Enantioselective synthesis of α-nitro-δ-ketosulfones via a quinine-squaramide catalyzed conjugate addition of α-nitrosulfones to enones

Article abstract of DOI:10.1039/c3cc45985c

Conjugate addition of α-nitrosulfones to vinyl ketones in the presence of 0.2 mol% of a quinine-squaramide organocatalyst afforded α-nitro-δ-ketosulfones possessing a tetrasubstituted chiral center in excellent yield and enantioselectivity in most cases. This strategy also offers a facile and convenient entry into γ-sulfonylhydroxamates that are one carbon homologs of potent enzyme inhibitors.

Full text of DOI:10.1039/c3cc45985c

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Enantioselective Synthesis of α-Nitro-δ-ketosulfones via a Quinine-
Squaramide Catalyzed Conjugate Addition of α-Nitrosulfones to Enones
Kalisankar Beraa and Irishi N. N. Namboothiri*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
δ-ketosulfones via Michael addition of α-nitrosulfones to enones
45 using an alkaloid derived squaramide catalyst. Transformations
of the product to quaternary γ-nitro-γ-sulfonyl hydroxamic acid
and γ-nitro-γ-sulfonyl carboxylic acid are also reported here.
Conjugate Addition of α-nitrosulfones to vinyl ketones in the
presence of 0.2 mol of quinine-squaramide
organocatalyst afforded α-nitro-γ-ketosulfones possessing a
%
a
tetrasubstituted chiral center in excellent yield and
10 enantioselectivity in most cases. This strategy also offers a
facile and convenient entry into γ-sulfonylhydroxamates that
are one carbon homologs of potent enzyme inhibitors.
Sulfonylhydroxamic acids are inhibitors of matrix metalla-
protease (MMP) and are effective for the treatment of cancer and
15 arthritis.1 For instance, β-sulfonylhydroxamate 1a, in which a
sulfonyl group is attached to a chiral center, is a potent MMP and
PDE (phosphodiesterase) inhibitor.2-3 The enantiopure compound
1a displays superior activity as compared to the racemic one and
the enantioselective synthesis of 1a involves the oxidation of
20 enantioenriched thioether precursor.3 Recently, sulfones attached
Figure 2 Catalysts Screened
50
The reaction conditions were optimized by performing
Michael addition of nitrosulfone 3a to phenyl vinyl ketone 2a
using several cinchona-based bifunctional organocatalysts and
solvents at different temperatures (Figure 2 and Table 1). At the
outset, the quinine-thiourea catalyst C1, recently reported from
to
a
tetrasubstituted chiral center4-6 received considerable
attention due to their wide range of biological activities, for
instance, against Alzheimer’s (as γ-secretase inhibitor 1b)4 and
glaucoma.6
55 our laboratory,15 was screened (entry 1). To our delight, the
Michael adduct 4a, a quaternary -nitrosulfone, was isolated in
good yield (96%) and enantioselectivity (90% ee) when 10 mol %
of C1 was employed in mesitylene at rt (entry 1). Other catalysts
such as quinine-thiourea C2, cinchonidine-thiourea C3 and
60 cinchonine-thiourea C4 were also quite effective in providing the
Michael adduct 4a in excellent yield and selectivity (entries 2-4).
Encouraged by these results, further improvement in the
enantioselectivity was explored by employing bifunctional
cinchona-squaramide catalysts C5-C7 with greater H-bonding
65 capability (entries 5-8).17 Cinchonidine-squaramide catalyst C5
provided the product 4a with lower selectivity (entry 5).
However, quinine-squaramide catalyst C618 furnished the
Michael adduct 4a with improved selectivity (94%) and excellent
yield (98%, entry 6). At this juncture, possible enhancement of
70 enantioselectivity by screening various solvents at different
temperatures in the presence of 10 mol% of C6 was investigated
(entries 8-17). Thus, toluene was identified as the best solvent
25
Figure 1 Potent Inhibitors of MMP, PDE and γ-Secretase
The sulfonyl group in organosulfones, viz. active methylene
sulfones, vinyl sulfones and other sulfone based nucleophiles and
electrophiles, takes part in a variety of synthetic transformations
30 and is an easily removable functional group.7
Enantioselective approaches to sulfones include catalytic
asymmetric Michael addition of various nucleophiles to -
unsubstituted vinyl sulfones8 and -substituted vinyl sulfones,9
nucleophilic addition of active methylene10 and methine11-12
35 sulfones to various electrophiles, and other miscellaneous
methods.13-14 However, to our knowledge, generation of sulfones
attached to a chiral carbon remains scarcely explored under
catalytic asymmetric conditions.12,14
o
which at -60 C provided Michael adduct 4a in 98% yield and
99% ee (entry 17). We were amazed to note that high yield (98%)
75 and selectivity (99% ee) were unaffected even when the catalyst
loading was gradually reduced to 5, 2 and 0.2 mol % albeit at the
expense of reaction rate (entries 18-20).
