2-Substituted tryptamines: Agents with selectivity for 5-HT6 serotonin receptors

Article abstract of DOI:10.1021/jm990550b

Several 2-alkyl-5-methoxytryptamine analogues were designed and prepared as potential 5-HT₆ serotonin agonists. It was found that 5-HT₆ receptors accommodate small alkyl substituents at the indole 2-position and that the resulting co

Full text of DOI:10.1021/jm990550b

J . Med. Chem. 2000, 43, 1011-1018
1011
2-Su bstitu ted Tr yp ta m in es: Agen ts w ith Selectivity for 5-HT6 Ser oton in
Recep tor s|
Richard A. Glennon,*,† Mase Lee,† J agadeesh B. Rangisetty,† Malgorzata Dukat,† Bryan L. Roth,‡
J ason E. Savage,‡ Ace McBride,‡ Laura Rauser,‡ Sandy Hufeisen,‡ and David K. H. Lee§
Department of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, Richmond, Virginia 23298,
Departments of Biochemistry, Psychiatry, and Neurosciences, School of Medicine, Case Western Reserve University,
Cleveland, Ohio 44106, and Allelix Biopharmaceuticals, Mississauga, Ontario L4V 1V7, Canada
Received November 4, 1999
Several 2-alkyl-5-methoxytryptamine analogues were designed and prepared as potential 5-HT6
serotonin agonists. It was found that 5-HT6 receptors accommodate small alkyl substituents
at the indole 2-position and that the resulting compounds can bind with affinities comparable
to that of serotonin. In particular, 2-ethyl-5-methoxy-N,N-dimethyltryptamine (8) binds with
high affinity at human 5-HT6 receptors (Ki ) 16 nM) relative to 5-HT (Ki ) 75 nM) and was a
full agonist, at least as potent (8: Kact ) 3.6 nM) as serotonin (Kact ) 5.0 nM), in activating
adenylate cyclase. Compound 8 displays modest affinity for several other populations of 5-HT
receptors, notably h5-HT1A (Ki ) 170 nM), h5-HT1D (Ki ) 290 nM), and h5-HT7 (Ki ) 300 nM)
receptors, but is otherwise quite selective. Compound 8 represents the first and most selective
5-HT6 agonist reported to date. Replacing the 2-ethyl substituent with a phenyl group results
in a compound that retains 5-HT6 receptor affinity (i.e., 10: Ki ) 20 nM) but lacks agonist
character. 2-Substituted tryptamines, then, might allow entry to a novel class of 5-HT6 agonists
and antagonists.
In tr od u ction
tion, mood-dependent behavior, and early growth pro-
cesses involving serotonin.9-11
Serotonin (5-hydroxytryptamine, 5-HT; 1a ) receptors
are classified as belonging to one of several different
families: 5-HT1-5-HT7. One of the newest populations
identified are the 5-HT6 receptors.1 5-HT6 serotonin
receptors are members of the G-protein superfamily, are
positively coupled to an adenylate cyclase second-
messenger system, and are found primarily in the
central nervous system.2 The exact clinical significance
of 5-HT6 receptors is unknown at this time. Of interest,
however, is that a number of typical and atypical
antipsychotic agents and tricyclic antidepressants bind
with high affinity at 5-HT6 receptors (i.e., with Ki values
of <100 nM).3-5 In rats prevented from expressing
5-HT6 receptors, the animals behave in a manner that
seems to involve an increase in cholinergic function; this
has led to speculation that one of the roles of 5-HT6
receptors may be to control cholinergic neurotransmis-
sion and that 5-HT6-selective antagonists could be
useful in the treatment of anxiety and memory defi-
cits.6,7 It has been further suggested that GABA-
containing neurons in the striatum and glutamate-
containing neurons in the hippocampus could be targets
of 5-HT actions mediated by 5-HT6 receptors.8 5-HT6
ligands might thus be of value in the treatment of
anxiety and related disorders. Other studies suggest
that 5-HT6 receptors might be involved in motor func-
Ro 04-6790 (2a ) and Ro 63-0563 (2b) represent the
first 5-HT6-selective antagonists.12 Several related struc-
tures have also been reported including SB-271046 (3,
R ) H).13 Repeated intracerebroventricular administra-
tion of antisense oligonucleotides to rats to prevent
expression of 5-HT6 receptors produces a behavioral
syndrome that consists of yawning, stretching, and
chewing;6,7 administration of Ro 04-6790 and Ro 63-0563
to naive animals produced a similar effect.12 [3H]Ro 63-
0563 has been developed as a radioligand for binding
studies.12b
| Presented in part at the Virginia Academy of Science Meeting,
Norfolk, VA, May 25-28, 1999.
* Correspondence to: Richard A. Glennon, Ph.D., Box 980540,
Department of Medicinal Chemistry, Virginia Commonwealth Uni-
versity, Richmond, VA 23298-0540. Phone: 804-828-8487. Fax: 804-
828-7404. E-mail: glennon@hsc.vcu.edu.
† Virginia Commonwealth University.
No 5-HT6 -selective agonists have yet been identified.
Various indolealkylamines, including the tryptamines
‡ Case Western Reserve University.
§ Allelix Biopharmaceuticals.
10.1021/jm990550b CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/11/2000

1012 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 5
Glennon et al.
Sch em e 1a
a
(a) i. nBuLi, THF, -78 °C, ii. CO2, iii. tBuLi, THF, -78 °C, iv. MeI; (b) Me2N-CH2CH2-NO2, CF3COOH; (c) LiAlH4, THF, ∆; (d)
NaBH3CN, 37% CH2O, MeCN, HOAc.
5-HT (1a ) and 5-methoxytryptamine (1b), and ergolines,
such as (+)lysergic acid diethylamide (LSD) and li-
suride, bind with high affinity. In fact, [125I]iodo-LSD
and [3H]LSD have been used to label 5-HT6 receptors.
