Abstract
A mild and efficient method has been developed for the chemoselective
synthesis of geminal diacetates (acylals)
from aldehydes using acetic anhydride in the presence of a catalytic
amount of ceric ammonium nitrate in excellent yield. Ketones are
found to be unaffected under the reaction conditions. The deprotections
of acylals by using water and ceric ammonium nitrate have also been achieved.
Key words
acylals - aldehydes - chemoselective - ceric
ammonium nitrate
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General Procedure
for Acylation : A solution of the aldehyde (1.13 mmol) in distilled
acetic anhydride (2.26 mmol) was stirred with CAN (10 mol%)
at r.t. under N2 . The progress of the reaction was monitored
by TLC. After completion of the reaction, the mixture was quenched
with sat. aq NaHCO3 solution (4 mL) and was extracted
with ether (3 × 25 mL). The combined organic layer was successively
washed with sat. aq NaHCO3 solution (3 × 10 mL),
water (15 mL) and brine (20 mL) and then dried (Na2 SO4 ).
The solvent was removed under reduced pressure and the residue obtained
that was column chromatographed over silica gel to obtain the pure
acylal.
14
General Procedure
for Deprotection : A mixture of the acylal (1.0 mmol), CAN (10
mol%), water (2 mL) in acetonitrile (2 mL) was stirred
at 70 °C under N2 . The progress of
the reaction was monitored by TLC. After completion of the reaction,
the mixture was diluted with brine (10 mL), extractited with ether
(3 × 20 mL). The combined ether extract
was washed with sat. aq NaHCO3 solution (3 × 10
mL) and dried (Na2 SO4 ). Solvent was removed
under reduced pressure and the residue obtained was column chromatographed
over silica gel to obtain the pure aldehyde.
15 All the acylals prepared and the aldehydes
obtained by the deprotection of the alylals were fully characterised
by 1 H NMR and IR study and comparing the spectral
data with those of authentic samples.