Catalyst-Free Cyclization- And Curtius Rearrangement-Induced Functional Group Transformation: An Improved Synthetic Strategy of First-in-Class ATX Inhibitor Ziritaxestat (GLPG-1690)

Article abstract of DOI:10.1021/acs.oprd.9b00511

A practical and highly efficient protocol for the production of Autotaxin (ATX) inhibitor Ziritaxestat (1) was described. The procedure began with a catalyst-free bicomponent cyclization for the construction of the imidazo[1,2-a]pyridine skeleton 16. Subsequently, a typical Curtius rearrangement of carboxylic acid 17 followed by nucleophilic attacking of 3,5-dichlorobenzyl alcohol 18f led to the carbamate analogue 19f. N-methylated 20 was readily deprotected through HBr/HOAc treatment, which further conveniently took part in an alternative K2CO3-induced N-alkylation reaction with 9 to give 10. 10 coupled directly with piperazine to furnish 13, which ideally circumvent the removal of the Boc group. As a result, the hypertoxic KCN and the hypertoxic and costly isonitrile 3 involved in the tricomponent cyclization were carefully avoided. Ultimately, a novel, scalable, and cost-effective route was favorably developed to afford Ziritaxestat in a 20.4% overall yield.

Full text of DOI:10.1021/acs.oprd.9b00511

Subscriber access provided by BIU Pharmacie | Faculté de Pharmacie, Université Paris V
Full Paper
Catalyst-free cyclization and Curtius rearrangement induced
functional group transformation: An Improved Synthetic
Strategy of First-in-Class ATX Inhibitor Ziritaxestat (GLPG-1690)
Hongrui Lei, Yu Yang, Changtao Li, Fang Jia, Nan Jiang, Ping Gong, and Xin Zhai
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.9b00511 • Publication Date (Web): 23 Apr 2020
Downloaded from pubs.acs.org on April 23, 2020
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the
course of their duties.

Page 1 of 13
Organic Process Research & Development
1
2
3
4
5
6
7
8
9
Catalyst-free cyclization and Curtius rearrangement induced functional
group transformation: An Improved Synthetic Strategy of First-in-Class
ATX Inhibitor Ziritaxestat (GLPG-1690)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Hongrui Lei, Yu Yang, Changtao Li, Fang Jia, Nan Jiang, Ping Gong, Xin Zhai*
Key Laboratory of Structure-Based Drug Design and Discovery, Ministry of Education, Shenyang
Pharmaceutical University, Shenyang 110016, China.
ABSTRACT
A practical and highly efficient protocol for the production of ATX inhibitor Ziritaxestat (1) was described. The
procedure was began with a catalyst-free bicomponent cyclization for the construction of the imidazo[1,2-
a]pyridine skeleton 16. Subsequently, a typical Curtius rearrangement of carboxylic acid 17 followed by
nucleophilic attacking of 3,5-dichlorobenzyl alcohol 18f led to the carbamate analog 19f. The N-methylated 20
was readily deprotected through HBr/HOAc treatment which further conveniently took part in an alternative
K2CO3 induced N-alkylation reaction with 9 to give 10. 10 coupled directly with piperazine to furnish 13 which
ideally circumvent the removement of Boc group. As a result, the hypertoxic KCN and the hypertoxic & costly
isonitrile 3 involved in the tricomponent cyclization were carefully avoided. Ultimately, the novel scalable and
cost-effective route was favorably conducted to afford Ziritaxestat in 20.4 % overall yield.
KEYWORDS: ATX inhibitor, Ziritaxestat, novel synthetic strategy, bicomponent cyclization, Curtius
rearrangement
INTRODUCTION
Ziritaxestat (GLPG-1690, Figure 1), a first-in-class small-molecule Autotaxin (ATX) inhibitor developed by
Galapagos, was now undergoing phase III clinical trials to treat patients with idiopathic pulmonary fibrosis (IPF)
or chronic obstructive pulmonary diseases (COPD).1-4
Figure 1. Chemical structure of Ziritaxestat (GLPG-1690).
To our great knowledge, there are no researches focusing on the synthetic procedure of Ziritaxestat other than
the developer Galapagos2,4. The reported synthetic routes from Galapagos were present in Scheme 1. In the
course of preparing 6, similar tricomponent cyclization protocols (method A & B) were utilized to construct the
imidazo[1,2-a] pyridine core. According to the original method A (1-4-6), two obvious disadvantages were
uncovered2. Firstly, the involvement of the hypertoxic KCN was an inevitable challenge to industrial
manufacturing; more importantly, the cyclization reaction suffered from low yield (29%). Alternatively, although
the 1-5-6 protocol (method B) gave an acceptable yield (56-78% in two steps), the involvement of 1,1,3,3-
tetramethylbutyl isonitrile (3, CAS No.: 14542-93-9) was an indeed troublesome matter. It should also be stated
that he isonitrile species is hypertoxic with a disgusting odour. Most of all, 3 is prohibitively expensive (5 g:
$ ~170; 500 g: $ ~4,096) which could even determine the total cost of the entire synthetic route1. Generally, both
ACS Paragon Plus Environment

