Novel 3-O-glycosyl-3-demethylthiocolchicines as ligands for glycine and γ-aminobutyric acid receptors

Article abstract of DOI:10.1021/jm061056s

New 3-O-glycosyl-3-demethylthiocolchicines containing natural and unnatural sugar moieties were prepared and tested on γ-aminobutyric acid (GABA) and strychnine-sensitive glycine receptors present in rat brain and spinal cord. Two different synthetic approaches were used with the readily available 3-O-demethylthiocolchicine (1b) and thiocolchicoside (2a). Glycosyl compounds 2a-g were obtained from 1b and 1-fluorosugars 4. 6′-Heterosubstituted glycosyl compounds 6-12 and the 6′-desoxy derivative 2h were prepared from 2a.

Full text of DOI:10.1021/jm061056s

J. Med. Chem. 2007, 50, 2245-2248
2245
Novel 3-O-Glycosyl-3-demethylthiocolchicines as Ligands for Glycine and γ-Aminobutyric Acid
Receptors
Maria Luisa Gelmi,*,§ Gabriele Fontana,‡ Donato Pocar,§ Guido Pontremoli,§ Sara Pellegrino,§ Ezio Bombardelli,‡
Antonella Riva,‡ Walter Balduini,† Silvia Carloni,† and Mauro Cimino†
Istituto di Chimica Organica “A. Marchesini”, Facolta` di Farmacia, UniVersita` di Milano, Via Venezian 21, 20133 Milano, Italy, Indena
S.p.a., Via Ortles 19, Milano, Italy, and Istituto di Farmacologia e Farmacognosia, Facolta` di Farmacia, UniVersita` di Urbino “Carlo Bo”,
Via S. Chiara 27, 61029 Urbino, Italy
ReceiVed September 5, 2006
New 3-O-glycosyl-3-demethylthiocolchicines containing natural and unnatural sugar moieties were prepared
and tested on γ-aminobutyric acid (GABA) and strychnine-sensitive glycine receptors present in rat brain
and spinal cord. Two different synthetic approaches were used with the readily available 3-O-demethylth-
iocolchicine (1b) and thiocolchicoside (2a). Glycosyl compounds 2a-g were obtained from 1b and
1-fluorosugars 4. 6′-Heterosubstituted glycosyl compounds 6-12 and the 6′-desoxy derivative 2h were
prepared from 2a.
Introduction
Thiocolchicoside (2a, Scheme 1) is a semisynthetic compound
obtained from natural colchicine, the main alkaloid of Colchicum
autumnale. In contrast to the parent thiocolchicine (1a), which
is characterized by antimitotic activity,1 2a is a muscle relaxing
agent and an anti-inflammatory drug1 whose action is due, at
least in part, to the activation of γ-aminobutyric acid (GABA)
receptors1 and to the inhibition of the tonic seizures induced
by picrotoxin. Recently, it has been proposed that the activity
Figure 1. Thiocolchicine derivatives.
of 2a may also be due to interaction with strychnine-sensitive
glycine receptors.1 2a generates a variety of metabolites,2 and
6, 8a, 9, 10, and 12 were tested for their interaction with [3H]-
two of them, the glucoronide3 and 1b, are reported to contribute
strychnine and [3H]muscimol binding sites in rat spinal cord
and cerebral cortex.
