Sadao Arai, Masanori Ishikura, Kiyoshi Sato, Takamichi Yamagishi. Synthesis of new azonia-helicenes with a quaternary nitrogen at the inner helix skeleton. Journal of Heterocyclic Chemistry,1995, 32, 3, 1081–1083.
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http://dx.doi.org/10.1002/jhet.5570320370