Discovery, characterization and biological evaluation of a novel (R)-4,4-difluoropiperidine scaffold as dopamine receptor 4 (D4R) antagonists

Article abstract of DOI:10.1016/j.bmcl.2016.10.049

Herein, we report the synthesis and structure–activity relationship of a novel series of (R)-4,4-difluoropiperidine core scaffold as dopamine receptor 4 (D₄) antagonists. A series of compounds from this scaffold are highly potent against the D₄receptor and selective against the other dopamine receptors. In addition, we were able to confirm the active isomer as the (R)-enantiomer via an X-ray crystal structure.

Full text of DOI:10.1016/j.bmcl.2016.10.049

Accepted Manuscript
Discovery, characterization and biological evaluation of a novel (R)-4,4-di-
fluoropiperidine scaffold as dopamine receptor 4 (D4R) antagonists
Daniel E. Jeffries, Jonathan O. Witt, Andrea L. McCollum, Kayla J. Temple,
Miguel A. Hurtado, Joel M. Harp, Anna L. Blobaum, Craig W. Lindsley, Corey
R. Hopkins
PII:
S0960-894X(16)31087-3
DOI:
http://dx.doi.org/10.1016/j.bmcl.2016.10.049
BMCL 24352
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Accepted Date:
23 September 2016
14 October 2016
Please cite this article as: Jeffries, D.E., Witt, J.O., McCollum, A.L., Temple, K.J., Hurtado, M.A., Harp, J.M.,
Blobaum, A.L., Lindsley, C.W., Hopkins, C.R., Discovery, characterization and biological evaluation of a novel
(R)-4,4-difluoropiperidine scaffold as dopamine receptor 4 (D4R) antagonists, Bioorganic & Medicinal Chemistry
Letters (2016), doi: http://dx.doi.org/10.1016/j.bmcl.2016.10.049
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Discovery, characterization and biological
evaluation of a novel (R)-4,4-
Leave this area blank for abstract info.
difluoropiperidine scaffold as dopamine
receptor 4 (D4R) antagonists
Daniel E. Jeffries, Jonathan O. Witt, Andrea L. McCollum, Kayla J. Temple, Miguel A. Hurtado, Joel M. Harp,
Anna L. Blobaum, Craig W. Lindsley, Corey R. Hopkins

Bioorganic & Medicinal Chemistry Letters
Discovery, characterization and biological evaluation of a novel (R)-4,4-
difluoropiperidine scaffold as dopamine receptor 4 (D4R) antagonists
Daniel E. Jeffriesa, Jonathan O. Witta, Andrea L. McColluma, Kayla J. Templeb,d, Miguel A. Hurtadob, Joel
M. Harpc, Anna L. Blobaumb,d, Craig W. Lindsleya,b,d, Corey R. Hopkinsa,b,d,e,
*
aDepartment of Chemistry, Vanderbilt University, Nashville, TN 37232, United States
bDepartment of Pharmacology, Vanderbilt University, Nashville, TN 37232, United States
cDepartment of Biochemistry, Vanderbilt University, Nashville, TN 37232, United States
dVanderbilt Center for Neuroscience Drug Discovery, Vanderbilt University, Nashville, TN 37232, United States
eDepartment of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, NE 58198, United States
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Accepted
Available online
Herein, we report the synthesis and structure-activity relationship of a novel series of (R)-4,4-
difluoropiperidine core scaffold as dopamine receptor 4 (D4) antagonists. A series of
compounds from this scaffold are highly potent against the D4 receptor and selective against the
other dopamine receptors. In addition, we were able to confirm the active isomer as the (R)-
enantiomer via an X-ray crystal structure.
2009 Elsevier Ltd. All rights reserved.
Keywords:
Dopamine 4 receptor
Difluoropiperidine
addiction
PD-LIDs
selectivity
produced some locomotor deficits that suggest the compound
reduces the overall benefit in the rat model.11,12 Despite the
promising results of these initial studies, there has been limited
effort over the past several years on the development of new and
selective D4 antagonists.
