Practical convergent laboratory-scale synthesis of a CCR5 receptor antagonist

Article abstract of DOI:10.1021/op200259t

An efficient laboratory-scale synthesis has been developed for the selective CCR5 antagonist 1. The convergent route has a longest linear sequence of nine steps (15 steps overall), and has overall yields of 18-25%. The route has enabled the preparation of 550 g of 1.

Full text of DOI:10.1021/op200259t

ARTICLE
pubs.acs.org/OPRD
Practical Convergent Laboratory-Scale Synthesis of a
CCR5 Receptor Antagonist
Jason B. Crawford,*,† Gang Chen, Bryon Carpenter, Trevor Wilson, Jenny Ji, Renato T. Skerlj,‡ and
Gary J. Bridger
AnorMED, Inc., 200-20353 64th Avenue, Langley, British Columbia, Canada V2Y 1N5
S Supporting Information
b
ABSTRACT: An efficient laboratory-scale synthesis has been developed for the selective CCR5 antagonist 1. The convergent route
has a longest linear sequence of nine steps (15 steps overall), and has overall yields of 18ꢀ25%. The route has enabled the
preparation of 550 g of 1.
Scheme 1. Medicinal Chemistry Synthesis of 1
’ INTRODUCTION
The human immunodeficiency virus (HIV) gains entry to
human white blood cells via a receptor-mediated process invol-
ving one of two human chemokine coreceptors, CXCR4 or
CCR5. The receptors are 7-transmembrane G-protein-coupled
receptors (GPCR) and are expressed on the surface of a variety
of cells, including CD4+ T cells. Inhibition of the viral interac-
tion with the chemokine receptor, through the use of specific
antagonists, has been clinically validated as a therapeutic ap-
proach to treat HIV infection for both CCR51 and CXCR42,3
antagonists. We have previously described the discovery and
development of the CXCR4 chemokine receptor antagonist
AMD070,4,5 an oral agent that entered clinical development
for the treatment of T-(CXCR4)-tropic HIV-1 infected patients.3
In addition we have recently disclosed our efforts to indentify
novel CCR5 antagonists.6 Similarly, the discovery and develop-
ment of a number of CCR5 antagonists, including maraviroc
(Selzentry),7 vicriviroc,8 TAK-779,9 and the experimental drug
R05114436,10 have been described. Herein, we describe the
practical laboratory-scale synthesis of a novel selective CCR5
antagonist AMD15548 (1).11
The medicinal chemistry route to 1 is shown in Scheme 1, and
was the starting point for process development. The single chiral
centre originated from D-alanine (methyl ester), and was incor-
porated into the piperidone synthon (2) over several steps
(discussed below and in Scheme 2). In the initial medicinal
chemistry routes, the nitrile was reduced and the primary amine
protected (BOC). Commercial 3-cyanothiophene was reduced
with lithium aluminum hydride (LAH) to provide 3-aminomethyl
thiophene (3), which was coupled with 2 via standard reductive
aminationconditionsto provide4 (afteramine deprotection). The
substituted nicotinic acid derivative 5 was synthesized in a two
step chlorination/hydrolysis from the costly commercial pyridone.
A peptide coupling using 1-hydroxybenzotriazole (HOBt) and
1-ethyl-3-(3-dimethylaminopropyl)carboxdiimide (EDCI) with
4 provided amide 5, and the substituted urea was installed via
reaction with 1,10-carbonyldiimidazole (CDI) and methoxyl-
amine to give 1.
Received: September 20, 2011
Published: October 29, 2011
r
2011 American Chemical Society
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Scheme 2. Synthesis of ketone intermediate 2
Scheme 3. Scale-up synthesis of 5
’ RESULTS AND DISCUSSION
Hofmann elimination/conjugate addition sequence14 under
mild conditions to form 2 directly. Purification did require silica
gel chromatography to produce 2 in >99% chemical purity and
ee. Yields of 2 from commercial D-alanine methyl ester were
typically in the 35% range for 300ꢀ400 g batches.
Preparation of Piperdone 2. We briefly investigated the
possibility of preparing 2 via an asymmetric conjugate addition
between a protected piperidone and crotononitrile,12 but aban-
doned the strategy due to lack of initial success with establishing
enantioselectivity, coupled with the limited practical availability
of isomerically pure crotononitrile. Following the Med Chem
route of chemical reduction of commercial D-alanine methyl
ester, the reaction could be carried out at 20-L scale via addition
of the ester to a solution of lithium aluminum hydride in THF at
<20 °C (Scheme 2). A Fieser-type workup13 was employed—the
hydrated aluminum salts formed granular white solids which
were easily held in suspension. A solution of di-tert-butyl
dicarbonate was added directly to the reaction mixture, and the
aluminum salts were removed by filtration to give the product 7
in 83ꢀ93% yields as a solid. The conversion to mesylate 8 was
straightforward and typically was used immediately to form
the nitrile with sodium cyanide at ∼50 °C. At the completion
of the reaction, filtration through silica gel removed any polar
impurities and/or salts, and the amine 10 was liberated via
treatment with aqueous acid. Coupling of amine 10 with the
commercially available piperidone salt 11 occurred via a
Preparation of Nicotinic Acid 5. The medicinal chemistry
route to 5 was only capable of supplying single gram amounts as
the commercial pyridone reagent was cost prohibitive (>$250/g)
and lead times were typically greater than 6 weeks. As an
alternative, based on availability and cost, 2-amino-4,6-dimethyl-
pyridine was selected as a reagent, and a route was developed
(Scheme 3). Rather than using the typical N-bromosuccinimide
(NBS) to brominate the pyridine, we instead employed 5,5-
dimethyl-1,3-dibromohydantoin15 (DBH). DBH gave a cleaner,
more regioselective bromination to give 12. With NBS, small
amounts of 3,5-dibrominated material (1ꢀ5%), as well as some
1
bromination of the methyl groups, could be detected by H
NMR.16 Once the reaction was complete, polar impurities were
removed via a filtration through silica gel, and 12 was isolated via
crystallization.
