Enzymatic desymmetrization route to ethyl [3-(2-amino-2-methylpropyl) phenyl]acetate

Article abstract of DOI:10.1021/op200108k

An efficient process to ethyl [3-(2-amino-2-methylpropyl)phenyl]acetate 6 has been developed. Key steps include a novel enzymatic desymmetrization of diester 2 and a Ritter reaction between alcohol 4 and chloroacetonitrile, followed by chemoselective deprotection with thiourea.

Full text of DOI:10.1021/op200108k

ARTICLE
pubs.acs.org/OPRD
Enzymatic Desymmetrization Route to
Ethyl [3-(2-Amino-2-methylpropyl)phenyl]acetate
Pieter D. de Koning,*,† Iain R. Gladwell,† Natalie A. Morrison,† Ian B. Moses,† Maninder S. Panesar,†
Alan J. Pettman,† Nicholas M. Thomson,† and Daniel R. Yazbeck‡
†Research API, Pfizer Global Research and Development, Sandwich Laboratories, Ramsgate Road, Sandwich, Kent CT13 9NJ, U.K.
‡Pfizer Global Research and Development, 10578 Science Center Drive, San Diego, California 92121, United States
ABSTRACT: An efficient process to ethyl [3-(2-amino-2-methylpropyl)phenyl]acetate 6 has been developed. Key steps include a
novel enzymatic desymmetrization of diester 2 and a Ritter reaction between alcohol 4 and chloroacetonitrile, followed by
chemoselective deprotection with thiourea.
’ INTRODUCTION
During the development of PF-00610355 (Figure 1), a novel
once-daily inhaled β2-adrenoreceptor agonist for the treatment
of asthma and COPD,1 we required significant quantities of
methyl [3-(2-amino-2-methylpropyl)phenyl]acetate 6a. Due to
the short timelines, we decided to use the Medicinal Chemistry
route (Scheme 1, steps a, c, and eÀg) to meet this initial demand;
Figure 1. Structure of PF-00610355, a novel once-daily β2-adrenor-
however, for convenience we decided to prepare the ethyl ester 6
eceptor agonist.
rather than the methyl ester 6a as the preceding step is conducted
in ethanol.1
and purity (84%) after crystallization from toluene/heptane.
Deprotection of chloroacetamide 5 with thiourea in a mixture
of acetic acid and ethanol afforded a solution of the intermediate
amino acid 6b, after filtration of the byproduct 8.3 Finally, 6b was
converted to the ethyl ester 6 by addition of concentrated sulfuric
acid to the ethanolic solution obtained after filtration.
’ RESULTS AND DISCUSSION
Prior to scaling up, we examined the route in the lab, which
highlighted some key concerns, the low-yielding (27%) and
laborious sequence used to access monoester 3 via equilibration
of the diester 2 with diacid 1 in acidic ethanol and the Ritter
reaction between chloroacetonitrile and alcohol 4, followed by
deprotection with thiourea.2 However, small-scale laboratory
runs and process safety testing indicated that these steps were
safe to operate and provided an acceptable quality product.
Although diacid 1 is commercially available, we chose to
prepare this through hydrolysis of the readily available dinitrile
7 (Scheme 1, step b). Treatment of the diacid 1 with 1.2 equiv of
ethanol in acidic THF (Scheme 1, step d) afforded an equilibrium
mixture of diacid 1, monoester 3, and diester 2 similar to that
obtained using the original process, thus removing the need to
prepare the diester 2 and then equilibrate it with diacid 1
(Scheme 1, steps a and c). This modified process afforded an
improved 39% yield of the monoester 3; however, the product
purity was moderate (typically 90%, the remainder being mainly
a mixture of diacid 1 and diester 2).
