Organocatalytic one-pot 1,4-/1,6-/1,2-addition sequence for the stereocontrolled formation of six consecutive stereocenters

Article abstract of DOI:10.1039/c4cc09730k

An unprecedented stereoselective organocatalytic one-pot 1,4-/1,6-/1,2-addition sequence between β-dicarbonyl compounds, β-nitroalkenes and 4-nitro-5-styrylisoxazoles sequentially catalyzed by low loading of a squaramide catalyst and an achiral base has b

Full text of DOI:10.1039/c4cc09730k

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COMMUNICATION
Organocatalytic
one-pot
1,4-/1,6-/1,2-addition
sequence for the stereocontrolled formation of six
consecutive stereocenters
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2014,
Accepted 00th January 2014
Pankaj Chauhan, Suruchi Mahajan, Gerhard Raabe and Dieter Enders*
DOI: 10.1039/x0xx00000x
www.rsc.org/
R
An unprecedented stereoselective organocatalytic one-pot 1,4-
1,6-/1,2-addition sequence between β-dicarbonyl compounds,
OH
OH
S
/
H
N
O
H
β-nitroalkenes and 4-nitro-5-styrylisoxazoles sequentially
catalyzed by low loading of a squaramide catalyst and an
achiral base has been developed. The protocol opens an
efficient entry to isoxazole bearing cyclohexanes with six
consecutive stereogenic centers including one tetrasubstituted
carbon in good yields and excellent diastereo- and
enantioselectivities.
HN
O
N
2
CO H
N
O
N
H
H
O
CO
2
H
H
2
N
O
E. ibotenic acid
F. Danazol
A. Oxacillin R = H
B. Cloxacillin R = 2-Cl
C. Dicloxacillin R = 2,6-Cl2
D. Flucloxacillin R = 2-Cl,6-F
Figure 1. Enantiopure drugs and bioactive natural products bearing an isoxazole
Over the last ten years, asymmetric organocatalytic cascade ring.
reactions have emerged as a powerful strategy for the synthesis of
4
complex molecules bearing multiple stereogenic centers in a highly example, compounds A-D are β-lactamase-resistant antibiotics,
1
stereocontrolled fashion. These one-pot organocatalytic reactions while an isoxazole containing natural product E is a powerful
5
were successfully employed for the creation of cyclohexane ring neurotoxin, which is used as a brain-lesioning agent . A synthetic
2
systems bearing up to six stereocenters. Most of these triple cascade androgenic steroid danazol D bearing an isoxazole ring suppresses
reactions are governed by more common 1,4-/1,4-/1,2 addition the production of gonadotrophins and also has some weak
6
sequences. Another important class of addition reactions involving androgenic effects. Moreover, isoxazoles serve as precursors for the
7
the enantioselective 1,6-addition to control the formation of a remote synthesis of various synthetically useful organic compounds. Thus,
stereocenter is more challenging and less explored in comparison to the development of efficient asymmetric methods for the synthesis
3
the other addition variants. Moreover, organocatalytic cascade of isoxazole ring containing molecules can provide a new series of
reactions using all possible types of addition reactions, i.e. 1,4-/1,6- potentially bioactive molecules.
/
1,2-addition reactions, are not known so far. Hence we took the
Recently, organo- and metal-catalyzed 1,6-additions to 4-nitro-
challenge to develop a new stereoselective one-pot organocascade 5-styrylisoxazoles emerged as an efficient method to generate
8
,9
sequence using 1,4-/1,6-/1,2-additions (Scheme 1).
Previous work:
enantiopure isoxazole derivatives bearing one or two stereocenters.
This work:
However, the 4-nitro-5-styrylisoxazoles remained less explored
9d,g
substrates in stereoselective cascade reactions.
Very recently,
1
,4-addition
1
,4-addition
O
Jørgensen’s group utilized 4-nitro-5-styrylisoxazoles in trienamine-
mediated asymmetric [4+2] cycloaddition reactions to afford
10
O
R1
R3
EWG
R1
R3
EWG
R2
R2
cyclohexene products bearing three vicinal stereocenters. Herein
EWG
we report a novel cascade reaction involving a 1,4-/1,6-/vinylogous
R5 EWG
R4
vinylogous
1,2-addition
1,6-addition
1
,2-addition sequence to access enantiopure cyclohexane rings
1
,2-addition
1,4-addition
R4
bearing as many as six contiguous stereogenic centers, sequentially
1
1
Scheme 1. Enantioselective strategies for the construction of cyclohexane rings catalyzed by low loading of a cinchona derived squaramide and an
bearing multiple stereogenic centers.
achiral base.
Initially, we started our investigation with a squaramide I (1
In addition, the isoxazole core is present in various important
mol%) catalyzed one-pot three component reaction between ethyl
naturally occurring and synthetic bioactive molecules (Figure 1). For
acetoacetate (1a), β-nitrostyrene (2a) and 4-nitro-5-styrylisoxazole
This journal is © The Royal Society of Chemistry 2014
Chem. Commun., 2014, 00, 1-3 | 1