The dopamine receptors are a class of G protein-coupled
receptors (GPCRs) that are prominent in the central nervous
system (CNS), with dopamine being the endogenous ligand for
this class of receptors.1,2 This family of receptors contains at
least five subtypes of receptors that are further subgrouped into
two families.3,4 The D1-like receptor family are coupled to Gs
which activates adenylyl cyclase resulting in the increase of
intracellular cAMP and contains the D1 and D5 receptors. The
D2-like receptor family are coupled to Gi which inhibits cAMP
and contains the D2, D3, and D4 receptors. Significant work has
been dedicated to the dopamine receptor class due to the
dopamine hypothesis of schizophrenia – resulting in numerous
approved medications as antipsychotics, predominantly non-
selective D2/D3 mixed receptor modulators.5
Recently, our laboratory has identified two series based on a
chiral morpholine scaffold that has produced potent and selective
D4 antagonists (Figure 1).13,14 The initial compound, ML398, 1,
was brain penetrant and active in
a
cocaine-induced
hypperlocomotion assay; however, the SAR was limited due to
synthetic access to the upper-right hand portion.13 Thus, the
right-hand phenethyl moiety was replaced with the alkoxymethyl
group which allowed for substantial synthetic modifications,
leading to 2, another potent, selective and brain penetrant D4
antagonist.14 Compound 2, was shown to drastically reduce
global AIM scores in a 6-OHDA mouse study for the reversal of
L-DOPA-induced dyskinesias.15
The D4 receptor received considerable attention for
schizophrenia; however, a key compound, L-745,870, failed in
the clinic due to a lack of efficacy.6-8 Nearly all of the
antagonists developed at this time were based on a piperazine
scaffold and the compounds possessed varying degrees of
selectivity. In addition to the potential as an antipsychotic, the D4
receptor has been implicated as a potential target for Parkinson’s
disease – especially for L-DOPA-induced dyskinesia’s9, and for
the treatment of substance abuse.10 L-745,870 has been shown to
offer a positive result in both rat and non-human primate models
of L-DOPA-induced dyskinesia’s, however, the compound
* Corresponding author. Tel. +1-402-559-9729; fax: +1-402-559-5643; email: corey.hopkins@unmc.edu

Figure 1.
Structures of previously disclosed chiral morpholine D4
anatgonists, ML398, 1, and alkoxymethyl morpholine, 2.
* Corresponding author. Tel. +1-402-559-9729; fax: +1-402-559-5643; email: corey.hopkins@unmc.edu

Next, we turned our attention to replacing the morpholine core
moiety with another novel group, a difluoropiperidine scaffold.
The synthesis of this new scaffold is shown in Scheme 1. The -
keto ester piperidine, 3, was alkylated with an appropriate
phenethyl halide, 4, (K2CO3, acetone, reflux) in moderate yield
(52-68%); which was then immediately decarboxylated (LiCl,
o
DMF, 150 C) yielding the alkylated piperidinone, 5.16 The
Table 1.
ketone was treated with Deoxo-Fluor®17,18 yielding the gem-
difluoropiperidine, 6, (48-64%) which was then deprotected
under reductive conditions (NH4HCO2, Pd/C, MeOH, reflux)
leaving the penultimate compound, 7, in good yields. The final
compounds were synthesized either via alkylation of the nitrogen
(ArCH2X, Cs2CO3) for 8 or via reductive amination (ArCHO,
Structure and D4 activity of the N-linked analogs.
o
D4.4 (%
IC50
Cmpd
R
Ki (nM)b
MP-CNBH3, W, 100 C) for 9. The racemic final compounds
inh.)a
(nM)b
were then separated by chiral-SFC to yield the desired chiral final
ML398,
1
2
compounds.19
130
56
36
14.3
80
760
210
8a
8b
80
85
790
300
220
82
8c
<20% inhibition at 10 M against D1,
D2L, D2S, D3 and D5
87
430
120
8d
79
55
93
87
8e
8f
510
140
380
8g
8h
Scheme 1. Reagents and conditions: (a) R-PhCH2CH2I, K2CO3, acetone,
reflux, 52-67%; (b) LiCl, DMF, 150 oC, 42-50%; (c) Deoxo-Fluor, EtOH
CH2Cl2, 48-64%; (d) NH4HCO2, Pd/C, MeOH, reflux, 90-96%; (e) ArCH2X,
Cs2CO3, THF, rt, 47-74%; (f) ArCHO, MP-CNBH3, CH2Cl2, W, 110 oC, 19-
48%.
