Facile and selective deprotection of allyl esters catalyzed by H-β zeolite

Article abstract of DOI:10.1081/SCC-120022476

Carboxylic acids are regenerated from their corresponding allyl or cinnamyl esters by H-β zeolite under environmentally safe, heterogeneous reaction conditions in excellent yields.

Full text of DOI:10.1081/SCC-120022476

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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 17, pp. 3017–3024, 2003
Facile and Selective Deprotection of Allyl Esters
Catalyzed by H-b Zeolite
Rajesh K. Pandey,1 Vijay S. Kadam,2 Rajesh K. Upadhyay,1
Mohan K. Dongare,2 and Pradeep Kumar1,
*
1Division ofOrganic Chemistry: Technology, and
2Catalysis Division, National Chemical Laboratory,
Pune, India
ABSTRACT
Carboxylic acids are regenerated from their corresponding allyl or
cinnamyl esters by H-b zeolite under environmentally safe, hetero-
geneous reaction conditions in excellent yields.
Key Words: Allyl ester; Carboxylic acid; H-b zeolite; Heterogeneous
catalysis; Deprotection.
*Correspondence: Pradeep Kumar, Division ofOrganic Chemistry: Technology,
National Chemical Laboratory, Pune—411 008, India; Fax: 0091-20-5893614;
E-mail: tripathi@dalton.ncl.res.in.
3017
DOI: 10.1081/SCC-120022476
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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3018
Pandey et al.
Functional group protection and deprotection strategies are quite
often a necessary requirement in the manipulation of multifunctional
organic molecules.[1] The carboxyl function is one of the most important
groups in organic molecules and its controlled manipulation during the
synthesis is ofgreat value to synthetic organic chemistry. In general,
carboxylic acids can be protected as anhydrides,[2] amides,[3] or esters.[4]
Allyl esters serve as important protecting groups for carboxylic acids due
to their outstanding stability under a variety ofreaction conditions and
ease ofpreparations either by the treatment ofcorresponding alcohol
with acid chloride or alkylation ofacid with the corresponding allyl
halide under base catalyzed conditions.[5]
A number ofmethods employed of r the deprotection ofallyl esters
[6]
include the use ofPd(OAc)
,
PdCl2(Ph3P)2,[7] Pd(Ph3P)4,[8] (Ph3P)3
2
RhCl,[9] Me2CuLi,[10] formic acid,[11] and excess ofiodine.
More
[12]
recently, sulphated SnO2,[13] natural kaolinitic clay and EPZG,[14]
montmorillonite K-10[15] have been developed for this purpose. Some
carboxylic acids protected as 3-buten-1-yl esters have been deprotected
via ozonolysis[16a] and b-elimination.[16b] Nevertheless there are sufficient
drawbacks to most of these procedures to justify the need for a general,
selective and practical method for deprotection of allyl esters.
Owing to the environmental and economic concerns in recent years,
there has been considerable upsurge ofinterest in the area ofzeolite
induced organic transformations.[17] In a series ofpublications rfom
our group,[18] we have exploited the catalytic potential ofzeolites ofr
various organic synthetic transformations e.g., thioacetalization of car-
bonyl compounds, sulfoxidation of thioethers, tetrahydropyranylation
ofalcohols, oxidative cleavage oftosylhydrazones, methoxymethylation
ofalcohols, acetylation ofaldehydes, addition ofthiols across double
bonds, chemo- and stereoselective epoxidation and oxidation ofufran
etc. In continuation, we wish to report an unprecedented, convenient
and heterogeneous catalytic methodology for deprotection of allyl ester
using H-b zeolite (Sch. 1).
O
O
R1
R2
H-beta zeolite
OH
O
toluene, Nu, reflux
R
R
Scheme 1.

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Allyl Esters Catalyzed by H-b Zeolite
RESULT AND DISCUSSION
3019
In a typical experimental procedure, when allyl esters were refluxed
in toluene in the presence ofcatalytic amount ofH-b zeolite, the cor-
responding acids were obtained in excellent yields (Table 1). The present
procedure is quite general as a wide range ofstructurally varied allyl
esters underwent deprotection smoothly under the reaction conditions
employed. The noteworthy feature of this methodology is that the
reaction is performed under anhydrous conditions and the formation
ofallyl cations by zeolite is ofllowed by the alkylation by aromatic
nucleus, toluene or anisole. It is evident from Table 1 that the presence
ofnucleophile (anisole) afcilitates the reaction whereas its absence
requires a little longer time to complete the reaction affording relatively
low yield ofthe corresponding acid. It is noted that the prenyl or allyl
esters are easily deprotected as compared to cinnamyl esters. Even
though cinnamyl esters took little longer time, the corresponding
acids were obtained in excellent yields (Table 1, Entries 3, 8). It is
important to note that allyl ester could be selectively deprotected in
the presence ofmethyl ester in mixed ester (Table 1, Entry 16).
Interestingly, the benzyl ester could be deprotected smoothly (Table 1,
Entry 14) while the normal esters remained unaffected under the reac-
tion conditions (Table 1, Entries 16, 18). The superiority ofthis proce-
dure can be clearly visualized in deprotection ofa variety ofsubstituted
aromatic allyl esters leading to the corresponding acids in excellent
yields (Table 1, Entries 6–13). In this connection it should be mentioned
that a recent literature report[14] which describes the selective regenera-
tion ofcarboxylic acids rfom their corresponding allyl or cinnamyl
esters employing a natural kaolinitic clay and EPZG failed with para
substituted esters. The effectiveness ofthis protocol is ufrther mani-
fested in its selectivity for the deprotection of aromatic allyl esters
whereas allyl esters derived from aliphatic acids failed to undergo reac-
tion (Table 1, Entries 17, 19, 20). It should be pointed out that these
results are in contrast with those observed by the other solid catalysts
already reported in literature.[13–15] The difference in the reactivity
pattern ofaromatic allyl esters rfom aliphatic ones could probably
be utilized in the selective manipulation ofesters in a synthetic
sequence. The observed efficient performance of H-b zeolite may be
attributed to its large pore opening, three dimensional channel system
[19]
and higher concentration ofacid sites.
The recovered catalyst was
reactivated for reuse by heating at 500ꢀC in the presence ofair. The
same catalyst was recycled for all the reactions without loss of activity
and selectivity.

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3020
Pandey et al.
Table 1. Deprotection ofallyl esters promoted by H- b zeolite.
Nucleophile
anisole (eq)
Reaction
time (h)
Yieldb
(%)
Entry
1
Allyl ester
Producta
O
O
O
2
2
90
OH
O
O
O
2
3
—
2
6.5
3
80
96
OH
O
O
O
OH
O
O
O
4
5
6
—
2
8
76
90
80
OH
O
O
O
2
OH
O
O
O
2
1.5
OH
O
N
O
N
2
2
O
O
O
O
7
8
—
2
5
70
90
87
70
89
OH
NO
2
NO
NO
2
O
O
3
OH
O
NO
2
2
O
O
9
2
1.5
6
O
N
O
N
O
O
OH
2
2
NO
NO
NO
2
2
O
10
11
—
2
O2
N
O N
2
OH
NO2
O
2
O
2
O
OH
Br
Br