Binaphthyl-modified quaternary phosphonium salts as chiral phase transfer catalysts: Application to asymmetric amination of β-keto esters

Article abstract of DOI:10.1055/s-0029-1216848

A chiral quaternary tetraalkylphosphonium salt has been successfully utilized for the first time as a phase-transfer catalyst for asymmetric amination of β-keto esters in high yield with high ee. Asymmetric amination of a cyclic five-membered β-keto ester

Full text of DOI:10.1055/s-0029-1216848

PRACTICAL SYNTHETIC PROCEDURES
2289
Binaphthyl-Modified Quaternary Phosphonium Salts as Chiral Phase
Transfer Catalysts: Application to Asymmetric Amination of b-Keto Esters
A
R
symmetric
A
mi
o
nation of
b
-K
n
etoEsters gjun He, Keiji Maruoka*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
Fax +81(75)7534041; E-mail: maruoka@kuchem.kyoto-u.ac.jp
Received 13 March 2009; revised 18 March 2009
PSP
No164
Abstract: A chiral quaternary tetraalkylphosphonium salt has been successfully utilized for the first time as a phase-transfer catalyst for
asymmetric amination of b-keto esters in high yield with high ee. Asymmetric amination of a cyclic five-membered b-keto ester is a valu-
able method for preparing a key intermediate for asymmetric synthesis of aldose reductase inhibitor AS-3201 (Ranirestat).
Key words: amination, asymmetric synthesis, phase-transfer catalysis, phosphonium salts
Ar
O
O
CF3
(S)-3b (1–5 mol%)
Br–
Bu
Bu
CO2t-Bu
CO2t-Bu
R2
NHCO2t-Bu
7–99% yield, 90–95% ee
R1
R1
*
+
P
t-BuO2CN NCO2t-Bu
base, toluene
Ar =
R2
N
t-BuO C
2
CF3
9
Ar
(S)-3b
Scheme 1 Asymmetric amination of b-keto esters
Introduction
asymmetric amination of b-keto esters.5,6 Such an asym-
metric transformation of cyclic five-membered b-keto es-
In contrast to the broad synthetic utility of chiral quaterna- ter 1 is quite valuable to prepare a key intermediate 2 for
ry tetraalkylammonium salts in asymmetric phase-trans- asymmetric synthesis of aldose reductase inhibitor AS-
fer catalysis, chiral quaternary tetraalkylphosphonium 3201 (Ranirestat) as shown in Scheme 2.7
1,2
salts have not been regarded as reliable phase-transfer cat-
We employed a binaphthyl structure as a basic chiral unit
alysts due to the facile formation of the corresponding
and first prepared the C -symmetric chiral quaternary tet-
2
3
ylides (Wittig reagents) under basic conditions. Indeed,
raalkylphosphonium bromide of type (S)-3a from the axi-
ally chiral dibromide (S)-4a (Scheme 3).
catalytic asymmetric synthesis utilizing chiral quaternary
tetraalkylphosphonium salts as phase-transfer catalysts
remains poorly studied, and only a few special examples
The potential of the catalyst was evaluated in the asym-
metric phase-transfer amination of tert-butyl 1-oxo-2-in-
danecarboxylate (5) using di-tert-butyl azodicarboxylate
1
d,4
have been reported so far with limited success. In this
context, we were interested in the possibility of using cer-
tain chiral quaternary phosphonium salts in asymmetric
phase-transfer catalysis. Here we wish to report a first ex-
ample on this subject by the successful application to
(
1.2 equiv) in toluene under basic conditions, giving the
corresponding amination product 6 (Scheme 4).
O
HN
O
O
NHCO t-Bu
2
O
CO2t-Bu
O
Br
N
CO2t-Bu
XN
O
XN
N
*
CO2Et
N
O
1
2
O
F
AS-3201 (Ranirestat)
Scheme 2 Projected synthesis of Ranirestat
SYNTHESIS 2009, No. 13, pp 2289–2292
x
x.
x
x
.
