Aryl acetoacetates, Pechmann coumarin

Aryl acetoacetates, which may be obtained by the reaction of phenols with diketene, are cyclized on treatment with sulfuric acid (54JCS854). The yields of coumarins are similar to those obtained by a Pechmann reaction on the phenol. 

In 1954, Lacey demonstrated that aryl acetoacetates 4 prepared from the condensation of a range of phenols with diketene could be cyclized in warm sulfuric acid to the corresponding coumarins (3) in yields comparable to those obtained from the corresponding phenols under conventional Pechmann condensation conditions, thereby establishing 4 as a viable intermediate in the Pechmann condensation. ... 

The intermediacy of o-hydroxycinnamic acid esters (5) in the Pechmann condensation was first proposed by Pechmann in 1884. In 1932, Robertson and co-workers provided support for this proposal through their examination of the reactions of w-methylanisole and dimethylresorcinol with ethyl acetoacetate under the conditions of the Pechmann condensation. Allowing these anisoles to stand with ethyl acetoacetate in sulfuric acid resulted in the formation of 4,7-dimethylcoumarin and 7-methoxy-4-methylcoumarin, respectively. 3 -(2-Methoxy-4-methylphenyl)-2-butenoic acid could likewise be converted to 4,7-dimethylcoumarin under similar conditions. These observations revealed cinnamate intermediates such as 5 to be viable intermediates in the Pechmann condensation and demonstrated that the formation of aryl acetoacetate intermediates such as 4 is not required for coumarin formation. 

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