Synthesis of novel HIV protease inhibitors (PI) with activity against PI-resistant virus

Article abstract of DOI:10.1016/j.bmcl.2007.07.040

A series of HIV protease inhibitors with modifications on the P3 position have been designed and synthesized. These compounds exhibit excellent antiviral activity against both the wild type enzyme and PI-resistant clinical viral isolates. The synthesis an

Full text of DOI:10.1016/j.bmcl.2007.07.040

Bioorganic & Medicinal Chemistry Letters 17 (2007) 5432–5436
Synthesis of novel HIV protease inhibitors (PI) with activity
against PI-resistant virus
Subharekha Raghavan,a,* Zhijian Lu,a Teresa Beeson,a Kevin T. Chapman,a
William A. Schleif,b David B. Olsen,b Mark Stahlhut,b Carrie A. Rutkowski,b
Lori Gabryelski,b Emilio Eminib and James R. Tataa
aDepartment of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
bDepartment of Antiviral Research, Merck Research Laboratories, West Point, PA 19486, USA
Received 15 June 2007; revised 10 July 2007; accepted 10 July 2007
Available online 31 July 2007
Abstract—A series of HIV protease inhibitors with modifications on the P3 position have been designed and synthesized. These
compounds exhibit excellent antiviral activity against both the wild type enzyme and PI-resistant clinical viral isolates. The synthesis
and biological activity of the compounds are described.
Ó 2007 Elsevier Ltd. All rights reserved.
Human immunodeficiency virus (HIV) protease cleaves
the gag and gag-pol polyproteins required by the infec-
tious virus to mature. Inhibition of HIV protease results
in immature virons that are incapable of replication.1
Protease inhibitors (PIs) used in combination with
reverse transcriptase inhibitors form the basis of
HAART therapy that has led to a significant reduction
in HIV-related morbidity. However, despite their clini-
cal success, prolonged use of PIs has resulted in emer-
gence of PI-resistant virus.2 Thus, there remains a
need to identify and develop next generation of drugs
that are effective against a broad panel of PI-resistant
mutant viruses.
constant, identified the 2,6-dimethyl benzyl group to be
optimal for antiviral activity. In order to explore the
SAR of the P3 portion of the molecule, we undertook
a study to find a surrogate for the thiazolidine ring
moiety. In this communication, we report that the thiaz-
olidine ring at P3 can be replaced with a spiro oxazoli-
dine moiety (Fig. 1).
The compounds described herein were prepared using
two different synthetic routes (Schemes 1 and 2). The
synthesis of the compounds of the structure 10 is out-
lined in Scheme 1. The epoxide 34 was treated with
NaH and p-methoxy benzyl alcohol to give the mono-
In a recent paper, we described a hybrid analog that
incorporated P10P30 portion of indinavir and the
P1P2P3 of JE-2147 that resulted in a new class of potent
HIV protease inhibitors exemplified by (1).3 This novel
class of compounds exhibited excellent antiviral activity
against wild type enzyme (NL4-3) in both the enzyme
inhibition assay (IC50), and the viral spread assay
(CIC95). In addition, this series also showed good antiviral
activity against a panel of HIV-resistant mutants.
Extensive SAR conducted around the P2 portion of
the molecule, while keeping the P10, P30, and P3 groups
HIV-1 protease inhibition Viral Spread Assay
IC50(nM)
CIC95 (nM)
NL4-3
NL4-3
0.60
0.03
45
15
Indinavir
P1'
P3
S
N
OH
OH
H
N
1
O
O
O
O
NH
P3'
P2
Keywords: HIV; Protease inhibitors.
*
Corresponding author. Tel.: +1 732 594 7153; fax: +1 732 594
9473; e-mail: subha_raghavan@merck.com
Figure 1. Hybrid of indinavir and JE-2147.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.07.040

