Preparation of sugar-derived 1,2-diamines via indium-catalyzed aza-Henry-type reaction: Application to the synthesis of 6-amino-1,6- dideoxynojirimycin

Article abstract of DOI:10.1016/j.tetlet.2013.02.041

The combination of (p-methoxyphenyl)imine, a bromonitroalkane, and zinc in the presence of catalytic indium allows straightforward access to β-nitroamine derivatives. The use of chiral sugar-derived imines furnished the corresponding β-nitroamines in high yields and stereoselectivities, from which the corresponding 1,2-diamines were easily obtained. The synthetic utility of the sugar-derived 1,2-diamines in the preparation of iminosugars was illustrated in a concise synthesis of 6-amino-1,6-dideoxynojirimycin.

Full text of DOI:10.1016/j.tetlet.2013.02.041

Tetrahedron Letters 54 (2013) 2156–2159
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Preparation of sugar-derived 1,2-diamines via indium-catalyzed
aza-Henry-type reaction: application to the synthesis of 6-amino-
1,6-dideoxynojirimycin
⇑
⇑
Raquel G. Soengas , Artur M. S. Silva
Department of Chemistry & QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The combination of (p-methoxyphenyl)imine, a bromonitroalkane, and zinc in the presence of catalytic
indium allows straightforward access to b-nitroamine derivatives. The use of chiral sugar-derived imines
furnished the corresponding b-nitroamines in high yields and stereoselectivities, from which the corre-
sponding 1,2-diamines were easily obtained. The synthetic utility of the sugar-derived 1,2-diamines in
the preparation of iminosugars was illustrated in a concise synthesis of 6-amino-1,6-dideoxynojirimycin.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 19 January 2013
Revised 12 February 2013
Accepted 13 February 2013
Available online 26 February 2013
Keywords:
Indium
Aza-Henry reaction
Bromonitroalkanes
Diamines
Iminosugars
OH
NH2
The aza-Henry (or nitro-Mannich) reaction involves nucleo-
philic addition of nitroalkanes to imines,1 and results in the forma-
tion of a carbon–carbon bond with concomitant generation of a b-
nitroamine, from which a wide variety of other organic compounds
can be derived by functional transformations of the nitro group.
Particularly interesting is the reduction of the nitro group to an
amine, giving rise to the corresponding 1,2-diamines,2 of great va-
lue in both synthesis and biology.3 The 1,2-diamino moiety is pres-
ent in many natural products that have relevant biological
properties4 and several synthetic diamine derivatives have also
been employed as medicinal agents, in particular as antitumoral
agents.5 Their use in organic synthesis has also increased exponen-
tially in the past few years, especially as intermediates in the syn-
thesis of heterocycles6 and in the field of catalytic asymmetric
synthesis.7 When applied to sugar-derived imines, the aza-Henry
reaction gives rise to homologated b-nitroamines, precursors of
the corresponding sugar-derived 1,2-diamines. The preparation of
sugar-derived vicinal diamines is an important objective in organic
synthesis, given their value in asymmetric catalysis8 and their po-
tential as intermediates in the preparation of iminosugars.
OH
H
N
H
N
N
HO
OH
HO
OH
HO
OH
OH
1
OH
3
OH
2
Figure 1. Biologically active iminosugars.
of metabolic pathways,10 the inhibition of glycosidases has become
a powerful strategy for the treatment of several important dis-
eases, such as diabetes, cancer, and viral infections, including
AIDS.11 Deoxymanno-jirimycin (DMJ) 1 (Fig. 1), for example, is a
potent inhibitor of glycosidases12 and some analogues, such as 2
are therapeutically useful drugs.13 Therefore, derivatives of DMJ
provide an opportunity for altering and hopefully increase the
specificity of inhibition of individual glycosidases.
We became interested in the synthesis of 6-amino derivatives of
iminosugars 3, not only for their biological interest but also for
their potential as stereodifferentiating agents in asymmetric catal-
ysis. For this purpose, we envisioned a synthetic strategy involving
an aza-Henry reaction of sugar imines as a key step for the forma-
tion of the intermediate 1,2-diamines. Consequently, we started
our investigations by developing a general, cheap, and scalable
procedure to prepare the starting sugar-derived b-nitroamines.
In connection with our interest in the application of Barbier-
type reactions to carbohydrate chemistry,14 we have recently de-
scribed an efficient preparation of b-nitroamines by the addition
Iminosugars, analogues of pyranoses in which the oxygen atom
of the heterocyclic ring is replaced by a nitrogen atom and the ano-
meric hydroxyl is absent, are almost always inhibitors of the corre-
sponding glycosidases.9 As glycosidases are involved in a number
⇑
Tel.: +351 234 370714; fax: +351 234 370084.
E-mail addresses: rsoengas@ua.pt (R.G. Soengas), artur.silva@ua.pt (A.M.S. Silva).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.02.041

