Development of a Novel Route for Incorporation of Carbon-14 into the Pyridine Ring of Rinskor Active

Article abstract of DOI:10.1021/acs.oprd.9b00302

Rinskor active is a new 6-arylpicolinate herbicide that is used for postemergent control of grass, broadleaf, and sedge weeds in rice and other crops. In order to register a new herbicide, carbon-14-labeled radiotracers of the active ingredient are requir

Full text of DOI:10.1021/acs.oprd.9b00302

Article
pubs.acs.org/OPRD
Cite This: Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Development of a Novel Route for Incorporation of Carbon-14 into
the Pyridine Ring of Rinskor Active
Pete Johnson*
Process Chemistry, Corteva Agriscience, 9330 Zionsville Road, Indianapolis, Indiana 46268, United States
S
* Supporting Information
ABSTRACT: Rinskor active is a new 6-arylpicolinate herbicide that is used for postemergent control of grass, broadleaf, and
sedge weeds in rice and other crops. In order to register a new herbicide, carbon-14-labeled radiotracers of the active ingredient
are required to track the parent along with the metabolites and degradants formed in the environment. This paper discusses the
novel route that was used to incorporate carbon-14 into the pyridine ring of Rinskor to support registration studies.
KEYWORDS: Rinskor active, auxin, herbicide, carbon-14, radiotracer
ingredient/hectare). It has a favorable environmental and
toxicology profile and shows rapid degradation in soil and
plants.
INTRODUCTION
■
The control of weeds is critical for increasing the quality and
yield of crops in the agricultural industry. Competition for
nutrients by weeds can severely inhibit the productivity of food
production. The discovery of 2,4-D, the first auxinic herbicide,
in the 1940s provided a novel solution for the control of key
weeds in many important crops (i.e., wheat, maize, rice). The
history of auxin herbicides at Dow Chemical and Corteva
Agriscience is long and interesting, and this is particularly true
of the picolinic acid-derived auxin herbicides. Picloram, the
first picolinic acid-derived auxin herbicide introduced by Dow,
was discovered in the late 1950s1 (Figure 1). Research in this
area of chemistry soon led to the discovery of clopyralid2 in the
1960s and eventually led to the discovery of aminopyralid3
almost 40 years later. The discovery of aminopyralid catalyzed
intense structure−activity relationship (SAR) studies around
the picolinic acid herbicides that eventually led to the
discovery of the 6-arylpicolinate Arylex active, which exhibits
very potent activity against a wide variety of targeted weed
species.4−6 Arylex was launched by Dow AgroSciences in 2014
for postemergent control of broadleaf weeds in cereals and
other crops.
Auxin herbicides cause physiological changes normally
associated with the natural plant hormone indole-3-acetic
acid.7 The phenotypic effects caused by auxin herbicides
include stem and leaf twisting due to uncontrolled growth,
abnormal root growth, and eventual plant death several weeks
after application. Dicot plants are usually more susceptible to
auxinic herbicides than monocot plants.8 It is important to
note that the complex mode of action associated with auxin
herbicides has resulted in low levels of resistance in
comparison with other herbicide modes of action.
To support registration of a new crop protection product,
over 100 core regulatory studies are conducted to evaluate its
potential effects on the environment and humans.9 These
studies include plant and mammalian metabolism, soil
degradation, hydrolysis, and UV photolysis. Identification of
a persistent metabolite or degradant from these studies might
trigger additional studies to understand its fate and evaluate
any safety concerns. To track and quantify the fate of an active
ingredient and the resulting metabolites/degradants, regulatory
agencies around the world require that studies be performed
with carbon-14-labeled analogues of the parent. The carbon-14
label needs to be incorporated at a metabolically stable
position (usually an aromatic ring) in the molecule so that the
radiotracer is not lost during the study. Placement of the label
is done in a defined position, which further aids in elucidation
of metabolites/degradants. If multiple aromatic rings are
present, it is preferred that each ring be labeled independently.
Key attributes of the carbon-14 radioisotope include the
following: (1) it has a long half-life (5730 years); (2) data
obtained from the studies are reliable, as all of the radioactivity
can be tracked; (3) it has an extremely low detection limit,
which allows for the identification of metabolites and
degradants at levels that could not be detected with a
nonlabeled parent; and (4) it is a low-energy β emitter that
does not pose an external dose hazard to researchers handling
the labeled material.
To support the regulatory studies of Rinskor, all three
aromatic rings had to be labeled independently. Herein we
describe the synthesis of an analogue of Rinskor with a carbon-
14-labeled pyridine ring.
Continued SAR study efforts around the 6-arylpicolinates
eventually led to the discovery of Rinskor active (1) (Figure
2).5,6 Rinskor is related to Arylex, with differences being a
fluorine atom at the 5-position of the pyridine ring and a
benzyl ester as opposed to the methyl ester. Rinskor is used for
the postemergent control of grass, broadleaf, and sedge weeds
in rice and other crops, which is a different market than that of
Arylex. Rinskor has a very low use rate (5−50 g of active
Special Issue: Corteva Agriscience
Received: July 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.oprd.9b00302
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Figure 1. Picolinic acid-related auxin herbicides.
methyl ester was a better coupling partner than the benzyl
ester in the Suzuki−Miyaura cross-coupling reaction. This
route to 8 was not only long (16 linear steps) but also low-
yielding overall (1%, 0.6 mCi, 4 mg). In addition, this route
introduced the radioisotope in the first step of a 16-step
sequence, which is undesirable from a handling and cost
perspective. To support additional registration studies of 1,
>100 mCi of a pyridine-labeled standard was required, and
therefore, this route was not amenable to support all of the
registration studies for 1. Ultimately, an alternative route had
to be identified quickly to prepare large quantities of 1.
Novel Cyclization Route to Rinskor (1). During our
investigations to identify new routes to access 1, a paper was
published on the preparation of polysubstituted 5-fluoropyr-
idines utilizing cascade cyclization of primary amines and
fluoroalkyl alkynylimines (Scheme 2).16
We envisioned that 1 could be accessed using this method
and proposed the retrosynthetic route shown in Scheme 3. The
key to the success of this route relied on identifying a suitable
amine protecting group for 9 and a propiolic acid derivative 11
to give alkynylimine 12. Cyclization of alkyne 12 with
benzylamine 13 would give rise to the desired framework for 1.
