Synthesis and cannabinoid-1 receptor binding affinity of conformationally constrained analogs of taranabant

Article abstract of DOI:10.1016/j.bmcl.2010.06.127

The design, synthesis, and binding activity of ring constrained analogs of the acyclic cannabinoid-1 receptor (CB1R) inverse agonist taranabant 1 are described. The initial inspiration for these taranabant derivatives was its conformation 1a, determined b

Full text of DOI:10.1016/j.bmcl.2010.06.127

Bioorganic & Medicinal Chemistry Letters 20 (2010) 4757–4761
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and cannabinoid-1 receptor binding affinity of conformationally
constrained analogs of taranabant
Ihor E. Kopka a, Linus S. Lin a, James P. Jewell a, , Thomas J. Lanza a, Tung M. Fong b, Chun-Pyn Shen b,
Zhege J. Lao b, Sookhee Ha c, Laurie G. Castonguay c, Lex Van der Ploeg b, Mark T. Goulet a,
,
William K. Hagmann a,
*
a Department of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
b Department of Metabolic Disorders, Merck Research Laboratories, Rahway, NJ 07065, USA
c Department of Molecular Modeling, Merck Research Laboratories, Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The design, synthesis, and binding activity of ring constrained analogs of the acyclic cannabinoid-1 recep-
tor (CB1R) inverse agonist taranabant 1 are described. The initial inspiration for these taranabant deriv-
atives was its conformation 1a, determined by 1H NMR, X-ray, and molecular modeling. The constrained
analogs were all much less potent than their acyclic parent structure. The results obtained are discussed
in the context of a predicted binding of 1 to a homology model of CB1R.
Received 22 April 2010
Revised 22 June 2010
Accepted 25 June 2010
Available online 1 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Cannabinoid
CB1R
Obesity
Conformationally constrained
There have been significant efforts toward identifying potent
selective antagonists/inverse agonists of the cannabinoid-1 recep-
tor (CB1R) as a treatment for obesity.1–4 Several compounds pro-
gressed to clinical studies and were found to be efficacious in
promoting sustained weight loss. One of these, rimonabant, had
been approved for marketing in the EU as a treatment for obesity.
Unfortunately, the long term safety profile of these agents was
found not to be consistent with treating an overweight but other-
wise healthy population and development of centrally acting CB1R
antagonists/inverse agonists was halted. In our efforts to identify
potent, selective CB1R modulators, we discovered taranabant 1
which was fully effective in preclinical and clinical evaluations of
weight loss.5–7 The acyclic nature of structure 1 is different from
that of other CB1R antagonists/inverse agonists which generally
contain two phenyl rings appended to a heteroaryl ring with addi-
tional substitution and/or appended fused rings. A detailed study
of the conformation of 1 revealed that it had a rigid structure 1a
(Fig. 1) which overlapped quite well with rimonabant, especially
in the region of the diphenylethyl substitution.8 In an effort to ex-
plore this conformation, we describe herein the preparation of a
series of simple methyl substitutions along the propyl backbone
as well as cyclic structures that overlapped well with 1a.
CN
CH3
O
CH3
CH3
O
N
H
N
CF3
Cl
1, taranabant
hCB1R IC50 0.74 nM
CN
Ha
Hd
e O
CH3
CH3
O
CH3
Hc
N
Hb
Cl
N
Hq
1a
CF3
Figure 1. Structure of taranabant and its conformation as determined from X-ray,
1H NMR, and molecular modeling studies.
The 3-bromophenyl derivative 2 (Fig. 2) was chosen as the acy-
clic reference compound. The methylated derivatives of 2 were
prepared by analogous methods previously described and are out-
lined below.9 Binding affinities were determined by inhibition of
* Corresponding author. Tel.: +1 732 594 7249; fax: +1 732 594 5966.
E-mail address: william.hagmann@merck.com (W.K. Hagmann).
Present address: Merck Research Laboratories, Boston, MA 02115, USA.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.06.127

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I. E. Kopka et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4757–4761
Br
Br
afford amine hydrochloride 17. The fibric acid that was typically
used was converted to its acylchloride and reacted with 17 in the
presence of N-methylmorpholine (NMM) to afford 4.
