A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

Crossed condensation of two different esters is not always practi­

cable because of the formation of mixed products, but it can be of

synthetic value, particularly where one of the esters used is incapable

of forming a carbanion, e.g. (C0 2 Et) 2 , HC0 2 Et and PhC0 2 Et.

The complexity of the alternative reactions that can take place from

the action of sodium on esters has already been referred to (p. 170).

Where the two ester groups are part of the same molecule and

cyclisation can therefore result, the condensation is known as a

Dieckmann reaction:

O

II

CH.—C—OEt

(CH»)„—CHi-CO,Et
(n=2or3)

CH. C—OEt

(^3) OEt
CH, C
(CH 2 )n—CHC0 2 Et
CHa C
•< >
L(CH»)n—C COsEt (CHa)n—C • CO,Et J
fOEt CH„ C
(CH^n—CHCOjEt
Here as in the simple Claisen ester condensation it is necessary, if
ethoxide is used as the catalyst, to be able to form the anion of the final
/3-keto ester in order to drive the overall reaction in the desired
direction.
Like all aldol-type reactions, the Claisen ester condensation is
reversible
O OA
II r*/>
Me—C—CH,CO]iEt+^0 OEt ^ Me-^CJ-CH, C0 2 Et
OEt

• t
  O •
  II - •
  Me—C+QCHjCOjEt
  OEt