As part of our ongoing research program to develop novel
40 catalytic methods for the asymmetric synthesis of quaternary
carbon centers, we have reported enantioselective synthesis of
quaternary α-nitro/aminophosphonates.15-16 Herein we report an
organocatalyzed enantioselective synthesis of quaternary α-nitro-
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Table 1 Catalyst Screening and Optimization of Reaction Conditionsa
enantioselectivities (entries 1-6). The enantioselectivity dropped
marginally (to 85% ee) in the case of 5a (entry 1) and
View Article Online
40 substantially (to 50% ee) when a nitrosulfone 3f possessing a
DOI: 10.1039/C3CC45985C
long alkyl chain was employed (entry 5). This is attributable to
the interference of the long, linear and flexible alkyl chain in 3f in
the Michael addition step (see Figure 3, vide infra). Since the rate
of the reaction was very slow at -60 oC in case of benzyl
45 substituted nitrosulfone 3g, the reaction was performed at rt
(entry 6).
Entry Cat solvent
T (oC) Time (h) Yield (%)b ee (%)c
1
2
3
4
5
6
7
8
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
xylene
toluene
PhCF3
CH2Cl2
(CH2)2Cl2
THF
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
4
5
8
11
96
94
95
93
95
98
98
97
97
95
86
97
91
92
92
98
98
98
98
98
90
88
C1
C2
C3
C4
C5
C6
C7
C6
C6
C6
C6
C6
C6
C6
C6
C6
C6
C6
C6
C6
91
87d
80
Table 2 Scope of Enones 2a
94
94
93
94
88
86
85
92
9
10
11
12
13
14
15
16
17
18e
19f
20g
Entry
R
Time (h)
Yield (%)b
ee (%)c
4
1
2
3
4
5
6
7
8
C6H5
11
10
16
20
5
98
99
98
99
98
99
99
97
99
98
99
96
97
86
98
99
96
99
96
>99
>99
99
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4-MeC6H4
4-OMeC6H4
3,4-(OMe)2C6H3
4-ClC6H4
4-CNC6H4
4-NO2C6H4
2-ClC6H4
3-BrC6H4
2-naphthyl
1-naphthyl
2-furyl
diethyl ether
MeCN
toluene
toluene
toluene
86
70
-20
-60
-60
-60
-60
96.4
99
98.5
98.6
99
3
2
toluene
toluene
14
5
14
15
12
7
98
9
>99d
99
a
The reactions were performed at 0.2 mmol scale. bAfter silica gel
10
11
12
13
14e
15
c
d
column chromatography. ee’s determined by chiral HPLC. opposite
72
99
97
91
e
enantiomer. 5 mol% catalyst. f 2 mol% catalyst. g 0.2 mol% catalyst and
5 0.5 mmol of 3.
2-thienyl
c-C6H11
PhCH=CH
The scope of the above reaction was subsequently investigated
by treating α-nitrosulfone 3a with various enones 2b-o under the
above optimized conditions, i.e. 0.2 mol % catalyst C6, in toluene
at -60C (Table 2). It is noteworthy that regardless of the steric
10 and electronic properties and position of substituents on the
aromatic ring of enones, the Michael adducts 4a-i were isolated
in excellent yields (97-99%) and selectivities (96-99% ee) over a
period of 2-20 h (entries 1-9). However, faster reaction rate was
observed in the case of enones possessing electron withdrawing
15 substituents such as NO2, CN, Br and Cl at unhindered positions
(entries 5-7 and 9) when compared to enones possessing electron
donating substituents such as Me and OMe (entries 2-4).
Polycyclic aromatic enones 2j-k, heteroaromatic enones 2l-m and
an aliphatic enone 2n were also well tolerated in terms of the
20 chemical and optical yields of the products (entries 10-14), except
that 1-naphthyl vinyl ketone 2k furnished the Michael adduct 4k
with lower selectivity (72% ee, entry 11). However, since the
reaction rate dramatically decreased with aliphatic enone 2n the
reaction had to be conducted at rt with 10 mol% of the catalyst
25 C6 (entry 14). A dienone 2o also furnished the Michael adduct 4o
in 98% yield and 97% ee (entry 15). Notably, the reaction is
highly regioselective in that α-nitrosulfone 3a selectively reacted
with the β-unsubstituted olefin moiety in the presence of a β-
substituted olefin moiety in enone 2o (entry 15).
10
15
97
a
b
The reactions were performed at 0.5 mmol scale. After silica gel
column chromatography. c Ee determined by chiral HPLC. d This reaction
50 was also performed at larger scale (vide infra). e Reaction performed at rt.
Table 3 Scope of Nitrosulfones 3 a
Entry R1
Time (h)
Yield (%)b ee (%)c
3
5
1
2
3
4
Et
allyl
n-Bu
n-C5H11
n-C7H15
benzyl
12
10
14
17
17
8
96
99
93
98
90
95
85
>99
95
95
50
96
3b
3c
3d
3e
3f
3g
5a
5b
5c
5d
5e
5f
5
6d
a
b
The reactions were performed at 0.5 mmol scale. After silica gel
c
d
column chromatography. Ee determined by chiral HPLC. Reaction
performed at rt.
