A synthesis of esters, amides, and sulfones bearing a 1-cyclopentenyl group at the α-position from cyclobutanones with one-carbon ring-expansion

Article abstract of DOI:10.1016/j.tetlet.2009.02.053

Treatment of 1-chlorovinyl p-tolyl sulfoxides, derived from cyclobutanones and chloromethyl p-tolyl sulfoxide, with lithium enolate of carboxylic acid tert-butyl esters, lithium enolate of carboxylic acid N,N-dimethylamides, or lithium α-carbanion of alky

Full text of DOI:10.1016/j.tetlet.2009.02.053

Tetrahedron Letters 50 (2009) 1961–1964
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Tetrahedron Letters
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A synthesis of esters, amides, and sulfones bearing a 1-cyclopentenyl group
at the a-position from cyclobutanones with one-carbon ring-expansion
a
a
a
b
b
Tsuyoshi Satoh a, , Yu Awata , Shingo Ogata , Shimpei Sugiyama , Masami Tanaka , Motoo Tori
*
a Department of Chemistry, Faculty of Science, Tokyo University of Science, Ichigaya-funagawara-machi 12, Shinjuku-ku, Tokyo 162-0826, Japan
b Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cho, Tokushima 770-8514, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of 1-chlorovinyl p-tolyl sulfoxides, derived from cyclobutanones and chloromethyl p-tolyl
sulfoxide, with lithium enolate of carboxylic acid tert-butyl esters, lithium enolate of carboxylic acid
Received 24 December 2008
Revised 6 February 2009
Accepted 9 February 2009
Available online 12 February 2009
N,N-dimethylamides, or lithium
adducts were treated with isopropylmagnesium chloride or ethylmagnesium chloride in dry toluene to
give esters, amides, and sulfones bearing a 1-cyclopentenyl group at the -position in moderate to good
a-carbanion of alkyl phenyl sulfones gave adducts in high yields. The
a
yields with one-carbon ring-expansion via magnesium carbenoid 1,2-CC insertion reaction. The magne-
sium carbenoid 1,2-CC insertion reaction proved to be highly stereospecific. The reaction mechanism and
origin of the specificity are described.
Keywords:
Cyclopentene
Sulfoxide-magnesium exchange
Magnesium carbenoid
1,2-CC insertion
Ó 2009 Elsevier Ltd. All rights reserved.
One-carbon ring-expansion
Carbenes and carbenoids are obviously one of the most interest-
ing highly reactive carbon species and are frequently used as ver-
satile intermediates in organic synthesis.1 Carbenes and
carbenoids show a variety of reactions, such as addition into a car-
bon–carbon double bond to give cyclopropanes, dimerization to
give olefins, and ylide formation with sulfides. Rearrangement
and insertion are other most striking reactions of carbenes and
carbenoids.2 Insertion reaction is very interesting and highly useful
for construction of molecules, because the reaction enables the for-
mation of a carbon–carbon bond between a carbene (or carbenoid)
carbon and a non-activated carbon or enables the formation of
olefins.
from 1-chlorovinyl p-tolyl sulfoxide 5, with i-PrMgCl and found
that the reaction resulted in the formation of an ester bearing a
1-cyclopentenyl group at the
a
-position 8 (X=COOtBu) in good
yields (Scheme 1). Obviously, 1,2-CC insertion reaction, instead of
the expected 1,3-CH insertion reaction, took place from magne-
sium carbenoid 7. As we recognized that this reaction is very useful
for synthesis of cyclopentenes bearing various functionalities at
the olefinic carbon 8 by assemblage of three components, cyclobu-
tanones, chloromethyl p-tolyl sulfoxide, and nucleophiles
(LiC(R2)HX), we further investigated this reaction. We report
herein a synthesis of esters, amides, and sulfones bearing a
1-cyclopentenyl group at the
a-position 8 from cyclobutanones
We recently reported a synthesis of bicyclo[n.1.0]alkanes 4 by
1,3-CH insertion reaction of magnesium carbenoid 3 as a key reac-
tion.3 Thus, 1-chlorovinyl p-tolyl sulfoxides 1 (n is one or over)
were synthesized from cyclic ketones with chloromethyl p-tolyl
sulfoxide. Addition reaction of 1 with lithium enolate of tert-butyl
acetate gave adducts 2 in high yields. Treatment of 2 with i-PrMgCl
in toluene resulted in the formation of magnesium carbenoid 3 via
the sulfoxide-magnesium exchange reaction. Magnesium carbe-
noid 1,3-CH insertion reaction of 3 took place smoothly to afford
4 in quantitative yields (Scheme 1).3
with one-carbon ring-expansion via 1-chlorovinyl p-tolyl sulfox-
ides 5 and magnesium carbenoids 7.
