Biaryl cannabinoid mimetics-Synthesis and structure-activity relationship

Article abstract of DOI:10.1016/j.bmcl.2007.04.059

Synthesis, in vitro biological evaluation, and structure-activity relationships of a biaryl cannabinoid mimetic 2 are reported. Variations in the substitution pattern yielded a number of agonists with low nanomolar affinity. Replacing the phenol group by

Full text of DOI:10.1016/j.bmcl.2007.04.059

Bioorganic & Medicinal Chemistry Letters 17 (2007) 3652–3656
Biaryl cannabinoid mimetics—Synthesis and
structure–activity relationship
Karin Worm,a,* Q. Jean Zhou,a Gabriel J. Stabley,b
Robert N. DeHavenb and Roland E. Dollea
aDepartment of Chemistry, Adolor Corporation, 700 Pennsylvania Drive, Exton, PA, USA
bDepartment of Pharmacology, Adolor Corporation, 700 Pennsylvania Drive, Exton, PA, USA
Received 2 March 2007; revised 10 April 2007; accepted 16 April 2007
Available online 25 April 2007
Abstract—Synthesis, in vitro biological evaluation, and structure–activity relationships of a biaryl cannabinoid mimetic 2 are
reported. Variations in the substitution pattern yielded a number of agonists with low nanomolar affinity. Replacing the phenol
group by a methyl morpholino acetate group led to compound 28, a 500-fold selective CB2 receptor agonist.
Ó 2007 Elsevier Ltd. All rights reserved.
D9-Tetrahydrocannabinol (THC, 1) and other classical
OH
cannabinoids display a wide range of physiological
effects including analgesic, anti-inflammatory, anti-
convulsive, and immunosuppressive activities.1 Two
cannabinoid receptors have been cloned. The CB1 recep-
tor is mainly expressed in the CNS and the CB2 receptor
is localized mainly in peripheral tissues.2 A number of
SAR studies have explored the lipophilic side chain
through variations in length, branching, spatial orienta-
tion or introduction of heteroatoms.3 It has been shown
that introducing a 10,10-dimethylheptyl- or 10,10-cyclo-
propylheptyl chain leads to enhanced affinity for both
cannabinoid receptors.4 Also it has been found that
the tricyclic moiety in 1 is not essential for high cannab-
inoid receptor affinity as demonstrated by the biaryl
phenol 2, a cannabinoid mimetic originally described
by researchers at Merck Frosst.5
HO
O
1, THC
2
R2
C6H13
C6H13
R1
R3
HO
HO
3
4
5
Previously 2 was synthesized in nine steps starting from
1,3-dimethoxy-5-(2-methyloctan-2-yl)benzene. This syn-
thesis included a low yielding monophosphonation step
(16%) resulting in a 4% yield overall.5 Scheme 1 illus-
trates our shortened synthesis resulting in a 42% yield
over six steps by adapting a procedure recently de-
scribed by Papahatjis4b starting from commercially
available (3-benzyloxyphenyl)acetonitrile 6. After dial-
kylation with methylbromide gas in 50% aq NaOH,
the nitrile intermediate was converted to aldehyde 7 with
DIBAL-H. Wittig reaction with pentyltriphenyl-
phosphonium bromide and NaH in DMSO and subse-
quent hydrogenation, followed by bromination, led to
advanced intermediate 8 in good overall yield. The final
To fully evaluate the potential of this underexplored but
synthetically more accessible non-classical cannabinoid
lead structure, we investigated modifications to the
side-chain R1 (3), top aryl group R2 (4), and phenol
replacements R3 (5).
Keywords: Cannabinoid CB1 receptors; Cannabinoid CB2 receptors;
Structure–activity relationships.
*
Corresponding author. Tel.: +1 484 595 1944; fax: +1 484 595
1551; e-mail: kworm@adolor.com
step consisted of
a
Suzuki reaction with 3,5-
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.04.059

