Protecting neighboring-group participation

Acyl protection should lead, as a consequence of neighboring-group participation and the anomeric effect, exclusively to a products. This has been proved in many experiments (Table XV) with Me3SiOTf as catalyst excellent yields could be obtained in cases where all other methods essentially failed (129). It could be shown that at least some of the reactions proceed via rapid orthoester formation (129), and this intermediate then rearranges under Me3SiOTf catalysis to the desired reaction product. 

Acyl-protected fucosyl donors have also been generated very successfully (Table XXXII) (58b). Their reaction with acceptors led via neighboring-group participation to products (58b, 178). 

Keywords Carbohydrate Chemistry, Carbohydrate Protection, Epimerization, Inversion, Dynamic, Regioselective Control, Neighboring Group Participation... 

The consecutive reduction and cyclization of O-benzoyl protected 5-0-methylhexose 0-(terf-butyldiphenylsilyl)oxime (104) with dimethylphenylsilane in trifluoroacetic acid afforded a iV-hydroxypyrrolidine (105) ring system in good yield (equation 44). The mechanism involves a cascade of neighboring group participation steps involving the 0-benzoyl protecting groups " ". ... 

The high a-selectivity is based on the absence of neighbor group participation. Protection of sugar 5 with acetyl groups and using the same epoxidation protocol leads to a 1 1 a ft mixture of the epoxides. 

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