Chemistry Reference
In-Depth Information
L-
rhamno
Scheme 5.1
Simmons-Smith cyclopropanation of allylic glycosides using l-rhamnal as chiral
auxiliary.
D-
gluco
1) 160 °C, DMF,
BnO
BnO
Bn
BnO
O
Py, H
2
O
O
Bn
BnO
O
Ph
HO
Ph
+
OTf
2) NaBH
4
OH
(7-12:1,
α
:
β
)
1g
5
(90%)
6
(80%)
Ms
2
O,
s
-collidine
CH
2
Cl
2
, 0 °C
BnO
Bn
BnO
O
7
(89%)
Scheme 5.2
Cleavage of the chiral auxiliary in the synthesis enantiopure cyclopropyl alcohols.
together with ring-contracted chiral auxiliary
6
in 80% yield. Subsequent dehydra-
tion via a mesylate derivative of
6
regenerated tri-
O
-benzyl-d-glucal
7
[7].
The synthetic utility of the approach has been demonstrated by the asymmetric
synthesis of all four isomers of coronamic acid, a cyclopropyl amino acid with
important agrochemical applications [8]. With these results it is clear that Charette's
chiral auxiliary mediated Simmons-Smith cyclopropanation can compete satisfac-
torily with the best methods in the literature for the synthesis of chiral non racemic
cyclopropanes [3]. Following the pioneering work of Charette, other Simmons-
Smiths reactions using carbohydrates as chiral templates have been developed,
directed mainly at simplifying the rather long route for regeneration of the chiral
auxiliary. To do so, Kang
et al.
coupled the alkene moiety via an acetal to fructo-
pyranose
8
as chiral template (Scheme 5.3). The acetals
9
were formed as a mixture
D-
fructo
O
O
O
O
CH(OEt)
2
O
1) aq. AcOH
O
O
Ph
R
2
ZnEt
2
/ CH
2
I
2
O
OR
1
O
OR
1
OR
1
OH
O
PPTS, C
6
H
6
∆
2) NaBH
4
(ClCH
2
)
2
≤
O
O
OH
O
HO
0 °C
H
H
9
10
11
8
R
2
R
2
Scheme 5.3
Cyclopropanation of alkenylidene acetals derived from fructopyranose.
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