Ethyl (nonafluorobutyl)sulfonylacetate (2) reacts stereoselectively wi
th substituted benzaldehydes 3 affording E)-3-aryl-2(nonafluorobutyl)s
ulfonyl-2-propenoates (E)-4, which are good dienophiles. The addition
to cyclopentadiene yields exclusively the exo-products 8.