Translocation of the 5-alkoxy substituent of 2,5-dialkoxyarylalkylamines to the 6-position: Effects on 5-HT2A/2C receptor affinity

Article abstract of DOI:10.1016/S0960-894X(02)00306-2

Positional modification of 2,5-dimethoxyamphetamine analogues has been studied. Specifically, the 5-alkoxy substituent was translocated to the 6-position of the phenyl nucleus. Methoxy groups were also constrained by incorporation into appended dihydrofuran and furan rings. 2,6-Dimethoxy-4-methylamphetamine had an approximately 3-fold lower affinity for the 5-HT_2A receptor compared to the parent 2,5-dimethoxy-4-methylamphetamine (DOM). The rigid compound based on the 2,3,5,6-tetrahydrobenzo[1,2-b;5,4-b′]difuran nucleus and the aromatic analogue containing the benzo[1,2-b;5,4-b′]difuran nucleus possessed an approximate 7- and 27-fold increase in affinity, respectively, compared to 2,6-dimethoxy-4-methylamphetamine, the non-rigid, positional isomer.

Full text of DOI:10.1016/S0960-894X(02)00306-2

Bioorganic & Medicinal ChemistryLetters 12 (2002) 1997–1999
Translocation of the 5-Alkoxy Substituent of
2,5-Dialkoxyarylalkylamines to the 6-Position:
Effects on 5-HT2A/2C Receptor Affinity
James J. Chambers, Deborah M. Kurrasch-Orbaugh and David E. Nichols*
Department of Medicinal Chemistry and Molecular Pharmacology, School of Pharmacy and Pharmacal Sciences, Purdue University,
West Lafayette, IN 47907-1333, USA
Received 11 February2002; accepted 12 April 2002
Abstract—Positional modification of 2,5-dimethoxyamphetamine analogues has been studied. Specifically, the 5-alkoxy substituent
was translocated to the 6-position of the phenyl nucleus. Methoxy groups were also constrained by incorporation into appended
dihydrofuran and furan rings. 2,6-Dimethoxy-4-methylamphetamine had an approximately 3-fold lower affinity for the 5-HT2A
receptor compared to the parent 2,5-dimethoxy-4-methylamphetamine (DOM). The rigid compound based on the 2,3,5,6-tetra-
hydrobenzo[1,2-b;5,4-b0]difuran nucleus and the aromatic analogue containing the benzo[1,2-b;5,4-b0]difuran nucleus possessed an
approximate 7- and 27-fold increase in affinity, respectively, compared to 2,6-dimethoxy-4-methylamphetamine, the non-rigid,
positional isomer. # 2002 Elsevier Science Ltd. All rights reserved.
Activation of the 5-HT2A (serotonin-2A) receptor byan
agonist ligand has been established as the keypharma-
cological action of hallucinogenic drugs.1 Structurally
diverse classes of drugs that activate this receptor
include certain substituted amphetamines, tryptamines,
and ergolines. The binding orientations in which the
different classes of ligands bring about their effects,
however, remain the subject of conjecture. To under-
stand better the different drug–receptor complexes of
these diverse ligands, construction and refinement of a
receptor model has been a major goal of our research.
Of these three main classes of 5-HT2A receptor agonist
families, the structure–activityrelationships (SARs) that
have been most thoroughlydeveloped are for the hallu-
cinogenic amphetamine class. Unfortunately, major
structural modifications are not accommodated bythe
target receptor and successful design of novel high-affi-
nitymolecular probes has been limited to relatively
minor alterations of the parent 2,5-dimethoxy-
amphetamine pharmacophore.
byan amphetamine agonist include a basic amine
approximately4 A (an ethyl group spacer) from the aryl
ring. This amine group is protonated at physiological
pH and believed to form an ionic interaction with
Asp155 in the third transmembrane segment of the
seven-helical bundle that comprises the 5-HT2A recep-
tor. The aryl ring typically bears small alkoxy-sub-
stituents at the 2- and 5-positions that are thought to
form hydrogen bonds with Ser159 and Thr160 in trans-
membrane segment 3, and Ser239 in transmembrane
segment 5, respectively.4,5 A bulky, hydrophobic sub-
stituent at the 4-position of the ligand is also character-
istic and thought to fill a hydrophobic void in the
binding site. As described in this work, the presence of a
6-alkoxysubstituent on the phenyl ring mayserve as a
surrogate for the 5-alkoxysubstituent in the ampheta-
mine 5-HT2A agonist class (Fig. 1).
DOM (2,5-dimethoxy-4-methylamphetamine, 1), con-
sidered a classical hallucinogenic amphetamine, is
The SARs of hallucinogenic amphetamines have been
discussed in detail elsewhere.2,3 Some general require-
ments for binding and activation of the 5-HT2A receptor
*Corresponding author. Tel.: +1-765-494-1461; fax: +1-765-494-
1414; e-mail: drdave@pharmacy.purdue.edu
Figure 1.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00306-2

