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Organosilyl anions
Chapter 11
Organosilyl anions
The preparation and fundamental chemistry 1 compounds has been well reviewed up to 1970.
of
organosilyl
metallic
11.1 Preparation The most flexible route to organosilyl anions utilizes organodisilanes, which suffer Si-Si bond fission when treated with an alkali metal, an alkali metal hydride, or a good nucleophile for silicon (Scheme 11.1). R 3S i M
R 3S i M
+
R 3S i N u
Scheme 11.1 2 3
Paradoxically, the requisite disilanes are o b t a i n e d ' by Wurtz coupling of an organohalogenosilane (Scheme 11.2). The first cleavage process, utilizing an alkali metal, requires at least one of the R groups to be aryl; within this limitation, a reasonable range (1), (2), (3), 4 5 (4), of triorganosilyl metals can be p r e p a r e d ' , often in good yield. 2R,SiCl
+ 2M
THF or EtoO ^ - > (HMPA)
R-J Si - SiR-* 4- 2ΜΧ
e.g.
2 M e 30S i C l + 2l_i
THF reflux,8h. Scheme 11.2
134
M e ^ S i - S i M e *0
97%
6
Preparation P h 3S i N a
68%
Ph 2MeSil_i
(1)
135
74%
(2)
P h 3S i L i
79%
PhMe 2SiLi
(3)
47%
(4)
Trialkylsilyl-metals cannot be obtained in this manner, but are available by 6 metal-metal exchange of disilylmercurials with lithium (Scheme 11.3). The dangers inherent in handling volatile mercury c o m p o u n d s make this route somewhat less than attractive. ( R 3S i ) 2H g
+
2Li
2 R 3S i L i
+
Hg
R = Me, Et Scheme 11.3 7
The second cleavage process employs either sodium or potassium hydride, and is well suited to the preparation of trialkylsilyl-metals (Scheme 11.4). An additional advantage is that if the triorganosilyl hydride is readily available, then cleavage of the S i - Η b o n d occurs under similar conditions. Either way, solutions of triorganosilyl-metals are formed in good yield and free from any by-products. M e 53 Si - S i M e 3
+
2MH5
DM Ε or HMPA
-, M e 3S i H
M e 3S i M +
r
2 Me^SiM
Μ = Να,Κ R 53 SiH
+
KH
PME or HMPA
3
Scheme 11.4
R = Et,Ph
The third process, that of cleaving a disilane with a good nucleophile for silicon, also gives access to a good range of trialkylsilyl-metals (Scheme 11.5) 8 and other more highly functionalized derivatives , when the presence of H M P A is not required. M e 3S i - S i M e 3
+
HMPA
MOMe
> - M e 3S i M
+
M e 3S i 0 M e
Μ = Να ,Κ ( M e O U S i - Si(OMe)? Me
+
THF
NaOMe
Me
> - ( M e 0 ) 2S i N a
. Λ _ Λ Scheme 11.5
+
(MeO)^SiMe
Me e
9
Trimethylsilyl-lithium itself is obtained by cleavage of hexamethyldisilane with methyl-lithium in H M P A , when a deep red solution of the reagent results (Scheme 11.6). M e 3, S i
SiMe, +
MeLi
HMPA 3
0 °C,5min Scheme 11.6
Me,SiLi
+
3
M e 4S i
4
136 Organosilyl anions A rather more specialized situation is seen in the cases of organopenta10 fluorosilicates. Catalysed a d d i t i o n of trichlorosilane to terminal alkenes and alkynes, followed by the addition of an aqueous solution of potassium 11 fluoride, produces highly reactive organopentafluorosilicates (Scheme 11.7), discussed further on p . 138. i 2 -
r
Scheme 11.7
11.2 Reactions Organosilyl anions are strong bases and good nucleophiles, and must be generated and reacted in aprotic media. 11.2.1 Alkylation This is generally successful with primary and secondary alkyl chlorides. Bromides and iodides give very poor yields, owing to competing electron transfer processes, the major product frequently being the coupled disilane 12 when H M P A is solvent . In D M E or T H F plus 18-crown-6, the dominant process becomes one of bimolecular nucleophilic substitution (Scheme 11.8), 7 and tetrasubstituted silanes can be obtained in excellent yields . Ph 3SiK M e3, S i N a
-I- P h C H 2C l +
n - C 12H 0 0 5-( 2B r
— THF 1
18-crown-6 Scheme 11.8
P h 3S i C H 2P h
70%
M e , S i ( C H - L C3H ,
75%
11.2.2 Reaction with carbonyl compounds This is a good route to silyl carboxylic acids, by reaction with C 0 2 , and to 13 silylmethanols by reaction with aliphatic aldehydes or ketones (Scheme 11.9). 1 R 3S i C 0 2H
1
R 3S i M
2
R C0R
3
1
2
R ,SiCR R Scheme 11.9
3
I OH
3
Reactions
137 1 4 51
With aromatic aldehydes and ketones, the initial adducts u n d e r g o Brook rearrangement (Chapter 5), and silyl ethers are isolated (Scheme 11.10). R 3S i M
+
ArCOAr
R 3S i C A r 2
- Q ^ A r , ' 2C H 0 S i M e 3
Scheme 11.10
Trimethylsilyl-lithium reacts with a/?-unsaturated ketones to give products 9 16 of exclusive 1,4-addition (Scheme 11.11). That these are kinetic products was demonstrated by separate generation of a product (5) of 1,2-addition; this species (5) proved to be completely stable under the conditions used for conjugate addition.