1360
70
8i
88
70
66
110
31
84
8j
8k
8l
The SAR evaluation started with the southern N-linked
portion of the molecule, keeping the upper right-hand phenethyl
portion constant (Table 1). The compounds were originally
evaluated at a single concentration (10 M) for the ability to
inhibit D4.4 using a radioligand (spiperone). Those compounds
that were able to produce 80% inhibition or greater were then
evaluated for an IC50 and Ki (www.EuroFins.com). The 4-
chlorophenyl, 8a, a direct comparator to ML398, 1, was active
(Ki = 210 nM); however, it was less potent than 1. Expanding
the steric bulk to the naphthalene derivative produced an
equipotent molecule (8b, Ki = 220 nM). Moving the 3,4-
dimethyl derivative produced the most potent compound of the
series to date (8c, Ki = 82 nM), as the racemic mixture. Both the
4-methoxy (8d) and 4-thiomethyl (8g) retained activity, 120 nM
and 140 nM, respectively, as did the 6-quinoline, 8h (Ki = 380
nM). The 3-fluoro-4-methoxyphenyl derivative, 8j, a direct
comparator to 2, was the most potent compound tested in this set
of analogs (Ki = 31 nM). A number of compounds were inactive
or weakly active (8n – 8v), analogs containing both electron
withdrawing and donating groups. The 5-methoxy-3-indole, 8w,
a moiety that previously was shown to impart significant activity
was moderately potent in this series (Ki = 200 nM). Dopamine
receptor selectivity on selected compounds reveals that the
compounds were selective for the D4 receptor (8c and 8cc) and
another showed slight activity against D2S (8w, 52% inhibition at
10 M).
88
77
300
8m
8n
51
6
8o
8p
10
8q
10
49
8r
8s

18
0
8t
8u
8v
D4.4 (%
inh.)a
Ki
Cmpd
ML398,
1
R
R1
IC50 (nM)b
130
(nM)b
48
91
36
720
200
56
14.3
2
8w
<50% inhibition @ 10 M vs. D1, D2L
D3 and D5; D2S = 52%
,
94
170
48
9a
94
75
230
230
64
63
9b
9c
66
8x
8y
54
3
9d
69
16
68
8z
76
66
88
67
89
370
100
430
9e
9f
8aa
8bb
1,540
9g
9h
9i
90
<50% inhibition @ 10 M vs. D1, D2L
D2S, D3 and D5
a% Inhibition at 10 M. bIC50 and Ki values were run in duplicate in a
radioligand binding assay using Spiperone at EuroFins
(www.EuroFins.com).
120
34
8cc
,
550
430
150
120
93
9j
88
91
94
95
640
480
25
180
130
7.0
9k
9l
The next round of SAR concentrated on a two-prong approach
by modification of the both areas of the molecule (Table 2). The
R group modification centered around both electron-donating
(OMe) and electron-withdrawing (F) groups. The southern R1
moieties consisted of the best groups that we have identified
previously. To this end, the 2-OMe phenyl R group produced
active compounds for all of the R1 groups evaluated, with the
exception of the 4-fluoro-3-indole, 9d, which was not active in
the alkoxymethyl morpholine series either.14 The 6-halogen-3-
indole (9a,b) and the 6-methoxy-3-indole (9e) were all sub-100
nM in activity. Moving to the 3-OMe phenyl group produced
less active compounds across the board (9f – i), although the best
compound, 9i, was similar in potency with its 9e counterpart (150
nM vs.100 nM, respectively). Much like the 3-OMe compounds,
the 4-OMe phenyl groups produced less active compounds
compared to the 2-OMe phenyl compounds (e.g., 9a vs. 9l).
