Combinatorial approach for the design of new, simplified chiral phase-transfer catalysts with high catalytic performance for practical asymmetric synthesis of α-alkyl-α-amino acids

Article abstract of DOI:10.1016/j.tetlet.2008.01.029

A very efficient, chiral phase-transfer catalyst (S)-2Db was prepared by taking advantage of the combinatorial approach from the known, easily available (S)-1,1′-binaphthyl-2,2′-dicarboxylic acid. This catalyst exhibited the high catalytic performance (0.

Full text of DOI:10.1016/j.tetlet.2008.01.029

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Tetrahedron Letters 49 (2008) 2026–2030
Combinatorial approach for the design of new, simplified chiral
phase-transfer catalysts with high catalytic performance for
practical asymmetric synthesis of a-alkyl-a-amino acids
Masanori Kitamura, Yuichiro Arimura, Seiji Shirakawa, Keiji Maruoka *
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
Received 5 December 2007; revised 22 December 2007; accepted 8 January 2008
Available online 11 January 2008
Abstract
A very efficient, chiral phase-transfer catalyst (S)-2Db was prepared by taking advantage of the combinatorial approach from the
known, easily available (S)-1,10-binaphthyl-2,20-dicarboxylic acid. This catalyst exhibited the high catalytic performance (0.01–
0.1 mol %) in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester compared to the existing chiral phase-transfer
catalysts, thereby allowing to realize a general and useful procedure for highly practical enantioselective synthesis of structurally diverse
natural and unnatural a-alkyl-a-amino acids.
Ó 2008 Elsevier Ltd. All rights reserved.
Development of truly efficient methods in organic syn-
thesis, especially in an enantiomerically pure form, has
become of great importance. However, despite their
numerous studies only a few catalytic systems have been
reported in asymmetric phase-transfer chemistry with
limited general applicability.1 In this context, though we
recently designed new, chiral spiro-type (R,R)- or (S,S)-
3,4,5-trifluorophenyl-NAS bromide (S,S)-1 (Ar = 3,4,5-
F3–C6H2)2 for effecting asymmetric alkylation of a-amino
acid derivatives,3,4 the multi-step [5 steps for right-hand
(S)-3,5-dihydro-4H-dinaphth[2,1-c:1020-e]azepine from (S)-
binaphthol; 11 more steps from (S)-binaphthol] pre-
paration of such spiro-type catalyst 1 constitutes severe
drawback, and the simplification of catalyst 1 is crucially
important to overcome the intrinsic problem in the chiral
phase-transfer process chemistry.
binaphthyl part (S)-3, an arylboronic acid (ArB(OH)2),
and a secondary amine (R2NH) as described previously,5
the appropriate modification of ArB(OH)2 and R2NH
parts should give a series of newly designed catalysts.
Hence, we began to study the substituent effect of Ar and
R moieties in detail by using combinatorial chemistry, since
variation of the substituents Ar and R would allow the
facile generation of large libraries of structures.
Ar
Br
N
Ar
(
S,S)-1
Br
Ar
Ar
Br
CO2Pri
Ar–B(OH)2
R
R
To simplify the structure of the original catalyst (S,S)-1,
we chose a basic structure of type 2 as a simplified chiral
phase-transfer catalyst. Since the catalyst (S)-2 can be read-
ily prepared from three components, that is, a chiral
+
N
R
HN
CO2Pri
Br
R
(S)-3
(S)-2
*
The requisite catalyst (S)-2 can be easily prepared from
the known, readily available (S)-1,10-binaphthyl-2,20-dicarb-
Corresponding author. Tel./fax: +81 75 753 4041.
E-mail address: maruoka@kuchem.kyoto-u.ac.jp (K. Maruoka).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.029

M. Kitamura et al. / Tetrahedron Letters 49 (2008) 2026–2030
2027
oxylic acid (S)-46 in 6-step sequence as outlined in Scheme
Ar
1.7 Thus, (S)-dicarboxylic acid (S)-4 was transformed with
i-PrBr, catalytic Bu4NꢀHSO4, and KFꢀ2H2O to the corres-
ponding diisopropyl ester (S)-5 in 95% yield. Treatment of
(S)-5 with freshly prepared Mg(TMP)2 in THF and subse-
quent addition of Br2 gave rise to (S)-3,30-dibromo-1,10-
binaphthyl-2,20-dicarboxylic ester (S)-3 in 91% yield.
