Phosphorus pentoxide as an efficient catalyst for the tetrahydropyranylation of alcohols under solvent-free conditions

Article abstract of DOI:10.1080/10426500490475157

A facile and efficient method for the preparation of tetrahydropyranyl ethers from alcohols is improved in solvent-free media. These reactions are catalyzed by P₂O₅ and afford various tetrahydropyranyl ethers in shorter reaction time, with good to excellent yields (75-95%) at room temperature. This method is also compatible with substrates containing acid-sensitive functional groups.

Full text of DOI:10.1080/10426500490475157

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PHOSPHORUS PENTOXIDE AS AN EFFICIENT CATALYST
FOR THE TETRAHYDROPYRANYLATION OF ALCOHOLS
UNDER SOLVENT-FREE CONDITIONS
a
a
Hossein Eshghi & Parvaneh Shafieyoon
a
Department of Chemistry, Sistan & Baluchestan University, Zahedan, Iran
Published online: 16 Aug 2010.
To cite this article: Hossein Eshghi & Parvaneh Shafieyoon (2004): PHOSPHORUS PENTOXIDE AS AN EFFICIENT CATALYST FOR
THE TETRAHYDROPYRANYLATION OF ALCOHOLS UNDER SOLVENT-FREE CONDITIONS, Phosphorus, Sulfur, and Silicon and the
Related Elements, 179:10, 2149-2152
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Phosphorus, Sulfur, and Silicon, 179:2149–2152, 2004
Copyright ꢀC Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490475157
PHOSPHORUS PENTOXIDE AS AN EFFICIENT
CATALYST FOR THE TETRAHYDROPYRANYLATION
OF ALCOHOLS UNDER SOLVENT-FREE
CONDITIONS
Hossein Eshghi and Parvaneh Shafieyoon
Department of Chemistry, Sistan & Baluchestan University,
Zahedan, Iran
(
Received February 2, 2004; accepted April 6, 2004)
A facile and efficient method for the preparation of tetrahydropyranyl
ethers from alcohols is improved in solvent-free media. These reactions
are catalyzed by P2O5 and afford various tetrahydropyranyl ethers in
shorter reaction time, with good to excellent yields (75–95%) at room
temperature. This method is also compatible with substrates containing
acid-sensitive functional groups.
Keywords: Alcohols; phosphorus pentoxide; protection; tetrahydropy-
ranyl ethers
INTRODUCTION
Tetrahydropyranyl (THP) ethers have been the most useful and repre-
1
sentative protecting groups of alcohols in synthetic organic chemistry.
Among various procedures available for the protection of hydroxy
groups, tetrahydropyranylation is the most frequently used process be-
cause of the remarkable stability of tetrahydropyranyl ethers under
a variety of conditions such as strong basic media, reactions involv-
ing Grignard reagents, oxidation, reduction, acylation reactions etc.,
and also the ease with which they can be removed. A variety of cata-
lysts have been developed for tetrahydropyranylation, such as protic
2
3
acids, Lewis acid and pyridinium p-toluene sulfonate, ion exchange
4
5
6
7
8
9
resins, clay materials, ZnCl2, SnCl2, 2H2O, AlCl3, 6H2O, In(OTf)3,
Sc(OTf)3, LiBr, CuCl4,
buthylammonium peroxydisulfate, etc. Although many of these meth-
ods and procedures are useful, there is still need for a simple, efficient,
1
0
11
12
13
14
SiO2/H2SO4,
Ph3P/Me3SiI, tetra-n-
15
Address correspondence to Hossein Eshghi, Department of Chemistry, Sistan &
Baluchestan University, Zahedan 98135-674, Iran. E-mail: heshghi@hamoon.usb.ac.ir
2
149

