Synthesis and monoamine oxidase B substrate properties of 1-methyl-4-heteroaryl-1,2,3,6-tetrahydropyridines

Article abstract of DOI:10.1016/S0968-0896(99)00225-4

Six analogues of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine [MPTP, (1)Scheme 1MAO-B catalyzed oxidation of MPTP.] bearing various heteroaryl groups at C-4 were synthesized and examined for their monoamine oxidase B substrate properties. The C-4 substituents include the 1-ethylpyrrol-2-yl, 1-propylpyrrol-2-yl, 1-isopropylpyrrol-2-yl, 1-cyclopropylpyrrol-2-yl, 3-ethylfuran-2-yl and 3-ethylthien-2-yl groups. The results provide information concerning steric and polar interactions between the C-4 substituent and the active site of MAO-B that are transmitted to the position of oxidation at C-6 of the tetrahydropyridinyl moiety. Copyright (C) 1999 Elsevier Science Ltd.

Full text of DOI:10.1016/S0968-0896(99)00225-4

Bioorganic & Medicinal Chemistry 7 (1999) 2835±2842
Synthesis and Monoamine Oxidase B Substrate Properties of
1-Methyl-4-heteroaryl-1,2,3,6-tetrahydropyridines
Jian Yu and Neal Castagnoli Jr.*
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212, USA
Received 25 March 1999; accepted 16 July 1999
AbstractÐSix analogues of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine [MPTP, (1)] bearing various heteroaryl groups at C-4
were synthesized and examined for their monoamine oxidase B substrate properties. The C-4 substituents include the 1-ethylpyrrol-
2-yl, 1-propylpyrrol-2-yl, 1-isopropylpyrrol-2-yl, 1-cyclopropylpyrrol-2-yl, 3-ethylfuran-2-yl and 3-ethylthien-2-yl groups. The
results provide information concerning steric and polar interactions between the C-4 substituent and the active site of MAO-B that
are transmitted to the position of oxidation at C-6 of the tetrahydropyridinyl moiety. # 1999 Elsevier Science Ltd. All rights
reserved.
Introduction
o-propylphenyl (6a, 86), o-isopropylphenyl (7a, 51)] are
poorer substrates, suggesting speci®c steric constraints in
the active site.8 The observation that the kcat/Km values for
the o-chlorophenyl analogue (8a, 1353) and o-methoxy-
phenyl analogue (9a, 233) are similar to the values for
the o-methylphenyl (4a) and o-ethylphenyl (5a) analogues,
respectively, argues that electronic factors may not play
a signi®cant role in determining the substrate properties
in this series of compounds. In general, the neurotoxicity
of these analogues correlated well with their MAO-B
substrate properties.17±19
The ¯avoenzyme monoamine oxidase B (MAO-B) cata-
lyzes the a-carbon oxidative deamination of a variety of
acyclic amines including the biogenic amine neuro-
transmitters.1±4 Some cyclic tertiary amines, such as the
parkinsonian inducing neurotoxin 1-methyl-4-phenyl-
1,2,3,6-tetrahydropyridine [MPTP (1)], are also good
MAO-B substrates.5±8 This bioactivation reaction gen-
erates the corresponding dihydropyridinium intermediate
2 which subsequently is oxidized to the pyridinium meta-
bolite MPP+ (3), the ultimate toxin that mediates the
neurodegeneration of dopaminergic nigrostriatal neurons
(Scheme 1).9±12
We have observed similar e€ects with a series of 1-
methyl-4-heteroaryl-1,2,3,6-tetrahydropyridinyl analo-
gues (Chart 1).13,20,21 For example, replacement of
hydrogen with a methyl group on an atom one bond
away from the point of attachment of a 2-pyrrolyl
(10a versus 11a), 2-furanyl (16a versus 17a) or 2-thie-
nyl (19a versus 20a) system leads to dramatic increases
(20 to 55 times) in kcat/Km values. In order to gain a
better understanding of the in¯uence of such steric
factors on substrate activity, we have extended this
investigation to additional 1-methyl-4-heteroaryl-
1,2,3,6-tetrahydropyridinyl derivatives. The structures
of the compounds examined in this study and their
MAO-B generated dihydropyridinium metabolites are
shown in Chart 1. The newly synthesized analogues
are 1-methyl-4-(1-ethylpyrrol-2-yl)-1,2,3,6-tetrahydro-
pyridine (12a), 1-methyl-4-(1-propylpyrrol-2-yl)-1,2,3,6-
tetrahydropyridine (13a), 1-methyl-4-(1-isopropyl-pyr-
rol-2-yl)-1,2,3,6-tetrahydropyridine (14a), 1-methyl-4-
(1-cyclopropylpyrrol-2-yl)-1,2,3,6-tetrahydropyridine (15a),
Extensive structure±activity relationship studies on the
biotransformation of 1-methyl-4-aryl-1,2,3,6-tetrahydro-
pyridinyl derivatives (structures a, Chart 1) to yield the
corresponding dihydropyridinium metabolites (structures
b, Chart 1) have been conducted in attempts to understand
the unique features of these cyclic tertiary allylamines that
lead to their unexpected MAO-B substrate properties and
neurotoxicity.8,13±16 Youngster et al. examined changes in
the MAO-B substrate properties of MPTP analogues
bearing substituents on the phenyl ring.8,17±19 Introducing
an o-methyl substituent (4a, kcat/Km=1275 min 1 mM
leads to an increase in kcat/Km relative to MPTP (523)
whereas larger alkyl substituents [o-ethylphenyl (5a, 295),
1
)
* Corresponding author. Tel.: +1-540-231-8202; fax: +1-540-231-
8890; e-mail: nacastagnoli@chemserver.chem.vt.edu
0968-0896/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(99)00225-4

2836
J. Yu, N. Castagnoli Jr. / Bioorg. Med. Chem. 7 (1999) 2835±2842
1-methyl-4-(3-ethylfuran-2-yl)-1,2,3,6-tetrahydropyridine
(18a) and 1-methyl-4-(3-ethylthien-2-yl)-1,2,3,6-tetra-
hydropyridine (21a).
