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[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
Author(s) -
Wollenweber Markus,
Etzkorn Markus,
Reinbold Jens,
Wahl Fabian,
Voss Torsten,
Melder JohannPeter,
Grund Clemens,
Pinkos Rolf,
Hunkler Dieter,
Keller Manfred,
Wörth Jürgen,
Knothe Lothar,
Prinzbach Horst
Publication year - 2000
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200012)2000:23<3855::aid-ejoc3855>3.0.co;2-a
Subject(s) - chemistry , bromine , stereochemistry , ene reaction , medicinal chemistry , organic chemistry
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 , E a ( 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 ( E 11 ) 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 [t 1/2 (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.