At first, 1-chlorovinyl p-tolyl sulfoxide 10 was synthesized from
cyclobutanone 94 with chloromethyl p-tolyl sulfoxide in good
overall yield (Table 1).5 Addition reaction of 10 with lithium eno-
late of tert-butyl acetate was carried out in the same manner as de-
scribed before6 to give the desired adduct 11 in a quantitative
yield. Finally, the adduct 11 was treated with 2 equiv of i-PrMgCl
in toluene at À78 °C and the temperature of the reaction mixture
was slowly allowed to warm to 0 °C for 2 h (Table 1, entry 1).
Although significant amount of the starting material remained,
the reaction mixture was rather clean and a product was obtained.
Initially, we expected that the product was bicyclo[2.1.0]pentane
derivative 13 (the product from the 1,3-CH insertion reaction of
magnesium carbenoid intermediate);3 however, the product had
an olefinic hydrogen (1H NMR spectrum; d 5.38, 1H, br s). Finally,
In continuation of our interest in the chemistry of magnesium
carbenoid 1,3-CH insertion reaction, we further investigated the
reaction mentioned above by using adduct 6 (X = COOtBu), derived
* Corresponding author. Fax: +81 3 52614631.
E-mail address: tsatoh@rs.kagu.tus.ac.jp (T. Satoh).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.053

1962
T. Satoh et al. / Tetrahedron Letters 50 (2009) 1961–1964
1,3-CH
insertion
COOtBu
COOtBu
LiCH2COOtBu
COOtBu
MgCl
Cl
i-PrMgCl
( )n
( )n
( )n
S(O)Tol
( )n
S(O)Tol
Cl
H
Cl
1
4
3
2
R2
R2
R2
R2
1,2-CC
insertion
R3MgCl
X
X
LiCHX
Cl
X
R1
R1
MgCl
R1
S(O)Tol
S(O)Tol
R1
Cl
Cl
6
5
7
8
X= COOtBu, CON(CH3)2, SO2Ph
Scheme 1.
Table 1
Optimization of the yield of this reaction was investigated and
A synthesis of tert-butyl acetate bearing a 1-cyclopentenyl group at the
from cyclobutanone 9 with one-carbon ring-expansion
a-position 12
the results are summarized in Table 1. Increasing the amount of
i-PrMgCl to 3 equiv resulted in 54% yield; however, some amount
of the starting material 11 still remained (entry 2). Finally, by the
use of 5 equiv of i-PrMgCl, all the starting material was consumed
and the product 12 was obtained in 73% yield (entry 3).7 Use of
higher reaction temperature gave worse results (entries 4 and 5).
Selection of THF as a solvent gave a similar result (compare the re-
sults in entries 3 and 6). Dichloromethane as the solvent gave mis-
erable result (entry 7). We concluded that the conditions in entry 3
are suitable for the reaction.
TolS(O)CH2Cl
Cl
EtO
EtO
EtO
EtO
O
in three steps
73%
S(O)Tol
9
10
COOtBu
S(O)Tol
LiCH2COOtBu
i-PrMgCl
Conditions
EtO
EtO
THF, -78 °C
quant.
Cl
11
Other two examples of this procedure are shown in Scheme 2.
Thus, 1-chlorovinyl p-tolyl sulfoxide 14, derived from cyclobuta-
none, was treated with lithium enolate of tert-butyl acetate to give
adduct 15 in a quantitative yield. 1-Chlorovinyl p-tolyl sulfoxide 17
having an acetal group gave adduct 18 without any problem. Both
adducts 15 and 18 were treated with 5 equiv of i-PrMgCl to give
the expected tert-butyl acetate bearing a 1-cyclopentenyl group
COOtBu
EtO
EtO
COOtBu
EtO
EtO
13
12
Entry
i-PrMgCl (equiv)
Conditions
12
at the a-position, 16 and 19, respectively, in about 80% yield.
