Cyclization of nitroacetamide derivatives with a tethered phenyl ring in triflic acid

Article abstract of DOI:10.1055/s-0033-1340960

N-(3-Hydroxypropyl)-2-nitro-N-(ω-phenylalkyl)acetami-des underwent intramolecular cyclization in triflic acid to afford the corresponding hydroxyimino six- to nine-membered benzofused lactams. The six-membered derivative slowly transformed into 2-(3-hydroxypropyl)isoquinolin-3-one. NMR spectroscopic analysis in situ provided information on the cationic species involved in the reaction, permitting a mechanism to be postulated. This reaction provides a novel and simple route to benzofused lactams.

Full text of DOI:10.1055/s-0033-1340960

LETTER
▌969
lCettyerclization of Nitroacetamide Derivatives with a Tethered Phenyl Ring in
Triflic Acid
Cyclization of Nitroacetamide Derivatives
Bamba Fanté,*a Yaya Soro,b Sorho Siaka,b Jérôme Marrot,c Jean-Marie Coustard†d
a
Laboratoire de Chimie Organique Structurale, Université Félix Houphouët-Boigny, 22 BP 582 Abidjan 22, Côte d’Ivoire
E-mail: fante_bamba1@yahoo.fr
b
Laboratoire des Procédés Industriels de Synthèse, de l’Environnement et des Énergies Nouvelles, Institut National Polytechnique Félix
Houphouët-Boigny, BP 991 Yamoussoukro, Côte d’Ivoire
c
Laboratoire ILV-UVSQ – UMR 8180 CNRS, 45 Avenue des États Unis, 78035 Versailles Cedex, France
d
Laboratoire IC2MP-UMR 7285 CNRS, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Received: 03.12.2013; Accepted after revision: 19.02.2014
This publication is dedicated to the memory of Professor Jean-Marie Coustard who supervised this work but passed away at the end of
August 2013. Rest in peace dear Professor.
In triflic acid or in HF–SbF5, ethyl 2-nitroacetate (1) un-
Abstract: N-(3-Hydroxypropyl)-2-nitro-N-(ω-phenylalkyl)acetam-
dergoes protonation and the loss of a molecule of water to
ides underwent intramolecular cyclization in triflic acid to afford the
afford a transient reactive hydroxynitrilium ion (protonat-
ed nitrile oxide) able to react with aromatics to form, for
corresponding hydroxyimino six- to nine-membered benzofused
lactams. The six-membered derivative slowly transformed into 2-
(3-hydroxypropyl)isoquinolin-3-one. NMR spectroscopic analysis instance, ethyl (2E)-2-hydroxyimmino-2-phenylacetate
(2, Ar = Ph; Scheme 1).4
in situ provided information on the cationic species involved in the
reaction, permitting a mechanism to be postulated. This reaction
provides a novel and simple route to benzofused lactams.
The configuration of the oxime is indirect evidence of the
existence of a reactive transient species with a CN triple
Key words: cations, protonation, cyclization, lactams, bicyclic
compounds
bond.5
The protonation/dehydration of N,S- or S,S-nitroketene
aminals 3 in triflic acid initially affords cations 4, then the
stable hydroxyiminium cations 5 that have been charac-
Nitroacetic acid and its derivatives are useful molecules
for the introduction of nitro groups when the nitration
methodology is not satisfactory.1 For instance, nitroacet-
amides are important precursors for the synthesis of
isatins2 and are used as a peptide synthons in the synthesis
of dipeptides.3
1
terized by H NMR and 13C NMR spectroscopy at low
temperature.6,7 They are able to react in situ either with an-
6
–
ions F– in HF–SbF5 or CF3SO3 in triflic acid8 or aromatic
rings.6–11 During the quenching of the acidic medium,
they may either be trapped by MeSH or MeOH,6 or form
a very reactive nitrile oxide able to react with a phenyl
ring by a cycloaddition process, to afford a conjugated cy-
clohexadiene (Scheme 2).9
O
OEt
Ar
NO2
OEt
TFSA, ArH
N
From a synthetic viewpoint, ethyl 2-nitroacetate (1) is a
versatile synthon for the preparation of nitroacetamides in
a one-step reaction with amines.12 Nitroacetamides can
also be prepared from nitroketene S,N-acetals,3,13 by ring
transformation of 3,5-dinitro-2-pyridones,14 by the reac-
O
OH
1
2
Scheme 1
H
+
Y
+
N
OH
O
NO2
a) NuH
b) – H
Z
Nu
+
Y
EtO
Y
NO2H
+
N
1
+
C
OH
H
+
Z
Z
H
5
Y
4
+
N
O–
+
C
Y
Z
NO2
Z
3
Scheme 2
SYNLETT 2014, 25, 0969–0974
Advanced online publication: 24.03.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340960; Art ID: ST-2013-D1105-L
© Georg Thieme Verlag Stuttgart · New York

