Retention of regiochemistry and chirality in the ruthenium catalyzed allylic alkylation of disubstituted allylic esters

Article abstract of DOI:10.1021/jo2007169

The regiospecific nucleophilic substitution during the ruthenium catalyzed allylic alkylation of 1,3-unsymmetrical disubstituted allylic esters was demonstrated. The nucleophile was selectively introduced at the position originally substituted with leavin

Full text of DOI:10.1021/jo2007169

NOTE
pubs.acs.org/joc
Retention of Regiochemistry and Chirality in the Ruthenium Catalyzed
Allylic Alkylation of Disubstituted Allylic Esters
Motoi Kawatsura,* Michinobu Sato, Hiroaki Tsuji, Fumio Ata, and Toshiyuki Itoh*
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, Koyama, Tottori 680-8552 Japan
S Supporting Information
b
ABSTRACT: The regiospecific nucleophilic substitution during the
ruthenium catalyzed allylic alkylation of 1,3-unsymmetrical disub-
stituted allylic esters was demonstrated. The nucleophile was selec-
tively introduced at the position originally substituted with leaving
group in the 2-DPPBA or ip-pybox ligated [RuCl2(p-cymene)]2
catalyzed allylic alkylation of 1,3-unsymmetrical disubstituted allylic
esters. The chirality of the optically active allylic esters was also
transferred to the alkylated products.
he transition metal-catalyzed allylic alkylation of allylic
substrates is one of the most well-studied carbonꢀcarbon
conversion) within 12 h. As shown in Table 1, the alkylation of 1a
in the presence of the ruthenium catalyst coordinated with PPh3,
which is our reported catalyst system,12 took place and provided
3 in 65% yield with a 93% regioselectivity (Table 1, entry 1). The
yield increased by changing the substrate from the allylic acetate
1a to the allylic carbonate 1b, and 2-(diphenylphophino)benzoic
acid (2-DPPBA)5f gave a better result than PPh3, although the
regioselectivity was slightly decreased (entries 1ꢀ3). We next
examined the reaction of 2, which is a regioisomer of 1a. When
PPh3 was used as the ligand, the reaction gave a complex mixture
(entry 4), but the expected regiospecific allylic alkylation reaction
proceeded using the 2-DPPBA ligated ruthenium catalyst and
afforded the corresponding alkylated product 4 in excellent yield
with over 89% regioselectivity (entries 5 and 6). These results
clearly indicated that the nucleophile was selectively introduced
at the position originally carried by the leaving group in the
[RuCl2(p-cymene)]2/2-DPPBA-catalyzed allylic alkylation of
the 1,3-unsymmetrical disubstituted allylic esters.
We next switched allylic substrates to other types of unsym-
metrical disubstituted allylic esters 5 and 6 and investigated the
alkylation reaction using the ruthenium catalyst and found that it
was regiospecific. As shown in Table 2, the reaction of 5 smoothly
proceeded by [RuCl2(p-cymene)]2 with a ligand, and a better
yield and improved regioselectivity were again obtained when the
ligand was changed from PPh3 to 2-DPPBA (entries 1 and 2).
Especially, the reaction of 5 using the 2-DPPBA ligated ruthe-
nium catalyst at 40 °C in THF exhibited a perfect regioselectivity
and afforded 7 in 88% yield (entry 3). As expected, the reaction of
6a, which is a regioisomer of 5, produced a product 8 as a single
regioisomer, but the yield was low (entries 4 and 5). Again, we
examined several types of ligands, such as monophosphines,
T
bond formation reactions,1 and the palladium-catalyzed allylic
alkylation of allylic esters with malonate anions is a representative
reaction system.2 However, within recent years, several alternative
transition metal catalysts containing metals such as iridium,3
rhodium,4 ruthenium,5 tungsten,6 iron,7 and molybdenum8 have
also been reported. The reaction of two types of 1,3-unsymme-
trical disubstituted allylic substrates potentially forms two types
of regioisomers, and the regioselectivity depends on the type of
the transition metal catalyst employed in the reaction. It has been
observed that the reaction of either of the 1,3-unsymmetrical
regioisomeric allylic substrates gives the same regioisomer in the
transition metal-catalyzed allylic alkylations because the reaction
generally proceeds via a π-allyl metal intermediate and the
regiochemistry of the starting allylic substrate is lost during the
reaction. However, several examples of an unusual regiochemical
outcome, which is the selective substitution at the position
originally substituted by the leaving group, were observed in
the reactions when using the palladium,9 rhodium,4a,10 or iron11
catalysts. We also observed the retention of the regiochemistry of
the allylic esters during the [RuCl2(p-cymene)]2/PPh3 catalyzed
allylic alkylation, but the allylic substrates were limited to the
monosubstituted allylic acetates.12,13 We now report the regios-
pecific nucleophilic substitution in the ruthenium-catalyzed
allylic alkylation of 1,3-unsymmetrical disubstituted allylic esters.
