Highly enantioselective aza-Henry reaction with isatin N-Boc ketimines

Article abstract of DOI:10.1039/c4cc04051a

A highly enantioselective aza-Henry reaction with isatin N-Boc ketimines using a Cu(ii)-BOX complex as a catalyst is described. The reaction, which does not require protection of the N1 atom, provides the corresponding nitroamines bearing a quaternary stereocentre with high yields and enantiomeric excesses (up to 99.9%).

Full text of DOI:10.1039/c4cc04051a

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Highly enantioselective aza-Henry reaction with
isatin N-Boc ketimines†
Cite this: Chem. Commun., 2014,
50, 9309
Melireth Holmquist, Gonzalo Blay* and Jose R. Pedro*
´
Received 27th May 2014,
Accepted 24th June 2014
DOI: 10.1039/c4cc04051a
www.rsc.org/chemcomm
A highly enantioselective aza-Henry reaction with isatin N-Boc ketimines enantiomerically enriched indolin-3-ones bearing a chiral quaternary
using a Cu(II)–BOX complex as a catalyst is described. The reaction, centre at the 2 position with good ee’s. Finally, Dixon10 has
which does not require protection of the N1 atom, provides the developed bifunctional thiourea catalysts incorporating an
corresponding nitroamines bearing a quaternary stereocentre with high iminophosphorane basic moiety, which catalysed the addition
yields and enantiomeric excesses (up to 99.9%).
of nitromethane to N-phosphinoyl ketimines with good yields
and enantioselectivities.
The nucleophilic addition of nitronate species to imine electrophiles
The oxindole skeleton featuring a quaternary stereocentre at
(aza-Henry or nitro-Michael reaction) is a carbon–carbon bond- C3 is present in numerous naturally occurring alkaloids and
forming method of prime importance in organic synthesis.1 This pharmaceutically active compounds.11 These compounds are
reaction leads to b-nitroamines, which are synthetically useful attractive targets in organic synthesis due to their usefulness as
compounds due to the presence of two vicinal nitrogen-containing drug candidates and as intermediates in alkaloid synthesis.12
functionalities in different oxidation states, providing opportunities Among the various chiral oxindoles, 3-amino-2-oxindoles bearing a
for selective manipulation of either.2,3 Addition of nitrocompounds stereogenic centre have been recognized as key structures in a
to prochiral imines gives rise to the formation of one or two new variety of biologically active compounds exhibiting significant
stereogenic centres. Therefore, tremendous effort has been dedicated pharmaceutical properties. A few examples are shown in Fig. 1,
to the development of asymmetric versions1 of this transformation which include the gastrin/cholecystokinin-B receptor antagonist
and excellent results have been obtained with aldimines through AG-041R,13 the orally active vasopressin V1b receptor antagonist
different procedures involving metal-catalysis4 or organo-catalysis.5 SSR149415,14 or the spirooxindole anti-malarial agent NITD609.15
However, the catalytic enantioselective aza-Henry reaction of the
less reactive ketimines, which leads to the direct formation of
a quaternary stereocentre,6 remains almost unexplored. A first
example by Feng et al. described the addition of nitromethane to
N-sulfonyl ketimines catalysed by a N,N 0-dioxide–copper(I) complex,
with fair to excellent enantioselectivities, but moderate yields.7
Furthermore, three organocatalytic procedures involving different
thiourea catalysts have been reported. Wang et al.8 carried out the
aza-Henry reaction with highly reactive trifluoromethyl ketimines
catalysed by chiral thiourea, which was applied to the synthesis of
the Anti-HIV Drug DPC 083, but that could not be extended to less
9
´
reactive methyl or phenyl ketimines. More recently, Aleman et al.
have carried out the addition of nitroalkanes to 2-aryl-3H-indol-
3-ones catalysed by thiourea–cinchona derivatives to obtain
´
`
´
`
Departament de Quımica Organica, Facultat de Quımica, Universitat de Valencia,
`
C/ Dr. Moliner 50, E-46100 Burjassot (Valencia), Spain. E-mail: jose.r.pedro@uv.es,
gonzalo.blay@uv.es; Fax: +34 963544328
† Electronic supplementary information (ESI) available: Experimental proce-
dures, characterization, NMR spectra and HPLC traces for all new compounds.
See DOI: 10.1039/c4cc04051a
Fig. 1 Examples of biologically active 3-aminooxindoles.
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Table 1 Addition of nitromethane to isatin N-Boc ketimine 1a (R = R0 = H).
