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A. K. Chakraborti, R. Gulhane / Tetrahedron Letters 44 (2003) 6749–6753
acetylation of endo-borneol was carried out with 1
equiv. of Ac2O for 2 h at room temperature in the
absence of solvent, 10 equiv. of Ac2O in THF was
required in the presence of Bi(OTf)3 to afford a com-
parable yield in 7 h. Acetylation of 1-adamantanol was
carried out with 1 equiv. of Ac2O in 0.25 h in the
absence of solvent whereas the Bi(OTf)3, Sc(OTf)3, and
TMSOTf-catalysed acetylations required 10 equiv. of
Ac2O in MeCN/DCM to afford comparable results in
3.5–8.5 h.9b
(F3CCO)2O (0.35 mL, 2.5 mmol) at room temperature
for 30 min under magnetic stirring in the presence of
InCl3 (5.5 mg, 0.025 mmol, 1 mol%) followed by 2-
hydroxynaphthalene (360 mg, 2.5 mmol) for an addi-
tional 10 min. The reaction mixture was extracted with
Et2O to afford 2%-naphthyl thiophene-2-carboxylate
(635 mg, 100%).
References
Further, studies were carried out during the InCl3-
catalysed acylations of 2-hydroxynaphthalene with
equimolar mixtures of (a) Ac2O and (EtCO)2O; (b)
Ac2O and (iPrCO)2O; (c) Ac2O and (tBuCO)2O; (d)
Ac2O and (PhCO)2O and (e) Ac2O and (F3CCO)2O.
Formation of 2-naphthyl acetate with 72, 87, 96, 100,
and 100% selectivity, respectively, demonstrated the
influence of the steric and electronic effects of R in
(RCO)2O on chemoselectivity.
1. (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis; Wiley: New York, 1999; (b) Hanson,
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To make the acylation generalised with respect to the
carboxylic acid, we planned to adopt a mixed anhy-
dride protocol. Thus, 2-hydroxynaphthalene was
treated separately with acetic acid, benzoic acid, phenyl
acetic acid, dihydrocinnamic acid, cinnamic acid, 4-
methoxycinnamic acid, 1-naphthoic acid, and thio-
phene-2-carboxylic acid in the presence of an equimolar
amount of (F3CCO)2O under the catalytic influence of
InCl3 affording the corresponding acylated products in
100, 98, 81, 100, 98, 90, 90, and 100% yields, respec-
tively, in 10–60 min at room temperature under neat
conditions. The exothermicity of the reaction proved to
be detrimental for the use of a stronger Lewis acid such
as In(OTf)3 or Sc(OTf)3, and in most of the cases an
intractable product mixture was obtained. The advan-
tage of InCl3 over In(OTf)3 and Sc(OTf)3 was demon-
strated by the fact that the reaction of
2-hydroxynaphthalene with an equimolar mixture of
(F3CCO)2O and thiophene-2-carboxylic acid resulted in
65 and 74% yields, respectively, with In(OTf)3 and
Sc(OTf)3.
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In conclusion, InCl3 is a new and highly efficient cata-
lyst for acylation of phenols, thiols, alcohols and
amines. The advantages include low cost, ease of hand-
ling; and with increasing environmental concern,21 the
solvent-free conditions employed in the present method
will make it ‘environment friendly’ and useful for indus-
trial applications.
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Typical procedure for acylation. 4-Nitrophenol (347.5
mg, 2.5 mmol) was treated with Ac2O (0.24 mL, 2.5
mmol) at room temperature for 30 min under magnetic
stirring in the presence of InCl3 (0.55 mg, 0.0025 mmol,
0.1 mol%). The reaction mixture was extracted with
Et2O to afford 4-nitrophenyl acetate (444 mg, 98%),
which was in full agreement with the spectral data (mp,
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Synlett 2000, 1652–1654.
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1
IR, H NMR and EIMS) of an authentic sample.
16. Nakae, Y.; Kusaki, I.; Sato, T. Synlett 2001, 1584–1586.
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Typical procedure for acylation with carboxylic acids
following the mixed anhydride protocol. 2-Thiophenecarb-
oxylic acid (320 mg, 2.5 mmol) was treated with