Chemistry Reference
In-Depth Information
204
nikkomycin Bz
O
OH
NH
2
NH
N
CO
2
H
N
O
O
CH
3
O
MeO
HO
OH
N-terminal
amino acid
C-terminal
amino acid
Figure 4.1
Unusual dipeptide natural product nikkomycin Bz (
204
).
omer. The N-terminal amino acid of nikkomycin Bz was prepared from
207
in
four steps, including reductive cleavage of the isoxazolidine N-O bond [49]. The
reaction sequence is summarized in Scheme 4.35.
L-gulo
L-gulonic-
γ
-lactone
HO
1.)
HO
O
O
CO
2
tBu
2.)
MeO
PMP
OH
HO
OH
O
O
178f
O
O
HO
O
O
O
OH
- H
2
O
O
205
TiCl
4
O
N
OH
toluene
N
dichloroethane
O
CO
2
Me
ent-192
ent-193a
ent-194a
O
O
N-terminal amino acid
of nikkomycin Bz
O
O
O
O
O
O
O
O
OH
NH
2
4 steps
H
O
N
CO
2
H
N
O
CH
3
O
MeO
O
207
O
206
208
O
PMP
H
75% from
ent-192
only diastereomer
PMP
Scheme 4.35
Asymmetric synthesis of the N-terminal amino acid of nikkomycin via
intramolecular 1,3-dipolar cycloaddition with l-gulosyl nitrone
206
as key step.
Apart from these examples, there have been miscellaneous reports of glycosyl
nitrones used as tools for asymmetric 1,3-dipolar cycloadditions. Brandi and cow-
orkers used nitrones from manno-configured oxime
175
and glyoxalic esters for
the preparation of 4-oxopipecolic acid using methylenecyclopropane as the dipo-
larophile. The reaction was successful, albeit the diastereoselectivity remained
modest [50]. The group of Fišera reported 1,3-dipolar cycloadditions of
N
-
aryl-maleimides to nitrones derived from d-glucopyranose, and while for most
examples the dr was around 70 : 30 in one case a dr of 95 : 5 was obtained [51].
Chiacchio has prepared isoxazolidinyl-thymidines as analogues of regular thymine
nucleosides by employing a modified ribose oxime for nitrone formation with
ethyl glyoxylate [52].
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