A practical synthesis of 5-lipoxygenase inhibitor MK-0633

Article abstract of DOI:10.1021/jo100561u

Practical, chromatography-free syntheses of 5-lipoxygenase inhibitor MK-0633 p-toluenesulfonate (1) are described. The first route used an asymmetric zincate addition to ethyl 2,2,2-trifluoropyruvate followed by 1,3,4-oxadiazole formation and reductive amination as key steps. An improved second route features an inexpensive diastereomeric salt resolution of vinyl hydroxy-acid 22 followed by a robust end-game featuring a through-process hydrazide acylation/1,3,4-oxadiazole ring closure/salt formation sequence to afford MK-0633 p-toluenesulfonate (1).

Full text of DOI:10.1021/jo100561u

pubs.acs.org/joc
A Practical Synthesis of 5-Lipoxygenase Inhibitor MK-0633
Francis Gosselin,*,† Robert A. Britton,† Ian W. Davies,‡ Sarah J. Dolman,†
Danny Gauvreau,† R. Scott Hoerrner,‡ Gregory Hughes,† Jacob Janey,‡ Stephen Lau,†
Carmela Molinaro,† Christian Nadeau,† Paul D. O’Shea,† Michael Palucki,‡ and
Rick Sidler‡
†Department of Process Research, Merck Frosst Centre for Therapeutic Research, 16711 Route
Transcanadienne, Kirkland, Queꢀbec, Canada H9H 3L1, and ‡Department of Process Research,
Merck Research Laboratories, P.O. Box 2000, Rahway, New Jersey 07065
francis_gosselin@merck.com
Received March 24, 2010
Practical, chromatography-free syntheses of 5-lipoxygenase inhibitor MK-0633 p-toluenesulfonate
(1) are described. The first route used an asymmetric zincate addition to ethyl 2,2,2-trifluoropyruvate
followed by 1,3,4-oxadiazole formation and reductive amination as key steps. An improved second
route features an inexpensive diastereomeric salt resolution of vinyl hydroxy-acid 22 followed by a
robust end-game featuring a through-process hydrazide acylation/1,3,4-oxadiazole ring closure/salt
formation sequence to afford MK-0633 p-toluenesulfonate (1).
Introduction
Leukotriene metabolism plays a central role in inflamma-
tory diseases such as asthma, chronic obstructive pulmonary
disease (COPD), and atherosclerosis.1 In particular, the acti-
vation of the enzyme 5-lipoxygenase (5-LO) and its associated
protein, 5-LO activating protein (FLAP), initiates a cascade
that transforms arachidonic acid into inflammatory leuko-
trienes.2 As part of a drug discovery program in our labora-
tories, MK-0633 p-toluenesulfonate (1) was identified as a
potent and selective inhibitor of 5-LO (Figure 1).3 Herein we
wish to report two practical, chromatography-free syntheses
FIGURE 1. Structure of MK-0633 p-toluenesulfonate.
that are suitable for the preparation of multikilogram amounts
of 5-lipoxygenase inhibitor MK-0633.
(1) Funk, C. D. Science 2001, 294, 1871.
(2) Evans, J. F.; Ferguson, A. D.; Mosley, R. T.; Hutchinson, J. H. Trends
Pharmacol. Sci. 2008, 29, 72.
Results and Discussion
^
(3) Ducharme, Y.; Blouin, M.; Brideau, C.; Chateauneuf, A.; Gareau, Y.;
ꢀ
ꢀ
Grimm, E. L.; Juteau, H. N.; Laliberte, S.; Mackay, D. B.; Masse, F.; Ouellet,
M.; Salem, M.; Styhler, A.; Friesen, R. ACS Med. Chem. Lett. DOI: 10.1021/
ml100029k. Published Online: April 13, 2010
Two convergent synthetic approaches to MK-0633 are
illustrated inScheme 1. The firstapproachthat weinvestigated
4154 J. Org. Chem. 2010, 75, 4154–4160
Published on Web 05/20/2010
DOI: 10.1021/jo100561u
r
2010 American Chemical Society

Gosselin et al.
JOCArticle
SCHEME 1. Retrosynthetic Analysis
SCHEME 2
and demonstrated on kilogram scale involved an asymmetric
BINOL-ethylzincate addition to trifluoropyruvate followed
by a late-stage reductive amination between 2-amino-1,3,4-
oxadiazole 3 and aldehyde 4 as key steps. A second genera-
tion and complementary approach to MK-0633 involved a
late-stage cyclization of mixed thiosemicarbazide 5. Both
approaches required the development of robust syntheses of
chiral R-trifluoromethyl R-hydroxy-acid intermediate 6. We
established the second route described in this report as a
potential manufacturing route to MK-0633.