Initially, we examined the key cyclization step to obtain 16
using 4-chlorobenzylamine (15) for route optimization and
screened various protecting groups for 12. After an extensive
optimization process (Scheme 4), we determined that 12
protected with N-trityl (R1) and diethyl acetal (R2) can
undergo the desired cyclization with 15 to give 16 using
cesium carbonate as the base and dimethyl sulfoxide (DMSO)
as the solvent at a reaction temperature of 80 °C.18 It is worth
mentioning that these conditions also resulted in expanding
the SAR by allowing novel analogues to be prepared by the
Discovery Chemistry group.17,18
Figure 2. The 6-arylpicolinate herbicide Rinskor active (1).
RESULTS AND DISCUSSION
■
First Synthetic Route to Rinskor-Py-2,6-14C (8). For
early-stage environmental fate and metabolism studies, only
small quantities (<10 mCi) of a radiotracer are required. For
incorporation of carbon-14 into the pyridine ring of 1, we
envisioned that we could use the route that we developed for
the preparation of the radiotracer picloram-2,6-14C (2),11
which can be accessed in 10 steps starting from K14CN (Figure
3). Electrolysis of 2 would afford aminopyralid-2,6-14C (3).
Incorporation of a fluorine at the 5-position of the pyridine
ring via electrophilic fluorination would give access to 4.
Finally, Suzuki−Miyaura cross-coupling would provide 8 in a
total of 16 linear steps.
The route used to access 2 was well-established,10,11 along
with the electrolysis step to achieve 3.12−14 For the challenging
key electrophilic fluorination step, a variety of electrophilic
fluorination reagents were explored, and Selectfluor (F-TEDA)
provided the desired product, albeit in low yield (Scheme 1).15
The low yield was attributed to incomplete reaction and
formation of 2, presumably resulting from pyridine ring
opening and chlorination of the starting material 3.15 The
fluorinated picolinic acid was then converted to methyl ester 5
for the subsequent Suzuki−Miyaura cross-coupling with
boronate ester 6. Methyl ester 7 was then hydrolyzed, and
the resulting acid was converted to the benzyl ester, Rinskor-
Py-2,6-14C (8). Of note, early results had shown that the
With optimal cyclization conditions in hand, we focused on
the synthesis of 117 to confirm that this route indeed would be
Figure 3. Retrosynthesis of Rinskor-Py-2,6-14C (8).
B
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Scheme 1. First Synthetic Route to Rinskor-Py-2,6-14C (8)
Scheme 2. Synthesis of Polysubstituted 5-Fluoropyridines via Cascade Cyclization
Scheme 3. Retrosynthetic Route to 1 Using a New Cyclization Approach
Scheme 4. Optimization of Cascade Cyclization for Rinskor
viable for incorporation of carbon-14 into the pyridine ring.
Alkyne intermediate 18 was prepared by a two-step process
starting from commercially available tritylamine (Scheme 5).
Treatment of tritylamine with trifluoroacetic acid, triphenyl-
phosphine, and triethylamine in carbon tetrachloride provided
imidoyl chloride 17 in good yield.19 Sonogashira cross-
coupling of imidoyl chloride 17 with propargyl aldehyde
diethyl acetal gave the key alkyne intermediate 18 in 65% yield.
The other key intermediate, benzylamine 13, was prepared by
a two-step process starting from benzaldehyde 19, which was
prepared by a published route.20 The benzaldehyde was
converted to O-methyl oxime derivative 20, which was then
C
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Scheme 5. Novel Cyclization Route to 1
Scheme 6. Labeling Options (a−d) for Incorporation of Carbon-14 into the Pyridine Ring
reduced to the desired benzylamine 13 with zinc in acetic
acid.21
the pyridine ring (Scheme 6). This was important not only for
yield optimization but also in finding vendors that could
prepare the desired intermediate with an acceptable cost.
Placing the label in the carbonyl of trifluoracetic acid would
incorporate the label at the 4-position of the pyridine ring (a),
labeling the alkyne would incorporate the label at the 2- or 3-
position (b or c), and labeling the benzylic carbon would
incorporate the label at the 6-position (d). We ruled out using
labeled benzylamine, as this reagent was used in excess to
achieve high cyclization yields. In the end, we found that
placing the label in the carbonyl of trifluoroacetic acid (a)
would offer the most accessible and cost-effective option.
Novel Cyclization Route to Rinskor-Py-4-14C (30). We
utilized a vendor to prepare 100 mCi (ca. 1.5 g) of the key
carbon-14-labeled alkyne intermediate 25 starting from
carbon-14-labeled trifluoroacetic acid using the same chemistry
that we developed to access 18 (Scheme 5). Treatment of
carbon-14-labeled alkyne 25 with 4-chloro-2-fluoro-3-methox-
With the key intermediates 13 and 18 in-hand, the final
steps to 1 were initiated (Scheme 5). Cyclization of
benzylamine 13 with alkyne 18 gave cyclized product 21 in
73% yield. The trityl and acetal protecting groups were
removed in one step to afford 4-aminopyridine-2-carboxalde-
hyde 22 in excellent yield. Selective chlorination at the 3-
position of the pyridine ring was achieved in good yield with
1,3-dichloro-5,5-dimethylhydantoin. Pinnick oxidation of 23
gave carboxylic acid 24 in 83% yield. Finally, carboxylic acid 24
was converted to the benzyl ester to give 1 in 91% yield. By
this route, 1 was accessed in seven linear steps starting from
trifluoroacetic acid in 40% overall yield. This novel route
developed was not only high-yielding but also robust and
therefore was used for the preparation of carbon-14-labeled 1.
In addition to the high overall yield, this new route also
offered several options for incorporation of a carbon-14 label in
D
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Scheme 7. Cyclization Route to Rinskor-Py-4-14C
ybenzylamine (13) in the presence of cesium carbonate
provided protected aminopyridine 26 in 85% yield (Scheme
7). Removal of the trityl and acetal protecting groups was
accomplished in a single step in 96% yield using sulfuric acid.
Selective chlorination of the pyridine ring was achieved with
1,3-dichloro-5,5-dimethylhydantoin to give 28 in 75% yield.
Oxidation of the aldehyde with sodium chlorite gave carboxylic
acid 29 in 83% yield. Finally, 29 was converted to the benzyl
ester to give the desired radiotracer, Rinskor-Py-4-14C (30), in
80% yield. This new route proved to be robust, which allowed
us to prepare several batches of 30 from 25 in reproducible
yields and deliver sufficient quantities of this key radiotracer to
support the registration of Rinskor.
a Fisher Scientific Nicolet 6700 FT-IR spectrometer. Melting
points were obtained using an OptiMelt automated melting
point system (Sanford Research Systems) and are uncorrected.