Suzuki–Miyaura cross coupling of (S)-3 with arylboronic
acid, ArB(OH)2 (Ar = Ph, 3,5-(CF3)2–C6H3, 3,4,5-F3–
C6H2) in the presence of catalytic Pd(OAc)2, PPh3, and
K2CO3 in DMF afforded (S)-3,30-diaryl-1,10-binaphthyl-
2,20-dicarboxylic ester (S)-6 in 57–68% yields. Reduction
of (S)-6 with LiAlH4 in THF and subsequent treatment
of the resulting crude alcohol (S)-7 with PBr3 in THF
furnished (S)-dibromide (S)-8 in moderate to high yields.
Reaction of (S)-8 with dialkylamine and K2CO3 in aceto-
nitrile led to the formation of catalysts (S)-2Aa–Di in high
yields.
To examine the substituent effect of catalyst (S)-2 on
the variation of the substituents Ar and R, we first pre-
pared a library of quaternary ammonium salts (S)-2Aa–
Di by combining use of 4 aryl substituents with 9 different
dialkylamines. The chiral amplitude of these simplified
phase-transfer catalysts (S)-2Aa–Di was screened efficiently
by using an in situ-generated method from 3,30-diarylated
(S)-binaphthyl dibromide (S)-8 (Ar = H, Ph, 3,5-(CF3)2–
C6H3, 3,4,5-F3–C6H2) in the asymmetric alkylation of
N-(diphenylmethylene)glycine tert-butyl ester 9 (Scheme
2). Thus, reaction of 9 with benzyl bromide (1.2 equiv)
and 50% aqueous KOH in toluene was carried out in the
R2NH (6 mol%)
K2CO3 (4.5 mol%)
CH2Br
dioxane reflux
CH2Br
Ar
(S)-8 (3 mol%)
in situ-generated
)-2Aa~Di (3 mol%)
O
O
Ph
Ph
Ph
Ph
(S
N
N
+
PhCH2Br
OBut
CH2Ph
R)-10
OBut
50% aq. KOH
H
toluene, 0 ºC
9
(
Scheme 2.
presence of 3 mol % of in situ-generated catalysts
(S)-2Aa–Di under argon atmosphere at 0 °C to furnish
benzylation product (R)-10 and the results are shown in
Table 1, which also includes the selected results by using
isolated, optically pure catalyst (S)-2 for comparison.
With bis(3,4,5-trifluorophenyl)-substituted catalyst (S)-
2D, we examined the substituent effect of R by changing
the number of straight alkyl chains as shown in Figure 1.
In the asymmetric benzylation of N-(diphenylmethyl-
ene)glycine tert-butyl ester 9, dimethylammonium salt,
(S)-2Da exhibited very low enantioselectivity, and use of
(CH3(CH2)nꢁ1)2 NH (n P 4) gave uniformly high asym-
metric induction.
