1-Pentyl-3-phenylacetylindoles, a new class of cannabimimetic indoles

Article abstract of DOI:10.1016/j.bmcl.2005.06.008

A new class of cannabimimetic indoles, with 3-phenylacetyl or substituted 3-phenylacetyl substituents, has been prepared and their affinities for the cannabinoid CB₁ and CB₂ receptors have been determined. In general those compounds with a 2-substituted phenylacetyl group have good affinity for both receptors. The 4-substituted analogs have little affinity for either receptor, while the 3-substituted compounds are intermediate in their affinities. Two of these compounds, 1-pentyl-3-(2-methylphenylacetyl)indole (JWH-251) and 1-pentyl-3-(3-methoxyphenylacetyl)indole (JWH-302), have 5-fold selectivity for the CB₁ receptor with modest affinity for the CB ₂ receptor. GTPγS determinations indicate that both compounds are highly efficacious agonists at the CB₁ receptor and partial agonists at the CB₂ receptor.

Full text of DOI:10.1016/j.bmcl.2005.06.008

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4110–4113
1-Pentyl-3-phenylacetylindoles, a new class of
cannabimimetic indoles
John W. Huffman,a,* Paul V. Szklennik,a Amanda Almond,a Kristen Bushell,a
Dana E. Selley,b Hengjun He,b Michael P. Cassidy,b Jenny L. Wileyb and Billy R. Martinb
aHoward L. Hunter Laboratory, Clemson University, Clemson, SC 29634-0973, USA
bDepartment of Pharmacology and Toxicology, Medical College of Virginia Campus, Virginia Commonwealth University,
Richmond, VA 23298-0613, USA
Received 12 May 2005; revised 30 May 2005; accepted 2 June 2005
Available online 6 July 2005
Abstract—A new class of cannabimimetic indoles, with 3-phenylacetyl or substituted 3-phenylacetyl substituents, has been prepared
and their affinities for the cannabinoid CB1 and CB2 receptors have been determined. In general those compounds with a 2-substi-
tuted phenylacetyl group have good affinity for both receptors. The 4-substituted analogs have little affinity for either receptor, while
the 3-substituted compounds are intermediate in their affinities. Two of these compounds, 1-pentyl-3-(2-methylphenylacetyl)indole
(JWH-251) and 1-pentyl-3-(3-methoxyphenylacetyl)indole (JWH-302), have 5-fold selectivity for the CB1 receptor with modest affin-
ity for the CB2 receptor. GTPcS determinations indicate that both compounds are highly efficacious agonists at the CB1 receptor
and partial agonists at the CB2 receptor.
Ó 2005 Elsevier Ltd. All rights reserved.
In the classical investigation of the structure–activity
relationships (SAR) of cannabimimetic aminoalkyl-
indoles, such as WIN-55,212-2 (1), it was found that a
3-(1-naphthoyl) substituent appended to the indole
nucleus provided greater affinity for the cannabinoid
CB1 receptor than a substituted benzoyl group.1 Nearly
simultaneously, we demonstrated that the N-aminoalkyl
group could be replaced by an alkyl group without loss
of cannabinoid activity. An n-pentyl group on the indole
nitrogen, as in JWH-018 (2), provided maximum affinity
for the CB1 receptor, and in vivo potency typical of tra-
ditional cannabinoids, such as D9-tetrahydrocannabinol
(3, D9-THC).2,3 Subsequently, we prepared a number of
N-alkyl 3-(1-naphthoyl)indole derivatives to develop
SAR for cannabimimetic indoles at both the CB1 and
CB2 receptors.4–7
hydro-1-naphthoyl) compound had moderate affinity
for the CB1 receptor and was quite potent in inhibiting
