The complete synthesis of favipiravir from 2-aminopyrazine

Article abstract of DOI:10.1007/s11696-018-0654-9

Favipiravir was first synthesized from an inexpensive and commercially available starting material, 2-aminopyrazine. The preferred route embedded within Scheme?4 consisted of seven steps, and was highlighted by the novel and efficient synthesis of 3,6-dichloropyrazine-2-carbonitrile 8. This intermediate was prepared in four successive steps which were regioselective chlorination of the pyrazine ring, bromination, Pd-catalyzed cyanation, and Sandmeyer diazotization/chlorination. This protocol eliminated the hazardous POCl₃ of previous synthetic methods and offered a better yield (48%) which was 1.3-fold higher than a recently published procedure. From intermediate 8, the subsequent nucleophilic fluorination, nitrile hydration and hydroxyl substitution efficiently afforded the target product. Another synthetic approach with the same starting material was also investigated to bypass the allergy-causing dichloro intermediate 8. However, the key step of monofluorination at the pyrazine C6 position of intermediate 19 or 22 was not achieved.

Full text of DOI:10.1007/s11696-018-0654-9

Chemical Papers
https://doi.org/10.1007/s11696-018-0654-9
ORIGINAL PAPER
The complete synthesis of favipiravir from 2‑aminopyrazine
Qi Guo1,2 · Mingshuo Xu1,2 · Shuang Guo1,2 · Fuqiang Zhu · Yuanchao Xie · Jingshan Shen
3
1
1,2
Received: 15 June 2018 / Accepted: 23 November 2018
©
Institute of Chemistry, Slovak Academy of Sciences 2019
Abstract
Favipiravir was frst synthesized ꢀrom an inexpensive and commercially available starting material, 2-aminopyrazine. The
preꢀerred route embedded within Scheme 4 consisted oꢀ seven steps, and was highlighted by the novel and eꢁcient synthesis
oꢀ 3,6-dichloropyrazine-2-carbonitrile 8. This intermediate was prepared in ꢀour successive steps which were regioselective
chlorination oꢀ the pyrazine ring, bromination, Pd-catalyzed cyanation, and Sandmeyer diazotization/chlorination. This
protocol eliminated the hazardous POCl oꢀ previous synthetic methods and oꢂered a better yield (48%) which was 1.3-ꢀold
3
higher than a recently published procedure. From intermediate 8, the subsequent nucleophilic ꢃuorination, nitrile hydration
and hydroxyl substitution eꢁciently aꢂorded the target product. Another synthetic approach with the same starting material
was also investigated to bypass the allergy-causing dichloro intermediate 8. However, the key step oꢀ monoꢃuorination at
the pyrazine C6 position oꢀ intermediate 19 or 22 was not achieved.
Keywords Favipiravir · 2-Aminopyrazine · Fluorination · Sandmeyer reaction
Introduction
bunyavirus, ꢃavivirus, alphavirus, and norovirus (Furuta
et al. 2013; Furuta et al. 2009; Jin et al. 2015). Moreover,
it is believed to be a promising therapeutic candidate ꢀor
Ebola virus inꢀection (Smither et al. 2014). In a retrospective
clinical study during the outbreak oꢀ Ebola virus, patients
who received T-705 had a signifcant viral load reduction
compared with the control group (Bai et al. 2016). Recently,
ꢀavipiravir and related structures have attracted extensive
attentions in the feld oꢀ antiviral and antiparasitic research
(Gao et al. 2018; Huchting et al. 2018; Huchting et al. 2017;
Klejch et al. 2018; Plebanek et al. 2017; Wang et al. 2016).
Favipiravir is a small pyrazine compound containing
Favipiravir (T-705, 6-ꢃuoro-3-hydroxypyrazine-2-carbox-
amide) is a novel anti-inꢃuenza drug which ꢀunctions to
selectively inhibit the RNA-dependent RNA polymerase
oꢀ inꢃuenza virus (Furuta et al. 2013). T-705 riboꢀurano-
syl triphosphate (T-705 RTP, Fig. 1) is the active ꢀorm,
generated ꢀrom the parent drug by a series oꢀ intracellular
enzymes (Sangawa et al. 2013; Naesens et al. 2013). Favip-
iravir also displays inhibitory activities against a number oꢀ
other pathogenic RNA viral inꢀections, such as arenavirus,
three substituents (–F, –OH, –CONH ) in place. Due to the
2
Electronic supplementary material The online version oꢀ this
article (https://doi.org/10.1007/s11696-018-0654-9) contains
supplementary material, which is available to authorized users.
hydroxyl group, this compound exhibits acidic properties
and could be tautomerized to the keto ꢀorm, shown in Fig. 1
(
El-Nahas and Hirao 1999). With respect to the synthesis,
three routes are now available. Route 1 is the original one
starting ꢀrom methyl 3-amino-6-bromopyrazine-2-carbox-
ylate 1, and consisted oꢀ fve steps, diazotization-alcoholy-
sis, Pd-catalyzed imine substitution/hydrolysis, aminolysis,
Schiemann ꢃuorination and demethylation (Scheme 1). The
frst step oꢀ diazotization-alcoholysis needs to be perꢀormed
in concentrated sulꢀuric acid and only gives a yield oꢀ 35%.
