462
3
Chemical Glycosylation Reactions
⊡ Scheme 2
⊡ Scheme 3
AW 300) [44], and Nafion ® -SAC resin [19] (see above) are highly valuable solid-supported
acid catalysts. This is nicely demonstrated in an avermectin B 1a analog synthesis [43]: Rhamnosylation of intermediate 5b with donor 5a gave target molecule 5c in quantitative yield
( > Scheme 5).
The solvent of choice in the glycosylation reactions is dichloromethane. In combination with
a solvent of low polarity and/or of low donicity (such as for instance cyclohexane, petroleum
ether, etc.), BF 3 OEt 2 as a mild catalyst and at low temperature, S N 2-type reactions could be
3
Chemical Glycosylation Reactions
⊡ Scheme 2
⊡ Scheme 3
AW 300) [44], and Nafion ® -SAC resin [19] (see above) are highly valuable solid-supported
acid catalysts. This is nicely demonstrated in an avermectin B 1a analog synthesis [43]: Rhamnosylation of intermediate 5b with donor 5a gave target molecule 5c in quantitative yield
( > Scheme 5).
The solvent of choice in the glycosylation reactions is dichloromethane. In combination with
a solvent of low polarity and/or of low donicity (such as for instance cyclohexane, petroleum
ether, etc.), BF 3 OEt 2 as a mild catalyst and at low temperature, S N 2-type reactions could be