-cyclopentyla-
mino derivative 1, while the less lipophilic amine 11
Table 1. HSV-1 antiviral activity and cytotoxicity of C5-substituted pyrazolopyridine analogs
7
N
N
R1
R2
5
N
N
N
H
a
b
Compound
Subst. R1
Subst. R2
5-Cl
5-c-PentylNH
IC50 (lM)
CC50 (lM)
9
F
>5
>40
18
10
11
12
13
14
15
16
17
18
19
20
F
0.53
5.07
1.35
0.83
2.73
3.63
4.27
0.21
0.92
0.33
0.56
F
5-NH2
20
F
5-(1-Pyrrolidinyl)
5-CH3O(CH2)2NH
5-i-PropylNH
5-Morpholino
5-Cl
>40
>40
>40
34
F
F
F
CH3O
>40
12.3
>40
>40
27
CH3O
CH3O
5-c-PentylNH
5-i-PropylNH
5-c-PentylNH
5-c-PentylNH
c-PropylCH2O
i-PropylCH2O
1c
F
CH3O
c-PropylCH2O
7-c-PentylNH
7-c-PentylNH
7-c-PentylNH
0.26
0.22
0.17
0.39
>160
>100
>40
21c
22c
ACV
Acyclovir
>200
a Vero cells, HSV-1, SC-16 strain. IC50 is the concentration at which 50% efficacy in the antiviral assay is observed using a capture hydrid method.11b
b CC50 is the concentration at which 50% cytotoxicity is observed.
c C-7 analogs included for comparison were prepared as described in Ref. 11b.

K. S. Gudmundsson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1157–1161
1159
X
Cl
N
N
N
N
N
N
N
N
N
N
N
N
N
F
N
N
F
F
F
a
Cl
N
N
Cl
a
H
H
9
10
23
N
N
H
N
H
N
H
N
H
37 X = Cl
38 X = SMe
39 X = SEt
40 X = SiPr
HN
24 Y = Cl
25 Y=c-PentylNH-
N
N
b or c
N
b
F
41 X=OEt
Y
N
Scheme 3. Reagents and conditions: (a) for 37: n-BuLi (5 equiv), THF,
À78 °C, then CCl4, (25%); for 38: n-BuLi (5 equiv), THF, À78 °C, then
MeSSMe (40%); (c) for 39: n-BuLi (5 equiv), THF, À78 °C, then
EtSSEt (70%); for 40: n-BuLi (5 equiv), THF, À78 °C, then i-PrSSiPr
(72%); (b) compound 39, m-CPBA (4 equiv), CH2Cl2, rt, 30 min. Then
NaOEt, EtOH, reflux, 2 h (40% for 2 steps).
N
H
Scheme 2. Reagents and conditions: (a) n-BuLi, THF, À78 °C, then
CCl4, (45%); (b) cyclopentylamine (neat), Pd(OAc)2, rac-BINAP,
Cs2CO3, 80 °C, 24 h (68% of 24); (c) cyclopentylamine (neat),
Pd(OAc)2, rac-BINAP, Cs2CO3, 130 °C, sealed tube, 24 h (37% of 25).
was less potent (Table 1). The potent anti-HSV activity
of 10 prompted us to synthesize a number of additional
5-substituted pyrazolopyridines (12–20) containing
either fluorine (12–15) or alkoxysubstituted (16–20) 2-
phenyl moiety using the synthesis outlined in Scheme
1. Anti-HSV activity for 12–20, along with activity for
Table 2. HSV-1 antiviral activity and cytotoxicity of disubstituted pyrazolopyridine analogs
R3
N
N
R1
R2
N
N
N
Subst. R1
Subst. (C5) R2
Subst. (C7) R3
IC50 (lM)
CC50 (lM)
a
b
Compound
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
50
52
54
55
56
ACV
F
Cl
Cl
Cl
7
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
20.6
32.5
>40
>40
>40
>40
>36
>40
>40
>40
>40
>200
F
c-PentylNH
c-PentylNH
c-PropylNH
i-PropylNH
i-PropylNH
Morpholine
Piperidine
Me2NH
Cl
0.33
0.24
0.33
1.5
F
c-PentylNH
Cl
Cl
F
F
F
i-PropylNH
Cl
Cl
0.55
0.18
0.15
0.38
5.3
F
F
F
Cl
Cl
OMe
OMe
OMe
OR4
OR4
F
Cl
Cl
c-PentylNH
c-PropylNH
c-PentylNH
c-PropylNH
Cl
0.22
0.34
0.43
0.42
1.36
0.17
0.07
0.37
0.16
0.08
0.22
0.04
0.21
1.8
Cl
Cl
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
MeO
F
MeS
EtS
F
F
i-PrS
EtO
F
F
MeO(CH2)2O
MeS
EtS
OMe
OMe
OMe
OMe
F
i-PrS
PhS
H
>36
0.13
0.32
0.07
0.17
0.39
F
MeO
MeO(CH2)2O
i-PrO
c-PentylNH
c-PentylNH
c-PentylNH
c-PentylNH
F
F
F
c-PropylCH2O
Acyclovir
a Vero cells, HSV-1, SC-16 strain. IC50 is the concentration at which 50% efficacy in the antiviral assay is observed using a capture hydrid method.11b
b CC50 is the concentration at which 50% cytotoxicity is observed.

