Polyfluorobicyclo[2,2,0]hex-2-enes as dienophiles; the effect of substituents on the double bond

Perfluorobicyclo[2,2,0]hex-2-ene (1) is a reactive dienophile and substitution of the vinylic fluorine atoms in (1) by hydrogen to give (3) increases the dienophilic reactivity towards furan; however, the dimethyl-, monomethoxy-, and dimethoxy-derivatives (2), (6), and (4) display no dienophilic reactivity towards furan.