Enantioselective Michael additions of β-keto esters to α,β-unsaturated carbonyl compounds catalyzed by a chiral biquinoline N,N′-dioxide-scandium trifluoromethanesulfonate complex

Article abstract of DOI:10.1016/S0040-4020(03)01139-6

A catalytic, enantioselective Michael addition of β-keto esters to α,β-unsaturated carbonyl compounds was achieved by using a chiral biquinoline N,N′-dioxide-scandium trifluoromethanesulfonate complex as a catalyst. The corresponding Michael adducts were

Full text of DOI:10.1016/S0040-4020(03)01139-6

Tetrahedron 59 (2003) 7307–7313
Enantioselective Michael additions of b-keto esters to
a,b-unsaturated carbonyl compounds catalyzed by a
chiral biquinoline N,N0-dioxide–scandium
trifluoromethanesulfonate complex
*
Makoto Nakajima, Satoshi Yamamoto, Yukiko Yamaguchi, Seiichi Nakamura
and Shunichi Hashimoto
Graduate School of Pharmaceutical Sciences, Hokkaido University, Kita-12 Nishi-6, Kita-ku, Sapporo 060-0812, Japan
Received 12 June 2003; revised 17 July 2003; accepted 23 July 2003
Abstract—A catalytic,0enantioselective Michael addition of b-keto esters to a,b-unsaturated carbonyl compounds was achieved by using a
chiral biquinoline N,N -dioxide–scandium trifluoromethanesulfonate complex as a catalyst. The corresponding Michael adducts were
obtained in high yields and with enantioselectivities up to 84% ee.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
reports using N-oxides as chiral ligands.4 In our pursuit to
develop N-oxide-mediated reactions,5 herein we describe
the details of a chiral biquinoline N,N0-dioxide–scandium
trifluoromethanesulfonate complex catalyzed enantio-
selective Michael addition of b-keto esters to a,b-
unsaturated carbonyl compounds.6
Catalytic, enantioselective Michael additions are funda-
mental carbon–carbon bond formation reactions in organic
synthesis because the products are versatile chiral building
blocks.1 Michael additions of prochiral b-keto esters to a,b-
unsaturated carbonyl compounds generate quaternary
stereogenic centers, which are still challenging for synthetic
organic chemists. Various chiral catalysts have been
developed for this type of Michael addition including
cinchona alkaloids,2a–c chiral alkoxide complexes,2d chiral
crown ether–metal alkoxide complexes,2e chiral transition
metal complexes,2f–k and chiral bimetallic lanthanoid
complexes.1b
2. Results and discussion
We have recently reported enantioselective conjugate
additions of thiols to cyclic enones and acyclic enals
catalyzed by a complex of chiral N-oxide 1 and cadmium
iodide.7 Some effective catalysts for enantioselective
conjugate addition of thiols also catalyze enantioselective
Michael addition of a,b-unsaturated carbonyl compounds
to enones.1b,2a,b This prompted us to investigate the Michael
addition of dibenzyl malonate to cyclohexenone employing
the 1–cadmium complex. According to the procedure for
the above conjugate addition,7 dibenzyl malonate was added
to a solution of cyclohexenone, N-oxide 1 (5 mol%) and
cadmium iodide (5 mol%) in toluene, but the Michael
adduct was not obtained at all. Then the Michael addition of
methyl 1-oxoindan-2-carboxylate (2a) to methyl vinyl
ketone (Eq. (1)), a reaction2 frequently investigated as a
probe for the enantioselective Michael addition was
examined. The reaction proceeded smoothly with
1–cadmium iodide complex in dichloromethane, but the
observed enantiomeric excess of the adduct was low
(Table 1, entry 1). Table 1 also shows other representative
results using various metal salts that resulted in a high yield
Although N-oxide is a functional group possessing a unique
electron-donating property, which allows it to form
complexes with a variety of metals,3 there are a limited
Keywords: enantioselectivities; a,b-unsaturated carbonyl compounds;
chiral biquinoline; enantioselection; catalyst; Michael addition; N-oxide;
scandium trifluoromethanesulfonate.
*
Corresponding author. Tel.: þ81-11-706-3766; fax: þ81-11-706-4981;
e-mail: nakajima@pharm.hokudai.ac.jp
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)01139-6

7308
M. Nakajima et al. / Tetrahedron 59 (2003) 7307–7313
which generated the corresponding adduct with a similar
enantioselectivity (isolated as methyl ester 4a, 75% yield,
30% ee).
Various b-keto esters were then evaluated as Michael
donors employing methyl vinyl ketone as an acceptor.
Reactions of methyl 1-oxo-1,2,3,4-tetrahydronaphthalene-
2-carboxylate (5), (10 h, 81%), benzyl 2-oxocyclopentane-
carboxylate (7), (3 h, 98%), and 2-acetyl-2-indanone (9)
(0.5 h, 86%) yielded racemic mixtures, which suggests that
indan-2-carboxylate skeleton is important in directing the
enantiocontrol.
ð1Þ
Table 1. Enantioselective Michael addition of 2a to MVK catalyed by
1-metal complex
Entry
MXn
Time (h)
Yield (%)a
ee (%)b
1
2
3
4
5
CdI2
48
24
24
1
75
98
82
98
83
13
9
18
39
9
Yb(OTf)3
Hf(OTf)4
Sc(OTf)3
ScCl3
24
Catalyst concentration: 0.5 mM.
a
Isolated yield.
Determined by Chiral HPLC (Daicel Chiralpak AD).
b
of the Michael adduct 3a with observable enantio-
selectivities. Among the surveyed metal salts, we found
that scandium trifluoromethanesulfonate8,9 gave 3a in a
quantitative yield with a moderate enantioselectivity of 39%
ee (entry 4).
