Practical Synthesis of a S1P Receptor 1 Agonist via a Guareschi-Thorpe Reaction

Article abstract of DOI:10.1021/acs.oprd.6b00210

A practical synthesis of S1P receptor 1 agonist ACT-334441 (1) through late-stage convergent coupling of two key intermediates is described. The first intermediate is 2-cyclopentyl-6-methoxyisonicotinic acid whose skeleton was built from 1-cyclopentylethanone, ethyl oxalate, and cyanoacetate in a Guareschi-Thorpe reaction in 42% yield over five steps. The second, chiral intermediate, is a phenol ether derived from enantiomerically pure (R)-isopropylidene glycerol ((R)-solketal) and 3-ethyl-4-hydroxy-5-methylbenzonitrile in 71% yield in a one-pot reaction. The overall sequence entails 18 chemical steps with 10 isolated intermediates. All raw materials are cheap and readily available in bulk quantities, the reaction conditions match with standard pilot plant equipment, and the route reproducibly afforded 3-20 kg of 1 in excellent purity and yield for clinical studies.

Full text of DOI:10.1021/acs.oprd.6b00210

Article
pubs.acs.org/OPRD
Practical Synthesis of a S1P Receptor 1 Agonist via a Guareschi−
Thorpe Reaction
Gunther Schmidt, Martin H. Bolli, Cyrille Lescop, and Stefan Abele*
Chemistry Process R&D, Actelion Pharmaceuticals Ltd., Gewerbestrasse 16, CH-4123 Allschwil, Switzerland
S
* Supporting Information
ABSTRACT: A practical synthesis of S1P receptor 1 agonist ACT-334441 (1) through late-stage convergent coupling of two
key intermediates is described. The first intermediate is 2-cyclopentyl-6-methoxyisonicotinic acid whose skeleton was built from
1-cyclopentylethanone, ethyl oxalate, and cyanoacetate in a Guareschi−Thorpe reaction in 42% yield over five steps. The second,
chiral intermediate, is a phenol ether derived from enantiomerically pure (R)-isopropylidene glycerol ((R)-solketal) and 3-ethyl-
4-hydroxy-5-methylbenzonitrile in 71% yield in a one-pot reaction. The overall sequence entails 18 chemical steps with 10
isolated intermediates. All raw materials are cheap and readily available in bulk quantities, the reaction conditions match with
standard pilot plant equipment, and the route reproducibly afforded 3−20 kg of 1 in excellent purity and yield for clinical studies.
ester was cleaved under aqueous acidic conditions to yield 2-
cyclopentyl-6-methoxyisonicotinic acid 5 as a white a solid.
This acid was coupled with hydroxybenzamidine 62 employing
O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetra-
fluoroborate (TBTU). Unfortunately, not only the hydrox-
ybenzamidine but also the phenol function was acylated leading
to considerable amounts (approximately 40%) of bis-acylated
product. The crude mixture was cyclized in dioxane at elevated
temperature to afford the oxadiazole 8 together with the phenol
ester byproduct A (Figure 2).
The crude product 8 was treated with aqueous NaOH to
cleave the ester of byproduct A. The product 8 was isolated in
44% yield (from 7) after crystallization from acetonitrile with
high purity (100% a/a, LC−MS). The free phenol group was
alkylated with excess (S)-3-chloro-1,2-propandiol under basic
conditions at 75 °C. The conversion reached only ca. 60%. The
API 1 was isolated in pure form after chromatography and
crystallization from EtOAc/heptane in a yield of 44%. The R-
isomer of API 1 was not detected by chiral HPLC analysis.
For the rapid delivery of API 1 in kilogram amounts a safe
and robust protocol for the Negishi reaction, an improved
coupling of the isonicotinic acid 5 with a suitable hydrox-
ybenzamide derivative, and a higher yielding method for the
introduction of the chiral side chain were required. The low
melting point of the API 1 (80 °C) and the good solubility in
most organic solvents posed a further challenge to the
purification and isolation.
INTRODUCTION
■
ACT-334441 (1) is a synthetic sphingosine 1-phosphate 1
receptor (S1PR1) agonist which was developed to treat
autoimmune diseases (Figure 1).1 The structure is similar to
ACT-209905, another S1PR1 agonist found at Actelion
Pharmaceuticals Ltd. The synthesis of ACT-209905 was
described earlier in this journal.2 The new compound ACT-
334441 (1) differs in the isonicotinic acid moiety, which is
decorated with a methoxy group and a cyclopentyl ring in the α
positions of the nitrogen and in the chiral side chain, while the
central part (oxadiazole and phenol moiety) is identical.
Therefore, this account mainly deals with the route finding
toward and the scale-up of the isonicotinic acid 5, the
introduction of the polar, chiral side chain, and the convergent
coupling to the active pharmaceutical ingredient (API) on
kilogram scale. Two routes to the isonicotinic building block 5
are presented. The first route consists of four steps including a
Negishi reaction as a key to introduce the cyclopentyl ring. This
synthesis was smoothly scaled up to several kilograms. The
second route comprises five steps and utilizes a Guareschi−
Thorpe reaction to construct the isonicotinic acid core. Both
routes are compared. The chiral side chain derived from L-
(−)-1,2-isopropylideneglycerol ((R)-solketal, 19) was effi-
ciently attached to the phenol moiety and served in addition
as an OH-protecting group.
MEDICINAL CHEMISTRY ROUTE
■
The first gram amounts of API 1 were synthesized in six steps
from 2-chloro-6-methoxyisonicotinic acid 2 (Scheme 1). First,
the chloro-isonicotinic acid 2 was protected as the methyl ester
in high yield with methanol under acidic conditions. The
cyclopentyl group was introduced via a Pd-catalyzed Negishi
coupling of methyl ester 3 with commercially available
cyclopentylzinc bromide at elevated temperature. The reaction
was carried out in full batch mode by dissolving the reagents
and the catalyst in dioxane and heating the resulting mixture to
85 °C. The pyridine derivative 4 was purified by flash
chromatography and isolated as an oil in 61% yield. The
RESULTS AND DISCUSSION
■
Isonicotinic Acid (5). We first addressed the scale-up of
isonicotinic acid 5 (Scheme 2). Although the starting material 2
of the medicinal chemistry route was available, it was deemed
too expensive for scale-up.3 Therefore, the less expensive
dichloroisonicotinic acid 9 was chosen as starting point.4 The
acid was converted to the methyl ester 10 in methanol and
Received: June 13, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
Figure 1. ACT-334441 (1) and retrosynthetic bond breaks revealing the key building blocks, isonicotinic acid 5 and hydroxybenzamidine 23.
Another similar S1PR1 agonist, ACT-209905,2 is shown for comparison.
