Novel synthesis of 4,5-diarylphenanthrenes via C2-C6 cyclization of benzannulated enyne-allenes

A new synthetic pathway to the 4,5-diarylphenanthrenes 8 having a helical twist in their structures was developed. The synthetic sequence involves condensation of the diketone 5 with 2 equiv of the lithium acetylides derived from the diacetylenes 4 followed by protonation to produce the propargylic alcohols 6. Reduction of 6 with triethylsilane in the presence of trifluoroacetic acid furnished the tetraacetylenic hydrocarbons 7 in nearly quantitative yields. Treatment of 7 with potassium tertbutoxide under refluxing toluene at 110 degreesC for up to 10 h then furnished the 4,5-diarylphenanthrenes 8. Apparently, the transformation from 7 to 8 involves initial prototropic isomerizations to form the benzannulated enyne-allenes 9. Two subsequent formal intramolecular Diels-Alder reactions via the biradicals 10 and 12 derived from the C-2-C-6 cyclizations then led to 13, which in turn underwent tautomerizations to give 8. The structure of Sa was established by the X-ray structure analysis, showing that the two phenyl substituents are bent away from each other and the central aromatic system is severely distorted with a helical twist. The existence of a helical twist in 8 imposed by the aryl groups at the 4- and 5-positions was also revealed with a set of AB IH NMR signals for the diastereotopic methylene hydrogens on the five-membered rings.