The Sandmeyer conversion of 12 to 1317 used pyridine-HCl/
catalytic concentrated HCl rather than stoichiometric amounts
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of HCl. A catalytic amount of HCl was added to the reaction
mixture after all other reagents were added, and it provided a
cleaner overall reaction profile, a faster and more predictable
reaction rate, and higher and more reproducible yields. For this
reaction, a range of 0ꢀ5% v/w concentrated HCl (relative to 12)
was investigated, with 2% being optimal (i.e. complete predict-
able reaction in 1.5 h). Silica gel filtration was used to remove
polar impurities, and the product 13 wasisolated asacrystallinesolid
in 54ꢀ60% yields following trituration. The regioselective
halogen/metal exchange, originally carried out on small scale
at ꢀ78 °C with n-BuLi, could be instead carried out on scale at
0 °C using the Grignard reagent isopropylmagnesium chloride
and LiCl to regioselectively obtain the acid 5, after quenching
with CO2. The noncryogenic conditions were reliable in our
hands and provided 5 in 400-g batches as a crystalline solid
isolated from water.
Preparation of Synthons 3 and 14. Commercial 3-cyano-
thiophene was readily reduced with LAH in THF to provide the
amine 3. The Fieser workup gave granular solids which could be
easily removed by filtration, and 3 could be isolated in >90% GC
purity through fractional vacuum distillation in 85ꢀ92% yields
(Scheme 4).
Similarly, a single transformation from p-nitrophenyl chloro-
formate and methoxylamine HCl provided 14, which was isol-
able as a crystalline solid via filtration directly from the reaction
mixture. In general, batches of 14 were produced within a week of
being required for subsequent coupling reactions.
thiophene 3 and ketone 2 provided the amine 15 in 82ꢀ90%
yields at 500-g scale. During the workup, the product 15 was
initially driven into the aqueous layer (at pH ≈ 4) via addition of
acid, and nonbasic impurities were efficiently removed in the
organic layer. A subsequent pH swing to 8 enabled the extraction
of 15 into dichloromethane. Crystallization of the residue from
hexanes and MTBE enabled isolation of 15 in >98% purity. The
reduction of the nitrile was carried out with solid LAH in MTBE,
which provided amine 4, effectively eliminating the need for an
intermediate BOC protection (as was used in the Med Chem
route). The amine 4 was coupled, without further purification,
with the acid 5 using EDCI and HOBt.11 As was the case with 15,
a pH swing was used to remove nonbasic impurities. Following
extraction and concentration, the product 6 could be isolated via
MTBE crystallization in >98% purity.
The incorporation of the urea functionality was accomplished
initially by using carbonyldiimidazole (with methoxylamine).11
The coupling yields were variable, however, and chromatogra-
phic purification was required. An improvement was the reac-
tion of the amine 6 with the carbamate reagent 14. After addi-
tion of water, and acidification to pH ∼1.5, the p-nitrophenol
side product was removed by a wash with ether. The product 1
was then isolated from the aqueous layer with dichloro-
methane at pH 8. A silica gel plug filtration permitted the pure
product 1 to be isolated as an amorphous freebase solid upon
concentration.
Overall, these routes were used to provide a single batch of
550 g of 1 for toxicological studies. It should be noted that the
procedures contained herein represent an initial streamlining
of the process with the goals of decreasing costs, removing
cryogenic conditions, decreasing the number of chromato-
graphic steps, and providing a basis for further scale-up efforts.
Significant further development would be required to enable a
safe and practical multikilogram process, including: calorimetric
hazard analysis, replacement of some process solvents (MTBE
for ether and heptanes for hexanes, for example), removal or
replacement of chemical desiccants, further isolation of crystal-
line solids from process streams (instead of concentration to
dryness) and further reduction or removal of chromatographic
steps. Further development was precluded by an organizational
restructure.
Assembly of Synthons to form 1. With all three synthons
in hand, the convergent assembly of 1 was achieved as shown
in Scheme 5. A reductive amination between aminomethyl
Scheme 4. Preparation of Remaining Synthons
Scheme 5. Assembly of 1
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’ CONCLUSIONS
organic layers were dried (sodium sulfate) and concentrated in
vacuo to afford 8 as an off-white solid. The yield of 8 was not
determined and was carried on to next reaction without purifica-
tion. 1H NMR of off-white solid 4.60 (bs, 1H), 4.20ꢀ4.22 (m, 1H),
4.12ꢀ4.16 (m, 1H), 3.95 (bs, 1H), 3.03 (s, 3 H), 1.44 (s, 9H),
1.23 (d, J = 6.9 Hz).
An efficient convergent laboratory-scale process to synthesize
the selective CCR5 antagonist 1 has been developed which
avoids cryogenic conditions and minimizes column purification
steps (two silica gel plug filtrations and two columns). The
sequence uses only one protecting group (Boc protection of
amine 7) and is thus efficient from an atom-economy perspec-
tive. The convergent route has a longest linear sequence of nine
steps (15 steps overall) and is based on strategic assembly around
a piperidone core (2), in which the aminothiophene 3, nicotinic
acid 5, and carbamate reagent 14 are incorporated sequentially.
Overall, the yields on scale are typically 18ꢀ25%, and the route
has been carried out to provide 550 g of 1.
Preparation of (R)-(2-Cyano-1-methyl-ethyl)-carbamic
Acid tert-Butyl Ester (9). Sodium cyanide (625 g, 12.75 mol)
was suspended in dimethyl sulfoxide (3.5 L) and warmed to
40 °C. (CAUTION: The solution of NaCN in DMSO could be
readily absorbed through the skin. Appropriate protective equip-
ment and safe disposal of all waste is required18—aqueous
streams containing cyanide should be kept alkaline to prevent
the release of HCN gas.) A solution of (R)-methanesulfonic
acid 2-tert-butoxycarbonylamino-propyl ester 8 (assuming 90%
conversion in previous step, 4.25 mol) in dimethyl sulfoxide
(1 L) was added to the suspension over 30 min while warming to
50 °C. Caution: an exotherm was observed when the reaction
temperature reached 50 °C and 600 mL of mesylate solution was
added. The temperature increased to 60 °C before it started to
decrease to 55 °C. The remainder of the mesylate solution was
added slowly to maintian a temperature of 55 °C. A thick orange
suspension formed. The suspension was cooled to 50 °C, stirred
for 4 h, then cooled further to 25 °C, and stirred an additional
10 h. The reaction was monitored by 1H NMR. To the orange
suspension was added water (4.25 L), and the temperature
increased slightly (25.9 to 34.0 °C). The dark-maroon solution
was extracted with diethyl ether (4 ꢁ 4 L). The combined
organic layers were washed with brine (2 ꢁ 1.5 L). The resulting
brine layers were then back extracted with diethyl ether (4 ꢁ
0.5 L). The combined diethyl ether layers were dried (magnesium
sulfate) and concentrated in vacuo to afford a dark-brown oil.
