Enantioselective Additions to Oxocarbenium Ions via Anion Binding

Significance

Enantioselective additions to oxocarbenium ions in the presence of chiral thiourea catalyst 1 are reported. Isochroman methyl acetals 2 are converted into the corresponding chlo­roisochromans 3 and used directly without purification in a one-pot, two-stage procedure. Thiourea catalyst 1 assisted chloride dissociation from these glycosyl chlorides generates a chiral oxocarbenium-chloride-thiourea complex 4 as the reactive electrophilic intermediate. Subsequent attack of silyl ketene acetals 5 as nucleophile leads to enantiomerically enriched products 6.