The first example for the asymmetric synthesis of allenes by the Doering-LaFlamme allene synthesis with enantiopure cyclopropylmagnesium carbenoids

Article abstract of DOI:10.1016/j.tetlet.2011.03.150

The reaction of lithium α-sulfinyl carbanion of enantiopure dichloromethyl p-tolyl sulfoxide with α,β-unsaturated carbonyl compounds gave optically active 1-chlorocyclopropyl p-tolyl sulfoxides having a carbonyl group with high asymmetric induction from the sulfur chiral center. Reduction of the carbonyl group followed by treatment with Grignard reagent, the 1-chlorocyclopropyl p-tolyl sulfoxides resulted in the formation of enantiopure allenic alcohols via the Doering-LaFlamme-type rearrangement of enantiopure cyclopropylmagnesium carbenoid intermediates. This is the first example for the asymmetric synthesis of allenes by the Doering-LaFlamme allene synthesis.

Full text of DOI:10.1016/j.tetlet.2011.03.150

Tetrahedron Letters 52 (2011) 3016–3019
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The first example for the asymmetric synthesis of allenes by the
Doering–LaFlamme allene synthesis with enantiopure
cyclopropylmagnesium carbenoids
Hitoshi Momochi, Takafumi Noguchi, Toshifumi Miyagawa, Naoki Ogawa,
⇑
Makoto Tadokoro, Tsuyoshi Satoh
Graduate School of Chemical Sciences and Technology, Tokyo University of Science, Ichigaya-funagawara-machi 12, Shinjuku-ku, Tokyo 162-0826, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of lithium
a-sulfinyl carbanion of enantiopure dichloromethyl p-tolyl sulfoxide with a,b-
Received 1 March 2011
Revised 28 March 2011
Accepted 31 March 2011
Available online 7 April 2011
unsaturated carbonyl compounds gave optically active 1-chlorocyclopropyl p-tolyl sulfoxides having a
carbonyl group with high asymmetric induction from the sulfur chiral center. Reduction of the carbonyl
group followed by treatment with Grignard reagent, the 1-chlorocyclopropyl p-tolyl sulfoxides resulted
in the formation of enantiopure allenic alcohols via the Doering–LaFlamme-type rearrangement of enan-
tiopure cyclopropylmagnesium carbenoid intermediates. This is the first example for the asymmetric
synthesis of allenes by the Doering–LaFlamme allene synthesis.
Keywords:
Optically active allene
Asymmetric synthesis
Ó 2011 Elsevier Ltd. All rights reserved.
Magnesium carbenoid
Cyclopropylmagnesium carbenoid
Doering–LaFlamme allene synthesis
1. Introduction
selective rearrangement of enantiopure cyclopropylmagnesium
carbenoids as a key reaction.
Allenes are quite interesting and highly important compounds
in organic, synthetic organic, and bioorganic chemistry.1 Allenyl
structure is frequently found in biologically active natural products
and pharmaceuticals.2 Allenes are characterized by a 1,2-diene
structure and some allenes have axial chirality. Asymmetric syn-
thesis of optically active allenes has received considerable atten-
tion in these days.3 In the several synthetic methods for allenes,
Doering–LaFlamme allene synthesis4 is characterized by the rear-
rangement of cyclopropylidenes (cyclopropylcarbenes)5 usually
generated from gem-dibromocyclopropanes with alkyllithium or
magnesium. Although this name reaction has been known for over
50 years, to the best of our knowledge, the asymmetric synthesis of
allenes through the Doering–LaFlamme allene synthesis has never
been achieved.
Thus, as shown in Scheme 1, addition reaction of
a,b-unsatu-
rated carbonyl compounds 1 with lithium -sulfinyl carbanion of
a
enantiopure (R)-dichloromethyl p-tolyl sulfoxide 2 resulted in
the formation of optically active 1-chlorocyclopropyl p-tolyl sulf-
oxides 3 with high asymmetric induction from the sulfur chiral
center. The carbonyl group of the cyclopropanes 3 was reduced
to alcohols 4, which were treated with i-PrMgCl at 0 °C to give
cyclopropylmagnesium carbenoid intermediates 5. Stereoselective
Doering–LaFlamme-type rearrangement took place from the mag-
nesium carbenoid intermediates 5 to afford enantiopure allenes 6
in good yields. In this Letter, aforementioned chemistry, determi-
nation of the absolute configuration of the products and the stereo-
chemistry of the rearrangement are described.
