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25.6 Cyclic Forms of Carbohydrates: Furanose Forms

25.6 Cyclic Forms of Carbohydrates: Furanose Forms. R. R. ••. ••. O. R"O. C. O. H. C. • •. ••. ••. ••. R'. R'. Recall from Section 17.8. Product is a hemiacetal. +. R"O H. R. O. C. O H. Cyclic Hemiacetals. R. OH.

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25.6 Cyclic Forms of Carbohydrates: Furanose Forms

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  1. 25.6Cyclic Forms of Carbohydrates:Furanose Forms

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

  3. R O C OH Cyclic Hemiacetals R OH • 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. C O

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

  5. CH O OH O 1 4 H 3 2 CH2OH D-Erythrose 1 • stereochemistry is maintained during cyclichemiacetal formation H H H 2 H OH 3 H OH H OH OH 4

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

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

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

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

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

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

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

  19. 25.7Cyclic Forms of Carbohydrates:Pyranose Forms

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  34. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH D-Glucose • OH group at anomeric carbon is axialin a-D-glucopyranose 1 1 b-D-Glucopyranose a-D-Glucopyranose

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

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

  37. HOCH2 HOCH2 OH H H H H H O O H OH H H OH OH OH OH 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. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH Mutarotation of D-Glucose 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D +112.2°

  41. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH Mutarotation of D-Glucose 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D +112.2° Final: [a]D +52.5°

  42. H H H H HOCH2 HOCH2 O O HO HO HO HO OH H OH OH H H H H H OH Mutarotation of D-Glucose 1 1 b-D-Glucopyranose a-D-Glucopyranose Initial: [a]D +18.7° Initial: [a]D +112.2° Final: [a]D +52.5°

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

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