The brown oil was taken up in warm diethyl ether (500 mL), and
hexanes (4 L) was added. The mixture was tumbled and warmed
to 50 °C to solubilize the mixture and then cooled slowly to 0 °C.
The resulting precipitate 9 (600 g) was collected on a glass frit
and dried under high vacuum. The mother liquor was taken up in
hexane (1 L) and a minimal amount of diethyl ether and passed
through a silica gel plug (600 g), eluting with hexane/ethyl
acetate (3:1). The product-containing fractions were concen-
trated to dryness and subjected to recrystallization from hexanes.
The resulting crystals (25 g) were collected by vacuum filtration
to afford a total of 625 g of 9. 1H NMR 4.67 (bs, 1H), 3.93ꢀ3.96
(m, 1H), 2.74 (dd, J = 16.5, 4.8 Hz, 1H), 2.52 (dd, J = 16.7,
4.1 Hz, 1H), 1.44 (s, 9H), 1.31 (d, J = 6.9 Hz).
’ EXPERIMENTAL SECTION
1H NMR (300 MHz) and 13C (75 MHz) spectra were
recorded using CDCl3 solvent with TMS as an internal standard
(unless otherwise noted). All reactions were conducted under an
inert (nitrogen) atmosphere, unless stated otherwise. HPLC was
performed using reverse-phase conditions with a C18 column
and acetonitrile/water mobile phase, and UV detection (220 or
254 nm).
Preparation of (R)-(2-Hydroxy-1-methyl-ethyl)-carbamic
Acid tert-Butyl Ester (7). A 1.0 M solution of lithium aluminum
hydride in THF (3.9 L, 3.9 mol) was cooled to 4 °C under argon
(typically <5 °C). D-Alanine methyl ester HCl (376.77 g,
2.79 mol) was added in portions (∼25 g/portion) over 90 min
at 5ꢀ20 °C. The addition funnel and walls of the reactor were
rinsed with THF (150 mL). The reaction was warmed over
20 min from 3.8 to 15.8 °C. The reaction was cooled to 13.3 °C
(10ꢀ15 °C target) and quenched with water (148 mL), over
60 min, maintaining a temperature below 22 °C. A 4 N sodium
hydroxide solution (226 mL) was added over 10 min, followed by
water (450 mL) over 5 min. A solution of di-tert-butyl dicarbo-
nate (534 g, 2.42 mol) in dry THF (1.2 L) was added to the
suspension, and stirring continued for an additional 18 h at
22 °C. The suspension was filtered through a glass frit, and the
collected solids were washed with ethyl acetate (6.5 L). The
resulting filtrate was concentrated in vacuo and taken up in
dichloromethane (1 L), dried over sodium sulfate, and reconcen-
trated to afford 453 g at 96.9 wt % (NMR estimate of solvent
content: corrected yield 439 g, 93 %) of 7 as an orange solid.
1H NMR 4.64 (bs, 1H), 3.74.3.79 (m, 1H), 3.61ꢀ3.66 (m, 1H),
3.49ꢀ3.54 (m, 1H), 2.67 (bs, 1H), 1.45 (s, 9H), 1.14 (d, J =
6.9 Hz, 3H).
Synthesis of (R)-Methanesulfonic Acid 2-tert-Butoxycar-
bonylaminopropyl Ester (8). To a solution of (R)-(2-hydroxy-
1-methylꢀethyl)-carbamic acid tert-butyl ester 7 (828.96 g,
4.73 mol) in dry dichloromethane (9.1 L) was added triethylamine
(1.155 L, 8.29 mol). The mixture was cooled (5.4 °C) and stirred
for 1 h. To the mixture above was added methane sulfonyl
chloride (400 mL, 5.2 mol) in dichloromethane (400 mL) slowly
over 45 min. The temperature was maintained below 17 °C.
Cooling was continued to 6 °C, and the mixture was stirred
an additional 40 min. The reaction was monitored by 1H NMR.
The mixture was quenched with water (3 L), the layers were
separated, and the aqueous layer was extracted with dichloro-
methane (3 L). The combined organics were washed with a
0.1 N HCl solution (2 L) followed by a saturated solution of
sodium bicarbonate (2.0 L). These two aqueous layers were back
extracted with dichloromethane (500 mL/each). The combined
Preparation of (R)-3-Amino-butyronitrile (10). To a cooled
(9 °C) solution of (R)-(2-cyano-1-methyl-ethyl)-carbamic acid
tert-butyl ester 9 (633 g, 3.44 mol) in tetrahydrofuran (1 L) was
added 6 N hydrochloric acid (1.7 L, 10.2 mol). Caution: it is
critical that no cyanide remain in 9 from the previous step as
HCN will form upon acid addition; bubbling occurs, and the
reaction is slightly exothermic (9.1 to 18.7 °C). The mixture was
cooled to 14 °C, the ice bath was removed, and the reaction was
warmed to 22 °C over 40 min. The reaction was warmed to 40 °C
over 1 h and stirred for an additional 50 min when an aliquot
1
was taken for H NMR. The reaction was cooled to ambient
temperature, and the tetrahydrofuran was removed in vacuo. The
resulting aqueous mixture was cooled to 10 °C. A 40% sodium
hydroxide solution (1.2 L) was added slowly (to pH > 12) at
<30 °C. Sodium chloride (300 g) was then added (to increase
subsequent extraction efficiency). The aqueous mixture was
extracted with a methanol/dichloromethane (1:9) solution
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(4 ꢁ 4 L). The combined organic layers were dried (sodium
sulfate) and concentrated in vacuo to afford an orange oil 10
(259.1 g, 90%). The material was used without further purifica-
tion. 1H NMR 3.22ꢀ3.33 (m, 1H), 2.47ꢀ2.62 (m, 2H), 1.17 (d,
J = 6.3 Hz, 3H).