The ꢀollowing step is a sequence oꢀ two reactions, which is
tedious and ineꢁcient ꢀor the synthesis oꢀ 3. Moreover, the
conversion oꢀ 4 to 5 involves the highly corrosive Olah’s
*
Yuanchao Xie
xieyuanchao@simm.ac.cn
1
CAS Key Laboratory ꢀor Receptor Research, Shanghai
Institute oꢀ Materia Medica, Chinese Academy oꢀ Sciences
(
CAS), 555 Zuchongzhi Road, Shanghai 201203,
People’s Republic oꢀ China
2
3
University oꢀ Chinese Academy oꢀ Sciences, No. 19A
Yuquan Road, Beijing 100049, People’s Republic oꢀ China
Topharman Shanghai Co., Ltd, No.388 Jialilue
Road, Zhangjiang Hitech Park, Shanghai 201203,
People’s Republic oꢀ China
Vol.:(0
1
123456789)
3

Chemical Papers
reagent (Furuta and Egawa 2000). Considering these disad-
vantages, this route is not welcomed ꢀor the scalable produc-
tion oꢀ ꢀavipiravir.
be easily replaced by hydroxyl group to ꢀurnish the fnal
product. Comparatively, Route 2 is attractive with respect
to step count, but not economically ꢀeasible ꢀor the prepara-
tion oꢀ 8 due to the low yield (18% yield ꢀor the nitration
oꢀ 6, shown in the experiment section) and the high cost
oꢀ the starting material 6. Route 3 was reported recently
with cheaper 3-aminopyrazine-2-carboxylic acid 11 as the
starting material, ꢀrom which, fve steps were required to
obtain 8 in a yield oꢀ 37%. In both oꢀ the routes, a substantial
Route 2 and Route 3 share the same crucial intermedi-
ate, 3,6-dichloropyrazine-2-carbonitrile 8, which is pre-
pared ꢀrom 3-hydroxypyrazine-2-carboxamide 6 or 3-ami-
nopyrazine-2-carboxylicacid 11 (Schemes 2 and 3). This
dichloro-intermediate is then transꢀormed to 9 by reacting
with KF, and subsequent nitrile hydration in concentrated
hydrochloric acid or treatment with an alkaline solution oꢀ
H O gives 10 (Beldar and Jordis 2009; Hara et al. 2010; Liu
excess oꢀ POCl and base are indispensable to completely
3
accomplish the ꢀunctional group transꢀormations. The large
2
2
et al. 2017). The C3 ꢃuorine oꢀ 10 is more reactive and could
Fig. 1 Chemical structures oꢀ ꢀavipiravir (T-705) and T-705 RTP
Scheme 1 Route 1 ꢀor the synthesis oꢀ ꢀavipiravir
Scheme 2 Route 2 ꢀor the synthesis oꢀ ꢀavipiravir
1
3

Chemical Papers
Scheme 3 Route 3 ꢀor the synthesis oꢀ ꢀavipiravir
quantity oꢀ POCl required in the procedure is a big concern
Na CO solution (20 mL) was added. The mixture was
2 3
3
with respect to both scale-up and waste disposal.
extracted with ethyl acetate (100 mL). The organic layer
was dried over anhydrous Na SO , decolorized with acti-
Herein, we reported an alternative approach to the syn-
2
4
thesis oꢀ intermediate 8 without using POCl . Starting ꢀrom
vated charcoal (0.5 g), and concentrated. The residue was
purifed by chromatography on silica gel eluting with
petroleum ether (PE)-ethyl acetate (EA) (10:1–2:1) to give
a light-yellow solid (1.38 g, yield 55%).
3
commercially available 2-aminopyrazine, the synthesis
was achieved in 48% yield over ꢀour steps. Following the
established procedures, the target compound was smoothly
obtained. Another strategy ꢀor the synthesis oꢀ ꢀavipiravir not
relying on the allergy-causing dichloro intermediate 8 was
also investigated. However, no good results were achieved.
Method B. To a suspension oꢀ 2-aminopyrazine (3 g,
31.6 mmol) in acetonitrile (50 mL) was added N-chloro-
N-methoxy-4-methylbenzenesulꢀonamide (TSA, 8.2 g,
3
4.7 mmol) in batches at room temperature. The reaction
mixture was heated at 40 °C ꢀor 4 h. Subsequently, the
solvent was evaporated in vacuum and the residue was
partitioned between ethyl acetate (50 mL) and saturated
aqueous Na CO solution (20 mL). The organic layer was
Experimental
Materials and methods
2
3
dried over anhydrous Na SO , decolorized with activated
2
4
1
H-NMR and 13C-NMR spectra were determined on a
charcoal (0.8 g), and concentrated. The crude product con-
taining ~ 5% dichlorinated impurity judged by TLC was
purifed by chromatography on silica gel eluting with PE/
EA (10:1–2:1) to give a light-yellow solid (3.3 g, yield
80%).
Brucker 400 Hz or Brucker 500 Hz instrument with TMS as
an internal standard. Multiplicities were reported as ꢀollows:
s=singlet, d=doublet, t=triplet, q=quartet, m=multiplet,
and br = broad. ESI–MS was determined on Finnigan™
LTQ™ (Thermo Fisher Scientifc, Bremen, Germany) lin-
ear ion trap mass spectrometer. All the reagents were pur-
chased ꢀrom commercial suppliers and used without ꢀurther
purifcation. All reactions were monitored by thin-layer
chromatography (TLC) on 25.4×76.2 mm silica gel plates
Method C. Reaction oꢀ 2-aminopyrazine (3 g, 31.6 mmol)
with N-chloro-N-methoxybenzenesulꢀonamide (BSA, 7.7 g,
34.7 mmol) ꢀollowing the same procedure oꢀ method B pro-
vided 3.1 g oꢀ 17 in a yield oꢀ 77%.