4-
a
T H F , HMPA
Me,SiLi
/
\ I
SiMe3
1.Me 3Snl_i
n e 0SiM 3 Bu Li
M e 3S n
y v
0SiMe3
SiMe7 (5)
Scheme 11.11
With cyclic a/?-unsaturated ketones, a strong preference for axial addition 17 is observed, as is a susceptibility to steric hindrance. A d d i t i o n of copper(I) iodide forms a silyl cuprate species, and allows higher reaction temperatures
SiMe 2Ph 0 P h 3S i L i
1 . Cul
2 RC0CI Scheme 11.12
R
SiPh3
75-90%
R = Me, Et, B u
f
138
Organosilyl anions
to be employed. This minimizes the effects of steric hindrance, and a wide range of a/?-unsaturated substrates can be smoothly converted into the corresponding y-ketosilanes (Scheme 11.12). The utility of this sequence in providing reversible protection to a/?-unsaturated ketones is discussed on p . 157. Silyl cuprates also react with aliphatic acid chlorides, to produce 18 19 acylsilanes . Their addition to terminal alkynes in a route to vinylsilanes was mentioned earlier in Chapter 7. 11.2.3 Reactions with ethers 20
Aryl alkyl ethers are cleaved by triphenylsilyl-lithium to give products of O-alkyl fission, albeit in rather modest yields. Cyclic ethers, on the other 21 h a n d , give good to excellent yields of ω-triorganosilyl-methanols (Scheme 11.13), with, when applicable, the regioselectivity expected from bimolecular 22 nucleophilic attack. Epoxides are themselves opened to ^-hydroxysilanes, although the synthetic utility of such opening when linked to the silyl-Wittig reaction (Chapter 12) was not recognized fully until recently. ArOMe
+
P h 3S i ü
A r 0 ~ Li+ -I- Ph 3SiMe
+
R 3S i L i
R 3S i <
(CH2)*
^OH
( C H 2) „ " = 0,1,2
Scheme 11.13
11.2.4 Organopentafluorosilicates Organopentafluorosilicates, described earlier on p . 136, react, in some cases exothermically, with a wide range of electrophilic species such as the halogens 10 23 24 and halogenoids , copper(II) halides , and m-chloroperbenzoic acid . In all cases, regioselectivity is readily attained via the initial addition of 54 - 8 6 %
2K*
Scheme 11.14
Reactions
139
trichlorosilane to the alkene. The cleavage reaction can show 25 26 stereoselectivity , allowing asymmetric synthesis of chiral alcohols and bromides from alkenes (Scheme 11.14). These sequences of reactions are unique in that they represent the first practical methods for the cleavage of an alkyl-silicon bond to give an alkyl halide or alkanol; one-electron transfer 27 processes are implicated in the cleavage step. 11 The species derived from alkynes can be allylated to produce 1,4-dienes in 10 28 reasonable yields (Scheme 11.15); b r o m i n a t i o n , methoxycarbonylation , 29 and thiocyanation processes have been described also. Br
Scheme 11.15
References 1 2 3 4 5 6 7 8
9 10
