Highly regioselective acetylation of primary hydroxy groups in thioglycoside derivatives with gluco- and galacto-configurations was achieved by treatment with aqueous or anhydrous acetic acid (60-100% AcOH) at elevated temperatures (80-118 °C), avoiding complex, costly and time-consuming manipulations with protective groups. Acetylation of both 4,6-O-benzylidene acetals and the corresponding diols as well as the unprotected tetraol with AcOH was shown to lead selectively to formation of 6-O-acetyl derivatives. For example, the treatment of phenyl 1-thio-β-d-glucopyranoside with anhydrous AcOH at 80 °C for 24 h gave the corresponding 6-O-acetylated derivative in 47% yield (71% based on the reacted starting material) and unreacted starting tetraol in 34% yield, which can easily be recovered by silica gel chromatography and reused in further acetylation.
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