With this information at hand, we further screened the
substituent effect of various Ar groups by preparing (S)-
2b catalysts derived from dibutylamine and various 3,30-
diarylated (S)-binaphthyl dibromide (S)-8 as shown in
Figure 2. Among mono-substituted phenyl derivatives, p-
substituted phenyl derivatives generally gave higher enantio-
selectivity than the corresponding m-substituted phenyl
analogues in the asymmetric benzylation of 9. In particu-
lar, p-(trifluoromethyl)phenyl and p-nitrophenyl deriva-
tives exhibited high enantioselectivity (94% ee). These
catalysts are more selective than various disubstituted
X
Ar
CO2R
CO2R
CO2Pri
CH2Y
b
(
S
)-2Aa
)-2Di
d
f
(
S
CO2Pri
CH2Y
Ar
X
(
(
S
S
)-4 (R = H)
)-5 (R = Pri)
(
(
S
)-3 (X = Br)
)-6 (X = Ar)
a
(
(
S
S
)-7 (Y = OH)
)-8 (Y = Br)
c
e
S
Table 1
F
Ar
CF3
Screening of in situ-generated catalyst (S)-2Aa–Di in the enantioselective
phase-transfer benzylation of glycine derivative 9a
Ph
H
F
(A)
(B)
F
(C)
(D)
CF3
Bu2NH (b)
Amine
Ar = H
Ar = Ph
Ar = 3,5-(CF3)2–
Ar = 3,4,5-F3–
(R2NH)
(A)
(B)
C6H3 (C)
C6H2 (D)
R2NH
Me2NH (a)
(C10H21)2NH (c)
(a)
(b)
12% ee
26% ee
43% ee
(60% ee)
58% ee
1% ee
7% ee
–27% ee
(–13% ee)
–17% ee
93% ee
(91% ee)
96% ee
(93% ee)
44% ee
31% ee
41% ee
78% ee
97% ee
(99% ee)
97% ee
(99% ee)
7% ee
43% ee
20% ee
81% ee
NH
(f)
(e)
NH
i
-Bu2NH (d)
(c)
O
O
O
O
NH
(d)
(e)
(f)
–9% ee
–7% ee
–23% ee
–19% ee
22% ee
5% ee
33% ee
26% ee
(28% ee)
3% ee
NH
N
(g)
H
(h)
(i)
(g)
Scheme 1. Reagents and conditions: (a) i-PrBr (10 equiv), BuNꢀHSO4
(20 mol %), KFꢀ2H2O (10 equiv), THF, reflux (95%); (b) (1) Mg(TMP)2
(4 equiv), THF, rt, (2) Br2 (8 equiv), ꢁ78 °C to rt (91%); (c) ArB(OH)2
(2.4 equiv), Pd(OAc)2 (5 mol %), PPh3 (15 mol %), K2CO3 (3 equiv),
DMF, 90 °C (57–68%); (d) LiAlH4 (3 equiv), THF, 0 °C to rt (59–82%);
(e) PBr3 (1 equiv), THF, 0 °C (65–87%); (f) R2NH (2 equiv), K2CO3
(1.5 equiv), CH3CN, reflux (34–88%).
(h)
(i)
22% ee
15% ee
2% ee
75% ee
6% ee
83% ee
(87% ee)
41% ee
a
The enantioselectivity in parentheses is obtained by using isolated,
optically pure (S)-2.

2028
M. Kitamura et al. / Tetrahedron Letters 49 (2008) 2026–2030
100
80
transfer catalysts. Indeed, asymmetric reaction of 9 with
benzyl bromide (1.2 equiv) and 50% aqueous KOH in
toluene was effected in the presence of only 0.01–0.1 mol %
of chiral catalyst (S)-2Db under argon atmosphere at
0 °C for 2–9 h to furnish benzylation product 10 almost
quantitatively with excellent enantioselectivity (98–99%
ee) (entries 1–3). However, use of 0.005 mol % of (S)-2Db
resulted in lowering both the chemical yield and the enantio-
selectivity (entry 4). A similar tendency is also observed in
the case of catalyst (S)-2Dc (entries 5 and 6).
Other selected examples are listed in Table 2. Several
characteristic features of the present alkylations follow:
(1) In contrast to the existing chiral phase-transfer cata-
lysts, the chiral phase-transfer catalysts (S)-2Cb, (S)-2Cc,
(S)-2Db or (S)-2Dc exhibited the high catalytic perfor-
mance (0.05–0.1 mol %), demonstrating the remarkable
efficiency and the practicability of the present approach
for the enantioselective synthesis of a-alkyl-a-amino acids.
(2) By using CsOHꢀH2O in place of 50% KOH, asymmetric
alkylation of 9 with simple alkyl halide such as ethyl iodide
proceeded smoothly at ꢁ20 °C to furnish the correspond-
ing a-alkyl-a-amino acids in good yield with high enantio-
selectivity (entry 21 vs 20).
