LETTER
1063
One Pot Deprotective Oxidation of O-Allyl Ethers Using 70% tert-Butyl
Hydroperoxide and Catalytic CrO3#
S. Chandrasekhar*, Pradyumna K. Mohanty, T. Ramachander
Indian Institute of Chemical Technology, Hyderabad - 500 007, India
Fax +91 (91) 407173387; E-mail: srivaric@iict.ap.nic.in
Received 20 March 1999
Scheme
Abstract: Deprotection of O-allyl ether and in situ oxidation of the
liberated free hydroxy group to carbonyl functionality is achieved In the first instance, readily available a-phenyl ethanol
in one pot using 70% tert-Butyl Hydroperoxide and catalytic CrO3. was protected as its allyl ether under standard reaction
Other sensitive groups viz., -OTHP, -OTBDMS and –OMOM
ethers survive under the reaction conditions.
9
conditions and subjected to the CrO -TBHP oxidation in
3
CH Cl (entry 1, Table). As anticipated, clean formation
2
2
Key words: protecting groups, allylation, oxidation, catalyst, one
pot reaction
of acetophenone was observed after 12 h in 77% isolated
yield. Similarly a-phenyl hexan-1-ol-o-allyl ether fur-
nished the corresponding keto compound (entry 2) in 74%
isolated yield. To check the capability of the reaction pro-
Chromium based oxidations have been widely explored tocol, a few substrates containing o-allyl group on one end
since becoming available to organic chemists, which is and other protective groups viz., OTHP ether (entry 5),
1
exemplified by plethora of literature. This is primarily OTBDMS (entry 6), and OMOM (entry 7) ethers on the
due to chemoselectivities achieved using these reagents in other end were exposed to CrO -TBHP and good yields of
3
oxidation. The various oxidations carried out using chro- o-allyl deprotection-oxidation products were obtained
2
mium reagents includes, benzylic and allylic oxidations,
without affecting the other protective groups. Similarly,
the menthol allyl ether (entry 8) was deprotectively oxi-
3
4
conversion of alcohols to carbonyl compounds and acids
5
besides others. However, the environmental hazards dised uneventfully. To check the efficacy of the CrO3-
caused by chromium metal prompted the use of this class TBHP on sugar substrates, where the allyl group is exten-
of reagents to be restricted totally or at least minimized. sively used as a protective group, the furanose substrate
Towards this end, various catalytic variations have been (entry 10) was reacted with CrO -TBHP and ulose deriv-
3
developed wherein CrO or CrO -amines are used in cat- ative (10b) was isolated in good yield without any damage
3
3
alytic amounts and peroxide/peracids are used as stoichi- at the 1,2,5,6-di-O-isopropylidene site. However, when
0
ometric oxidants. This combination of Cr-Peroxide is 1 -o-allyl ether (entry 11) was exposed, a mixture of alde-
6
substantially studied in these transformations. As part of hyde, alcohol and starting material were recovered in
an ongoing programme, new reagents and procedures for 30:40:30 ratio, this being a limitation to the present proto-
o
7
selective deprotective oxidation of 1 -O-silyl ethers and col.
8
deprotection of allyl ethers have been addressed by us. It
In summary, a new and efficient reagent system compris-
was further envisaged that allyl ethers would be cleaved
oxidatively and that proper choice of reagents would al-
ing of 70% tbhp and catalytic amount of CrO (10 mol%)
3
8
has been developed for the first time for one pot deprotec-
low further oxidation of liberated free alcohol to carbonyl
compounds, thus allowing us to achieve two transforma-
tions in one pot namely, deprotection and oxidation. Here-
in, we report our latest findings for the conversion of o-
allyl ethers to carbonyl compounds using catalytic
10
tion of o-allyl ethers and in situ oxidation.
Typical Procedure for the Chromium-Catalysed
Allyl Ether Oxidation by 70% TBHP:
amounts of CrO and excess of 70% TBHP. It is interest-
3
To a stirred solution of CrO (10 mol%, 24 mg) in meth-
3
ing to note that CrO , even though known for over several
3
ylene chloride (15 mL) was added successively 70% of
TBHP (888 mg, 9.87 mmol, 0.95 mL) and 1-(1-allyloxy-
ethyl)benzene 1a (400 mg, 2.46 mmol) in 5 mL of meth-
ylene chloride. The resulting mixture was stirred at
ambient temperature for 12 h. The mixture was filtered
through a plug of alumina, then washed successively with
years as a oxidant, is never used in deprotection of organic
functional groups.
7
0% t-BuOOH
O
O
Cat. CrO3
aqueous NaHCO and brine solution. Evaporation of the
3
R
R'
CH2Cl2, RT, 12 h
volatiles followed by chromatography using silica gel
R
R'
(
2% EtOAc in hexane as eluent) furnished 227 mg of pure
R=R'=alkyl, aryl
acetophenone (1b, 77%).
Synlett 1999, No. 07, 1063–1064 ISSN 0936-5214 © Thieme Stuttgart · New York
Chandrasekhar