Temp (°C)
Time (h)
Solvent
Yield (%)
Quite interesting results were obtained from the reaction
starting from unsymmetrical cyclobutanone (Scheme 3). Unsym-
metrical cyclobutanone, 2-(2-phenylethyl)cyclobutanone, 20 was
synthesized from cyclopropyl phenyl sulfide according to Piras’
method.8 1-Chlorovinyl p-tolyl sulfoxide 21 was synthesized from
20 with chloromethyl p-tolyl sulfoxide in three steps in 42% overall
yield as a mixture of two geometrical isomers, which were sepa-
rated by silica gel column chromatography to give E-isomer 21a
and Z-isomer 21b in a ratio of 21:79.9
1
2
3
4
5
6
7
2
3
5
5
5
5
5
À78ꢀ0
À78ꢀ0
À78ꢀ0
À40ꢀ0
0
2
2
2
1
0.5
2
2
Toluene
Toluene
Toluene
Toluene
Toluene
THF
35a
54a
73
51
54
70
À78ꢀ0
À78ꢀ0
CH2Cl2
45
a
Some amount of the starting material 11 was recovered.
Treatment of both vinyl sulfoxides 21a and 21b with lithium
enolate of tert-butyl acetate afforded adducts 22a and 22b, respec-
tively, as a single isomer in good yields. Relative stereochemistry of
the adducts was determined to be as shown in Scheme 3 based on
the previous study.10 The sulfoxide-magnesium exchange reaction
of 22a was, at first, carried out with i-PrMgCl; however, no reaction
was observed. Steric hindrance by the 2-phenylethyl group on the
the structure of the product was determined to be tert-butyl ace-
tate bearing 1-cyclopentene moiety 12 on the bases of IR, NMR,
and MS spectral data. Obviously, the product was produced by
magnesium carbenoid 1,2-CC insertion reaction. To the best of
our knowledge, this is the first example of one-carbon ring-expan-
sion by the magnesium carbenoid 1,2-CC insertion reaction.
COOtBu
LiCH2COOtBu
99%
COOtBu
S(O)Tol
i-PrMgCl (5 eq)
Cl
Toluene, -78 ~ 0 °C
77%
S(O)Tol
Cl
15
14
16
CH3
O
LiCH2COOtBu
99%
COOtBu
S(O)Tol
H3C O
H3C O
i-PrMgCl (5 eq)
Cl
COOtBu
H3C
O
Toluene, -78 ~ 0 °C
80%
S(O)Tol
Cl
18
17
19
Scheme 2.

T. Satoh et al. / Tetrahedron Letters 50 (2009) 1961–1964
1963
CH2CH2Ph
Cl
CH2CH2Ph
TolS(O)CH2Cl
3
O
In three steps
42%
S(O)Tol
20
21
( E : Z = 21 : 79 )
O
OtBu
CH2CH2Ph
COOtBu
CH2CH2Ph
PhCH2CH2
Cl
LiCH2COOtBu
S(O)Tol
EtMgCl (5 eq)
S
THF, -78 °C
74%
Toluene, -78 ~ 0 °C
40%
To l
Cl
S*
23
O
21a
22a
PhCH2CH2
O
CH2CH2Ph
Cl
CH2CH2Ph
OtBu
EtMgCl (5 eq)
LiCH2COOtBu
THF, -78 °C
Toluene, -78 ~ 0 °C
61%
S(O)Tol
COOtBu
Cl
S
Tol
81%
S*
24
21b
O
22b
Scheme 3.
cyclobutane ring was thought to be the reason for this difficulty.
Fortunately, 22a reacted smoothly with EtMgCl to afford the de-
sired ester bearing a 1-cyclopentene moiety 23 in 40% yield as a
single product. The same treatment of the diastereomer 22b with
EtMgCl gave the structural isomer 24 in 61% yield again as a single
product. Quite interestingly, these reactions are highly stereospe-
cific. The structure of two products, 23 and 24, was determined
by COSY, HSQC, HMBC, and NOESY spectra.11
would occupy the equatorial position. From this intermediate, the
1,2-CC insertion takes place from behind of the carbon–chlorine
bond to give the product 23. The situation of the reaction of 22b
with EtMgCl is thought to be the same. The magnesium carbenoid
intermediate derived from 22b must make the six-membered
intermediate B and again the 1,2-CC insertion reaction proceeds
from behind of the carbon–chlorine bond to afford 24.