trioxide with pyridine in dichloromethane to give 4-oxo-
4-(4-pyridyl)butanal (26) and 3-ethyl-4-oxo-4-(4-pyr-
1
idyl)butanal (27), respectively.24 The H NMR and 13C
NMR spectra indicate that 24 and 25 exist in equilibria
with their corresponding enol forms at room tempera-
ture. These mixtures were used directly in the subsequent
conversions.
Results and Discussion
Chemistry
The general synthetic pathways for the preparation of
the 4-(1-alkylpyrrol-2-yl)pyridines, 4-(3-ethylfuran-2-
yl)pyridine and 4-(3-ethylthien-2-yl)pyridine are given
in Schemes 3 and 4. Parr±Knorr cyclization25±27 of
4-oxo-4-(4-pyridyl)butanal (26) with the appropriate
amine furnished the desired 4-(1-alkylpyrrol-2-yl)pyr-
idines 28±31. Treatment of 28±31 with iodomethane
gave the corresponding N-methylpyridinium iodides 32±
35. These intermediates subsequently were reduced with
NaBH4 to give the desired 1,2,3,6-tetrahydropyridines
12±15 which were characterized as their oxalate salts
(Scheme 3).
Syntheses of the target compounds proceeded via the
corresponding 1,4-dicarbonyl precursors which were
prepared as shown in Scheme 2. Treatment of 4-bro-
mopyridine (22) with n-butyllithium at 78ꢀC in dry
diethyl ether followed by reaction of the resulting lithi-
ated intermediate with g-butyrolactone and a-ethyl-g-
butyrolactone,22,23 yielded the corresponding g-hydro-
xyalkylketones 24 and 25, respectively. These inter-
mediates were oxidized by the complex of chromium
Treatment of 3-ethyl-4-oxo-4-(4-pyridyl)butanal (27)
with sulfuric acid or phosphorus pentasul®de28 a€orded
4-(3-ethylfuran-2-yl)pyridine (36) or 4-(3-ethylthien-2-
yl)pyridine (37), respectively. Methylation of these pyr-
idines with iodomethane gave the corresponding N-
methylpyridinium iodides 38 and 39. Treatment with
NaBH4 converted the pyridinium intermediates to the
desired 1,2,3,6-tetrahydropyridines 18 and 21, which
were characterized as their oxalate salts (Scheme 4).
Scheme 1. MAO-B catalyzed oxidation of MPTP.
Chart 1. Structures of 1-methyl-4-aryl-1,2,3,6-tetrahyropyridines (a) and 1-methyl-4-aryl-2,3-dihydropyridinium species (b) discussed in the text.
Scheme 2. The synthesis of 1,4-dicarbonyl precursors 26 and 27. Reagents and conditions: (a) n-BuLi, Et2O, 78ꢀC, 1 h; (b) g-butyrolactone or a-ethyl-g-
butyrolactone, 78ꢀC, 3 h, then room temperature 1 day; (c) aq Na2CO3 solution, room temprature; (d) CrO3/pyridine, CH2Cl2, room temperature.

J. Yu, N. Castagnoli Jr. / Bioorg. Med. Chem. 7 (1999) 2835±2842
2837
Enzymology
footnotes. The plots of the initial velocities versus sub-
strate concentrations were linear in all cases, as were the
corresponding double-reciprocal plots from which kcat
and Km were calculated (Table 1). The kcat/Km values
used to estimate the substrate properties of this series of
The substrate properties of the newly synthesized tetra-
hydropyridines 12±15, 18 and 21 (50±500 mM) were
examined using MAO-B (0.09 mM) puri®ed from beef
liver.13 In each case the time dependent increase of a
chromophore consistent with that expected for the cor-
responding dihydropyridinium metabolite was observed
(see Chart 1 and Table 1). The enzyme kinetic para-
meters of the test compounds were then determined by
estimating the initial rates of formation (®rst 120 s) of
the dihydropyridinium metabolites at initial substrate
concentrations that bracketed the Km for that substrate.
Since not all of the dihydropyridinium metabolites were
available as synthetic standards, rates of product for-
mation were approximated using e values for the struc-
turally related compounds identi®ed in the Table 1
1
compounds ranged from 47 to 11,000 (min mM 1).
