Chemistry Reference
In-Depth Information
H
3
C
H
3
C
O
O
O
O
CO
2
Bu
CO
2
Bu
CO
2
Bu
CO
2
Bu
H
CO
2
Bu
O
+
+
O
O
+
O
O
O
92%
O
CH
3
CH
3
153
O
O
O
O
Aux
Aux
Aux
Aux
154a
154b
154c
154d
1.) benzonitrile, H
2
O
2
, 52%
2.) i) PhSeH; ii) H
2
O
2
, 76%
HO
HO
LiAlH
4
O
Aux
O
Aux
O
O
154a
155
156
OH
D-altrose
H
3
C
H
3
C
O
OH
HO
TFA
OsO
4
, 95%
HO
HO
HO
OH
O
O
O
HO
O
O
O
D-glucose
HO
O
HO
O
O
OH
OH
O
O
HO
O
CH
3
CH
3
HO
OH
HO
OH
HO
OH
157
O
158
158
OH
OH
α
-D-altropyranosyl-(1->3)-D-glucose
Scheme 4.24
Hetero-Diels-Alder reaction of carbohydrate diene
153
with butyl glyoxylate and
application in an alternative synthesis of disaccharides.
154d
and
154b
154c,
respectively (Scheme 4.24). These compounds may be
viewed as 2,3-unsaturated carbohydrate derivatives.
Product
154a
was subsequently used for the construction of disaccharide deriva-
tive
158
via a sequence of diastereoselective functionalization reactions [37].
Employing this strategy, the same authors also prepared derivatives of the blood
group A carbohydrate antigen [38]. Further studies were conducted on reactions
of cis-configured carbohydrate dienes [39] and dienes carrying an additional 4-
benzyloxy residue [40]. Even though these early examples of hetero-Diels-Alder
reactions on carbohydrate dienes were fraught with rather modest diastereoselec-
tivities and the preparation of di- and trisaccharides from the cycloaddition prod-
ucts required functionalization via multistep protocols, they impressively
demonstrate the potential of carbohydrates as asymmetric inductors.
Heterodienes fixed to carbohydrate scaffolds have been described by Marazano
[41]. Using the 1,2-dihydropyridine
N
-glycosides of d-glucose, d-xylose, and
d-arabinose carrying acetyl or pivaloyl groups, cycloadditions with methyl acrylate
were explored (Scheme 4.25). The isoquinuclidine derivatives
162,
which were
obtained after treatment of the initial hetero-Diels-Alder products with methyl
chloroformate, are interesting intermediates for alkaloid syntheses. To determine
the stereoselectivities of the reactions, products
162
were converted into the dias-
tereomeric compounds
163
and
164
by hydride reduction and treatment with
camphorsulfonyl chloride. The enantiomeric excesses were strongly dependent on
the carbohydrate scaffold and its protective groups. The bulkier pivalate derivatives
159b-161b
gave significantly better ees for than the acetates, and the d-gluco
architecture of dienes
159
was clearly superior to the d-xylo compounds
160.
While
+
Search WWH ::
Custom Search