Development of a scalable synthesis of a VEGFR inhibitor

Article abstract of DOI:10.1055/s-0032-1317554

Process development and salt selection for a novel VEGFR inhibitor are described. The overall convergent synthesis involved coupling of three key fragments, 2-chloronicotinoyl chloride, 4-isopropyl-3-methylaniline and 7-aminoisoquinoline. A cost-effective

Full text of DOI:10.1055/s-0032-1317554

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Development of a Scalable Synthesis of a VEGFR Inhibitor
Development of a Scalable Synthesis of a VEGFR Inhibitor
Ying Chen,*a Richard D. Crockett,a Xin Wang,b Robert D. Larsen,c Sheng Cui,*a Margaret M. Faul*a
a
Chemical Process Research and Development, Amgen Inc., One Amgen Center Drive, Thousand Oaks, CA 91320, USA
Fax +1(805)3754531; E-mail: ying@amgen.com; E-mail: scui@amgen.com; E-mail: mfaul@amgen.com
b
Isis Pharmaceuticals, Inc., 2855 Gazelle Court, Carlsbad, CA 92010, USA
c
Alcon Laboratories, Inc., 6201 South Freeway, Fort Worth, TX 76134, USA
Received: 27.09.2012; Accepted after revision: 19.10.2012
The retrosynthetic analysis of A is outlined in Scheme 1.
Amidation and amination were envisioned as the key late-
stage steps, bringing together three fragments (1, 2 and 3).
2-Chloronicotinoyl chloride (1) and 4-isopropyl-3-meth-
Abstract: Process development and salt selection for a novel
VEGFR inhibitor are described. The overall convergent synthesis
involved coupling of three key fragments, 2-chloronicotinoyl chlo-
ride, 4-isopropyl-3-methylaniline and 7-aminoisoquinoline. A cost-
effective and scalable synthesis of 7-aminoisoquinoline was also ylaniline (2) were both commercially available with rea-
achieved. A transition-metal-free SNAr process enabled the final C–
N coupling to afford the target molecule. A phosphate form of the
drug substance with improved physical properties was selected for
key intermediate 3. The coupling of these intermediates
further development and the corresponding crystallization process
was subsequently developed. Overall, a robust six-step route was
sonable cost. Our efforts initially focused on the
development of a scalable and cost-effective process for
and controlling the level of process impurities posed a
number of challenges for large-scale synthesis. This paper
will describe how these challenges were addressed during
the development of a robust process for preparing a phar-
maceutically acceptable salt form of compound A.
developed and demonstrated on multikilogram scales affording the
target compound in >30% yield and high purity (>99%).
Key words: amination, oxidation, amides, VEGFR inhibitor,
7-aminoisoqinoline
The medicinal chemistry approach3 to the key intermedi-
ate 3 is shown in Scheme 2. The synthesis began with pro-
tection of amino group of 4-nitrophenethylamine 4 as the
corresponding amide 5. Tetrahydroisoquinoline 6 was
then prepared through a Pictet–Spengler4 tetrahydroiso-
quinoline protocol by condensation of 5 with paraform-
aldehyde under strong acidic conditions. Cleavage of
trifluoroacetamide protecting group followed by one-pot
dehydrogenation and nitro group reduction under high
temperature and microwave irradiation furnished frag-
ment 3. This route was successfully utilized to provide
10–20 grams of 3 to support early development work.
However, it was recognized that this route would be un-
suitable for long-term supply due to challenges associated
with scaling up a microwave reaction and high cost of the
starting material 4-nitrophenethylamine 4 at the outset of
this development work. Moreover, not all the literature
Vascular endothelial growth factor (VEGF) is an impor-
tant signaling protein involved in both vasculogenesis (the
formation of the circulatory system) and angiogenesis (the
growth of blood vessels from pre-existing vasculature).1
VEGFR inhibitors are in broad use for the treatment of
metastatic renal-cell carcinoma, gastrointestinal stromal
tumors and hepatocellular carcinoma and in development
for a number of other oncology indications, including
colorectal cancer, non-small-cell lung cancer, pancreatic
cancer, thyroid malignancies, ovarian cancer, breast can-
cer and sarcomas.2 Recent efforts at Amgen led to the dis-
covery of the potent VEGFR inhibitor, compound A.3 To
supply toxicological and clinical studies, the development
of a scalable and cost-effective synthesis of compound A
was required.
O
O
N
H
Cl
N
NH
+
+
N
H2N
N
Cl
H2N
H3PO4
1
2
3
N
compound A
Scheme 1 General strategy towards the synthesis of compound A
SYNLETT 2013, 24, 0301–0304
Advanced online publication: 16.11.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1317554; Art ID: ST-2012-Y0825-C
© Georg Thieme Verlag Stuttgart · New York