CH3
CH
O
3O
H3C
CH3
CH3
N
CH3
CH3
N
N
H
N
H
The preparation of the benzylic methyl derivatives of 2 are out-
lined in Scheme 3. 3-Bromophenyl acetone9 was successively
alkylated, first with methyliodide followed by 4-chloro-benzyl-
chloride to yield 19. Reduction with sodium borohydride gave
the alcohol 20. Treatment with mesylchloride in the presence of
triethylamine (TEA) followed by reaction with sodium azide gave
a mixture of diastereomeric azides which were separated by col-
umn chromatography on silica gel eluted with ethyl acetate/hex-
anes. These diastereomers were separately reduced as noted
above to afford amine 22 and reacted with the acid chloride of
the pyridyl fibric acid to afford the isomers 5a (major, faster elut-
ing) and 5b (minor, slower eluting).
O
O
Cl
CF3 Cl
CF3
2 (+/-)
3 (+/-)
hCB1R IC50 22 nM
hCB1R IC50 0.59 nM
Br
Br
CH3
O
O
CH3
CH3
CH3
CH3
N
CH3
CH3
N
N
H
N
H
O
O
Cl
CF3 Cl
CF3
4 (+/-)
5a (+/-)
In order to confirm the speculations about the effect on confor-
mations by the methyl additions or transpositions in compounds
3–5, minimum energy conformations were determined for 2 and
3–5 and molecular overlays were determined (Fig. 3).12 All com-
pounds have good overlap in the region of the trifluoropyridyl
ether, but the addition/transposition of the backbone methyls on
the propyl group causes the phenyl rings to rotate away from that
hCB1R IC50 340 nM
hCB1R IC50 25 nM
5b (+/-)
hCB1R IC50 260 nM
Figure 2. The effect of backbone methyl substitution of 2 on CB1R binding potency.
binding of [3H]-CP55,490 to recombinant human CB1 or CB2
expressed on Chinese hamster ovary (CHO) cells.10 The reported
IC50 values are the result of two determinations (n = 2). The addi-
tion of a methyl group replacing Ha of 1a afforded the geminal di-
methyl analog 3 which lost binding potency compared to 2. This
should not be too surprising as a methyl group in the Ha position
of 1a is expected to eclipse the adjacent phenyl ring and move it
into a different and less favorable conformation. The transposition
of the methyl group in 4 to the Hb position takes away the contri-
e
bution of the CH3 group in stabilizing the conformation of 1a,
resulting in a larger loss of potency. The return of the stabilizing
influence of CH3 is evident in diastereomer 5a which improves
2 (green)
3 (salmon)
e
binding potency relative to 4. The other diastereomer 5b is much
less active. However, none of these methyl substitutions in 3–5 af-
ford compounds with the potency of 2, demonstrating how impor-
tant, and perhaps how sensitive, the conformation in 1a is to the
potency of these acyclic CB1R inverse agonists.
The preparation of tetramethyl derivative 3 is outlined in
Scheme 1. This synthetic route is particularly long and circuitous
as the tetramethyl amine 12 was not the intended target but was
rather an unexpected product in the preparation of a benzylic fluo-
rine analog derived from compound 10. 3-Bromobenzaldehyde
was homologated with cyanotrimethylsilane in the presence of
2 (green)
4 (salmon)
zinc chloride followed by esterification to afford
phenylacetate 6. Treatment of 6 with DAST displaced the hydroxyl
group to yield -fluoro-phenylacetate 7. Alkylation with 4-chloro-
a-hydroxy
a
benzylbromide followed by reduction/oxidation gave aldehyde 9.
Reaction with chiral N-tert-butylsulfinyl amine afforded chiral
sulfinyl imine 10.11 Treatment with methylmagnesium bromide
resulted in the introduction of two methyl groups, possibly
through an aziridine intermediate, to yield 12 without the benzylic
fluorine. Cleavage of the N–S bond with HCl gave the dimethyl
amine 12 which was coupled to the fibric acid9 to give the tetra-
methyl derivative 3.