DAVIS, D. D. and GRAY, C. E., Organometal. Chem. Rev. 6, 283 (1970) SAKURAI, H. and OKADA, Α., J. organometal. Chem. 36, C13 (1972) SEITZ, D. E. and FERREIRA, L., Synth. Communs. 9, 451 (1979) BROOK, A. G. and GILMAN, H., J. Am. chem. Soc. 76, 77, 278 (1954) GILMAN, H. and LICHTENWALTER, G. D., J. Am. chem. Soc. 80, 608 (1958) HENGGE, E. and HOLTSCHMIDT, N., J. organometal. Chem. 12, P5 (1968); VYAZANKIN, N. S., RAZUVAEV, G. Α., GLADYSHEV, Ε. N. and KORNEVA, S. P., J. organometal. Chem. 7, 353 (1967) CORRIU, R. J. P. and GUERIN, C , J. chem. Soc. chem. Communs 168 (1980) SAKURAI, H., OKADA, Α., KIRA, M. and YONEZAWA, K., Tetrahedron Lett. 1511 (1971); SAKURAI, H. and KONDO, F., J. organometal. Chem. 92, C46 (1975); WATANABE, H., HIGUCHI, K., KOBAYASHI, M., HARA, M., KOIKE, Y., KITAHARA, T. and NAGAI, Y., J. chem. Soc. chem. Communs 534 (1977) STILL, W. C , J. org. Chem. 41, 3063 (1976); ILSLEY, W. H., SCHAAF, T. F., GLICK, M. D. and OLIVER, J. P., J. Am. chem. Soc. 102, 3769 (1980) TAMAO, K., YOSHIDA, J., TAKAHASHI, M., YAMAMOTO, H., KAKUI, T., MATSUMOTO, H., KURITA, A. and KUMADA, M., J. Am. chem. Soc. 100, 290 (1978)
140
Organosilyl anions
11 YOSH1DA, J., TAMAO, K., TAKAHASHI, M. and KUMADA, M., Tetrahedron Lett. 2161 (1978) 12 See, for example, SAKURAI, H., OKADA, Α., UMINO, H. and KIRA, M., J. Am. chem. Soc. 95, 955 (1973) 13 GILMAN, H. and WU, T. C , J. Am. chem. Soc. 75, 2935 (1953); J. Am. chem. Soc. 76, 2502 (1954); GILMAN, H. and LICHTENWALTER, G. D., J. Am. chem. Soc. 80, 2680 (1958) 14 BROOK, A. G., J. Am. chem. Soc. 80, 1886 (1958); Accts chem. Res. 7, 77 (1974) 15 WRIGHT, A. and WEST, R., J. Am. chem. Soc. 96, 3214 (1974) 16 STILL, W. C. and MITRA, Α., Tetrahedron Lett. 2659 (1978) 17 AGER, D. J. and FLEMING, I., J. chem. Soc. chem. Communs 177 (1978) 18 DUFFAÙT, N., DUNOGUÈS, J., BIRAN, C , CALAS, R. and GERVAL, J., J. organometal. Chem., 161, C23 (1978) 19 FLEMING, I. and ROESSLER, F., J. chem. Soc. chem. Communs 276 (1980) 20 GILMAN, H. and TREPKA, W. J., J. organometal. Chem. 1, 222 (1964) 21 WITTENBERG, D., AOKI, D. and GILMAN, H., J. Am. chem. Soc. 80, 5933 (1958) 22 GILMAN, H., AOKI, D. and WITTENBERG, D., J. Am. chem. Soc. 81, 1107 (1959) 23 YOSHIDA, J., TAMAO, K., KURITA, A. and KUMADA, M., Tetrahedron Lett. 1809 (1978) 24 TAMAO, K., KAKUI, T. and KUMADA, M., J. Am. chem. Soc. 100, 2268 (1978) 25 TAMAO, K., YOSHIDA, J., MURATA, M. and KUMADA, M., J. Am. chem. Soc. 102, 3267 (1980) 26 HAYASHI, T., TAMAO, K., KATSURO, Y., NAKAE, I. and KUMADA, M., Tetrahedron Lett. 1871 (1980) 27 YOSHIDA, J., TAMAO, K., KUMADA, M. and KAWAMURA, T., J. Am. chem. Soc. 102, 3269 (1980); see also TAMAO, K., KAKUI, T. and KUMADA, M., Tetrahedron Lett. 4105 (1980) 28 TAMAO, K., KAKUI, T. and KUMADA, M., Tetrahedron Lett. 619 (1979) 29 TAMAO, K., KAKUI, T. and KUMADA, M., Tetrahedron Lett. I l l (1980)
Organosilyl anions
The preparation and fundamental chemistry 1 compounds has been well reviewed up to 1970.
