Reaction of 2-hydroxy- or 2-methoxy-substituted benzaldehydes or acetophenones with Meldrum’s acid in the presence of environmentally-friendly catalysts, like natural kaolinitic clays, EPZ10 and EPZG, without solvent using focused microwaves afforded 3-carboxycoumarins in excellent yields.
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Bandgar et al. (1999) studied this question.
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