We initially examined the allylic alkylation of the 1,3-unsym-
metrical disubstituted allylic acetates 1 and 2 by [RuCl2(p-
cymene)]2 with a ligand. Typically, the reaction was carried
out as follows: 5 mol % of [RuCl2(p-cymene)]2, 10 mol % of
phosphine ligand, and the allylic acetate 1 or 2 were allowed to
react with the dimethyl methylmalonate anion, which was
generated in situ from dimethyl methylmalonate and NaHMDS,
in toluene. Substrates 1 and 2 were usually consumed (100%
Received: April 7, 2011
Published: May 24, 2011
r
2011 American Chemical Society
5485
dx.doi.org/10.1021/jo2007169 J. Org. Chem. 2011, 76, 5485–5488
|

The Journal of Organic Chemistry
NOTE
Table 1. Ruthenium-Catalyzed Allylic Alkylation of 1,3-Di-
substituted Allylic Esters 1 and 2a
Table 2. Ruthenium-Catalyzed Allylic Alkylation of Disub-
stituted Allylic Esters 5 and 6a
entry
substrate
L
temp. (°C)
yield (%)
3:4b
1
2
3
4
5
6
1a
1a
1b
2
PPh3
100
100
100
60
65
87
93:7
entry
substrate
L
temp. (°C)
yield (%)
7:8b
2-DPPBA
2-DPPBA
PPh3
77:23
80:20
ND
89
1
2
3c
4
5
6
7
8
5
PPh3
60
60
66
86
88
39
45
70
45
81
96:4
97:3
<10
96
5
2-DPPBA
2-DPPBA
PPh3
2
2-DPPBA
60
11:89
5
40
>98:2
2
2-DPPBA
40
99
9:91
6a
6a
6b
6b
6b
100
100
100
100
100
2:>98
a Reaction conditions: allylic esters (1 or 2) (0.28 mmol), MeCH2-
(CO2Me)2 (0.42 mmol), NaHMDS (0.4 mmol, 1.0 M in THF), 5 mol %
of [RuCl2(p-cymene)]2, and 10 mol % L in toluene (0.4 mL). b Ratio was
determined by 1H NMR of the crude materials.
2-DPPBA
PPh3
2:>98
8:92
2-DPPBA
2:>98
ip-pybox
2:>98
a Reaction conditions: allylic esters (5 or 6) (0.28 mmol), MeCH2-
(CO2Me)2 (0.42 mmol), NaHMDS (0.4 mmol, 1.0 M in THF), 5 mol %
of [RuCl2(p-cymene)]2, and 10 mol % L in toluene (0.4 mL). b Ratio was
determined by 1H NMR of the crude materials. c THF was used as the
solvent.
bisphophines, bisoxazolines, bisimines, and pybox type ligands
etc., for the allylic alkylation of 6b, then determined that the
reaction of the allylic carbonate 6b by [RuCl2(p-cymene)]2 with
ip-pybox gave 8 in 81% yield with complete regioselectivity
(entry 8). We thus confirmed that the regiospecific nucleophilic
substitution had also occurred in the reactions of the regioiso-
meric allylic esters 5 and 6.
