1058
J . Org. Chem. 1998, 63, 1058-1063
Selective Ca ta lytic Tr a n sester ifica tion , Tr a n sth iolester ifica tion ,
a n d P r otection of Ca r bon yl Com p ou n d s over Na tu r a l Ka olin itic
Cla y
Datta E. Ponde, Vishnu H. Deshpande,* Vivek J . Bulbule, Arumugam Sudalai,* and
Anil S.Gajare
Division of Organic Chemistry: Technology, National Chemical Laboratory, Pune, 411 008 India
Received J uly 29, 1997
Transesterification and transthiolesterification of â-keto esters with variety of alcohols and thiols
and selective protection of carbonyl functions with various protecting groups catalyzed by natural
kaolinitic clay are described. The clay has been found to be an efficient catalyst in transesterifying
long chain alcohols, unsaturated alcohols, and phenols to give their corresponding â-keto esters in
high yields. For the first time, transthiolesterification of â-keto esters with a variety of thiols has
been achieved under catalytic conditions. Clay also catalyzes selective transesterification of â-keto
esters by primary alcohols in the presence of secondary and tertiary alcohols giving corresponding
â-keto esters. A systematic study involving the reactivity of different nucleophiles (alcohols, amines,
and thiols) toward â-keto esters is also described. Sterically hindered carbonyl groups as well as
R,â-unsaturated carbonyl groups underwent protection without the deconjugation of the double
bond. Chemoselective protection of aldehydes in the presence of ketones has also been achieved
over natural kaolinitic clay.
Sch em e 1
1. In tr od u ction
Transesterification is a process where an ester is
transformed into another through interchange of the
alkoxy moiety1 (Scheme 1). Since the reaction is an
equilibrium process, the transformation occurs essen-
tially by simply mixing two components. However, it has
been shown that the reaction is accelerated by Lewis acid
catalysts (such as boron tribromide,2 anhydrous alumi-
num trichloride embedded in polystyrene-divinyl ben-
zene3), Bronsted acid catalysts (such as hydrochloric,
phosphoric, sulfonic, sulfuric, or p-toluenesulfonic acid4),
or basic catalysts (such as metal alkoxides,5 metal
carbonates6).
Transesterification is more advantageous than the
ester synthesis from carboxylic acid and alcohol, due to
poor solubility of some of acids in organic solvents,
whereas the esters are commonly soluble in most of the
solvents. Some esters, especially methyl and ethyl esters,
are readily or commercially available and thus serve
conveniently as starting materials in transesterification.
Transesterification is applicable in the paint industry for
the curing of alkyl resin.1 It also plays an important role
in polymerization.1
It is apparent, however, that the reaction under the
acidic or basic conditions does not meet the requirements
of modern synthetic chemistry, which needs highly
efficient and selective reaction conditions. Also, efforts
may be continued to make the reaction catalytic, milder,
and more selective.
Among the catalysts developed, the distannoxane7 was
found to be an effective one for transesterification of
various types of esters; however, this catalyst is difficult
to prepare. Titanate-mediated8 transesterification method
is extremely mild but is found to be unsuccessful to give
allyl ester from an ethyl ester. This may be due to the
fact that transesterification of â-keto esters, especially
with allylic alcohol, is rather difficult as it is offset by
facile decarboxylation rearrangement. Although Taber’s9
DMAP-promoted method is effective for transesterifica-
tion of primary and secondary alcohols, it fails in the case
of long chain alcohols due to the decomposition of the
acetoacetate. A tin-based superacid10 was reported to be
effective for the synthesis of alkyl â-keto esters although
it failed with aromatic â-keto esters. Even though
aromatic â-keto esters could be transesterified with Hâ-
zeolite,11 it fails to chemoselectively transesterify primary
* Corresponding authors: Phone No. 337614, Fax No. +91-0212-
337614 E-mail: sudalai@dalton.ncl.res.in.
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Published on Web 01/30/1998