The purpose of our present study was to identify
potential agonists with enhanced selectivity for 5-HT6
receptors that might be useful for investigating this
population of receptors and receptor function.
tors and because 2-methyl 5-HT has been previously
considered a 5-HT3-selective agent, this seemed to be a
suitable starting point for the exploration of potentially
selective 5-HT6 agents.
Ch em istr y
The 2-methyl derivative 7 was prepared via a litera-
ture procedure17 from 2-methylindole using the Speeter-
Anthony method, and the 2-ethyl homologue 8 was
prepared by treatment of the free base of 7 with tBuLi
followed by the addition of MeI (Scheme 1). The 2-phen-
yl derivative 10 was prepared from 5-methoxy-2-phen-
ylindole as shown in Scheme 1. Reaction of 5-methoxy-
2-phenylindole with N,N-dimethylamino-2-nitroethyl-
ene afforded the nitrovinyl derivative 22; compound 22
was reduced to amine 23 using LiAlH4, and the amine
was reductively methylated with formaldehyde and
sodium cyanoborohydride.
5-HT and 5-methoxytryptamine have been demon-
strated to act as 5-HT6 agonists and produce a potent
dose-dependent increase in cAMP levels.14 Unfortu-
nately, these tryptamines are notoriously nonselective
and bind at multiple populations of 5-HT receptors.15
It has been demonstrated, however, that with appropri-
ate molecular modification tryptamine derivatives can
be developed that display enhanced selectivity for
different populations of 5-HT receptors.15
We began our investigation by exploring the structure-
affinity relationships for the binding of tryptamines at
5-HT6 receptors.16 We found that O-methylation of 5-HT
(1a : Ki ) 75 nM) to 5-methoxytryptamine (1b: Ki ) 88
nM) had little effect on affinity and that removal of the
hydroxyl group to give tryptamine (1c: Ki ) 180 nM)
only halved affinity.16 Most other changes led to sig-
nificant decreases in affinity. For example, lengthening
the alkyl chain by one methylene unit, conformational
restriction of the side chain as a 1,2,3,4-tetrahydropy-
rido[3,4-b]indole, replacement of the indolic nitrogen
atom with an sp3-hybridized carbon atom, and quater-
nization of the terminal amine all resulted in a dramatic
reduction in affinity (i.e., Ki > 5000 nM). On the other
hand, N-monomethylation and N,N-dimethylation re-
sulted in retention or a slight increase in affinity. More
importantly, we found that introduction of a 2-methyl
substituent was tolerated. That is, 2-methyl 5-HT (4:
Ki ) 46 nM) possessed an affinity at least comparable
to that of 5-HT itself. This is particularly noteworthy
because, with the exception of 5-HT3 receptors, 2-methyl
substitution is generally thought to reduce the affinity
of tryptamines for most populations of 5-HT receptors.
Indeed, 2-methyl 5-HT (4) is currently considered a
5-HT3-selective ligand. Interestingly, we have now
demonstrated that 2-methyl 5-HT binds at 5-HT6 recep-
tors (Ki ) 46 nM) with much higher affinity than it
displays for 5-HT3 receptors (Ki ) 1200 nM).16 Given
that 2-methylation of 5-HT is tolerated by 5-HT6 recep-
Many of the 5-methoxy-substituted tryptamines were
prepared from 5-methoxy-N,N-dimethyltryptamine (free
base of 11) (Scheme 2). Direct alkylation of 11 (free base)
under basic conditions with the appropriate alkyl halide
provided the N1-substituted derivatives 12-15. The
procedure is exemplified for compound 15. The 2-n-
propyl homologue 9 was also obtained from 11 (Scheme
2). The N1-position of 11 (free base) was protected with
a benzenesulfonyl group, and the resulting compound,
18, was treated with nPrI to give 19; hydrolysis of the
protecting group provided compound 9. Reaction of the
indolyl anion of 11 (free base) with anhydrous γ-buty-
rolactone afforded the N-butyrate 20. Attempts to
cyclize 20 using PPA at 100 °C were unsuccessful and
resulted in decomposition; however, substitution of
PPE18 for PPA gave the desired 21, which was reduced
with borane to give the desired 16 (Scheme 2). Com-
pound 17 was synthesized via debenzylation of its
known N-benzyl derivative.19
Resu lts a n d Discu ssion
2-Methyl 5-HT (4) is currently considered a 5-HT3-
selective ligand; on the other hand, it is known that
5-HT3 receptors do not readily accommodate a tryptamine
5-methoxy group. For example, 5-methoxytryptamine
(1b), the O-methyl ether of 5-HT (1a ), is completely
devoid of activity at 5-HT3 receptors.20 Hence, the first
compound that we examined was the simple 2-methyl

2-Substituted Tryptamines
J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 5 1013
Sch em e 2a
a
(a) NaH, DMF, rt, and Me2SO4, EtBr, nPrCl or iPrCl; (b) NaH, DMF, PhSO2Cl, rt; (c) BuLi, DME, nPrI, -10 °C; (d) Mg, MeOH, rt;
(e) γ-butyrolactone; (f) PPE, CHCl3, reflux; (g) B2H6/THF, rt.
