Evolution of a Scale-Up Synthesis to a Potent GluN2B Inhibitor and Its Prodrug

Article abstract of DOI:10.1021/acs.oprd.8b00120

This paper describes the efficient scale-up synthesis of the potent negative allosteric glutamate N2B (GluN2B) inhibitor 1 (BMS-986169), which relies upon a stereospecific S_N2 alkylation strategy and a robust process for the preparation of its

Full text of DOI:10.1021/acs.oprd.8b00120

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Full Paper
The Evolution of a Scale-up Synthesis to
a Potent GluN2B Inhibitor and its Prodrug.
James Kempson, Huiping Zhang, Michael K. Y. Wong, Jianqing Li, Peng Li, Dauh-
Rurng Wu, Richard A. Rampulla, Michael A. Galella, Marta Dabros, Sarah C. Traeger,
Vetrichelvan Muthalagu, Anuradha Gupta, Pirama Nayagam Arunachalam, and Arvind Mathur
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00120 • Publication Date (Web): 05 Jun 2018
Downloaded from http://pubs.acs.org on June 5, 2018
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The Evolution of a Scaleꢀup Synthesis to a Potent
GluN2B Inhibitor and its Prodrug.
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James Kempson,*,† Huiping Zhang,† Michael K. Y. Wong,† Jianqing Li,† Peng Li,† DauhꢀRurng
Wu,† Richard Rampulla,† Michael A. Galella,§ Marta Dabros,§ Sarah C. Traeger,† Vetrichelvan
Muthalagu, ‡ Anuradha Gupta, ‡ Pirama Nayagam Arunachalam,‡ and Arvind Mathur.†
† Discovery Chemistry and Molecular Technologies, BristolꢀMyers Squibb Research and
Development, Princeton, New Jersey, 08540, United States
§ Drug Product Science & Technology, Materials Science & Engineering, BristolꢀMyers Squibb
Research and Development, Princeton, New Jersey, 08540, United States
‡ Department of Discovery Synthesis, Biocon BristolꢀMyers Squibb Research Center (BBRC),
Bangalore 560099, India
*Corresponding author: james.kempson@bms.com
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KEYWORDS. NR2B, deoxyꢀfluorination, mesylate alkylation, prodrug
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ABSTRACT. This paper describes the efficient scaleꢀup synthesis of the potent, negative
allosteric glutamate N2B (GluN2B) inhibitor 1 (BMSꢀ986169) which relies upon a stereospecific
SN2 alkylation strategy and a robust process for the preparation of its phosphate prodrug 28
(BMSꢀ986163) from parent 1 using POCl3. A deoxyfluorination reaction employing bis(2ꢀ
methoxyethyl)aminosulfur trifluoride (DeoxoꢀFluor®) is also used to stereospecifically introduce
a fluorine substituent. The optimized routes have been demonstrated to provide APIs suitable for
toxicological studies inꢀvivo.
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INTRODUCTION
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Major depressive disorder (MDD) is a leading cause of disability in the developed world.1
Despite impressive gains in treating this disease with monoaminergic transporter blockers, a
significant number of patients are inadequately treated with current therapy. The development of
new drugs with different mechanisms of action may offer an approach to improved outcomes in
these patients. Clinical results with the NMDA (Nꢀmethylꢀdꢀaspartic acid) receptor blocker,
ketamine, have indicated that antagonism of NMDA receptors may represent a novel approach to
providing effective therapy for treatmentꢀresistant depression. Additional preclinical and early
clinical work has provided support for similar efficacy through negative allosteric modulation of
the GluN2Bꢀsubtype NMDA receptors. BMSꢀ986169 1 (Figure 1) and its phosphate prodrug
BMSꢀ986163 28 (Figure 2) were recently selected as promising GluN2B inhibitors worthy of
further evaluation as agents for MDD.2 To this end, reliable and scalable syntheses of 1 and 28
were required to support drug development activities that included pharmacologic assessment,
toxicological evaluation and formulation development. To meet these requirements, the
identification and execution of a scalable synthetic sequence for 1 was essential and is outlined
below. A practical method for the conversion of parent 1 to its prodrug 28 is also described.
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Figure 1. The structure of BMSꢀ986169
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RESULTS AND DISCUSSION
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First generation synthesis. A key disconnection common to all synthetic approaches is
highlighted in Scheme 1 and relies upon an alkylation strategy to unite the two appropriatelyꢀ
substituted fragments 2 and 3.
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Scheme 1. Retrosynthetic analysis of drug substance 1
The initial approach for the preparation of fragment 2 targeted X = Br and the synthesis
of the bromolactam (±)-7 is shown in Scheme 2. The synthesis of (±)-7 was achieved in two
steps using a modification to the reported procedure for a related bromolactam.3 Thus, starting
from acylation of the benzylamine 4 with the acid chloride 5,4 baseꢀpromoted cyclization then
afforded the bromolactam 7 as a racemic mixture. Attempts to separate the enantiomers were
successful using preparative chiral supercritical fluid chromatography (SFC); however, each
enantiomer quickly racemized back to (±)-7 during evaporation which thus necessitated the use
of the racemic material in this, our original approach, with resolution to now occur at a later
stage in the synthesis.
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Scheme 2. Synthesis of racemic bromolactam fragment, (±)-7
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The synthesis of the piperidine coupling partner is shown in Scheme 3 and utilized a
SuzukiꢀMiyaura reaction between the two commerciallyꢀavailable fragments 8 and 9,5 to produce
10 in 80% isolated yield. Hydroboration of the olefin in 10 was achieved using sodium
borohydride and boron trifluoride etherate6 which following oxidative workꢀup gave a racemic
mixture of the transꢀsecondary alcohol, (±)-11. This racemic mixture was separated by SFC on
chiral phase to afford each enantiomer of 11 with high e.e. With each antipode, 12 and 13, of the
chiral alcohol in hand, each was taken on into the chemistry outlined in Scheme 4.