However, moving to the electron-withdrawing fluorine groups,
the compounds were much more potent. The 4-fluorophenyl
group produced compounds that were sub-20 nM (9m – p), with
the 6-fluoro-3-indole (9m, Ki = 7.0 nM), the 6-methoxy-3-indole
(9o, Ki = 10.3 nM) and the imidazo[1,5-a]pyridine (9p, Ki = 9.7
nM) being the most potent from this series of racemic
compounds. The 3-fluorophenyl analogs were less potent than
the 4-fluorophenyl counterparts (e.g., 9m vs. 9q and 9p vs. 9t).
Moving to the 2-fluorophenyl moiety proved beneficial as well,
producing compounds that were potent in the single digit
nanomolar range (9w, Ki = 8.6 nM; 9z, Ki = 5.9 nM). The
difluoropiperidine scaffold has been shown to be a productive
scaffold change from the morpholine scaffold. Selectivity was
performed on select compounds which again showed variable
selectivity against the other dopamine receptors, with 9p and 9q
being selective and 9v and 9z showing modest activity against
D2S and D3.
9m
9n
65
17.9
96
97
37
35
10.3
9.7
9o
9p
<50% inhibition @ 10 M vs.
D1, D2L, D2S, D3 and D5
92
77
21
9q
9r
<50% inhibition @ 10 M vs.
D1, D2L, D2S, D3 and D5
93
45
12.5
44
94
78
160
9s
9t
59
96
9u
9v
55
15.3
<50% inhibition @ 10 M vs.
D1, D2L and D5; D2S, 57%, D3,
69%;
95
92
31
98
8.6
27
9w
9x
62
95
9y
9z
Table 2.
21
5.9
Structure and D4 activity of the bidirectional SAR evaluation.

77
64a
79b
230
23
63
(R)-8c
(S)-9bb
(R)-9bb
<50% inhibition @ 10 M vs.
D1, D5, and D2L;D2S, 66%: D3,
80%
6.3
58a
59b
92
150
42
9aa
(S)-9cc
(R)-9cc
a% Inhibition at 10 M. bIC50 and Ki values were run in duplicate in a
64
59
17.7
16.4
radioligand binding assay using Spiperone at EuroFins (www.EuroFins.com).
55a
66b
(S)-9dd
(R)-9dd
Up to this point, we have only synthesized and evaluated
racemic compounds; however, we were able to synthesize the
racemic compounds on scale (~150 mg) and then separate the
final compounds into the isolated enantiomers utilizing chiral
SFC protocols. In order to confirm the structure of the active
isomer, the intermediate 7 (R = H) was separated into the
corresponding (S) and (R) isomers and then the active isomer was
converted to the para-bromosulfonamide and then submitted for
a single crystal X-ray structure (Figure 2).20 The X-ray confirms
the active isomer as the (R)-enantiomer, which corresponds the
same isomer as the previously reported morpholine and
alkoxymethyl morpholine D4 antagonists.13,14
25a
53b
75b
(S)-9ee
(R)-9ee
88
24
89
320
(S)-9m
≤50% inhibition @ 10 M vs.
D1, D2L, D2S and D5; D3 = 52%
59d
7.4
2.1
<50% inhibition @ 10 M vs.
D1, D2S and D5; D2L = 53% and
D3 = 85%
(R)-9m
(S)-9o
64c
63d
620
170
7.0
1.9
<30% inhibition @ 10 M vs.
D1, D2S, and D5; D2L = 67% and
D3 = 52%
(R)-9o
73a
85c
3,200
890
(S)-9kk
(R)-9kk
21
5.8
<50% inhibition @ 10 M vs.
D1, D2S, D3 and D5; D2L = 51%
62b
83c
530
150
5.2
(S)-9r
(R)-9r
18.6
<50% inhibition @ 10 M vs.
D1, D2S and D5; D2L = 53% and
D3 = 85%
59a
84b
5,990
1,660
(S)-9v
(R)-9v
190
53
66b
490
140
<50% inhibition @ 10 M vs.
D1, D2S and D5; D2L = 57%; and
D3 = 54%
(S)-9w
(R)-9w
Figure 2. ORTEP drawing of the active enantiomer of the difluoropiperidine
85c
14.1
3.9
scaffold.
<50% inhibition @ 10 M vs.
