Ring-selective synthesis of O-heterocycles from acyclic 3-O-allyl-monosaccharides via intramolecular nitrone–alkene cycloaddition

Intramolecular 1,3-dipolar cycloadditions of nitrones formed from 3-O-allyl-d-glucose and d-altrose (both with threo-configuration at C-2,3) afford oxepanes selectively whereas the same reactions of nitrones derived from 3-O-allyl-d-allose and d-mannose (both with erythro-configuration at C-2,3) give tetrahydropyrans selectively.