302
Y. Chen et al.
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approaches5 were deemed efficient and cost-effective
since they suffered either expensive raw materials or mul-
tiple chromatography purifications, undesirable on large
scale. With this in mind, we started to investigate a num-
ber of alternative approaches that could make use of eco-
nomic raw materials and synthesize 3 by a more efficient
route with an acceptable cost.
N
O2N
b
a
10
NO2
NH
N
9
8
N
N
11
a
b
HN
CF3
NH2
O2N
O2N
c
d
O2N
O2N
O2N
⋅HCl
12
N
N
5
O
O2N
4
10
NH
c
d
13
N
CF3
O
NH
e
O2N
O
6
7
H2N
3
N
H2N
Scheme 3 Improved synthesis of 3. Reagents and conditions: (a)
NBS, NaOH, CH2Cl2; (b) KNO3, H2SO4; (c) (1) TsOH, i-PrOH; (2)
aq NaOH, 75%; (d) MnO2, PhCF3, 85%; (e) H2, Pd/C, EtOH, 76%.
3
Scheme 2 Original synthesis of 3. Reagents and conditions: (a)
(CF3CO)2O–DIPEA, 99%; (b) (CH2O)n, AcOH, H2SO4, 90%; (c)
LiOH, aq THF, 95%; (d) microwave, 220 °C, (HOCH2CH2)2O, Pd/C,
76%.
The final fragments assembling process began with ami-
dation of 2-chloronicotinoyl chloride (1) with 4-isopro-
pyl-3-methylaniline (2). Among all the solvents tested,
dichloromethane provided the best assay yield (>99%)
and full conversion was observed within 30 minutes even
at 5–10 °C probably due to the higher solubility of both 1
and 2 in CH2Cl2. The coupling product 14 was isolated by
crystallization from MTBE–heptanes with >99% purity
and 98% yield (Scheme 4).
An improved and practical approach to 7-aminoisoquino-
line (3) was developed. As shown in Scheme 3, the syn-
thesis began with partial oxidation of a low-cost starting
material, 1,2,3,4-tetrahydroisoqinoline (8) to provide di-
hydroisoquinoline 9 in 98% yield. The subsequent nitra-
tion was carried out with potassium nitrate in sulfuric acid
to provide a mixture of regioisomers 10 and 11 in 13:1 ra-
tio.6 Since no improvement in the ratio was observed by
optimizing the reaction conditions, we decided to explore
an isolation process to reject the undesired isomer. To our
delight, formation of the toluenesulfonic acid salt of 10
provided an effective control point for this purification
purpose. In practice crystallization of TsOH salt of 10, salt
break and crystallization afforded 10 in 75% overall yield
(steps b and c) with >99% purity. A screen of the subse-
quent oxidation conditions revealed that the highest assay
yield and cleanest reaction profile was obtained using
MnO2 as the oxidant. However, 5% amide impurity 13
was consistently formed during this reaction and proved
difficult to remove in downstream processing. Attention
then turned to identifying a solvent system where 12 and
13 would have very different solubility and oxidation re-
action can still proceed smoothly. Further studies showed
trifluoromethylbenzene met our requirements. Thus, the
insoluble amide impurity 13 produced in the reaction was
conveniently removed via in-line filtration, and the prod-
uct 12 was obtained with >98% purity and 85% yield. The
final reduction of nitro group was carried out by hydroge-
nation in the presence of 1% Pd/C catalyst and the addi-
tion of 10% AcOH was critical to improve the substrate
solubility and reaction kinetic rate. Ultimately this four-
step synthesis was successfully demonstrated on multi-
kilogram scales with 54% overall yield and >99% purity.
O
O
Cl
+
N
a
H
N
Cl HCl⋅H2N
N
Cl
14
1
2
O
c
b
N
compound A
H
N
NH
15
N
Scheme 4 Final coupling steps. Reagents and conditions: (a) Et3N,
CH2Cl2, 98%; (b) 3, LiHMDS, THF, 79%; (c) H3PO4, EtOH, 86%.
The Pd-catalyzed cross-coupling of aryl chloride and ani-
line has rapidly emerged as a popular method for the con-
struction of C–N bonds in contemporary organic
synthesis.7 Indeed, the original medicinal chemistry syn-
thesis was carried out using 5 mol% Pd2(dba)3/2-di-
cyclohexylphosphino-2′-(N,N-dimethylamino)biphenyl
Synlett 2013, 24, 301–304
© Georg Thieme Verlag Stuttgart · New York

