The anionic 5- endo - trig cyclization, a disfavored process according to the well-accepted Baldwin's rules, was facilitated by the introduction of two fluorine atoms at the terminal carbon atom. Extensive ab initio calculations of the reaction paths both by Gaussian 94 and Mulliken successfully revealed the role of two fluorine atoms as a stabilization of both transition states and product by their strong electronegative character.
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Yamazaki et al. (1999) studied this question.