Synthesis of 2′-β-C-methyl-neplanocin derivatives as anti-HCV agents

Article abstract of DOI:10.1016/j.tetlet.2008.04.115

The synthesis of 2′-β-C-methyl-neplanocin derivatives is described. The key intermediate cyclopentenyl alcohol 12 is prepared from sugar 5 in 12 steps. Coupling of 12 with appropriately protected purine, 7-deaza pyrimidine, uracil and pyrimidine bases via

Full text of DOI:10.1016/j.tetlet.2008.04.115

Tetrahedron Letters 49 (2008) 4149–4152
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Tetrahedron Letters
journal homepage: http://www.elsevier.com/locate/tetlet
Synthesis of 20-b-C-methyl-neplanocin derivatives as anti-HCV agents
a
b
b
a
Xibin Liao a, , Gabor Butora , David B. Olsen , Steven S. Carroll , Daniel R. McMasters ,
*
Joseph F. Leone a, Mark Stahlhut b, George A. Doss a, Lihu Yang a, Malcolm MacCoss a
a Department of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
b Department of Antiviral Research, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
a r t i c l e i n f o
a b s t r a c t
The synthesis of 20-b-C-methyl-neplanocin derivatives is described. The key intermediate cyclopentenyl
alcohol 12 is prepared from sugar 5 in 12 steps. Coupling of 12 with appropriately protected purine, 7-
deaza pyrimidine, uracil and pyrimidine bases via the Mitsunobu reaction followed by deprotection affor-
ded the target cyclopentenyl nucleosides (18–23, 27). The synthesized compounds were evaluated as
potential inhibitors of the hepatitis C virus (HCV) in vitro. Unfortunately, none of them show anti-HCV
activity below EC50 100 lM.
Article history:
Received 20 February 2008
Revised 17 April 2008
Accepted 22 April 2008
Available online 24 April 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Hepatitis C virus (HCV) is the pathogen associated with the
majority of sporadic and transfusion related non-A and non-B
hepatitis infections. Although HCV is often asymptomatic, it can
progress to chronic hepatitis, leading to liver cirrhosis and
hepatocellular carcinoma in up to 80% of patients.1,2 Since there
is no vaccine available, efficacious therapies are urgently needed
to combat this important viral disease.
Ourselves and others have previously demonstrated that some
C02-methyl modified non-natural nucleosides are potent inhibitors
of HCV replication. It has been demonstrated that the 20-b-C-
methyl derivatives are efficient chain terminators of HCV genome
replication without significant toxicity.3–5
NH2
NH2
N
N
N
N
N
N
O
N
HO
HO
Me
OH
HO
HO
OH
2'-C-Methylribonucleoside
(-)-Neplanocin A
2
1
NH2
Z
N
O
X
Carbocyclic nucleosides are biologically interesting molecules
that can display important antitumor and antiviral activities. Such
nucleosides are chemically more stable and are not subject to the
action of the enzymes that cleave the glycosyl linkage in conven-
tional nucleosides. Often, introduction of unsaturation can increase
the biological potency and specificity when compared to the corres-
N
N
N
N
Y
HO
HO
Me
Me
HO
OH
HO
OH
ponding saturated carbocyclic analogues.6 (ꢀ)-Neplanocin
A
Z =OH or NH2
X = N, or CH
Y = NH2, or H
3
4
(NPA, 2), originally isolated from the culture filtrate of the soil fun-
gus Ampullariella regularis, is a carbocyclic nucleoside with potent
antiviral and antitumor activity and is a powerful inhibitor of
S-adenosylhomocysteine hydrolase.7
Figure 1. C02-Methyl adenosines and (ꢀ)-neplanocin A.
In recent years, attention has been increasingly focused on
structural modifications of carbocyclic nucleosides. It has been dis-
covered that many synthetic analogues of NPA, including abacavir8
and carbovir,9 exhibit potent anti-viral activity. Combining the
structural features of both compounds 1 and 2, we speculated that
a 20-methyl modification on NPA such as 3 and 4 (Fig. 1) would
reduce cytotoxicity and introduce anti-HCV activity.
