Enantiospecific synthesis and pharmacological evaluation of a series of super-potent, conformationally restricted 5-HT2A/2C receptor agonists

Article abstract of DOI:10.1021/jm000491y

The affinity of ligands for either the 5-HT_2A or 5-HT_2C agonist binding site was enhanced by modification of the 2,5-oxygen substituents that are found in typical hallucinogenic amphetamines such as 4b (DOB). Restriction of the conformationally flexible 2,5-dimethoxy substituents into fused dihydrofuran rings generally resulted in increased potency relative to the parent 2,5-dimethoxy compounds. The pure enantiomers of these arylalkylamines were obtained by enantiospecific synthesis that involved acylation of the heterocyclic nucleus 7 with N-trifluoroacetyl-protected D- or L-alanyl chloride, followed by ketone reduction and N-deprotection. The enantiomers demonstrated modest stereoselectivity at the two receptors. Several general trends within these classes of new compounds were observed during their pharmacological investigation. For most pairs of optical isomers tested, the R-enantiomers of the compounds containing heterocycle 7 bound with only slightly higher affinity than their S-antipodes at the 5-HT_2A and 5-HT_2C receptors. Likewise, functional studies indicated that the R-enantiomers generally displayed increased potency compared to the S-enantiomers. Aromatization of the dihydrofuran rings of these arylalkylamines further increased affinity and potency. Only a few compounds were full agonists with most of them possessing intrinsic activities in the range of 60-80%. These compounds with a fully aromatic linear tricyclic nucleus are some of the highest-affinity ligands for the 5-HT_2A receptor reported to date.

Full text of DOI:10.1021/jm000491y

J . Med. Chem. 2001, 44, 1003-1010
1003
En a n tiosp ecific Syn th esis a n d P h a r m a cologica l Eva lu a tion of a Ser ies of
Su p er -P oten t, Con for m a tion a lly Restr icted 5-HT2A/2C Recep tor Agon ists
J ames J . Chambers, Deborah M. Kurrasch-Orbaugh, Matthew A. Parker, and David E. Nichols*
Department of Medicinal Chemistry and Molecular Pharmacology, School of Pharmacy and Pharmacal Sciences,
Purdue University, West Lafayette, Indiana 47907
Received November 17, 2000
The affinity of ligands for either the 5-HT2A or 5-HT2C agonist binding site was enhanced by
modification of the 2,5-oxygen substituents that are found in typical hallucinogenic amphet-
amines such as 4b (DOB). Restriction of the conformationally flexible 2,5-dimethoxy substit-
uents into fused dihydrofuran rings generally resulted in increased potency relative to the
parent 2,5-dimethoxy compounds. The pure enantiomers of these arylalkylamines were obtained
by enantiospecific synthesis that involved acylation of the heterocyclic nucleus 7 with
N-trifluoroacetyl-protected D- or L-alanyl chloride, followed by ketone reduction and N-
deprotection. The enantiomers demonstrated modest stereoselectivity at the two receptors.
Several general trends within these classes of new compounds were observed during their
pharmacological investigation. For most pairs of optical isomers tested, the R-enantiomers of
the compounds containing heterocycle 7 bound with only slightly higher affinity than their
S-antipodes at the 5-HT2A and 5-HT2C receptors. Likewise, functional studies indicated that
the R-enantiomers generally displayed increased potency compared to the S-enantiomers.
Aromatization of the dihydrofuran rings of these arylalkylamines further increased affinity
and potency. Only a few compounds were full agonists with most of them possessing intrinsic
activities in the range of 60-80%. These compounds with a fully aromatic linear tricyclic nucleus
are some of the highest-affinity ligands for the 5-HT2A receptor reported to date.
In tr od u ction
Over the past four decades, a vast base of information
has been gathered about the physiological role of the
neurotransmitter serotonin (5-HT, 5-hydroxytryptamine,
1). Serotonin is known to be important in physiological
systems ranging from hemodynamics and intestinal
motility to normal cognition and perceptual processes.
Receptors for this endogenous compound have been
shown to exist almost ubiquitously throughout the body
and especially in the gut and both central and periph-
eral nervous systems. Molecular biology techniques have
led to the discovery of numerous different receptor
subtypes for 5-HT, with the possibility of a different
physiological role for each.1 The 5-HT receptor subtype
of most interest to our laboratory is the 5-HT2A isoform.
Agonist activation of this receptor is believed to be
responsible for induction of the unique alterations in
dimethoxyamphetamine, 4b) represent a cross-section
of compounds that cause hallucinogenesis in humans
by putative activation of this receptor. The chemical
diversity of these compounds, coupled with the difficulty
of melding them into a common pharmacophore, has led
to much speculation about the functional topography of
the 5-HT2A receptor.
Although it is reasonably well-established that acti-
vation of the 5-HT2A receptor by an exogenous ligand
leads to hallucinogenesis, agonists that exhibit high
affinity for this receptor invariably also bind to the
5-HT2C receptor.1 This finding has raised questions
regarding the involvement of the 5-HT2C receptor in
hallucinogenesis.3 The 5-HT2A and 5-HT2C receptors
share a relatively high overall sequence homology and,
more importantly, are 80% homologous within the
transmembrane regions in which the agonist binding
consciousness produced by substances such as LSD (d-
lysergic acid N,N-diethylamide, 2) and psilocin (4-
hydroxy-N,N-dimethyltryptamine, 3).2
The molecular determinants for agonist binding to the
5-HT2A receptor have been of great interest to our
laboratory in order that we might gain a better under-
standing of the physiological role of serotonin at this
site. One of the factors complicating this research is that
structurally diverse classes of compounds retain the
ability to bind to and activate this receptor. Chemical
classes such as the ergolines (LSD, 2), tryptamines
(psilocin, 3), and amphetamines (DOB, 4-bromo-2,5-
* Address correspondence to: Dr. David E. Nichols, Dept. of
Medicinal Chemistry and Molecular Pharmacology, RHPH Pharmacy
Bldg., Rm. 506C, Purdue University, West Lafayette, IN 47907-1333.
Phone: (765) 494-1461. Fax: (765) 494-1414. E-mail: drdave@pharmacy.
purdue.edu.
10.1021/jm000491y CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/26/2001

1004 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 6
Chambers et al.
Sch em e 1a
site is thought to reside.4 Thus, it is believed that these
two receptors must also share structural features within
the agonist binding site. A truly selective agonist would
be an invaluable tool for investigating the differences
in neuropharmacology between these chemically quite
similar receptors.
Recent findings suggest that enough structural vari-
ance may exist between the agonist binding sites of the
5-HT2A and 5-HT2C receptors to confer moderate chemi-
cal selectivity. Additionally, animal behavioral models
of hallucinogenic activity have supported in vitro data
showing differential activity for the enantiomers of both
amphetamine and LSD analogues.5,6 We chose to ex-
plore further the importance of stereochemical selectiv-
ity in this receptor family through the synthesis and
pharmacological evaluation of enantiomerically pure
novel arylalkylamines.
a
(a) SOCl2, C5H5N, CH2Cl2; (b) Br2, ZnCl2, AcOH; (c) Mg,
EtMgBr, THF.
We have recently reported the synthesis and prelimi-
nary pharmacological analysis of several semirigid
analogues of DOB ((-4b) and related amphetamines.7-9
As a continuation of our studies of these extremely
potent 5-HT2A/2C receptor agonists, we present here the
enantiospecific synthesis and pharmacological evalua-
tion of a series of semirigid compounds. These com-
pounds incorporate either a tetrahydrobenzo[1,2-b;4,5-
b′]difuran heterocycle in place of the phenyl ring of
amphetamines, as in the 5 series, or the fully aromatic
benzo[1,2-b;4,5-b′]difuran counterpart, as in the 6 series.
Specifically, the tricyclic nucleus was substituted at the
4- and 8-positions to generate arylalkylamine analogues
of 2,5-dimethoxyamphetamine (2,5-DMA, 4a ), DOB
(4b), and 2,5-dimethoxy-4-trifluoromethylamphetamine
(DOTFM, 4c). The chiral side chain was derived from
the enantiomers of alanine. Both the R- and S-enanti-
omers of the six arylalkylamine structures were syn-
thesized and evaluated pharmacologically.
route afforded the optically pure ketone (R or S)-14,
which was then reduced with triethylsilane in trifluo-
roacetic acid to afford the protected arylalkylamine (R
or S)-15.13
The N-protected arylalkylamine (R or S)-15 was
hydrolyzed in base and then acidified to afford the
hydrochloride salt of the unsubstituted product (R or
S)-5a . The aromatic counterpart of this “non-para-
substituted” amphetamine analogue was synthesized by
oxidation of (R or S)-15 with 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ) to produce the aromatized
amide (R or S)-16.14 Subsequently, alkaline hydrolysis
and acidification was used to obtain the hydrochloride
salt of (R or S)-6a .
