Potassium ferrate supported on montmorillonite K-10: A mild and efficient reagent for oxidative deprotection of acetals and ketals

Article abstract of DOI:10.1007/s007060170061

A variety of acetals and ketals were oxidatively deprotected to their parent compounds using potassium ferrate(VI) supported on montmorillonite K-10 under non-aqueous conditions.

Full text of DOI:10.1007/s007060170061

Monatshefte fuÈr Chemie 132, 985±988 (2001)
Potassium Ferrate Supported on
Montmorillonite K-10: A Mild and Ef®cient
Reagent for Oxidative Deprotection of Acetals
and Ketals
Majid M. Heravi1;Ã, Mahmood Tajbakhsh2;Ã, Setareh Habibzadeh2,
and Mitra Ghassemzadeh3
1
Department of Chemistry, School of Sciences, Azzahra University, Vanak, Tehran, Iran
2
Department of Chemistry, University of Mazandaran, Babolsar, Iran
3
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Summary. A variety of acetals and ketals were oxidatively deprotected to their parent compounds
using potassium ferrate(VI) supported on montmorillonite K-10 under non-aqueous conditions.
Keywords. Oxidative deprotection; Potassium ferrate; Clay; Acetals; Ketals.
Introduction
The protection and deprotection of functional groups constitutes an important
feature in the synthesis of polyfunctional molecules, including the total synthesis of
natural products. The shielding of carbonyl groups from nucleophilic attack or other
kinds of unwanted reactions is common practice in multistage organic synthesis.
Although the protection of carbonyls can be achieved by a variety of protecting
groups; protection as cyclic or acyclic acetals have proved to be most promising [2].
Hence, the deprotection of acetals and ketals to their carbonyl compounds is of
practical signi®cance. The general methods employed for the deprotection of acetals
and ketals involve aqueous conditions using mineral acids [3]. Other methods
employ silica gel [4], phosphorus triiodide [5], titanium(IV) chloride [6], boron-
tri¯oride iodide [7], cerium(III) chloride [8], and silver and sodium bromate in the
presence of AlCl3 [9]. Recently, a repertoire concerning rapid and selective methods
for the cleavage of acetals using supported ammonium nitrate under microwave
irradiation has been reported [10]. Potassium ferrate (K2FeO4), a hexavalent iron
compound, has been introduced recently by P. Laszlo et al. as a novel oxidizing
reagent [11]. The use of a mineral support has attracted much attention because of
improved selectivity, reactivity, and associated ease of manipulation [12, 13].
Ã
Corresponding author. E-mail: mmheravi@azzahra.ac.ir

986
M. M. Heravi et al.
Herein we wish to report that potassium ferrate(VI) supported on montmor-
illonite K-10 can ef®ciently deprotect cyclic acetals and ketals to the corresponding
carbonyl compounds in high to excellent yields.
Results and Discussion
Potassium ferrate(VI) ꢀK2FeO4† can be prepared easily by oxidizing ferric nitrate
with sodium hypochlorite and subsequent treatment with potassium hydroxide [14].
The synthesis of the above reagent has been improved, and it has been claimed to be
a law polluting oxidizer [11]. Prompted by stringent environmental protection laws
in recent years, there has been increasing emphasis on the use and design of eco-
friendly supported reagents to reduce the amount of toxic waste and byproducts
arising from chemical processes [15].
Our approach to a clean and selective deprotection of acetals and ketals is to
make use of potassium ferrate(VI) supported on montmorillonite K-10. Combina-
tion of ferric nitrate with zeolite had been pioneered in our laboratory, resulting in
the design of zeofen as a versatile oxidizing agent [13b, 16]. Armed with this
experience we reasoned that using an iron salt as less toxic transition metal salt [17]
along with clay instead of Brùnsted (H2SO4; HNO3, etc.) and Lewis acids (AlCl3,
TiCl4, etc.) would help to maintain a non-polluted environment.
Scheme 1