Synthesis of Aminooxadiazole Intermediate. The synthesis
of 2-amino-1,3,4-oxadiazole 3 began with organometallic
addition to ethyl 2,2,2-trifluoropyruvate to form chiral ethyl
hydroxy-acid 6 (Scheme 2). Few methods have been reported
for asymmetric organometallic additions to R-keto esters.4
Furthermore, the challenging asymmetric synthesis of
chiral trifluoromethylated alcohols has received attention
recently.5-7
To install the tertiary alcohol center found in MK-0633,
we discovered and developed an asymmetric ethylzincate
addition to ethyl 2,2,2-trifluoropyruvate 8 (Scheme 2).8 Ad-
dition of EtMgCl to Et2Zn at -20 °C led to complete for-
mation of the corresponding chloromagnesium triethylzin-
cate as ascertained by 1H NMR analysis. Subsequent addi-
tion of Et3ZnMgCl to a slurry of (S)-(-)-1,10-bi(2-naphthol)
[(S)-BINOL] in 1,2-dichloroethane at -20 °C led to forma-
tion of a chiral (S)-BINOL-zincate complex.9 Ethyl 2,2,2-
trifluoropyruvate 8 was then added to this reagent over
4-6 h in order to obtain >95% conversion to the desired
adduct (S)-hydroxy ester 6 in 60-75% ee. Slow addition
(6) For cinchonine ammonium fluoride catalyzed enantioselective addi-
tion of CF3TMS to aldehydes and ketones, see: (a) Iseki, K.; Nagai, T.;
Kobayashi, Y. Tetrahedron Lett. 1994, 35, 3137. (b) Caron, S.; Do, N. M.;
ꢀ
Arpin, P.; Larivee, A. Synthesis 2003, 1693. In our hands, the analogous
reaction with ethyl 2-oxobutanoate afforded racemic 9a.
(7) For a biocatalytic approach, see: Shaw, N. M.; Naughton, A.;
Robins, K.; Tinschbert, A.; Schmid, E.; Hischier, M.-L.; Venetz, V.; Werlen,
J.; Zimmermann, T.; Brieden, W.; de Riedmatten, P.; Roduit, J.-P.;
(4) (a) DiMauro, E. F.; Kozlowski, M. C. J. Am. Chem. Soc. 2002, 124,
12668. (b) DiMauro, E. F.; Kozlowski, M. C. Org. Lett. 2002, 4, 3781. (c)
Jiang, B.; Chen, Z.; Tang, X. Org. Lett. 2002, 4, 3451. (d) Huddleston, R. R.;
Jang, H.-Y.; Krische, M. J. J. Am. Chem. Soc. 2003, 125, 11488. (e) Wieland,
L. C.; Deng, H.; Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2005,
127, 15453. (f) Kong, J.-R.; Ngai, M.-Y.; Krische, M. J. J. Am. Chem. Soc.
2006, 128, 718.
(5) For diastereoselective addition of CF3TMS to a chiral R-keto ester,
see: Song, J. J.; Tan, Z.; Xu, J.; Reeves, J. T.; Yee, N. K.; Ramdas, R.;
Gallou, F.; Kuzmich, K.; DeLattre, L.; Lee, H.; Feng, X.; Senanayake, C. H.
J. Org. Chem. 2007, 72, 292.
€
Zimmermann, B.; Neumuller, R. Org. Process Res. Dev. 2002, 6, 497.
(8) Gosselin, F.; Britton, R. A.; Mowat, J.; O’Shea, P. D.; Davies, I. W.
Synlett 2007, 2193.
(9) For reviews on the use of BINOL in asymmetric synthesis, see: (a)
Brunel, J. M. Chem. Rev. 2005, 105, 857. (b) Chen, Y.; Yekta, S.; Yudin,
A. K. Chem. Rev. 2003, 103, 3155.
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SCHEME 3. Synthesis of 2-Amino-1,3,4-oxadiazole 3
SCHEME 4. Synthesis of Aldehyde 4
of pyruvate 8 was required due to competing capture of
the highly reactive pyruvate as a stable hemiketal 9b (R =
(S)-BINOL) as ascertained by 19F NMR spectroscopic
analysis.10 In comparison, addition of pyruvate 8 over
5-10 min afforded at best 50% conversion to the desired
ethyl adduct 9a. Furthermore, the charge stoichiometry
of Et2Zn was found to be critical since excess reagent led
to reduction of pyruvate 8 to form racemic alcohol 9d.11
Pyruvate hydrate 9c, alcohol 9d, and (S)-BINOL were
removed in the basic aqueous workup. It was essential to
remove (S)-BINOL from the crude reaction mixture prior
to ester hydrolysis. Omitting removal of (S)-BINOL led to
poor yields of hydroxy-acid 6. Ester 9a was hydrolyzed
in situ by treating the crude product with aqueous KOH
at reflux. The crude acid 6 was solvent-switched to toluene
and the enantiomeric purity was upgraded to 88-94% ee
via formation of a diastereomeric salt with (S)-1-(2-naph-
thyl)ethylamine. Salt break and recrystallization of the
free acid in toluene gave (S)-6 in 49% yield over 3 steps
and 97.9-99.3% ee.
yield), EDC HCl/HOBt (68-72% yield), TBTU13 (46%
3
yield), PyBOP14 (68% yield).