1998
J. J. Chambers et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1997–1999
highlypotent (drug discrimination ED
50
kg in LSD-trained rats)6 and incorporates the aryl-sub-
stitution pattern that is typical of this drug class. As first
of 0.89 mmol/
material was then regioselectivelylithiated at the posi-
tion ortho to the aryl-oxygens and the resulting anion
quenched with DMF to afford 8. The substituted benz-
aldehyde 8 was treated with nitroethane utilizing piper-
idine acetate to catalyze the Knoevenagel condensation
yielding nitroalkene 9. A sample of this nitroalkene
was then reduced to the amine 3 using sodium alumi-
num hydride. Another sample of nitroalkene 9 was
treated with DDQ in dioxane at reflux to afford the
fullyaromatized nitroalkene 10. This nitroalkene was
then reduced with sodium aluminum hydride to the
amine 4.
7
reported byShulgin, 2,6-dimethoxy-4-methylampheta-
mine (2) is an amphetamine analogue in which the 5-
methoxygroup has been transposed to the 6-position. It
was found to retain manyof the in vivo qualitative
pharmacological characteristics of a hallucinogenic
amphetamine, albeit at a higher relative dose. Although
it is unclear how exactlyamphetamine analogues bring
about receptor activation, we believe that binding and
activation by 2 must be accompanied either bya shift in
binding orientation relative to 1 or, conversely, favor-
able interaction of the 6-methoxygroup of the ligand
with a different hydrogen bond donor residue in the
agonist binding site. Alternatively, it may be that
because compound 2 exhibits a symmetrical aryl sub-
stitution pattern, the kinetic likelihood of a successful
binding orientation is increased, thus compensating for
the absence of a 5-methoxygroup. Additionall,y the
non-bonded interactions between the alkylamine side
chain and the two adjacent ortho-methoxygroups con-
strain the side chain into a conformation we believe to
be optimal for binding. To investigate this substitution
pattern further, rigid analogues in which the 2,6-dioxy-
gen arrangement is incorporated into heterocyclic rings
(3 and 4) have been synthesized and tested in vitro (Fig.
2).
Pharmacological data from a previouslysnythesized
series of compounds (specifically, compounds based on
the tetrahydrobenzo[1,2-b;4,5-b0]difuran heterocycle)
that were structurallysimilar to the ligands reported in
this manuscript demonstrated that arylmethoxy group
rigidification resulted in a potencyincrease of up to
40-fold relative to the corresponding non-rigid
compounds.8À10 The locked alkoxysubstituents of the
rigid compounds presumablyresulted in a decreased
entropic barrier to binding and therebyenhanced affinity
for the 5-HT2A receptor. This conformational restric-
tion, we hypothesize, enabled the rigid compounds to
Semi-rigid analogues 3 and 4 effectivelyconstrain the
freelyrotating methoxygroups of the parent compound
2. This provides an analogue series with less conforma-
tional flexibilitythan 2 and more robust tools for testing
the validityof our 5-HT
receptor model. Docking of
2A
these rigidified analogues into the putative 5-HT2A
receptor binding site is deemed less ambiguous than
with flexible ligands due to the reduced number of pos-
sible conformers that the ligand can attain. This level of
certaintyenables more precise refinement of the recep-
tor model byusing an iterative approach in which
future drug design becomes increasinglybased on a vir-
tual screening method as the model is improved.
A sample of 2 was prepared as reported previously.7
The synthesis of 3 and 4 (Scheme 1) commenced with
dialkylation of commercially available orcinol (5) uti-
lizing excess 1-bromo-2-chloroethane and potassium
carbonate in acetone at reflux. Under these conditions
non-hydrated orcinol was superior to orcinol mono-
hydrate as an alkylation substrate (yields of the desired
product were 75 and 35%, respectively). Aromatic
dibromination was accomplished using bromine in ace-
tic acid and the product subjected to Grignard condi-
tions to effect ring closure and afford the substituted
tetrahydrobenzo[1,2-b;5,4-b0]difuran 7. This tricyclic
Scheme 1. Reagents and conditions: (a) BrCH2CH2Cl, K2CO3,
(CH3)2CO, Á, 2 days, 75%; (b) Br2, AcOH, 15 ꢀC ! rt, 24 h, 96%;
(c) Mg, CH3CH2MgBr (cat.), THF, 8 h, 76%; (d) (i) n-BuLi, Et2O,
À78 ! 0 ꢀC, 4 h; (ii) DMF, Et2O, 0 ꢀC ! rt, 15 h, 81%; (e)
CH3CH2NO2, piperidine acetate, Á, 1 h, 71%; (f) NaAlH4, THF, 4 h,
58–67%; (g) DDQ, dioxane, Á, 16 h, 59%.
Figure 2.

