Some protecting groups can protect two hydroxy groups on a carbohydrate at the same time. One such group protects the OH groups at carbon 4 and carbon 6 of β-D-mannose, as shown below. 

a. To what functional group does this protecting group belong? 
b. What reagent is employed in the synthesis of such protected compounds? 
c. When this blocking group is coupled with β-D-mannose, a new chiral center is formed. What is its location? 
d. Draw the compound's other stereoisomer and assess which stereoisomer should be more stable.

• A. a. The protecting group formed a hemiacetal group
  b. This can be produced using benzaldehyde with the sugar.
  c. The acetal carbon is the new chiral center
  d. The stereoisomer with the phenyl group at the axial position is more stable .
• B. a. The protecting group formed an aldehyde group
  b. This can be produced using benzoic acid with the sugar.
  c. The aldehyde carbonyl carbon is the new chiral center
  d. The stereoisomer with the phenyl group at the equatorial position is more stable .
• C. a. The protecting group formed an acetal group
  b. This can be produced using benzaldehyde with the sugar.
  c. The acetal carbon becomes the new chiral center
  d. The stereoisomer with the phenyl group at the equatorial position is more stable .
• D. a. The protecting group formed a hemiacetal group
  b. This can be produced using phenol with the sugar.
  c. The anomeric carbon is the new chiral center
  d. The stereoisomer with the phenyl group at the axial position is more stable.

Answer: (C)