Selective conversion of monoester 3 to the desired tertiary
alcohol 4 was achieved by reaction with 3 equiv of methyl
magnesium chloride in high yield (93%) but low purity (64%
by HPLC analysis). Due to this low purity, crystallization of the
alcohol 4 was not possible, and the crude product was taken
directly to the Ritter step. Slow addition of concentrated sulfuric
acid to a mixture of alcohol 4 and chloroacetonitrile in acetic acid
provided the desired chloroacetamide 5 in moderate yield (63%)
While this process worked reasonably well on lab scale (up to
∼150 g, ∼70% yield), it performed poorly on kilogram scale, and
isolation of 6 of suitable purity proved impossible due to the high
impurity burden. To recover as much material as possible from
the crude product, a salt screen was conducted, and fortuitously
di-p-toluoyl-(L)-tartaric acid (DTTA) in acetonitrile (MeCN)
was found to provide a crystalline salt 6c that afforded an
excellent purge of impurities. Importantly, given the variable
purity of the in-going 6, the process was tolerant of a wide
variation in acidÀbase stoichiometry, consistently providing the
1:1 salt 6c. The yield was usually around 70%, but this was highly
dependent on the quality of the in-going material, and in some
cases only 35À40% yield could be achieved. This enabled us to
recover sufficient 6 (as the DTTA salt 6c) from the crude reaction
mixtures to advance the development of PF-00610355; however,
this process was clearly not acceptable for future material demands.
After a careful analysis of the campaign, the following areas
were identified for further investigation.
1. The sequence used to prepare monoester 3. While this
process worked, it was low yielding and labor intensive, and
the moderate purity 3 obtained had a significant impact on
Received: April 20, 2011
Published: June 24, 2011
r
2011 American Chemical Society
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Scheme 1a Initial Route to 6
a Reagents and Conditions. (a) EtOH, H2SO4, reflux; (b) 6 M HCl, reflux; (c) 1 + 2, EtOH, 1,4-dioxane, HCl, reflux; (d) 1, EtOH (1.2 equiv), HCl,
THF, 50 °C; (e) MeMgCl, THF; (f) chloroacetonitrile, H2SO4, AcOH; (g) (i) thiourea, AcOH, EtOH, reflux; (ii) HCl, MeOH, reflux (to 6a); (h) (i)
thiourea, AcOH, EtOH, reflux (to 6b); (ii) H2SO4, EtOH, reflux (to 6); (iii) DTTA, MeCN (to 6c).
a
the downstream steps as the impurities and their fate
products were difficult to purge.
Table 1. Enzymes Showing Selectivity Towards Formation
of Monoester 3
2. The lack of purification points in the sequence resulted in
an unacceptably high impurity burden being carried into
the latter steps.
3. The deprotectionÀesterification sequence generated signif-
icant quantities of highly colored impurities and provided
poor quality material with inconsistent yield, necessitating
purification via crystallization of the DTTA salt 6c.
enzyme
diacid 1 (%)monoester 3 (%)diester 2 (%)
Mucor miehei Esterase
0
0
100
60
0
40
50
80
30
Rhizomucor miehei Lipase
Thermomuces lanoginosus Lipase
Penicillin G Acylase
0
50
0
20
Candida antarctica Lipase B
a Reactions screened at 10 g/L in pH 7.5 potassium phosphate buffer
with 10 wt % enzyme. Conversion was measured after 16 h.
54
16
From an examination of the literature, it was evident that
enzymatic hydrolysis potentially offered a highly selective route
to the desired monoester 3.4 Hydrolase enzymes, in particular,
pig liver esterase, have found extensive use in the desymmetriza-
tion of prochiral diesters to produce chiral monoesters; however,
only a few examples of their use for the selective monohydrolysis
of achiral diesters have been published.5
A wide range of readily available hydrolase enzymes were scr-
eened6 for the chemoselective monohydrolysis of diester 2, and
the best hits from this screen are summarized in Table 1. Im-
mediately apparent was that there were several enzymes that ap-
peared to offer significant selectivity for the desired monoester 3.
From this initial screen, Thermomuces lanoginosus lipase was
selected for further development as it was highly selective, in
addition to being cheap and readily available as a 100 KLU/g
solution (Lipolase 100 L). Furthermore, we had significant inter-
nal experience with the use of this enzyme and how best to
optimize the reaction conditions.7 Since this ultimately provided
a successful, cost-effective route to prepare monoester 3, none of
the other enzymes were investigated further.
sodium hydroxide until reaction completion (usually around 16 h
on lab scale, up to 48 h on larger scale).