D1, D2S and D5; D2L = 51% and
Our next set of analogs further establishes the (R)-enantiomer
as the active isomer since both enantiomers were made and tested
based on previously evaluated racemic compounds, as well as
new analogs (Table 3). For many of the analogs, the potency
resides completely in the (R)-enantiomer (8c, 9bb – 9ee);
however, in other compounds the (S)-enantiomer is active as a D4
antagonist (9a, 9kk, 9r, and 9v). The combination of the 4-
fluorophenyl and 6-methoxy-3-indole ((R)-9o, Ki = 1.9 nM) is
the most potent compound to date from this series of analogs. In
fact, the 6-methoxy-3-indole southern fragment produced a
number of potent analogs. Much like what was seen above, the
selectivity of the compounds showed modest activity against D3
and variable and D2L, with only (R)-9kk being the only
enantiospecific compound that is selective against all receptors.
D3 = 65%
d
a% inhibition a 10 M. b% inhibition at 1 M. c% inhibition at 0.1 M.
%
inhibition at 10 nM. e IC50 and Ki values were run in duplicate in a radioligand
binding assay using Spiperone at EuroFins (www.EuroFins.com).
Finally, having identified a number of potent analogs, we
further profiled selected compounds in a battery of Tier 1 in vitro
DMPK assays (Table 4). The intrinsic clearance (CLINT) was
assessed in liver microsomes and all of the compounds proved to
be unstable to oxidative metabolism in both species tested
(human and rat).21 In addition, utilizing an equilibrium dialysis
approach, the protein binding of the compounds was evaluated
and all compounds that were assessed were highly protein bound
(Fu < 0.015) in both species. Lastly, we assessed the ability of
these compounds to cross the blood-brain barrier (BBB) in a
rodent IV cassette experiment.22,23 The compounds evaluated are
shown in Table 4, and although the compounds show high
clearance in rats, many of the compounds were able to cross the
BBB with Kp values >1, with the exception of (R)-9dd and (R)-
9v.
Table 3.
Structure and D4 activity of the separated enantiomers.
Table 4.
D4.4
(% inh)
IC50
In vitro and in vivo DMPK analysis of select compounds.
Cmpd
R
R1
(nM)e
Ki (nM)e
Microsomal intrinsic
clearance
(mL/min/kg)
Plasma unbound
fraction (Fu)
>10,000
(S)-8c
D4 Ki
(nM)
Cmpd

hCLINT
108
111
70.5
99.0
rCLINT
1673
2488
1908
582
Human
0.005
0.001
0.001
0.001
0.004
0.018
0.003
0.005
0.005
Rat
12. Huot, P.; Johnston, T. H.; Koprich, J. B.; Aman, A.; Fox, S. H.;
Brotchie, J. M. J. Pharmacol. Exp. Ther. 2012, 342, 576.
13. Berry, C. B.; Bubser, M.; Jones, C. K.; Hayes, J. P.; Wepy, J.
A.; Locuson, C. W.; Daniels, J. S.; Lindsley, C. W.; Hopkins, C.
R. ACS Med. Chem. Lett. 2014, 5, 1060.
14. Witt, J. O.; McCollum, A. L.; Hurtado, M. A.; Huseman, E. D.;
Jeffries, D. E.; Temple, K. J.; Plumley, H. C.; Blobaum, A. L.;
Lindsley, C. W.; Hopkins, C. R. Bioorg. Med. Chem. Lett. 2016,
26, 2481.
63
0.004
0.014
0.001
0.003
0.011
0.009
0.002
0.002
0.050
(R)-8c
6.3
16.4
17.7
2.1
1.9
5.2
53
(R)-9bb
(R)-9dd
(R)-9kk
(R)-9m
(R)-9o
(R)-9r
298
300
156
957
1064
1301
(R)-9v
(R)-9w
3.9
15. Sebastianutto, I.; Maslava, N.; Hopkins, C. R.; Cenci, M. A.
Neurobiol. Dis. 2016, 96, 156.
16. Krapcho, A. P.; Weimaster, J. F.; Eldridge, J. M.; Jahngen, E.
G. E., Jr.; Lovey, A. J.; Stephens, W. P. J. Org. Chem. 1978, 43,
138.
