The Synthesis of 5,7-Dichlorocoumaran-3-one and Related Problems

Abstract

5,7-Dichlorocoumaran-3-one has been prepared from methyl 2-hydroxy-3,5-dichlorobenzoate by a three stage synthesis. It could not be prepared by (i) intramolecular cyclization of 2,4-dichlorophenoxy- acetic acid or 2,4-dichlorophenoxyacetyl chloride or (ii) by the reaction of ω-bromo-2-hydroxy-3,5-dichloroacetophenone with bases. The reaction of 2,4-dichlorophenoxyacetic acid with phosphorus pentoxide in benzene gave 2,4-dichlorophenol, 2',4'-dichlorophenyl 2,4-dichlorophenoxyacetate, and diphenylmethane. By the use of ¹⁴C-labelled acids the methylene carbon atom of the diphenylmethane was shown to be derived from the methylene carbon atom of 2,4-dichlorophenoxyacetic acid. A possible mechanism is proposed for this reaction.