55
The absolute configuration of the Michael adducts 4-5 was
unambiguously assigned as R by single crystal X-ray structure
analysis of a representative compound 4i (Figure 3, see also the
ESI). The proposed mechanism based on model studies involves
deprotonation of nitrosulfone 3 by the quinuclidine moiety of
30
Encouraged by the excellent results obtained from the reaction
of nitrosulfone 3a with a variety of enones 2 (Table 2), the scope
of the reaction was investigated further with other sterically and
electronically different nitrosulfones 3b-g (Table 3). Thus,
various alkyl, allyl and benzyl substituted nitrosulfones 3b-g
60 catalyst C6 and activation of enone 2 by the squaramide moiety
(Figure 3). In the favored approach II, the squaramide moiety
also co-ordinates with the nitro group, and the quinuclidine
moiety with the nitro and the sulfonyl groups, to enable Re-face
approach of the enone 2 towards the nitrosulfone anion affording
65 (R)-nitrosulfone 4 or 5. The alternative approach I appears
disfavored due to severe steric interactions between the phenyl
group of sulfone and the quinuclidine moiety of the catalyst.
35 were treated with a representative enone 2i under the optimal
reaction conditions (Table 3). In general, the Michael adducts 5a-
f
were obtained in excellent yields (90-99%) and
2
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45 3
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Our reaction conditions are suitable for the synthesis of
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drop in the yield or selectivity. This was demonstrated by the
synthesis of 2.9 g of 4i (97%) with 99% ee via Michael addition
of 1.5 g of nitrosulfone 3a to 2.2 g of vinyl ketone 2i (Table 2,
entry 9).
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DOI: 10.1039/C3CC45985C
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Figure 3. X-ray Structure of 4i and Proposed Mechanistic Model
Nitrosulfonylketones 4 and 5 in which the carbonyl group is at
the δ-position of the nitro and the sulfonyl group are excellent
6
M. F. Surgrue, A. Harris and I. Adamsoms, Drugs Today, 1997, 33,
283.
10 precursors for the enantioselective synthesis of carboxylic acid 7
and hydroxamic acid 8 (Scheme 2). Thus a representative
nitrosulfonylketone 4b was subjected to Baeyer-Villiger
oxidation using mCPBA-TFA to afford nitrosulfonyl ester 6 in
65 7 Selected books/reviews: (a) T. G. Back, K. N. Clary and D. Gao,
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93% yield.
15 nitrosulfonyl ester
Lithium hydroxide mediated hydrolysis of
afforded quaternary γ-nitro-γ-sulfonyl
70
Prilezhaeva, Russ. Chem. Rev., 2000, 69, 367.
6
8
Selected recent reviews: (a) A. R. Alba, X. Companyo and R. Rios,
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carboxylic acid 7 in 84% yield. More importantly, the ester 6 was
successfully transformed to hydroxamic acid 8 in 74% yield by
treating with hydroxylamine hydrochloride. The ee of the
intermediate 6 and the final product 8 matched well with the ee of
20 the starting compound 4b.
75 9
(a) S. Rajkumar, K. Shankland, J. M. Goodman and A. J. A. Cobb,
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2315.
O
O
O
LiOH.H2O
m-CPBA, TFA
SO2Ph
SO2Ph
Ar
SO2Ph
Ar
HO
O
THF:H2O (1:1)
rt, 10 min, 84%
DCM, rt, 18 h
93%
O2N
O2N
O2N
80
7
4b
Ar = 4-Me-C6H4
6
10 (a) J. L. García-Ruano, V. Marcos and J. Alemán, Chem. Commun.,
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K. A. Jørgensen, Org. Lett., 2011, 13, 4790.
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Beilstein J. Org. Chem., 2008, 4; (d) A.-N. Alba, X. Companyo´, A.
NH2OH.HCl
EtOH:DCM (7:3)
rt, 12 h, 74%, 99% ee
pyridine
O
HO
N
SO2Ph
H
O2N
8
85
Scheme 1 Enantioselective Synthesis of Hydroxamic Acid
In conclusion, conjugate addition of -nitrosulfones to vinyl
ketones afforded -nitro-δ-ketosulfones in excellent yield and
enantioselectivity in the presence of as low as 0.2 mol% quinine-
25 squaramide organocatalyst. The feasibility of scale up of the
enantioselective conjugate addition as well as
a practical
application of the products in the enantioselective synthesis of
carboxylic acids and hydroxamic acids have been successfully
demonstrated.
95
Moyano and R. Rios, Chem. Eur. J., 2009, 15, 7035; (e) S. Zhang, Y.
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30
INNN thanks DAE India for financial assistance and KSB
thanks CSIR India for a senior research fellowship.
a Department of Chemistry, Indian Institute of Technology Bombay,
Mumbai 400 076, India, irishi@iitb.ac.in
† Electronic Supplementary Information (ESI) available: See
35 DOI: 10.1039/b000000x/
100
105
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