Scheme 3. Synthesis and separation of piperidine fragment 12
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Hydrogenolysis of the benzyl ether in 12 afforded phenol 14 which after Bocꢀ
deprotection was used in the alkylation reaction with (±)-7, employing an excess of triethylamine
(5.0 equivalents) as base in warm (55 °C) acetonitrile to give 16 as a diastereomeric mixture in
49% yield after column chromatography. Subsequent treatment with diethylaminosulfur
trifluoride (DAST)7 led to 17 albeit in only 34% yield. Final SFC separation gave 1, with the
indicated stereochemistry assumed from Xꢀray data of related GluN2B active compounds which
had been synthesized utilizing the same synthetic route.2d
Scheme 4. Fragment union and elaboration to drug substance 1
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Although the process described in Scheme 4 was suitable for the preparation of milligram
quantities of 1 and was used extensively in the analog effort to prepare related compounds,
further scaleꢀup to supply material for toxicological evaluation presented several challenges.
Most notably: 1) the necessity of using racemic bromolactam, with only a modest (49%) yield in
the alkylation reaction; 2) two chiral SFC purification steps, one occurring at the very end of the
synthesis with 50% of the material discarded as the undesired isomer was at each of these steps;
and 3) a lateꢀstage fluorination (DAST) which proceeded in low yield. Taken together, these
inefficiencies resulted in a 2.2% yield over just 4 chemical steps starting from the racemic
alcohol (±)-11 with a projected amount of 100 g of (±)-11 needed to produce only 2 g of 1. This
prompted us to pursue a more convergent synthesis in which: 1) a chiral alkylation approach was
possible; and 2) earlier installation of the fluorine substituent with high yield (and confirmed
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stereospecificity).
The revised, second generation synthesis based upon these areas of
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optimization will now be discussed.
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Second generation synthesis. Based upon the observed racemization of the previously prepared
bromolactam, (±)-7, we decided to pursue a stereoselective synthesis starting from the readily
available (S)ꢀ3ꢀhydroxyꢀpyrrolidinꢀ2ꢀone8 (18) with a stereospecific SN2 reaction of the
appropriately functionalized carbinol anticipated to give the desired alkylation outcome. Several
strategies have utilized this disconnection as part of both medicinal chemistry efforts and natural
product syntheses. The Nielsen synthesis of peptide nucleic acids (PNAs) utilizes the chiral
hydroxyl group of a related lactam in a Mitsunobu coupling with adenine to give clean inversion
of configuration at the carbinol center. 9 Ganesh has also performed similar transformations to
other pyrrolidylꢀPNA analogs using an Nꢀalkylation displacement strategy of the –OMs
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pyrrolidinone.
Others have utilized azide displacement11 with complete inversion of
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stereochemistry including Snider in the synthesis of the immunosuppressant FR901483.
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this end, we were able to demonstrate the synthesis of the chiral mesylate 22 in four high
yielding steps (Scheme 5). This approach circumvented the need for SFC purification and we
were pleased to find that the chiral integrity of 22 (>99% ee) remained intact upon storage.
Scheme 5. Synthesis of chiral mesylate 22
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With the lactam fragment inꢀhand, we next sought to improve the synthesis of the
piperidine moiety. In order to conserve our supplies of the desired diastereoisomer 12, the initial
optimization work was carried out using the less valuable enantiomer 13. We decided to install
the fluorine substituent prior to the alkylation step since the previous fluorination reaction with
the fully elaborated molecule was low yielding (34%) and had proven challenging to optimize.
Thus, our initial attempts to perform the fluorination reaction began with the benzylꢀprotected
ether 13; however, treatment of 13 with 1.1 equivalents of DAST gave a surprisingly complex
mixture of compounds which proved difficult to separate and characterize. Interestingly, the
same reaction performed on the phenol 28 gave a much simplified reaction profile, providing
two chromatographicallyꢀseparable products each of which ionized with the same M + H ion in
LC/MS analysis. Further treatment with 4 M HCl led to the isolation of the Bocꢀdeprotected
amines which were isolated as the monoꢀHCl salts following recrystallization from isopropanol.
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Unambiguous structure confirmation of these two products together with the corresponding
starting amine was provided by single crystal Xꢀray crystallographic analysis.12 Interestingly,
the desired product 24 was formed with retention of configuration along with the unanticipated
rearrangement product 25, which was hypothesized to have originated through the intermediacy
of the reactive species depicted in Scheme 6 involving anchimeric participation of the phenol
ring. Nucleophilic attack of fluoride at the 3ꢀposition of this intermediate would then lead to the
desired product 24, while ring opening at the 4ꢀposition would lead to the observed sideꢀproduct
25 with the stereochemistry shown. A similar mechanism is presumably operating during the
deoxyfluorination reaction for a series of 3ꢀ(4ꢀfluoropiperidinꢀ3ꢀyl)ꢀ2ꢀphenylindole derivatives in
which nucleophilic assistance from an indole nitrogen would lead to the intermediacy of a
similar cyclopropane species. Interestingly in this case, the authors report the complete
regiospecific and enantioselective rearrangement product as the only outcome of this reaction. 13
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Scheme 6. Early installation of fluorine using DAST
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With the product structures confirmed for this reaction, we then proceeded to vary the
nature of the fluorine source along with reagent stoichiometry and reaction temperature in an
effort to alter the product distribution in favor of the desired product 24. Significant increases in
the yield of 24 were realized when the reaction was conducted at ꢀ78°C and allowed to warm to 0
°C (Table 1, entry 3). Interestingly, this observation did not hold when using the
aminodifluorosulfinium7 salts XtalFluorꢀM® and XtalFluorꢀE®, with 25 predominating in the
case of XtalFluorꢀM® (Table 1, entries 4 and 5). To our delight, DeoxoꢀFluor® was found to
give the best ratio of 24:25 (Table 1, entry 6) with a 75% isolated yield of 24 obtained when
performed on a 25 g scale. The observed enhanced selectivity with the use of DeoxoꢀFluor® is
somewhat surprising. A rationalization may be attributed to a stabilization effect of the proposed
carbocation species through interactions between the ether oxygens of the DeoxoꢀFluor® reagent.
