A New Formal Synthetic Route to Entecavir

Article abstract of DOI:10.1055/s-0037-1612215

We describe a new and straightforward approach to the formal synthesis of the hepatitis B virus inhibitor entecavir, an important hepatitis B drug, in ten steps overall. Key features of the route are a Morita-Baylis-Hillman reaction, a Sharpless asymmetric epoxidation, a reductive epoxide opening of an α,β-epoxy ketone, and a Riley selenium dioxide oxidation.

Full text of DOI:10.1055/s-0037-1612215

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
©
Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
letter
A
en
Synlett
L. Liu et al.
Letter
A New Formal Synthetic Route to Entecavir
Lixia Liua
Yongli Suna
Jiwu Wangb
Wentao Oua
N
O
Sharpless
epoxidation
Reductive
epoxide opening
O
O
N
NH
NH2
HO
N
TBSO
TBSO
HO
HO
O
HO
Entecavir
Xiaoji Wang*a
Shuangping Huang*b,c
a
School of Life Science, Jiangxi Science and Technology
Normal University, 330013 Nanchang, Jiangxi, P. R. of
China
School of Pharmacy, Jiangxi Science and Technology
b
Normal University, 330013 Nanchang, Jiangxi, P. R. of
China
College of Biomedical Engineering, Taiyuan University
c
of Technology, 030024 Taiyuan, Shanxi, P. R. of China
2012207455@tju.edu.cn
Received: 21.12.2018
Accepted after revision: 20.01.2019
Published online: 06.03.2019
carriers of HBV As part of our ongoing interest in research
on bioactive natural products and pharmaceuticals, we
4
DOI: 10.1055/s-0037-1612215; Art ID: st-2018-v0823-l
disclose a new, concise, and straightforward approach to
entecavir.
Abstract We describe a new and straightforward approach to the for-
mal synthesis of the hepatitis B virus inhibitor entecavir, an important
hepatitis B drug, in ten steps overall. Key features of the route are a
Morita–Baylis–Hillman reaction, a Sharpless asymmetric epoxidation, a
reductive epoxide opening of an α,β-epoxy ketone, and a Riley selenium
dioxide oxidation.
N
O
N
NH
HO
N
NH2
HO
Figure 1 Structure of entecavir
Key words entecavir, formal synthesis, epoxy ketones, Sharpless ep-
oxidation, ring cleavage, Riley oxidation
Our retrosynthetic analysis of entecavir is depicted in
Scheme 1.
Hepatitis B, also known as serum hepatitis, is caused by
hepatitis B virus (HBV) and is a serious and widespread dis-
ease that is particularly prevalent in Asia. According to a
modeling study involving a Delphi process and a dynamic
HBV transmission and progression model, it is possible that
about 300 million people may have been infected with
H
N
Boc2N
N
N
N
O
N
2
Cl
OH
N
NH
NH2
TBSO
TBSO
1
HO
N
HBV. In the absence of a response to chemotherapy, some
patients are likely to contract such diseases as liver cirrho-
sis or liver cancer, ultimately resulting in many deaths. Cur-
rently, hepatitis B can be treated with interferon or with an-
tiviral agents. Since its approval by the US Federal Drug Ad-
ministration in 2005, entecavir (BMS-200475; Figure 1), a
nucleoside analogue that inhibits the viral polymerase of
HBV, has become one of the most frequently used HBV in-
hibitors for the treatment of hepatitis B . Entecavir is con-
sidered to be one of the most effective drugs against HBV
HO
1
Entecavir
O
TBSO
TBSO
TBSO
O
4
3
2
because of its high efficacy and limited viral resistance. It
O
O
has therefore attracted considerable interest from the syn-
thetic chemistry community since Bristol-Myers Squibb
first accomplished a total synthesis of the drug in 1992.2
Because of its long synthetic route involving many chiral re-
agents, entecavir is expensive, placing a heavy burden on
HO
,3
5
6
Scheme 1 Retrosynthetic analysis of entecavir.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

