Use of ω-transaminase enzyme chemistry in the synthesis of a JAK2 kinase inhibitor

Article abstract of DOI:10.1021/op400133d

ω-Transaminase enzyme chemistry provides an excellent methodology to build synthetically useful chiral amines from their corresponding ketones. An application of this methodology, providing a long-term commercial manufacturing route to a JAK2 kinase inhibitor, is reported herein.

Full text of DOI:10.1021/op400133d

Article
pubs.acs.org/OPRD
Use of ω‑Transaminase Enzyme Chemistry in the Synthesis of a JAK2
Kinase Inhibitor
Lianne Frodsham, Michael Golden,* Susan Hard, Martin N. Kenworthy, David J. Klauber, Kevin Leslie,
Claire Macleod, Rebecca E. Meadows, Keith R. Mulholland, Julie Reilly, Christopher Squire,
Simone Tomasi, Denise Watt, and Andrew S. Wells
Pharmaceutical Development, AstraZeneca, Silk Road Business Park, Macclesfield, United Kingdom
ABSTRACT: ω-Transaminase enzyme chemistry provides an excellent methodology to build synthetically useful chiral amines
from their corresponding ketones. An application of this methodology, providing a long-term commercial manufacturing route to
a JAK2 kinase inhibitor, is reported herein.
chromatography and an additional rework. Despite excellent
enantioselectivity, the Rh-DuPhos catalyst used for the
reduction to produce the chiral amine 6 was expensive, and
the extremely water-soluble amine required BOC protection to
enable extraction into an organic solvent. The BOC group was
subsequently removed under anhydrous conditions. Coupling
of 6 and 7 (to form 1) was prone to significant and
unpredictable levels of epimerization (60−90% ee), such that
chiral chromatography was necessary to furnish the chirally
pure API 1.
INTRODUCTION
■
The API 1 is an orally active inhibitor of the JAK2 kinase1,2
with the potential to be a curative treatment for idiopathic
myelofibrosis (IMF) and polycythaemia rubra vera (PRV). The
compound was discovered by AstraZeneca, at its Boston
research site in North America, and was subsequently
developed at the Macclesfield facility in the UK.
The Medicinal Chemistry route (shown in Scheme 1) was
used for the production of the first manufacturing batch (400
g) in the large-scale laboratory at Macclesfield to supply the 1
month toxicological study. Very limited development work was
possible in the short timeframes available during preclinical
development, and the route was scaled with minimal improve-
ment work.
Production of the coupling partner 7 was relatively trivial:
yields of 90% were achieved from commercially available 3-
amino-5-methylpyrazole and 2,4,5-trichloropyrimidine. The
process was noteworthy only for the excellent selectivity for
displacement at the 4-position of the pyrimidine and was
subsequently scaled to 23 kg (input) with only minor changes.
The main part of the synthetic route (Scheme 1), however,
presented a number of challenging problems, not the least of
which was an overall yield to the API of only 2% from 2,4-
dichloro-5-fluoropyrimidine (2).
Water was difficult to remove during the workup and
contaminated the product of the dechlorination reaction
leading to 2-chloro-5-fluoropyrimidine (3). The next stage
was sensitive to water, so that 3 had to be reworked twice to
furnish suitable quality material.
Cyanation of 3 was initially achieved with an all-in process at
100 °C but was subject to a large exotherm (166 kJ/mol) (with
initiation at >100 °C). Large amounts of tar formation were
observed that, in combination with the metal residues, made
workup extremely difficult even on relatively small scale. The
reaction was also capricious in that it was prone to stalling
before completion and, once stopped, could not be restarted
through the addition of more reagents or catalyst. Filtration
through a silica plug and a rework were necessary to produce 4
of suitable quality.
RESULTS AND DISCUSSION
■
A further, larger campaign was run almost simultaneously with
the target of producing 5 kg of API for phase I clinical trials and
formulation. A commercial source of 2-chloro-5-fluoropyrimi-
dine (3) was found, and outsourcing of the subsequent two
stages furnished vinylacetamide 5, with modest improvements
in processing and yield. We introduced distillation of
cyanopyrimidine 4, leading to reduced tar levels in the
subsequent vinylacetamide 5 stage. Reduction to the chiral
amine 6 was scaled with little further work. Development of the
coupling step, however, was far from straightforward. The cause
of the epimerization during the reaction remained unknown,
and chiral chromatography was undesirable at the larger scale.