Organic Process Research & Development
Page 2 of 13
1
2
3
4
5
6
synthetic strategies mentioned above bore three independent introduction & removement of protecting groups
which accounted for the frustratingly poor efficiency.
Scheme 1. Reported Synthetic Route of Ziritaxestat1,2
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Given the conditions above, an alternative strategy should be proposed to construct the imidazo[1,2-a] pyridine
core. Typically, the 2-aminopyridine analogs and 2-haloketone esters were often combined to furnish an
alkylation/condensation bicomponent cyclization5-9. Also, 2-halo-1-phenacyl/alkyl pyridinium salts, pyridinium
azomethine ylides, and other derivatives have been explored as precursors for construction of the scaffold10. The
imidazopyridine nucleus have reportedly been synthesized from substituted imidazoles11, too. The
alkylation/condensation strategy turned out to feature ease of operation, low cost and high yields. As a consequence,
the bifunctional 5-bromo-3-methylpyridin-2-amine (2) and ethyl 2-bromo-3-oxopentanoate (15) were utilized
which efficiently brought the imidazo[1,2-a] pyridine skeleton 16. Both materials (2 & 15) are highly available
chemical intermediates with extremely low cost (500 g: $ <43).
Scheme 2. Schematic presentation of the preliminary synthetic route of Ziritaxestat (1)
With the aid of the catalyst-free alkylation/condensation cyclization strategy, a novel synthetic protocol was
forwarded that featured a typical functional group transformation (Scheme 2). After a routine saponification, the
carboxylic acid derivative 17 was undergoing a certain rearrangement to afford the methyl amine derivative 8.
Following the literature method, 8 was supposed to smoothly bring the final product 1 as reported. Herein, the
development of a practical manufacturing process for Ziritaxestat was discussed.
ACS Paragon Plus Environment

Page 3 of 13
Organic Process Research & Development
1
2
3
4
RESULTS AND DISCUSSION
5
6
7
8
Preparation of Key Intermediate 16 containing imidazo[1,2-a] pyridine skeleton. To construct the
imidazo[1,2-a] pyridine core, the bifunctional 5-bromo-3-methylpyridin-2-amine (2) and ethyl 2-bromo-3-
oxopentanoate (15) were combined to accomplish the alkylation/condensation bicomponent cyclization reaction.
Firstly, EtOH was chosen as the solvent and no further additives was added to the system12, 13 (Table 1, entry 1).
This condition led to 87% conversion ratio of 2 and an acceptable 69.1% yield. Although the unreacted 2 could be
easily recovered by treating with MeCN (2 is insoluble in MeCN), the relatively low conversion ratio was quite
unsatisfying. It was suggested in some papers that the involvement of base could drive the cyclization14, 15, and to
this end, several solvents were screened in the presence of NaHCO3 or TEA (entries 4-8). To our disappointment,
obviously decreased conversion ratios and yields were detected. When TEA was involved in EtOH, the reaction
efficiency also descended (entry 2).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Effects of Solvents and bases on the Bicomponent Cyclization Reaction
a
yield (%)b
69.1
solvent
EtOH
bases (equiv)
-
T (℃)
80
time(h)
5
conversion ratio (%)
87
entry
1
2
3
EtOH
H2O
TEA (2)
-
80
10
5
69
47
53
43
37
39
55
73
>99
91
58.3
31.1
33.1
25.7
21.9
25.3
38.7
57.7
78.5
75.3
100
100
100
100
70
4
H2O
NaHCO3 (2)
NaHCO3 (2)
TEA (2)
NaHCO3 (2)
NaHCO3 (2)
-
5
5
DMF
5
6
DMF
5
7
THF
5
8
1,4-dioxane
HOAc
n-BuOH
n-BuOH
100
100
100
100
5
9
5
10
11
-
3
TEA (2)
3
aStandard conditions: 2 (0.02 mol), 15 (0.024 mol), solvent (50 mL). bIsolated yield after workup.
Afterwards, HOAc, n-BuOH and H2O were tested successively as alternative solvents, and n-BuOH resulted to
be the preferred one which could realize >99% conversion ratio and 78.5% yield. It should be noted that the
utilization of TEA in n-BuOH also lowered down the reaction rate and decreased the conversion ratio to some
degree (91%). Economically, most n-BuOH involved in the reaction was proved to be recycled conveniently
through distillation.
ACS Paragon Plus Environment

Organic Process Research & Development
Page 4 of 13
1
2
3
4
5
6
Taking n-BuOH as the optimal solvent (without bases), 16 could be acquired in a kilogram level (yield 77.6%).
Subsequently, 16 was smoothly hydrolyzed with 4 M NaOH to result in the carboxylic acid intermediate 17 in
72.1% yield.
7
Scheme 3 Possible approaches for acquiring 8
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Preparation of the carbamate derivative 19 through a Curtius rearrangement reaction. With the carboxylic
acid 17 in hand, the following crux was the transformation of functional group from -COOH to amine (Scheme
3). Firstly, the amide intermediate 17a was prepared using NH4Cl as a synthon, however, the following Hofmann
rearrangement strategy (NaOH/Br2 or NaClO) was failed to bring 4. Alternatively, the Curtius rearrangement
reaction was thereby taken into consideration. Given the fact that NaN3 was highly explosive and hard for available,
the liquid material DPPA was selected for the generation of acyl azide intermediate. Various alcohols were utilized
in pursuit of acquiring stable carbamate intermediates16. The screening details were present in Table 2 and 3,5-
dichlorobenzyl alcohol 18f (entry 6) turned out to be the best choice with a relatively high yield (79.3%) and purity
(95.4%). It should be noted that the potential risk of DPPA may be reduced by slow addition (<500 mL/h) which
could allow the slow release of the N2. In the meantime, as an organophosphate ester, the toxicity of DPPA should
be carefully treated with.
Table 2. Screening of the alcohol reactant
a
Ar
-
time(h)
desired product
yieldb
57.2
HPLC (%)
81.3
entry
1
2
2.5
2
19a
19b
Ph-
59.5
79.2
3
4
3
2
19c
19d
45.7
61.1
71.8
84.7
ACS Paragon Plus Environment