Since chemical hydrolysis to the diacid 1 occurs at higher pH,
careful pH control is required. Similarly, using more concentrated
solutions of sodium hydroxide results in higher levels of diacid 1,
presumably resulting from chemical hydrolysis in localized high
pH areas during the addition of hydroxide to the reaction mixture.
As it proved simpler to separate residual diester 2 from monoester
3 (rather than diacid 1), the reaction was stopped at ∼95%
conversion to minimize the levels of diacid 1 present.
Once the reaction was complete, the pH was adjusted to ∼4,
and the monoester 3 was extracted into ethyl acetate. Removal of
the enzyme debris was required at this stage; otherwise, subse-
quent separations were extremely slow. On large scale, this was
achieved by filtration through a Gauthier filter (while successful
on lab scale, filtration through a Celite pad was not effective on
pilot plant scale). Purification of crude 3 was achieved through an
acidÀbase cycle (mainly to remove diester 2), and the product
was isolated as a toluene concentrate for use in the next step. The
yield was generally high on lab scale (around 80%), but initial
scale-up attempts were less successful, with only ∼60% yield of
lower purity product obtained (up to 10% diacid 1 was the main
impurity). This was largely due to pH overshoots during the
reaction resulting in chemical hydrolysis to diacid 1 and ineffec-
tive removal of enzyme debris resulting in poor phase separations
and significant material losses. Once these problems had been
identified and resolved, we were able to consistently obtain good
quality monoester 3 in around 80% yield.
These initial screening conditions were then scaled up to ∼1 g
input, and the concentration was increased to a more practical
250 g/L. Under these conditions, the reaction proceeded ex-
tremely slowly (50% conversion in 4 days); however, upon
examination of a range of different buffers, calcium acetate was
found to be extremely beneficial. Using a pH 7 calcium acetate
buffer on a 1 g scale, full conversion to monoester 3 was achieved
in ∼16 h at a concentration of 250 g/L.
After further process development, the optimal reaction con-
ditions were to suspend diester 2 in a mixture of Lipolase
(0.32 mL/g; approx 32 KLU/g 2) and 0.2 M Ca(OAc)2 buffer
(4 mL/g) at ambient temperature (around 20 °C) and then to
maintain the pH between 5.5 and 6.8 through addition of 1 M
To identify suitable purification points, all the intermediates
in the process were evaluated in both crystallization and salt
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Scheme 2a Optimized Route to 6
Esterification of chloroacetamide 5 in acidic ethanol afforded
the desired protected amino ester 9, which was isolated in excel-
lent purity (>95%) by crystallization from toluene/heptane. Sub-
sequent deprotection of 9 with thiourea in a mixture of ethanol
and acetic acid proceeded smoothly, and after filtration to
remove the precipitated byproduct 8, concentration afforded
crude 6 (as the acetate salt). Analysis of this indicated that the
crude material was virtually pure (in contrast to that prepared by
the previous route), and after partitioning between 2 M aq
Na2CO3 and DCM and concentration of the organic phase, pure
6 was obtained in 73% yield. While suitable quality 6 could be
obtained from this process, it was considered advantageous to
isolate a solid, so the di-p-toluoyl tartrate salt 6c identified
previously was prepared. This optimized procedure was then
scaled-up in our kilo-lab, delivering over 10 kg of tartrate salt 6c
in an excellent and reproducible (two batches) 64% overall yield
from 4 (Scheme 2).
a Reagents and Conditions. (a) Chloroacetonitrile, H2SO4, AcOH,
DCM; (b) H2SO4, EtOH, reflux; (c) (i) thiourea, AcOH, EtOH, reflux;
(ii) aq Na2CO3, DCM; (d) DTTA, MeCN.
screens. From these screens, crystallization of alcohol 4 was
found to be an important purification point. Crystallization from
toluene afforded a reasonable purge of impurities (purity in-
creased from ∼65% to ∼85%), as long as care was taken to
remove any residual THF and acetic acid, arising from hydrolysis
of isopropyl acetate, from the mixture. Fortunately, thiswas readily
accomplished by azeotropicdistillation from the extraction solvent
(isopropyl acetate, containing some THF) to toluene. On scale-
up, this modified isolation process delivered crystalline alcohol 4
in an acceptable 68% yield, although still of relatively low purity
(around 85%).
