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N
O
TBSO
O
O
N
TBSO
TBSO
TBSO
O
OMe
R
1
HO
R
1
OMe
34
O
ent-
33
O
35
O
TBSO
TBSO
O
OMe
K
O
O
R
1
X
Si
-face attack
36
Scheme 3.13
Diastereoselective synthesis of pyrazoline derivatives
ent-
33.
-hydroxy-
carboxylic acids has been carried out by Wang's group using diacetone-d-fructose
37
as chiral auxiliary [16]. Esterification with various glycolic acid derivatives car-
rying 11 different protective groups afforded the glycolate derivatives
39
in good
yields. A study of the enolization reaction shows that the use of LiHMDS in the
presence or absence of HMPA gave the best results depending on the reactivity
of the electrophile (Scheme 3.14).
An exhaustive study directed towards the asymmetric synthesis of
α
O
D-
fructo
O
O
O
O
Base
DCC
O
HO
O
+
OR
1
OR
1
OH
O
THF, additive
R
2
X
DMAP
O
O
O
O
O
39
38
37
11 examples
O
R
2
O
R
2
LiOH
THF / MeOH / H
2
O
HO
O
OH
O
OR
1
O
O
O
O
41
40
25-examples
up to 98% de
Scheme 3.14
Asymmetric synthesis of
α
-hydroxy carboxylic acids through aldol alkylation
using diacetone-d-fructose as chiral auxiliary.
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