2-Norbornene 2-Norbornylene 3,6-Endomethylenecyclohexene 498-66-8 Bicyclo[2.2.1]hept-2-ene Bicyclo[2.2.1]heptene NSC22453 Norbornene Norbornylene Norcamphene Norfenchene WLN: L55 A CUTJ