Having developed a reliable process to prepare sufficiently
pure alcohol 4, the focus then shifted to the conversion of alcohol
4 to amine 6. The RitterÀdeprotectionÀesterification sequence
was identified as one of the main sources of product degradation
and impurity generation in the initial synthesis.
In conclusion, a significantly improved process to monoester 3
has been developed through a novel enzymatic desymmetriza-
tion route. In addition, through a combination of step-reordering
and process development, an effective route to amino ester 6 has
been demonstrated on kilogram scale.
’ EXPERIMENTAL SECTION
Reactions Were Monitored by HPLC as Follows.
Column Phenomenex Luna 3μ Phenyl-hexyl 50Â2:0 mm
Injection 2 μL
Flowrate 0:8 mL=min
Solvents Solvent A-H2O : MeCN : TFA 1000 : 25 : 1
Solvent B-H2O : MeCN : TFA 25 : 1000 : 1
As a result of the high concentration of the Ritter reaction
(2 mL/g AcOH), there was a considerable scope for the gene-
ration of localized “hot spots” during the very exothermic addi-
tion of concentrated sulfuric acid to the reaction mixture; this was
thought to be one source of impurity generation. The subsequent
thiourea-mediated deprotection (in acetic acid/ethanol) gave
amino acid 6b that proved challenging to isolate and purify and
so was telescoped crude into the esterification, resulting in signi-
ficant impurity generation from thiourea-related residues, a pro-
blem that was exacerbated on scale.
To reduce the amount of acid used in the Ritter reaction and
allow for better temperature control, the use of an inert cosol-
vent, dichloromethane (DCM), was examined. Conducting the
reaction in 5 mL/g of DCM proved successful and enabled the
quantities of acid to be significantly reduced (3 equiv of AcOH
and 2 equiv of H2SO4 instead of 2 mL/g and 1.5 mL/g,
respectively). This reduction in acid level greatly simplified the
workup, and reasonable quality 5 (85À90% pure) was isolated in
65À70% yield after crystallization from toluene/heptane.
Recognizing that separating the deprotection and esterifica-
tion steps, preferably by isolating and purifying an intermediate,
might reduce the high impurity burden, the step-reordered
sequence (esterification, then deprotection) was examined
(Scheme 2), as purification of the intermediate protected amino
ester 9 was likely to be more facile than purification of amino acid
6b. In addition, a range of alternative bidentate reagents (for
example, guanidine and 1,2-ethylenediamine) were screened as
thiourea8 replacements in the deprotection step; however, none
of these offered a viable alternative and were not pursued.9
Detection Diode array at 210, 225, and 254 nm
1,3-Benzenediacetic Acid 1. 1,3-Phenylenediacetonitrile 7
(40 kg; 256.1 mol) was added to a mixture of 37% hydrochloric
acid (160 L) and water (160 L), and the resulting suspension was
heated to 100 °C. After 19 h, HPLC analysis indicated that the
reaction was complete. The reaction mixture was cooled to
20 °C, and the solid was isolated by filtration, washed with water
(2 Â 80 L), and dried under vacuum at 50 °C to give the diacid 1
as a fine white solid (45.55 kg; 92%), identical to commercially
available material.
1,3-Benzenediacetic Acid, 1,3-Diethyl Ester 21. Concen-
trated sulfuric acid (1.82 L) was charged to a suspension of 1
(45.55 kg; 234.6 mol) in EtOH (455.5 L), and the resulting mix-
ture was heated to reflux. After 20 h, HPLC analysis showed
complete reaction. The mixture was concentrated by distillation
under vacuum to remove ethanol, and then toluene (136.5 L)
was added. The toluene solution was washed with 5% aqueous
sodium hydrogen carbonate (91 L) to remove any residual 1 and
was then concentrated down to ∼100 L (roughly 1 mL/g
toluene) and was used directly in the next step. An aliquot was
concentrated to dryness to give 2 as a viscous liquid, and analy-
tical data were identical to that published in the literature (ref 1).
