378 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 378^379y
Selective Esterification of Nonconjugated
Carboxylic Acids in the Presence of Conjugated
or Aromatic Carboxylic Acids Under Mild
Conditionsy
Ramesh C. Anand,* Vimal and Archana Milhotra
Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi^110016, India
Nonconjugated carboxylic acids are selectively esterified in good yields in the presence of conjugated or aromatic
carboxylic acids by stirring over Amberlyst-15 in alcohol at room temperature.
Selective esteri¢cation of nonconjugated carboxyl groups in
the presence of conjugated or aromatic carboxyl groups
is an important reaction for the manipulation of functional
groups in multistep organic synthesis.1 In general
nonconjugated carboxyl groups should be more reactive
than conjugated ones in esteri¢cation reactions. However,
a survey of the literature2 reveals that such selective
esteri¢cation of carboxylic acids has been carried out under
reaction conditions or with carboxyl activating reagents
which are not mild enough to be of general and wide
applicability. Recently Spur and coworkers3 have reported
a methodology which is quite e¡ective at room temperature
but utilizes expensive reagents like 2,2-dimethoxypropane
and chlorotrimethylsilane.
a saturated and a conjugated or aromatic carboxylic acids
groups. It has also been noted that an aromatic carboxylic
acid is preferentially esteri¢ed over a conjugated carboxylic
acid attached to an aromatic ring under these conditions.
The procedure involves stirring of a 1 : 1 mixture of two
di¡erent carboxylic acid compounds or a compound con-
taining both functional groups in methanol or ethanol over
Amberlyst-15 at room temperature and the progress of
the reaction is monitored through TLC. The work up (after
an appropriate interval of time) involves simply decantation
of the solvent and evaporation followed by chemical separ-
ation of the residue. This methodology is quite inexpensive
in view of the ready reuse of Amberlyst. The examples
studied are given in Tables 1 and 2.
Here we report our observation that
a saturated
carboxylic acid can be selectively esteri¢ed at room tem-
perature in the presence of a conjugated or aromatic
carboxylic acid in good yield in the presence of
macroreticular ion exchange resin Amberlyst-15 using meth-
anol or ethanol as solvent. This result has been investigated
through competition experiments in (i) a mixture of two
di¡erent carboxylic acids (one nonconjugated and the other
conjugated or aromatic), (ii) a molecule containing both
Experimental
General Procedure for the Selective Esteri¢cation of Cyclohexane-
carboxylic Acid in the Presence of Benzoic Acid.öA solution of
cyclohexanecarboxylic acid (0.01 mol) and benzoic acid (0.01 mol)
in methanol (80 ml) containing Amberlyst-15 (50 g) was stirred
at room temperature and the progress of reaction monitored by
TLC. After 14 h, the solution was ¢ltered and the Amberlyst
washed with MeOHꢀ3 Â 30 ml). The combined solvent was evapor-
Table 1 Selective esterification of a mixture of two carboxylic acids with methanol
Yield of estera(%)
Entry
Acid A + acid B
Time/h
A
B
1
2
3
4
5
6
7
8
Caproic acid (A) benzoic acid (B)
13
14
12
14
10
10
12
24
82
78
85
81
78
82
75
81
0
0
0
0
0
5
4
0
Phenylacetic acid (A) benzoic acid (B)
Citronellic acid (A) geranic acid (B)
Cyclohexanecarboxylic acid (A) benzoic acid (B)
Succinic acid (A) maleic acid (B)
Caproic acid (A) crotonic acid (B)
Cyclohexanecarboxylic acid (A) 1-cyclohexenecarboxylic acid (B)
Benzoic acid (A) cinnamic acid (B)
aIsolated yield.
Table 2 Competition esterification of dicarboxylic acids in methanol
Yielda(%)
Entry Dicarboxylic acid
Time/h Non-conjugated monomethyl ester Conjugated monomethyl ester Dimethyl ester
1
2
Itaconic acid
2-(4-Hydroxycarbonyl
phenyl)acetic acid
12
12
94
88
0
0
0
0
3
Prop-1-ene-1,3-dicarboxylic acid 12
76
0
5
* To receive any correspondence (e-mail: rcanand@chemistry.iitd.
ernet.in).
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
ated and the residue was taken in CH2Cl2 (80 ml). The organic layer
was washed with aqueous NaHCO3 solution (20%, 30 Â 60 ml),
water ꢀ2 Â 60 ml), brine ꢀ2 Â 60 ml) and dried ꢀNa2SO4. The solvent
was removed completely to provide methyl cyclohexane carboxylate
Anand, Ramesh C.