2
0
0
9
Advanced online publication: 29.05.2009
DOI: 10.1055/s-0029-1216848; Art ID: Z04209SS
Georg Thieme Verlag Stuttgart · New York
©

2
290
R. He, K. Maruoka
PRACTICAL SYNTHETIC PROCEDURES
Ar
Ar
Quaternary Phosphonium Salts (S)-3a and (S)-3b; Phosphoni-
Br–
um Salt 3b; Typical Procedure
CH2Br
Bu2PH
8
A mixture of (S)-4b (0.490 g, 0.567 mmol, 1 equiv) and
+
Bu
Bu
9
P
dibutylphosphine (as a 0.12 M solution in Et O, 15 mL, 1.701
2
toluene
CH2Br
Ar
120 °C, 24 h
mmol, 3 equiv) in toluene (15 mL) was heated at 120 °C for 24 h
under argon, and then concentrated under vacuum. Purification of
the residue by column chromatography on silica gel (hexane–
EtOAc, 1:1, then CH Cl –MeOH, 50:1, 20:1, 10:1 as eluent) afford-
Ar
(
(
S)-4a Ar = Ph
S)-4b Ar = 3,5-(CF3)2C6H3
(S)-3a Ar = Ph
(S)-3b Ar = 3,5-(CF ) C H
2
2
27
3
2
6
3
ed (S)-3b as a white solid; yield: 0.527 g (99%); [a]D –26.78
c = 0.50, CHCl3).
(
Scheme 3 Synthesis of chiral quaternary phosphonium salts
IR (ATR): 2963, 2934, 2876, 2359, 2330, 1468, 1371, 1321, 1279,
–
1
1
175, 1134, 897, 847, 750, 710, 681 cm .
O
O
NHCO t-Bu
2
1
(
S)-3a,b (1–3 mol%)
H NMR (400 MHz, CDCl ): d = 8.07–7.99 (m, 10 H), 7.66 (t,
3
t-BuO2CN NCO2t-Bu
base, toluene
N
CO2t-Bu
J = 7.6 Hz, 2 H), 7.45–7.41 (m, 2 H), 7.12 (d, J = 8.8 Hz, 2 H), 4.17
(
J = 12.7 Hz, 2 H), 2.04 (q, J = 12.1 Hz, 2 H), 1.26–1.13 (m, 4 H),
CO2t-Bu
dd, J = 10.0, 15.6 Hz, 2 H), 3.25 (t, J = 16.0 Hz, 2 H), 2.52 (q,
*
CO t-Bu
2
5
6
0
.98–0.92 (m, 2 H), 0.74–0.70 (m, 8 H).
1
3
Scheme 4
C NMR (100 MHz, CDCl ): d = 141.4, 136.4, 136.4, 136.3, 136.3,
3
1
1
1
1
33.1, 133.0 (q, JC-F = 35 Hz), 133.0, 132.1–132.0 (m), 129.8 (br),
28.8, 128.6, 128.3, 126.5, 123.0 (q, JC-F = 274 Hz), 122.9, 122.9,
22.8, 122.8, 122.6–122.5 (m), 23.9 (d, JC-P = 5 Hz), 23.7 (d, JC-P
=
Scope and Limitations
0 Hz), 22.3 (d, JC-P = 48 Hz), 19.4 (dd, JC-P = 5, 41 Hz), 13.4 (d,
JC-P = 2 Hz).
After some preliminary experiments, an enantiomeric ex-
cess of more than 90% ee was attained by using 1 equiva-
3
1
P NMR (160 MHz, CDCl ): d = 51.2.
3
lent of b-keto ester and 1.2 equivalents of di-tert-butyl HRMS (ESI-TOF): m/z calcd for C46
azodicarboxylate in the presence of 3 mol% of catalyst
H F
12P+: 849.2514 ([M –
38
+
Br] ); found: 849.2506.
(
S)-3b and 1 equivalent of K HPO at –20 °C in toluene.
