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25.6 Cyclic Forms of Carbohydrates: Furanose Forms

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1 25.6 Cyclic Forms of Carbohydrates: Furanose Forms
1

2 Product is a hemiacetal.
Recall from Section 17.8 R"O C O H •• R R' C •• O • • R R' + R"OH Product is a hemiacetal. 5

3 Cyclic Hemiacetals C O R OH 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. 5

4 Carbohydrates Form Cyclic Hemiacetals
CH2OH 1 2 H OH O 1 2 3 4 3 4 equilibrium lies far to the right cyclic hemiacetals that have 5-membered rings are called furanose forms 2

5 stereochemistry is maintained during cyclic hemiacetal formation
D-Erythrose CH O CH2OH 1 H 2 H OH O 1 2 3 4 H H H OH 3 H OH H OH OH 4 stereochemistry is maintained during cyclic hemiacetal formation 2

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 3 2 3

12 anomeric carbon CH O CH2OH H H OH O H H H OH H OH H OH OH
D-Erythrose anomeric carbon CH O CH2OH 1 H 2 H OH O 1 2 3 4 H H H OH 3 H OH H OH OH 4 stereochemistry is variable at anomeric carbon; two diastereomers are formed 2

13 OH H O H OH O a-D-Erythrofuranose b-D-Erythrofuranose D-Erythrose 1 2
3 4 H OH O 1 2 3 4 a-D-Erythrofuranose b-D-Erythrofuranose 2

14 furanose ring formation involves OH group at C-4
D-Ribose CH O CH2OH H OH 1 2 3 4 5 furanose ring formation involves OH group at C-4 2

15 D-Ribose CH O CH2OH H OH 1 2 3 4 5 CH O H CH2OH OH HO 1 2 3 4 5 need C(3)-C(4) bond rotation to put OH in proper orientation to close 5-membered ring 2

16 D-Ribose CH O H HOCH2 OH 1 2 3 4 5 CH O H CH2OH OH HO 1 2 3 4 5 2

17 CH2OH group becomes a substituent on ring
D-Ribose CH O H HOCH2 OH 1 2 3 4 5 CH2OH group becomes a substituent on ring 2

18 CH2OH group becomes a substituent on ring
D-Ribose 5 CH O H HOCH2 OH 1 2 3 4 5 HOCH2 H OH O 1 2 3 4 b-D-Ribofuranose CH2OH group becomes a substituent on ring 2

19 25.7 Cyclic Forms of Carbohydrates: Pyranose Forms
1

20 Carbohydrates Form Cyclic Hemiacetals
CH2OH 1 2 3 4 5 5 OH O 4 1 3 2 H cyclic hemiacetals that have 6-membered rings are called pyranose forms 2

21 pyranose ring formation involves OH group at C-5
D-Ribose CH O CH2OH 1 2 3 4 5 H OH H OH H OH pyranose ring formation involves OH group at C-5 2

22 pyranose ring formation involves OH group at C-5
D-Ribose CH O CH2OH 1 2 3 4 5 CH O H CH2OH OH HO 1 2 3 4 5 H OH H OH H OH pyranose ring formation involves OH group at C-5 2

23 pyranose ring formation involves OH group at C-5
D-Ribose CH O H CH2OH OH HO 1 2 3 4 5 pyranose ring formation involves OH group at C-5 2

24 b-D-Ribopyranose H OH O HO CH O H CH2OH OH HO D-Ribose 5 1 4 3 2 1 2 3

25 D-Ribose H OH O 1 2 3 4 HO 5 OH H O 1 2 3 4 HO 5 a-D-Ribopyranose 2

26 pyranose ring formation involves OH group at C-5
D-Glucose CH O CH2OH 1 2 3 4 5 H HO OH 6 pyranose ring formation involves OH group at C-5 2

27 pyranose ring formation involves OH group at C-5
D-Glucose CH O CH2OH 1 2 3 4 5 H HO OH 6 OH CH O H CH2OH HO 1 2 3 4 5 6 pyranose ring formation involves OH group at C-5 2

28 D-Glucose CH O H CH2OH OH HO 1 2 3 4 5 6 need C(4)-C(5) bond rotation to put OH in proper orientation to close 6-membered ring 2

29 D-Glucose CH O H OH HOCH2 HO 1 2 3 4 5 6 CH O H OH CH2OH HO 1 2 3 4 5 6 need C(4)-C(5) bond rotation to put OH in proper orientation to close 6-membered ring 2

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

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

32 pyranose forms of carbohydrates adopt chair conformations
D-Glucose H OH O 1 2 3 4 HO HOCH2 5 6 b-D-Glucopyranose pyranose forms of carbohydrates adopt chair conformations 2

33 all substituents are equatorial in b-D-glucopyranose
D-Glucose 6 OH H HO HOCH2 O 6 HOCH2 5 OH 4 H O 5 H 4 1 OH H 2 3 1 HO 3 2 H H OH b-D-Glucopyranose all substituents are equatorial in b-D-glucopyranose 2

34 OH group at anomeric carbon is axial in a-D-glucopyranose
D-Glucose OH H HO HOCH2 O H OH HO HOCH2 O 1 1 b-D-Glucopyranose a-D-Glucopyranose OH group at anomeric carbon is axial in a-D-glucopyranose 2

35 Figure 25.5 CH O CH2OH H OH Less than 1% of the open-chain form of D-ribose is present at equilibrium in aqueous solution. 2

36 Figure 25.5 76% of the D-ribose is a mixture of the a and b- pyranose forms, with the b-form predominating OH H HO O b-D-Ribopyranose (56%) HO a-D-Ribopyranose (20%) H OH O 1 2

37 The a and b-furanose forms comprise 24% of the mixture.
Figure 25.5 The a and b-furanose forms comprise 24% of the mixture. HOCH2 H OH O HOCH2 OH H O b-D-Ribofuranose (18%) a-D-Ribofuranose (6%) 2

38 25.8 Mutarotation 10

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
OH H HO HOCH2 O H OH HO HOCH2 O 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D ° 2

41 Mutarotation of D-Glucose
OH H HO HOCH2 O H OH HO HOCH2 O 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D ° Final: [a]D +52.5° 2

42 Mutarotation of D-Glucose
OH H HO HOCH2 O H OH HO HOCH2 O 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D ° Final: [a]D +52.5° 2

43 Mutarotation of D-Glucose
OH H HO HOCH2 O H OH HO HOCH2 O 1 1 b-D-Glucopyranose a-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. 2

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