Intramolecular cyclization of 1-benzyl-2-(nitromethylene)pyrrolidines in triflic acid

Article abstract of DOI:10.1080/00397911.2014.937501

1-Benzyl-2-(nitromethylene)pyrrolidines in triflic acid undergo intramolecular cyclization to afford the corresponding 2,3-dihydro-1H-pyrrolo[1,2-b]isoquinolinium triflates and/or 10-amino-2,3-dihydro-1H-pyrrolo[1,2-b]isoquinolinium triflates, depending o

Full text of DOI:10.1080/00397911.2014.937501

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Intramolecular Cyclization of 1-Benzyl-2-
(nitromethylene)pyrrolidines in Triflic
Acid
Bamba Fanté a , Yaya Soro b , Mawa Koné a , Sorho Siaka b & Jean-
Marie Coustard c
a Laboratoire de Chimie Organique Structurale , Université Félix
Houphouët-Boigny , Abidjan , Côte d'Ivoire
b Laboratoire des Procédés Industriels de Synthèse, de
l'Environnement et des Energies Nouvelles , Institut National
Polytechnique Félix Houphouët-Boigny , Yamoussoukro , Côte
d'Ivoire
c Laboratoire IC2MP-UMR 7285 CNRS , Poitiers , France
Accepted author version posted online: 22 Jul 2014.Published
online: 26 Sep 2014.
To cite this article: Bamba Fanté , Yaya Soro , Mawa Koné , Sorho Siaka & Jean-Marie Coustard
(2014) Intramolecular Cyclization of 1-Benzyl-2-(nitromethylene)pyrrolidines in Triflic Acid, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
44:22, 3328-3336, DOI: 10.1080/00397911.2014.937501
To link to this article: http://dx.doi.org/10.1080/00397911.2014.937501
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Synthetic Communications1, 44: 3328–3336, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.937501
INTRAMOLECULAR CYCLIZATION OF 1-BENZYL-2-
(NITROMETHYLENE)PYRROLIDINES IN TRIFLIC ACID
1
Bamba Fante, Yaya Soro, Mawa Kone, Sorho Siaka, and
2
1
2
´
´
Jean-Marie Coustard3
1
´
Laboratoire de Chimie Organique Structurale, Universite Felix
´
ˆ
Houphoue¨t-Boigny, Abidjan, Cote d’Ivoire
2
´ ´
Laboratoire des Procedes Industriels de Synthese, de l’Environnement et des
Energies Nouvelles, Institut National Polytechnique Felix Houphoue¨t-Boigny,
`
´
ˆ
Yamoussoukro, Cote d’Ivoire
3Laboratoire IC2MP-UMR 7285 CNRS, Poitiers, France
GRAPHICAL ABSTRACT
Abstract 1-Benzyl-2-(nitromethylene)pyrrolidines in triflic acid undergo intramolecular
cyclization to afford the corresponding 2,3-dihydro-1H-pyrrolo[1,2-b]isoquinolinium tri-
flates and/or 10-amino-2,3-dihydro-1H-pyrrolo[1,2-b]isoquinolinium triflates, depending
on the nature of the aromatic substituent. Structures of products and reaction mechanisms
are discussed.
Keywords Intramolecular cyclization; isoquinolinium triflates; 2-(nitromethylene)
pyrrolidines; protonation; triflic acid
INTRODUCTION
In previous articles, we reported the behaviour and reactivity in superacidic
media (HF-SbF5 or triflic acid) of 1-heterosubstituted-1-methylthio-2-nitroethene
derivatives,[1,2] acyclic nitroketene S,S-acetals,[3] and nitroketene aminals.[4]
Received May 18, 2014.
Address correspondence to Bamba Fante, Laboratoire de Chimie Organique Structurale,
´
Universite Felix Houphoue¨t-Boigny, 22 BP 582 Abidjan 22, Cote d’Ivoire. E-mail: fante_bamba1@
´
´
ˆ
yahoo.fr
This publication is dedicated in memory of Professor Jean-Marie Coustard, who supervised this
work but passed away at the end of August 2013.
3328

