A Novel Tandem Cyclization of 2,7‐ or 2,8‐Bis‐Unsaturated Esters Mediated by (η2‐Propene)TiX2 (X: Cl or OiPr). A Facile Construction of Bicyclo(3.3.0)octane, ‐(4.3.0)nonane, and ‐(3.1.0) hexane Skeletons.