(
GF-254).
2‑Amino‑5‑chloropyrazine (17)
Preparation of 2‑amino‑5‑chloropyrazine (17)
Yield: 80% (method B), light-yellow solid, M.p.:128-
1
Method A. To a solution oꢀ 2-aminopyrazine (1.84 g,
130 °C (Lit. 129–130 °C, Palamidessi et al. 1964). H NMR
1
9.4 mmol) in acetonitrile (15 mL) was added NCS
(400 MHz, CDCl ): δ 8.01 (d, 1H, J=1.4 Hz), 7.76 (d, 1H,
3
1
3
(
2.59 g, 19.4 mmol) in batches at room temperature. The
J=1.4 Hz), 4.57 (s, 2H). C NMR (125 MHz, CDCl ): δ
3
+
35
reaction mixture was stirred ꢀor 5 h, and fltrated through
153.19, 141.32, 137.48, 130.82. EI-MS m/z: 129 (M , Cl
®
+
37
+
35
diatomaceous earth (Celite ). The flter cake was washed
100), 131 (M , Cl , 31), 102 (M , Cl , –H, –HCN, 100),
+
37
+
with ethyl acetate (20 mL × 2). The combined fltrate was
concentrated under vacuum, and then saturated aqueous
104 (M , Cl , –H, –HCN, 31), 67 (M , –H, –HCN, –HCl,
28).
1
3

Chemical Papers
Preparation of 2‑amino‑3‑bromo‑5‑chloropyrazine
18a)
0.15 mmol) were stirred in DMF (40 mL) at 120 °C ꢀor
10 h under nitrogen protection. Aꢀter cooling to room tem-
perature, 10% aqueous solution oꢀ Na S O (10 mL), water
(
2
2
3
1
7 (1.1 g, 8.5 mmol) was dissolved in dichloromethane
(60 mL) and ethyl acetate (200 mL) were added. The result-
ing mixture was stirred ꢀor 10 min, then fltered, and the
flter cake was washed with ethyl acetate (150 mL). The
organic layer was separated, washed with water (3×50 mL),
dried over anhydrous Na SO and concentrated. The residue
(
25 mL) ꢀollowed by the addition oꢀ NBS (1.52 g, 8.5 mmol)
in batches over a period oꢀ 15 min at room temperature.
When the reaction was completed, saturated aqueous
Na CO solution (10 mL) was added. Aꢀter stirring ꢀor
2
3
2
4
1
0 min, the mixture was partitioned between dichlorometh-
was purifed by chromatography on silica gel eluting with
PE/EA (10:1–2:1) to give a light-yellow solid (1.9 g, yield
85%).
ane (100 mL) and water (100 mL). The organic layer was
dried over anhydrous Na SO , decolorized with activated
2
4
charcoal (0.5 g), and concentrated to give a yellow solid
(
1.53 g, yield 87%).
6‑Chloro‑3‑aminopyrazine‑2‑carbonitrile (19a)
1
2
‑Amino‑3‑bromo‑5‑chloropyrazine (18a)
Yield: 85%, light-yellow solid, M.p.: 156–158 °C. H NMR
1
3
(
400 MHz, CDCl ): δ 8.23 (s, 1H), 5.34 (s, 2H). C NMR
3
1
Yield: 87%, yellow solid, M.p.: 108–110 °C. H NMR
(100 MHz, CDCl ): δ 154.98, 146.84, 137.41, 114.07,
3
1
+
35
+
37
(
(
400 MHz, CDCl ): δ 7.97 (s, 1H), 5.08 (s, 2H). H NMR
111.16. EI-MS m/z: 154 (M , Cl , 100), 156 (M , Cl , 35),
3
1
3
+
35
+
37
400 MHz, DMSO-d ): δ 8.10 (s, 1H), 6.99 (s, 2H).
C
127 (M , Cl , –H, –CN, 48), 129 (M , Cl , –H, –CN, 15).
6
NMR (125 MHz, CDCl ): δ 151.66, 140.26, 134.75, 123.27.
3
+
35
79
+
37
79
+
EI-MS m/z: 207 (M , Cl , Br , 76), 209 (M , Cl , Br
Preparation of 6‑bromo‑3‑aminopyrazine‑2‑
carbonitrile (19b)
3
5
81
+
37
81
and Cl , Br , 100), 211 (M , Cl , Br , 26), 128 (M ,
3
5
+
37
+
35
Cl , –Br, 75), 130 (M , Cl , –Br, 25), 101 (M , Cl , –Br,
+
37
–
HCN, 30), 103 (M , Cl , –Br, –HCN, 10).
