Chemistry Reference
In-Depth Information
R
1
m-CPBA
O
R
1
O
n
*R
n
*R
CHCl
3
R
2
O
O
R
2
O
R
3
R
3
26a-m
27a-m
D-
allo
D-
allo
D-
gluco
O
O
O
O
O
O
Ph
Ph
O
Ph
O
O
O
O
O
R
5
R
4
11
5
5
NHAc
NHAc
NHAc
OAc
OH
27c,
R
5
=H
(de: 68%)
27d,
R
5
=Me
(de: 72%)
27a,
R
4
=OMe
(de: 14%)
27b,
R
4
=NHAc
(de: 0%)
27e
(de: 20%)
Ph
R
6
D-
galacto
D-
allo
D-
altro
O
O
O
OEt
O
O
O
Ph
Ph
O
O
O
O
11
OMe
NHAc
HO
OBn
OH
OMe
OH
27f
(de: 28%)
27g
(de: 34%)
27h,
R
6
=H
(de: 26%)
27i,
R
6
=Me
(de: 74%)
D-
gluco
D-
xylo
R
8
R
7
O
OH
O
O
Ph
O
O
O
Ph
H
O
O
O
O
H
O
O
27j,
R
7
=H
(de: 56%)
27k,
R
7
=Me
(de: 60%)
27l,
R
8
=H
(de: 22%)
27m,
R
8
=Me
(de: 30%)
Scheme 5.7
Diatereoselective epoxidation of alkenylidene acetals derived from various sugar
auxiliaries.
-Unsaturated amides
29
and
30,
derived from carbohydrates, have been
obtained by condensation of sugar amines with the appropriate activated unsatu-
rated acid. These were subsequently treated with
m-
CPBA in dichloromethane
(Scheme 5.8).
The diastereoselectivity of the process was quite low at room temperature (26%
de) and enhanced at
α
,
β
10 °C (40% de). The formation of an additional five-
membered ring, by bridging the oxygen at C3 and the nitrogen at C2, enhanced
the rigidity of the auxiliary, resulting in a substantial improvement in the diastere-
oselectivity (56% to 94% de) [22]. Nevertheless, despite the rigidity of the chiral
auxiliary
30a-d,
the diastereoselectivity of the oxidation is in general quite disap-
pointing (Table 5.7, entries 4-6), which indicates the importance of a well-situated
hydrogen donor heteroatom in the chiral auxiliary for directing the oxygen trans-
fer. The
−
-epoxyamide can be separated from the sugar moiety with sodium
borohydride in THF, as the corresponding epoxy alcohol, and the chiral auxiliary
is recovered (Scheme 5.8).
α
,
β
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