An intramolecular nitro-Mannich route to functionalised tetrahydroquinolines

Article abstract of DOI:10.1016/j.tetlet.2012.08.062

A simple protocol for the diastereoselective synthesis of 3-nitrotetrahydroquinolines has been developed using an intramolecular nitro-Mannich reaction. In situ formation of an imine generated from treatment of 2-(2-nitroethyl)phenylamine with an aldehyde

Full text of DOI:10.1016/j.tetlet.2012.08.062

Tetrahedron Letters 53 (2012) 5707–5710
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Tetrahedron Letters
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An intramolecular nitro-Mannich route to functionalised tetrahydroquinolines
⇑
James C. Anderson , Adam Noble, Pascual Ribelles Torres
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple protocol for the diastereoselective synthesis of 3-nitrotetrahydroquinolines has been developed
using an intramolecular nitro-Mannich reaction. In situ formation of an imine generated from treatment
of 2-(2-nitroethyl)phenylamine with an aldehyde, in EtOH at room temperature, and subsequent addition
of NH4OH, led to the formation of trans-products in high yield and diastereoselectivity for 15 represen-
tative examples.
Received 10 July 2012
Revised 6 August 2012
Accepted 15 August 2012
Available online 23 August 2012
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords:
Nitro-Mannich
Tetrahydroquinoline
Diastereoselective
Cyclisation
Heterocycle
Synthetic routes to structurally diverse fused nitrogen hetero-
cycles are of interest due to their importance in medicinal chemis-
try and abundance in biologically active natural products. The
tetrahydroquinoline ring system is a common motif in many natu-
ral and unnatural biologically active compounds.1 Although many
methods exist for the synthesis of this ring system, the develop-
ment of new reactions and protocols to deliver stereochemically
pure products is continually needed to enable the synthesis of
more refined structures.2,3 We report here a concise and diastereo-
selective synthesis of 2-substituted-3-nitro-tetrahydroquinolines
that uses an intramolecular nitro-Mannich reaction as the key ste-
reodetermining step. The nitro-Mannich reaction is particularly
efficient for the formation of b-nitroamines with two contiguous
stereocentres, often with high levels of enantio- and diastereose-
lectivities.4–6 It has also been shown to be a useful reaction to syn-
thesise a range of heterocyclic structures.7 These useful building
blocks provide access to other valuable moieties through manipu-
lation of the nitro function, for example by the Nef reaction and ni-
tro reduction.8
We recently reported a reductive nitro-Mannich route to 2-
substituted-3-amino-tetrahydroisoquinolines that involved an
intramolecular palladium-catalysed amination reaction from a dia-
stereomerically pure 1,2-diamine with a pendant aryl bromide.9
We have also explored an alternative route that involves a base-
mediated intramolecular nitro-Mannich reaction. We postulated
that treatment of nitroalkane 1 with an aldehyde would give imine
2 which could then undergo an intramolecular nitro-Mannich reac-
tion forming 3-nitrotetrahydroquinoline 3 (Scheme 1).
NO
2
NO
R
2
RCHO
NH
2
2
N
1
NO
R
2
nitro-
Mannich
N
H
3
Scheme 1.
O
O
Ar
NO2
MeNO2
Ar
Ar'
cat (20 mol%)
N
R
N
Ar'
R
H
4
N
S
CF3
MeO
N
N
H
H
CF3
Scheme 2.
At the outset of this work there had been no reports of intramo-
lecular nitro-Mannich reactions of this type. However, during our
investigations the group of Xu reported an asymmetric organoca-
talysed Michael/nitro-Mannich tandem reaction sequence that
gave products 4 in excellent yield and enantioselectivity, and mod-
erate to good diastereoselectivity for a range of different imine
substituents (Scheme 2).10 This reaction was only applicable to
⇑
Corresponding author. Tel.: +44 (0)20 76795585.
E-mail address: j.c.anderson@ucl.ac.uk (J.C. Anderson).
0040-4039/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.062