The suspension oꢀ 18b (2.51 g, 10.0 mmol), NaCN (0.59 g,
1
2.0 mmol), CuI (0.95 g, 5.0 mmol) and Pd(PPh ) (0.14 g,
3 4
Preparation of 2‑amino‑3,5‑dibromopyrazine (18b)
0.1 mmol) were stirred in DMF (20 mL) at 120 °C ꢀor 10 h
under nitrogen protection. Aꢀter cooling to room tempera-
ture, 10% aqueous solution oꢀ Na S O (10 mL) and water
To a suspension oꢀ 2-aminopyrazine (4 g, 42.1 mmol)
in dichloromethane (100 mL) was added NBS (18.7 g,
2
2
3
(50 mL) were added. The mixture was extracted with ethyl
acetate (50 mL×3), washed with water (3×50 mL), dried
over anhydrous Na SO and concentrated. The residue was
1
05 mmol) in batches over a period oꢀ 1 h at room tempera-
ture. The reaction mixture was stirred at room temperature
or 5 h. When the reaction was completed, saturated aque-
ous Na CO solution (40 mL) was added. Aꢀter stirring ꢀor
2
4
ꢀ
purifed by chromatography on silica gel eluting with PE/
EA (10:1–2:1) to give an oꢂ-white solid (1.1 g, yield 55%).
2
3
1
5 min, the mixture was partitioned between dichlorometh-
ane (200 mL) and water (100 mL). The organic layer was
then dried over anhydrous Na SO , decolorized with acti-
6‑Bromo‑3‑aminopyrazine‑2‑carbonitrile (19b)
2
4
vated charcoal (1 g), and concentrated to give a yellow solid
Yield: 55%, oꢀꢀ-white solid, M.p.: 180–182 °C (Lit.
1
(
8.0 g, yield 76%).
183–184 °C, Sato et al. 1990a, b). H NMR (400 MHz,
1
DMSO-d ): δ 8.45 (s, 1H), 7.62 (s, 2H). H NMR (400 MHz,
6
1
3
2‑Amino‑3,5‑dibromopyrazine (18b)
CDCl ): δ 8.30 (s, 1H), 5.28 (s, 2H). C NMR (125 MHz,
3
DMSO-d ): δ 156.70, 150.49, 123.64, 115.45, 110.60.
6
+
79
+
81
Yield: 76%, yellow solid, M.p.: 112–114 °C (Lit.
EI-MS m/z: 198 (M , Br , 100), 200 (M , Br , 98), 171
1
+
79
+
81
1
13–114 °C, Sato et al. 1990a, b). H NMR (400 MHz,
(M , Br , –H, –CN, 17), 173 (M , Br , –H, –CN, 17),
1
+
+
DMSO-d ): δ 8.13 (s, 1H), 6.99 (s, 2H). H NMR (400 MHz,
119(M , –Br, 32), 92 (M , –Br, –H, –CN, 46).
6
1
3
CDCl ): δ 8.04 (s, 1H), 5.09 (s, 2H). C NMR (125 MHz,
3
CDCl ): δ 151.86, 143.06, 123.96, 123.65. ESI–MS m/z:
Preparation of 3,6‑dichloropyrazine‑2‑carbonitrile
(8)
3
−
2
49.2 [M − H] .
Preparation of 6‑chloro‑3‑aminopyrazine‑2‑
carbonitrile (19a)
To a solution oꢀ 19a (3 g, 20 mmol) in DCM (50 mL) at 0 °C
was successively added TiCl (2.2 mL, 20 mmol) and tert-
4
butyl nitrite (7.4 mL, 62 mmol). The reaction mixture was
stirred at room temperature ꢀor 3 h. When the reaction was
completed monitored by TLC, the solvent was evaporated
The suspension oꢀ 18a (3 g, 14.5 mmol), NaCN (0.85 g,
1
7.4 mmol), CuI (1.3 g, 7.3 mmol) and Pd(PPh ) (0.17 g,
3 4
1
3

Chemical Papers
and the residue was treated with water (50 mL) and then
extracted with ethyl acetate (2 × 50 mL). The extract was
dried over Na SO , and concentrated to give 8 as a white
(dd, J=41.6, 11.0 Hz), 114.04 (d, J=35.8 Hz), 110.62 (d,
+
+
J=9.1 Hz). EI-MS m/z: 141 (M , 100), 122 (M , –F, 50),
+
96 (M , –F, –CN, 50).
2
4
solid (2.8 g, yield 81%).
Preparation of 3,6‑diꢀuoropyrazine‑2‑carboxamide
(10)
3,6‑Dichloropyrazine‑2‑carbonitrile (8)
Yield: 81%, white solid, M.p.: 93–94 °C (Lit. 90–91 °C,
To a solution oꢀ 9 (1.4 g, 10 mmol) in THF (5 mL) was
added concentrated hydrochloric acid (30 mL). The reaction
mixture was heated at 60 °C ꢀor 1.5 h, then cooled to room
temperature and evaporated under vacuum. The residue was
purifed by chromatography on silica gel with PE/EA as elu-
ent to give 10 as a white solid (1.2 g, yield 75%).
1
13
Li 2017). H NMR (400 MHz, CDCl ): δ 8.60 (s, 1H).
C
3
NMR (125 MHz, CDCl ): δ 149.82, 147.65, 147.03, 128.90,
3
+
35
35
+
1
12.60. EI-MS m/z: 173 (M , Cl , Cl , 100), 175 (M ,
3
5
37
+
37
37
Cl , Cl , 65), 177 (M , Cl , Cl , 10).
