11/14/2016. NaBH 4. Superimposable on mirror image. (achiral) D-ribose. NaBH 4 NaBH 4. Not superimposable on mirror image.

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1 There are multiple ways to relate the structures of ketoses to aldoses. Reduction of a ketose with NaB 4 forms a new stereocenter and thus forms a mixture of two alditols. An aldopentose forms a chiral alditol upon reduction with NaB 4 that is also one of the alditols formed in reduction of D-ribulose. Identify this aldopentose. D-ribulose D-fructose This tells us that D-fructose, D-glucose, and D-mannose all have the same configuration at carbons 3, 4, and 5. D-ribose D-arabinose D-xylose A. B. C. D. none of the above D-glucose D-mannose 1 2 NaB 4 rotate 180 D-ribulose NaB 4 NaB 4 Superimposable on mirror image. (achiral) rotate 180 D-ribose Now, which alditol is chiral? D-arabinose Not superimposable on mirror image. (chiral) 3 4 1

2 Another reaction sequence used in relating structures of different monosaccharides is the Ruff degradation. Which pair of D-aldohexoses will form the same D-aldopentose upon Ruff degradation? D-erythrose D-glyceraldehyde D-threose D-glyceraldehyde This is how we can determine that the natural (D) form of erythrose and threose have the same configuration at the last stereocenter as D-glyceraldehyde. 5 D-glucose D-mannose D-galactose A. D-glucose and D-mannose B. D-glucose and D-galactose C. D-mannose and D-galactose D. None of the above 6 Remember hemiacetal formation: D-glucose D-arabinose D-mannose hemiacetal A hemiacetal is an intermediate in acetal formation. 4- and 5-hydroxy aldehydes (and ketones) form cyclic hemiacetals. D-galactose D-lyxose 7 8 2

3 Sugars form 5- and 6-membered ring cyclic hemiacetals and for most sugars these cyclic forms are much more stable than the acyclic forms. A 6-membered ring form is called a pyranose. A 5-membered ring form is called a furanose. These names come from the related structures pyran and furan. Carbohydrates exist primarily as cyclic hemiacetals. Glucose forms a 6-member ring by reaction of the C-5 oxygen atom with the aldehyde (C-1) carbon. 4 -pyran furan 9 D-glucose b-d-glucopyranose a-d-glucopyranose The b-anomer. O at C-1 to the left. The a-anomer. O at C-1 to the right. A new stereocenter is formed at C-1. The two stereoisomers formed are called anomers. 10 A more realistic view of a cyclic sugar is provided by a aworth formula. To go from a Fischer projection to a aworth formula. = 6 1 For pyranose sugars, draw a 6-member ring with oxygen in the upper right corner. The anomeric carbon (C-1 of an aldose) is on the far right. b-d-glucopyranose

4 Substituents on the right in the Fischer projection are drawn down in the aworth formula. Substituents on the left are drawn up. a-d-galactopyranose. b-d-glucopyranose The C 2 O group (C-6), is attached to C-5 of the ring and points up in a D-sugar. D-galactose 13 A. B. C. D. 14 Now for a furanose. b-d-ribofuranose. D-fructose a-d-fructofuranose D-ribose Draw a 5-member ring with oxygen at the top. The rest is the same as with a pyranose. A. B. C. D

5 b-d-ribofuranose. The structure of a pyranose sugar is more accurately represented by a chair form. D-ribose O O O b-d-glucopyranose = Notice all substituents are equatorial. Steric effects favor the b over the a anomer. A. B. C. D. There is an electronic effect involving the ring oxygen atom (the anomeric effect) that favors the a anomer (axial O). The two effects almost cancel so that glucose exists in solution as 64% b and 36% a A sugar with the O at the anomeric carbon replaced with OR is called a glycoside. Glycosides are stable under neutral and basic conditions but are hydrolyzed by aqueous acid. Simple glycosides are formed by reaction of a monosaccharide with an alcohol catalyzed by acid. 3 O methyl a-d-glucopyranoside C 3 O D-glucose methyl a-d-glucopyranoside methyl b-d-glucopyranoside C 3 O 2 O The squiggly line to the anomeric O indicates a mixture of anomers

6 methyl a-d-idopyranoside. D-idose 2. B 6. A 14. D 16. A 21. B Carbohydrate-related topics not covered on Exam 3: Disaccharides, oligosaccharides, polysaccharides Reducing vs. non-reducing sugars The anomeric effect A B C D