Additional examples for the synthesis of tert-butyl esters bear-
The mechanism of this interesting and highly stereospecific
magnesium carbenoid 1,2-CC insertion reaction can be explained
as follows (see Scheme 4). As the sulfoxide-magnesium exchange
reaction is known to proceed with retention of the configuration
of the carbon bearing the sulfinyl group,12 treatment of 22a with
ing 1-cyclopentenel moiety at the a-position and further extension
of this procedure to a synthesis of amides and sulfones are summa-
rized in Table 2. The addition reaction of 14 with tert-butyl propi-
onate and tert-butyl (4-methylphenyl)acetate gave adducts 25a
and 25b, respectively, in quantitative yields. Treatment of the ad-
ducts with 5 equiv of i-PrMgCl gave the expected products 26a
and 26b, in good to excellent yields (entries 1 and 2). The addition
reaction of 14 with lithium enolate of carboxylic acid N,N-dimeth-
ylamides13 gave adducts 25c and 25d both in quantitative yields.
*
EtMgCl gives magnesium carbenoid having R -configuration at
the carbon bearing the magnesium atom. The magnesium and car-
bonyl oxygen atom of the tert-butyl ester group must make six-
membered intermediate A, in which the bulkiest tert-butoxy group
O
CH2CH2Ph
OtBu
COOtBu
S*
PhCH2CH2
PhCH2CH2
OtBu
EtMgCl (5 eq)
Mg
O
Cl
R*
Toluene, -78 ~ 0 °C
Cl
S
To l
S*
Cl
23
A
O
22a
PhCH2CH2
PhCH2CH2
O
CH2CH2Ph
OtBu
R*
EtMgCl (5 eq)
OtBu
O
Toluene, -78 ~ 0 °C
R*
Mg
COOtBu
Cl
S
To l
S*
Cl
24
Cl
O
22b
B
Scheme 4.

1964
T. Satoh et al. / Tetrahedron Letters 50 (2009) 1961–1964
Table 2
Synthesis of esters, amides, and sulfones bearing a 1-cyclopentenyl group at the
a
-position 26 from 1-chlorovinyl p-tolyl sulfoxide 14 derived from cyclobutanone
R2
R2
i-PrMgCl (5 eq)
Nucleophile
Cl
X
X
Toluene, -78 ~ 0 °C
S(O)Tol
Conditions
S(O)Tol
Cl
14
25
26
Entry
Nucleophile
Conditions
25
26
Yield (%)
R2
X
Yield (%)
70
1
2
CH3CH2COOtBu
LDA, THF, À78 ꢀ À30 °C, 1 ha
LDA, THF,À78 °C, 15 minb
25a 99c
CH3
COOtBu
COOtBu
CH3
CH2COOtBu
CH3
25b 99
92
3
4
5
6
CH3CON(CH3)2
CH3CH2CON(CH3)2
CH3SO2Ph
LDA, THF,À78 °C, 15 mina
LDA, THF,À78 °C, 15 minb
LDA, THF,À78 °C, 15 mina
LDA, THF,À78 °C, 15 minb
25c 99d
25d 99e
25e 87
25f 60f
H
CH3
H
CON(CH3)2
CON(CH3)2
SO2Ph
77
81
78
96
CH3CH2SO2Ph
CH3
SO2Ph
a
Five equivalents of the nucleophile was used.
Ten equivalents of the nucleophile was used.
A 8:1 mixture of two diastereomers.
A 15:1 mixture of two diastereomers.
A 2:1 mixture of two diastereomers.
A 15:1 mixture of two diastereomers.
b
c
d
e
f
Beckwith, R. E. J. Chem. Rev. 2003, 103, 2861; (d) Davies, H. M. L.; Loe, O. Synthesis
2004, 2595; (e) Davies, H. M. L. Angew. Chem., Int. Ed. 2006, 45, 6422.
3. (a) Satoh, T.; Ogata, S.; Wakasugi, D. Tetrahedron Lett. 2006, 47, 7249; (b) Ogata,
S.; Saitoh, H.; Wakasugi, D.; Satoh, T. Tetrahedron 2008, 64, 5711.