Preparation of 6‑bromo‑3‑chloropyrazine‑2‑
carbonitrile (20)
3,6‑Diꢀuoropyrazine‑2‑carboxamide (10)
Reaction oꢀ 19b (2 g, 10 mmol) with TiCl (1.1 mL,
4
1
Yield: 75%, white solid, M.p.: 122–124 °C. H NMR
1
0 mmol) and tert-butyl nitrite (3.7 mL, 31 mmol) ꢀollow-
(
400 MHz, DMSO-d ) δ 8.57 (dd, J = 8.2, 1.7 Hz, 1H),
6
ing the above procedure ꢀor synthesizing 8 gave a white solid
1.8 g, yield 85%).
1
3
8
.21 (s, 1H), 7.98 (s, 1H). C NMR (125 MHz, DMSO-
(
d ): δ 162.55 (d, J=7.2 Hz), 157.05–155.08 (dd, J=246.4,
6
2
.5 Hz), 156.73–164.69 (dd, J=251.7, 2.5 Hz), 133.46 (dd,
6‑Bromo‑3‑chloropyrazine‑2‑carbonitrile (20)
J=43.2, 12.0 Hz), 131.50 (dd, J=26.9, 8.6 Hz). EI-MS m/z:
+
+
+
1
2
59 (M , 32), 116 (M , –CONH, 100), 115 (M , –CONH ,
1
2
Yield: 85%, white solid, M.p.: 98–100 °C. H NMR
5).
1
(
500 MHz, DMSO-d ): δ 9.08 (s, 1H). H NMR (400 MHz,
6
1
3
CDCl ): δ 8.71 (s, 1H). C NMR (100 MHz, CDCl ): δ
3
3
Preparation of 6‑ꢀuoro‑3‑hydroxypyrazine‑2‑
carboxamide (favipiravir)
1
50.45, 149.82, 138.22, 129.66, 112.58. EI-MS m/z: 217
+
35
79
+
37
79
35
81
(
M , Cl , Br , 45), 219 (M , Cl , Br and Cl , Br , 65),
+
37
81
+
35
+
2
21 (M , Cl , Br , 15), 138 (M , Cl , –Br, 100), 140 (M ,
3
7
+
35
+
10 (1.2 g, 7.5 mmol), sodium bicarbonate (3.8 g, 45.2 mmol)
were added into a mixture oꢀ 1,4-dioxane (10 mL) and
water (20 mL). The reaction mixture was heated at 60 °C
Cl , –Br, 30), 111 (M , Cl , –Br, –HCN, 15), 113 (M ,
3
7
Cl , –Br, –HCN, 4).
ꢀ
or 8 h. When the reaction was completed, hydrochloric
Preparation of 3,6‑diꢀuoropyrazine‑2‑carbonitrile
acid (30 mL, 3 M) was added to adjust the pH to 3–4. The
precipitate was collected, dried in vacuum to give a light-
yellow solid (0.83 g). The fltrate was extracted with ethyl
acetate (2 × 50 mL). The organic layer was washed with
(9)
A mixture oꢀ 8 (2.6 g, 15.0 mmol), KF (5.23 g, 90.0 mmol),
tetrabutylammonium bromide (1.93 g, 6.0 mmol) was pre-
dried under vacuum over phosphorus pentoxide. The mix-
ture was charged in a polytetraꢃuoroethylene bottle ꢀollowed
by the addition oꢀ dried DMSO (20 mL), and heated at 60 °C
brine (2 × 10 mL), dried over Na SO , and evaporated to
2
4
give another portion (0.14 g) oꢀ the product. The total yield
was 82%.
ꢀ
or 2.5–3 h. The reaction mixture was cooled to room tem-
perature, treated with water (80 mL) and extracted with
diethyl ether (100 mL). The organic layer was washed with
water (3×25 mL), dried over Na SO , and purifed by chro-
6‑Fluoro‑3‑hydroxypyrazine‑2‑carboxamide
(favipiravir)
2
4
matography on silica gel with PE/EA (50:1–10:1) as eluent
Yield: 82%, light-yellow solid, M.p.: 176–178 °C
1
to give 9 as a white solid (1.27 g, yield 60%).
(175–177 °C, Liu et al. 2017). H NMR (400 MHz,
DMSO-d ): δ 13.40 (s, 1H), 8.74 (s, 1H), 8.52 (s, 1H), 8.50
6
1
3,6‑Diꢀuoropyrazine‑2‑carbonitrile (9)
(s, 1H). H NMR (400 MHz, CDCl ): δ 12.35 (br, 1H),
3
1
3
8
.31 (d, 1H, J = 8.0 Hz), 7.43 (br, 1H), 5.89 (br, 1H).
C
Yield: 60%. white solid, M.p.: 58–60 °C (Lit. 56–57 °C,
NMR (125 MHz, DMSO-d ): δ 169.20, 160.21, 152.87
6
1
Li 2017). H NMR (400 MHz, CDCl ) δ 8.35 (dd, J=8.1,
(d, J = 243.8 Hz), 136.58, 122.40. ESI–MS m/z: 156.1
3
13
−
1
.4 Hz, 1H). C NMR (125 MHz, CDCl ): δ 159.73-157.69
[M − H] .
3
(
d, J = 255 Hz), 157.64-155.62 (d, J = 252.5 Hz), 135.09
1
3

Chemical Papers
Preparation of 3,5‑dibromo‑2‑hydroxylpyrazine
21b)
δ 160.73, 143.18, 134.78, 118.47, 113.87. ESI–MS m/z:
−
(
153.9 [M − H] .