We also have been interested in the synthesis of allenes by
using our original chemistry of sulfoxide–metal exchange reaction6
and some new synthetic methods were reported.7 In continuation
of our studies for the development of new synthetic method of al-
lenes, we describe herein the first example for the asymmetric syn-
thesis of allenes by the Doering–LaFlamme allene synthesis via the
2. Results and discussion
We previously reported the reaction of
a,b-unsaturated car-
bonyl compounds with -sulfinyl carbanion of dichloromethyl p-
a
tolyl sulfoxide derived from racemic 2 to result in the formation
of 1-chlorocyclopropyl p-tolyl sulfoxide in good yields with high
chiral induction from the sulfur chiral center.8 We carried out this
reaction with enantiopure (R)-29 in order to verify if optically ac-
tive 1-chlorocyclopropyl p-tolyl sulfoxide could be provided by
⇑
Corresponding author. Tel.: +81 3 5228 8272; fax: +81 3 5261 4631.
E-mail address: tsatoh@rs.kagu.tus.ac.jp (T. Satoh).
this reaction. The reaction of
a,b-unsaturated ketone 7 with (R)
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.150

H. Momochi et al. / Tetrahedron Letters 52 (2011) 3016–3019
3017
O
S
Tol
R3
Cl
H
R3
Cl
H
CHCl2
O
H
R2
R1
Reduction
R2
R1
(R)-2
R1
R3
Tol
Tol
S (R)
S
R2
LDA (or LiHMDS)
O
OH
1
O
O
R1 = OEt, alkyl, aryl
i-PrMgCl (5 eq)
4
3
R1 = H, alkyl, aryl
R3
H
OH
R1
H
Cl
R2
R1
R2
ClMg
R3
OMgCl
(R)-6
1 = H, alkyl, aryl
5
R
Scheme 1.
Table 1
Reaction of
the presence of a base at low temperature
-2 was selected as the representative example and the results are
summarized in Table 1.
a,b-unsaturated ketone 7 with (R)-dichloromethyl p-tolyl sulfoxide 2 in
To a mixture of enone 7 and (R)-2 in THF at ꢀ78 °C was added a
solution of sodium hexamethyldisilazide (NaHMDS; 1.2 equiv) and
the reaction mixture was slowly allowed to warm to room temper-
ature.8 This reaction afforded the desired cyclopropane 8 in 77%
yield; however, the optical purity was found to be 59% ee (entry
1). This low optical purity was thought to be due to the racemiza-
tion of the sulfur chiral center of dichloromethyl p-tolyl sulfoxide 2
with a base at low temperature.10 Slight modification of the condi-
tions of this reaction was made next. Thus, to a solution of LDA in
THF at ꢀ78 °C was added a solution of (R)-2 and enone 7, in order,
and the reaction mixture was slowly allowed to warm to room
temperature (entry 2). Significant improvement for the optical pur-
ity of the product 8 was observed. The reaction was started from
ꢀ85 °C with several bases and we found that LDA and LiHMDS
were suitable bases to this reaction (entries 3–6). We decided to
O
CH3
S
Tol
H3C
Cl
H
CHCl2
O
H
(R)-2
CH3
Ph
CH3
Tol
Ph
S (R)
Base (1.2 eq)
THF
CH3 CH3
O
O
7
8
Entry Base
Conditions
8
Diastereomeric ratioa Yield (%) % eeb
1
2
3
4
5
6
NaHMDS ꢀ78 °C to rtc Single isomer
77
80
78
93
96
58
59
94
94
99
99
59
LDA
ꢀ78 °C to rtd 8:1
NaHMDS ꢀ85 °C to rtc Single isomer
LDA
ꢀ85 °C to rtd Single isomer
ꢀ85 °C to rtd Single isomer
ꢀ85 °C to rtc 20:1
O
LiHMDS
KHMDS
S
Determined by 1H NMR.
a
b
c
Tol
CHCl2
Determined by HPLC with chiral stationary column, CHIRALCEL OD.
