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PREPARATION OF ENOL ACETATES FROM KETONES
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methyl lithium enol esters undergo facile regio- and stereospecific
transformation to lithium enolates.[5] Unfortunately relatively few methods
are available for the synthesis of enol esters. The most widely practiced
technique involves treatment of aldehyde and ketones under either acid or
basic conditions with the appropriate acid anhydride or chloride.[6]
In continuation of our interests on the chemistry of aliphatic nitro com-
pounds[7] such as simple nitroalkanes, nitroalkenes and a-nitroketones, we
were in search of a high yielding efficient routine methods of preparation of
enol acetates from which a-nitroketones are normally made by nitration.[8]
As a replacement of the obvious homogenous lewis acids such as BF3 Á OEt2
and Me3SiCl, which are normally used in several literature procedures along
with the appropriate anhydrides, we used heterogeneous and environment
friendly and cheap catalyst Montmorillonite KSF clay. We observed that in
several cases (Entry 1,2,3 & 6 in Table 1) ketones were efficiently converted
to their corresponding enol acetates even at RT. However in certain cases
heating the reaction mixture (Table 1) in the range between 35 to 140ꢀC was
necessary. Our results are summarized in Table 1.
It was further observed that aldehydes, esters as well as the lactones
are not converted to the corresponding enol acetates with this reagent indi-
cating its mild and selective nature. The reaction time can also be reduced
substantially from 2–3 h to 5 min by performing the reactions under low
energy microwave irradiation.
To a solution of cyclohexanone (1 g, 10. 2 mmol) in acetic anhydride
(10 mL) was added Montmorillonite KSF clay (0.5 g) and the mixture stirred
at RT while monitoring the reaction on TLC (1 : 20 ethyl acetate: hexane).
The catalyst was filtered off and the filtrate was poured into cold water
(50 mL) and extracted with ether (3 Â 25 mL). The organic phase was
washed successively with 5% NaHCO3 and water and dried over anhydrous
Na2SO4. Evaporation of the solvent gave a gum which was purified by
preparative TLC. Yield 1.34 g (94%).
ACKNOWLEDGMENT
The authors thankDirector RRL Jorhat for providing facilities for
this work.
REFERENCES
1. Larock, R.C.; Oertle, K.; Beatty, K.M. J. Am. Chem. Soc. 1980, 102,
1966 and references cited therein.