Page 5 of 13
Organic Process Research & Development
1
2
3
4
5
6
7
5
6
2
2
19e
19f
67.3
79.3
87.1
95.4
8
9
aStandard conditions: 17 (0.03 mol), DPPA (0.036 mol), TEA (0.075 mol), alcohols (0.036 mol), Toluene (50 mL), 110°C. bIsolated yield after
workup.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The Exploration on the Process of Preparing 10 from 19f. After the successfully preparation of carbamate
19f, the removement of the 3,5-diCl CBZ group was put on the agenda. In the process of preparing 10 from 19f,
there are two possible ways to get the desired product (Scheme 4). As an attempt, method A (19f-4-4a-10) was
first explored. 19f was deprotected by treating with 20% NaOH17 to give 4 in 37% yield. But the following N-
alkylation reaction with 9 made a complex mixture and no target product was obtained which left this method to
be abandoned.
Scheme 4. Possible Preparation Sequences of 10 from 19f
Alternatively, we put our attention on the 19f-20-8-10 procedure (method B) with the aim of acquiring 10 in an
efficient manner. Generally, the 3,5-diCl CBZ-protected intermediate 19f brought great convenience for the
subsequent methylation (91.1% yield) without introduction & removement of alternative protecting groups.
Subsequently, we focused our attention on the deprotection of the methylated intermediate 20.
To begin with, 20% NaOH was applied but unsatisfying conversion ratio and yield were observed (Table 3,
entry 6). Alternatively, other general de-protection conditions were screened as depicted in Table 3. Firstly, the H2
and NiCl2+NaBH4 protocol surprisingly led to 100% conversion18,19. However, to our great disappointment, the
products in entries 1 and 2 resulted to be the 6-des-bromo 8b. This was also the case when 19f was used in a
deprotection step (MW: 377.07). Next, the dioxane solution of HCl (g) and TMSI20 (entries 3 and 4) turned out to
be ineffective with no raw materials consumed. Then, Bu4NF was utilized with 53% conversion ratio but only
trace product was detected21.
Table 3. Condition Screening of the Deprotection Reaction to Afford 8
entry a
deprotecting agents
solvent
temp. (℃)
conversion ratio (%)
yield (%) b
ACS Paragon Plus Environment

Organic Process Research & Development
Page 6 of 13
1
2
3
4
5
6
7
8
9
1
2
Pd/C (0.1 equiv), H2
NiCl2+NaBH4
HCl(g)
MeOH
MeOH
1,4-dioxane
CH3CN
THF
60
25
25
25
66
100
25
25
25
25
100
100
0
0 (6-des-Br)
0 (6-des-Br)
3
0
4
TMSI
0
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
Bu4NF
53
<1
6
20% NaOH
HBr (8 equiv)
HBr (4 equiv)
HBr (6 equiv)
HBr (8 equiv)
EtOH
47
26.7
5.1
86.3
89.1
88.7
7
H2O
10
8
HOAc
HOAc
HOAc
97
9
100
100
10
aStandard conditions: 20 (0.02 mol); bIsolated yield after workup.
Finally, 40% HBr aqueous solution was tested with only 10% conversion ratio and 5.1% isolated yield. This
methodology did provide a promising way to generate 8. As an attempt, we altered the solution from H2O to HOAc
(33% HBr) 22a, 22b. 4 equiv. of HBr in HOAc was firstly applied and a surprisingly 97% conversion ratio was
achieved which shed a light on the finally victory in obtaining 8 efficiently. As expected, when the HBr amount
was raised to 6 equiv., the raw material was completed consumed and 89.1% yield was achieved. While 8 equiv.
HBr was used, the yield decreased slightly to 88.7%. Given the conditions mentioned above, 6 equiv. HBr was
finally utilized for a scalable production and the yield would not decrease.
Unexpectedly, the N-alkylation reaction between 8 and 9 in the catalysis of NaH was proved to be unstable.
With our observation, in most cases, the hydrolysis product of 9 (from -Cl to -OH, 9’) turned out to be the main
product with the effect of the strong base NaH. We alternatively utilized the K2CO3/MeCN system which finally
achieved a robust synthetic process and the N-alkylation between 8 and 9 to give 10 went thoroughly with 83.1%
yield in a scale-up batch.
The Preparation of 13 through Buchwald Coupling Reaction. The Buchwald coupling reaction between 10
and N-Boc piperazine to give 12 and the following Boc-removement with HCl (g, dioxane) went thoroughly with
78.6% overall yield in two steps2. But there is interest in getting rid of the Boc-removement step which could
obviously relieve the laborious works. Consequently, the free piperazine was utilized directly to experience the C-
N coupling with 10. Taking the conditions used in the C-N coupling reaction between 10 and N-Boc piperazine as
a reference, the same ingredients were applied and this system finally resulted in 83.0 % yield and only trace
(<0.3%) bidentate product 13b was detected which could be easily removed by a principle acid-base extraction.
Given the fact that the existing condition gave a relatively satisfying result, we subsequently examine the
potential effects in the use of each agent point by point. Alternative P-ligands, bases and the solvents were screened
separately using a variable-controlling approach. According to the results in Table 4, generally, the P-ligand made
a great influence in the yield of 13. According to the structure of the organophosphorus fragment in P-ligand, the
ratio of aliphatic alkyl group (e.g.: the t-Bu in JohnPhos) may account for the discrimination in different reaction
outcomes. When the three substituents on the P atom were all aromatic rings, the reaction systems generally
brought unsatisfying results (entries 2, 3). In terms of the base, Cs2CO3 performed worse than tBuONa and tBuOK
(entries 1, 4 and 5). Alternatively, the influence of solvents was slightly smaller (entries 1, 4, 5, 6 and 7).
Table 4. Condition Checking of the Buchwald Coupling Reaction
ACS Paragon Plus Environment