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Synlett
L. Liu et al.
Letter
Entecavir might be readily prepared from compound 1
through modified Mitsunobu coupling with purine 2 by fol-
treatment with diphenyldiselane and N-acetylcysteine (3.0
equiv) in an aqueous methanolic solution.10 On following
the reported procedure, numerous spots on the TLC plate
were observed in the presence of sodium hydroxide (entry
4). Treatment in methanol/aqueous borax buffer without
sodium hydroxide gave the epoxide-opening product 8 to-
gether with the elimination product 9 in yields of 16 and
44%, respectively (entry 5). Next, we considered the alkalin-
ity of the methanolic aqueous borax buffer solution, as we
expected that 9 would be more-readily produced under ba-
sic conditions. When the epoxide opening was conducted
in phosphate buffers of pH = 4.5 and 6.5 at room tempera-
ture, neither 8 nor 9 was observed on the TLC (entries 6 and
7). Fortunately, however, in the presence of borax, the de-
sired products 8 and 9 were obtained in similar yields in
methanolic phosphate buffers with nominal pH values of
4.5 and 5.5 (entries 8 and 9). Because we found that the ac-
tual pH of the system was 6.6 under the conditions of entry
9, we added acid to decrease the pH and to decrease the
amount of 9 that was formed. To our surprise, no reaction
occurred when we used 5% HCl (entries 10 and 11), but
changing to 5% H PO gave 8 (35%) and 9 (35%) when the
3h
lowing the reported procedure. We hypothesized that the
allylic hydroxy group in 1 might be constructed from the
protected diol 3 by allylic oxidation. We assumed that the
exocyclic double bond and the 1,3-diol moiety of com-
pound 3 could be established through olefination and re-
ductive epoxide opening of the α,β-epoxy ketone 4. The key
chiral center in compound 4 might be introduced by Sharp-
less epoxidation of the unsaturated α,β-ketone 5, which
might be obtained from cyclopent-2-en-1-one (6) by a
Morita–Baylis–Hillman reaction.
On the basis of this retrosynthetic analysis, commercial-
ly available cyclopent-2-en-1-one (6) was subjected to a
Morita–Baylis–Hillman reaction. In the presence of tribu-
tylphosphine, cyclopent-2-en-1-one (6) was added to a 37%
w/w aqueous solution of formaldehyde, stabilized with 7–
8
% MeOH, in a mixed MeOH–CHCl solvent, to give adduct 5
3
5
in 97% yield. Subsequently, Sharpless asymmetric epoxida-
tion was employed to introduce the chiral center. Next, the
6
effects were examined of changing the weight of 4 Å molec-
ular sieves, the temperature, the number of equivalents of
ligand, and the Lewis acid. Eventually, epoxide 7 was ob-
tained in a maximum yield of 87% and 8:1 er by using
3
4
system pH was adjusted to 6.0 or 6.2 (entries 12 and 13).
We were pleased to find that at a lower temperature of
(15 °C), the yield of the epoxide-opened product 8 in-
creased to 56% and the yield of 9 fell to 20% (entry 14). At-
tempts to reduce the amount of 9 formed by further de-
creasing the pH value failed, and no obvious changes in the
yields of 8 and 9 were observed when the pH of the system
was adjusted to 5.5 by using H PO (entry 15).
dipropyl L-(+)-tartrate [L-(+)-DIPT, 1.5 equiv], Ti(OiPr) (1.5
4
equiv), and tert-butyl hydroperoxide (TBHP; 3.0 equiv) in
the presence of 4 Å MS (0.4 g/mol) in CH Cl at −25 °C for 90
2
2
hours. To avoid the potential for free hydroxy group open-
ing of the epoxide, TBS-protection was conducted in the
presence of imidazole in DMF at 0 °C to generate intermedi-
ate 4 in almost quantitative yield (Scheme 2).
