Catalytic atropo -enantioselective preparation of axially chiral biaryl compounds

Article abstract of DOI:10.1246/cl.2009.246

The atriopo-enantioselective ring-opening of biaryl lactones with methanol was catalyzed by an optically active AgBF₄-phosphine complex to afford axially chiral biaryl compounds. The addition of triisobutylamine provided a dramatic rate acceler

Full text of DOI:10.1246/cl.2009.246

246
Chemistry Letters Vol.38, No.3 (2009)
Catalytic atropo-Enantioselective Preparation of Axially Chiral Biaryl Compounds
Tomoko Ashizawa and Tohru YamadaÃ
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522
(Received December 18, 2008; CL-081187; E-mail: yamada@chem.keio.ac.jp)
The atropo-enantioselective ring-opening of biaryl lactones
AgBF4 (20 mol%)
OMe
O
with methanol was catalyzed by an optically active AgBF4–
phosphine complex to afford axially chiral biaryl compounds.
The addition of triisobutylamine provided a dramatic rate accel-
eration in the reaction. Various types of axially chiral biaryl
compounds were obtained with high enantioselectivity.
(R)-BINAP (24 mol%)
THF, 0 °C
MeOH
O
O
OH
(25 equiv)
Dynamic Kinetic Resolution
R
1
R
2
Product
R = H
Additive
none
none
Time/h Yield/% ee/%
2a
24
163
5
quant.
NR
72
R = TBDMSO 2g
R = TBDMSO 2g
93
57
Et3N (1.0 equiv)
Axially chiral biaryl compounds are important building
blocks in various biologically active natural products1 and are
effective ligands for various enantioselective syntheses.2–4 In
view of their increasing importance, various synthetic methods
for axially chiral biaryl compounds have been reported to date.5
Bringmann et al. have developed an efficient synthetic method
for axially chiral biaryl compounds6 from biaryl lactones, which
are easily prepared by intramolecular aryl coupling. The biaryl
axis in these lactones is configurationally unstable and provides
atropo-enantiomers in equilibrium. Subsequent ring-opening
of these configurationally unstable biaryl lactones can be per-
formed stereoselectively, leading to axially chiral biaryl com-
pounds.
Various metal-assisted ring-opening reactions of biaryl lac-
tones have been reported6b and applied to the total synthesis of
numerous axially chiral natural products.6a,7 Considering the ef-
fectiveness of this lactone method, the catalytic atropo-enantio-
selective ring-opening of biaryl lactones is an ideal reaction to
prepare axially chiral biaryl compounds. Seebach et al. have
reported the first atropo-enantioselective ring-opening of biaryl
lactones catalyzed by (i-PrO)2Ti-TADDOLate, which can work
both as an optically active Lewis acid and as an O-nucleophile.8
Recently, the catalytic atropo-enantioselective reductive ring-
opening of various biaryl lactones using an optically active ꢀ-
ketoiminatocobalt(II) complex was disclosed by our group.9 In-
spired by these studies, we have explored the catalytic atropo-
enantioselective ring-opening of biaryl lactones with a simple
alcohol. We report here an optically active AgBF4–phosphine
complex10-catalyzed atropo-enantioselective ring-opening of
biaryl lactones with methanol, providing various axially chiral
biaryl compounds in high yields and high enantioselectivities.
We initially selected methanol as a simple nucleophile and
examined the catalytic atropo-enantioselective ring-opening of
biaryl lactone 1a in the presence of a variety of optically active
AgBF4–phosphine complexes. These studies indicated that the
AgBF4–(R)-BINAP complex is a desirable catalyst. Such condi-
tions delivered the corresponding biaryl compound 2a in quanti-
tative yield with 72% ee. With the catalytic atropo-enantioselec-
tive ring-opening of biaryl lactone 1a with methanol in hand,
the biaryl lactone 1g was subjected to atropo-enantioselective
ring-opening catalyzed by the AgBF4–(R)-BINAP complex,
but the corresponding biaryl compound 2g was not obtained.
