When the following aldohexose exists in its D-configuration, the total number of stereoisomers in its pyranose form is
Answer: 8
Solution
The given aldohexose has the structure shown. When it cyclizes to form the pyranose (six-membered ring) form, a new chiral center is created at the anomeric carbon (C-1). The aldohexose shown has 3 chiral centers in its open-chain form (at C-3, C-4, and C-5, noting that C-2 has two identical H atoms). When cyclization occurs to form the pyranose, the anomeric carbon (C-1) becomes a new chiral center, giving 2 anomers (α and β). Since we are considering only the D-configuration, and there are 3 chiral centers that can vary in the pyranose form (C-2, C-3, C-4 after accounting for the ring structure), the total number of stereoisomers = 23=8 (including both anomers for each configuration).