Several synthetic methodologies have been reported toward
NPA. Ohira and co-workers reported the so far shortest method
using a C–H insertion reaction of a methylidene carbene as a key
step.10 Based on this strategy, herein we describe the synthesis of
2-b-C-methyl-neplanocin A derivatives 3 and 4, and their antiviral
properties.
The intermediate 6 can be conveniently synthesized from 30,50-
O-(4-methylbenzoyl)-20-C-methyl-ribose11
5
according to the
reported procedure12 (Scheme 1). Protection of the 50-primary
hydroxyl of with tert-butyldiphenylsilyl (TBDPS) group,
* Corresponding author. Tel.: +1 732 594 2393; fax: +1 732 594 9473.
E-mail address: xibin_liao@merck.com (X. Liao).
6
a
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.115

4150
X. Liao et al. / Tetrahedron Letters 49 (2008) 4149–4152
TBPSO
TolO
HO
TBPSO
O
OH
OH
O
OH
O
O
OTES
b
a
TolO
OH
O
O
O
O
O
O
Tol = p-methylbenzoyl
5
7
6
8
c
TBPSO
e
TBPSO
TBPSO
TBPSO
OH
TBPSO
f
OH
OTES
O
OTES
O
d
O
O
O
O
O
O
O
O
O
O
11
9
10
12
8a
Scheme 1. Reagents and conditions: (a) (i) TBDPSCl, imidazole, DMF; (ii) [(dppp)NiCl2], DIBAL-H, toluene; (iii) NaBH4, MeOH, 82% for three steps; (b) (i) TESCl, imidazole,
DMF; (ii) Dess–Martin periodinane, DCM; (c) TMSC(Li)N2, THF, 0 °C (40–55% for 9 and 5–10% for 8a); (d) PPTS, MeOH; (e) PySO3, Et3N, DCM, DMSO; (f) NaBH4, CeCl3, MeOH.
followed by selective cleavage of the allyl ether using a slight
excess of diisobutylaluminum hydride in the presence of a catalytic
amount of [NiCl2(dppp)]13 afforded the hemiketal intermediate in
excellent yield. With the addition of an excess amount of DIBAL-
H, the hemiketal could be reduced further to give the correspond-
ing diol 7 in one pot with good yield.13 Selective protection of the
newly formed primary alcohol as a triethylsilyl (TES) ether,
followed by a Dess–Martin periodinane oxidation of the secondary
alcohol gave ketone 8. Cyclization via intramolecular C–H insertion
of the alkylidenecarbene derived from ketone 8 by exposure to
lithiotrimethylsilyl-diazomethane provided the desired product 9
with 40–54% yield, while epimerization at the 30 position gave rise
to byproduct 8a.14 In contrast to the previous report of Ohira who
obtained a 2.7:1 epimeric mixture of the cyclopentene derivative
during the synthesis of NPA,10 we only got a single cyclopentene
product 9. Selective TES deprotection of compound 9 using PPTS/
MeOH, followed by oxidation of alcohol 10 with trioxidesulfur-pyr-
idine complex provided ketone 11. Its selective reduction with
NaBH4 in the presence of cerium(III) chloride furnished key inter-
mediate 12 and its epimer 10 in a 4:1 ratio with 90% total yield.15
With the key intermediate 12 in hand, the base components of
the carbocyclic nucleosides were coupled with 12 using the Mits-
unobu reaction16 to obtain the desired nucleosides 13a, 14a,
21a–23a, and 25a (Schemes 2–4). To synthesize the purine ana-
logues (Scheme 2), the cyclopentenyl alcohol 12 was treated with
6-chloropurine and N2-acetylamino-6-chloropurine under stan-
dard Mitsunobu conditions to give 13a and 14a. Surprisingly we
also obtained the minor allylic migration byproducts 13b and
14b.17 When compared to reported values, the reaction rate as well
as the yield was decreased, which we attribute to steric hinderance
caused by the 20-C-methyl group. In addition, we observed the
effect of the 20-C-methyl group in previous steps when we reduced
11 to 12 and found a mixture of diastereomers 12 and 10 rather
