7,8,9,10-Tetrahydropyrrolo[2,1-a]isoquinolines in the search for new indolizine derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.08.054

7,8,9,10-Tetrahydropyrrolo[2,1-a]isoquinolines were obtained by 1,3-dipolar cycloaddition reactions of their corresponding N-ylides with olefinic (acrylonitrile) and symmetrical or non-symmetrical acetylenic dipolarophiles (methyl/ethyl propiolate, dimeth

Full text of DOI:10.1016/j.tetlet.2014.08.054

Accepted Manuscript
7,8,.9,10-Tetrahydropyrrolo[2,1-a]isoquinolines in searching for new indoli-
zine derivatives
Mino R. Caira, Marcel Mirel Popa, Constantin Draghici, Loredana Barbu,
Denisa Dumitrescu, Florea Dumitrascu
PII:
S0040-4039(14)01388-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.08.054
TETL 45021
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
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28 July 2014
13 August 2014
Please cite this article as: Caira, M.R., Popa, M.M., Draghici, C., Barbu, L., Dumitrescu, D., Dumitrascu, F.,
7,8,.9,10-Tetrahydropyrrolo[2,1-a]isoquinolines in searching for new indolizine derivatives, Tetrahedron Letters
(2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.08.054
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Graphical Abstract
7,8,9,10-Tetrahydropyrrolo[2,1-a]isoquinolines in
searching for new indolizine derivatives
Mino R. Caira, Marcel Mirel Popa, Constantin Draghici,
Loredana Barbu, Denisa Dumitrescu, Florea Dumitrascu

1
Tetrahedron Letters
journal homepage: www.els evier.com
7,8,.9,10-Tetrahydropyrrolo[2,1-a]isoquinolines in searching for new indolizine
derivatives
Mino R. Caira,a Marcel Mirel Popab,c∗, Constantin Draghicib, Loredana Barbu,b Denisa Dumitrescu,d Florea
Dumitrascub
aDepartment of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
bCenter for Organic Chemistry C.D. Nenitescu, Romanian Academy, Spl. Independentei 202B, Bucharest 060023, Romania
cFaculty of Applied Chemistry and Materials Science, 'Politehnica' University of Bucharest, Polizu Street 1-7, 011061, Bucharest, Romania
dFaculty of Pharmacy, “Ovidius” University, Aleea Universitatii nr.1, Campus Corp B, Constantza 900470, Romania
ARTICLE INFO
ABSTRACT
Article history:
Received
7,8,9,10-Tetrahydropyrrolo[2,1-a]isoquinolines were obtained by 1,3-dipolar cycloaddition
reactions of their corresponding N-ylides with olefinic (acrylonitrile) and symmetrical or non-
Received in revised form
Accepted
Available online
symmetrical
acetylenic
dipolarophiles
(methyl/ethyl
propiolate,
dimethyl
acetylenedicarboxylate). Also, stable 5,6,7,8-tetrahydroisoquinolinium dicyanomethylide was
isolated and characterized by X-ray diffraction analysis.
Keywords:
indolizine
2009 Elsevier Ltd. All rights reserved.
1,3-dipolar cycloaddition
pyrrolo[2,1-a]isoquinoline
dicyanomethylide
X-ray diffraction
The indolizine core has attracted much interest due to its
biological activity1-5 and physico-chemical properties such as
fluorescence.6-9 Also, benzoindolizine frameworks such as
pyrrolo[1,2-a]quinoline and pyrrolo[2,1-a]isoquinoline are found
as the building blocks of a number of alkaloids such as
gephyrotoxin10 and lamellarins,11,12 which are of medicinal
importance.11 For medicinal reasons the hydrogenated or partially
hydrogenated pyrrolo[2,1-a]isoquinoline framework was of
interest with a special focus on obtaining lamellarin analogs with
biological activity (e.g. lamellarin D)13 and similar compounds
possessing a dihydrogenated pyridine moiety.14
starting from the corresponding oxazolo[3,2-a]isoquinolinium
salt.
The
new
7,8,9,10-tetrahydropyrrolo[2,1-a]isoquinoline
derivatives were obtained by 1,3-dipolar cycloaddition of the
corresponding N-ylides with olefinic or acetylenic dipolarophiles.
The ylides were generated in situ from the corresponding
tetrahydroisoquinolinium bromides 3a-e, which were easily
obtained
in
80-90%
yields
by reacting 5,6,7,8-
tetrahydroisoquinoline (1) with different bromoacetophenones
2a-e in acetone at room temperature (Scheme 1).21
As the 1,3-dipolar cycloaddition reaction has proved to be a
very convenient route to the class of pyrroloazine compounds,15-
we have extended our studies to obtaining 7,8,9,10-
tetrahydropyrrolo[2,1-a]isoquinolines via the 1,3-dipolar
R
R
O
acetone
r.t.
