Clay supported cerium(IV) reagents for oxidative deprotection of tetrahydropyranyl ethers under non-aqueous conditions

Article abstract of DOI:10.1007/PL00010186

Primary and secondary tetrahydropyranyl ethers arc efficiently converted to the corresponding carbonyl compounds using clay supported [Ce(NO₃)₃]₂CrO₄ and [Ce(NO₃)₃]₃HIO₆ in dichloromethane.

Full text of DOI:10.1007/PL00010186

Monatshefte fuÈr Chemie 130, 1253±1256 (1999)
Clay Supported Cerium(IV) Reagents for
Oxidative Deprotection of Tetrahydropyranyl
Ethers under Non-Aqueous Conditions
Majid M. Heravi1;Ã, Hossein A. Oskooie2, Mitra Ghassemzadeh1,
and Fatemeh F. Zameni2
1
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
2
Department of Chemistry, Alzahra University, Tehran, Iran
Summary. Primary and secondary tetrahydropyranyl ethers are ef®ciently converted to the
corresponding carbonyl compounds using clay supported [Ce(NO3)3]2CrO4 and [Ce(NO3)3]3HIO6 in
dichloromethane.
Keywords. Oxidative deprotection; Non-aqueous conditions; BTNCC; TTNCPP.
An Ton adsorbierte Cer(IV)-Reagentien zur oxidativen EntschuÈtzung von
È
Tetrahydropyranylethern unter nichtwaûrigen Bedingungen
È
È
Zusammenfassung. Primare und sekundare Tetrahydropyranylether werden in Dichlormethan durch
an Ton adsorbiertes [Ce(NO3)3]2CrO4 oder [Ce(NO3)3]3HIO6 rasch und in guten Ausbeuten zu den
entsprechenden Carbonylverbindungen umgesetzt.
Introduction
Tetrahydropyranyl ethers are useful as protecting groups and important building
blocks for the synthesis of primary alcohols [1], allylic alcohols [2], and alkyl
halides [3]. More recently, the tetrahydropyranylation process (THP ether
synthesis) has been performed under environmentally friendly conditions with
concomitant minimum puri®cation requirement. The methods employed include
the use of heterogeneous catalysts such as montmorillonite K-10 [3], sulfonated
charcoal [4], H-Y zeolite [5], zinc chloride in alumina [6], envirocat-EPZG [7],
natural kaolinite clay [8], and zeolites HSZ [9]. However, the latter has been
claimed to effect protection as well as deprotection of hydroxy compounds. We
have recently described tetrahydropyranylation of various alcohols using bis-
trinitratocerium(IV) chromate and tris-trinitratocerium(IV) periodate in benzene
[10]. Direct oxidation of tetrahydropyranyl ethers to the corresponding carbonyl
Ã
Corresponding author

1254
M. M. Heravi et al.
Scheme
compounds is also a useful practical achievement; only few reports on this topic are
available [11±13].
Different salts of Ce(IV) are well known as versatile reagents in organic
synthesis Ceric ammonium nitrate has been used as an ef®cient catalyst for
protection and deprotection of alcohols with dihydropyran [14]. Bis-trinitrato-
cerium(IV) chromate (BTNCC) [15] and tris-trinitratocerium(IV) paraperiodate
(TTNCPP) [16] have been prepared as versatile oxidants [15±18]. Although
TTNCPP has been used for deprotection and direct oxidative deprotection of
trimethylsilyl ethers to their corresponding alcohols and carbonyl compounds [18],
there is no report on an application of these two catalysts for direct oxidative
deprotection of tetrahydropyranyl ethers. In this communication we wish to
disclose that montmorillonite K-10 supported BTNCC and TTNCPP in dichloro-
methane serve as two mild, ef®cient, and inexpensive catalysts for direct oxidative
deprotection of tetrahydropyranyl ethers to the corresponding carbonyl com-
pounds.
Results and Discussion
A variety of tetrahydropyranyl ethers were treated with catalytic amounts of clay
supported BTNCC or TTNCPP. The corresponding aldehydes or ketones were
obtained in high to excellent yields; the results are complied in Table 1.
Surprisingly, without montmorillonite K-10 the reaction is very slow, and only
negligible amounts of carbonyl compound are obtained. The reaction conditions
are particularly mild, and the work-up procedure is exceedingly simple and
reduced to a mere ®ltration.
The possible recycling of the catalysts was investigated in the reaction with 1b.
The catalysts were ®ltered, washed with dichloromethane, dried, and immediately
reused. After allowing the reaction mixture to re¯ux for 1 h in CH2Cl2, 2b was
obtained in 85% with BTNCC and 90% with TTNCPP. Recycling the catalysts ®ve
times afforded 2b in similar high yield.
In a typical reaction, a mixture of tetrahydropyranyl ether and catalytic
amounts of either clay supported BTNCC or TTNCPP was re¯uxed in dichloro-
methane. The progress of the reaction was monitored by GC. After the reaction
was complete, the catalyst was ®ltered off, the solvent was evaporated, and the
product was puri®ed by elution with chloroform over a silica gel pad and
evaporation of the solvent. A fairly wide range of tetrahydropyranyl ethers were
oxidatively deprotected to the corresponding carbonyl compounds in high yields.
The reactions proceed reasonably fast, and the conditions are mild enough not to
induce isomerizations or oxidations of double bonds.