Enantioselective Michael additions of various esters of
1-oxoindan-2-carboxylic acid to methyl vinyl ketone were
then investigated. The bulkiness of the ester substituent of
1-oxoindan-2-carboxylate had a pronounced effect on the
observed enantioselectivity. As shown in Table 3, the
enantioselectivities increased with the bulkiness of the ester.
tert-Butyl ester 2e gave the highest enantioselectivity of
84% ee (Table 3, entry 5)11. The addition of acrolein to 2e
affirmed the beneficial effect of tert-butyl ester (entry 6) and
is a rare example of the catalytic enantioselective Michael
addition of b-keto ester to a,b-unsaturated aldehyde.2a,12
Although scandium trifluoromethanesulfonate is insoluble
in dichloromethane, its complex with 1 and 2a dissolves in
dichloromethane to give a yellow solution. Other solvents
Table 2. Enantioselective Michael addition of 2a to MVK catalyzed by
1–Sc(OTf)3 complex
Entry 1/Sc(OTf)3 mMa Solvent Time (h) Yield (%)b ee (%)c
1
2
3
4
5
6
7
8
1
1
1
0.5
0.5
0.5
0.5
0.5
0.5
0.1
2.5
CH2Cl2
Toluene
THF
1
5
24
1
1
2
98
96
97
97
94
98
85
93
39
8
10
19
30
9
1
EtCN
The transition state model shown in Figure 1, in which
scandium trifluoromethanesulfonate forms a complex with
N-oxide 1 and b-keto ester 2a, may explain the predominant
formation of (R)-3e. In order to avoid steric repulsion with
the quinoline moiety, the bulky tert-butyl ester moiety
should be located on the si-face of the keto ester plane which
causes the methyl vinyl ketone to preferentially attack the
re-face.
0.1
2.5
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1
0.2
35
6
1
a
b
c
Concentration of catalyst.
Isolated yield.
Determined by Chiral HPLC (Daicel Chiralpak AD).
examined for the Michael addition afforded the product in
lower enantiomeric excess than dichloromethane (Table 2,
entries 2–4 vs. entry 1).10 The enantioselectivity was
strongly dependent on the ratio of N-oxide to scandium
(entries 5, 6 vs. entry 1) and the catalyst concentration
(entries 7, 8 vs. entry 1). These observations suggest a
variety of aggregation states for the scandium and N-oxide
complexes, but the details are unclear.
The Michael addition of 2a with various a,b-unsaturated
carbonyl compounds was investigated. Chalcone generated
a complex mixture of products, while a reaction was not
observed using methyl acrylate as an acceptor. The mild
reaction conditions allowed acrolein to react with 2a,
Figure 1. Proposed model for the enantioselective Micheal addition
catalyzed by 1–Sc(OTf)3 complex.

M. Nakajima et al. / Tetrahedron 59 (2003) 7307–7313
7309
Table 3. Enantioselective Michael addition of b-keto ester 2 catalyzed by 1–Sc(OTf)3 complex
Entry
Donor
R
Acceptor
Adduct
Yield (%)
ee (%)a
Configuration
[a]2D5 (Benzene)
1
2
3
2a
2b
2c
2d
2e
2e
Me
CH2Ph
iPr
CH2vCHCOMe
CH2vCHCOMe
CH2vCHCOMe
CH2vCHCOMe
CH2vCHCOMe
CH2vCHCHO
3a
3b
3c
3d
3e
4e
98
85
94
98
89
73g
39
38
47
69
84
75g
Rb
Rc
Rd
Rd
Rf
þ27.1
þ17.5
þ31.9
þ20.4
þ47.1
þ38.3h
4
CH(iPr)2
iBu
5e
6
iBu
Rd
Catalyst concentration: 0.5 mM.
a
Determined by HPLC analysis employing Daicel Chiralpak AD or Chiralcel OJ.
Assigned by optical rotation.
Assigned by optical rotation of 3b prepared from 3a.
Assigned by analogy.
Catalyst concentration: 0.1 mM.
Assigned by optical rotation after conversion to 3a.
Determined after conversion of aldehyde into methyl ester 4e.
Optical rotation of diester 4e.
b
c
d
e
f
g
h
Next, the present Michael addition was applied to the
enantioselective synthesis of biologically active com-
pounds. Griseofulvin13 is an antifungal agent that is used
to treat dermatomycoses in animals and humans. To date
only Pirrung,14 who applied an asymmetric sigmatropic
rearrangement of oxonium ylide for the construction of
spirocyclic structure, has reported the enantioselective
synthesis of griseofulvin. We plan to apply the present
Michael addition to the enantioselective synthesis of
griseofulvin.
methoxy group of the substrate and the quinoline ring of 1 in
Figure 1, though details are unclear.
As a model for synthesizing griseofulvin, the Michael
addition of tert-butyl 3-oxobenzo[b]furan-2-carboxylate 12,
an oxygen analogue of 2e was investigated. The reaction of
12 to methyl vinyl ketone proceeded smoothly to afford the
Michael adduct in 77% ee (Eq. (2)). Although the
diastereoselectivity was modest, the reaction of 3-penten-
2-one also proceeded smoothly to give a diastereomeric
mixture with a 76% ee of the major isomer, but the
stereochemistries of the newly formed chiral centers were
not determined. Encouraged by these findings, tert-butyl
oxofurancarboxylate 18 was synthesized from chloro-
dimethoxysalicylic acid 15 via a Dieckmann condensation
shown in Scheme 1 and the enantioselective Michael
addition of 18 was studied. The reaction proceeded
smoothly, but neither diastereoselectivity nor enantio-
selectivity were observed (Eq. (3)). This dramatic drop in
selectivities may be due to the steric repulsion of the
ð2Þ
ð3Þ
3. Conclusion
In conclusion, the potential of a chiral biquinoline N,N0-
dioxide–scandium trifluoromethanesulfonate complex to
catalyze enantioselective Michael additions of b-keto esters
to methyl vinyl ketone or acrolein was demonstrated.
1-Oxoindan-2-carboxylate skeleton is essential for the
enantiocontrol. The present reaction provides a rare
example of a catalytic enantioselective Michael addition
to a,b-unsaturated aldehyde.
Scheme 1. Preparation of Micheal donor 18. Reagents and condition:
(a) Me2NCH(OtBu)2, benzene, reflux, 2 h, 82%. (b) BrCH2COOtBu,
K2CO3, 2-butanone, reflux, 6 h, 97%. (c) tBuOK, THF, room temperature,
15 min, 98%.

7310
M. Nakajima et al. / Tetrahedron 59 (2003) 7307–7313
4. Experimental
4.3. A representative procedure for the enantioselective
Michael addition catalyzed by a scandium trifluoro-
methanesulfonate–1 complex
4.1. General
Melting points were measured using a Bu¨chi 535
melting point apparatus and were not corrected. Optical
rotations were obtained on a JASCO P-1030 digital
polarimeter. 1H NMR and 13C NMR spectra were recorded
on a JEOL EX-270 (1H, 270 MHz; 13C, 68 MHz)
spectrometer in deuteriochloroform. The chemical
shift values are in ppm relative to internal tetramethyl-
silane. The coupling constants (J) are in Hertz (Hz).