Scheme 1. Route Used by Medicinal Chemistry for the Synthesis of the First Grams of the API 1
Scheme 2. First-Generation Route for the Synthesis of
Building Block 5 for the Phase 1 Campaign
Figure 2. Phenol ester byproduct in the oxadiazole (8) formation step
of the Medicinal Chemistry route.
sulfuric acid at an elevated temperature in the presence of the
water scavenger trimethyl orthoformate.5 The ester crystallized
from methanol/water and was isolated as a white solid in
excellent yield (97%). One chloride of the dichloro
isonicotinate 10 was selectively substituted with methoxide
using NaOMe in methanol at reflux temperature.6 Again
product 3 crystallized from methanol/water and was isolated as
a white solid in excellent yield (90%) and good purity (100% a/
a, LC-MS). It was also possible to invert the reaction sequence
function.7 We chose the first route because the workup was
by first installing the methoxy group and then the ester
simpler and the yield was slightly higher. For the Negishi
B
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
Scheme 3. Second-Generation Route for the Synthesis of Isonicotinic Acid 5 for the Phase 2 Campaign
coupling we employed similar conditions as the precedent.
Cyclopentylzinc bromide was purchased as a 0.5 M solution in
THF from Rieke Metals.8 This reagent was also available in
larger quantities for further scale-up. The catalyst of choice was
[1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium(II)
complex with CH2Cl2 ((Pd(dppf)·DCM), and the loading was
0.008 equiv (1 g of catalyst per 33 g of substrate 3). Dioxane
was replaced by the less toxic THF. Since the Negishi coupling
was an exothermic reaction, we had to develop a dose
controlled process. The addition of isonicotinic acid 3 to the
organozinc bromide/Pd(dppf)·DCM mixture at elevated
temperature resulted in incomplete conversion. Next, we
added the cyclopentylzinc bromide solution (1.1 equiv) to
the starting material and catalyst in THF at 60 °C. The addition
was slightly exothermic and was easily controlled by the rate of
addition. The conversion to 4 after addition of the reagent was
usually 80%. After further stirring at elevated temperature, the
conversion went to completion. Interestingly, the use of 1 M
zinc reagent was less efficient, resulting in incomplete
conversion. A further challenge was the workup and purification
of the product. Most of the THF was distilled off. The residue
was diluted with tert-butyl methyl ether (TBME) and extracted
with aqueous HCl to remove the zinc salts. The TBME layer
was clarified by filtration over charcoal and Celite. This afforded
the crude ester 4 in good yield (95%) and purity (95% a/a, LC-
MS) as an orange oil. The hydrolysis of ester 4 with aqueous
NaOH in ethanol was straightforward and provided a further
purification possibility of the preceding Negishi coupling. The
acid 5 could easily be purified by extraction into the basic
aqueous layer and washing with TBME, back-extraction of 5
from the acidified aqueous solution, followed by crystallization
of 5 from acetonitrile. This route delivered the nicotinic acid 5
in only four steps with an overall yield of 64% as an off-white
solid with a purity of 100% a/a (LC-MS).
Phase 1 clinical studies. Still, the Negishi coupling presented a
major pitfall for further scale-up, specifically, the high dilution,
the high cost, and transportation restrictions of the organozinc
reagent, and the disposal of concomitant zinc waste. Therefore,
an alternative synthesis of the isonicotinic acid utilizing the
Guareschi−Thorpe reaction was studied (Scheme 3).9 This
strategy was already applied for the synthesis of ACT-209905.2
Thus, the reaction of cyclopentyl dioxobutanoate 14 with
cyanoacetamide was expected to provide the isonicotinic acid
framework, which could be further elaborated to the desired
building block 5. Unfortunately, the simple cyclopentylmethyl
ketone 13, the starting material of this route, was not available
for a reasonable price.10 Several approaches to 13 are reported
in the literature.11 All methods were not appropriate for our
needs, due to low yields, the use of expensive starting materials,
toxic reagents, or difficult-to-scale up protocols. A simpler and
scalable synthesis of cyclopentylmethyl ketone 13 was in
demand.
An early publication by Goldsworthy11e described the
reaction of ethyl acetoacetate and 1,4-dibromobutane in an
ethanolic NaOEt solution to form ethyl 1-acetyl-cyclo-
pentanecarboxylate, which was hydrolyzed and decarboxylized
to yield the desired cyclopentyl methyl ketone 13. Although the
process was low yielding and the decarboxylation reaction was
sluggish, we speculated that the decarboxylation of the
corresponding tert-butyl ester 12 under acidic conditions
would provide the ketone 13 in a cleaner and higher yielding
reaction.12 Following an alternative protocol for the cyclo-
alkylation,13 the reaction of tert-butyl acetoacetate with 1,4-
dibromobutane under phase transfer catalysis (PTC) in a polar
solvent like DMF or DMSO with K2CO3 at rt afforded the
desired keto ester 12. As a phase transfer catalyst we utilized
tetrabutyl ammonium iodide or bromide, or the ionic liquid
[Bmim]BF4, and obtained similar results. The reaction was
slow and did not reach full conversion even after several hours.
Extractive work up and distillation afforded the product 13 in a
This route was suitable for the production of 30 kg of
building block 5 to support the GMP manufacturing of API for
C
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
yield of 80% and a GC purity of 90% a/a. Nevertheless the
quality was sufficient for the following step. Since the slightly
exothermic reaction was carried out in full-batch mode, a safer
dose-controlled process was developed. To this end, NBu4Br,
1,4-dibromobutane, and 50% NaOH were heated to 50 °C. To
this mixture was added tert-butyl acetoacetate at 50 °C over a
period of 1 h. The reaction gave the product in good yields, but
a precipitate formed during the addition of the acetoacetate,
which made stirring difficult. Using less concentrated 32%
NaOH solved this problem. After the addition of the
acetoacetate and stirring for an additional hour, GC-MS
showed a clean reaction and disappearance of both starting
materials. The crude product 12 was introduced into the next
step as is with a GC-MS purity of 96% by addition to a 32%
HCl solution at 60 °C. The conversion to the ketone 13 was
completed in 2 h, and upon cooling to 40 °C, 13 separated
from the aqueous acidic layer. The neat 13 was washed with
brine, dried over MgSO4, and distilled at 40−55 °C and 30 to 7
mbar. The ketone 13 had to be dry for the next step. The yield
over the two steps was 60−75% with excellent purity (>99% a/
a, GC-MS).
The 2,4-diketoester 14 was formed in a Claisen-Schmidt
reaction by addition of a mixture of 13 and diethyl oxalate to a
slight excess of KOtBu in THF at −20 °C. Usually, the reaction
was complete after addition, and the product was formed in
high purity. A workup or purification was not required. For the
study of the envisioned Guareschi−Thorpe reaction, however,
diketoester 14 was purified as colorless liquid by distillation (95
°C, 5 × 10−2 mbar). 14 was reacted with cyanoacetamide in the
presence of triethylamine in ethanol at 65 °C for 1 h.14 Product
15 precipitated upon cooling and was isolated in 30% yield; the
solubility in ethanol called for a rework of the mother liquor to
get a second crop and a total yield of 60−70%. One of the
detected byproducts was tentatively attributed to the acid B
([M + 1] = 233) (Figure 3).15 The water formed during the
dilution with water and removal of THF by distillation, the
residue was added to 32% HCl and stirred at 100 °C for 22 h.
Upon cooling the desired product 16 precipitated as a white
solid in a yield of 69% and excellent purity (100% a/a, LC-MS).
