Stereoselection of sterically unbiased Diels–Alder dienes with spiro conjugation

1,3-Cyclopentadienes bearing a fluorene in spiro geometry are nonsterically biased dienes, which can be used to probe the facial selectivities of Diels–Alder reactions. Anti preference of spiro dienes (2 and 3) bearing 2- and 4-nitro and 2-methoxy substituents as Diels–Alder dienes is in striking contrast to facial preference of the related open-chain 5,5-diphenyl-cyclo-1,3-pentadienes.