Ta ble 1. Physicochemical Properties and 5-HT6 Receptor Affinities of Tryptamine Analogues
5-HT6 affinityb
compd
R
R5
OH
OCH3 CH3
CH3
R2
R1
yield (%)
RSa
mp (°C)
Ki (nM)
empirical formulac
4
5
6
7
H
H
CH3
CH3
H
H
H
H
H
H
H
H
46f
98
H
88
92
16
45
25
EtOH-Et2O 208
300
80
52
185
54
78
C13H18N2‚HCl
CH3 OCH3 CH3
A
242-245 dec
C14H20N2O‚C2H2O4
C15H22N2O‚C4H4O4
8 (EMDT) CH3 OCH3 C2H5
B-Et2O
123
146-147
d
9
CH3 OCH3 nC3H7
CH3 OCH3 C6H6
A
A
C16H24N2O‚C2H2O4
C19H22N2O‚C2H2O4
10
11
12
13
14
15
16
17
187-188
CH3 OCH3
CH3 OCH3
CH3 OCH3
CH3 OCH3
CH3 OCH3
CH3 OCH3
H
H
H
H
H
CH3
65
22
93
49
75
37
A
A
181-182
160-161
104
101
114-115
510
240
200
130
1030
168
C14H20N2O‚C2H2O4
C15H22N2O‚1.5C2H2O4
C16H24N2O‚C4H4O4
C16H24N2O‚C4H4O4
C17H24N2O‚1.15C2H2O4
C16H22N2O‚C2H2O4
C2H5
nC3H7
iC3H7
A-Et2O
A-Et2O
A
e
-CH2CH2CH2CH2-
EtOH-Et2O 224-226
a
Recrystallization solvents: EtOH represents absolute ethanol; Et2O, anhydrous ether; A, acetone; B, ethyl acetate. b Ki values represent
replicate determinations and SEM are (25%; for purpose of comparison, clozapine was determined to possess a Ki ) 5.3 ((0.4) nM. c All
compounds analyzed correctly to within 0.4% of theory for C, H, and N except where noted. Crystallized with 0.7 mol of H2O. e Crystallized
d
with 1 mol of H2O. f Ki value previously reported;16 included for purpose of comparison.
analogue of 5-methoxytryptamine (1b: Ki ) 88 nM),16
namely, 5-methoxy-2-methyltryptamine (5). Compound
5 (Ki ) 98 nM; Table 1) was found to bind at 5-HT6
receptors with an affinity comparable to that of 5-meth-
oxytryptamine. It was also found that 5 lacks affinity
for 5-HT3 receptors (Ki > 10000 nM). Compound 5 might
be a useful 5-HT6 ligand; however, given that 5 pos-
sesses a primary amine, its utility for future in vivo
studies might be hampered by its reduced ability to
penetrate the blood-brain barrier and/or due to its
potential for rapid metabolism by oxidative deamina-
tion. To address these problems, we sought to prepare

1014 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 5
Ta ble 2. Binding Profile of Compounds 7, 8, and 10a
Glennon et al.
Ki, nM ((SEM)
10
>10000
receptor population
7
8 (EMDT)
>10000
>10000
control (agent and Ki)
NET
SERT
6380 ((3190)
>10000
nortriptyline 6.3 ( 1.2
fluoxetine 3.5 ( 0.7
WAY 100,635 0.6 ( 1.5
ergotamine 0.8 ( 0.6
serotonin 0.5 ( 0.15
clozapine 9 ( 1
4700 ((1550)
h5-HT1A
h5-HT1D
h5-HT1E
r5-HT2A
r5-HT2C
h5-HT5A
h5-HT7
h5-HT6
200 ((60)
250 ((180)
1800 ((600)
170 ((54)
1470 ((310)
6225 ((70)
>10000
290 ((70)
520 ((180)
>10000
>10000
470 ((10)
675 ((180)
5160 ((930)
155 ((35)
20 ((5)
4020 ((640)
10450 ((2195)
145 ((34)
1810 ((490)
4620 ((650)
300 ((60)
16 ((4)
clozapine 23 ( 5
ergotamine 22 ( 3
clozapine 9 ( 2
60 (13)
clozapine 10 ( 3
a
Compounds displayed Ki values of >10000 nM at the following populations of receptors: histamine, NMDA, PCP, acetylcholine,
opiate, and vasopressin receptors; see Experimental Section for specific subpopulations examined. Ki values were >10000 nM for compounds
7 and 8 at hD1, rD2, rD3, rD4, and hD5 receptors and >10000 nM for 10 at hD1, rD2, and rD4 receptors; although 10 produced 70%
inhibition at 10000 nM at rD3 and hD5 receptors, it was not further evaluated. NET and SERT represent the norepinephrine and serotonin
transporters. Ki values for all three compounds at the dopamine transporter were >10000 nM.
several related derivatives that were somewhat more
lipophilic and/or that might be less prone to metabolism.
One approach to enhancing lipophilicity and hinder-
ing metabolism was to add N,N-dimethyl substituents
to the terminal amine; a second approach to enhancing
lipophilicity was to homologate the 2-position substitu-
ent. 2-Methyl-N,N-dimethyltryptamine (2-methyl DMT,
6: Ki ) 300 nM), an N,N-dimethyl analogue of 5 lacking
the 5-methoxy group, binds with severalfold lower
affinity than 5 itself. Reintroduction of the 5-methoxy
group, affording 2-methyl-5-methoxy DMT (7: Ki ) 80
nM), enhanced affinity. Homologation of the 2-methyl
substituent to an ethyl group (i.e., 8: Ki ) 52 nM) re-
sulted in a slight increase in affinity and in a compound
with affinity at least comparable to that of 5-HT itself.
Further homologation of the ethyl substituent to a 2-n-
propyl group (i.e., 9: Ki ) 185 nM) reversed this trend.
To explore the possibility of bulk tolerance, we examined
the 2-phenyl derivative 10 (Ki ) 54 nM) and found it to
bind with an affinity comparable to that of 8.
Another attempt to enhance lipophilicity was to
incorporate small alkyl substituents at the indole N1-
position. The idea was to subsequently incorporate a
2-alkyl substituent into whatever N1-substituted ana-
logue retained high 5-HT6 receptor affinity. N1-Methy-
lation of 5-methoxy DMT (11: Ki ) 78 nM) decreased
5-HT6 receptor affinity of the resulting compound by >6-
fold (12: Ki ) 510 nM). Homologation of the N1-methyl
group to an ethyl group (i.e., 13: Ki ) 240 nM) or
n-propyl group (i.e., 14: Ki ) 200 nM) doubled affinity,
but the compounds did not bind as well as 11. Branching
of 13 to the isopropyl derivative 15 (Ki ) 130 nM)
resulted in a further slight enhancement of affinity.