Arguments supporting both a hydrogen atom transfer
pathway (HAT: A ! C ! D)5±7 and a single elec-
tron transfer pathway (SET: A ! B ! C ! D)4,8±10
have been advanced to characterize MAO-B catalysis
(Scheme 5). Since a normal kinetic deuterium isotope
e€ect is observed on the MAO-B catalyzed oxidation of
MPTP to MPDP+ (Scheme 1), the rate determining
step for this transformation must involve carbon±
hydrogen bond cleavage. Consequently, resonance sta-
bilization of C via conjugation with the C-4 substituent
Scheme 3. Synthesis of 1-methyl-4-(1-alkylpyrrol-2-yl)-1,2,3,6-tetrahydropyridines (12±15). Reagents and conditions: (a) RNH2, CH3OH, room
temperature, 12 h ; (b) CH3I, room temperature; (c) NaBH4, CH3OH, 0ꢀC; (d) H2C2O4, Et2O, room temperature.
Scheme 4. Synthesis of 1-methyl-4-(3-ethylfuran-2-yl)-1,2,3,6-tetrahydropyridine (18) and 1-methyl-4-(3-ethylthien-2-yl)-1,2,3,6-tetrahydropyridine
(21). Reagents and conditions: (a) H2SO4, H2O/THF, 55ꢀC; (b) P2S5, toluene, 100ꢀC; (c) CH3I, room temperature; (d) NaBH4, methanol, 0ꢀC; (e)
H2C2O4, Et2O, room temperature.
Table 1. Kinetic parameters for the MAO-B catalyzed oxidations of 1-methyl-4-heteroaryl-1,2,3,6-tetrahydropyridines
1
1
1
C-4 substituent
l
max (nm)d
log P
kcat (min)
Km (mM)
kcat/Km (min mM
)
2-C4HNH (10a)a
424
420
420
421
422
420
384
399
399
386
400
393
1.27
1.73
2.26
2.79
2.61
1.62
1.90
2.36
2.89
2.40
2.87
3.40
85
360
360
209
121
418
31
348
336
60
1.80
0.20
0.16
0.38
1.28
0.14
0.20
0.04
0.03
0.20
0.05
0.04
4720
180013
2200
557
94
2-C4H3NCH3 (11a)a
2-C4H3NC2H5 (12a)a
2-C4H3NC3H7 (13a)a
2-C4H3NCH(CH3)2 (14a)a
2-C4H3NCH(CH2)2 (15a)a
2-C4H4O (16a)b
2878
15513
860021
11,000
30013
669021
4892
2-C4H2(3-CH3)O (17a)b
2-C4H2(3-C2H5)O (18a)b
2-C4H4S (19a)c
2-C4H2(3-CH3)S(20a)c
2-C4H2(3-C2H5)S (21a)c
337
222
a
Estimated using e=24,000 M 1, the value for the perchlorate salt of the 1-methyl-4-(1-methylpyrrol-2-yl)-2,3-dihydropyridinium species.13
Estimated using e=23,000 M 1, the value for the perchlorate salt of the 1-methyl-4-(1-furanyl)-2,3-dihydropyridinium species.13
Estimated using e=18,800 M 1, the value for the perchlorate salt of the 1-methyl-4-(1-thien-2-yl)-2,3-dihydropyridinium species.13
The lmax value of the corresponding dihydropyridinum species.
b
c
d

2838
J. Yu, N. Castagnoli Jr. / Bioorg. Med. Chem. 7 (1999) 2835±2842
Scheme 5. Proposed pathways for the MAO-B catalyzed oxidation of MPTP and analogues.
could lead to a decrease in ÁGy and increase in kcat. As a
possible indication of such stabilization, we have com-
Experimental
pared the lmax values and the relative rates of formation
for each of the dihydropyridinium metabolites. As shown
in Table 1, no meaningful correlation exists between these
two parameters. This analysis prompted a closer con-
sideration of polar and/or steric interactions as potentially
important factors in¯uencing the MAO-B substrate
properties of these tetrahydropyridinyl derivatives.
Caution! 1,4-Disubstituted tetrahydropyridines such as
MPTP are known or potential nigrostriatal neurotoxins
and should be handled using disposable gloves in a
properly ventilated hood. Detailed procedures for the
safe handling of MPTP have been reported.29
Chemistry
The kcat/Km values of the 4-pyrrol-2-yl analogues bear-
ing a methyl (11a), ethyl (12a) or cyclopropyl (14) group
on nitrogen are much larger than that observed with the
NH analogue 10a. These values follow the same trend
observed with the corresponding log P values. A simi-
lar, but less dramatic, correlation between log P and
kcat/Km is observed with the pyrrolyl (10a), furanyl (16a)
and thienyl (19a) set of analogues. These results suggest
that the region of the active site occupied by the phenyl
group of MPTP is a liphophilic pocket that can accom-
modate small substituents on the nitrogen atom of the
pyrrolyl moiety. Larger substituents, however, are not
well tolerated since the N-propyl (14a) and N-isopropyl
(15a) analogues are poorer MAO-B substrates than the
N-methyl, N-ethyl and N-cyclopropyl analogues even
though their log P values are higher.
