Chemistry Reference
In-Depth Information
With ribo-nitrones
176a
and
176b
derived from formaldehyde (
178a
) and acetone
(
178b
), respectively, 1,3-dipolar cycloaddition reactions with methyl methacrylate
(
179
) gave the isoxazolidines
(R)
- and
(S)
-
180
in excellent yields (Scheme 4.28).
In the case of formaldehyde nitrone
176a
the de was 67% while the acetone-derived
nitrone
176b
gave more than 95% de; in both cases the new stereocenter was
formed in the
(R)
-configuration. Cleavage of the products from the carbohydrate
scaffolds was possible under acidic conditions [43a, b]. The mannose-derived
nitrones
177a
and
177b
from formaldehyde and acetone, respectively, also reacted
smoothly with methyl methacrylate. The diastereoselectivities for the products
182
were on a similar level as in case of ribo-dipolarophiles
176
but the direction of
the asymmetric induction was reversed. Thus, products
181
were obtained in
enantiomeric configuration after cleavage from the manno-scaffold [43b].
H
3
C
H
3
C
CO
2
Me
CO
2
Me
D-ribo
178a:
CH
2
O
O
O
O
TrO
TrO
TrO
R
1
R
1
O
H
3
C
CO
2
Me
CO
2
Me
N
N
N
HClO
4
MeOH
O
O
O
H
3
C
178b:
179
R
1
R
1
R
1
R
1
H
3
C
C
H
3
+
175
R
R
1
O
H
- H
2
O
O
O
O
O
O
O
176
Me
Me
(R)-
180
Me
Me
Me
Me (S)-
180
181
a
R
1
= H
a
R
1
= H
R
1
= H
98%
91%
68%de (R)
(+)-
181a
67%ee
b
R
1
= Me
b
R
1
= CH
3
R
1
= CH
3
(-)-
181b
> 95%ee
>95%de (R)
D-manno
178a:
CH
2
O
178b:
H
3
C
H
3
C
H
3
C
H
3
C
H
3
C
H
3
C
O
O
O
O
O
O
H
3
C
CO
2
Me
O
H
3
C
CH
3
H
3
C
CH
3
H
3
C
CH
3
CO
2
Me
HClO
4
MeOH
H
3
C
1
7
9
O
O
O
O
O
O
H
3
C
C
H
3
R
R
1
58
O
O
O
O
R
1
R
1
R
1
H
R
1
R
1
- H
2
O
N
O
N
N
+
177
R
1
181
O
O
(S)-
182
(R)-
182
H
3
C
H
3
C
CO
2
Me
CO
2
Me
a
R
1
= H
R
1
= H
98%
75%
76%de (S)
(-)-
181a
80%ee
R
1
= CH
3
R
1
= CH
3
b
not given
(+)-
181b
90%ee
Scheme 4.28
Asymmetric 1,3-dipolar cycloadditions using
N
-glycosyl nitrones
176a,b
and
177a,b
derived from formaldehyde and acetone with methyl methacrylate (
179
) as
dipolarophile.
The reaction of ribo-configured oxime
175
with acetaldehyde (
178c
) yielded an
(E/Z)
-mixture of the nitrones
176c,
which upon 1,3-dipolar cycloaddition with
methyl methacrylate gave a mixture of four diastereomeric isoxazolidines (
180c
).
The two major diastereomers of this mixture had the same configuration at the qua-
ternary stereocenter of the isoxazolidine moiety and differed only in the configura-
tion at the stereocenter carrying the methyl group (Scheme 4.29) [43b]. Therefore,
with respect to the quaternary center, the asymmetric induction is independent of
the
(E)
- or
(Z)
-configuration of the nitrone
176c,
which, however, determines the
stereochemistry at the chiral carbon bearing the methyl group. The author also
described the reaction of non-activated alkenes with formaldehyde-derived nitrone
176a
(Scheme 4.29). Reactions of
176a
with 1-octene (
183
) and cyclohexene (
185
)
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