CHAPTER 28 HW: CARBOHYDRATES

Transcription

1 CAPTER 8 W: CARBYDRATES For all problems, consult the table of D-Monosaccharides provided in lecture if a structure or name is necessary. ISMERIC RELATINSIPS. Draw the following and answer the questions. Consult a table of D-monosaccharides for the names. Below is D-Talose Draw the enantiomer of D-Talose Draw a diastereomer of D-Talose that is an L sugar C C Switch every center C Keep at least, switch at least Must have C- on left C Enantiomer of the rd structure (D-Gulose) a. What is the name of the enantiomer of D-Talose? L-Talose b. What is the name of the diastereomer of D-Talose that you drew? L-Gulose (draw the enantiomer and check the table). Regarding the compounds below, C C D-Sorbose C C C a. What is the relationship between these two carbohydrates? Are they identical, enantiomers, diastereomers, constitutional isomers or not related (not isomers)? b. What is the name of the carbohydrate on the left? L-Sorbose Page

2 CYCLIC EMIACETALS AWRT PRJECTINS. Draw the following compounds as cyclic hemiacetals. For problem b, draw hemiacetals with two different sized rings. a. b.. Explain why the hemiacetal is favored in reaction (), but less so in reaction (). () () C the K would be <. C Reaction () has similar energetics as hydrate formation, which would be K~ for an unconjugated aldehyde. This K is a D G term, so takes into account both D and D S. Since D S is probably unfavorable ( reactants à product), the hemiacetal formation must have a favorable, or negative, D otherwise Reaction () is so similar to reaction () that it probably has a similar D (favorable). owever, a difference is that D S is probably ~zero since the reaction converts reactant à product. With D S no longer a hindrance to hemiacetal formation, the K is favorable in reaction () (Negative D leads to negative D G).. Describe each aworth projection by one of the following classifications: a -aldopyranose a - ketopyranose!(-ring) a - aldofuranose (-ring) a -ketofuranose b -aldopyranose b -ketopyranose! b -aldofuranose b -ketofuranose Carb β Aldose Ketose α C Type b -aldopyranose a -ketofuranose a -aldopyranose b -aldofuranose Page

3 . Draw aworth projections for each carbohydrate: a. a -D-idopyranose C C C up if D sugar α b. a b! forms of L-mannopyranose C D-mannose c. b -D-lyxopyranose C C L-mannose C C C down if L sugar C C d. a -D-sorbofuranose C C C C C α e. a b!forms of L-psicofuranose C C C C C D-psicose L-psicose C Page

4 f. b -D-ribulofuranose C C C Page

5 ISMERIC RELATINSIPS 8. What s the relationship between the carbohydrates in each set? Are they identical, enantiomers, diastereomers, constitutional isomers or not related (not isomers)? C Set C Relationship Diastereomers (or anomers) Constitutional Isomers (doesn t matter they re from the same sugar) MUTARTATIN 9. Explain how the a b forms of the carbohydrate below can interconvert. Draw the structure of a key intermediate in the process. The mutarotation process goes through a flat intermediate (the carbonyl in the open form, shown above), and when the attacks the carbonyl, it can attack from the top or bottom, which leads to the a or b forms. CAIR CNFRMATINS 0. Draw each carbohydrate in a chair conformation. Draw either the a or b form for problem b. Carb C C C Chair Page

6 MNSACCARIDE REDX REACTINS. Draw two five-carbon D-aldoses that would give an achiral product upon reaction with mild N. Briefly explain why the products are achiral. C C D-Ribose C C D-Xylose Both products have an internal mirror plane of symmetry, so they are achiral (meso compounds).. Give two carbohydrates that would give this product upon reaction with NaB and ethanol. C C C C NaB C C C C NaB C C C = C (Note you could also start with a certain ketose ). Give the major organic product of each reaction. If a significant mixture is expected, draw all products. a. Ag(N ) d. - b. c. C C C C NaB C C N 0 C C C C C C e. Ag(N ) - NaB C C C C C C Page

7 ACETAL FRMATIN YDRLYSIS. For the following process: S C C C a. Draw the curved arrow mechanism to show formation of one of the acetal products. S C C C C b. Use the mechanism to explain why two acetals can be formed. The mechanism goes through a flat intermediate (shown below), and the C can attack from either the top or bottom to make the two isomers. c. The only methyl ether formed is at C-. Explain why the anomeric carbon is more reactive in this mechanism than carbons -. When the anomeric alcohol becomes protonated and leaves, it forms a resonance stabilized cation (the intermediate shown in the previous problem), which is a low energy intermediate. Reaction of any of the other alcohols would form non-resonance stabilized cations.. Give the curved arrow mechanism to show formation of one of the acetal products. C S C C C C C C C C C S C C C C C C C C C C C C C Page 7

8 . Give the curved arrow mechanism for these reactions. a. C S (αβ) C C C (αβ) products b. C S C (αβ) C C C C (αβ) C C 7. Give the major organic product of these reactions. If a significant mixture is expected, draw all products. Note: Maintain the -membered ring in be. a. S C C C C C C b. C C C C S C (C ) C C C c. C S d. C C C S C C C Page 8

9 7 continued. e. C S C REACTIN F DISACCARIDES 8. Give the major organic product(s) for the reaction of this disaccharide with the reagents shown. NaB C C Ag(N ) - S x 9. Explain why the compound below does not react with Ag(N ) / -. nly hemiacetals are in equilibrium with the open chain form (carbonyl), while acetals are not (they need an on the anomeric carbon to start the opening mechanism). The anomeric carbons in this compound are part of acetal functional groups (R- C-R), not hemiacetals (R-C-). 0. Give the major organic product(s) for these reactions. a. C NaB C C b. C S C C Page 9