Trifluoroacetic Acid Promoted Ring Contraction in 2,3-Di-O-silylated O-Galactopyranosides and Hemiacetals

Abstract

AbstractA pyranose ring contraction of 2,3-di-O-silylated O-galactopyranosides with retention of aglycone promoted by anhydrous trifluoroacetic acid (TFA) in CH2Cl2 was demonstrated for the first time. In addition, TFA-promoted pyranose ring contraction of 2,3-bis-O-(triisopropylsilyl)-d-galactopyranose with formation of the corresponding anomeric triols in furanose form was successfully performed. A representative series of β-d-galactopyranosides with Me, Bn, allyl, or 3-(trifluoroacetamido)propyl aglycones has been investigated. TBDPS protective groups were found to be more stable than TIPS groups under conditions of TFA-promoted pyranose ring contraction. An easy access to 2,3-di-O-TBDPS-substituted allyl and benzyl galactofuranosides and 2,3-bis-O-(triisopropylsilyl)-β-d-galactofuranose may present an advantage in synthesis of selectively protected monosacharide building blocks, useful for the synthesis of biologically important oligosaccharides.