Abbreviations are as follows: s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet, br, broad. Infrared spectra
(IR) were recorded on a JASCO FT/IR-5300 spec-
trometer. Mass spectra were obtained on a JEOL JMS-
FABmate spectrometer by electron impact (EI) with an
ionization voltage of 70 eV. Michael donors 2a,2b 2b,15
5,16 7,17 and 918 were prepared by literature methods.
Esters 2c–2e were prepared from 2a with transesterifica-
tion catalyzed by dibutyltin oxide.19
A mixture of N,N0-dioxide 1 (0.026 mmol), scandium
trifluoromethanesulfonate (0.026 mmol) and b-keto ester
2e (0.52 mmol) in dichloromethane (5 ml) was sonicated for
5 min, resulting in a yellow solution. An a,b-unsaturated
carbonyl compound (2.6 mmol) was added to the solution
and the mixture was stirred at room temperature for 0.5 h.
The reaction mixture was washed with brine, dried over
Na2SO4, and concentrated in vacuo. The residue was
purified by column chromatography (silica gel 5 g,
hexane/AcOEt¼5:1) to yield the corresponding Michael
adduct. N,N0-Dioxide 1 was recovered by eluding with 10%
ethanol in dichloromethane without losing of optical purity.
The enantiomeric excess of the adduct was determined by
chiral HPLC.
4.3.1. Methyl 1-oxo-2-(3-oxobutyl)indan-2-carboxylate
(3a).2a HPLC: Daicel Chiralpak AD, hexane/
2-propanol¼9:1, 1 ml/min, tR: 16 (S) and 18 (R) min.
4.2. A representative procedure for the synthesis of
Michael donor by transesterification
[a]2D5¼þ27.1, [a] ¼þ29.8 (c 1.24, benzene) for 39% ee
25
577
rt
578
{lit. [a] ¼277 (c 2, benzene) for (S)-isomer}. IR
(CHCl3): n 1742, 1715 cm21
.
1H NMR (CDCl3): d
A solution of 2a (1.0 mmol), dibutyltin oxide (0.10 mmol)
and the corresponding alcohol (10 mmol) in toluene (10 ml)
was refluxed for 2 h. Evaporation of the solvent and
chromatography (silica gel 10 g, hexane/AcOEt¼20:1)
afforded the corresponding ester.
2.12 (3H, s), 2.19–2.27 (2H, m), 2.45–2.76 (2H, m),
3.04 (1H, d, J¼17.8 Hz), 3.67 (1H, d, J¼17.8 Hz), 3.70
(3H, s), 7.39–7.79 (4H, m). 13C NMR (CDCl3): d 28.6,
29.8, 37.8, 38.7, 52.7, 59.1, 124.8, 126.4, 127.9, 135.0,
135.5, 152.5, 171.5, 202.2, 207.3. MS (EI): m/z 260 (Mþ),
190.
4.2.1. Isopropyl 1-oxoindan-2-carboxylate (2c). Yield
68%. Colorless needles of mp 41–428C. IR (CHCl3): n
4.3.2. Benzyl 1-oxo-2-(3-oxobutyl)indan-2-carboxylate
(3b). HPLC: Daicel Chiralpak AD, hexane/2-propanol¼9:1,
1 ml/min, tR: 19 (S) and 22 (R) min. [a]2D5¼þ17.5 (c 0.66,
1713, 1732 cm21 1H NMR (CDCl3): d 1.28 (3H, d,
.
J¼6.6 Hz), 1.29 (3H, d, J¼6.6 Hz), 3.36 (1H, dd, J¼8.6,
17.3 Hz), 3.53 (1H, dd, J¼4.0, 17.3 Hz), 3.68 (1H, dd,
J¼4.0, 8.6 Hz), 5.09 (1H, sept, J¼5.9 Hz), 7.30–7.80 (4H,
m). 13C NMR (CDCl3): d 21.7, 30.2, 53.4, 69.2, 124.6,
126.7, 127.7, 129.2, 135.2, 153.6, 168.7, 199.6. MS (EI):
m/z 218 (Mþ), 176. HRMS: calcd for C13H14O3 218.0943
found 218.0950.
1
benzene) for 38% ee. IR (CHCl3): n 1714 cm21. H NMR
(CDCl3): d 2.08 (3H, s), 2.20–2.28 (2H, m), 2.48–2.66
(2H, m), 3.03 (1H, d, J¼17.2 Hz), 3.66 (1H, d, J¼17.2 Hz),
5.15 (2H, s), 7.23–7.79 (9H, m); 13C NMR (CDCl3): d
28.7, 29.9, 37.8, 38.8, 59.2, 67.1, 124.8, 126.3, 127.7,
127.9, 128.1, 128.4, 134.9, 135.3, 135.4, 152.3, 170.7,
201.9, 207.3. MS (EI): m/z 336 (Mþ), 266, 149. HRMS:
calcd for C21H20O4 (Mþ) 336.1361, found 336.1368.
Anal. calcd for C21H20O4: C, 74.98; H, 5.99. Found: C,
74.52; H, 6.38. The absolute configuration was determined
by the optical rotation of 3b ([a]2D5¼þ18.4 (c 0.65,
benzene)) prepared from 3a ([a]2D5¼þ27.1 (c 1.24,
benzene)) by transesterification with dibutyltin oxide and
benzyl alcohol.
4.2.2. 2,4-Dimethyl-3-pentyl 1-oxoindan-2-carboxylate
(2d). Yield 31%. Colorless needles of mp 62–638C. IR
(CHCl3): n 1713, 1731 cm21
.
1H NMR (CDCl3): d
0.82–0.93 (12H, m), 1.86–1.99 (1H, m), 3.39 (1H,
dd, J¼8.4, 17.0 Hz), 3.58 (1H, dd, J¼4.1, 17.0 Hz),
3.75 (1H, dd, J¼4.1, 8.4 Hz), 4.67 (1H, t, J¼6.5 Hz),
7.30–7.80 (4H, m). 13C NMR (CDCl3): d 16.9, 17.3,
19.4, 19.5, 29.4, 29.5, 30.5, 53.6, 84.2, 124.6, 126.5, 127.7,
135.2, 135.4, 153.5, 169.1, 199.6. MS (EI): m/z 274
(Mþ), 176. HRMS: calcd for C17H22O3 274.1569 found
274.1546.
