Development of an Enantioselective Novozym 435 Mediated Acetylation for the Preparation of (1S,3R)-3-Acetamidocyclohexane-1-carboxylic Acid

Article abstract of DOI:10.1021/acs.oprd.6b00146

Starting from a cheap and readily available starting material, a short and enantioselective route to (1S,3R)-3-acetamidocyclohexane-1-carboxylic acid was developed. The key steps were a rhodium catalyzed hydrogenation of 3-aminobenzoic acid and an enantioselective Novozym 435 mediated acetylation of racemic isopropyl 3-aminocyclohexanecarboxylate.

Full text of DOI:10.1021/acs.oprd.6b00146

Article
pubs.acs.org/OPRD
Development of an Enantioselective Novozym 435 Mediated
Acetylation for the Preparation of (1S,3R)‑3-Acetamidocyclohexane-
-carboxylic Acid
1
Staffan Karlsson*
Innovative Medicines, Pharmaceutical Sciences, Early Chemical Development, AstraZeneca R&D Gothenburg, SE-431 83 Mo
Sweden
̈
lndal,
*
S Supporting Information
ABSTRACT: Starting from a cheap and readily available starting material, a short and enantioselective route to (1S,3R)-3-
acetamidocyclohexane-1-carboxylic acid was developed. The key steps were a rhodium catalyzed hydrogenation of 3-
aminobenzoic acid and an enantioselective Novozym 435 mediated acetylation of racemic isopropyl 3-aminocyclohexanecarbox-
ylate.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
In one of our drug development programs, several hundreds of
grams of the enantiomerically pure carboxylic acid 1 were needed
Figure 1).
Enzymatic resolution by means of N-acetylation is a known
method for the preparation of enantiomerically pure acetamides
and it has been demonstrated that immobilized Candida
Antarctica B (e.g Novozyme 435) is a general and4
enantioselective enzyme for such types of transformations.
Thus, starting from a racemic mixture of cis-3-aminocyclohex-
anecarboxylic acid, cis-2 (Scheme 2), an enantioselective enzyme
mediated acetylation could potentially furnish the desired
enantiomerically pure acetamide 1 in a single step.
(
Starting from commercially available amino acid cis-2 and
using EtOAc as acyldonor and solvent, the enzymatic acetylation
mediated by immobilized Candida Antarctica B, Novozym 435
was attempted. Although acetylation was taking place to some
extent, the amino acid cis-2 had a very low solubility in organic
solvents resulting in a very slow process which was deemed
inconvenient and impractical to execute on a large scale. Instead,
the corresponding i-Pr ester cis-3 was prepared which gave an
easily handled nonviscous oil which has high solubility in most
organic solvents and also has a high chemical stability.
Fortunately, using the ester cis-3 and EtOAc as acyldonor/
solvent, the Novozym 435 catalyzed the formation of the
corresponding acetamide 4. No reaction took place in the
absence of Novozym 435. After an extractive workup, the
product 4 could be isolated essentially pure. A chiral HPLC
analysis of the mixture showed an ee of 90% at ∼50% conversion,
and by comparison with authentic material of known stereo-
chemistry, it was concluded that the desired 1S,3R-stereoisomer
Figure 1. Target molecule (1S,3R)-3-acetamidocyclohexane-1-carbox-
ylic acid 1.
Among various methods available to obtain enantiomerically
1
pure carboxylic acids of type 1, the two approaches depicted in
Scheme 1 were believed to be most suitable for large scale
purposes avoiding the handling of hazardous and toxic reagents.
Thus, resolution of BOC protected amino acid A through salt
2
formation followed by deprotection and acetylation would
furnish the desired compound 1. Alternatively, resolution of
3
BOC protected ester B mediated by a lipase followed by
deprotection, acetylation, and hydrolysis of the ester would
furnish the same compound 1.