1,3-Benzenediacetic Acid, 1-Ethyl Ester 31. Lipolase (9.4 L;
100 KLU/g solution) was added to a stirred solution of 0.2 M
calcium acetate (117.5 L), and the homogeneous solution was
stirred at room temperaturefor 30 min beforediester2 (∼1 mL/g
toluene solution; 29.35 kg of 2; 117.3 mol) was added. The
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Table 2. Solvent Timetable
mixture was extracted with iPrOAc (2 Â 18 L). The combined
iPrOAc extracts were washed with water (3 Â 18 L), after which the
solvent was exchanged to toluene by vacuum distillation. The volume
was adjusted to ∼20 L toluene by distillation; the solution was cooled
to 5 °C; and the resulting slurry was granulated for 2 h. The solid was
isolated by filtration, washing with toluene (3.59 L), and dried under
vacuum at 50 °C to give 4 as a white solid (2.29 kg; 68% un-
corrected). HPLC analysis indicated approximately 85% purity. Mp
58 °C. 1H NMR (400 MHz, DMSO-d6) δ: 1.06 (6H, s), 2.64 (2H,
s), 3.52 (2H, s), 4.28 (1H, s), 7.09 (3H, m), 7.20 (1H, s). 13C
NMR (100 MHz, DMSO-d6) δ: 29.1, 40.8, 49.4, 69.3, 126.7,
127.4, 128.7, 131.3, 134.1, 138.9, 172.7. MS: m/z 209 [M + H]+.
{3-[2-(2-Chloroacetylamino)-2-methylpropyl]phenyl}acetic
Acid 51. Chloroacetonitrile (1.63 kg, 21.62 mol) was added to a
slurry of alcohol 4 (3.00 kg, 14.41 mol) in DCM (15 L). Acetic
acid (2.6 kg, 43.23 mol) was added, maintaining the temperature
between 5 and 10 °C. The resulting solution was treated with
concentrated sulfuric acid (2.83 kg, 28.82 mol), maintaining the
temperature between 5 and 10 °C. The mixture was warmed to
20 °C, and after 90 min the reaction mixture was added to cold
water (30 L), maintaining the temperature below 10 °C. The
mixture was stirred for 30 min at 5À10 °C and then at 20 °C for
30 min. The layers were separated, and the aqueous layer was
extracted with further DCM (15 L). The combined DCM layers
were distilled down to 8 L volume at atmospheric pressure. The
concentrate was treated with n-heptane (27 L) and toluene (3 L)
and concentrated in vacuo to remove residual DCM. The re-
sulting slurry was granulated at 20 °C for 2 h, and then the solid
precipitate was isolated by filtration, washed with n-heptane (2 Â
3 L), and dried under vacuum at 40 °C to give 5 as an off-white
solid (3.76 kg; 92%). Mp 92 °C. 1H NMR (400 MHz, DMSO-d6)
δ: 1.22 (s, 6H), 2.97 (s, 2H), 3.53 (s, 2H), 3.97 (s, 2H), 7.01 (m,
2H), 7.11 (m, 1H), 7.21 (m, 1H), 7.62 (s, 1H), 12.27 (s, 1H). 13C
NMR (100 MHz, DMSO-d6) δ: 26.7, 40.7, 43.1, 43.4, 53.5, 127.1,
127.6, 128.6, 131.4, 134.4, 137.9, 165.4, 172.6. MS: m/z 284/286
[M + H]+. Anal. Calcd for C14H18ClNO3: C, 59.26; H, 6.39; Cl,
12.49; N, 4.94. Found: C, 59.60; H, 6.36; Cl, 11.94; N, 4.51.
