Syntheses of nucleosides with 2′-spirolactam and 2′- spiropyrrolidine moieties as potential inhibitors of hepatitis C virus NS5B polymerase

Article abstract of DOI:10.1016/j.tetlet.2014.05.051

To discover novel nucleosides as potential antiviral agents, 2′-spirolactam and 2′-spiropyrrolidine-containing nucleoside analogs were envisioned. Efficient synthetic routes were developed with an epoxide opening as the key step to establish the quaternar

Full text of DOI:10.1016/j.tetlet.2014.05.051

Tetrahedron Letters 55 (2014) 3813–3816
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Syntheses of nucleosides with 20-spirolactam and 20-spiropyrrolidine
moieties as potential inhibitors of hepatitis C virus NS5B polymerase
b
b
b
b
b
a
Qun Dang a, , Zhibo Zhang , Bingyu Tang , Yang Song , Ling Wu , Tongqian Chen , Stephane Bogen ,
⇑
Vinay Girijavallabhan a, David B. Olsen c, Peter T. Meinke a
a Discovery Chemistry, Merck Research Laboratories, 2000 Galloping Hill Road, Kenilworth, NJ 07033, USA
b Pharmaron Beijing Co. Ltd, 6 Tai-He Road, BDA, Beijing 100176, China
c Discovery Biology, Merck Research Laboratories, 770 Sumneytown Pike, West Point, PA 19486, USA
a r t i c l e i n f o
a b s t r a c t
To discover novel nucleosides as potential antiviral agents, 20-spirolactam and 20-spiropyrrolidine-
containing nucleoside analogs were envisioned. Efficient synthetic routes were developed with an
epoxide opening as the key step to establish the quaternary center at the 20 position, leading to the design
and synthesis of uridine analogs 8 and 21, prodrugs 13–16, and cytidine analog 11.
Ó 2014 Elsevier Ltd. All rights reserved.
Article history:
Received 7 April 2014
Revised 3 May 2014
Accepted 7 May 2014
Available online 21 May 2014
Keywords:
Nucleoside
Prodrug
HCV NS5B
Spiro-lactam
Spiro-pyrrolidine
Nucleoside analogs have a rich and vibrant history in the treat-
ment of human viral infections with many reaching the market to
produce profound impacts. For example, zidovudine (AZT) was
approved in 1987 to treat HIV infections,1 entecavir was approved
in 2005 to treat hepatitis B infections,2 and the recently approved
sofosbuvir to treat hepatitis C virus (HCV) infections3 (Fig. 1).
Discovery of novel nucleosides as HCV NS5B inhibitors proved
to be extremely challenging due to the daunting synthetic chal-
lenges they pose and the observations that many seemingly minor
structural modifications ablate useful biological activity.4,5 After
decades of efforts, while many nucleosides advanced into clinical
trials, including MK-608,6 NM-107,7 PSI-6130,8 and compound 19
(Fig. 2); only sofosbuvir reached the market recently. The
20-modifications have been the focus of most HCV nucleoside drug
discovery programs since it is one area that tolerates some varia-
tions without significant loss of activity against HCV.10,6,11–17
One interesting 20-modification is the introduction of a spirocy-
clic group, which has led to the discovery of the 20-spirocyclopro-
pane analog 1, as well as the 20-spirooxetane analog 218,19 both
showing anti-HCV activity. However, to date nucleosides function-
alized with 20-spirolactams and 20-spiropyrrolidines have not been
reported. It was envisioned that a 20-spirolactam or 20-spiropyrroli-
dine could mimic the 20-hydroxy group of MK-608 while the b-face
methylene could function the same way as the key b-methyl group
of many known nucleoside HCV NS5B inhibitors; the 20-spirooxe-
tane analog (2) inhibited HCV replication in a cellular assay,
O
O
O
N
N
N
NH
NH
NH
NH2
O
O
N
N
O
P
O
O
O
HO
NH2
NH2
HO
O
HN
NH2
N
NH2
N
O
N
N
N3
Zidovudine
F
OH
HO
Sofosbuvir
O
N
N
N
O
N
Entecavir
N
HO
O
O
O
O
O
HO
O
HO
HO
HO
Figure 1. Nucleoside analogs as antiviral drugs.
HO OH
MK-608
HO OH
NM-107
HO
F
PSI-6130
1
⇑
Corresponding author. Tel.: +1 908 740 3789.
E-mail address: qun_dang@merck.com (Q. Dang).
Figure 2. Clinically evaluated nucleoside HCV NS5B inhibitors.
http://dx.doi.org/10.1016/j.tetlet.2014.05.051
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