Oxidant-mediated cyclization of thiosemicarbazide 12
with I2/KI/aqueous NaOH proceeded to afford aminooxa-
diazole 3 in 66% yield (Scheme 3). Prolonged reaction time
led to decomposition of 3 under basic conditions. Other
oxidants gave inferior results: Na2WO4 2H2O/H2O2 (decom-
3
position), Br2 (55% yield), HIO3 (34% yield). Recrystalliza-
tion of (S)-aminooxadiazole 3 from MTBE removed colored
impurities and gave enantiomerically pure 3 as ascertained by
chiral SFC analysis.
Synthesis of Aldehyde Intermediate. The synthesis of the
aldehyde component began with the von Pechmann reaction
between resorcinol 13 and β-keto ester 14 in methanesulfonic acid
to afford hydroxy-coumarin 15 (Scheme 4).15,16 Reaction with
triflic anhydride gave triflate 16 and reductive carbonylation17
with Pd(OAc)2/dppp/Et3SiH/Et3N gave aldehyde 4 contami-
nated with the corresponding acid 17 (∼5A% HPLC) and
reduced coumarin 18 (∼10A% HPLC) as determined by LC-
MS analysis. These impurities were readily removed by a reslurry
of crude coumarin aldehyde 4 in acetone.
Reductive Amination End-Game. Although a plethora of
reaction conditions for imine formation were evaluated, we
found the reductive amination to be a challenging step
(Scheme 5).18 The difficulty observed in the formation of
the imine 20 was attributed to the poor reactivity of 2-amino-
1,3,4-oxadiazole 3 since aldehyde 4 readily formed imines
with more nucleophilic amines such as benzylamine. Reac-
tion conditions involving azeotropic removal of water with
benzene as solvent and 10 mol % pyridinium p-toluenesul-
fonate as acid catalyst for 18 h gave up to 80% conversion
and 70% yield on small scale. Extended reaction times led to
increased conversion but lower yields of imine suggesting
that 20 was thermally unstable. Imine formation with coumarin
Amidation of hydroxy-acid (S)-6 with HATU12 activation
and coupling with thiosemicarbazide 11 in THF gave hy-
drazide 12 in 91% yield (Scheme 3). A reslurry in acetone
partially removed residual 1-hydroxy-7-azabenzotriazole
(HOAt) and N,N,N,0N0-tetramethylurea coupling byproduct
and provided 12 in 99% yield, suitable for the oxidant-
mediated cyclization. Other peptide coupling reagents gave
inferior results in terms of yield and purity: CDI (40-59%
(10) Hemiketals of ethyl 2,2,2-trifluoropyruvate prepared by reaction
with the metal alkoxides of MeOH, 2-propanol, phenol, and BINOL
exhibited a singlet with a chemical shift between δ -85.4 and -85.8 ppm
by 19F NMR spectroscopic analysis with benzotrifluoride as internal stan-
dard (δ -61.5 ppm).
(11) In contrast, the reagent obtained from lithium triethylzincate,
formed by reaction of ethyllithium with diethylzinc, and (S)-BINOL gave
the opposite sense of stereoselectivity (52% ee).
(12) HATU: 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo-[4,5-
b]pyridinium hexafluorophosphate 3-oxide: (a) Carpino, L. A. J. Am. Chem.
Soc. 1993, 115, 4397. (b) Carpino, L. A.; El-Faham, A.; Minor, C. A.; Albericio,
F. J. Chem. Soc., Chem. Commun. 1994, 201. (c) Carpino, L. A.; Imazumi, H.;
El-Faham, A.; Ferrer, F.; Zhang, C.; Lee, Y.; Foxman, F. M.; Henklein, P.;
(14) PyBOP: O-benzotriazol-1-yl-oxytripyrrolidinophosphonium hexa-
fluorophosphate: Coste, J.; Le-Nguyen, D.; Castro, B. Tetrahedron Lett.
1990, 31, 205.
(15) von Pechmann, H.; Duisberg, C. Chem. Ber. 1884, 17, 929.
(16) The β-keto ester was prepared according to: Brooks, D. W.; Lu,
L. D.-L.; Masamune, S. Angew. Chem., Int. Ed. Engl. 1979, 18, 72–73.
(17) Reviewed in: Ashfield, L.; Barnard, C. F. J. Org. Process Res. Dev.
2007, 11, 39.
€
Hanay, C.; Mugge, C.; Wenschuh, H.; Klose, J.; Beyermann, M.; Bienert, M.
Angew. Chem., Int. Ed. 2002, 41, 441.