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DOI: 10.1039/C C09730K
Jou4Crnal Name
(
3a) (Table 1, entry 1). However our attempt to obtain the desired 61% yield and 91% ee. Further screening of different 4-nitro-5-
cyclohexane ring failed completely, and only the formation of the styrylisoxazoles bearing electron withdrawing and electron releasing
1
2
Michael adduct was observed. We envisaged that the squaramide substituents on the aryl ring as well as heteroaryl group provided a
catalyst was not enough active to generate a nitronate anion in the direct access to the corresponding cyclohexanes 4g-m in good yields
corresponding Michael adduct to initiate a domino 1,6-/vinylogous and high enantioselectivities (95-99% ee). The methyl acetoacetate
1
,2-addition sequence. Thus, a sequential reaction was performed and acetyl acetone were also tolerated under this one-pot protocol to
involving a squaramide I catalyzed Michael addition of the β- give rise to the respective products 4n and 4o in good yields and
ketoester 1a to the β-nitrostyrene 2a, followed by the addition of 3a excellent stereoselectivities. Employing pseudo-enantiomeric
a
and a catalytic amount of DBU (20 mol%) (entry 2). To our delight, amino-squaramide catalyst II successfully led to the formation of the
the desired cyclohexane 4a was obtained in 46% yield with excellent enantiomers of 4a-f, 4h and 4l in very good yields (51-69%) and
stereoselectivity (98% ee and >20:1 dr). Further optimization of the again excellent asymmetric inductions (>20:1 dr and 95-98% ee).
reaction conditions by screening different solvents (entries 3-5) and
bases (entries 6-11) showed that 30 mol% of DBU in CH Cl2
2
a
Table 2. Substrate scope .
provides a maximum yield of 62% and excellent stereoselectivity
N
O
(entry 6). The use of a quinidine derived squaramide catalyst II led
O
Me
Me
N
N
R3
O
Me OH
to the opposite enantiomer of the cyclohexane ent-4a with a similar
COR1
COR1
NO
2
Me
I or II
1
3
yield, ee and dr (entry 12).
or ent-4
(
1 mol%)
Cl , rt
24 h
DBU (30 mol%)
O
2
+
R3
R2
CH
2
2
CH Cl , rt
2
48 h
2
NO
2
a
R
2
Table 1. Optimizations of the reaction conditions.
NO
2
2
4
N
O
1
2
3
c
4
/
ent-4
R
R
R
yield
ee (%)
Me
N
N
b
O
O
Me OH
(%)
61
64
55
63
67
61
60
61
69
73
49
39
50
58
50
69
63
51
64
66
59
60
50
Ph
Me
CO
Ph
2
Et
CO
2
Et
3a NO2
4a
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
Ph
4-FC H4
Ph
Ph
Ph
Ph
Ph
Ph
98
99
99
93
97
91
98
97
97
99
95
96
97
97
96
96
97
95
98
95
96
97
96
Me
Ph
1
a
I or II (1 mol%)
base (x mol%)
solvent, rt
O
2
4
b
6
+
solvent, rt
Ph
NO
2
4c
4d
4e
4f
4-ClC
6 4
H
NO
2
2
a
4-MeC H4
6
4
a
OMe
OMe
6 4
4-MeOC H
H
H
O
N
2-Thienyl
N
4
g
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
6
4-FC
4-ClC
3-ClC
6 4
H
NH
HN
R = 3,5-(CF
3
)
2
C
6
H
3
CH
2
4h
4i
6 4
H
N
N
NH
R
HN
R
O
6 4
H
4j
4k
4l
6
4-MeC H4
O
I
O
II
b
c
d
6 4
2-MeC H
entry base (x mol%)
e
solvent
time (h)
yield (%)
ee (%)
4-MeOC H4
6
2-Thienyl
Ph
1
-
CH
2
Cl
2
24
-
46
-
4
m
2
DBU (20)
DBU (20)
DBU (20)
DBU (30)
DBU (30)
DBN (30)
TEA (30)
TBD (30)
DABCO (30)
Pipredine (30)
DBU (30)
CH
2
Cl
2
24+24
24+24
24+24
24+24
24+48
24+48
24+48
24+48
24+48
24+48
24+48
98
4
n
o
OMe
Me
3
4
5
CHCl
3
35
98
4
Ph
Toluene
THF
44
98
ent-4a
ent-4b
ent-4c
ent-4d
ent-4e
ent-4f
ent-4h
ent-4k
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
Ph
Ph
Ph
Ph
Ph
Ph
6
36
98
f
4-FC H4
6
7
8
9
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
Cl
2
62
98
f
f
f
4-ClC H4
6
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
36
97
4-MeC H4
6
traces
29
n.d.
98
4-MeOC H4
6
2-Thienyl
Ph
f
f
f
1
0
-
-
4-ClC H4
1
1
1
2
traces
58
n.d.
g
96
Ph
2-MeC H4
6
a
a
