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O
OH
Ph
H
Ph
Ti
R
1
28
26
R
1
(19-90% ee)
25, 29a-c
O
O
O
(Ph)
3
CO
HO
HO
HO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
HO
25
R
1
= diacetone-
D
-glucose
90% ee (
R
)
29a
R
1
=
D
-xylose acetonide
64% ee (
R
)
29b
R
1
= diacetone-L-idose
39% ee (
S
)
29c
R
1
= diacetone-
D
-allose
19% ee (
S
)
Scheme 7.8
Enantioselective allyltitanation of benzaldehyde with chiral
cyclopentadienyl(dialkoxy)titanium complexes
25, 29a-c.
Ti
O
OH
Ph
Ph
O
O
Ph
H
Ph
Ph
Ph
28
26
O
O
95% ee (
S
)
(
Si
side)
(
R,R
)-
30
Scheme 7.9
Allyltitanation of benzaldehyde with chiral complex
30,
derived from tartaric acid.
enantioface discrimination, rather than direct interactions of reactants with the
chiral ligand [19].
7.3.2
Allylsilicon Reagents
The asymmetric addition reaction of allylsilanes to carbonyl compounds promoted
by a Lewis acid, known as the Hosomi-Sakurai reaction, can be considered as one
of the most important methods developed for the stereoselective synthesis of chiral
homoallylic alcohols [20]. For this reason research efforts have concentrated on
the synthesis of chiral allylsilanes with the stereogenic center not residing in the
allyl moiety. In this sense, several allylsilyl ethers of l-arabinose
34
and
35
were
prepared by Shing, as indicated in Scheme 7.10, and used in the acid-mediated
reaction with aldehydes to determine steric and coordinative effects on the asym-
metric induction [21].
Table 7.3 summarizes the Lewis acid effect on the stereochemical outcome of
the reaction of allylsilanes
34
and
35
with aldehydes. As can be seen, BF
3
leads to
higher enantioselectivity than TiCl
4
or SnCl
4
, although homoallylic alcohols were
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