Synthesis and biological activity of novel heterocyclic ring systems: Imidazo[4′,5′:3,4]pyrido-[2,1-a]isoquinolines and imidazo[4,5-f][3]benzazecines

Article abstract of DOI:10.3987/COM-12-12574

Derivatives of two novel heterocyclic ring systems were synthesized and their affinities for dopamine receptors were measured. The compounds were obtained by reacting histamine with 2-(2-bromoethyl)benzaldehyde including an atypical Pictet-Spengler condensation, which afforded basic and not the usual neutral or acidic conditions. The resulting imidazo[4′,5′:3,4] pyrido[2,1-a]-isoquinoline derivative 4 was Boc protected at the most basic imidazole nitrogen, the isoquinoline nitrogen then quaternized by using methyl iodide and the tetracyclic isoquinolinium salt was both deprotected and cleaved under Birch conditions in one step to give a tricyclic imidazo[4,5-f][3] benzazecine derivative (3) by opening two 6-membered heterocycles towards one 10-membered. Radioligand binding studies showed a significant affinity of the moderately constrained 3 but not of 4 for dopamine receptors. Similar to the analogous indolo-benzazecine LE300, a preference of 3 for the D₁ receptor family was observed, but with some loss of affinity over all.

Full text of DOI:10.3987/COM-12-12574

HETEROCYCLES, Vol. 87, No. 3, 2013
551
HETEROCYCLES, Vol. 87, No. 3, 2013, pp. 551 - 558. © 2013 The Japan Institute of Heterocyclic Chemistry
Received, 29th August, 2012, Accepted, 29th January, 2013, Published online, 5th February, 2013
DOI: 10.3987/COM-12-12574
SYNTHESIS
AND
BIOLOGICAL
ACTIVITY
OF
NOVEL
HETEROCYCLIC RING SYSTEMS: IMIDAZO[4',5':3,4]PYRIDO-
[2,1-a]ISOQUINOLINES AND IMIDAZO[4,5-f][3]BENZAZECINES
Robert Otto, Christoph Enzensperger, and Jochen Lehmann*
Institute of Pharmacy, Friedrich-Schiller-University of Jena, Philosophenweg 14,
07743 Jena, Germany; E-mail: j.lehmann@uni-jena.de
College of Pharmacy, King Saud University Riyadh, Saudi Arabia
Abstract – Derivatives of two novel heterocyclic ring systems were synthesized
and their affinities for dopamine receptors were measured. The compounds were
obtained by reacting histamine with 2-(2-bromoethyl)benzaldehyde including an
atypical Pictet-Spengler condensation, which afforded basic and not the usual
neutral or acidic conditions. The resulting imidazo[4',5':3,4]pyrido[2,1-a]-
isoquinoline derivative 4 was Boc protected at the most basic imidazole nitrogen,
the isoquinoline nitrogen then quaternized by using methyl iodide and the
tetracyclic isoquinolinium salt was both deprotected and cleaved under Birch
conditions in one step to give a tricyclic imidazo[4,5-f][3]benzazecine derivative
(3) by opening two 6-membered heterocycles towards one 10-membered.
Radioligand binding studies showed a significant affinity of the moderately
constrained 3 but not of 4 for dopamine receptors. Similar to the analogous
indolo-benzazecine LE300, a preference of 3 for the D1 receptor family was
observed, but with some loss of affinity over all.
INTRODUCTION
Dopamine is a key neurotransmitter in the brain with various physiological functions, e.g. regulation of
locomotion, cognition, emotion and event prediction, and dopamine receptor antagonists play a crucial
role in the treatment of neuropsychiatric diseases. Azecine-type dopamine receptor antagonists, with
7-methyl-6,7,8,9,14,15-hexahydro-5H-indolo[3,2-f][3]benzazecine (1, LE 300) as a lead, represent a
chemically new class of potential antipsycotics with high affinities for dopamine receptors and a rather
unique selectivity profile, showing prevalence for the dopamine D1 subtype receptor family.1-3 Both, the

