 As originally described , the <unk> <unk> involves the activation of a <unk> ketone using a stoichiometric Lewis acid or <unk> acid promoter . The key step of the reaction mechanism involves a cationic 4π @-@ <unk> ring closure which forms the <unk> product ( See Mechanism below ) . As the reaction has been developed , variants involving substrates other than <unk> ketones and promoters other than Lewis acids have been subsumed under the name <unk> <unk> provided that they follow a similar mechanistic pathway . 