1
8b (3.0 g, 12 mmol) was added into acetic acid (20 mL)
Preparation of 6‑bromo‑3‑hydroxylpyrazine‑2‑
carbonitrile (22b)
and concentrated sulꢀuric acid (4 mL) under ice bath. To
this mixture was added a pre-prepared solution oꢀ NaNO
2
(
1.65 g, 24 mmol) in water (10 mL). The reaction mixture
Reaction oꢀ 21b (1 g, 4 mmol) with NaCN (230 mg,
4.8 mmol) catalyzed by CuI (380 mg, 2 mmol) and
Pd(PPh ) (50 mg, 0.04 mmol) according to the proce-
was stirred at room temperature ꢀor 3 h, and then poured
into ice water (100 g). The precipitate was collected by fl-
trated, washed with water and dried to give an oꢂ-white
solid (1.8 g, yield 61%).
3
4
dure ꢀor synthesizing 19b gave a gray solid (350 mg, yield
45%).
6‑Bromo‑3‑hydroxylpyrazine‑2‑carbonitrile (22b)
3,5‑Dibromo‑2‑hydroxylpyrazine (21b)
1
1
Yield: 45%, gray solid, M.p.: 168–170 °C. H NMR
Yield: 61%, oꢂ-white solid, M.p.: 182–184 °C. H NMR
13
1
(400 MHz, DMSO-d ): δ 8.46 (s, 1H). C NMR (125 MHz,
(
400 MHz, CDCl ): δ 11.23 (br, 1H), 7.53 (s, 1H). H
6
3
DMSO-d ): δ 151.12, 133.03, 129.18, 114.82, 99.21.
NMR (400 MHz, DMSO-d ): δ 13.16 (s, 1H), 7.94 (s, 1H).
6
6
−
1
3
ESI–MS m/z: 197.9 [M − H] .
C NMR (125 MHz, CD OD): δ 154.19, 136.73, 131.57,
3
−
1
14.62. ESI–MS m/z: 250.9 [M − H] .
Preparation of 3‑hydroxy‑6‑nitropyrazine‑2‑
carboxamide (7)
Preparation of 5‑chloro‑3‑bromo‑2‑hydroxylpyrazine
21a)
(
To a solution oꢀ 6 (4.0 g, 28.7 mmol) in concentrated sul-
ꢀ
uric acid (20 mL) under ice-cooling was added potassium
Reaction oꢀ 18a (3.0 g, 14.5 mmol) dissolved in acetic
nitrate (5.8 g, 57.5 mmol) in three batches. Aꢀter stirring at
0 °C ꢀor 4 h, the reaction mixture was poured into ice water
acid (20 mL) and concentrated sulꢀuric acid (4 mL) with
4
NaNO (1.9 g, 29 mmol) in water (10 mL) ꢀollowing the
2
(
200 g). The resulting precipitate was collected by fltration.
above procedure ꢀor synthesizing 21a gave a yellow solid
The solid was slurried in pure water (20 mL) and dried to
give a light-yellow solid (1 g, yield 19%).
(
1.6 g, yield 54%).
5‑Chloro‑3‑bromo‑2‑hydroxylpyrazine (21a)
3‑Hydroxy‑6‑nitropyrazine‑2‑carboxamide (7)
1
Yield: 54%, yellow solid, M.p.: 184–186 °C. H NMR
1
Yield: 19%, light yellow solid, M.p.: 182–184 °C. H NMR
(
400 MHz, DMSO-d ): δ 13.11 (br, 1H), 7.92 (s, 1H).
6
(400 MHz, DMSO-d ): δ 8.96 (s, 1H), 8.32 (s, 1H), 8.05 (s,
6
1
13
H NMR (500 MHz, CD OD): δ 7.58 (s, 1H). C NMR
13
3
1H). C NMR (125 MHz, DMSO-d ): δ 163.60, 156.92,
6
(
125 MHz, CD OD): δ 154.21, 136.07, 128.96, 127.76.
+
+
3
143.19, 138.63, 134.26. EI-MS m/z: 184 (M , 84), 141 (M ,
−
ESI–MS m/z: 206.9 [M − H] .
–
CONH, 10).
Preparation of 6‑chloro‑3‑hydroxylpyrazine‑2‑
carbonitrile (22a)
Results and discussion
Reaction oꢀ 21a (1.0 g, 4.8 mmol) with NaCN (280 mg,
The new synthetic approach to ꢀavipiravir starting ꢀrom
2-aminopyarzine 16 was outlined in Scheme 4. The preꢀerred
route (highlighted in red) consisted oꢀ seven steps, ꢀeatured
with a novel and eꢁcient method to the synthesis oꢀ 8 in
5
.8 mmol) catalyzed by CuI (460 mg, 2.4 mmol) and
Pd(PPh ) (55 mg, 0.05 mmol) according to the procedure
3
4
ꢀ
or synthesizing 19b gave a gray solid (0.5 g, yield 64%).
ꢀour transꢀormations.
With respect to the frst step, at the beginning, mono-chlo-
6‑Chloro‑3‑hydroxylpyrazine‑2‑carbonitrile (22a)
rination oꢀ 16 using NCS in CHCl was plagued by a very
3
1
low yield and large amounts oꢀ by-products (Cai et al. 2007).