Although, the salt- and enzymatic resolution depicted in
Scheme 1 are known methods in the literature and worked well
on a small scale, there were concerns in the scale up of these
3
routes: First, the enzymatic route was not scaleable and gave
5
was the one which preferentially was acetylated. Encouraged by
inconsistent results. Second, the salt resolution as described in
2
these results, optimization of the reaction to maximize the ee and
yield was investigated. Larger amounts of the i-Pr ester cis-3 was
required for these studies. Although commercially available, the
amino acid cis-2 is not easily accessible in larger quantities and to
a reasonable price. Instead kilogram quantities of amino acid 2 as
a mixture of cis/trans isomers (74% cis-isomer w/w) were
the literature but using ethanol as the solvent required three
sequential recrystallizations to obtain a satisfactory ee (>95% ee)
of the corresponding acid, and consequently, the yield was only
about 20%. Furthermore, using either of the approaches depicted
in Scheme 1 would require two extra steps (i.e deprotection
followed by acetylation) in the sequence to obtain the desired
acetamide. Thus, instead a short, robust and enantioselective
method to obtain the (1S,3R)-3-acetamidocyclohexane-1-
carboxylic acid 1 on a large scale was sought.
Received: April 22, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.oprd.6b00146
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
Scheme 1. Two Approaches for the Synthesis of Enantiomerically Pure Carboxylic Acid 1
Scheme 2. Enzyme-Mediated Acetylation
Table 1. Enantiomeric Excess (ee) of 4 as a Function of
a
Solvent/Acyldonor and Loading of Novozym 435
b
c
acyldonor/
solvent
Novozym conversion
time
ee 4
d
entry
435 (w/w)
(%)
(min) (%)
E
1
2
3
4
5
6
7
8
9
i-Pr acetate
EtOAc
20
20
20
20
20
20
5
46
44
43
47
0
135
220
93
93
93
94
67
60
58
85
n-Bu acetate
sec-Bu acetate
tert-Bu acetate
iso-Bu acetate
i-Pr acetate
i-Pr acetate
i-Pr acetate
444
220
1440
360
46
44
44
0
94
94
90
80
72
40
purchased, but this compound was also contaminated with
various other unknown impurities. It was hoped that these
impurities would not interfere in the subsequent steps and that
these could be removed through extractions or crystallization
downstreams in the synthetic sequence. Transformation to the
corresponding i-Pr ester 3 was easily accomplished after
treatment of 2 with a solution of HCl in 2-propanol at reflux
followed by an extractive workup (Scheme 3).
450
2.5
0
1440
1440
a
To a solution of 3 (100 mg) in 0.8 mL of solvent was added
Novozyme 435. The mixture was stirred at 20 °C at the specified time
given in the table. Determined using H NMR of the crude mixtures.
Determined using chiral HPLC analysis. Enantiomeric ratio (E)
calculated using the method of Sih et al.
b
1
c
d
6
Fortunately, in the subsequent step, it was found that the
desired 1S,3R-isomer of 3 was preferentially acetylated to give
compound 4 leaving the other three stereoisomers more or less
untouched. However, after an extractive workup, various
unknown impurities were detected which presumably originated
from the chemically impure starting material 2. To get rid of
these impurities, a subsequent crystallization of the crystalline
acetamide 4 was planned. Next, a study of the effect of
acyldonor/solvent on the reaction rate/enantioselectivity and
also the loading of the enzyme was performed (Table 1).
It was found that the choice of solvent/acyldonor had no or
little effect on the enantioselectivity, while the reaction rates
varied considerably (entries 1−6). Surprisingly, the more
sterically hindered isopropyl acetate gave a higher rate than
EtOAc, and the reaction reached almost 50% conversion within 2
h. Thus, isopropyl acetate was chosen as solvent/acyldonor for
further studies. The loading of the enzyme could be reduced to
of isopropyl acetate and using a loading of 5% w/w of Novozym
435 at 20 °C. Using these conditions and starting from 2.1 kg of
an impure cis/trans mixture of amine 3 (74% cis w/w), 731 g
(72% assay w/w, corrected yield 27%) of compound 4 with 87%
ee was isolated. After a solvent screen for crystallization of 4, it
was found that when a mixture of c-pentylmethyl ether/c-hexane
was used, the product 4 was obtained in >99% ee (379 g, 100%
w/w, 20% overall yield). Although the process above was
considered robust with consistency, the quite moderate yield
obtained in the enzymatic resolution was of some concern. It was
reasoned that the poor yield might be due to the additional
impurities other than the trans-isomers present in the starting
mixture of amine 3. To support this hypothesis, the enzymatic
acetylation starting from 59 g of the chemically pure cis-isomer
cis-3 was attempted instead. As anticipated, after 12 h of reaction
followed by extractions, the amide 4 was isolated in a significantly
better yield (50% corrected yield) and with 86% ee. One
crystallization from c-pentylmethyl ether/c-hexane improved the
ee to >99% and gave the product 4 in 40% overall yield. With
these encouraging results in mind, it was obvious that for long-
term supply of compound 4, a reliable supplier of the pure cis
isomers cis-2 or cis-3 had to be identified. Alternatively, a good
method to obtain cis-2 or cis-3 had to be developed in-house. As
pointed out earlier, suppliers of compound cis-2 are few, the
prices are high, and it is difficult to purchase in large quantities.
