I. (42 points) (1) (16 points) The titration curve for L-lysine is shown below.
Provide (i) the main structures in equilibrium at each of points A and B indicated
below ...
Name_______________________________
215 F10-Final Exam
Page 2
I. (42 points) (1) (16 points) The titration curve for L-lysine is shown below. Provide (i) the main structures in equilibrium at each of points A and B indicated below and (ii) the structure at the isoelectric point. Draw the structures using the Fischer projections.
(i) Structures in equilibrium at point A: O
O
OH
H3N
H 3N
O
O
H 3N
H
O
O
H3N
H
H3N
(ii) Structure at the isoelectric
Structures in equilibrium at point B:
H
O
O
H2N
H 3N
6
point:
H
H 3N
O
H 2N
H
H3N
6
4
(2) (16 points) Draw the full structure (including stereochemistry) for the tri-peptide Tyr-Phe-Arg (all L amino acids) in the predominant form it would exist at its isoelectric point. Use the Fischer projections. Structure at the isoelectric point:
The predominant form of this tripeptide at pH 1 has a net charge of: (circle one)
O O O H2N
H N H
H N H
O
+2
H
+1
0
-1
-2 4
NH2 N H
The predominant form of this tripeptide at pH 11 has a net charge of: (circle one)
NH2
+2 OH
+1
0
-1
-2
8
4
(3) (10 points) Complete the following reaction sequence by providing in each of the boxes the structure of the product [J. Org. Chem. 2010, 75, 3027]. O Ph
O O N O
O
O
2. N(CH2CH3)3
Ph
O
1. 4M aq HCl
O Ph
O
N
O
HO
O
N
DCC
NH
O
O N
O 5 + CO2 +
5 +
H N
H N O
Name_______________________________
215 F10-Final Exam Page 3
II. (42 points) (1) (16 points) Treatment of enone A with NaOH/H2O results in the formation of cyclohexanone (C) and acetone (D) via β-hydroxy ketone intermediate B. Provide in the box below a stepby-step mechanism using the curved arrow convention for the following transformation. You do not need to show Na+ for each step. OH NaOH/H2O
NaOH/H2O O
A
O
B
O
D
Mechanism for B -> C and D:
Na OH
O H
B
O OH
HO
A
O
C
Mechanism for A -> B:
O
+
H
O
OH
Na
O
O 2 pts for the intermediate 2 pts for each set of mech. arrows
B
O
O D
H OH
OH
6
C
O
2 pts for each intermediate 2 pts for each set of mech. arrows
O 10
(2) (26 points) Provide in the box below a step-by-step mechanism using the curved arrow convention for the following transformation. You do not need to show Na+ for each step. O NaOCH2CH3
CH3CH2O O
O
HOCH2CH3
Na
Mechanism: O
O
+ enantiomer
OCH2CH3
O
CH3CH2O
CH3CH2O O
O
O OCH2CH3
OCH2CH3
OCH2CH3
CH3CH2O O
Na
OCH2CH3
O O
CH3CH2O H
CH3CH2O O
H
OCH2CH3
CH3CH2O
O
H OCH2CH3
2 pts for each intermediate 2 pts for each set of mech. arrows
O CH3CH2O
O O
OCH2CH3
O
OCH2CH3 H OCH2CH3
O Na
O
OCH2CH3 26
Name_______________________________
215 F10-Final Exam Page 4
III. (56 points) Provide in the boxes below a step-by-step mechanism using the curved arrow convention for each of the following transformations. You may use H-A as your Brønsted acid and A- as its conjugate base. (1) (26 points) O
H N O 2N
OH
N H
HCl/H2O
O
NO2
O
H N O2N
OH
+
H N H H
Cl
NO2
OH O
Mechanism: H A H N O2N
H N
O O2N
O
NO2
O2N
NO2
OH
OH2
O
O2N
NO2
OH
N H
O
A H
O
OH H NO2
H
H
H
H
A H N
OH
NO2
OH
N
H
O
H N O2N
N
O
A
OH H OH
O
OH
N H
H A
OH
H N
2 pts for each intermediate 2 pts for each set of mech. arrows
O2N
NO2
OH
H N
OH
N H
OH
N H
H N
H
O
O2N H
Cl
O
H
OH
NO2
OH
N H
O
O
26
(2) (30 points) [Org. Lett. 2010, 12, 904] OCH3 HO
OCH3
OH OH
HO
O
TsOH (catalytic)
+ 2 CH3OH
O
Mechanism: H
H A
OCH3
OCH3
HO
H
A
OH
OCH3
OCH3
O
CH3O HO
OH
HO
O OH
OCH3
CH3O
OH HO
H A
O OH
CH3 O H HO
O OH HO
O O
2 pts for each intermediate 2 pts for each set of mech. arrows
H A
HO
O O 30
Name_______________________________
215 F10-Exam Exam
Page 5
IV. (38 points) Shown below is a trisaccharide recently isolated from Securidaca welwitschii [Helv. Chim. Acta 2010, 93, 2237]. Answer the following questions about this trisaccharide, 1.