Although the details of the reaction mechanism involved in the
retention of the regiochemistry in the 2-DPPBA ligated
[RuCl2(p-cymene)]2-catalyzed allylic alkyaltion are still not
clear, our present results may also support the fact that the
reaction proceeds with double inversion mechanism through the
σ-allyl or σ-enyl complexes like the rhodium-catalyzed reaction.4a,13
Further investigations about the mechanistic details and the
reactions of other types of substrates, such as (Z)-substrates,
will be the subject of a future study.
In conclusion, we examined the retention of the regiochem-
istry and chirality during the ruthenium-catalyzed allylic alkyla-
tion of the 1,3-unsymmetrical disubstituted allylic esters. The
regioselective introduction of a nucleophile at the position orig-
inally substituted with leaving group of the allylic esters and
chirality transfer occurred using the 2-DPPBA or ip-pybox ligated
[RuCl2(p-cymene)]2 catalysts.
We also examined the reaction of the enantiomerically en-
riched allylic esters (Scheme 1).5c,14 The reaction of the en-
antiomerically enriched monosubstituted allylic acetate (S)-9
(95% ee) using our previously reported catalyst12 ([RuCl2(p-
cymene)]2/2PPh3) gave a branch-type product (S)-10 with a
high regioselectivity (96% rs), but the enantiomeric excess of (S)-
10 decreased to 84% ee (Scheme 1, eq 1). Changing the ligand
from PPh3 to 2-DPPBA effectively retained the chirality of the
substrate, and we succeeded in obtaining (S)-10 with a 94% ee.
We further attempted the reactions of the enantiomerically
enriched 1,3-disubstituted allylic substrates, such as (S)-1b and
(S)-2, by RuCl2(p-cymene)/2-DPPBA catalyst (eq 2 and 3). In
both reactions, we observed an almost perfect chirality transfer
from the allylic substrates (S)-1b (99% ee) and (S)-2 (87% ee) to
the alkylated products (S)-(E)-3 (99% ee) and (S)-(E)-4 (86%
ee), respectively. Furthermore, we confirmed that the reaction of
the enantiomerically enriched (R)-5 (95% ee) gives a 95% ee of
(R)-7, which has a enantiomerically enriched quaternary carbon
center, in 83% yield with a perfect regioselectivity (>98% rs) at
room temperature (eq 4). It is obvious that in all the reactions,
the chirality of the allylic substrates was transferred to the
products in a regiospecific manner, and there are no epimeriza-
tion steps. Generally, the π-allylruthenium,5e,14,15 σ-allylruthe-
nium, or σ-enylruthenium14a complexes are possible reaction
intermediates in the ruthenium-catalyzed allylic alkylation.
’ EXPERIMENTAL SECTION
General and Materials. All manipulations were carried out under
a nitrogen atmosphere. Nitrogen gas was dried by passage through P2O5.
NMR spectra were recorded on a 500 MHz (for 1H) and 125 MHz (for
13C) instrument. Chemical shifts are reported in δ ppm referenced to an
internal SiMe4 standard for 1H NMR. Residual chloroform (δ 77.0 for
13C) was used as internal reference for 13C NMR. [RuCl2(p-cymene)]2,
2-DPPBA, ip-pybox and other reagents, including solvents, were pur-
chased from common commercial sources and used without further
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dx.doi.org/10.1021/jo2007169 |J. Org. Chem. 2011, 76, 5485–5488

The Journal of Organic Chemistry
NOTE
Scheme 1. Ruthenium-Catalyzed Allylic Alkylation of Chiral
Allylic Esters
obtained by the methylation of dimethyl 2-((S,E)-4-phenylbut-3-en-2-
yl)malonate21 (99% ee) with NaH, MeI in THF.