Thus, a good robust route to obtain the pure cis-isomers cis-2/3
2
.5% w/w with only a minor change in enantiopurity, but the
reaction time required increased significantly (entry 8). Although
1
H NMR measurement could be used as an off-line method for
determination of conversion, a real time FT-IR measurement
could also be used by looking at the specific absorption bands at
−1
1
530 and 1680 cm of 4 (Figure 2).
Figure 2 shows an initially fast reaction that slows down as it
approaches the end point for the enzymatic resolution. From the
results obtained above, the following conditions were chosen for
further scale-up studies: Amine 3 dissolved in 8 relative volumes
Scheme 3
B
DOI: 10.1021/acs.oprd.6b00146
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
−1
Figure 2. FT-IR monitoring (1680 cm ) of the conversion of 3 (dissolved in 8 relative volumes of isopropyl acetate) to 4 as a function of time. 8% w/w
1
of Novozym 435 was used and at 20 °C. The absolute values of conversion were determined using H NMR measurement. Absolute time on X-axis is
given in hours and peak height on Y-axis is given as absorption units (A.U.).
using simple starting materials was sought. This could potentially
be achieved by catalytic hydrogenation of the cheap and readily
available 3-aminobenzoic acid 5 (Scheme 4).
troublesome. Starting from the corresponding iPr-ester 6 instead
and using Rh as catalyst in 2-propanol at 80 °C, solved the
solubility problems and the product 3 was easily isolated as a 80/
2
0 cis/trans mixture containing also a few other unidentified
impurities (58% corrected yield, cis-isomer). Although the result
starting from the i-Pr ester 6 was encouraging, this starting
material is not easily accessible at reasonable prices. Attempts
were made to synthesize this from acid 5 using i-PrOH as solvent
and HCl as acid catalyst. However, due to the very low solubility
of 5 also in the presence of a large excess of HCl, the synthesis
was not straightforward and resulted in incomplete conversions.
Despite the low solubility, it was found that hydrogenation of 5 in
a solution of HCl in 2-propanol was possible. Since amino acid 2
had better solubility than 5 in such an acidic mixture and
consequently would be more prone to undergo acid catalyzed
esterification, a telescoped process where the hydrogenation and
the esterification of 5 was run in one pot was developed (Scheme
5).
Scheme 4. Hydrogenation Approach To Obtain 2/3
A literature survey showed that this transformation had been
accomplished using various catalysts such as Pd, Pt, Rh, Ru and7
Raney Nickel in a number of solvents such as water and alcohols.
Attempted catalytic hydrogenation using Pd, Pt or Rh as catalyst
in water, gave a complex mixture containing in addition to the
cis/trans isomers (∼80:20) of 2, also a lot of unknown
impurities. Running the reaction in EtOH instead, gave a better
result but due to the low solubility of the amino acid 2 in alcohols,
the removal of the catalyst and isolation of the product was
Thus, starting from amino acid 5 in an acidic (HCl) mixture of
2-propanol (100 mL) in the presence of 5% Rh on Al O (0.5
2
3
mol %) at 100 °C and under 8 bar of hydrogen, furnished in one
Scheme 5. Large-Scale Synthesis of (1S,3R)-3-Acetamidocyclohexanecarboxylic Acid 1
C
DOI: 10.1021/acs.oprd.6b00146
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
pot, a ∼80/20 cis/trans mixture of the HCl salt of 3 as a solid,
containing also various unidentified impurities (corrected yield
cis isomer = 48%). After a solvent screen it was found that this
The mixture was filtered, and the solid collected was washed with
isopropyl acetate (3 × 30 mL) followed by drying under reduced
8
pressure. This yielded the title salt (7.5 g, 33.8 mmol, 99% w/w,
1
salt, after one crystallization from isopropyl acetate furnished the
46%) as a colorless solid. H NMR (400 MHz, D O) δ 1.27 (d, J
2
desired HCl salt of cis-3 in 46% overall isolated yield and with
= 6.3 Hz, 6H), 1.29−1.57 (m, 4H), 1.89−2.14 (m, 3H), 2.2−2.36
>
99% cis/trans ratio. Extraction with a mixture of NaOH (aq.)
