Potassium dodecatangestocobaltate trihydrate (K5CoW12O40·3H2O): A mild and efficient catalyst for the tetrahydropyranylation of alcohols and their detetrahydropyranylation

Article abstract of DOI:10.1016/S0040-4039(01)00298-2

A simple, mild and effective method for tetrahydropyranylation of a variety of alcohols and cleavage of their tetrahydropyranyl ethers at ambient temperature in the presence of K₅CoW₁₂O₄₀·3H₂O as the catalyst with high turnovers is described.

Full text of DOI:10.1016/S0040-4039(01)00298-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2851–2853
Potassium dodecatangestocobaltate trihydrate
(K5CoW12O40·3H2O): a mild and efficient catalyst for the
tetrahydropyranylation of alcohols and their
detetrahydropyranylation
Mohammad H. Habibi,* Shahram Tangestaninejad, Iraj Mohammadpoor-Baltork,
Valiollah Mirkhani and Bahram Yadollahi
Chemistry Department, Isfahan University, Isfahan 81745-117, Iran
Received 29 January 2001; revised 4 February 2001; accepted 20 February 2001
Abstract—A simple, mild and effective method for tetrahydropyranylation of a variety of alcohols and cleavage of their
tetrahydropyranyl ethers at ambient temperature in the presence of K5CoW12O40·3H2O as the catalyst with high turnovers is
described. © 2001 Elsevier Science Ltd. All rights reserved.
The importance of selective introduction and removal
of protecting groups in organic synthesis is well estab-
lished. Tetrahydropyranylation is one of the most fre-
quently used methods for the protection of hydroxyl
groups in synthetic organic chemistry, in particular
natural product chemistry, because of its easy installa-
tion, general stability to most nonacidic reagents, and
easy removal under mild acidic conditions.1,2
purpose. However, some of these procedures suffer due
to the use of expensive and toxic reagents, high temper-
atures, strongly acidic conditions or formation of con-
siderable amounts of side products. Consequently, there
is a need to develop alternative methods for the protec-
tion of alcohols as well as deprotection of tetra-
hydropyranyl ethers.
Polyoxometalates have been proven to be good cata-
lysts in various oxidations. They are applied in bulk or
supported forms, and both homogeneous and heteroge-
neous catalysis is possible. Due to their acidic and
redox properties, heteropoly compounds (heteropoly
acids and salts) are useful and versatile catalysts in a
number of transformations. Since they exhibit weak
superacidic properties, they can be used in reactions
requiring electrophilic catalysis.21
A wide variety of catalysts have already been applied to
the tetrahydropyranylation of alcohols and phenols,
and their detetrahydropyranylation including the use of
protic acids3 (acetic acid, p-toluenesulfonic acid, boric
acid), Lewis acids4 (magnesium bromide in ether,
dimethylaluminium chloride), Lewis acids (copper sul-
fate and iron(III) chloride) supported on silica gel,5
electrogenerated acid,6 pyridinium p-toluenesulfonate,7
ion exchange resins8 (amberlyst H-15, Dowex 50W-X8,
Nafion-H), bentonitic earth,9 organotin phosphate con-
densates,10 triphenylphosphine dibromide (PPh3Br2)11
and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ).12
More recently, heteropolyacids,13 lithium chloride,14
expansive graphite,15 reductive cleavage using sodium
cyanoborohydride and BF3·OEt2,16 clay materials17
(montmorilonite K-10 and H-Y zeolite), ZnCl2,18
CuCl19 and NH4Cl20 have also been employed to this
Having these factors in mind, we now wish to report
that K5CoW12O40·3H2O22 is an excellent and effective
catalyst for tetrahydropyranylation and deprotection of
tetrahydropyranyl ethers to their parent alcohols
(Scheme 1).
* Corresponding author. Tel.: +98-311-7932707; fax: +98-311-689732;
e-mail: habibi@sci.ui.ac.ir
Scheme 1.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00298-2