3814
Q. Dang et al. / Tetrahedron Letters 55 (2014) 3813–3816
NH2
experiment, see Supplementary material section), and subsequent
epoxide opening with sodium azide gave the key 20-azido-20-alde-
hyde 5 in 70% yield, establishing the key quaternary center. Wittig
reaction of aldehyde 5 with Ph3P@CHCO2Et gave compound 6 in
81% yield. Reduction of both the azido group and the olefin under
hydrogenation conditions and subsequent cyclization gave lactam
7 in 70% yield, over the two steps. Removal of the TIPS protecting
group produced the desired uridine analog 8 with a 20-spirocyclic
lactam in 66% yield.
Conversion of uridine analog 8 to its corresponding cytidine
analog was achieved using the Sekine procedure21 (Scheme 2).
Treatment of lactam 8 with 2,4,6-tris(isopropyl)benzenesulfonyl
chloride (TPSCl) in the presence of DMAP and triethylamine led
to regio-selective O-sulfonate formation at the 4-position of uri-
dine, producing compound 9 in 67% yield. Aminolysis of compound
9 using an ammonia-saturated anhydrous THF solution gave
cytidine analog 10 in 90% yield, and a final removal of the TIPS pro-
tecting group using TBAF gave the desired cytidine 20-spirolactam
11 in 61% yield.
N
N
N
O
HO
Y
N
HO OH
MK-608
N
O
O
HO
HO
NH2
N
X: C=O, CH2
Y: OH, NH2
HN
X
N
O
O
HO
HO
2
O
Figure 3. Novel 20-spirocyclic amide and amine-containing nucleosides.
suggesting that some steric bulk is tolerated at the 20-position
(Fig. 3). Herein, our syntheses of these novel nucleosides are
reported.
To access the 20-spirolactam nucleoside scaffold, the Lukacs
azide-epoxide opening chemistry20 was selected as the key step
of our synthesis in order to install the quaternary center at the
20-position (Scheme 1). The synthesis started with the 30,50-TIPS
protected 20-ketone of uridine (3), which was prepared using the
Jonckers’ protocol.9 Under Lukacs’ conditions, addition of dichlo-
romethyl lithium to ketone 3 gave epoxide 4 in 27% yield (struc-
tural assignment of compound 4 was carried out by an NOESY
Nucleoside prodrugs are frequently utilized to improve efficacy
in the HCV replicon cellular assay, therefore it is important to dem-
onstrate that these 20-spirolactam and 20-spiropyrrolidine analogs
are amenable to common prodrug chemistry as well. Thus, the
classical McGuigan22 and the 30,50-cyclic phosphate23 prodrugs of
the uridine lactam 8 were prepared as illustrated in Scheme 3.
Treatment of lactam 8 with (iPr2N)2(iPrO)P in the presence of
4,5-dicyanoimidazole gave the cyclic phosphite 12 and subsequent
O
O
O
NH
N
NH
O
NH
O
N
O
4
N
a
O
O
b
Si
O
O
O
Si
O
Si
O
O
O
O
O
Si
O
Si
O
Si
O
O
O
N3
Cl
3
5
O
O
NH
O
NH
O
NH
N
N
d
c
N
O
e
Si
O
Si
O
O
O
HO
HO
O
O
O
O
Si
O
Si
CO2Et
N3
HN
HN
6
7
8
O
O
Scheme 1. Synthesis of compound 8. Reagents and conditions: (a) dichloromethane (4.0 equiv), LDA (3.5 equiv), THF, ꢀ70 °C to 25 °C, 12 h, 27%; (b) 15-crown-5 ether
(0.5 equiv), NaN3 (5 equiv), DMF, 30 °C, 2 h, 70%; (c) Ph3P@CHCO2Et (1.3 equiv), CH2Cl2, 35 °C, 2 h, 81%; (d) i. PdAC (10%), H2, EtOAc, 25 °C, 3 h; ii. HOAc–EtOAc (1:15, v/v),
25 °C, 2 h, 70%; (e) TREAT-HF (3.0 equiv), TEA (1.75 equiv), THF, 25 °C, 3 h, 66%.
O
OTPS
NH2
N
NH
O
N
O
N
N
b
N
a
O
Si
O
O
O
Si
O
O
O
O
Si
O
O
O
Si
O
Si
O
Si
HN
HN
HN
7
9
O
O
10
O
NH2
N
N
O
c
O
O
HO
HO
HN
11
Scheme 2. Synthesis of compound 11. Reagents and conditions: (a) TPSCl (3.0 equiv), DMAP (0.2 equiv), TEA (4.0 equiv), DCM, 15 °C, 1 h, 67%; (b) NH3 (saturated), THF, 15 °C,
12 h, 90%; (c) TBAF (2.0 equiv), THF, 15 °C, 1 h, 61%.