The possibility is to assume that the biaryl lactone 1g can not
be effectively activated by the AgBF4–(R)-BINAP complex,
Scheme 1. Catalytic atropo-enantioselective ring-opening.
because a bulky siloxy group of the biaryl lactone 1g could lead
to steric interactions with the ligand. Interestingly, the addition
of triethylamine drove the reaction to afford the corresponding
biaryl compound 2g in high yield with moderate enantioselec-
tivity (Scheme 1).
Intrigued by this finding, we hypothesized that triethylamine
served as a base for the activation of methanol, and thus inves-
tigated the effect of other bases on the reaction (Table 1). Al-
though the addition of carbonate salts 3a and 3b provided high
yields, low enantioselectivity was observed (Entries 1 and 2).
When DMAP (3c) and N-methylpyrrolidine (3d) were em-
ployed, the corresponding biaryl compound 2g was obtained in
low yield and low enantioselectivity after a long reaction time
(Entries 3 and 4). Bulky amine bases were effective additives
for the reaction (Entries 5–11). The addition of the most bulky
Table 1. Additive effects on the catalytic atropo-enantioselec-
tive ring-opening of biaryl lactone 1ga
Entry
Base (1.0 equiv)
Time/h Yield/% ee/%b
1
Cs2CO3
K2CO3
DMAP
3a
3b
3c
13
5
42
81
92
27
0
28
35
2c
3
Me
N
4
5
6
3d
3e
3f
47
17
24
38
88
77
29
45
53
Et
N
EtN
2
EtN
7
8
9
3g
3h
3i
23
5
81
93
92
54
57
60
2
Et3N
23
EtN
2
10
11
3j
14
29
89
84
66
70
i-PrN
2
3k
N
3
aReaction conditions: 0.10 mmol of substrate 1g, 2.5 mmol of
MeOH, 0.020 mmol of AgBF4, and 0.024 mmol of (R)-BINAP.
b
All reactions were carried out in THF at 0 ꢀC. Determined by
HPLC analysis. K2CO3 (0.5 equiv) was added.
c
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.3 (2009)
247
Table 2. Catalytic atropo-enantioselective ring-opening of var-
ious axially biaryl lactonesa
OMe
OH
OH
2 equiv LiAlH4
M
*
O
Temp Time Yield ee
THF, 0 °C
OH
Entry
1c
Product
R = H 2a
/°C
/h
93
/% /%b
2a
4a
–20
92
84
97
79
82
70
64
84
81% ee
91% yield, 80% ee
OMe
2
R = Cl 2b
R = Me 2c
20
20
47
71
74
O
OH
Scheme 2. Reduction of the axially chiral biaryl compound 2a.
3d
4d
R
produce the axially chiral biaryl compound 2i in 96% yield with
81% ee (Entry 11).
R = MeO 2d
–20
To determine the absolute configuration of 2a corresponding
to the AgBF4–(R)-BINAP complex, the axially chiral biaryl
compound 2a with 81% ee was reduced by LiAlH4
(Scheme 2). The corresponding reduced product 4a was obtained
in 91% yield with 80% ee. The absolute configuration of the diol
4a was revealed to be the M form by comparing its optical rota-
tion sign and HPLC retention time with reported values (see
Supporting Information).11 Therefore, the axially chiral biaryl
methyl ester 2a with the M configuration was assumed to corre-
spond to the AgBF4–(R)-BINAP complex. The mechanistic in-
vestigation and the examination of various nucleophiles are cur-
rently underway.
OMe
5
O
2e –20
73
89
84
OH
6
2f 40–50
42
42
49
77
50
46
OMe
O
OH
7e
2f
40
OMe
O
OH
8d,e
9d,e
R = TBDMSO 2g
R = i-PrMe2SiO 2h
–20
–20
139
115
82
55
80
76
T.A. thanks JSPS for research fellowships. We thank Dr.
Tsuyoshi Mita, Harvard University, for helpful discussions.
10
OMe
2i –20–rt 187
2i –20
66
21
96
63
81
O
MeO
OH
References and Notes
1
11e
For a general review, see: G. Bringmann, C. Gunther, M. Ochse,
¨
O. Schupp, S. Tasler, in Progress in the Chemistry of Organic
Natural Products, ed. by W. Herz, H. Falk, G. W. Kirby, R. E.