17. Singh, R. P.; Shreeve, J. M. Synthesis 2002, 17, 2561.
18. Stanton, M. G.; Hubbs, J.; Sloman, D.; Hamblett, C.; Andrade,
P.; Angagaw, M.; Bi, G.; Black, R. M.; Crispino, J.; Cruz, J. C.;
Fan, E.; Farris, G.; Hughes, B. L.; Kenific, C. M.; Middleton, R.
E.; Nikov, G.; Sajonz, P.; Shah, S.; Shomer, N.; Szewczak, A.
A.; Tanga, F.; Tudge, M. T.; Shearman, M.; Munoz, B. Bioorg.
Med. Chem. Lett. 2010, 20, 755.
Rodent IV Cassette (0.25 mg/kg, 0.25 h)
Plasma (ng/mL)
44.4
Brain (ng/g)
93.7
Kp
2.1
(R)-8c
28.6
25.3
14.9
13.2
9.2
39.0
54.8
30.4
100
3.5
N/A
1.3
1.9
3.3
1.1
0.4
1.3
(R)-9bb
(R)-9dd
(R)-9kk
(R)-9m
(R)-9o
(R)-9r
(R)-9v
(R)-9w
BLQ
19.6
25.5
30.3
43.5
23.5
38.9
Kp = brain:plasma total concentration
19. Representative synthesis of 2-iodoethyl benzene analogs
In conclusion, we have disclosed
a
novel 4,4-
(yields range 65-77%).
fluorophenyl)ethan-1-ol (1.12 g, 7.99 mmol) in DCM (45 mL)
at
oC was added imidazole (0.60 g, 8.8 mmol),
To
a
solution of 2-(4-
difluoropiperidine scaffold as a potent and selective dopamine
receptor 4 antagonist. The work builds upon our previous
disclosures of the chiral morpholine and chiral alkoxymethyl
morpholine scaffolds. Many of the compounds reported are very
potent (D4 Ki < 10 nM) with some showing full selectivity
against the other dopamine receptors. As we have shown
previously, the scaffold exhibits enantiopreference in activity
with the (R)-enantiomer being the more potent conformer.
Further, we were able to confirm via X-ray crystal structure that
the (R)-isomer is the active species. Although the compounds
possessed less than ideal in vitro DMPK properties, many are
highly brain penetrant which, when coupled with the potency,
could make these potential radioligands. Further profiling and
optimization will be reported in due course.
0
triphenylphosphine (2.31g, 8.80 mmol), and iodine (2.03 g, 8.00
mmol) in rapid succession. The mixture was allowed to stir at 0
oC for 10 min before being gradually warmed to room
temperature and let stir an additional 3 h. Once TLC confirmed
complete conversion the reaction was quenched with saturated
aqueous Na2S2O3 (90 mL) and the resulting solution extracted
with DCM (3 x 30 mL). The resulting organics were dried over
MgSO4 and concentrated in vacuo. The resulting white solid
was resuspended in 100 mL hexanes and filtered to remove the
byproduct triphenylphosphine oxide as a white precipitate.
Following filtration the crude material was purified using
Teledyne ISCO Combi Flash system (40 g column, solid
loading on silica, 100% hexanes, 20 minute run) to afford 1-
fluoro-4-(2-iodoethyl)benzene (1.43 g, 72%) as a clear oil.
Representative synthesis of methyl 1-benzyl-4-oxo-3-
phenethylpiperidine-3-carboxylate analogs (yields range 52-
67%). In a round bottom flask containing a magnetic stir bar
was added 1-fluoro-4-(2-iodoethyl)benzene (2.00 g, 7.99 mmol)
, methyl 1-benzyl-4-oxopiperidine-3-carboxylate HCl salt (1.86
g, 6.57 mmol), and potassium carbonate (3.63 g, 26.3 mmol)
which were dissolved in acetone (40 mL) and refluxed for 21
hours. After cooling to room temperature the crude material
was filtered and condensed in vacuo onto silica gel for
purification using Teledyne ISCO Combi Flash system (80 g
column, solid loading on silica, 0-15% EtOAc in hexanes, 30
minute run) to afford methyl 1-benzyl-3-(4-fluorophenethyl)-4-
oxopiperidine-3-carboxylate (1.3 g, 54%) as a yellow tinted,
viscous oil.
