 Once the <unk> has occurred , an <unk> cation is formed . As discussed extensively above , the typical course for this intermediate is elimination followed by <unk> <unk> . However , these two steps can be interrupted by various <unk> and <unk> , respectively . <unk> cation trapping has been developed extensively by <unk> G. West of the University of Alberta and his review covers the field . The <unk> cation can be trapped with <unk> and carbon <unk> and can also undergo <unk> <unk> with various <unk> partners . <unk> below is a cascade reaction in which successive cation trapping generates a <unk> core in one step with complete diastereoselectivity . 