Aꢀter a preliminary solvent screening, we determined ace-
tonitrile as the optimal reaction medium. However, ꢀurther
condition optimizations did not aꢂord signifcant improve-
ment in product yield, shown in Table 1. A stoichiometric
Yield: 64%, gray solid, M.p.: 158–160 °C. H NMR
1
(
400 MHz, DMSO-d ): δ 7.97 (s, 1H). H NMR (500 MHz,
6
1
3
CD OD): δ 8.25 (s, 1H). C NMR (125 MHz, CD OD):
3
3
1
3

Chemical Papers
Scheme 4 Our synthetic approach to ꢀavipiravir in this paper and the preꢀerred route was highlighted in red
Table 1 Screening oꢀ reaction conditions ꢀor the mono-chlorination oꢀ 16
Entry
Chlorinating Reagent
Equivalent (eq)
Time (h)
Temperature (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
NCS
NCS
NCS
NCS
BSA
TSA
TSA
TSA
TSA
0.9
1.0
1.1
1.2
1.1
0.9
1.0
1.1
1.2
8
8
8
R.T.
R.T.
R.T.
R.T.
50
50
50
44
55
50
48
77
51
76
80
54
8
12
12
12
12
12
50
50
amount oꢀ NCS was not suꢁcient to react with all the mate-
rial (Entry 2), and an increase in the amount oꢀ NCS would
lead to an overchlorinated product (Entry 3–4). Moreover,
the reaction mixture became dark brown and cloudy with
time, probably due to the ꢀormation oꢀ polymers (Cai et al.
increase oꢀ TSA ꢀrom 0.9 to 1.1 equiv signifcantly improved
the yield (Entry 6–8), oꢂering a maximum oꢀ 80%. Further
increase in the amount oꢀ TSA was detrimental to the yield
(Entry 9), which was exclusively attributed to the overchlo-
rinated by-product. The optimal ratio oꢀ TSA was 1.1 equiv,
also producing a small amount (~5%) oꢀ the overchlorinated
impurity, but could be removed at the stage or downstream
by chromatographic purifcation or recrystallization.
2
007). Recently, Pu et al. reported a chlorinating reagent
(
BSA) which provided an 80% yield oꢀ 17 (Pu et al. 2016).
Accordingly, we prepared BSA as well as the analog TSA,
and perꢀormed the reaction using the two reagents. 1.1
Equiv oꢀ BSA gave the product in a yield oꢀ 77% consistent
with the previous study (Table 1, Entry 5). However, BSA
was oil, not convenient to use. TSA was a solid and used
17 was amenable to bromination at the C3 position with
NBS in a yield oꢀ 87%, and subsequent replacement oꢀ the
bromine atom by the nitrile group smoothly aꢂorded 19a.
Sodium cyanide proved to be a good nitrile donor in the
coupling reaction with catalytic Pd(PPh ) . The reaction
ꢀor screening oꢀ reaction conditions. It was ꢀound that an
3
4
1
3

Chemical Papers
Table 2 Screening oꢀ reaction conditions ꢀor the preparation oꢀ 19a
and titanium tetrachloride (TiCl ) was applied to make this
4
transꢀormation on account oꢀ the simple workup and the
good yield (>80%). The residual TiCl would be converted
4
to the non-toxic titanium dioxide by reacting with water in
the post-processing.
Nucleophilic ꢃuorination oꢀ 8 with KF (6 equiv) and
Bu NBr (2 equiv) gave the diꢃuoro product 9 in 60% iso-
4
lated yield. Compound 19 bearing a C5 bromine group was
prepared with the same strategy, but it proved to be a very
poor substrate, only giving a small amount oꢀ the desired
product. For the ꢃuorination step, glass reaction vessel
should be avoided, otherwise the yield would be severely
decreased. The subsequent acid-mediated nitrile hydration
proceeded eꢁciently and the derived intermediate 10 was
Entry NaCN (eq) Pd(PPh ) (eq) Time (h) Tem-
Yield (%)
3
4
perature
(
°C)
1
2
3
4
5
6
7
8
1.2
1.2
1.1
1.2
1.2
1.5
1.2
1.2
0.01
0.01
0.01
0.01
0.01
0.01
0.005
0.02
14
14
10
10
10
10
10
10
80
47
52
79
85
72
82
74
82
100
120
120
150
120
120
120
subjected to anhydrous NaHCO solution to aꢂord the fnal
3
product in a yield oꢀ 62% over two steps. The diꢃuoro inter-
mediate 9 had a low melting point, and the general workup
procedure may easily cause yield loss. Recently, Liu et al.
reported a one-pot three-step protocol ꢀor the synthesis oꢀ
ꢀ
avipiravir ꢀrom intermediate 8 in good yield (60%). Since
intermediates 9 and 10 did not need to be isolated, this pro-
cess could minimize the yield loss (Liu et al. 2017).
yield was inꢃuenced by several ꢀactors, so we made a pre-
liminary investigation oꢀ the reaction condition (Table 2).
This coupling reaction should be perꢀormed at a temperature
oꢀ about 120 °C. Reducing the temperature to 100 °C or
Compound 8 is a potential allergy-causing substance,
which we had confrmed during the course oꢀ our studies.
