Tricyclo[2,2,2,0 2,6 ]octan und Tricyclo[2,2,2,0 2,6 ]oct‐7‐en Konjugation in Cyclopropyl‐äthylenen. Bicyclo[2. 2. 2]octan‐Reihe, 6. Mitteilung

3,5‐Dibromo‐tricyclo[2.2.2.0 2,6 ]octane (3), which is obtained by homoconjugative addition of bromine to bicyclo[2.2.2]octa‐2.5‐diene ( 1 ), is shown by NMR.‐spectroscopy to be a mixture containing ca. 72% of the trans stereoisomer 13 and ca. 28% of a cis stereoisomer, presumably the exo‐cis from 11 . Upon treatment with zinc in dimethylformamide these tricyclic dibromides 3 revert to the diene 1 . Hydrogenation over nickel produces bicyclo[2.2.2]octane ( 15 ) and a small amount of tricyclo‐[2.2.2.0 2,6 ]octane ( 4 ), which has been synthesized from bicyclo[2.2.2]octan‐2‐one ( 16 ) by a Bamford‐Stevens reaction. In addition to the diene 1 , Cope‐elimination of 2‐(dimethylamino‐N‐oxido)‐bicyclo[2.2.2]‐oct‐5‐ene ( 9 ) produces small amounts of the abnormal 1,3‐elimination product tricyclo[2.2.2.0 2,6 ]oct‐7‐ene ( 5 ). The latter compound has been prepared in high yield by a Bamford‐Stevens reaction on bicyclo[2.2.2]oct‐2‐en‐5‐one ( 23 ). In tricyclo[2.2.2.0 2,6 ]oct‐7‐ene ( 5 ) the plane of the double bond symmetrically bisects the cyclopropane ring. Strong conjugation of the cyclopropyl‐ethylene system is indicated by the UV.‐absorption maximum at 206 mμ (log ϵ = 3.71), which shows a bathochromic shift of 23 mμ with respect to cyclohexene.