2
150
H. Eshghi and P. Shafieyoon
TABLE I Conversion of Alcohols into THP-Ethers Catalyzed by P
2
O
5
Entry
1
Alcohols
Product
Yield %a,b
95
2
3
86
95
4
5
85
75
6
7
90
85
89
8
9
0
1
2
(CH3)3C-OH
(CH3)2CHCH2-OH
CH3(CH2)6CH2-OH
CH3CH(CH2)6CH3
OH
(CH3)3C-OTHP
(CH3)2CHCH2CH2-OTHP
CH3(CH2)6CH2-OTHP
CH3CH(CH2)5CH3
OTHP
90
85
95
95
1
1
1
a
Isolated yield.
b
All the compounds gave satisfactory spectral data (IR and 1H NMR).
and high-yielding alternate catalyst that works at ambient tempera-
ture, mild reaction conditions, and shorter reaction times. Phosphorus
pentoxide is a catalyst that meets all of these requirements.
In this article we wish to report a very simple and efficient method
for the tetrahydropyranylation of alcohols utilizing a catalytic amount
of P2O5 in solvent-free condition.
RESULTS AND DISCUSSION
The general reaction is illustrated in Scheme 1 and the results
are reported in Table I. All reactions were carried out by simply

Tetrahydropyranyl Ethers
2151
SCHEME 1
grinding the reagents at room temperature under solvent-free medium
for 10 min. Like primary alcohols, secondary alcohols and tertiary al-
cohols were readily transformed into the corresponding THP ethers in
good-to-excellent yields. Additionally, alcohols having other functional
groups underwent chemoselective reaction to give the THP ethers in
excellent yields with no undesired reactions. Especially, acid-sensitive
functional groups such as an allylic alcohols (entries 2 and 8), ben-
zylic alcohol (entry 1), and tetrahydrofuryl alcohol (entry 4) were sig-
nificantly unaffected under these reaction conditions. Ethylene glycol
(
entry 6) undergoes selective monoprotection in high yield in solvent-
free media.
1
The purity of the products was determined by H NMR, IR spectra,
and melting point. In all IR spectra the characteristic peaks of the OH
group of alcohols and the double bond of dihydropyrane disappeared,
1
and in the H NMR spectra the acetalic CH groups were observed at
around 4.3–4.8 as a triplet.
In conclusion, the reported procedure is an easy and novel method for
the preparation of tetrahydropyranyl ethers from alcohols in solvent-
free conditions. These reactions catalyzed by P2O5 afford various
tetrahydropyranyl ethers in shorter reaction time (10 min) with good-
to-excellent yields (75–95%) at room temperature. This method is also
compatible with substrates containing acid-sensitive functional groups.
We believe that this will be a useful addition to modern synthetic
methodologies.
EXPERIMENTAL
All melting points recorded are uncorrected open-capillary measure-
ments. IR spectra were recorded on a Shimadzu-IR 470 spectropho-
1
tometer. H NMR spectra were recorded on a Bruker-80 MHz instru-
ment using tetramethylsilane (TMS) as an internal standard. Column
chromatography was carried out on short columns of silica gel 60 (230–
400 mesh) in glass columns (2–3 cm diameter) using 15–30 g of silica
gel per 1 g of crude mixture.

2
152
H. Eshghi and P. Shafieyoon
Preparation of the Tetrahydropyranyl Ethers:
General Procedure
In a typical reaction, a mixture of alcohol (2 mmol), 3,4-dihydro(2H)-
pyrane (2 mmol), and phosphorus pentoxide (0.1 mmol) were ground
thoroughly in a mortar for 10 min. The completion of the reaction was
monitored by IR and TLC examination. After the completion of the
reaction, water (30 ml) was added to the mixture and extracted with
CH2Cl2 (2 × 20 ml). The extracts were combined and washed with 5%
sodium hydroxide solution (30 ml) and water (30 ml) and dried over
CaCl2. Evaporation of the solvent under vacuum gave tetrahydropy-
1
ranyl ethers with high purity (based on TLC, H NMR, IR, and melting
point).
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