1160
K. S. Gudmundsson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1157–1161
selected 7-substituted pyrazolopyridines11b (1, 21–22)
for comparison, is shown in Table 1. As can be deter-
mined from Table 1 the 5- or 7-substituted pyrazolopyri-
dines show similar potency. In general the C5 amines are
significantly more potent than the C5 chloro com-
pounds. Also the more lipophilic amines (e.g., cyclopen-
tylamine) are in general more potent than the smaller,
less lipophilic amines.
tained by m-CPBA oxidation of 39 to the sulfoxide, fol-
lowed by treatment with an alkoxide. Compounds 41
and 42 are better suited for development than the thiol
derivatives, which are potentially liable to in vivo oxida-
tion and thus risk of formation of a reactive metabolite.
Compounds 43–46 (Table 2) were prepared similarly.
Since the 5-amino derivatives (like 10) were displaying
increased activity over the chloro analogs (like 9), it be-
came of interest to look into ether substituents at the C5
position. Since transition metal catalyzed couplings of
alcohols with aryl halides are significantly less developed
than the amine coupling reaction, we sought an alterna-
tive synthetic route to access the 5-alkoxy derivatives.
Toward that end, the N-amino-4-methoxypyridine 47
was treated with diaryl acetylene 48 to give the 5-meth-
oxypyrazolopyridine 49 as outlined in Scheme 4. The
2-cyclopentyl amine substituent on the pyrimidine was
installed through oxidation of 49 to the sulfoxide and
subjected to thermal displacement to provide the desired
scaffold 50 as the 7-protio derivative. Lithiation of the 7
position as described above and sulfenylation proceeded
smoothly to provide a handle for further functionaliza-
tion. Oxidation and amine displacement again served
to provide the 5,7-disubstituted analog 52. Standard
demethylation with BBr3 gave the phenol and realkyla-
tion with assorted alkyl halides resulted in the 5-alkoxy
analogs 54–56. The above SAR study further delineates
the SAR of the pyrazolopyridine scaffold. Several mole-
cules with significantly (5- to 10-fold) better potency
than the current gold standard acyclovir were identified,
thus establishing the disubstituted pyrazolopyridines as
an exciting template for anti-HSV drug discovery.
Potent activity of both 7-substituted as well as 5-substi-
tuted pyrazolopyridines prompted us to consider mak-
ing 5,7-disubstituted pyrazolopyridines. As outlined in
Scheme 2 the C7 protio derivative 9 could be treated
with a strong base to selectively deprotonate the 7 posi-
tion.13 This could be further functionalized by trapping
the resulting anion with an electrophile, such as CCl4, to
give the 5,7-dichloropyrazolopyridine 23.
Use of n-BuLi for deprotonation did not seem to result in
significant C5 chloro exchange (the C5 protio derivative
was not isolated from the reaction mixture) nor did it ap-
pear to direct the deprotonation to the 4 or 6 positions.
Buchwald coupling conditions were then used to cleanly
replace the C7 chlorine (to give 24) without significant
reaction of the C5 chlorine as long as the reaction was
carried out under 100 °C. At higher temperatures both
the C5 and C7 chlorines could be replaced with alkyl-
amines to give 25. Compounds 26–36 were made in a
similar fashion as outlined in Scheme 2.
Additional 5,7-disubstituted derivatives were synthe-
sized from 10 as outlined in Scheme 3. Compound 10
was treated with excess n-BuLi followed by quenching
with a suitable electrophile. Especially potent were com-
pounds 38–40, obtained by quenching lithiated 10 with
alkyl disulfides. 7-Alkoxy derivatives 41 and 42 were ob-
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F
NH2
N+
N
N
N
F
a
OMe
48
N
N
OMe
47
49
N
SMe
SMe
X
N
N
N
F
b,c
Y
N
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51 X = SMe,
52 X = -NHcPentyl Y = -OMe
53 X = -NHcPentyl Y = -OH
54 X = -NHcPentyl Y = -OCH2CH2OMe
Y = -OMe
Y = -OMe
d
e,f
g
h
55 X = -NHcPentyl Y = -OiPr
56 X = -NHcPentyl Y = -OCH2cPr
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CPBA, CH2Cl2 0 °C; (c) cyclopentylamine (neat), 85 °C, 1 h (69%); (d)
LDA, THF À78 °C, then MeSSMe (91%); (e) m-CPBA, CH2Cl2 0 °C
(99%); (f) cyclopentylamine (neat), 130–145 °C, 24 h (65%); (g) BBr3,
CH2Cl2 À78 °C to rt, 3 d (66%); (h) Cs2CO3, MeCN, alkyl halide; for
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c-PrCH2Br (26%).

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