5708
J. C. Anderson et al. / Tetrahedron Letters 53 (2012) 5707–5710
Table 2
i) MeNO , 1M NaOH;
2
8M HCl
Effects of additives on the formation of 3a
CHO
NO
2
NaBH
4
ii) MsCl, DIPEA
NO
2
NO
81%
NO
NO
Ph
2
2
2
5
PhCHO
EtOH
+
N
H
Ph
N
H
cis-3a
additive
18 h, rt
NO
2
2a
1
NO
H , Pd/C
2
2
18 h, rt
trans-3a
NO
2
NH
1 99%
2
95%
Entry
Additive
Conversiona (%)
dr (trans:cis)a
1
2
3
4
5
6
7
AlCl3
CAN
TiCl4
Zn(OTf)4
DIPEA
Et3N
<10b
<10b
<10b
<10b
<10c
>95
—
—
—
—
—
60:40
90:10
Scheme 3.
aromatic imines and the reactions took 4–7 days to reach comple-
tion. In contrast, the protocol we report here is applicable to aro-
matic- and alkyl-imines, delivers a different diastereoisomer and
is complete in 36 h at room temperature.
NH4OH
>95
Determined by 1H NMR spectroscopy.
Imine recovered.
Degradation of the imine occurred.
a
b
c
To investigate the reaction we developed a synthesis of nitro
amine 1. Conceptually the most attractive route would have been
to subject 2-aminobenzaldehyde to a Henry condensation reaction
with nitromethane, followed by reduction. However, due to the
propensity of 2-aminobenzaldehyde to undergo polymerisation,
2-nitrobenzaldehyde (5) was used as the starting material. Henry
condensation and subsequent reduction to the nitroalkane with
NaBH4 proceeded in high yield (Scheme 3). Selective hydrogena-
tion of the aromatic nitro group with H2 over Pd/C could be
achieved by careful monitoring of the progress of the reaction by
thin-layer chromatography. The reaction reached completion in
1 h and gave a quantitative yield of 1. The faster rate of reduction
of the aromatic nitro group over the aliphatic nitro group is in
agreement with previous observations using a variety of different
reduction methods.11
Formation of imine 2a (R = Ph) was achieved simply by stirring
1 and benzaldehyde together in various solvents without a dessi-
cant (Table 1). After 18 h 1H NMR analysis showed >90% conversion
in all cases, but only trace amounts of tetrahydroquinolines 3a and
only in protic alcohol solvents (entries 1 and 2). The reactions were
then heated to 60 °C for 18 h leading to high conversions in EtOH
or MeOH, but only trace amounts in aprotic solvents. Unfortu-
nately the conversions of the reactions did not reflect the diaste-
reomeric ratios. Those performed in EtOH or MeOH (entries 1
and 2) were unselective, while those in aprotic solvents gave good
selectivities (Table 1). The assignment of diastereoisomers was
based upon inspection of 3J coupling constants between the vicinal
protons adjacent to the pendant phenyl ring and the nitro function,
3Jtrans ꢀ8 Hz versus Jcis ꢀ4 Hz.
3
The promising results in alcoholic solvents allowed us to inves-
tigate the effect of various additives on the selectivity of the reac-
tion. Initial formation of the imine in EtOH over 18 h was followed
by the introduction of a number of additives. Activation of the
imine nitrogen with various Lewis acids was attempted, but these
proved unsuccessful as no improvement to either the yield or
selectivity was observed (Table 2, entries 1–4). The addition of
Brønsted bases to activate the methylene adjacent to the aliphatic
nitro group proved to be more successful (entries 5–7). Triethyl-
amine gave complete conversion into tetrahydroquinoline 3a with
a slightly improved diastereomeric ratio (60:40, trans:cis). It was
found that the addition of aqueous NH3 (3.0 equiv) gave 3a in
excellent conversion and diastereoselectivity (90:10, trans:cis).
Work-up of the NH4OH reaction after only 3 h revealed 50%
conversion into 3a with a 1:1 ratio of diastereoisomers. Complete
conversion and a >90:10 (trans:cis) diastereoselectivity was rea-
lised after 18 h. This suggests the reaction proceeds via an initial
unselective and comparatively rapid ring-closure, followed by
slower epimerisation into the thermodynamically favourable
trans-3a over approximately 18 h. This contrasts with the work
of Xu et al. (Scheme 2) who found that the major product from
their intramolecular nitro-Mannich reactions possessed a cis-rela-
tionship between the NO2 group and the susbtituent derived from
the imine (R1, Fig. 1). Presumably the stability of this kinetic diaste-
reoisomer is due to the presence of the R2 substituent destabilising
the all trans-diastereoisomer due to a potential 1,3-diaxial interac-
tion between R1 and R2. In our system where R2 = H, epimerisation
occurs to give the thermodynamically more stable all equatorial
trans-3. The epimerisation process could occur either by a retro-ni-
tro-Mannich process or by deprotonation/reprotonation adjacent
to the nitro group, both of which are likely to be facilitated
Table 1
Effect of solvent on the intramolecular nitro-Mannich reaction
NO
NO
Ph
2
2
o
PhCHO
60 C
+
N
H
Ph
N
H
cis-3a
2a
1
solvent
18 h, rt
18 h
trans-3a
Entry
Solvent
% Conv. 18 h, rta
% Conv. 18 h, 60 °Ca
-
1
R
O
+
1
R
2a
3a (trans:cis)
2a
3a (trans:cis)
H
N
O
N
1
NO
2
R
H
1
2
3
4
5
6
7
EtOH
MeOH
CH2Cl2
PhMe
THF
95
90
3 (50:50)
7 (50:50)
12
4
88 (50:50)
96 (50:50)
3 (85:15)
3 (90:10)
2 (95:5)
HN
H
NO
2
epimerisation
HN
2
R
2
>95
>95
>95
>95
>95
0
0
0
0
0
97
97
98
98
97
R
2
R =H
stable diastereoisomer
2
Et2O
DME
2 (90:10)
3 (90:10)
trans-3
when R =-CH COPh
2
a
Determined by 1H NMR spectroscopy.
Figure 1. Explanation for the contrasting diastereoselectivity.

