[2.2.2.2]/[2.1.1.1]Pagodanes and [1.1.1.1]/[2.2.1.1]/[2.2.2.2]Isopagodanes: Syntheses, Structures, Reactivities − Benzo/Ene- and Benzo/Benzo-Photocycloadditions

The established route to the [1.1.1.1]/[2.2.1.1]pagodanes (1, 2) has been applied to the construction of the homologous [2.2.2.2] and [2.1.1.1] skeletons (3, 7). Application of this synthetic scheme to the iso[1.1.1.1]/iso[2.2.1.1]/iso[2.2.2.2] structures (4−6) failed though; the crucial [6+6]benzo/benzo photocycloaddition step in the face-to-face benzo/benzo intermediates (26a and b) − in contrast to the corresponding clean [6+2]benzo/ene-photocycloaddition (27a → 36) − did not take place. A bypass involving a stereoelectronically less demanding [2+2]ene/ene-photocycloaddition proved rewarding when double Birch reduction was achieved with the benzo/benzo substrates (26a and b), giving 28a and b. Domino-type [4+2]/[4+2] cycloadditions to the thermally rather labile "benzene-cyclodimers" [33a and b, Ea(33a) = 23.9 ± 1.5 kcal mol−1] allowed the subsequent completion of the isopagodane skeletons in standard manner. The attempts to convert the highly strained, yet thermally highly persistent, (iso)pagodanes (3−7) into one of the derived (iso)pagodadienes [A(A′), B(B′)] − the calculated energies (MMP2) are also given for the anti-Bredt isomers − using the proven bromine addition/fragmenting bromine elimination sequence were successful in only one case (5) and only when a nonstandard reaction sequence was used. X-ray structural analyses for (iso)pagodanes (3, 5, 6, 52 ≡ 3,10-dibromo 4) and a "pagodadiene" (56, B′2211) provided detailed structural information. Attempts to make use of the new "benzene-cyclodimer" 33a − differing from the structurally closely related isomer (E11) in its response to dienophiles − for the construction of an annelated [6]prismane remained fruitless. A regiospecific [6+6]benzo/benzo photocycloaddition in "janusene" (74) provided access to twofold and fourfold benzoannelated [2.2.2.2]pagodatetraenes (77, 80), via its thermally highly persistent [t1/2 (160 °C) ca. 25 min] "benzene-cyclodimer" (75, ca. 2:1 photoequilibrium). In compounds 77 and 80, the benzo/ene and benzo/benzo combinations, much less proximate yet more parallel than in "janusene", did not undergo [6+2]/[6+6] photocycloaddition under standard conditions.