evaporation was continued to near dryness. The solid was
triturated in hexaneꢀether (1:1, 0.5 L) until no clumps were
visible and the solids were isolated by filtration. The filter cake
was washed with hexaneꢀDCM (1:1, 0.3 L, slurry), and vacuum-
dried to give the product, 2-amino-4,6-dimethyl-5-bromopyri-
dine as white crystals (186 g, 74.4%). The mother liquor was
purified by flash chromatography (2 L silica, 1:1f2:3f1:2
hexaneꢀEtOAc as the eluent) to give another crop of product
Preparation of 1-Ethyl-1-methyl-4-oxo-piperidinium Io-
dide (11). To a solution of 1-methyl-4-piperidone (295 mL,
2.53 mol) in acetone (2.5 L) was added a solution of iodoethane
(245 mL, 3.05 mol) in acetone (0.5 L). The mixture was warmed
to 60 °C and stirred for 19 h under nitrogen (a white precipitate
begins to form in the solution after about 1 h). The solvent was
evaporated from the reaction vessel resulting in off-white solid.
The solid was resuspended in acetone (4.5 L) with stirring and
the suspension was cooled to 0 °C and stirred for 3.5 h. The solid
was collected by vacuum filtration, washed with acetone (2 L)
and air-dried for 30 min to afford an off-white solid 11 (539.74 g,
79%). 1H NMR 3.44ꢀ3.55 (m, 6H), 3.09ꢀ3.11 (m, 3H), 2.07
(bs, 4H), 1.36ꢀ1.40 (m, 3H).
1
(70 g, ∼14%). Overall yield of 12 was 420 g (85%). H NMR
6.25 (s, 1 H), 4.27 (br s, 2 H), 2.50 (s, 3 H), 2.28 (s, 3H).
Preparation of 2-Chloro-4,6-dimethyl-5-bromopyridine
(13). Pyridine HCl (520 g, 3 equiv, 3.34 mol) was suspended
3
in DCM (2 L) at RT. 2-Amino-4,6-dimethyl-5-bromopyridine
(12, 300 g, 1.49 mol) was added in one portion and was rinsed
down with DCM (0.5 L). CuCl (15 g, 0.1 equiv, 0.15 mol) was
added, followed by solid NaNO2 (308 g, 3 equiv, 3.34 mol) in
one portion. Concentrated HCl (6 mL, 0.04 equiv, 2% v/w vs
SM) was added slowly via a pipet, and the mixture was stirred
vigorously at RT. After 1.5 h, an aliquot of the reaction mix-
ture was analyzed by NMR (watch for the disappearance of
6.25 ppm signal) and showed a complete and clean reaction. The
reaction was carefully quenched with saturated NaHCO3 (1.2 L),
and the mixture was stirred for 15 min at RT. The layers were
separated, and the aqueous was extracted with DCM (0.6 L ꢁ 2).
The combined organic layers were dried with Na2SO4, and
concentrated by rotary evaporation. The residue was diluted
with hexane (0.5 L) and equilibrated with HCl (100 mL con-
centrated HCl in 1.5 L of water, final pH ∼7). The mixture was
extracted with ether (0.5 L ꢁ 4), and the combined extract was
washed with brine (0.5 L). The organic layer was dried with
Na2SO4 and filtered through a silica plug (1 L silica on a 2.5 L
glass frit). The silica plug was washed with ether (1 L), and the
filtrate was concentrated to dryness. The residue was triturated in
hexane (400 mL), filtered, and washed with hexane (4ꢀ500 mL).
The solid was dried under high vacuum to give 2-chloro-4,6-
dimethyl-5-bromopyridine (13) as off-white crystals (123 g,
38%). The mother liquor was concentrated and resubjected to
silica plug/crystallization to give another crop of product (53 g,
16%). 1H NMR 7.05 (s, 1 H), 2.65 (s, 3 H), 2.39 (s, 3 H).
Preparation of 6-Chloro-2,4-dimethylnicotinic Acid (5).
LiCl (anhydrous, 39.81 g, 1.2 equiv, 0.94 mol) was placed in a
flask equipped with mechanical stirring, followed by THF (0.5 L)
and i-PrMgCl (2 M in THF, 470 mL, 1.2 equiv, 0.94 mol, added
through a dropping funnel over 4 min). The mixture was stirred
at RT until LiCl had all dissolved. Solid 2-chloro-4,6-dimethyl-5-
bromopyridine (13, 172 g, 0.78 mol) was added under a strong
N2 flow over 2 min. The mixture was stirred for 2.5 h, after which
an aliquot NMR confirmed a complete reaction: Approximately
20 μL of reaction mixture was withdrawn with a gastight syringe,
quenched with MeOH-d4 in a NMR tube, and used directly. SM:
1H NMR (MeOH-d4) 7.25 (s, 1 H), 2.60 (s, 3 H), 2.41 (s, 1 H);
for exchanged product: δ 7.11 (s, 1H, 2.44 (s, 3 H), 2.33 (s, 3 H);
for proteo-debrominated material (if any): 7.11 (s, 1H), 7.08
(s, 1 H), 2.44 (s, 3 H), 2.33 (s, 3 H). The mixture was cooled in an
ice/water bath, and CO2 was introduced (after the internal
temperature had dropped below 10 °C) with vigorous stirring.
After 40 min, the reaction mixture was concentrated until
foaming started. The residue was cooled to ∼30 °C, and water
(175 mL, 1 wt equiv vs SM) was added. Concentrated HCl
(∼85 mL, 1.2 equiv, 0.93 mol) was added dropwise under
vigorous stirring. DCM (60 mL) was added, and rapid stirring
was continued for 0.5 h, during which time crystallization took
Preparation of (R)-3-(4-Oxo-piperidin-1-yl)-butyronitrile
(2). To a warm (40 °C) solution of (R)-3-amino-butyronitrile
10 (259.1 g, 3.08 mol) in ethanol (3 L) and water (1.5 L) was
added a solution of 1-ethyl-1-methyl-4-oxo-piperidinium iodide
11 (976.67 g, 3.62 mol) in water over 10 min. The mixture was
warmed to 67 °C over 2 h and stirred for an additional 2 h. The
1
reaction progress was monitored by H NMR. Once complete
(typically 2 h), the reaction was cooled to 30 °C, and the ethanol
was removed in vacuo. The resulting aqueous layer was diluted
with saturated sodium bicarbonate solution (1 L), and the
product was extracted with dichloromethane (4 ꢁ 4 L). The
combined dichloromethane layers were dried (sodium sulfate)
and concentrated in vacuo to afford 400 g of crude product.
