Organic Chemistry III

Professor Carl C. Wamser

Exam 3 Answer Key

1. (20 points) Write complete structures for each of the following, including stereochemistry.

a) sucrose

b) b-L-glucose (in a Haworth projection)

c) N,N-dimethyl-b-N-glycoside of D-2-deoxyribose

d) a C-1 monoglyceride of palmitoleic acid (C16, monounsaturated)

e) seryltyrosine (at pH 7)

2. (20 points) Write complete structures for each of the following, including stereochemistry.

a) BOC-N-protected cysteine (at pH 7)

b) the C-2 epimer of D-glucose in a b-pyranose chair form

c) isopentenyl pyrophosphate (at pH 7)

d) the polycyclic ring structure of a steroid (just the rings)

e) the fundamental structural features of a prostaglandin

3. (15 points) Complete each of the following reactions by adding the missing part: either the expected final major product, the necessary reagents and conditions, or the original starting material. Show the complete structure with stereochemistry if it is specific.

a)

b)

c)

d)

e)

4. (12 points) For the pentapeptide Ala-Arg-Cys-Pro-Trp indicate the peptides and/or amino acids that would be formed and/or affected upon treatment with each of the following reagents:

a) Edman’s reagent

releases Ala as the PTH derivative + Arg-Cys-Pro-Trp

b) carboxypeptidase

releases Trp + Ala-Arg-Cys-Pro

c) trypsin

cleaves at Arg , giving Ala-Arg + Cys-Pro-Trp

d) chymotrypsin

no reaction

e) partial hydrolysis to give dipeptides (list all four possibilities)

Ala-Arg , Arg-Cys , Cys-Pro , and Pro-Trp

5. (8 points) The synthetic sequence below is used to prepare an amino acid.

Which amino acid would be formed?

isoleucine

Is it possible to use this sequence of reactions to prepare the enantiomerically pure L-amino acid?

No

If so, explain what stereochemistry of the reactants would be required.
If not, explain why not.

The carbon that will become the L-stereocenter was originally a dicarboxylate ester, and when one is lost as CO2 in the final step, it could be either one.

6. (10 points) Show all the steps in the mechanism of the reaction shown below. Use electron-pushing arrows.
Show where an enzyme might use base catalysis and/or acid catalysis to facilitate this reaction.

7. (15 points) Write the structures of all the products that would be formed upon complete hydrolysis of each of the following. Write sugars in their open-chain form.

a)

b)

c)

d)

e)