Ethyl 2-{3-[2-(2-Chloroacetamido)-2-methylpropyl]phe-
nyl}acetate 9. A solution of chloroacetamide 5 (3.76 kg, 13.24
mol) in ethanol (30.1 L) was treated with concentrated sulfuric acid
(130 g, 1.31 mol) and heated at reflux for 90 min. The cooled
solution was adjusted to ∼pH 5 using 1.0 M aqueous sodium
hydrogen carbonate solution (2.0 kg). The mixture was concen-
trated down to 8 L volume in vacuo, diluted with toluene (11.7 L),
and concentrated down to 12 L volume in vacuo. The concen-
trate was diluted with toluene (25.8 L) and washed with water
(22.6 L), and the aqueous layer was re-extracted with further
toluene (15.0 L). The combined toluene layers were concen-
trated down to 8 L in vacuo. The concentrate was held at 35 °C,
and n-heptane (15.0 L) was added, maintaining the temperature
above 30 °C. The mixture was cooled, and the resulting slurry
was granulated at 20 °C for 2 h. The solid precipitate was isolated
by filtration, washed with n-heptane (2 Â 3.76 L), and dried in
a vacuum oven at 40 °C to give ethyl ester 9 as a white solid
(3.15 kg; 76%). Mp 52 °C. 1H NMR (400 MHz, DMSO-d6) δ:
1.19 (t, J = 7.0 Hz, 3H), 1.22 (s, 6H), 2.95 (s, 2H), 3.59 (s, 2H),
3.94 (s, 2H), 4.07 (q, J = 7.0 Hz, 2H), 7.00 (m, 2H), 7.09 (d, J =
7.6 Hz 1H), 7.20 (t, J = 8.0 Hz, 1H), 7.59 (s, 1H). 13C NMR (100
MHz, DMSO-d6) δ: 14.1, 26.6, 40.3, 43.2, 43.4, 53.5, 60.2, 127.1,
127.7, 128.8, 131.3, 133.8, 138.0, 165.4, 171.1. MS: m/z 312/314
[M + H]+. Anal. Calcd for C16H22ClNO3: C, 61.63; H, 7.11; Cl,
11.37; N, 4.49. Found: C, 61.79; H, 7.03; Cl, 11.04; N, 4.30.
time (mins)
solvent A (%)
solvent B (%)
0.00
0.50
100.0
100.0
25.4
0.00
0.00
74.6
0.00
8.00
12.50
100.0
mixture was stirred at ambient temperature for 48 h, at which
point HPLC analysis indicated ∼95% conversion of diester 2 to
monoester 3 and minimal levels of diacid 1. During this time, the
pH was checked regularly and maintained between 5.5 and 6.8
(target 6.5) by addition of 1 M sodium hydroxide aliquots
(approx 5 L). Upon reaction completion, the pH was adjusted
to 3.5 with 1 M hydrochloric acid, and EtOAc was added (117 L).
The biphasic mixture was then filtered through a Gauthier filter
and separated. The aqueous layer was extracted with EtOAc (2 Â
117 L). The combined organic phase was extracted with
saturated aqueous sodium hydrogen carbonate (3 Â 150 L)
(CAUTION À significant effervescence and foaming, particu-
larly with first extraction). The combined aqueous extracts were
adjusted to pH 2 by addition of 2 M hydrochloric acid
(CAUTION À effervescence and foaming), and the resulting
mixture was extracted with toluene (2 Â 147 L). The toluene
extract was then concentrated to ∼40 L (about 1 mL/g toluene)
and was used directly in the next step. Estimated yield from
HPLC assay was 19.7 kg (76%). A sample was concentrated to
dryness to give 3 as an oil, and the analytical data were identical to
that in the literature (ref 1).
1,3-Benzenediacetic Acid, 1-Ethyl Ester 3 (Alternative
Procedure). Ethanol (85.4 g; 1.85 mol; 1.2 equiv) and 37%
hydrochloric acid (30 mL) were added to a solution of 1 (300 g;
1.54 mol) in THF (3.0 L), and the resulting thin suspension was
heated to 50 °C. Once the equilibrium mixture of diacid 1, diester
2, and monoester 3 had been reached (around 5À6 h), the
solvent was exchanged to toluene (1.5 L) by distillation. The
resulting suspension was stirred for 15 min, and then the solid
(diacid 1) was removed by filtration and washing with toluene
(300 mL). The combined filtrate and washes were extracted with
saturated sodium hydrogen carbonate (1.35 L, then 2 Â 300 mL)
(CAUTION À effervescence, particularly with first extraction).
The combined aqueous phase was adjusted to pH 6 with 2 M
hydrochloric acid (CAUTION À effervescence), and the result-
ing slightly milky solution was extracted with tert-butyl methyl
ether (1.2 L, 2 Â 600 mL). The combined organic extracts were
washed with water (600 mL), dried over magnesium sulfate, and
concentrated to dryness under vacuum to give the product 3 as a
pale straw-colored oil (134 g; 39%).