than a single isomer 12. We also investigated this Mitsunobu reac-
tion under microwave-assisted conditions at 80 °C and noted that
the reaction was completed in only 1 minute (Scheme 3), com-
pared to thermal conditions, which usually required about 16
hours or longer. The mixture of 13a–b and 14a–b was converted
to the desired 4-amino derivatives 18a–b and 19a–b18 by ammo-
nolysis at elevated temperature followed by deprotection of the
tert-butyl diphenyl silyl and isopropylidene groups with 4 N HCl
in MeOH. Compounds 14a–b were converted to the corresponding
guanine nucleosides 20a–b by the previously reported method.19
The 7-deaza and 7-F-7-deaza purine analogues were prepared
using a similar procedure (Scheme 3). Treatment of 12 with 7-dea-
za-6-chloropurine, 7-F-deaza-6-chloropurine, and N2-acetylamino-
7-deaza-6-chloropurine, as described above, gave 21a, 22a, and
23a, respectively. The N2-acetyl group in 23a was removed using
aqueous NaOH to provide compound 24a. Compounds 21a, 22a,
and 24a were treated with methanolic ammonia at 80 °C (sealed
Cl
Cl
N
N
N
N
N
TBPSO
a
TBPSO
N
Y
N
O
Y
N
12
Me
Me
O
O
O
13 b: Y = H
13 a: Y = H
14 a: Y = NHAc
14 c: Y = NH2
X
14 b: Y = NHAc
14 d: Y = NH2
X
b
b
c
N
N
N
N
TBPSO
TBPSO
N
N
N
Y
N
Y
Me
O
Me
O
O
O
15 a : X = NH2, Y = H
16 a : X = NH2, Y = NH2
14 c: X = Cl, Y = NH2
17 a: X = OH, Y = NH2
15 b : X = NH2, Y = H
17 b : X = NH2, Y = NH2
14 d: X = Cl, Y = NH2
17 b : X = OH, Y = NH2
e
d
d
X
X
N
N
N
N
HO
N
HO
N
Y
N
Y
N
Me
OH
Me
OH
HO
HO
18 a : X = NH2, Y = H
19 a : X = NH2, Y = NH2
20 a : X = OH, Y = NH2
18 b: X = NH2, Y = H
19 b : X = NH2, Y= NH2
20 b : X = OH, Y = NH2
Scheme 2. Reagents and conditions: (a) Ph3P, DIAD, THF, rt, 30–75%; (b) 1 N NaOH,
MeOH; (c) methanolic ammonia, 18 h, 80 °C; (d) 1 N NaOH, reflux; (e) 4 N HCl,
MeOH, rt.

X. Liao et al. / Tetrahedron Letters 49 (2008) 4149–4152
4151
All the synthesized nucleosides were evaluated as potential
inhibitors of HCV using the cell-based bicistronic replicon assay,
modified for RNA quantitation by RNase protection.3 While none
of the analogs showed cytotoxicity below 100 lM, unfortunately
they were inactive in the inhibition of HCV replication when tested
up to 100 lM. A successful inhibitory action requires the nucleo-
sides to be converted to their corresponding nucleoside triphos-
phates; therefore, one possible reason for the inactivity of the
cyclopentenyl nucleoside is its inability to be phosphorylated by
the appropriate kinases.4
Cl
N
X
N
N
TBPSO
TBPSO
Y
OH
a
Me
O
O
O
O
21a : X = CH, Y = H
22a : X = CF, Y = H
12
In summary, a series of novel 20-b-C-methyl-neplanocin ana-
logues were successfully synthesized and their biological activity
was evaluated. Microwave irradiation improved a key Mitsunobu
coupling reaction. To the best of our knowledge, this represents
the first documented case of the synthesis of neplanocin analogues
under these conditions. Further structure–activity relationship and
other biological evaluations of these analogues are currently
underway, and will be reported in due course.
23a: X = CH, Y = NHAc
24a: X = CH, Y = NH2
b
Z
c
Z
X
N
N
X
N
N
TBPSO
d
Y
HO
N
Y
N
Me
Me
O
O
References and notes
HO
OH
21b : X = CH, Y = H, Z = NH2
22b : X = CF, Y = H, Z = NH2
23b: X = CH, Y = NH2, Z = NH2
21c : X = CH, Y = H, Z = NH2
1. Seeff, L. B.; Hoofnagle, J. H. National Institutes of Health Consensus Develop-
ment Conference Statement: Management of Hepatitis C, June 10–12, 2002,
Hepathology 2002, 36, S3–S20.