2852
M. H. Habibi et al. / Tetrahedron Letters 42 (2001) 2851–2853
As shown in Table 1, the treatment of a series of alcohols
with 3,4-dihydro-2H-pyran in the presence of only 0.01
mmol of the catalyst in acetone at room temperature,
afforded the corresponding tetrahydropyranyl ether in
high yields (60–100%) with high turnover per hour
(about 1400 for benzyl alcohol). Under the same reac-
tion conditions phenols reacted more slowly and the
corresponding tetrahydropyranyl ethers were obtained
in 5–55% yields (Table 1, entries 22–24).
In a typical procedure 3,4-dihydro-2H-pyran (2 mmol,
168 mg) dissolved in acetone (5 mL) was added to a
stirred solution of benzyl alcohol (1 mmol, 108 mg) and
K5CoW12O40·3H2O (0.01 mmol, 32 mg) in acetone (5
mL) at ambient temperature for 5 min. The solution
was first filtered through a short column of silica gel,
washed with a small amount of CH2Cl2 and concen-
trated to give tetrahydropyranyl ether, the product was
purified by column chromatography and the corre-
Table 1. Tetrahydropyranylation of alcohols catalyzed with K5CoW12O40·3H2O
Entry
Alcohols
THP–ethersa
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
C6H5CH2OH
C6H5CH2OTHP
5
15
10
5
5
5
97
75
80
97
97
97
95
98
95
98
85
80
85
86
88.5
96
76
100
68.5
82
60
5
2-NO2C6H4CH2OH
4-NO2C6H4CH2OH
4-ClC6H4CH2OH
2-CH3OC6H4CH2OH
3-CH3OC6H4CH2OH
4-CH3OC6H4CH2OH
C6H5CH2CH2CH2OH
C6H5CH(OH)CH3
C6H5CH2CH2OH
C6H5CHCHCH2OH
C6H5CH(OH)COC6H5
C6H11OH (Cyclohexanol)
CH3(CH2)6CH2OH
CH3(CH2)5CH2OH
CH3(CH2)3CH2OH
CH3(CH2)2CH2OH
a-Tetralol
2-NO2C6H4CH2OTHP
4-NO2C6H4CH2OTHP
4-ClC6H4CH2OTHP
2-CH3OC6H4CH2OTHP
3-CH3OC6H4CH2OTHP
4-CH3OC6H4CH2OTHP
C6H5CH2CH2CH2OTHP
C6H5CH(OTHP)CH3
C6H5CH2CH2OTHP
C6H5CHCHCH2OTHP
C6H5CH(OTHP)COC6H5
C6H11OTHP
CH3(CH2)6CH2OTHP
CH3(CH2)5CH2OTHP
CH3(CH2)3CH2OTHP
CH3(CH2)2CH2OTHP
a-TetralolTHP
5
5
9
10
5
10
10
10
5
10
5
15
5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2-Ethyl-1-hexanol
(−)-Menthol
Adamantanol
2-NO2C6H4OH
4-NO2C6H4OH
2-Ethyl-1-hexanolTHP
(−)-MentholTHP
AdamantanolTHP
2-NO2C6H4OTHP
4-NO2C6H4OTHP
5
15
15
60
30
15
25
55
CH3COC6H4OH
CH3COC6H4OTHP
a All tetrahydropyranyl ethers were identified by comparison of their physical and spectral data with authentic samples.
b GC yields.
Table 2. Deprotection of THP–ethers of alcohols catalyzed by K5CoW12O40·3H2O
Entry
THP–ethers
Alcohola
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
4-CH3OC6H4CH2OTHP
3-CH3OC6H4CH2OTHP
4-ClC6H4CH(OTHP)CH3
a-TetralolTHP
C6H4CH(OTHP)CH3
C6H5CH(OTHP)C6H5
Furfuryl alcoholTHP
2,5-CH3OC6H4CH2OTHP
C6H5CH2CH2OTHP
C6H5CHCHCH2OTHP
4-C6H5C6H5CH(OTHP)CH3
4-NO2C6H4CH2OTHP
C6H5CH2OTHP
4-CH3OC6H4CH2OH
3-CH3OC6H4CH2OH
4-ClC6H4CH(OH)CH3
a-Tetralol
C6H4CH(OH)CH3
C6H5CH(OH)C6H5
Furfuryl alcohol
2,5-CH3OC6H4CH2OH
C6H5CH2CH2OH
C6H5CH2CH2CH2OH
4-C6H5C6H5CH(OH)CH3
4-NO2C6H4CH2OH
C6H5CH2OH
60
60
45
50
45
100
90
90
150
150
60
40
80
60
100
120
100
100
100
100
100
95c
100
100
100
100
94c
100
100
100
100
100
9
10
11
12
13
14
15
16
C6H11OTHP
C6H5CHCHCH2OTHP
CH3(CH2)5CH2OTHP
C6H11OH (Cyclohexanol)
C6H5CHCHCH2OH
CH3(CH2)5CH2OH
a All alcohols were identified by comparison of their physical and spectral data with authentic samples.
b GC yields.
c Isolated yields.

