Acid catalyzes reversible ketal formation between acetone and suitably oriented hydroxyl groups. Glucose can equilibrate to its furanose form, allowing favourable 1,2- and 5,6-isopropylidene protection. The product is 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose, leaving the 3-OH free. Among the drawn structures, B has this furanose ring and the two correct acetals; the pyranose alternatives and different hydroxyl pairing are incorrect.
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