Page 7 of 13
Organic Process Research & Development
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
entrya
catalysts
Pd2 (dba)3
Pd2 (dba)3
Pd2 (dba)3
Pd2 (dba)3
Pd2 (dba)3
Pd2 (dba)3
Pd2 (dba)3
P-ligand
JohnPhos
XantPhos
BINAP
base
solvent
toluene
toluene
toluene
toluene
toluene
1,4-dioxane
DME
temp. (℃)
110
13b (%)
<0.3
yield (%) b
83.0
1
2
3
4
5
6
7
tBuONa
tBuONa
tBuONa
tBuOK
110
<0.1
31.7
110
<0.1
22.3
JohnPhos
JohnPhos
JohnPhos
JohnPhos
110
<0.3
79.5
Cs2CO3
tBuONa
tBuONa
110
<0.3
69.3
102
<0.3
63.2
85
<0.3
67.1
aStandard conditions: 10 (0.03 mol), 21 (0.06 mol), catalysts (0.0015 mol), P-ligand (0.003 mol), base (0.06 mol), solvent (40 mL). bIsolated
yield after workup.
To the end, this screening verified that the original condition turned out to be optimal and 82.7% yield could be
achieved in a 300-400 g scale.
Preparation of Desired Product 1. With the efficient preparation of the piperazine derivative 13, the final N-
alkylation reaction with 14 implemented smoothly in MeCN under the catalysis of K2CO3. After a principle acid-
base extraction, the final product Ziritaxestat was obtained in 83.7% yield with 99.60% purity without further
complex purification.
Scheme 5. Novel Synthetic Route to Ziritaxestat
ACS Paragon Plus Environment

Organic Process Research & Development
Page 8 of 13
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In this end, a novel 8-step synthetic protocol was established that featured a typical catalyst-free
alkylation/condensation strategy and an efficient functional group transformation (Scheme 5). The carboxylic acid
derivative 17 was undergoing a Curtius rearrangement followed by condensation with 3,5-dichlorobenzyl alcohol
18f to afford the carbamate 19f. The carbamate intermediate 19f offered a great convenience for the subsequent
methylation with ease of operation. After a dedicated condition screening campaign, the methylated carbamate 20
was readily deprotected by treatment with HBr/HOAc to give 8 as the substrate in a K2CO3 catalyzed N-alkylation.
The N-alkylation product 10 was ready for a Buchwald coupling reaction directly with piperazine to furnish 13
without the involvement of Boc-fragment. Finally, the desired product 1 was obtained through a N-alkylation
reaction between 13 and 14.
CONCLUSION
A novel synthetic strategy was exploited to resolve plenty of challenges to the preparation of Ziritaxestat (1)
and 20.4% overall yield was ultimately achieved. The new route was characterized by a catalyst-free bicomponent
cyclization which creatively avoided the utilization of the hypertoxic KCN or the hypertoxic & costly isonitrile 3.
Surprisingly, the implementation of Curtius rearrangement perfectly realized the functionality transformation from
-COOH to amine and offered a great convenience for the following methylation. Subsequently, the base alteration
from NaH to K2CO3 in the preparing of 10 generally furnished a safer and more stable process. More importantly,
laborious protecting group introduction & deblocking procedures were obviously relieved. From a commercial
perspective, with the advantages of mild experimental conditions, ease of operation, and cost-effective features,
the present work was proved to be quite suitable for large-scale preparation.
EXPERIMENTAL SECTION
General Information. All commercially available materials and solvents were used directly without further
purification unless otherwise noted. All melting points were obtained on a Büchi Melting Point B-540 apparatus
and were uncorrected. TLC analyses were performed on silica gel (200–300 mesh) from Qingdao Ocean Chemicals
(Qingdao, Shandong, China). Mass spectra (MS) were taken in Electrospray ionization (ESI) mode on Agilent
1100 LC–MS (Agilent, Palo Alto, CA, USA) and a mixture of MeCN/H2O (6:4) was utilized as the eluting system.
ACS Paragon Plus Environment

