Platinum-catalyzed ring opening of 1,2-cyclopropanated sugars with O-nucleophiles. Convenient synthesis of 2-C-branched carbohydrates

A new reaction is described catalyzed by Zeise's dimer that allows for ring opening of 1,2-cyclopropanated sugars with O-nucleophiles to give 2-C-bran ched carbohydrates. A number of O-nucleophiles can participate in the ring opening including alcohols, phenols, and water. A wide range of alcohols ha s been employed giving 2-C-branched glycosides ranging from simple methyl g lycosides to more complex disaccharides. A very high diastereoselectivity i s obtained at the newly formed C-1 stereocenter. The alpha -glycoside, favo red by the anomeric effect, is always the major product in the ring opening with alcohols regardless of the stereochemistry of the starting cyclopropa ne. When electron-rich phenols are employed as O-nucleophiles, rearrangemen t to the glycosyl arene has been observed. In general, the ring opening occ urs readily with unsubstituted sugar cyclopropanes to give 2-C-methyl carbo hydrates. However, cyclopropanes with ester or alkyl substituents are signi ficantly less reactive and some even completely inert to ring opening. When the ring opening is carried out with PdCl2(PhCN)(2) catalyst, a Ferrier-ty pe rearrangement occurs, giving 2,3-unsaturated glycosides containing a 2-C -alkyl substituent.