Abstract: The synthesis of 4-hydroxy-3-phenylcoumarin 3a from phenol1 and diethylphenylmalonate 2 yields the isocoumarin derivative 4 as a byproduct (1-2%). If 3a is heated with 2 or diphenylcarbonate 6 at 300_oC the coumarino-isocoumarin 4 is formed in 83% or 85% yield, respectively.

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