[3-(2-Hydroxy-2-methylpropyl)phenyl]acetic Acid 41. A
solution of monoester 3 (3.59 kg; 16.15 mol) in dry THF (36 L)
was prepared in a clean, dry, and nitrogen-inerted reactor. The
solution was cooled to 0À5 °C, and a solution of 1 M MeMgBr in
THF (56.53 L; 56.53 mol; 3.5 equiv) was added at such a rate as
to keep the temperature below 15 °C. (CAUTION À Exother-
mic reaction and methane evolution.) On completion of addition
(between 1 and 2 h), the cooling was removed, and the gray
suspension was warmed to 20 °C and held for 1 h, at which point
HPLC analysis indicated complete reaction. The slurry was
cooled to 0À5 °C, and then water (17.95 L) was added at such
a rate as to keep the temperature below 20 °C. The pH was
adjusted to ∼2 by addition of 5 M hydrochloric acid, and the
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Ethyl 2-[3-(2-Amino-2-methylpropyl)phenyl]acetate Hy-
drogen (2R,3R)-2,3-Bis[(4-methylbenzoyl)oxy]succinate 6c.
A solution of the chloroacetamide 9 (3.10 kg, 9.95 mol) in
ethanol (34.1 L) was treated with thiourea (0.91 kg, 11.93 mol)
and acetic acid (6.2 L), and the resulting mixture was heated at
reflux for 4 h. The mixture was cooled, and the precipitated solid
was removed by filtration and washed with ethanol (3.1 L). The
combined filtrate and wash were concentrated down to 8 L
volume in vacuo; toluene (31 L) was added; and the solution was
concentrated to 8 L in vacuo. This process was repeated with
toluene (24.8 L). The resulting mixture was treated with water
(9.3 L) and 2 M aqueous sodium carbonate solution (7.5 L), and
the product was extracted into DCM (31.0 and 15.5 L). The
combined DCM extracts were concentrated down to 8 L volume
at atmospheric pressure; MeCN (12.4 L) was added; and the
mixture was concentrated down to 8 L volume in vacuo. The con-
centrate was diluted with MeCN (24.8 L) and adjusted to 20 °C
before a solution of di-p-toluoyl-L-tartaric acid (3.65 kg, 9.45 mol)
in MeCN (18.6 L) was added. The resulting slurry was stirred for
15 h at 20 °C, and then the solid precipitate was isolated by
filtration, washed with MeCN (2 Â 6.2 L), and dried in a vacuum
ovenat50°C to givetartratesalt 6casa white solid(5.72 kg; 93%).
Mp 158 °C. 1H NMR (400 MHz, DMSO-d6) δ: 1.10 (s, 6H), 1.17
(t, J = 7.0 Hz, 3H), 2.34 (s, 6H), 2.78 (s, 2H), 3.63 (s, 2H), 4.06 (q,
J = 7.0 Hz, 2H), 5.61 (s, 2H), 7.03 (m, 2H), 7.15 (br. d, J = 7.8 Hz,
1H), 7.25 (t, J = 7.6 Hz, 1H), 7.30 (d, J = 8.2 Hz, 4H), 7.80 (d, J =
8.2 Hz, 4H), 8.30 (br s., 4H). 13C NMR (100 MHz, DMSO-d6) δ:
14.0, 21.1, 24.6, 24.6, 45.0, 53.4, 60.2, 72.4, 127.0, 127.7, 128.1,
128.9, 129.1, 129.3, 131.5, 134.2, 135.6, 143.6, 164.9, 168.2, 171.0.
MS: m/z 236 [M + H]+. Anal. Calcd For C34H39NO10: C, 65.69;
H, 6.32; N, 2.25. Found: C, 65.71; H, 6.32; N, 2.36.
’ ACKNOWLEDGMENT
We thank Stuart Field, Steve Fussell, Neal Sach, and Imelda
McCarthy for reaction screening; Watcharee Cooper, Isabelle
Lemaitre, and Guy Mathews for analytical support; Carlos
Martinez and Junhua Tao for their advice on screening and
optimization of enzymatic processes; Rob Bright and Fabrice
Salingue for their expertise and supervision during the transfer to
scale-up facilities; and David Entwistle for numerous fruitful
scientific discussions.