to the pharmacological activity of 2a, albeit the drug is much
more potent in vitro. Furthermore, it is not clear if the
glucuronide is generated by glucuronization of 1b or oxidation
of 2a itself. However, the rate of formation of the metabolites
Chemistry
Methods for the glycosylation of phenol derivatives with
activated sugars were studied,9-11 but some of them cannot be
applied to complex compounds where other functional groups
may be degraded. In particular, the reaction of 1b and aceto-
bromoglucose gives 2a in poor yield (22%).12 Our efforts on
the glycosidation reaction of 1b led to a significant improvement
of the glycosidation cited above with control of the stereochem-
istry. According to our knowledge,9 we performed the reaction
of 1,2,3,4,6-O-pentaacetyl-â-D-glucopyranose with the O-stannyl
derivative of 1b but the reaction failed. We then switched to
the coupling of 1b with glycosyl fluorides 4, known to be easy
to handle and usually used to promote high diastereoselec-
tion.13,14 The reaction of glucopyranosyl fluoride 4a with 1b in
MeCN in the presence of BF3‚Et2O and of 1,1,3,3-tetrameth-
ylguanidine (TMG)13 gave 5a (97%) as a single â-diastereomer
(Scheme 1). The deprotection of the hydroxy groups was done
with NaOH in EtOH, affording pure thiocolchicoside (â-2a)
(Scheme 1). Under ”one-pot“ conditions, â-2a was obtained
from 1b in 97% yield. This synthetic protocol allowed us to
prepare a series of thiocolchicine derivatives 2 (Scheme 1) by
reacting 1b with glycosyl fluorides 4: two hexoses (D-mannose
(4b) and D-galactose (4c)), the 6-deoxyhexose-L-rhamnose (4d),
and three pentoses (D-xylose (4e), D-arabinose (4f), D-lyxose
(4g)). Single â-diastereomers 5b-e were obtained from gly-
copyranosyl fluorides 4b-e. In the case of 4f and 4g, a mixture
of epimers 5f (â/R, 1 : 6) and 5g (â/R, 1 : 3) was formed,
seems to be crucial for the half-life of the drug and its overall
activity. Till now, few examples of thiocolchicoside analogues
have been reported.1-8 For this reason, we recently1 started a
program for the structural modification of 2a aimed at modulat-
ing the cleavability of the glycosidic bond and the susceptibility
of the glycosidic residue to oxidation. Our aim was to modulate
the glycoside residue diversity preserving the potency of the
parent drug. A first approach had been oriented to obtain
analogues of 2a, compounds 3, by coupling amino compound
1c and the corresponding glycosyl derivatives (Figure 1). The
displacement assay of [3H]strychnine and [3H]muscimol in rat
spinal cord (sc) and cerebral cortex (cx) showed mild but
correlated changes to the activity of the parent compound. We
here report the findings obtained by coupling 1b and a few
natural or unnatural sugars.
Two synthetic strategies were used with the readily available
1b and 2a. The reaction of 1b with sugars 4a-g gave the 3-O-
glycosyl derivatives 2a-g (Scheme 1). A series of 6′-hetero-
substituted compounds 6-12 and the 6′-desoxy derivative 2h
were prepared by taking advantage of the selective reactivity
of the 6′ hydroxy group of 2a (Scheme 2). Compounds 2a-h,
* To whom correspondence should be addressed. Phone: +390250314481.
Fax: +390250314476. E-mail: marialuisa.gelmi@unimi.it.
§ Universita` di Milano.
‡ Indena S.p.a.
† Universita` di Urbino “Carlo Bo”.
10.1021/jm061056s CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/06/2007

2246 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9
Brief Articles
Scheme 1. Synthesis of 3-O-Glycosyl-3-demethylthiocolchicines 2
Scheme 2. Synthesis of 3-O-Glycosyl-3-demethylthiocolchicines 6-12, 5h, 2h
respectively. The oxygen atoms were directly deprotected, and
5b-e were transformed into the corresponding pure â-glycosyl
derivatives 2b (70%), 2c (70%), 2d (70%), and 2e (70%) and
into the mixture of â,R-anomers 2f (77%) and 2g (77%). The
stereochemical outcome of the reaction of 1b and 4 is dependent
on the relative stereochemistry of 1,2-substituents of the fluoro
sugar. According to the mechanism proposed by Yamaguchi,13
the 1,2-trans derivatives were formed as single isomers in the
case of sugars 4a,c,e, having an axial fluorine and an equatorial
2-acetoxy group. In the case of 4f and 4g, where the above
groups are in the equatorial and axial positions, a small amount
of the 1,2-cis epimer was also detected, via oxonium intermedi-
ate.13 The 1,2-cis isomers were formed using sugars 4b and 4d
in which the above groups are in the trans position. In this case,
acetonitrile can form complexes with the oxonium cation, thus
affecting the orientation of the incoming O-nucleophile.15
The second part of this research was devoted to the direct
functionalization of the C-6′ position of 2a with heteroatoms
to give 6-12 and to prepare the 6′-desoxy derivative 2h
(Scheme 2). The reaction of 2a with bis(2-methoxyethyl)-
aminosulfur trifluoride (DAST) in CH2Cl2 at -40 °C gave the
6′-fluoro derivative 6 (40%). The reaction of 2a with p-
toluenesulfonyl chloride in CH2Cl2 and triethylamine (TEA) in
the presence of a catalytic amount of p-dimethylaminopyridine
(DMAP) afforded 7a (75%). The iodo derivative 8a (77%) was
obtained from 7a by using NaI in 2-butanone at 80 °C (16 h).
