reactions involving supramolecular catalysis through
noncovalent bonding as seen in enzymes. These biomi-
metic reactions can be effectively carried out in water
under neutral conditions without generating any toxic
waste products. Thus, mimicking biochemical conditions
with the reactions being carried out in water will be
superior to chemical selectivity. This background has
prompted us to attempt the deprotection of acetals of
various aromatic aldehydes using CDs in water, as this
is one of the most useful synthetic transformations
(Scheme 1).
The reactions were carried out by dissolving â-cyclo-
dextrin in water at 50 °C followed by the addition of
acetal and stirring at that temperature. The results are
summarized in Table 1. The yields were impressive and
realized between 80 and 90% in most cases. Benzalde-
hyde dimethyl acetal (entry-1) and substituted benzal-
dehyde acetals such as bromo, chloro, methyl, methoxy,
methylene dioxy, hydroxy, and OTBDMS (entries 2-12)
were all converted to the corresponding aldehydes in good
yields in reaction times ranging from 8 to 10 h, whereas
acetals with acetyl, allyloxy, and double-bond conjugation
(entries 13-15) have taken longer reaction times up to
12 h. These reactions can be effectively carried out with
only a catalytic amount of cyclodextrin, i.e., 0.1 mol of
CD per mole of the substrate. These reactions do not take
place in the absence of CD. Cyclodextrin can also be
recovered and reused. This methodology, apart from
having the advantage of neutral conditions and aqueous
medium, is also compatible in the presence of various
other functional groups such as OMe, OTBDMS, OAc,
allyloxy, methylenedioxy, and conjugated double bonds
to acetals. Though these reactions do take place with
acetals of aliphatic aldehydes, the yields are less than
satisfactory. For example, the acetals of octanal and
decanal react very slowly, taking up to 24 h and yielding
products in the range of 20% with the recovery of starting
materials. Here the role of CD appears to be to activate
the acetal group by hydrogen bonding, thereby facilitat-
ing its cleavage in the presence of water. This reaction
also takes place in the presence of R-CD, but â-CD is the
preferred catalyst because of its easy accessibility and
economic viability.
In conclusion, this work demonstrates cyclodextrin to
be a highly efficient catalyst for the first time for the
deprotection of aromatic acetals in water. This straight-
forward methodology using water as the reaction medium
and â-cyclodextrin as the reusable catalyst may find
potential applications in industry.
Exp er im en ta l Section
Ma ter ia ls. Acetals were either purchased commercially or
synthesized as reported in the literature.12
Gen er a l P r oced u r e for Dep r otection . In a typical proce-
dure, â-cyclodextrin (0.1 mmol) was dissolved in 20 mL of water
by heating at 50 °C, and acetal (1 mmol) in methanol (1 mL)
was added slowly; the mixture was stirred at this temperature
for the required length of time (Table 1). The reaction mixture
was cooled to room temperature and extracted with ethyl acetate
(3 × 20 mL). The combined organic extract was washed with
water (2 × 10 mL), dried over anhydrous sodium sulfate, and
concentrated under reduced pressure. The product obtained was
further purified by column chromatography on silica gel (100-
200 mesh) using 9:1 hexane/ethyl acetate. The products were
identified by comparison of their NMR, IR, TLC, and mixed TLC
analysis with those of authentic samples.
Ack n ow led gm en t. We thank Dr. J . S. Yadav for his
interest and CSIR, New Delhi, India, for a fellowship
to M.A.R.
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