To complete the synthesis of the remaining com-
pounds in this series, the amide (R or S)-15 from above
was brominated to afford the protected arylalkylamine
(R or S)-1711 followed by hydrolysis and conversion to
the hydrochloride salt of (R or S)-5b. Compound (R or
S)-17 was likewise oxidized to the fully aromatic com-
pound (R or S)-18 using DDQ and then hydrolyzed and
acidified to afford (R or S)-6b as the hydrochloride salt.
Brominated amide (R or S)-17 was utilized to produce
the final compounds (R or S)-5c and (R or S)-6c. The
aromatic bromine of (R or S)-17 was replaced by a
trifluoromethyl substituent by reaction with copper
iodide and tetramethylammonium trifluoroacetate15 to
afford (R or S)-19, which was hydrolyzed and acidified
to obtain the hydrochloride salt of (R or S)-5c. Finally,
oxidation of (R or S)-19 followed by hydrolysis and
acidification of the product afforded the hydrochloride
salt of (R or S)-6c. The Mosher amides16 of (R)- and (S)-
6c were analyzed by HPLC to ascertain that no race-
mization had occurred during any of the synthetic steps;
indeed this was found to be the case, with enantiomeric
purities of both compounds in excess of 99%. In addition,
it was noted that treatment of the tetrahydro compound
(R or S)-5b with elemental bromine could under certain
conditions lead to some oxidation to give small amounts
of aromatized side products. In view of the high phar-
macological activity that was found for (R or S)-6b,
HPLC analysis of the N-trifluoroacetyl derivatives of
(R)- and (S)-5b was used to verify that they contained
no significant amounts of such oxidation products, which
after hydrolysis might have contaminated (R or S)-5b
and confounded the pharmacological results.
Ch em istr y
An important intermediate in the synthetic scheme
employed to produce these compounds is the heterocyclic
nucleus tetrahydrobenzo[1,2-b;4,5-b′]difuran (7).10 Al-
though our previously reported synthesis of this com-
pound required only three steps and gave a useful
yield,11 the straightforward improvements reported here
(Scheme 1) provided 7 in 63% overall yield beginning
with commercially available 1,4-bis(2-hydroxyethoxy)-
benzene (8).
To effect the Friedel-Crafts acylation of 7, our
synthetic pathway (Scheme 2) required the acid chloride
of optically pure D- or L-alanine (11). (Only the R-
enantiomers are shown in the scheme.) Protection of the
amino group of (R or S)-11 to afford trifluoroacetamide
(R or S)-12 was accomplished following a previously
reported procedure.12 This very hygroscopic solid was
then converted to the acid chloride using oxalyl chloride
to afford (R or S)-13, which was then used without
purification in the subsequent aromatic acylation. This

5-HT2A/ 2C Receptor Agonists
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 6 1005
Sch em e 2a
a
(a) Et3N, CF3CO2Et, MeOH; (b) (COCl)2, CH2Cl2, C5H5N; (c) AlCl3, 7, CH2Cl2; (d) Et3SiH, CF3CO2H; (e) NaOH, MeOH, H2O; (f) HCl,
EtOH (anhyd); (g) DDQ, dioxane; (h) Br2, AcOH; (i) CF3CO2N(CH3)4, CuI, PhCH3, DMF.
Ta ble 1. Results of the Radioligand Competition Binding
Studies of Nonrigid (series 4) and Rigid (series 5 and 6)
Compounds at Cloned Rat 5-HT2A and 5-HT2C Receptors
Ta ble 2. Results of the Phosphoinositide Hydrolysis Studies of
Nonrigid (series 4) and Rigid (series 5 and 6) Compounds at
Cloned Rat 5-HT2A Receptors
Ki, nM (SEM)
percent maximal
EC50 at 5-HT2A
nM (SEM)
,
5-HT stimulation
(SEM)
at 5-HT2A
,
at 5-HT2C
125I]DOI
,
compd
X
compd
X
[3H]DOB
[
(R)-4a (2,5-DMA)
(()-4b (DOB)
(()-4c (DOTFM)
(R)-5a
(S)-5a
(R)-5b
(S)-5b
(R)-5c
(S)-5c
(R)-6a
H
Br
CF3
H
H
Br
Br
CF3
CF3
H
H
Br
Br
CF3
CF3
>10000
16 (3.1) @ 100 µM
79 (6.0)
86 (3.5)
99 (0.7)
62 (3.1)
(R)-4a (2,5-DMA)
(()-4b (DOB)
(()-4c (DOTFM)
(R)-5a
(S)-5a
(R)-5b
(S)-5b
(R)-5c
(S)-5c
(R)-6a
H
Br
CF3
H
H
Br
Br
CF3
CF3
H
H
Br
Br
CF3
CF3
2340 (326)
520 (34)
2.8 (0.68)
3.78 (1.12)
8.2 (2.4)
119 (31.8)
0.26 (0.025)
1.1 (0.20)
72 (3.6)
2.2 (0.33)
2.2 (0.26)
54.4 (5.40)
227 (24.5)
1.2 (0.26)
2.6 (0.33)
0.70 (0.090)
2.0 (0.17)
1.5 (0.090)
37.9 (5.40)
0.31 (0.042)
0.68 (0.083)
0.29 (0.030)
0.32 (0.030)
68.4 (12.6)
5650 (376)
2360 (210)
8.38 (1.86) 80 (7.8)
35 (1.0)
28 (6.7)
30 (6.2)
590 (20)
650 (68)
2.7 (0.55)
19 (1.6)
3.8 (0.53)
12 (1.4)
76 (5.0)
69 (3.9)
60 (1.2)
76 (3.2)
68 (2.3)
93 (9.9)
79 (1.5)
60 (3.9)
52 (6.0)
1.9 (0.32)
1.6 (0.19)
0.79 (0.24)
6.0 (0.17)
0.11 (0.014)
0.25 (0.046)
0.40 (0.13)
0.69 (0.098)
(S)-6a
(R)-6b
(S)-6b
(R)-6c
(S)-6a
(R)-6b
(S)-6b
(R)-6c
(S)-6c
(S)-6c
4). A similar potency trend is observed in the phospho-
inositide hydrolysis data (Table 2). One reasonable
explanation for this phenomenon is that the tethered
alkyl ether groups of the two rigid series are acting as
conformationally restricted mimics of the methoxy sub-
stituents in the analogous amphetamine analogues
(series 4). In this view, the conformationally restricted
compounds have higher affinity because they do not
incur the entropic penalty experienced when the com-
pounds with freely rotating methoxy groups bind.
Another possible explanation for the increase in binding
affinity for the benzo[1,2-b;4,5-b′]difuran analogues is
that the alkylamine side chain of these analogues is
likely to exist in an anti-periplanar conformation (in a
plane perpendicular to that of the aromatic system).
Solution NMR studies of amphetamine analogues indi-
cate that the alkylamine side chain resides in this out-
of-plane orientation under simulated physiological con-
ditions.17 If the out-of-plane rotamer is the biologically
active conformation, then these rigid analogues may
also derive increased affinity through the enhancement
of energetic accessibility to the anti-periplanar confor-
mation.
P h a r m a cology
Radioligand binding assays and phosphoinositide
hydrolysis were employed to pharmacologically charac-
terize this class of 5-HT2A/2C receptor agonists. As
indicated in Table 1, the 5-HT2A and 5-HT2C receptor
competition binding data were generated using [3H]-
DOB and [125I]DOI, respectively. As a measure of
functional activity, compounds were analyzed for their
ability to stimulate phosphoinositide hydrolysis in NIH-
3T3 cells expressing the 5-HT2A receptor. The results
of those assays are shown in Table 2.
Resu lts a n d Discu ssion
The pharmacological data generated by the testing
of these enantiomerically pure, rigid analogues have led
to some new ideas about the interactions of the 5-HT2A
receptor binding site with agonist ligands. Perhaps the
most interesting finding in Table 1 is that affinity for
both of the 5-HT2 receptor isoforms is generally in-
creased in the rigidified tetrahydrobenzo[1,2-b;4,5-b′]-
difuran compounds (series 5) compared to their corre-
sponding 2,5-dimethoxyamphetamine analogues (series

1006 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 6
Chambers et al.
