260
2
General Synthetic Methods
⊡ Scheme 67
⊡ Scheme 68
⊡ Scheme 69
As for the introduction of a selenium atom into carbohydrates, addition reactions to glycals
are also effective. Thus, the glucal 81 can be treated with phenylselenyl chloride and gives
mainly the trans-diaxial product 153 ( > Scheme 69) [112]. Similarly, azidophenylselenylation
of perbenzylated glucal 104 proceeds smoothly to give 2-Se-phenyl-2-selenoglycosylazides
154 and 155 ( > Scheme 69) [113].
4.4 Rearrangement Reactions
4.4.1 1,2-Migrations
In 1,2-trans-thioglycosides, DAST can mediate a 1,2-migration of the anomeric thio functionality through an episulfonium ion to give the 1,2-trans-product, as illustrated by the conversion
of 156 to 157 ( > Scheme 70) [101]. And under the same conditions 2-hydroxy-1-seleno gly-
2
General Synthetic Methods
⊡ Scheme 67
⊡ Scheme 68
⊡ Scheme 69
As for the introduction of a selenium atom into carbohydrates, addition reactions to glycals
are also effective. Thus, the glucal 81 can be treated with phenylselenyl chloride and gives
mainly the trans-diaxial product 153 ( > Scheme 69) [112]. Similarly, azidophenylselenylation
of perbenzylated glucal 104 proceeds smoothly to give 2-Se-phenyl-2-selenoglycosylazides
154 and 155 ( > Scheme 69) [113].
4.4 Rearrangement Reactions
4.4.1 1,2-Migrations
In 1,2-trans-thioglycosides, DAST can mediate a 1,2-migration of the anomeric thio functionality through an episulfonium ion to give the 1,2-trans-product, as illustrated by the conversion
of 156 to 157 ( > Scheme 70) [101]. And under the same conditions 2-hydroxy-1-seleno gly-