The product 2 was purified by flash chromatography on silica gel
(2 kg), eluting with ethyl acetate (12 L) to afford 2 as an orange
oil (297 g, 58%). The purity of 2 by gas chromatography (GC)
was 99.45%, and the chiral purity by GC was 99.58% ee. 1H NMR
3.20ꢀ3.27 (m, 1H), 2.81ꢀ2.85 (m, 4H), 2.38ꢀ2.58 (m, 6H),
1.22 (d, J = 6.9 Hz, 3H); 13C NMR 208.90, 118.79, 56.36, 48.53,
42.06, 22.33, 15.63.
Preparation of 2-Amino-4,6-dimethyl-5-bromopyridine
(12). 2-Amino-4,6-dimethylpyridine (150 g, 1.23 mol) was
dissolved in dichloromethane (3.6 L) at RT under a N2 blanket
with mechanical stirring. After all the solid had dissolved, cooling
was started. When the solution had cooled to e ꢀ5 °C, solid N,
N-dibromo-4,4,-dimethylhyndantoin (DBH, 176 g, 0.5 equiv,
0.62 mol) was added in ∼40-g portions with vigorous stirring.
The temperature was allowed to drop below ꢀ5 °C before
adding another portion: total addition time ∼30 min. The
reaction was stirred at for 1 h, maintaining the internal termpera-
ture at ꢀ5 °C. A reaction aliquot was withdrawn and quenched
with saturated Na2SO3; the mixture was extracted with DCM;
the organic layer was removed and concentrated; the residue was
dissolved in CDCl3 and the NMR taken, with the disappearance
of 6.13 ppm peak being diagnostic of reaction progress. Addi-
tional DBH was added based on the NMR integration of the
remaining starting material. After another 1 h, the reaction was
quenched with a Na2SO3 solution (15 g Na2SO3 in 2 L water),
and the reaction temperature was increased to 10 °C. NaOH
(10 M) was added until the equilibrated pH was 13ꢀ14
(∼100 mL used). The organic layer was removed, and the
aqueous layer was extracted with DCM (1 L ꢁ 2). The combined
organic layers were dried with Na2SO4 and concentrated on a
rotary evaporator. Spontaneous crystallization occurred, and the
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place. The solid was filtered, placed in a crystallizing dish, and
further dried in a vacuum oven at 60 °C for 2 d to give 6-chloro-
2,4-dimethylnicotinic acid (125.57 g, 87%). The mother liquor
was adjusted to pH 2 with concentrated HCl to give more solid,
which was filtered, dried, and then dissolved in MeOH (20 mL)
and diluted with DCM (40 mL). Hexane (200 mL) was added,
and the mixture was heated until all solids had dissolved. The
solution was cooled slowly to RT and further held in an ice/water
bath for 0.5 h. The crystals were filtered and dried in a vacuum
oven overnight to give another crop of product (9.31 g, 6%). 1H
NMR (MeOH-d4) 7.23 (s, 1 H), 2.51 (s, 3 H), 2.37 (s, 3 H).
94.8% chemical purity by GC.
Preparation of 3-(Aminomethyl)-thiophene (3). A solution
of lithium aluminum hydride in THF (1.0 M, 3.7 L, 3.7 mol) was
added to 0 °C THF (7 L). A solution of 3-cyanothiophene (402 g,
3.68 mol) in THF (600 mL) was then added over 20 min, at
<20 °C. A rinse of the dropping funnel with ∼50 mL of THF was
used to ensure all the nitrile was transferred. The reaction was
warmed to 50 °C over 2 h, and was maintained at that tem-
perature until the reaction was complete by 1H NMR: ∼2 h. The
reaction mixture was cooled to 10ꢀ15 °C and was quenched
through slow addition of the following reagents, in sequence
(at <30 °C: water (140 mL), 4 N NaOH (210 mL), and water
(420 mL). The fine white suspension of aluminum salts was
removed by filtration through a glass frit, and the filter cake was
washed with diethyl ether (4 ꢁ 1 L). The combined organic
fractions were then concentrated by rotary evaporation, and the
oily residue was purified by fractional distillation under vacuum
(product collected at 95ꢀ105 °C at 25ꢀ30 mmHg vacuum) to
give 355 g (85%) of 3 as a colourless oil. 1H NMR 7.31 (m, 1H),
7.15 (m, 1H), 7.03 (m, 1H), 3.68 (s, 1H).
Preparation of N-Methoxy-(4-nitrophenyl)-carbamate (14).
A solution of methoxylamine hydrochloride (137 g, 1.64 mol)
in water (3.0 L) was cooled to 0ꢀ2 °C. Sodium bicarbonate
(230 g, 2.73 mol) was added, and dichloromethane (1.8 L) was
added. A solution of 4-nitrophenyl chloroformate (300 g, 1.49 mol)
in dichloromethane (1.2 L) was then added over 20 min, at <10°C.
A white precipitate formed during the chloroformate addition. The
reaction mixture was then cooled to 0 °C, and was stirred for
20 min. The solids were collected by filtration, and the filter
cake was washed with water (1 L). The solids were dried under
vacuum (no heating) to give 14 (262 g, 88%). 1H NMR 8.27
(d, 2H, J = 9.1 Hz), 7.83 (br s, 1H), 7.36 (d, 2H, J = 9.1 Hz),
3.85 (s, 3H).