Representative synthesis of 1-benzyl-3-phenethylpiperidine-
4-one analogs (yields range 42-50%). A mixture of methyl 1-
benzyl-3-(4-fluorophenethyl)-4-oxopiperidine-3-carboxylate
(1.88 g, 5.09 mmol) and LiCl (2.16 g, 50.9 mmol) in DMF (51
mL) was refluxed for 4 h. After cooling to room temperature
1N HCl (1 mL) was added and the mixture allowed to stir for 5
additional min before being poured onto saturated aqueous
NaHCO3 (50 mL). The resulting solution was transferred to a
separatory funnel and diluted with EtOAc (125 mL) and washed
with 5% aq. LiCl (3 x 75mL). The organic layer was dried over
MgSO4 and condensed onto silica gel in vacuo for purification
using Teledyne ISCO Combi Flash system (40g column, solid
loading on silica, 0-20% EtOAc in hexanes, 25 minute run) to
afford 1-benzyl-3-(4-fluorophenethyl)piperidin-4-one (0.778 g,
50%) as a thick , yellow tinted oil.
References and notes
1. Jaber, M.; Robinson, S. W.; Missale, C.; Caron, M. G.
Neuropharmacol. 1996, 35, 1503.
2. Ye, N.; Neumeyer, J. L.; Baldessarini, R. J.; Zhen, X.; Zhang,
A. Chem. Rev. 2013, 113, PR123.
3. Girault, J.-A.; Greengard, P. Arch. Neurol. 2004, 61, 641.
4. Van Tol, H. H. M.; Wu, C. M.; Guan, H.-C.; Ohara, K.;
Bunzow, J. R.; Civelli, O.; Kennedy, J.; Seeman, P.; Niznik, H.
B.; Jovanovic, V. Nature 1992, 358, 149.
5. Tost, H.; Alam, T.; Meyer-Lindenberg, A. Neurosci. Biobehav.
Rev. 2010, 34, 689.
6. Patel, S.; Freedman, S. B.; Chapman, K. L.; Emms, F.; Fletcher,
A. E.; Knowles, M.; Marwood, R.; McAllister, G.; Myers, J.;
Patel, S.; Curtis, N.; Kulagowski, J. J.; Leeson, P. D.; Ridgill,
M. P.; Graham, M.; Matheson, S.; Rathbone, D.; Watt, A. P.;
Bristow, L. J.; Rupniak, N. M. J.; Baskin, E.; Lynch, J. J.;
Ragan, C. I. J. Pharmacol. Exp. Ther. 1997, 283, 636.
7. Bristow, L. J.; Kramer, M. S.; Kulagowski, J.; Patel, S.; Ragan,
C. I.; Seabrook, G. R. Trends Pharm. Sci. 1997, 18, 186.
8. Bristow, L. J.; Collinson, N.; Cook, G. P.; Curtis, N.; Freedman,
S. B.; Kulagowski, J. J.; Leeson, P. D.; Patel, S.; Ragan, C. I.;
Ridgill, M.; Saywell, K. L.; Tricklebank, M. D. J. Pharmacol.
Exp. Ther. 1997, 283, 1256.
9. Huot, P.; Johnston, T. H.; Koprich, J. B.; Fox, S. H.; Brotchie, J.
M. Pharmacol. Rev. 2013, 65, 171.
10. Di Ciano, P.; Grandy, D. K.; Le Foll, B. Adv. Pharmacol. 2014,
69, 301.
11. Huot, P.; Johnston, T. H.; Koprich, J. B.; Espinosa, M. C.;
Reyes, M. G.; Fox, S. H.; Brotchie, J. M. Behav. Pharmacol.
2015, 26, 101.
