An expedient and efficient method for the cleavage of dithioacetals to the corresponding carbonyl compounds using organic ammonium tribromide (OATB)

Article abstract of DOI:10.1055/s-2001-14579

A variety of dithioacetals of aldehydes or ketones 1 can be easily cleaved into the parent carbonyl compounds 2 at 0-5°C in very high yields by employing organic ammonium tribromides such as cetyltrimethyl-ammonium tribromide (CetTMATB) or tetrabutylammonium tribromide (TBATB) in dichloromethane.

Full text of DOI:10.1055/s-2001-14579

LETTER
785
An Expedient and Efficient Method for the Cleavage of Dithioacetals to the
Corresponding Carbonyl Compounds Using Organic Ammonium Tribromide
(OATB)
Ejabul Mondal, Gopal Bose, Abu T. Khan*
Department of Chemistry, Indian Institute of Technology, North Guwahati, Guwahati-781039, Assam, India
Fax +91 361 690762; E-mail: atk@iitg.ernet.in; atk@postmark.net
Received 22 March 2001
Dedicated to Professor U. R. Ghatak on the occasion of his 70th birthday
Table Cleavage of Various Dithioacetals by Organic Ammonium
Tribromides
Abstract: A variety of dithioacetals of aldehydes or ketones 1 can
be easily cleaved into the parent carbonyl compounds 2 at 0 5 °C
in very high yields by employing organic ammonium tribromides
such as cetyltrimethyl-ammonium tribromide (CetTMATB) or tet-
rabutylammonium tribromide (TBATB) in dichloromethane.
Key words: dithioacetals, deprotection, organic ammonium tribro-
mide, cetyltrimethylammonium tribromide, tetrabutylammonium
tribromide
Dithioacetal is well known as a protecting group1 as well
as an excellent acyl carbanion equivalent in organic syn-
thesis.2 Out of the various dithioacetals, diethyldithioace-
tal is extensively used as a protecting group in
carbohydrate chemistry to prepare acyclic derivatives of
monosacaccharides.3 Although a large number of methods
have been developed for the regeneration of the carbonyl
compounds from the corresponding dithioacetals, 1,4 there
still is a need to find better alternatives. Unmasking of the
carbonyl group sometimes requires long reaction times5
or toxic reagents such as mercury salts.6 Recently we have
reported7 the synthesis of various organic ammonium tri-
bromides (OATB) and their application for useful bromi-
nation reactions. The organic ammonium tribromide,
namely TBATB is also used for oxidative bromination.8
We have realized that all other organic ammonium tribro-
mides might be useful for other oxidation purposes.
In this communication, we wish to report organic ammo-
nium tribromides (OATB) are useful reagents for oxida-
tive cleavage of dithioacetals (Scheme).
a Products have been characterized by measuring IR, 1H NMR, mass
Scheme
spectra and elemental analyses of the samples. bIsolated yield.
The starting dithioacetals are usually prepared by the re-
action of the parent carbonyl compounds with ethanethiol
or 1,3-propanedithiol in the presence of catalytic boron
trifluoride etherate9 or concd hydrochloric acid, using
Synlett 2001 No. 6, 785–786 ISSN 0936-5214 © Thieme Stuttgart · New York