(m, 1H), 2.52 (tt, J = 3.3, 12.2 Hz, 1H), 3.22−3.32 (m, 1H), 5.01
13
and isopropyl acetate gave cis-3 as the free base as an easily
(hept, J = 6.3 Hz, 1H). C NMR (101 MHz, D O) δ 20.9 (2C),
2
9
handled nonviscous oil.
22.9, 27.4, 29.5, 32.0, 41.7, 49.4, 69.8, 176.6.
As described above, the enzymatic acetylation of cis-3 gave
after crystallization the product amide 4 in >99% ee and in 40%
yield. A standard saponification of this ester followed by
extraction and slurrying of the hard solid obtained in
methylisobutylketone (MIBK) gave the desired acid 1 as an
easily handled crystalline solid in 96% isolated yield. The entire
route to acid 1 was considered robust, and no further
optimization was undertaken.
(cis)-Isopropyl 3-Aminocyclohexanecarboxylate cis-3.
At 20 °C, to a mixture of (Cis)-3-(isopropoxycarbonyl)-
cyclohexan-1-aminium chloride cis-3 × HCl (14.0 g, 98% w/w,
62 mmol) in water (50 mL) and isopropyl acetate (150 mL) was
slowly added a solution of sodium hydroxide (2.72 g, 68.1 mmol)
in water (20 mL) until a pH of 10.9 was obtained. The aqueous
layer was extracted with isopropyl acetate (2 × 50 mL), and the
pooled organic layer was concentrated to give the title compound
as a colorless nonviscous oil (11.86 g, 97% w/w, 62 mmol, 100%
yield). Water was azeotropically removed using isopropyl acetate
CONCLUSION
■
1
(
1
2 × 20 mL). H NMR (400 MHz, CDCl ) δ 0.83−0.97 (m,
3
Starting from cheap and readily available 3-aminobenzoic acid 5,
an enzyme-mediated acetylation was developed as a key step for
the large scale preparation of (1S,3R)-3-acetamidocyclohexane-
carboxylic acid 1 in >99% ee and with 17% overall yield.
Furthermore, a practical large-scale method was developed for a
telescoped one-pot hydrogenation and esterification of 3-
aminobenzoic acid 5 to give (cis)-isopropyl-3-aminocyclohex-
anecarboxylate cis-3 in 46% yield.
H), 0.82−1.27 (m, 5H), 1.11 (d, J = 6.2 Hz), 6H), 1.67−1.82
(
(
(
6
1
m, 3H), 1.93−2.02 (m, 1H), 2.18 (tt, J = 3.3, 12.0 Hz, 1H), 2.56
13
tt, J = 3.6, 11.2 Hz, 1H), 4.88 (hept, J = 6.2 Hz, 1H). C NMR
101 MHz, CDCl ) δ 21.7 (2C), 24.3, 28.1, 36.0, 39.1, 42.7, 50.0,
3
+
+
7.2, 174.8. HRMS (ESI ): [M + H] m/z Calcd for C H NO
1
0
20
2
86.1494; Found 186.1490.
1S,3R)-Isopropyl 3-Acetamidocyclohexanecarboxy-
(
late 4. To a clear solution of (cis)-isopropyl 3-amino-
cyclohexanecarboxylate cis-3 (59.4 g, 93% w/w, 298.2 mmol)
in isopropyl acetate (480 mL) was added Novozym 435 (3 g).
The mixture was stirred at 20 °C for 12 h followed by filtration
and rinsing with isopropyl acetate (150 mL). The organic layer
was washed with 2 M HCl (aq., 200 mL). The aqueous layer was
extracted with isopropyl acetate (3 × 150 mL), and the pooled
EXPERIMENTAL SECTION
■
All materials were purchased from commercial suppliers and
used as such without further purification. RhAl O was purchased
from Johnson Matthey. Novozym 435 was purchased from
Novozymes A/S Denmark (activity 10000 PLU/g). All reactions
were performed under an atmosphere of nitrogen. Large-scale
reactions were performed using glass reactors equipped with an
overhead stirrer. IPCs were recorded using H NMR analysis of
the crude reaction mixtures. Assays were determined by H
NMR integration using benzyl benzoate as internal standard.