(13) TBTU: 1-[bis(dimethylamino)methylene]-1H-benzotriazolium tetra-
fluoroborate 3-oxide: (a) Dourtoglou, V.; Ziegler, J.-C.; Gross, B. Tetrahedron
Lett. 1978, 1269–1272. (b) Dourtoglou, V.; Ziegler, J.-C.; Gross, B. Synthesis
1984, 572–574. (c) Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D.
Tetrahedron Lett. 1989, 30, 1927–1930.
(18) A thorough investigation of milder imine formation conditions with
various dehydrating agents including Ti(OEt)4, Ti(OiPr)4, TiCl4, (MeO)3CH,
MgSO4, and CuSO4 in toluene, 1,2-dichloroethane, or THF led to poor
conversions to 20 (30-50%). Transimination via the N-TMS aldimine or
N-H imine derivatives also proved fruitless.
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SCHEME 5. Reductive Amination and Isolation of MK-0633 p-Toluenesulfonate
SCHEME 6. Hydrazide Synthesis
aldehyde 4 proceeded slower on scale and was attributed to
the poor solubility of 2-amino-1,3,4-oxadiazole (S)-319 and
afforded at best 65% conversion. Reduction with NaBH4 in
MeOH gave crude 2 in 64.5% assay yield. The stoichiometry of
NaBH4 relative to imine 20 was found to be critical since excess
hydride (>30 mol %) led to reduction of the coumarin ring to
afford 19 and 21 as byproducts (Scheme 5).20 Removal of
benzene from the reaction stream was done via azeotropic
removal with MeOH, followed by azeotropic removal of
MeOH with THF for the final step. Complete elimination of
benzene was achieved during the final salt formation step. The
removal of benzene was carefully monitored by HPLC analysis
throughout the process.
lization allowed robust purification from crude free base 2
obtained with purities ranging from 60A% to 74A% HPLC.
Thus, treatment of crude MK-0633 free base with p-toluene-
sulfonic acid hydrate in THF/heptane gave crystalline tosylate
salt (1) as a white powder in 82% yield, 99.69 wt %, 99.81A%
HPLC, 0.19A% HPLC benzyl alcohol 19, >99.9% ee,
<3 ppm residual Pd, PhH: not detected, <2 ppm).
Although this initial synthetic route provided kilogram
amounts of MK-0633 for preclinical evaluation, a number of
attributes made it impractical and difficult to scale-up
further than a few kilograms: (1) a highly work intensive
and inefficient synthesis of the chiral hydroxy-acid (S)-6; (2)
the expensive and not readily available peptide coupling
agent HATU was used in the preparation of thiosemicarba-
zide 12; (3) a sensitive cyclization to 2-amino-1,3,4-oxadiazole
(S)-3; and (4) a difficult reductive amination penultimate step.
To address these shortcomings, a second generation and
complementary approach to MK-0633 involved a late-stage
cyclization of mixed thiosemicarbazide 5 (Scheme 1).
A robust purification step was required to remove substan-
tial quantities of alcohol 19 (up to 26A% HPLC) formed in the
reductive amination step. We identified MK-0633 p-toluene-
sulfonate salt (1) as a stable, crystalline, and bioavailable API
form for safety assessment and clinical evaluation. Its crystal-
(19) Solid-state NMR analysis of (S)-3 indicated a different crystal form
vs material recrystallized from iPAc/heptane.
(20) The formation of byproduct 21 obtained from conjugate reduction
was attributed to residual palladium in the coumarin aldehyde.
Synthesis of Hydrazide Intermediate. Because of both eco-
nomic and operational considerations the stoichiometric BI-
NOL-zincate addition was deemed not viable for long-term
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SCHEME 7. Coumarin Isothiocyanate Synthesis
SCHEME 8. Synthesis End-Game
manufacturing of MK-0633. We therefore evaluated a number
of chemo- and biocatalytic approaches for the synthesis of ethyl
hydroxy-acid intermediate 6(see the Supporting Information for
discussion). Ultimately, we found that a simple route involving
diastereomeric salt resolution provided (S)-6 (Scheme 6). Addi-
tion of vinylmagnesium chloride to ethyl 2,2,2-trifluoropyruvate
8 followed by in situ hydrolysis of the ethyl ester gave racemic
acid 22 in 73% yield. Addition of 100 mol % (S)-phenylethyla-
mine to a solution of acid in toluene at 90 °C gave the desired
crystalline salt 23 in 43-45% yield and 92-94% ee. Attempted
resolution with a number of other chiral amines gave inferior
results. Hydrogenation of the olefin with either Pd/C or Pt/C
afforded ethyl hydroxy-acid (S)-6 in 88% yield after enantio-
meric purity upgrade via recrystallization from toluene. In stark
contrast, performing the diastereomeric resolution under a wide
variety of conditions with ethyl hydroxy-acid 6 afforded 66% ee
atbest(withD-alaninol as chiral amine). Activation of the acid to
its corresponding imidazolide with CDI and addition to 35%
aqueous hydrazine at rt gave hydrazide 24 in 78-82% isolated
yield after crystallization from toluene.