4.3.3. Isopropyl 1-oxo-2-(3-oxobutyl)indan-2-carboxyl-
ate (3c). HPLC: Daicel Chiralcel OJ, hexane/
2-propanol¼9:1, 1 ml/min, tR: 17 (S) and 25 (R) min.
[a]2D5¼þ31.9 (c 1.21, benzene) for 47% ee. IR (CHCl3): n
1
4.2.3. tert-Butyl 1-oxoindan-2-carboxylate (2e). Yield
52%. Colorless needles of mp 44–468C. IR (CHCl3): n
.
1711, 1731 cm21 1H NMR (CDCl3): d 1.47 (9H, s),
3.30 (1H, dd, J¼8.6, 17.3 Hz), 3.47 (1H, dd, J¼4.0,
17.3 Hz), 3.59 (1H, dd, J¼4.0, 8.6 Hz), 7.33–7.55 (4H,
m). 13C NMR (CDCl3): d 28.0, 30.3, 54.4, 82.0, 124.5,
126.6, 127.6, 135.2, 135.5, 153.7, 168.3, 200.0. MS (EI):
m/z 232 (Mþ), 176. HRMS: calcd for C14H16O3 232.1099
found 232.1094.
1712 cm21. H NMR (CDCl3): d 1.17 (6H, d, J¼5.9 Hz,
6H), 2.11 (3H, s), 2.17–2.23 (2H, m), 2.43–2.68 (2H, m),
3.01 (1H, d, J¼17.2 Hz), 3.63 (1H, d, J¼17.2 Hz), 5.01 (1H,
sept, J¼5.9 Hz), 7.37–7.77 (4H, m, 4H). 13C NMR
(CDCl3): d 21.5, 21.6, 28.5, 30.0, 37.9, 38.9, 59.3, 69.2,
124.7, 126.3, 127.8, 135.0, 135.3, 152.5, 170.4, 202.3,
207.5. MS (EI): m/z 288 (Mþ), 218, 176. HRMS: calcd for
C17H20O4 288.1361, found 288.1370. Anal. calcd for
C17H20O4: C, 70.81; H, 6.99. Found: C, 70.87; H, 7.14.

M. Nakajima et al. / Tetrahedron 59 (2003) 7307–7313
7311
4.3.4. 2,4-Dimethyl-3-pentyl 1-oxo-2-(3-oxobutyl)indan-
2-carboxylate (3d). HPLC: Daicel Chiralcel OJ, hexane/
2-propanol¼9:1, 1 ml/min, tR: 9 (S) and 24 (R) min.
[a]2D4¼þ20.4 (c 1.17, benzene) for 69% ee. IR (CHCl3): n
(EI): m/z 318 (Mþ), 262. HRMS: calcd for C18H22O5
318.1467, found 318.1460.
4.3.8. Methyl 1-oxo-2-(3-oxobutyl)-1,2,3,4-tetrahydro-
naphthalene-2-carboxylate (6).21 HPLC: Daicel Chiralpak
AD, hexane/2-propanol¼9:1, 1 ml/min, tR: 11 and 13 min.
IR (CHCl3): n 1730, 1688 cm21. 1H NMR (CDCl3): d 1.97–
2.18 (2H, m), 2.08 (3H, s), 2.44–2.51 (4H, m), 2.91–2.93
(2H, m), 3.61 (3H, s), 7.13–7.98 (4H, m). 13C NMR
(CDCl3): d 25.8, 27.6, 29.9, 31.6, 39.0, 52.4, 56.6, 126.8,
128.0, 128.7, 131.8, 133.6, 142.8, 172.3, 195.3, 207.6. MS
(EI): m/z 274 (Mþ), 204. HRMS: calcd for C16H18O4
274.1205, found 274.1194.
1
1711 cm21. H NMR (CDCl3): d 0.71 (6H, d, J¼4.6 Hz),
0.82 (6H, d, J¼3.3 Hz), 1.73–1.87 (2H, m), 2.13 (3H, s),
2.27 (2H, t, J¼7.6 Hz), 2.46–2.74 (2H, m), 3.07 (1H, d,
J¼17.2 Hz), 3.64 (1H, d, J¼17.2 Hz), 4.58 (1H, t,
J¼5.9 Hz), 7.38–7.79 (4H, m). 13C NMR (CDCl3): d
17.0, 17.2, 19.5, 19.6, 28.2, 29.4, 29.5, 30.0, 38.2, 38.8,
59.5, 84.1, 124.6, 126.2, 127.8, 135.2, 135.5, 152.4, 171.0,
202.4, 207.6. MS (EI): m/z 344 (Mþ), 229, 176; HRMS:
calcd for C21H28O4 344.1987, found 344.1991. Anal. calcd
for C21H28O4: C, 73.23; H, 8.19. Found: C, 73.21; H, 8.20.
4.3.9. Benzyl 2-oxo-1-(3-oxobutyl)cyclopentanecarboxyl-
ate (8).17 HPLC: Daicel Chiralpak AD, hexane/
2-propanol¼9:1, 1 ml/min, tR: 10 and 11 min. IR (CHCl3):
4.3.5. tert-Butyl 1-oxo-2-(3-oxobutyl)indan-2-carboxyl-
ate (3e). HPLC: Daicel Chiralpak OJ, hexane/
2-propanol¼9:1, 1 ml/min, tR: 10 (S) and 16 (R) min.
[a]2D5¼þ47.1 (c 1.03, benzene) for 84% ee. IR (CHCl3): n
1712, 1370, 1154 cm21. 1H NMR (CDCl3): d 1.39 (9H, s),
2.13 (3H, s), 2.15–2.21 (2H, m), 2.44–2.69 (2H, m), 3.00
(1H, d, J¼17.2 Hz), 3.60 (1H, d, J¼17.2 Hz), 7.37–7.77
(4H, m, 4H). 13C NMR (CDCl3): d 27.9, 28.5, 30.0, 38.0,
38.9, 59.9, 82.0, 124.6, 126.2, 127.7, 135.1, 135.2, 152.5,
170.0, 202.6, 207.0. MS (EI): m/z 246 (Mþ2tBu), 200, 176,
157. HRMS: calcd for C14H14O4 (Mþ2tBu) 246.0892,
found 246.0886. Anal. calcd for C18H22O4: C, 71.50; H,
7.33. Found: C, 71.51; H, 7.42. The absolute configuration
was determined by the optical rotation of 3a ([a]2D5¼þ72.0
(c 0.49, benzene)) from 3e ([a]2D5¼þ44.7 (c 1.23, benzene))
by treating with trifluoroacetic acid and then diazomethane.
