Formation of halogenated cyclopent-2-enone derivatives by interrupted Nazarov cyclizations

Allenyl vinyl ketones were exposed to titanium and indium halides in order to carry out Nazarov cyclizations. Whereas TiI4 and InX3 (X = Cl, Br, and I) led to cyclopentenones in which a halogen was trapped highly regioselectively, the reactions mediated by TiBr4 gave mixtures of brominated isomers. When the allenyl vinyl ketones were treated first with Br2 and then with TiBr4, doubly brominated cyclopentenones resulted, and treatment with I2 followed by TFA gave cyclopentenones bearing both hydroxyl and iodo groups.