J. J. Chambers et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1997–1999
1999
Table 1. Results of radioligand competition binding studies at cloned
rat receptorsa
6-fold increase and aromatization provides a 56-fold
increase in affinityover 2.
Compd
Ki, nM (SEM)
In summary, compounds 3 and 4 are novel 5-HT2A
receptor ligands because of their lack of a 5-oxygen
substituent on the amphetamine pharmacophore. These
data suggest that there mayexist an amino acid residue
that possesses the abilityto interact favorablywith the
6-oxygen substituent of these new compounds.
Although the ligands are not selective for either receptor
tested, theydo provide further data for the ongoing
refinement of our 5-HT2A receptor model.5
5-HT2Ab (ꢁ)-[125I]-DOI
5-HT2C (ꢁ)-[125I]-DOI
1 (DOM)
19 (2.0)6
49 (6.3)
6.3 (1.3)
1.8 (0.8)
nd
2
3
4
50 (12)
7.5 (0.6)
0.9 (0.1)
nd, no data.
aDOM (1) data obtained from rat prefrontal cortex homogenate,
others from cell culture.
bDOM (1) data from competition binding studies with [125I]-(R)-DOI,
others from (ꢁ)-[125I]-DOI.
Acknowledgements
form tighter hydrogen bonds with residues in the ago-
nist binding site bylocking the alkoxysubstituents into
an optimal conformation. Those previous studies also
demonstrated that aromatization of the dihydrofuran
rings increased binding affinity3- to 1500-fold relative
to the corresponding non-rigid compounds, presumably
byaiding in the partitioning of the drug into the
hydrophobic agonist binding site. In fact, increased
potencymaybe a result of both factors, optimal orien-
tation of the alkoxy oxygen and increased hydro-
phobicitythat favors partitioning into the hydrophobic
receptor binding domain. Due to the similaritybetween
the compounds discussed in this work and those in the
previous studies, we hypothesized that the 2,6-dioxy-
genated rigid analogues (3 and 4) would also possess
increased binding affinitydue to enhanced hdyrogen
bonding as well as enhanced hydrophobic interactions.
Indeed, the binding data in Table 1 provide support for
this hypothesis.
We would like to thank Dr. David Julius (UCSF) for
his kind donation of the 5-HT2A and 5-HT2C receptor
cell lines. This work was supported byGrant DA02189
from NIDA and bya grant from the Heffter Research
Institute. J.J.C. is supported bya grant from the Purdue
Research Foundation.
References and Notes
1. Aghajanian, G. K.; Marek, G. J. Neuropsychopharmacol-
ogy 1999, 21, 16 S.
2. Glennon, R. A. J. Med. Chem. 1987, 30, 1.
3. Shulgin, A. T.; Sargent, T.; Naranjo, C. Nature 1969, 221,
537.
4. Chambers, J. J.; Nichols, D. E. Great Lakes/Central
Regional American Chemical SocietyMeeting, 2001.
5. Chambers, J. J.; Nichols, D. E. J. Comput.-Aided Mol. Des.
Submitted for publication.
6. Nichols, D. E.; Snyder, S. E.; Oberlender, R.; Johnson,
M. P.; Huang, X. M. J. Med. Chem. 1991, 34, 276.
7. Shulgin, A. T. (Dow Chemical Co.) Substituted a-methyl-
b-phenylethylamines as central nervous system stimulants. Br.
Patent 1147379, 1969.
8. Monte, A. P.; Marona-Lewicka, D.; Parker, M. A.; Wain-
scott, D. B.; Nelson, D. L.; Nichols, D. E. J. Med. Chem.
1996, 39, 2953.
9. Parker, M. A.; Marona-Lewicka, D.; Lucaites, V. L.; Nel-
son, D. L.; Nichols, D. E. J. Med. Chem. 1998, 41, 5148.
10. Chambers, J. J.; Kurrasch-Orbaugh, D. M.; Parker, M. A.;
Nichols, D. E. J. Med. Chem. 2001, 44, 1003.
Examination of the pharmacological data reveals a
trend of increasing affinityfor the 5-HT 2A receptor with
rigidification and aromatization of the test compounds.
Constraint of the methoxy-substituents as dihydrofuran
rings (3) increases affinityfor the receptors approxi-
mately7-fold over the parent compound 2. Further,
aromatization of the dihydrofuran moieties to furan
rings (4) increases affinity27-fold over 2 at the 5-HT2A
receptor. A similar trend is observed at the 5-HT2C
receptor—constraint of the methoxygroups leads to a