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H. Firouzabadi et al.
LETTER
verted to the corresponding 1,3-dioxane with absolute
chemoselectivity in the presence of benzylacetone and ac-
etophenone, respectively (Table 2, entries 5-6).
In conclusion, mild reaction conditions, versatile and
good chemoselectivity, easy workup, and high yields of
the desired products are worthy to be mentioned as the
characteristics of the described methods.
General Procedure for the preparation of diethyl acetals.
To a mixture of carbonyl compound 1 (5 mmol) and
(EtO)3CH (10-12.5 mmol), ZrCl4 (0.05-0.1 mmol) was
added, and the resulting solution was stirred at room tem-
perature. After completion of the reaction (TLC or GC), a
cold aqueous solution of NaOH (10%, 25 mL) was added
and the mixture was extracted with CH2Cl2 (3 × 40 mL).
The organic extracts were washed with water (2 ×15 mL)
and dried over anhydrous Na2SO4. Evaporation of the sol-
vent under reduced pressure gave almost pure product(s).
Further purification was proceeded by vacuum distillation
to afford pure diethyl acetals 2 in good to excellent yields
(Table 1).
General procedure for the preparation of 1,3-dioxanes
via in-situ transacetalization; To a solution of carbonyl
compound 1 (5 mmol), 1,3-propanediol (7.5-10 mmol),
and (EtO)3CH (5 mmol) in dry CH2Cl2 (15 mL), ZrCl4
(0.05-0.15 mmol) was added and the resulting solution
was stirred at room temperature. After completion of the
reaction (TLC or GC), the reaction was quenched with a
cold aqueous solution of NaOH (10%, 25 mL) and the or-
ganic layer was separated and the residue was extracted
with CH2Cl2 (3 × 40 mL). The organic extracts were com-
bined together and washed with water (3 × 35 mL), and
dried over anhydrous Na2SO4. Evaporation of the solvent
under reduced pressure gave almost pure 1,3-dioxane 3.
Further purification was achieved by vacuum distillation
or recrystallization from the appropriate solvent to give
pure product(s) in good to excellent yields (Table 1).
Acknowledgement
We thank Shiraz University Research Council for support of this
work.
References
(1) a) Greene, T. W.; Wuts, P. G. M. Protective Groups in Orga-
nic Synthesis, 2nd ed.; Wiley: New York, 1991, pp 175-198. b)
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carried out efficiently in the presence of (EtO)3CH (1 eq),
1,3-propanediol (1.5-2 eq ) and ZrCl4 (2-3 mol%) in excel-
lent yields (Table 1, 3a-3e). Aromatic and aliphatic open
chain ketones as well as cyclic ketones were also convert-
ed to the corresponding 1,3-dioxanes under similar reac-
tion conditions in good to excellent yields (Table 1, 3f-3j).
In order to show the ability of the presented methods for
chemoselective acetalization of carbonyl compounds, we
have performed several competitive acetalization reac-
tions, whose results are demonstrated in Table 2. Benzal-
dehyde undergoes acetalization and transacetalization in
the presence of acetophenone, benzyl acetone and cyclo-
hexanone respectively (Table 2, entries 1-4). On the other
hand, cyclohexanone as a model for cyclic ketones is con-
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Synlett 1999, No. 3, 321–323 ISSN 0936-5214 © Thieme Stuttgart · New York