146 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 146±147$
Oxidative Deprotection of Tetrahydropyranyl
Ethers to Carbonyl Compounds with
4-(Dimethylamino)pyridinium and 2,2'-Bipyridinium
Chlorochromates under Non-aqueous Conditions$
Iraj Mohammadpoor-Baltork* and Bahram Kharamesh
Department of Chemistry, Esfahan University, Esfahan 81744, Iran
An ef®cient oxidation of tetrahydropyranyl ethers to their carbonyl compounds using 4-(dimethylamino)pyridinium and
2,2'-bipyridinium chlorochromates is described.
The tetrahydropyranyl group is one of the most useful
protective groups for alcohols in multi-step organic syn-
theses.1 Many catalysts have already been proposed for
the tetrahydropyranylation of alcohols and deprotection
of tetrahydropyranyl ethers to the parent alcohols.2±17
But there are only a few reports dealing with the direct
oxidation of tetrahydropyranyl ethers to their carbonyl com-
pounds.18,19 Consequently, there is a need to develop and
introduce new methods and reagents for such functional
group transformations.
We now report a new and ecient oxidative deprotection
of tetrahydropyranyl ethers to their carbonyl compounds
in high yields using 4-(dimethylamino)pyridinium and 2,2'-
bipyridinium chlorochromates.20,21 As shown in Table 1
these reagents are able to convert primary and secondary
tetrahydropyranyl ethers 1 to their corresponding aldehydes
and ketones 2 eciently in re¯uxing acetonitrile. Oxidation
of THP ethers in which a conjugated benzylic double
bond exists, in addition to the desired product is usually
accompanied with cleavage of the double bond to produce
the corresponding carbonyl compounds in 20±25% yield
(entries 9 and 10).
oxidize some corresponding alcohols under the same reac-
tion conditions. We found that the oxidation of alcohols is
very fast compared to THP ethers. Therefore, it was not
feasible to observe such alcohol intermediates in the oxi-
dation. It is worth mentioning that such intermediates were
not observed by TLC or GLC. It is possible that THP
ethers are ®rst cleaved to alcohols and then converted, via
very fast reactions, to the ®nal products.
In summary, the present methodology oers an attractive
and ecient method for the direct oxidation of tetrahydro-
pyranyl ethers to their carbonyl compounds.
Experimental
4-(Dimethylamino)pyridinium and 2,2'-bipyridinium chloro-
chromates were prepared by reported methods.20,21 Yields refer to
isolated products. All oxidation products were identi®ed by com-
parison of their physical data, IR and NMR spectra with those of
authentic samples. THP ethers were prepared according to described
procedures.3,12
General Procedure for the Oxidative Deprotection of THP Ethers
a round-
with 4-(Dimethylamino)pyridinium Chlorochromate.ÐIn
bottomed ¯ask (50 ml) equipped with a condenser and a magnetic
stirrer, a solution of THP ether (1 mmol) in MeCN (15 ml) was
prepared, 4-(Dimethylamino)pyridinium chlorochromate (0.775 g,
3 mmol) was added to this solution and re¯uxed for 25±50 min. The
progress of the reaction was monitored by GLC or TLC (eluent:
n-hexane±ethyl acetate, 20:1). After completion of the reaction,
silica gel (2 g) was added and the mixture was stirred at room
temperature for 5 min. The reaction mixture was ®ltered and the
solid material was washed with MeCN (15 ml). The ®ltrates were
combined and evaporated. The resulting crude material was puri®ed
on a silica-gel plate or silica-gel column with appropriate eluent.
In order to investigate whether THP ethers are directly
oxidized or initially cleaved to the corresponding alcohols
and then oxidized to carbonyl compounds, we attempted to
Table 1 Oxidative deprotection of THP ethers with DMAPÁ HCrO3Cl and Bipy Á HCrO3Cl
Yield (%) (t/min)
DMAP Á HCrO3Cl Bipy Á HCrO3Cl
Mp/8C [bp/8C/Torr]
Entry
Substrate
Product
Found
Reported
1
2
3
4
5
6
7
8
9
PhCH2OTHP (1a)
PhCHO (2a)
90 (25)
92 (30)
90 (30)
72 (50)
90 (40)
93 (30)
94 (30)
95 (30)
68 (30)
23
93 (20)
94 (25)
90 (15)
85 (35)
93 (35)
91 (25)
92 (30)
95 (30)
70 (20)
20
176±178 178±179
141±143/50 143/50
3-MeOC6H4CH2OTHP (1b)
4-MeOC6H4CH2OTHP (1c)
3-O2NC6H4CH2OTHP (1d)
4-O2NC6H4CH2OTHP (1e)
PhCH2CH2CH2OTHP (1f)
PhCH(Me)OTHP (1g)
3-MeOC6H4CHO (2b)
4-MeOC6H4CHO (2c)
3-O2NC6H4CHO (2d)
4-O2NC6H4CHO (2e)
PhCH2CH2CHO (2f)
PhCOMe (2g)
4-ClC6H4COMe (2h)
PhCH1CHCHO (2i)
PhCHO (2a)
247±248
57±59
248
57±59
106±108
96±98/12
200±202
231±232
246±248
105±108
97±98/12
202
232
248
4-CIC6H4CH(Me)OTHP (1h)
PhCH1CHCH2OTHP (1i)
10
3-O2NC6H4CH1CHCH(Ph)OTHP (1j)
3-O2NC6H4CH1CHCOPh (2j)
3-O2NC6H4CHO (2d)
4-PhC6H4COMe (2k)
a-Tetralone (2 l)
65 (30)
25
68 (30)
25
145±146
106±108
117±118
114±116
153±155
208±210
124±126
144±146
105±108
116±118
113±116/6
155
11
12
13
14
15
4-PhC6H4CH(Me)OTHP (1k)
a-Tetralol tetrahydropyranyl ether (1 l)
Cyclohexanol tetrahydropyranyl ether (1m) Cyclohexanone (2m)
( )-Menthol tetrahydropyrany l ether (1n)
Cholesterol tetrahydropyranyl ether (1o)
96 (35)
90 (40)
80 (30)
85 (40)
80 (40)
98 (30)
96 (25)
85 (30)
87 (30)
83 (30)
( )-Menthone (2n)
Cholest-5-en-3-one (2o)
207±210
125±127
*To receive any correspondence.
Evaporation of the solvent aorded pure carbonyl compound; yield
65±96% (Table 1).
General Procedure for the Oxidative Deprotection of THP Ethers
with 2,2'-Bipyridinium Chlorochromate.ÐTo a solution of THP ether
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).