Page 9 of 13
Organic Process Research & Development
1
2
3
4
5
6
7
8
9
1H and 13C NMR spectra were recorded with a Bruker ARX-400 spectrometer (Bruker Bioscience, Billerica, MA,
USA) using TMS as an internal standard. The reaction process, purity of Ziritaxestat and its intermediates was
determined by HPLC using a Schimadzu LC-10ATVP series instrument. The HPLC analysis data was reported in
relative area % and was not adjusted to weight %. The ingredients 9 and 14 were prepared according to the original
procedures and therefore were not discussed in the text.
Ethyl 6-bromo-2-ethyl-8-methylimidazo[1,2-a] pyridine-3-carboxylate (16). To a 50 L reactor was added n-
butanol (12 L), 15 (2.66 kg, 12 mol, 1.2 eq) and 2 (1.86 kg, 10 mol, 1.0 eq). The reaction mixture was carefully
warmed to 100 ℃ and kept for 3 h until <1% 2 remained by HPLC. The system was cooled to 25 ℃, the solvent
n-butanol was recycled by distillation. The residue was charged MeCN (2 L) after which the trace quantity of 2
was collected by filtration. Finally, the solvent was removed under a vacuum (maximum temperature 50 ℃) to
give a white solid 16 in 77.6% yield (2.41 kg). HPLC(%): 99.16%, MS: 311.32[M+1]+, 313.36[M+3]+, 1H NMR
(400 MHz, DMSO-d6) δ 9.20 (s, 1H), 7.55 (s, 1H), 4.37 (q, J = 7.1 Hz, 2H), 3.01 (q, J = 7.5 Hz, 2H), 2.52 (s, 3H),
1.36 (t, J = 7.1 Hz, 3H), 1.26 (t, J = 7.5 Hz, 3H). 13C NMR (101 MHz, DMSO) δ 160.94, 157.11, 145.49, 129.99,
128.29, 125.56, 112.44, 108.41, 60.89, 23.38, 16.71, 14.66, 13.90.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
6-bromo-2-ethyl-8-methylimidazo[1,2-a]pyridine-3-carboxylic acid (17). To a 50 L reactor were charged 16
(1.55 kg, 5 mol) and ethanol (4.2 L). The material was agitated into a solution. To the mixture was introduced 4 N
NaOH solution (6 L, 24 mol). After stirring at 60 ℃ for 2 h, the reaction was finished as determined by HPLC
analysis. The solvent was removed under vacuum and the residue was charged 6 L H2O and stirred to give a
homogenous solution. The pH of the solution was adjusted to 2 with 2N HCl solution. A light yellow solid 17 was
obtained through filtration in 72.1% yield (1.02 kg). HPLC(%): 99.49%, MS: 281.38[M-1]-, 1H NMR (400 MHz,
DMSO-d6) δ 9.28 (s, 1H), 7.52 (s, 1H), 3.02 (dd, J = 14.7, 7.2 Hz, 2H), 2.53 (s, 3H), 1.26 (t, J = 7.4 Hz, 3H). 13
C
NMR (101 MHz, DMSO) δ 162.48, 156.81, 145.24, 129.62, 128.06, 125.64, 112.93, 108.08, 23.03, 16.65, 13.99.
3,5-dichlorobenzyl (6-bromo-2-ethyl-8-methylimidazo[1,2-a]pyridin-3-yl)carbamate (19f). To a 20 L reactor
purged with nitrogen was added 17 (846.2 g, 3 mol, 1 eq), toluene (5.2 L), TEA (758.3 g, 7.5 mol, 2.5 eq) and
DPPA (990 g, 3.6 mol, 1.2 eq). Caution! The DPPA should be added slowly (<500 mL/h) under stirring. The
resulting slurry was warmed to reflux for 30 min before cooling to 30 ℃. 18f (638 g, 3.6 mol, 1.2 eq) was slowly
added and the mixture was heated to reflux for 2 h as determined by HPLC analysis. The solvent was concentrated
under vacuum followed by the introduction of water (3 L) and stirred. The crude product (cake) was collected
through filtration and MeCN was added subsequently. The resulting slurry was stirred for 1 h and filtered, rinsed
with MeCN (3*500 mL), dried to provide white solid 19f in 78.2% yield (1.067 kg). HPLC(%): 95.21%, MS:
456.40 [M+1]+, 458.41 [M+3]+, 1H NMR (400 MHz, DMSO-d6) δ 9.59 (s, 1H), 8.27 (s, 1H), 7.61 (s, 1H), 7.55 (s,
2H), 7.29 (s, 1H), 5.19 (s, 2H), 2.63 (q, J = 7.5 Hz, 3H), 2.48 (s, 3H), 1.20 (t, J = 6.6 Hz, 3H).13C NMR (101 MHz,
DMSO) δ 154.97, 142.02, 141.24, 140.19, 134.57(2C), 127.97(2C), 127.75, 126.68(2C), 121.53, 116.43, 106.65,
65.45, 20.22, 16.25, 13.73.
3,5-dichlorobenzyl (6-bromo-2-ethyl-8-methylimidazo[1,2-a]pyridin-3-yl)(methyl)carbamate (20). To a 20 L
reactor purged with nitrogen was added 19f (846.7 kg, 1.6 mol, 1 eq), DMF (5.3 L). The system was precooled to
0 ℃ and NaH (164 g, 4 mol, 2.5 eq) was added slowly. After a 1 h period of stirring, CH3I (458 g, 3.2 mol, 2 eq)
was added to the system and slowly warmed to 25 ℃ and held for 2 h. The mixture was cooled to 0 ℃ and slowly
added to a saturated solution of NH4Cl (10 L). The resulting slurry was stirred for 1 h, filtered, rinsed with water,
dried in vacuum to result the light yellow solid 20 in 91.1% yield (794.5 g). HPLC(%): 98.47%, MS: 470.38
[M+1]+, 472.40 [M+3]+, 1H NMR (400 MHz, DMSO-d6) δ 8.44 (s, 1H), 7.61 (s, 0.26H), 7.56 (s, 0.49H), 7.50 (s,
0.71H), 7.26 (s, 1H), 7.12 (s, 1.44H), 5.22 (s, 0.59H), 5.06 (s, 1.49H), 3.28 (s, 0.67H), 3.22 (s, 2.43H), 2.65 – 2.56
(m, 2H), 2.47 (s, 3H), 1.18 (t, J = 7.1 Hz, 3H). 13C NMR (151 MHz, DMSO) δ 154.98, 142.27, 141.09, 140.62,
134.45, 128.05, 127.89, 126.64(2C), 126.59, 126.28, 121.46, 120.70, 106.83, 65.65, 37.33, 20.40, 16.24, 13.51.
ACS Paragon Plus Environment