Dimethyl 2-Methyl-2-((E)-1-phenylbut-2-enyl)malonate
420. Yield: 99% (91:9 ratio) (Table 1, entry 6). Colorless oil. 1H
NMR (500 MHz, CDCl3): δ 1.42 (s, 3H), 1.66 (d, J = 6.9 Hz, 3H),
3.60 (s, 3H), 3.70 (s, 3H), 4.11 (d, J = 9.1 Hz, 1H), 5.50 (dq, J = 6.9,
15.9 Hz, 1H), 5.93 (dd, J = 9.1, 15.9 Hz, 1H), 7.19ꢀ7.28 (m, 5H). 13C
NMR (125 MHz, CDCl3): δ 18.1, 18.2, 52.3, 52.3, 53.6, 59.1, 126.9,
128.1, 128.8, 129.2, 129.4, 139.8, 171.4, 171.6.
þ
(S)-(E)-4: HRMS (ESI): m/z: calcd for C16H21O4 [M þ H]þ
277.1440, found 277.1429. IR (neat) 2952, 1732, 1452, 1434, 1242,
1111, 971 cmꢀ1. [R]D25 5.6 (c 0.5, CHCl3) (86% ee). Enantiomer ratio
was determined by HPLC using a Daicel CHIRALCEL OJ-H (hexane/
2-propanol = 99/1, flow: 0.4 mL/min, 254 nm, 35 °C, tR 18.7 min
(minor); tR 21.5 min (major)). The absolute configuration was deter-
mined to be S by comparison with the sign of optical rotation of (S)-4
24
([R]D þ8.6 (c 2.1, CHCl3) (99% ee)), which was obtained by the
methylation of dimethyl 2-((S)-1-phenylbut-2-enyl)malonate21 (89%
ee) with NaH, MeI in THF.
Dimethyl 2-Methyl-2-(2-phenylbut-3-en-2-yl)malonate 7.
1
Yield: 88% (>98:2 ratio) (Table 2, entry 3). Colorless oil. H NMR
(500 MHz, CDCl3): δ 1.48 (s, 3H), 1.71 (s, 3H), 3.59 (s, 3H), 3.61 (s,
3H), 5.02 (dd, J = 1.1, 17.8 Hz, 1H), 5.21 (dd, J = 1.1, 10.8 Hz, 1H), 6.88
(dd, J = 10.8, 17.8 Hz, 1H), 7.17ꢀ7.21 (m, 1H), 7.23ꢀ7.27 (m, 2H),
7.38ꢀ7.41 (m, 2H). 13C NMR (125 MHz, CDCl3): δ 19.5, 22.9, 49.0,
52.0, 52.0, 61.3, 114.3, 126.5, 127.3, 128.6, 143.8, 144.1, 171.6, 171.6. IR
(neat) 2999, 2952, 1732, 1433, 1259, 1111, 921, 755, 703 cmꢀ1
.
(R)-7: HRMS (ESI): m/z: calcd for C16H21O4þ [MþH]þ 277.1440,
22
found 277.1431. [R]D ꢀ103 (c 0.5, CHCl3) (95% ee). Enantiomer
ratio was determined by HPLC using a Daicel CHIRALCEL OJ-H
(hexane/2-propanol = 199/1, flow: 1.0 mL/min, 215 nm, 35 °C, tR 18.7
min (minor); tR 21.5 min (major)). The absolute configuration was
determined to be R by comparison with the sign of optical rotation of
24
(R)-7 ([R]D ꢀ16.2 (c 0.6, CHCl3) (66% ee)), which was obtained by
the methylation of dimethyl 2-((S)-2-phenylbut-3-en-2-yl)malonate22
(65% ee) with NaH, MeI in THF.
purification. Allylic esters (1aꢀb,16 2,17 5,18 and 6aꢀb18) and optically
active allylic esters [(S)-1b,16 (S)-2, (R)-5, and (S)-919] were prepared
according to the literature.
Alkylated Product 823. Yield: 81% (2:>98 ratio) (Table 2, entry
8). Colorless oil. 1H NMR (500 MHz, CDCl3): δ 1.39 (s, 3H), 1.97 (d,
J = 1.8 Hz, 3H) 2.72 (d, J = 7.5 Hz, 2H), 3.65 (s, 6H), 5.53 (dt, J = 1.8, 7.5
Hz, 1H), 7.13ꢀ7.17 (m, 1H), 7.20ꢀ7.26 (m, 4H). 13C NMR (125 MHz,
CDCl3): δ 16.1, 20.0, 34.8, 52.5, 53.9, 121.7, 125.8, 126.9, 128.2, 138.7,
143.7, 172.5.
