JOURNAL OF CHEMICAL RESEARCH 2011
RESEARCH PAPER 477
AUGUST, 477–479
Acid-washed bentonite: a new reagent for the deprotection of
tetrahydropyranyl ethers
Po.S. Poon, Ajoy K. Banerjee*, Liadis Bedoya, Jennifer Sanchez and Manuel S. Laya
Centro de Química, Instituto Venezolano de Investigaciones Científicas, (IVIC), Apartado-21827, Caracas-1020A, Venezuela
Acid-washed (pH 4 and pH 6) bentonite earth cleaves the tetrahydropyranyl ethers to the corresponding alcohols in
good yield under mild conditions.
Keywords: bentonite, dihydropyran, tetralone, dehydropyranylation
Tetrahydropyranylation is one of the most frequently used
methods to protect aliphatic and aromatic hydroxyl groups.
Tetrahydropyranyl ethers (THP) are usually transformed into
their parent alcohols under strongly acidic conditions.1,2 For
that reason several reagents have been utilised for the deprot-
ection of THP ethers.3,4 However some of these reagents suffer
from the disadvantages of high cost, high temperature require-
ment and being strongly acidic are undesirable for highly
acid-sensitive alcohols. Therefore efforts have been made to
accomplish dehydropyranylation under mild conditions which
are highly desirable for the protection of other functional
groups. Tonsil, a commercially available Mexican bentonite
earth which seems to have similar behaviour as a Lewis acid,
has been used for deprotection4 of tetrahydropyranyl ethers. In
the course of our studies on the synthesis of natural products
related to sesquiterpenes, we attempted to deprotect the tetra-
hydropyranyl ethers with the commercially available (Aldrich)
bentonite earth. We observed that the bentonite earth was
completely ineffective for dehydropyranylation but that
acid-washed bentonite earth was a suitable reagent for
dehydropyranylation.
dehydropyranylaton. To the best of our knowledge some of
the tetrahydropyranyl derivatives have never been subjected
to deprotection with other catalysts. We strongly believe that
the present method for dehydropyranyation constitutes an
important alternative for deprotection.
Experimental
The alcohol5 2a was prepared by reduction of 7-methoxy-1-tetralone
with sodium borohydride following the standard procedure. The tetra-
hydropyranyl derivatives 1b–k were prepared by the tetrahydropy-
ranylation of the commercially available alcohols 2b–k. The identity
of alcohols 2a–k, obtained by the dehydropyranylation of the tetrahy-
dropyranyl derivatives 1a–k, were confirmed by comparing the m.p.,
TLC and the spectral data of the commercially available alcohols.
Tetrahydropyranylation
A mixture of alcohol 2 (a–k) (4.62 mmol), 3,4-dihydropyran (17–
48 mL) and a catalytic amount of p-toluenesulfonic acid was stirred
for 1 h at room temperature. The progress of the reaction was followed
by TLC. After completion, potassium carbonate (1.2 mmol) was
added, stirred for 3 min, diluted with water and extracted with ether.
The organic extract was washed with water, dried and concentrated.
The residue was purified over a column of silica gel (eluant hexane-
ether 9:1) to obtain the tetrahydropyranyl ethers (1a–k) (Table 1). The
identity of all the tetrahydropyranyl derivatives was confirmed by
spectral data.5–8
To the best of our knowledge dehydropyranylation with
acid-washed bentonite has not been reported previously. We
now report for the first time its utility for deprotection.
Preparation of activated bentonite9,10
Results and discussion
Bentonite is a commercially available (Sigma-Aldrich) colloidal clay
which consists primarily of montmorillonite. The molecular formula
is H2Al2O6Si and molecular weight 180.1 g mol−1. The size of the
bentonite was 0.3 nm.
To the bentonite (5 g) in distilled water (20 mL), heated at 80 °C,
was added dropwise sulfuric acid (0.1 mL) and maintained at this
temperature for 12 h. The reaction mixture was cooled and then fil-
tered. Bentonite was washed several times with distilled water until it
reached pH=4. By similar treatment bentonite pH=6 was prepared.
Acid-washed bentonite was heated at reduced pressure in a furnace at
100 °C for 8 h, then cooled and crushed.
When a solution of tetrahydropyranyl ether 1 in acetone was
heated with acid-washed bentonite (see experimental) at 40–
50 °C, the corresponding alcohol 2 was isolated after filtration
and chromatography (Scheme 1).
The experimental procedure is simple and a wide range of
tetrahydropyranyl ethers (1a–k) afforded the corresponding
alcohols (2a–k). The yields ranged from excellent to poor
as can be observed in Table 1. The compound (1h) proved
extremely resistant to dehydropyranylation. Dehydropyranyl-
ation of the compound (1k) did not afford a good yield. It can
be observed that the functional groups such as methoxy, benzyl
ethers, and tertiary hydroxyl group remained unaffected with
this reagent. Acetone was found to be most suitable solvent for
the deprotection. We also observed (Table 1) a difference in
yield of the alcohol during the deprotection with acid-washed
bentonite (pH=4) and (pH=6). The progress of the reaction
was monitored by TLC. In conclusion, we have shown
that the acid-washed bentonite is a suitable catalyst for
Dehydropyranylation
Tetrahydropyranyl ether (0.37 mmol) in acetone (2 mL) and bentonite
(pH=4 or pH=6) were heated gently (40–50 °C) for 5–90 min. The
progress of the reaction was monitored by TLC. The reaction mixture
was cooled and filtered. The filtrate was chromatographed (eluant
hexane: ether 7: 3) over silica gel to obtain the respective alcohol
whose identity was confirmed by comparing its spectroscopic data
with alcohol commercially available.
Scheme 1 Deprotection of tetrahydropyranyl ethers with acid-washed bentonite.
* Correspondent. E-mail: abanerje@ivic.gob.ve