K. Worm et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3652–3656
3653
O
BnO
BnO
BnO
TBDMS
c, d, e
a, b
O
53%
96%
CN
CHO
CHO
O
7
6
7
11
a-c
d-f
Br
HO
HO
Br
f
g
HO
HO
72%
O
O
C4H9
C4H9
O
O
2
n
n
8
12
a: n = 0
b: n = 2
c: n = 4
Scheme 1. Reagents and condition: (a) 50% aq NaOH, CH3Br; (b)
DIBAL-H, aq H2SO4; (c) NaH, DMSO, PPh3C5H11Br; (d) H2, 10%
Pd/C; (e) Br2, CC14; (f) 3,5-dimethylphenylboronic acid 9, Pd(PPh3)4
cat., Cs2CO3, DMF, 90 °C.
13
h
i
dimethylphenylboronic acid 9, tetrakis-triphenylphos-
phine palladium, and cesium carbonate in DMF.
Scheme 2 describes utilizing this shortened approach to
substitute the 10,10-dimethyl group in 2 with carbocycles
of various ring sizes 10 to probe the steric limits of this
system. Using the same starting material 6, alkylations
and ring closures were accomplished using 1,2-dichloro-
ethane, 1,4-dibromobutane, and 1,5-dibromopentane,
respectively, followed by the chemistry described for
the synthesis of 2.6 Scheme 3 outlines the synthesis of
analogs 3 with modifications of the hydrocarbon tail
by varying length and substitution pattern which were
investigated next. Three different ester core structures
12a–c were prepared and converted to alcohols 14, acids
15, and amides 16. The solid phase synthesis approach
shown in Scheme 4 and the commercial availability of
a diverse set of aryl boronic acids allowed access to a
large number of analogs 4. Coupling of 8 to solid sup-
port followed by Suzuki coupling and cleavage from re-
sin yielded aryl- and heteroaryl-substituted analogs 19.7
HO
HO
O
R'
OH
n
n
14
15, R' = OH
j
16, R' = NHCH3
Scheme 3. Reagents and conditions: (a) MeI, DCM; (b) TBAF, THF;
(c) Br2, CC14; (d) NaH, DMSO, trimethyl phosphonoacetate or
4-(dimethoxyphosphoryl)but-2-enoic acid methyl ester; (e) H2, 10%
Pd/C; (f) Br2, CC14; (g) 3,5-dimethyl-phenylboronic acid, Pd(PPh3)4
cat., Cs2CO3, DMF, 90 °C; (h) NaBH4; (i) LiOH; (j) CH3NH2, TBTU,
DIEA, acetonitrile, 25 °C.
Br
O
a
b
Wang Br
The phenol group in 2 seemed to be essential for the can-
nabinoid binding affinity of this biaryl system, since
replacing it with hydrogen led to a striking loss of bind-
C6H13
17
X
X
R
R
c
HO
O
HO
BnO
a-f
C6H13
C6H13
C6H13
CN
18
19
n
6
10 a: n = 1
b: n = 3
Scheme 4. Reagents and conditions: (a) Compound 8, K2CO3, DMF,
80 °C; (b) boronic acids, Pd(PPh3)4 cat., Cs2CO3, DMF, 90 °C; (c)
TFA/DCM (1:1).
c: n = 4
Scheme 2. Reagents and condition: (a) LHMDS, HMPA, 1,2-dichlo-
roethane, 1,4-dibromobutane, 1,5-dibromopentane; (b) DIBAL-H, aq
H2SO4; (c) NaH, DMSO, PPh3C5H11Br; (d) H2, 10% Pd/C; (e) Br2,
CC14; (f) 3,5-dimethylphenyl boronic acid, Pd(PPh3)4 cat., Cs2CO3,
DMF, 90 °C.
ing.5 In order to further evaluate the effectiveness of the
phenol functionality as a H-bond donor, it was replaced
with a wide range of substituents displaying various