General Procedure for the Ruthenium-Catalyzed Allylic
Alkylation of 1, 2, 5, and 6. A typical procedure is given for the
reaction of 1b (Table 1, entry 3). To a solution of [RuCl2(p-cymene)]2
(8.6 mg, 0.014 mmol) and 2-DPPBA (8.6 mg, 0.028 mmol) in anhydrous
toluene (0.4 mL) were added 1b (58 mg, 0.28 mmol) and dimethyl
methylmalonate (61 mg, 0.42 mmol). NaHMDS (0.4 mmol, 0.4 mL of
1.0 M in THF) was slowly added at 0 °C, and the reaction mixture stirred
at 100 °C for 12 h. The reaction was quenched with water and extracted
with ethyl acetate. The combined organic layers dried over MgSO4 and
evaporated. The ratio of 3 to 4 was determined to be 80:20 by 1H NMR of
the crude materials. The residue was chromatographed on silica gel
(hexane/ethyl acetate = 99/1) to give 69 mg (89%) of 3 and 4.
Dimethyl 2-Methyl-2-((E)-4-phenylbut-3-en-2-yl)malonate
(320). Yield: 89% (Table 1, entry 3). Colorless oil. 1H NMR (500 MHz,
CDCl3): δ 1.15 (d, J = 5.6 Hz, 3H), 1.39, (s, 3H), 3.15 (dq, J = 5.6,
8.6 Hz, 1H), 3.70 (s, 3H), 3.73 (s, 3H), 6.14 (dd, J = 8.6, 16.1 Hz, 1H),
6.43 (d, J = 16.1 Hz, 1H), 7.19ꢀ7.22 (m, 1H), 7.27ꢀ7.34 (m, 4H). 13C
NMR (125 MHz, CDCl3): δ 16.1, 17.0, 42.1, 52.4, 57.8, 126.3, 127.3,
128.5, 130.5, 131.5, 137.3, 171.8. IR (neat) 2998, 2952, 1732, 1494,
Dimethyl 2-Methyl-2-((S)-1-phenylallyl)malonate (S)-109c.
Yield: 85% (4:96 ratio) (Scheme 1). Colorless oil. 1H NMR (500 MHz,
CDCl3): δ 1.43 (s, 3H), 3.62 (s, 3H), 3.71 (s, 3H), 4.15 (d, J = 8.7 Hz,
1H), 5.11 (d, J = 17.2 Hz, 1H), 5.14 (d, J = 10.3 Hz, 1H), 6.32 (ddd, J =
8.7, 10.3, 17.2 Hz, 1H), 7.22ꢀ7.28 (m, 5H). 13C NMR (125 MHz,
CDCl3): δ 18.4, 52.4, 52.4, 54.5, 58.8, 117.8, 127.1, 128.2, 129.5, 136.8,
20
139.0, 171.3, 171.5. [R]D22 22.1 (c 1.0, CHCl3) (94% ee) {lit.9c [R]D
46.4 (c 1.8, CHCl3) (86% ee)}. Enantiomer ratio was determined by
HPLC using a Daicel CHIRALCEL OJ-H (hexane/2-propanol = 99/1,
flow: 1.0 mL/min, 254 nm, 35 °C, tR 9.1 min (major); tR 10.5 min
(minor)). The absolute configuration was determined to be S by com-
parison with the sign of optical rotation of (S)-10 ([R]D24 23.8 (c 2.2,
CHCl3) (94% ee)), which was obtained by the methylation of dimethyl
2-((S)-1-phenylallyl)malonate24 (95% ee) with NaH, MeI in THF.