For the next steps, a direct transformation of isonicotinic acid
16 to the chloro ester 18 was tempting. Upon heating of 16 in
excess POCl3 at reflux and quench into methanol, the desired
product 18 was formed. However, the quench of the crude,
concentrated POCl3 reaction mixture into methanol was very
violent and exothermic. In addition, the subsequent aqueous
workup gave product contaminated with trimethyl phosphate
and some 2-chloro-6-cyclopentylisonicotinic acid E from
hydrolysis of ester 18. This chloro acid did not react as fast
as the ester 18 in the final methoxylation step and was not fully
purged in the isolation of methoxy isonicotinic acid 5.
Therefore, the acid 16 was first converted to the methyl ester
17 in methanol, trimethyl orthoformate, and catalytic H2SO4.
After addition to water at 50 °C, 17 crystallized and was
isolated by filtration in 97% yield. The direct methylation of 17
to 4 with methyl iodide in DMF with K2CO3 produced a 50:50
mixture of the N- and O-methylated regioisomers. No further
investigations to improve this reaction were carried out. The
triflate of pyridone 17 was reacted with KOMe in methanol
yielding only the isonicotinic acid 16 upon workup. The two-
step approach, i.e., chlorination−SNAr, seemed more promising.
The chlorination of ester 17 with POCl3 was sluggish and
required harsh conditions. In addition, the quench was violent
and the workup not scalable.16 Therefore, POCl3 was
exchanged with phenylphosphonic dichloride.17 This reagent
is convenient because of its high boiling point (258 °C)
allowing higher reaction temperatures than POCl3. The ester
17 was suspended in two equivalents of phenylphosphonic
dichloride and heated to 130 °C for 4 h reaching a clean
conversion to the product 18. The quench of the reaction
mixture had to be carefully designed to prevent ester hydrolysis.
To this end, the reaction mixture was diluted with acetonitrile
and quenched onto 10% trisodium citrate solution (pH 8−9)
and iPrOAc at 5−10 °C. The addition was only moderately
exothermic.18 The ester was stable under these conditions (pH
1). A black slimy product was formed troubling the following
extractive workup and necessitating several filtrations over
Celite. Still, the dark-brown byproducts were partially soluble in
iPrOAc and could not be removed completely during these
treatments. Changing the solvent to TBME gave a less colored
filtrate after Celite treatment. The slime was best removed by a
filtration over Celite after the washing step with Na2CO3
solution affording two clear layers and a smooth ensuing
aqueous extraction.19 The ester 18 was isolated as a brown oil
in excellent yield (97%) and purity (100% a/a, LC-MS).
The crude oil of 18 was treated with excess 25% KOMe in
methanol and heated to reflux leading to rapid substitution of
the chloride. Traces of water or KOH from the reagent
converted the chloro ester 18 to the chloro acid E, which was
much less reactive than the ester. Therefore, the mixture was
concentrated by distillation to reach an internal temperature of
92 °C to accelerate the reaction. The reaction required
approximately 7 h to reach a conversion of >99%. Chloro
acid E was not removed during workup and even enriched
during crystallization (up to 5%). Instead, it was easily removed
in the following coupling step (Scheme 5). The lactam
derivative 16 was formed as another minor impurity (<2% a/
a, LC-MS), but it was completely removed in the crystallization
of 5.20 In the optimized workup, water was added to the
Figure 3. Byproducts A−E in the Guareschi−Thorpe reaction route to
5 (according to LC-MS).
condensation could hydrolyze the ethyl ester. A second product
([M + 1] = 289) was detected and could correspond to the
ethyl ether of 15 (C). When isopropanol was used as a solvent,
this impurity was formed as well; the isopropyl ether could not
be detected. To circumvent these problems THF and KOtBu
(1.1 equiv) were chosen, the same conditions of the previous
step. To our delight, the reaction performed well, and these two
steps could be telescoped, the slight excess of base of the
reaction to ketopyruvate 14 being sufficient to drive the
Guareschi−Thorpe reaction to completion. The reaction
temperature of the Guareschi−Thorpe reaction was set to 22
°C, since the reaction was too slow at −20 °C. The in-process-
control (IPC) purity of the ethyl ester 15 was 63% a/a (LC-
MS) together with productive byproducts: while the impurity
with the mass 288 was not detected anymore, the acid was still
present together with a new impurity ([M + 1] = 251) which
was the amide D derived from nitrile hydrolysis of B. After
D
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
mixture at 80 °C containing the methyl ester 4, thus triggering
the hydrolysis to 5. After the removal of additional methanol,
acidification with 32% HCl, and extraction with isopropyl
acetate, the crude product was obtained quantitatively.
Crystallization from acetonitrile afforded the isonicotinic acid
5 with a yield of 86% and excellent purity (100% a/a, LC-MS).
A comparison of the two routes to 5 reveals a clear roadblock
for further scale-up for route 1 (Table 1). Notably, the zinc
removed easily under mild acidic conditions in the final step.
The synthesis sequence started with the activation of the free
hydroxy group of solketal as tosylate employing standard
conditions.22 The tosylate 20 was isolated as crude oil in a yield
of 95% and 99% a/a purity (GC-MS). Phenol 212 was cleanly
alkylated with 20 in DMF employing K2CO3 as base at 120 °C.
The liquid nitrile 22 was directly converted to the
hydroxamidine 23 by treatment with hydroxylamine·HCl in
methanol in the presence of NaHCO3. The right-hand side
building block 23 was isolated as a white solid after
crystallization from TBME and heptane. The reaction
procedures allowed to telescope all three steps, avoiding the
cumbersome isolation of oils. The overall yield for the sequence
was 71%, and the LC-MS purity of the product 23 was 100% a/
a.
Table 1. Comparison of the Two Approaches to Isonicotinic
Acid 5
route 1, Scheme 2
Negishi
route 2, Scheme 3
Guareschi−Thorpe
steps/isolated
intermediates
4/3
8/4
Synthesis of the API (1). With the two stable and well-
characterized building blocks 5 and 23 at hand, the stage was
set for the convergent coupling to the API 1 (Scheme 5). A
solution of the right-hand side building block 23 and Et3N in
DCM was dosed to the acid chloride of isonicotinic acid 5, the
latter being prepared using oxalyl chloride. Aqueous workup
and solvent switch from DCM to ethanol resulted in
precipitation of the coupled product 24 that was isolated as a
white solid in yields of 77−82% and a purity of 97% a/a
(HPLC). It was important to have an excellent quality at this
stage since an upgrade was difficult to achieve at the later
stages. For the formation of the oxadiazole ring, compound 24
was stirred in toluene at 110 °C for 4 h removing the generated
water with a Dean−Stark trap. Without removal of water, the
conversion was incomplete. The solvent was removed in the
reactor under reduced pressure to isolate the product as a
nonviscous yellow oil in quantitative yield. It was also possible
to perform a solvent switch from toluene to ethanol and to
telescope into the last step. Finally the protecting group had to
be removed to liberate the API. The ketal 25 was easily cleaved
with aqueous HCl at elevated temperature.23 Under strongly
acidic conditions the methoxy group was slowly hydrolyzed; the
ensuing impurity F (Figure 4) could not be purged, in contrast
to unreacted starting material 25, during the workup and final
crystallization.