However, none of these compounds displayed signifi-
cantly enhanced affinity. Compound 16 (Ki ) 1030 nM),
which may be viewed as a cyclic 1,2-disubstituted
analogue of 11, was also prepared for evaluation and
was found to bind with reduced affinity.
In a final attempt to enhance lipophilicity in the
2-substituted DMT series, the propyl group of 9 was
tethered to the DMT side chain to afford 17; compound
17 (Ki ) 168 nM) was found to bind at 5-HT6 receptors
with about 3-fold lower affinity than 8. Although
compound 17 possesses an asymmetric center and can
exist as a pair of optical isomers, no attempt was made
to examine the individual isomers because structurally
related agents have been shown to bind at 5-HT1D
receptors,21,22 and it was anticipated that the isomers
of 17 might lack the desired selectivity.
Bin d in g P r ofile. Compounds 7, 8, and 10 were
selected for examination of detailed binding profiles. All
three agents were examined at more than 30 different
receptor populations and produced <50% inhibition of
binding at a concentration of 10000 nM at most of these
populations. Where >50% displacement was observed,
Ki values were determined (Table 2). For these studies,
Ki values were redetermined for 7, 8, and 10. Com-
pounds 8 and 10 bind at human 5-HT6 receptors with
comparable affinity (Ki ) 16 and 20 nM, respectively)
and with an affinity similar to that of clozapine;
compound 7 binds with severalfold lower affinity (Ki )
60 nM). Although 7 and 8 appear relatively selective,
they also bind at h5-HT1A, h5-HT1D, h5-HT1E, and h5-
HT7 receptors, yet compound 8, in particular, still
displays 10-fold selectivity over 5-HT1A receptors and
nearly 20-fold selectivity over h5-HT1D and h5-HT7
receptors. Compound 10 is more selective and displays
735-fold selectivity over h5-HT1A receptors and >300-
fold selectivity over h5-HT1D receptors.
F u n ction a l Stu d ies. Compounds 7, 8, and 10 were
examined for their ability to activate adenylate cyclase.
Whereas compounds 7 and 8 behaved as full agonists
(Kact ) 7.9 ( 5.0 and 3.6 ( 1.3 nM, respectively) relative
to 5-HT (Kact ) 5.0 ( 3.0 nM), compound 10 showed no
agonist activity (see Figure 1). Compound 10 inhibited
5-HT-stimulated adenylate cyclase at 10000 nM sug-
gesting that it is an antagonist.
Su m m a r y
Molecular manipulation of a tryptamine template
revealed that 2-methyl substitution was tolerated by
5-HT6 receptors.16 Because 2-methyl 5-HT was previ-
ously considered to be a 5-HT3-selective ligand and by
taking advantage of the fact that 5-HT3 receptors do not
readily accommodate a 5-methoxy group, a series of

2-Substituted Tryptamines
J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 5 1015
to give a transparent solid. The flask was flushed with N2 and
dry THF (7 mL) was added. The reaction mixture was degassed
at -150 °C under reduced pressure of 1 mmHg, then allowed
to warm to -78 °C; 1.7 M tBuLi (2.8 mL, 4.8 mmol) was added
to give a bright yellow solution. The cooling bath was replaced
by an ice-salt bath and the reaction was kept at -20 °C for 45
min, then cooled to -78 °C, and MeI (0.3 mL, 4.81 mmol) was
added in a dropwise manner. The solution was kept at -78
°C for 3 h. The reaction mixture was acidified with a saturated
ethereal solution of HCl. Anhydrous Et2O was added to the
resulting suspension and the supernatant was decanted. The
residue was heated at 100 °C under reduced pressure for 20
min. The resulting residue was purified by flash chromatog-
raphy on silica gel (CH2Cl2/MeOH; 12:1) to give 0.17 g of a
bright yellow oil (16%): 1H NMR (CDCl3) δ 8.06 (s, 1H),
7.14 (d, 1H, J ) 8.67 Hz), 6.98 (s, 1H), 6.76 (dd, 1H, J ) 2.34,
8.73 Hz), 3.84 (s, 3H), 2.91-2.87 (m, 2H), 2.71 (q, 2H, J ) 7.38
Hz), 2.57-2.52 (m, 2H), 2.38 (s, 6H), 1.25 (t, 3H, J ) 7.38 Hz).
The maleate salt was prepared and recrystallized from an
EtOAc/Et2O mixture: mp 123 °C. Anal. (C15H22N2O‚C4H4O4)
C, H, N.
F igu r e 1. Typical dose-response curves for the effects of
compounds 7, 8, and 10 as 5-HT6 agonists in an adenylate
cyclase assay; serotonin was used as control. Each compound
was examined at five concentrations.
5-Met h oxy-2-n -p r op yl-N,N-d im et h ylt r yp t a m in e Ox-
a la te (9). Magnesium turnings (840 mg) and NH4Cl (77 mg,
1.44 mmol) were added to a solution of 19 (free base) (259 mg,
0.65 mmol) in MeOH (17 mL) and the mixture was allowed to
stir at room temperature for 1 h. Saturated NH4Cl solution
was added and the reaction mixture was extracted with CH2-
Cl2. The organic portion was dried (MgSO4) and the solvent
was removed under reduced pressure. The residue was purified
by flash chromatography on silica gel (CH2Cl2/MeOH; 9:1) to
give 75 mg (45%) of a bright yellow oil: 1H NMR (CDCl3) δ
7.71 (brs, 1H), 7.16 (d, 1H, J ) 8.67 Hz), 6.99 (d, 1H, J ) 2.43
Hz), 6.77 (dd, 1H, J ) 2.25, 8.73 Hz), 3.85 (s, 3H), 2.89-2.83
(m, 2H), 2.69 (t, 2H, J ) 7.56 Hz), 2.53-2.47 (m, 2H), 2.36 (s,
6H), 1.68 (tq, 2 H, J ) 7.28, 7.56 Hz), 0.98 (t, 3H, J ) 7.28
Hz). The oxalate salt was prepared and recrystallized from
acetone: mp 146-147 °C. Anal. (C16H24N2O‚C2H2O4‚0.7H2O)
C, H, N.