A typical experimental procedure of catalytic enantio-
selective benzylation of N-(diphenylmethylene)glycine tert-
butyl ester (9) is as follows (Table 2, entry 2): To a mixture
of glycine derivative (9) (88.6 mg, 0.3 mmol) and chiral
catalyst (S)-2Db (0.11 mg, 0.00015 mmol) in toluene
(1.0 mL)–50% KOH aqueous solution (1.0 mL) was added
benzyl bromide (43 lL, 0.36 mmol) dropwise at 0 °C. The
reaction mixture was stirred vigorously at the same temper-
ature for 2 h. The mixture was then poured into water
60
3,4,5-F3-C6H2
Br
(CH2)n-1CH3
40
20
0
N
(CH2)n-1CH3
3,4,5-F3-C6H2
(
S
)-2D
6
8
10
n = 1
2
3
4
[CH3(CH2)n-1]2NH
Fig. 1. Effect of the number of straight alkyl chain in (S)-2D on the
enantioselectivity in the asymmetric benzylation of 9.
phenyl derivatives. Among a variety of aryl substituents,
catalyst (S)-2Db possessing 3,4,5-trifluorophenyl substi-
tuents was found to give the best result in terms of enantio-
selectivity (99% ee).
The chemical behavior of the simplified phase-transfer
catalysts (S)-2Cb, (S)-2Cc, (S)-2Db, and (S)-2Dc was
examined by carrying out asymmetric alkylation of N-
(diphenylmethylene)glycine tert-butyl ester 9, and quite
surprisingly these types of catalysts are found to be by
far the most active catalysts among existing chiral phase-
3,4,5-F3-C6H2
C6F5
Ph
OCH3
(B)
(60% ee)
(G)
(75% ee)
(H)
(81% ee)
(E)
(71% ee)
(F)
(75% ee)
F
Cl
CN
OCF3
CF3
NO2
(I)
(M)
(N)
(94% ee)
NO2
(J)
(K)
(89% ee)
(L)
(82% ee)
(83% ee)
(90% ee)
(94% ee)
F
CF3
Cl
OCH3
CN
(O)
(Q)
(77% ee)
-Bu
(S)
(T)
(P)
(R)
(81% ee)
Ph
(75% ee)
(87% ee)
F
(89% ee)
CF3
(75% ee)
t
Cl
Cl
F
F
F
Ph
t-Bu
CF3
(W)
(67% ee)
(U)
(86% ee)
(V)
(91% ee)
F
(X)
(Y)
(C)
(89% ee)
(81% ee)
(91% ee)
Cl
Cl
Cl
F
F
(Z)
(D)
(99% ee)
(93% ee)
Fig. 2. Effect of aryl substituents of (S)-2b (R = Bu) on the enantioselectivity in the asymmetric benzylation of 9.

M. Kitamura et al. / Tetrahedron Letters 49 (2008) 2026–2030
2029
Table 2
Blackie Academic & Professional: London, 1997; Chapter 14; (b)
O’Donnell, M. J. In Catalytic Asymmetric Syntheses; Ojima, I., Ed.,
2nd ed.; WILEY–VCH: New York, 2000; Chapter 10; (c) Shioiri, T.;
Arai, S. In Stimulating Concepts in Chemistry; Vogtle, F., Stoddart, J.
F., Shibasaki, M., Eds.; WILEY-VCH: Weinheim, 2000; p 123; (d)
O’Donnell, M. J. Aldrichim. Acta 2001, 34, 3; (e) Maruoka, K.; Ooi, T.
Chem. Rev. 2003, 103, 3013; (f) O’Donnell, M. J. Acc. Chem. Res. 2004,
37, 506; (g) Lygo, B.; Andrews, B. I. Acc. Chem. Res. 2004, 37, 518; (h)
Maruoka, K.; Ooi, T.; Kano, T. Chem. Commun. 2007, 1487; (i) Ooi,
T.; Maruoka, K. Angew. Chem., Int. Ed. 2007, 46, 4222; (j) Ooi, T.;
Maruoka, K. Aldrichim. Acta 2007, 40, 77; (k) Hashimoto, T.;
Maruoka, K. Chem. Rev. 2007, 107, 5656.
