THE INFLUENCE OF THE AXIAL CHIRALITY OF DIBENZO[A,C] CYCLOOCTENE ON THE CONFIGURATION OF ITS PHOTO-DIELS-ALDER ADDUCTS

Triazole-diones and naphthoquinones are shown to add in a photochemica l [4 + 2] reaction to the strongly twisted title diene 1. With 1,4-nap hthoquinone, the process is also accompanied by a [2 + 2] cycloadditio n. When the pure atropisomer (-)-1 is irradiated in presence of 2,3-di chloro-1,4-naphthoquinone (9), the axial chirality of the diene is pre served. Moreover, it is found to exert complete control over the chira lity induced in the resulting spiro-dihydropyrane 10. Absolute configu rations are determined by X-ray crystallography. In absence of a photo -dienophile, the axially chiral, dextrorotatory 6-phenyldibenzo[a,c]cy clooctene ((+)-11) undergoes a stereospecific electrocyclization to gi ve levorotatory 4b,6a-dihydro-5-phenylcylobuta[l]phenanthrene ((-)-13) . Thus, only one out of two possible, disrotatory modes of ring closur e is preferred.