Treatment of 25c and 25d with i-PrMgCl resulted in the formation
of amides bearing a 1-cyclopentenyl group at the
around 80% yields (entries 3 and 4).
a-position
4. Michejda, C. J.; Comnick, R. W. J. Org. Chem. 1975, 40, 1046.
The addition reaction of 14 with lithium a-sulfonyl carbanions
5. (a) Kawashima, T.; Kashima, H.; Wakasugi, D.; Satoh, T. Tetrahedron Lett. 2005,
46, 3767; (b) Kashima, H.; Kawashima, T.; Wakasugi, D.; Satoh, T. Tetrahedron
2007, 63, 3953.
afforded the expected adducts 25e and 25f in somewhat lower
yields (entries 5 and 6). 1,2-CC insertion reaction of the magnesium
carbenoid generated from the adducts with i-PrMgCl proceeded
smoothly to give the desired sulfones bearing 1-cyclopentenyl
group at the a-position 26e and 26f in good to quantitative yields.
In conclusion, we have developed a new method for a synthesis
of tert-butyl carboxylates, carboxylic acid N,N-dimethyl amides,
6. Sugiyama, S.; Satoh, T. Tetrahedron: Asymmetry 2005, 16, 665.
7. tert-Butyl [4,4-bis(ethoxymethyl)cyclopent-1-enyl]acetate 12. To a solution of
LDA (1.3 mmol) in 0.6 mL of dry THF in a flame-dried flask under argon
atmosphere was added tert-butyl acetate (0.18 mL; 1.3 mmol) at À78 °C with
stirring. After the solution was stirred for 10 min, a solution of vinyl sulfoxide
10 (100 mg; 0.26 mmol) in THF (0.7 mL) was added. The reaction mixture was
stirred for 15 min and the reaction was quenched by adding saturated aq.
NH4Cl and the whole was extracted with CHCl3. The organic layer was dried
over MgSO4 and concentrated in vacuo. The product was purified by silica gel
column chromatography to afford 11 (123 mg; 99%) as a colorless oil; IR (neat)
2976, 2932, 2870, 1723 (CO), 1368, 1256, 1154, 1107 (COC), 1055 (SO), 811,
and sulfones bearing a 1-cyclopentenyl group at the
a-position
by assemblage of three components, cyclobutanones, chloromethyl
p-tolyl sulfoxide, and carboxylic acid derivatives and sulfones with
magnesium carbenoid 1,2-CC insertion reaction as the key reac-
tion. We believe that the magnesium carbenoid 1,2-CC insertion
reaction presented herein will be used widely in the one-carbon
ring-expansion of cyclobutane derivatives to cyclopentenes.
755 cmÀ1 1H NMR d 1.12 (3H, t, J = 7.1 Hz), 1.19 (3H, t, J = 7.0 Hz), 1.49 (9H, s),
;
2.20–2.29 (3H, m), 2.42 (3H, s), 2.66 (1H, d, J = 13.8 Hz), 2.96 (1H, d, J = 15.0 Hz),
3.05 (1H, d, J = 15.0 Hz), 3.36–3.41 (4H, m), 3.46–3.50 (4H, m), 5.42 (1H, s), 7.31
(2H, d, J = 8.3 Hz), 7.71 (2H, d, J = 8.3 Hz). MS m/z (%) 472 (M+, 0.7), 399 (34),
277 (59), 241 (15), 195 (47), 185 (27), 149 (32), 137 (100), 91 (28). Calcd for
C24H37ClO5S: M, 472.2050. Found: m/z 472.2050.
To a flame-dried flask under argon atmosphere was added dry toluene (0.2 mL)
followed by i-PrMgCl (in ether; 0.39 mmol; 5 equiv) at À78 °C. A solution of 11
(37 mg; 0.078 mmol) in toluene (0.2 mL) was added dropwise to the solution of
the Grignard reagent with stirring, and the reaction mixture was slowly
allowed to warm to 0 °C for 2 h. The reaction was quenched by adding
saturated aq. NH4Cl and the whole was extracted with CHCl3. The organic layer
was dried over MgSO4 and concentrated in vacuo. The product was purified by
silica gel column chromatography to afford 17 mg (73%) of ester 12 as colorless
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Re-
search No. 19590018 from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, and TUS Grant for Research
Promotion from Tokyo University of Science, which are gratefully
acknowledged.