S. Raghavan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5432–5436
5433
OBz
O
O
OH
O
OBz
O
O
HO
N
N
e
PMBO
N
HO
N
b,c
a
d
O
O
O
O
O
O
O
O
O
6
3
4
5
O
N
H
HN
OBz
O
OH
OH
H
OBz
O
N
N
N
N
8
Cl
N
g
O
O
O
O
O
O
O
NH
O
O
O
NH
7
9
10
Scheme 1. Reagents and conditions: (a) 4MeOC6H5CH2OH, NaH, THF reflux, 60%; (b) BzCl, py., 84%; (c) DDQ, THF-DCM, 78%; (d) PDC,
DMF, 70%; (e) (COCl)2, C6H6, DMF (cat.); (f) amine 8, TEA, DCM; (g) HCl, dioxane; 1 N NaOH, dioxane.
O
OTBS
O
O
N
OH
NH
HOOC
O
N
Boc
a, b
N
d,e
15
O
O
O
HN
O
O
c
O
O
HN
HO
HN
14
17
13
H2N
16
OH
OH
OH
H
O
OTBS
OH
N
N
H
N
N
18
g
O
O
O
HN
O
O
O
O
HN
f
O
19
20
Scheme 2. Reagents and conditions: (a) pyBrop, 2,6-dimethyl benzyl amine (91%); (b) TFA, DCM (100%); (c) pyBOP, HOAt, DIEA, 15, DCM, rt
(81%); (d) 1 N LiOH, H2O/THF, rt; (e) TBSOTf, DIEA, EtOAc, rt; H2O/THF; (f) 4-amino-3-chromanol, pyBOP, HOAt, DIEA, 59%; (g) TBAF,
THF, rt, 3 h, 75%.
protected diol 4. The secondary alcohol was protected as
the benzoate ester using BzCl/pyridine. Next, the PMB
ether was de-protected under oxidative conditions with
DDQ to give the primary alcohol 5, which was oxidized
to the acid 6 with PDC in a 60% yield. The acid 6 was
converted to the acid chloride 7 under standard condi-
tions using oxalyl chloride. Commercially available
Boc-protected L-proline was coupled to 2,6-dimethyl
benzyl amine using bromo-tris-pyrrolidino-phospho-
nium hexafluoro phosphate (pyBrop) and DIEA in
90% yield. Removal of the Boc group with TFA yielded
8, which upon treatment with the acid chloride 7 in
DCM gave the desired amide 9 in 50% yield. Removal
of the acetal group using HCl/dioxane followed by
saponification of the benzoate ester gave the final prod-
uct 10.
of viral infection in MT4 human T-lymphoid cells in-
fected with the NL4-3 isolate (CIC95).5b A subset of
compounds were also tested against highly PI-resistant
viral constructs that were engineered from viral isolates
of patients infected with multiple PI-resistant HIV.6
We first examined the effect of ring size on activity.
Accordingly, four-, five-, and six-membered-ring ana-
logs were synthesized (Table 1). Proline analog 10 was
the most active compound with an IC50 = 14 nM com-
pared to either 11 or 12. However, compared to the
thiazolidine analog 1, it was significantly less potent.
This result suggested to us, the importance of the gem-
dimethyl groups present on the thiazolidine ring for
activity.
We reasoned that the gem-dimethyl group could impart
a conformational constraint that helps orient the entire
benzyl amide group at the P2 position in the enzyme
binding pocket. In addition, it was felt that the gem-di-
All compounds were tested for their ability to inhibit the
wild type enzyme (NL4-3, IC50).5a In addition, the com-
pounds were tested for their ability to inhibit the spread

