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