970
B. Fanté et al.
LETTER
MeOH
Br
+
H2N
OH
H
N
OH
18–20 °C, 72 h
n
6a n = 1
n 8a–d
7
6b n = 2
6c n = 3
6d n = 4
MeS
MeS
NO2
+
NO2
NO2
9
SMe
O
N
OH
N
OH
n
n
10a–d
11a–d
Scheme 3
tion of nitroacetic acid chloride with amines,15a and from although this ratio depends upon factors such as the nature
α-bromoacetamide by nucleophilic bromide displacement of the solvent, acidity, substituents, and possible π-stack-
with nitrite.15b Approaches involving ring opening fol- ing.18c
lowed by intramolecular cyclization of phenyl-substituted
In triflic acid, the starting materials 11a–d were cleanly
transformed after three hours at 60 °C (Scheme 4) with
yields varying from 61–66% (Table 1) except for 11a for
lactams or cyclization of β,γ-unsaturated amides in triflic
acid have also been reported.16
In the present paper we report on the reactivity of N,N-di- which the reaction was more complex and afforded 12a
substituted nitroacetamides possessing a tethered phenyl along with the corresponding isoquinolin-2-one 13.
ring 11a–d in triflic acid. Nitroacetamides 11a–d were
Products, 12a–d and 13 result from an intramolecular cy-
prepared by a one-pot reaction between the corresponding
clization with the loss of a molecule of water (Scheme 4).
3-(ω-phenylalkylamino)propan-1-ol and 1,1-bis(methyl-
The reaction leads to a complex mixture when n = 1 be-
cause of the aromatization of the ring with the loss of the
oxime group leading to 13 (Scheme 4).
thio)-2-nitroethylene.13a The presence of the 3-aminopro-
panol group contributes to the stability of the cationic
species formed in trific acid and to the final isolated prod-
ucts.
Compounds 12a–d were characterized by the presence of
four CH aromatic carbons (DEPT 135), an oxime carbon
The reaction between 1-bromo-ω-phenylalkyl derivatives
(δC = 154.7–162.7 ppm) and an amidic carbonyl carbon
6a–d and two equivalents of 3-aminopropan-1-ol (7) in
(δC = 162.9–169.7 ppm). The presence of a deshielded ortho-
methanol afforded the 3-(ω-phenylalkylamino)propan-1-
aromatic proton (δH = 8.9 ppm for 12a) indicates the pres-
ols 8a–d. In a second step, 8a–d were condensed with 1,1-
ence of an E-configured oxime, with the phenyl ring and
bis(methylthio)-2-nitroethene (9) in refluxing tert-butanol
the oxime OH in a cis configuration. The structures were
(Scheme 3) with DMAP, to afford nitroacetamides 11a–d
further confirmed by X-ray crystallographic analysis of
(Scheme 3).13a
12b (Figure 1).19–22 It is worthy of note that the primary al-
Nitroacetamides 11a–d were isolated by column chroma- coholic function was not affected during this reaction.
tography on silica gel in low to medium yields (24–41%)
as indicated in Table 1.
In the crystal, the nonaromatic ring is in a pseudo-boat
conformation and the amidic function is planar. The tor-
The amidic function of the nitroacetamide is planar in the sion angle between the amidic C=O and the C=N oxime
solid state17 and the partial CN double bond character of bonds is close to 68° and the torsion angle between the ar-
the amidic function leads to the existence of two Z- and E- omatic ring and the oxime is close to 60° causing only
isomers in solution18a,b as indicated by the two sets of sig- weak electronic conjugation in the benzazepinic system.
nals in the NMR spectra (CDCl3), with a ratio close to 1:1;
Table 1 Yields of Products 8, 11, 12, and 13
Starting compounds
Yield of products 8a–d (%)a Yield of nitroacetamide 11a–d (%)a Yield of bicyclic products 12a–d and 13 (%)a
6a
6b
6c
6d
44
61
51
66
28
29
41
24
6 + 11
66
61
66
a Isolated yields after column chromatography.
Synlett 2014, 25, 969–974
© Georg Thieme Verlag Stuttgart · New York