All reagents were obtained from commercial sources
and were used directly. Melting points (mp) were deter-
mined using a Thomas±Hoover melting point apparatus
and are uncorrected. UV±vis spectra were recorded on a
Beckman 7400 spectrophotometer. Proton NMR spectra
were recorded on a Bruker WP 360-, 270- or 200-MHz
spectrometer. Chemical shifts (d) are reported in parts per
million (ppm) relative to tetramethylsilane as an internal
standard. The following abbreviations are used to
describe spin multiplicities when appropriate: b=broad,
s=singlet, d=doublet, t=triplet, q=quartet, m=multi-
plet, dd=doublet of doublets. Gas chromatography±
electron ionization mass spectrometry (GC±EIMS) was
performed on a Hewlett Packard 5890 GC ®tted with an
HP-1 capillary column (15 mÂ0.2 mm i.d., 0.33 mm ®lm
thickness) which was coupled to a Hewlett Packard
5870 mass-selective detector. Data were acquired using
an HP 5970 chemstation. The temperature program is
45ꢀC for 2 min and then with the ramp of 25ꢀC/min for
11.8 min. Normalized peak heights are reported as a
percentage of the base peak. High resolution±chemical
ionization mass spectrometry (HR±CIMS) and high
resolution±electron ionization mass spectrometry (HR±
EIMS) were performed on a VG 7070 HF instrument.
Elemental analyses, performed by Atlantic Microlab,
Inc., Norcross, GA, were within 0.17% of the theoretical
values calculated for C, H and N.
The substrate properties of the furanyl and thienyl ana-
logues are even more dramatically a€ected by the
introduction of a methyl or ethyl group at C-3. The kcat
/
Km values of the 3-methyl- and 3-ethylfuranyl analogues
17a and 18a were estimated to be 8620 and 11,000
1
min mM 1. These compounds are 50 and 70 times
more active substrates than the unsubstituted analogue
16a. The very low Km values for 17a and 18a must
re¯ect particularly favorable interactions with a lipo-
philic domain present in the active site of the enzyme.
Similar results were observed with 3-methyl (20a) and 3-
ethyl (21a) thienyl analogues.
4-Oxo-4-pyridin-4-yl-butyralcohol (24). To 4-bromo-
pyridine (3.95 g, 25 mol) in anhydrous diethyl ether
(50 mL) at 78ꢀC was added n-butyllithium (25 mmol)
in hexane dropwise. The mixture was stirred at 78ꢀC
for 1 h. Butyrolactone (2.2 g, 25 mmol) was added
dropwise to the mixture at 78ꢀC. The mixture was
stirred at this temperature for 4 h and then at room
temperature for another 6 h. The reaction mixture
was quenched with dilute Na2CO3 aqueous solution
(30 mL). The aqueous layer was extracted by ethyl
acetate (40 mL). The extracts were dried over MgSO4
These SAR results are analogous to those reported for a
related series of 4-heteroaryl MPTP analogues21 and
also to those generated with a series of MPTP analogues
in which the ortho-position of the phenyl ring was sub-
stituted with various alkyl groups.8 Attempts currently
are underway to construct a molecular model of the
active site of MAO-B that will accommodate all of the
available SAR results derived from 1,4-disubstituted
tetrahydropyridinyl derivatives.

J. Yu, N. Castagnoli Jr. / Bioorg. Med. Chem. 7 (1999) 2835±2842
2839
.
and the solvent was removed in vacuo. The residue
was puri®ed by column (silica gel, ethyl acetate) to a€ord
24 in 94.5% yield. See the Results and Discussion.
GC(tR=6.65 min)±EIMS m/z (%) 165 (10, M.+), 147 (3),
134 (5), 121 (100), 106 (100), 87 (10), 78 (98), 51 (72);
HR±CIMS calcd for C9H11NO2H+: 166.0868. Found:
166.0862.
Oxalate salt of 4-(1-ethylpyrrol-2-yl)pyridine (28 H2C2O4).
This was obtained in 93% yield. Mp 142.5±143ꢀC; GC
(tR=7.10 min)±EIMS m/z (%) 172 (100, M.+), 157 (39),
1
144 (22), 130 (22), 117 (22), 104 (9), 89 (35); H NMR
(DMSO-d6, 270 MHz) d 8.56±8.59 (dd, 2H, C-6 and C-2),
7.47±7.50 (dd, 2H, C-3 and C-5), 7.08±7.10 (dd, 1H, C0-5),
6.47±6.50 (dd, 1H, C0-4), 6.16±6.19 (dd, 1H, C0-3), 4.06±
4.17 (q, 2H, CH2), 1.10±1.29 (t, 3H, CH3); 13C NMR
(DMSO-d6, 360 MHz) d 162.03, 147.63, 141.87, 129.40,
126.32, 121.59, 112.60, 108.78, 42.19, 16.43. Anal. calcd
3-Ethyl-4-oxo-4-pyridin-4-yl-butyralcohol (25). This was
prepared with 4-bromopyridine and a-ethyl-g-butyro-
lactone22 using the same method. The crude product was
recrystallized in dichloromethane:diethyl ether in 53.5%
yield. See Results and Discussion. GC (tR=7.63 min)±
EIMS m/z (%) 192 (4, M.+), 160 (3), 149 (66), 124 (28),
106 (100), 78 (86), 51 (92). Anal. calcd for C11H15NO2: C,
68.37; H, 7.82; N, 7.25. Found: C, 68.27; H, 7.84; N, 7.16.
.
for C13H14N2O4 0.58H2C2O4: C, 54.06; H, 4.86; N, 8.90.
Found: C, 54.01; H, 5.03; N, 8.90.
.
