text:  Once the cyclization has occurred , an oxyallyl cation is formed . As discussed extensively above , the typical course for this intermediate is elimination followed by enolate tautomerization . However , these two steps can be interrupted by various nucleophiles and electrophiles , respectively . Oxyallyl cation trapping has been developed extensively by Fredrick G. West of the University of Alberta and his review covers the field . The oxyallyl 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 pentacyclic core in one step with complete diastereoselectivity . 
  