4.2 Cyclic Sugar Molecules

Question 1

What do Monosaccharides contain?

Answer 1

• both a hydroxyl group, which can serve as a nucleophile, and a carbonyl group which is the most common electrophile

Question 2

What can result from intramolecular rxns in monosaccharides?

Answer 2

• cyclic hemiacetals (from aldoses) and hemiketals (from ketoses)
• Regardless of whether hemiacetal or hemiketal is formed, the carbonyl carbon becomes chiral in this process, and is referred to as the anomeric carbon (which was the carbonyl carbon)

Question 3

Which cyclic molecules are stable in solution?

Answer 3

• sixmembered pyranose rings or five-membered furanose rings

Question 4

Anomers

Answer 4

• two molecules differ at the
  anomeric carbon
• cyclization of a sugar molecule: α or β are anomers

Question 5

When we convert the monosaccharide from its straight-chain Fischer projection to the Haworth projection which groups are up and which are down?

Answer 5

any group on the right in the
Fischer projection will point down

Question 6

What happens when hemiacetal rings are exposed to water?

Answer 6

• will cause them to spontaneously cycle between the open and closed form

Question 7

Mutarotation

Answer 7

• The spontaneous change of
  configuration about C-1
• occurs more rapidly when the reaction is catalyzed with an acid or base
• results in a mixture that contains both α- and β anomers at equilibrium concentrations (for glucose: 36% α, 64% β)

Question 8

Which configuration is favorable α- or β?

Answer 8

the α-anomeric configuration is less favored because the hydroxyl group of the anomeric
carbon is axial, adding to the steric strain of the molecule.