Cyclopropanation - Carbohydrates: Tools for Stereoselective Synthesis - page 111

Table 5.5

Epoxidation of propenyl-linked peracetylated N -acetyl glucosamine 22a-d.

D- gluco

R 1

R 1

AcO
Ac AcO

BnO
Bn BnO

m -CPBA

O O

O

R 2

R 2

O

O

NHAc CHCl 3

-15 °C

NHAc

R 3

R 3

22a-d

23a-d

Entry R 1 R 2 R 3 Epoxide Yield (%) de (%)

1 H H H 23a 41 80

2 Me H H 23b 57 95

3 H Ph H 23c 67 88

4 H Me Me 23d 52 43

peroxide in the epoxidation reaction takes place at the opposite side of the carbe-
noid species compared to the cyclopropanation reaction.
The epoxidation of propenyl-linked peracetylated N -acetyl glucosamine 22a-d
has been studied by the groups of Vega-Pérez and Takasu [19]. The use of m -CPBA
as oxidizing agent in CHCl 3 at 15 °C afforded the epoxides 23a-d with the (R)

absolute configuration in acceptable yields (42-71%) and moderate to good dias-
tereoselectivities (Table 5.5).
Better yields and diastereoselectivities were obtained by the use of conformation-
ally rigid, bicyclic, benzylidene acetals of glucosamine with a free or protected
hydroxyl group at C3 [20]. Oxidation of the allyl acetals 24, with a large amide
functionality at C2 of the sugar, with m -CPBA leads to the corresponding epoxides
25 in very high diastereoselectivity (up to 95% de) and good yields (Table 5.6). This
high facial control, and the fact that the major epoxide obtained has (R) -
configuration, implies that the hydrogen atom of the amide stabilizes the transi-
tion state of the oxygen transfer through hydrogen bonding with the attacking
peracid. Interestingly, even though the hydroxyl group at C3 is far from the alkene
function its protection decreases the diastereoselectivity of the oxidation. This
implies that besides the hydrogen bonding with amide function, the hydroxyl
group at C3 of the sugar helps in stabilizing the transition state for an efficient
oxygen transfer.
Alkenylidene acetals with five- and six-membered rings ( 26a-m ) related to those
already used in the Simmons-Smith cyclopropanation (Table 5.3) have also been
used as substrates in the diastereoselective epoxidation with m -CPBA (Scheme 5.7)
[21]. The final epoxides derived from N -acetyl-2-amino-2-deoxy-d-allopyranose,
d-altropyranose, d-galactopyranose, N -acetyl-d-glucosamine, d-glucofuranose, and
xylofuranose have been obtained with acceptable yields (62-90%) and low to mod-
erate diastereoselectivities (0-74% de).

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