2-alkyl-5-methoxytryptamines was synthesized for evalu-
ation at 5-HT6 receptors. Several compounds were
identified with affinities at least comparable to that of
5-HT itself (Ki ) 75 nM). In particular, 2-ethyl-5-
methoxy-N,N-dimethyltryptamine (EMDT; 8) possessed
high affinity (Ki ) 16 nM) and displayed reasonable
selectivity for 5-HT6 versus other receptors examined.
In functional studies, EMDT (8) was demonstrated to
behave as a 5-HT6 agonist (Kact ) 3.6 nM) with a potency
at least equivalent to that of 5-HT (Kact ) 5.0 nM).
EMDT is the most selective 5-HT6 agonist reported to
date. Also of interest is the 2-phenyl derivative 10
(MPDT: Ki ) 20 nM), which possesses a somewhat
different binding profile than 8; compound 10 lacks
agonist activity up to concentrations of 10000 nM and
may represent a novel 5-HT6 antagonist. Indeed, when
examined at the single concentration of 10000 nM, 10
behaved an antagonist. Hence, with the appropriate
substituents, 2-substituted tryptamines may provide
entry to new 5-HT6-selective agonists and antagonists.
5-Meth oxy-2-p h en yl-N,N-d im eth yltr yp ta m in e Oxa la te
(10). 5-Methoxy-2-phenylindole26 (3 g, 13.44 mmol) was added
to a stirred ice-cooled solution of 1-dimethylamino-2-nitro-
ethylene (1.56, 13.44 mmol) in trifluoroacetic acid (8 mL). The
resulting mixture was allowed to stir under N2 at room
temperature for 30 min and was then poured into ice/water.
The solution was extracted with EtOAc and the organic portion
was washed consecutively with saturated NaHCO3 solution,
H2O, then brine. The organic portion was dried (MgSO4) and
solvent was removed under reduced pressure. The residue was
recrystallized from CH2Cl2/hexane to give 2.36 g (60%) of 22
as a red powder: 1H NMR (acetone-d6) δ 8.82 (brs, 1H), 8.32
(d, 1H, J ) 13.44 Hz), 7.94 (d, 1H, J ) 13.35 Hz), 7.69-7.41
(m, 7H), 6.98-6.94 (m, 1H), 3.92 (s, 3H); IR (KBr) 1606, 1475,
1251 cm-1. A solution of 22 (2.00 g, 6.75 mmol) in dry THF
(20 mL) was added in a dropwise manner to a cooled (0 °C)
suspension of LiAlH4 (1.54 g, 40.5 mmol) in dry THF (40 mL)
under N2. The reaction mixture was heated at reflux for 1 h
and then allowed to stand at room-temperature overnight. The
resulting mixture was quenched with H2O then 15% NaOH
solution. Celite was added and the solution was filtered. The
solvent was removed under reduced pressure. The residue was
purified by flash chromatography on silica gel (CH2Cl2/MeOH;
9:1) to give 1.00 g (55%) of the primary amine 2323 as an oil:
1H NMR (CDCl3) δ 8.19 (brs, 1H), 7.59-7.58 (m, 2H), 7.49-
7.44 (m, 2H), 7.39-7.34 (m, 1H), 7.28-7.25 (m, 1H), 7.09 (d,
1H, J ) 2.37 Hz), 6.88 (dd, 1H, J ) 2.24, 8.75 Hz), 3.89 (s,
3H), 3.04 (brs, 4H); IR (KBr) 3397, 3347 cm-1. Sodium
cyanoborohydride (510 mg, 8.12 mmol) was added to a solution
of primary amine 23 (700 mg, 2.63 mmol) and 37% aqueous
CH2O in MeCN (10 mL) at room temperature. The resulting
mixture was adjusted to pH 5 with HOAc and was allowed to
stir at room-temperature overnight. A 15% solution of NaOH
was added to neutralize the mixture and the mixture was
extracted with CH2Cl2. The combined organic portion was
washed with saturated NaHCO3 solution and brine. The
Exp er im en ta l Section
Syn th esis. Melting points, determined with a Thomas-
Hoover melting point apparatus, are uncorrected. Proton
magnetic resonance spectra were obtained with a GE QE-300
or Varian Gemini 300 spectrometer; and tetramethylsilane was
used as an internal standard. Infrared spectra were recorded
on a Nicolet 5ZDX FT-IR. Elemental analysis was performed
by Atlantic Microlab Inc. and determined values are within
0.4% of theory. Flash chromatography was performed on silica
gel (Merck grade 60, 230-400 mesh 60 Å). Certain compounds
were previously reported in the literature but due to difficulty
in either preparing or purifying the reported salt, a different
salt was prepared. Specifically, compounds 7,17 10,23 and 1224
are known as their HCl salts but were isolated as their
monooxalate salts in the present investigation. Compound 6,
prepared earlier as a maleate salt,25 was isolated as its HCl
salt. All four of these compounds analyzed correctly for C, H,
and N.
2-Eth yl-5-m eth oxy-N,N-d im eth yltr yp ta m in e Ma lea te
(8). A 2.5 M solution of nBuLi (1.75 mL, 4.38 mmol) was added
in a dropwise manner to a stirred solution of 717 (free base)
(1.00 g, 4.33 mmol) in dry THF (7 mL) at -78 °C under N2.