An additional trend that can be observed in Tables 1
and 2 is that the benzo[1,2-b;4,5-b′]difuran-containing
compounds (series 6) bind with higher affinity and
exhibit increased potency relative to the corresponding
tetrahydrobenzo[1,2-b;4,5-b′]difurans (series 5), indicat-
ing that the compounds in series 6 possess more
favorable interactions with the agonist binding site. This
may be due to the increased hydrophobicity of the
extended tricyclic aromatic nucleus in 6a -c relative to
the tetrahydro congeners 5a -c and a resulting greater
tendency to partition into the hydrophobic receptor
binding site. It is also possible that the extended
aromaticity of the benzo[1,2-b;4,5-b′]difurans (series 6)
may result in enhanced affinity by increasing the
effective aromatic surface area on the ligand available
for favorable π-stacking interactions with the agonist
binding site, while still maintaining some (albeit weaker)
hydrogen-bond acceptor properties of the furan oxygen
atoms. It is interesting to note that although potency
is generally increased for the aromatic compounds
6a -c relative to the tetrahydro compounds 5a -c, the
intrinsic activity of these compounds remains largely
unchanged.
Exp er im en ta l Section
Ch em istr y. All reagents were commercially available and
were used without further purification unless otherwise in-
dicated. Dry THF and diethyl ether were obtained by distil-
lation from benzophenone-sodium under nitrogen immediately
before use. Melting points were determined using a Thomas-
Hoover apparatus and are uncorrected. 1H NMR spectra were
recorded using a 300-MHz Bruker ARX-300 NMR spectrom-
eter. Chemical shifts are reported in δ values ppm relative to
an internal reference (0.03%, v/v) of tetramethylsilane (TMS)
in CDCl3, except where noted. Chemical ionization mass
spectra (CIMS) using isobutane as the carrier gas were ob-
tained with a Finnigan 4000 spectrometer. Elemental analyses
were performed by the Purdue University Microanalysis
Laboratory and are within (0.4% of the calculated values
unless otherwise noted. Optical rotations were measured on
a Perkin-Elmer 241 polarimeter at the sodium D-line (λ ) 589
nm). Thin-layer chromatography was performed using J .T.
Baker flex silica gel IB2-F, plastic-backed sheets with fluo-
rescent indicator, visualizing with UV light at 254 nm and
eluting with 4:1 hexanes-ethyl acetate unless otherwise noted.
Column chromatography was carried out using silica gel 60,
230-400 mesh (J .T. Baker). All reactions were carried out
under an inert atmosphere of argon unless otherwise indicated.
1,4-Bis(2-ch lor oeth oxy)ben zen e (9).19 Pyridine (29.4 mL,
364 mmol) was added to a mechanically stirred solution of 1,4-
bis(hydroxyethoxy)benzene (8) (Aldrich; 30.0 g, 152 mmol) in
CH2Cl2 (300 mL) at 0 °C. Next, thionyl chloride (25.4 mL, 348
mmol) was added dropwise such that the internal temperature
did not exceed 5 °C.20 This mixture was allowed to warm
gradually to room temperature and was stirred overnight.
Aqueous 2 N HCl (500 mL) was added slowly and the layers
were separated. The aqueous layer was extracted with CH2-
Cl2 (3 × 75 mL) and the organic extracts were combined. The
pooled organic extracts were then washed with aqueous 2 N
HCl (2 × 200 mL), water (200 mL), 1 N NaOH (100 mL), and
brine (100 mL). The organic extracts were then dried (MgSO4),
filtered, and evaporated to leave a tan solid that was recrystal-
lized from ethanol to afford white crystals: 31.4 g, 88%; mp
The data in Tables 1 and 2 indicate that (with the
exception of the 5-HT2C data for (R)- and (S)-5c) the
R-enantiomer binds with higher affinity and exhibits
greater potency than the corresponding S-antipode.
However, the data also indicate that the degree of
enantioselectivity exhibited by the pairs of enantiomers
in the two rigid series is relatively modest, except for
the compounds lacking a substituent para to the side
chain, i.e., 5a and 6a . It might be inferred that a
hydrophobic substituent at that location induces a
conformational change in the ligand-binding domain
such that the region in the vicinity of the R-methyl
group is dramatically altered. It is known that the effect
of appending a hydrophobic substituent to the 4-position
of amphetamine analogues is to increase hallucinogenic
activity, presumably by increasing affinity and/or ef-
ficacy at the 5-HT2A receptor.18 This effect is dramati-
cally demonstrated in this new series of arylalkylamine
analogues in progressing from (R or S)-5a to the
brominated arylalkylamine (R- or S)-5b. Only affinity
is increased slightly when comparing (R or S)-5b to the
trifluoromethylated arylalkylamine (R or S)-5c.
1
90-91 °C; H NMR (CDCl3) δ 3.79 (t, 4 H, ArOCH2CH2Cl, J
) 6.0 Hz), 4.19 (t, 4 H, ArOCH2CH2Cl, J ) 6.0 Hz), 6.87 (s, 4
H, ArH); CIMS m/z 235 (M + H+).
1,4-Bis(2-ch lor oeth oxy)-2,5-dibr om oben zen e (10).11 Zinc
chloride (38.2 g, 280 mmol) was added to a solution of 9 (27.5
g, 117 mmol) in acetic acid (280 mL). After enclosing the
apparatus in aluminum foil to exclude light, bromine (39.3 g,
246 mmol) dissolved in acetic acid (55 mL) was added dropwise
to the suspension over 1.5 h. The reaction was allowed to stir
overnight, during which time a precipitate had formed. The
reaction was diluted with aqueous saturated sodium thiosul-
fate (500 mL) and extracted with CH2Cl2 (5 × 200 mL). The
organic layers were combined and washed with aqueous 1 N
NaOH (1 × 200 mL), brine (100 mL), dried (MgSO4), filtered,
and evaporated to leave an off-white solid. Recrystallization
from ethanol afforded a crystalline white product: 50.0 g, 91%;
Nevertheless, it must be noted that the nonsubsti-
tuted (R)-6a has rather remarkable affinity, being about
comparable to the most potent amphetamine derivatives
such as 4c, although its functional potency is still rather
low (EC50 ca. 600 nM). Therefore, the fully aromatic
system must compensate in some way for the lack of
this substituent, even though its absence is clearly
relevant to the high degree of stereoselectivity observed
in 6a versus 6b. These will be interesting effects to
study when a satisfactory three-dimensional model of
the 5-HT2A receptor is ultimately developed.
mp 120 °C (lit.11 mp 119 °C); H NMR (CDCl3) δ 3.83 (t, 4 H,
1
ArOCH2CH2Cl, J ) 6.3 Hz), 4.23 (t, 4 H, ArOCH2CH2Cl, J )
6.3 Hz), 7.14 (s, 2 H, ArH); CIMS m/z 391 (M + H+).
2,3,6,7-Tetr a h yd r oben zo[1,2-b;4,5-b′]d ifu r a n (7).10 To a
suspension of magnesium powder (Aldrich, -50 mesh, 99+%;
3.7 g, 152 mmol) in anhydrous THF (50 mL) was slowly added
EtMgBr (3.4 mL, 10 mmol, 3 M solution in Et2O).21 An
anhydrous THF solution (125 mL) of the dibrominated com-
pound 10 (20.0 g, 51.0 mmol) was then added dropwise such
that the internal reaction temperature did not exceed 35 °C.
Upon completion of the addition, the reaction was heated at
reflux for 3 h. The reaction was then cooled to room temper-
ature and carefully poured into cold 1 N HCl (200 mL). Upon
cessation of gas evolution, the mixture was extracted with Et2O
(3 × 300 mL). The organic layers were combined and washed
with aqueous 1 N NaOH (4 × 75 mL), brine (50 mL), dried
(MgSO4), filtered, and concentrated by rotary evaporation to
afford a tan solid. This was recrystallized from ethanol to
afford off-white, platelike crystals: 6.5 g, 79%; mp 155 °C (lit.11
In summary, tethering the 2,5-dimethoxy substitu-
ents of hallucinogenic amphetamines in the form of
tetrahydrofuran or furan rings has led to the most
potent 5-HT2A/2C receptor ligands yet reported. Our
anticipation that receptor isoform selectivity might arise
in these series has not been borne out, but these
compounds will be useful tools in further exploring the
role of these receptors in behavior and cognition.

