Silica gel-catalyzed migration of acetyl groups from a sulfur to an oxygen atom

During the chromatographic separation of 3-S-acetyl-1,2-O-isopropylidene-3-thio-α-d-allofuranose on silica gel, a migration of the acetyl group from S to O was observed to give 6-O-acetyl-1,2-O-isopropylidene-3-thio-α-d-allofuranose, whereas 3-S-acetyl-6-O-benzoyl-1,2-O-isopropylidene-3-thio-α-d-allofuranose gave 5-O-acetyl-6-O-benzoyl-1,2-O-isopropylidene-3-thio-α-d-allofuranose. No acetyl migration was observed, however, in the case of 3-O-acetyl-1,2-O-isopropylidene-α-d-allofuranose.