of
organosilyl
metallic
11.1 Preparation The most flexible route to organosilyl anions utilizes organodisilanes, which suffer Si-Si bond fission when treated with an alkali metal, an alkali metal hydride, or a good nucleophile for silicon (Scheme 11.1). R 3S i M
R 3S i M
+
R 3S i N u
Scheme 11.1 2 3
Paradoxically, the requisite disilanes are o b t a i n e d ' by Wurtz coupling of an organohalogenosilane (Scheme 11.2). The first cleavage process, utilizing an alkali metal, requires at least one of the R groups to be aryl; within this limitation, a reasonable range (1), (2), (3), 4 5 (4), of triorganosilyl metals can be p r e p a r e d ' , often in good yield. 2R,SiCl
+ 2M
THF or EtoO ^ - > (HMPA)
R-J Si - SiR-* 4- 2ΜΧ
e.g.
2 M e 30S i C l + 2l_i
THF reflux,8h. Scheme 11.2
134
M e ^ S i - S i M e *0
97%
6
Preparation P h 3S i N a
68%
Ph 2MeSil_i
(1)
135
74%
(2)
P h 3S i L i
79%
PhMe 2SiLi
(3)
47%
(4)
Trialkylsilyl-metals cannot be obtained in this manner, but are available by 6 metal-metal exchange of disilylmercurials with lithium (Scheme 11.3). The dangers inherent in handling volatile mercury c o m p o u n d s make this route somewhat less than attractive. ( R 3S i ) 2H g
+
2Li
2 R 3S i L i
+
Hg
R = Me, Et Scheme 11.3 7
The second cleavage process employs either sodium or potassium hydride, and is well suited to the preparation of trialkylsilyl-metals (Scheme 11.4). An additional advantage is that if the triorganosilyl hydride is readily available, then cleavage of the S i - Η b o n d occurs under similar conditions. Either way, solutions of triorganosilyl-metals are formed in good yield and free from any by-products. M e 53 Si - S i M e 3
+
2MH5
DM Ε or HMPA
-, M e 3S i H
M e 3S i M +
r
2 Me^SiM
Μ = Να,Κ R 53 SiH
+
KH
PME or HMPA
3
Scheme 11.4
R = Et,Ph
The third process, that of cleaving a disilane with a good nucleophile for silicon, also gives access to a good range of trialkylsilyl-metals (Scheme 11.5) 8 and other more highly functionalized derivatives , when the presence of H M P A is not required. M e 3S i - S i M e 3
+
HMPA
MOMe
> - M e 3S i M
+
M e 3S i 0 M e
Μ = Να ,Κ ( M e O U S i - Si(OMe)? Me
+
THF
NaOMe
Me
> - ( M e 0 ) 2S i N a
. Λ _ Λ Scheme 11.5
+
(MeO)^SiMe
Me e
9
Trimethylsilyl-lithium itself is obtained by cleavage of hexamethyldisilane with methyl-lithium in H M P A , when a deep red solution of the reagent results (Scheme 11.6). M e 3, S i
SiMe, +
MeLi
HMPA 3
0 °C,5min Scheme 11.6
Me,SiLi
+
3
M e 4S i
4
136 Organosilyl anions A rather more specialized situation is seen in the cases of organopenta10 fluorosilicates. Catalysed a d d i t i o n of trichlorosilane to terminal alkenes and alkynes, followed by the addition of an aqueous solution of potassium 11 fluoride, produces highly reactive organopentafluorosilicates (Scheme 11.7), discussed further on p . 138. i 2 -
r
Scheme 11.7
11.2 Reactions Organosilyl anions are strong bases and good nucleophiles, and must be generated and reacted in aprotic media. 11.2.1 Alkylation This is generally successful with primary and secondary alkyl chlorides. Bromides and iodides give very poor yields, owing to competing electron transfer processes, the major product frequently being the coupled disilane 12 when H M P A is solvent . In D M E or T H F plus 18-crown-6, the dominant process becomes one of bimolecular nucleophilic substitution (Scheme 11.8), 7 and tetrasubstituted silanes can be obtained in excellent yields . Ph 3SiK M e3, S i N a
-I- P h C H 2C l +
n - C 12H 0 0 5-( 2B r
— THF 1
18-crown-6 Scheme 11.8
P h 3S i C H 2P h
70%
M e , S i ( C H - L C3H ,
75%
11.2.2 Reaction with carbonyl compounds This is a good route to silyl carboxylic acids, by reaction with C 0 2 , and to 13 silylmethanols by reaction with aliphatic aldehydes or ketones (Scheme 11.9). 1 R 3S i C 0 2H
1
R 3S i M
2
R C0R
3
1
2
R ,SiCR R Scheme 11.9
3
I OH
3
Reactions
137 1 4 51
With aromatic aldehydes and ketones, the initial adducts u n d e r g o Brook rearrangement (Chapter 5), and silyl ethers are isolated (Scheme 11.10). R 3S i M
+
ArCOAr
R 3S i C A r 2
- Q ^ A r , ' 2C H 0 S i M e 3
Scheme 11.10
Trimethylsilyl-lithium reacts with a/?-unsaturated ketones to give products 9 16 of exclusive 1,4-addition (Scheme 11.11). That these are kinetic products was demonstrated by separate generation of a product (5) of 1,2-addition; this species (5) proved to be completely stable under the conditions used for conjugate addition.