Quantitation of radioactivity was performed using a
PerkinElmer Tri-Carb 2910 TR liquid scintillation analyzer
with PerkinElmer Ultima-Gold liquid scintillation cocktail.
High-performance liquid chromatography (HPLC) was
performed using an Agilent 1260 HPLC system. Chemical
purity was determined using an Agilent 1260 Infinity DAD UV
detector and radiochemical purity using an IN/US β-RAM 2
radioflow detector.
2,2,2-Trifluoro-N-tritylethanimidoyl Chloride (17). A 1
L three-neck round-bottom flask equipped with a mechanical
stirrer, addition funnel, and J-KEM temperature probe was
charged with tritylamine (24.90 g, 96 mmol) and carbon
tetrachloride (150 mL). The solution was cooled in an ice−
water bath (<5 °C) and treated in portions with trifluoroacetic
acid (6.13 mL, 80 mmol). The addition was done at such a rate
as to keep the temperature at or below 10 °C. Once the
mixture cooled below 5 °C, it was treated dropwise with
triethylamine (13.4 mL, 96 mmol; no exotherm was observed).
Once the addition of triethylamine was complete, the ice−
water bath was removed, and the addition funnel was replaced
with a reflux condenser. The reaction mixture was heated to 65
°C via a heating mantle. Triphenylphosphine (62.9 g, 240
mmol) was then added portionwise (5−10 g at a time) (note:
with each addition of triphenylphosphine, the temperature
dropped 2−3 °C and then rose to ca. 70 °C). The reaction
mixture was allowed to cool to 65−66 °C before addition of
triphenylphosphine. After all of the triphenylphosphine was
added, the reaction mixture was heated at 76 °C for 2 h,
allowed to cool to room temperature, and treated with hexanes
(400 mL). After 30 min of rapid stirring, the mixture was
filtered through a Buchner funnel. The collected solid was
resuspended in hexanes (400 mL), and the suspension was
stirred for several minutes and then filtered. The filtrates were
combined and concentrated in vacuo to give a solid, which was
slurried with hexanes (3 × 300 mL) and filtered. The solids
were removed by vacuum filtration through a fritted glass
funnel to give 14.32 g of a light-yellow liquid. The solvent was
removed in vacuo to give 30.37 g of a light-yellow solid. The
crude material was crystallized from acetonitrile (250 mL)
(note: crystals formed upon cooling). After the crystallization
CONCLUSIONS
■
A novel cascade cyclization of fluoroalkyl alkynylimines with
primary amines was optimized for the synthesis of a carbon-14-
labeled radiotracer of 1 with the label at the 4-position of the
pyridine ring. This new route provided the radiotracer of 1 in
five chemical steps in-house in an overall yield of 40% from
advanced carbon-14-labeled intermediate 25, which was
purchased from an external vendor. This new route was a
significant improvement over the original route (16 steps, 1%
yield overall from K14CN) and provided sufficient quantities of
the pyridine-labeled radiotracer to support environmental fate,
metabolism, and toxicology studies required for the registration
of Rinskor active.
EXPERIMENTAL PROCEDURES
■
General. N-[(5,5-Diethoxy-1,1,1-trifluoro(2-14C)pent-3-yn-
2-ylidene]-1,1,1-triphenylmethanamine was purchased from
Pharmaron (Cardiff, UK). NMR spectra were obtained on
either a Bruker Avance III HD 500 MHz, Bruker 400 MHz, or
Varian Gemini 300 MHz spectrometer. NMR chemical shifts
(δ) are reported in parts per million downfield from
tetramethylsilane as an internal reference. Mass spectra were
obtained using an LC−MS system (Waters Micromass ZQ).
High-resolution mass spectrometry (HRMS) was performed
on an Agilent 6210 TOF LC−MS system. GC−MS spectra
were obtained using a Hewlett Packard HP 6890 GC system
with an Agilent 5973 mass-selective detector. IR spectra were
obtained on neat samples using attenuated total reflectance on
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mixture was allowed to stand in a freezer for a couple of hours,
the crystals were removed by vacuum filtration and then
washed with cold acetonitrile. The material was vacuum-oven-
dried (40−50 °C) to give 21.97 g (73% yield) of the desired
product as fine white needles. Mp 144−145 °C; 1H NMR (400
MHz, CDCl3) δ 7.34−7.19 (m, 15H); 19F NMR (376 MHz,
CDCl3) δ −71.24; 13C NMR (101 MHz, CDCl3) δ 143.18 (s),
130.00 (d, J = 43.1 Hz), 129.07 (s), 128.11 (s), 127.46 (s),
116.80 (q, J = 278.0 Hz), 77.83 (s); GC−MS (EI) m/z 373
(M+), 338, 296, 243, 219, 193, 165, 143, 127, 119, 77; IR 3070,
1712, 1488, 1444, 1270, 1162, 1151, 946, 770 cm−1.
mixture was allowed to stir at room temperature under an
atmosphere of N2. After the reaction mixture was stirred at
room temperature overnight (17 h), an aliquot of the reaction
mixture was partitioned between EtOAc and 1 M HCl and
analyzed by TLC (80/20 hexanes/EtOAc). TLC analysis
indicated that all of the starting material had been consumed.
The reaction mixture was concentrated in vacuo, and the crude
mixture was taken up in EtOAc (150 mL) and washed with 2
M HCl (2 × 50 mL) and saturated NaCl (1 × 50 mL). The
organic phase was dried (Na2SO4), filtered, and concentrated
in vacuo to give 11.01 g of an off-white solid. The crude
material was recrystallized from hexanes (ca. 20 mL). The
crystals were removed by vacuum filtration and washed with
cold pentane. The solid was air-dried for several hours to give
8.20 g of the desired product as small white needles. A second
crop yielded an additional 1.06 g of the desired product for a
N-(5,5-Diethoxy-1,1,1-trifluoropent-3-yn-2-ylidene)-
1,1,1-triphenylmethanamine (18). A 500 mL three-neck
round-bottom flask equipped with a mechanical stirrer, reflux
condenser, and J-KEM temperature probe was charged with
propargylaldehyde diethyl acetal (5.13 g, 40 mmol), anhydrous
acetonitrile (125 mL), and imidoyl chloride 17 (14.95 g, 40
mmol). Potassium iodide (6.64 g, 40 mmol), potassium
phosphate (11.04 g, 52 mmol), and copper(I) iodide (2.29 g,
12 mmol) were combined, ground to a fine powder with a
mortar and pestle, and then added to the reaction mixture.