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Indeed, the greater stability of this reagent relative to DAST has been rationalized in a similar
manner owing to conformational effects of the reagent related to bonding between the ether
oxygens and sulfur.14 Despite the improved safety profile of DeoxoꢀFluor® relative to DAST,
the hazards associated with this class of reagent necessitated caution when using it on this scale.
Caution was taken to perform the reaction under anhydrous conditions and avoid the presence of
trace acid thus preventing the evolution of hydrogen fluoride gas. The waste stream was also
cautiously basified to pH 8.5 using 10% aqueous ammonium hydroxide. For this scale
application, the batchꢀtype use of DeoxoꢀFluor® proved a satisfactory solution in light of the
increased yield and drastically minimized byꢀproduct formation for this transformation. Further
process improvements could also be considered in the development of a continuousꢀflow process
to prevent reagent accumulation and circumvent the inherent problems associated with Deoxoꢀ
Fluor®.15
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Scheme 7. Optimization of fluorination reaction
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Table 1. Fluorine source, equivalence and temperature screen for optimization of regioꢀselective
fluorination
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entry
F source
equiv
temperature
24:25 HPLC
isolated yield of 24
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ratio
(%)
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DAST
DAST
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3.0
3.0
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3.0
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0°C
0 °C
2 : 1
2.6 : 1
13 : 1
1 : 2
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ꢀ78 °C to rt
ꢀ78 °C to rt
0 °C
XtalFluorꢀM®
XtalFluorꢀE®
DeoxoꢀFluor®
1.4 : 1
30 : 1
With these optimized conditions performing well on scale, the reaction was performed
using the desired enantiomer 14 to provide 26 in comparable overall yield. Bocꢀdeprotection of
26 with 4M HCl in dioxane led to 27, which was initially isolated as the monoꢀHCl salt.
Unfortunately, use of the amine hydrochloride salt in the alkylation reaction with the chiral
mesylate 22 resulted in a significant byꢀproduct resulting from chloride ion displacement of the
mesylate. Further alkylation of this chloroꢀcompound was not observed under the reaction
conditions and could be separated from the desired alkylation product during purification. No
attempt was made to determine the chiral integrity of this sideꢀproduct. This observation
necessitated free base generation prior to the alkylation reaction and was achieved by the
dropwise addition of triethylamine into a THF solution of the amineꢀsalt of 27. The
triethylammonium chloride that was formed was filtered off and the amineꢀcontaining filtrate
evaporated in vacuo and taken on directly into the final alkylation reaction. This was performed
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using potassium carbonate as the base with 1.1 equivalents of the amine 27 in acetonitrile to
secure the desired product 1 in 65% isolated yield. The stereochemistry of the alkylation
reaction was confirmed to be that in 1 since it was consistent with authentic material isolated
from the previous synthetic route, with all stereocenters definitively confirmed from a single
crystal Xꢀray analysis of the final product.
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Scheme 8. Optimized fluorination, deprotection and coupling with Xꢀray structure confirmation.
OH
OH
4 M HCl in dioxane
OH
Deoxo-Fluor
(75%)
BocN
BocN
OH
F
ClH.HN
(95%)
F
26
14
27
O
N
O
OMs
N
22
N
OH
CH3CN, DIPEA
(65%)
F
1
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Phosphate prodrug synthesis. To improve the solubility of 1, the discovery chemistry team
developed a prodrug strategy which identified the phosphate prodrug 28 (BMSꢀ986163, Figure
2) as a clinical candidate that offered a significant improvement in aqueous solubility.
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Figure 2. The structure of phosphate prodrug BMSꢀ986163 (28)
The original attempts to prepare 28 from parent 1 were disappointing. As shown in Scheme 9,
the reaction of 1 with phosphoryl chloride in the presence of DMAP and pyridine in
dichloromethane followed by hydrolysis with water gave the desired phosphate prodrug 28 in
only 8% isolated yield following reversed phase preparative HPLC. Alternatively, 1 was
converted to the bisbenzylphosphate 31 under standard Atherton–Todd (AT) reaction16
conditions in 95% yield. Debenzylation using H2 (balloon) over 10% Pd/C in MeOH as the
solvent led to the complete conversion of 31 to 28. However, the hydrogenation reaction
mixture became a thick suspension as the reaction proceeded and the product 28 was tightly
adsorbed on Pd/C. Excessive rinsing of the filter cake with a range of solvents, including water,
MeOH, CH2Cl2 and EtOAc, failed to recover most of the product 28, resulting in only an 18%
isolated yield. We then examined the debenzylation reaction using bromotrimethylsilane as a
mild reagent.17 Thus, the reaction of 31 with 4.4 equiv of Me3SiBr in acetonitrile at room
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temperature for 2 h gave complete conversion. After hydrolysis of the TMS ester moieties,
evaporation and multiple triturations with EtOAc and CH2Cl2, product 28 was isolated in 72%
yield with 94% HPLC purity. Further upgrades in the purity of 28 to meet API quality
requirements using chromatography proved challenging due to its low solubility in organic
solvents and the need for TFA as a modifier for preparative HPLC, which proved difficult to
remove.
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Scheme 9. Original attempted synthesis of prodrug 28
With the objective of developing a practical scalable synthesis of prodrug 28 with high
purity without using column chromatography for purification, we decided to reinvestigate the
reaction of 1 with phosphoryl chloride. It was quickly discovered that the conversion of 1 to
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dichlorophosphate 29 at 0 C in CH2Cl2 with 5 equiv each of POCl3, DMAP and pyridine went
smoothly and cleanly and was complete within 3 h (Scheme 9). However, the hydrolysis of 29
to mono chloride 30 and then to 28 was unexpectedly slow. After adding water and vigorously
stirring for several days, substantial amounts of 29 and 30 still remained in the reaction mixture
based on LCMS analysis. It was envisioned that the slow hydrolysis was most likely due to
immiscibility of water with CH2Cl2. By switching the solvent for this reaction to THF, we were
delighted to find that the hydrolysis was complete in less than 4 h with a clean reaction profile.