Oxalate salt of 4-(1-propylpyrrol-2-yl)pyridine (29
H2C2O4). This was obtained in 90% yield. Mp 115±
117ꢀC; GC (tR=7.45 min)±EIMS m/z (%) 186 (72,
.+), 157 (100), 144 (30), 130 (22), 117 (20), 104 (11),
M
1
4-Oxo-4-pyridin-4-yl-butyraldehyde (26). To a stirred
solution of pyridine (20 g, 240 mmol) in anhydrous
dichloromethane (300 mL), at room temperature, was
added chromium trioxide (12.0 g, 120 mmol) in por-
tions. The deep burgundy solution was stirred for 30
min. A solution of 24 (20 mmol) in 20 mL of dichloro-
methane was added in one portion. The mixture was
stirred for 12 h at room temperature. The solution was
decanted from the residue and the residue was washed with
dichloromethane (2Â200 mL). The solvent was removed
in vacuo and the residue was puri®ed by column (silica gel,
ethyl acetate). 26 was obtained in 65% yield. GC (tR=6.25
min)±EIMS m/z (%) 163 (1, M.+), 135 (44), 121 (34), 106
(76), 78 (100), 51 (80); 1H NMR (CDCl3, 360 MHz) d 9.89
(s, 1H, CHO), 8.81±8.83 (dd, 2H, C-2 and C-6), 7.75±7.77
(dd, 2H, C-3 and C-5), 3.29±3.32 (t, 2H, 3-CH2), 2.96±3.00
(t, 2H, 2-CH2); 13C NMR (CDCl3, 360 MHz) d 200.00,
197.56, 151.12, 142.36, 121.11, 37.46, 31.30; HR±CIMS
calcd for C9H9NO2H+: 164.0712. Found: 164.0714.
89 (28); H NMR (DMSO-d6, 270 MHz) d 8.55±8.58
(dd, 2H, C-2 and C-6), 7.47±7.50 (dd, 2H, C-3 and C-5),
7.06±7.08 (dd, 1H, C0-5), 6.47±6.49 (dd, 1H, C0-4), 6.14±
6.18 (dd, 1H, C0-3), 4.02±4.10 (t, 2H, CH2N), 1.53±1.64
(m, 2H, CH2), 0.70±0.77 (t, 3H, CH3); 13C NMR
(DMSO-d6, 360 MHz) d 162.03, 147.82, 141.90, 129.66,
127.10, 121.59, 112.61, 108.49, 48.96, 24.02, 10.70. Anal.
.
calcd for C14H16N2O4 0.58H2C2O4: C, 55.40; H, 5.26;
N, 8.52. Found: C, 55.37; H, 5.46; N, 8.62.
4-(1-Isopropylpyrrol-2-yl)pyridine
(30).
This was
obtained in 87% yield. GC (tR=6.78 min)±EIMS m/z (%)
186 (56, M.+), 171 (10), 144 (100), 117 (21), 89 (18);
1H NMR (CDCl3, 360 MHz) d 8.59±8.61 (dd, 2H, C-2
and C-6), 7.26±7.27 (dd, 2H, C-3 and C-5), 6.98 (dd,
1H, C0-5), 6.28 (m, 2H, C0-3 and C0-4), 4.52±4.59 (m,
1H, CH), 1.43 1.44 (d, 6H, CH3); 13C NMR (CDCl3,
360 MHz) d 150.13, 141.32, 131.34, 123.13, 119.88,
110.59, 109.20, 47.78, 24.31; HR±EIMS calcd for
C12H14N2: 186.1157. Found: 186.1152.
3-Ethyl-4-oxo-4-pyridin-4-yl-butyraldehyde (27). This
was prepared using the same methods as 26 in 59.7%
yield. GC (tR=7.26 min)±EIMS m/z (%) 191 (1, M.+),
163 (2), 149 (34), 134 (12), 106 (100), 78 (74), 51 (67); 1H
NMR (CDCl3, 360 MHz) d 9.78 (s, 1H, CHO), 8.81±8.82
(dd, 2H, C-2 and C-6), 7.76±7.77 (dd, 2H, C-3 and C-5),
3.78±3.86 (m, 1H, CH), 3.18±3.26 (dd, 1H, 2-CHH), 2.70±
2.76 (dd, 1H, 2-CHH), 1.68±1.78 (m, 1H, 10-CHH), 1.50±
1.60 (m, 1H, 10-CHH), 0.84±0.92 (t, 3H, CH3); 13C NMR
(CDCl3, 360 MHz) d 202.23, 200.30, 151.00, 142.89,
121.45, 45.08, 41.67, 25.02, 11.42; HR±EIMS calcd for
C11H13NO2H+: 192.1024. Found: 192.1022.
.
Oxalate salt of 4-(1-cyclopropylpyrrol-2-yl)pyridine (31
H2C2O4). This was obtained in 97% yield. Mp 150.5±
151.5ꢀC; GC (tR=7.87 min) EIMS m/z (%) 184 (100,
M
.+), 169 (29), 156 (22), 142 (11), 130 (13), 116 (23), 106
(33), 89 (39); 1H NMR (DMSO-d6, 360 MHz) d 8.55±8.57
(dd, 2H, C-2 and C-6), 7.74±7.77 (dd, 2H, C-3 and C-5),
7.07 (dd, 1H, C0-5), 6.61±6.63 (dd, 1H, C0-4), 6.11±6.14
(dd, 1H, C0-3), 3.73±3.74 (m, 1H, NCH), 0.95±1.0 (m, 2H,
CH2), 0.81±0.85 (m, 2H, CH2); 13C NMR (DMSO-d6, 360
MHz) d 162.03, 146.99, 141.82, 130.48, 127.20, 121.10,
112.80, 108.37, 30.04, 8.41. Anal. calcd for C14H14
.
