SHORT PAPER
393
Wet Alumina Supported Chromium(VI) Oxide: A Mild, Efficient
and Inexpensive Reagent for Oxidative Deprotection of Trimethylsilyl
andTetrahydropyranyl Ethers in a Solventless System
Majiid M. Heravi,* Dariush Ajami, Mitra Ghassemzadeh
Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335Ð186, Tehran, Iran
Received 17 July 1998; revised 10 October 1998
The reaction is conducted by mixing finely ground wet
aluminaÐchromium(VI) oxide with neat ethers. We dis-
covered that in the absence of wet alumina the reactions
do not proceed and in the presence of dry alumina the re-
actions are slow and a considerable amount of ethers and
alcohols were recovered unchanged in reactions at room
temperature, at elevated temperature or exposure to mi-
crowave irradiation for an extented period of time. By us-
ing wet aluminaÐchromium(VI) oxide in all cases the
reactions are completed upon simple mixing; gentle
warming affords the corresponding carbonyl compounds
in high to excellent yields (Table). The reactions are rela-
tively clean with no tar formation typical for many CrO3
oxidations. Interestingly, no overoxidation to carboxylic
acids is observed. The reaction of cinnamyl ethers (entry
Abstract: Primary and secondary trimethylsilyl and tetrahydro-
pyranyl (THP) ethers are converted to their carbonyl compounds ef-
ficiently using wet alumina supported chromium(VI) oxide under
solvent free condition.
Key words: wet alumina, deprotection oxidation, supported re-
agents, solventless system
The protection of certain functional groups and the depro-
tection of the protected derivatives constitute important
processes in the synthetic chemistry of polyfunctional
molecules including the total synthesis of natural prod-
ucts. Thus large numbers of protective groups have been
developed along with numerous methods for their remov-
1Ð3
al.
Direct oxidation of trimethylsilyl ethers to the corre- 9) gave a moderate yield of cinnamaldehyde (72%) and
sponding carbonyl compounds has attracted considerable benzaldehyde (28%) showing the carbonÐcarbon double
4
attention in recent years but the introduction of new bonds are prone to cleavage by this method.
methods, inexpensive reagents and environmentally
friendly conditions for such functional group tranforma-
tions is still in demand.
Oxidation of trimethylsilyl and tetrahydropyranyl ethers
with other supported oxidants is under investigation.
Tetrahydropyranyl (THP) is a useful protecting group for
alcohols in organic synthesis. Many catalysts have been
proposed for the tetrahydropyranylation of alcohols and
the cleavage of tetrahydropyranyl ethers to the parent al-
cohols.5 Since direct oxidations of tetrahydropyranyl
Ð7
4
,7
ethers are rare in the literature, introduction of new
methods are desirable.
Chromium-based reagents have been extensively used in
8
organic synthesis. The utility of chromium(VI) reagents
Trimethylsilyl and tetrahydropyranyl ethers were prepared accord-
ing to described procedures. All oxidation products are known
in the oxidative transformation is compromised due to
9,12
their inherent toxicity, cumbersome preparation and po- compounds, they are identified by comparison of physical and spec-
tential danger (ignition) in handling its complexes, diffi- troscopical data.
culties in terms of product isolation and waste disposal.
Introduction of reagents on solid supports had provided an
attractive alternative in organic synthesis in view of selec-
tivity and associated ease of manipulation.
Wet Alumina Supported Chromium(VI) Oxide
Neutral aluminium oxide (10 g, Aldrich, Brockmann IÐ150 mesh)
was shaken with distilled water (2 mL). This mixture (2.4 g) was
mixed with CrO (0.8 g, 8 mmol) using a pestle and mortar.
3
In continuation of our investigations on organic reactions
9
Ð12
in solventless systems,
we now report a facile oxida- Oxidative Deprotection of Trimethylsilyl Ethers (1aÐi)
and Tetrahydropyranyl Ethers (2aÐi); General Procedure
tive deprotection of trimethylsilyl and tetrahydropyranyl
ethers to carbonyl compounds using CrO Ðwet alumina
under solvent free conditions. Primary and secondary tri-
methylsilyl ethers 1aÐi and tetrahydropyranyl ethers 2aÐi
are converted to their corresponding carbonyl compound
In a watch glass, neat trimethylsilyl ether 1 (1 mmol), or tetrahydro-
pyranyl ether 2 (1 mmol), was mixed with the above catalyst
3
(1 mmol) with a spatula. An exothermic reaction started with dark-
ening of the orange color of the reagent and completion was con-
firmed by TLC (hexane: EtOAc; 8:2). The product was extracted
into CH Cl (2 × 20 mL) and is passed through a small bed of alu-
3
aÐi by wet alumina supported chromium(VI) oxide.
2
2
mina (1 cm) to afford the corresponding carbonyl compound (Ta-
ble).
Synthesis 1999, No. 3, 393Ð394 ISSN 0039-7881 © Thieme Stuttgart á New York