Chemistry Reference
In-Depth Information
unexpected role of the carbohydrate auxiliary in control of the diastereoselectivity
of
trans-
cyclopropanes.
5.2
Epoxidation
The epoxidation of C=C double bonds is surely one of the most useful transforma-
tions in organic chemistry [13]. The resulting epoxy functionality can be easily
transformed either by base-catalyzed and/or acid-catalyzed diastereoselective ring-
opening into a wide number of synthetic and biologically relevant molecules [14].
The powerfulness of the catalytic Sharpless-Katsuki [15] epoxidation of allylic
alcohols and the catalytic Jacobsen-Katsuki epoxidation of unfunctionalized
olefins [16] has completely supplanted other methodologies; thus, unsurprisingly,
investigations of the application of chiral auxiliaries in asymmetric epoxidation
have been very scarce [17]. As in the case of the Simmons-Smith cyclopropanation,
the use of carbohydrates in this area was dictated by the facility of controlling
their conformational behavior and the possibility of controlling electronic, steric,
and stereoelectronic interactions between the carbohydrate and the oxidants.
The alkene moiety has been linked to the carbohydrate auxiliary either as allyl
glycosides or through the nitrogen atom of amino sugars, that is, as amides.
Following their excellent results in the cyclopropanation reaction of allyl
glycosides, Charette's group studied the epoxidation of the same substrates as
for the cyclopropanation under various conditions. Surprisingly, the vanadium-,
molybdenum-, or titanium-catalyzed epoxidation of
trans
-2
′
-butenyl 3,4,6-
tri-
O
-benzyl-
-d-glucopyranoside
1b
produced the desired epoxide
21
in very low
yields (0-40%) and no diastereoselectivity [18]. The use of
m
-CPBA (
meta
-
chloroperoxybenzoic acid) as oxidizing agent afforded the diastereomeric epoxides
in 80% diastereoselectivity when the C2 hydroxyl was free, but with no selectivity
when it was protected (Scheme 5.6). This fact combined with the absolute configu-
ration of the major isomer suggests preferential electrophilic attack of the peracid
from the
Re
face (relative to the C2 position of the C=C double bond) of the allylic
substrate, assisted through hydrogen bonding between the OH group at the C2
center of the sugar and the peracid. Surprisingly, it is interesting to stress that the
major epoxide obtained (
21
) has the opposite configuration of that obtained in
cyclopropanation of the same substrate, indicating that oxygen delivery from the
β
D-
gluco
BnO
BnO
BnO
BnO
Bn
BnO
BnO
Bn
BnO
O
m
-CPBA
O
O
O
Me
O
Me
O
Me
+
O
OH
O
OH
OH
21
major (2
R
, 3
S
)
21
minor (2
S
, 3
R
)
1b
Scheme 5.6
Diastereoselective epoxidation of allylic glucopyranoside
1b.
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