3
4
Inspired by Engman’s proposal, we suggest a reasonable
mechanism for the epoxide opening of the α,β-epoxy ke-
tone 4 to give 8, as shown in Scheme 3. First, epoxide open-
ing by the attack of a benzeneselenolate ion at the α-posi-
tion relative to the carbonyl group gives an α-(phenylse-
lenenyl) β-hydroxy ketone, which is then attacked by
another benzeneselenolate ion. The resultant β-hydroxy ke-
O
O
a
b
OH
6
5
O
O
c
PhSe–
TBSO
HO
O
O
O
O
4
7
PhSe
PhSe–
Scheme 2 Reagents and conditions: (a) HCHO, PBu , MeOH–CHCl3
TBSO
3
TBSO
O
(
1:1.5), r.t., 2 h, 97%; (b) L-(+)-DIPT, Ti(OiPr) , TBHP, 4Å MS, CH Cl , –25
4
2
2
HO
℃, 90 h, 87%, er = 8:1; (c) TBSCl, imidazole, DMF, 0 ℃, 2 h, 98%.
4
–
PhSeSePh
With epoxide 4 in hand, we focused our efforts on the
subsequent epoxide opening. As shown in Table 1, a series
of conditions reported in the literature for opening the α,β-
epoxy ketone were examined. No product was obtained when
O
O
H+
7
8
the reaction was conducted in a Zn–HOAc or a Zn–NH Cl
TBSO
4
TBSO
system (Table 1, entries 1 and 2). An attempt to open the
HO
HO
8
α,β-epoxy ketone by hydrazine hydrate-induced reductive
9
Scheme 3 Proposed mechanism for the benzeneselenolate-induced
reductive epoxide opening of α,β-epoxy ketone 4
cleavage failed, and only the starting material was recov-
ered (entry 3). Finally, we carried out epoxide opening by
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L. Liu et al.
Letter
Table 1 Examination of the Epoxide-Opening Reaction
O
O
O
Epoxide
opening
TBSO
TBSO
+
TBSO
O
HO
8
9
4
Entry Conditions
Resulta
1
2
3
4
5
6
7
8
9
Zn, AcOH, THF, 50 °C
Zn, NH Cl, MeOH, 75 °C
NH NH ·H O, EtOH, r.t.
N-acetylcysteine, PhSeSePh, NaOH, MeOH–H
N-acetylcysteine, PhSeSePh, borax, MeOH–H
no reaction
no reaction
no reaction
decomposed
4
2
2
2
2
O, r.t.
b
2
O, r.t.
8 (16%) , 9 (44%)
N-acetylcysteine, PhSeSePh, MeOH–phosphate buffer (pH = 4.5), system pH = 2.3, r.t.
N-acetylcysteine, PhSeSePh, MeOH–phosphate buffer (pH = 6.5), system pH= 2.9, r.t.
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 5.5), r.t.
–c
–c
8 (30%, trans/cis = 2.8:1), 9 (40%)
8 (31%, trans/cis = 2.8:1), 9 (40%)
no reaction
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 4.5), system pH = 6.6, r.t.
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 4.5), addition of 5% HCl to adjust system pH to
1
0
1
2
3
4
5
5.5, r.t.
1
1
1
1
1
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 4.5),
addition of 5% HCl to adjust the system pH to 5.9, r.t.
no reaction
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 4.5), addition of 5% H
pH to 6.0, r.t.
3
PO
PO
4
4
to adjust the system
to adjust the system
to adjust the system
to adjust the system
8 (35%, trans/cis = 3:1), 9 (35%)
8 (35%, trans/cis = 3:1), 9 (35%)
8 (56%, trans/cis = 3.4:1), 9 (20%)
8 (52%, trans/cis = 3.3:1), 9 (23%)
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 4.5), addition of 5% H
pH to 6.2, r.t.
3
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 4.5), addition of 5% H
pH to 6.0, 15 °C
3
PO
PO
4
4
N-acetylcysteine, PhSeSePh, borax, MeOH–phosphate buffer (pH = 4.5), addition of 5% H
pH to 5.5, 15 °C
3
a
b
c
Yield of product isolated by chromatography on silica gel.