5-HT2A/ 2C Receptor Agonists
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 6 1007
mp 155-156 °C); 1H NMR (CDCl3) δ 3.13 (t, 4 H, ArOCH2CH2,
J ) 8.5 Hz), 4.52 (t, 4 H, ArOCH2CH2, J ) 8.5 Hz), 6.63 (s, 2
H, ArH); CIMS m/z 163 (M + H+).
saturated, aqueous NaHCO3 solution until evolution of gas had
ceased and the solution remained alkaline. The mixture was
then extracted with Et2O (4 × 75 mL), the organic layers
combined and dried (MgSO4), filtered and evaporated to leave
a tan solid. The product was triturated with hexanes and the
resulting suspension was then vacuum filtered to leave a white
(R)-N-Tr iflu or oa cetyla la n in e ((R)-12).12 The procedure
of Curphey was followed exactly except for a slight modification
of the acidic workup. Triethylamine (3.1 mL, 22 mmol) was
added to a solution of D-alanine ((R)-11; 99+%) (2.0 g, 22 mmol)
in MeOH (11 mL). After 5 min, ethyl trifluoroacetate (3.3 mL,
28 mmol) was added and the reaction was allowed to stir for
24 h. The solvent was removed by rotary evaporation and the
residue that remained was dissolved in H2O (35 mL) and
acidified with concentrated HCl (4 mL). After stirring for 15
min, the mixture was extracted with ethyl acetate (4 × 30 mL)
and the organic layers were combined and washed with brine
(25 mL), dried (MgSO4), filtered, and concentrated by rotary
evaporation to leave a clear oil. After being subjected to high
vacuum for 24 h, the clear oil solidified into a fluffy, hygro-
scopic solid: 2.7 g, 86%; mp 69 °C (lit.12 mp 70-71 °C); 1H
NMR (CDCl3) δ 1.58 (d, 3 H, CHCH3, J ) 7.5 Hz), 4.68 (p, 1
H, CHCH3, J ) 7.5 Hz), 6.87 (bs, 1 H, NH).
1
solid: 0.84 g, 92%; mp 139-140 °C; H NMR (CDCl3) δ 1.27
(d, 3 H, CHCH3, J ) 6.6 Hz), 2.75 (d, 2 H, ArCH2CH, J ) 6.0
Hz), 3.09 (t, 2 H, ArOCH2CH2, J ) 8.6 Hz), 3.17 (t, 2 H,
ArOCH2CH2, J ) 8.7 Hz), 4.16 (p, 1 H, ArCH2CH, J ) 6.5
Hz), 4.54 (t, 2 H, ArOCH2CH2, J ) 8.4 Hz), 4.56 (t, 2 H,
ArOCH2CH2), 6.56 (s, 1 H, ArH), 7.73 (bs, 1 H, NH); [R]D
)
+8.80° (c ) 0.081, CH3CN); CIMS m/z 316 (M + H+). Anal.
(C15H16F3NO3) C, H, N.
(S)-(-)-N-Tr iflu or oa cet yl-1-(2,3,6,7-t et r a h yd r ob en zo-
[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((S)-15). The
title compound was prepared from (S)-14 as described for
1
compound (R)-15: 94%; mp 139-140 °C; H NMR (CDCl3) δ
1.27 (d, 3 H, CHCH3, J ) 6.6 Hz), 2.75 (d, 2 H, ArCH2CH, J )
6.0 Hz), 3.09 (t, 2 H, ArOCH2CH2, J ) 8.6 Hz), 3.17 (t, 2 H,
ArOCH2CH2, J ) 8.7 Hz), 4.16 (p, 1 H, ArCH2CH, J ) 6.5
Hz), 4.54 (t, 2 H, ArOCH2CH2, J ) 8.4 Hz), 4.56 (t, 2 H,
(S)-N-Tr iflu or oa cetyla la n in e ((S)-12).12 The title com-
pound was prepared from L-alanine ((S)-11; 99%) as described
for compound (R)-12: 87%; mp 70 °C (lit.12 mp 70-71 °C); 1H
NMR (CDCl3) δ 1.58 (d, 3 H, CHCH3, J ) 7.5 Hz), 4.68 (p, 1
H, CHCH3, J ) 7.5 Hz), 6.87 (bs, 1 H, NH).
ArOCH2CH2), 6.56 (s, 1 H, ArH), 7.73 (bs, 1 H, NH); [R]D
)
-8.76° (c ) 0.069, CH3CN); CIMS m/z 316 (M + H+). Anal.
(C15H16F3NO3) C, H, N.
Rep r esen ta tive P r oced u r e for Hyd r olysis of Tr iflu o-
r oa cet a m id es: (R)-(-)-1-(2,3,6,7-Tet r a h yd r ob en zo[1,2-
b;4,5-b′]difu r an -4-yl)-2-am in opr opan e Hydr och lor ide ((R)-
5a ‚HCl). The protected amine (R)-15 (0.1 g, 0.3 mmol) was
dissolved in MeOH (20 mL) and cooled to 0 °C. Aqueous 5 N
NaOH (5 mL) was added to this solution and the mixture was
allowed to stir overnight and was then diluted with Et2O (50
mL). The phases were separated and the aqueous layer was
extracted with Et2O (3 × 15 mL). The organic layers were
combined, dried (MgSO4), filtered and evaporated to yield a
tan oil. This oil was taken up in anhydrous Et2O (15 mL) and
filtered through glass wool. A slight excess of ethanolic 1 N
HCl was added dropwise to the solution with vigorous stirring.
The flask was then placed in a freezer overnight and the
precipitate was collected by vacuum filtration. The resulting
solid was recrystallized from i-PrOH to afford white, fluffy
crystals: 0.073 g, 86%; mp 275-276 °C dec; 1H NMR (D2O) δ
1.28 (d, 3 H, CHCH3, J ) 6.9 Hz), 2.83 (d, 2 H, ArCH2CH, J )
6.9 Hz), 3.13 (dt, 4 H, ArOCH2CH2, J ) 7.8 Hz), 3.66 (m, 1 H,
ArCH2CH, J ) 6.9 Hz), 4.55 (dt, 4 H, ArOCH2CH2, J ) 9.0
Hz), 6.68 (s, 1 H, ArH); [R]D ) -11.78° (c ) 0.011, DMF); CIMS
m/z 220 (M + H+). Anal. (C13H18ClNO2) C, H, N.
(R)-(+)-N-Tr iflu or oa cetyl-2,3,6,7-tetr a h yd r o-4-a la n yl-
ben zo[1,2-b;4,5-b′]d ifu r a n ((R)-14). Trifluoracetamide-pro-
tected D-alanine ((R)-12) (4.09 g, 22 mmol) was added to a pre-
tared flask and placed under high vacuum for 24 h to remove
any residual moisture. The flask was then reweighed to record
an accurate weight of the starting material. Next, CH2Cl2 (100
mL) was added to the flask, followed by pyridine (2 drops) and
the solution was cooled to 0 °C. Oxalyl chloride (4.3 mL, 49
mmol) was then added via syringe and the reaction was
allowed to gradually warm to room temperature and stir for
3.5 h. The solvent and excess oxalyl chloride were removed by
rotary evaporation at 35 °C to afford acid chloride (R)-13,
which was not characterized. The heterocycle 7 (1.1 g, 6.6
mmol) was dissolved in CH2Cl2 (35 mL) and this solution was
added slowly via dropping funnel to a suspension of AlCl3 (2.4
g, 18.3 mmol) in CH2Cl2 (50 mL). This was followed by slow
addition of the acid chloride (R)-13 dissolved in CH2Cl2 (45
mL). The mixture was allowed to stir for 16 h, at which time
TLC showed only one yellow spot. The reaction mixture was
poured slowly onto ice to quench the reaction and the two
phases were then separated. The aqueous layer was extracted
with CH2Cl2 (4 × 75 mL) and the organic layers were combined
and washed with cold, aqueous 1 N HCl (75 mL), H2O (50 mL),
and then saturated NaHCO3 (2 × 50 mL). The organic solution
was then dried (MgSO4), filtered, and concentrated to leave a
yellow solid that was recrystallized from methanol to afford
fluffy, yellow crystals: 1.88 g, 86%; mp 209-210 °C; 1H NMR
(CDCl3) δ 1.46 (d, 3 H, CHCH3, J ) 6.8 Hz), 3.21 (m, 2 H,
ArOCH2CH2), 3.33 (p, 1 H, ArOCH2CH2, J ) 8.5 Hz), 3.61 (p,
1 H, ArOCH2CH2, J ) 9.0 Hz), 4.64 (m, 4 H, ArOCH2CH2),
5.62 (p, 1 H, CHCH3, J ) 6.8 Hz), 6.90 (s, 1 H, ArH), 7.64 (bs,
1 H, NH); [R]D ) +56.0° (c ) 0.01, CH3CN); CIMS m/z 330 (M
+ H+). Anal. (C15H14F3NO4) C, H, N.
(S)-(-)-N-Tr iflu or oa cet yl-2,3,6,7-t et r a h yd r o-4-a la n yl-
ben zo[1,2-b;4,5-b′]d ifu r a n ((S)-14). The title compound was
prepared from (S)-12 as described for compound (R)-14: 89%;
mp 210-211 °C; 1H NMR (CDCl3) δ 1.46 (d, 3 H, CHCH3, J )
6.8 Hz), 3.21 (m, 2 H, ArOCH2CH2), 3.33 (p, 1 H, ArOCH2CH2,
J ) 8.5 Hz), 3.61 (p, 1 H, ArOCH2CH2, J ) 9.0 Hz), 4.64 (m,
4 H, ArOCH2CH2), 5.62 (p, 1 H, CHCH3, J ) 6.8 Hz), 6.90 (s,
1 H, ArH), 7.64 (bs, 1 H, NH); [R]D ) -56.1° (c ) 0.01, CH3-
CN); CIMS m/z 330 (M + H+). Anal. (C15H14F3NO4) C, H, N.