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
3329
Scheme 1. Reactivity of 2-(nitromethylene)pyrrolidine in triflic acid.
Figure 1. 1-Benzyl-2-(nitromethylene)pyrrolidine derivatives 2a–d used in this study.
For instance, nonaromatic 1-methyl-2-(nitromethylene)pyrrolidine 1 in triflic
acid formed hydroxynitrilium ions that can be trapped in situ either with triflate
anion or benzene[1] (Scheme 1).
To widen the scope of these reactions in triflic acid and to synthesize new
pyrrolidines that may have some physiological interest, intramolecular reactions
were undertaken on 2-(nitromethylene)pyrrolidines with different para-substituted
N-benzyl groups 2a–d (Fig. 1).
RESULTS AND DISCUSSION
Starting Material
1-Benzylpyrrolidin-2-one derivatives 3 were prepared conventionally by alky-
lation[5] of pyrrolidin-2-one with the appropriate para-substituted bromobenzyle
derivative. Further treatment with dimethylsulfate followed by sodium ethoxide
led to the corresponding 1-benzyl-2,2-diethoxypyrrolidines 4,[6] which upon reaction
with nitromethane[7] afforded the expected 1-benzyl-2-(nitromethylene)pyrrolidine
derivatives 2 (Scheme 2). Column chromatography and crystallization from
CH2Cl2=hexanes afforded the expected products 2a–d. Yields are reported in
Scheme 2 without optimization.
Starting materials 2a–d were characterized from their NMR spectra, with the
vinylic protons in the range dH 6.69–6.87 ppm, the carbons bearing the nitromethylene
=
group ¼CH-NO2 at dC 109.7–109.9 ppm, and the other ethylenic carbons >C C at

´
B. FANTE ET AL.
3330
Scheme 2. 1-Benzyl-2-(nitromethylene)pyrrolidines 2a–d from 3a–d with yields (%) not optimized after
crystallization.
164.5–164.7 ppm. A single set of signals was observed by NMR, indicating the pres-
ence of a sole stereoisomer in solution, presumably the (E)-isomer, as previously
reported for 1-methyl-2-(nitromethylene)pyrrolidine by Rajappa et al.[8]
Reactions in Triflic Acid
Triflic acid (5 mL, 56.2 mmol) was placed under nitrogen in a 10-mL round-
bottomed flask equipped with a three-way valve and a Teflon-coated magnetic stir-
rer. Caution: Triflic acid is a strong corrosive acid that may react violently with
water. The system was cooled to ice-water bath temperature, and then the
1-benzyl-2-(nitromethylene)pyrrolidine 2 (1 mmol) was added. Compounds 2a–d dis-
solved easily under stirring. The reaction proceeded under nitrogen (Scheme 3) for 3
to 6 h, depending on the para-substituent. At the end of the reaction, the acidic
medium was quenched over CH2Cl2=MeOH (97=3) at ꢀ60 to ꢀ40 ꢁC and let warm
to about 0 ꢁC when brine (10 mL) and anhydrous Na2CO3 (6 g) were added. The
extraction was carried out promptly with CH2Cl2=MeOH (95=5).
The solvent was evaporated in vacuo to obtain crude product, which was pur-
ified using silica-gel column chromatography. The isolated products 5 and 6 were
crystallized from dichloromethane–petroleum ether. The yields were in the range
of 15 to 69% depending on the para-substituent as indicated in Table 1. The methoxy
compound gave the lowest yield, probably because of other intermolecular reactions
occurring in triflic acid, as previously observed in situ with 1-amino-2-nitroethylene
derivatives.[1] Compounds 5c and 6d were the sole products isolated from the
reactions with 2c and 2d respectively.
The products 5 and 6 were isolated as triflate salts. The triflate anion was
characterized from its CF3 group that resonates at dF ꢀ75 to ꢀ72 ppm and as a
Scheme 3. Intramolecular cyclization of 1-benzyl-2-(nitromethylene)pyrrolidines 2a–d in triflic acid.