Stress tests were performed to determine the best conditions
for the deprotection. The ketal 25 was dissolved in 2.5 vol
ethanol and 1.5 vol 2 N HCl and heated to 60 °C. IPCs after 60
and 120 min showed 0.8 and 0.7% of 25 and 0.21 and 0.7% of
the F, respectively. This showed that the reaction was
proceeding rapidly, but reaching full conversion would lead
to more byproduct. The reaction conditions were too harsh
leading to an unacceptable degradation of the API. Lowering
both the temperature to 50 °C and the acid concentration to 1
N HCl indicated 1.1% starting material and just 0.01%
byproduct F after 1 h. After 120 min, the content of starting
material was 0.7%, while 0.04% of F was formed. We chose
slightly milder conditions for scale-up by reducing the volume
of acid to 1.3 L/kg and lowering the temperature to 45 °C. In
addition, the reaction was run under reduced pressure (400
mbar, 45 °C) to remove acetone, and the reaction time was
extended to 4 h. Upon scale-up, the impurity F increased to
0.17%, while the starting material was at 0.5%. Extractive
aqueous workup with EtOAc afforded the API 1 as a yellow oil
in almost quantitative yield and high purity (98.8% a/a,
HPLC).
yield
63%
42%
metals
Pd-ligand, zinc
none
cost driver
Rieke reagent
(transport, waste)
phenylphosphonic
dichloride
robustness on scale
scale-up limitation
(Negishi)
all steps scalable, one
distillation
reagent makes it a costly endeavor. Although the number of
steps increased, the route 2 was chosen for Phase 2 clinical
supplies as all steps are robust and use cheap reagents.
Especially, no heavy metals are used, rendering it more
environmentally benign.
Synthesis of Hydroxybenzamidine 23. The strategy to
couple the nonprotected hydroxybenzamidine phenol 6 with
the isonicotinic acid 5 and to introduce the glycerol side chain
at the last stage of the reaction sequence was sensible from a
medicinal chemistry perspective to allow for the synthesis of
diverse lead compounds with a similar synthetic scheme (see
Scheme 1). It suffered from competing acylation at the phenolic
hydroxy group. Moreover, the alkylation of the phenol 8 with a
large excess of (S)-3-chloro-1,2-propandiol was low yielding
and gave low purity crude API. The alkylation of phenol 8 with
(S)-O-isopropylidene glycerol mesylate in DMF with NaH did
not take place at rt, while at elevated temperature the starting
material decomposed.21 The addition of KI or the use of
K2CO3 as a base did not improve the reaction.
We reckoned that an earlier introduction of the polar side
chain was beneficial, with the additional asset of the phenol
hydroxy group being protected. As a suitable chiral starting
material, we chose (R)-solketal (19, Scheme 4). The protection
group should be stable for the following steps and should be
Scheme 4. New Three-Step Telescoped Sequence to Right-
Hand Side Building Block 23 from (R)-Solketal 19
The API is very soluble in most common organic solvents
and has a melting point of 80 °C (DSC analysis). The crude
E
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
Scheme 5. Convergent Coupling of Key Building Blocks 5 and 23 to ACT-334441 (1)
(er) was determined to be 99.7:0.3. The production of 3.3 kg of
ACT-334441 was smooth, and the CRO was successfully
reproducing it under GMP to deliver 20 kg of API 1 for Phase 1
clinical supply.
Scheme 6 shows the entire synthetic sequence to the API 1
comprising 18 chemical transformations with 10 isolated
intermediates. The longest linear chain (13 chemical steps, 7
isolated intermediates) starts from tert-butyl acetoacetate (11)
and ends with the API 1 in 28% yield. Readily available 2-ethyl-
6-methyl aniline (26)24 and (R)-solketal (19) are the starting
materials for the synthesis of the right-hand side building block
23. (R)-Solketal is the most expensive raw material.
Figure 4. Demethylated API impurity F, formed during deprotection
of 25.
API could be crystallized from a mixture of EtOAc and n-
heptane in the presence of seed crystals. Without the addition
of seeds the product usually separated as an oil. The water
content must be controlled below 0.2% to prevent oiling out.
Two protocols were developed. (1) The slow addition of n-
heptane to a solution of 1 in EtOAc at 20 °C followed by the
addition of seeds, when the mixture got slightly turbid. (2) The
slow addition of a solution of the API 1 in EtOAc to a
suspension of a small quantity of 1 in n-heptane at 40 °C. After
the suspension was formed, an aging time of at least 0.5 h at 40
°C was implemented. Both protocols produced fine needle-like
particles. The particle size of the batches produced via the
second method (Figure 5a) was slightly larger than that of the
CONCLUSION
■
The success of the optimized large-scale synthesis of S1PR1
agonist ACT-334441 (1) resides on two pillars, i.e., (1) a
scalable access to the strategic building block, isonicotinic acid
5, and (2) a new advanced building block (23) decorated with
the chiral glycerol side chain. Both building blocks 5 and 23 are
stable and well-characterized solids. Whereas the first route to 5
using a Negishi coupling with cyclopentylzinc bromide could be
scaled up to 30 kg to support the API production for entry-
into-man, a novel strategy had to be devised for the installation
of the chiral side chain already for the first kilogram-makes. To
this end, a phenol intermediate (21) from the manufacturing of
another S1PR1 receptor agonist at Actelion Pharmaceuticals
Ltd. (ACT-209905) could be used for the alkylation with the
tosylate of (R)-solketal, leading, in a telescoped sequence to the
second strategic building block, the hydroxamidine 23. Three
high yielding steps lead to the API with high purity. As a
testimony to the robustness of this novel convergent scheme,
the scale-up to 100 g, 3.3 kg, and 20 kg of 1 proceeded well.
FDA’s recent imperative for quality by design (QbD) calls for a
robust process that delivers the API in a predictable and
efficient way, in consistent purity at acceptable costs. To avoid
later route changes and to profit from a longer learning curve,
QbD should start as early as possible. Hence, the route to
building block 5 was completely changed into more robust
steps devoid of the deficiencies of the Negishi step like narrow
operational windows and impediments on the environmental
and cost side. A novel access to cyclopentyl methyl ketone
(13)12 was decisive for the success of the Guareschi−Thorpe
approach to 5 that is the preferred route at larger scales.