Representative
phenethylpiperidine analogs (yields range 48-64%).
synthesis
of
1-benzyl-4,4-difluoro-3-
A

reaction
vessel
was
charged
with
1-benzyl-3-(4-
fluorophenethyl)piperidin-4-one (1.70 g, 5.46 mmol), EtOH
(0.06 mL), DCM (2.7 mL), and a stir bar before being briefly
purged with argon. Deoxo-Fluor (2.05 g, 9.29 mmol) was then
added and the reaction allowed to stir at room temperature for
22 h before being quenched with the dropwise addition of sat.
aqueous NaHCO3. The resulting solution was washed with
DCM (2 x 5 mL) and condensed onto silica gel in vacuo for
purification using Teledyne ISCO Combi Flash system (24g
column, solid loading, 0-20% EtOAc in hexanes, 25 minute
run)
to
afford
1-benzyl-4,4difluoro-3-(4-
fluorophenethyl)piperidine (0.924g , 51%) as a thick, yellow
tinted oil.
Representative synthesis of 3-phenylpiperidin-4-one analogs
(yields range
90-96%).
1-benzyl-4,4difluoro-3-(4-
fluorophenethyl)piperidine (75 mg, 0.23 mmol) was dissolved
in MeOH (4.5 mL) and to this solution was added ammonium
formate (70.9 mg, 1.13 mmol) and 10% palladium on carbon
(75 mg). The resulting solution was heated to 55 oC and
allowed to stir for 5 min at which time TLC confirmed complete
conversion. The reaction was cooled to room temperature and
the crude material was filtered over celite with EtOAc (10 mL)
then washed with sat. aqueous NaHCO3 (10 mL). The resulting
organics were passed through
a phase separator and
concentrated in vacuo to afford 3-(4-fluorophenethyl)piperidin-
4-one (52.9 mg, 94%) as a thick clear oil.
(R)-6-chloro-3-((4,4-difluoro-3-(4-
fluorophenethyl)piperidin-1-yl)methyl)-1H-indole (9kk-(R)).
Analytical separation was carried out on a Waters (Thar)
Investigator SFC: Chiral Technologies CHIRALPAK IF (5 um,
4.6 x 250 mm) column, 5-50% MeOH (0.1% DEA) in CO2 over
7 minutes at 3.5 mL/min, 40 oC, backpressure maintained at 100
bar. Preparative separation was carried out on a PIC Solution
Prep 100 SFC: Chiral Technologies CHIRALPAK IF (5 um, 20
x 250 mm) column, 10% MeOH (0.1% DEA) in CO2 at 80
mL/min, 40 oC, backpressure maintained at 100 bar. 1H NMR:
(600MHz, CDCl3) δ 8.04 (br.s, 1H), 7.65 (d, J = 8.5 Hz, 1H),
7.38 (d, J = 5.4 Hz, 1H), 7.09 (m, 2H), 6.94 (m, 2H), 6.89 (m,
2H), 3.68 (q, J = 13.3 Hz, 2H), 2.76 (br.s, 2H), 2.49 (m, 2H),
2.37 (br.s, 1H), 2.11 (br.s 1H), 2.03 (m, 1H), 1.95 (m, 4H). 13C
NMR: (150MHz, CDCl3) δ 161.5 (JCF = 242 Hz), 137.2 (JCF =
3.0), 136.7, 129.6 (JCF = 7.7 Hz), 128.2, 126.2, 123.8, 123.4
(JCF =241 Hz), 120.7, 120.4, 115.0 (JCF = 21 Hz), 113.3,
111.0, 54.5, 52.9, 50.0 (JCF = 9.6 Hz), 42.3 (JCF = 21 Hz),
33.7, 32.6, 27.2. LCMS: RT: 0.977 min., m/z = 407.1 [M + H]+,
>99% @ 215 and 254 nm. Specific O.R.: = - 14.2 (c = 1g/dL,
MeOH).
20. The structure data were deposited with Cambridge
Crystallographic Data Centre Deposition Number: CCDC
1413406.
21. Orbach, R. S. Drug Metab. Disp. 1999, 27, 1350.
22. Smith, N. F.; Raynaud, F. I.; Workman, P. Mol. Cancer Ther.
2007, 6, 428.
23. Bridges, T. M.; Morrison, R. D.; Byers, F. W.; Luo, S.; Daniels,
J. S. Pharmacol. Res. Perspect. 2014, 2, e00077.