the electrically induced contractions of the isolated
mouse vas deferens. The compound with a 3-(5,6,7,8-tet-
rahydro-1-naphthoyl) substituent had considerably less
affinity for the receptor, but was slightly more potent
than the 1,2,3,4-tetrahydro-1-naphthoyl analog in the
mouse vas deferens protocol. It was suggested that the
potency of these compounds is due to the presence of
a bicyclic substituent at C-3 of the indole, rather than
to specific aromatic interactions. However, there now
exists convincing evidence that cannabimimetic indoles,
including aminoalkylindoles, interact with the CB1
receptor primarily by aromatic stacking.8,9
There appeared to be two plausible explanations for
the greatly enhanced CB1 receptor affinities of the 3-
(1-naphthoyl)indoles. Either the presence of a second
aromatic ring increased the magnitude of stacking inter-
actions with the CB1 receptor or the geometry of the
naphthoyl indoles is such that the second aromatic ring
(carbons 5–8) is proximate to aromatic amino acids in
the receptor, which would increase the stacking interac-
tions. To gain evidence regarding this question, we pre-
pared a series of 1-pentyl-3-phenylacetylindoles (4,
Scheme 1). These indole derivatives include compounds
Among the compounds included in the study by
the Winthrop group were aminoalkylindoles with
3-(1,2,3,4-tetrahydro-1-naphthoyl) and 3-(5,6,7,8-tetra-
hydro-1-naphthoyl) substituents.1 The 3-(1,2,3,4-tetra-
Keywords: Cannabinoids; Structure–activity relationships; Cannabi-
noid receptors; Aminoalkylindole.
*
Corresponding author. Tel.: +1 86 4656 3133; fax: +1 86 4656
6613; e-mail: huffman@clemson.edu
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.008

J. W. Huffman et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4110–4113
4111
both with and without a C-2 methyl substituent (4,
R = CH3 or H). A variety of compounds were synthe-
sized, including those with methyl-, methoxy-, fluoro-,
chloro-, and bromophenyl substituents as well as the
unsubstituted analogs.
JWH-304) have uniformly low CB1 receptor affinity
(Ki = 179–3363 nM).
The 3-(2-substituted phenylacetyl)indoles have good to
high affinity for the CB1 receptor. The highest affinity
compounds are 1-pentyl-3-(2-chlorophenylacetyl)indole
(JWH-203), with Ki = 8.0 0.9 nM and 1-pentyl-3-
(2-bromophenylacetyl)indole (JWH-249) Ki = 8.4
Cannabimimetic indoles were synthesized from 1-
pentylindole (5, R = H) or 2-methyl-1-pentylindole (5,
R = CH3) and the appropriate phenylacetyl chloride
by the Okauchi modification of the Friedel–Crafts reac-
tion (Scheme 1).7,10 In this procedure the substrate in-
dole is stirred in dichloromethane with 1.5 equiv of
dimethylaluminum chloride at 0 °C for up to 1 h. To this
intermediate organoaluminum compound is added
1.5 equiv of the acyl halide.11 Evidence for the forma-
tion of an organoaluminum intermediate follows from
the observation that reaction of 1-pentylindole with
dimethylaluminum chloride and quenching with D2O
provided 3-deuterio-1-pentylindole.
1.8 nM.
1-Pentyl-2-methyl-3-(2-methoxyphenylace-
tyl)indole (JWH-306), the 1-pentyl-3-(2-fluoropheny-
lacetyl)indoles (JWH-311 and JWH-314), and the
1-pentyl-3-(2-methylphenylacetyl)indoles (JWH-251 and
JWH-252) have the lowest affinities of this group of
compounds with Ki = 23–39 nM. The other 3-(2-substi-
tuted phenylacetyl)indoles, JWH-204, JWH-305, and
JWH-250 have Ki = 11–15 nM.
