664
HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
Again, this produces a favourable tertiary carbocation. Loss of a proton gives the required alkene. Note that
potentially three different carbons could lose a proton. The reaction shown generates the most stable product; this
has the maximum number of alkyl substituents and also benefits from extended conjugation. We then get another
aldol-type reaction. The enolate anion is produced from the ethyl chloroacetate, and simple addition yields an
anion that is subsequently protonated.
O
O
O
CO 2 Et
Cl
O
OEt
O
OH
O
OEt
O
OH
O
O
OH
O O
O
H
O
O
H
O
S N 2 reaction
base hydrolysis of ester to acid
compare decarboxylation of
β-keto acid via 6-membered
H-bonded system
heat
H
OH
OH
H
≡
decarboxylation via H-bonded system
base-catalysed enol−keto tautomerism
However, the same anion has to be regenerated for the next reaction, in which an S N 2 displacement leads to
epoxide formation. Base hydrolysis of the ester produces the carboxylic acid, which decarboxylates on heating.
The mechanism for this reaction should be suggested by the structural change (loss of a carbon), and that this is
achieved merely by heating. What other systems do this? It is actually analogous to thermal decarboxylation of
a β-keto acid, a process where we invoke a cyclic mechanism within a hydrogen-bonded system. Think laterally
to see whether we can do the same here. We can, though any hydrogen bonding must involve a five-membered
ring rather than a six-membered ring. The sequence is completed by base-catalysed tautomerism of the enol to
a ketone.
Problem 16.21 (level 2; chapters 10, 6, and 7)
Propose a mechanism for all steps involved in the following synthesis of the ester of chrysanthemic acid, a
component of the natural insecticides, the pyrethrins:
O
O
O
CO 2 Et
O
O
CO 2 Et
Cl
EtO 2 C
NaOEt / EtOH
H 2 SO 4 / heat
NaOEt / EtOH
HCl / EtOH
ethyl chrysanthemate
CH 2 (CO 2 Et) 2
HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
Again, this produces a favourable tertiary carbocation. Loss of a proton gives the required alkene. Note that
potentially three different carbons could lose a proton. The reaction shown generates the most stable product; this
has the maximum number of alkyl substituents and also benefits from extended conjugation. We then get another
aldol-type reaction. The enolate anion is produced from the ethyl chloroacetate, and simple addition yields an
anion that is subsequently protonated.
O
O
O
CO 2 Et
Cl
O
OEt
O
OH
O
OEt
O
OH
O
O
OH
O O
O
H
O
O
H
O
S N 2 reaction
base hydrolysis of ester to acid
compare decarboxylation of
β-keto acid via 6-membered
H-bonded system
heat
H
OH
OH
H
≡
decarboxylation via H-bonded system
base-catalysed enol−keto tautomerism
However, the same anion has to be regenerated for the next reaction, in which an S N 2 displacement leads to
epoxide formation. Base hydrolysis of the ester produces the carboxylic acid, which decarboxylates on heating.
The mechanism for this reaction should be suggested by the structural change (loss of a carbon), and that this is
achieved merely by heating. What other systems do this? It is actually analogous to thermal decarboxylation of
a β-keto acid, a process where we invoke a cyclic mechanism within a hydrogen-bonded system. Think laterally
to see whether we can do the same here. We can, though any hydrogen bonding must involve a five-membered
ring rather than a six-membered ring. The sequence is completed by base-catalysed tautomerism of the enol to
a ketone.
Problem 16.21 (level 2; chapters 10, 6, and 7)
Propose a mechanism for all steps involved in the following synthesis of the ester of chrysanthemic acid, a
component of the natural insecticides, the pyrethrins:
O
O
O
CO 2 Et
O
O
CO 2 Et
Cl
EtO 2 C
NaOEt / EtOH
H 2 SO 4 / heat
NaOEt / EtOH
HCl / EtOH
ethyl chrysanthemate
CH 2 (CO 2 Et) 2