text:  As noted above , variants that deviate from this template are known ; what designates 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 propensity of the system to enter this conformation dramatically influences reaction rate , with α @-@ substituted substrates having an increased population of the requisite conformer due to <unk> strain . Coordination of an electron donating α @-@ substituent by the catalyst can likewise increase the reaction rate by enforcing this conformation . 
  