From our investigations, it became clear that, whilst the product
of the reaction itself appeared to be stable under the reaction
conditions, it was in fact the chiral amine 6 that was epimerising
as the reaction progressed, such that the % ee of API 1 slowly
decreased as the reaction progressed due to coupling of the
unwanted enantiomer of 6 (Figure 1).
Some potential mechanisms (imine formation, radical
mechanism) were proposed and ruled out through exper-
imentation. Computer modelling proved very useful to rule out
deprotonation by triethylamine followed by nonselective
reprotonation (confirmed also by experimentation: triethyl-
amine alone cannot racemise the amine). A more likely
mechanism, proposed by the modelling studies, proceeds via an
enamine or an ylide obtained from the action of triethylamine
Tar formation also hindered the reaction to form 5, which
was low yielding, operationally difficult, and in need of full
Received: May 24, 2013
© XXXX American Chemical Society
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Scheme 1. Medicinal Chemistry route
Figure 1. Chiral purity of amine (6) and product (1) during the reaction (using KHCO3).
and triethylammonium ion present in the buffered system
(Scheme 2). It was predicted that the epimerization should be
greatly reduced in the absence of such a buffer system. This
might be achieved through the use of a different base whose
conjugate acid would not remain in solution.
Changing the base from triethylamine to potassium
bicarbonate meant that the HCl was removed from the
solution (by precipitation of KCl). This led to a significant
reduction of the extent of epimerization, producing 1 of >94%
ee in the crude reaction mixture. Crystallisation as the maleate
salt (eutectic point at 92% ee) allowed chiral purification
without the need for chromatography and also gave excellent
processing properties (Scheme 3).
API 1 was crystallised (after neutralisation and extraction
into isopropyl acetate) by slow antisolvent addition of n-
heptane. Oiling prior to crystallisation was a major problem,
and the kinetics of the crystallisation were seen to be unusually
slow (additions of >24 h were necessary to prevent oiling).
Redesign of this final isolation would be required for robust
larger-scale manufacture, but it proved acceptable for the early
deliveries. With the improvements made to the process, the
second manufacturing campaign produced 7 kg of chirally pure
material although the overall yield from commercially available
starting material 3 was still only approximately 4%.
Recognising the difficulty of the synthesis to the chiral amine
6, further work was performed to try to optimise the existing
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Scheme 2. Postulated ylide and enamine mechanisms
Scheme 3. Coupling and purification stages
Scheme 4. Alternative approaches to the chiral amine (6)
low-yielding, operationally difficult vinylacetamide 5 step of the
medicinal chemistry route. A study utilising process analytical
technology (PAT) (UV−vis and mid-IR) was undertaken
which showed clean formation of the intermediate imine from
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Scheme 5. Proposed long-term manufacturing route
cyanopyrimidine 4 and corresponding high solution yield
(87%) of the ketone 10 formed by acidic quench. Problems in
the process were shown to occur during conversion of the
imine to vinylacetamide 5 by reaction with acetic anhydride,
although it was difficult to discern the nature of any
nonproductive pathways. Attempts to improve the solution
yield of vinylacetamide 5 by varying the mode and temperature
of addition were unsuccessful and offered no improvement over
the process used previously for manufacture (48% solution
yield). The purity of 5 was also not improved by any of the
process modifications tested which precluded isolation by
crystallisation without the need for chromatography.
cyanohydrin as cyanide source5 and BuOAc as solvent gave
clean conversion of 2-chloro-5-fluoropyrimidine to product
with controllable exotherm through the slow addition of the
(liquid) acetone cyanohydrin. A 70% solution yield was
obtained in the laboratory, (although a complete workup
procedure was not realised before the project was terminated,
and the chemistry was never scaled beyond the laboratory).
Higher solution yields (of 90%) were achieved using 2-bromo-
5-fluoropyrimidine.
Clean conversion of the cyanopyrimidine 4 to the ketone 10
was realised with relatively straightforward processing, via
conventional Grignard chemistry. This reaction was demon-
strated on 20 L scale with little problem.
With a promising route to the ketone in hand, numerous
strategies were explored to convert 10 to chiral amine 6 (see
Scheme 6).