The trans/cis ratio was not determined because of the low yield.
No 8 or 9 was observed by TLC.
tone enolate is subsequently protonated, with formation of
diphenyldiselane, to generate the desired β-hydroxy ketone
product 8. According to this proposed mechanism, it is rea-
sonable that the reaction does not proceed under more-
acidic conditions, because fewer benzeneselenolate ions
would be available.
87% yield when the reaction was conducted in the presence
of TiCl in CH Cl at −78 °C with subsequent stirring at room
4
2
2
temperature for two hours. We originally attempted to pre-
pare the allylic alcohol from 3 by a Riley allylic oxidation
13
with 1.0 equivalents of SeO (Scheme 4). However, allylic
2
alcohol isomers, an enone produced by further oxidation of
the allylic alcohols, and unreacted starting material were all
present, which resulted in numerous spots on the TLC plate.
We therefore explored alternative oxidation/reduction
strategies for preparing the desired alcohol 1. Treatment
Because, it was not possible to separate the desired
trans-isomer 8 from the corresponding cis-isomer, we used
a mixture of trans- and cis-8 in the next step. At this stage,
the hydroxy group in intermediate 8 was protected with a
TBS group under the same conditions as described previ-
ously to provide the trans-compound 10 in 80% yield. We
originally thought that the subsequent conversion of the
carbonyl group into a double bond would be achieved by a
Wittig reaction, but no reaction was observed and only the
starting material was recovered. We then attempted an ole-
with excess SeO and TBHP in the presence of 4 Å MS at
2
CH Cl2 at room temperature, followed by reduction with
2
lithium triethylborohydride in THF at −78 °C gave the key
intermediate 9 in 37% yield over two steps.3
g,14
The endgame for the synthesis of entecavir was the in-
troduction of the purine moiety through nucleophilic sub-
stitution of the hydroxy group with compound 2, with sub-
sequent removal of the Boc and TBS groups with treatment
11
fination by using the Nysted reagent. Treatment of ketone
0 with the Nysted reagent, by following the procedure de-
1
12
3h
veloped by Ogan et al., produced the exocyclic olefin 3 in
with HCl according to the reported procedure (Scheme 5).
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

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TBSO
L. Liu et al.
Letter
O
O
Supporting Information
b
a
Supporting information for this article is available online at
TBSO
https://doi.org/10.1055/s-0037-1612215.
S
u
p
p
orit
n
g Inform ati
o
n
S
u
p
p
orit
n
g Inform ati
o
n
HO
TBSO
8
10
References and Notes
d
path A
OH
TBSO
TBSO
TBSO
(1) Razavi-Shearer, D.; Gamkrelidze, I.; Nguyen, M. H. et al. Lancet
Gastroenterol. Hepatol. 2018, 3, 383.
SeO2 (1.0 equiv)
TBSO
c
1
3
(
2) Bisacchi, G. S.; Chao, S. T.; Bachard, C.; Daris, J. P.; Innaimo, S.;
Jacobs, G. A.; Kocy, O.; Lapointe, P.; Martel, A.; Merchant, Z.;
Slusarchyk, W. A.; Sundeen, J. E.; Young, M. G.; Colonno, R.;
Zahler, R. Bioorg. Med. Chem. Lett. 1997, 7, 127.
path B
(
3) (a) Zahler, R.; Slusarchyk, W. A. EP 0335355, 1989. (b) Innaimo,
S. F.; Seifer, M.; Bisacchi, G. S.; Standring, D. N.; Zahler, R.;
Colonno, R. J. Antimicrob. Agents Chemother. 1997, 41, 1444.