Additional careful optimization of the reaction conditions revealed that the DMAP was not
necessary, and exposure of 1 to 2.5 equivalents of TEA and 5 equivalents of POCl3 followed by
hydrolysis led to the complete conversion to 28 with greater than 98% solution HPLC purity.
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With the above optimized conditions in hand, we next turned our focus to exploring a
practical workꢀup and purification process using precipitation and crystallization (Scheme 10).
After completion of the reaction on a gram scale, the reaction mixture was a clear solution, even
after removal of THF, and the pH value of the resulting aqueous solution was less than one. This
indicated that the product existed as an HCl salt. Neutralization of the solution with 1N NaOH to
adjust the pH to between 1 and 5 led to the formation of a white suspension. Further increasing
pH >5 with 1N NaOH resulted in a clear solution, indicating formation of the phosphate sodium
salts. A series of further experiments confirmed that at pH = 1, the recovery yield was the
highest (>95%) after filtration, washing with water and drying. The product was a slightly
yellow free base isolated in amorphous form that had a HPLC purity of greater than 98%.
Scheme 10. Optimized prodrug formation
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C10
R
C15
C14
S
S
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In practical large scale preparations, both water and 1N NaOH were added at 0 C to
avoid potential side reactions during the hydrolysis step. Crystallization screening was also
performed in order to isolate a form suitable for formulation, and to remove small impurities and
residual THF as well as to facilitate the drying process. Thus, 80% EtOH was found to be the
superior crystallization solvent, affording a crystalline form that reflected an upgrade to the
purity, removal of the yellow color, with recovery of the majority of the material and that dried
rapidly at room temperature. The process was highly reproducible in tens of grams scale and no
racemization was observed.
CONCLUSION
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A scaleꢀup route has been developed for the synthesis of 1. As part of this effort, a high yielding,
4ꢀstep synthesis of a chiral pyrrolidine mesylate was developed together with a regioselective
fluorination reaction. The improved synthesis allowed confirmation of stereochemistry and
enabled preparation of API in quantities suitable for preclinical toxicological evaluation. Further
improvements to the synthesis are needed to enable large scale (kg) synthesis and will be focused
on the practical implementation of the fluorination reaction in addition to the selection of more
appropriate solvents that are devoid of the liabilties associated with dioxane and
dichloromethane. These optimizations will be reported in due course. In addition, a convenient,
oneꢀpot, high yielding and chromatographyꢀfree procedure using POCl3 was developed for the
large scale preparation of prodrug 28. The improved processes enabled the efficient preparation
of multiple batches of 28 at a purity level suitable for toxicological evaluation, including Ames
mutagenicity testing.
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EXPERIMENTAL SECTION
All reagents were obtained from the Aldrich Chemical Company and used without further
purification unless otherwise stated. All reactions were performed under a N2 atmosphere. All
glassware was dried and purged with N2 or Ar before use. All reactions were monitored using a
Shimadzu LCMS system under the following conditions: Phenomenex C18 column 10 ꢁm 4.6 X
50 mm. Solvent: A = 10% MeOH/90% H2O with 0.1% CF3CO2H; B = 90% MeOH/10% H2O
with 0.1% CF3CO2H. Gradient: 0ꢀ100% B over 4 min. Flow: 4 mL/min, Wavelength: 220 nm.
HPLC analyses were performed using a Shimadzu system (model SPD 10AV). All 1H NMR and
13C NMR spectra were recorded on a Bruker 300 MHz or 400 MHz spectrometer using DMSOꢀ
d6 or CDCl3 as the solvents.
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tertꢀButyl 4ꢀ(4ꢀ(benzyloxy)phenyl)ꢀ3,6ꢀdihydropyridineꢀ1(2H)ꢀcarboxylate (10). To a
solution of 1ꢀ(benzyloxy)ꢀ4ꢀbromobenzene (170 g, 646 mmol) in anhydrous CH3CN (1700 mL)
was added tertꢀbutyl4ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ5,6ꢀdihydropyridineꢀ1(2H)ꢀ
carboxylate (200 g, 646 mmol) in one portion followed by H2O (1700 mL). The resulting
solution was purged with N2 for 5 minutes before the subsequent addition of Na2CO3 (26.4 g,
249 mmol) and Pd(Ph3P)4 (3.55 g, 3.1 mmol) each in one portion with continuous purging with
N2. The mixture was heated to 88 °C overnight before cooling to room temperature, diluting
with EtOAc (2000 mL) and washing the organic layer with brine solution (1500 mL) and drying
(Na2SO4). Evaporation of the organics in vacuo afforded the crude material (390 g) which was
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purified by silica gel chromatography using 6ꢀ12% EtOAc in petroleum ether as the eluent to
1
afford 228 g of 10 as a white solid, yield 96.0%, 99.7% purity. H NMR (300MHz, CDCl3)
δ
7.49ꢀ730 (m, 5H), 7.27 (d, J = 10.7 Hz, 2H), 6.99ꢀ6.87 (m, 2H), 6.03ꢀ5.87 (m, 1H), 5.07 (s, 2H),
4.05 (d, J = 2.6 Hz, 2H), 3.62 (t, J = 5.7 Hz, 2H), 2.49 (br. s., 2H), 1.49 (s, 9H); MS m/z: [M+Hꢀ
Boc]+: 266.2; HPLC (XBridge 5ꢀ C18 4.6x50 mm, 4 mL/min, Solvent A: 10 % MeOH/water
with 0.2 % H3PO4, Solvent B: 90 % MeOH/water with 0.2 % H3PO4, gradient with 0ꢀ100 % B
over 4 minutes): 1.16 minutes.
tertꢀButyl (3S,4S)ꢀ4ꢀ(4ꢀ(benzyloxy)phenyl)ꢀ3ꢀhydroxypiperidineꢀ1ꢀcarboxylate (±)ꢀ(11).