Many of these processes exposed the inherent fragility of the
pyrimidine ring, and the formation of oily tars was common-
place. The only promising alternative identified was an
enzymatic approach using transaminases, the conditions of
which were by necessity very mild and provided the added
attraction of a single chemical processing step.
Seemingly fundamental problems with the synthetic route
therefore provoked an investigation into alternative routes to
chiral amine 6 (Scheme 4).
Attempts using such methodologies as Heck chemistry
(utilising vinyl butyl ether and hydrolysis)3 to give the ketone
10, palladium-catalysed insertion of nitroethane,4 and building
the pyrimidine ring, all suffered from low yields and required
extensive chromatography. The only seemingly viable alter-
native to emerge from this work centred on the Grignard
addition to the cyanopyrimidine 4 to deliver ketone 10, and
ultimately it was from this approach that the proposed long-
term manufacturing route was developed (Scheme 5).
Enzymatic transamination reactions are important and
abundant in nature and involve the transfer of an amino
group from α-amino acids to α-keto acids. Despite this,
examples of industrial applications6 for the transfer of an amino
Work to improve the operational difficulties of the existing
cyanation chemistry proved fruitful. A switch to acetone
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Scheme 6. Approaches for the conversion of ketone (10) to the chiral amine (6)
group (from a non-α-amino acid amine donor) to simple
ketones are not common within the literature. However, in
recent years some notable examples have been described such
as the preparation of sitagliptin by Merck.7
which extracted the acetophenone from the aqueous phase as it
was formed (see Scheme 7). This had the dual advantage of
avoiding enzyme inhibition and also driving the equilibrium
reaction such that only a small (and tolerable) excess of the
(S)-α-methylbenzylamine (1.1 equiv) was necessary to push the
reaction to completion. Scaling up to 20 L scale produced 500 g
of good quality amine 6. The relatively high loading of
formulated V. fluvialis enzyme (2.8 mL per 1 g ketone 10) did
lead to extended filtration times during processing, and
subsequent work sought to identify an enzyme that could be
used with a much lower loading. Forty-seven commercially
available transaminase enzymes were screened for activity, and
the Codexis enzyme, TA-P1-A06, was subsequently selected for
evaluation. This enzyme was successfully used to prepare amine
6 in 68% yield from ketone 10 with >99.5% ee with a loading of
0.025 g/g ketone 10.
An initial screen with ketone 10 highlighted almost complete
and clean conversion with excellent chiral selectivity using
Vibrio fluvialis enzyme and (S)-α-methylbenzylamine as the
amine donor, with pyridoxal phosphate cofactor in an aqueous
buffer. This clearly presented an opportunity to utilise this
methodology; a detailed discussion of the development of this
reaction step is included in a related publication,8 whilst an
overview is presented herein. Enzyme inhibition was observed
at higher concentrations of acetophenone byproduct, leading to
the use of high dilution (100 volumes). Under these conditions,
the reaction required greater than equimolar concentrations of
the (S)-α-methylbenzylamine (relative to the ketone 10), which
could not be separated from the product by the downstream
processing.
No further development work was performed on the
coupling stage (to produce 8) though the process was
successfully proven on 100 L scale. The purification process
to produce API 1 was subsequently redesigned such that
These issues were neatly solved (utilising Kim et al.
methodology9) by the use of an organic cosolvent (toluene),
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Scheme 7. Equilibrium of enzyme reaction
addition of KOH to a hot aqueous EtOH solution of maleate
salt 8 followed by cooling gave an excellent crystallisation,
without the oiling that plagued the isopropyl acetate/n-heptane
method, or the need for extraction. This procedure was
demonstrated on 100 L scale in the large scale laboratory.
The project was terminated in 2012, and whilst considered
viable as a long-term manufacturing route, the final route was
not scaled further. Laboratory yields of API suggest that an
overall yield from commercially available material could be
>30%.
DMSO) δ: 157.51 (s), 157.23 (s), 155.76 (s), 145.87 (s),
139.54 (s), 113.64 (s), 98.61 (s), 11.30 (s). HRMS Calcd for
C8H8Cl2N5: 244.0151; HRMS found [M + H]+: 244.0148.
Preparation of 5-Fluoropyrimidine-2-carbonitrile (4).
