 As noted above , variants that deviate from this template are known ; what <unk> a <unk> <unk> in particular is the generation of the <unk> cation followed by <unk> ring closure to an <unk> cation . In order to achieve this transformation , the molecule must be in the s @-@ trans / s @-@ trans conformation , placing the vinyl groups in an appropriate orientation . The <unk> of the system to enter this conformation dramatically influences reaction rate , with α @-@ substituted substrates having an increased population of the <unk> <unk> due to <unk> strain . <unk> of an electron <unk> α @-@ <unk> by the catalyst can likewise increase the reaction rate by enforcing this conformation . 