Those compounds with a 3-substituted phenylacetyl
group have CB1 receptor affinities intermediate between
those of the 2- and 4-substituted analogs. In particular,
1-pentyl-3-(3-methoxyphenylacetyl)indole (JWH-302,
Ki = 17 2 nM) and 1-pentyl-3-(3-chlorophenylace-
tyl)indole (JWH-237, Ki = 38 10 nM) have quite high
affinity for the CB1 receptor. The corresponding 2-
methylindoles (JWH-253 and JWH-303) have significant-
ly lower affinities than JWH-237 and JWH-302. Both
1-pentyl-3-(3-fluorophenylacetyl)indole (JWH-312) and
the corresponding 2-methylindole (JWH-315) have mod-
est and little affinity, respectively, for the CB1 receptor.
The affinities of the phenylacetylindoles for the CB1 recep-
tor were determined by measuring their ability to displace
[3H]CP-55,940 from its binding site in a membrane prep-
aration from rat brain,12 and CB2 receptor affinities were
determined by measuring the ability of the compounds to
displace [3H]CP-55,940 from a cloned human receptor
preparation.13 The results of these determinations are
summarized in Table 1. The receptor affinities for WIN-
55,212-2 (1) and D9-THC (3) are also included in Table 1.
The receptor affinities summarized in Table 1 indicate
that in general the 2-methylindoles have lower affinity
for the CB1 receptor than the 2-unsubstituted analogs.
This is a general trend in the cannabimimetic indole ser-
ies.1,3–5,7 The compounds with an unsubstituted pheny-
lacetyl group (JWH-167 and JWH-205) have modest
affinities (Ki = 90 17 nM and 124 23 nM, respective-
ly) for the CB1 receptor. The 4-substituted analogs
(JWH-208, JWH-209, JWH-201, JWH-202, JWH-313,
JWH-316, JWH-206, JWH-207, JWH-248, and
In general the CB2 receptor affinities of this class of in-
doles follow the same trend as their CB1 affinities (Table
1). That is, the 2-substituted phenylacetyl compounds
have the greatest affinity, followed by the 3-substituted
analogs. The 3-(4-substituted phenylacetyl)indoles have
negligible affinity for the CB2 receptor, and most of
the 2-methylindoles have lower CB2 receptor affinities
than the unsubstituted analogs. However, in the 1-pen-
tyl-3-(2-methylphenylacetyl)indoles the 2-methylindole
analog (JWH-252, Ki = 19 1 nM) has more than
Scheme 1.

4112
J. W. Huffman et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4110–4113
Table 1. Receptor affinities (mean SEM) of 1-pentyl-3-phenylacetylindoles
3-Substituent
R
Ki (nM)
CB1
1.9 0.1a
41 2b
90 17
124 23
CB2
Ratio CB1/CB2
WIN-55,212-2 (1)
0.28 0.16a
36 10a
159 14
6.8
1.1
D9-THC (3)
Phenylacetyl, JWH-167
Phenylacetyl, JWH-205
H
0.57
0.69
0.20
1.2
CH3
H
180
146 36
19
9
2-Methylphenylacetyl, JWH-251
2-Methylphenylacetyl, JWH-252
4-Methylphenylacetyl, JWH-208
4-Methylphenylacetyl, JWH-209
2-Methoxyphenylacetyl, JWH-250
2-Methoxyphenylacetyl, JWH-306
3-Methoxyphenylacetyl, JWH-302
3-Methoxyphenylacetyl, JWH-253
4-Methoxyphenylacetyl, JWH-201
4-Methoxyphenylacetyl, JWH-202
2-Fluorophenylacetyl, JWH-311
2-Fluorophenylacetyl, JWH-314
3-Fluorophenylacetyl, JWH-312
3-Fluorophenylacetyl, JWH-315
4-Fluorophenylacetyl, JWH-313
4-Fluorophenylacetyl, JWH-316
2-Chlorophenylacetyl, JWH-203
2-Chlorophenylacetyl, JWH-204