HO
A O
HO
OH O
α O
HO
O O
HO
C
OH
β
B
HO
1
OH
(1) (4 points) Label each of the glycosidic bonds in trisaccharide 1 using a small arrow ( → ). (2) (4 points) Classify each glycosidic linkage in 1 as α or β right next to each of the arrows you have shown. (3) (3 points) How many D-sugar units does trisaccharide 1 have? (circle one) 0 1 2 3 (4) (3 points) Is the hemi-acetal OH in trisaccharide 1 α or β?
(circle one)
(5) (3 points) Is trisaccharide 1 a reducing sugar? (circle one)
Yes
α
β
No
(6) (4 points) Describe the linkages between the following sugar units [e.g., α(1 -> 6)]: For A and B: α(1 -> 2)
For B and C: β(1 -> 4)
(7) (17 points) Draw Fischer projections for the open-chain forms of the three carbohydrates produced when trisaccharide 1 is hydrolyzed with a warm dilute aqueous acidic solution. Carbohydrate A
O
Carbohydrate B
O
H
H
H
O
OH
H
OH
HO
H
H
OH
HO
H
HO
H
OH
HO
H
H CH3
Carbohydrate C
H HO H
OH H OH CH2OH
CH3 6
H
6
5
Name_______________________________
215 F10-Final Exam
Page 6
V. (32 points) Complete the following reactions by providing in each of the boxes the structure of the reagent, intermediate, or product. Indicate stereochemistry for the product/intermediate and if more than one stereoisomer is formed, draw one structure and write “+ enantiomer” or “+ diastereomer.” (1) [Synthesis 2010, 3004] OCH3
O HO
O
HO
HO
TsOH (catalytic)
O + CH3OH
OH
OH 5
(2) O O Ph
α-anomer
Ph
O
O
O
HBr (gas)
O O
O
Ph
O O
Ph
Ph
CH2Cl2 (solvent)
O Ph
O
O O
Ph
NH2
O
(1 equiv)
O O
Ph
K2CO3
O Br Ph O
O
S N2
5
Ph
O
HO HO
4x
+ NH
O
NaOCH3 (catalytic)
O
OH O
OCH3
Ph
Ph
CH3OH (sovent)
OH
O
O
O O
NH
Ph
O O
O
Ph
5
5
(3) [Org. Lett. 2010, 12, 904] O
H
H
K
O
O
H
H
H
+ KCl
OCH3
OCH3 Cl
H
+ diastereomer
4
(4) [J. Am. Chem. Soc. 2010, 132, 14064]
HO
O
LDA (2 equiv) OCH2CH3
THF, -78 °C
Li
O
O
1. CH3I (1 equiv)
Li OCH2CH3
dilithium dianion
2. 4
Si
O
O OCH2CH3
Cl Si
+ diastereomer + LiI + LiCl
4
Name_______________________________
215 F10-Final Exam
Page 7
VI. (30 points) Complete the following reactions by providing in each of the boxes the structure of the product or intermediate. Indicate stereochemistry for the product/intermediate and if more than one stereoisomer is formed, draw one structure and write “+ enantiomer” or “+ diastereomer.” (1) [Synlett 2010, 3011] O N O
