Enantioselective synthesis of α-amino-γ-sulfonyl phosphonates with a tetrasubstituted chiral α-carbon via quinine-squaramide-catalyzed michael addition of nitrophosphonates to vinyl sulfones

Article abstract of DOI:10.1002/adsc.201300224

α-Nitro-γ-sulfonyl phosphonates with a key tetrasubstituted chiral α-carbon center have been synthesized for the first time in high yield and enantioselectivity through a quinine-squaramide-catalyzed conjugate addition of α-nitro phosphonates to aryl vinyl sulfones. Representative examples presented here for the transformation of nitrosulfonyl phosphonates to aminosulfonyl phosphonates, alkylation at the α-position of the sulfonyl group followed by desulfonation and scale-up of the conjugate addition highlight the practical applications of the methodology. Copyright

Full text of DOI:10.1002/adsc.201300224

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DOI: 10.1002/adsc.201300224
Enantioselective Synthesis of a-Amino-g-sulfonyl Phosphonates
with a Tetrasubstituted Chiral a-Carbon via Quinine-Squaramide-
Catalyzed Michael Addition of Nitrophosphonates to Vinyl
Sulfones
Kalisankar Beraa and Irishi N. N. Namboothiria,*
a
Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai 400 076, India
E-mail: irishi@iitb.ac.in
Received: March 15, 2013; Published online: April 30, 2013
Dedicated to Prof. A. M. Abdul Khader, Department of Chemistry, Mangalore University, on the occasion of
his 60th birthday and superannuation from distinguished service.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300224.
Abstract: a-Nitro-g-sulfonyl phosphonates with
a key tetrasubstituted chiral a-carbon center have
been synthesized for the first time in high yield and
enantioselectivity through a quinine-squaramide-
catalyzed conjugate addition of a-nitro phospho-
nates to aryl vinyl sulfones. Representative exam-
ples presented here for the transformation of nitro-
sulfonyl phosphonates to aminosulfonyl phospho-
nates, alkylation at the a-position of the sulfonyl
group followed by desulfonation and scale-up of the
conjugate addition highlight the practical applica-
tions of the methodology.
is one such group due to its versatility as a synthetic
handle and as a biological probe.[7] The role of sul-
fones in natural product synthesis and in functional
group transformations, including those involving
facile desulfonation, is well documented.[8] Various
biological activities of organosulfones have also been
extensively investigated.[9] For instance, the anti-pros-
tate cancer drug Bicalutamide and the anti-leprosy
drug Dapsone possess a sulfonyl group.
Although there are many reports on the catalytic
asymmetric Michael addition of various nucleophiles
to vinyl 1,1-bis-sulfones,[10,11] vinyl monosulfones have
been seldom used in such capacity.[12–14] Only three
nucleophiles, viz. a-aryl a-cyanoacetate,[12] 3-aryloxin-
doles[13] and a-branched aldehydes[14] have been suc-
cessfully added to aryl vinyl sulfones under catalytic
asymmetric conditions. Similarly, despite the fact that
different approaches to enantioenriched a-amino
phosphonates, which include resolution, auxiliary
based and catalytic approaches, are documented in
Keywords: amino phosphonates; amino sulfones;
asymmetric catalysis; Michael addition; squar-
ACHTUNGTRENNUNGamides
Amino phosphonates are regarded as transition state the literature,[15,16] asymmetric synthesis of a-amino
analogs of amino acids in various biological processes, phosphonates possessing a key g-sulfonyl group re-
for instance, in peptide bond hydrolysis.[1] The poten- mains unreported hitherto. We envisaged that this
tial of amino phosphonates as synthetic intermediates could be achieved via Michael addition of a-nitro
and as biological agents has prompted considerable phosphonates to vinyl sulfones in the presence of suit-
research in recent years.[2–4] a-Amino phosphonates, able chiral catalysts. This approach also generates tet-
in particular, exhibit anti-bacterial and anti-fungal ac- rasubstituted chiral a-carbon centers in an enantiose-
tivity as well as catalytic activity in organic transfor- lective fashion.[17]
mations.[5] A bioactive natural product K-26 possesses
As part of our efforts to develop novel catalytic
methods for the asymmetric synthesis of nitro/amino
an a-amino phosphonate moiety.[6]
Introduction of reactive functional groups in the phosphonates,[17,18] we embarked on the idea of Mi-
amino phosphonate chain in an asymmetric fashion chael addition of nitro phosphonate 2a to vinyl sul-
appeared an attractive approach to generate novel fone 1a (Table 1). Several Cinchona-based bifunction-
enantioenriched amino phosphonates with enhanced al organocatalysts were screened for our studies
or unexpected biological properties. A sulfonyl group (Figure 1 and Table 1).[19] At the outset, quinine-thio-
Adv. Synth. Catal. 2013, 355, 1265 – 1270
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Kalisankar Bera and Irishi N. N. Namboothiri
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Table 1. Catalyst screening and reaction condition optimiza-
tion.