2 4
Phosphonium Salt (S)-3a
2
7
With the optimal reaction conditions at hand, we further Yield: 99%; [a]D –38.05 (c = 0.50, CHCl3).
studied the generality of the asymmetric amination of sev-
eral five-membered cyclic b-keto esters under the influ-
IR (ATR): 3055, 2959, 2928, 2872, 1493, 1464, 1449, 1400, 1248,
–1
1
229, 897, 746, 704, 658 cm .
ence of chiral quaternary tetraalkylphosphonium bromide
1
H NMR (400 MHz, CDCl ): d = 8.06 (s, 2 H), 8.00 (d, J = 8.4 Hz,
H), 7.60–7.54 (m, 10 H), 7.51–7.44 (m, 2 H), 7.36–7.32 (m, 2 H),
.12 (d, J = 8.4 Hz, 2 H), 4.30 (dd, J = 9.6, 15.6 Hz, 2 H), 3.00 (t,
3
(
S)-3b as shown in Scheme 1 and Table 1. Electronic ef-
2
7
fect on the aromatic moiety in tert-butyl indanecarboxy-
late 5 was found to be not so sensitive on the J = 16.0 Hz, 2 H), 2.37 (q, J = 13.3 Hz, 2 H), 1.55 (dq, J = 4.0, 13.7
enantioselectivity (entries 1–4). In general, use of K CO3 Hz, 2 H), 1.20–1.06 (m, 4 H), 0.96–0.84 (m, 2 H), 0.68 (t, J = 7.6
2
Hz, 6 H), 0.31–0.25 (m, 2 H).
lowered the enantioselectivity. Notably, optically active
amination product 2 (X = CO t-Bu) derived from b-keto
ester 1 (X = CO t-Bu) is a key intermediate for aldose re-
ductase inhibitor AS-3201 (entries 10 and 11). Asymmet-
ric amination of functionalized acyclic b-keto ester 10 and
six-membered cyclic b-diketone 11 appears feasible (en-
tries 12–14).
1
3
C NMR (100 MHz, CDCl ): d = 139.3, 139.3, 139.2, 136.0, 136.0,
2
3
1
33.3, 133.2, 131.5, 131.4, 131.0, 130.9, 129.9, 129.5, 128.5, 128.5,
2
7
128.2, 127.5, 127.5, 127.5, 126.6, 126.6, 123.5, 123.4, 23.8 (d,
JC-P = 17 Hz), 23.3 (d, JC-P = 4 Hz), 21.6 (d, JC-P = 48 Hz), 17.8 (d,
JC-P = 41 Hz), 13.2.
3
1
P NMR (160 MHz, CDCl ): d = 50.4.
3
+
+
HRMS (ESI-TOF): m/z calcd for C H P : 577.3019 ([M – Br] );
found: 577.3026.
4
2
42
In conclusion, we have succeeded in designing a new,
chiral quaternary tetraalkylphosphonium bromide as
phase-transfer catalyst to realize the asymmetric amina-
Asymmetric Amination of b-Keto Esters; Compound 6; Typical
tion of cyclic b-keto esters and b-diketones. To the best of Procedure
our knowledge, this is the first successful example em- A mixture of substrate 5 (17.4 mg, 0.075 mmol), (S)-3b (2.1 mg, 3
mol%) and K HPO (13.0 mg, 0.075 mmol) in toluene (1 mL) was
cooled to –20 °C, to which was added di-tert-butyl azodicarboxy-
late (20.7 mg, 0.09 mmol). The mixture was stirred vigorously at the
ploying chiral quaternary tetraalkylphosphonium bromide
as a reliable phase-transfer catalyst in asymmetric synthe-
sis.
2
4
same temperature for 14 h, quenched with aq sat. NH Cl (10 mL)
4
and extracted with Et O (3 × 10 mL). The combined organic layers
2
were dried (Na SO ) and concentrated. Purification of the residue
Infrared (IR) spectra were recorded on a Shimadzu FT-IR 8200A
2
4
1
13
31
by column chromatography on silica gel with hexane–EtOAc (5:1)
spectrometer. H, C and P NMR spectra were measured on a Jeol
JNM-FX400 NMR instrument. High-performance liquid chroma-
tography (HPLC) was performed on Shimadzu 10A instruments us-
ing a Daicel CHIRALPAK AD-H, or OD-H, 4.6 mm × 25 mm
column. High-resolution mass spectra (HRMS) were performed on
a Bruker micrOTOF focus–KR. Optical rotations were measured on
a Jasco DIP-1000 digital polarimeter. All simple chemicals were
purchased and used as received.
