TMSOTf/CH2Cl2,16 or BiBr3/CH3CN;17 removal of aro-
matic acetate using ammonium acetate;18 deprotection
leaves the other one free for esterification.27 Selective
esterification of aliphatic carboxyl groups in the presence
of aromatic carboxyl groups in homophthalic acid and
similar systems has been reported by Rodriguez et al.28 as
well as Ram and Charles.29 The former method makes use
of a 2,2-dimethoxypropane (excess)/MeOH/TMSCl sys-
tem, and an ∼98% yield of homophthalic acid mono-
methyl ester was obtained on using 10 mol % of TMSCl
along with >10 mol equiv of 2,2-dimethoxypropane. As
of PMB ether using CeCl3 7H2O-NaI/CH3CN;19 re-
3
moval of aryl aldehyde 1,1-diacetate in the presence
of phenolic acetate using InBr3/PEG;20 and selective
deprotection of tert-butyl esters in the presence of -N-
(PhF) groups using ZnBr2 in CH2Cl2.21 Since the sub-
strates chosen in each of the above studies are different,
a comparative assessment of the efficiency of the catalysts
involved is difficult. The details presented in Table 3,
however, confirm that SmCl3 can offer promise in the
selective deprotection of acid labile protecting groups.
Comparison of esterification and deprotection conditions
(Tables 1 and 3) shows that the reaction time and temp
are the main factors that make the selectivity possible
between Boc/TBDMS/THP groups.
per the latter procedure, 10 mol % of NiCl2 6H2O cata-
3
lyzed the selective monoesterification of homophthalic
acid and itaconic acid to give 15b and 4b in 95% and
86% yields, respectively. Catalytic esterification using
MsOH supported on active carbon is another method that
can give good selectivity toward aliphatic carboxyl groups
in the presence of aromatic or conjugated ones, and an
82% yield of homophthalic acid monoethyl ester (15b)
and a 92% yield of itaconic acid monoethyl ester (4b)
were obtained upon using 1.2 g of the solid-supported
catalyst per 0.1 mol of the acids.30 In comparison, the
SmCl3-based method reported here requires only 1 mol %
of the catalyst for near-quantitative formation of mono
esters in these cases!
The most notable among the chemoselective esterifica-
tions include the following: esterification of R-hydroxy
carboxylic acids using boric acid22 or N-methyl-4-borono-
pyridinium halide;23 esterification of hydroxy acids using a
NaY faujasite-dimethyl carbonate system;24 Mitsunobu
conditions for chemoselective esterification between phe-
nolic acids and alcohols;25 and competitiveesterification of
sp3-Ctetheredcarboxylgroupsinthe presence of sp2- orsp-
C tethered ones using CBr4/MeOH.26 Ogawa et al. have
reported a monoesterification protocol for dicarboxylic
acids using CH2N2 in the presence of alumina as the solid
support; adsorption of one of the acid groups in these cases
In summary, we have unraveled the usefulness of
SmCl3 as an efficient catalyst in esterification and
deprotection reactions, which proceeded with excellent
chemoselectivity in the presence of competing functional
groups. Retention of Boc and peptidic amide groups
during esterification, selective esterification of an aliphatic
carboxylic acid group in the presence of aromatic or R,β-
unsaturated carboxyl groups, and selective deprotection of
a TBDMS or THP group in the presence of Boc and a
TBDMS group in the presence of THP are the key
highlights.
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Acknowledgment. Financial support of this work by
DST, India (Grants SR/S1/OC-13/2007) is gratefully ac-
knowledged. G.P. thanks IIT Madras and S.N. thanks
CSIR for research fellowships.
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Supporting Information Available. Experimental and
spectral details of selected compounds. This material is
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