3-Chlorophenylacetyl, JWH-237
3-Chlorophenylacetyl, JWH-303
4-Chlorophenylacetyl, JWH-206
4-Chlorophenylacetyl, JWH-207
2-Bromophenylacetyl, JWH-249
2-Bromophenylacetyl, JWH-305
4-Bromophenylacetyl, JWH-248
4-Bromophenylacetyl, JWH-304
29
23
3
3
7
CH3
H
1
179
570 127
1353 270
0.31
0.55
0.33
0.30
0.19
0.74
2.4
CH3
H
746 49
11
25
17
2
1
2
33
2
CH3
H
82 11
89 15
84 12
444 14
CH3
H
62 10
1064 21
1678 63
CH3
H
645
39
6
3
4
2.6
23
39
72
2
2
7
0.60
0.51
0.79
2.4
CH3
H
76
91 20
182 23
365 92
781 105
7.0 1.3
CH3
H
430 24
422 19
1.2
3.7
CH3
H
2862 670
8.0 0.9
1.1
CH3
H
13
1
25
106
1
2
0.52
0.36
0.85
0.78
0.43
0.42
0.52
1.6
38 10
117 10
389 25
1598 134
8.4 1.8
15 1.8
CH3
H
138 12
498 37
3723 10
CH3
H
20
29
2
5
CH3
H
1028 39
3363 332
657 19
2679 688
CH3
1.2
a Ref. 13.
b Ref. 12.
7-fold greater affinity for the CB2 receptor than the
unsubstituted compound (JWH-251, Ki = 146 36 nM).
cannabinoid full agonist. JWH-251 and JWH-302 both
stimulate GTPcS binding at CB1, with approximately
equal values of EC50 (29 nM) and are high efficacy ago-
nists with Emax of greater than 90% (Table 2). Although
the affinities of these compounds at CB2 are approxi-
mately one-fifth that of their affinities for the CB1 recep-
tor, both significantly stimulate GTPcS binding at the
CB2 receptor. Surprisingly, their potencies for CB2
receptor activation were similar to those seen with
CB1: for JWH-251, EC50 = 8.3 0.8 nM and for JWH-
302, EC50 = 24.4 6.9 nM. At the CB2 receptor, howev-
er, both compounds are partial agonists with Emax val-
ues of less than 50%.
In contrast to most cannabimimetic indoles, which tend
to show selectivity for the CB2 receptor,4,6,7,13 two of
these phenylacetylindoles show 5-fold selectivity for
the CB1 receptor. One of them, 1-pentyl-3-(2-methylph-
enylacetyl)indole, JWH-251, has good affinity for the
CB1 receptor (Ki = 29 3 nM) with modest affinity for
the CB2 receptor (Ki = 146 36 nM). The other, 1-pen-
tyl-3-(3-methoxyphenylacetyl)indole, JWH-302, also has
good affinity (Ki = 17 2 nM) for the CB1 receptor, and
fair affinity for the CB2 receptor (Ki = 89 15 nM). To
evaluate the efficacy of these compounds, their ability
to stimulate [35S]GTPcS binding at CB1 and CB2 was
determined.7,14 The results of these determinations
are summarized in Table 2, where the stimulation
produced at each receptor is normalized to a standard
The 1-pentyl-3-phenylacetylindoles constitute a new
class of cannabimimetic indoles, which in contrast to
most compounds of this general type show little selectiv-
ity for the CB2 receptor. Two of these indole derivatives,
Table 2. EC50 and Emax values (mean SEM) for stimulation by GTPcS binding of CB1 and CB2 for JWH-251 and JWH-302
a
a
Compound
CB1
CB2
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
1-Pentyl-3-(2-methylphenylacetyl)indole (JWH-251)
1-Pentyl-3-(3-methoxyphenylacetyl)indole (JWH-302)
29.0 5.5
29.3 0.8
97.6 1.5
91.5 2.9
8.3 0.8
24.4 6.9
47.0 2.4
33.5 2.9
a Stimulation values are from data normalized to stimulation produced by a maximally effective concentration of a standard full agonist: 10 lM
WIN-55,212-2 for CB1 and 3 lM CP-55,940 for CB2 receptors.