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
3331
Table 1. Yields of isolated products after reaction in triflic acid
Starting compounds
Z
Products 5a–d yield (%)
Products 6a–d yield (%)
2a
2b
2c
2d
H
Me
MeO
F
39
62
15
0
27
31
0
69
Note. After crystallization.
quadruplet (JCF Hz) at dC 120–122 ppm. Molecular and formula weights of the
cationic part were determined by high-resolution mass spectrometric (HR-MS)
analysis. NMR analysis of compounds 5 and 6 are characterized by a highly
deshielded proton that resonates as singlet at dH 9–10 ppm. Chemical shifts with
the same magnitude were previously reported on the aromatic carbon adjacent to
the quaternary nitrogen.[9]
For example, with compound 5a (Z H), the 13C NMR spectrum showed the
=
presence of six sp2 CH, at dC 147.62 (dH 10.02), 137.63 (dH 8.17), 131.54 (dH
8.49), 131.21 (dH 7.97), 128.43 (dH 8.27), and 122.60 ppm (dH 8.47 ppm), and two
quaternary sp2 carbons, at 140.09 and 150.16 ppm. The signal at dC 147.62 ppm is
broad and weak because of 15N coupling (triplet) and because of the fast relaxation
effect caused by the quadripolar moment of 14N nucleus, the most abundant iso-
tope.[10] Chemical shifts are also close to values reported for vasconine 7 (Fig. 2).[9]
Compounds 6 are characterized with a broad singlet of 2H intensity at dH
6.6 ppm. These protons are easily exchangeable with D2O. Two-dimensional (2D)
NMR indicated that the hydrogens are not bonded to a carbon. This broad singlet
was attributed to the NH2 group, in agreement with the formula weight of the catio-
=
2
nic part. Compound 6a (Z H) was also characterized by the presence of five sp CH
at dC 134.5 (dH 7.9), 132.3 (dH 9.16), 131.2 (dH 8.05), 130.6 (dH 8.28), and 122.9 ppm
(dH 8.47 ppm) and three quaternary carbons at 129.3, 131.6, and 141.3 ppm. The sig-
nal at 132.3 ppm (dH 9.16) is broad and weak because of the proximity of the
ammonium nitrogen, as previously indicated for compound 5a.
NMR Study and Reaction Mechanism
To have a better understanding of the reaction mechanism, 1-(4-methylbenzyl)-
2-(nitromethylene)pyrrolidine 2b in triflic acid was examined by NMR spectroscopy
Figure 2. Chemical shifts of compound 5a and vasconine 7.

´
B. FANTE ET AL.
3332
at 255 K. The starting material quickly disappeared to afford the observable cation
8b with a deshielded methylene (dC 71.6 and dH 5.37 ppm) next to the protonated
nitro group. These values are close to those previously observed for C,O-diproto-
nated nitroethylene derivatives in triflic acid.[1,11,12] After the disappearance of
kinetic cation 8b, cation 12b was observed as the main cation in this experiment.
The reference spectrum for cation 12b was prepared by dissolving 5b in triflic acid
(see the supplementary material). The mechanism for the formation of cation 8b is
probably similar to the one previously reported for nitroketene aminals.[4] At the
beginning of the reaction, 1-benzyl-2-(nitromethylene)pyrrolidines 2a–d undergo
double protonation: a C-protonation on the carbon bearing the nitro group and
an O-protonation of the nitro group. The O-protonation occurs through a fast pro-
ton exchange process with the acidic medium, as observed in fluorosulfonic acid at
ꢀ80 ꢁC.[13] Prototropic exchanges and the formal loss of a molecule of water lead
to the formation of the conjugated hydroxynitrilium ion 9.[2,3] As soon as it is
formed, as indicated in Scheme 4, 9 reacts with the tethered phenyl ring to afford
Scheme 4. Plausible mechanism for the formation of 5 and 6.

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
3333
the kinetic transient tricyclic cation 10. In the final step, cation 10 undergoes aroma-
tization either with the loss of the oxime group, or the formation of the amino group,
as indicated in Scheme 4. The driving force of the reaction is probably the aromatiza-
tion of the heteroaromatic ring.
The addition of triflate anion on the hydroxynitrilium ion 9 should not be
excluded, as previously observed,[1] and might explained the poor yield observed
with the methoxy derivative.
On neutralizing the reaction, compounds 5 and 6 were isolated as triflate salts
because of the presence of quaternary nitrogen in the isoquinolinium structure.
CONCLUSION
The current study is an application of the use of triflic acid and 2-(nitromethy-
lene)pyrrolidine derivatives in the field of heterocyclic synthesis to afford 2,3-
dihydro-1H-pyrrolo[1,2-b] isoquinolinium triflates and=or 10-amino-2,3-dihydro-
1H-pyrrolo[1,2-b] isoquinolinium triflates, depending on the nature of the substitu-
ent on the phenyl ring. The formed cations were studied by NMR in situ and a
mechanism was postulated for the observed reaction.
From a practical point of view, this reaction is a new, easy route to isoquino-
linium derivatives. These derivatives occurred as natural products, with some of
them having inhibitor enzyme activities and anti-inflammatory[14,15] or anticancer
properties.[16,17] Simpler isoquinolinium are also known to form ionic liquid
crystals.[18,19]
EXPERIMENTAL
Melting points were determined with a Buchi melting-point B545 apparatus
¨
using capillary tubes (temperature rate 2 ꢁC=min) and were not corrected. A Bruker
1
DPX 300 spectrometer, equipped with a low-temperature probe, was used for H,
19F, and 13C NMR spectra, which were recorded at 300.13, 282.37, and 75.47 MHz,
respectively. NMR spectra of cations were recorded in neat triflic acid at low tem-
perature in the presence of CD3OD contained in a sealed capillary tube placed inside
the NMR tube. NMR spectra of starting compounds and products were recorded at
room temperature and chemical shifts reported relative to Me4Si or CFCl3 for fluor-
ine. The reproducibility of 13C NMR shift was about ꢂ0.05 ppm, depending on cell
and concentration. Chemical assignments were made using DEPT 135, 2D, and
usual chemical shift assignments rules. Electron-impact ionization (70 eV) mass spec-
tra were obtained with a Finnigan Incos 500 instrument. HR-MS was performed by
the Faculty of Sciences-University of Picardie=Jules Verne, France. Column chroma-
tography was carried out on flash silica gel (20 to 45 mm particle size). Triflic acid was
purchased from Acros. No attempt was made to optimize the yields.
(E)-1-Benzyl-2-(nitromethylene)pyrrolidine (2a)
1-Benzylpyrrolidin-2-one (4.63 g, 26.46 mmol) and dimethylsulfate (2.60 mL,
27.40 mmol) were heated at 80 ꢁC for 15 h under nitrogen atmosphere. The reaction
mixture was then cooled at 50=60 ꢁC and a solution of sodium ethoxide (0.742 g of

