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| - Novel Synthesis of 4,5-Diarylphenanthrenes via C2−C6 Cyclizationof Benzannulated Enyne−Allenes
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| - A new synthetic pathway to the 4,5-diarylphenanthrenes 8 having a helical twist in their structureswas developed. The synthetic sequence involves condensation of the diketone 5 with 2 equiv of thelithium acetylides derived from the diacetylenes 4 followed by protonation to produce the propargylicalcohols 6. Reduction of 6 with triethylsilane in the presence of trifluoroacetic acid furnished thetetraacetylenic hydrocarbons 7 in nearly quantitative yields. Treatment of 7 with potassium tert-butoxide under refluxing toluene at 110 °C for up to 10 h then furnished the 4,5-diarylphenanthrenes8. Apparently, the transformation from 7 to 8 involves initial prototropic isomerizations to formthe benzannulated enyne−allenes 9. Two subsequent formal intramolecular Diels−Alder reactionsvia the biradicals 10 and 12 derived from the C2−C6 cyclizations then led to 13, which in turnunderwent tautomerizations to give 8. The structure of 8a was established by the X-ray structureanalysis, showing that the two phenyl substituents are bent away from each other and the centralaromatic system is severely distorted with a helical twist. The existence of a helical twist in 8imposed by the aryl groups at the 4- and 5-positions was also revealed with a set of AB 1H NMRsignals for the diastereotopic methylene hydrogens on the five-membered rings.
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