Organic Process Research & Development
Page 10 of 13
1
2
3
4
5
6
7
8
9
6-bromo-2-ethyl-N,8-dimethylimidazo[1,2-a]pyridin-3-amine (8). To a 10 L reactor was charged 20 (0.656 kg,
1.4 mol, 1 eq) and HBr/HOAc (33%, 2.04 kg, 8.4 mol, 6 eq). The material was stirred at 20 ℃ for 3 h with HPLC
indicating the finishing point. The mixture was added 5 L water and stirred for 30 min to give a homogenous
solution. The resulting solution was extracted with EtOAc (2 L) and the organic layer was discarded. The aqueous
layer was adjusted to pH =10 with 2 N NaOH solution. The resulting material was extracted with EtOAc (3×1500
mL) and the combined organic portions was concentrated under vacuum to give the light yellow solid 8 (332.7 g)
in 89.1% yield. HPLC(%): 97.27%, MS: 268.0 [M+1]+, 270.0 [M+3]+,1H NMR (400 MHz, CDCl3) δ 8.05 (s, 1H),
7.03 (s, 1H), 3.65 (m, 1H), 2.85 – 2.78 (q, 2H), 2.81 (s, 3H), 2.59 (s, 3H), 1.34 (t, J = 7.6 Hz, 3H). 13C NMR (101
MHz, CDCl3) δ 139.21, 129.72, 127.48, 127.38, 126.28, 120.41, 106.72, 35.69, 20.33, 16.56, 14.54.
2-((6-bromo-2-ethyl-8-methylimidazo[1,2-a]pyridin-3-yl)(methyl)amino)-4-(4-fluorophenyl)-thiazole-5-
carbonitrile (10). To a 5 L reactor was added 8 (267.3 g, 1.0 mol, 1 eq), 9 (100.1 g, 1.2 mol, 1.2 eq), K2CO3 (346.7
g, 2.5 mol, 2.5 eq) and anhydrous MeCN (1.6 L). The mixture was refluxed for 3 h with the indicating of HPLC
analysis. The mixture was cooled to 20 ℃ and the solvent were concentrated in vacuum. The crude product was
charged with 1.2 L methanol and the yellow product 10 was obtained through filtration in 83.1% yield (389.8 g).
HPLC(%): 97.48%, MS: 470.69 [M+1]+, 472.71 [M+3]+,1H NMR (400 MHz, DMSO-d6) δ 8.61 (s, 1H), 8.07 (dd,
J = 8.1, 5.6 Hz, 2H), 7.42 (t, J = 8.9 Hz, 2H), 7.38 (s, 1H), 3.59 (s, 3H), 2.66 (q, J = 7.6 Hz, 2H), 2.52 (s, 3H), 1.25
(t, J = 7.6 Hz, 3H).13C NMR (101 MHz, DMSO) δ 172.63, 164.76, 162.29, 160.70, 144.09, 141.74, 130.53, 130.45,
129.31, 128.57, 127.96, 122.42, 121.78, 116.69, 116.47, 114.75, 107.51, 87.99, 20.63, 16.28, 13.37.
2-((2-ethyl-8-methyl-6-(piperazin-1-yl)imidazo[1,2-a]pyridin-3-yl)(methyl)amino)-4-(4-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
fluorophenyl)thiazole-5-carbonitrile (13). To a 5 L reactor purged with nitrogen was added 10 (375.2 g, 0.8 mol),
dry toluene (3 L), piperazine (104.2 g, 1.2 mol), t-BuONa (154.3 g, 1.6 mol), JohnPhos (24.2 g, 0.08 mol) and
Pd2(dba)3 (22.1 g, 0.024 mol). The mixture was stirred to form a homogenous slurry and heated at 110℃ for 2 h.
When the end point comes, the mixture was filtered through a plug of silica gel, washed with EtOAc (500 mL).
The filtrate was concentrated and the resulting crude product was charged with n-hexane (2 L), stirred for 2 h,
filtered, and the filtrate was concentrated in vacuum to afford the faint yellow solid 13 in 82.7% yield (314.3 g).
HPLC(%): 99.70%, MS:476.62 [M+1]+,1H NMR (400 MHz, CDCl3) δ 8.12 (s, 2H), 7.28 (s, 1H), 7.11 (s,2H), 7.05
– 6.90 (m, 1H), 4.70 (s, 1H), 3.59 (s, 3H), 3.31 (m, 8H), 2.75 (s, 2H), 2.59 (s, 3H), 1.26 (s, 3H). 13C NMR (101
MHz, DMSO) δ 173.07, 162.25, 160.58, 142.71, 141.25, 140.01, 130.51, 130.42, 130.04, 129.36, 126.74, 122.45,
120.84, 116.63, 116.42, 114.85, 105.35, 87.75, 50.88(2C), 45.80(2C), 20.70, 16.67, 13.57.
Ziritaxestat; 2-((2-ethyl-6-(4-(2-(3-hydroxyazetidin-1-yl)-2-oxoethyl)piperazin-1-yl)-8-methylimidazo[1,2-
a]pyridin-3-yl)(methyl)amino)-4-(4-fluorophenyl)thiazole-5-carbonitrile (1). To a 5 L reactor was added 13
(237.8 g, 0.5 mol), MeCN (1.5 L), K2CO3 (172.5 g, 1.25 mol). The mixture was heated to reflux and held for 3 h.
When the reaction finished as indicated by HPLC, K2CO3 was removed through filtration after cooling and the
filtrate was concentrated in vacuum. Subsequently, the residue was dissolved with EtOAc (2L) and extracted with
4M HCl (2×1.5 L). The aqueous phase was combined and the pH was adjusted to 10 with 6 N NaOH. The resulting
turbid solution was extracted with EtOAc (2×1.5L) and the organic phase was washed with brine, dried over
Na2SO4, concentrated in vacuum to give the final product Ziritaxestat as a faint yellow solid in 83.7% yield (246.2
g). HPLC(%): 99.60%, MS: 589.92 [M+1]+, 611.94 [M+23]+, 587.93 [M-1]-, 1H NMR (400 MHz, DMSO-d6) δ
8.11 (dd, J = 8.5, 5.6 Hz, 2H), 7.43 (t, J = 8.8 Hz, 2H), 7.24 (s, 1H), 5.73 (s, 1H), 4.44 (s, 1H), 4.38 – 4.31 (m, 1H),
4.11 – 4.01 (m, 1H), 3.91 (dd, J = 9.2, 4.2 Hz, 1H), 3.65 – 3.59 (m, 1H), 3.58 (s, 3H), 3.52 (d, J = 8.3 Hz, 1H),
3.13 (s, 4H), 2.73 (s, 4H), 2.62 (dd, J = 15.1, 7.5 Hz, 2H), 2.48 (s, 3H), 1.23 (t, J = 7.6 Hz, 3H). 13C NMR (101
MHz, DMSO) δ 173.05, 164.73, 162.25, 160.58, 142.74, 139.95, 130.50, 130.42, 130.04, 129.35, 129.33, 126.82,
122.53, 120.89, 116.65, 116.43, 114.86, 105.94, 87.75, 60.73 (2C), 60.55, 58.15(2C), 52.61(2C), 49.15, 20.66,
16.66, 13.53.
ACS Paragon Plus Environment

