Wet Alumina Supported Chromium(VI) Oxide: A Mild, Efficient, and Inexpensive Reagent for Oxidative Deprotection of Trimethylsilyl and Tetrahydropyranyl Ethers

Article abstract of DOI:10.1007/PL00010215

Primary and secondary trimethylsilyl and tetrahydropyranyl ethers are efficiently converted to the corresponding carbonyl compounds using wet alumina supported chromium(VI) oxide.

Full text of DOI:10.1007/PL00010215

Monatshefte fuÈr Chemie 130, 337±339 (1999)
Wet Alumina Supported Chromium(VI) Oxide:
A Mild, Ef®cient, and Inexpensive Reagent for
Oxidative Deprotection of Trimethylsilyl and
Tetrahydropyranyl Ethers
Ã
Majid M. Heravi , Dariush Ajami, and Kourosh Tabar-Heydar
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Summary. Primary and secondary trimethylsilyl and tetrahydropyranyl ethers are ef®ciently converted
to the corresponding carbonyl compounds using wet alumina supported chromium(VI) oxide.
Keywords. Silyl ethers; Tetrahydropyranyl ethers; Deprotection; Oxidation; Carbonyl compounds.
Chrom(VI)oxid auf feuchtem Aluminiumoxid: Ein mildes, ef®zientes und billiges Reagens
zur oxidativen EntschuÈtzung von Trimethylsilyl- und Tetrahydropyranylethern
È
È
Zusammenfassung. Primare und sekundare Trimethylsilyl- und Tetrahydropyranylether werden
durch die Einwirkung von an feuchtem Aluminiumoxid adsorbiertem Chrom(VI)oxid in guten
Ausbeuten in die entsprechenden Carbonylverbindungen uÈbergefuÈhrt.
Introduction
The trimethylsilyl group is one of the most widely employed protecting groups in
organic synthesis and is often used to prepare silyl ethers as volatile derivatives of
alcohols and phenols [1]. The protection of hydroxy groups with 3,4-dihydro-2H-
pyran (DHP) also is a method which has found a broad ®eld of application in
organic synthesis [1b, 1c, 2].
Direct oxidation of trimethylsilyl ethers and tetrahydropyranyl ethers to the
corresponding carbonyl compounds has found considerable attention during recent
years [3a±g]. Although a large number of reagents are known for such trans-
formations [3a±g], there still appears a need either to improve the existing oxidation
methods or to introduce new reagents to permit better selectivity under milder
conditions and with easy work-up procedures. Wet alumina supported chromium(VI)
oxide ef®ciently ful®lls these requirements.
Ã
Corresponding author

338
M. M. Heravi et al.
Results and Discussion
In continuation of our investigations on organic reactions utilizing reagents
adsorbed on inorganic supporting materials [4±8], we mixed chromium(VI) oxide
with alumina and reacted this chromium supported reagent in CH2Cl2 with
trimethylsilyl and tetrahydropyranyl ethers. In the absence of wet alumina, the
reactions are slow, and considerable amounts of starting material and alcohols were
recovered unchanged in reactions at room temperature or even under re¯ux for an
extended period of time. As an example, the reaction of trimethylsilyl benzyl ether
with dry alumina CrO3 resulted in the formation of 30% benzylalcohol and 25%
benzaldehyde.
However, when alumina was premoistened and then mixed with chromium
oxide, trimethylsilyl benzyl ether was converted to benzaldehyde almost
quantitatively in CH2Cl2 at room temperature.
To assess the generality of this oxidative deprotection method, a variety of
trimethylsilyl and tetrahydropyranyl ethers were reacted under these conditions,
yielding the corresponding carbonyl compounds in good to excellent yields
(Table 1). Primary trimethyl silyl and tetrahydropyranyl ethers (allylic and non-
Table 1. Oxidative deprotection of trimethylsilyl and tetrahydropyranyl ethers with wet alumina
supported chromium(VI) oxide
Substrate
Reaction time Product
(min)
Yield
(%)
1a
1b
1c
1d
1e
1f
C6H5CH2OSiMe3
10
10
10
15
10
20
20
25
10
10
15
15
15
25
25
25
C6H5CHO
93b
91b
90b
89a
92a
85b
83b
83a
92b
90b
91b
84a
88a
83b
84b
83a
4-Me-C6H4CH2OSiMe3
2-NO2-5-Me-C6H3CH2OSiMe3
R,S-C6H5CH(Me)OSiMe3
(C6H5)2CHOSiMe3
4-MeC6H4-CHO
2-NO2-5-Me-C6H3CHO
C6H5COMe
(C6H5)2CO
c-C6H11OSiMe3
Cyclohexanone
2-Methylcyclohexanone
(ꢀ)-Menthone
1g
1h
2a
2b
2c
2d
2e
2f
2-Me-c-C6H10OSiMe3
(ꢀ)-Mentholtrimethylsilyl ether
C6H5CH2OTHP
C6H5CHO
4-Me-C6H4CH2OTHP
2-NO2-5-Me-C6H3CH2OTHP
R,S-C6H5CH(Me)OTHP
(C6H5)2CHOTHP
4-Me-C6H4CHO
2-NO2-5-Me-C6H3CHO
C6H5COMe
(C6H5)2CO
c-C6H11OTHP
Cyclohexanone
2-Methylcyclohexanone
(ꢀ)-Menthone
2g
2h
2-Me-c-C6H10OTHP
(ꢀ)-Menthol-THP
a
b
Yields based on GLC analysis; yields based on the isolation of the corresponding 2,4-dinitro-
phenylhydrazine derivative

Oxidation of Ethers by Wet Alumina Supported Chromium(VI) Oxide
339
allylic as well as benzylic) did not undergo overoxidation to the corresponding
carboxylic acids.
In conclusion, wet alumina supported chromium(VI) oxide represents a
convenient reagent for the one-pot oxidative deprotection of trimethylsilyl and
tetrahydropyranyl ethers to their corresponding carbonyl compounds.
Experimental
Trimethylsilyl and tetrahydropyranyl ethers were prepared according to known procedures [9, 2a].
All oxidation products were identi®ed by comparison of their physical data with those given in the
literature. Yields refer either to GC analysis or to isolation of the 2,4-dinitrophenylhydrazine
derivative. GC analysis was performed with a ®ssion 8000 gas chromatograph with FID using a 10%
carbowax 20 M on chromsorb W/A W column (1.8 mÂ6 mm). Wet alumina was prepared by shaking
neutral alumina oxide (10 g, Aldrich-Brockmann, 150 mesh) with distilled water (2 ml). The reagent
was prepared by mixing CrO3 (0.8 g, 8 mmol) with wet alumina (2.4 g) using pistle and mortar.
Oxidative deprotection of trimethylsilyl and tetrahydropyranyl ethers; general procedure
In a ¯ask (50 ml), a solution of 1 mmol of ether in 20 ml CH2Cl2 was prepared. Wet alumina supported
chromium(VI) oxide (0.06 g) was added, and the mixture was stirred magnetically for 10±25 min.
The progress of reaction was monitored by GC or TLC (eluent: hexane/EtOAc, 8:2). The mixture
was ®ltered, and the solid material was washed with CH2Cl2. The ®ltrate was evaporated on a rotary
evaporator, and the resulting crude material was puri®ed on a silica gel pad with the appropriate
eluent. Pure carbonyl compounds were obtained in yields of 83±93% (Table 1).
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Received May 18, 1998. Accepted (revised) August 10, 1998