The reaction of 7a with NaN3 in DMF (80 °C, 16 h) gave the
azido derivative 9 (75%). Its reduction with triphenylphosphine
in dioxane/MeOH afforded the 6′-amino derivative 10 (22%)
as the hydrochloride.
The more stable peracetyl tosylate 7b (65%) was obtained
by a ”one-pot“ procedure from 2a, via tosylate 7a, which was
then protected with Ac2O, TEA, and DMAP (catalytic amount).
The 6′-thioacetyl derivative 11 (70%) was prepared from 7b
with potassium thioacetate in MeCN. The deprotection of both
the oxygen and sulfur atoms with 1 N NaOH in EtOH at 25 °C
(1 h) gave disulfide 12 (45%). The thiol derivative was not
detected. Starting from the iodo derivative 8b, obtained in 65%
yield by reacting 7b with NaI in 2-butanone, the 6′-deoxy
derivative 5h was prepared by reduction with tributyltin hydride
and 2,2-azobisisobutyrylnitrile (AIBN) in CH2Cl2 at reflux. Its
hydrolysis with 1 N NaOH in EtOH gave 2h (65% overall
yield).
Effect of 3-O-Glycosyl-3-demethylthiocolchicines 2a-h, 6,
8a, 9, 10, and 12 on the Binding of [3H]Strychnine and [3H]-
Muscimol. The ability of the novel 3-O-glycosyl-3-demeth-
ylthiocolchicines to interact with strychnine-sensitive glycine
and GABA receptors, localized in rat sc and cx, was evaluated
in competition studies using synaptic membranes and the
selective radioligands [3H]strychnine and [3H]muscimol. Dis-

Brief Articles
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9 2247
Table 1. Relative Potency (IC50 (µM)) of
in anhydrous MeCN (100 mL). TMG (0.83 mL, 6.6 mmol) was
added and then BF3‚Et2O (2.2 mL, 17.6 mmol) (TLC, CH2Cl2/
MeOH, 10:2). After 2.5 h, aqueous NaHCO3 (30 mL) was added
and the solution was extracted with AcOEt (2 × 20 mL). The
organic layer was washed with aqueous KHSO4 (50 mL) and brine
(50 mL) and dried over MgSO4. As an example, 5a (830 mg, 97%)
was isolated as yellow solid after chromatography (SiO2, CH2Cl2/
MeOH, 1:0 to 0:1).
3-O-Glycosyl-3-demethylthiocolchicines 2a-h, 6, 8a, 9, 10, and 12
[3H]strychnine
(sc)
[3H]muscimol
(sc)
[3H]muscimol
(cx)
strychnine
glycine
GABA
â-2a
â-2b
â-2c
0.014 ( 0.001
15.8 ( 0.5
0.015 ( 0.002
1.9 ( 0.5
4.3 ( 0.8
5.9 ( 0.9
4.2 ( 1.1
5.0 ( 0.3
2.5 ( 0.6
4.0 ( 1.2
3.5 ( 1.1
2.0 ( 0.5
8.0 ( 1.0
8.2 ( 2.3
14.2 ( 0.5
7.8 ( 0.5
0.024 ( 0.003
3.4 ( 0.1
3.8 ( 1.2
8.3 ( 2.8
2.2 ( 0.3
3.99 ( 1.2
3.1 ( 1.3
5.6 ( 1.3
6.2 ( 0.8
3.8 ( 0.6
17.3 ( 0.8
7.8 ( 2.0
10.7 ( 1.9
>100
1.5 ( 0.1
2.3 ( 0.6
2.6 ( 0.4
3.0 ( 1.0
2.6 ( 0.6
0.5 ( 0.1
0.6 ( 0.04
1.9 ( 0.1
1.3 ( 0.1
>100
General Procedure for the Deacetylation Reaction. Synthesis
of Compounds 2a-g. Compound 5 (0.5 mmol) was dissolved in
EtOH (4 mL), and 1 N NaOH (2 mL) was added. The solution
was stirred at 25 °C (3 h). Compound 2 was obtained as yellow
solid after chromatography (SiO2, CH2Cl2/MeOH, 1:0 to 0:1) and
crystallization (MeOH/iPr2O). Yields are given on the starting
compound 1b.
