A mild chemoselective and rapid regeneration of alcohols from O-allyl ethers by LiCl/NaBH4

Article abstract of DOI:10.1081/SCC-120001520

An efficient chemoselective cleavage of O-allyl protected is developed employing LiCl/NaBH₄.

Full text of DOI:10.1081/SCC-120001520

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A MILD CHEMOSELECTIVE AND RAPID REGENERATION
OF ALCOHOLS FROM O-ALLYL ETHERS BY LiCl/NaBH4*
S. Raja Ram a , K. Purushothama Chary b , S. Salahuddin a & D. S. Iyengar
a Discovery Laboratory, Organic Division II, Indian Institute of Chemical Technology ,
Hyderabad, 500 007, India
b Institute fur Organische Chemie, University of Hannover , Scheiderberg 1B, Hannover,
30167, Germany
Published online: 21 Aug 2006.
To cite this article: S. Raja Ram , K. Purushothama Chary , S. Salahuddin & D. S. Iyengar (2002) A MILD CHEMOSELECTIVE
AND RAPID REGENERATION OF ALCOHOLS FROM O-ALLYL ETHERS BY LiCl/NaBH4*, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:1, 133-137, DOI: 10.1081/
SCC-120001520
To link to this article: http://dx.doi.org/10.1081/SCC-120001520
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SYNTHETIC COMMUNICATIONS, 32(1), 133–137 (2002)
A MILD CHEMOSELECTIVE AND RAPID
REGENERATION OF ALCOHOLS FROM
O-ALLYL ETHERS BY LiCl/NaBH4*
S. RajaRam,1 K. Purushothama Chary,1,2
y
S. Salahuddin,1 and D. S. Iyengar1,
1Discovery Laboratory, Organic Division II,
Indian Institute of Chemical Technology,
Hyderabad-500 007, India
2Institute fur Organische Chemie,
University of Hannover, Scheiderberg 1B, 30167,
Hannover, Germany
ABSTRACT
An efficient chemoselective cleavage of O-allyl protected is
developed employing LiCl/NaBH4.
The protection of alcohol functionality by the group is an important
synthetic step in the preparation of various compounds, more specifically in
carbohydrate chemistry.1 Recently various methods have been developed for
the cleavage of allyl group, some of the methods reported include AlCl3-N,N
dimethyl aniline,2 TiCl3-Mg-THF,3 PdCl2/CuCl/DMF-H2O/O2,4 NBS,5
*IICT Communication No. 4410
yCorresponding author. Fax: þ 91-40-7173387, 7173757; E-mail: iyengar@
iict.ap.nic.in
133
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

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134
RAJARAM ET AL.
8b
BCl3-Me2S,6 DDQ,7 NaBH4/I2,8a ZrCl4/NaBH4 and TMSCl/NaI.9 In
continuation of our research in exploring the novel methods for the produc-
tion of alcohols from respective O-ally ethers,8 we observed that LiCl/NaBH4
(1 eq.) reagent system in THF, cleaves both aromatic as well as aliphatic allyl
ethers smoothly, and chemoselectively under mild reaction conditions in
excellent yields furnishing the corresponding alcohols (Scheme 1).
Scheme 1.
The reagent system can be considered as a general deallylating agent
useful for aliphatic as well as aromatic allyl ethers. The regeneration of
alcohols from O-allyl ethers is visualized on the basis of insitu formation
of LiBH4.10 The added advantage of this reagent lies in its chemoselective
production of alcohols from respective O-allyl ethers in presence of some of
the reducible functionalities such as nitro (entry 5), benzyloxy (entry 7), Boc
(entry 10), and Cbz (entry 11). The obvious advantage of this reagent system
is its low volume requirement of the reagent compared to any other modified
borohydride reagent.
The deallylation can be rationalized by assuming the following cyclic
transition state. Probably the initial coordination of the lithium with
the ethereal oxygen followed by subsequent hydride delivery lead to the
deallylated alcohol (Scheme 2).
Scheme 2.
This method should find extensive application in organic synthesis
where selective deprotection of allyl ethers is often a special requirement.
Generality of the reaction is shown in the table.

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O-ALLYL BY LiCl/NaBH4
135
Table. Regeneration of Alcohols from O-allyl-Ethers Using LiCl/NaBH4
aAll reactions were completed in 30to 45 min period, at 35 ꢀC stirring.
bYields (%) refer to unoptimised, isolated and chromatographically pure products.
TYPICAL EXPERIMENTAL PROCEDURE
Into a two necked round bottom flask equipped with magnetic bead
and nitrogen balloon adapter was placed LiCl (1 eq), dry THF (25 ml) was

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RAJARAM ET AL.
syringed into the flask. The contents were cooled to 0ꢀC, and NaBH4 (1 eq)
was added in portion to the above solution. To the reagent system at 0ꢀC
was added a solution of allylated ether (1 eq) in THF. After complete addi-
tion, ice cooling was removed and contents were brought to room tempera-
ture (35ꢀC). The progress of the reaction was monitored by TLC, which
indicated the disappearance of the allylated ether in 30–45 min period.
Contents were cooled to 0ꢀC and quenched by dropwise addition of
5% aq. HCl solution, the solvent was evaporated under vaccum and the
remaining aqueous layer was extracted with ethyl acetate.
The organic fractions were combined, and washed successively with
brine, water and dried over anhydrous Na2SO4. Evaporation of the solvent
followed by purification of the crude product by column chromatography
using silicagel gave alcohol (isolated yields 70–92%). All the products thus
obtained were characterized by IR, 1H NMR, and Mass spectral data analy-
sis and comparing with authentic samples.
ACKNOWLEDGMENTS
The authors are thankful to Dr. K. V. Raghavan, Director IICT for
providing facilities and KPC is thankful to UGC, New Delhi for the award
of research fellowship.
REFERENCES
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137
b) Purushothama Chary, K.; Hari Mohan, G.; Iyengar, D.S. Chemistry
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Received in the Netherlands February 28, 2001

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