A selective Sc(OTf)3-catalyzed trialkylsilyl ether to acetyl ester exchange reaction with beta-l-idopyranoside and 3,4-O-isopropylidene-beta-D-galactopyranoside derivatives.
The selective silylation of monosaccharide building blocks is useful for preparing complex oligosaccharides. We now report that the diol, methyl (dimethylthexylsilyl 3-O-pivaloyl-beta-L-idopyranosyl)uronate, can be selectively silylated at the O-2 position by trialkylsilyl triflates. After protection of O-4, the O-2 silyl group can be selectively replaced by acetate by taking advantage of a trialkylsilyl-acetate exchange reaction catalyzed by Sc(OTf)3 in the presence of acetic anhydride. The high O-2 selectivity is shown for triethylsilyl (TES), tert-butyldimethylsilyl (TBS), and triisopropylsilyl (TIPS). The selective cleavage reaction only worked well for TES and TBS derivatives. A selection of silyl triflates and silyl chlorides were used as silylating reagents with ethyl 3,4-O-isopropylidene-1-thio-beta-D-galactopyranoside. In most cases, silylation afforded 2,6-di-O-silylated products in high yields. Studies on the cleavage reaction showed that only the primary silylated protecting groups were replaced by acetyl groups. This reaction worked with a variety of silyl protecting groups but not the tert-butyldiphenylsilyl (TBDPS) protecting group. Unfortunately, the 1-thioethyl group was also sensitive to the Sc(OTf)3, leading in these conditions to alpha/beta mixtures of the 1-acetates, which compromised the synthetic utility of this reaction for these compounds. The sequence presented here is a useful synthetic route to differentially protected L-iduronic acid building blocks.
Ke
Weijun
W
Institute for Biological Sciences, National Research Council, Ottawa, Canada ON K1A 0R6.
Whitfield
Dennis M
DM
D000085:Acetates / Q000737:chemistry; D004987:Ethers / Q000737:chemistry; D006601:Hexoses / Q000737:chemistry*; D008722:Methods; D009005:Monosaccharides / Q000737:chemistry*; D012821:Silanes / Q000737:chemistry*; D014574:Uronic Acids / Q000737:chemistry*
C453002:3,4-O-isopropylidene-galactopyranoside; D000085:Acetates; D004987:Ethers; D006601:Hexoses; D009005:Monosaccharides; D012821:Silanes; D014574:Uronic Acids; C500613:methyl (dimethylhexylsilyl 3-O-pivaloylidopyranosyl)uronate; C010997:idose