Additional acetonitrile (25 mL) was added, and the resultant
mixture was warmed to 60 °C with a heating mantle under an
atmosphere of N2. After the reaction mixture was stirred
overnight at 60 °C, an aliquot of the mixture was partitioned
between EtOAc and H2O and analyzed by thin-layer
chromatography (TLC) (95/5 hexanes/EtOAc) and GC−
MS. Analysis showed that the imidoyl chloride starting material
was still present along with the desired product and a minor
amount of a side product, the alkyne dimer. The reaction
mixture was treated with an additional 20 mol % propargyl
aldehyde diethyl acetal (1 g), KI (1.33 g), CuI (0.56 g), and
K3PO4 (2.20 g). The temperature of the reaction mixture was
raised to 70 °C. After an additional 3 h at 70 °C, TLC and
GC−MS still showed remaining imidoyl chloride starting
material. After the reaction mixture was stirred for 24 h total, it
was allowed to cool to room temperature, diluted with CH2Cl2
(400 mL), filtered, and washed sequentially with H2O (1 ×
150 mL) and saturated NaCl (1 × 150 mL). The organic layer
was dried (Na2SO4), filtered, and concentrated in vacuo to give
22.02 g of a yellow oil, which solidified upon standing in a
refrigerator. The crude material was dissolved in warm
hexanes, loaded onto a silica gel column, and chromatographed
using the following setup: Teledyne-ISCO CombiFlash
Companion, 330 g RediSep silica gel column, 0−60%
hexanes/CH2Cl2. Fractions containing the desired product
were combined and concentrated in vacuo to give 12.11 g
(65% yield) of the desired product as a white solid. Mp 84−86
°C; 1H NMR (400 MHz, CDCl3) δ 7.40−7.14 (m, 15H), 4.88
(d, J = 1.1 Hz, 1H), 3.77−3.54 (m, 1H), 3.46−3.54 (m, 4H),
1.13 (t, J = 7.0 Hz, 6H); 19F NMR (376 MHz, CDCl3) δ
−71.30; 13C NMR (101 MHz, CDCl3) δ 144.43 (s), 139.83
(q, J = 38.0 Hz), 129.53 (s), 127.90 (s), 127.24 (s), 118.70 (q,
J = 278.9 Hz), 98.53 (s), 90.82 (s), 78.29 (s), 74.04 (s), 61.20
(s), 14.94 (s); GC−MS (EI) m/z 465 (M+), 436, 394, 366,
346, 243, 165, 103, 75; IR 3059, 2978, 2886, 1640, 1490, 1446,
1312, 1136, 1052, 697 cm−1.
1
total yield of 9.26 g (80% yield). Mp 56−59 °C; H NMR
(400 MHz, CDCl3) δ 8.23 (s, 1H), 7.47 (dd, J = 8.6, 6.9 Hz,
1H), 7.18−7.09 (m, 1H), 3.99 (s, 3H), 3.96 (d, J = 1.2 Hz,
3H); 19F NMR (376 MHz, CDCl3) δ −134.45; 13C NMR
(126 MHz, CDCl3) δ 154.46 (d, J = 255.2 Hz), 144.63 (d, J =
13.1 Hz), 141.33 (d, J = 4.9 Hz), 129.70 (d, J = 3.6 Hz),
125.41 (d, J = 3.7 Hz), 120.82 (d, J = 3.3 Hz), 120.41 (d, J =
9.6 Hz), 62.37, 61.63 (d, J = 4.5 Hz); GC−MS (EI) m/z 217
(M+), 186, 175, 171, 159, 144, 129, 95, 81.
1-(4-Chloro-2-fluoro-3-methoxyphenyl)methanamine
(13). A 250 mL three-neck round-bottom flask equipped with
a mechanical stirrer, reflux condenser, and J-KEM temperature
controller was charged with methoxylamine 20 (5.88 g, 27.0
mmol), glacial acetic acid (100 mL), and zinc metal (8.63 g,
65.38 mmol, 5 equiv) (note: prior to use, the zinc metal was
stirred with 1 M HCl, washed with H2O followed by Et2O,
vacuum-oven-dried, and then ground with a mortar and
pestle). The reaction mixture was heated at 100 °C via a
heating mantle. After 60 min at 100 °C, an aliquot was filtered,
diluted with EtOAc, and analyzed by TLC (80/20 hexanes/
EtOAc) and GC−MS. Analysis showed that all of the starting
material had been consumed. The reaction mixture was
allowed to cool to room temperature, filtered over a plug of
Celite, and washed with acetic acid. The filtrate was
concentrated in vacuo to give a light-tan oil. The oil was
dissolved in H2O (100 mL) and washed with Et2O (1 × 30 mL
and 1 × 50 mL). The combined Et2O layers were extracted
with 1 M HCl (1 × 25 mL). The aqueous phases were
combined, and the pH was adjusted to ca. 10 using 50%
NaOH. The mixture was extracted with EtOAc (3 × 100 mL),
and the combined EtOAc extracts were washed with H2O (2 ×
100 mL) and saturated NaCl (1 × 100 mL). The organic layer
was dried (Na2SO4), filtered, and concentrated in vacuo to give
4.31 g of a light-yellow oil, which was purified via bulb-to-bulb
distillation to give 4.19 g (82% yield) of the desired product as
a colorless liquid. Bp 165−180 °C (1 mmHg); 1H NMR (400
MHz, CDCl3) δ 7.12 (dd, J = 8.3, 1.8 Hz, 1H), 7.00 (dd, J =
8.3, 7.3 Hz, 1H), 3.96 (d, J = 1.2 Hz, 3H), 3.88 (d, J = 1.2 Hz,
2H), 1.39 (s, 2H); 19F NMR (376 MHz, CDCl3) δ −134.64;
13C NMR (101 MHz, CDCl3) δ 154.39 (d, J = 248.6 Hz),
144.23 (d, J = 13.8 Hz), 130.75 (d, J = 13.9 Hz), 126.79 (d, J =
3.5 Hz), 124.98 (d, J = 3.9 Hz), 123.17 (d, J = 5.4 Hz), 61.52
(d, J = 4.7 Hz), 40.22 (d, J = 4.3 Hz); GC−MS (EI) m/z 189
(M+), 188, 173, 154 (base), 145, 139, 126, 111, 95, 81.