oil. IR (neat) 2977, 2853, 1731 (CO), 1369, 1256, 1146, 1111 (COC) cmÀ1 1H
;
NMR d 1.17 (6H, t, J = 7.0 Hz), 1.44 (9H, s), 2.20 (4H, s), 2.97 (2H, s), 3.33 (4H, s),
3.49 (4H, q, J = 7.0 Hz), 5.38 (1H, br s). MS m/z (%) 298 (M+, 9), 242 (15), 196
(16), 183 (12), 152 (10), 150 (55), 137 (48), 105 (43), 91 (55), 57 (100). Calcd for
C17H30O4: M, 298.2144. Found: m/z 298.2148.
References and notes
8. Bernard, A. M.; Frongia, A.; Secci, F.; Piras, P. P. Chem. Commun. 2005, 3853.
9. The geometry of the two compounds (21a and 21b) was determined from the
chemical shift of the hydrogen on the carbon bearing the 2-phenylethyl group;
the hydrogen cis to a sulfinyl group always shows lower d value in 1H NMR
compared to that trans to a sulfinyl group. (a) Satoh, T.; Kaneko, Y.; Yamakawa,
K. Bull. Chem. Soc. Jpn. 1986, 59, 2463; (b) Satoh, T.; Takano, K.; Ota, H.; Someya,
H.; Matsuda, K.; Koyama, M. Tetrahedron 1998, 54, 5557.
10. The face selectivity of the Michael addition followed by the protonation is
dominated by the sulfur chiral center (Ogata, S.; Masaoka, S.; Sakai, K.; Satoh, T.
Tetrahedron Lett. 2007, 48, 5017). The relative stereochemistry of the 2-
phenylethyl group was not determined.
1. Some monographs and reviews with regard to the chemistry of carbenes and
carbenoids: (a) Kirmse, W. Carbene Chemistry; Academic: New York, 1971; (b)
Dorwald, F. Z. Metal Carbenes in Organic Synthesis; Wiley-VCH: Weinheim,
1999; (c)Carbene Chemistry; Bertrand, G., Ed.; Marcel Dekker: New York, 2002;
(d) Kobrich, G. Angew. Chem., Int. Ed. Engl. 1972, 11, 473; (e) Stang, P. J. Chem.
Rev. 1978, 78, 383; (f) Burke, S. D.; Grieco, P. A. Org. React. 1979, 26, 361; (g)
Schaefer, H. F., III Acc. Chem. Res. 1979, 12, 288; (h) Wynberg, H.; Meijer, E. W.
Org. React. 1982, 29, 1; (i) Taylor, K. G. Tetrahedron 1982, 38, 2751; (j) Oku, A.;
Harada, T. J. Syn. Org. Chem. Jpn. 1986, 44, 736; (k) Oku, A. J. Syn. Org. Chem. Jpn.
1990, 48, 710; (l) Adams, J.; Spero, D. M. Tetrahedron 1991, 47, 1765; (m)
Padwa, A.; Weingarten, M. D. Chem. Rev. 1996, 96, 223; (n) Zaragoza, F.
Tetrahedron 1997, 53, 3425; (o) Kirmse, W. Angew. Chem., Int. Ed. 1997, 36,
1164; (p) Braun, M. Angew. Chem., Int. Ed. 1998, 37, 430; (q) Mehta, G.;
Muthusamy, S. Tetrahedron 2002, 58, 9477; (r) Muller, P. Acc. Chem. Res. 2004,
37, 243; (s) Knorr, R. Chem. Rev. 2004, 104, 3795.
11. Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of
Organic Compounds, 7th ed.; John Wiley and Sons: Hoboken, 2005.
12. (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.
13. Satoh, T.; Kamide, Y.; Sugiyama, S. Tetrahedron 2004, 60, 11805.
2. Some reviews with regard to the CH-insertion and the CC-insertion reaction: (a)
Padwa, A.; Krumpe, K. E. Tetrahedron 1992, 48, 5385; (b) Sulikowski, G. A.; Cha, K.
L.; Sulikowski, M. M. Tetrahedron: Asymmetry 1998, 9, 3145; (c) Davies, H. M. L.;