LETTER
Cyclization of Nitroacetamide Derivatives
971
HO
N
O
O
N
+
N
OH
N
OH
12a
13
n = 1
NO2
HO
OH
O
O
n = 2–4
TFSA
(+ TFSA for n = 3, 4)
N
1–4
OH
N
60 °C, 3 h
11a–d
1–3
12b–d
Scheme 4
rapid exchange rate with the medium.23 The oxygen atom
of the OH group is probably protonated as indirectly indi-
cated by the deshielding of the methylene carbon bearing
the alcohol (δ = 68.8 ppm as compared to δC = ca. 58 ppm
in the nonprotonated form 11b). The amide group is also
known to be readily O-protonated in superacids24a,b and
particularly in triflic acid where the DMF triflate salt can
be isolated.25 The major cationic species 14a is proposed
to be the Z-isomer due to the slight anisotropic magnetic
shielding effect of the phenyl ring in this conformation6–8
+
and because this places the tethered OH2 further from the
protonated nitro group. Given these assumptions, the
probable structures of the cations 14a and 14b are given
in Figure 2.
Figure 1 X-ray crystal structure of 12b
After 1.5 hours reaction time at 60 °C, cations 14a,b were
totally transformed into a stable cation 16b, characterized
by the presence of iminium and hydroxyiminium groups
at δC = 159.7 and 151.1 ppm, respectively. Protonation of
the alcoholic OH was not directly observed but can be
postulated because of the strong deshielding of the meth-
To have a better understanding of the reaction mecha-
nism, nitroacetamide 11b in triflic acid was examined by
NMR spectroscopy at 5 °C. Nitroacetamide 11b was
found to be protonated to give a stable trication at low or
close to room temperature (molar ratio triflic acid/α-nitro-
acetamides 11 close to 56:1). Two isomeric cations 14a
and 14b were formed as indicated by the two sets of NMR
signals (Z/E ratio close to 2:1). Six CH2 and three aromatic
CH were identified by DEPT 135 analysis (Figure 2). Un-
der these conditions the nitro group is known to be proton-
ated but the proton is not observed because of its very
+
ylene CH2OH2 (δC = 74.2 ppm as compared to δC = ca.
58.7 ppm in the starting compound 11b and δ = 60.96 ppm
in the final isolated product 12b). Trication 16b (Figure 3)
is stable for weeks in triflic acid, the protonated alcoholic
OH and the positively charged nitrogen at both ends of the
alkyl chain appearing to have a stabilizing effect, in sharp
Figure 2 DEPT 135 NMR spectra of trications 14a and 14b in triflic acid at 278 K
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 969–974

972
B. Fanté et al.
LETTER
Figure 3 13C NMR spectrum of trication 16b in triflic acid at 278 K
contrast to the degradation observed for long-chain ali- Scheme 5. The oxime configuration is consistent with the
phatic alcohols in superacid.26 The high stability of N-pro- mechanism of an addition on a triple bond, as predicted5
tonated oxime has also been reported in triflic acid.6–8,10,11 and in agreement with previously reported results.6–11 The
Protonation of the heteroatoms thus affords stable poly- mechanism also explains why only the E-configured trica-
protonated species 16b insensitive to further electrophilic tion 16 was formed. The sequence is completed by the in-
addition. Such a phenomenon has previously been pro- volvement of other transient superelectrophilic species as
posed to explain the selective reactions on polyfunctional- postulated previously for reactions in triflic acid.28
ized natural products in HF–SbF5.27
On neutralizing the reaction, compounds 12 and 13 were
The mechanism in Scheme 5 for the formation of com- isolated; compounds 12a,b being isolated as neutral com-
pounds 12 and 13 may be postulated. Substrate 11 under- pounds, whilst 12c,d were isolated as triflate salts.
goes multiple protonation and the loss of water to afford
the conjugated hydroxynitrilium ion 15. Once formed, 15
reacts with the tethered phenyl ring by electrophilic aro-
matic substitution to afford the observed trication 16b,
with the OH group of the formed oxime in the cis config-
uration relative to the aromatic ring, as described in
The conformation of the nonaromatic ring is probably re-
sponsible for the increased basicity of the lactam because
of the weak conjugation in the hydroxyimino benzofused
lactam ring (nonplanarity). In favor of this assumption is
the increased shielding of the aromatic proton close to the
hydroxyimino group: δH = 8.88 ppm in 12a (close to pla-
O
N
OH
13
n = 1
NO2
HO
H
N
NO2H
HO
N
N
O
O
O
TFSA
O
H
H
H
N
N
n
N
OH2
n
n
n
14
11
16
15
OH
OH2
OH2
n = 1–4
HO
N
O
N
OH
1–4
12
( + TFSA for n = 3, 4)
Scheme 5 Suggested mechanism for the formation of compounds 12 and 13
Synlett 2014, 25, 969–974
© Georg Thieme Verlag Stuttgart · New York

