Chm 122
Spring 2007
Problem Set 3
Due February 28, 2007 5pm
Name:
I agree to obey the WFU Honor Code.
Methylenomycin A is a potent antibiotic isolated from Streptomyces violaceoruber. Answer the
following questions about methylenomycin A.
O
S
S
R
O
CO 2H
1. Identify the stereogenic centers in Methylenomycin A. Determine whether each is R or S as
drawn above. Clearly mark R or S next to the stereocenter. See above
2. How many stereoisomers of this molecule are theoretically possible? How many are
practically possible? If these two questions have different answers, explain. Four. Three
stereocenters implies that 8 stereoisomers are possible. However, the epoxide (three
membered ring containing oxygen) must have a cis ring fusion. Thus, the relative
stereochemistry of the two S centers are fixed and the number of stereocenteres halved.
3. Draw the enantiomer of methylenomycin A. Draw a diastereomer of methylenomycin A.
Draw the enantiomer of this diastereomer. Draw a constitutional isomer of these molecules.
O
O
O
O
CO2H
CO2H
enantiomer
methylenomycin
O
O
enantiomers
O
O
CO2H
CO2H
diastereomer
diastereomer
O
constitutional isomer - among many others
O
CO2H
How many units of unsaturation does methylenomycin A contain?
5 units of unsaturation (two rings, three double bonds)
4. Indicate the relationship of each pair of molecules. Choose from enantiomers, diastereomers,
same, meso, and constitutional isomers. Write your choice clearly on the line to the right of the
pair. If there is a difference in the stability of the molecules, indicate the more stable one.
same
H
OH
CH3
H3C
Cl
H
OH
Cl
constitutional
Cl
Cl
OH
OH
same/meso
OH
OH
HO
HO
enantiomers
O
O
enantiomers
OH
OH
OH
OH
Cl
Cl
HO
Br
rotate 120 degrees around C2-C3 bond
to bring Cl front, methyl to side.
Br
O
HO
enantiomers
O
diastereomers
5. Assign absolute configurations to the following stereogenic centers.
H
Cl
CH3
Cl
S
H OH
R
Cl
H
R
H
HO
S
S
6. Identify the stereocenters in the following molecule by placing an asterisk next to each and
indicate how many stereoisomers are possible for this structure (including the one shown). (1)
*
*
*
*
25 = 32
*
7. Each structure below has a more stable resonance structure. Draw it. (1)
O
OCH3
H
O
H
OCH3
H
H
8. Draw the most stable chair conformation of the following cyclohexanes. Name them. (1)
OH
OH
O
O
9. Draw the Newmann projection looking down the indicated bond with the starred carbon in
front. Draw each staggered conformation and indicate which is the low energy conformation.
Draw a potential energy diagram for rotation about this bond. Is the molecule chiral? (Yes)
Give the absolute configuration at any chiral centers (R).
H3C
CH3
H3C
R
H3C
*
H
CH3
OH
H
CH3
H3C
CH3
CH3
CH3
CH3
CH3
HO
CH3
H3C
H
H
H
OH
H
CH3
highest, tBu/Me gauche and
Me/Me gauche (Me bigger than
OH)
Middle, tBu/Me gauche
CH3
OH
H3C
lowest, no tBu gauche interactions
largest subst. on first carbon antiperiplaner to largest on second
H carbon
H
CH3
H3C
CH3
OH
CH3
CH3
OH
H3C
defined as 180
H
H3C
H
H3C
H
defined 120
CH3
CH3
0
H
60
120
180
240
300
360
23. Draw the Newmann projection for the most stable conformation of ethylene glycol. Why is
this the most stable? What sort of inter- and intramolecular interactions are possible for ethylene
glycol that aren’t for most hydrocarbons?
HO
OH
H
H
O
H
H
O
H
H
the gauche conformation
is stabilized by an
internal H bond that is
not possible in the
anti-staggered
conformation