The role of fluorine substitution in the structure-activity relationships (SAR) of classical cannabinoids

Article abstract of DOI:10.1016/j.bmcl.2007.01.006

A facile synthesis of 1-fluoro-1-deoxy-Δ⁸-THC analogs with side chains seven carbons in length, in the alkane/ene/yne- series (6, 5, and 4), was achieved from 1-fluoro-3,5-dimethoxybenzene (1). In vitro studies show that substitution by a fluorine has a significant detrimental effect on CB1 binding which is supported by in vivo testing. The implications of these results on the SAR of classical cannabinoids are discussed.

Full text of DOI:10.1016/j.bmcl.2007.01.006

Bioorganic & Medicinal Chemistry Letters 17 (2007) 1504–1507
The role of fluorine substitution in the structure–activity
relationships (SAR) of classical cannabinoids
Peter J. Crocker,a Anu Mahadevan,a Jenny L. Wiley,b
Billy R. Martinb and Raj K. Razdana,*
aOrganix Inc. 240, Salem Street, Woburn, MA 01801, USA
bDepartment of Pharmacology and Toxicology, Medical College of Virginia, Virginia Commonwealth University,
Richmond, VA 23298, USA
Received 8 December 2006; revised 3 January 2007; accepted 3 January 2007
Available online 13 January 2007
Abstract—A facile synthesis of 1-fluoro-1-deoxy-D8-THC analogs with side chains seven carbons in length, in the alkane/ene/yne-
series (6, 5, and 4), was achieved from 1-fluoro-3,5-dimethoxybenzene (1). In vitro studies show that substitution by a fluorine has a
significant detrimental effect on CB1 binding which is supported by in vivo testing. The implications of these results on the SAR of
classical cannabinoids are discussed.
ꢀ 2007 Published by Elsevier Ltd.
Fluorine’s high electronegativity and small size are
among the special properties that contribute to its impor-
tance in medicinal chemistry.1 The effects of fluorine sub-
stitution on the biological behavior of biologically active
molecules have been used effectively in drug design, espe-
cially after the successful use in steroids and the antican-
cer drug 5-fluorouracil.1 As a result, the presence of
fluorine in drugs is now quite common. We were there-
fore interested in examining the role of fluorine substitu-
tion in classical cannabinoids. It is well known in the
SAR of classical cannabinoids that substitutions in the
C-1, C-3, and C-9 positions play an important role2,3 in
the interaction with CB1 cannabinoid receptors. Hydro-
gen-bonding interactions of the hydroxyl group at C-1
and the presence of a hydroxymethyl at C-9 are of partic-
ular interest in this respect. The effect of substituting
fluorine for a hydroxyl is especially interesting since the
fluorine can only accept hydrogen-bonds, whereas
hydroxyl groups can both accept and donate hydrogen-
bonds. Our molecular modeling studies4 suggested that
the phenolic hydroxyl of THC corresponded to the ter-
minal hydroxyl of anandamide. Another reason which
prompted us to carry out this study was the finding in
our laboratory5,6 that substituting a fluorine atom for
the 20-hydroxyl(O-585) in anandamide (AEA) increased
its CB1 binding affinity 10-fold. A similar increase was
found in the 2-methyl-20-F-AEA (O-689) compared to
2-methyl-AEA (O-680). Earlier studies by Charalam-
bous et al.7 and Martin et al.8,9 had shown that substitu-
tion of fluorine for hydrogen on C-50 (or C-500) of the
pentyl side chain had relatively little effect on the phar-
macological activity of the THCs (tetrahydrocannabi-
nols) or CBD (cannabidiol). This was attributed to the
fact that both fluorine and hydrogen atoms occupy com-
parable volumes although their electrostatic properties
are different. Tius and co-workers10 reported on C-9
difluoro and monofluoro-THC analogs and found them
to have marginal anti-inflammatory activity. They also
introduced fluorine probes in the aromatic ring of nabi-
lone and found diminished affinity11a for the CB1 recep-
tor thus confirming the hypothesis that the phenolic
hydroxyl group is involved in a hydrogen bonding inter-
action with the receptor. A (ꢀ)-50-18F-D8-THC analog
was studied12 for brain distribution in a primate, using
positron emission tomography (PET) technique, but
the results were inconclusive due to low binding affinity
of the ligand.
With this background, we synthesized 1-fluoro-1-deoxy-
D8-THC analogs and examined their pharmacological
activity. We prepared the alkane/ene/yne series of 1-flu-
oro-THCs (4, 5, and 6) with side chains seven carbons in
length, which previous SAR studies indicated to be near
Keywords: Fluorine substitution; Tetrahydrocannabinols; CB1 binding
affinity.
*
Corresponding author. Tel.: +1 781 932 4142; fax: +1 781 933
6695; e-mail: razdan@organixinc.com
0960-894X/$ - see front matter ꢀ 2007 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2007.01.006

