week5_questions (Key)

week5_questions (Key) - Chemistry S-20 Week 5 Carbohydrates...

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Carbohydrates O OH HO HO OH OMe O OH HO HO OH OH 1. When glucose is treated with methanol and acid, the methyl glucoside is formed. Provide a complete curved-arrow mechanism for this transformation: 2. The correct stereochemistry of only one stereocenter is indicated on the above structure. The actual stereochemistry of methyl glucoside is such that, when it is in a chair form, all of the substituents fall in the equatorial positions, except for the –OMe, which is found in the axial position. Given this information, draw a clear, correct representation of methyl glucoside in a chair conformation, with all substituents properly indicated. In addition, explain why the –OMe group preferably adopts an axial conformation; a diagram will be essential. 3. When glucose is in its open-chain form, its stereochemistry can best be represented using a Fischer projection. On the skeleton Fischer projection shown at right, fill in the H and OH groups of the glucose "backbone" with the correct stereochemistry as determined from your structure shown above. CH 3 OH, H + O OH HO HO OH OH 2 O OH HO HO OH O OH HO HO OH OMe H O OMe HO OH OH OH O OMe This is an example of the anomeric effect. The –OMe goes axial in order that the lone pair fromther ingOcandonatein to * C–O of the –OMe group. * C–O O H H OH HO H H OH H OH CH 2 OH H + HOCH 3 Sol Chemistry S-20 Week 5 152
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