Preparation of (R)-3-[4-(Thiophen-3-ylamino)-piperidin-
1-yl]-butyronitrile (15). 3-(Aminomethyl)-thiophene (274 g,
2.42 mol) was added to dichloromethane (10 L). The solution
was cooled to 8ꢀ10 °C, and then acetic acid (139 mL, 4.84 mol)
was added (temperature rose to 14 °C). Sodium triacetoxybor-
ohydride (770 g, 3.63 mol) was added, rinsing forward with
100 mL of dichloromethane. The reaction mixture was cooled to
8ꢀ10 °C, then (R)-3-(4-oxo-piperidin-1-yl)-butyronitrile (6,
422 g, 2.53 mol) in dichloromethane (600 mL) was added over
20 min, at <20 °C. The reaction was stirred at 20ꢀ22 °C until
complete by 1H NMR (∼2 h). Water (5 L) was added over 5 min
(effervescence), and then conc. HCl (465 mL) was added
(target pH of aqueous layer was 4.0). The organic and aqueous
layers were separated, and the aqueous layer was washed with
dichloromethane (2 L). To the aqueous layer was added 10 M
sodium hydroxide (1300 mL) to achieve a target pH of 8. The
aqueous layer was then extracted with dichloromethane (6 ꢁ
2.5 L). The combined organic fractions were washed with a solution
of 10 M NaOH and saturated sodium chloride (10: 1 ratio, 2 L
total). The organic fraction was dried with sodium sulfate,
filtered, and concentrated. The residue was taken up in a 1:1
mixture of methyl tert-butyl ether and hexane (400 mL total) and
was warmed slightly to solubilize. The solution/suspension was
cooled to 4 °C overnight. The product 15 was isolated by
filtration, and the crystals were washed with a mixture of hexanes
and MTBE (3:2 ratio, 3 ꢁ 400 mL) to give 380 g as a first crop.
The washings and mother liquor were concentrated and taken up
in MTBE and hexanes (1:1 mixture, 500 mL), warming to 30 °C
to solubilize. The solution was cooled to 0ꢀ4 °C overnight to
give a second crop of crystals (140 g). Total yield of 15: 520 g
(82%). If desired, the filtrate can be subjected to a repeat of the
concentration/recrystallization cycle to obtain slightly more
1
material (8ꢀ10% increase in yield). H NMR 7.29 (m, 1H),
7.12 (s, 1H), 7.04 (d, 1H, J = 4.9 Hz), 3.83 (s, 2H), 3.04 (m, 1H),
2.83 (m, 2H), 2.54 (m, 2H), 2.20ꢀ2.38 (m, 3H), 1.90 (m, 2H),
1.35 (dq, 2H, J = 11.2, 3.4 Hz), 1.19 (d, 3H, J = 6.7 Hz). Purity of
combined batches >99% by HPLC.
Preparation of (R)-3-[4-(Thiophen-3-ylamino)-piperidin-
1-yl]-butyl-1-amine (4). To a nitrogen-purged reactor (equipped
with overhead stirring, a dropping funnel and condensor) was added
lithium aluminum hydride (116.1 g, 3.05 mol), followed by slow
addition of MTBE (6 L). The mixture was warmed to 45 °C, and
then a solution of nitrile 15 (536.5 g, 2.04 mol) in MTBE (0.5 L)
was added slowly over 45 min, at <52 °C. The mixture was stirred
a further 60 min—at which point an aliquot examined by NMR
determined that the reaction was complete. The mixture was
cooled over 60 min to 30 °C and was quenched via a sequential
addition of the following: water (116 mL), 4 N NaOH (174 mL),
water (350 mL) at <50 °C. The suspension was cooled to 30 °C
and then was filtered, washing the aluminum salts with MTBE
(4 ꢁ 800 mL). The filtrate was concentrated to give 4 as a
colourless oil (538 g, 87%) which was used directly in the next
reaction. 1H NMR 7.25 (dd, 1H, J = 4.9, 4.0 Hz), 7.11 (m, 1H),
7.02 (dd, 1H, J = 4.9, 0.8 Hz), 3.82 (s, 2H), 7.73 (m, 4H), 2.48
(m, 1H), 2.35 (dt, 1H, J = 11.2, 2.3 Hz), 2.11 (dt, 1H, J = 11.2,
2.3 Hz), 1.88 (m, 2H), 1.63 (hex, 1H, J = 7.1 Hz), 1.36 (m, 6H),
0.94 (d, 3H, J = 6.6 Hz).
Preparation of 6-Chloro-N-{(R)-3-[4-(thiophen-3-yl)-pi-
peridin-1-yl]-butyl}-2,4-dimethylnicotinamide (6). To a so-
lution of amine 4 (538 g, 2.01 mol) in dichloromethane (12 L)
was added carboxylic acid 2 (381 g, 2.01 mol). HOBt (285 g, 2.11
mol) and EDCI (404 g, 2.11 mol) were then added, rinsing
forward with 0.5 L dichloromethane. (Caution: HOBt has been
reported to have explosive properties, especially in anhydrous
forms. The reagent should contain >10% water by mass to be
considered desensitized19.) The mixture was stirred under
nitrogen at ambient temperature for 16 h. Water (7 L) was then
added, and while the mixture was stirred, the pH of the aqueous
layer was adjusted to 6 with concentrated HCl (∼200 mL). The
layers were then separated, the organic layer was washed with
water (5 L), and the combined aqueous layers were washed with
dichoromethane (3 L). The aqueous layer (containing product)
was then adjusted to pH 10 with 10 M NaOH solution (380 mL),
and the mixture was extracted with dichloromethane (3 ꢁ 3 L),
adjusting the pH of the aqueous layer between extractions.
The combined dichloromethane fractions were concentrated to
∼1.5 L volume and were filtered through silica gel (2 kg) which had
been preconditioned with 10% methanol in dichloromethane.
The product was eluted with 1% NH4OH, 10% methanol in
dichloromethane (monitored by TLC). Fractions containing
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Organic Process Research & Development
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product were concentrated. To the viscous oil was added
MTBE (1.5 L), and the mixture was warmed to 30ꢀ35 °C with
vigorous stirring. A crystalline suspension began to form, and
then hexanes (1 L) was added. The suspension was stirred for
another 20 min and then was allowed to stand for 30 min before
being filtered. The product 6 was collected as a pale-yellow,
crystalline solid (528 g, 61%). It should be noted that the mother
liquor can be harvested, rechromatographed, and recrystallized
to provide a further 10ꢀ20% yield, if desired. 1H NMR 8.77 (m,
1H), 7.08 (m, 1H), 7.02 (m, 2H), 3.86 (m, 1H), 3.70 (s, 2H),
3.33 (m, 1H), 2.83 (m, 2H), 2.44 (m, 1H), 2.51 (s, 3H), 2.37 (dt,
1H, J = 9.8, 1.8 Hz), 2.33 (m, 1H), 2.30 (s, 3H), 1.73 (m, 3H),
1.53 (m, 1H), 0.94 (d, 3H, J = 6.6 Hz), 0.74 (m, 3H). Purity
99.1% by HPLC.