To avoid using this intermediate, another synthetic route was
investigated. Retrosynthetically, compound 23 or 24, iꢀ avail-
able, could be a ꢀavorable precursor oꢀ ꢀavipiravir. Along this
line, we made our eꢂort to synthesize the two compounds
(Scheme 5). Compound 22a and 22b were synthesized ꢀrom
18 in two steps via Sandmeyer hydroxylation reaction and
Pd-catalyzed cyanation oꢀ 21. However, reaction oꢀ the two
compounds with KF or CsF under the same condition did
not give the desired product and most oꢀ the starting material
was remained in the reaction mixture. Based on the reaction
results ꢀrom 19, 8, 20 and 22, it could be concluded that the
ꢃuoride preꢀerred to replace the chlorine atom instead oꢀ the
bromine atom; the adjacent Cl oꢀ the nitrile group oꢀ 8 was
frst substituted, which would ꢀacilitate the ꢃuorination at the
8
0 °C remarkably decreased the yield even with a prolonged
reaction time (Entry 1–2). 1 mol % oꢀ Pd(PPh ) were suꢀ-
3
4
fcient ꢀor the coupling reaction (Entry 4 and Entry 7–8), and
the optimal amount oꢀ NaCN was 1.2 equiv (Entry 3–4 and
Entry 6). In theory, the chlorine atom at the C6 position oꢀ
1
9a could also be replaced by the nitrile group in the cou-
pling reaction, so it would be better to control the amount
oꢀ NaCN in the range oꢀ 1.2–1.5 equiv. Generally, the best
condition was Entry 5 with 1.2 equiv oꢀ NaCN, 1 mol % oꢀ
Pd(PPh ) and 120 °C oꢀ reaction temperature, which pro-
3
4
vided a yield oꢀ 85% (Entry 4).
With compound 19a in hand, it was easy to prepare the
key intermediate 8 via Sandmeyer reaction. t-Butyl nitrite
Scheme 5 Another synthetic approach to ꢀavipiravir proposed in this paper
1
3

Chemical Papers
C6 position; and mono-ꢃuorination at the C6 position oꢀ 19
or 22 was very diꢁcult.
Hara T, Norimatsu N, Kurushima H, Kano T (2010) Method ꢀor pro-
ducing dichloropyrazine derivative. European Patent Oꢀꢀice
WO2010087117 (issued August 2, 2012)
Huchting J, Winkler M, Nasser H, Meier C (2017) Synthesis oꢀ T-705-ri-
bonucleoside and T-705-ribonucleotide and Studies oꢀ chemical
stability. ChemMedChem 129:652–659. https://doi.org/10.1002/
cmdc.201700116
Conclusion
Huchting J, Vanderlinden E, Winkler M, Nasser H, Naesens L, Meier C
(2018) Prodrugs oꢀ the phosphoribosylated ꢀorms oꢀ hydroxypyra-
zinecarboxamide Pseudobase T-705 and its de-ꢃuoro analogue
T-1105 as potent inꢃuenza virus inhibitors. J Med Chem 61:6193–
In summary, we investigated the synthesis oꢀ ꢀavipiravir
starting ꢀrom an inexpensive and commercially available
2
-aminopyrazine. A ꢀacile route ꢀor the synthesis oꢀ 8 was
6
210. https://doi.org/10.1021/acs.jmedchem.8b00617
developed in ꢀour steps. This new approach was ꢀree ꢀrom
Jin Z, Tucker K, Lin X, Kao CC, Shaw K, Tan H, Symons J, Behera I,
Rajwanshi VK, Dyatkina N, Wang G, Beigelman L, Deval J (2015)
Biochemical evaluation oꢀ the inhibition properties oꢀ ꢀavipiravir and
the hazardous POCl and provided a good yield. The pur-
3
pose oꢀ fnding a novel synthesis to ꢀavipiravir not via inter-
mediate 8 was not achieved, but due to the allergy-causing
problems oꢀ 8, alternative protocols should be considered.
2
′-C-methyl-cytidine triphosphates against human and mouse noro-
virus RNA polymerases. Antimicrob Agents Chemother 5912:7504–
516. https://doi.org/10.1128/aac.01391-15
7
Klejch T, Pohl R, Janeba Z, Sun M, Keough D, Guddat L, Hockova D
(2018) Acyclic nucleoside phosphonates with unnatural nucle-
obases, ꢀavipiravir and allopurinol, designed as potential inhibi-
tors oꢀ the human and Plasmodium falciparum 6-oxopurine phos-
phoribosyltransꢀerases. Tetrahedron 74:5886–5897. https://doi.
org/10.1016/j.tet.2018.08.014
Acknowledgements This work was supported by National Science
Foundation ꢀor Young Scientists oꢀ China (Grant no. 21502209). The
authors grateꢀully acknowledged Topharman Shanghai Co., Ltd ꢀor
collaboration.
Li M (2017) Synthetic method oꢀ ꢀavipiravir. State Intellectual Property
Oꢁce oꢀ the P.R.C. CN107226794A (issued October 3, 2017)
Liu F-L, Li C-Q, Xiang H-Y, Feng S (2017) A practical and step-eco-
nomic route to Favipiravir. Chem Pap 7111:2153–2158. https://doi.
org/10.1007/s11696-017-0208-6
Compliance with ethical standards
Conflict of interest On behalꢀ oꢀ all authors, the corresponding author
states that there is no conꢃict oꢀ interest.
Naesens L, Guddat LW, Keough DT, van Kuilenburg AB, Meijer J, Vande
Voorde J, Balzarini J (2013) Role oꢀ human hypoxanthine guanine
phosphoribosyltransꢀerase in activation oꢀ the antiviral agent T-705
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