Synthesis of a Three-Bladed Propeller-Shaped Triple [5]Helicene.

Three-bladed propeller-shaped triple [5]helicene was synthesized using eliminative and oxidative photocyclization reactions, which proceeded in 37 and 63% yields, respectively. Chromatographic purification gave a mixture of diastereomers, and the PPM and PMM isomers were gradually converted to the thermodynamically more stable PPP and MMM isomers at room temperature. The activation parameters for the racemization of the PPP and MMM isomers were determined, and the structure of the triple [5]helicene was determined by X-ray crystallographic analysis.