1
n 1740, 1722 cm21. H NMR (CDCl3): d 1.82–2.00 (4H,
m), 2.08 (3H, s), 2.10–2.18 (1H, m), 2.30–2.49 (4H, m),
2.58–2.71 (1H, m), 5.15 (2H, s), 7.32–7.40 (5H, m). 13C
NMR (CDCl3): d 19.5, 27.1, 29.8, 34.3, 37.9, 38.7, 59.0,
67.0, 128.0, 128.3, 128.6, 135.5, 171.1, 207.6, 214.5.
4.3.10. 2-Acetyl-2-(3-oxobutyl)-1-indanone (10). HPLC:
Daicel Chiralcel OJ, hexane/2-propanol¼9:1, 1 ml/min, tR:
1
36 and 44 min. IR (CHCl3): n 1716, 1701 cm21. H NMR
(CDCl3): d 2.12 (3H, s), 2.23 (3H, s), 2.24–2.31 (2H, m),
2.39–2.42 (2H, m), 2.86 (1H, d, J¼17.8 Hz), 3.77 (1H, d,
J¼17.8 Hz), 7.37–7.76 (4H, m). 13C NMR (CDCl3): d 26.1,
26.2, 28.5, 30.0, 38.6, 67.3, 124.5, 126.4, 127.8, 135.1,
135.5, 152.7, 203.0, 203.8, 206.9. MS (EI): m/z 244 (Mþ),
201, 145. HRMS: calcd for C15H16O3 244.1099, found
244.1085. Anal. calcd for C15H16O3: C, 73.75; H, 6.60.
Found: C, 73.18; H, 6.98.
4.3.6. Methyl 2-(methoxycarbonylethyl)-1-oxoindan-2-
carboxylate (4a). To the crude Michael adduct (from
92 mg of 2a) in MeOH–water (9:1, 1 ml) was added
NaHCO3 (0.81 g, 9.6 mmol). To this mixture, bromine
(0.1 ml, 1.9 mmol) was added over 30 min with vigorous
stirring at room temperature.20 After stirring for 2 h, the
excess bromine was decomposed with solid Na2S2O3. The
mixture was filtered and the filtrate was extracted with
AcOEt. The organic layer was washed with brine, dried
over Na2SO4, and concentrated in vacuo. The residue
was purified by column chromatography (silica gel 10 g,
hexane/AcOEt¼5:1) to give 4a as a yellow oil (87 mg,
65%). HPLC: Daicel Chiralpak AD, hexane/
2-propanol¼9:1, 1 ml/min, tR: 16 (S) and 18 (R) min. IR
(neat): n 1741, 1712 cm21. 1H NMR (CDCl3): d 2.20–2.49
(4H, m), 3.07 (1H, d, J¼17.2 Hz), 3.65 (3H, s), 3.70 (3H, s),
3.70 (1H, d, J¼17.2 Hz), 7.41–7.79 (4H, m). 13C NMR
(CDCl3): d 29.6, 29.7, 37.2, 51.7, 52.8, 59.4, 124.9, 126.4,
128.0, 135.0, 135.5, 152.5, 171.2, 173.1, 201.8. MS (EI):
m/z 276 (Mþ), 244. HRMS: calcd for C15H16O5 276.0997,
found 276.1004.
4.3.11. tert-Butyl 2-(2-tert-butoxy-2-oxoethyl)benzoate
(11). To a solution of tert-butyl salicylate (400 mg,
2.06 mmol) and potassium carbonate (300 mg, 2.16 mmol)
in 2-butanone (1 ml) was added tert-butyl bromoacetate
(442 mg, 2.16 mmol) and the mixture was refluxed for 2 h.
The reaction was quenched with water and the whole
mixture was extracted with chloroform. The organic layer
was washed with 5% NaOH aq and brine successively and
dried over Na2SO4. Evaporation of the solvent and
chromatography (silica gel 20 g, hexane/AcOEt¼20:1)
afforded 11 (585 mg, 92%) as a pale yellow oil. IR
1
(CHCl3): n 1719, 1753 cm21; H NMR (CDCl3): d 1.44
(9H, s), 1.56 (9H, s), 4.56 (2H, s), 6.82 (1H, d J¼7.6 Hz),
6.97 (1H, m), 7.36 (1H, m), 7.70 (1H, d J¼7.9 Hz). 13C
NMR (CDCl3): d 27.8, 28.1, 66.8, 80.9, 81.9, 113.9, 121.0,
123.0, 131.2, 132.3, 157.2, 165.1, 167.4. MS (EI): m/z 308
(Mþ), 251. HRMS: calcd for C17H24O5 308.1624, found
308.1622.
4.3.7. tert-Butyl 2-(methoxycarbonylethyl)-1-oxoindan-
2-carboxylate (4e). HPLC: Daicel Chiralpak AD, hexane/
2-propanol¼9:1, 1 ml/min, tR: 9.0 (S) and 9.8 (R) min.
[a]2D2¼þ38.3 (c 0.69, benzene) for 75% ee. IR (neat): n
1742, 1709 cm21 1H NMR (CDCl3) d 1.38 (9H, s),
.
2.13–2.46 (4H, m), 3.01 (1H, d, J¼17.2 Hz), 3.62 (1H, d,
J¼17.2 Hz), 3.63 (3H, s), 7.33–7.78 (4H, m). 13C NMR
(CDCl3): d 27.8, 29.4, 29.7, 37.0, 51.8, 52.5, 59.9, 125.0,
126.3, 128.6, 135.4, 135.8, 152.5, 171.0, 173.0, 201.5. MS
4.3.12. tert-Butyl 3-oxo-2,3-dihydrobenzo[b]furan-2-car-
boxylate (12). To a suspension of potassium tert-butoxide
(218 mg, 1.95 mmol) in toluene (10 ml) was added 11
(300 mg, 0.974 mmol) and the mixture was stirred for
30 min at room temperature. The reaction was quenched
with NH4Cl satd aq. and the whole was extracted with
AcOEt. The organic layer was washed with brine and dried
over Na2SO4. Evaporation and chromatography (silica gel
10 g, hexane/AcOEt¼30:1) afforded 12 (208 mg, 91%) as a

7312
M. Nakajima et al. / Tetrahedron 59 (2003) 7307–7313
pale yellow viscous oil. IR (CHCl3): n 1659, 1744 cm21. 1H
NMR (CDCl3): d 1.66 (9H, s), 7.45 (2H, d, J¼7.1 Hz), 7.72
(2H, d, J¼7.1 Hz), 8.20 (1H, brs); 13C NMR (CDCl3): d
27.6, 83.0, 97.9, 112.5, 120.3, 122.8, 126.8, 128.8, 150.3,
153.2, 162.1. MS (EI): m/z 234 (Mþ), 178. HRMS: calcd for
C13H14O4 234.0892, found 234.0891.