A solution of the base was added to a solution of 7 and (R)-2.
(R)-2
n-C4H9
Cl
H
d
O
H
To
a
solution of the base was added
a
solution of (R)-2 followed by a
LiHMDS
CH3
OEt
solution of 7.
Tol
EtO
C4H9-n
S (R)
THF, -85 °C to
r.t., 39 %
CH3
CH3
CH3
H
O
H3C
Cl
H
H3C
Cl
O
12
NaBH4
i-PrMgCl (5 eq)
CH3
CH3
Ph
13 (98%ee)
Tol
(S) Ph
Tol
S (R)
EtOH, 0 °C
97%
S
Toluene, 0 °C,
30 min
i-PrMgCl
(5 eq)
n-C4H9
Cl
H
O
OH
NaBH4 (3 eq)
CH3
CH2OH
O
O
9a (1' = S)
9b (1' = R)
Tol
8
THF-MeOH
reflux, 96%
S (R)
Toluene,
0 °C, 10 min
(9a:9b = 10:1)
CH3
O
OH
Ph
14
H3C
Cl
ClMg
H
H
CH3
Ph
OMgCl
n-C4H9
Cl
ClMg
H
H
CH2OH
CH3
H3C
CH3
74%
CH3
CH2OMgCl
CH3
66%
n-C4H9
11 (99% ee)
10
(R)-16
15
[α]D28 = -138.3
[α]D26 -22.4 (c = 0.28, CHCl3)
(lit. 13, [α]D26 -25.1)
(c= 0.26, EtOH)
Scheme 2. Synthesis of enantiopure allene 11 from enantiopure 1-chlorocyclopro-
pyl p-tolyl sulfoxide 8 by reduction followed by treatment with i-PrMgCl via
cyclopropylmagnesium carbenoid intermediate 10.
Scheme 3. Asymmetric synthesis of optically active allene (R)-16 from
unsaturated ester 12 and (R)-2 by the Doering–LaFlamme allene synthesis.
a,b-

3018
H. Momochi et al. / Tetrahedron Letters 52 (2011) 3016–3019
optical purity as a single product;8 however, the chemical yield of
Cl
13 was not satisfactory. The ester group was reduced with NaBH4
in THF–CH3OH12 to give alcohol 14 in quantitative yield. After
the optical purity was improved to 99% by recrystallization, 14
was treated with i-PrMgCl to give optically active allene 16 in
66% yield. Although the optical purity of produced 16 could not
be determined, comparing the sign of the specific rotation of 16
with that of the reported (R)-2-methylocta-2,3-dien-1-ol13 the
absolute configuration of allene 16 was unambiguously deter-
mined to be R.
n-C4H9
Cl
H
ClMg
H
CH3
CH2OMgCl
ClMg
CH2OMgCl
n-C4H9
CH3
15'
anticlockwise rotation
15
H
CH2OH
CH3
-MgCl2
n-C4H9
(R)-16
With both the absolute configuration of the starting material, 1-
chlorocyclopropyl p-tolyl sulfoxide 14, and the produced allene 16
in hand, we propose the stereochemistry of the ring-opening of the
cyclopropylmagnesium carbenoid intermediate 15 as shown in
Scheme 4. As the sulfoxide–magnesium exchange reaction was
proved to take place with retention of the configuration of the car-
bon bearing the sulfinyl group,14 treatment of 14 with i-PrMgCl
afforded magnesium carbenoid intermediate 15. Structure of 15
is rewritten as 150 and the Doering–LaFlamme-type rearrangement
(ring-opening of the cyclopropylmagnesium carbenoid) gave (R)-
16 as shown in Scheme 4. From the correlation of the structure be-
tween 150 and (R)-16, the ring-opening is proved to proceed with
anticlockwise rotation of the carbon–carbon bond between the
carbons bearing the methyl group and the magnesium. This rear-
rangement is thought to be concerted process; as the optical purity
was perfectly retained (see also Table 3).