19
+
Br
+
N
N
Br
O
cycloaddition reaction
of the corresponding 5,6,7,8-
3a-e
2a-e
1
tetrahydroisoquinolinium N-ylides by different approaches.
Scheme 1. Synthesis of tetrahydroisoquinolinium bromides 3a-e
Herein we present the synthesis and characterization of new
partially hydrogenated pyrrolo[2,1-a]isoquinoline derivatives,
which might be of potential synthetic interest for medicinal
purposes and/or optical applications.
The synthetic route to the new 7,8,9,10-tetrahydropyrrolo[2,1-
a]isoquinolines was investigated in order to obtain optimum
conditions and simple work-up procedures. Scheme 2 depicts the
synthesis of the new compounds by different pathways. The
reaction mechanism of the 1,3-dipolar cycloaddition briefly
involves the generation of the N-ylide in situ by the action of a
base,15 or other deprotonation agent acting as a base,16 on the
corresponding cycloiminium bromide and subsequent
To our knowledge only one example of
a 7,8,9,10-
tetrahydropyrrolo[2,1-a]isoquinoline has been reported by
Babaev et al.,20 but it was obtained by an alternative method
———
Corresponding author address: mirelupb@gmail.com

2
Tetrahedron Letters
cycloaddition of the N-ylide to the olefinic or acetylenic
dipolarophile, thus leading to the generation of new condensed
five-membered ring systems (Scheme 2).
7
6
6a
5
8
4
10b
N
9
3
COAr
10a
10
2
1
MeOOC
H
4
i
iii
ii
N
N
COAr
+
COAr
N
CH2COAr
3
Br
NC
H
EtOOC
7
H
iv
5
Et
i
:
H
COOMe
COOEt
O
ii
:
H
/Et3N/CH2Cl2
N
COAr
/Et N/DMF/TPCD
CH2 =CHCN
MeOOC
iii :
iv :
3
/Et N/CH Cl
COOMe
3
2
2
MeOOC
COOMe
6
Scheme
2.
Synthesis
of
the
new
7,8,9,10-
tetrahydropyrrolo[2,1-a]isoquinolines 4-7
Table 1. The new tetrahydroisoquinolinium salts 3a-e and the
new 7,8,9,10-tetrahydropyrrolo[2,1-a]isoquinolines 4-7
Thus compounds 4 were obtained22 using 1,2-epoxybutane as
the reaction medium and agent for the generation of the ylide in
situ starting from the bromides 3 in the presence of methyl
acetylenecarboxylate. The reaction was also carried out by using
methylene chloride as the solvent and triethylamine as the base
for the generation of the ylides, as in the case of compounds 5
and 6 obtained by reacting 5,6,7,8-tetrahydroisoquinolinium
bromides 3 with ethyl propiolate or DMAD (Scheme 2).
R1
R2
N
COAr
+
Br
4-7
N
i, ii, iii or iv
CH2COAr
3a-e
R1
R2
R1
-
R2
-
Ar
m.p. (oC) Yield (%)
213-215 93
C6H5
3a
3b
3c
3d
3e
4a
4b
4c
5a
5b
5c
6a
6b
6c
7a
-
-
4-C6H5C6H4 219-222 90
4-BrC6H4 260-262 87
Compounds 7 were obtained using acrylonitrile as the olefinic
dipolarophile and TPCD (tetrakispyridineCo(II)dichromate)15 as
the oxidant in order to direct the reaction to the desired aromatic
compounds.
-
-
-
-
4-MeOC6H4 195-198 89
3-O2NC6H4 222-224 84
-
-
It is noteworthy that either reaction pathway works for each
dipolarophile employed with yields ranging from medium to
good, and could be also extended to other dipolarophiles. The
new compounds are presented in Table 1.
COOMe
COOMe
COOMe
COOEt
COOEt
COOEt
H
H
H
H
H
H
C6H5
135-136 45
4-MeOC6H4 129-132 60
3-O2NC6H4 190-192 52
4-BrC6H4
137-139 65
4-MeOC6H4 140-142 54
3-O2NC6H4 148-150 69
COOMe COOMe C6H5
152-154 50
58
COOMe COOMe 4-C6H5C6H4 98-100
COOMe COOMe 4-BrC6H4
163-166 65
149-152 60
CN
7b CN
CN
H
H
H
4-BrC6H4
4-MeOC6H4 175-177 63
3- O2NC6H4 189-191 55
7c

3
3.327(3)-3.446(3) Å, π-stacking (minimum centroid···centroid
distance 3.575 Å) and a C-H···π(phenyl) interaction with the
H···centroid distance being 2.78 Å.
