Oxidative deprotection of cyclic acetals and trimethylsilyl ethers by γ-picolinium chlorochromate under nonaqueous conditions

Article abstract of DOI:10.1023/A:1022526521167

Deprotection of cyclic acetals and oxidative desilylation of trimethylsilyl ethers into the corresponding carbonyl compounds with γ-picolinium chlorochromate (γ-PCC) under nonaqueous conditions at room temperature is described. Oxidation of aldehydes to carboxylic acids was not observed in any case.

Full text of DOI:10.1023/A:1022526521167

Russian Journal of Organic Chemistry, Vol. 38, No. 11, 2002, pp. 1671 1673. From Zhurnal Organicheskoi Khimii, Vol. 38, No. 11, 2002,
pp. 1728 17230.
Original English Text Copyright
2002 by Salehi, Khodaei, Goodarzi.
Oxidative Deprotection of Cyclic Acetals and Trimethylsilyl
Ethers by -Picolinium Chlorochromate Under Nonaqueous
Conditions*
P. Salehi1, M. M. Khodaei2, and M. Goodarzi2
1Department of Phytochemistry, Medicinal Plants Research Institute, Shahid Beheshti University,
Evin, Tehran, Iran, E-mail:p-salehi@cc.sbu.ac.ir
2Department of Chemistry, Razi University, Kermanshah, Iran
Received November 2, 2001
Abstract Deprotection of cyclic acetals and oxidative desilylation of trimethylsilyl ethers into the cor-
responding carbonyl compounds with -picolinium chlorochromate ( -PCC) under nonaqueous conditions at
room temperature is described. Oxidation of aldehydes to carboxylic acids was not observed in any case.
The protection of certain functional groups and
their deprotection constitute important processes in
synthetic reactions [1]. Although there are several
methods for deprotection of acetals [2 18] and silyl
ethers [19 27], many of them suffer from low yields,
unavailability and instability of the reagent, severe
reaction conditions, and oxidation of recovered
aldehydes. Therefore new mild and convenient
methods are still in demand.
ketones such as acetophenone, p-methylacetophenone,
and benzyl methyl ketone were also deprotected in
79 89% yield (Table 1, runs nos.3 5). Cyclo-
hexanone as a saturated cyclic ketone as well as
Table 1. Deprotection of acetals to carbonyl compounds
with -picolinium chlorochromate at room temperature
Recently we introduced -picolinium chloro-
chromate ( -PCC) as a new and efficient reagent for
oxidation of alcohols and oximes into carbonyl com-
pounds under nonaqueous conditions [28, 29]. We
now report on a mild and convenient method for
deprotection of cyclic acetals and oxidative deprotec-
tion of trimethylsilyl ethers to the corresponding
carbonyl compounds in good yields with the use of
-PCC.
Different types of cyclic ketals and acetals were
subjected to deprotection by -PCC in dichloro-
methane at room temperature (Scheme 1, Table 1).
Benzaldehyde and p-chlorobenzaldehyde were
recovered from the corresponding 1,3-dioxolanes in
good yields (Table1, runs nos. 1, 2). Different methyl-
Scheme 1.
*
The article was submitted in English.
1070-4280/02/3811-1671$27.00 2002 MAIK Nauka/Interperiodica

1672
SALEHI et al.
to oxidative deprotection and the corresponding
Scheme 2.
carbonyl compounds were isolated in good yields
(Table 2, runs nos. 9 11).
Mild reaction conditions, good yields of products,
simple work-up procedure, and stability and ease of
the reagent preparation are among the advantages of
this procedure which make it an attractive supplement
to the previously reported methods.
butyraldehyde and 2-heptanone as saturated linear
aldehydes and ketones were obtained by deacetaliza-
tion of the corresponding acetals in satisfactory
amounts (Table 1, runs nos. 6 8). No excessive
oxidation of aldehydes was observed in any case.
EXPERIMENTAL
All the products are known compounds and were
identified by comparison of their spectral data (IR,
1H NMR) and by melting points of their 2,4-2,4-di-
nitrophenylhydrazones, in particular mixed with
authentic samples. The reaction progress was
monitored by thin layer chromatography on silica gel
Polygram SIL G/UV 254 plates. The yields are
derived from the weight of the corresponding 2,4-di-
nitrophenylhydrazones.
We have also studied the application of -PCC to
oxidative deprotection of trimethylsilyl ethers
providing aldehydes and ketones (Scheme 2, Table2).
Benzylic silyl ethers with different substituents
were reacted with -PCC at room temperature to
afford the corresponding carbonyl compounds with
no excessive oxidation of the aromatic aldehydes
(Table 2, runs nos. 1 7). Cinnamaldehyde as an allyl
model compound formed safely under the reaction
conditions in 76% yield (Table2, run no. 8). The
other saturated trimethylsilyl ethers were subjected
General procedure for deprotection of acetals to
carbonyl compounds with -PCC. -Picolinium
chlorochromate (25 mmol) was added to acetal
(5 mmol) in dichloromethane (20 ml). The mixture
was stirred at room temperature until acetal was no
longer detectable. The progress of reaction was
monitored by TLC (eluent toluene ethyl acetate,
2: 1). Hydrochloric acid (20%, 25 ml) was added to
the resulting mixture. The product was extracted into
ethyl ether (3 30 ml). On evaporation of the solvent
the corresponding carbonyl compound was obtained
in 71 89% yield and was characterized as 2,4-di-
nitrophenylhydrazone derivative.
Table 2. Oxidative deprotection of trimethylsilyl ethers
with -picolinium chlorochromate at room temperature
providing carbonyl compounds
Reaction
Substrate
product
1
2
3
4
p-ClC6H4CH2OSiMe3
p-MeOC6H4CH2OSiMe3
p-BrC6H4CHOSiMe3
5
5
5
4
p-ClC6H4CHO
88
p-MeOC6H4CHO 78
p-BrC6H4CHO 85
m-PhOC6H4CHO 82
General procedure for oxidative deprotection of
trimethylsilyl ethers with -PCC. A mixture of tri-
methylsilyl ether (5 mmol) and -PCC (25 mmol) was
prepared in dichloromethane. The solution was stirred
at room temperature for an appropriate time (see
Table 2). The completion of reaction was confirmed
by TLC (eluent n-hexane ethyl ether, 5: 1). The
reaction mixture was treated with HCl (20%, 25 ml)
followed by extraction with diethyl ether (3 30 ml).
The organic layer was separated and dried with
Na2SO4 and then concentrated to obtain the carbonyl
compounds in 60 90% yield. The latter were isolated
as the corresponding 2,4-dinitrophenylhydrazones.
m-PhOC6H4CH2OSiMe3
OSiMe3
5
6
C6H5CHC6H5
-C10H7CHMe
3
6
C6H5COC6H5
-C10H7COCH3
90
85
OSiMe3
O OSiMe3
O O
7
C6H5 C C C6H5
4
5
C6H5 C C C6H5 95
H
OSiMe3
CHO
8
9
C6H5CH=CHCH2
CH3(CH2)5CH2OSiMe3 10 CH3(CH2)5CHO
C6H5CH= CH
76
65
10 CH3(CH2)4CHOSiMe3
12 CH3(CH2)4C= O 65
ACKNOWLEDGMENT
CH3
CH3
11 cyclo-C6H11OSiMe3
15 cyclo-C6H10= O
60
We are grateful to Razi University Research
Council for the financial support of this work.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 11 2002

OXIDATIVE DEPROTECTION OF CYCLIC ACETALS
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