25.6 Cyclic Forms of Carbohydrates: Furanose Forms

1
25.6Cyclic Forms of CarbohydratesFuranose Forms
2
Recall from Section 17.8

R"OH

• Product is a hemiacetal.

3
Cyclic Hemiacetals
R
OH
C
O

• Aldehydes and ketones that contain an OH group
  elsewhere in the molecule can undergo
  intramolecular hemiacetal formation.
• The equilibrium favors the cyclic hemiacetal if
  the ring is 5- or 6-membered.

4
Carbohydrates Form Cyclic Hemiacetals
1
2
3
4

• equilibrium lies far to the right
• cyclic hemiacetals that have 5-membered ringsare
  called furanose forms

5
D-Erythrose
1
H
H
H
2
H
OH
3
H
OH
H
OH
OH
4

• stereochemistry is maintained during
  cyclichemiacetal formation

6
D-Erythrose
1
2
3
4
7
D-Erythrose
1
2
1
4
turn 90
3
2
3
4
8
D-Erythrose

• move O into position by rotating about bond
  between carbon-3 and carbon-4

1
4
2
3
9
D-Erythrose
1
1
4
4
2
2
3
3
10
D-Erythrose

• close ring by hemiacetal formation between OH at
  C-4 and carbonyl group

1
4
2
3
11
D-Erythrose
1
1
4
4
2
2
3
3
12
D-Erythrose
anomeric carbon
1
H
H
H
2
H
OH
3
H
OH
H
OH
OH
4

• stereochemistry is variable at anomeric
  carbontwo diastereomers are formed

13
D-Erythrose
a-D-Erythrofuranose
b-D-Erythrofuranose
14
D-Ribose

• furanose ring formation involves OH group at C-4

15
D-Ribose

• need C(3)-C(4) bond rotation to put OH in proper
  orientation to close 5-membered ring

16
D-Ribose
17
D-Ribose

• CH2OH group becomes a substituent on ring

18
D-Ribose
5
b-D-Ribofuranose

• CH2OH group becomes a substituent on ring

19
25.7Cyclic Forms of CarbohydratesPyranose Forms
20
Carbohydrates Form Cyclic Hemiacetals
5
OH
O
1
4
H
2
3

• cyclic hemiacetals that have 6-membered ringsare
  called pyranose forms

21
D-Ribose
OH
H
H
OH
H
OH

• pyranose ring formation involves OH group at C-5

22
D-Ribose
OH
H
H
OH
H
OH

• pyranose ring formation involves OH group at C-5

23
D-Ribose

• pyranose ring formation involves OH group at C-5

24
D-Ribose
b-D-Ribopyranose
25
D-Ribose
a-D-Ribopyranose
26
D-Glucose

• pyranose ring formation involves OH group at C-5

27
D-Glucose

• pyranose ring formation involves OH group at C-5

28
D-Glucose

• need C(4)-C(5) bond rotation to put OH in proper
  orientation to close 6-membered ring

29
D-Glucose

• need C(4)-C(5) bond rotation to put OH in proper
  orientation to close 6-membered ring

30
D-Glucose
b-D-Glucopyranose
31
D-Glucose
b-D-Glucopyranose
a-D-Glucopyranose
32
D-Glucose
b-D-Glucopyranose

• pyranose forms of carbohydrates adopt chair
  conformations

33
D-Glucose
6
HOCH2
6
5
OH
H
4
O
5
H
1
4
H
OH
2
3
HO
1
H
2
3
OH
H
b-D-Glucopyranose

• all substituents are equatorial in
  b-D-glucopyranose

34
D-Glucose
1
1
a-D-Glucopyranose
b-D-Glucopyranose

• OH group at anomeric carbon is axialin
  a-D-glucopyranose

35
Figure 25.5

• Less than 1 of the open-chain form of D-ribose
  is present at equilibrium in aqueous solution.

36
Figure 25.5

• 76 of the D-ribose is a mixture of the a and b-
  pyranose forms, with the b-form predominating

37
Figure 25.5

• The a and b-furanose forms comprise 24 of the
  mixture.

b-D-Ribofuranose (18)
a-D-Ribofuranose (6)
38
25.8Mutarotation
39
Mutarotation

• Mutarotation is a term given to the change in
  the observed optical rotation of a substance with
  time.
• Glucose, for example, can be obtained in either
  its a or b-pyranose form. The two forms have
  different physical properties such as melting
  point and optical rotation.
• When either form is dissolved in water, its
  initial rotation changes with time. Eventually
  both solutions have the same rotation.

40
Mutarotation of D-Glucose
1
1
a-D-Glucopyranose
b-D-Glucopyranose
Initial aD 18.7
Initial aD 112.2
41
Mutarotation of D-Glucose
1
1
a-D-Glucopyranose
b-D-Glucopyranose
Initial aD 18.7
Initial aD 112.2
Final aD 52.5
42
Mutarotation of D-Glucose
1
1
a-D-Glucopyranose
b-D-Glucopyranose
Initial aD 18.7
Initial aD 112.2
Final aD 52.5
43
Mutarotation of D-Glucose
1
1
a-D-Glucopyranose
b-D-Glucopyranose

• Explanation After being dissolved in water, the
  a and b forms slowly interconvert via the
  open-chain form. An equilibrium state is reached
  that contains 64 b and 36 a.