Ammonium chlorochromate adsorbed on montmorillonite K-10: Oxidative deprotection of tetrahydropyranyl ethers using microwaves in a solventless system

Article abstract of DOI:10.1007/s007060170102

Tetrahydropyranyl ethers were rapidly and selectively oxidized to the corresponding carbonyl compounds by clay supported ammonium chlorochromate under solvent free conditions using microwaves.

Full text of DOI:10.1007/s007060170102

Monatshefte fuÈr Chemie 132, 651±654 (2001)
Ammonium Chlorochromate Adsorbed on
Montmorillonite K-10: Oxidative Deprotection
of Tetrahydropyranyl Ethers Using Microwaves
in a Solventless System
Majid M. Heravi1;Ã, Rahim Hekmatshoar1, Yahya S. Beheshtiha1,
and Mitra Ghassemzadeh2
1
Department of Chemistry, School of Sciences, Azzahra University, Tehran, Iran
2
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Summary. Tetrahydropyranyl ethers were rapidly and selectively oxidized to the corresponding
carbonyl compounds by clay supported ammonium chlorochromate under solvent free conditions
using microwaves.
Keywords. Ammonium chlorochromate; Montmorillonite; Tetrahydropyranyl ethers; Microwave.
Introduction
Introduction and removal of protective groups constitute important processes in the
synthesis of polyfunctional molecules including the total synthesis of natural
products [1]. Tetrahydropyranylation of hydroxy groups has been recognized as a
useful and representative method for the protection of alcohols and phenols due to
the remarkable stability of tetrahydropyranyl (THP) ethers under a variety of
conditions such as strongly basic media, Grignard reagents, and oxidation and
reduction by inorganic hydrides [2]. Many catalysts have already been proposed for
the tetrahydropyranylation of alcohols and the cleavage of THP ethers to their
parent alcohols [3]. However, there are only few reports dealing with the direct
oxidation of THP ethers to the corresponding carbonyl compounds [4]. Montmoril-
lonite K-10 clay has been widely used as a catalyst for a variety of acid catalyzed
organic reactions [5] and as a mineral solid support [6].
Recently, there has been a growing interest in employing reagents impregnated
on mineral supports using microwaves in dry media [7]. Organic solvents are not
only expensive, but often also ¯ammable, toxic, and environmentally hazardous.
In addition, solvent free reactions under microwave irradiation are especially
appealing as they provide an opportunity to work in open vessels and on a large
scale [8].
Ã
Corresponding author

652
M. M. Heravi et al.
Montmorillonite K-10 supported ammonium chlorochromate has been used for
the oxidation of alcohols to carbonyl compounds [9]. In continuation of our ongoing
efforts to develop environmentally benign solventless methods using solid supports
and microwave activation [10], we envisioned the applicability of montmorillonite
K-10 supported ammonium chlorochromate for the oxidative deprotection of THP
ethers under solvent free conditions using microwaves. Herein we report our results.
Results and Discussion
Ammonium chlorochromate adsorbed on montmorillonite K-10 was prepared
according to a previously reported procedure [9]. Among the various solid supports
examined such as alumina, zeolites, or silica gel, montmorillonite K-10 was found
to work best. The reaction was performed by mixing the ®nely ground supported
reagent with neat THP ethers. This mixture was irradiated with microwaves to
afford the desired compounds in a very fast reaction in excellent yields (Table 1,
Scheme 1). It is noteworthy to mention that in the absence of clay the reaction is
very sluggish, and considerable amounts of alcohols are recovered unchanged even
after extended microwave irradiation. Moreover, in the absence of support the
isolation of products from the ensuing residues is dif®cult.
The oxidative deprotection of benzyltetrahydropyranyl ether is representative
of the general procedure employed. Montmorillonite K-10 supported ammonium
chlorochromate mixed with benzyltetrahydropyranyl ether was irradiated to afford
exclusively benzaldehyde in 94% yield within 20 s. An over-oxidation to the corre-
sponding carboxylic acid was not observed. The same reaction in solution requires
heating to 50ꢀC for a period of 1 h.
In conclusion, the present methodology offers an attractive, fast, effecient, and
environmentally friendly procedure for the direct oxidative cleavage of THP ethers
to the corresponding carbonyl compounds.
Table 1. Oxidative deprotection of THP ethers with montmorillonite K-10 supported ammonium
chlorochromate under microwave irradiation in a solventless system
Reaction time
sec
Yield
Substrate
Product
%
PhCH2OTHP
20
20
PhCHO
94
92
80
88
78
88
89
90
88
85
4-MeC6H4CH2OTHP
2-NO2-5-Me-C6H3CH2OTHP
PhCH(Me)OTHP
4-MeC6H4CHO
2-NO2-5-MeC6H3CHO
PhCOMe
50
50
PhCH=CHCH2OTHP
PhCH(OTHP)Ph
100
100
120
120
120
100
PhCH=CHCHO
PhCOPh
Cyclohexanol-THP
2-Methylcyclohexanol-THP
Benzoin-THP
Cyclohexanone
2-Methylcyclohexanone
Benzil
( )-Menthol-THP
( )-Menthone