M. H. Habibi et al. / Tetrahedron Letters 42 (2001) 2851–2853
2853
sponding benzyltetrahydropyranyl ether was obtained
in 97% yield.
5. (a) Caballero, G. M.; Gros, E. G. Synth. Commun. 1995,
25, 395; (b) Fadel, A.; Salaun, J. Tetrahedron 1985, 41,
1267.
Deprotection of THP–ethers was also investigated. As
shown in Table 2, these ethers are converted to their
parent alcohols in excellent yields.
6. Torri, S.; Inokuchi, T.; Kondo, K.; Ito, H. Bull. Chem.
Soc. Jpn. 1985, 58, 1347.
7. Myasita, N.; Yoshikoshi, A.; Grieco, P. A. J. Org. Chem.
1977, 42, 3772.
In a typical deprotection reaction, a solution of
benzyltetrahydropyranyl ether (1 mmol, 192 mg) with
K5CoW12O40·3H2O (0.01 mmol, 32 mg) in methanol (5
mL) was stirred at ambient temperature for 80 min.
Work-up as mentioned earlier for tetrahydro-
pyranylation gave the pure benzyl alcohol in 100%
yield.
8. (a) Bongini, A.; Cardillo, G.; Orena, M.; Sandri, S.
Synthesis 1979, 618; (b) Beier, R.; Mundy, B. P. Synth.
Commun. 1979, 9, 271; (c) Olah, G. A.; Husain, A.;
Singh, B. P. Synthesis 1983, 892.
9. Cruz-Almanza, R.; Perez-Felores, F. J.; Avila, M. Synth.
Commun. 1990, 20, 1125.
10. Otera, J.; Niibo, Y.; Chikada, S.; Nozaki, H. Synthesis
1988, 328.
In conclusion, our protocol provides a simple, fast and
effective method for the protection and deprotection of
tetrahydropyranyl ethers. In addition, recovery and
industrial applicability of the catalyst, easy work-up,
low reaction times and high yields are worthy advan-
tages of this procedure.
11. Wagner, A.; Heitz, M.-P.; Mioskovski, C. J. Chem. Soc.
Chem. Commun. 1989, 1619.
12. Raina, S.; Singh, V. K. Synth. Commun. 1995, 25, 2395.
13. Molnar, A.; Beregszaszi, T. Tetrahedron Lett. 1996, 37,
8597.
14. Maiti, G.; Roy, S. C. J. Org. Chem. 1996, 61, 6038.
15. Zhang, Z.-H.; Li, T.-S.; Jin, T.-S.; Wang, J.-X. J. Chem.
Res. (S) 1998, 152.
16. Srikrishna, A.; Sattigeri, J. A.; Viswajanani, R.; Yelmag-
Acknowledgements
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P.; Dinesh, C. U.; Reddy, R. S.; Pandey, B. Synthesis
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The authors are thankful to Isfahan University
Research Council for financial support of this work.
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22. The synthesis of potassium dodecatangestocobaltate tri-
hydrate (K5CoW12O40·3H2O) starts with the preparation
of sodium tangestodicobalt(II)ate from cobaltous acetate
(2.5 g, 0.01 mol) and sodium tangestate (19.8 g, 0.06 mol)
in acetic acid and water at pH 6.5 to 7.5. The sodium salt
is then converted to potassium salt by treatment with
potassium chloride (13 g). Finally cobalt(II) complex is
oxidized to cobalt(III) complex by potassium persulfate
(10 g) in 40 mL of 2 M H2SO4. The crystals of
K5CoW12O40·20H2O were dried at 200°C, after recrystal-
ization with methanol, potassium dodecatangestocobal-
tate trihydrate (K5CoW12O40·3H2O) was obtained.
4. (a) Kim, S.; Park, J. H. Tetrahedron Lett. 1987, 28, 439;
(b) Ogawa, Y.; Shibasaki, M. Tetrahedron Lett. 1984, 25,
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