Synthesis and reactions of 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene and 5,6-didehydrodibenzo[a,e]cyclooctene

Syntheses of 5,6,11,12-tetradehydrodibenzo[a,e]-cyclooctene (5) and 5,6-didehydrodibenzo[a,e]cyclooctene (6) by dehydrobromination with potassum t-butoxide of 5,6,11,12-tetrabromo-5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene (9) and 5,6-dibromo-5,6-dihydrodibenzo[a,e]cyclooctene (8), respectively, are described. Compounds 5 and 6 are unusual, essentially planar, conjugated anti-aromatic 8-membered ring systems, the reactions of which have been studied. The diacetylene 5 readily underwent Diels-Alder reaction with 1,3-diphenylisobenzofuran and furan to give the bis-adducts 13 and 14, respectively. [2+2]Cycloaddition between 5 and dichloroketene and subsequent hydrolysis of the gemdichloride yielded 2,3,6,7-dibenzobicyclo [6.2.0]deca-2,6,8-trien-4-yne-9,10-dione (21). The monoacetylene 6 readily underwent Diels-Alder reaction with 1,3-diphenylisobenzofuran and furan to give the mono-adducts 24 (endo and exo isomers) and 25, respectively. 1,3-Dipolar cycloaddition of phenylazide to 6 led to the 1-phenyl-1H-triazole derivative 27, whereas [2+2]cycloaddition between 6 and dichloroketene and hydrolysis yielded 2,3,6,7-dibenzobicyclo[6.2.0]deca-2,4,6,8-tetraene-9,10-dione (30). Spectral, as well as chemical and electrochemical reductions, of 5 and 6 have been studied. For instance, treatment of the monoacetylene 6 with a potassium mirror gave the aromatic 10π-electron dianion 34.