4-
a
T H F , HMPA
Me,SiLi
/
\ I
SiMe3
1.Me 3Snl_i
n e 0SiM 3 Bu Li
M e 3S n
y v
0SiMe3
SiMe7 (5)
Scheme 11.11
With cyclic a/?-unsaturated ketones, a strong preference for axial addition 17 is observed, as is a susceptibility to steric hindrance. A d d i t i o n of copper(I) iodide forms a silyl cuprate species, and allows higher reaction temperatures
SiMe 2Ph 0 P h 3S i L i
1 . Cul
2 RC0CI Scheme 11.12
R
SiPh3
75-90%
R = Me, Et, B u
f
138
Organosilyl anions
to be employed. This minimizes the effects of steric hindrance, and a wide range of a/?-unsaturated substrates can be smoothly converted into the corresponding y-ketosilanes (Scheme 11.12). The utility of this sequence in providing reversible protection to a/?-unsaturated ketones is discussed on p . 157. Silyl cuprates also react with aliphatic acid chlorides, to produce 18 19 acylsilanes . Their addition to terminal alkynes in a route to vinylsilanes was mentioned earlier in Chapter 7. 11.2.3 Reactions with ethers 20
Aryl alkyl ethers are cleaved by triphenylsilyl-lithium to give products of O-alkyl fission, albeit in rather modest yields. Cyclic ethers, on the other 21 h a n d , give good to excellent yields of ω-triorganosilyl-methanols (Scheme 11.13), with, when applicable, the regioselectivity expected from bimolecular 22 nucleophilic attack. Epoxides are themselves opened to ^-hydroxysilanes, although the synthetic utility of such opening when linked to the silyl-Wittig reaction (Chapter 12) was not recognized fully until recently. ArOMe
+
P h 3S i ü
A r 0 ~ Li+ -I- Ph 3SiMe
+
R 3S i L i
R 3S i <
(CH2)*
^OH
( C H 2) „ " = 0,1,2
Scheme 11.13
11.2.4 Organopentafluorosilicates Organopentafluorosilicates, described earlier on p . 136, react, in some cases exothermically, with a wide range of electrophilic species such as the halogens 10 23 24 and halogenoids , copper(II) halides , and m-chloroperbenzoic acid . In all cases, regioselectivity is readily attained via the initial addition of 54 - 8 6 %
2K*
Scheme 11.14
Reactions
139
trichlorosilane to the alkene. The cleavage reaction can show 25 26 stereoselectivity , allowing asymmetric synthesis of chiral alcohols and bromides from alkenes (Scheme 11.14). These sequences of reactions are unique in that they represent the first practical methods for the cleavage of an alkyl-silicon bond to give an alkyl halide or alkanol; one-electron transfer 27 processes are implicated in the cleavage step. 11 The species derived from alkynes can be allylated to produce 1,4-dienes in 10 28 reasonable yields (Scheme 11.15); b r o m i n a t i o n , methoxycarbonylation , 29 and thiocyanation processes have been described also. Br
Scheme 11.15
References 1 2 3 4 5 6 7 8
9 10
