Ch 27 - Carbohydrates

Question 0

What are epimers?

Answer 0

2 diastereomers that differ in configuration around ONE stereogenic ctr only

Question 1

How to use OH to label sugars w/many stereogenic ctrs as D or L?

Answer 1

• OH on stereogenic ctr farthest from carbonyl on RIGHT ➡ D (natural)
• OH on stereogenic ctr farthest from carbonyl on LEFT ➡ L

Question 2

What is the anomeric carbon?

Answer 2

• Cyclization of sugar ➡ new stereogenic ctr at hemiacetal C

* former aldehyde C

Question 3

Two types of anomeric C’s?

Answer 3

• alpha anomer - OH ⬇

* beta anomer - OH ⬆

Question 4

3 steps to draw haworth projection (flat rings) from acyclic aldohexose? (also know reverse)

Answer 4

1) draw flat 6 mem. ring, O in upper right corner. Add CH2OH on first counterclockwise C (D: ⬆, L: ⬇)
2) first C clockwise from O is anomeric C, add its OH (alpha: ⬇, beta: ⬆)
3) add other subs (right: ⬇, left: ⬆)

Question 5

2 Steps to convert Haworth projection ➡ chair form?

Answer 5

1) draw chair w/O as ⬆ atom
2) add subs: ⬆ in Haworth: ⬆ in chair, ⬇ in Haworth: ⬇ in chair (OH’s equatorial, H’s axial; hemiacetal OH: ⬆ eq. (beta), ⬇ axial (alpha)

Question 6

Hemiacetal rxn ➡ glycoside (acetal), reagent?

Answer 6

ROH/HCl

Question 7

Mechanism for glycoside formation? (4 steps)

Answer 7

1) protonate hemiacetal OH w/acid
2) H2O leaves ➡ carbocation
3) alcohol attacks C+
4) Cl- deprotonates alcohol

Question 8

Glycoside hydrolysis rxn? (know mechanism!)

Answer 8

• Use mild acid (H3O+) and water ➡ cyclic hemiacetal + alcohol
• reverse mechanism of glycoside formation

Question 9

Monosaccharides ➡ ether, Reagent? (also: mechanism of Williamson ether synthesis?)

Answer 9

Ag2O/xs RX

1) deprotonate alcohol
2) alkoxy attacks RX (Sn2)

Question 10

Monosaccharides ➡ esters, reagent?

Answer 10

Ac2O or Acl/pyridine

Ac = acetyl group

Question 11

Red of carbonyl group of Aldose ➡ alditol (primary alcohol), reagent?

Answer 11

NaBH4/CH3OH

Question 12

Oxi of Aldose ➡ aldonic acids (carbonyl ➡ COOH) or aldaric acids (carbonyl & CH2OH ➡ COOH at both ends), reagents?

Answer 12

• aldonic acids (3): Ag2O/NH4OH, Cu2+, Br2

* aldaric acids: HNO3/H2O

Question 13

How to remove 1C from aldose? Reagents used?

Answer 13

Wohl degradation: 
1) NH2OH 
2) Ac2O, NaOAc
3) NaOCH3
• C1-C2 bond cleaved, C2 becomes new CHO

Question 14

How to add 1C from aldose? Reagents used?

Answer 14

Kiliani-Fischer Synthesis:

1) NaCN, HCl
2) H2/Pd, BaSO4
3) H3O+

Question 15

Mechanisms to know for exam!

Answer 15

• glycoside formation

* glycoside hydrolysis