J. C. Anderson et al. / Tetrahedron Letters 53 (2012) 5707–5710
5709
Table 3
5 (Scheme 4). Subsequent intramolecular nitro-Mannich reaction
using benzaldehyde under our standard conditions gave 3-nitro-
tetrahydroquinoline 8 in 92% isolated yield. This example illus-
trates the tolerance of additional ortho-substitution adjacent to
the amine function of 1.
In conclusion the intramolecular nitro-Mannich methodology
presented here provides a convenient method for the synthesis
of 3-nitrotetrahydroquinolines in high yield and high selectivity
for the trans-diastereoisomer. Limitations do exist to the availabil-
ity of 2-aminonitroalkenes from the corresponding 2-nitroaldehy-
des. The reaction should be amenable to control of absolute
stereochemistry, through one of the many strategies that currently
exist for asymmetric nitro-Mannich reactions.4–6 These products
should serve as useful chiral building blocks for further functionali-
sation towards biologically active targets.
Scope of the intramolecular nitro-Mannich rection with respect to the aldehyde
NH4OH
(3 equiv.)
NO2
R
RCHO
EtOH
NO2
R
1
N
H
18 h, rt
N
2
18 h, rt
trans-3
Entry
R
Product
Yielda (%)
dr (trans:cis)b
1
2
3
4
5
6
7
8
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
82
94
70
72
80
86
84
83
89
92
93
92
90:10
90:10
90:10
85:15
85:15
90:10
90:10
90:10
90:10
>95:5
95:5
4-CF3-C6H4
4-Br-C6H4
4-NO2-C6H4
4-Me-C6H4
4-MeO-C6H4
3-Cl-C6H4
3-MeO-C6H4
3,5-Cl2-C6H3
2-Me-C6H4
2-CF3-C6H4
2-Cl-C6H4
2-Br-C6H4
Cyclohexyl
CO2Et
9
Acknowledgments
10
11
12
13
14
15
We thank the EPSRC, GSK and the Ministerio de Educación
(Spain) for funding, Dr. L. Harris for mass spectra and Ms. J. Max-
well for microanalytical data.
>95:5
>95:5
90:10
60:40
90
65
0 (70)c
a
b
c
Isolated yield.
Determined by 1H NMR spectroscopy.
Conversion determined by 1H NMR spectroscopy.
Supplementary data
Supplementary data (experimental procedures for all
compounds) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2012.08.062.
by the presence of excess base. The stability of the kinetic cis–
trans-products generated by Xu et al. would be favoured by their
catalytic, rather than stoichiometric, use of base in their reaction
conditions, which are likely to slow any epimerisation process.
With the formation of 3-nitrotetrahydroquinoline 3a in excel-
lent yield and diastereoselectivity, the scope of the intramolecular
nitro-Mannich reaction was investigated with respect to the alde-
hyde reaction partner (Table 3). The products 3-nitrotetrahydro-
quinolines 3a–o were formed in good to excellent yields and
with excellent diastereoselectivites for all electron-rich, electron-
poor and ortho-substituted aldehydes. The reaction proved less tol-
erant of alkyl aldehydes, with cyclohexyl carboxaldehyde giving
product 3n in a lower yield of 65%, but still with good diastereose-
lectivity (entry 14). A lower conversion (70%) was observed with
the ethyl glyoxylate and 3o was formed with a significantly lower
trans:cis ratio of 60:40 (entry 15). The lower decreased conversion
into 3o was attributed to the decreased stability of this product
which underwent complete degradation during purification by col-
umn chromatography.
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(>95:5 trans:cis)
Scheme 4.

5710
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