Figure 5. Microscope pictures of 1 showing the different particle sizes
of batches obtained by two different crystallization protocols described
in the text.
batches emerging from the first method (Figure 5b). The
particle size distribution (PSD) showed that 90% of the
particles were smaller than 59 and 18 μm for the second and
first protocol, respectively. The filtration of the suspension was
rather smooth causing no problems. The yield was 81%, and
the HPLC purity increased from 98.8% a/a (crude) to 99.5% a/
a with single impurities <0.23% a/a. The enantiomeric ratio
F
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
Scheme 6. Synopsis of the Final Route to ACT-334441 (1) with Second-Generation Synthesis of Isonicotinic Acid 5
mp 80 °C (DSC); 1H NMR (CDCl3): δ 7.45 (s, 1 H), 7.24 (s,
1 H), 4.00 (s, 3 H), 3.96 (s, 3 H).
EXPERIMENTAL SECTION
■
General. 1 vol or 1 wt means 1 L of solvent or 1 kg of
Methyl 2-Cyclopentyl-6-methoxyisonicotinate (4). Me-
thoxyisonicotinate 3 (183 g, 1.0 equiv) and Pd(dppf)·DCM
(4.9 g, 0.008 equiv) were placed in a 4 L double-jacketed
reactor. The reactor was inertized three times with nitrogen/
vacuum, and degassed THF (0.6 L) was added. The mixture
was heated to 65 °C. 0.5 M cyclopentylzinc bromide solution in
THF (2 L, 1.1 equiv) was added in portions of approximately
200 mL at 65 °C. The color turned from red to violet and then
to black. The addition of the cold Rieke reagent led to cooling
of the mixture to approximately 60 °C. When the temperature
reached 66 °C another portion of Rieke reagent was added.
After complete addition, a sample was analyzed by LC-MS
indicating a conversion of ca. 80%. The mixture was stirred at
reflux for 40 min. The mixture was concentrated, and solvent (2
L) was removed at a jacket temperature of 110 °C and 1 atm.
The residue was cooled to 20 °C and diluted with TBME (2 L).
The mixture was extracted with 1 N HCl (1 L). The TBME
layer was filtered over a pad of Celite (50 g) and Norit (50 g).
The filtrate was washed with water (0.5 L). The organic layer
was concentrated to dryness to yield an orange oil. Yield: 205 g
(95%). Purity (LC-MS method 1): 95% a/a, tR 1.92 min; [M +
reagent with respect to 1 kg of the reference starting material.
Compounds are characterized by 1H NMR (400 MHz, Bruker)
or 13C NMR (100 MHz, Bruker); the internal standard for
quantitative NMR was 1,4-dimethoxybenzene. Details for the
HPLC, LC-MS, and GC-MS methods are listed in the
Supporting Information. Unless stated otherwise, yields are
given as is.
Methyl 2,6-Dichloroisonicotinate (10). Dichloroisonicotinic
acid 9 (300 g, 1.0 equiv), trimethyl orthoformate (171 mL, 1
equiv), conc. H2SO4 (25 mL, 0.3 equiv), and MeOH (1.55 L)
were heated to 60 °C for 18 h. The solution was cooled to 10
°C, and to the resulting suspension was added water (1.5 L).
The suspension was filtered and the product washed with water
(0.5 L). The white solid 10 was dried in a tray drier at 55 °C.
Yield: 311 g (97%). Purity (LC-MS method 1): 100% a/a, tR
1
1.475 min; [M + 1]+ = 206; mp 81 °C (DSC); H NMR
(CDCl3): δ 7.83 (s, 2 H), 4.00 (s, 3 H).
Methyl 2-Chloro-6-methoxyisonicotinate (3). Isonicotinic
ester 10 (286 g, 11.0 equiv), MeOH (1.4 L), and 5.4 M
NaOMe in MeOH (283.5 mL, 1.1 equiv) were heated to reflux
for 3.5 h. The suspension was cooled to 0 °C, and a solution of
25% HCl (86 mL) in water (1.7 L) was added. The suspension
was filtered. The white solid was washed with water (300 mL)
and dried in a tray drier at 55 °C. Yield: 251 g (90%). Purity
(LC-MS method 1): 99% a/a, tR 1.557 min; [M + 1]+ = 202;
1
1]+ = 236; H NMR (CDCl3): δ 7.28 (s, 1 H), 7.09 (s, 1 H),
3.95 (s, 3 H), 3.92 (s, 3 H), 3.16 (quint, J = 8.1 Hz, 1 H), 2.03
(m, 2 H), 1.81 (m, 4 H), 1.67 (m, 2 H).
G
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
2-Cyclopentyl-6-methoxyisonicotinic Acid (5). From 4
(Route 1). A mixture of methyl ester 4 (205 g, 1.0 equiv),
EtOH (1 L), 32% NaOH (0.2 L), and water (0.2 L) was heated
at 70 °C for 10 min. The reaction mixture was concentrated at
70 °C and reduced pressure. 1 L of solvent was removed. To
the black residue was added TBME (0.5 L) and water (1.5 L) at
20 °C. The mixture was filtered over Celite (50 g). The
aqueous layer was separated, and the pH was adjusted to 1 with
25% HCl (0.3 L). The aqueous layer was extracted with iPrOAc
(0.75 and 0.5 L). The combined organic layers were washed
with water (1 L) and concentrated to dryness to obtain crude
acid 5 (181 g). The crude product was dissolved in acetonitrile
(0.87 L) at 80 °C. The solution was cooled to 20 °C and
filtered. The product was washed with acetonitrile (30 mL),
and the off-white solid was dried at 50 °C. Yield: 148 g (77%).
Purity (LC-MS method 1): 100% a/a; tR 1.60 min; [M + 1]+ =
222; mp 118 °C (DSC); 1H NMR (d6-DMSO): δ 13.50 (br s, 1
H), 7.25 (s, 1 H), 6.98 (s, 1 H), 3.88 (s, 3 H), 3.18 (m, 1 H),
2.01 (m, 2 H), 1.72 (m, 6 H). 13C NMR (100 MHz, d6-
DMSO): δ 166.6, 164.8, 164.2, 142.0, 113.8, 107.7, 53.7, 47.1,
33.4, 25.9.
From 18 (Route 2). A mixture of methyl isonicotinate 18
(157 g, 1.0 equiv) and a 25% solution of KOMe in MeOH
(1.98 L, 10.2 equiv) was heated to 100 °C jacket temperature.
MeOH (0.57 L) was removed by distillation until the internal
temperature reached 92 °C. The mixture was further stirred for
4 h. The mixture was further concentrated (1.2 L solvent
removed). To the residue was added water (1.8 L) at 80 °C.
The mixture was further concentrated under reduced pressure
(0.23 L solvent removed). The residue was cooled to 10 °C,
and 32% HCl (700 mL) was added below 15 °C. The mixture
was extracted with iPrOAc (1 L). The organic layer was washed
with water (2 × 0.5 L) and concentrated to dryness to yield a
brown oil (146 g, 101%). The crude product was recrystallized
from acetonitrile (0.43 L) to yield a light brown solid. Yield:
124 g (86%). Purity (LC-MS method 1): 100% a/a, tR 1.60
min, [M + 1]+ = 222.
was removed). The reactor was charged with 32% HCl (5 L)
and heated to 50 °C. The residue was added to the HCl
solution at 44−70 °C (Caution: strong foaming). The mixture
was stirred at 100 °C for 22 h at 135 °C jacket temperature and
approximately 400 mbar (2.7 L solvent was removed). The
suspension was diluted with water (2.5 L) and cooled to 10 °C.
The suspension was filtered, and the cake was washed with
water (2.5 L) and acetone (3 L). The product was dried at 60
°C under reduced pressure to obtain an off-white solid. Yield:
255 g (69%). Purity (LC-MS method 2): 100% a/a; tR 0.96
min, [M + 1]+ = 208; 1H NMR (d6-DMSO): δ 12.67 (br, 2 H),
6.63 (s, 1 H), 6.38 (s, 1 H), 2.89 (m, 1 H), 1.98 (m, 2 H), 1.63
(m, 6 H).