LETTER
Cyclization of Nitroacetamide Derivatives
973
HO
H
N
HO
H
H
HO
H
N
N
N
N
H
OH
R
OH
R
O
R
OH
R
HNO
N
– H+
prototropic
exchange
N
H
H
O-protonated 13
16a
R = (CH2)3OH2+
Scheme 6
(E)-N-Benzyl-N-(3-hydroxypropyl)-2-nitroacetamide [(E)-11a]
1H NMR(300 MHz, CDCl3): δ = 1.77 (m, 2 H, CH2CH2CH2), 3.62
(m, 4 H, NCH2CH2 and CH2OH), 4.62 (s, 2 H, CH2Ph), 5.58 (s, 2 H,
CH2NO2), 7.17–7.20 (m, 1 H, arom. p-H), 7.24–7.43 (m, 4 H, arom.
o-H and m-H). 13C NMR (75 MHz, CDCl3): δ = 30.33
(CH2CH2CH2), 44.36 (NCH2CH2), 48.72 (CH2Ph), 58.26 (CH2OH),
77.58 (CH2NO2), 128.22 (p-CH), 128.52 (o-CH), 129.19 (m-CH),
136.49 (ipso-C), 163.09 (C=O). ESI-HRMS: m/z [M + Na]+ calcd
for C12H16N2O4 + Na: 275.1008; found: 275.1005.
narity) to δ = 7.66 ppm in 12b (θ ≈ 60° as indicated Figure
1). After several hours in methanol at room temperature,
12c,d slowly isomerized to the Z-isomers. A plausible
mechanism to explain the formation of 13 is illustrated in
Scheme 6, wherein the trication 16a would undergo de-
protonation followed by prototropic exchanges leading to
loss of the oxime, to afford the O-protonated isoquinolin-
2-one 13.
Synthesis of (1E)-1-Hydroximino-3-(3-hydroxypropyl)-4,5,6,7-
tetrahydro-3-benzazonin-2-one (12d)
In conclusion, the present study extends previous works
on the use of hetero-substituted nitroethene derivatives
and 2-nitroesters in the field of heterocyclic synthesis. In
triflic acid at 60 °C for three hours, nitroacetamides 11
lead to conjugated hydroxynitrilium ions which undergo
an electrophilic aromatic substitution to afford the corre-
sponding hydroxyimino-benzofused lactams (isoquinoli-
none, benzazepinone, benzazocinone, and benzazoninone).
The six-membered ring hydroxyimino lactam slowly
eliminates the oxime group to afford the corresponding
isoquinolin-2-one. From a practical viewpoint, this reac-
tion affords a simple route to benzofused lactams such as
benzazocin-2-one and benzazonin-2-one derivatives that
have been reported as potential angiotensin converting en-
zyme inhibitors, antihypertensive agents, and for the treat-
ment of neuropsychological disorders.29
Nitroacetamide 11d (294.00 mg, 1.00 mmol) was dissolved in triflic
acid (5.00 mL, 56.20 mmol) at 2 °C under nitrogen (molar ratio tri-
flic acid/α-nitroacetamides 11 ca. 6:1). The solution was stirred at
60 °C reaction was complete, monitoring by TLC (3 h). At the end
of the reaction, the solution was cooled below 0 °C and then poured
into 50 mL of CH2Cl2–MeOH (90:10) at –60 to –40 °C. The result-
ing solution was allowed to warm with gentle stirring and, when the
temperature was close to 0 °C, cold brine (10.00 mL) and then an-
hydrous Na2CO3 (6.00 g) were added. The organic phase was sepa-
rated, and the aqueous phase was further extracted with CH2Cl2–
MeOH (90:10; 4 × 20 mL). The combined organic phases were