â-2d
â-2e
2fa
2ga
â-2h
â-6
â-8a
â-9
â-10
â-12
â-2a: yield 97%; mp 259 °C (mp 220 °C, lit.16); [R]25 -253°
D
(c 1.0, MeOH).
4.3 ( 0.9
4.7 ( 1.4
4.7 ( 0.3
â-2b: yield 47%; mp 210-212 °C; [R]25D -182° (c 0.4, MeOH).
â-2c: yield 44%; mp 214-216 °C; [R]25D -168° (c 0.4, MeOH).
r-2d: yield 49%; mp 165-168 °C; [R]25D -220° (c 0.4, MeOH).
a Mixture of anomers.
â-2e: yield 43%; mp 193 °C; [R]25 -201° (c 1.0, MeOH).
D
2f: mixture of anomers (â/R, 1:6; 55%).
placement curves were constructed for each product and
compared with the potency of strychnine, glycine, GABA, and
2a. Representative competition studies illustrating the ability
of increasing concentrations of selected compounds to displace
[3H]strychnine and [3H]muscimol from their specific binding
sites in the two regions of the central nervous system (CNS)
are in Figure FS1 of Supporting Information. The quantitative
evaluation of the displacement curves obtained by the calculation
of the specific IC50 is reported in Table 1. The data demonstrate
that all the compounds were able to compete with both receptors.
On [3H]strychnine binding, all compounds except 8a showed a
good potency, which like 2a is in the low micromolar range.
Exceptions to this pattern were 2f and 2g, which appear to be
the most potent compounds (IC50 is 3-fold lower than for 2a).
Therefore, the absence of a substituent on C-5 gave a notable
improvement to the activity. In [3H]muscimol binding, the
compounds maintain the activity of 2a with the exception of
8a, 10, and 12, which are at least 1 order of magnitude less
active than 2a. Compound 6, in which 6′-OH has been replaced
for a fluorine, was equipotent to 2a on all the receptors. Such
a result is of great interest because the activity has been
maintained while abolishing the susceptibility to direct oxidation
to the glucuronyl derivative, making 6 a good candidate for
further development.
2g: mixture of anomers (â/R, 1:3; 53%).
6′-Fluorothiocolchicoside 6. At -40 °C under N2 and stirring,
DAST (0.14 mL, 1.03 mmol) was added in 30 min to a suspension
of 2a (100 mg, 0.17 mmol) in dry CH2Cl2 (2.0 mL). The mixture
was stirred (30 min, -40 °C) and then at 25 °C (3 h) (TLC, CH2-
Cl2/MeOH, 10:2). After the mixture was cooled at -20 °C, MeOH
(0.5 mL) and NaHCO3 (100 mg) were added. The solvent was
evaporated, and the mixture was chromatographed (SiO2, CH2Cl2/
MeOH, 1:0 to 0:1) to afford pure 6 (39 mg, 40%) as a yellow
solid: mp 178-180 °C (MeOH/iPr2O); [R]25 -197° (c 0.5,
D
MeOH).
6′-Tosylthiocolchicoside 7a. Under N2 and stirring at 0 °C, 2a
(1 g, 1.72 mmol), TEA (0.5 mL, 3.61 mmol), DMAP (315 mg,
3.15 mmol), and TsCl (380 mg, 1.99 mmol) were added to dry
pyridine (15 mL). After 20 h at 25 °C, the solvent was evaporated
and the residue was dissolved in AcOEt/BuOH (3:1, 15 mL),
washed with H2O (3 × 10 mL), aqueous NaHCO3 (3 × 10 mL),
and brine (3 × 10 mL), and then dried over MgSO4. Yield 924
mg, 75%; mp 155 °C (MeOH/iPr2O); [R]D -84.3° (c 0.3, MeOH).
6′-Tosylthiocolchicoside Triacetate 7b. To a solution of 7a (see
above) were added TEA (2.2 mL, 15.78 mmol), DMAP (200 mg,
1.64 mmol), and Ac2O (2.5 mL, 24.49 mmol) under N2 at 25 °C.