Charged were 1,1′-bis(diphenylphosphino)ferrocene)-
palladium(II) chloride (303 mg, 0.01 mol equiv), 2-chloro-5-
fluoropyrimidine (7.0 g 1.00 mol equiv), triethanolamine (5.56
mL, 1.1 mol equiv), butyl acetate (19.74 mL 3.0 rel vols), and
zinc dust (292 mg, 0.12 mol equiv) to the reaction vessel. The
mixture was degassed and inerted thoroughly with nitrogen. It
was heated to 105 °C, and acetone cyanohydrin (3.57 mL, 1.10
mol equiv) was charged at 105 °C over 2 h via syringe pump.
Then the mixture was cooled to ambient temperature and
filtered; washing with butyl acetate yields an orange solution
(48 mL, 70% yield based on solution assay) (90% solution yield
can be achieved using 2-bromo-5-fluoropyrimidine as input).
CONCLUSION
■
We report a manufacturing route to the API 1, utilising ω-
transaminase enzyme chemistry to construct the key chiral
amine coupling partner. Examples of this methodology, though
known in the literature, are limited to mostly lab-scale
procedures, and few examples6,7 exist of their application on
scale. Development work of this enzymatic stage, utilising an
elegant two-phase reaction mixture to avoid inhibition of the
enzyme (and therefore concentrate the process), has made this
a commercially viable step. This paper demonstrates a useful
addition to the application of ω-transaminase enzyme method-
ology.
1H NMR (400 MHz, CDCl3) δ: 9.17 (d, 2H, J = 0.88 Hz). 13
C
NMR (100 MHz, DMSO) δ: 157.10 (d, J = 270.9 Hz), 147.51
(d, J = 22.6 Hz), 139.60 (d, J = 6.0 Hz), 115.57 (s). GC/MS:
m/z 123 M+ (and fragments consistent with the proposed
structure).
Preparation of 1-(5-Fluoropyrimidin-2-yl)ethanone
(10). A solution of 2 M MeMgCl (268 mL, 0.81 mol) in
tetrahydrofuran was added to a solution of 5-fluoropyrimidine-
2-carbonitrile (4) (82.5 g, 0.65 mol) in 2-methyltetrahydrofur-
an (600 mL) at −40 °C. On complete reaction, the reaction
mixture was warmed to −25 °C and transferred into a solution
of aqueous hydrochloric acid (475 mL, 1.98 mol). On complete
reaction, the phases were separated, and the aqueous phase was
extracted further with 2-methyltetrahydrofuran (300 mL). The
organic phases were combined and concentrated by evapo-
ration before n-heptane was added to crystallize the product as
EXPERIMENTAL SECTION
■
Preparation of 2,5-Dichloro-N-(5-methyl-1H-pyrazol-
3-yl)pyrimidin-4-amine (7). 3-Amino-5-methylpyrazole
(12.9 kg, 1.05 mol equiv) was charged to a vessel and dissolved
in IMS (67.3L, 2.9 rel vols) to give a yellow solution.
Meanwhile, 2,4,5-trichloropyrimidine (23.2 kg, 1.0 mol equiv),
IMS (67.3 L, 2.9 rel vols) and triethylamine (16.6 kg, 1.3 mol
equiv) were charged to a second vessel and agitated. The 3-
amino-5-methylpyrazole solution was then added slowly at 20−
25 °C, and the mixture was left stirring at 20−25 °C for 20 h.
Water (227.4 L, 9.8 rel vols) was slowly added and the stirred
mixture held for 1 h before being filtered under vacuum. The
solid was then washed with water (78.9 L, 3.4 rel vols) before
being placed in a vacuum oven at 40 °C to dry. The product
was isolated as a white crystalline solid (27.9 kg, 90% yield). 1H
NMR (400 MHz, DMSO) δ: 2.26 (s, 3H), 6.29 (s, 1H), 8.32
(s, 1H), 9.70 (s, 1H), 12.31 (s, 1H). 13C NMR (100 MHz,
1
a light-brown crystalline solid (73.2 g, 80%). H NMR (400
MHz, DMSO) δ: 9.09 (d, 2H, J = 0.86 Hz), 2.68 (s, 3H). 13C
NMR (100 MHz, DMSO) δ: 195.81 (s), 157.15 (d, J = 269.2
Hz), 156.73 (d, J = 5.6 Hz), 146.18 (d, J = 21.1 Hz), 27.59 (s).
19F NMR (367.5 MHz, CDCl3) δ: −132.2 (s).