(c) Pendri, Y. R.; Chen, C.-P. H.; Patel, S. S.; Evans, J. M.; Liang, J.;
Kronenthal, D. R.; Powers, G. L.; Prasad, S. J.; Bien, J. T.; Shi, Z.;
Patel, R. N.; Chan, Y. Y.; Rijhwani, S. K.; Singh, A. K.; Wang, S.;
Stojanovic, M.; Polniaszek, R.; Lewis, C.; Thottathil, J.;
Krishnamurty, D.; Zhou, M. X.; Vemishetti, P. WO 2004052310,
O
OH
TBSO
TBSO
TBSO
TBSO
11
Scheme 4 Reagents and conditions: (a) TBSCl, imidazole, DMF, 0 ℃, 2 h,
0%; (b) Nysted’s reagent, TiCl , CH Cl , –78 ℃ to r.t., 2 h, 87%; (c)
SeO , TBHP, 4Å MS, CH Cl , r.t., 30 h; (d) LiBHEt , THF, –78 ℃, 37% from 3
8
4
2
2
2
2
2
3
2004. (d) Ziegler, F. E.; Sarpong, M. A. Tetrahedron 2003, 59,
9013. (e) Zhou, M. X.; Reiff, E. A.; Vemishetti, P.; Pendri, Y. R.;
Singh, A. K.; Prasad, S. J.; Dhokte, U. P.; Qian, X.; Mountford, P.;
Hartung, K. B.; Sailes, H. WO 2005118585, 2005. (f) Rawal, R. K.;
Singh, U. S.; Gadthula, S.; Chu, C. K. Curr. Protoc. Nucleic Acid
Chem. 2011, 47, 14.7.1. (g) Zhou, B.; Li, Y. Tetrahedron Lett. 2012,
53, 502. (h) Liu, X.; Jiao, X.; Wu, Q.; Tian, C.; Li, R.; Xie, P. Tetra-
hedron Lett. 2012, 53, 3805. (i) Velasco, J.; Ariza, X.; Badía, L.;
Bartra, M.; Berenguer, R.; Farràs, J.; Gallardo, J.; Garcia, J.;
Gasanz, Y. J. Org. Chem. 2013, 78, 5482. (j) Hyun, Y. E.; Kim, H.-
R.; Choi, Y.; Jeong, L. S. Asian J. Org. Chem. 2017, 6, 1213.
N
O
OH
e
N
NH
NH2
TBSO
HO
N
TBSO
H
N
1
BocHN
N
HO
Entecavir
HN
N
O
2
Scheme 5 Reagents and conditions: (e) See Ref. 3(h).
(k) Wang, S.-c.; Zhang, X.-q.; Gu, H.-m.; Zhu, X.-y.; Guo, Y.-j. Org.
Prep. Proced. Int. 2017, 49, 568. (l) Xu, H.; Wang, F.; Xue, W.;
Zheng, Y.; Wang, Q.; Qiu, F.; Jin, Y. Org. Process Res. Dev. 2018,
22, 377. (m) Gioti, E. G.; Koftis, T. V.; Neokosmidis, E.; Vastardi,
E.; Kotoulas, S. S.; Trakossas, S.; Tsatsas, T.; Anagnostaki, E. A.;
Panagiotidis, T. D.; Zacharis, C.; Tolika, E. P.; Varvogli, A.-A.;
Andreou, T.; Gallos, J. K. Tetrahedron 2018, 74, 519.
In conclusion, we have accomplished a formal synthesis
of entecavir by a new and straightforward approach, in
which the chiral center is introduced by a Sharpless asym-
metric epoxidation. The use of a catalytic chiral reagent
should decrease production costs in comparison with those
of other synthetic routes. The other key features of the
route include a Morita–Baylis–Hillman reaction, a reductive
epoxide opening of an α,β-epoxy ketone, and a Riley seleni-
um dioxide oxidation. We made a thorough study of the
benzeneselenolate-induced reductive epoxide opening of
an α,β-epoxy ketone to generate a β-hydroxy ketone, and
we obtained a moderate yield of the epoxide-opened prod-
uct in a mixture of buffer solvents. The present strategy
provides an alternative approach to the synthesis of similar
compounds. Further studies on the application of this ap-
proach to natural products and pharmaceuticals are in
progress and will be reported in due course.