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HETEROCYCLES, Vol. 87, No. 3, 2013
annulated azecine ring itself, but also a moderately rigidized phenylethyl- or heteroarylethyl-amine partial
structures are considered to be essential for the biological activity,4 so further variations of these scaffolds
are interesting. The azecine 1 and its precursor quinolizine 2 have been obtained in our group by a
Bischler-Napieralski cyclization but synthesis has been recently improved by performing a Pictet-
Spengler condensation of tryptamine and 2-(2-bromoethyl)benzaldehyde (Scheme 1).5
Me
NH2
MeI
Br
N
N
+
N
O
N
N
H
H
Na°, liq. NH3
H
2
1, LE300
Scheme 1. Synthesis of the lead compound 1 (LE 300)5
Changing the biogenic amines from tryptamine to histamine should give the novel heterocycles 4 and 3.
RESULTS AND DISCUSSION
Compound 3 combines structural aspects of both histamine and phenylethylamine in one heterocycle and
hence might have interesting biological activities. We refluxed the histamine free base with
2-(2-bromoethyl)benzaldehyde6 in dichloromethane for 24 h (Scheme 2) which was expected to form the
imidazo[4',5':3,4]pyrido[2,1-a]isoquinoline 4 by Pictet-Spengler cyclization, since many analogous
indole- or thiophene-ethylamines have reacted this way.7 But in this particular case of histamine, the
reaction stopped at the intermediate Schiff base 5 which did not undergo any further spontaneous ring
closure (Scheme 2).
Br
HN
NH2
HN
N
O
+
N
N
4
NH3
Br
HN
N+
N
5
Scheme 2. Synthesis of 4 stops at the intermediate imminium salt 5 under non-basic conditions

HETEROCYCLES, Vol. 87, No. 3, 2013
553
Pictet-Spengler condensations generally undergo various steps and the final cyclization of the
intermediate Schiff base occurs either spontaneously or can be promoted by heating under acidic
conditions.7 With histamine and 2-(2-bromoethyl)benzaldehyde these conditions did not lead to the fully
cyclized product. We tried various solvents and pH ranges from neutral to acidic. Even with very strong
acids like TFA in dichloromethane -these conditions easily induce cyclizations with tryptamine5- no
reaction of 5 towards 4 was observed. But surprisingly, we detected the desired cyclized isoquinoline 4 by
GC/MS analysis after preparing a sample of 5 for GC/MS analysis by adding aqueous ammonia solution
and extracting with ethyl acetate. So we found and reconfirmed after repeating and upscaling that in this
particular case a Pictet-Spengler condensation was achieved not under acidic but rather under basic
conditions. Actually, a base-promoted (excess of Et3N) Pictet-Spengler reaction for serotonin has been
reported8 and other authors reacted histamine with carbonyl compounds in ethanolic KOH.9 So we
applied these conditions from the beginning by reacting histamine and 2-(2-bromoethyl)benzaldehyde in
ethanol with an excess of Et3N, or alternatively by using ethanolic KOH. In both cases only traces (~
0.4%) or no 4 at all were detected (GC/MS). Since the preparation of the intermediate 5 under non-acidic
conditions showed to be simple, we just dissolved 5 in water and added aqueous conc. ammonia.
Cyclization took place within less than 5 min. and 4 could be extracted with ethyl acetate.
Annulated isoquinolines as precursors for the N-methylated azecines were usually quaternized at the
quinolizine nitrogen and then the central C-N bond cleaved with sodium in liq. ammonia. To prevent any
alkylation of 4 at the imidazole, a Boc group was introduced giving the tert-butyl
imidazo[4',5':3,4]pyrido[2,1-a]isoquinoline-3(5H)-carboxylate 6. It is not obvious at which of the
imidazole nitrogens the Boc-group is attached, but only one regioisomer was formed. Comparison with
1
calculated spectra (ACD-Chem-Sketch, H-NMR Predictor) suggests that the BOC group is at the more
exposed position 3. The chemical shift for the proton attached to carbon 12b was calculated with = 5.12
ppm for the 3-Boc isomere and = 4.72 ppm for the 1-Boc isomere and it was found at = 5.04 ppm. So
there is some preference for the 3-Boc compound. This preference is confirmed by the 13C shift for
imidazole carbon 3a which was calculated with = 139.63 ppm for 1-Boc and = 124.35 ppm for 3-Boc
and was found to be = 123.9 ppm. The position of the Boc group at the intermediates is not relevant in
view of the desired target compounds; nevertheless, not only steric considerations but also NMR data
strongly suggest the 3-Boc intermediates. The Boc-protected isoquinoline 6 was quaternized with methyl
iodide yielding the corresponding isoquinolinium iodide 7 (Scheme 3). When performing the finalizing
cleavage of the quinolizine C-N-bond towards the azecine ring under Birch conditions the Boc group
fortunately was also cleaved off although it is described to be stable against Na in liq. ammonia.10,11