3654
K. Worm et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3652–3656
H-bond strengths, H-bond orientations/conformations,
and charge distributions. The synthesis is summarized
in Scheme 5. The phenol replacement analogs 5 were
prepared from 2 through the trifluoromethanesulfonate
intermediate 20. Cross-coupling with benzophenone
imine catalyzed by Pd2(dba)3/dppf with NaOtBu as base
followed by hydrolysis gave the aniline 21. Pd-catalyzed
CO insertion with hexamethyldisilazane yielded analog
23, which was then reduced by LiAlH4 to generate com-
pound 26. A similar CO insertion catalyzed by
Pd(OAc)2 in MeOH/DMSO gave the methyl ester ana-
log 22, which was in turn reduced to a benzylic alcohol
25. Analog 22 was also hydrolyzed to a carboxylic acid
24, which was then coupled with isopropyl amine with
assistance of TBTU to give amide 27. Esterification of
25 with 2-bromoacetyl chloride followed by amination
with morpholine generated not only the desired product
28, but also a side product 29.8
R
b, c
a
2
21, R=NH2
C6H13
20, R = OTf
e, f
d
23, R= CONH2
26, R= CH2NH2
h
g
24, R=COOH
22, R=COOMe
25, R= CH2OH
i
h
27, R=CONHiPr
j, k
O
N
O
29, R=
28, R=
N
+
O
O
All the compounds9 were evaluated in CB1 and CB2
binding studies.10a Active compounds were tested in
[35S]GTPcS binding and behaved as full agonists relative
to the maximal effect of WIN55212-2.10b The results of
the in vitro binding assays for side-chain analogs 3
and aryl modifications 4 are presented in Table 1. Con-
necting the two methyl groups into a cyclopropyl ring
leads to some loss of activity, in contrast to what was
Scheme 5. Reagents and conditions: (a) (Tf)2O/pyridine, DCM; (b)
Ph2CNH, Pd2(dba)3/dppf, NaOtBu, toluene, 80 °C, 16 h; (c) H2NOH-
HC1, KOAc, MeOH, 25 °C, 1.5 h; (d) CO/Pd(OAc)2/dppf, TEA,
MeOH/DMSO; (e) PdCl2(PPh3)2, HMDS, DMF, CO, 95 °C, 16 h; (f)
2 N aq H2SO4; (g) LiOH/H2O/MeOH, rt; (h) LAH/THF, 0–25 °C; (i)
TBTU, DIEA, (CH3)2CHNH2, 0–25 °C; (j) 2-bromoacetyl chloride,
DCM; (k) morpholine.
Table 1. Side chain and top modificationsa,b
R2
C6H13
HO
HO
R1
4
3
Compound
R1
KiCB1
(nM)
KiCB2
(nM)
Ratio
CB1/CB2
Compound
R2
KiCB1
(nM)
KiCB2
(nM)
Ratio
CB1/CB2
C6H13
2
2.7
2.3
55
1
18a
2,6-Dimethylphenyl
1.7
4.6
49
18
0.1
0.7
2
C6H13
C6H13
C6H13
10a
10b
10c
13a
13b
13c
15a
13
0.2
0.5
1
18b
18c
18d
18e
18f
18g
18h
2,5-Dimethylphenyl
2,3-Dimethylphenyl
3,4-Dimethylphenyl
2-Methoxyphenyl
6.6
20
2.9
5.5
1.7
16
1.8
600
1.7
34
830
1.0
7.8
580
1700
0.7
2
CO2CH3
160
120
24
10
20
2
(CH2)2CO2CH3
(CH2)4CO2CH3
COOH
5.6
14
2,3-Dimethoxyphenyl
3-Methoxyphenyl
4
91
0.2
0.8
>1000
>1000
n.d.
3,4-Dimethoxyphenyl
1400

K. Worm et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3652–3656
3655
Table 1 (continued)
Compound R1
KiCB1 KiCB2 Ratio
(nM)
Compound R2
KiCB1 KiCB2 Ratio
(nM)
(nM)
CB1/CB2
(nM)
CB1/CB2
(CH2)4CO2OH
CH2OH
15b
14a
14b
14c
16a
16b
16c
>1000 >1000
n.d.
18i
2-Aminophenyl
5.2
17
0.3
0.3
0.3
0.6
n.d.
n.d.
1
2200
1100
39
600
56
4
18j
3-Aminophenyl
4-Aminophenyl
3-Cyanophenyl
4-Cyanophenyl
15
44
(CH2)3OH
20
10
n.d.
6
18k
181
18m
18n
18o
320
27
1000
45
(CH2)5OH
3.6
CONHCH3
>1000 >1000
>1000
>1000
>1000
20
(CH2)2CONHCH3
(CH2)4CONHCH3
840
96
140
39
Phenyl-3-carboxylic acid >1000
2
3-Pyridyl
23
a Values are geometric means computed from at least three separate determinations.
b For assay description, see Ref. 10.
Table 2. Phenol replacementsa,b
observed for THC analogs.4b Increasing the ring size re-
stores affinity and substitutions up to a six-membered
ring in 3 are well accepted, indicating some tolerance
for steric bulk in the C10-subsite. In general, longer side
chains give better binding, as seen in THC analogs.
Introduction of a carboxylic acid group results in loss
of affinity for both receptors (Ki > 1000 nM). The CB2
receptor seems more tolerant toward polar groups in
R1, accepting alcohol, ester, and amide functionalities.
Ester 13b and alcohol 14b exhibit a 20-fold selectivity
for the CB2 receptor. In compound 4 activity is retained
relative to compound 2 when the top aryl ring is substi-
tuted in the 2 and/or 3 positions, with the 2-position
being preferred. Substitution in the 4-position reduces
receptor affinities by about two orders of magnitude
compared to substitution in the 2-position and again
the introduction of a carboxylic acid group leads to a
loss of affinity at both receptors. Aryl analogs 4 did
not show significant selectivity for either receptor with
the exception of 18a with a modest 10-fold selectivity
for the CB1 receptor.
C6H13
R3
5
Compound R3
KiCB1
(nM)
KiCB2 Ratio
(nM)
CB1/CB2
22
–OH
2.7
320
2.3
34
1
3011
315
21
–OCH3
–H
–NH2
9
>30000 2138
>10
2
80
230
51
22
23
–COOCH3
–CONH2
–COOH
–CH2OH
–CH2NH2
110
320
2
370
1
24
25
>1000
360
1500
>1000 n.d.
480
380
0.8
4
26
O
27
28
>1000
370
>1000 n.d.
N
H
O
The results for analogs 5 are summarized in Table 2.
Replacing the phenol group in 2 leads generally to a signif-
icant loss of potency at both receptors with the methoxy
and amino compounds 30 and 21 retaining some binding
affinity to CB2. Introducing a methyl morpholino acetate
considerably enhances the CB2 binding affinity of the ben-
zylic alcohol precursor 25 leading to the discovery of 28
with a selectivity for the CB2 receptor of 500-fold.
O
0.81
660
500
6
N
O
N
29
3900
O
a Values are geometric means computed from at least three separate
determinations.
b For assay description, see Ref. 10.
Overall the side-chain SAR was comparable to that re-
ported for THC analogs.3,4 This general observation
does not extend to the introduction of a cyclopropyl
ring, which leads to an increase in affinity for THC ana-
logs4b but to a loss of affinity in the biaryl system (10a).
The high selectivity for the CB2 receptor observed in 28