our delight, the Michael adduct 3a, with a tetrasubsti-
tuted chiral a-carbon center, a-nitro-g-sulfonyl phos-
phonate, was isolated in good yield (91–92%) and
enantioselectivity (83–87% ee, entries 1 and 2). Other
catalysts such as cinchonidine-thiourea C3, cincho-
nine-thiourea C4, quinine-thiourea C5 and dihydro-
quinine-thiourea C6 were also quite effective (en-
tries 3–6), but C5 was the best giving the Michael
adduct 3a in 97% yield and 91% ee (entry 5). At this
juncture, possible enhancement of enantioselectivity
by modifying C5 was explored. Thus, C7 where the
thiourea moiety in C5 is replaced by a squaramide
moiety with greater H-bonding capability,[20] not only
decreased the reaction time from 12 h to 4 h at room
temperature in mesitylene, but also improved the se-
lectivity to 93% while maintaining excellent yield
(96%, entry 7). Further solvent screening (entries 8–
15) helped us identify ethylene dichloride as the best
solvent (95% yield and 96% ee, entry 13). High yield
(92–94%) and selectivity (96% ee) were maintained
even with lower catalyst loading (2–5 mol%), albeit at
the expense of reaction rate (entries 16 and 17). Final-
ly, lowering the reaction temperature to 08C, though
considerably decreased the reaction rate, allowed us
to achieve the highest yield (96%) and selectivity
(98% ee, entry 18).
Entry C[a] Solvent
Temp.
[oC]
Time Yield
[h]
ee
[%][b]
[%][c]
1
2
3
4
5
6
7
8
C1 mesitylene r.t.
C2 mesitylene r.t.
C3 mesitylene r.t.
C4 mesitylene r.t.
C5 mesitylene r.t.
C6 mesitylene r.t.
C7 mesitylene r.t.
12
12
15
15
15
15
4
4
4
4
5
5
5
4
5
12
48
72
91
92
91
87
97
94
96
95
94
91
90
94
95
94
91
92
94
96
87
83
85
88[d]
91
87
93
95
95
93
96
96
96
92
90
96
96
98
C7 toluene
C7 benzene
C7 xylene
C7 CH2Cl2
C7 CHCl3
r.t.
r.t.
r.t.
r.t.
r.t.
9
10
11
12
13
14
15
C7
C7 THF
C7 MeCN
G
16[e] C7
E
17[f]
18
C7
C7
ACHTUNGTRENNUNG
A
0
Under the above optimized conditions, that is, in
the presence of 10 mol% C7, in ethylene dichloride at
08C, phenyl vinyl sulfone 1a was treated with a wide
range of a-substituted nitro phosphonates 2a–m
(Table 2). It is noteworthy that the Michael adducts
3a–m were isolated in excellent yields (85–99%) and
selectivities (90–98% ee) over a period of 2–7 days.
[a]
Catalyst.
[b]
[c]
[d]
[e]
[f]
After silica gel column chromatography.
The ee values were determined by chiral HPLC.
Opposite enantiomer.
5 mol% catalyst.
2 mol% catalyst.
urea C1 and dihydroquinine-thiourea C2, recently re- Although there was no major dependence of the yield
ported from our laboratory,[17] were screened and, to and selectivity on the nature of a-substituent, the re-
Figure 1. Bifunctional organocatalysts screened.
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Enantioselective Synthesis of a-Amino-g-sulfonyl Phosphonates with a Tetrasubstituted Chiral a-Carbon
Table 2. Scope of nitrophosphonates 2.