Catalytic enantioselective phase-transfer alkylation of glycine derivative 9
catalyzed by (S)-2Cb, (S)-2Cc, (S)-2Db, and (S)-2Dca
Entry Catalyst
(mol %)
R–X
Cond. Yieldb % eec
(°C, h) (%)
(config)d
1
2
3
4
5
6
7
8
9
(S)-2Db (0.1)
PhCH2Br
0, 2
99
98
92
51
94
79
89
87
9
99 (R)
99 (R)
98 (R)
57 (R)
99 (R)
98 (R)
91 (R)
91 (R)
90 (R)
93 (R)
77 (R)
98 (R)
82 (R)
97 (R)
83 (R)
91 (R)
98 (R)
88 (R)
88 (R)
91 (R)
99 (R)
(S)-2Db (0.05)
(S)-2Db (0.01)
(S)-2Db (0.005)
(S)-2Dc (0.05)
(S)-2Dc (0.01)
(S)-2Cb (0.1)
(S)-2Cb (0.05)
(S)-2Cb (0.01)
(S)-2Cc (0.05)
(S)-2Cc (0.01)
(S)-2Db (0.05)
(S)-2Db (0.01)
(S)-2Dc (0.05)
(S)-2Cb (0.01)
(S)-2Cc (0.05)
(S)-2Db (0.05)
(S)-2Db (0.01)
(S)-2Cb (0.05)
(S)-2Db (0.1)
(S)-2Db (0.1)
0, 2
0, 9
0, 48
0, 4
0, 24
0, 4
2. Ooi, T.; Kameda, M.; Maruoka, K. J. Am. Chem. Soc. 2003, 125,
5139.
0, 5
0, 48
0, 48
0, 48
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10
11
12
13
14
15
16
17
18
19
20
21
a
85
51
87
62
99
60
59
88
28
80
12
67
CH2@CHCH2Br 0, 3
0, 48
0, 5
0, 48
0, 48
0, 4
0, 48
0, 46
0, 72
–20, 1
HC„CCH2Br
CH3CH2Ie
CH3CH2Ie,f
Unless otherwise specified, the reaction was carried out with 1.2 equiv
of RX in the presence of cat. (S)-2 in 50% aqueous KOH/toluene (volume
´
´
5629; (l) Belokon, Y. N.; Bespalova, N. B.; Churkina, T. D.; Cısarova,
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Kocovsky, P.; Kochetkov, K. A.; Larionov, O. V.; Lyssenko, K. A.;
ratio = 1:1) under the given reaction conditions.
b
Isolated yield.
c
Enantiopurity of (R)-10 was determined by HPLC analysis of the
´
North, M.; Polasek, M.; Peregudov, A. S.; Prisyazhnyuk, V. V.;
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hexane–isopropanol as solvent.
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Absolute configuration was determined by comparison of the HPLC
retention time with the authentic sample independently synthesized by the
reported procedure.2
e
Use of 5 equiv of alkyl halide.
Use of CsOHꢀH2O as base.
f
and extracted with ether. The organic extracts were washed
with brine and dried over Na2SO4. Evaporation of sol-
vents and purification of the residual oil by column
chromatography on silica gel (ether/hexane = 1:10 as
eluant) gave the benzylation product (R)-10 (113 mg,
0.294 mmol, 98% yield) as a colorless oil. The enantiomeric
excess was determined by chiral HPLC analysis (DAICEL
CHIRALCEL OD, hexane/isopropanol = 100:1, flow
rate = 0.5 mL/min, retention time; 14.8 min (R) and
28.2 min (S)).
´
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Acknowledgments
This work was partially supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. M.K. is
grateful to the Japan Society for the Promotion of Science
for Young Scientists for a Research Fellowship.
References and notes
1. Recent representative reviews on asymmetric PTC: (a) Shioiri, T. In
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2030
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