P. J. Crocker et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1504–1507
1505
optimum length for cannabinoid activity. The synthe-
sis13a is shown in Scheme 1. Commercially available 1-
fluoro-3,5-dimethoxybenzene (1) was demethylated with
boron tribromide (2.5 equiv, CH2Cl2, ꢀ78 ꢁC for
10 min, then warmed to room temperature for 1 h) to af-
ford 5-fluororesorcinol which was condensed (TsOH
catalytic amount, benzene, reflux Dean–Stark trap,
2 h) with cis-p-menth-2-en-1,8-diol to afford a mixture
from which two THC isomers were isolated (silica gel
chromatography) in approximately equal amounts
(ꢁ10% each). These isomers differed in the relative sub-
stitution pattern on the aromatic THC ring, one isomer
being the desired 1-fluoro-3-hydroxy-D8-THC (2), the
other being the undesired 3-fluoro-1-hydroxy-D8-THC
(3). The structures were assigned on the basis of shifts
found for the aromatic protons (H-2 and H-4) when
their NMRs were taken in CDCl3 and C6D6, as reported
by Arnone et al.13b The desired isomer was then activat-
ed as a triflate before palladium (0)-catalyzed coupling14
to 1-heptyne to afford 1-fluoro-3-(1-heptynyl)-D8-THC
(4). This in turn was reduced to both the cis-alkene
(5, 1 atm H2, Lindlar’s catalyst, 1 drop of quinoline,
alcohol)15 and the alkane (6, 1 atm H2, 5% Pd–C, alco-
hol) in both cases without reducing the D8-double bond.
The target compounds were all characterized16 on the
to both D9-THC and the parent analog (O-964). Similar
findings were reflected in the tetrad tests and the TF (it)
tests. Based on these results and the previous literature
studies related to the effect of fluorine substitution in
the C-50, C-9, and C-11 positions, it can be concluded
that substitution by a fluorine, especially at C-1 posi-
tion, has a detrimental effect on CB1 binding which is
supported by in vivo testing. This is in contrast to our
findings in the anandamide analogs (see above).
These findings have several implications in connection
with the SAR of THCs particularly in connection with
the role of the C-1 hydroxyl group of THC with the
CB1 receptor and the determination of the common
pharmacophore in the THC/AEA molecular modeling
overlaying studies. With regard to the former, previous
studies19 have attempted to determine the nature of the
hydrogen-bonding by preparing and testing specific ana-
logs of D9- and D8-THCs. However, the interpretation of
the data obtained is controversial. So far the studies
which support the hypothesis that the phenolic hydroxyl
group is involved in a hydrogen-bonding interaction with
the CB1 receptor are by Tius et al.11a and Song and Bon-
ner.11b The latter authors prepared a mutant CB1 recep-
tor in which lysine 192 was replaced by an alanine and
examined the binding affinity of various CB1 agonists.
Based on this study they arrived at the same conclusions
as Tius et al.11a Our results suggest that the hydroxyl
group of THC is functioning as a hydrogen-bond donor
in its interaction with the CB1 receptor and not solely as
a hydrogen-bond acceptor.11c,11d In our molecular mod-
eling studies4 of overlaying THC with AEA, we had used
the pharmacophore fit involving the pyran oxygen of D9-
THC with the carbonyl oxygen of AEA thereby leaving
the phenolic hydroxyl of THC as the counterpart to
the terminal hydroxyl of anandamide. Using this align-
ment, reasonably good COMFA correlations were
obtained for THC and AEA analogs. However, because
of our findings in the AEA series, we had expected
enhanced binding affinity for the C-1-fluoro-substitut-
ed-THCs. Our results do not support the pharmaco-
phore fit we used and lend support to the Tong
model20 which uses the superposition of the hydroxyl
1
basis of their H NMR, high resolution mass spectrum,
TLC, and GLC analyses. In vitro binding assays for
CB1 and CB2 receptors were determined and their in vi-
vo activity was examined in the tetrad tests.17 For tail-
flick (TF) test, we also tested them by intrathecal (it)
route18 in order to investigate if there were any potential
differences in receptor interaction or activation based on
different routes of administration. The results are shown
in Table 1, and for comparison purposes we have also
included the activity of the parent 1-hydroxy-THCs,
(O-964) for compound 4, and (O-1317) for compound
5, respectively. It is quite clear from Table 1 that substi-
tution of a fluorine at C-1 in THCs has a significant det-
rimental effect on the CB1 binding affinity. The alkane
analog
6 has 38-fold less binding affinity than
D9-THC, whereas the alkene analog 5 has 7-fold less
than D9-THC and 331-fold less than its parent analog
(O-1317). The yne analog 4 is 245-fold and 277-fold less
F
OH
F
a, b
+
H3CO
OCH3
O
F
O
OH
1
2
3
11
c, d
F
10
F
8
F
10a
1
e
f
6a
6'
2'
4'
3
6
O
O
4
7'
O
1'
5'
3'
5
6
4
5
Scheme 1. Reagents and conditions: (a) BBr3, CH2Cl2, ꢀ78–25 ꢁC, 1 h; (b) p-menthenediol, TsOH, benzene, reflux, 2 h; (c) Tf2O, py, CH2Cl2, 0 ꢁC,
1 h; (d) 1-heptyne, Pd(Ph3)4, piperidine, 80 ꢁC, 2 h; (e) Lindlar’s catalyst (quinoline, 1 drop), H2, ethanol, 25 ꢁC; (f) Pd/C, H2, ethanol, 25 ꢁC.