After 1 h, CH2Cl2 (30 mL) was added and the organic layer washed
with H2O (3 × 15 mL), aqueous NaHCO3 (3 × 15 mL), and brine
(20 mL) and then dried over MgSO4. After chromatography (SiO2,
CH2Cl2/MeOH, 1:0 to 0:1), 7b (890 mg, 65%) was obtained: mp
142-144 °C (iPr2O); [R]25 -94.2° (c 0.58, MeOH).
D
Conclusions
6′-Iodothiocolchicoside 8a. NaI (32 mg, 0.214 mmol) and 7a
(110 mg, 0.142 mmol) in 2-butanone (1 mL) was heated at 80 °C
(16 h). The solvent was evaporated and the mixture chromato-
graphed (SiO2, CH2Cl2/MeOH, 1:0 to 0:1), giving 8a (70 mg, 77%)
as a yellow solid: mp 110-112 °C (MeOH/iPr2O); [R]D -61.5°
(c 0.52, MeOH).
Two series of new 3-glycosyl-3-O-demethylthiocolchicines
containing natural and unnnatural sugar moieties were prepared
from 1b and 2a. The condensation of 1b with 1-fluorosugars 4
afforded 3-O-glycosylthiocolchicines 2a-g in good yield and
with high diastereocontrol. By functionalization of the C-6′
position of 2a, 6′-heterosubstituted compounds 6-12 and the
6′-desoxy derivative 2h were prepared. Their biological activities
were evaluated on GABA and strychnine-sensitive glycine
receptors present in rat brain and spinal cord. Arabinosyl, lixosyl,
and 6′-fluoro derivatives 2f, 2g, and 6 display a higher activity
on the binding of [3H]strychnine and a similar potency on the
binding of [3H]muscimol compared with 2a, suggesting that the
new compounds may be considered good candidates for further
studies aimed at elucidating their involvement in myorelaxation.
6′-Iodothiocolchicoside Triacetate 8b. Pure 8b (341 mg, 65%)
was prepared as described for 8a from 7b (533 mg, 0.67 mmol)
and NaI (120 mg, 0.80 mmol) in 2-butanone (3 mL) (6 h) after
chromatography (SiO2, CH2Cl2/MeOH, 5:1): mp 216-218 °C
(iPr2O); [R]D -95.8° (c 0.5, CHCl3).
6′-Azidothiocolchicoside 9. 7a (500 mg, 0.69 mmoli) and NaN3
(90 mg, 1.39 mmol) in DMF (5 mL) were heated at 80 °C (16 h).
After cooling, the mixture was taken up with AcOEt/BuOH (3:1,
10 mL), washed with H2O (3 × 10 mL), aqueous NaHCO3 (3 ×
10 mL), and brine (3 × 10 mL), and dried over Na2SO4. Compound
9 (300 mg, 75%) was obtained as a yellow solid after chromatog-
raphy (SiO2, CH2Cl2/MeOH, 1:0 to 0:1) and crystallization: mp
165-167 °C (MeOH/iPr2O); [R]D -141.3° (c 0.4, MeOH).
6′-Aminothiocolchicoside Hydrochloride 10. To a solution of
PPh3 (389 mg, 1.48 mmol) and 9 (300 mg, 0.49 mmol) in dioxane/
MeOH (4.2 mL/0.8 mL) at 25 °C, after 1 h, NH4OH (1.1 mL, 35%)
Experimental Section
General Procedure for the Condensation of 3-O-Demethyl-
thiocolchicine 1b with Fluorosugars 4a-g. At 25 °C, under N2
and stirring, 1b (2.2 mmol) and sugar 4 (3.3 mmol) were suspended

2248 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9
Brief Articles
was added. After 12 h, the solvent was evaporated and 1 N HCl (5
mL) was added. The aqueous solution was washed with toluene (3
× 5 mL) and evaporated, and the crude was taken up with benzene
(3 × 5 mL) until dryness was obtained. Compound 10 (60 mg,
22%) was isolated as an orange solid: mp 148-150 °C (MeOH/
iPr2O).