5434
S. Raghavan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5432–5436
Table 1. Antiviral activity as a function of ring size
n
Enzyme inhibition
IC50 (nM) NL4-3
Viral spread assay
CIC95 (nM) NL4-3
Indinavir
0.6
45
OH
OH
(n)
H
0, 11
1, 10
2, 12
90
14
42
1000
1000
1000
N
O
N
O
O
O
N
H
methyl group might have some positive van der Waals
interactions, accounting for the high affinity to the en-
zyme. To study this further the corresponding carbon
(20) and oxygen analogs (24) of the thiazolidine were
synthesized.
3-hydroxy-3-methyl-butanoic acid (b-hydroxy-L-valine)
21 upon treatment with aqueous formaldehyde gave
the desired oxazolidine.10 The nitrogen was protected
with Boc group to give 22. This was elaborated using
the chemistry described in Scheme 2 to 24.
Scheme 2 outlines the synthesis of these compounds. (2S)-
3,3-Dimethyl-N-(Boc)proline 13 was prepared using a lit-
erature procedure.7 The carboxyl group was coupled to
2,6-dimethyl benzyl amine with pyBrop/DIEA in 91%
yield (Scheme 2). After removal of the Boc group with
TFA, 14 was coupled to the acid 153,8 using pyBOP to give
the desired amide 16 in 81% yield. Hydrolysis of the lac-
tone 16 with aqueous 1 N LiOH in dioxane followed by
protection of the alcohol as the tert-butyl dimethyl silyl
ether with TBSOTf/DIEA gave the desired acid 17. This
was coupled with cis amino chromanol9 18 to afford 19.
Removal of the TBS group with 1 M TBAF solution pro-
vided the final product 20 in 75% yield.
In vitro antiviral activity for the compounds is summa-
rized in Table 2. As anticipated, we observed a 100-fold
improvement in activity for 20 compared to 10 for the
wild type enzyme. More significant was the superior
Boc
Boc
TMSO
N
Boc
N
TMSO
N
O
c
a, b
O
MeOOC
O
27
NH2
OH
26
25
HO
Boc
f
d,e
i,j
HO
g,h
HO
NHBoc
OH
N
O
O
30
28
29
O
k,l
O
Synthesis of the oxazolidine intermediate 23 is outlined
in Scheme 3. Commercially available (S)-(ꢀ)-2-amino-
NBoc
NH
HOOC
31
HN
O
32
O
OH
O
NH
O
c,d
a,b
N Boc
O
HOOC
NH2
Scheme 4. Reagents and conditions: (a) AllylMgBr, THF, ꢀ20 °C to
rt, 75%; (b) TMSOTf, Et3N, DCM, 55%; (c) Schrock’s catalyst, C6H6,
reflux 95%; (d) HCOONH4, HCOOH, EtOH, Pd/C, rt, 87%; (e) 1 M
TBAF, THF, 83%; (f) TsOH, MeOH rt, 100%; (g) Jones oxidation,
ꢀ15 °C to rt, 74%; (h) HCl, 1,4-dioxane; (i) HCHO (37% aq), 2 N
NaOH; (j) Boc2O, NaOH 48% over three steps; (k) pyBrop, DIEA,
2,6-dimethyl benzyl amine, 69%, (l) CH3SO3H, DCM, 78%.
HN
HO
21
22
23
Scheme 3. Reagents and conditions: (a) HCHO (37% aqueous), 2N
NaOH, 24 h, rt; (b) Boc2O; 70% over 2 steps; (c) 2,6-dimethyl benzyl
amine, pyBrop, DIEA, 67%; (d) CH3SO3H, DCM, 93%.
Table 2. Influence of the gem-dimethyl group on antiviral activity
HIV protease inhibition IC50 (nM)
Viral spread assay CIC95 (nM)
WT
K-60C
61
V-18C
43.6
WT
K-60C
1000
V-18C
Indinavir
0.6
45
890
X
OH
OH
H
N
N
X@CH2, 20
X@O, 24
0.14
0.17
1.6
2.2
0.4
1.3
15
15
500
500
31
62
O
O
O
O
NH

S. Raghavan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5432–5436
5435
Table 3. Antiviral activity of spiro cyclopentane-oxazolidine analogs with different P2 substituents
HIV-1 protease inhibition IC50 (nM)
Viral spread assay CIC95 (nM)
K-60C V-18C
WT
K-60C
V-18C
WT
HO
H
HO
O
N
O
O
N
O
O
NH
R
Indinavir
0.6
0.1
61
—
43.6
0.5
45
8
1000
125
890
62
33
Cl
34
0.11
0.5
1.0
62.5
250
500
Cl
35
36
0.2
0.1
1.5
0.4
3.2
0.8
250
125
500
250
1000
250
N
O
N
S
antiviral activity observed for both 20 and 24 against
mutant enzymes, especially V18-C.
ing the benzyl group at the P2 position with the
smaller 5-membered heterocycle such as the isoxazole
35 or the isothiazole 36 resulted in a significant loss in
activity particularly against PI-resistant virus isolates.
This result was surprising to us because earlier observa-
tions by our group had shown that these groups syner-
gized well with the thiazolidine ring.3a The decrease in
activity of 35 and 36 in the viral spread assay could be
due to poor cell permeability.
Encouraged by these findings we examined the effect of
placing a larger group at the P3 position. Thus, the gem-
dimethyl group was replaced with a spiro-cylopentane
moiety (33) (Scheme 4).
Treatment of commercially available D-serine methyl es-
ter 25 with an excess of allylmagnesium bromide in
THF gave the desired tertiary alcohol in 75% yield, which
was protected as the TMS ether to give 26. Upon treat-
ment of 26 with Schrock’s catalyst in refluxing benzene,
it smoothly underwent olefin metathesis to give the cyclo-
pentene 27 in excellent yield.11 Hydrogenation of the dou-
ble bond followed by de-protection of the silyl group
afforded the alcohol 28. Next, the acetonide group was re-
moved under acid catalyzed conditions with p-toluene
sulfonic acid to give the diol 29. Jones oxidation followed
by removal of the Boc group gave the amino acid 30.
Treatment with formaldehydefollowed by Boc protection
as described in Scheme 3 gave the desired amino acid. This
material was transformed to 33 using the chemistry de-
scribed in Scheme 1. We also synthesized three additional
analogs that varied the groups at the P2 position. The bio-
logical results are summarized in Table 3.
In summary, we designed and synthesized a new series of
HIV protease inhibitors that were active against HIV-
resistant clinical isolates. In particular, we found that a
spiro cyclopentane-oxazolidine moiety to be an ideal
replacement for the thiazolidine ring while maintaining
the activity against both wild type and PI-resistant
isolates.
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5436
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