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D. Krehan et al. / Bioorg. Med. Chem. 11 (2003) 4891–4896
in agreement with the receptor model proposed by
Chebib et al.13
3-ol (THAZ),15 4-(2-aminoethyl)-5-methylisoxazol-3-ol
(AEMI),16 THIP17 and 5,6,7,8 - tetrahydro - 4H - iso-
xazolo[5,4-c]azepin-3-ol (THIA).17
The homologue of THIP, THAZ, contains a larger
seven membered ring and is therefore a more flexible
molecule. THAZ is a moderately potent antagonist
(Ki=23 mM) at GABAC r1 receptors. Being equipotent
with THIP at this receptor, THAZ may be able to attain
a conformation similar to that of THIP in order to fit
into the GABAC binding pocket. In contrast, AEMI,
which is an open ring analogue of THAZ, is markedly
more flexible, and a conformation of AEMI comparable
with those of THIP and THAZ is unlikely to be ener-
getically favourable. Additionally, the methyl group in
the 5-position of the 3-isoxazolol ring of AEMI may
protrude into an area of steric hindrance causing AEMI
to be some seven times weaker than THAZ (Table 1).
Melting points were determined in open capillary
tubes and are corrected. Elemental analyses were per-
formed at Analytical Research Department, H. Lund-
beck A/S, Denmark, or by Mr. J. Theiner,
Department of Physical Chemistry, University of
Vienna, Austria, and are within Æ0.4% of the calcu-
lated values unless otherwise stated. Nuclear Magnetic
Resonance Spectra (NMR) were recorded on
300 MHz Varian Gemini spectrometer.
a
4-Aminocarbonyl-1-benzylpyridiniumbromide (1). To a
solution of isonicotinamide (10.0 g, 81.9 mmoles) in
ethanol (250 mL) was added benzyl bromide (29.2 mL,
245.6 mmoles) and the reaction mixture was refluxed
(90ꢀ C) for 4 h. After cooling, diethylether (250 mL) was
added, and the reaction mixture was then placed at 4ꢀ C
overnight for crystallization to provide crude 1 as white
crystals (19.3 g, 80%). An analytical sample was
recrystallized (methanol): mp 243.5–246.0ꢀ C (decom-
THIA, an isomer of THAZ (Fig. 1) shows no activity at
GABAC r1 receptors and no affinity for the GABAA
receptors. THIAmay not be able to adopt a conform-
ation where the amino group is in the right position to
interact with the receptor.
1
posed); H NMR (D2O) d 8.93 (2H, d, J=6.6 Hz), 8.19
(2H, d, J=6.6 Hz), 7.35 (5H, s), 5.73 (2H, s), 4.65 (3H,
s); 13C NMR (D2O) d 166.62, 148.65, 145.47, 132.15,
130.11, 129.62, 129.30, 126.45, 66.33. Analysis
(C13H13BrN2O) C, H, Br, N.
Conclusion
TPMPAhas so far been the most useful antagonist for
studying GABAC receptors. However, Aza-THIP,
which has now been shown to be a specific competitive
GABAC antagonist, may be a supplement to TPMPAas
a reference GABAC compound. TPMPAshows strong
antagonist activity at GABAC receptors and, in contrast
to Aza-THIP, weak activity at both GABAA (Table 1)
and metabotropic GABAB receptors.6
1-Benzyl-1,2,3,6-tetrahydropyridine-4-carboxylic
acid
amide (2). Asolution of crude 1 (19.3 g, ca. 65.8
mmoles) in methanol (350 mL) was cooled to 0ꢀ C.
Sodium borohydride (2.62 g, 69.1 mmoles) was added
over 3 h and the mixture was then allowed to stir at
0ꢀ C for 30 min and at 25ꢀ C for 16 h. The mixture
was evaporated in vacuo and the residue was extrac-
ted with CH2Cl2. The combined organic phases were
dried (MgSO4) and evaporated in vacuo to give crude
2 (8.87 g, 62%). An analytical sample was dissolved
in 2 M hydrochloric acid, evaporated in vacuo and
crystallized (methanol) to provide analytically pure
hydrochloride of 2: mp 236.0–238.0ꢀ C (decomposed);
1H NMR (D2O) d 7.34 (5H, m), 6.38–6.35 (1H, m),
4.23 (2H, s), 3.74–3.67 (2H, m), 3.50 and 3.07 (1H,
two broad signals), 2.50–2.43 (2H, m); 13C NMR
Using a series of compounds related to THIP, we have
shown that only minor structural changes affect their
potency as GABAC antagonists. The acidic character of
the 3-isoxazolol ring of Iso-THIP (pKa 3.0) is more
pronounced than that of the 3-isoxazolol ring of THIP
(pKa 4.4) (Table 1), suggesting that this difference in
potency may be explained by a stronger electrostatic
interaction of Iso-THIP than of THIP with the receptor.
The structural and protolytic similarity between THIP
and Aza-THIP suggests that the latter compound, like
THIP, is capable of penetrating the blood-brain-barrier.
Thus, although Aza-THIP is only moderately potent as
a GABAC antagonist, it may be a useful compound for
behavioural pharmacological studies.
(D2O) d 170.50, 131.25, 130.25, 130.13, 129.38,
128,75, 126.00, 59.00, 49.38, 48.00, 21.25. Analysis
(C13H17ClN2O) C, H, Cl, N.
4-Aminocarbonyl-1,2,3,6-tetrahydropyridine-1-carboxylic
acid methyl ester (3). Methyl chloroformate (6.13 mL,
79.7 mmoles) was added through a condenser to crude 2
(8.62 g, ca. 39.9 mmoles). The mixture was stirred for 7
h, evaporated in vacuo and extracted continuously with
CH2Cl2 for 23 h. The CH2Cl2 phase was dried (MgSO4),
evaporated in vacuo and the residue recrystallized (eth-
anol–diethylether) to give 3 (3.22 g, 44%). An analytical
sample was recrystallized (ethyl acetate): mp
Experimental
Chemistry
The following compounds were synthesized by published
procedures: 4,5,6,7-tetrahydropyrazolo[5,4-c]pyridin-3-ol
(Aza-THIP),14 1-methyl-4,5,6,7-tetrahydropyrazolo[5,4-
c]pyridin-3-ol (1-Me-Aza-THIP),14 2-methyl-4,5,6,7-tet-
rahydropyrazolo[5,4-c]pyridin-3-ol (2-Me-Aza-THIP),14
4,5,6,7 - tetrahydroisoxazolo[3,4 - c]pyridin - 3 - ol (Iso-
THIP),14 5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-
ꢀ
1
128.0–130.5 C; H NMR (CDCl3) d 6.60 (1H, s), 6.07
(2H, broad s), 4.19–4.04 (2H, m), 3.73 (3H, s), 3.65–3.49
(2H, m), 2.47–2.31 (2H, m); 13C NMR (CDCl3) d
169.00, 156.60, 131.50, 130.50, 52.85, 43.90, 40.00,
24.00. Analysis (C8H12N2O3) C, H, N.