´
B. FANTE ET AL.
3334
sodium in ethanol, 14 mL) was added dropwise. The resulting mixture was stirred
under nitrogen for 4 h at 50=60 ꢁC. Then nitromethane dried over CaCl2 (3 mL,
55.0 mmol) was added, and the resulting solution was refluxed for 10 min. After cool-
ing, the solvent was evaporated under reduced pressure. The residue was dissolved in
CH2Cl2 and filtered, and the solvent was evaporated in vacuo. The resulting oil was
purified by column chromatography on silica gel (CH2Cl2=AcOEt, 99=1). Further
crystallization from CH2Cl2=petroleum ether afforded compound 2a as white
crystals (0.424 g, 28%).
1
Mp 94.9 ꢁC (CH2Cl2=petroleum ether). H NMR (300 MHz, CDCl3): d ¼ 2.10
(m, 2H, CH2-CH2-CH2), 3.56 (m, 4H, CH2-CH2-CH2), 4.40 (s, 2H, CH2-Ph), 6.86 (s,
1H, ¼CH-NO2), 7.08 (d, J ¼ 7.3 Hz, 2H, o-H), 7.37 (m, 3H, p-H, m-H). 13C NMR
(75 MHz, CDCl3): d ¼ 20.85 (CH2-C¼), 35.04 (CH2-CH2-CH2), 51.37 (CH2-Ph),
54.56 (N-CH2-CH2), 109.88 (¼CH-NO2), 127.60 (2 ꢃ ar-CH), 128.68 (ar-CH),
129.54 (2 ꢃ ar-CH), 134.24 (ipso-C), 164.72 (>C¼CH-; C-2). HRMS (ESI): m=z
[MþNa]þ calcd. for C12H14N2O2Na: 218.0953; found: 241.0959.
2,3-Dihydro-1H-pyrrolo[1,2-b]isoquinolin-4-ium
Trifluoromethanesulfonate (5a)
(E)-1-Benzyl-2-(nitromethylene)pyrrolidine 2a (218 mg, 1 mmol) was dissolved
in triflic acid (5 mL, 56.2 mmol) at 2 ꢁC under nitrogen atmosphere. The reaction was
monitored by thin-layer chromatography (TLC; CH2Cl2=MeOH, 97=3). After the dis-
appearance of the starting compound, the acidic solution was poured into 50 mL of a
mixture of CH2Cl2=MeOH (90:10) at ꢀ60 to ꢀ40 ꢁC and let warm. When the tempera-
ture was close to 0 ꢁC, brine (10 mL) and anhydrous Na2CO3 (6 g) were added. The
extraction was carried out promptly with CH2Cl2=MeOH, 95=5 (4 ꢃ 20 mL). The
organic phase was dried over MgSO4 and the solvent was evaporated under reduced
pressure. The crude product was purified by column chromatography using
CH2Cl2=MeOH (95=5) as eluent, followed by precipitation from a mixture of
dichloromethane=petroleum ether, to yield 5a (124 mg, 39%) as white powder.
Mp 73–74 ꢁC (CH2Cl2=petroleum ether). 1H NMR (300 MHz, acetone-d6):
d ¼ 2.66 (m, 2H, CH2-CH2-CH2), 3.67 (t, J 7.5 Hz, 2H, CH2-C¼), 5.16 (t, J
7.4 Hz, 2H, CH2-N), 7.98 (t, J 7.2 Hz, 1H, H-9), 8.18 (t, J 7.3 Hz, 1H, H-8), 8.26
(d, J 8.4 Hz, 1H, H-7), 8.47 (d, J 9.5 Hz, 1H, H-10), 8.49 (s, 1H, H-6), 10.02 (s,
1H, H-5). 13C NMR (75 MHz, CD3COCD3): d ¼ 23.90 (CH2-CH2-CH2), 31.99
(CH2-C¼), 60.06 (CH2-N), 122.61 (qd, JCF 321.6 Hz, CF3SO3ꢀ), 122.60 (C-6),
128.25 (C-5a), 128.43 (C-7), 131.21 (C-9), 131.54 (C-10), 137.63 (C-8), 140.09
(C-9a), 147.62 (C-5), 150.16 (C-10a). 19F NMR (282 MHz, CD3COCD3): d ¼
ꢀ73.9 ðCF3SO3ꢀÞ. HRMS (EI): m=z ½M-ðCF3SO3ꢀÞꢄ calcd. for C12H12N:
170.0969; found 170.0961.
10-Amino-2,3-dihydro-1h-pyrrolo[1,2-b]isoquinolin-4-ium
Trifluoromethanesulfonate (6a)
From 2a, the second product 6a (91 mg, 27%) was obtained as powder.
Mp 153–154 ꢁC (CH2Cl2=petroleum ether). 1H NMR (300 MHz, CD3COCD3):
d ¼ 2.66 (m, 2H, CH2-CH2-CH2), 3.67 (m, 2H, CH2-C¼), 5.16 (m, 2H, CH2-N), 6.65