’ REFERENCES
(1) Glossop, P. A.; Lane, C. A. L.; Price, D. A.; Bunnage, M. E.;
Lewthwaite, R. A.; James, K.; Brown, A. D.; Yeadon, M.; Perros-Huguet,
C.; Trevethick, M. A.; Clarke, N. P.; Webster, R.; Jones, R. M.; Burrows,
J. L.; Feeder, N.; Taylor, S. C. J.; Spence, F. J. J. Med. Chem. 2010,
53, 6640.
(2) Jirgensons, A.; Kauss, V.; Kalvinsh, I.; Gold, M. R. Synthesis
2000, 1709.
(3) Bradley, P. A.; Carroll, R. J.; Lecouturier, Y. C.; Moore, R.;
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(4) For recent reviews, see: (a) Hudlicky, T.; Reed, J. W. Chem. Soc.
Rev. 2009, 38, 3117. (b) de Maria, D.; Garcia-Burgos, C. A.; Bargeman,
G.; van Gemert, R. W. Synthesis 2007, 1439.
(5) (a) Nair, R. V.; Shukla, M. R.; Patil, P. N.; Salunkhe, M. M. Synth.
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(6) Yazbeck, D. R.; Tao, J.; Martinez, C. A.; Kline, B. J.; Hu, S. Adv.
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(7) Martinez, C. A.; Hu, S.; Dumond, Y.; Tao, J.; Kelleher, P.; Tully,
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Ethyl [3-(2-Amino-2-methylpropyl)phenyl]acetate 61 (Alter-
native Procedure). A mixture of chloroacetamide 5 (151.4 g,
534 mmol), thiourea (48.7 g, 640 mmol), and acetic acid (303 mL)
in ethanol (1.5 L) was heated to reflux under a nitrogen atmosphere.
After 5 h the reaction mixture was cooled to 25 °C, and the resul-
ting suspension was concentrated to dryness in vacuo. Toluene
(900 mL) was added to the residue, and the resulting suspension
was concentrated to dryness. This process was repeated with fresh
toluene (900 mL). The residue was then suspended in ethanol
(1.5 L), stirred for 1 h, and then filtered. The filtrate was cooled in an
ice bath (temp 0À5 °C), and concentrated sulfuric acid (227 mL)
was added at such a rate as to keep the temperature below 15 °C.
Once the addition was complete, the mixture was stirred for 1 h at
20 °C. The resulting solution was concentrated in vacuo to remove
most of the ethanol, and then aqueous sodium hydrogen carbonate
was added to adjust the pH to around 9. The mixture was filtered,
and the solid residue was washed with water (300 mL) and EtOAc
(1.0 L). The combined biphasic filtrate and washes were separated,
and the aqueous layer was extracted with EtOAc (1.0 L and
500 mL). The combined EtOAc phase was dried over magnesium
sulfate, filtered, and concentrated in vacuo to give amino ester 6 as a
brown oil (89.5 g; 71%). 1H NMR (400 MHz, DMSO-d6) δ: 1.01
(s, 6H), 1.17 (t, J = 7.0 Hz, 3H), 2.64 (s, 2H), 3.62 (s, 2H), 4.07
(q, J = 7.0 Hz, 2H), 7.06 (m, 3H), 7.21 (m, 1H). 13C NMR (100
MHz, DMSO-d6) δ: 14.0, 28.2, 40.7, 48.7, 50.7, 60.1, 127.0, 127.7,
128.9, 131.5, 133.8, 138.0, 171.0.
(8) A recent MSDS lists the following hazards for thiourea: H302
Harmful if swallowed, H351 Suspected of causing cancer, H411 Toxic to
aquatic life with long lasting effects, H361d Suspected of damaging the
unborn child. While the reagent can be used with appropriate precau-
tions, an alternative, safer, reagent would be preferable.
(9) The best alternative was ethylenediamine which afforded ∼30%
deprotection, along with significant degradation.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: pieter.de.koning@pfizer.com.
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dx.doi.org/10.1021/op200108k |Org. Process Res. Dev. 2011, 15, 871–875