Some bioactive oxindoles feature 3,30 spiranic moieties which
incorporate two nitrogen atoms onto their cyclic structures as for
instance in the DP2 receptor antagonist spirohydantoin16 or in the
spirocyclic thiourea with p53/Hdm2 antagonist activity.17 In
fact, the presence of these two nitrogen atoms is responsible for
the increased potency and bioavailability of the DP2 receptor
antagonist.16b
Because of the significance of chiral 3-amino-oxindoles in
medicinal chemistry, the development of procedures for their
enantioselective synthesis has received considerable attention
in recent years.18 Together with the electrophilic amination of
oxindoles,19 the asymmetric addition of carbon nucleophiles to
isatin ketimines represents a straightforward procedure for the
synthesis of this kind of compounds. Accordingly, examples of
catalytic enantioselective Strecker,20 Mannich,21 Friedel–Crafts22 and
Screening of conditionsa
Entry
Cu(II)
BOX
3a yield (%)
eeb (%)
1
Cu(OTf)2
Cu(OTf)2
Cu(AcO)2
CuCl2
Cu(BF4)2ꢀH2O
Cu(BF4)2ꢀH2O
Cu(BF4)2ꢀH2O
Cu(BF4)2ꢀH2O
Cu(BF4)2ꢀH2O
BOX1
BOX1
BOX1
BOX1
BOX1
BOX1
BOX2
BOX3
BOX4
21 (51)c
44 (20)c
97
98
100
100
98
96
94
99.2
97.0
27.0
85.0
99.6
98.2
6.0
2d
3
4
5
6e
7
8
9
8.0
ꢁ89.9
a
Cu(II) salt (10 mol%), BOX (10 mol%), i-Pr2NH (13 mol%), MeNO2 (9.2 eq.),
r.t. b Determined by HPLC with chiral stationary phases. c Yield of isatin in
brackets. d Reaction carried out in the presence of 4 Å M.S. e Cu(BF4)2
(5 mol%), BOX1 (5 mol%), i-Pr2NH (7 mol%), MeNO2 (9.2 eq.), r.t.
homoenolate addition23 reactions featuring isatin imines as electro- BOX3 bearing aliphatic substituents on the oxazoline ring gave
philes have been reported in the literature. In this communication, almost racemic compounds while BOX4 gave compound 3a with
we describe our results on the development of a highly enantio- good yield and high ee.
selective aza-Henry reaction with isatin ketimines (Scheme 1), a
Next, the scope of the addition of nitromethane to substituted
reaction that has not been explored yet to a great extent. Zhou isatin ketimines was studied (Table 2). Whilst the recently reported
et al. pioneered this work using a quinine-derived bifunctional procedures24,25 needed isatin derivatives bearing an alkyl group at
catalyst obtaining modest to good enantioselectivities.24 Also the N1-atom that makes their deprotection difficult, our method
during the preparation of this manuscript, the enantioselective gave high yields and enantioselectivities with N1-unprotected isatin
addition of nitromethane to 1-methylisatin N-Boc ketimines using ketimines. Furthermore, better results were also obtained with N1-
a pyBidine-NiCl2 catalyst has been reported by the group of Arai.25 protected isatin ketimines, especially with those isatin derivatives
Because of the commercial availability and high performance having electron-withdrawing groups attached to this position. Thus,
of bis-oxazoline (BOX) ligands in metal-mediated asymmetric we obtained excellent results with the 1-carbamoylated imine 1e
reactions,26 we used this kind of ligands combined with Cu(II) (Table 2, entry 5) while low yield (19%) and ee (57%) have been
salts in our study.27 The addition of nitromethane (2a) to the reported for the reaction of the related 1-acetyl isatin N-Boc ketimine
N-Boc imine of isatin (1a, R = R1 = H) in the presence of 10 mol% with the pyBidine-NiCl2 catalyst.25 N-Boc imines of isatins substi-
of the BOX1–Cu(OTf)2 complex gave a low yield (21%) of the aza- tuted on the aromatic ring were also suitable substrates for this
Henry product 3a although with excellent ee (99.2%) together reaction (Table 2, entries 6–14), which gave the corresponding N-Boc
with 51% yield of isatin resulting from hydrolysis of the imine nitroamines 3 with good yields and excellent enantiomeric excesses
(Table 1, entry 1). As isatin formation could not be prevented above 93%, except in the cases of imine 1h bearing a bulky bromide
even when performing the reaction in the presence of molecular at the C5 position (Table 2, entry 8) and imine 1j bearing a strong
sieves (Table 1, entry 2), we thought that this side reaction may
be caused by the strongly acidic copper triflate. The use of less
Table 2 Addition of nitromethane to isatin N-Boc ketimines 1. Scope of
acidic copper(II) salts avoided imine hydrolysis but affected the
enantioselectivity (Table 1, entries 1–5). The best result was obtained
with copper(II) tetrafluoroborate hydrate, which allowed us to obtain
compound 3a in quantitative yield and 99.8% ee (Table 1, entry 5).