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
3335
(broad s, 2H, NH2), 7.92 (dt, J 7.2 Hz; 1.2 Hz, 1H, H-8), 8.05 (dt, J 7.0 Hz; 1.0 Hz,
1H, H-7), 8.28 (d, J 8.3 Hz, 1H, H-9), 8.48 (d, J 8.6 Hz, 1H, H-6), 9.17 (s, 1H, H-5).
13C NMR (75 MHz, CD3COCD3): d ¼ 22.99 (CH2-CH2-CH2), 29.88 (CH2-C¼),
60.74 (CH2-Nþꢅ), 122.54 (qd, JCF 321.6 Hz, CF3SO3ꢀ), 122.98 (ꢆCH, C-6),
127.21 (C-5a), 129.35 (C-9a), 130.63 (ꢆCH, C-9), 131.18 (ꢆCH, C-7), 131.58
(C-10a), 132.32 (bs,>Nþ¼CH-; C-5), 134.49 (CH, C-8), 141.32 (C-NH2).
19F NMR (282 MHz, CD3COCD3): d ¼ ꢀ74.1 ðCF3SO3ꢀÞ. HRMS (EI): m=z [M-
(CF3SO3-)] calcd. for C12H13N2: 185.1074; found 185.1074.
Cation in Triflic Acid 12b
1H NMR (300 MHz, 298 K, CD3OD): d ¼ 1.98 (t, J 7,5 Hz, 2H, H-2), 2,04 (s,
3H, CH3), 2.92 (t, J 7.5 Hz, 2H, H-1), 4.26 (t, J 7.5 Hz, 2H, H-3), 7.16 (d, J 8,6 Hz,
1H, H-6), 7.27 (s, 1H, H-9), 7.45 (s, 1H, H-10), 7.47 (d, J 8,6 Hz, 1H, H-7), 8.58 (s,
1H, H-5). 13C NMR (75 MHz, 298 K, CD3OD): d ¼ 21.72 (CH3), 22.71 C-2), 30.80
(C-1), 58.77 (C-3), 121.29 (C-6), 126.14 (C-5a), 126.29 (C-10), 129.32 (C-7), 133.75
(C-9), 140.21 (C-8), 144.12 (C-5), 148.66 (C-9a), 150.40 (C-10a).
FUNDING
`
We acknowledge CNRS=France for financial support, the Ministere des
Affaires Etrangeres, France, for fellowship funds (Y. S. and S. S.), and the Ministere
`
`
´
de l’Enseignement Superieur Cote d’Ivoire for fellowship funds (B. F.).
ˆ
SUPPORTING INFORMATION
1
Full experimental details and H and 13C NMR spectra for this article can be
accessed on the publisher’s website.
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