Cycloaddition Reactions of the Methano[10]annulene Derivative 9,9-Dichloro-1,4-dihydro-4a,8a-methanonaphthalene

9,9-Dichloro-1,4-dihydro-4a,8a-methanonaphthalene (4) adds the electron-deficient dienophiles 4-phenyl-1,2,4-triazoline-3,5-dione, maleic anhydride and dimethyl acetylenedicarboxylate to the α-face to give adducts (5)-(7) respectively; the addition of the alkyne requires Lewis acid catalysis. Inverse electron-demand addition of 3,6-diphenyl-1,2,4,5-tetrazine to the monoene component of (4) in hydrophilic solvent (ethane-1,2-diol) is thwarted; the CCl2 bridge is ejected and the 1,4-dihydronaphthalene formed is captured to give (ultimately) phthalazine (16). Dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate adds to (4) to give the methanobenzophthalazine (17b).