Enantiomeric purity of 4 was determined using HPLC [Lux C2
2
3
organic layer was washed with Na CO solution (aq., sat, 50 mL).
2
3
The aqueous layer was extracted with isopropyl acetate (100
mL), and the pooled organic layer was then washed with water
1
1
(
50 mL) followed by concentration to give the title crude
compound as a colorless solid (42.59 g, 80% w/w, 150 mmol,
0% corrected yield) with 86% ee. Water was azeotropically
5
removed using isopropyl acetate (2 × 200 mL). To the above
crude compound was added c-pentylmethyl ether (70 mL) and c-
hexane (140 mL). The suspension obtained was heated to 70 °C
after which a homogeneous solution was obtained. The mixture
was slowly allowed to attain 20 °C in the oil bath. Seeding crystals
were added at 50 °C which initiated crystallization. The resulting
suspension was stirred for 2 days followed by filtration and
washing of the solid with 33% c-pentylmethyl ether/c-hexane (2
× 30 mL). After drying under reduced pressure at 40 °C, the title
compound was obtained as a colorless solid (26.8 g, 99% w/w,
column (4.6 × 150 mm), 15% 2-propanol in CO 120 bar as
2
eluent]. High resolution mass spectrometry was performed on a
Waters LCTP instrument. NMR measurements were performed
using a Bruker Avance III spectrometer. FT-IR measurements
were performed using a Mettler Toledo React IR 15 instrument
integrated with a DiComp (diamond) FT-IR probe.
(
cis)-3-(Isopropoxycarbonyl)cyclohexanaminium
Chloride (cis-3 × HCl). To a suspension of 3-aminobenzoic
acid 5 (10 g, 72.9 mmol) in 2-propanol (100 mL) was added a
solution of HCl (5−6 M in 2-propanol, 16 mL) followed by the
addition of Rhodium (5% on Al O , 0.75 g, 0.36 mmol), and the
20
118 mmol, 40% yield), ee >99%. [α]
= +58 (c = 1, CH
H NMR (400 MHz, CDCl ) δ 0.99−1.12 (m, 1H), 1.18 (d, J =
3
CN).
2
3
D
3
1
mixture was put under 8 bar pressure of hydrogen at 100 °C for 3
days after which full conversion of the hydrogenation had been
obtained. Additional HCl (5−6 M in 2-propanol, 5 mL) was
added, and the mixture was allowed to stir at 70 °C in a sealed
steel vessel for an additional 2 days after which full conversion to
the corresponding isopropylester had been obtained. The
mixture was allowed to attain 20 °C and filtered through a
Celite filter followed by rinsing with 2-propanol (2 × 10 mL) and
last water (2 × 20 mL). The filtrate was concentrated to give a
colorless solid. Water was azeotropically removed using 2-
propanol (50 mL). The crude mixture was suspended in 100 mL
of hot (70 °C) isopropyl acetate. The mixture was then slowly
allowed to attain 20 °C and was stirred for an additional 15 min.
6.3 Hz, 6H), 1.2−1.44 (m, 3H), 1.75−1.91 (m, 3H), 1.93 (s,
3H), 2.09−2.23 (m, 1H), 2.34 (tt, J = 3.6, 11.8 Hz, 1H), 3.65−
3.87 (m, 1H), 4.94 (hept, J = 6.3 Hz, 1H), 5.74 (d, br, J = 7.3 Hz,
1
3
1H). C NMR (101 MHz, CDCl ) δ 21.8 (2C), 23.5, 24.1, 28.3,
3
+
32.5, 35.1, 42.4, 47.6, 67.6, 169.3, 174.7. HRMS (ESI ): [M +
H] m/z Calcd for C H NO 228.1600; Found 228.1589.
+
12
22
3
(1S,3R)-Isopropyl 3-Acetamidocyclohexanecarboxy-
late 4. Starting from an impure cis/trans mixture of 3, in a 25
L glass reactor, to a solution of isopropyl 3-aminocyclohex-
anecarboxylate 3 (2120 g, 74% cis isomer w/w, 8.47 mol) in
isopropyl acetate (17 L) was added Novozym 435 (106 g). The
mixture was stirred (150 rpm) at 20 °C for 18 h after which a 51%
D
DOI: 10.1021/acs.oprd.6b00146
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
conversion had been obtained. The mixture was filtered followed
by rinsing with isopropyl acetate (1 L). An aqueous solution of
ACKNOWLEDGMENTS
I thank our Separation Science Laboratory group for
determination of enantiomeric purity and Dr. Tomas Leek for
HRMS analyses. I also thank Dr. Hans Emtenas and Dr. Peter
̈
Smith for their comments on the manuscript.