Methyl 2-Cyclopentyl-6-hydroxyisonicotinate (17). A
mixture of 16 (1.52 kg, 1.0 equiv), MeOH (15.2 L), trimethyl
orthoformate (1.56 L, 2 equiv), and conc. H2SO4 (471 mL, 1.2
equiv) was heated at 95 °C jacket temperature for 18 h at
approximately 800 mbar (10 L solvent was removed). The
residue was cooled to 20 °C and added to water (7.6 L) at 50
°C. The suspension was diluted with water (3.8 L), cooled to
10 °C, and filtered. The cake was washed with water (3.8 L).
The product was dried in a tray in a fume hood for 4 days to
obtain a yellowish solid. Yield: 1568 g (97%). Purity (LC-MS
1
method 2): 100% a/a, tR 1.16 min, [M + 1]+ = 222; H NMR
(d6-DMSO): δ 11.98 (br, 1 H), 6.63 (m, 1 H), 6.39 (s, 1 H),
3.83 (s, 3 H), 2.91 (m, 1 H), 1.99 (m, 2 H), 1.72 (m, 2 H), 1.58
(m, 4 H).
Methyl 2-Chloro-6-cyclopentylisonicotinate (18). 17 (150
g, 1.0 equiv) and phenylphosphonic dichloride (90% assay, 214
mL, 2.0 equiv) were stirred at 130 °C for 4 h. The reaction
mixture was cooled to 22 °C and diluted with CH3CN (300
mL). The reaction mixture was added to a mixture of TBME
(1.5 L) and a 10% solution of trisodium citrate (1.5 L) below 8
°C over a period of 40 min. The mixture was warmed to 22 °C,
and the layers were separated. The TBME layer was extracted
with 0.5 M Na2CO3 solution (1 L). A brown precipitate was
removed by filtration over Celite prior phase separation. The
organic layer (pH 8) was washed with 0.5 N HCl (1 L) and
water (1 L). The TBME layer was concentrated to dryness to
yield a brown oil. Yield: 157.1 g (97%). Purity (LC-MS method
1): 100% a/a, tR 1.88 min, [M + 1]+ = 240; 1H NMR (CDCl3):
δ 7.69 (s, 1 H), 7.67 (s, 1 H), 3.97 (s, 3 H), 3.23 (m, 1 H), 2.12
(m, 2 H), 1.80 (m, 6 H). 13C NMR (100 MHz, CDCl3): δ
168.2, 164.8, 151.4, 140.3, 121.0, 119.5, 52.9, 47.8, 33.6, 25.7.
(R)-4-((2,2-Dimethyl-1,3-dioxolan-4-yl)methoxy)-3-ethyl-
N-hydroxy-5-methylbenzimidamide (23). To a solution of
DMAP (28 g, 0.1 equiv), (R)-2,2-dimethyl-1,3-dioxolane-4-
methanol (19, 300 g, 1.0 equiv), and Et3N (306 mL, 0.97
equiv) in toluene (1 L) was added a solution of p-TsCl (424 g,
0.98 equiv) in toluene (1.25 L) at 20 °C. After stirring for 2 h,
the reaction mixture was washed with water (690 mL), a
mixture of water (630 mL) and 1 N HCl (170 mL), a mixture
of water (630 mL) and sat. NaHCO3 solution (60 mL), and
water (690 mL). The organic phase was concentrated at 70 °C
under reduced pressure to yield a yellow oil. To the residue was
added DMF (1 L) and phenol 21 (291.9 g, 0.9 equiv), followed
by K2CO3 (528 g, 1.9 equiv) (Caution: gas evolution). The
reaction mixture was heated to 120 °C (Caution: exothermic
reaction). The reaction mixture was stirred at 120 °C for 3 h.
After full conversion (IPC by LC-MS), the reaction was diluted
with iPrOAc (4 L) at 20 °C and washed with water (3 × 4 L).
The organic layer was concentrated at 70 °C under reduced
pressure to yield a yellow oil. To the residue was added MeOH
1-Cyclopentylethanone (13). A mixture of 1,4-dibromobu-
tane (273 g, 1.0 equiv) and Bu4NBr (20 g, 0.05 equiv) in 32%
NaOH (1 L) was heated to 50 °C. tert-Butyl acetoacetate (0.2
kg, 1.0 equiv) was added keeping the maximum temperature
below 55 °C. The mixture was stirred at 50 °C for 5 h. After
layer separation, the organic layer was washed with 1 N HCl
(0.5 L). The organic layer was added to 32% HCl (0.3 L) at an
external temperature of 60 °C. The mixture was stirred at 60 °C
for 3.5 h and cooled to 40 °C. The organic layer was separated,
washed with brine (0.15 L), and dried with MgSO4 (8 g). The
mixture was filtered, and the product was purified by distillation
(external temperature: 70 °C, head temperature: 40−55 °C,
30−7 mbar) to obtain a colorless liquid. Yield: 107 g (75%).
Purity (GC-MS): 99.8% a/a; GC-MS: tR 1.19 min; [M + 1]+ =
1
113. H NMR (CDCl3): δ 2.86 (m, 1 H), 2.15 (s, 3 H), 1.68
(m, 8 H).
2-Cyclopentyl-6-hydroxyisonicotinic Acid (16). A 10 L
double-jacketed reactor was charged with KOtBu (0.22 kg, 1.1
equiv) and THF (3 L). The solution was cooled to −20 °C. A
mixture of diethyloxalate (260 g, 1 equiv) and 13 (200 g, 1.78
mol, 1.0 equiv) was added below −18 °C. The reaction mixture
was stirred at −10 °C for 30 min and then warmed to 15 °C.
To the mixture was added cyanoacetamide (0.18 kg, 1.2 equiv).
The mixture was stirred at 22 °C for 20 h. Water (0.6 L) was
added, and the reaction mixture was concentrated at 60 °C
under reduced pressure on a rotary evaporator (3.4 L solvent
H
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
(3 L), NH2OH·HCl (150 g, 2.16 equiv), and solid NaHCO3
(181.5 g, 2.16 equiv) at 20 °C. The mixture was stirred at 20 °C
for 1 h (Caution: gas evolution). The mixture was heated at
reflux overnight. After full conversion (IPC by LC-MS), the
reaction mixture was diluted with water (0.5 L) and
concentrated at 100 °C jacket temperature and reduced
pressure (2.41 L solvent was removed). TBME (2 L) was
added to the residue, and the mixture was washed with water (2
× 1.5 L). The organic layer was concentrated at 80 °C (0.91 L
solvent removed). TBME (2 L) was added to the residue. The
mixture was washed with water (1 L). The organic layer was
concentrated at 75 °C (2 L solvent removed). The mixture was
cooled to 40 °C resulting in a white suspension. To the residue
was added n-heptane (2 L). The suspension was filtered. The
product was dried at 40 °C and 10 mbar to afford a white solid.
