Chemistry Reference
In-Depth Information
O
HO
O
1. Ac
2
O, NEt
3
, DMAP (cat)
1. R
1
OH,AcCl (cat),
r.t.
O
O
O
O
HO
O
2. MeOCH
2
OMe, P
2
O
5
CH
2
Cl
2
, r.t.
3. K
2
CO
3
/MeOH, r.t.
2. Me
2
C(OMe)
2
acetone, r.t.
HO
HO
HO
OR
1
OH
OR
1
32
33
L-arabinose
31
CH
2
=CHCH
2
SiMe
2
Cl
imidazole, THF
CH
2
=CHCH
2
SiMe
2
Cl
imidazole, THF
O
O
O
O
O
O
O
OR
1
O
Si
OR
1
Si
34a,
R
1
= Me (94%)
34b,
R
1
= Bn (95%)
35a
, R
1
= Me (94%)
35b
, R
1
= Bn (98%)
Scheme 7.10
Synthesis of allylsilyl ethers of l-arabinose,
34
and
35.
obtained with only moderate enantioselectivity in all cases. Allylsilane reagent
34b,
which carries the bulkiest group, gave the highest enantioselectivity with
n
-
heptanal in the presence of BF
3
[21].
The results show that a sterically demanding group in the chiral auxiliary could
have an important influence on the asymmetric induction, despite being remote
from the reaction site. All the reactions of allylsilane
34b
with different aliphatic
aldehydes in the presence of BF
3
gave similar results [36-45% ee
(S),
with moder-
ate to high chemical yields (54-72%)]. According to the
(S)
configuration of the
new stereogenic center of the homoallylic alcohols, it has been proposed that all
reactions proceed via an intermediate where the
Re
face of the complex [22] of the
Table 7.3
Effect of Lewis acid on the stereochemical outcome of the reaction of allylsilanes
34
and
35
with
aldehyde
36.
R
2
R
2
O
OH
O
O
Lewis Aci
d
O
n
-C
6
H
13
H
n
-C
6
H
13
O
Si
OR
1
40-74% yield
37
9-41% ee
34, 35
36
Silane
34a (R
1
=
Me,
R
2
=
Me)
34b (R
1
=
Bn,
R
2
=
Me)
35a (R
1
=
Me,
R
2
=
H)
35b (R
1
=
Bn, R
2
=
H)
Lewis acid
BF
3
TiCl
4
SnCl
4
BF
3
TiCl
4
SnCl
4
BF
3
TiCl
4
SnCl
4
BF
3
TiCl
4
SnCl
4
Yield (%)
48
40
71
63
65
74
57
47
55
77
62
71
ee (%)
25
15
9
41
11
9
13
6
0
33
15
9
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