Photochemical transformations, 85 - [2.2.2.2]/[2.1.1.1]pagodanes and [1.1.1.1]/[2.2.1.1]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 a pplied to the construction of the homologous [2.2.2.2] and [2.1.1.1] skelet ons (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-photoc ycloaddition (27a --> 36) - did not take place. A bypass involving a stereo electronically less demanding [2+2]ene/ene-photocycloaddition proved reward ing when double Birch reduction was achieved with the benzo/benzo substrate s (26a and b), giving 28a and b. Domino-type [4+2]/[4+2] cycloadditions to the thermally rather labile "benzene-cyclodimers" [33a and b, E-a(33a) = 23 .9 +/- 1.5 kcal mol-l] 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 deri ved (iso)pagodadienes [A(A'), B(B')] - the calculated energies (MMP2) are a lso given for the anti-Bredt isomers - using the proven bromine addition/fr agmenting bromine elimination sequence were successful in only one case (5) and only when a nonstandard reaction sequence was used. X-ray structural a nalyses for (iso)pagodanes (3, 5, 6, 52 = 3,10-dibromo 4) and a "pagodadien e" (56, B'(2211)) provided detailed structural information. Attempts to mak e use of the new "benzene-cyclodimer" 33a - differing from the structurally closely related isomer (Ell) in its response to dienophiles - for the cons truction of an annelated [6]prismane remained fruitless. A regiospecific [6 +6]benzo/benzo photocycloaddition in "janusene" (74) provided access to two fold and fourfold benzoannelated [2.2.2.2]pagodatetraenes (77, 80), via its thermally highly persistent [t(1/2) (160 degreesC) ca. 25 min] "benzene-cy clodimer" (75, ca. 2:1 photoequilibrium). In compounds 77 and 80, the benzo /ene and benzo/benzo combinations, much less proximate yet more parallel th an in "janusene", did not undergo [6+2]/[6+6] photocycloaddition under stan dard conditions.