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HETEROCYCLES, Vol. 87, No. 3, 2013
Boc
N
Boc
N
Me
I-
Me
HN
HN
N+
N
MeI
N
N
K2CO3
Boc2O
NH3
Na°
N
N
N
N
4
6
7
3
Scheme 3. Synthesis of target azecine
BIOLOGICAL ACTIVITIES
The affinities of the target compounds 3 and 4 for stably cloned human dopamine receptors were
evaluated by radioligand binding experiments using a protocol which we have described previously.12
The Ki values (low value means high affinity) are given in Table 1.
Table 1. Affinities (Ki values) for the human cloned dopamine receptors D1-D5, measured by
radioligand binding experiments
Compounds
Ki [nM] (Radioligand binding studies)
HEK D1
> 10 000
HEK D2L
HEK D3
CHO D4.4
HEK D5
> 10 000
N
N
H
2
1, LE300
4
> 10 000a
> 10 000
> 10 000
Me
N
1.9±0.5b
> 10 000
44.7±15.8b
> 10 000
40.35b
74.9±50b
> 10 000
7.5±0.3b
> 10 000
N
H
HN
N
N
N
> 10 000
Me
HN
N
5035±
2577c
3
769.5±36.5c
> 10 000
> 10 000
366±211d
aValues from Ref.[3], bvalues from Ref.[2], cvalues are the mean of two experiments, performed in triplicate (± SEM),
dvalues are the mean of three experiments, performed in triplicate (± SD)

HETEROCYCLES, Vol. 87, No. 3, 2013
555
The azecine 3 is the more active compound compared to its precursor 4, which is in line to all previous
data comparing annulated isoquinoline and azecine derivatives. Target compound 3 is less affine
compared to LE300 but still offers submicromolecular affinities for D1 and D5.
EXPERIMENTAL
Melting points are uncorrected and were measured in open capillary tubes, using a Gallenkamp melting
point apparatus. 1H- and 13C-NMR spectral data were obtained from a Bruker Advance 250 spectrometer
(250 MHz) and Advance 400 spectrometer (400 MHz). TLC was performed on silica gel F254 plates
(Merck). MS data were determined by GC/MS, using a Hewlett Packard GCD-Plus (G1800C) apparatus
(HP-5MS column; J&W Scientific). High resolution mass spectrometry (HRMS) data were determined
on a TSQ Quantum Access Mass Spectrometer (Therma Electron Corporation). Purities of the
compounds were determined by elemental analysis, performed on a Hereaus Vario EL apparatus. All
values for C, H, and N were found to be within ± 0.4. All compounds showed > 95% purity.
2-[2-(1H-Imidazol-5-yl)ethyl]-3,4-dihydroisoquinolinium bromide hydrobromide (5)
b
In a 250 mL round bottom flask 610 mg (5 mmol) of histamine free base
H
N
1'
a
(purchased from Sigma Aldrich) and 1.4 g (6 mmol) of 2-(2-bromoethyl)-
benzaldehyde6 were dissolved in 130 mL CH2Cl2 and refluxed for 24 h. The
mixture was cooled to room temperature and the precipitated light yellow solid
was filtered off, dried under reduced pressure and recrystallized from isopropanol
to give 5 as white crystals. (950 mg, 49%); mp 213 °C; 1H NMR (DMSO-d6, 250
2'
HBr
Br
N
N
3'
3
4
1
MHz): δ (ppm) 3.24 (t, J = 8.0 Hz, 2H, Hb), 3.35 (t, J = 6.9 Hz, 2H, H4), 4.12 (t, J = 8.0 Hz, 2H, Ha),
4.31 (t, J = 6.9 Hz, 2H, H3), 7.53 (m, 2H, H5 and H6), 7.63 (s, 1H, H4’), 7.82 (m, 2H, H8 and H7), 9.12
(s, 1H, H2’), 9.36 (s, 1H, H1); 13C NMR (DMSO-d6): δ (ppm) 22.76, 24.92, 48.35, 58.78, 117.71, 125.07,
128.56, 128.73, 128.82, 134.01, 135.09, 137.23, 138.19, 167.38; Anal. Calcd for C14H17Br2N3: C, 43.4; H,
4.43; N, 10.9. Found: C, 43.1; H, 4.62; N, 10.8. HRMS calcd for C14H16N3: 226.13442. Found: 226.13426.
3,4,5,7,8,12b-Hexahydroimidazo[4',5':3,4]pyrido[2,1-a]isoquinoline (4)
4
In a 100 mL round bottom flask 810 mg 5 (2.1 mmol) were dissolved in 20 mL
water and conc. ammonia solution was added to adjust the pH at 10. After 5 min,
the mixture was extracted with EtOAc (3 x 15 mL). The combined organic phases
were dried over Na2SO4, evaporated and dried under reduced pressure to give 4 as
a yellow amorphous solid. (320 mg, 67%) For analytical purpose a dihydrochloric
salt was formed by dissolving 4 in isopropanol and adding conc. HCl
H
N
3
2
6
N
N
7
1
12
b
8
12
11
9
10