Pd(OAc)2/Et2Zn/dppf. Attempted cyanation of triflate 16 in
the absence of a catalyst either failed or led to competing
hydrolysis of the sulfonate group.21 Reduction of the nitrile
with Raney cobalt in the presence of ammonia under 300 psi
of H2 at 90 °C gave complete conversion to the desired
7-aminomethyl-coumarin 26. A modified protocol with Raney
cobalt in AcOH/H2O as solvent was later devised due to
instability of the coumarin amine free base in NH3/MeOH.
Formation of isothiocyanate 7 was initially performed with
carbon disulfide and p-toluenesulfonyl chloride in THF.22
Due to serious safety concerns toward handling hazardous
CS2 in the pilot plant we sought alternative conditions for the
formation of 7. We initially found that use of thiophosgene in
CH3CN/Et3N gave a mixture of desired 7 and the correspond-
ing coumarin thiourea dimer 27 (30-70%). Use of inorganic
base in CH3CN/water gave improved profile and yield. Ulti-
mately we found that powdered CaCO3 gave best results
affording isothiocyanate 7 in 84% yield on kilogram scale
after simple precipitation of product via addition of water.23
Under these reaction conditions <1% of the corresponding
coumarin thiourea dimer 27 was formed.
Synthesis of Isothiocyanate Intermediate. Coumarin tri-
flate 16 was converted to the corresponding coumarin nitrile
25 by using Pd2dba3/dppf as catalyst and NaCN as cyanide
source (Scheme 7). The desired coumarin nitrile 25 was
isolated in 91% yield after simple precipitation from the
reaction mixture. Alternatively, we found that the cyanation
proceeded in 97% yield by using a catalyst prepared from
(21) The corresponding 7-bromo-coumarin afforded nitrile 25 upon
reaction with CuCN but required elevated temperatures (150 °C, 60 h) to
reach complete conversion.
(22) Wong, R.; Dolman, S. J. J. Org. Chem. 2007, 72, 3969.
(23) Dyson, G. M.; Hunter, R. F. Recl. Trav. Chim. Pays-Bas 1926, 45,
422.
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Through-Process End-Game. The end-game chemistry pro-
ceeded efficiently without isolation of intermediates (e.g., a
through-process; Scheme 8). Thus, acylation of hydrazide 24
with isothiocyanate 7 in THF at rt and cyclization of thiose-
micarbazide 5 with TsCl/pyridine24 was followed by aqueous
workup, carbon treatment, and solvent-switch to isopropyl
acetate (iPAc) to afford crude MK-0633 free base 2. Addition
of 2 (∼74A% HPLC) to a solution of p-toluenesulfonic acid
monohydrate (1.0 equiv) in iPAc at 60 °C gave crystalline
MK-0633 p-toluenesulfonate salt (1) (91.4% yield, 99.82A%
HPLC) as a white powder.
suspended in CH3CN (23.4 L) and water (18.7 L) at rt to afford a
white slurry that stirred well. Mixing caused an endotherm to
9.2 °C. A solution of thiophosgene (1.40 L, 18.37 mol, 120 mol %)
in CH3CN (1.40 L) was added dropwise over 45 min. Caution:
proper venting is required due to gas evolution! The internal
temperature rose from 9.2 to 24.5 °C during the addition of
thiophosgene and gave a thick cream-colored slurry. The batch
was stirred for 15 min and HPLC analysis showed >99%
conversion to isothiocyanate 7. Water (23.4 L) was added drop-
wise over 30 min and the slurry was filtered. The cake was washed
with water (3 ꢀ 18.7 L) and dried on the frit for 3 h. The solids were
dried under high vacuum/N2 sweep at 30 °C. Isothiocyanate 7 was
obtained as an off-white solid: 4.0 kg, 84% yield; mp 127-128 °C;
1H NMR (DMSO-d6) δ 7.62 (m, 2H), 7.49 (d, 2H, J = 8.0 Hz),
7.42 (dd, 2H, J = 9.0, 8.5 Hz), 7.36 (d, 1H, J = 8.5 Hz), 6.47
(s, 1H), 5.10 (s, 2H); 13C NMR (DMSO-d6) δ 163.9, 161.9, 159.4,
153.6, 153.5, 139.3, 131.0, 130.8, 127.3, 123.2, 118.1, 116.0, 115.8,
115.5, 115.2; 19F NMR (DMSO-d6) δ -112.8; HRMS calcd for
C17H11FNO2S [M þ H] 312.0495, found 312.0489. HPLC analy-
sis: column, Zorbax Extend C18 (4.6 ꢀ 150 mm, 5 μ); eluent 0.1%
aqueous NH4OH/CH3CN; 5-40% over 8 min, hold 2.5 min, to
90% over 5 min, hold 5.5 min, post-time 4.5 min; flow = 1 mL/
min; detection = 210 nm; temp = 40 °C; inj., 10 μL, coumarin
amine 26 tR = 11.4 min, coumarin thiourea dimer 27 tR = 16.0 min,
coumarin isothiocyanate 7 tR = 16.3 min.
