CH402X
(c) An example of the Sharpless asymmetric epoxidation of allylic alcohols is illustrated below
X)H
HO
-s>C°2Et
HO
(i)
100% conversion
>90% ee
'C02Et
Explain the roles of the titanium complex, the peroxide and the diester in the
reaction.
[10%]
(ii)
Using a Sharpless asymmetric epoxidation as the key step, design a synthesis of the
molecule 9 from reagents 10 and 11.
[20%]
Ph
^ ^U
Ph
HN'
10
HO
11
(d) In the following reaction, the oc,p-unsaturated aldehyde 12 is reduced enantioselectively to
13, using the enantiomerically pure amine 14 as catalyst.
10mol%: Ph""^MX^Ph
H 14
n
O
H
1.1 equivalents:
12
(i)
-""N
Me
13
Amine 14 catalyses the reaction by reacting with 12 to form a reactive intermediate.
Draw the structure of this intermediate.
[10%]
(ii)
Illustrate the mechanism of hydride transfer to the reactive intermediate formed
between 12 and 14, and explain how the asymmetric reduction is controlled.
[10%]
Section A continues on the next page
CH402X
3 . Answer ALL parts.
(a) Using appropriate illustrations of the likely intermediate complex formed, explain why both
the E- and Z-isomers of 15 are hydrogenated to the same major enantiomer of product 16 in
the reaction shown below.
Catalyst:
o
h
A.
Q
-cr
or
OMe
Z-15
Or
E-15
OMe
H2
O
A NHH
o
R-16
[25%]
(b) Enantiomerically pure 17 can be prepared using a three step sequence illustrated below:
Catalyst:
1©
C02Me
100%ee
Asymmetric
hydrogenation
V1 4'
(i)
O
A
/C02Me
O ^NMe
Give a mechanism for the conversion of 18 into 19 and explain how asymmetric
induction is achieved in the process.
[25%]
(ii)
Provide a mechanism for the conversion of 20 into 17.
(iii) Explain how the process could be modified to generate an enantiomerically pure
diastereoisomer of 17.
(Question 3 continues on next page)
Section A continues on the next page
1
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(c) Using a mechanism, illustrate how the oxazaborolidine 21 catalyses the formation of the
diol product 22 in the reaction shown below.
10mol%
catalyst:
Ph
Ph Ph
O
B—Me
21
HO H
Ph
Ph
BH3 (2.0 molar equivalents)
HO H
Ph
22
[20%]
(d) Using catalyst 21, propose a synthesis of amine 23 from the starting material 24.
MeN-
MeO
MeO
[20%]
End of Section A.
Section B continues on the next page