786
E. Mondal et al.
LETTER
literature procedures. The deprotection step is achieved10
by treating the protected compounds 1 with cetyltrimeth-
ylammonium tribromide (CetTMATB, Method A) or tet-
rabutylammonium tribromide (TBATB, Method B) in
dichloromethane at 0 5 °C. The reaction is completed
within a short period. The results are summarized in the
Table. The products are compared with the authentic com-
pound by co-IR, mix melting point as well as character-
(2) a) Corey, E. J.; Seebach, D. Angew. Chem. 1965, 77, 1134.
b) Metzner, P.; Thuillier, A. Sulfur Reagents in Organic
Synthesis, Academic Press, New York, 1994, pp. 12-19.
(3) a) Khan, A. T.; Sharma, P.; Schmidt R. R. J. Carbohydr.
Chem. 1995, 14, 1353. b) Khan, A. T.; Ahmed, W.; Schmidt,
R. R. Carbohydr. Res. 1996, 280, 277.
(4) a) March, J. Advanced Organic Chemistry, John Wiley and
Sons Inc., New York, 1992, p 375 and references therein.
(5) Olah, G. A; Mehrotra, A. K.; Narang, S.C. Synthesis 1982,
151.
(6) a) Wolfrom, M. L.; Weisblat, D. I.; Zophy, W. H.; Waisbrot,
S. W. J. Am. Chem. Soc. 1941, 63, 201. b) Kozikowski, A. P.;
Scripko, J. G. J. Am. Chem Soc. 1984, 106, 353.
(7) Chaudhuri, M. K.; Khan, A. T.; Patel, B. K.; Dey, D.;
Kharmawaphlang, W.; Lakshmiprabha, T. R.; Mandal, G. C.
Tetrahedron Lett. 1998, 39, 8163.
1
ized by H NMR, mass spectra and elemental analyses.
Interestingly, we have not observed any aromatic ring
bromination or any other bromination by this procedure.
In conclusion, we have devised a simple and useful meth-
od for the regeneration of parent carbonyl compounds
from the corresponding dithioacetals by employing cetyl-
trimethyl-ammonium tribromide (CetTMATB) or tet-
rabutylammonium tribromide (TBATB) under very mild
reaction conditions. Due to its operational simplicity, gen-
erality and efficacy, this method is expected to have wider
applicability for the cleavage of dithioacetals. A similar
cleavage might also be possible by using other organic
ammonium tribromides as well as other dithioacetals,
which is under investigation.
(8) Berthelot, J.; Fournier, F.; Fournier; M. Synth. Commun.
1985, 15, 213.
(9) Grobel, B. T.; Seebach, D. Synthesis 1977, 357.
(10) A typical procedure - Method A: To a well stirred solution
of diethyldithioacetal 2a (0.073 g, 0.3 mmol) in CH2Cl2 (7
mL) at 0-5 °C was added 0.157 g (0.3 mmol) of cetyltrimethyl
ammonium tribromide (CetTMATB) and stirring was
continued at the same temperature. The reaction was
completed within 5 min as monitored by TLC. The reaction
mixture was quenched by adding a 5% solution of sodium
metabisulfite (1 10 mL), and finally extracted with CH2Cl2
(2 15 mL). The organic layers were washed with water
(2 20 mL) and dried over anhydrous Na2SO4. Evaporation of
solvent gave the crude product, which was finally purified by
column chromatography on silica gel (eluent: hexane/EtOAc,
9:1). The product was obtained as colourless liquid 0.038 g
(93%).
Acknowledgement
We are grateful to Professor Mihir K. Chaudhuri in our department
for providing the method for the preparation of the reagents as well
as for his encouragement. The authors acknowledge the financial
support from Council of Scientific and Industrial Research, New
Delhi [Grant No. 01(1541)/98/EMR- II to A.T.K]. The authors are
also thankful to the Director, I.I.T. Guwahati for providing general
facilities for this work.
Method B: To a stirred solution of diethyldithioacetal 2a (0.2
g, 0.83 mmol) in CH2Cl2 (7 mL) at 0-5 °C was added 0.4 g
(0.83 mmol) of tetrabutylammonium tribromide (TBATB).
The reaction was completed within 30 min as monitored by
TLC. Usual work up and purification procedures were
followed. The product was obtained as colourless liquid 0.106
g (94%).
References and Notes
(1) a) Greene, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis, John Wiley and Sons Inc., New York, 2nd
edn.1991, pp.198-207. b) Metzner, P.; Thuillier, A. Sulfur
Reagents in Organic Synthesis, Academic Press, New York,
1994, pp. 12-19. c) Kocienski, P. J. Protecting Groups,
George Thieme Verlag, Stuttgart, 1994.
Article Identifier:
1437-2096,E;2001,0,06,0785,0786,ftx,en;D00101ST.pdf
Synlett 2001, No. 6, 785–786 ISSN 0936-5214 © Thieme Stuttgart · New York