(
4) Huang, S.; Liu, D.; Tang, L.; Huang, F.; Yang, W.; Wang, X. Synlett
2015, 26, 2019.
(
5) Ito, H.; Takenaka, Y.; Fukunishi, S.; Iguchi, K. Synthesis 2005,
3035.
(6) Bailey, M.; Staton, I.; Ashton, P. R.; Markó, I. E.; Ollis, W. D. Tetra-
hedron: Asymmetry 1991, 2, 495.
7) Barrero, A. F.; Herrador, M. M.; Quílez del Moral, J. F.; Arteaga,
P.; Meine, N.; Pérez-Morales, M. C.; Catalán, J. V. Org. Biomol.
Chem. 2011, 9, 1118.
8) Singh, V.; Porinchu, M. Tetrahedron 1996, 52, 7087.
(9) Salvador, J. A. R.; Leitão, A. J. L.; Sá e Melo, M. L.; Hanson, J. R.
Tetrahedron Lett. 2005, 46, 1067.
(
(
(
(
10) Engman, L.; Stern, D. J. Org. Chem. 1994, 59, 5179.
11) (a) Nysted, L. N. US 3865848, 1975. (b) Anon Aldrichimica Acta
1993, 26, 14.
(
(
(
12) Ogan, M. D.; Kucera, D. J.; Pendri, Y. R.; Rinehart, J. K. J. Labelled
Funding Information
Compd. Radiopharm. 2005, 48, 645.
13) Gioiello, A.; Sardella, R.; Rosatelli, E.; Sadeghpour, B. M.;
Natalini, B.; Pellicciari, R. Steroids 2012, 77, 250.
14) {[(1R,3S,4S)-4-methyl-5-methylenecyclopentane-1,3-
diyl]bis(oxy)}bis[tert-butyl(dimethyl)silane] (1)
This work was financially supported by the National Science Founda-
tion of China (21062088, 21562020) and the Science and Technology
Plan Project of Jiangxi Province (No. 20151BBG70028,
20142BBE50006).
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A solution of the protected diol 11 (1.2 g, 3.2 mmol) in THF (14
mL) was treated by dropwise addition of a 1.0 M solution of
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

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Synlett
L. Liu et al.
Letter
2
0
LiBHEt3 in THF (6.5 mL, 6.4 mmol) at –78 ℃, and the mixture
was stirred for 15 min. The reaction was then quenched by
(2.8 mmol, 87%); mp 63–65 °C, [α]D –8.18 (c 1.25, CHCl3).
1
H NMR (400 MHz, CDCl ): δ = 0.03 (d, J = 4.9 Hz, 6 H), 0.08 (s, 6
3
addition of sat. aq NH Cl, and the resultant mixture was stirred
at r.t. for another 20 min. The mixture was then poured into sat.
aq potassium sodium tartrate (Rochelle salt), and the aqueous
H), 0.88 (s, 18 H), 1.82 (d, J = 13.6 Hz, 1 H), 1.93–2.04 (m, 1 H),
2.74 (m, 1 H), 3.30 (dd, J = 10.2, 9.0 Hz, 1 H), 3.56 (dd, J = 10.3,
5.1 Hz, 1 H), 4.35 (d, J = 9.6 Hz, 2 H), 5.12 (s, 1 H), 5.38 (s,
4
13
phase was extracted with Et O (3 × 20 mL). The organic layers
1 H). C NMR (100 MHz, CDCl ) = –5.4, –5.3, –4.7, 18.0, 18.4,
2
3
were combined, washed with brine, dried (Na SO ), and con-
25.9, 26.0, 42.2, 55.1, 64.8, 75.4, 111.7, 154.4. HRMS (ESI): m/z
2
4
+
centrated. The residue was purified by column chromatography
silica gel, PE–EtOAc (10:1)] to give a clear solid; yield: 1.04 g
[M + Na] calcd for C19H40NaO Si : 395.2408; found: 395.2406.
3
2
[
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E