Chemistry Letters 2001
1159
Technology (DST), New Delhi for financial support [Grant
No.: SP/S1/G-35/98 to A.T.K]. E. M. and G. B. are grateful to
CSIR, New Delhi for the research fellowships and P. R. Sahu to
DST for the fellowship. The authors are thankful to the
Director, I.I.T. Guwahati for providing general facilities for this
work and also grateful to Prof. Mihir K. Chaudhuri in our
Department for his encouragement.
References and Notes
1
2
3
K. Jarowicki and P. Kocienski, Contemporary Organic
Synthesis, 2, 315 (1995).
A. T. Khan, W. Ahmed, and R. R. Schmidt, Carbohydr.
Res., 280, 277 (1996).
T. W. Greene, P. G. M. Wuts, in “Protective Groups in
Organic Synthesis,” John Wiley and Sons, Inc., New York,
3 rd ed., (1999), pp 329–344 and references therein.
L. F. Fieser and M. Fieser, in “Reagents for Organic
Synthesis,” John Wiley & Sons, New York (1967), Vol. 1,
p 654; L. F. Fieser and M. Fieser, in “Reagents for Organic
Synthesis,” John Wiley & Sons, New York (1969), Vol. 2,
p 182.
4
5
6
a) E. Vedejs and P. L. Fuchs, J. Org. Chem., 36, 366
(1971). b) T. L. Ho, H. C. Ho, and C. M. Wong, J. Chem.
Soc., Chem. Commun., 1972, 791.
A. Kamal, E. Laxman, and P. S. M. M. Reddy, Synlett,
2000, 1476 and references therein.
7
8
G. Mehta and R. Uma, Tetrahedron Lett., 37, 1897 (1996).
a) E. J. Corey and B. W. Erickson, J. Org. Chem., 36, 3553
(1971). b) G. A. Olah, A. K. Mehrotra, and S. C. Narang,
Synthesis, 1982, 151. c) G. S. Bates and J. O’Doherty, J.
Org. Chem. 46, 1745, (1981).
9
a) H. Vilter, Phytochemistry, 23, 1387 (1984). b) A. Butler
and J. V. Walker, Chem. Rev., 93, 1937 (1993).
10 U. Bora, G. Bose, M. K. Chaudhuri, S. S. Dhar, R.
Gopinath, A. T. Khan, and B. Patel, Org. Letters, 2, 247
(2000).
This method has been further extended for the preparation of
open chain aldehydic compounds 2p and 2q from compound 1p
and 1q, respectively. The probable mechanism for the cleavage
of thioacetals can be explained as follows. Vanadium pentoxide
reacts with H2O2 to generate reactive peroxovanadate (V) inter-
mediates,13 which oxidize bromide (Br–) to the Br+. The reactive
11 A typical deprotection procedure: Into a stirred solution of
vanadium pentoxide (0.018 g, 0.1 mmol) in water (1.0 mL)
is added 30% hydrogen peroxide solution (0.6 mL, 5
mmol) at 0–5 °C and stirring is continued. The color of the
solution is changed to the clear dark brown-red after 25
min, and ammonium bromide (0.1 g, 1.0 mmol) is added
into it. After 10 min of stirring at the same temp, the sub-
strate 4-nitrobenzaldehyde diethyldithioacetal (1e) (0.257
g, 1 mmol) is added slowly by dissolving in dichloro-
methane (5 mL). Stirring is continued further and 50%
conversion is over within 30 min as monitored by TLC.
Then further 0.6 mL (5 mmol) of H2O2 is added and the
reaction is completed within another 55 min. The reaction
mixture is finally extracted with CH2Cl2 (20 mL × 2) and
the organic layers are dried over anhydrous Na2SO4. The
organic extract is concentrated in vacuo to give crude prod-
uct, which is finally purified by column chromatography
on silica gel (eluent: hexane/EtOAc, 9:1). The product 2e
is obtained as light yellowish crystalline solid 0.145 g
(96%), m.p. 105–106 °C (lit. 104–106 °C).
–
bromonium ion can undergo further oxidation to Br2 or Br3 ,
which might exist in solution. Then the reactive species Br+
reacts with dithioacetals to form bromosulfonium complex,
which is finally hydrolyzed by water to the parent carbonyl com-
pound.
In conclusion, we have devised a simple and useful catalyt-
ic method for the regeneration of parent carbonyl compounds
from their corresponding dithioacetals or ketals chemoselec-
tively using ammonium bromide, V2O5 and H2O2 under very
mild conditions. It is significant to note that neither olefin and
allylic position, nor aromatic ring is brominated under the
experimental conditions. Due to its operational simplicity, gen-
erality, efficacy, and cost effective, this method is expected to
have much wider applicability for the cleavage of thioacetals
chemoselectively. A similar deprotection reaction also might
be possible by using other alkali metal bromide, which is under
investigation.
12 G. A. Olah, S. C. Narang, and G. F. Salen, Synthesis, 1980,
657.
13 M. J. Claugue and A. Butler, J. Am. Chem. Soc., 117, 3475
(1995).
The authors acknowledge the Department of Science and