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HETEROCYCLES, Vol. 87, No. 3, 2013
dropwise until the pH reached 5. The formed salt was collected and recrystallized from acetone/water. mp
275 °C; 1H NMR (DMSO-d6, 400 MHz): δ (ppm) 3.04 (m, 2H, H8), 3.19 (m, 2H, H4), 3.51 (m, 2H, H5),
3.71 (m, 2H, H7), 6.04 (s, 1H, H12b), 7.32-7.36 (m, 3H, H9-11), 7.50 (d, J = 7.1 Hz, 1H, H12), 9.06 (s,
1H, H2); 13C NMR (DMSO-d6): δ (ppm) 17.77, 23.68, 46.26, 46.80, 54.40, 123.90, 124.58, 127.65,
128.21, 128.95, 129.09, 129.50, 131.41, 135.81; Anal. Calcd for C14H17Cl2N3 + 1/3 H2O: C, 55.3; H,
5.85; N, 13.8. Found: C, 55.1; H, 5.58; N, 13.6. HRMS Calcd for C14H16N3: 226.13442. Found:
226.13519.
Tert-butyl 4,7,8,12b-tetrahydroimidazo[4',5':3,4]pyrido[2,1-a]isoquinoline-3(5H)-carboxylate (6)
Me
To a solution of 330 mg 4 (1.47 mmol) in 50 mL isopropanol:water (5:6) 223 mg
K2CO3 (1.61 mmol) and 344 µL of di-tert-butyl dicarbonate were added. The
mixture was stirred at room temperature for 17 h, then concentrated under reduced
pressure and extracted with EtOAc (3x 20 mL). Organic layers were combined,
dried over Na2SO4 and evaporated. The resulting yellow oil was purified by
column chromatography (Silicagel, MeOH : CH2Cl2 (1:9)) to give 112mg (23%)
Me
Me
O
O
4
5
N
N
2
6
N
7
1 12
b
8
12
11
1
of 6 as light yellow oil. H NMR (CDCl3, 250 MHz): δ (ppm) 1.59 (s, 9H,
9
10
tert-butyl), 2.79-3.23 (m, 8H, H4-8), 5.04 (s, 1H, H12b), 7.07-7.26 (m, 3H, H9-
13
11), 7.92 (d, J = 7.2 Hz, 1H, H12), 8.02 (s, 1H, H2); C NMR (CDCl3): δ (ppm) 22.64, 26.54, 27.93,
47.70, 28.28, 57.29, 85.18, 123.92, 125.81, 126.43, 128.70, 128.80, 133.48, 135.18, 136.20, 139.17,
147.81; Anal. Calcd for C19H23N3O2 *1.2 MeOH: C, 66.7; H, 7.70; N, 11.5. Found: C, 67.1; H, 7.84; N,
11.1.
3-(Tert-butoxycarbonyl)-6-methyl-3,4,5,7,8,12b-hexahydroimidazo[4',5':3,4]pyrido[2,1-a]isoquinolin-6-
ium iodide (7)
Me
In a 50 mL round bottom flask 64 µL (1 mmol) of methyl iodide was added to 100
Me
Me
mg 6 (0.3 mmol) dissolved in 30 mL acetone and the mixture was stirred at room
O
O
temperature for 72 h. The resulting solid was filtered off and dried under reduced
4
N
5
1
pressure to give 138 mg (96 %) of 7 as white crystal solid. mp 209 °C; H NMR
Me
2
N+
6
I-
N
1
(DMSO-d6, 400 MHz): δ (ppm) 1.57 (s, 9H, tert-butyl), 3.20-3.26 (m, 4H,
7
8
12
b
H4+H8), 3.32 (s, 3H, N-CH3), 3.72-3.92 (m, 4H, H5+H7), 5.91 (s, 1H, H12b),
12
11
13
7.25-7.38 (m, 3H, H9-11), 7.60 (d, J = 7.4 Hz, 1H, H12), 8.27 (s, 1H, H2); C
9
10
NMR (DMSO-d6): δ (ppm) 19.49, 23.25, 27.86, 49.32, 66.05, 86,47, 121.25,
127.26, 128.42, 128.73, 129.24, 133.74, 138.83, 147.27. Anal. Calcd for