Ce(IV) Mediated Deprotection of Tetrahydropyranyl Ethers
1255
Table. 1. Oxidative deprotection of tetrahydropyranyl ethers with BTNCC or TTNCPP
Substrate
Catalyst
Reaction
time (h)
Product
Yielda
(%)
1a
1b
1c
1d
1e
1f
C6H11OTHP
BTNCC
BTNCC
BTNCC
BTNCC
BTNCC
BTNCC
1
Cyclohexanone
PhCHO
90
88
80
60
90
72
PhCH2OTHP
1.5
1.5
1.5
1
CH2=CHCH2OTHP
C7H15CH2OTHP
C3H7CH2OTHP
2-Octyltetrahydro-
pyranylether
CH2=CHCHO
C7H15CHO
C3H7CHO
2
2-Octanone
1g
(-)-Mentholtetrahydro- BTNCC
pyranylether
2
(-)-Menthone
90
1h
1a
1b
1c
1d
1e
1f
4-MeOC6H4CH2OTHP BTNCC
1.5
1.2
1.5
1
4-MeOC6H4CHO
Cyclohexanone
PhCHO
88
92
90
68
85
88
75
C6H11OTHP
TTNCPP
TTNCPP
TTNCPP
TTNCPP
TTNCPP
TTNCPP
PhCH2OTHP
CH2=CHCH2OTHP
C7H15CH2OTHP
C3H7CH2OTHP
2-Octyltetrahydro-
pyranylether
CH2=CHCHO
C7H15CHO
1.2
1.5
1.2
C3H7CHO
2-Octanone
1g
(-)-Mentholtetrahydro- TTNCPP
pyranylether
1.5
1
(-)-2-Menthone
90
65
1h
4-MeOC6H4CH2OTHP TTNCPP
4-MeOC6H4CHO
a
Yield based on the isolation of 2,4-dinitrophenylhydrazine derivatives
In conclusion, clay supported BTNCC and TTNCPP represent two convenient
reagents for the one-pot oxidative deprotection of tetrahydropyranyl ethers to the
corresponding carbonyl compounds. The notable advantages of this methodology
are mild conditions (re¯ux in CH2Cl2), fairly fast reaction times (1±1.5 h), good to
high yields, (60%±90%), and reusability of catalysts. We believe that this method
could serve as a useful addition to modern synthetic methodologies.
Experimental
Products were characterized by IR and NMR spectroscopy and by direct comparison with authentic
samples. BTNCC and TTNCPP were prepared according to reported procedures [15, 16].
Tetrahydropyranyl ethers were prepared according to Ref. [10].
Oxidative deprotection of tetrahydropyranyl ethers; General procedure
To a solution of tetrahydropyranyl ether 1 (1 mmol) in CH2Cl2 (20 ml) in a 50 ml round bottomed
¯ask equipped with a condenser and a magnetic stirrer, a mixture of montmorillonite K-10 (200 mg)
and the appropriate catalyst (0.25 mmol) was added. The reaction mixture was re¯uxed for 1±1.5 h.
The progress of the reaction was monitored by GC or TLC (eluent: hexane/EtOAc ˆ 9:1). After
completion of the reaction, the catalyst was ®ltered off and washed with CH2Cl2. The ®ltrate was
evaporated, and the resulting crude material was puri®ed on a silica gel plate or a silica gel column
with the appropriate eluent. Pure carbonyl compounds 2a±h were obtained in 60±90% yield
(Table 1).

1256
M. M. Heravi et al.: Ce(IV) Mediated Deprotection of Tetrahydropyranyl Ethers
Acknowledgments
M. M. Heravi is grateful to Prof. H. Firouzabadi whose friendship and advice have been a constant
source of encouragement throughout this work.
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Received October 13, 1998. Accepted October 26, 1998