Scheme 4. A proposed mechanism for the stereochemistry of the rearrangement of
cyclopropylmagnesium carbenoid intermediate 15.
use the conditions mentioned in entries 4 and 5 throughout in this
study. The absolute configuration of the product 8 was proved to
be as shown in Table 1 based on the results in previous study.8
The ketone carbonyl group of 8 was reduced with NaBH4 to give
alcohol 9 in quantitative yield as a 10:1 mixture of two diastereo-
mers (Scheme 2). In order to confirm the absolute configuration
and the stereochemistry of the newly generated hydroxyl group,
the main product 9a was separated and its X-ray analysis was car-
ried out.11 Absolute configuration of 9a was confirmed to be as
shown in Scheme 2 and the absolute configuration of the carbon
bearing the hydroxyl group was determined to be S.
Next, 9a was treated with excess i-PrMgCl in toluene at 0 °C for
30 min to afford the desired allene 11 in 74% yield. Gratifyingly, the
optical purity of 11 was proved to be over 99% ee by HPLC with chi-
ral column. It should be noted that at this stage the absolute ste-
reochemistry (axial chirality) of 11 was unclear. Later, the
absolute configuration of 11 was proved to be R (see below).
Encouraged by this result, we planned the experiment for deter-
mining the absolute configuration of the produced allenes and the
stereochemistry of the rearrangement of the cyclopropylmagne-
sium carbenoid intermediate (Scheme 3). Thus, the reaction of
Next, in order to confirm the generality of the procedure de-
scribed above, the addition reaction of several a,b-unsaturated car-
bonyl compounds with (R)-2 was carried out and the results are
summarized in Table 2. Lithium diisopropylamide was found to
be the suitable base to the reaction of (R)-2 with
a,b-unsaturated
ketones and both chemical yield and enantiomeric excess of 3 were
excellent (entries 1–3). When the reaction was carried out with
,b-unsaturated esters, chemical yield was moderate in one case
(entry 5); however, the optical purity of all the products was found
a
to be excellent (entries 4–6).
a,b-unsaturated ester 12 with (R)-2 was carried out with LiHMDS
Finally, the produced 1-chlorocyclopropyl p-tolyl sulfoxides
to give the desired 1-chlorocyclopropyl p-tolyl sulfoxide 13 in high
3 derived from a,b-unsaturated carbonyl compounds were
Table 2
Reaction of a,b-unsaturated carbonyl compounds 1 with (R)-dichloromethyl p-tolyl sulfoxide 2 in the presence of a base at low temperature
O
S
Tol
R3
Cl
H
CHCl2
O
H
R2
(R)-2
R1
R3
Tol
S (R)
R1
Base (1.2 eq)
R2
O
THF, -85 °C to r.t.
O
1
3
Entry
1
Base
3
R1
R2
R3
Diastereomeric ratioa
Yield (%)
% ee
1
2
Ph
CH3CH2
CH3
CH3
CH3CH2
PhCH3CH2
LDA
LDA
10:1
>99:1
96
94
96b
98b
O
c
3
LDA
>99:1
90
—
Ph
4
5
6
EtO
EtO
EtO
PhCH2CH2
PhCH2CH2
PhCH2CH2
i-Pr
CH3CH2
PhCH2CH2
LDA
>99:1
>99:1
>99:1
97
65
79
98b
99b
95d
LiHMDSe
LiHMDSe
a
Determined by 1H NMR.
Determined by HPLC with chiral stationary column, CHIRALCEL OD.
Could not be obtained by HPLC.
Determined by HPLC with chiral stationary column, CHIRALCEL AD.
1.4 equiv of base was used.
b
c
d
e

H. Momochi et al. / Tetrahedron Letters 52 (2011) 3016–3019
3019
Table 3
Synthesis of enantiopure allenes (R)-6 from optically active 1-chlorocyclopropyl p-tolyl sulfoxides 3 via cyclopropylmagnesium carbenoid intermediates 5
R3
Cl
H
R3
Cl
H
i-PrMgCl (5 eq)
R2
NaBH4 (3 eq)
EtOH, 0 °C
R2
Tol
R1
Tol(O)S
R1
S (R)
Toluene, 0 °C,
30 min
O
OH
O
4
3
R3
Cl
H
OH
R1
R2
R1
H
.
