A pyranose ring contraction
of ethyl 1-thio-β-d-galactopyranosides has been discovered
that proceeds with retention of aglycon under mildly acidic conditions
(aq TFA in CH2Cl2). Key factors for success
of this rearrangement are the presence of bulky silyl (TIPS or TBDPS)
substituents at both O-2 and O-3 and a free hydroxy
group at C-4 (derivatives with acid-labile protective groups at O-4
will also engage in this reaction). The rearrangement cleanly proceeds
for 2,3-di-O-TIPS derivatives with two hydroxy groups
at C-4 and C-6, acid-labile TES groups at O-4 and O-6, or one acyl
substituent (Bz, ClAc) at O-6. A possibility to switch the direction
of the debenzylidenation reaction in 4,6-O-benzylidene-2,3-di-O-TIPS/TBDPS derivatives by the choice of an acid (TFA,
which cleanly gives furanose, versus AcOH, which cleaves benzylidene
acetal only) may present an advantage in the divergent synthesis of
selectively protected glycosyl donors (either in furanose or pyranose
form) useful for the synthesis of biologically important oligosaccharides.
Highly regioselective acetylation of primary hydroxy groups in thioglycoside derivatives was achieved by treatment with aqueous or anhydrous acetic acid (60–100%) at elevated temperatures (80–118 °C), avoiding manipulations with protective groups.
Novel glycosyl donors with a triisopropylsilyl (TIPS) nonparticipating group at O-2 were introduced for use in 1,2-cis-fucosylation. Coupling of 2-O-TIPS-substituted thiofucosyl donors with N-trifluoroacetyl-β-D-glucosamine mono-and disaccharide derivatives was found to lead exclusively to α-linked oligosaccharides. No remote participation in 2-O-TIPS thiofucosyl donors seems to be required to favor α-selective fucosylation.
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