Abstract
The di-π-methane photoisomerization of dimethyl 9-acetoxy- and 9- hydroxy-9,10-dihydro-9,10-ethenoanthracene-11,12-dicarboxylate has been investigated. The 9-acetoxy compound (28) rearranges regiospecifically to give dimethyl 4b-acetoxy-4b,8b,8c,8d- tetrahydrodibenzo[a,f]cyclopropa[cd]pentalene-8b,8d-dicarhoxylate (29), i.e. a dibenzosemibullvalene. Rearrangement of the 9-hydroxy compound (16) gives one of two keto diesters (17) and (18) depending on the solvent used. The keto diesters have been shown to be indeno[1,2- alinden-9-ones epimeric at C 10 and are probably formed by further rearrangement of one of the expected hydroxydibenzosemibullvalenes (31). The regiospecific rearrangement of the 9-acetoxy and 9-hydroxy compounds (28) and (16) by alternative pathways is explained in terms of the effect of hydrogen bonding and the electronegativity of the 9- substituents upon the reaction profile.
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