The new compounds were characterized by NMR
spectroscopy, and for the representative compounds 3e and 5b,
single crystal X-ray diffraction analysis was performed.23
The NMR spectra of all the compounds presented the
expected signals. The intermediate salts 3 were also investigated
by NMR and X-ray (3a) analysis and their structures were
confirmed.
Figure 1 shows the X-ray structure of the new salt 3e. This is
the
first
structural
elucidation
of
a
5,6,7,8-
tetrahydroisoquinolinium salt in which there are no substituents
other than hydrogen at positions 5-8. The dihedral angle C7-C8-
C9-C10 is 58.6(3)°, indicating a half-chair conformation for the
partially saturated ring, while the plane of the ring N1ꢀC6 is
nearly orthogonal to the phenyl ring [interplanar angle 82.2(1)°].
Crystal cohesion is maintained by three C-H···Br- hydrogen
bonds and π-stacking of inversion-related phenyl rings with a
centroid···centroid distance of 3.834 Å.
Figure 2. X-ray structure of 5b with thermal ellipsoids for
non-H atoms drawn at the 50% probability level. Dotted lines
represent intramolecular C-H···O hydrogen bonds.
The isolation and characterization of stable dicyanopyridinium
ylides has been of interest for many years since their first
synthesis,24 as their reactions have been employed in obtaining
indolizines.25,26 Thus, by reacting 5,6,7,8-tetrahydroisoquinoline
with tetracyanomethyleneoxide, we obtained the corresponding
dicyanomethylide 9 (Scheme 3), as the first stable N-ylide of a
tetrahydroisoquinoline, which was investigated by NMR
spectroscopy and X-ray analysis.
O
EtOAc
r.t.
Figure 1. X-ray structure of 3e with thermal ellipsoids for
non-H atoms drawn at the 50% probability level.
N+
NC
CN
CN
+
N
C
CN
NC
1
The most characteristic H-NMR signals of compounds 4-7
CN
9
were the H-5 and H-6 hydrogens which appeared as two doublets
with J5,6= 7.1 Hz. In the case of the unsymmetrical dipolarophiles
(ethyl propiolate, methyl propiolate and acrylonitrile), the
regioselectivity of the reaction was proved by the singlet
assigned to H-2 at ~7.60 ppm and confirmed by X-ray diffraction
analysis. The main spectral 13C-NMR features were the signals of
the CO groups in the benzoyl and carboxylate moieties for
compounds 4, 5 and 6, and the CN group for compounds 7.
Carbon C-1 appeared highly shielded in compounds 7 due to the
shielding effect of the attached CN group.
Scheme
3.
Synthesis
of
novel
5,6,7,8-
tetrahydroisoquinolinium dicyanomethylide 9
The NMR spectrum showed the expected signals. In the
aromatic region two multiplets with integrals of 2:1
corresponding to the three aromatic protons in the pyridine
moiety were present. The 13C-NMR spectrum also exhibited the
expected signals, the main one being that of the exocyclic
carbanion C atom which appeared at 59 ppm [the signal was
resolved by adding chromium(III) acetylacetonate (Cr(acac)3) to
the sample].
Figure 2 shows the X-ray structure of compound 5b, a
representative of the new 7,8,9,10-tetrahydropyrrolo[2,1-
a]isoquinolines. The condensed ring system is rare, and as
alluded to earlier, the only other known compound in this class,
the X-ray structure of which had been reported, is 2-(4-
bromophenyl)-5-methoxy-7,8,9,10-tetrahydropyrrolo[2,1-
The IR spectrum showed two distinct bands for the two CN
groups at 2129 and 2171 cm-1. The stable ylide 9 could be
employed in further 1,3-dipolar cycloaddition reactions with the
aim of conferring structural variations on the 7,8,9,10-
tetrahydropyrrolo[2,1-a]isoquinoline framework.