The catalyst load could be reduced to 5 mol% without an appreci-
able effect (Table 1, entry 6). Other commercially available BOX
ligands were also tested (Table 1, entries 7–9). Ligands BOX2 and
the reactiona
Entry
1
R
R1
t (h)
3
Yield (%)
eeb (%)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1a
H
H
H
H
H
22
16
16
16
18
15
16
22
15
15
21
21
22
15
22
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3a
99
99.6
99.8
99.9
99.9
99.9
96.3
97.7
81.3
97.5
6.6
96.2
98.1
93.1
98.7
95.8
Me
Bn
MOM
Boc
H
H
H
H
H
H
H
82 (14)c
79 (13)c
84 (14)c
91
H
5-Me
5-MeO
5-Br
5-Cl
5-NO2
6-Cl
7-Cl
7-F
94
97
95
87
92
92
94
86
9
10
11d
12
13
14d
15e
H
H
H
4,7-Me2
H
89
98
a
1 (0.25 mmol), Cu(BF4)2 (10 mol%), BOX1 (10 mol%), i-Pr2NH
b
(13 mol%), MeNO2 (9.2 eq.), r.t. Determined by HPLC with chiral
c
stationary phases. Yield of the isatin nitroaldol product in brackets.
d
Cu(BF4)2 (5 mol%), BOX1 (5 mol%), i-Pr2NH (7 mol%), MeNO2
e
Scheme 1 Aza-Henry reaction with isatin N-Boc ketimines.
9310 | Chem. Commun., 2014, 50, 9309--9312
(9.2 eq.), r.t. Reaction carried out with 3.0 mmol of 1a.
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Table 3 Addition of larger nitroalkanes to isatin N-Boc ketimines 1a
stereoisomeric oximes 11 that were dehydrated with SOCl2/Et3N to
give amino nitrile 12 in 84% yield. These kinds of compounds are
useful synthetic intermediates for spirocyclic oxindoles.28 Treatment
of nitrile 12 with dry HCl in MeOH gave amino ester 13 in 61% yield
with a decreased ee (92%).
The absolute stereochemistry of compounds 12 and 13 obtained
in this way was achieved upon comparison of their optical rotation
values and HPLC traces with those reported in the literature.20a,28
The configuration of the quaternary stereocentre in compound 3a
was determined to be S by chemical correlation. For the rest of the
aza-Henry products 3, 6 and 7, it was achieved upon the assumption
of a uniform stereochemical mechanism.
Entry
1
R
R2
t (h) 6–7 Yieldb (%) drc
eed (%)
1
2
3
5
5
6
1a
1a
H
H
Me 22
Et 16
6a
7a
6k
7k
6l
68 (25)
75 (20)
95
91
91
91 : 09 99.8/99.1
91 : 09 99.6/98.0
60 : 40 91.7/88.8
85 : 15 87.8/78.8
76 : 24 98.9/99.9
88 : 12 99.8/99.8
1k 6-Cl Me 16
1k 6-Cl Et 16
7-Cl Me 18
7-Cl Et 15
1l
1l
7l
89
In conclusion, we have developed a highly efficient procedure for
the enantioselective aza-Henry reaction with isatin N-Boc ketimines
catalysed by Cu(II)–BOX complexes. Compared with the precedents
recently appeared in the literature,24,25 our method provides higher
yields and enantioselectivities (above 93% ee in most of the cases).
Interestingly, the protection of the N1 atom is not required,
although in our case the presence of either electron-donating or
electron-withdrawing groups at this position does not affect the
enantioselectivity of the reaction, making our procedure of much
more general application. The use of commercially available
ligands represents an additional advantage.
a
Cu(BF4)2 (10 mol%), BOX1 (10 mol%), i-Pr2NH (13 mol%), R2CH2NO2
b
(9.2 eq.), r.t. Yield of the isatin nitroaldol product in brackets.
c
d
Determined by 1H NMR. Determined by HPLC with chiral stationary
phases. Enantiomeric excesses for the major and minor diastereomers.
electron withdrawing nitro group at the same position (Table 2, entry
10). Remarkably, the reaction can be scaled up at least up to a
3.0 mmol scale maintaining the high yield and high enantioselec-
tivity (Table 2, entry 15).
The results of a few examples of the aza-Henry reaction with
larger nitroalkanes are provided in Table 3. Nitroethane (4) and
nitropropane (5) reacted with imines 1a, 1k, and 1l to give the
corresponding products 6 and 7, respectively, with good yields,
fair to good diastereomeric ratios (dr), and excellent enantio-
meric excesses for both diastereomers.
Some modifications of nitroamine 3a are outlined in Scheme 2.
Deprotection of the N-Boc amino group in compound 3a was carried
out with trifluoroacetic acid in dichloromethane at room tempera-
ture to give the free nitro amine 8 with good yield. Reduction of the
nitro group was achieved with NaBH4/NiCl2 to obtain the mono-
protected diamine 9 in 99% yield, which upon removal of the Boc
protecting group with HCl in diethyl ether/methanol gave the free
bis-amine 10 in 65% yield. On the other hand, reduction of the nitro
group with SnCl2 allowed obtaining 75% yield of a mixture of two
´
Financial support from the Ministerio de Economıa y Compe-
˜
titividad (Gobierno de Espana) and FEDER (EU) (CTQ2009-13083)
and from Generalitat Valenciana (ISIC 2012/001) is acknowledged.
´
M. H. thanks the GV for a pre-doctoral grant (Santiago Grisolıa
Program).
Notes and references
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Scheme 2 Examples of functional modification of compound 3a.
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9312 | Chem. Commun., 2014, 50, 9309--9312
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