2
2
as eluent afforded 6 as a colorless oil; yield: 37.0 mg (99%); [a]D
+
91.88 (c = 0.91, CHCl ); 91% ee.
3
HPLC Analysis: Daicel Chiralpak AD-H, hexane–EtOH (9:1), flow
rate: 1.0 mL/min, l = 254 nm, t : 6.1 min (minor) and 8.4 min (ma-
R
jor).
+
HRMS (ESI-TOF): m/z calcd for C H N O + Na : 485.2258 ([M
2
4
34
2
7
+
+
Na] ); found: 485.2264.
Synthesis 2009, No. 13, 2289–2292 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Asymmetric Amination of b-Keto Esters
2291
Table 1 Asymmetric Amination of b-Keto Esters and b-Diketone with Chiral Phase-Transfer Catalyst (S)-3ba
Entry
1
Substrate
Base (equiv)
K HPO (1)
Conditions (°C, h)
–20, 14
Yield (%)b
99
ee (%)c
91
O
2
4
CO2t-Bu
O
5
MeO
2
3
K HPO (5)
–40, 70
–40, 5
97
99
90
77
2
4
CO2t-Bu
K CO (1)
2 3
7
O
4
K HPO (1)
–20, 22
99
89
2
4
CO2t-Bu
Cl
8
5
6
7
8
9
K HPO (1)
–20, 40
–20, 10
–40, 16
–20, 2
42
99
99
99
99
83
85
95
90
92
2
4
O
K HPO (5)
2
4
CO2t-Bu
K HPO (5)
2
4
K CO (1)
2
3
K CO (1)
–40, 2
2
3
9
O
CO2t-Bu
1
1
0
1
K HPO (1)
–20, 40
–20, 18
99
99
92
90
XN
2
4
K HPO (5)
2
4
O
1
(X = CO t-Bu)
2
O
CO2t-Bu
–20, 84d
–20, 84e
1
2
K HPO (5)
99
73
Ph
2
4
F
1
1
0
1
O
1
1
3
4
COC4F9
K HPO (5)
55
75
87
88
2
4
d
K HPO (5)
–20, 96
2
4
a
Unless otherwise specified, the reaction was carried out with 1.2 equiv of di-tert-butyl azodicarboxylate in the presence of 3 mol% of (S)-3b
and base in toluene under the given reaction conditions.
b
Isolated yield.
c
Enantiopurity of the products was determined by HPLC analysis using a chiral column (DAICEL Chiralcel OD-H or AD-H) with hexane–i-
PrOH or hexane–EtOH as solvent.
d
Use of 5 mol% of (S)-3b and 10 equiv of azodicarboxylate.
Use of 5 equiv of azodicarboxylate.
e
Acknowledgment
(2) For representative reviews on asymmetric phase-transfer
catalysis, see: (a) O’Donnell, M. J. In Catalytic Asymmetric
Synthesis; Ojima, I., Ed.; VCH: Weinheim, 1993, Chap. 8.
This work was partially supported by a Grand-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
(
b) Shioiri, T. In Handbook of Phase-Transfer Catalysis;
Sasson, Y.; Neumann, R., Eds.; Blackie Academic &
Professional: London, 1997, Chap. 14. (c) O’Donnell, M. J.
Phases-The Sachem Phase Transfer Catalysis Review 1998,
Issue 4, 5. (d) O’Donnell, M. J. Phases-The Sachem Phase
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Angew. Chem. Int. Ed. 1999, 38, 1583. (f) Shioiri, T.; Arai,
S. In Stimulating Concepts in Chemistry; Vogtle, F.;
Stoddart, J. F.; Shibasaki, M., Eds.; Wiley-VCH: Weinheim,
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Synthesis 2009, No. 13, 2289–2292 © Thieme Stuttgart · New York