3656
K. Worm et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3652–3656
Meeting of the American Chemical Society, Washington,
DC; American Chemical Society: Washington, DC, 2005;
MEDI 88.
was not paralleled in the tricyclic system, suggesting a
divergence in the SAR and promising areas for future
exploration. In summary, these synthetically more
accessible biaryl cannabinoid mimetics may represent a
valid starting point for the development of more selec-
tive ligands. The results of a combinatorial approach
examining substitutions in the R1, R2, and R3 positions
conducted in our laboratories will be reported
subsequently.
9. New compounds were fully characterized by 1H NMR and
LC/MS.
10. Binding assays were performed by modification of the
method of (a) Pinto, J. C.; Potie, F.; Rice, K. C.;
Boring, D.; Johnson, M. R.; Evans, D. M.; Wilken, G.
H.; Cantrell, C. H.; Howlett, A. Mol. Pharmacol. 1994,
46, 516, Receptor binding assays were performed by
incubating 0.2–0.6 nM [3H]CP55940 with membranes
prepared from cells expressing cloned human CB1 or
CB2 receptors in buffer consisting of 50 mM Tris–HCl,
pH 7.0, 5.0 mM MgCl2, 1.0 mM ethylene glycol-bis(2-
aminoethylether)-N,N,N0,N0-tetraacetic acid (EGTA),
and 1.0 mg/ml fatty acid-free bovine serum albumin.
After incubation for 60 min at room temperature for
CB2 binding or 120 min at 30 °C for CB1 binding, the
assay mixtures were filtered through GF/C filters that
had been pre-soaked overnight in 0.5% (w/v) poly(eth-
yleneimine) and 0.1% BSA in water. The filters were
rinsed six times with 1 ml each of cold assay buffer,
30 ll of MicroScint 20 (Perkin-Elmer) was added to
each filter, and the radioactivity on the filters was
determined by scintillation spectroscopy in a TopCount
(Perkin-Elmer). Nonspecific binding was determined in
the presence of 10 lM WIN55212-2.; The [35S]GTPcS
binding method is a major modification of the method
by (b) Selley, D. E.; Stark, S.; Sim, L. J.; Childers, S. R.
Life Sci. 1996, 59, 659, CB2-mediated stimulation of
[35S]GTPcS binding was measured in a mixture con-
taining 100–150 pM [35S]GTPcS, 150 mM NaCl, 45 mM
MgCl2, 3 lM GDP, 0.4 mM dithiothreitol, 1.0 mM
EGTA, 1.0 mg/ml fatty acid-free bovine serum albumin,
25 lg of membrane protein, and agonist in a total
volume of 250 ll of 50 mM Tris–HCl buffer, pH 7.0, in
96-well Basic FlashPlates (Perkin-Elmer). After incuba-
tion at room temperature for 6 h, the plates were
centrifuged at 800g at 4 °C for 5 min and the radioac-
tivity bound to the membranes was determined by
scintillation spectrometry using a TopCount (Perkin-
Elmer). The extent of stimulation over basal [35S]GTPcS
binding was calculated as a percentage of the stimula-
tion by 10 lM WIN55212-2. Basal [35S]GTPcS binding
was determined in the absence of agonist. Generally, the
stimulation by 10 lM WIN55212-2 was between 50%
and 100% over basal binding. Full agonists stimulate
binding to the same maximal extent as WIN55212-2.
11. Zhou, Q. J.; Worm, K.; Dolle, R. E. J. Org. Chem. 2004,
69, 5147.
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DeHaven, R. N. Abstracts of Papers, 230th National