(R)-(+)-N-Tr iflu or oa cet yl-1-(2,3,6,7-t et r a h yd r oben zo-
[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((R)-15). Ke-
tone (R)-14 (0.94 g, 2.9 mmol) was dissolved in trifluoroacetic
acid (9.4 mL) and then triethylsilane (2.3 mL, 14.5 mmol) was
added. The mixture was heated at reflux for 6 h, at which time
TLC showed no starting material. The reaction was cooled to
room temperature followed by the very slow addition of
(S)-(+)-1-(2,3,6,7-Tetr a h yd r oben zo[1,2-b;4,5-b′]d ifu r a n -
4-yl)-2-a m in op r op a n e Hyd r och lor id e ((S)-5a ‚HCl). The
title compound was prepared from (S)-15 as described for (R)-
1
5a ‚HCl: 84%; mp 275-276 °C dec; H NMR (D2O) δ 1.28 (d,
3 H, CHCH3, J ) 6.9 Hz), 2.83 (d, 2 H, ArCH2CH, J ) 6.9
Hz), 3.13 (dt, 4 H, ArOCH2CH2, J ) 7.8 Hz), 3.66 (m, 1 H,
ArCH2CH, J ) 6.9 Hz), 4.55 (dt, 4 H, ArOCH2CH2, J ) 9.0
Hz), 6.68 (s, 1 H, ArH); [R]D ) +11.72° (c ) 0.011, DMF); CIMS
m/z 220 (M + H+). Anal. (C13H18ClNO2) C, H, N.
Rep r esen ta tive P r oced u r e for Ar om a tiza tion of Su b-
stitu ted 2,3,6,7-Tetr a h yd r oben zo[1,2-b;4,5-b′]d ifu r a n s to
Ben zo[1,2-b;4,5-b′]d ifu r a n s: (R)-(+)-N-Tr iflu or oa cetyl-1-
(b en zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((R)-
16). A solution of DDQ (0.42 g, 1.85 mmol) in dioxane (8 mL)
was slowly added to a solution of protected amine (R)-15 (0.20
g, 0.63 mmol) in dioxane (10 mL).14 The solution was heated
at reflux for 24 h, at which time TLC indicated reaction com-
pletion. The reaction was then cooled to room temperature and
the precipitate that formed was removed by vacuum filtration.
The filter cake was washed thoroughly with CH2Cl2 and then
the solvents were removed by rotary evaporation. The brown
oil that remained was subjected to column chromatography
(4:1 hexanes-ethyl acetate as eluent) to afford a white solid
product: 0.134 g, 64%; mp 150-151 °C; 1H NMR (CDCl3) δ
1.29 (d, 3 H, CHCH3, J ) 6.6 Hz), 3.38 (dd, 2 H, ArCH2CH, J
) 6.0, 3.0 Hz), 4.48 (m, 1 H, ArCH2CH, J ) 6.9 Hz), 6.54 (bs,
1 H, NH), 6.85 (d, 1 H, ArH, J ) 2.4 Hz), 6.90 (d, 1 H, ArH, J

1008 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 6
Chambers et al.
) 2.4 Hz), 7.58 (s, 1 H, ArH), 7.64 (d, 1 H, ArH, J ) 2.4 Hz),
7.67 (d, 1 H, ArH, J ) 2.1 Hz); [R]D ) +3.06° (c ) 0.006, CH3-
CN); CIMS m/z 312 (M + H+). Anal. (C15H12F3NO3) C, H, N.
6.3 Hz), 2.78 (dd, 2 H, ArCH2CH, J ) 7.2, 3.3 Hz), 3.14 (t, 2
H, ArOCH2CH2, J ) 8.7 Hz), 3.23 (t, 2 H, ArOCH2CH2, J )
8.7 Hz), 3.63 (m, 1 H, ArCH2CH, J ) 6.9 Hz), 4.61 (q, 4 H,
ArOCH2CH2, J ) 8.7 Hz); [R]D ) -10.82° (c ) 0.0089, DMF);
CIMS m/z 297 (M + H+). Anal. (C13H17BrClNO2) C, H, N.
(S)-(-)-N-Tr iflu or oa cetyl-1-(ben zo[1,2-b;4,5-b′]d ifu r a n -
4-yl)-2-a m in op r op a n e ((S)-16). The title compound was
prepared from (S)-15 by the general procedure described for
(S)-(+)-1-(8-Br om o-2,3,6,7-tetr a h yd r oben zo[1,2-b;4,5-b′]-
d ifu r a n -4-yl)-2-a m in op r op a n e H yd r och lor id e ((S)-5b ‚
HCl). The title compound was prepared from (S)-17 by the
general procedure described for compound (R)-5a ‚HCl: 78%;
1
compound (R)-16: 58%; mp 152-153 °C; H NMR (CDCl3) δ
1.29 (d, 3 H, CHCH3, J ) 6.6 Hz), 3.38 (dd, 2 H, ArCH2CH, J
) 6.0, 3.0 Hz), 4.48 (m, 1 H, ArCH2CH, J ) 6.9 Hz), 6.54 (bs,
1 H, NH), 6.85 (d, 1 H, ArH, J ) 2.4 Hz), 6.90 (d, 1 H, ArH, J
) 2.4 Hz), 7.58 (s, 1 H, ArH), 7.64 (d, 1 H, ArH, J ) 2.4 Hz),
7.67 (d, 1 H, ArH, J ) 2.1 Hz); [R]D ) -3.04° (c ) 0.006, CH3-
CN); CIMS m/z 312 (M + H+). Anal. (C15H12F3NO3) C, H, N.
1
mp 282-283 °C; H NMR (D2O) δ 1.27 (d, 3 H, CHCH3, J )
6.3 Hz), 2.78 (dd, 2 H, ArCH2CH, J ) 7.2, 3.3 Hz), 3.14 (t, 2
H, ArOCH2CH2, J ) 8.7 Hz), 3.23 (t, 2 H, ArOCH2CH2, J )
8.7 Hz), 3.63 (m, 1 H, ArCH2CH, J ) 6.9 Hz), 4.61 (q, 4 H,
ArOCH2CH2, J ) 8.7 Hz); [R]D ) +10.80° (c ) 0.0089, DMF);
CIMS m/z 297 (M + H+). Anal. (C13H17BrClNO2) C, H, N.
(R)-(-)-1-(Ben zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r o-
p a n e Hyd r och lor id e ((R)-6a ‚HCl). The title compound was
prepared from (R)-16 by the general procedure described for
compound (R)-5a ‚HCl: 81%; mp 269 °C dec; 1H NMR (D2O) δ
1.29 (d, 3 H, CHCH3, J ) 6.9 Hz), 3.36 (d, 2 H, ArCH2CH, J )
6.9 Hz), 3.85 (m, 1 H, ArCH2CH, J ) 6.9 Hz), 6.94 (d, 1 H,
ArH, J ) 1.2 Hz), 6.98 (d, 1 H, ArH, J ) 1.2 Hz), 7.64 (s, 1 H,
ArH), 7.77 (s, 1 H, ArH), 7.78 (s, 1 H, ArH); [R]D ) -9.54° (c
) 0.0102, DMF); CIMS m/z 216 (M + H+). Anal. (C13H14ClNO2)
C, H, N.
(S)-(+)-1-(Ben zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r o-
p a n e Hyd r och lor id e ((S)-6a ‚HCl). The title compound was
prepared from (S)-16 by the general procedure described for
compound (R)-5a ‚HCl: 82%; mp 270 °C dec; 1H NMR (D2O) δ
1.29 (d, 3 H, CHCH3, J ) 6.9 Hz), 3.36 (d, 2 H, ArCH2CH, J )
6.9 Hz), 3.85 (m, 1 H, ArCH2CH, J ) 6.9 Hz), 6.94 (d, 1 H,
ArH, J ) 1.2 Hz), 6.98 (d, 1 H, ArH, J ) 1.2 Hz), 7.64 (s, 1 H,
ArH), 7.77 (s, 1 H, ArH), 7.78 (s, 1 H, ArH); [R]D ) +9.58° (c
) 0.0102, DMF); CIMS m/z 216 (M + H+). Anal. (C13H14ClNO2)
C, H, N.
(R)-(+)-N-Tr iflu or oa cet yl-1-(8-b r om o-2,3,6,7-t et r a h y-
d r oben zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((R)-
17). The protected amine (R)-15 (1.1 g, 3.5 mmol) was dissolved
in acetic acid (70 mL) and the flask was covered with
aluminum foil to exclude light.11 This solution was cooled to
15 °C and a solution of bromine (0.54 g, 3.5 mmol) in acetic
acid (12 mL) was added dropwise. The reaction was allowed
to warm to room temperature and stir for 4.5 h, at which time
a precipitate had formed and TLC indicated the absence of
starting material. The reaction was quenched by the addition
of H2O (100 mL) and extracted with CH2Cl2 (5 × 50 mL). The
organic layers were combined, dried (MgSO4), filtered, and
evaporated to afford 1.0 g (79%) of the product as a white solid.