Page 11 of 13
Organic Process Research & Development
1
2
3
4
5
6
AUTHOR INFORMATION
Corresponding Author
7
8
9
*E-mail address: zhaixin_syphu@126.com (Xin Zhai).
ORCID
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Xin Zhai: 0000-0002-7428-2405
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information
Structural characterization Spectra (1H and 13C NMR spectra, MS spectra), HPLC purity spectra of 1 and
involving intermediates or impurities.
ACKNOWLEDGMENT
This work was supported by National Natural Science Foundation of China (No. 81872751), Development
Project of Ministry of Education Innovation Team (No. IRT1073) and Youth Backbone Talent Training Project of
Shenyang Pharmaceutical University (No. ZQN2018008).
REFERENCES
(1) Joncour, A.; Desroy, N.; Housseman, C.; Bock, X.; Bienvenu, N.; Cherel, L.; Labeguere, V.; Peixoto, C.; Annoot, D.;
Lepissier, L.; Heiermann, J.; Hengeveld, W. J.; Pilzak, G.; Monjardet, A.; Wakselman, E.; Roncoroni, V.; Tallec S. L.; Galien,
R.; David, C.; Vandervoort, N.; Christophe, T.; Conrath, K.; Jans, M.; Wohlkonig, A.; Soror, S.; Steyaert, J.; Touitou, R.;
Fleury, D.; Vercheval, L.; Mollat, P.; Triballeau, N.;Aar, E. v. d.; Brys, R.; Heckmann, B. Discovery, Structure-Activity
Relationship and Binding Mode of Imidazo[1,2-a]pyridine Series of Autotaxin Inhibitors. J. Med. Chem. 2017, 60, 7371-7392.
(2) The discovery synthesis of GLPG-1690 has been reported: Desroy, N.; Housseman, C.; Bock, X.; Joncour, A.; Bienvenu,
N.; Cherel, L.; Labeguere, V.; Rondet, E.; Peixoto, C.; Grassot, J. M.; Picolet, O.; Annoot, D.; Triballeau, N.; Monjardet, A.;
Wakselman, E.; Roncoroni, V.; Tallec, S. L.; Blanque, R.; Cottereaux, C.; Vandervoort, N.; Christophe, T.; Mollat, P.; Lamers,
M.; Auberval, M.; Hrvacic, B.; Ralic, J.; Oste, L.; Aar, E. v. d.; Brys, R.; Heckmann, B.; Discovery of 2‑[[2-Ethyl-6-[4-[2-(3-
hydroxyazetidin-1-yl)-2-oxo-ethyl]piperazin-1-yl]-8-methylimidazo[1,2-a]pyridin-3-yl]methylamino]-4-(4-
fluorophenyl)thiazole-5-carbonitrile (GLPG1690), a First-in-Class Autotaxin Inhibitor Undergoing Clinical Evaluation for the
Treatment of Idiopathic Pulmonary Fibrosis. J. Med. Chem. 2017, 60, 3580-3590.
(3) Croasdell, G.; European Respiratory Society 2015 Annual Congress. Amsterdam, The Netherlands-September 26-30, 2015,
Drugs Fut. 2015, 40, 691.
(4) Desroy, N.; Heckmann, B.; Brys, R.; Joncour, A.; Peixoto, C.; Bock, X.; Housseman, C. Compounds and pharmaceutical
compositions thereof for the treatment of inflammatory disorders. PCT/EP2014/062241, PCT/EP2014/054440.
(5) Samala, G.; Nallangi, R.; Devi, P. B.; Saxena, S.; Yadav, R.; Sridevi, J. P.; Yogeeswari, P.; Sriram, D. Identification and
development of 2-methylimidazo[1,2-a]pyridine-3-carboxamides as Mycobacterium tuberculosis pantothenate synthetase
inhibitors. Bioorg. Med. Chem. 2014, 22, 4223-4232.
(6) Bhagat, S. B.; Telvekar, V. N. NBS mediated protocol for the synthesis of N-bridged fused heterocycles in water,
Tetrahedron Lett. 2017, 58, 3662-3666.
(7) Zhang, Y.C.; Jiang, F.; Shi, F.; Organocatalytic Asymmetric Synthesis of Indole-Based Chiral Heterocycles: Strategies,
Reactions, and Outreach, Acc. Chem. Res. 2020, 53, 425-446.
ACS Paragon Plus Environment