J. W. Huffman et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4110–4113
4113
3. Wiley, J. L.; Compton, D. R.; Dai, D.; Lainton, J. A. H.;
Phillips, M.; Huffman, J. W.; Martin, B. R. J. Pharmacol.
Exp. Ther. 1998, 285, 995.
JWH-251 and JWH-302, are moderately selective for the
CB1 receptor and are full agonists at this receptor. Selec-
tive agonists for the CB1 receptor are relatively rare and
although these compounds are also partial agonists at
the CB2 receptor, they may serve as the prototypes for
additional CB1 receptor selective agonists. In addition,
the high CB1 receptor affinities of several of these com-
pounds combined with the efficacies of JWH-251 and
JWH-302 suggest that the increased potency of cannab-
imimetic 3-(1-naphthoyl)indoles relative to their benzoyl
congeners is caused by their molecular geometry rather
than the presence of a second aromatic ring.
4. Aung, M. M.; Griffin, G.; Huffman, J. W.; Wu, M.-J.;
Keel, C.; Yang, B.; Showalter, V. M.; Abood, M. E.;
Martin, B. R. Drug Alcohol Depend. 2000, 60, 133.
5. Huffman, J. W. Curr. Med. Chem. 1999, 6, 705.
6. Huffman, J. W. Curr. Pharm. Des. 2000, 6, 1323.
7. Huffman, J. W.; Zengin, G.; Wu, M.-J.; Lu, J.; Hynd,
G.; Bushell, K.; Thompson, A.; Bushell, S.; Tartal, C.;
Hurst, D. P.; Reggio, P. H.; Selley, D. E.; Cassidy, M.
P.; Wiley, J. L.; Martin, B. R. Bioorg. Med. Chem. 2005,
13, 89.
8. Reggio, P. H.; Basu-Dutt, S.; Barnett-Norris, J.; Castro,
M. T.; Hurst, D. P.; Seltzman, H. H.; Roche, M. J.;
Gilliam, A. F.; Thomas, B. F.; Stevenson, L. A.; Pertwee,
R. G.; Abood, M. E. J. Med. Chem. 1998, 41, 5177.
9. Huffman, J. W.; Mabon, R.; Wu, M.-J.; Lu, J.; Hart, R.;
Hurst, D. P.; Reggio, P. H.; Wiley, J. L.; Martin, B. R.
Bioorg. Med. Chem. 2003, 11, 539.
Acknowledgments
The work at Clemson was supported by Grants
DA03590 and DA15340 to J.W.H., that at Virginia
Commonwealth University by Grant DA03672 to
B.R.M. and DA05274 to D.E.S., all from the National
Institute on Drug Abuse.
10. Okauchi, T.; Itonaga, M.; Minami, T.; Owa, T.; Kitoh,
K.; Yoshino, H. Org. Lett. 2000, 2, 1485.
11. The phenylacetyl indoles were formed in unoptimized
yields of 32–72%. All target compounds have mass
spectral, 1H, and 13C NMR data consistent with the
assigned structures. All compounds gave acceptable
microanalytical or high-resolution mass spectral data.
12. Compton, D. R.; Rice, K. C.; De Costa, B. R.; Razdan,
R. K.; Melvin, L. S.; Johnson, M. R.; Martin, B. R.
J. Pharmacol. Exp. Ther. 1993, 265, 218.
13. Showalter, V. M.; Compton, D. R.; Martin, B. R.; Abood,
M. E. J. Pharmacol. Exp. Ther. 1996, 278, 989.
14. Selley, D. E.; Stark, S.; Sim, L. J.; Childers, S. R. Life Sci.
1996, 59, 659, Chinese hamster ovary (CHO) cells stably
expressing the human CB1 or CB2 receptor were employed
in this determination.
References and notes
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