Similarly , β @-@ substitution directed inward restricts the s @-@ trans conformation so severely that E @-@ Z isomerization has been shown to occur in advance of cyclization on a wide range of substrates , yielding the trans cyclopentenone regardless of initial configuration . In this way , the Nazarov cyclization is a rare example of a stereoselective pericyclic reaction , whereas most electrocyclizations are stereospecific . The example below uses triethylsilyl hydride to trap the oxyallyl cation so that no elimination occurs . ( See Interrupted cyclizations below )