[6+6]photocycloadditions in face-to-face benzo/pyridazino substrates - : En route to azapagodanes

In two specifically constructed rigid, proximate, “face-to-face” benzo/pyridazino systems (shortest π,π distances ca. 3 A) photoequilibration with the photo[6+6]cycloadducts has been established by 254-nm irradiation (ratios ca. 2:1). The failure to observe such “benzene/heteroarene” photodimers for differently substituted benzo/pyridazino analogues is related to unfavorable UV absorption characteristics of the respective pair of photoisomers as determined by their acceptor/donor substituents. The highly strained photo[6+6]cycloadducts are sufficiently thermally persistent toenable the addition of standard dienophiles, thus openingaccess to novel azapagodane-type cage molecules. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)