title: Regioselective reductive openings of 4,6-benzylidene acetals: synthetic and mechanistic aspects.
abstract: The use of benzylidene acetals as protecting groups in carbohydrate chemistry is utterly important. The main advantage of benzylidene acetal is the ability for regioselective openings. 4,6-benzylidene acetal can be opened selectively under reductive conditions to yield either free 4-OH or 6-OH. There are a plethora of methods available for regioselective openings, but only a few of these are widely used. In recent years, the mechanism has been investigated for borane mediated openings and it seems likely that the regioselectivity is determined by borane, rather than Lewis acid. When borane is activated by Lewis acids, borane is the most electrophilic species that consequently coordinates to the most nucleophilic oxygen of the acetals, usually O-6. This results in the formation of 6-O-benzyl ethers. If borane is not activated, Lewis acid is the most electrophilic species that thus adds to O-6 and hence generates the 4-O-benzyl ether.
authors.affiliation: Center for Analysis and Synthesis, Lund University, PO Box 124, SE-221 00 Lund, Sweden.
authors.forename: Markus
authors.initials: M
authors.lastname: Ohlin
authors.forename: Richard
authors.initials: R
authors.lastname: Johnsson
authors.forename: Ulf
authors.initials: U
authors.lastname: Ellervik
mesh_terms: D000080:Acetals / Q000138:chemical synthesis*; D001597:Benzylidene Compounds / Q000138:chemical synthesis*; D001880:Boranes / Q000737:chemistry; D002236:Carbohydrate Conformation; D002625:Chemistry, Organic / Q000379:methods*; D058116:Lewis Acids / Q000737:chemistry; D010084:Oxidation-Reduction; D013237:Stereoisomerism
chemical_list: D000080:Acetals; D001597:Benzylidene Compounds; D001880:Boranes; D058116:Lewis Acids