Boron trifluoride etherate (146 mL, 1149 mmol) was added dropwise over 30 minutes to a
cooled (0 °C) solution of NaBH4 (43.5 g, 1149 mmol) in anhydrous THF (1200 mL). The
reaction mixture became a white turbid suspension after the addition at which point the reaction
mixture was allowed to warm to room temperature and stirred for 30 minutes. A solution of 10
(140 g, 383 mmol) in anhydrous THF (600 mL) was added over a period of 15 minutes and the
mixture stirred at room temperature for 2 hours. The reaction mixture was quenched by the slow
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addition (2 h) of H2O (140 mL) followed by EtOH (140 mL). NaOH (10% w/w aqueous
solution, 770 mL) followed by H2O2 solution (70 mL) were added and the reaction mixture
heated to 70 °C overnight. The reaction mixture was allowed cool to room temperature before
the addition of CH2Cl2 (3000 mL) and cooling to 10 °C. The pH of the mixture was adjusted to
6ꢀ7 using 3N HCl (700 mL) and the organic layer was separated. The aqueous layer was
extracted with CH2Cl2 (2 x 500 mL), the organic layers combined, dried (Na2SO4) and
evaporated in vacuo to afford crude material (150 g) which was triturated with petroleum ether
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(500 mL) to afford (±)-(11) (140 g, 365 mmol, 95% yield) as a white solid. H NMR (400 MHz,
DMSOꢀd6)
δ 7.47ꢀ7.42 (m, 2H), 7.42ꢀ736 (m, 2H), 736ꢀ7.28 (m, 1H), 7.14 (d, J = 9.0 Hz, 2H),
6.92 (d, J = 9.0 Hz, 2H), 5.07 (s, 2H), 4.74 (d, J = 5.5 Hz, 1H), 4.10 (br. s., 1H), 3.94 (br. s., 1H),
3.46ꢀ335 (m, 1H), 2.47ꢀ231 (m, 2H), 1.70ꢀ1.61 (m, 1H), 1.55ꢀ1.45 (m, 2H), 1.42 (s, 9H).; MS
m/z: [M+HꢀtertBu]+: 328.3.
SFC Separation of (±)-(11) to give 12 and 13:
tertꢀButyl (3S,4S)ꢀ4ꢀ(4ꢀ
(benzyloxy)phenyl)ꢀ3ꢀhydroxypiperidineꢀ1ꢀcarboxylate, (±)-(11) (140 g) was separated into the
individual enantiomers using preparative supercritical fluid chromatography under the following
conditions: A Thar SFCꢀ250 instrument was utilized with a LuxꢀCelluloseꢀ2 (250 x 21 mm), 5
ꢀm column eluting with 60% CO2 and 40% of a solution of 0.3% diethylamine in MeOH at a
flow rate of 100.0 g/min. Sample was injected at a concentration of 74 mg/mL. Analytical SFC
was carried out on LuxꢀCelluloseꢀ2 (250 x 4.6 mm), 5 ꢀm column eluting with 55% CO2 and
45% of a solution of 0.3% diethylamine in MeOH at a flow rate of 3.0 g/min. The recovery was
50.0 g of 12/peak 1 (desired) with a retention time of 2.49 minutes, and 45g of 13/peak 2
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(undesired) with a retention time of 3.08 minutes. Analytical data matched those from the
racemate.
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tertꢀButyl (3S,4S)ꢀ3ꢀhydroxyꢀ4ꢀ(4ꢀhydroxyphenyl)piperidineꢀ1ꢀcarboxylate (14).
A
solution of 12 (100 g, 261 mmol) in anhydrous MeOH (1500 mL) was treated with 10% Pd on C
(10 g) in a pressure bottle. H2 gas at 50 psi was introduced and the reaction mixture was stirred
for 16 h. The mixture was filtered through celite and concentrated in vacuo to afford 14 (74 g,
247 mmol, 95% yield) as a white solid which was sufficiently pure to carry forward without
1
further purification (purity 99%). H NMR (400 MHz, DMSOꢀd6)
δ 9.10 (br. s., 1H), 7.01 (d, J
= 8.5 Hz, 2H), 6.65 (s, 2H), 4.70 (d, J = 5.0 Hz, 1H), 4.09 (br. s., 1H), 3.93 (br. s., 1H), 3.17 (s,
2H), 2.79ꢀ2.63 (m, 1H), 2.34 (br. s., 1H), 1.68ꢀ1.57 (m, 1H), 1.44 (br. s., 1H), 1.42 (s, 9H); MS
m/z: [M+HꢀtertBu]+: 238.0. HPLC (XBridge 5ꢀ C18 4.6x50 mm, 4 mL/min, Solvent A: 10%
MeOH/H2O with 0.2 % H3PO4, Solvent B: 90 % MeOH/H2O with 0.2 % H3PO4, gradient with 0ꢀ
100 % B over 4 minutes): 2.45 minutes.
tertꢀButyl (3S,4S)ꢀ3ꢀfluoroꢀ4ꢀ(4ꢀhydroxyphenyl)piperidineꢀ1ꢀcarboxylate (26).
