for various times. The reaction mixture was cooled, and then
poured into crushed ice and water. Scratching gave the solid which
was filtered off, dried in air and recrystallised from ethanol to give
the coumarins 6–10 (68.0–82.0% yield) in a pure form.
The authors thank the CSIR and the UGC for providing
financial support for this work.
Received, 13th August 1996; Accepted, 25th October 1996
Paper E/6/05672E
cyclization,9 Graebe–Ullmann syntheses10 and heterocyclic
fused quinones.11
Here we report the use of microwaves in a quick and
simple preparation of substituted coumarins. Substituted
phenols and their derivatives were treated with methyl aceto-
acetate in the presence of H2SO4 under microwave irradia-
tion for various times in a common household microwave
oven. The results of a comparative study of the synthesis of
coumarins by classical methods at room temperature,12a–e
heating using a water bath and microwave irradiation are
shown in Table 1. From these results it is clear that the
reaction time is reduced from several hours to only a few
minutes by using microwave irradiation, indicating that
microwaves play an important role in rate enhancement in
the Pechmann reaction.The products are also obtained in
excellent yields.
We also carried out the reaction on a solid support using
K-10 montmorillonite clay under microwave irradiation.
Under these conditions reactions generally proceed with
greater selectivity and under milder conditions than homo-
geneous reactions. When substituted phenols and methyl
acetoacetate supported on K-10 montmorillonite clay were
exposed to microwave irradiation for different times and at
different power levels, the products were obtained in poor
yields.
References
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Experimental
General Procedure.sA mixture of phenols 1–5 (1 mmol) and
methyl acetoacetate (1 mmol, 0.116 g) was taken in a Borosil 250
ml beaker and to it was added conc. H2SO4 (2 ml). The beaker was
covered with a watch glass and exposed to microwave irradiation