2. Alter, M. J.; Kreuszon-Moran, D.; Nainan, O. V.; McQuillan, G. M.; Gao, F. X.;
Moyer, L. A.; Kaslow, R. A.; Margolis, H. S. New Engl. J. Med. 1999, 341, 556.
3. Carroll, S. S.; Tomassini, J. E.; Bosserman, M.; Getty, K.; Stahlhut, M. W.; Eldrup,
A. B.; Bhat, B.; Hall, D.; Simcoe, A. L.; LaFemina, R.; Rutkowski, C. A.; Wolanski,
B.; Yang, Z.; Migliaccio, G.; De Francesco, R.; Kuo, L. C.; MacCoss, M.; Olsen, D. B.
J. Biol. Chem. 2003, 278, 11979.
22c : X = CF, Y = H, Z = NH2
23c: X = CH, Y = NH2, Z = NH2
Scheme 3. Reagents and conditions: (a) Ph3P, DIAD, THF, rt, 20–30%; or microwave
80 °C, 1 min, 40–50%; (b) 1 N NaOH, MeOH; (c) methanolic ammonia, 18 h, 80 °C;
(d) 4 N HCl, MeOH, rt.
4. Eldrup, A. B.; Allerson, C. R.; Bennett, C. F.; Bera, S.; Bhat, B.; Bhat, N.;
Bosserman, M. R.; Brooks, J.; Burlein, C.; Carroll, S. S.; Cook, P. D.; Getty, K. L.;
MacCoss, M.; McMasters, D. R.; Olsen, D. B.; Prakash, T. P.; Prhavc, M.; Song, Q.;
Tomassini, J. E.; Xia, J. J. Med. Chem. 2004, 47, 2283; Olsen, D. B.; Eldrup, A. B.;
Bartholomew, L.; Bhat, B.; Bosserman, M. R.; Ceccacci, A.; Colwell, L. F.; Fay, J. F.;
Flores, O. A.; Getty, K. L.; Grobler, J. A.; LaFemina, R. L.; Markel, E. J.; Migliaccio,
G.; Prhavc, M.; Stahlhut, M. W.; Tomassini, J. E.; MacCoss, M.; Hazuda, D. J.;
Carroll, S. S. Antimicrob. Agents Chemother. 2004, 48, 3944.
Bz
O
N
TBPSO
O
TBPSO
OH
N
5. For recent anti-HCV Reviews: Zapf, C. W.; Bloom, J. D.; Levin, J. I. Annu. Rep. Med.
Chem. 2007, 42, 281; De Clercq, Erik. Nat. Rev. Drug Discov. 2007, 6, 1001; De
Francesco, Raffaele; Carfi, Andrea Adv. Drug Deliv. Rev. 2007, 59, 1242; Roenn,
R.; Sandstroem, A. Curr. Top. Med. Chem. 2008, 8, 533.
a
Me
O
O
O
O
6. Marquez, V. E.; Lim, M.-I. Med. Res. Rev. 1986, 6, 1; Marquez, V. E. Adv. Antiviral
Drug Des. 1996, 2, 89; Shuto, S.; Obara, T.; Saito, Y.; Andrei, G.; Snoeck, R.; De
Clercq, E.; Matsuda, A. J. Med. Chem. 1996, 39, 2392.
25a
12
7. Yaginuma, S.; Muto, N.; Tsujino, M.; Sudate, Y.; Hayashi, M.; Otani, M. J.
Antibiot. 1981, 34, 359; Bennett, L. L., Jr.; Brochman, R. W.; Rose, L. M.; Allan, P.
W.; Shaddix, S. C.; Shealy, Y. F.; Clayton, J. D. Mol. Pharmacol. 1985, 27, 666;
Wolfe, M. S.; Borchardt, R. T. J. Med. Chem. 1991, 34, 1521; De Clercq, E.
Antimicrob. Agents Chemother. 1985, 28, 84.