Preparation of (1S)-1-(5-Fluoropyrimidin-2-yl)-
ethanamine (6) on 20 L scale. (S)-α-methylbenzylamine
(450 mL, 1.10 mol equiv) was added to a solution of
monobasic potassium phosphate (87.4 g, 0.20 mol equiv) in
water (6.75 L, 15.0 rel vols). The pH of the solution was
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adjusted to pH 7.5 by the addition of acetic acid (variable
amount). Pyridoxal phosphate (4.3 g, 0.005 mol equiv) was
added, followed by 1-(5-fluoropyrimidin-2-yl)ethanone (10)
(450 g, 1.0 mol equiv), a buffered solution of V. fluvialis
transaminase enzyme (174.6 KU), and toluene (2.25 L, 5.0 rel
vols). The reaction mixture was adjusted to pH 7.5 with
potassium carbonate (variable amount) and then held at 29 °C
for 18 h. The reaction was filtered and the organic layer
discarded. Potassium carbonate (887.7 g, 2.0 mol equiv) was
added to the aqueous phase followed by a solution of di-tert-
butyl dicarbonate (771.0 g, 1.1 mol equiv) in 2-methyltetrahy-
drofuran (3.6 L, 8.0 rel vols). The mixture was filtered and the
aqueous layer extracted with further 2-methyltetrahydrofuran
(1.8 L, 4.0 rel vols). The organic layers were combined and
concentrated at atmospheric pressure to a contents temperature
of 98 °C. 5−6N hydrochloric acid in isopropanol (1.5 L, 2.7
mol equiv) was added at ambient temperature. The reaction
mixture was heated to 40 °C to precipitate the product, which
pyrimidine-2,4-diamine; Maleic Acid (8). 2,5-Dichloro-N-
(5-methyl-1H-pyrazol-3-yl)pyrimidin-4-amine (7) (100 g, 1.0
mol equiv), (1S)-1-(5-fluoropyrimidin-2-yl)ethanamine (6)
(90.96 g, 1.25 mol equiv), and potassium bicarbonate (164.2
g, 4.0 mol equiv) were stirred in tert-amyl alcohol (331 mL, 4.1
rel vols) at 100 °C for 50 h. The resulting slurry was cooled to
50 °C and washed twice with water (400 mL, 4.0 rel vols) at 50
°C. Isopropanol (800 mL, 8.0 rel vols) was added before a
solution of maleic acid (47.6 g, 1.0 mol equiv) in isopropanol
(400 mL, 4.0 rel vols) is charged. Further isopropanol (200 mL,
2.0 rel vols) was added before cooling to 0 °C, filtering, and
washing with MTBE (300 mL, 3.0 rel vols). The product was
dried at 50 °C and isolated as a white solid (153 g, 81% yield).
1H NMR (500 MHz, DMSO) δ: 1.54 (d, 3H, J = 7.0 Hz), 2.22
(s, 3H), 5.25 (m, 1H), 6.21 (s, 2H), 7.93 (s, 1H), 8.77 (s, 2H).
13C NMR (100 MHz, CDCl3) δ: 167.3, (s), 166.4 (s), 158.6
(s), 157.7 (d, J = 260.1 Hz), 154.8 (s), 152.4 (s), 145.0 (d, J =
20.7 Hz), 144.7 (s), 140.5 (s), 130.9 (s), 102.4 (s), 96.1 (s),
52.5 (s), 20.6 (s), 11.4 (s). 19F NMR (367.5 MHz, CDCl3) δ:
−140.9 (s). HRMS Calcd for C14H15ClN8F: 349.1092; HRMS
found [M + H]+: 349.1080.
1
was isolated as a crystalline solid (451 g, 79%). H NMR (400
MHz, DMSO) δ: 9.03 (d, 2H, J = 1.08 Hz), 4.54 (q, 1H, J =
6.90 Hz), 1.60 (d, 3H, J = 6.90 Hz). 13C NMR (100 MHz,
DMSO) δ: 162.57 (d, J = 5.2 Hz), 157.15 (d, J = 269.2 Hz),
145.52 (d, J = 20.7 Hz), 50.72 (s), 18.81 (s). 19F NMR (367.5
MHz, DMSO) δ: −138.7 (s). HRMS Calcd for C6H9FN3:
142.0775; HRMS found [M + H]+: 142.0774. Enantiomeric
excess was determined by chiral HPLC (CrownPak CR+,
aqueous perchloric acid, >99% ee).
