Tetrahydropyranylation and depyranylation of alcohols catalyzed by aqueous zinc tetrafluoroborate

Article abstract of DOI:10.1081/SCC-200063950

Aqueous solution of zinc tetrafluoroborate as an effective catalyst for tetrahydropyranylation and depyranylation of alcohols has been described. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200063950

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Tetrahydropyranylation and
Depyranylation of Alcohols
Catalyzed by Aqueous Zinc
Tetrafluoroborate
Samimul Islam a , Adinath Majee a & A. T. Khan b
a Department of Chemistry, Visva‐Bharati University,
Santiniketan, West Bengal, India
b Department of Chemistry, Indian Institute of
Technology, North Guwahati, Guwahati, India
Version of record first published: 15 Aug 2006.
To cite this article: Samimul Islam , Adinath Majee & A. T. Khan (2005):
Tetrahydropyranylation and Depyranylation of Alcohols Catalyzed by Aqueous Zinc
Tetrafluoroborate, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 35:13, 1789-1793
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Synthetic Communicationsw, 35: 1789–1793, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200063950
Tetrahydropyranylation and
Depyranylation of Alcohols Catalyzed by
Aqueous Zinc Tetrafluoroborate
Samimul Islam and Adinath Majee
Department of Chemistry, Visva-Bharati University, Santiniketan,
West Bengal, India
A. T. Khan
Department of Chemistry, Indian Institute of Technology,
North Guwahati, Guwahati, India
Abstract: Aqueous solution of zinc tetrafluoroborate as an effective catalyst for
tetrahydropyranylation and depyranylation of alcohols has been described.
Keywords: Aqueous zinc tetrafluoroborate, depyranylation, tetrahydropyranylation
INTRODUCTION
Tetrahydropyranylation is one of the methods of choice for the protection of
hydroxyl groups because of the stability of tetrahydropyranyl ethers in a
variety of conditions. There are several methods for tetrahydropyranylation
and depyranylation of alcohols.[1] Some reagents that can catalyze both trans-
formations are p-toluenesulphonic acid (PTSA),[2] pyridinium p-toluene
sulfonate (PPTS),[3] ammonium chloride,[4] ZrCl4,[5] I2,[6] LiBr,[7] acetonyldi-
phenyl phosphonium bromide,[8] tetrabutyl ammonium tribromide,[9]
potassium dodecatangestocobaltate trihydrate,[10] and indium triflate.[11]
Many of these reagents have been used under dry reaction conditions.
Received in India February 28, 2005
Address correspondence to Adinath Majee, Department of Chemistry, Visva-
Bharati University, Santiniketan, West Bengal 731235, India. E-mail: adinathm@
yahoo.com
1789

1790
S. Islam, A. Majee, and A. T. Khan
Scheme 1.
In this article we describe the use of aqueous Zn(BF4)2 for both tetrahydropyr-
anylation and depyranylation reactions.
RESULTS AND DISCUSSION
Tetrahydropyranylation (Scheme 1) of alcohols was carried out using a 40%
aqueous solution of zinc tetrafluoroborate in dichloromethane at room
temperature.
Table 1. Tetrahydropyranylation and depyranylation of alcohols
Protection
Deprotection
Time
(min)
Yield
(%)a
Time
(min)
Yield
(%)a
Entry
Alcohol
Cetyl alcohol
Decyl alcohol
Ref
13
1
2
5
95
90
65
45
90
90
15
12
3
4
10
87
13
25
92
15
89
13
60
72
5
6
90
90
78
85
13, 14
120
120
87
90
13, 15
7
8
Cholesterol
PhCH2OH
40
25
90
95
13, 15
15
90
40
95
80
9
50
95
13, 6a
90
85
(continued)

Tetrahydropyranylation and Depyranylation of Alcohols
1791
Table 1. Continued
Protection
Deprotection
Time
(min)
Yield
(%)a
Time
(min)
Yield
(%)a
Entry
10
Alcohol
Ref
30
92
13, 6a
90
85
11
12
25
5
84
93
13, 6a
13
120
45
90
92
13
14
65
20
90
85
13
13
40
30
82
80
15
16
55
90
80
92
13
13
60
75
85
120
1
aAll are pure isolated products and were characterized by IR and H NMR.
Typical Procedure for Tetrahydropyranylation
To a well-stirred solution of appropriate alcohol (1 mmol) in dichloromethane
(3 ml), 1.2 equivalent of dihydropyran and 0.33 mmol of zinc tetrafluoroborate
(40% aqueous solution) were added at room temperature. After completion of
the reaction (monitored by TLC), it was quenched by two drops of saturated
sodium bicarbonate solution and diluted with water. The reaction mixture
was extracted with dichloromethane and washed with brine. The organic layer

1792
S. Islam, A. Majee, and A. T. Khan
Scheme 2.
was dried over anhydrous sodium sulphate, evaporated, and the crude product
was purified on basic alumina using 2% ethyl acetate in hexane.
As is evident from Table 1, a wide range of alcohols were converted to
the corresponding tetrahydropyranyl (THP) ethers in high yields using this
methodology. The reactions with sterically hindered molecules such as
menthol (entry 6), cholesterol (entry 7), and carbohydrates (entries 14, 15, 16)
occurred satisfactorily. Acid-sensitive functional groups, such as ketal and
anomeric methoxy remained unaffected under the present reaction conditions.
For depyranylation (Scheme 2), the reaction conditions were modified
and two molar ratio of zinc tetrafluroborate with methanol and dichloro-
methane was used. Compared to the protection, the deprotection reaction
required more time.
Typical Procedure for Depyranylation
To a well-stirred solution of the THP ethers (1 mmol) in dichloromethane and
methanol (2 : 1) (3 ml), 0.66 mmol of zinc tetrafluoroborate (40% aqueous
solution) was added at room temperature. After the usual workup as
described previously, the crude product was purified on silica gel using 2%
ethyl acetate in hexane.
In conclusion the present methodology provides another alternative for
the preparation of tetrahydropyranyl ethers and their subsequent deprotection
in high yields.
ACKNOWLEDGMENT
We are thankful to the Department of Chemistry, Visva-Bharati University,
and the Department of Chemistry, Indian Institute of Technology Guwahati,
for providing general facilities to carry out the research work. We also
express our sincere thanks to B. C. Ranu for his constant encouragement.
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