Facile deprotection of aromatic tert-butyl and allylic esters under microwave irradiation conditions

Article abstract of DOI:10.1081/SCC-200026661

A simple and efficient method for the deprotection of tert-butyl and allylic esters is described, which occurs under microwave irradiation with p-toluenesulfonic acid in solvent-free conditions.

Full text of DOI:10.1081/SCC-200026661

w
SYNTHETIC COMMUNICATIONS
Vol. 34, No. 16, pp. 3017–3020, 2004
Facile Deprotection of Aromatic tert-Butyl
and Allylic Esters Under Microwave
Irradiation Conditions
*
Jong Chan Lee, Eun Sang Yoo, and Jin Seo Lee
Department of Chemistry, Chung-Ang University, Seoul, South Korea
ABSTRACT
A simple and efficient method for the deprotection of tert-butyl and allylic
esters is described, which occurs under microwave irradiation with
p-toluenesulfonic acid in solvent-free conditions.
Key Words: Deprotection; Esters; Microwave; Solvent-free.
INTRODUCTION
Protection of carboxylic acids into carboxylic esters are fundamental
procedures in numerous organic synthesis. Among others, transformation of
carboxylic acids to tert-butyl, allyl, and prenyl (3-methylbut-2-enyl) esters
*
Correspondence: Jong Chan Lee, Department of Chemistry, Chung-Ang University,
Seoul 156-756, South Korea; Fax: þ82-2-825-4736; E-mail: jclee@cau.ac.kr.
3
017
DOI: 10.1081/SCC-200026661
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

3
018
Lee, Yoo, and Lee
[1]
received much attention due to their ease of preparation and high stabilities.
A variety of useful methods for deprotection of tert-butyl esters have been
reported. For example, deprotection of tert-butyl esters can be efficiently
[2]
mediated by use of various reagents, such as trifluoroacetic acid, nitric
[3]
[4]
acid, and ZnBr2, in dichloromethane solvent. On the other hand, the
reported methods for deprotection of allylic esters still remain far less in com-
parison with those of tert-butyl esters. Recently, several efficient methods for
deprotection of allylic esters have been developed using reagents such as
[
5,6]
[7]
[8]
[9]
palladium(II) acetate,
iodine, sulfated tin oxide, and K-10 clay.
However, generally these methods make use of toxic and expensive reagents
and performed for extended period of times, which limit their practical
utilities.
Recently, microwave promoted organic synthesis coupled with solvent-
free conditions utilized in wide variety of organic synthesis.
[10]
In connection
with our program on the development of microwave accelerated organic
synthetic method, we wanted to effect deprotection of carboxylic esters in
solvent-free conditions. In this paper, we describe the efficient microwave
promoted deprotection of tert-butyl and allylic esters by treatment with
p-toluenesulfonic acid (Sch. 1).
Treatment of aromatic tert-butyl esters with p-toluenesulfonic acid mono-
hydrate (2.0 equiv.) under microwave irradiation for 3–4 min smoothly furn-
ished carboxylic acids in high yields. When we also tested several aromatic
allyl and prenyl esters under the same conditions, their corresponding
carboxylic acids were produced in very good yields. The results are summar-
ized in Table 1. As shown in Table 1, it seems like that the electron density in
aromatic ring did not much affect the efficiency of the present method. On
examination of different protic organic acids (trifluoroacetic acid, trifluoro-
methanesulfonic acid, and chlorosulfonic acid) for the present method, we
found p-toluenesulfonic acid as the reagent of choice in terms of yields and
cleanness of reactions.
In summary, a simple, efficient, and rapid method for the deprotection of
aromatic tert-butyl and allylic esters has been developed by combination of
p-toluenesulfonic acid and microwave irradiation in solvent-free conditions.
Scheme 1.

Deprotection of tert-Butyl and Allylic Esters
3019
Table 1. Yields in deprotection of aromatic carboxylic esters.
a
Deprotection yield (%)
Substrate
R ¼ t-butyl
R ¼ allyl
R ¼ prenyl
9
9
9
8
7
8
6
3
2
6
3
0
97
82
90
82
86
70
72
80
80
84
90
84
a
Isolated yield.
The application of the present method to other carboxylic esters is currently
underway.
EXPERIMENTAL
A commercial domestic microwave oven (Samsung RE-50M) with 700 W
1
power output was used for all reactions. H NMR spectra were obtained on a
Varian Gemini 2000–300 MHz spectrometer. Infrared (IR) spectra were
recorded on Jasco FT-IR 5300 spectrophotometer.
General Procedure
A mixture of carboxylic ester (1.0 mmol) and p-toluenesulfonic acid
monohydrate (0.380 g, 2.0 mmol), was placed in a 50 mL of glass tube. The
reaction mixture was inserted in an alumina bath inside a domestic microwave
oven and irradiated (700 W) six to eight times for a period of 30 sec with
1
0-sec intervals. After completion of the reaction, the product was dissolved
in ethyl ether (30 mL) and extracted with aqueous 10% sodium carbonate
solution (2 ꢀ 30 mL). The aqueous layer was neutralized with dilute HCl
and the product was extracted with ethyl ether (2 ꢀ 20 mL). The combined
organic layer was dried with sodium sulfate and evaporated to give the
carboxylic acid.

3
020
Lee, Yoo, and Lee
ACKNOWLEDGMENT
This research was supported by the Creative Initiative Research Program
of Chung-Ang University in 2004.
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Received in Japan February 23, 2004

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