TIPS group-assisted isomerization of benzyl protected d-manno- and d-glucopyranose to d-fructofuranose derivatives

Base-promoted (MeONa in MeOH or imidazole in DMF) isomerization of a series of 3,4,6-tri-O-benzyl-d-gluco- and d-mannopyranose derivatives with triisopropylsilyl (TIPS) substituents was studied. The presence of a bulky TIPS group at O-1 or O-2 was shown to be favorable for the isomerization of benzyl protected d-gluco- and d-mannopyranose derivatives to d-fructofuranose derivatives, in which the bulky silyl group occupies less sterically hindered primary position. The highest yield (33%) of the fructofuranose derivative was achieved when 3,4,6-tri-O-benzyl-2-O-triisopropylsilyl-d-mannopyranose was treated with MeONa in MeON at 50 °C.

Trifluoroacetic acid-promoted ring contraction in 2,3-di-O-silylated O-galactopyranosides and hemiacetals

A pyranose ring contraction of 2,3-di-O-silylated O-galactopyranosides with retention of aglycone promoted by anhydrous trifluoroacetyc 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 bilding blocks, useful for the synthesis of biologically important oligosaccharides.