R2
ClMg
R3
OMgCl
5
(R)-6
Entry
3
4
(R)-6
R1
R2
CH3
CH3
R3
Yield (%)
Diastereomeric ratio
Yield (%)
% ee
[a]
D
a
1
2
Ph
CH3CH2
CH3CH2
PhCH2CH2
96
99
10:1
>99:1
72
79
99b
99c
ꢀ144.5
ꢀ74.3
O
3
90
>99:1
76
99c
ꢀ33.3
Ph
d
4
5
OEt
OEt
PhCH2CH2
PhCH2CH2
(CH3)2CH
PhCH2CH2
99
84
—
—
86d
57d
99b
99b
+34.0
+28.5
d
a
b
c
All specific rotations were measured in ethanol at room temperature.
Determined by HPLC with chiral stationary column, CHIRALCEL IA.
Determined by HPLC with chiral stationary column, CHIRALCEL OD-H.
R1 = H.
d
4. (a) Chinoporos, E. Chem. Rev. 1963, 63, 235; (b) Li, J. J. Name Reactions; Springer-
Verlag: Berline, 2002; (c) Hassner, A.; Stumer, C. Organic Syntheses Based on
Name Reactions, 2nd ed.; Pergamon: Amsterdam, 2002.
5. Kirmse, W. Carbene Chemistry; Academic Press: New York, 1971.
6. (a) Satoh, T. The Chemical Record 2004, 3, 329; (b) Satoh, T. Chem. Soc. Rev. 2007,
36, 1561; (c) Satoh, T. In The Chemistry of Organomagnesium Compounds;
Rappoport, Z., Marek, I., Eds.; John Wiley and Sons: Chichester, 2008; pp 717–
769; (d) Satoh, T. Yakugaku Zasshi 2009, 129, 1013.
7. (a) Satoh, T.; Itoh, N.; Watanabe, S.; Koike, H.; Matsuno, H.; Matsuda, K.;
Yamakawa, K. Tetrahedron 1995, 51, 9327; (b) Satoh, T.; Kuramochi, Y.; Inoue,
Y. Tetrahedron Lett. 1999, 40, 8815; (c) Satoh, T.; Sakamoto, T.; Watanabe, M.
Tetrahedron Lett. 2000, 43, 2043; (d) Satoh, T.; Kurihara, T.; Fujita, K.
Tetrahedron 2001, 57, 5369; (e) Satoh, T.; Hanaki, N.; Kuramochi, Y.; Inoue,
Y.; Hosoya, K.; Sakai, K. Tetrahedron 2002, 58, 2533; (f) Satoh, T.; Sakamoto, T.;
Watanabe, M.; Takano, K. Chem. Pharm. Bull. 2003, 51, 966; (g) Satoh, T.; Gouda,
Y. Tetrahedron Lett. 2006, 47, 2835; (h) Satoh, T.; Noguchi, T.; Miyagawa, T.
Tetrahedron Lett. 2008, 49, 5689.
recrystallized to give enantiopure 3. The enantiopure 3 were
reduced with NaBH4 and the resultant alcohols 4 were treated with
i-PrMgCl to give optically pure (R)-allenes 6 in up to 86% yield
(Table 3).
In conclusion, asymmetric synthesis of allenes from a,b-unsat-
urated carbonyl compounds with optically pure dichloromethyl
p-tolyl sulfoxide 2 in up to three steps via the rearrangement of
enantiopure cyclopropylmagnesium carbenoids was established.
This is the first example for the asymmetric synthesis of allenes
by the Doering–LaFlamme allene synthesis. The results presented
herein will contribute greatly to the asymmetric synthesis of al-
lenes and to the chemistry of magnesium carbenoids.
8. Miyagawa, T.; Tatenuma, T.; Tadokoro, M.; Satoh, T. Tetrahedron 2008, 64, 5279.
9. Noguchi, T.; Miyagawa, T.; Satoh, T. Tetrahedron: Asymmetry 2009, 20, 2073.
10. Satoh, T.; Momochi, H.; Noguchi, T. Tetrahedron: Asymmetry 2010, 21, 382.
11. Crystal data for Ent-9a (the X-ray crystallographic analysis was carried out with
enantiomer of 9a derived from (S)-2): C21H25ClO2S, M = 376.92, Orthorhombic,
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Re-
search No. 19590018 and 22590021 from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, and TUS
Grant for Research Promotion from Tokyo University of Science,
which is gratefully acknowledged.