8. Daluge, S. M.; Good, S. S.; Feletto, M. B.; Miller, W. H.; St. Clair, M. H.; Boone, L.
R.; Tisdale, M.; Parry, N. R.; Reardon, J. E.; Dornsife, R. E.; Averett, D. R.;
Krenitsky, T. A. Antimicrob. Agents Chemother. 1997, 41, 1082; Weller, S.;
Radomski, K. M.; Lou, Y.; Sterin, D. S. Antimicrob. Agents Chemother. 2000, 44,
2052.
b
X
N
X
N
O
TBPSO
O
HO
N
N
d
Me
Me
OH
O
O
HO
9. Vince, R.; Hua, M. J. Med. Chem. 1990, 33, 17.
10. Ohira, S.; Sawamoto, T.; Yamato, M. Tetrahedron Lett. 1995, 36, 1537.
11. Bio, M. M.; Xu, F.; Waters, M.; Williams, J. M.; Savary, K. A.; Cowden, C. J.; Yang,
C.; Buck, E.; Song, Z. J.; Tschaen, D. M.; Volante, R. P.; Reamer, R. A.; Grabowski,
E. J. J. J. Org. Chem. 2004, 69, 6257; Xu, F.; Simmons, B.; Savary, K.; Yang, C.;
Reamer, R. A. J. Org. Chem. 2004, 69, 7783.
12. Butora, G.; Olsen, D. B.; Carroll, S. S.; McMasters, D. R.; Schmitt, C.; Leone, J. F.;
Stahlhut, M.; Burlein, C.; MacCoss, M. Bio. Med. Chem. 2007, 15, 5219.
13. Taniguchi, T.; Ogasawara, K. Angew. Chem., Int. Ed. 1998, 37, 1136. Note: during
the large scale synthesis, even though with the addition of excess DIBAL, we
observed that the hemiketal could not be consumed completely, just worked
up and treated the crude mixture with NaBH4 to give the desired diol.
14. Rodriguez, J. B. Tetrahedron 1999, 55, 2157–2170.
27a : X = OH
27b : X = NH2
26a : X = OH
26b : X = NH2
c
Scheme 4. Reagents and conditions: (a) Ph3P, DIAD, THF, rt, 28%; (b) 1 N NaOH,
MeOH; (c) 2,4,6-triisopropylbenzenesulfonyl chloride, DMAP, Et3N, MeCN, 0 °C–rt,
24 h then 30% NH4OH, rt, 5 h; (d) 4 N HCl, MeOH, rt.
tube, 16–36 h) to afford 21b, 22b, and 23b; deprotection of these
derivatives with 4 N HCl in methanol at room temperature pro-
vided the free carbocyclic nucleosides 21c, 22c, and 23c in high
yields ranging from 95% to 99%.
15. Note: The stereochemistry of cyclopentenol 10 and 12 was determined on the
basis of nuclear Overhauser effect (1D-NOE).
16. Nokami, J.; Matsuura, H.; Takahashi, H.; Yamashita, M. Synlett 1994, 491–493.
17. The allylic migration product has been reported for the first time from the
similar synthesis of NPA or NPA’s derivatives. This observation could be
explained by invoking a SN20 reaction mechanism for the Mitsunobu coupling
reaction with allylic alcohols. For a reference pertinent to this issue, see: Shull,