Oxidative Deprotection of Acetals and Ketals
987
Table 1. Oxidative cleavage of acetals and ketals using K2FeO4 supported on montmorillonite K-10
Substratea
Productb
t/min
Yield/%c
M.p./ꢁC or b.p./ꢁC
Found
Reported [18]
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
10
10
15
15
15
10
10
15
15
15
15
95
85
90
92
91
90
93
92
88
82
88
177±178 (760 torr)
178±179 (760 torr)
247
(760 torr)
248
238
(760 torr)
(760 torr)
235±236 (760 torr)
56±57
57±59
151±153 (760 torr)
153
(760 torr)
(10 torr)
(760 torr)
120
(10 torr)
121
1g
1h
1i
2g
2h
2i
201±202 (760 torr)
116±118
202
116±118
75±77
78±80
76±77
1j
1k
2j
2k
77±79
111±112 (6 torr)
113±116 (6 torr)
a
All substrates were synthesized according to known procedures; b all Products were characterized by
comparison of their physical and spectroscopic data with those of authentic samples; c yield are based
on isolated products
The reaction between potassium ferrate(VI) supported on montmorillonite K-10
and the acetal proceeded smoothly and afforded the corresponding carbonyl
compound in high yield. It is noteworthy to mention that the reaction remained
incomplete when the solid support was not used even with a higher ratio of reagent
(2:8) as well as at a higher temperatures (re¯uxing in acetonitrile) and longer
reaction times (5 h). Overoxidation was also not observed.
The acetal of cinnamaldehyde was transferred to the parent aldehyde by this
method, showing that the carbon-carbon double bond is not prone to cleavage by the
reagent. To assess the generality of the method, a variety of acetals and ketals were
deprotected in high to excellent yields (Scheme 1, Table 1).
In conclusion, we have developed a selective, simple, inexpensive, ef®cient, and
environmentally benign protocol for oxidative deprotection of acetals and ketals.
Experimental
All starting materials and products were known; their physical and spectroscopic data were compared
with those of authentic samples and found to be identical. Acetals and ketals were synthesized
following a reported procedure [17]. The reagent (K2FeO4) was prepared according to the literature
[11] and impregnated on montmorillonite K-10.
Oxidative deprotection of acetals and ketals with potassium ferrate(VI) supported
on montmorillonite K-10; general procedure
In a round-bottomed ¯ask (50 cm3) equipped with a magnetic stirrer and a condenser, a solution of
2 mmol acetal or ketal in (20 cm3) CH2Cl2 was prepared. Potassium ferrate (4 mmol) was mixed
intimately with 1 g montmorillonite K-10 and added to this solution. The reaction mixture was
re¯uxed and magnetically stirred for 10±15 min. The progress of the reaction was monitored by TLC
(eluent: petrol ether:ethyl acetate ˆ 8:2). The reaction mixture was ®ltered and washed with CH2Cl2.

988
M. M. Heravi et al.: Oxidative Deprotection of Acetals and Ketals
The combined ®ltrate was evaporated to dryness. The crude residue was passed through a silica pad
using a suitable solvent to regenerate the corresponding carbonyl compounds.
Acknowledgements
The authors are grateful for partial ®nancial assistance by the research council of Mazandaran
University.
References
[1] a) Green TW, Wutz PGM (1991) Protective Groups in Organic Synthesis, 2nd edn. Wiley, New
York, p 68; b) Reese CB (1973) Protective Groups in Organic Chemistry, chap 3. Plenum,
London
[2] Kocienskio PJ (1994) Protecting Groups. Thieme, New York
[3] Baudin G, Bondon D, Pietrasanto Y, Pucci B (1978) Tetrahedron 34: 3269
[4] Huet F, Le Chevealier A, Pellet M, Conia JM (1978) Synthesis 63
[5] Denis JN, Krief A (1980) Angew Chem Int Ed Engl 19: 1006
[6] Balme G, Gore J (1983) J Org Chem 48: 3336
[7] Mandal AK, Shroti PY, Ghogare AD (1985) Synthesis 221
[8] Marcantoni E, Nodili F (1997) J Org Chem 62: 4183
[9] Mohammadpoor-Baltork I, Nourozi AR (1999) Synthesis 487
[10] Mishram HM, Sumittra G, Reddy GS, Ganesh YSS, Yadav JS (1999) Synth Commun 29: 2807
[11] Delaude L, Laszlo P (1996) J Org Chem 61: 6360
[12] a) Mckillop A, Young DW (1979) Synthesis 401, 481; b) Balogh M, Laszlo P (1993) Organic
Chemistry Using Clays. Springer, Berlin; c) Clark JH (1994) Catalysis of Organic Reactions by
Supported Inorganic Reagents. VCH, New York
[13] a) Heravi MM, Ajami D, Ghassemzadeh M, Mojtahedi MM (1999) Tetrahedron Lett 40: 561; b)
Heravi MM, Ajami D, Aghapoor K, Ghassemzadeh M (1999) J Chem Soc Chem Commun 833;
c) Heravi MM, Beheshtiha YS, Ghassemzadeh M, Hekmat-Shoar R, Sarmad N (2000) Monatsh
Chem 131: 187
[14] Thompson GW, Ockerman LT, Schreyer JM (1951) J Am Chem Soc 73: 1379
[15] Khouv CB, Dartt CB, Lalenger JA, Davis ME (1994) J Catal 149: 195
[16] Heravi MM, Ajami D, Ghassemzadeh M, Tabar-Hydar K (2001) Synth Commun (in press)
[17] a) Maity G, Roy S (1993) Synth Commun 25: 395; b) Mistens FAJ Synthesis 1981: 501;
c) Bhalerao UT, Joju Davis K (1996) Synth Commun 26: 3081
[18] Aldrich, Catalogue Handbook of Fine Chemicals
Received January 23, 2001. Accepted (revised) March 14, 2001