2-(4-Chloro-2-fluoro-3-methoxyphenyl)-6-(diethoxy-
methyl)-3-fluoro-N-tritylpyridin-4-amine (21). A 100 mL
round-bottom flask equipped with a magnetic stir bar was
4-Chloro-2-fluoro-3-methoxybenzaldehyde O-Methyl
Oxime (20). A 250 mL round-bottom flask equipped with a
magnetic stir bar was charged with aldehyde 19 (10.00 g, 53.0
mml), ethanol (50 mL), and pyridine (50 mL). To this
solution was added methoxylamine hydrochloride (4.87 g, 58.3
mmol, 1.1 equiv, 25−30 wt % solution in water). The resultant
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charged with alkyne 18 (2.33 g, 5.0 mmol) and anhydrous
DMSO (25 mL) under an atmosphere of N2. Once all of the
alkyne dissolved, benzylamine 13 (2.84 g, 15 mmol, 3 equiv)
was added to the solution. The resultant light-yellow solution
was stirred for ca. 2 min at room temperature and then treated
with cesium carbonate (4.07 g, 12.5 mmol, 2.5 equiv) in one
portion. The reaction flask was placed in a preheated bath at
80 °C. After 2 h at 80 °C, an aliquot of the reaction mixture
was partitioned between EtOAc and H2O and analyzed by
TLC (80/20 hexanes/EtOAc). Analysis indicated that all of
the alkyne starting material had been consumed. The reaction
mixture was allowed to cool to room temperature, diluted with
EtOAc (150 mL), and washed with H2O (3 × 50 mL) and
saturated NaCl (1 × 50 mL). The organic layer was dried
(Na2SO4), filtered, and concentrated in vacuo to give 4.63 g of
an orange solid, which was dissolved in a minimal amount of
CH2Cl2, loaded onto a silica gel column, and chromatographed
using the following setup: Teledyne-ISCO CombiFlash
Companion, 80 g RediSep silica gel column, 0−60%
hexanes/EtOAc. Fractions containing the desired product
were combined and concentrated in vacuo to give 2.243 g
(73% yield) of the desired product as a peach-colored solid.
13.9 Hz), 128.34 (d, J = 3.5 Hz), 125.90 (d, J = 3.9 Hz),
125.50 (d, J = 3.6 Hz), 123.59 (dd, J = 14.2, 3.4 Hz), 108.35
(d, J = 5.9 Hz), 61.59 (d, J = 4.3 Hz), 40.10; HRMS-ESI (m/z)
[M + H]+ calcd for C13H9ClF2N2O2 298.0321, found
298.0322.
4-Amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxy-
phenyl)-5-fluoropyridine-2-carbaldehyde (23). A 100 mL
round-bottom flask equipped with a magnetic stir bar and
reflux condenser was charged with aminopyridine 22 (0.851 g,
2.85 mmol), acetonitrile (30 mL), and 1,3-dichloro-5,5-
dimethylhydantoin (0.309 g, 1.567 mmol, 0.55 equiv). The
resultant light-yellow mixture was stirred at room temperature
for ca. 5 min and then heated to reflux under an atmosphere of
N2. After refluxing for <30 min, the reaction mixture turned
into a yellow homogeneous solution. After 60 min at reflux, an
aliquot of the reaction mixture was partitioned between EtOAc
and H2O and analyzed by TLC (80/20 hexanes/EtOAc) and
HPLC. On the basis of the analysis, all of the starting material
had been consumed. The reaction mixture was allowed to cool
to room temperature, diluted with EtOAc (150 mL), and
washed with H2O (1 × 50 mL), dilute sodium bisulfite (1.0 g
of NaHSO3/50 mL of H2O, 1 × 50 mL), and saturated NaCl
(1 × 50 mL). The organic layer was dried (Na2SO4), filtered,
and concentrated in vacuo to give 1.00 g of a tan solid. The
solid was dissolved in acetone and treated with 3 g of Celite.
The solvent was removed in vacuo, and the residue was placed
in a solid load cartridge and chromatographed using the
following setup: Teledyne-ISCO CombiFlash Companion, 40
g RediSep silica gel column, 5−100% hexanes/EtOAc.
Fractions containing the desired product were combined and
concentrated in vacuo, and the light-tan solid was stirred with
2/1 H2O/CH3CN (15 mL). After 3 h of stirring, the solid was
vacuum-filtered and washed with 1/1 H2O/CH3CN (10 mL).
The solid was air-dried for 1 h, transferred to a 25 mL round-
bottom flask, and treated with CH3CN (4 × 5 mL), and the
mixture was concentrated in vacuo to give 749 mg (79% yield)
of the desired product as a light-tan solid. Mp 192−196 °C; 1H
NMR (400 MHz, DMSO-d6) δ 9.97 (s, 1H), 7.51 (dd, J = 8.5,
1.2 Hz, 1H), 7.36 (dd, J = 8.5, 7.0 Hz, 1H), 7.15 (s, 2H), 3.94
(s, 3H); 19F NMR (376 MHz, DMSO-d6) δ −129.20 (d, J =
27.9 Hz), −134.34 (d, J = 28.0 Hz); 13C NMR (101 MHz,
DMSO-d6) δ 190.27, 154.38, 151.87, 147.07, 144.47, 144.02−
143.45 (m), 141.98 (d, J = 13.9 Hz), 137.06 (d, J = 13.7 Hz),
128.65 (d, J = 3.5 Hz), 125.89 (d, J = 3.3 Hz), 125.61 (d, J =
3.6 Hz), 122.91 (dd, J = 13.8, 4.1 Hz), 116.14 (d, J = 3.8 Hz),
61.62 (d, J = 4.3 Hz), 38.86; HRMS-ESI (m/z) [M + H]+
calcd for C13H8Cl2F2N2O2 331.9931, found 331.9930.
1
Mp 177−180 °C; H NMR (400 MHz, CDCl3) δ 7.40−7.08
(m, 17H), 6.34 (d, J = 6.5 Hz, 1H), 5.90 (d, J = 4.5 Hz, 1H),
5.09 (s, 1H), 3.98 (d, J = 1.0 Hz, 3H), 3.30 (dq, J = 9.4, 7.1
Hz, 2H), 3.16 (dq, J = 9.4, 7.0 Hz, 2H), 1.03 (t, J = 7.0 Hz,
6H); 19F NMR (376 MHz, CDCl3) δ −128.23 (d, J = 33.8
Hz), −146.89 (d, J = 33.1 Hz); 13C NMR (126 MHz, CDCl3)
δ 154.92, 153.17 (d, J = 5.3 Hz), 152.90, 146.71 (d, J = 250.6
Hz), 144.45 (d, J = 14.1 Hz), 143.96, 141.44 (d, J = 9.7 Hz),
137.60 (d, J = 14.5 Hz), 129.02 (d, J = 3.3 Hz), 128.90, 128.36,
127.41, 125.83, 125.21 (d, J = 3.7 Hz), 108.89, 102.27, 71.36,
65.91, 61.52, 15.09; HRMS-ESI (m/z) [M + H]+ calcd for
C36H33ClF2N2O3 614.2148, found 614.2156.