The synthesis of the transposed methyl derivative 4 is outlined
in Scheme 2. 3-Bromo-phenylacetate was alkylated with 4-chloro-
benzylchloride and then with methyliodide to afford 14. The ester
in 14 was reduced with lithium aluminum hydride, followed by
sulfonylation with mesylchloride (MsCl) and displacement with
sodium azide to yield 16. The azide 16 was reduced with hydrogen
in the presence of platinum oxide and Boc anhydride; the latter
was necessary for purification of the desired product. The N-t-Boc
group was subsequently removed with 4 M HCl in dioxane to
2 (green)
5a (salmon)
5b (yellow)
Figure 3. Overlay of minimum energy conformation of compound
compounds 3, 4, 5a, and 5b.
2 with

I. E. Kopka et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4757–4761
4759
Br
Br
Br
CN
CO2Et
NH2
CN
CO2Et
24
a
b
a,b
c
d
CO2CH3
CO2CH3
CO2Et
25
CHO
23
OH
F
7
6
O
S
Cl
Br
Br
Br
Cl
O
F
F
F
N
c
d
e
NH
26
CO2CH3
e,f
CHO
NH
28
H
g
h
27
Cl
Cl
Cl
9
10
8
Cl
O
Br
Cl
Br
S
NH2
CH3
HN
Scheme 4. Synthesis of azetidine derivative 28. Reagents and conditions: (a)
BrCH2Ph-4-Cl, LiHMDS, THF, DMI, À78 °C; (b) W2 RaNi, NH3, EtOH, 20 psi H2, (c)
t-BuMg, Et2O, 24 °C; (d) DIBAL, reflux, 2 h; (e) PyBOP, DIPEA, CH2Cl2, R-CO2H, 20 h.
i
j
CH3
3
CH3
CH3
Cl
Cl
12
O
O
11
O
b
a
O
O
O
Scheme 1. Preparation of tetramethyl derivative 3. Reagents and conditions: (a)
TMS-CN, ZnCl2, CH2Cl2, 0 °C; (b) HCl (g), MeOH, rt; (c) DAST, CH2Cl2, 0 °C; (d)
KHMDS, BrCH2Ph-4-Cl, THF, À78 °C; (e) LAH, Et2O, À78 °C; (f) PCC, CH2Cl2, 0 °C; (g)
(R)-t-Bu-S(O)NH2, Ti(OEt)4, THF, rt; (h) CH3MgBr, CH2Cl2, À78 °C; (i) HCl (g), MeOH,
dioxane, EtOAc, rt; (j) PyBOP, NMM, CH2Cl2, R-CO2H, 20 h.
30
29
Cl
Cl
NH
O
d
c
e
NH
33
Br
Br
Br
31
32
CH3
CO2CH3
CO2CH3
CO2CH3
b
c
a
Cl
Cl
Scheme 5. Synthesis of pyrrolidine derivative 33. Reagents and conditions: (a)
BrCH2Ph-4-Cl, LiHMDS, THF, DMI, À78 °C; (b) 5% Ni(COD)2, 8.5% BINAP, 2.15 equiv
NaHMDS, 15% ZnBr2, tol, 20 h, 75 °C; (c) Al(CH3)3, NH3, DCE, 6 h, 75 °C; (d) DIBAL,
tol, reflux, 2 h; (e) PyBOP, DIPEA, CH2Cl2, RCO2H, 20 h.
Cl
Cl
13
14
Br
Br
Br
CH3
CH3
CH3
NH2 HCl
h
OH
d,e
N3
f,g
CHO
+
CN
a
b
4
Cl
Cl
Cl
Cl
17
15
16
CN
34
Scheme 2. Preparation of benzylic methyl derivative 4. Reagents and conditions:
(a) KHMDS, ClCH2Ph-4-Cl, THF, À78 °C; (b) KHMDS, CH3I, THF, À78 °C; (c) LAH,
Et2O, À50 °C; (d) MsCl, TEA, EtOAc, À10 °C to >0 °C; (e) NaN3, DMF 120 °C to
>150 °C, 21 h; (f) (Boc)2O, Pt2O, H2, EtOAc; (g) 4 M HCl, dioxane; (h) NMM, CH2Cl2,
R-COCl, rt.