Table 3. Scope of vinyl sulfones 1.[a–c]
Entry
R
Time [d]
3
Yield [%][a] ee [%][b]
1
2
3
Et
Me
n-Pr
nBu
3
2
5
6
4
6
7
4
7
7
5
5
7
3a 96
3b 93
3c 93
3d 98
3e 90
3f 99
3g 88
3h 85
3i 89
3j 95
3k 95
3l 99
3m 97
98
97
97
97
90
98
94
94
90
96
97
96
97
4
5[c]
6
iBu
n-C5H11
c-C6H11
c-C3H5
PhCH2
PhCH2CH2
n-C7H15
n-C9H19
7[c,d]
8[c,d]
9[c]
10
11
12
13
ACHTUNGTRENNUNG(CH2)3CO2Et
[a]
After silica gel column chromatography.
The ee values were determined by chiral HPLC.
Reaction performed at room temperature.
20 mol% catalyst.
[b]
[c]
[d]
action rate decreased with nitro phosphonates 2e and
2g–i bearing sterically demanding R groups (entries 5
and 7–9). Therefore, these reactions had to be con-
ducted at room temperature. In the case of nitro-
phosphonates 2g and 2h, catalyst loading also had to
be increased to 20 mol% to complete the reaction
(entries 7 and 8).
[a]
1b: Ar=4-ClC6H4, 1c: Ar=4-BrC6H4, 1d: Ar=phenyl-
AHCTUNGTRENNGtUN etrazolyl.
Yields after silica gel column chromatography.
The ee value was determined by chiral HPLC.
[b]
[c]
Excellent yields and enantioselectivities observed
in the synthesis of a-nitro-g-sulfonylphosphonates 3 moiety of C7 (Figure 2). Si-face addition of the nitro
derived from phenyl vinyl sulfone 1a and a variety phosphonate anion to the activated vinyl sulfone af-
nitro phosphonates 2 (Table 2) prompted us to extend fords nitro sulfone 3 or 4.
the scope of this reaction to other vinyl sulfones 1b–
Our reaction conditions are suitable for the enan-
e (Table 3). Thus, the reaction of vinyl sulfone 1b with tioselective synthesis of nitro phosphonates 3–4 on
nitro phosphophonate 2a provided the desired Mi- a gram scale. This was demonstrated by the synthesis
chael adduct 4a in excellent yield (98%) and enantio- of 1.96 g of 3c (96%) with 98% ee via Michael addi-
selectivity (98% ee) in 36 h at 08C. To our delight, tion of 1.2 g of nitrophosphonate 2c to 1.26 g of vinyl
aryl vinyl sulfones 1b and 1c also reacted with nitro- sulfone 1a (Scheme 1).
phosphonate 2a to afford the Michael adducts 4b and
Nitrosulfonyl phosphonate 3 or 4 is an excellent
4c in similar yields and selectivity, but in longer reac- precursor for the enantioselective synthesis of amino-
tion times (60 h) in the case of 1c. The heteroaryl sulfonyl phosphonate 5 which in turn is amenable for
vinyl sulfone 1d underwent Michael addition with ni- alkylation a to sulfonyl group and desulfonation with
trophosphonates 2a, 2i and 2m even at À408C, due to complete stereochemical integrity (Scheme 2). Thus,
the electron-withdrawing nature of tetrazole moiety, a representative nitrosulfonyl phosphonate 3c was
to afford the desired products 4d–f in excellent yields, subjected to nitro group reduction using Zn-HCl to
albeit in moderate selectivity (74–79% ee).
afford aminosulfonyl phosphonate 5 in 95% yield. N-
The absolute configuration of the Michael adducts Benzoylation of 5 using benzoyl chloride and Et3N
3 and 4 was unambiguously assigned as S by single gave the amide 6 in 74% yield. Subsequent LDA
crystal X-ray structure analysis of a representative mediated benzylation at the a position of sulfonyl
compound 3c (Figure 2, see also the Supporting Infor- group in 6 proceeded well to provide 7 in 75% yield.
mation). The proposed transition state involves acti- Desulfonation of 7 using Na-Hg in EtOH took place
vation of sulfone by the squaramide moiety and de- under mild conditions to deliver amidophosphonate 8
protonation of nitro phosphonate by the quinuclidine in excellent yield (81%).
Adv. Synth. Catal. 2013, 355, 1265 – 1270
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Kalisankar Bera and Irishi N. N. Namboothiri
COMMUNICATIONS
Figure 2. X-ray structure with absolute configuration of 3c and proposed TS for the formation of 3.
benzenoid and lower (74–79%) with a tetrazolyl
group. Nitro group reduction and sulfonyl group as-
sisted functionalization followed by desulfonation as
well as scale up of the enantioselective conjugate ad-
dition have been successfully carried out.