DAVIS, D. D. and GRAY, C. E., Organometal. Chem. Rev. 6, 283 (1970) SAKURAI, H. and OKADA, Α., J. organometal. Chem. 36, C13 (1972) SEITZ, D. E. and FERREIRA, L., Synth. Communs. 9, 451 (1979) BROOK, A. G. and GILMAN, H., J. Am. chem. Soc. 76, 77, 278 (1954) GILMAN, H. and LICHTENWALTER, G. D., J. Am. chem. Soc. 80, 608 (1958) HENGGE, E. and HOLTSCHMIDT, N., J. organometal. Chem. 12, P5 (1968); VYAZANKIN, N. S., RAZUVAEV, G. Α., GLADYSHEV, Ε. N. and KORNEVA, S. P., J. organometal. Chem. 7, 353 (1967) CORRIU, R. J. P. and GUERIN, C , J. chem. Soc. chem. Communs 168 (1980) SAKURAI, H., OKADA, Α., KIRA, M. and YONEZAWA, K., Tetrahedron Lett. 1511 (1971); SAKURAI, H. and KONDO, F., J. organometal. Chem. 92, C46 (1975); WATANABE, H., HIGUCHI, K., KOBAYASHI, M., HARA, M., KOIKE, Y., KITAHARA, T. and NAGAI, Y., J. chem. Soc. chem. Communs 534 (1977) STILL, W. C , J. org. Chem. 41, 3063 (1976); ILSLEY, W. H., SCHAAF, T. F., GLICK, M. D. and OLIVER, J. P., J. Am. chem. Soc. 102, 3769 (1980) TAMAO, K., YOSHIDA, J., TAKAHASHI, M., YAMAMOTO, H., KAKUI, T., MATSUMOTO, H., KURITA, A. and KUMADA, M., J. Am. chem. Soc. 100, 290 (1978)
140
Organosilyl anions
11 YOSH1DA, J., TAMAO, K., TAKAHASHI, M. and KUMADA, M., Tetrahedron Lett. 2161 (1978) 12 See, for example, SAKURAI, H., OKADA, Α., UMINO, H. and KIRA, M., J. Am. chem. Soc. 95, 955 (1973) 13 GILMAN, H. and WU, T. C , J. Am. chem. Soc. 75, 2935 (1953); J. Am. chem. Soc. 76, 2502 (1954); GILMAN, H. and LICHTENWALTER, G. D., J. Am. chem. Soc. 80, 2680 (1958) 14 BROOK, A. G., J. Am. chem. Soc. 80, 1886 (1958); Accts chem. Res. 7, 77 (1974) 15 WRIGHT, A. and WEST, R., J. Am. chem. Soc. 96, 3214 (1974) 16 STILL, W. C. and MITRA, Α., Tetrahedron Lett. 2659 (1978) 17 AGER, D. J. and FLEMING, I., J. chem. Soc. chem. Communs 177 (1978) 18 DUFFAÙT, N., DUNOGUÈS, J., BIRAN, C , CALAS, R. and GERVAL, J., J. organometal. Chem., 161, C23 (1978) 19 FLEMING, I. and ROESSLER, F., J. chem. Soc. chem. Communs 276 (1980) 20 GILMAN, H. and TREPKA, W. J., J. organometal. Chem. 1, 222 (1964) 21 WITTENBERG, D., AOKI, D. and GILMAN, H., J. Am. chem. Soc. 80, 5933 (1958) 22 GILMAN, H., AOKI, D. and WITTENBERG, D., J. Am. chem. Soc. 81, 1107 (1959) 23 YOSHIDA, J., TAMAO, K., KURITA, A. and KUMADA, M., Tetrahedron Lett. 1809 (1978) 24 TAMAO, K., KAKUI, T. and KUMADA, M., J. Am. chem. Soc. 100, 2268 (1978) 25 TAMAO, K., YOSHIDA, J., MURATA, M. and KUMADA, M., J. Am. chem. Soc. 102, 3267 (1980) 26 HAYASHI, T., TAMAO, K., KATSURO, Y., NAKAE, I. and KUMADA, M., Tetrahedron Lett. 1871 (1980) 27 YOSHIDA, J., TAMAO, K., KUMADA, M. and KAWAMURA, T., J. Am. chem. Soc. 102, 3269 (1980); see also TAMAO, K., KAKUI, T. and KUMADA, M., Tetrahedron Lett. 4105 (1980) 28 TAMAO, K., KAKUI, T. and KUMADA, M., Tetrahedron Lett. 619 (1979) 29 TAMAO, K., KAKUI, T. and KUMADA, M., Tetrahedron Lett. I l l (1980)