Synthesis of 2‑Phospha[7]helicene, a Helicene with a Terminal Phosphinine Ring

A synthetic strategy toward phosphahelicenes
containing a terminal
phosphinine ring has been explored. The 4-phenyl-6-methyl-2-phospha[7]­helicene
was prepared from starting 2-bromobenzo­[c]­phenanthrene
in 12% overall yield in 12 steps. The synthetic approach involves
introduction of the phosphorus function prior to photocyclization
forming the final helicene skeleton, followed by the formation of
a phosphorus hexacycle. The structure of the first phosphahelicene
with a terminal phosphinine ring was confirmed by X-ray crystallography.