Reaction conditions: 0.5 mmol of 1, 0.5 mmol of 2, 1 mol% of I (entry 1-
b
17) or II, 1.0 mmol of 3 and 30 mol% of DBU (0.1 M in CH Cl ). Yield of
2 2
c
isolated product after column chromatography. Enantiomeric excess of the
major diastereomer determined by HPLC analysis on a chiral stationary
phase.
Reaction conditions: 0.2 mmol of 1a, 0.2 mmol of 2a, 1 mol% of I, 0.24
b
mmol of 3a and x mol% of base (0.1 M in solvent). Time in hours for both
c
reaction steps. Yield of isolated 4a after column chromatography.
d
Enantiomeric excess of the major diastereomer (>20:1 dr) determined by
e
HPLC analysis on a chiral stationary phase. All the reactants were added in
f
g
one step. 2 Equivalents of 3a were used. ee Value of ent-4a synthesized by
using catalyst II.
The absolute configuration of the products 4a-o can be
assigned as (1S), (2S), (3R), (4S), (5S) and (6R) on the basis of the
13
Once equipped with optimized reaction conditions, we
evaluated the substrate scope at a 0.5 mmol scale of the β-dicarbonyl
compounds and the β-nitrostyrenes (Table 2). The various
nitroalkenes bearing electron withdrawing and electron donating
groups gave rise to the corresponding isoxazole products 4b-e in 55-
X-ray crystallographic analysis of 4a (Figure 2).
To demonstrate the practical and preparative application of
this new organocascade 1,4-/1,6-/1,2-addition sequence, we
performed a gram-scale reaction between 1a
,
2a and 3a using a
(Scheme 2). The
lower loading (0.5 mol%) of the squaramide
I
6
7% yield and excellent stereoselectivities (>20:1 dr and 93-99%
desired product 4a was obtained in 57% yield with unchanged
ee). The nitroalkenes bearing a heteroaromatic group also worked
ee and dr values. The enantiomeric purity could be enriched to
well in this cascade sequence to provide the desired product 4f in
>
99% ee after a single crystallization of the product.
2
| J. Name., 2014, 00, 1-3
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.1039/C4CC09730K
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5
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Figure 2. X-ray structure of 4a
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N
O
O
Me
Me
N
N
O
Me OH
Ph
CO
.0 mmol
2
Et
3a
CO
2
Et
NO
2
Me
Ph
3. For selected reviews and highlights on 1,6-addition reactions, see: (a) A. G.
Csaký, G. de la Herrán and M. C. Murcia, Chem. Soc. Rev., 2010, 39, 4080;
8
(
16 mmol
(3.68 g)
1a
O
2
1.04 g)
I (0.5 mol%)
CH Cl , rt, 48 h
Ph
Ph
(b) A. T. Biju, ChemCatChem, 2011, 3, 1847; (c) E. M. P. Silva and A. M. S.
+
DBU (30 mol%)
CH Cl , rt, 48 h
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2
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2
2
NO
2
NO
2
4
2.34 g) 57% yield
>20:1 dr, 98% ee
a
(
8
.0 mmol
1.19 g)
2
a
(
(g) L. Dell’Amico, Ł. Albrecht, T. Naicker, P. H. Poulsen and K. A.
Scheme 2. Gram-scale 1,4-/1,6-/1,2-addition sequence.
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7
.
1
Support from the European Research Council (ERC 8. For selected examples of 4-nitro-5-styrylisoxazoles in non-enantioselective
1
,6-addition reactions, see: (a) S. Chimichi, F. D. Sio, D. Donatic, P. S.
Advanced Grant 320493 “DOMINOCAT”) is gratefully
acknowledged. We thank BASF SE for the donation of
chemicals.
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*Notes and references
Institute of Organic Chemistry, RWTH Aachen University Landoltweg 1, 52074
Aachen (Germany)
E-mail: enders@rwth-aachen.de; Fax: +49 (241) 8092127
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†
Electronic Supplementary Information (ESI) available: See
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