HETEROCYCLES, Vol. 87, No. 3, 2013
557
C20H26IN3O2 129.24, 133.74, 138.83, 147.27. Anal. Calcd for C20H26IN3O2 + 0.8 H2O: C, 49.9; H, 5.77;
N, 8.7. Found: C, 49.5; H, 5.67; N, 9.1.
6-Methyl-4,5,6,7,8,13-hexahydro-3H-imidazo[4,5-f][3]benzazecine (3)
A mixture of 120 mg (0.25 mmol) 7 and 50 mL of freshly condensed liquid
4
H
3
N
ammonia were stirred at – 40 °C and small pieces of sodium metal were added
portionwise until the mixture maintained a deep blue color for 7 min. The reaction
was quenched by adding dropwise a saturated solution of NH4Cl in water until the
Me
2
N
N
7
8
13
1
12
11
blue color disappeared. The mixture was stirred at room temperature under
nitrogen and all ammonia was allowed to evaporate under a stream of nitrogen. To
9
10
the remaining solids 30 mL of water were added and the emulsion was extracted with CH2Cl2 (3x20 mL).
The combined organic layers were evaporated to dryness to yield the crude product. The pale yellow oil
upon trituration with a small amount of Et2O to give 38 mg (63%) of white crystals which were dried
under reduced pressure. mp 130 °C; 1H NMR (CDCl3, 250 MHz): δ (ppm) 2.21 (s, 3H, N-CH3), 2.55-2.70
(m, 6H, H5-8), 2.83 (t, J = 6.1 Hz, 2H, H4), 4.07 (s, 2H, H13), 7.03-7.18 (m, 3H, H9-11), 7.33 (d, J = 4.1
13
Hz, 1H, H12), 7.36 (s, 1H, H2); C NMR (CDCl3): δ (ppm) 26.48, 30.83, 34.23, 45.70, 57.12, 58.80,
126.32, 126.47, 130.17, 130.27, 132.76, 138.79, 139.89. Anal. Calcd for C15H19N3 *0.04 CH2Cl2: C, 73.8;
H, 7.86; N, 17.2. Found: C, 74.1; H, 7.56; N, 16.9. HRMS calcd for C15H20N3: 242.1657226. Found:
242.16549.
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