Clay supported Bis(trimethylsilyl)chromate: Oxidative deprotection of tetrahydropyranyl ethers under solvent-free conditions using microwaves

Article abstract of DOI:10.1007/PL00010252

Tetrahydropyranyl ethers arc rapidly and selectively oxidized to the corresponding carbonyl compounds by montmorillonite K-10 supported bis(trimethylsilyl)chromate under solvent-free conditions using microwaves.

Full text of DOI:10.1007/PL00010252

Monatshefte fuÈr Chemie 130, 709±712 (1999)
Clay Supported
(trimethylsilyl)chromate:
Oxidative Deprotection of Tetrahydropyranyl
Ethers under Solvent-Free Conditions using
Microwaves
Ã
Majid M. Heravi and Dariush Ajami
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Summary. Tetrahydropyranyl ethers are rapidly and selectively oxidized to the corresponding
carbonyl compounds by montmorillonite K-10 supported bis(trimethylsilyl)chromate under solvent-
free conditions using microwaves.
Keywords. Clay supported reagent; Oxidation; Deprotection; Dehydropyranylation.
An Ton adsorbiertes (trimethylsilyl)chromat: Oxidative EntschuÈtzung von
È
Tetrahydropyranylethern unter losungsmittelfreien Bedingungen und
Bestrahlung mit Mikrowellen
Zusammenfassung. Tetrahydropyranylether werden rasch und selektiv durch an Montmorillonit
È
K-10 adsorbiertes Bis(trimethylsilyl)chromat unter losungsmittelfreien Bedingungen und Einwir-
kung von Mikrowellen oxidativ zu den entsprechenden Oxoverbindungen gespalten.
Introduction
The protection of certain functional groups and the deprotection of the protected
derivatives constitute important processes in the synthetic chemistry of polyfunc-
tional molecules including the total synthesis of natural products. The
tetrahydropyranyl group is one of the most useful protective groups for alcohols
in multi-step organic synthesis [1], and considerable efforts have been made for the
development of new methods for their introduction and removal [2±17]. However,
there are only few reports dealing with the direct oxidation of tetrahydropyranyl
ethers to the corresponding carbonyl compounds [18, 19]. Consequently, there is a
need to develop and introduce new methods and reagents for this transformation.
The use of supported reagents [20] has attracted interest because of improved
selectivity, reactivity, and associated ease of manipulation. Since polar reactants
adsorbed on the surfaces of various mineral carriers absorb microwave energy, a
variety of reagents supported on such surfaces can be utilized for the enhancement
Ã
Corresponding author

710
M. M. Heravi and D. Ajami
of organic reactions using a simple microwave (MW) oven [21, 22]. Microwave
enhanced chemical reactions, especially on inorganic solid supports and under
solvent-free conditions [20e, 21] have attracted attention recently. They offer
several advantages over conventional homogeneous and heterogeneous reactions
with respect to high reaction rates and yields.
Results and Discussion
Montmorillonite K-10 supported bis(trimethylsilyl)chromate has been used for the
oxidation of a variety of compounds [23, 24]. We report herein a direct oxidative
deprotection of tetrahydropyranyl ethers to carbonyl compounds using montmor-
illonite K-10 supported bis(trimethylsilyl)chromate under solvent free conditions
and accelerated by microwave irradiation. We examined various mineral supports
such as clays, silica gel, or alumina and found that montmorillonite K-10 provides
the best results in terms of formation of pure products. The yields are much higher
than those reported for related reactions [18, 19]. In the absence of support, the
reaction is very slow, the yield is low, and the isolation of products from the
ensuing residues is dif®cult. The procedure involves simple mixing of neat
tetrahydropyranyl ethers with montmorillonite K-10 supported bis(trimethylsilyl)-
chromate and irradiating the mixture in a MW oven for 20±120 sec in the solid
state. The results of this rapid direct oxidative deprotection protocol are
summarized in Table 1.
The oxidative deprotection of benzyltetrahydropyranyl ether is representative
of the general procedure employed. Montmorillonite K-10 supported bis(tri-
methylsilyl)chromate mixed with benzyltetrahydropyranyl ether was irradiated to
afford exclusively benzaldehyde. An overoxidation to carboxylic acid was not
observed. The same reaction could be completed in 1 h at a comparable
temperature of 55ꢀC in an oil bath.
Table 1. Oxidative deprotection of THP ethers with montmorillonite K-10 supported BTSC
Substrate
Time (s)
Product
Yield (%)
PhCH2OTHP
20
20
PhCHO
92
90
85
90
75
85
88
85
80
88
4-MeC6H4CH2OTHP
2-NO2, 5-Me-C6H3CH2OTHP
PhCH(Me)OTHP
4-MeC6H4CHO
2-NO2, 5-MeC6H3CHO
PhCOMe
40
60
PhCH=CHCH2OTHP
PhCH(OTHP)Ph
100
120
100
120
120
100
PhCH=CHCHO
PhCOPh
Cyclohexanol-THP
2-Methylcyclohexanol-THP
Benzoin-THP
Cyclohexanone
2-Methycychohexanone
Benzil
(±)-Menthol-THP
(±) Menthone

Oxidative Deprotection of Tetrahydropyranyl Ethers
711
In summary, the presented methodology offers an attractive, rapid, and ef®cient
method for the direct oxidative cleavage of tetrahydropyranyl ethers to the
corresponding carbonyl compounds.
Experimental
All compounds used are known; their physical data were compared with those of authentic samples
and found to be identical.
Preparation of bis(trimethylsilyl)chromate supported on montmorillonite K-10
To a solution of 6.4 ml (0.03 mol) hexamethyldisiloxane in 20 ml of dry CH2Cl2, 3 g (0.03 mol) CrO3
were added. The reaction mixture was stirred in a 50ꢀC bath for 5 h. Solid CrO3 dissolved, and the
dark red mixture became homogeneous. Montmorillonite K-10 (13 g) pre-dried at 120ꢀC overnight
and activated in the microwave oven for 3 min was added to the warm reaction mixture, and the
resulting mixture was stirred for further 5 h. The solvent and other volatile components were
removed under reduced pressure to afford 19 g of the supported chromium oxidant.
Oxidative deprotection of tetrahydropyranyl ethers; typical procedure
Montmorillonite K-10 supported bis(trimethylsilyl)chromate (0.93 g, equivalent to 1.5 mmol) was
thoroughly mixed with 0.192 g benzyltetrahydropranyl ether (1 mmol), and the material was placed
in a beaker inside the microwave oven (900 W) for 20 s. After completion of the reaction as
monitored by TLC (hexane:AcOEt ˆ 8:2) the product was extracted into CH2Cl2, the solvent was
removed, and the residue was chromatographed on a silica gel column using hexane:AcOEt ˆ 8:2 to
afford 92% benzaldehyde.
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Received July 6, 1998. Accepted (revised) October 12, 1998