’ ASSOCIATED CONTENT
S
Supporting Information. NMR spectra for 1, 4, 6, 14 and
b
15 are provided. This information is availablefree of chargevia the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
jcrawford@cdrd.ca.
Present Addresses
†Centre for Drug Research and Development, Suite 364-2259
Lower Mall, University of British Columbia, Vancouver, BC,
Canada V6T 1Z4.
‡Genzyme Corp., 153 2nd Ave., Waltham, MA 02451, United States.
Preparation of 6-Chloro-N-{(R)-3-[4-(3-methoxy-1-thio-
phen-3-ylmethyl-ureido)-piperidin-1-yl]-butyl}-2,4-dimethy-
lnicotinamide (1). Dichloromethane (2 L) was added to a
reactor, and the solvent was cooled to 15 °C. Amine 6 (500 g,
1.15 mol) was then added, rinsing forward with dichloro-
methane (1 L). Carbamate reagent 14 (256 g, 1.21 mol) was
then added, again rinsing forward with dichloromethane (0.5 L).
The mixture formed a pale-yellow suspension. Diisopropylethy-
lamine (240 mL, 1.37 mol) was added over 5 min, rinsing
forward with dichloromethane (0.5 L). At the completion of
addition of the DIPEA, all the solids dissolved. After 60 min, the
’ ACKNOWLEDGMENT
We thank the analytical staff at AnorMED for their method
development and analyses, and scientists at Genzyme Corp. for
their proofreading and assistance with manuscript preparation.
’ REFERENCES
1
(1) (a) Selzentry (Maraviroc) was approved for the treatment of
adults infected with CCR5-tropic HIV-1. For Phase 3 clinical results, see:
Gulick, R. M.; Lalezari, J. L.; Goodrich, J.; Clumeck, N.; DeJesus, E.;
Horban, A.; Nadler, J.; Clotet, B.; Karlsson, A.; Wolhfeiler, M.; Montana,
J. B.; McHale, M.; Sullivan, J.; Ridgway, C.; Felstead, S.; Dunne, M. W.;
van der Ryst, E.; Mayer, H. N. Engl. J. Med. 2008, 359 (4), 1429–1441.
(b) Vicriviroc demonstrated potent virologic suppression load in a Phase
2 clinicial trial: Gulick, R. M.; Su, Z.; Flexner, C.; Hughes, M. D.; Skolnik,
P. R.; Wilkin, T.; Gross, R.; Krambrink, A.; Coakley, E.; Greaves, W. L.;
Zolopa, A.; Reichman, R.; Godfrey, C.; Hirsch, M.; Kuritzkes, D. R.
J. Infect. Dis. 2007, 196 (2), 304–312.
(2) (a) Hendrix, C. W.; Collier, A. C.; Lederman, M. M.; Schols, D.;
Pollard, R. B.; Brown, S.; Jackson, J. B.; Coombs, R. W.; Glesby, M. J.;
Flexner, C. W.; Bridger, G. J.; Badel, K.; MacFarland, R. T.; Henson,
G. W.; Calandra, G. J. Acquired Immune Defic. Syndr. 2004, 37, 1253. (b)
Franzen, S.; Bridger, G.; Whitcomb, J. M.; Toma, L.; Stawiski, E.; Parkin,
N.; Petropoulos, C. J.; Huang, W. Antimicrob. Agents Chemother. 2008,
52, 2608.
(3) Moyle, G.; DeJesus, E.; Boffito, M.; Wong, R. S.; Gibney, C.;
Badel, K.; MacFarland, R.; Calandra, G.; Bridger, G.; Becker, S. Clin.
Infect. Dis. 2009, 48, 798–805.
(4) Skerlj, R. T.; Bridger, G. J.; Kaller, A.; McEachern, E. J.;
Crawford, J.; Zhou, Y.; Atsma, B.; Langille, J.; Nan, S.; Veale, D.; Wilson,
T.; Harwig, C.; Hatse, S.; Princen, K.; De Clercq, E.; Schols, D. J. Med.
Chem. 2010, 53, 3376.
(5) Crawford, J. B.; Chen, G.; Gauthier, D.; Wilson, T.; Carpenter,
B.; Baird, I. R.; McEachern, E.; Kaller, A.; Harwig, C.; Atsma, B.; Skerlj,
R. T.; Bridger, G. J. Org. Process Res. Dev. 2008, 12, 823–830.
(6) (a) Skerlj, R.; Bridger, G.; Zhou, Y.; Bourque, E.; Langille, J.; Di
Fluri, M.; Bogucki, D.; Yang, W.; Li, T.; Wang, L.; Nan, S.; Baird, I.;
Metz, M.; Darkes, M.; Labrecque, J.; Lau, G.; Fricker, S.; Huskens, D.;
Schols, D. Bioorg. Med. Chem. Lett. 2011, 21, 2450–2455. (b) Labrecque,
J.; Metz, M.; Lau, G.; Darkes, M.; Wong, R. S. Y.; Bogucki, D.; Carpenter,
B.; Chen, G.; Li, T.; Nan, S.; Schols, D.; Bridger, G. J.; Fricker, S. P.;
Skerlj, R. T. Virology 2011, 413, 231. (c) Metz, M.; Bourque, E.;
Labrecque, J.; Danthi, S. J.; Langille, J.; Harwig, C.; Yang, W.; Darkes,
M. C.; Lau, G.; Santucci, Z.; Bridger, G. J.; Schols, D.; Fricker, S. P.;
Skerlj, R. T. J. Am Chem. Soc. 2011, 131, 16477–16485. (d) Skerlj, R.;
Bridger, G.; Zhou, Y.; Bourque, E.; McEachern, E.; Langille, J.; Harwig,
C.; Veale, D.; Yang, W.; Li, T.; Zhu, Y.; Bey, M.; Baird, I.; Sartori, M.;
Metz, M.; MacFarland, R.; Mosi, R.; Nelson, K.; Bodart, V.; Wong, R.;
reaction was complete by H NMR aliquot. The reaction was
quenched with water (4 L), and the pH of the aqueous layer was
adjusted to 1ꢀ2 (target 1.5) with slow addition of concentrated
HCl (225 mL). Diethyl ether (8 L) was added, and the organic
and aqueous layers were separated. The aqueous layer was
washed with diethyl ether (3 ꢁ 4 L) to remove the yellow
p-nitrophenol. Dichloromethane (3 L) was added to the aqu-
eous layer, and the pH adjusted to 7 with 4 M NaOH solution
(270 mL). Saturated aqueous sodium bicarbonate (1 L) was
added, and the pH adjusted to 8 with 4 M NaOH (100 mL).