Deprotection of THP ethers
653
Scheme 1
Experimental
Ammonium chlorochromate supported on montmorillonite K-10 was prepared by a reported method
[9]. Yields refer to isolated products. The obtained compounds were identi®ed by comparison of
their physical data with authentic samples. THP ethers were prepared according to described
procedures [3b].
Oxidative deprotection of tetrahydropyranyl ethers; general procedure
Montmorillonite K-10 supported ammonium chlorochromate was thoroughly mixed with 0.192 g
benzyltetrahydropyranyl ether (1 mmol), and the mixture was placed in a beaker inside a microwave
oven (900 W) for 20 sec. After completion of the reaction as monitored by TLC (hexane:AcOEt ˆ
8:2) the product was extracted with CH2Cl2, the solvent was removed, and the residue was
chromatographed over silica gel using hexane:AcOEt (8:2) to afford 94% benzaldehyde.
Acknowledgments
Financial support for this project from the research council of Azzahra University is gratefully
acknowledged.
References
[1] a) Green TW, Wuts PGM (1991) Protective Groups in Organic Synthesis, 2nd edn. Wiley, New
York; b) Kocienski PG (1994) Protective Groups. Thieme, Stuttgart
[2] Hoyer S, Laszlo P (1986) Synthesis 685 and references cited therein
[3] a) Oskooie HA, Abasszadeh MR, Zameni F, Heravi MM (1998) Synth Commun 28: 2281; b)
Heravi MM, Ajami D, Ghassemzadeh M (1999) Synth Commun 29: 1013; c) Ballini R, Bigi F,
Corloni R, Sartori G (1999) Tetrahedron Lett 38: 4169; d) Li TS, Zhang Jin TS (1999) Synth
Commun 29: 181 and references cited therein
[4] a) Heravi MM, Ajami D (1998) J Chem Res 718; b) Heravi MM, Ajami D, Ghassemzadeh M
(1999) Tetrahedron Lett 40: 561
[5] a) Theory RKG (1974) The Chemistry of Clay in Organic Reactions. Hilger, London; b) Dewan
SK, Varma U, Malik SD (1995) J Chem Res 21; c) Heravi MM, Tajbakhsh M, Mohajerani B,
Ghassemzadeh M (1999) Z Naturforsch 54b: 541; d) Varma RS, Dahiya R, Kumar S (1997)
Tetrahedron Lett 38: 2039
[6] a) Heravi MM, Ajami D, Tabar-Hydar K (1998) Monatsh Chem 129: 1305; b) Heravi MM,
Kiakojoori R, Tabar-Hydar K (1999) Monatsh Chem 130: 581; c) Heravi MM, Ajami D (1999)
Monatsh Chem 130: 709
[7] a) Varma RS, Meshram HM (1997) Tetrahedron Lett 38: 5427; b) Varma RS, Dehiya R, Saini RK
(1997) Tetrahedron Lett 38: 8819 and references cited therein
[8] a) Cablewaski T, Faux AF, Strauss CR (1994) J Org Chem 59: 3408; b) Bangell L, Cablewaski T,
Strauss CR, Trainer RW (1996) J Org Chem 61: 7355

654
M. M. Heravi et al.: Deprotection of THP ethers
[9] Heravi MM, Ajami D, Tabar-Hydar K (1998) J Chem Res 656
[10] a) Heravi MM, Ajami D, Aghapoor K, Ghassemzadeh M (1999) J Chem Soc Chem Commun
833; b) Heravi MM, Ajami D, Hektmat-Shoar R, Beheshtiha Y, Assadollah K, Ghassemzadeh M
(2000) Z Naturforsch 55b: 431; c) Heravi MM, Ajami D, Tabar-Hydar K (1999) Synth Commun
29: 163; d) Mirza-Aghayan M, Heravi MM (1999) Synth Commun 29: 785; e) Heravi MM,
Ajami D, Tabar-Hydar K (1999) Synth Commun 29: 1009
Received July 25, 2000. Accepted September 7, 2000