Conclusion
In conclusion, we have developed a practical, chromato-
graphy-free route to MK-0633 featuring a through-process
hydrazide acylation/1,3,4-oxadiazole ring closure/salt for-
mation sequence to afford multikilogram amounts of MK-
0633 p-toluenesulfonate (1). It is notable that although major
efforts were directed toward developing practical asym-
metric routes to chiral R-hydroxy-acid intermediate 6, in
the end a simple diastereomeric salt resolution emerged to
provide 6 with improved productivity and ease of operation.
(1S)-N-{[4-(4-Fluorophenyl)-2-oxo-2H-chromen-7-yl]methyl}-
5-[1-hydroxy-1-(trifluoromethyl)propyl]-1,3,4-oxadiazol-2-amine
(2). A visually clean 100L round-bottomedflaskequipped witha
mechanical stirrer, a thermocouple, and a N2 inlet was charged
with solid isothiocyanate 7 (2.00 kg, 6.43 mol) and hydrazide 24
(1.28 kg, 6.62 mol). The solids were dissolved in THF (32 L) and
the amber solution was let stir at rt overnight. Internal tempera-
ture was 20 °C. HPLC analysis indicated >99% conversion to
the desired mixed thiosemicarbazide 5 after ∼15 h: mp 198.1-
Experimental Section
(2S)-2-Hydroxy-2-(trifluoromethyl)butanohydrazide (24).
Hydroxy-acid (S)-6 (3.18 kg, mol) was dissolved in THF (8 L).
The solution was added over 1 h to a stirred slurry of CDI (3.6 kg,
mol) in THF (8 L). Caution: Gas evolution, ensure good venting
of CO2! After stirring for 1 h, the resulting homogeneous solution
was added over 1 h to a stirred biphasic solution of 35 wt %
aqueous hydrazine (3.9 L) in THF (8 L). The reaction was stirred
at rt for 18 h and then diluted with iPAc (28 L). A portion of
aqueous hydrazine separated from the organic layer (∼1.7 kg)
was drained off. The organic layer was then washed with citric
acid (20% aqueous, 15 L then 3 L) and saturated aqueous
NaHCO3 (10 L). The layers were cut and solid NaCl was added
to the combined aqueous layers until saturation. The aqueous
layer was then back-extracted with iPAc (2 ꢀ 13 L). The
combined aqueous layers were extracted with iPAc (5 L). The
organic layers were combined, line-filtered, and concentrated.
The solution was solvent-switched to toluene at 25 °C and the
resulting gelatin-like slurry was filtered and washed with toluene
(8 L) to give 24 as a crystalline solid (2.78 kg, 97.7A%, 96.3
wt %, 99.5% ee, 78% yield): mp 98-99 °C, [R]20D -26.7 (c 1.69,
MeOH); 1H NMR (DMSO-d6) δ 9.25 (br s, 1H), 6.61 (br s, 1H),
4.49 (br s, 2H), 2.02 (m, 1H), 1.68 (m, 1H), 0.82 (t, 3H, J =
7.0 Hz); 13C NMR (DMSO-d6) δ 165.4, 124.8 (q, J = 285 Hz),
77.3 (q, J = 26 Hz), 24.8, 6.7; 19F NMR (DMSO-d6) δ -78.6.
Chiral GC analysis: column, Restek Rt-gammaDEX-sa, 30 m ꢀ
0.32 mm, 0.25 μm film (RTx-1701); injector, split 12ꢀ, cup linear
with Siltek; inj., 1.0 μL at 230 °C; det., FID at 250 °C; carrier gas,
He = 3.4 mL/min, flow 24 psi, 180 °C isothermal; (S)-hydrazide
tR = 7.9 min, (R)-hydrazide tR = 9.3 min; HRMS calcd for
C5H10F3N2O2 [M þ H] 187.0694, found 187.0688.
1
198.9 °C; H NMR (400 MHz, DMSO-d6) δ 10.31 (br s, 1H),
9.75 (br s, 1H), 7.99 (br s, 1H), 7.61 (dd, 2H, J = 5.6, 8.8 Hz),
7.37 (m, 4H), 7.23 (d, 1H, J = 8.8 Hz), 6.91 (s, 1H), 6.40 (s, 1H),
4.85 (m, 2H), 1.98 (m, 1H), 1.75 (m, 1H), 0.90 (t, 3H, J = 7.2 Hz);
13C NMR (125 MHz, DMSO-d6) δ 163.8, 161.9, 159.7, 153.8,
153.7, 144.5, 131.1, 131.0, 130.9, 126.4, 123.5, 123.1, 116.9,
115.9, 115.8, 114.8, 114.3, 77.9 (q, J = 26.1 Hz), 46.2, 25.5;
19F NMR (375 MHz, DMSO-d6) δ -77.8, -113.1. HRMS calcd
for C22H19F4N3O4S [M þ H] 498.1111, found 498.1114. IR
(cm-1, KBr pellet): 3303, 2980, 1701, 1617, 1543, 1510, 1420,
1374, 1282, 1239, 1194, 1005, 841; [R]20 -4.8 (c 1.08, EtOH).