Cl
O
N (CH2)3Br
O
c
O
(CH2)3
N
CN
CN
O
35
36
Br
Br
Br
Cl
Cl
O
O
O
O
CH3
O
Ac
CH3
CH3
NH
N
b
c
d,e,f
CH3
a
g
CH3
38
Cl
37
18
19
Br
Br
Br
Cl
i
Cl
CH3
CH3
CH3
N3
CH3
g,h
OH
CH3
NH2 HCl
h
NH
40
CH3
i
d,e,f
5a
Cl
Cl
Cl
39
(5b)
21
20
22
Cl
Scheme 3. Preparation of dimethyl derivatives 5a and 5b. Reagents and conditions:
(a) Cs2CO3, CH3I, AcCN, rt; (b) CsOH, CH2Cl2, rt; (c) NaBH4, MeOH, rt, 10 min; (d)
MsCl, TEA, EtOAc, 0 °C, 10 min; (e) NaN3, DMF, 120 °C, 10 h; (f) diastereomers
separated onsilica gel chromatography; (g) (Boc)2O, Pt2O, H2, EtOAc, rt; (h) 4 M HCl,
dioxane, rt; (i) NMM, CH2Cl2, R-COCl, rt.
Scheme 6. Preparation of piperidine derivative 40. Reagents and conditions: (a)
piperidine, EtOH, reflux; (b) NaBH4, 3:1 pyr:MeOH, reflux; (c) LiHMDS, THF, DMI,
À78 °C; (d) concd HBr, heat; (e) adjust pH to 6.5; (f) Ac2O, heat; (g) NaOH, EtOH; (h)
DIBAL, tol, reflux 2 h; (i) PyBOP, DIPEA, CH2Cl2, RCO2H, 20 h.

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I. E. Kopka et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4757–4761
seen with the parent molecule 2. Compounds 3 and 5a appear to
have better overlays with the conformation of 2 than do com-
pounds 4 and 5b. These results are consistent with the order of
binding potencies of 3–5 relative to 2 (Fig. 2).
amide and Ser383 of CB1R. Alkylation of that nitrogen through
cyclization would eliminate that interaction and may introduce
unfavorable steric interactions. Simple modeling studies did not
distinguish which ring size would be optimum so 4- to 6-mem-
bered rings were prepared as shown in Schemes 1–3.
The four-membered ring azetidine derivative was prepared as
outlined in Scheme 4. Ethyl a-cyanophenylacetate 23 was treated
with lithium hexamethyldisilazide (LiHMDS) in the presence of
1,3-dimethyl-2-imidazolidone (DMI) in THF and was alkylated
with 4-chlorobenzylbromide to yield cyano ester 24. Hydrogena-
tion of 24 with Raney nickel catalyst in the presence of ammonia
reduced the cyano group to the aminoester 25. Treatment with
t-butylmagnesium chloride as a base in ether effected ring closure
to azetidinone 26. DIBAL reduction afforded the azetidine 27 which
was coupled to the fibric acid to afford amide 28.
Further examination of the conformation of 1a suggested that
the proton at Hb is near to Hq on the amide nitrogen. No NOE
was reported between these two protons in the 1H NMR studies,
but the overall conformation of 1a places them in a spatial rela-
tionship analogous to 1,3 hydrogens in a cyclohexane chair confor-
mation. Closing a ring between these two positions (the arrow
between Hb and Hq in 1a, Fig. 1) would not be expected to change
the overall conformation of the acyclic backbone. However, dock-
ing studies with taranabant into the CB1R homology model
suggested an important hydrogen bond between the NH of the
The pyrrolidine analog was prepared as outlined in Scheme 5.
-Butyrolactone was treated with LiHMDS, DMI in THF and alkyl-
O
O
CH3
CH3
CH3
CH3
c
N
N
ated with with 4-chlorobenzylbromide to afford lactone 29. The
phenyl ring was introduced employing the nickel–BINAP catalyzed
method described by Spielvogel and Buchwald to yield 30.13 The
lactone was converted to the lactam 31 by treatment with trimeth-
ylaluminum/ammonia followed DIBAL reduction to yield pyrroli-
dine 32. Coupling with the fibric acid gave the five-membered
ring analog 33.