A
solution of 14 (25.0 g, 85.0 mmol) in anhydrous CH3CN (500 mL) was cooled to 0 °C under a
N2 atmosphere. To the stirred solution was added bis(2ꢀmethoxyethyl)aminosulfur trifluoride
(DeoxoꢀFluor®, 50% solution in toluene, 94 mL, 256 mmol) dropwise via an addition funnel
over 65 min. After the addition was complete, the reaction mixture was stirred for 30 minutes at
0 °C and then allowed to warm to room temperature and stirred for an additional 2 h. Saturated
NH4Cl solution (500 mL) was added and the mixture extracted with CH2Cl2 (2 x 750 mL). The
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organic layers were combined, dried (Na2SO4) and concentrated in vacuo to afford crude product
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which was purified by silica gel chromatography eluting with 0ꢀ15% acetone in hexanes to
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afford 26 (16.6 g, 56 mmol, 66% yield, 98% purity). HNMR (400 MHz, CDCl3)
δ 7.15 (d, J =
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8.6 Hz, 2H), 6.83 (dt, J = 8.6, 2.0 Hz, 2H), 4.59ꢀ4.48 (m, 1H), 4.47ꢀ437 (m, 1H), 4.23ꢀ4.12 (m,
1H), 2.88ꢀ2.68 (m, 3H), 1.96ꢀ1.84 (m, 1H), 1.80ꢀ1.66 (m, 1H), 1.51 (s, 9H); 19F NMR (471 MHz,
CDCl3)
δ
ꢀ184.00; MS m/z: [M+HꢀtertBuꢀF]+: 220.0. HPLC (XBridge 5ꢀ C18 4.6x50 mm, 4
mL/min, Solvent A: 10 % MeOH/H2O with 0.2 % H3PO4, Solvent B: 90 % MeOH/H2O with 0.2
% H3PO4, gradient with 0ꢀ100 % B over 4 minutes): 4.07 minutes.
4ꢀ((3S,4S)ꢀ3ꢀFluoropiperidinꢀ4ꢀyl)phenol (27). A solution of 26 (36.4 g, 123 mmol) in
anhydrous dioxane (300 mL) was treated with HCl (4M in 1,4ꢀdioxane, 175 mL, 700 mmol).
The reaction mixture was allowed to stir at room temperature for 6 h and then evaporated in
vacuo to provide the HCl salt of 27 (25.3 g). Without further isolation, the HCl salt was
suspended in anhydrous THF (1000 mL) and Et3N added dropwise (75 mL). After stirring for 1
h, the reaction mixture was filtered and concentrated in vacuo to afford 27 (21.4 g, 105 mmol,
1
85% yield) as an offꢀwhite solid (purity 99%). H NMR (400 MHz, MeODꢀd4) δ 7.19 ꢀ 7.09 (m,
2H), 6.84 ꢀ 6.75 (m, 2H), 4.91 (ddd, J = 15.0, 10.0, 5.0 Hz, 1H), 4.78 (td, J = 10.0, 4.6 Hz, 1H),
3.83 ꢀ 3.57 (m, 1H), 3.55 ꢀ 3.34 (m, 2H), 3.29 ꢀ 3.25 (m, 1H), 3.23 ꢀ 2.96 (m, 3H), 2.23 ꢀ 2.10 (m,
1H), 2.07 ꢀ 1.93 (m, 1H), 1.18 (t, J = 7.0 Hz, 1H); 19F NMR (471MHz, CDCl3)
δ ꢀ182.25; MS
m/z: [M+H]+: 196.2. HPLC (XBridge 5ꢀ C18 4.6x50 mm, 4 mL/min, Solvent A: 10 %
MeOH/H2O with 0.2 % H3PO4, Solvent B: 90 % MeOH/H2O with 0.2 % H3PO4, gradient with 0ꢀ
100 % B over 4 minutes): 0.25 minutes.
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(S)ꢀ3ꢀ((tertꢀButyldimethylsilyl)oxy)pyrrolidinꢀ2ꢀone (19). DMAP (3.59 g, 29.4 mmol),
imidazole (121 g, 1780 mmol) and TBDMSꢀCl (161 g, 1068 mmol) were each added to a stirred
solution of (S)ꢀ3ꢀhydroxypyrrolidinꢀ2ꢀone (18) (90 g, 890 mmol) in anhydrous CH2Cl2 (1780
mL) maintained under a N2 atmosphere at room temperature. The reaction mixture was allowed
to stir at room temperature overnight before quenching with saturated NaHCO3 solution (2000
mL) and extracting with CH2Cl2 (3 x 500mL). The combined organic extracts were dried
(Na2SO4), evaporated in vacuo and purified by silica gel chromatography using 0ꢀ50% EtOAc in
petroleum ether as an eluent to afford 19, (173 g, 801 mmol, 90% yield) as an oil (purity 98%).
1HꢀNMR (400 MHz, CDCl3) δ 6.60 (br s, 1H), 4.27 (t, J = 7.7 Hz, 1H), 3.42 ꢀ 3.35 (m, 1H), 3.25
(dt, J = 9.6, 7.4 Hz, 1H), 2.36 (dtd, J = 12.8, 7.4, 3.1 Hz, 1H), 2.03 (dq, J = 12.6, 8.1 Hz, 1H),
1.81 (s, 1H), 0.94 ꢀ 0.90 (m, 9H), 0.15 (d, J = 6.8 Hz, 6H); MS m/z: [M+H]+: 216.1. HPLC
(XBridge 5ꢀ C18 4.6x50 mm, 4 mL/min, Solvent A: 10 % MeOH/H2O with 0.2 % H3PO4,
Solvent B: 90 % MeOH/H2O with 0.2 % H3PO4, gradient with 0ꢀ100 % B over 4 minutes): 2.25
minutes.