D
HPLC analysis: 4.6 mm ꢀ150 mm Eclipse XDB Phenyl column,
gradient elution (0.1% H3PO4/CH3CN from 65:35 to 10:90 over
50 min, hold 10 min; run time = 60 min), flow rate = 1.0 mL/
min, detection = 210 nm, T = 25 °C; sample preparation = an
aliquot was withdrawn and diluted with CH3CN/water (75/25).
A 10 μL sample was injected, thiosemicarbazide 5 tR = 22.68
min. Chiral HPLC: Chiralpak AS-RH (4.6 mm ꢀ 150 mm);
isocratic method, CH3CN/water = 37:63, flow rate = 1.0 mL/
min, detection = 210 nm, T = 20 °C; sample preparation = an
aliquot was withdrawn and diluted with CH3CN/water (37/63),
a 5 μL sample was injected: tR = 21.7 min (major), 18.8 min
(minor). The reactor was equipped with a reflux condenser and
p-toluenesulfonyl chloride (1.47 kg, 7.72 mol) and pyridine
(1.09 L, 13.51 mol) were charged. The batch was heated to reflux
(internal temperature = 67 °C) for ∼18 h. HPLC analysis
indicated >95% conversion to MK-0633 free base 2. The heat
was shut off and the batch was cooled to 35 °C. The batch was
line-transferred into 1 N aqueous HCl (30 L) and then diluted
with iPAc (30 L). The layers were cut and the top organic layer
was washed with water (2 ꢀ 30 L). The crude solution was
combined with a batch of the same size to afford a total of ∼6 kg
of MK-0633 free base in iPAc/THF. The solution was treated
with Darco KB-B (1.5 kg, 25 wt %) for 2 h at 18-19.7 °C and
4-(4-Fluorophenyl)-7-(isothiocyanatomethyl)-2H-chromen-2-one
(7). A visually clean 100 L round-bottomed flask equipped with a
mechanical stirrer, a dropping funnel and a N2 inlet was charged
with coumarin amine hydrochloride 26 (4.68 kg, 15.31 mol) and
powdered CaCO3 (2.30 kg, 22.97 mol, 150 mol %). The solids were
(24) It is noteworthy that the corresponding semicarbazide analogue
afforded only 20% assay yield of MK-0633 under the same conditions. We
recently reported on the dramatically improved reactivity of thiosemicarba-
zides vs semicarbazides in the synthesis of aminooxadiazoles: Dolman, S. J.;
Gosselin, F.; O’Shea, P. D.; Davies, I. W. J. Org. Chem. 2006, 71, 9548.
J. Org. Chem. Vol. 75, No. 12, 2010 4159

Gosselin et al.
JOCArticle
then filtered on solkafloc. The filter cake was washed with iPAc
(3 ꢀ 10 L) and concentrated. The batch was solvent-switched to
iPAc (3 ꢀ 20 L): mp 92.9-94.3 °C; [R]D -4.5 (c 1.08, EtOH); 1H
NMR (400 MHz, DMSO-d6) δ 8.54 (t, 1H, J = 6.0 Hz), 7.60 (m,
2H), 7.38 (m, 4H), 7.31 (d, 2H, J = 8.4 Hz), 7.23 (s, 1H), 6.41 (s,
1H), 4.51 (d, 2H, J = 6.0 Hz), 2.08 (m, 1H), 1.97 (m, 1H), 0.88 (t,
3H, J = 7.2 Hz); 13C NMR (125 MHz, DMSO-d6) δ 164.1,
159.6, 156.1, 153.7, 153.6, 143.8, 131.0, 130.9, 126.8, 123.5,
117.4, 116.0, 115.8, 115.4, 114.7, 73.7 (q, J = 26.1), 45.4, 26.1;
pTsOH H2O. After 20% of the crude MK-0633 free base
3
solution was added the addition was stopped. MK-0633 tosy-
late 1 seeds (1 g) were added and the mixture was stirred for
10 min. Addition of crude free base was then continued. After
the end of the addition, the batch was gradually cooled to rt
over 2 h, then let stir at rt for 16 h and the slurry was filtered.