6′-Thioacetylthiocolchicoside Triacetate 11. To a solution of
7b (650 mg, 0.82 mmol) in dry MeCN (10 mL) under N2 at 0 °C,
MeCOSK (298 mg, 2.63 mmol) was added. The suspension was
refluxed for 2 h. After cooling, CH2Cl2 (30 mL) was added and
the solution washed with brine (3 × 20 mL) and H2O (3 × 20 mL)
and dried over MgSO4. Chromatography on silica gel (CH2Cl2/
MeOH, 1:0 to 0:1) gave pure 11 (477 mg, 70%) as a red-orange
solid: mp 140-142 °C (iPr2O); [R]D -117.5° (c 0.4, CHCl3).
6′-Disulfurthiocolchicoside 12. Compound 11 (300 mg, 0.36
mmol) and 1 N NaOH (1.5 mL) in EtOH (2 mL) was stirred at 25
°C (1 h). The mixture was chromatographed (SiO2, CH2Cl2/MeOH,
1:0 to 0:1) to give 12 (178 mg, 45%): mp 260 °C (MeOH/iPr2O);
[R]D -262.5° (c 0.16, MeOH).
6′-Desoxythiocolchicoside 2h. To a solution of 8b (100 mg, 0.12
mol) and AIBN (62 mg, 0.37 mol) in CH2Cl2 (4 mL) at reflux,
Bu3SnH (4 × 0.15 mL) was added (4 times in 2 h), and heating
was continued (15 h). After solvent evaporation 5h was obtained
and then stirred in EtOH (1 mL) and 1 N NaOH (1 mL) for 1 h.
Compound 2h (55 mg, 65%) was obtained as a yellow solid after
chromatography (SiO2, CH2Cl2/MeOH, increasing polarity): mp
185-187 °C (MeOH/iPr2O); [R]D -223° (c 0.36, MeOH).
[3H ]Strychnine Binding. The synaptosomal membrane fraction
from rat sc17 was stored at -80 °C and used within 2 weeks. The
binding assay was performed in a final volume of 1.2 mL of 50
mM sodium-potassium phosphate buffer (pH 7.1) containing 2
nM [3H]strychnine (New England Nuclear, specific activity of 25.7
Ci/mmol), increasing concentrations of cold strychnine, glycine,
or colchicoside derivatives, and membranes at a final protein
concentration of 0.2-0.4 mg/1.2 mL. The assay was carried out at
4 °C for 10 min and rapidly filtered through Watman GF-B glass
fiber filters. The filters were rapidly rinsed with NaCl (5 mL, 0.15
M). Nonspecific binding was determined in the presence of 0.1
mM unlabeled strychnine.
of elemental analysis results for 2 and 6-12. This material is
available free of charge via the Internet at http://pubs.acs.org.
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culture of Centella asiatica. Phytochemistry 1993, 33, 817-820.
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[3H]Muscimol Binding. The interaction of 2a with GABAA
receptors was tested in a [3H]muscimol binding assay.18 Membranes
were obtained by sc brainstem and cx of adult Sprague-Dawley
rats19 and incubated with 5 nM [3H]muscimol and increasing
concentrations of GABA, 2a, or other colchicosides, in 50 mM
Tris-citrate buffer, pH 7.1, in a final volume of 1 mL. After 30
min of incubation at 4 °C, the samples were rapidly filtered through
Watman GF-B glass fiber filters and washed (three times) with
ice-cold buffer (5 mL). Nonspecific binding was determined in the
presence of 200 µM unlabeled GABA. Radioactivity was deter-
mined with a Wallach 1409 liquid scintillator counter with 50%
efficiency.
Protein Determination. Protein content was determined by using
the Bradford dye-binding procedure from Bio-Rad Laboratories.
Data Analysis. Each experimental point was run in triplicate,
and each displacement curve used for the determination of IC50
represents the average of three curves obtained from three
independent experiments. Results were analyzed by nonlinear fitting
using the computer program Prism (GrapPad Software). The F test
was used to assess if the fitting using a two-site model equation
was better (P < 0.05) than that obtained with a one-site model.
(19) Balduini, W.; Cimino, M.; Depoortere, H.; Cattabeni, F. Characteriza-
tion of [3H]thiocolchicoside binding sites in rat spinal cord and
cerebral cortex. Eur. J. Pharmacol. 1999, 376, 149-157.
Supporting Information Available: Figure FS1 of binding
assay results, spectroscopic data for 2 and 5-12, and Table TS1
JM061056S