a]isoquinoline (Cambridge Structural Database refcode
XIDKEB).20 However, some structural features of 5b are
distinctly different from those reported for XIDKEB. For
example, at the low temperature of the X-ray analysis, the
partially saturated ring in 5b is ordered, adopting a half-chair
conformation with a C8-C9-C10-C11 dihedral angle of 61.5(4)°,
with the five formal single C-C bonds being close to 1.50 Å
(consistent with the results for 3e above), whereas the equivalent
dihedral angle in XIDKEB has a magnitude of only ~33°, with
the distance equivalent to C9-C10 being abnormally low at ~1.33
Å. (These discrepancies are possibly due to disorder in the latter
case, so further detailed comparison is not warranted). In
addition, the overall conformation of 5b is uniquely maintained
by the two intramolecular C-H···O hydrogen bonds shown in
Figure 2. The crystal structure is further stabilized by three
intermolecular H-bonds with C···O distances in the range
Figure
3
shows the X-ray structure23 of the new
dicyanomethylide 9. The saturated residue again adopts a half-
chair conformation, as found for 3e and 5b, with a dihedral angle
magnitude of 62.5(2)°. The six atoms comprising the
dicyanomethylide unit are co-planar, with a maximum deviation
from their least-squares plane of 0.006(2) Å (atom N2). The
interplanar angle between this unit and the aromatic ring is
19.4(1)°. Pertinent bond lengths (listed in the caption to Figure 3)
indicate a highly symmetrical arrangement about the N2+-C11-
axis. These are in accord with those reported in the crystal of
pyridinium-1-dicyanomethylide.27 While the two C≡N bond
lengths in 9 do not differ significantly (difference ~2.6 combined
standard deviations), the environments of atoms N13 and N15 in
the crystal are distinct, atom N13 being the acceptor of an
intermolecular hydrogen bond [C3-H3···N13i (i = x, -1+y, z)

4
Tetrahedron Letters
10. Popa, M. M, Caira, M. R.; Dumitrascu, F. in Bioactive
with H···N 2.47 Å, C···N 3.165(3) Å and C-H···N angle
Heterocycles: Synthesis and Biological Evaluation; Ameta, K. L.,
Ed.; NovaScience Publishers: New York 2013, pp. 19-40.
11. Li, Q.; Jiang, J.; Fan, A.; Cui, Y.; Jia, Y. Org. Lett. 2011, 13, 312.
12. Shen, L.; Yang, X.; Yang, B.; He, Q.; Hu, Y. Eur. J. Med. Chem.
2010, 45, 11..
131°], and N15 instead not engaged in hydrogen bonding. We
infer that this structural difference may account for the
appearance of two distinct infrared peaks in the range expected
for the CN stretching vibration. In the crystal, the pyridinium
rings of two inversion-related molecules of 9 engage in offset π-
stacking with a centroid···centroid distance of 3.688 Å.
13. Fan, H.; Peng, J.; Hamann, M. T.; Hu, J.-F. Chem. Rev. 2008, 108,
264.
14. Wang, H.-T., Lu, C.-D.; Tetrahedron Lett. 2013, 54, 3015.
15. Dumitrascu, F.; Mitan, C. I.; Draghici, C.; Caproiu, M. T.;
Raileanu, D. Tetrahedron Lett. 2001, 44, 8379.
16. Dumitrascu, F.; Georgescu, E.; Georgescu, F.; Popa, M. M.;
Dumitrescu, D. Molecules 2013, 18, 2635.
17. Xia, Z.; Przewloka, T.; Koya, K.; Ono, M.; Chen, S.; Sun, L.
Tetrahedron Lett. 2006, 47, 8817.
18. Caira, M. R.; Georgescu, E.; Georgescu, F.; Albota, F.;
Dumitrascu, F. Monatsh. Chem. 2011, 142, 743.
19. Shang, Y.; Zhang, M.; Yu, S.; Ju, K.; Wang, C.; He, X.
Tetrahedron Lett. 2009, 50, 6981.
20. Babaev, E. V.; Efimov, A.V.; Tsisevich, A. A.; Nevskaya, A. A.;
Rybakov, V. B. Mendeleev Commun. 2007, 17, 130.
21. The 5,6,7,8-tetrahydroisoquinoline 1 (8 mmol) and substituted
bromoacetophenone 2 (8 mmol) were stirred in acetone for 6 h.
Figure 3. X-ray structure of 9 with thermal ellipsoids for non-
H atoms drawn at the 50% probability level. Selected bond
distances (Å): C1-N2 1.354(3), N2-C3 1.356(3), N2-C11
1.418(3), C11-C12 1.397(3), C11-C14 1.396(3), C12-N13
1.146(3), C14-N15 1.157(3).
The precipitated compounds
obtained were filtered and were used further in the synthesis of
compounds 4-7. 2-[2-Phenyl-2-oxoethyl]-5,6,7,8-
3 (see Supporting Information)
tetrahydroisoquinolinium bromide (3a). Colorless crystals, with
m.p. 213-215 oC, 93%. Anal. Calcd. C17H18BrNO: C 61.46, H
5.46, Br 24.05, N 4.22. Found: C 61.75, H 5.72, Br 24.42, N 4.43.