O
N
O
N
CF3
CF3
28 (+/-)
33 (+/-)
hCB1R IC50 810 nM
Cl
Cl
hCB1R IC50 1300 nM
O
The synthesis of the piperidine analog began with condensation
of 4-chlorobenzaldehyde and phenylacetonitrile to yield trans-
stilbene derivative 34 (Scheme 6). Reduction with sodium cyano-
borohydride afforded diphenylethylene derivative 35 which was
treated with lithium hexamethyldisilylamide followed by alkyl-
ation with phthalylamidopropylbromide to yield 36. Treatment
of 36 with concentrated hydrogen bromide with heating removed
the protecting group which was followed by cooling and pH
adjustment to 6.4 which affected ring closure. To facilitate purifica-
tion, the crude lactam was treated with acetic anhydride to acylate
CH3
N
CH3
N
CH3
O
CH3
CH3
O
N
H
O
N
CF3
Cl
Cl
CF3
40 (+/-)
hCB1R IC50 2000 nM
41
hCB1R IC50 0.54 nM
Figure 4. Comparison of acyclic and cyclic taranabant analogs.
Figure 5. Docking of minimum energy conformations of compounds 28, 33, 40, and 41 into the CB1R homology model.

I. E. Kopka et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4757–4761
4761
the NH group to form 37. Treatment with sodium hydroxide to re-
move the acetyl group (38) followed by DIBAL reduction gave
piperidine 39. Again, coupling with the fibric acid gave the desired
product 40.
derivatives was careful examination of its conformation 1a, deter-
mined by 1H NMR, X-ray, and molecular modeling. However, our
observations were the constrained analog were all much less
potent than their acyclic parent structures. With respect to
designing novel, constrained derivatives of potent molecules, the
present study illustrates, once again, the need to consider potential
bound conformations and interactions and not just solution and/or
solid state structures.
The binding potencies of the 4- to 6-membered ring derivatives
28, 33, and 40 are shown in Figure 4. The acyclic analog 41 is
shown for comparison. Obviously, all of these conformationally
constrained analogs lost considerable potency for binding to
CB1R relative to the acyclic derivative 41 despite very good molec-
ular overlays. Part of this loss of potency may be for the same rea-
son that the methylated derivatives 4 and 5 lost binding activity;
that is, the methyl group at the benzylic position (replacing Hb)
causes an unfavorable interaction with the chlorobenzyl group
which causes a change in its position. The methylene groups of
28, 33, and 40 may do the same thing.
Earlier efforts had shown that N-methylation of the acyclic
structure caused ꢀ5-fold loss of potency (data not shown),
although the reason for that loss is less obvious since replacement
of Hq would appear to be benign in conformation 1a. Greater in-
sights as to the reasons for these losses in CB1R activity of the
methylated and ring constrained analogs may come less from ana-
lyzing the apparent conformation of 1a but more so from examin-
ing the possible binding modes of compounds 28, 33, and 40 in a
CB1R homology model (Fig. 5).5,14,15 Docking of compounds was
performed with ICM software. An important interaction that per-
tains to the compounds described herein would seem to be a
hydrogen bond formed between the NH of the amide and Ser383.
Mutation of this amino acid residue led to a large loss of binding
activity for taranabant but not for rimonabant. N-methylation or
the ring structures shown in Figure 4 would eliminate this impor-
tant interaction. In addition, the carbons of the appended, con-
strained rings of all three compounds appear to point back
towards Thr197, making unwanted interactions. It appears that
docking of these constrained ring structures into the CB1R homol-
ogy model was more predictive of their relative binding potencies
than was the analysis of conformations of the acyclic parent
structure.
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compound 2.
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In summary, we have described the design, synthesis, and bind-
ing activity of ring constrained analogs of the acyclic CB1R inverse
agonist, taranabant 1. The initial inspiration for these taranabant
15. Lange, J. H. M.; Kruse, C. G. Drug Discov. Today 2005, 10, 693.