Preparation of Optically Active Allylic Acetate (S)-2. Allylic
acetate (S)-2 was obtained by treatment of the corresponding chiral
alcohol25 with acetic anhydride and pyridine. Experimental procedure:
To a solution of chiral alcohol (577 mg, 3.9 mmol), pyridine (0.5 mL,
5.8 mmol) and catalytic amount of 4-dimethylaminopyridine in CH2Cl2
(8 mL) was added acetic anhydride (0.45 mL, 4.7 mmol). The mixture
was stirred at room temperature for 12 h, quenched with saturated
CuSO4 solution, and extracted with ether. The organic phase was
1449, 1262, 1106, 971, 747 cmꢀ1
.
þ
(S)-(E)-3: HRMS (ESI): m/z: calcd for C16H21O4 [M þ H]þ
27
277.1440, found 277.1433. [R]D ꢀ28.8 (c 0.4, CHCl3) (99% ee).
Enantiomer ratio was determined by HPLC using a Daicel CHIRAL-
CEL OJ-H (hexane/2-propanol = 99/1, flow: 1.0 mL/min, 254 nm,
35 °C, tR 15.5 min (major); tR 20.9 min (minor)). The absolute con-
figuration was determined to be S by comparison with the sign of optical
25
rotation of (S)-3 ([R]D ꢀ35.2 (c 1.0, CHCl3) (99% ee)), which was
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NOTE
washed with water and brine, dried over MgSO4, and evaporated. The
crude product was purified by silica gel column chromatography
(hexane/EtOAc = 95/5) to give 580 mg (78%) of chiral (S)-2 as
colorless oil. [R]D27 5.2 (c 1.8, CHCl3) (87% ee). Enantiomer ratio was
determined by HPLC using a Daicel CHIRALCEL OJ-H (hexane/2-
propanol = 99/1, flow: 1.0 mL/min, 254 nm, 35 °C, tR 6.7 min (minor);
tR 7.2 min (major)).
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(S)-1-Phenylbut-2-enyl Acetate ((S)-2). 1H NMR (500 MHz,
CDCl3): δ 1.72 (d, J = 5.7 Hz, 3H), 2.09 (s, 3H), 5.67 (ddq, J = 1.4, 6.8,
15.1 Hz, 1H), 5.76 (dqd, J = 0.7, 5.7, 15.1 Hz, 1H), 6.22 (d, J = 6.8 Hz,
1H), 7.27ꢀ7.36 (m, 5H). 13C NMR (125 MHz, CDCl3): δ 17,7, 21.3,
76.3, 126.8, 127.9, 128.4, 129.5, 129.6, 139.7, 170.0. IR (neat) 3033,
2918, 1735, 1452, 1370, 1237, 1017, 962 755, 697 cmꢀ1. HRMS (ESI):
m/z: calcd for C12H15O2þ (M þ Hþ) 191.1072, found 191.1066.
Preparation of Optically Active Allylic Acetate (R)-5. Allylic
acetate (R)-5 was obtained by treatment of the corresponding chiral
alcohol26 with acetyl chloride and n-butyllithium. Experimental proce-
dure: To a solution of chiral allyl alcohol (413 mg, 2.79 mmol) and
catalytic amount of 1,10-phenanthroline in THF was added 2.6 M n-
butyllithium in hexane (1.3 mL, 2.6 mmol) at ꢀ78 °C and stirred for
30 min. To this reaction mixture was added acetyl chloride (0.24 mL,
2.62 mmol) at ꢀ78 °C and stirred for 10 min. The mixture was allowed
to warm to room temperature and stirred for 90 min. The reaction was
quenched with saturated sodium bicarbonate solution and extracted
with EtOAc. The organic layer was washed with saturated sodium bi-
carbonate solution and brine, dried over MgSO4 and evaporated. The
crude product was purified by silica gel column chromatography
(hexane/EtOAc = 99/1) to give 147 mg (28%) of chiral (R)-5 as color-
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2007, 4283–4285.
(13) Recently, Kazmaier also reported the ruthenium-catalyzed
allylic alkylation of optically active branch-type allylic esters with
chelated enolates by [RuCl2(p-cymene)]2/2PPh3, see: Bayer, A.;
Kazmaier, U. Org. Lett. 2010, 12, 4960–4963.