Care should be taken to use not more than 1.0 equivalent of
bromine in this procedure and to recrystallize the product if
any impurities are observed, since we have noted that bromine
can oxidize the dihydrofuran moieties to furans under some
circumstances. An analytical sample was recrystallized from
ethyl acetate-hexane: mp 201-202 °C; 1H NMR (CDCl3) δ
1.27 (d, 3 H, CHCH3, J ) 6.3 Hz), 2.71 (dd, 2 H, ArCH2CH, J
) 7.2, 1.5 Hz), 3.20 (t, 4 H, ArOCH2CH2, J ) 8.7 Hz), 4.13 (p,
1 H, ArCH2CH, J ) 6.6 Hz), 4.59 (t, 2 H, ArOCH2CH2, J ) 8.4
Hz), 4.65 (t, 2 H, ArOCH2CH2, J ) 8.4 Hz), 7.47 (bs, 1 H, NH);
[R]D ) +6.6° (c ) 0.00445, CH3CN); CIMS m/z 394 (M + H+).
Anal. (C15H15BrF3NO3) C, H, N.
(R)-(+)-N-Tr iflu or oa cetyl-1-(8-br om oben zo[1,2-b;4,5-b′]-
d ifu r a n -4-yl)-2-a m in op r op a n e ((R)-18). The title compound
was prepared from (R)-17 by the general procedure described
for compound (R)-16: 68%; mp 215-216 °C; 1H NMR (CDCl3)
δ 1.26 (d, 3 H, CHCH3, J ) 6.7 Hz), 3.35 (dd, 2 H, ArCH2CH,
J ) 6.4, 3.3 Hz), 4.46 (m, 1 H, ArCH2CH, J ) 6.8 Hz), 6.41
(bs, 1 H, NH), 6.92 (d, 1 H, ArH, J ) 2.2 Hz), 6.99 (d, 1 H,
ArH, J ) 2.2 Hz), 7.69 (d, 1 H, ArH, J ) 2.2 Hz), 7.73 (d, 1 H,
ArH, J ) 2.3 Hz); [R]D ) +17.7° (c ) 0.0075, CH3CN); CIMS
m/z 390 (M + H+). Anal. (C15H11BrF3NO3) C, H, N.
(S)-(-)-N-Tr iflu or oa cetyl-1-(8-br om oben zo[1,2-b;4,5-b′]-
d ifu r a n -4-yl)-2-a m in op r op a n e ((S)-18). The title compound
was prepared from (S)-17 by the general procedure described
for compound (R)-16: 64%; mp 214-215 °C; 1H NMR (CDCl3)
δ 1.26 (d, 3 H, CHCH3, J ) 6.7 Hz), 3.35 (dd, 2 H, ArCH2CH,
J ) 6.4, 3.3 Hz), 4.46 (m, 1 H, ArCH2CH, J ) 6.8 Hz), 6.41
(bs, 1 H, NH), 6.92 (d, 1 H, ArH, J ) 2.2 Hz), 6.99 (d, 1 H,
ArH, J ) 2.2 Hz), 7.69 (d, 1 H, ArH, J ) 2.2 Hz), 7.73 (d, 1 H,
ArH, J ) 2.3 Hz); [R]D ) -17.2° (c ) 0.0075, CH3CN); CIMS
m/z 390 (M + H+). Anal. (C15H11BrF3NO3) C, H, N.
(R)-(-)-1-(8-Br om ob en zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-
a m in op r op a n e Hyd r och lor id e ((R)-6b‚HCl). The title com-
pound was prepared from (R)-18 by the general procedure
described for compound (R)-5a ‚HCl: 87%; mp 290 °C dec; 1H
NMR (D2O) δ 1.23 (d, 3 H, CHCH3, J ) 6.6 Hz), 3.21 (d, 2 H,
ArCH2CH, J ) 6.9 Hz), 3.74 (m, 1 H, ArCH2CH, J ) 6.9 Hz),
6.84 (d, 1 H, ArH, J ) 2.1 Hz), 6.96 (d, 1 H, ArH, J ) 2.4 Hz),
7.73 (d, 1 H, ArH, J ) 2.4 Hz), 7.76 (s, 1 H, ArH, J ) 2.4 Hz);
[R]D ) -20.1° (c ) 0.005, DMF); CIMS m/z 294 (M + H+). Anal.
(C13H13BrClNO2) C, H, N.
(S)-(+)-1-(8-Br om oben zo[1,2-b;4,5-b′]difu r an -4-yl)-2-am i-
n op r op a n e Hyd r och lor id e ((S)-6b‚HCl). The title com-
pound was prepared from (S)-18 by the general procedure
described for compound (R)-5a ‚HCl: 85%; mp 291 °C dec; 1H
NMR (D2O) δ 1.23 (d, 3 H, CHCH3, J ) 6.6 Hz), 3.21 (d, 2 H,
ArCH2CH, J ) 6.9 Hz), 3.74 (m, 1 H, ArCH2CH, J ) 6.9 Hz),
6.84 (d, 1 H, ArH, J ) 2.1 Hz), 6.96 (d, 1 H, ArH, J ) 2.4 Hz),
7.73 (d, 1 H, ArH, J ) 2.4 Hz), 7.76 (s, 1 H, ArH, J ) 2.4 Hz);
[R]D ) +20.8° (c ) 0.005, DMF); CIMS m/z 294 (M + H+). Anal.
(C13H13BrClNO2) C, H, N.
(R)-(+)-N-Tr iflu or oa cetyl-1-(8-tr iflu or om eth yl-2,3,6,7-
t et r a h yd r ob en zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r o-
p a n e ((R)-19). Toluene (25 mL) was added to a flask fitted
with a Dean-Stark trap and containing tetramethylammo-
nium trifluoroacetate (1.2 g, 6.3 mmol), copper iodide (1.4 g,
7.1 mmol), and (R)-17 (0.50 g, 1.3 mmol).15 This mixture was
brought to reflux for 4 h to azeotrope residual H2O present in
the starting materials. Next, anhydrous DMF (7.9 mL, 101.5
mmol) was added slowly with concomitant removal of toluene
to bring the reaction temperature to 145 °C for 4 h. The
reaction was monitored by 19F NMR for the appearance of a
second singlet. Upon completion, the reaction was cooled to
room temperature, diluted with H2O (100 mL) and extracted
with CH2Cl2 (4 × 50 mL). The organic layers were combined,
dried (MgSO4), filtered, and evaporated to leave a white solid
that was subjected to column chromatography (4:1 hexanes-
ethyl acetate as eluent) to afford the title compound as white
crystals: 0.35 g, 72%; mp 178-179 °C; 1H NMR (CDCl3) δ 1.28
(d, 3 H, CHCH3, J ) 6.3 Hz), 2.77 (t, 1 H, ArCH2CH, J ) 13.5
(S)-(-)-N-Tr iflu or oa cet yl-1-(8-b r om o-2,3,6,7-t et r a h y-
d r oben zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((S)-
17). The title compound was prepared from (S)-15 by the
general procedure described for compound (R)-17: 82%; mp
201-202 °C; 1H NMR (CDCl3) δ 1.27 (d, 3 H, CHCH3, J ) 6.3
Hz), 2.71 (dd, 2 H, ArCH2CH, J ) 7.2, 1.5 Hz), 3.20 (t, 4 H,
ArOCH2CH2, J ) 8.7 Hz), 4.13 (p, 1 H, ArCH2CH, J ) 6.6
Hz), 4.59 (t, 2 H, ArOCH2CH2, J ) 8.4 Hz), 4.65 (t, 2 H,
ArOCH2CH2, J ) 8.4 Hz), 7.47 (bs, 1 H, NH); [R]D ) -6.9° (c
) 0.00445, CH3CN); CIMS m/z 394 (M + H+). Anal. (C15H15
BrF3NO3) C, H, N.