Moore, Springer, Vienna, 2001, Vol. 82, pp. 1–249.
For reviews on enantioselective synthesis using BINOL, see: J. M.
Brunel, Chem. Rev. 2005, 105, 857, and references therein.
For examples of enantioselective synthesis using BINAP, see:
a) R. Noyori, H. Takaya, Acc. Chem. Res. 1990, 23, 345. b) R.
Noyori, Acta Chem. Scand. 1996, 50, 380.
aReaction conditions: 0.10 mmol of substrate, 2.5 mmol of
MeOH, 0.020 mmol of AgBF4, and 0.024 mmol of (R)-BINAP.
All reactions were carried out in THF. Determined by HPLC
b
analysis. c(S)-H8-BINAP was used instead of (R)-BINAP. d(R)-
H8-BINAP was used instead of (R)-BINAP. Triisobutylamine
(1.0 equiv) was added.
2
3
e
cyclic amine 3e gave the product 2g in high yield, but enantio-
selectivity remained low at 45% (Entry 5). The enantioselectiv-
ities were improved with bulky acyclic amine bases such as
isobutylamine derivatives 3i–3k (Entries 9–11). Specifically,
the addition of triisobutylamine 3k provided the product 2g in
84% yield and 70% ee (Entry 11).
4
For examples of enantioselective synthesis using phosphoric acid,
see: a) T. Akiyama, J. Itoh, K. Yokota, K. Fuchibe, Angew. Chem.,
Int. Ed. 2004, 43, 1566. b) D. Uraguchi, M. Terada, J. Am. Chem.
Soc. 2004, 126, 5356.
G. Bringmann, A. J. P. Mortimer, P. A. Keller, M. J. Gresser, J.
Garner, M. Breuning, Angew. Chem., Int. Ed. 2005, 44, 5384.
a) G. Bringmann, M. Breuning, S. Tasler, Synthesis 1999, 525.
b) G. Bringmann, M. Breuning, R.-M. Pfeifer, W. A. Schenk, K.
Kamikawa, M. Uemura, J. Organomet. Chem. 2002, 661, 31. c) G.
Bringmann, S. Tasler, R.-M. Pfeifer, M. Breuning, J. Organomet.
Chem. 2002, 661, 49.
5
6
Encouraged by these studies, the generality and scope of this
atropo-enantioselective reaction were investigated (Table 2).
From biaryl lactones 1a, 1b, and 1c, the corresponding biaryl
methyl esters were obtained in high yields with good-to-high
enantioselectivity (Entries 1–3). The biaryl lactone 1d with a
methoxy group also afforded the axially chiral biaryl compound
2d in high yield with 84% ee (Entry 4). Enantioenriched biphen-
yl compound 2e can also be produced from the corresponding
biaryl lactone 1e (Entry 5). The binaphthyl compound 2f was
obtained in high yield with moderate enantioselectivity by the
addition of triisobutylamine (Entry 7). In the reaction of the
biaryl lactones with bulky siloxy groups, it is crucial to activate
methanol with the triisobutylamine additive. In the presence of
triisobutylamine, biaryl lactones 1g and 1h afforded products
2g and 2h in good-to-high yields with 80% ee and 76% ee, re-
spectively (Entries 8 and 9). The reaction of the biaryl lactone
1i was also accelerated by the addition of triisobutylamine to
7
8
K. Ohmori, M. Tamiya, M. Kitamura, H. Kato, M. Oorui, K.
Suzuki, Angew. Chem., Int. Ed. 2005, 44, 3871.
D. Seebach, G. Jaeschke, K. Gottwald, K. Matsuda, R. Formisano,
D. A. Chaplin, M. Breuning, G. Bringmann, Tetrahedron 1997, 53,
7539.
9
T. Ashizawa, S. Tanaka, T. Yamada, Org. Lett. 2008, 10, 2521.
10 For reviews on enantioselective synthesis using chiral silver com-
plexes, see: a) A. Yanagisawa, H. Yamamoto, J. Synth. Org.
Chem. Jpn. 2005, 63, 888. b) M. Naodovic, H. Yamamoto, Chem.
Rev. 2008, 108, 3132.
11 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) February 7, 2009; doi:10.1246/cl.2009.246