■
3.8 M HCl (3 L) was added. The aqueous layer was extracted
with isopropyl acetate (3 × 1 L). The pooled organic layer was
extracted with water (500 mL), Na CO solution (aq., sat, 1 L),
2
3
and last water (500 mL) followed by polish filtration and
concentration to give the crude title compound as a colorless
solid (731 g, 72% w/w, 27% corrected yield, 87% ee). Water was
azeotropically removed using isopropyl acetate (2 × 1.2 L). To
the above crude compound was added c-pentylmethyl ether (2
L) and c-hexane (4 L). The suspension was heated to 45 °C after
which a homogeneous solution was obtained. The mixture was
slowly allowed to attain 16 °C. Seeding crystals were added at 40
REFERENCES
■
(
1) (a) Yu, H.; Sheng-Liang, Y.; Jin, Y.; Jin, Z.; Jin-Gen, D. Chin. J.
Chem. 2006, 24, 795−799. (b) Hongyu, T.; Baoguo, S. Huaxue Tongbao
2
004, 67, 934−937. (c) Allan, R. D.; Johnston, G. A. R.; Twitchin, B.
Aust. J. Chem. 1981, 34, 2231−2236. (d) Murahashi, S.-I.; Taniguchi, Y.;
Imada, Y.; Tanigawa, Y. J. Org. Chem. 1989, 54, 3292−3303. (e) Winkler,
M.; Knall, A. C.; Kulterer, M. R.; Klempier, N. J. Org. Chem. 2007, 72,
7
2
1
423−7426. (f) Yeung, Y.-Y.; Hong, S.; Corey, E. J. J. Am. Chem. Soc.
006, 128, 6310−6311. (g) Boaz, N. W. Tetrahedron: Asymmetry 1999,
0, 813−816. (h) Liang, J.; Cochran, J. E.; Dorsch, W. A.; Davies, I.;
°C which initiated a rapid crystallization. The suspension was
stirred for 3 days followed by filtration and washing of the solid
using 33% c-pentylmethyl ether/c-hexane (3 × 500 mL). After
drying at 43 °C under reduced pressure, the title compound was
obtained as a colorless solid (379 g, 100% w/w, 20% yield) with
ee >99%. Analytical data were in accordance with those given
above.
Clark, M. P. Org. Process Res. Dev. 2016, 20, 965−969.
(
6
(
2) Amorín, M.; Castedo, L.; Granja, J. R. Chem. - Eur. J. 2005, 11,
543−6551.
3) Badland, M.; Bains, C. A.; Howard, R.; Laity, D.; Newman, S. D.
Tetrahedron: Asymmetry 2010, 21, 864−866.
(4) (a) Gonzalez-Sabín, J.; Gotor, V.; Rebolledo, F. Tetrahedron:
Asymmetry 2005, 16, 3070−3076. (b) Gonzalez-Sabín, J.; Gotor, V.;
(
1S,3R)-3-Acetamidocyclohexanecarboxylic Acid 1.
With a mantle temperature set at 20 °C, to a solution of
1S,3R)-isopropyl 3-acetamidocyclohexanecarboxylate 4 (426.3
́
́
Rebolledo, F. Tetrahedron: Asymmetry 2004, 15, 481−488. (c) Gotor, V.
Bioorg. Med. Chem. 1999, 7, 2189−2197. (d) Skupinska, K. A.;
McEachern, E. J.; Baird, I. R.; Skerlj, R. T.; Bridger, G. J. J. Org. Chem.
(
g, 98% w/w, 1.837 mol) in MeOH (500 mL) was slowly (10
min) added a solution of sodium hydroxide (3.8 M, aq., 726 mL,
2
003, 68, 3546−3551. (e) Gonzal
Tetrahedron: Asymmetry 2002, 13, 1315−1320. (f) Choi, Y. K.; Kim, M.
J.; Ahn, Y.; Kim, M.-J. Org. Lett. 2001, 3, 4099−4101. (g) Lopez-Serrano,
P.; Jongejan, J. A.; van Rantwijk, F.; Sheldon, R. A. Tetrahedron:
chez, V. M.; Rebolledo, F.;
́
ez-Sabín, J.; Gotor, V.; Rebolledo, F.
