A convenient and selective method for preparation of enol acetates from ketones using montmorillonite KSF clay

Article abstract of DOI:10.1081/SCC-120013734

The reaction of ketones with stoichiometric amounts of acetic anhydride and Montmorillonite KSF clay at room temperature afforded the corresponding enol acetates in excellent yields. It has also been observed that by performing the reactions under microwave irradiation the reaction time can be dramatically reduced.

Full text of DOI:10.1081/SCC-120013734

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A CONVENIENT AND SELECTIVE METHOD FOR
PREPARATION OF ENOL ACETATES FROM KETONES USING
MONTMORILLONITE KSF CLAY
Biswajit Kalita a , Maitreyee Sarma Bezbarua a & Nabin C. Barua b
a Regional Research Laboratory , Jorhat, Assam, 785 006, India
b Regional Research Laboratory , Jorhat, Assam, 785 006, India
Published online: 16 Aug 2006.
To cite this article: Biswajit Kalita , Maitreyee Sarma Bezbarua & Nabin C. Barua (2002) A CONVENIENT AND SELECTIVE
METHOD FOR PREPARATION OF ENOL ACETATES FROM KETONES USING MONTMORILLONITE KSF CLAY, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:20, 3181-3184, DOI: 10.1081/
SCC-120013734
To link to this article: http://dx.doi.org/10.1081/SCC-120013734
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 20, pp. 3181–3184, 2002
A CONVENIENT AND SELECTIVE
METHOD FOR PREPARATION OF ENOL
ACETATES FROM KETONES USING
MONTMORILLONITE KSF CLAY
Biswajit Kalita, Maitreyee Sarma Bezbarua,
*
and Nabin C. Barua
Regional Research Laboratory,
Jorhat-785 006, Assam, India
ABSTRACT
The reaction of ketones with stoichiometric amounts of acetic
anhydride and Montmorillonite KSF clay at room tempera-
ture afforded the corresponding enol acetates in excellent
yields. It has also been observed that by performing the reac-
tions under microwave irradiation the reaction time can be
dramatically reduced.
In organic synthesis, enol esters are considered to be extremely
valuable intermediates, especially as synthons for the preparation of
a-acyloxy or a-halo carbonyl compounds via epoxidation or halogenation
respectively.[1] They also result carbon–carbon bond formation on photo-
lysis,[2] reduction,[3] acylation and rearrangements.[4] Upon treatment with
*Corresponding author. Fax: þ91-0376-370011; E-mail: ncbarua@csir.res.in
3181
DOI: 10.1081/SCC-120013734
Copyright & 2002 by Marcel Dekker, Inc.
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3182
KALITA, BEZBARUA, AND BARUA
Table 1. Preparation of Enol Acetates from Ketones
Reaction
React.
Yield
(%)b
Entry
1
Substrate
Time (h) Temp. (ꢀC)
Producta
3
2
23
23
88
2
3
94
4
23
89
4
5
4.5
1.5
23
70
93
83
6
2.5
75
81
7
8
3
4
35
83
81
120
9
2
140
65
72
10
cholestan-3-one
75
cholest-2-en-3-ol 97
acetate
aProducts were characterized by IR, NMR (300 MHz), MS, and by
direct comparison wit authentic materials; bYield refers to yields of
pure isolated products.

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PREPARATION OF ENOL ACETATES FROM KETONES
3183
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.

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3184
KALITA, BEZBARUA, AND BARUA
2. Wexler, A.; Ba lchunis, R.J.; Sweton, J.S. J. Chem. Soc., Chem.
Commun. 1975, 601–602.
3. Schmitt, G.; Warwel, S.; Hominga, E.; Meltzow, W. Justus Liebigs
Ann. Chem. 1972, 763, 75.
4. Yong, F.G.; FrostickJr., E.C.; Sanderson, J.J.; Hauser, C.R. J. Am.
Chem. Soc. 1950, 72, 3635.
5. House, H.O.; Trost, B.M. J. Org. Chem. 1965, 30, 2502.
6. (a) Villotti, R.; Djerassi, C.; Ringold, H.J. J. Am. Chem. Soc. 1959, 81,
4556; (b) Liston, A.J. J. Org. Chem. 1966, 31, 2105; (c) Chowdhury,
P.K.; Sharma, R.P.; Baruah, J.N. Tetrahedron Letters 1983, 24, 32.
7. (a) Kalita, D.; Khan, A.T.; Saikia, A.; Barua, N.C. Synthesis 1998,
975; (b) Kalita, D.; Khan, A.T.; Barua, N.C.; Bez, G. Tetrahedron
1999, 55, 5177; (c) Bez, G.; Bezbarua, M.S.; Saikia, A.K.; Barua,
N.C. Synthesis 2000, 537–540.
8. Rank, W. Tetrahedron Letters 1991, 39, 5353.
Received in the Netherlands September 28, 2001