[3+3] Cyclizations of 1,3-bis(trimethylsilyloxy)-1,3-butadienes—a new approach to diverse CF3-substituted fluorenes, dibenzofurans, 9,10-dihydrophenanthrenes and 6H-benzo[c]chromenes

Trifluoromethyl-substituted fluorenes, dibenzofurans, 9,10-dihydrophenanthrenes and 6H-benzo[c]chromenes were prepared by formal [3+3] cyclizations of 1,3-bis(trimethylsilyloxy)-1,3-butadienes. The reactions proceeded with very good regioselectivity. The product distribution depends on the type of 1,3-dielectrophile employed and can be explained by electronic reasons.