After stirring the reaction mixture for 5 min, the cooling bath
was removed and CO2 gas was passed into the solution for 10
min. The solvent was removed at 0 °C under reduced pressure

1016 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 5
Glennon et al.
organic portion was dried (MgSO4) and solvent was removed
under reduced pressure. The residue was purified by flash
chromatography on silica gel (CH2Cl2/MeOH; 9:1) to give 195
mg (25%) of 10 (free base) as a white powder: 1H NMR (CDCl3)
δ 8.05 (brs, 1H), 7.56-7.53 (m, 2H), 7.49-7.44 (m, 2H), 7.39-
7.34 (m, 1H), 7.29-7.25 (m, 1H), 7.11 (d, 1H, J ) 2.25 Hz),
6.87 (dd, 1H, J ) 2.52, 8.73 Hz), 3.89 (s, 3H), 3.13-3.08 (m,
2H), 2.72-2.66 (m, 2H), 2.39 (s, 6H). Although the HCl salt
has been previously reported,23 difficulties in its purification
led to isolation of the product as its oxalate salt: mp 187-
188 °C after recrystallization from acetone. Anal. (C19H22N2O‚
C2H2O4) C, H, N.
5-Meth oxy-1-(2-pr opyl)-N,N-dim eth yltr yptam in e Male-
a te (15). A mixture of 5-methoxy-N,N-dimethyltryptamine (11;
free base) (500 mg, 2.29 mmol) and 60% NaH (100 mg, 2.52
mmol) was heated at 100 °C under N2 until evolution of H2
gas ceased. The resultant mass was dissolved in anhydrous
DMF (3 mL) and 2-bromopropane (0.25 mL, 2.84 mmol) was
added to the solution at 0 °C. The reaction mixture was allowed
to stir at room temperature for 3 h. Brine was added and the
reaction mixture was extracted with CH2Cl2. The organic
portion was dried (MgSO4) and the solvent was removed under
reduced pressure. The residue was purified by flash chroma-
tography on silica gel (CH2Cl2/MeOH; 10:1) to give 294 mg of
a bright yellow oil (49%): 1H NMR (CDCl3) δ 7.23 (d, 1H, J )
8.94 Hz), 7.04 (d, 1H, J ) 2.46 Hz), 7.01 (s, 1H), 6.86 (dd, 1H,
J ) 2.46, 8.88 Hz), 4.59-4.54 (m, 1H), 3.86 (s, 3H), 2.95-2.89
(m, 2H), 2.66-2.61 (m, 2H), 2.36 (s, 6H), 1.48 (d, 6H, J ) 6.72
Hz). The maleate salt was prepared and recrystallized from
an acetone/Et2O mixture: mp 101-102 °C. Anal. (C16H24N2O‚
C4H4O4) C, H, N.
1H NMR data for compounds 13 and 14 are as follows: 13
(CDCl3) δ 7.22 (d, 1H, J ) 9.0 Hz), 7.06 (d, 1H, J ) 2.5 Hz),
6.95 (s, 1H), 6.88 (dd, 1H, J ) 2.5, 6.0 Hz), 4.10 (q, 2H, J )
7.5 Hz), 3.88 (s, 3H), 3.01-2.97 (m, 2H), 2.76-2.73 (m, 2H),
2.45 (s, 6H), 1.44 (t, 3H, J ) 7.5 Hz); 14 (CDCl3) δ 7.19 (d, 1H,
J ) 8.85 Hz), 7.04 (d, 1H, J ) 2.37 Hz), 6.90 (s, 1H), 6.87-
6.83 (m, 1H), 3.98 (t, 2 H, J ) 7.08 Hz), 3.86 (s, 3H), 2.93-
2.87 (m, 2H), 2.64-2.59 (m, 2H), 2.35 (s, 6H), 1.82 (q, 3H, J )
7.2 Hz), 0.91 (t, 2H, J ) 7.4 Hz).
6,7,8,9-Tetr a h yd r o-2-m eth oxy-10-(N,N-d im eth yla m in o-
eth yl)p yr id o[1,2-a ]in d ole Oxa la te (16). A solution of 1.0
M borane/THF (2 mL, 2 mmol) was added in a dropwise
manner to ice-bath cooled 21 (290 mg, 1.01 mmol) under N2.
The reaction mixture was allowed to stir at room temperature
for 2 h. Acetone (3 mL) was added, and the reaction mixture
was heated at reflux for 1 h to quench the unreacted borane
reagent. The solvent was removed under reduced pressure. A
15% solution of NaOH was added and the mixture was
extracted with CH2Cl2, and the CH2Cl2 portion was washed
with H2O, then brine. Solvent was removed under reduced
pressure. The residue was purified by flash chromatography
on silica gel (hexane/EtOAc; 4:1) to give 207 mg (75%) of a
light yellow oil: 1H NMR (DMSO-d6) δ 7.34 (d, 1 H, J ) 8.85
Hz), 7.21 (s, 1H), 7.11 (s, 1H), 4.08 (t, 2H, J ) 6.65 Hz), 3.79
(s, 3H), 3.40-3.35 (m, 2H), 3.30-3.25 (m, 2H), 3.06-3.01 (m,
2H), 2.83 (s, 6H), 1.76-1.69 (m, 2H), 1.40-1.31 (m, 2H). A
small portion was converted to its oxalate salt: mp 114-115
°C. Anal. (C17H24N2O‚1.15C2H2O4‚H2O) C, H, N.
1H, ArH), 6.90 (d, 1H, ArH), 6.70 (dd, 1H, ArH), 3.80 (s, 3H,
OCH3), 3.40 (t, 1H, CH), 3.15 (d, 1H, CH), 3.00 (t, 1H, CH),
2.82 (s, 6H, 2× CH3), 2.63-2.73 (m, 2H, CH2), 2.33 (m, 1H,
CH), 1.8-2.0 (m, 3H, CH2-CH). Anal. (C16H22N2O‚C2H2O4) C,
H, N.