1H-NMR (300 MHz, CDCl3, δ): 1.83-1.88 (m, 4H, 2CH2); 2.90,
2.98 (2t, J = 6.0 Hz, 4H, 2CH2); 6.90 (s, 2H, CH2-N); 7.41-7.47
(m, 2H, H-3′, H-5′); 7.55-7.58 (m, 1H, H-4′); 7.64 (d, J = 6.3 Hz,
1H, H-4); 8.07-8.11 (m, 2H, H-2′, H-6′); 8.89 (d, J = 6.3 Hz,1H,
H-3); 9.02 (s, 1H, H-1). 13C-NMR (75 MHz, CDCl3, δ): 21.0
(2CH2); 26.2, 29.7 (2CH2); 65.9 (CH2-N); 127.3 (C-4); 128.8,
129.0 (C-2′, C-3′, C-5′, C-6′); 133.5, 137.9, 142.2 (C-4a, C-8a, C-
1′); 134.7 (C-4′); 145.6 (C-1); 158.4 (C-3); 190.8 (COAr).
In conclusion, we have investigated the efficient synthesis of
new 7,8,9,10-tetrahydropyrrolo[2,1-a]isoquinolines 4-7 formed
via 1,3-dipolar cycloaddition reaction of the corresponding
ylides. The compounds were characterized by NMR spectroscopy
as well as X-ray diffraction in the case of the representative
compounds 3e and 5a. In addition, we were interested in
investigating the stable ylides corresponding to 5,6,7,8-
tetrahydroisoquinoline, and we subsequently isolated, and
characterized by X-ray analysis, the corresponding
tetrahydroisoquinolinium dicyanomethylide 9. The new
compounds could be of relevant synthetic and medicinal interest
and show promising optical properties.
22. To isoquinolinium bromide
3 (3 mmol) was added methyl
propiolate (4 mmol) and the reactiom mixture was stirred under
reflux in 1,2-epoxybutane for 12 h. After the evaporation of the
solvent the residue was treated with ethanol and the crystalline
compound 4 was filtered and washed with a small amount of cold
ethanol on the filter paper. Compounds 4 were crystallized from
ethanol. Methyl 3-benzoyl-7,8,9,10-tetrahydropyrrolo[2,1-
a]isoquinoline-1-carboxylate (4a) Yellow crystals, m.p. 135-136
oC, 45% yield. Anal. Calcd. C21H19NO3: C 75.66, H 5.74, N 4.20.
Found: C 75.85, H 5.51, N 4.48. 1H-NMR (300 MHz, CDCl3, δ):
1.83-1.87 (m, 4H, 2CH2); 2.84, 3.20 (2t, J = 6.0 Hz, 4H, CH2);
3.82 (s, 3H, Me); 6.79 (d, J = 7.1 Hz, 1H, H-6); 7.47-7.54 (m, 3H,
H-3′, H-4′, H-5′); 7.72 (s, 1H, H-2); 7.77-7.80 (m, 2H, H-2′, H-6′);
9.76 (d, J = 7.1 Hz,1H, H-5).13C-NMR (75 MHz, CDCl3, δ): 21.9,
22.6 (2CH2); 27.5, 29.9 (2CH2); 51.6 (Me); 128.4, 129.0 (C-2′, C-
3′, C-3′, C-6′); 117.9, 126.2, 131.3, 131.4 (C-2, C-5, C-6, C-4′);
107.2, 121.5, 128.1, 137.9, 139.2, 140.4 (C-1, C-3, C-6a, C-10a,
C-10b, C-1′); 164.9 (COOMe); 185.1 (COAr). (For compounds 5-
7 see the Supporting Information associated with this paper).
23. Crystallographic data (excluding structure factors) for the
structures in this paper (3e, 5b and 9) have been deposited with
the Cambridge Crystallographic Data Centre as supplementary
publication nos. CCDC 3e: 1000508; 5b: 1000509; 9: 1000510.
Copies of the data can be obtained, free of charge, on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-
(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
MRC is grateful to the NRF (Pretoria) and the University of
Cape Town for research support. MP gratefully acknowledges
financial support from the Sectorial Operational Programme
Human Resources Development 2007-2013 of the Ministry of
European
Funds through
the
Financial
Agreement POSDRU/159/1.5/S/134398.
Supplementary data
Supplementary data associated with this article can be found
in the online version at DOI:
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