N2O4 0.38 H2O: C, 59.83; H, 5.29; N, 9.97. Found: C,
59.85; H, 5.15; N, 9.73.
General procedures for the synthesis of the oxalate salts
of 4-(1-alkylpyrrol-2-yl)pyridine
.
Oxalate salt of 4-(3-ethylfuran-2-yl)pyridine (36 H2C2O4).
To 4-oxo-4-pyridinylbutanal (20 mmol) in 40 mL
methanol was added the corresponding primary amines
(4 equiv.). The mixture was stirred at room temperature
for 1 day. The solvent was removed in vacuo. After
chromatography (silica gel, ethyl acetate), a yellow oil of
4-(1-alkylpyrrol-2-yl)pyridine was obtained. To this
yellow oil (5 mmol) in dry diethyl ether (15 mL) was
added the oxalic acid (6 mmol) in 5 mL diethyl ether.
The mixture was stirred for 4 h and the precipitate was
®ltered and recrystallized in methanol/diethyl ether.
To 3-ethyl-4-oxo-4-(4-pyridyl)butanal 27 (1.00 g, 5.23
mmol) in 20 mL THF and 10 mL H2O was added 5 mL
H2SO4 dropwise. The mixture was stirred for 15 h at
55ꢀC. THF was evaporated in vacuo. The remaining
solution was basi®ed and extracted with diethyl ether
(4Â40 mL). The extracts were dried over MgSO4 and
the solvent was removed in vacuo. The crude product
was chromatographed (silica gel, 10:1.5 ethyl acetate:
hexane) to give 36 (0.875 g, 96.6%). Treatment of 36 in
dry diethyl ether with oxalic acid (1.2 equiv) a€orded a

2840
J. Yu, N. Castagnoli Jr. / Bioorg. Med. Chem. 7 (1999) 2835±2842
precipitate, which was recrystallized in methanol:diethyl
ether to give a yellow solid 36 H2C2O4 in 97% yield. Mp
General procedures for the synthesis of oxalate salt of 1-
methyl-4-(1-alkylpyrrol-2-yl)-1,2,3,6-tetrahydropyridine
.
170±171ꢀC (decomposed); GC (tR=6.85 min)±EIMS m/z
(%) 173 (100, M.+), 158 (87), 144 (11), 130 (93), 115 (13),
103 (26), 89 (11); 1H NMR (DMSO-d6, 200 MHz) d 8.61±
8.63 (dd, 2H, C-2 and C-6), 7.82 (d, 1H, C0-5), 7.57±7.60
(dd, 2H, C-3 and C-5), 6.65 (d, 1H, C0-4), 2.69±2.81 (q,
2H, CH2), 1.18±1.26 (t, 3H, CH3); 13C NMR (DMSO-d6,
360 MHz) d 161.40, 148.56, 144.21, 143.96, 138.70, 128.86,
To 4-(alkylpyrrol-2-yl)pyridine (5 mmol) in dry acetone
(10 mL) was added methyl iodide (6 equiv) at room
temperature. The mixture was stirred overnight. The sol-
vent was removed in vacuo. To the residue was added
methanol (25 mL). Sodium borohydride (2.5 equiv) was
added in portions to this stirred solution at 0ꢀC. The
mixture was stirred for an additional 1 h at room tem-
perature and the solvent was subsequently removed in
vacuo. The residue was taken up in 15 mL of water and
the aqueous solution was extracted with diethyl ether
(4Â30 mL). The combined organic layers were dried over
MgSO4, ®ltered, and concentrated to 15% of the original
volume. Treatment with oxalic acid (1.2 equiv) in 10 mL
of diethyl ether precipitated the crude oxalate salt, which
was recrystallized from the appropriate solvent.
.
118.76, 114.24, 18.67, 13.75. Anal. calcd for C13H13O5N
0.45H2C2O4: C, 54.93; H, 4.61; N, 4.59. Found: C, 54.95;
H, 4.76; N, 4.58.
1-Methyl-4-(3-ethylfuran-2-yl)pyridinium iodide (38). To
4-(3-ethylfuran-2-yl)pyridine (0.484 g, 2.8 mmol) in 5 mL
dry acetone was added methyl iodide (5 equiv). The
solution was stirred overnight. After ®ltration, the crude
product was recrystallized from methanol:diethyl ether
to a€ord a yellow solid 38 (0.37 g, 42%). Mp 162.5Ð
163.5ꢀC; H NMR (DMSO-d6, 200 MHz) d 8.82±8.85
Oxalate salt of 1-methyl-4-(1-ethylpyrrol-2-yl)-1,2,3,6-
1
.