Q. Dang et al. / Tetrahedron Letters 55 (2014) 3813–3816
3815
of 31P NMR.15 Alternatively, stereo-controlled synthesis of McGui-
gan prodrugs 15 and 16 was carried out using the method devel-
oped by Ross et al.24 Treatment of lactam 8 with the two chiral
O
O
O
NH
O
NH
O
NH
O
N
b
N
N
O
O
O
phosphoramidites of phenol, pentafluorophenol and L-alanine iso-
O
O
P
O
O P
O
propyl ester in the presence of tetramethylpiperidinyl magnesium
chloride-lithium chloride (TMPMgCl–LiCl)25 gave the McGuigan
prodrugs 15 and 16, in 10% and 14% yields, respectively.
P
O
O
O
O
O
HN
O
HN
HN
O
12
O
13
14
O
O
The preparation of the 20-spirocyclic amine analog (21) proved
to be much more challenging. Direct reduction of the lactam 8
under various conditions (e.g. LiAlH4 2.0 equiv, THF, 25 °C or
40–75 °C; no reaction occurred) failed to produce requisite compound
21, thus several protecting groups were introduced to enhance the
possibility of success. However, as evident from Scheme 4,
protecting the 50-hydroxy with a trityl group or protecting the 30
and 50-hydroxy groups as TBS ethers did not enable the successful
reduction of the lactam (e.g. LiAlH4, THF, 0 °C to reflux: no reaction
or messy; BH.3SMe2, THF, 0 °C to reflux, 6 h: no reaction). Therefore,
a new synthetic route was designed to accomplish the synthesis of
the 20-spirocyclic amine analog 21.
a
O
O
NH
O
F
F
F
F
NH
O
O
P
O
P
N
O
O
N
H
F
N
PhO
HN
O
O
O
HO
OPh
HO
HN
TMPMgCl-LiCl, DMF, 0 o
10%
C
HO
HN
CO2Pr-i
15
O
8
O
O
TMPMgCl-LiCl, DMF, 0 o
14%
C
NH
O
O
P
N
O
F
F
PhO
HN
O
P
O
O
Wittig reaction of aldehyde 5 with Ph3P@CHCHO gave the
desired compound 18 in 76% yield (Scheme 5). Reduction of
the 20-azide group under hydrogenation conditions gave the
20-spirocyclic imine 19, which was subjected to sodium triacetoxy-
borohydride reduction to give the TIPS-protected 20-spirocyclic
amine 20, 34% yield over the two steps. Final TIPS removal using
TBAF gave the desired 20-spirocyclic amine 21 in 65% yield.
In summary, 20-spirolactam- and spiropyrrolidine-containing
nucleosides were designed as potential inhibitors of HCV NS5B.
New synthetic routes have been developed to access these novel
nucleoside analogs and their prodrugs. The 20-spirolactam scaffold
was demonstrated on the uridine and cytidine nucleoside series,
while the 20-spiropyrrolidine scaffold was exemplified on the uri-
dine series. Testing of these novel nucleosides against HCV should
provide important SAR information to help us to expand the under-
standing of the field; results will be reported in due course.
N
H
O
F
HO
HN
O
OPh
CO2Pr-i
F
F
16
O
Scheme 3. Synthesis of prodrugs 13–16. Reagents and conditions: (a) (iPr2N)2
(iPrO)P (1.3 equiv), 1H-imidazole-4,5-dicarbonitrile (2.5 equiv), MeCN–NMP
(2:1, v/v), 0 °C, 12 h; (b) I2 (1.0 equiv), THF–pyridine–water (39:10:1), 0 °C, 1 h,
13, 8%; 14, 19%.
O
O
O
NH
O
NH
O
NH
O
N
N
a, b
N
O
O
O
R1O
R2O
HO
HO
R1O
R2O
X
HN
HN
HN
8
O
O
21
: R1 = Tr, R2 = H
: R1 = R2 = TBS
17a
17b
Supplementary data
Scheme 4. Attempted preparation of compound 21. Reagents and conditions:
(a) TrCl (1.43 equiv), pyridine, 25 °C, quantitative; (b) TBSCl (2.1 equiv), imidazole
(3.0 equiv), DMF, 25 °C, 90%.
Supplementary data (experimental procedures and character-
ization data for all new compounds) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.05.051.
O
N
O
N
NH
NH
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O
O
O
O
a
b
Si
O
5
Si
O
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O
O
O
N3
O
N
Si
Si
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18
19
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Scheme 5. Preparation of compounds 21. Reagents and conditions: (a) Ph3P@CHCHO
(1.0 equiv), DCM, 25 °C, 6 h, 76%; (b) PdAC (10%), H2, EtOAc, 25 °C, 3 h; (c) NaBH(OAc)3
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