SHORT PAPER
1671
Selective Transesterification of Aliphatic Acids in the Presence of Aromatic
Acids Using Silica Gel Supported NaHSO4 Catalyst1
Biswanath Das,* Bollu Venkataiah
Organic Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad-500 007, India
Fax (91)407173387, (91)407173757; E-mail: biswanathdas@yahoo.com
Received 9 April 2000; revised 16 June 2000
Table Preparation of Ethyl Esters of Carboxylic Acids by Treat-
ment with EtOAc in the Presence of NaHSO4 SiO2 Catalysta
Abstract: Acidolysis of EtOAc in the presence of silica gel support-
ed NaHSO4 catalyst has been applied for the protection of aliphatic
nonconjugated carboxylic acids through the formation of esters.
Yield (%)b
Entry Acid
Time
(h)
Key words: carboxylic acids, acidolysis, NaHSO4 SiO2 catalyst,
selective protection
Ester derived Ester derived
from aliphatic fromaromatic
carboxylic
acid group
carboxylic
acid group
Esterification is a general method for protection of a car-
boxylic group. Acidolysis of an ester in the presence of a
carboxylic acid can be utilized for esterification of the lat-
ter.2
1
2
Phenylacetic acid
6
6
95
96
4-Hydroxyphe-
nyl acetic acid
3
4
2-Hydroxyphe-
nyl acetic acid
6
92
95
3-Phenyl pro-
panoic acid
5.5
EtOAc is an easily available ester, which can be applied
for this purpose. It is generally utilized for transesterifica-
tion of aliphatic hydroxyl groups in the presence of a cat-
alyst. Recently the catalyst, silica gel supported NaHSO4
(NaHSO4 SiO2)3 has been applied for acetylation of ali-
phatic hydroxyl groups over aromatic hydroxyl groups.
Several diols (containing both aliphatic and aromatic hy-
droxyl groups) were treated with EtOAc in the presence of
NaHSO4 SiO2 to afford monoacetates derived from alco-
holic hydroxyl groups. We now report on esterification of
various carboxylic acids (both aliphatic and aromatic) by
treatment with EtOAc in the presence of the same catalyst
(Table). Only the nonconjugated aliphatic carboxylic ac-
ids afforded the corresponding ethyl esters. The experi-
mental procedure for esterfication is very simple, the
reaction proceeds at room temperature under mild reac-
tion conditions and the catalyst can be easily prepared
from the readily available materials.3 The aliphatic non-
conjugated carboxylic acids were converted to the corre-
sponding ethyl esters in very high yields, while the
aromatic and conjugated carboxylic acids did not show
any conversion under similar conditions. The selective es-
terification of aliphatic nonconjugated carboxylic acids
has been demonstrated by taking carboxylic acids with
both types of carboxylic groups in the same molecule and
also by competition experiments (Table).
5
6
7
Stearic acid
5.5
10
6
94
0
Cinnamic acid
2-Carboxy phe-
nyl acetic acid
96
0
0
8
2-Carboxyphe-
nyl propanoic
acid
5.5
95
9
Benzoic acid
8
0
0
10
3,5-Dinitroben-
zoic acid
8.5
11
12
13
14
3-Chlorobenzoic
acid
8.5
0
0
0
0
Phenylacetic acid 6.8
and benzoic acid
93
93
94
Phenylacetic acid
and cinnamic acid
9
4-Hydroxyphe-
nyl acetic acid
and benzoic acid
8
15
2-Hydroxyphe-
nyl acetic acid
and 3-chloroben-
zoic acid
8.5
91
0
In summary, we have developed a novel, mild, efficient
and general methodology for selective protection of ali-
phatic nonconjugated carboxylic acids in the presence of
aromatic and conjugated carboxylic acids by applying ac-
idolysis of a common solvent, EtOAc, in the presence of
a All the products gave satisfactory analytical and spectroscopic (1H
NMR and MS) data
b Yields are based on isolated pure products.
Synthesis 2000, No. 12, 1671–1672 ISSN 0039-7881 © Thieme Stuttgart · New York