Formation of 2,2-dimethylchroman-4-ones during the photoinduced rearrangement of some aryl 3-methyl-2-butenoate esters. A mechanistic insight

A series of (hetero)aryl 3-methy-2-butenoate esters upon irradiation undergo a photo-Fries rearrangement leading to the expected ortho- and para-regioisomers and phenols. Consecutively, the ortho-regioisomers under photochemical conditions followed by a thermal 6π-electrocyclization and/or thermal (intramolecular oxa-Michael addition) conditions cyclizise to give the corresponding 2,2-dimethylchroman-4-one derivatives depending on the proticity of the solvent. The photo-Fries reaction occurs from the singlet excited state and provides the preparation in situ of o-hydroxyphenone synthons that are key intermediates in the synthesis of chroman-4-ones.