C.-W. L u¨ et al.
lowest yield was observed (Table 3, entries 5, 6). High yields were obtained when
the strong electron withdrawing substituents were present in the meta- and para-
positions on the ring of aromatic aldehydes. For example, 3-nitrobenzaldehyde and
4
-nitrobenzaldehyde gave the best results (Table 3, entries 13, 14). On the other
hand, when employing ortho-substituted aromatic aldehyde as substrate, the steric
hindrance was dominant than other factors in the product formation. Poor yields or
no products were obtained for the ortho-substituted benzaldehydes, such as
2
-chlorobenzaldehyde, 2-bromobenzaldehyde, and 2-nitrobenzaldehyde. Unexpect-
edly, 2-tolualdehyde and 2-methoxybenzaldehyde gave 78 and 92 % yields in this
three-component reaction (Table 3, entries 4, 7). Unfortunately, aliphatic aldehyde
had a low reactivity in this reaction (Table 3, entry 15). Finally, when 1-methylin-
dole was used as starting material, a lower yield than indole was obtained (Table 3,
entries 16, 17).
Conclusions
A glycerol and water-mediated procedure for the synthesis of 5-[(indol-3-yl)-
arylmethyl]-2,2-dimethyl-1,3-dioxane-4,6-dione derivatives from the condensation
of indole with Meldrum’s acid and aldehyde under ultrasonic irradiation has been
developed. This green synthetic pathway has many advantages, such as using
inexpensive green solvents, catalyst-free, mild reaction conditions and tolerant the
substrates with diverse functional groups.
Acknowledgments We are grateful for financial support from the National Natural Science Foundation
of China (21403100, 21576128) and Liaoning Province, and the Doctoral Scientific Research Foundation
(20141100).
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