5-Chloro-N2-[(1S)-1-(5-fluoropyrimidin-2-yl)ethyl]-N4-
(5-methyl-1H-pyrazol-3-yl)pyrimidine-2,4-diamine (1). 5-
Chloro-N2-[(1S)-1-(5-fluoropyrimidin-2-yl)ethyl]-N4-(5-
methyl-1H-pyrazol-3-yl)pyrimidine-2,4-diamine maleic acid (8)
(130.0 g, 1.0 mol equiv) was charged to a vessel with ethanol
(494 mL, 3.8 rel vols), water (611 mL, 4.7 rel vols), and 49%
aqueous potassium hydroxide (45.88 g, 1.5 mol equiv). The
mixture was heated to 60 °C to dissolve the substrate, screened
through an in-line filter, and washed with 50% aqueous ethanol
(195 mL, 1.5 rel vols). The solution was then seeded with 1
(0.975 g, 0.0075 rel wt) and then cooled to crystallise the
product. A further portion of water (650 mL, 5.0 rel vol) was
added to increase the recovery. The product was then filtered
and washed with a mixture of water (260 mL, 2.0 rel vol) and
ethanol (130 mL, 1.0 rel vol) before drying at 40 °C to
constant weight. The product was isolated as a white crystalline
solid (81.1 g, 87% yield). 1H NMR (500 MHz, CDCl3) δ: 1.61
(d, 3H, J = 7.03 Hz), 2.32 (s, 3H), 5.39 (q, 1H, J = 7.03 Hz),
7.61 (s, 1H), 7.94 (s, 1H), 8.59 (s, 2H). 13C NMR (126 MHz,
CDCl3) δ: 167.4 (d, J = 6.0 Hz), 159.6 (s), 156.6 (d, J = 263.9
Hz), 154.5 (s), 145.0 (s), 144.5 (s), 140.2 (s), 103.3 (s), 94.8
(s), 52.9 (s), 21.1 (s), 12.4 (s). 19F NMR (471 MHz, CDCl3) δ:
−140.1 (s). HRMS Calcd for C14H15ClN8F: 349.1092; HRMS
found [M + H]+: 349.1093.
Preparation of (1S)-1-(5-Fluoropyrimidin-2-yl)-
ethanamine (6) using Codexis TA-P1-A06. (S)-α-Methyl-
benzylamine (18.9 mL, 0.15 mol) was added to a solution of
monobasic potassium phosphate (3.6 g, 0.27 mol) in water
(300 mL). The pH of the solution was adjusted to pH 7.5 by
the addition of acetic acid (7.0 mL, 0.12 mol). Pyridoxal
phosphate (0.16 g, 0.0007 mol) was added, followed by 1-(5-
fluoropyrimidin-2-yl)ethanone (10) (20.0 g, 0.13 mol), TA-P1-
A06 enzyme (0.47 g), and toluene (100 mL). The reaction
mixture was adjusted to pH 7.5 with potassium carbonate (1.84
g, 0.013 mol) and then held at 29 °C for 18 h. Celite (4.0 g)
was added, and the reaction mixture was filtered. A mixture of
toluene (40 mL) and water (100 mL) was added to the
reaction vessel, and the reaction mixture was stirred and
discharged as a wash to the filter bed. The aqueous phase was
separated and the organic phase discarded. Potassium
carbonate (36.8 g, 0.27 mol) was added to the aqueous phase
followed by a solution of di-tert-butyl dicarbonate (31.9 g, 0.15
mol) in 2-methyltetrahydrofuran (160 mL). The mixture was
heated to 40 °C for 18 h and the resulting biphasic mixture
filtered. The organic layer was separated and evaporated to
dryness. The residue was dissolved in MTBE (58 mL) and a
solution of 5−6N hydrochloric acid in isopropanol (72.0 mL,
0.36 mol) was added. The reaction mixture was heated to 40
°C for 24 h. An off-white suspension was formed. The reaction
mixture was cooled to 25 °C and heptane (60 mL) was added.
After 1h the reaction mixture was filtered. The isolated solid
was washed with 2-methyltetrahydrofuran (40 mL) and dried in
vacuo to give the amine 6 as a monohydrochloride salt (16.1 g,
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: michael.golden@astrazeneca.com
Notes
The authors declare no competing financial interest.
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