R. G. Soengas, A. M. S. Silva / Tetrahedron Letters 54 (2013) 2156–2159
2157
of bromonitroalkanes to sugar imines.15 The indium promoted
addition of bromonitroalkanes15a constitutes a very convenient
alternative to the classic aza-Henry reaction, as it is very simple
from the experimental point of view and performs better in terms
of yields and substrate scope. However, an important drawback of
this procedure would be its high cost, derived from having to use
stoichiometric amounts of indium. Having to scale up the aza-
Henry reaction to obtain preparative quantities of the intermediate
b-nitro-amines, the use of a catalytic amount of indium would be
desirable. We decided to study the addition of bromonitroalkanes
to imines using catalytic amounts of indium metal in the presence
of a cheap secondary reducing agent.16
In a preliminary experiment, the addition of bromonitrome-
thane 5a to N-(cyclohexylmethylene)-4-methoxybenzenamine 4a
as the model was studied with different additives (Scheme 1,
Table 1).
The reaction can be mediated by stoichiometric indium, giving
high conversions (entry 1).15a Neither aluminum nor zinc alone
was effective in promoting the reaction (entries 2 and 5, respec-
tively). Manganese activated by TMSCl also failed to produce the
desired 2-nitroamine (entry 7), while tin(II) chloride afforded good
yields of the addition product (entry 9).17
Table 1
Screening conditions for the indium-catalyzed addition of bromonitromethane 5a to
imine 4a
Entry
Conditions
Conversion (%)
1
2
3
4
5
6
7
8
9
1 equiv In
5 equiv Zn
80
0
68
79
0
35
0
41
72
78
5 equiv Zn/0.12 equiv In
10 equiv Zn/0.12 equiv In
5 equiv Al
5 equiv Al/0.12 equiv In
5 equiv Mn/5 equiv TMSCl
5 equiv Mn/5 equiv TMSCl/0.25 equiv In
2 equiv SnCl2
10
2 equiv SnCl2/0.12 equiv In
O
O
Br
NO2
Br
NO2
5b
Br
NO2
5a
5c
Figure 2. Bromonitroalkanes 5a–c.
On the other hand, when those metals were combined with cat-
alytic amounts of indium, the desired nitroamine 6a was obtained
in all cases. The best catalytic system consisted of a catalytic
amount of indium combined with an excess of Zn dust (entries 3
and 4). The most favorable conditions to achieve the indium cata-
lyzed Henry-type reaction are the use of a catalytic amount of in-
dium (as low as 0.12 equiv) together with a 10 mol excess of zinc
dust (entry 4). Reducing the zinc to 5 equiv resulted in a lower con-
version (entry 3). The reaction failed in the absence of indium,
which excludes the direct involvement of zinc as promoter. As in
the case of the indium-catalyzed addition of the bromonitroalk-
anes to aldehydes,16 the proposed mechanism involves the regen-
eration of the indium reactive species due to the reducing
character of zinc.
Aluminum and manganese, on the other hand, were less effec-
tive than zinc, giving low yields of nitroalcohol 6a (entries 6 and
8, respectively). Finally, the system tin(II) chloride/catalytic indium
afforded similar results than both stoichiometric indium and the
system zinc/catalytic amount of indium. In this case, as tin(II) chlo-
ride alone can promote the reaction and yield the desired nitro-
amine 6a albeit in slightly lower yield, the mechanism of the
reaction is not clear. However, we decided to focus on the zinc/in-
dium system, as zinc is considerably cheaper than tin(II) chloride.
With the established optimal conditions we then investigated
the generality of the procedure. Thus, to a sonicated mixture of
an appropriate bromonitroalkane 5a–c18 (0.6 mmol) (Fig. 2), in-
dium (0.055 mmol), and zinc (5.00 mmol) in THF (1 mL) the corre-
sponding p-methoxyphenylimine derivative 4a–f (0.50 mmol) was
added and the mixture was sonicated for 6 h. In all cases, the de-
sired b-nitroamines 6b–g were isolated in good yields (Scheme 2,
Table 2).
Scheme 2. Indium-catalyzed addition of bromonitroalkanes 5a–c to imines 4a–f.
easily available. Thus, the above conditions were tested in the reac-
tion of sugar-derived imines 4g and 4g with bromonitroalkanes
5a–c. When the reaction was carried out in the presence of a (p-
methoxy-phenyl)imine 4g and 4g, indium, and zinc under the con-
ditions described above, the corresponding nitroamines 6h–n were
obtained in all cases in good yields as isomeric mixtures from
which the major anti isomers were isolated as pure compounds
(Scheme 3, Table 3). The excellent anti diastereoselectivity can be
explained in terms of a lowering of the C–N antibonding orbital,
which would bring about increased stabilization of a Felkin–Anh
antiperiplanar nucleophilic addition of the organoindium species
to the imine carbonyl (Fig. 3).19 These results are in accordance
with those observed in the stoichiometric indium-mediated
reaction.15a
Given the excellent results obtained in the preparation of sugar
b-nitroamines, we aimed next at the preparation of the corre-
sponding orthogonally protected 1,2-diamines, valuable interme-
diates according to our synthetic objectives. Although there are
numerous protocols for the reduction of nitro compounds to
amines,20 the reduction of b-nitroamines is complicated by the
inherent instability of this function.21 SmI2 has been widely used
as a mild reductant for the conversion of b-nitroamines to 1,2-dia-
mines.22 b-Nitroamines have also been reduced with Zn in the
presence of a proton source23 and with catalytic amount of In with
Zn as a stoichiometric reductant.24 The reduction of the nitro group
was evaluated for model nitrosugars 6h, 6i, and 6k. Treatment of
these nitroamines with SmI2/H2O in the presence of pyrroli-
dine25,15a afforded the corresponding 1,2-diamines 7h, 7i, and 7k
in good yield and without any loss of the stereochemical integrity
(Scheme 4, Table 4). On the other hand, when the reduction was
carried out with Zn powder in the presence of diluted hydrochloric
acid as the proton source, 1,2-diamines 7h, 7i, and 7k were also ob-
tained, albeit in lower yield.15a It is somehow surprising that the
acetonide protecting groups are stable toward the addition of
Once we found a general and economic procedure for the prep-
aration of racemic b-nitroamines, this methodology was applied to
sugar-derived imines to furnish the corresponding chiral b-nitro-
amines, from which the corresponding 1,2-diamines would be
PMP
NHPMP
NO2
N
[M]
+
Br
NO2
H
THF
4a
5a
6a
Scheme 1. Indium-catalyzed addition of bromonitromethane 5a to imine 4a.