B. K.; Sakai, T.; Nichols, J. B.; Koreeda, M. J. Org. Chem. 1997, 62, 8294–8303.
18. Data for compound 18a: 1H NMR (600 MHz, CD3OD) d 8.20 (s, 1H), 7.98 (s, 1H),
5.90 (m, 1H), 5.46 (m, 1H), 4.44 (s, 1H), 4.31 (dd, J = 15 Hz, 2H), 0.92 (s, 3H);
Compound 18b: 1H NMR (500 MHz, CD3OD) d 8.24 (s, 1H), 8.22 (s, 1H), 6.65 (d,
J = 5.5 Hz, 1H), 6.20 (d, J = 5.5 Hz, 1H), 4.25 (s, 1H), 4.04 (d, J = 10.5 Hz, 1H), 3.98
The synthesis of the cytosine and uracil derivatives of 20-b-C-
methyl-neplanocin A was carried out using a similar strategy,
(Scheme 4). Briefly, the cyclopentenol 12 was coupled with N-Bz
uracil followed by deprotection of the benzoyl group, tert-butyl
diphenyl silyl group, and isopropylidene groups to yield compound
27a. Compound 26a was converted to the corresponding cytosine
analogue 27b by the reported method.20

4152
X. Liao et al. / Tetrahedron Letters 49 (2008) 4149–4152
(d, J = 10.5 Hz, 1H), 1.41 (s, 3H); Compound 19a: 1H NMR (500 MHz, CD3OD) d
(d, J = 5.0 Hz, 1H), 6.42 (d, J = 5.0 Hz, 1H), 5.87 (s, 1H), 5.41 (s, 1H), 4.46 (s, 1H),
4.36 (dd, J = 16.0, 16.0 Hz, 2H), 0.86 (s, 3H); Compound 27a: 1H NMR (500 MHz,
CD3OD) d 7.32 (d, J = 4.5 Hz, 1H), 5.67 (s, 1H), 5.66 (d, J = 4.5 Hz, 1H), 5.40 (s,
1H), 4.34 (s, 1H), 4.28 (dd, J = 13.0, 13.0 Hz, 2H), 1.10 (s, 3H); Compound 27b:
1H NMR (500 MHz, CD3OD) d 7.64 (br s, 1H), 6.08 (br s, 1H), 5.68 (s, 1H), 5.42 (s,
1H), 4.38 (s, 1H), 4.25 (dd, J = 13.5, 13.5 Hz, 2H), 1.13 (s, 3H).
7.56 (s, 1H), 5.82 (s, 1H), 5.23 (s, 1H), 4.40 (s, 1H), 4.25 (dd, J = 7.5 Hz, 2H), 0.84
(s, 3H); Compound 19b: 1H NMR (500 MHz, CD3OD) d 7.80 (s, 1H), 6.50 (d,
J = 6.0 Hz, 1H), 6.14 (d, J = 6.0 Hz, 1H), 4.12 (s, 1H), 3.98 (d, J = 11 Hz, 1H), 3.71
(d, J = 11 Hz, 1H), 1.36 (s, 3H); Compound 20a: 1H NMR (500 MHz, CD3OD) d
7.62 (s, 1H), 5.92 (m, 1H), 5.24 (m, 1H), 4.45 (s, 1H), 4.28 (dd, J = 14, 14 Hz, 2H),
0.93 (s, 3H); Compound 20b: 1H NMR (500 MHz, CD3OD) d 7.82 (s, 1H), 6.60 (d,
J = 5.5 Hz, 1H), 6.20 (d, J = 5.5 Hz, 1H), 4.21 (s, 1H), 4.02 (d, J = 10.5 Hz, 1H), 3.84
(d, J = 10.5 Hz, 1H), 1.42 (s, 3H); Compound 21c: 1H NMR (500 MHz, CD3OD) d
8.31 (s, 1H), 7.26 (s, 1H), 6.95 (s, 1H), 5.87 (s, 1H), 5.73 (s, 1H), 4.52 (s, 1H), 4.35
(br s, 2H), 0.84 (s, 3H); Compound 22c: 1H NMR (500 MHz, CD3OD) d 8.24 (s,
1H), 7.12 (s, 1H), 5.80 (s, 1H), 5.71 (s, 1H), 4.46 (s, 1H), 4.32 (dd, J = 17.0,
17.0 Hz, 2H), 0.85 (s, 3H); Compound 23c: 1H NMR (500 MHz, CD3OD) d 6.64
19. Eldrup, A. B.; Prhavc, M.; Brooks, J.; Bhat, B.; Prakash, T. P.; Song, Q.; Bera, S.;
Bhat, N.; Dande, P.; Cook, P. D.; Bennett, C. F.; Carroll, S. S.; Ball, R. G.;
Bosserman, M. R.; Burlein, C.; Colwell, L. F.; Fay, J. F.; Flores, O. A.; Getty, K. L.;
Lafemina, R. L.; Leone, J.; MacCoss, M.; McMasters, D. R.; Tomassini, J. E.; Von
Langen, D.; Wolanski, B.; Olsen, D. B. J. Med. Chem. 2004, 47, 5284.
20. Song, G. Y.; Paul, V.; Choo, H.; Morrey, J.; Sidwell, R. W.; Schinazi, R. F.; Chu, C.
K. J. Med. Chem. 2001, 44, 3985.