over Na2SO4, evaporation of the solvent and chromato-
graphy (silica gel 5 g, hexane/AcOEt¼5:1) afforded 16
(108 mg, 82%) as colorless prisms of mp 568C. IR (CHCl3):
n 1601, 1649, 2980 cm21. 1H NMR (CDCl3): d 1.90 (9H, s),
3.85 (3H, s), 3.94 (3H, s), 6.04 (1H, s), 12.62 (1H, s). 13C
NMR (CDCl3): d 28.2, 55.9, 56.0, 82.9, 87.8, 98.9, 101.7,
159.6, 159.9, 160.8, 170.1. MS (EI): m/z 288 (Mþ), 232.
HRMS: calcd for C13H17ClO5 288.0764 found 288.0760.
Anal. calcd for C13H17ClO5 C, 54.08; H, 5.93 found C,
54.09; H, 6.02.
4.3.13. tert-Butyl 3-oxo-2-(3-oxobutyl)-2,3-dihydro-
benzo[b]furan-2-carboxylate (13). A mixture of N,N0-
dioxide 1 (4.7 mg, 0.015 mmol), scandium trifluoro-
methanesulfonate (9.0 mg, 0.015 mmol) and b-keto ester
18 (70 mg, 0.30 mmol) in dichloromethane (10 ml) was
sonicated for 5 min to give a yellow solution. Methyl vinyl
ketone (0.1 ml, 1.3 mmol) were added to the solution and
the mixture was stirred at room temperature for 1 h. The
reaction mixture was washed with brine, dried over Na2SO4,
filtered and concentrated in vacuo. The residue was purified
by column chromatography (silica gel 5 g, hexane/
AcOEt¼10:1) to give 13 (82 mg, 89%) as a viscous oil.
HPLC: Daicel Chiralpak AD, hexane/2-propanol¼40:1,
1 ml/min, tR: 21.4 (min) and 23.3 (maj) min. IR (CHCl3): n
4.3.16. tert-Butyl 6-(2-tert-butoxy-2-oxoethoxy)-3-chloro-
2,4-dimethoxybenzoate (17). To a suspension of 16
(800 mg, 2.77 mmol) and potassium carbonate (402 mg,
2.91 mmol) in 2-butanone (2 ml) was added tert-butyl
bromoacetate (595 mg, 3.05 mmol) and the mixture was
refluxed for 6 h. The reaction was quenched with water and
the whole mixture was extracted with chloroform. The
organic layer was washed with 5% NaOH aq. and brine
successively and dried over Na2SO4. Evaporation of the
solvent and chromatography (silica gel 50 g, hexane/
AcOEt¼20:1) afforded 17 (1.08 g, 97%) as colorless
needles of mp 160–1618C. IR (CHCl3): n 1718,
1
1720, 1745 cm21. H NMR (CDCl3): d 1.36 (9H, s), 2.04
(3H, s), 2.2–2.6 (4H, m), 7.0–7.2 (2H, m), 7.5–7.6 (2H, m).
13C NMR (CDCl3): d 27.0, 27.6, 37.1, 83.8, 90.8, 113.3,
119.6, 122.5, 124.8, 138.3, 138.4, 164.3, 172.1, 196.0,
206.3. MS (EI): m/z 304 (Mþ), 248. HRMS: calcd for
C17H20O5 304.1310 found 304.1325.
1
1753 cm21. H NMR (CDCl3): d 1.51 (9H, s), 1.54 (9H,
s), 3.83 (3H, s), 3.91 (3H, s), 4.51 (2H, s), 6.31 (1H, s). 13C
NMR (CDCl3): d 28.0, 28.7, 56.2, 56.3, 70.4, 81.8, 82.1,
93.0, 108.2, 114.1, 152.4, 156.1, 157.0, 164.0, 166.9. MS
(FAB): m/z 425 (MþþNa), 402. HRMS: calcd for
C19H27ClO7–Na 425.1343, found 425.1336. Anal. calcd
for C19H27ClO7 C, 56.64; H, 6.77, found C, 56.60; H, 6.81.
4.3.14. tert-Butyl 3-oxo-2-(1-methyl-3-oxobutyl)-2,3-
dihydrobenzo[b]furan-2-carboxylate (14). A mixture of
N,N0-dioxide 1 (4.4 mg, 0.014 mmol), scandium trifluoro-
methanesulfonate (8.4 mg, 0.014 mmol) and b-keto ester 13
(65 mg, 0.28 mmol) in dichloromethane (10 ml) was
sonicated for 5 min, resulting in a yellow solution.
3-Penten-2-one22 (0.1 ml, 1.3 mmol) was added to the
solution and the mixture was stirred at room temperature for
1 h. The reaction mixture was washed with brine, dried over
Na2SO4, and concentrated in vacuo. The residue was
purified by column chromatography (silica gel 5 g,
hexane/AcOEt¼10:1) to give 14 (76 mg, 86%) as a
inseparable mixture of diastereomers. HPLC: Daicel
Chiralcel OD, hexane/2-propanol¼100:1, 1 ml/min, tR:
17.1 (minor enantiomer of major diastereomer), 18.4
(minor enantiomer of minor diastereomer), 21.9 (major
enantiomer of major diastereomer), 24.6 (major enantiomer
of minor diastereomer). IR (CHCl3): n 1720, 1745 cm21. 1H
NMR (CDCl3): d 0.79 (3H£1/3, d J¼7.0 Hz), 1.12 (3H£2/3,
d J¼7.0 Hz), 1.44 (9H£1/3, s), 1.46 (9H£2/3, s), 2.06
(3H£2/3, s), 2.18 (3H£1/3, s), 2.3–2.7 (2H, m), 3.1–3.3 (H,
m), 7.0–7.2 2, m), 7.6–7.7 (2H, m). 13C NMR (CDCl3): d
13.7, 14.2, 14.6, 21.0, 27.7, 29.0, 30.1, 30.2, 33.9, 34.0,
43.9, 45.4, 60.4, 84.0, 94.8, 113.2, 113.4, 120.2, 120.3,
122.4, 122.5, 124.7, 124.8, 138.4, 138.6, 171.1, 172.5,
172.7, 195.7, 206.0, 206.3. MS (EI): m/z 318 (Mþ). HRMS
calcd for C18H22O5 318.1467, found 318.1451.