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(S)ꢀ3ꢀ((tertꢀButyldimethylsilyl)oxy)ꢀ1ꢀ(4ꢀmethylbenzyl)pyrrolidinꢀ2ꢀone (20). (S)ꢀ3ꢀ((tertꢀ
butyldimethylsilyl)oxy)pyrrolidinꢀ2ꢀone (19) (86.3 g, 401 mmol) was dissolved in anhydrous
THF (800 mL) and the mixture was cooled to 0 °C under a N2 atmosphere. NaH (26.2 g, 601
mmol) was added portionwise and the reaction mixture was allowed to stir for 30 minutes before
the dropwise addition of 1ꢀ(bromomethyl)ꢀ4ꢀmethylbenzene (93 g, 501 mmol) in anhydrous THF
(800 mL). The reaction mixture was allowed to stir at 0 °C for 1 h, the cooling bath was removed
and mixture was allowed to warm to room temperature overnight. The reaction mixture was
cautiously quenched with ice water (500 mL) and extracted with EtOAc (3 x 500 mL). The
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combined organic layers were washed with brine (1000 mL) and dried (MgSO4). Evaporation of
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the solvent in vacuo gave the crude product which was purified by silica gel chromatography
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using 0ꢀ20% ethyl acetate in hexanes to afford 20 (90.3 g, 283 mmol, 71%) as a white solid
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(99% purity). HꢀNMR (400 MHz, CDCl3)
δ 7.15 (s, 4H), 4.42 (s, 2H), 4.37 (t, J = 7.6 Hz, 1H),
3.32ꢀ3.18 (m, 1H), 3.10 (dt, J = 9.7, 7.5 Hz, 1H), 2.36 (s, 3H), 2.29 (dtd, J = 12.6, 7.6, 3.1 Hz,
1H), 1.97ꢀ1.84 (m, 1H), 0.95 (s, 9H), 0.20 (d, J = 10.3 Hz, 6H). MS m/z: [M+H]+: 320.3. HPLC
(XBridge 5ꢀ C18 4.6x50 mm, 4 mL/min, Solvent A: 10 % MeOH/H2O with 0.2 % H3PO4,
Solvent B: 90 % MeOH/H2O with 0.2 % H3PO4, gradient with 0ꢀ100 % B over 4 minutes): 4.35
minutes.
(S)ꢀ3ꢀHydroxyꢀ1ꢀ(4ꢀmethylbenzyl)pyrrolidinꢀ2ꢀone (21). HCl (4M in 1,4ꢀdioxane, 707
mL, 2828 mmol) was added in one portion to a solution of 20 (180 g, 566 mmol) in anhydrous
CH2Cl2 (600 mL) at room temperature maintained under a N2 atmosphere. A slight exotherm of
10 °C was noted. The reaction mixture was allowed to stir at room temperature for 2 h and then
evaporated in vacuo. The residue was taken up in CH2Cl2 (1000 mL) and washed with saturated
NaHCO3 solution (1000 mL), brine (1000 mL), dried (MgSO4) and concentrated in vacuo. The
crude product was purified by silica gel chromatography using 40ꢀ100% EtOAc in hexanes to
1
afford 21 (110 g, 533 mmol, 94% yield) as a white solid (99% purity). HꢀNMR (400 MHz,
CDCl3)
δ 7.26ꢀ7.02 (m, 4H), 4.43 (d, J = 3.5 Hz, 2H), 4.41ꢀ4.37 (m, 1H), 3.66 (d, J = 2.6 Hz,
1H), 334ꢀ3.05 (m, 2H), 2.41(dddd, J = 12.8, 8.4, 6.6, 2.2 Hz, 1H), 2.34 (s, 3H), 1.93 (dq,J = 12.8,
8.8 Hz, 1H). MS m/z: [M+H]+: 206.2. HPLC (XBridge 5ꢀ C18 4.6x50 mm, 4 mL/min, Solvent
A: 10 % MeOH/H2O with 0.2 % H3PO4, Solvent B: 90 % MeOH/H2O with 0.2 % H3PO4,
gradient with 0ꢀ100 % B over 4 minutes): 3.15 minutes.
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(S)ꢀ1ꢀ(4ꢀMethylbenzyl)ꢀ2ꢀoxopyrrolidinꢀ3ꢀyl methanesulfonate (22). Et3N (92 mL, 658
mmol) was added to a cooled (0 °C) solution 21 (90.0 g, 438 mmol) in anhydrous CH2Cl2 (2200
mL) maintained under a N2 atmosphere. Methanesulfonyl chloride (38 mL, 482 mmol) was
added dropwise and the reaction mixture was allowed to stir at 0 °C for 1 h before quenching
with saturated NaHCO3 solution (1000 mL). The mixture was allowed to warm to room
temperature and the aqueous layer was separated and extracted with CH2Cl2 (2 x 500 mL). The
combined organic layers were dried (MgSO4) and evaporated in vacuo to give crude product
which was purified by silica gel chromatography using 0ꢀ50% EtOAc in hexanes to afford 22
(122 g, 429 mmol, 98% yield) as a white solid (purity 97%). 1HꢀNMR (400 MHz, CDCl3) δ 7.27
(s, 1H), 7.15 (q, J = 8.1 Hz, 4H), 5.23 (dd, J = 8.4, 7.3 Hz, 1H), 4.44 (s, 2H), 3.37 ꢀ 3.31 (m, 4H),
3.22 (dt, J = 10.0, 7.3 Hz, 1H), 2.58 ꢀ 2.49 (m, 1H), 2.35 (s, 3H), 2.22 (ddt, J = 13.9, 8.9, 7.1 Hz,
1H), 1.60 (s, 1H); MS m/z: [M+H]+: 284.2. HPLC (XBridge 5ꢀ C18 4.6x50 mm, 4 mL/min,
Solvent A: 10 % MeOH/water with 0.2 % H3PO4, Solvent B: 90 % MeOH/water with 0.2 %
H3PO4, gradient with 0ꢀ100 % B over 4 minutes): 3.45 minutes. Analytical SFC conditions:
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CO2/(MeOH/CH2Cl2, 1/1) 80/20, 3mL/min, 40 C, 220nm, WhelkꢀO (RR) (0.46x25cm , 5ꢀm),
140 barBPR were used to confirm the ee>99% compared to a racemic standard derived from
(R,S)ꢀ3ꢀhydroxyꢀ1ꢀ(4ꢀmethylbenzyl)pyrrolidinꢀ2ꢀone.
(R)ꢀ3ꢀ((3S,4S)ꢀ3ꢀFluoroꢀ4ꢀ(4ꢀhydroxyphenyl)piperidinꢀ1ꢀyl)ꢀ1ꢀ(4ꢀ
methylbenzyl)pyrrolidinꢀ2ꢀone
(1).