The aqueous and organic layers were separated, and the aqueous
layer washed with dichloromethane (3 L). The combined
organic layers were dried with sodium sulfate and were con-
centrated. The residue was reconstituted in dichloromethane
(50 mL) and was filtered through silica gel (1 kg), which had
been preconditioned with 1% NH4OH, 5% MeOH in ethyl
acetate. The filtration was monitored by TLC, eluting first with
1% NH4OH, 5% MeOH in ethyl acetate, and then 25% MeOH,
10% NH4OH in ethyl acetate, and was intended to remove high-
polarity baseline residual reagents and impurities. The fractions
containing product were then concentrated, taken up in metha-
nol, and concentrated again (to remove ethyl acetate). The
product was dried under vacuum in a 40 °C oven for 48 h to
produce an amorphous light-yellow, foamy solid (1) in a yield
of 563 g (96%). 1H NMR (CDCl3) δ 8.76 (d, 1H, J = 5.5 Hz),
7.38 (dd, 1H, J = 5.0, 3.0 Hz), 7.12 (m, 1H), 7.03 (m, 2H), 6.93
(m, 1H), 4.22 (m, 1H) 3.84 (m, 1H), 3.68 (m, 2H), 3.66 (s, 3H),
3.26 (m, 1H), 2.83 (m, 2H), 2.72 (m, 1H), 2.57 (t, 1H, J = 11.4
Hz), 2.53 (s, 3H), 2.26 (m, 3H), 2.16 (t, 1H, J = 11.4 Hz),
1.76ꢀ1.66 (m, 3H). 1/54 (m, 1H), 1.02 (m, 1H), 0.98 (d, 3H,
J = 6.6 Hz), 0.88 (m, 1H). 13C NMR 167.4, 159.4, 155.7, 150.4,
147.9, 139.1, 133.2, 127.9, 126.4, 122.8, 121.7, 64.6, 60.9, 52.6,
52,1, 43.6, 41.2, 40.5, 31.0, 30.0. 22.4, 19.1, 13.8. MS m/z 508
(m + H). HPLC purity 99.84%. Chiral HPLC purity >99.5% ee.
Anal. Calcd For C24H34N5ClO3S: C, 56.74; H, 6.74; N, 13.78;
Cl, 6.98; S, 6.31. Found: C, 56.94; H, 6.82; N, 13.76; Cl, 7.14;
S, 6.23.
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Fricker, S.; Huskens, D.; Schols, D. Bioorg. Med. Chem. Lett. 2011, 21,
6950–6954.
(7) (a) Haycock-Lewandowski, S. J.; Wilder, A.; Ahman, J. Org.
Process Res. Dev. 2008, 12, 1094–1103. (b) Ahman, J.; Birch, M.;
Haycock-Lewandowski, S. J.; Long, J.; Wilder, A. Org. Process Res. Dev.
2008, 12, 1104–1113.
(8) Tagat, J. R; McCombie, S. W.; Nazareno, D.; Labroli, M. A.;
Xiao, Y.; Steensma, R. W.; Strizki, J. M.; Baroudy, B. M.; Cox, K.;
Lachowicz, J.; Varty, G.; Watkins, R. J. J. Med. Chem. 2004, 47,
2405–2408.
(9) Baba, M.; Nishimura, O.; Kanzaki, N.; Okamoto, M.; Sawada, H.;
Iizawa, Y.; Shirashi, M.; Aramaki, Y.; Okonogi, K.; Ogawa, Y.; Meguro,
K.; Fujino, M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96 (10), 5698–5703.
(10) Huang, X.; O’Brien, E.; Thai, F.; Cooper, G. Org. Process Res.
Dev. 2010, 14, 592–599.
(11) Zhou, Y.; Borque, E.; Zhu, Y.; Langille, J.; Metz, M.; Yang, W.;
McEachern, E.; Harwig, C.; Baird, I. R.; Li, T.-S.; Skerlj, R. T. WO/2006/
138350 A2,2006.
(12) Fadini, L.; Togni, A. Tetrahedron: Asymmetry 2008, 19 (22),
2555–2562.
(13) Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; Wiley:
New York, 1967; pp 581ꢀ595.
(14) Kuehne, M. E.; Muth, R. S. J. Org. Chem. 1991, 56, 2701–2712.
Tschaen, D. M.; Abramson, L.; Cai, D.; Desmond, R.; Dolling, U.-H.;
Frey, L.; Karady, S.; Shi, Y.-J.; Verhoeven, T. R. J. Org. Chem. 1995,
60, 4234–4330.
(15) Chen, W.; Stephenson, E. K.; Cava, M. P.; Jackson, Y. A. Org.
Synth. 1992, 70, 151.
(16) It should be noted that freshly recrystallized NBS gave slightly
cleaner reaction profiles than commercially sourced reagent (which was
typically yellow to slightly orange), so it is possible that the side reactivity
could be due, in part, to bromine contaminant.
(17) For comparison, see: (a) Sibley, G. E. M.; Downham, R.; Payne,
L. J.; Law, D.; Oliver, J. D.; Birch, M.; Davies, G. M. WO 2009/134081
A1. (b) Zhou, Y.; Bridger, G. J.; Skerlj, R. T.; Bogucki, D.; Yang, W.;
Bourque, E.; Langille, J.; Li, T.-S.; Metz, M. U.S. Patent 7,498,346 B2,
March 3, 2009.
(18) Example procedures for destruction of cyanide waste solutions:
Parga, J. R.; Shukla, S. S.; Carrillo-Pedroza, F. R. Waste Manage.
(Amsterdam, Nether.) 2003, 23, 183–191.
(19) Wehrstedt, K. D.; Wandrey, P. A.; Heitkamp, D. J. Hazard.
Mater. 2005, A126, 1–7.
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