 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 nucleophiles and electrophiles , respectively . <unk> cation trapping has been developed extensively by Fredrick G. West of the University of Alberta and his review covers the field . The <unk> cation can be trapped with heteroatom and carbon nucleophiles and can also undergo cationic cycloadditions with various tethered partners . Shown below is a cascade reaction in which successive cation trapping generates a <unk> core in one step with complete diastereoselectivity . 