4.3.17. tert-Butyl 7-chloro-4,6-dimethoxy-3-oxo-2,3-
dihydrobenzo[b]furan-2-carboxylate (18). To a solution
of potassium tert-butoxide (168 mg, 1.50 mmol) in tetra-
hydrofuran (10 ml) was added 17 (300 mg, 0.745 mmol)
and the mixture was stirred for 1 h at room temperature. The
reaction was quenched with NH4Cl sat aq and the whole was
extracted with AcOEt. The organic layer was washed with
brine and dried over Na2SO4. Evaporation and chromato-
graphy (silica gel 10 g, hexane/AcOEt¼2:1) afforded 18
(240 mg, 98%) as a pale yellow viscous oil. IR (CHCl3): n
1
1709, 1741 cm21. H NMR (CDCl3): d 1.49 (9H, s), 3.95
(3H, s), 3.99 (3H, s), 5.05 (1H, s), 6.10 (1H, s). 13C NMR
(CDCl3): d 27.7, 56.2, 56.8, 83.8, 89.7, 91.3, 97.4, 103.7,
157.8, 162.6, 164.3, 169.3, 188.0. MS (FAB): m/z 328 (Mþ),
272 (Mþ2tBu). HRMS: calcd for C15H17ClO6 328.0713
found 328.1711.
4.3.18. tert-Butyl 7-chloro-4,6-dimethoxy-3-oxo-2-(1-
methyl-3-oxobutyl)-2,3-dihydrobenzo[b]furan-2-car-
boxylate (19). A mixture of N,N0-dioxide 1 (4.3 mg,
0.014 mmol), scandium trifluoromethanesulfonate (8.2 mg,
0.014 mmol) and b-keto ester 18 (90 mg, 0.27 mmol) in
dichloromethane (10 ml) was sonicated for 5 min, resulting
in a yellow solution. 3-Penten-2-one22 (0.1 ml, 1.3 mmol)
was added to the solution and the mixture was stirred at
room temperature for 1 h. The reaction mixture was washed
with brine, dried over Na2SO4, and concentrated in vacuo.
The residue was purified by column chromatography (silica
gel 8 g, hexane/AcOEt¼25:1) to give 19 (92 mg, 81%) as an
inseparable mixture of diastereomers. HPLC: Daicel
Chiralcel OD, hexane/2-propanol¼100:1, 1 ml/min, tR:
4.3.15. tert-Butyl 5-chloro-2,6-dimethoxy-6-hydroxy-
benzoate (16). A solution of 3-chloro-4,6-dimethoxy-
salicylic acid 1523 (106 mg, 0.456 mmol) and N,N-
dimethylformamide di-tert-butyl acetal24 (0.50 ml,
2.09 mmol) in benzene (2 ml) was refluxed for 2 h. The
mixture was diluted with AcOEt and then successively
washed with water, NaHCO3 satd aq., and brine. Drying
16.0, 19.3, 24.0, 25.3 min. IR (CHCl3):
n 1720,

M. Nakajima et al. / Tetrahedron 59 (2003) 7307–7313
7313
1745 cm21
.
1H NMR (CDCl3): d 0.82 (3H£1/2, d,
5. (a) Nakajima, M.; Sasaki, Y.; Shiro, M.; Hashimoto, S.
Tetrahedron: Asymmetry 1997, 8, 341–344. (b) Nakajima, M.;
Sasaki, Y.; Iwamoto, H.; Hashimoto, S. Tetrahedron Lett.
1998, 39, 87–88. (c) Nakajima, M.; Saito, M.; Shiro, M.;
Hashimoto, S. J. Am. Chem. Soc. 1998, 120, 6419–6420.
(d) Nakajima, M.; Saito, M.; Hashimoto, S. Chem. Pharm.
Bull. 2000, 48, 306–307. (e) Nakajima, M.; Saito, M.;
Hashimoto, S. Tetrahedron: Asymmetry 2002, 13,
2449–2452. (f) Nakajima, M.; Saito, M.; Uemura, M.;
Hashimoto, S. Tetrahedron Lett. 2002, 43, 8827–8829.
6. Nakajima, M.; Yamaguchi, Y.; Hashimoto, S. Chem.
Commun. 2001, 1596–1597.
J¼6.6 Hz), 1.13 (3H£1/2, d, J¼6.6 Hz), 1.45 (9H£1/2, s),
1.14 (9H£1/2, s), 2.07 (3H£1/2, s), 2.18 (3H£1/2, s),
2.2–2.3 (1H, m), 2.6–2.7 (1H, m), 3.2–3.3 (1H, m), 3.96
(3H£1/2, s), 3.98 (3H£1/2, s), 4.01 (3H, s), 6.10 (1H£1/2,
s), 6.11 (1H£1/2, s). 13C NMR (CDCl3): d 13.5, 14.4, 27.7,
30.2, 33.7, 43.8, 45.4, 56.3, 56.9, 75.9, 83.9, 89.2, 89.5,
95.9, 96.1, 104.6, 104.7, 157.5, 157.7, 164.0, 164.3, 168.9,
169.1, 190.2, 190.4, 194.4, 206.0, 206.3. MS (FAB): m/z
412 (Mþ). HRMS: calcd for C20H25ClO7 412.1289, found
412.1276.
7. (a) Saito, M.; Nakajima, M.; Hashimoto, S. Chem. Commun.
2000, 1851–1852. (b) Saito, M.; Nakajima, M.; Hashimoto, S.
Tetrahedron 2000, 56, 9589–9594.
Acknowledgements
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Science, Sports and Culture of Japan, Novartis Foundation
for the Promotion of Science, and the Akiyama Foundation.
8. (a) Kobayashi, S. Eur. J. Org. Chem. 1999, 15–27. (b) Cotton,
S. A. Polyhedron 1999, 18, 1691–1715.
9. Examples of chiral scadium trifluoromethanesulfonate com-
plexes: (a) Kobayashi, S.; Araki, M.; Hachiya, I. J. Org. Chem.
1994, 59, 3758–3759. (b) Kitajima, H.; Katsuki, T. Synlett
1997, 568–570. (c) Sanchez-Blanco, A. I.; Gothelf, K. V.;
Jørgensen, K. A. Tetrahedron Lett. 1997, 38, 7923–7926.