2.757 mol). The reaction temperature increased to 30 °C during
this addition.The mixture was stirred for 2 h at 20 °C after which
full conversion to the title acid had been obtained. To the mixture
was added a 37% aqueous solution of HCl (∼250 mL) until pH
́
Asymmetry 2001, 12, 219−228. (h) San
́
∼
1 was obtained. EtOAc (1 L) was added which resulted in 2
Gotor, V. J. Org. Chem. 1999, 64, 1464−1470.
clear phases. The aqueous layer was extracted with EtOAc (4 ×
00 mL). The pooled organic layer was concentrated to a
colorless solid. Water was azeotropically removed using EtOAc
2 × 500 mL). Methylisobutylketone (MIBK, 700 mL) was
(5) Starting from commercially available 1S,3R-3-aminocyclohexane-
carboxylic acid (TCI, Lot QVG2I-JA), esterification of this using 2-
propanol/HCl followed by acetylation using AcCl/Pyr, furnished
compound 4 which showed the same retention time on chiral HPLC
with that of a sample obtained from the sequence described in Scheme 3.
5
(
added. The mixture was heated to to 60 °C, and the suspension
was then slowly allowed to attain 20 °C followed by stirring for an
additional 18 h. The mixture was filtered, and the solid collected
was washed with MIBK (350 mL) followed by drying under
reduced pressure at 40 °C for 2 h. This yielded the title acid as a
colorless solid (355.0 g, 92% w/w, 1.763 mol, 96% yield) with
(
1
(
6) Chen, C.-S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J. J. Am. Chem. Soc.
982, 104, 7294−7299.
7) (a) ref 3.. (b) Greenstein, J. P.; Wyman, J., Jr. J. Am. Chem. Soc.
1
938, 60, 2341−2347. (c) Reference 2. (d) Fotouhi, N.; Gillespie, P.;
Guthrie, R. W.; Pietranico-Cole, S. L.; Yun, W. (Hoffmann-La Roche
Inc., Nutley, NJ) Diaminopropionic acid derivatives. U.S. Patent US
2
0
1
NaCl as the major impurity. [α]D = +58.4 (c = 1, CH CN). H
6,331,640, Dec 18, 2001. (e) Rodríguez-Vaz
L. F.; Amorín, M.; Granja, J. R. Eur. J. Org. Chem. 2013, 2013, 3477−
493. (f) Mazur, R. H.; Tyner, D. A.; Hallinan, E. A. (G. D. Searle & Co.)
Cyclohexyl and phenyl substituted enkephalins. U.S. Patent US
́
quez, N.; Salzinger, S.; Silva,
3
NMR (400 MHz, MeOD) δ 1.09−1.5 (m, 4H), 1.82−1.91 (m,
3
2
H), 1.93 (s, 3H), 1.94−2.01 (m, 1H), 2.08−2.2 (m, 1H), 2.40
13
(
tt, J = 3.5, 12.3 Hz, 1H), 3.68 (tt, J = 3.8, 11.8 Hz, 1H). C NMR
4
,407,746, Oct 4, 1983.
(
1
1
101 MHz, DMSO) δ 22.8, 24.0, 28.1, 32.0, 35.1, 41.8, 47.1,
−
(8) Using a small excess of HCl, it was found that the hydrogenation
68.4, 176.1. HRMS (ESI ): [M − H] m/z Calcd for C H NO
9
14
3
was complete within three days whereas the esterification only had
reached ∼20% conversion. To obtain full conversion of the
esterification, additional HCl was added followed by heating for an
additional two days.
84.0979; Found 184.0982.
ASSOCIATED CONTENT
■
(9) It was found that a high pH (>10.9) was required to convert the
* Supporting Information
S
HCl salt to the corresponding free amine. Extractions with Na CO (aq.,
2
3
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.oprd.6b00146.
sat) solutions sometimes gave incomplete conversions, and con-
sequently, some of this HCl salt ended up in the organic layer as
impurity.
1H NMR, 13C NMR, HRMS, and chiral HPLC analyses
(
PDF)
AUTHOR INFORMATION
■
*
Corresponding Author
E-mail: staffan.po.karlsson@astrazeneca.com.
Notes
The authors declare no competing financial interest.
E
DOI: 10.1021/acs.oprd.6b00146
Org. Process Res. Dev. XXXX, XXX, XXX−XXX