(dd, 2H, C-2 and C-6), 8.11±8.14 (m, 3H, C-3, C-5 and
C0-5), 6.85 (d, 1H, C0-4), 4.28 (s, 3H, NCH3), 2.82±2.94
(q, 2H, CH2), 1.21±1.29 (t, 3H, CH3); 13C NMR
(DMSO-d6, 360 MHz) d 147.61, 145.28, 143.03, 142.80,
135.77, 120.30, 115.66, 46.84, 19.13, 13.41. Anal. calcd
for C12H14INO: C, 45.73; H, 4.48; N, 4.44. Found: C,
45.56; H, 4.54; N, 4.36.
tetrahydropyridine (12a H2C2O4). This was obtained in
80% yield. Mp 146.5±147.7ꢀC; GC (tR=7.15 min)±EIMS
m/z (%) 190 (100, M.+), 176 (16), 161 (63), 146 (29), 132
(83), 117 (43), 108 (55), 94 (96); 1H NMR (DMSO-d6, 200
MHz) d 6.82 (dd, 1H, C0-5), 6.06 (m, 1H, C0-3), 5.98±6.01
(m, 1H, C0-4), 5.65 (b, 1H, C-5), 3.90±4.0 (q, 2H, N0-CH2),
3.75 (d, 2H, C-6), 3.26±3.32 (t, 2H, C-2), 2.80 (s, 3H,
NCH3), 2.60 (b, 2H, C-3), 1.22±1.29 (t, 3H, CH3); 13C
NMR (DMSO-d6, 360 MHz) d 164.39, 131.22, 127.31,
122.89, 116.13, 108.18, 107.13, 51.19, 49.67, 41.76, 41.66,
1-Methyl-4-(3-ethylthien-2-yl)pyridine (37). To 3-ethyl-
4-oxo-4-(4-pyridyl)butanal 27 (0.955 g, 5 mmol) in dry
toluene (20 mL) was added powdered phosphorus
pentasul®de (5.5 g, 25 mmol). The mixture was stirred
for 2 h at 100ꢀC under N2. The mixture was cooled and
treated with water (25 mL) and the aqueous layer was
adjusted to pH 8. The aqueous layer was extracted with
ether (3Â40 mL). The combined organic layer was
washed with dilute Na2CO3 solution, dried over MgSO4
and evaporated. The residue was separated through
column (silica gel, ethyl acetate:hexane, 10:2). 0.284 g
yellow oil was obtained in 30% yield. GC (tR=6.98
min)±EIMS m/z (%) 189 (71, M.+), 174 (100), 147 (20),
130 (15), 115 (6), 102 (7); 1H NMR (CDCl3, 360 MHz) d
8.58±8.62 (dd, 2H, C-3 and C-5), 7.34±7.36 (m, 3H, C-2,
C-6 and C0-5), 7.03±7.04 (d, 1H, C0-4), 2.72±2.78 (q, 2H,
CH2), 1.24±1.30 (t, 3H, CH3); 13C NMR (CDCL3, 360
MHz) d 150.00, 142.50, 142.07, 134.30, 129.85, 125.66,
123.34, 22.169, 15.28; HR±EIMS calcd for C11H11NS:
189.0612. Found: 189.0613.
.
26.18, 16.61. Anal. calcd for C14H20N2O4 0.15H2C2O4:
C, 58.46; H, 6.96; N, 9.54. Found: C, 58.49; H, 7.03; N,
9.65.
Oxalate salt of 1-methyl-4-(1-propylpyrrol-2-yl)-1,2,3,6-
.
tetrahydropyridine (13a H2C2O4). This was obtained in
76% yield. Mp 177.5±178ꢀC; GC (tR=7.49 min)±EIMS
m/z (%) 204 (89, M.+), 189 (13), 175 (41), 161 (27), 146
1
(35), 132 (50), 117 (46), 104 (28), 94 (100); H NMR
(DMSO-d6, 200 MHz) d 6.80±6.82 (dd, 1H, C0-5), 6.06±
6.09 (m, 1H, C0-3), 5.98±6.01 (m, 1H, C0-4), 5.65 (b, 1H,
C-5), 3.85±3.91 (t, 2H, N0CH2), 3.75 (d, 2H, C-6), 3.28±
3.32 (t, 2H, C-2), 2.80 (s, 3H, NCH3), 2.60 (b, 2H, C-3),
1.55±1.72 (m, 2H, N0CHCH2), 0.72±0.82 (t, 3H, CH3); 13
C
NMR (DMSO-d6, 360 MHz) d 164.39, 131.51, 127.45,
123.66, 116.38, 108.22, 106.89, 51.23, 49.71, 48.51, 41.83,
26.35, 24.08, 10.84. Anal. calcd for C15H22N2O4: C, 61.21;
H, 7.53; N, 9.52. Found: C, 61.12; H, 7.49; N, 9.55.
1-Methyl-4-(3-ethylfuran-2-yl)pyridinium iodide (39). To
4-(3-ethylthien-2-yl)pyridine (0.19 g, 1 mmol) in 5 mL
dry acetone was added methyl iodide (5 equiv). The
solution was stirred overnight. After ®ltration, the crude
product was recrystallized from methanol:diethyl ether
to a€ord a yellow solid 39 in 45% yield. Mp 164±165ꢀC;
1H NMR (DMSO-d6, 360 MHz) d 8.89±8.90 (dd, 2H, C-
2 and C-6), 8.13±8.15 (dd, 2H, C-3 and C-5), 7.97±7.98
(d, 1H, C0-5), 7.30 (d, 1H, C0-4), 4.31 (s, 3H, NCH3),
2.84±2.88 (q, 2H, CH2), 1.22±1.27 (t, 3H, CH3); 13C
NMR (DMSO-d6, 360 MHz) d 148.86, 147.12, 145.30,
131.56, 131.36, 130.65, 124.90, 46.98, 22.17, 14.55. Anal.
calcd for C12H14INS: C, 43.52; H, 4.26; N, 4.23. Found:
C, 43.55; H, 4.31; N, 4.17.
