718 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 718±719$
Oxidative Deprotection of Tetrahydropyranyl Ethers
to Carbonyl Compounds with Montmorillonite K-10
Supported Bis(trimethylsilyl) Chromate under
Non-aqueous Conditions$
Majid M. Heravi* and Dariush Ajami
Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran
A variety of tetrahydropyranyl ethers are oxidized to the corresponding carbonyl compounds in excellent yields by
montmorillonite K-10 supported bis(trimethylsilyl) chromate in dichloromethane at room temperature.
Tetrahydropyranylation is one of the most frequently used
processes to protect hydroxy groups.1 Because of the
remarkable stability of tetrahydropyranyl ethers toward a
variety of conditions such as strongly basic media, reactions
involving Grignard reagents and lithium alkyls, reduction
with hydride, oxidation, oxidative alkylation and acylation
reactions, etc., tetrahydropyranylation is one of the
methods of choice to protect a hydroxy group in a multi-
step organic synthesis.2 Many catalysts have already been
proposed for the tetrahydropyranylation of alcohols and
the cleavage of tetrahydropyranyl ethers to the parent
alcohols.3,4 Direct oxidation of tetrahydropyranyl ethers
to their carbonyl compounds is a useful practical achieve-
ment. This oxidation is rather rare in the literature.5,6
Consequently introduction of new methods and inexpensive
reagents for such a functional group transformation is still
in demand.
Recently we have introduced montmorillonite K-10
supported bis(trimethylsilyl) chromate for oxidation7,8 and
oxidative deprotection of trimethylsilyl ethers.9 We now
report a new and convenient method for the oxidative
deprotection of tetrahydropyranyl ethers to their carbonyl
compound in high to excellent yields using montmorillonite
K-10 supported bis(trimethylsilyl) chromate. At room tem-
perature this reagent is able to convert primary and second-
ary tetrahydropyranyl ethers to their corresponding
aldehydes and ketones (Scheme 1, Table 1).
Overoxidation of oxidative deprotection of primary tetra-
hydropyranyl ethers was not observed. Tetrahydropyranyl
cinnamyl ether was oxidatively deprotected to cinnamalde-
hyde in excellent yield. In this reaction the formation of
benzaldehyde was not observed, showing that the carbon±
carbon double bond is not prone to cleavage with this
reagent.
In order to investigate whether THP ethers are directly
oxidized or initially cleaved to the corresponding alcohols
and oxidized to carbonyl compounds we attempted to
oxidize some corresponding alcohols under the same
reaction conditions, when we found that the oxidation of
alcohols is very fast compared to THP ethers. However we
could observe such alcohol intermediates by TLC or GLC.
Thus we could conclude that THP ethers are ®rst cleaved to
alcohols and then converted via fast reactions to the ®nal
products. It is also noteworthy that with nsupported
bis(trimethylsilyl) chromate no reaction takes place at all.
In conclusion the present methodology oers an attractive
and ecient method for the direct oxidation of tetrahydro-
pyranyl ethers to their carbonyl compounds.
Experimental
Supported montmorillonite K-10 was prepared by the reported
method.7,9 Yields refer to isolated product. Products were identi®ed
by comparison of their physical data. THP ethers were prepared
according to described procedures.3,4,10
.
Oxidative Deprotection of Tetrahydropyranyl Ethers. General
Procedure.ÐIn a round-bottomed ¯ask (50 ml) equipped with a
condenser and a magnetic stirrer a solution of THP ether (1 mmol)
in CH2Cl2 (15 ml) was prepared. Supported montmorillonite K-10,
bis(trimethylsilyl) chromate [0.8 g, equivalent to 1.2 mmol of
chromium(VI) oxidant] was added to this solution and re¯uxed for
10±25 min. Reaction progress was monitored by GLC or TLC
(eluent hexane±ethyl acetate 8:2). After completion of the reaction,
the mixture was ®ltered and solid material was washed with
Scheme 1
Table 1 Oxidative deprotection of THP ethers with montmorillonite K-10 supported bis(trimethylsilyl) chromate in CH2Cl2 at reflux
temperature
Mp/8C [Bp/8C (Torr)]
Entry
Substrate
t/min
Product
Yield(%)
Found
Reported11
1
2
3
4
5
6
7
8
9
PhCH2OTHP
10
10
25
25
25
25
25
20
20
PhCHO
92
90
88
82
85
80
82
85
82
175±176/760
58±59
65±66
200±201/760
47±48
247±248/760
154±155/760
164±165/760
208±210/760
178±179/760
59±61
66±67
202/760
49±51
248/760
155/760
165/760
208±210/760
4-MeC6H4CH2OTHP
2-NO2-5-MeC6H3CH2OTHP
PhCH(Me)OTHP
Ph2CHOTHP
.
PhCH CHCH2OTHP
CyclohexylTHP
4-MeC6H4CHO
2-NO2-5-MeC6H3CHO
PhCOMe
PhCOPh
.
PhCH CHCHO
Cyclohexanone
2-Me-CyclohexylTHP
( )-MenthylTHP
2-Me-Cyclohexanone
( )-Methone
*To receive any correspondence (e-mail: CCERCI@neda.net.ir).
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
CH2Cl2. The ®ltrates were combined and evaporated. The resulting
crude material was further puri®ed on a silica gel plate or silica
gel column with appropriate eluent to aord the corresponding
carbonyl compound (Table 1).
Heravi, Majid M.