-
(R)-(-)-1-(8-Br om o-2,3,6,7-tetr a h yd r oben zo[1,2-b;4,5-b′]-
d ifu r a n -4-yl)-2-a m in op r op a n e Hyd r och lor id e ((R)-5b ‚
HCl). The title compound was prepared from (R)-17 by the
general procedure described for compound (R)-5a ‚HCl: 75%;
1
mp 282-283 °C; H NMR (D2O) δ 1.27 (d, 3 H, CHCH3, J )

5-HT2A/ 2C Receptor Agonists
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 6 1009
procedure described for compound (R)-5a ‚HCl: 71%; mp 283
Hz), 2.78 (q, 1 H, ArCH2CH, J ) 12.9 Hz), 3.13 (t, 2 H,
ArOCH2CH2, J ) 8.7 Hz), 3.34 (t, 2 H, ArOCH2CH2, J ) 8.7
Hz), 4.17 (m, 1 H, ArCH2CH, J ) 6.9 Hz), 4.59 (t, 2 H, ArOCH2-
CH2, J ) 9.0 Hz), 4.67 (t, 2 H, ArOCH2CH2, J ) 8.7 Hz), 7.40
(bs, 1 H, NH); [R]D ) +1.79° (c ) 0.013, CH3CN); CIMS m/z
384 (M + H+). Anal. (C16H15F6NO3) C, H, N.
(S)-(-)-N-Tr iflu or oa cetyl-1-(8-tr iflu or om eth yl-2,3,6,7-
t et r a h yd r ob en zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r o-
p a n e ((S)-19). The title compound was prepared from (S)-17
by the procedure described for compound (R)-19: 70%; mp
178-179 °C; 1H NMR (CDCl3) δ 1.28 (d, 3 H, CHCH3, J ) 6.3
Hz), 2.77 (t, 1 H, ArCH2CH, J ) 13.5 Hz), 2.78 (q, 1 H, ArCH2-
CH, J ) 12.9 Hz), 3.13 (t, 2 H, ArOCH2CH2, J ) 8.7 Hz), 3.34
(t, 2 H, ArOCH2CH2, J ) 8.7 Hz), 4.17 (m, 1 H, ArCH2CH, J
) 6.9 Hz), 4.59 (t, 2 H, ArOCH2CH2, J ) 9.0 Hz), 4.67 (t, 2 H,
ArOCH2CH2, J ) 8.7 Hz), 7.40 (bs, 1 H, NH); [R]D ) -1.83° (c
) 0.013, CH3CN); CIMS m/z 384 (M + H+). Anal. (C16H15F6-
NO3) C, H, N.
(R)-(-)-1-(8-Tr iflu or om eth yl-2,3,6,7-tetr a h yd r oben zo-
[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e H yd r och lo-
r id e ((R)-5c‚HCl). The title compound was prepared from (R)-
19 by the general procedure described for compound (R)-5a ‚
HCl: 84%; mp 314-315 °C; 1H NMR (D2O) δ 1.27 (d, 3 H,
CHCH3, J ) 7.2 Hz), 2.87 (m, 2 H, ArCH2CH, J ) 6.9 Hz),
3.18 (t, 2 H, ArOCH2CH2, J ) 9.0 Hz), 3.33 (t, 2 H, ArOCH2CH2,
J ) 8.6 Hz), 3.69 (m, 1 H, ArCH2CH, J ) 6.6 Hz), 4.61 (q, 2 H,
ArOCH2CH2, J ) 8.7 Hz), 4.64 (q, 2 H, ArOCH2CH2, J ) 8.7
Hz); [R]D ) -13.33° (c ) 0.0097, DMF); CIMS m/z 288 (M +
H+). Anal. (C14H17ClF3NO2) C, H, N.
(S)-(+)-1-(8-Tr iflu or om eth yl-2,3,6,7-tetr a h yd r oben zo-
[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e H yd r och lo-
r id e ((S)-5c‚HCl). The title compound was prepared from (S)-
19 by the general procedure described for compound (R)-5a ‚
HCl: 84%; mp 314-315 °C; 1H NMR (D2O) δ 1.27 (d, 3 H,
CHCH3, J ) 7.2 Hz), 2.87 (m, 2 H, ArCH2CH, J ) 6.9 Hz),
3.18 (t, 2 H, ArOCH2CH2, J ) 9.0 Hz), 3.33 (t, 2 H, ArOCH2CH2,
J ) 8.6 Hz), 3.69 (m, 1 H, ArCH2CH, J ) 6.6 Hz), 4.61 (q, 2 H,
ArOCH2CH2, J ) 8.7 Hz), 4.64 (q, 2 H, ArOCH2CH2, J ) 8.7
Hz); [R]D ) +12.90° (c ) 0.0097, DMF); CIMS m/z 288 (M +
H+). Anal. (C14H17ClF3NO2) C, H, N.
1
°C dec; H NMR (D2O) δ 1.28 (d, 3 H, CHCH3, J ) 6.6 Hz),
3.41 (d, 1 H, ArCH2CH, J ) 6.9 Hz), 3.42 (d, 1 H, ArCH2CH,
J ) 6.3 Hz), 3.87 (m, 1 H, ArCH2CH, J ) 6.6 Hz), 7.08 (d, 1 H,
ArH, J ) 2.7 Hz), 7.13 (p, 1 H, ArH, J ) 1.8 Hz), 7.87 (d, 1 H,
ArH, J ) 2.4 Hz), 7.90 (d, 1 H, ArH, J ) 2.1 Hz); [R]D ) +33.38°
(c ) 0.0106, DMF); CIMS m/z 284 (M + H+). Anal. (C14H13
ClF3NO2) C, H, N.
-
Ch r om a togr a p h ic Stu d ies. All HPLC analyses were
performed on a Rainin Rabbit HPX high-performance liquid
chromatograph with a Knauer variable wavelength detector
set to observe at 254 nm. A 20-µL injection loop was used for
sample injection. A flow rate of 1 mL/min of dichloromethane
was used as the mobile phase through a 250- × 4.6-mm Alltech
Silica column. Chromatographic parameters reported for each
solute are averages of triplicate analyses.
Rep r esen ta tive P r oced u r e for Am in e Der ivitiza -
tion :22 N-Tr iflu or oa cetyl-1-(8-br om o-2,3,6,7-tetr a h yd r o-
ben zo[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((R or
S)-17). To confirm that the samples of product (R)- and (S)-
5b‚HCl that were used in pharmacological evaluation were
not contaminated with side products of the aromatic bromi-
nation reaction used to make (R or S)-17, a sample of (R)-5b‚
HCl was converted to the N-trifluoroacetyl-protected com-
pound. Trifluoroacetic anhydride (0.010 mL, 0.072 mmol) was
added dropwise to a dichloromethane (0.4 mL) solution of (R)-
5b‚HCl (0.010 g, 0.030 mmol), triethylamine (0.014 mL, 0.105
mmol) and 4-(dimethylamino)pyridine (0.0003 g, 0.0024 mmol).
The reaction was stirred for 4 h and then diluted with
dichloromethane (4 mL), washed with aqueous 2 N HCl (2 mL),
saturated NaHCO3 (2 mL), dried (MgSO4), filtered, and
evaporated to leave a clear oil. HPLC analysis of this oil
indicated that it was 99.5% pure and had a retention time of
7.10 min ((0.01 min). This procedure was repeated for a
sample of (S)-5b‚HCl. HPLC analysis of this product indicated
a 99.3% purity and a retention time of 7.11 min ((0.02 min).
None of the minor impurities observed in the HPLC data trace
corresponded to the fully aromatic compound (R)-18 which was
likewise analyzed by HPLC and found to have a retention time
of 4.98 min ((0.03 min) and a purity of 99.2%. These results
indicate that careful addition of bromine to the reaction
discussed above and recrystallization of the product led to pure
compounds and that the pharmacological data were not
confounded by potent impurities.
N-(1-Meth yl-2-(8-tr iflu or om eth ylben zo[1,2-b;4,5-b′]d i-
fu r a n -4-yl)eth yl)-(R)-MTP A Am id es. To confirm that the
integrity of the chiral center remained intact throughout the
reactions used in the synthetic scheme, of which several might
have potentially racemized the stereocenter, products (R)- and
(S)-6c‚HCl were converted to their corresponding MTPA
amides using (R)-(-)-R-methoxy-R-(trifluoromethyl)phenyl-
acetyl chloride16 ((R)-MTPA-Cl: Fluka) to provide the pair of
diastereomers for HPLC analysis. This was accomplished by
the general procedure22 used to make the trifluoroacetyl-
protected compound discussed above, using 2.4 equiv of (R)-
MPTA-Cl in place of the trifluoroacetic anhydride. The oils that
were obtained were dissolved in dichloromethane to give equal
concentrations of each solute. To be certain baseline separation
could be accomplished using the HPLC conditions discussed
above, an equimolar mixture of the pure amides was prepared
by combining equal volumes of the two amide solutions and
performing HPLC analysis on this mixture. Excellent resolu-
tion was obtained, with approximately 2.2 min of baseline
separation between the two eluted diastereoisomers. The
2R,1′S-amide was analyzed by HPLC and found to be 99.4%
pure, with a retention time of 11.69 min ((0.03 min). A very
small peak that corresponded to the 2R,1′R-amide was ob-
served however, which accounted for the other 0.6% of the
injected sample. The 2R,1′R-amide was also analyzed by HPLC
and found to be 100.0% pure, with a retention time of 13.95
min ((0.01 min). These results indicated that racemization
had not occurred in the synthesis of these products. Further,
because products (R)- and (S)-6c‚HCl were subjected to all
possible reaction conditions in the synthetic scheme, one may
(R)-(+)-N-Tr iflu or oa cet yl-1-(8-t r iflu or om et h ylb en zo-
[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((R)-20). The
title compound was prepared from (R)-19 by the general pro-
cedure described for compound (R)-16: 74%; mp 170-171 °C;
1H NMR (CDCl3) δ 1.27 (d, 3 H, CHCH3, J ) 6.3 Hz), 3.35 (d,
2 H, ArCH2CH, J ) 6.0 Hz), 4.48 (m, 1 H, ArCH2CH, J ) 7.5
Hz), 6.39 (bs, 1 H, NH), 7.68 (d, 1 H, ArH, J ) 2.4 Hz), 7.07
(p, 1 H, ArH, J ) 1.8 Hz), 7.74 (d, 1 H, ArH, J ) 2.4 Hz), 7.77
(d, 1 H, ArH, J ) 2.1 Hz); [R]D ) +7.46° (c ) 0.0025, CH3CN);
CIMS m/z 380 (M + H+), 360. Anal. (C16H11F6NO3) C, H, N.