4-Amino-6-(4-chloro-2-fluoro-3-methoxyphenyl)-5-
fluoropyridine-2-carbaldehyde (22). A mixture of trityl-
protected aminopyridine 21 (2.117 g, 3.44 mmol), acetonitrile
(15 mL), water (15 mL), and 1 N H2SO4 (7.5 mL) was placed
in an oil bath and heated at 80 °C. After 2 h at 80 °C, an
aliquot of the reaction mixture was partitioned between EtOAc
and 10% NaHCO3. On the basis of HPLC and TLC (80/20
hexanes/EtOAc), only a trace amount of the starting material
remained. The reaction mixture was allowed to cool to room
temperature, vacuum-filtered, and washed with 2/2/1
CH3CN/H2O/1 N H2SO4. The filtrate was transferred to a
separatory funnel, diluted with EtOAc (150 mL), and treated
with 10% NaHCO3. A white precipitate formed in the aqueous
layer that did not readily extract into the organic phase. The
layers were separated, and the aqueous layer was extracted
sequentially with EtOAc (3 × 50 mL), CH2Cl2 (1 × 50 mL),
and EtOAc (1 × 50 mL). The combined organic layers were
washed with saturated NaCl (1 × 50 mL), dried (Na2SO4),
filtered, and concentrated in vacuo to give 1.04 g of a light-tan
solid, which was slurried with hexanes (20 mL). After 2 h of
stirring, the solid was vacuum-filtered and washed with
hexanes. The solid was air-dried to give 0.941 g (92% yield)
of the desired product as a light-tan solid. Mp 191−193 °C; 1H
NMR (400 MHz, DMSO-d6) δ 9.79 (s, 1H), 7.49 (dd, J = 8.5,
1.5 Hz, 1H), 7.39−7.33 (m, 2H), 6.84 (s, 2H), 3.94 (d, J = 0.8
Hz, 3H); 19F NMR (376 MHz, DMSO-d6) δ −129.20 (d, J =
27.4 Hz), −139.73 (d, J = 27.3 Hz); 13C NMR (101 MHz,
DMSO-d6) δ 192.77, 154.43, 151.92, 148.95 (d, J = 4.0 Hz),
148.70, 146.15, 143.98 (dd, J = 32.7, 13.4 Hz), 139.49 (d, J =
4-Amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxy-
phenyl)-5-fluoropyridine-2-carboxylic Acid (24). A 25
mL round-bottom flask equipped with a magnetic stir bar and
reflux condenser was charged with aldehyde 23 (400 mg, 1.201
mmol) and tert-butyl alcohol (6 mL). Rapid stirring and
warming failed to dissolve all of the aldehyde. An additional 2
mL of t-BuOH was added, but this still failed to form a
homogeneous solution. The mixture was treated with H2O (2
mL), 2-methyl-2-butene (1 mL), sodium hydrogen phosphate
dihydrate (341 mg, 2.402 mmol, 2 equiv), and sodium chlorite
(326 mg, 3.60 mmol, 3 equiv). The mixture was stirred at
room temperature for 5 min and then placed in an oil bath at
85 °C. After 60 min of stirring at 85 °C, the reaction mixture
turned into a light-yellow homogeneous solution. After the
reaction mixture was stirred at 83 °C overnight (12 h), an
aliquot of the mixture was partitioned between EtOAc and 1 M
G
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HCl and analyzed by HPLC and LC−MS. Analysis showed
that all of the starting material had been consumed. The
reaction mixture was allowed to cool to room temperature,
diluted with EtOAc (15 mL), and washed with 1 M HCl (1 ×
5 mL), H2O (1 × 5 mL), and saturated NaCl (1 × 5 mL). The
organic layer was dried (Na2SO4), filtered, and concentrated in
vacuo to give a cream-colored solid, which was slurried with
Et2O (5 mL). After 3 h of stirring, the solid was vacuum-
filtered, washed with Et2O, and dried under vacuum to give
348 mg (83% yield) of the desired product as a white solid. Mp
reaction flask was placed in an oil bath at 80 °C. After 60 min
of stirring at 80 °C, an aliquot of the reaction mixture was
partitioned between EtOAc and H2O and analyzed by TLC
(80/20 hexanes/EtOAc) and HPLC. Analysis indicated that all
of the alkyne starting material had been consumed. The
reaction mixture was allowed to cool to room temperature,
transferred to a 100 mL round-bottom flask, and diluted with
EtOAc (30 mL) and CH2Cl2 (5 mL). The resultant mixture
was washed with H2O (3 × 10 mL) and saturated NaCl (1 ×
10 mL). The organic layer was dried by passage through a plug
of Na2SO4, washing with CH2Cl2. The filtrate was concen-
trated in vacuo to give a mustard-colored solid, which was
dissolved in warm CH2Cl2 (3 mL), loaded onto a silica gel
column, and chromatographed using the following setup:
Teledyne-ISCO CombiFlash Rf, 40 g RediSep silica gel
column, 0−60% hexanes/EtOAc. Fractions containing the
desired product in >95% radiochemical purity were combined
and concentrated in vacuo. The residue was dried under high
vacuum (40−50 °C) to give 792 mg (85% yield) of the desired
product as a light-tan solid. HPLC analysis showed ca. 95%
radiochemical purity (β-RAM) and >98% chemical purity at
254 nm. HRMS-ESI [M + H]+ calcd for C36H33ClF2N2O3
614.22148, found 614.2078.
4-Amino-6-(4-chloro-2-fluoro-3-methoxyphenyl)-5-
fluoro(4-14C)pyridine-2-carbaldehyde (27). A solution of
26 (1.54 g, 2.52 mmol) in CH3CN (10 mL), H2O (10 mL),
and 1 N H2SO4 (5 mL) was placed in an oil bath at 80 °C.
After 2 h of stirring at 80 °C, an aliquot of the reaction mixture
was partitioned between EtOAc and 10% NaHCO3 and
analyzed by TLC (80/20 hexanes/EtOAc) and HPLC.
