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