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order morpholino (
i
)
diethylamino (
c
)
> dibenzylamino (
g
), ranging between 65% and 38%. The results were better
when the amine was part of a morpholine ring. Basicity seems to have a strong
influence on the reaction, since the presence of Ts as substituent on the amino
groups decreased the reaction rate and provided a low ee. This result is in contrast
with previous work in which ligand
9
(identical to
14k
) provided high conversion
and enantioselectivity in the addition of diethylzinc to benzaldehyde in the pres-
ence of titanium Lewis acids.
Thus, the best ligand for this reaction in terms of enantioselectivity was
14i
with
an
>
pyrrolidino (
h
)
>
diisopropylamino (
e
)
>
-d-
gluco
configuration and a 2-morpholino residue. This study clearly shows
the possibilities of tuning carbohydrate ligands through a systematic variation of
configuration and substituents at the different positions of the carbohydrate, as
well as the remarkable influence of substituents neighboring the coordination
sites on the enantioselectivity.
Ligands
19-27
[14, 15] (Schemes 13.7 and 13.8) constitute a family of structur-
ally related ligands that contain pyridine or quinoline moieties incorporated in a
fructopyranose or glucofuranose backbone, whose main structural features are as
α
D-fructose
R
2
CO/H
+
R
1
R
1
R
1
R
1
O
O
O
O
O
O
Br
N
R
R=H, Br
N
R
O
O
N
R=, H, Me, Ph
N
R
2
OH
OH
O
O
R
1
R
1
BuLi, Et
2
O
-78°C
BuLi, Et
2
O
-78°C
R
1
R
1
R R
R
2
O
O
19a
R
1
= CH
3
, R
2
= H (45%)
19b
R
1
= -(CH
2
)
5
- R
2
= H (36%)
19c
R
1
= CH
3
, R
2
= Ph (97%)
19d
R
1
= CH
3
, R
2
= Br (45%)
19e
R
1
= -(CH
2
)
5
- R
2
= Br (44%)
O
20a
R
1
= CH
3
, R
2
= H (80%)
20b
R
1
= -(CH
2
)
5
-, R
2
= H (77%)
20c
R
1
= -(CH
2
)
5
-, R
2
= Ph (53%)
20d
R
1
= CH
3
, R
2
=6-Me-2-pyridyl
O
O
O
R
R
18a
R= CH
3
18b
R= -(CH
2
)
5
-
from
20d
BuLi, Et
2
O, -78°C
then
18a
0°C
from
19d, 19e
NiCl
2
/PPh
3
Zn, DMF
R
R
O
O
N
R
R
O
O
BuLi, Et
2
O, 0°C
N
N
N
N
O
O
O
O
O
O
OH
HO
O
O
OH
HO
O
O
R R
O
O
O
O
R
R
R
O
O
O
O
O
R
22a
R= CH
3
(55%)
22b
R= -(CH
2
)
5
- (52%)
23
(32%)
O
N
OH
O
R
R
21a
R= CH
3
(83%)
21b
R= -(CH
2
)
5
- (79%)
Scheme 13.7
Synthesis of pyridyl-, quinolyl-, and bipyridyl-/alcohols
19-23.
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