2158
R. G. Soengas, A. M. S. Silva / Tetrahedron Letters 54 (2013) 2156–2159
Table 2
Synthesis of 2-nitroamines 6b–g
Entry
4
R1
5
R2
R3
6
Yield (%)
1
2
3
4
5
6
4a
4b
4c
4d
4e
4f
C6H11
p-OMe-C6H4
C6H5
5b
5c
5a
5b
5a
5b
CH3
CH3
—
H
CH3
H
CH3
6b
6c
6d
6e
6f
78
35
59
80
69
68
2,2-Dimethyl-1,3-dioxane
H
CH3
H
CH3
CH3(CH2)6
iPr
iBu
6g
Table 4
Synthesis of 1,2-diamines 7h, 7i, and 7k
Entry
6
R1
7
Yield (%)
87a
69b
O
BnO
1
6h
7h
O
O
Scheme 3. Indium catalyzed addition of bromonitroalkanes 5a–c to chiral sugar
imines 4g–j.
84a
62b
O
TBSO
2
3
6i
7i
O
O
Table 3
Synthesis of enantiopure 2-nitroamines 6h–l
85a
62b
O
Entry
4
R1
5
6
Yielda (%)
anti/synb
O
O
6k
7k
O
BnO
OBn
1
4g
5a
6h
76
4/1
a
SmI2/H2O/pyrrolidine.
Zn/HCl/2-propanol.
O
O
b
2
3
5a
5b
6i
6j
79
79
4/1
5/1
O
TBSO
4h
4i
H2N
CbzHN
O
O
i
O
O
O
O
PMPHN
PMPHN
4
5
5a
5c
6k
6l
71
73
4/1
6/1
O
O
O
O
O
O
BnO
BnO
8
7k
OBn
O
6
7
5b
5c
6m
6n
73
75
5/1
6/1
O
ii
4j
O
CbzHN
H2N
Cl
H2
N
O
O
H3N
Cl
iii
O
a
Isolated yields of pure b-nitroamines after column chromatography.
HO
OH
Determined by 300 MHz 1H NMR analysis of the corresponding crude products.
b
O
BnO
OH
9
10
Scheme 5. Reagents and conditions: (i) ClCbz, NaHCO3 (sat. aq), EtOAc, rt, 2 h, 91%;
(ii) CAN, CH3CN, rt, 1 h, 79%; (iii) (a) TFA/H2O 1:1, rt, 14 h; (b) NaHCO3, THF, 40 °C,
24 h; (c) Pd–C, NH4CO2, MeOH, 50 °C, 16 h; (d) AcCl, MeOH, 16 h, 39%.
MeO
In
N
O
R
O
R
1 M aqueous HCl, which is probably due to the excess of zinc
quickly consuming the protons before most of acetonide
decomposed.
To illustrate the synthetic utility of these sugar-derived 1,2-dia-
mines in the field of azasugar synthesis, our strategy was first ap-
N
O
H
H
Figure 3. Felkin–Ahn model for the attack on sugar imines 4g–j.
plied to the synthesis of D-glucose 6-amino derivative of 1-DNJ 10.
This iminosugar plays an important role in glycosidase studies, not
only because of its weak inhibition properties, but also because it is
a synthetic precursor of a wide range of glycosidase inhibitors.26
However, very few syntheses of 6-amino-1-deoxynojirimycin have
been reported.27 We have developed a short route to azasugar 10,
synthesized from diamine 7k in five steps (Scheme 5).
Thus, protection of the primary amino group on diaminosugar
7k with benzyloxycarbonyl chloride under Schotten–Baumann
conditions and removal of the PMP protecting group of resulting
PMP
PMP
[Red]
HN
R1
6h, 6i, 6k
HN
R1
7h, 7i, 7k
NO2
NH2
Scheme 4. Reduction of b-nitroamines 6h, 6i, and 6k.

