Chemistry Reference
In-Depth Information
This means that a
reverse Claisen reaction
can
occur if a
since the nucleophile is the same as the leaving
group (see Section 10.7). Aqueous base treatment
of a
-ketoester is treated with base. This is
most likely to
β
occur if we
attempt
to hydrolyse
-ketoester will, however, result in both ester
hydrolysis and a reverse Claisen reaction, and poses
a
β
the
β
-ketoester to give a
β
-ketoacid using aqueous
base.
Note
that
the
alcoholic
base
used
for
the
problem
if
one
only
wants
to
hydrolyse
the
Claisen
reaction
does
not
affect
the
ester
group,
ester.
O
base causes reverse
Claisen reaction as well
as ester hydrolysis
CO
2
H
CO
2
Et
CH
2
Ph
CO
2
H
NaOH
CH
2
Ph
O
O
CO
2
Et
CH
2
Ph
CO
2
H
CH
2
Ph
H
+
acid only hydrolyses ester
b
-ketoester
b
-ketoacid
The reverse Claisen reaction is common, especially
with
-diketones, the products of a mixed
Claisen reaction between an ester electrophile and
a ketone nucleophile (see Box 10.13), behave sim-
ilarly
Cleavage of
β
-ketoesters, such as one gets from
the Dieckmann reaction (see Section 10.8). If one
only wants to hydrolyse the ester, it thus becomes
necessary to use the rather less effective acid-
catalysed hydrolysis method (see Section 7.9.2).
cyclic
β
towards
base,
and
a
reverse
Claisen
reac-
tion
ensues.
Again,
this
is
prevalent
with
cyclic
systems.
reverse Claisen
+ ester hydrolysis
NaOH
O
O
O
O
Claisen
CO
2
Et
CO
2
H
CH
3
CH
3
CH
3
b
-diketone
Nevertheless, as we shall see in Section 10.9, it is
also possible to exploit the reverse Claisen reaction
to achieve useful transformations.
β
-ketoacid, but these compounds are especially
susceptible to loss of carbon dioxide, i.e.
decar-
boxylation
. Although
-ketoacids may be quite sta-
ble, decarboxylation occurs readily on mild heating,
and is ascribed to the formation of a six-membered
hydrogen-bonded transition state. Decarboxylation is
represented as a cyclic flow of electrons, leading
to an enol product that rapidly reverts to the more
favourable keto tautomer.
β
10.9 Decarboxylation reactions
Hydrolysis of the ester function of the
β
-ketoester
Claisen
product
under
acidic
conditions
yields
a