Oxalate salt of 1-methyl-4-(1-isopropylpyrrol-2-yl)-1,2,
.
3,6-tetrahydropyridine
(14a H2C2O4).
This
was
obtained in 91% yield. Mp 180±180.5ꢀC; GC (tR=7.38
min)±EIMS m/z (%) 204 (100, M.+), 189 (20), 175 (24),
161 (39), 146 (46), 133 (17), 118 (30), 104 (14); 1H NMR
(DMSO-d6, 270 MHz) d 6.94 (s, 1H, C0-5), 6.02±6.03
(m, 1H, C0-4), 6.98 (m, 1H, C0-3), 5.60 (b, 1H, C-5),
4.43±4.48 (m, 1H, N0CH), 3.75 (d, 2H, C-6), 3.30 (t, 2H,
C-2), 2.80 (s, 3H, NCH3), 2.58 (b, 2H, C-3), 1.31±1.34
(d, 6H, CH3); 13C NMR (DMSO-d6, 360 MHz) d
164.39, 132.03, 127.85, 118.41, 118.01, 107.53, 107.17,
51.26, 49.76, 46.71, 41.84, 26.99, 23.87. Anal. calcd for
.
C15H22N2O4 0.07H2C2O4: C, 60.50; H, 7.42; N, 9.32.
Found: C, 60.50; H, 7.44; N, 9.25.

J. Yu, N. Castagnoli Jr. / Bioorg. Med. Chem. 7 (1999) 2835±2842
2841
Oxalate salt of 1-methyl-4-(1-cyclopropylpyrrol-2-yl)-1,
.
prepared in 100 mM sodium phosphate bu€er (pH 7.4).
A 480±495 mL aliquot of each solution was added to
the sample cuvette which was placed in the spectro-
photometer and maintained at 37ꢀC. After a 3 min
equilibration period, 5 mL of the MAO-B enzyme pre-
paration was added (®nal MAO-B concentration was
0.09 mM). The rates of oxidation of each substrate were
estimated by monitoring the absorbance of the corre-
sponding dihydropyridium metabolite every 5 s for 0.5
min. The Km and kcat values were calculated from
Lineweaver±Burk double-reciprocal plots. Duplicate
analyses gave kcat/Km values that di€ered less than 9.4%.
2,3,6-tetrahydropyridine (15a H2C2O4). This was
obtained in 71% yield. Mp 183±183.5ꢀC; GC (tR=8.05
min)±EIMS m/z (%) 202 (100, M.+), 187 (11), 173 (42),
1
159 (30), 144 (41), 130 (36), 118 (41), 94 (55); H NMR
(DMSO-d6, 270 MHz) d 6.80 (d, 1H, C0-5), 6.08 (m, 2H,
C0-3 and C0-4), 5.93±5.95 (m, 1H, C-5), 3.77 (s, 2H, C-6),
3.38 (m, 1H, N0CH), 3.28±3.32 (t, 2H, C-2), 2.80 (s, 3H,
NCH3), 2.68 (b, 2H, C-3), 0.94-0.98 (m, 2H, CH2CH2),
0.86 (m, 2H, CH2CH2); 13C NMR (DMSO-d6, 360 MHz)
d 164.39, 132.70, 126.76, 123.72, 115.25, 108.43, 106.63,
51.28, 49.68, 41.80, 29.96, 25.24, 8.25. Anal. calcd for
C15H20N2O4: C, 61.63; H, 6.90; N, 9.58. Found: C, 61.53;
H, 6.88; N, 9.66.
Acknowledgements
Oxalate salt of 1-methyl-4-(3-ethylfuran-2-yl)-1,2,3,6-
.
tetrahydropyridine (18a H2C2O4). This was obtained in
This work was supported by the National Institute of
Neurological and Communicative Disorders and Stroke
Grant NS 28792, and the Harvey W. Peters Research
Center. The authors thank Dr. S. Mabic for providing
the log P values listed in Table 1.
86% yield. Mp 148.5±149ꢀC; GC (tR=6.96 min)±EIMS
m/z (%) 191 (26, M.+), 176 (6.5), 162 (6.5), 148 (3.3), 133
(15), 105 (20), 91 (14), 83 (45), 70 (100); 1H NMR
(DMSO-d6, 200 MHz) d 7.57 (d, 1H, C0-5), 6.47±6.48 (d,
1H, C0-4), 5.92 (b, 1H, C-5), 3.76 (d, 2H, C-6), 3.26±3.29
(t, 2H, C-2), 2.79 (s, 3H, NCH3), 2.71 (b, 2H, C-3), 2.50±
2.51 (m, 2H, CH2), 1.10±1.17 (t, 3H, CH3); 13C NMR
(DMSO-d6, 360 MHz) d 164.32, 146.40, 141.24, 126.36,
123.20, 115.76, 113.11, 51.01, 49.25, 41.85, 23.09, 18.38,
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