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Development of a Scalable Synthesis of a VEGFR Inhibitor
303
(DavePhos) in THF at 65 °C affording compound 15 in tion of free base 15 was achieved in 79% yield and >99%
55% isolated yield after chromatography purification. The purity.
low yield was mainly attributed to incomplete reaction.
A free base was initially used by the medicinal chemistry
Additionally a more critical issue identified was the Pd re-
team to enable screening and preliminary toxicology stud-
moval after the coupling reaction. The residual Pd in the
ies, but the poor stability, bioavailability and solubility of
final product A remained high (>100 ppm) even after sev-
this form precluded its use for the further development.
After extensive high throughput and subsequent manual
eral treatments, using different solid scavengers, during
the workup of the coupling reaction. In addition, the Pd
salt and polymorph screenings, a more crystalline, stable
levels cannot be further reduced in the following salt for-
and water-soluble phosphate salt was identified for the
mation step. With the concern of intrinsic binding capabil-
long-term development. A robust crystallization was sub-
ity of final product to Pd, we turned our attention to
sequently designed on the basis of solubility of the free
develop a transition-metal-free SNAr reaction to build the
base and salt. In practice, 1.1 equivalents of H3PO4 was
C–N bond. Extensive base and solvent screening revealed
slowly added to a free-base solution in EtOH at 80 °C and
that treatment of 3 with 3.0 equivalents of LiHMDS in
1% seed was added to the reaction mixture to induce crys-
tallization. The resulting salt was isolated by filtration in
THF, followed by addition of aryl chloride 14, provided
>99% conversion. However varied amounts (0.5–3%) of
86% yield and >99% purity.
an impurity (17) with m/z = 283 (Scheme 5)8 was found in
In summary, a non-chromatography and robust chemical
many development lots and rejection of this impurity
process to synthesize a VEGFR inhibitor was developed
proved to be extremely difficult in this step and the fol-
to support the preclinical and clinical studies. Process re-
lowing phosphate formation step under a variety of crys-
search efforts also led to the development of a practical
tallization conditions. Therefore, in an attempt to
and cost-effective synthesis of 7-aminoisoquinoline (3).
The whole process was demonstrated on multikilogram
scales with >30% overall yield and >99% purity.
completely avoid its formation during the reaction, we de-
cided to investigate the mechanism of this impurity for-
mation. On the basis of the literature example9 that the
imino radical can be formed in tert-butoxide-catalyzed
autoxidation of 1(or 2)-aminonaphthalene, an oxidative
coupling mechanism was proposed in Scheme 5. The fact
that this impurity can be prepared in ca. 60% isolated
yield by sparging air into the reaction mixture would serve
as additional evidence in support of this mechanism. Ulti-
mately the level of this impurity can be decreased to
<0.1% by implementing a rigorous degassing procedure
prior to adding the base LiHMDS. After quenching the re-
action with water and solvent-switch to MeOH, the free
base 15 was isolated by crystallization from MeOH–H2O
(2:1). Using this process, a multikilogram-scale prepara-
Acknowledgment
Authors would like to acknowledge Maosheng Chen, Minhui Ma,
Mike Ronk and Duncan Smith for their analytical support.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
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H2N
N
strong base and O2
N
N
N
N
3
N
H2N
17 m/z = 283
3
–HN
N
N
–HN
16
16
N
N
NH–
NH2
N
N-
NH
HN
N
N
HN
N
Scheme 5 Proposed mechanism for the formation of 17 with m/z = 283
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 301–304

304
Y. Chen et al.
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