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5-Fluorocyclopentadiene 2 has been prepared by the reaction of cyclopentadienylthallium and the F+ source, 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate). In their reactions with compound 2, dieneophiles form adducts having exclusively syn orientation.
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MCCLINTON et al. (1992) studied this question.
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