(d) Kodama, H.; Ito, J.; Hori, K.; Ohta, T.; Furusawa, I.
J. Organomet. Chem. 2000, 603, 6–12. (e) Evans, D. A.;
Sweeney, Z. K.; Rovis, T.; Tedrow, J. S. J. Am. Chem. Soc.
2001, 123, 12095–12096. (f) Sugihara, H.; Daikai, K.; Jin,
X. L.; Furuo, H.; Inanaga, J. Tetrahedron Lett. 2002, 43,
2735–2739.
References
1. (a) Rossiter, B. E.; Swingle, N. M. Chem. Rev. 1992, 92,
771–806. (b) Shibasaki, M.; Sasai, H.; Arai, T. Angew. Chem.
Int. Ed. Engl. 1997, 36, 1236–1256. (c) Christoffers, J. Eur.
´
J. Org. Chem. 1998, 1259–1266. (d) Leonard, J.; Dıez-Barra,
E.; Merino, S. Eur. J. Org. Chem. 1998, 2051–2061.
¨
(e) Krause, N.; Hoffmann-Roder, A. Synthesis 2001, 171–196.
10. Higher and lower temperatures resulted in lower selectivities
(08C: 91% yield, 5% ee, 408C: 98% yield, 20% ee).
11. For the reaction of tert-butyl ester 2e, a lower concentration
resulted in a slightly better enantioselectivity (0.5 mM: 88%
yield, 80% ee, 0.1 mM: 89% yield, 84% ee).
2. (a) Wynberg, H.; Helder, R. Tetrahedron Lett. 1975,
4057–4060. (b) Hermann, K.; Wynberg, H. J. Org. Chem.
1979, 44, 2238–2244. (c) Kobayashi, N.; Iwai, K. J. Am.
Chem. Soc. 1978, 100, 7071–7072. (d) Tamai, Y.; Kamifuku,
A.; Koshiishi, E.; Miyano, S. Chem. Lett. 1995, 957–958.
(e) Cram, D. J.; Sogah, G. D. J. J. Chem. Soc., Chem. Commun.
1981, 625–628. (f) Brunner, H.; Benedikt, H. Angew. Chem.
Int. Ed. Engl. 1984, 23, 312–313. (g) Desimoni, G.; Quadrelli,
P.; Righetti, P. P. Tetrahedron 1990, 46, 2927–2934.
(h) Botteghi, C.; Paganelli, S.; Schionato, A.; Boga, C.;
Fava, A. J. Mol. Catal. 1991, 66, 7–21. (i) Sawamura, M.;
Hamashima, H.; Ito, Y. J. Am. Chem. Soc. 1992, 114,
8295–8296. (j) Suzuki, T.; Torii, T. Tetrahedron: Asymmetry
2001, 12, 1077–1081. (k) Hamashima, Y.; Hotta, D.;
Sodeoka, M. J. Am. Chem. Soc. 2002, 124, 11240–11241.
3. Karayannis, N. M.; Pytlewski, L. L.; Mikulski, C. M. Coord.
Chem. Rev. 1973, 11, 93–159.
12. (a) Yamaguchi, M.; Shiraishi, T.; Hirama, M. J. Org. Chem.
1996, 61, 3520–3530. (b) Kumaraswamy, G.; Sastry, M. N.
V.; Jena, N. Tetrahedron Lett. 2001, 42, 8515–8517.
13. Oxford, A. E.; Raistrick, H.; Simonart, P. Biochem. J. 1939,
33, 240–248.
14. Pirrung, M. C.; Brown, W. L.; Rege, S.; Laughton, P. J. Am.
Chem. Soc. 1991, 113, 8561–8562.
15. Hashimoto, S.; Miyazaki, Y.; Shinoda, T.; Ikegami, S.
J. Chem. Soc., Chem. Commun. 1990, 1100–1102.
16. Brown, D. S.; Marples, B. A.; Smith, P.; Walton, L.
Tetrahedron 1995, 51, 3587–3606.
¨
17. Christoffers, J.; Robler, U.; Werner, T. Eur. J. Org. Chem.
2000, 701–705.
4. (a) Diana, M. B.; Marchetti, M.; Melloni, G. Tetrahedron:
Asymmetry 1995, 6, 1175–1179. (b) O’Neil, I. A.; Turner,
C. D.; Kalindjian, S. B. Synlett 1997, 777–780. (c) Dyker, G.;
18. Sartori, G.; Maggi, R.; Bigi, F.; Porta, C.; Tao, X.; Bernardi,
G. L.; Ianelli, S.; Nardelli, M. Tetrahedron 1995, 51,
12179–12192.
¨
Holzer, B.; Henkel, G. Tetrahedron: Asymmetry 1999, 10,
¨
3297–3307. (d) Derdau, V.; Laschat, S.; Hupe, E.; Konig,
19. Otera, J.; Yano, T.; Kawabata, T.; Nozaki, H. Tetrahedron
Lett. 1986, 27, 2383–2386.
W. A.; Dix, I.; Jones, P. G. Eur. J. Inorg. Chem. 1999,
1001–1007. (e) Miura, K.; Katsuki, T. Synlett 1999, 783–785.
(f) Liu, B.; Feng, X. M.; Chen, F. X.; Zhang, G. L.; Cuui, X.;
Jing, Y. Z. Synlett 2001, 1551–1554. (g) Tao, B.; Lo,
M. M.-C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 353–354.
(h) Malkov, A. V.; Orsini, M.; Pernazza, D.; Muir, K. W.;
Langer, V.; Meghani, P.; Kocovsky, P. Org. Lett. 2002, 4,
1047–1049. (i) Shimada, T.; Kina, A.; Ikeda, S.; Hayashi, T.
Org. Lett. 2002, 4, 2799–2801. (j) Denmark, S. E.; Fan, Y.
J. Am. Chem. Soc. 2002, 124, 4233–4235.
20. Lichtenthaler, F. W.; Jarglis, P.; Lorenz, K. Synthesis 1988,
790–792.
21. Brunner, H.; Kraus, J. J. Mol. Catal. 1989, 49, 133–142.
22. Arisawa, M.; Torisawa, Y.; Nakagawa, M. Synthesis 1995,
1371–1372.
23. Maillard, J.; Vincent, M. Bull. Soc. Chim. Fr. 1966,
2520–2524.
24. Widmer, U. Synthesis 1983, 135–136.