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Rh
III
LG
Nu
Rh
I
Nu
k
1
A
k
2
Rh
III
k
2
Rh
III
LG
Nu
Rh
I
Nu
k
1
ent-
A
Scheme 8B.67.
Postulated mechanism for the Rh-catalyzed allylic alkylations.
TABLE 8B.47. Allylic Alkylations with Methyl Sodio(Phenylsulfonylacetate)
OCO
2
Me
NaCH(CO
2
Me)(SO
2
Ph )
CO
2
Me
CO
2
Me
+
R
RhCl(PPh
3
)
3
/P(OMe)
3
,
THF, 30°C
R
SO
2
Ph
SO
2
Ph
b
l
Allylic Substrate
Ratio
Entry
R
ee (%)
Yield (%)
b
:
l
ee (%)
1
BnOCH
2
94
86
99:1
92
2
Me
97
86
36:1
95
3
Bn
94
86
9:1
92
4
TBSOCH
2
>
99
78
3:1
99
8B.6.1.1.1. Substrate - Controlled Asymmetric Allylic Alkylations
Based on this mecha-
nistic aspects, the easiest way for asymmetric allylations is the use of chiral allyl sub-
strates, especially if symmetric nucleophiles such as malonates are used. Herewith, only
one stereogenic center (in the allyl moiety) has to be controlled. The situation with
unsymmetric nucleophiles is different, which generally give mixtures of diastereomers
(in addition to regioisomers).
Evans and Kennedy also investigated the reactions of phenylsulfonyl acetates [276].
Depending on the allylic substituent R, regioselectivity of up to 99%, high yield, and
excellent chirality transfer to the branched product could be obtained (Table 8B.47).
The branched product was obtained as a 1:1 mixture of diastereomers, and the linear
product was formed as racemate. This can be explained by the high acidity of the sulfonyl
acetates, which can epimerize under the reaction conditions used.
This problem could be solved by Kazmaier and Stolz using chelated ester enolates
[271]. The unsaturated amino acids obtained are confi gurationally stable, and as a result