Analysis showed that a minor amount of starting material
(10−15%) was present. The reaction mixture was treated with
an additional 0.5 mL of 1 N H2SO4 and heated for 2 h at 80
°C. On the basis of HPLC analysis, only a trace amount of the
starting material remained. The reaction mixture was allowed
to cool to room temperature, filtered, and washed with 2/2/1
CH3CN/H2O/1 N H2SO4. The filtrate was neutralized with
10% NaHCO3. The resultant mixture was extracted with
EtOAc (2 × 20 mL), CHCl3 (2 × 10 mL), EtOAc (2 × 10
mL), and CHCl3 (1 × 5 mL). The combined organic extracts
were dried by passage through a plug of Na2SO4. The filtrate
was concentrated in vacuo to give a light-tan solid, which was
slurried with 85/15 hexanes/EtOAc (25 mL). After 2 h of
stirring, the mother liquor was removed via a filter stick. The
residual solid was washed with hexanes (2 × 3 mL), and the
hexane washes were removed via filter stick. The solid was
slurried with 1/1 CH3OH/CH2Cl2 (ca. 30 mL). An aliquot of
this slurry was diluted with CH3CN and analyzed by HPLC.
HPLC analysis showed >95% chemical (254 nm) and
radiochemical purity. The solvent was removed in vacuo, and
the solid was dried under high vacuum (40−50 °C) to give
0.720 g (96% yield) of the desired product as a light-tan solid.
HRMS-ESI [M + H]+ calcd for C13H9ClF2N2O2 298.0321,
found 298.0314.
1
192−196 °C; H NMR (400 MHz, DMSO-d6) δ 13.66 (s,
1H), 7.47 (dd, J = 8.5, 1.4 Hz, 1H), 7.31 (dd, J = 8.4, 7.1 Hz,
1H), 7.04 (s, 2H), 3.93 (s, 3H); 19F NMR (376 MHz, DMSO-
d6) δ −129.10 (d, J = 28.3 Hz), −138.56 (d, J = 28.4 Hz); 13
C
NMR (101 MHz, DMSO) δ 165.91 (s), 153.06 (d, J = 252.5
Hz), 146.20 (d, J = 4.3 Hz), 144.70 (d, J = 256.1 Hz), 143.81
(d, J = 13.9 Hz), 141.37 (d, J = 14.1 Hz), 136.02 (d, J = 13.2
Hz), 128.43 (d, J = 3.2 Hz), 125.90 (d, J = 2.9 Hz), 125.51 (d,
J = 3.5 Hz), 123.03 (dd, J = 13.8, 3.9 Hz), 112.17 (d, J = 2.7
Hz), 61.60 (d, J = 4.0 Hz); LC−MS (ESI+) 349 (M + H),
(ESI−) 347 (M − H).
Benzyl 4-Amino-3-chloro-6-(4-chloro-2-fluoro-3-me-
thoxyphenyl)-5-fluoropyridine-2-carboxylate (Rinskor,
1). A 50 mL round-bottom flask containing a magnetic stir
bar was charged with carboxylic acid 24 (519 mg, 1.49 mmol)
and anhydrous DMSO (10 mL). To this solution was added
powdered potassium carbonate (311 mg, 2.97 mmol, 2.0 equiv,
325 mesh) followed by benzyl bromide (0.23 mL, 1.93 mmol,
1.3 equiv). The resultant mixture was stirred at room
temperature under an atmosphere of N2. After the reaction
mixture was stirred overnight (17 h), an aliquot of the mixture
was partitioned between 1 M HCl and EtOAc and analyzed by
HPLC and TLC. Analysis showed that only a trace amout of
the starting material remained. The reaction mixture was
diluted with EtOAc (30 mL) and washed with H2O (3 × 10
mL) and saturated NaCl (1 × 10 mL). The organic phase was
dried (Na2SO4), filtered, and concentrated in vacuo to give a
light-yellow solid, which was dissolved in CH2Cl2 (2 mL),
loaded onto a silica gel column, and chromatographed using
the following setup: Teledyne-ISCO CombiFlash Companion,
24 g RediSep silica gel column, 0−100% hexanes/EtOAc.
Fractions containing the desired product were combined and
concentrated in vacuo to give 593 mg (91% yield) of the
desired product as a white solid. 1H NMR (400 MHz, CDCl3)
δ 7.50−7.42 (m, 2H), 7.42−7.31 (m, 3H), 7.26 (d, J = 3.6 Hz,
2H), 5.43 (s, 2H), 4.92 (s, 2H), 3.98 (d, J = 1.2 Hz, 3H); 19F
NMR (376 MHz, CDCl3) δ −128.20 (d, J = 32.8 Hz),
−137.74 (d, J = 34.5 Hz); 13C NMR (101 MHz,) δ 164.62,
154.24 (d, J = 254.5 Hz), 147.39, 144.96, 144.82, 144.38 (d, J
= 4.5 Hz), 140.43 (d, J = 13.6 Hz), 138.05 (d, J = 13.8 Hz),
135.49, 130.29 (d, J = 3.1 Hz), 128.78, 128.76, 125.85, 125.78
(d, J = 4.4 Hz), 123.00 (d, J = 14.3 Hz), 115.83, 68.14, 62.00
(d, J = 4.4 Hz); LC−MS (ES+) 439 (M + H), (ES−) 437 (M −
H).
2-(4-Chloro-2-fluoro-3-methoxyphenyl)-6-(diethoxy-
methyl)-3-fluoro-N-trityl(4-14C)pyridin-4-amine (26). To
a solution of N-[(5,5-diethoxy-1,1,1-trifluoro(2-14C)pent-3-yn-
2-ylidene]-1,1,1-triphenylmethanamine (25) (50.0 mCi at 33
mCi/mmol, 1.52 mmol) in DMSO (7 mL) was added
benzylamine 13 (0.86 g, 4.55 mmol, 3 equiv) under an
atmosphere of N2. The resultant light-yellow solution was
stirred at room temperature for 2 min and then treated with
cesium carbonate (1.234 g, 3.79 mmol, 2.5 equiv). The
4-Amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxy-
phenyl)-5-fluoro(4-14C)pyridine-2-carbaldehyde (28).