dried over MgSO4, filtered, and the solvent removed under reduced
pressure. The residual oil was purified by column chromatography
(CH2Cl2–MeOH, 95:5) to afford 12d as its triflate (280.00 mg,
66%) as colorless crystals.
Mp 55.6 °C. IR (KBr): 3066 5 (s), 2951, 2866, 2363, 1734 (s), 1617
(m), 1459 (m), 1437 (m), 1275 (s), 1250 (s), 1170 (s), 1031 (s), 763
(m), 639 (m) cm–1. 1H NMR (300 MHz, CDCl3): δ = 1.69 (br s, 4 H,
CH2CH2CH2C≤), 2.06 (q, J = 5.7 Hz, 2 H, CH2CH2OH), 2.69 (t,
J = 6.3 Hz, 2 H, CH2C≤), 2.94 (m, 4 H, CH2NHCH2), 4.45 (br s, 2
H, CH2OH), 7.16 (d, J = 7.5 Hz, 1 H, o-H), 7.31–7.45 (m, 3 H,
arom. H). 13C NMR (75 MHz, CD3OD): δ = 24.27
(CH2CH2CH2C≤), 25.67 (CH2CH2OH), 26.18 (CH2CH2C≤), 33.05
(CH2C≤), 43.87 (CH2CH2CH2OH), 46.07 (CH2N), 64.67 (CH2OH),
Typical Procedures
Synthesis of N-Benzyl-N-(3-hydroxypropyl)-2-nitroacetamide
(11a)
1,1-Bis(methylthio)-2-nitroethylene (9, 1.00 g, 6.06 mmol) was
added to a solution of N-benzylamino-3-propan-1-ol (8a, 1.00 g,
6.06 mmol) and DMAP (0.74 g, 6.06 mmol) in t-BuOH (20 mL).
The resulting solution was heated to reflux for 24 h under nitrogen
atmosphere monitoring by TLC (CH2Cl2–MeOH, 97:3). After cool-
ing to r.t., the solvent was removed in vacuo to afford an oily residue
that was purified by flash chromatography on silica (CH2Cl2–
MeOH, 98:2) to give nitroacetamide 11a (oil, 0.42 g, 28%) as a mix-
ture of Z- and E-isomers in a ratio close to 1:1.
–
122.11 (qd, JC,F = 318.4 Hz, CF3SO3 ), 127.93 (C-10), 129.94 (C-
8), 130.88 (C-11), 131.09 (C-9), 132.52 (C-11a), 140.51 (C-7a),
151.64 (C=NOH), 165.33 (C=O). 19F NMR (282 MHz, acetone-d6):
–
δ = –79.24 (CF3SO3 ). HRMS (EI): m/z [M + H]+ calcd for
C15H21N2O3: 277.1552; found: 277.1556.
Acknowledgement
(Z)-N-Benzyl-N-(3-hydroxypropyl)-2-nitroacetamide [(Z)-11a]
1H NMR(300 MHz, CDCl3): δ = 1.77 (m, 2 H, CH2CH2CH2), 3.28
(t, J = 6.7 Hz, 2 H, NCH2CH2), 3.62 (m, 2 H, CH2OH), 4.49 (s, 2 H,
CH2Ph), 5.27 (s, 2 H, CH2NO2), 7.17–7.20 (m, 1 H, p-H), 7.24–7.43
(m, 4 H, arom. o-H and m-H). 13C NMR (75 MHz CDCl3): δ = 30.14
(CH2CH2CH2), 43.42 (CH2N), 51.64 (CH2Ph), 59.20 (HOCH2),
77.40 (CH2NO2), 126.53 (p-CH), 128.79 (o-CH), 129.79 (m-CH),
135.07 (ipso-C), 162.45 (C=O).
The authors thank CNRS/France for financial support, the ‘Mini-
stère des Affaires Etrangères’ France, for Fellowship funds (S.S.
and S.Y.) and the ‘Ministère de l’Enseignement Supérieur’ Ivory
Coast (B.F.).
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett. Included are
1
full experimental details, representative H and 13C NMR spectra
and X-ray data. SnuIpgfrio
m
p
nrtiSatnoIufrip
m
g
to
iorat
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 969–974

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LETTER
(19) Sheldrick, G. M. SHELXL-97, Program for the Refinement
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