(S)-(-)-N-Tr iflu or oa cet yl-1-(8-t r iflu or om et h ylb en zo-
[1,2-b;4,5-b′]d ifu r a n -4-yl)-2-a m in op r op a n e ((S)-20). The
title compound was prepared from (S)-19 by the general pro-
cedure described for compound (R)-16: 75%; mp 170-171 °C;
1H NMR (CDCl3) δ 1.27 (d, 3 H, CHCH3, J ) 6.3 Hz), 3.35 (d,
2 H, ArCH2CH, J ) 6.0 Hz), 4.48 (m, 1 H, ArCH2CH, J ) 7.5
Hz), 6.39 (bs, 1 H, NH), 7.68 (d, 1 H, ArH, J ) 2.4 Hz), 7.07
(p, 1 H, ArH, J ) 1.8 Hz), 7.74 (d, 1 H, ArH, J ) 2.4 Hz), 7.77
(d, 1 H, ArH, J ) 2.1 Hz); [R]D ) -7.38° (c ) 0.0025, CH3CN);
CIMS m/z 380 (M + H+), 360. Anal. (C16H11F6NO3) C, H, N.
(R)-(-)-1-(8-Tr iflu or om eth ylben zo[1,2-b;4,5-b′]d ifu r a n -
4-yl)-2-a m in op r op a n e Hyd r och lor id e ((R)-6c‚HCl). The
title compound was prepared from (R)-20 by the general
procedure described for compound (R)-5a ‚HCl: 69%; mp 282
1
°C dec; H NMR (D2O) δ 1.28 (d, 3 H, CHCH3, J ) 6.6 Hz),
3.41 (d, 1 H, ArCH2CH, J ) 6.9 Hz), 3.42 (d, 1 H, ArCH2CH,
J ) 6.3 Hz), 3.87 (m, 1 H, ArCH2CH, J ) 6.6 Hz), 7.08 (d, 1 H,
ArH, J ) 2.7 Hz), 7.13 (p, 1 H, ArH, J ) 1.8 Hz), 7.87 (d, 1 H,
ArH, J ) 2.4 Hz), 7.90 (d, 1 H, ArH, J ) 2.1 Hz); [R]D ) -33.49°
(c ) 0.0106, DMF); CIMS m/z 284 (M + H+). Anal. (C14H13
ClF3NO2) C, H, N.
-
(S)-(+)-1-(8-Tr iflu or om eth ylben zo[1,2-b;4,5-b′]d ifu r a n -
4-yl)-2-a m in op r op a n e Hyd r och lor id e ((S)-6c‚HCl). The
title compound was prepared from (S)-20 by the general

1010 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 6
Chambers et al.
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deduce that all the other products are likewise enantiomeri-
cally pure. The small amount of the 2R,1′R-amide found in
the 2R,1′S-amide most likely arose as a consequence of the
slightly lower purity of the L-alanine starting material.
(3) Roth, B. L.; Willins, D. L.; Kristiansen, K.; Kroeze, W. K. 5-Hy-
droxytryptamine2-Family Receptors (5-Hydroxytryptamine2A
,
P h a r m a cology Meth od s. 1. Cell Cu ltu r e. NIH-3T3 fi-
broblast cells stably transfected to express either the rat
5-HT2A or 5-HT2C receptor23 were maintained in minimum
essential medium, containing 10% dialyzed fetal bovine serum
(Gibco BRL) and supplemented with L-glutamine, Pen/Strep,
and Geneticin. The cells were cultured at 37 °C in a water
saturated atmosphere of 95% air and 5% CO2. For radioligand
binding assays, cells were split into 100-mm2 culture dishes
when they reached 90% confluency. Upon reaching 100%
confluency in the culture dishes, the cells were washed with
sterile filtered phosphate-buffered solution and left to incubate
in serum-free Opti-MEM for 5 h. After this incubation, the
cells were harvested by centrifugation (15000g, 20 min) and
placed immediately in a freezer at -80 °C until the assay was
performed. For phosphoinositide hydrolysis experiments the
cells were seeded into 24-well plates and assays were per-
formed when 70% confluency was achieved.
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assays, 0.125-6 nM [3H]DOB or [125I]DOI were used. The total
volume of the assay was 250 µL. Nonspecific binding was
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Determinants of Affinity for the 5-HT2A Receptor. Ph.D. Thesis,
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expressing cells) or 10 µM mianserin (rat 5-HT2C-expressing
cells). Competition binding experiments were carried out in a
total volume of 500 µL with either 1.0 nM [3H]DOB or 0.20
nM [125I]DOI. Previously harvested cells were resuspended and
added to each well containing assay buffer (50 mM Tris, 0.5
mM EDTA, 10 mM MgCl2; pH ) 7.4), radioligand, and new
compound (or in the case of the saturation assays, cinanserin
or mianserin). Incubation was carried out at 25 °C for 60 min
and terminated by rapid filtration using a prechilled Packard
96-well harvester with GF/B Uni-filters that had been prein-
cubated for 30 min in 0.3% polyethylenimine. The filters were
rinsed using chilled wash buffer (10 mM Tris, 154 mM NaCl)
and left to dry overnight. The following day, Microscint-O was
added and radioactivity was determined using a TopCount
(Packard) scintillation counter. GraphPad Prism (GraphPad
Software, San Diego, CA) was used to analyze the saturation
and competition binding curves.
3. P h osp h oin ositol Hyd r olysis Stu d ies in NIH-3T3
Cells Exp r essin g th e 5-HT2A Recep tor . Accumulation of
inositol phosphates was determined using a modified version
of a previously published protocol.24 Briefly, cells expressing
the rat 5-HT2A receptor were labeled for 18-20 h in CRML
medium containing 1.0 µCi/mL [3H]myo-inositol. After pre-
treating the cells with 10 µM pargyline/10 mM LiCl for 15 min,
the cells were placed in the presence of agonist for 30 min at
37 °C, under an atmosphere of 95% O2 and 5% CO2. The assay
was terminated by aspirating the medium and adding 10 mM
formic acid. After incubation for 16 h at 4 °C, the [3H]inositol
phosphates were separated from the cellular debris on Dowex-1
ion-exchange columns and eluted with 1.0 M ammonium
formate and 0.10 M formic acid. The vials were counted for
tritium using a TriCarb scintillation counter (Packard Instru-
ment Corp.).
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1988, 950-952.
(22) J ackson, P. T.; Kim, T.-Y.; Carr, P. W. Diastereomeric Selectivity
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graphic Media. Anal. Chem. 1997, 69, 5011-5017.
(23) J ulius, D.; Huang, K. N.; Livelli, T. J .; Axel, R.; J essell, T. M.
The 5HT2 receptor defines a family of structurally distinct but
functionally conserved serotonin receptors. Proc. Natl. Acad. Sci.
U.S.A. 1990, 87, 928-932.
Ack n ow led gm en t. We thank S. Frescas and A.
Monte for their helpful contributions in optimizing the
synthesis of the benzodifuran nucleus. We also thank
Dr. David J ulius (UCSF) for his kind donation of the
5-HT2A/2C cell lines. This work was supported by Grant
DA02189 from NIDA and by a grant from the Heffter
Research Institute.
Refer en ces
(24) Berg, K. A.; Clarke, W. P.; Sailstad, C.; Saltzman, A.; Maayani,
S. Signal Transduction Differences Between 5-Hydroxytryptamine
Type 2A and Type 2C Receptor Systems. Mol. Pharmacol. 1994,
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J M000491Y