(S)ꢀ1ꢀ(4ꢀMethylbenzyl)ꢀ2ꢀoxopyrro1idinꢀ3ꢀ
ylmethanesulfonate (22) (20.32 g, 71.7 mmol) in anhydrous CH3CN (190 mL) was added
dropwise over a period of 4 h to a solution of 27 (14.0 g, 71.7 mmol) and iPrEt2N (31.3 mL, 179
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mmol) in anhydrous CH3CN (180 mL) at 80 °C maintained under a N2 atmosphere. After the
addition was completed, the reaction mixture was stirred at 80 °C for 16 h before cooling to
room temperature. The reaction volume was reduced in vacuo to ca. 80 mL. Saturated NH4Cl
solution (500 mL) was added before extracting with CH2Cl2 (3 x 500 mL). The organic layers
were combined, dried (Na2SO4) and concentrated in vacuo to give the crude productwhich was
purified by silica gel chromatography using 0ꢀ20% of solvent B in solvent A, where Solvent
B=20% MeOH/CH2Cl2and solvent A = CH2Cl2. Fractions containing the product were
combined. Evaporation of the solvents gave 18.6 g of 1 with 97% purity by LC/MS analysis
(conditions B). The product was further purified by preparative SFC chromatography using a
Ce114 (3 x 25 cm) 5 ꢀm column and eluting with 45% MeOH in CO2 at 150 mL/min injecting
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1.5 mL of a 80 mg/mL solution. Concentration of the desired fractions afforded 1 (17.5 g, 20.4
1
mmol, 65% yield) as a white powder (99.5% purity). HꢀNMR (500MHz, CDCl3)
δ 7.23 ꢀ 7.11
(m, 5H), 6.92 (d, J = 8.5 Hz, 2H), 6.18 (br. s., 1H), 4.79 ꢀ 4.55 (m, 1H), 4.57 ꢀ 4.33 (m, 2H), 3.72
(t, J = 8.7 Hz, 1H), 3.46 ꢀ 3.30 (m, 1H), 3.30 ꢀ 3.09 (m, 2H), 2.82 (d, J = 8.5 Hz, 1H), 2.73 ꢀ 2.56
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(m, 2H), 2.49 (d, J = 2.5 Hz, 1H), 2.36 (s, 3H), 2.21 ꢀ 1.98 (m, 2H), 1.87 (br. s., 2H); C NMR
(126MHz, CDCl3)
δ 172.4, 155.0, 137.5, 133.0, 132.8, 129.4, 128.6, 128.2, 115.6, 91.6 (d, J =
173.5 Hz), 65.0, 54.5 (d, J = 25.4 Hz), 48.3, 47.7 (d, J = 17.3 Hz), 46.7, 43.6, 31.5, 21.1, 19.2.;
19F NMR (471MHz, CDCl3) ꢀ182.75; HRMS (ESI): calcd for C23H28O2N2F (M + H)+:
δ
383.2129. Found: 383.2118. Anal. Calcd. for C23H28O2N2F: C, 72.23; H, 7.12; N, 7.32; Found:
C, 72.17; H, 7.06; N, 7.30. Analytical chiral SFC purity determination was performed on 1 and
the result was compared to an initial method development run using a mixture of 1 and the three
other potential stereoisomeric impurities. A diastereoisomeric purity = 100% was determined
using this method; [α]D20 = +6.09 (c 1.15, MeOH).
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4ꢀ((3S,4S)ꢀ3ꢀFluoroꢀ1ꢀ((R)ꢀ1ꢀ(4ꢀmethylbenzyl)ꢀ2ꢀoxopyrrolidinꢀ3ꢀyl)piperidinꢀ4ꢀyl)ꢀ
phenyldihydrogen phosphate (28, BMSꢀ986163). Anhydrous THF (320 mL) was added to fourꢀ
necked, 2 L round bottom flask equipped with a mechanical stirrer, a thermometer and a N2 inlet
and the vessel flushed with N2. The solvent was cooled below 0 °C with an ice/iPrOH bath.
POCl3 (19.5 mL, 209 mmol) was added followed by the addition of a suspension of 1 (16 g, 41.8
mmol) in THF (320 mL) (a clear solution formed) and Et3N (14.59 mL, 105 mmol) (portionwise
addition, slightly exothermic). The reaction mixture was stirred below 0 °C for 80 mins at which
point HPLC analysis indicated a greater than 99.7% conversion. H2O (320 mL) and 1N NaOH
(160 mL) were added slowly while maintaining the solution below 20 °C. The ice bath was
removed and the reaction mixture was allowed to warm to rt over a period of 18 h. THF (550
mL) was removed in vacuo at room temperature and the remaining suspension, which indicated a
pH of <1, was transferred to a 2 L, 4 necked flask equipped with a mechanical stirrer. The flask
was rinsed with a small amount of H2O and cooled with an ice bath. The suspension was titrated
with 1N NaOH (400 mL) with stirring until the pH was adjusted to 1 . The white suspension was
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stirred at 5 C for 1 h, filtered, the filter cake washed with ice cooled H2O (100 mL) and dried
overnight by suction to give the wet product 28 (20.65g) as a slightly yellow solid.
Crystallization from 80% EtOH (1 L) cooled from reflux to 10 °C afforded a suspension which
was filtered, washed with a small amount of EtOH and dried uder vacuum suction for 24 h to
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give 28 as a white crystalline solid (19.35 g, 33.8 mmol, 93% yield and 99.7% purity). [α]D
=
+2.86 (c = 2.86, DMSO); 1HꢀNMR (500 MHz, DMSOꢀd6) δ 7.27 (d, J = 8.5 Hz, 2H), 7.18 ꢀ 7.13
(m, 2H), 7.13 ꢀ 7.08 (m, 4H), 4.68 (dtd, J = 48.2, 9.9, 5.2 Hz, 1H), 4.38 (d, J = 14.8 Hz, 1H),
4.29 (d, J = 14.8 Hz, 1H), 3.66 (br t, J = 8.5 Hz, 1H), 3.53 ꢀ 3.42 (m, 1H), 3.22 ꢀ 3.15 (m, 1H),
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