The filter cake was suspended in iPAc (40 L), filtered, resus-
pended in iPAc (40 L) and filtered. The batch was dried under
low vacuum on the frit for 4 h. The solids were then transferred
in trays to a vacuum oven at 30 °C. The solids were dried to a
constant weight under low vacuum/N2 sweep. MK-0633 tosylate
salt (1) was obtained as a white solid (6.64 kg, 91.4% yield): mp
164-165 °C; [R]20D - 0.86 (c 10.0, EtOH); 1H NMR (500 MHz,
DMSO-d6) δ 8.58 (1 H, t, J = 6.2 Hz), 7.62 (2 H, dd,
J = 8.3, 5.4 Hz), 7.49 (2 H, d, J = 7.8 Hz), 7.47-7.38 (4 H, m),
7.33 (1 H, d, J = 8.3 Hz), 7.13 (2 H, d, J = 7.7 Hz), 6.44 (1 H, s),
4.53 (2 H, d, J = 5.6 Hz), 2.30 (3 H, s), 2.17-2.05 (1 H, m),
2.03-1.93 (1 H, m), 0.90 (3 H, t, J = 7.37 Hz); 13C NMR (125
MHz, DMSO-d6) δ 164.1, 162.9 (d, J = 246.8 Hz), 159.6, 156.1,
153.7, 153.6, 145.5, 143.7, 137.7, 131.1 (d, J = 3.5 Hz), 130.9 (d,
J = 8.7 Hz), 128.1, 126.8, 125.4, 124.5 (q, J = 286.6 Hz), 123.5,
117.4, 115.9 (d, J = 22.0 Hz), 115.4, 114.7, 73.7 (q, J = 28.6 Hz),
45.4, 26.1, 20.8, 7.0; 19F NMR (375 MHz, DMSO-d6) δ -79.7,
-113.1; HRMS calcd for C22H18F4N3O4 [Mþ H] 464.1228, found
464.1246. IR (cm-1, NaCl thin film) 3324, 3010, 2977, 1735, 1716,
1618, 1510, 1428, 1215, 1178. HPLC analysis: eclipse XDB-phenyl
column 4.6 mm ꢀ 15 cm (0.1% aq H3PO4/CH3CN 65:35 to 10:90
over 50 min, 1.0 mL/min, 210 nm, 25 °C); MK-0633 (1) tR = 16.86
min. Chiral HPLC analysis: Chiralpak AD-H column 4.6 mm ꢀ 25
cm (EtOH/hexane 60:40, hold 15 min, 0.5 mL/min, 300 nm, 30 °C);
(S)-enantiomer tR = 9.5 min; (R)-enantiomer tR = 11.5 min.
19F NMR (375 MHz, DMSO-d6) δ -79.8, -113.1; IR (cm-1
,
KBr pellet) 3309, 3068, 2945, 1701, 1617, 1510, 1420, 1372, 1329,
1280, 1238, 1190, 1160, 1108, 1079, 1014, 841. HRMS calcd
for C22H17F4N3O4 [M þ H] 464.1233, found 464.1228. HPLC
analysis: 4.6 mm ꢀ 150 mm Eclipse XDB Phenyl column,
gradient elution (0.1% H3PO4/CH3CN from 65:35 to 10:90 over
50min, hold10min; runtime= 60 min), flowrate= 1.0 mL/min,
detection = 210 nm, T = 25 °C;samplepreparation = an aliquot
was withdrawn and diluted with CH3CN/water (75/25). A 10 μL
sample was injected. HPLC tR = 17.65 min. Chiral HPLC
retention time: major enantiomer tR = 14.00 min, minor enan-
tiomer tR = 19.75 min.
(1S)-N-{[4-(4-Fluorophenyl)-2-oxo-2H-chromen-7-yl]methyl}-
5-[1-hydroxy-1-(trifluoromethyl)propyl]-1,3,4-oxadiazol-2-ami-
nium 4-Methylbenzenesulfonate (MK-0633 Tosylate, 1). The
crude solution of MK-0633 free base 2 (5.4 kg according to
HPLC after concentration) in iPAc (15.5 L) was diluted in iPAc
(40 L) and was stirred at rt for 16 h. Stirring was stopped and
solids settled rapidly at the bottom of the flask. The super-
natant was filtered and analyzed by HPLC and 1H NMR
spectroscopy. HPLC assay of the solution indicated 54.4 kg
at 9.72% wt in iPAc for a total mass 5.3 kg of free base 2. 1H
NMR analysis showed a 1:8.74 wt ratio of free base: iPAc. Thus
the free base solution already contained 52.89 L of iPAc.
Supporting Information Available: Experimental proce-
dures and characterization for compounds (S)-3, 4, (S)-6, 12,
15, 16, (S)-22, 25, and 26 and copies of 1H, 13C and 19F NMR
spectra of selected compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
p-Toluenesulfonic acid H2O (2.17 kg, 11.42 mol) was dissolved
3
in iPAc (13.2 L). The slurry was heated to 60 °C and stirred until
complete dissolution of pTsOH H2O (∼20 min). The solution
3
of free base 2 was added over 2 h to the hot solution of
4160 J. Org. Chem. Vol. 75, No. 12, 2010