Yield: 497.4 g (71% with respect to 19). Purity (LC-MS
method 1): 100% a/a, tt 1.16 min, [M + 1]+ = 309; mp 117 °C
(DSC); 1H NMR (d6-DMSO): δ 9.46 (s, 1 H), 7.36 (m, 2 H),
5.69 (s, 2 H), 4.42 (m, 1 H), 4.11 (m, 1 H), 3.80 (m), 2.63 (m,
2 H), 2.25 (s, 3 H), 1.35 (d, J = 18.6 Hz, 6 H), 1.17 (t, J = 7.5
Hz, 3 H).
126.6, 122.4, 112.1, 109.7, 106.1, 74.5, 73.3, 66.7, 53.7, 47.6,
33.3, 26.8, 25.9, 25.4, 22.9, 16.5, 14.8.
(S)-3-(4-(5-(2-Cyclopentyl-6-methoxypyridin-4-yl)-1,2,4-
oxadiazol-3-yl)-2-ethyl-6-methylphenoxy)propane-1,2-diol
(1). To a 30 L steel enamel reactor was added 25 (2.28 kg, 1.0
equiv) and EtOH (5 L). The solution was heated to 45 °C, and
1 N HCl (3 L, 0.75 equiv) was added. The resulting mixture
was stirred at 45 °C at 1 atm for 1 h and at 45 °C for further 3 h
under reduced pressure (400 mbar). The mixture was
neutralized with 32% NaOH (300 mL, 0.75 equiv) and
concentrated at 60 °C under reduced pressure until the
minimum stirring volume was reached (approximately 2 L).
The residue was diluted with EtOAc (20 L) and washed with
water (2 × 10 L). The organic layer was concentrated at 60 °C
under reduced pressure to yield a yellow oil. Yield: 2053 g
(98%). Purity (HPLC): 99.16% a/a, tR 11.69 min. A second
batch was produced. Yield: 1907 g (98%). Purity (HPLC):
98.79% a/a, tR 11.686 min.
Crystallization of 1. Both batches were combined (3.96 kg)
and dissolved in EtOAc (5.5 L). To the 30 L reactor was added
a portion of solid 1 (14 g from a preceding batch) and n-
heptane (30 L). The suspension was heated to 40 °C, and the
API solution in EtOAc was added at 40 °C over 1 h. The
suspension was further stirred for 0.5 h, cooled to 20 °C, and
filtered over a 30 L glass nutsche. The product was washed with
n-heptane (6 L) and dried on the nutsche applying a gentle N2
stream for 2 d. Yield: 3.3 kg (83%). Purity (HPLC): 99.51% a/
a; assay (1H NMR): 99.40% w/w; er (HPLC method 2): (S):
(R) = 99.7:0.3, tR 10.70 min (S-isomer), 14.5 min (R-isomer);
mp 80 °C (DSC); 1H NMR (d6-DMSO): δ 7.78 (s, 2 H), 7.53
(s, 1 H), 7.26 (s, 1 H), 4.98 (d, J = 4.6 Hz, 1 H), 4.65 (s, 1 H),
3.94 (s, 3 H), 3.86 (m, 2 H), 3.75 (m, 1 H), 3.50 (t, J = 5.4 Hz,
2 H), 3.28 (m, 1 H), 2.75 (d, J = 7.5 Hz, 2 H), 2.35 (s, 3 H),
2.03 (m, 2 H), 1.81 (m, 4 H), 1.69 (m, 2 H), 1.22 (t, J = 7.5
Hz, 3 H). 13C NMR (CDCl3): δ 174.3, 168.9, 165.8, 164.4,
157.4, 137.7, 133.6, 131.7, 128.4, 126.7, 122.5, 112.0, 106.0,
73.9, 71.1, 63.8, 53.7, 47.5, 33.3, 25.9, 22.9, 16.4, 14.8.
(R)-N-((2-Cyclopentyl-6-methoxyisonicotinoyl)oxy)-4-
((2,2-dimethyl-1,3-dioxolan-4-yl)methoxy)-3-ethyl-5-methyl-
benzimidamide (24). To a 30 L steel-enamel reactor was
added 5 (1.27 kg, 1.0 equiv), DMF (17 mL), and DCM (18 L).
To the suspension was added oxalyl chloride (534 mL, 1.1
equiv) at 20 °C over 30 min. The mixture was stirred for 30
min. After full conversion (IPC by LC-MS), a solution of 23
(1.77 kg, 1.0 equiv) and Et3N (1.78 L, 2.2 equiv) in DCM (4
L) was added to the acid chloride at <30 °C over 20 min. After
stirring for 15 min the reaction was confirmed complete by LC-
MS analysis. The reaction mixture was washed with water (7
L). Solvent (18 L) was removed at 55 °C under reduced
pressure. EtOH (26 L) was added, the suspension cooled to 0
°C and filtered. The filter cake was washed with EtOH (7 L).
The solid was dried on a rotary evaporator at 50 °C to yield an
off-white solid. Yield: 2.26 kg (77%). A second batch was
produced. Yield: 2.22 kg (82%). Purity of both batches (LC-MS
method 1): 100% a/a, tR 1.89 min, [M + 1]+ = 512; mp of both
ASSOCIATED CONTENT
■
1
S
* Supporting Information
batches 142 °C (DSC); H NMR (CDCl3): δ 7.43 (s, 2 H),
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.oprd.6b00210.
7.34 (s, 1 H), 7.12 (s, 1 H), 5.16 (s, 2 H), 4.52 (quint, J = 5.8
Hz, 1 H), 4.21 (dd, J1 = 8.3 Hz, J2 = 6.9 Hz, 1 H), 3.98 (s, 3 H),
3.96 (m, 1 H), 3.83 (m, 2 H), 3.19 (m, 1 H), 2.70 (m, 2 H),
2.33 (s, 3 H), 2.06 (m, 2 H), 1.85 (m, 4 H), 1.71 (m, 2 H), 1.46
(d, J = 21.3 Hz, 6 H), 1.25 (t, J = 7.6 Hz, 3 H); 13C NMR
(CDCl3): δ 165.2, 164.2, 163.0, 158.0, 157.0, 139.8, 137.7,
131.7, 127.6, 126.6, 125.8, 113.4, 109.7, 107.4, 74.5, 73.3, 66.6,
53.6, 47.5, 33.4, 26.8, 25.9, 25.4, 22.9, 16.4, 14.9.
Analytical methods and characterization data of 1 and
intermediates (PDF)
AUTHOR INFORMATION
■
Corresponding Author
(R)-5-(2-Cyclopentyl-6-methoxypyridin-4-yl)-3-(4-((2,2-di-
methyl-1,3-dioxolan-4-yl)methoxy)-3-ethyl-5-methylphenyl)-
1,2,4-oxadiazole (25). A mixture of 24 (2.15 kg, 1.0 equiv) in
toluene (10 L) was heated to reflux for 4 h. Water was collected
in a Dean−Stark apparatus. The solution was concentrated to
dryness at 70 °C under reduced pressure to yield a yellow oil.