20
less oil. [R]D 1.2 (c 0.6, CHCl3) (95% ee). Enantiomer ratio was
determined by HPLC using a Daicel CHIRALCEL OJ-H (hexane/2-
propanol = 49/1, flow: 1.0 mL/min, 215 nm, 35 °C, tR 11.8 min (major);
tR 15.1 min (minor)).
(R)-2-Phenylbut-3-en-2-yl Acetate ((R)-5). 1H NMR (500 MHz,
CDCl3): δ 1.88 (s, 3H), 2.08 (s, 3H), 5.24 (dd, J = 0.8, 10.8 Hz, 1H), 5.27
(dd, J = 0.8, 17.5 Hz, 1H), 6.27 (dd, J = 10.8, 17.5 Hz, 1H), 7.23ꢀ7.27 (m,
1H), 7.32ꢀ7.38 (m, 4H). 13C NMR (125 MHz, CDCl3): δ 22.2, 25.4, 83.1,
114.4, 125.1, 127.2, 128.2, 141.5, 143.7, 169.4. IR (neat) 2988, 1743, 1447,
1244, 1016, 944, 765, 700 cmꢀ1. HRMS (ESI): m/z: calcd for C12H15O2þ
(M þ Hþ) 191.1072, found 191.1066.
(14) (a) Hermatschweiler, R.; Fernꢀandez, I.; Pregosin, P. S.; Watson,
E. J.; Albinati, A.; Rizzato, S.; Veiros, L. F.; Calhorda, M. J. Organome-
tallics 2005, 24, 1809–1812. (b) Fernꢀandez, I.; Hermatschweiler, R.;
Breher, F.; Pregosin, P. S.; Veiros, L. F.; Calhorda, M. J. Angew. Chem.,
Int. Ed. 2006, 45, 6386–6391.
’ ASSOCIATED CONTENT
(15) Burger, E. C.; Tunge, J. A. Chem. Commun. 2005, 2835–2837.
(16) Takashima, Y.; Kobayashi, Y. J. Org. Chem. 2009, 74, 5920–
5926.
(17) Alickmann, D.; Fr€ohlich, R.; Maulitz, A. H.; W€urthwein, E.-U.
Eur. J. Org. Chem. 2002, 1523–1537.
S
Supporting Information. Copies of NMR Spectra and
b
HPLC analysis. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) Kawatsura, M.; Uozumi, Y.; Ogasawara, M.; Hayashi, T. Tetra-
hedron 2000, 56, 2247–2257.
’ AUTHOR INFORMATION
(19) Matsuda, H.; Ando, S.; Morikawa, T.; Kataoka, S.; Yoshikawa,
M. Bioorg. Med. Chem. Lett. 2005, 15, 1949–1953.
(20) Malkov, A. V.; Baxendale, I. R.; Mansfield, D. J.; Koꢁcovskꢀy, P.
J. Chem. Soc., Perkin Trans. 1 2001, 1234–1240.
(21) Hayashi, T.; Yamamoto, A.; Hagihara, T. J. Org. Chem. 1986,
51, 723–727.
(22) Hou, X.-L.; Sun, N. Org. Lett. 2004, 6, 4399–4401 and refer-
ences cited therein.
(23) Miura, T.; Shimada, M.; Murakami, M. J. Am. Chem. Soc. 2005,
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(24) Fr€olander, A.; Lutsenko, S.; Privalov, T.; Moberg, C. J. Org.
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(25) Sasaki, M.; Ikemoto, H.; Kawahata, M.; Yamaguchi, K.; Takeda,
K. Chem.—Eur. J. 2009, 15, 4663–4666.
(26) Stymiest, J. L.; Bagutski, V.; French, R. M.; Aggarwal, V. K.
Nature 2008, 456, 778–782.
Corresponding Author
*E-mail: kawatsur@chem.tottori-u.ac.jp; titoh@chem.tottori-u.ac.jp.
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