Reaction of 4-hydroxycoumarin derivatives with activated dimethyl sulphoxide

Abstract

The Swern reaction of 3-alkyl-4-hydroxycoumarins affords in high yield α-chloro-α-alkyl-o-hydroxyacetophenone derivatives, resulting from the halogenodecarbonylation of the pyranone ring. On a model compound, other activators of DMSO (TFAA, P₄O₁₀, DCC, SO₃–pyridine) gave mixtures of methylthiomethyl derivatives, accompanied by a dimeric product in the case of P₄O₁₀. The formation of the halogenated acetophenones and the dimeric product can be rationalized assuming the initial formation of a chromandionyl sulphonium salt, followed by nucleophilic displacement by the chloride counterion or by the unchanged 4-hydroxycoumarin. The resulting 3,3-disubstituted chromandiones are then hydrolytically decarboxylated during the aqueous work-up.