Yield: 2116 g (102%). A second batch was produced. Yield:
2280 g (103%). 1H NMR (CDCl3): δ 7.87 (d, J = 6.3 Hz, 2 H),
7.50 (s, 1 H), 7.30 (s, 1 H), 4.55 (quint, J = 5.8 Hz, 1 H), 4.23
(dd, J1 = 8.4 Hz, J2 = 6.5 Hz, 1 H), 4.01 (m, 4 H), 3.90 (m, 2
H), 3.24 (m, 1 H), 2.77 (m, 2 H), 2.40 (s, 3 H), 2.09 (m, 2 H),
1.88 (m, 4 H), 1.73 (m, 2 H), 1.50 (s, 3 H), 1.48 (d, J = 22.0
Hz, 6 H), 1.32 (t, J = 7.5 Hz, 3 H); 13C NMR (CDCl3): δ
174.3, 169.0, 165.8, 164.4, 157.7, 137.8, 133.7, 131.8, 128.3,
*E-mail: stefan.abele@actelion.com. Telephone: +41 61 565 67
59.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Patric Dorrwachter and Marco Tschanz for their
̈
̈
skillful experimental work and Dr. Thomas Weller for the most
fruitful and seamless collaboration with Medicinal Chemistry.
́
We are grateful to Aurelien Henriou, Elvire Fournier, and
Laurent Denu for analytical support. Dr. Martin Kesselgruber is
thanked for professionally outsourcing these protocols to DSM
(now Patheon) that successfully produced the GMP batch.
I
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
(22) De Ferra, L.; Anibaldi, M.; Ammirati, E. WO2005/049549A1,
2005.
(23) Kocienski, P. J. Protecting Groups; Georg Thieme Verlag:
REFERENCES
■
(1) (a) Bolli, M. H.; Lescop, C.; Mathys, B.; Morrison, K.; Mueller,
C.; Nayler, O.; Steiner, B. WO/2011/007324A1, 2011. (b) Bolli, M.
H.; Lescop, C.; Birker, M.; de Kanter, R.; Hess, P.; Kohl, C.; Nayler,
O.; Rey, M.; Sieber, P.; Velker, J.; Weller, T.; Steiner, B. Eur. J. Med.
Chem. 2016, 115, 326−341.
(2) Schmidt, G.; Reber, S.; Bolli, M. H.; Abele, S. Org. Process Res.
Dev. 2012, 16, 595−604.
(3) Aldrich catalogue: 2-chloro-6-methoxyisonicotinic acid [CAS No.
15855-06-8]; 1 g of CHF163.
Stuttgart, 1994.
(24) Aniline 26 is the starting material for the bulk manufacturing of
the herbicide (S)-metolachlor.
(4) Aldrich catalogue: 2,6-dichloropyridine-4-carboxylic acid [CAS
No. 5398-44-7]; 1 g of CHF66.4.
(5) Elhaïk, J.; Pask, C. M.; Kilner, C. A.; Halcrow, M. A. Tetrahedron
2007, 63, 291−298.
(6) Markley, L. D.; Soper, J. M. US 4474602, 1984.
(7) (a) Adamczyk, M.; Akireddy, S. R.; Reddy, R. E. Org. Lett. 2000,
2, 3421−3423. (b) Bolli, M.; Lehmann, D.; Mathys, B.; Mueller, C.;
Nayler, O.; Steiner, B.; Velker, J. WO2008/029371A1.
(8) http://www.riekemetals.com.
(9) (a) Baron, H.; Remfry, F. G. P; Thorpe, J. F. J. Chem. Soc., Trans.
1904, 85, 1726−1761. (b) Guareschi, I. Gazz. Chim. Ital. 1919, 49,
124−133. Guareschi, I. Mem. Reale Accad. Sci. Torino II 1896, 46, 1−
30.
(10) Aldrich catalogue: 250 mg of CHF414.
(11) Selection of methods for the synthesis of 1-cyclopentyletha-
none: (a) Cyclopentyl Grignard, Ac2O, −78 °C (43%): Overberger,
C. G.; Lebovits, A. J. Am. Chem. Soc. 1954, 76, 2722−2725. (b)
Chromium oxide and cyclopentylmethyl alcohol (66%): Field, G. F.;
Vermeulen, J. R.; Zally, W. J. US5001140A1, 1991. (c) Cyclo-
pentanecarboxylic acid and methyl lithium, −78 °C (81%): Hanack,
M.; Fuchs, K. A.; Collins, C. J. J. Am. Chem. Soc. 1983, 105, 4008−
4017. (d) From cyclopentanone in five steps (47%): Zhang, P.; Li, L.-
C. Synth. Commun. 1986, 16, 957−965. (e) From sodium acetoacetate
in two steps (34%): Goldsworthy, L. J. J. Chem. Soc. 1934, 377−378.
(12) Schmidt, G. WO2013/175397A1, 2013.
(13) Muthusamy, S.; Gnanaprakasam, B. Tetrahedron Lett. 2005, 46,
635−638.
(14) (a) Bardhan, J. C. J. Chem. Soc. 1929, 2223−2232.
(b) Libermann, D.; Rist, N.; Grumbach, F.; Cals, S.; Moyeux, M.;
Rouaix, A. Bull. Soc. Chim. Fr. 1958, 687−694. (c) Blackwood, R. K.;
Hess, G. B.; Larrabee, C. E.; Pilgrim, F. J. J. Am. Chem. Soc. 1958, 80,
6244−6249. (d) Ichiba, A.; Emoto, S. Sci. Pap. Inst. Phys. Chem. Res.
(Tokyo) 1941, 38, 347. (e) Pieper, P. A.; Yang, D.; Zhou, H.; Liu, H. J.
Am. Chem. Soc. 1997, 119, 1809−1817. (f) Tracy, A. H.; Elderfield, R.
C. J. Org. Chem. 1941, 06, 70−76.
(15) Only one tautomeric form is displayed for the pyridones. Most
probably, both the hydroxypyridine and the lactam tautomer are
present in solution.
(16) Even with 10 equiv of POCl3 and refluxing for 4 h the
conversion only reached 95%. Addition of DMF, dimethylaniline, or
bases like Et N or Hunig’s base did not accelerate the reaction. The
̈
3
workup proved difficult: POCl3 was partially distilled off; the residue
was dissolved in DCM and quenched onto methanol (very violent and
exothermic). The mixture was evaporated to dryness, and the residue
was taken up in DCM. Insoluble material was removed by filtration
over Celite. This workup gave the product in a yield of 109% and a
purity of 98% a/a (2% 17).
(17) PhP(O)Cl2 was recommended for the chlorination of α-
pyridones: Robison, M. M. J. Am. Chem. Soc. 1958, 80, 5481−5483.
(18) Other buffers like dibasic potassium phosphate or disodium
phosphate were also suitable.
(19) The Celite filtration was not effective for the quenched reaction
mixture. This was not of concern since the phase separation was good
and the slime remained in the organic layer.
(20) This compound could be formed by chlorine displacement of
18, due to the attack of the present potassium isonicotinate, followed
by hydrolysis of the ester.
(21) Wissner, A.; Sum, P. E.; Schaub, R. E.; Kohler, C. A.; Goldstein,
B. M. J. Med. Chem. 1984, 27, 1174−1181.
J
DOI: 10.1021/acs.oprd.6b00210
Org. Process Res. Dev. XXXX, XXX, XXX−XXX