1506
P. J. Crocker et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1504–1507
Table 1. Binding affinity and tetrad tests of 1-fluoro-1-deoxy-THCs
Compound
Structure
CB1 (nM)
40.7 1.7a
CB2 (nM)
36.4 10b
Tetrad testsd (ED50, mg/kg)
TF (it) (lg/mouse)
SA
1.0
TF
1.4
RT
1.4
D9-THC
29 (24–36)c
OH
4
>10,000
>4550
—
8% at 30 mg
3.68
4% at 30 mg
3.24
ꢀ2.2ꢁ at 30 mg
156.9 (114.1–216)
—
(O-964)
36 0.8
2.96
4.1
5
285 34
0.86 0.09
1557 203
40
8
0.51
5.8
99.5 (62.5–158)
—
OH
(O-1317)
—
0.09
0.09
0.13
6.9
6
1508
31.1
21.5
36% at 100 lg
a See Ref. 9.
b See Ref. 6.
c The results are presented as ED50 (95% confidence limits in parentheses); see Ref. 18.
d Behavioral evaluation (tetrad tests); SA (spontaneous activity), TF (tail-flick), RT (rectal temperature), RI (ring immobility) were carried out in
mice. The ED50 data are given in mg/kg. For details, see Refs. 17 and 8. RI test was not carried out for any of the compounds and is therefore not
given.
of the AEA to the cyclohexyl at C-9 of 9-nor-9b-OH-
hexahydrocannabinol (HHC). The difference in the
CB1 binding affinity9 between 11-F-D8-THC and its par-
ent 11-OH-D8-THC was marginal, Ki = 107 nM versus
55 nM, respectively. It is interesting to note that the al-
kene analog 5 bound to CB1 receptors significantly bet-
ter than the alkane and alkyne analogs. A similar pattern
was observed in the 11-hydroxy-THC series by Makri-
yannis and co-workers.21 The alkene analog also showed
some CB2 selectivity in binding (7-fold) as found in the 1-
deoxy-D8-THC-DMH series.22,23 Moreover, the loss of
CB2 affinity in analog 6 was unexpected given the obser-
vation that 1-deoxy-THC analogs retain CB2 affinity.22
These findings suggest that electrostatic properties at
C-1 are crucial for CB2 receptor affinity.
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This study presents some interesting conclusions: (a)
substitution of 1-hydroxyl group in D8-THC by a fluo-
rine results in a significant decrease in its interaction
with the CB1 receptor, (b) the 1-hydroxyl in D8-THC
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Acknowledgments
The authors are grateful to NIDA Grants DA-05488
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1.71 (br s, 3H), 2.49 (t, J = 7 Hz, 2H), 2.6–3.1 (m, 2H),
5.43 (br d, J = 4 Hz, 1H), 6.41 (dd, J = 11.9, 1.8 Hz, 1H),
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1
16. Selected spectroscopic data: H NMR (100 MHz, CDCl3)
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2.5 Hz, 1H), 6.18 (d, J = 3 Hz, 1H); TLC, Rf 0.4 (1:4
EtOAc/hexanes); (b) compound 4: d 0.91 (t, J = 6 Hz, 3H),
1.09 (s, 3H), 1.38 (s, 3H), 1.71 (br s, 3H), 2.37 (t,
J = 6.6 Hz, 2H), 2.58–3.07 (m, 2H), 5.43 (br d, J = 5 Hz,
1H), 6.52–6.70 (m, 2H); TLC Rf 0.5 (1:19 EtOAc-
hexanes); GLC >84%; HRMS (CI) calcd for C23H30FO
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(s, 3H), 1.72 (br s, 3H), 2.33 (q, J = 6 Hz, 2H), 2.6–3.1 (m,
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6.23 (br d, J = 12 Hz, 1H), 6.52 (dd, J = 12.2, 1.6 Hz, 1H),
6.54 (br s, 1H); TLC Rf 0.2 (5·hexanes); GLC 90%;
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