1-Be n ze n e su lfon yl-5-m e t h oxy-N ,N -d im e t h ylt r yp t a -
m in e Oxa la te (18). A mixture of 5-methoxy-N,N-dimethyl-
tryptamine (11; free base) (4.35 g, 19.93 mmol) and 60%
NaH (0.87 g, 21.75 mmol) was heated at 100 °C under N2
until evolution of H2 gas ceased. The resultant mass was
dissolved in anhydrous DMF (21 mL) and benzenesulfonyl
chloride (2.8 mL, 21.94 mmol) was added in a dropwise man-
ner at 0 °C. The reaction mixture was allowed to stir at room-
temperature overnight. Saturated NaHCO3 solution was added
and the mixture was extracted with CH2Cl2. The organic
portion was dried (MgSO4) and the solvent was removed under
reduced pressure. The residue was purified by flash chroma-
tography on silica gel (CH2Cl2/MeOH; 9:1) to give 4.39 g of
an oil (61%): 1H NMR (CDCl3) δ 7.89-7.87 (m, 1H), 7.83 (d,
2H, J ) 8.0 Hz), 7.51 (t, 1H, J ) 7.8 Hz), 7.34 (s, 1H), 6.93-
6.92 (m, 2H), 3.82 (s, 3H), 2.80 (t, 2H, J ) 7.8 Hz), 2.59 (t, 2H,
J ) 7.8 Hz), 2.33 (s, 6H); IR (CHCl3) 1357, 1115 cm-1. The
oxalate salt was prepared and recrystallized from an acetone/
Et2O mixture: mp 224-226 °C. Anal. (C19H22N2O3S‚C2H2O4)
C, H, N.
1-Ben zen esu lfon yl-5-m eth oxy-2-n -p r op yl-N,N-d im eth -
yltr yp ta m in e Oxa la te (19). A 2.5 M solution of nBuLi (1.4
mL, 3.5 mmol) was added in a dropwise manner to a stirred
solution of 18 (free base) (1.00 g, 2.79 mmol) in DME (4 mL)
at -10 °C under N2. The resulting solution was allowed to stir
for an additional 10 min at -10 °C, and then nPrI (0.35 mL,
3.59 mmol) was added. The reaction mixture was allowed to
stir for 1 h at -10 °C. Saturated NaHCO3 solution was added
and the reaction mixture was extracted with CH2Cl2. The
organic portion was washed with brine and dried (MgSO4); the
solvent was removed under reduced pressure and the residue
was purified by flash chromatography on silica gel (CH2Cl2/
MeOH; 30:1) to give 0.19 g (17%) of a bright yellow oil: 1H
NMR (CDCl3) δ 8.06 (d, 1H, J ) 8.79 Hz), 7.62 (d, 2H, J )
8.22 Hz), 7.51-7.46 (m, 1H), 7.38-7.33 (m, 2H), 6.95 (brs, 1H),
6.89-6.85 (m, 1H), 3.85 (s, 3H), 2.96-2.89 (m, 4H), 2.63-2.57
(m, 2H), 2.48 (s, 6H), 1.73 (q, 2H, J ) 7.51 Hz), 1.00 (t, 3H, J
) 7.51 Hz); IR (CHCl3) 1355 cm-1. The oxalate salt was
prepared and recrystallized from acetone: mp 175-176 °C.
Anal. (C22H28N2O3S‚C2H2O4) C, H, N.
6,7,8,9-Tetr a h yd r o-2-m eth oxy-10-(N,N-d im eth yla m in o-
eth yl)p yr id o[1,2-a ]in d ol-9-on e Oxa la te (21). A mixture of
5-methoxy-N,N-dimethyltryptamine (11; free base) (2.00 g,
9.17 mmol) and 60% NaH (0.41 g, 10.1 mmol) was heated at
100 °C under N2 until evolution of H2 gas ceased. The resultant
mass was dissolved in anhydrous DMF (25 mL) and anhydrous
γ-butyrolactone (1.4 mL, 18.2 mmol) was added in a dropwise
manner at room temperature. The reaction mixture was
heated at reflux for 20 h, cooled to 0 °C, and acidified by the
addition of a saturated ethereal solution of HCl. Additional
Et2O was added to the resulting suspension and the super-
natant was decanted. The residue was dissolved in PPE (52.5
mL) and CHCl3 (100 mL) and the reaction mixture was heated
at reflux for 3 h under N2. The resulting mixture was
neutralized by the addition of 15% NaOH solution, at ice-bath
temperature, and extracted with CH2Cl2. The organic portion
was dried (MgSO4) and solvent was removed under reduced
pressure. The residue was purified by flash chromatography
on silica gel (CH2Cl2/MeOH; 20:1) to give 0.52 g (20%) of 21
(free base) as a yellow oil: 1H NMR (DMSO-d6) δ 7.35 (d, 1H,
J ) 8.79 Hz), 7.18 (s, 1H), 6.88 (d, 1H, J ) 8.85 Hz), 4.06 (t,
2H, J ) 6.60 Hz), 3.80 (s, 3H), 3.42-3.36 (m, 2H), 3.17-3.12
4-(Dim eth yla m in om eth yl)-6-m eth oxy-1,2,3,4-tetr a h y-
d r oca r ba zole Oxa la te (17). Sodium metal (1.0 g) was added
portionwise over a 30-min period to a stirred solution of
4-(dimethylaminomethyl)-9-benzyl-6-methoxy-1,2,3,4-tetrahy-
drocarbazole hydrochloride19 (4.0 g, 0.01 mol) in liquid NH3
(300 mL). NH4Cl (3.0 g) was added until the blue color of the
mixture dissipated. The NH3 was evaporated, H2O (50 mL)
was added, and the mixture was extracted with CH2Cl2 (3 ×
50 mL). The combined organic portion was washed with H2O
(50 mL), brine (50 mL), dried (MgSO4) and evaporated to give
an oil. The oil was purified by column chromatography (CHCl3/
MeOH; 9:1) and converted to an oxalate salt. The oxalate salt
was recrystallized from anhydrous Et2O/absolute EtOH to give
1.8 g (37%) of the desired target as a white powder: mp 224-
226 °C; 1H NMR (CDCl3, free base) δ 8.10 (s, 1H, NH), 7.20 (t,
(m, 2H), 2.85 (s, 6H), 2.66-2.62 (m, 2H); IR (CHCl3) 1648 cm-1
.