P212121
V = 2027.19(19) Å3,
(MoK
) = 3.02 cmꢀ1, T = 293 K, Radiation = 0.71073 Å, R1 = 0.0405 for I >2.0
(#19),
a = 7.4258(4) Å,
Z = 4, F(0 0 0) = 800,
b = 12.5418(7) Å,
c = 21.7666(12) Å,
Dcalcd = 1.235 g cmꢀ3
,
l
a
r
(I), wR2 = 0.1059 for all data (4621 reflections), GOF = 1.077 (231 parameters),
crystal dimensions 0.20 ꢁ 0.20 ꢁ 0.10 mm3, absolute structure parameter
0.02(6). The single crystals were mounted on glass fibers. Diffraction data
were measured on a Bruker APEX CCD-detector X-ray diffractometer with
References and notes
monochromated MoKa radiation from a rotating anode source apparatus. The
1. (a) Some monographs and reviews concerning chemistry, synthesis, and
synthetic uses of allenes: The Chemistry of Ketenes, Allenes, and Related
Compounds, Part 1 and 2; Patai, S., Ed.; John Wiley and Sons: Chichester,
1980; (b) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes and
Cumulenes, A Laboratory Manual; Elsevier: Amsterdam, 1981; (c) Schuster, H.
F.; Coppola, G. M. Allenes in Organic Synthesis; John Wiley and Sons: New York,
1984; (d) Pasto, D. J. Tetrahedron 1984, 40, 2805; (e) Zimmer, R. Synthesis 1993,
165; (f) Sydnes, L. K. Chem. Rev. 2003, 103, 1133; (g) Miesch, M. Synthesis 2004,
746; (h) Krause, N.; Hoffmann-Roder, A. Tetrahedron 2004, 60, 11671; (i) Ma, S.
Chem. Rev. 2005, 105, 2829; (j) Ma, S. Aldrichim. Acta 2007, 40, 91; (k)
Brummond, K. M.; DeForrest, J. E. Synthesis 2007, 795; (l) Miura, T.; Murakami,
M. J. Synth. Org. Chem. Jpn. 2010, 68, 745.
absolute structure of Ent-9a is determined by the comparison with the
parameter (1.03(10)) of the inversion symmetry structure because the crystal
is constructed from all light atoms. The data reduction, structure solution and
refinement, and all the necessary computational data processes were
performed using APEX, SAINT, SHELXTL programs. Crystallographic data
excluding structures have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication numbers CCDC
813610 for Ent-9a. A copy of the data can be obtained free of charge from
CCDC, 12 Union road, Cambridge CB2 1EZ. UK [DIRECT LINE: +44 1223 762910,
fax: +44 (0) 1223-336033 or e-mail: deposit@ccdc.cam.ac.uk.
12. Soai, K.; Oyamada, H.; Takase, M.; Ookawa, A. Bull. Chem. Soc. Jpn. 1984, 57,
1948.
2. Hoffmann-Roder, A.; Krause, N. Angew. Chem., Int. Ed. 2004, 43, 1196.
3. Some selected recent reviews concerning the asymmetric synthesis of allenes:
(a) Araki, Y.; Konoike, T. J. Synth. Org. Chem. Jpn. 2000, 58, 956; (b) Ogasawara,
M. J. Synth. Org. Chem. Jpn. 2008, 66, 100; (c) Ogasawara, M.; Watanabe, S.
Synthesis 2009, 1761; (d) Ogasawara, M. Tetrahedron: Asymmetry 2009, 20, 259;
(e) Ruano, J. L. G.; Marcos, V.; Aleman, J. Synthesis 2009, 3339.
13. Ma, S.; Yu, F.; Gao, W. J. Org. Chem. 2003, 68, 5943.
14. (a) Satoh, T.; Kobayashi, S.; Nakanishi, S.; Horiguchi, K.; Irisa, S. Tetrahedron
1999, 55, 2515; (b) Hoffmann, R. W.; Holzer, B.; Knopff, O.; Harms, K. Angew.
Chem., Int. Ed. 2000, 39, 3072; (c) Satoh, T.; Matsue, R.; Fujii, T.; Morikawa, S.
Tetrahedron 2001, 57, 3891; (d) Hoffmann, R. W. Chem. Soc. Rev. 2003, 32, 225.