To a 100 mL round-bottom flask containing 27 (720 mg,
2.41 mmol) and equipped with a magnetic stir bar and a reflux
condenser were added CH3CN (30 mL) and 1,3-dichloro-5,5-
dimethylhydantoin (261 mg, 1.36 mmol, 0.55 equiv). The
resultant light-yellow mixture was stirred at room temperature
for 5 min and then placed in an oil bath and heated to reflux
under an atmosphere of N2. After 60 min at reflux, an aliquot
H
DOI: 10.1021/acs.oprd.9b00302
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of the reaction mixture was partitioned between EtOAc and
H2O and analyzed by TLC (80/20 hexanes/EtOAc) and
HPLC. Analysis indicated that all of the starting material had
been consumed. The reaction mixture was allowed to cool to
room temperature, diluted with EtOAc (45 mL), and washed
with H2O (1 × 15 mL), dilute sodium bisulfite (1.0 g of
NaHSO3/50 mL of H2O, 1 × 15 mL), and saturated NaCl (1
× 15 mL). The organic phase was dried by passage through a
plug of Na2SO4. The filtrate was concentrated in vacuo to give
a light-yellow solid, which was slurried with 95/5 hexanes/
EtOAc (15 mL). After 60 min of stirring, the mother liquor
was removed via filter stick. The solid was washed with 95/5
hexanes/EtOAc (2 × 3 mL) followed by 2/1 H2O/CH3CN.
The solid was treated with CH3CN and concentrated in vacuo
(4 × 10 mL) to give a light-tan solid. A small amount of this
material was dissolved in 1/1 CH3OH/CH2Cl2 and analyzed
by HPLC. Analysis showed ∼92% chemical and radiochemical
purity.
The poor recovery can be attributed to the low solubility of
the desired compound. The aqueous phase and washes were
combined and concentrated in vacuo. The residual water phase
was extracted with EtOAc (1 × 30 mL and 1 × 10 mL) and
CHCl3 (1 × 10 mL). The combined organic extracts were
washed with 0.5 N NaOH (1 × 10 mL) and saturated NaCl (1
× 10 mL) and then dried by passage through a plug of Na2SO4.
The filtrate was concentrated in vacuo to give a light-tan solid,
which was slurried with acetone. The solvent was removed in
vacuo to give 0.612 g of a light-tan solid. The solid was stirred
for ca. 15 min with 95/5 hexanes/EtOAc (10 mL). The
mother liquor was removed via a pipet, and the solid was
washed with 95/5 hexanes/EtOAc (1 × 3 mL). The solid was
dried under high vacuum to give 0.600 g (75% yield) of the
desired product as a light-tan solid. HRMS-ESI [M + H]+ calcd
for C13H8Cl2F2N2O2 331.993, found 331.9928.
solid. HRMS-ESI [M + H]+ calcd for C13H8Cl2F2N2O3
347.9880, found 347.9876.
Benzyl 4-Amino-3-chloro-6-(4-chloro-2-fluoro-3-me-
thoxyphenyl)-5-fluoro(4-14C)pyridine-2-carboxylate
(Rinskor-Py-4-14C, 30). To a 100 mL round-bottom flask
containing a magnetic stir bar and 29 (519 mg, 1.49 mmol)
was added anhydrous DMSO (10 mL). To this solution was
added powdered potassium carbonate (411 mg, 2.97 mmol, 2
equiv, 325 mesh) followed by benzyl bromide (0.23 mL, 1.93
mmol, 1.3 equiv). The resultant mixture was stirred at room
temperature under an atmosphere of N2. After the reaction
mixture was stirred overnight (16 h), an aliquot of the mixture
was partitioned between 1 M HCl and EtOAc and analyzed by
HPLC and TLC. Analysis showed that all of the starting
carboxylic acid had been consumed. The reaction mixture was
diluted with EtOAc (30 mL) and washed with H2O (3 × 10
mL) and saturated NaCl (1 × 10 mL). The organic layer was
dried (Na2SO4), filtered, and concentrated in vacuo to give a
light-yellow solid, which was dissolved in CH2Cl2 (2 mL) and
loaded onto a silica gel column. The crude material was
chromatographed using the following setup: Teledyne-ISCO
CombiFlash Rf, 40 g RediSep silica gel column, 0−75%
hexanes/EtOAc. Fractions containing the desired product in
>97% radiochemical purity where combined and concentra-
teed in vacuo. The residue was dried under high vacuum (50−
60 °C) to give a constant weight of 525 mg (80% yield) of the
desired product as a white solid. The radiochemical purity was
97.8% as determined by HPLC with a specific activity of 32.9
mCi/mmol. HRMS-ESI [M
+
H]+ calcd for
C20H14NCl2F2N2O3 438.0350, found 438.3044.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.oprd.9b00302.
4-Amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxy-
phenyl)-5-fluoro(4-14C)pyridine-2-carboxylic Acid (29).
To a 50 mL round-bottom flask containing 28 (600 mg, 1.80
mmol) and equipped with a magnetic stir bar and a reflux
condenser was added tert-butyl alcohol (11 mL). The mixture
was treated with H2O (3 mL), 2-methyl-2-butene (1.5 mL),
sodium hydrogen phosphate dihydrate (511 mg, 3.60 mmol, 2
equiv), and sodium chlorite (489 mg, 3.60 mmol, 3 equiv).
The mixture was stirred at room temperature for 5 min and
then placed in an oil bath at 80 °C. After the reaction mixture
was stirred overnight at 80 °C (16 h), an aliquot of the mixture
was partitioned between EtOAc and 1 M HCl. Analysis by
HPLC and TLC (80/20 hexanes/EtOAc) indicated that all of
the starting material had been consumed. The reaction mixture
was allowed to cool to room temperature and diluted with
EtOAc (25 mL). The resultant mixture was washed with 1 M
HCl (1 × 10 mL), H2O (1 × 10 mL), and saturated NaCl (1 ×
10 mL). The organic layer was dried by passage through a plug
of Na2SO4. The filtrate was concentrated in vacuo to afford a
very faint yellow solid, which was slurried with Et2O (8 mL)
for 3 h, and then the mother liquor was removed via a filter
stick. The solid was washed with Et2O (2 × 2 mL), and the
washes were removed via a filter stick. The solid was slurried
with EtOAc, and a small aliquot of the slurry was dissolved in
CH3CN and analyzed by HPLC. HPLC analysis showed ca.
95% chemical and radiochemical purity. The solvent was
removed in vacuo, and the solid was dried under high vacuum
to give 519 mg (83% yield) of the desired product as a white
NMR spectra of compounds 1, 13, 17, 18, and 20−24
and HPLC chromatograms of radiotracers 26−30
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: pete.johnson@corteva.com.
ORCID
Pete Johnson: 0000-0001-6856-4310
Notes
The author declares no competing financial interest.
ACKNOWLEDGMENTS
■
The author thanks Jim Renga, Greg Whiteker, Natalie
Giampietro, and Chris Galliford for many helpful discussions
and process improvements in the development of this novel
route used to prepare Rinskor-Py-4-14C and Jeff Godbey for
mass spectral analysis of the radiotracers prepared in this
report.
REFERENCES
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DOI: 10.1021/acs.oprd.9b00302
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