404 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 404^405y
A Selective Catalytic Method of Enol-acetylation
under Microwave Irradiationyz
Dipok J. Kalita, Ruli Borah and Jadab C. Sarma*
Natural Products Chemistry Division, Regional Research Laboratory, Jorhat 785006, Assam,
India
Six-membered cyclic ketones on treatment with acetic anhydride and a catalytic amount of iodine under
microwave irradiation give the corresponding enol acetates in good yield.
Table 1 Enol-acetylation reactions under microwave irradiation
In pursuance of our interest on iodine as a catalyst for vari-
ous reactions such as
acylation,1 acetylation,2
Power
setting
acetalization,3 etc. we had studied the e¡ect of iodine on
the reaction of acetic anhydride with ketones. When
cyclohexanone was re£uxed in THF with acetic anhydride
and iodine for 8 h an indication of a less polar spot was
observed in TLC. The same reaction when heated under
microwave irradiation for 5 min furnished a quantitative
yield of the corresponding enol acetate. To generalise the
reaction, several examples were performed, and results
are given in Table 1.
Entry
Substrate
O
t / min
Product
OAc
Yield (%)
1
5
2
90
1
2
O
OAc
2
3
5
2
3
75
72
Enol esters of ketones are extremely valuable
intermediates in synthetic organic chemistry. Various
manipulations, such as epoxidation, halogenation,
photolysis, etc. on such intermediates a¡ord di¡erent
important products.4 This is also a reaction of preparative
value for 17a-hydroxycorticoids including cortison.5 Despite
their importance as valuable intermediates, only a limited
number of methods are available for the preparation of enol
acetates.
10
O
OAc
3
4
5
6
Earlier methods of enol acetylation of ketones described
mainly the use of acetic anhydride with acidic catalysts such
as perchloric acid,6 toluene-p-sulfonic acid7 or acetic anhy-
dride with basic catalysts such as potassium carbonate,8
triethyl amine,9 etc. Another method utilized isopropenyl
acetate and toluene-p-sulfonic acid10 as the reagent of
choice. Ladjama and Riehl11 used KH^AcCl for enol acety-
lation of aldehydes while Limat and Schlosser12 reported
recently the use of acyl £uoride for the same purpose. In
two di¡erent reports Choudhury et al.13 and Caliez et al.14
used chlorotrimethylsilane as a catalyst for enol acetylation
of ketones with acetic anhydride.
Earlier methods of enol acetylation used either strong
acids or bases as catalysts which may be detrimental to deli-
cate substrates. Our present method is fast and utilizes a
mild non-toxic catalyst. Moreover the present reagent sys-
tem is selective to six-membered cyclic ketones only. It does
not react with open-chain or ¢ve-membered cyclic ketones
(entries 9 and 10 in Table 1) or with a; b-unsaturated acyclic
ketones (entry 8) under these conditions. Longer reaction
times or use of higher microwave power on these substrates
leads to decomposition only. In contradiction to an earlier
report,13 carvone under the reaction conditions gives
carvacrol instead of an enol acetate or carvacrol acetate.
4
5
2
50
O
O
OAc
OH
5
6
10
3
2
85
75
O
OAc
5
7
8
Cholestan-3-one
5
5
2
2
2-cholestene-3-ol acetate 90
α-Ionone
NRa
α-Ionone
O
O
5
5
2
2
NR
NR
9
Experimental
IR spectra were recorded on
spectrophotometer, NMR spectra on a Varian 360L instrument
and mass spectra on a INCOS 50 GC-MS instrument.
a Perkin Elmer 237B IR
O
O
10
* To receive any correspondence.
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
z Dedicated to Dr R. P. Sharma on his 60th Birthday.
NR no reaction.
Kalita, Dipok J.