Organic Process Research & Development
Page 12 of 13
1
2
3
4
5
(8) Wang, Y.; Lu, H.; Xu, P.F.; Asymmetric Catalytic Cascade Reactions for Constructing Diverse Scaffolds and Complex
Molecules, Acc. Chem. Res. 2015, 48, 1832−1844.
(9) Mei, G.J.; Shi, F.; Catalytic asymmetric synthesis of spirooxindoles: recent developments, Chem. Commun., 2018, 54,
6607–6621.
6
7
8
9
(10) Vega, J.A.; Vaquero, J.J.; Alvarez-Builla, J.; Ezquerra, J.; Hamdouchi, C. A new approach to the synthesis of 2-
aminoimidazo[1,2-a] pyridine derivatives through microwave-assisted N-alkylation of 2-halopyridines, Tetrahedron. 1999, 55,
2317-2326.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(11) Knolker, H.J.; Hitzemann, R. Imidazole derivatives, part IX selective reactions of functionalized imidazo[1,2-a]pyridines:
stereospecific synthesis of 5,6-dihydroimidazo[1,2-a]pyridines. Tetrahedron Lett. 1994, 35, 2157-2160.
(12) Ismail, M. A.; Brun, R.; Wenzler, T.; Tanious, F. A.; Wilson, W. D.; Boykin, D. W. Novel Dicationic Imidazo[1,2-
a]pyridines and 5,6,7,8-Tetrahydro-imidazo[1,2-a]pyridines as Antiprotozoal Agents. J. Med. Chem. 2004, 47, 3658-3664.
(13) Kummerle, A.E.; Schmitt, M.; Cardozo, S. V. S.; Lugnier, C.; Villa, P.; Lopes, A. B.; Romeiro, N. C.; Justiniano, H.;
Martins, M. A.; Fraga, C.A.M.; Bourguignon, J.J.; Barreiro, E. J. Design, Synthesis, and Pharmacological Evaluation of N‑
Acylhydrazones and Novel Conformationally Constrained Compounds as Selective and Potent Orally Active
Phosphodiesterase‑4 Inhibitors. J. Med. Chem. 2012, 55, 7525−7545.
(14) Starrett, J. E.; Montzka, T. A.; Crosswell, A. R.; Cavanagh, R. L. Synthesis and Biological Activity of 3-Substituted
Imidazo[l,2-a]pyridines as Antiulcer Agents. J. Med. Chem. 1989, 32, 2204-2210.
(15) Roslan, I. I.; Ng, K. H.; Wu, J. E.; Chuah, G. K.; Jaenicke, S. Synthesis of Disubstituted 3‑Phenylimidazo[1,2‑a]pyridines
via a 2‑Aminopyridine/CBrCl3 α‑Bromination Shuttle. J. Org. Chem. 2016, 81, 9167−9174.
(16) Maiti, A.; Reddy, P. V. N.; Sturdy, M.; Marler, L.; Pegan, S. D.; Mesecar, A. D.; Pezzuto, J. M.; Cushman, M. Synthesis
of Casimiroin and Optimization of Its Quinone Reductase 2 and Aromatase Inhibitory Activities. J. Med. Chem. 2009, 52,
1873–1884.
(17) Sahoo, H.; Mandal, A.; Selvakumar, J.; Baidya, M. Remote C-H Selenylation of 8-Amidoquinolines via Copper-Catalyzed
Radical Cross-Coupling. Eur. J. Org. Chem. 2016, 2016, 4321-4327.
(18) Saroha, M.; Aggarwal, K.; Bartwal, G.; Khurana, J. M. Development of a novel protocol for chemoselective deprotection
of N/O-benzyloxycarbonyl (Cbz) at ambient temperature. Monatshefte fur Chemie-Chemical Monthly, 2018, 149, 2231–2235.
(19) Masataka, I.; Nobuaki, T.; Kazuharu N.; Keiichiro F. Total synthesis of hydrocinchonidine and hydrocinchonine via photo-
oxygenation of an indole derivative. J. Chem. Soc. Perkin. Trans. 1. 1988, 1277-1281.
(20) McCarthy, A. R.; Pirrie, L.; Hollick, J. J.; Ronseaux, S.; Campbell, J.; Higgins, M.; Staples, O. D.; Tran, F.; Slawin, A.
M. Z.; Lain, S.; Westwood, N. J. Synthesis and biological characterisation of sirtuin inhibitors based on the tenovins, Bioorg.
Med. Chem. 2012, 20, 1779–1793.
(21) Jacquemard, U.; Valerie, B.; Lefoix, M.; Routier, S.; Merour, J.Y.; Coudert, G. Mild and selective deprotection of
carbamates with Bu4NF. Tetrahedron 2004, 60, 10039–10047.
(22) (a) Homer, R, B.; Moodie, R. B.; Rydon, H. N. The Kinetics and Mechanism of the Removal of the N-Benzyloxycarbonyl
Group from N-Benxyloxycarbonyl glycine Ethyl Ester and Related Compound t3 in Acetic Acid containing Hydrobromic Acid
or Sulphuric Acid. J. Chem. Soc., 1965, 4403-4409; (b) Van, T. T.; Koiro, E.; Grzoska, Z. Synthesis of γ-Tetrazole Analogues
of L-Glutamic Acid and Its Derivatives. Tetrahedron, 1977, 33, 2299-2302.
ACS Paragon Plus Environment

Page 13 of 13
Organic Process Research & Development
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
244x51mm (300 x 300 DPI)
ACS Paragon Plus Environment