A small sample was converted to the oxalate salt: mp 191-
192 °C dec. Anal. (C19H24N2O6‚1.6C2H2O4) C, H, N.
5-HT6 R a d ioliga n d Bin d in g Assa y. The binding assay
employed human 5-HT6 receptors stably transfected to HEK
293 human embryonic kidney cells with [3H]lysergic acid
diethylamide (70 Ci/mmol; DuPont NEN) as radioligand. All

2-Substituted Tryptamines
J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 5 1017
assays were conducted in triplicate using polypropylene 1 mL/
well plates (Anachemia). The radioligand was diluted in
incubation buffer in borosilicate glass vials and protected from
light. Competing agents (1 mM stock solutions) were dissolved
in DMSO or saline and stored at -20 °C in 1.2-mL polypro-
pylene tubes (ElKay). Dilutions of compounds were made using
incubation buffer in 96-well polypropylene plates and mixed
by multichannel pipetting >25 times. Serial dilutions (1 in 4)
started at a final concentration of 10000 nM. Final concentra-
tions >10000 nM were individually prepared from the 1 mM
stock solution. Nonspecific binding was defined by 100 µM
serotonin creatinine sulfate (Research Biochemicals) prepared
fresh in incubation buffer at the time of each determination
and protected from light. Reactions volumes were as follows:
200 µL of incubation buffer (50 mM Tris, 0.5 mM EDTA, 10
mM MgCl2), pH 7.4 at 22 °C, 100 µL of test agent or serotonin
(100 µM) or buffer (for total binding), 100 µL of [3H]lysergic
acid diethylamide (2 nM final concentration), and 100 µL of
membrane preparation (15 µg of protein). The incubation was
initiated by the addition of membrane homogenate and the
plates were vortexed (Baxter S/P multitube mixer). The plates
were incubated, with protection from light, by shaking (Gy-
rotop water bath/shaker model G76, speed 2) at 37 °C for 60
min. The binding reaction was stopped by filtration. The
samples were filtered under vacuum over 96-well glass fiber
filters (Packard Unifilter GF/B), presoaked in 0.3% PEI in 50
mM Tris buffer (4 °C, pH 7.4) for at least 1 h, and then washed
six times with 1 mL of cold 50 mM Tris buffer (pH 7.4) using
the Packard Filtermate 196 harvester. The Unifilter plates
were dried overnight in a 37 °C dry incubator. The Unifilter
bottoms were sealed and 35 µL of Packard MicroScint-0 was
added. The plates were allowed to equilibrate for 1 h and were
then sealed using a Packard TopSeal P with the Packard Plate
Micromate 496. Plates were counted in a Packard TopCount
4.1 by liquid scintillation spectrometry. Each well was counted
for 3 min. The test agents were initially assayed at 1000 and
100 nM. If the compound was active (defined as causing at
least 80% inhibition of [3H]lysergic acid diethylamide binding
at 1000 nM), it was further tested for determination of a Ki
value. The range of concentrations was chosen such that the
middle concentration would produce approximately 50% in-
hibition.
determinations. Data represent the mean of N ) 4 separate
determinations.
Ack n ow led gm en t. Supported in part by MH57635
and MH01366 (B.L.R.) and the NIMH Psychoactive
Drug Screening Program.
Refer en ces
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Recep tor Scr een . Assays for the following receptors were
performed by the NIMH Psychoactive Drug Screening Pro-
gram: (1) serotonin receptors: h5-HT1A, h5-HT1B, h5-HT1D, h5-
HT1E, r5-HT2A, r5-HT2C, h5-HT6, h5-HT7; (2) dopamine recep-
tors: hD1, rD2, rD3, rD4, hD5; (3) muscarinic acetylcholine
receptors: hm1, hm2, hm3, hm4, hm5; (4) nicotinic acetylcholine
receptors: R2/â2, R2/â4, R3/â2, R3/â4, R4/â2, R4/â2, R4/â4; (5)
vasopressin receptors: hV1, hV2, hV3; (6) opiate receptors:
hµ, hδ, rκ; (7) transporters: hSERT, hNET, rDAT; (8) rNMDA;
(9) rPCP; and (10) rH1-histamine. Detailed on-line protocols
for the binding assays are described at: http://meds20785.
cwru.edu/myweb/protocol.htm. For screening purposes, the
ability of 10 µM of each compound (dissolved in 10% DMSO)
was incubated with the appropriate receptor preparation and
percent inhibition determined for duplicate determinations
each performed in duplicate. Where >50% inhibition of specific
binding was measured, Ki determinations were then measured
by competition binding assays in which concentrations from
1 to 100000 nM were incubated in duplicate. For each Ki value
the data represent the mean ( SD of computer-derived
estimates for N ) 4 separate determinations. h5-HT6 receptor
assays were performed exactly as previously described.3,4
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confluency and 18 h prior to assay the medium was replaced
with DMEM containing dialyzed 10% fetal calf serum. For the
assay, the medium was aspirated and replaced with fresh
DMEM without serum and incubated with various concentra-
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trichloroacetic acid (TCA). The TCA extract was used for cAMP
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