R. G. Soengas, A. M. S. Silva / Tetrahedron Letters 54 (2013) 2156–2159
5. Michalson, E. T.; Szmuszkovicz, J. Prog. Drug Res. 1989, 33, 135–145.
2159
diamine 8 on treatment with cerium ammonium nitrate, yielded
orthogonally protected diamine 9. Acidic hydrolysis of the aceto-
nide protecting group, followed by basic treatment and catalytic
hydrogenation gave the desired azasugar 10, isolated as its dihy-
drochloride salt after treatment with methanolic hydrochloric acid.
In summary, we have demonstrated that zinc can be used in the
addition of bromonitroalkanes to imines in the presence of a cata-
lytic amount of indium to afford the corresponding b-nitroamines.
This economic protocol can be easily scaled up to preparative
amounts and performs better than the classical base-catalyzed
reaction in terms of yields and substrate scope. When applied to
sugar-derived imines, this methodology allows the preparation of
carbohydrate b-nitroamines, from which the corresponding 1,2-
diamines were easily derived. As a first practical application of
the novel orthogonally protected carbohydrate 1,2-diamines, we
reported here a new route to iminosugar 6-amino-1,6-dideoxynoj-
irimycin. This approach is shorter and more efficient than the pre-
viously reported synthetic routes.
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Acknowledgments
Thanks are due to the University of Aveiro, Fundação para a
Ciência e a Tecnologia (FCT) and FEDER for funding the Organic
Chemistry Research Unit (project PEst-C/QUI/UI0062/2011) and
the Portuguese National NMR Network (RNRMN). R.S. thanks the
FCT for a ‘Investigador Auxiliar’ position.
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Supplementary data
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Supplementary data (experimental procedures for the prepara-
tion of nitroamines 6a–n, diamines 7h, 7i, 7k and iminosugar 10)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2013.02.041.
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