Scheme 1. Scale up of the synthesis of nitro phosphonate 3c.
Experimental Section
Michael Addition of Diethyl 1-Nitrobutylphos-
phonate 2c to Phenyl Vinyl Sulfone 1a
To a solution of diethyl 1-nitrobutylphosphonate 2c (1.2 g,
5.0 mmol) and catalyst C7 (315 mg, 0.5 mmol) in ethylene
dichloride was added phenyl vinyl sulfone 1a (1.26 g,
7.5 mmol) at 08C. The reaction mixture was stirred at 08C
for 6 d. The solvent was evaporated under reduced pressure
and the residue was purified by silica gel column chromatog-
raphy using petroleum ether-EtOAc (15–55%) as eluent to
afford
(S)-diethyl
3-nitro-1-(phenylsulfonyl)hexan-3-yl-
phosphonate 3c as a colorless solid; yield: 1.96 g (4.8 mmol,
˜
96%; 98% ee); mp 82–848C; IR (film): n=2971 (w), 2923
(m), 2851 (w), 1642 (w), 1549 (s), 1448 (w), 1305 (m), 1260
(m), 1148 (s), 1087 (m), 1045 (s), 1020 (s), 978 (m), 750 (m);
1H NMR (400 MHz, CDCl3): d=0.92 (t, J=7.3 Hz, 3H),
1.13–1.27 (m, 1H), 1.30 (t, J=7.1 Hz, 3H), 1.31 (t, J=
7.1 Hz, 3H), 1.33–1.43 (m, 1H), 1.93–2.04 (m, 1H), 2.19–
2.30 (m, 1H), 2.33–2.44 (m, 1H), 2.59–2.73 (m, 1H), 3.40
(dd, J=9.6, 7.2 Hz, 2H), 4.11–4.23 (m, 4H), 7.57 (t, J=
7.4 Hz, 2H), 7.65–7.70 (m, 1H), 7.91–7.94 (m, 2H);
13C NMR (100 MHz, CDCl3): d=14.0, 16.5 (d, J=4.0 Hz),
16.9 (d, J=7.0 Hz), 26.7, 37.9, 51.7 (d, J=2.0 Hz), 64.4 (d,
J=7.0 Hz), 64.9 (d, J=7.0 Hz), 92.1 (d, J=149.0 Hz), 128.3,
129.6, 134.2, 138.8; 31P NMR (162 MHz, CDCl3): d=14.9;
MS (ES+, Ar): m/z (%)=409 ([MH+1]+, 22), 408 (MH+,
100), 362 (40), 361 (47), 333 (45), 207 (14); HR-MS (ES+,
Ar): m/z=408.1230, calcd. for C16H27NO7PS (M+H)+:
408.1246; [a]2D5: 5.808 (c=0.5 in CHCl3); HPLC (Chiralpak
IA; petroleum ether/i-PrOH=80/20, flow rate 1 mLminÀ1,
l=216 nm): tR (major)=10.4 min, tR (minor)=9.1 min;
Scheme 2. Synthesis of tetrasubstituted chiral a-carbon a-
amino-g-sulfonyl phosphonate 5 and its further transforma-
tions.
In conclusion, conjugate addition of a-nitro phos-
phonates to aryl vinyl sulfones afforded a-nitro-g-sul-
fonyl phosphonates in excellent yield (90–99%) and
good to excellent enantioselectivity in the presence of
a quinine-squaramide organocatalyst. The enantiose-
lectivities were excellent (90–98%) when aryl was 97% ee. X-ray data: C16H26NO7PS, M=407.41,
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Enantioselective Synthesis of a-Amino-g-sulfonyl Phosphonates with a Tetrasubstituted Chiral a-Carbon
orthorhombic, space group P2(1)2(1)2(1), a=9.100(2) ꢁ,
b=14.581(4) ꢁ, c=15.200(4) ꢁ, a=908, b=908, g=908, V=
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Acknowledgements
INNN thanks DAE India for financial assistance. KSB
thanks CSIR India for a senior research fellowship. The au-
thors thank Prof. Maheswaran Shanmugam for help with X-
ray analysis.
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and the references cited therein.
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Adv. Synth. Catal. 2013, 355, 1265 – 1270