1. Thermally, compound 1 rearranges to give compounds 2 and 3, but NOT 2' and 3'
C8H17
D
C8H17
C8H17
H
Me
Me H
DH
D
Me
OH
OH
OH
1
2'
2
C8H17
C8H17
HD
Me D
Me
H
OH
OH
3'
3
a. provide an arrow pushing mechanism for the rearrangement
b. Use your knowledge of the Woodward-Hoffman rules and molecular orbital theory to explain the
stereospecificity of the reaction. A complete answer will identify the symmetry of the frontier molecular
orbital that controls the rearrangement, as well as a molecular orbital description of the transition state for
the rearrangement.
a
C8H17
C8H17
antarafacial
[1,7]-deuteride shift
H
D
Me
DH
Me
OH
OH
2
1
C8H17
C8H17
D
H
Me
antarafacial
[1,7]-hydride shift
Me D
H
OH
1
OH
3
b
consider only the business end of the molecule. Draw the transition state
HO
7e–
H
Me
Me H
Me H
Me H
C8H17
Thus, we have a 7 carbon,
7 electron pi system.
The symmetry of the HOMO
is 'unlike' (ungerade)
The 1,7 shift cannot take place on the top face of the molecule as we observe for
the 1,5 shift. This would result in an unfavorable anti-bonding interaction as shown
below:
antibonding
bonding
'Top face' or 'suprafacial' delivery is not
allowed for a 1,7-shift.
H
HOMO
However, the reaction can still proceed because of the flexibility of the 7 carbon
bridge. Distortion of the bridge can put the like ends of the HOMO within reach of
the hydrogen s-orbital. Thus the concerted [1,7]- shift can occur by an 'over-under
mechanism'.
bonding
'over/under' or 'antarafacial' delivery is
allowed for a 1,7-shift.
H
bonding
2. Vitamin D is made in the skin when dehydrocholesterol absorbs UV light between 270–300 nm. (Despite what your
advisor may tell you, sunshine is good for you!) Dehydrocholesterol-d3 (3) photochemically rearranges to give an
intermediate 4, which thermally rearranges to give vitamin D-d3 (5).
D3C
H
hν
[4]
Δ
3
HO
D
CD2
H
HO
dehydrocholesterol-d3
5
vitamin D-d3
a. what is the structure of 4? provide an arrow pushing mechanism for the formation of 4
b. Why is light necessary for the conversion of 3 to 4? Use your knowledge of the Woodward-Hoffman rules and
molecular orbital theory to explain in detail. A complete answer will identify the symmetry of the frontier molecular
orbital that controls the rearrangement, and provide a detailed description of the rearrangement using molecular orbital
theory.
c. Provide an arrow pushing mechanism for the rearrangement of 4 to 5.
a,c mechanisms
D3C
D2C
step 1
HO
D
CD2
H
D
H
step 2
HO
electrocyclic ring opening
HO
[1,7]-hydride shift
part b begin by simplfy the structure of dehydrocholesterol
OH
OH
H3C
H CH3
Ring
Ring
CH3
H H3C
Ring
Ring
H
the electrocyclic ring opening to give a 1,3,5-hexatriene is a 6pi, 6 electron process that is controlled by the HOMO.
Under photochemical conditions, the HOMO has "unlike" (ungerade) symmetry.
therefore, the HOMO has 'unlike' symmetry at the ends. For hexadiene, this is the photochemical homo.
thermal
HOMO
has 'like'
symmetry
photochemical HOMO
has 'unlike' symmetry
hν
Under photochemical conditions, this is a conrotatory ring opening
Ring
CH3
Ring
Ring
H
H H3C
Ring
OH
H3C
Why do we need light? Under thermal conditions, the HOMO has 'like' (gerade) symmetry, and therefore
would require a disrotatory ring opening to be allowed under Woodward-Hoffman rules. The triene would
contain a trans-cyclohexene, which is too strained!
dis
Ring
CH3
H
Ring
H
H C
Ring 3
Ring
H3C
OH